Influence of Catalyst Deposition on Charge Carrier Trapping at Catalyst-Semiconductor Interfaces

The production of solar fuels via photoelectrochemical water splitting is an attractive option for satisfying the world’s demand for clean energy. The success of this solar-to-chemical energy conversion depends highly on the identification of a stable and efficient photoanode for water oxidation. Bismuth vanadate (BiVO₄) is a promising photoanode material because of its small band gap, its stability over large pH ranges and favorable location of its valence band edge relative to the water oxidation potential. However, its surface is not catalytically active for water oxidation and its performance is limited. This problem can be mediated by the integration of an effective water oxidation catalyst such as cobalt phosphate (Co-Pi). Nevertheless, defects created at catalyst-semiconductor interfaces can lead to charge carrier trapping, preventing charge transfer to the catalyst and lowering conversion efficiency. This work provides a quantitative analysis of the accumulation of photogenerated holes at catalyst-semiconductor interfaces as a function of semiconductor doping and electrocatalyst deposition. In situ transient photocurrent and photovoltage behavior of a BiVO₄ photoanode coated with a Co-Pi electrocatalyst were measured in both a standard aqueous electrolyte and in one containing a sacrificial reagent. Associated time constants were analyzed to determine deleterious carrier trapping at these interfaces. Comparisons were also made with nominally undoped BiVO₄, and molybdenum doped BiVO₄ with and without catalyst. Results presented here provide an important step to identifying parameters affecting the quality of BiVO₄/Co-Pi interface and solidify a method of analyzing similar types of interfaces of importance to the development of artificial photosynthesis.