gold nanocomposites

gold nanocomposites

Journal of Electroanalytical Chemistry Journal of Electroanalytical Chemistry 585 (2005) 206–213 www.elsevier.com/locate/jelechem New pathway for th...

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Journal of

Electroanalytical Chemistry Journal of Electroanalytical Chemistry 585 (2005) 206–213 www.elsevier.com/locate/jelechem

New pathway for the controllable synthesis of gold nanoparticles on platinum substrates and their derivatives of polypyrrole/gold nanocomposites Yu-Chuan Liu b

a,*

, Chung-Chin Yu

b

a Department of Chemical Engineering, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Tao-Yuan, Chung-Li City, Taiwan, Republic of China Department of Environmental Engineering, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Tao-Yuan, Chung-Li City, Taiwan, Republic of China

Received 16 May 2005; received in revised form 19 July 2005; accepted 16 August 2005 Available online 21 September 2005

Abstract The synthesis of gold nanoparticles with controllable particle sizes and densities on platinum substrates was first developed by sonoelectrochemical methods in this study. First, an Au substrate was cycled in a deoxygenated aqueous solution containing 0.1 M HCl from 0.28 to +1.22 V vs. Ag/AgCl at 500 mV/s with different scans. The durations at the cathodic and anodic vertexes are 10 and 5 s, respectively. After that Au-containing complexes were found existing in this aqueous solution. Subsequently, the Au working electrode was immediately replaced by a Pt electrode and different cathodic overpotentials and reduction time were employed under controlled sonication to synthesize Au nanoparticles on the Pt electrode. Then pyrrole (Py) monomers were encouragingly found to be autopolymerized to form polypyrrole (PPy)/Au nanocomposites on the Pt substrate due to the electrochemical activity of some unreduced species of the positively charged Au-containing complexes, which were incorporated into the deposited Au nanoparticles during the sonoelectrochemical reduction in a specific condition.  2005 Elsevier B.V. All rights reserved. Keywords: Sonoelectrochemical methods; Gold nanoparticles; Polypyrrole; Autopolymerization; Nanocomposites

1. Introduction Recently, nano-structured materials have been the focus of scientific research [1,2] due to their unusual properties of optical [3], chemical [4], photoelectrochemical [5], and electronic [6] properties. The control of different shapes, such as rods [7], wires [8] and cubes [9], and sizes [10] has been demonstrated with a wide range of metals and synthetic procedures since metals nanoparticles including clusters exhibit size- and shape-dependent chemical and physical properties [11,12]. The number of potential applications for nanoparticles, especially in the field of proteins detection [13] and catalysts modification [14], are rapidly grow*

Corresponding author. Tel.: +886 3 4515811x540; fax: +886 3 4514814/2 86638557. E-mail address: [email protected] (Y.-C. Liu). 0022-0728/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.08.011

ing because of their unique electronic structure and extremely large surface areas. The developed methods for nanoparticles fabrication include chemical reduction [15], sonochemical reduction [16], laser ablation [17], annealing from high-temperature solutions [18], metal evaporation [19], and Ar+ ion sputtering [6], etc. Among conducting polymers (CPs), polypyrrole (PPy) is one of the most widely studied CPs because it is easily synthesized and it offers reasonably high conductivity and has fairly good environmental stability. Therefore, PPy is widely used in batteries [20,21], supercapacitors [22], sensors [23,24], anhydrous electrorheological fluids [25], microwave shielding [26] and corrosion protection [27]. There are two methods generally used to synthesize PPy, chemical and electrochemical polymerizations. The principal advantage of the electrochemical method is related to the better conducting properties and long-term stability

Y.-C. Liu, C.-C. Yu / Journal of Electroanalytical Chemistry 585 (2005) 206–213

of conductivities [28,29]. Recently, Henry et al. [30] reported that chlorauric acid can act as an oxidant to oxidize chemically polymerized PPy to produce composite PPy colloids. Burke and Hurley [31] reported that gold surfaces can be superactivated by a combination of thermal and cathodic pretreatment. They display inexplicably high catalytic activity for some reactions in the form of oxide-supported microparticles. Palys et al. [32] reported that if the roughened Ag electrode is prepared under daylight conditions, the unreduced species (e.g., Agþ n clusters) can remain on the silver surface and encourage the oxidation of 1,8-diaminonaphthalene. Hepel [33] also reported that PtCl2 4 anions were trapped inside the PPy matrix during the electropolymerization of pyrrole. In the next step followed by solution exchange, PtCl2 anions were reduced 4 to Pt(0) particles with an average size of 10 nm. The methanol is electrocatalytically oxidized at these finely dispersed Pt nanoparticles in PPy films. All of these studies reveal that fine metals or metal-containing complexes with nano-structures demonstrate specially electrocatalytic activities. However, the detailed exploration and the mechanism of the catalytic activity for the electrooxidation pathway for the polymerization of conducting polymers were less observed in the literature. In collecting the Raman signals of organic compounds that are present in a system at very low concentration levels, surface-enhanced Raman scattering (SERS) spectroscopy was employed to enhance the detection [34,35]. Since SERS is a surface plasmon-mediated phenomenon, the magnitude of the effect is highly dependent on the excitation conditions, nature, and geometry of the metallic nano-structures [36]. As shown in the literature, various substrates, such as spherical nanoparticles [37], nanowires [38] and nanorods [39], have been employed for SERS. In the studies of SERS, the electrochemical oxidation–reduction cycles (ORC) procedure [40] is a better way to produce SERS-active metal substrates because a controllable and reproduced surface roughness can be easily generated [41,42]. In the previous studies of the preparations of metals nanoparticles in aqueous solutions [43–45], we reported the size-controlled synthesis of Au, Ag, and Au–Ag alloy nanoparticles from their bulk substrates by sonoelectrochemical methods. In this work, we first use an electrochemical ORC roughening procedure to obtain gold-containing complexes in a 0.1 M HCl aqueous solution from a gold substrate. Subsequently, the Au working electrode was immediately replaced by a Pt electrode and a cathodic overpotential of 0.2 V was applied under controlled sonication to synthesize Au nanoparticles on the Pt electrode. Then pyrrole monomers were originally autopolymerized on the deposited Au nanoparticles due to the electrochemical activity of unreduced species of the positively charged Au-containing complexes inside the nanoparticles. Since PPy can be used in sensors and Au nanoparticles are known catalysts, therefore, the prepared PPy/Au nanocomposites on Pt substrates would be useful. Meanwhile, Au

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nanoparticles with controllable particles sizes and densities on Pt substrates were synthesized by changing the experimental conditions. 2. Experimental 2.1. Chemical reagents Pyrrole (Py) was triply distilled until a colorless liquid was obtained and was then stored under nitrogen before use. The HCl reagent (p.a. grade) purchased from Acros Organics was used as received without further purification. All of the solutions were prepared using deionized 18 MX cm water. 2.2. Preparation of gold nanoparticles on platinum substrates All of the electrochemical experiments were performed in a three-compartment cell at room temperature, 22 C, and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). First, a sheet of gold with a bare surface area of 4 cm2, a 2 · 4 cm2 platinum sheet, and a KCl-saturated silver–silver chloride (Ag/AgCl) were employed as the working, counter, and reference electrodes, respectively. Before the ORC treatment, the gold electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 lm of alumina slurry to a mirror finish. Then the electrode was cycled in a deoxygenated aqueous solution of 100 mL containing 0.1 M HCl from 0.28 to +1.22 V vs. Ag/AgCl at 500 mV/s with different scans of 50, 100 and 200 ones. The durations at the cathodic and anodic vertexes are 10 and 5 s, respectively. After these roughening procedures, Au- and Cl-containing complexes were left in these aqueous solutions. In this work, solutions containing low, middle and high concentrations of Au complexes were corresponded to the ORC procedures with 50, 100 and 200 scans, respectively. Immediately, the gold working electrode was replaced by a platinum substrate with a bare surface area of 0.238 cm2 without changing the electrolyte, and different cathodic overpotentials of 0.2, 0.4 and 0.6 V, corresponding to low, middle and high cathodic overpotentials, respectively, from the open circuit potentials (OCP) of ca. 0.81 V vs. Ag/AgCl were applied under a controlled sonication and slight stirring for different time of 1, 5 and 10 min, corresponding to short, middle and long reduction time employed, respectively, to synthesize Au nanoparticles on the platinum substrates. After the deposition of Au nanoparticles, the Pt electrode was rinsed throughout with deionized water, and finally dried in a vacuum-dryer with dark atmosphere for 1 h at room temperature for subsequent use. The ultrasonic irradiation was performed by using an ultrasonic generator (model XL2000, Microson) and operated at 20 kHz with a barium titanate oscillator of 3.2 mm diameter to deliver a power of 80 W. The distance between the barium titanate oscillator rod and the electrode is kept at 5 mm.

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A deoxygenated aqueous solution containing 0.5 M pyrrole was instantly dropped onto the Au nanoparticlesdeposited Pt substrate. Then it was placed in a desiccator with nitrogen and dark atmospheres for 2 h. Finally, the sample was rinsed throughout with deionized water and dried in a vacuum-dryer with dark atmosphere at room temperature before test. 2.4. Characteristics of Au nanoparticles and PPy/Au nanocomposites on Pt substrates Ultraviolet–visible absorption spectroscopic measurements were carried out on a Perkin–Elmer Lambda 25 spectrophotometer in 1 cm quartz curvettes. The surface morphology of the Au nanoparticles-deposited Pt substrate was obtained from scanning electron microscopy (SEM, model S-4700 Type II, Hitachi). For the XPS measurements, a Physical Electronics PHI 1600 spectrometer with monochromatized Mg Ka radiation at 15 kV 250 W and an energy resolution of 0.1–0.8% DE/E was used. To compensate for surface charging effects, all XPS spectra are referred to the C 1 s neutral carbon peak at 284.6 eV. Raman spectra were obtained using a Renishaw in Via Raman spectrometer employing a He–Ne laser of 1 mW radiating on the sample operating at 632.8 nm and a charge couple device (CCD) detector with 1 cm1 resolution. 3. Results and discussion 3.1. Synthesis of polypyrrole/gold nanocomposites In ORC treatment, the chloride electrolyte was selected because, as for silver, this facilitates the metal dissolution– deposition process that is known to produce SERS-active roughened surfaces [46]. As shown in a previous study [40], after the ORC procedure of roughening the Au substrate, it would leave some unreduced species, possibly positively charged Au clusters, on the Au surface. Meanwhile, the Au-containing complexes were also present in the solution after roughening the Au substrate [43]. As shown in Fig. 1, after roughing the Au substrate by the ORC treatment with 100 scans, the absorbance maximum of Au-containing complexes appears approximately at 309 nm, which is markedly different from that of zero-valent Au located at ca. 520 nm [47] and can be confirmed from the XPS analysis [43]. After applying a cathodic overpotential of 0.2 V from the OCP of ca. 0.81 V vs. Ag/AgCl under a controlled sonication of 80 W and slight stirring for 1 min to synthesize Au nanoparticles on Pt substrates in solutions containing different concentrations of Au complexes, the isolated spherical Au nanoparticles with different particle sizes and densities were even deposited on the Pt substrates, as demonstrated in Fig. 2. In a high concentration solution, the Au nanoparticles on Pt show larger particle sizes and

Absorbance / a.u.

2.3. Autopolymerization of pyrrole on the gold nanoparticles deposited on the Pt substrates

200

300

400

500

600

700

800

Wavelength / nm Fig. 1. UV–Vis absorption spectrum of Au-containing complexes after roughing the Au substrate by the ORC treatment with 100 scans.

a denser distribution, as shown in Fig. 2(a). In contrast, as shown in Fig. 2(b), prepared in a low concentration solution, the deposited Au nanoparticles with a mean diameter of ca. 10 nm demonstrate a thin distribution on the Pt substrate. Fig. 2(c) is a blank experiment showing no Au nanoparticles deposited on the Pt electrode. The detailed effects of preparation conditions on the resulting Au nanoparticles on Pt substrates will be discussed later. As shown in the previous report [43], Au nanoparticles, ranging from 2 to 15 nm in diameters, can be prepared in solutions with the similar preparation conditions, but using a sonication of 100 W. In this work, less power of 80 W was used to provide the chance, for the Au-containing complexes, not only obtaining electrons from the cathodic electrode but also depositing on it. However, sonication power used less than 60 W would cause serious aggregation of Au particles on the Pt electrode. Therefore, the deposited Au particles are not in a nano-structured morphology but in a film one. It indicates that different delivery power can be employed to control the deposition of Au nanoparticles on the Pt electrode. Meanwhile, larger Au nanoparticles deposited on the Pt electrode in this study reveal that unreduced positively charged Au clusters may exist inside the Au nanoparticles by incorporating them into the elemental Au nanoparticles during the sonoelectrochemical reduction. Further XPS analyses will confirm this phenomenon. Fig. 3 displays the XPS Au 4f7/2  5/2 core-level spectra of Au nanoparticles deposited on Pt substrates by using sonoelectrochemical reductions for 1 min in solutions containing different concentrations of Au complexes. As shown in spectrum b, the doublet peaks located at 84 and 87.7 eV can be assigned to Au(0) [30]. It means that

XPS intensity / a.u.

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209

a

b

80.0

83.0

86.0

89.0

92.0

95.0

Binding energy / eV Fig. 3. XPS Au 4f7/2  5/2 core-level spectra of gold nanoparticles deposited on platinum substrates by sonoelectrochemical reductions at a cathodic overpotential of 0.2 V for 1 min in solutions containing different concentrations of Au complexes. (a) In a high concentration of Au complexes. (b) In a low concentration of Au complexes.

Fig. 2. SEM images of gold nanoparticles deposited on platinum substrates by sonoelectrochemical reductions at a cathodic overpotential of 0.2 V for 1 min in solutions containing different concentrations of Au complexes. (a) In a high concentration of Au complexes. (b) In a low concentration of Au complexes. (c) A blank experiment of a mechanically polished Pt substrate without the deposition of Au nanoparticles.

the Au nanoparticles prepared in a low concentration solution are completely reduced and are of an elemental state. It is notable, as comparing spectrum a, representing the Au nanoparticles prepared in a high concentration solution,

with spectrum b, representing the elemental nanoparticles Au(0), that there are extra oxidized components of Au shown in the higher binding energy side. The oxidized Au shown in spectrum a can be assigned to monovalent Au(I) and trivalent Au(III) at 85.2 and 86.7 eV, respectively [48]. No further deconvolution was made, and the term of positively charged Au was adopted in this study. This phenomenon is interesting. The positively charged Au particles may serve as oxidants for the polymerization of pyrrole monomers since this polymerization is an oxidative one. It is possible to use the deposited Au nanoparticles with positively charged Au to further prepare PPy/ Au nanocomposites on the Pt substrate. Fig. 4 shows the results. The polymer layer of autopolymerized PPy on the Au nanoparticles is not visible to the naked eye and even hard to be resolved on SEM. However, as shown in Fig. 2, the surface morphology is a typical aspect of a rough surface with a good Raman activity, which demonstrates a microstructure smaller than 100 nm [49,50]. Thus, we can use SERS spectrum to identify the autopolymerized PPy on the Au nanoparticles. Spectrum a in Fig. 4 shows the SERS spectrum of the autopolymerized PPy film deposited on the Au nanoparticles with positively charged Au. It is a typical Raman spectrum of polypyrrole deposited on metal substrates, as shown in the literature [51,52]. Thus, it certifies that the autopolymerized PPy can be produced, which is distinguishable from the known chemical or electrochemical method, on the Au nanoparticles due to the special activity of the unreduced complexes. For comparison, a blank experiment is also preformed on

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3.2. Controllable synthesis of gold nanoparticles

Intensity / a.u.

Generally, the advantages of electrochemical methods over the chemical ones in preparing metal nanoparticles are the high purity of the produced particles and the

a b c 0

400

800

1200

1600

2000

-1

Wavenumber / cm

Fig. 4. Raman spectra with (curve a) and without (curve b) the autopolymerization of PPy on gold nanoparticles deposited on platinum substrates by sonoelectrochemical reductions at a cathodic overpotential of 0.2 V for 1 min in solutions containing a high and a low concentrations, respectively, of Au complexes. Curve c represents the Raman spectrum of the positively charged Au nanoparticles on Pt prepared from a high concentration of Au complexes.

the deposited Au nanoparticles without the unreduced positively charged Au complexes prepared in a low concentration solution, as shown in spectrum b of Fig. 4. It indicates that no polymerization of PPy happens on the Au(0) nanoparticles. As described in Section 2, after the dropping of pyrrole monomers for 2 h, the sample was rinsed throughout with deionized water and dried in a vacuum-dryer with dark atmosphere at room temperature before test. Therefore, the sample is free of pyrrole monomers in the SERS analysis. Fig. 4(c) shows the Raman spectrum of the positively charged Au nanoparticles on Pt prepared from a high concentration of Au complexes. It is similar with that of the elemental Au nanoparticles on Pt prepared from a low concentration of Au complexes shown in Fig. 4(b), which demonstrates a featureless curve as compared with the SERS spectrum of the autopolymerization of PPy on Au nanoparticles deposited on the Pt substrate. Conclusively, a solution containing a high concentration of positively charged Au complexes can be obtained by using a high scans of 200 times in the ORC procedure for roughening the Au substrate in 0.1 M HCl. Then Au nanoparticles with unreduced positively charged Au complexes could be deposited on a Pt substrate by sonoelectrochemical reductions at a cathodic overpotential of 0.2 V for 1 min in this solution. Finally, the autopolymerized PPy can be formed on the Au nanoparticles to prepare PPy/ Au nanocomposites due to the special activity of the unreduced complexes.

Fig. 5. SEM images of gold nanoparticles deposited on platinum substrates by sonoelectrochemical reductions at a cathodic overpotential of 0.6 V for different time in solutions containing middle concentrations of Au complexes: (a) for short time; (b) for middle time; (c) for long time.

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easy control of particle size by adjusting applied potentials or current densities [53,54]. In this work, we investigate the effects of electrochemical preparation conditions, including the reduction time, the concentration of positively charged Au complexes, and the cathodic overpotential, on the particles sizes and distribution of Au nanoparticles deposited on the Pt substrates. Fig. 5 shows the effect of the reduction time in sonoelectrochemical reduction on the characteristics of the depos-

ited Au nanoparticles on the Pt substrate. It is clear and reasonable that longer reduction time is contributive to producing Au nanoparticles with larger particle sizes and a denser distribution on the Pt substrate, because the later deposition of Au nanoparticles would occur on the formerly deposited Au nanoparticles and on the space between the formerly deposited isolated Au nanoparticles. Also the larger particle sizes may be related with the growth of particles.

Fig. 6. SEM images of gold nanoparticles deposited on platinum substrates by sonoelectrochemical reductions at a cathodic overpotential of 0.6 V for short time of 1 min in solutions containing different concentrations of Au complexes. (a) In a high concentration of Au complexes. (b) In a middle concentration of Au complexes. (c) In a low concentration of Au complexes.

Fig. 7. SEM images of gold nanoparticles deposited on platinum substrates by sonoelectrochemical reductions at a cathodic overpotential of 0.6 V for long time of 10 min in solutions containing different concentrations of Au complexes. (a) In a high concentration of Au complexes. (b) In a middle concentration of Au complexes. (c) In a low concentration of Au complexes.

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Fig. 6 shows the effect of the concentration of positively charged Au complexes in sonoelectrochemical reduction for short time of 1 min on the characteristics of the deposited Au nanoparticles on the Pt substrate. As shown in Fig. 6(a), the deposited Au is formed as a film, not isolated

nanoparticles, in a high concentration solution. Decreasing the concentration of Au complexes, the characteristics of the deposited isolated Au nanoparticles are smaller and thinner on the Pt substrates. However, the effect of concentration is quit insignificant if long time of 10 min is used in reduction, as shown in Fig. 7. Fig. 8 shows the effect of the cathodic overpotential in sonoelectrochemical reductions on the characteristics of the deposited Au nanoparticles on the Pt substrate. It indicates that a lower overpotential of 0.2 V would be of benefit for obtaining smaller Au nanoparticles. In electrochemical reduction, a larger overpotential means a larger driving force available to produce larger Au nanoparticles on the Pt substrate. Conclusively, the shorter reduction time, the lower cathodic overpotential and the lower concentration of positively charged Au complexes in electrolytes, are of benefit for producing smaller and thinner Au nanoparticles deposited on the Pt substrates. In contrast, to produce larger Au nanoparticles, the opposite preparation conditions should be employed. 4. Conclusion Au nanoparticles with unreduced positively charged Au complexes could be deposited on a Pt substrate by sonoelectrochemical reductions at a cathodic overpotential of 0.2 V for 1 min in a solution containing a high concentration of positively charged Au complexes. Then the autopolymerized PPy can be formed on the Au nanoparticles to prepare PPy/Au nanocomposites due to the special activity of the unreduced complexes. The Au nanoparticles sizes and distribution on the Pt substrates can be controlled by adjusting the concentration of positively charged Au complexes in solutions, the time and the cathodic overpotential used in reductions. Generally, shorter reduction time, lower overpotential and concentration, are of benefit for obtaining smaller and thinner Au nanoparticles on Pt substrates. Acknowledgments The authors thank the National Science Council of the Republic of China (NSC-92-2214-E-238-001) and Vanung University for their financial support. References

Fig. 8. SEM images of gold nanoparticles deposited on platinum substrates by sonoelectrochemical reductions at different cathodic overpotentials for middle time of 5 min in solutions containing high concentrations of Au complexes. (a) At a high overpotential of 0.6 V. (b) At a middle overpotential of 0.4 V. (c) At a low overpotential of 0.2 V.

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