Non-corroding α-alumina@TiO2 core–shell nanoplates appearing metallic gold in colour

Abstract

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)_mCl_n]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.