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

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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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 andtechnology. 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 technicalapplications, for example: gas sensors, solar cells, thin-film batteries, gate electrodesfor electronic devices and photocatalysts. In recent years, most studies of doped TiO2materials have concentrated on their photocatalytic activities, especially for environmentalpollution treatment solutions. Due to its wide band-gap, doped TiO2 film’s photocatalyticeffect is negligible in the range of visible light. Visible photocatalytic effects can beavailable if the band-gap of doped TiO2 materials is narrowed. Impurity doping is themost commonly used method to reduce band-gap and extend the light absorption range ofdoped TiO2 materials from the UV to the visible region and N is one of the most effectiveReceived October 12, 2014. Accepted October 26, 2014.Contact Duong Quoc Van, e-mail address: vandq@hnue.edu.vn150 A DFT study on structural and electronic properties of N-doped anatase TiO2 layersdopants [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 bulkform properties and they used commercial software such as Materials Studio. In this paper, our research concentrates on the N-doped TiO2 layers, which can beconsider ideal films. Calculations were performed using Quantum Espresso (QE) [3], anintegrated suite of Open-Source computer code for electronic-structure calculations andmaterials modeling at the nanoscale, based on the density-functional theory (DFT), planewaves and pseudopotentials. This research can serve as a guidance to understand TiO2thin films properties and can be considered as a first step in the use of Quantum Espresso.2. Content2.1. Computational methods Quantum Espresso, a free and open-source code, was used for calculations.Generalized Gradient Approximation (GGA) are used for the exchange-correlationfunctional and the parametrization of the Perdew-Burke-Ernzerhof (PBE) correlationpotential for homogeneous electron gas was employed [4]. The interaction potentialsbetween ionic cores and valence electrons (3s2 3p6 3d2 4s2 for Ti, 2s2 2p4 for O and 2s2 2p3for N) are described by the Vanderbilt ultrasoft pseudopotential [5]. All calculations were performed for unit cells of N-doped TiO2 layers whichcontains 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 energyfor the plane-wave representation of the wavefunctions in the geometry optimization wasset at 40 Rydberg.2.2. Results and discussions2.2.1. 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 Afollowing lattice parameters: a = 3.7893 A, ˚ and c = 5 × 3.7893 A ˚ (see Figure1a). The value of was selected to confirm the convergence of total energy of TOO-L andto avoid an interaction between two layers in ...