Surface
Modification of TiO<sub>2</sub> Nanocrystals by WO<sub><i>x</i></sub> Coating or Wrapping: Solvothermal Synthesis and Enhanced Surface
Chemistry
TiO<sub>2</sub> 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><sub>w</sub>) 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><sub>0.16</sub>, the as-prepared materials were constituted
by an anatase core surface-modified by WO<sub><i>x</i></sub> monolayers. This structure persisted up to 500 °C, without
any WO<sub>3</sub> phase segregation. For <i>R</i><sub>w</sub> up to <i>R</i><sub>0.64</sub>, the anatase core was initially
wrapped by an amorphous WO<sub><i>x</i></sub> gel. Upon
heat treatment, the WO<sub><i>x</i></sub> phase underwent
structural reorganization, remaining amorphous up to 400 °C and
forming tiny WO<sub>3</sub> nanocrystals dispersed into the TiO<sub>2</sub> host after heating at 500 °C, when part of tungsten
also migrated into the TiO<sub>2</sub> 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 TiO<sub>2</sub> and WO<sub>3</sub> analogously prepared. They
showed that even the simple surface modification of the TiO<sub>2</sub> host resulted in a 3 orders of magnitude response improvement with
respect to pure TiO<sub>2</sub>
