Advanced Materials Research Vols. 79-82 (2009) pp 1245-1248
© (2009) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMR.79-82.1245
Online: 2009-08-31
Electronic Structure and Optical Properties of Non-metals (N, F, P,
Cl ,S)-doped Cubic NaTaO3 by Density Functional Theory
Peilin Han1,a, Xiaojing Wang1,b,*, YanHong Zhao1,c, Changhe Tang1,d
College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhehaote 010021, P.
R. China.
a
[email protected], [email protected]
c
[email protected], [email protected]
Keywords: NaTaO3, Electronic Structure, Optical Property, Density Functional Theory
Abstract. Electronic structure and optical properties of non-metals (N, S, F, P, Cl) -doped cubic
NaTaO3 were investigated systematically by density functional theory (DFT). The results showed
that the substitution of (N, S, P, Cl) for O in NaTaO3 was effective in narrowing the band-gap
relative to the F-doped NaTaO3. The larger red shift of the absorption edge and the higher visible
light absorption at about 520 nm were found for the (N and P)-doped NaTaO3. The excitation from
the impurity states to the conduction band may account for the red shift of the absorption edge in an
electron-deficiency non-metal doped NaTaO3. The obvious absorption in the visible light region for
(N and P)-doped NaTaO3 provides an important guidance for the design and preparation of the
visible light photoactive materials.
Introduction
Exploitation of new energy sources has gained considerable attention due to the worldwide
energy shortage and environment pollution [1-2]. Water splitting using a photocatalyst is one of
effective methods for directly obtaining clean and high energy-containing H2 [3]. However, the
number of the reported photocatalysts in a stoichiometric amount with a reasonable activity is very
limited. It is well documented that the perovskite-type alkali tantalates, ATaO3 (A = Li, Na, and K)
show reasonable activities for water splitting under UV irradiation [4]. However, the alkali
tantalates are usually less active under visible-light irradiation. Modifying the crystal structure using
foreign elements can alter the surface morphology, microstructure, and particle size as well, which
directly affects the photocatalytic properties. In this work we investigated the electronic structures
and optical absorption properties of (N, S, F, P, Cl)-doped cubic NaTaO3 by density-functional
theory (DFT) with an aim to obtain some useful information for materials design of the highly
photocatalytic activities.
*
Corresponding author. Telephone: +86-471-4994406. Fax number: +86-471-4992981.
E-mail: [email protected]
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Multi-Functional Materials and Structures II
Calculation Method
NaTaO3 crystallizes in a layered perovskite structure of space group Pm(3)m. The lattice
constant is 3.9313 Å. The supercells (2×2×2) for NaTaO3 were constructed in this calculation.
Lattice parameters and atomic positions of NaTaO3 were optimized by minimizing the total energy
with CASTEP code [5]. Wave functions of valence electrons were expanded to a basis set of plane
waves within specified cutoff energy (Ecut=330 ev). The supercell models of X (N, S, F, C, P,
Cl)-doped NaTaO3 were built by substituting an X atom for either O atom in NaTaO3. Atomic
positions of X-doped NaTaO3 were optimized based on the theoretical lattice constants of NaTaO3
without imposing any symmetry.
Results and Discussion
The structures of pure and X-doped NaTaO3 were calculated by optimizing the lattice
parameters and atomic positions. The brillouin zone in high-symmetry direction was optimized. An
indirect band gap of 1.75 eV was obtained, while (N, S, F, P, Cl)-doped NaTaO3 turned into a direct
band gap semiconductor, which would be a significant advantage for the better photocatalytic
performance due to the effective excitation. It is obvious that a new state appeared near the top of
valence band (VB) for the N, P-doped samples. Fig. 1 is the partial density of states (PDOS). The
VB mainly consists of O 2p and Ta 5d orbitals in the range from -6.0 to 0 eV. The DOS peaks at the
top of valence band are constructed by O 2p orbital, and the contribution of Ta orbital is almost
negligible. On the other hand, the CB below 5.5 eV is mainly composed of the Ta 5d orbital and the
contributions of Na atom and O atom orbital were small. Ta 5d orbitals were divided into two peaks
in the range of 1.5-3.5 eV and of 3.5-5.5 eV as a result of the crystal field splitting in the octahedral
TaO6 environment. For the X-doped NaTaO3, the conduction band below 5.5ev is mainly composed
of the Ta 5d orbital mixed with the O 2p orbital (Fig.2). In the (N, P, S)-doped NaTaO3, except for
the O 2p orbital, the electrons in p orbitals of N, P, S atoms also contributed to the VB at the higher
energy level. The mixing of X and O p orbital resulted in the VB moving up to the higher energies
and the band gaps decreased. While for the (F, Cl)-doped NaTaO3, the 2p orbitals of F atom are
located at the lower energies. The mixing shifted to the lower energies for both VB and CB. The
shift of VB is relatively bigger than CB, which results in an increase in band gap in F-doped
NaTaO3. While for Cl-doped NaTaO3, the shift of CB is relatively bigger, which results in a
decrease of band gap as compared to pure phase NaTaO3.
Advanced Materials Research Vols. 79-82
1247
a
Ta
O
TDOS
-40
-30
-20
s
-10
0
Energy( ev)
p
10
20
d
Fig.2 Profiles of DOS and PDOS for NaTaO3
Fig.3 Profiles of PDOS of X-doped
The optical constants, calculated with ε1 (ω) and ε2 (ω) as input, were presented in Fig. 3 along
with (100) in cubic phase. D1(2.93 eV), D2(4.45 eV), and D3(8.64 eV) come mainly from the
electron transition from the upper valence band peak of O2p to CB of Ta 5d in cubic NaTaO3. The
band of D1, D2, and D3 disappeared in the (N and P)-doped NaTaO3, which can be explained by
the oxygen vacancies because of the inequivalent dopants. The existence of the electronic hole in
valence band makes the transition disappear between the top VB to the bottom CB. The
(F,Cl)-doped NaTaO3 also contain inequivalent dopants. However, the F and Cl doped- NaTaO3
held a distinctive feature, that is, the Fermi level turned into CB due to the more electrons of F and
Cl atom than Oxygen. It means that CB moves to the lower energies while that part of Ta 5d was
not found in the total state density profile, thus D3 peak disappeared originated from the excitation
between the VB (O2p) to the CB (the upper of the Ta5d) due to the vanish of the second part of Ta
5d. While for the isoelectronic compounds by S-doped, a half filled band was not provided, so an
changed concentration of oxygen vacancies is not allowed. But the D3 peak was not found due to
the same reason as F,Cl doped samples do. The absorption curves obtained from the imaginary part
of the dielectric constant was presented in Fig.4. F-doped enlarged the forbidden band width as
discussed in the last section and the blue shift was found in the optical absorption. The absorbed
peaks shift toward the lower energies due to the narrowed band gap for the doping by non-metal N,
P, S, and Cl element. The N, P-doping clearly showed a contribution to the absorption in the visible
region.
1248
Multi-Functional Materials and Structures II
Fig.3 Calculated dielectric functions of
N, P, F, CL, S-doped and pure NaTaO3.
Fig.4.The absorption spectra of pure
NaTaO3 and X-doped NaTaO3.
Conclusions
The electronic structure and optical properties of several different nonmetal-doped NaTaO3 were
investigated by density functional method. The dielectric function and the absorption properties
were presented. The calculated results showed that the substitution of O atom by the non-metal may
induce some change of energy band structure due to the inequivalent dopants. The band gap was
narrowed for the N, P, S, Cl-doped NaTaO3 while it was enlarged for the F-doped NaTaO3. The
absorption edge shifted to the lower energies due to the narrowed band gap. The obvious absorption
in the visible light region showed that the non-metal doping may provide a potential method to
prepare the photocatalyst of effective solar energy absorption.
Acknowledgements
This work was supported by the Chinese National Science Foundation (20863004) and Inner
Mongolia National Science Foundation (200607010202).
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Multi-Functional Materials and Structures II
10.4028/www.scientific.net/AMR.79-82
Electronic Structure and Optical Properties of Non-Metals (N, F, P, Cl, S)-Doped Cubic NaTaO3 by
Density Functional Theory
10.4028/www.scientific.net/AMR.79-82.1245
DOI References
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