A systematic study on geometry, electronic structure and vibrational properties of N-doped TiO2 anatase cluster, within the framework of the density functional theory, has been performed in this work. The calculations confirmed that the most structures in substitutional model consist of a two-coordinate bridge structure and a three-coordinate hollow structure. The calculated results can well explain the red shift in N-doped TiO2 observed in experiments. The study provides an illustration for the N-doped anatase from the viewpoint of chemical bonding theory.National Natural Science Foundation of China [20503021]; National Basic Research Program of China [2007CB815301

Cao, Fei

Lin, Meng-Hai

Tan, Kai

Zhang, Qiang-Er

张乾二

曹飞

林梦海

谭凯

Xiamen University Institutional Repository

28 卷 8 期                            结  构  化  学（JIEGOU HUAXUE）                     Vol. 28,  No. 8 2009. 8                                   Chinese  J.  Struct.  Chem.                              998~1002  A Density Functional Study of  N-Doped TiO2 Anatase Cluster①  CAO Fei  TAN Kai② LIN Meng-Hai  ZHANG Qiang-Er  (State Key Laboratory of Physical Chemistry of  Solid Surface & Center for Theoretical Chemistry,  Departmental of Chemistry, College of Chemistry and Chemical  Engineering, Xiamen University, Xiamen 361005, China)  ABSTRACT  A systematic study on geometry, electronic structure and vibrational properties of N-doped TiO2 anatase cluster, within the framework of the density functional theory, has been performed in this work. The calculations confirmed that the most structures in substitutional model consist of a two-coordinate bridge structure and a three-coordinate hollow structure. The calculated results can well explain the red shift in N-doped TiO2 observed in experiments. The study provides an illustration for the N-doped anatase from the viewpoint of chemical bonding theory. Keywords: N-doped TiO2, DFT, HOMO-LUMO gap  1  INTRODUCTION  As an important photocatalyst, titanium dioxide has some evident advantages in applications because of its high efficiency, low price and high thermal sta- bility[1, 2]. However, titanium dioxide photocatalyst can only absorb UV light which is a small fraction of the sun’s energy due to its wide band gap (3.2 eV for anatase). Thus, attempts to increase the photoac- tivity of titanium dioxide catalysts to cover the range of visible light are quite attractive and important.  Some research work is devoted to the application of doping treatments of titania with metal or metal- ions[3, 4]. Recently, non-metal-doped TiO2 was found to be most effective to extend the photoresponse of TiO2 to the visible region[5～11]. Asahi et al.[7] showed that TiO2-xNx, in which N was substituted on the lattice O by sputtering methods, exhibits enhanced photoactivity in the visible spectral range. Since then, more experimental[12, 13] and theoretical[14,15] resear- ches on the N-doped TiO2 have been reported. These theoretical calculations under the plane-wave pseu- dopotential method (LDA or GGA) have evidenced that the substitutional doping of N for O in the anatase TiO2 was effective because N p states con- tribute to the band gap narrowed by effective mix- ing with O 2p states. Furthermore, the synthesized TiO2-xNx nanoparticles are photoactive, with absor- bance that extends into the visible region up to 600 nm. In this regard, ab initio cluster model maybe help to provide information for the N-doped anatase system.  The aim of this work is to investigate the effect of N-doping on anatase electron structure and to find cluster models suitable for modeling the electronic structures of doping anatase nano system. We will present the geometric structures and relative energetic and molecular properties of anatase that are modified by N-doping. Moreover, it provides a bonding analysis to make clear how doping N affects the system. Vibrational properties and                                                                                                 Received 25 September 2008; accepted 20 November 2008 ① Supported by the National Natural Science Foundation of China (No. 20503021) and National Basic Research Program of China (2007CB815301) ② Corresponding author. Tan Kai, born in 1971, associate professor. E-mail: ktan@xmu.edu.cn   2009 Vol. 28               结  构  化  学（JIEGOU HUAXUE）Chinese  J.  Struct.  Chem.                  999  HOMO-LUMO gap are also reported.   2 COMPUTATIONAL DETAILS  TiO2-xNx clusters were computed with the hybrid density functional theory at the B3LYP level[16, 17]. The standard 6-31G(d) basis set[18] for O and N atoms and the small-core RECP (relativistic effec- tive core potential) plus valence double-ζ basis set (LanL2DZ)[19] for Ti were employed. For open shell systems, the spin-unrestricted algorithm was employed. All calculations were performed by using the Gaussian03 program[20]. Geometry optimizations were performed with the BFGS algorithm with no constraints of freedom. Reported stationary points have been confirmed to be local minima by harmonic vibrational analyses at the same theoretical level. The optical energy gap has been performed within the framework of time-dependent density functional theory, employing the hybrid nonlocal exchange-correction functional of Becke, Lee, Yang and Parr (B3LYP).  3  RESULTS AND DISCUSSION  3. 1  Cluster model Clusters (n = 10, 17, 21, 29) cut out from the anatase structures were considered, as shown in Fig. 1. The information about the stability of the cluster models can be obtained by investigating their energies and HOMO-LUMO gaps (Table 1). For the bare Ti17O34 cluster, a HOMO-LUMO gap of 3.58 eV is obtained, with a TDDFT lowest transition of 3.00 eV. Therefore, we simulated the N-doped TiO2 anatase system with (TiO2)n (n = 17, 21) according to the following reasons: (1) Nanostructured anatase contenting 17 or 21 TiO2 molecules with a size about 1 nm. (2) When N replaces one oxygen atom in this cluster model, the stoichiometry is TiO2-xNx with x = 0.05～0.06, which is comparable to that used in the experiment. (3) Suitable band gap and inexpensive computing costs.   Table 1.  HOMO-LUMO Gap, TDDFT Optical Band Gap and  Relative Energies of the Cluster Models of Anatase  Anatase Symmetry HOMO-LUMO gap (eV) TD gap (eV) E(TiO2)/n (kcal·mol-1) Ti10O20 C2 3.91 2.97 −105.25 Ti17O34 D2d 3.58 3.00 −100.65 Ti21O42 D2d 3.69 3.13 −109.21 Ti29O58 S4 3.33 2.79 −113.28   Fig. 1.  Optimized geometries of (TiO2)n cluster models, n = 10, 17, 21, 29  1000                  CAO F. et al.: A Density Functional Study of N-Doped TiO2 Anatase Cluster               No. 8  3.  2  Geometry and stability of the nitrogen      doping in anatase clusters (TiO2-xNx) Different possible doping geometries for TiO2-xNx are given from the optimization. The different doping geometries and energies are given in Table 2.  The lowest energy Ti17O33N cluster found by our calculations is a bridge structure, as shown in Fig. 2, cluster 17c which has the Ti–N bond lengths to be 1.77 and 2.02 Å. The next lowest-energy cluster (17b) is a 3-coordinate hollow structure with only 3.30 kcal/mol higher in energy. The lowest energy Ti21O41N cluster is a 3-coordinate hollow structure (21b) and the next lowest-energy cluster (21c) a bridge structure. This indicates that these two structures have relative stability compared with other structures. The Ti–O bond lengths in anatase clusters are from 1.67 to 2.16 Å, and optimized Ti–N bond lengths are 1.73 to 2.24 Å, indicating replacing an O atom with a N atom does not lead to significant structural changes.   Table 2.  Geometrical Parameters, Relative Energies and Electronic                                 Properties for Ti17O33N and Ti21O41N Clusters  Cluster Structure mode  Ti-N bond length (Å) N atomic spin Gap (eV) Relative energies(Kcal/mol)Ti17O33N a (1c-chain) R1 = 1.75 1.33 1.89 10.66  b (3c-hollow) R1 = R2 = 2.12, R3 = 2.01 0.65 2.80 3.30  c (2c-bridge) R1 = 1.77, R2 = 2.07 0.90 2.75 0.00  d (2c-bridge) R1 = 1.82, R2 = 2.02 0.89 2.38 11.98 Ti21O41N a (1c-chain) R1 = 1.73 1.00 2.29 11.73  b (3c-hollow) R1 = R2 = 2.06, R3 = 2.01 0.91 2.24 0.00  c (2c-bridge) R1 = 1.77, R2 = 2.24 1.00 2.75 2.76  d (2c-bridge) R1 = 1.80, R2 = 2.12 0.89 2.40 14.55   Fig. 2.  Optimized geometries of TiO2-xNx  3. 3  Electronic structures and frequencies  To analyze the electronic properties of N-doped anatase clusters, we calculated different structural modes. The calculated HOMO-LUMO gap and atomic spin of Ti17O33N and Ti21O41N structures obtained at the same theoretical level as that used in the optimization processes are listed in Table 2. The calculated energy gap of pure anatase cluster is about 3.60 eV. Asahi et al.[7] show Ti 3d states to be present throughout the valence band except the very low binding energy region of the valence band, which is attributed almost solely to O 2pπ states. For N-doped anatase cluster, the calculated results show little changes of LUMO, which is completely consistent with the experimental observation that the conduction band edge remains unchanged by nitrogen-doping. However, the HOMO is mainly attributed to N 2pπ states, as seen in Fig. 3. It  2009 Vol. 28               结  构  化  学（JIEGOU HUAXUE）Chinese  J.  Struct.  Chem.                  1001  indicates that N impurities introduce some energy levels above the top of O 2p valance state resulting in a reduction of energy gap by about 0.8 eV in contrast with that of pure anatase cluster. The results are consistent with a previous study and provide a good explanation for the red shift phenomena observed in experiments[8].    Fig. 3.  Model diagrams showing HOMO and LUMO of Ti17O33N and Ti21O41N  In addition, the spin localization on N is demon- strated by the Mulliken population analysis which shows a spin density ≈ 1 on the N atom. EPR investigations of N-doped anatase reveal the pre- sence of paramagnetic centers consisting of single N atoms localized at the bulk oxide lattice[21]. To investigate the vibrational spectra of doping effect, we also computed vibrational spectra of pure anatase and N-doped anatase, as shown in Fig. 4. Similar to the pure anatase cluster, the dominant vibrations occur at 448, 553, 888, 904, 942 and 1013 cm−1.   Fig. 4.  Computed vibrational spectra of pure anatase cluster  (Ti17O34) and N-doped anatase cluster (Ti17O33N)  4  CONCLUSION  N-Doped TiO2 clusters have been studied by using the density functional theory. 3-Coordinate hollow structure and 2-coordinate bridge structure have relative stability compared with other structures. N-doped does not cause significant structural changes. The calculated results show that localized N 2p states are located above the O 2p state, which is responsible for the red shift observed in experi- ments. This localized state leads to a reduction of energy gap of about 0.8 eV. Our calculated results are in good agreement with the experimental data. 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A Density Functional Study of N-Doped TiO2 Anatase Cluster

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A Density Functional Study of N-Doped TiO2 Anatase Cluster

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