Reduced Graphene Oxide-Hierarchical ZnO Hollow Sphere

Mar 16, 2012 - Fabrication of Au/Graphene-Wrapped ZnO-Nanoparticle-Assembled Hollow Spheres with Effective Photoinduced Charge Transfer for Photocatal...
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Reduced Graphene Oxide-Hierarchical ZnO Hollow Sphere Composites with Enhanced Photocurrent and Photocatalytic Activity Qiu-Ping Luo, Xiao-Yun Yu, Bing-Xin Lei, Hong-Yan Chen, Dai-Bin Kuang,* and Cheng-Yong Su* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China S Supporting Information *

ABSTRACT: The reduced graphene oxide (RGO)-hierarchical ZnO hollow sphere composites are prepared through a simple ultrasonic treatment of the solution containing graphene oxide (GO), Zn(CH3COO)2, DMSO, and H2O. The GO is reduced to RGO effectively, and the ZnO hollow spheres consisting of nanoparticles are uniformly dispersed on the surface of RGO sheets during the ultrasonic process. The optimum synergetic effect of RGO-ZnO composites is found at a RGO mass ratio of 3.56%, and the photocurrent and photodegradation efficiency on methylene blue of RGO-ZnO composites are improved by five times and 67%, respectively, compared with those of pure ZnO hollow spheres. The enhancements of photocurrent and photocatalytic activity can be attributed to the suppression of charge carriers recombination resulting from the interaction between ZnO and RGO.

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INTRODUCTION Since the water splitting on the semiconductor-based photochemical electrode reported by Fujishima and Honda,1 the photocatalytic process of organic pollutants under ultraviolet (UV) irradiation has been attracting increasing attention during the past decades because of the great potential for environmental purification and converting photon energy into chemical energy.2 The photocatalysis mechanism of wide band gap semiconductor is as follows: With UV irradiation, the electrons are excited from the valence band (VB) of semiconductors (e.g., TiO2, ZnO) to the conduction band (CB) and form electron−hole pairs, which are responsible for the photocatalytic activity of the semiconductors;3 simultaneously, a variety of reactive oxidation species (ROSs) are formed, which oxidize the organic compounds.4,5 A major limitation for achieving high photocatalytic efficiency is the quick recombination of photogenerating electron−hole pairs, which is faster than the surface redox reactions so that the quantum efficiency of photocatalysis is seriously reduced.2,6 Therefore, a number of efforts have been attracted to inhibit the recombination of electron−hole pairs and improve charge transport via coupling the wide band gap semiconductor photocatalysts with other materials, such as semiconductor-noble metal composite,7 quantum dot-semiconductor composite,8 C−N doped semiconductor,9 carbon nanotube (CNTs),6 or fullerene (C60)semiconductors composites,10 and so on. Graphene is another allotrope of carbon besides zerodimensional (0D) fullerenes and one-dimensional (1D) CNTs, and it can be seen as a two-dimensional (2D) single atomic © 2012 American Chemical Society

layer of graphite. As the newest member of carbon family, graphene possesses a unique 2D layer structure of sp2hybridized carbon atoms and many unique properties that predisposes it for use in photoconversion devices. The 2D structure of graphene transmits 95% of visible and UV light as a result of minimizing light blocking,11 what’s more, its potentially ballistic electron transport capability results in effectively zero conduction resistance for storing and transporting electrons within the graphene structure.12 It has been demonstrated that assembling metal or metal oxide nanomaterials (Au, Pt, TiO2, ZnO, SnO2, etc.) on graphene sheets can exhibit enhanced efficiencies in excitonic solar cells and photocatalytic reactions due to excellent electron accepting property of graphene.13−15 Among these wide band gap semiconductors used in photoelectrochemical and photocatalytic application, ZnO has been getting increased attention because of its suitable band gap at 3.37 eV at room temperature and the high electron mobility of ∼115−155 cm−2 V−1 s−1.16,17 Recently, a few publications focus on the study of the RGOZnO nanoparticles composites and exhibit interesting photocatalytic effect.18−20 It is well known that the photocatalytic performance strongly depends on the morphology of nanomaterials. Recently, hierarchically structured ZnO hollow spheres consisting of nanoparticles have confirmed that the photoelectrochemical performance of dye-sensitized solar cells can be Received: November 24, 2011 Revised: March 13, 2012 Published: March 16, 2012 8111

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infrared (FTIR) spectra were recorded using a FTIR Analyzer (Nicolet/Nexus 670). Raman spectra were recorded on a laser micro-Raman spectrometer with an excitation of 514 nm laser light. X-ray photoelectron spectroscopy (XPS) measurements were taken by using a spectrometer (Thermo Fisher Scientific, ESCALAB 250) with a monochromic Al Ka source at 1486.7 eV, at a voltage of 15 kV and an emission current of 10 mA. The morphologies of products were characterized using a fieldemission scanning electron microscope (FESEM, Quanta 400F) and a transmission electron microscope (TEM, JEOL-2010). The diffuse reflectance absorption spectra (DRS) were recorded on a UV−vis spectrophotometer (Hitachi (Shimadzu) UV3150) equipment, and the recorded range of the spectra is 300−800 nm. The electrochemical impedance spectroscopy (EIS) measurements were performed with a Zennium electrochemical workstation (ZAHNER) with the frequency range from 10 mHz to 100 kHz. Photoelectrochemical Characterizations. The photoelectrochemical characteristics were measured in a Zennium electrochemical workstation (ZAHNER) using a standard threecompartment cell under UV light irradiation (500 W mercury lamp). Typically, 5 mg of ZnO hollow spheres or RGO-ZnO composite was added to 2 mL of ethanol and then grinded for 15 min. Afterward, the viscous paste was binded onto the FTO glass via doctor-blading technique, which formed the ZnO/FTO or RGO-ZnO/FTO glass working electrode and then dried at 70 °C in air for 6 h. A Pt wire and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. Na2SO4 (0.01 M) aqueous solution was employed as electrolyte. Photocatalytic Experiments. Typically, 20 mg of photocatalysts (ZnO hollow spheres or RGO-ZnO composites) was added to 100 mL of aqueous solution of the methylene blue (MB) dyes (1 × 10−5 mol L−1). Prior to the irradiation, the suspensions were magnetically stirred in the dark for 30 min; afterward, the photoreaction vessel was exposed to the UV irradiation (500 W mercury lamp) under ambient conditions and stirring. The distance between UV light and the photoreaction vessel is 5 cm. At given time intervals, the photoreacted suspension (∼3 mL) was analyzed by recording variations of the absorption peak maximum in the UV−vis spectra of MB using a UV−vis spectrophotometer (Hitachi (Shimadzu) UV-3150).

improved because of the superior light scattering and facile transport for the electrolyte.16 Similarly, hierarchically structured ZnO hollow spheres possess large surface area, feasible to the transport of organic pollutants as well, which can potentially enhance the photocatalytic performance. Although superior photoelectrochemical and photocatalytic performance can be expected for the RGO-hollow structured ZnO composites, to our knowledge, there are no reports on this kind of hybrid materials. On the basis of previous successful preparation of hierarchically structured ZnO hollow spheres,21 herein, RGO-ZnO composites (hierarchical ZnO hollow spheres on the surface of RGO sheets) are further synthesized through a one-step ultrasonic process in a solution containing GO and zinc acetate. The novel hierarchical RGO-ZnO composites show enhanced photocurrent and photocatalytic performance because of the large surface area of hierarchical structured ZnO hollow spheres as well as excellent electron transport and reduced electron− hole pair recombination resulting from the graphene.

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EXPERIMENTAL SECTION All chemicals were analytical reagent grade and used as received without further purification. Synthesis of Graphene Oxide. Graphene oxide (GO) was synthesized from natural graphite powder using modified Hummers’ method.22 In brief, 0.5 g of graphite was mixed with 0.5 g of NaNO3, 23 mL of H2SO4, and 3 g of KMnO4 in an ice bath under strong stirring. After the mixture was stirred at room temperature for 120 h, 46 mL of water was added and the mixture kept at 95 °C for 1 h. The unreacted KMnO4 was removed by the addition of 10 mL of H2O2. The oxidized graphite was purified by washing with 10% HCl and a mix-solvent (H2O/EtOH 1:5). Exfoliation was accomplished by sonicating 1 mg/mL graphite oxide aqueous solution for 120 min and then centrifuged at 5000 rpm for 10 min to obtain GO solution. Synthesis of RGO-ZnO Composites. 2.5 mL of aqueous solution containing an appropriate amount of GO was added to a beaker containing 1.098 g of zinc acetate (Zn(Ac)2) and 47.5 mL of DMSO under stirring; then, ultrasonication was performed on the mixture by an ultrasonic apparatus (Sonics: VCX-500) for 15 min under ambient conditions (Scheme 1).

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Scheme 1. Scheme Presentation of the RGO-ZnO Composites via an Ultrasonic Procedure

RESULTS AND DISCUSSION The mass content of RGO in the RGO-ZnO composites was evaluated by TGA. The TGA curves (Figure 1) of the RGO-ZnO composites show weight loss from room temperature to 250 °C, which may be due to the desorption of surface bound water.23

The products (referred by RGO-ZnO) were washed by deionized water and ethanol. Characterizations. Thermogravimatric analysis (TGA) was performed under air using a Netzsch TG209F3 TGA system. The samples were heated from room temperature to 800 °C with an acceleration of 10 °C/min. X-ray diffraction (XRD) measurements were taken on a D8-Advanced Bruker-AXS diffractometer by using Cu Kα irradiation. Fourier transform

Figure 1. TGA curves of RGO-ZnO composites with different amount of RGO. 8112

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The weight loss from 250 to 700 °C could be attributed to the removal of oxygen-containing groups and the decomposition of carbon framework from the composites. On the basis of the TGA analysis, the RGO mass ratios of the three RGO-ZnO composites are 2.06, 3.56, and 7.43%, which are calculated from the weight loss of 250 to 700 °C and shown in Figure 1. The morphology of RGO-ZnO 3.56% was characterized by FESEM and TEM (Figure 2). Clearly, the stratiform RGO

reflection peak of GO is absent after ultrasonic treatment resulting from the reduction of GO during the ultrasonic process and breach of the regular stack of GO by the intercalation of metal oxide.25 Obviously, the diffraction peaks of RGO-ZnO composites are in good agreement with the hexagonal wurtzite structure (P63mc, a = 3.2495 Å, c = 5.2069 Å, JCPDS no. 361451) of ZnO, which does not have any changes in comparison with the ZnO hollow spheres prepared in the absence of GO (Figure 3a). Hence, the present ultrasonic reaction not only produces the hierarchical ZnO hollow spheres but also simultaneously reduces the GO so that the RGO-ZnO composites are obtained. The effectiveness of the ultrasonic reduction of GO was further evaluated with FTIR spectra, Raman spectroscopy, and XPS. In the FTIR spectrum of the GO (Figure 3b), the strong and broad absorption located on 3401 cm−1 can be assigned to the stretching vibrations of O−H. The broad absorption peaks at around 1056 and 1228 cm−1 and the peak at 1724 cm−1 are attributed to the characteristic stretching vibration of C−O, and CO of COOH groups situated at the edges of GO sheets, respectively. The absorption due to the O−H blending vibration, epoxide groups, and skeletal ring vibrations is visible around 1621 cm−1,26 whereas for the RGO-ZnO 3.56% prepared via ultrasonic reaction, the absorption peaks of O−H group (3401 cm−1) and epoxide groups (1621 cm−1) decrease dramatically in intensity, and the absorption peaks of C−O, and CO oriented from COOH groups at 1724, 1228, and 1056 cm−1 are absent. These results indicate that the GO has been reduced to a great extent (theoretical model predicts that complete reduction may be difficult27). Furthermore, the absorption band in the range of 440−590 cm−1 for the RGOZnO 3.56% can be assigned to the stretching vibration of Zn− O, which is blue-shifted from 410 cm−1 of Zn−O in the bulk ZnO. The blue shift probably results from the coordination among the residual hydroxyl, epoxy groups on the RGO sheets, and ZnO hollow spheres. The Raman spectra (Figure 3c) of GO and RGO-ZnO 3.56% samples show two characteristic bands: the G band (1598 cm−1) and the D band (1350 cm−1). As we know, the G band is the response of the in-plane stretching motion of symmetric sp2 C−C bonds, whereas the D band results from the disruption of the symmetrical hexagonal graphitic lattice.28 Compared with GO, RGO-ZnO 3.56% displays a slight increase in ID/IG (0.8711 vs 0.7386), which is regarded as a decrease in the size of sp2 domains upon reduction29 and the interaction between ZnO hollow spheres and the graphene sheets.3,30 The peak at 2711 cm−1 corresponds to the overtone of the D band, and the

Figure 2. FESEM images (a,b) and TEM images (c,d) of RGO-ZnO 3.56%.

covered with numerous ZnO hollow spheres is observed, as shown in Figure 2a,b. The hierarchical hollow structure of ZnO sphere is displayed in the inset of Figure 2a, which is about 400−500 nm in diameter and is in good agreement with the previous report in the absence of RGO.21 The TEM image (Figure 2c) further shows that the ZnO hollow spheres are composed of nanoparticles with diameter of ∼15 nm. HRTEM image (Figure 2d) shows the intimate contact between ZnO hollow spheres and RGO, which probably makes electronic interaction between the components.24 XRD analysis was employed to investigate the crystal phase of GO, hierarchical ZnO hollow spheres, and RGO-ZnO 3.56%. As shown in Figure 3a, the peak of GO at around 2θ = 10.2° corresponds to the (001) reflection. However, this (001)

Figure 3. XRD patterns of GO, ZnO hollow spheres, and RGO-ZnO 3.56% (a); FTIR spectra of GO and RGO-ZnO 3.56% (b); and Raman patterns of GO and RGO-ZnO 3.56% (c). 8113

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peak at 2933 cm−1 associates with the D + G band.31 After ultrasonic reaction, the appearance of the bands at ∼3000 cm−1 indicates the increased disorder in RGO-ZnO 3.56% compared with GO, which is in agreement with the increase in ID/IG.32 The peak located at the low frequency (∼440 cm−1) is assigned to the ZnO nonpolar optical phonons. This peak shifts to lower frequency compared with pure ZnO hollow spheres (not shown here) due to the interaction of ZnO and RGO sheets.33 The reduction effectiveness of GO via the ultrasonic reaction is further confirmed by the XPS C1s spectra (Figure 4). As is

changed. However, the absorption intensity increases for RGO-ZnO compared with RGO (Figure S1 of the Supporting Information); it can be ascribed to the increase in surface electric charge of ZnO in the composite.24 To investigate the electronic interaction between RGO and ZnO, the photocurrent transient response (PCTR) measurements for hierarchical ZnO hollow spheres, and RGO-ZnO electrodes are employed. All films are prepared using doctorblading technique with a thickness of ∼4 μm, and the photocurrent response was measured in 10 s on−off cycles. Figure 6 shows that the fast and uniform photocurrent

Figure 4. XPS spectra of GO (a) and RGO-ZnO 3.56% (b) in the C1s region.

well known, the C−C and C−H bonds usually appear between 284.8 and 285 eV, whereas the C−O (C−OH, C−O−C), C O, and C(O)OH bands locate at +1.3 to 1.7, +2.5 to 3, and +4 to 4.5 eV, respectively.28,34 In brief, from the C1s XPS spectra of GO (Figure 4a), a considerable degree of oxidation with three components that corresponds to carbon atoms in different functional groups can be seen: the nonoxygenated ring C, the C in C−O bonds, and the carboxylate carbon (O−CO).35 After ultrasonic reaction, a notable decrease in oxygen content is clearly visible; in addition, the peak corresponding to the C−O bond has disappeared. The oxygen loss mainly results from the loss of C−O, and the relative ratio of C(O)OH decreases remarkably as well. As discussed above, it is evident that O and C atomic ratio for RGO-ZnO 3.56% is much lower than that for GO. These results are consistent with the FTIR and Raman spectrum characterizations. The considerable deoxygenation by the ultrasonication will enhance the conductivity of RGO sheets25 and achieve more effective antirecombination during the phtotocatalytic process, as demonstrated by Hoffman.36 The optical properties of the hierarchical ZnO hollow spheres and RGO-ZnO 3.56% were probed by using UV−vis diffuse reflectance spectra (Figure 5). The absorption edge of the RGO-ZnO composites remains the same as the bare ZnO hollow spheres, indicating that the band gap has not been

Figure 6. Photocurrent response of ZnO hollow spheres and RGOZnO 3.56%.

responses for each switch-on and switch-off event for these electrodes. When the illumination is stopped, the photocurrent decreases back to zero immediately. The photocurrent generation of the RGO-ZnO composites increases along with the increase in the mass ratio of RGO from 0 to 3.56%; however, further increase in the mass ratio of RGO (7.43%) leads to a decrease in photocurrent intensity (shown in Figure S2 of the Supporting Information). It is worth noting that the photocurrent of the RGO-ZnO 3.56% (∼35 μA cm−2) is significantly higher than that of the pure ZnO hollow spheres (∼6 μA cm−2). Furthermore, the photocurrent is much better that the previous report (90 nA) for the ZnO nanoparticle− RGO composite.37 Because the photocurrent from the illuminated semiconductor film (ZnO) is determined by the speed of excited electrons withdrawn from semiconductor to FTO and the recombination at the electrolyte/film interface;28 therefore, in this case, the photocurrent enhancement of RGOZnO composites with the mass ratio of RGO increases up to 3.56% can be attributed to the two factors: (1) the higher separation efficiency of the photoinduced electron−hole pairs and lower recombination rate, which resulted from the interaction of ZnO and RGO (this deduction can be supported by the open-circle voltage decay (OCVD)) and (2) the decrease in the solid-state interface layer resistance and the chargetransfer resistance on the surface is demonstrated by the electrochemical impedance measurements and is discussed in the later sections. The photocatalytic activity of ZnO hollow spheres and RGOZnO composites is evaluated by the degradation of MB under UV light irradiation. The normalized temporal concentration changes (C/C0) of MB during photodegradation are proportional to the normalized maximum absorbance (A/A0).38 It is clear that RGO-ZnO 3.56% shows a remarkable enhancement in the photodegradation of MB compared with ZnO hollow spheres (Figure 7). Under UV light illumination for 1.5 h, almost all of the initial MB dyes were decomposed by RGO-ZnO

Figure 5. UV−vis diffused reflectance spectra (DRS) of ZnO hollow spheres and RGO-ZnO 3.56%. 8114

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FTO glass surface and external circuit to produce photocurrent.39 In addition, the photogenerated electrons can react with the adsorbed O2 to form radicals such as ·O2−, ·OH for the photodegradation of MB,40 and the remaining holes in ZnO can take part in the redox reactions in the photocatalytic process.2 The more ROSs, the better the photocatalytic activity. The enhancements of photoelectrochemical performance and photocatalytic activity for RGO-ZnO composites compared with bare ZnO hollow spheres are further discussed. As shown in Figure 5, the light absorption of RGO-ZnO composite for the visible region (400−800 nm) exhibits an obvious enhancement compared with ZnO hollow spheres, thus improving the photocurrent (Figure 6) and photodegradation (Figure 7) of MB. Besides the light absorption, the charge transportation and separation were also considered to be key factors on improving the photocatalytic activity.41 Graphene has been reported to be a competitive candidate for the acceptor material due to the unique 2D π-conjugation structure.42 Previous study demonstrated that the excited electrons of TiO2 could transfer from the TiO2 conduction band to graphene via a percolation mechanism.43 Similarly, in the present work, the photogenerated electrons would transfer from the conduction band of ZnO to RGO, and the RGO serves as an acceptor of electrons and suppresses charge recombination effectively, resulting in higher photocurrent density and more charge carriers to form reactive species, promoting the photodegradation of MB dyes. The recombination property can be characterized using OCVD measurement.34 The electron recombination kinetics have been investigated by monitoring the transient Voc as a function of time upon switching off the light. After the illumination is switched off with a shutter under a steady voltage, the Voc, which relates to the electron lifetime, decays sharply because of electron recombination.44,45 The Voc decay, which is a function of time measured using PCTR based on ZnO hollow spheres and RGO-ZnO 3.56%, is displayed in Figure 8a. The Voc decay rate of RGO-ZnO 3.56% is lower than that of ZnO hollow spheres, revealing slower recombination kinetics for the former compared with the later anode. Figure 8b further plots the calculated τn, which acted as a function of Voc for the two anodes. The lifetime of photogenerated electrons (τn), which is defined by the average time that the photogenerated electrons exist before they recombine, can be calculated by the following equation: τn = −(kBT/e)(dVoc/dt)−1, where kB is the Boltzmann constant and T is the temperature.46,47 It is evident that τn of the RGO-ZnO 3.56% is longer than that of ZnO hollow spheres. The longer electron lifetime indicates a slower electron recombination rate and more electrons surviving

Figure 7. Photocatalytic degradation of methylene blue (MB) under the irradiation of UV light over ZnO hollow spheres and RGO-ZnO 3.56%.

3.56%; however, ∼60% of the initial dyes still remained in the solution for ZnO hollow spheres. Compared with ZnO hollow spheres, all of the RGO-ZnO composites with different amount of RGO content exhibit enhanced photocatalytic degradation efficiency of MB (Figure S3 of the Supporting Information), and the RGO-ZnO 3.56% shows the best photocatalytic activity; further increasing the mass ratio of RGO leads to a decrease in photocatalytic activity. The schematic illustration of the charge transfer, enhanced photocurrent, or photocatalytic activity by RGO-ZnO composites is summarized in Scheme 2. Under the illumination Scheme 2. Mechanical Illustration of Enhanced Photocurrent Density and Photocatalytic Activity for RGOZnO Composites

of UV light, ZnO was excited to form the photogenerated electron−hole pairs, which separated at the interfaces of ZnO and RGO sheets. The electrons can transport along the superior-electron-conduct RGO sheets from inner region to the

Figure 8. Open-circuit voltage−decay curve and according electron lifetime in ZnO hollow spheres and RGO-ZnO 3.56%. Voltage−decay measurement (a) and electron lifetime determined from open-circuit voltage−decay measurement (b). 8115

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enhancement of the light absorption, which was verified by the open-circle voltage decay, electrochemical impedance measurements, and UV−vis diffuse spectra. The present study provides a low-cost, convenient method for assembling various nanostructured semiconductor-RGO composites in applications of water purification as well as optoelectronic fields at a large scale.

from the back-reaction, which can improve both the photocurrent (Figure 6) and photocatalytic activity (Figure 7).45 AC impedance spectroscopy is a powerful technique to determine the kinetic parameters of an electrochemical system.48 The EIS measurement was employed to obtain more detailed information about the charge transport within the films. Figure 9

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis diffuse absorbance spectra (DRS) of RGO-ZnO composites, photocurrent response of ZnO and RGO-ZnO composites, and liquid-phase photocatalytic degradation of MB under the irradiation of UV light over ZnO hollow spheres and RGO-ZnO composites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 20 84113015. Fax: +86-20-84113015. E-mail: [email protected], [email protected]. Notes

Figure 9. Nyquist plots for ZnO hollow spheres and RGO-ZnO 3.56% at an open circuit between 10 mHz and 100 kHz.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial supports from the National Natural Science Foundation of China (20873183, 21073239, U0934003), the Fundamental Research Funds for the Central Universities, the Research Fund for the Doctoral Program of Higher Education (20100171110014), and the Open Project Program of Key Laboratory of Photochemical Conversion and Optoelectronic materials (Technical Institute of Physics and Chemistry, Chinese Academy of Science).

shows the Nyquist plots of ZnO hollow spheres and RGO-ZnO 3.56% films. The semicircle at the high frequencies represents the capacitance and the resistance of the solid-state interface layer, which is formed at the highly charged state and results from the passivation reaction between the electrolyte and the surface of the electrode; the double-layer capacitance and the chargetransfer resistance correspond to the second semicircle at the medium frequencies, whereas the Warburg impedance is usually presented by a straight sloping line at the low-frequency region.49 At the high and medium frequencies region, both of the semicircles of RGO-ZnO 3.56% are smaller than that of ZnO hollow spheres, which indicates a decrease in the solid-state interface layer resistance and the charge-transfer resistance on the surface.38 Hence, a higher rate in the photocatalytic activity of RGO-ZnO 3.56% would be achieved. The above photoelectric and photocatalytic investigations reveal that the introduction of RGO can enhance the photocurrent response and photocatalytic performance of RGO-ZnO composites compared with ZnO hollow spheres. However, there is an optimal RGO content (3.56%) for RGO-ZnO composites. Both the photocurrent and the photodegradation efficiency of RGO-ZnO composites would decrease when the mass ratio of RGO reaches 7.43%. This is mainly resulting from the conductivity improvement of the composites because the increase in RGO cannot compensate for the intrinsically lower photogeneration rate of the ZnO hollow spheres; namely, the photogenerated electron-hole pairs will decrease. (More RGO decreases the relative amount of ZnO.)

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REFERENCES

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CONCLUSIONS

In summary, a simple ultrasonic procedure was demonstrated to prepare RGO-hierarchical ZnO hollow spheres composites. In this study, RGO-ZnO 3.56% shows superior photocurrent response and photodegradation efficiency of MB under illumination of UV light in comparison with the pure ZnO hollow spheres. These improvements can be attributed to the enhanced charge transportation and separation, the suppressed charge carriers recombination, longer electron lifetime, and the 8116

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The Journal of Physical Chemistry C

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