Article pubs.acs.org/JPCC
Electronic and Photocatalytic Properties of Ag3PC4VI (C = O, S, Se): A Systemic Hybrid DFT Study Zuju Ma,†,‡ Zhiguo Yi,† Jing Sun,† and Kechen Wu*,† †
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ABSTRACT: Recently developed photoelectrode Ag3PO4 exhibits extremely high quantum yield (up to 90% at 420 nm) of O2 generation from water oxidation, but it can only absorb part of visible light and cannot reduce water to release H2. It is still a challenge to develop the high performance water splitting photocatalysts toward broader solar spectrum response. We theoretically investigated the electronic and photocatalytic properties of Ag3PC4VI (C = O, S, Se) by the hybrid density functional method. The experimental band gap of Ag3PO4 (2.45 eV) was well reproduced by this level of theory (2.49 eV). We found that orthorhombic Ag3PSe4 presents not only an ideal direct band gap (2.09 eV) to harvest a large part of solar light in the visible range but also suitable reduction potential (−0.275 V vs normal hydrogen electrode, NHE) for water reduction. Ag3PS4 has a relatively larger band gap of 2.88 eV, but it has a rather negative reduction potential (−0.53 V vs NHE). The analysis of the density of states and the single-state charge densities suggested that the weakened P−CVI bonds and the disappearance of Ag−Ag interaction from Ag3PO4 to Ag3PC4VI(C = S, Se) result in the great change of the conduction band minimum from the highly delocalized Ag s orbitals to the strong mixing of P−Se(S) orbitals. The potential problems of stability and photocorrosion for Ag3PC4VI(C = S, Se) were also discussed. Our theoretical results demonstrated that Ag3PC4VI(C = S, Se) both are potential candidates for the photocatalytic hydrogen generation from water. A highest quantum yield of ∼93% at 420 nm has been reported for the Pt-PdS/CdS photocatalyst in the presence of sacrificial reagents of S2−/SO32−.12 Although chalcogenide semiconductors usually have a disadvantage of photocorrosion, some tactics, such as incorporation of metal sulfides into interlayer and mesoporous materials,15,16 hybridizing with conductive polymer,17 combining with the cocatalysts,12 and forming solid solution18 have been used to stabilize the metal sulfides and improve the performance of hydrogen production. In addition, some multicomponent metal sulfides have been reported to be more stable and show higher photocatalytic activity under visible light than binary metal sulfides.10,19−21 Recently, Yi et al. reported a new use of the cubic Ag3PO4 semiconductor, which shows extremely high quantum yield (up to 80−90%) of O2 generation from water oxidation at wavelengths less than 480 nm.22 A series of compounds have been used as cocatalysts to further improve the photochemical activity and stability of Ag3PO4, such as AgBr,23 Ag,24,25 Fe3O4,26 TiO2,27 SnO2,28 and Ag nanowires.29 However, this novel photoactive material cannot reduce water to generate H2 because the CBM (conduction-band minimum) of Ag3PO4 is
1. INTRODUCTION Owing to the depletion of fossil-fuel resources as well as the increasingly serious environmental problems resulted from the consumption of the fossil fuels, hydrogen, as one of the clean and renewable energy carriers, has reattracted much attention over the past decade.1 Hydrogen production from photoinduced water splitting via a photocatalytic material is one of the most attractive methods for solving the cheap and environmental friendly energy problems.2−4 For an efficient visible-light-sensitive photocatalyst for hydrogen generation from water, on the one hand, a band gap around 2.0 eV is preferable in order to utilize the maximum portion of the solar visible light. On the other hand, suitable band edge position for reducing water is needed.5 At present, only a few semiconductors can simultaneously meet the two criteria. The reported metal oxides photocatalysts, such as n-type TiO2,6 NaTaO3,7 Sr2NbO7,8 La2Ti2O7,9 etc., exhibit high photocatalytic activities for hydrogen generation from water splitting, however, because of their large band gaps, these metal oxides photocatalysts are only active under UV light which constitutes only 5% of the solar spectrum.10 Some chalcogenides as visible-light photocatalysts have been extensively studied because of their more suitable band edge positions than oxides and excellent performance for photocatalytic hydrogen production, such as CdS11,12 and ZnSe.13,14 © 2012 American Chemical Society
Received: September 20, 2012 Revised: November 6, 2012 Published: November 6, 2012 25074
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more positive than the reduction potential of hydrogen. Moreover, it can only weakly absorb part of visible light because of the indirect band gap (2.45 eV).30,31 Given the fact that metal sulfides and selenides usually possess more suitable band edge positions and greater optical absorption32−34 than oxides, we report our comprehensive study of the potential of Ag3PC4VI (C = S, Se) to work as solar H2-evolution photocatalysts by examining the electronic structure, redox potentials, charge separation, light absorption abilities, etc. and make a comparison with Ag3PO4. Our results suggest that Ag3PC4VI (C = S, Se), two known compounds that have been neglected for a long time,35−38 might be a new class of potential visible-light-driven photocatalysts for hydrogen generation.
Figure 1. (a) Crystal structure of Ag3PO4 with P4̅3n space group. (b) Crystal structures of Ag3PS4 and Ag3PSe4 with Pmn21 space group. AgC4VI (C = O, S, Se) and PC4VI (C = O, S, Se) tetrahedral units are indicated by the gray and pink tetrahedrons, respectively.
sulfur and heating.35 Ag3PSe4 has been synthesized by the reaction of Ag powder, P2Se5, and Se in a molar ratio of 1:1:1 at 500 °C.38 All the three compounds have normal tetrahedral structures where each atom is tetrahedrally coordinated. Specifically, as depicted in Figure 1, both the silver and phosphorus atoms are coordinated to four chalcogen atoms forming AgC4VI and PC4VI tetrahedral units, respectively. The chalcogen atom is shared by three AgC4VI tetrahedrons and one PC4VI tetrahedron. In Ag3PO4, AgO4 tetrahedron is heavily disordered, which is attributed to the inductive effect of the highly electronegative PO4 units.30 The cell volume and atom positions are relaxed by minimizing the total energy. As shown in Table 1, the GGA/PBE optimized structures agree well with
2. CALCULATION METHODS For the metal oxides with s-like conduction bands, the band gaps are usually seriously underestimated by the standard density functional theory (DFT) methods due to the selfinteraction error as well as the missing discontinuity in the exchange-correlation potential. For example, the error exceeds 2 eV for ZnO.39,40 Previous studies have confirmed that the band gap of Ag 3 PO 4 calculated by the local density approximation (LDA) and generalized gradient approximation (GGA) are far smaller than the experimental value,30,31,41 because the CBM of Ag3PO4 is mainly composed of highly delocalized Ag s states. Taken into account the effects of nonlocal exchange, our first-principles calculations were performed using the plane-wave pseudopotential method based on hybrid-DFT with PBE0 formalism, which is implemented in the CASTEP code.42 The PBE0 method is based on the Perdew−Burke−Ernzerhof (PBE)43 functional but with 25% of the GGA exchange replaced by Hartree−Fock (HF) exchange, as shown in the following form PBE0 E XC =
1 HF 3 Ex + ExPBE + EcPBE 4 4
Table 1. Lattice Parameters, Relaxed Average Bond Distances, Band Gaps Eg, Effective Masses of the Electron (me*) and the Hole (mh*) in the Unit of Free-Electron Mass (me), and mh*/me* of Ag3PC4VI (C = O, S, Se) (Experimental Values Are Shown in Parentheses) symmetry lattice
(1)
Here, ExHF, ExPBE, and EcPBE are the HF exchange, the PBE exchange, and the PBE correlation energies, respectively. This hybrid functional method has been proven to be more efficient than the LDA, GGA, and LDA+U approaches in the calculation of the band structure of Ag 3PO 4 .30,31,41 The valence configurations of the pseudopotentials are 4d105s1 for Ag, 2s22p4 for O, 3s23p4 for S, 4s24p4 for Se, and 3s23p3 for P. An energy cutoff of 850 eV and Monkhorst-Pack k-point mesh of 3 × 3 × 3 were found to get convergent lattice parameters. The GGA/PBE43 and LDA/CA-PZ44,45 exchange and correlation functions were adopted in geometric optimization. On the basis of these optimized structures, the hybrid-DFT method PBE0 was then applied to calculate the electronic structures and optical properties. It was found that the electronic structure of Ag3PO4 based on the PBE optimized structure tends to be more accurate than that based on the LDA optimized structures. Therefore, we only show the results obtained based on the PBE optimized structures.
methods
Ag3PO4
Ag3PS4
Ag3PSe4
PBE
P43̅ n 6.15 (6.005a)
Pmn21 a = 7.82 (7.65b) b = 6.93 (6.86b) c = 6.62 (6.51b) dP−S = 2.08 (2.05b) dAg−S = 2.60(2.56b) dAg−Ag = 3.71(3.61b) 2.88 1.01 0.538 2.405 4.470
Pmn21 a = 8.23 (7.69c) b = 7.21 (6.66c) c = 6.91 (6.38c) dP−Se = 2.25 (2.36c) dAg−Se = 2.67(2.38c) dAg−Ag = 3.95(3.80c) 2.09 0.41 0.618 2.167 3.506
parameters (Å)
bond
PBE
distances (Å)
Eg (eV) me* (me) mh* (me) mh*/me* a
PBE0 PBE PBE0 PBE0 PBE0
dP−O = 1.55 (1.56a) dAg−O = 2.44(2.36a) dAg−Ag = 3.07(3.00a) 2.49 (2.45d) 0.21 0.472 2.126 4.504
Reference 46. bReference 37. cReference 38. dReference 22.
experiments, except for the orthorhombic Ag3PSe4 where there is a slight deviation from experiment.38 The LDA/CA-PZoptimized Ag3PSe4 also shows an enlarged structure as PBE, confirming that the PBE-optimized structure of Ag3PSe4 is reliable. For comparison of the bonding character, relaxed bond distances by the PBE approach for Ag3PC4VI (C = O, S, Se) are given in Table 1 with experimental values. Ag3PO4 has a much shorter P−O bond than the P−S(Se) bond of Ag3PS4(Ag3PSe4), indicating the formation of the rigid
3. RESULTS AND DISCUSSION 3.1. Geometry Structure and Bonding Character. The compounds studied in this paper include cubic Ag3PO4 with P43̅ n symmetry,46 and orthorhombic Ag3PC4VI (C = S, Se) with Pmn21 symmetry,35−38 as shown in Figure 1. Ag3PS4 can be obtained by high-temperature reaction of the elements or conversion of Ag2P2S6 by adding a proper amount of silver and 25075
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Figure 2. Electronic structure of Ag3PO4. (a) Band structure. The Fermi level is set to the zero of energy. (b) PDOS. (c) The single-state charge densities of the CBM at 0.008 e/Å3. The red, violet, and cyan particles denote the O, P, and Ag atoms, respectively. (d) The enlarged plots of the PDOS which is close to the CBM.
Figure 3. Electronic structure of Ag3PS4. (a) Band structure. The Fermi level is set to the zero of energy. (b) PDOS. (c) The single-state charge densities of the CBM at 0.008 e/Å3. The yellow, violet, and cyan particles denote the S, P, and Ag atoms, respectively. Red arrows highlight the antibonding character of the P−S orbitals. (d) The enlarged plots of the PDOS which is close to the CBM.
Se) present direct band gaps with the gaps located at the G point. The reciprocal-space fractional coordinates for the kpoints used are the following: G = (0, 0, 0), M = (1/2, 1/2, 0) and Z = (0, 0, 1/2). As shown in Table 1, the band gap of 0.21 eV for Ag3PO4 determined by stand DFT (GGA/PBE) is seriously underestimated in comparison with the experimental value of 2.45 eV.22 On the contrary, the hybrid-DFT method (PBE0) gives a much more credible band gap of 2.489 eV, which agrees well with the experimental value.22 For Ag3PC4VI (C = S, Se), our theoretical results show that the selenide has a smaller band gap of 2.09 eV than the sulphide (2.88 eV), which is attributed to the lower energy of Se 4s4p than S 3s3p in the CBM. The same trend can be found in AgGaC2VI (C = S, Se) and CuGaC2VI (C = S, Se).47 Although there is no experimental value to make a comparison, it is reasonable to believe that both the selenide and the sulphide are visible light sensitive semiconductors. To better understand the nature of band structures, the projected density of states (PDOS) for Ag3PC4VI (C = O, S, Se) calculated by PBE0 are presented in Figures 2b, 3b, and 4b,
tetrahedral units PO4 with strong P−O covalent bonds in Ag3PO4. The Ag−CVI (C = O, S, Se) bonds are also elongated by about 0.1−0.2 Å after substituting S/Se atoms for O atoms, which is attributed to the larger ionic radius of S2−/Se2− compared with O2−. The Ag−Ag distance in Ag3PO4 is about 3 Å, which is much smaller than that in Ag3PC4VI (C = S, Se). Umezawa et al. demonstrated the short Ag−Ag distance in Ag3PO4 results in the formation of the metallic Ag−Ag bond, which contributes to the dispersive conduction bands and a small effective mass of electron.41 These metallic Ag−Ag bonds disappear after replacing O atoms with S/Se atoms, leading to a great change of the conduction bands, as discussed below. 3.2. Band Structures and Density of States. We show the band structures of Ag3PC4VI (C = O, S, Se) determined by PBE0 in Figures 2a, 3a, and 4a, respectively. The corresponding band gaps calculated by PBE and PBE0 are listed in Table 1. One can see the band structures are strongly symmetry dependent. Cubic Ag3PO4 shows an indirect band gap with the VBM (valence-band maximum) and CBM located at the M and G points, respectively, while orthorhombic Ag3PC4VI (C = S, 25076
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Figure 4. Electronic structure of Ag3PSe4. (a) Band structure. The Fermi level is set to the zero of energy. (b) PDOS. (c) The single-state charge densities of the CBM at 0.008 e/Å3. The orange, violet, and cyan particles denote the Se, P, and Ag atoms, respectively. Red arrows highlight the antibonding character of the P−Se orbitals. (d) The enlarged plots of the PDOS which are close to the CBM.
respectively. The corresponding extended plots of PDOS at the energy near the CBM are shown in Figures 2d, 3d, and 4d. In Ag3PO4, Ag 4d and O 2p orbitals predominate the VBM, while the CBM is composed mainly of the delocalized Ag 5s orbitals. These characters are consistent with previous theoretical studies on Ag3PO4.30,41 When substituting S(Se) atoms for O atoms, Ag 4d orbitals have been moved to lower energy at about −4 eV below the VBM, but the main character of the VBM does not change, which is still composed of Ag 4d orbitals and S/Se p orbitals. We note that there are almost no Ag 4d orbitals in the CB of all three compounds, indicating the formation of nonbonding states in the VB. This character is beneficial for the transfer of photogenerated electrons in the CB, due to that sp states are less localized than d states. We can see the nonbonding character from the single-state charge densities of the VBM, as shown in Figure 5. Ag3PSe4 has a
(C = S, Se) (∼7 eV) are much smaller than that of Ag3PO4 (∼12 eV). Second, the main components of the CBM change from the highly delocalized Ag 5s orbitals to the strong mixing of S(Se) p, P s, P p, Ag s, and Ag p orbitals. These changes could be well understood as a result of the weakened P−CVI (C = O, S, Se) bonds and the extended Ag−Ag distances in Ag3PC4VI (C = S, Se). From Table 1, we can see that Ag3PC4VI (C = S, Se) have longer P−CVI bonds than that of Ag3PO4. This suggests weaker interactions between the P atoms and the CVI atoms in Ag3PC4VI (C = S, Se). The weakened P−CVI bond will decrease the energy of P−CVI antibonding orbitals in the CB, which can be clearly seen from the PDOS (Figures 3d and 4d) and the single-state charge density of the CBM (Figures 3c and 4c). The antibonding characters are highlighted by the red arrows in Figures 3c and 4c. This well explains the shrunken CB and the appearance of P p, S(Se) p orbitals at the CBM. The other factor that affects the change of CB is the Ag−Ag interaction. In Ag3PO4, a large amount of hybridizations between Ag s and Ag s occur at the CBM. As shown in Figure 2c, the highly delocalized Ag 5s orbitals embrace the PO4 tetrahedral units, which is beneficial for the photogenerated electrons transfer. In contrast, for Ag3PS4 (Ag3PSe4), the hybridizations between Ag s and Ag s can certainly be neglected because of the much longer Ag−Ag bond. Therefore, the Ag s orbitals cannot be the dominant component of the CBM in Ag3PS4 (Ag3PSe4). The relatively more dense energy bands composed of P, S(Se), and Ag orbitals close to the CBM for Ag3PS4 (Ag3PSe4) would result in a larger effective mass of electron than that in Ag3PO4, which is unfavorable for the charge transfer but is favorable for the photons absorption. 3.3. Charge Mobility and Separation. The separation and diffusion rate of the photogenerated charge carriers is one of the most important factors determining the photocatalytic activity.5 Here, we assess the recombination rate of the electron−hole pairs by computing the relative effective masses of electron and hole in perfect crystals. A lower carrier effective mass corresponds to a higher carrier mobility. A larger value of mh*/me* (me* and mh* are the effective mass of electron and hole, respectively) suggests a greater difference in the electron− hole mobility and thus a lower recombination rate of the electron−hole pairs. The effective mass of electrons and holes in the unit of free-electron mass (me) are estimated by fitting
Figure 5. The single-state charge densities of the VBM at 0.008 e/Å3 for (a) Ag3PO4 and (b) Ag3PS4. The red, violet, cyan, and yellow particles denote the O, P, Ag and S atoms, respectively. The red arrows highlight that Ag d orbitals do not hybridize with CVI (C = O, S, Se) p orbitals but form nonbonding states in VB.
similar contour map as that of Ag3PS4; thus only one of them is presented in Figure 5. One can see that Ag 4d orbitals do not hybridize with CVI (C = O, S, Se) p orbitals, as highlighted by the red arrows in Figure 5, even though they both occupy the VBM. In contrast with the VB, the CB vary greatly from Ag3PO4 to Ag3PC4VI (C = S, Se). First, the widths of the CB of Ag3PC4VI 25077
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the energy-momentum dependence of the states at the CBM and VBM in Brillouin zone along G-M for P4̅3n space group and G-Z for the Pmn21 space group. Table 1 lists the results. The me* values (0.47−0.62 me) of Ag3PC4VI (C = O, S, Se) are much smaller than that of TiO2 (about 1 me),48 indicating that the electrons in the conduction bands can transfer more rapidly to the surface. Our calculated me* of Ag3PO4 (0.47 me) is slightly larger than the previous theoretical result (0.42 me) by the LDA+U method,41 indicating that the hybrid-DFT tends to predict a less sharp CBM peak than LDA+U. For the value of mh*/me*, Ag3PC4VI (C = S, Se) have slightly smaller values than that of Ag3PO4 (4.47 for Ag3PS4, 3.51 for Ag3PSe4, and 4.50 for Ag3PO4), as evidenced by the less dispersive conduction bands in Ag3PC4VI (C = S, Se) than that in Ag3PO4 discussed above. Nevertheless, these ratios from 3.51 to 4.50 are all much larger than that of some other ternary oxides photocatalysts such as BiVO4 (mh*/me* is about 1),40 suggesting that Ag3PC4VI (C = O, S, Se) have excellent band edge characteristics for carrier separation. 3.4. Redox Potentials. Besides a low recombination rate of electron−hole pairs, suitable redox potentials and strong light absorption are also prerequisites for a high-efficiency visiblelight-sensitive photocatalyst. The redox ability is assessed by aligning the VBM and CBM with respect to the water oxidation/reduction potential level. The CBM and VBM positions can be theoretically predicted from the formula31 ECBM = X − Ee − E VBM = ECBM + Eg
1 Eg 2
generation from water oxidation. This is consistent with the experimental observations.22 It is of interest to note that for Ag3PC4VI (C = S, Se), the CBM potentials are higher than the reduction potential of hydrogen, and the VBM potentials are lower than the oxidation potential of O2/H2O. This indicates that both water oxidation and reduction reactions are thermodynamically feasible for Ag3PC4VI (C = S, Se). For Ag3PSe4, it has not only a much smaller band gap (2.09 eV) than that of TiO2 (3.20 eV) but also a comparable CBM potential (−0.275 V) to that of TiO2 (−0.29 V), suggesting its potential applications in photocatalytic hydrogen generation. The results that Ag3PS4 has a more negative CBM potential than Ag3PSe4 (−0.53 V vs −0.275 V) indicate that the water reduction might thermodynamically be more feasible for Ag3PS4 than for Ag3PSe4. It should be stressed that the band alignment is only a thermodynamic requirement for water splitting but not a sufficient condition. They might not be active for water oxidation or overall water splitting into H2 and O2. The acidity of the solution, the stability of the materials, as well as the chemical kinetics should be considered in practice. For example, S2−(Se2−) in Ag3PS4 (Ag3PSe4) might be self-oxidized by photogenerated holes if no sacrificial reagent is employed.10,11,20,21 It is still a challenge to predict accurately a photocatalyst. Nevertheless, the reliable DFT calculations can be very helpful to explore the complex structure−property relationship and design the new photocatalyst from thermodynamic point of view.4,30,31,51−56 3.5. Optical Absorption. The optical absorption spectra of Ag3PC4VI (C = O, S, Se) were calculated by the Fermi golden rule within the dipole approximation from PBE0 wave functions. The imaginary part of the dielectric function due to direct interband transitions is given by the expression57,58
(2) (3)
where X is the absolute electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap from PEB0 approach. The Mulliken electronegativities of a compound is the geometric mean of the electronegativities of the constituent atoms.49 They are calculated to be 5.96, 5.41, and 5.27 eV for Ag3PO4, Ag3PS4, and Ag3PSe4, respectively.50 We show the CBM and VBM of these compounds in Figure 6. Ag3PO4 presents positive CBM potential (0.215 V vs NHE) and cannot reduce H+ to release H2 from water. However, the electrode potential of Ag/Ag3PO4 is higher than that of Ag/AgNO3 (∼0.8 V), and therefore AgNO3 can be used as a sacrificial reagent to consuming the photogenerated electrons of Ag3PO4. The sufficiently positive VBM potential of Ag3PO4 exhibits strong potential for O2
ε2(ℏω) =
2e 2π Ωε0
∑ |⟨ψkc|ur|ψkv⟩|2 δ(Ekc − Ekv − E) k ,v ,c
(4)
where Ω, ω, u, ν, and c are the unit-cell volume, photon frequencies, the vector defining the polarization of the incident electric field, valence bands, and conduction bands, respectively. The real part of the dielectric function is obtained from ε2 by a Kramers−Kronig transformation. The absorption coefficient η (ω) can be obtained based on ε1 and ε2.59 The MonkhorstPack scheme with k-point grids of 4 × 4 × 4, 5 × 5 × 5, and 7 × 7 × 7 were used to calculate the optical properties of Ag3PO4. The results show that a 4 × 4 × 4 k-point grid was enough to obtain a convergent optical absorption spectrum (see the inset of Figure 7). Therefore the same set was applied to other compounds. It can be seen clearly from Figure 7 that the absorption coefficients of sulfides and selenides are much larger than that of oxides, which is consistent with previous reports on the potential solar absorber Cu3 PSe 4 . 32 The stronger absorptions for the sulfides and selenides are attributed to the larger DOS around the CBM and the direct band gap character. Take the case of Ag3PSe4, for example, the strong mixing of Se p, P s, and Ag p orbitals in the conduction bands of Ag3PSe4 form more dense energy bands than the highly delocalized Ag s orbitals in Ag3PO4 (Figures 2 and 4). Thus, Ag3PSe4 can absorb more photons than Ag3PO4. In addition, the direct band gap character is favorable for electron transition owing to the involvement of phonons is unnecessary. In comparison with Ag3PS4, the greater absorption for Ag3PSe4 in the visible light
Figure 6. The CBM and VBM potentials of Ag3PC4VI (C = O, S, Se). The data of anatase TiO2 was obtained from ref 62. 25078
dx.doi.org/10.1021/jp3093447 | J. Phys. Chem. C 2012, 116, 25074−25080
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Figure 7. The direction-averaged optical absorption coefficient spectra for Ag3PC4VI (C = O, S, Se) calculated by the PBE0 approach. The inset shows the optical absorption coefficient spectra of Ag3PO4 calculated using Monkhorst-Pack k-point grid of 4 × 4 × 4, 5 × 5 × 5, and 7 × 7 × 7.
region is primary due to the absorption curve being shifted to lower energies by its smaller band gap.
4. CONCLUSIONS In conclusion, we systematically studied the electronic structures and potential photocatalytic properties of Ag3PC4VI (C = O, S, Se) by hybrid density functional method. We found that the limitations of Ag3PO4 (positive CBM potential and weak optical absorption) can be greatly improved by substituting isovalent sulfur or selenium atoms for oxygen atoms. In particular, Ag3PSe4 presents not only a negative conduction-band minimum potential to reduce water (−0.275 V vs NHE) but also an idea band gap (2.09 eV) as well as very strong optical absorption abilities to absorb the maximum portion of the solar visible light. Ag3PS4 has a relatively larger band gap of 2.88 eV, but it has a rather negative reduction potential (−0.53 V vs NHE). This systematical theoretical study on the electronic structures and optical properties of Ag3PC4VI (C = O, S, Se) is expected to not only lead to the finding of new photocatalysts with much higher activity but also enrich the knowledge of Ag-based ternary chalcogenides, which have long been experimentally investigated due to their applications in optoelectrics and nonlinear optics.60,61 Experimental efforts for further studying these two materials are needed.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant Nos. 20973174 and 91122015). REFERENCES
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