Crucial Role of Donor Density in the Performance of Oxynitride Perovskite LaTiO2N for Photocatalytic Water Oxidation
Graphical Improving photocatalysts : Sufficient donor density in LaTiO 2 N photocatalysts is essential to generate an efficient internal electric field for charge separation and for high oxidizing power of the holes. On the basis of this understanding, LaTiO 2 N photocatalysts were modified to give a quantum efficiency for water oxidation of 25 % at 450 nm, which is higher than that of conventional LaTiO 2 N. LaTiO 2 N photocatalysts were prepared by thermal ammonolysis of flux-synthesized La 2 Ti 2 O 7 and La 2 TiO 5 , and were investigated for water oxidation. Though LaTiO 2 N derived from La 2 TiO 5 appears defect-free by UV/Vis/near-IR and electron paramagnetic resonance (EPR) spectroscopy, its performance is much lower than that of conventional La 2 Ti 2 O 7 -derived LaTiO 2 N with defects. It is shown by Mott–Schottky analysis that La 2 TiO 5 -derived LaTiO 2 N has significantly lower donor density; this can result in insufficient built-in electric field for the separation of photogenerated electrons and holes. The lower donor density is also consistent with the smaller difference between the Fermi level and the valence-band maximum, which accounts for a lower oxidative power of the holes. In light of this discovery, the donor density was increased substantially by introducing anion vacancies through annealing in Ar. This resulted in improved performance. The CoO x -assisted La 2 TiO 5 -derived LaTiO 2 N annealed at 713 °C has a higher quantum efficiency (25 %) at 450 nm than high-performance conventional CoO x /LaTiO 2 N (21 %).
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