Figure 7: The schematic of charge transfer from dye to
Semiconductor TiO2
The direct photoinjection is usually indicated by appearance of an
additional peak in excitation spectrum where no additional peaks are
observed in indirect photoinjection
[39, 40,
81-83]. In this work a
(TiO2)96QD having 288 atom and size 2.7
nm is utilized to model the photoinjection in
(TiO2)96-D complex where D = D1, D1A,
D2, D2A, D3 and D3A. In order to prepare the series of complexes, the
dyes were adsorbed on the surface of
(TiO2)96 in dissociative monodentate
mode in which OH and O of the anchoring group were attached with
respective oxygen and titanium atoms on surface of
(TiO2)96[39,
84]. SCC-DFTB approach was utilized in
order to get optimized geometries of
dye-(TiO2)96 complex with parameters
utilized in these simulations are org/trio 0-1. The relaxed geometries
of the complex were obtained by allowing the dye and its anchoring
TiO2 units to take part in geometry optimization whereas
the rest of TiO2 units were kept fixed in order to
lessen the computational cost. UV-Vis excitation spectra were then
obtained for the optimized complex at SCC-DFTB level of theory in order
to model the photoinjection in the mentioned complexes. The spectra
obtained for all six complexes show small red shift and broadening of
peaks in comparison to the bare dyes. The behavior of all dyes is
changed after absorption on (TiO2)96whereas the extent of red shift in the adsorbed dyes is also different.
The values of red shift for D1 and D1A are 0.26 eV and 0.21 eV
respectively. D2 exhibited largest red shift of 0.85 eV whereas D2A, D3
and D3A show red shift of 0.14 eV, 0.04 eV and 0.1 eV respectively. The
spectra of the adsorbed dyes shows a red shift that attribute to
hybridization of orbits in density orbit stabilization
[84]. This behavior points to the
indirect photoinjection in these dyes. The excitation spectra of bare
dyes and dyes attached to TiO2QD are given in figure 8.