Detailed study of defect layer at the surface of BaTaO₂N: a potential candidate for visible light photocatalytic water splitting

BaTaO₂N has a valence and conduction band position which makes the material a promising candidate for photocatalytic water splitting. However, the material shows a very low solar to hydrogen (STH) efficiency. It is generally assumed 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. The bulk of the material is highly crystalline and has a very low defect density. Here we show with high resolution transmission electron microscopy (HR TEM) the presence of an amorphous layer at the surface BaTaO₂N which may be a cause of the low efficiency. We further demonstrate that 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) in the energy region close to the valence band (VB) edge, whereas the VBXPS results are compatible with the optical band gap of the material. However, the DOS of BaTaO₂N samples etched with H₂SO₄ solution as analyzed with electron spectroscopy are consistent with the optical band gap for both UPS and VBXPS measurements, indicating that after removal of the amorphous layer BaTaO₂N is a suitable bulk material candidate for photocatalytic water splitting with visible light.