Synthesis of ZnO decorated graphene nanocomposite ...

Synthesis of ZnO decorated graphene nanocomposite for enhanced photocatalytic properties S. Gayathri, P. Jayabal, M. Kottaisamy, and V. Ramakrishnan Citation: Journal of Applied Physics 115, 173504 (2014); doi: 10.1063/1.4874877 View online: http://dx.doi.org/10.1063/1.4874877 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Doping concentration driven morphological evolution of Fe doped ZnO nanostructures J. Appl. Phys. 116, 164315 (2014); 10.1063/1.4900721 Electrodeposition of hierarchical ZnO/Cu2O nanorod films for highly efficient visible-light-driven photocatalytic applications J. Appl. Phys. 115, 064301 (2014); 10.1063/1.4863468 Photocatalytic and antibacterial properties of Au-TiO2 nanocomposite on monolayer graphene: From experiment to theory J. Appl. Phys. 114, 204701 (2013); 10.1063/1.4836875 Theoretical and experimental approach on dielectric properties of ZnO nanoparticles and polyurethane/ZnO nanocomposites J. Appl. Phys. 112, 054106 (2012); 10.1063/1.4749414 Visible light photocatalytic property of Zn doped V2O5 nanoparticles AIP Conf. Proc. 1447, 351 (2012); 10.1063/1.4710024

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. IP: 14.139.128.20 On: Tue, 23 Feb 2016 09:45:35

JOURNAL OF APPLIED PHYSICS 115, 173504 (2014)

Synthesis of ZnO decorated graphene nanocomposite for enhanced photocatalytic properties S. Gayathri,1 P. Jayabal,1 M. Kottaisamy,2 and V. Ramakrishnan1,3,a) 1

Department of Laser Studies, School of Physics, Madurai Kamaraj University, Madurai 625021, Tamilnadu, India Department of Chemistry, Thiagarajar College of Engineering, Madurai 625014, Tamilnadu, India 3 Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram 695016, Kerala, India 2

(Received 20 February 2014; accepted 22 April 2014; published online 2 May 2014) Zinc oxide/Graphene (GZ) composites with different concentrations of ZnO were successfully synthesized through simple chemical precipitation method. The X-ray diffraction pattern and the micro-Raman spectroscopic technique revealed the formation of GZ composite, and the energy dispersive X-ray spectrometry analysis showed the purity of the prepared samples. The ZnO nanoparticles decorated graphene sheets were clearly visible in the field emission scanning electron micrograph. Raman mapping was employed to analyze the homogeneity of the prepared samples. The diffuse-reflectance spectra clearly indicated that the formation of GZ composites promoted the absorption in the visible region also. The photocatalytic activity of ZnO and GZ composites was studied by the photodegradation of Methylene blue dye. The results revealed that the GZ composites exhibited a higher photocatalytic activity than pristine ZnO. Hence, we proposed a simple wet chemical method to synthesize GZ composite and its application on photocatalysis was C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4874877] demonstrated. V I. INTRODUCTION

Currently, organic dyes and their waste products from the industries, such as textile, paper, and plastic, contaminate the surroundings by the discharge of the toxic, nonbiodegradable, and carcinogenic materials.1 The chemicals found as pollutants in waste water effluents from industrial sources must be removed before their discharge to the environment.2 Also, such pollutants may be found in ground water and so it is essential to remove the pollutants to achieve acceptable drinking water quality. The removal of such organic pollutants in waste water using semiconducting materials as photocatalysts has attracted a lot of attention in environmental protection.3 Photocatalytic oxidation is an inexpensive process, since it involves only with non-degradable photocatalytic materials and a natural or an artificial light source.4 In general, photocatalysis is based on the light absorption of semiconductor oxide photocatalyst, such as TiO2 and ZnO, to excite the electrons from valence band to conduction band and create electron–hole pairs.5 ZnO is a n-type, wide band gap (Eg ¼ 3.37 eV) semiconductor used in several applications such as nano-scale electronic and optoelectronic devices.6–9 Due to its higher photosensitivity and wide band gap, ZnO can be considered as a promising photocatalyst alternative to TiO2.10 However, there are still some problems such as high recombination rate of photogenerated electron– hole pairs that limit the use of pure ZnO for photocatalytic applications.11 Therefore, to enhance the photocatalysis efficiency, it is essential to slow down the recombination of the charge carriers. Several works have been done to reduce the recombination rate by linking the photocatalysts with other materials such as noble metals, semiconductors, and carbon materials.12 a)

E-mail: [email protected]

0021-8979/2014/115(17)/173504/9/$30.00

Graphene is a two-dimensional, single atomically thin honeycomb lattice of sp2-bonded carbon and it has outstanding electrical and mechanical properties.13 These fascinating properties of graphene would make it an exceptional electron-transport material in the photocatalysis process than C60 or graphite-like carbon.14–16 It has been shown that graphene combined with nanoparticles, such as TiO2, ZnO, SnO2, and CdS, show enhanced photocatalytic reduction, higher efficiency in solar cells, and fuel cells.17 Herring et al.18 have demonstrated the photocatalytic activity of zinc oxide/graphene (GZ) nanocomposites prepared by microwave synthesis. Ahmad et al.19 investigated photocatalytic performance of GZ composites synthesized via solvothermal method. Li et al.1 synthesized flower-like ZnO nanoparticles attached on graphene oxide (GO) sheets and studied its use for photocatalytic degradation. Lv et al.20 demonstrated photocatalytic degradation of Methylene blue (MB) by ZnO–reduced GO (RGO)–carbon nanotube (CNT) synthesized by microwave-assisted reaction. However, to the best of our knowledge, in-situ synthesis of ZnO decorated on graphene sheets (GZ composite) by one step chemical method has not been reported so far. In the present work, we report a simple one-step synthesis of ZnO anchored on graphene sheets through chemical reduction of GO in the presence of zinc acetate. In order to know the structural and elemental analysis, the synthesized GZ composites were examined by x-ray diffraction (XRD), micro-Raman, energy dispersive x-ray (EDX), and field emission scanning electron micrograph (FE-SEM). In addition to the SEM analysis, we have carried out Raman mapping of GZ composites for the first time so as to understand the distribution of ZnO on graphene surface. The photocatalytic activity of the GZ composites and bare ZnO for the degradation of MB was studied.

115, 173504-1

C 2014 AIP Publishing LLC V


173504-2

Gayathri et al.

J. Appl. Phys. 115, 173504 (2014)

II. EXPERIMENTAL

E. Characterization

A. Materials

Powder XRD of the synthesized samples was recorded by PANalytical X-ray diffractometer with CuKa radiation ˚ ). The structural morphology of the samples was (k ¼ 1.54 A observed using a field emission scanning electron microscope (Nova NanoSEM NPE206). The elemental analysis was carried out using EDX spectrometry (Brucker Quantax 200 AS). Micro-Raman and Raman mapping measurements were performed at room temperature in 180 back scattering geometry by using HORIBA Jobin Yvon LabRAM HR 800 equipped with a charge coupled device (CCD) detector and an automated XY motorized stage, where 632.8 nm line from He-Ne laser was used as the excitation source. The room temperature photoluminescence (PL) was measured by LabRAM HR 800 using the 325 nm excitation line of He-Cd laser and 40 objective. The UV–Visible diffuse reflectance spectra (DRS) were recorded on a Shimadzu UV-2450 Spectrophotometer.

Graphite powder was purchased from Alfa Aesar. 37% hydrochloric acid (HCl), 98% sulfuric acid (H2SO4), hydrogen peroxide (H2O2), potassium permanganate (KMnO4), zinc acetate dihydrate (Zn(CH3COO)22H2O), and hydrazine hydrate (N2H4) were purchased from Merck. All chemicals were used as received without further purification. Deionized water was used throughout this study. B. Synthesis of ZnO nanoparticles

The ZnO nanoparticles were synthesized by dissolving 0.2M zinc acetate dihydrate in water. Then, the solution was stirred for 3 h at room temperature to get transparent solution. Appropriate amount of hydrazine hydrate was added to the above solution and stirred for another 3 h at 80 C. The resulting precipitate was centrifuged and washed with water several times and dried. Finally, the powder was annealed at 450 C for 4 h. C. Synthesis of graphene oxide

The graphite oxide was synthesized by modified Hummer’s method.21 Briefly, 1 g of graphite powder was added to 80 ml of H2SO4 and stirred in an ice bath. After 30 min, 6 g of KMnO4 was slowly added to the above solution and stirred. Then, the ice bath was replaced by a water bath (30–35 C) and the solution was stirred overnight. 100 ml of water was slowly added and stirred for 30 min. A mixture of H2O2 (5 ml) and water (100 ml) was slowly added to the above solution and stirred for 10 min. The solution gradually turns to yellow color from dark brown. The solution was then filtered to get the precipitate. The filtered yellow cake (graphite oxide) was initially washed (3–4 times) using HCl and then washed several times in H2O until the neutral pH is reached. The synthesized graphite oxide was then dispersed in water and sonicated for 4 h to get GO. D. Synthesis of GZ composite

In the typical synthesis, dried GO powder was dispersed in 90 ml of water (10 mg/ml) and sonicated for 30 min to obtain GO suspension. Subsequently, 300 mg of zinc acetate was added to the GO suspension and stirred for 3 h. Then, 30 ll of N2H4 was added and the resulting mixture was stirred for another 3 h at 80 C. The solution turned to black from brown color indicating the reduction of GO to graphene. The precipitate was then centrifuged (3000 rpm) for 30 min and washed several times using water and alcohol. Later, the obtained powder was dried at 100 C in a water bath for 2 h and the dried powder was annealed at 450 C for 4 h. The final powder is designated as GZ-1. In order to optimize the graphene and ZnO composition to get better photocatalytic efficiency, the process was repeated with varying mass of zinc acetate, viz., 600 mg and 900 mg and the final products were designated as GZ-2 and GZ-3, respectively.

F. Photocatalytic activity experiments

The photocatalytic properties of the prepared samples were estimated by monitoring the photodegradation of MB in a home-made apparatus with a Newport-66901 Xe lamp (300 W, 200–800 nm) as the radiation source. In each experiment, 50 mg of photocatalyst was dispersed in 100 ml of MB aqueous solution (1 105M). The suspension was then magnetically stirred in dark for 30 min. Later, the solution was transferred to Erlenmeyer flask and exposed to the UV-visible light irradiation and at a regular time interval of 20 min, 5 ml of the exposed solutions was sampled, centrifuged, and the supernatant was collected for analysis of the MB concentration. The photocatalytic degradation process was monitored using a UV–Vis spectrophotometer (Shimadzu UV-2450) to record the characteristic absorption peak (660 nm) of MB. III. RESULTS AND DISCUSSION

ZnO nanoparticles were decorated on graphene sheets by a facile reaction between Zn2þ and OH ions in the aqueous solution. The schematic illustration of the formation mechanism of GZ composites is presented in Fig. 1. When dissolving zinc acetate powder into GO solution, Zn2þ ions will be adsorbed onto the surfaces of GO sheets due to their bonding with the O atoms of the negatively charged oxygencontaining functional groups via electrostatic force.1,22 Once the hydrazine hydrate solution is added to the above suspen2 may bond with the functional sion, ZnðOHÞ2 4 and ZnO groups such as carboxyl and hydroxyl groups (introduced on graphite oxide during the process of oxidation) of GO sheets by intermolecular hydrogen bonds.23,24 At the same time, the GO will be reduced to graphene. ZnO anchored on the graphene sheets was obtained after the annealing treatment. The obtained products were characterized by several techniques to ensure the formation of GZ composite. The XRD patterns of graphene, pristine ZnO, and GZ composites are shown in Fig. 2. All the diffraction peaks in the XRD pattern of ZnO nanoparticles can be indexed to


173504-3

Gayathri et al.

J. Appl. Phys. 115, 173504 (2014)

FIG. 1. Schematic illustration of the formation mechanism of GZ composites.

hexagonal wurtzite ZnO and the planes were in good agreement with the JCPDS Card 36-1451. In the XRD pattern of graphene, two broad bands at 26 and 42 are assigned to (002) and (100) planes of graphene, respectively. It is clearly visible from the XRD pattern of GZ composites that as the concentration of ZnO was increased, the intensity of the

FIG. 2. XRD pattern of graphene, pristine ZnO, and GZ composites.

peaks corresponding to ZnO phase increased as well.25 The plane (002) which corresponds to graphene is very weak in the XRD pattern of GZ-1.18 The (002) plane gives the most dominant peak of graphite as seen in the XRD pattern of graphite shown in the Fig. S1.26 A weak broad band of graphene (002) reflection peak at 26 appeared on the XRD pattern of GZ-2, which indicates that the ZnO nanoparticles are anchored on the surface of graphene. The absence of graphene peak in GZ-3 was due to the higher concentration of the ZnO precursor and therefore, the graphene surface may be covered by ZnO. Thus, XRD results confirmed the formation of ZnO on graphene surface. The absence of other peaks except ZnO and graphene revealed the quality of the prepared samples. Raman spectroscopy is one of the sensitive tools to characterize carbon based nanostructures.27 Raman spectra of ZnO, GO, and GZ composites are presented in Fig. 3. The bare ZnO nanoparticles showed the standard Raman modes and they were observed at 331 (second-order vibration related to the E2(high)-E2(low)), 438 (E2(high)), and 581 cm1 (E1(LO)).8 According to the selection rules for Raman scattering, in the backscattering geometry, ZnO compounds with the hexagonal structure should exhibit phonons of the symmetry E2(high) at 438 cm1.28 Therefore, the E2(high) mode observed in the Raman spectra of the prepared ZnO [Fig. 3(a)] confirmed the formation of hexagonal wurtzite ZnO and this is in good agreement with the XRD result. The Raman spectrum of GO shown in Fig. 3(b) has two broad peaks at 1335 (D band) and 1586 cm1 (G band) corresponding to the breathing mode of k-point photons of A1g symmetry and the first-order scattering of the E2g phonon of sp2 carbon atoms, respectively.29 Although the structural defects are greater in the graphene lattice (D/G ratio of GO), these defects act as helpful nucleation sites for ZnO. This results in less aggregated ZnO particles due to


173504-4

Gayathri et al.

J. Appl. Phys. 115, 173504 (2014)

FIG. 3. Micro-Raman spectra of (a) ZnO, (b) GO, (c) GZ-1 (d) GZ-2, and (e) GZ-3 composites.

FIG. 4. EDX spectra of ZnO and GZ composites.


173504-5

Gayathri et al.

their strong interaction with defect sites of graphene sheets.18 Figures 3(c)–3(e) show the Raman spectra of GZ-1, GZ-2, and GZ-3 composites, respectively. The Raman spectra of GZ composites displayed the modes of ZnO, defect peak (D-band), and high intense G band. As the concentration of ZnO was increased, the intensity of E2 (high) mode increased as well. The D and G bands of GZ-1 resemble the profile of multi-layer graphene. Although the graphene plane was not observed in the XRD pattern of GZ-3, the micro-Raman spectrum showed the Raman modes of graphene. Hence, the formation of wurtzite hexagonal ZnO and GZ composites was confirmed by the micro-Raman analysis. The synthesized samples were subjected to EDX analysis in order to confirm the chemical composition of the product. The EDX spectra of pristine ZnO and GZ composites are shown in Fig. 4. It is very clear from Fig. 4 that our synthesized samples are free from elemental impurities and it consists of Zn, O, and C. Hence, pure GZ composites can be prepared by simple chemical method. The FE-SEM images of ZnO and GZ composites are presented in Fig. 5. The uniformly distributed rod-like ZnO nanoparticles are clearly visible in the SEM image and it is shown in Figs. 5(a) and 5(b). The SEM images of GZ composites at different concentrations show nearly similar morphological features. By increasing the ZnO concentration, the distribution of ZnO on graphene also increased. From

FIG. 5. SEM images of (a) and (b) ZnO, (c) and (d) GZ-1, (e) and (f) GZ-2, and (g) and (h) GZ-3 (scale bar: 4 lm for (a), (c), (e), (g) and 2 lm for (b), (d), (f), (h)).

J. Appl. Phys. 115, 173504 (2014)

Figs. 5(e)–5(h), it is clear that the ZnO nanoparticles are agglomerated and no individual particles are visible in the SEM image. The graphene surface was marked on the SEM images of GZ composites. At the highest ZnO concentration (GZ-3), the ZnO nanoparticles stick on the surface of graphene sheets and it can be clearly seen from Figs. 5(g) and 5(h). A similar morphology was observed for the sample GZ-2 and its corresponding SEM image is shown in Figs. 5(e) and 5(f). But a different situation was observed for the GZ-1 sample. In contrast to GZ-2 and GZ-3, at the lower concentration of ZnO (GZ-1), the graphene sheets clasped together and appeared like multi-layer graphene and ZnO was not clearly visible in the SEM image (Figs. 5(c) and 5(d)). However, the presence of ZnO in GZ-1 can be clearly seen from the XRD, micro-Raman, and EDX results. In the case of GZ-2 and GZ-3, the anchoring of ZnO on both sides of graphene sheets weakened the van der Waals force between the graphene layers and so the graphene sheets did not combine together during the process of annealing. Conversely, in the case of GZ-1, the graphene sheets stacked together due to annealing treatment of high temperature (450 C) and showed nearly graphite like morphology. These results suggest that GZ composites can be prepared with tunable ZnO density. In order to analyse the homogeneity of the synthesized samples, Raman mapping measurements were carried out. Rectangular mapping for the area of (95 90) lm2 was made for all the samples. The automated XY stage coupled with the micro-Raman spectrometer moved (20 20) steps covering the selected area and as a result, 400 Raman spectra were collected and their corresponding Raman intensity map was generated. The sequential collection of more number of spectra in the selected region of the synthesized samples would provide the homogeneity of our samples. Figure 6 shows the Raman intensity maps of GZ composites obtained when plotting the peak intensity of E2(high) band at 438 cm1 as a function of the spatial location. In the Raman map of GZ-1, the ZnO concentration is very low as expected. The bright yellow (high intense) colour in the map represents the ZnO and the dark yellow colour in the map is attributed to the graphene. It is clear from the map that very few ZnO particles are anchored on the surface of graphene and the Raman profile of the dark area resembled the profile of multi-layer graphene which may be due to annealing treatment. In the case of GZ-2, the concentration of ZnO seemed to be greater than graphene as the brighter domain is larger than the darker domain. Hence, the Raman map of GZ-2 suggests that the groups of agglomerated ZnO nanoparticles are anchored on the graphene sheets. From the Raman intensity map of GZ-3, it is clear that the ZnO particles are uniformly anchored on the graphene surface. The Raman maps of GZ-2 and GZ-3 displayed quite similar results. On the other hand, a completely different result was obtained for the sample GZ-1 as the concentration of the ZnO precursor is relatively lower than the other samples. The homogeneity of the synthesized GZ composites and the distribution of ZnO on graphene surface were successfully analysed by Raman mapping technique. The UV-Visible DRS absorption spectra of the prepared GZ composites and ZnO are shown in Fig. 7(a). The


173504-6

Gayathri et al.

J. Appl. Phys. 115, 173504 (2014)

FIG. 7. (a) UV-Visible DRS and (b) PL spectra of GZ composites and bare ZnO.

FIG. 6. Raman intensity maps of (a) GZ-1, (b) GZ-2, and (c) GZ-3.

absorption edge observed at 390 nm indicates the existence of highly crystalline ZnO nanostructure.30 It is clear from the spectra that the absorbance of the GZ composites has increased in visible light region and showed red shift of absorption edge when compared to bare ZnO. The maximum absorption of visible light is observed for GZ-1 composite which can be visibly noticed from the absorption spectra. However, with the increase of ZnO content, the absorbance in the visible light gradually decreased. The observed results are in well agreement with the previous reports.12,31 This increased visible light absorbance of GZ composites may be due to the increase of surface electric charge of the oxides in the GZ composites and the alteration of the process of electron–hole pair formation during irradiation.11 The obtained results reveal the importance of graphene introduction on the optical properties of ZnO and suggest that the incorporation of graphene on metal oxides improves the absorption of visible-light. The bandgap of ZnO and GZ composites was estimated from the absorption edge of their respective absorption spectra. The bandgap of ZnO, GZ-3, GZ-2, and

GZ-1 was estimated as 3.17, 2.90, 2.78, and 2.67 eV, respectively. The observed red shift in the absorption edge of GZ composites compared to bare ZnO indicates that the synthesized GZ composites can be used as photocatalysts and the composites could absorb more photons (visible light) than bare ZnO. As a result, the red shift observed in the absorption spectrum might enhance the photocatalytic efficiency.5 The room temperature PL spectra of GZ composites and bare ZnO are shown in Fig. 7(b). The PL spectrum of pristine ZnO clearly shows a broad yellow-green emission band in the region of 420–600 nm with a peak centred at 490 nm, which is generally due to oxygen vacancies in ZnO lattice.32 Luminescence quenching of the emission of ZnO is observed when it is anchored on the graphene surface. Also, the PL spectra of GZ composites were blue shifted from that of pristine ZnO. As ZnO is a good electron donor and graphene is a good electron acceptor, the PL quenching is ascribed to the electron transfer from the conduction band of ZnO to graphene.33,34 As a result, the effective electron transfer may reduce the recombination of photogenerated electron–hole pairs35 and therefore the synthesized GZ composites are expected to show better photocatalytic activity than bare ZnO. The photocatalytic activity of the prepared photocatalysts, i.e., GZ composites and bare ZnO was tested by using


173504-7

Gayathri et al.

J. Appl. Phys. 115, 173504 (2014)

FIG. 8. The time dependent UV-Visible absorption spectra of MB dye solution in the presence of (a) ZnO, (b) GZ-3, (c) GZ-2, and (d) GZ-1.

MB in aqueous solution as contaminant. MB is an intensely absorbing organic dye in the visible region at 660 nm. The time dependent UV-Visible absorption spectra of MB dye solution in the presence of photocatalysts are shown in Fig. 8. The absorbance of MB dye decreased with increasing irradiation time. After the irradiation time of 240 min, the MB dye was completely degraded in the presence of ZnO. With the photocatalysts GZ-1, GZ-2, and GZ-3, the dye was degraded at the irradiation times of 140, 100, and 160 min, respectively. The photocatalytic degradation of MB in the presence of ZnO and GZ composites under light irradiation is shown in Fig. 9. The synthesized GZ composites exhibited better photocatalytic performance than bare ZnO. GZ-2 showed excellent enhancement, i.e., 2.4 times higher than that of the bare ZnO. From the PL spectra of the synthesized samples, we expected degradation efficiency of the samples in the order of GZ-1 > GZ-2 > GZ-3 > ZnO. The observed PL quenching is attributed to the effective electron transfer from the conduction band of ZnO to graphene which may reduce the recombination of photogenerated electron–hole pairs33 and therefore the synthesized GZ composites are expected to show better photocatalytic activity than bare ZnO. But, the degradation efficiency of the dye using various photocatalysts was in the order of GZ-2 > GZ-1 > GZ-3 > ZnO. As expected, the

sample GZ-2 showed better photocatalytic performance than ZnO and GZ-3. Even though GZ-1 displayed good performance than ZnO and GZ-3, the performance was not as expected. This result might be attributed to the following

FIG. 9. The photocatalytic degradation of MB in the presence of ZnO and GZ composites under light irradiation.


173504-8

Gayathri et al.

J. Appl. Phys. 115, 173504 (2014)

had promoted the absorption in the visible region. Quenching of luminescence was observed for the GZ composites compared with bare ZnO. Finally, the photocatalytic activity of the synthesized samples was evaluated by using MB dye as contaminant. GZ composites showed markedly enhanced photocatalytic performance than ZnO as expected, and the concentration dependent study of ZnO on graphene surface was made to understand photocatalytic efficiency. ACKNOWLEDGMENTS

The authors acknowledge UGC-UPE for providing micro-Raman facility and also DST-FIST, for the XRD instrumentation facility. The authors are grateful to DSTSERB for the financial support and providing the Raman mapping facility. 1

FIG. 10. Schematic representation of photocatalytic mechanism.

reason. As the concentration of ZnO is very low in GZ-1 (one third of graphene), the graphene sheets clasped together and appeared like multi-layer graphene because of the heat treatment and so the sample appeared like ZnO on multi-layer graphene (SEM image). So, the two dimensional planar structure might have been lost, consequently the surface area decreases. Hence, we suggest that for chemically derived ZnO/graphene composite, GZ-2 would be the optimum concentration for photocatalysis application. The results are comparable with the literatures.1,18–20 According to the results shown above, mechanism of MB degradation is proposed and shown in Fig. 10. This remarkable enhancement of photocatalytic activity of GZ composites could be attributed to the strong interaction between ZnO and defect sites of graphene.18 After the irradiation of light, the electrons from the valence band of ZnO may be excited to its conduction band and consequently to graphene. The molecules of MB can be transferred to the surface of the GZ composites (i.e., adsorption of dye) by means of p-p conjugation between dye and aromatic regions of graphene.11 The increase in number of holes initiates an oxidative pathway and therefore the adsorbed dye can be oxidized. As a result, the photoactive radicals generated during the reaction produce CO2, H2O, and other intermediates and thereby leading to the degradation of MB. These results revealed that the contaminants could be removed successfully by the photocatalysts, which were synthesized by a simple, inexpensive chemical method.

IV. CONCLUSION

ZnO decorated on graphene sheets was successfully synthesized by a facile chemical route. XRD and micro-Raman results confirmed the formation of hexagonal wurtzite ZnO and GZ composites. The tube-like morphology of ZnO anchored on the surface of graphene sheets was revealed by the SEM analysis. The homogeneity of the GZ composites was successfully analyzed by Raman mapping technique. The DRS showed that the formation of ZnO on graphene surface

B. Li, T. Liu, Y. Wang, and Z. Wang, “ZnO/graphene-oxide nanocomposite with remarkably enhanced visible-light-driven photocatalytic performance,” J. Colloid Interface Sci. 377, 114–121 (2012). 2 D. Beydoun, R. Amal, G. Low, and S. McEvoy, “Role of nanoparticles in photocatalysis,” J. Nanopart. Res. 1, 439–458 (1999). 3 S. O. Fatin, H. N. Lim, W. T. Tan, and N. M. Huang, “Comparison of photocatalytic activity and cyclic voltammetry of zinc oxide and titanium dioxide nanoparticles toward degradation of methylene blue,” Int. J. Electrochem. Sci. 7, 9074–9084 (2012). 4 J. Ru, Z. Huayue, L. Xiaodong, and X. Ling, “Visible light photocatalytic decolourization of C. I. Acid Red 66 by chitosan capped CdS composite nanoparticles,” Chem. Eng. J. 152, 537–542 (2009). 5 C. Wanga, J. Yan, X. Wua, Y. Song, G. Cai, H. Xu, J. Zhu, and H. Li, “Synthesis and characterization of AgBr/AgNbO3 composite with enhanced visible-light photocatalytic activity,” Appl. Surf. Sci. 273, 159–166 (2013). 6 J. Yu and X. Yu, “Hydrothermal synthesis and photocatalytic activity of zinc oxide hollow spheres,” Environ. Sci. Technol. 42, 4902–4907 (2008). 7 T. S. Velayutham, W. H. A. Majid, W. C. Gan, A. K. Zak, and S. N. Gan, “Theoretical and experimental approach on dielectric properties of ZnO nanoparticles and polyurethane/ZnO nanocomposites,” J. Appl. Phys. 112, 054106 (2012). 8 K. J. Chen, T. H. Fang, F. Y. Hung, L. W. Ji, S. J. Chang, S. J. Young, and Y. J. Hsiao, “The crystallization and physical properties of Al-doped ZnO nanoparticles,” Appl. Surf. Sci. 254, 5791–5795 (2008). 9 Z. H. Wang, D. Y. Geng, Z. Han, and Z. D. Zhang, “Characterization and optical properties of ZnO nanoparticles obtained by oxidation of Zn nanoparticles,” Mater. Lett. 63, 2533–2535 (2009). 10 D. Chu, Y. Masuda, T. Ohji, and K. Kato, “Formation and photocatalytic application of ZnO nanotubes using aqueous solution,” Langmuir 26, 2811–2815 (2010). 11 Z. Chen, N. Zhang, and Y. J. Xu, “Synthesis of graphene–ZnO nanorod nanocomposites with improved photoactivity and anti-photocorrosion,” CrystEngComm 15, 3022–3030 (2013). 12 T. Xu, L. Zhang, H. Cheng, and Y. Zhu, “Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study,” Appl. Catal., B 101, 382–387 (2011). 13 A. K. Geim and K. S. Novoselov, “The rise of graphene,” Nature Mater. 6, 183–191 (2007). 14 T. N. Lambert, C. A. Chavez, B. H. Sanchez, P. Lu, N. S. Bell, A. Ambrosini, T. Friedman, T. J. Boyle, D. R. Wheeler, and D. L. Huber, “Synthesis and characterization of titania-graphene nanocomposites,” J. Phys. Chem. C 113, 19812–19823 (2009). 15 G. Williams, B. Seger, and P. V. Kamat, “TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide,” ACS Nano 2, 1487–1491 (2008). 16 H. Zhang, X. Lv, Y. Li, Y. Wang, and J. Li, “P25-graphene composite as a high performance photocatalyst,” ACS Nano 4, 380–386 (2010). 17 B. Li and H. Cao, “ZnO@graphene composite with enhanced performance for the removal of dye from water,” J. Mater. Chem. 21, 3346–3349 (2011).


173504-9 18

Gayathri et al.

N. P. Herring, S. H. Almahoudi, C. R. Olson, and M. S. El-Shall, “Enhanced photocatalytic activity of ZnO–graphene nanocomposites prepared by microwave synthesis,” J. Nanopart. Res. 14, 1277 (2012). 19 M. Ahmad, E. Ahmed, Z. L. Honga, J. F. Xu, N. R. Khalid, A. Elhissi, and W. Ahmed, “A facile one-step approach to synthesizing ZnO/graphene composites for enhanced degradation of methylene blue under visible light,” Appl. Surf. Sci. 274, 273–281 (2013). 20 T. Lv, L. Pan, X. Liu, and Z. Sun, “Enhanced photocatalytic degradation of methylene blue by ZnO–reduced graphene oxide–carbon nanotube composites synthesized via microwave-assisted reaction,” Catal. Sci. Technol. 2, 2297–2301 (2012). 21 W. S. Hummers and R. E. Offeman, “Preparation of graphitic oxide,” J. Am. Chem. Soc. 80, 1339 (1958). 22 X. Chen, Y. He, Q. Zhang, L. Li, D. Hu, and T. Yin, “Fabrication of sandwich-structured ZnO/reduced graphite oxide composite and its photocatalytic properties,” J. Mater. Sci. 45, 953–960 (2010). 23 J. Wang, Z. Gao, Z. Li, B. Wang, Y. Yan, Q. Liu, T. Mann, M. Zhanga, and Z. Jiang, “Green synthesis of graphene nanosheets/ZnO composites and electrochemical properties,” J. Solid State Chem. 184, 1421–1427 (2011). 24 Y. L. Chen, Z. A. Hu, Y. Q. Chang, H. W. Wang, Z. Y. Zhang, Y. Y. Yang, and H. Y. Wu, “Zinc oxide/reduced graphene oxide composites and electrochemical capacitance enhanced by homogeneous incorporation of reduced graphene oxide sheets in zinc oxide matrix,” J. Phys. Chem. C 115, 2563–2571 (2011). 25 A. R. Marlinda, N. M. Huang, M. R. Muhamad, M. N. Anamt, B. Y. S. Chang, N. Yusoff, I. Harrison, H. N. Lim, C. H. Chia, and S. V. Kumar, “Highly efficient preparation of ZnO nanorods decorated reduced graphene oxide nanocomposites,” Mater. Lett. 80, 9–12 (2012). 26 See supplementary material http://dx.doi.org/10.1063/1.4874877 for XRD spectrum of graphite.

J. Appl. Phys. 115, 173504 (2014) 27

A. C. Ferrari, “Raman spectroscopy of graphene and graphite: Disorder, electron–phonon coupling, doping and nonadiabatic effects,” Solid State Commun. 143, 47–57 (2007). 28 A. G. Milekhin, N. A. Yeryukov, L. L. Sveshnikova, T. A. Duda, E. I. Zenkevich, S. S. Kosolobov, A. V. Latyshev, C. Himcinski, N. V. Surovtsev, S. V. Adichtchev, Z. C. Feng, C. C. Wu, D. S. Wuu, and D. R. T. Zahn, “Surface enhanced Raman scattering of light by ZnO nanostructures,” J. Exp. Theor. Phys. 113, 983–991 (2011). 29 Q. Zheng, G. Fang, F. Cheng, H. Lei, W. Wang, P. Qin, and H. Zhou, “Hybrid graphene–ZnO nanocomposites as electron acceptor in polymerbased bulk-heterojunction organic photovoltaics,” J. Phys. D: Appl. Phys. 45, 455103 (2012). 30 X. Liu, L. Pan, T. Lv, T. Lu, G. Zhu, Z. Sun, and C. Sun, “Microwaveassisted synthesis of ZnO–graphene composite for photocatalytic reduction of Cr(VI),” Catal. Sci. Technol. 1, 1189–1193 (2011). 31 X. Bai, L. Wang, R. Zong, Y. Lv, Y. Sun, and Y. Zhu, “Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization,” Langmuir 29, 30973105 (2013). 32 M. Koyano, P. QuocBao, L. T. ThanhBinh, L. H. Ha, N. N. Long, and S. Katayama, “Photoluminescence and Raman spectra of ZnO thin films by charged liquid cluster beam technique,” Phys. Status Solidi A 193, 125–131 (2002). 33 Y. Yang and T. Liu, “Fabrication and characterization of graphene oxide/ zinc oxide nanorods hybrid,” Appl. Surf. Sci. 257, 8950–8954 (2011). 34 D. I. Son, B. W. Kwon, J. D. Yang, D. H. Park, W. S. Seo, H. Lee, Y. Yi, C L. Lee, and W. K. Choi, “Charge separation and ultraviolet photovoltaic conversion of ZnO quantum dots conjugated with graphene nanoshells,” Nano Res. 5, 747–761 (2012). 35 M. Dutta, S. Sarkar, T. Ghosh, and D. Basak, “ZnO/graphene quantum dot solid-state solar cell,” J. Phys. Chem. C 116, 2012720131 (2012).
