Surface Modification of TiO₂ Nanocrystals by WO_<i>x</i> Coating or Wrapping: Solvothermal Synthesis and Enhanced Surface Chemistry

TiO₂ anatase nanocrystals were prepared by solvothermal processing of Ti chloroalkoxide in oleic acid, in the presence of W chloroalkoxide, with W/Ti nominal atomic concentration (<i>R</i>_w) ranging from 0.16 to 0.64. The as-prepared materials were heat-treated up to 500 °C for thermal stabilization and sensing device processing. For <i>R</i>_0.16, the as-prepared materials were constituted by an anatase core surface-modified by WO_<i>x</i> monolayers. This structure persisted up to 500 °C, without any WO₃ phase segregation. For <i>R</i>_w up to <i>R</i>_0.64, the anatase core was initially wrapped by an amorphous WO_<i>x</i> gel. Upon heat treatment, the WO_<i>x</i> phase underwent structural reorganization, remaining amorphous up to 400 °C and forming tiny WO₃ nanocrystals dispersed into the TiO₂ host after heating at 500 °C, when part of tungsten also migrated into the TiO₂ structure, resulting in structural and electrical modification of the anatase host. The ethanol sensing properties of the various materials were tested and compared with pure TiO₂ and WO₃ analogously prepared. They showed that even the simple surface modification of the TiO₂ host resulted in a 3 orders of magnitude response improvement with respect to pure TiO<sub>2