The cross-substitution effect of tantalum on the visible-light-driven water oxidation activity of BaNbO2N crystals grown directly by an NH3-assisted flux method

Abstract

Various perovskite-type transition metal oxynitride photocatalysts have been intensively investigated for photocatalytic water splitting under visible light. The band gap engineering and the formation of solid solution have been demonstrated to be one of the beneficial approaches to achieve a high solar energy conversion efficiency. As a member of the niobium-based oxynitride family, BaNbO₂N has a small band gap energy of 1.79 eV and light absorption up to 690 nm. Here, we have investigated the cross-substitution effect of tantalum on the photocatalytic water oxidation activity of BaNbO₂N crystals grown directly by an NH₃-assisted flux method. It was found that with increasing the amount of tantalum substituted for niobium in BaNb_1−xTa_xO₂N (x = 0, 0.25, 0.50, 0.75, and 1), the average crystal size and the intensity of background absorption gradually reduced and the O₂ evolution rate increased monotonically for BaTaO₂N crystals due to the shift of the top of the valence band to a more positive side and the lowered densities of mid-gap states associated with defects. The CoO_x/flux-grown-BaTaO₂N/Ta photoanode exhibited a photocurrent of about 0.85 mA cm⁻² at 1.2 V_RHE, which decreased gradually by about 35% after 24 h, evidencing that the flux-grown BaTaO₂N crystals are highly stable for photoelectrochemical water oxidation. BaTaO₂N modified with CoO_x (2 wt% Co) as an O₂ evolution cocatalyst exhibited an apparent quantum yield (AQY) of 0.24% at 420 nm for an O₂ evolution reaction in the presence of Ag⁺ ions as sacrificial electron acceptors. In addition, the results from the first-principles DFT calculations conform to the experimentally obtained data on the photocatalytic water oxidation activity of the BaNb_1−xTa_xO₂N crystals.