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Xanthommatin-Based Electrochromic Displays Inspired by Nature Amrita Kumar, Thomas L. Williams, Camille A. Martin, Amanda M. Figueroa-Navedo, and Leila F. Deravi* Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States
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ABSTRACT: Color is a signature visual feature in nature; however, the ability to trigger color change in the presence of different environmental stimuli is unique to only a handful of species in the animal kingdom. We exploit the natural colorchanging properties of the predominant pigment in arthropods and cephalopodsxanthommatin (Xa)and describe its utility as a new broad-spectrum electrochromic material. To accomplish this goal, we explored the spectroelectrochemical properties of Xa adsorbed to an indium-doped tin oxide-coated substrate chemically modified with poly(3,4-ethylene dioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS). We identified a synergistic role between PEDOT:PSS and Xa that contributed to its absorption profile, which could be modulated across multiple cycles. By varying the ratio of the two electroactive components, we also altered the perceived visible color of Xa-based devices, which cycled from different shades of red to yellow under reducing and oxidizing potentials, respectively. Together, our data illustrate the utility of Xa-based devices as new broad-spectrum electrochromic materials. KEYWORDS: electrochromic device, spectroelectrochemistry, redox, xanthommatin, poly(3,4-ethylene dioxythiophene)−poly(styrenesulfonate)
1. INTRODUCTION Natural dyes and pigments often have functions that extend beyond simple pigmentation. For instance, the chlorophyll in chloroplasts absorbs light at 430 and 660 nm, producing a green color in plants but is also used in conjunction with water to generate adenosine triphosphate, ultimately using absorbed light to convert carbon dioxide and water into glucose1−4 On the other hand, eumelanin, found in most living organisms, is black with a broadband absorption spanning the ultraviolet and visible (UV−vis) spectrum that also converts UV radiation into nonionizing heat.5,6 Xanthommatin (Xa), a phenoxazonebased pigment derived from tryptophan via the ommochrome biosynthesis pathway, is redox-active and absorbs UV−vis light to display yellow or red colors that are dependent on their local microenvironments.7−13 Xa exists in several natural systems including butterflies, dragonflies, and cephalopods to give rise to a broad spectrum of color.8−12,14,15 For instance, in squid Doryteuthis pealeii different ratios of Xa and its decarboxylated form combine and contribute to a range of colors that span the UV−vis range that may assist in communication, defense, or signaling.15,16 On the other hand, the ratio of the reduced and oxidized forms of Xa changes throughout the life-cycle of dragonfly genera Crocothemis and Sympetrum to yield red and yellow colors, respectively, as a result of sex-specific maturation in these insects. Such redox-dependent color changes are also achievable in vitro upon application of chemical-oxidizing (sodium nitrite) and -reducing (ascorbic acid) agents.8 Given © XXXX American Chemical Society
the unique redox-dependent color-changing features of Xa, we asked whether it would be possible to leverage their natural color-changing features for material applications, specifically to create a new, functional electrochromic device (ECD). Because of their fabrication simplicity, high contrast ratio, fast switching responses, and low energy consumption, ECDs are ideal systems for a number of commercial applications including smart windows, antiglare mirrors, eye-wear, displays, wearable fabrics, energy storage, and camouflage.17,18,22 A general method to build ECDs is to sandwich a gel electrolyte between two electrodes with the electrochromic (EC) material either bound to the electrode surface or embedded within the electrolyte.17,18,21 These materials generally display two or more distinct colors as they undergo variations in their redox state during electron transfer.17,18 Thus, we reasoned Xa with its two distinct redox-dependent colors would be an ideal candidate for new EC materials. The use of redox-active pigments and dyes in ECDs is not uncommon. In fact, a wide variety of materials utilizing inorganic compounds (e.g., Prussian blue,23 vanadium oxide,24 nickel oxide,25 tungsten oxide26), small organic molecules (e.g., viologen27), and conductive polymers18,28 [e.g., polyaniline, polyimide, poly(3,4-ethylenedioxythiophene), poly(3,4-ethylReceived: August 16, 2018 Accepted: November 21, 2018
A
DOI: 10.1021/acsami.8b14123 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Bruker TopSpin 3.5 Software, where all values were reported in δ units parts per million (ppm) measured relative to DMSO (2.50 ppm). Data for 1H NMR are reported as follows: chemical shift (δ ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, m = multiplet), coupling constant (Hz), and integration. 1H NMR (500 MHz, DMSO): δ 8.38 (d, J = 3.85 Hz, 3H), 8.05 (t, J = 4.47 Hz, 1H), 7.83 (d, J = 4.01 Hz, 2H), 7.70 (s, 1H), 6.67 (s, 1H), 4.45 (d, J = 4.96 Hz, 1H), 3.90 ppm (m, J = 5.42 Hz, 2H) (see Figure S1). The concentrated TFA which was added to the sample to reduce the number of exchangeable protons was observed as a broad peak centered between 9 and 10 ppm. The Xa structure was also confirmed with liquid chromatography mass spectrometry (LC−MS) obtained using a H-Class Acquity UPLC system coupled to a Xevo G2-S QToF mass spectrometer (Waters Corp, Milford, MA), where we observed the theoretical m/z of Xa [M + 1] at 424.0775 with observed m/z at 424.07223. 2.3. Electrochemical Studies. Cyclic voltammetry (CV) and other electrochemical redox experiments were carried out using Gamry Interface 1000B potentiostat. Current−voltage (I−V) measurements were reported using 3 mL of 0.25 mg/mL Xa in 0.1 M PBS, previously adjusted to a pH of 1.70−2.20 using 12 M HCl. The solution was then titrated with microliter amounts of 5 M NaOH to gradually increase the pH during the runs. After each addition, the pH of the solution was determined, and CV was used to characterize the redox behavior of the pigment by using a GC as working electrode, silver−silver chloride (saturated KCl) as reference electrode, and platinum as counter electrode. The voltammograms were scanned using −0.2 to +0.6 V up to 100 cycles. 2.4. Preparation of PEDOT:PSS-Coated ITO Substrate. ITOcoated PET/glass slides were spin-coated with a water dispersion of PEDOT:PSS (2−3% wt/v) for 30 s at 1500 rpm using a G3P-8 Spin Coater. DMSO [5% (wt/wt)] was added into the dispersion as a dopant to enhance the conductivity.38 The coated substrate was annealed at 60 °C for an hour. 2.5. Electrodeposition of Xa on PEDOT:PSS-Coated ITO Surface. As Xa was more soluble in acidic methanol, we used 0.5% (v/v) HCl/MeOH to reconstitute a stock solution of Xa to 3 mL (0.12 mg/mL of Xa). Here, a 0.1 M LiTRIF/methanol was used as the electrolyte. A three-electrode electrochemical cell featuring ITOcoated PET as the counter electrode and Ag/AgCl (saturated KCl) as the reference electrode was used to determine the oxidation-reduction potentials of Xa on bare ITO and modified ITO. The reference electrode was shorted with the counter electrode for all SEC studies. 2.6. ECD Assembly. ECDs were prepared by facing two 2 cm × 2 cm conductive ITO−PET substrates; among them, one was coated with the Xa−PEDOT:PSS mixture. Next, a mixture of PC (2 g), 1butyl-3-methylimidazolium tetrafluoroborate (1.6 g), and hydroquinone (0.003 g) was prepared as the electrolyte.39 An adhesive transparent tape was used on whole periphery, leaving two small holes to inject the electrolyte. 2.7. Characterization. 2.7.1. Fourier Transform Infrared Spectroscopy. A PerkinElmer spectrum 100 Fourier transform infrared (FTIR) spectrometer with horizontal attenuated total reflectance (ATR) accessory using an AMTIR crystal (from PIKE technologies) was used to characterize the infrared vibrational modes of Xa. After Xa was deposited onto the modified electrode, the substrate was washed with methanol, air-dried, then analyzed using FTIR. A background spectrum was also collected for the ITO modified with PEDOT:PSS for each substrate prior to analysis. OPUS v.7.2 (Bruker, Germany) was used for band integration, where the area was calculated from the base of each specified peak. 2.7.2. Scanning Electron Microscopy. A Hitachi S4800 SEM was used to image the Xa-deposited films. 2.7.3. Colorimetric Analysis. An Ocean Optics spectrophotometer was used for spectral transmittance and reflection measurements. For colorimetric measurements, a 45° diffuse reflectance probe and D65 standard illuminant were used. A white standard (WS-1) was used as a reference before measuring color coordinates of each sample. 2.7.4. Optical Contrast, Switching Speed, and Stability. Optical contrast was measured as the % transmittance (% T) difference
enedioxypyrrole)] show interesting color-changing properties under the application of a biased potential. Metal oxide EC materials typically exhibit two colors associated with their different redox states; however, this color change occurs over a slow switching timescale and lacks diversity in the presented color. On the other hand, conductive polymers generate fast electronic responses (typically on the order of ∼second per cm2 area ECDs), as they change their color over the visible through infrared regions. These colors can also be tuned by directly modifying the monomeric units of the polymer backbone,18,28 where broad absorption (e.g., achromatic/ neutral color) can be achieved by optimizing the ratio of electron-donating and -accepting groups in the polymer backbone29−33 or by blending or building multilayered polymeric structures with complimentary colors to absorb the entire visible range.34−36 In this paper, we describe the preparation and spectroelectrochemical (SEC) characterization of Xa films on an indiumdoped tin oxide (ITO) surface modified with poly(3,4ethylene dioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS) designed to not only enhance the adsorption of Xa onto the substrate but also to expand its dynamic color range during activation. Whereas PEDOT exhibits a dark blue color and an absorption profile centered at 610 nm during dedoping,18,28 Xa has an absorbance maximum (λmax) centered at 500 nm in its reduced state and 430 nm in its oxidized state.13 By combining the two materials and applying subtractive color theory to balance their absorption intensities, we can achieve different hues on demand within a single platform that spans the entire visible spectrum to ultimately achieve an achromatic coloran important feature of ECDs.32
2. EXPERIMENTAL SECTION 2.1. Materials. Ascorbic acid, sodium nitrite, lithium trifluoromethane sulfonate (LiTRIF), poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonate (PEDOT:PSS), dimethyl sulfoxide (DMSO), propylene carbonate (PC), 1-butyl-3-methylimidazolium tetrafluoroborate, and hydroquinone were purchased from Fisher Scientific. 3-Hydroxykynurenine (3-OHK), potassium hexacyanoferrate [K3Fe(CN)6], ITO-coated polyethylene terephthalate (PET), and glass slides (sheet resistance 10−15 Ω sq−1) were purchased from Sigma-Aldrich. ACS-grade sodium hydrogen phosphate (Na2HPO4) was purchased from Alfa Aesar (Ward Hill, MA, USA). ACS-grade sodium phosphate monobasic (NaH2PO4) was purchased from AMRESCO (Solon, OH, USA). High-performance liquid chromatography-UV grade methanol (CH3OH) was purchased from PharmcoAAPER (Brookfield, CT, USA). ACS-grade sodium hydroxide (NaOH) and ACS-grade hydrochloric acid (HCl) were purchased from EMD Millipore (Jaffrey, NH, USA). The water used in this experiment was purified using a Milli-Q water system (Darmstadt, Germany) with a gamma gold 0.22 μm Millipak. Oasis weak ion exchange sample extraction cartridges (1 cc30 mg) were purchased from Waters. Glassy carbon (GC), silver−silver chloride (saturated KCl), and platinum electrodes were purchased from Gamry (Warminster, PA, USA). 2.2. Synthesis of Xa. Xa was synthesized via an oxidative cyclization of 3-OHK using potassium hexacyanoferrate in a phosphate buffered saline solution (PBS, pH = 7.4) according to a previous report.37 The product was purified using an OASIS WAX solid phase extraction (1 cc30 mg, Waters) and collected using 0.5% (v/v) hydrochloric acid in methanol (HCl−MeOH). The product was verified using nuclear magnetic resonance (NMR) spectroscopy. The 1H NMR spectrum was recorded at ambient temperature on a 500 MHz Varian NMR spectrometer in 4% Optima LC/MS trifluoroacetic acid (TFA, Fisher Chemical) in 99.9% atom % D DMSO-d6 (Sigma-Aldrich). The NMR data were analyzed by using B
DOI: 10.1021/acsami.8b14123 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. (a) Proposed chemical structures of Xa in both redox states. (b) Variations in redox behaviors associated with an increasing pH using a fixed scan rate of 100 mV/s. (i) pH of the Xa-electrolyte bath was varied, and CVs were collected on the third cycle; (ii) half-wave potentials of Xa (triangle, n = 3) plotted from (i) with an R2 = 0.998 ± 0.001. The potentials are compared to the control potassium hexacyanoferrate (circle), which was used to synthesize Xa. (c) The peak oxidative current of Xa was recorded at various scan rates in two different sodium phosphate solutions buffered at pH 1.7 and 7.4. The slopes taken from the linear fit of the log of the peak current versus the log of the scan rate at pH 1.7 (1.10 ± 0.1) and pH 7.4 (1.08 ± 0.1) indicate a linear relationship associated with surface-bound redox processes, rather than the square root relationship associated with diffusion-limited processes. Plotting the peak oxidation current at pH 1.7 and 7.4 (inset) against scan rates confirms linear fits with slopes of 25.3 ± 0.4 nA/(mV/s) at pH 1.7 and 10.5 ± 0.2 nA/(mV/s) at pH 7.4, again indicating surface-bound processes. between two color states (ΔT % = % Toxidized − % Treduced) at a specified wavelength. Because the human eye is most sensitive at 555 nm,35 all transmittance values were calculated at this wavelength. To calculate the switching speed, contrast ratio, and stability, a single wavelength spectrophotometry method was used to monitor the transmittance change, whereas a square wave potential was applied for a reversible redox switch. Switching speed was then calculated based on time required to achieve 90% of maximum contrast during oxidation and reduction. 2.7.5. Color Efficiency. Color efficiency (CE) was calculated by measuring the optical density change (ΔO.D.) at a specified wavelength as a function of the charge density change per unit area using the following equations. CE =
ΔO. D. charge density change per unit area
ΔO. D. = log
(Tox ) (Tred)
slope of −0.062, which followed Nernstian behavior (59 mV/ pH) (Figure 1b(ii), N = 3, R2 = 0.998). These experimental data indicated that protons were indeed involved in the redox behavior of Xa and that the number of protons was constant in our reaction within the specified pH range. We next monitored the pH-dependent redox behavior of 3 mL of potassium hexacyanoferrate in PBS buffer (2.1 mg/mL) (Figure 1b(ii), circle). As a common oxidizing agent and the catalyst used in our synthesis, the potassium hexacyanoferrate control exhibited a negligible redox response to pH when compared to the synthesized Xa, further supporting that the observed redox behaviors were specific to Xa. To further investigate this redox process, we next measured the peak oxidative currents (ip) as a function of voltage sweep scan rate (v). We found that ip increased proportionally with v at both pH conditions, suggesting the reaction was not diffusion-controlled (Figures 1c and S2a(i,ii)).40 This relationship was also confirmed by plotting the log ip versus log v (Figure 1c, inset), where a slope of ∼1 was calculated, indicating that Xa had likely adsorbed to the GC electrode during the CV.40 When recorded up to 100 cycles at a scan rate of 100 mV/s, the CVs were also reversible but with an observed increase in current intensity and a gradual shift in oxidation onset (Figure S2b(i,ii)). These characteristics were attributed to the deposition of Xa onto the electrode surface, which is a common feature also observed in other systems.41 We observed that the adsorption of Xa to the GC electrode was stable; even after the electrode was moved to a buffer without Xa, the reversible redox signal was maintained. Next, we investigated the redox-dependent color changes of Xa within a SEC cell. To accomplish this, we first transitioned our setup from the GC working electrode to a cuvette-sized
(1)
(2)
The amount of ejected or injected charge was measured using a chronocoulometry method, and the change in transmittance values was monitored while the charge passed during the redox reaction.
3. RESULTS AND DISCUSSION Even though its exact mechanism for electron transfer remains unknown, Xa is believed to undergo a reversible reduction to dihydroxanthommatin, gaining two electrons and two protons during its redox-dependent color change (Figure 1a).13 To test this, we first explored how proton concentration influenced the redox behavior of Xa using CV. We calculated a redox potential of 310 mV under acidic (pH ≈ 1.70) conditions and −31 mV under neutral (pH ≈ 7.40) conditions (Figure 1b(i)). When the half-wave potentials (E1/2) were plotted as a function of buffer pH, we observed a linear relationship with a C
DOI: 10.1021/acsami.8b14123 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) ATR−FTIR spectra during electrodeposition of Xa on the modified ITO electrode measured as a function of time. (b) Integrated area under the growing chemical bands A, B, and C analyzed from (a). (c) Representative optical images of the Xa before and after electrodeposition on the PEDOT:PSS-modified ITO working electrode and subsequent color change at variable applied potentials. (d) Corresponding absorption spectra of oxidized and reduced Xa films on the modified working electrode. (e) Potential step absorptiometry (at 555 nm) of the electrodeposited Xa film during chronoamperometric potential steps from −1.5 to +1.5 V (pulse width 6 s).
band B), and 1152−1344 cm−1 (referred to as band C) (Figure 2a). The presence of these spectral features also correlated well with previous reports of Xa extracted from squid,44 where the broad profile of the bands suggested the contribution of several vibrational modes. One of the most prominent vibrational modes showed a broad profile because of CH stretches from the aromatic rings from 3000 to 3500 cm−1 (in band A region). We also observed characteristic N−H stretching at 3515 cm−1 from the single primary amine in Xa and OH stretches from the carboxylic acid groups at 3787 cm−1. The band B doublet was attributed to the N−H scissor mode, which was observed at 1644 and 1709 cm−1, along with a weak vibrational mode corresponding to aromatic ketone groups (CO) that shifted toward 1768 cm−1 throughout the duration of the electrodeposition. The broad profile of band C included characteristic vibrational modes from aromatic C−N and C−O stretches, as well as aromatic C−C stretching in Xa. When the areas under these bands were integrated and normalized, we calculated a 22, 20, and 80% increase from the integrated peak areas associated with bands A, B, and C, respectively, over the 20− 95 min deposition period (Figure 2b). Together, these data suggested that Xa had indeed deposited and accumulated on the modified ITO surface. With Xa adsorbed to the modified ITO, we next evaluated its ability to controllably change colors. We applied a reducing potential (−1.5 V) and observed a significant color change from the original oxidized yellow color to red (Figure 2c). The absorption spectra of the Xa film was then measured within the SEC cell at each redox state (+1.5 and −1.5 V vs ground), where we observed a shift in λmax from 460 to 530 nm associated with the oxidized and reduced states of Xa (Figure 2d). The peak broadening and shoulder present in the near infrared region when films were biased at the reducing potential were associated with PEDOT:PSS. In each case, the colors observed on the films were distinctly different from the oxidized and reduced forms of Xa in solution (without the
ITO-coated transparent electrode. To assist in the SEC characterization, we also attempted to generate a thin film of Xa onto the ITO working electrode. We first tried electrodeposition under a fixed oxidative potential (+500 mV for 30 min) to effectively replicate the Xa deposition on the GC electrode; however, no Xa was detected on the bare ITO substrate. Instead, the dye remained in solution with an oxidation peak at +420 mV (see Figure S3a,b). We next modified the ITO substrate with a layer of PEDOT:PSS, where we hypothesized that PEDOT:PSS would promote a higher affinity of Xa to the substrate by either facilitating π−π interactions or the generation of charged polarons under a positive potentialboth could facilitate the electron transfer and subsequent adsorption between PEDOT:PSS and Xa during the deposition.42,43 We observed that the modified ITO substrate contributed to an oxidative potential that was ∼40 mV lower than that of the bare ITO substrate and that this property did indeed facilitate charge transfer that assisted in Xa adsorption during electrodeposition (Figure S3c). Furthermore, we observed a linear relationship of both peak cathodic and anodic currents as a function of sweep scan rate (v), supporting that the Xa had adsorbed to the modified ITO substrate in a process similar to that observed on the GC electrode (Figure S3d).40 To show the specificity and selectivity of Xa on the modified ITO substrate only, a pattern of PEDOT:PSS was made on ITO then electrodeposited with Xa (Figure S4). It was clear that after applying a positive potential for 30 min, Xa deposited only on the area that was coated with PEDOT:PSS. To track the electrodeposition of Xa on the modified ITO over time, we followed changes in its chemical signature using time-dependent ATR−FTIR spectroscopy, where spectra were collected in 15 min intervals during the deposition cycle. We observed an increase in intensity of distinctive chemical signatures over time that included stretches from 3018 to 3281 cm−1 (referred to as band A), 1575−1752 cm−1 (referred to as D
DOI: 10.1021/acsami.8b14123 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces presence of PEDOT:PSS), where more narrow absorption spectra for both redox states were observed (Figure S5). When taken together, these results suggested that the presence of PEDOT:PSS on the ITO substrate contributed to the broadened absorption profile of Xa under the reducing potentials. However, without the PEDOT:PSS, no Xa was deposited, indicating an important synergy between the two EC materials to achieve changes in color that spanned the visible spectrum. The EC response time (% T at 555 nm) of the deposited Xa film at ±1.5 V (pulse width 6 s) was calculated from Figure 2e, where the time required to switch from 45.9% T (reduced state) to 60.2% T (oxidized state) was 2.1 and 3.6 s, respectively. To tune the visible color of these films, we next sought to build a device that leverages the broad-spectrum colorchanging features of the Xa-modified films but within a simplified formulation. Because electrodeposition generally requires a lot of material to achieve uniform deposition, where the success rate toward large-area, defect-free devices was low, alternative approaches including solution casting or mixing of the EC material into or within a gel-based electrolyte have been implemented to template ECDs.15,16,18−22 Other methods to enhance the performance of EC materials within a device incorporate blends of two polymers or the assembly of multilayered films to introduce extra ionic moieties that could facilitate charge transfer.19,45 We chose to build an EC film by drop-casting a layer of Xa followed by a second coat comprising PEDOT:PSS onto the modified ITO to create a composite layer on the conductive substrates (Figure S6). The device was then assembled by sandwiching a liquid electrolyte (details in experimental) between a bare ITO electrode and the PEDOT:PSS/Xa-modified ITO electrode (Figure 3a). To evaluate the spectral tunability of these materials, we varied the ratio between PEDOT:PSS and Xa. For these studies, we drop-casted 200 μL of methanol varying the concentration of Xa (from 0.5 to 5.5 mg/mL), whereas the amount of PEDOT:PSS remained constant. The color of each film activated at the different redox states (Figure 3b) was measured using the International Commission of Illumination (CIE) 1931 xy-chromaticity diagram (Figure 3c).35,46 The first system (O1 and R1, Figure 3b), which contained 0.03 mg Xa/ cm2, generated a neutral color in both oxidized (x = 0.3150, y = 0.3185) and reduced (x = 0.3005, y = 0.3155) states (Figure 3c, black square and circle, respectively). On the other hand, by significantly increasing the amount of Xa (0.27 mg Xa/cm2) in the films, we were able to achieve richer colors in both oxidized (O4; x = 0.4586, y = 0.4449) and reduced (R4, x = 0.4500, y = 0.3905) conditions (Figure 3c, blue square and circle, respectively). The spectra of each redox state were also collected and are plotted in Figure 3d, where we extrapolated the optical contrast (ΔT % = % Toxidized − % Treduced) between the two color states to calculate the CE for all four devices (reported in Table 1). We observed that by increasing the concentration of Xa in the ECDs, more charge injection or ejection per unit area was necessary to accompany the higher change in optical density. This trend matched well with previously reported EC materials such as tungsten oxide and viologen, which showed similar concentration-dependent changes in CE.18,47,48 In our materials, the richest yellow (O4) and red (R4) films contributed to an optical contrast of 21.5% (67.4% T to 45.9% T for oxidized and reduced conditions, respectively) and CE of 104 cm2 C−1, indicating
Figure 3. (a) Schematic diagram of an assembled multilayered ECD containing Xa. (b) Optical images of four separate ECDs were assembled by varying the ratio between Xa and PEDOT:PSS and switched between oxidized (O1, O2, O3, O4) to reduced (R1, R2, R3, R4) state and their corresponding (c) CIE color coordinates and (d) transmittance spectra in both states. (e) Potential step absorptiometry (at 555 nm) of the ECD during chronoamperometric potential steps between ±1.5 V up to 500 cycles (pulse width 25 s). (f) The % T zoomed at the (i) initial and (ii) final six cycles.
Table 1. Optical Contrast (% ΔT) and CE at 555 nm for Four ECDs Prepared by Varying the Ratio between PEDOT:PSS and Xa ECD1 ECD2 ECD3 ECD4
% Toxidized
% Treduced
% ΔT
CE (cm2 C−1)
85.2 78.9 71.7 67.3
75.7 61.0 52.6 45.9
9.5 17.9 19.1 21.5
62 78 99 104
that indeed ECDs assembled using Xa could reversibly switch into two distinct colors at low operational voltages. For any ECD, switching reversibility and response time are important parameters in assessing device performance.18,47,48 To test this in our films, we monitored the % T at 555 nm in a fresh device (Xa deposited on modified ITO at 1 mg/mL), while cycling ±1.5 V with a 25 s pulse over 400 min (Figure 3e). We calculated the % contrast loss over time and observed a loss of only 1.7% T at 320 min (from 14.5 to 12.8% without any background correction). After that time, we observed an increase in contrast loss approaching ∼7%, which we attributed to the leakage of our ionic liquid-based electrolyte from the device over time (Figure 3f).49 Future efforts will be directed at overcoming these technical limitations, as we further refine E
DOI: 10.1021/acsami.8b14123 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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these materials for practical applications. In spite of these issues, the switching response (
