DOI: 10.1002/chem.201400403

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& Click Chemistry

Plasmon-Enhanced Photocurrent Generation from Click-Chemically Modified Graphene Sandeep G. Yenchalwar,[a, b] Rami Reddy Devarapalli,[a, b] Ashvini B. Deshmukh,[a] and Manjusha V. Shelke*[a, b, c]

aminopropyltrimethoxysilane (APTMS) linker. Cyclic voltammetry (CV) and impedance measurements also suggest fast electron transfer on account of the low resistance offered by the click-modified rGO surface whereby introduction of triazoles offers the efficient bridge between the donor AuNPs and acceptor rGO.

Abstract: The visible-light response of Au nanoparticles (AuNPs) assembled on rGO through different molecular bridges was investigated by transient photocurrent generation. We prepared rGO with two self-assembled monolayers (SAMs), one linear and the other with aromatic triazoles through a click cycloaddition reaction. A fivefold photocurrent enhancement was observed for triazole linkers over the

Introduction

based chemistry on its surface so as to recover the electrical strength and practical utilisation of graphene in the devices. Self-assembled monolayers (SAMs), which are ultrathin molecular films, can be used to modify the surface and influence the electronic properties of graphene.[7] Metal nanoparticles and composites, particularly of Au and Ag, are potential candidates for non-linear optics,[8] surface-enhanced Raman scattering (SERS)-based sensors,[9] energy harvesting and photocatalysis.[10] Plasmonically active Au nanoparticles have been investigated for their enhanced photoluminescence and photocurrent in hybrids like TiO2/Au,[11] ZnO/Au[12] and so forth. A plasmon-enhanced photocurrent in dye-sensitised solar cells (DSSCs)[13] and adsorbed photochromic molecules such as PdPc (palladium phthalocyanine)[14] and porphyrins[15] was achieved by anchoring AuNPs on self-assembled monolayers. Localised surface plasmons (LSPs) on the AuNPs significantly improve photocurrent signals by intense photon absorption and they induce a large electromagnetic (EM) field adjacent to its surface. Covalently immobilised AuNPs on the rGO surface through organic linkers has been suggested as a means of achieving controllable transport in memory devices.[16] Covalent bonding facilitates and controls the transport of electrons and thus can effectively charge and discharge electrons onto the Au nanoparticles by means of linker molecules. Such molecular conducting junctions are necessary for controlled, efficient electron transfer and excitation energy transfer between two electrodes in which one acts as a donor and other one acts as an acceptor. Conjugated p systems are of special interest in this respect. As a spacer or bridge, such systems significantly affect the rate of electron transfer, which is dependent on the length and electronic structure of the bridge, the donor–bridge energy gap for injection, the spectral density coupling of molecules and the electrode.[17] Aromatic triazoles have been used as a potential connecting bridge to facilitate photoelectron transfer through the involvement of

Graphene is an impressive material with extraordinary thermal properties, electrical conductivity, mechanical properties and high surface area.[1] It finds a wide range of applications in electronics,[2] energy storage[3] and sensing devices.[4] It has a two-dimensional lattice of carbon atoms arranged in a honeycomb crystal structure with a near-zero bandgap and a linear energy-momentum dispersion relation for both electrons and holes. The photoresponse of graphene and its derivatives has recently begun to be studied for potential applications in photovoltaics and photodetection. Pristine graphene lacks persistent photoconductivity owing to the fast recombination of the photogenerated electrons and holes.[5] An electron energy bandgap could be opened by means of oxidation/functionalisation of graphene. Graphene is not a suitable surface for chemical modification as it lacks functional groups, which also restricts its fine dispersion in aqueous solution. Graphene oxide (GO) and partially reduced graphene oxide (rGO) have numerous electroactive sites owing to oxygen functionalities on the basal plane and the edges; they are also called functionalised graphene.[6] This allows non-covalent and covalent[a] S. G. Yenchalwar, R. Reddy Devarapalli, A. B. Deshmukh, Dr. M. V. Shelke Physical and Materials Chemistry Division CSIR-National Chemical Laboratory, Pune-411008, MH, India Fax: (+ 91) 2025902636 E-mail: [email protected] [email protected] [b] S. G. Yenchalwar, R. Reddy Devarapalli, Dr. M. V. Shelke Academy of Scientific and Innovative Research (AcSIR) AnusandhanBhawan, 2 Rafi Marg, New Delhi-110001 (India) [c] Dr. M. V. Shelke CSIR-Network Institute of Solar Energy CSIR-National Chemical Laboratory, Pune-411008, MH (India) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201400403. Chem. Eur. J. 2014, 20, 1 – 9

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Full Paper a conjugative p network in intramolecular processes.[18] The Cu-catalysed azide–alkyne cycloaddition reaction between terminal alkynes and azides is famously known as “click chemistry” or Huisgen 1,3-dipolar cycloaddition. It has found wide applications in organic chemistry, polymer chemistry, supramolecular chemistry, drug discovery and so on.[19] Click chemistry is one of the strategies that can be implemented easily on various surfaces: for example, silicon,[20] semiconductor NPs and metal NPs,[21] graphene,[22] graphene oxide[23] and so forth. In this work, we explore the influence of AuNPs assembled on different SAMs on the rGO surface and subsequent plasmonic enhancement in the photocurrent generated from the chemically modified rGO/AuNP hybrid. In particular, we have shown that copper-mediated click chemistry leads to 1,2,3-triazoles, a perfectly suited linker between rGO and AuNPs for efficient charge transfer. The interfacial properties of the covalently modified rGO considerably change upon gold nanoparticle immobilisation. After photoexcitation of Au nanoparticles, efficient charge separation and electron transfer occurs to the graphene through the molecular linkers. The maximum photocurrent is achieved when a triazole interfaces with the gold nanoparticles, thus indicating the necessity of p conjugation for the charge transfer.

Scheme 1. Schematic of the GO surface functionalisation and immobilisation of AuNPs.

Results and Discussion GO was first synthesised by acid-mediated oxidation and exfoliation of graphite powder.[24] Reduction of aqueous GO solution was carried out under UV light. In this type of synthesis hydrated electrons from water act as a reductant. Also, GO significantly absorbs UV radiation to generate photoexcited electrons; relaxation of these electrons causes phonon vibration in the graphene lattice through thermal energy transfer. This creates localised heating and simultaneous removal of oxygen functionalities from the surface. As-prepared rGO was spray-deposited on a fluorine-doped tin oxide (FTO) substrate. The surface properties of the spraydeposited rGO were analysed by atomic force microscopy (AFM) and SEM (see Figures S1–S2 in the Supporting Information). The surface of rGO deposited on FTO was then modified chemically by covalent reactions. Scheme 1 shows the process for the SAM formation and subsequent AuNP attachment onto the rGO/FTO substrates. Two types of organic linkers were attached onto the rGO surface by hydrosilylation, that is, aminopropyltrimethoxysilane (APTMS) and azidopropyltriethoxysilane (AzPTES). The azide-functionalised rGO surface was further subjected to click chemistry with propargylamine. Both amine-terminated SAMs were then used as anchoring sites for AuNPs. These chemically modified rGO substrates as well as UV-irradiated GO were characterised by FTIR (Figure 1a). The GO contained oxygen functionalities such as alcoholic, carbonyl, C=C and epoxy groups abundantly on its surface, which was confirmed from the various IR vibrational modes at 3363, 1732, 1630 and 1059 cm 1. The IR intensity of OH and C=O groups is decreased upon irradiation with UV light, which indicates the reduction of GO sheets albeit still with sufficient functionalities &

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Figure 1. A) ATR/FTIR spectra of a) GO, b) rGO, c) N3/rGO, d) click/rGO, e) APTMS/rGO, f) AuNPs(click)/rGO and g) AuNPs(APTMS)/rGO. B) Raman spectra of a) GO, b) rGO, c) AuNPs(APTMS)/rGO and d) AuNPs(click)/rGO.

available over their surface for further chemical modification (Figure 1A (a and b)). The silanisation of rGO is based on silane chemistry in which OH or COOH groups are necessary for covalent attachment onto the surface. Briefly, the silane reacts with the above-mentioned functional groups by hydroxylation. Such SAMs have excellent stability under ambient atmosphere, which simultaneously passivates the electronic defects on the graphene edges and surface.[7] The FTIR spectra of azide- and APTMS-modified rGO show a characteristic Si-O-C peak at 1100 cm 1 and the azide vibrational mode at 2090 cm 1 as in Figure 1A (c and e). The peak at 1260 cm 1 belongs to C N stretching. The additional peaks at 740 and 890 cm 1 could be assigned to Si-O-Si and Si-O-C symmetric stretching modes. 2

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Full Paper For the click reaction on the azide-modified surface, we have carried out 1,3-dipolar addition of propargylamine in the presence of Cu as catalyst and sodium ascorbate as a reducing agent. The versatility of the click reaction is that it can be performed under ambient conditions owing to the inertness of azides and alkynes towards molecular oxygen and various solvents including water. CuI generated after the reduction formed a complex with propargylamine by reacting with a terminal alkyne, which then selectively added to azide to form 1,2,3-triazole. The complete disappearance of the azide vibrational mode (2090 cm 1) can be seen in the IR spectra (Figure 1A (d)), thus indicating successful click attachment of the propargylamine on the azide-terminated GO surface, whereas the triazole ring peak appears at 1066 cm 1 after the click reaction. The click reaction on the N3-rGO ultimately terminates the rGO surface with the amine end, which electrostatically immobilises citrate-capped AuNPs. The appearance of peaks at 1460 and 1574 cm 1 represent C N stretching and N H bending modes of amide groups, which further confirms the covalent attachment of AuNPs onto the functionalised rGO surface.[25] Raman spectroscopy (633 nm excitation wavelength) was used to investigate the structural changes and interaction effects of Au nanoparticles with the graphene surface. The Raman spectrum (Figure 1b) displays two distinct peaks: the G band for the graphitic mode that arises from the first-order scattering of the E2g phonon at the Brilliouin zone centre and the in-plane breathing modes of the sixatom ring (A1g phonon), which requires defects for its activation of GO, respectively. The intensity of the D band (at 1320 cm 1) is higher than the G band (1590 cm 1) with the ID/IG ratio of 1.83 in GO, thereby suggesting the higher defect concentration on the GO surface, which is considerably decreased in rGO Figure 2. a–c) HRTEM images of rGO(click)/AuNPs. d) The SAED pattern. (ID/IG  1.63). The ID/IG ratio indicates the average size charge transfer because of the linkers used for the covalent asof sp2 domains in the graphene. In our case the decrease in sembly of AuNPs. the ratio is attributed to desorption of the oxygen functionaliFurthermore, transmission electron microscopy was carried ties, that is, the removal of defects with the introduction of sp2 out to investigate the morphology and adsorption of AuNPs domains at different levels and subsequent reduction of the on the click-modified GO surface. Figure 2a–d shows the GO surface.[26] The ratio for APTMS and the click-modified rGO HRTEM images of the as-prepared rGO/Au. AuNPs with an surface was found to be 1.56 and 1.49, respectively. AuNPs deaverage diameter of 12 nm were observed on the surface of posited on the APTMS and click-functionalised rGO surface the rGO, which was confirmed from the particle-size distribushows enhanced Raman signals of the rGO bands. The gold tion calculated from Figure 2a. A lattice spacing of 0.246 nm nanoparticles interfaced with APTMS showed a 97 % enhancewas measured, which corresponds to (111) and other planes ment in the D band, whereas a 426 % enhancement was obfor face-centred cubic (fcc) Au nanocrystals. The XRD pattern served for the click-modified surface. The enhancement could for the GO, rGO and functionalised rGO with AuNPs is shown be attributed to electromagnetic (excitation of LSPs) or chemiin Figure S3 of the Supporting Information. The peaks at 9.98 cal mechanisms (charge-transfer complex formation).[27] The (001) and 24.5 (002) are the characteristic of GO and rGO, reenhanced Raman intensity of the D and G bands in the Raman spectively. The other peaks such as 38 (111), 44.3 (200) and spectra of the AuNPs on the click-modified rGO surface can be 64.4 (220) in the XRD and selected-area electron diffraction attributed to the involvement of the resonating SPs on the sur(SAED) patterns also reveal the fcc (JCPDS no. 04-0784) nature face of the Au nanoparticles, which would induce concentratof the gold nanoparticles. ed electromagnetic fields around the nanoparticle as well as Chem. Eur. J. 2014, 20, 1 – 9

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Full Paper cyclic voltammograms of various electrodes at a scan rate of 100 mV s 1 using 10 mm potassium ferrocyanide (as a fast electron-transfer probe) with 0.1 m KCl as a supporting electrolyte. The cathodic (ipc) and anodic (ipa) peaks for the [Fe(CN)63 /4 ] redox system on the FTO/rGO electrode are well defined and sharp; they are observed at 0.268 and 0.054 V, respectively. This suggests that the rGO is a highly conducting layer owing to high-quality sp2 network. For other modified electrodes a shift is observed for both the peaks: the anodic peak shifts positively and the cathodic peak shifts negatively. For the rGO(APTMS)/ AuNPs electrode, the cathodic Figure 3. XPS spectra of C and N before and after click functionalisation of rGO: C1s peak in a) GO, b) rGO, and anodic peaks are found at c) azide-modified rGO, and N1s peak in d) azide and e) after click reaction. 0.494 and at 0.138 V, whereas for the rGO(click)/AuNPs elecA high-resolution XPS survey further demonstrated the suctrode, they are at 0.320 and 0.003 V, respectively. The shift in cessful click functionalisation of the azide-terminated GO surthe peak values suggests that the reaction kinetics on the difface. GO samples show the considerable oxygen moieties presferent electrodes are different. It becomes sluggish for the ent in the carbon backbone of the GO in Figure 3a. The carbon rGO(APTMS)/AuNPs electrode relative to rGO(click)/AuNPs. 1s shows four deconvoluted peaks at 284.5, 286.4, 287.5 and The peak potential separations (DEp) for [Fe(CN)63 /4 ] are 288.5 eV that correspond to C=C, C-O, C=O, and O=C O, refound to be 0.214, 0.632 and 0.323 V, which correspond to spectively.[24] Most of the oxygen functionalities were removed, rGO, rGO(APTMS)/AuNPs and rGO(click)/AuNPs, thus indicating the facile heterogeneous electron-transfer kinetics of the redox which is evident from Figure 3b for rGO, and from the successcouple at rGO and AuNPs deposited on the rGO surface. The ful integration of amine moieties on the rGO surface as indicatstructure of the bridge between the AuNPs and rGO affects ed by the peak at 285.7 eV that belongs to the C N bond (Figthe interfacial electronic structure and charge-transfer rate. The ure 3c). The XPS analysis of carbon shows the considerable inlower DEp value for clicked rGO might be attributed to the low crease in the intensity of the C=C bond, which suggests the restoration of the hexagonal network of graphene owing to the barrier offered by the triazole ring conjugated with rGO, reduction of GO to rGO. whereas aminosilane offers greater resistance to the electron Figure 3d reveals the presence of a nitrogen peak in the transfer, which hinders the electron transport.[18a, 29] form of azide, which can be deconvoluted into two nitrogen (ratio of 2:1) peaks at 400 and 403.7 eV, thereby confirming the successful azide functionalisation of the graphene oxide Impedance measurements surface. However, Figure 3e shows the XPS data obtained for the same surface after the click reaction whereby the absence Figure 4b illustrates the electrochemical impedance characterof an azide peak at 403 eV along with the broadening of the N istics of the [Fe(CN)63 /4 ] couple on the interface of different peak at 399.5 eV in the XPS spectra confirms the formation of electrodes. The Nyquist plots as obtained consist of a small triazoles. The high-resolution scan of this broad peak shows semicircle at higher frequencies that corresponds to the the presence of three N atoms in different environments, charge-transfer resistance-limiting process and a straight line namely, NH2 at 399.5 eV, N=N of the triazoles at 400.5 eV and at lower frequencies that indicates the diffusion process. The charge-transfer resistance value for the FTO/rGO electrode is NH4 + at 401.7 eV.[28] low (  27 W) because of the direct interaction of redox species with the rGO conducting surface. However, gold assembled on Electrochemical experiments the functionalised rGO electrodes shows the slight increase in CV studies of the electrodes the Rct values. The AuNPs assembled on click-modified surface A cyclic voltammetry study was subsequently carried out to offer less resistance to the charge transfer (Rct  52 W) than the understand the reversibility of electrochemical reactions at difAPTMS-modified surface (Rct  76 W), which can be attributed ferent electrode interfaces. Figure 4a displays the resulting to the aromatic triazoles ring assisting or providing the neces&

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Figure 5. Linear sweep voltammograms (irradiated with light) and photocurrent responses of a) FTO/rGO, b) FTO/rGO(APTMS)/Au and c) FTO/rGO(click)/ Au electrodes versus Ag/AgCl as a reference electrode using 1 m Na2SO4 as an electrolyte and applying 0 V bias versus Eoc.

Figure 4. a) CV and b) EIS measurements of rGO and functionalised rGO after AuNP attachment in 10 mm of potassium ferrocyanide and 0.1 m KCl versus Ag/AgCl (EIS frequency range  10 kHz–0.1 Hz).

sary conducting link for the electron transfer onto the electrode surface. We further explored the influence of immobilised gold nanoparticles on the chemically modified rGO surface for photocurrent generation. The photocurrent time profiles were recorded at 0 V against the open circuit potential (Eocp) in which Ag/AgCl and Pt foil served as a reference electrode and as a counter electrode, respectively. The normalised photocurrent responses for rGO, rGO(APTMS)/AuNPs and rGO(click)/AuNPs electrodes irradiated with white light in the visible range (l > 400, 100 mW cm 2) are plotted in Figure 5. The photocurrent was prompt and became stable in the time interval of the pulse illumination, that is, an on/off cycle. When irradiated with white light, a cathodic photoresponse was shown by rGO electrodes, which indicated the p-type conductivity (photocurrent  0.54 mA cm 2). The other samples also retained the same type of conductivity. The click-modified rGO exhibited a cathodic photocurrent five orders of magnitude higher (3.62 mA cm 2) than APTMS/rGO (0.70 mA cm 2). Chem. Eur. J. 2014, 20, 1 – 9

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To ascertain the plasmonic effect in photocurrent generation, the click-modified samples were irradiated with light of specific wavelengths. When the wavelength of incident light matched the surface plasmon band of the gold nanoparticles (543 nm; Figure S4a in the Supporting Information), it showed the highest photocurrent (2.36 mA cm 2) relative to the other wavelength. This can be attributed to the SPs generated on the AuNPs by light irradiation; this induces a dipole that enhances the electric field, thus leading to separation and generation of hot electrons. These hot electrons were then injected to the conduction band of the graphene, which enabled the large photocurrent. Also, the conjugated triazole moiety serves as a better linker than the unconjugated one, thereby facilitating charge transfer from AuNPs to graphene and the functionalised rGO surface for better electronic conductivity and interfacial shuttling of electrons from the AuNPs to the rGO surface through the aromatic triazole linker. In the original DRS spectra we see the shift and broadening of the SPR peak owing to the aggregation of AuNPs.[30] After subtracting the contribution of 5

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Full Paper gap up to 0.73 eV. The Fermi level of the AuNPs (  0.5 V versus the normal hydrogen electrode (NHE))[32] lies well above the LUMO levels of the functionalised rGO. The energy barrier between the Fermi level of AuNPs and click-modified rGO is small (0.44 eV) relative to the APTMS/rGO (0.79 eV), thereby improving the charge injection rate to the click-modified rGO through the triazole moiety. It is worth mentioning that the presence of bifunctional linkers between semiconductor and AuNPs could affect the electronic structure at the interface as well as the electron-transfer rate.[33] In such a system, the photoexcited electron can relax by any one of the two mechanisms, namely, tunnelling or resonant charge-transfer mechanisms. We believe that resonant charge transfer is probably the more suitable mechanism in our case. The charge transfer involves the resonant transitions in the Fermi level of the metal and the modified energy level of the rGO owing to functionalisation. We found that the clickmodified rGO surface is suitable for such a process to occur, which is evident from the energy-level diagram. The results shows that the selective p-conjugated covalent attachments on the rGO remarkably enhance the photoelectrical transport properties of the rGO by modifying its energy levels relative to the linear unconjugated ones.

rGO/FTO (Figure S4b in the Supporting Information), two peaks at the SPR position of 543 nm and a new plasmonic coupling band or low-energy shoulder at 644 nm began to arise. Depending on the average size, anisotropic nature and degree of nanoparticle agglomeration, the intensity of the second plasmonic peak is much lower than the peak at the SPR position for the gold nanoparticle.[31] The results of the photocurrent measurements are shown in the top panel of Figure 6 and

Conclusion This work demonstrates enhanced plasmonic photocurrent generation from the rGO/AuNP hybrid with different linker molecules. The enhanced plasmonic photocurrent was achieved with gold nanoparticles assembled on click-functionalised rGO whereby AuNPs act as a plasmonic antenna and triazoles act as an effective electron-transferring linker. The organic modification of the graphene surface essentially alters the energy levels to result in opening of the bandgap. The nature of SAMs influences the electron-transport properties in the hybrid in which aromatic triazoles show the fast electrical conduction. Also, by comparison, the CV and electron impedance spectroscopy (EIS) data at the interface of the electrodes suggest the conjugated triazole ring to be an excellent bridge for the fast charge transport to the graphene surface. The inclusion of p conjugation near metal nanoparticles allows an efficient pathway for the relaxation of hot electrons, thereby ensuring a large photocurrent from the hybrid.

Figure 6. Wavelength-dependent photocurrent measurements of AuNPs assembled on a click-modified rGO surface showing enhancement in the photocurrent at the SPR wavelength of AuNPs. At the bottom is the energylevel diagram depicting the HOMO–LUMO of chemically modified rGO and possible resonant electron transfer from SPR levels of the AuNPs to rGO.

clearly indicate the maximum current at the SPR position of gold nanoparticles; when excited with 530 nm and 470 nm, which is due to the high photonic absorption of the composite, whereas the low absorption and the resulting less photocurrent at 665 nm are consistent with the results of the diffuse reflectance spectra. To support our results, we further determined how the energy levels of rGO were affected by the chemical functionalisation (shown in Figure 6). On the basis of the onset of the oxidation and reduction potential values, the calculated HOMO– LUMO levels for the rGO were found to be at 5.46 and 5.02 eV, which gives a bandgap of 0.44 eV. For APTMS/rGO with values at 5.14 and 5.48 eV, the bandgap of rGO is reduced to 0.34 eV, whereas for click-functionalised rGO, the energy levels lie at 4.79 and 5.52 eV, which opens a band&

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Experimental Section Synthesis of GO and rGO Graphene oxide (GO) was prepared from natural graphite by the modified Hummers method. As-prepared GO (50 mg, 1 mg mL 1) was dispersed in water and irradiated with UV light (using a source of 3 mW, 250–400 nm cutoff) for 6 h. The dark brown colour of the GO solution indicated the formation of partially reduced graphene oxide. As-synthesised rGO was centrifuged at 12 000 rpm and redispersed in the ethanol and used for further studies.

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Spray deposition and functionalisation of rGO Commercially available fluorine-doped tin oxide (FTO)-coated glass plates (resistivity: 7 W cm 2 ; Solaronix, Switzerland) were cut into pieces of 1  1 cm dimensions and used in this study. FTO-coated glass plates were pre-cleaned by acetone, isopropyl alcohol and finally with water under ultrasonication and dried in N2 gas. The dispersion of rGO in ethanol was spray coated with air blowing on the conducting surface of the preheated FTO at 150 8C until the desired thickness was achieved. Finally, the rGO layer on the FTOcoated glass was heated at 150 8C for 2 h until complete removal of solvent and to ensure film adhesion and better contact of rGO sheets onto the FTO surface.

Acknowledgements S.G.Y. and R.R.D. acknowledge the UGC and CSIR, India respectively, for research fellowships. Keywords: gold · graphene · nanoparticles · photophysics · surface plasmon resonance

The FTO/rGO plates were immersed in a solution of APTMS and AzPTES in ethanol for 12 h for amine and azide functionalisation, respectively. After the functionalisation of the FTO/rGO plates with amine and azide, the plates were washed with ethanol and water and dried under N2 gas.

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The click attachment of propargylamine was performed on the azide-terminated FTO/rGO electrodes. The electrodes were kept in the solution that contained propargylamine (50 mL), copper sulfate pentahydrate (CuSO4·5 H2O, 1 mm) and sodium ascorbate (1 mm) in ethanol (10 mL) for 12 h at room temperature. The electrodes were then washed with ethanol and water several times until the excess amount of physically adsorbed reactants were removed from the substrate and dried under N2 gas.

Synthesis of gold nanoparticles Gold nanoparticles were prepared by the citrate reduction method. Briefly, HAuCl4 (20 mL, 1 mm) was heated in a water bath at 120 8C for 15 min and citrate solution (20 mL, 1 mm) was added to the HAuCl4 solution at once. The yellow solution of the gold ions gradually turned wine red, thus indicating the formation of gold nanoparticles. Heating was continued for 15 min and then the mixture was allowed to cool to room temperature.

Attachment of gold nanoparticles on the functionalised FTO/rGO The functionalised FTO/rGO plates were immersed in the as-synthesised gold nanoparticle solution (10 mL, pH  6.4) for 30 min. After that, the plates were washed in a water stream to remove physically adsorbed gold nanoparticles and then dried in N2 gas.

Electrochemical measurements CV and EIP measurements: All the CV and EIP measurements were carried out in a three-electrode configuration. The electrochemical measurements were carried out in an electrolyte solution that contained 0.1 m KCl in water, and 10 mm potassium ferrocyanide was used as a redox probe (after purging with N2 gas). In the three-electrode system, FTO plates coated with rGO and functionalised rGO with AuNPs acted as a working electrode, Ag/AgCl as a reference electrode and Pt foil was used as a counter electrode to characterise the electrochemical activity of the electrodes. The contacts were made by soldering copper wire on the conducting side of the FTO plate. I–V measurements and transient photocurrent measurements were carried out by exposing a fixed area (5 mm  5 mm) of each electrode to illumination with a solar simulator equipped with a xenon lamp under AM 1.5 conditions with an intensity of 100 mW cm 2 in degassed 1 m Na2SO4 solution as a sacrificial electron donor. Wavelength-dependent studies were carried out by exposing the electrodes to the light-emitting diode Chem. Eur. J. 2014, 20, 1 – 9

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FULL PAPER & Click Chemistry

Bridge group: The visible-light response of Au nanoparticles (AuNPs) assembled on reduced graphene oxide (rGO) through different molecular bridges was investigated by transient photocurrent generation (see figure). Click-modified rGO shows a greater plasmon-enhanced photocurrent, thus revealing the importance of p conjugation in the photoelectron transfer.

Chem. Eur. J. 2014, 20, 1 – 9

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S. G. Yenchalwar, R. Reddy Devarapalli, A. B. Deshmukh, M. V. Shelke* && – && Plasmon-Enhanced Photocurrent Generation from Click-Chemically Modified Graphene

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Plasmon-enhanced photocurrent generation from click-chemically modified graphene.

The visible-light response of Au nanoparticles (AuNPs) assembled on rGO through different molecular bridges was investigated by transient photocurrent...
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