Bacterial Cellulose Nanofiber-Supported Polyaniline Nanocomposites

Article pubs.acs.org/JPCC

Bacterial Cellulose Nanofiber-Supported Polyaniline Nanocomposites with Flake-Shaped Morphology as Supercapacitor Electrodes Huanhuan Wang, Enwei Zhu, Jiazhi Yang, Peipei Zhou, Dongping Sun,* and Weihua Tang* Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

ABSTRACT: Bacterial cellulose (BC) nanofiber-supported polyaniline (PANI) nanocomposites have been synthesized via in situ polymerization of aniline onto BC nanofibers scalfold. Optimized preparation conditions were employed to achieve higher conductivity. The resultant BC/PANI nanocomposites were fully characterized in terms of structure, morphology, and thermal stability. The flake-like morphology of BC/PANI nanocomposites was observed using a field-emission gun scanning electron microscope. By manipulating the ordered flake-type nanostructure, BC/PANI nanocomposites achieved outstanding electrical conductivity as high as 5.1 S/cm. The as-prepared BC/PANI nanocomposites demonstrated a mass-specific capacitance of 273 F/g at 0.2 A.g−1 current density in supercapacitor application, the highest value reported so far for polymer-supported PANI composites.

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INTRODUCTION Conducting polymers−cellulose composites with nanostructure have received growing interest in recent years due to their better performance or new properties compared to their conventional counterpart, which have largely potential applications such as batteries, sensors, antistatic coating, corrosion protection, and electrical devices.1−4 Among conducting polymers, polyaniline (PANI) is one of the most promising materials because of its facile synthesis, good environmental stability, simple doping/dedoping chemistry, and controllable electrical conductivity.5−7 Cellulose is an attractive natural material due to its unique characteristics such as abundance in nature, renewability, biodegradability, biocompatibility, and good mechanical properties.8−10 The most common cellulose scaffold for preparation of polymers− cellulose conducting nanocomposites include cellulose pulp,11 cellulose derivatives,12,13 cotton cellulose,1,14 microcrystalline cellulose,15 and bacterial cellulose (BC) membrane.16−19 As a special kind of cellulose, BC was produced by fermentation of bacteria (Acetobacter xylinum, E. coli, etc.) in static or agitated culture.20 Although sharing similar molecular structure as natural cellulose, BC has attracted ever-increasing interest as a bioscaffold owing to its specific ultrafine network © 2012 American Chemical Society

structure and particular properties such as sufficient porosity, high purity and crystallinity, good mechanic properties, high water holding capability, excellent biodegradability, and biocompatibility.21,22 Recently, some research groups have explored the preparation of BC/PANI conducting nanocomposites by in situ polymerization of aniline (An) nanoparticles onto BC membrane to obtain relatively low electrical conductivity ranging from 1.61 × 10−4 to 1.3 S/cm.17−19 Such low conductivity was attributed to the poorly controlled morphology of composites during chemical preparation and followed a drying step with BC membrane as scaffold. In this paper, highly conductive BC/PANI nanocomposites with conductivity hitting 5.1 S/cm have been developed with BC nanofibers as the scaffold. The as-prepared nanocomposites were characterized by FTIR, XRD, XPS, BET, TGA, and FESEM techniques. The preparation conditions were optimized to obtain the highest electrical conductivity with well-controlled composite morphologies. With a surface area of 33.969 m2/g, the BC/PANI nanocomposites were further evaluated for their Received: February 2, 2012 Revised: May 20, 2012 Published: May 25, 2012 13013

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Figure 1. Effect of preparation conditions on the conductivity of BC/PANI nanocomposites: (a) mass ratio of BC to aniline, (b) molar ratio of APS to aniline, (c) molar ratio of HCl to aniline, (d) volume ratio of DMF to H2O, (e) temperature, and (f) time.

Advance, Bruker, Germany, Cu Kα radiation, 40 kV, diffraction angle 10−45°), XPS (PHI-5300, Perkin-Elmer), BET (NOVA 1000, Quanta Chrome, America), TGA (Mettler Toledo SDTA815e, 20 °C/min heating rate, 30 mL/min N2 flow rate), and field-emission gun scanning electron microscopy (FE-SEM) (Hitachi S4800, 15 kV). For XPS, a Mg Kα excitation source (1253.6 eV) with high resolution and wide scan (71.55 eV analyzer pass energy) was used, with core-level signals obtained at a photoelectron take off angle (α) of 45° with respect to the sample surface. All binding energies were calibrated against the C1s binding energy at 284.6 eV.18 Conductivity Meseaurement. The conductivity of the samples was measured with a conventional four-point probe technique (RTS-8, Probes Tech., China) at ambient temperature. The flat surfaces of samples were prepared by pressing the polymers at 15 KPa. The conductivity of pure PANI disk was determined to be ∼10° S/cm.16,17 According to the fourpoint probe method, resistivity can be calculated with ρ = 2πS(V/I), where S is the probe spacing (mm), which was kept constant, I is the supplied current in microamperes, and the corresponding voltage is measured in millivolts. Conductivity can be computed using σ = 1/ρX. Electriochemical Measurement. Cyclic voltammetry and chronopotentiometry were carried out on a CHI660D electrochemical workstation (Shanghai, China) with a threeelectrode system. The working electrode was prepared by mixing the active material with 15 wt % acetylene blank and 5 wt % polytetrafluoroeyhylene (based on the total electrode mass) to form a slurry. Then the slurry was cast on stainless steel mesh. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The electrolyte was 1 M H2SO4 solution, which is a strong electrolyte to ensure the high ionic strength. Cyclic voltammetry was performed in the voltage range from −0.2 to 0.8 V at 1, 5, and 10 mV/s scan rates. In addition, the galvanostatic charge−discharge experiment was carried out in

applicability for supercapacitor application, and a high special capacitance of 273 F/g was obtained at 0.2 A·g−1 current density. In addition, the growth mechanism of PANI on BC scaffold was proposed.

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EXPERIMENTAL SECTION Materials. All chemicals of analytical grade were procured from Sigma-Aldrich (Shanghai, China). Aniline was distilled prior to use. The pristine bacterial cellulose (BC) nanofibers were produced from agitated culture and purified as reported.23 Due to the high molecular weight, BC fibers did not dissolve or even swell in DMF or water, so that BC fiber-supported Pd nanoparticles were successfully employed as a robust catalyst for Heck coupling in DMF and aqueous Suzuki reaction even at 80 °C.21,24 Preparation of BC/PANI Nanocomposites. Conducting BC/PANI nanocomposites were prepared by the following method: Wet BC nanofibers (0.5 g, water content 98.4%) were fully dispersed in a mixed solvent of DMF and distilled water (1:2 v/v) at 25 °C. The resultant suspension was added with aniline monomer (11 mmol, 1 mL) with vigorous stirring to allow the monomer to self-assembe onto the surface of BC nanofibers via hydrogen bonding, after which the reaction system was cooled to the as-set temperature. Then the mixture of oxidant (ammonium peroxide sulfate, 11 mmol, 2.51 g) and dopant (hydrochloric acid, 13.2 mmol, 1 M HCl) was added into the suspension dropwise at the same temperature with mechanical stirring (ca. 350 rpm). The solution color changed from ivory to dark green after polymerization was initiated. An overnight reaction was allowed to ensure completion of polymerization. The resultant precipitate was filtered and sequentiallly washed with copious amounts of acetone, distilled water, and 1 M HCl until the filtrate was clear. The as-prepared nanocomposites were dried overnight with a freeze−thaw process to afford the title product as dark green solids. Characterization. BC and BC/PANI composites were characterized with FT-IR (Bomen MB154S), XRD (D8 13014

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the potential range from −0.2 to 0.8 V with an applied current density of 0.2, 0.4, 0.6, and 0.8 A/g.

the aliphatic C−H stretching vibration.22 Moreover, the characteristic bands of PANI in BC/PANI composites clearly observed at 1536 and 1448 cm−1 are assigned to the C−C stretching of the quinoid and benzenoid rings, respectively. In addition, the bands at 1294, 1164, and 849 cm−1 correspond to the C−N in-plane ring bending modes, the C−H in-plane bending, and the C−H out-of-plane bending modes, respectively.18,19 All these suggested that the BC scafold was fully coated with a PANI layer. This can be further confirmed from XRD and XPS studies. Formation of polycrystalline polymer can be revealed with Xray diffraction measurement, and highly ordered systems generally display metallic-like conductivity. Figure 3 shows

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RESULTS AND DISCUSSION Preparation of BC/PANI nanocomposites for the highest electrical conductivity was optimized by sequentially fine tuning the reaction parameters including the feeding mass ratio of BC/ An, molar ratio of oxidizer APS/An, molar ratio of dopant HCl/An, composition of mixture solvent of DMF/H2O, reaction temperature, and reaction time. A further correlation study of electrical conductivity with morphology was conducted to secure the optimized synthetic protocols for controllable morphology and highest conductivity. The electrical conductivity of BC/PANI composites is strongly dependent upon reaction conditions (Figure 1). It was obvious that optimized synthetic protocols for BC/PANI conductive nanocomposites could be easily obtained as follows: feeding mass ratio of BC/An as 1:10, molar ratio of APS/An as 1:1, molar ratio of HCl/An as 1.2:1, volume ratio of DMF/H2O as 1:2, reaction temperature as 0−10 °C, and reaction time longer than 4 h. After the freeze-thaw process, the BC/PANI nanocomposites prepared under optmum reaction conditions were weighted and the content of PANI was found to be 86 wt %. The conductivity of BC/PANI nanocomposite films could be obtained to be as high as 5.1 S/cm, which is over 100 times improved when compared with the reported highest values from the latest literature.17−19 With the BC/PANI nanocomposite obtained under optimized reaction conditions (same for all following BC/PANI composites) at hand, its structure was first confirmed by comparing its FTIR spectra with that of pristine BC (Figure 2).

Figure 3. X-ray diffraction patterns of pure BC and BC/PANI nanocomposite with 86 wt % PANI.

the XRD patterns of pristine BC and PANI−BC nanocomposite samples. In the XRD pattern of pure BC with cellulose I structure, three main peaks located at 14.6°, 16.8°, and 22.6° can be identified in both spectra for (1i0), ̅ (110), and (200) deffraction planes, respectively.22,26 Three broad diffraction peaks of BC/PANI nanocomposite can be seen at 14.8°, 20.8°, and 25.2°, indicative of amorphous PANI.18,27,28 It obviously noted that the crystalline structure of BC was fully hindered by the coating of amorphous PANI shell, since BC remained unchanged even in DMF.24 The nature of the interactions between polymer coating and the BC nanofiber substrate is further elucidated by XPS. Figure 4 shows the XPS and BET results of the surface chemical composition of BC/PANI conducting films. PANI can be present in one of three oxidation states: leucoemeradine, emeraldine, and pernigraniline corresponding to fully reduced, reduced/oxidized, and fully oxidized states, respectively.18 The N 1s core-level spectra of PANI in the protonated state were deconvoluted into three components. The binding energies of N 1s situated at 398.1, 399.2, and 400.7 eV correspond to the quinoid amine (−N), benzenoid amine (−NH−), and positively charged nitrogen (−N+−), respectively.18,29−31 The doping level was determined from the Cl/N ratio of Cl 2p and N 1s core-level XPS spectra. The results revealed that more chlorine anions were incorporated into the polyaniline structure from DMF/H2O solution than from pure H2O, which agrees with the elemental analysis. XPS analyses further reinforced the suggestion that hydrogen bonding occurred between the nitrogen lone pairs (N̈ ) of the polymer coating with the −OH groups of the underlying cellulose substrate.18,32

Figure 2. FTIR spectra of pure BC and BC/PANI nanocomposite with 86 wt % PANI.

In the case of pure BC,22 a broad band at 3403 cm−1 is attributed to O−H stretching vibration. The band at 2897 cm−1 represents the aliphatic C−H stretching vibration. A sharp and steep band observed at 1063 cm−1 is due to the presence of C− O−C stretching vibrations. It is worth noting the small peak at 1647 cm−1, resulting from carbonyl functional groups as a result of natural aging of cellulose.25 In the case of BC/PANI nanocomposites, a band at 3432 cm−1 attributed to the N−H stretching vibration became intensified, indicating that the BC scalfold was sufficiently coated with PANI. Futher evidence of PANI coating onto BC fibers can be found for the dispearance of the sharp band at 1063 cm−1 (C−O−C stretching vibrations) and the band at 2897 cm−1, which is assigned to 13015

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Figure 4. X-ray photoelectron spectra of pure BC and BC/PANI nanocomopposite with 86 wt % PANI.

occurred from 320 °C because of polyaniline backbone degradation. Both pure PANI and BC/PANI nanocomposites produced significant amount of residues. The BC/PANI composites displayed higher amount of residue (59.0%) than pure BC (only 8.9% residue), which can be explained by the weight ratio of BC to PANI. All results demonstrated that thick PANI coating acted as a protective barrier on the surface of BC against thermal degradation. Similar results were obtained for polyaniline-coated bacterial cellulose composite films in the latest literature,17−19 which was also attributed to the interactions between BC and PANI. The morphology evolution with reaction media is shown in Figure 6. Pristine BC nanofibres boast an average diameter of about 30 nm and a length ranging from micrometers up to dozens of micrometers. With coating of PANI, the fiber-like morphology changed into a flake-structured morphology with high densification and aggregation of BC/PANI flakes (Figure 6c−f), mainly due to strong hydrogen bonding between sequentially formed PANI layer. The weakening of such intramolecular interactions via DMF resulted in well-separated and homogeneous BC/PANI fibers (Figure 6d−f),20 with the best morphology obtained at DMF/H2O (1:2, v/v). We infer that DMF played an important role in diffusion of aniline monomer in miscible two-phase systems, resulting in abundant aniline on the surface of BC nanofibers and in the 3-D networks for polymerization. When increasing the concentration of DMF, a better separated flake morphology of composites was observed (Figure 6c−f), resulting from better dispersion of An monomer into 3-D networks of BC fibers for in situ polymerization with the aid of DMF. Besides reaction media, reaction temperature was also crucial in controlling the well-ordered morphology of BC/PANI nanocomposites. As displayed in Figure 7, well-separated BC/ PANI flakes were clearly observed with for specimens from 0 °C (Figure 7a). At higher temperature, aggregation of flakes occurred to form larger dimension BC/PANI flakes (Figure 7c and 7d), which resulted from the accelerated polymerization with increased temperature. More importantly, the single fibers found in the FE-SEM images (pointed by the arrow in Figures 6f and 7d) indicated PANI layers were coated onto BC

Surface area measurement indicated that BC/PANI composites achieved an almost 11-fold improvement in the Brunauer−Emmett−Teller (BET, nitrogen, 77 K) surface area than pristine BC fibers. In particular, BC/PANI composites prepared from the optimized reaction conditions had a surface area of 33.969 m2·g−1, which is 37% higher than that of composites prepared from pure water. This resuslt agrees well with the morphology in Figure 6. TGA provides better understanding of the thermal stability and amount of PANI coating on BC nanofibers. As shown in Figure 5, the TGA curve of pure PANI showed two main steps

Figure 5. TG curves of pure PANI (doped by HCl), BC, and BC/ PANI nanocomposite with 86 wt % PANI (doped by HCl).

of weight loss, i.e., the first at 150 °C due to removal of dopant HCl and the second at approximately 400 °C resulting from structural decomposition of PANI polymers.9 The TG curve of pure BC nanofibers showed a three-stage weight loss profile:17 initial weight loss at 50−100 °C attributed to evaporation of the moisture inside BC, weight loss at 250−420 °C due to removal of molecular fragments such as O−H and CH2−OH groups, and the last weight loss at 420−800 °C explained by decomposition of cellulose backbone. In contrast to pure BC, the onset decomposition temperature of BC/PANI nanocomposites was at 173 °C due to removal of dopant ion and the side chain of PANI. In addition. the third stage in weight loss 13016

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Figure 6. BC nanofibers SEM image (a) and TEM image (b); FE-SEM images of BC/PANI nanocomposites with 86 wt % PANI prepared from (c) pure H2O, (d) DMF/H2O (1:2, v/v), (e) DMF/H2O (2:2, v/v), and (f) DMF/H2O (3:2, v/v).

important redox feature of conducting polymers. Measurements were calibrated using ferricyanide as the standard.33 The characteristic redox behavior of PANI34,35 is clearly observed with oxidation peaks at 0.52 and 0.7 V and a reduction peak at 0.18 V vs SCE. The results indicate that the BC/PANI nanocomposite has obvious capacitance performance, with a couple of Faradaic peaks relating to the protonation/ deprotonation processes. It shoud be mentioned that the CV curves are not rectangular in shape, maybe due to the fact that BC/PANI composites were not porous enough for high performance. Further study may be needed to improve their morphology. Galvanostatic charge/discharge for BC/PANI nanocomposite was performed with an applied current density of 0.2, 0.4, 0.6, and 0.8 A/g in a voltage range from −0.2 to 0.8 V (Figure 9b). The resulting curve was close to an ideal straight line, and the sweep was caused by pseudocapacitiance behavior.36 The specific capacitance (Cs) vaules may be calculated from the charging and discharging curves according to Cs = (I × Δt)/ (ΔV × m), where Δt is the discharge time, ΔV ≈ 1 V, and m is the mass of active material (∼5 mg).37 Specific capacitance values as high as 190−273 F/g were gained for BC/PANI nanocomposites at a current density ranging from 0.2 to 0.8 A/ g, which were comparable to that of composites composed of 20 wt % of multiwalled carbon nanotubes and 80 wt % of PANI (305 F/g)36 and that of graphene−PANI nanocomposites (∼300 F/g).38 The high Cs may be attributed to the good dispersion of PANI coating around BC nanofibers under optimum reaction conditions, which could help to provide a largely electrolyte-accessible surface area to improve utilization of PANI particles for redox reactions. Furthermore, the electrochemical stability of BC/PANI nanocomposites was also investigated (Figure 9c). It is found that BC/PANI electrode retained about 94.3% (234.7 F/g) of initial capacitance after 1000 cycles. This good stability may be ascribed to the well-ordered BC/PANI composites and strong interaction between the BC core and the PANI shell.

Figure 7. FE-SEM images of BC/PANI nanocomposites prepared at different reaction temperatures: (a) 0, (b) 10, (c) 15, and (d) 25 °C.

nanofibers via in situ polymerization to form a core−shell structure. According to the discussions above, the growth mechanism of BC/PANI nanocomposites may be speculated as in Figure 8. The coating of PANI shell on BC nanofibers occurs by two steps: (a) aniline dispersion and self-assembly onto BC nanofibers with the aid of DMF and (b) in situ polymerization of aniline with APS along with doping by HCl. Potential applications of as-prepared BC/PANI nanocomposites were explored by fabricating the samples into supercapacitor electrodes and characterized with cyclic voltammograms (CVs) and galvanostatic charge/discharge measurements. The CV response of BC/PANI carried out at varied scan rates in the potential range from −0.2 to 0.8 V using 1 M H2SO4 solution is shown in Figure 9a. The nanocomposites showed a high degree of electroactivity, with transitions from reduced to oxidized forms demonstrating the 13017

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Figure 8. Schematic illustration of the process of preparation of BC/PANI in DMF/H2O mixed solution.

Figure 9. Cyclic voltammogram and galvanostatic charge/discharge for the electrochemical character of BC/PANI nanocomposites with potential (vs SCE) from −0.2 to 0.8 V: (a) 1, 5, and 10 mV/s; (b) specific capacitance at different current density, and (c) cycle life of BC/PANI at 0.4 A/g in 1 M H2SO4 solution.

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CONCLUSIONS

Highly conductive BC/PANI nanocomposites with flakestructured morphology were prepared by in situ polymerization of self-assembed aniline onto BC nanofibers. The composites morphology evolved strongly with the reaction parameters including the feeding ratio of BC/An, concentration of oxidant and dopant, reaction duration, reaction temperature, and reaction media. With the optimized reaction protocols, the structurally defined BC/PANI composites exhibited outstanding electrical conductivity (ca. 5.1 S/cm) and a high surface area (∼33.969 m2/g). The as-prepared BC/PANI nanocomposites had a specific capacitance hitting 273 F/g at 0.2 A·g−1 current density in the supercapacitor, the best reported so far for cellulose-supported PANI composites. The electric conductivity, thermal stability, and well-controlled microstructure of BC/PANI composites pave the way toward promising applications in various electronic devices.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their thanks to support from the National Natural Science Foundation of China (Grant No. 21074055), the Doctoral Fund of the Ministry of Education of China (No. 20103219120008), the Foundation of Key Laboratory of Luminescence and Optical Information (Grant No. 2010LOI04), and NUST Research Funding (No. 2010ZDJH04).

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