Two-step synthesis and visible-light-driven photocatalytic water oxidation activity of AW(O,N)3 (A = Sr, La, Pr, Nd and Eu) perovskites

To expand the family of transition metal oxynitride perovskites, the two-step synthesis of a series of tungsten-based metal oxynitride perovskites (EuW(O,N)(3), NdW(O,N)(3), PrW(O,N)(3), LaW(O,N)(3), and SrW (O,N)(3)) and their visible-light-driven photocatalytic water oxidation activity with the assistance of CoOx (2 wt% Co) cocatalyst were studied in this work. The XRD results revealed that the cubic perovskite LnW(O,N)(3) (Ln = Pr, Nd, and Eu) and SrW(O,N)(3) phases and tetragonal perovskite LaW(O,N)(3) phase were successfully synthesized by nitriding their corresponding oxide precursors at 900 degrees C for 10-25 h under an NH3 flow, with minor secondary phases in only PrW(O,N)(3) and NdW(O,N)(3). The highly porous structures of EuW(O,N)(3), LaW(O,N)(3), and SrW(O,N)3 were formed from the segregation of nanocrystals with average sizes of 140, 92, and 160 nm, respectively. The surfaces of the NdW(O,N)(3) and PrW(O,N)(3) crystal structures were covered with plate-like crystals which can be identified as W5N4. No clear absorption edges were observed in the UV-Vis diffuse reflectance spectra of the tungsten-based metal oxynitrides owing to the extensive amount of reduced tungsten species (W5+ and W4+) or metallic tungsten and anion deficiency. Within 5 h of the photocatalytic water oxidation half-reaction, the CoOx-loaded SrW(O,N)(3) crystal structures exhibited the highest photostability and O-2 evolution rate of 3.3 mu mol h(-1) compared with CoOx-loaded LnW(O,N)(3) (Ln = La, Pr, Nd, and Eu) crystal structures due possibly to the highest O/N ratio and more positively positioned top of valence band of SrW(O,N)(3). The present work is expected to stimulate research into the development of more stable and efficient tungsten-based metal oxynitride perovskites in the future. (C) 2016 Elsevier Inc. All rights reserved.