Charge Doping of Large-Area Graphene by Gold-Alloy NanoparticlesMaria Antoaneta Bratescu*

EcoTopia Science Institute, Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8603, Japan

Nagahiro Saito

Department of Materials, Physics and Energy Engineering, Green Mobility Collaborative Research Center, Nagoya University,Furo-cho Chikusa-ku, Nagoya 464-8603, Japan

*S Supporting Information

ABSTRACT: We present a facile, one-step, and surfactant-free methodfor direct synthesis and loading of stable gold and gold-alloynanoparticles (NPs) on large-area graphene using an electrical dischargein a liquid environment, termed solution plasma. We observed a chargedoping of graphene by the gold NPs, which depends on the particles’chemical composition, even if the NPs contain a few percent of trivalentsp metals, such as indium (In) or gallium (Ga). Raman and electronenergy loss spectroscopy (EELS) methods show that graphene is dopedwith electrons (n-type) in the case of gold NPs and with holes (p-type)in the case of gold-alloy NPs. The Raman band shift indicates that theamount of the transferred electrons from the gold NPs to graphene is −2× 10−4 electrons per unit cell. The gold-alloy NPs receive from graphene(2 and 4) × 10−5 electrons per unit cell if the gold NPs contain In andGa, respectively. In the EELS spectra, the decrease in the intensity of the 1s-π* transition and the shift of the π* peak to higherenergy confirm the depopulation of the antibonding states caused by the electron transfer from graphene to the gold-alloy NPs.

■ INTRODUCTION

Graphene is a good conductor with high transparency,flexibility, and strength, and it is environmentally stable.1

Nanoparticles (NPs) are useful materials especially due to theirsurface plasmon resonance (SPR) and catalytic activity. TheNP size, composition, and dielectric surroundings can changethe optical and electrical properties and the catalytic activity.2,3

Graphene decorated with NPs shows enhanced catalyticactivity; for example, PtAu alloy NPs on graphene have beenused for formic acid oxidation,4 and graphene-supported Pt andPt−Ru NPs were found to be an efficient electrocatalyst formethanol and ethanol oxidation.5 In these cases, the synthesisof alloy NPs required the use of surfactants that were alsoinvolved in the catalysis process.Recently, graphene and graphene-based hybrid nanoassem-

blies gained considerable attention in optoelectronic devicessuch as displays, touch screens, light-emitting diodes, and solarcells, which require materials with low sheet resistivity and hightransparency.6−8 Graphene can fulfill multiple functions inlight-conversion systems as the transparent conductive window,photoactive material, channel for charge transport, andcatalyst.8 For example, a single-layer graphene/n-Si Schottkyjunction exhibits high solar-power conversion efficiency,6 orchemical vapor deposition (CVD) graphene with indium tinoxide electrodes on polyethylene terephthalate substrate maybe used as a transparent conductive electrode in an organic

photovoltaic cell, which demonstrates the great potential of theCVD graphene films for flexible optoelectronics devices.7

To be used in electronics and optics, graphene must be incontact with other materials, which can change its electrical andoptical properties. The substrate, charge impurities, doping withchemical functional groups, and metal contacts can shift theposition of the Fermi level of graphene.9−12 The mechanismresponsible for the changes in the phonon dispersion ofgraphene on Ni(111) surface, that is, the suppression of theKohn anomalies (KAs), was explained by the hybridization ofthe π bands of graphene with the metallic d bands.9 Ramanspectroscopy showed the presence of the excess charges ongraphene both under the dopants monitored in an electro-chemically top-gated graphene transistor and also in theabsence of intentional doping.10,13 A detailed correlationanalysis of Raman G and 2D modes has demonstrated theeffects of mechanical strain and charges substrate-mediated ingraphene.14,15 Density functional theory (DFT) was used toexplain how graphene was doped by adsorption on metalsubstrates, and it was found that even in the case of weakadsorption the position of the Fermi level moves away from theconical points due to the doping of graphene with electrons orholes.16

Received: September 19, 2013Revised: November 25, 2013Published: November 26, 2013

Article

pubs.acs.org/JPCC

© 2013 American Chemical Society 26804 dx.doi.org/10.1021/jp409368c | J. Phys. Chem. C 2013, 117, 26804−26810

A major issue is to understand the charge-transferphenomenon to control the doping of graphene. Furthermore,graphene with plasmonic NPs can offer a new perspective forlight conversion systems by optimization of visible-lightabsorption via the SPR of the NPs, followed by electronexchange between graphene and NPs and electron transportthrough graphene.17

We present a facile, one-step, and surfactant-free method forthe synthesis and loading of stable gold and gold-alloy NPs onlarge-area graphene without NP deterioration using an electricaldischarge in a liquid solution, termed solution plasma (SP). Weinvestigated the charge-transfer process between graphene andgold-alloy NPs by Raman spectroscopy and electron energyloss spectroscopy (EELS) in high-resolution transmissionelectron microscopy (HRTEM).

■ EXPERIMENTAL METHODSGraphene was synthesized on 100 mm × 400 mm × 25 μmarea Cu foils (Alfa Aesar), in a quartz hot furnace at 1000 °C, inH2 and CH4 gases at 60 Pa total pressure over 30 min.18 Thebase pressure was 0.5 Pa. The quartz furnace was carefullycleaned of any carbon or copper traces from the previousdeposition by using a diluted solution of HF (KantoChemicals).The experimental setup of the SP is explained in detail in ref

19. The Cu-graphene foil was introduced in the 1 mM indiumor gallium nitrate (Sigma-Aldrich) solution in SP between twogold rod electrodes to synthesize the gold-alloy NPs directly onthe Cu-graphene surface. The SP was maintained at 1000 V, 2A, with a repetition rate of 15 kHz and a pulse width of 1 μsduring 3 min.19−21 The gold NPs on the Cu-graphene surfacewere produced by gold electrode erosion in a 1 mM KNO3(Sigma-Aldrich) solution for 3 min under the same dischargeconditions.The Cu-graphene foils without and with NPs were washed

with water, dried, and spin-coated with PMMA (poly(methylmethacrylate)) (MW = 950 kDa, in 4% anisole, MicroChem) at3000 rpm during 1 min. The graphene films without and withNPs were removed from the Cu foils by slowly etching in a∼0.2 M aqueous solution of Fe(NO3)3 (Sigma-Aldrich) for∼24 h. The graphene films covered with PMMA wererepeatedly washed in water and water−ethanol solution andcarefully lifted from water and transferred to the targetsubstrates, with the PMMA coating on top. The PMMA filmwas removed using acetone. The final substrates were dried invacuum at 60 °C for 4 h.After synthesis, the colloidal solutions in a 1 cm optical

absorption path length cuvette and the graphene without andwith NPs on glass substrates were characterized by UV−visspectroscopy (UV−vis−NIR 3600 spectrometer, Shimadzu) inthe spectral range 400−750 nm, with 0.5 nm spectralresolution. Zeta potential of the suspended NPs in aqueoussolutions was measured with Photal ELS-7300K, OtsukaElectronics.Raman spectroscopy was performed with an inVia Raman

Microscope, Renishaw, with 1 cm−1 spectral resolution, 0.05cm−1 repeatability, with a laser at 532 nm, and a spatialresolution of 1 μm for the 20× objective. Sample scanning wasrepeated on different places on the surface within an area of∼400 μm2, in steps of 10 μm. The calculation of more than 300Raman spectra for each sample in a map, that is, the fwhm, theG and 2D band position, and the ratio I2D/IG, was performedwith a homemade Matlab program. The Raman measurements

were carried out before loading the NPs on the Cu-graphenesurface to confirm the quality of the graphene.The morphology of the gold-alloy NPs on the suspended

graphene was observed by TEM (JEM − 2500SE, Jeol) with200 kV accelerating voltage. The samples for TEM analysiswere prepared by placing the graphene without or with NPs ona holey Cu TEM grid. The PMMA was carefully removed bywashing the grid with acetone and vacuum drying, as previouslydescribed. The composition of the NPs on the graphene surfacewas analyzed by electron-dispersive spectroscopy (EDS) inbright-field mode and EELS.The HRTEM and EELS were performed by recording the

images close to the Scherzer defocus, and the sample heightwas adjusted to keep the objects focused in the optimum lenscurrent. A beam current density of ∼10 A cm−2 used forHRTEM observation using a CCD camera brings at most atemperature increase of a few degrees; current was reported tohave little influence on the samples.21

Using a Gatan Imaging Filtering device for EELS operated byFilter Control and Digital Micrograph software, we recordedthe EELS spectra with 0.2 eV per pixel and 2 mm aperture,which corresponds to a collection angle of 10 mrad. Thespectra calibration was checked before and after eachmeasurement using the zero loss peak.The crystal structure of the gold-alloy NPs on graphene

transferred on glass substrates was analyzed by X-raydiffractometry (XRD, SmartLab, Rigaku), equipped with CuKα radiation source (λ = 0.154056 nm), using an X-ray powderdiffraction method. The identification of the crystalline phasesin the gold-alloy NPs structure was carried out using integratedX-ray powder diffraction software − PDXL qualitative analysis,from Rigaku.

■ RESULTS AND DISCUSSION

The gold-alloy NPs were directly synthesized and loaded on theCu-graphene foil surface in a one-step process in an aqueoussalt metal SP between two gold electrodes. The SP is a usefuland simple method for the metal NPs synthesis because thisnonequilibrium plasma can provide extremely rapid reactionsdue to the reactive chemical species, radicals, and UV radiationproduced in atmospheric pressure plasma at room temper-ature.20,21 The gold-alloy NPs were produced by thesimultaneously processes of gold electrode erosion and themetal reduction from ion to neutral form, without anysurfactant.19 In this way, the SP method offers the possibilityto directly load the gold-alloy NPs on large-area graphene. Thegraphene with NPs was then transferred onto differentsubstrates without deterioration, maintaining indium andgallium in the NPs composition.XRD with PDXL software determines the crystal structure

and composition of the gold-alloy NPs on graphene transferredon glass and silicon substrates. The AuIn NPs contain 5 wt %Au and 95 wt % Au−In phase with 4 wt % In, and the AuGaNPs contain 3 wt % Au and 97 wt % Au−Ga phase with 12 wt% Ga (Figure S1 in the Supporting Information).19,22 Thismeans that the AuIn and AuGa NPs contain ∼4 wt % In and∼12 wt % Ga, respectively. The EDS maps confirm thepresence of In and Ga in the NPs composition (Figure S2 inthe Supporting Information). In the NP composition and onthe graphene surface, nitrogen and oxygen atoms, which mayinduce charge doping in graphene, were not detected by EDSand EELS.11

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Visible absorption spectra of a ∼1 cm2 area of graphene withNPs on glass were used to measure the SPR. The SPR peaks at573.0 nm of the AuIn NPs and at 571.5 nm of the AuGa NPsare red-shifted when compared with the SPR peak of the goldNPs on graphene at 543.0 nm (Figure 1a−d). In solution the

Au, AuIn, and AuGa NPs have the SPR peak at 520.0, 535.0,and 560.0 nm, respectively, which are blue-shifted comparedwith their corresponding SPR peak on the surface. This is dueto the NPs’ change in shape from spherical in solution toellipsoidal on surface (Figure 1e).2,19 The two peaks in theabsorption spectra at 476 and 669 nm are produced by lightinterference on the layer. (See the Supporting Information.)Around 700 nm, the absorbance of the graphene without NPsis 0.050 ± 0.002, which would correspond to a bilayer graphenebecause one graphene layer absorbs πα = 2.3% fraction of theincident visible light, where α (= 1/137).23 In the case ofgraphene with NPs, the background caused by the NPs on thesurface increases the absorbance to 0.066 ± 0.002, 0.065 ±0.002, and 0.081 ± 0.002 for Au, AuIn, and AuGa NPs,respectively (Figure 1). The SPR absorption peaks are weak(∼0.01) because the surface coverage of the gold NPs on thegraphene is much smaller than the maximum possible surfacecoverage.Figure 2 shows a set of typical Raman spectra of graphene on

a SiO2/Si substrate (a) without NPs and (b) with Au NPs andgold-alloy NPs (c) with 4 wt % In and (d) 12 wt % Ga, derivedfrom their corresponding map shown in the left side of the

Figure. The analysis of more than 300 spectra over a ∼1 cm2

area, allows us to determine Raman peak positions and shiftsfor graphene samples decorated with gold and gold-alloy NPs.The G and 2D band frequency shifts induced by the gold NPsare opposite to those of the gold-alloy NPs on graphene. Figure2e−g shows the statistical analysis of the position of G band,2D band, and the ratio between the intensities of 2D and Gbands, respectively. In the case of graphene without NPs, theposition of the G band is at 1585 ± 2 cm−1 and the position ofthe D band is at 2678 ± 3 cm−1. Despite the relatively highsensitivity to charge doping, the analysis using the peakfrequencies of the G and 2D bands has revealed a significantamount of discrepancies among many reported works regardingthe accuracy of quantification.13,24 In addition, typicalfrequency variation in a given top-quality graphene can bemore than several cm−1 because of the native charge dopingand mechanical strain.10,14 Large-area CVD-grown grapheneshows even larger variation in the Raman bands frequenciesinduced by transfer and interaction with a target substrate.15,25

Furthermore, a typical surface hole-doping concentration ofabout nini ≈ 1.5 × 1013 cm−2 obtained in experiments10,25,26

corresponds to an important Fermi energy level shift (EFini ≈ 0.5

V). In the present experiment graphene without NPs was

Figure 1. UV−vis spectroscopy of the NPs on graphene and insolution. Visible absorption spectra of graphene transferred on glass(a) without NPs and (b) with gold NPs. The SPR peak in theabsorption spectra of graphene on glass with (c) AuIn NPs and (d)AuGa NPs is red-shifted when compared with Au NPs on graphene.The two peaks at 476 and 669 nm are due to the light interference onthe layer, corresponding to the interference orders 2 and 3. (See theSupporting Information). Panel e represents the optical absorptionspectra through 1 cm optical absorption path of the gold and gold-alloy NPs in water solution after synthesis.

Figure 2. Set of Raman maps and typical spectra of the graphene on Sisubstrate. The spectra were derived from the Raman map data shownon the left side of the Figure. These images contain the microscopeimage of the sample and two maps of the intensity distribution of the2D band (green) and G band (red). The spectra were normalized tothe 2D band intensity. (a) Raman maps and spectra of graphenewithout NPs. Raman maps and spectra of the graphene decorated with(b) gold NPs and gold-alloy NPs with (c) 4 wt % In and (d) 12 wt %Ga. The black vertical dashed lines indicate the position of the G bandat 1585 cm−1 and the D band at 2678 cm−1 for the undoped graphenelayer. The shifts of the Raman bands are indicated by the short(colored) dash lines. Statistical analysis of the position of (e) G band,(f) 2D band, and (g) ratio between the intensities of 2D and G bandsfor graphene without and with NPs. The standard deviation, σ, for Gband position is ±2 and ±3 cm−1 for graphene without and with NPs,respectively. For 2D band position, σ is ±3 and ±5 cm−1 for graphenewithout and with NPs, respectively.

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assumed to have a surface charge concentration nini and a Fermienergy level shift EF

ini, which are considered the initial dopingpoint.The NPs on the graphene produce a change of the

equilibrium lattice parameter, which causes the stiffening orthe softening of the phonons and a rapid modification of thephonon dispersion curve (dynamic effects), close to KAs.13,27,28

In graphene, the KAs have been proven in field-effect transistor(FET) experiments, where the applied gate voltage modulatesthe charge doping of graphene and the measured frequencyshift of the G band obeys KAs in accordance with the gate-voltage-induced Fermi-level variations.13,27 In the presentexperiment, the observed frequency shift of the G band waspositive and also negative, with respect to initial doping, whenthe gold or gold-alloy NPs were loaded on the graphenesurface. The downshift of the G-band frequency is explainedwithin the nonadiabatic approach considering nini.Using the dynamic approach in the context of the DFT

calculation, the G-band frequency shift with respect to zerodoping (Dirac point) (ΔωG) in dependence with Fermi energylevel EF is given by the relation:24,29

ω α α ω ω

ω

ℏΔ = ′| | + ′ℏ · | | |−ℏ

| | + ℏ |

E E

E

( /4) ln( ( /2)

/( /2) )G F a

0F a

0

F a0

where α′/(2πℏc) = 35.8 cm−1/(eV), c is the speed of light, ℏ isthe reduced Planck constant, and ωa

0 is the adiabatic zerodoping phonon frequency. In this relation, there are twologarithmic divergences for EF = ±ℏωa

0/2, and for |EF| ≫ ℏωa0/

2 the frequency shift increases as α′|EF|, as in the presentexperiment. The Fermi energy in graphene changes as: EF =ℏ|νF|(πn)

1/2, where |vF| = 1.1 × 108 cm s−1 is the Fermi velocityand n is the change of the surface electron concentration incm−2 with respect to zero doping graphene.13 In a simpleanalytical model, the number of electrons per graphene unit celltransferred from (or to) metal NPs due to the change of theFermi level is given by: N = sign(ΔEF)D0ΔEF2/2, where D0 =0.09 per (eV2 unit cell), is a proportionality constant factor,considering the graphene density of states to be linear withenergy, within ±1 eV, and sign(x) is sign of x.16 ΔEF producedby the NPs loaded on the graphene is related to the

experimental G-band frequency shift (ΔωGexp) relative to the

initial G-band frequency shift (ΔωGini), which is induced by nini

as:

ω ω αΔ ± |Δ | = ′ ± |Δ |E E( )Gini

Gexp

Fini

F

The excess charge on graphene caused by transferred electronsor holes from the NPs to graphene was calculated from thechange of the Fermi level relative to EF

ini and ΔωGexp. In the case

of graphene with gold NPs, ΔωGexp is −2 cm−1, which means a

decrease in n with −3.5 × 1012 cm−2 and a movement of EFwith ΔEF= −0.06 eV. The average amount of the transferredelectrons from the gold NPs to the graphene unit cell is N = −2× 10−4.The ΔωG

exp shifts are +0.7 and +1 cm−1 in the case of theAuIn and AuGa NPs, respectively, caused by the transfer ofelectrons from graphene to the gold-alloy NPs. This producesan increase in n with (1.3 and 1.9) × 1012 cm−2, whichcorrespond to an increase in EF with ΔEF = 0.02 and 0.03 eV,respectively. The hole doping of graphene unit cell is N = (2and 4) × 10−5 if the gold NPs contain In and Ga, respectively.The variation of the 2D band frequency with doping is

mainly due to the charge transfer: an electron doping producesa down shift of the frequency, and a hole doping causes anupshift of the 2D band frequency.10 We compared the abovecalculated results of EF derived from ΔωG

exp with those obtainedfrom the 2D band shifts (Δω2D

exp), using the experimental datareported in refs 13 and 14. For graphene with gold NPs, Δω2D

exp

is −2 cm−1, which means that EF moves downward with −0.03eV. In the case of graphene with AuIn NPs and AuGa NPs,Δω2D

exp are +7 and +6 cm−1, and EF moves upward with 0.07 eV.The uncertainties in the calculation of ΔEF, n, and N aredifficult to assess due to the high standard deviation values inRaman measurements.In all Raman spectra, the 2D band was fitted with one

Lorentz function with an average full width at half-maximum(fwhm) of 35 ± 5 cm−1, which corresponds to monolayergraphene.30,31 This result agrees with UV−vis spectroscopymeasurements because the absorbance was measured over alarge graphene area (∼1 cm2), which incorporates defects,edges, and folding, making the absorbance greater than 0.023.

Figure 3. Low-magnification TEM images of the suspended graphene without and with the NPs on holey carbon TEM grid. (a) Suspendedgraphene without NPs with identification of the number of layers. One (right) and two (left) graphene layers are easily distinguished by the SAEDpatterns. Suspended graphene with (b) Au, (c) AuIn, and (d) AuGa NPs.

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The Raman spectra analysis was done for those spectra thatcorrespond to a single-layer graphene. Likewise, in the Ramanspectra, the dispersion of the data for the G, 2D position, andthe intensity ratio between these bands is generated by thesurface defects, the edges,14,25,32,33 and the nonuniformity inthe NPs distribution on the surface, as can be seen in theRaman maps from Figure 2 and in the low-magnification TEManalysis from Figure 3. In some of the Raman spectra, the Dband was detected with a very low intensity, about 50 timessmaller than those of the G band, especially after the loading ofthe NPs on the graphene surface. The D band is due to thebreathing modes of sp2 atoms, requires a defect to be Ramanactive, and usually can be detected at the edges.10

The ratio between the 2D band and the G band intensities(I2D/IG), which is an important parameter to characterize themonolayer,30 has an average value ∼3 for the graphene withoutNPs and decreases to ∼1.5 to 2 after the NPs were synthesizedon the graphene. A similar decrease in I2D/IG was measuredwhen graphene was doped with electrons or holes by applyingan external voltage in an electrochemical cell.13,27

The suspended graphene without and with NPs has beenobserved by TEM with low magnification (Figure 3). One ortwo graphene layers without NPs were easily distinguished byselected area electron diffraction (SAED) patterns (Figure 3a).

Figure 4 displays the HRTEM images of the suspendedgraphene without and with one NP, where the graphenehexagonal crystal structure can be distinguished and the crystalstructure of the NPs can be identified. The gold NPs aremultiple twinned particles (MTPs) with icosahedral morphol-ogy and single-nanotwinned face-centered cubic configurations(Figure 4b−d).21In the present experiment, EELS measures the excitation of

the carbon K-shell electron (1s electron) to empty antibondingπ* and σ* states at ∼285 and ∼290 eV, respectively.34 The firstpeak is a sharp and narrow peak, while the second peak is a verybroad signal. The fine structure of the EELS signal givesinformation about the difference in the carbon doping.The EELS carbon-edge signals of graphene without and with

various NPs are shown in Figure 5. Each spectrum correspondsto an HRTEM image presented in Figure 4. The carbon edge(CE), the position of the peak corresponding to electrontransition 1s-π* (π*), and the relative ratio between the signalintensities of 1s-π* and 1s-σ* transitions (r) are influenced bythe charge doping of graphene by the NPs.For the suspended graphene without NPs CE and π* are at

284 and 284.8 eV, respectively, and r is 0.60. If the gold NPs areloaded on the graphene, then the CE and π* are found at lowerenergy, at 283.4 and 284.6 eV, respectively, and the value r

Figure 4. HRTEM images of the suspended graphene layer (a) without NPs, (b) with one gold NP, and with one gold-alloy NP containing (c) 4 wt% In and (d) 12 wt % Ga. The insert B represents a magnified image of the region A. The graphene hexagonal crystal structure can be distinguished.The crystal planes of the NP are identified in the panels b−d.

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decreases at 0.46. In the case of the graphene with the gold-alloy NPs, the CE and π* shift at higher energy, and r is lessthan that of the graphene without NPs. For the AuIn NPs,these values are at CE = 284.4, π* = 286.8 eV, and r = 0.50, andfor the AuGa NPs, CE = 285, π* = 287.3 eV, and r = 0.45.In graphene decorated with gold-alloy NPs, the shift of π* to

higher energy is due to the increase in the carbon 1s bindingenergy caused by the greater net charge per carbon atom. If theelectrons are transferred from graphene to the gold-alloy NPs,then the π* states are depopulated and the intensity of the 1s-π* transition decreases.35,36 The transferred electrons from thegold NPs to graphene cause a decrease of the carbon 1s bindingenergy and consequently a shift of CE and π* energies to lowervalues. However, in the case of the gold NPs, a clear change ofthe 1s-π*transition intensity has not been observed. The EELSdata confirm the type of the electron transfer between grapheneand the NPs without giving a quantitative evaluation because ofthe low resolution of the experiment (0.2 eV per pixel).The Fermi level shift depends on the separation distance, the

difference in the work functions, and the chemical interactionbetween graphene and the NP.16,37 The work function of theNPs differs from that of the bulk metal because it is affected bythe size of the NPs38 and the presence of indium or gallium inthe composition.39 The work function of the Au, AuIn, andAuGa NPs becomes 5.52, 5.45, and 5.40 eV, respectively, if theNPs size is <20 nm (Supporting Information).40

Considering the experimental results from the Ramanspectroscopy analysis, the calculated separation distancebetween the graphene and the NPs is ∼0.32 nm in the caseof the gold NPs and ∼0.33 nm in the case of the gold-alloyNPs. (See Figure S3 in the Supporting Information.)16

The small amount of In or Ga in the gold-alloy NPscomposition induces changes in their electrical properties dueto a difference in the electron density between gold and theother metal. In a binary gold alloy, the charge transfer is animportant component in the surface metal−metal bonds thatinvolve dissimilar elements. Therefore, the variation in thecoordination number of a metal or in the geometricalarrangement of its neighbors can produce changes in theorbital hybridization that increase its electronegativity.41

The magnitude of zeta potential of the suspended NPs inwater at pH 7 was −40.11, +42.58, and +48.15 mV for Au,AuIn, and AuGa, respectively (Supporting Information). Theincreased electronegativity of the gold-alloy NPs can explainwhy they accept electrons from graphene differently from thegold NPs.

■ CONCLUSIONS

We investigated large-area graphene decorated with gold andgold-alloy NPs directly synthesized onto the graphene surfacein an aqueous SP without using any surfactant. After thetransfer of the graphene with NPs on different substrates, theNPs maintained indium and gallium in their composition.We observed that graphene may be doped n- or p-type. The

EELS results were consistent with Raman spectroscopy results;that is, the electrons and holes are transferred from the goldand gold-alloy NPs to graphene, respectively. Raman spectros-copy is a microanalysis method and provides average values ofthe Fermi level shift. EELS data confirm on the nanoscale rangethe type of charge transferred between graphene and the NPs.We found that the average charge transferred from graphene

unit cell was −2 × 10−4 e and ∼4 × 10−5 e for gold and gold-alloy NPs, respectively.

■ ASSOCIATED CONTENT

*S Supporting InformationXRD analysis of graphene with NPs transferred on glass,STEM-BF EDS analysis data of the gold-alloy NPs onsuspended graphene, the explanation about UV−vis data, thework function of NPs, the dependence of the Fermi level shifton the equilibrium separation distance, and a comment aboutzeta potential of NPs. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: maria@rd.numse.nagoya-u.ac.jp.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was partially supported by Challenging ExploratoryResearch Kakenhi project sponsored by Ministry of Education,Culture, Sports, Science, and Technology of Japan Science andTechnology Agency.

Figure 5. Carbon edge of the EELS spectra of suspended (a) graphenelayer, (b) graphene with gold NPs, and graphene with gold-alloy NPscontaining (c) In and (d) Ga. The excitation of the carbon K-shellelectron to empty antibonding π* and σ* states is at ∼285 and ∼290eV, respectively. The carbon edge at 284 eV is shifted depending onthe graphene charge doping, which indicates the sign of the Fermilevel change. The black dotted vertical line indicates the position of π*peak for graphene. For each case, the position of π* is indicated by acolored arrow. The positions of CE are marked with colored dotsarrows.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp409368c | J. Phys. Chem. C 2013, 117, 26804−2681026809

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp409368c | J. Phys. Chem. C 2013, 117, 26804−2681026810