Solar hydrogen is one ideal and sustainable energy source to replace fossil fuel. Solar Photovoltaic (PV) cells normally generate electricity using sunlight, but it is renewable only as long as our sun shines. Converting sunlight into electricity is an efficient way to address energy crisis but harvesting solar energy in the form of chemical energy is a sustainable solution for fueling tomorrows. Storing energy in the form of hydrogen bond is more efficient not only because of its high energy density and but also it is a clean energy source. Hydrogen can be generated in a number of ways, including but not limited to steam reforming, thermolysis, and electrolysis. Photoelectrochemical (PEC) water splitting is one of the most promising methods for solar-to-chemical energy conversion. In order to address the need for clean and renewable energy, recent trends in global CO2 emissions and energy production are analyzed, and the photoelectrochemical properties of multi-metal oxide based thin films are presented. Bismuth vanadate (BiVO4), barium bismuth niobate (Ba2BiNbO6), and antimony vanadate (SbVO4) were investigated for use as photoelectrodes in PEC water splitting for solar hydrogen production. This dissertation starts with synthesis, deposition, and characterization of antimony vanadate and Sb alloyed bismuth vanadate thin films to observe their photoelectrochemical ability to split water. Antimony doping in bismuth vanadate thin films prompts to modify valence and conduction band edges of bismuth vanadate. It has been found that Sb alloying with less than 20% wt. improves the electron conductivity and consequently leads to significant enhancement of photocurrents without creating secondary phases. The hole mobility is further improved by incorporating NaF and metallic Ni on the surface of the electrode. The NaF incorporation is believed to reduce electron effective mass and therefore increased electron mobility by suppressing scattering centers. As a result, antimony doped thin films exhibited much improved performance in PEC water splitting as compared to pure sputtered BiVO4. The metallic Ni deposition on the surface of Sb-doped BiVO4 acted as electrode corrosion inhibitor. But we found that Ni topping can enhance the stability of electrode in strong acidic solutions at the cost of reducing its optical absorption and hence lowering its photon-to-electron conversion efficiency. However, surface modification of thin films using various stack structure and oxides coatings helped to enhance their stability along with the oxygen evolution catalysis. Large area Bi-based quaternary oxides (Ba2Bi1.4Nb0.6O6 and Ba2BiNbO6) were deposited using RF sputtering deposition and the effects of surface-modification was also investigated using various electrochemical methods. Thin film uniformity was obtained by incorporating oxygen gas in the sputtering plasma. Photoelectrochemical thin films with higher stability in aqueous solution and better corrosion resistant were fabricated, analyzed, and tested. Capacitance-voltage measurement was used to measure the chemical kinetics of interfacial electron transfer of the system. Charge-carrier mobility was extremely limited by the rate of recombination, while the surface chemistry was altered by using Oxygen Evolution Reaction (OER) catalysts. Using the OER catalysts significantly reduced the surface recombination losses thereby extending hole carrier lifetime. Finally, a novel, high-throughput, combinatorial approach for the material synthesis and screening of mixed-metal oxides for photoanode design was developed. This methodology relies on controlling stoichiometric ratio of different sputtering yield metal oxides. After fabrication, the photoelectrochemical properties of oxide electrodes can be fully characterized by using various optical and electrochemical technique.

Photoelectrochemical (PEC) water splitting is a highly promising process for converting solar energy into hydrogen energy. The book presents new cutting-edge research findings in this field. Subjects covered include fabrication and characteristics of various electrode materials, cell design and strategies for enhancing the properties of PEC electrode materials. Keywords: Renewable Energy Sources, Solar Energy Conversion, Hydrogen Production, Photoelectrochemical Water Splitting, Electrode Materials for Water Splitting, Transition Metal Chalcogenide Electrodes, Narrow Bandgap Semiconductor Electrodes, Ti-based Electrode Materials, BiVO4 Photoanodes, Noble Electrode Materials, Cell Design for Water Splitting.

Photoelectrochemical (PEC) water splitting is a highly promising process for converting solar energy into hydrogen energy. The book presents new cutting-edge research findings in this field. Subjects covered include fabrication and characteristics of various electrode materials, cell design and strategies for enhancing the properties of PEC electrode materials. Keywords: Renewable Energy Sources, Solar Energy Conversion, Hydrogen Production, Photoelectrochemical Water Splitting, Electrode Materials for Water Splitting, Transition Metal Chalcogenide Electrodes, Narrow Bandgap Semiconductor Electrodes, Ti-based Electrode Materials, BiVO4 Photoanodes, Noble Electrode Materials, Cell Design for Water Splitting.

The first chapter in this thesis presents an introduction and background motivation for artificial photosynthesis using transition metal oxide semiconductors. Also included is a section on some fundamental concepts of electrochemistry with semiconductors for the reader that may be unfamiliar with this research area. The second and third chapters are devoted to copper tungstate (CuWO4), an n-type semiconductor with a band gap of 2.0 eV that exhibits great promise as the photoanode in a z-scheme water-splitting device. The second chapter is in regards to CuWO4 thin films deposited via reactive-ion co-sputtering, while the third chapter presents a novel technique for the preparation of nanostructured CuWO4 with the aim of addressing some fundamental limitations when using 3rd-row transition metal oxide materials. In the second chapter, a detailed systematic study into the co-sputter growth conditions of CuWO4 will be presented with the aim of understanding the optimal growth parameters for photoelectrochemical applications. Structural and electronic characterization of the thin films will be presented to demonstrate the quality of the growth process. A thickness series was performed to determine the optimal thickness for maximizing photocurrent density. The photocurrent density reported in this thesis is the highest current density thus reported in the literature for CuWO4 at the thermodynamic water oxidation potential. A two-electrode experiment was performed in order to determine the feasibility of utilizing CuWO4 in a z-scheme device. A number of oxygen evolution reaction catalysts were deposited onto the surface of CuWO4 thin films and their effect on the overall current density will be discussed. Long-duration potentiostatic measurements were carried out over a wide range of pH values to ascertain the stability of the material, and a discussion into possible degradation mechanisms will be discussed. Finally, the efficacy of CuWO4 as a water oxidation catalyst will be demonstrated and discussed. The third chapter in this thesis shall discuss two novel approaches for the formation of nanostructured CuWO4 with the aim of overcoming the inherently poor minority carrier mobility that has thus far slowed limited photoelectrochemical applications of the material. In the first approach, anodic aluminum oxide nanotemplates were utilized in an attempt to electrochemically deposit CuWO4 nanowires into the pores. In the second approach, a novel nitric acid treatment on tungsten thin films was utilized to develop a nanostructured surface followed by incorporation of copper using an combined physical vapor deposition and subsequent annealing process. The overall results of both techniques will be discussed. The fourth and final chapter in this thesis is a report on the growth and characterization of a nickel iron oxide alloy material to serve as a photoanode. Thin films were grown via reactive-ion co-sputtering of nickel and iron metal targets in the presence of oxygen. Optical, structural, electronic, and photoelectrochemical characterization was performed and the results shall be discussed. Two appendices complete the work. The first appendix is a list of characterization and deposition instruments utilized throughout this body of work. The second appendix is a collection of Mathematica® programs developed during the course of the author's Ph. D. studies in order to aid in data collection and analysis.

Photoelectrochemical (PEC) cell is a device generated hydrogen fuel through an environmentally friendly method. The earliest report should date back to 1972. Honda and Fujishima first demonstrated solar water splitting by using titanium dioxide as photoanode in the cell. Then extensive efforts have been devoted to improving the solar-to-hydrogen (STH) conversion efficiency and decreasing the cost. However, current the efficiency of PEC device was limited on finding out a suitable photoanode material. The ideal photoanode material should have a good bandgap, favorable bandgap position, chemically stable and low cost. Therefore, this thesis would focus on studying different photoanode materials including GaN, TiO2 and Fe2O 3 to achieve high PEC water oxidation performance. In this thesis, I will first designed GaN nanowires on carbon cloth via a chemical vapor deposition (CVD) method and demonstrated significant photoactivity for photoelectrochemical water oxidation. In addition, our group used to report a facile and general strategy to fundamentally improve the performance of TiO2 nanowires for PEC water splitting. However, there are some concerns about the real effects under higher hydrogen treated temperature as well as the stability of oxygen vacancies in TiO2. Therefore I investigated the effect of hydrogenation temperature and the stability of oxygen vacancies in TiO2 photoanodes. Furthermore, there are few reports about the study on the long term stability of TiO2 photoanode even though most scholars used to think TiO 2 belongs to one of the most stable photoanode materials. So I carried out the first long term photostability measurement on various phases TiO 2 photoanodes including rutile, anatase and mixed phased and found TiO 2 photoanodes were not stable as people expected. Then I investigated the mechanism of the instability of TiO2 and carried out two strategies to stabilize TiO2 materials in the PEC cell. Finally, I created a facile acid treated method on hematite to substantially enhance the PEC activity. I found the enhanced photocurrent is due to improved efficiency of charge separation as well as potential passivation of surface electron traps.

Metal Oxide Semiconductors Up-to-date resource highlighting highlights emerging applications of metal oxide semiconductors in various areas and current challenges and directions in commercialization Metal Oxide Semiconductors provides a current understanding of oxide semiconductors, covering fundamentals, synthesizing methods, and applications in diodes, thin-film transistors, gas sensors, solar cells, and more. The text presents state-of-the-art information along with fundamental prerequisites for understanding and discusses the current challenges in pursuing commercialization and future directions of this field. Despite rapid advancements in the materials science and device physics of oxide semiconductors over the past decade, the understanding of science and technology in this field remains incomplete due to its relatively short research history; this book aims to bridge the gap between the rapidly advancing research progress in this field and the demand for relevant materials and devices by researchers, engineers, and students. Written by three highly qualified authors, Metal Oxide Semiconductors discusses sample topics such as: Fabrication techniques and principles, covering vacuum-based methods, including sputtering, atomic layer deposition and evaporation, and solution-based methods Fundamentals, progresses, and potentials of p–n heterojunction diodes, Schottky diodes, metal-insulator-semiconductor diodes, and self-switching diodes Applications in thin-film transistors, detailing the current progresses and challenges towards commercialization for n-type TFTs, p-type TFTs, and circuits Detailed discussions on the working mechanisms and representative devices of oxide-based gas sensors, pressure sensors, and PH sensors Applications in optoelectronics, both in solar cells and ultraviolet photodetectors, covering their parameters, materials, and performance Memory applications, including resistive random-access memory, transistor-structured memory devices, transistor-structured artificial synapse, and optical memory transistors A comprehensive monograph covering all aspects of oxide semiconductors, Metal Oxide Semiconductors is an essential resource for materials scientists, electronics engineers, semiconductor physicists, and professionals in the semiconductor and sensor industries who wish to understand all modern developments that have been made in the field.

This book outlines many of the techniques involved in materials development and characterization for photoelectrochemical (PEC) – for example, proper metrics for describing material performance, how to assemble testing cells and prepare materials for assessment of their properties, and how to perform the experimental measurements needed to achieve reliable results towards better scientific understanding. For each technique, proper procedure, benefits, limitations, and data interpretation are discussed. Consolidating this information in a short, accessible, and easy to read reference guide will allow researchers to more rapidly immerse themselves into PEC research and also better compare their results against those of other researchers to better advance materials development. This book serves as a “how-to” guide for researchers engaged in or interested in engaging in the field of photoelectrochemical (PEC) water splitting. PEC water splitting is a rapidly growing field of research in which the goal is to develop materials which can absorb the energy from sunlight to drive electrochemical hydrogen production from the splitting of water. The substantial complexity in the scientific understanding and experimental protocols needed to sufficiently pursue accurate and reliable materials development means that a large need exists to consolidate and standardize the most common methods utilized by researchers in this field.