Effects of Substitutional Dopants on the Photoresponse of a Polyoxotitanate Cluster

In this paper, using a simple method, 17 isostructural polyoxotitanates (POTs) were synthesized, including the pristine [Ti₁₂O₁₆(OⁱPr)₁₆], the monodefected [Ti₁₁O₁₃(OⁱPr)₁₈], and the heterometal-doped [Ti₁₁O₁₄(OⁱPr)₁₇(ML)] (M = Mg, Ca, Zn, Cd, Co, or Ni; L = Cl, Br, I, or NO₃). The electronic structures of these POTs were determined by UV-vis spectroscopy and DFT calculations. Upon UV irradiation of the POTs, electron spin resonance showed the formation of Ti^III under anaerobic conditions and superoxide (O₂^•-) in the presence of O₂. The photoactivities of the POTs were then probed with Ti^III production and short-circuit photocurrent experiments. The photophysical processes were studied using steady-state and transient photoluminescence. The results show that within the very similar structures, the deexcitation processes of the photoexcited POTs can be greatly affected by the dopants, which result in enhanced or decreased photoactivities. Co and Ni doping enhances the absorption of the visible light accompanied by serious loss of UV photoactivities. On the other hand, a Ti vacancy (in [Ti₁₁O₁₃(OⁱPr)₁₈]) does not reduce the band gap of a POT but improves the UV photoactivities by serving as surface reaction site. The POTs were then used as molecular models of titanium oxide nanoparticles to understand some important issues relevant to doped titanate, i.e., coordination environment of the dopant metal, electronic structure, photoactivities, and photophysical processes. Our present findings suggest that for solar energy harvesting applications of titanium oxides like photocatalysis and solar cells substitution of titanium atoms by transition metal ions (like Co and Ni) to extend the absorption edges may not be an efficient way, while loading of Ti vacancies is very effective.