Book Description

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.