Journal of Alloys and Compounds 615 (2014) 843–848 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom Photochemical decoration of silver nanoparticles on graphene oxide nanosheets and their optical characterization Nguyen Thi Lan a, Do Thi Chi a, Ngo Xuan Dinh a, Nguyen Duy Hung a, Hoang Lan a, Pham Anh Tuan b, Le Hong Thang c, Nguyen Ngoc Trung d, Nguyen Quang Hoa e, Tran Quang Huy f, Nguyen Van Quy g, Thanh-Tung Duong h, Vu Ngoc Phan a, Anh-Tuan Le a,⇑ a Department of Nanoscience and Nanotechnology, Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Nam b Vietnam Metrology Institute, 08 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Viet Nam c School of Materials Science and Engineering, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Nam d School of Engineering Physics, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Nam e Department of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam f Laboratory for Ultrastructure and Bionanotechnology (LUBN), National Institute of Hygiene and Epidemiology (NIHE), No Yecxanh Street, Hai Ba Trung District, Hanoi, Viet Nam g International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Nam h Department of Materials Engineering, Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea a r t i c l e i n f o Article history: Received 24 April 2014 Received in revised form 13 June 2014 Accepted July 2014 Available online 11 July 2014 Keywords: Ag-GO nanohybrid Green synthesis Optical properties a b s t r a c t Nanohybrid materials based on silver nanoparticles (Ag-NPs) and graphene oxide (GO) are attracting considerable research interest because of their potential many applications including surface-enhanced Raman scattering, catalysis, sensors, biomedicine and antimicrobials In this study, we established a simple and effective method of preparing a finely dispersed Ag-GO aqueous solution using modified Hummer and photochemical technique The Ag-NPs formation on GO nanosheets was analyzed by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy The average size of Ag-NPs on the GO nanosheets was approximately 6–7 nm with nearly uniform size distribution The Ag-GO nanohybrid also exhibits an adsorption band at 435 nm because of the presence of Ag-NPs on the GO nanosheets Photoluminescence emission of the Ag-GO nanohybrid was found at 400 and 530 nm, which can be attributed to the interaction between the luminescence of exploited GO nanosheets and localized surface plasmon resonance from metallic Ag-NPs The observed excellent optical properties of the as-prepared Ag-GO nanohybrid showed a significant potential for optoelectronics applications Ó 2014 Elsevier B.V All rights reserved Introduction Graphene, which consists of a one-atom-thick sheet of sp2bonded carbon atoms in a hexagonal two-dimensional lattice, is attracting considerable research interests because of its remarkable physicochemical properties Such properties include a high specific surface area, mechanical strength, and thermal and electrical conductivities, as well as extraordinary electronic properties and electron transport capabilities [1] These excellent properties make graphene a promising nanomaterial for various technological applications, ranging from biosensor, energy to optoelectronic devices [1] ⇑ Corresponding author E-mail address: tuan.leanh1@hust.edu.vn (A.-T Le) http://dx.doi.org/10.1016/j.jallcom.2014.07.042 0925-8388/Ó 2014 Elsevier B.V All rights reserved A specific class of graphene research deals with graphene oxide (GO), GO sheets are chemically synthesized graphene sheets that are modified with oxygen-containing functional groups Oxygenated groups in GO can strongly affect the electronic, mechanical, and electrochemical properties of GO, thereby resulting in differences between GO and pristine graphene In comparison with the pristine graphene, the existence of these oxygen functional groups can also provide advantages such as hydrophilicity and controllable electronic properties for using GO in various technological applications [2–4] Silver nanoparticles (Ag-NPs) are attractive objects for the scientific community in materials science because Ag-NPs posses many advantages such as good conductivity, catalytic and widespectrum antimicrobial activity against various micro-organisms and localized surface plasmon resonance (LSPR) effect [5,6] 844 N.T Lan et al / Journal of Alloys and Compounds 615 (2014) 843–848 To explore the combined advantageous properties of Ag-NPs and GO sheets, Ag-GO nanohybrids have been intensively studied [7,8] The Ag-NPs have an important role in many applications such as surface-enhanced Raman scattering, catalyst, and sensors, as well as biomedical and antimicrobial applications Insertion of Ag-NPs into the GO nanosheet is important for further exploration of Ag-NPs properties and applications For example, Wei et al [9] reported that introduction of Ag-NPs into GO sheets indicate that the antibacterial performance of Ag-GO nanohybrids were enhanced compared with Ag-NPs and GO materials alone The Ag-GO nanohybrids also show non-toxic effect on rat skin [9] Other reports [10–12] showed excellent antimicrobial activity for Ag-GO nanohybrids To date, several solution-based routes have been developed to synthesize the Ag-NPs on the GO nanosheets such as microwave irradiation, hydrogen reduction in supercritical CO2, surfacemodification method using thiol groups, and citrate-modified chemical reduction, [7–12] However, some challenges and problems remain in preparing highly dispersed metallic Ag nanoparticles of regular size on GO nanosheets and in controlling stable dispersions of Ag-GO suspension in aqueous solution because of Ag-GO agglomeration To overcome this problem, we introduce a simple and effective method for preparation of Ag-GO nanohybrids via a two-step process, in which aqueous dispersions of GO nanosheets are produced using a modified Hummer technique and the Ag-NPs are then decorated on GO nanosheets by a photochemical technique In this study, we demonstrate an easy synthesis method for effective decoration of the Ag-NPs on the GO nanosheet using modified Hummer and photochemical techniques UV irradiation was used to improve the uniform dispersions of Ag-NPs on the GO nanosheets during the reduction process by glucose with oleic acid as a capping agent The analyzed results suggest that presence of Ag-NPs on the surface of GO nanosheets and the interaction between Ag-NPs and functional groups on the edge of the GO nanosheets were ascribed to the electron transfer from metallic Ag to the GO nanosheets Two emission peaks in photoluminescence of Ag-GO nanohybrids were also observed at 400 and 530 nm, which are attributed to the interaction between luminescence of exploited GO nanosheets and localized surface plasmon resonance from metallic Ag-NPs Photoluminescence intensity of Ag-GO nanohybrid increased at peak $400 nm with increasing concentration of Ag-NPs because of surface plasmon-enhanced luminescence Experimental procedures 2.1 Chemicals Analytical-grade silver nitrate (AgNO3, 99.9%), sodium hydroxide (NaOH), ammonium hydroxide (NH3, 25%), potassium permanganate (KMnO4, 99.9%), hydrogen peroxide (H2O2, 30%), sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), nitric acid (HNO3, 63%), oleic acid, and glucose that were used in this study were purchased from Shanghai Chemical Reagent Co Ltd Graphite (nature coal powder) was fabricated from coal in Vietnam 2.2 Synthesis of graphene oxide (GO) by modified Hummer method First, GO nanosheets were synthesized from coal powder by modified Hummer method as described previously [13] Briefly, g of coal powders were mixed with HNO3 and KMnO4 at a volume ratio of 1:2:1.5, respectively, and then the mixture were converted to exploited graphite (EG) under microwave 800 W for In this reaction, the mixture of g of EG, g of KMnO4, and g of NaNO3 was added slowly to 160 mL of 98% H2SO4 at °C in ice-water bath and then stirred for 30 Ice-water bath was removed, and then temperature was increased slowly to 45 °C and continuously stirred for h Deionized water was added slowly to the mixture which was stirred until purple fumes were inhibited By increasing reaction temperature to 95 °C and stirring the mixture for h, the resulting product of the GO nanosheets was obtained with yellow–brown color The GO nanosheets were then treated by H2O2 30% and HCl 10% solution to eliminate KMnO4, MnO2, and other metal ions that remained in the GO solution The final GO products were purified by filtering, washing several times by ultrasonic vibration, centrifugation with deionized water, and removal of ultrafine carbon powder that was not oxidized 2.3 Synthesis of Ag-GO nanohybrid by modified photochemical method The Ag-NPs were then deposited on the GO nanosheets by modified Tollens process as reported elsewhere [14] Fig shows the schematic of a two-step process to synthesize the Ag-GO nanohybrid In a typical experiment, 1.7 g (10 mmol) of AgNO3 was dissolved in 100 mL of deionized water The AgNO3 solution was then precipitated with 0.62 g (15.5 mmol) of sodium hydroxide (Aldrich, >99%) The obtained precipitate, which is composed of Ag2O, was filtered and dissolved in 100 mL of aqueous ammonia (0.4% w/w, 23 mmol) until a transparent solution of silver ammonium complex [Ag(NH3)2]+(aq) formed Up to 2.5 g (8.9 mmol) of oleic acid was then added dropwise into the complex, and the resulting solution was gently stirred for h at room temperature until the complete homogeneity of the reaction mixture was achieved As to the synthesis of Ag-GO nanohybrid, resulting complex mixture was mixed with GO suspension (3 mg/mL) while stirring for 30 and followed by the addition of g (11.1 mmol) of glucose The reduction process of the silver complex solution (in quartz glass) was initiated with UV irradiation A UV lamp (k = 365 nm, 35 W) was used as a light source to stimulate the reduction process After 12 h of UV irradiation, the Ag-NPs were deposited on the GO nanosheets to form the Ag-GO nanohybrid 2.4 Characterization techniques Transmission electron microscopy (TEM, JEOL-JEM 1010) was conducted to determine the morphology and distribution of the Ag-NPs on the GO nanosheets The samples for TEM characterization were prepared by placing a drop of colloidal solution on a formvar-coated copper grid that was dried at room temperature The composition of the Ag-GO nanohybrid was characterized by energy-dispersive X-ray (5410 LV JEOL) The crystalline structure of the prepared Ag-NPs and Ag-GO nanohybrid was analyzed by X-ray diffraction (XRD, Bruker D5005) using Cu Ka radiation (k = 0.154 nm) at a step of 0.02° (2h) at room temperature The background was subtracted using linear interpolation method The chemical functional groups of GO and Ag-GO were characterized using FTIR measurements, samples were collected with one layer coating in potassium bromide and compressed into pellets, and spectra in the range of 400–4000 cmÀ1 were recorded with Nicolet 6700 FT-IR instrument Raman measurement was conducted using 633 nm of HeANe laser excitation The UV–vis absorbance spectra were recorded using a HP 8453 spectrophotometer, and the absorption spectrum of all suspension samples in 10 mm path length quartz cuvettes was 300–900 nm The photoluminescence spectra of GO, Ag, and Ag-GO were measured using Nanolog, Horiba The photoluminescence spectra were obtained with 300 nm excitation Results and discussion 3.1 Formation of GO nanosheet and Ag-GO nanohybrid Fig shows the TEM images of (a) Ag-NPs and (b–d) Ag-GO nanohybrids at different magnifications The Ag-NPs are finely dispersed (Fig 2a), the average size of the Ag-NPs was $5 nm (see inset of Fig 2a) No aggregation of silver particles was also observed, indicating the important role of UV irradiation for controlling stably uniform dispersions in Ag-NP synthesis process Fig 2b–d clearly show the presence of a large number of Ag-NPs that are anchored to the GO surfaces The adhered nanoparticles have quasi-spherical morphologies and are dispersed uniformly on the GO nanosheets In these TEM images, most nanoparticle diameters are $7 nm (see inset of Fig 2a) The wrinkles of the GO nanosheets (Fig 2c) are also observed, revealing that the GO nanosheets are thin Based on TEM analysis, no aggregation of Ag-NPs is found on the surface of GO nanosheets The small sizes and fine dispersions of Ag-NPs on GO nanosheets enable potential for various technological applications The formation of the Ag-NPs on GO nanosheets is further confirmed by XRD analysis Fig shows the XRD patterns for GO nanosheets and GO-Ag nanohybrid samples The GO nanosheets exhibited a broad peak at 10.9° corresponding to the (0 2) interlayer spacing of 0.81 nm, which indicates that the ordinal structures of graphite have been exploited and that oxygen-containing functional groups have been inserted into the interspaces After N.T Lan et al / Journal of Alloys and Compounds 615 (2014) 843–848 845 Fig A schematic protocol for a two-step process to synthesize the Ag-GO nanohybrid Fig TEM images of (a) Ag-NPs and (b–d) Ag-GO nanohybrids at different magnifications decorating the Ag-NPs, three distinct diffraction peaks appear at 2h = 38.2°, 44.4°, and 64.5°, which correspond to the (1 1), (2 0), and (2 0) crystalline planes of metallic Ag (JCPDS No 040783) These observations confirm that the metallic Ag-NPs are effectively anchored to the surface of GO nanosheets TEM and XRD analyses revealed that the GO, Ag-NPs, and Ag-decorated GO nanosheets were formed These obtained results suggest that the Ag-NPs are successfully decorated on the GO nanosheets using two-step process In the present study, the mechanism for Ag-GO formation can be understood as follows: after mixing silver ammonia complex with GO nanosheets, the positively charged Ag[(NH3)2]+ can be easily attached to the negatively charged oxygen functional groups on the GO When Ag-GO formation occurs by adding glucose to the mixture, the aldehyde groups of glucose release electrons to reduce silver ammonia complex into silver nanoparticles The Ag-NPs can be deposited into the GO nanosheets because of the electrostatic interaction between silver ammonia complex and GO nanosheets UV irradiation is performed during the reduction process to control uniform dispersions of Ag-NPs on the GO nanosheets 846 N.T Lan et al / Journal of Alloys and Compounds 615 (2014) 843–848 Fig XRD patterns for GO nanosheets and GO-Ag nanohybrid samples Fig Raman spectra of (a) GO and (b) Ag-GO nanohybrids On the basis of TEM and XRD studies as well as earlier reports [15,16], a possible mechanism of silver nanoparticle formation and growth under the applied experimental conditions was suggested The UV irradiation causes excitation of [Ag(NH3)2]+ ions followed by electron transfer from the glucose molecule to Ag+, thus producing Ag0 atoms which then form clusters and seeds: GO nanosheets [2,3] However, a noticeable decrease in the intensity of the adsorption bands of the oxygenated functional groups was found in the FTIR spectrum of the Ag-GO nanohybrid This finding results mainly from both presence of the Ag-NPs on the surface of GO nanosheets and a slight reduction of GO by glucose during the synthesis process of Ag-GO The decrease of OAH stretch absorption intensity in the hybrids is attributed to interactions between silver ions and hydroxyl group of GO The variation of the other peak in the case of Ag-GO demonstrates the interaction between silver ions and oxygen functional groups on both basal planes (hydroxyl group OH) and edges (carboxyl group CAOH) of the GO nanosheets through the formation of a coordination bond or through simple electrostatic attraction The FTIR results demonstrate that the GO nanosheets have been successfully exfoliated, and strong interactions may exist between Ag-NPs and the remaining hydroxyl and carboxyl groups on the surface of the GO Fig shows the Raman spectra of (a) GO and (b) Ag-GO nanohybrids For the case of GO (Fig 5), two characteristic prominent peaks were observed at 1360 cmÀ1 (D band) and at 1591 cmÀ1 (G band) Compared with GO, the Raman spectra of Ag-GO indicates that the D band and the G band are slightly shifted to 1338 and 1595 cmÀ1, respectively The D band represents edges, other defects, and disordered carbon because of vibration of sp3-bonded carbon atoms and impurities, whereas the G band arises from the zone center E2g mode, assigning to the ordered sp2-bonded C atoms A significant frequency shift (about 22 cmÀ1) toward a smaller wavenumber of the D-band is found in Ag-GO sample compared with the GO indicating a higher level of disorder of the graphene layers and increased numbers of defects because of the partial reduction of GO by glucose during the synthesis of the Ag-GO nanohybrid The spectra showed that the carbon framework of GO is modified by reduction reaction process of Ag-NPs This finding is consistent with the FTIR result and that from previous works [7,10] Besides, the ratio of intensity of the D band to that of the G band (ID/IG) also increased The ID/IG values are approximately 0.77 and 0.92 for GO and GO-Ag respectively The ratio value of ID/IG represents the degree of disorder and the average size of the sp2 domains The observed increasing ID/IG-value suggested a decrease in in-plane size of graphene upon the reduction process The partial reduction of GO could cause fragmentation along the reactive sites and might yield new graphitic domains, leading to smaller sizes and higher number of graphene than that of GO before the reduction [7,11] In the present study, the higher increased ID/IG-value of the Ag-GO nanohybrid than that of the GO is likely attributed to the surface-enhanced Raman scattering from the intense local electromagnetic fields of Ag-NPs that accompanies plasmon resonance effect The FTIR and Raman results suggest that the attachment hv _ ẵAgNH3 ị2 ỵ ỵ RCHOH ! Ag0 ỵ 2NH3 ỵ Hỵ ỵ RCOH nAg0 ! Agn ị0 ; where RCHOH represents glucose in cyclic form The use of UV irradiation leads to the substantially simultaneous formation of a large amount of silver nuclei which then started to grow This situation results in small dimensions and stably uniform dispersions of the finally obtained silver NPs on the GO nanosheets The remaining silver ions are adsorbed on the surface of already formed nanoparticles and attract oppositely charged oxygen functional groups on the GO sheets through an electrostatic interaction to keep the reduced silver nanoparticles staying on the GO [11] 3.2 Chemical groups in GO and Ag-GO nanohybrid To elucidate the chemical attachment of Ag-NPs on the GO nanosheets, FTIR and Raman analyses were conducted Fig shows the FTIR spectra of (a) GO and (b) Ag-GO nanohybrids For the case of GO (Fig 4), the presence of adsorption bands at 3493 cmÀ1 corresponds to the AOH stretching vibration Other peaks of oxygen functional groups were also detected including CO2 groups at 2359 cmÀ1, C@C bonding of aromatic rings of the GO carbon skeleton structure at 1647 cmÀ1, and OAH deformations of the CAOH groups at 1383 cmÀ1 These oxygen functional groups could be located on both basal planes and edges of the Fig FTIR spectra of (a) GO and (b) Ag-GO nanohybrids N.T Lan et al / Journal of Alloys and Compounds 615 (2014) 843–848 847 of Ag-NPs on the GO nanosheets and the interaction between the Ag and the functional groups of the GO nanosheets were ascribed to the electron transfer from the metallic Ag to the GO nanosheets 3.3 Optical characterization of GO, Ag, and Ag-GO To explore optical characterizations of the prepared Ag-GO nanohybrid, we conducted UV–vis and photoluminescence (PL) analyses Fig shows (a) the UV–vis spectra of GO, Ag-NPs, and Ag-GO nanohybrid and (b) the UV–vis spectra of Ag-GO nanohybrid at different Ag concentrations One peak (Fig 6a) at 305 nm comes from n ? p* transitions of C@O bond in sp3 hybrid regions, and another prominent peak at $393 nm is ascribed to the CAOH bond, whereas the presence of absorption peak at $726 nm is attributed to band edge absorption feature [17] Obviously, the Ag-NP and AgGO samples display strong absorption peaks at 428 and 435 nm, respectively, because of the surface plasmon resonance (SPR) effect of Ag-NPs The appearance of characteristic surface plasmon band at 435 nm indicates the formation of Ag-NPs on GO nanosheets The SPR phenomenon occurs when the incident light interacts with valence electrons at the outer band of Ag-NPs, leading to oscillation of electrons along with the frequency of the electromagnetic source [18] However, the absorption band is shifted to longer wavelength with increased concentration of Ag-NPs (Fig 6b) The shifting of the absorption peak toward longer wavelength for higher concentration of Ag-NPs indicates the formation of larger Ag nanoparticles with different shapes and sizes [8,18] The surface plasmon band shifts are strongly dependent on particle size, shape, chemical surrounding, and adsorbed species on the surface and dielectric medium, whereas the plasmon peak and full width at half maximum depends on the extent of colloid aggregation [18,19] Fig shows (a) the PL emission spectra of GO, Ag-NPs, and Ag-GO nanohybrid and (b) the PL emission spectra of Ag-GO Fig (a) The PL emission spectra of GO, Ag-NPs, and Ag-GO nanohybrid and (b) the PL emission spectra of Ag-GO nanohybrid at different concentration of Ag nanohybrid at different Ag concentrations The PL emission spectra of GO aqueous suspension show two emission peaks at 412 and 530 nm Previous experiments verified that GO fluorescence is due to electron–hole recombination from conduction band bottom and nearby localized electronic states to wide-range valance band In view of atomic structure, the GO emission is predominantly resulting from electron transitions among/between the nonoxidized carbon region (AC@CA) and the boundary of oxidized carbon atoms (CAO, C@O, or O@CAOH) [15,20] The Ag-NP aqueous suspension also displays a maximum emission peak at 400 nm The visible luminescence of Ag-NP colloid is ascribed to excitation of electrons from occupied D bands into states above the Fermi level Subsequent electron–phonon and hole–phonon scattering processes lead to energy loss and finally photoluminescent–radiative recombination of an electron from an occupied sp band with the hole [18] PL spectrum of Ag-GO showed two emission peaks at 400 and 530 nm, which were attributed to the interaction between luminescence of atomically layered GO and localized surface plasmon resonance from metallic Ag-NPs Compared with the GO, the first peak position is shifted to 400 nm, and emission of peak at 400 nm is attributed to the plasmon resonance of Ag-NPs, whereas the second peak did not change at $530 nm With increasing concentration of Ag-NPs, the PL intensity of Ag-GO was also increased at peak $400 nm because of surface plasmon-enhanced luminescence [21,22] The effect of surface plasmon interaction of the Ag-NPs with the GO surface was also probed by UV–vis observation The signal at about 332 nm in the PL spectra is the Raman signal of water, whereas the signals at $606 and $668 nm are attributed to the second mode of lamp and water, respectively Conclusions Fig (a) The UV–vis spectra of GO, Ag-NPs, and Ag-GO nanohybrid and (b) the UV–vis spectra of Ag-GO nanohybrid at different concentration of Ag An easy and effective method of preparing Ag-GO aqueous solution was presented The fine dispersions of Ag-NPs on the GO 848 N.T Lan et al / Journal of Alloys and Compounds 615 (2014) 843–848 nanosheets were altered by tuning the UV irradiation The results also indicated that attraction of Ag-NPs on the GO nanosheets and the interaction between the Ag-NPs and the functional groups of the GO nanosheets were ascribed to the electron transfer from the metallic Ag to the GO nanosheets The Ag-GO emission was predominantly caused by the interaction between luminescence of atomically layered GO and localized surface plasmon resonance from the metallic Ag-NPs Acknowledgements This work was supported by Vietnam’s National Foundation for Science and Technology Development (NAFOSTED) through a fundamental research project (Code: 103.44-2012.60) The authors would like to thank P.T Huy at AIST for providing GO samples and also thanks to N.D Cuong at AIST for proof reading and useful discussions References [1] K.S Novoselov, A.K Geim, S.V Morozov, D Jiang, Y Zhang, S.V Dubonos, I.V Grigorieva, A.A Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669; A.K Geim, K.S Novoselov, The rise of graphene, Nat Mater (2007) 183–191 [2] Daniela C Marcano, Dmitry V Kosynkin, Jacob M Berlin, Alexander Sinitskii, Zhengzong Sun, Alexander Slesarev, Lawrence B Alemany, Wei Lu, James M Tour, Improved synthesis of graphene oxide, ACS Nano (2010) 4806–4814 [3] Da Chen, Hongbin Feng, Jinghong Li, Graphene oxide: preparation, 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bonding of aromatic rings of the GO carbon skeleton structure at 1647 cmÀ1, and OAH deformations of the CAOH groups at 1383 cmÀ1 These oxygen functional groups could be located... interaction between silver ions and oxygen functional groups on both basal planes (hydroxyl group OH) and edges (carboxyl group CAOH) of the GO nanosheets through the formation of a coordination bond