Highly efficient preparation of ZnO nanorods decorated reduced graphene oxide nanocomposites

Highly efficient preparation of ZnO nanorods decorated reduced graphene oxide nanocomposites

Materials Letters 80 (2012) 9–12 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matl...

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Materials Letters 80 (2012) 9–12

Contents lists available at SciVerse ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Highly efficient preparation of ZnO nanorods decorated reduced graphene oxide nanocomposites A.R. Marlinda a, N.M. Huang a,⁎, M.R. Muhamad b, M.N. An'amt c, B.Y.S. Chang a, N. Yusoff a, I. Harrison d, H.N. Lim e, C.H. Chia f, S. Vijay Kumar a a

Low Dimensional Materials Research Centre, Physics Department, University of Malaya, 50603 Kuala Lumpur, Malaysia Chancellery Office, Multimedia University, Jalan Multimedia, 63100 Cyberjaya, Selangor, Malaysia Faculty of Agro Industry & Natural Resources, Universiti Malaysia Kelantan, Karung Berkunci 36, Pengkalan Chepa, 16100 Kota Bharu, Kelantan, Malaysia d School of Chemical and Environmental Engineering, Faculty of Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, Semenyih, Selangor, Malaysia e Chemistry Department, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia f School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia b c

a r t i c l e

i n f o

Article history: Received 8 March 2012 Accepted 13 April 2012 Available online 20 April 2012 Keywords: Graphene oxide Zinc oxide Nanocomposites Carbon materials

a b s t r a c t A one-step method for the synthesis of zinc oxide/reduced graphene oxide (ZnO/rGO) nanocomposites by a hydrothermal technique is reported. This simple method involves a hydrothermal treatment of a solution comprising graphene oxide (GO), Zn(CH3COO)2.2H2O, NaOH and NH3.H2O. The concentration of GO as a starting material plays an important role in the density distribution of ZnO nanorods on the rGO sheets and on the percentage of the formation of ZnO/rGO nanocomposites. The resulting rod-like ZnO nanoparticles formed on the rGO sheets, in high density, has a potential in the gas sensing application. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Graphene, a single two-dimensional (2D) carbon sheet with a hexagonal lattice structure, has unique electrical, thermal and mechanical properties. These properties can be enhanced by the addition of nanoparticles of inorganic compounds to form a nanocomposite [1–3]. Zinc oxide, with band gap energy of 3.37 eV, is a well-known semiconductor and has a significant number of applications. Zinc oxide nanoparticles have been used in the fabrication of solar cells [4], gas sensors [5], transparent conductors [6],catalysis [7] and coatings [8]. Graphene-based metal oxide nanocomposites consist of the highly conductive carbon film serving as an anchor for the metal oxide nanocrystals and are emerging as a class of new exciting materials [9]. There are several reports on the syntheses of graphene-metal oxide nano-composites using techniques such as ultrasonic spray pyrolysis [10], hydrothermal [11], solvothermal [12] and microwave-assisted reduction [13]. However, there has been no report on the preparation of ZnO nanorod, in high density, on graphene sheets. In the present work, the preparation of ZnO/rGO nanocomposites via a simple hydrothermal method is reported. Moreover, the effect of

⁎ Corresponding author. Tel.: + 60 3 8921 3257; fax: + 60 3 58911088. E-mail address: [email protected] (N.M. Huang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.04.061

varying the concentration of graphene oxide in the starting materials on the nanocomposite has also been investigated. 2. Experimental 2.1. Preparation of ZnO/rGO nanocomposite The starting materials used in the synthesis of the nanocomposites were 20 ml of graphene oxide (GO) solution prepared using the simplified Hummers method [14,15], and 2 ml of 0.1 M of Zn(OH)2 prepared from 2.75 g of (Zn(CH3COO)2.2H2O) and 1 g NaOH dissolved in 25 ml of 25% NH3.H2O. The mixture was gradually added with 0.1 M of NaOH solution to increase the pH to ~10 while stirring the solution in an oil bath at 60 ± 2 °C until a cloudy dark brown liquid was obtained. Subsequently, the solution was subjected to hydrothermal treatment for 24 h at 180 °C. The product was obtained by washing with de-ionized water and ethanol, and then dried at 60 °C. The amount of GO in the solution was varied so that the effects of the GO concentration on the nanocomposites could be found. 2.2. Characterizations The surface morphology, structure and composition of the nanocomposites were characterized by a field emission scanning electron microscope, FESEM ( FEI Nova NanoSEM 400 operate at 10.0 kV), X-ray diffractometer, XRD (Philips D5000) using Cu Kα


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radiation (λ = 1.4506 Å) and thermal gravity analyzer, TGA (TA instruments Q500) over the temperature range of 20–1000 °C using a scan rate of 20 °C/min. Optical absorption properties, over the spectral region of 190–900 nm, were assessed using a UV-Visible spectrophotometer (Thermoscientific Evolution 300).

3. Results and discussion In Fig. 1A, the XRD spectrum of the GO is similar to the GO reported by other methods [16]. It does not contain the sharp graphite (002) peak at 26.7° but has a new lower intensity peak centered at 11.1°, which corresponds to a d-spacing of 8.29 Å, which is attributed to the (001) reflection of GO. This result suggests that the graphite was fully oxidized [17] and the distance between the carbon sheets has increased due to the insertion of the inter-planar groups. Consequently, the GO sheets are thicker than the graphite sheets because of the presence of covalently bound oxygen atoms and the displacement of the sp 3hybridized carbon atoms above and below the original graphene sheets [16]. The XRD spectra of pure ZnO and ZnO/rGO composites (Fig. 1B) are dominated by the hexagonal ZnO reflection peaks (JCPDS file no. 01-089-0510). No other reflection peaks, including the (001) GO reflections, are observed indicating the formation of high purity ZnO

Fig. 1. (A) XRD patterns of graphite flake (i) and GO (ii) and (B) XRD pattern of pure ZnO (i), ZnO/rGO nanocomposites with 0.2 mg/ml GO (ii), 0.6 mg/ml GO (iii) and 1.0 mg/ml GO (iv).

and the reduction, by hydrothermal process, of the GO to rGO. As the concentration of GO increases, the intensity of the reflection peaks increased. The absence of the graphite (002) reflection peak at 26.7° indicates that the surface of the rGO was fully covered with ZnO nanoparticles for all samples [18]. In the absence of GO, ZnO appears as rod-like shape with an approximated diameter and length of 90 nm and 3 μm respectively (Fig. 2a). The FESEM images of ZnO/rGO composites at different concentrations of GO show similar morphological features with different levels of density of ZnO deposited on the rGO, depending on the initial GO concentration. By increasing the GO concentration from 0.2 mg/ml to 1.0 mg/ml, the density of ZnO nanorods decreased. At the highest GO concentration of 1.0 mg/ml, the rGO sheet is visibly adhered with ZnO nanorods on both sides of the translucent sheets (Fig. 2d). On the other hand, at the lower concentration of GO solution, the rGO sheet was fully enveloped by ZnO nanorods (Fig. 2b and c). These results indicate the formation of ZnO graphene composites with tunable density. Fig. 3A shows the UV–vis absorption spectra of the as synthesized GO, ZnO and ZnO/rGO nanocomposites. The GO sample has an absorption peak at 227 nm, attributed to the π → π transition of the c c bond, while the ZnO sample shows a strong absorption peak at 368 nm. The absorption spectra of the nanocomposites have two main areas of interest. Firstly, there is now an absorption peak at 266 nm and is attributed to the excitation of a π-plasmon of the graphitic bond [19]. This shift in peak position provides evidence for reduction of surface functional groups on the GO sheets. Secondly, all the ZnO/rGO nanocomposites have an absorption peak at 368 nm caused by the absorption of the ZnO nanoparticles. Comparing the nanocomposites synthesized using different GO concentrations, the relative intensity of the carbon related absorption (at 266 nm) to the ZnO absorption increases with GO concentration indicating, as expected, a relative higher amount of rGO in these samples. ZnO is a very stable structure and the TGA of the fabricated nanoparticles without using GO is expected to show little mass change over 120 °C (Fig. 3Ba). There is however a significant weight loss at 120 °C which is explained by the loss of water absorbed on the surface of the ZnO nanoparticles. For the GO starting material, the TGA curve shows mass loss occurring between 175 and 200 °C. This is caused by the removal of oxygen-containing groups [20]. The TGA of rGO shows weight loss over a similar temperature range but the amount is less than GO sample indicating the rGO is more thermally stable than GO. This observation strengthens the hypothesis that the hydrothermal process has significantly reduced the oxygenic groups on the surface of GO. As for the ZnO/rGO composites, there is a mass loss at > 800 °C regardless of the concentration of GO used. This mass loss is due to the decomposition of the ZnO deposited on the rGO surface. It is well known that ZnO has a lower decomposition temperature when it is heated in the presence of a catalyst such as carbon. In this case, rGO acted as a catalyst to reduce the thermal decomposition temperature of ZnO from 1900 °C to approximately 800 °C, which is similar to the reported value [21,22]. Based on this fact, it is expected that all the nanocomposites would diminish at 800 °C. However, ZnO/rGO prepared at the concentration of 0.2 mg/ml of GO solution, only experienced a total mass loss of 35 wt.%, while the nanocomposites prepared at the concentration of 0.6 mg/ml and 1.0 mg/ml of Go solution had 55 wt.% and 95 wt.% of total mass losses, respectively. The remaining white powder for all the nanocomposites was confirmed to be ZnO by the XRD analysis (results not shown). These results indicate the presence of free ZnO nanorods in the samples of nanocomposites that were not attached with rGO. From the TGA results, we conclude that the ZnO/rGO nanocomposites formed using 1.0 mg/ml of GO solution had the highest amount of ZnO attached to the rGO sheets as the remaining ZnO in the crucible is less than 5 wt. %.

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Fig. 2. FESEM micrographs of pure ZnO (a), and ZnO/rGO composites with different concentrations of GO by 0.2 mg/ml (b), 0.6 mg/ml (c) and 1.0 mg/ml (d).

4. Conclusions A facile and scalable chemical reduction method based on the hydrothermal processing route for the synthesis ZnO/rGO nanocomposites was successfully carried out. The morphology, optical properties and percentage of formation of the ZnO/rGO nanocomposites were greatly influenced by the concentration of GO. Acknowledgements This work was financially supported by the High Impact Research Grant (UM.C/625/1/HIR/030) from the University of Malaya, High Impact Research Grant (UM.C/625/1/HIR/MOHE/05) from the Ministry of Higher Education Malaysia and Postgraduate Research Fund, PPP (PV039-2011A). References

Fig. 3. (A) UV–vis spectra and (B) TG curves of pure ZnO (a), and ZnO/rGO composites with different concentrations of GO at 0.2 mg/ml (b), 0.6 mg/ml (c) and 1.0 mg/ml (d).

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