Langmuir 2006, 22, 10915-10918
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Micelle-Assisted One-Pot Synthesis of Water-Soluble Polyaniline-Gold Composite Particles Zhangquan Peng,† Limin Guo,† Zhonghua Zhang,‡ Bernd Tesche,§ Thorsten Wilke,† Daniel Ogermann,† Shuhua Hu,† and Karl Kleinermanns*,† Institute for Physical Chemistry, Heinrich-Heine UniVersity Du¨sseldorf, 40225 Du¨sseldorf, Germany, Key Lab of Liquid Structure and Heredity of Materials, Shandong UniVersity, Jingshi Road 73, Jinan, 250061, P. R. China, and Max-Plank-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany ReceiVed July 21, 2006. In Final Form: October 13, 2006 A micelle-based method to synthesize dispersed polyaniline (PANI)-Au composite particles by direct oxidation of aniline using AuCl4- as the oxidant is presented. The obtained composite particles have a core-shell structure, where Au nanoparticles of 20 nm mean diameter are encapsulated by PANI of well-defined tetrahedron shape with 150 nm average edge length. The polaron band of the dispersed PANI-Au composite particles is centered at 745 nm and is rather narrow compared to the broad 835 nm absorption of PANI synthesized by the IUPAC procedure. The surface plasmon absorption of Au nanoparticles normally centered at around 520 nm is absent in the composite particles with oxidized PANI. Our results point to a strong electronic interaction between the encapsulated Au nanoparticles and the shell of oxidized PANI. Films and pellets produced from these composite particles show a twofold higher conductivity than IUPAC PANI.
1. Introduction Composites from conducting polymers and metal nanoparticles are attractive materials as they combine the properties of low dimensional organic conductors and high surface area materials.1,2 Composites composed of polyaniline (PANI) and noble metal nanoparticles (such as Au, Pt, and Pd) are currently of great research interest due to PANI’s numerous applications arising from its good environmental stability and tunable electrical and optical properties3 as well as the unique optical and catalytic properties of metal nanoparticles.4 There are mainly two paths for the preparation of PANI-metal nanoparticle composites, namely, electrochemical and chemical methods. For example, electrochemically incorporating metal nanoparticles into PANI is often achieved by the electrochemical reduction of the corresponding metal complex ions (such as AuCl4- and PtCl6-), which act as counterions in the predeposited PANI film via an ion exchange procedure.5 Chemical preparation of PANI-metal nanoparticle composites often follows one of the two routes: (i) chemical polymerization of PANI around the preformed particles,6 or (ii) a “one-pot” approach in which the aniline monomers or their derivatives act as reductant for the metal ions.7 Under certain conditions, the products of the one-pot method are in colloidal form, enabling them to disperse well in solution. The preparation of colloidal PANI is also one of the attractive alternatives to overcome its poor processability due to its insolubility in common organic solvents and water. * Corresponding author. Fax: +49-221-8115195. E-mail: kleinermanns@ uni-duesseldorf.de. † Heinrich-Heine University Du ¨ sseldorf. ‡ Shandong University. § Max-Plank-Institut fu ¨ r Kohlenforschung. (1) Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887. (2) Sih, B. C.; Wolf, M. O. Chem. Commun. 2005, 3375. (3) Handbook of Conducting Polymers; Doblhofer, K., Rajeshwar, K., Eds.; Marcel Dekker: New York, 1998. (4) Metal Nanoparticles: Synthesis, Characterization and Applications; Feldheim, D. L., Colby, A. F., Jr., Eds.; Marcel Dekker: New York, 2002. (5) Hatchett, D. W.; Josowicz, M.; Janata, J.; Baer, D. R. Chem. Mater. 1999, 11, 2989. (6) Sarma, T. K.; Chattopadhyay, A. J. Phys. Chem. A 2004, 108, 7837. (7) Dai, X.; Tan, Y.; Xu, J. Langmuir 2002, 18, 9010.
In this paper, we report a micelle-based method to synthesize dispersed PANI-Au composite particles by the direct oxidation of aniline using AuCl4- as the oxidant. Micelle-assisted synthesis has previously been used to prepare polymer particles in which the particle sizes were determined by the micellar “reactor” sizes.8 In addition to size control, enhancement of growth rate has been observed due to the high local concentration of the monomer in the micellar reaction system.9 The resultant PANI is soluble in water or organic solvents.10 By taking advantage of the micellar reaction system, we obtained colloidal PANI doped with Au nanoparticles by the one-step chemical oxidative polymerization of aniline in the presence of micelles of sodium dodecyl sulfate (SDS). The PANI-Au composite particles disperse well in water and have uniform morphology and well-defined inner structures. 2. Experimental Section The PANI-Au composite particles were produced as follows. Procedure I: In a conical flask 50 mL of 12.5 mM SDS/0.1 M HCl solution and 2.5 mL of 25 mM aniline/0.1 M HCl were mixed with magnetic stirring. A 0.5 mL portion of 25 mM HAuCl4 was added to the solution and magnetically stirred for 3 h until PANI was generated with a characteristic deep green color. To generate enough material for film and pellet preparation, we used the same procedure described above at higher concentrations of 0.125 M SDS, 0.15 M HCl, 25 mM aniline, and 5 mM HAuCl4. The microscopic and spectroscopic results for PANI-Au composites obtained from the lower and higher concentrated solutions were similar. Procedure II: For comparison of results, PANI without Au was synthesized using the same procedure described above (0.125 M SDS, 0.15 M HCl, 25 mM aniline) and 32.5 mM (NH4)2S2O8 for oxidation (instead of HAuCl4). Procedure III: Alternatively, oxidized PANI was synthesized without SDS using the IUPAC convention (0.2 M aniline hydrochloride with 0.25 M (NH4)2S2O8).11 IR Spectra of PANI-Au composites and IUPAC PANI demonstrated around 50% doping each, because the quinoid (NdQdN) and benzenoid (N-B-N) (8) Kinlen, P. J.; Liu, J.; Ding, Y.; Graham, C. R.; Remsen, E. E. Macromolecules 1998, 31, 1735. (9) Kuramoto, N.; Michaelson, J. C.; McEvoy, A. J.; Gratzel, M. Chem. Commun. 1990, 1478. (10) Kuramoto, N.; Tomita, A. Polymer 1997, 38, 3055. (11) Stejskal, J.; Gilbert, R. G. Pure Appl. Chem. 2002, 74, 857.
10.1021/la062135+ CCC: $33.50 © 2006 American Chemical Society Published on Web 11/07/2006
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Figure 1. UV-vis spectra of colloidal PANI-Au composites in aqueous SDS solution prepared by direct oxidation of aniline with AuCl4- (method I) compared to PANI synthesized by oxidation of aniline with (NH4)2S2O8 (method II). stretching vibrations were observed with similar intensity at 1568 and 1486 cm-1, respectively (spectra not shown here). For the preparation of PANI-Au composite films and pellets, ∼100 mL of acetonitrile was added to the green dispersion to destroy the micelles. The obtained suspension was filtered, and the precipitate was washed with 0.15 M HCl and acetone to remove residual monomer, SDS, oxidant, and its decomposition products as well as other low-molecular-weight organic intermediates and oligomers. A thorough final washing with acetone also prevents the aggregation of PANI precipitate during drying so that the product is obtained as a fine powder.11 PANI without Au from procedure II and III was washed and filtered using the same process. PANI-Au films were produced by spraying the green suspension onto filter paper using a homemade centrifuge sprayer. The suspension was dropped on a rotating dish containing two steps so that only the smaller particles made it to the top of the dish to form a fine spray. A quite homogeneous film was obtained by this method. For the production of pellets, the suspension was freeze-dried, and the obtained solid was compressed at 750 MPa to a pellet. The electrical conductivity of the pellets, which were 13 mm in diameter and 1-1.5 mm thick, was measured by the four-probe method.12 Optical absorption spectra of the products were recorded by using a Cary 300 UV-vis spectrophotometer operated at a resolution of 1 nm. The morphology of the PANI-Au composite particles shown in Figures 2 and 3 was examined with a LEO 1530 VP scanning electron microscope (SEM) operated at 20 kV, and a Philips CM 20 transmission electron microscope (TEM) operated at 200 kV. The SEM images of bulk PANI films with and without Au in Figure 4 were recorded with a HITACHI S 3500 N. The samples were prepared for SEM imaging by casting the product solution onto the surface of an evaporated Au film without further sputter-coating of a conductive thin layer due to the good electrical conductivity of the products. The TEM samples were prepared by adding droplets of the product solution onto carbon-coated copper grids (Cu-400CK, Pacific Grid-Tech) and allowing them to dry in air. Spatially resolved energy-dispersive X-ray (EDX) spectroscopy was performed by casting the product solution onto the surface of a glass substrate to reveal the elemental composition of the composites.
3. Results and Discussion The addition of AuCl4- into aniline-doped SDS micelles leads to a clear, green solution, where colloidal PANI molecules doped with Au nanoparticles are well dispersed and stabilized by SDS micelles (see Experimental Section). About one week after the chemical reaction, a tiny Au deposit was formed on the bottom of the reaction cell. This observation suggests that part of the reduced Au does not strongly interact with the PANI in solution. In order to understand the structure and stability of the products, we first examined the spectroscopic properties of the dispersed products in water. Figure 1 shows that PANI synthesized via (12) Prokes, J.; Krivka, I.; Tobolkova, E.; Stejskal, J. Polym. Degrad. Stab. 2000, 68, 261.
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procedure II absorbs at λmax(H2O)/nm 320 (π-π* transition), 430 (polaron band), and 835 (region of interchain excitation), with a long tail extending into the near-infrared region (intrachain excitation), consistent with the typical spectra of PANI in the protonated emeraldine form.13,14 The bands associated with the PANI-Au products are located at λmax(H2O)/nm 340 (π-π* transition), 426 (polaron band), and 745 (region of interchain excitation), without a substantial tail to long wavelengths (little intrachain excitation). Earlier studies have shown that the intensity of the intrachain band decreases with a decrease in molecular weight and level of protonation.13 It is well-known that colloidal Au has a strong surface plasmon resonance absorbance in the visible region, even in dilute concentrations.4 However, we did not observe any absorption band corresponding to colloidal Au in neither isolated nor aggregated form.15 To know whether the reduced Au nanoparticles coexist with the dispersed PANI in water and to learn about the morphology and inner structure of the obtained products, we performed SEM and TEM measurements. The SEM micrograph in Figure 2A indicates that PANI-Au composites are tetrahedronshaped particles that are characterized by a wide size distribution (50-300 nm) with an average particle edge length of about 150 nm. Complimentarily, the TEM investigation (Figure 2B,C) revealed the inner structure of the products. PANI-Au coreshell composites are formed with encapsulated Au nanoparticles, which are distinguishable by dark cores surrounded by gray shells of PANI. The selected-area electron diffraction (SAED) pattern consisting of spots superimposed on the diffraction rings is shown in Figure 2D and provides structural information with regard to the Au nanoparticles. From the diffraction pattern it can be concluded that the Au nanoparticles inside PANI are crystalline. This pattern is consistent with diffraction by many different crystallographic orientations. Whether the polymer is amorphous is still unclear, although SEM demonstrates that the composite particles are of well-defined tetrahedron shape. During the TEM imaging, small Au nanoparticles can move within the PANI matrix, as indicated in Figure 3. The motion of two small Au nanoparticles toward a larger one was marked by arrows. The position of the large nanoparticle within the PANI matrix did not change during this motion. This observation suggests that the interaction between small Au nanoparticles and the surrounding PANI is not strong and/or the inner part of the PANI is flexible or porous. Prolonged irradiation of the composites by the electron beam can cause their partial degradation, which may also contribute to the observed motion of the small particles. This motion is consistent with our earlier observation that a tiny Au deposit was observed on the bottom of the reaction cell 1 week or so after the oxidation reaction was finished. The leaking of small Au nanoparticles from the PANI matrix might result from the thermal motion of the loosely trapped nanoparticles and must be a very slow process when the PANI-Au composites are dispersed in solution. On the basis of the TEM results, we are now able to further interpret the spectra in Figure 1. Au nanoparticles of ∼20 nm diameter normally show an intense plasmon absorption at around 520 nm.15 The absence of this absorption in Figure 1 can be attributed either to the fact that the surface plasmon is delocalized within the polymer film so that it is no longer possible to optically (13) (a) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989, 32, 263. (b) Neoh, K. G.; Young, T. T.; Looi, N. T.; Kang, E. T.; Tan, K. L. Chem. Mater. 1997, 9, 2906 (14) (a) Kinyanjui, J. M.; Hatchett, D. W.; Smith, J. A.; Josowicz, M. Chem. Mater. 2004, 16, 3390. (b) Wang, Y.; Liu, Z.; Han, B.; Sun, Z.; Huang, Y.; Yang, G. Langmuir 2005, 21, 833. (15) Peng, Z.; Walther, T.; Kleinermanns, K. Langmuir 2005, 21, 4249.
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Figure 3. TEM bright-field micrographs of PANI-Au composite particles with several Au nanoparticles encapsulated in the PANI tetrahedron. The motion of the small Au nanoparticles toward the larger one is indicated by an arrow.
A physical mixture of Au nanoparticles of around 20 nm size and PANI synthesized according to method II also did not exhibit the 520 nm Au absorption. If green PANI-Au composites were deprotonated in an alkaline medium to obtain the blue PANI base, the Au nanoparticle surface plasmon absorption at 520 nm was observed again. Similar results have been reported by Leroux et al.16b Therefore, we can conclude that Au nanoparticles closely neighbored to PANI emeraldine salt chains are necessary to effectively eliminate the 520 nm plasmon absorption and to produce the band at 745 nm. The observed strong electronic PANI-Au interaction points to a changed electrical conductivity of PANI-Au compared to that of conventional conducting PANI. Therefore, we measured the conductivity of both PANI-Au composite films and pellets produced as described in the Experimental Section. We obtained conductivities of 0.23 ( 0.08 S cm-1 for PANI (method III) and 0.46 ( 0.06 for PANI-Au composites (method I) in 5-cm-long film strips of around 1 µm thickness obtained from various samples of five different synthesis/preparation procedures each. The conductivity of compressed PANI-Au pellets from various samples was measured by using the four-probe method. Conductivities of around 4 S cm-1 for PANI (method III) and 8 S cm-1 for PANI-Au (method I) were obtained. It is wellknown that PANI films generally exhibit lower conductivities than PANI pellets.17 Figure 4 shows SEM images of PANI-Au (method I) and PANI (method II; method III results in similar images) on a glass substrate. EDX analysis of the elemental composition of PANIAu composites exhibits Au contents of around 1.5% for a film directly obtained from the PANI-Au suspension and 0.6% upon centrifuge-spraying the suspension onto the glass substrate (PANI-Au film). Our results show that both films and pellets produced from PANI-Au composites have an approximately 2-fold higher conductivity than PANI synthesized by the IUPAC procedure. Hence, less than 1% Au doping leads to a considerable increase in conductivity and change in the absorption spectrum of PANI. We could not measure the electrical conductivity of the physical mixture of PANI and Au nanoparticles successfully because here the film preparation process leads to separation between PANI and gold nanoparticles and the precipitation of gold. One obvious advantage of the PANI-Au core-shell structure is its enhanced ability to trap the Au nanoparticles. Synthesis of PANI with different morphologies by using AuCl4- as the oxidant in the absence of SDS micelles in an aqueous medium has been reported by several groups. For example, Kinyanjui et al. reported a simple method for the oxidation of aniline by AuCl4- and the simultaneous formation of bulk quantities of a PANI-Au composite.14a Wang and coworkers demonstrated the synthesis of PANI nanofibers by the polymerization of aniline using AuCl4- as the oxidant,14b where very uniform PANI nanofibers with a diameter of ∼35 nm and aggregated gold nanoparticles precipitated from the liquid phase during the reaction have been obtained. By using AuCl4- as the oxidant to directly oxidize aniline in the presence of SDS micelles, we thought that it would be possible to obtain PANI-Au composites in the form of nanometer-sized particles in an aqueous medium. SDS micelles lead to the preconcentration of aniline at the SDS/water interface and solubility of the obtained composite particles. The subsequent addition of HAuCl4 leads to the
excite the localized surface plasmons (LSPs) or to a dramatic decrease in the surface plasmon lifetime because the PANI dielectric function is complex in its oxidized state (the imaginary part induces the quenching and damping of the LSP).16
(16) (a) Raether, H. In Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Hohler, G., Ed.; Springer Tracts in Modern Physics: Berlin, 1988. (b) Leroux, Y. R.; Lacroix, J. C.; Chane-Ching, K. I.; Fave, C.; Felidj, N.; Levi, G.; Aubard, J.; Krenn, J. R.; Hohenau, A. J. Am. Chem. Soc. 2005, 127, 16022. (17) Roy, R.; Bhattacharyya, A.; Sen, S. K.; Sen, S. Phys. Status Solidi A 1998, 165, 245.
Figure 2. (A) SEM micrograph of tetrahedron-shaped PANI-Au composite particles on an evaporated gold film. (B) TEM brightfield micrograph of colloidal PANI-Au composites. (C) TEM brightfield micrograph of an isolated PANI-Au particle at higher magnification. (D) SAED pattern obtained from the PANI-Au composites.
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Figure 5. Schematic illustration of the formation of PANI-Au nanocomposites in the presence of SDS micelles.
Figure 4. SEM images of a PANI-Au composite synthesized by procedure I (A) and of PANI synthesized by procedure II (B) on a glass substrate.
formation of aniline oligomers and small Au nanoparticles at the micelle/water interface; both products most probably in close contact with each other. As the reaction proceeds, the growing Au nanoparticles will aggregate into larger particles because, at high ionic strength of the supporting electrolyte (0.1 M HCl), the van der Waals attraction between the particles will overtake their Coulomb repulsion, leading to the formation of larger Au particles.18 The attachment of PANI on the Au nanoparticle surface provides a steric barrier against further aggregation. PANI on the Au nanoparticle surface has the ability to form complexes with the SDS molecules, which also contribute to the solubility and stability of the composite particles.19 The large size and the size variability indicate that a large number of such micelles with dissolved anilinium ions contact during the reaction process. (18) Isrealachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (19) Kuramoto, N.; Teramae, K. Polym. AdV. Technol. 1998, 9, 222. (20) Li, D.; Kaner, R. B. J. Am. Chem. Soc. 2006, 128, 968. (21) Granot, E.; Katz, E.; Basnar, B.; Willner, I. Chem. Mater. 2005, 17, 4660.
The presence of a high concentration of SDS and low concentration of anilinium ions might prevent the formation of larger particles, thereby producing nanometer-sized particles only. A scheme of the possible synthesis mechanism of the composite particles is shown in Figure 5. Additionally, one interesting observation is that the morphology of the composite particle could be controlled to some extent by the amount of energy input into the reaction cell. Specifically, if we mix all precursors and leave the reaction cell with only slight agitation, the products are large PANI particles (several hundred nanometers) with large gold nanoparticles (in some cases, triangular or disk-shaped Au) enclosed. When stirring was performed at very high speed, tiny composite nanoparticles were obtained with smaller Au nanoparticles incorporated, but, in this case, quite a lot of reduced gold was not embedded by PANI and formed a golden film on the bottom of the reaction cell. The agitation effect on the morphology of nanoscale PANI has recently been reported by Li and co-workers.20
4. Conclusions In summary, we were able to synthesize PANI-Au composite particles by direct oxidation of aniline with HAuCl4 in SDS micellar medium. The obtained PANI-Au composites are watersoluble and have a well-defined tetrahedron shape with an average edge length of ∼150 nm. The PANI matrix is formed around individual or multiple Au nanoparticles. A modest increase in the conductivity of PANI-Au composite films and pellets relative to IUPAC PANI was observed. Our findings suggest that PANIAu composites and electrodes modified with this material may find interesting applications in electrochemical sensors and conducting polymer coatings.21 Acknowledgment. Z.P. and Z.Z. acknowledge funding by the Alexander von Humboldt Foundation (AvH). The authors are grateful to Dr. Christoph Somsen (Ruhr Universita¨t Bochum, Bochum) for his help with TEM measurements. LA062135+
