Origin of Visible Light Photocatalytic Activity of Ag3AsO4 from First-Principles Calculation

Hindawi Publishing Corporation International Journal of Photoenergy Volume 2014, Article ID 639509, 5 pages http://dx.doi.org/10.1155/2014/639509 Research Article Origin of Visible Light Photocatalytic Activity of Ag3AsO4 from First-Principles Calculation Yan Gong, Hongtao Yu, and Xie Quan Key Laboratory of Industrial Ecology and Environmental Engineering of Ministry of Education, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China Correspondence should be addressed to Xie Quan; Received 12 December 2013; Revised 18 February 2014; Accepted 4 March 2014; Published 21 May 2014 Academic Editor: Yuexiang Li Copyright © 2014 Yan Gong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Recently a novel sliver oxide Ag3 AsO4 has been found to be an excellent photocatalyst with strong oxidation capability for pollutant degradation under visible light. But the origin of its high visible light photocatalytic activity was unclear which hindered further research of Ag3 AsO4 . For clarifying that, the electronic structure and optical properties of Ag3 AsO4 have been analyzed by the hybrid density functional method. The results reveal that the Ag3 AsO4 presents a narrow band gap with strong oxidation ability of the valence bands maximum edge and the highly delocalized charge distribution of the conduction bands minimum is beneficial for the carriers transfer to surface to participate in the photocatalytic reaction. These results provide clear explanations of the excellent visible light photocatalytic performance of the Ag3 AsO4 from microscopic aspect. And it is significant to design novel materials with high photocatalytic performance. 1. Introduction Semiconductor photocatalysts are finding increasing applications in high-efficiency solar cells [1], water/air purification [2], and water splitting [3, 4]. For the purpose of effectively utilizing solar energy, ideal semiconductor photocatalysts should at least have the two characteristics. One important aspect is a narrow band gap for utilizing the visible light which dominates 43% of the solar light and almost 100% of indoor light. The other one is a suitable band edge position for water splitting and environment pollutants decomposition or other target reactions. In the past decades, overwhelming attention has focused on designing visible-light response photocatalysts, such as BiVO4 [5], CdS [6], and WO3 [7]. However, their low visible-light photocatalytic activity is of course a great inhibition to use as a highly efficient photocatalysts. Since Yi’s group has reported the strong oxidation power of Ag3 PO4 under visible light [8], a series of sliver based oxides has aroused wide attention on account of their excellent photocatalytic abilities under visible light region, such as Ag3 VO4 [9], AgSbO3 [10], and Ag2 O [11]. Among them, as a novel sliver oxide photocatalyst, Ag3 AsO4 , has been found to be an excellent photocatalyst with powerful oxidation ability under visible light by Tang’s group recently [12]. It has a band gap of 1.6 eV which can fulfill the high absorption capacity of visible light. And the potential of valence band edge is about 2.22 eV that the photogenerated holes own strong oxidation to decompose the pollutants efficiently. Despite important insight being achieved, the mechanisms involved in photocatalysis are not yet clear. Fulfilling this goal requires the assistance of theoretical investigations such as electronic structure calculations. Therefore, a systemic investigation about microscopic mechanism of photocatalysis is vitally important for understanding the excellent photocatalytic performance of Ag3 AsO4 . To understand the superior photocatalytic activity of Ag3 AsO4 from its intrinsic properties, the first-principles calculations on its electronic structure and optical properties were carried out in our work. As a common problem, the band gaps of the semiconductors are usually underestimated by the conventional DFT methods due to the selfinteraction error as well as the missing discontinuity in the exchange-correlation potential. For instance, the band gap error exceeds 2 eV for ZnO [13, 14] and Ag3 PO4 [15]. This is adverse to analysis of the redox ability of the semiconductor. 2 The hybrid-DFT with PBE0 formalism has been successfully used as an available method to calculate the band gap accurately, such as Ag3 PO4 [16]. So in our work, the hybridDFT method PBE0 was applied to calculate the electronic structures and optical properties. The results revealed that the hybrid-DFT method is more precise for the calculation of the electronic and band structures of Ag3 AsO4 . Furthermore, we analyzed the relations of these microscopic factors to photocatalytic activities of Ag3 AsO4 . International Journal of Photoenergy while the interaction between silver and oxygen is formed mainly by ionic bond. Previous study showed that the short Ag–Ag distance results in the formation of the metallic Ag– Ag bond, which contributes to the dispersive conduction bands and a small effective mass of electron [20]. The length of Ag–Ag bond is 3.112 Å which is much smaller than that in Ag2 O (3.30 Å) and AgNbO3 (3.90 Å), but a little larger than AgPO3 (2.95 Å). So the shorter Ag–Ag distance in the Ag3 AsO4 indicates the metallic Ag–Ag bond which has remarkable influence of the band structure of the Ag3 AsO4 . 2. Computational Methods In this paper, our first-principles calculations were performed using the plane-wave pseudopotential method based on hybrid-DFT with PBE0 formalism, which was implemented in the CASTEP code [17]. Three-dimensional periodic boundary conditions were employed to simulate an infinite solid. The generalized gradient approximation (GGA) in the PBE0 hybrid functional formalism was applied combined with norm-conserving pseudopotentials. To achieve the accurate density of the electronic states, a 4 × 4 × 4 Monkhorst-Pack grid [18] was used for Brillouin-zone sampling. A plane-wave basis set with a cutoff of 400 eV was used. Geometric optimization was achieved and the convergence criterion for the force between atoms was 3 × 10−2 eV/Å, the maximum displacement was 1 × 10−3 Å, and the total energy and the maximal stress were 1 × 10−5 eV/atom and 5 × 10−2 GPa, respectively. The self-consistent convergence accuracy was set at 1.0 × 10−6 eV/atom, and the valence configurations of the pseudopotentials are 4d10 5s1 for Ag, 4s2 4p3 for As, and 2s2 2p4 for O, respectively. 3. Results and Discussion 3.1. Geometry Structure and Bonding Character. The geometry optimization crystal structure of Ag3 AsO4 is a cubic structure with P4-3n symmetry which is shown in Figures 1(a) and 1(b). Figure 1(b) presents the polyhedron configurations of the Ag3 AsO4 . It clearly shows that both the (...truncated)