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I. F. Elegbeleye et al.
(DSSCs) [1, 2]. The interest in TiO 2 as semiconductor for dye sensitized solar cell
is motivated by its non-toxicity and excellent stability upon illumination [1, 3–5].
However, the major drawback limiting its usage is its inability to absorb visible
and infra-red photons of the solar spectrum because of its wide conduction band gap
(3.0–3.2 eV) [5–7]. In DSSCs, dye molecules chemisorbed on the surface of TiO 2 are
used to harness light, and the consequent photoexcited electrons are injected into the
conduction band of TiO 2 [8–10]. An efficient photosensitizer must have good excited
state properties and intense absorption in the visible and near infra-red region of the
solar spectrum. Several dye molecules have been employed as sensitizer for DSSC;
the highest efficiency in excess of 13% has been achieved in cells with nanostructured
TiO 2 semiconductor sensitized by Ruthenium (II) polypyridyl complex(N3) [11–15].
A significant number of studies has been done towards the surface modification
of TiO 2 crystals with atoms/sensitizing dye molecules to step-wise reduce the band
gap and further enhance their activities in the visible and near infra-red region of
the solar spectrum. Polymorphs of TiO 2 have been a model for such studies, aimed
at improving photocurrent yield and light harvesting in DSSCS [3, 16]. Energy
band modulation by elemental doping, monodoping, codoping with nonmetals and
transition metals, and adsorption of dye molecules on TiO 2 surfaces, have been
tested; the results showed improved spectral response and enhanced photocatalytic
performances of TiO 2 [7, 17]. Surfaces of rutile and anatase polymorphs have been
greatly exploited and have been a prototypical model for basic studies on TiO 2 oxide
[18, 19]. Relatively limited work has been done on the brookite form of TiO 2, in
contrast to rutile and anatase polymorphs that has been greatly exploited [4].
A recent study on TiO 2 brookite suggested that it is a good photocatalyst and may
exhibit higher photocatalytic activity than both rutile and anatase [4]. The absorption
edge of brookite observed in a prior study was also reported to be broad and to
extend to the visible region of the solar spectrum, in contrast to steep edges in
the visible region observed for rutile and anatase polymorphs of TiO 2 [19]. Since
brookite surfaces have not been studied and are reported to have better photocatalytic
properties, it is of keen interest to study the interactions of promising ruthenium (N3)
dye molecules with surfaces of brookite TiO 2 for optimization of photon current
density in dye sensitized solar cells.
2 Computational Procedures
Optimization of the ground state geometries of the TiO 2 brookite/ruthenium complex
in the gas phase was performed using DFT with the hybrid B3LYP exchange correlation functional and the LANL2DZ pseudopotential for Ti atom; the 6–311+G (d,
p) basis set was used for the C, N, O and H atoms. All DFT/TD-DFT calculations on
the ruthenium (N3) complex were done using Gaussian 03 quantum chemical package [20]. The UV-Vis simulated absorption spectrum of the TiO 2 /dye complex was
computed by Time dependent (TD) DFT in vacuo with the same functional and basis
set. 80 singlet to singlet electronic transitions were considered for the excitations,
I. F. Elegbeleye et al.
(DSSCs) [1, 2]. The interest in TiO 2 as semiconductor for dye sensitized solar cell
is motivated by its non-toxicity and excellent stability upon illumination [1, 3–5].
However, the major drawback limiting its usage is its inability to absorb visible
and infra-red photons of the solar spectrum because of its wide conduction band gap
(3.0–3.2 eV) [5–7]. In DSSCs, dye molecules chemisorbed on the surface of TiO 2 are
used to harness light, and the consequent photoexcited electrons are injected into the
conduction band of TiO 2 [8–10]. An efficient photosensitizer must have good excited
state properties and intense absorption in the visible and near infra-red region of the
solar spectrum. Several dye molecules have been employed as sensitizer for DSSC;
the highest efficiency in excess of 13% has been achieved in cells with nanostructured
TiO 2 semiconductor sensitized by Ruthenium (II) polypyridyl complex(N3) [11–15].
A significant number of studies has been done towards the surface modification
of TiO 2 crystals with atoms/sensitizing dye molecules to step-wise reduce the band
gap and further enhance their activities in the visible and near infra-red region of
the solar spectrum. Polymorphs of TiO 2 have been a model for such studies, aimed
at improving photocurrent yield and light harvesting in DSSCS [3, 16]. Energy
band modulation by elemental doping, monodoping, codoping with nonmetals and
transition metals, and adsorption of dye molecules on TiO 2 surfaces, have been
tested; the results showed improved spectral response and enhanced photocatalytic
performances of TiO 2 [7, 17]. Surfaces of rutile and anatase polymorphs have been
greatly exploited and have been a prototypical model for basic studies on TiO 2 oxide
[18, 19]. Relatively limited work has been done on the brookite form of TiO 2, in
contrast to rutile and anatase polymorphs that has been greatly exploited [4].
A recent study on TiO 2 brookite suggested that it is a good photocatalyst and may
exhibit higher photocatalytic activity than both rutile and anatase [4]. The absorption
edge of brookite observed in a prior study was also reported to be broad and to
extend to the visible region of the solar spectrum, in contrast to steep edges in
the visible region observed for rutile and anatase polymorphs of TiO 2 [19]. Since
brookite surfaces have not been studied and are reported to have better photocatalytic
properties, it is of keen interest to study the interactions of promising ruthenium (N3)
dye molecules with surfaces of brookite TiO 2 for optimization of photon current
density in dye sensitized solar cells.
2 Computational Procedures
Optimization of the ground state geometries of the TiO 2 brookite/ruthenium complex
in the gas phase was performed using DFT with the hybrid B3LYP exchange correlation functional and the LANL2DZ pseudopotential for Ti atom; the 6–311+G (d,
p) basis set was used for the C, N, O and H atoms. All DFT/TD-DFT calculations on
the ruthenium (N3) complex were done using Gaussian 03 quantum chemical package [20]. The UV-Vis simulated absorption spectrum of the TiO 2 /dye complex was
computed by Time dependent (TD) DFT in vacuo with the same functional and basis
set. 80 singlet to singlet electronic transitions were considered for the excitations,