A TDDFT study on TiO2/Graphene Hybrids

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HNUE JOURNAL OF SCIENCE Natural Sciences 2018, Volume 63, Issue 11, pp. 77-81 This paper is available online at http://stdb.hnue.edu.vn DOI: 10.18173/2354-1059.2018-0073 A TDDFT STUDY ON TiO2/GRAPHENE HYBRIDS Duong Quoc Van Faculty of Physics, Hanoi National University of Education Abstract. In this research, ab-initio calculations have been used to study the geometry, electronic structures, adsorption energy and the formation of TiO 2 /Graphene hybrids. The unit cells of TiO2/Graphene have been built, optimized and calculated using GGA-PBE parametrization of exchange-correlation functional. Negative values of adsorption energies show that TiO2/Graphene hybrids are stable and formed naturally. To identify the interactions between TiO2 and Graphene, electronic structures of hybrid models were calculated and analyzed; reconfirmed by calculated results of the changes of electron density distributions. Calculated band-gap, density of states (DOSs) and partial density of states (PDOS) of models imply that the hybrid models have higher photocatalytic activity in visible range than neat TiO2. The research suggests another useful method to improve the visible photocatalytic activity of TiO2 material. Keywords: TiO2/Graphene, adsorption energy, PDOS, band structure, charge density. 1. Introduction TiO2 is one of the most widely studied materials in sciences and technology, especially for photocatalytic degradation of organic compounds [1]. Non-toxic, chemical and physical stability, low cost and easy to produce make TiO2 become one of the most widely used photocatalytic material, especially in water and air treatment [2]. The application of TiO2 is limited by two drawbacks: large band-gap value (~3.2 eV) and high electron-hole pairs recombination rate [3]. Recently, an efficient method was used to enhance the efficiency of photocatalytic processes of TiO2 is compositing with other carbon-like materials such as carbon nanotubes (CNTs) or Graphene [4]. Gholamvande et al. [5] prepare TiO2/Graphene composites using chemical reduction and prove that the sample can completely photodegrade famotidine (C8H15N7O2S3) solution with concentration 100 mg/L in 200 minutes while the neat TiO2 sample only photodegrade about 20%. Pan et al. [6] has successfully synthesize TiO2/Graphene composites using hydrothermal method. The samples have been used to photodegrade methylene blue (MB) solution and show the higher efficiency than neat TiO2 nanoparticles and nanowires. The formation of TiO2/Graphene has been modeled and calculated by Bukowski [7] but the photocatalytic and the formation mechanisms of TiO2/Graphene composites have not been clearly explained. Received August 30, 2018. Revised November 5, 2018. Accepted November 12, 2018. Contact Duong Quoc Van, e-mail address: vandq@hnue.edu.vn 77 Duong Quoc Van In this research, simple model of TiO2/Graphene has been built, optimized to calculate the adsorption energy, band-gap value, density of states and charge density difference; the results can be used to predict the formation and photocatalytic activity of TiO2/Graphene composites. 2. Content 2.1. Computational methods All calculations based on Time Dependent Density Functional Theory (TDDFT) were performed using DMol3 module in Materials Studio software [8]. Generalized Gradient Approximation (GGA) were used for the exchange-correlation functional with the parametrization presented by Perdew, Burke and Ernzerhof (PBE) [9]. The Monkhorst-Pack scheme k-point grid sampling [10] was set at 2×2×1 in the supercell. The electron-ion interactions were modeled using norm-conserving pseudopotentials within the Density Functional Semi-core PseudoPotentials (DSPP) method; the valance configurations of the atoms were 3s23p63d24s2 for Ti, 2s22p4 for O and 2p2 for C. The convergence threshold for self-consistent iterations was set at 10-6 eV. The energy change, maximum force and maximum displacement tolerances in the geometry optimization processes were set at 1×10−5 Ha/atom, 0.002 Ha/Å and 0.005 Å, respectively. 2.2. Results and discussion 2.2.1. Vacuum slabs convergence Figure 1. Unit cell of pseudoperiodic lattice of graphene Figure 2. The dependence of total energy of unit cell of graphene pseudo-periodic lattice on the vacuum slabs thickness Graphene is a 2-dimensional material with parallelogram shaped unit cell, it can not be calculated in DMol3. A new pseudo-periodic lattice was created by laying different graphene sheets in space, paralleled and equidistant from each other. The unit cell of new material lattice is shown in Figure 1: a = b are lattice parameters of graphene, c is the distance between two graphene sheets (or vacuum slabs thickness). The value of c must be chosen to avoid the interactions between two closest sheets in the infinite models. The convergence of total energy of a unit cell when c varies from 1 to 50 Å is shown in Figure 2. The total energy of the unit cell converges when c is larger than or equal to 10 Å; the value of c = 25 Å is chosen for latter calculations. The geometry optimization result shows that graphene has lattice parameter a = b= 2.4481 Å,  =  = 90o and  = 120o; in a good agreement with Dharmendar et al. [11]. 78 A TDDFT study on TiO2/Graphene Hybrids 2.2.2. Calculation for TiO2/Graphene Hybrid * Models of TiO2/Graphene Hybrid The model of TiO2/Graphene hybrid was created by putting a (TiO2)n cluster at a distant to graphene surface to form a new material (denoted as GTO). The selected cluster is (TiO2)5 - large enough to represent for TiO2 but small enough to improve calculation efficiency. The selected graphene sheet has size of 7×7 pristine graphene unit cells, contain 98 C atoms. The initial supercell of GTO has a = b = 17.220 Å, C = 25 Å,  =  = 90o and  = 120o as shown in Figure 3; the red spheres represent for Ti atoms and grey spheres represent for O atoms. The values of  and b are chosen to minimum the interaction between two nearest (TiO2)5 clusters in infinite lattice. * Adsorption Energies of TiO2/Graphene Hybrid The adsorption energy of a hybrid model can be calculated as following formula: Figure 3. Unit cell of TiO2/Graphene hybrid (1) Eads = EGTO - (Egraphene - ETiO2 ) Where EGTO is total energy of TiO2/Graphene hybrid model, Egraphene is total energy of graphene sheet and ETiO2 is total energy of corresponding (TiO2)5 cluster. Two models of (TiO2)/Graphene material with different initial positions of (TiO2)5 clusters have been built. In the first model, the plane of (TiO2)5 cluster was parallel to the graphene sheet while in the second model they are perpendicular to each other. The total energy of former hybrid model is -131778.312 eV and of the latter one is -131779.315 eV, indicates that the perpendicular model is more stable and was used for latter calculations. The calculated adsorption energy of perpendicular model is -1.717 eV, implies that the TiO2/Graphene model are able to form. The large value of formation energy suggests that TiO 2Graphene bonds are chemical and the absorption process of TiO2 on graphene is chemisorption, implies that hybrid materials are stable and formable in reality. * Electronic Structures of TiO2/Graphene Hybrid a b c Figure 4. Projected density of states of (a) GTO, (b) Graphene and (c) TiO2 cluster Figure 4 shows the total density of states (DOS) and projected density of states (PDOS) of GTO, graphene and TiO2 cluster models calculated using DMol3, respectively. Figure 4a shows that TiO2/Graphene is a small band-gap semiconductor, the calculated band-gap of TiO2/Graphene hybrid is 1.074 eV, smaller than the value of 3.2 eV of TiO2 [12]. The reduction of band-gap of 79 Duong Quoc Van TiO2 is caused by the presence of new mid-gap states, which can be related to the localization of hybrid orbitals in TiO2/Graphene system. PDOSs of graphene and TiO2 in Figure 4a and 4b shows that graphene absorbs long wavelength light and make the absorption edge of TiO2 shift to the visible region of electromagnetic spectrum. Figure 4c also shows that 2p and 3d orbitals play the most important roles in the formation of TiO2 band structure, is consistent with previous result [13]. * Charge Density Difference of TiO2/Graphene Hybrid (a) (b) Figure 5. (a) Structure and (b) charge density difference in the (TiO2)5 cluster adsorbed on graphene To have more information on electronic properties and chemical bonding of TiO2/Graphene hybrid, the charge density distribution and difference of the model before and after hybridization. The charge difference was calculated as following equation: (2)   GTO  (  graphene  TiO ) 2 where GTO is charge density of TiO2/Graphene hybrid model, graphene and TiO2 are the charge densities of initial Graphene sheets and (TiO2)5 clusters, respectively. Figure 5a shows the optimized structure of (TiO2)5 cluster adsorbed on a graphene sheet. The shortest distance from (TiO2)5 cluster to the graphene sheet is 3.137 Å; larger than stable distance between TiO2 and CNTs in corresponding hybrid model [14]. The charge density difference of (TiO2)5 cluster adsorbed on graphene was shown in Figure 5b. The reds are the regions where the electron density increase and the blues are the regions where the electron density decrease, is consistent with other calculations [15, 16]. It can be seen that photo-induced electrons in graphene injected into TiO2, reduce the rate of electron-hole recombination and improve the photocatalytic activity of TiO2/Graphene material. The mechanisms of electron-hole recombination in TiO2/Graphene and TiO2/CNTs materials [14] are reversed, indicate that graphene and CNTs play different roles in the improvement of TiO2-based materials. 3. Conclusions TDDFT calculations were successfully used to investigate the electronic properties of (TiO2)5/Graphene hybrid model. The calculated adsorption energy is negative, suggests that TiO2/Graphene materials are easily to form and the adsorption of TiO 2 on graphene is chemisorption. The density of states of TiO2/Graphene material shows the appearance of new 80 A TDDFT study on TiO2/Graphene Hybrids hybrid levels in the mid-gap of TiO2 while the charge density difference indicates the injection of photo-induced electrons from graphene into TiO2. These results show that the presence of graphene decrease both of band-gap value and the recombination rate of electron-hole pairs of TiO2, increasing the photocatalytic activity under visible light. This study suggests another efficient method to improve the visible photocatalytic activity of TiO2. REFERENCES [1] A. Fujishima, X. Zhang, 2006. Titanium dioxide photocatalysis: present situation and future approaches, Comptes Rendus Chimie, 9 (5-6), pp. 750-760. [2] A. Mills, S. L. Hunte, 1997. An overview of semiconductor photocatalysis, Journal of Photochemistry and Photobiology A - Chemistry, 108, pp. 1-35. [3] S. M. Gupta, M. Tripathi, 2011. 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