Ammonolysis with N2-diluted NH3 suppresses Ta(iv) defects in BaTaO2N and enhances photocatalytic water oxidation

Abstract

BaTaO₂N stands out among oxynitride photocatalysts because of its ability to capture visible light and to drive the photoelectrochemical water oxidation reaction. However, its solar energy conversion performance is limited by electron–hole recombination at Ta(IV) defects in the material. These defects are formed by overreduction of the Ta(V) oxide precursor by excess ammonia under the high temperature conditions during ammonolysis. Here we show for the first time that Ta(IV) defect concentrations can be lowered by conducting the ammonolysis reaction in mixed NH₃/N₂ gas. The obtained BaTaO₂N samples crystallize in the cubic CaTiO₃ structure type and form 200–300 nm faceted nanocrystals, based on X-ray diffraction, scanning electron microscopy, and HRTEM. Electron paramagnetic resonance spectra observe the Ta(IV) defects at g = 1.999 and confirm an 11-fold reduction for the product synthesized in mixed (0.13 : 1.0 vol) NH₃/N₂ gas, equivalent to 1.14 × 10¹⁶ cm⁻³ Ta(IV) ions. This optimized BaTaO₂N has nearly twice the photocatalytic oxygen evolution activity (AQE of 6.78% at 400 nm) of a reference material made with 1.0 atm ammonia and 78% higher photoelectrochemical water oxidation photocurrent (0.9 mA cm⁻² at 1.23 V vs. RHE) under simulated sunlight. According to X-ray photoelectron spectroscopy, remaining Ta(IV) defects are concentrated in the surface region of the BaTaO₂N particles, where >50% of all Ta ions are found in the +4 oxidation state. This surface Ta(IV) population can be directly observed in Vibrating Kelvin Probe Surface Photovoltage Spectra (VK-SPV) via its 1.2–1.4 eV photovoltage onset. It suggests that the surface Ta(IV) ions contribute empty d-states 0.5–0.7 eV below the BaTaO₂N conduction band edge. These findings highlight how the energetics and concentrations of Ta(IV) defects influence the photoelectrochemical water oxidation ability of BaTaO₂N. Additionally, the work establishes ammonolysis with diluted NH₃ as a new tool to minimize defects in BaTaO₂N and to raise its solar energy conversion efficiency toward its theoretical limit. Because of its simplicity, the reduced ammonia pressure strategy will likely be applicable to other oxynitrides, which generally suffer from overreduction problems during ammonolysis.