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 cells (PEC) are devices which convert solar energy into consumable chemical energy by splitting water into oxygen and hydrogen. Photocatalytic activity at a semiconductor oxide surface forms the backbone of the PEC and thus the quest for high activity oxide materials and improving the cells efficiency is a widely explored field of research. Metal oxide semiconductors with band gaps in the visible spectrum are actively sought as photocatalytic electrode materials. The major advantages are that oxides are nontoxic, stable, and inexpensive. However, their overall efficiency is usually limited by short carrier diffusion length due to structural defects, limited light absorptivity and sluggish kinetics at the interface. To overcome these limitations crystalline semiconductor oxides synthesized by high temperature techniques are desired. A direct liquid injection chemical vapor deposition technique has been employed to synthesize films of Fe2O3 (hematite) and BiVO4 (bismuth vanadate) for use as photocatalysts. The high temperature synthesis technique is optimized to obtain good quality crystalline smooth films on fluorine doped tin oxide substrates and their photoelectrochemical characteristics have been studied. It is observed that the interlayer oxide material used for growth of the Fe2O3 and BiVO4 has a significant role in their photoactivity.The interlayer oxide serves as an efficient electron transport layer and also influences the grain characteristics of the film. For hematite it is observed that a n-type metal oxide interlayer (e.g. Nb2O5 or TiO2) helps improve the photoactivity as compared to a p-type oxide (NiO). BiVO4 has a poor electron diffusion length, and a WO3 interlayer improves the photocurrent in BiVO4 films by improving the charge collection efficiency. The low absorption coefficient of hematite requires a dense electrode for greater light absorption; however, the electrode thickness is limited by the poor hole diffusion length (~4 nm). Plasmonic metal nanostructures of gold (Au), silver (Ag), and copper (Cu), which are known to concentrate and scatter broad range wavelengths of incident light, are promising for enhancing the light absorption cross-section of a semiconducting material. Gold nanoparticles embedded in hematite films have been synthesized. About three times higher light absorption and photocurrent enhancement are obtained. A thickness-dependent study of photoactivity indicates a greater enhancement of gold-embedded hematite thin films compared to thicker films due to reduced charge transport distance and optimal local field enhancement effect. The embedded structure also has the advantage of consistent performance and protection of plasmonic nanostructures from electrochemical corrosion, resulting in long cycles of operation.

A comprehensive and timely overview of this important and hot topic, with special emphasis placed on environmental applications and the potential for solar light harvesting. Following introductory chapters on environmental photocatalysis, water splitting, and applications in synthetic chemistry, further chapters focus on the synthesis and design of photocatalysts, solar energy conversion, and such environmental aspects as the removal of water pollutants, photocatalytic conversion of CO2. Besides metal oxide-based photocatalysts, the authors cover other relevant material classes including carbon-based nanomaterials and novel hybrid materials. Chapters on mechanistic aspects, computational modeling of photocatalysis and Challenges and perspectives of solar reactor design for industrial applications complete this unique survey of the subject. With its in-depth discussions ranging from a comprehensive understanding to the engineering of materials and applied devices, this is an invaluable resource for a range of disciplines.

Explore green catalytic reactions with this reference from a renowned leader in the field Green reactions—like photo-, photoelectro-, and electro-catalytic reactions—offer viable technologies to solve difficult problems without significant damage to the environment. In particular, some gas-involved reactions are especially useful in the creation of liquid fuels and cost-effective products. In Photo- and Electro-Catalytic Processes: Water Splitting, N2 Fixing, CO2 Reduction, award-winning researcher Jianmin Ma delivers a comprehensive overview of photo-, electro-, and photoelectron-catalysts in a variety of processes, including O2 reduction, CO2 reduction, N2 reduction, H2 production, water oxidation, oxygen evolution, and hydrogen evolution. The book offers detailed information on the underlying mechanisms, costs, and synthetic methods of catalysts. Filled with authoritative and critical information on green catalytic processes that promise to answer many of our most pressing energy and environmental questions, this book also includes: Thorough introductions to electrocatalytic oxygen reduction and evolution reactions, as well as electrocatalytic hydrogen evolution reactions Comprehensive explorations of electrocatalytic water splitting, CO2 reduction, and N2 reduction Practical discussions of photoelectrocatalytic H2 production, water splitting, and CO2 reduction In-depth examinations of photoelectrochemical oxygen evolution and nitrogen reduction Perfect for catalytic chemists and photochemists, Photo- and Electro-Catalytic Processes: Water Splitting, N2 Fixing, CO2 Reduction also belongs in the libraries of materials scientists and inorganic chemists seeking a one-stop resource on the novel aspects of photo-, electro-, and photoelectro-catalytic reactions.

The handbook comprehensively covers the field of inorganic photochemistry from the fundamentals to the main applications. The first section of the book describes the historical development of inorganic photochemistry, along with the fundamentals related to this multidisciplinary scientific field. The main experimental techniques employed in state-of-art studies are described in detail in the second section followed by a third section including theoretical investigations in the field. In the next three sections, the photophysical and photochemical properties of coordination compounds, supramolecular systems and inorganic semiconductors are summarized by experts on these materials. Finally, the application of photoactive inorganic compounds in key sectors of our society is highlighted. The sections cover applications in bioimaging and sensing, drug delivery and cancer therapy, solar energy conversion to electricity and fuels, organic synthesis, environmental remediation and optoelectronics among others. The chapters provide a concise overview of the main achievements in the recent years and highlight the challenges for future research. This handbook offers a unique compilation for practitioners of inorganic photochemistry in both industry and academia.

Oxide semiconductors, including titanium dioxide (TiO2), are increasingly being considered as replacements for silicon in the development of the next generation of solar cells. Oxide Semiconductors for Solar Energy Conversion: Titanium Dioxide presents the basic properties of binary metal oxide semiconductors and the performance-related properties

Photoelectrochemical (PEC) water splitting is a promising approach for synthesizing chemical fuels from sunlight. First demonstrated by Fujishima and Honda in 1972, PEC cell components and design strategies have proliferated in recent years. Regardless of the specific device architecture, however, any efficient PEC device requires (1) a high yield of energetic photogenerated carriers and (2) a mechanism for extracting these photogenerated carriers, (3) a corrosion-resistant anode at the pH and operating potential of the device, and (4) an effective catalyst for water oxidation. This dissertation addresses these challenges in the context of a photoanode, where water oxidation is tightly coupled to one of the light-absorbing elements of the PEC device. Metal-insulator-semiconductor (MIS) junctions are a promising photoanode design that electronically couples a high-quality semiconductor to an efficient water oxidation catalyst. The photovoltage produced by an MIS junction depends on the strength of the built-in field, or Schottky barrier height. This built-in field, in turn, depends on the difference in work function between the semiconductor and the metal, taking charges and interface fields into account. For optimal performance, a high work function metal induces a field that sweeps photogenerated holes from an n-type semiconductor to the electrolyte interface for water oxidation. In addition to generating large photovoltages, the ideal Schottky contact to an n-type semiconductor photoanode must also catalyze water oxidation and protect the underlying semiconductor from corrosion. In this work, I use atomic layer deposition (ALD) to fabricate alloys of TiO2 and transition metal oxides (specifically RuOx and IrOx) that function as the "M" of an MIS photoanode. Alloying TiO2 with these noble metal oxides combines the corrosion resistance of TiO2 with the high work function and catalytic activity of RuOx and IrOx. These alloys represent an ultra-thin analogue to the dimensionally stable anode used industrially for chlorine evolution. By investigating the chemical and electronic properties of these alloys, I unravel some of the key design principles for corrosion resistant Schottky contacts in MIS photoanodes. First, I demonstrated that ALD TiO2 protects the underlying silicon from corrosion and stabilizes RuOx and IrOx during water oxidation in acid. Second, I found that the electronic properties of TiO2 could be altered by alloying with metal oxides that have the desired work function. TiO2 makes a poor Schottky contact to n-type silicon, and its conductivity is difficult to control. Alloying TiO2 with high work function, conductive metal oxides like RuOx or IrOx not only enables high photovoltages but also guarantees high conductivity. By comparing the electronic properties of TiO2-RuOx alloys with TiO2-IrOx alloys, I also determined that the density of states at the alloy/SiO2 interface was critical for charge transport through the MIS junction. Finally, I gained insight into the relationship between catalytic activity and stability for RuOx and IrOx, two of the most commonly used water oxidation catalysts in acid. While IrOx is more stable than RuOx, its catalytic activity nonetheless degrades slowly over time. Though I used silicon as a model semiconductor, this ALD alloying approach may be particularly valuable for semiconductors that must rely on MIS junctions to generate large photovoltages (because forming a p-n junction is problematic). ALD enables unusually precise control over both the film thickness and composition. The ability to create graded structures by ALD presents a unique opportunity to control the composition these protection layers as a function of depth, placing valuable metal atoms where they are needed most—at the electrode/electrolyte interface for catalysis and at the insulator/metal interface for efficient tunneling. As such, ALD is capable of addressing many of the challenges associated with fabricating carrier selective contacts in photoelectrochemical and photovoltaic devices.