(Invited) Integrated Semiconductor/Catalyst Assemblies for Sustained Photoanodic Water Oxidation

Photoelectrochemical conversion of solar energy to chemical fuel relies on the availability of semiconductors that are stable under aqueous conditions, possess bandgaps suitable for harvesting light from the sun, and can be integrated with active catalysts. Regardless of the desired reduced product, water oxidation is necessary to provide an abundant source of protons and electrons for fuel formation. However, semiconductor (photo)corrosion under oxidizing conditions is a central challenge. Here, we provide an overview of different approaches to achieving durable semiconductor photoanodes and highlight two specific examples of how catalyst integration can be used to improve chemical stability. In the first example, we show that plasma-assisted atomic layer deposition (PE-ALD) of cobalt oxide onto silicon enables efficient photoelectrochemical water splitting. These integrated assemblies are stable under aqueous conditions, including at pH 14. Structure and composition, which play crucial roles in performance, are investigated via a combination of X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, grazing incidence wide angle X-ray scattering, and transmission electron microscopy. Together, these methods reveal that amorphous catalyst coatings are beneficial for enhancing stability and improving catalytic activity. In the second example, semiconductors that possess at least a modicum of native stability are investigated. These semiconductors are often polycrystalline thin film metal oxides possessing lateral inhomogeneities, poorly understood interactions with electrolyte environment, and relatively unknown optoelectronic properties. One such material is bismuth vanadate (BiVO₄), a 2.4-2.5 eV bandgap n-type semiconductor. We have utilized a suite of spectroscopic and microscopic methods to probe local compositional non-uniformities, understand the role of catalyst integration on chemical stability, and determine electronic structure and photocarrier dynamics that govern charge extraction efficiency. These measurements provide valuable insight into basic properties – often defined by local nonuniformities – that affect ultimate performance of functional photoanodes. The examples presented here highlight that, although few materials are inherently stable under conditions required for artificial photosynthesis, conformal catalyst deposition provides a viable approach to sustained operation in harsh environments.