Remarkable Ability To Modulate Light Transmittance and Block Heat

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Remarkable Ability To Modulate Light Transmittance and Block Heat in the Bleached State Combined in One Electrochromic Material: Highly Crystalline Polyaniline Natalia Gospodinova,* Taras Skorokhoda, and Volodymyr Lobaz Institute of Macromolecular Chemistry AS CR, 162 06 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: It is revealed that chain ordering strongly affects optical and electrochromic properties of polyaniline. Polyaniline film with 70% crystalline phase combines the remarkable ability to modulate the light transmittance (coloration efficiency of 150 cm2/C with a contrast ratio approaching 10 at λ = 633 nm) with an unprecedented “bright-cool” bleached state, where the polymer is highly transparent to light but has low transmittance for the most intense near-infrared irradiation. Compared to the recently developed dual-band electrochromic materials in which modulation of the near-infrared absorption is realized through plasmonic electrochromism of WO3−x nanoparticles, the highly crystalline polyaniline film demonstrates in the “bright-cool” regime a 10 times lower transmittance at 900 nm (1.6 vs 16%) with the same level (≈60%) of transmittance in the visible region. The coloration efficiency found here is almost 3-fold higher than commonly communicated for polyaniline.

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Aldrich was cleaned ultrasonically in acetone, ethanol, and distillated water before using. Supported and free-standing highly crystalline PANI films were obtained according to the procedure described by us earlier.11−13 Briefly, to obtain the supported film the deposition of PANI on FTO (or glass) support was performed during polymerization at 0 °C in an aqueous solution containing 0.04 mol L−1 aniline and ammonium peroxydisulfate, 5 mol L−1 formic acid, and 6.4 mol L−1 NaCl. The support was immersed in the reaction medium before beginning the polymerization. After completion, the PANI-covered support was washed with 5 mol L−1 formic acid and dried at ambient conditions. PANI precipitated during polymerization was separated from reaction medium, washed, and dried on the glass support to obtain free-standing films. Two-dimensional WAXS measurements were performed using a BM26 of ESF (European Synchroton Facility, Grenoble, France). The strips of the film were mounted with the film surface parallel to the Xray beam: in this configuration the X-ray beam penetrates the film thickness. Supported films were detached from glass support in 5 mol L−1 formic acid solution and transferred to Teflon support before drying. The X-ray profile was indexed by the whole pattern fitting procedure. The degree of crystallinity of PANI was calculated from the diffraction curve corrected for the background and the Lorentz factor.14 The dc conductivity was measured using the standard four contact method. Four parallel gold contacts were deposited on the surface of a strip of the film by evaporating gold metal under high vacuum (10−6 mbar). The measurements were carried out in the temperature range 4−300 K in a helium gas flow cryostat (Oxford Instruments CF 1200 D). The electron paramagnetic resonance (EPR) spectrum of the film was obtained by using a Bruker ER 200 EPR instrument at a frequency

INTRODUCTION The traditional electrochromic materials developed for smart windows applications modulate the transmittance in the visible range of the solar spectrum.1−4 The two most commonly studied classes of electrochromics are transition metal oxides, for example, WO3,1 and conducting polymers, such as polyanilines (PANI), polypyrroles, and polythiophenes.2−7 A recently developed concept of dual-band electrochromic materials that independently modulate the lighting and heating fluxes is based on the coupling of traditional electrochromics with metal oxide nanocrystals (tin-doped indium oxide, aluminum-doped zinc oxide, and WO3−x), which modulate NIR transmittance through electrochemical charging and discharging of the free electrons responsible for their localized surface plasmon resonance (LSPR) absorption.8−10 For example, WO3−x nanocrystals exhibit LSPR absorption peak at 875 nm.9 The ability of such materials to simultaneously absorb heat and be transparent to light (realization of so-called “bright-cool” regime) should significantly decrease the energy consumption of buildings. Here, it is shown that a PANI film with 70% crystalline phase provides both an unprecedented “bright-cool” bleached state and remarkably high electrochromic efficiency in the visible range of the solar spectrum. The decisive role of chain ordering on the optical and electrochromic properties of PANI is highlighted.

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EXPERIMENTAL SECTION

Analytic grade aniline, ammonium peroxydisulfate, formic, sulfuric, nitric, hydrochloride acids, and sodium chloride were used as received (Lach-ner, Czech Republic). Fluorine-doped tin oxide (FTO)-coated glass slide with surface resistance of about 10 Ω/sq purchased from © XXXX American Chemical Society

Received: November 30, 2017 Revised: March 3, 2018

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DOI: 10.1021/acs.macromol.7b02543 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules of 9.5 GHz at ambient conditions. For both measurements the freestanding films with thickness of 20 μm were used. Highly crystalline PANI films deposited on FTO support during synthesis were used for spectral and spectroelectrochemical measurements. The spectroelectrochemical measurements were performed using an AvaSpec-ULS2048L spectrophotometer (Avantes) coupled with the AUTOLAB PGSTAT302N potentiostat. The photoelectrochemical cell PECC-2 (Zahner Scientific Instruments) with platinum wire as a counter electrode and Ag/AgCl as a reference one was used. The measurements were made in ambient conditions with 5 mol L−1 formic acid as an electrolyte. All the spectra were normalized by subtraction of the spectrum of the cell with electrolyte and bare FTO support.

represents the distance between nuclei planes; the chains in the lattice adopt a planar zigzag conformation. Location of the peak corresponding to the shortest distance between nuclei planes in the equator signifies perpendicular orientation of the nuclei with respect to the film surface (more details about crystalline lattice and chain orientation are given in Figure S1). Both supported and free-standing films are characterized by the same crystalline lattice, degree of crystallinity (70%), and chain orientation. A remarkable feature of the highly crystalline PANI films is that the repeating distance along the chain (parameter c of the crystalline lattice) corresponds to four aniline motifs, in contrast to the two-motif repeating unit in the “common” semicrystalline PANI obtained by chemical or electrochemical synthesis.15 It is considered that a dimeric repeating unit is characteristic of the polaron (radical cation) configuration, in which both rings have an intermediate (semiquinone) oxidation state, whereas a tetrameric elementary unit composed of rings with low and high oxidation states represents the bipolaron (dication or localized polaron) organization.16−21 Thus, it can be supposed that the bipolaron configuration dominates in the highly crystalline PANI film. It should be mentioned that in contrast to the experimental studies that suggest a predominant role for the polaron configuration in the conducting PANI,16−19 the theoretical studies demonstrate that the spineless bipolaronic state is more stable.20,21 The highly crystalline PANI film is characterized by the room temperature conductivity of 0.6 S cm−1 (Figure 1b), close to that of the “common” PANI. The location of the longwavelength maximum in the optical spectrum at 590 nm (Figure 1c, black spectrum) rather than at approximately 800 nm, which is typical for polaron defects,16−18,22 can be explained by the predominant bipolaron configuration in the highly crystalline PANI. The detection of an EPR signal (inset in Figure 1b) as well as a low-energy tail in the UV−vis−NIR spectrum (Figure 1c, black spectrum) indicates, most probably, the presence of a low amount of isolated polarons in the polymer. As suggested in ref 18, the absorption of isolated polarons is red-shifted with respect to that of the polarons in the polaron lattice. Remarkably, the film that was immersed in aqueous acid (as well as the “wet” film after the synthesis) exhibits spectral features consistent with the polaronic form of PANI (red spectrum in Figure 1c). This transition is completely reversible in multiple cycles of “immersion drying” and is independent of the used acid (HCl, HCOOH, HNO3, H2SO4, H3PO4). As supposed, transformation of bipolarons to polarons occurs via an internal redox process.16−19 We hypothesize that the presence of the aqueous acid as an electrolyte is necessary for the realization of this redox process. Such an electrolyte could be either “external” (PANI immersed in a solution) or “internal” (trapped in the amorphous phase of the dry polymer). Taking into account that highly crystalline PANI contains a substantially lower percent of the amorphous phase (≈30%) than the “common” semicrystalline PANI (higher than 50%),15 the probability of the internal redox process occurring in the dry sample should be noticeably lower in the first case. The presumed higher amount of “internal” electrolyte in the amorphous phase of “common” semicrystalline PANI, compared to the highly crystalline polymer, can explain the predominant polaron configuration in the dry sample. It should to be noted, however, that the magnetic susceptibility measurements on “common” semicrystalline PANI are

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RESULTS AND DISCUSSION A high level of ordering and a strong chain orientation in the highly crystalline film can be clearly seen from the twodimensional WAXS spectrum presented in Figure 1a. The diffraction peaks are indexed as triclinic crystal system with following lattice constants: a = 12.27 Å, b = 3.69 Å, c = 19.24 Å, α = 94.6°, β = 90°, and γ = 97.2°, where d-spacing a corresponds to the distance between two neighboring chains related by intercalated water molecules and chloride anions, c is the distance along to the chain (four aniline motifs), and b

Figure 1. (a) Equatorial and meridian patterns extracted from the twodimensional WAXS spectrum of the highly crystalline film, where the film surface is parallel to the X-ray beam. (b) Temperature dependence of the conductivity with EPR spectrum (inset). (c) UV−vis−NIR spectrum of the highly crystalline PANI film in the dry state (black) and immersed in aqueous 5 mol L−1 formic acid (red). B

DOI: 10.1021/acs.macromol.7b02543 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules sensitively dependent on moisture.23,24 It was supposed that small amounts of moisture in combination with some residual acid in the sample have the effect of increasing the delocalization length of charge carriers. It should be emphasized that the highly crystalline PANI film immersed in aqueous acid demonstrates not only a strong redshift of the long-wavelength maximum but also a 1.6-fold increase in the absorption intensity (red spectrum in Figure 1c). Furthermore, despite evident similarities between the spectra of the dry highly crystalline PANI film and that polarized at 0.7 V (see the black spectrum in Figure 1c and Figure 2, top) indicating predominant bipolaron configuration

range with a very low transmittance toward the most intense NIR irradiation (1.6 and 5.7% at 900 and 1000 nm, respectively), clearly exhibiting a so-called “bright-cool” bleached state. “Common” PANI at low potentials is typically transparent for both light and heat, demonstrating only negligible absorption with maximum at approximately 800 nm.4−7 The unusually high intensity of the NIR absorption in the bleached state can be explained by the high degree of chain ordering in the film. The red-shift in the long-wavelength absorption maximum observed during reduction of the highly crystalline film is most probably related to the consecutive isolation of polarons. It should be underlined that compared to the dual-band materials based on WO3−x nanocrystals,9 the highly crystalline PANI film demonstrates in the “bright-cool” regime a 10 times lower transmittance at 900 nm (1.6 vs 16%) with the same level (≈60%) of transmittance in the visible region. The electrochromic performance of the film in the visible region was characterized by the electrochromic contrast (change in the transmittance between the colored and bleached states, ΔT = Tb − Tc), optical contrast ratio (ratio of the optical densities of the colored and bleached states, CR = Ac/Ab), and coloration efficiency, CE (ratio of the change in the optical density to the corresponding injected/ejected charge, CE = Ac − Ab/ΔQ). Electrochromic switching from the bleached (at 0 V) to the colored state (at 0.7 V) was studied by monitoring the transmittance change at 633 nm in response to potential step chronoamperometry (Figure 3). Coloration efficiency was estimated within the first 95% of overall optical change.

Figure 2. UV−vis−NIR spectra of the highly crystalline PANI film at different applied potentials (top); evolution of the transmittance in the visible and NIR ranges of the solar spectrum in response to an applied potential at selected wavelengths (bottom); inset: view through the film polarized at 0 and 0.7 V. All measurements were performed in aqueous 5 mol L−1 formic acid.

in both cases, the latter demonstrates an almost 2-fold increase in the absorption intensity and a red-shift in the absorption maximum by 30 nm. We explain this phenomenon by an improvement in the overall chain ordering under immersion of the film in the aqueous acid. Namely, formation of the watermediated interchain hydrogen bonds can induce “ordering” in the amorphous zones interconnecting the crystallites. As has been revealed by us earlier,11−13 water-mediated hydrogen bonds play a crucial role in the creation of crystalline order in the PANI films. As proven for 3-hexylthiophene,25 reinforcement of the interchain interactions leads to a substantial increase in the intensity of light absorption and a red-shift in the absorption maximum. The evolution in the transmittance of the highly crystalline PANI film at wavelengths representing visible (500 and 633 nm) and most intense NIR irradiation (900 and 1000 nm) in response to an applied potential is presented in Figure 2 (bottom). A consecutive shift in the long-wavelength absorption maximum from 830 to 905 nm is observed upon reduction of the film from 0.4 V (corresponding to open circuit condition) to 0 V (Figure 2, top). As seen, the film polarized at 0 V has a transmittance of approximately 60% over the visible

Figure 3. Evolution of the transmittance of the highly crystalline PANI film at λ = 633 nm (top) and the corresponding injected/ejected charge (bottom) in response to potential step chronoamperometry.

As seen, a high light transmittance in the bleached state and a very low transmittance in the colored state (0.3%) give rise to an unprecedentedly high (approaching 10) optical contrast ratio. For comparison, a transmittance of 20−40% in the colored state and an optical contrast ratio of 3−5 are typically measured at the same wavelength for PANI.4−7 Similar values C

DOI: 10.1021/acs.macromol.7b02543 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules have been found for polythiophenes10 and niobium oxide9 used as light-modulating materials in dual-band electrochromic materials. Poly(3,4-ethylenedioxythiophene), one of the most efficient electrochromic materials, displays in the colored state a transmittance of approximately 4% at 633 nm with an optical contrast ratio of approximately 4.3 Furthermore, the coloration efficiency found here (150 cm2/C at 633 nm) is almost 3-fold higher than that commonly communicated for PANI.5−7 This unprecedented performance is related both to the high optical contrast ratio and the high efficiency of the electrode processes. As has been shown by us earlier,11,13 the highly crystalline PANI film during contact with a Fe-containing needle in aqueous acid demonstrates a propagation of reduction (discoloration) over an area exceeding by almost 300 times diameter of the needle. Furthermore, a high switching stability of the highly crystalline PANI film with an electrochromic contrast loss of only 8% after 2000 cycles (see Figure S2) indicates that a major portion of the redox processes contributes to the color modification of the film.

D. Anokhin for help in performing the X-ray diffraction analysis and the interpretation of the obtained data.

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(1) Granqvist, C. G. Electrochromics for Smart Windows: Oxide based Thin Films and Devices. Thin Solid Films 2014, 564, 1−38. (2) Beaujuge, P. M.; Reynolds, J. R. Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices. Chem. Rev. 2010, 110, 268−320. (3) Sonmez, G.; Sonmez, H. B.; Shen, C. K. F.; Wudl, F. Red, Green, and Blue Colors in Polymeric Electrochromics. Adv. Mater. 2004, 16, 1905−1908. (4) Watanabe, A.; Mori, K.; Iwasaki, Y.; Nakamura, Y.; Niizuma, S. Electrochromism of Polyaniline Film Prepared by Electrochemical Polymerization. Macromolecules 1987, 20, 1793−1796. (5) Wei, H.; Yan, X.; Wu, S.; Luo, Z.; Wei, S.; Guo, Z. Electropolymerized Polyaniline Stabilized Tungsten Oxide Nanocomposite Films: Electrochromic Behavior and Electrochemical Energy Storage. J. Phys. Chem. C 2012, 116, 25052−25064. (6) Zhang, J.; Tu, J.; Zhang, D.; Qiao, Y.; Xia, X.; Wang, X.; Gu, C. Multicolor Electrochromic Polyaniline−WO3 Hybrid Thin Films: One-pot Molecular Assembling Synthesis. J. Mater. Chem. 2011, 21, 17316−17324. (7) Wei, H.; Zhu, J.; Wu, S.; Wei, S.; Guo, Z. Electrochromic Polyaniline/Graphite oxide Nanocomposites with Endured Electrochemical Energy Storage. Polymer 2013, 54, 1820−1831. (8) Runnerstrom, E. L.; Llordes, A.; Lounis, S. D.; Milliron, D. J. Nanostructured Electrochromic Smart windows: Traditional Materials and NIR-Selective Plasmonic Nanocrystals. Chem. Commun. 2014, 50, 10555−10572. (9) Kim, J.; Ong, G. K.; Wang, Y.; LeBlanc, G.; Williams, T. E.; Mattox, T. M.; Helms, B. A.; Milliron, D. J. Nanocomposite Architecture for Rapid, Spectrally-Selective Electrochromic Modulation of Solar Transmittance. Nano Lett. 2015, 15, 5574−5579. (10) Barile, C. J.; Slotcavage, D. J.; McGehee, M. D. Polymer− Nanoparticle Electrochromic Materials that Selectively Modulate Visible and Near-Infrared Light. Chem. Mater. 2016, 28, 1439−1445. (11) Gospodinova, N.; Dorey, S.; Anokhin, D.; Ivanov, D.; Romanova, J.; Kolev, H. Centre National de la Recherche Scientifique and Universite de Haute-Alsace, Method of Preparing Polyaniline Films and Highly Self-oriented Films Obtained. US Patent US8986790 B2, 2015. (12) Gospodinova, N.; Ivanov, D.; Anokhin, D.; Mihai, I.; Vidal, L.; Brun, S.; Romanova, J.; Tadjer, A. Unprecedented Route to Ordered Polyaniline: Direct Synthesis of Highly Crystalline Fibrillar Films with Strong π-π Stacking Alignment. Macromol. Rapid Commun. 2009, 30, 29−33. (13) Gospodinova, N.; Musat, V.; Kolev, H.; Romanova, J. New Insight into the Redox Behavior of Polyaniline. Synth. Met. 2011, 161, 2510−2513. (14) Alexander, L. E. X-Ray Diffraction Methods in Polymer Science; R.E. Krieger: Huntington, NY, 1979. (15) Pouget, J. P.; Jozefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. X-ray Structure of Polyaniline. Macromolecules 1991, 24, 779−789. (16) Macdiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Polyaniline: a New Concept in Conducting Polymers. Synth. Met. 1987, 18, 285−290. (17) Ginder, J. M.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Insulator-to-Metal Transition in Polyaniline. Solid State Commun. 1987, 63, 97−101. (18) Bernard, M. C.; Hugot-Le Goff, A.; Joiret, S.; Arkoub, H.; Saıdani, B. Influence of the nature of substituent on the charge mechanisms in substituted polyanilines (SPANI, POMA) studied by Raman and optical spectroscopies. Electrochim. Acta 2005, 50, 1615− 1623. (19) Kon’kin, A.; Shtyrlin, V.; Garipov, R.; Aganov, A.; Zakharov, A.; Krinichnyi, V.; Adams, P.; Monkman, A. EPR, Charge Transport, and

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CONCLUSIONS In summary, this work reveals that chain ordering has a strong impact on optical and electrochromic properties of PANI. The PANI film with 70% crystalline phase exhibits almost 3-fold higher coloration efficiency at λ = 633 nm than “common” PANI. Moreover, such a film exhibits an exceptionally high NIR absorption in the bleached state. Until now, the ability to simultaneously absorb heat and be transparent to light has been demonstrated only for dual-band electrochromic materials in which traditional electrochromics are coupled with metal oxide nanocrystals absorbing in the NIR. We assume that the high intensity of light and NIR absorption (originating from bipolarons and isolated polarons, respectively) as well as the high efficiency of electrode processes are related to the unprecedented chain ordering in the polyaniline films. In this way, the present work sheds new light on the relationships between the chain ordering and the fundamental properties of PANI and contributes to the general knowledge of electrochromic materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02543. Figures S1 and S2 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +420 296 809 268; Fax +420 296 809 410 (N.G.). ORCID

Natalia Gospodinova: 0000-0001-5568-1892 Volodymyr Lobaz: 0000-0003-0479-2837 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Czech Republic Grand Agency (no. 15-14791S) and by the European Regional Development Fund (CZ.2.16/3.1.00/21545). The authors acknowledge Dr. D

DOI: 10.1021/acs.macromol.7b02543 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Spin Dynamics in Doped Polyanilines. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 075203. (20) Kuwabara, M.; Shimoi, Y.; Abe, S. Polaron versus bipolaron in conducting polymers: A density matrix renormalization group study. J. Phys. Soc. Jpn. 1998, 67, 1521−1524. (21) Petrova, J. N.; Romanova, J. R.; Madjarova, G. K.; Ivanova, A. N.; Tadjer, A. V. Fully Doped Oligomers of Emeraldine Salt: Polaronic versus Bipolaronic Configuration. J. Phys. Chem. B 2011, 115, 3765− 3776. (22) Cao, Y.; Smith, P.; Heeger, A. J. Spectroscopic studies of polyaniline in solution and in spin-cast films. Synth. Met. 1989, 32, 263−281. (23) Kahol, P. K.; Guan, H.; McCormick, B. Moisture Effect on the Magnetic State in Polyaniline. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 44, 10393−10395. (24) Javadi, H. H. S.; Angelopoulos, M.; MacDiarmid, A. G.; Epstein, A. J. Conduction Mechanism of Polyaniline - Effect of Moisture. Synth. Met. 1988, 26, 1−8. (25) Brown, P. J.; Thomas, D. S.; Kohler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Effect of interchain interactions on the absorption and emission of poly 3-hexylthiophene. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 064203.

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DOI: 10.1021/acs.macromol.7b02543 Macromolecules XXXX, XXX, XXX−XXX