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 TiIII under anaerobic
conditions and superoxide (O2•–) in the presence of O2. The photoactivities of the POTs
were then probed with TiIII 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.