A study of the optical properties of metal-doped polyoxotitanium cages and the relationship to metal-doped titania

摘要

To what extent the presence of transition metal ions can affect the optical properties of structurally welldefined, metal-doped polyoxotitanium (POT) cages is a key question in respect to how closely these species model technologically important metal-doped TiO_2. This also has direct implications to the potential applications of these organically-soluble inorganic cages as photocatalytic redox systems in chemical transformations. Measurement of the band gaps of the series of closely related polyoxotitanium cages [MnTi_(14)(OEt)_(28)O_(14)(OH)_2] (1), [FeTi_(14)(OEt)_(28)O_(14)(OH)_2] (2) and [GaTi_(14)(OEt)_(28)O_(15)(OH)] (3), containing interstitial Mn(II), Fe(II) and Ga(III) dopant ions, shows that transition metal doping alone does not lower the band gaps below that of TiO_2 or the corresponding metal-doped TiO_2. Instead, the band gaps of these cages are within the range of values found previously for transition metal-doped TiO_2 nanoparticles. The low band gaps previously reported for 1 and for a recently reported related Mn-doped POT cage appear to be the result of low band gap impurities (most likely amorphous Mn-doped TiO_2).