Nowadays clean and renewable energy supply is one of the biggest challenges for the mankind. Hydrogen is often proposed as a prospective fuel of the future, but there are numerous scientific and technological problems to be solved on the way to the hydrogen economy. One of them is hydrogen production. The most efficient way of hydrogen production would be direct water splitting using solar energy. Photoelectrode materials for solar water splitting cells must fulfill a number of requirements: an appropriate band gap, band edge positions, high specific surface area, long term chemical and mechanical stability, low cost of fabrication. In this work, porous thin film electrodes for photoelectochemical solar water splitting were developed by the dealloying approach. The search for materials with optimal physical and photoelectrochemical properties inevitably involves fabrication and characterization of a large number of samples. In order to accelerate this search, combinatorial and high-throughput methods were used for fabrication and investigation of the thin film materials libraries.

The rapidly increasing global demand for energy, combined with the environmental impact of fossil fuels, has spurred the search for alternative sources of clean energy. One promising approach is to convert solar energy into hydrogen fuel using photoelectrochemical cells. However, the semiconducting photoelectrodes used in these cells typically have low efficiencies and/or stabilities. This dissertation will describe engineering of metal-oxide-semiconductor (MIS) photoelectrodes for application in solar water splitting. First, we show that a silicon-based photocathode with an epitaxial oxide capping layer can provide efficient and stable hydrogen production from water. In particular, we grow a thin epitaxial layer of strontium titanate (SrTiO3) directly on Si (001) by molecular beam epitaxy. Photogenerated electrons can be easily transported through this layer because of the conduction band alignment and lattice match between single crystalline SrTiO3 and silicon. The approach is used to create a metal-insulator-semiconductor photocathode that under broad-spectrum illumination at 100 mW/cm2 exhibits a maximum photocurrent density of 35 mA cm2 and an open circuit potential of 450 mV; there was no observable decrease in performance after 10 hours of operation in 0.5 M H2SO4. Then, we propose and demonstrate a general method to decouple the two roles of the insulator by employing localized dielectric breakdown. This approach allows the insulator to be thick, which enhances stability, while enabling low-resistance carrier transport as required for efficiency. This method can be applied to various-oxides, such as SiO2 and Al2O3. In addition, it is suitable for silicon, III-V, and other optical absorbers for both photocathodes and photoanodes. Finally, the thick metal-oxide layer can serve as a thin-film antireflection coating, which increases light absorption efficiency.

This outstanding thesis provides a wide-ranging overview of the growth of titanium dioxide thin films and its use in photo-electrochemicals such as water splitting. The context for water splitting is introduced with the theory of semiconductor-liquid junctions, which are dealt with in detail. In particular plasmonic enhancement of TiO2 by the addition of gold nanoparticles is considered in depth, including a thorough and critical review of the literature, which discusses the possible mechanisms that may be at work. Plasmonic enhancement is demonstrated with gold nanoparticles on Nb-doped TiO2. Finally, the use of temperature and pressure to control the phase and morphology of thin films grown by pulsed laser deposition is presented.

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.

The increasing need for carbon free energy has focused renewed attention on solar energy conversion. Although photovoltaic cells excel at directly converting of solar energy to electricity, they do not directly produce stored energy or fuels that account for more than 75% of current energy use. Direct photoelectrolysis of water has the advantage of converting solar energy directly to hydrogen, an ideal non-carbon and nonpolluting energy carrier, by replacing both a photovoltaic array and an electrolysis unit with one potentially inexpensive device. Unfortunately no materials are currently known to efficiently photoelectrolyze water that are, efficient, inexpensive and stable under illumination in electrolytes for many years. Nanostructured semiconducting metal oxides could potentially fulfill these requirements, making them the most promising materials for solar water photoelectrolysis, however no oxide semiconductor has yet been discovered with all the required properties. We have developed a simple, high-throughput combinatorial approach to prepare and screen many multi component metal oxides for water photoelectrolysis activity. The approach uses ink jet printing of overlapping patterns of soluble metal oxide precursors onto conductive glass substrates. Subsequent pyrolysis produces metal oxide phases that are screened for photoelectrolysis activity by measuring photocurrents produced by scanning a laser over the printed patterns in aqueous electrolytes. Several promising and unexpected compositions have been identified.

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.