NH3-assisted chloride flux-coating method for direct fabrication of visible-light-responsive SrNbO2N crystal layers

Abstract

Perovskite-type SrNbO₂N crystal layers were prepared on niobium substrates by using an NH₃-assisted chloride flux-coating method. The optimization of synthesis parameters (holding temperature and strontium source : flux molar ratio) was performed using a NaCl–KCl flux. By choosing the optimal synthesis conditions, platelet SrNbO₂N crystals were grown over the entire substrate surface, and each SrNbO₂N platelet has a single-crystalline structure in a cubic symmetry. The optimal crystal layer possesses a well-adhered SrNbO₂N/NbN_x/Nb structure and absorbed photons with wavelengths up to 680 nm. In addition, the optimal SrNbO₂N/NbN_x photoelectrode was used for photoelectrochemical water oxidation (2H₂O → 4H⁺ + 4e⁻ + O₂↑), as one half of the water splitting reaction. A photocurrent density of 113 μA cm⁻² at 1.23 V_RHE was recorded on the SrNbO₂N/NbN_x electrode without any additional co-catalyst loading and treatment under simulated sunlight, due to the higher crystallinity of SrNbO₂N and higher interface-adhesion of the SrNbO₂N/NbN_x/Nb structure, which suppress the recombination of photogenerated electrons and holes at the defects and lead to an increase of the photogenerated electron collection efficiency in a niobium substrate, respectively. This study is the first to address the fabrication of quaternary oxynitride crystal layers on a conductive substrate using an NH₃-assisted flux-coating method.