Electronic properties of beta-TaON and its surfaces for solar water splitting

Recently, oxynitrides materials such as beta-TaON has been using as a photoanode material in the field of photo catalysis and is found to be promising due to its suitable band gap and charge carrier mobility. Computational study of the crystalline beta-TaON in the form of primitive unit cell, supercell and its N, Ta, and O terminated surfaces are carried out with the help of periodic density functional theory (DFT). Optical and electronic properties of all these different species are simulated, which predict TaON as the best candidate for photo catalytic water splitting contrast to their Ta2O5 and Ta3N5 counterparts. The calculated bandgap, valence band, and conduction band edge positions predict that beta-TaON should be an efficient photoanodic material. The valence band is made up of N 2p orbitals with a minor contribution from O 2p, while the conduction band is made up of Ta 5d. Turning to thin films, the valence band maximum; VBM (-6.4 eV vs. vacuum) and the conduction band minimum; CBM (-3.3 eV vs. vacuum) of (010)-O terminated surface are respectively well below and above the redox potentials of water as required for photocatalysis. Charge carriers have smaller effective masses than in the (001).N terminated film (VBM -5.8 and CBM -3.7 eV vs. vacuum). However, due to wide band gap (3.0 eV) of (010)-O terminated surface, it cannot absorb visible wavelengths. On the other hand, the (001)-N terminated TaON thin film has a smaller band gap in the visible region (2.1 eV) but the bands are not aligned to the redox potential of water. Possibly a mixed phase material would produce an efficient photoanode for solar water splitting, where one phase performs the oxidation and the other reduction.