Aliovalent gallium dopants remove Ti3+ defects and improve photocatalytic and photoelectrochemical water oxidation properties of LaTiO2N

Abstract

LaTiO₂N is a promising semiconductor for the water splitting reaction due to its 2.1 eV band gap and stability against corrosion. However, its solar energy conversion is limited by Ti³⁺ recombination defects introduced during ammonolysis. Here we show for the first time that Ti³⁺ defects in LaTiO₂N can be suppressed with incorporation of 2, 5, and 10% aliovalent gallium (Ga³⁺) dopants during synthesis via the layered La₂Ti₂O₇ intermediate. Electron paramagnetic resonance (EPR) spectroscopy on the solid powders confirms a reduction in the Ti³⁺ donor density from 2.97 × 10¹⁷ cm⁻³ for the non-doped material to ∼6.24 × 10¹⁶ cm⁻³ for 5% Ga-doped LaTiO₂N. The remaining Ti³⁺ defects are concentrated near the LaTiO₂N surface, according to X-ray photoelectron spectroscopy. The defect reduction shifts the optical absorption edge from 2.02 to 2.09 eV and eliminates a broad absorption band at 1050 nm from the optical absorption spectra. It also removes a 1.0–1.7 eV sub-band gap photovoltage signal from surface photovoltage spectra. This suggests that empty Ti³⁺ d-orbitals are located 1.0–1.7 eV above the LaTiO₂N valence band edge. Removing these recombination states with increasing Ga³⁺ content enhances the photoconversion efficiency of LaTiO₂N during water oxidation. The optimized 2 wt% CoO_x-loaded 5% Ga-doped LaTiO₂N material has a 16% AQE (400 nm) for O₂ production from aqueous silver nitrate solution and a ∼2.1 mA cm⁻² water oxidation photocurrent at 1.23 V under 100 mW cm⁻² Xe arc lamp illumination. The water oxidation photocurrent is stable during a 50 min test, and the Faraday efficiency for O₂ generation is 97%, confirming short-term corrosion stability of LaTiO₂N. Overall, these results provide a better understanding of the distribution, concentration, and impact of Ti³⁺ defects on the optical, photovoltage, and photoelectrochemical properties of LaTiO₂N. In combination with other defect control strategies, aliovalent Ga³⁺ doping can help bring the solar energy conversion efficiency of LaTiO₂N closer to the theoretical limit.