A DFT study on N-doped TiO2 anatase

In order to understand the photocatalytic mechanisms of N-doped TiO2 with different doping positions of N, this research performed ab-initio calculations based on density functional theory (DFT) without and with Hubbard U correction, concentrate on the electronic structure of the materials.
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A DFT study on N-doped TiO2 anatase JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0045 Mathematical and Physical Sci., 2016, Vol. 61, No. 7, pp. 157-164 This paper is available online at http://stdb.hnue.edu.vn A DFT STUDY ON N-DOPED TiO2 ANATASE Duong Quoc Van, Le Minh Thu, Nguyen Manh Nghia and Nguyen Minh Thuy Faculty of Physics, Hanoi National University of Education Abstract. In order to understand the photocatalytic mechanisms of N-doped TiO2 with different doping positions of N, this research performed ab-initio calculations based on density functional theory (DFT) without and with Hubbard U correction, concentrate on the electronic structure of the materials. The adopted value of effective Hubbard U for Ti 3d is 8.18 eV; the corresponding calculated band-gap value of TiO2 with this value of U is 3.201 eV, in a good agreement with experiment value. The calculated results show that substitutional doping is easier to form than interstitial doping in the N-doped TiO2 material. Band-gap of defective models are decreasing, lead to the shifting of edges of absorption to the visible light region. Keywords: N-doped TiO2 , substitutional, interstitial, oxygen vacancy, DFT and DFT + U.1. Introduction Since the first demonstration of Fujishima and Honda in 1972 [1], TiO2 has been consideredas one of most important material due to nontoxic, chemical and physical stability, low cost andphotocatalytic activity. However, band-gap value of pure anatase TiO2 is approximates 3.2 eV andonly absorbs ultraviolet light with wavelengths shorter than 387 nm. The limitation due to wideband-gap lead to the ineffective of TiO2 photocatalytic activity in the region of visible light. Toreduce this limitation, modified TiO2 has been studied for the extending of optical absorption tothe visible light region. After Asahi et al. [2] reported that doping N into TiO2 lead the optical absorption extendsto the visible region, N-doped TiO2 has been studied widely all over the world. N-doped TiO2 hasbeen prepared in different shapes such as powders [3], films [4-7] and studied for photocatalyticactivity in visible light region. The shifting of absorption edge of N-doped TiO2 to the visibleregion was hypothesized by Asahi et al. [2] from the hybrid of N 2p and O 2p and narrowedband-gap of N-doped TiO2 whereas Irie et al. [3] considered the reason from the appearance ofN 2p doped level above the valence band. Effects of oxygen vacancies and its contribution to theabsorption spectra of N-doped TiO2 were reported [8-10]. Theoretical calculations of Valentinet al. [11] showed that the energy costs to form oxygen vacancies in TiO2 reduced when nitrogenis doped. Rumaiz et al. [12] indicated that the defect level of oxygen vacancies above the valenceband maximum is the reason for the knee formation in the optical absorbance spectra of N-dopedTiO2 . Effects of interstitial and substitutional N-doping states were reported by Lee et al. [13],Received September 30, 2016. Accepted October 27, 2016.Contact Duong Quoc Van, e-mail address: vandq@hnue.edu.vn 157 Duong Quoc Van, Le Minh Thu, Nguyen Manh Nghia and Nguyen Minh Thuysuggested that the interstitial N-doping states with the oxygen deficiency were more effective forphotocatalysis than the substitutional N-doping states with the oxygen deficiency. All of thesetheoretical calculations have been investigated for the mechanisms of photocatalytic activity ofN-doped TiO2 , most of them greatly underestimated the band gap of TiO2 due to the adoption ofconventional density functional theory (DFT) method. Wu et al. [14] used Generalized GradientApproximation (GGA) with Hubbard [15] parameter (GGA+U) to investigate the mechanism ofphotocatalytic activity in N-doped TiO2 . In this research, we both use GGA and GGA+U approaches to investigate electronicstructure of the N-doped anatase TiO2 with oxygen vacancies systematically to comprehend themechanisms of photocatalytic activities of materials.2. Content2.1. Computational methods In this research, a 2 × 2 × 1 supercell of pure anatase TiO2 containing 16 Ti atoms and32 O atoms (labeled as TOO) was used to studied (Figure 1a). The effects of N doping or Oremoving in TiO2 on structural and electronic properties were investigated using the modifiedsupercells as shown in the Figure 1 (b-f). The substitutional N-doping supercell (labeled as TON-s)was constructed by substituting one O atom with one N atom, the interstitial N-doping supercell(labeled as TON-i) was constructed by embedding one N atom into the interspace and the oxygenvacancy T ...