A DFT study on structural and electronic properties of n doped anatase TiO2 layers

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In this research, the structural and electronic properties of N-doped anatase TiO2 layers were evaluated using the density functional theory (DFT). The results show that doping positions of N atoms cause different effects on the size and shape of unit cells of models.

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Nội dung Text: A DFT study on structural and electronic properties of n doped anatase TiO2 layers

JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2014, Vol. 59, No. 7, pp. 150-156 This paper is available online at http://stdb.hnue.edu.vn A DFT STUDY ON STRUCTURAL AND ELECTRONIC PROPERTIES OF N-DOPED ANATASE TiO2 LAYERS Duong Quoc Van1 , Nguyen Minh Thuy1 and Nguyen Huy Viet2 1 Faculty of Physics, Hanoi National University of Education 2 Institute of Physics, Vietnam Academy of Science and Technology, Hanoi Abstract. In this research, the structural and electronic properties of N-doped anatase TiO2 layers were evaluated using the density functional theory (DFT). The results show that doping positions of N atoms cause different effects on the size and shape of unit cells of models. Calculated band structures of doped layers show the appearance of acceptor levels in the band-gaps and the decrease of band-gap values, corresponds to pure layer values. Density of states (DOS) and projected density of states (PDOS) of doped layers show that N 2p orbital play the key role in the appearance of acceptor levels in forbidden bands. Ti 3d and O 2p orbitals still play the most important roles in the DOS of N-doped TiO2 layers. Keywords: TiO2 layers, N-doped, DFT, structural properties, electronic structure. 1. Introduction Thin films have been used in many types of equipments in life, science and technology. Various materials have been used to prepare thin films such as Si, ZnO, InGaZnO and more. Doped TiO2 films have been used in many kinds of technical applications, for example: gas sensors, solar cells, thin-film batteries, gate electrodes for electronic devices and photocatalysts. In recent years, most studies of doped TiO2 materials have concentrated on their photocatalytic activities, especially for environmental pollution treatment solutions. Due to its wide band-gap, doped TiO2 film’s photocatalytic effect is negligible in the range of visible light. Visible photocatalytic effects can be available if the band-gap of doped TiO2 materials is narrowed. Impurity doping is the most commonly used method to reduce band-gap and extend the light absorption range of doped TiO2 materials from the UV to the visible region and N is one of the most effective Received October 12, 2014. Accepted October 26, 2014. Contact Duong Quoc Van, e-mail address: vandq@hnue.edu.vn 150
A DFT study on structural and electronic properties of N-doped anatase TiO2 layers dopants [1]. To understand the mechanism of photocatalytic activity in doped TiO2 , ab-initio calculations based on the density-functional theory (DFT) have been performed [2]. Most of these studies have same disadvantages: they were performed for TiO2 bulk form properties and they used commercial software such as Materials Studio. In this paper, our research concentrates on the N-doped TiO2 layers, which can be consider ideal films. Calculations were performed using Quantum Espresso (QE) [3], an integrated suite of Open-Source computer code for electronic-structure calculations and materials modeling at the nanoscale, based on the density-functional theory (DFT), plane waves and pseudopotentials. This research can serve as a guidance to understand TiO2 thin films properties and can be considered as a first step in the use of Quantum Espresso. 2. Content 2.1. Computational methods Quantum Espresso, a free and open-source code, was used for calculations. Generalized Gradient Approximation (GGA) are used for the exchange-correlation functional and the parametrization of the Perdew-Burke-Ernzerhof (PBE) correlation potential for homogeneous electron gas was employed [4]. The interaction potentials between ionic cores and valence electrons (3s2 3p6 3d2 4s2 for Ti, 2s2 2p4 for O and 2s2 2p3 for N) are described by the Vanderbilt ultrasoft pseudopotential [5]. All calculations were performed for unit cells of N-doped TiO2 layers which contains 4 Ti atoms, 7 O atoms and 1 N atom (in different doping positions). A 5 × 5 × 2 k-point mesh was used in the Brillouin zone sampling for all models [6]. Cutoff energy for the plane-wave representation of the wavefunctions in the geometry optimization was set at 40 Rydberg. 2.2. Results and discussions 2.2.1. Structural properties * Building N-doped TiO2 layers Unit cells of un-doped TiO2 layers (hereafter referred as TOO-L) were built with ˚ b = 9.6072 A following lattice parameters: a = 3.7893 A, ˚ and c = 5 × 3.7893 A ˚ (see Figure 1a). The value of was selected to confirm the convergence of total energy of TOO-L and to avoid an interaction between two layers in the periodic system [7]. Un-optimized unit cells of N-doped TiO2 layers were received from unit cells of the un-doped TiO2 layer by replacing one (of eight) O atom with an N atom. There are 8 different O atoms in TOO-L, therefore there are 8 different models, all of them having the ˚ b = 9.5102 A same lattice parameters: a = 3.6704 A, ˚ and c = 18.9468 A. ˚ 151
Duong Quoc Van, Nguyen Minh Thuy and Nguyen Huy Viet * Structural properties of N-doped TiO2 layers Most of the ab-initio calculations started with structural optimization. In this study, all unit cells of N-doped TiO2 layers were optimized using the variable-cell relaxation method. All atoms in the unit cell are moved to minimize the forces and stresses on them. The optimized structures were received when the forces, the stresses of atoms and also the total energies of unit cells were minimized. To avoid the effect of size limitation in the z-axis, the value of was kept constant during the relaxation process. After relaxation, 8 optimized unit cells of doped layers are received and denoted TON-L-05 to TON-L-12 (all of them will be denoted as TON-Ls), due to positions of the doped N atoms, respectively. Table 1. Lattice parameters of N-doped TiO2 layers before and after optimization Structural information of doped layers before and after optimization is shown in Table 1. Depending on their lattice parameters, optimized unit cells can be divided into 2 groups: G1 and G2. The G1 group has 4 unit cells: TON-L-05, TON-L-06, TON-L-09 and TON-L-10; their lattice parameters have significant changes after optimization. The G2 group has 4 unit cells: TON-L-07, TON-L-08, TON-L-11 and TON-L-12; their lattice parameters do not change much compared to the un-optimized values. Optimized unit cells show different changes in their total energies and volumes after optimization. For the G1 group, both total energy and volume are increase while for the G2 group, the total energy is increased and the volume is decreased such that structures in the G2 group are not as stable as those in the G1 group. Optimized translation units of TOO-L and TON-Ls are shown in Figure 1.The results show significant distortions in the optimized translation units of the G1 group (compared to un-doped TiO2 ) while there is a slightly change in G2. Lattice distortions of translation units (similar to that of unit cells) can be explained as a consequence of doping. The atomic radius and the ionic radius of N2− ion and O2− are very close, and this makes it easy to place N atoms into TiO2 layer lattice but it still causes lattice distortion. 152
A DFT study on structural and electronic properties of N-doped anatase TiO2 layers When N is doped into TiO2 , doped layers can transform to different structures but they will exist in more stable structures. This means that N-doped TiO2 layers tend to transform to structures in the G1 group. For more evidence, the band structure and DOS of two groups will be analyzed. Figure 1. Optimzed translation units of un-doped (a), N-doped TiO2 layers in G1 group (b-e) and G2 groups (g-i) 2.2.2. Bands structure Calculated results show that the band structures of G1 are similar as are the band structures of the G2 group. Figure 2 shows the bands structures of un-doped TiO2 , TON-L-05 and TON-L-08 layers calculated using ab-initio calculations, and the Generalized Gradient Approximation (GGA) for the xc-functional and the parametrization of the Perdew-Burke-Ernzerhof (PBE) correlation potential (hereafter referred to as GGA-PBE). Figure 2. Band structure of un-doped (a) and N-doped TiO2 layers (b, c) The bands structures show that un-doped and doped TiO2 are indirect band-gap semiconductors. A doping effect was shown with the narrowing of band-gap of the 153
Duong Quoc Van, Nguyen Minh Thuy and Nguyen Huy Viet N-doped layers. The band-gap values of G1 are larger than those of G2. The band-gap energies of G1 layers are around 2.45 eV whereas corresponding values for G2 are around 2.0 eV, much smaller than the un-doped TiO2 layer value (∼ 2.55 eV [7]). Recently, Franco et al. [8] estimated that the band-gap of un-doped TiO2 film is around 3.2 eV whereas corresponding values for N-doped TiO2 films depend on the doping concentration and vary from 2.1 eV to 2.6 eV. Wang et al. [18] proved that those values are 3.2 eV and 2.8 eV, respectively. Wei Quin et al. [9] prepared N-doped TiO2 films using a micro-plasma oxidation method and showed that the band-gap values of all samples are around 2.8 eV. Baoshun Liu et al. [10] used a radio frequency reactive magnetron sputtering method to prepare N-doped TiO2 films, finding the band-gap values of 3.11 eV, 2.90 eV and 2.70 eV for samples with N concentrations of 2.35%, 6.70% and 12.6%. It is easy to see that our results are in qualitative agreement with these experimental values. Calculations showed a narrowing of the band-gap of TiO2 layers when N was doped, and a band-gap value decrease when the doping concentration is increased. Bands structures of two groups also show the appearance of acceptor levels on the top of the valence bands. In the G1 group, the separation gap between the acceptor level and valance bands is about 0.5 eV, in good agreement with our previous results [7]; for the G2 group, the acceptor levels overlap on the valance bands. In the band structure of the N-doped TiO2 models, it can be seen the contribution of N is due to the acceptor level in the top of the valence band. The contribution of this level in the G1 group differs from that in the G2 group as can be seen on the band structure of TON-L-05 and TON-L-08. 2.2.3. Density of states * Density of States Figure 3. Density of States (DOS) of 1-layer N-doped TiO2 layers Figure 3 shows the total density of states of un-doped and N-doped TiO2 layers calculated using GGA-PBE. Changing values of band-gap are easily seen here in addition to the varying shapes of total DOS. The contribution of N to the density of states of N-doped TiO2 layers can be seen 154
A DFT study on structural and electronic properties of N-doped anatase TiO2 layers very clearly in Figures 3b and 3c. For layers in G1, acceptor levels are stronger and clearly separate while in G2 they are weaker and overlap with valence bands. * Contribution of partial orbitals Figure 3a shows the partial density of states (PDOS) of un-doped TiO2 layer. Figures 3b and 3c show the PDOS of TON-L-05 (representing G1) and TON-L-08 (representing G2) calculated using GGA-PBE in the band-gap (the range of energy -5 eV to +5 eV). They show that Ti 3d and O 2p still play the most important roles in the band structure of TiO2 , similar with results for un-doped TiO2 bulk. The upper valence bands and lower conduction bands shows a strong hybridization of Ti 3d and O 2p electrons. The acceptor levels on the top of the valence bands are composed predominantly by N 2p orbitals, in agreement with previous results [7]. Figure 4. Contribution of Ti 3d, O 2p and N 2p to total DOS of 1-layer N-doped TiO2 layers 3. Conclusion Quantum Espresso has been successfully installed and a detailed setup for ab-initio calculations using QE has been set. Quantum Espresso has been used to calculate the structural and electronic properties of N-doped TiO2 layers. The calculated results show that N doping leads to a change in lattice parameters or a distortion of unit cells. Band structures of N-doped TiO2 layers show the narrowing band-gap compared to the un-doped sample, which can be considered to be the effect of acceptor levels which appear in the top of the valence bands. Calculated PDOSs prove that the contribution of N to band structures of N-doped TiO2 layers are dominated by the N 2p orbital. The results also show that Ti 3d and O 2p play the most important role in the creation of a band-gap in TiO2 layers. This work serves as a first step in our theoretical study on optical properties of N-doped TiO2 thin films using DFT calculations. Acknowledgments. This work was supported by the Ministry of Education and Training Grant, No. B2014-17-46. 155
Duong Quoc Van, Nguyen Minh Thuy and Nguyen Huy Viet REFERENCES [1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, 2001. Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science, Vol. 293, No. 5528, pp. 269-271. [2] C. D. Valentin et al., 2007. N-doped TiO2 : theory and experiment. Chem. Phys, p. 339. [3] P. Giannozzi et al., 2009. Quantum Espresso: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter, 21, 395502. [4] P. Perdew, K. Burke and M. Ernzerhof, 1996. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 77, 3865. [5] D. Vanderbilt, 1990. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 41, 7892. [6] H. J. Monkhorst and J. D. Pack, 1976. Special points for Brillouin-zone integrations. Physical Review B, Vol. 13, No. 12, pp. 5188-5192. [7] Duong Quoc Van, Nguyen Minh Thuy, Dang Thi Thu Hoai, Nguyen Huy Viet, 2013. Electronic Structure of Ideal N-doped TiO2 Films. The 8th National Conference of Solid States Physics and Materials Science. [8] L. M. Franco et al., 2012. Photocatalytic activity of nitrogen-doped and undoped titanium dioxide sputtered thinfilms. Superficies y Vacío 25(3), pp. 161-165. [9] W. Qin, S. Lu, X. Wu, S. Wang, 2013. Dye-sensitized Solar Cell Based on N-Doped TiO2 Electrodes Prepared on Titanium. Int. J. Electrochem. Sci., 8, pp. 7984-7990. [10] B. Liu, L. Wen, X. Zhao, 2008. The structure and photocatalytic studies of N-doped TiO2 films prepared by radio frequency reactive magnetron sputtering. Solar Energy Materials & Solar Cells 92, pp. 1-10 156