Conductive Polymer Nanotubes for Electrochromic Applications - ACS

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Conductive Polymer Nanotubes for Electrochromic Applications Fei Hu, Bin Yan, Gang Sun, Jian-Long Xu, Yingchun Gu, Shaojian Lin, Sihang Zhang, Baicang Liu, and Sheng Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00472 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Nano Materials

Conductive Polymer Nanotubes for Electrochromic Applications Fei Hua, Bin Yana, Gang Sunb, Jianlong Xuc, Yingchun Gua, Shaojian Lina, Sihang Zhanga, Bai cang Liud and Sheng Chen*, a

aFunctional

Polymer Materials Laboratory, College of Light Industry, Textile and Food

Engineering, Sichuan University, Chengdu, 610065, China

bFiber

and Polymer Science, University of California-Davis, Davis, California 95616,

United States

cInstitute

of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou,

Jiangsu 215123, China

dInstitute

of New Energy and Low-Carbon Technology, Sichuan University, Chengdu,

Sichuan 610207, PR China

ACS Paragon Plus Environment

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KEYWORDS

poly (3, 4-ethylenedioxythiophene), nanotubes, halloysites, in-situ polymerization, templates, electrochromism

ABSTRACT

Conducting polymers nanotubes are indispensable materials for the development of electrochromic applications. However, tailorable and efficient production of conducting polymer nanotubes in a cost-effective manner has remained a daunting challenge by far. Here we develop a template-assisted strategy that employs the natural-abundant halloysites nanotubes as growth templates and is applicable to producing PEDOT nanotubes. The conformal growth of PEDOT would inherit the morphological features of the halloysites template, thereby, endowing the obtained architectures with tubular-like structure. Thus-derived PEDOT nanotubes with an excellent dispersion stability can be


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easily cast into film. A systematic characterization confirms that PEDOT nanotubes film possesses superior electrochromic performance when compared with unmodified PEDOT.

1. INTRODUCTION

There has been much work on the development of conducting polymer-based electrochromic devices for sensors,1 smart windows,2 signal transduction,3 and flexible displays4 since these devices have many attributes such as flexible, thin, low-power consuming, lightweight, and inexpensive devices. Poly (3, 4-ethylenedioxythiophene) (PEDOT) have been extensively explored as ideal organic materials for electrochromic devices due to the high contrast ratios as well as the availability of diverse colors. In these electrochromic devices, PEDOT is generally applied as compact film forms, which greatly retard ions from diffusing into/out of the films during redox processes, thereby resulting in a low color-switching rate as well as the color contrast. The fast electrochromic response can be achieved by reducing the film thickness since the color-switching rate is determined by the diffusion rate of counter ions into the film during the redox process.


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However, this leads to insufficient coloration and color contrast of the electrochromic device.

Nanotubular structure of conductive polymer is one of the ideal structures that can enhance the performance by improving charge-transport rate as well as increasing surface area.5 Desired tubular structures have been prepared by various methods, such as template methods, interfacial polymerization, reverse microemulsion and so on.6-8 Among the methods aforementioned, template method have particularly fascinated scientists due to its simplicity and diverse applicabilities. However, all the templates adopted to date are synthetic materials, which could be labor-intensive, energy consuming and expensive to produce.9-13 It is worth noting that template methods often rely on electropolymerization which is practically impossible to obtain nanotubes in a large area (>cm2), thereby greatly limiting their throughput and restricting their widespread applications.14

Herein, we present a facile and feasible template strategy of PEDOT nanotubes by chemical oxidation polymerization. A specific type of silicate minerals, halloysite


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nanotubes (HNT), has been selected as the template due to its high natural abundance, low cost, and unique tubular structure.15 The growth of PEDOT would inherit the morphological features of the template after the EDOT monomers were absorbed onto the surface of HNT. Therefore, the resultant PEDOT nanotubes have significantly increased surface areas that guarantee sufficient contact between active materials and electrolyte, which contributes to the full utilization of the PEDOT. Meanwhile, the thinwalled nanotubular structure and abundant voids enables ions to diffuse into/out of the conductive polymer, which results in fast color switching rate. The prepared PEDOT nanotubes film demonstrated improved electrochemical and electrochromic properties when compared with the electrochromic film based unmodified PEDOT. This strategy can be easily scaled up and can be extended to prepare other conducting polymer nanotubes.

2. EXPERMENTAL SECTION 2.1. Materials. Halloysites were purchased from Guangzhou shinshi Metallurgical Chemical Co., Ltd. 3,4-ethoxylene dioxy thiopheneand and ammonium peroxydisulfate (APS) were obtained from Chengdu Chron Chemicals Co,.Ltd. Indium tin oxide (ITO)


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glass (3 × 2 cm2 and 4 × 3 cm2 in size and sheet resistance Rs ≈ 8.0 Ω/m2) was purchased from South China Xiang Science & Technology Co., Ltd. 2.2. Purification of HNTs. Halloysites were purified according to a reported procedure.16 About 250 g of halloysite nanotubes powder and 500 mL of deionized water were mixed in the flask and stirred for 2 h. Sodium hexametaphosphate [(NaPO3)6] (1.25 g) was added gradually under continuous stirring, and then 10 wt % NaOH aqueous solution was added to adjust the pH value to between 8 and 9. The mixture was further stirred for 24 h and then left to stand for 6 h. The supernatant was decanted and then centrifuged at 3000 rpm for 5 min. The supernatant was decanted again for removing much long unbundled HNTs, and then a centrifugation at 7000 rpm for 10 min was further performed. Afterward, the obtained precipitates were repeatedly washed with deionized water and centrifuged successively for at least three centrifugation cycles until the decantate became neutral. Finally, the product was collected by vacuum freeze-drying method. 2.3. Preparation of HNT@PEDOT Nanocomposites. 0.05 g EDOT was dissolved in 52.5 mL water followed by adding 0.15 g HNT powder. Then the mixed solution was


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magnetically stirred at room temperature for 30 min to obtain good dispersion. Then 0.183 g (NH4)2S2O8 dissolved in 5 ml water was added the system to initiate the polymerization of EDOT. After polymerization for 4 h, the dark blue suspensions of HNT@PEDOT could be observed. 2.4. Preparation of PEODT Nanotubes. The HNT@PEDOT nanocomposites obtained were dispersed in water containing conc. HCl and conc. HF and then immerged overnight. The PEDOT nanotubes produced were washed with water for several times until neutral. The yield of PEDOT nanotubes is about 78%. 2.5. Preparation of Films Electrodes. The conductive ITO glass substrates were ultrasonically washed with petroleum ether, acetone, ethanol, and deionized water in sequence to remove contaminants from the surface. The cleansed ITO glass substrates were dried and directly used. The film electrodes were prepared by drop coating 50 μL of aqueous dispersion onto the ITO glass with an active area = 2.0 × 1.5 cm2 and dried at room temperature. 2.6. Characterizations. Morphologies and structure were investigated using scanning electron microscopy (SEM) (JSM-5900LV, JEOL) and transmission electron microscopy


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(TEM) (Tecnai G2 F20 S-TWIN, FEI). The vibrational spectra of the materials were recorded by Fourier transform infrared spectra (FTIR) spectrometer (Shimadzu Corp.). The optical transmission spectra, optical contrast and response of the films were measured by a UV−vis spectrometer (Beijing Purkinje General Instrument Co., Ltd.).

3. RESULTS AND DISCUSSION 3.1. Template-assisted Synthesis and Characterization of PEDOT nanotubes.


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Figure 1. (a) Schematic illustration of the preparation of PEDOT nanotubes; (b-d) TEM images of HNT, HNT@PEDOT and PEDOT nanotubes.

The PEDOT nanotubes was prepared with the synthetic procedure illustrated in Figure 1a. Firstly, the prime HNT (Figure S1) were purified according to the reported procedure.16 Then, 0.15 g purified HNT (Figure 1b) and 0.05 g EDOT monomer were added into 52.5 mL of water followed by 30 min stirring at room temperature to obtain a good dispersion. After that, 0.183 g (NH4)2S2O8 dissolved in 5 mL water was added into the system to initiate the polymerization of EDOT. The whole system was maintained at room temperature for 4 hours. Then, the composites and etching solution (HCl/HF = 1:1, mass


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ratio) were put into a plastic beaker for 12h. Finally, the product were washed with deionized water several time until neutral.

The TEM images of purified HNT, HNT@PEDOT composites and PEDOT nanotubes were presented in Figure 1b-d. The TEM image reveals the smooth external surface of HNT, which shows variation in terms of diameter and length (Figure 1b). Addition, we found that the HNT have a cylindrical shape and contain a transparent central area that runs longitudinally along the cylinder, indicating that the nanotube are hollow and openended. Figure 1c clearly reveals a typical coaxial core-shell structure of the HNT@PEDOT composites where the HNT template as the core encapsulated in the PEDOT shell. On the contrary, only grainy PEDOT were obtained by following a similar process but without the introduction of HNT (Figure S2). After etching the template halloysite, the TEM analysis of as-obtained PEDOT verifies the existence of tubular structure (Figure 1d). It could be concluded that the intact wrapping of HNT by PEDOT leads to the replication with respect to the morphological features of templates. Further, the product etched from composites with thin shell layer turns out to be flake rather than


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tubes, as shown in Figure 2a. If the etching process is accompanied by violent agitation or vibration, tubular PEDOT structure could also be destroyed (Figure 2b). The appropriate thickness of the PEDOT shell and the static etching condition are critical to obtaining the PEDOT nanotubes (Figure 2c).17

Figure 2. TEM of the product under different condition: (a) monomer: HNT = 1: 3, under stirring; (b) monomer: HNT = 1: 1, under stirring; and (c) monomer: HNT = 1: 1, completely static.

The chemical structure of HNT, HNT@PEDOT, and PEDOT nanotubes were confirmed by using Fourier transform infrared (FTIR) spectroscopy and X-ray powder diffraction (XRD) spectroscopy. As shown in Figure 3a, the bands at 1517 and 1476 cm-1 are assigned to the asymmetric stretching mode of C = C and inter-ring stretching mode of C-C. The bands at 1201, 1140, 1090 and 1000 cm-1 should be C-O-C bending vibration


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in ethylenedioxy group and the bands at 976, 909, 838, and 686 cm-1 are attributed to stretching vibrations of the C-S-C bond in thiophene ring. Meanwhile, no peak at 1186 and 890 cm-1 belonging to the =C–H in-plane and out-of-plane deformation of EDOT monomer was found. It can be confirm that EDOT monomers have been successfully polymerized on the HNT.18 Furthermore, compared with the FTIR spectrum of HNT, no HNT bands at 3695, 3620 and 468 cm-1 were identified in FTIR spectrum of PEDOT nanotubes,19 thus confirming that the HNT templates have been completely etched after the treatment with the HF/HCl mixture.

The X-ray diffraction pattern of PEDOT was shown in Figure 3b. As we can see that there are peaks around 2θ of 13.1° and 26.6°, which can be assigned to (100) and (020), respectively. Meanwhile, the peak around 26.6° with full-width half-maximum (FWHM) of 3.8°, which is the featured properties in PEDOT whether obtained by chemical oxidation and electrochemical polymerization. Compared with other XRD data reported for PEDOT,20-23 the XRD in this work shows much higher order and clearer lattice parameters, which is indicative of highly ordered structure.24 Further, the shift of the (020) peak to a


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somewhat higher angle (26.6°) indicates that the face-to-face distance between the stacked PEDOT nanotubes sheets becomes shorter. The face-to-face distance (3.4 Å) between the PEDOT sheets of the nanotube product is considerably shorter than that (3.79 Å) of grainy PEDOT. It is reported that a similar face-to-face distance between PEDOT becomes shorter by p-doping. The doping seems to decrease the molecular interaction between the stacked π-conjugated polymers molecules.25,26 In our work, the p-doping may be ascribed to the HF and HCl secondary doping for partial PEDOT chain segment.

Figure 3. Structure characterization of HNT, HNT@PEDOT, and PEDOT nanotubes by (a) FTIR spectra and (b) XRD spectra.


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As shown in Figure 4a, a distinct hollow feature with the weaker contrast in the core of the structure can be viewed, and the size of the cavity is coincident with the diameter of HNT templates (Figure S3). In addition, different from the rigid HNT, the PEDOT nanotubes display some clear wrinkles on the surfaces, which should be very effective to increase the electroactive sites.27 We also found that the obtained PEDOT nanotubes demonstrate an excellent suspension stability in the aqueous solution. After settling for 12 hours, no apparent precipitation or separation was observed in the PEDOT nanotubes aqueous dispersion; by contrast, for the HNT@PEDOT nanocomposites, a clear deposition phenomenon occurred in less than 2 hours (Figure 4b). This excellent dispersion stability of the PEDOT nanotubes in aqueous makes it possible for the preparation of the large area films by using various casting methods, such as, doctor blading, spray coating, screen printing, drop casting, slot-die coating, etc.


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Figure 4. (a) TEM of PEDOT nanotubes; (b) The dispersion stability (about 12 hours from left to right): HNT@PEDOT (upper) and PEDOT nanotubes (bottom); (c) The dimensions comparison of PEDOT nanotubes; (d) N2 adsorption/desorption isotherm of HNT@PEDOT and PEDOT nanotubes, Inset: corresponding pore size distribution.

The dimensions of PEDOT nanotubes compared with other references are summarized in Figure 4c. The as-prepared PEDOT nanotubes possess average diameter about 87


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nm and wall thickness about 11 nm, which is the smallest value for PEDOT nanotubes reported so far. The thin-walled nanotubular structure enables ions to diffuse into/out of the conductive polymer easily, which results in fast switching rate.

The surface area and porosity of resulting PEDOT nanotubes were determined by measuring nitrogen adsorption and desorption isotherms since these parameters are key to electrochemical and electrochromic performances. Obviously, the N2 volume adsorbed of PEDOT nanotubes becomes larger in comparison with that of HNT@PEDOT, which is considered to be caused by the removal of template (Figure 4d).28 In general, the mesopore volume, which is associated with the mesopore diameter, increases a little after the removal of template, proving that the removal of template (inset of Figure 4d). Furthermore, tubular structure possesses mesoporous with a wide pore size distribution ranging from 20 to 60 nm, revealing a hierarchically mesoporous structure. According to our conjecture, these multi-mesoporous structures mainly comprise two parts: 1) PEDOT nanotubes stacked with each other, forming the interspaces as shown in TEM images; 2) Tubular channels of the nanotubes. Thus, the nanotube channels and a large number of


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voids with various sizes construct a hierarchical porous structure. The hierarchical porous structure can serve as the diffusion pathway in the electrochemical redox process, which would be discussed later.

Figure 5. Electrochemical and electrochromic performance characterization. (a) Cyclic voltammetry curves of PEDOT NT film at different potential scan rates; (b) Cyclic voltammetry curves of HNT@PEDOT NT and PEDOT NT films at 100 mV/s; (c) In-situ optical response at 520 nm of the grainy PEDOT, HNT@PEDOT and PEDOT NT films in the bleached and colored states. (d) The variation of the in situ optical density vs. the


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charge density for grainy PEDOT, HNT@PEDOT, and PEDOT Nanotubes; (e) The photos of the PEDOT Nanotubes film casted on ITO/glass at bleached and colored states.

3.2.

Electrochemical and Electrochromic Performance Evaluation

Cyclic voltammetry (CV) can be used to study the electrochemical reactions of the films during ion insertion/extraction. Figure 5a shows the CV curves of the PEDOT nanotubes film with a scan rate of 10, 25, 50, 100, and 150 mV/s, which was recorded between −0.4 V to +0.6 V in the 0.1 M LiClO4/acetonitrile solution. The CV curve was attributed to the reaction of PEDOT, which involves anion diffusion into and out of the polymer matrix to compensate for the charge of the polymer film. Similar to the results reported previously,29 the CVs displayed broad anodic and cathodic peaks. The increased current densities for PEDOT nanotubes with favorable adhesion on the electrode surface as the increasing scan rates indicate the non-diffusional redox behaviors even at a higher scan rate (Novel copolymers based on PEO bridged thiophenes and 3,4-ethylenedioxythiophene: Electrochemical, optical, and electrochromic properties)(Figure 5a). As shown in Figure 5b, PEDOT nanotubes possess highest current density when comparing to that of the


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HNT@PEDOT and grainy PEDOT film. The improved electrochemical activity should be attributed to the larger specific surface area of PEDOT nanotubes film which provide more electroactive sites.30 After the removal of nonconductive HNT, the PEDOT nanotubes film becomes more porous, which provide more diffusion pathway for electrolyte ions and better ion/electron transport properties, resulting in lower oxidation potential.31 Thus, the PEDOT nanotubes film gives the obviously lower anodic peak potential comparing to the grainy film. Electrochemical impedance spectroscopy (EIS) were conducted to further understand the electrochemical behaviours of the PEDOT nanotubes (Figure S3). The characterizations confirmed that the PEDOT nanotubes film possesses both fast ion diffusion and low charge transfer resistance, which may provide advantages for excellent electrochemical performances.30 It is worth mentioning that there are a number of contacts between the adjacent nanotubes shown in Figure 1d. This contact could facilitate the transfer charges, enhancing the overall electrical activity of the PEDOT nanotubes.31

To explore the potential application of the PEDOT nanotubes nanocomposites as electrochromic materials, the electrochromic performance of the grainy PEDOT,


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HNT@PEDOT and PEDOT nanotubes films are prepared and investigated (Figure 5c and d). The electrochromic parameter values were calculated from Figure 5c and listed in Table S2. The transmittance variation (ΔT) of PEDOT nanotubes film (about 67% at 520 nm) is obviously higher than that of the grainy PEDOT and HNT@PEDOT composite films. Meanwhile, the value is much higher than most of the recently reported performance of the PEDOT-based EC material (Table S3). This signifies that the tubular structure of PEDOT could provide easier access to a larger number of ions and thus more PEDOT units can effectively switch during the redox process. Meanwhile, the coloration time tc and bleaching time tb of PEDOT nanotubes film were found to be 1.2 s and 0.7 s, respectively, which are also faster than most of the results reported in the literatures. The tubular structure plays a significant role in influencing the electrochromic performance of the PEDOT as it could increase contact area and reduce transport pathway. The photographic images of PEDOT nanotubes film in a bleached state and a colored state are shown in the Figure 5e.


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The coloration efficiency (CE) is defined as the change in optical density (OD) per unit charge (Q) inserted into or extracted from the EC films. A high CE value provides large optical modulation with small charge insertion or extraction. This can be calculated from the following formulas:32

∆𝑂𝐷 = 𝑙𝑜𝑔 (𝑇𝑏 𝑇𝑐)

CE = ∆𝑂𝐷 𝑄

As shown in Figure 5d, the CE values are 88, 127, and 136 cm2 C-1 for the grainy PEDOT,

HNT@PEDOT,

and

PEDOT

nanotubes

films,

respectively,

further

demonstrating the excellent EC performance of the PEDOT nanotubes film.

Figure 6. The model of ions insertion/extraction.


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The superior electrochromic performance of the PEDOT nanotubes film can be attributed to the unique nanostructure as shown in Figure 6. It is well known that electrochromic properties mainly depend on the diffusion extent of ions in EC film and the efficiency of electrochemical redox reaction. In the dense grainy PEDOT film, the redox reaction area is confined to a thin surface layer due to the limited diffusion of ions, thus reducing the corresponding optical contrast and adding the response time. However, the PEDOT nanotubes film possesses porous morphology and thin PEDOT shell layer. The large number of pores with various sizes in the nanocomposites film could provide effective channels for ions to contact with the active materials of the whole shell layer, thus resulting in fast kinetics of electrochemical reactions. Meanwhile, the tubular structure inherited from HNT could act as a reservoir and a transport path for ions to provide smooth and convenient ion transfer, thereby enhancing the accessibility of the electrolyte to the above porous regions. The larger surface area (about 58 m2/g) than pure PEDOT (about 25 m2/g) also can provide abundant reaction sites of active materials that guarantee sufficient contact between active materials and ions, which contributes to the full utilization of the PEDOT. In other words, this behavior can be explained by


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considering that the more open morphology produced by the porous structure allows more ions to be contained within the film, resulting in a more highly doped state. Therefore, the PEDOT nanotubes film exhibited especially high changes in visible transmittance. Based on these reasons, the as-prepared PEDOT nanotubes film on ITO/glass exhibit much higher electrochromic performance than grainy PEDOT film.

4. CONCLUSIONS In summary, we have reported a facile and feasible chemical oxidative polymerization strategy to prepare the PEDOT nanotubes. The growth of PEDOT on halloysites template has enabled the production of conductive polymer with tubular structure and abundant voids, which are beneficial to fast ion diffusion and mass transport. The characterizations indicate the PEDOT nanotubes film noticeably improve the electrochemical and electrochromic performance compared to that of the grainy PEDOT films. The strategy we reported could pave the way for developing conducting polymer nanotubes with enhanced properties.


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ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

The SEM images of Halloysite nanotubes and PEDOT without the introduction of HNTs; diameter range of Halloysites; Nyquist plots of the grainy PEDOT and PEDOT NTs nanoparticle films; pore texture of obtained samples; electrochromic parameters of different films at 520 nm; and A partial list of PEDOT-based films reported in literatures.

AUTHOR INFORMATION Corresponding Author * Sheng Chen. E-mail: [email protected]. ORCID Sheng Chen: 0000-0002-9428-3675

Fei Hu: 0000-0001-5996-8524

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51003064, No. 61705152 and No. 21876119) and the Sichuan Province Science and Technology Foundation (No. 2017GZ0429 and No. 2014JY0146). We would like to thank the Analytical & Testing Center of Sichuan University for Structured Illumination Microscopy Work and we would be grateful to Hui Wang for her help of SEM image.

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ToC graphic：


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Conductive Polymer Nanotubes for Electrochromic Applications - ACS

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