Atomic Layer Deposition (ALD) for Photoelectrochemical Water Splitting
Abstract: Photoelectrochemical water splitting for hydrogen production by semiconductors has been extensively investigated in recent decades for the conversion and storage of the intermittent solar energy. However, the conversion efficiency is still relatively low because of the poor charge utilization efficiency for photoelectrodes, which partly attributes to the inferior charge separation (such as Fe2O3, BiVO4) and the inclination to photocorrosion (such as ZnO, Si). This work describes the design and fabrication of several photoelectrodes with enhanced charge separation for Fe2O3 photoanodes and enhanced stability for ZnO photoanode and Si photocathode, using atomic layer deposition (ALD) as a surface engineering tool. Highly-oriented Fe2O3/ZnFe2O4 nanocolumnar arrays with a well-defined morphology are prepared by depositing conformal ALD ZnO layer onto Fe2O3 nanocolumnar arrays and post-annealing. Similarly, three-dimensional Fe2O3/Fe2TiO5 heterojunction is fabricated through annealing FeOOH/TiO2 nanorods, where TiO2 thin film is deposited by ALD. These two heterojunction structures promote the charge separation at the interface, leading to a more effective charge carrier utilization and an improved photoelectrochemical performance for water oxidation. On the other hand, ALD Ta2O5 as a transparent protective layer greatly enhances the stability of ZnO photoanode and p-Si photocathode. The optical property of Ta2O5 ensures its transparency to sunlight, prohibiting the generation of high energy oxidative holes from Ta2O5 that will attack the underlying material. Owing to the excellent anti-corrosion performance, the charge carrier utilization can be greatly enhanced. Our work demonstrates the great advantages of ALD in surface engineering and enlightens us that constructing well-organized photoelectrodes and efficient surface engineering play important roles in promoting charge carrier utilization and improving the performance of photoelectrodes for solar water splitting.
Biography: Tuo Wang received his BS from Tianjin University and PhD from the University of Texas at Austin, both in chemical engineering. After gaining another year of research experience as a postdoctoral associate, he joined Lam Research Corp. as a process development engineer in Tualatin, Oregon. Since August 2012, he has been an associate professor in chemical engineering in Tianjin University. His research included ALD of high-k dielectrics and epitaxial growth of perovskite thin films, as well as PECVD of carbon-based etching hardmasks. In recent years, he has employed thin film deposition techniques in photoelectrochemical water splitting and CO2 reduction for controllable fabrication of photoelectrodes, which has been demonstrated to be effective in constructing highly efficient carrier transfer pathways, improving the light absorption of photoelectrodes, and enhancing the efficiency of surface reactions. He is the author of about 60 peer-reviewed papers, including JACS, AIChE J, Angew Chem Int Ed, Adv Mater, Chem Sci, and an invited review for Chem Soc Rev. He serves as the associate editor for Renewable Energy.