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Strong acid doping for the preparation of conductive polyaniline nanoflowers, nanotubes, and nanofibers
Journal Pre-proof Strong Acid Doping for the Preparation of Conductive Polyaniline Nanoflowers, Nanotubes, and Nanofibers
H. Noby, A.H. El-Shazly, M.F. Elkady, M. Ohshima PII:
S0032-3861(19)30854-7
DOI:
https://doi.org/10.1016/j.polymer.2019.121848
Reference:
JPOL 121848
To appear in:
Polymer
Received Date:
14 June 2019
Accepted Date:
28 September 2019
Please cite this article as: H. Noby, A.H. El-Shazly, M.F. Elkady, M. Ohshima, Strong Acid Doping for the Preparation of Conductive Polyaniline Nanoflowers, Nanotubes, and Nanofibers, Polymer (2019), https://doi.org/10.1016/j.polymer.2019.121848
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Strong Acid Doping for the Preparation of Conductive Polyaniline Nanoflowers, Nanotubes, and Nanofibers H. Nobya*, A. H. El-Shazlyb,d, M. F. Elkadyb,e, M. Ohshimac a Materials
Engineering and Design, Faculty of Energy Engineering, Aswan University, Aswan, Egypt. Chemical and Petrochemicals Engineering Department, Egypt-Japan University of Science and Technology, New Borg El-Arab, Alexandria, Egypt. c Chemical Engineering Department, Kyoto University, Kyoto, Japan 615-8510. d Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt. e Fabrication Technology Department, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research and Technology Applications, Alexandria 21934, Egypt. *Corresponding author: [email protected] b
Abstract High-pressure CO2-supported aniline polymerization was thoroughly investigated with a focus on the effect of acid doping on the morphology of the prepared polyaniline (PANI). The polymerization was conducted using HCl at three different concentrations and H2SO4 as the doping agents. Self-assembled HCl-doped PANI nanoflowers (PANNFL) were synthesized for the first time by high-pressure CO2-supported polymerization in the presence of 2 M HCl. Furthermore, HCl-doped PANI nanotubes (HCl-PANNTs) and nanofibers (HCL-PANNFs), as well as sulfuric aciddoped PANI nanofibers (SUA-PANNFs), were successfully prepared by changing either the acidity of the reaction or the doping agent. With 0.1 M HCl doping, circular and rectangular cross-sectional HCl-PANNTs with a wall thickness of 40 nm were prepared. Upon increasing the acid concentration to 2 M HCl, the morphology of the PANI structures transformed from nanotubes to nanoflowers while retaining the same wall thickness. Further increasing the HCl concentration resulted in a mixture of nanosheet structures, and the amount of nanoflowers decreased. Additionally, 1.1 M H2SO4 doping produced nanofibers (SUA-PANNF) with an average fiber diameter of 45 nm. All the prepared PANIs had partially crystallized structure in the emeraldine salt form. Additionally, the crystallinities of the prepared HCl-PANI structures were higher than that of the SUA-PANI structure. It was clearly observed that the type of acid dopant and the acid concentration affected the polymerization yield (PY) and could control the electrical conductivity (EC). The highest EC, i.e., 3.7 S/cm, was exhibited by PANI prepared with 5 M HCl.
1.
Keywords: Self-assembled Polyaniline; CO2-Supported Polymerization; Strong Acid Doping; Nanoflowers; and Electrical Conductivity. other properties of PANI in a wide range of applications 1. Introduction owing to the resulting high surface area [17–19]. Polyaniline (PANI) and its derivatives have been Although current strong acid doped PANIs (SA-PANIs) receiving a substantial amount of attention from have shown granular, nanofibrous, or nanorod structures researchers because of their unique properties, which has depending upon the acidic conditions [12, 20–23], they enabled them to be utilized in interdisciplinary might not have had sufficiently large surface areas. applications [1–8]. The electrical conductivity (EC) of Specifically, the initial pH of the doped aniline solution, PANI, which is one of the most attractive properties of i.e. the acid to aniline molar ratio, was found to be crucial PANI, is crucial, especially for electrochemical and for the PANI morphology [20, 23, 24]. Thus, a growing energy applications. To obtain a high EC, PANI has been challenge exists regarding the preparation of highly produced by doping with strong acids [9–16]. The EC electrically conductive self-assembled SA-PANIs with changes with changing PANI morphology, such as advanced morphologies. polyaniline nanofibers (PANNFs), nanorods (PANNRs), Self-assembled PANNTs prepared from aniline/strong nanotubes (PANNTs), nanowalls, nanosheets acid solutions, as an example of PANIs with an advanced (PANNSHs), nanoflowers (PANNFLs), or any other morphology, has been reported several times [25, 26]. three-dimensional (3-D) hierarchical structures. These Zhang et al. [25] showed that PANNTs could be prepared three-dimensional hierarchical structures have improved
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without a template by setting the reaction temperature at 0-5 °C and changing the pH. Aside from the nanotube structure, it has been shown that strong acid doping can produce PANNFLs and improve the performance of PANI for electrochemical applications [27]. Reports on the synthesis of PANNFLs with or without templates are limited. With the aid of2. templates, PANNFLs have been produced using a chemical polymerization process [28-32]. However, there are few reported studies on self-assembled PANNFLs; Liu et al. [27] reported a self-assembled and conductive PANNF, which was prepared with a weak acid (tartaric acid). Wang et al. [33] prepared a self-assembled ptoluene sulfonic acid (p-TSA)-doped PANNFLs by tuning the molar ratio of aniline to p-TSA. Although these works represent remarkably interesting outcomes, most of them did not show high ECs. Self-assembled strong acid doped PANNFLs (SA-PANNFLs) by means of interfacial polymerization have also been reported [34,35]. However, self-assembled PANNFLs by means of conventional chemical oxidation polymerization and without the use of intercalating organic solvents for interfacial polymerization have not yet been reported. High-pressure and supercritical CO2-supported polymerization techniques have also been applied for preparing PANI and its derivatives in a chemical oxidation polymerization process [26, 36–41]. The ability of the CO2-supported polymerization method to prepare PANNFs [38, 40], PANNTs [26, 36, 39], and PANNRs [17, 41] have been successfully proven. Moreover, CO2supported polymerization has been shown to change the morphology of PANI from nanoparticles to PANNRs or PANNTs while maintaining the same reaction conditions, i.e., temperature and pH, like those of the conventional polymerization method [10, 15, 16]. The doping acid in the conventional aniline polymerization process has been shown to change not only the morphology but also the other characteristics of the resulting PANI [20, 25]. For CO2-supported polymerization technique, the effect of the doping acid on the characteristics of the prepared PANI has not yet been investigated. In this study on the CO2-supported polymerization process, the aniline to doping acid ratio, as a neverstudied parameter in the mentioned technique, was thoroughly investigated. Consequently, solutions with three molar ratios of HCl to aniline and one of H2SO4 to aniline were prepared to examine different SA-PANI structures. SEM observation clearly showed that the HClPANNTs, HCl-PANNFLs, HCl-PANNFs, HCl-PANI
nanosheets (HCl-PANNSHs), and H2SO4 doped PANNFs (SUA-PANNFs) were produced under high-pressure CO2-supported chemical oxidation polymerization. Among these obtained PANI structures, to the best of our knowledge, the preparation of self-assembled HCl-doped PANNFLs is reported for the first time in this study.
2. Experimental 2.1. Materials Aniline C6H7N (99%), ammonium peroxodisulfate (NH4)2S2O8 (APS, 98%), 36.5% HCl, H2SO4, methanol, and ethanol (Wako Pure Chemical Industries, Japan) were purchased and used without further purification. For the preparation of aqueous solutions and washing processes, distilled water was used.
2.2. Preparation of PANI PANIs were prepared with two types of strong acids (HCl and H2SO4) at different concentrations. The high-pressure CO2-supported polymerization was conducted as described in previous [26] using three different concentrations of HCl (0.1, 2, and 5 M) and one concentration of H2SO4 (1.1 M), as an alternative dopant (as shown in Fig. 1). Anilinium salt solutions were prepared from 25 ml of a 0.2 M of aniline solution mixed with the abovementioned acids at different concentrations. The initial pH of the prepared solution was measured by (HORIBA, Benchtop F-51 pH meter, Japan). For the prepared HCl/aniline solutions, the initial pH was 1, -0.3, and -0.69 for 0.1 M, 2 M, and 5 M HCl/aniline solutions, respectively. For the prepared H2SO4/aniline solution, the initial pH was 0. It was known that some strong acid solutions could give a pH values of zero or even minus in case of high acidity [42, 43]. The anilinium salt, anilinium hydrochloride or anilinium sulfate, solution was poured into a high-pressure autoclave. The autoclave was pressurized to 6 MPa at room temperature by a CO2 pump (Jasco, PU-2080) and kept under the CO2 pressure for 100 mins. To initiate polymerization, 25 ml of a 0.25 M APS aqueous solution was injected into the autoclave using a high-pressure pump (Shimadzu LC-10AT VP, HPLC). After 120 min of polymerization, the CO2 pressure was released by quickly opening the release valve. The precipitated dark green polymeric droplets were collected, separated by 5000 rpm centrifugation for 10 mins, and washed several times
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using diluted HCl, ethanol, methanol and distilled water before drying overnight at 60 °C.
2.3. Characterization Field Emission scanning electron microscopy (FESEM, JEOL JSM 6340 FS, Japan) and transmission electron microscopy (TEM, JEOL JEM-1010, Japan) observations were conducted to identify the morphology of the prepared PANI. The chemical groups of the produced PANI were evaluated using Fourier-transform infrared spectroscopy (FTIR) spectrometer (Vertex 70, Bruker Scientific Instruments, Germany). X-ray diffraction (XRD, Ultima IV/285/DX, Ultima IV, Japan) was used for the crystallinity measurements. The obtained PANI was weighted to calculate the polymerization yield (PY). The calculation was performed based on the following equation [40]; 𝑃𝑌 =
𝑊𝑓 𝑊𝑜
∗ 100
Figs. 2(b) show TEM image in which PANI nanotubes and nanorods can be clearly observed. The nanorods could be formed during the formation of the nanotube structures [26]. The main reason for obtaining a mixture of PANNTs and PANNRs may have been the gradual increase in acidity during polymerization (as shown in Fig. 3), where H2SO4 was released as a byproduct [20– 22].
(1)
where Wf and Wo are the weights of the produced PANI and the aniline in the monomer solution, respectively.
To measure the electrical conductivity (EC), a four-probe system (Signatone, PRO4-440N) was utilized. Prior to the EC measurements, the PANI powder was compressed on a hydraulic press for 5 mins under a load of 5 ton into a disc-shaped plate of 2 mm in thickness and 12 mm in diameter. The EC measurements were conducted 5 times at room temperature for each sample by changing the pin positions on the disc surface, and then the values were averaged.
Fig. 1 Schematic diagram of the steps of the PANI preparation process using the high-pressure CO2supported polymerization method.
3. 3. Results and Discussion 3.1. Morphology 3.1.1. HCl-doped PANI According to what obtained in our previous study [26], when the 0.1 M HCl/aniline mixed solution was utilized in the presence of high-pressure CO2, self-assembled HCl-PANNTs were obtained as shown in Figs. 2(a and b). Nanotubes with a 100 - 150 nm outer diameter and 40 nm average wall thickness were clearly observed in the images. Furthermore, from the shape of the PANNT cross-sections, some of the nanotubes were circular while others were rectangular. Self-assembled PANNTs have rarely been obtained from a strong acid/aniline solution.
Fig. 2 (a) SEM and (b) TEM of the PANNTs prepared using 0.1 M HCl/aniline solution. The CO2-supported polymerization of PANI prepared with 2 M HCl produced a novel 3D structure consisting of nanoflowers (Fig. 4). The nanoflower structures consisted of numerous self-arranged nanowalls. From the SEM images in Figs. 4(a, b, and c), the thickness of the nanowalls was estimated to be 40 nm. PANNFLs alone could not be observed but were obtained in a mixture with nanofibers (i.e., PANNF) that had an average diameter of 70 nm (Fig. 4d). To the best of our knowledge, self-
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assembled HCl doped PANNFLs have not been previously reported. The heterogeneity of the morphology was associated with the change in acidity as the polymerization reaction progressed [20–22]. However, nanofibers structure is also recognized as having high surface area, which are desired for several applications [45, 46].
Fig. 3 Chemical oxidation polymerization of aniline in an acidic medium. Further increasing the initial acidity by means of a 5 M HCl solution in the presence of high-pressure CO2 produced PANI with a nanosheet morphology, as shown in Fig. 5. The PANNSHs had an average thickness of 40 nm. These nanosheets occasionally formed a nanoflowerlike shape. Thus, it could be considered that increasing the acid concentration leads to dispersion of the PANI nanosheets, i.e., degradation of the nanoflower structure and restriction of the growth in the Z-direction, which results in the flatter structure. This can be clearly seen in Fig. 5a, where a flower-shaped morphology with decreased depth can be seen. 3.1.2. H2SO4-doped PANI When a 1.1 M H2SO4/aniline solution was used, PANI nanofibers (PANNFs) with small diameters were obtained. From the SEM image (Fig. 6) and the TEM image (Fig. 7), the average diameter of the PANNFs was estimated to be 45 nm. The nanofiber morphology was preferred because of its exceptionally high surface area. In addition, the produced nanofibers had a uniform shape without any other morphologies. This indicated that the PANI prepared from the H2SO4/aniline solution was not affected by the change in acidity.
Fig. 4 SEM images of the PANI structures prepared with an aqueous 2 M HCl solution: (a, b, c) nanoflower morphology and (d) nanofiber morphology.
Fig. 5 SEM images of the mixed PANI nanosheets prepared with an aqueous 5 M HCl solution at magnifications of (a) x18,000 and (b) x24,000.
Fig. 6 SEM images of the PANNFs prepared with an aqueous 1.1 M H2SO4 solution at different magnifications (a) x20,000 and (b) x80,000.
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Fig. 7 TEM image of the PANNFs prepared with an aqueous 1.1 M H2SO4 solution. Three assumptions regarding the formation of the different PANI structures can be given [25]. First, micelles of the monomer contribute to the structural arrangement. Second, aniline oligomers nucleate and act as a template to form the PANI structures. Finally, the final PANI structure is determined from the PANI oligomers, and para-coupled structures are formed during the polymerization process. Additionally, the effect of the acidity change during the early stages of the reaction on the PANI structure should not be neglected. The oligomers that form in the early stages of polymerization are strongly considered to be the main factor in determining the PANI structures. This can be seen clearly in Figs. 3(a and b), where nanotubes were formed as an extension of the nanorods. Another factor in the formation of such structures is the presence of high-pressure CO2, which plays a role in altering the solution properties, i.e., producing no surface tension or expansion of the aniline solution [38]. Expansion of the anilinium salt solution may improve and increase the amount of interactions in the APS solution. As described in a previous report [20], aniline oxidation is proportional to the surface area and hydrophilicity of the solution interface; thus, the formation of PANI was modified by the changes in the surface area and hydrophilicity of the solution interface due to the presence of CO2.
3.2. XRD analysis Figure 8 shows the XRD patterns of the produced PANI. The obtained XRD peaks agree with the peaks of the emeraldine salt structure [26, 31]. The XRD pattern of the 0.1 M HCl doped PANI sample showed three peaks. The peak at 2θ of 19.26° was attributed to the periodicity parallel to the PANI polymer chain. The peak at 25.82° could be ascribed to the periodicity perpendicular to the PANI chain. The peak at 6.44° arose from the interaction
between the Cl ions and the polymer chains. When PANI was prepared with 2 M HCl, these three peaks became narrower and increased in intensity. Additionally, two new peaks appeared in the spectra at 8.8° and 20.7°. However, upon further increasing the HCl molarity, the peaks at 6°, 19.5°, and 25° decreased in intensity. In contrast, the intensity of the peaks at 8.8° and 20.7° increased along with the appearance of a new peak at 15.8°. It was concluded that PANI with a higher degree of crystallinity was formed as the doping acid concentration increased. For the 1.1 M H2SO4/aniline solution, three main peaks were observed at 6.66°, 19.44°, and 25.44°. The intensities of these three peaks were lower than those of the HCl-doped PANI. Thus, it can be said that the PANI prepared with H2SO4 was less crystalline than those obtained with HCl doping. Moreover, (excluding the results of 0.1 M HCl-doped PANI), the intensity of the peak at approximately 25° was always higher than that of the peak at approximately 19.5°. This indicated that the doped structure was the primary structure in comparison with the amount of the linear polymer structure at high doping acid concentrations.
3.3. FTIR analysis FTIR spectra of the PANI samples prepared with different aniline/acid solutions (0.1 M HCl and 1.1 M H2SO4) are shown in Fig. 9. The characteristic peaks of PANI emeraldine salt can be observed. For the PANNTs prepared with 0.1 M HCl, the characteristic peaks of the benzenoid unit and quinonoid unit were observed at 1469 cm-1 and 1570 cm-1, respectively. Moreover, the intensity of the benzenoid peak was stronger than that of the quinonoid peak, which indicated that the produced PANNTs had not been fully oxidized and were in the emeraldine salt form. A band could be seen at 1417 cm-1 especially in the HCl-doped PANI may attributed to phenazine-like structures [47]. Additionally, the peaks at 695 and 1298 cm-1 were ascribed to the aromatic CــــH out-of-plane bending and CــــN stretching of the secondary aromatic amine, respectively. The characteristic peaks of the HCl-PANI spectrum had high intensities in comparison with those of the SUA-PANI spectrum. The peak at 1039 cm-1 was noticeably clearer for the HCl doped PANI that that of the SUA-PANI, which could support the presence of strong acid doping for the HCl-PANI structure. This may support the existence of a high EC in case of the HCl-PANI than the SUA-PANI. Also, according to the HCl doping, the produced HCl-PANI is negatively charged, which may
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improve the attraction of the positively charged materials as in chemicals sensing and water purification applications. From the FTIR spectra, the oxidation ratio of PANI could be calculated by measuring the ratio of the peak intensities of the quinonoid to benzenoid units (i.e., C=N and CــN) [44]. Any PANI with a ratio of these two peaks that is less than one has more benzenoid units than quinoid units within its polymer chain.
the fact that the crystallinity of PANI increased with increasing acid concentration, as illustrated in Fig. 8. In addition, the SUA-PANI sample showed a lower EC of 1.2 S/cm compared with that of the HCl-doped PANI. Based on the EC values of the prepared HCl-doped PANI, it could be concluded that HCl doping facilitated the production of PANI with higher electrical conductivity.
3.5. Polymerization Yield The polymerization yield (PY) of PANI was also influenced by the type of acid dopant and the acidity during the polymerization process. As shown in Fig. 11, the PY decreased with increasing HCl concentration. Accordingly, the PY reached an exceedingly small value when 5 M HCl was utilized. This may be related to the slow reaction rate of aniline polymerization in highly acidic media [23].
Fig. 8 XRD curves for the PANI samples prepared with 1.1 M H2SO4 and 0.1, 2, and 5 M HCl.
Fig. 10 ECs for the obtained PANI structures using different acid dopants and concentrations.
Fig. 9 FTIR spectra for the PANI samples prepared with 1.1 M H2SO4 and 0.1 M HCl.
3.4. Electrical conductivity As expected, the EC was significantly influenced by the initial acidity and the type of acid dopant, as illustrated in Fig. 10. The EC increased from 1.5 to 3.7 S/cm when the HCl concentration was increased from 0.1 M to 5 M. As expected, the increase in acidity increased the conductivity of PANI. These results could be attributed to
Fig. 11 PYs of the different prepared PANI structures.
Conclusions The effect of the doping acid and the doping acid concentration on the self-assembled and strong aciddoped PANI morphology under high-pressure CO2 was investigated. Self-assembled HCl-doped PANNFLs were
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produced successfully for the first time using the highpressure CO2-assisted polymerization. These nanoflowers were formed by the self-assembly of nanowalls with an average wall thickness of 40 nm. However, the nanoflowers did not show a uniform structure and they were mixed with nanofiber structures. This could be attributed to the rise of acidity caused by the release of H2SO4 as a reaction byproduct during the polymerization process. Additionally, increasing the molarity of HCl to 5 M produced PANI with the scattered nanosheets morphology. PANNFs with an average diameter of 45 nm were obtained when aniline was prepared with 1.1 M H2SO4. The ECs of the prepared PANI samples increased significantly when the doping acid concentration was increased. However, the polymerization yield decreased with the increasing acid concentration. The high-pressure CO2-supported polymerization method developed here shows a promising behavior to produce distinct strong acid doped PANI morphologies with the ability of controlling the PANI electrical conductivity.
Declarations of interest None Acknowledgements The authors acknowledge the Egyptian Ministry of Higher Education for financial supports and Egypt-Japan University (E-JUST) for providing the facilities. Dr. Ahmed ElMohammady and all the members of Chemical and Petrochemicals Engineering lab, Egypt-Japan University (E-JUST) were greatly acknowledged for their continuous support and effective help.
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Manuscript title: "Strong Acid Doping for the Preparation of Conductive Polyaniline Nanoflowers, Nanotubes, and Nanofibers"
Highlights Doping acid type was studied in the High-pressure CO2 polymerization for the first time. Self-assembled HCl-doped polyaniline nanoflowers were prepared successfully. Self-assembled polyaniline nanotubes and nanofibers were also prepared. The electrical conductivity could be controlled by adjusting the doping acid concentration. Increasing the doping acid concentration affect the polymerization yield negatively.
H. Noby, A.H. El-Shazly, M.F. Elkady, M. Ohshima PII:
S0032-3861(19)30854-7
DOI:
https://doi.org/10.1016/j.polymer.2019.121848
Reference:
JPOL 121848
To appear in:
Polymer
Received Date:
14 June 2019
Accepted Date:
28 September 2019
Please cite this article as: H. Noby, A.H. El-Shazly, M.F. Elkady, M. Ohshima, Strong Acid Doping for the Preparation of Conductive Polyaniline Nanoflowers, Nanotubes, and Nanofibers, Polymer (2019), https://doi.org/10.1016/j.polymer.2019.121848
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Strong Acid Doping for the Preparation of Conductive Polyaniline Nanoflowers, Nanotubes, and Nanofibers H. Nobya*, A. H. El-Shazlyb,d, M. F. Elkadyb,e, M. Ohshimac a Materials
Engineering and Design, Faculty of Energy Engineering, Aswan University, Aswan, Egypt. Chemical and Petrochemicals Engineering Department, Egypt-Japan University of Science and Technology, New Borg El-Arab, Alexandria, Egypt. c Chemical Engineering Department, Kyoto University, Kyoto, Japan 615-8510. d Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt. e Fabrication Technology Department, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research and Technology Applications, Alexandria 21934, Egypt. *Corresponding author: [email protected] b
Abstract High-pressure CO2-supported aniline polymerization was thoroughly investigated with a focus on the effect of acid doping on the morphology of the prepared polyaniline (PANI). The polymerization was conducted using HCl at three different concentrations and H2SO4 as the doping agents. Self-assembled HCl-doped PANI nanoflowers (PANNFL) were synthesized for the first time by high-pressure CO2-supported polymerization in the presence of 2 M HCl. Furthermore, HCl-doped PANI nanotubes (HCl-PANNTs) and nanofibers (HCL-PANNFs), as well as sulfuric aciddoped PANI nanofibers (SUA-PANNFs), were successfully prepared by changing either the acidity of the reaction or the doping agent. With 0.1 M HCl doping, circular and rectangular cross-sectional HCl-PANNTs with a wall thickness of 40 nm were prepared. Upon increasing the acid concentration to 2 M HCl, the morphology of the PANI structures transformed from nanotubes to nanoflowers while retaining the same wall thickness. Further increasing the HCl concentration resulted in a mixture of nanosheet structures, and the amount of nanoflowers decreased. Additionally, 1.1 M H2SO4 doping produced nanofibers (SUA-PANNF) with an average fiber diameter of 45 nm. All the prepared PANIs had partially crystallized structure in the emeraldine salt form. Additionally, the crystallinities of the prepared HCl-PANI structures were higher than that of the SUA-PANI structure. It was clearly observed that the type of acid dopant and the acid concentration affected the polymerization yield (PY) and could control the electrical conductivity (EC). The highest EC, i.e., 3.7 S/cm, was exhibited by PANI prepared with 5 M HCl.
1.
Keywords: Self-assembled Polyaniline; CO2-Supported Polymerization; Strong Acid Doping; Nanoflowers; and Electrical Conductivity. other properties of PANI in a wide range of applications 1. Introduction owing to the resulting high surface area [17–19]. Polyaniline (PANI) and its derivatives have been Although current strong acid doped PANIs (SA-PANIs) receiving a substantial amount of attention from have shown granular, nanofibrous, or nanorod structures researchers because of their unique properties, which has depending upon the acidic conditions [12, 20–23], they enabled them to be utilized in interdisciplinary might not have had sufficiently large surface areas. applications [1–8]. The electrical conductivity (EC) of Specifically, the initial pH of the doped aniline solution, PANI, which is one of the most attractive properties of i.e. the acid to aniline molar ratio, was found to be crucial PANI, is crucial, especially for electrochemical and for the PANI morphology [20, 23, 24]. Thus, a growing energy applications. To obtain a high EC, PANI has been challenge exists regarding the preparation of highly produced by doping with strong acids [9–16]. The EC electrically conductive self-assembled SA-PANIs with changes with changing PANI morphology, such as advanced morphologies. polyaniline nanofibers (PANNFs), nanorods (PANNRs), Self-assembled PANNTs prepared from aniline/strong nanotubes (PANNTs), nanowalls, nanosheets acid solutions, as an example of PANIs with an advanced (PANNSHs), nanoflowers (PANNFLs), or any other morphology, has been reported several times [25, 26]. three-dimensional (3-D) hierarchical structures. These Zhang et al. [25] showed that PANNTs could be prepared three-dimensional hierarchical structures have improved
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without a template by setting the reaction temperature at 0-5 °C and changing the pH. Aside from the nanotube structure, it has been shown that strong acid doping can produce PANNFLs and improve the performance of PANI for electrochemical applications [27]. Reports on the synthesis of PANNFLs with or without templates are limited. With the aid of2. templates, PANNFLs have been produced using a chemical polymerization process [28-32]. However, there are few reported studies on self-assembled PANNFLs; Liu et al. [27] reported a self-assembled and conductive PANNF, which was prepared with a weak acid (tartaric acid). Wang et al. [33] prepared a self-assembled ptoluene sulfonic acid (p-TSA)-doped PANNFLs by tuning the molar ratio of aniline to p-TSA. Although these works represent remarkably interesting outcomes, most of them did not show high ECs. Self-assembled strong acid doped PANNFLs (SA-PANNFLs) by means of interfacial polymerization have also been reported [34,35]. However, self-assembled PANNFLs by means of conventional chemical oxidation polymerization and without the use of intercalating organic solvents for interfacial polymerization have not yet been reported. High-pressure and supercritical CO2-supported polymerization techniques have also been applied for preparing PANI and its derivatives in a chemical oxidation polymerization process [26, 36–41]. The ability of the CO2-supported polymerization method to prepare PANNFs [38, 40], PANNTs [26, 36, 39], and PANNRs [17, 41] have been successfully proven. Moreover, CO2supported polymerization has been shown to change the morphology of PANI from nanoparticles to PANNRs or PANNTs while maintaining the same reaction conditions, i.e., temperature and pH, like those of the conventional polymerization method [10, 15, 16]. The doping acid in the conventional aniline polymerization process has been shown to change not only the morphology but also the other characteristics of the resulting PANI [20, 25]. For CO2-supported polymerization technique, the effect of the doping acid on the characteristics of the prepared PANI has not yet been investigated. In this study on the CO2-supported polymerization process, the aniline to doping acid ratio, as a neverstudied parameter in the mentioned technique, was thoroughly investigated. Consequently, solutions with three molar ratios of HCl to aniline and one of H2SO4 to aniline were prepared to examine different SA-PANI structures. SEM observation clearly showed that the HClPANNTs, HCl-PANNFLs, HCl-PANNFs, HCl-PANI
nanosheets (HCl-PANNSHs), and H2SO4 doped PANNFs (SUA-PANNFs) were produced under high-pressure CO2-supported chemical oxidation polymerization. Among these obtained PANI structures, to the best of our knowledge, the preparation of self-assembled HCl-doped PANNFLs is reported for the first time in this study.
2. Experimental 2.1. Materials Aniline C6H7N (99%), ammonium peroxodisulfate (NH4)2S2O8 (APS, 98%), 36.5% HCl, H2SO4, methanol, and ethanol (Wako Pure Chemical Industries, Japan) were purchased and used without further purification. For the preparation of aqueous solutions and washing processes, distilled water was used.
2.2. Preparation of PANI PANIs were prepared with two types of strong acids (HCl and H2SO4) at different concentrations. The high-pressure CO2-supported polymerization was conducted as described in previous [26] using three different concentrations of HCl (0.1, 2, and 5 M) and one concentration of H2SO4 (1.1 M), as an alternative dopant (as shown in Fig. 1). Anilinium salt solutions were prepared from 25 ml of a 0.2 M of aniline solution mixed with the abovementioned acids at different concentrations. The initial pH of the prepared solution was measured by (HORIBA, Benchtop F-51 pH meter, Japan). For the prepared HCl/aniline solutions, the initial pH was 1, -0.3, and -0.69 for 0.1 M, 2 M, and 5 M HCl/aniline solutions, respectively. For the prepared H2SO4/aniline solution, the initial pH was 0. It was known that some strong acid solutions could give a pH values of zero or even minus in case of high acidity [42, 43]. The anilinium salt, anilinium hydrochloride or anilinium sulfate, solution was poured into a high-pressure autoclave. The autoclave was pressurized to 6 MPa at room temperature by a CO2 pump (Jasco, PU-2080) and kept under the CO2 pressure for 100 mins. To initiate polymerization, 25 ml of a 0.25 M APS aqueous solution was injected into the autoclave using a high-pressure pump (Shimadzu LC-10AT VP, HPLC). After 120 min of polymerization, the CO2 pressure was released by quickly opening the release valve. The precipitated dark green polymeric droplets were collected, separated by 5000 rpm centrifugation for 10 mins, and washed several times
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using diluted HCl, ethanol, methanol and distilled water before drying overnight at 60 °C.
2.3. Characterization Field Emission scanning electron microscopy (FESEM, JEOL JSM 6340 FS, Japan) and transmission electron microscopy (TEM, JEOL JEM-1010, Japan) observations were conducted to identify the morphology of the prepared PANI. The chemical groups of the produced PANI were evaluated using Fourier-transform infrared spectroscopy (FTIR) spectrometer (Vertex 70, Bruker Scientific Instruments, Germany). X-ray diffraction (XRD, Ultima IV/285/DX, Ultima IV, Japan) was used for the crystallinity measurements. The obtained PANI was weighted to calculate the polymerization yield (PY). The calculation was performed based on the following equation [40]; 𝑃𝑌 =
𝑊𝑓 𝑊𝑜
∗ 100
Figs. 2(b) show TEM image in which PANI nanotubes and nanorods can be clearly observed. The nanorods could be formed during the formation of the nanotube structures [26]. The main reason for obtaining a mixture of PANNTs and PANNRs may have been the gradual increase in acidity during polymerization (as shown in Fig. 3), where H2SO4 was released as a byproduct [20– 22].
(1)
where Wf and Wo are the weights of the produced PANI and the aniline in the monomer solution, respectively.
To measure the electrical conductivity (EC), a four-probe system (Signatone, PRO4-440N) was utilized. Prior to the EC measurements, the PANI powder was compressed on a hydraulic press for 5 mins under a load of 5 ton into a disc-shaped plate of 2 mm in thickness and 12 mm in diameter. The EC measurements were conducted 5 times at room temperature for each sample by changing the pin positions on the disc surface, and then the values were averaged.
Fig. 1 Schematic diagram of the steps of the PANI preparation process using the high-pressure CO2supported polymerization method.
3. 3. Results and Discussion 3.1. Morphology 3.1.1. HCl-doped PANI According to what obtained in our previous study [26], when the 0.1 M HCl/aniline mixed solution was utilized in the presence of high-pressure CO2, self-assembled HCl-PANNTs were obtained as shown in Figs. 2(a and b). Nanotubes with a 100 - 150 nm outer diameter and 40 nm average wall thickness were clearly observed in the images. Furthermore, from the shape of the PANNT cross-sections, some of the nanotubes were circular while others were rectangular. Self-assembled PANNTs have rarely been obtained from a strong acid/aniline solution.
Fig. 2 (a) SEM and (b) TEM of the PANNTs prepared using 0.1 M HCl/aniline solution. The CO2-supported polymerization of PANI prepared with 2 M HCl produced a novel 3D structure consisting of nanoflowers (Fig. 4). The nanoflower structures consisted of numerous self-arranged nanowalls. From the SEM images in Figs. 4(a, b, and c), the thickness of the nanowalls was estimated to be 40 nm. PANNFLs alone could not be observed but were obtained in a mixture with nanofibers (i.e., PANNF) that had an average diameter of 70 nm (Fig. 4d). To the best of our knowledge, self-
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assembled HCl doped PANNFLs have not been previously reported. The heterogeneity of the morphology was associated with the change in acidity as the polymerization reaction progressed [20–22]. However, nanofibers structure is also recognized as having high surface area, which are desired for several applications [45, 46].
Fig. 3 Chemical oxidation polymerization of aniline in an acidic medium. Further increasing the initial acidity by means of a 5 M HCl solution in the presence of high-pressure CO2 produced PANI with a nanosheet morphology, as shown in Fig. 5. The PANNSHs had an average thickness of 40 nm. These nanosheets occasionally formed a nanoflowerlike shape. Thus, it could be considered that increasing the acid concentration leads to dispersion of the PANI nanosheets, i.e., degradation of the nanoflower structure and restriction of the growth in the Z-direction, which results in the flatter structure. This can be clearly seen in Fig. 5a, where a flower-shaped morphology with decreased depth can be seen. 3.1.2. H2SO4-doped PANI When a 1.1 M H2SO4/aniline solution was used, PANI nanofibers (PANNFs) with small diameters were obtained. From the SEM image (Fig. 6) and the TEM image (Fig. 7), the average diameter of the PANNFs was estimated to be 45 nm. The nanofiber morphology was preferred because of its exceptionally high surface area. In addition, the produced nanofibers had a uniform shape without any other morphologies. This indicated that the PANI prepared from the H2SO4/aniline solution was not affected by the change in acidity.
Fig. 4 SEM images of the PANI structures prepared with an aqueous 2 M HCl solution: (a, b, c) nanoflower morphology and (d) nanofiber morphology.
Fig. 5 SEM images of the mixed PANI nanosheets prepared with an aqueous 5 M HCl solution at magnifications of (a) x18,000 and (b) x24,000.
Fig. 6 SEM images of the PANNFs prepared with an aqueous 1.1 M H2SO4 solution at different magnifications (a) x20,000 and (b) x80,000.
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Fig. 7 TEM image of the PANNFs prepared with an aqueous 1.1 M H2SO4 solution. Three assumptions regarding the formation of the different PANI structures can be given [25]. First, micelles of the monomer contribute to the structural arrangement. Second, aniline oligomers nucleate and act as a template to form the PANI structures. Finally, the final PANI structure is determined from the PANI oligomers, and para-coupled structures are formed during the polymerization process. Additionally, the effect of the acidity change during the early stages of the reaction on the PANI structure should not be neglected. The oligomers that form in the early stages of polymerization are strongly considered to be the main factor in determining the PANI structures. This can be seen clearly in Figs. 3(a and b), where nanotubes were formed as an extension of the nanorods. Another factor in the formation of such structures is the presence of high-pressure CO2, which plays a role in altering the solution properties, i.e., producing no surface tension or expansion of the aniline solution [38]. Expansion of the anilinium salt solution may improve and increase the amount of interactions in the APS solution. As described in a previous report [20], aniline oxidation is proportional to the surface area and hydrophilicity of the solution interface; thus, the formation of PANI was modified by the changes in the surface area and hydrophilicity of the solution interface due to the presence of CO2.
3.2. XRD analysis Figure 8 shows the XRD patterns of the produced PANI. The obtained XRD peaks agree with the peaks of the emeraldine salt structure [26, 31]. The XRD pattern of the 0.1 M HCl doped PANI sample showed three peaks. The peak at 2θ of 19.26° was attributed to the periodicity parallel to the PANI polymer chain. The peak at 25.82° could be ascribed to the periodicity perpendicular to the PANI chain. The peak at 6.44° arose from the interaction
between the Cl ions and the polymer chains. When PANI was prepared with 2 M HCl, these three peaks became narrower and increased in intensity. Additionally, two new peaks appeared in the spectra at 8.8° and 20.7°. However, upon further increasing the HCl molarity, the peaks at 6°, 19.5°, and 25° decreased in intensity. In contrast, the intensity of the peaks at 8.8° and 20.7° increased along with the appearance of a new peak at 15.8°. It was concluded that PANI with a higher degree of crystallinity was formed as the doping acid concentration increased. For the 1.1 M H2SO4/aniline solution, three main peaks were observed at 6.66°, 19.44°, and 25.44°. The intensities of these three peaks were lower than those of the HCl-doped PANI. Thus, it can be said that the PANI prepared with H2SO4 was less crystalline than those obtained with HCl doping. Moreover, (excluding the results of 0.1 M HCl-doped PANI), the intensity of the peak at approximately 25° was always higher than that of the peak at approximately 19.5°. This indicated that the doped structure was the primary structure in comparison with the amount of the linear polymer structure at high doping acid concentrations.
3.3. FTIR analysis FTIR spectra of the PANI samples prepared with different aniline/acid solutions (0.1 M HCl and 1.1 M H2SO4) are shown in Fig. 9. The characteristic peaks of PANI emeraldine salt can be observed. For the PANNTs prepared with 0.1 M HCl, the characteristic peaks of the benzenoid unit and quinonoid unit were observed at 1469 cm-1 and 1570 cm-1, respectively. Moreover, the intensity of the benzenoid peak was stronger than that of the quinonoid peak, which indicated that the produced PANNTs had not been fully oxidized and were in the emeraldine salt form. A band could be seen at 1417 cm-1 especially in the HCl-doped PANI may attributed to phenazine-like structures [47]. Additionally, the peaks at 695 and 1298 cm-1 were ascribed to the aromatic CــــH out-of-plane bending and CــــN stretching of the secondary aromatic amine, respectively. The characteristic peaks of the HCl-PANI spectrum had high intensities in comparison with those of the SUA-PANI spectrum. The peak at 1039 cm-1 was noticeably clearer for the HCl doped PANI that that of the SUA-PANI, which could support the presence of strong acid doping for the HCl-PANI structure. This may support the existence of a high EC in case of the HCl-PANI than the SUA-PANI. Also, according to the HCl doping, the produced HCl-PANI is negatively charged, which may
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improve the attraction of the positively charged materials as in chemicals sensing and water purification applications. From the FTIR spectra, the oxidation ratio of PANI could be calculated by measuring the ratio of the peak intensities of the quinonoid to benzenoid units (i.e., C=N and CــN) [44]. Any PANI with a ratio of these two peaks that is less than one has more benzenoid units than quinoid units within its polymer chain.
the fact that the crystallinity of PANI increased with increasing acid concentration, as illustrated in Fig. 8. In addition, the SUA-PANI sample showed a lower EC of 1.2 S/cm compared with that of the HCl-doped PANI. Based on the EC values of the prepared HCl-doped PANI, it could be concluded that HCl doping facilitated the production of PANI with higher electrical conductivity.
3.5. Polymerization Yield The polymerization yield (PY) of PANI was also influenced by the type of acid dopant and the acidity during the polymerization process. As shown in Fig. 11, the PY decreased with increasing HCl concentration. Accordingly, the PY reached an exceedingly small value when 5 M HCl was utilized. This may be related to the slow reaction rate of aniline polymerization in highly acidic media [23].
Fig. 8 XRD curves for the PANI samples prepared with 1.1 M H2SO4 and 0.1, 2, and 5 M HCl.
Fig. 10 ECs for the obtained PANI structures using different acid dopants and concentrations.
Fig. 9 FTIR spectra for the PANI samples prepared with 1.1 M H2SO4 and 0.1 M HCl.
3.4. Electrical conductivity As expected, the EC was significantly influenced by the initial acidity and the type of acid dopant, as illustrated in Fig. 10. The EC increased from 1.5 to 3.7 S/cm when the HCl concentration was increased from 0.1 M to 5 M. As expected, the increase in acidity increased the conductivity of PANI. These results could be attributed to
Fig. 11 PYs of the different prepared PANI structures.
Conclusions The effect of the doping acid and the doping acid concentration on the self-assembled and strong aciddoped PANI morphology under high-pressure CO2 was investigated. Self-assembled HCl-doped PANNFLs were
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produced successfully for the first time using the highpressure CO2-assisted polymerization. These nanoflowers were formed by the self-assembly of nanowalls with an average wall thickness of 40 nm. However, the nanoflowers did not show a uniform structure and they were mixed with nanofiber structures. This could be attributed to the rise of acidity caused by the release of H2SO4 as a reaction byproduct during the polymerization process. Additionally, increasing the molarity of HCl to 5 M produced PANI with the scattered nanosheets morphology. PANNFs with an average diameter of 45 nm were obtained when aniline was prepared with 1.1 M H2SO4. The ECs of the prepared PANI samples increased significantly when the doping acid concentration was increased. However, the polymerization yield decreased with the increasing acid concentration. The high-pressure CO2-supported polymerization method developed here shows a promising behavior to produce distinct strong acid doped PANI morphologies with the ability of controlling the PANI electrical conductivity.
Declarations of interest None Acknowledgements The authors acknowledge the Egyptian Ministry of Higher Education for financial supports and Egypt-Japan University (E-JUST) for providing the facilities. Dr. Ahmed ElMohammady and all the members of Chemical and Petrochemicals Engineering lab, Egypt-Japan University (E-JUST) were greatly acknowledged for their continuous support and effective help.
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Manuscript title: "Strong Acid Doping for the Preparation of Conductive Polyaniline Nanoflowers, Nanotubes, and Nanofibers"
Highlights Doping acid type was studied in the High-pressure CO2 polymerization for the first time. Self-assembled HCl-doped polyaniline nanoflowers were prepared successfully. Self-assembled polyaniline nanotubes and nanofibers were also prepared. The electrical conductivity could be controlled by adjusting the doping acid concentration. Increasing the doping acid concentration affect the polymerization yield negatively.