Journal of Electroanalytical Chemistry 886 (2021) 115113 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem Study on the electrochromic properties of polypyrrole layers doped with different dye molecules Maryam Bayat a, Hossein Izadan a,⇑, Sara Santiago b, Francesc Estrany c,d, Mohammad Dinari e, Dariush Semnani a, Carlos Alemán c,d,⇑, Gonzalo Guirado b,⇑ a Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b Departament de Química, Universitat Autònoma de Barcelona, 08193 Barcelona, Cerdanyola del Vallès, Spain c Departament d’Enginyeria Química, EEBE, Universitat Politècnica de Catalunya, C/ Eduard Maristany 10-14, Ed. I2, 08019 Barcelona, Spain d Barcelona Research Center for Multiscale Science and Engineering, Universitat Politècnica de Catalunya, Eduard Maristany 10-14, 08019 Barcelona, Spain e Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran A R T I C L E I N F O A B S T R A C T Keywords: Three dye molecules (Dye) of Acid Brilliant Scarlet 3R (AR18), Amido Naphthol Red G (AR1), Indigo Carmine Polypyrrole (IC), as well as sodium dodecyl sulfate (SDS) as dopant agents were used for electrochemical synthesis of poly- Electrochromic pyrrole (PPy) layers onto indium doped tin oxide (ITO) coated polyethylene terephthalate (PET) electrode. The Spectroelectrochemical morphology, electrochemical, optical, and spectroelectrochemical properties of the layers were investigated. Dye molecules The study of the electrochemical behavior showed that the presence of each AR18, AR1, or IC with SDS, Dopant had shown an excellent synergistic effect on the electrochemical stability of layers. The morphological charac- terization of the PPy/dopant(s) using atomic force microscopy (AFM) showed that the surface roughness in the PPy/IC-SDS layer was 39% and 32% less than the PPy/AR18-SDS and PPy/AR1-SDS, respectively. The absorp- tion spectrum of PPy/dopant(s) in the UV–Vis-NIR wavelength range showed the formation of polaron and bipolaron in PPy chains. Also, the optical bandgap energy of PPy/dopant(s) decreased, and the fully doped state in all PPy films was observed. Spectroelectrochemical properties of the films showed that the simultane- ous use of each dye molecule and the surfactant as dopant in PPy layers demonstrated proper electrochemical and optical stability and satisfactory electrochromic parameters. For example, the color contrast of PPy/AR18- SDS was 50%, while this parameter in control sample (PPy doped with lithium perchlorate) was 21%. Also, the cathodic and anodic coloration efficiency showed a 6-fold increase in PPy/Dye-SDS compared to PPy/ClO-4. In general, according to the results it is likely that by increasing the number of anion groups in the dye molecules and decreasing their dimensions as dopant agents, the electrochemical and electrochromic properties of the resultant layer would be improved. 1. Introduction solution electrochromes, solution-solid electrochromes, and solid elec- trochromes. In the first group, are the compounds that both states, the Electrochromism is a process in which an electroactive material initial and the oxidized or reduced state, are soluble in the electrolyte changes its color due to electron transfer (oxidation/reduction). Mate- during the color switching. In the second group, one of the states is rials that are capable of reversible color change due to the application insoluble while the other dissolves in the media. In the third type, of external potential are called electrochromic materials [1]. Platt in the materials are solid in both oxidation states that is to say, before 1961 stated for the first time that a color shift of hundreds of and after applying a voltage. A wide range of inorganic and organic angstroms in some dyes is also possible when applying an external chemicals are used in electrochromic processes, including but not lim- electric potential [2]. Electrochromic materials can be classified into ited to inorganic oxides, Prussian Blue, metal hexacyanometallates, three main categories according to their solubility of their redox states: metal phthalocyanines, viologens, and polymers [1,3]. ⇑ Corresponding authors at: Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran (H. Izadan); Departament d’Enginyeria Química, EEBE, Universitat Politècnica de Catalunya, C/ Eduard Maristany 10-14, Ed. I2, 08019 Barcelona, Spain (C. Alemán); Departament de Química, Universitat Autònoma de Barcelona, 08193 Barcelona, Cerdanyola del Vallès, Spain (G. Guirado). E-mail addresses: maryam.bayat@alumni.iut.ac.ir (M. Bayat), izadan@iut.ac.ir (H. Izadan), sara.santiago@uab.cat (S. Santiago), francesc.estrany@upc.edu (F. Estrany), dinari@iut. ac.ir (M. Dinari), d_semnani@iut.ac.ir (D. Semnani), carlos.aleman@upc.edu (C. Alemán), gonzalo.guirado@uab.cat (G. Guirado). https://doi.org/10.1016/j.jelechem.2021.115113 Received 12 January 2021; Received in revised form 14 February 2021; Accepted 22 February 2021 Available online 9 March 2021 1572-6657/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Electrochromic polymers are a group of conducting polymers that PPy doped with polyanions is provided with stability and mechanical are capable of color change under external voltage. These include properties [41,45,46]. polyaniline, polypyrrole (PPy), polyfuran, polycarbazole, and poly- Very limited studies using dyes as dopants in PPy, have concluded thiophene derivatives [1,3–6]. Electrochromic polymers have that the electrochromic properties improved because of the conjugated achieved significance, in part because of their easy control of forma- properties of such compounds [47–52]. Moreover, investigations tion by spin coating, self-assembly, drop casting or electropolymeriza- showed that dyes can also be placed between polymer chains, creating tion techniques, among others, to form solid and robust films [7–9]. nano-pathways for better electron transfer. In addition, the previous Although conducting polymers exhibit, in general, less electrical con- study [53] showed that simultaneous use of dye molecule (AR18) ductivity than metals, the conductivity as well as some other physical and surfactant (SDS) as a dopant agent in polypyrrole had improved and chemical properties of such compounds, can be altered using the electrochemical properties of the polypyrrole layer produced. dopants agents [10]. The present study aims to determine the effect of the molecular The theory of polaron and bipolaron can interpret the conductivity structural properties of the dye on the properties of PPy layers. There- of conducting polymers. When polymers are oxidized or reduced, a fore, three dyes, which differ in the size and the number of negatively polaron is formed, that is, the formation of a radical cation or radical charged groups, have been used separately as dopants with SDS of PPy. anion is created, respectively. It is known as bipolaron when polarons The morphology and the electrochemical, optical, and electrochromic are doble charged. Partial doping produces polarons, while doping at a properties of the resulting PPy films have been investigated. It should higher level produces bipolarons. The formation of polarons and bipo- be noted that the Acid Brilliant Scarlet 3R (AR18) and Amido Naphthol larons, lows the energy gap (Eg) between the valence band (HOMO) Red G (AR1) dyes (Fig. 1) have similar dimensions, but a different and conduction band (LUMO) that results in the overlapping of the number of negatively charged groups. In contrast, AR1 and Indigo Car- bands where the electrons are free to move in the conduction band mine (IC) dyes have the same number of negatively charged groups, thus, increasing the conductivity. The doping process increases the but different molecular sizes. polymer’s electrical conductivity by creating moving charges in the polymer structure [11,12]. Conducting polymers are semi-conductors 2. Experimental in the neutral (non-doped) state, but in redox states the bandgap energy is reduced, and the electrical conductivity increases. For exam- 2.1. Materials ple, in the case of PPy, the bandgap energy is 3.16 eV in the neutral state, while the bandgap energy decreases to 1.72 eV in the fully doped Pyrrole (Py) from Aldrich was double distilled and kept in a refrig- state [13,14]. The doping process in polymers consists of adding elec- erator in a dark BG Bottle before use. Anhydrous lithium perchlorate tron donor or acceptor species to their molecular structure. The pres- (LiClO4) of analytical reagent grade was used as the electrolyte and ence of dopants causes a significant change in the molecular dopant for the control sample (Sigma-Aldrich). AR1 from Sigma- structure of the polymer compared to the non-doped state [15]. Aldrich, AR18 from Panreac, IC from Merck, and sodium dodecyl sul- Oxidation produces a polymer with a positive charge, which is known fate (SDS) from Sigma were used as dopants. All of the solutions were as p-doping. On the other hand, reduction produces a polymer with a made using Milli-Q water. negative charge, which is known as n-doping [16,17]. PPy and its derivatives are among the most well-known electroac- 2.2. Electropolymerization of doped PPy tive and conducting polymers. The oxidation potential for pyrrole (0.8 V) is lower than for other aromatic monomers; thus it is synthe- Polypyrrole (PPy) films were electropolymerized on ITO-PET sized both chemically and electrochemically in a wide range of organic sheets using ClO− 4 (control sample) AR1, AR18, IC, and SDS as dopant and aqueous solvents [18–21]. Indeed, PPy is one of the few conduct- agent. In the case of the PPy/ClO-4 chronoamperometry was applied to ing polymers that can be produced in aqueous solutions. Additionally, a solution consisted of lithium perchloride and the pyrrole monomer PPy has been considered more than other conducting polymers due to and water. Whereas, the solutions prepared for obtaining PPy/IC- its high conductivity, ease, and flexibility in production, stability, and SDS films contained water, dye molecule (IC), surfactant (SDS), and satisfactory mechanical properties [22,23]. Hence, PPy has various pyrrole monomer. The PPy/dopant films were prepared on the conduc- applications, such as in electronic and electrochromic systems tive electrode (ITO-PET). by chronopotentiometry (CP), applying con- [14,24–26], as capacitors [27,28], in sensors [29–32], in light batter- stant currents of 1.5 mA/cm2 for PPy/ClO− 2 4 , and 1.0 mA/cm for PPy/ ies [33–35], and in separating membranes [36,37]. AR18, PPy/SDS, PPy/AR18-SDS, PPy/AR1-SDS, and PPy/IC-SDS for Although there are various methods for the synthesis of conducting 150–200 s. After producing the films, an aqueous solution of lithium polymers, electrochemical polymerization is preferable for various rea- perchloride was used as an electrolyte to investigate their electrochem- son, such as easy control of the thickness, and high polymerization ical and electrochromic behavior. speed [38]. The use of dopants affects the general features and thermal The volume of each synthesis solution was 10 mL, and the concen- stability of PPy [39–41]. The presence of dopant anions, which must tration of the materials in the aqueous synthesis solution used is shown be electrochemically stable, creates a charge balance in the polymer in Table 1. All PPy films were synthesized using a three-electrode one- structure. The use of aqueous solutions in the synthesis of PPy signif- compartment cell at room temperature. After preparing the aqueous icantly increases the selectivity of the dopants, which can be small synthesis solutions with a certain amount of monomer (Py) and or large compounds anions. The type, size, three-dimensional struc- dopants, the oxygen was replaced by injecting nitrogen (99.995% in ture, charge, and electronic structure of the dopant, the deposition purity) for 10 min. ITO-PET sheets were used as a working electrode conditions, the electrode substrate, and solvent used determine the with 1 × 1.5 cm2 area with 60 Ω/sq surface resistance, while stain- properties of PPy film produced by electrochemical synthesis less-steel sheets (AISI 316L) were used as a counter electrode with a [23,42–44]. Polyelectrolytes or anionic surfactants are examples of surface of 1.5 × 2 cm2. The Ag|AgCl electrode that contained a satu- large dopant anions. Unlike small dopants, which enter and exit from rated KCl aqueous solution was used as a reference electrode. the polymer matrix processes, large anions usually remain immobile The electropolymerization of PPy is affected by various parameters, and stable in the polymer structure. In the latter case, cations present including the electrochemical method used, the current or potential in the electrolyte solution balance the charge during electrochemical applied, the deposition duration, the concentration of each material, processes (i.e., the penetration and escape of cations during the elec- the dopant, and the type of solvent. These factors were optimized in trochemical reduction and oxidation of PPy, respectively). Moreover, a previous work [53]. In the present work, the optimal conditions were 2 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 1. Chemical structure of dye Acid Red1, Acid Red18, and Indigo Carmine. The size of molecules was measured with ChemDraw software. Table 1 Concentration of dopants used for the electrodeposition of PPy films on the ITO-PET sheet. Electropolymerization of PPy was performed by CP at 1–1.5 mA/cm2 for 150–200 s duration. The concentration of Py monomer was 0.1 M in all cases. PPy film [ClO− 4 ] (M) [SDS] (M) [AR18] (mM) [AR1] (mM) [IC] (mM) Thickness (µm) PPy/ClO− 4 0.1 – – – 5.0 ± 0.3 PPy/SDS – 0.1 – – 5.0 ± 0.3 PPy/AR18 – – 0.5 – 5.0 ± 0.2 PPy/AR18-SDS – 0.1 0.5 – 5.1 ± 0.2 PPy/AR1-SDS – 0.1 – 0.5 4.9 ± 0.2 PPy/IC-SDS – 0.1 – – 0.5 5.0 ± 0.2 used for all produced films. All PPy/dopant films have almost the same The optical characterization and the electrochromic performance of thickness and a good electrochemical response. PPy/dopants films were recorded by in situ spectroelectrochemistry in the absorption mode. 2.3. Equipment 3. Results and discussion Electrochemical analyses were performed using an Autolab PGSTAT302N potentiostat/galvanostat (Ecochimie, the Netherlands) 3.1. Electrochemical behavior and a conventional three-electrode system. Autolab devices were equipped with NOVA software and interfaced to the PC system. The PPy/dopants films electrodeposited on ITO-PET sheets were range of potential was selected between − 1.0 V and + 1.0 V vs washed with distilled water and dried in the vacuum oven at room Ag|AgCl, using 0.1 M LiClO4 aqueous solution as electrolyte. The scan temperature. The successful incorporation of the dyes was investigated rate was 100 mV/s in all cases. by FTIR, Raman and energy-dispersive X-ray (EDX) spectroscopies and Electrochromic parameters and absorption spectra of PPy layers scanning electron microscopy (SEM). The most successful evidences were determined using in-situ spectroelectrochemical techniques. A were achieved by EDX, as is shown below in the morphological study. VSP100 potentiostat model (EC-Lab V9.51 software) was coupled to Unfortunately, the noise added by oxidized PPy, the charged sulfonyl an L12090 Hamamatsu spectrophotometer (Bio-Kine 32 V4.46 soft- groups, the sodium, and the low amount of dyes as dopant agents ware). The electrodes used in the spectroelectrochemical studies make difficult by FTIR and Raman spectroscopies. However, Raman included the PPy/dopant coating on ITO sheets as working electrode, spectra allowed to identify some shoulders that were attributed to a Pt wire counter electrode, and a saturated calomel electrode (SCE) as the sulfonyl and azo groups of the dyes. This is illustrated in Fig. S1, reference electrode. Finally, a 0.1 M LiClO4 aqueous solution was used which compares the spectra and micrographs recorded by µ-Raman as the electrolyte in all measurements. for PPy/SDS, PPy/AR18-SDS and PPy/AR18. Despite the remarkable Surface morphology of films was evaluated using a Zeiss Neon 40 differences found in the superficial morphology, the spectra recorded analytical field emission scanning electron microscope (FESEM). The for the three samples are very similar. However, the band at applied voltage for observation was 5 kV. Energy dispersive X-ray 1047 cm−1 (SO–3) and the shoulders at 1454 cm−1 (N@N) and (EDX) spectroscopy was performed with the same scanning electron 1534 cm−1 (Azo-benzyl) suggest the incorporation of the dye. microscope. The electrochemical behavior of the films was determined by cyclic Topographical analyses of the films were conducted by atomic voltammetry (CV), using a LiClO4 0.1 M aqueous solution as elec- force microscopy (AFM) using a VEECO Dimension 3100 model, with trolyte. The voltammograms of all PPy/dopants, with exception of a Molecular Imaging PicoSPM using a NanoScope IV controller under PPy/IC-SDS, were reported in a previous work [53]. Fig. 2 compares ambient conditions. The tapping mode AFM was operated at constant the voltammogram of PPy/IC-SDS with those recorded for deflection. The row scanning frequency was set to 1 Hz. AFM measure- PPy/AR18-SDS and PPy/AR1-SDS after 25 consecutive redox cycles. ments were performed on various parts of the layers, which provided Since during successive cyclic in CV diagrams of each layer, the area reproducible images similar to those displayed in this work. The statis- between the oxidation and reduction curves is almost constant, so tical application of the NanoScope Analysis software was used to deter- the doping of the dye molecule and the surfactant in the polymer mine the root mean square roughness (Rq), which is the average height chains is definite. Also, the comparison of CV diagrams of the layers deviation taken from the mean data plane. showed that the electrochemical activity of PPy/AR18-SDS is higher A Dektak 150 stylus profilometer (Veeco, Plainview, NY, USA) than that of PPy/AR1-SDS and PPy/IC-SDS, which is probably due to measured the thickness of layers. the number of anionic groups and dimension of the dye. As the number 3 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 each layer, including the method and the concentration of materials, are the same, just only the type of dye molecule is different between the PPy layers. The diagrams of the current density of PPy/AR1-SDS and PPy/IC- SDS films and the voltammograms after 2 and 25 redox cycles are shown in Fig. 3. The presence of AR1 and IC dyes improves the elec- trochemical behavior of the PPy layers, with the electrostability increasing 14% and 10% for PPy/AR1-SDS and PPy/IC-SDS, respec- tively, after 25 consecutive redox cycles. This self-electrostabilizing behavior has been attributed to the presence of the dye, which facili- tates the transfer of electrons among the polymer chains. The limit of current density in each redox cycle was higher for PPy/IC-SDS than for PPy/AR1-SDS, which has been associated to the larger size of IC. 3.2. Morphological and topographical characterization Fig. 2. Cyclic voltammograms for PPy/AR18-SDS, PPy/AR1-SDS, and PPy/IC- Fig. 4 (left) shows representative scanning electron microscopy SDS layers after 25 redox cycles in 0.1 M LiClO4 aqueous solution. Scan rate: (SEM) images of PPy/SDS, PPy/AR18-SDS, PPy/AR18 and PPy/IC. 100 mV/s. Although all samples displayed a globular morphology, this is more compact for films without SDS. Thus, the size of the globules is higher for PPy/SDS and PPy/AR18-SDS than for PPy/IC and, especially, PPy/ of anionic groups increases and the dimension of the dye decreases, AR18. Energy dispersive X-ray (EDX) spectra, which are included in the redox capacity of the PPy film increases, suggesting that the num- Fig. 4 (right), show the presence of sulfur and sodium, demonstrating ber of charge carriers in the polymer matrix increases with the nega- the successful incorporation of the SDS and the dyes as dopant agents. tively charged density groups. Also, the distance between the Thus, the characteristic signal of sulfur, which comes from the sulfate polymer chains decreases by reducing the size of the dye, enhancing of SDS and the sulfonate of dyes, is clearly detected in all spectra. the charge transfer rate, and the electrochemical activity. Therefore, Instead, the sodium signal is only observed for SDS-containing films. the properties of the PPy/dopant film are affected by two factors: This has been attributed to the content of the different dopant agent, the number of negatively charged groups in the dopant, and its dimen- which in turn is related to the flexibility of SDS and the rigidity of sions. According to Fig. 1, the AR18 has a smaller size in comparison to AR18 and IC. Thus, the entrance of flexible SDS in the polymeric AR1 and IC. Besides, AR18 has three anionic groups, while AR1 and IC matrix is easier than that of rigid dyes and, therefore, the content of have only two. These results are while all the conditions of synthesis at the former is expected to be higher than that of the latter. Accordingly, Fig. 3. Variation of the current density versus the time (left). Voltammograms for PPy/AR1-SDS and PPy/IC-SDS after 2 and 25 redox cycles (right). 4 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 4. Representative SEM micrograph (left) and EDX spectrum (right) of (a) PPy/SDS, (b) PPy/AR18-SDS, (c) PPy/AR18 and PPy/IC. 5 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 5. The 3D (a) and 2D (b) AFM images for PPy/SDS, PPy/AR18, PPy/AR18-SDS, PPy/AR1-SDS, and PPy/IC-SDS films. 6 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Table 2 Surface Roughness (Rq) for different PPy films. This data was measured with NanoScope Analysis software of AFM images. PPy/SDS PPy/AR18 PPy/AR18-SDS PPy/AR1-SDS PPy/IC-SDS Rq (nm) 766 576 1434 1364 1030 where h is the Plank constant, c is the speed of light and λonset is the col- lision point of the tangent line to the absorption spectra in π  π  transmissions. The results, which are listed in Table 3 show two λmax for each film, confirm the formation of PPy. Thus, the absorption in the range of 400–500 nm is related to π-π* transfers in the neutral and polaron state, while absorption at λmax > 700 nm is related to bipolaron mode in PPy chains [51]. The optical bandgap energy of PPy was 3.16 eV in the neutral state. This value decreases to 2.26, 1.76, and 1.46 eV for the polaron, bipo- laron, and the fully doped state, respectively [14]. The values of εg dis- played in Table 3 indicate that PPy is in fully doped and bipolaron states in all films prepared in this work, no significant difference among the different PPy/dopants films being detected. 3.4. Spectroelectrochemical characterization Fig. 6. UV–Vis-NIR spectra for PPy/ClO-4, PPy/SDS, PPy/AR18, PPy/AR18- SDS, PPy/AR1-SDS, and PPy/IC-SDS films. The spectroelectrochemical properties of PPy layers electrode- posited on transparent ITO-PET electrodes are studied using a 0.1 M aqueous solution of LiClO4 as supporting electrolyte. A UV–Vis spec- trometer, in the absorbance mode, is coupled and synchronized to a Table 3 Energy bandgap (ɛg ) and maximum absorption wavelength (λmax) of PPy layers. potentiostat and CVs with scan swept 0 V/ −1 V/ +1 V and 10 mV/s scan rate is carried out. Fig. 7 corresponds to the 3D and System λmax (nm) ɛ g (eV) 2D absorption spectrum for each PPy layer and significant differences PPy/ClO-4 426, 936 1.3 are found for each sample. In general, when the potential is shifted PPy/AR1-SDS 462, 930 1.37 from − 1 V to + 1 V, the absorbance decreases at λmax 410 nm range PPy/SDS 462, 999 1.41 while it increases in the range of 650–1100 nm. This spectroscopic PPy/IC-SDS 462, 924 1.44 response is likely to be due to a change in the PPy chains from the PPy/AR18-SDS 428, 1003 1.45 PPy/AR18 502, 914 1.75 polaron form to the bipolaron one [51]. Therefore, the higher the number of bipolarons, the lower is the energy bond in the polymer, which resulted in the wavelength shift to larger values when oxidized, and the higher is the conductivity of the polymer (Fig. 6) [14]. The potential sweep of the voltametric measurement, at slow scan the sodium signal detected in PPy/SDS and PPy/AR18-SDS spectra rate (10 mV/s), allowed determining that the intensity and width of corresponds to the sodium ions necessary for neutralizing the sulfate the peaks were gradually changed at each small variation of potential. groups of exceeding SDS molecules. Moreover, the λmax and shape of each spectrum was different, indicat- The topographical characterization of the films was performed with ing that the PPy layers have unique colors at each voltage. Thus, it is AFM (Fig. 5), the surface roughness (Rq) being measured with NanoS- possible to have different colors in the PPy layers and achieve a multi- cope software (Table 2). Results indicate that the type of dopant affects color electrochromic layer with different dopants in the PPy chains. the surface characteristics of the produced films. The presence of a sur- Furthermore, the comparison of the PPy layers showed that the factant as a dopant in PPy/SDS increases the Rq compared to the pres- absorption intensity at λmax for thin films of 5 μm thickness, in oxidized ence of the dye molecule as a dopant in PPy/AR18, which may be or reduced form, was high, so that the color change was recognizable related to the opening of the polymer structure in the presence of to the naked-eye for all layers except the control layer (PPy/ClO-4). But SDS. The combination of a dye and SDS in PPy/AR18-SDS has a syner- as in Fig. 7, the absorption spectrum changed by applied voltage in gistic effect on Rq, these results being consistent with those of Girotto each layer. This change in the absorption spectrum is mean creating and de Paoli [48]. The comparison of films with different dyes as a a different color in the layer. To better understand these color changes, dopant shows that Rq decreases with increased linear structure and the color calculations were performed at − 1, 0, and 1 voltage. amount of negatively charged groups: Rq for PPy/IC-SDS < Rq for According to Table S1, the dopant type is very effective in the final PPy/AR1-SDS < Rq for PPy/AR18-SDS. color of the layer. Hence, in the next section, the electrochromic parameters were investigated in PPy/dopant for a better comparison 3.3. Optical characterization of layers. To investigate the optical behavior of the PPy/dopant films, absorption spectra were recorded in the UV–Vis-NIR wavelength range 3.5. Electrochromic parameters in natural states. As shown in Fig. 6, the absorption spectra changed with the presence of the different dopants. The optical bandgap energy Several electrochromic parameters - color contrast, response time, of the PPy was calculated using Eq. (1) [53]: coloration efficiency, and cyclability- that describe the spectroelectro- chemical responses of prepared PPy films, are determined in this sec- h  c 1240 tion, since the final applications of the PPy films, markedly rely on ɛg ¼ ¼ onset ð1Þ λonset λ these electrochromic properties. Apart from the intrinsic properties 7 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 7. Spectroelectrochemical behavior of PPy layers on ITO-PET sheets with different dopants in 0.1 M aqueous solution of LiClO4 as supporting electrolyte during a 10 mV/s scan rate using CV in the scan sweep 0 V/+1 V/-1 V. The variation of absorption to wavelength to potential/ time in 3D (a) and 2D (b). 8 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 7 (continued) of the electrochromic material (i.e., the PPy films), these parameters (≈ 580–980 nm; dark green and blue), and the color contrast at a are strongly influenced by the ionic intercalation, necessary to com- 780 nm wavelength was very close to the color contrast at λOxidized max . pensate charges when the material is oxidized or reduced, since the In general, on comparing the electrosynthetized films PPy/dopant, amount and speed of electron and ion transfer between polymer they show enhanced color contrast with respect to the control sample chains, or interface between the polymer layer and electrolyte solution (PPy/ClO4) achieving an improvement of 29%. This result suggests 36. For this reason, the use of stabilizers may help to enhance the ionic that the type of dopants used, individually or simultaneously, has high intercalation, and therefore the electrochromic performance. positive effects on the properties of the PPy layers. In contrast to pre- In the present study, all the PPy films were synthesized using the vious reports, the synthetized PPy/dopant films obtained in this study, CP method that allowed reproducible thicknesses and comparable show greater color contrast as shown in Table 5. Presumably, this fact films to be obtained. Ultimately, the electrochromic performance for is because of the suitability of the synthesis method and the proper each case can be compared in order to determine the effect of each placement of the dopants between the polymer chains, which eventu- additive in the final performance. For the spectroelectrochemical ally led to the high order of the polymer chains. study, the chronoamperometric (CA) method was used to determine Studies of the dye molecule as dopant in PPy (PPy/AR18) increases the electrochromic parameters for the PPy/dopants films (Fig. 8). the color contrast of the layers by 30%. According to Ferreira [50] and Tavoli [51], the reason for this improvement in the color contrast is because the dye molecule forms a nano-bridge between the PPy 3.5.1. Color contrast chains, which leads to an increase in the order of the PPy structure The color contrast is defined as the change in percentage of trans- (as shown in XRD analysis). Ion and charge transfer in the PPy struc- mittance at λmax between the oxidized and reduced states of PPy tures were improved due to the formation of the nano-bridge. The ðΔT% ¼ TReduced  TOxidized ). The color contrast is influenced by the type presence of large conjugated bonds in the dye molecule can also allow of dopant, the synthesis conditions (such as temperature) that affect to establish electronic interaction with the π bonds of PPy chains, the ordination of the polymer chains, and the thickness of the polymer which can change the electronic properties of PPy. Finally, the pres- layer [14,50–52]. When the PPy coatings are very thin, the transmit- ence of the dye molecule can keep the structure of PPy chains constant tance, which is proportional to the concentration of the species, is in the quinoid forms. This is associated with the formation of polaron small, and therefore the color contrast as well. On the other hand, and bipolaron in PPy chains, thus the color contrast of PPy layers was thicker layers are more opaque, and this makes it difficult to effec- increased [49–51]. It is worthy to notice that the presence of three tively change the color between the oxidized and reduced states negatively charged groups in the case of AR18 instead of two sulfonate [50]. This can be explained as the undoped form of PPy the neutral groups (IC and AR1) could help the formation of nano-bridges between state- is an electrical insulator material [22]. To determine the color PPy chains, therefore better color contrasts are obtained for the system contrast at different wavelengths in the oxidized and reduced states PPy/AR18. Besides, the 41% of color contrast can be related to the sta- (Table 4), [54] a constant potential of − 1 V is applied for 60 s fol- bilization of the reduced and oxidized form for the system PPy/SDS lowed by + 1 V for 60 s. The 2D and 3D spectra are shown in since SDS is a well-known stabilizing agent. Fig. 8. The color contrast was determined at different wavelengths in A comparison of the color contrast of the layers (for example, at the oxidized and reduced states (as shown in Table 4) [54]. Some of 780 nm) showed that the simultaneous use of the dye molecule and these wavelengths were: the wavelength of the maximum absorption the surfactant as a dopant (PPy/AR18-SDS layers) produces a higher spectrum in the neutral state of the polymer layers, the maximum color contrast than the separate use of the dopants for the elec- wavelength in the oxidized or reduced states [54], the wavelength of trochromic films. This increase confirms the synergistic effect of the π  π  transmission in polymer chains [55], and the wavelength in dye molecule and the surfactant and also the effect of one does not which the most significant absorption difference was observed interfere with the mechanism of the second. The comparison of the between oxidized and reduced states [56]. Also, if the maximum vari- color contrast obtained for the three films PPy/AR18-SDS, PPy/IC- ation of transmittance is higher than the visible wavelength range SDS, and PPy/AR1-SDS showed that the presence of AR18 dye mole- (>780 nm), it is referred to as the NIR optical contrast [57,58]. cule in the PPy structure had a better effect on the color contrast of Therefore, in this study, the optical contrast of the produced layers the film. The AR18 dye molecule had 22% and 71% more color con- is obtained at the λmax of the absorption spectrum in the oxidation and trast than the layers containing the IC and AR1 Dye, respectively. reduction states. Also, the optical contrast was obtained for all films at The difference in the color contrast of the layers is likely to be related a constant wavelength in the visible region (Table 4). to the configuration of the dye molecule (i.e., the dimensions, the The data in Table 4 show that the calculated color contrast is differ- number of anion groups, and how the anion groups were placed in ent depending on the λmax , since the molar extinction coefficient, which the dye structure). Each of these factors affects the placement of the measures how strongly a substance absorbs light, is characteristic at dye molecule, the mode of electron transmission, and the amount of each wavelength. Therefore, the color contrast calculated at λOxidized max electron transfer between the polymer chains. (≈ 410 nm; yellow) of the PPy/dopant was higher than the λReduced max 9 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 8. Spectroelectrochemical behavior of PPy layers on ITO-PET electrodes with different dopants in 0.1 M LiClO4 aqueous solution as supporting electrolyte. (a) 3D and (b) 2D spectrum for the samples when a constant potential of − 1.0 V vs SCE for 60 s is applied to obtain the reduced state (red dashed line) and + 1.0 V for next 60 s for the oxidized state (blue dashed line). 10 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 8 (continued) Table 4 Table 6 Color contrast for PPy layers at various wavelengths. Anodic and cathodic response times for electrochromic layers at a different wavelength. System λReduced max λOxidized max ΔT%atλReduced max ΔT%atλOxidized max ΔT%atλ780nm (nm) (nm) System τc ðsÞatλReduced max τa ðsÞatλOxidized max τc ðsÞatλ410nm τa ðsÞatλ780nm PPy/ClO-4 412 581 16 21 26 PPy/ClO-4 3.8 6 3.8 3 PPy/SDS 413 927 29 41 41 PPy/SDS 12.9 7.5 12.5 9.2 PPy/AR18 413 864 11 54 53 PPy/AR18 21.5 12.1 21.8 15 PPy/AR18-SDS 409 976 25 50 55 PPy/AR18-SDS 47.8 17.2 47.8 20.2 PPy/IC-SDS 413 871 14 44 45 PPy/IC-SDS 48.5 29.8 48 31.3 PPy/AR1-SDS 419 861 13 33 23 PPy/AR1-SDS 45 21.9 44.9 22.5 in the structure. The data obtained for the response time of PPy layers Table 5 showed that the type of dopants have an effect on the response time. In Comparison of color contrast and response time of different studies that used Dye. general, the smaller the size of dopant (for example, in PPy/ClO-4- layer), the shorter the response time of the layer, because the path System ΔT% τc ðsÞ τa ðsÞ references of electrons is shorter, with response times below 60 s being obtained PPy-DS-IC 37 (700) 8 [48,49] for all the samples. The presence of the dye molecule as a dopant (PPy/ PPy/DS – 50 [49] AR18) increased the response time compared to the use of the SDS sur- PPy-RB 41 (795) 14 [50] factant as a dopant (PPy/SDS), due to the presence of conjugated PPy-DR 23 (795) 9 [50] PPy-ECR 27 (800) <1.5 [52] bonds in the dye molecule structures. For the samples containing the PPy-ClO-4 7 (800) >2 [51] dye and surfactant molecules (PPy/AR18-SDS) a slow response, in PPy-IC 35 (700) 12 3 [52] the order of seconds, is determined. Finally, a comparison between 15 (530) [52] the response times of PPy films containing AR18, AR1, and IC mole- PPy-IC-Au(nanoparticles) 44 (700) 10 4 [52] cules, shows that the fastest time response is obtained for PPy/AR18 22 (530) [52] because the dye molecule has more anion groups. In addition, it is known that the length of AR18 molecule is 13 Å, being smaller than the AR1 or IC, 13.3 Å and 15 Å, respectively. There 3.5.2. Response time seems to be a correlation between the response time and the dopant The response time is defined as the time required to change 90% of size, since, in the case of PPy/AR1-SDS, the response time is shorter the transmittance of an electrochromic substance at a specific λ than PPy/IC-SDS layers. Therefore, these results indicate that two fac- [56–61]. For the response time calculations, CA measurements are per- tors affect the response time; the number of anion groups and the formed by applying − 1.0 V, and a subsequent + 1.0 V vs SCE for 60 s. aspect ratio of the dopants structure. According to Fig. 1, the AR18 Note that the response time was usually measured between the color- structure has both factors; hence PPy/AR18-SDS has a shorter response ing and bleaching states. However, in this study, coloring was seen in time compared to PPy/AR1-SDS or PPy/IC-SDS. If the number of anion both oxidized and reduced states. Therefore, the response time of the groups is the same in different dopants, the dimensions of the dopant electrochromic layer is different at each redox state, thus in this paper, structure increase their influence on the properties. In the case of the two response times were defined; an anodic response time (τa Þ and a two dyes, AR1 and IC, it was observed that PPy/IC-SDS exhibits a cathodic response time (τc Þ In order to facilitate the calculation of longer response time than PPy/AR1-SDS. Thus, the dimension of the the response times, a part of the respective λmax Oxidized and λReduced shown max structure determines the length of electron passages between the in Table 4 for each sample, two more λ were fixed at 780 nm and PPy chains. 410 nm. However, the main improvement of the current synthesis method- The results of the response times in the four wavelengths are shown ology presented in this work does not rely on the enhancement of the in Table 6, whereas the response time diagrams for all PPy/dopants at response times, since, as shown in Tables 5 and 6, slower response λ ¼ 780 nm and λ = 410 nm, are shown in Fig. 9. Note that the times were obtained. response times were shorter at λmax Oxidized or λReduction for each sample with max respect to the ones calculated at 780 and 410 nm. Furthermore, the 3.5.3. Coloration efficiency anodic response times in PPy layers are more concise than the cathodic The coloration efficiency (η) is defined as the change in the optical response times, which means that the cationic state of the doped PPy is density (ΔOD) per unit of electric charge applied (Qd) to the elec- more stable than their anionic state because of the SO-3 groups present trochromic material (Equation (2)). The ΔOD is calculated with the 11 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 9. The anodic (at 780 nm) and cathodic (at 410 nm) response time diagrams for (a) PPy/ClO4, (b) PPy/SDS, (c) PPy/AR18, (d) PPy/AR18-SDS, (e) PPy/AR1- SDS, and (f) PPy/IC-SDS layers at same wavelength in the visible range. Table 7 Coloration Efficiency of PPy / dopant films produced. System Tc/TaatλReduced max Tc/TaatλOxidized max Tc/Taatλ780nm Qc (mC) Qa (mC) ηc (cm2/C) ηc (cm2/C) PPy/ClO-4 0.75 1.33 1.4 20 34 70 41.2 PPy/SDS 0.52 2.68 2.5 27 50 277.8 150 PPy/AR18 0.77 2.48 2.41 27 61 267.8 118.5 PPy/AR18-SDS 0.60 2.34 2.35 18 28 391.7 251.8 PPy/IC-SDS 0.73 2.18 2.18 39 23 167.7 284.4 PPy/AR1-SDS 0.74 1.9 1.86 24 35 323.5 159.4 ratio of the logarithm of Tc % (transmittance of the reduced state) and The electrodeposition synthesis is fully controlled and optimized, Ta% (transmittance of the oxidized state) at λmax [14]. Hence, in our all the resulting PPy films are 3 cm2 of surface area. Table 7 shows case the transmittance values are measured at λReduced max , λOxidized max , and the total charge required for the oxidation or reduction. In these stud- 780 nm (Table 7) for PPy films with and without dopants, with the ies, CA measurements are carried out by applying − 1.0 V and + 1.0 V Tc/Ta at λOxidized ;ratio being very close to one in all the cases. vs SCE for 60 s each. This time was stipulated as that necessary to max ensure the fully oxidized or reduced state of the polymer and thus ΔOD log ½Tc =Ta  η¼ ¼ ð2Þ obtain more accurate results. PPy is an electrochromic material in both Qd Q=A 12 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 Fig. 10. Optical stability for (a) PPy/ClO-4,(b) PPy/SDS, (c) PPy/AR18, (d) PPy/AR18-SDS, (e) PPy/AR1-SDS, and (f) PPy/IC-SDS layers for oxidation (-1 V) and reduction (+1 V) state with 5 s residence time. Table 8 Comparison of the color contrast of PPy films after 25 cycles. of the presence of the surfactant, which helps the dye molecule enter between PPy chains. Therefore, the participation of the dye in the con- System ΔT%1ndcycle ΔT%25thcycle jugate system of the PPy increases the coloration efficiency of the PPy PPy/ClO-4 26 5 films. Another possible reason for the increase in the coloration effi- PPy/SDS 41 41 ciency when the dye and the surfactant are simultaneously used, is PPy/AR18 53 52 because of an increase in the charge of the PPy structure. Subse- PPy/AR18-SDS 55 55 quently, the electron transport along the conjugated system is favored, PPy/IC-SDS 45 45 PPy/AR1-SDS 23 23 and facilitates the color change when an electric potential is applied, since less total charge is needed in both the cathodic and anodic pro- cess. It is worth noting, that while there are slight differences in the the oxidation and reduction states, thus the cathodic coloration effi- Tc/Ta ratio between PPy layers, there is a noticeable difference in col- ciency (ηc ) and anodic coloration efficiency (ηa ) are presented in oration efficiency. According to Equation (2), the coloration efficiency Table 7. depends on Qd, so as shown in Table 7, the Qa is greater than Qc for As shown in Table 7, the use of large anions (surfactants and dyes) cases other than PPy/IC-SDS. as dopants in the PPy layer has a more significant effect on the col- As a result, the coloration efficiency is different for each PPy/- oration efficiency of the electrochromic layers compared to small dopant for both, ηc and ηa, and in general ηc is higher than ηa in all anions. In addition, the simultaneous use of the dye molecule and electrochromic films except in case of PPy/IC-SDS film. Presumably, the surfactant had a beneficial synergistic effect on the coloration higher ηc values, compared to ηa, are obtained because of the higher efficiency. This positive effect was observed in all the three dyes, espe- order of the polymer chains in the reduced state (i.e., PPy and dopants cially the AR18 molecule in both ηc and ηa. The reasons for this posi- exhibit intermolecular interaction with π bonds that favors the tive effect could be due to the opening of the PPy structure, as a result rearrangement). 13 M. Bayat et al. Journal of Electroanalytical Chemistry 886 (2021) 115113 The PPy/AR18-SDS layer had a higher cathodic and anodic col- ing - review & editing. Gonzalo Guirado: conceptualization, oration efficiency than other layers, which is likely to be due to the investigation, methodology, supervision, funding, Visualization, Writ- presence of more anionic groups and smaller molecule dimensions in ing - original draft, Writing - review & editing. AR18. With these results, it has been demonstrated that several factors influence the electrochromic performance of PPy films, and the appro- Declaration of Competing Interest priate selection of dopants plays an important role (e.g., number of anion groups, the distribution of anion groups in the dopant structure, The authors declare that they have no known competing financial the dimensions of the dopant structure, and the existence of conju- interests or personal relationships that could have appeared to influ- gated bonds) in order to enhance the coloration efficiency. ence the work reported in this paper. 3.5.4. Fatigue resistance Acknowledgements The ability of the electrochromic material to undergo, and be stable, upon consecutive switching potentials, is known as elec- The authors thank the Iran Ministry of Science, Research, and Tech- trochromic fatigue resistance [53]. For this study, cycles of − 1.0 V nology for monetary support for the performance of this work at UPC, (5 s) and + 1.0 V (5 s) vs SCE are applied and repeated 25 times this Spain during a research visit. The authors acknowledge MINECO/ is shown in Fig. 10. The color contrast of the films after 25 cycles indi- FEDER (RTI2018-098951-B-I00, CTQ2015-65439-R and PID2019- cates that all the electrochromic layers exhibit excellent optical stabil- 106171RB-I00) and the Agència de Gestió d’Ajuts Universitaris i de ity, except the PPy/ClO-4 film (Table 8). Therefore, the use of SDS and Recerca (2017SGR359). Support for the research of C.A. was received dyes as dopants in the PPy electrosynthesis clearly has an effect on through the prize, “ICREA Academia” for excellence in research, improving the fatigue resistance and thus the optical stability. In gen- funded by the Generalitat de Catalunya. eral, the results show that there is a close relationship between the electrochemical stability and optical stability; hence the samples that Appendix A. Supplementary data exhibit good electrochemical stability also have convenient optical stability. Supplementary data to this article can be found online at https://doi.org/10.1016/j.jelechem.2021.115113. 4. Conclusions References In this study, PPy electrochromic films were produced using dif- ferent dopant agents, such as surfactants (SDS), and three different [1] R.J. Mortimer, D.R. Rosseinsky, P.M.S. Monk, Electrochromic Materials and Devices, 1st ed., WILEY-VCH, Weinheim, Germany, 2005. Dyes (AR18, AR1, and IC). The results showed that the use of a [2] J.R. Platt, Electrochromism, a possible change of color producible in dyes by an Dye helps to improve the electrochemical behavior, optical character- electric field, J. Chem. 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