Multifunctional Reduced Graphene Oxide Wrapped Circular Au

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Cite This: ACS Appl. Nano Mater. 2018, 1, 3945−3955

Multifunctional Reduced Graphene Oxide Wrapped Circular Au Nanoplatelets: Enhanced Photoluminescence, Excellent SurfaceEnhanced Raman Scattering, Photocatalytic Water Splitting, and Non-Enzymatic Biosensor Sumit Majumder,*,†,‡ Biswarup Satpati,‡ Sanjay Kumar,*,† and Sangam Banerjee‡ †

Department of Physics, Jadavpur University, Kolkata 700032, India Surface Physics and Materials Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India

ACS Appl. Nano Mater. 2018.1:3945-3955. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/28/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Herein, we demonstrate the synthesis and multifunctional properties of reduced graphene oxide (RGO)-wrapped Au nanoplatelets. We have characterized the sample by field emission scanning electron microscope (FESEM), high-resolution transmission electron microscope (HRTEM), energydispersive X-ray spectroscopy (EDS), high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF), electron energy-loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS) studies. It has been shown that the RGO wraps a large number of 2D circular Au nanoplatelets (diameter ∼15 nm). We have examined the optical property of the sample using Raman, UV−vis, and PL spectroscopic techniques. Large enhancement in intensity of Raman spectra was observed due to the surface enhanced Raman scattering (SERS) resulting from the Au nanoplatelets. The collective sway of surface plasmon resonance and fluorescence resonance energy transfer effect owing to Au gives rise to giant enhancement in intensity of photoluminescence emission spectrum. Upon visible-light irradiation, photocurrent flows through the sample due to interband 6sp transition within the Au nanoplatelets and it exhibits photocatalytic water splitting effect. The sample displays excellent nonenzymatic hydrogen peroxides (H2O2) and ascorbic acid (AA) sensing property. The value of sensitivity for H2O2 is 280.28 μA mM−1 cm−2 in the linear range of 1 μM to 0.8 mM and that for AA is 314.07 μA mM−1 cm−2 in the linear range of 25−300 μM. The lowest detection limit of both H2O2 and AA is 6.8 μM at S/N= 3. So, the sample can be used for multifunctional applications in SERS as substrate, photocatalytic water splitting, photodetectors, and nonenzymatic biosensing. KEYWORDS: RGO-wrapped circular 2D Au plasmonic nanoplatelets, giant PL enhancement, interfacial effect, SPR, visible-light-assisted photocatalysis, nonenzymatic biosensor High-quality fluorescent materials can be developed by noble metal NPs like Pt, Pd, Au, and Ag because of their outstanding light absorption property.14−16 Actually, metal and metal oxide based nanocomposites having widespread applications as gas sensor, biosensor and catalyst exhibit outstanding mechanical, optical, electrical, magnetic and photocatalytic properties.17−21,3 In case of noble metal based nanocomposites, energy transfer from metal surface to material in contact with the metal by surface plasmon resonance (SPR) process caused by collective oscillation of electrons through illumination of light wave of specific frequency appreciably improves the optical characteristics of those materials and leads to enhancement in intensity of their Raman and photoluminescence (PL) spectra.22−25

1. INTRODUCTION In the recent past, multifunctional nano/microstructural materials and their core−shell structures have attracted considerable attention due to their large-scale prospective applications.1−7 Graphene, low-cost, sp2-bonded, two-dimensional sheets of carbon atoms, has received enormous attention in recent years.8−11 Graphene exhibits large specific surface and it is biocompatible. It also possesses strong mechanical strength and excellent thermal and electrical conductivity. In contrast, noble metal nanostructures, a special class of functional materials, shows exceptional physical properties.11−13 Interestingly, noble metal nanoparticles (NPs) and graphene composites not only unite the qualities of each component but exhibit fascinating structural, electromagnetic, and electrochemical properties that are pole apart from their ingredients.11 In this context, we have synthesized reduced graphene oxide (RGO)-wrapped noble metal (Au) nanoplatelets to explore its multifunctionality. © 2018 American Chemical Society

Received: May 2, 2018 Accepted: July 10, 2018 Published: July 10, 2018 3945

DOI: 10.1021/acsanm.8b00730 ACS Appl. Nano Mater. 2018, 1, 3945−3955

Article

ACS Applied Nano Materials

sample shows large enhancement in the intensity of Raman peaks, which can be attributed to the surface enhanced Raman scattering (SERS) produced by the Au nanoplatelets. The sample exhibits giant enhancement of PL emission intensity with respect to GO and RGO due to increase in radiative recombination rate because of the presence of surface states modified by Fluorescence resonance energy transfer (FRET) and coupling between FRET and SPR mechanism.3 Electron accrual instigated by interfacial charge transfer in RGO wrapped Au nanoplatelets system also plays an important role here.3 The sample exhibits visible-light-assisted photocatalytic water splitting property and it displays excellent nonenzymatic hydrogen peroxide (H2O2) and ascorbic acid (AA) sensing activity.

In the literature, a plethora of reports are available on fabrication of RGO-Au biosensors and they exhibit excellent biosensing activity.26−31 Accurate, selective and susceptible detection of hydrogen peroxide (H2O2) and ascorbic acid (AA) is of immense importance in clinical diagnosis, biopharmaceuticals and industrial fields.32−35 Enzyme free amperometric sensing has evolved as an alternative choice for detection of H2O2 and AA as it not only removes the demerits of enzyme-based sensors like complicated fabrication technique and low thermal/chemical stability but also because of its excellent sensitivity, good selectivity, very short response time and low cost of instrumentation.5 Diabetes can be easily diagnosed through detection of H2O2. On the other hand, H2O2 is corrosive and toxic agent that causes irreversible cell damages and triggers cancer, neurological, and cardiovascular disorder.34,5 Further, AA is a very good antioxidant. Vegetables and citrus fruits have good amount of AA. It is mainly used for getting relief from cold, hepatic disease and cancer.35,36 AA is an essential nutrient that helps in the repair of tissue. Normal doses of AA are safe during pregnancy but large doses may cause gastrointestinal upset, headache, sleeping trouble, and flushing of the skin. A few research groups have worked on reduced graphene oxide (RGO)−gold (Au) syetems.37−40 The majority of researchers have used cumbersome method for the synthesis RGO-Au nanoparticle system and they have used attractive linkers to form bonds between graphene oxide (GO) and Au nanoparticles. The method adopted by us for synthesis of RGO-wrapped Au nanoplatelets is very simple and we have not used any linker or bonding molecules. Au/rGO nanocomposites can be utilized as efficient SERS substrate to monitor plasmon-driven surface catalytic reaction for formation of DMAB from 4-nitrobenzenethiol and signature of enhancement in reactivity is found in the time-dependent SERS spectra of the sample.41 Lv et al. have successfully synthesized hemin-graphene-gold nanocomposite, which exhibits excellent peroxidase-like activity.42 Amine-functionalized RGO-Au nanorods embedded in silicate matrix exhibit very good electrochemical activity toward sensing of nitric oxide (NO).43 Qin et al., have synthesized RGO-Au nanocomposite and studied H2O2, glucose and hydrazine sensing activity of this sample.44 According to literature survey, only a few reports are available on visible-light-assisted photocatalytic activity of graphene and Au-based nanocomposites45−50 and there is no report on visible-light-induced photocatalysis of RGO-wrapped Au nanoplatelets. In this background, investigation on optical property, photocatalytic response, and nonenzymatic amperometric sensing activity of RGO-wrapped Au nanoplatelets appears to be very much attractive. We have successfully fabricated novel RGO-wrapped circular 2D Au nanoplatelets by simple chemical process without using any surfactant and characterized it systematically by field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), field-emission transmission electron microscopy (EFTEM), energy-dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), thickness mapping, and X-ray photoelectron spectroscopy (XPS). We have examined the optical property of the sample using Raman, UV−vis, and PL spectroscopic techniques. We have extensively investigated the electrochemical sensing property of the sample toward sensing of H2O2 & AA along with its visible light irradiated photocatalytic water splitting activity. The

2. EXPERIMENTAL SECTION Materials. Graphite powder, sulfuric acid (98 wt %), hydrogen peroxide (30%), sodium nitrate (NaNO3), potassium permanganate (KMnO4 ), chlorauric acid (HAuCl 4), and trisodium citrate (Na3C6H5O7) were purchased from loba chemicals. All these reagents were used without further purification. Preparation of GO Solution. GO was prepared by chemical exfoliation of the natural flake graphite by a modified Hummer’s method. The mass concentration of the obtained aqueous GO dispersion was estimated to be 2 mg/10 mL. The GO synthesized by this method is sometime few layers instead of a single layer. Synthesis of RGO-Wrapped Circular 2D Au Nanoplatelets. The RGO wrapped circular Au nanoplatelets were synthesized by boiling a mixture of 5 mg of GO suspended in 1 mM HAuCl4 (25 mL) solution and 20 mL of sodium citrate (23 wt %) added with 100 mL of doubly distilled water following the method described elsewhere.51 Synthesis of RGO. We have prepared RGO by following the same method of synthesis of RGO-Au without using 1 mM HAuCl4 (25 mL) solution. Characterization Techniques. The morphological characterization of samples was performed by FEI, INSPECT F50 fieldemission scanning electron microscope (FESEM), and FEI, TF30, ST high resolution transmission electron microscope (HRTEM) equipped with Fischione (model: 3000) dark-field detector. The HRTEM was operated at 300 kV. Purity of the samples was checked by probing the chemical composition using BRUKER EDS system attached with the FESEM and HRTEM equipment. The presence of oxygen functional groups on the surface of samples was probed by EELS study using a Gatan Quantum SE (model: 963 SE) post column equipment attached with HRTEM instrument. The atomic force microscopic (AFM) images of the samples were recorded by a Brukar Multimode 8 instrument. The valence state of constituent elements of the samples was determined by Omicron Multiprobe XPS system equipped with monochromatic Al Kα X-ray source (1486.7 eV, model: XM 500). The UV−vis and PL data of GO, RGO and RGOAu were recorded by using JASCO V-630 spectrophotometer and JASCO FP-6700. The Raman spectra of GO, RGO and RGO-Au were recorded by using WITEC alpha 300R equipment coupled with a diode laser source of wavelength 532 nm. For Raman spectroscopic study, a thin film of the sample over Si substrate developed by the drop cast technique was used to reduce the noise level.1 Electrochemical performance of RGO- Au sample was evaluated by using a CHI 660C (CH Instruments, USA) electrochemical workstation. Details of working electrode fabrication and methodology adopted for electrochemical experiment has provided in our earlier paper.5

3. RESULTS AND DISCUSSION Growth Mechanism. The detailed procedure of fabrication of RGO wrapped circular 2D Au nanoplatelets is schematically illustrated in Figure 1. The mixture of KMnO4, H2SO4 and graphene was vigorously ultrasonicated to oxidize 3946


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sample (Figure 2b). However, this aggregation of Au nanoplatelets will not influence the catalytic and biosensing activity of the samples significantly because both catalysis and biosensing experiments are performed by dispersing the sample in liquid medium. The EDS profiles of the GO and RGO-wrapped Au in the energy range from 0 to 3 keV recorded by EDS equipment attached with FESEM and TEM instruments are shown in Figure S1 and Figure S2, respectively. The X-ray peaks for only carbon (C) and oxygen (O) are obtained in the EDS profile of GO. The X-ray peaks corresponding to C, O, and Au have been observed in EDS survey spectrum of RGO-wrapped Au sample. The signal for O in the EDS profile of RGO wrapped Au is very low as here GO has been reduced to RGO. The analysis of EDS profile has revealed that the weight and atomic percentages of Au in RGO wrapped Au are 6.94 and 0.45%, respectively. The TEM EDS mapping (Figure S3) of RGO wrapped Au comprising of STEM image, Au-L map, C−K map, and combined map of RGO-Au, confirms formation of RGO-Au nanocomposite. No signal linked with probable impurities has been observed in EDS pattern of the samples and this confirms that both samples are chemically pure. TEM, HRTEM, HAADF-STEM, Elemental Mapping, and Thickness Mapping Study. TEM image of bare freestanding graphene oxide (GO) nanosheets is presented in Figure 3. The high-angle annular dark-field scanning trans-

Figure 1. Schematic diagram of growth mechanism of few-layered RGO-wrapped circular 2D Au nanoplatelets.

graphene and this leads to formation of multilayer graphene oxide (GO) flakes. After that, we have prepared ultrathin few layered or single layered GO by ultrasonication of multilayered GO flakes. When graphene oxide sheet (GO), having oxygen containing functional groups like carbonyl (CO), epoxy (C−O−C), carboxyl (COOH), or hydroxyl (OH) attached at the surfaces and edges of GO sheet comes in contact with Au nanoparticles then, it offers a large contact surface for Au nanoparticle and serves as the nuclei for crystalline growth of Au. During this period, GO gets further reduced and becomes RGO. The active sites in RGO help in fastening Au nanoparticles and this leads to formation of 2D Au discs. Finally, RGO wraps the circular 2D Au nanoplatelets. The entire scenario of formation of RGO wrapped circular 2D Au nanoplatelets from graphite powders and HAuCl4 is depicted in Figure 1. The active RGO sheets of the RGO-wrapped Au nanoplatelets may be beneficially used as catalytic sites. FESEM and EDS Study. The morphology of the synthesized GO and RGO wrapped Au nanoplatelets are characterized by SEM study. The FESEM micrographs of GO and RGO wrapped Au nanoparticles are presented in Figure 2a, b, respectively. Here, FESEM image shows GO sheet with

Figure 3. (a, b) STEM-HAADF image, (c) HRTEM image, and (d) SAED pattern of few-layered graphene oxide (GO) nanosheets.

mission electron microscopic (HAADF-STEM) images illustrating stacked and ripple like structure of GO are depicted in Figure 3a, b. Because of such structure, GO will provide a high surface area for loading of Au nanoplatelets. Although it is difficult to distinguish graphene oxide from the background because the monolayer/few-layer graphene sheet is extremely thin, we can still identify the transparent graphene oxide sheets from the fringe and wrinkles. Figures 3c, d depict HRTEM image and SAED pattern of GO nanosheet, respectively. As per the HRTEM image the lattice constant of GO is 0.47 nm. From the SAED pattern, it can be inferred that GO nanosheets possess hexagonal symmetry. On other hand, Figure 4a−e shows TEM and HRTEM images of RGO-wrapped Au nanocomposite, respectively, at different scales revealing the detailed morphology and

Figure 2. FESEM image of (a) bare GO sheet and (b) RGO-wrapped Au nanoparticles.

some folded layers. In Figure 2b, well-defined nanoparticles of Au with average diameter of 15−20 nm are noticed. The surfaces of the Au nanoplatelets are distinctly wrapped with gauze-like RGO sheets and this has resulted in RGO-wrapped Au. Both agglomerated and individual Au nanoplatelets have been distinctly observed in the FESEM micrographs of the 3947


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bright field TEM image, (b) gold M jump ratio map, (c) carbon K jump ratio map, and (d) combined (gold and carbon) map of EFTEM measurement. The green and red colors represent the elements gold and carbon, respectively. In combined map (Figure 5d), we have observed that red colored carbon i.e., RGO are present on the green colored Au nanoplatelet surface by wrapping on it. This confirms the presence of wrapping of RGO nanosheet on circular Au nanoplatelets surface. Now to confirm the 2D nanoplatelets - like morphology of the circular Au particle, we have taken EFTEM images of the RGO wrapped Au nanoplatelets. It can be seen from Figures 6a−c, that RGO has wrapped most of the circular Au Figure 4. (a−d) TEM, (e) HRTEM, and (f) SAED pattern of RGOwrapped Au nanoplatelets (red color circles are drawn for clarity).

crystallinity of the sample. The TEM image of RGO-Au shows that circular Au nanoplatelets are uniformly distributed and wrapped by RGO nanosheets. The size of the obtained Au nanoplatelets is estimated from HRTEM images. The Au nanoplatelets have narrow size distribution and average diameter of Au nanoplatelets is ∼15 nm. Uniform distribution of Au on RGO nanosheets was achieved and no obvious aggregation is detected. The lattice constant observed for Au platelet in RGO-Au sample is 0.22 nm, which is close to lattice constant of Au. The prominent spots in the SAED pattern suggest that the RGO-Au sample is well crystallite in nature (Figure 4f). Figure 4f also indicates that the particles are not single crystal but are composed of twins. In Figures 4a−c, we see that the Au particles are firmly attached to RGO sheets. It may be noted that for preparation of TEM sample, the RGO wrapped Au nanoplatelets have been treated with ultrasonic wave for a long time. It may therefore be inferred that the binding between RGO and Au in RGO-Au sample is very strong. The Au−N edge at 83 eV was selected at a slit width of 8 eV to examine the wrapping of RGO nanosheets on Au nanoplatelets by recording the energy filtered TEM (EFTEM) elemental map of the RGO-Au sample and the relevant images are presented in Figure 5. Figure 5 shows (a)

Figure 6. (a−c) Relative thickness map using EFTEM, showing Flattop morphology RGO-wrapped circular Au nanoplatelets. (d) Line profile showing the relative thickness.

nanoplatelets having the average diameter of ∼15 nm. The surface of each nanoplatelet exhibits flat-top morphology. Figure 6d displays the relative thickness line profile along the pink line of one circular Au nanoplatelet. The 2D Au nanoplatelet selected for thickness analysis exhibits flat-top morphology. We have chosen a large particle (avg. diameter 25 nm) of Au nanoplatelet for morphology analysis. EELS Study. The EELS spectra of RGO wrapped Au nanoplatelets at chosen probe points (P1, P2, and P3) as shown on the HAADF-STEM image along with the corresponding HAADF-STEM image are shown in Figure S4. We have studied EELS spectra correspond to carbon Kedge and oxygen K-edge peaks. Here three different probe points have been fixed on surfaces of carbon grid (P1), RGO portion (P2), and RGO-Au portion (P3). The EELS spectrum of carbon grid (P1) contains three peaks centered at 284.7, 292.5, and 325 eV. The EELS spectrum of both RGO (P2) and RGO-Au (P3) contain peaks at 285.1, 293, and 325 eV. The carbon peak at around 285 and 292 eV can be attributed to 1s core level to the π* band and 1s core level to σ* band transitions, respectively.52 The relative intensities of 1s to π* transition and 1s to σ* transition increase from P1 to P2 and P3 may be due to RGO formation.52 Both of these features are characteristic of graphene layers. The peak at 325 nm suggests the presence of sp3 bonds or structural defects. These sp3 bonds are found in RGO as structural defects in sp2 network and remain as interlayer bridges which link the (001) graphite plane. Moreover, the peak at around 540 eV corresponds to the presence of oxygen (very low intensity) in RGO (P2) and

Figure 5. EFTEM mapping: (a) bright-field TEM image, (b) gold M jump ratio map, (c) carbon K jump ratio map, and (d) combined map. 3948


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that the RGO wrapped Au nanocomposite contains RGO and metallic Au nanoparticles. Raman Spectroscopy Study. The structural changes in carbon network can be precisely determined by Raman spectroscopic technique.57 The Raman spectra of GO, RGO, and RGO-wrapped Au nanocomposites are presented in Figure 7. The characteristic peaks for D and G bands of GO have

RGO-Au (P3), which is absent in carbon grid (P1). The intensity of the peaks has been substantially diminished due to low oxygen content of RGO-Au sample. This low oxygen content of RGO-Au sample shows that good amount of GO has been reduced and converted into RGO during the synthesis of RGO wrapped Au nanoplatelets. AFM Study. The Atomic Force Microscopy (AFM) has been employed to investigate the thickness, surface morphology and height profiles of GO and RGO-Au samples. The tapping mode AFM images of 2D, 3D and height profiles of the GO and RGO-Au, are shown in Figures S5 and S6, respectively. In the 3D image, the highest point of the sample surface and the valley or sample pores are represented by light pink and dark red colors, respectively. The average thickness of bare GO sample is about 2.5 nm, which confirms formation of few layers GO sheets. The thickness of RGO-Au nanoplatelets is 50 nm. In EFTEM thickness map (Figures 5 and 6), the 2D nanoplatelet-like morphology of the circular Au particle in RGO-wrapped Au nanoplatelets has been clearly observed and the average diameter of Au is found to be ∼15 nm. Thus, thickness of RGO-Au nanoplates has considerably increased due to wrapping of RGO over Au nanoplatelets. Interestingly, the grafted Au on the surface of RGO is not uniform, which may be due to the presence of randomly distributed epoxy groups on the sample’s surface. The values of thickness, roughness, and surface roughness of both GO and RGO-Au samples are provided in Table S1. As RGO has wrapped a large number of Au nanoplatelets, an individual Au nanoplatelet is not observed in the AFM image. XPS Study. We have recorded the XPS spectrum of RGOAu sample to assess the chemical composition of the sample and determine chemical valence state of the constituent elements in the RGO-Au. Only the characteristic bands associated with the constituent elements (C, O, and Au) of RGO-Au have been observed in the XPS spectrum of RGO-Au sample. Figure S7 depicts high resolution XPS spectrum corresponding to (a) Au 4f orbital, (b) Au 4d orbital (c) fitted C 1s peak, and (d) fitted O 1s peak of RGO-Au sample. The values of binding energy of 4f7/2 and 4f5/2 orbitals of metallic Au0 are 84.0 and 87.7 eV, respectively.53 The high-resolution XPS spectrum corresponding to Au 4f orbital exhibits two peaks for Au 4f7/2 and Au 4f5/2 centered at 83.8 and 87.8 eV, respectively. Thus, the values of binding energies corresponding to these two XPS bands of RGO-Au sample match well with the reported values of binding energies of metallic Au0.53 Moreover, the XPS bands obtained at binding energies of 345.6 and 349.5 eV represent the Au 4d5/2 and Au 4d3/2 (of metallic Au), respectively, which corroborates well with previously reported values of binding energies of metallic Au.54 This suggests that during reaction of GO with HAuCl4, GO has successfully reduced Au3+ ions to produce metallic Au0 nanoparticles. C 1s band in the XPS spectrum of RGO-Au has been deconvoluted into components due to three different carbon containing functional groups. The main peak of C 1s band at 283.3 eV has resulted from C−C bonding of sp2 network of GO and other two low intensity components at 284.8 and 286.6 eV have originated from C−O and CO.55 The XPS spectrum corresponding to O 1s peak of RGO-Au contains two components of C−O−C at 530.5 eV and OC at 532.4 eV.56 The results of XPS study are consistent with the EDS, EELS, and Raman studies of the RGO-Au sample and confirm

Figure 7. Raman spectra of bare GO, RGO, and RGO-wrapped Au nanoplatelets.

been obtained at 1364 and 1615 cm−1, respectively.57 The RGO sample displays two characteristic peaks at 1351 and 1601 cm−1, which can be assigned to characteristic D and G bands, respectively.57 D-band of GO and RGO corresponds to the breathing mode of A1g κ-point phonons of disordered sp3 carbon framework of graphite, while the G-band appears due to the tangential second order stretching of the E2g vibrational mode of in-plane sp2 carbon atoms and represents the degree of graphite formation.58−60 The peaks for D and G bands of RGO-Au sample has shifted toward the right with respect to both GO and RGO and the corresponding peaks appear around 1375 and 1632 cm−1, respectively. This right shift arises due to charge transfer from RGO to Au nanoplatelets and indicates strengthening of electron−phonon coupling·61 It is well-known that intensity ratio of D to G Raman bands (ID/ IG) is used to estimate the degree of structural disorder in graphite related systems and its value for GO is smaller than that of RGO.59,60,62 The estimated values of ID/IG for GO, RGO, and RGO-Au are 0.95, 1.02 and 1.1, respectively. This indicates that during formation of RGO wrapped Au nanoplatelets, GO has been reduced to RGO and sp2 carbon framework has got reestablished,62 which corroborates with the findings of EELS study. When RGO is prepared by reduction of GO, the size of the reestablished carbon framework becomes smaller that than of the original sp2 carbon framework of graphite, which gives rise to significant enhancement of ID/ IG of RGO compared to GO.62 Moreover, Raman spectrum of RGO-Au exhibits two overtones of D and G bands at 2679 and 3218 cm−1, respectively, and a mixed overtone of Raman bands has been observed around 2935 cm−1.63,64 These three peaks have not been observed in the Raman spectra of GO and RGO. The intensity of D and G bands of RGO-Au system has been significantly enhanced with respect to bare GO & RGO, as a result of surface enhanced Raman scattering (SERS) from the Au nanoplatelets because of increase of local electromagnetic field caused by the Au nanoplatelets attached with RGO and interfacial transfer of charge between the Au 3949


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Figure 8. (a) UV−vis spectra of bare GO, RGO, and RGO-Au samples. Photoluminescence (PL) spectra of bare (b) GO, RGO, and (c) RGO-Au samples.

Figure 9. (a) Photoelectrochemical performance (PEC) study of both GO and RGO-Au samples under visible-light illumination. (b) Schematic diagram of overall reaction corresponds to the splitting of water by visible light. (c) Schematic diagram of interaction of Au nanoplatelets with visible light in RGO-Au. (d) Electrochemical impedance spectroscopy (EIS) study of RGO-Au.

and RGO-Au are 7.09 × 1010, 4.81× 1010, and 1.01 × 1011, respectively, at 1375 cm−1, whereas SERS enhancement factor of GO, RGO, and RGO-Au are 3.91 × 1010, 2.25 × 1010, and 1.43 × 1011, respectively, at 1632 cm−1. Optical Property. We have recorded the UV−vis spectra of bare GO, RGO, and RGO-Au samples and those are presented in Figure 8a. The main UV−vis peak lying at 224 nm for GO and at 285 nm for both RGO and RGO wrapped Au

nanoplatelets and RGO. Modification of the Raman polarizability by direct electronic interaction between Au and RGO has also played a role in this enhancement. The Raman intensity of RGO-Au enhances 2.07 times relative to GO and 2.05 times relative to RGO at 1375 cm−1, whereas this enhancement is about 1.80 times relative to GO and 1.79 times relative to RGO at 1632 cm−1. Moreover, the SERS enhancement factor (taking 1 × 10−6 M R6G) of GO, RGO, 3950


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Figure 10. (a) Cyclic voltammetry (CV) study with different molar concentrations of H2O2, (b) Amperometric J−t curve of the sample for the H2O2 sensing study and linear response of H2O2 concentration with the current values (inset). (c) Cyclic voltammetry (CV) study with different molar concentrations of AA. (d) Amperometric J−t curve of the sample for the AA sensing study and linear response of AA concentration with the current values (inset).

nanoplatelets can be ascribed to the π−π* transition of C−C and CC bonds in sp2 carbon network.65,66 The shoulder peaks at 260 nm for GO, at 360 nm for RGO and RGOwrapped Au nanoplatelets are assigned to n-π* transition of the C = O bond in sp3 carbon framework.65 The UV−vis peaks of RGO and RGO-Au samples show red shift compared to GO. In the UV−vis spectrum, RGO-Au sample shows presence of the characteristic surface plasmon resonance (SPR) band of metallic Au centered at 540 nm, which gives evidence in favor of formation of Au nanoplatelets in it.61 Room temperature photoluminescence (PL) spectra of GO, RGO and RGO-Au samples at an excitation wavelength of 310 nm are presented in Figure 8b, c. Both GO and RGO exhibit a broad PL emission band centered at 430 nm because of band edge emission along with a sharp and strong peak at around 350 nm due to presence of active sites like the epoxide groups in the sample which often leads to formation of isolated sp2 domains, i.e., graphene islands. PL intensity of RGO reduces relative to GO due to increase of disorder in carbon framework. On the other hand, the PL peaks of RGO-Au have a high intensity and they exhibit red shift with respect to GO and RGO. The RGO-Au sample has shown two emission peaks at 467 and 535 nm. The peak at 467 nm has appeared due to the presence of RGO in the RGO-Au sample. This peak has been shifted toward longer wavelength because of the presence of large size Au nanoplatelets, which perhaps have reduced electron−hole pair recombination gap.25 The peak at 535 nm has originated due to the localized surface plasmon resonance

(SPR) created from the interaction between RGO and Au.25 The PL intensity of RGO-Au sample has been increased by 26 times compared to PL intensity of bare GO sample and 35 times compared to that of RGO. By comparing the PL spectra of bare GO, RGO and RGO-Au, it can be inferred that a giant enhancement in PL spectrum intensity of RGO-Au has taken place. This can be attributed to the formation of so-called “hot spot” due to confinement of local electromagnetic field, coupling between surface plasmon and excitons, and fluorescence resonance energy transfer process (FRET).61,67,25 Photoelectrochemical Performance Study. The photoelectrochemical performance of bare GO and RGO-Au electrode were investigated by immersing the electrodes in a photoelectrochemical cell containing 1 M Na2SO4 electrolyte by illuminating the working electrode using a 150 W mercury vapor lamp. The data was recorded under light “on−off” conditions using Ag/AgCl as reference electrode. Figure 9a shows amperometric current versus time (I-t) plot of both bare GO and RGO-Au samples at a bias voltage of 0.85 V under light “off-on” conditions. The GO sample does not respond to light illumination whereas RGO-Au sample shows visible-light assisted excellent photocatalytic water splitting effect. The photocurrent reaches to its steady state value when charge transfer from Au nanoparticle to RGO is maximum. The current returns almost instantaneously to its original value, when the light is turned off. Au nanoparticles provide more free charges (both electrons and holes), which enhances the catalytic effect of RGO-Au sample than bare GO. 3951


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the CV plots in absence of both H2O2 and AA. These figures clearly indicate that the current densities at the reduction potential (−0.42 V) for H2O2 and that at oxidation potential (−0.16 V) of AA significantly increase with increase in molar concentration of the corresponding solutions. This suggests that RGO-Au-coated ITO working electrode can act as an efficient sensor for detection of H2O2 and AA. The amperometric sensing studies for H2O2 and AA has been conducted at -0.42 V (the reduction potential of H2O2) and −0.16 V (oxidation potential of AA), respectively, and the results are depicted in Figures 10b, d. When H2O2 is successively added in the electrolyte, stable current steps have been generated in the current density versus time plot (Figure 10b). A same feature has been observed for AA (Figure 10d). The calibration plots for H2O2 and AA are shown in the inset of Figure 10b, d, respectively. The calibration curve for H2O2 and AA can be fitted by the equations J = −280.282 μA mM−1 cm−2 CH2O2 + 32.1214 with a correlation coefficient R2 = 0.96681 for H2O2 and J = 314.07 μA mM−1 cm−2 CAA − 9.26157 with correlation coefficient R2 = 0.97133 for AA, respectively. It has been found that the linearity ranges for H2O2 and AA are 1 μM to 0.8 mM and 25 μM to 300 μM, respectively. The values of sensitivity for H2O2 and AA are 280.28 μA mM−1 cm−2 and 314.07 μA mM−1 cm−2, respectively. The lowest detection limit of both H2O2 and AA is 6.8 μM at S/N = 3. For evaluating the electrochemical sensing performance of a sample, it is customary to monitor the electrochemical reduction signal of it.70 The RGO-Au/ITO electrode is probing H2O2 by way of detecting subsequent reduction by products. Thus, the working electrode is indirectly detecting the presence of H2O2. The mechanism by which the working electrode is detecting H2O2 is described in Figure S9 and can be expressed by the following equation.70

Electrolysis of water molecules can be explained by taking in account the oxidation of water at the surface of working electrode. The holes (h+) created in the valence band of the sample oxidizes water to form oxygen followed by generation of H+ ions (4h++H2O → O2+4H+), which then captures the conduction band electron under influence of the photo voltage generated by the second photosystem and produces hydrogen (4H++4e−(electrons) → 2H2).68 The reaction mechanism of the splitting of water by visible light is depicted in Figure 9b. If metallic NPs are exposed to light irradiation, then the conduction (free) electrons at the surface of NPs can absorb energy and efficiently transfer it to molecule adsorbed on the surface and activate it for catalytic reaction rather than disseminate their energy to the entire particle. When Au nanoplatelets absorb light then interband electron transitions (from 5d to 6sp) take place because of localized surface plasmon resonance (LSPR) effect, as depicted in Figure 9c. Sarina et al., proposed such a scheme for Au nanoparticle.69 Further, upon light illumination, electrons in the d-band of Au get excited and they are transferred to RGO surface due to the presence of electric field at RGO-Au junction. This has resulted in higher photoconductivity and current density in RGO-Au increases by 27 times than that of bare GO. Therefore, photodetection properties of RGO-Au sample can be greatly enhanced compared to bare GO by incorporation of Au nanoplatelets in the RGO-Au nanocomposite. The Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) measurement of RGO-Au/ITO electrode in dark and visible light at frequency ranging between 1 Hz to 100 kHz in 0.1 M Na2SO4 solution with an ac signal of amplitude 5 mV are depicted in Figure 9d. The high and low frequency ranges of the Nyquist plots are composed of a semicircular arc and a straight line and these plots can be modeled by an equivalent circuit presented in the inset of Figure 9d. Details of the equivalent circuit and methodology adopted for analyzing the Nyquist plot have been described in our previous work.3 The smaller value of charge transfer resistance (Rct) under visible light illumination than that in the dark indicates that interfacial charge transfer between the RGO and Au in RGO-Au nanoplatelets occurs more efficiently under visible light illumination compared to that in dark condition. So, we can use RGO-Au sample for switching or photodetection applications purposes. Electrochemical Cyclic Voltammetry Study. Electron accumulation and interfacial charge transfer of an electrode/ electrolyte interface can be verified by cyclic voltametry (CV) measurement. Figure S8 depicts CV plots of bare GO-coated ITO (GO/ITO), RGO-coated ITO (RGO/ITO) and RGOAu-coated ITO (RGO-Au/ITO) electrodes at a scan rate 0.04 V/s. Figure S8 indicates that the value of current in RGO-Au/ ITO electrode increases relative to RGO/ITO electrode and GO/ITO electrode. This result implies that electron accumulation takes place when Au nanoparticles are in contact with GO sheet in RGO-Au nanocomposite. It has been found that the electrochemical response of RGO-Au is better than that of bare GO and RGO. So, we have performed a biosensing study for RGO-Au sample only. Hydrogen Peroxide (H2O2) Sensing and Ascorbic Acid Sensing Study. The non-enzymatic electrochemical sensing property of H2O2 and AA conducted in 0.1 M PbS solution using RGO-Au/ITO as working electrode is shown in Figure 10. Figure 10a, c describes the comparative CV plots for different concentrations of H2O2 and AA solutions along with

H 2O2 + e− → OHad + OH− OHad + e− → OH− OH− + 2H+ → 2H 2O

The mechanism of detection of AA is briefly described in Figure S9. The RGO-Au/ITO working electrode adsorbs AA molecule from the electrolytic mixture. Then, by hydrolysis process the AA gets oxidized and converted to dehydroascorbic acid. This reaction mechanism is described in detail in the literature.71 Reproducibility and Long-Term Stability. Three identical working electrodes have been prepared for examining the reproducibility of data for sensing of H2O2 and AA. The long-term stability was evaluated for a period of 3 weeks by investigating the electrochemical performance after every 1 week. The results of reproducibility and long-term stability studies are presented in a nutshell in Figure S10. The standard deviations of sensitivity of H2O2 and AA for repeated use are 0.226 and 0.227%, respectively. For H2O2, the current density reduces to 85 and 75% of its initial value after the time of one and 3 weeks, respectively while the current density drops to 80 and 65% of its initial value for AA after one and 3 weeks, respectively. Thus, reproducibility and long-term stability of the RGO-Au/ITO electrode toward sensing of H2O2 and AA is very good. The analytical performance (sensitivity, detection limit, and linear response range) of the RGO-Au/ITO electrode has been 3952


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UPE program of UGC and the FIST and PURSE program of DST, Government of India are also acknowledged for SEM and AFM measurement. We also gratefully acknowledge Prof. Manabendra Mukherjee (Professor, SPMS Division SINP, India) Mr. Gautam Sarkar (Scientific assistant, SINP, India), and Dr. Dhrubojyoti Roy (NPDF Fellow, SINP, India) for XPS measurement. We gratefully acknowledge Mr. Gourab Bhattacharjee (SPMS Division, SINP, India) for TEM analysis. We gratefully acknowledge Dr. Partha Pratim Ray, Mr. Arka Dey, and Mr. Sayantan Sil (Physics Department, JU, India) for AFM measurement.

compared with other H2O2 and AA sensors reported earlier and is summarized in Tables S2 and S3, respectively. From Tables S2 and S3, it is clear that the proposed H2O2 and AA sensors show excellent electrochemical activity compared to previously reported H2O2 and AA sensors.

4. CONCLUSION In summary, we have successfully synthesized for the first time RGO-wrapped circular 2D Au nanoplatelets by a two-step chemical process without using any surfactant. The Raman spectra of the RGO-Au sample shows large enhancement compared to both GO and RGO, which can be ascribed to SERS caused by Au nanoplatelets. The giant enhancement in intensity of PL emission has been obtained in RGO-Au sample due to increasing radiative recombination rate and coupling effect of SPR and FRET mechanism. Accumulation of electron by interfacial transfer of charge between RGO and Au nanoplatelets is probed by CV analysis and the sample exhibits improved current response. Moreover, the electrochemical response of the RGO-Au modified ITO working electrode for H2O2 and AA sensing is excellent among all the H2O2 and AA nonenzymatic sensing electrodes and this is due to having more catalytic sites in RGO-Au. Hence, RGO-Au-coated ITO working electrode can act as an appropriate platform for non enzymatic H2O2 and AA sensing. So, we can use RGOwrapped Au nanoplatelets for multifunctional applications such as in visible-light-assisted catalysis, photodetection, light emission devices, and nonenzymatic biosensing purposes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00730. EDS spectrum, TEM-EDX mapping, STEM-HAADF image, EELS spectra, AFM images, XPS spectra and comparative CV plots, sensing mechanism, reproducibility, stability of RGO-Au toward H2O2 and AA, table of AFM results, and table of comparative analytical performance of the proposed H2O2 and AA biosensor with other H2O2 and AA biosensors reported previously (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.K.). *E-mail: [email protected] (S.M.). ORCID

Sumit Majumder: 0000-0002-1475-946X Biswarup Satpati: 0000-0003-1175-7562 Sanjay Kumar: 0000-0002-0584-0901 Sangam Banerjee: 0000-0002-2840-2760 Author Contributions

S.M. conducted experiments and analyzed the results under the supervision and mentorship of S.B. and S.K. B.S. performed TEM measurement. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (S.M.) gratefully acknowledges UGC, Government of India, for providing a research fellowship. The 3953


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Multifunctional Reduced Graphene Oxide Wrapped Circular Au

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