Controlling Nb(IV) Defects in SrNbO(2)N Oxygen Evolution Photocatalyst by Ammonolysis With Dinitrogen-Ammonia Mixtures.

Strontium niobium oxynitride (SrNbON) is a promising, corrosion resistant semiconductor for the visible light-driven water splitting reaction, a non-photovoltaic pathway to green hydrogen fuel. However, SrNbON materials made by ammonolysis usually contain Nb defect states that cause electron-hole recombination. Here, we demonstrate that such defects can be minimized by synthesizing SrNbON from metal oxides in a mixed 13%:87% (vol) NH/N atmosphere. According to electron paramagnetic resonance (EPR), SrNbON made in pure NH contains paramagnetic impurities with g = 2.002 and 2.195, which can be assigned to lattice and surface Nb 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 SrNbON contains five times fewer lattice Nb defects (8.95 × 10 cm), based on the integrated EPR signal intensity, and supports a water oxidation photocurrent of 1.07 mA cm 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 LaTiON and BaTaON, dilution of NH during synthesis appears generally beneficial to transition metal oxynitrides.