Solid State Electrochromic Devices of Plasma Modified WO3 Hybrids

Article pubs.acs.org/IECR

Solid State Electrochromic Devices of Plasma Modified WO3 Hybrids Melek Kiristi,† Ferhat Bozduman,‡ Aysegul Uygun Oksuz,*,† Lutfi Oksuz,*,‡,§ and Ahmed Hala∥ †

Department of Chemistry and ‡Department of Physics, Faculty of Arts and Science, Suleyman Demirel University, 32000 Isparta, Turkey § Center for Plasma-Aided Manufacturing and Department of Engineering Physics, University of WisconsinMadison, Madison, Wisconsin 53706, United States ∥ King Abdulaziz City for Science and TechnologyKACST, The National Centre for Mathematics and Physics, 11442 Riyadh, Saudi Arabia S Supporting Information *

ABSTRACT: Electrochromic (EC) properties of tungsten trioxide (WO3) was improved with preparing hybrids of tungsten trioxide−titanium dioxide (WO3−TiO2) and tungsten trioxide−poly(3,4-ethylenedioxythiophene) (WO3−PEDOT) by a rotating capacitively coupled radio frequency (rf 13.56 MHz) plasma reactor. Energy-dispersive X-ray spectroscopy mapping results indicated that TiO2 and PEDOT were coated homogeneously onto the surface of the WO3 powders. Thin films of hybrid powders have been prepared by the physical vapor deposition method of the electron beam evaporation technique. Redox potentials, optical contrast at 700 nm, and durability during 2000 cycles of EC devices were investigated, comparatively. Hybrids of WO3 indicated excellent coloration efficiency (cm2 C−1) and switching speed values compared with untreated WO3. The coloration efficiency values were found to be 85.88 and 41.61 cm2 C−1 of WO3−TiO2 and WO3−PEDOT, respectively. The switching speed of WO3 (13.3 s, from bleached state to colored state) increased to 1.4 s for WO3−TiO2. speed18 to obtain inorganic−organic hybrid EC material. As for an inorganic−inorganic hybrid, the wide gap semiconductor TiO219 was used. The solid-state EC devices of the hybrids were compared with that of untreated WO3 and demonstrated that the rf plasma modified WO3−TiO2 and WO3−PEDOT have higher electrochemical activity, faster color switching time, and coloration efficiency.

1. INTRODUCTION Tungsten oxide (WO3) is one of the most favored electrochromic (EC) materials due to its robust behavior compared to organic or polymeric electrochromes.1−3 However, inherent drawbacks have been reported related to its single color changes (between blue and transparent) and slow response or switching time under applied voltages.4−6 Multicomposites or hybrids of WO3 were proposed in order to overcome the challenges and improve the EC properties.7,8 Until now, WO3 has been doped with conjugated polymers and/or other metals using several methods to combine synergistic properties of the materials.9−11 For EC applications, Zhang et al. produced an organic−inorganic hybrid of WO3/polyaniline thin film by embedding WO3 nanorods in a polyaniline matrix.12 In a different work, Cai et al. reported the inorganic−inorganic hybrid composition of WO3 coated titanium dioxide (TiO2) nanorods by electrodeposition.13 The hybridization process has been mainly achieved by solution based wet synthesis methods. The wet processing methods have some difficulties such as complex synthesis steps, nonhomogenous surfaces, impurities, and long processing times.14,15 In contrast, plasma coating is a solvent-free (dry), nontoxic, single-step process that provides thickness control ranging from tens of angstrom to micrometers.15 In addition, constant rotating movement of the Pyrex plasma chamber supplies uniform modification of the target powder samples.16,17 Radio frequency (rf) rotating plasma coating of nanocomposite powder has only been recently reported. Uygun Oksuz et al. introduced uniform rf plasma nanocoating of polythiophene onto TiO2 nanopowders.17 In this study, WO3 powders were coated homogeneously using poly(3,4-ethylenedioxythiophene) (PEDOT) and TiO2 by an rf rotating plasma modification method. PEDOT was preferred because of its good environmental stability and fast color switching © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. The rf Plasma Coating of TiCl4 and 3,4-Ethylenedioxythiophene (EDOT) onto WO3 Powders. The plasma modification process was carried out in a capacitively coupled, 13.56 MHz rf rotating plasma reactor (Figure 1). In order to modify dry WO3 powder (Aldrich, 99.9%), vapors of the dopant agents of TiCl4 (99%) and EDOT (98%) (Aldrich) were sent into the rotating chamber and plasma was created through 50 W rf power. Steady state plasma coating parameters were applied for both modification during 48 h and the base pressure of 20−30 mTorr. 2.2. Thin Film Deposition and Fabrication of SolidState Electrochromic Devices. Thin films were deposited onto indium tin oxide (ITO) coated glass substrates (transmittance of 85%, sheet resistance of 30 Ω/sq) by the electron beam evaporation technique. The pelletized hybrids and untreated WO3 targets were used as evaporation material and were heated using an electron beam that was collimated from the dc heated cathode, a tungsten filament. The distance between the Received: Revised: Accepted: Published: 15917

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Figure 1. The rf rotating plasma setup.

target area and the rotating panel substrate was 60 cm. Experimental parameters were kept constant for all processes. They were 40 mA emission current, 6 kV voltage, and 10−6 Torr base pressure. Deposited thin films were transparent in the visible wavelengths. For solid-state EC fabrication, transparent gel electrolyte of lithium perchlorate (LiClO4), poly(methyl methacrylate) (PMMA), acetonitrile (ACN), and propylene carbonate (PC) at a ratio of 3:7:70:20 by weight percent was used.20 EC devices were prepared by sandwiching the gel electrolyte between ITO electrodes and the WO3 film-coated sides of ITO. The effective area of the devices was 15 cm2, and their four edges were sealed using silicon. Characterization is given in the Supporting Information.

Figure 2. XRD patterns of WO3, WO3−PEDOT, and WO3−TiO2 films.

pressure of 20 mTorr. The wavelengths of atomic and molecular spectral lines were determined from standard references and are cited in Tables S1 and S2 in the Supporting Information. The parameters of the plasma modification process were optimized according to the energy-dispersive X-ray spectroscopy (EDS) results. Plasma treatment was applied to WO3 powders until traces of dopant agents appeared in the atomic composition (Table 1). For instance, the amounts of sulfur (S) for PEDOT and titanium (Ti) for TiCl4 plasma were used as indicators for the process. In the OES of TiCl4 plasma modification, TiO species were dominant during plasma reactions (Supporting Information, Figure S1). This means that titania oxidized to TiO2 during plasma reactions and an inorganic−inorganic hybrid of WO3− TiO2 was obtained. This phenomen is also confirmed by EDS analysis (Table 1). TiCl4 modification has resulted in an atomic percent ratio of 93:7 of W:Ti. The amount of Cl was significantly reduced. TiCl4 precursor is usually used for TiO2 synthesis by interacting with oxygen to regenerate chlorine for recycling according to reaction 1:

3. RESULTS AND DISCUSSION 3.1. Optical Emission Spectroscopy (OES) Results. Excited plasma phases of TiCl4 and EDOT were analyzed by OES in low pressure plasma with the external measuring probe. During plasma treatment a variety of characteristic lines were observed (Figures S1 and S2 in the Supporting Information). It is assumed that nitrogen peaks originated as a result of the medium Table 1. Elemental Compositions of Hybrids

TiCl4 + O2 → TiO2 + 2Cl 2

WO3−PEDOT

at. % ratio

WO3−TiO2

at. % ratio

carbon oxygen sulfur tungsten chlorine

22.575 62.085 0.124 15.216 −

carbon oxygen titanium tungsten chlorine

27.032 59.405 1.066 11.887 0.620

(1)

Atmospheric pressure plasma was also used to produce TiO2 powders from TiCl4 and air.21 In this study, TiO2 coating onto WO3 particles was achieved by low pressure rf plasma treatment. After PEDOT plasma coating onto WO3, the atomic percent ratio of W:S reached 9:1. In the experimental spectra of PEDOT modification, the −CH line, which was observed at 315.5 nm, is

Figure 3. AFM images of WO3, WO3−TiO2, and WO3−PEDOT films (deposited on ITO). 15918

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attributed to the growth of the plasma polymerization process onto the WO3 nanoparticles15 (Supporting Information, Figure S2). It is concluded that plasma polymerization of EDOT on spherical particles of WO3 was achieved. As a result, the inorganic−organic hybrid of WO3−PEDOT was obtained after the process. Neither a precursor nor an auxiliary agent was used in plasma processes. Homogeneity of TiO2 and PEDOT plasma modifications onto WO3 particles was confirmed by EDS elemental mapping results in the Supporting Information (Figures S3 and S4). 3.2. Structure and Morphology. X-ray diffraction (XRD) patterns of WO3, WO3−TiO2, and WO3−PEDOT films on ITO substrates were collected (Figure 2). The peaks at 30.5, 35.4, 51, and 60.5° show similar profiles and were dominated by peaks from the ITO substrate.22,23 The peak at around 2θ ∼ 21.5° belongs to the WO3 structure24 and overlapped the peaks of PEDOT25 and anatase TiO226 in the hybrid structures. Note that the broad diffraction peaks for the hybrids are not very strong because of the back signals of ITO substrate suppression.27 Another reason is the tiny amount of dopants coated around the WO3 molecule during the plasma modification process. However, the plasma coatings affected the crystallinity degree of the WO3 molecule and increased the amorphous layer. It is reported that the WO3 structure is composed of a core of the WO3 crystals surrounded by amorphous WO3 layers.24,28 The amorphous layer of WO3 provides interaction between inner crystalline WO3 and the electrolyte medium.29 Therefore, increasing the amorphous layer around WO3 is crucial for the diffusion of Li+ and electrons. On the other hand, a higher crystallinity degree affects the long life durability of EC molecules related to the more stable proton insertion sites.7,8 As seen from atomic force microscopy (AFM) images, the surface roughnesses of WO3−TiO2 and WO3−PEDOT thin films are higher than that of untreated WO3 film (Figure 3). This wavy surface formation provides interpenetrating pathways for ions under applied voltages and creates proton-capturing sites.7 This feature is very important for electroactivity and the optical variations of the EC materials. Scanning electron microscopy (SEM) images (Figure S5 in the Supporting Information)

Figure 4. CV curves of WO3, WO3−TiO2, and WO3−PEDOT films in 1 M LiClO4 (in PC) at a potential scanning rate of 50 mV/s versus Ag/AgCl.

Figure 5. Optical transmittance spectra of solid-state EC devices of WO3, WO3−TiO2, and WO3−PEDOT under potentials of +3 and −3 V, respectively.

Figure 6. Color switching speed of EC devices.

Table 2. Electrochromic Parameters of Solid-State Devicesa

a

λ = 700 nm

Tbleached (%)

Tcolored (%)

ΔOD

Q (C cm−2)

η (cm2 C−1)

tb (s)

tc (s)

WO3 WO3−TiO2 WO3−PEDOT

68.41 70.49 66.53

17.68 3.63 6.03

0.58 1.28 1.04

0.027 0.015 0.025

21.48 85.88 41.6

13.3 1.4 1.5

6.7 10.1 9.5

tb, color switching time from bleached state to colored state; tc, color switching time from colored state to bleached state. 15919

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illustrate the surface structures as well as thicknesses of the thin films. 3.3. Electrochemical and Electrochromic Properties. Cyclic voltammetry (CV) was employed in order to compare the electroactivities and to determine the oxidation−reduction peak potentials of the WO3 and hybrid structures. The measurements were carried out in LiClO4 (1 M in PC) at a potential scanning rate of 50 mV/s. The potential was swept from cathodic (−0.6 V) to anodic (+1.2 V) versus Ag/AgCl (Figure 4). It is known that the ion insertion−extraction process (W6+ ↔ W5+) is responsible for EC color switching from blue (reduced) to transparent (oxidized) states.3 Voltammetric cycles of hybrid films exhibit higher current densities than that of the WO3 current peak at −0.233 V, and their onset potentials of the cathodic current shifted significantly in the positive direction. Therefore, lower voltages are capable of providing Li+ ion insertions to hybrid films.9 WO3−TiO2 hybrid has a cathodic current peak at +0.522 V, and it shows a similarity with electrodeposited WO3−TiO2 composite films.13 WO3− PEDOT has the highest exchange current density at +0.573 V. The chemical structure of plasma polymerized PEDOT is different from than that of conventionally polymerized PEDOT.30 Therefore, the characteristic oxidation peak of PEDOT was not observed, clearly. However, plasma PEDOT coating increased the amorphous layer of WO3 and enhanced the ion-capturing sites.7 Thus, higher cathodic current densities of the hybrid films show the abundance of electroactive species depending on the increased surface roughness. Figure 5 shows the visible in situ optical transmittance spectra of solid-state EC devices of WO3, WO3−TiO2, and WO3− PEDOT under applied potentials of ±3.0 V, respectively. The color of the EC devices can switch between colorless (bleached state) and deep blue (colored state) with the electric potential variation. The transmittance variations (ΔT% = Tbleached − Tcolored) were obtained as 66.86% of WO3−TiO2 and 60.03% of WO3−PEDOT at 700 nm. These values are higher than that of WO3 (50.73%) and also those of previously reported studies for WO3.7,9,29 The color switching speeds of the EC devices were figured out through the in situ coloration/bleaching transmittance response at a wavelength of 700 nm and applied potential of ±3.0 V for 30 s (Figure 6). It is determined as the time to reach 90% of the final change in transmittance between the steady bleached and colored states3 and is given in Table 2. The color switching times of solid-state EC devices of WO3−TiO2 and WO3−PEDOT from the bleached state to the colored state are found to be 1.4 and 1.5 s (for the reverse process, it takes longer times for bleaching of 10.1 and 9.5 s), respectively. These values are much faster than those of the solid-state EC device of WO3 (13.3 s) and the other previously reported EC device. As is well-known, the color switching time depends on the diffusion rates of the Li+ intercalation/deintercalation process. Li+ ions and electrons are inserted into the EC layer during the cathodic scan. Their movement rate depends on the surface features such as surface area, crystallinity, and effect of dopants. Results showed that the rf plasma hybridization made easier Li+ ion insertion into the film for the coloration. Because of the good interaction force between Li+ and the hybrid film surface, deintercalation of Li+ ions resulted in a longer bleaching time.3 Chronamperometric (CA) characterization was used to test the stability and repeatability of the solid-state EC devices during 2000 cycles at atmospheric conditions (Figure 7). After subjecting the samples during 2000 cycles, the peak currents remained

Figure 7. CA measurements of solid-state devices during 2000 cycles against an applied cyclic potential of ±3 V with the time interval set to 0.01 s at 50 mV/s scan rate.

stable and were not affected much by the air exposure, particularly for the WO3. EC devices of hybrids showed weaker stability, relatively. These results agree with previous studies and explained that the cyclic stability of the EC device is related to the crystallinity of the WO3.3,7,13 However, the cyclic stability of the hybrids was damaged relatively because of the decreased crystallinity after modification depending on the unstable proton-capturing sites.7 15920

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The coloration efficiency (CE, η) of the EC devices was obtained from the equation of ΔOD/Q in units of cm2 C−1, where Q [C cm−2] is per unit of charge inserted into (or extracted from) the film for a measurement area of 15 cm2. The charges exchanged during the cathodic (proton insertion, Qca) and anodic (proton extraction, Qa) cycles were calculated by integration of the curves of current density vs time. Differential optical density (ΔOD = [log Tbleached/Tcolored]) is defined by the logarithm of the ratio of colored (Tcolored) to bleached (Tbleached) transmittance at the wavelength of 700 nm. Table 2 summarizes the corresponding values. Solid-state EC devices of WO3−TiO2 and WO3−PEDOT indicated better CE performance than the untreated WO3. The highest CE of WO3−TiO2 reveals the large optical modulation with a small charge insertion.7 The rf plasma hybridization approach has boosted EC performances of the materials.

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4. CONCLUSIONS Electrochromic hybrids of WO3−TiO2 and WO3−PEDOT were obtained by a single-step rf rotating plasma process. Amorphous coatings occurred around the WO3 molecule as a result of plasma hybridization. The increased surface area and amorphous characters provided better interaction between the electrode area and Li+ ions. Tuned electrochemical activity enhanced optical contrasts and color switching speeds of the hybrid EC devices. Color switching times (from bleached state to colored state) were obtained as 1.4 and 1.5 s, and coloration efficiencies were 85.88 and 41.6 cm2 C−1 for WO3−TiO2 and WO3− PEDOT, respectively. These values are better than those of the EC device of untreated WO3 (13.3 s and 21.48 cm2 C−1). Despite increasing the amorphous structure by plasma modification, the durability factor of solid-state ECs stayed stable during 2000 cycles, relatively.

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ASSOCIATED CONTENT

S Supporting Information *

Characterization methods, OES graphs (Figures S1 and S2), atomic and molecular OES lines (Tables S1 and S2), SEM−EDS mapping for hybrid materials (Figures S3 and S4), and SEM images of thicknesses (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: lutfi[email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Funding

The authors would like to acknowledge the TUBITAK/ TEYDEB (9100036) and Suleyman Demirel University Fund (Project No. 3599D213) that financially supported this work. Notes

The authors declare no competing financial interest.

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REFERENCES

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