Surface Energies and Electronic Structures of TiO 2 Surfaces Surrounded by Fluorine Atoms

1. Surface Energies and Electronic Structures of TiO2 Surfaces Surrounded by Fluorine Atoms Results and Discussion Introduction Conclusions Computational Methods C.H. Sun, H.G. Yang, S.Z. Qiao, G. Liu, J. Zou, S.C. Smith,H.M. Cheng and G.Q. (Max) Lu* ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology & School of Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Email: maxlu@uq.edu.au Centre for Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, Australia CMM Centre, School of Engineering, The University of Queensland, QLD 4072, Australia Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Figure 3: Calculated total (black) and projected density of states (PDOS, with cyan, red and blue lines for F, O and Ti). PDOS ranging from -18.0 eV to 1.0 eV contributed by O1 and O2 are also shown in the insets with black and red symbols, respectively, where O1 and O2 refer to three- and two-coordinated oxygen atoms. The current interest in titanium dioxide (TiO2) for advanced photochemical applications has prompted a number of studies to analyze its surface stability and reactivity with an aim to synthesize highly active surfaces [1-3]. Unfortunately, surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. So it is an open challenge to synthesize highly active surfaces through controlling their surface chemistry. For instance, in the case of anatase TiO2, (101) is the most stable and frequently observed one among various low-indexed surfaces, due to its relatively low surface free energy (γ=0.44 J/m2 [4]). However, the high stability/low reactivity of the majority (101) surface makes it often difficult to understand the observed reactivity of anatase. A typical example is that recent sum frequency generation studies of the coadsorption of water and methanol on anatase nanoparticles show the presence of hydroxyls and methoxy groups, with the latter being strongly bound to the surface and capable toreplacing surface hydroxyls [5-6], which cannot be resulted by (101) surfaces on which both water and methanol can only molecularly adsorbed [7-9]. Gong et al. believed that the minority (001) surface exhibits a high reactivity and can account for the above results based on detailed density functional theory (DFT) calculations [10]. Thereby, anatase TiO2 single crystals with a large percentage of {001} facets are predicted to have high reactivity, which underlines the importance of developing effective approaches to control the stabilities of different surfaces. Previously, we successfully synthesized single crystals of anatase TiO2 with a large percentage of reactive facets, with using titanium tetrafluoride (TiF4) aqueous solution and hydrofluoric acid as the precursor and crystallographic controlling agent, respectively [11]. In this poster, the effect of fluoride atoms on the surface stability and electronic structures of anatase TiO2 will be studied in the framework of DFT, which can help us to understand the function of fluoride atoms for the morphology controlling of anatase TiO2. Surface free energies and equilibrium morphology • According to early experimental studies, the surface of anatase TiO2 with the equilibrium morphology consists of two facets, {001} and {101}, whose surface free energies are calculated. The calculated values of surface free energies of (001) and (101) of anatase TiO2 terminated by different adsorbates are presented in Figure 2. Based on these results, it is found that: (i) among 12 non-metal-terminated surfaces and the clean surfaces, the surfaces terminated by different nonmetals present different surface free energies, suggesting that the surface chemistry and stabilities can be adjusted using different nonmetal adsortates; and (ii) the relative stabilities of (001) and (101) vary with the adsorbates. Although in most cases, (101) is more stable than (001), however, with the termination of Si and F, (001) is more stable, indicating that the percentages of these surfaces may be controllable using suitable morphology controlling agents. The surface free energies and equilibrium morphologies of anatase TiO2 surrounded by nonmetals have been studied with the frameworks of density functional theory with the exchange-correlation functional of GGA. Two conclusions can be summarized from our calculations: (i) fluorine atoms can effectively stabilize the surfaces, and F-termined (001) is more stable than (101), so the addition of fluorine species is helpful for the synthesis of large single crystals of TiO2 with high percentage of reactive {001} facets; and (ii) the stabilization effect associated with fluorine atoms is coming from the Ti-F (attractive) and O-F (repulsive) interactions according to the calculated electronic structures of F-surrounded surfaces. Acknowledgement Figure 2: The optimized ratios of B/A and percentage of {001} facets (S001/S) are also shown. e: calculated surface free energies, indicated by γ. f: Plots of the optimized value of B/A and the percentage S001/S. As indicated in the inset diagram, two independent parameters A and B denote lengths of the side of the bipyramid and the side of the square {001} ‘truncation’ facets, respectively. In each case, stoichiometric slab models (1×1), consisting at least 6 atomic layers, are employed. In clean situations, all low-indexed surface contain fivefold Ti on two slides of each slab, which is saturated by X (X=H, B, C, N, O, F, Si, P, S, Cl, Br, I). During the structural optimization, all atoms are relaxed without any constraint before the total energies are calculated. All calculations have been carried out using density functional theory (DFT) within the generalized-gradient approximation (GGA) [12], with the exchange-correlation functional of Perdew-Burke-Ernzerhof (PBE) [13-14]. This has been implemented in the Vienna ab initio simulation package (VASP) [15-16], which spans reciprocal space with a plane-wave basis, in this case up to a kinetic energy cutoff of 450 eV. The financial support from the ARC Centre of Excellence for Functional Nanomaterials Australia and from the University of Queensland to this work is acknowledged. We also acknowledge Shenyang National Laboratory for Materials Science, Institute of Metal Research, for allowing the access to the high performance computing clusters. Electronic structures of clean and fluorated surfaces • To understand the stabilization effect resulted by fluorine atoms, we calculated the electronic structures of F-surrounded (001) and (101), with clean surfaces as the reference. The calculated results were shown in Figure 3. In the case of clean surfaces, it is found that electrons contributed by Ti3d delocalized in the whole range of the higher valence band (VB), and the lower VB (mainly O2p) is decomposed into two peaks due to the cleavage of the surfaces. With the formation of Ti-F, however, localized states of Ti3d were observed and the gasp between O2s peaks of saturated (three-coordinated, O1) and unsaturated (two-coordinated, O2) decrease remarkably, as shown in Fig. 3(c) and (d), suggesting that fluorine atoms stabilize not only titanium atoms but also unsaturated oxygen atoms, which is consistent with the shortening of the bond length of Ti1-O1 based on our optimized geometries. In fact, from the calculated DOS, both Ti3d and O2p can interact with F2p, and Ti-F (attractive) and O-F (repulsive) interactions can be expected, thus a new balance is constructed, which stabilize unsaturated Ti and O atoms on the surfaces. This is the essential effect of fluorine atoms for the synthesis of large single crystals with high percentage of {001} facets. References: [1] K.I. Hadjiivanov and D.G. Klissurski, Chem. Sov. Rev. 25, 61, 1996. [2] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95, 735, 1995. [3] A. Hagfelt and M. Grätzel, Chem. 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Figure 1: Slab models and calculated surface free energies of anatase TiO2(001) and (101) surfaces. As indicated in the inset diagram, two independent parameters A and B denote lengths of the side of the bipyramid and the side of the square {001} ‘truncation’ facets, respectively. a & b: Clean, unrelaxed (001) and (101); c & d: unrelaxed (001) and (101) surfaces surrounded by X atoms.