Download Chapter 3 Fe2O3- the material for gas sensors

Chapter
3
Fe2O3- the material for gas sensors
3.0 Introduction ……………………………………………………………………….. 97
3.1 Iron [Fe] …………………………………………………………………………… 97
3.1.1
Thermal properties …………………………………………………………. 98
3.1.2
Electrical properties ………………………………………………………... 98
3.1.3
Chemical properties …………………………………………………………98
3.1.4
Ionization potential ………………………………………………………… 99
3.1.5
Oxidation states ……………………………………………………………. 99
3.1.6
Crystal structure ……………………………………………………………. 99
3.2 Iron Compounds ………………………………………………………………….. 99
3.2.1
Haemoglobin ………………………………………………………………..99
3.2.2
Iron (II) sulfate ……………………………………………………………...99
3.2.3
Iron (III) chloride …………………………………………………………. 100
3.2.4
Iron (III) oxide ……………………………………………………………. 100
3.3 Reactions …………………………………………………………………………. 100
3.3.1
Reaction with air ………………………………………………………….. 100
3.3.2
Reactions with acids………………………………………………………. 101
3.4 Chemiresistive sensor material- Iron Oxide …………………………………… 101
3.5 Gas-sensing metal oxides ………………………………………………………... 102
References ……………………………………………………………………………. 106
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Figure captions
Sr. No.
Descriptions
Page No.
Fig. 3.1
Unit cell of α-Fe2O3 rhombohedral crystal structure.
101
Fig. 3.2
Corundum crystal structure.
101
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Chapter
3
Fe2O3- the material for gas sensors
3.0 Introduction
From the viewpoint of the basic research in the field of gas sensor, being a
transition metal, iron (III) oxide is one of the important compound for the study. The
existence of amorphous Fe2O3 and four polymorphs (alpha, beta, gamma, epsilon) is well
established [1]. The most frequent polymorphs, the hexagonal corundum structure
“alpha” and cubic spinel structure “gamma”, have been found in nature as hematite and
maghemite minerals. The other polymorphs, the cubic bixbyite structure “beta” and
orthorhombic structure “epsilon”, as well as nanoparticles of all forms, have been
synthesized and extensively investigated in recent years [1,2].
Gamma and epsilon type Fe2O3 are ferromagnetic; α-Fe2O3 is
an
antiferromagnetic while beta type Fe2O3 is a paramagnetic material. On account of the
attractive scientific and industrial applications of α-Fe2O3 nanoparticles, novel methods
for their synthesis and new approaches in their characterization have been reported in
recent years. It has been reported [3, 4] that different techniques of preparation lead to
different phases or mixtures of phases and different degrees of size control. Furthermore
the correlation of the preparation route with size and magnetic properties, such as the
moment and coercivity, is still a subject of debate.
This chapter briefly introduces the origin of iorn oxide and their concerning
parameters necessary for understanding in the field of research.
3.1
Iron [Fe]
At. no.
:
26 (Neutron 30)
At. wt.
:
55.845 (2) g/mol
Group No
:
8
Group Name :
Transition metals
Block
:
d-block
Period
:
4
State
:
solid at 298 K
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Colour
:
lustrous, metallic, greyish tinge
Classification :
Metallic
Boiling Point :
3134K (2861°C)
Melting Point :
1811K (1538 °C)
Density
7.86g/cm3
:
Source: Ninety percent of all mining of metallic ores is for the extraction of iron!
Industrially, iron is produced starting from iron ores, principally haematite (nominally
Fe2O3) and magnetite (Fe3O4).
Electrons per shell
3.1.1
3.1.2
3.1.3
:
2,8,14,2
Electron Configuration:
[Ar] 3d6 4s2
Ground state
:
5
Atomic Volume
:
7.1 cm3 mol-1
Electronegativity
:
1.83
Magnetic ordering
:
Ferromagnetic
D4
Thermal properties
Enthalpy of Atomization :
414.2 kJ mol-1 @ 25°C
Enthalpy of Fusion
14.9 kJ mol-1
:
Enthalpy of Vaporisation :
340.2 kJ mol-1
Heat Capacity
:
25.10 J mol-1 K-1
Thermal Conductivity
:
80.4 W m-1 K-1
Thermal Expansion
:
11.8 µm m-1 K-1
Electrical properties
Electrical resistivity
:
9.7 x 10-8 Ω m
Electrical conductivity
:
0.0993 106/cm Ω
Chemical properties
Electrochemical Equivalent :
0.69455 g Ah-1
Electron Work Function
:
4.7 eV
Valence Electron Potential
:
67 (-eV)
98
3.1.4
3.1.5
3.1.6
Ionization potential
First
:
7.87
Second
:
16.18
Third
:
30.651
Main
:
Fe+2, Fe+3
Other
:
Fe-2, Fe-1, Fe0, Fe+1, Fe+4, Fe+5, Fe+6
:
Body centered cubic
Oxidation States
Crystal Structure
Structure
a = 286.65 pm
b = 286.65 pm
c = 286.65 pm
α = 90°
β = 90°
γ = 90°
3.2 Iron Compounds
3.2.1
Haemoglobin C2952H4664N812O832S8Fe4
The iron-containing oxygen-transport metalloprotein in the red cells of the blood
in mammals and other animals, Hemoglobin in vertebrates transports oxygen from the
lungs to the rest of the body, such as to the muscles, where it releases the oxygen load.
Hemoglobin also has a variety of other gas-transport and effect-modulation duties, which
vary from species to species, and which in invertebrates may be quite diverse.
3.2.2
Iron(II) sulfate FeSO4.H2O
In horticulture it is used as a lawn conditioner and moss killer, traditionally
referred to as sulphate of iron. Ferrous sulfate is also used to treat iron-deficiency anemia.
Side effects of therapy may include nausea and epigastric abdominal discomfort after
taking iron. These side effects can be minimized by taking ferrous sulfate at bedtime.
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Ferrous sulfate can also be used to color concrete. It is best used for newly cured
concrete. Mix with water until saturated and spray onto concrete. The color will range
from yellow to rust.
3.2.3
Iron(III) chloride - FeCl3
Highly Corrosive : Toxic
Most widely used for etching copper in the production of printed circuit boards.
Iron(III) chloride is also used as a catalyst for the reaction of ethylene with chlorine,
forming ethylene dichloride (1,2
(1,2-Dichloroethane), an important
ant commodity chemical,
which is mainly used for the industrial production of vinyl chloride,, the monomer for
making PVC. It is also commonly used by knife craftsmen and swordsmiths to stain
blades, as to give a contrasting effect to the metal, and also tto
o view metal layering or
imperfections. Iron(III) oxide
3.2.4
Iron(III) oxide - Fe2O3
Used: in magnetic storage, for example in the magnetic layer of floppy disks.
A very fine powder of ferric oxide is known as jeweller's rouge, red rouge, or
simply rouge. It is used to put the final polish on metallic jewellery and lenses, and
historically as a cosmetic.
3.3 Reactions
3.3.1
Reaction with air
On heating with oxygen th
the result is formation of the iron oxides Fe2O3 and Fe3O4.
4Fe(s) + 3O2(g)
2Fe2O3(s) ……………(1)
3Fe(s) + 2O2(g)
2Fe3O4(s) …………....(2)
Iron reacts with excess of fluorine, chlorine and bromine to form Fe (III) halides.
2Fe(s) + 3F2(g)
2FeF3(s) …………….(3)
2Fe(s) + 3Cl2(g)
2FeCl3(s) …………..(4)
2Fe(s) + 3Br2(g)
2FeBr3(s) ………….(5)
Fe(s) + I2(s)
FeI2(s) …………………(6)
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3.3.2
Reactions with acids
Iron metal dissolves readily in dilute sulphuric acid in the absence of oxygen to
form solutions containing the aquated Fe(II) ion together with hydrogen gas.
Fe(s) + H2SO4(aq)
Fe2+(aq) + SO42-(aq) + H2(g) …………….(7)
If oxygen is present, some of the Fe(II) oxidizes to Fe(III). The strongly oxidizing
concentrated nitric acid, HNO3, reacts on the surface of iron and passivates the surface.
3.4 Chemiresistive sensor material
material- Iron Oxide
In Fe-O
O system, three different polymorphic forms , FeO, Fe3O4 and Fe2O3 exist.
Also Fe2O3 has typical two modifications: α Fe2O3 (hematite, corundum-type
corundum
hexagonal
lattice, a=5.035Ǻ and c=13.750
c=13.750Ǻ) and γFe2O3 (meghaemite).
Fig. 3.1: Unit cell of α-Fe2O3 rhombohedral
hedral crystal structure.
structure
Fig. 3.2: Corundum crystal structure.
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α- Fe2O3 has a complex defect structure in which three types of defect species,
namely, oxygen vacancies, fe3+ interstitials and Fe2+ interstitials are present. The presence
of these defects gives rise to semiconducting properties. Loss of oxygen leaves behind
extra electrons and produce an n-type semiconductor, while extra oxygen (entering the
lattice as O2) creates a deficit of electron (i.e. introduces electronic holes) which produces
p-type behavior. Thus in α- Fe2O3 a transition from n- to p- type response or vice versa
can be introduce by the change in the gas concentration, by appropriate dopant and/ or by
the operating temperature. Fe2O3 is known to show high sensitivity towards organic
gases. [5,6.]. Various metal oxides have been doped into Fe2O3 to make it selective for
particular gas. It has been demonstrated that ZnO doped Fe2O3 works as a selective NH3
sensor working at room temperature. When doped with Pt, Pd or RuO2, Fe2O3 sensor
detects acetone, which finds utility in medical diagnostics [6]. Au or Zn doped Fe2O3 is
reported to sense CO and NO2[7]. γ-Fe2O3 is known to be a good sensing element for H.
3.5
Gas-sensing metal oxides
It is observed that almost any metal oxide could be a basis for solid-state gas
sensor. For this purpose we need only to prepare this metal oxide as a sufficiently fine
dispersed porous substance with properties controlled by surface states. While selecting
the metal oxide we must understand their electronic structure [8]. In ref [8] the following
information along with the issue of choice of the material for gas sensing application is
thoroughly explained.
Metal oxides exhibit a very wide range of electro-physical properties [9-13]. Their
electrical behavior ranges from the best insulators (e.g., Al2O3 and MgO) through wideband gap and narrow-band gap semiconductors (TiO2, SnO2 and Ti2O3, respectively) to
metals (V2O3, Nax WO3, and ReO3), and superconductors (including reduced SrTiO3).
The range of electronic structures of oxides is so wide that metal oxides were divided into
two following categories:
• Transition-metal oxides (Fe2O3, NiO, Cr2O3, etc.) and
• Non-transition-metal oxides, which include
i)
pre-transition-metal oxides (Al2O3, etc.) and
ii)
post-transition- metal oxides (ZnO, SnO2, etc.)
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The fact that valence orbitals of the metal atoms are of s- and p-symmetry is the
common feature of the non-transition-metal oxides. With transitional metal oxides,
however, the d atomic orbitals assume crucial importance. Many of the complications
with transition-metal oxides stem from this difference, because of the different bonding
properties, associated with d orbitals. These complexities include the existence of
variable oxidation states, the frequent failure of the band model, and the crystal field
splitting of the d orbitals [9].
In Ref. [9] the following explanation of the difference in behavior of nontransition and transition-metal oxides was given. The non-transition-metal oxides contain
elements that with some exceptions have only one preferred oxidation state. Other states
are inaccessible, because too much energy is needed to add or remove an electron from
the cations when they are coordinated with O2− ligands.
Transition-metal oxides behave differently because the energy difference between
a cation dn configuration and either a dn+1 or dn−1 configurations is often rather small. The
most obvious consequence is that many transition elements have several stable oxides
with different compositions. It is also much easier than with nontransient-metal oxides to
make defects, having different electron configurations. A as result of high defect
concentration the bulk and surface chemistry of transition-metal oxides is very
complicated. Trends in the stability of different oxidation states are very important in
surface chemistry, as they control both the types of defect that may be formed easily, and
the type of chemisorptions that may take place [9, 10, 12]. The d0 configuration
represents the highest oxidation state that can ever be attained: thus pure TiO2, V2O5, etc.,
cannot gain any more oxygen, although they can lose oxygen to form defects or other
bulk phases. On the other hand, dn oxides with n≥1 are potentially susceptible to
oxidation, as well as reduction. The stability of high oxidation states declines with atomic
number increase across a given series.
In contrast to transition-metal oxides, pre-transition-metal oxides (MgO, etc.) are
expected to be quite inert, since they can neither be reduced nor oxidized easily. In terms
of electronic structure, this is related to the large band gap, which means that neither
electrons nor holes can easily be formed. It means that these oxides are good isolators.
However oxides, characterized by exceedingly high resistance, are not promising material
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for resistive gas sensors, because of the difficulties, encountered in electrical conductivity
measurements. Such big difference in pre-transition, and transition-metal oxides’
behavior means that transition-metal oxides are more sensitive to the change of outside
ambient. Therefore it seems that this type of oxides could be more preferable for the use
in gas sensors. However, in practice the transition-metal oxides are not being used for
conductometric gas sensor design. As it will be shown in the next sections, structure
instability and nonoptimality of other parameters important for conductometric gas
sensors, such as Eg and electroconductivity, considerably limit their field of use. As it is
known many transition-metal oxides have small band gap. Moreover, the band model
predicts that majority of oxides, having a partially filled d band – that is for dn with 0 < n
< 10 – should be metallic. These expectations are frequently not fulfilled because of
intervention of various types of electron–electron and electron–lattice interactions.
Nevertheless, “simple” metallic behavior is found with a number of oxides of elements in
the 4d and 5d series (ReO3, RuO2). Some oxides of the 3d series (Ti2O3, VO2, Fe2O3) also
have high conductivity in the metallic range.
Only transition-metal oxides with d0 and d10 electronic configurations find their
real gas sensor application. As we know, the post-transition-metal oxides, such as ZnO
and SnO2 have cations with the filled d10 configuration. The d0 configuration is found in
binary transition-metal oxides such as TiO2, V2O5, WO3, and also in perovoskites such as
ScTiO3, LiNbO3, etc. These compounds share many features with the non-transition
metal oxides. They have a filled valence band of predominantly O 2p character, and gap
between valence band and an empty conduction band. Typical band gaps are 3–4 eV.
Unlike transition-metal oxides with 0 < n < 10, stoichiometric, post-transition-metal
oxides ZnO, SnO2, and d0 transition-metal oxides may be reduced, but not oxidized. The
post-transition oxides ZnO, In2O3, SnO2, as well as majority of transition-metal oxides
are active in “redox” reactions since the electron configuration of the solid may be
altered. However, the reaction with oxidizing species such as O2 is expected only with
samples that have been bulk reduced or where the surfaces have been made oxygen
deficient [14]
At that the reduction of post-transition oxides as a rule leads to the formation of
free carriers, which greatly increase the metal oxide conductivity, a fact that is crucial for
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sensor applications. However, limited use of pure transition-metal oxides (1 < n < 10) for
conductometric gas sensor fabrication does not mean that transition-metal oxides are not
of interest of gas sensor designers. On the contrary, unique surface properties, plus high
catalytic activity make them very attractive for various sensor applications, such as
properties’ modification of more stable and wide band gap oxides, and forming of more
complicated nanocomposite materials [15]. For example, for optical waveguide gas
sensors, where the change of optical refraction index is more important than the change
of electroconductivity, transition-metals oxides such as WO3 (H2 and alcohol detection),
Mn2O3, Co3O4, and NiO (CO detection) [16] are the most attractive.
***
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and Properties, Clarendon Press, Oxford, 1992, pp. 66–68.
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