Local structure investigation of WO_x cluster modified on titanium-substituted hydroxyapatite for promoting charge separation under UV illumination

Abstract

The local structure of WO_x clusters modified on Ti-doped hydroxyapatite (Ti-HAp) photocatalyst surface was analyzed using X-ray absorption fine structure (XAFS) measurements. W L₁- and L₃-edge X-ray absorption near-edge structure (XANES) spectra analyses revealed that WO_x clusters include WO₄ and WO₆ structures and that WO₆ amounts increase concomitantly with increasing WO_x cluster amounts. Results of analyses of W L₃-edge extended X-ray absorption fine structure (EXAFS) spectra confirmed this finding, demonstrating that the average coordination number of W surrounded by O increases from 4 to 5 with increasing WO_x cluster amount. The number of W–O–W bonds increased concomitantly with increasing amounts of modified WO_x clusters, indicating the formation of WO_x cluster aggregates. Assuming that two-dimensional WO_x monolayer domains were formed and that the domain is square, the average WO_x cluster structure was calculated as changing from a monomeric to a trimeric or tetrameric structure with increasing WO_x loading amount. The cluster size obtained from the model is consistent with those found in our earlier experimental study. The cluster structure represented here demonstrated that the modified WO_x clusters included a low coordination structure. Its electronic structure became different from that of doped W or bulk WO₃ and a suitable electron transition path from Ti 3d to W 5d was created, showing high photocatalytic activity.