Probing the electronic and geometric structures of photoactive electrodeposited Cu2O films by X-ray absorption spectroscopy

Abstract

Cu2O is an attractive photocathode for important renewable energy reactions such as water splitting and CO2 reduction. Electrodeposition is commonly used to deposit Cu2O films on conductive substrates due to its simplicity and consistency. However, structural descriptors, linking electrodeposition parameters, film structure and the catalytic properties are elusive. A variety of Cu2O films reported by many research groups would often display vastly different electronic properties and catalytic activity, while appear indistinguishable under common characterisation tools. In this work, we take a systematic look into electrochemically deposited Cu2O and investigate the impact of deposition parameters towards the bulk and surface chemistry of the deposited film. Specifically, we employ high resolution XANES for thorough quantitative analysis of the Cu2O films, alongside more common characterisation methods like XRD, SEM and Raman spectroscopy. Photoelectrochemical (PEC) studies reveal an unexpected trend, where the highest PEC activity appears to correlate with the amount of Cu2+ content. Other factors which also affect the PEC activity and stability are film thickness and crystallite grain size. Our study shows that the use of high resolution XANES, though not perfect due to possible self-absorption issue, is apt for extracting compositional descriptor in concentrated thin film samples from the pre-edge energy position analysis. This descriptor can serve as a guide for future development of more active Cu2O based films for wide range of PEC processes as well as for solar cell applications.