Preparation of mixed-phase titanium dioxide nanocomposites via solvothermal processing

Mixed-phase titanium dioxide nanocrystals with varying phase composition were prepared by a low-temperature solvothermal process. We have re-examined the effect of hydrochloric acid on the formation of rutile phase and found that the proportion of the rutile phase in synthesized mixed-phase materials did not increase monotonically with increasing acidity. Rather, there was an optimum HCl/Ti molar ratio for rutile formation when titanium tetra-isopropoxide was used as the titanium precursor. At high HCl/Ti ratios, Cl- and H2O may act as charge-shielding agents, inhibiting the rutile formation during the solvothermal process. A low H2O/Ti molar ratio was necessary for preparing anatase-rutile composites, because the formation of photocatalytically inactive brookite phase was favored at relatively high H2O/Ti molar ratios. In addition, we found the solvothermal processing to be a possible approach to control interparticle connection. Mixed-phase TiO2 nanocrystals synthesized at relatively high H2O/Ti molar ratios possessed abundant surface hydroxyl groups and tended to form micrometer-sized aggregates. We suggest that hydrogen bonding can be utilized to bring TiO2 nanocrystals together, creating solid-solid interfaces upon calcination, thereby potentially facilitating interparticle charge transfer in photocatalytic processes.