Defect Layer at the Surface of BaTaO2N and Implications for Photocatalysis

BaTaO2N has a valence and conduction band position which make the material a promising candidate for photocatalytic water splitting. However, the material shows a very low solar to hydrogen (STH) efficiency. It is generally assessed that the reason for the low STH is the presence of defects at the surface of the material which act as recombination centers for the excited electrons and holes. Here we show with high resolution transmission electron microscopy (HR TEM) an amorphous layer at the surface BaTaO2N which may be a cause of low STH. We demonstrate the amorphous layer can be removed through etching. Subsequently, the valence band structure is analyzed with UV photoelectron spectroscopy (UPS) and valence band X-ray photoelectron spectroscopy (VBXPS), where the latter has a significantly larger probing depth than the first. UPS reveals that non-etched BaTaO2N has a low density of states (DOS) whereas the VBXPS results are compatible with the optical band gap of the material. However, the DOS of BaTaO2N samples etched with H2SO4 solution were consistent with the optical band gap for both UPS and VBXPS measurements, indicating that after removal of the amorphous layer BaTaO2N is a suitable candidate for photocatalytic water splitting.