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A freestanding electrochromic copolymer for multicolor smart window
Journal Pre-proofs A freestanding electrochromic copolymer for multicolor smart window Xiaofang Liu, Tianmin Cao, Wenqian Yao, Lanlan Shen, Jingkun Xu, Fengxing Jiang, Yukou Du PII: DOI: Reference:
S0021-9797(20)30297-6 https://doi.org/10.1016/j.jcis.2020.03.016 YJCIS 26131
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
3 December 2019 5 March 2020 5 March 2020
Please cite this article as: X. Liu, T. Cao, W. Yao, L. Shen, J. Xu, F. Jiang, Y. Du, A freestanding electrochromic copolymer for multicolor smart window, Journal of Colloid and Interface Science (2020), doi: https://doi.org/ 10.1016/j.jcis.2020.03.016
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A freestanding electrochromic copolymer for multicolor smart window Xiaofang Liua,b, Tianmin Caob, Wenqian Yaob, Lanlan Shenb, Jingkun Xub, Fengxing Jiangb,*, Yukou Dua,* a
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China. b
Department of Physics, Jiangxi Science and Technology Normal University, Nanchang 330013, P. R. China.
Abstract: Electrochromic
devices
with
low-cost,
energy-saving
advantages,
and
controllable color switching have gained widely attention. Yet, electrochromic materials are limited for smart window due to challenges such as difficulty freestanding, monotonous color change, slow switching capability, and low optical contrast. In this work, a freestanding copolymer based on Poly(N-vinylcarbazole) (PVK) and 3, 4-ethoxylenedioxythiophene (EDOT) are designed. The copolymer synthesized by the good secondary film-forming of PVK not only contains the self-standing property of PVK, but also possesses excellent electrical and electrochemical properties of EDOT. The freestanding copolymer is used to create the multicolor: brown, dark brown, purple, blue. A high optical contrast of up to 39.1% and a color efficiency of up to 107.00 cm
2
C-1 prove a significant coloration and
bleaching effect, which is satisfactory for the application of electrochromic devices. Further, an electrochromic device based on P(PVK-co-EDOT) as coloring materials is constructed. This work contributes new ideas into the design of electrochromic smart
materials. Keywords: Copolymer, electrochemistry, electrochromics 1.
Introduction Electrochromic (EC) devices exhibited different optical features when different
bias voltages were applied and shown a broad range of applications in electronic displays, smart windows, goggles, and anti-glare rearview mirrors [1-3]. Therefore, EC materials were required to possess various advantages, including low power consumption, low applied potential, memory effect under open circuit and tunable color depth [4]. Moreover, over the devices comprising glass substrates, flexible EC devices would have significant merits including light-weight, formability, etc. [5, 6]. Such flexible devices would present the specific advantage of versatile shapability. Their light weight and adaptability to curved shape would make it easy to integrate into building and automotive glass, allowing them to be added as layers to existing windows without the need to completely replace smart windows [7-9]. However, as result of the limited availability of transparent conductive films and EC materials with the require optoelectronic properties and durability, it remains challenging to develop EC devices. Furthermore, most of EC materials limited its use in devices due to their brittleness and high-temperature handing requirements [10, 11]. Electronic devices have attracted widespread attention, and the development of high-performance electrochromic materials paly great significance roles in future electronic devices. EC materials include viologens, transition metal oxides, conducting polymers, and Prussian blue (PB) systems [10, 12]. Among them,
conducting polymers, such as polythiophenes (PThs), polyaniline, and polypyrrole for optoelectronic purposes are widely studied as electrochromic active materials. However, it is very difficult to produce the freestanding films such as polyaniline, because of its infusibility, rigidity of polymer chains, poor mechanical properties and poor solubility. And the electrochemical and EC performances (monotonous color changes, slow switching capability, low optical contrast etc.) of conducting polymers need to be improved and meet the requirement for practical applications [13, 14]. For instance, Zhang et al. [15] reported that the sprayed monodisperse silica/polyaniline (SiO2/PANI) core/shell nanosphere film was used as an electrochromic layer, showing low optical contrast and low speed response. The poly(N-isopropylacrylamide) hydrogel made by Wang et al. [16] exhibited low optical contrast of about 25%. Herein,
polycarbazole
and
its
derivatives
which
possess
outstanding
environmental stability, photoconductivity, and EC properties have attached extensive attentions owing to their potential industrial applications in rechargeable batteries, light emitting diodes, and electroluminescent applications [17-20]. Comparing with polymers of carbazole and other carbazole derivatives, Poly(N-vinylcarbazole) (PVK) has elicited considerable interest due to its good film-forming properties, high glass transition temperature characteristic and optical properties [19, 21]. PVK has the advantage of containing pendant electroactive monomer units in a polyvinyl polymer backbone with the ability to carry out secondary polymerization via a large number of carbazole units [22]. Xu et al. [22-24] had manufactured a freestanding poly[poly(N-vinyl carbazole)] (PPVK) film, which had high-quality with excellent
thermal stability and high electrical conductivity. However, there have been no reports on the EC property of PPVK films obtained by secondary polymerization. The reason is that PPVK cannot exhibit obvious electrochromic property. In this work, we prepare freestanding films by electrochemical copolymerization. Moreover, to improve EC property, PVK is allowed to copolymerize with suitable monomers. EC active materials could be easily prepared by introducing suitable comonomers by electrochemical copolymerization [25, 26]. Owing to the low oxidation potential, good electron donor property, high conductivity, optical transparency, and high stability of EDOT, the forthputting of EDOT as a co-monomer almost dominated the studies of electrochemical copolymerization [4, 25-27]. Furthermore, the high-quality freestanding P(PVK-co-EDOT) films are successfully prepared
by
electrochemical
copolymerization
on
ITO
substrates.
The
electrodeposited EC films exhibit multicolor range from brown, dark brown, purple, and blue. Moreover, the constructed EC device based on P(PVK-co-EDOT) as coloring materials can be switched between blue and purple. 2.
Experimental
2.1. Materials Tetrabutyl-ammonium hexafluorophosphate ([Bu4N] PF6, 98%) was purchased from Energy Chemical Industry Co., Ltd. and used directly as received. Before use, tetrahydrofuran (THF) was purified by distillation with calcium hydride under reduced pressure. 3,4-ethylenediophene (EDOT, 98%) were obtained from Shanghai J&K Scientific Ltd. Poly(N-vinylcarbazole) (PVK) were products of Tokyo Chemical
Industry Co., Ltd. Poly(vinylidene fluoride) (PVDF) was purchased from Fluorochem Co., Ltd. 2.2. Electrochemical Measurements and Electrochemical Polymerization The electrochemical properties tests of monomers and polymers were carried out in a three-compartment system by an electrochemical workstation (Princeton V3) at room temperature. An Ag/AgCl wire for preparing a reference electrode was obtained via electrodeposition on silver wire for a specific period of time (100 ~120s) at a specific potential (1.5 V) with a mass fraction of 50% of HCl (aq) as an electrolyte. Both the working and counter electrodes were Pt wire (diameter: 1.0 mm). A glass having an area of 3 cm2 coated with ITO was used as a working electrode. The
films
were
synthesized
on
the
conductive
electrode
by
electrodeposition, a Pt foil having an area of 4.5 cm2 as the counter electrode and an Ag/AgCl wire as the reference electrode, respectively. PPVK, P(PVK-co-EDOT), and PEDOT were all obtained at constant potential of 1.50 V via electrodeposition. The electrolyte solutions were in anhydrous THF containing 0.05 M [Bu4N] PF6. 2.3. Spectroelectrochemical and Electrochromic Experiments The spectroelectrochemical and electrochromic behavior of as-prepared films were measured in a Princeton V3 potentiostat-galvanostat adapted to a Cary 50 UV/Vis/NIR spectrophotometer in a three-compartment system. Working electrode was the ITO glass (2.5 cm × 0.8 cm active area), the counter electrode was a Pt wire and the reference electrode was an Ag/AgCl wire, respectively. Prior to operation, the supporting electrolyte containing 0.05 M
Bu4NPF6 in anhydrous THF were deaerated with a dry nitrogen stream for 30 minutes. According to literature [28], the following equation is used to calculate the coloration efficiency (CE) of the polymer at a specific wavelength (λmax): CE = △OD / Qd The optical contrast ( Δ T%) at the λ max between de-doped and doped states can be related to the optical density (ΔOD) , which is calculated based on equation:
△OD = log (Tox / Tred) 2.4. Preparation of Electrochromic Device Assembly. The gel electrolyte was prepared by adding [Bu4N] PF6 (0.34 g), dehydrated PVDF (1.30 g) into a flask with three necks and dissolved in THF (17 mL) with stirring, following by vigorous stirring at 60~65°C for 12 h. Finally, the transparent PVDF-based gel electrolyte was obtained. The P(PVK-co-EDOT) with the best electrochromic properties was obtained in the monomer ratio of 1:1 as a color changing material. The transparent gel electrolyte was poured onto the copolymer deposited on the ITO by electrodeposition, carefully pressing another ITO against each other to form an electrochromic device. Finally, the device was sealed by copper foil and epoxy-resin glue. The schematic illustration of the preparation of flexible film and device was shown in Figure 1 and Figure 6. 2.5. Characterization A Bruker Vertex 70 Fourier transform infrared (FT-IR) spectrometer of polymers were measured in the range of 300-4000 nm. The microstructure and texture of the
as-prepared films were researched by atomic force microscopy (AFM, Veeco Multimode, and Plainview, NY). The Perkin-Elmer Pyris Diamond (TG/DTG) thermal analyzer were carried out to obtain thermogravimetric analysis data under a nitrogen stream. 3.
Results and discussion
3.1. Electrochemical behaviour
(Inset Figure 1) The reversibility of electron transfer within the period of electrochemical polymerization is demonstrated by using a highly successful technique, cyclic voltammetry (CV). The electrochemical polymerization of EDOT and its electrochemical copolymerization with PVK are implement via CV scanning. Figure S1 shows the corresponding products of the electrochemical copolymerization between PVK and EDOT. As can be seen from Figure 1A-C, during the homo-polymerization and copolymerization, the first cycles (red lines) are different from each other, which prove that the composition of the initial monomers is different. The two monomers used for electrochemical copolymerization had the similar values of onset oxidation potential (PVK~1.13 V and EDOT~0.78 V vs. Ag/AgCl), which allow to achieve good copolymers through alternating units in the backbone [29]. During the electrochemical copolymerization process, the onset oxidation potential of mix-monomer was slightly negatively shifted compared to homo-monomer. The introduction of EDOT into mixed monomers leads to a reduction at the oxidation potential, which may be result from the electron-donating effect of two adjacent
oxygen atoms in the heterocyclic of EDOT [26]. Moreover, although the CV scanning were carried out for the same number of cycles (10 cycles), the increase of the current intensity in copolymerization were much more compared with homo-polymerization of PVK. As shown in Figure 1A and 1C, the redox peaks of PVK appeared from 0.78 to 1.20 V and of EDOT appeared from -1.35 to 0.52 V, respectively. One can see the apparent differences of CVs among the mixed monomers in Figure 1B and Figure S2A, S2C, implying the achievement of the copolymerization. For mix-monomers, the redox range of 0.10 ~ 0.80 V (1:1), 0 ~ 0.75 V (2:1) and 0 ~ 1.00 V (3:1) could be observed. Moreover, as shown in Figure 1A-C and Figure S2A, S2C, those are observed that the electrochemical property of EDOT are positively inherited by the copolymer, which the contribution of EDOT can be observed during the electrochemical copolymerization process. During both homo-polymerization and copolymerization, the formation of polymer film on the surfaces of the conductive electrode are demonstrated by the formation of new signals in each cycle and their increase in current densities. Simultaneously, a dark blue copolymer was formed on the ITO surface. To investigate the electrochemical redox activity of homo-polymers PPVK and PEDOT as well as copolymer P(PVK-co-EDOT), CVs were examined in supporting electrolyte (devoid of monomer) for electrochemical behavior of polymer (Figure 1D-F and Figure S2B, S2D). Homo-polymers PPVK and PEDOT were found to oxidize at 0.40 and 0.70 V respectively, while copolymers P(PVK-co-EDOT) (the PVK/EDOT monomer feed ratio of 3:1) were oxidized at 0.25 V. Under identical, the
considerable differences confirmed the successful formation of copolymers in oxidation potential between homo-polymers and copolymers [30]. When increasing the content of EDOT component in the membrane, the redox current was widened comparing to PPVK. The higher oxidation resistance of EDOT can explain this. It can be clearly seen that although the current values and the scan rates of the as-polymers in all experimented are greatly different, copolymer synthesized here have good electrochemical
redox
activity.
From
the
illustration
of
Figure
1E,
P(PVK-co-PEDOT), the metallic dark blue freestanding films in its oxidized state are produced. 3.2. FT-IR characterization and thermal properties
(Inset Figure 2) FT-IR spectroscopy was performed in order to characterize the structures of PPVK, P(PVK-co-EDOT), and PEDOT. According to Figure 2A and Figure S3, the essential characterization of PEDOT were that its IR main bands centered at 3644, 3430, 1516, and1090 cm-1 were considered to arise from aromatic C-H vibrating, conjugated C-O stretching, thiophene ring stretching, and C-H vibration as well as deformation, respectively. The peaks observed at 1330 and 748 cm-1 represented the C-H in-plane bending deformation in vinylidene groups and the C-H out-plane bending deformation of C-H bond of benzene ring, which demonstrates the presence of the PVK sell on the EDOT [21]. The peaks at 1482-1448 cm-1 were ascribed to the ring vibration of the N-vinylcarbazole moiety. As shown in Figure 2A and Figure S3, the intensity of characteristic PPVK peaks reduced with reducing PVK in the
backbone. We note in the vibrational spectra of copolymer P(PVK-co-EDOT) the presence of the main characteristic bands of the PEDOT, as well as those of PPVK, which indicate that the new skeletons with alternating PVK and EDOT are formed by polymerization of mix-monomers. Moreover, the new peaks of copolymer were that the new main band observed at 838 and 981 cm-1 was obtained from phenyl ring substitution bands and substitute aromatic ring of PVK. This result further indicates the successful electrosynthesis of P(PVK-co-EDOT). The thermogravimetric experiments were performed under a nitrogen stream at a heating rate of 10 K min-1 to examine the thermal stability of the as-prepared polymer by TG and DTG analyses. In Figure 2B and Figure S4, the thermal degradation curves were the function of the temperature range of 300 to 1050 K, which indicate that polymers exhibited a similar weight loss process. It can be observed that copolymer have higher thermal stability than PPVK and PEDOT. Eventually, P(PVK-co-EDOT) film had a residual weight of 40.8% (the monomer feed ratio of PVK/EDOT=3:1) at 1050 K, which showed better thermal stability than PPVK (25.7%). In summary, the above results indicate that the good thermal properties of the copolymer will be beneficial to application in electrochromism. 3.3. Film Morphology (Atomic Force Microscopy) of polymer
(Inset Figure 3) Atomic force microscopy (AFM) was a typical surface technique suitable for analyzing the nanoscale properties and other properties of thin films of conducting polymers [31]. [Figure 3(a–d)] and [Figure 3(e–h)] show the AFM height images with
the corresponding surface roughness curves and the AFM phase images of homo-polymers and copolymers, respectively. In the characteristic picture of the polymer, there is a significant difference in height between the dark and the bright patterns with the maxima values of 120.7, 80.6, 154.5, and 146.0 nm for the PPVK, P(PVK-co-EDOT) (2:1), P(PVK-co-EDOT) (3:1), and PEDOT films, respectively. The root-mean-square surface roughness of PPVK, P(PVK-co-EDOT) (2:1), P(PVK-co-EDOT) (3:1), and PEDOT films are estimated to be 102.79, 74.57, 38.30, and 27.43 nm, respectively. The AFM images of the film show that the surface is uniform and compactness. The as-prepared film appeared as a tightly packed spherical particle on the surface. The average particle diameter of the copolymer is slightly larger than the homo-polymer. Furthermore, the phase images show in Figure 3(e–h) the clearer difference in the morphology between the homo-polymer and the copolymer. As can be seen from Figure 3(e, h), the dark and the bright boundaries in the PPVK and PEDOT films were very clear with a maximum 215.2 deg and 211.0 deg phase lag, respectively. It is worth noting that after copolymerization, Figure 3(f, g) shows a more uniform surface structure with a smaller phase lag of 88.0 deg and 151.5 deg compared to the PPVK and PEDOT films. Interestingly, the optimal electrochromic properties of copolymer (the PVK/EDOT monomer feed ratio of 2:1) produced an obvious change in the minimum phase lag in Figure 3(f), which indicates that the formation of large particles on the surface of the polymer. This should be interpreted as an indirect reflection of large particles that contribute to electron transport within the polymer [32].
3.4. Spectroelectrochemistry
(Inset Figure 4) The spectroelectrochemistry was a powerful technique used to estimate the effect of optical behavior when applied to different potentials and provide information about its
polaronic
/
bipolaronic
states
and
the
electronic
structure.
The
spectroelectrochemistry of all the homo-polymers and copolymers were performed at different potentials using the supporting electrolyte containing 0.05 M Bu4NPF6 in THF solvent (Figure 4 and Figure S5A, S5C). As can be seen from Figure S5A and Figure 4C, the strong absorbance bands of PPVK and of PEDOT can be observed at about 346 and 591 nm in neutral state, respectively. On the contrary, the max of P(PVK-co-EDOT) had a red-shifts of 224 nm compared to PVK, which can be attributed to the increase in the conjugation length of the copolymers and the introduction of electron-donating EDOT in copolymer backbone [33]. This further confirms the successful formation of the copolymer between EDOT and PVK by electrochemical copolymerization. PPVK polymer had no obvious absorption between neutral state and oxidized state (in the ranges of 400-1100 nm). However, as a result of the generation of polarons/bipolarons, the copolymers showed the maxima absorption peaks at 570 and 900 nm (Figure 4A and B). In neutral state, the maximum absorption wavelength (max) of the copolymer was 570 nm. After oxidation, the intensity of this band started to decrease and shifted to a lower wavelength (about 470 nm). Similarly, at a maximum of approximately 800 nm, a new band started to form, demonstrating that
the polarons are generated. After further oxidation, a new band over beyond 1000 nm was formed, indicating the generation of bipolarons. More importantly, the copolymers showed a multi-electrochromic property: it had brown, dark brown, purple and blue colors at different states (Figure 4-below). In addition, it can be observed that the colors of the original PEDOT were measured under different applied potentials is different from the color of the copolymer. As described in the previous literature [28], the color of PEDOT was an opaque dark blue under its neutral state and exhibited a transparent sky blue after oxidation. 3.5. Electrochromic switching
(Inset Figure 5) In order to study the electrochromic switching process, homo-polymer and copolymer were studied on ITO/glass via chronoabsorptiometry. It is important to determine the change in optical contrast (∆T%) of the polymer film between its oxidized and neutral states. In the chronoabsorptiometric measurement, while applying sequential square-wave bias pulses (which can quickly switch between the reduced and oxidized states of the film), monitor the transmittance of the film is monitored (Figure 5 and Figure S5B, S5D). For PPVK polymers, the intensity of the % transmittance was observed to be almost zero. The use of PEDOT in electrochromic devices has grown resulting in stable electrochromic effects [34]. Adding the EDOT unit to the copolymer helped stabilize the transmittance, and could measure its ∆T%. As shown in Figure 5 and Figure S5, the highest value of the copolymers obtained in the monomer ratio of 2:1
was found to be 39.1%. Compared with PEDOT (37.6%), the optical contrast of the copolymer was improved, which indicate that copolymerization with EDOT has a positive effect on the ∆T%. Furthermore, response times (t-ox and t-red) could be extracted from the optical transmittance data. The optical switching times was one of the principal parameters for judging the superiority of electrochromic materials. The switching time was measured as the time to reach 95% optical contrast of the entire switch, and the results are shown in Figure S6. The copolymers P(PVK-co-EDOT) (the PVK/EDOT monomer feed ratio of 1:1) showed a fast switching of about 3.4 s, while allowing the transitions from the reduced states to the oxidized states. However, in the reverse transition (i.e. from the oxidized state to reduced state), the response time was reduced to 2.8 s. The reduced process was about twice lower than PEDOT (4.2 s). Also, it can be observed that the response time of copolymers P(PVK-co-EDOT) is shorter than that of the PPVK and the response time of reduction of copolymers P(PVK-co-EDOT) are slightly shorter than that of PEDOT. A good electrochromic material should have excellent CE, which means that the charge or energy injected into the polymer can modulate optical and effectively change the optical density. Table 1 summaries the basic optical properties of these polymer films. At 95% of the optical density, it is worth noting that the CE of the copolymer is improved compared to PEDOT (53.88 cm2 C-1). The copolymer P (PVK-co-EDOT) (the PVK/EDOT monomer feed
ratio of 1:1) showed the highest CE values of 107.00 cm2 C-1. The highest CE of the copolymer is almost twice that of PEDOT. This suggest that the conjugated-non-conjugated structures may exhibit a lower band gap and contribute
to
the
improvement
of
electrochromic
properties.
The
P(PVK-co-EDOT) with the best electrochromic properties is obtained with a monomer ratio of 1:1 as the color changing material for device construction.
(Inset Table 1) 3.6. Electrochromic devices
(Inset Figure 6) In order to further exhibit the promising application of P(PVK-co-EDOT) electrode in flexible electrochromic displays, electrochromic devices were manufactured
according
to
the
schematic
(Figure
6),
in
which
P(PVK-co-EDOT) (the PVK/EDOT monomer feed ratio of 1:1) was used as a color changing material. ITO is an excellent material for the counter electrode since it is almost colorless in the oxidized and neutral state and was easily reversibly oxidized and reduced without discoloration [8, 35, 36]. The copolymers are prepared by electropolymerization method. The photographs of devices and color change of films are shown in Figure 6. And when the square-wave potentials were applied between −0.9 V and +0.5 V, the electrochromic device and films both presented an electrochromism with a color reversibly changing from blue to purple. 4. Conclusions
In summary, a freestanding film P(PVK-co-EDOT) has been successfully synthesized via electrochemical deposition, which shows better thermal stability and superior electrochemical activity. The as-prepared copolymer’s the electrochemical and optical behavior are investigated for different monomer feed ratios. Compared with homo-polymers, P(PVK-co-EDOT) possesses a better
electrochemical
activity
and
presents
more
excellent
spectroelectrochemical and electrochromic properties. The optical contrast ratio values of P(PVK-co-EDOT) is higher than that of PEDOT and PPVK. Simultaneous, the CE values of P(PVK-co-EDOT) (107.00 cm2 C-1) is about twice higher than that of PEDOT (53.88 cm2 C-1). Moreover, the electrochemical properties and electrochromic properties of the copolymer are improved. Furthermore, the electrochromic devices are prepared by reversibly switching the color of the polymer films when the potential applied to the polymer-modified electrode changes. It is very clear that it can be accounted as a promising material to be used in smart window applications. Acknowledgements Funding support by Natural Science Foundation of Jiangsu Higher Education Institutions of China (18KJA150008), Natural Science Foundation of Jiangsu Province (BK20181428), the National Natural Science Foundation of China (51762018, 51863009, and 51873136). References [1] S. Gong, W. Schwalb, Y. Wang, Y. Chen, Y. Tang, J. Si, B. Shirinzadeh, W. Cheng, A wearable and highly sensitive pressure sensor with ultrathin gold
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Electrochromic Smart Windows Can Achieve an Absolute Private State through Thermochromically Engineered Electrolyte, Adv Energy Mater 9 (2019) 1900433. [17] M. Li, J. Zhang, H.J. Nie, M. Liao, L. Sang, W. Qiao, Z.Y. Wang, Y. Ma, Y.W. Zhong, K. Ariga, In situ switching layer-by-layer assembly: one-pot rapid layer assembly via alternation of reductive and oxidative electropolymerization, Chem Commun (Camb) 49 (2013) 6879-6881. [18] E. Contal, C.M. Sougueh, S. Lakard, A. Et Taouil, C. Magnenet, B. Lakard, Investigation of Polycarbazoles Thin Films Prepared by Electrochemical Oxidation of Synthesized Carbazole Derivatives, Front Mater 6 (2019) 131. [19] R. Liguori, A. Botta, A. Rubino, S. Pragliola, V. Venditto, Stereoregular polymers with pendant carbazolyl groups: Synthesis, properties and optoelectronic applications, Synthetic Met 246 (2018) 185-194. [20] E.G. Cansu-Ergun, Covering the More Visible Region by Electrochemical Copolymerization of Carbazole and Benzothiadiazole Based Donor-Acceptor Type Monomers, Chinese J Polym Sci 37 (2018) 28-35. [21] J. Jang, Y. Nam, H. Yoon, Fabrication of Polypyrrole- Poly(N-vinylcarbazole) Core-Shell Nanoparticles with Excellent Electrical and Optical Properties, Adv Mater 17 (2005) 1382-1386. [22] J. Xu, W. Zhou, H. Peng, S. Pu, J. Wang, L. Yan, Electrosyntheses of Freestanding and Conducting Poly[poly(N-vinyl-carbazole)] Films in Tetrahydrofuran Containing Additional Boron Trifluoride Diethyl Etherate, Polym J 38 (2006) 369-375. [23] G. Ye, J. Xu, X. Ma, Q. Zhou, D. Li, X. Liang, X. Duan, W. Zhou, Enhancing effect of boron trifluoride diethyl etherate electrolytes on capacitance performance of electropolymerized poly[poly(N-vinyl-carbazole)] films, J Solid State Electr 21 (2016) 81-90. [24] F. Jiang, F. Ren, W. Zhou, Y. Du, J. Xu, P. Yang, C. Wang, Freestanding poly[poly(N-vinyl carbazole)]-supported Pt-based catalysts with enhanced performance for methanol electro-oxidation in alkaline medium, Fuel 102 (2012) 560-566. [25] X. Liu, Y. Hu, L. Shen, G. Zhang, T. Cao, J. Xu, F. Zhao, J. Hou, H. Liu, F. Jiang, Novel copolymers based on PEO bridged thiophenes and 3,4-ethylenedioxythiophene: Electrochemical, optical, and electrochromic properties, Electrochim Acta 288 (2018) 52-60. [26] Y. Hu, F. Jiang, B. Lu, C. Liu, J. Hou, J. Xu, Freestanding oligo(oxyethylene)-functionalized polythiophene with the 3,4-ethylenedioxythiophene building block: electrosynthesis, electrochromic and thermoelectric properties, Electrochim Acta 228 (2017) 361-370. [27] Y. Hu, X. Liu, F. Jiang, W. Zhou, C. Liu, X. Duan, J. Xu, Functionalized Poly(3,4-ethylenedioxy bithiophene) Films for Tuning Electrochromic and Thermoelectric Properties, The journal of physical chemistry. B 121 (2017) 9281-9290. [28] E. Tutuncu, M. Icli Ozkut, B. Balci, H. Berk, A. Cihaner, Electrochemical and optical characterization of a multielectrochromic copolymer based on
3,4-ethylenedioxythiophene and functionalized dithienylpyrrole derivative, Eur Polym J 110 (2019) 233-239. [29] R. Yue, S. Chen, C. Liu, B. Lu, J. Xu, J. Wang, G. Liu, Synthesis, characterization, and thermoelectric properties of a conducting copolymer of 1,12-bis(carbazolyl)dodecane and thieno[3,2-b]thiophene, J Solid State Electr 16 (2011) 117-126. [30] S. Singhal, A. Patra, Layer-by-layer versus copolymer: Opto-electrochemical properties of 1,3,5-Tris(N-carbazolyl)benzene and EDOT based polymers, J Electroanal Chem 19 (2019) 113296. [31] Y. Jia, C. Liu, J. Liu, C. Liu, J. Xu, X. Li, L. Shen, Q. Jiang, X. Wang, J. Yang, F. Jiang, Efficient enhancement of the thermoelectric performance of vapor phase polymerized poly(3,4-ethylenedioxythiophene) films with poly(ethyleneimine), J Polym Sci Pol Phys 57 (2019) 257-265. [32] R. Pernites, A. Vergara, A. Yago, K. Cui, R. Advincula, Facile approach to graphene oxide and poly(N-vinylcarbazole) electro-patterned films, Chem Commun (Camb) 47 (2011) 9810-9812. [33] L. Wang, D. Zhao, C. Liu, G. Nie, Low-potential facile electrosynthesis of freestanding poly(1H-benzo[g]indole) film as a yellow-light-emitter, J Polym Sci Pol Chem 53 (2015) 2730-2738. [34] G. Lacerda, C. Calado, H. Calado, Electrochromic and electrochemical properties of copolymer films based on EDOT and phenylthiophene derivatives, J Solid State Electr 23 (2019) 823-835. [35] M. De Keersmaecker, A.W. Lang, A.M. Osterholm, J.R. Reynolds, All Polymer Solution Processed Electrochromic Devices: A Future without Indium Tin Oxide?, ACS Appl Mater Interfaces 10 (2018) 31568-31579. [36] M. Sezgin, O. Ozay, S. Koyuncu, H. Ozay, F. Baycan Koyuncu, A neutral state colorless phosphazene/carbazole hybride dendron and its electrochromic device application, Chem Eng J 274 (2015) 282-289.
Figure 1. Electrochemical deposition of (A) 0.03 M PVK, monomer mixtures with PVK/EDOT= (B) 3:1 and (C) 0.03 M EDOT in 0.05 M [Bu4N] PF6 / THF at 50 mV s-1 on a platinum wire. Cyclic voltammograms of (D) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT= (E) 3:1 and (F) PEDOT on a platinum wire in 0.05 M [Bu4N] PF6 / THF solution at different scanning rates. The scanning rates refer to 25, 50, 100, 150, 200, 250, and 300 mV s-1. Insider: the digital photo image of the obtained copolymer.
Figure 2. FTIR-ATR spectra (A) of (a) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT = (b) 1:1, (c) 2:1, (d) 3:1, and (e) PEDOT films at the wavelength range of 600-1800 nm. TG curves (B) of (a) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT = (b) 1:1, (c) 2:1, (d) 3:1, and (e) PEDOT films under a nitrogen atmosphere.
Figure 3. AFM images of (a, e) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT = (b, f) 2:1, (c, g) 3:1, and (d, h) PEDOT films; (a–d) are height images and (e–h) are phase images.
Figure 4. Spectroelectrochemistry of P(PVK-co-EDOT) obtained with PVK/EDOT= (A) 2:1, (B)3:1 and (C) PEDOT on an ITO glass. Below are the corresponding photos of copolymer under oxidized/reduced states.
Figure 5. (a) Current densities and (b) electrochromic switching of P(PVK-co-EDOT) obtained with PVK/EDOT= (A) 2:1, (B)3:1 and (C) PEDOT on an ITO glass.
Figure 6. Digital photos of the electrochromic device.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
28
Table 1. Electrochromic parameters of PPVK, P(PVK-co-EDOT) and PEDOT films at their maximum absorption wavelength. Feed
max
Polymers
Response time (s)
CE
∆T% Oxidation
Reduction
(cm2 C-1)
21.9
3.4
2.8
107.00
565
39.1
4.0
2.6
101.08
3:1
575
26.0
4.4
4.6
64.13
PEDOT
0:1
585
37.6
1.2
4.2
53.88
PPVK
1:0
-
-
-
-
-
P(PVK -co-EDOT)
Ratio
(nm)
1:1
530
2:1
29
30
Declaration of interest statement I hereby declare, on behalf of myself and my co-authors, that: the paper submitted is an original work and not under consideration for publication by any other journal, and that all authors are aware of the submission and agree to its publication. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
31
Credit Author Statement Xiaofang
Liu: Conceptualization,
Methodology,
Investigation,
Writing
-
original
draft. Tianmin Cao: Formal analysis, Writing - review & editing. Wenqian Yao: Software, Formal analysis. Lanlan Shen: Writing - review & editing. Jingkun Xu: Conceptualization, Visualization. Fengxing Jiang: Conceptualization, Visualization, Investigation, Resources. Yukou Du: Resources, Supervision, Funding acquisition.
32
S0021-9797(20)30297-6 https://doi.org/10.1016/j.jcis.2020.03.016 YJCIS 26131
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
3 December 2019 5 March 2020 5 March 2020
Please cite this article as: X. Liu, T. Cao, W. Yao, L. Shen, J. Xu, F. Jiang, Y. Du, A freestanding electrochromic copolymer for multicolor smart window, Journal of Colloid and Interface Science (2020), doi: https://doi.org/ 10.1016/j.jcis.2020.03.016
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A freestanding electrochromic copolymer for multicolor smart window Xiaofang Liua,b, Tianmin Caob, Wenqian Yaob, Lanlan Shenb, Jingkun Xub, Fengxing Jiangb,*, Yukou Dua,* a
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China. b
Department of Physics, Jiangxi Science and Technology Normal University, Nanchang 330013, P. R. China.
Abstract: Electrochromic
devices
with
low-cost,
energy-saving
advantages,
and
controllable color switching have gained widely attention. Yet, electrochromic materials are limited for smart window due to challenges such as difficulty freestanding, monotonous color change, slow switching capability, and low optical contrast. In this work, a freestanding copolymer based on Poly(N-vinylcarbazole) (PVK) and 3, 4-ethoxylenedioxythiophene (EDOT) are designed. The copolymer synthesized by the good secondary film-forming of PVK not only contains the self-standing property of PVK, but also possesses excellent electrical and electrochemical properties of EDOT. The freestanding copolymer is used to create the multicolor: brown, dark brown, purple, blue. A high optical contrast of up to 39.1% and a color efficiency of up to 107.00 cm
2
C-1 prove a significant coloration and
bleaching effect, which is satisfactory for the application of electrochromic devices. Further, an electrochromic device based on P(PVK-co-EDOT) as coloring materials is constructed. This work contributes new ideas into the design of electrochromic smart
materials. Keywords: Copolymer, electrochemistry, electrochromics 1.
Introduction Electrochromic (EC) devices exhibited different optical features when different
bias voltages were applied and shown a broad range of applications in electronic displays, smart windows, goggles, and anti-glare rearview mirrors [1-3]. Therefore, EC materials were required to possess various advantages, including low power consumption, low applied potential, memory effect under open circuit and tunable color depth [4]. Moreover, over the devices comprising glass substrates, flexible EC devices would have significant merits including light-weight, formability, etc. [5, 6]. Such flexible devices would present the specific advantage of versatile shapability. Their light weight and adaptability to curved shape would make it easy to integrate into building and automotive glass, allowing them to be added as layers to existing windows without the need to completely replace smart windows [7-9]. However, as result of the limited availability of transparent conductive films and EC materials with the require optoelectronic properties and durability, it remains challenging to develop EC devices. Furthermore, most of EC materials limited its use in devices due to their brittleness and high-temperature handing requirements [10, 11]. Electronic devices have attracted widespread attention, and the development of high-performance electrochromic materials paly great significance roles in future electronic devices. EC materials include viologens, transition metal oxides, conducting polymers, and Prussian blue (PB) systems [10, 12]. Among them,
conducting polymers, such as polythiophenes (PThs), polyaniline, and polypyrrole for optoelectronic purposes are widely studied as electrochromic active materials. However, it is very difficult to produce the freestanding films such as polyaniline, because of its infusibility, rigidity of polymer chains, poor mechanical properties and poor solubility. And the electrochemical and EC performances (monotonous color changes, slow switching capability, low optical contrast etc.) of conducting polymers need to be improved and meet the requirement for practical applications [13, 14]. For instance, Zhang et al. [15] reported that the sprayed monodisperse silica/polyaniline (SiO2/PANI) core/shell nanosphere film was used as an electrochromic layer, showing low optical contrast and low speed response. The poly(N-isopropylacrylamide) hydrogel made by Wang et al. [16] exhibited low optical contrast of about 25%. Herein,
polycarbazole
and
its
derivatives
which
possess
outstanding
environmental stability, photoconductivity, and EC properties have attached extensive attentions owing to their potential industrial applications in rechargeable batteries, light emitting diodes, and electroluminescent applications [17-20]. Comparing with polymers of carbazole and other carbazole derivatives, Poly(N-vinylcarbazole) (PVK) has elicited considerable interest due to its good film-forming properties, high glass transition temperature characteristic and optical properties [19, 21]. PVK has the advantage of containing pendant electroactive monomer units in a polyvinyl polymer backbone with the ability to carry out secondary polymerization via a large number of carbazole units [22]. Xu et al. [22-24] had manufactured a freestanding poly[poly(N-vinyl carbazole)] (PPVK) film, which had high-quality with excellent
thermal stability and high electrical conductivity. However, there have been no reports on the EC property of PPVK films obtained by secondary polymerization. The reason is that PPVK cannot exhibit obvious electrochromic property. In this work, we prepare freestanding films by electrochemical copolymerization. Moreover, to improve EC property, PVK is allowed to copolymerize with suitable monomers. EC active materials could be easily prepared by introducing suitable comonomers by electrochemical copolymerization [25, 26]. Owing to the low oxidation potential, good electron donor property, high conductivity, optical transparency, and high stability of EDOT, the forthputting of EDOT as a co-monomer almost dominated the studies of electrochemical copolymerization [4, 25-27]. Furthermore, the high-quality freestanding P(PVK-co-EDOT) films are successfully prepared
by
electrochemical
copolymerization
on
ITO
substrates.
The
electrodeposited EC films exhibit multicolor range from brown, dark brown, purple, and blue. Moreover, the constructed EC device based on P(PVK-co-EDOT) as coloring materials can be switched between blue and purple. 2.
Experimental
2.1. Materials Tetrabutyl-ammonium hexafluorophosphate ([Bu4N] PF6, 98%) was purchased from Energy Chemical Industry Co., Ltd. and used directly as received. Before use, tetrahydrofuran (THF) was purified by distillation with calcium hydride under reduced pressure. 3,4-ethylenediophene (EDOT, 98%) were obtained from Shanghai J&K Scientific Ltd. Poly(N-vinylcarbazole) (PVK) were products of Tokyo Chemical
Industry Co., Ltd. Poly(vinylidene fluoride) (PVDF) was purchased from Fluorochem Co., Ltd. 2.2. Electrochemical Measurements and Electrochemical Polymerization The electrochemical properties tests of monomers and polymers were carried out in a three-compartment system by an electrochemical workstation (Princeton V3) at room temperature. An Ag/AgCl wire for preparing a reference electrode was obtained via electrodeposition on silver wire for a specific period of time (100 ~120s) at a specific potential (1.5 V) with a mass fraction of 50% of HCl (aq) as an electrolyte. Both the working and counter electrodes were Pt wire (diameter: 1.0 mm). A glass having an area of 3 cm2 coated with ITO was used as a working electrode. The
films
were
synthesized
on
the
conductive
electrode
by
electrodeposition, a Pt foil having an area of 4.5 cm2 as the counter electrode and an Ag/AgCl wire as the reference electrode, respectively. PPVK, P(PVK-co-EDOT), and PEDOT were all obtained at constant potential of 1.50 V via electrodeposition. The electrolyte solutions were in anhydrous THF containing 0.05 M [Bu4N] PF6. 2.3. Spectroelectrochemical and Electrochromic Experiments The spectroelectrochemical and electrochromic behavior of as-prepared films were measured in a Princeton V3 potentiostat-galvanostat adapted to a Cary 50 UV/Vis/NIR spectrophotometer in a three-compartment system. Working electrode was the ITO glass (2.5 cm × 0.8 cm active area), the counter electrode was a Pt wire and the reference electrode was an Ag/AgCl wire, respectively. Prior to operation, the supporting electrolyte containing 0.05 M
Bu4NPF6 in anhydrous THF were deaerated with a dry nitrogen stream for 30 minutes. According to literature [28], the following equation is used to calculate the coloration efficiency (CE) of the polymer at a specific wavelength (λmax): CE = △OD / Qd The optical contrast ( Δ T%) at the λ max between de-doped and doped states can be related to the optical density (ΔOD) , which is calculated based on equation:
△OD = log (Tox / Tred) 2.4. Preparation of Electrochromic Device Assembly. The gel electrolyte was prepared by adding [Bu4N] PF6 (0.34 g), dehydrated PVDF (1.30 g) into a flask with three necks and dissolved in THF (17 mL) with stirring, following by vigorous stirring at 60~65°C for 12 h. Finally, the transparent PVDF-based gel electrolyte was obtained. The P(PVK-co-EDOT) with the best electrochromic properties was obtained in the monomer ratio of 1:1 as a color changing material. The transparent gel electrolyte was poured onto the copolymer deposited on the ITO by electrodeposition, carefully pressing another ITO against each other to form an electrochromic device. Finally, the device was sealed by copper foil and epoxy-resin glue. The schematic illustration of the preparation of flexible film and device was shown in Figure 1 and Figure 6. 2.5. Characterization A Bruker Vertex 70 Fourier transform infrared (FT-IR) spectrometer of polymers were measured in the range of 300-4000 nm. The microstructure and texture of the
as-prepared films were researched by atomic force microscopy (AFM, Veeco Multimode, and Plainview, NY). The Perkin-Elmer Pyris Diamond (TG/DTG) thermal analyzer were carried out to obtain thermogravimetric analysis data under a nitrogen stream. 3.
Results and discussion
3.1. Electrochemical behaviour
(Inset Figure 1) The reversibility of electron transfer within the period of electrochemical polymerization is demonstrated by using a highly successful technique, cyclic voltammetry (CV). The electrochemical polymerization of EDOT and its electrochemical copolymerization with PVK are implement via CV scanning. Figure S1 shows the corresponding products of the electrochemical copolymerization between PVK and EDOT. As can be seen from Figure 1A-C, during the homo-polymerization and copolymerization, the first cycles (red lines) are different from each other, which prove that the composition of the initial monomers is different. The two monomers used for electrochemical copolymerization had the similar values of onset oxidation potential (PVK~1.13 V and EDOT~0.78 V vs. Ag/AgCl), which allow to achieve good copolymers through alternating units in the backbone [29]. During the electrochemical copolymerization process, the onset oxidation potential of mix-monomer was slightly negatively shifted compared to homo-monomer. The introduction of EDOT into mixed monomers leads to a reduction at the oxidation potential, which may be result from the electron-donating effect of two adjacent
oxygen atoms in the heterocyclic of EDOT [26]. Moreover, although the CV scanning were carried out for the same number of cycles (10 cycles), the increase of the current intensity in copolymerization were much more compared with homo-polymerization of PVK. As shown in Figure 1A and 1C, the redox peaks of PVK appeared from 0.78 to 1.20 V and of EDOT appeared from -1.35 to 0.52 V, respectively. One can see the apparent differences of CVs among the mixed monomers in Figure 1B and Figure S2A, S2C, implying the achievement of the copolymerization. For mix-monomers, the redox range of 0.10 ~ 0.80 V (1:1), 0 ~ 0.75 V (2:1) and 0 ~ 1.00 V (3:1) could be observed. Moreover, as shown in Figure 1A-C and Figure S2A, S2C, those are observed that the electrochemical property of EDOT are positively inherited by the copolymer, which the contribution of EDOT can be observed during the electrochemical copolymerization process. During both homo-polymerization and copolymerization, the formation of polymer film on the surfaces of the conductive electrode are demonstrated by the formation of new signals in each cycle and their increase in current densities. Simultaneously, a dark blue copolymer was formed on the ITO surface. To investigate the electrochemical redox activity of homo-polymers PPVK and PEDOT as well as copolymer P(PVK-co-EDOT), CVs were examined in supporting electrolyte (devoid of monomer) for electrochemical behavior of polymer (Figure 1D-F and Figure S2B, S2D). Homo-polymers PPVK and PEDOT were found to oxidize at 0.40 and 0.70 V respectively, while copolymers P(PVK-co-EDOT) (the PVK/EDOT monomer feed ratio of 3:1) were oxidized at 0.25 V. Under identical, the
considerable differences confirmed the successful formation of copolymers in oxidation potential between homo-polymers and copolymers [30]. When increasing the content of EDOT component in the membrane, the redox current was widened comparing to PPVK. The higher oxidation resistance of EDOT can explain this. It can be clearly seen that although the current values and the scan rates of the as-polymers in all experimented are greatly different, copolymer synthesized here have good electrochemical
redox
activity.
From
the
illustration
of
Figure
1E,
P(PVK-co-PEDOT), the metallic dark blue freestanding films in its oxidized state are produced. 3.2. FT-IR characterization and thermal properties
(Inset Figure 2) FT-IR spectroscopy was performed in order to characterize the structures of PPVK, P(PVK-co-EDOT), and PEDOT. According to Figure 2A and Figure S3, the essential characterization of PEDOT were that its IR main bands centered at 3644, 3430, 1516, and1090 cm-1 were considered to arise from aromatic C-H vibrating, conjugated C-O stretching, thiophene ring stretching, and C-H vibration as well as deformation, respectively. The peaks observed at 1330 and 748 cm-1 represented the C-H in-plane bending deformation in vinylidene groups and the C-H out-plane bending deformation of C-H bond of benzene ring, which demonstrates the presence of the PVK sell on the EDOT [21]. The peaks at 1482-1448 cm-1 were ascribed to the ring vibration of the N-vinylcarbazole moiety. As shown in Figure 2A and Figure S3, the intensity of characteristic PPVK peaks reduced with reducing PVK in the
backbone. We note in the vibrational spectra of copolymer P(PVK-co-EDOT) the presence of the main characteristic bands of the PEDOT, as well as those of PPVK, which indicate that the new skeletons with alternating PVK and EDOT are formed by polymerization of mix-monomers. Moreover, the new peaks of copolymer were that the new main band observed at 838 and 981 cm-1 was obtained from phenyl ring substitution bands and substitute aromatic ring of PVK. This result further indicates the successful electrosynthesis of P(PVK-co-EDOT). The thermogravimetric experiments were performed under a nitrogen stream at a heating rate of 10 K min-1 to examine the thermal stability of the as-prepared polymer by TG and DTG analyses. In Figure 2B and Figure S4, the thermal degradation curves were the function of the temperature range of 300 to 1050 K, which indicate that polymers exhibited a similar weight loss process. It can be observed that copolymer have higher thermal stability than PPVK and PEDOT. Eventually, P(PVK-co-EDOT) film had a residual weight of 40.8% (the monomer feed ratio of PVK/EDOT=3:1) at 1050 K, which showed better thermal stability than PPVK (25.7%). In summary, the above results indicate that the good thermal properties of the copolymer will be beneficial to application in electrochromism. 3.3. Film Morphology (Atomic Force Microscopy) of polymer
(Inset Figure 3) Atomic force microscopy (AFM) was a typical surface technique suitable for analyzing the nanoscale properties and other properties of thin films of conducting polymers [31]. [Figure 3(a–d)] and [Figure 3(e–h)] show the AFM height images with
the corresponding surface roughness curves and the AFM phase images of homo-polymers and copolymers, respectively. In the characteristic picture of the polymer, there is a significant difference in height between the dark and the bright patterns with the maxima values of 120.7, 80.6, 154.5, and 146.0 nm for the PPVK, P(PVK-co-EDOT) (2:1), P(PVK-co-EDOT) (3:1), and PEDOT films, respectively. The root-mean-square surface roughness of PPVK, P(PVK-co-EDOT) (2:1), P(PVK-co-EDOT) (3:1), and PEDOT films are estimated to be 102.79, 74.57, 38.30, and 27.43 nm, respectively. The AFM images of the film show that the surface is uniform and compactness. The as-prepared film appeared as a tightly packed spherical particle on the surface. The average particle diameter of the copolymer is slightly larger than the homo-polymer. Furthermore, the phase images show in Figure 3(e–h) the clearer difference in the morphology between the homo-polymer and the copolymer. As can be seen from Figure 3(e, h), the dark and the bright boundaries in the PPVK and PEDOT films were very clear with a maximum 215.2 deg and 211.0 deg phase lag, respectively. It is worth noting that after copolymerization, Figure 3(f, g) shows a more uniform surface structure with a smaller phase lag of 88.0 deg and 151.5 deg compared to the PPVK and PEDOT films. Interestingly, the optimal electrochromic properties of copolymer (the PVK/EDOT monomer feed ratio of 2:1) produced an obvious change in the minimum phase lag in Figure 3(f), which indicates that the formation of large particles on the surface of the polymer. This should be interpreted as an indirect reflection of large particles that contribute to electron transport within the polymer [32].
3.4. Spectroelectrochemistry
(Inset Figure 4) The spectroelectrochemistry was a powerful technique used to estimate the effect of optical behavior when applied to different potentials and provide information about its
polaronic
/
bipolaronic
states
and
the
electronic
structure.
The
spectroelectrochemistry of all the homo-polymers and copolymers were performed at different potentials using the supporting electrolyte containing 0.05 M Bu4NPF6 in THF solvent (Figure 4 and Figure S5A, S5C). As can be seen from Figure S5A and Figure 4C, the strong absorbance bands of PPVK and of PEDOT can be observed at about 346 and 591 nm in neutral state, respectively. On the contrary, the max of P(PVK-co-EDOT) had a red-shifts of 224 nm compared to PVK, which can be attributed to the increase in the conjugation length of the copolymers and the introduction of electron-donating EDOT in copolymer backbone [33]. This further confirms the successful formation of the copolymer between EDOT and PVK by electrochemical copolymerization. PPVK polymer had no obvious absorption between neutral state and oxidized state (in the ranges of 400-1100 nm). However, as a result of the generation of polarons/bipolarons, the copolymers showed the maxima absorption peaks at 570 and 900 nm (Figure 4A and B). In neutral state, the maximum absorption wavelength (max) of the copolymer was 570 nm. After oxidation, the intensity of this band started to decrease and shifted to a lower wavelength (about 470 nm). Similarly, at a maximum of approximately 800 nm, a new band started to form, demonstrating that
the polarons are generated. After further oxidation, a new band over beyond 1000 nm was formed, indicating the generation of bipolarons. More importantly, the copolymers showed a multi-electrochromic property: it had brown, dark brown, purple and blue colors at different states (Figure 4-below). In addition, it can be observed that the colors of the original PEDOT were measured under different applied potentials is different from the color of the copolymer. As described in the previous literature [28], the color of PEDOT was an opaque dark blue under its neutral state and exhibited a transparent sky blue after oxidation. 3.5. Electrochromic switching
(Inset Figure 5) In order to study the electrochromic switching process, homo-polymer and copolymer were studied on ITO/glass via chronoabsorptiometry. It is important to determine the change in optical contrast (∆T%) of the polymer film between its oxidized and neutral states. In the chronoabsorptiometric measurement, while applying sequential square-wave bias pulses (which can quickly switch between the reduced and oxidized states of the film), monitor the transmittance of the film is monitored (Figure 5 and Figure S5B, S5D). For PPVK polymers, the intensity of the % transmittance was observed to be almost zero. The use of PEDOT in electrochromic devices has grown resulting in stable electrochromic effects [34]. Adding the EDOT unit to the copolymer helped stabilize the transmittance, and could measure its ∆T%. As shown in Figure 5 and Figure S5, the highest value of the copolymers obtained in the monomer ratio of 2:1
was found to be 39.1%. Compared with PEDOT (37.6%), the optical contrast of the copolymer was improved, which indicate that copolymerization with EDOT has a positive effect on the ∆T%. Furthermore, response times (t-ox and t-red) could be extracted from the optical transmittance data. The optical switching times was one of the principal parameters for judging the superiority of electrochromic materials. The switching time was measured as the time to reach 95% optical contrast of the entire switch, and the results are shown in Figure S6. The copolymers P(PVK-co-EDOT) (the PVK/EDOT monomer feed ratio of 1:1) showed a fast switching of about 3.4 s, while allowing the transitions from the reduced states to the oxidized states. However, in the reverse transition (i.e. from the oxidized state to reduced state), the response time was reduced to 2.8 s. The reduced process was about twice lower than PEDOT (4.2 s). Also, it can be observed that the response time of copolymers P(PVK-co-EDOT) is shorter than that of the PPVK and the response time of reduction of copolymers P(PVK-co-EDOT) are slightly shorter than that of PEDOT. A good electrochromic material should have excellent CE, which means that the charge or energy injected into the polymer can modulate optical and effectively change the optical density. Table 1 summaries the basic optical properties of these polymer films. At 95% of the optical density, it is worth noting that the CE of the copolymer is improved compared to PEDOT (53.88 cm2 C-1). The copolymer P (PVK-co-EDOT) (the PVK/EDOT monomer feed
ratio of 1:1) showed the highest CE values of 107.00 cm2 C-1. The highest CE of the copolymer is almost twice that of PEDOT. This suggest that the conjugated-non-conjugated structures may exhibit a lower band gap and contribute
to
the
improvement
of
electrochromic
properties.
The
P(PVK-co-EDOT) with the best electrochromic properties is obtained with a monomer ratio of 1:1 as the color changing material for device construction.
(Inset Table 1) 3.6. Electrochromic devices
(Inset Figure 6) In order to further exhibit the promising application of P(PVK-co-EDOT) electrode in flexible electrochromic displays, electrochromic devices were manufactured
according
to
the
schematic
(Figure
6),
in
which
P(PVK-co-EDOT) (the PVK/EDOT monomer feed ratio of 1:1) was used as a color changing material. ITO is an excellent material for the counter electrode since it is almost colorless in the oxidized and neutral state and was easily reversibly oxidized and reduced without discoloration [8, 35, 36]. The copolymers are prepared by electropolymerization method. The photographs of devices and color change of films are shown in Figure 6. And when the square-wave potentials were applied between −0.9 V and +0.5 V, the electrochromic device and films both presented an electrochromism with a color reversibly changing from blue to purple. 4. Conclusions
In summary, a freestanding film P(PVK-co-EDOT) has been successfully synthesized via electrochemical deposition, which shows better thermal stability and superior electrochemical activity. The as-prepared copolymer’s the electrochemical and optical behavior are investigated for different monomer feed ratios. Compared with homo-polymers, P(PVK-co-EDOT) possesses a better
electrochemical
activity
and
presents
more
excellent
spectroelectrochemical and electrochromic properties. The optical contrast ratio values of P(PVK-co-EDOT) is higher than that of PEDOT and PPVK. Simultaneous, the CE values of P(PVK-co-EDOT) (107.00 cm2 C-1) is about twice higher than that of PEDOT (53.88 cm2 C-1). Moreover, the electrochemical properties and electrochromic properties of the copolymer are improved. Furthermore, the electrochromic devices are prepared by reversibly switching the color of the polymer films when the potential applied to the polymer-modified electrode changes. It is very clear that it can be accounted as a promising material to be used in smart window applications. Acknowledgements Funding support by Natural Science Foundation of Jiangsu Higher Education Institutions of China (18KJA150008), Natural Science Foundation of Jiangsu Province (BK20181428), the National Natural Science Foundation of China (51762018, 51863009, and 51873136). References [1] S. Gong, W. Schwalb, Y. Wang, Y. Chen, Y. Tang, J. Si, B. Shirinzadeh, W. Cheng, A wearable and highly sensitive pressure sensor with ultrathin gold
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Figure 1. Electrochemical deposition of (A) 0.03 M PVK, monomer mixtures with PVK/EDOT= (B) 3:1 and (C) 0.03 M EDOT in 0.05 M [Bu4N] PF6 / THF at 50 mV s-1 on a platinum wire. Cyclic voltammograms of (D) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT= (E) 3:1 and (F) PEDOT on a platinum wire in 0.05 M [Bu4N] PF6 / THF solution at different scanning rates. The scanning rates refer to 25, 50, 100, 150, 200, 250, and 300 mV s-1. Insider: the digital photo image of the obtained copolymer.
Figure 2. FTIR-ATR spectra (A) of (a) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT = (b) 1:1, (c) 2:1, (d) 3:1, and (e) PEDOT films at the wavelength range of 600-1800 nm. TG curves (B) of (a) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT = (b) 1:1, (c) 2:1, (d) 3:1, and (e) PEDOT films under a nitrogen atmosphere.
Figure 3. AFM images of (a, e) PPVK, and P(PVK-co-EDOT) obtained with PVK/EDOT = (b, f) 2:1, (c, g) 3:1, and (d, h) PEDOT films; (a–d) are height images and (e–h) are phase images.
Figure 4. Spectroelectrochemistry of P(PVK-co-EDOT) obtained with PVK/EDOT= (A) 2:1, (B)3:1 and (C) PEDOT on an ITO glass. Below are the corresponding photos of copolymer under oxidized/reduced states.
Figure 5. (a) Current densities and (b) electrochromic switching of P(PVK-co-EDOT) obtained with PVK/EDOT= (A) 2:1, (B)3:1 and (C) PEDOT on an ITO glass.
Figure 6. Digital photos of the electrochromic device.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
28
Table 1. Electrochromic parameters of PPVK, P(PVK-co-EDOT) and PEDOT films at their maximum absorption wavelength. Feed
max
Polymers
Response time (s)
CE
∆T% Oxidation
Reduction
(cm2 C-1)
21.9
3.4
2.8
107.00
565
39.1
4.0
2.6
101.08
3:1
575
26.0
4.4
4.6
64.13
PEDOT
0:1
585
37.6
1.2
4.2
53.88
PPVK
1:0
-
-
-
-
-
P(PVK -co-EDOT)
Ratio
(nm)
1:1
530
2:1
29
30
Declaration of interest statement I hereby declare, on behalf of myself and my co-authors, that: the paper submitted is an original work and not under consideration for publication by any other journal, and that all authors are aware of the submission and agree to its publication. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
31
Credit Author Statement Xiaofang
Liu: Conceptualization,
Methodology,
Investigation,
Writing
-
original
draft. Tianmin Cao: Formal analysis, Writing - review & editing. Wenqian Yao: Software, Formal analysis. Lanlan Shen: Writing - review & editing. Jingkun Xu: Conceptualization, Visualization. Fengxing Jiang: Conceptualization, Visualization, Investigation, Resources. Yukou Du: Resources, Supervision, Funding acquisition.
32