Indian Journal of Chemistry
Vol. 46A, June 2007, pp. 895-898
DFT studies on anionic hetero atom (N or/and S) substitution in TiO2
M Sathish, M Sankaran, B Viswanathan* & R P Viswanath
National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
Email: [email protected]
Received 17 January 2007; revised 24 April 2007
The variation in the band structure, band gap and stability of the hetero atom substituted TiO2 have been studied. Study
shows that S doping has considerable role in the reduction of the band gap, Eg. In addition, it has been shown that sulphur
doping at the oxide ion position is preferred.
Hetero atom doped TiO2 has received considerable
attention as a material. The reason for this increased
attention is due to shift of the absorption of light to
the visible region in the doped systems as compared
to the undoped system which absorbs only in UV
region. TiO2 is a well-known photocatalyst, because
of its favourable band edge positions and stability
against photocorrosion. However, it is active only for
UV light. The solar spectrum consists of only 4-5%
UV light, which restricts TiO2 for harvesting solar
light. Various attempts like doping by metal ion and
dye sensitization have been made to activate the TiO2
in the visible region, but none of these methods have
been successful. The main drawback of metal ion
doped systems is insufficient overlapping of valence
and conduction band orbitals with the doped
metal/metal ion orbitals, which results in insignificant
alteration in the resulting band structure of the doped
TiO2. Also, the stability of the metal ion doped TiO2
against photocorrosion and solubility of dyes are the
added drawbacks associated with these methods.
Hetero atom doped TiO2 has several advantages.
Various preparation methods, and characterization
techniques have been reported for the non-metallic
hetero atom doped TiO2 particularly for N and S
doped TiO21-4. The knowledge of the chemical state of
the doped hetero atom and its contribution to the band
gap reduction in hetero atom doped TiO2 are essential
for further developments in this direction. With this in
view, a few density functional theory (DFT)
calculations have been performed and reported in
literature5-8. Asahi et al. 5 have studied the effect of
hetero atom (F, N, C, S and P) doping in TiO2 and the
results show that substitutional doping of N atom
reduces the band gap. Umebayashi et al.6,7 have
studied the band gap narrowing of anatase and rutile
TiO2 by substitution doping of S in the oxygen
position. Similarly, Ohno et al.8 have also studied the
substitution of S in the Ti position. The presence of S
as anionic (S2-) and cationic (S6+) forms in the TiO2
lattice have been reported, both by XPS studies and
theoretical results in the former and latter case,
respectively.
The present study has been carried out to
understand and explain the band gap narrowing in the
doped TiO2. The study is focused on both (i) cluster
model DFT study and (ii) first principle band
calculations using super cell approach. Sulphur
doping is also possible at the oxide ion sites in TiO2.
This study attempts to examine this possibility as well
as the consequences that could result from such a
replacement.
Computational Model and Methodology
Cluster model (Ti5O14H8) used for the quantum
chemical calculations was taken from the crystal
lattice of anatase TiO2 which consist of 5 Ti atoms
and 14 oxygen atoms and the edge position is
saturated with hydrogen to avoid the edge effect
(Fig. 1). The effect of N and S substitution in the
oxygen position of the cluster has been studied. All
DFT calculations have been carried out by Becke
three parameter hybrid functions with the LYP
correlation function (B3LYP) and an effective core
potential basis set of 6-31g (d, p) level9,10 using the
Gaussian 98 program11. In the calculations, the
geometry of the cluster has been optimized by
Universal Force Field (UFF 1.02) approach using
896
INDIAN J CHEM, SEC A, JUNE 2007
Fig. 1—Model of the cluster (Ti5O14H8). [The position of the
replaced oxygen is shown by an arrow].
Cerius2 software12. Using force field optimized
parameters DFT single point energy and bond
population analysis calculations have also been
carried out.
To study the details of the band gap engineering
due to doping of N and S in the crystal lattice, DOS
(density of states) have been calculated by utilizing
primitive unit cell of the TiO2 anatase crystal structure
(Fig. 2). The doping effects are modelled by replacing
one oxygen atom with one doping atom. The planewave-based DFT calculation13-15 was carried out using
CASTEP program in Materials Studio supplied by
Accelerys with the core orbital replaced by ultrasoft
pseudopotentials, and a kinetic energy cutoff of 300
eV. All the electronic band structures and the optical
absorption spectra were calculated on the
corresponding optimized crystal geometries. The
Generalized Gradient Approximation (GGA) with the
PW91 exchange correlation function has been
adopted.
Results and Discussion
The total density of states for hetero atom (N, S, N
and S) doped and undoped TiO2 have been computed.
The band gap reduction can be seen in the case of
hetero atom doped TiO2 when compared to undoped
TiO2. The reduction in the band gap is attributed to
the orbital mixing of hetero atom with oxygen 2p and
Ti 3d orbitals. It is well-known that the valence and
conduction bands of TiO2 are mainly formed due to
the major contribution by completely filled oxygen 2p
orbitals and the empty Ti 3d orbital, respectively. Due
to energetic equality, the 2p and 3d orbital of the
doped hetero atom contribute significantly to both, the
valence and conduction bands, by mixing with Ti 3d
and oxygen 2p orbitals. Bonds between atoms with a
large electronegativity difference (greater than or
Fig. 2—Model of the primitive TiO2 crystal (The position of the
replaced oxygen is shown by an arrow. For N, S co-doping,
another oxygen which has bonded on the same Ti at 180° is
replaced).
equal to 1.7) are usually considered to be ionic, while
values between 1.7 and 0.4 are considered as polar
covalent. The value of electronegativity difference
between Ti and oxygen is 1.9 while that for Ti and
nitrogen is 1.5 and that for Ti and sulphur is 1.04.
Therefore, when substituting O with N or S, the
electronegativity difference) is reduced significantly,
which will increase the covalent character of the
bond. The increase in the covalent nature results in
reduction of the band gap due to the destabilization of
filled 2p orbital and stabilizing unfilled 3d orbital of
anion and the cation respectively. The density of
states of N doped TiO2 shows broadening in the top of
the valence band and bottom of the conduction band.
It is seen from Fig. 3 that both the valence and
conduction band structures have been altered as a
result of doping by N and S (curves b, c and d). Ohno
et al.8 considered the possibility of one sulphur atom
replacing one Ti atom. Their results showed that this
substitution results in extra occupied states below and
above the valence band. They have argued that the
extra DOS on the top of the valence band resulting
from S substitution are responsible for the photo
response of doped TiO2 in the visible region. In the
case of sulphur doped TiO2 (curve c), the bottom of
the conduction band has been altered significantly.
This may be due to the formation of additional energy
levels at the bottom of the conduction band.
The energy of S 3p orbital is higher compared to
that of N 2p and O 2p, and extent of overlapping with
O 2p will be lesser than that obtained with N 2p. In
SATHISH et al.: DFT STUDIES ON HETERO ATOM SUBSTITUTION IN TIO2
addition, it is speculated that the overlapping of empty
S 3d orbital with the Ti 3d orbital in addition to 3p
orbital results in conduction band broadening. The
cluster model study shows that contribution of S
orbital in the conduction band is more than in the
valence band; 9.16% and 2.36% for the former and
latter cases, respectively, which supports our
speculation. This is further confirmed from the N, S
co-doped TiO2 system (curve d), where it can be seen
that both the conduction and valence bands are altered
by the orbital mixing of doped N and S atoms. There
is no difference in the conduction and valence bands
between the sulphur doped and N, S co-doped TiO2,
though the top of the valence band is destabilized like
N-doped TiO2. The band gap are in the order: TiO2 >
N-TiO2 > S-TiO2 ≥ N, S co-doped TiO2. This
observation shows good correspondence with band
gap calculated from the cluster model calculation
(Table 1) and the literature reports6,7. Also, the
magnitude of band gap reduction can be explained
based on the electronegativity values16: sulphur is
more electronegative than nitrogen atom and shows
Fig. 3—Total density of states for (a) undoped TiO2; (b) N-doped
TiO2 (c) S-doped TiO2 and (d) N, S co-doped TiO2.
more covalent nature, and thus shows more reduction
in the band gap than N-TiO2. Similarly, in N, S doped
TiO2, two oxygen atoms are replaced by N and S,
which results in more covalent character and more
reduction in the band gap.
It can also be seen from Fig. 3, that the total width
of the valence and conduction band (shown as A
and B) increases significantly for the doped TiO2 as
compared to that of the undoped TiO2. Between the S
and N doped TiO2, the former has higher band width
both in valence and conduction band than the latter.
This also suggests that 3s, 3p and 3d orbitals of S are
mixing with TiO2 valence and conduction band
orbitals and thereby decrease the band gap. The band
gap, orbital contribution and the stabilization energy
obtained from the cluster (Ti5O14H8) model DFT
studies are given in Table 1.
It can be seen from the table that, when an oxygen
atom in the Ti5O14H8 cluster (shown by arrow in
Fig. 1) is replaced by N or S, the band gap is reduced.
The band gap of undoped TiO2 cluster is 2.00 eV,
which is reduced to 1.31 and 1.71 eV for the N and S
doped TiO2 clusters respectively. Experimental
studies reported in literature for N and S doped TiO2
also show reduced values for the band gap as
compared to that of the pure TiO217-19. The stability of
the cluster is also reduced for the N and S doped TiO2,
while the magnitude is more for the S doped TiO2
compared to N doped TiO2 due to the larger size of
the S atom as compared to that of N or O. The %
orbital contribution of Ti in the conduction band is
almost the same for the doped and undoped clusters,
whereas the valence band contribution increases in the
order: undoped < N doped < S doped TiO2. This may
be due to the mixing of Ti 3d orbital with N 2p and S
3p orbital. The oxygen orbital contribution in the
conduction band is more or less constant for the all
the three clusters. It decreases for the N doped cluster
due to the significant contributions of N 2p orbital in
the valence band. In S doped cluster, the oxygen
Table 1—Band gap, stabilization energy and % ε orbital contribution of pure and hetero atom (N, S) doped Ti5O14H8 cluster
Cluster
VB
Ti (%)
O (%)
N (%)
S (%)
H (%)
ΔE (Hartree)
B.G. (eV)
Ti5O14 H8
CB
8.3
89.61
1.92
-6.3859
2.00
83.5
15.78
0
VB
22.55
31.99
45.06
0
-6.2595
1.31
897
Ti5O13 N H8
CB
77.87
16.76
0.26
4.9
VB
36.54
60.72
2.36
0
-6.1961
1.17
Ti5O13 S H8
CB
78.41
12.24
9.16
0
898
INDIAN J CHEM, SEC A, JUNE 2007
TiO2. The magnitude of band gap reduction increases
with decrease in the electronegativity difference
between Ti and hetero atom. The results of this study
show that S doping has an impact on the band gap
reduction. The doping of S atom in the TiO2 replaces
possibly the oxygen atoms, and not the Ti atoms in
the lattice.
Acknowledgement
We thank Department of Science and Technology
(DST), New Delhi, for research funding and
University Grants Commission (UGC) New Delhi, for
a fellowship to one of the authors (MS).
References
Fig. 4—Total density of states for (a) Undoped TiO2; (b) S-doped
in oxygen position in TiO2, and, (c) S doped in the Ti position in
TiO2.
contribution is higher than that of the N doped cluster,
but still lower than in the undoped TiO2. This can be
attributed to the inefficient mixing of S 3p orbital with
O 2p orbital.
Figure 4 shows the density of states of pure TiO2,
and sulphur doped in oxygen and Ti positions in TiO2.
It can be seen from the figure that when oxygen atom
is replaced by S, the valence band and conduction
band structure is altered significantly in addition to
the increase of the bandwidth. Formation of additional
energy levels at the bottom of the conduction band
has been observed, while when S replaces Ti, both the
valence and conduction bands are stabilized. Though
the band gap appears to be reduced to almost the same
value as in the case of others, due to the small number
of states, the light absorption will be restricted. This
observation suggests that S replaces the oxygen and is
doped as S2- in the TiO2 lattice rather than as S6+ by
replacing titanium in the TiO2 lattice.
The above study shows that the band gap reduction
in the TiO2 may be achieved by substitutional doping
with non-metallic hetero atoms like N and S in the
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Sato S, Chem Phys Lett, 123 (1986) 126.
Chen X & Burda C, J Phys Chem B, 108 (2004) 15446.
Sakthivel S & Kisch H, Chem Phys Chem, 4 (2003) 487
Umebayashi T, Yamaki T, Tanaka S & Asai K, Chem Lett,
32 (2003) 330.
Asahi R, Morikawa T, Ohwaki T, Aoki K & Taga T, Science
293 (2001) 269.
Umebayashi T, Yamaki T, Itoh H & Asai K, Appl Phys Lett,
81 (2002) 454.
Umebayashi T, Yamaki T, Yamamoto S, Miyashita A,
Tanaka S, Sumita T & Asai K, J Appl Phys, 93 (2003) 5156.
Ohno T, Akiyoshi M, Umebayashi T, Asai K, Mitsui T &
Matsumura M, Appl Catal, 265 (2004) 115.
Becke A D, J Chem Phys, 98 (1993) 5648.
Lee C, Yang W & Parr R G, Phys Rev B, 37 (1988) 785.
Gaussian98, Rev A.9, (Gaussian Inc., Pittsburgh, PA) 1998.
Rappe A K, Casewit C J, Colwell K S, Goddard W A & Skiff
W M, J Am Chem Soc 114 (1992) 10024.
Blaha P, Schwarz K, Sorantin P & Trickey S B, Comput
Phys Commun, 59 (1990) 399.
Perdew J P, Burke K & Ernzerhof M, Phys Rev Lett, 77
(1996) 3865.
Kohn W & Sham L J, Phys Rev, 140 (1965) 1133.
Viswanathan B, Bull Cat Soc India, 2 (2003) 71.
Sathish M, Viswanathan B, Viswanath R P & Gopinath C S,
Chem Mater, 17 (2005) 6349.
Madhusudan Reddy K, Baruwati B, Jayalakshmi M, Mohan
Rao M & Manorama S V, J Solid State Chem, 178 (2005)
3352.
Diwald O, Thompson T L, Zubkov T, Goralski Ed G, Walck
S D & Yates, J T Jr, J Phys Chem B, 108 (2004) 6004.