NaTaO3 Photoanode for Bias-Free Water Splitting: A Photo-Electrochemical and Kelvin Probe Surface Photovoltage Study

NaTaO3 powder is well-known for its extraordinary water splitting capabilities, but its potential as photoelectrode material is little studied. We find that an existing facile synthesis method for NaTaO3 thin films on Ta provides such photoelectrodes with good electrical back contact, enabling photoelectrochemical and Kelvin probe photovoltage measurements. Connected to a Pt counter electrode, the NaTaO3 acts as a photoanode, driving a water splitting photocurrent under UV illumination without bias. Cyclic voltammetry under UV shows a steep water oxidation photocurrent onset at −0.3 VRHE, while directly below this potential water reduction sets in. Photocurrent spectroscopy shows an absorption onset of 310 nm, which agrees with reported value of the NaTaO3 band gap. Without bias, the incident photon to current efficiency reaches 12% at 250 nm. Kelvin probe surface photovoltage measurements show relaxation over multiple days, indicating the presence of trap states. In addition, isolated NaTaO3 nanocrystals are grown on Pt and frequency modulation Kelvin probe force microscopy with and without UV is used to visualize the photovoltage between the two materials. These NaTaO3 photoelectrodes provide a new platform for detailed research and development of NaTaO3-based water splitting systems.