The goal of this thesis is to investigate the properties of metal-oxide thin films on fluorine-doped tin oxide (FTO)-coated glass substrates, prepared by using radio- frequency (RF) reactive magnetron sputtering for photoelectrochemical (PEC) applications. Metal-oxide thin films as a photoelectrode are of special interest for PEC systems to produce hydrogen in an aqueous solution by solar energy due to their low cost and potential stability.

The following list represents some of the accomplishments and results of this work:

• Narrowing of N-incorporated ZnO (ZnO:N) was achieved by reactive sputtering in a O₂/N₂ mixture ambient, and ZnO:N films with various bandgaps were realized by varying N concentration, which was controlled successfully by varying the RF powers.
• Nitrogen incorporation narrows the bandgap of ZnO and shifts the optical absorption into the visible-light region. As a result, the ZnO:N films exhibit higher photocurrents than ZnO films.
• p-type ZnO thin films with significantly reduced bandgaps were synthesized by heavy Cu incorporation.
• ZnO thin films deposited in pure argon ambient lead to polycrystalline films. However, the presence of N₂ in the deposition chamber ambient promotes the formation of aligned nanorods at temperatures above 300°C and these films exhibit enhanced photocurrents.
• Proper Ar/N₂ ratio in the chamber ambient plays a significant role in the formation of aligned nanorods in ZnO thin films.
• Bandgap-reduced p-type ZnO thin films with various carrier concentrations are realized by Cu and Ga co-doping.
• ZnO thin films co-doped with Ga and N showed significantly enhanced crystallinity and improved N incorporation compared to ZnO doped solely with N and exhibited dramatically improved PEC response.
• Ga and N co-doped ZnO films exhibited enhanced N incorporation and photocurrents as the substrate temperature was increased.
• Controlling O₂/N₂ gas flow rate in the chamber ambient is critical for Ga and N co-doped ZnO thin films; otherwise, it will result in phase separation.
• Synthesized porous ZnO nanocoral structures demonstrated a 10-fold increase in PEC response as compared to compact ZnO films.
• ZnO:N, ZnO:(Ga,N), and ZnO:(Al,N) films deposited under a N2/O2 plasma showed n-type behavior due to substitutional N₂ molecules that act as shallow double donors.
• Significantly reduced bandgaps enhanced crystallinity, and PEC responses were observed for Al and N co-doped ZnO thin films.
• N incorporation in the Al and N co-doped ZnO films were successfully controlled by varying the N₂/O₂ gas flow rate and RF powers.
• Bandgap-reduced solid solution of ZnO and GaN (ZnO:GaN) that exhibited improved PEC responses were synthesized.
• It was found that the Al and N co-doped ZnO and ZnO and GaN solid solution deposited under N₂/Ar gas flow failed to incorporate the N in the films; N₂/O₂ gas flow succeeded in incorporating N in the films.
• CoAl₂O₄—Fe₂O₃ p-n nanocomposite electrodes exhibited much improved photoresponses as compared to p-type CoAl₂O₄ only.
• Ternary cobalt-based spinal oxides as PEC catalysts are limited by the poor transport properties induced by small polaron mobility.
• p-type Cu-Ti-oxide, Cu-W-oxide, and Cu-Sn-oxide films were synthesized.