Research ArticleReceived:14May2009Accepted:8August2009Published online in Wiley Interscience:7October2009 ()DOI10.1002/jrs.2481Surface-enhanced Raman scattering of silylated graphite oxide sheets sandwiched between colloidal silver nanoparticles and silver pieceXiaoqi Fu,Fengli Bei,∗Xin Wang,∗Xujie Yang and LudeLuGraphite oxide(GO)was successfully silylated by3-mercaptopropyltrimethoxysilane.The surface-enhanced Raman scattering spectrum of the silylated GO sheets sandwiched between colloidal silver nanoparticles and silver piece is presented.The Raman signal shows a104enhancement compared to that of bulk GO.The large Raman enhancement is most likely a result of electromagnetic(EM)coupling between the colloidal silver nanoparticles(localized surface plasmon)and the silver piece (surface plasmon polariton),creating large localized EMfields at their interface,where the silylated GO sheets reside in this sandwich architecture.Copyright c 2009John Wiley&Sons,Ltd.Supporting information may be found in the online version of this article.Keywords:graphite oxide;silylation;surface-enhanced Raman scattering;sandwichIntroductionGraphite oxide(GO)is a well-known layer-structured com-pound containing hydroxyl and epoxide functional groups on the top and bottom surfaces of each sheet and carboxyl and carbonyl groups at sheet edges.[1]Owing to the pres-ence of the hydrophilic polar groups in the interlayer,GO exhibits swelling and intercalation properties and allows in-terlayer accommodation of several materials such as aliphatic hydrocarbons,[2,3]metal ions,[4]as well as hydrophilic molecules and polymers.[5]By intercalation of these materials,GO possesses remarkable properties(electronic,[6]chemical,[7]and thermodynamic[8]).In particular,surface properties of GO have attracted considerable attention recently.[9,10]Surface-enhanced Raman scattering(SERS) is an effective tool to investigate interfacial processes at the molecular level,and the Raman signal can be enhanced by many orders of magnitude for molecules adsorbed on metals,which offer the opportunity to study GO sheets by Raman spectroscopy.SERS of amorphous carbon,[11]fullerenes,[12]and carbon nanotubes[13] has been studied in previous works.Kneipp et al.[14]have observed SERS of single-walled carbon nanotubes in contact with fractal silver colloidal clusters with an enhancement of approximately 12orders of magnitude.GO has an analogous structure to carbon nanotubes,and it is expected to show a similar SERS effect.In this study,we silylated GO with3-mercaptopropyl-trimethoxysilane(MPTS)and used the‘free’mercapto groups of the resulting silylated GO for assembly of sub-monolayers of silylated GO sheets on a silver piece.Another free mercapto group on the side of the resulting silylated GO reacted with colloidal Ag to form a sandwich structure.Finally,we investigated the SERS effect of this system in detail.ExperimentalNatural graphite powder(44µm)was provided by Qingdao Zhongtian Company.MPTS(95%)was purchased from Alfa.The other reagents were obtained from Sinopharm Chemical Reagent Co.,Ltd.GO was prepared from natural graphite by oxidation with fuming nitric acid and KClO3,according to Brodie’s method.[15] This oxidation procedure was repeated forfive times,and the composition was55.58%carbon,2.66%hydrogen and41.76% oxygen,based on the elemental analysis of hydrogen and carbon. The obtained GO was silylated by MPTS in a similar manner as reported by Matsuo et al.[16]GO(100mg)and butylamine(1ml) were mixed in a sealed glass vial in the presence of dry toluene (10ml).The mixture was sonicated and then heated at60◦C under nitrogen.After1h,MPTS(2ml)was added to the solution.The resulting solution was sonicated again and allowed to reflux at 110◦C for24h.The precipitate was centrifuged and then washed with ethanol and acetone several times.The obtained sample was dried at60◦C under vacuum for12h.The silylated GO was dispersed in N,N-dimethylformamide (DMF)under sonication for0.5h.A silver piece pretreated by polishing with1.0and0.3µm alumina slurry was immersed in the dispersion of the silylated GO.After24h,the sample was washed extensively with DMF and distilled water,and then submerged into colloidal Ag or Au(prepared by citrate reduction according∗Correspondence to:Xin Wang and Fengli Bei,Key Laboratory for Soft Chem-istry and Functional Materials,Nanjing University of Science and Technology, Ministry of Education,Nanjing210094,China.E-mail:wxin@;beifl@Key Laboratory for Soft Chemistry and Functional Materials,Nanjing University of Science and Technology,Nanjing210094,ChinaJ.Raman Spectrosc.2010,41,370–373Copyright c 2009John Wiley&Sons,Ltd.370SERS of silylated GO sheets sandwiched between colloidal silver nanoparticles and silverpieceFigure1.XRD patterns of(a)GO,(b)MPTS–GO and(c)butylamine-GO. to Lee and Meisel’s method[17])for24h.The resulting sample was rinsed profusely with ethanol and distilled water and then examined in air by Raman spectroscopy.The Raman spectra were recorded on a Renishaw Invia Raman microscope excited by an argon ion laser beam(514.5nm,20mW). The laser power at the sample position was10mW with a spot size of ca.1–2µm.The data acquisition time used in the measurement was25s.Replicate measurements on different areas were made three times to verify that the spectra were a true representation of each experiment.X-raydiffraction(XRD)patternswere recordedon a Bruker D8Advanced X-ray diffractometer using Cu Kαradiation (λ=0.1542nm).Fourier transform infrared spectra(FT-IR)were recorded on a Bruker Vector22spectrometer(KBr pellets).1H nuclear magnetic resonance(1H-NMR)spectra were obtained on a Bruker300instrument.Thermogravimetric analyses(TGA)and differential thermal analyses(DTA)were performed on a Shimadzu PTG-60thermogravimetric analyzer from50to800◦C in airflow. Elemental analyses were performed using a Perkin Elmer240C elemental analyzer.In addition,afield emission scanning electron microscope(FESEM,LEO-1550,5kV)equipped with an energy dispersive X-ray spectrometer(EDS)was used to characterize the samples.Results and DiscussionFigure1shows the XRD patterns of GO and MPTS-GO.As Fig.1(b) shows,the diffraction peak at around2θ=13◦corresponding to the(001)reflection of GO almost disappeared in the pattern of MPTS-GO.Recent researches have shown that,[18,19]if the regular stacks of GO or graphene are destroyed,for example, by exfoliation or intercalation,their diffraction peaks become weak or even disappear.In addition,the diffraction of MPTS-GO apparently differs from that of butylamine-GO(Fig.1(c)),which has an obvious peak at2θ=9.4◦induced by the intercalation of butylamine into interlayer of GO.These results indicate that MPTS has interacted with GO and destroyed the interlayer. Successful synthesis of MPTS-GO was further confirmed by the IR spectrum(Supporting Information,Fig.1S)and1H-NMR spectrum (Supporting Information,Fig.2S),which indicated that MPTS reacted with GO,losing the methoxy group,and was covalently bonded to the GO layer by Si–O bonding.The composition of MPTS-GO was62.42%carbon and3.43%hydrogen based on elemental analysis data,and10.25%silicon calculated from the weight of residual SiO2after TG measurement(Supporting Information,Fig.3S).Figure2(a)shows the proposed formation processes of Ag piece/GO/colloidal Ag sandwiches.MPTS reacted with GO by losing the methoxy group and then bonding to the GO layer.The resulting silylated GO sheets with the free mercapto groups were spontaneously formed on the silver piece.Another free mercapto group on the side reacted with colloidal Ag to form the Ag piece/GO/colloidal Ag sandwich structure.It should be noted that the lamellar structures of GO sheets and the size of Ag particles remained(Fig.2(b)),although the GO layers were exfoliated and destroyed after silylation.XRD pattern of MPTS-GO adsorbed on the silver piece is shown in Fig.3.The diffraction peaks appearing at38.3,44.4,64.7and77.5◦are all ascribed to the characteristic diffraction of silver(JCPDS04–0783).No obvious diffraction peaks below14◦corresponding to GO or graphite were observed,which indicates that MPTS-GO adsorbed on the silver piece was inaFigure2.(a)Scheme showing a proposed formation processes of Ag piece/GO/colloidal Ag sandwiches.SEM images of(b)MPTS-GO and(c)Agpiece/GO/colloidal Ag sandwiches.The inset shows the EDS image.J.Raman Spectrosc.2010,41,370–373Copyright c 2009John Wiley&Sons,/journal/jrs371372X.Fu et al.Figure 3.XRD pattern of MPTS-GO adsorbed on a silverpiece.Figure 4.Raman spectra of (a)Ag piece/GO/colloidal Ag sandwiches and (d)MPTS-GO on the silver piece;Raman spectra of (b)bulk GO and (c)bulk MPTS-GO.Raman intensities of (b),(c)and (d)were multiplied by 10.monolayer or sub-monolayer form,and not a multilayer one.After the silver piece adsorbed with the silylated GO sheets was submerged into colloidal Ag,Ag nanoparticles were adsorbed on the surfaces of GO sheets,as can be seen from Fig.2(c).The upper insert is the EDS image,in which the relative content of oxygen compared with carbon is obviously lower than that of MPTS-GO.It is probably due to the fact that the epoxide groups of GO were reduced by colloidal Ag solution.Efforts to interpret it and fabricate a regular distribution of single-layer GO sheets will be made in our future work.Raman spectra of MPTS-GO on the silver piece and the SERS spectrum of Ag piece/GO/colloidal Ag are shown in Fig.4,together with the normal Raman spectra of GO and MPTS-GO.The Raman spectrum of GO shows the well-known main groups of bands:the tangential stretching mode (G band)in the range of 1500–1600cm −1as well as the disorder band (D band)in the range of 1200–1500cm −1.A weak band at 1107cm −1was also observed,which we assigned to the ring-breathing mode.In thespectrum of MPTS-GO (Fig.4(c)),the relative intensity of the G band compared to the D band was weaker than that of GO.A new band at 1614cm −1was observed,which can be assigned to the D band.[20]In Fig.4(d),the intensities of the G and D bands were too weak to be seen clearly.However,the ring-breathing band was enhanced obviously and shifted to 1123cm −1.In addition,a new band at 239cm −1assignable to the stretching mode of Ag–S was observed,which indicates that MPTS-GO was adsorbed on the silver piece through the sulfur atoms.In the SERS spectrum (Fig.4(a)),it is obvious that the number of bands in the range of 1200–1600cm −1,as well as the intensities of bands,were more in comparison with those of GO and MPTS-GO.The profile of the G band fits a complex band with two main peaks at 1577and 1545cm −1.Furthermore,we observed that the D band becomes much broader and splits into more bands with full peak shapes (1434,1375,1325,1294and 1173cm −1).In the range of 590–870cm −1,new bands appear at 818,706,665and 632cm −1.Atpresent,we cannotfindsatisfactoryassignmentsin the literature for these new bands.It may be related to the C–C and C–O bonds.In addition,it should be noted that the ring-breathing band at 991cm −1,as well as the stretching of Ag–S at 239cm −1,exhibits a dramatic increase.From the experimental findings in our studies,the SERS signal of silylated GO sheets comes essentially from the formed Ag piece/GO/colloidal Ag sandwiches.The classical electromagnetic (EM)mechanism of SERS shows that the large enhancements result predominantly from the concentration of the EM optical fields at ‘hot spots’,which may be junctions and interstices between metal nanoparticles.[21,22]As two nanoparticles approach each other,their transition dipoles get coupled,and the EM field around each nanoparticle is enhanced greatly.Recent studies [23,24]have demonstrated that the interaction is not only between particles but also between particles and the metallic substrate.Therefore,in our experiment,the enhancement of the SERS spectrum of silylated GO sheets can be attributed to the EM coupling of the colloidal Ag nanoparticles and the surface of the Ag piece,most probably due to the interactions of the localized surface plasmon of colloidal Ag nanoparticles and the surface plasmon polariton of the Ag piece.To further evaluate the enhancement efficiency of this system,we quote a simple formula,[25,26]EF =(I SERS /I bulk )×(N bulk /N SERS ),to calculate the enhancement factor (EF).I SERS and I bulk are Raman intensities of the Ag piece/GO/colloidal Ag sandwiches and bulk GO,respectively,whereas N SERS and N bulk are the corresponding number of samples.Choosing the G band (1577or 1545cm −1)in the SERS spectrum as the typical band,the ratio of the intensities (I SERS /I bulk )is about 10.In our experiment,the confocal depth of laser beam is about 2µm.Assuming that two layers of Ag piece/GO/colloidal Ag are adsorbed on the silver piece under the beam spot area,the calculated N bulk /N SERS is about 103.Therefore,the EF of this system is estimated to be about 104.ConclusionIn summary,silylated GO sheets were self-assembled on a silver piece and then Ag piece/GO/colloidal Ag sandwiches were formed by submerging the piece into colloidal Ag.The EF values derived from this sandwich architecture were estimated to be as large as 104.The plasmon coupling between the silver nanoparticles and the surface of silver piece is most likely responsible for the observed enhancement.This research extends the capability of/journal/jrs Copyright c2009John Wiley &Sons,Ltd.J.Raman Spectrosc.2010,41,370–373SERS of silylated GO sheets sandwiched between colloidal silver nanoparticles and silver pieceSERS investigations to GO products and points to a promising future for the fabrication of thinfilms with smart properties. 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