Hydrothermal synthesis of anatase nanocrystals with lattice and surface doping tungsten species

We have synthesized anatase nanocrystals of 8–15 nm containing lattice and surface doping tungsten through hydrothermal reactions. We used phosphotungstate–titania nanocomposites with various compositions as the reagents. The homogeneous distribution of tungsten in the reagent solids appears to strongly influence the synthesis results. X-Ray diffraction and Raman spectroscopy data showed that the samples contained brookite impurities when the tungsten content was low (up to W/(W + Ti) = 3.7%) but pure anatase was obtained when the tungsten content was high (up to W/(W + Ti) = 26%). The tungsten content in the anatase nanocrystals is reflected in the lattice parameters, the Ti(W)–O bond distances, the Raman peak positions and peak widths. Tungsten atoms are present as lattice doping and surface coating (wolframyl group). The relative proportion between these species changes with the tungsten content: at low tungsten-content, the lattice doping tungsten is the dominating species that expands the lattice. Around the critical concentration of W/(W + Ti) = 13%, the amount of surface wolframyl group increases while the lattice doping appears to have reached a saturation. A formation mechanism that explains most of the observations was proposed.