Download sol-gel chemistry of transition metal oxides

Prog. Solid St. Chem. Vol, 18, pp. 250341, 1988
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Copyright © 1989 Pergamon Press plc
SOL-GEL C H E M I S T R Y OF T R A N S I T I O N
METAL OXIDES
J. Livage, M. H e n r y and C. S a n c h e z
Laboratoire de Chimie de la Mati6re Condensde, CNRS (UA 302), Universit6
Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France
i.
The sol-gel
ceramics.
polynterization
processing"
chemistry"
"powder"
process provides
Starting from
is often
route,
Homogeneous
precursor
These
approach to
an
reactions
broadly used to
These processes
occur
describe
offer
the preparation
oxide network is
in
solutions
the synthesis
many advantages
of glasses and
obtained via inorganic
and
the
term "sol-gel
of inorganic oxides
as compared to
by "wet
the conventional
such as :
multi-component
solutions
Temperatures
a new
molecular precursors,
reactions.
methods.
INTRODUCTION
systems
can
be
easily
obtained
by
mixing
the molecular
1,2
required for material processing
can be noticeably
lowered leading to
unusual
glasses or ceramics 3
The
rheological
properties
of
composites by such techniques
This
explains
technological
attention
devoted mainly
Ceramics
why
sols
the
sol-gel
process
the synthesis
has
to the
product,
emphasis which relates
morphology.
present
The
paper
16. Unfortunately,
Therefore,
different
is an
of an
chemistry of transition metal
topic can be quite
tetravalent
cations,
The most
salt
complicated because of the
versatile
the
chemistry
extensively
process
of
the
powder
molecular
transformation,
or a
in
depending
metal organic
JPSSC 18:4-A
259
sol-gel
Two
on whether the precursor
compound.
the first section of
numerous molecular
of
silica
for transition metal oxide precursors.
and even oxo-hydroxides
for the
and condensation
studied in the case of
the pH or the concentration.
precursors
way from
gel formation and
is based on hydroxylation
ions is described
oxides, hydroxides
Composites"
a real mastery of the sol-gel
in the literature
inorganic
depending on the oxidation state,
now
of transition metal oxide gels.
available
routes are usually described
are
Glasses and Glass-
Glasses and
in the sol-gel to material
These reactions have been
aqueous solution
meetings
is the ability to go all the
successively
involved
and applications
much less data is
much scientific and
14,15
The chemistry of the sol-gel process
molecular precursors.
so
Workshop on
chemical reactivity to
reviews
the aggregation phenomena
the physical properties
films or
allowing a better control of the whole process and
materials.
would require an
precursors,
received
Processing of Ceramics,
through Chemistry"
of "tailor-made"
formation of fibers,
Several international
One unique property of the sol-gel process
the molecular precursor
the
the "International
"Ultrastructure
11-13 and "Better Ceramics
allow
last decade.
topic, namely
from Gels" 7-10,
gels
as spinning 4, dip-coating 5 or impregnation 6
during the
to this
or
The
species which
Moreover,
aqueous
this paper.
This
can exist
in the case of non
can be obtained 17,18
synthesis
of oxides are undoubtely
260
J. Livage et al.
metal alkoxides
which are
Hydrolysis
condensation
section.
and
These alkoxides
be handled
with great
modification
metal
as
20
with
understood
properties
metal
alkoxides
such
in
a dry
environment
silicon,
As
a
and
result
are described
They
metal
the
atom's
latter
must
are often stabilized via chemical
ability
property,
to
exhibit
of the
several
coordination
expansion
evolution during the sol to gel and gel to solid transitions
need to be
before
a
real
mastery
of
the
sol-gel
process
can be reached.
of a gel and its response to heat treatment are very sensitive
already created during the sol stage. Therefore
determines
in the second
is due to the lower electronegativity
the
of
and
19
as water
occurs when the metal alkoxide reacts with water.
Structural
fully
transition
reagents
appear to be much more reactive than silicon alkoxides.
care,
states.
spontaneously
of
toward nucleophilic
This high chemical reactivity
compared
coordination
very reactive
the main properties
polymerized,
structures
colloidal particles
21
can
formed
which
aggregation
silica polymers have been extensively
of
range
colloidal
The
structure
aggregates
and its ability for
By varying the chemical conditions
be
The
the formation of colloidal
of the resulting powder
which the powder can be sintered.
to the
often
the extent to
under which silica
is
from randomly branched polymers
SiO 2
particles
and
the growth
studied during the last few years. They usually
to
of
give
rise to very tenuous objects which have very low densities even for large radii of gyration
and can be described as fraetal aggregates
Monodispersed
exhibit anisotropic
anisotropic
transition
22
metal oxide
colloids are
shapes 23. Particle-particle
aggregates
in
which
all
ordered aggregates,
called "tactoids"
lead to anisotropic
coatings
individual
24 will
currently synthesized which can
interactions
then lead to the formation of
particles
be described
that behave as host structures
for intercalation
Sols and gels are usually considered as intermediates
and ceramics.
Therefore,
drying and densification
be fully understood 26-28.
applications
are very important processes
metal oxide-based materials
will be reviewed briefly at the end of the paper.
solid
are actually diphasic
network.
Specific
together with their
antistatic
coatings
electronic
interface have been
or electrochromic
A survey of the literature
processing
and
solvent molecules
properties
observed.
arising
They lead
and
gel route. These
The fourth section shows that
ionic
of glasses
that need to
the properties
obtained via the
materials made of
These
They can
25
in the processing
The present paper does not intend to describe
of transition
metal oxide gels
are mutually oriented.
in the third section.
transition
trapped
in a
from the two phases
to new applications
such as
devices 29
shows that most studies are concerned with the
sol-gel
of silicates 30. Fewer papers have been published about A1203, TiO 2 or ZrO 2
very few papers deal with other transition metal oxides 29. Therefore,
is mainly concerned with transition elements of
described here can be extended
"f" (rare-earths)
the present
the "d" group. However,
to other elements belonging
to
and
article
most of the
ideas
the "p" (B, AI, P . . . .
) or
groups.
2. AQUEOUS CHEMISTRY OF INORGANIC PRECURSORS
The aqueous chemistry of inorganic salts is quite complicated
rence
of
hydrolysis
precipitates.
reactions
The hydrolysis
will start by considering
Bjerrum at the
of
convert
the
ions
salts can involve the
the hydrolysis
beginning of the
Pfeiffer 33 proposed
which
of metal
20th century 31
the concept of
"aquo-acidity"
to
cation,
cations,
new
owing to the
ionic
species
occuror to
the anion or
even both. We
which was first
studied by N.
At the same
time, A. Werner
which describes
cation
32 and P.
hydrolysis
as
Sol-Gel Chemistry of Transition Metal Oxides
261
the stepwise removal of protons from hydrating water molecules.
L.G. Sillen 34 , the
formation of polynuclear
However, until the work
hydrolysis products was
author proposed a mechanism of hydrolysis in which hydroxyl groups are added to the
which leads to the formation of condensed species.
of
almost ignored. This
cation
Iso o and heteropoly oxometalates are now
well known 3S , and detailed experimental data on the hydrolysis of cations can be found
the
literature
18,36
Unfortunately,
it
is
quantitatively on a theoretical basis. However,
difficult
to
account
for
these
a model was recently proposed which
in
data
allows
the calculation of the partial charge distribution of any complex in order to predict their
chemical reactivity.
When
two atoms combine,
a partial electron
transfer occurs so
that
each atom acquires a positive or negative partial charge 6 i . It is usually assumed that the
electronegativity Xi of an atom changes linearly with its charge 38
xi = x~ + ~i~i
:
(1)
where X~ ° is the electronegativity of the
neutral atom and N~ is the "hardness"
which may
be defined as 37.
.~ - k/fT
k
is
a
scale).
constant
that
According
Sanderson 39 ,
to
the
(2)
depends
the
charge
on
the
principle
transfer
electronegativity scale (k = 1.36 for Pauling's
of
electronegativity
should
stop
when
equalization
the
constituent atoms become equal to the mean electronegativity X given by 37
= Zi P i / ~
Zi
where Pi corresponds
(pi/~)
to the stoichiometry
of the i th
atom in the
:
compound and z
Electronegativity actually corresponds
is the
to the electronic
is nothing else than the
chemical potential equalization
partial charge 6 i can be deduced from eq.
in the equilibrium
well-known
state. The
(1),(2) and (3) leading to:
6~ = (~ - x[)Ik~
be easily
all
(3)
chemical potential and electronegativity equalization
6 i can
of
+ kz
total charge of the ionic species.
thermodynamic principle of
stated by R.T.
electronegativities
(4)
calculated knowing
the electronegativity
X~ of
all neutral atoms, the
stoichiometric composition for the ionic species and its charge z. The Partial Charge Model
can be applied to both inorganic and metal-organic precursors.
and is easy to handle.
its
present
ideas
It corresponds to a thermodynamic approach and leads to a relatively
good quantification of inductive effects. However,
In
It is based on simple
form,
the
Partial
Charge
several limitations do arise, namely :
Model does not take into account the real
structure of the chemical species.
- Resonance effects and ~ overlapping are not included.
It is difficult
to account for coordination
variations which occur during
the chemical
process.
Nevertheless,
reactions
involved
this
in
the
model
can
sol-gel
be
process
applied successfully to describe the chemical
and
provides
a useful guide for inorganic
polymerization reactions.
2.1. Hydrolysis of metal cations
2. i.I.
Formation
of
inorganic
precursors.
When
dissolved
in pure water,
becomes solvated by the surrounding water molecules according to :
Mz ÷
+ :0
~ M +-- 0
a cation M z+
262
J. Livage et al.
In the case of transition metal ions,
partially covalent
orbital of the
bond. A
partial charge
water molecule
to
this solvation leads to the formation of
transfer occurs
the empty d
from the
orbitals of the
filled 3a I
transition metal ion.
positive partial charge on the hydrogen atoms
then increases and the water molecule,
whole,
the
becomes
more
following reactions
acidic.
occur
[M
N
be
the
(coordination number).
(-OH), and
number
of
the
an
2.1.2.
The
chromium,
[M(OH2)N] z+
precursor
hydroxo-aquo
[M(OH),] ("'z)"
non complexing aqueous medium
can
be
an
,
it
is
aquo
bound to the cation M z+
an
oxo-hydroxo
[M(OH)h(OH2)N.h ](z'h)+
diagram.
Let
us
consider
When h=0, the
"oxo-ion"
complex
(h<N)
a
or
pre-
[MON](2N'Z)"
[MOx(OH)N.x ](N+x'z)"
an
hydroxo complex
gives
typical
:
transition metal such as
two stable oxidation states, namely Cr(Vl) and Cr(lll).
characterized
[CrO (OH)z]°
rise
to
For Cr(III) however,
h=7
h=8
only
Only three
18 .
h=6
[CrO~'] 2"
oxo-hydroxo
or
oxo
complexes but never to aquo
five precursors have been reported 18 o
]3*
h=0
]2+
h=l
[Cr (OH)2 (OH2)4 ]+
h=2
[Cr(OH)3 (0H2)3 ]0
h=3
[Cr(OH)4 ]"
h=4
[Or(OH) (OH 2)s
consequence,
in aqueous solutions
[CrO] (OH) ]
[Or(OH2)6
a
:
(h=N).
which exhibits
As
covalently
molar ratio of hydrolysis.
for h=2N
either
complex
"charge-pH"
Cr(VI)
a
the
(=0).
molecules
while
Cr(Vl) precursors have been experimentally
complexes.
of the electron transfer,
The rough formula for any inorganic precursor can then be written as
cursor is an "aquo-ion"
(h>N),
in a
oxo ligands
water
[MONH2,.h ](z'h)+, where h is defined as the
If o<h<2N,
magnitude
[M-OH] (z'l)+ + H + = [M=O] (z'2)+ + 2H +
then be considered
( O H 2 ) , hydroxo ligands
Let
on
The
as
:
OH2 ]z+ =
-
Three kinds of ligands must
ligands
Depending
a
bonding
Cr(III)
forms only aquo, aquo-hydroxo
or hydroxo complexes
in
aqueous solutions but never oxo-complexes,
These
observations
qualitative
17,40
way
can
using
be
summed
pH of the aqueous solution.
can
defined
namely
valent
hydroxo
Such a
cations
and/or
whole range of
(z>+5) form
+1
:
Three domains
"aquo"
[MONH2N.h ](z'h)+,
[MON](2N'z)'.
+3
[M(OH2)N] z+ ,
and
"oxo"
diagram shows that
(z<+4)
hydroxo
give
rise
complexes
to
oxo-hydroxo
and/or
7
14 pH
(z=+4) are on
the border line,
lowaquo-
over
pH, while high-valent
the
cations
oxo complexes
over the same range of pH. Tetravalent
0
of
the
"hydroxo"
OH-
as a function
the formal charge z of the cation M z+ and
be
0 2-
a
diagram
as shown in figure i. This diagram gives
the nature of the precursors
+8
up in
a "charge-pH"
cations
and therefore
lead to a large number of possible precursors.
Fig. i : The "charge-pH"
diagram.
Sol-Gel Chemistry of Transition Metal Oxides
2.1.3.
Quantitative
the magnitude
"charge-pH"
diagram
Using the model,
predicted
analysis.
of charge
37
can thus be
acidic
Under
O-H bond arising
The Partial
transfer between
established
or basic
acidic
Charge Model
ligands
reaction
as long
close
the reaction
atom's
as 8(OH)>O
=
order
aquo)
agreement
with
to be considered
to calculate
and cations
M z÷ . A
experimental
in an aqueous
large polarization
~+
S" 6 ÷
M - O - H + H20
This occurs
can be used in
(oxo, hydroxo,
forms of a given cation
conditions
from the metal
in
263
solution
data.
can also be
is the cleavage
of
the
:
M - 0" + H30 +
in the
[MONH2N.p] (z'p)+
precursor,
leading
to the
:
[MONH2N ]z+ + PH20 = [MONH2N.p] (z'p)+
The limiting
condition
6(OH)=0
- mean electronegativity
leads to the following
X = ~(OH)
+ pH30 +
relations
:
= 2.71
z - n&(H)-6(M)
charge conservation
p =
1-8(H)
Partial
charges
~(H) and ~(M) can thus be calculated
p = 1.45z
Relation
hydrolysis
size of the
depends
that the
on
number
the formal
X~ of the metal.
cation M z+ which
three possible
These
(2N-p>2N)
can thus be
: the
taken into account.
function
When applying
to initiate hydrolysis.
the
(5)
any acidic behavior.
X~
2N-p
Couple
4
1,78
-i,I
[Ru04]°
Mn
+7
4
1,63
0,5
[MnO4]'/[MnO3(OH)]°
Cr
+6
4
1,59
2,1
[CrO2(OH)2]°/[CrO(OH)3
V
+5
6
1,56
8,4
Ti
+4
6
1,32
10,2
[TiO(OH2)5]2+/[Ti(OH)(OH2)5]
Zr
+4
8
1,29
15,1
[Zr(OH)(OH2)7]3+/[Zr(OH2)8]
4÷
occurs
for
3+
]+
[VO2(OH2)4]+/[VO(OH)(OH2)4]
Fe
+3
6
1,72
11,2
[Fe(OH)(OH2)5]2+/[Fe(OH2)6]
Mn
+2
6
1,63
12,7
[Mn(OH2)6] 2+
Ag
+i
2
1,68
4,3
[Ag(OH2)e] ÷
1 - Some inorganic
(2N-p<0)
0<p<2N,
: the
are
precursors
[MON](2N'z)"
Cr(VI),
V(V),
solutions.
: under acidic
in equilibrium
where
Ti(IV)
conditions,
case corresponds
E(p)
3÷
example
any basic behavior
is RuO 4 .
two species
corresponding
the whole part
to
of p. Typical
and Fe(III).
___+
as soon as a hydroxyl
to S(OHaq)=-i
A typical
indicates
reaction
of the metal atom
q = i + 1.25z
does not exhibit
conditions,
the limiting
(M-OH)aq
occurs
2+
in their most acid forms.
precursor
by H3 O+ in aqueous
(0<2N-p<2N)
from the low polarization
This reaction
relation
This situation
N
are Mn(VII),
and the
of
:
does not exhibit
Z
Under basic
limiting
i)
spontaneous
number N
are a direct
+8
and h=E(p+l)
arising
the coordination
two parameters
(cf Table
through
Ru
and cannot be protonated
examples
removed
M
ii) p>2N,
h=E(p)
(5)
p
for example.
Table
iii)
protons
[M(OH2)H] z+ precursor
A base such as OH" must be added in order
Ag + and Mn 2+ cations
of
charge z,
last
cases have to be considered
i) p<0,
to :
- 0.45 N - 1 . 0 7 ( 2 . 7 1 - X ~ ) / ~
(5) shows
directly
electronegativity
leading
in the
is the cleavage
of the M-O bond
:
M +aq + OHaq
ion
can be formed through
[MONH2N.q]a (z-q)+
q
- 0.92(2.49-X~)/~
precursor,
solvation.
leading
(6)
The
to 37 :
264
J. Livage et al.
(2N-q) corresponds
to the number of protons
that cannot be removed from the precursor
at very high pH. Two cases can be encountered when applying relation
M
Z
N
X~
2N-q
Couple
Ru
+8
5
1.78
-0.5
[RuO~] 2"
Mn
+7
4
1.63
-i.I
[Mn04]"
Cr
+6
4
1.59
0.2
[CrO~]/[CrO3(OH)]"
V
+5
4
1.56
1.4
[VO3(OH)]2"/[VO2(OH2)]
Ti
Zr
+4
5
1.32
1.29
5.0
[MO(OH)4]2"/[M(OH)5]"
Fe
+3
4
1.72
3.8
[FeO(OH)3]2"/[Fe(OH)4]"
Mn
+2
3
1.63
3.1
[Mn(OH)B]'/[Mn(OH)2(OHz)]
Ag
+I
2
1.68
2.3
[Ag(OH)2]'/[Ag(OH)(OH2)] °
Table 2 - Some inorganic precursors
i) q>2N (2N-q<0)
:
Mn(VII).
ii)
(0<2N-q<2N)
Fe(lll))
2.1.4.
O<q<2N
: twe
at very high pH. These
or hydroxe-aquo
Initiation of
condensation
through two simple mechanisms
i) If
the preferred
condensation
occurs via a
of M is an
oxo-ion
species corresponding
may be exo-hydroxe
complexes
even
2).
°
in their most basic forms.
The most basic form
examples are Ru(VlII),
equilibrium
(6) (cf.Table
[MON](2N'z)'.
to h=E(q)
complexes
Typical
and h-E(q+l)
(V(V), Ti(IV),
are in
Zr(IV) and
(Mn(ll), Ag(1)).
Condensation
reactions.
in
aqueous
solutions
can occur
that can be related to the coordination unsaturation
coordination
substitution
is
:
already fulfilled in the molecular precursor,
reaction.
In this
case an entering group
OX and a
leaving group OY must be present around M :
X
~M - 0 - M
M - OX + M - OY
+ OY
in order to keep the coordination number of the metal unchanged.
ii) If
the preferred
coordination
addition reactions become possible
M
-
OX
An increase of the coordination
In aqueous solutions,
the "charge-pH"
- Oxo-ions
diagram
[MON](2N'z)"
addition
-
+
M
•
three kinds of precursors have to be considered according to
the partial charge on
but very
show no nucleophilic
Other
leaving
precursors
ligands
substitution
coordination
the
hydroxo
precipitates).
As a consequence
Condensation
oxo ligands are
therefore occurs
very
only via
is unsaturated.
molecule
property and act
[MONH2N.h] Cz'h)+
(hydroxo
domain
groups.
positive while the
charge en M is usually strongly
is slightly
positive
only as leaving
positive
(6(H20)>0).
groups.
Aquo
Condensation
(6(M)>>0),
ligands thus
cannot
occur
because no entering group is available.
reactions
sphere.
M is usually slightly
(6(0)<<0).
poor leaving
on the H20
with such precursors
X
~ M - 0 - M - OY
OY
-
number occurs so that no OY group need to be eliminated.
[M(OH2)N] z+ : the partial
while the charge
in the molecular precursor,
(cf. Fig.l).
when the precursor
aquo-ions
not fulfilled
:
partial charge on 0 is strongly negative
good nucleophiles
is
or
can
Following
in
aquo)
thus
are
begin
the
order
:
both
present
as
"charge-pH"
to
This can be done by :
get
nucleophilic
soon
ligands
around the metal.
as
one
hydroxo
diagram this means
condensed
species
(oxo or hydroxo)
Condensation
through
ligand appears
in the
that we must move
(oligomers,
sols,
and
gels
into
or
Sol-Gel Chemistry of Transition Metal Oxides
- adding a base or an oxidizer
to an aquo precursor
265
:
[Fe(OH2)6] 3+ + 3 OH"
~ [Fe(OH)3(OHz)3] ° + 3 HzO
[Mn(OH2)6] 2+ + H202
, [Mn(OH)4(OH2)2 ]° + 2 H + + 2 H20
- adding an acid or a reducing agent to an oxo precursor
[WO412"
2[Mn04]"
+ 2 H3 O+
, [WO2(OH)2(OH2)2] °
+ 3 H202 + 6 H20
- or even v i a thermohydrolysis
[Fe(OH2)6] 3+ + H20
In this case,
hydrolysis
2.2.
the temperature
reaction
Condensation
2.2.1. Mechanism.
hydroxo
or
precursors
=
, 2[Mn(OH)4(OH2)2 ]° + 3 0 Z + 2 OH"
of an aquo precursor
the enthalpy change All
via olation
According
"oi"
to the literature
bridge
M-OH-M.
Such
41,
"olation"
a condensation
leads
process
N. Basically
(SN) in w h i c h M - O H is the nueleophile
~M--OH~
M
~+ ~+
+..~.~M~jOH 2
H
M--O--M
~
9H2
~
M/OH-M
~,~+ (~+ / 0 H-------~ ~+ ~+
H20-- M - *~M-OH2
a nucleophilic
+ H20
2(OH)1
+ H20
3(0H)I
H
.
M
o
M
+ 2 H20 2(0H)2
/oH
H
+ H20
OH
,~_
2( OH )3
H
z+
[M(OH2)N ] z * - - -
[M(OH2)N_I ]
* H20
SN 1
Li* N a*K*RIoCs*
B e 2.
I
I
in 3*
I
I
1
Fig.2.
I
,
2
I
~
3
Ti3*Ni 2+
II
i
4
O l a t i o n mechanisms
H g 2+
Zn 2. Cd 2*
I
V2" Fe3. C J *
i
,r ,r
TI 3*
J
G a 3*
o
Mg2 ÷
I
AI 3+
II
d + F,Z*Mn=*A,g* C~*C,u =* Mn3÷
J
5
,
I I
r ,
6
I
7
I,
8
Several kinds
b r i d g e s will be
cO~
~o~_
H20
,~+
to
M\
~ M ~ .
I
of a
occurs w i t h hydroxo-aquo
and H20 the leaving group.
_
M/
to the formation
it corresponds
of bridges can occur as shown in figure 2 . Following Baran 4 2
I
of the
18
[M(OH)x(OH2)N.x ](z'x)+ where x <
substitution
:
[Fe(OH)(OH2)5] 2+ + H30 +
has to be increased because
is positive
:
I
I log ~(s")
9
and lability of some aquo-ions.
10
symbolized
266
J. Livage et al.
as × (OH)y where x is the number of metal
of bridges
between these
x metal
atoms linked by one "oi" bridge and y
atoms. As
oxygen cannot
form more
the number
than four covalent
bonds, the limiting value for x is 3.
In all
kinetics of
cases an aquo ligand must be removed from the coordination sphere. The
olation therefore
strongly depends
on the
lability of
the M-OH 2 bond. This
lability depends mainly on the charge, size, electronegativity and electronic configuration
of the M atom as shown in figure 2
radius, the
faster the
43,44
M-OH 2 bond
transition elements whose
the smaller the charge and the larger the
"
will be
broken. In
electronic configuration is
d8 (Ni 2+) are kinetically inert
octahedral coordinations
owing to their high
45. For
these elements
addition, it
is well
ionic
known that
d3 (CrB+ ,V2+) ,d6 low spin
(Co3+) or
crystal field stabilization energy
the rate
constant for
in
solvent exchange
ranges typically between 10 .4 and 10 .6 s "I 46
In other cases, olation can be extremely fast especially for low valent precursors
(O~_z-h<2) and is limited only by diffusion (k>10 ? M" I s" I). Rates are much slower for highly
charged
precursors
(z-h~2),
particularly
when
the
size
of
the
cation is small. The
dimerization rate constant k of the Fe 3+ precursors is rather low 47 :
2(0H)I:
H
o}
[Fe(OH)(OH2)5] 2+ + [Fe(OH2)6] 3+ = [(H20)sFe-O-Fe(OH2)5] 5+ + H20 (k=2.5.10 "2 M'Is "I)
2(0H)2: 2[Fe(OH)(OH2)5] 2÷ = [(H20)4Fe
e(OH2) 4
]4+
+ 2 H20
(k = 10"1-103 M'Is "I)
0
while it is much faster for VO 2÷ or Cu 2+ 4?,48
.
H
2(0H)I:
[VO(OH)(OH2)4 ]+ + [VO(OH2)5] 2÷ = [(H20)40V-O-VO(OH2)4] 3+ + H20
2(0H)2:2[VO(OH)(OH2)41+
=
[(H20)30
(k = i M'Is "I )
0(0H2)3 ]2+ + 2 H20
(k = 104 M I s "I)
-OH
2(0H)2 : 2[Cu(OH)(OH2)5 ]+ =
2.2.2. Polycations.
[(H20)4C o,CU(OH2)412÷~
/0~
~
Charged precursors
solid phase. This is mainly due to
(z-h ~I)
+ 2 H20
(k = 108 M'Is "I)
cannot condense
indefinitely to
the fact that the nucleophilic strength of
form a
the hydroxo
group 6(OH) varies during the condensation process. In the typical dimerization reaction of
Cr(lll)
:
2[Cr(OH)(OH2)5] 2+ =
[(H20)4Cr \
~
°I
Cr(OH2)4] 4+ + 2 H20
O
OH groups are negatively charged in the monomer (6(OH)=-0.02) while they become
positively
charged in the dimer (6(OH)~+0.OI). The partial charge of hydroxo groups can change in sign
during the condensation process,
owing to the departure
of donor water molecules.
chemical stand point, this means that OH loses its nucleophilic power in this
compound.
Condensation
is
then
condensed polycations can however
limited
to
dimers
be formed if the
mainly
for
From a
polycationic
entropic reasons. More
nucleophilic strength of the
starting
monomer is higher. As an example, let us consider the dimerization of Ni(ll) species :
2[Ni(OH)(OH2)31÷ =
[(H20)2Ni\
{
i(OH2)212+ + 2 H20
O
6(OH)=-0.07 in the monomer
and °0.03 in the
charged and keeps some nucleophilic
dimer. The hydroxo group
remains
character. Condensation can proceed further
negatively
towards a
Sol-Gel Chemistry of Transition Metal Oxides
tetramer
whose presumed
structure
is shown in figure
2[Ni2(OH)2(OH2)4
The partial
stage
charge
becomes
in agreement
]2+
=
6(0H)=+0.06
with experiments
267
3 (structure
E)
:
[Ni4(OH)4(OH2)4] 4+ + 4 H20
in this tetramer
and condensation
stops
at this
49
A
(A)
[M2 (OH) (OH2)× ]3+
M = Mn 2+, Co 2+
(B)
Ni2+
18
x](2z'2)+
[M 2(OH) 2(OH2)
M = VO 2+, Cr3+,
Fe3+,
(C)
[Cr2(OH)(OH2)I0 ]5+ 50
(D)
[M 4 (OH)6 (OH 2)12 ]6+
M = Cr 3+ 51
(E)
[M4 (OH)4 (OH 2 )4 ]4+
Ti3+,
Cu2+
18
C
D
M = Co 2+ , Ni 2+ 49
(F)
oM ®oH
Figure
OHio
3
[M4 (OH)8 (OH 2 )I 6 ]8+
M = Zr 4+ , Hf 4+ 52,53
Fig.3.
gives
other examples
of transition
Transition
metal
metal polycations,
polycations
show that in each case the partial
charge
positive.
is thus able to explain why condensation
infinite
The Partial
network
hydrolysis
Charge Model
is formed.
on the hydroxo
18'49"53.It
These polycations
and condensation
reactions
must then
of monomeric
Precursor
Table
by the
groups
for several
V(IV)
which
prototropic
h=2
means
transfer
+ 0.68
[V(OH)2(OH2)4 ]2+
+ 0.01
- 0.07
+ 0.87
[Hf(OH)2(OH2)6 ]2+
- 0.06
+ 0.81
of h = 2 precursors
precursors
Charge Model.
Table
is
6+
OH
metal
ligand
thus
formed can
and cannot be easily
make a
pretenated
can also be easily
As
groups
is quite
condensation
can occur
strength
low for
is
of OH
Ti(IV)
and
inhibited,
a
:
OH2
strong
again.
metals
elements.
O
OH
The oxo
in
the nucleophilic
strength
difficult.
the two geminal hydroxo
of tetravalent
tetravalent
3 compares
The nucleophilic
condensation
between
for
an
as end points
6(M)
[Zr(OH)2(OH2)6 ]2+
precursors.
that
6(OH)
weakly
stops before
in a given range of pH.
+ 0.88
of oxo-aquo
Partial
precursors
- 0.01
strength
to zero or
be considered
[Ti(OH)2(OH2)4] 2+
3 - Nucleophilic
The formation
explained
ligand is close
is easy to
double
bond with
As a c o n s e q u e n c e
the highly
the stable
electrophilic
form of the
h~2
268
J. Livage et al.
precursor
is
an
oxo-aquo
precursor
precursor [M(OH)2(OH2)4] 2÷ in
does
not
occur
nucleophilic
with
enough
zirconium
to
[MO(OH2)5] 2+
good agreement with
and
initiate
hafnium.
further
rather
than a geminal dihydroxo-aquo
experiments 54,55,56. Such
Hydroxo
a mechanism
groups in the h-2 precursors are
condensation.
Therefore cyclic tetramers with
2(OH) 2 bridges are formed rather than monomeric oxo-aquo ions 52,53
2.2.3. Precipitation and
gelation. Zero charged
solid phase through infinite
precursors (h-z) are
condensation of "oi" groups.
able to nucleate
The final term of
a
this process
must then be a hydroxide M(OH)z provided oxolation does not occur. In order to know whether
oxolation
has
to
be
taken
into
account
when
considering
aquo-hydroxo
precursors
[M(OH)h(OH2)N.h] (z'h)+ or hydroxides M(OH)z, let us consider the following equilibrium :
6÷
--M--
6"
6÷
--
This reaction is basically a 1,3
=
the water molecule
positive or negative
.-
electrophilic rearrangement where a proton jumps
two adjacent hydroxo ligands, with at least
partial charge of
--M--~.
6
one of them being in a bridging
created by this
between
position. The
prototropic transfer can
be either
:
i) 6(H20 ) < 0 : There is a
net attractive force between the cation M(6 +) and
aquo llgand (6"). Water elimination is thus prevented and the reverse prototropic
the
transfer
occurs reforming the "oi" bridge which was originally broken. In such a situation the
"oi"
bridge remains stable and oxolatlon does not occur.
ii) 6(H20 ) > 0 : There is a
net repulsive force between the cation M(6 +) and
aquo ligand (6+). Water can be removed and the reverse transfer becomes impossible
to the irreversible
formation of an
oxo bridge. In
such a situation
the
leading
the "oi" bridge
is
unstable and oxolation can compete with olation.
Table 4
gives the
calculated values
hydroxo precursors and hydroxides.
can be isolated 57
such
conditions an
of 6(H20)
for some
transition metal aquo-
It is seen that as soon as 6(H20)<0 an hydroxide
M(OH) z
This is no more the case when 6(H20)>0 for oxolation can now occur.
oxy-hydroxide can be
obtained with
trivalent
Crystalline
phases known
Soluble precursor
6(H20 )
Solid hydroxide
formed by pure
olation
[Mn(OH)2(OH2)4] °
- 0,02
Mn(OH) 2
- 0,06
Mn(OH)2,MnO
[Fe(OH)2(OH2)4] °
- 0,01
Fe(OH) 2
- 0,02
Fe(OH)2,FeO
[M(OH)2(OH2)4] °(*)
- 0,003
M(OH) 2
- 0,01
M(OH)2, MO
[Sc (OH)3 (OH 2 )3 ] °
- 0,05
[Y(OH)3 (OH2)3 ] °
Sc(OH)3
6(H20)
- 0,i0
Y(OH)3
In
elements while hydrous
Y(OH)~ YOOH
Sc(OH)~,ScO.OH
Y203 , Sc203
[V(OH)3(OH2)3 ]°
+ 0,01
V(OH) 3
+ 0,02
VO.OH, V203
[Cr(OH)3(OH2)3 ]°
+ 0,01
Cr(OH) 3
+ 0,03
CrO.OH,Cr203
[Mn(OH)3(OH2)3 ]°
+ 0,02
Mn(OH) 3
+ 0,04
MnO.OH, Mn203
[Fe(OH)3(OH2)3 ]°
+ 0,03
Fe(OH) 3
+ 0,07
FeOOH, Fe203
[Co(OH)3(OH2)3 ]°
+ 0,03
Co(OH) 3
+ 0,08
CoOOH
[TiO(OH)2(OH2)3 ]°
+ 0,01
TiO(OH)2
+ 0,02
TiO 2
[VO(OH)2(O~)3 ]°
+ 0,05
VO(OH) 2
+ 0,12
VO2
[Zr(OH)4(OH2)4] °
+ 0,002
Zr(OH) 4
+ 0,005
ZrO 2
[Hf(OH)4(OH2)4] °
+ 0,01
Hf(OH) 4
+ 0,03
HfO 2
(*)M = Co, Ni, Cu.
Table 4 - Stability of hydroxides M(OH) z deduced from the Partial Charge Model.
Sol-Gel Chemistry of Transition Metal Oxides
oxides are obtained only
these
oxy
hydroxides
considered as the
with tetravalent elements. However,
are
formed
final term of
under
very
specific
nucleation and growth
269
it must be pointed
conditions.
out that
They should not be
processes which would
lead to the
oxide MOz/2 if 6(HzO)>0.
The formation of a gel rather than a precipitate from aquo-hydroxo inorganic
pre58.
cursors is a rather complicated process which depends critically upon many parameters
A pH-gradient is induced by the
-
(NH2)2CO,
gelifying agent which may be NaOH, NH 3 ,
NaHCO 3 , Na2CO 3 ,
or any hydroxyl exchanger.
The concentration of both reagents may be quite different.
The addition mode and the speed of agitation of the solution must be controlled.
-
- The order of mixing of the reactants and the geometry of the vessel play
a role.
- The temperature can either favor or inhibit gel formation.
The
chemical
composition
of
the
aqueous
solution
can
induce modification
of the
precursors at a molecular level.
All these
involve
mainly
consequence,
parameters must
olation
be taken
reactions
Other
metals
that
gelatinous precipitates
These precipitates
finally
are
because nucleation
diffusion-controlled
and growth
processes.
As
colloidal gels are obtained which are not very stable when prepared in a
form. Metals that lead to stable "oi"
59
into account
which
do
to
stable
the
well defined.
oxide
MOz/2
multivalents elements such
as Mn, Fe
solution,
or even
the solid
bridges give rise to well defined hydroxides
form
MOx/2(OH)z.x.YH20 when
are not
leading
not
phase,
a
pure
M(OH) z
hydroxo bridges lead to hydrated amorphous
a base
They lose
60,61,62
is added
to the
aquo precursors.
water continuously through oxolation
Other
and Co because
complications
can
electron transfers may
at the oxide-water
interface.
arise
with
occur in the
The following
examples
will briefly show how these different reactions may be analyzed.
2.2.4. Sols and gels of divalent metal oxides. We will consider mainly Co 2+, Ni 2+ and
cations because other
divalent metals
(V 2÷ ,
Cr 2+ , Mn 2+ and
Fe 2+) are easily
Cu 2+
oxidized in
aqueous solution.
Green transparent nickel hydroxide gels can be obtained by dissolving the
precipitated hydroxide in tartric
proportions
(>0.5 M) 63. Similar
acid and adding sodium
results are obtained when
glycerol and treated by an alcohol
dessication,
freshly
or potassium hydroxide in
molar
nickel acetate is dissolved
solution of potassium hydroxide 64
After
in
dialysis and
the solid phase is Ni(OH)2 and not NiO showing the stability of the ol bridges
in this system. No structural characterization has been undertaken for these gels.
Owing to the easy
results are obtained
from pink to purple
oxidization of Co 2+ in strongly alkaline solutions,
with cobalt.
In
this case gelation
to green and after
many days to brown
Co 5÷ obviously occurs under such conditions
is slower and
63
different
the color changes
Oxidation of Co 2+
towards
:
3 Co 2+ + 3 H20 + 1/2 02
, Co304 + 6H +
This reaction was used by Sugimoto and Matijevie to produce monodispersed Co304 sols 6 5
In
this case it is interesting to point out that sols can be obtained only in the presence
of
acetate ions. No
salts (nitrate,
precipitation is observed
conditions when other
Co (II)
chloride and sulfate) are used.
Copper hydroxide gels are more
difficult to produce and the
must be fulfilled in order to make them 66,67
i) The
under the same
starting precursor
must be
following conditions
.
copper (II)
acetate. Nitrates,
always give rise to gelatinous precipitates.
ii) The added base must be diluted ammonia without any excess.
chlorides or sulfates
270
J. Livage et al.
iii) A small amount of sulfate ions must be added in order to get a stable gel.
These gels are highly
anisotropic
and show interesting
aggregation phenomena which
have been studied in our laboratory.
Copper
(II) hydrous-oxide
sulfate or a mixture of copper
tartrate complexes
(Fehling's
sols can also be made by heating a solution of
(II)
solution) with glucose, uniform copper
can be obtained with various particle
2.2.5.
by
Cr(III)
potassium hydroxide
sulfate,
70,71
copper
(I) hydrous oxide sols
shapes and sizes 69
Sols and gels of trivalent metal oxides.
treating
nitrate or
nitrate and potassium phosphate 68. By heating
nitrate,
Hydrous
chloride
chromic
or
Highly vibrant monolithic
oxide gels
can
be made
acetate precursors with ammonia or
gels can be produced only when acetate
ions are present in excess 70-72. The color of these gels is blue-grey when NH 3 is used and
bottle-green with KOH .
NH 3 . These
gels are
microcrystalline
This difference may well
amorphous
to
X-rays, but
be due to complexation
small fractions
Cr(OH) 3 can sometimes be detected 73,74,?5.
that the gels have the stoichiometry
[Cr(OH)3(OH2)3].nH20
between Cr 3+ and
of crystalline
EXAFS measurements
this
oxolation
is
~-Cr203
agreement with the predicted
ageing
chromium
ions are necessary
such
as
or
of
is
acetates or oxalates.
aquo-hydroxo
precursors
symmetry.
monodispersed
different
precipitates
added
:Fe 3+
to
are
ions towards nucleophilie
exhibits
in the same
Some sulfate and
phosphate
sols 78
similar eleetronegativity
obtained instead
of gels
such as chlorides,
no
crystal
field
25oc 79. In contrast,
symmetry.
This implies
and thus olation rates
the rate of dimerization
80. As
77,78
(table 4). By
and Cr(NO3) 3 at high
and
when a base
sulphates,
nitrates,
stabilization
the
in an
olation is fast as shown by the rate of dimerization
substitution
drastic way. In agreement,
Cr2(SO4) 3
This may be correlated with the rate of olation of
the [Fe(OH)(OH2)5] 2+ ion : k- 450 M'Is "I at
crystal field stabilization
The final term
such as CrOOH, which is in
in the h ~ 3 precursor
despite
precursors
(3d 5)
Consequently,
phase
KCr(SO4)2.16H20,
Fe 3+ is quite
Gelatinous
NaOH
intermediate
sols can be prepared
perchlorates,
octahedral
as
in order to obtain
number.
NH 3
such
chromic oxide
The behaviour
coordination
no
instability of ol bridges
salts
temperature hydrous
with
shown
and that hydroxyl groups condense
to form Cr-O-Cr bonds without decreasing the coordination number of Cr 3+ 7 6
of
CrOOH and
have
10 .5 M I s "I
at 25°C
monolithic
condensation
is slow, gels are easily
gels
Cr 3+ (3d 3) shows a
of
high
a low reactivity
of Cr 3+
must slow down in
a rather
of the [Cr(OH)(OH2)(C204)2]2"ion
are preferentially
formed when
formed with Cr 3+ while only gelatinous
is k =
the rate of
precipitates
are obtained with Fe 3+ .
These
gelatinous
intermediate between ~-FeOOH
precipitates
(goethite)
are
amorphous and
and o-Fe203
has been proposed for a compound whose composition
is supposed to be an amorphous
at pH>I0 while ~-Fe203
6(H20 )
Cr(OH) 3
in
the
can
hydrolysis
be
h=3
[Fe(OH)3(OH2)3 ]°
Another
to have a composition
crystal structure
is close to 2Fe203.FeOOH.4H2 O83 . The gel
of this material 84,85. Upon aging, ~-FeOOH
is obtained at pH<4 86,87.
detected.
kinetics
form
seem
(haematite) 81'82. A
precursor,
difference
of the aquo-ion 88,89
no
microerystalline
between
Fe 3+
Fe(OH) 3 similar to
and
Cr 3+ lies in the
.
[Or(OH2)6] 3+ + H20 = [Cr(OH)(OH2)5] 2+ + H3 O+
kl :
[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2
kl -
3. i0 zs" I
%
6.1 104s "I
+ H3 O÷
[Fe(OH)(OH2)5 ]2+ + H20 = [Fe(OH)2(OH2)4] + + }{30÷
As a result,
acidic ferric
solutions are highly unstable
The mechanism of
is formed
In agreement with the high partial charge
spontaneous
hydrolysis.
this precipitation
and appears
to proceed as follows 98,99,100
:
~
and
1.4 105s'I
precipitate
was extensively
studied
through
90-97
Sol-Gel Chemistry of Transition Metal Oxides
- The h=l precursor
~-FeOOH
[Fe(OH)(OH2)5 ]2+ can
undergo a dimerization
phase through mixed olation/oxolation
- At room temperature
composition
is
the h=2
precursor
[Fe403(OH)4 ]2n+n with
polycation gives rise
and 2(0H)2
seem to be present in
the
atoms
iron
are
in
a
a
molecular
solutions.
near the surface.
octahedrally
coordinated.
However,
weight around 104 g/mole
diameter which are
Mixed oxo-hydroxo
this polycation.
Upon ageing,
A
coordination
other
bridges 2(O)i,
in
the
responsible
for the gelatinous
a base,
are formed
sulfate
salt
ions a basic
precipitates
2(0)2,
These needles
105,106
~-
then undergo
of chloride
while in
The synthesis
atoms are
fibrous tactoids
In the presence
~-FeOOH 103,104
2(0H)I
101 , in which
occurs leading to
rod-like particles which can form
rather than
for the
core and in an octahedral
aggregation
aspect of the precipitate.
fl-FeOOH precipitates
(n=25)~ This
results 102 suggest that all iron
or adding
giving
whose mean
responsible
structure was proposed
FeOOH needles with the same diameter as the original polycation.
an oriented aggregation process
the
reactions.
2-4 nm in
tetrahedral
coordination
reaction and nucleate
[Fe(OH)2(OH2)5] + can form a polycation
to spheres about
brown-red color of the colloidal
271
ions
the presence
of this
of
Fe-polycation has
been reviewed 107
- At high temperature,
the h=2 precursor
into ~-Fe203
that may exhibit various morphologies
particles
does not form a polycation.
It nucleates
directly
108,109
Iron oxide sols or gels can also be made through the oxidation of Fe(II) precursors
or the
reduction of
Fe(III)
phases thus formed can be
i)
Magnetite
salts.
Fe304
Fe304 can
Depending on
, ?-Fe203
nucleation
neutral
copreeipitation
to a green
leads
or
weakly
acidic
magnetite
111
Fe304 .
Fe(II)/Fe(III)
precursors
aqueous mixture
of ferric chloride and
to an ammonia solution.
are
An
alkaline
hydroxide.
An
perchloric
acid,
peptization
acidic
Such
in
a
order
mechanism
to
sol
is
is
centrifuged
then
obtained
and
when
peptized
reactions
are characterized
with
other
Chemisorption
electron
by an
at the interface
trapping
by
Desorption
or in-situ oxidation of
Fe 3+ migration
vacancies
(cf 5.4.2).
electrons
the
fast
washing with
the
precipitate
is
by
adding
distilled
stirred with aqueous
water.
Fe(ll)
solid
In
all cases
ratio is lower than 0.15114'115
or other oxidizing agents
and Fe(lll)
This transformation
116-118
All these
interface,
coupled
ions inside the particle.
surface Fe 3+ cations
in
oetahedral
phase which are normally deloealized.
this reactive Fe(ll)
core towards
formed and can be
with tetramethylammoni~1
occurs, while
the surface
The final product of these processes
iii) Finally,
FeOOH 111,119
from the
from
with the
charge compensation
creation of oxygen
is aggregated 7-Fe203
particles.
oxidation of Fe(II) by H202 leads to either crystalline
or amorphous phases 120
an
agitation,
peptization
induces a reduction of
positions
leads to
between
113. Typically,
decantation without
electron transfer at the water-solid
transfer
involved when mixed
is instantaneously
for the oxidation of Fe304 is ~-Fe203116
H3 O÷, Fe 3+, Fe(OH)3 , Ag ÷
leads
particles
by
is possible only when the Fe(II)/Fe(III)
is induced by air,
is probably
is added, under strong
or magnetic
made
of the green-rust
obtain ferrofluids
ferrous chloride
and
are oxidized snd
thus formed, with ferrous precursors
Surface oxidation
centrifugation
ii) The final term
an
used
ferrofluid
much
inside the gel phase 111,112
some Fe(II) precursors
A black gelatinous precipitate
isolated from the solution by
water.
mechanism
conditions,
of the ferric hydroxo complexes
colloidal
mechanism 110. The situation appears
takes place near the surface of Fe(OH) 2 particles
product called green-rust
to
and is
of Fe(OH) 2 .
the growth involves a contact-recrystallization
Under
the solid
reduction of ~-Fe203 with hydrazine
recrystallization
more complex when it is made by slow-oxidation
- Under basic conditions,
conditions,
or 6-FeOOH.
be made through
formed following a dissolution
the experimental
6-
272
J. Livage et al.
2.2.6.
Sols and
gels of tetravalent
Ti, Zr or Hf can be
sols.
However,
their
rather difficult
aqueous
solution
neutralization
of
these
structure
growth mechanism
to obtain.
TiO 2 gels
124,125,126
of chloride
colloidal
present 61 , 62,128
or nitrate
remains
Sols of
involves
precursors
precursors
so
mixed
NH 4+
bridges
sols and gels such as VO 2 ,CrO 2 or MnO2,
133 ,
glucose,
by reduction
fructose
or
to an
The structure
bridges
seem to be
ZrO 2 for which a
and/or 3(OH)1
obtained
gels are
carbonate
ZrO 2 gels can be made by
oxo/hydroxo
amorphous
form stable
that clear
adding sodium
with urea or by peptization.
but
study concerns
readily
of tetravalent
can therefore
olation
made by
atoms linked through 3(O)1
MnO 2 are
Mn2+132 ,
mainly
have been
unknown,
The only structural
With other dioxide
Na2S204131 ,
Hydroxo-aquo
These cations
or by acid peptization 127 . Similarly,
gels
with zirconium
be neglected.
metal oxides.
easily hydrolyzed 121'122'123.
sheet-like
was proposed
redox reactions
129
cannot
of KMnO 4 with As(OH)3130 ,
galactose
134.Gels
have also been
formed 59.
2.3.
Condensation
2.3.1.
via oxolation
Mechanism.
Oxolation
leads to the
formation
of oxo bridges
M-O-M between
cations M. Such a condensation
process
is observed when no aquo ligand
coordination
metal.
Typically,
sphere
of
[MOx(OH)N.x ](N+x'z)"
the
where x<N.
this
Two basic mechanisms
occurs
have
is available
for oxo-hydroxo
to be
two metal
considered
in
the
precursors
for oxolation
reactions.
i)
addition
When
the
metal
be removed
and chains
examples
are
by
given
[M4OIz(OH)4] 4" .
2(0)2
is
(AN) with M-OH and/or M-O as nucleophiles
need not
agreement
coordination
The
[MO3(OH)]"
kinetic
with a pure
or face bridges
or cycles
constants
2(0)3
are easily
(M
of
-
such
138
fully
can occur,
are formed
species
addition mechanism
not
saturated,
as shown in figure 4. Ligands
very rapidly
W,Mo)
which
reactions
According
nucleophilic
135,136,13Z.
form
cyclic
are larger
Typical
tetramers
than lOSM-~s "I in
to this mechanism,
edge bridges
formed.
/
<
/ \O
+
O -- M -
'
- ~O-~
-%
A
-
2(O)3
ii) When the metal coordination
already
fully
saturated,
philic
substitution
must
with M-OH as a nucleophile
or
s
HzO
as
leaving
reaction
can
be
two basic
steps
6"
M-OH
of small polymers
nucleophilic
(A)(B)(C)
addition
chains
according
mechanism.
; (D) cycles.
to a
leading
to
This
into
addition
by
the
:
leading
bridge
:
H
M-O-M-OH
,
water molecule
groups.
decomposed
2(0H)I
6+
+ M-OH
followed
Fig. A. Formation
occur
and OH"
:
- a nucleophilic
to an unstable
is
nucleo-
a
fl-elimination
departure
of one
Sol-GelChemistryof TransitionMetalOxides
6+
H
-
M
-
O
~"
OH
-
M
-
,
M
-
O
-
M
H20
+
This basic m e c h a n i s m will be called ANflE i in order to indicate
prototropie
transfer w i t h i n the transition
The first step
attack
273
the two step process
and the
state.
can be catalyzed by bases w h i c h strongly favour the nucleophilic
:
M - OH + OH"
, M - O" + H20
M - O" + M - OH
, M - O - M + OH"
This m e c h a n i s m will be called AN~E 2 in order to indicate a c o n c e r t e d elimination.
The second step can be catalyzed by acids w h i c h strongly
the leaving group
,
charge of
the "oi"
As a w a t e r molecule
H
[ M - O - M - OH2] + + H20
1
H20
M - O - M
removal.
of
:
H
6"
M - 0 - M - OH + H 30 +
The positive
favor the elimination
+ H 30 + c
H
]+
[ M - O - M
+ H 20
bridge
increases
greatly
is e l i m i n a t e d
its
from the transition
acidity,
favoring
proton
state this m e c h a n i s m will be
called AN~E I .
These mechanisms
wide range of pH.
Moreover,
explain
why,
as a p r o t o n
in contrast
to olation,
has to be transferred
the rate limiting step can be either the proton transfer
leaving group
(ANflE I and AN~E2).
Oxolation kinetics
oxolation
before
(AN~Ei)
thus strongly
occurs ever a
elimination
or the elimination
depends
occurs,
of
M and the pH. The r e a c t i o n rate u s u a l l y goes through a m i n i m u m around the isoleetric
of the solution
precursor,
(precursor
[HCrO4]
while
[MOz. N(OH)2N.Z] ° predominent
the d i m e r i z a t i o n
+ [HerO4]"
the p o l y m e r i z a t i o n
reaction of Cr(VI)
= [Cr207 ]2" + H20
of vanadates
in solution).
are shown b e l o w
M---OH
+
3(0)I
and
2.3.2.
Polyanions.
precursors,
is the
One
of
o x o l a t i o n may not go b e y o n d
loss of n u c l e o p h i l i e
the
fact that even
The different
and cannot p r o c e e d as fast
types of bridges
that
can
M--0--M +
H~O
2 (°)1
/~O--M +
H20
3 (0)1
+
H20
4 (O)I
+
H2°
4 (°)I
common. They can be found in
142
and in [Cu40Cl 6 (Ph3PO) 4 ] complexes
4(0)I
[M30(OAc)6 (0H2)3 ]÷ (M = Cr,Fe,Ru)
k = 5 102 M'Is "I
is a slow process
:
HO---M
/~OH + HO--M
Isolated
.
~ = 3.1 104 M'Is "I
= [V309] 3" + 2 H20
an ANflE m e c h a n i s m
as o l a t i o n as it is never under diffusion control.
be formed via oxolation
the h = 7
138,139,140
leads to 141.
[VO3(OH)] 2" + [V204(OH)3]"
following
point
k = i M'Is "I and k ~ 5.10 .4 M'Is "I
[VO3 (OH) ]2" + [VOa(OH)2 ]" = [V206 (OH) ]3" + H20
Oxolation
Considering
can be w r i t t e n as follows
the
on b o t h the metal
bridges
main
are
differences
w h e n the charge
not
very
between
is zero
aquo-hydroxo
(x=z-N),
a limited degree of polymerization.
and oxo-hydroxo
condensation
through
This is again due to
strength of hydroxo groups after c o n d e n s a t i o n has o c c u r r e d
:
the
274
J. Livage et
2[CrO2(OH)2] °
) [(HO)O2Cr-O-CrO2(OH)]
6(OH) = -0.01
This dimer behaves
called
as condensation
"polyacids".
consider
as an example
h=5 precursors
ionization
acid
deprotonation
atoms
such species
are
often
can be obtained.
Let
us
that can be formed by the polycondensation
of
are
required
deprotonation
) [H6V10028 ]° + 12 H20
-0,09
leads to a polyoxy-ion
higher pH further
species
:
6(OH)=
vanadium
+ H+
takes place,
on M, more or less condensed
I0 [VO(OH)3]°
Ten
:
[Cr206 (OH) ]" = [Cr207] 2" + H +
the decavanadic
[VO(OH)31%
to form polyanions
[Cr205(0H) 2]° = [Cr206(OH)]"
must occur before
Depending
° + H20
8(OH) = +0.04
as an acid and can lose protons
{
However,
al.
6(OH) = +0.003
to
make
the
hydroxo
group
[HzVI0028 ]4" whose structure
occurs
leading
positive.
Spontaneous
is shown in figure
5G. At
to :
[H2V10028 ]4" = [V10028 ]6" + 2 H*
Figure
35,143-161.
in figure
5 shows also the
These polyanions
6 137when
more open structures
structures
are probably
the rate of
of some well known transition
formed through
the AN~E reaction
particularly
a mixed AN/AN~E
is fast.
when the reaction
Geometric
rate is slow
metal polyanions
mechanism
as shown
constraints
(figure
lead to
7) 35,162-171
A
B
o
o
~\ Oo
AN
Fig.5.
(A)
Structures
(C) [M6019] 8"
(D)
of compact
[W4012(OH)4 ]4" , (B)
isopolyanions
[W4016] 8" 35,143
Fig.6.
anions
Mechanism
according
M = Nb, Ta 144,145
Formation
[M6019 ]2"
M = W 146, Mo14?,148
[M4012 (OH)4 ]4-
[MzO24] 6"
M = W 35,143,
successive
Mo 149,150
(E) ~-[Mo8026] 4" 151-154
(F)
[MOsO26(OH)2 ]6" 155,156
(G)
[MI0028] 6" M = V157"159;
(H)
[Au206] 6" 161
and
of
the
the isopolyanion
formation
of isopoly-
and Glemser
[M4011(OH)5] 3"
(B)
addition
protonation.
[M4OI2(OH)4] 4"
Nb 16°
of
to Tytko
(A)
tetramers
of
(C)
tetramer
through
137
and
through
[MO3(OH)]'tetrahedra
Structure
and
growth
of
the
(D) of
an AN~E mechanism.
Sol-Gel Chemistry of Transition Metal Oxides
275
¢
Fig.7. Structures of non compact
isopolyanions.
(A) [M207] 2
M = Cr 1 6 2
[M207] 4
Mo 35
M = V 16S
(B) [Cr3010 ]2" 164
(C) [VsO9 I s
16s
(D) [Cr4013 ]2" 165
(E) [V4012] 4
ss,16s
(F) ~-[Mo8026 ]4" 153,154,166
V
(G) [H2W12042 ]I0" 167
(H) [H2W12040 ]6" 168
(I) [W10032 ]4" 169,170
(j) [Mo360112(OH2)1618-
171
It should be noted that the formation of most isopolyanions involves a change in the
coordination of the metal from 4 to 6. This change occurs because protonation increases the
electrophilic strength of
+0.50
octahedral
towards
the
the metal M
coordination
metal.
This
is
as shown in
preferred
explains
why
table 5. As
because
pyrovanadates
6(M) becomes larger
than
it allows a larger charge transfer
(precursor
h=7
[VO3(OH)]2" ) and
metavanadates
(precursor h=6 [VO2(OH)2]" ) have a tetrahedral structure 35'143 while vanadium
172
oxide gels and decavanadates (precursor h=5 [VO(OH)3] °) have an octahedral structure
With niobium in
the h=6 precursor,
the higher positive
charge explains why
niobium must
keep octahedral coordination even at very high pH (hexaniobate ion) :
6[NbO2(OH)2 ]"
With Mo(VI) and
both
coordinations
, [H2Nb6019 ]6" + 5 H20
W(VI), the h=7 precursors
while
coordination. Thus for
h=6
precursors
Mo(VI) a great
[M03(OH)]" are on
[MO2(OH)2] °
variety of polyanions
element can have two different coordinations as in
Precursor
appear
the border line
unstable
can be formed
in
between
tetrahedral
in which this
~-[Mo8026 ]4"
X
6(0)
[V04] 3
1.583
-0.74
6(OH)
6(M)
pK
[HV0412"
2.056
-0.57
-0.59
+0.29
14.4
[H2V04]
2.378
-0.44
-0.30
+0.48
8.95
[H3V04] °
2.611
-0.35
-0.09
+0.62
3.74
[Mo0412
2.046
-0°57
[I{Mo04 ]"
2.431
-0.42
-0.25
[H2MoO4 ]°
2.693
-0.32
-0.02
[WO4 ] 2
2.055
-0.57
[HWO 4 ]
2.439
-0.42
-0.25
+0.50
3.50
[H2WO 4]°
2.701
-0.31
-0.01
+0.64
4.60
[Nb04 ] 3
1.550
-0.77
[H2Nbo4]-
2.027
-0.58
+0.01
+0.29
+0.51
3.89
+0.67
3.61
+0.27
+0.07
-0.61
+0.38
Table 5 - Variation of partial charges with protonation for some tetrahedral inorganic
precursors.
JPSSC 18:4-B
276
J. Livage et al.
At very low pH, positively
low nucleophilic
strength
charged
can occur leading to acidic polycations
Tetrahedral
species
Hydrated phases can nucleate
the h=z
only
through
oxo bridges
This will no longer be the case if hydration occurs.
which have very different
slow AN~E
an acid is
are formed owing to the
Condensation
such as [Mo20(OH2)x] 2+ or [Mo20(OH)(OH2)×] ÷ 173
oxide MOz/2 upon heating.
precursor,
clear gels when
(cf.2.2.2).
such as [CrO2(OH)2] ° cannot condense beyond a certain point,
leading to the formation of polyacids.
into the anhydrous
oxo-aquo precursors
of hydroxo groups
added.
mechanisms
Some of
structures
and can be
transformed
Moreover as coordination becomes saturated
are
involved,
leading
the probable growth
in
to the formation of
mechanisms
for these gels
will now be described.
2.3.3.
Sols and gels of pentavalent
adding nitric acid to a vanadate
best method
however,
product quite
is
rapidly,
to use
without
thus be prepared by ion exchange
176,177.
hours.
The freshly prepared
Decavanadic
polymeric
species
acid
metal
a proton
exchange resin
or washing
gels can be
if the
which yields
a relatively pure
acid solutions can
predominates
solutions
and turns dark-red within
a few
below 10"3M and transforms
above 10"3M 176. Aggregation
vanadium concentration
vanadium reduction occurs during the polycondensation
The
175. Polyvanadic
acid is yellow
(M.W.-lOO0g/mole)
made by
oxide V205 174
in a resin from sodium or ammonium metavanadate
decavanadic
is observed
Vanadium pentoxide
dialysis
(M.W.=2.106g/mole)
finally gelation
oxides.
salt or by hydration of the amorphous
into
occurs above 2.10"2M
is larger
than 0.i
and
M. Some
process and about 1% of the
vanadium
ions are in the V(IV) oxidation state as shown by ESR 178
The fibrous
.............
gel
is
nature of
well
the
established
(figure 8). Electron and Xray diffraction
have
shown
fibers
flat
studies 172
that
actually
ribbons
about
wide and IOA thick.
ding
to
these
look like
the
i00 A
Accor-
2D structure
observed along the ribbons,
V205 layers
are formed
by
fibrils 27A wide linked together side by side.
molecules
can
be
Water
inter-
calated leading to a gel or
Fig.8.
Fibrous texture of V205 gels.
a colloidal
solution.
xerogel obtained by
these gels at room temperature has a water content about 1.6 H20 per V205 which
to one interfoliar water layer
179. Swelling of this xerogel
The
drying
correspond
can be followed by SAXS
and
around pH-2 leads to the formation of the
h=5
SANS 180
Acidification
of vanadate
precursor which can be formulated
structure,
ligand
is
vanadium
thus
is highly electrophilic
expected
and
This can be achieved in two ways
i) tetrahedral'h=5
solutions
[VO(OH)3]°.
precursors
transition
In this monomeric precursor with
(6(V)
- + 0.62). Addition of any
towards an octahedral
coordination
tetrahedral
nucleophilic
must occur.
:
are acidic species
:
[VO(OH)3 ]°
6(0)=-0.35
=
[VO2(OH)2 ]" + H +
6(0)=-0.44
Sol-Gel Chemistry of Transition Metal Oxides
Addition and condensation
formation of decavanadic
of several
acid as shown in 1.3.2.
(10-x)[VO(OH)3]°
ii) when
x =
0 in
equilibrium,
than h=5 precursors
towards fiber formation.
(x~4)
precursors
lead
to the
:
water
(6(0)=-0.35)
The first step
the V(V) atom from four to six
nucleophilic
+ x[VO2(OH)2 ]" = [H6.×V10028] x" + 12 H20
the previous
better nucleophiles
such tetrahedral
277
molecules
(6(0)=-0.40)
appear to be
and figure 9 shows a possible
corresponds
to an increase
in the
pathway
coordination
through the addition of two water molecules.
of
An octahedral
complex is formed with a long V-OH 2 bond along the z axis, opposite to the short V=0 double
bond. The other water molecule has an hydroxo ligand in a trans position.
readily leading to a chain compound whose stoichiometry
this case olation
leaving group
formed,
occurs before oxolation
(6(H20)=+0.I0)
condensation
unstable
2(0H)I
and
a good
because
nucleophile
through oxolation can occur
bridges
into
stable
double chains leads to a fibre-like
3(0)I
corresponds
the same
to [VO(OH)3(OH2)] ~.
complex contains both
(6(OH)=-0.14).
Once the
transform
Further condensation between these
structure as evidenced by electron diffraction
o
and fibrous polymeric
/}'k"~',OH
can thus
manner.
OH2
In
a good
chains are
between two chains in order to
bridges.
The coexistence
o
Olation can occur
172
of decavanadic
species
be understood
acid
in V205 gels
in a
very simple
Both species are in an equilibrium
which can be shifted in either
direction
by varying the V(V) concentration.
Niobium
differently
and
from vanadium
known while NbCI 5
molecules
structure 57
VOCI 3 is
-
i
5.75~
has
occurred
of
formation
through
olation
of
V205
(SN)
NbCI 5
or
Peptization
2.3.4.
Sols
TaCI 5
of
with
and
ammonia
precipitate
gels
of
or
by
hexavalent
acidification
washing
metal
or
oxides.
acid to
acidification
with a proton exchange
can be performed
free of foreign ions 1 8 3 , 1 8 4
obtained
which becomes
within a few hours.
progressively
Light-yellow
complexes
diffraction has
they
2(CI)2
mixed
precursors
remain octahedrally
amorphous
gelatinous
or tantalate.
to sols and gels 181
tungstic acid is usually
solution 1 8 2
resin in order to
As with vanadium,
obtain colloidal
a clear yellow-colored
solution
is
and then to a precipitate
are obtained when the tungsten
concentration
are dark-yellow at higher concentrations
shown that the light-yellow
of
is not possible
inorganic
leads
turbid and turns to a gel
precipitates
is low (< 0.5M) while the precipitates
ray
After exchange,
and
are formed through hydrolysis
Colloidal
a sodium tungstate
2(0)I
an alkali-niobate
dialysis
obtained by adding hydrochloric
solutions
of
complex
even at very high pH.
precipitates
this
why
condensation
formation
Consequently,
h=5 precursor.
of
tetrahedral
through
because
coordinated
and
(AN~E) from the monomeric
double
infinite octahedrally
Therefore,
aquo-hydroxo-oxo
mainly
octahedral
Nb=O and Ta=O
is an
with Nb(V)and Ta(V)
oxolation
an
is undimeric
stable, which explains
a monomerie
bridges.
fibers
VCI 5
coordinated polymer in which
b
3.60
Fig.9. Mechanism
with
Also,
while NboCI 3
:
is stable as
Nb2CIIo
bonds are not
ODH
tantalum behave quite
xerogel corresponds
(>0.7M). X-
to W03.2 H20 hydrate
278
J. Livage et al.
while
the
WO3.H20
dark-yellow
hydrate.
particles
thus
plate-like
one
is
The colloidal
obtained have
a
shape and are able to
form long range ordered tactoids
184 (figure i0).
As in the case of vanadium,
the
h=6
precursor
acidification
[MO2(OH)2 ]°
formed
around
is
able
its coordination
(tetrahedral)
by
pH=2
to change
number from
to 6
4
(oetahedral)
owing to the high partial charge
Fig.10.
Lamellar structure
through
of WO3.H20 xerogels obtained
polycondensation
of
on
[WO(OH)4(OH2)]°.
tungsten
can occur
i)Tetrahedral
h=6 species are acidic:
(6-x)[MO 2(0H)2] ° + x [MO 3(OH)]
-
ii) If x ~ O, water molecules
precursor
has
two
relative to the short
oxolation
f=
oxo
ligands,
two
M=O double bonds.
+ H+
lead to isopolyanions
M = Mo,W
+ 5 H20
ean enter into the coordination
In
•
' [H4.xW10032 ]x" + 8 H20
, [Hz-xM6019 ]x-
water
:
6(0)=-0.42
of these tetrahedral precursors
(10-x)[WO2(OH)2] ° + x [WO3(OH)]"
ligands
again in two ways
[MO 2(OH) 2] ° = [MO 3(OH)]"
6(O)=-0.31
Addition and condensation
atom (6(W)=+0.64).
Addition of nucleophilic
molecules
this case condensation
leading to linear or cyclic species because
sphere. As the h=6
can be added in a trans position
can
occur only through
the functionality
of the precursor
is
2 :
n[MO2(OH)2(OH2)z] °
, [MO3(OH2)2]"
No
I
+ n H20
precipitation
o
This explains
H ~*
occurs
because
low
molecular weight cycles are easily formed.
W ",,o
% //? %"oH
? JVo~
Ho/i\o.
0"2
H206-
rise to
/
why Mo(Vl)
precipitates
exchange techniques
Mo(VI)
thus
in
185,186,167
a similar
seems to
coordination
188 with two water
have an
preventing
precipitation.
is
not
observed
another
possibility
other oxo
the
same
through
prevented.
hydrogen
behavior
W(VI) because
(figure
ligand remains
ii)
is the
in
2.2.2. The
stable and
the
in the trans position plays
role
can
molecules
Such
with
proposed
as
before.
[WO(OH)4(OH2)] ° h=6
which
way as
octahedral
of one water molecule by the
water molecule
WO3.H20 layers
give
ion-
are used
behaves
reverse mechanism
of formation of
does not
gels when
Cr(VI), but
disso-ciation
Fig.ll. Mechanism
nor
grow
in
oxolation
An oetahedral
precursor
a
is
formed
bidimensional
because
olation
way
is
The sheets thus formed can make
bridges
structure of
leading
tungstic
to the layered
hydrates
WO3.2H~O
Sol-Gel Chemistry of Transition Metal Oxides
279
and WO3.H20 189 shown in figure I0. The water dissociation process is very slow with Mo(VI)
but occurs upon
ageing or heating
leading to isostructural
hydrates MoO3.2H20190'191 and
MoO3.H20 192,193,194 or to ~-MoO3.H20 a white-colored hydrate 195,196
2.4. Role of the anions
In our p r e v i o u s d i s c u s s i o n on the
anion
was
completely
neglected.
The
h y d r o l y s i s of cations,
metal
atom
the role of
the counter
was a s s u m e d to be s u r r o u n d e d b y aquo,
h y d r o x o or oxo species only. This s i t u a t i o n occurs w h e n p H m o d i f i c a t i o n s are o b t a i n e d
an ion exchange resin. However,
in m o s t cases a
2Fm
A
t'
Q
't
"
le
counter a n i o n is p r e s e n t w h e n an inorganic
,
C
"
,.,%
i'
e ......
B
,
,IV r~
_
. . .
• ..
i
,I
II'itl I
, .
~'.
11
~
•
llll~ i
•
i. , ' -
..
,
~ A
.~.~
~/~'~fM~
g
• •
~V~'~A:k~;::
t'
D
:
~I
~td._. /
itY/"~"ik
.f
t -i,.-
e- Y
~ ~-~
F
Fig. 12. Various morphologies of particles as a function of the type of
counter-ions present in solution according to E. Matijevic.
(A) Cl"
(~.Fe203)
(E) H2PO ~
(B) CIO4 108
(~_Fe203)
(F) Cl" 23
(C) NO3 108
(~_Fe203)
(G) HSO 4 105 (Fe3(OH)s(SO4)2.2H20)
23
(D) CI'/EtOH 109(~_Fe203)
with
23
(~_Fe203)
(~-FeOOH)
;?
" !
280
J. Livage et al.
salt is dissolved
into water.
In some cases organic or inorganic anionic
to the solution in order to control the precipitation process.
anions besides hydroxide
oxides 23,197-202.
the precipitate
ions
play a decisive role
in homogeneous
Some anions are strongly coordinated
while others
can be
removed by
Many
techniques
monodispersed
predict the
colloidal
morphology
are
now
available
particles.
of
However
these particles.
physical role. At the beginning of the
giving rise to a
is expected
change
double
to be different.
layer
composition
In
produce
a large number of well-defined
if not impossible,
to play
both a
chemical
they are able to coordinate
chemical reactivity
the
metal
most cases anions strongly
still difficult,
Once colloidal
and
other
of
(figure 12) 23'105'108'109'201
Anions seem
process,
new molecular precursor whose
condensation
the
to
it is
precipitation
to metal cations and thus end up in
leaching.
affect the particle morphology and colloid stability
species are added
It is well known that
species
the
and
a
metal ion
toward hydrolysis
are formed,
to
the
and
anions
ionic strength of the solution therefore
modifying aggregation processes.
This section will attempt to describe
chemistry
of
composition
inorganic
precursors
and the structure of
the morphology
of the
in
order
particles
cannot
be a
and therefore physico-chemical
the present time,
it
is difficult
to make
units and growth through aggregation.
show
formed
negatively
when
of metal
both
how they can orient the chemical
The following discussion
unique function
of the
shows that
chemistry
involved
factors must also play a decisive role. At
a clear difference between
growth by monomeric
As a result, no attempt will be made to correlate
cations.
positively
charged anions X"
associations
Associated
charged
aquo precursor
The full
the
static dielectric
present in
Therefore
203,204,
substitution.
coordinated
:
to
the
X-
hydroxo-
of the anionic species
However,
(e=80) which favors
the dissociation
a nucleophilic
the question
X"
arises
consider
hydrolyzed
an
associated
cation.
species
Ionic
1(z.h.1) +
H20
[M(OH)h(X)(OH2)N .h lJaq..
+. H20
.-
of
ionic species.
they
It
ligand. Therefore we have to
whethe~ the M-X bond is stable against both ionic dissociation
us
N.M.R 205,206 or
Water actually plays a double role. It behaves as a solvent with a high
constant
Let
M-X
if such species remain stable in an aqueous medium or whether
also ~ a-donor molecule which reacts as
i)
Such
cation in an aquo or
the coordination
can
[M(OH)h(OH2)N.h] (z'h)+ and
an aqueous solution.
optical spectroscopy
with such a precursor occurs via a nucleophilic
readily dissociate.
[M(OH)h(X)(OH2)~.h.I ](z'h'1)+
cations
coordination N of a metal
is already satisfied.
whether one can predict
species
hydrolyzed
are simultaneously
have been clearly shown by
ray scattering 207,208.
reaction
the aqueous
observed by TEM and the chemical role of anions.
2.4.1 Complexation
be
to
colloidal particles.
during nucleation
morphology
the chemical role of anions in
in
and hydrolysis.
which
dissociation
is
check
a monovalent
corresponds
[M(OH)h(OH2)N
anion X'is
to the following
h 1(z'h)+aq. + X'aq.
(7)
A partial charge transfer occurs between M and X within the M-X chemical bond leading to
modification
of the negative charge of the anion. Two possible cases arise
- X" is more electronegative
than H20 ligands
(x(H20) - 2.49).
Electrons
by X and the overall transfer goes from the precursor to X, increasing
of the anion (6(X)<-!)
the aqueous
right and
are attracted
the negative
The M-X bond become~ more ionic and the high dielectric
solvent favors
the associated
ion-pair formation.
species are
not stable
Equilibrium
(7)
against ionic
a
:
charge
constant of
is displaced
toward the
dissociation.
X'does not
exhibit any ability to complex with the metal cation.
- X
is less eleotronegative
the precursor.
than H20 ligands.
The negative charge of the
Electrons
anion decreases
can be transferred
(6(X)>-I)
from X
giving rise to a
to
more
Sol-Gel Chemistry of Transition Metal Oxides
covalent M-X bond
which is not
dissociated by the
solvent.
toward the left and the anion X" remains coordinated
The ability of an
therefore
bond. This electron transfer leads
after
(6(X)) eomplexation,
Ax=x+6(X).
which
Equilibrium
to a charge variation Ax
A rough estimate of how much equilibrium
of the anion, before
(7) is displaced
6(X)
=
1
+
-
x
In the case of monovalent
when
Ax<O.
Its
electronegativity
electrostatic
anions
of
form
: Ax = 1 + 6(X). Anion X" does
increases
decreases.
only. Entropic
~-hydroxy
acids)
presence
associated
The
above
increase
with
Ax>O
of a large excess of water molecules.
hydrolysis
This
transferred
],aq
( z - h" - I ) + +
equilibrium
6(HX)<O
remains
H2 0
goes
by
the
a
on
are in the
they must also be stable
transition
6(HX)>0
associated
:
lead to two possibilities
:
considerations,
positively
nucleophilic
species
is not
charged
Complexation
M z÷
against
According
the previous
rolysis
in
by a
the
negatively
:
It
(9) is displaced
becomes possible
could be
and the
stable however in the
of [Fe(OH2)613+
let us consider
anion
X" such
as CIO4,
ligands and should be
species has to be stable against
3÷
leading to the non
hydrolyzed
calculations
performed on both coordinated
.%
According
to
:
+ Xaq.
+
Depending on the strength of the acid HX in aqueous solution,
reprotonated
has
aqueous
NO3, HSO4, H2PO ~ or
to [Fe(X)(OH2)4] 2+ species.
. = [Fe(OH2)6]aq
species
now whether this
such anions behave as bidendate
=
the hydrolyzed
[Fe(OH2)6] 3+ precursor.
Table 6
species can be
reports charge
species using the Partial Charge Model.
x-
ClO~
2.86
2.76
2.64
2.49
2.24
6(X)
-0.92
-0.84
-0.50
-0.34
+0.40
+0.08
+0.18
+0.50
+0.66
+1.40
6(HX) I -0.52
-0.42
-0.15
+0.02
+0.70
.so i
He%
Table 6 . Partial Charges $(X) and 6(HX) in [Fe(X)(OH2)4]2+
species respectively,
charged HX species
Equilibrium
molecules
The hydrolysis
giving rise
this complexed
: [Fe(X)(OH2)4]
i
a proton can be
to the metal.
hydrolysis.
monovalent
water molecules
Ax
which
(9)
such as DMSO.
to the literature,
discussion,
Ionic dissociation
+ HXaq "
[M(OH)h+ I(HX)(OH2)N.h.I ](z'h'1)+
cation.
water
As an illustration,
be complexed
able to replace two
by
of Fe 3+ aqueous precursors.
been described previously.
can
substitution
stable towards
presence of aprotic solvents
state
:
towards the left and the anion X" remains coordinated
,y
based
to form complexes and
processes 209-213
[M(OH)h(X)(OH2)N.h.I ](z'h'1)+
Therefore,
towards the X group
: from purely electrostatic
attracted
precursor
are
with chelating
ability
[ M ( O H ) h + l ( O H 2 ) N . h . l , a q](z-h-l)+
"
=
through
from a water molecule
charge considerations
CH3COO'.
considerations
effects observed
their
not complex
i.e. when the
:
[M(OH)h(X)(OH2)N.h.1
2.4.2.
with Ax when Ax>0,
and resonance
can
species
toward the left can be
i00
complexes
anion
and
Model:
:
such species are often used to control precipitation
ii)The
Again,
the
interactions
(E.D.T.A.,
therefore
to
(x)
(8)
-
x
anions this leads to
ability
will
within the M-X
with the Partial Charge
made by looking at the relative charge variation of the anion
- -
is displaced
complexes with a cation M z+
of electron transfer from X to M
can be easily calculated
ax
(7)
to the metal atom.
anionic species X x" to form
depend mainly on the magnitude
281
CH3 COl
and [Fe(OH)(HX)(OH2)3 ]2+
as a function of the mean electronegativity
X of the anion Xaq.
282
J. Livage et al.
According
electronegativity
ion pair
to
table
of X"
formation.
becomes possible
On
6,
the
M-X
bond
decreases.
The complexed
the other
hand,
6(HX)
becomes
less
and
species then become
increases
so
less
ionic when the
more stable towards
that hydrolytic
dissociation
as soon as 6(HX)>0.
'~,x
-+1.0
\
-1.0-
-
-+0.5
A©O
0.5
-
CI-
- -0.5
+ 0.5
--1,0
+1.0-
~(HX)~
X" monovalent
Fig.13. Variation of Ax - I+6(X) and 6(HX) versus X for some
anions in
[Fe(X)(OH2)4] 2÷ precursors.
An electronegativity
6(HX) versus X
~(HX)<0)
for
as shown in
for intermediate
the
example
negativities
(6(HX)>0).
experimental
(Ax<0)
in
figure
while
13.
hydrolytic
HCO3,
conditions.
the hydrolysis
figure 13. Anion
electronegativities
shown
Therefore,
range can be estimated graphically
CI"
and
ratio h of the precursor,
to the metal
only, roughly speaking,
between
Ionic
CH3CO0"
coordinate
Fe 3+
ions
because
of
ionic
and
(Ax>0,
2.55<~<2.90
for higher electro-
occurs for lower electronegativities
cannot
give
stable
complexes
in these
for a given anion X" also depends on
on the pH of the solution.
Table
performed on hydrolyzed neutral species corresponding
One can see that highly electronegative
to
dissociation prevails
dissociation
i.e.
Ax=l+6(x)
X" remains coordinated
The ability to form complexes
partial charge calculations
if we plot
anions such as perchlorates
dissociation.
7 reports
to h=2.
are not
able
They behave as counter ions.
Sol-Gel Chemistry of Transition Metal Oxides
However,
occurs
the
hydrolysis
under
highly
experimental
h can
acidic
observations
highly concentrated
are able
ratio
conditions
207,208
HCIO 4 214
to coordinate
pH. As a consequence,
the
be decreased
as
A
by lowering
shown
similar
metal cation when
in
the
table
behavior
On the other hand,
283
6
pH and complexation
and
in agreement with
has been reported for Ti(IV)
less eleetronegative
the hydrolysis
ratio
anions such as
is high,
in
HC03
i.e. at high
they behave mostly as counter ions except under basic conditions.
X
~
6(x)
Ax
CIO4
2.86
-1.26
-0.26
-0.94
6(HX)
HSO4
2.64
-0.92
+0.08
-0.65
HC03
2.49
-0.72
+0.28
-0.45
Table 7. Partial charges 6(X) and 6(HX) in the neutral species
[Fe(OH)2X(OH2)2 ]°,
I,=3
I
2.0
2.1
2.2
I
2.3
AooFig.14.
Electronegativity
monovalent
As
a
increases
anions
behave
dissociation),
anions
of colloids
behave
aquo-hydroxo
complex.
described
as
counter
for
except
at
very
ions
or
molecules
have
a
ratio h for which X"
which anions form stable
strong
They can even remain
the pH, can
inorganic precursors
each
allowing
element,
a
pH.
Anions having a low
(because
of
hydrolytic
mean electronegativity
over the whole range of pH.
effect
on
both hydrolysis
and
of the structure and morphology
coordinated
to the metal
cation up to
such as those observed when
be complexing
can be
or
is
description
ranges
Fe 3+
obtained
not (cf. figure 12).
easily extended to any
electronegativity
quantitative
pH of the
electronegative
of SO~'. On the other hand, pure ~-Fe203
depending on
For
low
when the
highly
having a
giving rise to basic salts
Fe 3÷
3.0
C,O;
low electronegativities
anions however,
therefore
in the presence
This discussion concerning
previously
range
They will induce deep modifications
which,
2.9
NO~
are able to form stable complexes
and precipitates.
ions are precipitated
ions
pH. Some
will
the end of the precipitation
with other anions
2.8
[Fe(OH)h(X)(OH2)4.h ](z'h)+
electronegativity
counter
at high
(sulfates)
condensation processes.
2.7
HSO;
shifts towards
close to that of H20 (X=2.49)
Such
HCO;
2.6
as shown in figure 14. As a consequence,
as
also
except
c,-
the
metal cations
electronegativity
2.5
range as a function of the hydrolysis
guide,
aqueous solution
usually
2.4
anions form stable complexes
rough
complexes with
h:, .=0
I':211
I
may
other
be computed as
of complexation
phenomena
in
aqueous solutions.
2.4.3. Hydrolysis
and condensation
of Fe 3+ . Anion complexation
new precursors
whose chemical reactivity
Fe 3+
species
aqueous
by
a
strong
N(CH2COO)2 ]4" , has been studied carefully.
measured before 89,99,203,215
Hydrolysis
can be noticeably
chelating
ligand
different.
The
modification
and condensation
constants
of
were
leading to the following results
:
[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2" + H30 +
[Fe(OH2) 2 EDTA]"
to
such as EDTA,[(OOCCH2)2N-CH2-CH2-
Both hydrolysis
and after 216 complexation
of metal cations leads
~
+ H20 = [Fe(OH)(OH2)EDTA] 2" + H3 O+
= i0 "5
~
= 10 .25
:
284
J. Livage et al.
Condensation :
2[Fe(OH)(OH2)5] 2+ = [Fe2(OH)2(OH2)8] 4+ + 2 H20
K d = 6.10 .4
2[Fe(OH)(OH2)2EDTA]2"
K~ - 102.95
It
condensation
= [Fe2(OH)2(EDTA)2] 4" + 2 H20
can
be
seen
that
hydrolysis
is
is
favored.
This
is
due
mainly
prevented
to
EDTA
by
complexation
modifications
charge
while
induced
by
complexation. A partial charge calculation shows that (table 8):
6(Fe)
6(H)
[Fe(OH2)6] 3+
Precursor
+ 0.59
+ 0.34
6(OH)
[Fe(OH2)2(EDTA)]"
+ 0.43
+ 0.20
[Fe(OH)(OH2)5] 2÷
+ 0.55
+ 0.30
- 0.01
[Fe(OH)(OH2)(EDTA)] 2"
+ 0.40
+ 0.17
- 0.25
Table 8 : Complexation of h-0 and h-i aquo precursors of Fe 3+ by EDTA 4" (EDTA=C10HI208N2).
The positive partial
charge on the
protons of the
precursors decreases upon complexation. The
water molecules in
the non hydrolyzed
EDTA modified precursor is therefore
a weaker
acid and the deprotonation of coordinated water molecules is more difficult.
The condensation process
group onto the
begins with a
nucleophilic attack by
positively charged metal
atom. This process
the negatively charged
therefore is easier
OH
as 6(OH)
becomes more negative and 6(M) more positive. Table 8 shows that EDTA complexation leads to
a decrease of
6(Fe) and an
increase of 6(OH).
which factor will prevail. However, a
Therefore, it is
not obvious to determine
rough estimate of the condensation ability
could be
given by the product 6(M).6(OH) that varies from -5.10 .3 up to -10 "I upon complexation. The
larger variation
comes from
6(OH) and
dimerization of
the modified
precursor should be
easier as confirmed by the equilibrium constants.
2.4.4. Formation of basic salts. Complexing
anions coordinated to the dissolved metal
ion
do not only change the charge distribution within the aqueous precursor. They can also play
a role as network formers in the structure of condensed phases. Some of them end up in
the
solid giving rise to the precipitation of basic salts.
Figure 14
shows the
structures which can nucleate from the h-2 aqueous zirconium
precursor [Zr(OH)2(OH2)6] 2+. All these structures have been experimentally determined by Xray diffraction 2 1 7 - 2 2 1
Non-eomplexing anions (~-0%) such
displace water molecules. They are not
as CI" or CIO4 are not able
involved in the formation of condensed
hydrous zirconia ZrOz.nH20 can precipitate at high pH. A cyclic tetramer
is
formed
via
2 (OH)2
bridges
in
which
to
species and
[Zr(OH)2(OH2)4]~ +
zirconium is surrounded by four terminal water
molecules and four bridging OH groups (square antiprism). Complexation occurs with
nitrate
which exhibits a weak complexing ability (~-4%). Two terminal water molecules are
replaced
by one NO3 group and a chain polymer [Zr(OH)2(NO3)(OH2)z] ~ is formed in
remains in eight-fold coordination (dodecahedron).
remain as terminal groups : they do
It
should
which
zirconium
be mentioned that nitrates
not link chains together and should not
be considered
as network formers. Sulfates have a higher complexing ability (~-32%). Thus, they are
to replace all coordinated
zirconium
network
is
eightfold
formers,
complexation
is
water molecules leading to
coordinated
bridging
expected
three
with
(square
different
HPO~"
(a-50%)
[Zr(OH)2SO4] n species in which
antiprism).
together.
Stronger
(~-53%) ions. Chromate compounds
exhibit a layered structure in which [Zr3(OH)6CrO4]~ n+ sheets are linked together by
tetrahedra.
Zirconium
exhibit
both
eightfold
the
Moreover, SO~'anions behave as
[Zr(OH)2 ]2n+,n chains
or CrO~"
able
(dodecahedron)
CrO~"
and sevenfold (pentagonal
bipyramid) coordinations. Another structure was suggested for the phosphate derivative 219
Sol-Gel Chemistry of Transition Metal Oxides
285
Stronger
complexation
mate ions
ved
if
of chro-
(~=64%) can be obserh
is
reduced to 1.5.
[Zr4(OH)6(CrO4~lSn+-.n chains
zirconium
b
nation
in
sevenfold coordi-
(pentagonal
linked
bipyramid)
together
tetrahedra
with
are
by
Cr04"
found
in
the
resulting compound.
O H20
C
2.4.5.
Monodispersed
chromium
hydrous oxide sols. The
produc-
tion of monodispersed powders is
of the utmost importance for the
ceramic
industry.
Therefore
great efforts have been made
order to control nucleation
growth
processes
that
the formation of a
d
in
and
lead to
precipitate.
It appears that the
fundamental
requirement for the
preparation
of
monodispersed
particles
aqueous solutions is to
in
control
of the rate of generation of the
o>
solutes species that are precursors
to
precipitates
222,223
The goal is to reach a
critical
supersaturation of the
particle
Fig. 15. X-ray structures of some basic salts of zirconium. forming species so that only one
(A) {[Zr4(OH)s(OH2)1618+,8CI04 " ) 52,53
: 6(CI04)=.I.I 7
burst
nucleation
occurs.
Care
(B) ([Zr(OH)2(NO3)(OH2)2]n+,nNO" ) 217 :
6 (NO3 )=-0.96
must be taken to avoid secondary
(C)
~ (HS04)=-0.37
nucleation 2 0 1
[Zr(OH)2(SO4)(OH2) ] 218 :
(D) [Zr(OH)2(S04) ] 217,219
(E)
[Zr(OH)2(Cr04)
] 220
[Zr(OH)2(H2P04)2
.
.
] 221
:
well
been
6 (HerO 4)=-0.47
Matijevic et al. who showed that
ions 78 but not
anions
have
conditions,
specific
but
obtained, and
polymeric
in the presence
a
in
no solid
chemical
complexes
and
225,226,227
or to
the
materials
Moreover,
role
obtained
analysis
are
formed
from
the
the
of
but
containing
or CH3CO0"
nucleation
these
prior
indicates
species
that
not
ions.
oxide
can be
or phosphate
It appears that
complexing
only solute hydrolysis products are
Electron microscopy
the
hydrous
sulfate 224,225
process. Under identical experimental
anions,
to
E.
formation
sulfates
shows that strands of
of spherical particles 226
are bound in both solute chromium
in the spherical chromium hydroxide particles
the role of sulfates seems to be restricted to the nucleation step,
of polymeric
mainly neutral species, the
22Z : [Cr(OH)3(OH2)3]o,
in
particles precipitate.
polymeric
the cross-linking
of CI', NO3
absence
are
Therefore,
of solutions
by
spherical amorphous particles of
6 (H2 P04 )=- 0.50
hydrothermal ageing
illustrated
has
6 (HS04)=-0.68
chromium
generated by
This effect
chromium hydroxide
chains. As nucleation
following monomeric precursors have
[Cr(OH)2(HSO4)(OH2)2 ]° . Polymeric species
condensation
of
these
monomers.
involves
to be taken into
account
giving rise to
embryos
At high sulfate concentrations,
286
J. Livage et al.
condensation
involves mostly modified precursors.
through olation and
that in
linked together by
such polymers,
concentrations,
important
HSO~ remains
condensation
This gives rise to chain polymers
sulfate bridges.
coordinated
between
modified
to
show
atom. At lower sulfate
non modified precursors becomes more
:
in this
case the Cr-HSO 4
lose their complex-forming
processes
results
occur
225
transformed
At
As
Metal
synthesis
of
including
the
decreases
from
unsaturated
However,
a consequence,
alkoxides
metal
METAL
:
free from sulfate ions, in agreement
sulfate
concentrations,
chromium
the
ORGANIC
M(OR) n
oxides.
lanthanides
oxide
both
precipitation
polymeric basic salt
are
They
19.
versatile
are
The
MOLECULAR
known
number
with
condensation
displaces
the
should be progressively
Therefore,
(Fe, Co,
the chemistry
and d O transition metals
of electron-rich
at
relatively
transition metal alkoxides
following
of
the usual methods
silicates
Unfortunatly,
there
is
from
a
transition metal alkoxides.
lack
of
but at much higher prices
data
the
has been
concerning
insoluble
being widely
Many
Co,
have to be prepared in
the
extensively
the hydrolysis
19
The sol-gel
studied
28,30,231
and condensation
chemical reactivity of these alkoxides,
will be compared to the chemical reactivity of the corresponding
Si(OR) 4 . The main differences
arise from the following two points
- The lower electronegativity
of transition
elements
has
(V, Mn, Fe,
for the synthesis of metal alkoxides
silicon alkoxides
Therefore,
(Ti, Zr)
low cost (Si, Ti, AI, Zr).
Ni, Cu, Y, Nb, Ta)
laboratory
of
metal alkoxides
alkoxides which are already
applications
processing
saturated or
reactions which lead to the formation of
available
Otherwise,
sol-gel
the highest oxidation state
others can be found for small-scale
230
the
to the soft d n late transition metals have
Ni, Cu...) 229. Some
used in industry are commercially
for
stability of transition metal alkoxides
of main group elements
228. Moreover,
precursors
almost all transition metal elements,
table. The alkoxy group OR (R =
while those corresponding
long been restricted by oligomerization
species
molecular
is a hard ~-donor and stabilizes
alkoxides
been much less studied
PRECURSORS
for
and
left to right across periodic
organic group)
are rather well-known,
polymeric
sulfate ions
into hydrous Cr203 .
3.
the metal.
and
= [Cr2(OH)6(OH2)4] ° + HSO4 + H3 O+
intermediate
simultaneously.
previous equilibrium.
ionic (6(HSO4)<-I)
ability giving rise to ion pair formation as follows
Hydrous chromium oxide should then precipitate,
Matijevic's
[Cr2(OH)5(HSO4 )(0H2)4] ° + 2H20
bond becomes more
[Cr2(OH)5(HSO4)(OH2)2 ]°
Ti(OR)4,
formed
charge calculations
the chromium
and
[Cr(OH)2(HSO4)(OH2)2] ° + [Cr(OH)3(OH2)3] ° ~
However,
Partial
of
mostly
silicon alkoxides
:
leads to a much higher
electrophilic
character of the metal.
-
The possibility
exists for most
that full coordination
is
transition metals to
usually not satisfied
coordination
expansion.
As a result,
transition metal alkoxides
care,
in
the absence
of moisture.
are
in
exhibit several coordinations
the molecular precursor,
much more reactive.
They readily
which
They must be handled
form precipitates
so
allows
with
rather than gels when
water is added.
3.1. Hydrolysis
and condensation
Electronegative
alkoxo
of metal alkoxides
groups
(OR)
make
the
metal
atom
highly
prone
to
Sol-Gel Chemistry of Transition Metal Oxides
287
nucleophilic attack. Metal alkoxides are therefore extremely reactive with water leading te
the formation
follows
of hydroxides
or hydrous
oxides. The
overall reaction
can be
written as
:
M(OR)n + nH20 ----+ M(OH)n + nROH
This reaction is
actually much more
namely hydrolysis and condensation,
metal alkoxides.
solution,
and
Hydrolysis
a
complex than it
of the alkoxide
reactive
M-OH
might seen. Two
chemical processes,
are involved in the formation of an oxide network
occurs upon adding
group
hydroxo
water or a
from
water/alcohol
is generated. A three steps mechanism is
usually proposed in the literature 20,37
,
H-I + M-OR(a)
/
H~ :
H/O --+ M-OR
(b)
M-0H + R0H
H0-M +-- O ~
(c)
kH
(d)
The first step (a) is a nucleophilic addition of a water molecule to the positively charged
metal atom M. This leads to a transition
increased by
intermediate
one. The
(c). A
tively charged
better
second step
proton from
oxygen of
leaving
group
involves a
proton transfer
the entering water
an adjacent
which
state (b) where the coordination number of M
OR group.
should
be
the
within (b) leading to the
molecule is transfered
The third
most
has
step is
to the nega-
the departure of the
positively charged species within the
transition state (c).
The whole process,
distribution governs
(a) to (d), follows a nucleophilic substitution mechanism.
the thermodynamics
of this
reaction which
will
Charge
be highly favored
when:
The nucleophilic character
the metal atom are strong
of the entering molecule
and the electrophilic character
of
: 6(0)<<0 and 6(M)>>O.
The nucleofugal character of the leaving molecule is high
On the other hand,
: 6(ROH)>>O.
the rate of the nucleophilic substitution depends on :
The coordination unsaturation
of the metal atom in the alkoxide given by
the difference
between the maximum coordination number N of the metal atom in the oxide and its
state z. The larger (N-z), the
lower the activation energy associated to
oxidation
the nucleophilic
addition of step (a) should be.
The ability of the proton to be transferred within the intermediate
the proton,
(b). The more
acidic
the lower the activation energy associated with this transfer will be.
Condensation is also a complex process and can occur as soon as hydroxo groups are
generated.
Depending on
considered namely
experimental conditions,
: alcoxolation,
i) Alcoxolation
competitive mechanisms have
to be
is a reaction by which a bridging oxo group is formed through
elimination of an alcohol molecule.
The
with M replacing H in the entering group
mechanism is basically the same as
(a)
~
the
for hydrolysis
:
M-~ + M-OR --+ M-Ok:--+ M-OR
Consequently,
three
oxolation and elation.
~
M-O-M +-- O~/*" ~
(b)
(c)
M-O-M+
~
ROH
(d)
the thermodynamics and kinetics of this reaction are governed by the
same parameters as for hydrolysis.
ii) Oxolation follows the same mechanism
as alcoxolation,
but the R group
leaving species is a proton
M-O + M-OH
(a)
M-0:--+ M-OH
~ (b)
The leaving group is thus a water molecule.
I
M-O-M '~--:0~
(c)
~
:' M-O-M + H20
(d)
of the
288
J. Livage et al.
iii)
satisfied
Olation
in
can
the alkoxide
through the
elimination
occur
when
(N-z~O).
In
of
The thermodynamics
distribution.
The
since no proton
metal coordination
These
involved
in
structure
four
and
contribution
M - O - M + H20
this nucleophilic
is
strongly
structure
hydrolysis
that silicon
are
resulting
temperature)
not very
into contact
metal alkoxides
higher
transition
metals
very unstable
a dry
than
carefully,
in
processing
of transition metal oxides 20
Alkoxide
~(M)
and
Zr(OEt)4
Ti(OEt)4
+ 0.65
+ 0.63
the
relative
carefully adjusting
(nature of the
and external
water.
metal atom
(water/alkoxide
ratio,
On
than
it is well known
the other hand,
transition
reaction is observed as soon
rough estimate of
This
as
the partial charge
the partial positive
charge
is much
explains why transition metal
19,20,181,232. They
stabilizing agents
+ 0.53
on
The
elements are more electropositive
silicon.
Nb(OEt)s
depend
internal
shows that
for
an oxide network.
is much easier. Actually
with water. A
towards hydrolysis
environment
since the
parameters.
reactive with
(table 9) in
are
strongly
and a strongly exothermic
is
into
can be optimized by
Since transition
the alkoxide
alkoxides
oxide
are related to both
distribution
for
brought
6(M)>>0.
state and
oxolation and olation) may be
precursor
of the molecular precursor)
react vigorously
the charge
character of the
: 6(0)<<0 and
the transition
alcoxolation,
molecular
of transition metal alkoxides
alkoxides
metal alkoxides
a
the
metal atom.
ROH
the reaction rate is usually quite fast.
These contributions
solvent,
are governed by
when the nucleophilic
strength of the metal are high
of
conditions which
3.1.1. Nature of the
silicon,
substitution
favored
(hydrolysis,
of
be formed
H20 or
:
,
reactions
concentration,
groups can
be either
M - OH + M +-- O
transformation
and alkyl groups,
catalyst,
latter can
M - O - M + ROH
of each reaction.
the experimental
bridging hydroxo
This
transfer is involved within
morphology
of the metal atom is not
,
is not saturated,
the
molecule.
in the medium
entering group and the electrophilic
Moreover,
coordination
M - OH + M +-- O
of
reaction
full
this case
a solvent
depending on the water concentration
the
must be handled very
are often
added in
the sol-gel
Ta(OEt)5 VO(OEt)3
W(OEt)6
Si(OEt)4
+ 0.49
+ 0.43
+ 0.32
+ 0.46
I
1
Table 9 : Positive partial charge on M for some metal ethoxides.
Another peculiarity
of transition
metal alkoxides
of the metal readily occurs upon hydrolysis.
Hydrolysis
higher than for Si(OR)4 where the fourfold coordination
survey of the literature
ranging between 10 .4
pH=7 gives ~ = 5
metal alkoxides,
Ti(OR) 4
239-243
Hydrolyzed
concerning hydrolysis
and 10 .6 M'Is "I
is that
coordination
rates are thus expected to be
Extrapolation
minimum value of
which
is
at
~ = l O ' 3 M ' I s "I at pH-7
least
five
orders
can be
of magnitude
(~)
of this
10 .9 M'Is "1 234. Although very little data is available
a
constant at
roughly estimated for
larger than for Si(OR)4.
alcoxolation
, (RO)3M-O-M(OR)2OH
+ ROH
oxolation
M(OR)30H + HOoM(OR)3
A
gives values
for most transition
species such as M(OR)3(OH ) (M - Si,Ti) can undergo two condensation
M(OR)3OH + RO-M(OR)2OH
much
of silicon is already satisfied.
rates of Tetraethoxysilane
at pH-3 233-238
expansion
, (RO)3M-O-M(OR)3
+ H20
processes
:
Sol-Gel Chemistry of Transition Metal Oxides
The charge distribution
and R = Et
calculated within the transition states M2(OR)6(OH) 2 for M = Si, Ti
are given in table i0 :
M
6(M)
cases the
species must
+0.64
-0.36
-0.25
+0.02
-0.34
-0.21
+0.13
for two transition states during condensation.
are highly
negatively
charged metal atom. After
be removed.
6(EtOH)
+0.33
Table i0
shows that
partial charge while ethanol carries
alkoxides
6(H20)
Si
hydroxo groups
attack of the positively
6(OH)
Ti
Table i0 : Charge distribution
In both
289
in both
a positively
This conclusion
244 and for Tetraethoxysilane
alcoxolation
oxoalkoxides
which can be isolated as
Their structures has been
3 Ti(OEt)4
single crystals.
leads to well
NMR
defined
resolved
Nb 248 and Zr 249 and are shown in figure 16:
+ 4 Ti(OEt)3(OH )
2 Nb(OEt)3(OH)2
has been
by 29Si
In the case of transition metal alkoxides,
for M = Ti 2 4 7
negative
of hydrolyzed
233,245,2:46
by X-ray diffraction
chsrged
retains a
condensation
rather than oxolation.
by IR absorption
allowing a nucleophilic
cases, water
a positive one. Therefore
should proceed via alcoxolation
checked for Tetramethoxysilane
charged
proton transfer,
~ TiFO4(OEt)20
+ 6 Nb(OEt)4(OH )
5Zr(OMe) 4 + 8 Zr(OMe)3(OH )
+ 4 EtOH
, NbsO6(OEt)10
The
main
compounds
+ i0 EtOH
+ 8 MeOH
, Zr1308(OMe)36
feature of these molecular
is
that
the
usual metal atom
coordination number is always satisfied.
The low coordination
in non-hydrolyzed
xides must be
rates of
a
alke-
correlated with their
condensation.
rather slow process
where
of the metal atom
transition metal
global
high
Condensation
is a
for silicon alkoxides,
rate
M-ls-1
kc=10 "4
been measured for TEOS 250,251 . Such
constants
Ti(OR)4,
are
difficult
of the oxide. However,
tant of
B
about 30
precipitated
a global rate cons-
s "I was
from
Conden-
is also extremely fast
by
following
deduced
from
linear
turbidity
: r(nm.min'1)=0.9
[Ti] 4"I
This means that for [Ti]=0.1 M the growth-
structures
metal oxo-alkoxides
(]3) NbsO10(OEt)20
of some transition
: (A) TizO 4 (OEt)20
; (C) Zr1308(OMe)36
3.1.2. Nature of the organic ligand.
were
243
the
evidenced
rate is about 1.8 n m s "I
alkoxides
found for TiO 2
Ti(OEt)4
sation of Ti(OPri)4
measurements 242
Fig.16. X-ray
measure for
precipitation
as
growth-rate
C
to
owing to the rapid
has
rate
measured
by
authors
which is
rather
Similar behavior has been found
the
hydrolysis
ethanol where the
rates
of
for
Zr(oPrn)4
time t elapsed
mixing and precipitation
in
between
is given by 252
.
t-1(s'1)=0.9[H20] 3 [Zr(OPrn)4] I (mol.l "I)
The hydrolysis
several
p
high.
rate constants
234-237
who
for a series of
silicon
pointed out that the rate of
290
J. Livage et al.
hydrolysis
decreases with
these results can
alkoxides also becomes
shows that
increasing size of
be extended to
the alkyl groups
transition metal alkoxides
(table Ii). It
: hydrolysis
seems that
of
titanium n-
slower when the size of the alkyl group increases 239,253.
the partial
charge distribution
in the
alkoxide depends
Table
ii
on the alkyl group,
giving rise to more or less polar M-OR bonds 37
R
6(Ti)
-6 (OR)
6(H)
6(Si)
6(OR)
6(H)
~ I 0 2 M " Is I [H+] I
571
CH 3
+0.66
-0.16
+0.12
+0.36
-0.09
+0.14
CzH 5
+0.63
-0.16
+0.i0
+0.32
-0.08
+0.ii
n-C4~
+0.61
-0.15
+0.09
+0.30
-0.08
+0.09
1.9
0.83
n-C6HI 3 +0.60
-0.15
+0.08
+0.29
-0.07
+0.08
n-CgHI 9 +0.59
-0.15
+0.07
+0.28
-0.07
+0.08
Ti(OR) 4
Si(OR) 4
in Ti(OR) 4 and Si(OR) 4 n-alkoxides.
Table Ii : Charge distribution
The positive
partial charge
length of the alkyl chain. The
decrease,
of the
sensitivity
in agreement with experiments
of the hydrogen
difficult,
235
atom decreases
in
metal atom
(M ~
Si,Ti)
decreases with the
of the alkoxide towards hydrolysis
235,239,253.
the same way.
Moreover,
Experimental
Proton transfer should
charge
then become more
results are often explained in terms of steric hindrance
tertiary > secondary >
titanium butoxides,
normal 239. A reverse
the hydrolysis
for Si(OBuS)4
236,23?.
The hydrolysis
shielding,
slow
the
but
becomes
distribution
much
faster
alone cannot
for
and
effects
Gomory
253
aryloxides Ti(OR)4
resistant
to
affect
also
studied
rate of metal
hydrolysis
(R = C6H5, C6HsCH2,
m-CH3C6H4).
W(OC6H5) 6 and W(OEt)6
255
more
water
with
than
aliphatie
reaction
of the first OR
Si(OEt)4
alkoxides.
of metal-organic
of
effect
of
the
and charge
I
precursors.
a number of alkoxides and
The
same behavior
silicon phenoxide
255,256
aryloxo group increases
are
more
is observed with
Si(OC6H5) 4 appears
This difference
to be
in reactivity between
transition metals and silicon may he explained by two competing effects
-I inductive
group is
Inductive +
They concluded that aryloxides
alkoxides.
On the other hand,
than
and 500 h
the same way
be taken into account.
the hydrolysis
the
hydrolysis
reactive
may
behaves
following ones 254. Steric hindrance
explain the hydrolysis
effects of the alkyl chain should also probably
Mesomeric
i.e. hydrolysis
It
order
silicon butoxides
236 h for Si(OBun)(oBut)3
of titanium tetra-tert-amyloxide
as required on the basis of effective
as well.
rate are in the
behavior was observed for
where the measured gelation time is 32 h for Si(OBun)4,
- The
should then
the positive partial
which is an effect that could be related to the decrease of the kinetic constant
has been shown that, for isomeric
Bistan
0.3
:
the positive charge on the
metal atom.
- The mesomeric
+E
effect
of
the aromatic ring increases
groups and reduces the positive charge of
For silicon phenoxide
silicon
aryloxides
that the
atom
highly
mesomerie
+E effect
the -I inductive effect is probably the strongest
rendering
hydrolytic
Condensation
and J.D. Mackenzie
length of
stability
to
nucleophilic
attack. Meanwhile,
predominent.
also depends
upon the
This is
for transition metal
also supported by
nature of
the fact
the aryloxo groups,
i.e.
groups are more stable than classical phenoxides.
is also strongly affected by the nature of the alkyl chain. K.C. Chen
showed
the alkyl
prone
the metal atom. The magnitude
of the metal d-orbitals.
effects should be
ortho-and para-substituted
~-donor ability of aryloxo
of this
strongly depends on the availability
the
the
that the gelation time
chain 257
For transition
of silicon alkoxides
metal alkoxides,
increases
with the
under neutral
or basic
Sol-Gel Chemistry of Transition Metal Oxides
conditions,
and without any chemical modification,
the chain length precipitates
when R=Bu n 239,258,259
or R=Am n 260
precipitation cannot be avoided,
Pr i . However,
molecular
formed. Precipitation of TiO 2
that
even under mild hydrolysis conditions, when R = Et, Pr n or
: M.W.=5600 g/mole
This corresponds to molecular
from
formed
Experiments performed in our laboratory confirm
Analytical ultracentrifuging
weights
Depending on
240,242 while linear polymers seem to be
stable sols can be obtained when Ti(OBun)4
the same conditions.
mean
gelation is never possible.
or polymer colloids are
Ti(OR) 4 is observed when R=Et 240 ,241 pr i
291
or Ti(OAmt) 4 are hydrolyzed
of these
sols
for R=Bu n
under
leads to the following
and M.W.=3800 g/mole for R=~n t .
species containing at most
several tens of titanium
atoms.
This supports the formation of small polymeric species whose degree of condensation depends
on the R group. The larger the R group is, the smaller
The
morphology
main
and
characteristics
crystalline
of
phases),
oxide
obtained
the resulting polymer.
powders
via
(particle
size,
surface area,
hydrolysis and condensation of metal
alkoxides strongly depend on the identity of the alkyl group. For example, both anatase and
rutile phases can
be present in
ratio rutile/anatase can be
precursor
261~262
The
same
condensation of zirconium
particle
size
of
the
monoelinic-tetragonal
a TiO 2 powder
phenomenon
has
alkoxides 263. The
resulting
the
one of
oxidation
coordination number N.
by
using
found
These
recently
in
turn
z
- Oligomerization
features of
of
the
metal
The full coordination
its
nucleophilic ligands.
metal occurs via
the main
state
monomerie alkoxides M(OR)z.
number
been
The
metal-organic
in the hydrolytic
the morphology and
affect
the
the
sintering and
transformation of ZrO 2.
into account
compounds
of a gel.
molecular weight of the
alkyl group affects
materials.
3.1.3. Molecular structure of the alkoxide.
not take
obtained after calcination
varied by changing the
Consequently,
vacant
In non
d
: The above discussion
transition metal
is
generally
of the metal
alkoxides.
does
In such
smaller than its normal
is therefore not
satisfied in
the metal atom tends to increase its coordination
orbitals
to
accept
polar solvents one finds
alkoxy-bridging which leads
oxygen or nitrogen lone pairs from
that coordination expansion of
to the formation
of more or
the
less condensed
oligomers in which the metal attains a higher coordination number. This oligomerization
basically a nucleophilic addition of a negatively charged OR group to a positively
is
charged
metal atom M. It corresponds to an alcolation reaction which could proceeds as follows:
The degree of association depends on the nature of the metal atom. Within a
group, the molecular complexity
According to
Bradley 264
increases with the atomic
alkoxides should
consistent with all atoms attaining
divalent transition
given
(table 12).
smallest possible
structural unit
their higher coordination number. The
insolubility of
metal alkoxides
adopt the
size of the metal
(Cu, Fe,
Ni,Co,Mn) may
thus be
attributed to their
highly polymeric nature 265
Ti(OEt)4
Zr(OEt)4
Hf(OEt) 4
Covalent radii (•)
Compound
1.32
1.45
1.44
Th(OEt) 4
1.55
Molecular complexity
2.9
3.6
3.6
6.0
Table 12 : Degree of oligomerization for some transition metal ethoxides as a function of
metal size.
The molecular complexity also depends on the nature of the alkoxy group. It
decreases
JPSSC 18:4-C
with
increasing
branching
and
bulkiness
of
the
OR group because of steric
292
J. Livage et al.
hindrance
effects 19. The molecular
molecular weight measurements
for the
oligomerization
experiments
266,26z.
edge
that
show
pentacoordinated
3.09A
were
oligomers
titanium alkoxides
titanium
is
was recently
tetracoordinated
in Ti(OEt)4 , Ti(OBu")4
found
in
formed, while
Ti...Ti correlations.
the
in
hydrolysis
of
and (OAm t )
In the case of silicon alkoxides
formation
:
Metal
is performed.
These
far from being chemically
structure
alkoxides
However,
these last alkoxides
groups lead
Bradley observed that while Ti(OEt)4
the same was no longer
Si(OR)4 precursors
about
showing no
are
and
always
solvents before
the parent alcohol and
As a general rule,
nature of the solvent has to be
remains trimeric
dilution
as EtOH 19. This was due
which causes dissociation
are
taken into
in an inert solvent such
true in a polar solvent such
of the alcohol
of
showing that
the oxidation state z = 4 of Si
Therefore
to the alkoxide.
account.
while it is
to monomers
often dissolved in organic
the
benzene,
absorption
Ti(OAmt)4
19
are
inert with respect
properties
and
solvents usually correspond to
should lead to lower association.
the nucleophilic
Direct evidence
Moreover Ti...Ti distances
spectra
bulky (OPr i )
and exhibit a tetrahedral
Solvate
19,228
provided by X-ray absorption
Ti(OPri)4
and Ti(OPrn)4.
EXAFS
its usual coordination number N - 4 are identical.
monomeric
is usually estimated from
or by mass spectrometry
The shape and intensity of the prepeak observed before the
clearly
are
of
complexity of metal alkoxides
in solution
and solvation of
as
to
the
oligomer as follows:
2[Ti3(OEt)12 ] + 6 EtOH = 3[Ti2(OEt)6,
These experiments
point out a
bridging is not the only method for coordination
addition of a solvent
donor molecule
acids and react with Lewis bases
molecular
19
structure
Zirconium
dissolved
in
the
is also
for
parent
generally
instance,
alcohol
to expand its
groups.
of such solvates
than
a
in
in ethanol
(6(Si)=+0.32;
N-z=0)
Hydrolysis/Condensation
hydrolysis
precursors
rates
than
expected
when
Zr(OPrn)4
of
condensation
propanol
bridges
Zr(OPrn)4
is dissolved
formation
bonds.
which
is
rates
leads
of
and
Zr(OPrn)4
than cyclohexane
This is due to the
instead of
alkoxo
Therefore,
stable Zr(OPri)4.iPrOH
(6(Ce)-+0.75 ; N-z=4) 270 solvates can
(6(Ti)-+0.63
; N-z=2)
Alkoxy
bridges
from
a
be
can only be observed
for Si(OEt)4
appear to be more stable towards
given
alkoxide,
on the solvent used. Therefore,
to
different molecular
different hydrolysis
completely different oxide materials.
in
n-propanol.
This
can
monoliths
much
2Z2
faster
owing to the presence
when
As
a
Precipitation
however be avoided when
aprotic solvent such as cyclohexane,
ZrO 2
are
and leads to the
result,
the alkoxide
hydrolysis
and
is dissolved
into
of solvate bonds in the former and
alkoxy
in the latter.
The same
phenomenon was observed
have been synthesized by controlled
also occurs when Ti(OPri)4
Ti(OEt)4
:
dissolved
gels
complexity when
No solvate has ever been characterized
Starting
in a non-polar
polymeric
molecular
the
solvent
19
can be obtained depending
are
occurs
2ZI
reactions
solvate
of solvation,
an inert solvent.
the
behave as Lewis
increases with the positive charge of the metal atom
while Ti(OEt)4.EtOH
in the solution at low temperature
Because
Alkoxy
process of
then
coordination with alcohol molecules
; N-z=3) 268,269 and Ce(OPi)4.iPrOH
isolated as single crystals,
The alternative
reduced
and its tendency to acquire a higher coordination number.
(6(Zr)=+0.64
alkoxides.
depends on the nature of the
exhibit
rather
tendency of zirconium
The stability
expansion.
found. Metal alkoxides
leading to solvate formation.
of alkoxide precursors
alkoxides,
2 EtOH]
very important property of metal
exhibits an
monomeric.
Therefore,
is dissolved
oligomeric
hydrolysis
for titanium alkoxides.
hydrolysis
in iPrOH
structure
through
is much faster
Monodispersed
of Ti(OEt) 4 in EtOH
240,241.
but the monodispersity
ethoxy bridges while
for this
is
TiO 2 powders
Precipitation
lost 240-242
Ti(OPrl)4
latter precursor
than
remains
for the
Sol-Gel Chemistry of Transition Metal Oxides
former. Condensation,
being
a fast process
powders can be obtained with Ti(OEt) 4
rates. This is not possible with
in both cases,
where
293
means that monodispersed
hydrolysis rates are lower
TiO 2
than condensation
Ti(OPri)4 where hydrolysis and condensation rates
are of
the same order of magnitude.
All these experiments
taken
into
account
in
show that the molecular structure of the precursor has to be
order
to
describe
demonstrated by Bradley et al. 254 who
of
alkoxides.
They
proposed
chemical
reactivity.
This
was
first
carefully studied the hydrolysis of a large
different
behavior of transition metal alkoxides
its
structural
(Ti, Zr,
number
models to account for the hydrolysis
Nb, Ta, Ce). For each model the
molecular
complexi1~y deduced from ebulliometric experiments can be related to the hydrolysis ratio
assuming a sixfold coordinated metal atom. The structure of the molecular precursor is
supposed
species.
to
be
modified
upon
hydrolysis,
Four structural models names paqb
results
h
not
and condensation occurs between oligomerized
have been proposed to account for
experimental
:
The p3q4 model based on non solvated trimeric units Ti3(OR)12
-
The p2q3 model based on solvated dimeric units M2(OR)8(ROH)2
-
(R=Et, Pr n, Bun).
(M=Zr, Ce, Ti).
- The plq3 model based on solvated monomers M(OR)4(ROH) 2 (M=Ti, Zr, Ce).
- And the plq2 model based on solvated monomers M(OR)5(ROH)
of pentaalkoxides
(M =
Ta, Nb).
The pioneer work of Bradley emphasizes
the fact that both alcolation and
formation can play a decisive role in hydrolysis/condensation
solvate
reactions.
3.1.4. Hydrolysis ratio. The main external parameter is the hydrolysis ratio h which can be
defined as :
h
Bradley showed
that for
[H20]
[M(OR) z ]
a given
(i0)
model paqb
a mathematical
relation can
between the average condensation degree n and the hydrolysis ratio 271
I/n = i/a - I/h
This
relation
hydrolysis
shows
ratio.
experiments.
that
condensation
However,
For instance,
the trimeric
(ii)
could
quantitative
be established
.
be
adjusted
predictions
show
by a careful control of the
some
discrepencies
structure proposed for Ti(OEt) 4 in
which titanium
atoms have a sixfold coordination 254
does not agree with XANES-EXAFS experiments
which suggest a fivefold coordination
in Ti(OR) 4 (R = Et,
to the X..ray data
on single crystals giving
TiTO4(OEt)20
does not
(figure 15) 247,274
domains could be considered in a rough qualitative analysis
:
- h<l
by alcolation
: In
reactions.
this domain
condensation is
mainly governed
266,267
Bun). This may explain why
first hydrolysis product predicted from Bradley's model, Ti604(OEt)16,
with
the
correspond
Three main
and alcoxolation
The functionality of precursor towards alcoxolation is always smaller than
one,
while for alcolation it could go up to z-i (i.e. three for a tetravalent metal). Under such
conditions,
as
long
as
processes,
an infinite network is seldom obtained.
hydrolysis
alcolation
remains
or
carefully
alcoxolation
which can be isolated as single
lead
Gelation or precipitation cannot
controlled
(no
local excess of water).
occur
Both
to molecular transition metal oxo-alkoxides
crystals from the solution (figure 16).
Alcolation cannot
occur with silicon alkoxides owing to the fact that N-z=0. However, molecular compounds can
be
for~ed
through
alcoxolation
(dimers,
trimers,
tetramers .... )
which
have
been
characterized by 29Si NMR in solution 246
Oxo-alkoxides
are the organic counterparts of polyanions and polycations which can
be obtained in aqueous solutions under
careful control of the pH. Moreover,
the structure
J. Livageetal.
294
of
these
molecular
isostructural
clusters
with
Mo706~__
close
is
to
their
Nb8010(OEt)20
and
inorganic
has
analogs.
the
same
Ti704(OEt)20
is
structural units as
paratungstate Z [ H 2 W 1 2 0 4 2 1 1 0 "
- ishs z : Charge calculations are reported
the
hydrolysis
ratio.
Ti(OPri)4
was
in table 13, in order to describe the
chosen
as
an
example
because
role of
of its monomeric
structure. Table 13 gives the results obtained for h~0,1,2,3 or 4, in the transition states
Ti(OH)h(OR)4.h(OH2)
and hydrolyzed species Ti(OH)h(OR)4. h :
Precursor
Ti (OPri)4
h
6(OPr i )
6(OH)
6(priOH)
6(H20)
6(Ti)
0
-0.15
Ti(OPri )4 (OH2)
I
-0.08
-0.38
+0.01
-0.28
+0.62
Ti (OPr i )30H
1
-0.08
-0.38
+0.02
-0.28
+0.62
2
-0.00
-0.36
+0.i0
-0.26
+0.64
2
+0.04
-0.36
+0.15
-0.25
+0.64
3
+0.13
-0.34
+0.25
-0.22
+0.65
3
+0.28
-0.32
+0.41
-0.18
+0.67
Ti(OPri)(OH)3(OH2)
4
+0.38
-0.30
+0.52
-0.16
+0.68
Ti(OH) 4
4
+0.01
+0.76
Ti(OPri)3(OH)(OH 2 )
Ti(OPri)2(OH)2
Ti(OPri)2(OH)2(OH 2)
Ti(OPri)(OH) 3
-0.19
Table 13 : Influence of the hydrolysis ratio h upon the charge distribution in monomeric
precursors.
This table shows that the first
6(OR)<0
and
6(Ti)>0.
alcoxolation may
As
occur in
alcoxolation should be
steps of hydrolysis (h<2) can readily
previously
discussed,
this domain.
Owing to
favored thermodynamically
competition
the positive
.
between
occur when
oxolation
partial charge
Under such conditions,
and
on ~PrOH,
chain polymers
can be obtained according to the following simplified scheme :
n M(OH)(OR)
3 ~
...-
0
-
I I I IZ Ii
R-
O -
R-
0
-
R-
0
-
O -
0
-
...
+ nROH
Such polymers were first obtained by Boyd and Winter with Ti(OBun)4 239,258. Under
similar conditions, spinnable sols
were synthetized by Kamiya
et al. 275,276, from
which
SiO 2 or Tie 2 fibers could be drawn.
Upon further hydrolysis, the partial charge of the OR group becomes more and
more
positive. This means that the prototropic transfer could become the rate limiting step.
a consequence, hydrolysis
may not go
to completion even
when h=4. This
prediction is in
agreement with experimental data showing that the fourth alkoxy group is very difficult
remove
via
hydrolysis
or
239,241,258-260
alcoxolation
Therefore,
As
condensation
to
via
oxolation becomes highly competitive when the full coordination is already satisfied (TMOS,
TEES).
However,
elation,
can
6(M)>>0 and
in
occur
the
case
of
transition
preferentially
N-z>>0) are
because
fulfilled. The
metal
the
alkoxides, the alternative pathway,
required charge conditions (6(OH)<<0,
formation of
elated polymers
in this
domain is
strongly supported by the fact that upon ageing, solvent is released via syneresis.
-
h>z :
excess
Cross-linked polymers, particulate
of
water
is
added
to
the
gels or precipitates
alkoxide.
hydrolytic ratio strongly affects the mean
monodispersed
powders
based
on
for Si, Ti, Zr alkoxides. Using an
Ti02,
ZrO 2
and
Ta205
have
experiments,
can be
excess of
been obtained via
controlled precipitation of Ti(OEt)4 240, Zr(OPrn)4 279 and Ta(OEt) 5 280. As
is an extremely fast process in these
when an
size and weight of macromolecules which
formed. This observation seems to be general
water,
can be obtained
It has been observed 263,277,278 that the
precipitation
it is highly probable that elation
not oxolation is the predominent pathway for condensation.
and
Sol-Gel Chemistry of Transition Metal Oxides
3.1.5. Role of the catalyst. Another
295
way to control hydrolysis and condensation
is to adjust the pH of the water used to perform hydrolysis.
processes
This can be done with an
acid
such HCI or HNO3, or a base such as NH 3 or NaOH.
- Acid catalysis
: Negatively charged OR groups can be easily protonated by H3 O+ ions :
M - OR + H30 +
Under such conditions,
group can no
longer be the
, M +--:O~
the prototropic
+ H20
transfer and the departure of
rate limiting steps.
As a consequence,
the leaving
all OR groups
hydrolyzed as long as enough water is added. Hydrolysis rates can thus be greatly
by
using
an
acid
234,235,281,282.
catalyst.
This
seems
to
be
a general conclusion for all alkoxides
In the presence of H30 + , condensation occurs between these rapidly
hydrolyzed species M(OH)x(OR)z.x.
HO-
can be
improved
formed
Let us consider a typical polymer such as :
- 0 - . . . - 0 -
- 0 - . . . - 0 -
A
- 0 - . . . - 0 -
-OR
C
(charge calculations performed on different moieties of this polymer
(A,B,C,D) are gathered
in table 14 :
SITE
6(OR)
6(Ti)
A
-0.01
+0.70
B
+0.22
+0.76
C
+0.04
+0.71
D
-0.08
+0.68
Table 14 : Charge distribution along a titanium oxo polymer.
It is easily
D>>A>C>>B.
seen that reactivity
towards protonation decreases
in the order
OH groups are thus preferentially generated at the end of chains which leads
rather linear polymers
283,284. The control
acid catalysts together
of gelation rates
with substoichiometric hydrolysis
is thus possible
ratio. Under such
the condensation
process.
more acidic conditions
Protonation
of the
(close to [H÷]=[Ti]),
hydroxo group
to
by using
experimental
conditions spinnable sols 275,276 or monolithic gels 244,277 can be reproducibly
It must be pointed out that
:
obtained.
strongly inhibit
becomes possible,
leading to
mixed aquo-hydroxo species such as those encountered with inorganic precursors.
The use of hyperacid catalysts such as trifluoromethanesulfonic
trifluoroacetic acid (CF3COOH)
may
be
completely
different
acid (CF3SO3H)
is also possible. However in this case,the reaction
and
involves
extremely
reactive
intermediates
or
pathway
such
as
sililenium ion (>Si ~) 285
- Base catalysis
: Under acidic conditions hydrolysis and condensation can be
uncorrelated
233. This is no longer the case with basic catalysts. Using NH 3 as a catalyst,
that
hydrolysis
of
silicon
nucleophilic activation
Conversely,
alkoxides
of silicon
was
activated
through the
using NaOH as a catalyst,
234,235
This
coordination of
could be due to a
the nitrogen
Bradley 281 showed that hydrolysis of
more difficult than under neutral or acidic conditions.
it was shown
lone pair.
Ti(OBuS)4 was
In this case, nucleophilic addition
of OH" can occur which decreases the positive charge of the titanium atom.
Using
NH 3
highly nucleophilic
or
NaOH,
condensation
species such as M - O"
M
-
OH
+
:B
-
M
is
always activated through the formation of
:
-
O"
+
BH +
(B
=
OH',
NH3)
This reactive condensation precursor will attack the more positively charged metal
296
J. Livage et al.
atom. According
to table
linked polymers
conditions,
obtained.
14, the
order of
reactivity will
depending upon the hydrolitic
ratio, non-spinnable
This is also the case if olation is a competitive
3.1.6. Other physical parameters.
The hydrolysis
most important external parameters
concentration,
be B>>C A>D.
are expected to be formed in agreement with literature
nature
of the
and
Under
sols or particulate
temperature
such
gels are
pathway for condensation.
ratio and nature of the catalysts
in sol-gel processing.
solvent,
Strongly cross-
283,284.
However,
can
are
other parameters
also play
the
such as
a decisive role in
reactions pathways.
Dilution,
for
instance,
processes when acid catalysts
could
help
to
and high hydrolytic
separate
hydrolysis
and condensation
ratios are used. This has been shown
for
TEOS by several authors using 29Si NMR 233,238,286,287
Another effect of dilution is to prevent growth through aggregation.
Yoldas, 263'277'2T8
for Ti(OR)4
the m e a n polymer size decreases
and Zr(OR)4
systems.
This
According
as the precursor concentration
is intimately
linked to
the occurence
to
increases
of sol-gel
transition which is strongly affected by aggregation processes.
Solvent effects are much more
(formamide,
propylene
for hydrolysis
bonds.
carbonate
subtle.
and water in
and condensation
reactions
Solvents having a high
large excess)
through
can
dielectric
induce different pathways
the cleavage
of the
polar M
It is usually assumed that cleavage occurs at the M - 0 bond 288,289.
may not
be the
reactive
intermediates
case
Increasing
processes.
As
a
when
tertiary alkoxides
such as carbocations
the
temperature
consequence,
temperature may be
reactive
this
conditions-highly
hydrolysis
precursors
the sol-gel transition.
such as transition
lowered in order to slow down hydrolysis
In such
activates both
poorly
increased to activate
strongly reactive precursors
are hydrolyzed.
- O - C
However,
not be neglected.
may
generally
with
constant
metal alkoxides,
and condensation processes
such
and condensation
as
Si(OR)4 , the
On the other
the
hand,
temperature
for
must be
as shown by Rehspringer
et al. 290 in BaTiO 3 processing.
3.2. Chemical modification
of metal alkoxides
One of the main drawback or advantages
of transition metal alkoxides
reactivity with water.
They must be handled with great care,
is
rather
usually
additives
than
gelation.
A
survey of literature
are almost always used in order to improve the sol-gel process
materials.
agents,
observed
Such additives
291,292
nucleophilic XOH
precursor 20
or
can
drying
molecules
be solvents 257, acidic
control
that
chemical
react with
additives
M(OR) n + x XOH = M(OR)n.x(OX)x
The chemical reactivity of the
on the following
-
282
better
stabilizing
293,294. In most cases they are
giving rise
to a new molecular
+ xROH
alkoxide with nucleophilic
species mainly
power of the metal atom increases when its electronegativity
- The ability of the metal atom to
(N-z) between
increase
its coordination
its usual coordination
state z. For a given group,
- The nucleophilic
and obtain
depends
:
The electrophilic
difference
shows that chemical
or basic catalysts
the alkoxide
is their high
in a dry box and precipitation
number N in
decreases.
that can be estimated as
the oxide and
(N-z) increases when going down the periodic
the
its oxidation
table.
strength of the chemical modifiers.
Addition or substitution
reactions
differently with respect to hydrolysis
lead to
new molecular precursors
and condensation.
The charge distribution
which react
among
the
Sol-Gel Chemistry of Transition Metal Oxides
metal atom and its
ligands
is modified leading
occur when the coordination
nucleophilic
hydrolysis
reactions
and
funetiennality
behavior of
together
condensation.
of
with
It
a
to enthalpy changes while
Both effects
be
noted
mixed alkoxide
compounds M(OR)z
while addition reactions
of
the
is not
Substitution
it unchanged.
Thus,
and condensation.
Less electronegative
hydrolysis while
more electronegative
reactions.
particles becomes more anisotropic which promotes
Molecular modifications
particle
morphology,
(monomer
can
k
depends on the hydrolysis
different products
sol-gel
transition
decrease
the
promotes
ligands are first
ones (the
a
and
modifiers)
the growth of the
the formation of polymeric
= [C0k(f2-2f)]'1
then be varied
concentration),
gels.
such as gelation
time,
in polymer chemistry is
(12)
in order to
(bimoleeular
rate). A good
in table 15. It suggests
The
the
(12) 295,296
t
Three parameters
etc...
reactions
As a consequence,
have a strong effect on parameters
porosity,
usually given by equation
for
deduced from
substitution
rather quickly
should be mainly removed during condensation
ligand reactivity
often simply
decoupling between hydrolysis
removed upon
of the
that the chemical reactivity and the
and M(OX)z.
leave
entropy changes
lead to a modification
differentiation
should
a M(OR)z.x(OX)x
the parent
functionnality
number increases.
297
optimize
condensation
the sol-gel
rate)
process,
namely C O
and f (functionality which
rule of thumb for the sol-gel chemist
that, depending on the relative hydrolysis
is reported
and condensation
rates,
can be obtained.
Hydrolysis
rate
Condensation
rate
Result
SLOW
SLOW
COLLOIDS/SOLS
FAST
SLOW
POLYMERIC GELS
FAST
FAST
COLLOIDAL GEL
OR GELATINOUS
PRECIPITATE
SLOW
FAST
CONTROLLED
PRECIPITATION
Table 15 : Products obtained according to the relative rates of hydrolysis
and
condensation.
3.2.1. Alcohol
interchange.
following equilibrium
Metal alkoxides
react with a variety of alcohols
M(OR) z + x R'OH = M(OR)z.x(OR')x
In general,
R
group
the facility for interchange
decreases
;
exhibit
faster
exchange
illustrated by NMR experiments
place at room temperature
rates
233,245
isopropanol molecule
hours.
Conversely,
using 29Si
takes
VO(OPri)3
takes
place
alcohelysis
are
alkoxides.
reaction also
transition metal
This
point
the metal probe. Recent 29Si NMR
17a), that
can
take
:
+ x EtOH
exchange between ethoxy group and
catalysis
performed
can be
measurements
and solvent molecules
) Si(OEt)4.x(HOPri)x
NMR (figure
alkoxy
on a
in our
time scale of
laboratory have
about twenty
shown that the
:
+ x (HOAm t)
instantaneously
reactions
silicon
of the
. The following reaction has been studied 233
experiments
following exchange reaction
More particularly,
silicon alkoxides
place under acidic
51V NMR
facility for the interchange
atom 19
than
performed on
Si(OEt) 4 + x HOPr i
been shown,
The
nature of the metal
have clearly shown that exchange between
It has
the
+ x ROH
increases when the steric hindrance
OMe>OEt>OPri>OBu t "
depends strongly on the
alkoxides
to set up
:
at
widely
room
~ VO(OPr~)3.×
temperature
used for the
(OAmt) x + x HOPr ~
(figure 17b) without a catalyst.
synthesis of
metal alkoxides.
Such
It is well
298
J. Livage et al.
known that hydrolysis
and
conden-
sation rates depend on the
nature
of the alkyl group. Therefore,
should be
possible
rate
gelation
of
alkoxide
by
to
it
adjust the
of
a
using
given
different
solvents 257
¢o
Similar experiments
performed in
titanium
A
"
B ~
i
i
J
-80
Fig.17.
J
i
-100
'llil
. .
stoichiometrie
(H20/Ti~2)
PPml
obtained
the
other
Ti(OAmt)4.
few
Ti(OPri)4
is dissolved
minutes
with no
acid catalyst after I0
chloride
alkoxldes.
giving rise
Metal
to halide
alkoxides
alkoxides.
which
be
into
to
calculations
20
are
known
Chloride
TiCI 4 + 3 EtOH
The reactivity of
metal chlorides
, TiCI2(OEt)z.EtOH
decreases
i.e. when going down the periodic
pushed to completion with
only partial
substitution
alkoxides can be considered
easy to synthesize
and
:
groups
hydrolyzed
partial
charge
to react with halogen or
alkoxides
can
also be
very
19
+ 2 HCI
with increasing electropositive
character of
Under the same conditions
TiCI2(OEt)2.EtOH
i.e. alcoholates
to be chemical modifications
can be used as
of
table. The reaction of SiCI 4 with EtOH can
the formation of Si(OEt) 4 .
while ThCI 4 forms only addition compounds
AmtOH
alkoxide
(OAm t )
first
according
when
Formation
occur
in
On
gelation occurs
a
easily obtained through the reaction of metal chlorides with alcohols
ZrCI 4 undergo
with
hand,
20 hours.
hydrogen halides
the metal,
solutions
within
should
water
colloidal
in propanol-2 with an acid catalyst after
minutes.
Metal
are
of
a
Ti(OPri)4
a mixed Ti(OPri)2(OAmt) 2
in iPrOH
preci-
added to
prior to hydrolysis.
(a) 29Si NMR spectrum of a solution of Si(OEt) 4
(b) 51V NMR spectrum of a solution of VO(OAmt)3
3.2.2.
TiO 2
amount
is
while stable
__.,
I , I , I , i
8()
- 400 -500 -600 -700 O'
J
-120
alkoxides.
with
pitates are readily formed when
iJl
....
have been
our laboratory
297
be
TiCI 4 and
and ZrCI3(OEt).EtOH
: ThCI4.4EtOH
298
19
Such
chloride
of the alkoxides.
They
are very
molecular precursors
for the
sol-gel processing of
transition metal oxides.
Niobium pentoxide
metal organic,
Nb(OEt)5,
gels are
precursors.
quite difficult
to obtain
occurs rather than gelation.
Niobium chloride alkoxides
dissolved into an alcohol 181
:
NbcI 5 + 3 ROH
Solutions
of these chloride
from inorganic,
Both are highly reactive with water and
, NbCI2(OR)3
are
NbCI 5 , or
precipitation
readily formed when NbCI 5
is
+ 3 HCI
alkoxides are quite stable.
They can be stored
in a
dry environment without any special care. Gels can be easily obtained through hydrolysis
these solutions with an excess of water. The rate of gelation depends on the alcohol
It is much
PriOH,
faster the longer
the alkyl chain.
Gelation occurs within
of
used.
a few seconds with
a few hours with EtOH and several days with MeOH 181
Electrochromie
Tungsten hexaehloride
WO 3
layers have
is dissolved
been made
from tungsten
in ethanol where upon
chloride alkoxides
a violent reaction the
20
solution
Sol-Gel Chemistry of Transition Metal Oxides
299
turns blue. The chemical reaction can be written as follows 19 .
WCI 6 + 2 EtOH
Reduction of W(VI)
solutions
of oxychloride
alkoxides
for making electrochromic
3.2.3. Acetic acid.
an alkoxide 2 9 9
to
Stable metal alkoxo-acylates
increase
:
M = Si 275,
hydrolysis
AI 300, Ti 301,302
rates
while homogeneous
of acetic acid 20,301
An exothermic
leads to a clear
and
infra-red
spectra
A stoichiometric
that
CH3CO0"
coordination
is not satisfied.
intermediate
is
with
Therefore,
charged
calculation
charged
NMR
The
more
hydrolysis
titanium and thereby slow
(6=+0.61)
in Ti(OBun)4
down the
which
leads
to
+ H20
ligands.
-
giving
in this intermediate
the
charged
(6=+0.1).
substituted alkoxide
addition of H20:
charged
(6=-0.6) while
(OBu n) groups are then removed first upon hydrolysis
266
As
acetate groups are not immediately
groups
located
the slower gelation
removed
of Ti(OBun)3(OAc)
is smaller than
around
the smaller the
occurs.
titanium,
In
agreement
Acetylacetone
is known to be a rather strong
have already
been reported in the literature 3 0 3
contains
19,303
Therefore,
as a stabilizing
Ti(OPri)4
a reactive
hydroxyl group
acetylacetone
has
often
agent for metal alkoxide
266, Ti(OBun)4
which acetylacetone
is used
been stabilized up
to high pH
full
......~.. Ti(OR)3(OAc)(OH 2)
the functionality
(OAc)
therefore
with
and its
addition of AcOH is possible
The charge distribution
shows that AcO remains negatively
(6=+0.2).
~-diketones
literature
291,305,
can
+ BuOH
are first removed upon
(6--0.7) while BuOH is positively
experiments
and many metal ~-diketonates
alkoxides
13 C
while
with this
the gelation time strongly increases as the molar ratio HOAc/Ti approaches
Chelating
form of
shows
ligand (chelating and
of this new precursor begins via a nucleophilie
functionality will be and
3.2.4.
a bidentate
Ti(OBun)3OAc
nucleophilic
removed
or condensation,
Ti(OBun)4.
precursor
based on the Partial Charge Model are in agreement
then
Hydrolysis
through hydrolysis
analysis,
as
bonded much longer to
Ti(OBun)3(OAc)
of
,
: Ti(OBun)4(AcOH).
shows that AcOH is negatively
agreement
behaves
Titanium has a high positive charge
(Buno) is positively
on the Ti(OBun)4
from 5 to 6 upon acetic acid addition 266
show that acetate groups are bonded to titanium
+ AcOH
Calculations
A charge distribution
even
place when acetic acid is added to Ti(OBun)4 , which
show that (BunOH) groups
rise to the
Ti(OBun)3(OAc).
obtained in the
to a few minutes or
:
while chelating acetates remain
molecule
ZrO 2 gels are
chemical reaction takes place for a one to one ratio which
Ti(OBun)4
these experiments.
metal
readily occurs when pure water is added
X-ray absorption experiments
Infra-red and NMR experiments
gelation process 301
is
to the complexing ability of the acetate ligand.
indicate
be written as follows
catalysis
acid is currently used to decrease the
transparent TiO 2 or
number of Ti increases
bridging).
that
acetic
or Zr 20,263. Acid
Gelation times then increase up
and IH NMR of the modified precursor
alcohol
used
effect was actually observed with transition
reaction takes
solution.
that the coordination
in
and
A reverse
days. This can be attributed
An
Stable
They can be kept for months and
can be formed when acetic acid is added to
such as Ti(0R) 4 or Zr(OR) 4 . Precipitation
to the alkoxide
presence
are thus obtained.
layers by dip-coating 20
gelation time of Si(OR)4282.
alkoxides
+ 1/2 CI 2 + 2 HCI
Acetic acid is often used as an acid catalyst in the sol-gel processing of
metal alkoxides M(OR)n
known
, WCI3(OEt)2
to W(V) can be avoided by using WOCI 4 , instead of WCI 6 .
306 or Al(OBuS)3
to improve the process
with acetylacetone
which reacts
been
precursors
2 301
chelating
ligand
The enolic
readily with metal
reported
in
the
: W(OEt) 6 304,
sol-gel
Zr(OPri)4
292. Patents have even been obtained
307,308.
309
Recently,
in
TiO 2 colloids
have
X-ray absorption experiments
show
300
J. Livage et al.
that Ti(OPri)4
acetylacetone
is a four fold coordinated monomer.
is mixed to Ti(OPri)4
with infra-red experiments
that the coordination
show
number
that acac ligands are
increases up to 5
which can be written as follows
Ti(OPri)4
+ acaeH
turns to 6
correlations
visible
condensed species.
acac ligands
Precipitation
or
diameter are obtained.
modification
-
, Ti(OPri)3(acac)
in
the
EXAFS
was
not
showing
that
even when
observed.
These colloids are
a large
Small
of H202. According
to R. Roy
et al. 310
aerohydrogels
appear to
have a
leads to
first. All
excess of water is added.
colloidal
much smaller than
Ti...Ti
hydrolysis
show that (OPr i ) groups are hydrolyzed
: Some papers 181,292,305
that
suggests
takes place
+ priOH
particles
about 5 nm in
those obtained without
(15 nm) which shows that this new ligand prevents
Hydrogen peroxide
XANES
reaction
when
together
added to the new precursor.
spectrum
completely removed,
gelation
bonded to titanium.
A stoichiometric
as soon as water is
NMR and I.R. spectra
cannot be
20
reaction takes place
IH and 13C NMR spectra,
:
Titanium coordination
become
An exothermic
in a one to one ratio.
condensation
report the formation of gels in the
the reaction of
H202 with alkoxides
fibrillar microstructure.
acac
20
Monolithic
presence
results in
Nb202 gels can
also be easily obtained when H202 is added to NbCI 5 rather than H20 181. In both cases,
resulting gels exhibit a yellow-orange
311,312
Complex
during gelation.
polymerization
color
processes
have been found for peroxotitanium
can be formed as follows
distribution
(6(O2)~-0.89).
It
Ti(OEt)202.H20
negatively
molecule
+ H202
groups more facile.
the functionality
Organically
steric
gels.
In
calcinated
ORMOSILS,
A transition state
is added to the new precursor.
while the
, Ti(OEt).O2(OH)
or
in order to
and the departure
alkoxy
of
alkoxy
removed upon hydrolysis
which
explain the
can
to new,
modified
(ORMOSILS)
Si-C
derivatives
Therefore,
compounds
or o-hydroxyacids
with
have been
bonds are
formed
reactivity
cannot be extended to transition metal oxides
destroyed upon hydrolysis.
using polyhydroxylated
react
silicates
non-hydrolyzable
network formers depending on the chemical
Such a modification
effects.
leading
EtOH
+ EtOH
a value close to 2. This could
Organically
or polyethanolamine)
species
The
positively charged
species such as :
groups are not
these compounds,
M-C bond would be
be performed
hindrance
to Ti
group is negatively charged
to titanium.
observed by TEM 310
306,318-320. These
condensation
bound
attack of water molecules
these peroxo
modified
of the organic group 3 1 7
derivatives
+ 2 EtOH
peroxy
molecule
of the alkoxide to
314-316
because the more ionic
These
strongly
, Ti(OEt)202.H20
which behave as network modifiers
polyethyleneglycol
the
giving rise to hydrolyzed
+ H20
Moreover,
recently developed
7
peroxy species
increase the positive partial charge of the metal atom and the
fibrillar microstructure
acids).
be
that
remains bound
groups, making both the nueleophilic
could however
, Ti(OEt)202
shows
therefore
peroxy group
O~" ligands
numbers up to
compounds 312. Let us suppose that
could be formed when one water
charged
Ti(OEt)202
3.2.5.
are thus involved
its coordination 311 . Coordination
(IV)
calculation
should
can be removed,
decreases
peroxy compounds
:
Ti(OEt)4
charge
involving
the
species
Peroxy ions 022" are known to be strong chelating ligands that are able to
react with the metal atom and increase
A
arising from the formation of peroxy
metal
alkoxides
such
(glycolic,
giving
Organic
modification
as polyols
salicylic
rise
(glycerol,
or
mandelic
to mixed alkoxide
appear to be very stable because of chelate
they
are
mixed organic-lnorganic
obtain a ceramic powder
not
removed
materials.
290. They can also
and offer a wide range of new possibilities.
during
These
hydrolysis
compounds
can
be used as such,
and
and
be
like
Sol-Gel Chemistry of Transition Metal Oxides
Electrolyte
gels
have been
made via
the reaction
301
of a
polyol
(glycerol)
and a
carboxylic
acid (acetic acid) with a titanium alkoxide Ti(OBun)4
321. More or less
gels
are
obtained
(Ti-OH2C-CHOH-CH20-Ti)
inorganic
(Ti-O-Ti)
Layers deposited
(o=5.10 .4
upon
hydrolysis
bridges are
from these
Scm'1).
Such
in
which
formed.
They
gels exhibit
gels
have
both
organic
remain stable
high proton
been
used
even upon
conductivities
as electrolytes
viscous
heating at
and
80°C.
at room temperature
for making electrochromic
display devices 321
Reactions with maeromolecules
to other
organically modified
such as
TiO 2 gels.
cellulosic
In such
322 or polysaccharides
compounds
a
production
in different amounts.
of high viscosity
gelation of cellulosics
Application
fluids for hydraulic
or textile materials
to
lead
good control of the cross
linking of hydroxy group is achieved by merely mixing the non-hydrolyzed
polymeric material
323
of these materials
alkoxide with
are manyfold
the
:
fracturing 323
make water repelant or
flame retardant
fabrics 324,325
Very
few
papers
concerning
metals have been described 326,327.
modified
with
modifier
in
vinyl
a one
partial hydrolysis
deposited
good example
acetylacetone.
to one
A
ratio. A
onto
a
Ti(OBun)4
giving
A
involving transition
gels organically
alkoxide first reacts with the organic
process
and radical polymerization
as a catalyst.
substrate
copolymers
is provided with TiO 2
double polymerization
of the alkoxy groups
using azobisisobutyronitrile
be
organic-inorganic
A
viscous product
photochromic
is
then initiated via
of the vinyl
functions
is obtained that can
easily
coatings which turn blue upon U.V.
irradiation 20
4. ORDERED AGGREGATION AND INTERCALATION
All colloidal
spontaneously.
This
colloidal aggregates
geometry
is
systems have an
is a
random process
usually exhibit
often used
to describe
measured by Small Angle X-ray
slope
of
the
aggregation"
in-built tendency to become unstable
scattering
such aggregates
(SAXS) or
curve
in
governed by
structures.
328. The
Neutron Scattering
the
Porod
or "diffusion limited aggregation"
the observed fractal dimensions.
Brownian motion,
disordered open
region
are
and as
a result,
concept of fractal
fractal dimension
(SANS).
329,330.
then computed
Most of the studies published
The
and aggregate
can be
It is deduced from
in order to account
in the literature
deal
silica gels. Transition metal oxide colloids however may exhibit a large variety of
201 and two possibilities
have to be considered for aggregation
i) if for any mutual orientation
is
less
than
ultimately
kT,
all
of the colloidal particles,
collisions
will
be non-elastic
the
Models such as "cluster
for
with
shapes
24
the potential
energy maximum
and the multi-particle
aggregate
formed will be completely disordered and isotropic.
ii) if for a particular
maximum is less than
and, therefore
anisotropic
colloidal particles
that exhibit
orientation of the two colliding particles,
kT, while it is
aggregate
in excess of kT
is bound to result.
are strongly anisotropic
specific properties
such as
the potential
for other orientations,
(platelets,
Such aggregates
an
energy
ordered
usually occur when
rods). They lead to sols or
streaming birefringence,
rheopexy or
gels
chemical
intercalation.
4.1. Anlsotropic
Aggregates
4.1.1.
formation.
Tactoid
Electrostatic
repulsion
between
charged colloidal particles
302
J. Livage et al.
usually prevents
to a long-range
a crystal.
aggregation
and flocculation.
Such systems,
known as "colloidal
spherical colloidal particles
the same order
colloidal crystals
such a long-range
anisotropic
wavelength
then appear
iridescent.
shape
is
such as
isotropic
the
case
rods or
phase.
called "tactoids".
of
colloidal
platelets.
Interactions
light
is of
scattering.
are a well-known
24,332
colloids
lead
to the layers.
be called "smectic"
particles
Colloidal
These
example
of
solutions
solid particles
are
anisotropic
mutually oriented giving rise to the
so-
dispersed
which
have
a
are
phase and
the
are randomly
sediments
of these non-spherical
quite strong in
into
Two main types of orientations
to
that exhibit a strongly
into a concentrated,
between
Colloidal particles
phase called "atactosol"
the so-called
to visible
The natural opals
concentrated phase where colloidal particles
perpendicular
in
ordering.
interesting
Platelike
can lead
array as
or SiO 2 . The distance between particles
giving rise
particles may separate under suitable conditions
a dilute,
these interactions
crystals" 331, are observed with monodispersed
such as latex
as the optical
More
In some cases,
ordering in which charged colloids are placed along a periodic
the isotropic dilute
have been observed
periodicity
along
:
the
axis
The tactoids which are formed by such oriented aggregates
tactoids.
They are
"schiller layers".
characterized by a brilliant
Tungstic acid
or ~-FeOOH
luster giving
are typical
may
rise to
examples of such
systems 333,334
Rodlike particles
called
"nematic"
are arranged with their
tactoids.
typically ellipsoidal
The
best
tactoids 335
cylinder and
V205,
particles.
colloidal
energy between
may be
sols that give rise to
these tactoids results from
a viscous
the
that
lead
taetoid and the surrounding
droplet.
increases with the
Below
solution remains
a
the
anisotropy
phenomenon
of
of "rheopexy"
critical
concentration
such
of the sol. In the case
337.
V205
particles.
enhancing their
into ordered,
changed
into
structures
mean
distance
332
between
colloidal particles
sheets
Ordered,
On careful addition of
reversible
giving rise
Smectic tactoids
in
V205
can be progressively
electrolytes,
fraction of their original size, maintaining
aggregates
which
layers deposited from
colloidal particles
solvent.
properties.
336 arising
also explain the amazing
aggregates
to crystalloids
of ~-FeOOH can,by
individual
layers
can
in
and even
then be changed
which all particles
drying slowly, be
are maintained,
readily
reminiscent
of
like mica 338
4.1.2. Anisotropie
anisotropic
shrink to a
irreversible
solid
Vanadium pentoxide
it is observed that gelation of the sol is accelerated
internal anisotropy.
are mutually oriented
can
1013
; on rolling a test tube, containing a liquified thixotropic
The
gradually
This
of
are not formed and the
a shear stress is applied.
reduced by suitable changes of the dispersion medium.
V205 tactoids
tactoids
as thixotropy or streaming birefringence
colloidal
sol, between the palms of the hands,
significantly
concentration,
isotropic unless
sols exhibit unusual properties
the
to each other. They
V205
they can reach a length of about 250 ~m. Such tactoids are made of approximately
colloidal
from
are
; the interaction between rodlike particles
the interfacial
dilute sol that lead to a spherical
The size of tactoids
examples
The special shape of
competition between two energy terms
to a
main axis parallel
known
Anisotropic
One of
can
coatings
the best known
gels that have been extensively
ordered colloids.
be preserved and even
can
therefore
examples
be
is undoubtely
The spontaneous
enhanced,
obtained
the V205
orientation
of
upon slow removal
of
that
exhibit
layers
specific
deposited from
studied during the last decade 339
V205 gels are made of entangled fibers
(figure 8). Electron microscopy
these fibers actually look like flat ribbons approximately
shows
I x i0 x 102 nm in size.
that
X-ray
Sol-Gel Chemistry of Transition Metal Oxides
303
and electron diffraction experiments 172 show that these ribbons exhibit a
structure defined by a unit
modified
upon
swelling
cell
and
: a = 27.0
seems
to
A and
be
b =
closely
two-dimensional
3.6 A . This 2D structure
related
is not
to the layered structure of
orthorhombic V205 . Fibers are built of basic blocks containing I0 vanadium atoms along
a direction.
Some strongly bound water molecules,
giving rise to the
corrugated structure of the
or OH groups,
ribbons 340
link these blocks
X-ray absorption
is, however,
no evidence
for a long V-O
together
experiments
show that vanadium is surrounded by five oxygen ate,ms with a short V=0 distance
in crystalline V205 . There
the
(1.58 A) as
bond between adjacent
layers 341
Under ambient conditions,
n=l.8. Thermal analysis 179,342
or under vacuum,
occurs
and
the
down to a
thermal
the water
shows that
content of V2Os.nH20 xerogels corresponds
water can be removed reversibly
composition V205,0.5H20.
dehydration
crystalline V205 . According to
process
exchange hydrogen bonds with
no
to
upon heating
Below this value further
becomes
condensation
longer reversible,
leading to
the nature of water
molecules
For high water content (n>l.8) water
molecules
infra-red and Raman studies
depends on the water stoichiometry 343,344.
,
the oxide network while
for low water contents
(n=0.5) they
are directly bonded to vanadium atoms 345, in agreement with ESR and ENDOR experiments 178
V205
j ,;
001
can
003
be
of
a
Reflection
one-dimensional
gels
by
X-ray
geometry Xtypical
order corresstacking
of the ribbons one upon another along
15
20
25
30
~
o
a
direction
179
substrate
can be
002
indexed as
]
20
115
18).
25
~
30
and polarized EXAFS
The basal spacing
of the
001 peaks,
amount
increases with the
O
of water in the sample
: d=8.7 A
a
under
xerogel
(V205,0.5H20)
xerogel
:
tions
b) n = 0.5, basal spacing d =
with
8.7 A
dried
dried
and
d=ll.5
under
similar
layered
increase
for
vacuum
A
for
a
ambient condi-
(V205,1.8 H20 ). By
the 2.8 A
the
was
E.S.R.
d, deduced from the position
a) n = 1.8, basal spacing d = 11.5
to
the
peaks
001 (figure
346 spectroscopies.
Fig.18. X-ray diffraction pattern of a V205.nH20
attributed
to
demonstrated by
178,infra_red 345
II II t,0ob
I
10
perpendicular
All diffraction
The anisotropy of these coatings
003
also clearly
was
detected
ponding to the turbostratic
10
layer
from
ray diffraction patterns are
U
5
deposited
easily
diffraction.
,
S
layers
exhibit an anisotropic structure that
comparison
clay systems,
of the d-spacing
reversible intercalation of one water molecule layer between the
V205 ribbons.
The
swelling
process
followed by X-ray diffraction
of
V2Os.nH20
xerogels
and Wide Angle Neutron
at
low water contents
Scattering 180. A stepwise
(n<20) was
swelling
process was first observed up to about n=6. The basal distance d increases by steps of
corresponding
to the
thickness of
a single
water layer.
In this domain,
between the oxide ribbons remain quite strong and the swelling process can be described
the intercalation of
water molecules into
a layered host
2.8
interactions
lattice. Beyond n=6,
as
the basal
304
J. Livage et al.
spacing
d
increases
progressively
describe the water uptake process
and their mean distance
colloidal solutions.
state chemistry
and
a
continuous
swelling seems more appropriate
(figure 19). Interactions
between particles become weaker
increases continuously with the amount of water added,
The composition V205.6H20
(n<6) and colloidal chemistry
to
corresponds
as in
usual
to a turning point between solid
(n>6).
Ln(I)
~d(A)
40
%
k7 xI'"'' /
++i % .HI.# n=192
+~.
.i ~#tll,p,"~.
f [~,.,.~+~++%+,
,j~/~ I1~4'd'~'~+ltlln.
#l 155
IE ~,.
'+++ ~+l+,~++._+~°,.
30
20
__1]_
_.~_1]._.
.%
WATER
LAYERS
%1-
-1~-
10
5
10
Fig. 19. Variation
2.25 4.5
0.0
15
nil20
of the basal distance d as
Fig.20.
n= Sl
~,~. ~.~
6.75 9.0
10"2Q[~-1)
Scattering curve of V205.nD20
a function of the water content of a
gels
V20 s.nH20 layer.
amount of water.
800
Lna(A) _
n f120/V205
300
'
100 50
.
.
.
.
20 10 5
.
1
.
as
a
function
Small Angle Neutron Scattering
I ~(,~)
of the
experiments
were also performed to follow the swelling
347
process
to higher water
content
Scattering curves for V 205.nD20 samples in
:~
TrSenC~.
".
~
2510::
the concentration
exhibit maxima
range 80<n<200
in the
clearly
angular dependence
(figure 20). The d-values corresponding
First
F~j_ T:ainr:~tIOn
"'- ~ 50
~"30
~'"
to
these maxima are plotted in figure 21 as a
function of the volume fraction ¢ of V205 .
2I Regl/e
-6
-5
Fig,21.
I
-4
I
-3
I
-2
I
-1
Swelling of V205.nH20
gels
Ln~
They can be described by : in(d) = kln(¢).
lo
0
Assuming
additivity
of
fractions of V20 s and water,
about
-I
observed
in
the
as a function of the oxide
regime should correspond
volume fraction.
procedure
fact,
divided into two parts.
The gel looks
the
of
this
the slope
plate-like
swelling
particles.
range
powder and the
swelling procedure
is
In
can
The first part (regime I) has a slope of -0.9 for n<6 (or d<25
like an hydrated
of
concentrated
to a ID
concentration
volume
characterized
be
A).
by an
increase of the basal spacing by steps of about 2.8 A. The second part has a slope of about
-I.i
for
10<n<80
(or
50<d<250
~).
The
gel
is
in
a thixopic elastic state. A first
transition occurs in the range 25-50 ~, where the gel becomes an inelastic,
The
slope
thixotropic
-0.60
observed
in
the
more
diluted
regime
pasty material.
II, where the gel turns from a
liquid to a slightly viscous one, suggests a 2D swelling.
The range between I00
Sol-Gel Chemistry of Transition Metal Oxides
and 200A corresponds
305
to a second transition range, where the swelling process progressively
turns from ID to 2D.
Such behavior
V205-ribbons
can
be described using
the following model.
obtained from the extrapolation
approximately
of the one dimensional
the same (7.4 A).
interparticular
distance perpendicular
swelling becomes
two dimensional•
to the largest
As a result,
governed by the increase of
surface of the
particles.
that reflects
the
is
the
When the
to the width of the particles,
d-spacing
ordering
in
7-Fe203
colloids.
Ferrimagnetic
about i0 nm in diameter can be prepared by increasing
and Fe 3+ salts.
the
interparticular
Aggregation
the magnitude
surface charges
of
dipolar interactions
not likely to contribute
to the primary aggregation process.
in colloidal
superparamagnets 348
Colloidal
correlative
aggregates
aggregates
and
and
of surface charges
formed in an acidic medium
higher pH (figure 22B).
on
the sign and
the medium. Magnetic
interactions
However,
(=lOkT) and are
magnetic
ordering has
that seem to behave as superferromagnets
rather
can be frozen in place by adding a water soluble polymer
dispersion
electron microscopy
shows the effect
the acidity of
in
a
toluene-polymer
M6ssbauer
mixture,
spectroscopy
on the aggregation
(figure 22A), while small chains
Much longer strings
around the point of zero charge
(n=50)
in
experiments.
state
Fe 2÷
in an acidic or a
depends mainly
(=kT) are much smaller than electrostatic
been observed
surfaction
in relation to
iron oxide particles
the flocculate
of the colloidal particles
114
spinel
the pH of an aqueous mixture of
Stable sols are then obtained by peptizing
basic medium 1 1 3
by
the
value
increases more slowly with water content.
4.1.3. Magnetic
or
The
swelling regime to the dry state
In regime I, swelling is
mean distance between ribbons reaches values comparable
distance
The thickness of
was estimated around 8.8 A by X-ray and neutron powder diffraction.
: small
than
349
order to perform
Figure
clusters
22 clearly
(n=5) are
(n=15) are observed at slightly
or large compact aggregates
are found
(figure 22C).
o
/'.
A
.~;: '
•,
-.%.
%
.. ,.%.
•
l-
• r...,< z.: ?~ .~,
:
ij :j ;i ::: !:
.:
-
~.
-
VELOCITY (ram/s)
Fig.22.
Electron micrographs
and M6ssbauer
spectra
sols.
(300K) of 7-Fe203
colloids
in frozen-in
306
J. Livage et al.
Small clusters
These
give M6ssbauer
spectra
spectra A
typical
of
superparamagnetic
relaxation.
The
particle magnetic
coupling.
size.
are
Because
coiling in
of
change
Strings
neighboring
the strings
effects
in
the
charges
can
ordering
4.2. Intercalation
positive
separate the
interactions.
of
host
surface
charges
to the aqueous sol.
in cationic
Magnetic
sols. Anions and
between
species into
its
Host lattices must
superferromagnetic
ordering
: NO3"<CIO4"<SO42<HPO42
host lattices
last decade.
basic
Intercalation
structural
is a reversible
integrity during the course of
This
versatile
crystalline
already
temperature,
4.2.1.
host
compounds
days. Gels are
gels
exhibit
very few
In the case of orthorhombic
network and the weak V-O bonds
a
layered
related to that
Therefore,
stepwise swelling process
very
therefore
layered
V205
V205
framework rather than a van der Waals host. Li+ions
bonds between the layers are
was
to the
between
352
pentoxide
oxide.
350
to small cations such as Li + 351
orthorhombic
ribbons is closely
V205 . Vanadium-oxygen
intercalation.
seems to be restricted
are inserted into the channels of the
layers persist in LixV205
decreases
with layered structures has
exhibit a strong 2D anisotropy,
actually behaves as a three-dimensional
possible.
then
: super-
In agreement,
retains
intercalation
crystalline
leading
particles
interactions
oxides are able to give rise to reversible
the
coupling
especially those due to exchange coupling
attention during the
matrix
planes is observed.
Vanadium
spectrum of
Exchange
forward and backward reactions while expansion of the lattice perpendicular
structure of the
is
of V205 ~els
guest
received ever increasing
for instance,
Spectrum C
likely becomes operative,
iron oxide colloids.
must vanish.
properties
Intercalation
The
on their
average number of first
can also be obtained by adding an electrolyte
compensate
when the complexing ability of the anions increases
process.
to the
gets around 3 neighbors.
magnetic moments of all elementary
are then expected to decrease,
ferromagnetic
weak evolution of the
to each other.
hydration water molecules
particles
undergo
(A~B) depending
is nearly identical to the M6ssbauer
ordering 349. The
Large aggregates
Anionic
a
concentration.
which
small clusters case and of branching and
is related
between facing spins at the surface of adjacent particles
tend to be parallel
to
same features
and was interpreted as enhanced interparticle
to superferromagnetic
particles
corresponds
It occurs when one particle
It
the aggregate
uniaxial
A~B
change A~B
typical of large, compact aggregates.
bulk 7-Fe203
depending on
exhibit the
case, the
neighbors per particle.
or B
magnetically
reversible
observed
in
during the first
structure
for
structure
planes of orthorhombic
however much weaker in the gel than
intercalation
the
in which the internal 2D
of the (a,c)
case
of
guest
of water intercalation
hydration stages. V205
intercalation.
in
species becomes
leading to a
gels actually offer
Intercalation
reactions
a
involving
are quite slow. They usually require heating under reflux for several
much more reactive
species,
full intercalation
can be performed,
at room
within a few hours or even minutes 25
Intercalation
of
V205.nH20 xerogel and
metal
metal
cations.
metal cations occurs
solution
of
chlorides
diffusion
in the gel phase.
353
It can be
The
Ion
exchange
as soon as
rate
of
the stacking of
that guest species are
intercalated.
the gel is
ionic
V205 ribbons.
The
The d increase
protons
of the hydrous
dipped into an aqueous
H+/M + exchange
monitored by measuring
decreases when protons are released 354. Intercalation
order arising from
between
is controlled by
the pH of the solution
does not affect the
basal spacing d
monodimensional
increases,
however depends on the
which
showing
nature of the
Sol-Gel Chemistry of Transition Metal Oxides
intercalated
cation.
It varies with the charge/ionic
307
radius ratio,
related to the hydration
enthalpy U h of the cation. All data gathered are centered around two values
=13.6 A (figure
layers
353
having a
layers
23) suggesting
Intercalated
low U h
occurs
observations
value
with
that
the M + cation
species
(mainly
cations
containing
monovalent
having
a
Uh
d
one or two water
water layer are obtained with cations
cations)
high
can be explained as resulting
is intercalated with
one
: d=ll A and
whereas
value
intercalation
with two water
(mainly divalent cations).
These
from the competition between two energy terms
:
- The energy required to separate V205 layers increases with the basal spacing variation.
The energy
required to remove
water molecules
increases with the charge/radius
from
the solvation sphere
of the cation
ratio.
It
is
interesting
to
point
out
that
V205
o
d(A)
xerogels
Li + Ca2+C~2+Fe2+ Mg2+
water.
~-27X
N
=
1
=
2
I e/r
3
1=
as a
function of the
intercalated
stops
after
layers.
layered structure
of V205 gels.
that
into the
alkali ions in the presence
charge
the
nature
of
the
of non
solvent.
For water,
is
of
the
Two
solvation
to
observed
of one or two
due
to the
cations
charged
further
V205
swelling by
These results
have
recently extended to the intercalation
stages
have
An increase of
of
of
the basal
radius ratio, but also
been
the intercalation
A
which
intercalated
negatively
aqueous organic solvents 355
they correspond
occur in
cations.
probably
the
is
a colloidal
metal
is
preventing
spacing d is observed which depends not only on the charge/ionic
distances.
of
additional water molecules.
been
process
or
intercalation
This
thus
gel
process
the
attracts
ribbons
a
phenomenon does not
swelling
positive
swelling
to
solutions
water
charge/radius
ratio of cations
Such a
aqueous
limited
H 4÷
non-limited
leading
solution.
Fig.23. Variation of the basal distance
d
A
observed,
the
Cs y R I ~ .
can be dissolved when dipped into pure
on
deduced from interlayer
either one or
two solvent
layers together with the metal cation.
Sodium intercalation
temperature 356
has been recently used to synthesize vanadium bronzes at
A V205,1.8 H20 xerogel
into an aqueous solution of NaCI(IM).
compound characterized
crystallization
usual
solid
of the
state
crystallization
4.2.2.
Intercalation
H20
and
monoclinic
Na0.33V205
The
bronze
anisotropy
tunnels therefore
as reversible
of
molecular
cathodes
ions.
ranging from 1 to 18, have been intercalated
diffraction patterns
then dipped
by a basal spacing d=10.9 A. Water is then removed upon heating,
along these
properties
substrate,
leading to a Na0.33V205,1.8
reactions.
occurs,
occurs at 320°C
of
the
layer
of the bronze so that the tunnels present
Ionic diffusion
remarkable
is deposited onto a glass
Intercalation
low
exhibit a serie of
is
instead of 700°C
conserved
by
even after
in the structure remain parallel.
becomes
easier and
in lithium batteries
Alkylammonium
such bronzes
exhibit
356
ions,
CnH2n+1 N+ (CH3)3, with n
into the layered structure of V205 gels. X-ray
001 peaks typical of the turbostratic
stacking of
the V205 ribbons 357
Figure 24 reports
the
variation of the basal
carbon atoms in the alkyl chain. The
volume can be
easily deduced from
spacing d as a
orientation
of the alkyl chain within the
this variation.
layer plane is given by 1.27sin ~=Ad/An.
function of the number
The
angle ~ between
The 1.27 factor corresponds
n of
interlayer
the chain and the
to the projection of a
C-C bond onto the main axis of the alkyl chain. Three domains are clearly seen:
Domain
JPSSC 18:4-D
I
(e=0)
corresponds
to
short
alkyl
chains
(n<6).
Alkylammonium
ions are
308
J. Livage et al.
dool (.~)
4(]
3C
I
20
.~-":/
--
10
~90 °
IT
o
~ ~ 53
I
, -/~- i:1
o
ooi
•
j
7
.....
I
i
I
2
I
4
I
6
I
8
I
10
Fig.24. Variation of
Ii
12
the
i
14
I
16
I
n
"v
a
basal distance d
b
Fig.25.
Position of
C
alkylammonium
ions
of V205 .xH 20 gels as a function of n number
between the V205 layers as deduced from
of carbon
figure 24 :(a) n<6, a=O °
atoms of the
intercalated
alkyl chain of the
alkylammonium
intercalated parallel
ions.
to the
separate the V205 layers
- Domain III (==90 ° )
(b) 6<n<12,
layer planes
to long alkyl
layers
to one
required to
alkylammonium
Waals interactions
ions are aligned parallel
(figure 25c).
situation where both energy terms,
are of the same order of magnitude.
another,
but
they cannot
layer
Alkyl chains
stand perpendicular
to the
(figure 25b).
Cobalticinium
and ferricinium molecular
gels. In both cases,
the
ribbon structure
not modified upon
is
basal distance
stacking is even observed,
spacing suggests
that
4.2.3.
Swelling
in an
aqueous solution
layers.
spacing d
solvents
(DMSO) lead
to a
at
This appears
taken into
ribbons
carbonate)
single layer
all. The
to
account,
namely
to the
the intercalation
water is replaced
increases
by steps
(square
form
ID
the basal
layers and somehow
of one
to several
by an organic
when the
a double-layer
compound
solvent.
gel is
(d=16.5
difficult problem as
solid-solid,
intercalation.
root
of
the
intercalate
A). Some
reliable intercalation
gels
water
The basal
dipped into a polar
The
cohesive
(d=21.5
criteria
appears
density)
(DMF) do not
interactions
and solvent-solvent
best parameter
others
is therefore opened.
or even Gutman's
energy
A) while
solvents however
many different
solid-solvent
nor relative permittivities
predict possible
6
~d=4.4 A increase of
perpendicular
of the
The first stages of the swelling process of V205
described as
problem of
be a rather
Neither dipole moments
parameter
rings are
improvement
while the internal 2D structure of the ribbons remains unchanged 179. Some
(propylene
intercalate
solvents.
can be
between V205
A noticeable
into V205
basic V205
sheet structure of the ribbons.
process occurs when
organic solvent,
about 13.2 ~ 358. The
especially with Co(C5H5); . The
in organic
The same
ions have also been intercalated
increases up to
intercalation.
cyclopentadienyl
inserted into the corrugated
one to
the energy
(n>12). Van der
to the layer planes
to an intermediate
separation and Van der Waals interactions,
aligned parallel
chains
Therefore,
in a direction perpendicular
- Domain II (a=42 °) corresponds
are still
to minimize
(figure 25a).
corresponds
between alkyl chains become predominant.
to one another
in order
0°<=<90 ° ; (c) n>12, a=90 °
have to be
interactions.
donnor numbers allow
to
which
be the Hildebrand
is
related to the
Sol-Gel Chemistry of Transition Metal Oxides
vaporization
energy and the
molar volume of the
309
solvent 359. No intercalation
is usually
observed in V205 gels when 6 is smaller than 13 call/2cm "3/2.
In some cases
between the
gel and
however,
a chemical
the organic
proceeds via an ion exchange process.
been intercalated
into V205 .nH20
ion
Pyridine,
benzidine
and alkylamines
gels 360,361
involving interlayer
water molecules.
organic bases leads
ions.
black flocculate
organized,
Intercalation
is obtained
insoluble
to be non reversible
solid,
V(IV)
Although
of
for instance have
(TTF)
reaction
acid character of these
of pyridinium,
has also
(TTFxV205
benzidinium
or
been reported.
A
is
an ill-
a large amount of water 362. The process
with x<l.8)
appears
described as intercalation.
or solvent
A
severe reduction of
(ethanol) presumably
occurs.
of water increases
shown with other reducing reagents 363
ions as it was previously
occurs
and intercalation
vanadium reduction occurs,
the Bronsted
and the resulting material
that contains
gel is destroyed
some
to the formation
tetrathiofulvalene
and can hardly be
is formed
is mainly governed by a proton transfer,
This denotes
vanadium ions by the organic molecules
structure of the
electron exchange)
molecular
gels. Protonation
alkylammonium
(proton or
A
infra-red studies show that intercalation
of
reaction
compound.
and the amount
The
layered
with the amount
of
5. PHYSICAL PROPERTIES AND APPLICATIONS OF TRANSITION METAL OXIDE GELS.
The sol-gel process
The gel state is then
materials.
examples
Drying
and densification
can be found in
of
materials
science.
water)
Gels
are
or
(usually
to be water-oxide
composites.
from
intimate
of
states
the chemical
mixing
giving rise
xerogels
trapped
both
inside
Therefore
phases.
to mixed
are
an
surface of the oxide particles.
the
oxide,
giving
diffusion within the
electrochemical
rise
H3 O÷
of
phase and ion
or
is that
so that
states. A strong
oxide network.
in
the
in which solvent
Such materials
they exhibit specific properties
can be
arising
Electronic properties
Water molecules
due to
a
are adsorbed at the
depending on the
acidity
Ionic properties
arising from ion
thus be expected.
Both phases are
involved in the
metal
oxide
diffusion through the
gels.
Electron diffusion occurs
liquid phase.
Because
electron transfer at the oxide-water
of the very
interface can
be
properties.
properties
5.1.l. Small polaron hopping.
states,
of gels or
OH" species.
transition
large interface between both phases,
systems
to
will not be
Transition metal ions often exhibit several
greatly enhanced leading to specific photochemical
metal oxides
diphasic
valence compounds.
liquid phase can
properties
through the solid
5.1. Electronic
metal oxide gels are used
They can be more or less ionized,
to
these
of gels. Many
fibers 4. Such applications
hopping process within the solid phase can be observed.
of
synthesis
fibers.
of
in order to show that they can lead to new applications
considered
the
films or
The discussion will rather be focused on the physical properties
molecules
valence
ceramics,
stage in the processing
in which transition
optical coatings 5 or
xerogels before calcination
field
quickly follow
the literature
make ceramic powders 2,364
described here.
is mainly used for making glasses,
nothing more than an intermediate
A general condition for semieondueting
the metal
ions should
conduction can take
electron-phonon
coupling is
leading to the formation of a so-called
the unpaired electron and
be capable
place by electron
of existing
the polar oxide network
transition
in several
transfer from low
usually observed in
"small polaron"
behavior of
valence
to high valence
transition metal oxides
365. The strong interaction between
leads to a polarization
of the lattice
310
J. Livage et al.
and a displacement
of the oxygen
these
are
distortions
trapped in its own
ions around the
limited
to
the
low valence transition
nearest neighbors,
potential well 366. A
"small-polaron"
metal ion. When
the unpaired electron becomes
is formed,
characterized
by its
binding energy Wp which is usually about 0.5 eV for most transition metal oxides 367. Small
polaron
hopping
between
two
neighboring
potential energy. This is achieved by
the hopping process.
sites
occurs
when
both
sites have the same
lattice distortions and phonons must be
involved in
Conduction then has the character of a thermally activated process
given by Wh=I/2 Wp 368 . The hopping
which the activation energy W h should be
depends on two factors
in
rate however
:
- A phonon term corresponding to the probability for both sites to have the same
potential
energy.
- An electronic term corresponding to the
probability for the electron to tunnel from
one
site to the other during this coincidence.
A detailed analysis of small polaron diffusion is rather difficult and can be found in many
review papers 368. A general formula for electrical conductivity in transition metal oxides
was proposed by Austin and Mott 366.
e2
a = w -RkT
where
W
c(l-c) e x p ( - 2 o R ) e x p ( - ~ )
(13)
:
- u is a phonon frequency related to the Debye temperature 8 by hv-k0.
-
R is the distance between transition metal ions.
c
is the
ratio of
ion concentration
in the
low valence
state reported
to the total
concentration of transition metal ions.
a
is
the
rate
of
the
electronic
wave function decay, exp(-2~R) corresponds to the
tunnelling transfer.
W is the thermal activation energy of the hopping process.
One of the most
striking features of the
small polaron conductivity is
that the
thermal activation energy W decreases with the temperature.
At
high
temperature
multiphonon process.
(T>8/2),
the
small
polaron
The activation energy is
hopping is activated by an optical
given by W = W h + i / 2 W d ,
where
W d corresponds
to a disorder term in the case of non-crystalline oxides 369
-
As the
temperature is lowered
drops continuously
to
the phonon spectrum
zero, leading
to a
freezes out and
decrease in
the polaron term W h
the observed activation energy W
below 8/2. A detailed analysis of the electrical conductivity variation in this temperature
range was proposed by Schnakenberg 370
- At very low temperature
(T<0/4) an acoustical phonon assisted hopping takes place and the
activation energy becomes W - 1/2 W d .
5.1.2. Semiconducting V205 xerogels.
The semiconducting properties of V205
from
studied
gels
have
been
antistatic coatings
extensively
in the
near room temperature,
electronic and ionic
because
photographic industry
appears
to
The
be
transient regime
their
371,372,373.
layers deposited
potential
Such
application as
xerogels, when dried
still contain some water and care must be taken in order to separate
contributions
to the
electrical conductivity 3?4
conductivity can be observed when the xerogel is
atmosphere.
of
water content
purely
ohmic,
is observed
then corresponds
both
when a
a.c.
and
Purely electronic
under vacuum or in the presence of a
to V205,0.5
d.c.
d.c. voltage
H20. The electrical behavior
conductivities
is applied
are identical and no
across the sample 375. The
room-temperature conductivity depends on the amount of reduced vanadium ions. It
quite
fast
with
the
V 4+
concentration
376
Some
dry
discrepancies
are
increases
observed in the
Sol-Gel Chemistry of Transition Metal Oxides
literature concerning
as high as a=0.1
the conductivity
depends on the way electrodes
with the very
of
Scm "I at 300K have
plotted as log(aT) versus
(T "I) is
with
of
process.
low
mobility
Theoretical
the experimental
models
dependence
conductivity,
carriers,
suggested by Mott
of the d.c.
variation,
together
is typical of a small polaron hopping
or Schnakenberg 3 7 0 fit quite
366
agree
10 .5 and 10 .6
The temperature
charge
somehow
values ranging between
shown in figure 26. The non-linear
the
Conductivities
seems that this value
onto the sample. All results however do
the charge carriers,
em2V'Is "I are currently reported 373
the
V205 layers deposited from gels.
been reported 371. It
are deposited
low mobility of
311
well with
results 371,373
hn ~T.101
-.4
N
'7 E
ii
. .~
-.7
O
U
.,a
\\
[
,HI .... ..... ,
.
i
i
3.6
3.2
Fig.26.
i
4.0
4i.
4
,
4.8
Temperature
electrical
,
5.2
L
5,6
6i.
0 10
0~
/
T(K"1)
dependence
of the
conductivity
of V205
0
V (volts)
Fig.27.
layers deposited from gels.
The
small-polaron
According
to the theory,
Wopt=4 Wt h 369
leV,
hopping
i.e.
can
be
the optical activation energy
\\
VH
20
VTH
characteristic
the switching effect in a
Wop t should roughly correspond
Therefore,
to
to a broad absorption around
most mixed valence
compounds
study of V205 gels was performed by
J.
They found an optical gap of 2.2eV close to the gap of crystalline V205.
An Urbach tail was observed on the
of V 4+ . Moreover,
low energy side, whose slope increases with
the absorption due to the optically
in the near infra-red
from
.
thermally or optically activated.
transfer then usually corresponds
in the red part of the optical spectrum.
predicted
.
Intensity-Potential
either
exhibit a typical blue color. An optical absorption
Bullot et al. 377
.
V205,1.8 H20 xerogel.
process
Intervalence
I
\
.
10
showing
\\
region.
conductivity
The absorption band
data.
It
depends
the amount
induced polaron hopping was
maximum
on
the
(0.geV)
is close
V 4+ concentration
detected
to the energy
and suffers a
redshift when the amount of V 4+ increases.
A threshold
378
switching
Two gold electrodes
of the layer.
electrodes.
The
device is formed by
After a few cycles,
is shown in figure 27.
close to 500
process was
#A and the
observed in
0.i mm apart were evaporated
the
V205
applying a high voltage
device starts switching.
The threshold voltage
on/off ratio 400.
layers deposited from gels
in a coplanar geometry at the surface
is around
of about IOOV between
A typical I-V
25 V, the minimum holding
These values depend
on the way
made. On/off ratio as high as 800 were obtained but these results are hardly
Optical microscopy
voltage
is applied.
shows that some filaments
These filaments
switching effect should be due to
could explain why switching
the metal-insulator
the device is
to the formation of VO 2
transition of VO 2 around
is no longer observed above this temperature
.
current
reproducible.
grow between the electrodes when the
correspond presumably
the
characteristic
forming
and the
60°C. This
312
J. Livage
et al.
5.2. Ionic properties
5.2.1.
Particle
xerogels
hydrates.
are hydrous
From
oxides.
defined as "particle hydrates"
al. 379. Following
a chemical
stand point,
They correspond
transition metal
to the general
according to the classification
these authors,
particle hydrates
"S". The structure within a particle
and the particles
linked together to form
surface of the particles
the
inter-particle
composite
solid. A protonation
the liquid-like
dissociation
a
at the
Additional
connected,
viscous
water molecules
occupy
liquid region through the
makes
region acidic or basic depending on the nature of the oxide network.
Acidic
is
promoted
equilibrium exists at the oxide-water
by
large metal
such as
content
produce
oxide
The full coordination
This
dissociation by
features
to
separated
is that of the anhydrous
agglomerates.
is preserved by water molecules.
region
can be
suggested by W.A. England et
consist of charged particles
by an aqueous solution
are
oxides gels or
formula MOx.nH20 and
small
atoms of
good ion exchange
favours
metal
densifieation
atoms
of
low charge.
properties
and
high
positive
charge
Particle hydrates
or fast
particle
interface.
and
exhibit some common
proton conduction 380.
hydrates
can
often
basic
be
The liquid
compressed
into
transparent pellets by cold pressing.
5.2.2.
few
Ion exchange.
decades.
materials,
The
ion-exchanging
materials,
resistant
exchanger.
Scheme
Dissociation,
(15)
widely studied during the last
energy,
hydrometallurgy,
reinforced attempts
to chemicals,
to find new,
temperature
and radiation.
They can compete with
can be represented schematically
M-OH
=
M + + OH"
(14)
M-OH
=
M-O" + H +
(15)
acid solutions where the hydrous
corresponds
near the isoelectric
to
a
point of
cationic
amphoteric
occurs in both ways which enables simultaneous
overall charge on the
exchanger
oxides,
development
a characteristic
pH value for
surface is zero. This
The
oxygen/metal
depends on the preparation
general
formula
for
ratio R per particle
MOz.nH20 -
where the
brackets
correspond
in
titration.
ion
or ThO 2
processes.
from positive
any particular
.
a basic medium.
such as ZrO2, TiO 2
to negative
oxide at which
the zero point of
The shape of the pH
the
charge
titration
of the hydrous oxide.
tetravalent
M(IV)
hydrous
can be written as follows 379
molar formula
This region is
basic,
of the
oxide
having
a
mean
.
[MOa(OH)b(H20)R.(a+b)] 4.(2a+b)
to the
aqueous region of the material.
SA, if (2a+b)>4, where
a
organic
as follows 379
of both ion exchange
pH value is called
(ZPC) and is readily determined by potentiometric
curve, however,
Hydrous oxides
commercial
oxide acts as an anionic
ion
The resultant charge on the particles may be switched reversibly
by changing pH. There is
high-purity
highly selective
Hydrous oxides of polyvalent metals behave as cation
Their dissociation
(14) takes place in
have been
nuclear
as ion-exchangers.
(clays) products.
or anion exchangers.
Scheme
in
etc. has
candidates
inorganic
ion-exchangers
development
water purification,
appear to be good
or natural
Inorganic
rapid
+ S
solid particle
and S is the
S = Sb, if (2a+b)<4 and acidic;
S =
:
S A - (2a+b-4)H3 O+ + [(2+n-R)-(2a+b-4)]H20
SB
The two equilibria may
~.
-
(4-2a-b)OH"
Cation exchange corresponds
from an external solution
constant ~ .
+ [(2+n-R)-(4-2a-b)]H20
be represented by acidic
Cation exchange
to the replacement
S'. This equilibrium
and basic dissociation
constants,
of the H3 O÷ ions in S by M ÷ ions
is characterized by a
coming
relative formation
is then favored by a high pH, a large concentration
or a large value of KAK N . For hydrous oxides, K M apparently
K A and
of M + ions
increases with decreasing
the
Sol-Gel Chemistry of Transition Metal Oxides
size
of
M ÷.
Therefore,
cation
exchange
of
alkali
313
metals
decreases
in the order
Cs+>Rb+>K+>Na+>Li+ "
The exchange properties
of silica gels
oxide gels find applications
in
and purification
Among the
of water.
isolation,
are already well
known.
removal and treatment of
main characteristics
Transition metal
radioactive materials
of these
compounds
stability in strong radiation fields and retention of ion exchange properties
5.2.3 Fast proton conduction.
The development
stimulus
side due
cells,
from
the practical
storage batteries
rather good proton conductors
and
applications
devices.
a
kink
below
decreases
0°C,
faster
molecules
dependence
higher
of the conductivity
to
the
activation
fuel-
extensively
studied
of hydrous oxides usually
are
observed.
exhibits
Conductivity
then
Above 60°C, some water
drops. An analysis of the literature
usually ranges between
be
and 1H NMR.
freezing of included water.
energies
leave the network and the conductivity
that proton conductivity
low-temperature
therefore proton diffusion has been
corresponding
and
in
Hydrous oxides have been shown to
during the last decade, mainly by a.c. conductivity measurements
The temperature
above 100°C.
of solid state proton conductors has received
to possible
and electrochromic
are their
i0 "6 Scm "I and 10 .4 Scm "I with
shows
activation
energies around 0.30 eV. These values do not depend strongly on the nature of the oxide and
can be accounted for by proton diffusion through water molecules
small colloidal particles.
of the hydrous
Therefore,
oxide MOx.nH20 , i.e. as
Quite different values
framework hydrates
are found for
such as antimonic
A high proton conductivity
well as a high proton mobility.
contain high-valent
classical
cations.
drift of H3 O+ or
last process has a
two conduction
conductivity
a function of the water pressure above
hydroxides
necessitates
motion in
phase between the particles,
available.
in
One
content
the sample.
(~=6.10 "8 Scm "~)
or
(a=7.5.10 "3 Scm "I) 381
a large concentration
of mobile protons as
in highly acidic oxides which
aqueous solutions
via the tunnelling of a
are
AI(OH)3.H20
The first factor is optimised
much higher probability
pathways
such as
acid Sb205,nH20
Proton
adsorbed at the surface of
increases quite fast with water
usually occurs
via the
proton through an hydrogen bond.
the case of solid
is entirely
hydrous oxides.
However,
within the interconnected
while the other is via the surface of the isolated
This
liquid
particles.
It is not clear at this point whether proton diffusion occurs through the liquid or at
surface.
Nevertheless,
good proton
mobility should
be expected
in those
the
hydrous oxides
which have a large water content and a high oxidation state.
Proton NMR relaxation
According
to the authors,
times have been measured for several hydrous oxides 382,383
protons are found in three different environments
i) at the surface as hydroxyl
ii) in acid solution in micropores
iii) in acid solution in macropores
Pores
result
from
macropores
should
be
constrained
and viscous.
of various viscosity.
conductivity
:
groups.
the
almost
(diameter < i00 A).
(diameter > 103 A).
agglomeration
liquid-like
Proton conduction
while
of
oxide
that
particles.
the
The solution in the
micropores
will be more
involves chemical exchange between
environments
It must be pointed out however,
in
that there is no simple link
between
and NM_R data.
5.2.4 Mixed conduction
in V205 gels.
379. When prepared from gels,
layers.
Therefore
However,
it is
they
Hydrous oxides appear
they are easy to compress
would
not clear whether
be
very
good candidates
the measured conductivity
water adsorbed at the surface of the sample.
to be good
proton conductors
into pellets or to deposit as
for solid-state
arises from the
Proton conductivity
thin
ionic devices.
bulk or from
must then be studied as
a
314
J. Livage et al.
function of
the water
stoichiometry
Figure 28 shows the dependence
and
related to
a function of the water pressure
above the sample. As shown
water content increases by steps corresponding
layers between the ribbon-like
a continuous
swelling
the water
adsorption
isotherms 384
of the water content of a V205 layer deposited from gels
colloidal particles.
is observed that can
by X-ray diffraction
to the intercalation
lead,
as
180, the
of one to several water
For a relative humidity
if enough water is added,
larger than 80%
to
a colloidal
solution.
Electronic
conductivity
xerogel corresponds
predominates
water pressures.
The
water content of
the
to VZ05, 0.5 H20 and the basal distance to d-8.7 A. This means that
water remains intercalated
activation
at low
energy
for
between the ribbon-like
conductivity
decreases
vanadium oxide particles.
with
the temperature.
no
The thermal
Such a behavior
is
typical of small polaron hopping between V4÷ and V 5+ ions in the oxide network 373
10
E
½
/
%
/
c
z
I
0.25
relative
I
0.5
I
0.75
i_
J.
0.25
I~umidity P/P$ (H20)
Fig.28. Water adsorption
0.5
0.75
relative h u m i d i t y
Fig.29. Variation
isotherm
of a V205.nil20 xerogel.
of
the
Ir
P/Ps (H20)
a.c.
conductivity
humidity of the surrounding
- Conductivity
177,385.
The
increases quite quickly with the water pressure
A high a.c. conductivity
water
corresponds
content
of
the
is
V205,1.6
H20
and
atmosphere.
above the sample
is observed in ambient conditions
xerogel
to the intercalation
(figure 29)
(o=10 .2 Scm "I at 300K).
the
basal distance d=ll.5 A
of one water layer. The log(aT) vs f(T "I ) curve shows
Arrhenius behaviors with a
kink around -10°C, typical
hydrates
study of this xerogel
177. A dielectric
of a
V205,nH20 gel as a function of the relative
suggests three different behaviors
of proton conductivity
in a broad frequency
for the intercalated water 386
range
in
two
particle
(105-1010Hz)
.
- A low frequency effect due to proton diffusion.
- Two dielectric
relaxations
due to water
molecules which are strongly or weakly bound
to
the ribbons.
- A dielectric
relaxation which should be due to a fast rotation of H30 + ions.
It has to be pointed out that both curves
in figure 28 and figure 29 are quite similar.
plateau is observed around ambient conditions
that corresponds
first water layer. This gives rise to the sigmoidal
looks like
a type
adsorption process
II Brunauer
isotherm.
to the intercalation
shape of the conductivity
Such an
isotherm is
typical of
of
A
the
isotherm that
a multilayer
in which the first layer is much more strongly bound than the
following
ones.
This study was extended to framework hydrates
(Ce(HPO4)2.nH20) . All these compounds have
Conductivity
occurs either in a layer or on a surface.
drawn between the bonding
of water molecules
strongly bound water molecules
give a
(HUO 2PO 4 ,nH20) and particle hydrates
a bidimensional
character
as a
The main difference
responsible
common feature.
then has
for proton conduction.
well ordered lattice through which proton
The
to be
most
diffusion
Sol-Gel Chemistry of Transition Metal Oxides
will be solid-like.
network,
As soon as
are less tightly
bound to the
solid
they become disordered and give rise to a liquid like behavior 384
The example of
xerogel,
the water molecules
315
V205 layers shows
cannot be fully
into account. Mixed
that the electrical
described unless both the
conduction occurs in
conductivity of a
solid and the liquid
V205 gels. Electron
hopping is observed
water content while proton diffusion predominates
as soon as the swelling process
This could account for
literature and explains one
some discrepancies
in the
gel, or
phases are taken
at low
begins.
of the main
advantages of V205 antistatic coatings that keep their electrical properties under both dry
or humid atmospheres 387
5.3 Electrochemical properties
5.3.1 Electrochromic
display devices.
Electrochromie layers
based on
amorphous WO 3 thin
films have been extensively studied during the last decade 388 . Such films can exhibit
stable states, one is
transparent while the other
one is blue. Reversible
two
coloration and
bleaching can be easily obtained in an electrochemical cell. A double injection process
observed that can be described as follows
is
:
WO 3 + xe" + xM + = MxWO 3 (M+ = H + , Li + )
Electrochromic WO 3 layers have been used to make display devices 389, rear-view mirrors 390
or smart windows 391
Amorphous
WO 3
thin
films
are
usually
deposited
by
vacuum
evaporation
or
sputtering, however sol-gel derived eleetrochromic layers have also been made recently 392
Several techniques
during the
organic
for the
last few
precursors
Tungstic
acid
preparation of
years. Amorphous
such
as
colloidal
tungsten
solutions
WO 3 films
WO 3 .nH20 can
from solutions
be formed
hexaphenoxide
have
been
393
or
obtained
have been published
upon hydrolysis
by ion exchange from an aqueous
solution of sodium tungstate 183
Peroxotungstic acid coated films were
for
395
electrochromic
applications
They
are
obtained
by
also investigated
dissolution
precipitated tungstic acid in an hydrogen peroxide solution. More recently,
chlorides were obtained upon dissolution of WOCI 4 into an alcohol 20
obtained
which
WO3.nH20
(n=l,2)
can
be
easily
layers have
deposited
and
recently been
of metal-
tungsten ethoxide 302,394
of a freshly
tungsten alkoxo
Stable solutions
are
hydrolyzed by dip-coating 20. Crystalline
deposited from
gels and colloidal solutions.
They appear to be strongly anisotropic as shown by X-ray diffraction and infrared dichroism
184
This
W03.1~20
lamellar
films
structure
chemically
of
the
intercalate
film
favors
long-chain
the intercalation of guest species.
alkylammonium
and
electrochemically
intercalate Li + ions. They can therefore be used for making display devices.
Other transition metal
deposited via the
oxides also exhibit
sol-gel process.
TiO 2
from white to blue reversibly. V205 films
yellow to green upon an applied voltage of
electrochromic properties and
films have been
made from Ti(OBun)4.
can be
They
turn
deposited from a polyvanadic acid sol turn
from
± 1.5V. They have a memory effect of
more than
20 hours 396
The sol-gel technique offers many advantages for making electrochromic devices
Thin layers can be easily deposited under ambient conditions by dip-coating,
or spraying.
According
sensitive to
Large surfaces can be coated at low cost 5
to
the
literature,
the method
eleetrochromic
of preparation
It
characteristics
has been
of
WO 3 films are very
reported that sputtered films are
easier to color and bleach than evaporated ones. This is probably due to the smaller
content of
faster
:
spin-coating
the latter.
coloration
397
Water has
Sol-gel
to be
incorporated into
deposited
films
always
WO 3 films
contain
water
in order to obtain
some
water
making
316
J. Livage et al.
electrochemical
ion
diffusion
adsorb water when
placed in an
layers
described
could
be
electrochromic
their porous,
easier.
as
xerogels
all-gel
devices
and electrolyte)
cathodes
insertion of
Most work was
have
recently
for
lithium
However,
also
experiments
Li + ions per V205
The high
to
active layers
long
for
few
ago
can be
403
disordered
partially dehydrated
carbonate
lithium
at 230°C.
to
experiments
Amorphous V205
cathode 402
show
and 2 V (vs.
decreases with
x-l.6. The
oxidation.
xerogels was
made using a triple
solution as an electrolyte.
However,
as
the
The remaining
the
Li + ions
are
Two LixV2Oscompounds
process appears
to be
An X-ray study
of the
I-D order is restored during the
water appears
to be
The
discharge
none of the inflections
average cycling efficiency during
is accompanied
Li/V205 . Gels are
using a V205 xerogel
negative electrode 404. This xerogel was
electrode was detected.
(vs Li), with
performance
were
study was published recently,
swelling by the solvent.
5.4. Interfaclal
400,401.
a reversible
of Li + ions into V2Os.I.6H20
x-l.l and
linear between 3.5-2.2V
useful in the processing
as
with
Therefore vanadate glasses
reversibly between 3.5 V
upon electrochemical
used in order to avoid
insertion of i.i
properties
3D framework rather
stacking of the V205 ribbons is destroyed upon insertion,
adverse effect on the lithium counter
However this good
applications.
can be achieved
is applied to the V205 electrode.
material.
metallic
of crystalline V205 . The
for such
that no phase transition occurs.
oxidation cycle. Another electrochemical
and
on the
studied 399
exhibit layered structures.
for lithium batteries
inserted
corresponding
layer shows that the well-ordered
cathode
that
oxide behaves as a
Electrochemical
and Li + ions are removed
a
this
intercalation
intercalated when a negative voltage
to
based
the open circuit voltage continuously
electrode device and a LiCiO4-propylene
upon reduction
batteries,
performed with LiAsF 6 in a cyclic ether as an electrolyte
electrochemical
years
Lithium
: values up to 600 Wh/kg have been reported
interesting
Contrary to crystalline V205,
Reversible
the
all
have been extensively
changes should be limited.
the amount of inserted Li + , suggesting
as
which
to be good candidates
cathodes
exhibits
that up to 1.8
rise
in
is hoped that better reversibility
as reversible
Electrochemical
giving
batteries.
as already mentioned,
splat-cooling
reversible
made
metal chalcogenides
amorphous oxides for which structural
have been suggested
are formed
been
a host lattice
energy densities
than a Van der Waals host 352. It
a
tungsten hydroxy-oxides.
very long memory 321. They open the way
(V205, V6012... ) also appear
They offer high stoichiometric
reported
and a
Li + ions into
focused on transition
Vanadium oxides
Li+/Li).
hydrated
that such hydrated
devices.
Reversible
made by
amorphous WO 3 films always
has been shown
are deposited from gels. Such cells exhibit a rather
response time, a good cycling behavior,
new micro-ionic
for V205 3 5 2
of
It
and the short response time of these layers was attributed
spongy structure 398
Multi-layer
reversible
evaporated
reversibility
(electrochromic
5.3.2
Moreover,
ambient atmosphere.
by a modest
quite suitable for
strongly bound,
A polymeric
curve appears
and plateaux
to be
energy density
was
nearly
characteristic
the first 46 cycles
making layers and
as no
electrolyte
:
is 99.7%.
420 Wh/kg for
should be quite
of thin film micro-batteries.
properties
5.4.l.Photochemistry
of
colloidal
contact with an aqueous solution,
until the electrochemical
semiconductors.
When
a semiconductor
is brought into
an electron transfer occurs at the oxide-water
potentials
of both phases
become equal. As a result of this transfer,
(Fermi level and mean redox
interface
potential)
the oxide surface becomes charged with
respect
Sol-Gel Chemistry of Transition Metal Oxides
to the solution.
depends on
charge layer
transfer
This charge is actually
the doping
is typically
can
electrolyte
be
junction.
opposite
Electron-hole
oxide-water
such
separation
Moreover,
so
charge
carriers
at
can
can be coupled
semiconductor
of the semiconductorof energy larger than
field of
space charge region.
while electrons move
Redox reactions
particle
oxide
best
the
reach
without
the
For n-type
toward the bulk.
The
are thus expected at
the
an
optically
junction
is
already
transparent
particles
and offer
the
surface.
Light-induced
of bulk
than
cannot be
and charge carrier
are small,
charge separation and redox
diffusion.
catalysts
a larger
junction model
diffusion theory 407. When the particles
intervention
properties
aqueous
or
reactions
of pollutants
414. Transparent
of
transition metal
Thus a single
colloidal
so that different
organic
oxide colloids
regions
medium.
This
TiO 2 sols are usually
413 or photodegradation
produced via hydrolysis
in acid aqueous solutions 415
Particles
characteristics.
ligand such as acetylacetone
that remain stable up to pH i0. Moreover,
to a strong
efficiency of TiO 2 sols.
of
TiCI 4 in
a few i00 A
in
However these
only absorb U.V. light and are not stable above pH 3. Chemical modification
with a strong chelating
WO 3
performed
oxide can lead to a large variety of
such as water splitting 412, photocatalysis
of Ti(OPri)4
Fe203 4 0 8
although most of the work has been
diameter are obtained which exhibit good photochemical
atom gives rise
been widely
may be smaller
no longer effective
can be treated with appropriate
photochemical
water or hydrolysis
have
out using colloidal particles with a
function either as anodes or cathodes 405
The photochemical
in
suspensions
carried
the size of colloidal
or MnO 2 411 have been widely studied,
TiO 2
model
that the semiconductor-electrolyte
separation
of the same particle
on
the
which
field created by the electron
bending
within
rim. Such solutions are
interface 4 0 6
Charge
of
are
mobility should be described by usual
409,410
band
properties
studies
the space charge thickness
reactions
The electric
typical
p-type semiconductors.
diameter smaller than i0
both
i000 A.
toward the surface
photochemical
However,
applied.
the space
oxide,
interface 405,406
The
studied.
of
weakly doped
pairs can be created when photons
electron-hole
holes move
is observed for
oxide-water
the
over a region the thickness
For a
are adsorbed at the surface of the oxide. The electrical
provides
semiconductors,
by
distributed
the semiconductor.
more than
described
the band gap (h~>Eg)
junction
level of
317
sols
of the alkoxide
was reported to give transparent
sols
charge transfer from the (acac) ligand to the
absorption of visible
light that improves
These modified colloids are
Ti
the photochemical
stronger reducing agents than
other
TiO 2 colloids 416
A photoelectrochemical
electrolyte
and connected
cell
consists
electrically by a
with one face in contact with the electrolyte
wire by an ohmic contact.
they are collected by
that does
not react
corresponding
called the
immersed into
electrode
chemically with
to the counter electrode,
the electrolyte
potential Vfb.
407 that behaves
This is
the electrons
as a photoanode.
a very
41T. In
- Metastable
shorting
usually a metal
such a cell, the potential
(i.e. the point of zero charge)
important parameter
generated upon illumination
that gives
of
is
an
the n-type
Very few papers actually report the sol-gel
despite the fact that this process
can offer many advantages:
- Oxide layers of large area can be easily deposited onto a metallic
of rutile 418
semiconductor
The majority carriers move toward the bulk of the electrode where
of photoelectrodes
at relatively
is a
an aqueous
and the other face connected to the
to zero excess charge in the semiconductor
flat band
semiconductor
electrodes
wire. The main
the wire and transferred
estimate of the reducing power of
processing
of two
substrate
and sintered
low temperature.
crystalline
phases can be obtained such as anatase in the case of TiO 2 instead
318
J. Livage et al.
- Oxides
little
such as
TiO 2 actually
interest
electrical
for
conductivity
such
mixing the appropriate
solutions
unless
gap (Eg=3.2eV).
means
are
Therefore
found
to
as Cr 3+ or AI 3+ .
Doping can be made
they are of
enhance both their
This can be obtained by
at a molecular
of molecular precursors which yields highly
level
by
homogeneously
419,420
Photoelectrochemical
-
wide band
and light response into the visible region.
doping with impurities
doped photoanodes
have a
photoelectrolysis
and
photochemical
421,422
experiments
flat band potential values that can give useful information
(colloid or gel) interface.
Moreover,
permit such measurements
to characterize
as
the water oxide
the d.C. photocurrent variation as a function of
the
incident wave length gives an access to the band profile of gels or xerogels.
5.4.2. Electron transfer at the
oxides
Fe304 colloids
interface.
Interfacial
properties
do not usually take into account the bulk of the solid network.
metal oxides is usually
leading to positively
described as arising from
charged particles.
the protonation
Acid peptization
of surface
Electron hopping occurs in mixed valence
and the whole oxide network may be involved in surface charge modifications.
become
ratio. As
especially
important
an example,
for
electron delocalization
between the reduced
sites are equally occupied
ions
only.
delocalization
ions exhibit
It
a
mixed
an average
almost the same lattice parameter,
iron
deficient.
It
can
be
cubic inverse spinel
structure.
Octahedral
+2.5. The
but it
described
forms.
where
consequence,
molecular
label octahedral
sites.
sites
electron
are occupied by
hopping
octahedrally
formula
7-Fe203
leads
coordinated
may thus
be written
is also a spinel oxide
contains no Fe 2+ ions and
as
from
similarity
compound
sites. As a
results
structural
ions while tetrahedral
valence
charge of
where brackets
been shown to involve the
Such behavior
close
(7-Fe203)
by Fe 2+ and Fe 3+
is
colloids have
Fe304 together with
Fe304 exhibits a
over the octahedral
Fe3+[Fe~'5+]O4
of Fe304
(Fe304) and oxidized
oxides
Such a process
particles having a large surface/volume
surface of the particle.
in mixed valence
At room temperature
Fe 3+
colloidal
redox reactions
whole spinel lattice and not only the
of
M-OH groups
Such a description may become no longer valid when
metal ions exhibit several valence states.
can
of metal
Fe3+[Fe~3DI/3]04
octahedral
to
iron
as
with
sites are
where D corresponds
to iron
vacancies.
It has been shown that oxidation
conditions.
colloids
spinel
One of the possible pathways
in a weakly acidic medium
framework
significant
while
structural
protons
consumed
changes and the overall
per Fe 2+ ion released in solution.
bonds
are
weakened,
formation of iron vacancies
electron localization
118.
The potential
flux
neutrality
is
vacancies
renewed:
new superficial
protons
the
locally maintained
leaving
adsorb
towards
within
from
Fe(II)/Fe(III)s0.15,
the
iron
the
by outward
solution
proceeds
is found to be 2
of Fe-OH hydroxyl
without
H + consumed
groups appears
octahedral
species
leading to soluble
the surface
(figure 30c).
to
sites is
(figure 30b).
[Fe(OH2)6] ++ with
Simultaneously,
As Fe 2+ ions are released in solution,
leaving
(figure
a
positively
migration of
(figure
30d).
ions are coordinated by
leading to a
under anaerobic
unpaired Fe 3÷ ions in the core of the particle
surface
lattice
desorption
energy at protonated
occurs,
and bare oxygen at
at the surface leaves
occurs
Fe2+
and formation of -O-Fe-OH~
hydrolysis
which are acceptor states for mobile electrons.
electron
can occur
is outlined in figure 30 for Fe304
stoichiometry
Surface protonation
lowered leading to electron localization
O-Fe 2+
into 7-Fe203
(pH=2).In this case all Fe 2÷ ions are released from the
are
be the driving force of the process.
As
of Fe304
for the process
30c).
stable cationic sol
iron ions
an
charged core. Charge
towards the surface
The surface is thus progressively
water molecules
Peptization
while bared
occurs
as
oxygen
soon as
while at the end of the process the
Sol-Gel Chemistry of Transition Metal Oxides
colloid is
converted into
partially
reversible.
?-Fe203 . It
Once
Fe 2÷
readsorbed just by raising the
Fe304
anymore.
After
must be
have
been
pH up to 5 or
adsorption
of
pointed out
removed
319
that this
from
Fe304
transformation is
colloids,
6, but the particle does
Fe 2÷
at
the
octahedral Fe 3+ and delocalization over the v-Fez03
surface
they can be
not transform into
electron
transfers towards
lattice occurs but no migration of iron
ions inside the particle is observed. This process leads to an epitaxial growth of a
layer and adsorption stops as soon as all octahedral sites within the core have an
charge of +2.5. As no significant
proton diffusion towards the
the lattice
iron diffusion occurs,
core is involved in
it is supposed
Fe304
average
that simultaneous
order to maintain charge
balance within
116
(~
H H
H H
H
0~0~0~0~0~0~0
0¢0~0¢0~0e0
OeOeO~OeO®OeO
H ;H H I H
'-'
..... 6 , i 6 6
o®
oAo o
H IH H: H
,
,
~Fe 25*
H IH H H Y H
E......6 H ~ H 6 6H"E)'H 6 ' ' ' ~
O~OuO~O¢OPO~O
0 / ( 0 ¢ 0 ~ 0 a~"O@0
OoOeOeOo/OeOeO
......; 6 d d 6 6 .......
=
(~3
H
D
6'=
o
H
H
H
H
oFe 3÷
"Fe2*
uFe vacancy
(~[Fe (OH2)612÷
hopping
I electrons
H
........6 6,6 6
6,6-o/O.6.,6.o.6.6.oTo}
I ooo,,o,,ooo,,o •
Od 0 ~ 0 cO-'6 O ~ 0 • 0
ODO®O ° O n O ~ O ~ O
Fig.30. Schematic process of the transformation Fe304 ~ 7-Fe2O 3 in weakly acidic medium.
(A) Configuration of the octahedral sublattice in Fe304.
(B) Protonation and electron localization at surface sites.
(C) Desorption of Fe 2÷ and migration of iron towards the surface.
(D) Fe 2÷ content has decreased and vacancies have appeared.
This transformation Fe304 ----+ 7-Fe203
(aerobic oxidation,
same
electronic
Fe 3+ adsorption,
process:
electron
transfer within the particle.
same but adsorption phenomena
conditions that
conditions
transfer
The intrinsic
through
the
structural transformation is likely to be
that induce electron transfer
by
delocalization.
In agreement
with this result,
inhibited in weak acid medium,
explains
the
outstanding
another divalent
Fe 2÷ may
cation such
of
as Co 2÷ prevents electron
surface hydrolysis of
and Co 2÷ adsorption onto 7-Fe203
behavior
be quite
the
outward
fundamental role of electron delocalization in surface phenomena is nicely corroborated
Fe 2+ by
the superficial
at the interface and
The
that replacing
behavior of
the
interface relayed by electron
different.
the fact
rule the
is also observed under various
etc...) 116"118. Analysis of this reaction reveals
spinel
iron
Fe2CoO 4 is strongly
is also very limited.
This
oxide colloids which may be used as
colloidal electron exchanger in aqueous solutions.
6. M O N O G R A P H
The
present
monograph
provides
transition metal oxide sols and gels.
atomic number. Multicomponent
a
brief
review of the published literature on
The main elements are classified according
to their
systems are discussed separately at the end of the monograph.
320
J. Livage et al.
This review mainly points out the nature of the precursors,
the main applications
the experimental
procedure
and
of the resulting materials.
6.1. Transition metal oxide gels
6.1.1. Titanium
oxide. TiO 2
dissolving sodium titanate
as ~ C 0 3 ,
can
(NH4)2CO 3 or
be
easily
conditions
been known
Na2CO 3 in order to
obtained
60,423
gels have
through
The
for a
in concentrated hydrochloric
avoid high pH gradients
thermohydrolysis
colloidal
long time.
particles
of
are
TiCI 4
the
parameters
attention on processing
Sol and
which
:
crystalline
Monolithic
244,277.
chemical modification
of titanium alkoxides 20 .
acetic acid 266,309,420,
acetylacetone
60,423
have
powders.
metal-organic
ratios
This modification
routes using
from Ti(OR) 4 (R= Et,
(l<h<4)
or colloids can
266,309 or
Some authors
while others have focused their
coatings or monodispersed
hydrolysis
TiO 2 sols
and have anatase or rutile
are devoted to
TiO2.based gels
such
TiO(N03) 2 under acidic
TiO 2 gels can be synthesized
Bu s ) using substoichiometric
(HCI, HNO3)
formation
Most recent studies
Ti(OR) 4 alkoxides precursors.
catalysts
gel
in order to obtain fibers,
gel formation
Bu n , Pr I , Pr n ,
influence
124,125,126
or
structure depending on the pH and the nature of the counter-ions
studied
They can be made by
acid, then adding a weak base
and inorganic acid
also be obtained
after a
is performed mainly
with
hydrogen peroxide 310. A good review
of
gel synthesis using inorganic precursors was published by Woodhead 424
- TiO 2
fibers
controlled.
ethanol
:
using
or gels
sols
can
offers
an
in ethanol
sols or transparent
allow fibers
be
substoichiometric
acetylacetone
hydrolyzed
TiO 2 sols
Spinnable
made
hydrolysis
alternative
route.
TiO 2
coatings
inorganic
:
sols.
Coatings
on
Dispersable
sols
can
of Ti(OPri)4
of cellulose 428,429.
be
either inorganic or
TiOS04 430,431
about 0.4
available
: Monodispersed
diameter are
(NH4OH) catalysts
is
leading to
freezing
have been
obtained
made by
using
dip coating in various
phase-transfer
or extraction
have been
obtained
and peptization by HCI or HNO 3 in the presence
submicronic
The
TiO 2
of Na2SO 4 121
obtained.
powders can be obtained
inorganic route involves
With
by using
thermohydrolysis
In both cases monodispersed
alkoxides precursors,
two main
of
spheres
routes are
:
i) Controlled precipitation
of
Ti(OEt) 4 in EtOH with
These powders can easily be doped
Hydrolysis
of
Ti(OEt) 4
or
an excess of water
with Nb(OEt) 5 and Ta(OEt) 5 434
can be used in order to improve the monodispersion
ii)
modified precursor Ti(OPri)2(acac)2
(HCI) or basic
in
with
to depend mainly on the firing temperature.
organic precursors.
or TiCI 4 in the presence
~m in
modification
porosity for ultrafiltration
or Ti(OBun)4
Porosity appears
- Powders for ceramics
Chemical
of TiCI 4 with KOH followed by dialysis 426
various metals
techniques 427. Membranes with controlled
through hydrolysis
The
276
is carefuly
(HCI) of Ti(OPri)4
can also be obtained by unidirectional
of a gel made through partial neutralization
-
drawn when viscosity
acid hydrolysis
ratio
in the presence of acidic
monoliths 425. Fibers
to be
through
Ti(OPri)4
240,241,432,433
Hydroxypropyl
cellulose
435
aerosols leads to monodispersed
spheres whose
diameter can be varied from 0.06 to 0.6 ~m 122,243,436
Other TiO2-based powders
Ti(OPri)4
in
iprOH 242,437,438
Ti(OR)4/ROH mixtures
can be obtained either by
in iPrOH
436 or
precipitation
anionic exchangers
have been synthesized
Spherical TiO 2 powders with diameters
obtained from gels made from inorganic 440,441
of
by spray techniques using
(R - Et, Pr i , Bu n ) 261. This leads to dense TiO 2 ceramics when
around 900°C. TiO2-based
of Ti(OR) 4 439
for ceramics
and Ti(OBui)4
through acid
heated
hydrolysis
in the range of 1-2000 ~m can also be
or organic precursors
441
Sol-Gel Chemistry of Transition Metal Oxides
6.1.2.
Vanadium
pentoxide.
synthesised by different
- Acidification
V205
been
of sodium or ammonium metavanadate
- Acidification
174-176
known
for
a
long time and can be
solutions by hydrochloric
or nitric acid
174
of sodium or ammonium
metavanadate
solutions with a proton exchange
resin
of amorphous V205 prepared by splat cooling into water 442-444
Pouring the molten V205 oxide directly
-
have
routes.
followed by washing or dialysis
Dissolution
gels
321
Hydrolysis
of vanadium oxoalkoxides
into water 445
VO(OR)3
(R = Et, Pr i , Pr n , Bu n , Am t) in the
presence
of excess water 446,447
The structure
and properties
6.1.3. Chromium oxide.
Monodispersed
by thermohydrolysis
Monolithic
salts
sols of hydrous chromic oxide
of various Cr(III)
of sulphate or phosphate
Cr(IIl)
of V205 gels have been reviewed recently 339,448
salts
(CrCI3, Cr(NO3)3,
green or
blue-grey hydrous
(CrCI 3, Cr(N03) 3,
Cr2(SO4)3,
gels was
studied by
Catalysis
is one of the main applications
Hydrosols
Infra-Red
also be
Cr(OEt)3.EtOH
6.1.4. Manganese
condensation
EXAFS,
can
chromic
oxide gels are
Cr(OOCCH3)3)
are treated
sols
TEM
easily formed when
by an aqueous basic
The structure
of these
and magnetic measurements
74,76
hydrolysis
and
condensation
of CrCI3.4EtOH ,
in ethanol 450
Pure hydrosols
of Mn(OH)2
can be
obtained via hydrolysis
and
in ethanol 450
Hydrous MnO 2 sols and colloids
Transparent
the presence
of these gels 449
obtained via
and Cr(OEt)3
oxides.
of Mn(OEt)2
spectroscopy,
are readily obtained through the reduction of KMnO 4
with reducing agents such as As(OH)3130 ,
134.
in
ions 78
solution of NH40H or KOH with an excess of acetate ions 70,71,74,76
CrCI3.3EtOH,
have been synthetised
KCr(S04)2)
of
prepared by 7-irradiation
manganese
Na2S204131,
(IV)
Mn2+ 132,411,451,
oxides
of KMnO 4 solutions 4 1 1
and
manganese
No structural
NH~ 133 or
glucose
III oxides can also be
studies have been performed
on these sols.
6.1.5.
Hydrous
through
ferric
controlled
perchlorate)
oxide.
Monodispersed
thermolysis
108
Chlorides
of
first
Fe(III)
lead
to
ferric oxide sols can be obtained
(chloride,
monodispersed
give rise
Fe203 particles
are directly formed with nitrates or perchlorates
a water/ethanol
mixture,
110. In
109
~-Fe203
Monodispersed
108
of a
reducing agent
monodispersed
such as
if Fe2(S04) 3 is used as a Fe 3+ precursor,
Fe3(SO4)2(OH)5.2H20
Fe20 B sols can
hydrogen peroxide
peptization
as
perchloric
acid
obtained through hydrolysis
Gelatinous
113,114
and condensation
precipitates
variety of techniques
Pure
of hydrous
107: dialysis,
and oxidation
hydrosols
or hydrazine,
of
iron oxide
studied 90-97
of precipitates
ammonium hydroxide
of Fe(OEt)3
hydrous
the
of
:
sols
7-
hydrous
or a strong acid
ferric oxide can also be
in ethanol 450
ferric oxide
hydrolysis
cubic ~-
are obtained
Fe304 sols 110 occurs.
that occurs upon aging have been extensively
be obtained through
in
monodispersed basic salts are obtained
Fe304 using weakly polarizing bases such as tetramethyl
such
ellipsoidal ~-
discs
and Fe4(SO4)(OH)I 0 105. The formation of these hydrous
and the aggregation process
and
If FeCI 3 is aged
~-Fe203
Fe2+/Fe 3+ ratio can be adjusted until the formation of monodispersed
Finally,
sulphate
fl-FeOOH is more rapidly formed and leads to monodispersed
while with triethanolamine
the presence
spheres.
nitrate,
acicular ~-FeOOH which, upon
further aging,
Fe203 particles,
to monodispersed
hydrous
salts
can be
obtained using
of inorganic precursors
through
a large
dilution,
322
J. Livage et al.
ionic exchange, phase
transfer extraction with
with a weak base such as NaHCO 3
long chain organic
amines, neutralization
100, peptization of a precipitate 452 or
decomposition of
ferrous oxalate by hydrogen peroxide 120
6.i.6. Cobalt
nickel and
copper oxides.
Monodispersed
synthesized by aging CoOOH precipitated from
cubic Co304
particles have been
cobalt acetate 65. Pure hydrous cobalt
oxide
hydrosols are obtained through hydrolysis and condensation of Co(OEt) 3 in ethanol 450
Ni(OH)2 and
Co(OH) 2 gels
tartrate precipates 63
can be
synthetized upon
Green Ni(OH)2 gels
dialysis of
nickel or cobalt
can also be obtained through neutralization
of
nickel(II) acetate dissolved in glycerol with alcoholic KOH 64
Monodispersed Cu20 sols are formed upon ageing copper (If) tartrate in the presence
of glucose 69
Ellipsoidal CuO or Cu(OH)2 particles can be obtained upon ageing
copper(II)
nitrates or sulphates 68
Sky-blue
copper(II)
copper acetate with
hydroxide
ammonia in the
gels
can
presence of a
be
obtained through neutralization of
small amount of
sulfate ions 66,6z
or
through neutralization of CuCI 2 with NaOH 58
6.1.7. Hydrous yttrium oxide.
Sols and gels can easily be obtained from yttrium nitrate by
ion exchange
Structural characterization
techniques 453
EXAFS, SAXS, light
scattering and TEM
of such
454. Peptization of
sols has
been done by
yttrium hydroxide precipitates
also leads to colloidal solutions 455
6.1.8. Zirconium oxide. Monolithic ZrO 2 gels can be synthesized from Zr(OR) 4 (R = Et,
Pr n , Bu n) using
substoichiometric hydrolysis ratios
(HCl, HN03) 263. Stabilization of Zr(oPrn)4
with
acetic
acid
263,
acetylacetone
acid catalysts
via chemical modification 20 can be
291,305
solvents also leads to monolithic gels
(l<h<4) and inorganic
Pr I ,
performed
or hydrogen peroxide 305. Using different
upon mild hydrolysis of Zr(oPrn)4 272
or Zr(OBun)4
456. ZrO2 gels obtained from inorganic precursors were reviewed by Woodhead 424. Structural
studies have been performed on amorphous ZrO 2 gels 129,441
ZrO 2 fibers : Two main methods have been used to get ZrO 2 fibers :
- Extrusion and calcination of zirconium acetate 457
- Unidirectional freezing of aqueous solutions made from ZrOCI 2 426,458
ZrO 2 coatings: They
have been made
from colloidal solutions
alkoxides 429,459,460,461. Chemical modification
inorganic precursors 42? or
by acetic
mainly by dip-coating
acid 429,459,460,
acac or
etac 461
and ethylene
glycol 461
using
of Zr(oPrn)4
allows a
better
control of the viscosity. The dip-coating process can thus be easily optimized.
Powders for ceramics :
powders
from
455,462,463,464
Controlled
Thermohydrolysis
inorganic
The
precursors
formation
precipitation
279,364,433,466,
6.1.9. Niobium
of
of
is the cheapest way to obtain
such
as
ZrOCI2,
monodispersed
zirconium
ZrO 2
alkoxides,
ZrO(NO3)2,
sols
Zr(OPri)4
monodispersed ZrO 2
ZrCI 4
was
followed
or
or
ZrO(S04)
by TEM 465
Zr(oPrn)4
in
EtOH
allows the synthesis of submieronie monodispersed ZrO 2 based powders.
and tantalum
pentoxldes. Monodispersed
Ta205 powders
through controlled precipitation of Ta(OEt)5 in an ethanol/butanol-i
can be
synthesized
(1:4) mixture with
an
excess of water (h=3-10) 280
Ta205 sols can be used to make storage capacitor dielectrics for
microelectronies
by hydrolysis-condensation of Ta(OEt)5 in ethanol or toluene with an acid catalyst such
HCI or CH3COOH
232,467. Thin films
coating technique 467
1750 A thick
were obtained from
such sols by
as
a spin
Sol-Gel Chemistry of Transition Metal Oxides
Various methods
- Hydrolysis
can be used in order to synthesize Nb205
of NbcI 5 followed by a
in order to remove chloride
- Hydrolysis
of chloride-alkoxides
of niobium ethoxide
can be easily obtained
order to
alkoxides
321
or
acetylacetone
tungstic acid can be obtained when acidification
by acidification
devices 183
alkoxides.
by washing or dialysis 468,469.
of a
Electrochromic
make display
peroxide
alcohols.
solution with HCI is followed
exchange column 1 8 3 , 4 7 0
:
such as Nb(0R)3CI 2.
in various
Tungsten oxide WO 3. Colloidal
sodium tungstate
gels 181
careful washing and the addition of hydrogen
ions.
- Hydrolysis
6.1.10.
323
sodium tungstate
thin films have been
These sols
In
this
can also
case,
a
of a
A pure
solution through
sol
a proton-
deposited from WO 3 sols
in
be made from tungsten chloride-
chemical
stabilization
of
W(OEt) 6 by
in butanol must be made 304
6.1.11.
Noble
elements
such as Au(lll).
metal
oxides.
Gelification
Na Au(OH)4 with an inorganic acid 4 7 1
hexachloroiridate
can
also be achieved with noble transition
Hydrous Au203 gels have been synthesized
(III)
or
(IV)
Colloids
at
pH=7
through acidification
IrO2.xH20 can be prepared by hydrolysis
472.
Finally,
synthesized by dissolving KRuO4and poly(styrene/maleic
colloidal
anhydride)
RuO2.2H20
of
can
be
(i:i) in water and adding
aqueous H202 at pH=7 473. The main application of these noble transition metal colloids
in the field of photocatalytic
of
is
materials 474
6.2. Materials
The sol-gel process
glasses.
Only materials
as coprecipitation,
freeze drying,
6.2.1. Ferroelectric
for high dielectric
or Ti(OPri)4
is especially
suitable for making multicomponent
derived from sol-gels
ceramics.
are rewiewed here.
Barium
titanate BaTiO 3 is the most
and barium alkoxides
often studied material
The alkoxide route using Ti(OEt) 4
such as Ba(OEt) 2 290, Ba(oPrn)2
CH30 H 478 have been mainly used to obtain thin films 478 or monolithic
be substituted by strontium
through
controlled
strontium titanate
modification
as Sr(N03)2,
(HN03)
can be
Lead
capacitors.
of
a
tungstic acid
complex
powders which can
titanate
PbTiO 3
can
H2WO 4 and
also
486
The substitution
Films of various
spin
Titanium
487,
488,489
Ti(OBun)4
can
be
and
488,489,
used
in
and
Zr(oPrn)4
are
performed under
butanol-2.
dielectric
acidic
They lead to
applications
constant ceramic
and Ti(OPri) 4
483,484,485.
of some titanium
compositions
is introduced
electrooptic
gel from Pb(OAc)2.nH20
zirconium
W doped
obtained by mixing Zr(OBun)4,
high
precursors
Thin films have
by zirconium
lead
been
to the so
are obtained by dip-coating 487
usually
488,489,
introduced
Zr(OEt)4
introduced as lead acetate or lead ethyl-2 hexanoate 487,488
JPSSC 18:4-E
is
obtained
through chemical
Strontium
Sr(OH) 2 in
(HN03) hydrolysis
called PZT compositions.
coating
hydrolysis
in
Barium may
479,480
as high as 40,000
Ba(OH) 2 and
be
gels 2 9 0
SrTi(OPri)6
be used for piezoelectric
using acid catalyzed
made by spin-coating
alkoxide
formulations
It can be made as a monolithic
in methoxyethanol
Ti(OPri)4
double
Sb(OEt) 3, La(N03)3,
290,475
476 or Ba(OH)2
Strontium titanate powders can be
synthesized giving c values
Finally
Ti(OBun)4 , Nb(OEt) 5,
ceramics.
with ethylene glycol and citric acid 481
tungsten as
conditions.
fine perovskite
482
in these
precipitation
of Ti(OBun)4
or
such
spray drying and liquid drying will not be considered.
constant ceramic capacitors.
476-478
ceramics
Other wet techniques
487
;
as alkoxides
while
or
:
lead is
324
J. Livage et al.
Alkali niobates
and tantalates
are also
very important
which can be obtained by the sol-gel process. LiNbO 3 490,491
ferroelectric materials
; Nal. x Li x NbO3 492 ; KNbO 3 ;
KTaO 3 and K(Ta,Nb)O 3 476 have been made mainly from alkoxides.
6.2.2. Magnetic
ceramics.
Spherical
synthesized by ageing Fe(II),
mixed cobalt
Co(II) and Ni(II) hydroxides
sulfate ions 493. Ferromagnetic NiFe204
containing Nickel(II)
and nickel
ferrite particles have been
in the presence of
films are deposited by dip-coating
ethyl-2-hexanoate
and Fe(III) ethyl-2-hexanoate
nitrate or
from solutions
487
Barium ferrite powders BaFe12 O19 can be obtained from a goethite gel and
in ethanol 494. Through hydrolysis-condensation
Ba(OR) 2
of Ba(OEt) 2 and Fe(OEt) 3 magnetic
Ba2Fe204
can also be synthesized 495
6.2.3. Other ceramics. Many binary systems have been made by the sol-gel process
.
_
TiO2_AI203
ZrO2.AI203
(Ti(OPri)4/iproH)
(Zr(OPrn)4/EtOH)
:
432 and AI2TiO 5 from organometallic precursors 496
432, and AI203-ZrO 2 composites made by dispersing ZrO 2 fibers
in AI203 gel (Al(OBuS)3/qINO3 or HCI) 497
Y3AI5012
made
from
yttrium
and
aluminium
alkoxides 480 or Y(NO3) 3 and Al(OPri)3
in
ethanol with base catalyst which leads to a translucent gel 498
-
Y203-AI205
transparent gels from Al(OBuS)3
- LaYO 3 thermomechanic
- TiO2-CeO 2
and yttrium
acetate hydrolyzed at pH 5.5 499
ceramics made by basic (NH 3) hydrolysis of La(OEt) 3 and Y(OEt) 3 500
films made
by
dip-coating from
a solution
containing Ti(OPri)4,
CeCI 4 and
acetic acid 501
-
ZrO2-Ce203
ZrO2-Cr203
TiO2-V205
thermomechanic
ceramics from Ce(acac) 3 and Zr(OBun)4
in ethanol 502
thin films by solvent extraction 503
fibrous gels from VO(OEt)3 or decavanadic acid sols 447
6.2.4. Glasses
and vitroceramics.
based on TiO2-SiO 2
Low thermal
systems. TEOS and
Ti(OBun)4
504,505,506
conditions
(HCI, CH3COOH , PTSA).
and
expansion coefficient
titanium alkoxides Ti(OEt) 4
Ti(OPri)2(acac)2
425
are
mixed
glasses are
315,504,
mainly
Ti(OPri)4 2 7 5
and hydrolyzed under acidic
Alkali resistant glasses are obtained in the SiO2-ZrO 2 system. Gels can be made by
mixing TEOS and zirconium
n-propoxide 275,315 Zr(acac) 4
nm thick can be obtained from Si(OEt)4/Zr(oPrn)4
95% relative humidity 509.
507 or ZrO(NO3) 2 508. Coatings 50
mixtures hydrolyzed under an atmosphere of
Monolithic films or fibers
can be obtained when
hydrolysis
is
performed under acidic conditions 275
Other vitreous compositions
SiO2-ZrO 2
studied included :
photoresponsive
tetrahydrofuran
and
polymers
other solvents
made
by
such
as :
polymerization
benzene,
of
Zr(OBun)4
in
ethanol, CS 2 and acetone with
freshly crushed silica gel 456
Si02-Fe203 made from TEOS and Fe(OEt)3 2?5
Si02-Y203 high
SiO2-TiO2-ZrO 2
Ti(OBun)4
temperature glasses made from Y(NO3) 3 and TEOS/EtOH
films
made
by
from
ethanolic
(1:3) 510
solution containing TEOS,
and Zr(OPrn)4 with HCI and/or formamide 511
Si02-TiO2-ZrO 2 glasses obtained
Zr(oPrn)4
dip-coating
through hydrolysis of a
mixture of TEOS, Ti(OBun)4
and
in ethanol 512
SiO2-ZrO2-AI203-Na20
Zr(oPrn)4, Al(OBuS) 3
alkali
and NaOEt
ethylene glycol and pentanol)
acid 513
resistant
glasses
mixtures at
followed by
obtained
h<l.8 with
through
hydrolysis of TEOS,
chemical modifiers
(acetic acid,
a peptization process with nitric or
perch~oric
Sol-Gel Chemistry of Transition Metal Oxides
SiO2-TiO2-AI203-Li20
low
hydrolysis of a Si(OMe)4,
thermal
expansion
coefficient
Ti(OPri)4 , Al(OBuS)3 and LiOMe
325
glasses
made
through
the
alkoxide mixture in methanol
in
the presence of a DCCA such as formamide 514
Na20-B203-V205-SiO 2
gels
made
by
mixing
Si(OEt) 4,
methanol using NH 3 as a catalyst and a wet atmosphere
VO(OEt) 3,
B(OBun)3 and NaOMe in
Similar
colored
6.2.5. Catalysts. The sol-gel process offers many advantages for making catalysts.
Powders
gels
for hydrolysis 5 1 5
are obtained when Co(OAc) 2 or ~i(OAc) 2 is used instead of VO(OEt) 3 .
with
high
surface
area
and
optimized
pore
size
distributions can be obtained. Since
homogeneous mixing can be made at the molecular scale, the chemical reactivity of the oxide
surface can be greatly enhanced.
- Hydrogen adsorption is achieved at the surface of chromium oxide gels which provide
catalysts for the dehydro-cyclization
slow
precipitation
with
dilute
gelation from chromic acetate
agents
such
as
dugar
or
of paraffins 516. These
ammonia
acid 449 .
The
glass column
with
acid by alcohol or
highest
other reducing
recorded rate for the dehydro-
from chromium oxide gels obtained
sol-gel techniques 517
Hydrolysis
chloride with ammonia and hexamethylenetetramine.
into a
by
through reduction
Chromium oxide microspheres for catalyzing fluorination processes can
also be obtained using
particles
gels have been prepared
dilute chromium nitrate solutions, and by
and reduction of chromic
oxalic
cyclization of n-heptane was obtained
with oxalic acid 449
from
good
using ethyl-2
narrow
size
hexanol as
distributions
is achieved by mixing
chromium
Gelation occurs by injecting the solution
the extraction
have
solvent. Highly
dispersed
been obtained from Ti(OPr~)4 and cobalt
nitrate dissolved in ethyleneglycol 518. The average size of the particles can be varied in
the range 30-120 A by diluting the
alkoxide precursor during the synthesis. This leads
to
modified catalytic activity for the hydrogenation of propionaldehyde.
If
drying is
performed in
hypercritical conditions,
a highly
porous material
called
"aerogel" is obtained. Aerogels exhibit better catalytic properties (activity, selectivity,
resistance to desactivation)
made from Ti(OPri)4 or
than usual xerogel
catalysts 519,520. Anatase
Ti(OBun)4 allow partial oxidation,
TiO 2 aerogels
at room temperature under
irradiation, of paraffins, olefins and alcohols into ketones and aldehydes 521
or NiO/SiO2/AI203
U.V.
NiO/AI203
aerogels made from nickel acetate in methanol, Al(OBuS)3 and Si(OMe) 4 are
almost 100% selective towards partial oxidation
of paraffins or olefins. Isobutene can
converted into methacroleine and acetone 519,522
also allow the conversion
be
Similar aerogels and Cr203/AI203 aerogels
of olefins into nitriles
520, while Fe203/SiO 2 and
Fe203/AI203
aerogels exhibit Fisher-Tropsch reaction rates two or three orders of magnitude higher than
those
of
the
conventional
reduced
aerogel form can be made from
for
electrochemical
TiO2-SiO2,
generator
ZrO2-Si02,
iron
catalysts
523. Reduced oxides such as MoO 2 in
Mo(acac)3 in methanol/ammonia solution. They have
MgO-TiO2,
catalysts
520
MgO-ZrO2)
been used
Finally, mixed oxide aerogels (TiO2-ZrO 2 ,
made
from Ti(OBun)4 , Zr(OPri)4 , Si(OMe) 4 and
Mg(OMe) 2 precursors, can replace SiO 2 or AI203 as substrates for catalysts 519,520
-
The
transition metal
various
glassy
oxide catalytic
substrates.
Amorphous
phase can
TiO 2
also be
coatings
used as
a coating on SiO 2 or
made from an ethanolic solution of
Ti(OBun)4 can be deposited onto glass spheres of the Si02/AI203/CaO/K20/MgO/Na20
system and
treated by a solution of Pd(CBHs) 2 in pentane 524. The catalytic activity of such catalysts
towards olefin hydrogenation is
comparable with that of
mono-atomic layer of amorphous niobium oxide can
the best conventional systems.
be deposited onto the surface of SiO 2
reacting surface silanol groups with Nb(OEt5) in dry hexane followed by chemical
with H20 and
ethanol.
02 5 2 5
Such
a catalyst is
active and selective
A
by
treatment
for ethene formation
from
326
J. Livage et al.
6.2.6. High Te Superconducting Ceramics. A tremendous effort has been applied to synthesize
high temperature superconducting ceramics by
the sol-gel process. The versatility
process
ceramics,
will
allow
one
to
obtain
dense
fibers
of this
or films from sols or gels
intermediates. Bulk ceramics and thick films have already been made by solution techniques.
In the case of
the 90K superconducting phase
through coprecipitation 526-528
Some
other
solution
ethyleneglycol
dissolve
and
copper
, by controlled
453,529,530,
hydroxides and acetates
processes
citrates
alkoxides
YBa2Cu307 such materials have
have
very
precipitation with colloidal
solutions of neodecanoates
included
535,534
or
few
the
control
metacrylates
processes
been achieved
531,
of
mixtures of
532
ethylhexanoates
the
rheology by use of
535. Because of the difficulty to
have been described using alkoxides 536
However, some soluble alkoxides like Cu(OCH2CH2NEt2) 2 o'r Cu(OCH2CH2OBu)2 have been recently
successfully used 537
Unfortunately,
these precursors decompose through oxides and barium carbonates around
500°C. The reaction leading to the pure material occurs then only around 850°C upon a
long
time, and the sintering effect is weak. Although better homogeneity is achieved, the transport properties of the
superconducting ceramics obtained up
to date by sol-gel
are still dominated by the grain boundaries and they do not show
than
conventional
ones.
The
best
results
better
processes
critical currents
have actually been obtained with carbon-free
precursors like nitrates 538-541 or hyponitrites
537 that decompose easily to oxides
heating. The superconducting phase can then be obtained around 650=C, yielding
upon
submicronic
grains with, unfortunately, a poor diamagnetic signal intrinsic to the small grain size 537 .
7.
Interest in the
CONCLUSION
sol-gel process began
about 20 years
ago 542. Many
significant
results have been obtained since then and products such as optical coatings or fibers
have
already appeared on the market. However, the future of the sol-gel technology still depends
on whether it
will be able
materials 543. Therefore a
to make better
and cheaper materials
real mastery of the
process
or even completely
is required
new
from both scientific
knowledge and technological expertise point of view. One of the main advantage of the
gel process is the ability to go all
sol-
the way from the molecular precursor to the
product,
making possible to synthesize tailor-made materials. However, many parameters are
involved
along
the
process
:
chemistry
during
hydrolysis
and
condensation of the precursors,
physical chemistry of aggregation, gelation, drying and finally physics to account for
properties
of
the
material.
Each
step
has
to
the
be optimized depending on the required
application.
The
sol-gel
chemistry is one of
process
is
based
the main points for
on
inorganic
polymerization
further development of the
reactivity of silica precursors is beginning to
reactions.
Thus,
process. The chemical
be rather well understood but this is
not
yet the case for transition metal oxides. Six chemical reactions are mainly involved in the
sol-gel
process,
alcoxolation. More
namely
;
hydrolysis,
reliable experimental
modification, olation, alcolation, oxolation and
data and
chemical species involved in these reactions have
accurate characterization
of all
to be obtained before a real science
inorganic polymerization can be established. Many efforts are being made in order to
the existing techniques to this problem. The ideal method should be able to give
and dynamical
information all
particles and
gels. Spectroscopies
the way,
Scattering, high resolution liquid
from small
such as
the
molecular species
X-ray absorption,
and solid state N.M.R.
of
adapt
"in-situ"
to large colloidal
IR-Raman, X-ray or Neutron
have been used during
the last
Sol-Gel Chemistry of Transition Metal Oxides
few years.
They appear to give significant
chemical reactivity
results.
327
From a theoretical
point of view,
the
of a molecular MX n precursor mainly depends on the polarity of the
M-X
bond and the nature of the solvent ROH. As shown in this paper,
reactions
can
be
described
on
the
X considerations
electronegativity
basis
(X=-#) 37.
with ROH when its mean electronegativity
in the reverse case (x(MXn)>X(ROH)).
that alkoxides
moisture.
cannot be
Inorganic
electronegative
and
chemical reactivity
not react
gels have
used in
with most
low electronegativity
reacts
so
presence of
They are
quite
ROH reagents.
Their
are formed,
Aggregation
been obtained
polymeric
from inorganic
scattering
aggregation and gelification
observed scattering
the ultrastructure
parameters
gels are made
330
curves,
should
precursors
176
are
Computer models
of
the
particle-particle
gels
are
almost exclusively
obtained
from
metal
salt aqueous solutions 2
be considered with
experiments
and the fractal
such as
or polymeric
gels are typically formed from metallic
Small-angle
many
20 and
chemistry does not play such an important
occurs which dictate
Thus, either colloidal
the literature,
so that they react with
variety of chemical reactions
On
for the sol-gel process.
simplified classification
544
or
(X(H20) = 2.49)
in the
aqueous solutions.
as readily
They offer a large
interactions.
alkoxides while colloidal
such a
#
while solvation occurs
readily hydrolyzed
it mainly depends on physico-chemical
or particle-solvent
alkoxides
and are
therefore will
sol-gel process.
to
potential
is quite low and they are not easily modified by chemical additives.
Once the colloidal particles
However,
(x(MXn)<x(ROH)),
Water has a high electronegativity
then be
are therefore versatile precursors
278. According
chemical
of these
A molecular precursor MEn chemically
precursors must
chemical reagents besides water.
oxide. However,
electronic
is smaller
easily handled
the other hand, metal alkoxides have a
role in the
of
the thermodynamics
care. Polymeric V205
and colloidal
SiO 2 from
currently done
have been proposed
geometry of aggregates
silicon
in order
to study
to account for
is still a
the
matter of
debate 22
The functionality
f
of the molecular
329. As for organic polymers
precursors
is sometimes
this will be a very important parameter.
taken into account
A
three-dimensionnal
network is usually obtained when f is larger than 2, while chain polymers
f
is
close
functionality
to
2.
of
the precursors.
Gelation
colloidal particles
colloids
can
coatings.
However,
is often
sometimes
be
processes
should
As mentioned
governed by
obtained
therefore
in this
the chemical
which
lead
be
paper,
are expected when
strongly dependent on the
the
shape of
the primary
conditions.
Strongly
anisotropic
to ordered aggregation
it must be pointed out that, up to now, it remains
and anisotropic
impossible
to
relate
chemistry and morphology.
Despite
the
present
lack
of
knowledge,
processing of transition metal oxides will continue
unique advantages
for making monodispersed
5 , fibers 4 or even completely
the main drawbacks
densification.
it
may
to grow
process remains
the
drying 545 and the so-called DCCA
almost exclusively
used for
than for
long time required
necessary basis for the development
gels or xerogels,
interesting physical
properties
or electrochromic
of the sol-gel process.
and
can be
display devices.
used for
namely
Anyway,
to make.
hypercritical
546. They
transition metal oxides.
can actually be considered as liquid-solid
one of
for drying and
are difficult
this problem,
analysis of the drying process has been also recently proposed 2 8
coatings
317. However,
(Drying Control Chemical Additives)
silica rather
It offers
ceramics 2,
materials
Rapid drying causes cracking and monolithic materials
Two general approaches have been proposed to circumvent
batteries
assumed that the sol-gel
powders 23, multicomponent
new mixed organic-inorganic
of the sol-gel
be
in the near future.
have been
Theoretical
This should provide
the
as shown in this paper,
composites.
making antistatic
They exhibit
coatings,
some
micro-
328
J. Livage et al.
Acknowledgments
:
We
are
greatly
indebted
to
providing the reproduction of electron micrographs
for providing
electron micrograph
of figure
Prof.
E. MATIJEVIC for authorizing and
of figure 12 and to D r .
i0. Special
thanks are
K. CHEMSEDDINE
also due to D r J.P.
JOLIVET and P. BARBOUX for helpful discussions and preparation of the final manuscript.
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