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Controlled Electrodeposition of Gold Nanoparticles onto Copper-Supported Few-Layer Graphene in Non-Aqueous Conditions

Anna K. Farquhara, Paula A. Brooksbya*, Robert A.W. Dryfeb*, Alison J. Downarda*

a) MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand

b) School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom

Corresponding Authors:

alison.downard@canterbury.ac.nz

paula.brooksby@canterbury.ac.nz

robert.dryfe@manchester.ac.uk

Abstract

Graphene-Au hybrid materials show promise for applications ranging from biosensing to field emission devices. Electrodeposition is an inexpensive, fast and technically simple method for controlled deposition of nanoparticles but its use with graphene prepared by chemical vapor deposition (CVD) presents some problems. Cu foil is commonly used to catalyze the CVD process and the resulting graphene is most conveniently handled while retained on the Cu support. However Cu is able to spontaneously reduce Au salts in aqueous solution and hence deposition of nanoparticles via galvanic displacement occurs simultaneously with electrodeposition, and control of the growth process is lost. We show here that Au nanoparticles can be controllably electrodeposited onto Cu-supported few layer graphene (FLG) from N,N-dimethylformamide (DMF) solutions of a [AuCl4]- salt because spontaneous deposition of Au nanoparticles does not occur in this medium. Deposition occurs by the instantaneous nucleation mechanism when driven by an applied potential enabling the Au nanoparticle density to be controlled by the deposition conditions, predominantly the deposition potential. Following nucleation, nanoparticle growth is diffusion controlled. Our results demonstrate that the growth rate is similar in the presence and absence of an applied potential and control of growth time is key to controlling nanoparticle size.

Keywords: N,N-Dimethylformamide; chronoamperometry; controlled potential electrolysis; composite; hybrid

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1. Introduction

Graphene, a two-dimensional material comprised of a single sheet of sp2-hybridized carbon atoms [1-3], has numerous exciting properties which allow for a range of potential applications, from sensors to energy storage materials [2]. An ongoing area of research is the development of graphene-metal hybrid materials which give composites with additional properties [4]. Often, graphene is decorated with noble metal nanoparticles such as Au, Ag, Pt, and Pd [5]. Metal nanoparticles with sizes less than 100 nm may display different properties to the bulk material, hence hybrid graphene-nanoparticle composites offer the opportunity of combining the advantageous properties of both the graphene and the metal nanoparticles, as well as novel properties that result from interactions between two components.

Au nanoparticles are a popular choice for the development of graphene-metal nanoparticle hybrid materials [6]. Valcárcel et al. recently demonstrated that Au-decorated reduced graphene oxide (RGO) can function as a substrate for surface enhanced Raman spectroscopy (SERS), and used this for the detection of the antibiotic metronidazole [7]. Zhang et al. thermally evaporated Au nanoparticles onto graphene and demonstrated enhanced function as a field emission device [8]. Lee and coworkers used an in-situ chemical vapor deposition method for the preparation of Au nanoparticles encapsulated within few-layer graphene and utilized the material as a non-enzymatic glucose sensor [9]. Yong et al. used a graphene-gold system in plasmonic biosensing [10]. They found that by transferring graphene to an Au thin film, they could achieve a four-fold increase in sensitivity, compared to the Au film alone. They then showed a further order of magnitude increase by incorporating Au nanoparticles on to the graphene/Au surface. As a final example, Chen and Ju deposited Au nanoparticles onto nitrogen-doped graphene, and demonstrated a high sensitivity and selectivity for the electrochemical detection of hydrogen peroxide [11].

Clearly, graphene-Au nanoparticle composites have applications in a wide range of fields. Many of these applications are dependent on the size and morphology of the Au deposits, which are dictated by the deposition method. The traditional routes to make graphene-, graphene oxide (GO)- and RGO-Au composites use ex situ (mixing) or in situ (growth) methods to deposit particles, or wrap particles within, the graphene, GO or RGO sheets [6]. Solution phase methods have the advantage of simplicity, however spontaneous in situ growth methods can be difficult to control. In contrast, electrodeposition is a particularly useful method for introducing metal nanoparticles onto conducting substrates as varying the deposition parameters can readily control the morphology of the nanoparticles [12]. The majority of work involving Au electrodeposition onto graphene materials has been carried out using RGO supported on a conducting substrate such as glassy carbon (GC) [13-15]. RGO typically has a small particle size and a significant number of residual oxygen groups whereas graphene prepared via chemical vapor deposition (CVD) has a low oxygen content [16] and can have a large surface area [17, 18]. CVD graphene is usually grown on a Cu foil catalyst [19] and is most easily handled before removal from the solid support. Hence it is desirable to undertake further functionalization procedures on the Cu-supported graphene.

Electrodeposition of Au nanoparticles is commonly carried out in aqueous conditions. For graphene supported on Cu this presents a significant challenge. In water, the standard electrode potential of the Au/[AuCl4]- couple (+1.00 V vs. SHE) is more positive than that of the Cu/Cu2+ couple (+0.34 V vs. SHE) [20]. Thus the Cu support is able to reduce Au(III), resulting in galvanic displacement of Cu for Au [21]. Furthermore, Jung et al. [22] showed that RGO itself can the reduce [AuCl4]- to Au(0) in aqueous solution. It is therefore possible that graphene is also able to spontaneously reduce [AuCl4]-. The operation of the galvanic processes makes it difficult to control the size of the Au deposits formed during electrodeposition in water, as both processes will occur simultaneously. Navarro et al. [23] recently demonstrated that applying a negative potential to the graphene/Cu working electrode prior to adding the metal salt precursor could prevent the spontaneous reaction and demonstrated this with Ag and Pd nanoparticles. An alternative approach for nanoparticle growth on graphene/Cu is to use non-aqueous conditions. The standard electrode potential of the Au/[AuCl4]- couple in organic solvents is typically more negative than in water, due to the difference in solvation energy of the [AuCl4]- ion [24]. For example, in 1,2 dichloroethane, the potential was calculated to be −1.14 V vs SHE [24]. Under these conditions, the chemical reduction of [AuCl4]- by Cu is thermodynamically non-spontaneous, which allows better control of the deposit morphology, and prevents the Cu foil from being oxidized, and the graphene delaminating, during the deposition.

The goal of this work was to combine the advantages of electrodeposition with the convenience of working with graphene on its Cu support, to prepare few layer graphene (FLG) decorated with Au nanoparticles of controlled size and density. We show that in N,N-dimethylformamide (DMF), spontaneous deposition of Au nanoparticles on Cu-supported FLG does not occur, allowing exclusively potential-driven initiation of the deposition. After electrodeposition of Au, a PMMA-free method [16] was used to transfer the modified graphene from the Cu support to Si wafer for analysis. Electrodeposition from DMF is thus a simple and convenient strategy for controlled deposition of Au nanoparticles on FLG.

2. Experimental

2.1 Reagents and substrates

All chemicals were used as received. MilliQ water, resistivity > 18 MΩ.cm, was used to prepare electrolyte solutions and for cleaning samples. Tetraoctylammonium tetrachloroaurate (TOA)[AuCl4] was prepared by mixing TOACl and HAuCl4.3H2O in ethanol, followed by recrystallization [25]. FLG was grown on a Cu foil coupon by atmospheric pressure (AP) CVD as described previously [16]. Si/SiOx wafers were cleaned in acetone and MilliQ water prior to use and then dried under N2.

2.2 Electrochemical deposition of Au nanoparticles and transfer of FLG

Electrochemical depositions and measurements were made using an Eco Chemie Autolab PGSTAT302N potentiostat running Nova software. The working electrode was a 1 cm × 1 cm FLG/Cu coupon, with a Cu foil electrical contact. The Cu coupon has FLG on both sides, and both sides were exposed to the deposition solution. A Pt wire was used as a pseudo-reference electrode and a Pt mesh as the counter electrode. Electrodepositions were carried out in a solution of 0.36 mM (TOA)[AuCl4] and 0.1 M tetrabutylammonium perchlorate (TBAClO4) in DMF. Prior to deposition experiments, a cyclic voltammogram (CV) was obtained (scan rate = 100 mV s-1) in a separate solution, and with added ferrocene (Fc), using a Pt wire-working electrode to confirm the peak potential for [AuCl4]- reduction and E1/2 for the Fc/Fc+ redox couple. E1/2(Fc/Fc+) was 0.10 V vs the Pt pseudo-reference electrode and it was assumed that the same E1/2 applies at FLG/Cu allowing all potentials to be reported vs Fc/Fc+. Note that a FLG/Cu working electrode was not used to obtain CVs of the Fc/Fc+ couple to avoid possible complications from oxidation of Cu. Depositions of Au nanoparticles were carried out at fixed potentials of −0.4, −0.6 or −0.8 V for times ranging from 30 s to 10 min. After Au deposition, the FLG/Cu coupon was washed with fresh DMF and floated on 0.5 M aqueous ammonium persulfate solution for 15 min. The FLG was released from one side of the Cu coupon; this FLG was discarded. The coupon was then refloated, Cu-side down, on ammonium persulfate solution to remove the Cu. The ammonium persulfate solution was removed and replaced with MilliQ water to wash the sample [16]. For analysis, the FLG/Au sheet was collected onto a 1 × 1 cm2 Si/SiOx substrate, then dried in air for 1 h and at 60oC for 30 min [16]. Where the results of duplicate experiments are reported, samples were prepared separately beginning with FLG grown in different furnace runs.

2.3 Microscopy and spectroscopy

Atomic force microscopy AFM (Multimode 8 from Bruker, USA) topographical images were collected in peak force quantitative nanomechanical mapping (QNM) mode with scan assist activated with a Si cantilever (SNL-10). Square images were collected with 512 samples per line at a scan rate of 0.65 Hz. Particle height and density were measured using NanoScope Analysis software. The particle height was determined using the section tool. A line profile was drawn across a nanoparticle, and the height of the nanoparticle was taken as the vertical distance from immediately before the nanoparticle to the highest point on the particle. The heights of 15 individual nanoparticles over a 1 μm × 1 μm area were measured, and the average reported. Particle density was calculated using the particle analysis tool over a 2.5 μm × 2.5 μm area.

Scanning electron microscopy (SEM) images were collected using an FEI Quanta 650 ESEM-FEG operating in high vacuum at 20.0 kV.

Raman Spectroscopy was carried out using a Renishaw RM 264N94 (532 nm laser) spectrometer operating at power ≤ 1 mW. For each sample 3 spectra were recorded at 3 separate points on the sample, meaning the results listed are an average of 9 spectra for each sample.

3. Results and Discussion

The FLG used in this study was comprised of 3-4 layers, as confirmed by a UV transparency of approximately 92% [26]. The Raman spectrum (Supplementary Data, Fig. S1) of an unmodified FLG sheet transferred to a Si/SiOx wafer shows the expected G- (1584 cm-1) and 2D- (2698 cm-1) bands [27]. The D-band (1350 cm-1) indicates that a number of defects are present, most likely in the form of point defects and grain boundaries between graphene domains [28]. An AFM image of unmodified FLG on Si/SiOx is given in Fig. 1A. The FLG shows wrinkles and folds, which result from the transfer process. Water can become trapped between the FLG and the substrate, enhancing these features [29].

In order to tightly control the size and density of Au nanoparticles deposited on FLG/Cu, electrodeposition must be undertaken in a solvent in which spontaneous nucleation of particles does not occur. Initial experiments investigated the suitability of DMF for this purpose. DMF was selected because it is a good solvent for electrochemistry and is able to dissolve the tetraoctylammonium (TOA) salt of [AuCl4]-. Previous work demonstrated that just 1 min immersion of Cu-supported graphene in an aqueous solution of HAuCl4 at open-circuit potential led to a significant coating of Au nanoparticles [20]. In contrast, there was no evidence of nanoparticle deposition after 5 min immersion of FLG/Cu in a solution of 0.36 mM (TOA)[AuCl4] and 0.1 M TBAClO4 in DMF, without the application of a potential. AFM images of FLG transferred to a Si/SiOx wafer before and after immersion in the (TOA)[AuCl4] solution are shown in Fig. 1. No new features are seen after immersion. In the high resolution images (Figs. 1B and D), the numerous small approximately circular features, less than 1 nm in height, are most likely carbon based structures, for example spheres and amorphous particles [16]. The larger high features are the consequence of water trapped between the Si/SiOx substrate and the FLG [29, 30]. The similarity between the images before and after immersion in the DMF solution of (TOA)[AuCl4] confirms that electroless deposition of Au nanoparticles does not occur in DMF.

Fig. 1

Fig. 2 shows CVs obtained at FLG/Cu in 0.1 M TBAClO4 - DMF in the absence and presence of (TOA)[AuCl4]. The broad reduction peak at -0.70 V (vs. Fc/Fc+) is assigned to reduction of [AuCl4]-.

Fig. 2

Reduction of [AuCl4]- to Au(0) has been shown to proceed via a one-step, three-electron process (eqn 1) [31, 32] or a two-step process (eqns 2 and 3) [33, 34].

[AuCl4]- + 3e- → Au(0) +4Cl-(1)

[AuCl4]- + 2e- → [AuCl2]- + 2Cl- (2)

[AuCl2]- + e- → Au(0) + 2Cl-(3)

A one-step process is observed in aqueous media whereas the two-step process has been reported for ionic liquids and some organic solvents. In dimethylsulfoxide (DMSO) the first reduction peak is seen at −0.3 V (vs. Fc/Fc+) [34] and hence CVs were obtained over a wider potential range to establish whether there is a second reduction step in DMF. GC, rather than FLG/Cu, was used as the working electrode for these experiments to avoid Cu oxidation. As shown by the CV in Fig. S2, there is only one reduction peak in the range 0.9 to -1.1 V (vs. Fc/Fc+) indicating a one-step, three-electron reduction for [AuCl4]- in DMF.

Electrodeposition of Au nanoparticles onto FLG/Cu was investigated using constant potential electrolysis. Three potentials were chosen: −0.4, −0.6, and −0.8 V which correspond to prior to the reduction peak (2), after the first shoulder, and after the peak, respectively. Typical chronoamperometric transients for the deposition at −0.6 V and −0.8 V are shown in Fig. S3. A selection of AFM images of FLG after electrodeposition and transfer to a Si/SiOx wafer are shown in Fig. 3. AFM images corresponding to all other deposition conditions can be found in Fig. S4. As noted above, the large high features are most likely due to the effect of water trapped between the Si/SiOx substrate and the FLG [29, 30].

Fig. 3

Figs. 4A and B show plots of particle density and height vs. time, obtained from the AFM images in Figs. 3 and S4. Particle height rather than diameter is reported as a measure of particle size because it is not possible to accurately determine the diameter of sub-50 nm particles using our AFM system. Fig. 4C shows particle density vs. particle height.

Fig. 4A, B, and C

The results shown in Fig. 4 are similar to those obtained by Penner et al. [35, 36] for Au deposition on highly ordered pyrolytic graphite (HOPG), and Macpherson et al. [37] for Au deposition on carbon nanotubes, and are consistent with a kinetically-controlled, instantaneous nucleation mechanism [23]. The increase in nanoparticle density when the applied potential is increased from −0.4 to −0.6 V (Fig. 4A) is the expected result of a higher reduction rate at the more negative potential which generates a greater density of nuclei [23, 35, 37, 38]. The current-time transients in Fig. S3 which show a rapid increase in current to a constant value, are also consistent with a mechanism in which the nucleation time is significantly shorter than the growth phase. However the increase in particle density with deposition time, especially apparent for the samples prepared at −0.6 V suggests that some nucleation continues during the growth phase.

Fig. 4A shows that the samples prepared by 10 min deposition at −0.6 and −0.8 V have the same nanoparticle densities. In fact, deposition at −0.8 V led to nanoparticle aggregation as is clearly evident in the SEM image of Fig. 5. Aggregation makes it difficult to accurately determine the particle density and hence the value shown in Fig. 4A underestimates the number of nanoparticles nucleated.

The AFM images of Fig. 3 reveal that Au nanoparticle densities are not uniform across the FLG surface. This suggests that particle nucleation is not favorable at some areas however we have not investigated the origin of this effect.

For nanoparticle growth through an instantaneous nucleation mechanism, formation of nuclei is followed by a diffusion controlled growth phase, which is independent of applied potential. As a consequence, the size of nanoparticles should depend on deposition time only. However the data of Fig. 4B reveal that at a given deposition time, there is a small decrease in average particle size as the deposition potential increases. This is attributed to the increase in particle density with potential, which results in a lower flux of [AuCl4]- ions to each particle and therefore a slower growth rate [37]. Differences between the diffusion fields for isolated and closely-spaced nanoparticles also accounts for the observation that the size distribution (indicated by the error bars in Fig. 4B) of nanoparticles deposited at −0.4 and −0.6 V, increases with time. As particle growth proceeds, the effects of different fluxes of [AuCl4]- ions on nanoparticle growth rate is amplified, resulting in a broadening size distribution [35].

Fig. 4C shows particle density vs particle height for the three different electrodeposition potentials investigated. The area between the three sets of data points indicates particle density and height combinations that should be accessible through selection of the appropriate potential and deposition time.

AFM and SEM imaging (Fig. 5) of the electrodeposited samples reveals that the nanoparticles are spherical or hemispherical. Although it is not possible to determine from the images whether the nanoparticles are spherical or hemispherical, it is likely that they are spherical. Metal nanoparticle growth on graphitic surfaces is expected to be the thermodynamically favorable Volmer-Weber type growth [39, 40]. A weak interaction between the metal and the substrate results from the large difference in hydrophilicity of the two materials. Consequently, the nanoparticles grow in a manner that maintains a minimum contact area with the substrate, resulting in spherical-shaped deposits [12].

Fig. 5

Such weak interactions between graphene and metallic particles might be expected to lead to weak adhesion of the Au nanoparticles. However, for Au nanoparticles deposited on HOPG, Stimming et al. [41] suggested that edge and defect sites could increase the adhesion of gold nanoparticles. A similar effect may be operative for FLG. Furthermore, the polymer-free process used here to transfer the modified FLG from its Cu support to a second substrate, minimizes the opportunities for dislodging nanoparticles

An interesting observation concerning Au nanoparticle growth is that once nucleation and growth is initiated, Au nanoparticles continue to grow in the absence of an applied potential. Fig. 6 shows an AFM image of a FLG sample for which a potential of -0.6 V was applied for 1 min, and then the FLG/Cu working electrode was disconnected but left in contact with the deposition solution for a further 9 min. The nanoparticle density for this sample was 51 μm-2, similar to that of the sample electrolyzed for 1 min at -0.6 V, suggesting that nucleation only occurred while the potential was applied. However, the average particle height (15.3 nm) was significantly greater than for the sample deposited for 1 min at -0.6 V and approaches that of the sample deposited at -0.6 V for 10 min. This suggests a similar growth rate, with and without applied potential, that is, growth controlled by diffusion of [AuCl4]- to the particles. For growth to continue, [AuCl4]- must be reduced to Au(0). The most likely reducing agent is DMF: it has been reported that colloidal Ag and Au nanoparticles can be formed via reduction of the corresponding metal precursor by DMF at elevated temperatures [42, 43]. Our results suggest that at room temperature, spontaneous reduction of [AuCl4]- and nucleation of Au(0) on FLG cannot occur in DMF in the absence of an applied potential, but once nucleated, DMF can reduce [AuCl4]- at the Au nanoparticle surface leading to nanoparticle growth. The practical consequence of this phenomenon is that the FLG must be removed from the deposition solution immediately after completion of electrodeposition to ensure control over particle size.

Fig. 6

4. Conclusion

Electrodeposition from DMF solution is a convenient method for depositing Au nanoparticles on Cu-supported CVD graphene. Retaining the graphene on the Cu foil catalyst greatly simplifies its handling, while the uncontrolled, spontaneous nucleation of nanoparticles that occurs in aqueous conditions is avoided through use of DMF as the electrodeposition medium. Control of the electrodeposition potential and time allows control of the nanoparticle density and size: in this work particles with heights ranging from 5-25 nm were deposited, with densities ranging from 25 -95 μm-2. We expect that different combinations of particle size and density might be accessible by varying the concentration of Au(III) salt in the deposition medium. In future work, the extension of the methodology to other noble metal nanoparticles will be investigated as a straightforward route to graphene-nanoparticle hybrid materials with interesting functional properties.

Acknowledgements

R.D. thanks the EPSRC (grant ref. EP/K016954/1) for support. A.K.F, P.A.B. and A.J.D. thank the Royal Society of New Zealand Marsden Fund (UOC1307) for support. A.K.F. thanks the University of Canterbury for a doctoral scholarship. The authors thank Dr. Patrick Hill at the University of Manchester for SEM imaging.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at

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Figure Captions

Fig. 1. AFM images of FLG on a Si/SiOx wafer before (A) & (B) and after (C) & (D) immersion for 5 min in 0.36 mM (TOA)[AuCl4] and 0.1 M TBAClO4 in DMF with no applied potential.

Fig. 2. CV obtained at FLG/Cu in 0.1 M TBAClO4 DMF in the absence (red) and presence (blue) of 0.36 mM (TOA)[AuCl4]. Scan rate = 100 mV s-1.

Fig. 3. AFM images of FLG on Si/SiOx after electrodeposition of Au nanoparticles for (A) 5 min at -0.4 V; (B) 10 min at -0.4 V; (C) 5 min at -0.6 V; (D) 10 min at -0.6 V; (E) 10 min at -0.8 V. Scale bars: 1.25 μm

Fig. 4. Plots of (A): particle density with deposition time. (B) nanoparticle height with deposition time. (C) nanoparticle density with particle height at the deposition time indicated. Error bars show the standard deviation of particle height over a 1 µm × 1 µm area; hollow and solid symbols of the same colour show the results of duplicate experiments under the conditions indicated.

Fig. 5. SEM image of FLG on Si/SiOx after electrodeposition of Au nanoparticles for 10 min at −0.8 V.

Fig. 6. AFM image of FLG on Si/SiOx after electrodeposition of Au nanoparticles at -0.6 V for 1 min followed by 9 min in the deposition solution at open circuit potential. Scale bar 1.25 μm.

Fig. 1

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Fig. 3

Fig. 4A

Fig. 4 B

Fig 4 C

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