Charge Transport through Polyaniline Incorporated Electrically

Article pubs.acs.org/JPCC

Charge Transport through Polyaniline Incorporated Electrically Conducting Functional Paper Kartick L. Bhowmik,†,‡ Krishna Deb,† Arun Bera,† Ranendu K. Nath,‡ and Biswajit Saha*,† †

Department of Physics, National Institute of Technology Agartala, Jirania 799046, India Department of Chemistry, Tripura University, Suryamaninagar 799022, India

‡

S Supporting Information *

ABSTRACT: The hopping charge transfer between polarons and bipolarons and metal-like band structure in polaron lattice of polyaniline makes it of high scientific worth. With above views polyaniline has been deposited on paper by a low cost chemical method to make the paper electrically conductive and functional. The prepared conductive papers were characterized using X-ray diffraction measurement, FTIR, UV−vis−NIR spectrophotometric measurement, and electrical conductivity measurement. Impressive electrical properties with good optical transparency were obtained with a highest light transmittance of 94.0% at 550 nm and electrical sheet resistance of 8.92 × 105 Ω/□. In addition, repeated cycles of deformation of the electrically conductive polyaniline incorporated flexible paper do not produce any noticeable change in its electrical conductivity and optical absorbance. The electrically conductive papers are of significant interest in preparing flexible and foldable circuits. The polyaniline-deposited conductive papers are of low cost, and these are environmentally stable.

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INTRODUCTION In the recent advent of rapidly developing material science there is a challenge to reduce the price of devices without compromising their performance. Conducting (conjugated) polymers have attracted considerable attention for both fundamental interest and technological applications. Polymer based functional materials are therefore in extensive study to explore their outstanding properties. Different polymers such as polypyrrole,1 polyaniline,2 polyacetylene,3 poly(p-phenylene-vinylene) (PPV),4 poly(3,4ethylene dioxythiophene) (PEDOT),5 etc., are now attracting the researcher’s attentions because of their excellent physical properties. The unique combination of electrical, electrochemical, thermo-electrical, optical, and other properties of polymers and their composites makes them good candidates for development of supercapacitors,6,7 sensors,8 electronic devices,9 batteries,10 and electrochromic and electroluminescent devices.11,12 These compounds are interesting from the scientific standpoint, concerning the fundamentals of charge transfer processes. Polymers13 and their composites with carbon nanomaterials14−18 are also of a particular interest. The organic conducting polymers are now regarded as a new class of electronic materials. Polyaniline is one of the most promising polymers of this class due to its good chemical and environmental stability, ease of the synthesis, availability, low cost of the monomer, etc. Besides, polyaniline has attracted great deal of attention in the past two decades because of its potential applications in various fields such as electronic material,19,20 gas sensor,21 adsorbent of pollutant of water,22 and pH sensor.23 © 2016 American Chemical Society

In this context, paper based devices may appear as an important candidate particularly with organic polymers. The flexibility of the conducting papers is of great advantage in designing flexible and foldable electronic devices. In this communication we report the synthesis of polyaniline on paper and its characterizations. A number of synthesis processes have been employed by different researchers to synthesize polyaniline. Such important synthesis process include solution polymerization,24 interfacial polymerization,25,26 seeding polymerization,27 vapor phase self-assembling polymerization,28,29 photoinduced polymerization,30,31 plasma polymerization,32,33 sonochemical synthesis,34,35 and electrochemical synthesis36,37 of polyaniline. In this work we have synthesized polyaniline by oxidation of aniline monomer. Usually polyaniline is prepared by oxidation of aniline in solution using appropriate chemical oxidizers38−40 or by electrochemical oxidation.41 In this work the oxidation of aniline was carried out using ferric chloride as oxidizing agent. The prepared conducting and stable polyaniline coated paper has been characterized by analyzing its X-ray diffraction pattern, UV−vis-NIR transmittance spectra, FTIR spectra, and electrical I−V measurements. The results indicate the formation of crystalline and electrically conducting polyaniline coated paper that can be employed to explore paper based cost-effective devices. Received: September 4, 2015 Revised: February 19, 2016 Published: March 3, 2016 5855

DOI: 10.1021/acs.jpcc.5b08650 J. Phys. Chem. C 2016, 120, 5855−5860

Article

The Journal of Physical Chemistry C

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EXPERIMENTAL DETAILS

Synthesis of Polyaniline Coated Paper. Polyaniline was synthesized on clean butter paper (4 cm × 2 cm) through polymerization of aniline monomer. FeCl3 (Merck, Germany) was used as oxidant to polymerize the aniline vapor in a reaction chamber. Twenty percent (w/w) FeCl3 solution was prepared in methanol (Merck, Germany), and to this solution, a butter paper strip was dipped and gently taken out in such a way that a FeCl3 socked butter paper was formed in ambient conditions. Before placing it inside the reaction chamber the paper was allowed a few minutes for the solvent to evaporate. Then the butter paper with the oxidant (FeCl3) film was introduced in the reaction chamber. In the reaction chamber double distilled aniline monomer (Merck, Germany) was taken in a beaker and was maintained at a temperature of 60 °C. The vapor pressure of aniline at 60 °C was ∼10 mmHg. Inside the reaction chamber the aniline monomer vapor comes into contact with the ferric chloride coated butter paper, and gradually a green color was found to appear on the paper, which was due to polymerization of aniline. This process was allowed for 10 min, and the film was then removed from the chamber and washed several times with methanol to remove the biproduct of ferrous chloride and the unreacted ferric chloride and aniline monomer. Finally, the film was dried in oven at 60 °C for 5 min. In this self-assembly process pure polyaniline films are obtained. The thickness and the conductivity of the films can be optimized by judiciously varying the oxidant concentration or the deposition time or the evaporation temperature of aniline. The advantages of vaporphase polymerization are its simplicity and controllability in terms of time of interaction and temperature. It is a solvent-free process and polymer is synthesized by introducing monomers to a surface in its vapor phase. Thus, this process appears considerably useful in preparing conducting polymer and its nanocomposites on special substrate, such as porous structure substrates and paper. The vapor-phase polymerization process also leads to the formation of ordered polymers and sometimes crystalline ones. Characterization of Polyaniline Coated Paper. The prepared polyaniline coated papers were characterized for its structural properties by X-ray diffraction (XRD) pattern analysis obtained from an X-ray diffractometer (XRD, Bruker, D-8 Advance). The Fourier transform infrared spectroscopic measurements (FTIR) of the prepared sample were carried out using FTIR (PerkinElmer). The optical transmittances were measured using a UV−vis−NIR spectrophotometer (Shimadzu UV-3101PC). Electrical properties were studied by conventional four-probe technique using Keithley instrument (model 2400).

Figure 1. Formation and molecular structure of polyaniline.

Figure 2. X-ray diffraction pattern of polyaniline coated paper before washing in alcohol and after washing in alcohol (inset).

crystalline phase of polyaniline deposited on paper is very interesting from a scientific viewpoint of preparing electrically conducting functional paper with excellent flexibility as shown in Figure 3. The (0 2 0) reflection at 2θ = 21.8° is caused by the layers of polymer chains alternating distance. The (200) reflection at 2θ = 25.8° attributed to the periodicity parallel to the polymer chain. The crystallite size (T) of the highest intense crystalline peak was determined from the Scherrer relationship42

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RESULTS AND DISCUSSION Structural Properties. The synthesized polyaniline is in emeraldine salt form with chemical structure as shown in Figure 1. The polyaniline deposited paper was investigated through Xray diffraction measurements to study the crystallinity of the polyaniline deposited on the paper. The X-ray diffraction pattern shown in Figure 2 depicts that the polyaniline appears in polycrystalline form on paper. The X-ray diffraction pattern reveals three intense and sharp peaks. The characteristic peaks of polyaniline occurred at 21.8°, 25.6°, and 34.2° correspond to (020), (200), and (322) Millar planes of polyaniline. It indicates the successful synthesis of polyaniline on paper. This

Figure 3. Photograph of flexible polyaniline coated conducting paper. 5856

DOI: 10.1021/acs.jpcc.5b08650 J. Phys. Chem. C 2016, 120, 5855−5860

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The Journal of Physical Chemistry C T=

Kλ β cos θ

(1)

where K is the shape factor for the average crystallite (∼0.9), λ is the wavelength of the incident X-ray, and B is the full width at half-maxima of the crystalline peak in radian. It is obtained as 6.5 nm. The interchain separation length (R) corresponding to the highest intense crystalline peak was obtained 2.6 Å as calculated using the relationship given by Klug and Alexander43

R=

5λ 8 sin θ

(2)

The crystallinity of polyaniline makes the system more ordered and thereby assisting the polaron bipolaron charge transport over a long-range of polymer chain. The crystallinity of polyaniline thus helps increasing the overall mobility of charge transport system. FTIR Studies. The FTIR spectra of polyaniline sample prepared through polymerization of aniline by FeCl3 are shown in Figure 4. The bonding configurations of polyaniline have

Figure 5. Transmittance spectra of polyaniline coated conducting paper.

is due to its good crystalline nature. The energy band gap is calculated using the fundamental relationship (αhυ)1/ n = A(hν − Eg )

(3)

where A is a constant, Eg is the band gap of the material, and the exponent n depends on the type of transition: n = 1/2, 2, 3/ 2, and 3 corresponding to allowed direct, allowed indirect, forbidden direct, and forbidden indirect, respectively. Taking n = 1/2, we have calculated the direct band gap from (αhν)1/n vs hν plot (Figure 6) by extrapolating the linear portion of the

Figure 4. FTIR spectra of polyaniline and FTIR spectra of the paper (inset).

been obtained from the characteristic peaks in FTIR spectra. The peak at 3442 cm−1 refers to the N−H stretching vibration.44 The characteristic band of PANI at around 1642 cm−1 is assigned to the CO stretching vibration of quinone.45 The bands at 1560 and 1480 cm−1 can be ascribed to the CC stretching vibration of the quinoid and benzenoid rings, respectively.46 The strong band at 1304 cm−1 in the FTIR spectra of polyaniline is the characteristics of C−N stretching vibrations.44 The peak at 1136 cm−1 corresponds to vibration mode of the −NH•+ structure, which is formed during protonation.47 The band corresponding to out of plane bending vibration of C−H bond of p-substituted benzene ring appears at 806 cm−1 indicating that anilines were polymerized by endto-end connection way.48 Optical Properties. The optical transmittance spectra of the polyaniline incorporated paper as obtained from UV−vis− NIR spectroscopic measurements are shown in Figure 5. The polyaniline exhibits its characteristic band of absorbance minimum centered at 547 nm corresponding to the optical energy gap of 2.27 eV. The reasonably sharp fall in the transmittance spectra just below 547 nm at the absorption edge

Figure 6. Band gap of polyaniline from (αhν)2 vs hν plot.

graph to hν axis. The intercept on the hν axis determines the direct band gap. The energy band gap of 2.83 eV was calculated for the polyaniline incorporated paper. The exciton binding energy (Eb) for the polyaniline was determined from the relationship3 E b = Eg − Eo

(4)

For polyaniline, band gap (Eg) = 2.83 eV and optical band gap (Eo) = 2.27 eV, which gives Eb = 0.56 eV. This reasonably large exciton bonding in the polyaniline film favors the formation of polarons/bipolarons and thus supports the p-type conductivity model. 5857

DOI: 10.1021/acs.jpcc.5b08650 J. Phys. Chem. C 2016, 120, 5855−5860

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The Journal of Physical Chemistry C Electrical Properties. Transport properties of electrically conducting polymers involve a number of charge transport mechanisms due to their complex structural and morphological forms. Hopping of charge carriers along and between polymer chains and tunneling between high-conductive crystallites embedded into amorphous matrix and electron phonon interactions play significant roles in the charge transfer mechanism of such polymers. The prepared polyaniline is a mixed oxidation state of polymer consisting of benzoid and oxidized quinoid units. The oxidation state of polyaniline is denoted as 1 − y, whereby the value of y determines the existence of three distinct oxidation states. In the fully reduced state called leucoemeraldine (LE), 1 − y = 0. For the half oxidized emeraldine salt (ES) where 1 − y = 0.5 and for fully oxidized pernigraniline (PE) state 1 − y = 1. The ES state is regarded as the most useful form of polyaniline due to its high stability at room temperature, it is composed of two benzoid units and one quinoid unit that alternate, and it is known to be a semiconductor. The electrical conductivity of the polyaniline coated papers was found with sheet resistance of 8.92 × 105 Ω/□. More interestingly PANI is a p-type semiconductor, and this makes it very attractive because most of the outstanding semiconductors are of n-type. Thus, there is always a necessity for an exceptionally good p-type semiconductor. The p-type conductivity of polyaniline coated paper can be confirmed from the positive sign of the Seebeck coefficient. The Seebeck coefficient at a given temperature is calculated from the eq 5 by linearly fitting ΔV versus ΔT as shown in Figure 7(inset).

ΔV = −SΔT

(5)

Figure 8. SEM image of the polyaniline incorporated paper (a) before bending and (b) after repeated bending.

the formation of polarons and bipolarons increase rapidly contributing to higher values of current through the sample.49 The electrical conduction of polyaniline deposited on paper can be realized from the charge transfer mechanism involved in such system of materials as shown in Figure 9. Polarons (or

Figure 7. Room temperature IV characteristics of polyaniline coated paper and temperature difference vs Seebeck voltage plot (inset).

The negative Seebeck voltage at the hot end indicates the holes are the main carriers. The I−V characteristics plot of the polyaniline coated paper is shown in Figure 7. This electrical behavior of the film retains even after repeated cycle of bending. This is because the repeated cycle of bending does not cause any change in the polyaniline incorporated paper as it can be realized from SEM images shown in Figure 8. The I−V characteristics plot has been found nonohmic in nature. This nonlinear increase in current with applied voltage is explained by the conduction mechanism of polyaniline. Here charge conduction is not only by free carriers (electron and hole) such as in intrinsic semiconductors but it also involves the formation of polarons and bipolarons. As the applied voltage is increased,

Figure 9. Charge transfer mechanism through polaron and bipolaron in polyaniline coated paper. 5858

DOI: 10.1021/acs.jpcc.5b08650 J. Phys. Chem. C 2016, 120, 5855−5860

The Journal of Physical Chemistry C

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bipolarons) are (a type of) charge carrier defect states that are important in conducting polymers such as polyaniline.50−53 A polaron is an electronic carrier self-trapped in a potential well produced by the deformation of the molecule it occupies.54 Its energy is determined by electron−phonon coupling through Coulomb interactions.55,56 In polaron structure, a cation radical of one nitrogen acts as a hole, which can transfer an elemental positive charge. The electron from the adjacent nitrogen (neutral) jumps to this hole, and it becomes electrically neutral initiating motion of the holes through a resonance process. Thus, p-type conductivity is achieved in polyaniline. From Xray diffraction studies it is clear that the polyaniline loaded on paper is crystalline. Therefore, in this system the crystalline polyaniline chains may be regarded as to be loaded on the amorphous matrix of paper. Thus, the charge transport mechanism also involves variable range hopping of charge carriers in the network of chains embedded in the amorphous matrix of paper. The electrical properties were found to remain unaffected along with its optical properties after washing the polyaniline coated paper in ethanol several times. This confirms the strong adhesion of polyaniline with paper.

CONCLUSIONS Paper with outstanding electrical conductivity and flexibility are of great importance as a functional material for recent development of material science. This article thus reports a convenient chemical process of polyaniline synthesis on paper, which can be used for large scale production of conducting polymer on paper. The deposition was carried out through a chemical process of oxidation of aniline vapor using FeCl3. The X-ray diffraction pattern confirms the crystalline phase of polyaniline deposited on amorphous paper. Substantially high p-type conductivity of polyaniline has been reported in this article. The conduction mechanism includes the formation of polaron and bipolaron in the polymer network. This polyaniline incorporated paper with p-type electrical conductivity might appear as a candidate for low cost and large scale applications in polymer based devices. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08650. Photographic view of the reaction chamber used for vapor phase polymerization of aniline (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +919436569904. Author Contributions

The authors K.L.B., K.D., and A.B. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (B.S.) acknowledges the Department of Science and Technology (DST), Government of India for partial financial support (Grant No. SERB/F/7430/2013-14) under Fast Track Young Scientist award scheme. 5859

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