Understanding Oxide Interfaces in Photoelectrochemistry with XPS

Initial materials development efforts on emerging photoabsorbers tend to focus on bulk properties, such as band gap, carrier lifetime, and carrier diffusion length. Once a promising absorber has been identified, the next challenge is to engineer the interface in order to facilitate efficient charge transfer and ensure good (photo)chemical stability. One of the main tools to study such interfaces is X-ray photoelectron spectroscopy (XPS), which can provide information on both chemical (oxidation states, local environment) and physical (band bending) properties. I will show three examples, with increasing complexity, where XPS has provided key information on (photo)electrochemical interfaces. In the first example, we used lab-based XPS to study ultrathin MnO_x films on silicon as oxygen evolution catalysts. We find that films thinner than 1.5 nm are not OER active due to electrostatic catalyst-support interactions that prevent the electrochemical oxidation of Mn close to the Si/MnO_x interface [1]. In the second example, we use hard X-ray photoelectron spectroscopy (HAXPES) to study the a-SnWO₄/NiO_x interface. The NiO_x prevents passivation of the a-SnWO₄ absorber and introduces favorable upward bend bending, but also oxidizes part of the Sn²⁺ to Sn⁴⁺. This results in a thin SnO₂ layer that pins the Fermi level and reduces the photovoltage [2]. Finally, we use ambient pressure HAXPES to study a ‘real’ BiVO₄/electrolyte interface. We previously reported the formation of an ultrathin BiPO₄ layer at this interface, but more recent results reveal that this formation depends on the surface structure of BiVO₄ and can be avoided [3]. With these examples, I hope to illustrate some of the recent developments in this classical surface science technique and how these can help to understand complex interfaces in photoelectrochemical systems.<br/> <br/>[1] P. Plate et al, <i>ACS Appl. Mater. Interfaces</i> 13, 2428-2436 (<b>2021</b>).<br/>[2] P. Schnell et al, <i>Adv. Energy Mater.</i> 2003183 (<b>2021</b>).<br/>[3] M. Favaro et al., <i>J. Phys. D. Appl. Phys.</i> 54 (16), 164001 (<b>2021</b>).