2020 年 128 巻 10 号 p. 798-804
The local structure of WOx 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 WOx clusters include WO4 and WO6 structures and that WO6 amounts increase concomitantly with increasing WOx 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 WOx cluster amount. The number of W–O–W bonds increased concomitantly with increasing amounts of modified WOx clusters, indicating the formation of WOx cluster aggregates. Assuming that two-dimensional WOx monolayer domains were formed and that the domain is square, the average WOx cluster structure was calculated as changing from a monomeric to a trimeric or tetrameric structure with increasing WOx 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 WOx 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.