Hydrogen produced from water using renewable energy has been investigated as a next-generation energy carrier. Photoelectrochemical (PEC) water splitting is a potential candidate for clean hydrogen production with reasonable cost. Considering utilization of sunlight as an energy source, photocatalysts need to absorb a wide range of visible light. SrNbO₂N is an attractive material with an absorption edge of 690 nm and works as a photoanode in a PEC system.¹ Photoelectrochemical properties of SrNbO₂N particles are greatly affected by their oxide precursors and nitridation conditions. Therefore it is thought that understanding nitridation mechanism is a critical issue to activate PEC properties of oxynitrides. In this study, a novel oxide precursor SrNbO₃ and subsequently nitrided SrNbO₂N was investigated.

SrNbO₃ oxide precursor was synthesized by a solid state reaction under an Ar flow.² A mixture of Nb and Sr₅Nb₄O₁₅ which was synthesized by calcining a stoichiometric mixture of SrCO₃ and Nb₂O₅ in advance was heated under an Ar flow at 1773 K for 20 h. Obtained oxide precursors were nitrided under an ammonia flow at 1173 K for different time. Photoelectrodes of synthesized particles were fabricated by particle transfer method.³Nitridation process of oxide precursors were investigated based on X-ray diffraction, diffuse reflectance spectra and elemental analysis as which were combined oxygen-nitrogen analysis with inductively coupled plasma atomic emission spectroscopy (ICP-AES) as well as PEC properties.

SrNbO₃ and SrNbO₂N nitrided for more than 20 h both had a perovskite-type structure. At the initial stage of nitridation, however, SrNbO₃ was separated to Sr₅Nb₄O₁₅ and SrNbO_3-xN_y then gradually nitrided to SrNbO₂N as shown in Fig. 1. This phase separation appeared due to introduction of N atoms to oxidize Nb⁴⁺ to Nb⁵⁺. According to progression of nitridation, photocurrent densities improved. This study shows the effect of oxide precursor on the PEC performance of resulting oxynitrides. Nitridation mechanism affected by the crystal structure of oxide precursors is also discussed.

1) M. Kodera, H. Urabe, M. Katayama, T. Hisatomi, T. Minegishi and K. Domen, J. Mater. Chem. A, 2016, 4, 7658–7664.

2) X. Xu, C. Randorn, P. Efstathiou and J. T. Irvine, Nat. Mater., 2012, 11, 595–598.

3) T. Minegishi, N. Nishimura, J. Kubota and K. Domen, Chem. Sci., 2013, 4, 1120–1124.