Controlling Nb(IV) Defects in SrNbO₂N Oxygen Evolution Photocatalyst by Ammonolysis With Dinitrogen-Ammonia Mixtures

Strontium niobium oxynitride (SrNbO₂N) is a promising, corrosion resistant semiconductor for the visible light-driven water splitting reaction, a non-photovoltaic pathway to green hydrogen fuel. However, SrNbO₂N materials made by ammonolysis usually contain Nb⁴⁺ defect states that cause electron-hole recombination. Here, we demonstrate that such defects can be minimized by synthesizing SrNbO₂N from metal oxides in a mixed 13%:87% (vol) NH₃/N₂ atmosphere. According to electron paramagnetic resonance (EPR), SrNbO₂N made in pure NH₃ contains paramagnetic impurities with g = 2.002 and 2.195, which can be assigned to lattice and surface Nb⁴⁺ defects. These states also cause broad optical absorptions centered at 800 and 1020 nm, respectively, and the lattice defect produces a 1.55-1.63 eV signal in surface photovoltage spectra. The improved SrNbO₂N contains five times fewer lattice Nb⁴⁺ defects (8.95 × 10¹⁵ cm^-3), based on the integrated EPR signal intensity, and supports a water oxidation photocurrent of 1.07 mA cm^-2 at 1.23 V versus RHE under simulated sunlight and an apparent quantum efficiency of 5.1% at 400 nm during photocatalytic oxygen evolution. Based on earlier results with LaTiO₂N and BaTaO₂N, dilution of NH₃ during synthesis appears generally beneficial to transition metal oxynitrides.