Alternatives to fossil fuel-based energy must be developed as fossil fuel sources deplete worldwide. Solar energy is an attractive choice for energy generation but is difficult to implement on a large scale since it can only be harnessed intermittently. One way to alleviate this concern is to use sunlight to directly catalyze the formation of fuels that can be stored and utilized when necessary. Hydrogen gas is one such solar fuel that is carbon-neutral and environmentally benign that can be generated from the sunlight-driven process of photoelectrochemical water splitting. For photoelectrochemical water splitting to be a commercially competitive way of storing solar energy, semiconductor materials which can efficiently absorb sunlight, convert them into photo-excited electron-hole pairs, and use the electrons and holes to perform water reduction and water oxidation must be developed. Water oxidation reaction is the more kinetically limiting of the two half-reactions for water splitting, making the development of efficient photoanodes for the water oxidation reaction critical for maximizing the overall efficiency of generating hydrogen. From a technoeconomic perspective, oxide-based photoanodes are the most promising to use as photoanodes because of their solution-based, scalable synthesis routes. However, they currently suffer from low efficiencies for solar water oxidation due to poor solar spectrum absorbance and charge transport limitations. The work presented herein was conducted to increase the efficiency of oxide-based photoanode BiVO4, as well as to investigate other promising photoanodes for photoelectrochemical water splitting. Studies on BiVO4 focused on improving its charge transport properties by doping at the Bi-site with lanthanide ions, developing a new tandem device architecture which increased light harvesting capabilities, and evaluating its long-term chemical stability in near-neutral aqueous electrolytes. Studies were also conducted on electrochemically synthesized PbCrO4, Pb2CrO5, CoV2O6, and BiMn2O5 to evaluate their utility as photoanodes for water oxidation via new electrochemical synthesis routes. The studies described herein will guide future work on the development of future oxide-based photoelectrodes to bring photoelectrochemical water splitting technologies closer to commercial realization.

Cleavage of water to its constituents (i.e., hydrogen and oxygen) for production of hydrogen energy at an industrial scale is one of the "holy grails" of materials science. That can be done by utilizing the renewable energy resource i.e. sunlight and photocatalytic material. The sunlight and water are abundant and free of cost available at this planet. But the development of a stable, efficient and cost-effective photocatalytic material to split water is still a great challenge. To develop the effective materials for photocatalytic water splitting, various type of materials with different sizes and structures from nano to giant have been explored that includes metal oxides, metal chalcogenides, carbides, nitrides, phosphides, and so on. Fundamental concepts and state of art materials for the water splitting are also discussed to understand the phenomenon/mechanism behind the photoelectrochemical water splitting. This book gives a comprehensive overview and description of the manufacturing of photocatalytic materials and devices for water splitting by controlling the chemical composition, particle size, morphology, orientation and aspect ratios of the materials. The real technological breakthroughs in the development of the photoactive materials with considerable efficiency, are well conversed to bring out the practical aspects of the technique and its commercialization.

To better utilize solar energy for clean energy production, efforts are needed to overcome the natural diurnal variation and the diffuse nature of sunlight. Photoelectrochemical (PEC) hydrogen generation by water splitting is a promising approach to harvest solar energy. Hydrogen gas is a clean and high energy capacity fuel. However, the solar-to-hydrogen conversion efficiency is determined mainly by the properties of the materials employed as photoanodes. Improving the power-conversion efficiency of PEC water splitting requires the design of inexpensive and efficient photoanodes that have strong visible light absorption, fast charge separation, and lower charge recombination rate. In the present study, PEC characteristics of various semiconducting photoelectrodes such as TiO2, WO3 and CuWO4 were investigated. Due to the inherent wide gap, such metal oxides absorb only ultraviolet radiation. Since ultraviolet radiation only composes of 4% of the sun's spectrum, the wide band gap results in lower charge collection and efficiency. Thus to improve optical absorption and charge separation, it is necessary to modify the band gap with low band gap materials.The two approaches followed for modification of band gap are doping and sensitization. Here, TiO2 and WO3 based photoanodes were sensitized with ternary quantum dots, while doping was the primary method utilized to investigate the modification of the band gap of CuWO4. The first part of this dissertation reports the synthesis of ternary quantum dot - sensitized titania nanotube array photoelectrodes. Ternary quantum dots with varying band gaps and composition (MnCdSe, ZnCdSe and CdSSe) were tethered to the surface of TiO2 nanotubes using successive ionic layer adsorption and reaction (SILAR) technique. The stoichiometry of ternary quantum dots was estimated to beMn0.095Cd0.95Se, Zn0.16Cd0.84Se and CdS0.54Se0.46. The effect of varying number of sensitization cycles and annealing temperature on optical and photoelectrochemical properties of prepared photoanodes were studied. The absorption properties and surface morphology of the sensitized tubes was analyzed using UV-visible spectroscopy and scanning electron microscopy. The phase composition was determined using X-Ray diffraction and X-ray photoelectron spectroscopy techniques. Electrodes were also evaluated for their stability using inductively coupled plasma optical emission spectrometry. Results show that the sensitization of TiO2 nanotubes with MnCdSe (8.79 mA/cm2), ZnCdSe (12.70 mA/cm2) and CdSSe (15.58 mA/cm2) resulted in up to a 30 fold increase in photocurrent compared to unsensitized nanotubes (0.4 mA/cm2). In the second part, the application of WO3 as photoanode for water splitting was explored. The porous thin films of WO3 films were sensitized with ternary quantum dots (ZnCdSe) using the SILAR technique. The structural, surface morphological and optical properties of the sensitized WO3 thin films were studied. PEC characteristics of the sensitized films were found to be 120 fold increase (8.53 mA/cm2) in comparison to that of unmodified WO3 films (0.07 mA/cm2). In the last part of this dissertation, CuWO4 was investigated as the potential photoanode material. The band gap of CuWO4 was estimated using density functional theory (DFT) calculations. The band structure was obtained using the first-principles plane wave self-consistent field (pwscf) method and the effect of nickel dopant on the band gap and optical properties of CuWO4 was evaluated. Theoretical calculations showed that doping led to a decrease in band gap. The validity of the theoretical approach was evaluated by experimentally synthesizing Ni-doped CuWO4 electrodes. Experimental results showed that the band gap indeed decreases when CuWO4 was doped with Ni, and thus validated the DFT approach. Ternary quantum dots were found to increase the PEC activity of TiO2 and WO3 based photoelectrodes by 120 fold. In addition, a method of computing band gap of semiconductor using DFT modeling was developed and validated with experimental results.

Research for the development of more efficient photocatalysts has experienced an almost exponential growth since its popularization in early 1970’s. Despite the advantages of the widely used TiO2, the yield of the conversion of sun power into chemical energy that can be achieved with this material is limited prompting the research and development of a number of structural, morphological and chemical modifications of TiO2 , as well as a number of novel photocatalysts with very different composition. Design of Advanced Photocatalytic Materials for Energy and Environmental Applications provides a systematic account of the current understanding of the relationships between the physicochemical properties of the catalysts and photoactivity. The already long list of photocatalysts phases and their modifications is increasing day by day. By approaching this field from a material sciences angle, an integrated view allows readers to consider the diversity of photocatalysts globally and in connection with other technologies. Design of Advanced Photocatalytic Materials for Energy and Environmental Applications provides a valuable road-map, outlining the common principles lying behind the diversity of materials, but also delimiting the imprecise border between the contrasted results and the most speculative studies. This broad approach makes it ideal for specialist but also for engineers, researchers and students in related fields.

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.

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.

Dieses klar strukturierte Fachbuch legt den Schwerpunkt auf praktische Anwendungen von Nanokompositen und Nanotechnologien im Rahmen einer nachhaltigen Entwicklung. Es zeigt, wie Nanokomposite zur Lösung von Energie- und Umweltproblemen beitragen können, bietet zusätzlich einen breiten Überblick über Anwendungen im Energiebereich und behandelt eine einzigartige Auswahl an Umweltthemen. Der erste Teil beschäftigt sich mit Anwendungen wie Lithium-Ionen-Batterien, Solarzellen, Katalyse, Gewinnung von Wärme und Energie aus Abfällen mithilfe der Thermoelektrizität und Wasserspaltung. Der zweite Teil beleuchtet in einzigartiger Weise ökologische Themen, darunter Atommüllmanagement sowie die Abscheidung und Speicherung von Kohlendioxid. Dieses Fachbuch vermittelt auf erfolgreiche Weise Grundlagenwissen für Einsteiger als auch die neuesten Erkenntnisse für erfahrene Wissenschaftler, Ingenieure und Forscher aus der Industrie.