UC Santa Cruz

Photoelectrochemical (PEC) cell is a device generated hydrogen fuel through an environmentally friendly method. The earliest report should date back to 1972. Honda and Fujishima firstdemonstrated solar water splitting by using titanium dioxide as photoanode in the cell. Then extensive efforts have been devotedto improving the solar-to-hydrogen (STH) conversionefficiency and decreasing thecost. However, current the efficiency of PEC device was limited on finding out a suitable photoanode material. The ideal photoanode material should have a good bandgap, favorable bandgap position, chemically stable and low cost. Therefore, this thesis would focus on studying different photoanode materials including GaN, TiO2 and Fe2O3 to achieve high PECwater oxidation performance. In this thesis, I will first designed GaN nanowires on carbon cloth via a chemical vapor deposition (CVD) method and demonstrated significant photoactivity for photoelectrochemical water oxidation. In addition, our group used to report a facile and general strategy to fundamentally improve the performance of TiO2 nanowires for PEC water splitting. However, there are some concerns about the real effects under higher hydrogen treated temperature as well as the stability of oxygen vacancies in TiO2. Therefore I investigated the effect of hydrogenation temperature and the stability of oxygen vacancies in TiO2 photoanodes. Furthermore, there are few reports about the study on the long term stability of TiO2 photoanode even though most scholars used to think TiO2 belongs to one of the most stable photoanode materials. So I carried out the first long term photostability measurement on various phases TiO2 photoanodes including rutile, anatase and mixed phased and found TiO2 photoanodes were not stable as people expected. Then I investigated the mechanism of the instability of TiO2 and carried out two strategies to stabilize TiO2 materials in the PEC cell. Finally, I created a facile acid treated method on hematite to substantially enhance the PEC activity. I found the enhanced photocurrent is due to improved efficiency of charge separation as well as potential passivation of surface electron traps.