Performance Enhancement of ZnO Photocatalyst via Synergic Effect

ZnO1–x/graphene hybrid photocatalyst was prepared via a facile in-situ reduction of graphene ...... Powder Technology 2017 315, 73-80 .... R. Parame...
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Performance Enhancement of ZnO Photocatalyst via Synergic Effect of Surface Oxygen Defect and Graphene Hybridization Xiaojuan Bai, Li Wang, Ruilong Zong, Yanhui Lv, Yiqing Sun, and Yongfa Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la4001768 • Publication Date (Web): 19 Feb 2013 Downloaded from http://pubs.acs.org on February 25, 2013

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Figure 1. Apparent rate constants for the photocatalytic degradation of MB over ZnO1−x and ZnO1−x/graphene photocatalysts under the irradiation: (a) visible light (λ > 420 nm); (b) UV light (λ = 254 nm). 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125

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200 °C for 8 h under N2 flow (rate = 60 mL/min). All investigated electrodes were of similar thickness (0.8−1.0 μm). Instrumentation. High-resolution transmission electron microscopy (HRTEM) images were obtained by a JEOL JEM-2011F field emission transmission electron microscope with an accelerating voltage of 200 kV. To avoid electron beam-induced damage, lowintensity beam was used for collecting selected area electron diffraction (SAED) patterns. X-ray diffraction (XRD) patterns of the powders were recorded at room temperature by a Bruker D8 Advance X-ray diffractometer. The diffuse reflectance absorption spectra (DRS) of the samples were recorded in the range from 250 to 800 nm using a Hitachi U-3010 spectroscope equipped with an integrated sphere attachment, and BaSO4 was used as a reference. Raman spectra were recorded on a microscopic confocal Raman spectrometer (Renishaw 1000 NR) with an excitation of 514.5 nm laser light. Fourier transform infrared (FTIR) spectra were carried out using PerkinElmer spectrometer in the frequency range of 4000−450 cm−1 with a resolution of 4 cm−1. The electron spin resonance (ESR) signals of radicals spin-trapped by spin-trap reagent 5,5′-dimethyl-1-pirroline-Noxide (DMPO) (purchased from Sigma Chemical Co.) were examined on a Bruker model ESR JES-FA200 spectrometer equipped with a quanta-Ray Nd:YAG laser system as the irradiation source (λ = 365/ 420 nm). To minimize experimental errors, the same type of quartz capillary tube was used for all ESR measurements. The ESR spectrometer was coupled to a computer for data acquisition and instrument control. Magnetic parameters of the radicals detected were obtained from direct measurements of magnetic field and microwave frequency. Electrochemical and photoelectrochemical measurements were performed in a three-electrode quartz cells with 0.1 M Na2SO4 electrolyte solution. Platinum wire was used as counter and saturated calomel electrode (SCE) used as reference electrodes, respectively, and ZnO1−x and GHZ1.2 films electrodes on ITO served as the working electrode. The photoelectrochemical experiment results were recorded with an electrochemical system (CHI-660B, China). The intensity of light was 1 mW cm−2. Potentials are given with reference to the SCE. The photoresponses of the photocatalysts as UV light on and off were measured at 0.0 V. Electrochemical impedance spectra (EIS) were measured at 0.0 V. A sinusoidal ac perturbation of 5 mV was applied to the electrode over the frequency range of 0.05−105 Hz. Photocatalytic Experiments. The photocatalytic activities of the as-prepared samples for the degradation of methylene blue (MB) in solution were tested under UV and visible light. An 11 W UV lamp (λ = 254 nm) and 175 W metal halide lamp (λ > 420 nm) were used as light resources, and the average light intensity was 0.93 and 2.5 mW/ cm2, respectively. 50 mg of ZnO1−x/graphene-1.2 wt % (GHZ1.2) photocatalysts was dispersed in a 100 mL of MB aqueous solution (1 × 10−5 M). Prior to the irradiation, the suspensions were magnetically stirred in the dark for 60 min to reach the absorption−desorption equilibrium. At given time intervals, 3 mL of the liquid was sampled and analyzed by recording variations in the absorption band (663 nm)

EXPERIMENTAL SECTION

Materials. Graphene oxide (GO) was synthesized by the modified Hummmers’ method,43 and graphene was prepared according to the literature.44 ZnO nanopowder (particle diameter 10−20 nm, surface area 10.4 m2 g−1) were commercially available. Titanium dioxide (TiO2) nanopowder is P25 (Degussa Co., Ltd., Germany). All chemicals used were analytical reagent grade without further purification. Synthesis of ZnO1−x Surface Oxygen Defect Samples. ZnO1−x with surface oxygen defect samples were prepared by reduction of ZnO nanopowder as follows: (1) the temperature-programmed reduction (TPR) measurement using H2 gas was performed in a specially designed quartz tube with 0.050 g of nanometer ZnO sample, and then the tube was put in a cylindrical electric furnace. The temperature of the furnace was controlled by a programmable regulator with the thermocouple. A thermal conductivity detector (TCD) was used to detect H2 consumption during the hydrogenated treatment process. (2) The ZnO sample was pretreated by helium (He) gas from room temperature to 130 °C with a ramping rate of 10 °C/min for 1 h. Then it cooled to the room temperature again keep He gas purring into. (3) The H2/Ar mixture gas (mole ratio = 1:9) was introduced into the homemade quartz tube with ZnO samples, and the samples were heated from 20 to 465 °C in H2/Ar mixture gas for 5 h with a gas flow rate 25 mL/min and a heating ramping rate of 10 °C/min. (4) Finally, the samples were cooled down to room temperature naturally, keeping the H2/Ar mixture gas in. Sample Preparation. Appropriate GO was well dispersed in distilled water, and then the disperstion was ultrasonicated for 60 min to get GO exfoliated. The obtained brown dispersion was then subjected to 30 min of centrifugation at 5000 rpm to remove any unexfoliated GO. The obtained exfoliated GO was then dispersed in 100 mL of water, and 0.5 g of ZnO1−x was added into the GO dispersion. Then the ZnO1−x and GO mixture was dispersed by ultrasonication for 30 min and stirred for 48 h. The reduction of GO to graphene is performed according to the literature.45 In a typical procedure, appropriate amounts of hydrazine solution (35 wt % in water) and ammonia solution (28 wt % in water) were added to the above dispersion. After being vigorously shaken or stirred for a few minutes, the dispersion was put in a water bath (95 °C) for 3 h. An opaque powder was obtained after evaporation at 60 °C for 12 h. ZnO1−x/graphene photocatalysts with different mass ratio from 0.4% to 3.2% were prepared according to above method. The ZnO1−x/ graphene-X wt % hybrid photocatalysts were marked as GHZ-X, X label as ZnO1−x/graphene mass ratio 0.4, 0.8, 1.2, 1.6, 2.0, and 3.2. The ZnO1−x/graphene mechanical mixture-1.2 wt % was marked as GMZ1.2. ZnO1−x and GHZ1.2 electrodes were prepared as follows: 4 mg of as-prepared photocatalyst was suspended in 2 mL of ethanol to produce slurry, which was then dip-coated onto a 2 cm × 4 cm indium−tin oxide (ITO) glass electrode. Electrodes were exposed to UV light for 10 h to eliminate ethanol and subsequently calcined at B

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Figure 2. Photoresponses of ZnO1−x and GHZ1.2 electrodes under the irradiation of (a) visible light (λ > 420 nm) and (b) UV light (λ = 254 nm). [Na2SO4] = 0.1 M. 176 177

in the UV−vis spectra of MB using a Hitachi U-3010 UV−vis spectrophotometer.

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RESULTS AND DISCUSSION Photocatalytic Activity and Photocurrent Response. The photocatalytic activities of the ZnO1−x/graphene samples, using photodegradation of methylene blue (MB) as model reaction under visible light and UV light, are shown in Figure 1a,b. Figure 1a shows the MB photodegradation rate constants on ZnO1−x and ZnO1−x/graphene photocatalysts under visible light irradiation. The photocatalytic activity trend follows the order GHZ1.2 > GHZ0.8 > GHZ0.4 > GHZ1.6 > GHZ2.0 > GHZ3.2. While the sample of GHZ1.2 shows the highest reaction rate constant of 0.606 h−1, which is 4.6 times that of ZnO1−x. More graphene leads to a significant decrease of photocatalytic activity. Clearly, when the weight ratio of graphene is 3.2 wt %, the activity of GHZ3.2 is even lower than pristine ZnO1−x. Although graphene is beneficial for charge separation of the ZnO1−x/graphene photocatalyst, it will shade ZnO1−x at too much addition. Therefore, due to the balance between charge separation and light harvesting, the photocatalytic activity of ZnO1−x/graphene first increases and then decreases with the increasing of graphene addition. ZnO1−x presents an apparent reaction rate constant k of 0.131 h−1 under the irradiation of visible light. In addition, among ZnO, ZnO1−x, GHZ1.2, P25, and GMZ1.2, ZnO and ZnO1−x show rather poor photocatalytic activity due to their limited visible photoresponse. That is, only 27% and 57% of MB diminished after 5 h. Whereas the GHZ1.2 composite shows remarkable improvements in the photodegradation rate, 97% dye is decomposed after the same time, indicating 1.9 times that of P25. Compared to GMZ1.2, the activity of GHZ1.2 exhibits 7 times, which may imply the charge transfer between ZnO1−x and graphene. Figure 1b shows MB degradation rate over ZnO1−x and ZnO1−x/graphene photocatalysts under UV light irradiation. The k is 0.159 min−1 for ZnO1−x while 0.189 min−1 for GHZ1.2, which is 1.2 and 1.8 times that of ZnO1−x and ZnO, respectively. Furthermore, the photocatalytic activity trend follows the order GHZ1.2 > ZnO1−x > P25 > GMZ1.2 > ZnO. The higher photocatalytic activities of GHZ1.2 than GMZ1.2 indicate that the interaction for interconnected interface is crucial, which could significantly increase the separation of photogenerated charge and inhibit the recombination of photoinduced electron−hole pairs. As shown in Figure S1, no

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MB degradation can be observed either under visible light without catalyst or with catalyst in the dark; then the obvious visible activity and enhanced UV light activity of GHZ1.2 can be confirmed reasonably. Also, the total organic carbon (TOC) removal percentage is helpful to verify the superior photocatalytic activity of GHZ1.2. As shown in Figure S2, it is 95% for GHZ1.2 while only 79% for ZnO1−x. The photoresponses of ZnO, ZnO1−x, and GHZ1.2 electrodes were irradiated under both visible (Figure 2a) and UV light (Figure 2b). ZnO electrode showed weak photocurrent response under visible and UV light irradiation relative to ZnO1−x and GHZ1.2. The photocurrent of ZnO1−x under visible and UV light irradiation can be enhanced by 2 and 3.5 times after graphene hybridization, implying an enhanced separation efficiency of photoinduced electrons and holes. Photocorrosion Inhibition of ZnO1−x Photocatalyst. The durability of the ZnO1−x/graphene photocatalyst-1.2 wt % (GHZ1.2) for the degradation of MB under visible light was also performed (Figure S3). The photodegradation of MB was monitored for four consecutive cycles, for every 5 h. After every cycle, the samples were filtrated and washed thoroughly with water, and then fresh MB solution was added. The C/C0 constants of MB degradation of GHZ1.2 declined from 95% to 85%. There no significant decrease in photodegradation activity during the four consecutive cycles, indicating the good stability of GHZ1.2 photocatalyst. The durability experiment of ZnO1−x/graphene under UV light irradiation was also performed. As shown in Figure S12, the results of photostability experiment show that the photocatalytic activity of GHZ1.2 exhibits only 7% decrease after running for four cycles (about 44 h irradiation); the photostability of GHZ1.2 is higher than that of pure ZnO. The result indicates that GHZ1.2 is a stable UV-light photocatalyst. Photocorrosion is one of the barriers for ZnO application, and it is anticipated that ZnO could become an excellent photocatalyst if the photocorrosion can be suppressed.35 To further investigate the inhibitation of photocorrosion, XRD patterns and TEM images were performed before and after photocatalytic reaction. Figure S4 shows the XRD patterns of GHZ1.2 before and after 20 h photocatalytic reaction. The XRD patterns of GHZ1.2 show no obvious difference before and after 20 h photocatalytic reaction, indicating that graphene hybridized ZnO1−x is photostable and the photocorrosion is successfully inhibited. The TEM images of GHZ1.2 photocatalyst before and after 20 h reaction are shown in Figure S5.

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Before reaction, the GHZ1.2 is consisted of agglomerated spherical particles with diameters of 30 nm. After 20 h photocatalytic reaction, it does not exhibite any changes in morphology. The TEM result is in good agreement with the result of XRD, further revealing that the presence of graphene on the ZnO1−x surface could effectively inhibit the photocorrosion of ZnO1−x during the photocatalytic reaction. Furthermore, the good photostable property may be due to not only the presence of graphene layer but also the oxygen defects of ZnO1−x, while the two effects could introduce a disordered layer on the ZnO surface which can prevent ZnO molecular structure from destroying. The additional potential advantage of these engineered disorders is that they provide trapping sites for photogenerated carriers and prevent them from rapid recombination, thus promoting electron transfer and inhibitation of photocorrosion.42 Hybrid Structure and Optical Properties. The absorption range of light plays an important role in the photocatalysis, especially for the visible light photodegradation of contaminants. As shown in Figure 3, which shows the UV−vis diffuse

respectively. The enhancement of adsorption could be contributed to the π−π stacking between MB and graphene.35 Furthermore, it is known that the introduction of surface disorders in ZnO1−x nanoparticles could provide trapping sites for photogenerated carriers and prevent them from rapid recombination, thus promoting electron transfer and photocatalytic reactions.42 The conclusion that the oxygen defects with engineered surface disorders play a key role in enhancing the photocatalytic efficiency has been demonstrated by the previous reports.39,41,42 Therefore, the synergetic effect of graphene and oxygen defects is responsible for the enhanced photocatalytic efficiency. As mentioned above, the charge transfer and interaction at the interface may be responsible for enhancing the photocatalytic activity Then the transmission electron microscopy (TEM) analysis is carried out to get further confirmation, as shown in Figure S7a,b. It is observed that GHZ1.2 and ZnO1−x present similar particle size (d = 30 nm) and morphology, while the graphene is hardly seen. Figures 4a and 4b show the high-

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Figure 4. HRTEM images of (a) ZnO1−x and (b) GHZ1.2 photocatalysts.

resolution transmission electron microscopy (HRTEM) images of ZnO1−x and GHZ1.2, respectively. As shown in Figure 4a and the inset image, the surface layer of ZnO1−x exhibits few disordered structure after hydrogenation. The decoration effect of graphene can be deduced by the close connection among ZnO1−x particles and graphene, as can be seen in Figure 4b and the inset image. Graphene hybridization causes no obvious change in lattice structure of ZnO1−x; however, it results in obvious disordered surface layer, as well as a surrounding a layer of 0.667 nm, which was close to the scale of monolayer CCG (chemically converted graphene) (about 0.78 nm).45 Therefore, it can be estimated that monolayer or submonolayer graphene film is absorbed on the surface of GHZ1.2. The FTIR and Raman spectra of graphene and ZnO1−x/ graphene are shown in Figures 5a and 5b, respectively. Figure 5a present typical fingerprint groups of GO, including carboxylic species, hydroxyl species, and epoxy species (C O, 1734 cm−1; OH deformation, 1400 cm−1; the C−OH stretching, 1230 cm−1; C−O−C (epoxy group) stretching, 1061 cm−1; skeletal ring stretch, 1624 cm−1).46 The absorption band at around 1580 cm−1 clearly shows the skeletal vibration of the graphene sheets, indicating the formation of graphene structure.37 The hybridization between ZnO1−x and graphene sheets resulted in some changes in the IR spectrum. In the FTIR spectrum of GHZ1.2, the broad absorption at about 443 cm−1 is broader than the corresponding peak of pristine ZnO, which usually shows a distinct absorption band around 464 cm−1 or lower wavenumbers). Since oxygen defects generally

Figure 3. Diffuse reflectance absorption spectra of ZnO1−x and GHZ1.2 photocatalysts. The inset image shows the digital picture of ZnO1−x, GHZ1.2, and GMZ1.2 suspensions.

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reflectance spectroscopy (DRS) of ZnO, ZnO1−x, and GHZ1.2, there is not too obvious red-shift of ca.10−15 nm in the absorption edge of GHZ1.2 powder compared to ZnO1−x and pristine ZnO. The change in the reflectance spectra at ∼3.02 eV (410 nm) suggests that the energy band gap of the GHZ1.2 is simply associated with broad contribution from absorbance of graphene. In addition, the introduction of graphene over ZnO1−x induces the increased light absorption intensity in the visible region.37 The stronger absorption intensity in UV and visible regions for GHZ1.2 than ZnO1−x is a key factor for higher photocatalytic activity. It can also be clearly observed that ZnO1−x with disorders structure showed enhanced intensity of light absorption. The inset image shows the suspensions of ZnO1−x, GHZ1.2, and GMZ1.2. Clearly, GHZ1.2 exhibited deeper color than GMZ1.2, indicating different interactions between ZnO1−x and graphene for GHZ1.2 and GMZ1.2. Compared with that of pure ZnO1−x, GHZ1.2 shows an enhanced adsorptivity (Figure S6). After adsorption equilibrium, 85% and 70% of MB remained in the solution with pure ZnO1−x and GHZ1.2 photocatalyst, D

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Figure 5. (a) FTIR and (b) Raman spectra of graphene and ZnO1−x/graphene photocatalysts.

Figure 6. EIS Nynquist plots of the ZnO1−x and GHZ1.2 photocatalysts with light on/off cycles under the irradiation of (a) visible light (λ > 420 nm) and (b) UV light (λ = 254 nm). [Na2SO4] = 0.1 M. 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375

decrease the strength of the Zn−O bond and result in a redshift of the IR absorption peak, this observation could be ascribed to the variation of type and density for the surface oxygen defects.39 Moreover, the intensity of the IR absorption peak (I ∝ EV) would change with variation in the oxygen defects.39 Accordingly, the oxygen defects in GHZ1.2 may be more than in ZnO1−x, which play a key role in enhancing the photocatalytic efficiency. Figure 5b shows the Raman spectroscopy of graphene and GHZ-X samples. The Raman spectrum of graphene is usually characterized by two main features: the G mode (usually around 1590 cm−1) arising from sp2 C atoms and the D mode (around 1350 cm−1) arising from sp3 C atoms, which is attributed to local defects and disorders, particularly the defects located at the edges of graphene and graphite platelets.47 The Raman spectra of GHZ-X samples all exhibit two peaks around 1350 and 1590 cm−1. The D band at around 1350 cm−1 indicates the presence of surface defects.27 It is notable that the Raman peaks of graphene shift gradually as the mass ratio of graphene and ZnO1−x changes. Compared with graphene, the D band was slightly red-shifted by 7 cm−1 of the nanocomposites, indicating the defects could increase as graphene increased, while the G band showed a blue-shift of 2 cm−1. These shifts in the Raman peak could be attributed to the chemical interaction between ZnO1−x and graphene. Raman

spectroscopy is also utilized to investigate the single-, bi-, and multilayer characteristics of graphene and graphene oxide layers.48 For instance, it was shown that the G band of the single-layer graphene, located at 1592 cm−1, shifts about 2 cm−1 into higher wavenumbers indicated more contribution of singlelayer graphene sheets in the ZnO1−x/graphene sample. In general, the ID/IG intensity ratio is a measure of disorder degree and average size of the sp2 domains in graphite materials.48 Hence, a higher ID/IG peak intensity ratio indicate more defects and disorders of the graphitized structures containing the disorders caused at the edges of the carbon platelets. Compared with graphene (ID/IG = 0.75), the increased ID/IG intensity ratio for GHZ1.2 (ID/IG = 1.03) is observed, which implies a decrease in the size of the in-plane sp2 domains and formation of the defects and disorders in the graphene sheets. Above results are consistent with the results in FTIR characterization, revealing the restablishment of the conjugated graphene network (sp2 carbon). The XRD patterns of the ZnO1−x and GHZ1.2 are shown in Figure S8. It is obvious that all the diffraction peaks for ZnO1−x and GHZ1.2 are consistent with JCPDS No. 89-0510, and no change happens when ZnO1−x is hybridized by graphene, possibly due to the low amount and relatively low diffraction intensity of graphene. E

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Proposed Mechanism. Based on the above analysis, the enhancement of photocatalytic activity could be contributed to chemical contact and charge transfer between graphene and ZnO1−x. The interface charge separation efficiency of photogenerated electrons and holes is a crucial factor for photocatalytic activity. The interface charge separation efficiency can be investigated by the typical electrochemical impedance spectra (presented as Nyquist plots). Figure 6 shows the EIS Nynquist plots of ZnO1−x and GHZ1.2 without and with irradiation. The radius of the arc on the EIS spectra reflects the interface layer resistance occurring at the surface of electrode. This smaller arc radius implies the higher efficiency of charge transfer.49 It is observed that, with the introduction of graphene, the arc radius of GHZ1.2 is smaller than that of ZnO1−x, in the cases of both with and without irradiation, which indicated more rapid charge transfer. This result demonstrates that the introduction of graphene to ZnO1−x can dramatically enhance the separation and transfer efficiency of photogenerated e−h pairs through an interfacial interaction between ZnO1−x and graphene. The photoluminescence (PL) emission spectra of asprepared samples are shown in Figure 7. It can be clearly

seen that three main emission peaks at around 420, 446, and 490 nm, which are attributed to defect states of oxygen for ZnO, ZnO1−x, and GHZ1.2.50 Indeed, the emission intensity of PL spectra for ZnO1−x decreased slightly while GHZ1.2 decreased largely, suggesting that the introduction of graphene and oxygen defects could quench the fluorescence from the ZnO nanoparticles. The quenching mechanism of the PL spectra may because electron transfers from the excited ZnO nanoparticles. It may be possible to increase the rate of electron transfer and thus the interfacial interaction between the ZnO1−x nanoparticles and graphene.51 Therefore, graphene and surface oxygen defect are promising in enhancing the photocatalytic activity in terms of prolonging electron−hole pair lifetime, inhibiting the recombination and accelerating the transfer rate of electrons. Because of the interactions between the excited ZnO1−x and graphene, as demonstrated earlier, such emission quenching represents interfacial charge-transfer processes.52 In general, efficient charge separation and inhibited e−h recombination are benefit for enhanced photocatalytic activity. Graphene, as a conjugative π structure material, is responsible for the efficient charge separation and transportation. The surface defects may serve as charge carrier traps as well as adsorption sites where the charge transfers to the adsorbed species and prevents the e−h recombination, whereas bulk defects only act as charge carrier traps where e−h recombine in photocatalytic process.41 Ming Kong and co-workers reported that both surface and bulk defects in TiO2 nanocrystals play very important roles in photocatalysis, while decreasing the relative concentration ratio of bulk defects to surface defects significantly improves the e−h separation efficiency and thus enhance the photocatalytic activity.41 Therefore, to combine graphene and surface defects together can be expected to obtain enhanced photocatalytic activity. However, little report has focused on this topic. Here, we try to probe this using ESR (electron spin resonance) spin-trap technique and trapping experiments of radicals and holes. The ESR spin-trap technique is employed to monitor the reactive radical species generated during the irradiation with DMPO as radical scavenger; the results are shown in Figure S9. Under visible and UV light irradiation, the signals of the formed hydroxyl radical and superoxide radical species for ZnO1−x and GHZ1.2 samples in H2O and DMSO was all obviously observed, respectively. The signal peak of carbon radical is

Figure 7. Room-temperature PL emission spectra of ZnO, ZnO1−x, and GHZ1.2 photocatalysts (λex = 277 nm).

Figure 8. Kinetic curves for the photocatalytic degradation of MB over GHZ1.2 with the addition of hole and radical scavenger under the irradiation of visible light (λ > 420 nm) and UV light (λ = 254 nm). F

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only found in GHZ1.2 and exhibits more rapid enrichment of radicals (see Figures S10 and S11). The carbon free radical plays a decisive role in stabling and prolonging the lifetime of other active and oxidative radicals (OH• and O2•−), although it has no activity for degradation of pollutants. The carbon free radical derives from defects structure of graphene film in ZnO1−x/graphene. Thus, the enhanced photocatalytic activity of GHZ1.2 compared to ZnO1−x is mainly due to the larger amount and longer lifetime of oxidative radicals (OH• and O2•−) which enriched and prolonged by the more stable carbon free radical. The theory is faithfully confirmed by the above electron spin resonance (ESR) spectra results. To reveal the photocatalytic mechanism further, the main oxidative species in the photocatalytic process are detected through the trapping experiments of radicals using tBuOH as hydroxyl radical scavenger53 and EDTA-2Na as holes radical scavenger.54 As shown in Figure 8, the photocatalytic activity of GHZ1.2 decreases slightly by the addition of hydroxyl radical scavenger, while reduced largely with the addition of hole capture, indicating that hydroxyl radicals are not the main oxidative species on GHZ1.2. On the basis of the above analysis, it can be concluded that the enhancement of the photocatalytic activity of the ZnO1−x/ graphene hybrid may be mainly attributed to synergic effect between oxygen defects which serve as absorption active sites and graphene which has superior electrical conductivity. Then a possible mechanism for the synergic photocatalysis is proposed, as shown in Scheme 1. The ZnO1−x with surface oxygen defect

recombination of electrons and holes. As the oxidation reaction might occur on the surface of Oi″ defects, the synergic effect between the oxygen defects, which serve as adsorption active sites, and graphene, which could dramatically improve the separation efficiency of photogenerated electrons and holes, enhanced the photocatalytic efficiency significantly.

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CONCLUSIONS ZnO1−x/graphene hybrids are prepared via the simple in-situ reduction method. The hybrid photocatalysts showed obviously superior photocatalytic activity for the photodegradation of methylene blue under the irradiation of visible and UV light. The visible and UV light photocatalytic activity of ZnO1−x/ graphene-1.2 wt % was 4.6 and 1.2 times as that of ZnO1−x sample, respectively. The superior photocatalytic activity of the hybrid could be attributed to the synergic effect between graphene and ZnO1−x surface defect layer, which enhanced separation efficiency of photoinduced electron−hole pairs mainly resulting from the promotion of HOMO orbit of graphene and the Oi″ defect level of ZnO1−x in ZnO1−x/ graphene. This work can provide important inspirations for developing of graphene hybridized defect-type materials.

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ASSOCIATED CONTENT

S Supporting Information *

ESR spectra, XRD patterns, TEM images, TOC spectrum, and synthesis details of graphene oxide and graphene. This material is available free of charge via the Internet at http://pubs.acs.org.

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Scheme 1. Schematic Drawing Illustrating the Mechanism of Charge Separation and Photocatalytic Process over ZnO1−x/ Graphene Photocatalysts under Light Irradiation

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +86-10-62787601; Tel +86-10-62787601.

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ACKNOWLEDGMENTS This work was partly supported by Chinese National Science Foundation (20925725 and 50972070), National Basic Research Program of China (2013CB632403), National High Technology Research and Development Program of China (2012AA062701), and Special Project on Innovative Method from the Ministry of Science and Technology of China (2009IM030500).

can be excited by visible and UV light due to the narrow of the energy band gap, resulting from the generation of the surface defect level induced by surface oxygen-defect states. The surface defect level, which can enhance the ecb−−hvb+ separation rate in ZnO1−x, is introduced by Oi″ defect states, resulting from the rise of the VB position, which energy is referred to in Zheng et al.38 After ZnO1−x photocatalyst produce photogenerated electron−hole pairs, the photogenerated holes on ZnO1−x could transfer easily to graphene via the well-developed interface due to the energy difference between HOMO orbit (the highest occupied molecular orbital) of graphene and the Oi″ defect level of ZnO1−x, which serve as photogenerated holes’ shallow trappers to restrain the recombination of photogenerated electrons and holes. Meanwhile, the CB position of ZnO1−x was lower than LUMO orbit (the lowest unoccupied molecular orbital) of graphene; the photogenerated electron on graphene could transfer easily to the CB of ZnO1−x, which work as electron acceptors and can trap the photogenerated electrons temporarily to reduce the surface

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The authors declare no competing financial interest.

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