Preparation and characterization of graphene

Preparation and characterization of graphene supported palladium nanoparticles for Direct Methanol Fuel Cells K.Priya, N.Rajasekar School of Eelctrical Engineering, VIT University, Vellore

C.Santhosh and Andrews Nirmala grace Centre for Nanotechnology Research, VIT University, Vellore

[email protected], [email protected]

[email protected], [email protected]

Abstract— Graphene found to be one of the best alternative to carbon nanotubes due to its superior properties and distinctive nanostructure. Graphene supported palladium electrocatalysts (Pd/G) is prepared using a facile microwave reduction method; which assists reduction of graphene oxide and further leads to formation of palladium nanoparticles. The structural characteristics of the prepared materials are characterized using FE-SEM, EDAX, XRD, FTIR spectroscopy and Raman spectroscopy. Electrocatalytic activity of the prepared electrocatalysts with respect to Methanol Oxidation Reaction (MOR) is analyzed using cyclic voltammetry. In addition, Vulcan carbon supported palladium electrocatlyst (Pd/G) is also prepared via same method for the evaluation of Pd/G. Further, electrocatalytic performance of prepared materials is investigated. Experiments are also carried out at different loading of palladium onto graphene matrix. This prepared graphene supported electrocatalysts having high electrocatalytic can be considered to be a better substitute for fuel cells anode material. Keywords: Palladium electrocatalysts; Graphene support; Vulcan carbon support; cyclic voltammetry.

I.

INTRODUCTION

Renewable energy plays a vital role in meeting energy demands of a nation. Among the resources present, fuel cell research is considered to be more important since it offers various advantages such as high efficiency, cleaner and better energy conversion ratio. Recently, the Direct Methanol Fuel Cells (DMFCs) brought consideration among researchers around the world since it offers several unique advantages such as low pollution, light weight, high power density, easy liquid handling and operating temperature is low [1-3]. There are two main reaction occurs in DMFCs are cathodic oxygen reduction reaction (ORR) and anodic methanol oxidation reaction (MOR) respectively. The best suited catalyst for ORR and MOR of DMFCs is Platinum (Pt) because of its activity and stability. Due to higher cost of Pt catalyst, the manufacturing costs of fuel cells are too high. This found to be the major obstacle and limits its commercialization. Moreover, Pt catalysts can be readily poisoned at low temperatures with CO [4]. Therefore, an alternative to platinum catalyst is need of the hour. Palladium (Pd) could be a best alternative to Platinum because of similar lattice constant and electronic configuration.

It is important to mention that, several factor affects catalyst performance some of them are preparation method, pretreatment of the materials and supporting materials [5]. The supporting material is one of the crucial factors that determines particle size and distribution of the metal nanoparticles located on its surface, and it significantly influences utilization, catalytic performance, and stability [6]. In view of practical considerations, the following features are highly desired; 1. decreased use of platinum, 2. catalysts loading onto the surface of supporting materials for performance enhancement. In the literature, many carbon materials such as Vulcan XC-72 carbon [7], carbon nanofibers [8] and mesoporous carbon [9] have been examined as catalyst supports. One of the popular support for catalysts is Vulcan XC-72 carbon; however, it processes poor corrosion resistance under working conditions of DMFCs [10]. A common strategy adopted to build well dispersed metal nanoparticles on CNTs is by applying various treatment methods by modifying CNTs surface with desired functional groups. However, this may lead to lowering of overall electrocatalytic activity and stability, damage the integrity and intrinsic conductivity of the tubes [7]. Therefore, identifying suitable supporting material for lower loading of Pt having high catalytic performance and increased utilization assumes significance. On the other hand, Graphene with basic structure of all graphitic forms, have conceived larger attention of researchers because of its ability to improve catalytic properties and its unusual electronic properties [11]. Graphene surface are rich in oxygen functional groups [12] which can efficiently disperse any metal nanoparticles and has the ability to remove accumulated carbonaceous species (e.g., CO) thus resulting in higher electrocatalytic activity. Despite its typical electronic properties, higher specific surface area and lower cost, graphene can be a better catalyst support for DMFCs [13]. From the author’s previous experience [14-17], in this paper, an attempt has been made to use graphene and Vulcan XC-72 as supporting materials for preparation of Pd catalysts during oxidation of methanol. For preparation of graphene supported palladium electrocatalyst, facile microwave reduction technique was followed. The morphology of the prepared materials was tested using field emission scanning electron microscopy (FE-SEM), FTIR and Raman analysis, X-ray diffraction (XRD) analysis, energy dispersive X-ray spectroscopy (EDAX), and cyclic voltammetry.

II. EXPERIMENTAL SECTION A. Preparation of Graphene Oxide (GO) Modified Hummers method [18] is followed in this work for preparation of Graphene oxide. The procedure adopted is as follows: 1 gm of graphite powder was mixed with 23 ml of 98% sulphuric acid and this mixture is stirred over a period of 1 day followed by the addition of 100 mg of sodium nitrate. This blend is stirred for about 30 minutes and brought to temperature of less than 5ºC with the help of an ice bath. Further, 3 gms of potassium permanganate (KMnO4) was added to the blend in step wise addition. This combination was later heated to 35-40ºC and stirred continuously for another 30 minutes. Then 46 ml of water was further added to the slurry over 25 minutes interval. At last, 140 ml and 10 ml of water, hydrogen peroxide (30%) were added to the blend respectively. The solution was centrifuged to remove the raw graphite. To extract graphite oxide from the solution, it was dried in a hot air oven. Obtained graphite oxide was added with deionized water (0.5 mg/ml) and subsequently sonicated for about 2 hours to extract exfoliated graphene oxide.

and dried in air. 0.0545 mg/cm2 catalyst loading was made on the electrode surface. The cyclic voltammetry was carried out in nitrogen saturated 0.5M H2SO4 solution, to find out the electrochemical surface area of the (ECSA) of the coated electrocatalysts. III. RESULTS AND DISCUSSION A. Microstructural Analysis The following Fig. 1 illustrates the microstructural characterization of Pd/G electrocatalysts by means of is FESEM and EDAX analysis tools. The FE-SEM images of obtained GO (Fig. 1(a)) and prepared Pd/G electrocatalyst under different magnifications (Fig. 1(b & c)) are depicted. From the images it was noticed that uniformly dispersed Pd nanoparticles can be clearly seen onto the graphene matrix. The Fig. 1(d) depicts the EDAX spectrum of Pd/G electrocatalyst. Furthermore, a broader area obtained to ensure the existence of Pd nanoparticles onto graphene matrix and the composition of Pd element is found to be 40 %.

B. Preparation of palladium electrocatalysts (Pd/G and Pd/VC Simple microwave route reduction is followed in this work for preparation of Palladium electrocatalyst supported on graphene (Pd/G) [19] and its procedure is as follows: 0.05 M PdCl2 concentration with 1 ml aqueous solution is added to 25 ml of Ethylene Glycol (EG). This mixture, after stirring, is slowly mixed with 0.25 ml of 0.4 M KOH solution. 40 mg of GO prepared using previous step is now added to the mixture. Later this mixture is stirred and ultrasonicated for about 10 minutes. At last, the solution is heated in microwave oven for about 50 sec. It is then washed several times using water and dried over night using hot air oven at 100 oC. For synthesizing various loading of Pd/G, concentration of palladium is varied from 0.07M to 1M at same grapheme support loading. For comparative study, palladium supported via Vulcan carbon is prepared following the above procedure with same amount of loading. C. Electrochemical measurements Electrochemical measurements were carried out with the help of CH Instruments Electrochemical Workstation. A three electrode test cell system is used for this purpose. In this test system, platinum disk having surface area of 0.0314 cm2 is taken as working electrode, platinum wire forms the counter electrode and standard calomel (SCE) as a reference electrode. Electrochemical properties of the Pd/G electrode was investigated in 0.5M CH3OH + 0.5M H2SO4 using cyclic voltammetry with voltage range of -0.3V to +1.0V and scan rate of 50 mV/s. It is important to mention here, that all measurements were done at room temperature. Working electrode was prepared by applying following steps: First, surface of palladium working electrode was washed using millipore water and acetone; followed by catalyst solution preparation by sonicating 0.6 mg of catalyst with 10 µl of Iso propyl alcohol (IPA) and water together with 2 µl of Nafion solution (5 wt%, DuPont). Finally 2 µl of 0.6 mg/22 µl catalyst solution has been added to the Pt electrode surface

Fig. 1 Micrographic images of FE-SEM GO (a), Pd/G (b & c) and EDAX of Pd/G (d)

In general the metal precursor is responsible for the metal nanoparticles size. Since the reduction rate of metal nanoparticles is determined by the precursor used. Furthermore, in this reduction process ethylene glycol is used as a precursor. Owing to its high dielectric constant value of 41.4 (at 298 K) and dielectric loss, fast heating can takes place easily [20]. In addition, occurrence rapid heating under homogeneous microwave irradiation speed up the process and offers uniform environment for nucleation and development of nanoparticles and hence, the formation of homogeneous and smaller Pd metal nanoparticles. Hence, obtained nanoparticles results with larger surface area and further it shows better electrocatalytic activity with respect to MOR. B. FTIR Spectroscopic analysis FTIR spectroscopy is one of the characterization tool to estimate the reduction of GO. Fig. 2 illustrates the FTIR spectroscopic patterns of graphite, GO and Pd/G. From the

FTIR spectrums it is inferred that, in GO the main groups that containing oxygen is found in the broad range of 3000-3700 per cm and the peak located at 3406 per cm. In addition, this peak is assigned to stretching of O-H group. The absorption peak at 1065 per cm can be allocated to C-O-C group stretching vibration. The -COOH group deformation is assigned at 1387 per cm. The peaks for skeletal vibrations of C-C bonding and stretching vibrations of C=O are assigned to 1618 per cm and 1718 per cm respectively. These peaks indicate that the GO was successfully oxidized from raw graphite [20-21].

symmetrical peak of 2D band if exactly positioned at 2700 per cm which indicates presence of few graphene layers [29]. In Fig. 3(a) existence of peaks at 1436 per cm and 1598 per cm are subsequent bands of D and G of exfoliated GO respectively. In addition to it, 2D band of GO is depicted in Fig. 3(b). The peak positioned at 2693 per cm indicates the presence of multiple layers of graphene. For Pd/G electrocatalyst, the corresponding bands of D and G are positioned at 1393 per cm and 1574 per cm respectively.

Fig. 2 FTIR patterns of graphite, GO and Pd/G

Fig. 3 Raman spectrum of GO (a & b) and Pd/G (c & d).

All the above peaks are observed in microwave reduced Pd/G and they are blue shifted. A significant decrease in IR intensity was also observed. Functional groups that contain oxygen such as carboxyl, carbonyl, hydroxyl, alkoxy and epoxy, etc., are reduced effectively and became functionalized with Pd. A new peak at 1245 per cm indicates stretching of C-OH. The skeletal vibrations of graphene can be found at 1603per cm, showing the presence of graphene sheets in the prepared electrocatalysts [22-23].

From Fig. 3(b & d) it can be inferred that, the 2D band shift from 2693 per cm to 2688 per cm confirms the existence of few layers of graphene in Pd/G. In order to estimate the reduction of GO, the intensity (ID/IG ratio) need to be calculated. The value of ID/IG ratio for GO is 0.85 and for Pd/G is 0.76 conforming that GO was successfully reduced [26, 30]. In addition to that above results, the D, G and 2D bands of Pd/G was blue shifted when compared that of GO [18].

C. Raman Spectroscopic analysis Raman spectroscopic analysis is the best non- destructive technique to analyze the vibrational motion of carbon materials. Fig. 3 depicts the Raman spectrum of GO (a & b) and Pd/G (c & d). The Raman spectral range for carbon material lies between 800-2000 per cm [24]. There are three specific regions for graphene 1) G band, which is responsible for E2g phonon that caused ordered sp2 carbon atoms presented out of plane and it is specified between the limits 1500 and 1600 per cm. 2) D band, which is responsible for A1g phonon that are disordered due to defect or impurities present at the edges of carbon and this is specified in the range of 1200-1500 per cm [25-26]. 3) 2D band is associated with number of layers in the graphene. For an ideal monolayer of graphene, the Lorentizian peak is exactly centered at 2640 per cm [28]. It will be altered to the range 2655-2679 per cm if the graphene layers are loosely arranged [27-29]. Besides,

D. XRD data analysis The XRD pattern corresponding to graphite, GO and the prepared catalyst are shown in Figure 4(a) and (b) respectively. Figure 4(a) shows XRD diffraction peaks at 26.4, 42.2, 44.3, 54.5 and 77.3º for raw graphite having hexagonal lattice at (002), (100), (101), (004), (110) planes respectively. In addition, its inter-planar distance is found to be 0.338 nm. Further, XRD diffraction patterns of GO and microwave reduced Pd/G are presented in figure 4(b). From the figure it is evident that in case of GO, graphite diffraction peak intensity has been reduced and the (26.4º) XRD peak that corresponds to (002) was shifted to 10.09º. Further, its interplanar distance in GO is increased to 0.876 nm, whereas it was 0.338 nm in case of graphite. This increase could be attributed due to the presence of oxygen, carboxyl and other functional groups [12]. In GO, a broader area of small elevated diffraction intensity peak at 21.6º is found to be interpreted in terms of short range order in stacked graphene sheets.

double layer charging current is observed. Cathodic current is recorded when electrode potential is below 0.3 V. This happens because of adsorption of hydrogen on palladium surface and even at lower potentials the dissolution of hydrogen occurs in the bulk of palladium [31]. The real surface area of Pd-based catalysts was estimated from CV at 0.5 M H2SO4 with hydrogen absorption region charge integration. To compute the ECSA (electrochemical active surface area) for different catalysts the following equation was used [32]:

Fig. 4 XRD patterns of (a) raw graphite and (b) exfoliated GO, Pd/G and Pd/VC.

This indicates that graphite material is converted to graphene oxide. Cubic (fcc) lattice pattern is observed in case of Pd/G and diffraction patterns that corresponds to palladium oxides are not observed in any case. Other peaks such as 40.01º, 46.54 º and 67.9º have its corresponding (111), (200) and (220) planes of palladium crystalline. In case of Pd/VC, the presence of (111) plane at 40.08° specifies the presence of palladium nanoparticles. E. Electrochemical performance of Pd/G and Pd/C Cyclic voltammogram (CV) experiments were carried out, initially in 0.5 M H2SO4 to estimate the electrochemical activity of Pd/G catalysts. The potential range was varied from -0.3V to +1.0 V at constant 50 mV/sec scan rate. Since electrode surface is sensitive to fuel oxidation, measurement of surface area can be one of the important parameter to analyze the catalystic property of an electrocatalyst. All solutions used for this experiment nitrogen purged. Cyclic voltammogram curves were recorded for 0.5M H2SO4 and are presented in Fig. 5(a).

AEL (m 2 / gPd ) =

QH 0.21 × 10 −3 C × g Pd

Where,AEL indicates the electrochemically obtained Pd electrode surface.QH indicates the amount of exchange of charges through electroadsorption. Further the ECSA is calculated for various catalysts and is given in Table 1. TABLE 1 COMPARISON OF ACTIVE SURFACE AREAS BASED ON ELECTRODE ABSORPTION Catalyst/ Electrode

QHa (mC)

SELb (cm2)

Catalyst Loading (mg/cm2)

AELc (m2/g catalyst)

Bare Pt disk

0.018

0.0857

0.0545

0.157

Pd/VC

0.247

1.176

0.0545

2.16

Pd/G

0.507

2.414

0.0545

4.43

SEL: amount of real surface area. From the table it is evident that the peak value of hydrogen adsorption and desorption occurs with Pd/G catalyst compared to that of the Vulcan supported catalyst which indicates that, Pd/G has relatively larger active surface area. This occurs due to the high distribution rate and smaller size of Pd nanoparticles on graphene matrix. F. Electrochemical Impedance measurements

Fig. 5 CV results of the prepared electrocatalysts in 0.5 M H2SO4 (a) and 0.5 M H2SO4 + 0.5 M CH3OH (b) solutions with 50 mVs-1 scanrate.

Based on potential three regions are identified: 1. the range of voltage from -0.3V to +0.1V refers to hydrogen adsorption or desorption region, 2. double-layer region having range between 0.3V to 0.8 V, 3. Voltage above 0.7V is called surface oxide formation or stripping region at ca. 0.75 oxidation process started at palladium surface. During the cathodic sweep, the region between ca. 0.3 V and 0.5 V,

The evaluation of the performance of prepared Pd/G is analyzed regarding fuel oxidation CV results are obtained and hence for the comparison Pd/VC is considered. The CV was taken (Fig. 5(b)) for both electrocatalyst with 50 mVs-1 scan rate in solution that contains 0.5 M H2SO4 + 0.5 M CH3OH. Initially CV was taken for the plain 0.5 M H2SO4 solution for comparison and subsequently the results obtained in 0.5 M H2SO4 + 0.5 M CH3OH for methanol oxidation. Furthermore, as a result of two finely defined anodic peaks observed which is responsible for methanol oxidation. The occurrence of first peak during forward scan is produced by oxidation newly derived absorbed groups from methanol molecules. The occurrence of second peak during reverse scan is due to elimination of carbonaceous groups that are not oxidized entirely at forward scan [33]. From the voltammogram analysis, the electrooxidation of Pd/G electrocatalyst initiates at +0.5 V. The value of current increase slowly till it reaches 0.5 V and beyond 0.5V it increases rapidly.

TABLE 2 CYCLIC VOLTAMMOGRAM READINGS FOR PREPARED ELECTROCATALYST

section, which is named as (Pd/G) Pd/G-1, Pd/G-2, and Pd/G3 for convenience.

Catalytic activity analysis Catalysts/Electrode IP (mA)

EP (V)

EOP (V)

Pd/G

0.1418

0.579

0.29

Pd/VC

0.0394

0.584

0.36

Bare Pt disk

0.0214

0.588

0.46

Consequently, the methanol oxidation current covers limit between 0.5 V and 0.75 V. During the oxidation process alcohol experiences a dissociative adsorption and further carboxyl intermediates for instance (CHnO)ads [n = 1=3] and steadily adsorbed CO intermediate groups [33-34]. In addition, these intermediates further need to be oxidized with groups containing oxygen (such as H2O and OHads in liquid systems) to obtain CO2 intermediates and hence, becomes adsorbate. Therefore, an electrocatalyst must be able to adsorb these above mentioned species during electroxidation process of alcohol. Moreover, this prepared Pd/G becomes a better electrocatalyst by showing bifuntional character, thus generating a higher electrocatalytic current towards methanol oxidation. The following formula used for calculating catalytic efficiency (γ) of prepared electrocatalysts (Pd/G and Pd/VC) on unchanged Pt electrodes :

γ =

Fig. 6 Chronoamperometry results of the Pd/G and Pd/VC electrocatalysts at 0.6 V.

100 * [ I p ( Pd / G or Pd / VC ) − I p ( BarePtdisk )] I p ( BarePtdisk )

Where, IP refers to oxidation peak current values of prepared catalyst.Calculated catalytic efficiency for Pd/G and Pd/VC are 562 and 84.11 respectively. H. Chronoamperometry study on prepared electrocatalysts The chrornoamperometry was run to analyze the long term performance of the prepared electrocatalysts mainly for Pd/G and Pd/VC. Fig. 6 depicts the chronoamperometric results at constant potential of 0.6 V against SCE for the electrocatalysts Pd/G and Pd/VC in the solution of 0.5 M H2SO4 + 0.5 M CH3OH. The rapid decay in current shows the poisoning of electrocatalysts; this happens during methanol oxidation reaction and is because of the formation of intermediate and some poisoning species. In addition, Pd/G electrocatalyst maintains the highest current even after a long time operation of methanol oxidation. The experimental study indicates that Pd/G electrocatalyst found to be stable and poisoning-tolerant electrocatalysts and the current for Pd/G catalyst is three orders in magnitude higher than Pd/VC catalyst. I. Effect of various loading of Palladium To investigate the effect of various loading Pd on the graphene matrix experiments are further carried out and results were plotted in terms of oxidation current (Fig. 7). For this, various Pd/G catalysts were synthesized at different molar concentrations (0.22:1, 0.31:1 and 4.45:1) of Pd with the same graphene loading as explained in the experimental

Fig. 7 Cyclic voltammetry results obtained for various concentrations at 50 mVs-1scan rate.

From Fig. 7(a), it was observed that, with an increase in Pd concentration, there is a considerable increase in peak current (IP). But for abundant higher concentration of Pd, the current peak went down; because abundant of Pd content blocked the surface of the graphene. Pd/G-2 showed highest activity among all the other catalysts. Fig. 7(b) shows chronoamperometry results that were obtained with constant voltage of 0.6 V for different concentrations. From the graph it can be observed that Pd/G-2 has better response compared to other two electrocatalysts. IV. CONCLUSION In this paper, a simple microwave reduction method was followed for the preparation electrocatalysts. In order to analyze the structural characteristics of the prepared materials FE-SEM, EDAX, XRD were taken. In addition to that, methanol oxidation reaction of the prepared electrocatalysts was examined using cyclic voltammetry. FTIR spectroscopy analysis and Raman spectroscopy have been carried out to obtain information about reduction of GO and presence of graphene layers. Besides, Graphene supported palladium showed superior electrocatlytic performance towards methanol

oxidation reaction compared to that of commercial (Vulcan) carbon support. More analysis on the above results proved that graphene supported palladium electrocatalyst possess better ECSA over (Vulcan) carbon supported Pd electrocatalyst. The chronoamperometry results suggest that, graphene supported electrocatalyst showed better stability compared to other supports. Hence, it is concluded that graphene supported electrocatalysts with superior electrochemical performance found to be the promising anode material in fuel cells. REFERENCES [1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11] [12]

[13]

[14]

[15]

S.K. Kamarudin, W.R.W. Daud, S.L. Ho, U.A. Hasran, “Overview on the challenges and developments of micro-direct methanol fuel cells (DMFC)”, J. Power Sources, Vol. 163, pp. 743–754. Jan 2007. N. Wongyao, A. Therdthianwong, S. Therdthianwong, “Performance of direct alcohol fuel cells fed with mixed methanol/ethanol solutions”, Energ. Convers. Manage,Vol. 52, pp. 2676–2681. Jul 2011. S.K. Kamarudin, F. Achmad, W.R.W. Daud, “Overview on the application of direct methanol fuel cell(DMFC) for portable electronic devices”, Int. J. Hydrogen Energ. Vol. 34, pp. 6902 – 6916. Aug 2009. Y. Li, L. Tang, J. Li, ‘Preparation and electrochemical performance for methanol oxidation, of pt/graphene nanocomposites’, Electrochem. Commun, Vol. 11, pp. 846–849. Apr 2009. M.H.M.T Assumpc¸ R.F.B. De Souza, D.C. Rascio, J.C.M. Silva, M.L. Calegaro, I. Gaubeur, T.R.L.C. Paixa, P. Hammer, M.R.V. Lanza, M.C. Santos, ‘A comparative study of the electrogeneration of hydrogen peroxide using Vulcan and Printex carbon supports’, Carbon, Vol. 49, pp. 2842-2851. Jul 2011. S. Basri, S.K. Kamarudin, W.R.W. Daud, Z. Yaakub, ‘Nanocatalyst for direct methanol fuel cell (DMFC)’, Int. J. Hydrogen Energ. Vol. 35, pp. 7957-7970. Aug 2010. N. Alexeyeva, A. Sarapuu, K. Tammeveski, F.J. Vidal-Iglesias, J. SollaGullon, J.M. Feliu, ‘Electroreduction of oxygen on Vulcan carbon supported Pd nanoparticles and Pd–M nanoalloys in acid and alkaline solutions’, Electrochim. Acta Vol. 56, pp. 6702 – 6708. Jul 2011. T. Maiyalagan, K. Scott, ‘Performance of carbon nanofiber supported Pd–Ni catalysts for electro-oxidation of ethanol in alkaline medium’, J. Power Sources Vol. 195 pp. 5246–5251. Aug 2010. S.H. Joo, C. Pak, D.J. You, H.I Seol-Ah Lee, Lee, J.M. Kimb, H. Chang, D. Seung,‘Ordered mesoporous carbons (OMC) as supports of electrocatalysts for direct methanol fuel cells (DMFC): Effect of carbon precursors of OMC on DMFC performances’, Electrochim. Acta Vol. 52, pp. 1618–1626. Dec 2006. J. Moreira, P. Del Angel, A.L. Ocampo, P.J. Sebastian, J.A. Montoya, R.H. Castellanos, ‘Synthesis, characterization and application of a Pd/Vulcan and Pd/C catalyst in a PEM fuel cell’, Int. J. Hydrogen Energ. Vol.29, pp. 915 – 920. Aug 2004. A.K. Geim, K.S. Novoselov, ‘The rise of graphene’, Nat. Mater. Vol. 6, pp.183 – 191.2007. S.M. Choi, M.H. Seo, H.J. Kim, W.B. Kim, ‘Synthesis of surfacefunctionalized graphene nanosheets with high Pt-loadings and their applications to methanol electrooxidation’, Carbon Vol. 49, pp. 904-909. Mar 2011. H. Yang, F. Li, C. Shen, D. Han, Q. Zhang, L. Niu, A. Ivasha, ‘Covalent functionalization of chemically converted graphene sheets via silane and its reinforcement’, J. Mater. Chem. Vol. 19, pp. 4632-4638. May 2009. V. Selvaraj, A. Nirmala Grace, M. Alagar, ‘Electrocatalytic oxidation of formic acid and formaldehyde on nanoparticles decorated single walled carbon nanotubes’, J. Colloid.Interf. Sci. Vol. 333, pp. 254–262. May 2009. A. Nirmala Grace, K. Pandian, ‘Pt, Pt–Pd and Pt–Pd/Ru nanoparticles entrapped polyaniline electrodes – A potent electrocatalyst towards the oxidation of glycerol’, Electrochem. Commun. Vol. 8, pp. 1340–1348. Aug 2006.

[16] A. Muneendra Prasad, C. Santhosh, A. Nirmala Grace,‘Carbon nanotubes and polyaniline supported Pt nanoparticles for methanol oxidation towards DMFC applications’, Appl. Nanosci. Vol.2, pp. 457466, Jan 2012. [17] K.Priya, A. Nirmala Grace, “Graphene supported Pd/G nanoparticles as high performance electrocatalysts for direct methanol fuel cell” Proceedings of International conference on thin flims & Applications, Mar 2012. [18] S.Liu, L. Wang, J. Tian, W. Lu, Y. Zhang, X. Wang, X. Sun, ‘Microwave-assisted rapid synthesis of Pt/graphene nanosheet composites and their application for methanol oxidation’, J. Nanopart. Res. Vol.13, pp. 4731-4737. Jun 2011. [19] Z. Liu, L. Hong, M.P. Tham, T.H. Lim, H. Jiang, ‘Nanostructured Pt/C and Pd/C catalysts for direct formic fuel cells’, J. Power Sources, Short Communications Vol. 161, pp. 831-835. Oct 2006. [20] Y. Zhang, Yong-e Gu, S. Lin, J. Wei, Z. Wang, C. Wang, Y. Du, W. Ye, ‘One-step synthesis of PtPdAu ternary alloy nanoparticles on graphene with superior methanol electrooxidation activity’, Electrochim. Acta. Vol.56, pp. 8746– 8751. Oct 2011. [21] Y.F. Huang, C.W. Lin, ‘Polyaniline-intercalated graphene oxide sheet and its transition to a nanotubes through a self-curling process’, Polymer Vol.53, pp. 1079-1085. Feb 2012. [22] T. Rattana, S. Chaiyakun, N. Witit-anun, N. Nuntawong, P. Chindaudom, S. Oaew, C. Kedkeaw, P. Limsuwan, ‘Preparation and characterization of graphene oxide Nanosheets’, Procedia Engineering Vol. 32, pp. 759 – 764. 2012. [23] Z. Qianqian, B. Tang, H. Guoxin, ‘High photoactive and visible-light responsive graphene/titanate nanotubes photocatalysts: Preparation and characterization, J. Hazard. Mater. Vol. 198, pp. 78– 86. Dec 2011. [24] A.C. Ferrari, ‘Raman spectroscopy of graphene and graphite: Disorder, electron– phonon coupling, doping and nonadiabatic effects’, Solid State Commun. Vol. 143, pp. 47–57. Jul 2007. [25] J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, ‘Reduction of graphene oxide via L-ascorbic acid’,Chem. Commun. Vol. 46, pp. 1112- 1114. Jan 2010. [26] T.V. Cuong, V.H. Pham, Q.T. Tran, S.H. Hahn, J.S. Chung, E.W. Shin, E.J. Kim, ‘Photoluminescence and Raman studies of graphene thin films prepared by reduction of graphene oxide’, Mater. Lett. Vol. 64, pp. 399401. Feb 2010. [27] O. Akhavan, ‘Graphene Nanomesh By ZnO Nanorod Photocatalysts’, Acs Nano Vol. 4, pp. 4174-4180. Jun 2010. [28] C. Faugeras, A. Nerriere, M. Potemski, A. Mahmood, E. Dujardin, C. Berger,W.A. de Heer, ‘Few-layer graphene on SiC,pyrolitic graphite and graphene: A Raman scattering study’, Phys. Lett. Vol. 92, pp. 011914. Jan 2008 [29] W. Can, Z. Liang, Q. Wen-ming, L. Li-cheng, ‘Preparation of graphene nanosheets through detonation’, New Carbon Mater. Vol. 26, pp. 21–25. Jan 2011. [30] S. Stankovich, D.A. Dikin, R.D. Piner,K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, ‘Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide’, Carbon Vol. 45, pp. 1558-1565. Jun 2007. [31] Y. Wang, Z.M. Sheng, H. Yang, S.P. Jiang, C.M. Li, ‘Electrocatalysis of carbon black or activated carbon nanotubes-supported Pd–Ag towards methanol oxidation in alkaline media’, Int. J. Hydrogen Energ. Vol. 35, pp. 10087–10093. Oct 2010. [32] A.N. Salanov, A.I. Titkov, V.N. Bibin, ‘ Mechanisms of oxygen adsorption and desorption on polycrystalline palladium’, Kinet. Catal Vol.47, pp. 430– 436. Jun 2006. [33] A.R. Siamaki, A.E. Rahman, S. Khder, V. Abdelsayed, M. Samy ElShall, B.F. Gupton, ‘Microwave-assisted synthesis of palladium nanoparticles supported on graphene:A highly active and recyclable catalyst for carbon–carbon cross- coupling reactions’, J. Catal. Vol. 279, pp. 1–11. Apr 2011. [34] X. Liang, T.S. Zhao, J.B. Xu, L.D.Zhu, ‘Mechanism study of the ethanol oxidation reaction on palladium in alkaline media’, Electrochim. Acta Vol.54, pp. 2203–2208. Mar 2009.