Development of a Novel Photoanode, a Zinc Oxide and Manganese Oxide Composite, for Visible Light-Driven Water Splitting

Abstract

A sustainable energy supply is arguably one of the most pressing problems facing society today, so there is a huge demand for clean energy technologies. Photocatalytic water splitting, the disassociation of water into H2 and O2 in the presence of a photocatalyst, offers environmentally friendly and low-cost production of hydrogen by solar energy. Consequently there is motivation to improve and develop new photocatalysts for this process, but many photocatalysts that have been investigated are limited in that they do not absorb visible light, are inefficient, or are unstable. This senior thesis aims to produce a photocatalyst that is efficient under visible light irradiation. We report our research for developing a novel photoanode (an oxidation photocatalyst): a zinc oxide, ZnO, and manganese oxide, Mn2O3, composite. ZnO has a large bandgap, so it is ineffective under visible light, but this can perhaps be improved by combining it with manganese oxide. By employing two synthesis methods, i.e., sputter deposition and deposition annealing, we synthesized thin film composite photocatalysts. ZnO and Mn2O3 composites (97% Zn) produced from deposition annealing showed photocurrent densities of up to 54 μA/cm2 at 1.0 V vs. Ag/AgCl, which is well below reported photocurrent densities for other ZnO systems. Further tests show that Mn2O3- containing samples absorb visible light and the 97% Zn composite shows photoactivity under visible light, which are critical steps toward developing visible light photocatalysts. There is little knowledge of the role of Mn2O3 in photocatalytic water splitting, and photocurrent density in these unoptimized samples is modest. But with further research, these composites of zinc oxide and manganese oxide have the potential to be developed further as visible light photocatalysts.