Electronic structure and photocatalytic water oxidation activity of RTiNO2 (R = Ce, Pr and Nd) perovskite nitride oxides

Three perovskite nitride oxides CeTiNO2, PrTiNO2, and NdTiNO2 have been synthesized and their electronic structures and photocatalytic activities characterized and compared to LaTiNO2. All three compounds have band gaps that fall in the range 2.0 − 2.1 eV, very similar to LaTiNO2, which enables them to absorb a significant fraction of the visible spectrum. Photocatalytic oxygen evolution studies under visible light irradiation in the presence of a sacrificial electron acceptor (Ag+) show that the activity of NdTiNO2 (16 μmol/g/hr) is comparable to that of LaTiNO2 (17 μmol/g/hr), while PrTiNO2 (11 μmol/g/hr) and CeTiNO2 (5 μmol/g/hr) have activities that are only 65% and 30% that of LaTiNO2. X-ray photoelectron spectroscopy measurements reveal the presence of partially occupied f-orbital states that lie in the band gap for CeTiNO2, and near the valence band maximum for PrTiNO2. As evidenced by time resolved IR kinetic decay, these localized f-orbital states act as electron-hole recombination centers that inhibit the photocatalytic activities of both compounds. On the other hand, NdTiNO2, where the f-orbital energies fall below the valence band maximum, does not suffer from this effect.