One-step synthesis of a Si/CNT–polypyrrole composite film by electrochemical deposition

Abstract

A Si/CNT–polypyrrole thin film was prepared by electrochemical deposition from an aqueous solution containing Si/CNT composite colloids. The proposed co-deposition mechanism involves the rapid electropolymerization of polypyrrole nanowires and the electrophoretic deposition of Si/CNT particles. The CNT coating on the Si particles enables electrophoresis of the Si particles and promotes the incorporation of the particles into the polypyrrole film.

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Over the past few decades, the field of conducting polymers has rapidly grown and consistently received great attention because of their unique properties of organic polymers combined with the electrical properties of semiconductors.^1,2 Polypyrrole is one of the most studied conducting polymers due to its good environmental stability, flexibility and high electrical conductivity.^3–5 Moreover, since the pyrrole monomer is readily oxidized and water soluble, polypyrrole can be easily prepared via chemical or electrochemical polymerization.^6–8 Recently, the fabrication of polypyrrole–metal composite materials was attracted considerable interests, and hence diverse composite structures were developed: metal particles incorporated in a polypyrrole matrix, metal–polypyrrole core–shell nanoparticles, metal–polypyrrole hybrid nanowires and metal–polypyrrole coaxial nanocables.^9–13 Owing to the function of the polypyrrole matrix, the electrochemical, physical properties and the environmental stability of the metal particles can be enhanced.¹⁴ In addition, the metal particles in the composite materials would improve the chemical and physical properties of polypyrrole, such as its Raman activity, conductivity and mechanical and electrical properties.¹⁵

In a recent communication,¹⁶ we reported an electrochemical approach for synthesizing polypyrrole nanowires using a cathodic electropolymerization method¹⁷ that utilizes an electrochemically generated oxidizing agent, the nitrosonium ion (NO⁺). The cathodic electropolymerization method allows the direct deposition of polypyrrole nanowires on oxidizable metal substrates that are not stable under the typical anodic electropolymerization conditions.¹⁷ In addition, because both the electropolymerization of pyrrole and the reduction of the metal ion occur in a similar cathodic potential range, SnO₂–polypyrrole hybrid nanowires can also be synthesized using a single-step process based on cathodic electrodeposition.¹⁸

Herein, we suggest a novel synthesis route for metal–polypyrrole composite nanostructures by cathodic deposition with the use of colloidal metal particles. This synthetic approach involves the rapid electropolymerization of polypyrrole nanowires simultaneously with the electrophoretic deposition of metal particles, thereby forming a composite thin film composed of metal particles incorporated in the polypyrrole nanowire matrix. Based on this approach, we prepared Si/CNT–polypyrrole composite thin films by electrochemical deposition and investigated the deposition mechanism of the Si/CNT–polypyrrole composite.

The Si/CNT composite particles were prepared by ball-milling multi-walled carbon nanotubes (CNT) and Si powder with a size of 5–20 μm in a mass ratio of 3 : 7. The weight ratio of milling balls to reagent powders was 5 : 1. The rotation speed of the miller was set to 1725 rpm, and the milling time was 3 h. A Si/CNT–polypyrrole composite thin film was prepared by electrochemical deposition at a potential of −0.6 V_SCE for 10 min from an aqueous solution containing 0.2 M NaNO₃, 0.8 M HNO₃, 0.25 M pyrrole and 0.5 g L⁻¹ Si/CNT powder. The electrodeposition was conducted using a three-electrode cell: a Cu sheet was used as the working electrode (substrate), a stainless steel plate pre-coated with polypyrrole as a counter electrode, and a saturated calomel electrode (SCE, 0.241 V vs. SHE) as a reference electrode. The solution was agitated with a magnetic stirrer at approximately 800 rpm during the deposition. All experiments were conducted at room temperature (25 °C). After being rinsed with distilled water, the deposits were dried under vacuum for 12 h.

The surface morphology and microstructure of the thin film were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The ball-milled Si/CNT powders were observed by transmission electron microscopy (TEM). To prepare the TEM specimens, the ball-milled composite particles were dispersed in ethanol through sonication for 30 min. The chemical bonding states of the polypyrrole nanowires were investigated by X-ray photoelectron spectroscopy (XPS). To elucidate the deposition mechanism of the Si/CNT–polypyrrole nanowire composites, the zeta-potential of the Si/CNT composites and Si powder were measured in di water at 25 °C using a photal ELS-Z2 zeta-potential analyzer (Otsuka Electronics Co. Ltd. Japan). The pH of the suspensions were adjusted by adding HNO₃.

Fig. 1 shows SEM images on pure Si powder, CNT and Si/CNT composites prepared by ball-milling for 3 h, respectively. As shown in Fig. 1a, pristine Si particles were equiaxed and irregular in shape, with a size of 5–20 μm. To reduce the particle size and to coat the particles with CNT, the Si powder was ball-milled with CNT that had a length of over 10 μm and a diameter of approximately 10–20 nm (Fig. 1b). After 3 h milling, the particle size of the Si/CNT composites decreased from 5–20 μm to several hundred nanometers with a broad distribution in the particle size (Fig. 1c) because the particles were crumbled and crushed by milling balls.^19,20 In addition, the high-resolution TEM image in Fig. 1d clearly demonstrates that the crushed Si particles were densely covered with the flattened CNT after ball-milling.

Fig. 1 SEM images of (a) Si powder and (b) CNT. (c) SEM image and (d) TEM image of Si/CNT composites prepared by ball-milling for 3 h.

The coating of Si particles with CNT was clearly confirmed by the XRD pattern of the Si/CNT composites. As shown in Fig. 2c, the Si/CNT composites had an additional peak at 25.7° that is corresponding to the (002) plane of carbon from the CNT, whereas the pure Si particles did not have any peak at 25.7°. The peak intensities corresponding to the (111), (200) and (311) planes of Si in the Si/CNT composites were diminished due to the reduced particle size after ball-milling, which is in accordance with the SEM images in Fig. 1c. Therefore, these results demonstrated that CNT-coated Si particles were successfully fabricated by the ball-milling process.

Fig. 2 XRD patterns for (a) CNT, (b) Si powder and Si/CNT composites prepared by ball-milling for 3 h.

Fig. 3 shows the surface morphologies on a Si–polypyrrole thin film and a Si/CNT–polypyrrole thin film cathodically deposited at −0.6 V_SCE for 10 min from an aqueous solution containing 0.2 M NaNO₃, 0.8 M HNO₃, 0.25 M pyrrole and 0.5 g L⁻¹ Si powder or Si/CNT composites. When pure Si powder was added to the electrolyte, the electrodeposit appeared to form a black thin film at low magnification as shown in Fig. 3a, but at high magnification, the film exhibited a porous network structure composed of fine nanowires with a high degree of interlocking (Fig. 3b). The nanowires were formed uniformly over the whole surface, with an apparent thickness of approximately 80–130 nm and an average length of approximately 1 μm. These dimensions of the nanowire are very similar to those of the pure polypyrrole thin film prepared by cathodic electropolymerization in the previous study.¹⁶ However, it should be noted that very few Si particles were embedded in the polypyrrole film. In contrast, a Si/CNT–polypyrrole thin film contained a large number of incorporated particles in the polypyrrole nanowire network. As shown in Fig. 3c, numerous Si/CNT particles were co-deposited within the polypyrrole nanowires. The high-magnification image in Fig. 3d shows that the Si/CNT composites were completely covered with polypyrrole nanowires, indicating that the Si/CNT particles were tightly attached to the substrate by the mechanical support of the nanowires.

Fig. 3 SEM images of (a and b) a Si–polypyrrole composite thin film and (c and d) a Si/CNT–polypyrrole composite thin film.

Fig. 4 shows the XPS spectra of C_1s and N_1s for the Si/CNT–polypyrrole composite thin film. Notably, the peaks for C_1s (Fig. 4a) and N_1s (Fig. 4b) for the Si/CNT–polypyrrole composites are identically in agreement with those for the pure polypyrrole nanowires prepared by cathodic electropolymerization.¹⁶ The results clearly confirm that the polymerization of pyrrole stably takes place on the Cu substrate and indicate that such growth kinetics of the polypyrrole nanowires were not affected by the co-deposition of Si/CNT particles.

Fig. 4 High-resolution XPS spectra of the Si/CNT–polypyrrole nanowire composite thin film: (a) C_1s and (b) N_1s.

To better understand the deposition mechanism of the Si/CNT–polypyrrole composite thin film, the zeta-potential of the Si/CNT composites and the Si particles were measured as a function of pH (pH 0.8, 2.5, 5 and 6.5). As shown in Fig. 5, the zeta potential of pure Si powder without CNT coating were near 0 regardless of pH, indicating that the bare Si particles have no electrical charge density in the water. On the other hand, the Si/CNT composites exhibit a typical dependence of the zeta potential on pH. The Si/CNT composites had positive zeta potentials in the pH range from 0.8 to 2.5 and negative values at pH higher than 5. The isoelectric point of the Si/CNT composites can be estimated as approximately pH 3–4. Since this behavior of the zeta potential is considerably similar to that of CNT in water,^21,22 it can be deduced that electrophoretic properties of the Si/CNT composites are attributed to the CNT on the surface of the Si particles. It should be emphasize that the electrolyte pH in this study was adjusted 0.95 because a sufficiently strong acidic condition is essential to the cathodic electropolymerization of pyrrole.^16,17 This implies that the Si/CNT composites have a positive zeta potential in the electrolyte.

Fig. 5 Zeta potential of Si powder and Si/CNT composites as a function of pH.

It is likely that the successful deposition of the Si/CNT composites is mainly due to the CNT coating on the Si particles which enables electrophoresis of the Si/CNT composites during electrochemical deposition. According to Guglielmi's mechanism,²³ the electrochemical co-deposition of inert particles is a two-step process involving the loose adsorption and strong adsorption of the particles. The first step is loose physical adsorption of the inert particles on the cathode in the absence of an electrochemical reaction of the metal ions adsorbed on the particles. The second step is the strong electrochemical adsorption of the particles as the result of the applied electrochemical field accompanied with the electrochemical reaction of the metal ions. While the particles are strongly adsorbed on the cathode, those particles can be embedded in the growing metal layer due to the electrodeposition of metal ions in the electrolyte. Since the polypyrrole nanowires should grow on the surface and around the adsorbed Si particles for the Si particles to be incorporated in the polypyrrole film, the electrophoretic properties of colloids are essential to fabricate a metal–polypyrrole composite thin film.

Based on the results of this study, we propose a novel deposition mechanism for a Si/CNT–polypyrrole thin film which involves the rapid electropolymerization of pyrrole and electrophoresis of metal particles. Scheme 1a presents the cathodic electropolymerization process of pyrrole. Under a cathodic potential, NO⁺ is generated by the reduction of NO₃⁻. Because NO⁺ is a superior electron transfer agent that can act as a strong oxidant, the pyrrole monomers are strongly oxidized by NO⁺, leading to the formation of a pyrrole radical cation.¹⁷ The pyrrole radical cation is thermodynamically unstable, and hence the polymerization of the radical cations occurs spontaneously. As we demonstrated in the previous study,¹⁶ when the reactivity of the radical cations is sufficiently low, chain-growth polymerization favorably takes place on the substrate, resulting in the growth of polypyrrole nanowires on the deposited polypyrrole nanospheres (Scheme 1b). If inert particles dispersed in an electrolyte have the electrophoretic properties, the particles can move to and be adsorbed on the cathode by the applied electrochemical field (Scheme 1c). While particles are adsorbed on the substrate, the polypyrrole nanowires grow rapidly not only on the Cu substrate but also around the particles, and thus, the particles become embedded in the nanowire networks generated by the growing nanowires (Scheme 1d).

Scheme 1 A schematic illustration of the mechanism of the synthesis of a Si/CNT–polypyrrole composite thin film by electrochemical deposition.

Conclusions

A Si/CNT–polypyrrole thin film was successfully prepared by a one-step electrochemical method with the use of Si/CNT composite colloids. The co-deposition mechanism involves the rapid electro-polymerization of polypyrrole nanowires and the electrophoretic deposition of Si/CNT particles. The present work may contribute to the electrochemical synthesis of new composite materials that may be used for various applications.

Acknowledgements

This work was supported by the Center for Inorganic Photovoltaic Materials (no. 2012-0001167) grant funded by the Korea government (MSIP) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Project no. NRF-2010-0024752).

Notes and references

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