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Materials Chemistry and Physics 139 (2013) 450e458

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Catalytic properties of grapheneemetal nanoparticle hybrid prepared usingan aromatic amino acid as the reducing agent

Bimalendu Adhikari, Arindam Banerjee*

Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

h i g h l i g h t s

* Corresponding author. Fax: þ91 332473 2805.E-mail address: bcab@iacs.res.in (A. Banerjee).

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.01.020

g r a p h i c a l a b s t r a c t

< Synthesis of reduced graphene ox-ide (RGO) nanosheet using an aminoacid.

< The amino acid (Dopa) can reducenoble metal salt (Au3þ/Agþ) to metalnanoparticle (MNP).

< Single step and eco-friendly syn-thesis of RGO-MNP nanohybrid us-ing Dopa.

< Characterization of RGO, MNP andRGOeMNP nanohybrid.

< RGO-MNP nanohybrid acts as a cat-alyst for the reduction of aromaticnitro.

a r t i c l e i n f o

Article history:Received 18 June 2012Received in revised form3 January 2013Accepted 12 January 2013

Keywords:Chemical synthesisNanostructuresAtomic force microscopy (AFM)Electron microscopy (TEM and SEM)Composite materialsMetals

a b s t r a c t

An easy and single step process of making reduced graphene oxide nanosheet from graphene oxide (GO)in water medium has been demonstrated by using a naturally occurring non-proteinaceous amino acid(2,4-dihydroxy phenyl alanine, Dopa) as a new reducing agent and stabilizing agent. This amino acid hasalso been used to reduce the noble metal salt (AuCl3/AgNO3) to produce the corresponding noble metalnanoparticles (MNP) without using any external reducing and stabilizing agents. So, this amino acid hasbeen used to reduce simultaneously GO to RGO and noble metal salts to produce corresponding MNP toform RGOeMNP nanohybrid system in a single step in water medium and also in absence of any externaltoxic reducing and stabilizing agents. Different techniques UVeVisible absorption spectroscopy, X-raydiffraction, transmission electron microscopy, atomic force microscopy and others have been used tocharacterize the reduction of GO to RGO, metal salts to produce corresponding MNPs and the formationof RGOeMNP nanohybrid systems. Moreover, this metal nanoparticle containing RGOeMNP nanohybridsystem acts as a potential catalyst for the reduction of aromatic nitro to aromatic amino group.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Graphene has become an interesting nanomaterial from the timeof its discovery due to its fascinating physical, chemical andmaterialproperties and promising applications [1,2]. Making of graphene bythe chemical route is the most powerful approach because of its

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several merits including scalability in production and economicallycheap. However, the reduced form of graphene oxide (GO) hasa strong tendency to aggregate through pep stacking interactions.So, a stabilizer of reduced graphene oxide (RGO) is required toincrease its stability and dispersibility inwater/organicmedium. Thesynthesis of graphene by the chemical reduction of graphene oxidewas first showed by Ruoff and co-workers and they used dime-thylhydrazine or hydrazine as a reducing agent and a polymer/sur-factant as a stabilizing agent [3]. Various reducing agents includinghydrazine [3], p-phenylenediamine [4], benzyl amine [5], sodium

Fig. 1. Illustration of the reduction of GO to RGO nanosheet in water by using Dopa as a reducing as well as stabilizing agent.

B. Adhikari, A. Banerjee / Materials Chemistry and Physics 139 (2013) 450e458 451

borohydride [6], HI/AcOH [7], tolleng’s reagent [8], hydroquinone [9],and others [10,11] were reported for the reduction of GO to RGO.However, most of these above mentioned chemicals are poisonous.So, there is a real need for the easy, eco-friendly and scalable prep-aration of graphene. There are only a few methods for eco-friendlysynthesis of reduced graphene oxide from GO [12e22]. In an ear-lier work, Loh and his co-workers demonstrated a green synthesis ofgraphene using hydrothermal dehydration techniques [12]. Guo andcoworkers reported the green chemical reduction of graphene oxideto graphene nanosheets using L-ascorbic acid [13]. Protein (BSA) canact as a reducing agent for eco-friendly synthesis of reduced gra-phene oxide in alkaline medium at higher temperature [16]. To thebest of our knowledge, the chemical synthesis of graphene using anaromatic amino acid has not been reported yet. Here, we reporta simple and environmentally friendly approach to prepare reducedgraphene oxide nanosheet by using Dopa as a reducing agent aswellas stabilizing agent (Fig. 1). There are several advantages in ourmethod: (1) water medium, (2) scalable in production, (3) non-toxicreducing and stabilizing agents and (4) easily attainable reactioncondition.

Construction of graphene based different nanohybrid systemsincluding graphene�metal nanoparticle (MNP) has become a highlyexpanding area in modern research due to various promising appli-cations of thesematerials in catalysis, energyconversion, fuel cells andin otherfields [23,24,25,26,27�49]. Kamat and Seger have extensivelystudied the graphene-metal/metal oxide nanoparticles hybrid [26].El-Shall and coworkers have demonstrated a synthesis of grapheneemetal NPs hybrid using microwave irradiation [27]. To the best of our

Fig. 2. Photographs of aqueous dispersion GO (a) only GO, (b) GO with ammonia and(c) GO with Dopa and ammonia after heating.

knowledge, there is no report for the single step, in situ and eco-friendly synthesis of RGO-MNPs nanohybrid system using a singleamino acid. We report here for the first time simultaneous reductionof GO to RGO and noble metal salts (AuCl3, AgNO3) to correspondingMNPs tomake the RGO-MNP nanohybrid system inwater in presenceof an amino acid Dopa (2,4-dihydroxy phenyl alanine) using singlestep, in situ and eco-friendly method. Au3þ/Agþ ions have been in situreduced by the Dopa to form graphene-metal nanoparticle hybridsystem without the requirement of any external reducing agents. Asa single step, in situmethodhas been used for the preparation of RGO-MNP hybrid by single reducing agent, the nascently formed Au/Agnanoparticles can be homogeneously and uniformly fabricated on thesurface of the RGO nanosheet. Interestingly, this RGO-MNP nano-hybrid can act as a catalyst for the reduction of aromatic nitro to ar-omatic amino compounds with a high catalytic efficiency.

2. Experimental section

2.1. Materials

Graphite was purchased from Alfa Aesar. Dopa was purchasedfrom SigmaeAldrich. Sodium nitrite, potassium permanganate,concentrated sulfuric acid, hydrogen peroxide, sodium hydroxideand ammoniawere purchased from SRL, India. Thewater used in allexperiments was prepared with ultrapurewater from aMilli-Q Plussystem (Millipore).

2.2. Synthesis of graphene oxide

GO was synthesized from the graphite powder with a size<30 mmusing a modified Hummer’s method [14]. Initially, graphitepowder (0.5 g) was dispersed in conc. H2SO4 (20 mL) and thenNaNO3 (0.25 g) was added to it. The mixture was then cooled to0 �C. After that KMnO4 (1.5 g) was slowly added to the mixture, sothat the temperature was below 20 �C during the KMnO4 addition.Then the mixture was transferred to a water bath (35 �C) andstirred for 30 min. As a result, a brownish paste was obtained.The temperature was raised to 90 �C when water (30 mL) wasadded to the mixture and then the temperature was maintained for15 min. Next warmwater (80 mL) was added to the whole mixture.To this mixture, H2O2 (0.5 mL of 30%) was added and it reduced theremaining permanganate. The warm mixture was filtered andwashed thoroughly with warm water. The material was dispersedin distilled water (100 mL) by sonication. Then the mixture wascentrifuged at 3000 rpm for 15 min. The filtrate was resuspendedby using sonication and again centrifuged at 20,000 rpm. Theresuspension (with distilled water) and centrifugation process wasrepeated for several times until the final pH of the aqueous sus-pension of GO was reached to 6. Finally centrifuged semi solid GOwas collected and freeze dried in vacuum.

Fig. 3. UV/Vis absorption spectra of GO and RGO showing the reduction of GO usingDopa.

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2.3. Synthesis of RGO using Dopa

Reduced graphene oxide (RGO) was synthesized through thereduction of GO by Dopa in water at an elevated temperature(w80 �C). A mixed aqueous solution containing GO (0.1 mg mL�1),Dopa (0.5 mg mL�1) and alkali [aqueous NaOH (0.3 mg mL�1) or

Fig. 4. (aec) Transmission electron microscopic (TEM) images of the as-prepared RGO nano

NH3 (0.35 mg 30% NH3 per 1 mL final solution)] was sonicated for15 min and then the mixture was heated at w80 �C for 30 min. Alarge amount of blackish RGO aqueous dispersion was easilyobtained.

2.4. Synthesis of MNP using Dopa

For the synthesis of AgNPs using Dopa as a reducing agent, atfirst a basic aqueous solution of Dopa was prepared by mixing1:2M (Dopa:NaOH) Dopa and NaOH inwater. Then, AgNO3 solutionwas added into the basic aqueous solution of Dopa. Equimolaramount of AgNO3 was used with respect to the basic solution ofDopa. The resultant mixture was sonicated for 2 min. Immediately,Dopa stabilized AgNP was obtained.

2.5. Synthesis of RGOeMNP nanocomposite using Dopa

For the synthesis of RGOeAgNP nanocomposite system, at firsta mixed aqueous solution containing GO (0.1 mg mL�1), Dopa(0.55 mg mL�1) and NaOH (0.35 mg mL�1) was sonicated for15 min. Then, AgNO3 solution (final concentration 0.05 mg mL�1)was added into the above mentioned mixture and it was sonicatedfor 2min. After that, the resultant mixturewas heated atw80 �C for30 min. As a result of that RGOeAgNP nanocomposite wasobtained.

2.6. Catalytic study

For catalytic study, 0.1 mg of RGO-AuNP nanohybrid was dis-persed in 3 mL aqueous solution of p-nitrophenol or p-nitroaniline

sheet with increasing magnification from a to c; (d) SAED image on a RGO nanosheet.

Fig. 5. (a) AFM image of as-prepared RGO nanosheets and (b) height profile image ofa graphene sheet.

Fig. 7. Raman spectra of GO before and after reduction.

B. Adhikari, A. Banerjee / Materials Chemistry and Physics 139 (2013) 450e458 453

(0.1 mM) at room temperature. Then a freshly prepared aqueoussolution of NaBH4 (0.3 mL; 0.1 M) was added to the mixture. Thereduction of aromatic nitro was monitored by using UVeVis ab-sorption spectroscopy.

Instrumentation.

Fig. 6. XRD patterns of (a) graphite powder (b) GO and (c) as-prepared RGO.

2.7. UV/Vis spectroscopy

UV/Vis absorption spectra of the GO and RGO nanosheet (free ofDopa) were recorded by using a Varian Cary 50 Bio UV/Visspectrophotometer.

2.8. Transmission electron microscopy (TEM) study

Morphology of as-prepared RGO was investigated usinga transmission electron microscope (TEM). The sample was pre-pared by placing a drop of the water-dispersed RGO on a TEM grid(300 mesh Cu grid) coated with Formvar and carbon film. The gridwas then allowed to dry under vacuum at 30 �C for two days. Im-ages were taken by a JEOL electron microscope operated at anaccelerating voltage of 200 kV.

2.9. Atomic force microscopy (AFM) study

Morphology of as-prepared RGO was investigated using a tap-ping-mode atomic force microscope (AFM). AFM study was doneby placing a small amount of water-dispersed RGO on mica. Thematerial was then allowed to dry in air by slow evaporation firstand then under vacuum at room temperature for two days. Imageswere recorded by exploiting an Autoprobe CP Base Unit di CP-IIinstrument (model no. AP-0100).

2.10. X-ray diffraction (XRD)

XRD of dried thin flim of raw griphite powder, GO, RGO, metalnanoparticles and RGO-metal nanoparticle composites samples onglass slide were performed separately by using an X-ray diffrac-tometer (Bruker D8 Advance) equipped with a conventional CuKaX-ray radiation (l ¼ 1.54 �A) source and Bragg diffraction setup(Seifert 3000P).

2.11. Raman spectra

For Raman spectra, dried GO and dried RGO samples wereplaced on a glass slide and measured by irradiating with laser lightat 632.81 in a Horiba Jobin Yvon instrument (LABRAM HR 800).

2.12. X-ray photoelectron spectroscopy (XPS)

XPS analysis of dried pure GO and dried RGO have been carriedon an ESCALAB MK II X-ray photoelectron spectrometer.

Fig. 8. (a) UVeVis absorption spectra for the formation of RGOeAgNP as indicated in the figure, D denotes Dopa; (b) UVeVis absorption spectra for the formation of Au/AgNP byDopa in basic aqueous medium; (c) XRD pattern of RGO-AgNP composite.

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3. Results and discussion

3.1. Synthesis and characterization of reduced graphene oxide

Stable aqueous dispersion of reduced graphene oxide (RGO)nanosheet was prepared by chemical reduction of GO at 80 �C withthe use of an amino acid Dopa as a reducing and stabilizing agent.Dopa was added into an aqueous dispersion of GO with a smallamount of aqueous ammonia solution (or sodium hydroxide). Thenthe resultant mixture was first sonicated and heated at 80 �C gentlyfor a fewminutes. The GOwas reduced in presence of Dopa at 80 �Cwithin 30 min (Fig. 1). A gradual color change was observed fromyellowish brown to dark black during the reduction process. Theformation of dark black dispersion in water may be the primaryindication of the reduction of GO to RGO [21]. The as-prepared RGOhas a good stability in water medium. Photographs of aqueousdispersion of GO before and after the reduction are shown in theFig. 2. It can be mentioned that in this study, the presence of base isessential for the reduction of GO to RGO. In presence of a base, Dopais transformed into Dopaquinone via the involvement of twoelectrons in a redox reaction and this is discussed later. To the bestof our knowledge, there is no previous report of using Dopa isa reducing agent for the reduction of GO to RGO. So, it can betermed as a new reagent tomake RGO fromGO. The aromatic groupin Dopa is responsible for the dispersion of RGO into water.

UVevis spectroscopy is used to follow the reduction of GO toRGO and it is shown in the Fig. 3. GO has two peaks with absorption

maxima 230 and 300 nm. It is interesting to note that these twoabsorption peaks completely disappeared after the reduction usingDopa with appearance of a new absorption peak at 266 nm (Fig. 3).The change of the UVeVis absorption provides a hint about thereduction of GO to RGO nanosheet using Dopa.

The high-resolution transmission electron microscopic (HR-TEM) image and a selected area electron diffraction (SAED) patternof the as-prepared chemically reduced graphene oxide nanosheetare shown in the Fig. 4. TEM image exhibits the formation of largeRGO nanosheets after the reduction with Dopa. It is observed fromSAED image that a well-defined six-fold-symmetry diffractionpattern is present. This indicates the formation of RGO from GOwith the signature of crystalline state of the graphene [15].

AFM image of the as-prepared RGO nanosheets using Dopa isexhibited in the Fig. 5 and this shows the individual presence ofnanosheet in the dispersion. The thickness of as-prepared RGOnanosheets is measured from the height profile of the AFM imageand it is found to vary from 1 nm to 3 nm. This suggests that theformation of RGO sheets with varying thickness frommonolayer toa maximum trilayer. It can be stated that the capping agent Dopaadsorbs on the surface of RGO sheet and it can enhance the thick-ness of the as-prepared RGO nanosheet as the Dopa has a thicknessof its own [14].

The reduction of GO to RGO nanosheet has been characterizedusing X-ray diffraction (XRD) study and the result is depicted in theFig. 6. A diffraction peak centered at 2q ¼ 10.92� is observed for GOand this suggests the entire oxidation of GO from graphite powder

Fig. 9. TEM images of (i-a) Au nanoparticles decorated on the surface of the RGO nanosheet and (i-b) an enlarged version of image i-a with high resolution showing the almostuniform fabrication of Au nanoparticles on RGO sheets; (ii-a) Ag nanoparticles decorated on the surface of the RGO nanosheet and (ii-b) an enlarged version of image ii-a with highresolution showing the almost uniform fabrication of Ag nanoparticles on RGO sheets.

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[7]. It has been seen that the d-spacing value of GO (0.78 nm) ismore than that of the graphite (0.34 nm). This is due to the presenceof oxygenated functional groups on the surface of carbon sheets inGO. After the reduction, the diffraction peak at 2q ¼ 10.92� for GOhas been shifted to 25� for RGO. This suggests that most oxygenfunctionalities have been removed after reduction from GO to RGO.

The structural change of graphene oxide before and after thereduction has been monitored using Raman spectroscopic study(Fig. 7). Two fundamental vibrations have been seen for GO: one at1331 cm�1 and another at 1590 cm�1 and these correspond to the Gband and D band respectively. The G band peak for GO (1590 cm�1) isshifted toward longer wavenumber compared to that of graphite(1580 cm�1) [7]. The G-band of as-prepared RGO has appeared at1573 cm�1. This indicates the recovery of the hexagonal network ofcarbonatoms inRGOwithsomedefects. It isobserved that the integralarea ratios of D/G band for RGO (1.22) is higher than that of GO (1.02).This suggests thatmore sp2 domains are formed after the reduction ofGO to RGO [7]. Moreover, a 2D peak at 2684 cm�1 is observed sug-gesting the multilayer properties of the as-prepared RGO.

The reduction of GO to RGO by using Dopa is also confirmed bythe XPS study. Fig. S1 (in the Supplementary data) shows the C1sdeconvoluted spectra of the as-prepared RGO and the pure GO. ForGO, different peaks centered at 284.6, 286.7, 288.11 and 289.2 eVare observed, corresponding to the binding energies of C]C and/orCeC in aromatic rings, C�OH (epoxy and alkoxy), C]O andOeC]Ogroup respectively. The intensities of all C1s peaks of the carbonbinding to oxygen decreased considerably for the as-prepared RGO

using chemical reduction by Dopa. This indicates that most of theseoxygen-containing functional groups are removed after thereduction using Dopa. On the contrary, there is an increase in in-tensity of the peak at 284.6 eV corresponding to the binding en-ergies of C]C and/or CeC in aromatic rings from GO to the as-prepared RGO nanosheet after the reduction using Dopa. Thissuggests that after reduction, the percentage of C]C and/or CeC inaromatic rings is increased significantly indicating a restoration ofthe aromatic nature in the as-prepared RGO nanosheet [7,13].

3.2. Synthesis and Characterization of RGO-MNP hybrid

The presence of MNP on graphene surface in a nanocompositesystem shows specific electronic and catalytic properties [23].Though there are some reports of the preparation of various MNPegraphene composite systems [36e56], the major challenge still re-mains to make uniformly distributed, high-dispersible MNP on thegraphene surface inwatermedium [36e42]. The presence of oxygencontaining functional groups on the surface of GO makes GO dis-persible in aqueous solution, providing reactive sites for chemicalmodification or functionalization using surface chemistry. However,RGO has a poor dispersibility due to pep stacking interactions andmaking individual RGO sheets almost inaccessible for modificationor functionalization. As a result of that, generally RGOeMNP com-posites aggregate themselves and they are precipitated out from themedium. This makes them poor dispersibility of these types ofnanocomposite systemandmaking themunsuitable for application.

Fig. 10. (a) Schematic representation of RGOeAuNP nanocomposite catalyzed hydrogenation reactions. (b) UVeVis spectroscopy to monitor the reduction of (i) p-nitrophenol to p-aminophenol catalyzed by RGOeAuNP (ii) p-nitroaniline to p-diphenyleneamine catalyzed by RGOeAuNP where p-nitroaniline in water shows a characteristic absorption peak at381 nm and reduction of p-nitroaniline to p-phenylenediamine resulted in the disappearance of the absorption peak at 381 nm.

B. Adhikari, A. Banerjee / Materials Chemistry and Physics 139 (2013) 450e458456

To overcome this aggregation, there are some existing methods forthe preparation of RGOeMNP composites, in which graphene isattached to a substrate prior to the incorporation ofMNP [38,50,51].However, there are some difficulties associatedwith thesemethods.There is a real need for the development of eco-friendly, single step,in situ, facile synthesis of water dispersible graphene-MNP. In thisstudy a single aromatic amino acid has been used to reduce simul-taneously both GO to RGO and metal salts to corresponding MNP tomake a nanocomposite system without the help of any externalreducing and stabilizing agents.

For the synthesis of RGO-MNP composites, appropriate amountof Dopa was mixed into the aqueous dispersion of GO with smallamount of dilute NaOH. The mixture was sonicated for a fewminutes and then metal salts were added to it and sonicated again.Just after the addition of metal salts, MNPwas formed immediately.The mixture was gently heated at 80 �C for 30 min. As a resultcorresponding metal nanoparticle decorated RGO was obtained inwater having good dispersibility. The presence of ortho catecholmoiety in the structure of Dopa (Fig. 1) is responsible for thereduction of both GO and metal salts. After reduction Dopa istransformed into Dopaquinone via the involvement of two electronredox reaction (Fig. S2, Supplementary data) [57].

The formation of RGOeMNP has been characterized by usingUVeVis absorption spectroscopy, TEM and XRD studies. Thespontaneous reduction of Agþ/Au3þ by the amino acid Dopa hasbeen monitored through UVeVis absorption spectroscopy. Fig. 8a

illustrates the formation of RGOeAgNP composites from GO inpresence of Dopa. The appearance of a broad surface plasmonresonance band centered at 420 nm suggests the formation of Agnanoparticles. Agþ ions are efficiently and spontaneously reducedby the amino acid Dopa in basic aqueous medium (Fig. 8b). Theabsorption spectra for RGO-AuNP composite is shown in the Fig. S3(Supplementary data). XRD pattern for the RGOeAg nanoparticlescomposite has shown diffraction peaks at 2q ¼ 38.1, 44.3, 64.6 and77.5. These values are consistent with those for Ag nanoparticles(Fig. 8c) corresponding to the (111), (200), (220), (311), and (222)Miller indices of Ag, respectively.

TEM experiments were performed to examine the morphologyof theRGO-Ag/AuNP composite systems. Fig. 9a and Fig. 9b show thecorresponding TEM images of RGO-Au and RGO-Ag compositesrespectively. It is evident from these images that Au/Ag NPsare uniformly decorated on the RGO nanosheets to make nano-composite systems. Average sizes of these Au and Ag nanoparticlesare found to be 35 nm and 25 nm respectively. Particle size distri-bution shows most of these AuNPs are within the range 32e38 nm(Fig. S4, Supplementary data). The presence of RGO nanosheets andAu/Ag NPs are separately characterized using selected-area electrondiffraction (SAED) study (Fig. S5, Supplementary data).

The catalytic property of RGO-AuNP composite was investigatedfor the transformation of aromatic nitro to aromatic amino group:(i) reduction of p-nitrophenol to p-aminophenol by NaBH4 and (ii)the reduction of p-nitroaniline to p-aminoaniline by NaBH4 [58,59].

Table 1Conditions for the catalytic transformation of p-nitrophenol to p-aminophenol.

Nature of AuNPbased catalyst

Catalyst (metal)loading

Time ofconversion

Medium and temperature Concentrationof NaBH4

Concentrationofp-nitrophenol

RGOeAuNP bydopa [Our study]

0.03 mg mL�1 2 min Aqueous solution and 25 �C 0.09 (M) 0.09 (mM)

AuNPeGO[59] 5.0 mol% 25 min Aqueous solution and 25 �C 50 equivalentAuNPebohemite film[60] Film size 1.25 cm � 0.8 cm � 1.9 mm 32 min Aqueous solution and 25 �C 10 (mM) 1 (mM)Au/ILs/PPyNTs hybrid[61] 0.1 (mM) 14 min Aqueous solution and 22 �C 13 (mM) 0.17 (mM)PANIeAuNP[62] Composite membrane (2 cm � 2 cm) 110 min Aqueous solution 0.1 (M) 0.1 (mM)

B. Adhikari, A. Banerjee / Materials Chemistry and Physics 139 (2013) 450e458 457

For catalytic experiment, freshly prepared aqueous solution ofNaBH4 was added into diluted aqueous solution of p-nitrophenol orp-nitroaniline (concentration 1�10�4M). A color change from lightyellow to yellowish greenwas observed after the addition of NaBH4.Then RGOeAuNP composite was added into the as-obtained solu-tion and the reaction mixture has been kept stirring. These catalyticreductions were monitored by UVeVis absorption spectroscopy(Fig. 10). In the case of reduction of p-nitrophenol, a decrease in theUVeVis absorption peak at 400 nm was observed with the con-comitant appearance of a new peak at 298 nm, because of theformation of p-aminophenol (Fig. 10). It can be noted that inabsence of RGO-AuNP catalyst, there is no change in color of p-aminophenol solution and no change in UVevis absorption posi-tion is observed. It can be mentioned that graphene alone does notshow any catalytic property. In the case of reduction of p-nitro-aniline, a decrease in the UVevis absorption peak at 381 nm wasobserved with the concomitant appearance of a new peak at304 nm because of the formation of p-aminophenol (Fig. 10). Bothof these catalytic transformations occurred efficiently with a com-pletion time of 2 min. Our RGO-AuNP nanocomposite catalyst takesmuch less time for the reduction compared with other previouslyreported AuNP based catalytic systems (Table 1) [59e62]. In thisstudy, RGO act as a good support for anchoring the NPs on thesurface of the sheet and these NPs act as a good catalyst for thereductive transformation. Here, it is expected that p-nitrophenol orp-nitroaniline can be adsorbed on the surface of RGO via pepstacking interactions and this results in the enhancement of theeffective concentration of p-nitrophenol or p-nitroaniline near tothe AuNPs on RGO, leading to an efficient contact between them.

4. Conclusion

This study demonstrates an easy, single step and eco-friendlymethod to make RGO and Au/AgNP simultaneously from respec-tive precursors to form a nanohybrid system using an aromaticamino acid based new reducing agent inwater medium. The aminoacid 2,4-dihydroxy phenyl alanine, Dopa acts as a new reducingagent to form GO to RGO and Au/Ag salts to Au/Ag NPs in a singlestep. Metal nanoparticles are fabricated uniformly and homoge-neously on the RGO nanosheet as it is evident from TEM images.Interestingly, the RGO-AuNP hybrid system acts as a catalyst for thereduction of aromatic nitro to aromatic amino group with a highefficiency. Creation of graphene-MNP nanohybrid system usinga new reducing agent holds future promise to use variousgraphene-NP based nanohybrid system in an environmental suit-able manner for interesting applications in catalysis, battery elec-trodes, optoelectronics, sensing platforms, and others.

Acknowledgement

B.A. thanks the CSIR, New Delhi, India, for financial assistance.We acknowledge unit of nanoscience project at IACS, funded by DSTNew Delhi for support.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.matchemphys.2013.01.020.

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