Mechanism of Superior Visible-Light Photocatalytic Activity and

Article pubs.acs.org/JPCC

Mechanism of Superior Visible-Light Photocatalytic Activity and Stability of Hybrid Ag3PO4/Graphene Nanocomposite Liang Xu,† Wei-Qing Huang,*,† Ling-Ling Wang,† Gui-Fang Huang,*,† and Ping Peng‡ †

Department of Applied Physics, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, China ‡ School of Materials Science and Engineering, Hunan University, Changsha 410082,China ABSTRACT: Electronic structure and photocatalytic performance of Ag3PO4(100) surface interfaced with graphene (GR) have been investigated using density functional theory. The interactions between GR and Ag3PO4(100) surface would induce the charge distribution fluctuations of GR sheet. A large builtin potential formed at the interface can prompt charge transfer from Ag3PO4 into GR in the ground electronic state, thus improving the stability of the hybrid Ag3PO4(100)/GR. Due to C 2p states forming the upper part of valence band, the band gap is reduced to about 0.53 eV, resulting in a strong absorption in the entire visible region and thus superior photocatalytic activity. An absorption peak located near 440 nm in the calculated absorption spectrum can be rationalized by the experimental results of electron spin resonance. These results provide theoretical evidence supporting the experimental findings and pave the way for the applications of Ag3PO4-based nanocomposites.

1. INTRODUCTION Since photoinduced decomposition of water on TiO2 electrodes was discovered, semiconductor-based photocatalysts have attracted extensive interest in both scientific and industrial communities due to their potential applications in energy and environmental related fields, such as hydrogen production and water and air purification.1−6 Recently, significant effort has been devoted to silver orthophosphate (Ag3PO4), a novel highly active visible-light photocatalyst, which has extremely high photooxidative capabilities for O2 generation from water splitting and superior photodegradation rate of organic dyes that is dozens of times faster than that over commercial TiO2−xNx under visible light irradiation.7,8 Particularly, its remarkable quantum yields of up to 90% indicate that Ag3PO4 has very low recombination rates of photoexcited carriers. Therefore, Ag3PO4 has been becoming one of the most promising photocatalysts in harvesting solar energy for environmental purification and clean energy production.9 To further enhance photocatalytic activity of Ag3PO4, different strategies have been developed, from the control of its size to morphology, such as spheres, 10,11 rhombic dodecahedrons,12 concave trisoctahedrons and tetrahedron,13 cubes,12,14 2D dendritic morphology,15 nanorods,16 tetrapods,17 and flower-like nanospheres.18 The photocatalytic properties of Ag3PO4 with different morphologies depend mainly on its exposed facets with distinct surface energies. Martin et al. demonstrated that in comparison to rhombic dodecahedron {110} and cubic {100} structures, tetrahedral Ag3PO4 crystals show extremely high activity for water photooxidation, with an initial oxygen evolution rate exceeding 6 mmol h−1 g−1, 10 times higher than either {110} or {100} facets. Moreover, the © XXXX American Chemical Society

internal quantum yield for water photooxidation is almost unity at 400 nm, and greater than 80% from 365 to 500 nm, achieved by {111} terminated tetrahedrons.19 The low structural stability of pure Ag3PO4 is, unfortunately, the major obstacle for its practical applications. Wang et al. found that the photocatalytic activity of Ag3PO4 can be enhanced as Ag nanoparticles deposited on Ag3PO4 because the Ag3PO4 decomposition could capture the photogenerated electrons and thus prevent the recombination of electron−hole pairs within the Ag3PO4 samples at the initial stage of photocatalytic reactions. However, the photoactivity decreases with increasing Ag contents due to the formation of Ag layers on the surface of Ag3PO4. The Ag layers would shield light absorption, thus inhibiting the transfer of holes from the valence band of Ag3PO4 to the interface between photocatalyst and solution, and also hinder the contact of dye molecules with Ag3PO4, and accordingly, the photocatalytic activity deteriorates gradually.11 This deterioration of the Ag3PO4 photocatalytic activity due to photocorrosion largely limits its practical application as a recyclable highly efficient photocatalyst. As a potentially attractive approach, composites of Ag3PO4 with other materials can help to improve the stability of Ag3PO4 photocatalysts and maintain or even enhance their activities without using sacrificial reagents. Many Ag3PO4 composite systems, such as TiO2/Ag3PO4,20,21 Fe3O4/Ag3PO4,10 CeO2/ Ag3PO4,22 AgX/Ag3PO4 (X = Cl, Br, I),12 and SnO2/Ag3PO4,23 have been fabricated and have demonstrated better photoReceived: April 7, 2014 Revised: May 24, 2014

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dx.doi.org/10.1021/jp5034273 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. (a) Top and (b) side view of the simulating interface between graphene and the cubic Ag3PO4(100) surface model. Gray, red, purple, and blue spheres represent C, O, P, and Ag atoms, respectively. The symbols on the right side are used to designate the central location of corresponding atomic layers, respectively. For example, O-3 is the central location of the third layer of oxygen from the surface.

appropriate Hubbard U values for Ag 4d, O 2p, and P 3p are 7.2, 7.0, and 7.0 eV, respectively. A Morkhost−Pack mesh of k points, 2 × 2 × 1 and 4 × 4 × 1 points, is used, respectively, to sample the two-dimensional Brillouin zone for geometry optimization and for calculating the density of states. The cutoff energy for plane waves is chosen to be 340 eV, and the convergence tolerance of force on each atom during structure relaxation is set at 0.01 eV/Å. The strong light absorption is one of fundamental premises for a high-efficiency photocatalyst. The frequency-dependent dielectric matrix is calculated for pure Ag 3 PO 4 and Ag3PO4(100) and for the hybrid Ag3PO4/GR nanocomposite by the Fermi golden rule within the dipole approximation. The imaginary part of the dielectric function due to direct interband transitions is given by the expression

catalytic activity and much more stability than pure Ag3PO4. More recently, Ag3PO4/graphene (GR) composite has been found to have high photocatalytic stability (almost no loss of photocatalytic activity after four recycles), as well as superior visible-light-driven photocatalytic performance. For example, it has the efficiency of nearly 100% in 2 min for rhodamine B degradation under visible-light irradiation,24 making it possible for practical applications. The underlying mechanisms for these behaviors have, however, not yet been unraveled. In this work, the structural and electronic properties of hybrid Ag3PO4/GR composite have been investigated using large-scale density functional theory (DFT) computations to understand the mechanisms of its high photocatalytic activity and stability. We find that the band gap is reduced to 0.53 eV due to interfacing with GR, resulting into a strong absorption in the entire visible region and thus superior photocatalytic activity. The electrostatic potential distribution in the interface, where the potential at GR is higher than that in Ag3PO4, can effectively inhibit the reducibility of Ag+ ions in the Ag3PO4(100) surface and therefore improving its stability. This study would provide some new insight into optimizing the photocatalytic properties of Ag3PO4-based composites.

ε2(ℏω) =

2e 2π Ωε0

∑ |⟨ψkc|u·r|ψkv⟩|2 δ(Ekc − Ekv − E) k ,v ,c

(1)

where Ω, ω, u, v, and c are the unit-cell volume, photon frequency, 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 on the basis of ε1 and ε2.

2. COMPUTATIONAL DETAILS To construct the complex of Ag3PO4 and GR, we choose a stoichiometric cubic Ag3PO4(100) slab because it is the most stable one among the low index surfaces. In a supercell (12.01 × 12.25 × 23 Å3), a 5 × 6 single GR layer containing 60 carbon atoms sits on a 2 × 2 seven atomic layer stoichiometric Ag3PO4(100) surface slab containing 64 atoms with three bottom layers fixed at bulk position and is followed by a 15 Å thick vacuum layer to avoid artificial interaction. The whole system contains totally 124 atoms with 736 valence electrons. The local density approximation (LDA) with inclusion of the van der Waals (vdW) interaction is chosen because long-range vdW interactions are expected to be significant in such a complex.25 However, LDA has been known generally to underestimate the energy gap of semiconductor, resulting into an overestimate for photoinduced electrons transfer in photocatalytic process.26 To correct this band gap problem, all of the theoretical calculations are performed using the DFT/ LDA+U method implemented in the plane wave basis CASTEP code.27 We performed extensive tests to determine the appropriate U parameters for Ag 4d states, which reproduced the correct energy gap (2.48 eV) for cubic Ag3PO4. The

3. RESULTS AND DISCUSSION 3.1. Geometric Structure. The interaction between the Ag3 PO 4 surface and GR determines the stability and mechanisms of the electron transfer, as well as the photocatalytic efficiency of the composite. The Ag3PO4/GR geometry and separation characterize the strength of the interfacial interaction. Parts a and b of Figure 1 present the top and side views of the Ag3PO4(100)/GR interface model used in our calculations, respectively. Geometry optimizations have first been performed for the system using the conjugate gradient method. The equilibrium distance between the GR sheet and the top of the Ag3PO4 (110) surface is calculated to be 2.65 Å, which is about equal to those between the GR sheet and other materilas (2.6528 and 2.617 Å29 for TiO2(110)/GR, 2.85 Å for TiO2(001)/GR,30 2.422−2.866 Å for ZnO(0001)/GR31). After optimization, the GR sheet is still flat, indicating that the Ag3PO4−GR interaction is indeed vdW rather than covalent, in accordance with the others’ results.29,31 Closer inspection of the atomic position at the interface reveals that due to the B



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Figure 2. Band structures for (a) bulk Ag3PO4, (b) Ag3PO4 (100), (c) graphene, and (d) Ag3PO4(100)/GR. The horizontal dashed line indicates the Fermi level. The calculated (e) HOMO and (f) LUMO of Ag3PO4(100)/GR with an isovalue of 0.0075 e/Å3. The blue and yellow isosurfaces indicate the positive and negative wave functions, respectively.

of surface states at the bottom of CB. Therefore, the absorption edge of Ag3PO4(100) can extend to a longer wavelength in visible-light region. Figure 2c shows that the monolayer graphene sheet is a zero-gap semiconductor, and therefore graphene cannot be used directly in many applications such as photocatalysis, field-effect transistors. In the hybrid Ag3PO4(100)/GR, interestingly, the CB minimum appears at the one point within the AB line, whereas the VB maximum is located at G point, as shown in Figure 2d. Thus, the Ag3PO4(100)/GR is an indirect-gap semiconductor. As is known, the electron−hole recombination in a direct band gap semiconductor does not involve any phonons because there is no need for momentum change for the electron. In contrast, in an indirect gap semiconductor, the excited electron located in the CB needs to undergo a change in momentum state before it can recombine with a hole in the VB, as a result, conservation of momentum demands that the electron−hole recombination must be accompanied by the emission of a phonon, because it is not possible to make this recombination by the emission of a photon alone. Therefore, the recombination of the excited electron−hole pairs in the Ag3PO4(100)/GR composite would partly be inhibited due to its indirect band gap character. Moreover, the Ag3PO4(100)/ GR has an Eg of 0.53 eV (Figure 2d) due to the Fermi level down-shifting by 0.655 eV with respect to the Dirac point of GR. Such small Eg of the Ag3PO4(100)/GR would cause its absorption spectrum covering the entire visible region, even infrared light. Overall, both the small Eg and indirect band gap could dramatically enhance the photocatalytic performance of Ag3PO4/GR complex. To understand the electronic structure of the Ag3PO4(100)/ GR composite, we evaluate the total density of states (TDOS) of the composite and local density of states (LDOS) of O, P, and Ag atoms in each layer, as displayed in Figure 3. It is clear that the upper part of VB is only consisted of C 2p states, which greatly broadens the upper VB of the Ag3PO4(100)/GR composite. The broadening of the upper VB due to the

interaction between the Ag3PO4 surface and GR, the Ag atoms in the top layer (Ag-1) are pushed downward about 0.06 Å by the GR sheet, whereas the O atoms in the top layer (O-1) slightly move upward about 0.02 Å. The rearrangements of atoms in the top two layers of Ag3PO4(100) indicate that the electron transfer occurs at the interface, which will be discussed later. The stability of the hybrid Ag3PO4/GR can be assessed by the interface adhesion energy, which is defined as Ead = Ecomb − EGR − E Ag PO4 (100) 3

(2)

where Ecomb, EGR, and EAg3PO4(100) represent the total energy of the relaxed Ag3PO4(100)/GR, pure GR sheet, and clean Ag3PO4(100) surface, respectively. By this definition, negative Ead suggests that the adsorption is stable. The interface binding energy is calculated to be −0.76 eV for this model interface, which indicates a rather strong interaction between GR and Ag3PO4(100) surface, and the high thermodynamical stability. 3.2. Band Structure and Density of States. To investigate the effect of GR hybridization on the electronic properties of Ag3PO4, the band structures for pure Ag3PO4, Ag3PO4(100) surface, GR sheet, and hybrid Ag3PO4(100)/GR are calculated and shown in Figure 2. In pure Ag3PO4, the valence band (VB) maximum is at the M point and the conduction band (CB) minimum is located at G point in the Brillouin zone, and the indirect band gap (Eg) between M and G is 2.48 eV (Figure 2a), in agreement with the experimental value (2.45 eV).7 Therefore, Ag3PO4 is an indirect band gap semiconductor. Compared with the top of VB, the bottom of CB is very dispersive, suggesting that the photogenerated electrons possess smaller effective masses, which promote the separation of electron−hole pairs during the reaction process and, therefore, good photocatalytic activity. Though its (100) surface is exposed, Ag3PO4 becomes a direct band gap semiconductor and the Eg decreases to 2.15 eV, as shown in Figure 2b. The reduction of band gap arises from the presence C



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producing well-separated electron−hole pairs. Moreover, the hybridization between Ag 4d, O 2p, and C 2p states near the Fermi level would dramatically affect the optical properties of the hybrid Ag3PO4(100)/GR. 3.3. Charge Transfer and Mechanism Analysis. Such a strong change of the DOSs also indicates a substantial charge transfer between the involved constituents. This can be visualized by three-dimensional charge density difference (Δρ = ρAg3PO4(100)/GR − ρAg3PO4(100) − ρGR, where ρAg3PO4(100)/GR, ρAg3PO4(100), and ρGR are the charge densities of the composite, Ag3PO4(100) surface, and free-standing GR in the same configuration, respectively), as shown in Figure 4b. We observe a significant variation of the charge density at the interface due to the adsorption of the GR. A strong charge accumulation is found just above the Ag atoms in the top layer (Ag-1), whereas the regions of charge depletion appear both on the lower side of the GR (facing the surface) just above the O atoms in the top layer (O-1) and on the higher side of the Ag atoms in the top layer in the ground electronic state. Part c of Figure 4 plots the planar averaged charge density difference along the direction perpendicular to the Ag3PO4(100) surface, which offers quantitative results of charge redistribution. The positive values represent electron accumulation, and negative values indicate electron depletion. One can see that the largest efficient electron accumulation localized above the Ag atoms in the top layer is about 15.0 × 10−4 e/Å3, the largest local efficient electron depletion above the O atoms in the top layer is about −17.5 × 10−4 e/Å3, and the efficient electron accumulation above the GR sheet is about 2.5 × 10−4 e/Å3). This clearly demonstrates that the electron transfer occurs in the interface due to the coupling of GR and Ag3PO4(100) surface. To quantitatively analyze the charge variation at the interface, the Mulliken population analysis of the plane-wave pseudopotential calculations has been performed on the GR sheet, Ag3PO4(100) surface, and Ag3PO4(100)/GR composite. Figure 5 shows the results of the Mulliken charge on different atoms, in which several typical values are denoted. The 3-fold coordinated O in the top layer of Ag3PO4(100) surface and Ag3PO4(100)/GR composite have a Mulliken charge of −1.0 and −0.96, respectively, indicating that the electron of O atoms in the top layer is reduced in the composite. The 2-fold coordinated Ag in the top layer of Ag3PO4(100) surface and Ag3PO4(100)/GR composite have a Mulliken charge of +0.58 and +0.85, respectively. The charge variation demonstrates that

Figure 3. Total and local density of states for Ag3PO4(100)/GR and each kind of atom in each layer, respectively. The vertical dashed line indicates the Fermi level.

presence of C 2p states would increase the mobility of photogenerated carriers, which improves the photocatalytic activity of the hybrid Ag3PO4(100)/GR. A large amount of hybridization between Ag 5s and Ag 5s states from the Ag atoms in different layers, especially the Ag-2 layer, forms the lower part of CB. These highly dispersive Ag 5s-Ag 5s hybrid bands without the “contamination” of d states are advantageous for the carrier transfer. Due to the interaction with GR, the electronic states of the top two layers (O-1 and Ag-1) change significantly; i.e., O 2p and Ag 4d states extend toward Fermi level, as shown in the second arrow panels in Figure 3. These states obviously hybride with C 2p states at the upper part (−1.0−0 eV) of VB. As is known, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of pure Ag3PO4 are dominated by O 2p and Ag 5s electrons, respectively. In the case of hybrid Ag3PO4(100)/GR, some O 2p orbitals are added to the LUMO, but the HOMO will be dominated by GR, as shown in Figure 2e,f. Under visible light irradiation, electrons in the HOMO can be directly excited to the CB of Ag3PO4(100)/GR,

Figure 4. (a) Profile of the planar averaged self-consistent electrostatic potential for the Ag3PO4(100)/GR as a function of position in the zdirection. (b) 3D charge density difference for the Ag3PO4(100)/GR nanocomposite with an isovalue of 0.006 e/Å3. Blue and yellow isosurfaces represent charge accumulation and depletion in the space. (c) Profile of the planar averaged charge density difference for the Ag3PO4(100)/ GR as a function of position in the z-direction. The horizontal lines denote the central location of each atomic layer. D



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Figure 5. Charge distribution maps of (a) Ag3PO4(100)/GR and (b) Ag3PO4 (100), with a isovalue of 0.7 e/Å3. Gray, red, purple, and blue spheres represent C, O, P, and Ag atoms, respectively.

Figure 6. Calculated (a) imaginary part of the dielectric function and (b) absorption spectra of the bulk Ag3PO4 (red dash line), cubic Ag3PO4(100) surface (green dot-dashed line), and the hybrid Ag3PO4(100)/GR (blue solid line) for the polarization vector perpendicular to the surface.

function of position in the z-direction is displayed in Figure 4a. In the Ag3PO4 lattice (from Ag-1 to O-4 layer), the periodic lattice potential is clear although it has some distortion due to the atoms in first four layers (Ag-1/O-1, P-1/Ag-2) having a slight movement compared to their positions in bulk Ag3PO4. Because the potential at GR is high, a potential well is formed between GR and Ag3PO4(100) surface, which is markedly different from that at the semiconductor−semiconductor interface, such as the SrTiO3−TiO2 interface.41 The existence of a potential well can effectively hinder the recombination of photogenerated charge carriers in the Ag3PO4(100)/GR composite. The most striking feature in Figure 4a is that a large built-in potential of around 48 eV is formed, and therefore the electrons could be pumped from the Ag3PO4(100) slab to the GR sheet, resulting in the net efficient electrons accumulation at GR. Moreover, the built-in potential is large enough to drive efficient charge separation in the composite. It is reasonable to deduce, therefore, that the existence of built-in potential at the Ag3PO4(100)/GR interface would be the primary mechanism for improving the photocatalytic activity and stability of Ag3PO4/GR photocatalyst. 3.4. Optical Properties. The improved photocatalytic activity of the Ag3PO4/GR composite under visible light irradiation has been experimentally demonstrated.24 To explore the mechanism of enhanced visible light absorption, the calculated imaginary part of the dielectric function and UV− vis absorption spectra of pure Ag3PO4, Ag3PO4(100) surface, and the hybrid Ag3PO4(100)/GR composite are illustrated in Figure 6, respectively. Figure 6 displays that the optical absorption for pure Ag3PO4 occurs at about 2.48 eV, which is attributed to the intrinsic transition from the O 2p to Ag 5s orbitals. For pure Ag3PO4(100) surface, the absorption edge

the Ag atoms in the top layer of the Ag3PO4(100)/GR composite would lose more electrons than the Ag3PO4(100) surface. This would be the physical mechanism of enhanced stability of Ag3PO4/GR photocatalyst. Although the C atom in the pure graphene has a Mulliken charge of zero electrons, those C atoms in the Ag3PO4(100)/GR composite have different Mulliken charges because the arrangement of atoms under various C atoms is different. For examples, the C atom directly over the Ag atom has a Mulliken charge of −0.08, and its next one has a Mulliken charge of −0.03, whereas the C atom right above the O atom has a Mulliken charge of +0.01. As a result, the charge distribution fluctuations appear in GR sheet due to the interactions between GR and Ag3PO4(100) surface. The charge redistribution in the GR sheet is distinct from that in hybrid TiO2/GR, where each C atom loses some electron,28 or vice versa.30 This might be attributed to the different atom configurations in the interface (one is metal Ag atom, the other is nonmetal O atom). A further charge analysis based on the Bader method32 demonstrates that there is an average charge transfer of about 0.008 e per C atom from Ag3PO4 to GR, and each Ag atom loses 0.508 e and each O atom obtains 0.708 e in the Ag3PO4/GR composite. The charge redistribution in the Ag3PO4/GR composite, as shown in Figure 4b, is a novel electron-transfer, partly different from those in TiO2/GR,28,30,33−35 ZnO/GR,31 TiO2/carbon nanotube,36 C60/TiO2,37 metal/GR,38,39 and semiconductor-metal composites,40 in which the charge merely transfers from one constituent to another. The origin of such an interface charge redistribution can be traced to the electrostatic potential distribution in whole system. The profile of the planar averaged self-consistent electrostatic potential for the Ag3PO4(100)/GR composite as a E



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Figure 7. Maps of the electron and hole density distributions for the VB and CB states with an isovalue of 0.006 e/Å3 for the hybrid Ag3PO4/GR. Here, VBM and CBM are determined by the highest-occupied and lowest-unoccupied bands, respectively.

shifts to 2.15 eV due to the presence of surface states, also corresponding to the intrinsic transition. Previous investigations found that the electron transition leads to the formation of Ag layers on the surface of Ag3PO4, i.e., photocorrosion, which largely limits its practical application as a recyclable highly efficient photocatalyst. Moreover, the absorption intensities of both pure Ag3PO4 and Ag3PO4(100) surface are weak in the visible-light region, as shown in Figure 6. The two serious limitations can be improved dramatically by the GR combination. As can be clearly seen from Figure 6, the red shift of absorption edge is as large as about 1.50 eV for Ag3PO4(100)/GR compared to that for pure Ag3PO4 and Ag3PO4(100) surface. The large red shift is caused by the transition from the C 2p to Ag 5s states (Figure 3). Furthermore, the absorption intensity of Ag3PO4(100)/GR is enhanced significantly in the UV−vis light region, especially two peaks characterized by resonant-like absorption (I and II) appear at about 300 and 440 nm respectively, owing to the coupling of Ag3PO4 and GR sheet. After careful considerations of the positions of levels and needed energy related to the two peaks, we propose a possible transition mechanism as follows. Under irradiation of light, the normal photoexcitation in pure Ag3PO4 is from the O 2p states to the Ag 5s orbitals. Because the Ag 5s states in the bottom of the CB are low (Figure 3), the values of s−p matrix elements in eq 2 are small, which results in very weak optical absorption in the visible-light region (Figure 6b). In the Ag3PO4/GR composite, C 2p states are predominant components of the top of the VB and hybridize with O 2p and Ag 4s orbitals at the bottom of the CB, as is stated above. Therefore, new visible light absorption bands from the transitions between the highlying VB states and the low-lying CB states will appear due to C 2p states involved. Under visible-light irradiation, electrons in GR as shown in VB-2, VB-1, and VBM are most likely to be directly excited into surface C 2p (CBM), Ag 5s (CB+1), and O 2p (CB+2) orbitals (Figure 7), respectively. The energy required for these three transitions corresponds to that of peak II (∼440 nm). The states (in particular, C 2p and Ag 5s states) involved in these three transitions are large (Figures 3 and 7); therefore, the values of s−p and p−p matrix elements in eq 2 are big, leading to the large intensity of the peak II. The transitions of peak I are more complicated than those of peak

II. We can only speculate that peak I may be attributed to the transitions likely from VB-2 to CB+6, VB-3 to CB+5, VB-4 to CB+4, and VB-5 to CB+3 (Figure 7). The strong absorption peak II in the visible light region is fundamental to the enhancement of photocatalytic activity of the Ag3PO4/GR composite. The electron spin resonance (ESR) results found that certain visible light irradiation is crucial to the generation of •OH and O2−• radical species.24 Therefore, the light with a wavelength of ∼440 nm might be the most appropriate visible light for generation of radical species in the Ag3PO4/GR composite. With the light irradiation, photogenerated electrons would transfer from GR to the CB of the composite, leading to an efficient hole accumulation in the GR. Thus, the GR sheet is a sensitizer to Ag3PO4 in this composite, just like in TiO2/ GR.28 During the process of photocatalytic reaction, abundant • OH radicals are continuously produced on the GR due to the reaction of H2O and active holes, whereas O2−• radicals can be continuously generated due to a large amount of electrons accumulated at the Ag3PO4. The two kinds of radicals on the surface of Ag3PO4/GR composite has been directly confirmed by ESR results.24 This continuous process not only accelerates the separation of photogenerated electron−hole pairs but also effectively prevents the reduction of Ag+ to Ag atom in the photocatalytic process. As a result, the hybrid Ag3PO4/GR composite has superior visible-light-driven photocatalytic activity and stability. It should be pointed out that because the charge transfer in the photocatalysis is a dynamical process, the studies on mechanisms for charge separation and energy losses in composite, particularly, in the GR-based composites where charge separation competes with energy losses that can result in rapid electron−hole annihilation inside metallic GR, are of great importance. Recently, Long et al.29 investigated the mechanisms of the photoinduced interfacial charge transfer, energy relaxation, and energy transfer in a hybrid GR-TiO2 system in real time and at the atomistic level using a mixed quantum-classical approach that combines TD-DFT with NAMD. They found that the electron injection from GR into TiO2 occurs on an ultrafast time scale due to strong donor− acceptor coupling, and the photoinduced electron transfer is several times faster than the electron−phonon energy relaxation (i.e., charge separation is efficient in the presence of electron− F



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(5) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. SemiconductorBased Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503−6570. (6) Xiang, Q.; Yu, J. Graphene-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2013, 4, 753−759. (7) Yi, Z. G.; Ye, J. H.; Kikugawa, N.; Kako, T.; Ouyang, S. X.; StuartWilliams, H.; Yang, H.; Cao, J. Y.; Luo, W. J.; Li, Z. S.; Liu, Y.; Withers, R. L. An Orthophosphate Semiconductor with Photooxidation Properties Under Visible-Light Irradiation. Nat. Mater. 2010, 9, 559−564. (8) Bi, Y.; Ouyang, S.; Umezawa, N.; Cao, J.; Ye, J. Facet Effect of Single-Crystalline Ag3PO4 Sub-microcrystals on Photocatalytic Properties. J. Am. Chem. Soc. 2011, 133, 6490−6492. (9) Huang, G. H.; Ma, Z. L.; Huang, W. Q.; Tian, Y.; Chao, J.; Yang, Z. M.; Wan, Z.; Pan, A. Ag3PO4 Semiconductor Photocatalyst: Possibilities and Challenges. J. Nanomater. 2013, 371356, 1−8. (10) Li, G.; Mao, L. Magnetically Separable Fe3O4-Ag3PO4 Submicrometre Composite: Facile Synthesis, High Visible Light-driven Photocatalytic Efficiency, and Good Recyclability. RSC Adv. 2012, 2, 5108−5111. (11) Wang, W. G.; Cheng, B.; Yu, J. G.; Liu, G.; Fan, W. H. VisibleLight Photocatalytic Activity and Deactivation Mechanism of Ag3PO4 Spherical Particles. Chem.Asian J. 2012, 7, 1902−1908. (12) Bi, Y. P.; Ouyang, S. X.; Cao, J. Y.; Ye, J. H. Facile Synthesis of Rhombic Dodecahedral AgX/Ag3PO4 (XCl, Br, I) Heterocrystals with Enhanced Photocatalytic Properties and Stabilities. Phys. Chem. Chem. Phys. 2011, 13, 10071−10075. (13) Jiao, Z.; Zhang, Y.; Yu, H.; Lu, G.; Ye, J.; Bi, Y. Concave Trisoctahedral Ag3PO4 Microcrystals with High-Index Facets and Enhanced Photocatalytic Properties. Chem. Commun. 2013, 49, 636− 638. (14) Bi, Y.; Hu, H.; Ouyang, S.; Jiao, Z.; Lu, G.; Ye, J. Selective Growth of Metallic Ag Nanocrystals on Ag3PO4 Submicro-Cubes for Photocatalytic Applications. Chem.Eur. J. 2012, 18, 14272−14275. (15) Bi, Y.; Hu, H.; Jiao, Z.; Yu, H.; Lu, G.; Ye, J. Two-Dimensional Dendritic Ag3PO4 Nanostructures and Their Photocatalytic Properties. Phys. Chem. Chem. Phys. 2012, 14, 14486−14488. (16) Dong, P.; Wang, Y.; Li, H. H.; Li, H.; Ma, X.; Han, L. ShapeControllable Synthesis and Morphology-Dependent Photocatalytic Properties of Ag3PO4 Crystals. J. Mater. Chem. A 2013, 1, 4651−4656. (17) Wang, H.; He, L.; Wang, L.; Hu, P.; Guo, L.; Han, X.; Li, J. Facile Synthesis of Ag3PO4 Tetrapod Microcrystals with an Increased Percentage of Exposed {110} Facets and Highly Efficient Photocatalytic Properties. CrystEngComm 2012, 14, 8342−8344. (18) Yang, Z. M.; Tian, Y.; Huang, G. F.; Huang, W. Q.; Liu, Y. Y.; Jiao, C.; Wan, Z.; Yan, X. G.; Pan, A. Novel 3D Flower-like Ag3PO4 Microspheres with Highly Enhanced Visible Light Photocatalytic Activity. Mater. Lett. 2014, 116, 209−211. (19) Martin, D. J.; Umezawa, N.; Chen, X. W.; Ye, J. H.; Tang, J. W. Facet engineered Ag3PO4 for efficient water photooxidation. Energy Environ. Sci. 2013, 6, 3380−3386. (20) Yao, W.; Zhang, B.; Huang, C.; Ma, C.; Song, X.; Xu, Q. Synthesis and Characterization of High Efficiency and Stable Ag3PO4/ TiO2 Visible Light Photocatalyst for the Degradation of Methylene Blue and Rhodamine B Solutions. J. Mater. Chem. 2012, 22, 4050− 4055. (21) Rawal, S. B.; Sung, S. D.; Lee, W. I. Novel Ag3PO4/TiO2 Composites for Efficient Decomposition of Gaseous 2-Propanol Under Visible-Light Irradiation. Catal. Commun. 2012, 17, 131−135. (22) Yang, Z. M.; Huang, G. F.; Huang, W. Q.; Wei, J. M.; Yan, X. G.; Liu, Y. Y.; Jiao, C.; Wan, Z.; Pan, A. Novel Ag3PO4/CeO2 Composite with High Efficiency and Stability for Photocatalytic Applications. J. Mater. Chem. A 2014, 2, 1750−1756. (23) Zhang, L.; Zhang, H.; Huang, H.; Liu, Y.; Kang, Z. Ag3PO4/ SnO2 Semiconductor Nanocomposites with Enhanced Photocatalytic Activity and Stability. New J. Chem. 2012, 36, 1541−1544. (24) Yang, X.; Cui, H.; Li, Y.; Qin, J.; Zhang, R.; Tang, H. Fabrication of Ag3PO4-Graphene Composites with Highly Efficient and Stable

phonon relaxation). Considerable effort has also been paid to the dynamical process of charge transfer in other hybrid systems.42−44 More recently, the modifications to the electronic structure of graphene induced upon interfacing with disparate types of materials have been reviewed.45 In this work, the reasonable explanations of the calculated results in the ground electronic state for both enhanced photocatalytic activity and stability of the hybrid Ag3PO4/GR are clear and are expected to be robust. To deeply reveal the mechanisms of electron transfer, relaxation, and recombination dynamics, and the effect of these processes on the photocatalytic properities in hybrid Ag3PO4/GR, more experimental and theoretical studies are needed.

4. CONCLUSION The electronic structure and mechanisms of superior visiblelight-driven photocatalytic activity and stability have been investigated in the hybrid Ag3PO4/GR nanocomposite under the framework of DFT. The Ag3PO4−GR interaction is vdW rather than covalent, although the atoms in the top two layers of Ag3PO4(100) are rearranged due to the effect of GR. The charge distribution of GR sheet has been obviously altered by the interactions between GR and Ag3PO4(100) surface. A large built-in potential is formed at the interface, and it can prompt charge transfer from Ag3PO4 into GR in the ground electronic state, thus improving the stability of the hybrid Ag3PO4(100)/ GR. The band structure of the Ag3PO4/GR is greatly modified due to the presence of C 2p states in the upper VB and its energy gap is reduced to 0.53 eV. This leads to a significant enhancement of photocatalytic activity under visible light irradiation. The resonant-like absorption peak near 440 nm in the calculated absorption spectrum might be the most appropriate visible light for generation of radical species (O2−• and •OH) in the Ag3PO4/GR composite, just as found in the ESR experiment. These results provide explanations for the superior photocatalytic activities and stability of Ag3PO4/ GR nanocomposite found in experiments.

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AUTHOR INFORMATION

Corresponding Authors

*W.-Q. Huang: e-mail, [email protected]. *G.-F. Huang: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Hunan Provincial Natural Science Foundation of China (Grant No. 12JJ3009), Science and Technology Plan Projects of Hunan Province (2013SK3148).

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