Fabrication of ITNPs nanocomposite by two-stepchronoamperometry electrode
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Applied Surface Science 349(2015)805–810Contents lists available at ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scFabrication of ITO-rGO/Ag NPs nanocomposite by two-stepchronoamperometry electrodeposition and its characterization as SERS substrateRong Wang a ,b ,c ,Yi Xu a ,b ,∗,Chunyan Wang b ,d ,Huazhou Zhao a ,b ,Renjie Wang a ,b ,Xin Liao a ,b ,Li Chen b ,d ,Gang Chen b ,daChemistry and Chemical Engineering College,Chongqing University,Shapingba,Chongqing 400044,ChinabKey Disciplines Lab of Novel Micro-Nano Devices and System Technology,and School of Optoelectronics Engineering,Chongqing University,Shapingba,Chongqing 400044,China cAnalytical and Testing Center,Sichuan University of Science &Engineering,Zigong,Sichuan 643000,China dSchool of Optoelectronic Engineering,Chongqing University,Shapingba,Chongqing 400044,Chinaa r t i c l ei n f oArticle history:Received 10March 2015Received in revised form 5May 2015Accepted 11May 2015Available online 19May 2015Keywords:ITO-rGO/Ag NPs Nanocompositechronoamperometry electrodeposition SERSEnhance factorsa b s t r a c tA novel composite structure of reduced graphene oxide (rGO)–Ag nanoparticles (Ag NPs)nanocomposite,which was integrated on the indium tin oxide (ITO)glass by a facile and rapid two-step chronoamperom-etry electrodeposition route,was proposed and developed in this paper.SERS-activity of the rGO/Ag NPs nanocomposite was mainly affected by the structure and size of the fabricated rGO/Ag NPs nanocompos-ite.In the experiments,the operational conditions of electrodeposition process were studied in details.The electrodeposited time was the important controllable factor,which decided the particle size and surface coverage of the deposited Ag NPs on ITO glass.Under the optimized conditions,the detection limit for rhodamine6G (R6G)was as low as 10−11M and the Raman enhancement factor was as large as 5.9×108,which was 24times higher than that for the ITO–Ag NPs substrate.Apart from this higher enhancement effect,it was also illustrated that extremely good uniformity and reproducibility with low standard deviation could be obtained by the prepared ITO-rGO/Ag NPs nanocomposite for SRES detection.©2015Published by Elsevier B.V.1.IntroductionSince the surface enhanced Raman spectroscopy (SERS)phe-nomenon was first reported in the 1979s [1–3],SERS equipped with high sensitivity and specificity for chemical and biological molecules detection,has received increasing attention [4–6].Raman scattering is an inelastic scattering process that is inher-ently weak [7],therefore,fabrication of high SERS-active substrate is of great importance for SERS detection.The Raman enhancement effect of SERS substrate is closely related to its composition and nanostructure [8],and a great deal of research effort has been devoted to the development of new SERS substrates.Up to now,different types of materials have been used as SERS-substrate,which mainly include noble metals [9–11],transition metal∗Corresponding author at:Chemistry and Chemical Engineering College,Chongqing University,Shapingba,Chongqing 400044,China.Tel.:+862365111022;fax:+862365104131.[12],semiconductor [13],and new emerged carbon nanomaterials [14,15].Unfortunately among these,only noble metal has exhibited a significantly reinforcing effect on Raman signal.However hybrid nanocomposite consist of noble metals and the other materials provide the potential in various applications [16–18].Recently,the noble metals based hybrid multifunctional SERS substrates have become a hot research topic for the further development of SERS applications.The SERS signal intensity of the analytes not only is dominated by enhancement effect but also rely on the amount of molecules in enhancement region,for instance,insufficient molecule adsorp-tion would causes low electromagnetic chemical enhancement effect.[19]Therefore,a good SERS substrate ought to possess the characteristics of high enhancement effect and efficient adsorp-tion.Ag nanostructures is the most favorable SERS substrate due to their excellent ability to enhance the local electromagnetic field [17],but its affinity for some target molecules is unsatisfactory [19].On the other hand,as novel SERS materials,graphene oxide and reduced graphene oxide (rGO)were found that could strongly/10.1016/j.apsusc.2015.05.0670169-4332/©2015Published by Elsevier B.V.806R.Wang et al./Applied Surface Science349(2015)805–810suppressfluorescence and enhance chemical Raman(10–100fold) signals[20–22],at the same time,which had the inherent abil-ity to adsorb variety of organic molecules,especially the aromatic molecules[23].Therefore there are increasing studies focused on the investigation of SERS efficiencies of graphene oxide/Ag hybrid substrates.Study show that compared to pure silver substrates [24],GO/Ag NPs hybrids substrates could further improved SERS performance owning to combination the beneficial properties of Ag NPs and GO,In recent years,several methods have been car-ried out to synthesize nano-hybrid structures based GO and Ag, including in situ method[25,26]and self-assembly method[19].The in situ method is usually relative simple,however the particle size and morphology cannot be controlled good enough[25].The self-assembly is an ordinary method to fabricate GO/Ag NPs hybrids, because loading ration and morphology of the nanoparticles are tunable.Nevertheless,linker molecules are usually required in this method,which is undesirable for most applications[27].In addi-tion,S elf-assembly method is time-consuming.For example,GO/Ag NPs nanocomposite were fabricated by Wei Fand[28]and Xiao-juan Liu[19]using self-assembly method which spent2days. Since the performances of GO/Ag NPs SERS substrate are signif-icantly affected by the particle size and distribution,it is vitally important to develop effective and controllable methods that could fabricate Ag-graphene oxide hybrid materials without any foreign molecules.Electrochemical deposition technique can be used for large-scale preparing of Ag NPs and rGO nanocomposite with the advantages of low cost and simple operation.Some researcher reported that rGO–Ag NPs hybrid material prepared by cyclic voltammetry method were used in biosensor[29].But there are few reports on preparing rGO–Ag nano-hybrid SERS-active sub-strate by electrochemical.Study showed that Ag NPs dispersed on rGO surface was more conducive to improve SERS sensitiv-ity than rGO coverage Ag NPs[30].Herein,we demonstrate a facile,rapid,harmless and low cost two-step chronoamperometry electrodeposition method to fabricate ITO-rGO/Ag NPs SERS sub-strate.Because oxidation reduction potential of graphene oxide is more negative than silver ammonia ion,the reduction of graphene oxide and the growth of Ag nanoparticles on ITO is under con-trol at different potential respectively,the size and densities of Ag NPs decorated on rGO sheets surface could be simply adjusted by changing the electrodeposition time.The SERS sensitivity and reproducibility of the as-fabricated substrates was investigated with model molecule R6G.The obtained ITO-rGO/Ag NPs nanocom-posite exhibited a strong SERS signal for low concentration of R6G molecules due to combine the advantages of electromag-netic enhancement caused by the Ag NPs as well as chemical enhancement and high probe molecules absorption ability of rGO.2.Experimental2.1.MaterialsITO glasses were purchased from Shenzhen huanan Technol-ogy Co.Ltd.(Shenzhen China),which sheet resistance less than 10 /square.Flake nature Graphite powder and Silver nitrate (AgNO3,99.7%)were obtained from Aladdin-Reagent Co.Ltd. (Shanghai,China).Sulfuric acid(H2SO4,98%),potassium per-manganate(KMnO4,99.9%),hydrogen peroxide(H2O2,30%), hydrochloric acid(HCl,37%),Ammonia solution(NH3,25%)and sodium hydroxide(NaOH,99.99%)were purchased from Chongqing Chemical Reagent Co.Ltd.Ultra-pure water(18.2M cm,produced by a Milli-Q system)was used throughout this work.2.2.Fabrication of ITO-rGO/Ag SERS nanocomposite substratesGO was synthesized from natural graphite powder by using a modified Hummers’method[31].Typically,1.0g offlake nat-ural graphite powder and0.5g of sodium nitrate were added to 23mL of cooled(0◦C)concentrated H2SO4(98%).Then3.0g of potassium permanganate was gradually added into above solu-tion under vigorous stirring,so that the temperature of the mixture was maintained below20◦C.After that,the ice bath was replaced by a water bath and the mixture was heated to35±3◦C for3h under continuous stirring.After this,46mL of ultra-pure water was slowly added into the pasty suspension,and the reaction was kept at98◦C for30min.The reaction was terminated by sequen-tial adding100mL of ultra-pure water for1.5h under continuous stirring.Subsequently,the excess potassium permanganate and manganese dioxide were removed by treatment with10mL of30% hydrogen peroxide.The solid product was separated by centrifu-gation at3000rpm for10min after the color of the suspension turned bright yellow.Then resulting graphite oxide wasfiltered and washed with5%hydrochloric acid and ultra-pure water to remove the free SO42−.Finally,the resultant solid was dried in air at0◦C overnight and ground into afine powder.The graphite oxide disper-sion(1.0mg/mL)was treated ultrasonically(at40Hz with power of 320W)for about1h to ensure that most graphite oxide is exfoliated into single layer GO.The electrodeposition of ITO-rGO/Ag NPs was conducted in a conventional three-electrode electrochemical cell in the aqueous electrolyte solution including of5mM silver ammonia[Ag(NH3)2]+ and0.1mg/mL GO with0.01M KNO3as the supporting electrolyte at room temperature.The chronoamperometry was performed on VersaSTAT3electrochemical workstation(Versastat3Applied Research Princeton,USA)using a three-electrode electrochemical cell,an ITO(0.5×1cm2,10 cm−2)as the working electrode,Pt plate as the counter electrode and a saturated calomel electrode (SCE)as the reference electrode.A salt bridgefilled with agar and potassium nitrate was utilized to connect the saturated potassium nitrate solution for preventing the contamination of chloride ion from the SCE.The electrodeposition process contains two steps:In thefirst step,the potentials was set to be−1.0V for10s,Then,the second step,the growth process of Ag was extended at potential of −0.2V for long time.The effects of electrodeposition time on SERS performance of nanostructures were investigated.For comparison, the ITO–Ag SERS substrate was prepared in the same electrodeposi-tion conditions at the absence of GO.After deposition,the working electrode was washed with ultra-pure water and keep in ultra-pure water to avoid Ag NPs oxidized in air.Which was dried with nitrogenflow before used as SERS substrates.2.3.CharacterizationThe crystal phase of the substrates and graphene oxides were determined by X-ray powder diffraction using Cu/K␣radiation ( =1.5406˚A)(XRD,Bruker,D2PHASER),The diffraction pattern was collected in the2Ârange8–90◦at room temperature.The surface morphology and microstructure of the substrates were characterized withfield emission scanning electron microscope (FESEM;FEI Nova Nano SEM400operated at10.0kV)with energy dispersive spectrometer(EDS),The Raman spectra were collected on Raman microscope(Lab RAM HR,HORIBA Jobin Yvon,Germany) with the633-nm line of a He–Ne laser as the excitation source.The laser power at the sample was set to4.5mW.All the spectra were collected with a10X objective(NA=0.25,Olympus)correspond-ing to a laser spot on the sample of1m in diameter.The spectral acquisition times was2s.Each measurement was repeated at least three times.R.Wang et al./Applied Surface Science349(2015)805–810807Fig.1.Schematic illustration of the fabrication strategy of ITO-rGO/Ag NPs SERS substrate and its application in SERS.(Step1:Ag nucleus and reduced graphene oxide were electrodeposited on the ITO substrate at the potentials of−1.0V for10s.Step2:AgNP was grown up by electrodeposited at the potentials of−0.2V).2.4.SERS measurementsIn the SERS experiments,R6G was used as the probe molecule. For the SERS studies,the as-fabricated SERS substrates were immersed in1mL of R6G solutions with different concentrations (10−7,10−8,10−9,10−10,10−11and10−12M)for30min,and then, the substrates were thoroughly washed with Milli-Q water and subsequently drying with N2.To evaluate the EF values of SERS, the Raman intensity of R6G(10−3M)was also acquired on a highly resistiveflat Si wafer as the reference for comparison.3.Results and discussion3.1.Design of ITO-rGO/Ag NPs SERS nanocompositeIn this paper,two-step chronoamperometry electrodeposition technology was used to prepare ITO-rGO/Ag NPs SERS nanocom-posites substrates.Electrochemical reduction potential of graphene oxide was at−0.9V,which was more negative than that of [Ag(NH3)2]+.Thefirst-step potential of chronoamperometry elec-trodeposition was set at−1.0V holding for10s,Ag nucleation and reduction of GO were deposited simultaneously,and then the second-step potential was set at−0.2V holding for a long time. The size and gap of the Ag NPs electrodeposited on ITO electrode could be change by adjusting the second-step chronoamperometry electrodeposition time.The fabrication procedure was illustrated schematically in Fig.1.3.2.Fabrication and characterization of the ITO-rGO/Ag NPs nanocompositeThe surface morphology and microstructure of ITO-rGO/Ag NPs substrates were characterized by FESEM images in Fig.2a–e.It was evident that Ag NPs attached on the surface of ITO glass were stag-gered distributed evenly in two sizes.It was found that the two sizes Ag NPs were initially formed atfirstly step electrodeposited (Fig.2a),which acted as nuclei for subsequently growth.As the deposition time increases,Ag NPs size continued growing and the gap between adjacent nanoparticles decreased correspondingly. These results demonstrated that the size and density of Ag NPs could be controlled by adjust growth deposition time.The corre-sponding energy-dispersive X-ray(EDX)spectroscopy(Fig.2f)of ITO-rGO/Ag NPs substrate showed the peaks corresponding to C,O, and Ag elements,confirming the existence of silver and rGO sheets on ITO surface.The composition of as-prepared GO and ITO-rGO/Ag NPs was characterized by X-ray diffraction as shown in Fig.3.The sharp peak at2Â=10.2◦was due to(001)plane of GO.In Fig.3c,the characteristic peaks at2Â=38.101◦,44.600◦,64.678◦and77.549◦were related to the(111),(200),(220),and(311)planes of fcc Ag(JCPDS04-0783)respectively.The arrows as marked showed that Ag NPs exhibit high crystallinity.Nevertheless,for Fig.3b, only(111),(200)plane of Ag could be found easily,which was due to the less silver.The sharp diffraction peak of Ag(111)with high intensity indicated that the deposited nano Ag had a tendency to grow with the surfaces dominated by the lowest energy(111) Fig.2.SEM images of ITO-rGO/Ag NPs SERS substrate electrodeposited at−1.0V for10s and at−0.2V for0s(a),100s(b),300s(c),500s(d),700s(e);EDS images of ITO-rGO/Ag NPs SERS substrate electrodeposited at−1.0V for10s and at−0.2V500s(f).808R.Wang et al./Applied Surface Science 349(2015)805–810Fig.3.XRD spectra of GO (a)and ITO-rGO/Ag NPs electrodeposited at −1.0V for 10s and at −0.2V for 0s (b),500s (c).facet [32].The XRD in Fig.2indicated that the nano Ag had been successfully electrodeposited on ITO substrate,while the GO peak is not observed for all prepared ITO-rGO/Ag NPs substrate which indicated that GO had been successfully reduced by electrodepo-sition.This might also be due to the low content of GO in the nanocomposite and the growth of silver on the reduced GO sheet could prevent the stacking of the rGO layers [33].So,Raman spec-troscopy was used to further characterize the structural changes of graphene-based materials,including disorder and defect struc-tures.The Raman spectra of GO,ITO-rGO/Ag NPs composites were shown in Fig.4,respectively.The Raman spectra of GO showed two peaks at 1333cm −1and 1592cm −1,which correspond to the D band (symmetrical A1g mode)and the G band (E2gode of sp 2carbon atoms)respectively [34].In comparison to GO,the Raman spectrum of rGO/Ag NPs indicated that the D and G bands shifted to1360cm −1and 1586cm −1.In addition,the Raman spectrum of rGO/Ag NPs showed a slightly greater I(D)/I(G)intensity ratio (1.14)than that of GO (0.95).D/G intensity ratio increased after electrochemical reduction that suggested smaller sp 2domains were formed upon reduction of the exfoliated GO [35].This observation also verified that the GO was reduced on the ITO substrate by electrochemicalmethod.Fig.4.Raman spectra of ITO-rGO/Ag NPs (a)and GO (b).3.3.SERS measurement based on the ITO-rGO/Ag NPs nanocompositeFor Ag NPs on substrate,good distribution and high cover density were very important to surface-based applications.The SERS effects of ITO-rGO/Ag NPs substrate for 10−7M R6G with different Ag particle size and density (shown in Fig.2a–e,respec-tively)under different electrodeposition condition were shown in Fig.5.R6G was used as the probe molecule,because it has been well-characterized by SERS and most of the prominent Raman bands have been assigned.In Fig.5A,the predominant bands were located at 609cm −1,771cm −1,1180cm −1,1309cm −1,1358cm −1,1506cm −1,1570cm −1,1646cm −1,which were consistent with lit-erature reports for R6G Raman spectra of molecules [36,37],and indicated the presence of R6G on substrate.Although the pos-itions of Raman peaks of all samples were same,their intensities were different.Enhancement of the peak intensity compared to different electrodeposition conditions were shown in Fig.5B.At first,the SERS intensity of R6G increased with electrodeposition time prolong and reached a maximum at the electrodeposition of 500s,because the size and density of Ag NPs were increased the gap between Ag NPs became smaller,leading to stronger coupling between the Ag NPs.And then,the SERS intensities became sta-ble with the further increase of electrodeposition time,which was due to the combination effects by the stronger coupling between Ag NPs and smaller area of exposed rGO for adsorption of R6G [27].These results demonstrated the ITO-rGO/Ag NPs substrate with high density of Ag NPs suitable particle size and exposed rGO area for molecule adsorption could be conductive to SERS-active substrate.The most important element to a new SERS-active substrate was sensitivity.Because of the electromagnetic enhancement effect of Ag NPs as well as chemical enhancement and strong adsorption of rGO,such ITO-rGO/Ag NPs SERS substrate could be potentially used as SERS substrates for sensitive analysis.In order to evaluate SERS performance of the ITO-rGO/Ag NPs substrates and determine how the rGO/Ag NPs nanocomposite enhance the SERS signal in comparison with a pure silver surface,both substrates were inves-tigated.The SERS spectra was shown in Fig.6,It was found that the detection limit were as low as 10−11M and 10−10M for ITO-rGO/Ag NPs substrates and ITO–Ag NPs substrates pared with ITO–Ag NPs substrates,SERS performance of ITO-rGO/Ag NPs substrate had definite advantage.To estimate the enhancement ability of the ITO-rGO/Ag NPs substrates,Enhancement factor was calculated by comparing the intensity of the 771cm −1peak in the SERS spectrum with that in the normal Raman spectrum of 10−3M R6G,which did not show any chemical effect by rGO.The apparent surface enhancement factor (EF)was calculated according to the formula [38]:EF =I SERS SERS ×C RSRSwhere I SERS and I RS were peak intensities,while C SERS and C RS con-centrations of R6G for SERS and Raman measurements.As a result,the EF was calculated to be 5.9×108for ITO-rGO/Ag substrates,which was 24fold higher than the ITO–Ag substrate with EF of 2.4×107.The result indicated that the rGO sheets indeed amplified the SERS signals of the R6G probe molecules.It is known that poor reproducibility of Raman signals in tra-ditional SERS analysis is the main obstacle to use of SERS as a routine analytical technology.For practical purposes,SERS sub-strate exhibit not only high enhancement ability but also good reproducibility both across a single substrate and between dif-ferent substrate.To investigate the homogeneity of ITO-rGO/Ag NPs substrate,SERS spectra of 10−7M R6G were collected from 5spots selected randomly on single ITO-rGO/Ag NPs substrateR.Wang et al./Applied Surface Science 349(2015)805–810809Fig.5.(A)SERS spectra of 10−7M R6G absorbed on the ITO-rGO/Ag NPs substrate electrodeposited at −1.0V for 10s and at −0.2V for 0s (a),100s (b),300s (c),500s (d),700s (e),separately;(B)Relationship between the Raman intensity and electrodeposited time at the potentials of −0.2V.Fig.6.SERS spectra of different concentrations of R6G aqueous solution collected on the ITO-rGO/Ag NPs substrate (A)and on the ITO–Ag substrate(B).Fig.7.SERS spectra of 10−7M R6G in the 5spots collected on single ITO-rGO/Ag NPs substrate (A)and five ITO-rGO/Ag NPs substrate (B).under same laser power,and the Raman spectra were shown in Fig.7A.Obviously,SERS spectra of R6G were enhanced greatly at each point,and the difference among the five SERS signal inten-sities was small with relative standard deviation (RSD)of 9.3%at the 1642cm −1.To evaluate the reproducibility of the ITO-rGO/Ag NPs substrate,five ITO-rGO/Ag NPs substrates were fabricated under identical experimental conditions.The SERS spectra of R6G molecules with a concentration of 10−7M on these five ITO-rGO/Ag NPs substrates were shown in Fig.7B.The RSD were 15.2%for substrate-to-substrate.The results indicated that the substrates had excellent reproducibility and homogeneity.4.ConclusionsIn summary,a new ITO-rGO/Ag NPs hybrid SERS substrate was fabricated by two-step chronoamperometry electrodeposi-tion route.The distribution density of Ag NPs on rGO surface was depended on Ag NPs growth electrodeposition pared to ITO–Ag NPs substrate,the as-prepared ITO-rGO/Ag NPs SERS sub-strates exhibited better SERS-active,and detectable concentration for R6G were lowered to 10−11M and the EF value were raised to 5.9×108,owing to the plenty of “hot spots”as well as the chemi-cal enhancement and strong adsorption of rGO.More importantly,810R.Wang et al./Applied Surface Science349(2015)805–810large-scale production of ITO-rGO/Ag NPs SERS substrate could be easily realized by the proposed fabrication strategy with good reproducibility and homogeneity.The as-prepared ITO-rGO/Ag NPs SERS substrates would have great potential applications in detec-tion of chemistry and biomolecules.AcknowledgementsThis work wasfinancially supported by the National Natural Science Foundation of China(nsfc:21375156),Ministry of Sci-ence and Technology863Plan(2015AA021104,2015AA021107), key project of central university basic scientific research business 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