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Xiaojuan Chen , Youzhi Dai , Jing Guo , Tanhua Liu , and Xingyan Wang .... Longhai Feng , Lirong Kong , Zhenyuan Ji , Yi Wang , Xiaoping Shen , Shiqin...
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Reduced Graphene Oxide Grafted Ag3PO4 Composites with Efficient Photocatalytic Activity under Visible-Light Irradiation Bo Chai,* Jing Li, and Qian Xu School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, P. R. China ABSTRACT: The design and synthesis of highly efficient visible-light-driven photocatalysts through a facile, environmentally friendly, and economical method have become a key aim in the photocatalytic field. In this study, reduced graphene oxide grafted Ag3PO4 (RGO/Ag3PO4) composites with enhanced photocatalytic activity were prepared by the in situ deposition of Ag3PO4 nanoparticles on the surface of RGO sheets. The as-prepared RGO/Ag3PO4 composites were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV−vis diffuse-reflectance absorption, Fourier transform infrared, and Raman spectrometry, and photoluminescence. The photocatalytic degradation of Rhodamine B (RhB) over RGO/Ag3PO4 composites was investigated and optimized, suggesting that the optimal amount of RGO in the composites was 3 wt %. The significantly enhanced photocatalytic activity and stability of RGO/Ag3PO4 composites could be attributed to the excellent electron-accepting and -transporting properties of RGO. and stability.27−34 Considering these superior properties of graphene, it is expected that the photocatalytic activity and stability of the Ag3PO4 photocatalyst could be further improved by the modification of graphene. Very recently, several groups have reported the synthesis of graphene-based Ag 3 PO 4 photocatalysts with enhanced photocatalytic activity. Chen et al. have prepared the Ag3PO4/graphene oxide (Ag3PO4/GO) composites by a liquid-phase deposition method.35 Dong et al. have fabricated Ag3PO4/reduced GO (RGO) nanocomposites by a facile chemical precipitation approach in N,N-dimethylformamide solvent.36 Liang et al. have synthesized GO/Ag3PO4 composites through a solution approach via electrostatic interaction.37 Yang et al. have developed the Ag3PO4/graphene composites by a facile and effective hydrothermal method.38 Liu et al. have prepared GO-enwrapped Ag3PO4 composites through an ion-exchange method of CH3COOAg and Na2HPO4 in the presence of GO sheets.39 Fu’s groups have synthesized the novel Ag/Ag3PO4/graphene triple heterostructure composites in situ by the redox reaction between Ag+ and graphene.40 Therefore, the combination of graphene and Ag3PO4 could be regarded as an ideal system for achieving enhanced photocatalytic activity and stability. Although there have been some reports on RGO/Ag3PO4 composites, as a promising hybrid material for photocatalysis, the exploration of RGO/Ag3PO4 composites still has a long way to go, especially through a facile, environmentally friendly, and economical method. In this work, RGO/Ag 3 PO 4 composites were prepared by the in situ deposition of Ag3PO4 particles on the surface of RGO sheets at room temperature and applied to the photocatalytic degradation of a Rhodamine B (RhB) solution under visible-light irradiation. Conspicuously enhanced photocatalytic activity was achieved

1. INTRODUCTION Over the past few decades, the design and synthesis of highly efficient visible-light-driven photocatalysts have attracted considerable attention because of their potential applications in the water splitting for hydrogen production, removal and degradation of organic pollutants, and reduction of CO2 into renewable chemical fuels.1−3 To date, some novel and visiblelight-responsive semiconductor photocatalysts have been explored with the aim of improving the photocatalytic activity such as BiOX (X = Cl, Br, I),4,5 Bi2MoO6,6 Bi2WO6,7 AgX@Ag (X = Cl, Br, I),8,9 C3N4,10,11 InMO4 (M = V, Nb, Ta),12 and so on. Recently, silver orthophosphate (Ag3PO4) has been reported as a new visible-light-driven photocatalytst for the oxidation of water and photodecomposition of organic compounds.13−16 These works demonstrate that Ag3PO4 is a suitable candidate for the application in photocatalytic fields. However, pure Ag3PO4 will be photocorroded and decompose to weakly active silver during the photocatalytic reaction process, which is the main hindrance for the practical application of Ag3PO4 as a recyclable and highly efficient photocatalyst. To further improve the photocatalytic performance and stability of Ag3PO4, combining Ag3PO4 with a metal or other semiconductor to form hybrid materials is an effective way to promote the separation of photogenerated charge carriers and thus enhance the photocatalytic activity and stability. In this case, various coupled systems of Ag3PO4 such as AgBr/Ag3PO4,17,18 Ag/Ag3PO4,19 Ag3PO4/TiO2,20 Ag3PO4/ ZnO,21 Ag3PO4/SnO2,22 C3N4/Ag3PO4,23 Bi2MoO6/Ag3PO4,24 carbon nanotubes/Ag3PO4,25 and carbon quantum dots/ Ag3PO426 composites have been developed to enhance the photocatalytic activity and stability of Ag3PO4. Graphene, as a new carbon material, has recently attracted intense scientific interest owing to its special two-dimensional (2D) structure, large specific surface area, excellent mobility of charge carriers, and high chemical and thermal stability. These properties make it become an ideal support of the photocatalyst to enhance the transfer and separation of photogenerated electrons and holes, then enhancing the photocatalytic activity © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8744

December 4, 2013 March 17, 2014 May 6, 2014 May 6, 2014 dx.doi.org/10.1021/ie4041065 | Ind. Eng. Chem. Res. 2014, 53, 8744−8752

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working at 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed on a VG Multilab 2000 with Al Kα source operation at 300 W. The UV−vis diffuse-reflectance absorption spectrometry (DRS) spectra were obtained by a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere using BaSO4 as the reference sample. The Fourier transform infrared (FTIR) spectra of the samples were recorded on a Thermo Nicolet Avatar 360 spectrometer using conventional KBr pellets. Raman spectra were performed using a Renishaw RM-1000 spectrometer equipped with a 514.5 nm Ar+ laser for Raman excitation. The photoluminescence (PL) spectra were measured at room temperature on a Varian Cary Eclipse fluorescence spectrophotometer with the excitation wavelength at 320 nm. 2.5. Photocatalytic Activity Measurement. The photocatalytic activity of RGO/Ag3PO4 composites was evaluated by the degradation of a RhB aqueous solution under visible-light irradiation. A total of 50 mg of photocatalysts was added to 100 mL of a RhB solution with the initial concentration of 1.0 × 10−5 mol·L−1. A 500 W xenon lamp (Changzhou Yuyu ElectroOptical Device Co. Ltd., China) with a 420 nm cutoff filter provided visible-light irradiation. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption−desorption equilibrium. At given irradiation time intervals, 4 mL of each of the suspensions was collected and then the slurry samples including the photocatalysts and a RhB solution were centrifuged (10000 rpm, 10 min) to remove the photocatalysts. The TU-1810 spectrometer was used to measure the concentration changes of RhB solution with the wavelength of 554 nm.

by comparing with pristine Ag3PO4 in the present study. Moreover, the effect of mass ratios of RGO in the RGO/ Ag3PO4 composites on the photocatalytic activity was explored comparatively. The possible mechanism for enhanced photocatalytic activity was proposed based on the obtained experimental results.

2. EXPERIMENTAL SECTION 2.1. Preparation of Graphene Oxide (GO). GO was synthesized from natural graphite by a modified Hummers method.32 Graphite (2.5 g) and NaNO3 (1.25 g) were mixed with 60 mL of H2SO4 (95%) in a 500 mL flask. The mixture was stirred for 30 min in an ice bath. Under vigorous stirring, potassium permanganate (7.5 g) was added to the suspension. The rate of addition was carefully controlled to keep the reaction temperature below 20 °C. The mixture was stirred at room temperature overnight. Then, 75 mL of distilled water was slowly added under vigorous stirring. The reaction temperature was rapidly increased to 98 °C with effervescence, and the color changed to brown-yellow. The diluted suspension was stirred at 98 °C for 24 h. Afterward, 25 mL of 30% H2O2 was added to the mixture. For purification, the mixture was washed with a 5% HCl aqueous solution and distilled water several times, until the pH value of the washed solution was about 6.0. After filtration and drying under vacuum, solid GO was obtained. 2.2. Preparation of Reduced GO (RGO). RGO was prepared by the chemical reduction of GO with sodium borohydride.32 A total of 0.3 g of GO was dispersed in 50 mL of distilled water. Then, 0.5 g of sodium borohydride was added, and the mixture was heated at 100 °C for 24 h. After the reaction was completed, the solid RGO was centrifuged and repeatedly washed with distilled water, and the final product was dried in a vacuum at 80 °C for 24 h. 2.3. Preparation of RGO/Ag3PO4 Composites. RGO/ Ag3PO4 composites were prepared as follows: an appropriate amount of RGO was added to 30 mL of distilled water with sonication for 1 h to make RGO totally disperse. A total of 0.26 g of AgNO3 was dissolved in 1.5 mL of concentrated ammonia to form a transparent silver−ammino complex solution. Afterward, the later solution was added to the former one, and the resulting solution was stirred for 30 min before the addition of 0.183 g of Na2HPO4·12H2O. The pH value of the mixture was adjusted to 7.30 by a 0.1 M HNO3 solution. The suspensions were stirred for 4 h in the dark at room temperature. The resultant powders were washed with ethanol and distilled water several times and dried in a vacuum at 80 °C for 12 h to obtain the RGO-hybridized Ag3PO4 samples. For comparison, a series of RGO/Ag3PO4 composites with different mass ratios of RGO (from 0.5 to 6 wt %) in the composites were prepared by the addition of an appropriate amount of RGO. Similarly, the pristine Ag3PO4 was also prepared by following the same procedure as that above, except for the addition of RGO. For comparison, a 3 wt % RGO/Ag3PO4 composite is prepared by a mechanical mixing method, denoted as the “control sample”. 2.4. Material Characterization. The products were characterized by X-ray diffraction (XRD) patterns using a Bruker D8 Advance X-ray diffractometer with Cu Kα irradiation (λ = 0.154178 nm) at 40 kV and 40 mA. The morphology of the samples was investigated by a JSM-6700F microscope. Transmission electron microscopy (TEM) measurement was conducted using a JEOL JEM 2100F microscope

3. RESULTS AND DISCUSSION 3.1. XRD Analysis. The XRD patterns of GO, RGO, Ag3PO4, and the 3 wt % RGO/Ag3PO4 composite are shown in Figure 1. As can be seen from the diffraction pattern of GO,

Figure 1. XRD patterns of GO, RGO, Ag3PO4, and the 3 wt % RGO/ Ag3PO4 composite.

there is a strong and sharp diffraction peak at around 11° corresponding to the (002) reflection of GO, indicating that pristine graphite was fully oxidized into GO sheets.33 As for RGO, the sharp diffraction peak disappears and a very broad peak at about 26° is observed, confirming the formation of RGO by the chemical reduction of sodium borohydride.33 For the pattern of bare Ag3PO4, all of the diffraction peaks could be well indexed to the cubic phase of Ag3PO4 (JCPDS no. 060505), which is in agreement with previous reports.38,39 In addition, the 3 wt % RGO/Ag3PO4 composite exhibits a XRD 8745

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Figure 2. FESEM images of RGO, Ag3PO4, and the 3 wt % RGO/Ag3PO4 composite and TEM image of the 3 wt % RGO/Ag3PO4 composite: (a) RGO; (b) Ag3PO4; (c and d) 3 wt % RGO/Ag3PO4 composite.

could be attributed to P5+ in PO43−.42 The O 1s XPS spectrum illustrated in Figure 3d could be deconvoluted into two peaks at 530.4 and 532.1 eV, which correspond to the oxygen element of the Ag3PO4 crystal lattice and the hydroxyl or carboxyl groups on the surface of RGO, respectively.43 As shown in Figure 3e, the high-resolution C 1s spectrum of GO can be fitted with three peaks. The peak located at 284.6 eV is usually assigned to adventitious carbon and graphitic carbon from GO. Another two weak peaks positioned at 286.6 and 288.4 eV corresponded to hydroxyl carbon (C−OH) and carboxyl carbon (OC−O), respectively.34 In contrast to the GO spectrum, the peak for C−OH in RGO decreases remarkably. This implies that GO is partially reduced and some oxygencontaining functional groups are removed during a chemical reduction process.34 As for the C 1s spectrum of the 3 wt % RGO/Ag3PO4 composite, the peaks are similar to that of RGO. The XPS results confirm the coexistence of RGO and Ag3PO4 in the composites. On the basis of XPS analysis, the RGO surface contains some functional groups (hydroxyl and carboxyl) and the positively charged [Ag(NH3)2]+ complex can easily absorb onto the surface of negatively charged RGO sheets. With excess NH3· H2O in the system, free Ag+ ions can hardly be obtained, and no Ag3PO4 precipitation can be acquired after the addition of a Na2HPO4 solution to the mixture.16 Thus, the pH value of a mixture solution is adjusted to neutral conditions by dilute HNO3. Furthermore, as the Na2HPO4 aqueous solution is added, the [Ag(NH 3 ) 2 ] + complex has been gradually decomposed by H+ ions and released Ag+ ions to react with PO43− ions, which leads to crystallization of Ag3PO4 particles on the surfaces of RGO.

pattern similar to that of bare Ag3PO4. No diffraction peaks corresponding to RGO are observed in the RGO/Ag3PO4 composite, which may be due to the small amounts and relatively low diffraction intensity of RGO in the sample.38 3.2. Microstructure Analysis. The field-emission scanning electron microscopy (FESEM) images of RGO, Ag3PO4, and the 3 wt % RGO/Ag3PO4 composite are shown in Figure 2a−c. As can be seen from Figure 2a, the RGO nanosheets are curled and corrugated, which is consistent with the previous report.41 The pristine Ag3PO4 (Figure 2b) exhibits irregular particle morphology with diameters of 100−500 nm. The FESEM image of the 3 wt % RGO/Ag3PO4 composite is shown in Figure 2c. It seems that large amounts of Ag3PO4 particles are distributed on the surface of RGO sheets. In Figure 2d, the TEM image of the 3 wt % RGO/Ag3PO4 composite further shows that Ag3PO4 particles are attached on the surface of RGO, which is in good agreement with the FESEM observation. This structure might provide enough contact surface area between the RGO sheets and Ag3PO4 particles and also presumably facilitate charge-carrier transport. 3.3. XPS Analysis and Formation Process. XPS measurement was performed to determine the chemical composition and valence state of various species. The peak positions in all of the XPS spectra are calibrated with C 1s at 284.6 eV. Figure 3a exhibits the XPS survey spectrum of the 3 wt % RGO/Ag3PO4 composite. As expected, it contains silver, phosphorus, oxygen, and carbon elements. The peaks (Figure 3b) with binding energies of 367.6 and 373.5 eV in the highresolution XPS spectrum of Ag 3d are assigned to Ag 3d5/2 and Ag 3d3/2, indicating the existence of Ag+ in the composite.42 The XPS peak of P 2p is found at 132.4 eV (Figure 3c), which 8746

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Figure 3. XPS spectra of the 3 wt % RGO/Ag3PO4 composite: (a) XPS survey spectrum; (b) high-resolution Ag 3d spectrum; (c) high-resolution P 2p spectrum; (d) high-resolution O 1s spectrum; (e) high-resolution C 1s spectra of GO, RGO, and the 3 wt % RGO/Ag3PO4 composite.

3.4. Raman and FTIR Analysis. Raman spectra were further carried out to affirm the presence of RGO in the RGO/ Ag3PO4 composites. Figure 4 displays a comparison of the Raman spectra of GO, RGO, Ag3PO4, and the 3 wt % RGO/ Ag3PO4 composite. In the case of GO, there are two typical bands located at 1364 and 1606 cm−1, which correspond to disordered sp2 carbon (D band) and well-ordered graphite (G band), respectively. Compared with GO (ID/IG = 0.79), an increased D/G intensity ratio (ID/IG = 0.89) is observed in RGO, suggesting the reduction of GO to RGO.38 As for Ag3PO4, the sharp peak centered at about 912 cm−1 is attributed to the PO43− symmetric stretching vibration.25,36 For the RGO/Ag3PO4 composite, all of the Raman bands for Ag3PO4 and the graphitized structures (D and G bands) can be observed. Furthermore, the D/G intensity ratio is about 0.88, which is similar to that of bare RGO. The Raman results support the presence of RGO and Ag3PO4 in the composites. Figure 5 depicts the FTIR spectra of RGO, Ag3PO4, and the 3 wt % RGO/Ag3PO4 composite. The broad peak at about

Figure 4. Raman spectra of GO, RGO, Ag3PO4, and the 3 wt % RGO/ Ag3PO4 composite.

3420 cm−1 in the spectrum of RGO corresponds to the stretching vibration mode of −OH, and the physically adsorbed 8747

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Figure 6. DRS spectra of bare Ag3PO4 and the 3 wt % RGO/Ag3PO4 composite.

Figure 5. FTIR spectra of RGO, Ag3PO4, and the 3 wt % RGO/ Ag3PO4 composite.

H2O also contributes to this broad peak. The peak at 1720 cm−1 is attributed to the CO stretching of COOH groups. The peak at around 1610 cm−1 is related to the bending vibration of −OH and the skeletal vibration of RGO sheets. The broad absorption band centered at 1142 cm−1 is ascribed to the stretching vibration band C−O−C of epoxy and alkoxy. The peak at 1390 cm−1 is assigned to the tertiary C−OH group stretching vibration.36,37 For bare Ag3PO4, the two peaks at 3420 and 1650 cm−1 are related to the −OH stretching and bending vibrations of adsorbed H2O molecules. The peak at around 540 cm−1 is ascribed to the OP−O bending vibration, while the peaks at 850 and 1097 cm−1 are due to the symmetric and asymmetric stretching vibrations of P−O−P rings. One strong band featured at 1383 cm−1 derives from the stretching vibration of doubly bonded oxygen (PO) and the harmonics of the above modes.37 In the case of the RGO/ Ag3PO4 composite, the characteristic bands for Ag3PO4 still remain, but two new peaks emerge at 1575 and 1260 cm−1, which are attributed to the skeletal vibration of the graphene sheets and C−O stretching vibration of epoxy, proving the existence of RGO in the composite. In addition, the characteristic peak assigned to the stretching vibration of the P−O−P group also shifts to a higher wavenumber of 1122 cm−1 compared with that of 1097 cm−1 in bare Ag3PO4, suggesting that the interaction between Ag3PO4 and RGO has already appeared.37 The interaction between Ag3PO4 and RGO may benefit the photogenerated electron transfer and then enhance the photocatalytic activity of composites. 3.5. DRS Spectra. Figure 6 shows the DRS spectra of pristine Ag3PO4 and the 3 wt % RGO/Ag3PO4 composite. As can be seen, Ag3PO4 and the 3 wt % RGO/Ag3PO4 composite present the sharp absorption edge at about 506 nm, which are attributed to the intrinsic band-gap absorption of Ag3PO4. The band gap of Ag3PO4 is estimated to be 2.45 eV according to the equation of Eg = 1240/λg, where Eg is the band-gap energy of the semiconductor and λg is the optical absorption edge of the semiconductor. The RGO/Ag3PO4 composite presents more intensive absorption over the whole visible-light region compared with the spectrum of Ag3PO4, which could be ascribed to the introduction of RGO. 3.6. Photocatalytic Activity. Figure 7 shows the photocatalytic activities of pristine Ag3PO4 and RGO/Ag3PO4 composites with different mass ratios for degradation of the RhB solution. For comparison, the blank test and mechanical mixing sample were also conducted under the same reaction conditions. It could be seen that the degradation percentage of

Figure 7. Comparison of the photocatalytic activities of samples for degradation of the RhB solution.

RhB is very low in the absence of photocatalysts under visiblelight irradiation for 20 min, indicating that self-degradation of RhB is not obvious. Pristine Ag3PO4 exhibits weak photocatalytic activity with a degradation percentage of 45% after 20 min of visible-light irradiation. The RGO/Ag3PO4 composites greatly enhance the photocatalytic activities with degradation percentages of RhB of about 69%, 84%, 92%, and 82% for 0.5, 1, 3, and 6 wt % RGO/Ag3PO4 composites during 20 min of visible-light illumination, respectively. The mechanical mixing sample also displays an enhanced degradation percentage compared with bare Ag3PO4, suggesting that combining Ag3PO4 with RGO may enhance the photocatalytic performance. However, the photocatalytic activity of the mechanical mixing sample is lower than that of as-prepared RGO/Ag3PO4 composites, which is attributed to intimate contact between RGO and Ag3PO4 by the in situ deposition of Ag3PO4 particles on the surface of RGO sheets, thus remarkably enhancing the separation efficiency of photogenerated electrons and holes. As for a series of RGO/Ag3PO4 photocatalysts, the photocatalytic activity of composites increases with increasing RGO contents and the 3 wt % RGO/Ag3PO4 composite shows the highest photocatalytic performance. With further enhancement of the RGO amount, the photocatalytic performance decreases. The decrease in the photocatalytic activity at a higher mass ratio of RGO in the composites is considered to increase the absorbing and scattering of photons by RGO in the photoreaction system, leading to a decrease in the absorption efficiency of light for Ag3PO4 particles. A similar phenomenon has been observed in the case of TiO2/RGO composites.34 8748

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Ag3PO4 composite still reaches 88% in 30 min after four recycling runs, indicating that it has considerable photostability. FESEM and XRD studies were performed for the 3 wt % RGO/Ag3PO4 composite after four successive experimental runs, and the results are shown in Figure 10. In contrast to the as-prepared 3 wt % RGO/Ag3PO4 composite with a relatively smooth surface of Ag3PO4 particles (Figure 2c), the surface of Ag3PO4 for the used 3 wt % RGO/Ag3PO4 composite has rough morphology (Figure 10a), suggesting that Ag3PO4 in the composite is slightly decomposed. The XRD pattern (Figure 10b) of the recycled 3 wt % RGO/Ag3PO4 is essentially similar to that of the original one; there is no obvious deviation in the locations of these peaks, and the mild decrease of the peak intensities indicates moderately declined crystallinity after photocatalytic reaction. The results indicate that the RGO/ Ag3PO4 composites have slight photocorrosion during photocatalysis. To make the reaction mechanism clear, isopropyl alcohol (IPA), triethanolamine (TEOA), and p-benzoquinone (BQ) were respectively introduced as the scavengers of hydroxyl radicals (•OH), holes (h+), and superoxide radicals (•O2−) to examine the effects of reactive species on the photocatalytic degradation of RhB.44 The concentrations of IPA, TEOA, and BQ in the reaction system were 10, 10, and 1 mmol·L−1, respectively. In Figure 11, we could see that BQ and TEOA lead to a remarkable suppression of the degradation efficiency of RhB, whereas IPA hardly exhibits a restraining effect on the degradation efficiency. The results indicate that h+ and •O2− play a more important role than •OH in the photocatalytic degradation of RhB. On the basis the above results, a proposed mechanism is discussed to explain enhancement of the photocatalytic activity of the RGO/Ag3PO4 composite. As is well-known, efficient charge separation is crucial for enhancement of the photocatalytic activity. RGO has proven to be an effective electron acceptor and transporter in previous investigations.27−34 The RGO sheets could facilitate charge transfer and suppress the recombination of electron−hole pairs in the RGO-based photocatalysts. Therefore, it is believed that a reinforced charge separation might be achieved in the RGO/Ag3PO4 composites. As shown in Figure 12, when the RGO/Ag3PO4 composite is illuminated under visible light, the photogenerated electrons on the conduction band (CB) of Ag3PO4 could be effectively transferred to the RGO sheets, thus inhibiting the charge recombination on the surface of Ag3PO4 and improving the photocatalytic activity. Efficient electron transfer from Ag3PO4 to RGO sheets also sustains the stability of the RGO/Ag3PO4 composite by keeping electrons away from Ag3PO4 and inhibiting photocorrosion. As a result, these well-separated electrons could be trapped by the dissolved O2 to yield •O2−. On the other hand, it has been reported that the valence band (VB) potential of Ag3PO4 and the standard redox potentials of • OH/H2O are 2.64 and 2.72 V (vs NHE), respectively.18,45 This suggests that the photogenerated holes on the VB of Ag3PO4 could not react with H2O to form •OH. Consequently, the degradation of RhB would be the reaction with photogenerated holes directly, which is consistent with the previous report.15 The better separation of photogenerated electrons and holes in the RGO/Ag3PO4 composite is confirmed by PL emission spectra of single Ag3PO4 and the 3 wt % RGO/Ag3PO4 composite. It is well-known that the PL signals of semiconductor materials result from the recombination of photo-

Generally, the photocatalytic degradation of RhB could be considered as a pseudo-first-order reaction with low concentration, and its kinetics could be expressed as follows: −ln(C /C0) = kt

(1)

where k is the degradation rate constant and C0 and C are the absorption equilibrium concentration of RhB and the concentration of the pollution at a reaction time of t, respectively.25 As shown in Figure 8, the rate constants (k) of

Figure 8. Pseudo-first-order kinetic curves of RhB degradation over the different samples.

different samples are 0.03253, 0.07996, 0.11172, 0.12846, 0.10496, 0.05187, and 0.00058 min−1 for the pristine Ag3PO4 and 0.5, 1, 3, and 6 wt % RGO/Ag3PO4 composites, mechanical mixing sample, and the sample without catalysts, respectively. Under the same experimental conditions, the kinetic constant of 3 wt % RGO/Ag3PO4 is about 3.9 times as large as that of pristine Ag3PO4. The stability of a photocatalyst is important for its assessment and application. The recycled runs for the photocatalytic degradation of RhB over bare Ag3PO4 and the 3 wt % RGO/Ag3PO4 composite were performed to evaluate their photocatalytic stability. After every 30 min of photodegradation, the separated photocatalysts were washed with distilled water and dried. As shown in Figure 9, after four runs of the photodegradation of RhB, the photocatalytic activity of the 3 wt % RGO/Ag3PO4 composite displays a slight decrease, whereas the bare Ag3PO4 presents obvious deterioration. The photocatalytic degradation efficiency of the 3 wt % RGO/

Figure 9. Stability studies on the photocatalytic degradation of a RhB solution over bare Ag3PO4 and the 3 wt % RGO/Ag3PO4 composite under visible-light irradiation. 8749

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Figure 10. (a) FESEM image of the recycled 3 wt % RGO/Ag3PO4 composite. (b) XRD patterns of the 3 wt % RGO/Ag3PO4 composite before and after the recycling photocatalytic experiments.

Figure 13. PL emission spectra of bare Ag3PO4 and the 3 wt % RGO/ Ag3PO4 composite.

Figure 11. Effect of reactive species on the photocatalytic degradation process of a RhB solution over the 3 wt % RGO/Ag3PO4 composite.

nation of photogenerated charge carriers is inhibited. The result of PL verifies that the RGO/Ag3PO4 composite could effectively separate photogenerated electron−hole pairs.

4. CONCLUSIONS In summary, visible-light-driven RGO/Ag3PO4 composites were prepared by the in situ deposition of Ag3PO4 particles on the surface of RGO sheets and applied to the photocatalytic degradation of a RhB solution. The enhanced photocatalytic activity and stability is attributed to the excellent electronaccepting and -transporting properties of RGO, which is favorable for the separation of photogenerated electrons and holes. Moreover, photocatalytic mechanism investigations demonstrated that h+ and •O2− played a key role in the RGO/Ag3PO4 composite under visible-light irradiation. The resulting RGO/Ag3PO4 composite may be a promising efficient photocatalyst for the degradation of organic pollutants in the industrial and engineering field.

Figure 12. Proposed mechanism of enhanced photocatalytic activity for the RGO/Ag3PO4 composite.

induced charge carriers. In general, the lower PL intensity indicates a decrease in the recombination rate of photogenerated charge carriers. As shown in Figure 13, the strong emission peak is located at about 370 nm for the bare Ag3PO4, which could be attributed to the recombination of the chargetransfer transition between the O 2p orbital and the empty d orbital of central Ag+.37 The emission at around 500 nm corresponds to the photoexcited electrons on the CB recombining with holes on the VB, which is approximately equal to the band gap of Ag3PO4. Compared with single Ag3PO4, the emission peak intensity of the RGO/Ag3PO4 composite decreases moderately, indicating that the recombi-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+86)27-83943956. Fax: (+86)27-83943956. Notes

The authors declare no competing financial interest. 8750

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51302200).

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