Effects of Solvent and Doping Acid on the Morphology of Polyaniline

May 4, 2010 - Various morphologies of polyaniline (PANI), such as microflakes stacked by 1D nanofiberes, porous microwebs, hemispheres, and nests pile...
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J. Phys. Chem. C 2010, 114, 9264–9269

Effects of Solvent and Doping Acid on the Morphology of Polyaniline Prepared with the Ice-Templating Method Hui-yan Ma,† Yun-wu Li,† Sheng-xue Yang,† Fei Cao,† Jian Gong,*,†,‡ and Yu-lin Deng‡ Key Laboratory of Polyoxometalates Science of Ministry of Education, Northeast Normal UniVersity, Changchun, Jilin, 130024, China, and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0620 ReceiVed: February 19, 2010; ReVised Manuscript ReceiVed: April 16, 2010

Various morphologies of polyaniline (PANI), such as microflakes stacked by 1D nanofiberes, porous microwebs, hemispheres, and nests piled by nanoparticles, were prepared through an ice-templating method, using different doping acids assisted by secondary solvent. The structure and morphology of these PANI were characterized by Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) patterns, and scanning electron microscope images (SEM). Although the ice-templating method has been used for preparing conducting polymer materials with unique structures in recent years, to the best our knowledge, this is the first report on synthesizing PANI doped with polyoxometalates (POMs) using the ice-templating method assisted by secondary solvent to direct the polymerization of aniline. In this paper, the effects of doping acid and the secondary solvent on the morphology of PANI were investigated in detail. All the results showed that just the doping acids, POMs with unique nucleophilic oxygen-enriched surfaces, multi-hydrogen proton, and the binding capacity with the secondary solvent play a major role in determining the formation of various morphologies of PANI. A possible mechanism for the formation of the different morphologies of PANI was proposed. The gas-responses to ammonia were examined at room temperature. Compared with PANI porous microwebs, hemispheres, or nests piled by nanoparticles, the PANI microflakes stacked by nanofibers showed the best performance in both sensitivity and time response due to their small nanofiber diameter, high surface area, and porous nature, which will have potential application in the area of chemical sensors. 1. Introduction The design and manipulation of the microstructure of conducting polymer is one of the most challenging tasks in materials science, which have immense impact on chemical and biological research.1-3 Although it is still difficult to prepare fully predictable polymer micro-/nanostructures based on the rational design, recent rapid developments in micelle and methodology chemistry provide more and more knowledge and possibilities for the creative synthesis of desired conducting polymer micro-/nanostructures.4-10 Especially in recent years, PANI micro-/nanostructures have been well-developed and systematically discussed in comprehensive reviews.11-13 It is found that the morphology of PANI micro-/nanostructures is strongly affected by the nature, species, and concentration of the dopant, solvent, surfactant, oxidant, and monomer, as well as the various kinds of template and synthesis methods.14-22 By changing these parameters, various PANI micro-/nanostructures from one-dimensional (1D) to three-dimensional (3D), such as single crystalline nanoneedle,18 aligned arrays,23 nanotubes,5,7,24,25 nanofibers,8,19,22 nanosheets,26a leaves,26b,27 rose-like plates,28 and hollow spheres,29-31 have been successfully synthesized. Among these parameters, the use of template has been one of the most attracted attentions due to the predictability of product morphologies.4-7,24,25 And undoubtedly, the exploitation of more various green templates has recently witnessed explosive growth in the areas of PANI synthetic methodology.32-34 The cry* To whom correspondence should be addressed. E-mail: gongj823@ nenu.edu.cn or [email protected]. † Northeast Normal University. ‡ Georgia Institute of Technology.

ochemical synthesis method, as unique methods, has received extensive interest.35,36 The use of ice crystals grown in situ has been proved to be an efficient way to dramatically influence the structural morphology of products.37-41 More importantly, another distinct merit is that the ice-templating method is a highly biocompatible, economical, and environmentally friendly method for the generation of highly pure materials with unique structures compared with the traditional synthetic method.37,38 On the basis of the aforementioned considerations, in the present paper, we chose the ice template as an in situ grown template to direct the polymerization of aniline monomer via use of POMs as doping acids meanwhile assisted by secondary solvent, i.e., diethyl ether (DE). We consider this method as an ice-templating method assisted by secondary solvent. Our aim is to synthesize new PANI micro-/nanostructures using the icetemplating method and investigate the effect of different acids and the assisted secondary solvent on the PANI ultimate morphology. Herein, in order to compare, four various morphologies of PANI synthesized by the original ice-remplating method32 or the ice-templating method assisted by secondary solvent are first taken into account together, i.e., (1) POMsdoped PANI hemispherical containers, (2) POMs-doped PANI microflakes stacked by one-dimensional nanofibers, (3) HCldoped PANI porous microwebs, and (4) HCl-doped PANI nests piled with nanoparticles. Results indicate that the four different morphologies of PANI are highly dependent on the nature of the acid used and the secondary solvent. The possible formation process and preliminary growth mechanism for the different morphologies are investigated and proposed. Furthermore, the gas sensitivity of the PANI with different morphologies also has been studied.

10.1021/jp101525q  2010 American Chemical Society Published on Web 05/04/2010

Morphology of Polyaniline 2. Experimental Section 2.1. Materials. Iron(III) chloride hexahydrate (ferric chloride; FeCl3 · 6H2O), hydrochloric acid (HCl), and other reagents were used as received. In our experiment, aniline (Beijing Chemical Co.) was distilled twice under reduced pressure before use. FeCl3 was used as a milder polymerization initiator. A much lower reaction rate than that by (NH4)2S2O8 (APS) used in most polymerizations could guarantee that the polymerization of aniline completely takes place in the frozen ice phase after water freezes.18-20 H4SiW12O40 (SiW12) was prepared and characterized according to the literature.42 2.2. Measurements. The morphologies of the resulting PANI products were observed with an XL-30 ESEM FEG scanning electron microscope operated at 20 kV. Fourier transform infrared (FT-IR) spectra were obtained by using an Alpha Centauri 560 Fourier transform infrared spectrophotometer (frequency range 4000-400 cm-1) with a KBr pellet. X-ray diffraction (XRD) patterns were performed on a D/Max IIIC X-ray diffractometer by using a Cu KR radiation source. Scans were made from 3° to 60° (2θ) at a speed of 2 deg min-1. 2.3. Polymerization. (1) Preparation of POMs-doped PANI hemispherical containers: In a typical experiment, 0.10 g of aniline, 0.50 g of SiW12, 10 mL of diethyl ether, and 35 mL of distilled water were mixed. Then 0.50 g of FeCl3 · 6H2O was dissolved in the above mixture, and the resulting solution was stirred another 1 min at room temperature to ensure complete mixing. After that, the mixture was allowed to stand in a freezer at -18 °C for 20 days. After the frozen ice thawed, the products remained. The green precipitate was filtered and washed with distilled water and ethanol several times until the filtrate became colorless and then dried in a vacuum at 50 °C for 24 h. (2) Preparation of POMs-doped PANI microflakes stacked by one-dimensional nanofibers: Please see our previous report.32 (3) Preparation of HCl-doped PANI porous microwebs. In a typical synthesis, 0.10 g of aniline, 0.10 g of HCl, and 40 mL of distilled water were mixed. Then 0.35 g of FeCl3 · 6H2O as a mild oxidant was dissolved into the above mixture. The resulting solution was stirred another 1 min at room temperature to ensure complete mixing. After that, the mixture was allowed to stand in a freezer at -10 °C for 20 days. Finally, after the frozen ice thawed, the products left. The green precipitate was filtered and washed with distilled water and ethanol several times until the filtrate became colorless and then dried in a vacuum at 50 °C for 24 h. (4) Preparation of HCl-doped PANI nests piled with nanoparticles. The whole operation procedure was similar to the synthesis of the POMs-doped PANI hemispherical container, but with HCl instead of SiW12. 3. Results and Discussion 3.1. Morphology and Characterization of the PANI. Figure 1, from left to right, shows SEM images of the four various morphologies of PANI, hemispheres, microflakes, microwebs, and nests. As shown in Figure 1, PANI microflakes could be obtained by an original ice-templating method.32 However, when HCl was chosen and SiW12 was replaced as a doping acid, the morphology of PANI underwent great change, and porous microwebs were obtained. Interestingly, when we added DE into the aniline/oxidant/SiW12 water solution, pure PANI hemispheres were obtained. Once the doping acid was replaced by HCl, PANI hemispheres disappeared and PANI nests piled with nanoparticles appeared. Here, in order to investigate and understand the formation process of the four various PANI morphologies, discussion is divided into two parts

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Figure 1. SEM images of PANI hemisphere, microflakes, microweb, and nest (from left to right, scale bar: 100, 5, 2, and 50 µm) obtained with the ice-templating method. (DE: diethyl ether; and SiW12: H4SiW12O40).

Figure 2. SEM images of PANI microflakes doped with SiW12 at low magnification (A) and at higher magnification (B) and PANI porous microweb doped with HCl at low magnification (C) and at higher magnification (D) prepared by an original ice-templating method.

as follows. One is the original ice-templating method and another is the ice-templating method assisted by DE. With the original ice-templating method, PANI microflakes can be synthesized in the presence of SiW12 as doping acid (Figure 2A). The SEM image in larger magnification scale reveals that these microflakes consist of one dimension nanofibers with a uniform diameter around 25 nm (Figure 2B).32 However, when HCl replaces SiW12 as doping acid, the morphology of the resulting PANI is interesting and looks like microwebs as shown in panels C and D of Figure 2, which exhibits many multiple pores (Figure 2C). The enlarged image reveals that the microwebs are composed of nanoparticles rather than nanofibers (Figure 2D). Obviously, the major difference in morphology is caused by the different nature of HCl and SiW12. It is noteworthy that the excellent properties of POMs may play a key role in determining the formation of microflakes morphology. As a unique class of metal oxide complexes with nucleophilic oxygen-enriched surfaces and multi-hydrogen proton, POMs has been proved to be indicative for the synthesis of many nanostructured materials.43-46 Furthermore, they are completely hydrophilic and very soluble in polar solvents because of their negative charges and water ligands that are chemically bonded to the external surface of POMs.47

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Figure 4. SEM images of PANI hemispheres (A) doped with SiW12 and nest (B) doped with HCl prepared with an ice-templating method assisted by DE. (The inset shows the high magnification images.) Figure 3. The interaction patterns between H4SiW12O40 and H2O.

The structure of SiW12 has been discussed in detail in many previous reports.42,48 Utilizing hydrogen bonds between hydroxy groups and protons or terminal oxygen atoms of POMs to synthesize new POMs structure is very popular with chemists and always receives some finer results.49-52 In our studies, compared with the PANI doped with HCl, the morphology of PANI doped with SiW12 shows microflake. Herein, we consider that it may be protons or nucleophilic oxygen-enriched surfaces of the POMs interacting with water molecules of the ice crystal through forming the hydrogen bonds between the hydroxy groups of water and the proton or terminal oxygen atoms of the POMs, which is similar to previous reports.52 As shown in Figure 3, the hydroxy groups of water molecules and protons or terminal oxygen atoms of SiW12 unit form a O(SiW12) · · · H-O (H2O) (Figure 3, left) or O (H2O) · · · H-Oc(SiW12) (Figure 3, right) hydrogen bond. Meanwhile, the multi-hydrogen proton of SiW12 can offer aniline additional protons to form anilinuim dodecatungstosilicate through a nonspecific acid-base-type reaction.8c,31 Furthermore, aniline groups are always chosen as the organic groups for their hydrogen bonding ability and possible use as ligands, and arylimido groups could bond to cations, or bond to cluster oxo ligands.53 These studies show one fact that aniline monomer can be captured by POMs easily and also provide us some sufficient evidence that POMs can be an interfacial adhesion between aniline and water molecules, which directs aniline molecules growing along the ice template and forms microflake morphology.54 However, HCl cannot provide nucleophilic oxygen-enriched surfaces or more protons to water molecules (ice) when it forms aniline hydrochloride with aniline. As a result, the aniline hydrochloride that is correspondingly free among ice crystals will polymerize freely and produce a porous microweb structure. The difference caused by doping acid can also be observed in the following discussion on the morphology of PANI synthesized by the ice-templating method assisted by DE. DE was used as a secondary reagent to adjust the polymerization system due to its weak polarity and “appropriate” solubility in the water phase. It can be released from water when water is frozen into ice at -18 °C.55 As a result, it forms spherical droplets in the ice phase, which will change the morphology of PANI. Figure 4 shows us the different SEM images of PANI obtained by the ice-templating method assisted by DE. Pure PANI hemispheres with SiW12 as doping acid in the presence of DE can be observed (Figure 4A). When HCl is adopt as doping acid instead of SiW12, we only obtain some spherical nests encased among agglomerate nanoparticles (Figure 4B). These nests piled by nanoparticles have smooth but incompact inner surface (Figure 4B, inset). What is the reason for this difference? As is commonly known, aniline is

Figure 5. Possible mechanism for the formation of the different morphologies of PANI.

hydrophobic. However, aniline molecules in SiW12 solution will be captured by SiW12 to form anilinuim dodecatungstosilicate. More importantly, when the water is frozen to solid ice gradually, most of the POMs are gradually extruded from ice crystals into the DE spherical droplets because POMs are easily form etherate with DE in a strong acid system.42,55 The etherate with high density will sink and mainly accumulate in the bottom of the spherical container, which is very common in the synthesis process of POMs.42 Because aniline molecules captured by POMs are taken into spherical droplets and mainly also accumulated in the bottom of droplets with POM etherate, hence, the polymerization takes place in the bottom of the spherical droplets and produces hemispherical morphology with a thick bottom shell like a bowl. On the contrary, although HCl can also form aniline hydrochloride with aniline when HCl replaces SiW12, the aniline hydrochloride cannot enter the diethyl ether spherical droplets like anilinuim dodecatungstosilicate because HCl will not react with DE once it forms aniline hydrochloride with an aniline molecule. The supplementary experiment has been done as follows: We added HCl liquid to aniline monomer to form aniline hydrochloride. Then, we poured DE onto the aniline hydrochloride solid. There was an obvious layer between DE and aniline hydrochloride, and the interface was retained without change for a long time under magnetic stirring. The experiment shows that the aniline hydrochloride is hydrophilic. Thus, in our polymerization system, although DE solvent exists, aniline cannot transfer into DE spherical droplets once aniline forms aniline hydrochloride, and only

Morphology of Polyaniline

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Figure 6. FT-IR spectra (left) of PANI microweb (A), nest (B), hemisphere (C), and microflakes (D) and XRD patterns (right) of PANI microflakes (A), hemisphere (B), nest (C), and microweb (D).

polymerizes on the outside of spherical droplets. As a result, the polymerization reaction takes place in the water phase rather than in the spherical droplets, and only retains a track of the spherical nest. Figure 5 shows the mechanism of forming different morphologies of PANI. Molecular structures of the PANI synthesized with different synthetic conditions are characterized by FT-IR spectra and XRD patterns as shown in Figures 6. The FT-IR peaks of all products are approximately identical (Figure 6, left). That is to say, although the shapes of products are different, the structure of PANI belongs to the same type. As one can see, both of the spectra of the HCl-doped PANI microwebs, nests (curves A and B) and SiW12-doped PANI hemispheres, and microflakes (curves C and D) show four (at ca. 1140, 1298, 1486, and 1575 cm-1) characteristic absorption peaks of PANI. The first two correspond to the aromatic C-H in-plane bending and the C-N stretching of secondary aromatic amine, while the last two are associated with benzenoid rings, and the CdC stretching deformation of quinoid. These characteristic peaks are identical to those prepared in a common method.56 SiW12 with a Keggin structure consists of one {SiO4} tetrahedron surrounded by four {W3O13} sets formed by three edge-sharing octahedra. There are four kinds of oxygen atoms in the SiW12, which are Oa (oxygen in {SiO4} tetrahedron), Ob (corner-sharing oxygen between different {W3O13} sets), Oc (edge-sharing oxygen bridge within {W3O13} sets), and Od (terminal oxygen atom). Compared with curves A and B, curves C and D show an additional four characteristic peaks of SiW12 around 778 (ascribes to W-Oc-W), 879 (W-Ob-W), 914 (Si-Oa), and 964 cm-1 (WdOd), which demonstrate that SiW12 has been doped into the PANI structure in PANI microflakes and hemispheres.57 The X-ray diffraction patterns of these PANI products with different morphologies are shown in Figure 6 (right). One broad band centered at 2θ ) 26.0° is observed, which indicates that these PANIs are still amorphous, although they show different micromorphologies as shown in SEM images. However, there is a strong and narrow peak centered at 2θ ) 7.0° in the XRD patterns of PANI hemispheres and microflakes (curves A and B) synthesized by using SiW12 as doping acid while this peak is absent in the PANI nests and microwebs (curves C and D) synthesized by using HCl as doping acid. The peak centered at 2θ ) 7.0° is related to the PANI repetition unit.58a This result suggests that doping with SiW12 leads to a more ordered structure than that with HCl, which is consistent with previous reports.58b

Figure 7. Resistance changes of PANI products to 100 ppm of NH3: (A) microflakes, (B) hemisphere, (C) microweb, and (D) nest.

3.2. Gas Sensitivity of the Sensor. The structure of the sensor used in our experiments has been reported in previous literature.33 PANI films were fabricated by using a drop-coating technique. The films were deposited onto a porcelain tube that consisted of a 1 × 1 mm2 Pt electrode. The substrates were equipped with integrated pastes that were obtained by adding an organic vehicle for improving the adhesion of the layers to the substrates of the above powders. Then the films were dried for 1 h under vacuum at 40 °C. The PANI device was placed in an air-proof test box (about 27 L). Next 6 V dc voltages were applied. The box and the device were flushed with highpurity N2 continuously until the electrical resistance reaches a steady value. Then, a certain amount of volatile solvent was injected into the test box with a syringe. The changes in electrical resistance were monitored and recorded automatically with a computer. The gas sensitivity of the PANI was defined as the ratio of R/R0, in which R0 is the initial resistance of the PANI before exposure to the test gas and R is the time-dependent resistance of the PANI exposed to the test gas. In Figure 7, we show the response of different morphologies of PANI products upon exposure to 100 ppm concentrations of NH3, respectively. The microflakes have the highest increase in resistance when exposed to 100 ppm NH3 (Figure 6A, the resistance average value R/R0 if the gas is NH3, ∼27). Compared with other PANI products (the resistance average value R/R0 if the gas is NH3: hemispheres, ∼7.5 (Figure 6B); microwebs, ∼4.7 (Figure 6C); nests, ∼3.1 (Figure 6D)), the PANI microflakes stacked by nanofibers give a significant best gas sensitivity to NH3 gas.

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This is consistent with its porous nature, high surface area, and small nanofiber diameter (the diameters of the fibers in the microflakes are in the range 22-32 nm with an average diameter of 26.2 nm),32 as well as the thin and strongly adherent structure of the nanofiber microflakes, which make gas molecules easily diffuse into and out of the nanofibers. Therefore, the PANI microflakes stacked by nanofibers not only bear diaphanous morphology but also show better performance in both sensitivity and time response of NH3 in comparison with other PANI products.33,34,54 4. Conclusions In summary, four kinds of PANI morphologies, hemispheres, microflakes, microwebs, and nests are prepared by the original ice-templating method and the ice-templating method assisted by DE, respectively. The studies on the effect of the doping acid, SiW12, and the secondary solvent, diethyl ether, on the PANI ultimate morphology indicate that SiW12 with unique nucleophilic oxygen-enriched surfaces, multi-hydrogen proton, and binding capacity with diethyl ether plays a major role in determining the formation of various morphologies of PANI. A possible formation mechanism of the PANI with different morphologies has been discussed. Although the detailed formation process is not very clear and still needs more investigation, there is no doubt that a simple and easily controlled route is successfully provided to prepare different PANI morphologies. Our results further prove that the ice-templating method is a highly biocompatible, economical, and environmentally friendly method for the generation of highly pure materials with unique morphologies. In future work we will be exploring these characteristics of doping acids and additive reagents and taking full advantage of them to design and synthesize new morphology of materials, which is expected to have more potential applications in the area of sensor development. Acknowledgment. This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University and the Science Foundation of Jilin Province (20070505). References and Notes (1) Ottenbrite, R. M.; Fadeeva, N. ACS Symp. Ser. 1994, 545, 1. (2) (a) Rubinson, J. F. ACS Symp. Ser. 2002, 832, 2. (b) Aussawasathien, D.; He, P.; Dai, L. ACS Symp. Ser. 2006, 918, 46. (3) Tan, S. X.; Zhai, J.; Wan, M. X.; Meng, Q. B.; Li, Y. L.; Jiang, L.; Zhu, D. B. J. Phys. Chem. B 2004, 108, 18693. (4) (a) Feng, X.; Mao, C.; Yang, G.; Hou, W.; Zhu, J. Langmuir 2006, 22, 4384. (b) Tian, S.; Liu, J.; Zhu, T.; Knoll, W. Chem. Mater. 2004, 16, 4103. (5) Pan, L.; Pu, L.; Shi, Y.; Song, S.; Xu, Z.; Zhang, R.; Zheng, Y. AdV. Mater. 2007, 19, 461. (6) (a) Zhang, Z.; Sui, J.; Zhang, L.; Wan, M.; Wei, Y.; Yu, L. AdV. Mater. 2005, 17, 2854. (b) Zhang, Z.; Deng, J.; Sui, J.; Yu, L.; Wan, M.; Wei, Y. Macromol. Chem. Phys. 2006, 207, 763. (7) (a) Anilkumar, P.; Jayakannan, M. Macromolecules 2007, 40, 7311. (b) Anilkumar, P.; Jayakannan, M. J. Phys. Chem. C 2007, 111, 3591. (8) (a) Huang, J. X.; Kaner, R. B. Angew. Chem., Int. Ed. 2004, 43, 5817. (b) Li, D.; Kaner, R. B. J. Am. Chem. Soc. 2006, 128, 968. (c) Huang, J. X.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851. (9) (a) Dong, H.; Jones, W. E., Jr. Langmuir 2006, 22, 11384. (b) Zhang, X. Y.; Manohar, S. K. J. Am. Chem. Soc. 2005, 127, 14156. (c) Zhang, X. T.; Zhang, J.; Song, W. H.; Liu, Z. F. J. Phys. Chem. B 2006, 110, 1158. (10) (a) Zang, J. F.; Li, C. M.; Bao, S. J.; Cui, X. Q.; Bao, Q. L.; Sun, C. Q. Macromolecules 2008, 41, 7053. (b) Zhong, W. B.; Liu, S. M.; Chen, X. H.; Wang, Y. X.; Yang, W. T. Macromolecules 2006, 39, 3224. (11) (a) Tran, H. D.; Li, D.; Kaner, R. B. AdV. Mater. 2009, 21, 1487. (b) Skotheim, T. A. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986.

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