Non-corroding α-alumina@TiO2 core–shell nanoplates appearing metallic gold in colour
Non-corroding α-alumina@TiO2 core–shell nanoplates exhibiting a lustrous metallic gold colour were synthesized using sol–gel solution chemistry with controlled reaction conditions for the formation of anatase and rutile TiO2 nanocrystals. Due to the small mismatch of lattice parameters between rutile TiO2 and two-dimensional α-alumina nanoplates, the rutile TiO2 could be conformably grown on the surface of the α-alumina nanoplates, however, the lustring effect was deteriorated by back scattering due to the formation of rough needle-shaped rutile TiO2. The surface roughness of the TiO2 nano-shell was controlled by controlling the pH of the reactant solution because rutile phase TiO2 nanocrystals tend to be formed in more acidic conditions, whereas anatase phase TiO2 is likely to be promoted in less acidic conditions, of which anatase small nanocrystals smooth the surface of the TiO2 nano-shell during the crystal growth. The successful formation of a smooth TiO2 nano-shell via controlled crystal growth was associated with the fact that the intermediate complex of [Ti(OH)mCln]2− (m + n = 6) tends to cause corner sharing in more acidic conditions and edge sharing in less acidic conditions. Apparently the α-alumina@TiO2 core–shell nanoplates prepared in more and less acidic conditions showed a metallic gold colour, however, the core–shell nanoplates synthesized in less acidic conditions exhibited a better lustring effect than those from the more acidic conditions due to the formation of a smooth TiO2 shell.