Colloid Polym Sci (2010) 288:1097–1103 DOI 10.1007/s00396-010-2238-2

ORIGINAL CONTRIBUTION

Electrodeposition of gold nanoparticles from ionic liquid microemulsion Chaopeng Fu & Haihui Zhou & Ding Xie & Lu Sun & Yifan Yin & Jinhua Chen & Yafei Kuang

Received: 3 February 2010 / Revised: 13 April 2010 / Accepted: 30 April 2010 / Published online: 14 May 2010 # Springer-Verlag 2010

Abstract Gold nanoparticles were electrodeposited directly for the first time from a new electrolyte system: water-in-ionic liquid (W/IL) microemulsion. The electrochemical behavior of Au(Ш) in W/IL microemulsion was investigated. The cyclic voltammetry (CV) result of Au(Ш) shows a pair of redox peak. The effect of precursor apparent concentration on the reduction peak current density is similar to that in homogeneous solution such as aqueous solution. The effect of scan rate on the reduction peak current density is different from that in homogeneous solution. Linear-sweep voltammograms result for a rotating disk electrode in the W/IL microemulsion suggests that the reduction is kinetically limited and not transport limited. And also the activation energy of the reaction was calculated to be 26.7 KJ mol−1. The gold electrodeposits were characterized by scanning electron microscopy and X-ray diffraction. It is found that the gold electrodeposits are face-centered cubic and nanosized. Furthermore, the potential mechanism for the electrode reaction was proposed. In addition, the electrochemical properties of the gold nanoparticles were researched through the electro-oxidation of glycerol. The CV and electrochemical impedance spectroscopy studies demonstrate that the gold nanoparticles electrodeposited from W/IL microemulsion have much higher electro-catalytic activities than bare gold for glycerol oxidation. C. Fu : H. Zhou : D. Xie : L. Sun : Y. Yin : J. Chen : Y. Kuang (*) College of Chemistry and Chemical Engineering, Hunan University, Changsha, China 410082 e-mail: [email protected] H. Zhou : J. Chen State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, China 410082

Keywords Ionic liquid microemulsion . Electrochemical behavior . Electrodeposition . Gold nanoparticles . Electrocatalysis

Introduction Recently, nanoparticles synthesis has been an active research field, because nanoparticles exhibit unusual and excellent physical and chemical properties different from those of relatively larger particles [1–3]. Materials with nanostructure have exhibited outstanding applications in many various areas such as electrocatalysis [4, 5], biosensor [6, 7], microelectronic devices [8, 9], and photocatalysis [10, 11]. A variety of methods have been used to prepare nanosized materials [12– 14]. One promising technique employed in synthesizing nanoparticles is microemulsion method. Microemulsions are unique class of thermodynamically stable isotropic dispersions of two or more immiscible liquids which are stabilized by an adsorbed surfactant film at the liquid–liquid interface [15]. In this system, the surfactant-stabilized nanopools can be used as microreactors which can limit particles nucleation, growth and agglomeration [16, 17], then particles with controllable size, good size distribution, and chemical composition can be obtained in such a medium through modulating synthesis conditions [2, 17, 18]. A large number of nanosized materials were prepared by reverse microemulsion method, though most of the preparation is via chemical reduction or chemical oxidation through adding reductant or oxidant into reverse microemulsions [18, 19]. More recently, some investigations reported that ionic liquids (ILs) could substitute water or oil to form novel microemulsions in the presence of surfactant. The novel ionic liquid (IL) microemulsions have both the advantages of ILs and conventional microemulsions, which can over-

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come the inability of ILs to dissolve a number of chemicals including some hydrophilic substances and then broaden the utilization of ILs [20]. Some papers demonstrated that IL could substitute water to form nonaqueous IL microemulsion and exist as nanosized polar domains dispersed in p-xylene or triethylamine with the aid of surfactant [21, 22]. And also, Gao et al. prepared and characterized TX-100/H2O/1-butyl3-methylimidazolium hexafluorophosphate (bmimPF6) microemulsion using different techniques. Their research results showed that water domains existed in the water-inbmimPF6 microemulsion, which could dissolve salts [23]. Compared with conventional reverse microemulsion (waterin-oil), water-in-IL microemulsions are more effective for electrochemical research because both IL and aqueous solution are conductive, which make IL microemulsions have better conductivity. Electrochemical deposition which is an economical and convenient choice for preparing uniform and size controllable nanomaterials has never been reported in IL microemulsions by other researchers. Recently, our group has described the comparison of electrodeposition in IL microemulsions and found the difference in water-in-IL (W/IL) and IL-in-oil microemulsions [24]. However, the discussion of the electrodeposition was not in detail. The present article is an extension of our previous investigation on electrochemical research in IL microemulsion. In the present paper, the electrochemical behavior of Au(Ш) on glassy carbon electrode was discussed, and gold nanoparticles were electrodeposited in water-in-IL microemulsion for the first time, and the electrochemical property of the gold nanoparticles obtained from the special medium was also reported.

Materials and methods Materials TX-100 [CH3C(CH3)2CH2C(CH3)2C6H4(OCH2CH2)9H] (M= 646.86 gmol−1, d25 =1.07 gmL−1) was obtained from Alfa Aesar. Hydrogen tetrachloroaurate hydrate (HAuCl4·3H2O, 99.9%), glycerol and sodium hydroxide were analytical grade and provided by Sinopharm Group Chemical Reagent Co., Ltd. bmimPF6 (>98%) was synthesized according to the method reported in the literature [25] and dehydrated at 80 °C under vacuum until the weight was consistent. Double distilled water was used throughout this study.

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the samples were mixed and stirred, a transparent and stable W/IL microemulsion was obtained. The W/IL microemulsion used in the present study contained 7.3 wt.% H2O, 40.6 wt.% TX-100, and 51.8 wt.% bmimPF6. Cyclic voltammetry (CV) experiments were carried out with a CHI model 760C electrochemical workstation (Shanghai Chenhua Instrument Factory, China). Linear-sweep voltammograms were conducted with a scan rate of 10 mV s−1 at different rotation rates using a Pine Instrument Company, Model AFMSRCE Rotator. The temperature was controlled by a thermostatic bath (HH-S, Yuhua Instrument Co., China). Measurements were carried out in a three-electrode cell, which consisted of a glassy carbon working electrode (Φ=3 mm), a platinum counter electrode and an Ag/AgCl reference electrode. Prior to each experiment, the glassy carbon electrode was polished to a mirror finish using alumina powder and then was ultrasonicated in ethanol, dilute HNO3 aqueous solution, and double distilled water for 5 min, respectively. Electrodeposition experiments were performed with galvanostatic system (Handan Instrument Factory, China) under 2 mA cm−2 for 10 min in a two-electrode cell, which consisted of a pretreated glassy carbon substrate and a Pt foil counter electrode. The microstructure of the gold deposits was investigated by both X-ray diffraction (XRD; Bruker D8 Advance Diffractometer, Cu Kα1) and scanning electron microscopy (SEM; JSM-6700F, JEOL Company Japan). Electrochemical properties of gold nanoparticles The glycerol electro-oxidation experiments were conducted by CV and electrochemical impedance spectroscopy (EIS) with CHI 760C electrochemical workstation in an aqueous solution of 0.1 mol L−1 glycerol and 1 mol L−1 NaOH. The three-electrode cell consisted of a working electrode, a platinum counter electrode and a saturated calomel electrode. Before experiments, the obtained Au-modified glassy carbon (Au/GC) electrode was washed thoroughly with ethanol and water to remove the impurities and then dried. For CV, the potential was scanned from −0.3 to 0.6 V, and the scan rate was 50 mV s−1. For EIS, the frequency ranged from 100 kHz to 0.05 Hz with an excitation signal of 5 mV. The active surface areas measurements were taken by CV between 0 and +1.5 V in 0.5 mol L−1 H2SO4 by integration of the cathodic reduction peak.

Electrodeposition and characterization of gold nanoparticles

Results and discussion

Hydrogen tetrachloroaurate hydrate was dissolved in water phase prior to mixing with TX-100/IL phase. The concentration of hydrogen tetrachloroaurate hydrate was 2.2, 2.9, 4.4, 5.9, 8.8 mmol L−1, respectively. It is apparent concentration which is based on the total volume of microemulsions. After

Electrochemical behavior of Au(Ш) in W/IL microemulsion Since gold received much attention in catalysis and biosensor fields, the electrochemical behavior of Au(Ш)

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in aqueous solutions has been widely reported [26, 27]. IL microemulsions, as a new kind of electrolyte, have special structure compared with homogeneous solutions such as aqueous solutions and ILs. The interface between electrode and IL microemulsion is different from that between electrode and aqueous solution or IL. And also, it can be predicted that the electrochemical reaction behavior of Au (Ш) in these systems are different. CV measurements were carried out in the IL microemulsion system. Figure 1 depicts the steady-state CV curves of GC electrode in W/IL microemulsion without (curve a) and with (curve b) HAuCl4 at a scan rate of 50 mV s−1. A pair of redox peaks can be observed clearly in curve b while there is no peak in curve a. A cathodic reduction current peak appears at about 0.059 V in the presence of Au(Ш) corresponding to the deposition of metallic gold, and an anodic current peak is also present during the positive potential scan related to the oxidation of gold electrodeposits. This means that Au(Ш) can be electro-reduced from the W/IL microemulsion system. It is necessary to mention that usually there is a current crossover between the cathodic and anodic branches in aqueous solution which is indicative of nucleation growth kinetics [26]. However, the current crossover is not observed in the present CV curves, which suggests that the nucleation characteristic of gold in W/IL microemulsion is different from that in aqueous solution. To obtain more kinetics information about the electroreduction of Au(Ш), the effects of scan rate (v), Au(Ш) concentration and reaction temperature on the reduction peak current (ip) were investigated. The effect of Au(Ш) apparent concentration on ip is shown in Fig. 2a. The expected increase of ip as the Au(Ш) concentration increasing can be observed. It is obviously found that ip is proportional to Au(Ш) concen-

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tration. In Bard's report, whether the electrode reaction was reversible, quasireversible or totally irreversible, the linear relationship between ip and concentration could be always observed [28]. In W/IL microemulsion, most of the HAuCl4 was dissolved in the water phase. The microcosmic distribution of the Au(Ш) concentration is asymmetric and incontinuous, although the effect of Au(Ш) concentration on the reaction kinetic is the same as that in homogeneous solution such as aqueous solution. The relationship between ip and v1/2 in the system was investigated. From Fig. 2b, it can be observed that the plot of ip versus v1/2 is not linear, which indicates that the reduction reaction of Au(Ш) in W/IL microemulsion is not completely diffusion controlled. Bard also documented that whether the electrode reaction was reversible, quasireversible or totally irreversible, the linear relationship between peak current and v1/2 could be always observed [28]. However, the linear relationship does not exist for the present electrode reaction. This reveals that the effect of scan rate on the reaction kinetic in W/IL microemulsion is different from that in homogeneous solution. Linear-sweep voltammograms (LCVs) were also performed at a rotating disk electrode under steady-state conditions. Figure 3a illustrates the LCVs for a rotating disk GC electrode in W/IL microemulsion containing HAuCl4 at a scan rate of 10 mV s−1 with different rotation rates (ω). It can be observed that a current plateau appears instead of a “peak” under each rotation rate. Apparently, the current plateau value (limited current iL) increases with increasing rotation rate. Figure 3b shows the Koutecky– Levich (K-L) plot (1/iL versus ω-1/2) for the reduction of Au (Ш) in W/IL microemulsion. It can be seen that the plot shows a linear relationship between 1/iL and ω-1/2 with a nonzero intercept, implying that the reduction is kinetically limited and not transport limited [29–31]. The relationship between reduction peak current and reaction temperature was researched. It is found that the peak current increases with increasing the temperature from 10 to 45 °C. Since the peak current is proportional to the rate constant at a given experimental condition, the activation energy of the electrode reaction can be estimated using the Arrheniuslike expression [32–34] (see Eq. 1).
ln ip ¼ B  EA =RT

Fig. 1 CV curves of GC electrode in W/IL microemulsion without (a) and with (b) HAuCl4 at a scan rate of 50 mV s−1

ð1Þ

Where EA is the activation energy for the reaction, B is a constant, R is the molar gas constant and T is the temperature (K). The ip used in this equation is the reduction peak current obtained from the CV data. Figure 4 depicts the linear relationship between ln(ip) and 1/T with a high correlation coefficient. EA was calculated from the slope of the line to be 26.7 KJ mol−1. The value of the activation energy is low, indicating a less dependency on

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Fig. 2 a The reduction peak current density as a function of the Au (Ш) apparent concentration (2.2, 2.9, 4.4, 5.9, 8.8 mmol L−1) in W/IL microemulsion with a scan rate of 50 mV s−1. b The reduction peak

current density as a function of the square root of scan rate in W/IL microemulsion with the Au(Ш) apparent concentration of 4.4 mmol L−1. Scan rate: 20, 50, 80, 100, 120, 150, 200 mV s−1

temperature for the electroreduction of Au(Ш) in W/IL microemulsion [32, 33]. For the results presented above, we could not find any other similar study for comparison. The special electrochemical behavior of Au(Ш) in W/IL microemulsion may be due to the particular structure characteristic of the microemulsion. In W/IL microemulsion, Au(Ш) is dissolved in water which is dispersed and incontinuous. Only when the water droplets containing Au(Ш) collide with the electrode by Brownian motion, Au(Ш) can reach the electrode surface and the electrode reaction can take place, which is different from homogeneous solution. Thus, the

special electrochemical behavior of Au(Ш) in the W/IL microemulsion is obtained.

Fig. 3 a LCVs for a rotating disk GC electrode in W/IL microemulsion containing HAuCl4 at a scan rate of 10 mV s−1 with different rotation rates: 900, 1,600, 2,000, 2,500, 3,000 rpm. b K-L plot for the reduction of Au(Ш) in W/IL microemulsion

Electrodeposition of gold nanoparticles Microemulsion route is one of the most promising methods for synthesizing nanostructure materials because droplets in microemulsion can be used as microreactors [2, 17]. Electrodeposition was carried out in the electrode/IL microemulsion system with Au(Ш) at a fixed content. Figure 5 shows SEM image of the gold electrodeposits obtained from W/IL microemulsion under 2 mA cm−2 current density for 10 min. It can be observed that the electrodeposits are composed of nanosized gold particles

Fig. 4 Arrhenius plot of ln(ip) versus 1/T in W/IL microemulsion containing 4.4 mmol L−1 Au(Ш) at the temperature from 10 to 45 °C

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Fig. 5 SEM image of the gold electrodeposits obtained from W/IL microemulsion containing 4.4 mmol L−1 Au(Ш) under 2 mA cm−2 current density for 10 min

with the diameter of about 25 nm. Figure 6 shows the XRD pattern of the electrodeposits obtained from the W/IL microemulsion under 2 mA cm−2 current density for 10 min. The peaks at 38.06°, 44.24°, 64.42°, and 77.62° of the scattering angles 2θ in Fig. 6 correspond to the gold (111), (200), (220), and (311) planes, respectively, which suggests that the gold is indexed as face-centered cubic [35, 36]. The XRD result also approves that gold can be electrodeposited in W/IL microemulsion. On the basis of the information we have gathered, the cathodic reduction process of Au(Ш) at the electrode/IL microemulsion interface can be explained as follows. In the electrode system, HAuCl4 was dissolved in water which

Fig. 6 XRD pattern of the electrodeposits obtained from W/IL microemulsion containing 4.4 mmol L−1 Au(Ш) under 2 mA cm−2 current density for 10 min

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was the dispersed phase of the microemulsion, the water nanopools containing Au(Ш) collide with the electrode surface continuously because of Brownian motion. When the water nanopools collide with the electrode, Au(Ш) can reach the electrode surface and the electrode reaction can take place. Once the water nanopools leave the electrode surface after colliding, the electrode reaction ceases. During the reaction process, the high dispersive surfactant-covered water nanopools offer unique microenvironments as nanoreactors, and the surfactant-stabilized nanoreactors provide a cage-like effect that limits particle nucleation [24]. In other words, when the particle size approaches that of the water nanopool, the surfactant molecules will adsorb on the particle surface to inhibit growth and agglomeration of the particle. Consequently, the electrodeposition in W/IL microemulsion is completely different from that in aqueous solution and the morphology of the electrodeposits obtained from W/IL microemulsion reveals granular with nanostructure. Electrochemical properties of the electrodeposits The active surface areas of the gold nanoparticles electrode and bare gold electrode were evaluated by accepting the charge of 0.386 mC cm−2 as the charge necessary to form a monolayer of electroadsorbed O in the form of AuO allowing for the double-layer charging [37]. According to the CVs conducted in 0.5 mol L−1 H2SO4 solution for the two different gold electrodes, the active surface area of the gold nanoparticles electrode was calculated to be 0.072 cm2, compared with 0.035 cm2 for the bare gold electrode. Furthermore, we investigate the electro-catalytic oxidation of glycerol on the prepared gold nanoparticlesmodified electrode, which is important in both electrosynthesis and fuel cells technology [38, 39]. Figure 7 presents the steady-state voltammograms of the gold nanoparticles electrode and bare gold electrode recorded in 0.2 mol L−1 glycerol and 1.0 mol L−1 NaOH aqueous solution at 50 mV s−1. It can be seen that both the CV curves have two oxidation peaks, one oxidation peak is observed in the positive potential scan and the other is observed in the negative potential scan, which means that glycerol is electro-oxidized on both the gold nanoparticles electrode and bare gold electrode. However, both the oxidation peak current densities on the gold nanoparticles electrode are higher than those on the bare gold electrode and both the oxidation peak potentials on the gold nanoparticles electrode are more negative than those on the bare gold electrode. The results described above clearly show that the gold nanoparticles electrode has better electro-catalytic performance than the bare gold electrode owing to high specific surface area of the gold nanoparticles electrode.

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To further elaborate the difference between electrooxidation of glycerol on the gold nanoparticles electrode and that on the bare gold electrode, electrochemical impedance spectroscopy was carried out, which is a sensitive and powerful measurement to study electrochemical performance [40]. Figure 8 shows the Nyquist spectra of the gold nanoparticles electrode and the bare gold electrode recorded at 0.2 V in 0.2 mol L−1 glycerol and 1.0 mol L−1 NaOH aqueous solution. It can be seen from Fig. 8 that both curves exhibit negative impedance arcs in

the second quadrant. Such a negative faradaic impedance is often observed in system containing adsorbed intermediates, which indicates the presence of an inductive component. The appearance of negative faradaic impedance may be due to the formation of chemisorbed hydroxyl species, which enhances the oxidative removal of the adsorbed CO intermediate [41]. The diameter of the negative impedance arc for the gold nanoparticles electrode is smaller than that for the bare gold electrode, indicating more inductive component and less intermediate adsorbed species poisoning, which is consistent with the CV results presented in Fig. 7 [40, 41]. That is to say the gold nanoparticles electrode has better anti-poisoning ability than the bare gold electrode for the electro-oxidation of glycerol. Increasing the onset potential from 0.2 to 0.4 V, it can be seen from Fig. 9 that the impedance plots appearing in the first quadrant change from negative to positive, signifying the electron transfer kinetics. The diameter of the impedance arc for the gold nanoparticles electrode is also smaller than that for the bare gold electrode, indicating faster electron transfer and smaller charge transfer resistance [41–43]. At the potential of 0.4 V, the intermediates such as CO adsorbed on the gold surface are completely removed and the catalyst active sites occupied by intermediates are recovered. The electron transfer is faster for the gold nanoparticles, which suggests that more active sites are recovered. Overall, the impedance results are in good agreement with the CV data presented above, and the conclusion can be drawn that the gold nanoparticles electrodeposited from W/IL microemulsion have much higher electro-catalytic activities than bare gold.

Fig. 8 The Nyquist plots of the impedance spectroscopy on the gold nanoparticles electrode (a) and the bare gold electrode (b) in 0.2 mol L−1 glycerol and 1.0 mol L−1 NaOH aqueous solution at the onset potential of 0.2 V

Fig. 9 The Nyquist plots of the impedance spectroscopy on the gold nanoparticles electrode (a) and the bare gold electrode (b) in 0.2 mol L−1 glycerol and 1.0 mol L−1 NaOH aqueous solution at the onset potential of 0.4 V

Fig. 7 CV curves of glycerol electro-oxidation on the gold nanoparticles electrode obtained from W/IL microemulsion (a) and the bare gold electrode (b) in 0.2 mol L−1 glycerol and 1.0 mol L−1 NaOH aqueous solution at 50 mV s−1

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Conclusions In the report, a novel method for preparing nanostructured materials was developed. Gold nanoparticles were electrodeposited directly from W/IL microemulsion. The electrochemical behavior of Au(Ш) on glassy carbon electrode in W/IL microemulsion was discussed. The CV result shows that Au(Ш) can be reduced directly from this new electrode system. The effect of precursor apparent concentration on the reduction peak current density is the same as that in homogeneous solution such as aqueous solution. The effect of scan rate on the reduction peak current density is different from that in homogeneous solution. The results also suggest that the electrode reaction is not completely diffusion controlled and the peak current density increases linearly with increasing the apparent concentration of Au (Ш). Linear-sweep voltammograms result for a rotating disk electrode suggests that the reduction of Au(Ш) is kinetically limited and not transport limited. And also, the activation energy of the reaction is calculated to be 26.7 KJ mol−1. The SEM image demonstrates that the gold electrodeposits are nanosized with a diameter of about 25 nm. The XRD result proves that the gold electrodeposits are face-centered cubic. Furthermore, the electrochemical properties of the gold nanoparticles were researched. The CV study of glycerol oxidation presents that the oxidation peak current densities on the gold nanoparticles electrode are higher than those on bare gold electrode and the oxidation peak potentials on the gold nanoparticles electrode are more negative than those on bare gold electrode. The EIS studies at different onset potentials demonstrate that the gold nanoparticles electrode has better antipoisoning ability and smaller charge transfer resistance than bare gold electrode for the electro-oxidation of glycerol. In a word, the gold nanoparticles electrodeposited from W/IL microemulsion have high electrochemical activities. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 20673036, J0830415) and Hunan Provincial Natural Science Foundation of China (Grant No. 09JJ3025).

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