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Chem Soc Rev TUTORIAL REVIEW

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Amphoteric oxide semiconductors for energy conversion devices: a tutorial review Kalpana Singh,a Janusz Nowotnyb and Venkataraman Thangadurai*a In this tutorial review, we discuss the defect chemistry of selected amphoteric oxide semiconductors in conjunction with their significant impact on the development of renewable and sustainable solid state energy conversion devices. The effect of electronic defect disorders in semiconductors appears to control the overall performance of several solid-state ionic devices that include oxide ion conducting solid oxide fuel cells (O-SOFCs), proton conducting solid oxide fuel cells (H-SOFCs), batteries, solar cells, and chemical (gas) sensors. Thus, the present study aims to assess the advances made in typical n- and

Received 23rd September 2012

p-type metal oxide semiconductors with respect to their use in ionic devices. The present paper briefly

DOI: 10.1039/c2cs35393h

outlines the key challenges in the development of n- and p-type materials for various applications and also tries to present the state-of-the-art of defect disorders in technologically related semiconductors such as TiO2, and perovskite-like and fluorite-type structure metal oxides.

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Key learning points

Defect chemistry, Brouwer diagram and doping strategies for the n- and p-type metal oxide semiconductors. The functional properties of solid state materials depend intricately on defect chemistry. Desired semiconducting properties of oxides can be achieved through change in oxygen partial pressure and doping strategies. Defect models need to be understood deeply to fine tune the semiconductor oxides for solar cells, solid oxide fuel cells and gas sensors. Innovative porous materials engineering is needed to get thermally stable nanostructured n- and p-type semiconductors for solid state ionic devices.

1. Introduction Industrialization has improved the standard of living of developed countries in terms of transport, health care, agriculture, manufacturing and technology. Inadvertently, it has also increased the concentration of greenhouse gases through combustion of fossil fuels, which in turn has adversely affected the earth’s ecological balance through global warming. At the same time, increase in prosperity along with ever growing population has increased the world energy demand quite high, with its current growth rate reaching 1.7%. Consequently, there is general agreement between scientists and policy makers to limit the use of fossil fuels and meet the increasing global energy demand with the use of alternative energy sources. Even with lucrative advantages, use of alternative energy sources in the energy and transport sector is well below the a

University of Calgary, Department of Chemistry, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. E-mail: [email protected]; Fax: +1 403 210 9364; Tel: +1 403 210 8649 b University of Western Sydney, Solar Energy Technologies, Penrith NSW 2751, Australia

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level that is needed to have a significant impact on climate change and energy security. This is because these environmentally friendly sources can only be intensively used when the energy conversion and storage devices exhibit high efficiency, durability at low cost, and therefore, surpasses the conventional energy supply from fossil fuels and coal. If these criteria are not met, economic and market forces will determine their future, which in turn will not be beneficial for earth’s ecological balance and energy security. In view of this, semiconducting metal oxides are attractive candidates for various alternative energy conversion devices due to their low cost, and their interesting electrical properties. Electrical conduction behavior of semiconducting metal oxides commonly changes at different oxygen partial pressures (p(O2)), making them promising component materials for a wide range of environmentally friendly applications, including SOFCs, sensors, solar cells, photoelectrochemical cells for production of solar-hydrogen fuel and solar-driven water purification systems. Thus, the focus of this review will be to highlight recent progress in use of amphoteric semiconducting metal oxides for energy conversion devices and to discuss the main technological challenges towards their implementation in

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Tutorial Review the applications. The review will begin with a brief introduction to defect disorder and electrical conductivity behavior of a few technologically relevant semiconductors, including TiO2, perovskite-type (ATiO3 where A = alkaline earth ions), and fluorite-type (CeO2), and later focusing on application challenges.

2. Semiconducting metal oxides In transition metal oxides, the oxidation state of the metal ion generally drives the conduction towards n-type (electron, e 0 ) or p-type (hole, h ). For example, oxides like TiO2, Nb2O5, and CeO2 that have central metal ions in their highest oxidation state show n-type conductivity (due to reduction of metal ions). Whereas NiO and MnO show p-type conductivity, due to the oxidation state of the central metal ion, which is in its lowest

Kalpana Singh is pursuing her PhD studies in the Department of Chemistry at the University of Calgary under the supervision of Professor V. Thangadurai. She obtained her bachelor’s and master’s degrees in Chemistry from University of Pune, India, in 2006 and 2008, respectively. She then worked as a project assistant under the supervision of Dr C. V. V. Satyanaryana for two years (2008– 2010) in the Heterogeneous Kalpana Singh Catalysis Division of National Chemical Laboratory, Pune, India. Her current research is focused on developing novel sulfur and coke tolerant mixed conductors and solid oxide ion electrolytes for solid oxide fuel cells (SOFCs).

Dr Janusz Nowotny is Professor of Solar Energy Technologies, University of Western Sydney. His research concerns oxide semiconductors for the conversion of solar energy into chemical energy. He was an organizer of over 200 international meetings, including NATO Advanced Research Workshop on Nonstoichiometric Compounds at Rottach-Egern, Germany, and NATO Summer School, Oleron, France. He is the Janusz Nowotny founder and coordinator of two workshop series: Nonstoichiometric Compounds (1980–1990) and Ceramic Interfaces (1993–2001). His association, as Professor and Visiting Professor, included U-Bordeaux, U-Grenoble, Tokyo Institute of Technology, U-Burgogne, Max-Planck-Institute for Solid State Research, U-Paris-Sud, U-Nancy, U-Marseille and U-NSW.

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Chem Soc Rev oxidation state. A natural consequence of the above statement is that oxides such as MgO and Al2O3 that cannot change their oxidation state are electronic insulators. Thus, defect disorder of metal oxides is intimately related to the change in oxidation state and semiconducting behavior. The following section discusses the effect of the partial pressure of oxygen p(O2) on the defect disorder and the related semiconducting properties of metal oxides. 2.1 Some basic definitions related to point defects and electronic conductivity Point defects in oxides play an important role in determining electrical properties, such as electrical conductivity, thermoelectric power, and a work function. Point defects in solids can be either stoichiometric or non-stoichiometric. There are two types of stoichiometric defects: (i) Schottky defect and (ii) Frenkel defect. In the Schottky defect, equivalent amounts of cation and anion vacancies are available, while, in the case of the Frenkel defect, both a vacancy and an interstitial ion pair are present. A non-stoichiometric defects include metal excess, metal deficiency, anion excess, and impurity defects. Defect disorder of non-stoichiometric compounds has been ¨ger–Vink or frequently represented in the form of the Kro Brouwer diagram, which allows for semiquantitative representation of the concentration (or electrical conductivity) as a function of oxygen activity.1 Table 1 lists selected examples of ¨ger–Vink notation in defect the most commonly used Kro chemistry of metal oxides. Fig. 1 shows the idealized schematic representation of the effect of p(O2) on electrical conductivity for an amphoteric oxide semiconductor (AOS).2 Those oxides that can show both n-type and p-type conductivity in response to changes in p(O2) are usually termed amphoteric semiconductors. When a semiconducting metal oxide (MO) is exposed to high oxygen pressures p(O2) at elevated temperatures, there are two

Dr Thangadurai is an Associate Professor of Chemistry at the University of Calgary in Canada. He has received his PhD from the Indian Institute of Science (IISc), India. He did his postdoctoral research at the University of Kiel, Germany. He received a prestigious postdoctoral research fellowship from the Alexander von Humboldt (AvH) Foundation, Venkataraman Thangadurai Bonn, Germany. He received his Habilitation degree from the University of Kiel. His present research activities include developing novel solid electrolytes and electrodes for solid oxide fuel cells (SOFCs), proton conducting solid oxide fuel cells (H-SOFCs), all-solid-state Li ion batteries, and gas sensors.

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Table 1 Kro ¨ ger–Vink notations for commonly occurring defects in metal oxides

¨ger–Vink notation Kro

Defect 2+

V00M V O Mn i O00i DM D0M e0 h

Vacancy at the metal site (M ) Vacancy at the oxide site (O2) Interstitial metal with effective positive charge (Mn+) Interstitial oxide ion Dopant/impurity metal (D3+) with effective positive charge at the metal site (M2+) Dopant/impurity metal (D+) with effective positive charge at the metal site (M2+) Free electron Free hole

loss is compensated by oxygen vacancies (V O ) and cation interstitials (M00i ) as shown by eqn (3) and (4), respectively. 1 0 O OðsÞ Ð O2ðgÞ þ VOðsÞ þ 2eðsÞ 2

ð3Þ

1 0 MOðsÞ Ð O2ðsÞ þ M iðsÞ þ 2eðsÞ 2

ð4Þ

The total electrical conductivity (s) of semiconductors can be simply considered as a sum of ionic and electronic (electrons and holes) contributions, i.e. stotal = sionic + sn(electrons) + sh(holes)

Fig. 1 Schematic Brouwer diagram for a binary amphoteric (MO1d) oxide.2

possibilities for oxygen to go into the lattice, as represented by the defect equilibrium reaction. 1 O2ðgÞ Ð O00iðsÞ þ2hðsÞ 2

ð1Þ

1 O2ðgÞ Ð V00MðsÞ þ2hðsÞ þ O OðsÞ 2

ð2Þ

From eqn (1) and (2) it can be seen that the excess oxygen in the lattice can be compensated by oxygen interstitials (O00i ) and cation vacancies (V00M ), respectively. In both cases, neutral oxygen goes into the lattice by picking up two electrons from the metal valence band to become an oxide ion, thereby creating two holes. Hence, material becomes a p-type electronic conductor with increasing p(O2) and is termed oxidation-type semiconductor. In an oxygen deficient atmosphere, there is loss of neutral oxygen, which leaves behind two electrons in the lattice for n-type conduction. Hence, material becomes an n-type electronic conductor with decreasing p(O2) and the compound is termed reduction-type semiconductor. In the present case, oxygen

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(5)

The electrical conductivity (s) and charge carriers are related by the expression: s = nqm where q is the elementary charge (1.6 1019 C) and m is the electrical mobility and can be related to diffusion coefficient (D) by the Nernst–Einstein equation: qD ð6Þ m¼ kT The total conductivity of the semiconductor could now be expressed as: 2 2 2 q Dionic q Delectrons q Dholes þ nelectrons þ nholes stotal ¼ nionic kT kT kT ð7Þ

3. Defect chemistry and electrical properties of semiconducting metal oxides 3.1

Case study on TiO2

TiO2 is one of those oxides which find their special place in the minds of researchers, because of its widespread applications, including in the pigment industry, photovoltaics, photoelectrochemical solar cells, gas sensing, photocatalysis and as self-cleaning building material and a semiconductor. It is a non-stoichiometric compound with the chemical formula TiO2d (where d is oxygen non-stoichiometry) and commonly exhibits three different polymorphs that include rutile, anatase, and brookite. Fig. 2 represents the rutile structure of TiO2, which is the thermodynamically stable form of TiO2. The literature on TiO2 is full of reports discussing its defect disorder and functional physical and chemical properties.3–12 These studies

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Fig. 2 Idealized crystal structure of TiO2 in rutile form (ICSD: No-9161). showed that oxide ion vacancies ½V , O , Ti interstitials ½Tii 0000 and Ti ion vacancies ½VTi are the intrinsic point defects in ¨ger–Vink notation for TiO2, along with the classiTiO2. The Kro cal notation, is summarized in Table 2.12 Extrinsic defects can be incorporated into the TiO2 lattice through aliovalent doping, which can lead to the creation of acceptors and donors. Formation of intrinsic defects in TiO2 can be represented by equilibria ((8)–(11)):

1 0 O O Ð VO þ 2e þ O2 2

ð8Þ

þ 3e0 þ O2 2O O þ TiTi Ð Tii

ð9Þ

þ 4e0 þ O2 2O O þ TiTi Ð Tii

ð10Þ

0000 O2 Ð 2O O þ VTi þ 4h

ð11Þ

nil Ð e0 þ h

ð12Þ

where electron is represented as e 0 located on Ti3+ ions and h represents hole. Literature studies on TiO2 indicate that electrical conductivity of TiO2 can be divided into three regimes of p(O2).3–5 Strongly reduced regime with a slope of log s vs. log p(O2) equal to 1/6. The related charge neutrality condition requires that 2½V O ¼ n. Reduced regime with a slope of log s vs. log p(O2) equal to 1/4. The related charge neutrality condition requires that 0000 ½V O ¼ 2½VTi .

Extremely oxidized regime with a slope of log s vs. log p(O2) equal to +1/4. The related charge neutrality condition requires 0000 that ½V O ¼ 2½VTi . In the strongly reduced and reduced regimes, TiO2 shows n-type conductivity due to movement of electrons and can be represented as TiO2d. Whereas, in the oxidized regime, it shows p-type semiconductivity and its chemical formula can be written as TiO2+d. Accordingly, by changing partial pressure of oxygen in a controlled way, n-type, p-type and mixed conduction properties can be obtained in TiO2. Also, temperature plays an important role in extending or narrowing the n-type or p-type p(O2) range. Under oxidized conditions of p(O2), the minima corresponding to n–p transition get shifted to higher p(O2) values as temperature is increased, which means the range of n-type conductivity can be increased by increasing the temperature.3 The effect of oxygen non-stoichiometry on the band gap of TiO2 was shown by Cronemeyer,6 where the band gap of oxidized TiO2+d (3.05 eV) was much greater than the band gap of reduced TiO2d (0.73 eV). The defect disorder of donor Nb-doped TiO2 can also be divided into the following three regimes:7 Strongly reduced regime with a slope of log s vs. log p(O2) equal to 1/6. Then, the charge neutrality requires that 2½V O ¼ n. Reduced regime I where electrical conductivity was found to be independent of oxygen activity, and could be explained by charge neutrality condition ½NbTi ¼ n. Reduced regime II with a slope of log s vs. log p(O2) equal to 1/4. The charge neutrality requires that ½NbTi ¼ 4½V0000 Ti . In both undoped and Nb-doped TiO2, the slope of the log s versus log p(O2) is the same, indicating that Nb doping does not affect the electrical conductivity of TiO2 in the strongly reduced regime. On the other hand, if Nb concentration is increased in TiO2, it results in a decrease of the p(O2) corresponding to the region between the strongly reduced regime and the reduced regime. 3.1.1 Reproducibility. Selected literature data on the effect of p(O2) on the electrical conductivity for undoped TiO2 at 1273 K are shown in Fig. 3.3,5,8–10 As seen, data exhibit a substantial scatter in absolute values and in the p(O2) dependence. The scatter is reflective of the following effects: Effect of microstructure: the electrical properties of single crystals differ from those of polycrystalline specimens due to the presence of grain boundaries. Their chemical composition and the related electrical properties are entirely different from that of the bulk phase as a result of segregation. Therefore, the

Table 2 The Kro ¨ ger–Vink and the traditional notations of defects for TiO21,12

Traditional notation TiTi4+ TiTi3+ VTi Tii3+ Tii4+ OO2 VO OO

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¨ger–Vink notation Kro

Description

Ti Ti e0 V0000 Ti Ti i Ti i O O V O h

4+

Ti ion in the titanium lattice site Ti3+ ion in the titanium site Titanium vacancy Ti3+ in the interstitial site Ti4+ in the interstitial site OO2 ion in the oxygen lattice site Oxygen vacancy OO ion in the oxygen lattice site (quasi-free electron hole)

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Fig. 3 Effect of oxygen partial pressure on the electrical conductivity for undoped TiO2 at 1273 K.3,5,8–10

electrical conductivity of polycrystalline TiO2 depends on the density of grain boundaries. Effect of impurities: the presence of impurities, especially of aliovalent ions forming donors and acceptors, has a substantial effect on electrical properties even at the level of several parts per billion. Oxygen content: the electrical properties are extremely sensitive to the oxygen activity in the lattice and the related concentration of intrinsic defects. These may differ from specimen to specimen. Therefore, the determination of well-defined and well reproducible data requires the following points to be addressed: The effect of microstructure must be either well defined or minimized. The latter can be achieved for single crystals, which are free of grain boundaries. The effect of p(O2) on electrical properties of single crystal TiO2 is shown in Fig. 4.11 The effect of impurities may be minimized either for high purity specimens or for solid solutions involving high concentration of aliovalent ions. The effect of oxygen is well defined when the crystal is well equilibrated with the gas phase of controlled oxygen activity. In the case of TiO2, however, two types of the gas–solid equilibrium should be considered. The equilibrium with respect to the defects that exhibit high mobility including oxygen vacancies and titanium interstitials may be reached within 10–20 min at 1273 K,8 while the equilibrium with respect to defects that

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Fig. 4 Effect of titanium vacancies, formed due to prolonged oxidation, on the n–p transition point that is related to the minimum of electrical conductivity (from the plot of conductivity versus oxygen activity) and zero value of thermoelectric power for single crystal TiO2 at 1123 K.11

exhibit very low mobility, such as titanium vacancies, may be reached within 5000–6000 h at 1273 K. 3.1.2 Effect of titanium vacancies. It has been shown that the diffusion coefficient for titanium vacancies is approximately four orders of magnitude lower than that for oxygen vacancies and titanium interstitials.11 Therefore, under the commonly applied processing conditions, lasting up to an hour at approximately 1273 K, titanium vacancies may be considered as acceptor-type dopants. The influence of titanium vacancies on the electrical properties is reflective in Fig. 4, representing the effect of oxygen activity on both electrical conductivity and thermoelectric power at 1123 K. As seen, oxidation of the n-type TiO2 single crystal (curve 1) results in a decrease of conductivity, which reaches a minimum and subsequently increases. The minimum is approximately related to the n–p transition, which is observed at p(O2) = 8.3 kPa. The changes in the electrical conductivity are associated with the changes in thermoelectric power between 600 mV K1 and +400 mV K1 for n- and p-type TiO2. The entire experimental cycle of the determination lasts approximately 1 hour. As seen in Fig. 4, the shapes of both log s vs. log p(O2) and S vs. log p(O2) dependencies taken after 2470 h of prolonged oxidation (curve 2) are different: the minimum of the electrical conductivity is shifted to a lower value of oxygen activity; p(O2) = 0.7 kPa;

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Tutorial Review the associated thermoelectric power changes between 650 mV K1 and +800 mV K1 for n- and p-type TiO2. The observed shift in the n–p transition point is commonly related to the effect of acceptor-type dopants, which in this case are titanium vacancies introduced during the prolonged oxidation.11 3.1.3 Defect disorder diagram of TiO2. Knowledge of the equilibrium constants related to the predominant defects may be used to derive a full defect disorder diagram in the form of the plot of the concentration of the reversible defects as a function of p(O2). The diagram, showing the concentration of defects in pure TiO2, as a function of oxygen activity at 1273 K, is represented in Fig. 5.12 This diagram may be used for prediction of the experimental conditions required for the imposition of either n- or p-type semiconducting properties, as well as mixed conduction. The latter is related to the n–p transition range. The diagram as shown in Fig. 5 allows the following conclusions to be made: Oxidation of TiO2 results in an initial decrease in the concentration of oxygen vacancies, which subsequently assume the level that is practically independent of oxygen activity. In the strongly reduced regime, the oxygen vacancies are compensated by electrons (electronic compensation). As oxygen activity increases, oxygen vacancies are mainly compensated by titanium vacancies (ionic compensation). Oxidation of TiO2 in the n-type regime results in a decrease in the concentration of electrons and an increase in the concentration

Fig. 5 Effect of oxygen activity on the concentration of defects for undoped/ pure TiO2.12

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Chem Soc Rev of holes, which, intersect at the n–p transition point. Increase of oxygen activity above this point results in the formation of p-type TiO2. These data indicate that pure TiO2 is an amphoteric semiconductor. Desired non-stoichiometry and the related semiconducting properties may be modified by a change in oxygen activity in the crystal. This may be achieved by equilibration of the TiO2 specimen with controlled oxygen activity.

3.2

Defect chemistry of SrTiO3

SrTiO3 (ABO3) crystallizes in the cubic perovskite structure, where A and B-sites are occupied by divalent Sr2+ and tetravalent Ti4+, respectively (Fig. 6). The filled valence band from O-2p orbitals and the empty conduction band from Ti-3d orbitals make the electronic energy band of SrTiO3. All of the electrical conductivity studies on undoped SrTiO3 indicate the n-type conduction process under reducing conditions, in accordance with the oxygen deficiency model. The n-type electronic conductivity of SrTiO3 can be increased through donor doping. In the Sr titanates, increasing temperature results in increasing n-type conductivity range. On the other hand, there are very few theoretical and experimental studies confirming p-type conductivity in acceptor (Sc3+ and In3+)-doped SrTiO3.13–16 Fig. 7 shows the typical conductivity plot of undoped SrTiO3 and acceptor-doped SrTiO3 as a function of p(O2).17–19 The same as TiO2, according to the partial pressure of oxygen, the electrical conductivity of SrTiO3 can be divided into three regions, namely n-type, ionic and p-type conductivity. Dopants play a vital role in varying defect chemistry and electrical conductivity of SrTiO3. For example, Fe-doped SrTiO3 (Fig. 7) shows p-type electrical conductivity at high p(O2) and n-type conductivity at low p(O2).19 At low oxygen partial pressures, log conductivity shows 1/4 dependence, indicating n-type conductivity, whereas, at high p(O2), the curves of log (s) show +1/4 dependence on log p(O2), reflecting p-type conductivity (Fig. 7). Increasing iron concentrations results in an increase of the p-type conductivity. At intermediate activity of oxygen, the conductivity curves

Fig. 6 Idealized structure of cubic perovskite-type (ABO3) SrTiO3 (ICSD-23076).

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Fig. 7 Effect of p(O2) on the electrical conductivity of undoped, Fe-doped and Y-doped SrTiO3 at 1073 K.17–19

show a flat region, indicating ionic conductivity, which is independent of p(O2). 3.3

BaTiO3 and CaTiO3

The defect chemistry and conduction property of BaTiO3 are similar to those of SrTiO3. The conductivity of BaTiO3 is higher than that of SrTiO3 in the p-type regime, but about the same in the n-type regime.20 The effect of oxygen activity on electrical properties of BaTiO3 is shown in Fig. 8 in terms of electrical conductivity and thermoelectric power in the range 1090–1310 K.21 As seen, the electrical conductivity exhibits n- and p-type properties at low and high oxygen activity, respectively. The slopes of the log s vs. log p(O2) dependencies are reflective of the related defect disorder, which are considered elsewhere.21 These slopes are consistent with the slopes of S vs. log p(O2) within the respective regimes. As seen, the minima of the electrical conductivity are consistent with oxygen activities at which thermoelectric power assumes the zero value (S = 0). It was shown that the slopes of log s vs. log p(O2) within the n–p transition regime, 1/4 and +1/4, are governed by the ionic charge compensation and charge neutrality requires: 0000 ½V O ¼ 2½VTi

ð13Þ

The slope within the strongly reducing regime is consistent with the charge neutrality condition: 2½V O¼n

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Fig. 9 Electrical conductivity as a function of p(O2) at 1223 K in CaTiO3.22

ð14Þ

Donor doping in BaTiO3 also results in an increase of conductivity in the n-type region and decrease of conductivity in the p-type region. In analogy to SrTiO3 and BaTiO3, CaTiO3 also exhibits amphoteric semiconducting properties. As seen from the plot of log s vs. log p(O2) represented in Fig. 9 at 1223 K, CaTiO3 also exhibits a minimum of electrical conductivity at the intermediate p(O2).22 The same as SrTiO3, acceptor-doping

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Fig. 8 Effect of p(O2) on electrical conductivity and thermopower for undoped BaTiO3 single crystals.21

in CaTiO3 with Fe leads to increase in p-type conductivity. Table 3 gives the extracted log p(O2) range for the n- and p-type conductivity of selected titanates and pure TiO2. 3.4

Defect chemistry of CeO2

Metal oxides, including CeO2, ZrO2, and ThO2, exhibit fluorite (CaF2) structure as shown in Fig. 10. Depending on the temperature

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Table 3 Extracted log p(O2) range for the n- and p-type conductivity of titanium dioxide and selected titanates

Compound

T (K)

Log10 p(O2) range for n-type (atm)

Log10 p(O2) range for p-type (atm)

Log10 p(O2) range for the ionic domain (atm)

Ref.

TiO2

1038 1123–1173 1223 1273–1323 1273 1773 1387 1273 985 1166 1073 1073

23 to 3 18.75 to 2.89 18.48 to 1.24 16.96–0 15–0 8.5–0 15–0 18.27 to 3.09 25 to 8 18.81 to 2.86 20–0 20 to 13

3–0 — — — — — — — 3–0 2.86–0 — 10–0

— 2.89–0 1.24–0 — — — — 3.09–0 — — — 13 to 10

5 9 9 9 8 8 5 23 3 3 19 19

5% Cr-doped TiO2 1% Cr-doped TiO2 Sr0.995La0.005TiO3 SrTi0.995Fe0.005O3

and p(O2), fluorites also show ionic, n- and p-type electronic conductivity. For example, when pure CeO2 oxide deviates from its stoichiometry at high temperatures (above 773 K) and low p(O2), it becomes an n-type semiconductor.24 The n-type conductivity arises due to the reduction of tetravalent Ce4+ to trivalent Ce3+ under reducing conditions of low p(O2), described by the following equation: 1 0 3þ 0 O O þ 2CeCe Ð O2 ðgÞ þ VO þ 2CeCe fCeCe4þ ¼ CeCe g ð15Þ 2 Fig. 11 shows the variation of electrical conductivity as a function of p(O2) for undoped CeO2 and Y-doped CeO2.25,26 At elevated temperatures, pure ceria is a n-type semiconductor between 1020 and 1010 atm with a slope of about 1/6. In the case of acceptor-doped CeO2, as shown in Fig. 11, the plot of conductivity against p(O2) can be divided into the n-type region and ionic conductivity region between 1020 to 1010 and 1010 to 1 atm, respectively. In an oxidizing environment, a less number of electrons are transferred between Ce4+ and Ce3+ making ceria a poor electronic conductor compared to other mixed conductors, based on perovskites. As the temperature is increased, the ionic conductivity range decreases and the n-type conductivity range increases for acceptor-doped ceria. Thus, at low temperature and high p(O2), the conductivity remains independent of p(O2). Therefore, the temperature range and the type of conductivity in doped

Fig. 11 Conductivity of undoped and Y-doped ceria against different activities of oxygen at 1073 K.25,26

ceria play a deciding role for the final applications. For example, rare-earth-doped ceria is a promising electrolyte for low temperature SOFCs, because as the temperature is increased beyond 873 K, the n-type conductivity range is extended and it becomes unsuitable as an electrolyte. Mixed conductivity of ceria, under oxidizing conditions, can be improved by doping easily reducible cations like Pr4+ and Mo6+. The n-type conductivity in CeO2 could be due to the polaron transport mechanism. Like titanates, the p-type conduction process in fluorites can only be achieved through extremely careful measurements and under high p(O2) conditions.

4. Applications of amphoteric metal oxide semiconductors 4.1

Fig. 10 Crystal structure of cubic CeO2 (ICSD-28709).

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Solid oxide fuel cells (SOFCs)

A solid oxide fuel cell (SOFC) is a high temperature (773–1273 K) solid state energy conversion device that directly converts chemical energy of fuels such as H2, CH4 and other hydrocarbons into electrical energy with high efficiency. Even though an SOFC is not a renewable energy device like solar cells, it is indeed

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Chem Soc Rev complementary to renewable energy devices. Each single cell of the SOFC consists of three main solid-state parts, an anode, a cathode and an electrolyte. At the cathode, oxygen from the air gets reduced to form oxide ions (O2), and these O2 ions travel through the oxide ion conducting electrolyte (yttrium-doped zirconia, YSZ) towards the anode for fuel oxidation. The electrons produced at the anode due to fuel oxidation reach the cathode through the external circuit. Ni-YSZ cermet is the state-of-the-art anode material for the SOFC, where Ni provides electronic conductivity and catalytic property, whereas YSZ serves as an ionic conductor. However, when hydrocarbons (methane, ethane, butane) are used instead of pure hydrogen, the Ni-YSZ anode suffers from performance degradation as a result of coke and sulfur poisoning. Metal oxides belonging to perovskite and fluorite phases are in high demand for overcoming poisoning effects in SOFC anodes due to their mixed ionic–electronic conducting (MIEC) behavior. In the case of the cathode, MIEC oxides are used to carry out oxygen reduction reaction. Oxides that exhibit high n-type conductivity under reducing conditions serve as anodes. On the other hand, those oxides that exhibit high p-type conductivity under oxidizing conditions work as cathodes. 4.1.1 SOFC cathodes. Pervoskite based La1xSrxMnO3 (LSM) is the conventional material for the SOFC cathode, showing predominate p-type electronic conductivity. Along the same line, several perovskites related p-type oxides including (La,Sr)FeO3d (LSF), (La,Sr)CoO3d (LSC), (La,Sr)(Fe,Co)O3d (LSCF), (La,Sr)(Fe,Mn)O3d (LSMF), and (Sm,Sr)CoO3d (SSC) are being considered as cathodes for SOFCs.27 LSM exhibits intrinsic p-type conductivity due to changes in the Mn valence. In the high p(O2) region, the electrical conductivity and Seebeck coefficient are independent of p(O2) and remain unchanged, while, in the low p(O2) region, the electrical conductivity decreases with a decrease in p(O2) and the Seebeck coefficient increases with the decrease in p(O2). Since the mobility of holes decreases in the low p(O2) region, the conductivity also decreases for LSM. Perovskite oxides of the type A1xA 0 xBO3 (A = La, Pr, Ce; A 0 = Sr; B = Mn, Fe, Co, Ni, Ga, Mg) have been explored as cathodes for SOFCs. Between the 1011 and 1013 atm range, LaMnO3 and PrMnO3 oxides showed p-type conductivity with p(O2) dependence of +1/4.28 The effect of temperature on conductivity of p-type cathode materials is shown in Fig. 12, where semiconducting behavior is evident as conductivity increases with increasing temperature.29–31 The electrical conductivity tests on La0.8Sr0.2FeO3 in the oxygen partial pressure range from 1019 to 0.5 atm and temperatures between 1023 K and 1223 K showed that holes are the majority charge carriers from 1023 to 1223 K, showing p-type conductivity of B467 S cm1 at 1023 K.32 La-deficient La0.7Sr0.25FeO3 showed an area specific polarization resistance (ASR) of 0.1 Ocm2 at 800 1C in air on the YSZ electrolyte.33 4.1.2 SOFC anodes. Ni-YSZ anodes were found to be intolerant towards sulfur and coke poisoning, due to which single-phase oxides with n-type conductivity have been studied as alternative anode materials. Out of several dopants, Y-doping in SrTiO3 shows the highest conductivity of 64 S cm1 at 1073 K18 because of which, it has been extensively studied as anode material for SOFCs. In the La1xCaxCr1yTiyO3d series, compounds with Ca/Ti > 1

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Fig. 12 Arrhenius plots of electronic conductivity for p-type cathodes for SOFCs. The solid points are experimental data, and the solid lines are guides to eyes.29–31

showed p-type conductivity and Ca/Ti o 1 showed n-type semiconductivity.34–36 Fig. 13 shows the temperature dependence of the conductivity of typical anodes for SOFC applications.35,37 However, high conductivity is not the only criteria for anode applications, for e.g., when high conducting oxide La2Sr4Ti6O19–d (60 S cm1 in reducing condition) was tested as anode, its performance was low due to high electrode polarisation.38 In view of minimisation of carbon coking, rare earth-doped ceria has shown promising results due to MIEC and catalytic activity for fuel oxidation reaction.39 Irvine’s group40 studied Nb-based TiO2 materials as SOFC anodes, which also showed n-type conductivity under fuel cell conditions with slope 1/6 when log s is plotted against log p(O2). Apart from titanates, fluorites, tungsten bronzes, and double pervoskites have also been studied for SOFC anode applications.

Fig. 13 Arrhenius plots of electronic conductivity for typical n-type anode materials for SOFCs.35,37

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Chem Soc Rev Table 4 Conductivity values in air (oxidizing) and under H2 fuel (reducing) conditions for a few oxides considered as electrode materials for the SSOFC approach

Oxide

T (K)

s in aira (S cm1)

s in H2a (S cm1)

Ref.

LSCM La0.75Sr0.25Cr0.5Mn0.3Ni0.2O3d LaCrO3d Sr2Fe1.5Mo0.5O6d La0.8Sr0.2Sc0.2Mn0.8O3 La1/3Sr2/3(Ti1xFex)O3d Pr0.7Ca0.3Cr1xMnxO3d La0.7Ca0.3CrO3d La0.7Ca0.3Cr0.97O3d

1173 1073 1073 1053 1123 1223 1223 1073 1123

38 20 1 550 45 0.9 10 50 62

3 0.5 0.3 310 6 0.1 1 1.6 3.3

41, 46 47, 48 43 49 50 45 51 43 44

a

Fig. 14 Proposed structure of a symmetrical solid oxide fuel cell (SSOFC) (AOS represents amphoteric oxide semiconductor).

From the above examples, it becomes clear that n- and p-type single phase oxides fulfill the role of anode and cathode quite beautifully. However, it should be kept in mind that the type and value of electrical conductivity is not the only criteria for these oxides to be used as electrodes. Thermal and chemical compatibility with the electrolyte, long term stability and catalytic activity towards electrochemical oxidation of fuel also play a prominent role. Nevertheless, it is hard for a single material to fulfill all the requirements simultaneously. A new approach towards the SOFC configuration can be achieved with the use of amphoteric semiconductor oxides, where the same oxide will simultaneously serve the function of the cathode and anode (Fig. 14). The latter approach is widely termed symmetrical solid oxide fuel cell (SSOFC). One of the first reports on the SSOFC, approach came in the year 2006 on redox stable La0.75Sr0.25Cr0.5Mn0.5O3d (LSCM), which is stable and conductive under both oxidizing and reducing conditions.41,42 The SSOFC approach will bring plenty of benefits, first being the fabrication step, which can be performed in a single thermal treatment step. Second being the minimization of the chemical and thermal compatibility problem. Thirdly, the problem of sulfur and coke poisoning can be minimized by just switching the gas flow on both sides of the SSOFC. However, there are several conditions, which need to be fulfilled by oxide before it can work as a symmetrical electrode in SSOFCs. In addition to requirements for being an anode and cathode, it should also possess high electronic conductivity under both cathode and anode conditions, high catalytic activity for fuel oxidation and oxygen reduction, and stability in the air and under reducing conditions. The effect of non-stoichiometry on the electrical conductivity can be easily seen in the case of stoichiometric La0.7Ca0.3CrO3d, which showed a very high conductivity of 62 S cm1 at 1123 K in air when it was made Cr (La0.7Ca0.3Cr0.97O3d) deficient.43,44 As discussed earlier, the n-type conductivity range for titanates is quite high, making it difficult to obtain p-type conductivity in an oxidizing atmosphere. Thus, to make them attractive candidates for SSOFCs, p-type dopants like Fe are introduced.

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Approximate values.

To validate this point, Canales-Vazquez et al.45 proposed the titanate system, (La, Sr)(Ti1xFex)O3d. As seen from Section 3.2, Fe-doping in titanates tends to introduce p-type conductivity,19 thus in the (La, Sr)(Ti1xFex)O3d system both n-type and p-type conductivity is present. The most promising conductivity values were 0.1 S cm1 under reducing conditions, and 0.9 S cm1 in the air, for the member with x = 0.5. Many more oxide semiconductors have also been studied for the SSOFC configuration, including LSCM, Sr2Fe1.5Mo0.5O6d and LSCF. Table 4 lists the conductivity data for a few oxides in the air and under H2 fuel conditions as electrode materials for the SSOFC approach. 4.2

Photovoltaics and photocatalysis

Solar energy comes in the forefront as an alternative to fossil fuels due to its abundance and renewability. The solar power is so abundant that even if some part is captured, it may solve most of the world power needs. Electron–hole pairs are generated when photons from sunlight fall on semiconductor material, thus generating electric potential difference between junctions of two different materials. In this way, a photovoltaic cell produces energy from solar energy. Conventionally, Si is used as semiconductor material for solar cell applications. However, oxide semiconductors like TiO2 have been studied for applications in the next generation solar cells due to their non-toxicity and interesting electrical properties. Solar energy can also be converted into fuels such as H2, CO, and methanol through photolysis and photocatalysis. H2 production through photolysis of water became a huge hit after the primary work of Fujishima and Honda52 on a semiconducting n-type TiO2 based electrochemical cell. Since then, many semiconducting oxides have been studied for photolysis of water. The wide band gap of 3.2 eV of TiO2 makes it absorb only UV light and thus it only absorbs 4% of solar spectrum. Approaches to lower the band gap include making nanostructures, doping and coupling with p-type oxides such as Cu2O in a p–n junction.53 For photoanode applications, new classes of semiconductor oxides which absorb visible light are investigated. p-Type Cu2O is another semiconducting oxide which has found its place in H2 production by photolysis. However, Cu2O gets easily photoreduced

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Chem Soc Rev to Cu; thus it is combined with n-type semiconductors like TiO2 and ZnO. In this way, self-photoreduction of Cu2O is minimised since electrons get transported to TiO2 or ZnO.53 a-Fe2O3 is an attractive photoanode material for producing solar fuels, due to its smaller band gap of 2.1 eV. On the other side, due to its indirect band gap transitions and slow kinetics towards oxygen evolution reaction, its efficiency is quite low. WO3 and ZnO have also been used as photocatalysts for water splitting electrochemical cells.53 Even though many semiconducting oxides are available for water splitting and photovoltaic applications, still, none of them show an acceptable rate of performance for commercial use. In the end, nanostructure engineering and doping need to be employed for achieving improved performance. 4.3

Semiconductor based gas sensors

The research field of ceramic-based gas sensors is developing due to the rising level of environment pollution and related health hazards. As seen from previous discussions, the electrical properties of most of the oxides change with change in p(O2), hence they can offer new opportunities to develop materials for sensors. One of the widely studied semiconducting oxides for oxygen sensors is TiO2. In order to improve the sensitivity and response of oxygen sensors, Cr-doped TiO2 was used.54 Fergus55 has reported a very careful review regarding perovskite oxides for semiconductor-based oxygen sensors covering ATiO3 (A = Ca, Sr and Ba) and Pb(Zr0.5Ti0.5)TiO3. One of the important criteria for sensors is fast response time; in view of this one of the potential materials studied is CeO2, where fast diffusing oxygen vacancies ½V O influence the response time. Apart from oxygen sensors, titanates, ferrites, cobaltates, nickelates and stannates have been studied for CO sensing. Thus, amphoteric semiconductors can be used for a variety of sensors; however, their applications get limited due to several strict requirements like fast response, high sensitivity and linearity of the response. 4.4

Ordered mesoporous semiconducting metal oxides

The discovery of mesoporous MCM-41 and SBA-15 and their enormous applications have inspired researchers of the 21st century towards synthesising high surface area metal oxides. Mesoporous oxides can be prepared either by soft template or hard template methods. The soft template method employs sol–gel chemistry, whereas the hard template method works by infiltrating pores of mesoporous silica materials with an inorganic oxide precursor and then removing the negative replica as metal oxide. It is a well known fact that having high surface area and ordered pore structures, metal oxides can provide better functional properties in various applications, as compared to their bulk counterparts. For example, as compared to bulk ceria, ordered mesoporous ceria (5 nm in size) showed better photovoltaic response, due to enhanced electronic transport as a result of high surface area.56 In the case of disordered mesoporous n-type TiO2 film, solar conversion efficiency was enhanced by 50% as compared to randomly oriented films in liquid dye sensitized solar cells (DSSCs).57 In another study, when ordered mesoporous n-type SnO2 was coated with ultrathin

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Tutorial Review TiO2 and Al2O3, the solar conversion efficiency was enhanced by 3 orders of magnitude.58 In the case of gas sensors also, mesoporous oxides show enhanced performance. SnO2 coated mesoporous SnO2 showed excellent H2 sensing properties.59 However, poor thermal stability of mesoporous oxides remains a challenge for application as electrodes in high temperature devices such as SOFCs. Applications and synthesis of mesoporous oxides are studied in great detail in ref. 60.

5. Summary and outlook This review shows how defects play a pivotal role in changing the electrical properties of oxides which in turn significantly influence functional properties of alternative energy conversion devices. A significant amount of research has been done in the area of amphoteric oxide semiconductors right from simple oxides like NiO, ZnO to complex oxides such as perovskites. We have seen how material design of amphoteric oxides leads to the development of innovative strategies, which can be employed for energy conversion devices. The sulphur and coke poisoning limitation of Ni-YSZ can be overcome by n-type oxide semiconductors. However, further improvement in electrode performance of oxide semiconductors is still needed to match Ni-YSZ performance. In the case of cathode materials for SOFCs, p-type conducting perovskite-based oxides are studied including LSF, LSC, LSCF, LSMF, and SSC. We have also discussed the SSOFC approach which takes its principle from the amphoteric nature of semiconducting oxides. Semiconductor oxygen sensors use n-type and p-type conducting oxides by measuring the change in conductivity due to change in p(O2). Semiconductor sensors have been shown to be sensitive to various gases like O2, CO, hydrocarbons, and NOx. SrTiO3 is commonly used for oxygen sensing, while cobaltites and ferrites find their applications in sensing hydrocarbons and NO2. For developing sensors with wide operating range, fast response time and excellent chemical stability, improved understanding of defect chemistry is needed to steer new materials for a variety of applications. From this review, it was also found that apart from TiO2, semiconductor oxides having a narrow band gap also find applications as photoanodes. The crux of the matter is that, though literature is full of promising reports, major breakthroughs in material design are required to develop alternative energy sources for desired applications. Knowledge of defect chemistry is also rather essential to develop materials with reliable and reproducible property.

Acknowledgements This work was supported through funding to the NSERC Solid Oxide Fuel Cell Canada (SOFCC) Strategic Research Network from the Natural Science and Engineering Research Council (NSERC) and other sponsors listed at www.sofccanada.com. The present paper is largely based on Kalpana Singh’s independent study course (CHEM 701) at the Department of Chemistry under Professor Thangadurai, University of Calgary, as part of her graduate program which involves the development of novel oxide ion electrolytes and anodes for solid oxide fuel cells (SOFCs).

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