Gold Fibrous

Feb 16, 2011 - The diameter of composite nanofibers can be controlled in the range of 200−80 nm by simply tuning the speed of mechanical stirring du...
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LETTER pubs.acs.org/Langmuir

Highly Uniform Self-Assembled Conducting Polymer/Gold Fibrous Nanocomposites: Additive-Free Controllable Synthesis and Application as Efficient Recyclable Catalysts Jie Han, Jie Dai, Liya Li, Ping Fang, and Rong Guo* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002 Jiangsu, P. R. China

bS Supporting Information ABSTRACT: Uniform poly(2-aminothiophenol) nanofibers embedded with highly dispersed gold nanoparticles have been fabricated through a facile templateless one-step method. The diameter of composite nanofibers can be controlled in the range of 200-80 nm by simply tuning the speed of mechanical stirring during materials synthesis. Results from our work will provide insight into the shape-controlled synthesis of other nanomaterials by simply introducing mechanical agitation. Removal of gold nanoparticles in composite nanofibers leads to polymer nanotubes with continuous or incontinuous nanocavities depending on mechanical stirring speeds. Furthermore, morphology-dependent catalytic performances of such composites are also investigated.

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omposites composing conducting polymer and noble metal nanoparticles have aroused considerable attention in recent years. Conducting polymer, typical polyaniline (PANI), is unique due to its reversible acid/alkali doping/dedoping property, tunable chemical structures through redox, and abundant amine/ imine functional groups.1 Noble metal nanoparticles, especial gold nanoparticles, have generated great interest in the past decades due to their special optical, electronic, and catalytic performances.2 Incorporation of gold nanoparticles within the PANI matrix to form composites may endow them either new or enhanced chemical properties that can be exploited for chemical or biological applications.3 Morphology and size control over PANI/gold nanoparticle composites and structure-related applications have been the research focus. Actually, their vast potential applications in fields such as catalysts,4 sensors,5 digital memory devices,6 and so forth have already been uncovered recently, where structures of composites play the key role. Until now, many chemical methods have been proposed for making PANI/gold nanoparticle composites, which may be classified as two-step and one-step methods. Two-step strategy can be first reminded in which PANI or gold nanoparticles as cores will be synthesized in advance and then another component is deposited on the surface of the cores. Because of the simplicity for synthesis of gold particles with controllable size, gold nanoparticles are commonly used as cores for further deposition of PANI shells.7 PANI as matrix for deposition of r 2011 American Chemical Society

gold or other noble metal nanoparticles has also been discovered recently, where PANI is often used as reactive supporter for noble metal nanoparticles; that is, PANI itself can also act as reducing agent for reduction of noble metal ions to metallic states in addition to the function of supporter for noble metal nanoparticles. The redox activity toward gold ions has been found earlier,8 and it has been remotivated in the past few years. For instance, PANI irregular particulates have been first used as reactive supporter for palladium, gold, and silver nanoparticles.9 Thereafter, PANI nanostructures of typical nanofibers have been thoroughly investigated, and simple and reproducible methods have been well-established by groups of Kaner and others.10 Due to special properties of PANI nanofibers such as large surface area and high suspension ability, PANI nanofibers have been exploited as reactive supporter for noble metal nanoparticles. For instance, gold nanoparticle decorated PANI nanofibers have been synthesized, and their application as nonvolatile plastic memory device and catalysts for 4-nitrophenol (4NP) reduction have been demonstrated;4c,6 PANI nanofibers decorated with palladium nanoparticles and application as semi-heterogeneous catalyst for Suzuki-Miyaura cross-coupling reaction in water have been described;11 PANI nanofibers decorated with platinum Received: January 20, 2011 Revised: February 9, 2011 Published: February 16, 2011 2181

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Figure 1. (a, b) FE-SEM and (c, d) TEM images of PATP/gold nanoparticle composite nanofibers. Inset shows corresponding HR-TEM image. Synthetic conditions: [monomer] = 0.024 mol/L; [HAuCl4] = 0.006 mol/L, without surfactant and stirring.

or platinum/palladium hybrid nanoparticles have been successfully synthesized; and further usage for constructing electrochemical devices with high performance has been reported.12 In addition, PANI derivatives as reactive supporter for gold nanoparticles and applications as catalysts for alcohol oxidation and Suzuki-Miyaura cross-coupling reaction in water have also been reported.13 One-step method to PANI/gold composites refers that two components of PANI and gold nanoparticles are formed simultaneously. In comparison with the two-step method, obvious advantages of simplicity and reproducibility make one-step more attractive. Recent reports have tried to obtain PANI/gold composites; however, shape-controlled syntheses of uniform structures are seldom seen. For example, if aniline monomer and gold salt are mixed together in acidic conditions, gold particles of micrometers in size apart from PANI nanofibers are commonly seen.14 Addition of surfactant in aniline and gold salt systems may lead to gold/PANI core/shell composites but only in limited conditions, where gold size always exceed tens of nanometers that will restrict their applications as catalysts.15 Up to now, it is still a challenge to shape-controlled fabrication of uniform PANI/gold composites by a facile one-step method with controlled gold size to meet catalytic applications. Herein, a simple and reproducible templateless one-step method is proposed for the successful synthesis of uniform fibrous nanocomposites from a PANI derivative of poly(2-aminothiophenol) (PATP) and gold nanoparticles. A thiol group introduced in aniline monomer can significantly suppress the growth of gold nuclei and also function as stabilizer for gold nanoparticles. When HAuCl4 and 2-aminothiophenol monomer are mixed in acidic

aqueous solution together, participation of products is clearly observed. Figure 1 shows typical FE-SEM and TEM images of products. It is clearly seen in Figure 1a that products are nanofibers without exception with diameter of about 200 nm and length of tens of micrometers. The ruptured surfaces in Figure 1b disclose their solid interiors, which are also proved by the TEM image in Figure 1c. Figure 1d gives a high-magnification TEM image of two individual nanofibers. Clear observation of each individual nanofiber reveals that each nanofiber is decorated with highly dispersed black dots which should be ascribed to gold nanoparticles that come from reduction of HAuCl4. It should be noted that all of the nanofibers are interspersed by gold nanoparticles as proved by examining more than 100 nanofibers, which proves high uniformity of PATP/gold nanoparticle composite nanofibers. The inset in Figure 1d displays a high-resolution TEM image of gold nanoparticles with size of about 2 nm and narrow size distribution. It is noticeable that gold nanoparticles are uniformly embedded in polymer matrix, which is comparable to that of PANI/gold nanocomposites by two-step method where gold nanoparticles are always deposited on outer surfaces of PANI matrix.6,8-13 PATP/gold composites are further confirmed by FTIR, XRD, XPS, and TG techniques. As shown in Figure 2a, the intense peaks in the range of 3200-3500 cm-1 are attributed to the NH stretching vibrations; the weak absorption band at 2615 cm-1 comes from S-H stretching vibrations; the characteristic peaks at 1609 and 1475 cm-1 result from the CdC stretching deformation of quinonoid and benzenoid rings, respectively. The results confirm the formation of conducting polymer PATP. In the XRD pattern (Figure 2b), the four obvious main peaks correspond to (111), (200), (220), and (311) Bragg reflection of 2182

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Figure 2. (a) FTIR, (b) XRD and (c, d) XPS spectra, and (e) TG curve of PATP/gold nanoparticles composite nanofibers.

gold,14b indicating the presence of gold nanoparticles in products. The broad band centered at 2θ = 30° reveals the amorphous feature of PATP polymer. From XRD and FTIR, we can confirm the successful synthesis of polymer PATP and gold nanoparticles. Besides, the measured XPS spectrum (Figure 2c) shows clearly the signatures of C, N, and S for PATP polymer and Au for gold nanoparticles. The XPS signature of the Au 4f doublet (4f7/2 and 4f5/2, Figure 2d) confirms that gold nanoparticles only exist in their metallic state.16 As the gold contents in aqueous phase after polymerization are almost neglectable as determined by atomic absorption spectroscopy (AAS), then we can conclude that HAuCl4 has been transformed into gold nanoparticles completely. In addition, thermal stability of composites was also conducted by TG analysis. It is seen in Figure 2e that a rapid drop of weight is observed beginning from about 200 °C and weight loss continues to about 1000 °C, which is higher than the commonly seen decomposition temperature of conducting polymer.17 This result confirms strong interactions between polymer and gold nanoparticles. The effect of mechanical agitation on aggregation of nanoparticles during synthesis has been realized in recent years, which is proved to be of great value in reproducibly synthesizing nanoparticles with well-controlled size and shape.10b Herein, the stirring speed in reaction system is also investigated, and some interesting results are found. As given in Figure 1, randomly arranged individual PATP/gold nanoparticle composite nanofibers with diameter of about 200 nm are observed. If mechanical agitation is introduced at low stirring speed of 100 rpm, nanofibers of about 150 nm in diameter are found (Figure 3a). Corresponding TEM image in Figure 3b reveals uniform PATP/

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gold nanoparticle composite nanofibers. When increasing stirring speed from 100 to 500 rpm, it is found that parallel arrayed nanofiber bunches typically consisting of several nanofibers are found (Figure 3c). This novel phenomenon is also verified by the TEM image shown in Figure 3d, where a single nanofiber is about 100 nm in diameter. Further increase of stirring speed to 1200 rpm, large-area parallel arrayed nanofibers typically consisting of tens of nanofibers are discovered (Figure 3e), and the diameter of individual nanofibers is estimated to be 80 nm (Figure 3f). Then it is concluded that the diameter of individual nanofibers decreased with increasing stirring speed. In contrary to evolution of nanofiber diameter, the length of individual nanofibers is found to increase with increasing stirring speed as displayed in Figures 1 and 3. From the above analysis, it is deduced that stirring is more favorable for heterogeneous nucleation of PATP/ gold nanoparticle composite nanofibers, meanwhile it is preferred for orientated attachment of individual nanofibers. In addition to shape evolution of composite nanofibers, the size of gold nanoparticles embedded in PATP nanofibers is almost unchanged at 2 nm, which indicates that nucleation and growth of gold nuclei are almost unaffected by stirring speed. Mechanical agitation-controlled size and orientation of one-dimensional polymer nanostructures are seldom reported,18 and we believe our results will provide insight into the shape-controlled synthesis of other nanomaterials by simply introducing mechanical agitation. Preliminary experiments illustrate that size of PATP/gold nanoparticle composite nanofibers also can be tuned by introducing a surfactant (see the Supporting Information, Figure S1). Uniform composite nanofibers are also obtained when F127 surfactant is introduced at a concentration of 1.58  10-4 mol/L. In comparison with Figure 1, the diameter of individual nanofibers decreases from 200 to 100 nm. When increasing F127 concentration to 1.58  10-3 and 6.34  10-3 mol/L, the diameter of nanofibers decreases to 80 and 50 nm, respectively. Except for changes in nanofiber size, gold nanoparticles embedded in PATP nanofibers remained almost unchanged with increasing surfactant concentration. When using other nonionic surfactants such as Brij 30 or TX-100, the same trends can be identified; however, this is not applicable to an ionic surfactant such as CTAB. This is possibly due to altered charge interactions between monomer and HAuCl4 introduced by ionic surfactant. As compared with size control of PATP/gold nanoparticle composite nanofibers through stirring, a significant difference is the regularity or arrangement of individual nanofibers in stirring-speed-controlled way. According to experimental results as detailed above, a tentative explanation for fibrous growth of uniform PATP/gold nanoparticle composite nanofibers is then proposed. Fibrous growth of PANI nanofibers has been deeply investigated recently. It is reported that the intrinsic fibrous growth of PANI is favored and the secondary growth is suppressed by improving the traditional synthetic conditions. For example, interfacial polymerization,10a stirring or shaking-controlled polymerization,10b,c and diluting polymerization10c strategies have been proposed for successful synthesis of PANI nanofibers. However, most of the methods are not applicable to synthesis of PANI derivatives. In particular, catalytic growth of PANI nanofibers by gold nanoparticles has also been proposed.14b,19 This method has been proved to be applicable to synthesis of poly(o-phenylenediamine) and poly(otoluidine) nanofibers.20 However, clear phase separation of polymer and gold particles is observed. When replacing HAuCl4 with commonly used oxidant ammonium peroxydisulfate (APS) 2183

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Figure 3. (a, c, d) FE-SEM and (b, d, f) TEM images of PATP/gold nanoparticle composite nanofibers synthesized under different stirring speeds (rpm): (a, b) 100, (c, d) 500, and (e, f) 1200. Inset shows corresponding magnified FE-SEM images. Other synthetic conditions: [monomer] = 0.024 mol/L; [HAuCl4] = 0.006 mol/L, without surfactant.

while leaving other synthetic conditions unchanged, only irregular particulates are obtained (see the Supporting Information, Figure S2). Results confirm that gold nanoparticles generated during the polymerization process may act as catalysts for fibrous growth of PATP polymer. In addition, results show that high HAuCl4 concentration is not favorable for catalytic growth of one-dimensional nanofibers (see the Supporting Information, Figure S3), which may result from the fact that gold nanoparticles generated are at relatively high concentration which inhibits their catalytic effect. This is also consistent with the results reported.20b Due to strong interactions between gold nanoparticles and polymer, almost all gold nanoparticles are incorporated into polymer, which is in contrast with references where microsized bare gold particles apart from polymer are always seen.14,15b,19,20a

Sacrificial template synthesis of nanomaterials is a hot topic in nanotechnology.21 Bearing this in mind, it is believed that PATP nanotubes can be attained after removal of gold nanoparticles embedded in PATP matrix. It is interesting to find that PATP nanotubes with incontinuous or continuous cavities can be fabricated depending on stirring speed during removal processes. As given in Figure 4a, when PATP/gold nanoparticle composites are immersed in KI/I2 saturated solution without stirring for 12 h, novel PATP nanotubes with incontinuous nanocavities can be identified. Clearly, the observation reveals almost complete removal of gold nanoparticles embedded. Results from XPS further confirm the complete removal of gold species. The size of the nanocavities is in the range of 20-50 nm, and such nanocavities almost spread all over PATP nanofibers. It is clear 2184

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Figure 4. TEM images of PATP nanotubes derived from PATP/gold nanoparticle composite nanofibers (Figure 1) after removal of gold nanoparticles using saturated KI/I2 solution: (a-c) without stirring, (d, e) with mechanical stirring (1200 rps).

that there is typically just one array of nanocavities in PATP nanotubes (Figure 4b), there are seldom two (Figure 4c). If mechanical stirring is introduced, PATP nanotubes with continuous hollowness as normally seen in ref 22 can be observed (Figure 4d). The fraction of interiors of PATP nanotubes is obviously larger than that of nanotubes obtained by soft-template methods,22 and comparable to that of nanotubes fabricated by hard-template methods.23 Although the reasons involved in PATP nanotube formation affected by mechanical stirring are still unclear at present, PATP nanotubes with incontinuous spherical nanocavities as a novel nanostructure that are fabricated by mechanical-stirring-controlled synthesis will find promising application in drug-controlled release.24 Catalytic activities of PATP/gold nanoparticle composite nanofibers on the reduction of 4NP are examined. In the absence of gold catalysts, the mixtures of 4NP and NaBH4 shows an absorbance band at λmax = 400 nm corresponding to the 4NP ions in alkaline conditions. This peak remains unaltered with time, indicating that the reduction did not take place in the absence of a catalyst.25 However, the addition of a small amount of PATP/gold nanoparticle composite nanofibers to the above reaction mixture causes a fading and ultimate bleaching of the yellow color of 4NP in aqueous solution. Time-dependent absorbance spectra of this reaction mixture show the disappearance of

the peak at 400 nm that is accompanied by a gradual development of a new peak at 300 nm corresponding to the formation of 4-aminophenol (4AP) (Figure 5a). PATP/gold nanoparticle composite nanofibers with fiber diameters of 200 nm (Figure 1) and 80 nm (Figure 3e, f) are chosen as catalysts for evaluating the effect of fiber diameter on catalytic activity, considering the same gold size of 2 nm. It is seen that the reduction of 4NP to 4AP will be finished in 35 min with large fiber diameter while that in 12 min with low fiber diameter (Figure 5b). Upon the addition of gold nanocatalysts, a certain period of time (defined as tads) was required for the 4NP to adsorb onto the catalyst’s surface before reduction could be initiated.25a As shown in Figure 5b, tads is about 10 min for large PATP nanofibers while it is about 2 min for small PATP nanofibers. It is seen that the reaction catalyzed by smaller PATP fibers shows the shorter adsorption time and faster reaction rate. The inbreak of 4NP to gold catalysts embedded in smaller PATP fiber will be faster, which leads to the shorter adsorption time. Due to the larger surface area of PATP nanofibers with smaller size, the amount of gold catalysts near fiber surfaces is higher, which results in faster reaction rate in the case smaller PATP fiber diameter. For comparison, the catalytic activity of PATP/gold nanoparticle composite nanofibers with fiber diameter of 80 nm obtained by surfactant-mediated route (Supporting Information 2185

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gold nanoparticles embedded in PATP nanofibers, where interiors of PATP nanotubes with continuous or incontinuous nanocavities can be controlled depending on mechanical stirring. PATP/gold nanoparticle composite nanofibers show good catalytic activities and reusability toward the reduction of 4NP in the presence of NaBH4, where the reaction catalyzed by smaller nanofibers (80 nm) shows shorter adsorption time and faster reaction rate due to large exposure of gold catalysts on fiber surfaces. Results of our work show the profound impact of mechanical stirring on the resulting materials morphology and catalytic performance. This will provide insight into the shapecontrolled synthesis of other nanomaterials by simply introducing mechanical agitation. It is believed that PATP/gold composites as-synthesized will find promising applications in fields of catalysis and sensors.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed experimental procedures, and FF-SEM and TEM images of PATP/gold nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author Figure 5. (a) Successive UV-vis absorbance spectra of the reduction of 4NP by NaBH4 in the presence of PATP/gold nanoparticle composite nanofibers as imaged in Figure 1. (b) Normalized absorbance at the peak position for 4NP (400 nm) as a function of time in the presence of PATP/gold nanoparticle composite nanofibers.

Figure S1c, d) on the reduction of 4NP is also investigated. However, the yellow color remained almost unchanged for several days, indicating that such composites show no catalytic activity on the reduction of 4NP (Figure 5b). Then, it is speculated that surfactant introduced in PATP nanofibers may act as a catalyst inhibitor. In addition, the reusability of PATP/gold nanoparticle composite nanofibers as heterogeneous catalyst involved in the reduction of 4NP is also considered. We have recently reported that gold nanoparticles supported on surfaces of polyaniline nanofibers show catalytic activity toward the reduction of 4NP in the presence of NaBH4, where obvious loss in catalytic activity with increasing cycles is obtained due to loss of gold catalysts during centrifugation and purification processes.4c However, as for PATP/gold nanoparticle composite nanofibers with a fiber diameter of 80 nm (Figure 3e, f), no obvious catalytic loss is evidenced within six cycles. Due to the incorporation of gold nanoparticles in polymer matrix, such PATP/gold nanoparticle composite nanofibers show good stability and recyclability, which shows their potential in catalysis. In summary, PATP composite nanofibers with highly dispersed gold nanoparticles (2 nm in size) have been synthesized by mixing monomer and HAuCl4 together in aqueous solution without template or surfactant. This is a facile, reproducible, and controllable one-step strategy to conducting polymer/gold nanoparticle composite nanofibers. More interesting, mechanical stirring tunes the diameter of composite nanofibers in the range of 200-80 nm, meanwhile parallel arrangements of individual composite nanofibers can be obtained at high stirring speeds. Furthermore, PATP nanotubes can be obtained by removing

*E-mail: [email protected].

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