Electronic structure and optical properties of Ag3PO4

Electronic structure and optical properties of Ag3PO4 photocatalyst calculated by hybrid density functional method J. J. Liu, X. L. Fu, S. F. Chen, and Y. F. Zhu Citation: Applied Physics Letters 99, 191903 (2011); doi: 10.1063/1.3660319 View online: http://dx.doi.org/10.1063/1.3660319 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/99/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Synergistic effect of V/N codoping by ion implantation on the electronic and optical properties of TiO2 J. Appl. Phys. 115, 143106 (2014); 10.1063/1.4871192 Research on the effect of crystal structures on W-TiO2 nanotube array photoelectrodes by theoretical and experimental methods J. Appl. Phys. 114, 084308 (2013); 10.1063/1.4819304 Band-engineered SrTiO3 nanowires for visible light photocatalysis J. Appl. Phys. 112, 104322 (2012); 10.1063/1.4767229 The electronic and optical properties of Eu/Si-codoped anatase TiO2 photocatalyst Appl. Phys. Lett. 100, 102105 (2012); 10.1063/1.3692750 Electronic structure of cation-codoped TiO 2 for visible-light photocatalyst applications from hybrid density functional theory calculations Appl. Phys. Lett. 98, 142103 (2011); 10.1063/1.3574773

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APPLIED PHYSICS LETTERS 99, 191903 (2011)

Electronic structure and optical properties of Ag3PO4 photocatalyst calculated by hybrid density functional method J. J. Liu,1,a) X. L. Fu,2 S. F. Chen,2,a) and Y. F. Zhu3 1

Department of Physics and Electronic Information, Huaibei Normal University, Anhui, Huaibei 235000, China 2 Department of Chemistry and Material Science, Huaibei Normal University, Anhui, Huaibei 235000, China 3 Department of Chemistry, Tsinghua University, Beijing 100084, China

(Received 2 September 2011; accepted 21 October 2011; published online 8 November 2011) The electronic structure and optical properties of Ag3PO4 were studied by hybrid density functional theory. The results indicated that the band gap is 2.43 eV, which agrees well with the experimental value of 2.45 eV. The conduction bands of Ag3PO4 are mainly attributable to Ag 5s and 5p states, while the valence bands are dominated by O 2p and Ag 4d states. The highest valence band edge potential was 2.67 V (vs. normal hydrogen electrode), which has enough driving force for photocatalytic water oxidation and pollutants degradation. The optical absorption C 2011 American Institute spectrum showed that Ag3PO4 is a visible light response photocatalyst. V of Physics. [doi:10.1063/1.3660319] Semiconductor-based photocatalysts exhibit high photocatalytic activities for decomposing organic pollutants as well as splitting water into hydrogen and oxygen.1–4 Among them, titanium dioxide (TiO2) shows the strong redox potential and high activity; however, its band gap (3.2 eV) is too wide that only absorbs ultraviolet light and utilizes about 3%-5% solar energy. One way by doping other elements can improve its absorption in visible light and increase utilization of solar energy, but dopants will serve as recombination sites of electron-hole and reduce the quantum efficiency. The other way is development of visible-light-responsive photocatalysts. The attempts are mostly focused on oxides of cations with d0 configuration such Zr4þ, Ta5þ, or Nb5þ, as well as oxides or nitrides of d10 configuration such as Ga3þ, In3þ, and Sn4þ, which show better photocatalytic activities.5–10 Recently, a few silver based photocatalysts are found that own high photocatalytic activation in the visible light region, such as Ag3VO4,11 AgGaO2,12 AgAlO2,13 AgSbO3,14 and so on. Especially, Ye et al. found cubic structure Ag3PO4 which exhibits extremely high photooxidative capabilities for O2 evolution from water, and its quantum efficiencies achieve up to nearly 90% under visible light irradiation.15 Ag3PO4 has a suitable band gap of 2.45 eV that can absorb visible light; moreover, its value band edge is more positive than the oxidation potential of EH(O2/H2O) (1.23 V vs. normal hydrogen electrode (NHE)), and photogenerated holes have strong oxidation ability and can decompose organic pollutants efficiently. For the purpose of understanding the excellent high photocatalytic activation of Ag3PO4, the first-principles calculations about its electronic and band structures were carried out in recent works.16,17 The local density approximation (LDA) and LDA þ U approaches were adopted in these studies, and the calculated band gaps of 0.36 and 1.30 eV are far less than the experimental value of 2.45 eV. It is a well known problem that the LDA approach underestimates the band gap of semiconductor. The fundamental reasons have a)

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0003-6951/2011/99(19)/191903/3/$30.00

been attributed to the missing discontinuity in the exchangecorrelation potential and the self-interaction error within the LDA. Even if using LDA þ U approach, the band gap is still underestimated about 47%. Since the photocatalytic properties of a semiconductor highly depend on its electronic and band structures, developing an approach to, instead of underestimated, LDA method becomes essential. Furthermore, the analysis of the redox capacity of the photocatalyst will be also benefit from this method. Hybrid density functional theory (DFT) using PBE0 formalism, which is obtained by mixing a fixed amount of the Fock exchange, can effectively remedy the drawbacks of LDA. Hybrid-DFT method not only gives a better structural representation, but also significantly improves the calculated band gap. Therefore, PBE0 formalism is good candidate for the calculation. In this article, both the hybrid-DFT and the standard generalized gradient approximation (GGA) were used to calculate the band structure, density of state (DOS), and optical properties of Ag3PO4. Compared with experimental results, the results indicated that the hybrid-DFT method is more suitable for the calculation of the electronic and band structures of Ag3PO4. The calculations were performed using the plane-wave pseudopotential method based on hybrid-DFT, as implemented in the CASTEP code.18 Three dimensional periodic boundary conditions were used to approximate an infinite solid. The GGA in the PBE0 hybrid functional formalism was applied combined with norm-conserving pseudopotentials. According to the Monkhorst-Pack grid, the K points in the Brilliouin Zone were set to 4 4 4, and the plane-wave basis cutoff energy was taken to be 500 eV for Ag3PO4. Geometric optimization was achieved by using convergence ˚ thresholds of 1.0 105 eV/atom for total energy, 0.03 eV/A 3 ˚ for maximum force, 0.05 GPa for pressure, and 1.0 10 A for maximum displacement. The self-consistent calculations were carried out with a total energy convergence tolerance of less than 1.0 106 eV/atom, and the valence electronic configurations for Ag, P, and O were 4d105s1, 3s23p3, and 2s22p4, respectively.

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The crystal structure of Ag3PO4 has a cubic structure with space group P4-3 n. Its basic structural unit is constructed by PO4 tetrahedron and AgO4 tetrahedron. The P atom is four-fold coordinated with surrounding oxygen atoms forming a PO4 tetrahedron. Ag atom is surrounded by four oxygen atoms forming a AgO4 tetrahedron. The opti˚ and mized cell parameters are a ¼ b ¼ c ¼ 6.010 A a ¼ b ¼ c ¼ 90 , and agree well with the experimental data of ˚ and a ¼ b ¼ c ¼ 90 .19 The deviations a ¼ b ¼ c ¼ 6.026 A are less than 0.8%. The PO and AgO bond length are cal˚ , respectively. For the presculated to be 1.518 and 2.386 A ence of PO4 tetrahedral structure, the length of PO bond is smaller than that of AgO bond, suggesting the stronger interaction between P and O in the lattice. A Mulliken bond population analysis indicates the PO bond population is 0.63, and the AgO is 0.11. Therefore, the interaction between phosphorus and oxygen is achieved mainly by covalent bond, while the interaction between silver and oxygen is formed mainly by ionic bond. The electronic structures of Ag3PO4 were performed using hybrid-DFT approaches. The band structures of bulk Ag3PO4 along the high symmetry directions in the Brillouin zone are shown in Figures 1(a) and 1(b). The Fermi level indicated by dashed line is set as zero. The valence bands maximum (VBM) was at the M point and the conduction bands minimum (CBM) was located at G points in the Brillouin zone for both approaches. As shown in Figure 1(b), the direct band gap is 2.61 eV in G point and the indirect band gap between M and G is 2.43 eV. Therefore, Ag3PO4 is an indirect band gap semiconductor. The band structure of Ag3PO4 using the standard DFT method has been calculated and the indirect band gap between M and G is 0.70 eV. Calculations show that the value obtained by PBE0 approach agrees well with the experimental value (2.45 eV). Compared with the top of valence band (VB), the bottom of conduction band (CB) is well dispersive, which indicates that the photogenerated electrons possess smaller effective mass and, therefore, higher migration ability. To further elucidate the composition and the nature of the electronic band structures, the total DOS (TDOS) of

FIG. 1. (a) Band structure of Ag3PO4 calculated using the PBE0 approach. (b) A magnified view of band structure near the Fermi level.

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Ag3PO4 and partial DOS (PDOS) for Ag, P, and O elements are calculated by PBE0 approach, which were shown in Fig. 2. Hybridization of sp is occurred between the 3s and the 3p states of P atom, which then forms a bonding states with the O 2s and O 2p states at energy band about 23 eV and 21 eV. From 10 eV to 0 eV, the valence bands are mainly occupied by O 2p and Ag 4d states, and a small amount of P 3p and O 2s states. The distribution of Ag 4d states is sharp, and no splitting phenomenon is observed. Near the Fermi level, the VBM is derived from O 2p and Ag 4d states. The dispersion of O 2p is wider than that of Ag 4d states, which is beneficial to the transition of electrons from valence band to conduction band. Above the Fermi level, the CBM is dominated by Ag 5s and Ag 5p states, which form the anti-bonding state. The redox ability of Ag3PO4 is assessed by determining the energy positions of valence and conduction bands. The positions can be calculated according to the equation as follows using Mulliken electronegativity and the band gap value: 1 EVB ¼ v Ee þ Eg ; 2

(1)

ECB ¼ EVB Eg ;

(2)

where EVB and ECB are the VB and CB edge potentials, respectively, v is the Mulliken electronegativity of Ag3PO4, which is calculated about 5.96 eV, and Ee is the energy of

FIG. 2. TDOS and PDOS of Ag3PO4 using PBE0 approach.


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FIG. 3. Calculated VBM and CBM potentials of Ag3PO4.

free electrons on the hydrogen scale (4.5 eV). Eg is the calculated band gap. Fig. 3 shows the valence and conduction band edge potentials of Ag3PO4 calculated using these equations. The VBM potential of Ag3PO4 is 2.67 V, more positive than O2/H2O (1.23 V). Therefore, Ag3PO4 has the ability to oxidized H2O to produce O2 or oxidation pollutants. Whereas the CBM potential of Ag3PO4 is 0.24 V, which is lesser than Hþ/H2 (0 V) and cannot reduce Hþ to H2. The calculations consistent with experimental results.15 Fig. 4 shows the optical absorption coefficients of Ag3PO4 calculated by PBE0 approach. Form curve, the optical absorption is observed in visible light region(1.6–3.1 eV), which is according with experiment.15 So Ag3PO4 is a visible light response photocatalyst. The optical band gap of a semiconductor could be deduced according to the following equation: aðhmÞ ¼ Cðhm Eg Þn=2 ;

(3)

FIG. 4. Calculated absorption coefficient of Ag3PO4 using PBE0 approach.

well with the experimental value of 2.45 eV. The valence bands of Ag3PO4 are mainly composed by O 2p and Ag 4d states, while the conduction bands are dominated by Ag 5s and Ag 5p states. The calculated VBM potential located at 2.67 V (vs. NHE), while the CBM potential at 0.24 V by PBE0 approach. Therefore, Ag3PO4 has the ability to oxidized H2O to produce O2 or oxidation pollutants, but cannot reduce Hþ to H2. The optical absorption spectrum showed that Ag3PO4 is a visible light response photocatalyst. This work was supported by the Natural Science Foundation of China (Nos. 20973071, 51172086, 21103060, and 11104100) and the Young Scientists Funds of Huaibei Normal University (No. 700283). 1

where a is the absorption coefficient, hm is the photon energy, C is a proportionality constant related to the material, and Eg is the band gap energy of the semiconductor. The value of the index n decides the characteristics of the transition in a semiconductor. Ag3PO4 absorption spectrum experiments revealed that an indirect optical band gap is 2.36 eV and a direct optical transition is 2.43 eV.15 So Ag3PO4 is an indirect band gap semiconductor. According to Eq. (3), when n ¼ 4, it corresponds to the indirect optical absorption, and the calculated indirect optical band gap is 2.51 eV; when n ¼ 1, it corresponds to the direct optical absorption, and the calculated direct optical band gap is 2.63 eV. The calculated optical band gaps using PBE0 approach from absorption spectra were close to the experimental values. The calculated indirect optical band gap (2.51 eV) is slightly larger than the value (2.43 eV) obtained by band structure calculations. Due to the participation of phonon, the probability of electron indirect transition between the VBM and CBM is small. It is the main reason for this difference. Moreover, Ag 5s and Ag 5p states in CB are hybridized which made the electron energy dispersed and the absorption intensity weaken. In conclusion, the band structures, density of states, and optical properties of Ag3PO4 were calculated using PBE0 approach. The calculated band gap is 2.43 eV, which agrees

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