Preparation, Characterization, and Electrochromic ... - ACS Publications

Research Article www.acsami.org

Preparation, Characterization, and Electrochromic Properties of Nanocellulose-Based Polyaniline Nanocomposite Films Sihang Zhang,† Gang Sun,‡ Yongfeng He,† Runfang Fu,† Yingchun Gu,† and Sheng Chen*,† †

Functional Polymer Materials Laboratory, College of Light Industry, Textile and Food Engineering, Sichuan University, Chengdu 610065, China ‡ Fiber and Polymer Science, University of CaliforniaDavis, Davis, California 95616, United States S Supporting Information *

ABSTRACT: On the basis of nanocellulose obtained by acidic swelling and ultrasonication, rodlike nanocellulose/ polyaniline nanocomposites with a core−shell structure have been prepared via in situ polymerization. Compared to pure polyaniline, the nanocomposites show superior film-forming properties, and the prepared nanocomposite films demonstrate excellent electrochemical and electrochromic properties in electrolyte solution. Nanocomposite films, especially the one prepared with 40% polyaniline coated nanocomposite, exhibited faster response time (1.5 s for bleaching and 1.0 s for coloring), higher optical contrast (62.9%), higher coloration efficiency (206.2 cm2/C), and more remarkable switching stability (over 500 cycles). These novel nanocellulose-based nanorod network films are promising novel electrochromic materials with excellent properties. KEYWORDS: polyaniline, nanocellulose, in situ polymerization, nanocomposite, electrochromism by chemical polymerization processes,13 as pure PANI chains are quite rigid and have a very low solubility in water solution or other conventional organic solvents.14 In addition, the PANI molecules in the neat film are closely packed, which hinders the ionic transportation and thus greatly undermines the final electrochromic response of the materials.15 To overcome these limits, some approaches have been developed, such as controlling the repeat-unit structure, adjusting electrodes, electrospinning, formation of hybrid materials, and so on.16,17 Among these methods, hybridizing PANI with other materials has been extensively exploited as an effective method to improve the film-forming and electrochromic properties of PANI. Silva et al.15 prepared hybrid materials based on polyaniline, chitosan, and organically modified clay via in situ polymerization, which showed good electrochromic properties and good adhesion onto ITO glass electrodes as compared with pristine PANI. Luo et al.18 reported a kind of graphene/PANI hybrid hollow sphere which showed high specific capacitance and good cycling stability. Shi et al.19 developed a new route to construct a supramolecular complex of PANI/cellulose, and a highly homogeneous structure and improved mechanical properties of the resultant composite films were obtained. Xia et al.20 had explored TiO2/PANI core/shell nanorod array

1. INTRODUCTION Electrochromism is a reversible optical change in materials upon redox reaction in the presence of an external potential.1 Electrochromic materials have attracted considerable attention due to their potential applications in displays, electrochromic windows, electronic books, antiglare rearview mirrors, etc.2,3 Inorganic oxides such as tungsten trioxide (WO3) and iridium dioxide (IrO2) were first reported to be used as electrochromic materials.4 After decades of research, inorganic electrochromic materials have been widely developed for commercial applications.5 However, inorganic electrochromic materials suffer from many disadvantages, such as long response time, high cost, high oxidative potential, and poor cycling stability.6 Recently, organic electrochromic materials have been extensively studied for their electrochromic properties owing to their low cost, short response time, long cycling stability, and low operation potential.7−9 With the continuing development of organic electrochromic materials, many conducting polymers, such as polyaniline (PANI), polythiophene, polypyrrole, and their derivatives, have been studied for electrochromic application.10 Among the numerous conducting polymers, PANI has attracted the extensive interest of researchers because of the relative low cost of the monomer and its good environmental stability, ease of synthesis, and tunable properties.11 PANI shows an obvious color contrast between the bleached state and colored states in the visible wavelength range (400−800 nm) at different applied voltages.12 However, it is difficult to prepare a neat PANI film © 2017 American Chemical Society

Received: February 25, 2017 Accepted: April 27, 2017 Published: April 27, 2017 16426

DOI: 10.1021/acsami.7b02794 ACS Appl. Mater. Interfaces 2017, 9, 16426−16434

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration for preparation of NC/PANI nanocomposite films. and then quenched by diluting with 200 mL of cold deionized water. The suspension was washed by centrifuging (Jintan Medical Instrument Factory) at 4000 rpm for 10 min several times, until the pH of the supernatant was neutral. Then acid-swollen cellulose fibers were obtained. About 150 mL of a suspension containing acid-swollen cellulose fibers was placed in a JY98-III DN ultrasonic generator (Ningbo Scientz Biotechnology Co., Ltd.) of 20−25 kHz frequency and equipped with a cylindrical titanium alloy probe tip (1.5 cm). The subsequent ultrasonication was conducted for 30 min at an output power of 1000 W, resulting in a nanocellulose fiber suspension. The suspension was filtered to remove large-scale cellulose fibers and impurities. To prevent overheating, ultrasonic treatment was carried out in an ice/water bath and stopped for 2 s every 2 s. The final concentration of the suspension was 0.75 wt %, which was obtained by rotary evaporation. The NC/PANI nanocomposites were prepared according to our reported procecdure with a modification.29 A typical example is described as follows: An aniline (ANI) solution was first prepared by dissolving aniline (0.08 g) in 6.314 g of concentrated HCl (37 wt %), and then it was mixed with a certain amount of the freshly prepared NC suspension to obtain 64 mL of a homogeneous reaction mixture dispersion. The obtained aqueous dispersion was put in a vial in a lowtemperature water bath (10 °C). After stirring for 10 min, 0.132 g of APS dissolved in 24 mL of HCl (1 mol/L) solution was added with vigorous shaking to initiate polymerization of aniline. After polymerization at 10 °C for 4 h, a dark green suspension was formed. Finally, the suspension was washed several times by centrifugation with deionized water. According to different contents of ANI, the NC/ PANI composites containing 20%, 40%, 60%, or 80% ANI were named PN20, PN40, PN60, or PN80, respectively. As a reference, the pure PANI was prepared by the same method, except that no nanocellulose was introduced. 2.3. Preparation of NC/PANI Nanocomposite Film Electrodes. The ITO substrate was rinsed with petroleum ether, acetone, ethanol, and deionized water in sequence to remove contaminants from the surface. The cleansed ITO substrate was dried and directly used as the substrate for the NC/PANI nanocomposite films. The NC/PANI thin film electrodes were prepared by dip-coating about 0.1 mL of NC/PANI aqueous dispersion (solid content of 0.2 wt %) onto the ITO glass with an active area = 1.0 × 1.5 cm and dried at room temperature. The thickness of all the film samples is about 5.2 ± 0.4 μm measured by micrometer. The area density of all film samples was calculated to be about 1.33 ± 0.10 g/m2. The typical process for the fabrication of a NC/PANI film electrode is illustrated in Figure 1. 2.4. Characterizations. The morphology of the nanocellulose and nanocomposites was studied using field emission scanning electron microscopy (FE-SEM) (JSM-5900LV, JEOL) and transmission

heterostructures as electrochromic materials, which showed four color modes and fast optical switching speed. Cellulose is one of the most important biomaterials and chemical raw materials for human beings due to its low cost, biodegradability, sustainability, and abundance.21 Currently, intense research and development are focused on the isolation and application of nanoscale cellulose fibers.22 Nanocellulose (NC), which can be prepared from natural cellulose, with size ranging from a few to tens of nanometers in one dimension, has some unique properties, including its renewability, excellent mechanical properties, high specific surface area, biodegradability, and biocompability,23 rendering it good reinforcement for natural and synthetic polymer matrices.24−26 Moreover, nanocellulose, rich in hydroxyl groups, has good affinity with a wide variety of polymers, including conducting polymers.27,28 Therefore, combining cellulose nanowhiskers and PANI is a promising strategy to develop novel functional PANI nanocomposite films and apply them in electrochromic devices. In the present work, we report a facile process for preparing nanocellulose/polyaniline (NC/PANI) nanocomposites in a core−shell structure and their improved electrochromic properties. The NC was prepared by a combination method of mild acidic swelling and intensive ultrasonication. The NC/ PANI nanocomposites containing a variety PANI contents ranging from 20% to 80% have been successfully developed through in situ polymerization of aniline in a nanocellulose suspension, and the electrochromic properties of the NC/PANI nanocomposite films on ITO glass were measured. Compared with the pure PANI film, the NC/PANI nanocomposite films showed better film-forming properties and significantly enhanced electrochromic behaviors.

2. EXPERIMENTAL SECTION 2.1. Materials. Cotton pulp was provided by Yibin Grace Group Co. (NH4)2S2O8, H2SO4, HCl, aniline, and ammonium persulfate (APS) were analytical reagents, purchased from Kelong Chemical Reagent Co. Transparent indium tin oxide (ITO) coated glass was used as a substrate with a sheet resistance of 15 Ω/□, provided by Foshan Meijingyuan Glass Technology Co., Ltd. Deionized water was used throughout the experiment. 2.2. Preparation of NC and NC/PANI Nanocomposites. Cotton pulp board was first cut into small pieces that then were added to 17 mL of 64 wt % sulfuric acid at 40 °C, stirred for 25 min, 16427


Research Article


Figure 2. SEM images of NC (a), PN20 (b), PN40 (c), PN60 (d), PN80 (e), and pure PANI (f) and TEM images of NC (g) and PN40 (h).

Figure 3. FTIR spectra of raw cellulose, NC, pure PANI, and PN40. electron microscopy (TEM) (Tecnai G2 F20 S-TWIN, FEI). Fourier transform infrared spectra (FTIR) were recorded on a Tracer100 spectrometer (Shimadzu Corp.). Thermal gravimetric analysis (TGA) was measured under a nitrogen atmosphere with a DTA-60 thermal analyzer (Shimadzu Corp.). The electrochromic properties of the NC/PANI nanocomposite film were measured by using a CS150 electrochemical workstation (Wuhan Corrtest Instrument Co., Ltd., at room temperature in a homemade colorimetric cell whose volume is 3 × 4 × 3 cm at room temperature). A three-electrode system was used, consisting of the NC/PANI-coated ITO glass as the working electrode (W-E), platinum as the counter electrode (C-E), and a saturated calomel electrode as the reference electrode (R-E), and 0.25 M HCl solution was used as the electrolyte. Ultraviolet−visible (UV−vis) absorption spectra were recorded on a TU-1900 UV−vis spectrometer (Beijing Purkinje General Instrument Co., Ltd.).

Figure 2b−f show the morphologies of nanocellulose-based composites containing (b) 20% PANI, (c) 40% PANI, (d) 60% PANI, (e) 80% PANI, and (f) pure PANI (100% PANI). The TEM image of PN40 is shown in Figure 2h. Figure 2h shows the rodlike nanocellulose as the matrix materials and polyaniline coating the surface of the nanocellulose to form the core− shell structure. The lengths of nanocomposite particles range from 100 to 300 nm. The average diameters of PN40 composites are about 38.82 nm, which are higher than that of the pure cellulose nanowhiskers. It is further proved that the PANI was coated on the surface of cellulose nanowhiskers to form the core−shell structure of nanocomposites. Figure 2f shows that the structure of pure PANI film prepared by chemical oxidative polymerization without nanocellulose is relatively uniform and compact. The particles of polyaniline are small and stacked together compactly. From Figure 2b−e, the average diameters of PN20, PN40, PN60, and PN80 are measured to be about 32.92 ± 3.52, 38.82 ± 4.48, 54.14 ± 4.63, and 55.11 ± 8.60 nm by Nano Measurer software. There are a lot of small holes which can be found in the NC/PANI composite film, which is probably due to the network structure formed by rodlike nanocomposites. This should be good for the injection and extraction of electrolyte ions during the electrochemical redox process. Figure 2d,e shows that when the content of PANI in the composites is higher, extra ANI monomers were self-polymerized to form an increased amount

3. RESULTS AND DISCUSSION 3.1. Morphology. The typical morphologies of nanocellulose are shown in Figure 2a,g. The pure nanocellulose film is composed of randomly oriented rodlike cellulose nanowhiskers. The diameters of rodlike nanocellulose range from 10 to 35 nm, and the average diameter is about 28.36 ± 4.03 nm, measured by Nano Measurer software. Their lengths of them are about 100−300 nm. The diameter distribution of obtained nanocellulose is comparatively narrow. The results show that the nanocellulose was successfully prepared by combining swelling and ultrasonic treatments. 16428


Research Article


Figure 4. (a) TGA curves of raw cellulose, NC, pure PANI, and PN40, respectively. (b) CV curves of pure PANI, PN80, PN60, and PN40, respectively. Electrolytic solution was 0.25 M HCl and scan rate 100 mV/s. (c) CV curves of PN40 in 0.25 M HCl at different potential scan rates: 5, 10, 20, 50, 100, and 200 mV/s. (d) The relationships between the oxidation peak current density vs potential scan rate.

of the C−H band of the para-disubstituted benzene ring appears at 793 cm−1.32 The characteristic peaks of NC (3341 cm−1) and PANI (1567, 1484, 1293, and 793 cm−1) are both present in the spectrum of the NC/PANI nanocomposites. The absorption peak at 3412 cm−1 in the spectrum of pure NC is shifted to 3270 cm−1 in the spectrum of the NC/PANI nanocomposite. These indicate that the nanocellulose is successfully activated by acids and the intermolecular hydrogen bands are broken.29 Thus, more hydroxyl groups in NC are accessible and it helps to form the uniform dispersion of PANI on the surfaces of NC. Combining SEM, TEM, and FTIR analyses, it could be concluded that PANI was successfully formed on the surface of NC upon in situ polymerization. 3.3. Thermal Stability. TGA curves of raw cellulose, NC, pure PANI, and NC/PANI were recorded from 50 to 500 °C at heating rate of 10 °C/min, as shown in Figure 4a. All the TGA curves could be divided into three stages. During the initial stage from 50 to 120 °C, the moisture present in the samples containing cellulose vaporized, and the dopant HCl in PANI was removed.33 The main weight losses of raw cellulose and NC occur at the second stage from 120 to 400 °C. The onset temperatures of thermal−oxidative degradation of NC and PN40 were observed at 162 and 133 °C, respectively, lower than that of raw cellulose (above 300 °C). The low degradation temperature of NC means that the decomposition of nanocellulose would require less thermal energy, which could be caused by the smaller transverse dimension of nanofibers and, consequently, more surface areas being exposing to heat as compared to the raw cellulose. Additionally, surface carboxyl groups and sulfate groups are expected to cause the lower degradation temperature for NC by direct solid-to-gas phase transitions from decarboxylation of surface carboxyl groups.34 A massive weight loss of NC/PANI nanocomposite is observed at

of polyaniline particles in solution rather than on the surface of NC. In contrast, when the content of PANI in the composites is low, almost all ANI monomers were polymerized on the surface of NC to form rodlike nanocomposites with core−shell structure, as shown in Figure 2b,c, which are beneficial to improve the film-forming properties of the nanocomposites. 3.2. Chemical Structure. The FTIR spectra of raw cellulose, NC, pure PANI, and PN40 nanocomposites are shown in Figure 3. The obtained NC presents the usual cellulose IR band, and the typical features of PANI have already been described in the literature.30,31 For the FTIR spectra of raw cellulose and NC, the characteristic broad band of the O− H group in cellulose appears at 3341 cm−1. The peak around 2900 cm−1 is due to the asymmetrically stretching vibration of C−H, and the band at 1649 cm−1 resulted from the H−O−H bending of the absorbed water. The peak at 1371 cm−1 is attributed to the O−H bending, and the characteristic peak at 1164 cm−1 corresponds to the C−O antisymmetric bridge stretching. A strong band at 1061 cm−1 is due to the C−O−C pyranose ring skeletal vibration. Comparing the FTIR spectra of pure cellulose and NC, there is no new absorption peak that appears for NC, which indicates that no new functional groups were generated during the acid-swelling treatment and ultrasonication process and that the original molecular structure of cellulose is maintained. For the FTIR spectra of PANI and NC/PANI nanocomposites, the peaks around 1484 and 1567 cm−1 originate from the stretching vibration of N−A−N and NBN structures, respectively (where A and B represent benzenoid and quinoid moieties in the PANI chains). The peaks at 1126 and 1293 cm−1 have been observed because of the vibration of C−H in the benzene ring and the stretching of the C−N band. The peak corresponding to the out-of-plane bending vibration 16429


Research Article


Figure 5. UV−vis absorption spectra of pure PANI (a), PN80 (b), PN60 (c), PN40 (d), and PN20 (e) films measured at different potentials (−0.2 to +1.0 V with a step size of 0.2 V) in 0.25 M HCl solution. (f) Photographic images of pure PANI and PN40 films at different applied potentials.

NC contents, the first anodic peak potential (Epa1) decreases initially and then increases. On the contrary, the second cathodic peak potential (Epc2) increases initially and then decreases. The results indicate that when the content of NC in the nanocomposite (e.g., PN80) is low, the pores in the nanocomposite film (as shown in Figure 2e) facilitate the transmission of ions, and PANI formed on the surface of NC can be oxidized at lower potential or reduced at higher potential. However, with the increase of nonconductive NC content (e.g., PN60 and PN40), films become more dense and uniform and their resistance increases also, which results in higher oxidation potential and lower reduction potential. The current densities of NC/PANI films are larger than that of pure PANI film. With the increase of NC content, the current densities of NC/PANI nanocomposite films increase obviously, the redox peaks become sharp and narrow, and a larger area of closed CV curves appears, which exhibits the higher specific capacitances of NC/PANI nanocomposite electrodes. These may be due to the special structure of NC/ PANI nanocomposite films. The nanoscale architectures of the NC/PANI composites with the network interwoven structures significantly enhance the interface areas of electrode materials and electrolyte, which makes the electrolyte ions easily pass

155 °C, but the decarboxylation of the NC should be at 180 °C. Another possible reason is the weakened inter- and intramolecular hydrogen bonding of NC in the NC/PANI nanocomposite.35 The final stage in weight loss is observed in the region of 400−500 °C, where most samples have been carbonized. Compared with raw cellulose (7.4%), PANI, NC/ PANI, and NC showed much higher char residues, which are 81.1, 47.2, and 37.2%, respectively. The result shows that the thermal stability of PN40 is better than that of NC and weaker than that of pure PANI. 3.4. Electrochromic Properties of Pure PANI and NC/ PANI Nanocomposite Films. 3.4.1. Cyclic Voltammetric (CV) Analysis. Figure 4b displays CV curves of PN40, PN60, and PN80 nanocomposite films and pure PANI film from −0.4 to +1.2 V, performed in 0.25 M HCl solution at a scan rate of 100 mV/s. As the content of PANI is low, PN20 film shows a poor electrochromic response with very low current and an irregular CV curve. This is due to nanocellulose being an insulator, and the addition of a large amount of nanocellulose might lead to the poor conductivity of the composite film and the difficulty in ion transport during the electrochromic redox process.36 So PN20 is not shown in Figure 4b. Comparing the CV curves of pure PANI and NC/PANI, with the increase of 16430


Research Article

ACS Applied Materials & Interfaces through the films formed by loose fibrous NC/PANI. So with the addition of NC in the composites, fluency conductive paths are obtained, which results in the rapid transportation of the electrolyte ions during charge/discharge processes and the improved electrochemical properties of the NC/PANI nanocomposites.37−39 Figure 4c displays cyclic voltammograms of PN40 nanocomposite film, which were performed in 0.25 M HCl solution at different potential scan rates (5−200 mV/s). The CV curves were recorded from −0.4 to +1.2 V. There are two pairs of redox peaks of PANI (A1/C1 and A2/C2) in all the CV curves, typical redox characteristics of PANI, appearing in all the CV curves. The peaks (A1/C1) around +0.2 and −0.1 V are associated with the redox process between leucoemeraldine base (LB) and emeraldine salt (ES) states of PANI, which transform reversibly from yellow to green. The peaks (A2/C2) around +0.7 and +0.5 V are associated with the redox process between ES and pernigraniline base (PB) salt states of PANI, which transform reversibly from green to blue or purple,40 and the phenomenon is in accordance with the result in the literature.41 From Figure 4d, it is clear that the current densities of redox peaks increased with potential scan rates in the region of 5−200 mV/s. For Figure 4d1, the current densities show a linear relationship with potential scan rates in the region of 0− 50 mV/s, indicating that redox processes are controlled by counterion insertion−deinsertion processes.42,43 However, the current densities show a new linear relationship as the scan rates are higher than 50 mV/s, and the redox processes tend to be limited by ion diffusion from the electrolyte solutions to the electrode surfaces.44 3.4.2. Spectroelectrochemistry. The UV−vis spectra of pure PANI, PN80, PN60, PN40, and PN20 films in 0.25 M HCl solution at different potentials is shown in Figure 5a−e. With the increase of applied potentials, the absorption around 423 nm increases from −0.2 to +0.2 V and then drops from +0.2 to +1.0 V. This can be explained by the formation of bipolaronic species at higher potentials, which results in decreasing polaronic population. A distinct blue-shift of the maximum absorption wavelength at around 700 nm of the NC/PANI occurs with increasing potentials from −0.2 to +1.0 V. At lower potentials (−0.2 to 0 V), the absorbance band around 825 nm makes the nanocomposite films exhibit pale yellow color, as PANI is in a reduced state. At medium potentials (+0.2 to +0.6 V), two absorbance bands around 770 and 423 nm result in the green color of the nanocomposite films, which is partially oxidized in the intermediate state. At higher potentials (+0.8 to +1.0 V), a characteristic absorbance band around 665 nm is related to the π−π* transition in the quinoid ring of PANI, which is totally oxidized as the pernigraniline structure form. The phenomenon is in accordance with the results in the literature.45−47 The nanocomposite film displays blue or dark blue color. The results are consistent with the results of cyclic voltammetry. The pure PANI film and the NC/PANI nanocomposite films show similar properties in the UV−vis spectra. In comparison with the UV−vis spectra of all of the films, including pure PANI, PN80, PN60, PN40, and PN20, the absorbance modulations (ΔA) of all the films at λ = 665 nm are 0.28, 0.37, 0.59, 0.64, and 0.09, respectively. Except for PN20, all the NC/PANI composite films have larger ΔA than pure PANI. PN40 film has the largest ΔA compared with all the other films. The contrast ratio of the NC/PANI films increases with the content of NC in the composite, except for PN20. It is indicated that the electrochromic properties of PANI are

improved by the combination of NC. However, the UV−vis spectra of PN20 showed almost no change at different potentials. This is because nanocellulose is an insulative material, and the high cellulose content of PN20 may lead to poor conductivity and a low electrochromic property. Photographic images of pure PANI and PN40 films at a bleaching state (−0.2 V), intermediate state (+0.4 V), and coloring state (+0.8 V) are shown in Figure 5f. At −0.2 V, the pure PANI and PN40 films are fully reduced and present almost transparent color. At +0.4 V, the pure PANI and PN40 films change to partially oxidized and show light green color. When increasing the applied potential to +0.8 V, the pure PANI and PN40 films become fully oxidized and show very dark blue and dark blue color, respectively. The color difference is due to the partially excessive oxidation of the pure PANI film at +0.8 V.43 However, for the nanocomposite film, the excessive oxidation is avoided because of its excellent ion transport efficiency, and nanocellulose shows a protective effect in NC/ PANI composite films that depresses the formation of the highly oxidized PANI.36,48 From Figure 5f, it is obvious that the film uniformity and color contrast of PN40 are better than that of pure PANI, indicating that the combination of NC and PANI improves the film-forming properties and electrochromic behaviors of PANI. 3.4.3. Electrochromic Performance. Color-switching response tests of pure PANI and NC/PANI nanocomposite films in 0.25 M HCl solution were performed by the transmittance change at 665 nm (the maximum absorption peak wavelength), as shown in Figure 6. The optical transmittance spectra were

Figure 6. Kinetic optical transmittance curves at 665 nm of PN20, PN40, PN60, PN80, and pure PANI films, switched between −0.2 and +0.8 V for 4 s at each step in 0.25 M HCl solution.

measured with switched potentials between −0.2 and +0.8 V for 4 s at each step. The optical contrast value (ΔT) is the difference between colored states (Tc) and bleached states (Tb). The response time is defined as the time required for 90% transmittance change in the coloring process (τc) and bleaching process (τb).49 The optical contrast and the response time listed in Table 1 are important electrochromic parameters for electrochromic materials. The response times of the samples were all lower than 4 s, which meets the requirement of electrochemic devices (0.1−10 s). It is evident that the thickness of the shell increased as the content of PANI in the composite films increased. For PN20 and PN40, with the thickness of the shell increased, the electrochromic behaviors of composite film increased. This is due to the conductivity of 16431


Research Article


3.4.4. Cycling Stability. The electrochemical stability of pure PANI and PN40 films was investigated by single-wavelength spectrophotometry at λ = 665 nm for cycles 1−5 and 501−505, as shown in Figure 7. The current−time curves and corresponding transmittance-cycle number curves were obtained with switched potentials between −0.2 and +0.8 V with a 4 s delay at each state. The electrochromic parameter values were calculated from Figure 7 and are listed in Table 2. The processes of coloration-switching response of pure PANI and PN40 are different. Coloration efficiency (CE), which is also called electrochromic efficiency, is defined as the change in optical density (ΔOD) induced by unit charge density (Q).51 The formulas for calculation are

Table 1. Electrochromic Parameters of PN20, PN40, PN60, PN80, and Pure PANI at 665 nm sample

PANI content (%)

Tc/Tb

ΔT (%)

τc (s)

τb (s)

PN20 PN40 PN60 PN80 PANI

20 40 60 80 100

53.5/56.6 19.2/82.1 21.5/78.8 32.8/78.5 33.4/70.8

3.1 62.9 57.3 45.7 37.4

3.5 1.0 1.2 1.1 1.5

3.4 1.5 1.0 1.6 1.5

composite films gradually increasing. For PN40, PN60, PN80, and pure PANI, with the thickness of the shell increased, most of the PANI self-polymerized together instead of onto the surfaces of nanocellulose, which is harmful to ion diffusion and formation of a porous structure. The electrochromic performances of composite films gradually decreased. This result is consistent with the results of Figure 2. Table 1 shows that the response times τc and τb of PN40 and PN60 are lower than that of pure PANI. The lower response time of the nanocomposite films is expected and is attributed to the porous film structures formed by core−shell nanorods and the rapid ion transportation performance,38 as shown in Figures 2 and 5. The optical contrast (ΔT) of PN40 reached about 63%, while it is only about 37% for pure PANI film. The contrast ratio and response speed of PN40 are significantly better than those of PN20. This is due to the remarkably higher electrical conductivity of PN40 than of PN20, which is in accordance with the results in the literature.31 Except for PN20, the transmittances at the bleached states of the NC/PANI nanocomposite films are larger than that of pure PANI. In addition, the electrochromic performances of NC/PANI composite films are also better than many of PANI-based composite films reported in the literature (Supporting Information, Table S1). The improved contrast is due to the improved film-forming properties of the composites and the porous space among the nanocomposites, which can make the ion diffusion easier and provide larger surface area for redox reactions.50

CE = (ΔOD)/Q

(1)

ΔOD = log(Tb/Tc)

(2)

where Tb and Tc refer to the transmittance of the films in its bleached and colored state, respectively. CE is an important criterion to evaluate the electrochromic performance. The calculated CE values of PN40 films are 206.2 cm2/C (cycles 1− 5) and 176.3 cm2/C (cycles 501−505), which are much higher than those of pure PANI (28.4 cm2/C in cycles 1−5 and 22.2 cm2/C in cycles 501−505). The higher CE values of NC/PANI nanocomposite films indicate that a small amount of charges injected per area is necessary to obtain an obvious change of color. These results demonstrated that, concerning energy economy, NC/PANI nanocomposites are promising materials for application in electrochromic devices. After 500 switched cycles, the change of response times of pure PANI is larger than those of PN40. And the ΔT and CE values of pure PANI decrease by 39.8% and 21.8%, respectively, while those of PN40 decrease only by 21.3% and 14.5% respectively. The cycling stability of PN40 film is also better than many reported results in the literature (Supporting Information, Table S1). The above results reveal that NC/PANI nanocomposite films exhibit better electrochromic properties and cycling stability, which further proves the advantages of the novel core/shell

Figure 7. Potential, chronoamperometry, and transmittance curves of pure PANI film (a) and PN40 nanocomposite film (b) with applied potential of −0.2 and +0.8 V for 500 cycles, respectively. 16432


Research Article

ACS Applied Materials & Interfaces Table 2. Electrochromic Parameters of Pure PANI and PN40 Films at 665 nm for Cycles 1−5 and 501−505 sample

PANI content (%)

cycles

Tc (%)

Tb (%)

ΔT (%)

Q (mC/cm2)

CE (cm2/C)

τc (s)

τb (s)

PANI

100

PN40

40

1−5 501−505 1−5 501−505

33.4 39.3 19.2 25.0

70.8 61.8 82.1 74.5

37.4 22.5 62.9 49.5

11.5 8.8 3.1 2.7

28.4 22.2 206.2 176.3

1.5 2.0 1.0 1.2

1.5 2.0 1.5 1.6

Organic Electrochromic Devices. Sol. Energy Mater. Sol. Cells 2017, 163, 200−203. (3) Chotsuwan, C.; Asawapirom, U.; Shimoi, Y.; Akiyama, H.; Ngamaroonchote, A.; Jiemsakul, T.; Jiramitmongkon, K. Investigation of The Electrochromic Properties of Tri-block Polyaniline-polythiophene-polyaniline Under Visible Light. Synth. Met. 2017, 226, 80−88. (4) Deb, S. K. A Novel Electrophotographic System. Appl. Opt. 1969, 8, 192−195. (5) Song, X.; Dong, G.; Gao, F.; Xiao, Y.; Liu, Q.; Diao, X. Properties of NiOx and Its Influence upon All-thin-film ITO/NiOx/LiTaO3/ WO3/ITO Electrochromic Devices Prepared by Magnetron Sputtering. Vacuum 2015, 111, 48−54. (6) Tong, Z.; Li, N.; Lv, H.; Tian, Y.; Qu, H.; Zhang, X.; Zhao, J.; Li, Y. Annealing Synthesis of Coralline V2O5 Nanorod Architecture for Multicolor Energy-efficient Electrochromic Device. Sol. Energy Mater. Sol. Cells 2016, 146, 135−143. (7) Medrano-Solís, A.; Nicho, M. E.; Hernández-Guzmán, F. Study of Dual Electrochromic Devices Based on Polyaniline and Poly (3hexylthiophene) Thin Films. J. Mater. Sci.: Mater. Electron. 2017, 28, 2471−2480. (8) Nunes, M.; Araújo, M.; Fonseca, J.; Moura, C.; Hillman, R.; Freire, C. High-Performance Electrochromic Devices Based on Poly[Ni(salen)]-Type Polymer Films. ACS Appl. Mater. Interfaces 2016, 8, 14231−14243. (9) Carbas, B. B.; Kivrak, A.; Kavak, E. Electrosynthesis of A New Indole Based Donor-acceptor-donor Type Polymer and Investigation of Its Electrochromic Properties. Mater. Chem. Phys. 2017, 188, 68−74. (10) Beaujuge, P. M.; Reynolds, J. R. Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices. Chem. Rev. 2010, 110, 268−320. (11) Wu, X.; Zhang, W.; Wang, Q.; Wang, Y.; Yan, H.; Chen, W. Hydrogen Bonding of Graphene/polyaniline Composites Film for Solid Electrochromic Devices. Synth. Met. 2016, 212, 1−11. (12) Zhang, W.; Ju, W.; Wu, X.; Wang, Y.; Wang, Q.; Zhou, H.; Wang, S.; Hu, C. Structure, Stability and Electrochromic Properties of Polyaniline Film Covalently Bonded to Indium Tin Oxide Substrate. Appl. Surf. Sci. 2016, 367, 542−551. (13) Liu, D. Y.; Sui, G. X.; Bhattacharyya, D. Synthesis and Characterisation of Nanocellulose-based Polyaniline Conducting Films. Compos. Sci. Technol. 2014, 99, 31−36. (14) Bhadra, S.; Singha, N. K.; Khastgir, D. Effect of Aromatic Substitution in Aniline on the Properties of Polyaniline. Eur. Polym. J. 2008, 44, 1763−1770. (15) Silva, R. C.; Sarmento, M. V.; Nogueira, F. A. R.; Tonholo, J.; Mortimer, R. J.; Faez, R.; Ribeiro, A. S. Enhancing The Electrochromic Response of Polyaniline Films by The Preparation of Hybrid Materials Based on Polyaniline, Chitosan and Organically Modified Clay. RSC Adv. 2014, 4, 14948−14955. (16) Pud, A.; Ogurtsov, N.; Korzhenko, A.; Shapoval, G. Some Aspects of Preparation Methods and Properties of Polyaniline Blends and Composites with Organic Polymers. Prog. Polym. Sci. 2003, 28, 1701−1753. (17) Fatyeyeva, K.; Pud, A. A.; Bardeau, J. F.; Tabellout, M. Structure−property Relationship in Aliphatic Polyamide/polyaniline Surface Layered Composites. Mater. Chem. Phys. 2011, 130, 760−768. (18) Luo, J.; Ma, Q.; Gu, H.; Zheng, Y.; Liu, X. Three-dimensional Graphene-polyaniline Hybrid Hollow Spheres by Layer-by-layer Assembly for Application in Supercapacitor. Electrochim. Acta 2015, 173, 184−192. (19) Shi, X.; Zhang, L.; Cai, J.; Cheng, G.; Zhang, H.; Li, J.; Wang, X. A Facile Construction of Supramolecular Complex from Polyaniline

nanocomposites. The improved electrochromic properties are mainly attributed to the core/shell structure, improvement of the film-forming properties, and the porous space among the nanorod composites, which can reduce the oxidation potential, make ion diffusion easier, and provide larger surface area for redox reactions.36,46,52 Besides, the presence of nanocellulose might have retarded the formation of the highly oxidized PANI and hence improved the durability.36,48

4. CONCLUSIONS A series of NC/PANI nanocomposites containing different contents of PANI have been prepared successfully via in situ polymerization. The uniform porous films were constructed by the interwoven core−shell NC/PANI nanorods with an average diameter of over 30 nm and average length of about 200 nm. The unique core−shell structure enhanced the transmission rate of electrolyte ions and the redox reaction rate of the electrochromic materials. The NC/PANI nanocomposite films showed significant color changes (yellow, green, and blue) at different electrical potentials. As compared with pure PANI film, the NC/PANI nanocomposite films exhibited higher optical contrast values, higher coloration efficiencies, lower response times, and better electrochromic stabilities. The film with a composition of PN40 demonstrated the highest transmittance difference (ΔT 62.9%) and the fastest response speed for coloring and bleaching (1.0 and 1.5 s) and was preferred as an electrochromic material.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02794. A comparison of PANI-based electrochromic films properties (Table S1) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 28 85401805. E-mail: [email protected]. ORCID

Sheng Chen: 0000-0002-9428-3675 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) (51003064) and Sichuan Province Science and Technology Foundation (2014JY0146).

■

REFERENCES

(1) Mishra, S.; Pandey, H.; Yogi, P.; Saxena, S. K.; Roy, S.; Sagdeo, P. R.; Kumar, R. Interfacial Redox Centers as Origin of Color Switching in Organic Electrochromic Device. Opt. Mater. 2017, 66, 65−71. (2) Tsuneyasu, S.; Watanabe, Y.; Nakamura, K.; Kobayashi, N. In Situ Measurements of Electrode Potentials of Anode and Cathode in 16433


Research Article

ACS Applied Materials & Interfaces and Cellulose in Aqueous System. Macromolecules 2011, 44, 4565− 4568. (20) Xia, X.; Chao, D.; Qi, X.; Xiong, Q.; Zhang, Y.; Tu, J.; Zhang, H.; Fan, H. J. Controllable Growth of Conducting Polymers Shell for Constructing High-quality Organic/inorganic Core/shell Nanostructures and Their Optical-electrochemical Properties. Nano Lett. 2013, 13, 4562−4568. (21) Ioelovich, M. Cellulose as a Nanostructured Polymer: A Short Review. Bioresources 2008, 3, 1403−1418. (22) Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of NatureBased Materials. Angew. Chem., Int. Ed. 2011, 50, 5438−5466. (23) Islam, M. T.; Alam, M. M.; Patrucco, A.; Montarsolo, A.; Zoccola, M. Preparation of Nanocellulose: A Review. AATCC J. Res. 2014, 1, 17−23. (24) Nishino, T.; Takano, K.; Nakamae, K. Elastic Modulus of the Crystalline Regions of Cellulose Polymorphs. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1647−1651. (25) Liu, D. Y.; Yuan, X. W.; Bhattacharyya, D.; Easteal, A. J. Characterisation of Solution Cast Cellulose Nanofibre - Reinforced Poly(lactic acid). eXPRESS Polym. Lett. 2010, 4, 26−31. (26) Lin, N.; Yu, J.; Chang, P. R.; Li, J.; Huang, J. Poly(butylene succinate)-Based Biocomposites Filled with Polysaccharide Nanocrystals: Structure and Properties. Polym. Compos. 2011, 32, 472−482. (27) Johnston, J. H.; Kelly, F. M.; Burridge, T.; Borrmann, T. Hybrid Materials of Conducting Polymers with Natural Fibres and Silicates. Int. J. Nanotechnol. 2009, 6, 312−328. (28) Richardson, M. J.; Johnston, J. H.; Borrmann, T. Electronic Properties of Intrinsically Conducting Polymer-Cellulose Based Composites. Curr. Appl. Phys. 2006, 6, 462−465. (29) Chen, S.; Sun, G. High Sensitivity Ammonia Sensor Using a Hierarchical Polyaniline/Poly(ethylene-co-glycidyl methacrylate) Nanofibrous Composite Membrane. ACS Appl. Mater. Interfaces 2013, 5, 6473−6477. (30) Mo, Z. L.; Zhao, Z. L.; Chen, H.; Niu, G. P.; Shi, H. F. Heterogeneous Preparation of Cellulose−Polyaniline Conductive Composites with Cellulose Activated by Acids and Its Electrical Properties. Carbohydr. Polym. 2009, 75, 660−664. (31) Luong, N. D.; Korhonen, J. T.; Soininen, A. J.; Ruokolainen, J.; Johansson, L. S.; Seppälä, J. Processable Polyaniline Suspensions Through in Situ Polymerization onto Nanocellulose. Eur. Polym. J. 2013, 49, 335−344. (32) Kulkarni, M. V.; Viswanath, A. K.; Marimuthu, R.; Seth, T. Spectroscopic, Transport, and Morphological Studies of Polyaniline Doped with Inorganic Acids. Polym. Eng. Sci. 2004, 44, 1676−1681. (33) Wang, H.; Zhu, E.; Yang, J.; Zhou, P.; Sun, D.; Tang, W. Bacterial Cellulose Nanofiber-Supported Polyaniline Nanocomposites with Flake-Shaped Morphology as Supercapacitor Electrodes. J. Phys. Chem. C 2012, 116, 13013−13019. (34) Jiang, F.; Hsieh, Y. L. Chemically and Mechanically Isolated Nanocellulose and Their Self-assembled Structures. Carbohydr. Polym. 2013, 95, 32−40. (35) Hu, W.; Chen, S.; Yang, Z.; Liu, L.; Wang, H. Flexible Electrically Conductive Nanocomposite Membrane Based on Bacterial Cellulose and Polyaniline. J. Phys. Chem. B 2011, 115, 8453−8457. (36) Chen, W. K.; Hu, C. W.; Hsu, C. Y.; Ho, K. C. A Study on The Electrochromic Properties of Polyaniline/Silica Composite Films with An Enhanced Optical Contrast. Electrochim. Acta 2009, 54, 4408− 4415. (37) Chu, J.; Kong, Z.; Lu, D.; Zhang, W.; Wang, X.; Yu, Y.; Li, S.; Wang, X.; Xiong, S.; Ma, J. Hydrothermal Synthesis of Vanadium Oxide Nanorods and Their Electrochromic Performance. Mater. Lett. 2016, 166, 179−182. (38) Yu, H.; Li, Y.; Zhao, L.; Li, G.; Li, J.; Rong, H.; Liu, Z. Novel MoO3-TiO2 Composite Nanorods Films with Improved Electrochromic Performance. Mater. Lett. 2016, 169, 65−68. (39) Kateb, M.; Safarian, S.; Kolahdouz, M.; Fathipour, M.; Ahamdi, V. ZnO-PEDOT Core-shell Nanowires: An Ultrafast, High Contrast

and Transparent Electrochromic Display. Sol. Energy Mater. Sol. Cells 2016, 145, 200−205. (40) Ninh, D. H.; Thao, T. T.; Long, P. D.; Dinh, N. N. Characterization of Electrochromic Properties of Polyaniline Thin Films Electropolymerized in H2SO4 Solution. Open J. Org. Polym. Mater. 2016, 6, 30−37. (41) Xiong, S.; Phua, S. L.; Dunn, B. S.; Ma, J.; Lu, X. Covalently Bonded Polyaniline-TiO2 Hybrids: A Facile Approach to Highly Stable Anodic Electrochromic Materials with Low Oxidation Potentials. Chem. Mater. 2010, 22, 255−260. (42) Gazotti, W. G., Jr.; Faez, R.; De Paoli, M.-A. Electrochemical, Electrochromic and Photoelectrochemical Behavior of A Highly Soluble Polyaniline Derivative: Poly(o-methoxyaniline) Doped with Functionalized Organic Acids. J. Electroanal. Chem. 1996, 415, 107− 113. (43) Malta, M.; Gonzalez, E. R.; Torresi, R. M. Electrochemical and Chromogenic Relaxation Processes in Polyaniline Films. Polymer 2002, 43, 5895−5901. (44) Zhao, L.; Zhao, L.; Xu, Y.; Qiu, T.; Zhi, L.; Shi, G. Polyaniline Electrochromic Devices with Transparent Graphene Electrodes. Electrochim. Acta 2009, 55, 491−497. (45) Xia, X. H.; Tu, J. P.; Zhang, J.; Wang, X. L.; Zhang, W. K.; Huang, H. A Highly Porous NiO/polyaniline Composite Film Prepared by Combining Chemical Bath Deposition and Electropolymerization and Its Electrochromic Performance. Nanotechnology 2008, 19, 465701−465708. (46) Cai, G.; Tu, J.; Zhou, D.; Zhang, J.; Xiong, Q.; Zhao, X.; Wang, X.; Gu, C. Multicolor Electrochromic Film Based on TiO2@ Polyaniline Core/Shell Nanorod Array. J. Phys. Chem. C 2013, 117, 15967−15975. (47) Shreepathi, S.; Holze, R. Spectroelectrochemical Investigations of Soluble Polyaniline Synthesized via New Inverse Emulsion Pathway. Chem. Mater. 2005, 17, 4078−4085. (48) Hwang, T.; Quilitz, M.; Schmidt, H. Comparison of The Electrochrornic Properties of Films Made of Silica/polyaniline Composite Nanoparticles and Films Made of Pure Polyaniline. J. New Mater. Electr. Sys. 2007, 10, 237−242. (49) Lakshmanan, R.; Raja, P. P.; Shivaprakash, N. C.; Sindhu, S. Fabrication of Fast Switching Electrochromic Window Based on Poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) Thin Film. J. Mater. Sci.: Mater. Electron. 2016, 27, 6035−6042. (50) Cai, G. F.; Tu, J. P.; Zhou, D.; Zhang, J. H.; Wang, X. L.; Gu, C. D. Dual Electrochromic Film Based on WO3 /Polyaniline Core/shell Nanowire Array. Sol. Energy Mater. Sol. Cells 2014, 122, 51−58. (51) Thakur, V. K.; Ding, G.; Ma, J.; Lee, P. S.; Lu, X. Hybrid Materials and Polymer Electrolytes for Electrochromic Device Applications. Adv. Mater. 2012, 24, 4071−4096. (52) Fu, X.; Jia, C.; Wan, Z.; Weng, X.; Xie, J.; Deng, L. Hybrid Electrochromic Film Based on Polyaniline and TiO2 Nanorods Array. Org. Electron. 2014, 15, 2702−2709.

16434


Preparation, Characterization, and Electrochromic ... - ACS Publications

Recommend Documents