(Invited) Understanding Photoelectrocatalysis on Epitaxial Oxide Surfaces

The intermittent nature of renewable energy sources requires a clean, scalable means of converting and storing energy. One Earth abundant storage option is water electrolysis, storing energy in the bonds of O 2 and H 2 . Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces can convert sunlight to chemical fuels without external circuitry, such as “splitting” water into O 2 and H 2 upon illumination. The efficacy of conversion depends in part on the location of the semiconductor band edges, dictating absorption, and also the rectifying properties of semiconductor–electrolyte junctions, which drives the separation of electron–hole pairs. We will present studies of model oxide La (1-x) Sr x FeO 3 photoelectrodes grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) that display a known crystallographic orientation, surface area, path for charge transport, and strain. Aliovalent doping can tune the transition metal redox properties as well as the material band gap and conductivity, resulting in a rich phase space of possible water-oxidation photoanodes. Comparing the photooxidation of a fast redox couple to the slugging water oxidation reaction demonstrates the potential of electronic states that must be filled prior to oxygen evolution. Measurements of the surface speciation in a humid environment demonstrate that Sr incorporation also promotes hydroxylation, suggesting that water oxidation proceeds through the oxidation of Fe III -OH states, which is facilitated by Sr incorporation and results in an increased photovoltage. Surface-sensitive measurement of the valence band and unoccupied states (via O K-edge X-ray absorption spectroscopy) under humid conditions further support this picture. This fundamental insight builds understanding necessary for the design of active, earth-abundant photocatalysts that can be integrated into PEC devices for efficient conversion of solar energy into chemical fuels.