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Prog. Solid St. Chem. Vol, 18, pp. 250341, 1988 Printed in Great Britain. All rights reserved 007%6786/88 $0.00 + .50 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. 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