Hydrothermal synthesis of TiO2 anatase nanocrystals using hexaprismatic-shaped oxo-carboxylate complexes

The molecular structural chemistry of titanium oxo-carboxylate complexes is critically reviewed with particular emphasis on the understanding of the various characterised molecular shapes. Among the four crucial parameters thought to have a structural influence on these shapes (nature of the OR alkoxy group, molar ratio C=O/Ti, stoichiometric ratio S=XCOOH/Ti and chemical nature of the carboxylic X moiety), it was concluded that the nature of R group, the C ratio and the steric hindrance of X were of minor importance. From the available data, it was also concluded that a high S ratio seems to favour corner-sharing versus edge-sharing of TiO6 octahedra and that it was not possible to establish a clear correlation between the pKa value of the carboxylic acid and the observed molecular shape. During this study, a new titanium oxo-carboxylate complex was obtained by reacting titanium isopropoxide with phenylacetic acid (PAAH, X=PhCH2 at S=3/2). This hexaprismatic Ti6O6(OPri)6(PAA)6 complex was characterised by single-crystal X-ray diffraction and used as a molecular precursor of TiO2 anatase nanocrystals after hydrolysis by NMe4OH and autoclaving at T=200 deg C. The size and size distribution of these nanocrystals were found to decrease as the R=Ti/NMe4 ratio was increased. Nucleation and growth of anatase nanocrystals was found to be deeply modified by the presence of phenylacetate ions in the solution, but there was no evidence of an influence of the molecular anisotropy of the molecular precursor on the final shape of the nanocrystals.