Basic Principles for Observing the Photosensitizer Role of Graphene

Sep 23, 2013 - Basic Principles for Observing the Photosensitizer Role of Graphene in the Graphene–Semiconductor Composite Photocatalyst from a Case...
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Basic Principles for Observing the Photosensitizer Role of Graphene in the Graphene−Semiconductor Composite Photocatalyst from a Case Study on Graphene−ZnO Min-Quan Yang†,‡ and Yi-Jun Xu*,†,‡ †

State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, P. R. China ‡ College of Chemistry and Chemical Engineering, New Campus, Fuzhou University, Fuzhou 350108, P. R. China S Supporting Information *

ABSTRACT: The graphene−ZnO (GR−ZnO) nanocomposites have been fabricated via a facile, low-temperature in situ wet chemistry process, during which the reduction of graphene oxide and intimate interfacial contact between ZnO nanoparticles and GR nanosheets are achieved simultaneously. The GR−ZnO nanocomposites exhibit visible light photoactivity toward reduction of Cr(VI) in aqueous solution under ambient conditions. The large band gaps of GR−ZnO composites suggest that the visible light irradiation cannot photoexcite electrons in the valence band (VB) to conduction band (CB) of ZnO, thus ruling out the possibility that the visible light photoactivity of GR−ZnO is induced by the band gap photoexcitation of ZnO. Instead, under visible light irradiation, the GR sheet in GR−ZnO can be photoexcited from ground-state GR to excited-state GR* to generate electrons, which then inject into the CB of ZnO to make the GR−ZnO composites exhibit visible light photoactivity. Our results provide new robust proof of the role of GR as a macromolecular photosensitizer for semiconductors. Along with the fact that the photosensitization efficiency of GR is generally low, we propose that three basic principles are required to experimentally observe the exact photosensitizer role of GR in the GR−semiconductor composites under visible light. We hope that this work could boost further study on the photosensitizer role of GR and on how to improve the photosensitization efficiency of GR and inform ongoing interest in deep thinking of the microscopic charge carrier transfer pathway across the interface between GR and the semiconductor.

1. INTRODUCTION Graphene (GR)-based semiconductor composite photocatalysis has attracted a lot of attention due to its potential for converting solar energy into chemical energy,1−18 which is now considered to be one of the promising approaches to solve the problems of the world energy crisis and environmental issues.18−27 Given that GR possesses excellent electron conductivity and high specific surface area,28−30 it is highly desirable to use GR as a two-dimensional (2D) platform to accept and shuttle electrons photogenerated from semiconductors upon light irradiation, thus enhancing the photoactivity of semiconductors.11,12 So far, regarding various GR− semiconductor composite photocatalysts available in the literature, the enhancement of semiconductor photoactivity is usually ascribed to the fact that GR is able to accept and shuttle photogenerated electrons from semiconductors, by which the life span of photoexcited electron−hole pairs is prolonged, thereby resulting in the photoactivity improvement of GR− semiconductor composites.1−15,31−46 Besides the role of GR in accepting/shuttling photogenerated electrons from semiconductors, recent theoretical calculations suggest that GR can potentially act as a photosensitizer for semiconductor under visible light irradiation.47,48 For example, Du and co-workers have used largescale density functional theory (DFT) calculations to study the © 2013 American Chemical Society

GR/rutile TiO2(110) interface and found that electrons in the upper valence band (VB) can be directly excited from GR to the conduction band (CB) of TiO2 under visible light irradiation, which is in agreement with experimental wavelength-dependent photocurrent generation of the GR/TiO2 photoanode.48 However, only the photocurrent measurement cannot be sufficient enough to prove whether or not GR acts as a photosensitizer for a semiconductor during an actual photocatalytic process. Experimentally, Manga et al. have demonstrated that the ultrafast electron transfer can occur within 0.2 ps in the GR− titania hybrid film composites under visible light irradiation, which was proven by the femtosecond transient absorption spectroscopy using laser pulse excitation at 400 nm.49 Because titania is transparent to visible light (400 nm), they suggest that the electron transfer mechanism is driven by the absorption of visible light by the GR nanosheets and the subsequent injection of photoexcited electrons into the conduction band (d-orbital) of titania. However, this statement is not convincing because the band gap narrowing of titania resulting from its hybridization with GR is not considered. In other words, even the Received: August 22, 2013 Revised: September 16, 2013 Published: September 23, 2013 21724

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semiconductor ZnO instead of band gap photoexcitation of ZnO. That is, under visible light irradiation, GR can be photoexcited and transfer electrons to the conduction band of ZnO, thus transforming ZnO to be a visible light photocatalyst for reduction of Cr(VI). In particular, we show the key importance of specific interfacial interaction on observing the photosensitization role of GR in the as-prepared GR−ZnO composite photocatalyst. It is hoped that this work could inform ongoing interest in deep thinking of the microscopic charge carrier transfer pathway across the interface between GR and semiconductor and on how to improve the photosensitization efficiency of GR toward specific solar energy conversion.

slight band gap narrowing of titania in GR−titania can make it respond to 400 nm visible light irradiation, thereby inducing photogenerated charge carriers. In such a case, it is difficult to determine the exact transfer direction of charge carriers across the interface between GR and titania. Robust experimental evidence should be based on a GR−semiconductor composite system for which the semiconductor cannot be band gap photoexcited, and under this circumstance, the role of GR is able to be adequately investigated. Very recently, our group has experimentally observed the role of GR as an organic dye-like macromolecular photosensitizer for wide band gap ZnS.50 For the GR−ZnS composites under visible light irradiation (λ > 420 nm), the photogenerated electrons from GR can transfer to the conduction band of ZnS, leading to the visible-light-responsive photoactivity for selective oxidation of alcohols and alkenes. In contrast, the photosensitizer role of GR cannot be observed in the GR−ZnS composite that is prepared by a simple mechanical mixing of GR and ZnS nanoparticles, which indicates that the intimate interfacial contact between GR and ZnS is a key factor for observing the photosensitizer role of GR in the GR−ZnS composite. In addition, it has also been reported that the photogenerated electrons from the GR sheet can directly inject to the conduction band of wide band gap ZnWO4,51 endowing the GR−ZnWO4 composites with enhanced visible light photoactivity for degradation of methylene blue (MB). However, this work does not exclude the possibility that photogenerated electrons, resulting from the dye (such as methylene blue or rhodamine B) photosensitization for ZnWO4 under visible light irradiation, can transfer to the conduction band of ZnWO4 or the GR sheet, which will enhance the visible light photoactivity for dye degradation over GR−ZnWO4. In other words, it is not a proper choice to choose the degradation of methylene blue or rhodamine B as a probe reaction to study the photosensitizer role of GR in the GR−semiconductor composite system because the photosensitization effect of such dyes for semiconductors can enhance the visible light photoactivity.36,38,52 Thus, it is clear that three basic preconditions are required if one aims to experimentally observe the exact photosensitizer role of GR in the GR−semiconductor composites under visible light. The first one is that, for the GR−semiconductor composites under visible light irradiation, the semiconductor by itself should be unable to be band gap-photoexcited. The second one is that the proper interfacial contact between GR and the semiconductor should be achieved because it will affect the charge carrier transfer efficiency across the interface between GR and the semiconductor. The third one is to choose suitable probe reactions that should avoid the selfinduced photosensitization effect, e.g., organic dye photosensitization for the semiconductor. On the basis of the above principles, we herein have designed the GR−ZnO nanocomposite system to show the photosensitizer role of GR on semiconductors. The ZnO nanoparticles are in situ grown on the GR nanosheet to ensure the strong interfacial bonding between ZnO and GR for facilitating the electron transfer between them. The photocatalytic reduction of Cr(VI) in aqueous solution is adopted to show the photosensitizer role of GR in the as-synthesized GR−ZnO nanocomposites to avoid the self-induced photosensitization effect of the reaction substrate. It is proven that the visible light photoactivity for reduction of Cr(VI) observed in the GR− ZnO composite originates from the photosensization of GR for

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Materials. Graphite powder, hydrochloric acid (HCl), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), concentrated sulfuric acid (H2SO4, 98%), ethanol (C2H5OH), potassium permanganate (KMnO4), N,N-dimethylformamide (DMF), hydrogen peroxide (H2O2, 30%), nitric acid (HNO3, 65%), and zinc acetate dihydrate [Zn(CH3COO)2·2H2O] were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the reagents were used as received without further purification. The deionized water used in the catalyst preparation was from local sources. Synthesis. (a) Synthesis of Graphene Oxide (GO). GO was synthesized from natural graphite powder by a modified Hummers’ method, which was also used in our previous works.3,12,13 The detail of the typical process was presented in the Supporting Information. (b) Preparation of Blank ZnO and GR−ZnO Nanocomposites. The synthesis of blank ZnO and GR−ZnO nanocomposites was based on a modified in situ wet chemistry method as originally reported by Choi et al.53 In a typical synthesis of the composites, a varying amount of GO was uniformly dispersed in 120 mL of DMF with the aid of ultrasonication for 2 h to form a homogeneous GO suspension. Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] (0.92 g) was dissolved in 120 mL of DMF, then the GO solution was added under continual ultrasonication to form a stable precursor and retained for another 15 min. Subsequently, the mixed solution was heated to 95 °C and maintained at that temperature for 5 h. After that, the products were cooled to room temperature, and the precipitants were collected by centrifugation and then rinsed with ethanol several times to remove the residue of DMF. After drying the products at 60 °C, the final GR−ZnO nanocomposites with different weight addition ratios of GR, namely, 1, 5, 10, and 30% GR−ZnO nanocomposites, were obtained. For the synthesis of blank ZnO, the procedure was the same as that for the preparation of GR−ZnO nanocomposites without the addition of GO. (c) Fabrication of GR−Blank−ZnO and GR. For comparison, the 10% GR−Blank−ZnO nanocomposite was also synthesized by a simple ex situ hard integration method.54,55 The protocol was described as the following: 22 mg of GO was ultrasonicated in 60 mL of deionized water and 20 mL of anhydrous ethanol solution to disperse it well; after that, 0.2 g of blank ZnO nanoparticles that were synthesized as described in the above procedure were added into the GO solution. The mixed solution was aged with vigorous stirring for 1 h to obtain a homogeneous suspension. Then, this suspension was transferred to a 120 mL Teflon-sealed autoclave and 21725

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maintained at 180 °C for 24 h. Under such a solvothermal condition, the solvent of ethanol−water has the strong power to reduce GO to GR, and the deposition of ZnO onto the GR sheet can also be simultaneously achieved.54,55 The resulting composites were recovered by filtration, washed by water and ethanol for several times, and finally dried at 60 °C in an oven to get the final 10% GR−Blank−ZnO nanocomposite. In addition, the bare GR sample was prepared using the same solvothermal procedure in the absence of ZnO. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns of the samples were collected on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation in the 2θ range from 5° to 80° with a scan rate of 0.02° per second. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Ultraviolet−visible (UV−vis) diffuse-reflectance spectra (DRS) were recorded on a Cary-500 UV−vis− NIR spectrometer in which BaSO4 powder was used as the internal standard to obtain the optical properties of the samples over a wavelength range of 200−800 nm. The morphology of the samples was determined by field emission scanning electron microscopy (FE-SEM) on a FEI Nova NANOSEM 230 spectrophotometer. Transmission electron microscopy (TEM) images were collected by using a JEOL model JEM 2010 EX microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermo Scientific ESCA Lab 250 spectrometer which consists of a monochromatic Al Kα as the X-ray source, a hemispherical analyzer, and sample stage with multiaxial adjustability to obtain the surface composition of the samples. All of the binding energies were calibrated by the C 1s peak at 284.6 eV. The time-resolved photoluminescence decay was performed on an Edinburgh FLS920 spectrofluorometer (Edinburgh, UK) with a 405 nm laser beam as the excitation source. 2.3. Catalyst Activity. The photocatalytic activities of the samples were evaluated by the photoreduction of Cr(VI) in an aqueous solution under the irradiation of visible light. The photocatalytic activity experiments were performed at ambient temperature and pressure. A 300 W xenon lamp (PLS-SXE 300C, Beijing Perfectlight Co., Ltd.) equipped with a filter to cut off light of wavelength below 400 nm (λ > 400 nm) was used as the visible light to trigger the photocatalytic reactions. Typically, 60 mL of 15 mg/L K2Cr2O7 aqueous solution containing 30 mg of photocatalyst was magnetically stirred in the dark for 2 h to establish the adsorption−desorption equilibrium before the irradiation. During illumination, a 4 mL sample solution was drawn from the system at a certain time interval. After the removal of the catalyst particles by centrifugation, the residual amount of Cr(VI) in the solution was analyzed on the basis of its characteristic optical absorption at 372 nm, using a Varian UV−vis spectrophotometer (Cary50. Varian Co.) to measure the change of Cr(VI) concentration with irradiation time based on Lambert−Beer’s law. The percentage of degradation is indicated as C/C0. Here, C is the absorption of Cr(VI) solution at each irradiation time interval of the main peak of the absorption spectrum, and C0 is the absorption of the initial concentration when the adsorption− desorption equilibrium was achieved. For comparison, the photocatalytic activity of the GR, blank ZnO, and GR−Blank− ZnO nanocomposites was also performed under identical experimental conditions.

3. RESULTS AND DISCUSSION The phase purity of ZnO in samples is confirmed by the XRD patterns as shown in Figure 1. It can be seen that for the

Figure 1. XRD patterns of the as-synthesized blank ZnO, GR−ZnO with different weight addition ratios of GR, and 10% GR−Blank− ZnO.

obtained blank ZnO, GR−ZnO nanocomposites with different weight addition ratios of GR and the sample of 10% GR− Blank−ZnO, they show similar XRD patterns. All the samples exhibit analogous diffraction peaks in terms of ZnO framework. The dominant peaks located at ca. 31.7, 34.4, 36.3, 47.6, 56.6, 62.9, 66.4, 68.0, 69.1, 72.6, and 77.0° are indexed to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) crystallographic planes of hexagonal ZnO (JCPDS file no.36-1451).56−58 Notably, no diffraction peaks for GR (Figure S1, Supporting Information) can be observed in the nanocomposites of GR−ZnO and 10% GR−Blank−ZnO. One possible reason might be due to the low amount and relatively low diffraction intensity of GR in comparison with the diffraction intensity of ZnO, and the other is probably due to the disappearance of the layer-stacking regularity after reduction of graphene oxide, indicating that the coupling of ZnO with GR can prevent the restacking of the graphene layers effectively.10,32 Table 1 summarizes the average crystallite sizes of blank ZnO, the samples of GR−ZnO with different addition ratios of Table 1. Summary of the Average Crystallite Size of Blank ZnO, GR−ZnO with Different Weight Addition Ratios of GR, and 10% GR−Blank−ZnO samples

average crystallite size (nm)

Blank ZnO 1% GR−ZnO 5% GR−ZnO 10% GR−ZnO 30% GR−ZnO 10% GR−Blank−ZnO

19.6 19.2 18.8 19.2 19.4 23.3

GR, and 10% GR−Blank−ZnO. For all the samples, the average crystallite sizes of ZnO particles are calculated from the (101) peak of the XRD patterns by the Scherrer formula: D = 0.89λ/β cos θ, where D is the average crystallite size, λ the Cu Kα wavelength (0.15406 nm), β the half-width of the peak in radians, and θ the corresponding diffraction angle.59 The average crystallite sizes of ZnO particles in blank ZnO and GR−ZnO nanocomposites are close to each other. However, for 10% GR−Blank−ZnO, the average crystallite size of ZnO 21726

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The current DRS result also indicates that there is no absolutely direct correlation between the shift of light absorption edge and interfacial interaction manner for GR−semiconductor composites. Field-emission scanning electron microscopy (FE-SEM) has been used to directly analyze the morphology of the assynthesized blank ZnO, GR−ZnO, and GR−Blank−ZnO nanocomposites. Figure 3a,b displays the typical SEM images of blank ZnO; it can be observed that the blank ZnO sample is composed of a large quantity of aggregated small nanoparticles. Figure 3c,d shows the SEM images of GR−ZnO (here, taking 10% GR−ZnO as an example), from which it can be seen that the nanocomposite maintains the two-dimensional sheet-like structure of graphene, and a large number of ZnO nanoparticles densely and tightly spread on the surface of GR. Also, the ZnO nanoparticles seem to be strongly bound to the GR sheets, which could be due to the “GO-structure-directing” growth of ZnO nanoparticles on the GR sheets during an in situ wet chemistry process.3 For the 10% GR−Blank−ZnO nanocomposite that is synthesized by a simple ex situ hard integration method, as shown in Figure 3e,f, it is seen that a great amount of ZnO particles are dispersed on the surface of GR in a poor interfacial contact manner. To further obtain the microscopic morphology and structure information of the samples, the transmission electron microscopy (TEM) analysis on the bare GO, 10% GR−ZnO, and 10% GR−Blank−ZnO has been performed, as shown in Figure 4. Figure 4a shows a general view of the bare GO nanosheets, indicating that the GO sheets are very clean. Figure 4b−f shows the TEM and high-resolution TEM (HR-TEM) images of 10% GR−ZnO nanocomposite. It can be seen that the nanosized ZnO particles with main particle size of approximately 12−29 nm have a uniform and dense dispersion on the surface of GR sheet support, demonstrating that the GR sheet and semiconductor ZnO ingredient have a strong interfacial contact for 10% GR−ZnO prepared by an in situ wet chemistry approach. In distinct contrast, for the sample of 10% GR−Blank−ZnO obtained from the simple ex situ hard integration of GR with blank ZnO nanoparticles, the interfacial and spatial contact of GR and ZnO is clearly poor, as evidenced by the TEM images in Figure 4g,h. This demonstrates that the strong interfacial contact between the GR sheet and ZnO ingredient cannot be obtained by the simple hard integration process. Figure 4i reveals that the size of ZnO nanoparticles in 10% GR−Blank−ZnO is mainly centered on 15−40 nm. The selected area electron diffraction (SAED) patterns of 10% GR− ZnO and 10% GR−Blank−ZnO in Figure 4c and h both display six distinct diffraction rings, which can be indexed to (100), (002), (101), (102), (110), and (103) crystal planes of ZnO. The SAED patterns indicate that the 10% GR−ZnO and 10% GR−Blank−ZnO nanocomposites both possess the polycrystalline structure, which is in agreement with the result of XRD analysis. The above optical measurement of GR−ZnO nanocomposites and 10% GR−Blank−ZnO demonstrates that the band gaps of the samples are close to the intrinsic band gap (ca. 3.2 eV) of blank ZnO. Therefore, in principle, the as-synthesized blank ZnO and the GR−ZnO composite samples cannot be band gap-photoexcited under visible light irradiation (λ > 400 nm), and thus they should not exhibit visible light photoactivity. In fact, when we perform the photocatalytic reduction of Cr(VI) in aqueous solution over the samples of blank ZnO and 10% GR−Blank−ZnO, they do not display visible light

particles is slightly larger than that in the GR−ZnO nanocomposites. The UV−vis diffuse reflectance spectra (DRS) measurement has been performed to determine the optical property of the samples. As shown in Figure 2a, it can be seen that the presence

Figure 2. UV−vis diffuse reflectance spectra (DRS) of the samples of blank ZnO, GR−ZnO with different addition ratios of GR, and 10% GR−Blank−ZnO (a); the plots of transformed Kubelka−Munk function versus the energy of light (b).

of different amounts of GR has a significant influence on the optical property of light absorption for the GR−ZnO and GR− Blank−ZnO nanocomposites. With the addition of GR, it induces the increased light absorption intensity in the visible light region ranging from 390 to 800 nm in comparison with blank ZnO, which can be ascribed to the broad background absorption of GR in the visible light region. In addition, it should be noted that the GR−ZnO and GR−Blank−ZnO nanocomposites all display the sharp absorption edge onset at ca. 385 nm. This result means that the introduction of GR into the matrix of ZnO has no significant influence on the absorption edge of GR−ZnO and GR−Blank−ZnO. Figure 2b shows the plot of the transformed Kubelka−Munk function versus the energy of light. The band gap values of the samples are all roughly estimated to be around 3.2 eV, confirming that the coupling of GR with ZnO via the present in situ lowtemperature wet chemistry process does not narrow the band gap of semiconductor ZnO to the visible light region. This also means that the as-obtained GR−ZnO nanocomposites cannot be band gap-photoexcited under visible light irradiation. It is particularly worth noting that the absence of a red shift of light absorption edge to a higher wavelength for GR−ZnO is remarkably different from previous GR−semiconductor (e.g., TiO2) nanocomposite photocatalysts,3,13,55 clearly suggesting that the light absorption properties of GR−semiconductor are strongly dependent on the variation of preparation methods. 21727

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Figure 3. FE-SEM images of the samples of blank ZnO (a, b), 10% GR−ZnO (c, d), and 10% GR−Blank−ZnO (e, f).

photoactivity because no apparent reduction of Cr(VI) has been observed, as shown in Figure 5a. However, when the GR− ZnO nanocomposites prepared by an in situ process are used to perform the same photocatalytic reaction, it is quite interesting that they obviously exhibit visible light photoactivity. Among the GR−ZnO nanocomposites with different GR addition ratios, 10% GR−ZnO exhibits the best visible light photoactivity. The whole reduction process is monitored by measuring the time-dependent UV−vis absorption spectra of the reaction mixture solution. As shown in Figure 5b, during the reaction process, the absorption intensity of Cr(VI) at 372 and 272 nm decreases gradually with the irradiation time, indicating the successful reduction of Cr(VI). Besides, it is clear that the addition ratios of GR are crucial to determine the photocatalytic performance of the GR−ZnO nanocomposites, indicating the synergetic effect between ZnO and GR. It is also notable that there is no reduction of Cr(VI) in aqueous solution over the bare GR, implying that GR by itself cannot directly act as photocatalyst for reduction of Cr(VI) under visible light irradiation. Blank experiments in the absence of catalysts and/or visible light irradiation have also been carried out, and no reduction of Cr(VI) is observed, which ensures that the reaction is really driven by a photocatalytic process, as shown in Figure S2 (Supporting Information). On the basis of the above results, it can be seen that the visible light irradiation cannot photoexcite the electron transfer from the valence band (VB) to the conduction band (CB) of ZnO, but the GR−ZnO nanocomposites prepared by an in situ wet chemistry process exhibit photoactivity toward reduction of Cr(VI) under visible light irradiation (λ > 400 nm). Thus, we infer that the observed visible light photoactivity should be derived from the photosensitization effect of GR for ZnO under visible light irradiation. Such a photosensitizer role of GR in the GR−semiconductor composite photocatalyst has been recently

revealed by the GR−ZnS nanocomposites. That is, upon the visible light irradiation, the ground-state GR is able to be promoted to its excited-state GR*, during which the photogenerated electrons then inject into the CB of ZnO that consequently transforms the wide band gap ZnO to a visible light photocatalyst. To confirm the presence of electrons photogenerated in the current GR−ZnO photocatalyst, the controlled experiment with adding S2O82− (K2S2O8) as a quenching agent for photogenerated electrons60,61 over the optimal 10% GR−ZnO has been performed, as shown in Figure 6. It is clear to see that when the S2O82− scavenger for electrons is added into the reaction system the photoreduction of Cr(VI) driven by electrons is remarkably inhibited, which proves the existence of photogenerated electrons over 10% GR−ZnO under visible light irradiation. To further confirm the photogenerated electron transfer between GR and ZnO, the time-resolved photoluminescence decay study on the 10% GR−ZnO nanocomposite using laser pulse excitation at 405 nm has been performed. As shown in Figure S3 (Supporting Information), the photoluminescence lifetime of 10% GR− ZnO is determined to be ca. 0.87 ns, thus demonstrating the charge carrier transfer in 10% GR−ZnO under visible light irradiation. As discussed above, because ZnO in 10% GR−ZnO is unable to be band gap-photoexcited under visible light irradiation, the charge carriers should result from the GR to GR*, which subsequently inject into the conduction band (CB) of ZnO. In addition, we have also carried out the controlled experiment of photocatalytic reduction of Cr(VI) in aqueous solution over 10% GR−ZnO by utilizing monochromatic light with the wavelength in the range of 400−700 nm, as shown in Figure 7. The result indicates that the energy of incident light irradiation has an important influence on the photoactivity of 10% GR−ZnO, which could be ascribed to the different energy 21728

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Figure 4. TEM images of a bare GO sheet (a), 10% GR−ZnO (b−c), 10% GR−Blank−ZnO (g, h); high-resolution TEM (HR-TEM) images of 10% GR−ZnO (d, e); size distribution of ZnO nanoparticles in 10% GR−ZnO (f) and 10% GR−Blank−ZnO (i); insets in panels (c) and (h) are the selected area electron diffraction (SAED) pattern of 10% GR−ZnO and 10% GR−Blank−ZnO, respectively.

temperature thermal treating process.63 Thus, to exclude the formation of the Zn−C bond between ZnO and GR in our assynthesized GR−ZnO nanocomposite, the X-ray photoelectron spectra (XPS) analysis has been performed in the following. Figure 8a displays the C 1s XPS spectrum of original GO, which indicates the presence of abundance various oxygencontaining functional groups on the GO surface. For the 10% GR−ZnO nanocomposite, it can be observed from Figure 8b that, after coupling ZnO with GO, a large amount of oxygencontaining functional groups have been removed, suggesting the sufficient reduction of GO to GR during the current in situ low-temperature wet chemistry process.53 In addition, we have not found the typical XPS peak with the binding energy located between 280 and 284 eV, which generally can be assigned to the Zn−C bond.62−64 Therefore, the C 1s XPS analysis result further proves that the photoactivity of 10% GR−ZnO for reduction of Cr(VI) under visible light irradiation is not caused by the formation of the Zn−C bond (i.e., carbon doping) but induced by the photosensitization of GR for the wide band gap semiconductor ZnO. The absence of the Zn−C bond for our as-prepared GR−ZnO nanocomposite could be ascribed to the low-temperature preparation method because it is noted that

level of GR*. The higher energy of incident light could photoexcite the ground-sate GR to a higher-energy excitation level (GR*) in GR−ZnO, and in this case, more photogenerated electrons can inject into the CB of ZnO. As a result, the GR−ZnO nanocomposite displays higher photoactivity for reduction of Cr(VI) under the irradiation of higher energy of incident visible light, as pictorially illustrated in Scheme 1. It is clear that a specific interfacial interaction/contact manner between GR and ZnO is required to experimentally observe the photosensitizer role of GR in the GR−semiconductor composite photocatalysts. In addition, the effect of carbon doping should be considered because it has a significant influence on the photochemical properties of semiconductors, and some reports have shown that carbon doping can transform the wide band gap semiconductors to visible light driven photocatalysts. For example, Cho et al. have fabricated the carbon-doped ZnO nanocomposite via a hydrolysis approach, followed by a high-temperature calcination treatment.62 The obtained C-doped ZnO nanocomposites exhibit visible light photoactivity due to the formation of the Zn−C bond. Fu et al. have also demonstrated that the introduction of GR into the matrix of ZnO can form the Zn−C bond after a high21729

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Figure 7. Photocatalytic performance of 10% GR−ZnO for reduction of Cr(VI) in aqueous solution by utilizing monochromatic light with the wavelength in the range of 400−700 nm.

Scheme 1. Illustration of the Proposed Reaction Mechanism for Photocatalytic Reduction of Cr(VI) in Aqueous Solution over the 10% GR−ZnO Nanocomposite under the Irradiation of Different Monochromatic Light in the Wavelength Range of 400−700 nm

Figure 5. Photocatalytic performance of bare GR, blank ZnO, GR− ZnO with different weight addition ratios of GR, and 10% GR− Blank−ZnO in the reduction of Cr(VI) aqueous solution under visible light irradiation (λ > 400 nm) (a) and UV−vis absorption spectra of Cr(VI) aqueous solution taken at regular time intervals over the 10% GR−ZnO nanocomposite (b).

Zn 2p doublet for 10% GR−ZnO shifts to the lower binding energy, as shown in Figure 8f. The negative shift (−0.4 eV) could be caused by the strong interfacial interaction between GR and ZnO nanoparticles in the GR−ZnO nanocomposites, which would promote electron transfer from the GR sheet to ZnO nanoparticles. A similar phenomenon has also been observed for the Au−graphene nanocomposite.66,67 Therefore, the strong, specific interfacial interaction of GR and ZnO in the GR−ZnO nanocomposites is concluded to be the substantial reason for different photocatalytic performance for reduction of Cr(VI) between the 10% GR−ZnO nanocomposite and the sample of 10% GR−Blank−ZnO with poor interfacial interaction, which is also in line with our previous viewpoint that an intimate interfacial contact is needed for the realization of the photosensitizer role of GR for wide band gap semiconductor ZnS.50 In addition, the possible contribution of different oxygen vacancies associated with semiconductor ZnO to the observed visible light photoactivity of our GR−ZnO composite for reduction of Cr(VI) should be considered. It has been well demonstrated that, similar to the case of TiO2 with oxygen vacancies,68 the introduction of oxygen vacancies (depending on the preparation methods, types, and concentration of oxygen vacancies) could lead to two different results. The first possible result is to induce the distinct band gap narrowing of ZnO owing to the introduction of oxygen vacancies, which can be directly reflected by the optical DRS spectra.69 Under this situation, ZnO with the band gap narrowing to the visible light region is able to be band-gap-photoexcited under visible light irradiation and thus exhibits visible light photoactivity. It is clear that this case can be excluded in the current research work because, as discussed above in Figure 2, we do not observe any

Figure 6. Controlled experiment over the 10% GR−ZnO nanocomposite with and without S2O82− (K2S2O8) as a scavenger for photogenerated electrons in photocatalytic reduction of Cr(VI) aqueous solution under visible light irradiation (λ > 400 nm).

the formation of the Zn−C bond as reported in previous research works is generally observed for the carbon−ZnO composite prepared via a high-temperature thermal treating process.62,63 Furthermore, Figure 8c shows the C 1s XPS spectrum of 10% GR−Blank−ZnO, which indicates the efficient reduction of GO to GR after a hydrothermal reduction treatment of blank ZnO solid nanoparticles and GO. With regard to 10% GR− ZnO, two binding energy peaks at 1021.2 eV (Zn 2p3/2) and 1044.3 eV (2p1/2), as shown in Figure 8d, correspond to the Zn2+ (ZnO). Notably, compared with the characteristic peaks for Zn2+ at 1021.6 and 1044.7 eV of the samples of blank ZnO (Figure S4, Supporting Information) and 10% GR−Blank− ZnO (Figure 8e),56,65 it is found that the XPS signature of the 21730

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Figure 8. C 1s X-ray photoelectron spectra (XPS) of the original GO (a), 10% GR−ZnO (b), and 10% GR−Blank−ZnO (c); Zn 2p XPS spectra of 10% GR−ZnO (d), 10% GR−Blank−ZnO, (e) and comparison of the Zn 2p XPS spectra between 10% GR−ZnO and 10% GR−Blank−ZnO (f).

nanocomposite under visible light irradiation. Alternatively, even if there is the photoexcitation of electrons from the VB of ZnO to the impurity states near the VB or CB of ZnO, the asgenerated electron−hole pairs will recombine very quickly, which is in agreement with the negligible photoactivity for degradation of phenol and methyl orange over the 10% GR− ZnO nanocomposite under visible light irradiation. Furthermore, it is interesting to note that, although there are electrons photogenerated from GR photosensitization in 10% GR−ZnO under visible light, we also do not observe any visible light photoactivity of 10% GR−ZnO toward oxidation of alcohols and alkenes, which is remarkably different from the GR−ZnS nanocomposite photocatalyst as reported previously from our group.50 This could be ascribed to the difference of lifetime and transfer efficiency of electrons photogenerated from the photosensitizer of GR in GR−ZnS and GR−ZnO. It is worth noting that, although electrons cannot directly participate in the oxidation reaction, they can activate molecular oxygen in the photocatalytic reaction system. As a result, the activated oxygen species (e.g., superoxide radical species) are able to induce an oxidation reaction. However, the fact that we do not observe

obvious band gap narrowing for the as-prepared GR−ZnO nanocomposites. The second possible result is that the introduction of oxygen vacancies does not induce the obvious band gap narrowing of ZnO to the visible light region, but they can form the impurity states near the valence band (VB) or conduction band (CB) of ZnO and act as active centers or trap centers for electrons or holes photogenerated from ZnO under UV light irradiation.70 As a result, the introduction of oxygen vacancies will increase or decrease the UV light photoactivity of ZnO. However, this possibility can also be excluded in the current work because our photocatalytic reaction for reduction of Cr(VI) is performed under visible light irradiation. In fact, we have done the control experiments for degradation of phenol and methyl orange that do not have a photosensitization effect for semiconductor ZnO over the 10% GR−ZnO nanocomposite under visible light irradiation. The results show that no degradation of phenol or methyl orange is observed under the present experimental conditions, suggesting that no effective holes with strong oxidative power for degradation of phenol or methyl orange are generated from band gap photoexcitation of ZnO in the 10% GR−ZnO 21731

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visible light photoactivity for oxidation of alcohols and alkenes over the current GR−ZnO nanocomposite clearly suggests the difference of lifetime and transfer efficiency of electrons photogenerated from the photosensitizer of GR across the interface in GR−ZnS or GR−ZnO. These results also suggest that the reduction of Cr(VI) by directly capturing electrons more easily occurs over the GR−ZnO nanocomposite than the aerobic oxidation of alcohols and alkenes which requires the activation of molecular oxygen by photogenerated electrons. The above discussions strongly highlight that the photosensitizer role of GR in the composite of GR−semiconductor seems to be an interesting but complicated issue. On the basis of the previous reports50 and current results, we think that it strongly depends on the specific interfacial interaction manner because this will directly influence the lifetime and transfer efficiency of electrons photogenerated from the GR photosensitizer across the interface between GR and the not-band gap-photoexcited semiconductor under visible light irradiation. In addition, because the GR photosensitizer-induced visible light photoactivity is low which could be ascribed to the low lifetime of photogenerated electrons from GR photoexcitation, the choice of suitable probe reactions is also important which will directly determine whether or not we can distinctly observe the photoactivity experimentally. Accordingly, we propose that three basic principles are required if one aims to experimentally observe the exact photosensitizer role of GR in the GR− semiconductor composites under visible light: (i) for the GR−semiconductor composites under visible light irradiation, the semiconductor by itself is unable to be band gap-photoexcited; (ii) the proper interfacial interaction manner between GR and the semiconductor should be achieved because it will affect the charge carrier lifetime and transfer efficiency across the interface between GR and the semiconductor; (iii) the suitable probe reactions should be chosen, which not only should avoid the self-induced photosensitization effect (e.g., organic dye photosensitization for semiconductor71) but also can effectively capture the photogenerated electrons from GR excitation in the GR−semiconductor composites under visible light irradiation. In particular, since the visible light photoactivity induced by the GR photosensitizer role in GR−semiconductor composite photocatalysts is generally low, we think that the reduction probe reaction driven by directly capturing electrons is more suitable and relatively easier to occur than the oxidation probe reaction for experimentally observing the GR-photosensitizerinduced visible light photoactivity. Finally, on the basis of the above discussions, a possible reaction mechanism for photocatalytic reduction of Cr(VI) in aqueous solution over the GR−ZnO nanocomposite has been proposed, as pictorially illustrated in Figure 9. Under visible light irradiation, GR in the GR−ZnO composite is photoexcited from the ground-state GR to the excited-state GR* during which electrons are photogenerated. The GR* in the excited state then injects electrons into the conduction band (CB) of ZnO. The photogenerated electrons in the reaction system can be accepted by Cr(VI) in the reaction system, by which the reduction of Cr(VI) is reached.

Figure 9. Illustration of the proposed reaction mechanism for photocatalytic reduction of Cr(VI) in aqueous solution over the GR−ZnO nanocomposite under visible light irradiation (λ > 400 nm).

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CONCLUSION In summary, we have prepared a series of GR−ZnO nanocomposites with different weight addition ratios of GR by a facile, low-temperature in situ wet chemistry method. It has been demonstrated that the as-obtained GR−ZnO nanocomposites with intimate interfacial contact can perform as visible light driven photocatalysts for catalytic reduction of Cr(VI) in aqueous solution under ambient conditions, while the semiconductor ZnO is unable to be band gap-photoexcited. The result proves that the visible light photoactivity of GR− ZnO is ascribed to the photosensitizer role of GR for semiconductor ZnO, for which GR is photoexcited to generate electrons that then transfer to the conduction band of ZnO. In contrast, for the GR−Blank−ZnO nanocomposite prepared by a simple ex situ hard integration method, the sample fails to display visible light photoactivity for catalytic reduction of Cr(VI). The comparison results reveal that the proper and specific interfacial interaction between GR and ZnO is a crucial factor for observing the role of GR as a macromolecular photosensitizer for a wide band gap semiconductor under visible light. It is hoped that this work could contribute to an indepth understanding of the photosensitizer role of GR, the microscopic charge carrier transfer pathway across the interface between GR and the semiconductor, and importantly promote ongoing interest in how to improve the photosensitization efficiency of GR toward specific solar energy conversion.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental detail for synthesis of graphene oxide (GO); XRD pattern of the graphene (GR); controlled experiments for photocatalytic reduction of Cr(VI) aqueous solution under visible light irradiation (λ > 400 nm) over the 10% GR−ZnO nanocomposite in the absence of catalysts and/or visible light irradiation; time-resolved photoluminescence decay of the 10% GR−ZnO nanocomposite and Zn 2p X-ray photoelectron spectrum (XPS) of blank ZnO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 591 83779326. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 21732

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ACKNOWLEDGMENTS The support by the National Natural Science Foundation of China (NSFC) (20903023, 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), Program for Returned High-Level Overseas Chinese Scholars of Fujian Province, and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged.

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