Efficient and Scalable Synthesis of Pure Polypyrrole Nanoparticles

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Efficient and Scalable Synthesis of Pure Polypyrrole Nanoparticles Applicable for Advanced Nanocomposites and Carbon Nanoparticles Xin-Gui Li,*,† Ang Li,†,‡ Mei-Rong Huang,*,† Yaozu Liao,†,‡ and Yong-Gen Lu§ Institute of Materials Chemistry, Key Laboratory of AdVanced CiVil Engineering Materials, College of Materials Science and Engineering, Tongji UniVersity, Shanghai 200092, People’s Republic of China, Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095-1569, United States, and State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua UniVersity, Shanghai 201620, People’s Republic of China ReceiVed: August 7, 2010; ReVised Manuscript ReceiVed: September 29, 2010

Pure polypyrrole (PPy) nanoparticles that are well-applicable for nanocomposite and nanocarbon precursor were productively synthesized by necessarily unstirred oxidative polymerization of pyrrole in acidic aqueous media at 0 °C without any template. The species and concentration of acid and oxidant have been carefully investigated to optimize the polymerization yield, conjugated structure, size, and conductivity of the PPy particles. Laser particle-size analysis, field-emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy all revealed that the PPy particles produced in still acid media have narrow size distribution and uniform spheroid morphology. Homogeneous nucleation and static repulsion are proposed as the formation and self-stabilization mechanisms of the PPy nanoparticles. Combination of HNO3 medium and (NH4)2S2O8 oxidant is optimal for the synthesis of PPy nanoparticles possessing maximal yield of 87.2%, small diameter, and high conductivity which has been confirmed by a strong UV-vis band due to a large π-conjugated chain structure. This quiescent polymerization could be simply scaled up or down to synthesize a larger or smaller amount of PPy nanoparticles without compromising their yield, structure, and properties. Furthermore, the conductivity of the nano-PPy could reach 2.8 S/cm upon doping in 2.0 M HClO4. Simultaneous thermogravimetry-differential thermal analysis technique demonstrates that the PPy nanoparticles at 1000 °C can be efficiently carbonized into carbon nanoparticles with narrower size distribution, smaller diameter of 62 nm, and much higher conductivity of about 21 S/cm. In particular, the conductivity will dramatically be enhanced to 219 S/cm and even 370 S/cm at the carbonization and graphitization temperatures of 1300 and 2300 °C in nitrogen and argon, respectively. A conductive nano-PPy/cellulose diacetate nanocomposite film with low percolation threshold down to 0.2 wt %, good conductivity stability for at least 8 weeks, and potential bioapplicability was simply fabricated. The present synthesis requires no external templates and provides a facile and direct route to scalable synthesis of PPy exclusive nanoparticles with high yield, controllable size, strong re-dispersibility, high purity, adjustable conductivity, and high nanocarbon yield. Introduction Polypyrrole (PPy) is one of the most important conducting polymers with great potential applications because of its ease of synthesis, outstanding conductivity, superior redox properties, biocompatibility, and good environmental stability.1-3 The PPy in many biological environments can be used to modulate cellular activities, including cell proliferation,4 migration,5 and DNA synthesis6 via electrical stimulation. The external stimulation of cultured cardiac myocytes was carried out using a PPycoated microelectrode.7 A lack of the necessary strength of pure free-standing PPy film limits its widely practical application. Combining with other polymers to form composites with various desired properties is the key to such a knotty problem. Recently, PPy nanocomposites have been fabricated by adding a small amount of nanosized PPy into matrix polymers such as poly* To whom correspondence should be addressed. Tel.: +86-21-65799455. Fax: +86-21-65799455. E-mail: [email protected] (X.-G.L.); [email protected] (M.-R.H.). † Tongji University. ‡ University of California, Los Angeles. § Donghua University.

(vinyl alcohol)8 and polylactide.9,10 Note that the PPy/poly(vinyl alcohol) nanocomposites would ineluctably lose necessary strength when meeting with water, and few solvents could dissolve polylactide and homogeneously disperse nanosized PPy in polylactide solution, leading to nonuniformity and relatively high percolation threshold of the nanocomposites. Therefore, nanostructured PPy with high self-stability and good redispersibility even in viscous polymer concentration has attracted more attention because of additionally novel chemical and physical properties endowed by nanoeffect and its potential vital application in nanocomposites, sensors, biomedicines, supercapacitors, and molecular memory devices.10-14 Up to now, many approaches such as hard and soft template techniques have been employed for the synthesis of nanostructured PPy with various morphologies.15-17 Unfortunately, sophisticated preparation is inevitable with both methods by which the nano-PPy fabricated does not possess satisfactory self-stability and redispersibility. Thus, the template-free methods including copolymerization and electrochemical synthesis have received a lot of interest over the past decade from different scientific and industrial fields. The process ability and functionality of PPy

10.1021/jp107435b  2010 American Chemical Society Published on Web 10/22/2010

Efficient Synthesis of Pure Polypyrrole Nanoparticles are significantly improved via co-polymerizing with other monomers carrying self-stabilized functional groups.18-22 However, the imperfect conductivity and relatively high cost of functionalized co-monomer restrict the extensive applications of the co-polymerization method to some extent. On the other hand, electrochemical polymerization leads to formation of a nanosized PPy film on working electrode. Though the size and morphology of the PPy film are controllable, the electropolymerization is not suitable for mass production.23 To date, the facile method of productively synthesizing nanostructured PPy without any surfactants or external stabilizer still faces the challenge. Here we report a facile chemical route to high-quality PPy nanoparticles in the absence of any external traditional templates. The key to the formation and stabilization of the PPy nanoparticles is unstirred chemical oxidative polymerization of pyrrole (Py) in quiescent acidic aqueous media at 0 °C. The effect of polymerization media and oxidant on the yield, structure, and properties of nano-PPy has been systematically optimized. PPy products have productively been synthesized, consisting of almost exclusively uniform nanoparticles with narrow size distribution, high conductivity, strong re-dispersibility even in viscous media, good nanocomposite capability, and high yield of conducting nanocarbon at 1000 °C at the same time. A unique and scalable methodology to effectively prepare self-stable and re-dispersible PPy nanoparticles was successfully developed. Experimental Section Materials. Pyrrole (Py), (NH4)2S2O8, FeCl3, K2Cr2O7, HNO3, HCl, HClO4, p-toluenesulfonic acid, aniline-2,5-disulfonic acid, dodecylbenzenesulfonic acid, iodine, ethanol, acetone, cellulose diacetate (CDA), NaCl, NaHCO3, KCl, K2HPO4, CaCl2, MgCl2, and Na2SO4 were of analytical reagent grade and used as received. Fabrication of PPy Nanoparticles. A typical procedure for the preparation of PPy nanoparticles in an acidic polymerization medium is as follows: Aqueous HNO3 solution (0.35 M, 75 mL) was added with Py (0.175 mL, 2.5 mmol) to a 250 mL glass flask in a glaciofluvial bath at 0 °C. (NH4)2S2O8 (570 mg, 2.5 mmol) was dissolved separately in HNO3 (0.35 M, 25 mL) to prepare an oxidant solution that was kept in the same glaciofluvial bath. The oxidant solution was then added dropwise to the Py monomer solution at a rate of 1 drop (ca. 60 µL) every 1 s at 0 °C over about 20 min. Then the reaction mixture was kept still for 6 h in the glaciofluvial bath at 0 °C. After the reaction, the virgin HNO3-doped PPy formed was isolated from the reaction mixture by centrifugation and washed with an excess of distilled water to remove residual oxidant and watersoluble oligomers. Note that a thin top layer, side layer, and bottom layer of the PPy on the reactor wall were not included in the target PPy product. Ultrasonic dispersion, washing, and centrifugation were successively repeated four or five times until the top liquid layer in the centrifuge tube became pellucid. The purified black precipitate was divided into two parts, one of which was dispersed in water for direct solution measurements by UV-vis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and laser particle-size analysis and direct application for nanocomposite preparation, and the other, left to dry under an infrared lamp at 40 °C for 2 days for evaluation of the polymerization yield, infrared (IR), X-ray diffraction (XRD), and thermogravimetry-differential thermal analysis (TG-DTA). The nominal oxidative polymerization of Py for the synthesis of PPy nanoparticles is shown in the inset in Figure 1.

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Figure 1. Influence of HCl (-) and HNO3 (--) concentrations on the apparent polymerization yield, bulk electrical conductivity, numberaverage diameter (Dn), and polydispersity index PDI (Dw/Dn) in pure water of the PPy nanoparticles with a fixed (NH4)2S2O8/Py monomer molar ratio of 1 in still aqueous acids at 0 °C for 6 h. The inset is the nominal oxidative polymerization of Py for the synthesis of PPy nanoparticles.

Redoping of PPy Nanoparticles. A 100 mL aliquot of aqueous solution of acid re-dopant at a certain concentration was mixed with 20 mg of PPy nanoparticle powder in a 250 mL glass flask and then magnetically stirred at room temperature for 24 h. The fully re-doped PPy was isolated from the mixture by centrifugation and washed with distilled water two or three times and then left to dry under an infrared lamp at 40 °C for 2 days. I2 vapor re-doping was carried out by the following procedure: 2 g of I2 powder was placed in one side of a glass tube, and 20 mg of PPy powder was placed as a thin layer in the other side. The glass tube was tightly sealed and then heated at 60 °C in an oven for a week. Preparation of Carbon Nanoparticles. Carbon nanoparticles were simply prepared by heat treatment of purified dry PPy nanoparticles at a temperature range from 25 to 1000 °C (or 1300 °C) at a heating rate of 10 °C/min (or 20 °C/min) in a nitrogen flow with a TG-DTA instrument. Preparation of PPy Nanocomposite Film. A 2.5 mL aliquot of acetone dispersions of PPy nanoparticles were well-mixed with 5 mL of 4.8 wt % CDA acetone solution by ultrasonically dispersing them for 2 h to prepare several groups of composite solutions. Then the composite solution was cast onto a poly(tetrafluoroethylene) plate. After drying in an oven at 25 °C for 48 h, a 20 µm thick film was peeled off from the substrate to form a free-standing film for performance evaluation. Preparation of PPy Nanocomposite Scaffold. A 1 g amount of CDA was well-mixed with the acetone dispersions of PPy nanoparticles (10 mL) to prepare several groups of composite solutions. Then the composite solution was cast into a watch glass whose size determines the thickness of the scaffold. After stirring mildly for 2 h in the oven at 25 °C, 3 g of NaCl was added into the viscous solution used as hole making reagent. After 12 h drying, a 1.5 mm thick scaffold was obtained. After

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TABLE 1: Synthesis and Characteristics of PPy Particles under Different Polymerization Conditions Py polymerization

PPy particles

oxidant/Py molar ratio

media

stirring

yield (%)

Dn (nm)

PDI

electrical conductivity (S/cm)

FeCl3/Py ) 0.5 FeCl3/Py ) 2 (NH4)2S2O8/Py ) 1 (NH4)2S2O8/Py ) 2 (NH4)2S2O8/Py ) 1

0.35 M HNO3 2.0 M HCl 2.0 M HCl 2.0 M HCl 2.0 M HCl

no no no no yes

1.80 15.5 72.2 73.4 77.6

2126 2193 285 354 3484

1.19 1.16 1.17 1.29 1.21

1.87 × 10-6 3.64 × 10-4 8.49 × 10-6 2.12 × 10-8 9.19 × 10-6

soaking in the deionized water for 12 h to dissolve away the NaCl, a porous PPy/CDA scaffold was gained. Measurements. UV-vis spectra of PPy particle dispersions in ethanol were obtained on a Lambda 35 UV/vis spectrophotometer (Perkin-Elmer Instruments) in the wavelength range of 190-900 nm at a scanning rate of 400 nm min-1. IR reflection-absorption spectra of the PPy pellets were recorded on a Nicolet Magna 550 IR spectrometer at 2 cm-1 resolution. Wide-angle X-ray diffractograms were obtained by using a Rigaku D/max2550VB3+/PC model X-ray diffractometer with Cu KR radiation (wavelength ) 0.15406 nm) over a 2θ range from 3 to 90°. Simultaneous thermogravimetry and differential thermal analysis measurements were performed at heating rates of 10 and 20 °C/min in nitrogen atmosphere in a temperature span of 25-1300 °C and sample masses of 33.76 and 22.25 mg, respectively, by NETZSCH STA 449C Jupiter thermogravimetric apparatus. The size and size distribution of the asprepared PPy particles in water were analyzed on a Beckman Coulter LS230 laser particle-size analyzer (LPA). The size and morphology of the particles were further carefully observed by Quanta 200FEG field-emission scanning electron microscope (FE-SEM), Hitachi model H600 transmission electron microscope, and a Multimode Nanoscope IIIa atom force microscope. The bulk electrical conductivity of PPy pellets and nanocomposites was calculated by measuring the resistance and thickness of the pellets or nanocomposites between two round-disk steel electrodes with a diameter of 1 cm with a multimeter and a thickness gauge at 15 °C. The bulk electrical conductivity of the PPy scaffold was calculated by measuring the resistance between the beginning and the end of the long side of the cuboid plate with two steel probe electrodes and corresponding parameters including length, width, and thickness. The relative error for various measurements is less than 5%.

and polymerization medium.26 As shown in Figure 1, with increasing acid concentration from 0 to 3 M, the apparent yield of PPy increased first and then decreased persistently, exhibiting a maximum of 81.8 and 87.2% at 0.5 and 0.25 M in HCl and HNO3, respectively. Actually, the yield obtained in pure water without any H+ is comparably high (81.5%), that is attributed to the strong polymerizability of Py monomer. A small amount of H+ could create a suitable acidic environment, activating the monomer further and simultaneously enhancing its solubility. Oppositely, excessive H+ would help generate a large amount of nuclei in homogeneous media rather than grow into polymers. The formation of many oligomers will lead to a degressive yield. Generally, the polymerization yield in HNO3 is higher than that in HCl due to a little higher molecular weight of NO3- attaching to the PPy chains through doping than Cl-. The dependence of polymerization yield on the oxidant species and oxidant/monomer ratio has been summarized in Table 1 and Figure 2. It is apparent that the PPy using FeCl3 as oxidant has much lower yield than that using (NH4)2S2O8 regardless of acid species because of much lower solubility in acid media and much weaker oxidizability of FeCl3. It can be seen that the (NH4)2S2O8/Py monomer ratio strongly influences the polymerization yield. The oxidant appears to be in a state of shortage when the oxidant/monomer molar ratio is less than 0.8, but the oxidant is basically sufficient if the oxidant/monomer molar ratio is higher than 1.

Results and Discussion Synthesis of PPy Nanoparticles. Ammonium persulfate as preferred oxidant was added dropwise to a Py solution in aqueous HCl and HNO3 respectively to synthesize the PPy nanoparticles. The concentration and nature of the acid medium were properly regulated to optimize the rate and yield of polymerization (Figure 1). When the synthesis progressed under acidic environment, a light blue PPy was generated in about 3 min and the polymerization rate became increasingly fast as the reaction progressed; subsequently the whole solution turned opaquely black in 5 min. Although no remarkable change of the polymerization rates was detected as the acid concentration increased, not until 20 min was the polymerization observed when using pure water as polymerization medium. It is reasonable to infer that sufficient H+ is necessary for (NH4)2S2O8 to oxidize Py and then initiate oxidative polymerization. Py monomer is so polymerizable that a very small amount of H+ is sufficient for chain initiation and propagation even at low temperature.24,25 Various polymerization parameters dramatically influence the synthetic yield, such as oxidant species, oxidant/monomer ratio,

Figure 2. Influence of (NH4)2S2O8/Py molar ratio on the apparent synthetic yield, bulk electrical conductivity, number-average diameter Dn, and size PDI Dw/Dn of PPy particles in pure water determined by LPA in a fixed 2.0 M HCl at 0 °C for 6 h.

Efficient Synthesis of Pure Polypyrrole Nanoparticles


Figure 3. UV-vis absorption spectra of fine PPy particles (dispersed in ethanol) prepared in seven HCl concentrations at a fixed (NH4)2S2O8/ monomer molar ratio of 1 (a), six HNO3 concentrations at a fixed (NH4)2S2O8/monomer molar ratio of 1 (b), and at six (NH4)2S2O8/pyrrole molar ratios in 2.0 M HCl (c) at the same polymerization temperature 0 °C for 6 h; variation of the UV-vis spectra in ethanol of the carbon nanoparticles (d) carbonized at 1000, 1300, and 2300 °C on the basis of PPy nanoparticles synthesized in two acid media.

Structure of PPy Nanoparticles. The macromolecular, supramolecular, and morphological structures of PPy particles were analyzed by UV-vis and IR spectroscopy, wide-angle X-ray diffraction (XRD), LPA, SEM, TEM, and AFM methods. UV-Vis Spectra. Up until now, seldom have there been any reports on UV-vis spectra of absolutely insoluble PPy particles without any other additives or macromolecular dopants. Fortunately, ethanol dispersions of the PPy particles obtained in this study demonstrate significantly distinct UV-vis absorption spectra in Figures 3 and S1 (Supporting Information), strongly suggesting that the PPy particles are very fine nanoparticles with an excellent dispersibility in ethanol because it is well-known that the PPy particles are insoluble in ethanol and the microparticles or larger particles of PPy in ethanol will not give any significant UV-vis absorption spectra. There are four remarkable absorptions around 230, 380, 500, and 900 nm. The band at 230 nm is mainly attributed to the π-conjugated structure of Py rings. With the variation of acid concentration, oxidant/ monomer ratio, or dispersion temperature, the band at 230 nm changes slightly, but the other three bands change dramatically. With enhancing acid concentration from 0 to 3.0 M, the three bands at 380, 500, and 900 nm all increase first and then decline, displaying maximal intensity of 380 and 500 nm in 1.5 M HCl, maximal intensity of 900 nm in 0.5 M HCl, and maximal intensity of 380, 500, and 900 nm in 0.35 M HNO3. Similarly, the band intensity in a wavelength range between 380 and 900 nm also increases first and then reduces with changing of the oxidant/monomer molar ratio from 0 to 2, illustrating the maximum at the oxidant/monomer molar ratio of 0.5. In particular, the relative intensity of the 900 nm band in Figure 4 is exactly in agreement with the relative conductivity discussed below. That is to say, the 900 nm band is attributable to the large π-conjugated structure of PPy chains. Note that the UV-vis spectrum reduces monotonically with elevating dispersion temperature; i.e., the large π-conjugated length of PPy chains would become shorter because of more vigorous thermal

Figure 4. Variation of 900 nm band intensity of UV-vis spectra of fine PPy particles (dispersed in ethanol) with HCl and HNO3 concentrations at a fixed (NH4)2S2O8/monomer molar ratio of 1 and with (NH4)2S2O8/pyrrole molar ratios in 2.0 M HCl at the same polymerization temperature of 0 °C for 6 h.

motion of the PPy chains at higher temperature. On the basis of the theory of energy band and polarization, these experimental phenomena could be explained as follows: On the basis of the chain structures and band models of three typical PPy states having different doping and oxidizing degrees,27,28 neutral PPy is an insulator whose electronic band is characterized by an empty bandgap.27 Upon the coaction of oxidizing and doping, electrons would be extracted from the PPy backbone and the electric charges become delocalized, forming the polaron. The presence of a polaron introduces two localized electronic levels in the gap: an occupied bonding polaron level (BPL) and an empty antibonding polaron level (APL). Two polarons are unstable when they combine together.28 It should be noticed that the two electronic levels of bipolaron, i.e., bonding/antibonding bipolaron levels, BBL and ABL, get

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Figure 5. (a) IR reflection spectra of PPy particles prepared in 0.5 M HCl and 0.5 M HNO3 and corresponding nanocarbon nanoparticles and (b) the area ratio of bands around 1556 (1571) over 1479 (1485) cm-1.

closer and the bipolaron bonding state is empty, which is dissimilar to the polaron case. Therefore, the band at about 380 nm associates with interband π-π* transition whose intensity is related to neutral PPy content. The band at about 500 nm represents a transition from valence bond to the antibonding polaron or bipolaron state because of the polaron absorption. The band at 900 nm or even higher wavelength should be assigned to a transition from the valence bond to the bonding polaron or bipolaron state mainly due to the bipolaron absorption.29,30 The intensity and position of the bands in the UV-vis spectra reveal the corresponding structure of PPy. The band intensity at 380 nm is directly proportional to the amount of the PPy neutral state. The red shift at about 500 nm is attributable to the reduction of the energy difference between valence bond and antibonding level from polaron state to bipolaron state, indicating the existence of more bipolaron of PPy. The high band intensity around 900 nm further reflects the presence of the PPy in a state of more polarons or bipolarons, remarkably enhancing the conductivity. In short, the complicated variation of the UV-vis spectra in a wavelength from 380 to 900 nm results from the combination of nanoeffect, polarization, and conjugated structure of PPy particles. Figure 3d depicts UV-vis absorption spectra of the PPy-based carbon nanoparticles dispersed in ethanol. Upon heat treatment at 1000 °C, PPy can convert into a carbon-like structure, thus achieving high conjugacy and an almost overlapped bonding and antibonding levels that have been confirmed by a mergence of the bands at 500-900 nm. It is surprisingly noted that much stronger absorbance between 380 and 900 nm of the carbon nanoparticles obtained at 1300 °C than 1000 °C significantly indicates much larger π-conjugacy, which has been further proved by much higher conductivity of the nanocarbon achieved at 1300 °C discussed below. Satisfactory UV-vis spectra of the carbon nanoparticles in ethanol indirectly suggest a good re-dispersibility of the carbon nanoparticles in liquid media. IR Spectra. IR reflection spectra of PPy particles are shown in Figure 5. The broad peak centered at 3250 cm-1 is assigned to N-H stretching. The strong band at 1556/1571 cm-1 is predominantly due to CdC and C-C in-ring-stretch mode, the weak band at 1479/1485 cm-1 originates from CdC and C-N stretch mode, the shoulder bands at 1296 cm-1 are attributed to C-C in-ring stretch and C-N deformation modes, the strong bands at 1186/1211 and 1043 cm-1 are mostly ascribed to C-H in-plane and C-C out-of-plane deformation vibration, respectively, the strong band at 912/931 cm-1 is due to C-H out-ofplane deformation vibration, and the medium band at 790/787 cm-1 might result from C-H and N-H out-of-plane degrees of freedom.31 The IR spectrum of the PPy particles obtained in

Figure 6. Wide-angle X-ray diffractograms of (a) PPy nanoparticles from 0.5 M HNO3, (b) PPy nanoparticles from 0.5 M HCl, (c) carbon nanoparticles based on the PPy nanoparticles from 0.5 M HCl through a high-temperature charring from 25 through 1000 at 10 °C/min, (d) carbon nanoparticles based on the PPy nanoparticles from 0.5 M HCl through a high-temperature charring from 25 through 1300 at 20 °C/ min; and (e) graphite nanoparticles based on the PPy nanoparticles from 0.5 M HCl through a high-temperature graphitization from 25 through 2300 °C at ca. 38 °C/min and held at a constant 2300 °C for 30 min.

HCl is partly different from that in HNO3, indicating that the macromolecular structure of the PPy varies with acid medium, which has been verified by the UV-vis spectra in Figure 3. By measuring the “effective conjugation” of PPy from IR spectra,32 the degree of electron delocalization is directly proportional to the ratio of peak areas at about 1479 over 1556 cm-1.33 It is clear from delicate IR spectra in Figure 5b that the conductivity is indeed directly proportional to the A1479/A1556 ratio. By the way, the IR spectrum of the PPy-based nanocarbon hardly ever demonstrates any significant absorption in the whole wavenumber region, revealing that most of the PPy chains have transformed into a carbon network structure during the charring process from 25 through 1000 °C in nitrogen. Wide-Angle X-ray Diffraction. The wide-angle X-ray diffractograms in Figure 6 reveal that the PPy particles formed in this study are essentially amorphous, indicating that the acid medium has little influence on the supramolecular structure of PPy particles. A broad diffraction peak centered at 2θ ) 23.5°

Efficient Synthesis of Pure Polypyrrole Nanoparticles corresponding to a d-spacing of 0.38 nm is characteristic of amorphous PPy caused by the scattering from PPy chains at the interplanar spacing.34 This case may be attributed to a crosslinked structure formed by a nondominant R-β bonding during the Py polymerization that will be discussed below. It is worthy of note that the X-ray diffractogram of PPy-based nanocarbon obtained at up to 1000 °C in N2 displays different characteristics, having a sharper diffraction at larger 2θ ) 24.6° with a d-spacing of 0.362 nm, accompanied by an additional weak diffraction at 43.4° and much stronger diffraction at around 2θ ) 3°, in good agreement with the UV-vis and IR spectral results. Moreover, the X-ray diffractogram of PPy-based nanocarbon obtained at up to 1300 °C in nitrogen displays not only stronger diffraction characteristics at 2θ around 3, 25.20, and 43.26°, but also seven additional sharp diffractions at 2θ of 25.52° (strong, d002), 35.06 (strong, d200), 43.26 (strong, d210), 52.46, 57.38, 68.10, and 76.79° that may be attributable to β carbon nitride crystals.35,36 This significantly indicates a transformation of PPy macromolecular chains to graphite-like carbonnetwork structure together with the formation of a small amount of β carbon nitride crystals. Especially, the PPy-based nanographite obtained at up to 2300 °C in argon only demonstrates extremely strong and sharp diffractions at 2θ around 3 and 26.04° with a d-spacing of 0.342 nm corresponding to graphite d002 that implies a regularly layered graphite structure with the same interlayer distance of 0.342 nm between adjacent carbonnetwork layers, together with three very weak diffractions at 42.64° with a d-spacing of 0.212 nm corresponding to graphite d100,35 53.90° (d-spacing, 0.170 nm), and 77.78° (d-spacing, 0.123 nm). In particular, the crystallite size calculated on the basis of the half-height width of the strongest diffraction at 26.04° by Scherrer equation is 16.13 nm, indicating that the fine and uniform graphite nanocrystals have grown well, as displayed below (Figure 10g-i). This strongly suggests a substantial transformation of PPy macromolecular chains to well-developed and regularly layered graphite carbon-network structure. Size and Morphology of PPy Particles. The size and size distribution of the PPy particles prepared in various polymerization media, oxidant/monomer ratio, and oxidant species are summarized in Figures 1 and 2 and Table 1. The concentration and species of the acid media and oxidant have a tremendous effect on particle size and its polydispersity. The number-average diameter Dn sharply decreases upon introducing acid into pure water medium, while the Dn of the PPy particles varies slightly with changing acid concentration from 0.25 to 3.0 M. The variation of oxidant from FeCl3 to (NH4)2S2O8 will greatly reduce the particle size from 2193 to 354 nm. FE-SEM images in Figure S2 (Supporting Information) confirmed that the PPy particles synthesized in still pure water with FeCl3 as oxidant generally have large size (>1000 nm) and irregular shape. If using quiescent ethanol as polymerization medium with FeCl3 as oxidant, the PPy particles obtained are smaller (ca. 250 nm) but still irregular, as shown in Figure S3 (Supporting Information). If using K2Cr2O7/(NH4)2S2O8 as composite oxidant even in quiescent 2.0 M HCl, only irregular PPy particles were obtained, as shown in Figure S4 (Supporting Information). Besides, changing stirring to unstirring also drastically diminishes the particle size from 3484 to 285 nm. Note that the size of the PPy particles obtained in HNO3 is always smaller than that in HCl, whereas the PPy particles produced in HCl have a lower size polydispersity index Dw/Dn possibly due to smaller Cl- anion than NO3- anion. In particular, the PPy particles prepared in 2.0 M HNO3 have the smallest Dn of down to 237

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nm and also very small polydispersity index of 1.181, whereas the PPy particles formed in acid-free pure water are the largest because of the absence of H+ in the polymerization medium. It is indicated that the existence of H+ is one of the key factors to control the particle size in a nanolevel, because of the stabilization from a proper protonization of H+ onto the active Py units in the PPy nanoparticulates in Scheme 1. With increasing acid concentration, the protonization of H+ on the Py units tends to be saturated, resulting in a nearly unchanged size and distribution of the PPy nanoparticles. The highly charged, the self-stable, and uniform PPy nanoparticles have been obtained satisfactorily. With increasing (NH4)2S2O8/Py molar ratio from 0.3 to 2, the size of the PPy particles declines first and then increases, exhibiting a minimum Dn of 260 nm at (NH4)2S2O8/Py ratio of 0.5 in 2.0 M HCl. At a lower oxidant concentration, for example, (NH4)2S2O8/Py molar ratio of 0.5, less Py radicals are formed, the polymerization proceeds in a comparably slow and ordered way, and thus smaller particles are generated accompanied with some unpolymerized monomer and oligomers inevitably, which is consistent with the low yield discussed above. When excessive oxidant was added, more monomers would be oxidized into radicals to tempestuously take part in polymerization. Accordingly, both the particle size and yield increase simultaneously. LPA is used here mainly to reveal the size distribution of the PPy nanoparticles because the LPA data were statistically treated by an equivalent-sphere model.37 Therefore, exact morphology and size of the PPy particles were determined by FE-SEM. Parts a-c of Figure 7 are FE-SEM images of PPy particles synthesized in 2.0 M HCl without stirring. Clearly, the synthetic products are fine spheroidic particles with average diameter of about 130-150 nm. The gold-clad layer thickness is about 15 nm on one side, and then the real diameter of the particles is about 100-120 nm. It has been revealed in Table 1 that Dn of the PPy particles synthesized in 2.0 M HCl under stirring is about 3.5 µm, which is much bigger than that without stirring. Parts d-f of Figure 7 reveal that the PPy synthesized under stirring exists as a similar particle aggregate that is more compact and cannot be well-broken into much smaller nano-

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Figure 7. SEM images of PPy particles synthesized in 2.0 M HCl without stirring (a-c) and under stirring (d-f) by chemical oxidative polymerization of Py with a fixed (NH4)2S2O8/monomer molar ratio of 1 for 6 h. The scale bars represent the lengths of 5 µm (a), 1 µm (b), 500 nm (c), 2 µm (d), 1 µm (e), and 400 nm (f).

particles in water even under ultrasonication. In contrast, the PPy synthesized in quiescent HCl seems a looser aggregate that can facilely be disintegrated and re-dispersed into nanoparticles even in pure water under ultrasonic treatment. It is interesting that even if changing Py monomer concentration from 25 mM up to 50 mM or down to 10 mM in the same other polymerization conditions such as quiescent 100 mL of 2.0 M HCl at 0 °C for 6 h, the PPy obtained is all fine nanoparticles with about the same synthetic yield of 68.8-72.2%, number-average diameter of 285-298 nm, size-polydispersity index of 1.16-1.20, and bulk electrical conductivity of (4.0-8.5) × 10-6 S/cm (Figure S5 of the Supporting Information). That is to say, this quiescent polymerization could be simply scaled up or down to efficiently synthesize a larger or smaller amount of PPy nanoparticles in the same amount of polymerization media without compromising their yield, structure, and properties. Why does the chemical oxidative polymerization of Py in quiescent aqueous acid afford re-dispersible self-stable nanoparticles as products? All the experimental phenomena can be explained by two nucleation ways.38 The nuclei can be formed either homogeneously in the parent phase or heterogeneously on other species such as reactor surface.39 Heterogeneous nucleation requires less energy and then occurs much more often than homogeneous nucleation. As Py polymerization proceeds, a PPy aggregate layer consisting of a considerable amount of nuclei produced by heterogeneous nucleation will always be deposited on the reactor and the solution surface, which is similar to the aniline polymerization.40 Whether the reaction solution is stirred or not, the film will appear, however, the area and integrality of which increase without stirring. Therefore, a possible mechanism of the formation and self-stabilization of the PPy nanoparticles is proposed in Scheme 1. Under vigorous agitation in the cases of II′-III′, (1) the PPy film cannot exist stably and there are always preferential sites for heterogeneous nucleation; (2) the heterogeneous nuclei spread throughout the solution, and the opportunity of the polymerization of many Py molecules onto the heterogeneous nuclei becomes much more; and (3) this results in the fast formation of a large amount of indispersible aggregate of PPy nanoparticles. In contrast, in the absence of stirring (II-III), the heterogeneous PPy aggregate layer at the interface between the polymerization medium and air or reactor would not enter the bulk medium to participate in

Figure 8. FE-SEM images of PPy nanoparticles synthesized in still 1.0 M HCl aqueous solution at 0 °C for 6 h at 25 mM Py and a few drops of aqueous dispersion of purified nanosized polyaniline as template with (NH4)2S2O8/monomer molar ratio of 1 by using a JEOL FE-SEM JSM-6700F at UCLA. The nanosized polyaniline used here was synthesized in quiescent 2.0 M HNO3 at 15 °C for 6 h. The scale bars represent the length of 1 µm.

further polymerization; as a result, only the homogeneous nucleation is predominant in the bulk phase of polymerization medium. Thereafter the polymerization proceeds on such homogeneous nuclei in a slow and ordered way. In addition, with the help of the micelle effect and the electrostatic repulsion of the doping quaternary ammonium cation the PPy nanoparticles with a good re-dispersibility are successfully and facilely obtained. In particular, nearly monodisperse PPy particles have successfully been synthesized in still 1.0 M HCl aqueous solution at 25 mM Py and a few drops of aqueous dispersion of nanosized polyaniline as template with (NH4)2S2O8/aniline molar ratio of 1 regardless of their slightly large size of 400 nm with gold coating and 370 nm without gold coating in Figure 8. That is to say, the existence of a very small amount of nanosized polyaniline is beneficial to the formation and stabilization of uniform spheroidal sub-microparticles of PPy. It is revealed from FE-SEM images in Figures 9 and 10 that the spheroidal PPy nanoparticles synthesized in 0.35 M HNO3 and 0.50 M HCl have the diameters of ca. 150 and 230 nm, respectively, with gold coating and ca. 120 and 200 nm, respectively, without gold coating. TEM images in Figure S6 (Supporting Information) suggest that PPy particles synthesized both in HCl (a) and HNO3 (b) are basically spherical and have the diameters of around 167 and 100 nm, respectively. Upon carbonization at high temperature of up to 1000 and 1300 °C, the PPy nanoparticles would be converted into smaller carbon


Figure 9. FE-SEM images of PPy nanoparticles (a, b) synthesized in 0.5 M HCl and corresponding carbon nanoparticles (c, d) through carbonization in nitrogen at the temperature of up to 1300 °C. The scale bars represent the lengths of 2 µm (a), 200 nm (b), 2 µm (c), and 500 nm (d).

nanoparticles with diameters of ca. 66 (without gold coating) and 62 nm in Figures 10d-f and S6c as well as ca. 135 nm

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19251 (without gold coating) in Figure 7 because the nanoparticles would shrink after the elimination of the dopant and most of the hydrogen atoms in the PPy nanoparticles with the heat treatment at elevated temperatures. As shown in Figure S7 (Supporting Information), AFM observation reveals that the diameter of PPy nanoparticles prepared in 0.35 M HNO3 ranges from 40 to 90 nm with an average diameter of about 65 nm and that of the corresponding carbon nanoparticles ranges between 30 and 70 nm with an average diameter of about 50 nm. In particular, the PPy nanoparticles usually have smaller size if using HNO3 as polymerization medium than HCl, and the carbon nanoparticles also have smaller size than their PPy nanoparticle precursor. Summarily, LPA, SEM, TEM, and AFM methods have completely confirmed the successful preparation of fine PPy nanoparticles and their based carbon nanoparticles with good re-dispersibility and intrinsic functionalities by a very facile technique just keeping the acidic polymerization medium still. Bulk Electrical Conductivity of PPy Nanoparticles. As shown in Figures 1 and 2, the bulk electrical conductivity of the PPy particles increases first and then decreases as the concentration of HCl and HNO3 rises from 0 to 3.0 M. The maximum conductivities of 0.037 and 0.068 S/cm are attained in 0.5 M HCl and 0.35 M HNO3 as polymerization media, respectively. This conductivity variation coincides exactly with the intensity of the 900 nm band of UV-vis spectra (Figures 3d and 4) associated with the bipolaron concentration and conjugated length. Although the doping level would be higher

Figure 10. FE-SEM images of PPy nanoparticles synthesized in 0.35 M HNO3 (a-c), corresponding carbon nanoparticles through carbonization in nitrogen at the temperature of up to 1000 °C (d-f), and corresponding graphite nanoparticles through graphitization in argon at the temperature of up to 2300 °C (g-i). The scale bars represent the lengths of 5 µm (a), 1 µm (b), 100 nm (c), 5 µm (d), 1 µm (e), 100 nm (f), 5 µm (g), 1 µm (h), and 500 nm (i).

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SCHEME 2: Protonation and Possible Polymerization Routes of Pyrrole by Two Coupling Modes

in an acid medium of higher concentration, the lower conductivity of PPy can be explained by different coupling mode and corresponding spatial structure shown in Scheme 2. It is known that the R-position on the Py ring is a preferred protonation site which possesses higher proton affinity or reactivity than the β-position and even the N-position;41 i.e., R substitution is predominant during the Py polymerization.42 On the basis of the theory of resonance structure, there are abundant electrophile H+ among the reaction system and the electrophilic substitution will occur on both R- and β-positions. If attacked on β-position, there will be two resonance structures. However, if substituted reaction occurs on R position, there will be three resonance structures, indicating that more stable structure can be formed in such a case. Afterward, with the oxidative effect of (NH4)2S2O8 the predominant R-Py radical and minor β-Py radical can be formed. The R-R coupling makes a major contribution to coupling reaction, resulting in a linear alternating PPy with a high degree of large π-conjugation.11 On the other hand, as the acid concentration increases, the doping level gets higher accompanied with a greater amount of bipolaron. Meanwhile, a slightly cross-linked PPy chain would be formed because of the R-β coupling which provides access for charge to delocalize among PPy chains, improving the conductivity to some extent. Nevertheless, severe cross-link induced by excessive protonic acid will tremendously break the linearity and planarity of PPy chains and then reduce conjugated structure, consequently leading to a low conductivity. Noteworthily, the PPy synthesized in HNO3 has higher conductivity (0.051 S/cm) than that (0.037 S/cm) in HCl at the same acid concentration of 0.5 M. It can be inferred that the PPy doped by HNO3 has a little higher degree of electron delocalization than that by HCl, which is also confirmed by IR analysis (Figure 5b). In addition, exclusive R-R coupling formed favorably under lower acid concentration will lead to a rigid planar structure with a bigger spatial size. In comparison, the twisted spatial structure formed by some R-β coupling under high acid concentration tends to generate small and stable PPy spheres with low surface energy which can further support the positive effect of protonic acid to prepare PPy nanoparticles. As for the effect of oxidant on conductivity, it is seen from Table 1 that the conductivity is generally higher at a higher oxidant/Py molar ratio of 2 when FeCl3 is used as oxidant rather than (NH4)2S2O8 because the overoxidation of the PPy by (NH4)2S2O8 would happen in this case. Contrarily, the conductivity is much higher at a lower oxidant/Py molar ratio of around

0.5 if using (NH4)2S2O8 rather than FeCl3. Anyway, much lower yield and usually lower conductivity caused by using FeCl3 especially at the lower oxidant/monomer ratio significantly indicate that FeCl3 is not an effective or optimal oxidant for productive synthesis of highly conducting PPy particles. The amount of oxidant has a tremendous effect on the PPy conductivity. For example, as (NH4)2S2O8/monomer molar ratio increases from 0.3 to 2, the conductivity augments first and then decreases, as shown in Figure 2, displaying a maximum conductivity of 0.0364 S/cm and a minimum particle size at an optimal (NH4)2S2O8/monomer molar ratio of 0.5. This has been further proved by the maximal UV-vis absorbance of 500 and 900 nm bands at the optimal (NH4)2S2O8/monomer ratio of 0.5 in Figure 4c. The electrical conductivity enhancement of re-doped PPy nanoparticles was summarized in Figure 11. Dodecylbenzenesulfonic acid (DBSA), aniline-2,5-disulfonic acid (ADA), and I2 vapor are primarily attempted as re-dopant, unfortunately decreasing the conductivity to some extent. p-Toluenesulfonic acid (TSA), HCl, FeCl3, H2SO4, and HNO3 as re-dopants can slightly enhance the conductivity. Only 2.0 M HClO4 can significantly (41.2 times) improve the conductivity up to 2.8 S/cm. Moreover, 30 min carbonization at the temperature from

Figure 11. Electrical conductivity enhancement of PPy nanoparticles synthesized in 0.35 M HNO3 after re-doping or heat treatment. Only virgin doped PPy nanoparticles (0.037 S/cm) synthesized in 0.5 M HCl are subject to heat treatment at the temperature of up to 2300 °C in argon.



Figure 12. TG, DTG, and DTA plots at a heating rate of 20 °C/min of PPy nanoparticle powder prepared in 0.5 M HCl with a fixed (NH4)2S2O8/monomer molar ratio of 1 at 0 °C for 6 h.

25 through 2300 °C in argon can dramatically (10 000 times) elevate the conductivity up to 370 S/cm because of the formation of highly conducting graphite-like carbon network structure, as discussed below. Thermal Properties of PPy Nanoparticles as Precursor of Carbon Nanoparticles. TG, DTG, and DTA curves of PPy nanoparticle powder prepared in 0.5 M HCl and 0.35 M HNO3 are plotted as a function of temperature in Figures 12 and S8 (Supporting Information). The degradation process can be divided into three steps: the first stage with the weight loss of 4-5% from 55 to 130 °C due to the loss of water in the PPy, the second step from 130 to 350 °C due to an elimination of dopant or dedoping process, and the third step above 350 °C due to the exothermic thermal degradation (350-720 or 895 °C) and subsequent endothermic carbonization (750-1000 at 10 °C/min or 895-1300 at 20 °C/min) of the PPy backbones. The char yields at 1000 (or 1300) °C of the PPy powder prepared in 0.5 M HCl and 0.35 M HNO3 are 55.1 (or 40.0) and 50.7%, respectively, which are apparently higher than those of other PPy reported earlier.43,44 More importantly, the PPybased char formed thus is novel carbon nanoparticles with smaller size, narrower size distribution, much higher conductivity up to 21.0 S/cm (at 1000 °C in nitrogen), 219 S/cm (at 1300 °C in nitrogen regardless of the existence of a small amount of insulating carbon nitride crystals), and 370 S/cm (at 2300 °C in argon) (Table 2 and Figure 11), and stronger re-dispersibility in ethanol than initial PPy nanoparticles, which has been further verified by the UV-vis spectra in Figure 3d, IR spectra in Figure 5, and X-ray diffraction results in Figure 6. It can be speculated that the conductivity of the carbon nanoparticles would be further greatly enhanced if successively elevating the carboniza-

Figure 13. (a) Effect of PPy nanoparticle content on the electrical conductivity of as-prepared PPy/CDA nanocomposite film and scaffold with length × width × thickness of 30 × 10 × 1.5 mm, respectively. (b) Relationship between the conductivity of the PPy/CDA nanocomposite film and soaking time in a simulated body fluid (pH 7.25) containing 142 mM Na+, 5.0 mM K+, 1.5 mM Mg2+, 2.5 mM Ca2+, 147.8 mM Cl-, 4.2 mM HCO3-, 1.0 mM HPO42-, and 0.5 mM SO42-.

tion/graphitization temperature to 3600 °C because the graphite structure in the carbon nanoparticles will tend to be complete at this extremely high temperature. Nanocomposite of PPy Nanoparticles. One of the most attractive advantages of PPy as novel biomaterials is its unique ability to couple electrical stimulation and cell growth, which hopefully leads to a new strategy in tissue engineering. To prepare biodegradable conducting materials with enough strength, PPy nanocomposite film has been prepared using degradable CDA as matrix and less harmful acetone as solvent. As shown in Figure 13, two series of uniform PPy/CDA nanocomposite films with a thickness of about 20 µm exhibit the same percolation threshold of 0.2 wt %, regardless of much higher conductivity when the re-doped PPy nanoparticles with the highest conductivity of 2.8 S/cm were used as conductive dispersed phase. Moreover, a PPy/CDA porous scaffold with a thickness of about 1.5 mm has been successfully fabricated, which further confirms the practicability of the PPy composite in the biomaterial aspect. NaCl is used as a hole-making reagent, and acetone is used to solve CDA and disperse PPy in order to

TABLE 2: Variation of Size, Size Distribution, and Conductivity from PPy Nanoparticles to Corresponding Carbon Nanoparticles at the Carbonization Temperature of up to 1000-2300 °C PPy nanoparticles f carbon nanoparticles by LPA polymerization media

Dn (nm)

Dw/Dn

by FE-SEM average diameter (nm)

bulk electrical conductivity (S/cm)

0.5 M HCl 0.35 M HNO3

338f125 260f82.3

1.37f1.21 1.47f1.22

200f135 125f66

0.037f21.0-219 0.068f17.5-370

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guarantee the safety of such scaffold material when used in biotic experiment. The scaffold can be cut into small pieces with free-form shape and size, as expected, having slightly higher percolation threshold of 0.45 wt % PPy than the nanocomposite film. An eight inorganic ions containing acellular simulated body fluid (SBF) that is similar to that of human extracellular fluid45 was synthesized to study the electroconductivity stability of asprepared biodegradable composite materials in vitro. As shown in Figure 13b, the nanocomposite films containing 0.2 wt % PPy nanoparticles exhibit basically stable conductivity after an obvious decline of the conductivity during the initial week soaked in SBF solution mainly because of the water absorption, dedoping, and ions adhesion. Water absorption would swell and hydrolyze the CDA matrix and then affect the electrical path formed by dispersed PPy nanoparticles. Slight dedoping and deprotonation of PPy would be unavoidable in the SBF. Besides, the conductivity test was carried out only after drying the film. During this drying process some nonconducting ions would adhere onto the film and then lower the conductivity. Particularly, the conductivity of the nanocomposite film containing redoped PPy nanoparticles can remain almost constant for at least 8 weeks. Conclusions PPy nanoparticles have facilely and productively been synthesized by chemical oxidative polymerization of Py in necessarily quiescent aqueous acid in the absence of any external stabilizer. The polymerization yield, size, morphology, and conductivity of the PPy nanoparticles have been successfully optimized by controlling the species and concentration of the acid and oxidant. The optimal polymerization medium for the synthesis of the PPy nanoparticles is 0.35 M HNO3 with highest conductivity of 0.037 S/cm and minimum particle size. (NH4)2S2O8/ Py molar ratio of 0.5 is the optimal condition when 2.0 M HCl is used as polymerization medium. The presence of a small amount of nanosized polyaniline is beneficial to the synthesis of regular and nearly monodisperse sub-microparticles of PPy. Another advantage of the synthetic methodology of PPy nanoparticles is that this quiescent polymerization could be simply scaled up or down to synthesize a larger or smaller amount of PPy nanoparticles without compromising their yield, structure, and properties. A 2.0 M amount of HClO4 is proved to be the most effective dopant which can enhance the PPy conductivity more than 41 times compared with the virgin doped state. The PPy obtained in 0.5 M HCl has high nanocarbon yield of 55.1% at 1000 °C. Furthermore; novel carbon nanoparticles formed thus demonstrate the highest conductivity up to 370 S/cm and good re-dispersibility in ethanol. Intrinsically conductive nano-PPy/CDA nanocomposites with long-term conductivity stability, potential bioapplicability, and biodegradability but a very low PPy content have been simply fabricated. Acknowledgment. The project was supported by the National Natural Science Foundation of China (Grant 50773053). We would like to thank Professor Dr. Richard B. Kaner, University of California, Los Angeles, CA, for his wonderful help. Supporting Information Available: UV-vis spectra in ethanol of PPy nanoparticles at three testing temperatures, FESEM images of PPy particles synthesized in still pure water and ethanol at 0 °C for 6 h at 25 mM Py and a FeCl3/Py monomer molar ratio of 2 and in still 1.0 M HCl at 0 °C for 6 h at 25 mM Py and Kr2Cr2O7/(NH4)2S2O8 molar ratio of 1 as

Li et al. composite oxidant, influence of pyrrole concentration on the synthetic yield, bulk electrical conductivity, number-average diameter, and polydispersity index of PPy particles, TEM images of PPy nanoparticles synthesized in 1.5 M HCl, 0.35 M HNO3, and 0.35 M HNO3-PPy-based carbon nanoparticles, AFM images of PPy nanoparticles obtained in 0.35 M HNO3 and PPy nanoparticles based carbon nanoparticles, and TG, DTG, and DTA plots of PPA nanoparticle powder. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, X. G.; Huang, M. R.; Duan, W.; Yang, Y. L. Chem. ReV. 2002, 102, 2925–3030. (2) Wang, L. X.; Li, X. G.; Yang, Y. L. React. Funct. Polym. 2001, 47, 125–139. (3) Guimard, N. K.; Gomez, N.; Schmidt, C. E. Prog. Polym. Sci. 2007, 32, 876–921. (4) Ateh, D. D.; Navsaria, H. A.; Vadgama, P. J. R. Soc., Interface 2006, 3, 741–752. (5) Li, X. F.; Kolega, J. J. Vasc. Res. 2002, 39, 391–404. (6) Wong, J. Y.; Langer, R.; Ingber, D. E. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 3201–3204. (7) Nishizawa, M.; Nozaki, H.; Kaji, H.; Kitazume, T.; Kobayashi, N.; Ishibashi, T.; Abe, T. Biomaterials 2007, 28, 1480–1485. (8) Li, Y.; Neoh, K. G.; Cen, L.; Kang, E. T. Langmuir 2005, 21, 10702–10709. (9) Shi, G. X.; Zhang, Z.; Rouabhia, M. Biomaterials 2008, 29, 3792– 3798. (10) Shi, G. X.; Rouabhiab, M.; Wang, Z. X.; Dao, L. H.; Zhang, Z. Biomaterials 2004, 25, 2477–2488. (11) Jang, J. AdV. Polym. Sci. 2006, 199, 189–259. (12) An, K. H.; Jeong, S. Y.; Hwang, H. R.; Lee, Y. H. AdV. Mater. 2004, 16, 1005–1009. (13) Fan, L. Z.; Maier, J. Electrochem. Commun. 2006, 8, 937–940. (14) Pillai, R. G.; Zhao, J. H.; Freund, M. S.; Thomson, D. J. AdV. Mater. 2008, 20, 49–53. (15) Pedrosa, V. A.; Luo, X. L.; Burdick, J.; Wang, J. Small 2008, 4, 738–741. (16) Jang, J.; Yoon, H. Langmuir 2005, 21, 11484–11489. (17) Jang, J.; Yoon, H. Small 2005, 1, 1195–1199. (18) Tran, H. D.; Shin, K.; Hong, W. G.; D’Arcy, J. M.; Kojima, R. W.; Weiller, B. H.; Kaner, R. B. Macromol. Rapid Commun. 2007, 28, 2289– 2293. (19) Kim, J. W.; Cho, C. H.; Liu, F.; Choi, H. J.; Joo, J. Synth. Met. 2003, 135-136, 17–18. (20) Li, X. G.; Hou, Z. Z.; Huang, M. R.; Moloney, M. G. J. Phys. Chem. C 2009, 113, 21586–21595. (21) Li, X. G.; Wei, F.; Huang, M. R.; Xie, Y. B. J. Phys. Chem. B 2007, 111, 5829–5836. (22) Li, X. G.; Huang, M. R.; Xie, Y. B. Synthesis, Properties and Application of Conducting PPY Nanoparticles. In NanoScience in Biomedicine; Shi, D., Ed.; Springer Press: Berlin, 2009; pp 568-587. (23) Jerome, C.; Jerome, R. Angew. Chem., Int. Ed. 1998, 37, 2488– 2490. (24) West, R. X.; Zeng, Q. Langmuir 2008, 24, 11076–11081. (25) Zhou, M.; Pagels, M.; Geschke, B.; Heinze, J. J. Phys. Chem. B 2002, 106, 10065–10073. (26) Zhong, W. B.; Liu, S. M.; Chen, X. H.; Wang, Y. X.; Yang, W. T. Macromolecules 2006, 39, 3224–3230. (27) Cˇabala, R.; Sˇkarda, J.; Kamloth, K. P. Phys. Chem. Chem. Phys. 2000, 2, 3283–3291. (28) Bre´das, J. L.; Scott, J. C.; Yakushi, K.; Street, G. B. Phys. ReV. B 1984, 30, 1023–1025. (29) Zang, J. F.; Li, C. M.; Bao, S. J.; Cui, X. Q.; Bao, Q. L.; Sun, C. Q. Macromolecules 2008, 41, 7053–7057. (30) Kim, D. Y.; Lee, J. Y.; Moon, D. K.; Kim, C. Y. Synth. Met. 1995, 69, 471–474. (31) Kostic, R.; Rakovic, D.; Stepanyan, S. A.; Davidova, I. E.; Gribov, L. A. J. Chem. Phys. 1995, 102, 3104–3109. (32) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3892–3898. (33) Song, M. K.; Kim, Y. T.; Kim, B. S.; Kim, J.; Char, K.; Rhee, H. W. Synth. Met. 2004, 141, 315–319. (34) Wynne, K. J.; Street, G. B. Macromolecules 1985, 18, 2361–2368. (35) Ma, Y. W.; Jiang, S. J.; Jian, G. Q.; Tao, H. S.; Yu, L. S.; Wang, X. B.; Wang, X. Z.; Zhu, J. M.; Hu, Z.; Chen, Y. Energy EnViron. Sci. 2009, 2, 224–229. (36) Wang, E.; Chen, Y.; Guo, L. Sci. China, Ser. A: Math., Phys., Astron. 1997, 40, 967–970. (37) Li, X. G.; Li, A.; Huang, M. R. Chem.sEur. J. 2008, 14, 10309– 10317.

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Efficient and Scalable Synthesis of Pure Polypyrrole Nanoparticles

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