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Mar 26, 2015 - 1 Physics Department, College of Science and Mathematics, Western Mindanao ... Mindanao State University-Iligan Institute of Technology,.

Effect of supercritical carbon dioxide treatment on the polarons of HCl-doped polyaniline films

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Phys. Status Solidi C 12, No. 6, 576– 579 (2015) / DOI 10.1002/pssc.201400262

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current topics in solid state physics

J. G. Fernando*,1,2, R. M. Vequizo**,2,3, M. K. G. Odarve3, B. R. B. Sambo3, R. M. Malaluan4, and L. A. M. Malaluan4 1

Physics Department, College of Science and Mathematics, Western Mindanao State University, 7000 Zamboanga City, Philippines School of Graduate Studies, College of Science and Mathematics, Mindanao State University-Iligan Institute of Technology, 9200 Iligan City, Philippines 3 Department of Physics, College of Science and Mathematics, Mindanao State University-Iligan Institute of Technology, 9200 Iligan City, Philippines 4 Department of Chemical Engineering, College of Engineering, Mindanao State University-Iligan Institute of Technology, Iligan City 9200, Philippines 2

Received 28 September 2014, revised 4 February 2015, accepted 12 February 2015 Published online 26 March 2015 Keywords polyaniline, polarons, supercritical carbon dioxide, treatment ** Corresponding author: e-mail [email protected] ** e-mail [email protected]

In situ HCl-doped polyaniline films were synthesized on glass substrates via chemical oxidative polymerization of aniline using ammonium peroxydisulfate as oxidant. The films were treated with supercritical carbon dioxide (SCCO2) at 30 MPa and 40 °C for 30 minutes. The physicochemical and electrical properties of the films were determined to establish the effect of supercritical carbon dioxide treatment on the polaron structures of polyaniline films. The treatment removed limited number of polarons

which affected the properties of polyaniline. The treated films remained conducting despite that the conductance decreased substantially. The treatment enhanced the quinoid character of the films and altered the HOMOLUMO transitions. In addition, SC-CO2 treatment was unable to remove the PANI precipitations adhering on the surface of the films. A mechanism on the effect of SC-CO2 interaction to HCl-doped PANI films was proposed.

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1 Introduction Intrinsically conducting polymer (ICP) is a class of material that exhibits properties that are common to both metals and conventional polymers [1]. It has attracted considerable interests because of its high conductivity, low energy optical transition, low ionization potential and high electron affinity [2]. Among ICPs, polyaniline (PANI) is considered superior due to its good thermal stability, tunability, less expensive monomer and ease in synthesis [3]. One of the disadvantages of polyaniline, however, is its poor processability because it is insoluble in common solvents and infusible [4]. The use of supercritical fluid, which exhibits gas-like diffusivity and liquid-like density [5], may improve the processability of polyaniline. Among supercritical fluid components, carbon dioxide is widely used because of its convenient supercritical temperature and pressure (7.38

MPa and 31.1 °C) [6]. Despite that supercritical carbon dioxide (SC-CO2) was used to prepare a number of polymeric products by blending different polymers [7] and infusing additives [8], none has been reported in polyaniline. SC-CO2 was only recently used as a solvent in synthesizing polyaniline and its composites [9-11]. For SC-CO2 to be a viable fluid for processing polyaniline, SC-CO2 must not substantially disrupt the polaron structures. The polarons are formed when polyaniline are protonated with a suitable dopant and known to play a crucial role in facilitating charge transport through the radical cation acting as a hole [3, 12]. Since many technological application possibilities depend on the presence of polarons [13-15], it is of practical importance to investigate the effect of SC-CO2 treatment on the polaron structures of polyaniline. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Contributed Article Phys. Status Solidi C 12, No. 6 (2015)

In this study, we synthesized HCl-doped polyaniline thin films on glass substrates via chemical oxidative polymerization of aniline using ammonium peroxydisulfate (APS) as the oxidant. The post-polymerized films were treated with supercritical carbon dioxide and their electrical and physico-chemical properties were studied to establish the effect of SC-CO2 treatment on the polaronic structures. 2 Experimental 2.1 Film synthesis Polyaniline films on glass substrates were prepared by oxidative chemical polymerization of aniline in an acidic medium using ammonium peroxydisulfate as oxidant. Aniline (1.83 ml) and ammonium peroxydisulfate (5.71 g) were dissolved separately to different molarities of hydrochloric acid solutions (0.2, 0.6, and 1.0 M). The substrates were then placed on the reaction vessel where aniline and ammonium peroxydisulfate were mixed to start the polymerization. The samples were then removed after 30 minutes and rinsed successively with 0.2 M HCl, ethanol and water. 2.2 Supercritical carbon dioxide treatment The air-dried films were treated with supercritical carbon dioxide for 30 minutes using the supercritical carbon dioxide extraction machine (Akico). The pressure and temperature were maintained at 40°C and 30 MPa, respectively. 2.3 Characterization The morphologies of the samples were examined using scanning electron microscope. The UV-Visible absorption spectra of the films were measured in the range 400 to 900 nm by Lambda 35 UVVis spectrometer (Perkin Elmer, UK). Fourier transform infrared spectra (FTIR) with a 1 cm-1 resolution were obtained in the range 650-4000 cm-1 by using Spectrum 100 FTIR spectrometer (Perkin Elmer, UK). The conductance was determined using two-probe method. 3 Results and discussion 3.1 Surface morphology and composition The SEM micrographs of the samples (Fig. 1) show that the grown PANI films have sufficiently covered the glass substrates. A mixture of dendritic nanorods and granular nanostructures were observed on the surface of the films doped with 0.2 and 0.6 M HCl while circular patches composing of granular nanostructures were seen on a film synthesized with highest HCl concentration. The surface of the treated films contains PANI precipitations which SC-CO2 treatment was unable to remove. We also observed that the treatment deteriorated the surface quality of the films doped with 0.6 and 1.0 M HCl since a number of voids formed on the surface. However, voids found on the surface of the film doped with 1.0 M HCl were greater in number and well dispersed throughout the sample as evident in SEM micrographs at lower magnification. EDS scans of the samples revealed the presence of C, Cl and N, the typical elemental composition of polyaniline www.pss-c.com

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in doped state [16]. We calculated the Cl/N ratio to estimate the relative amount of dopants that incorporated into the imine nitrogen. The Cl/N ratios for treated films (2.53, 2.45 and 2.81% for 0.2, 0.6 and 1.0 M, respectively) are lower than the corresponding untreated films (4.62, 2.51 and 2.93% for 0.2, 0.6 and 1.0 M, respectively). The reduction in the Cl/N ratio in treated films indicates that SC-CO2 partially deprotonated the films, thereby dismantling the polaron structures associated to the removed dopants.

Figure 1 SEM images of untreated and SC-CO2 treated PANI films doped with different HCl molarities.

Figure 2 UV-Vis absorbance spectra of untreated (a) and SCCO2 treated (b) PANI films doped with different HCl molarities.

3.2 UV-Vis spectra The UV-Vis absorption spectra of untreated films (Fig. 2) peaked at nearly 700 nm and extended to the near-infrared spectra. These absorption profiles indicate that the PANI films are in doped state [17, 18], which confirms the result in the elemental composition analysis. This can be further verified by the absence of absorption bands at 615 nm, which is associated to the PANI base [19]. Moreover, the intensity of the absorption bands above 800 nm, which is linked to the presence of po© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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J. G. Fernando et al.: Effect of SC-CO2 treatment on the polarons of polyaniline films

larons [20], was observed to increase with increasing HCl molarity. This result shows that that films synthesized with higher HCl concentrations are more protonated and contain more number of polarons. The absorption spectral shapes of SC-CO2 treated films are almost identical to the untreated films suggesting that the films remained in doped state after the treatment. However for treated films doped with 0.6 and 1.0 M HCl, the intensity of the absorption band at 800 nm were found lower because of their reduced number of polarons. In addition, the high absorbance in the 800 to 900 nm range indicates that SC-CO2 treatment removed only a fraction of the polarons while leaving most of them undisturbed. Hence, SC-CO2 has no adverse effect on the polaron population due to its limited solvating power for polar molecules such as the HCl dopant. On the other hand, the absorption intensity at 800 nm in treated film doped with 0.2 M HCl appeared higher than the untreated film. This could not be attributed to reprotonation since like in other films its relative amount of Cl decreased after the treatment. Instead, this could be due to the increase in the film thickness caused by swelling when the film was subjected to SC-CO2 [21]. The HOMO-LUMO transitions, referred hereto as bandgaps were determined from the absorption spectra using the Tauc relation [22]. The (αhν)2 was plotted against the photon energy hv (eV) and the linear part of the graph was extrapolated towards the energy axis to find the bandgap. The bandgaps of untreated films doped with 0.2, 0.6 and 1.0 M HCl are 2.24, 2.34 and 2.41 eV, respectively. These bandgap values are close to the reported experimental value of 2.21 eV for doped polyaniline [24]. The bandgaps of SC-CO2 treated films doped with 0.6 and 1.0 M HCl molarities, which are 2.30 and 2.33 eV, respectively tend to be lower than the corresponding untreated films. The change in the bandgaps is possibly due to the reduced level of protonation in treated films. However, the bandgap of the film doped with 0.2 M HCl increased to 2.28 eV that may be due to stronger interaction of carbon dioxide with the polymer chains that led to the modification of the polymer structure [23]. 3.3 Electrical characterization All samples exhibited ohmic behaviour and their conductance were determined by taking the slope of their respective linear IV plots. As seen in Fig. 3, the electrical conductance of the untreated films increased with HCl concentration. This is due to the preferential protonation of imine nitrogen atoms by HCl which forms the polarons, the known charge carriers in polyaniline [3]. After the treatment, the conductance of the films decreased substantially. This shows that the removal of small amount of polarons can significantly alter the electrical properties of the films. It is possible that the polaron lattice in the films was disrupted after the treatment, which makes it difficult for the electrons to flow across the film through the hopping mechanism. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3 Plot of conductance of untreated and SC-CO2 treated films.

3.4 FTIR spectroscopy Figure 4 shows the FTIR spectra of untreated films. The maxima corresponding to C=C stretching of the quinoid (Q) and benzenoid (B) rings are located at 1571 and 1492 cm-1, respectively. Another band appears at 1303 cm-1 belonging to C-N stretching of secondary aromatic amine. The conducting nature of the films was confirmed by the presence of two peaks at 1243 and 1148 cm-1 that correspond to C-N+° stretching and vibrational mode of –NH+=, respectively [25]. The intense peaks found below 1140 cm-1 are linked to the glass substrates.

Figure 4 FTIR spectra of untreated (a) and SC-CO2 treated (b) polyaniline films doped with different HCl molarities.

For SC-CO2 treated films, the main characteristic absorption bands of polyaniline were also observed. The maxima of these bands are not shifted with respect to untreated films except for the film protonated with 0.2 M HCl in which the –NH+= band shifted by 12 cm-1 to 1136 cm-1. The presence of C-N+° stretching vibration band at 1248 cm-1 and the broad absorption band above 1600 cm-1 suggests that SC-CO2 treated films remained protonated even after SC-CO2 treatment which is in agreement with our UV-Vis results. To examine the effect of SC-CO2 treatment on the oxidation level of PANI films, the integrated intensity ratios of quinoid (1571 cm-1) and benzenoid peaks (1492 cm-1) were calculated. The Q/B ratios of treated and untreated films (Fig. 5) are close to unity which implies that the films are all in emeraldine oxidation state [26]. However, the Q/B ratios for treated films are higher which could acwww.pss-c.com

Contributed Article Phys. Status Solidi C 12, No. 6 (2015)

count for the change in the bandgaps of untreated films doped with 0.6 and 1.0 M HCl.

Figure 5 Q/B ratio of treated and untreated PANI films doped with different HCl molarities.

We propose the following mechanism to consolidate the obtained results: when the films were treated with SCCO2, the supercritical fluid dissolved into the polymer matrix that caused the films to swell. The increased free volume allowed the fluid to penetrate deeper into the matrix removing HCl from the polymer backbone. However, only a fraction of the HCl dopant was removed since SC-CO2 has limited solvating power for polar molecules. The benzenoid ring next to the site where HCl was removed was converted back to quinoid ring enhancing the quinoid character and reducing the bandgap of the films. Although most of the polarons are left intact by the treatment, the small amount of polarons removed from the backbone is enough to lower the conductance of the treated films. 4 Conclusion The polaron structures are less affected by the SC-CO2 fluid because of its poor solvating power for HCl. Despite that the films remained conducting after the treatment the conductance decreased as compared to the untreated films. Also, the treatment enhanced the quinoid character of the films which altered the HOMOLUMO transition of the films. Voids started to appear in film doped with 0.6 M HCl and increased in number at higher HCl molarity.

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Acknowledgements We would like to thank the Commission on Higher Education for the scholarship grant of the first author and the CHED-PHERNET GIA Project for funding the project.

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