Ammonolysis Under NH ₃ –Limiting Conditions as a Pathway to Improved LaTiO ₂ N Water Splitting Photoanodes

LaTiO2N is a promising intermediate band gap semiconductor for the water splitting reaction, a pathway to hydrogen fuel from solar energy. However, the photoelectrochemical (PEC) activity of the material is hindered by defects, particularly Ti(III) species, which promote photocarrier recombination. These defects are formed during the high-temperature ammonolysis reaction. Here we show that improved LaTiO2N materials can be synthesized under NH3-limiting conditions by introducing N2 to lower the NH3 partial pressure to 0.13 atm. This reduces the Ti(III) defect density in the material from 6.06 × 1016 to ∼4.61 × 1015 cm–3, by a factor of 13, based on electron paramagnetic resonance (EPR) spectroscopy. Any remaining Ti(III) defects are localized at the LaTiO2N surface, according to X-ray photoelectron spectroscopy (XPS), due to the formation of a depletion layer in the semico. Optical absorption spectra of the improved LaTiO2N reveal a blue-shifted band gap absorption edge and a suppressed sub-band gap absorption. Defect removal also reduces a sub-band gap surface photovoltage feature visible in the 1.0 atm reference material. The improved LaTiO2N supports a 1.57 mA cm–2 water oxidation photocurrent at 1.23 V RHE under simulated sunlight conditions, and an enhanced quantum efficiency of 4.5% (400 nm) for photocatalytic oxygen evolution from aqueous silver nitrate solution. Stable PEC operation is observed for over 55 min. This confirms that ammonolysis under NH3-limiting conditions improves the solar energy conversion properties of LaTiO2N. The ability to control metal ion defects in oxynitrides by varying the ammonia partial pressure during ammonolysis might be generally useful for the preparation of metal nitrides and oxynitrides.