Oxidation−Reduction Reaction Driven Approach for Hydrothermal

8588

J. Phys. Chem. C 2009, 113, 8588–8594

Oxidation-Reduction Reaction Driven Approach for Hydrothermal Synthesis of Polyaniline Hollow Spheres with Controllable Size and Shell Thickness Yan-Sheng Zhang, Wei-Hong Xu, Wei-Tang Yao, and Shu-Hong Yu* DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, The People’s Republic of China ReceiVed: NoVember 29, 2008; ReVised Manuscript ReceiVed: April 1, 2009

Different polyaniline (PANI) micro/mesostructures have been synthesized by one-pot polymerization of aniline using hydrogen peroxide (H2O2) as oxidant and Fe3+ as catalyst under hydrothermal conditions. Well-defined PANI hollow spheres with relatively uniform sizes and controllable shell thickness can be prepared in case of low concentrations of monomer and oxidant. The oxidation-reduction reaction between the benzenoid unit and O2 is a driving force for the formation of hollow spheres. This approach provides a unique route for the preparation of well-defined PANI hollow spheres in the absence of any sacrificial templates and organic surfactants and can be potentially extended to synthesize other polymer hollow spheres. 1. Introduction Polyaniline (PANI) has been known for more than a century and is unique among inherently conducting polymers for its high conductivity, excellent environmental stability, and modified properties between the oxidation and protonation state.1 In recent years, various PANI micro/nanostructures have been the research focus for their chemical and physical properties different from the corresponding bulk forms.2 Among them, the fibrillar morphology appears to be the intrinsic nature of PANI synthesized by chemical3 or electrochemical polymerization processes.4 Shape control synthesis of PANI particles with nonfibrillar morphologies has been a hot research topic. Hollow polymer spheres or capsules, which have potential applications in reactors, pigments, catalysts, sensors, carriers, combinatorial analytics, and photochemistry, have especially attracted a lot of attention.5 These hollow polymer spheres are mainly prepared by coating the surfaces of colloidal templates with layers of the desired materials, followed by removal of the templates by means of dissolution, evaporation, or thermolysis. The sacrificial hard or interfacial soft templates are the most commonly used approaches for polymer hollow spheres.6 In the case of PANI, most of the hard templates, such as polystyrene,7 polyelectrolyte,8 metal oxide,9 and SiO2,10 are not involved in the reaction and must be selectively removed from the final products, and this step sometimes results in the destruction of the structures of PANI. The interfacial soft templates provide the two-phase interfaces where the polymerization process takes place. The liquid-liquid interfacial templates, such as micelles,11 lipid vesicles,12 and emulsions,13 have been used to prepare polymer hollow spheres. A large quantity of surfactants or functionalized organic molecules is usually required, and thus the cost is naturally expensive. Generally, the liquid-liquid interfacial templates are limited to the demand of the compatibility of polymer monomer and interface. In addition, these templates lack control over size, geometry, and structure of the products. Thus, seeking for new type templates should be performed to solve these problems. * To whom correspondence should be addressed. Fax: + 86 551 3603040. E-mail: [email protected].

Gaseous bubbles are actually gas-liquid interfacial soft templates.14 Two groups have demonstrated the controlled synthesis of polypyrrole microcontainers through the electrochemical deposition of polypyrrole onto surfactant-stabilized O215 or H216 gaseous bubbles released from electrolysis of H2O. In comparison with electrochemical polymerization, chemical polymerization has the advantage of being a simple process capable of synthesizing products in large quantity and simplifying the reaction setup. Recently, several groups have reported many template-free approaches for the preparation of polymer hollow spheres employing various novel mechanisms. Im et al. prepared polystyrene (PS) hollow particles with controllable holes in their surfaces through solvent swelling mechanism.17 Guo and coworkers18 prepared various polymer hollow spheres using droplet self-templating mechanism. Tan et al. prepared poly(o-anisidine) and PANI hollow spheres based on a diffusion-related process,19 and the mesostructure evolution of as-prepared samples undergoes a gradual process from solid spheres to hollow spheres. Herein, we report an oxidation-reduction reaction driven hydrothermal approach that has been developed for synthesis of PANI hollow spheres by one-pot polymerization of aniline using hydrogen peroxide (H2O2) as oxidant and Fe3+ as catalyst. The oxidation-reduction reaction between the benzenoid unit in the PANI structural chain and O2 is a driving force for the formation of the hollow spheres. 2. Experimental Section 2.1. Materials and Synthesis. Aniline monomer, hydrogen peroxide (H2O2) (30 v/v % solution in water), ferric chloride hexahydrate (FeCl3 · 6H2O), and phosphoric acid (H3PO4) were all AR grade (Shanghai Chemical Reagent Co. Ltd., China). Aniline monomer was distilled before use under reduced pressure, and the other reagents were used as received without further purification. All the water used was deionized water (18.2 Ω · cm-1). In a typical experimental procedure, 1 mmol (0.1 mL) of aniline monomer was added into 40 mL H3PO4 of aqueous solution (0.4 M) with vigorous stirring at room temperature for several minutes to form a uniform solution. Then 1 mmol (0.12

10.1021/jp810491u CCC: $40.75  2009 American Chemical Society Published on Web 04/27/2009

Synthesis of Polyaniline Hollow Spheres

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8589

Figure 1. FE-SEM and TEM images of five PANI samples with different morphologies prepared from different amounts of ANI, H2O2, and Fe3+: (a and b) microspheres and mesospheres (5 mmol ANI + 20 mmol H2O2 + 0.04 mmol FeCl3); (c and d) a mixture of mesospheres and rod/tubes (2.5 mmol ANI + 10 mmol H2O2 + 0.02 mmol FeCl3); (e) PANI-A (1 mmol ANI, 4 mmol H2O2, and 0.008 mmol FeCl3); (f) PANI-B (1 mmol ANI, 2 mmol H2O2, and 0.004 mmol FeCl3); (g) PANI-C (1 mmol ANI, 1 mmol H2O2, and 0.002 mmol FeCl3); (h) The dependence of diameter (outer diameter and shell thickness) vs the oxidant amount used for synthesis of the samples PANI-A, PANI-B, and PANI-C.

mL) of H2O2 and 0.02 mL of FeCl3 (0.1 M) aqueous solution were mixed with the above solution in turn. After fully stirring, the mixture was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed quickly and maintained at 140 °C for 6 h in a digital temperature-controlled oven. Then, the autoclave was cooled to room temperature immediately by water cooling for several minutes. The precipitate was filtered and washed with deionized water and ethanol, respectively, until the filtrate became colorless. At last, the sample was dried in vacuum at 60 °C at least 6 h, in preparation for characterization. To understand the influence of the monomer amount on the morphology of PANI, a set of control experiments were performed with the amount of ANI ranging from 5, to 2.5, to 1

mmol, while the molar ratios of H2O2 to ANI (4:1) and H2O2 to Fe3+ (500:1) were kept constant. The controlled experiments concerned with other important synthetic parameters, such as the amount of catalyst, the concentration of doping acid, and reaction temperature, were also performed. 2.2. Characterization. The obtained samples were characterized by different techniques. The FE-SEM images were taken on a field emission scanning electron microscope (FE-SEM) (JEOL JSM-6700F, 15 kV). Transmission electron microscope (TEM) observation was performed on a Hitachi model H-800 TEM at an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) was performed at an accelerating voltage of 200 kV (JEOL-2010). Fourier transform infrared (FT-IR) spectra were

8590

J. Phys. Chem. C, Vol. 113, No. 20, 2009

Figure 2. TEM images of three PANI samples with different shell thicknesses prepared from different amounts of H2O2 and Fe3+, while the amount of ANI was kept constant (1 mmol): (a) PANI-A (4 mmol H2O2 and 0.008 mol FeCl3); (b) PANI-B (2 mmol H2O2 and 0.004 mol FeCl3); (c) PANI-C (1 mmol H2O2 and 0.002 mol FeCl3).

Zhang et al.

Figure 4. FE-SEM images of two PANI samples prepared. The concentration of the doping acid and reaction temperature are (a) 0.8 M H3PO4 and (b) 120 °C, respectively, while all other experiments parameters are kept constant as those for the sample PANI-A.

Figure 5. XRD patterns of the PANI samples: (a) PANI-A (1 mmol ANI, 4 mmol H2O2, and 0.008 mmol FeCl3); (b) PANI-B (1 mmol ANI, 2 mmol H2O2, and 0.004 mmol FeCl3); (c) PANI-C (1 mmol ANI, 1 mmol H2O2, and 0.002 mmol FeCl3).

Figure 3. TEM images of five PANI samples prepared by using different amounts of H2O2 and Fe3+, while the amount of ANI (1 mmol) was kept constant: (a and b) 4 mmol H2O2 (in the absence of catalyst Fe3+); (c) 2 mmol H2O2 and 0.02 mmol FeCl3; (d) 1 mmol H2O2 and 0.01 mmol FeCl3; (e) 2 mmol H2O2 and 0.04 mmol FeCl3; (f) 1 mmol H2O2 and 0.02 mmol FeCl3.

measured on a Bruker Vector-22 FT-IR spectrometer from 4000 to 500 cm-1 at room temperature. UV-vis-NIR absorption spectra were obtained using a UV-2550 spectrometer (Shimadzu). X-ray power diffraction (XRD) analyses were carried out on a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite-monochromatized Cu KR radiation (λ ) 1.54056 Å), and the operation voltage and current were maintained at 40 kV and 40 mA, respectively. Thermal gravimetric analysis (TGA) was carried out on a Perkin-Elmer Diamond TG thermal analyzer with a heating rate of 10 °C · min-1 and a flowing N2 of 60 mL · min-1. The electrical conductivity of the pressed sample pellet at room temperature was measured by a standard four-point-probe technique. 3. Results and Discussion 3.1. Morphology-Controlled Synthesis of PANI Particles. A set of control experiments were performed to investigate the roles of several important synthetic parameters, i.e., the amount of monomer, the oxidant, molar ratio of monomer to oxidant, and molar ratio of H2O2 to Fe3+) on the morphology of these PANI particles. The amount of monomer ANI was found to be

the dominant parameter for morphology control. When the monomer amount was 5 mmol, different-sized microspheres were obtained (Figure 1, parts a and b). A magnified image shown in Figure 1b indicates that the wall thickness of one typical hollow microsphere is about 120 nm. As a whole, these microspheres have different diameters and wall thicknesses. In contrast, with the monomer amount decreasing to 2.5 mmol, the rods or tubes with diameters from tens of nanometers to several micrometers and mesospheres were observed in the product (Figure 1, parts c and d). Further decreasing the monomer amount leads to the formation of well-defined PANI hollow spheres with relatively uniform sizes. If the amount of the monomer is kept as 1 mmol, even the initial molar ratio of the monomer to the oxidant is varied from 1:4 to 1:1, only welldefined PANI hollow spheres can be obtained finally (Figure 1e-g, and Figure 3). For convenience, these well-defined PANI hollow spheres with relatively uniform sizes were labeled with PANI-A (1 mmol ANI, 4 mmol H2O2, and 0.008 mmol FeCl3), PANI-B (1 mmol ANI, 2 mmol H2O2, and 0.004 mmol FeCl3), and PANI-C (1 mmol ANI, 1 mmol H2O2, and 0.002 mmol FeCl3) in turn. The sample PANI-A mainly consists of vessels originated from bubble coalescence, whereas the other two samples consist of nearly monodispersed spheres (Figure 2a). The outer diameters of these three samples are in the range of 400-450 nm for the sample PANI-B and 450-500 nm for the samples PANI-A and PANI-C, respectively (Figure 2). Figure 1h presents the relationship of sphere size and oxidant amount. The average shell thickness of each sample is 32 nm (PANI-A), 76 nm (PANI-B), and 155 nm (PANI-C), respectively. These relatively uniform hollow spheres may exhibit different proper-

Synthesis of Polyaniline Hollow Spheres

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8591

Figure 6. (a) FT-IR spectra of PANI samples in KBr pellets. (b) UV-vis-NIR spectra of samples dissolved in m-cresol solution.

ties from those of hollow spheres with wide size distribution, making them attractive from both scientific and technological viewpoints.5 3.2. Roles of the Catalyst Fe3+ Ions and Other Parameters. For chemical oxidative polymerization, a chemical oxidant is usually used in solution, and this oxidant is expected to influence the structure and property of PANI. Different oxidants,1 such as (NH4)2S2O8, FeCl3, and HAuCl4, have been used to prepare PANI. H2O2 alone is not a good oxidant for preparing PANI,20 which the standard redox potential (Eox ) 1.77 V) is high enough for aniline polymerization (Eox ) 0.5 V), but not so high enough for the polymerization process to take place vigorously, and the induction time is very long.21 With addition of a small amount of ferrous or ferric ions as catalysts, the reaction activity of H2O2 can be improved greatly.21 During the polymerization process, Fe3+ (Eox ) 0.771 V) first acts as a milder oxidant, and its reductive product is Fe2+, which plays the actual role as a catalyst. As a well-known Fenton-type catalyst, Fe2+ can be used to speed up the decomposition of H2O2 to give · OH or other radical species,21 which act as the oxidants in the polymerization of PANI. Meanwhile, the catalytic function of Fe3+ or Fe2+ can also favor the rapid decomposition of H2O2 to O2 bubbles.22 In the absence of Fe3+, aggregated solid PANI spheres in the size range of 750-800 nm (Figure 3, parts a and b) were obtained instead of hollow spheres in the range of 400-450 nm (Figure 2a). If H2O2 alone is used as oxidant, its conversion into radical ions is slow,21 and thus the polymerization rate is extremely low, which does not favor the rapid generation of the aniline radical cations. The appropriate amount of Fe3+ ions can control the polymerization rate. Under conditions using constant amount of ANI monomer (1 mmol) but using different amounts of H2O2, the influence of amount of catalyst Fe3+ became more complicated. When the amount of H2O2 was 2 mmol, hollow PANI spheres with a size of 350-400 nm could still be obtained in the presence of 0.02 mmol Fe3+ (Figure 3c), and the hollow PANI spheres in size range of 300-350 nm tend to aggregate with each other if the amount of Fe3+ ions increases up to 0.04 mmol (Figure 3e). Nevertheless, when the amount of H2O2 was 1 mmol, hollow spheres was seldom found in the presence of 0.01 mmol Fe3+ ions and most of the particles were solid spheres with a size of 370-420 nm (Figure 3d). Further increasing the amount of Fe3+ to 0.02 mmol, no hollow spheres were observed any more and only solid spheres with a size of 280-330 nm were obtained (Figure 3f). These phenomena can be explained by the fact that the decomposition rate of H2O2 increases with increasing the amount of the catalyst, and a considerable fraction of O2 diffuses into the atmosphere. Thus, it can be seen that the amount of Fe3+ ions can also have influence on the size of PANI hollow spheres.

Figure 7. TG curves of the three PANI samples performed in N2 atmosphere.

The concentration of the doping acid, H3PO4, and the reaction temperature are two other important factors in determining the morphology of the particles. As shown in Figure 4, two samples were prepared under the conditions of different concentrations of the doping acid, or reaction temperature, respectively. There were many small solid particles coexisting with hollow spheres when the concentration of H3PO4 was increased up to 0.8 M. The increasing viscosity of the reaction solution did not favor the fast material transmission. If the temperature decreased to 120 °C (Figure 4b) or 90 °C (see Supporting Information Figure S1), a few rods were found, which were also observed in the chemical polymerization of aniline at 100 °C reported by Huang and Kaner.23 The suitable concentration of doping acid and high temperature (above 140 °C) are favorable for the formation of only well-defined PANI hollow spheres. 3.3. Structural Characterization, Thermal Stability, and Conductivity of the PANI Hollow Spheres. The different structures of these relatively uniform PANI samples (the PANI-A, PANI-B, and PANI-C samples in the above sections) have been characterized with XRD, FT-IR, UV-vis, and TGA. The X-ray diffraction patterns in Figure 5 show that typical amorphous features of the PANI hollow spheres. With increasing of the shell thickness of the hollow spheres, the diffraction intensity increases from sample A to C, which is actually in agreement with the increased shell thickness observed by TEM (Figure 2) and also is possibly due to the different polymerization degree under different conditions. The FT-IR spectra are shown in Figure 6a. The two characteristic peaks at 1594 and 1497 cm-1 can be assigned to the stretching vibration of quinoid ring and benzenoid ring, respectively.24 The band at 1312 cm-1 attributes to C-H stretching vibration with aromatic conjugation.25 The peak at 1182 cm-1 results from the NdQdN (Q denotes quinoid ring) stretching mode and is an indication of electron delocalization

8592

J. Phys. Chem. C, Vol. 113, No. 20, 2009

Zhang et al.

Figure 8. TEM images of PANI hollow spheres synthesized with 1 mmol ANI, 4 mmol H2O2, and 0.08 mol FeCl3 at 140 °C after different periods: (a) 2 h; (b) 4 h; (c) 6 h. (d) FT-IR and (e) UV-vis-NIR spectra of these PANI samples at different periods; (f) the oxidation-reduction reaction between the benzenoid unit and O2.

in PANI.26 The peak near 3358 cm-1 corresponds to the hydrogen-bonded N-H stretching vibration.27 The absorption around 829 cm-1 is due to the bending vibrations of the C-H bonds within the 1,4-disubstituted aromatic ring. The relatively strong peaks at 758 and 690 cm-1, which can be assigned to 1,3-coupling and 1,2,3-coupling of the aromatic ring, respectively, indicate that the samples have the branched and crosslinked chain structures.26 In comparison with the other two samples, the peak at 1497 cm-1 in PANI-C increases remarkably, indicating that more benzenoid units reserve with the oxidants amount decreasing.28 The UV-vis-NIR spectra of the PANI samples are shown in Figure 6b. For PANI-A and PANI-B, there are no obvious peaks on the spectra except one broad peak at about 570 nm assigned to the benzenoid to quinoid excitonic transition.29 According to the results about the annealing effect on the structure of PANI reported by other groups,30 this strong blue shift is most likely caused by the cross-linking reaction and the decrease of chain length. For the sample PANI-C, except the weak peak at 570 nm, another distinct peak at 410 nm is observed, which can be attributed to the polaron-π* transition and indicates the conversion from insulating to conductive state.31 This is similar to that reported previously,21b which suggests that the PANI-C structure is nearly identical to the

emeraldine salt form of PANI. Therefore, the sample PANI-C has highest conductivity among these three samples. This is consistent with conductivity measurements by the four-probe method, showing that the room-temperature conductivity is 6.0 × 10-7 S/cm for PANI-A, 1.0 × 10-5 S/cm for PANI-B, and 3.6 × 10-5 S/cm for PANI-C, respectively. The thermal stability of these three samples was examined by TGA. As shown in Figure 7, the small fractions of weight loss before 120 °C are attributed to the loss of moisture in these PANI samples.32 The weight loss at 120-400 °C was due to the evaporation or decomposition of a few unstable oligoaniline.33 The sharp weight loss beginning at 400 °C presumably corresponded to thermal decomposition of the molecular main chains. The thermal stability of PANI-C is quite different from that of PANI-A and PANI-B. The poor thermal stability of the samples PANI-A and PANI-B may be due to the larger hollow cavity of these two samples, which allows a more rapid dissociation of volatile decomposition products. The good thermal stability of PANI-C is probably due to its high compactness and the reason that the benzenoid units have better thermal stability than the quinoid units.25 3.4. Formation Mechanism of Hollow PANI Spheres. The morphology evolution of the sample PANI-A was studied to understand the detailed fabrication process of hollow spheres.

Synthesis of Polyaniline Hollow Spheres

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8593

SCHEME 1: Schematic Illustration of the Possible Formation Mechanism of PANI Particles with Different Morphologies

There are mainly small hollow or solid spheres ranging in size from 140 to 180 nm in the product after reaction for 2 h, which can be regarded as original spheres. Several original hollow spheres seemed to fuse into one larger peachlike hollow sphere (Figure 8a). The sizes are in the range of 350-400 nm and 450-500 nm for the samples prepared after reaction for 4 and 6 h, respectively (Figure 8, parts b and c). With the reaction time prolonging, the coalescence and fusion process will make smaller original spheres to form larger hollow spheres. During this evolution process, with O2 bubbles fusing into hollow spheres gradually, the benzenoid unit (reduction unit) number in the structural chain is decreasing, and meanwhile the quinoid unit (oxidation unit) number is increasing as shown in Figure 8d. The oxidation-reduction reaction between the benzenoid unit and O2 is the internal driving force for the formation and increase of hollow cavity, which is very different from that reported gas-bubble template approaches.14-16 Scheme 1 presents schematic illustration for the scenario of the morphology changes for the samples prepared under different conditions. This phenomenon can be explained by the feature of the monomer aniline and reaction condition. Due to its hydrophobic benzene-ring and hydrophilic NH2 group, aniline monomer can be regarded as an amphiphilic molecule. The anilines or anilinium cations (the salt form of aniline in acid aqueous solution) could form micelles in acid solution, which might act as building units to fabricate various PANI structures.34 The micelles could become spheres through accretion35 or rods/ tubes through elongation34,36 depending on the detailed reaction conditions. In our experiments, the monomer amount and reaction temperature are two important factors for morphology control. These micelles will first react with oxidants, resulting in the formation of original spheres. Then these original spheres will coalesce with each other, soluble micelles, or react with new-produced O2 bubbles driven by the oxidation-reduction reaction. And this coalescence or fusion process is rapid and unorganized, leading to the broad size distribution of PANI spheres (see Figure 1, parts a and b, Scheme 1a). Under suitable reaction condition (the aniline amount 2.5 mmol), a considerable fraction of the micelles will assemble with each other into onedimensional structures through elongation with an appropriate continuous supply of the building units along one-dimensional

direction (see Figure 1, parts c and d, Scheme 1b), and the residual fraction will assemble into spheres through accretion. As further decreasing the amount of monomer to 1 mmol, it is difficult to form a continuous supply of the building units due to the low concentration of micelles, and most of the micelles will assemble into original spheres through accretion. Afterward, these original spheres will react with newly produced O2 bubbles driven by the oxidation-reduction reaction, or coalesce with each other and soluble micelles, resulting in the formation of relatively uniform PANI hollow spheres (see Figure 1, parts e and f, Scheme 1c). The choice of low concentrations of monomer is the key factor for the synthesis of these relatively uniform hollow spheres. Not only the reactant concentration, but also low reaction temperature, can facilitate the occurrence of the elongation process in the present reaction system. In the case of PANI-A (1 mmol ANI, 4 mmol H2O2, and 0.008 mol FeCl3), only hollow spheres could be obtained at 140 °C. If the polymerization occurred at lower temperature (120 or 90 °C) as mentioned above, the formation of fibrous structures besides the hollow spheres was enhanced by lower reaction temperature.34 4. Conclusion In summary, an oxidation-reduction reaction driven hydrothermal approach has been developed for synthesis of PANI hollow spheres with controllable sizes and thickness, i.e., onepot polymerization of aniline using hydrogen peroxide (H2O2) as oxidant and Fe3+ as catalyst. The morphologies of the PANI particles are more sensitive to the concentration of monomer. The use of low concentrations of monomer and oxidant favors for the formation of well-defined PANI hollow spheres with relatively uniform sizes. The shell thickness and structures of these hollow spheres with relatively uniform sizes could be adjusted by the amounts of H2O2 and FeCl3. The oxidationreduction reaction between the benzenoid unit and O2 is the driving force for the formation hollow particles. This approach provides a unique route for the preparation of well-defined PANI hollow spheres in the absence of any sacrificial templates and organic surfactants and could be potentially extended to synthesize other conducting polymer hollow spheres.

8594

J. Phys. Chem. C, Vol. 113, No. 20, 2009

Acknowledgment. This research was supported by the Natural Science Foundation of China (Grant Nos. 50732006, 20621061, 20671085, 20701035), 2005CB623601, and the Partner-Group of the Chinese Academy of Sciences-the Max Planck Society. Supporting Information Available: TEM images. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Chandrasekhar, P. Conducting Polymer, Fundamentals and Application: A Practical Approach; Kluwer Academic Publishers: Boston, MA, 1999. (b) Wallace, G. G.; Spinks, G. M.; Kane-Maguire, L. A. P.; Teasdale, P. R. ConductiVe ElectroactiVe Polymers: Intelligent Materials Systems, 2nd ed.; CRC Press, 2003. (2) See examples: (a) Zhang, Z. M.; Sui, J.; Zhang, L. J.; Wan, M. X.; Wei, Y.; Yu, L. M. AdV. Mater. 2005, 17, 2854. (b) Zhong, W. B.; Deng, J. Y.; Yang, Y. S.; Yang, W. T. Macromol. Rapid Commun. 2005, 26, 395. (c) Pan, L. J.; Pu, L.; Shi, Y.; Song, S. Y.; Zhou, X.; Zhang, R.; Zheng, Y. D. AdV. Mater. 2007, 19, 461. (d) Song, G. P.; Han, J.; Guo, R. Synth. Met. 2007, 157, 170. (e) Wang, J. X.; Wang, J. S.; Zhang, X. Y.; Wang, Z. Macromol. Rapid Commun. 2007, 28, 84. (f) Yan, Y.; Yu, Z.; Huang, Y. W.; Yuan, W. X.; Wei, Z. X. AdV. Mater. 2007, 19, 3353. (3) Huang, J. X.; Kaner, R. B. Chem. Commun. 2006, 4, 367. (4) Liang, L.; Liu, J.; Windisch, C. F.; Exarhos, G. J.; Lin, Y. H. Angew. Chem., Int. Ed. 2002, 41, 3665. (5) (a) Hollow and Solid Spheres and Microspheres: Science and Technology Associated With Their Fabrication and Application. MRS Symposium Proceedings; Materials Research Society: Pittsburgh, PA, 1995; p 372. (b) Meier, W. Chem. Soc. ReV. 2000, 29, 295. (c) Shchukin, D. G.; Sukhorukov, G. B. AdV. Mater. 2004, 16, 671. (d) Peyratout, C. S.; Dahne, L. Angew. Chem., Int. Ed. 2004, 43, 3762. (6) Caruso, F. AdV. Mater. 2001, 13, 11. (7) (a) Niu, Z. W.; Yang, Z. Z.; Hu, Z. B.; Lu, Y. F.; Han, C. C. AdV. Funct. Mater. 2003, 13, 949. (b) Feng, X. M.; Mao, C. J.; Yang, G.; Hou, W. H.; Zhu, J. J. Langmuir 2006, 22, 4384. (8) Shi, X. Y.; Briseno, A. L.; Sanedrin, R. J.; Zhou, F. M. Macromolecules 2003, 36, 4093. (9) Zhang, Z. M.; Deng, J. Y.; Sui, J.; Yu, L. M.; Wan, M. X.; Wei, Y. Macromol. Chem. Phys. 2006, 207, 763. (10) Fu, G. D.; Zhao, J. P.; Sun, M.; Kang, E. T.; Neoh, K. G. Macromolecules 2007, 40, 2271. (11) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 127, 8274.

Zhang et al. (12) (a) Hotz, J.; Meier, W. AdV. Mater. 1998, 10, 1387. (b) Krafft, M. P.; Schiedknecht, L.; Marie, P.; Giulieri, F.; Schmutz, M.; Poulain, N.; Nakache, E. Langmuir 2001, 17, 2872. (13) (a) Wei, Z. X.; Wan, M. X. AdV. Mater. 2002, 14, 1314. (b) Huang, K.; Meng, X. H.; Wan, M. X. J. Appl. Polym. Sci. 2006, 100, 3050. (c) Zhu, Y.; Hu, D.; Wan, M. X.; Jiang, L.; Wei, W. AdV. Mater. 2007, 19, 2092. (14) Wang, X.; Peng, Q.; Li, Y. D. Acc. Chem. Res. 2007, 8, 635. (15) Qu, L. T.; Shi, G. Q.; Chen, F. E.; Zhang, J. X. Macromolecules 2003, 36, 1063. (16) Bajpai, V.; He, P. G.; Dai, L. M. AdV. Funct. Mater. 2004, 14, 145. (17) Im, S. H.; Jeong, U.; Xia, Y. N. Nat. Mater. 2005, 4, 671. (18) (a) Han, J.; Song, G. P.; Guo, R. AdV. Mater. 2006, 18, 3140. (b) Han, J.; Song, G. P.; Guo, R. Chem. Mater. 2007, 19, 973. (19) Tan, Y. W.; Bai, F.; Wang, D. S.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2007, 19, 5773. (20) Pron, A.; Genoud, F.; Menardo, C.; Nechtschein, M. Synth. Met. 1988, 24, 193. (21) (a) Sun, Z. C.; Geng, Y. H.; Li, J.; Wang, X. H.; Jing, X. B.; Wang, F. S. Synth. Met. 1997, 84, 99. (b) Sun, Z. C.; Geng, Y. H.; Li, J.; Wang, X. H.; Jing, X. B.; Wang, F. S. J. Appl. Polym. Sci. 1999, 72, 1077. (22) Salem, I. A.; Elhag, R. I.; Khalil, K. M. S. Transition Met. Chem. 2000, 25, 260. (23) Huang, J. X.; Kaner, R. B. Angew. Chem., Int. Ed. 2004, 43, 5817. (24) Furukawa, Y.; Ueda, F.; Hyodo, Y.; Harada, I.; Nakajima, T.; Kawagoe, T. Macromolecules 1988, 21, 1297. (25) Zeng, X. R.; Ko, T. M. Polymer 1998, 39, 1187. (26) Pan, L. J.; Pu, L.; Shi, Y.; Sun, T.; Zhang, R.; Zheng, Y. D. AdV. Funct. Mater. 2006, 16, 1279. (27) Zheng, W.; Angelopoulos, M.; Epstein, A. J.; MacDiarmid, A. G. Macromolecules 1997, 30, 7634. (28) Asturias, G. E.; MacDiarmid, A. G. Synth. Met. 1989, 29, E157. (29) Wei, Y.; Hsueh, F. K.; Jang, G. W. Macromolecules 1994, 27, 518. (30) Cruz-Silva, R.; Romero-Garcia, J.; Angulo-Sanchez, J. Z.; FloresLoyola, E.; Farias, M. H.; Castillon, F. F.; Diaz, J. A. Polymer 2004, 45, 4711. (31) Huang, W. S.; MacDiarmid, A. G. Polymer 1993, 34, 1833. (32) (a) Amano, K.; Ishikawa, H.; Kobayashi, A.; Satoh, M.; Hasegawa, E. Synth. Met. 1994, 62, 229. (b) Matveeva, E. S.; Calleja, R. D.; Parkhutik, V. P. Synth. Met. 1995, 72, 105. (33) Guo, Q. X.; Yi, C. Q.; Zhu, L.; Yang, Q.; Xie, Y. Polymer 2005, 46, 3185. (34) Zhang, Z. M.; Wei, Z. X.; Wan, M. X. Macromolecules 2002, 35, 5937. (35) Kim, B. J.; Oh, S. G.; Han, M. G.; Im, S. S. Langmuir 2000, 16, 5841. (36) Harada, M.; Adachi, M. AdV. Mater. 2000, 12, 839.

JP810491U