It is widely accepted within the community that to achieve a sustainable society with an energy mix primarily based on solar energy we need an efficient strategy to convert and store sunlight into chemical fuels. A photoelectrochemical (PEC) device would therefore play a key role in offering the possibility of carbon-neutral solar fuel production through artificial photosynthesis. The past five years have seen a surge in the development of promising semiconductor materials. In addition, low-cost earth-abundant co-catalysts are ubiquitous in their employment in water splitting cells due to the sluggish kinetics of the oxygen evolution reaction (OER). This review commences with a fundamental understanding of semiconductor properties and charge transfer processes in a PEC device. We then describe various configurations of PEC devices, including single light-absorber cells and multi light-absorber devices (PEC, PV-PEC and PV/electrolyser tandem cell). Recent progress on both photoelectrode materials (light absorbers) and electrocatalysts is summarized, and important factors which dominate photoelectrode performance, including light absorption, charge separation and transport, surface chemical reaction rate and the stability of the photoanode, are discussed. Controlling semiconductor properties is the primary concern in developing materials for solar water splitting. Accordingly, strategies to address the challenges for materials development in this area, such as the adoption of smart architectures, innovative device configuration design, co-catalyst loading, and surface protection layer deposition, are outlined throughout the text, to deliver a highly efficient and stable PEC device for water splitting.

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Photoelectrochemical devices for solar water splitting – materials and challenges

Oxide ion conductors find important technical applications in electrochemical devices such as solid-oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Na0.5Bi0.5TiO3 (NBT) is a well-known lead-free piezoelectric material; however, it is often reported to possess high leakage conductivity that is problematic for its piezo- and ferroelectric applications. Here we report this high leakage to be oxide ion conduction due to Bi-deficiency and oxygen vacancies induced during materials processing. Mg-doping on the Ti-site increases the ionic conductivity to ~0.01 S cm(-1) at 600 °C, improves the electrolyte stability in reducing atmospheres and lowers the sintering temperature. This study not only demonstrates how to adjust the nominal NBT composition for dielectric-based applications, but also, more importantly, gives NBT-based materials an unexpected role as a completely new family of oxide ion conductors with potential applications in intermediate-temperature SOFCs and opens up a new direction to design oxide ion conductors in perovskite oxides.

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A family of oxide ion conductors based on the ferroelectric perovskite Na0.5Bi0.5TiO3

Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4% at room temperature. This behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.

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Self‐Regulation Mechanism for Charged Point Defects in Hybrid Halide Perovskites

Advancement of gas sensor technology over the past few decades has led to significant progress in pollution control and thereby, to environmental protection An excellent example is the control of automobile exhaust emissions, made possible by the use of oxygen gas sensors Since early 1970's there have been sustained studies on oxygen sensors and has led to development of sensors for various applications with varying performance characteristics Solid electrolyte based potentiometric, amperometric and metal oxide based semiconducting resistive type sensors are used for high temperature applications For solution-based pollution monitoring, dissolved oxygen sensors based on Clark electrodes have played a major role More recently, for biological and medical applications, optical oxygen sensors are beginning to have an impact In this review, we focus on both high temperature as well as dissolved oxygen sensors and compare the different methods of oxygen sensing, discuss underlying principles, and outline the designs and specific applications

Oxygen sensors: Materials, methods, designs and applications

While most of crystalline wide gap oxides are both stoichiometric and insulating, a handful of them including ZnO and ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ are naturally anion-deficient and electron conductors. Even fewer of the oxides are naturally cation-deficient and hole conductors, the arch-type of which is ${\mathrm{Cu}}_{2}\mathrm{O}$. Based on first principles calculation of equilibrium nonstoichiometry and defect stability, we explain why the ${\mathrm{Cu}}^{(I)}({d}^{10})$ oxide-based materials are both $p$-type and naturally cation-deficient, and why cation vacancies lead to delocalized, conductive states, whereas in other oxides (e.g., ZnO and MgO), they lead to localized, nonconductive states.

Origins of the p -type nature and cation deficiency in Cu 2 O and related materials

1. Introduction Reference 2. A Few Useful Crystal Structures Introduction Close-Packed Structures Structures for Eight-Coordinate Cations Structures for Ternary Compounds Conclusion References Problems 3. Lattice Defects and the Law of Mass Action Introduction Lattice Defects as Part of the Equilibrium State The Law of Mass Action Another View of Mass Action Lattice Disorder in Elemental Solids Summary References 4. Intrinsic Ionic Disorder Lattice Defects and Reference States Conservation Rules Defect Notation Major Types of Intrinsic Ionic Disorder General Comments on Intrinsic Ionic Disorder References Problems 5. Extrinsic Ionic Disorder Introduction The AgCl-CdCl[2 System The CaF[2-CaO System The TiO[2-Nb[2O[5 System Summary of Important Points Schematic Representation of Defect Concetrations Summary of Extrinsic Ionic Disorder References Problems 6. Defect Complexes and Associates Introduction Complexes Containing an Impurity Center and an Ionic Defect Intrinsic Ionic Defect Associates The Effect of Impurities on the Concentrations of Defect Complexes and Associates References 7. Ionic Transport Introduction Basic Concepts of Diffusion Ionic Conduction in Crystalline Solids Intrinsic and Extrinsic Ionic Conduction Fast Ion Conductors References 8. Intrinsic Electronic Disorder Introduction The Development of Energy Bonds The Mass-Action Approach The Fermi Function Holes, Waves, and Effective Masses Electronic Conductivity Hopping Mechanisms The Band Structure of Compounds Chemistry and the Band Gap Summary References 9. Extrinsic Electronic Disorder Introduction Interactions with the Gaseous Ambient The Choice of Compensating Defect The Chemical Consequences of Electronic Compensation The Interactions of Impurity Centers with Electrons and Holes The Situation for Compounds Summary References 10. Intrinsic Nonstoichiometry Introduction Nonstoichiometry in Pure Crystalline Compounds Nonstoichiometry and Equilibrium Defect Concentrations The Hypothetical Compund MX with Schottky Disorder Summary of the Kroeger-Vink Diagram for MX Conclusion of the Discussion of MX A More Complex Kroeger-Vink Diagram Summary of the Kroeger-Vink Diagrams for Intrinsic Nonstoichiometry Enthalpy Relationships Conclusion Reference Problems 11. Extrinsic Nonstoichiometry Introduction A Simple Example: Donor-Doped MX Enthalpy Relationships A More Complex Example: Acceptor-Doped M[2O[3 General Considerations Nonstoichiometric Reactions in the Impurity-Controlled Region Problems 12. Titanium Dioxide Introduction The Amount of Nonstoichiometry The Equilibrium Electrical Conductivity of Undoped TiO[2 The Seebeck Coefficient of Undoped TiO[2 Ionic Conduction in TiO[2 The Effect of Dopants on TiO[2 General Comments on the Defect Chemistry of TiO[2 References Problems 13. Cobalt Oxide and Nickel Oxide Introduction Cobaltous Oxide, CoO Nickelous Oxide, NiO Summary References 14. Barium Titanate Introduction General Expectations The Equilibrium Conductivity of Undoped BaTiO[3 Insulating Properties of BaTiO[3 Acceptor-Doped BaTiO[3 Ionic Conduction in BaTiO[3 Donor-Doped BaTiO[3 Trivalent Dopants in BaTiO[3 Summary References 15. Order versus Disorder Block Structures Summary References Index

Donald Morgan Smyth

Papers

The Defect Chemistry of Metal Oxides