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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Prominent Electrochromism Achieved Using Aluminum Ion Insertion/extraction in Amorphous WO Films 3
Junji Guo, Mei Wang, Xungang Diao, Zhi-Bin Zhang, Guobo Dong, Hang Yu, Fa-Min Liu, Hao Wang, and Jiang Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05692 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018
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The Journal of Physical Chemistry
Prominent Electrochromism Achieved Using Aluminum Ion Insertion/extraction in Amorphous WO3 Films Junji Guo1,3 , Mei Wang1 ,∗ Xungang Diao2 ,† ZhiBin Zhang3 ,‡ Guobo Dong1 , Hang Yu1 , Famin Liu1 , Hao Wang4 , and Jiang Liu5 1
Electrochromic Center, School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, P. R. China 2 School of Energy and Power Engineering, Beihang University, Beijing 100191, P. R. China 3 Solid State Electronics, Ågströmlaboratoriet, Uppsala University, Sweden 75121, Uppsala 4 The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, P. R. China and 5 Jiangsu Fanhua Glass Co., Ltd. Hai’an, Nantong, Jiangsu Provience, P. R. China Abstract: Although monovalent lithium has been successfully used as coloring ions in electrochromic applications, however, it still faces the challenges of low safety, high-cost and limited reserves. Herein we demonstrate that the amorphous WO3 films intercalated with Al3+ ions could exhibit desired wide optical modulation (∼63.0%) and high coloration efficiency (∼72.0 cm2 A−1 , which is >100% higher than that with Li+ or Na+ ), benefiting from the three-electron redox properties of aluminum. Due to the strong electrostatic force and large atomic weight, the charge exchange processes for Al3+ ions are limited only to the near-surface region, and consequently bring about enhanced electrochromic stability. Our findings provide in-depth insights into the nature of electrochromism and also open up a new route towards scalable electrochromic devices using sputtering technique and earth−abundant materials. I. INTRODUCTION
As one of the most promising inorganic electrochromic (EC) materials, tungsten trioxide WO3 has evoked wide interest in today’s smart windows technology for energy efficient architectures.1,2 So far most of the research on WO3 -based applications worldwide have focused on amorphous phases due to their higher chromic rates and efficiencies.3,4 The process leading to the optical absorption in amorphous tungsten oxide a-WO3 can be represented by the Faughnan model: WO3 + xMκ+ + xκe− Mx WO3 ,
(1)
where Mκ+ is an ion of valency κ, e− is charge compensating electron and xκ is the intercalation level for charge.5 During coloration, insertion ions break up the W=O bonds, and then chemically bond with the lattice oxygens. The compensating electrons are localized in the W 5d orbitals and polarize their surroundings to induce lattice distortion. The spatial separation between insertion ions and the compensating electrons creates small polarons, and the optical absorption originates from the polaron hopping between two adjacent non-equivalent W sites: W4+ (i) + W5+ (j) + ~ω45 → W5+ (i) + W4+ (j)
(2)
W5+ (i) + W6+ (j) + ~ω56 → W6+ (i) + W5+ (j),
(3)
and
with the photon energy ~ω45 ≈1.75 eV and ~ω56 ≈1.05 eV, respectively.6−8 The relative number of W4+ , W5+ and W6+ sites
∗ These
authors contributed equally. address:
[email protected] ‡ Electronic address:
[email protected] † Electronic
varies with the charge intercalation level, and the polaron hoppings between different W sites give rise to the corresponding optical absorptions.9 The electrochromic performances of a-WO3 strongly depend on the radius and valency of the coloring ions.10−13 Coloration efficiency, a convenient metric to assess and compare how well EC materials are performing, is defined as the optical absorbance [A, given by log(1/T )] change obtained for a certain amount of transferred electric charge per unit area.14 For monovalent species [H+ (0.029 nm), Li+ (0.059 nm) and Na+ (0.095 nm)], the ion with small ionic radius owns a relatively high charge density. The higher charge density is, the more bipolaronic W4+ sites could be obtained on the same charge intercalation level, owing to the higher probability of contact between the W sites and electrons. As a result, the transition probability between W4+ and W5+ sites can be improved.10−12 The multivalent species, especially Al3+ (0.054 nm), have ability to involve more electrons in one redox couple, so that lead to a higher content and ratio of bipolaronic W4+ sites with a certain xκ as compared to the monovalent ones.12,13 As to the EC response rate (defined as the color-change per unit time, which is mainly dependent on ionic conductivity), monovalent species have the advantage of high ionic conductivity with an associated low energy barrier for solid-state diffusion, particularly for the lighter H+ and Li+ .10,11 Normally, multivalent species have extremely low mobility because of their high activation energy for interfacial charge-transfer and strong Coulombic interaction with surrounding atomic frameworks.15 Thanks to the low-dimensional nanostructures, however, a larger ion diffusion coefficient (1.59× 10−10 cm2 /s) for Al3+ in crystalline W18 O49 nanowires was estimated by Tian etc., which was comparable to that for protons.12 Regarding the EC stability, the acidic H+ has a strong tendency to corrode the surface of a-WO3 film, and the larger radius Na+ would result in excess lattice expansion due to obscuring stress effects.10,11,16 Although EC devices based on earth-scarce lithium exhibit long-desired cycle life, unfortunately the Li+ -containing electrolyte salts are normally expensive, toxic and easy to react with air/moisture.17 It was reported
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Binding energy (eV) Figure. 1: (a) GIXRD profile and top-view SEM image of a-WO3 film. (b) Deconvolution of W 4f and W 5p XPS spectra, and corresponding fitting analysis results.
that the application of cheap and stable aluminum salts in electrochromism would yield enhanced EC performances, owing to the increase in degree of crystal structure stability caused by the Al3+ ion-inserting-induced strong electrostatic force.12,13 Therefore, it would be of great importance to select appropriate guest species as the coloring ions for a-WO3 , in order to guarantee comprehensive EC performances. In this paper, in situ analysis methods were developed to reveal the electrochemical and electrochromic behaviors of sputtered aWO3 film cycled in nonaqueous Li+ -, Na+ - and Al3+ containing electrolytes. We illustrate that charge exchange with Al3+ ions insertion/extraction is limited only to the near-surface region, which is essentially different from the deep-diffusion in bulk region for Li+ and Na+ . Arising from the three-electron redox properties and short diffusion path length, the film cycled in Al3+ exhibits desired high coloration efficiency, high optical contrast and excellent EC stability. Our findings may contribute toward a fundamental understanding of the electrochromism and provide a significant step in realization of more economical, more excellent electrochromic devices.
II. EXPERIMENTAL SECTION
Tungsten oxide thin films were deposited in a Ar (99.99%)+O2 (99.99%) gas mixture at room temperature. Base pressure of the sputter main chamber was under 1.6×10−3 Pa. The gas flow rates,
in standard sccm, were 27 for Ar and 9 for O2 . Sputtering took place from a 100-mm-diam metallic W target onto 3 × 4 cm2 glass substrates (precoated with 120−nm−thick indium tin oxide having a sheet resistance of 17 Ω/square), with the target-substrate separation ∼7 cm. The 3 × 1 cm2 insulating tapes were attached to the edges of these substrates prior to sputtering in order to reserve the electrodes for electrochemical measurements. Before the deposition, the W-target was pre-sputtered for 20 min using Ar alone with shutter above the gun closed. A stable discharge was realized by setting the sputtering-power and -pressure to 260 W and 6.6 Pa, respectively. The thickness d of the film was ∼260 nm, as measured by high precision step profiler (Dektak II, Bruker Corp., Germany). Crystal structure was determined by Grazing incidence X-ray diffraction (GIXRD) using a Rigaku D/Max 2200 diffractometer with Cu-Kα (λ=0.15406 nm) radiation source at glancing incidence angle 0.3◦ . Surface morphology analysis was performed using the high−resolution FEI−Phillips XL30 S−FEG scanning electron microscope (SEM). Prior to imaging by SEM, the film was sputter-coated with ∼10 nm gold for improving electrical conductivity. Chemical bonding state of tungsten was determined by x−ray photoelectron spectroscopy (XPS, Physical Electronics PHI−5600 system) using a monochromatic Al anode. Analysis of XPS data were performed using the Tougaard background subtraction method, and the spectra was fitted by assuming a Gaussian-Lorentzian (30:70) line shape. Electrochromic measurements were performed using a CHI660C electrochemical working station (Chenhua, Shanghai, China) equipped with the three−electrode electrochemical cell. The working, counter and reference electrode was the a-WO3 film on substrate, platinum plate and KCl saturated Ag/AgCl, respectively. In order to exclude possible effects of water molecules and anions, nonaqueous electrolytes possessing the same anion were prepared by dissolving 1 M LiClO4 , NaClO4 and Al(ClO4 )3 in propylene carbonate solvent, respectively. The in-situ optical responses were measured at 550 nm by UV−visible−NIR spectrometer (Evolution 100, Thermo Electron Corporation), using the cell filed with electrolytes as the basic reference.
III. RESULTS AND DISCUSSION
The electrochromic properties of WO3 are strongly dependent on crystal structure and stoichiometry. Figure 1(a) presents the GIXRD pattern of as-deposited WO3 thin film. Only broad diffuse halo peaks were found, which was consistent with the uniform surface morphology (insert) indicating naturally amorphous phase of the film. This is because the thermal energy is not high enough to produce the octahedral WO3 crystals at room temperature.18,19 Compared with the densely packed atomic structure of crystalline, amorphous film could provide higher surface accessibilities, which is more favorable for charge exchange.5,20 Figure 1(b) displays the XPS W 4f and W 5p core level spectra of the WO3 thin film. According to previous reports, the main peaks at 35.8 eV (W 4 f7/2 ) and 38.0 eV (W 4 f5/2 ) are correspond to WO3 , while the second doublet peaks at 36.4 eV (W 4 f7/2 ) and 35.4 eV (W 4 f5/2 ) are originated from W2 O5 .21,22 From the fitting
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Figure. 2: (a)-(c) CV curves of a-WO3 films in nonaqueous Li+ -, Na+ - and Al3+ containing electrolytes at scan rates ranging from 0.07 to 0.35 V/s versus Ag/AgCl. (d)-(f) Evolutions of the redox peaks versus square root of scan rate. (g)-(i) Corresponding time-resolved in situ optical responses recorded at 550 nm.
peak areas, the relative content of WO3 and W2 O5 was estimated to be 95.5% and 4.5%, respectively. It has been reported that the existence of oxygen vacancy W5+ defects in the film play a crucial role on optoelectronic properties, especially for amorphous form.6,7,20 In Figure 2(a)-(c), we compared the cyclic voltammetry (CV) characteristics of a-WO3 intercalated with Li+ -, Na+ - and Al3+ ions, respectively, at different scan rates ν. It can be clearly observed that the film intercalated with Al3+ exhibited much narrower CV loops and lower electrochemistry activities as compared to that with the monovalent Li+ and Na+ at the same scan rate, which is likely due to the relatively high electrode-electrolyte interfacial resistances for Al3+ insertion/extraction.13 Regardless of the type of electrolyte used, no distinguishing characteristic peak except the broad ill-defined anodic peak was observed in the CV curves for these three ions, because of the broad distribution of energetically different types of intercalation sites in host structure.23 The higher the scan rates, the more current passes through the film, and thus the higher cathodic (i pc ) and anodic (i pa ) peak current intensities. As illustrated in Figure 2(d)-(f), approximately a linear relationship was obtained for both i pc and i pa in these electrolytes by plotting the peak current intensities against the square root of scan rate, indicating the diffusion processes were semiinfinite. The effective ion diffusion coefficients D for Li+ , Na+
and Al3+ can be estimated from the Randles-Servcik equation: D1/2 =
i pc/pa , 2.69 × 105 × S × C0 × n3/2 ν1/2
(4)
with the concentration of the cations in electrolyte C0 and the number of involved electrons n (taking the value of the valency of cations).23 The D values obtained for Al3+ (10−13 cm2 /s) was several orders of magnitude lower than that of Li+ and Na+ (10−11 10−10 cm2 /s) because of both the strong electrostatic force and large atomic weight.12,13 Further, the slower charge-transfer kinetics of Al3+ was further confirmed by the chronoamperometry (CA) test (Supplementary Discussion S1). The charge transfer rate of Li+ , Na+ and Al3+ was, respectively, 46.4, 37.8 and 13.5 mC/s, agreeing well with the results of effective diffusion coefficient. It should be noted that the extremely low diffusibility of Al3+ ions in solids would limit the participation of a-WO3 only to the near-surface region. Figure 2(g)-(i) show the corresponding in situ optical responses recorded at 550 nm during CV cycles. The electrochromic modulation in Al3+ at ν=0.07 V/s was about 62.9%, nearly as high as that in Li+ ∼67.0%, much higher than that in Na+ ∼47.8%. When the scan rate increases, the resulting shallow ion diffusion depth would reduce the accessible inner a-WO3 , thus decreasing the EC modulation span.24 It was noteworthy that the EC modulation change in Al3+ was less sensitive to the scan rate compared with that in Li+ and Na+ .
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Figure. 3: (a) Cathodic branch of the CV curves of a-WO3 films in nonaqueous Li+ -, Na+ - and Al3+ containing electrolytes at a scan rate of 0.07 V/s versus Ag/AgCl. The corresponding time-resolved in situ (b) optical responses and (c) Coloration efficiencies at 550 nm. (d) Average coloration R t2 efficiency ηave (t) calculated from t η(t)dt/(t2 − t1 ) at different scan rates. 1
Coloration efficiency, η = S × δA/δQ [with A=log(1/T ) the optical absorbance], is a qualified analytical index for electrochromic performance.12,14,20,21 In the existing literatures, the value of η was extracted approximately as the slope of the line fitted to the linear region of the plots of δA versus the charge density at a certain wavelength. Considering that the instantaneous current I(t) is directly proportional to the differential capacity in the cyclic voltammogram scan, I(t) can be express as ν · δQ/δE or δQ/δt. Thus the coloration efficiency can conveniently be redefined as a function of time, namely η(t) = lim S × [δA(t)/δt]/(δQ/δt) = S × A0 (t)/I(t). t→0
(5)
The total instantaneous current I(t) in CV curves can be described as the combination of capacitive effects [k1 (t)ν, including faradaic pseudocapacitive current and non-faradaic double-layer capacitive current] and diffusion-limited ions insertion [k2 (t)ν1/2 , namely faradaic diffusion current] according to I(t) = k1 (t)ν + k2 (t)ν1/2 .25,26 The solid lines in Figure 3(a) and (b) display the cathodic CV half-cycles and corresponding in situ optical responses for inserting Li+ , Na+ and Al3+ into a-WO3 film at a scan rate of 0.07 V/s, respectively. The dynamic behaviors of instantaneous current I(t) roughly exhibited mirror image relationship with the optical absorbance derivative A0 (t) [the dashed curves in Figure 3(b)]. Compared with monovalent species, especially in low potential range at low scan rate, the high valance ions are easier to be adsorbed or accumulated on the surface of films, thereby increasing the pro-
portion of non-faradaic double-layer capacitive current.27 This is apparent from Figure 3(c), which shows a extremely low amplitude of coloration efficiency η(t) for Al3+ at the beginning of the coloring process. As stated previously the three-electron redox properties of Al3+ would lead to higher transition probability between W4+ and W5+ sites compared with monovalent Li+ and Na+ on the same charge intercalation level xκ, and thereby enhancing the coloration efficiency. During the negative-going potential scan from -0.25 to -1.20 V, the amplitude of η(t) for Al3+ first reached a maximum ∼-50.0% around -0.81 V and then decreased with further decreasing potential, while that for the other two ions showed only slight increase and almost tends to a constant ∼-30.0% in this range. The amplitude of η(t) increased dramatically with increasing potential for all ions at the beginning of the positive potential scan. This feature might be assigned to the decrease in the proportion of non-faradaic double-layer capacitive current. A quantitative analysis of the capacitive contribution to the current response clearly indicates that the higher the scan rate, the greater the proportion of capacitive current for monovalent species (Supplementary Discussion S2). This resulted in a gradual decrease in average coloration efficiency ηave (t) with the increase of ν for Li+ and Na+ , as shown in Figure 3(d). In addition, a higher scan rate would also lead to a relative shallow diffusion depth with high concentration of the intercalant.28,29 Consequently, a significant increase in ηave (t) for Al3+ was observed when the scan rate is increased from 0.07 to 0.35 V/s, due to the higher content and ratio of bipolaronic W4+ sites were formed per unit diffusion depth at high scan rates. Noteworthy, the ηave (t) of amorphous WO3 (-71.8
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Figure. 4: (a)-(c) CA curves evolution of a-WO3 in nonaqueous Li+ -, Na+ - and Al3+ containing electrolytes by applying a triangular wave periodic potential step cycling from -1.5 to 1.5 V versus Ag/AgCl, 10 s for each state. (d) Trapped charges calculated at different CA cycles. (e)-(g) Corresponding in situ optical responses at 550 nm evolution on cycling. (h) Optical contrast versus cycle number derived from optical response data.
cm2 /A) in Al3+ at 0.42 V/s was comparable to that of crystalline W18 O49 (68.0 cm2 /C) in the amplitude.12 Upon extended electrochemical cycling, the constant host-guest interactions would bring about irreversible guest-trapping and host framework damage, leading eventually to degraded electrochromism in optical modulation.4,18,30−33 The interaction forms and intensities of guest with host are mainly determined by the host frameworks, types of insertion guests as well as the operation conditions. To study the effect of coloring ionic radius and valency on the EC stability of a-WO3 films, the real-time detections of the amount of trapped ions were performed using Chronoamperometry technique in Li+ , Na+ and Al3+ [as shown in Figure 4(a)(c), respectively], and the results were summarized in Figure 4(d). The large amount of trapped ions during the first few CA cycles should be assigned to the initial metallization of a-WO3 films.4,30,32 For smaller radius dimensions Li+ , along with the progression of CA cycles the increment of trapped ions density for each cycle dropped from initial ∼6.0×1019 cm−3 to a stable value of ∼3.6×1019 cm−3 around the 60th cycle. These trapped Li+ ions could not be extracted under the same potential operation and yield accumulation of charges in a-WO3 film upon cycling, leading to the corresponding transmittance degradation in the bleached states [Figure 4(e)]. In contrast, severe transmittance
degradation were observed in both the bleached and colored states for the film cycled in larger radius dimensions Na+ [Figure 4(f)]. These effects could be assigned to the destruction of ion diffusion channels induced by excess lattice expansion, so that a further Na+ ions insertion is prevented.18,32 Consequently, the increment of trapped ions density for each CA cycle decreased dramatically from initial value of ∼4.3×1019 cm−3 to almost zero around the 100th cycle. In the case of trivalent Al3+ ions, there was no obvious transmittance degradation for both the bleached and colored states [Figure 4(g)], and the optical modulation was almost keep in a steady value of ∼63.0% even after the 300 cycles [Figure 4(h)], indicating that the film cycled in Al3+ was more stable compared to that in Li+ and Na+ . Four explanations can be advanced for the outstanding EC stability in Al3+ : (i) After the initial metallization, the increment of trapped ions density for each CA cycle was only about 1.5×1019 cm−3 , which was less than that of Li+ ; (ii) Compared with Li+ and Na+ ions, trivalent Al3+ ions have a smaller radius dimensions, and thus decreasing the extent of lattice expansion; (iii) The strong electrostatic forces which stem from the Coulombic interactions between Al3+ ions and host frameworks might, to some degree, stabilize the crystal structure; (iv) Owing to the lower diffusion coefficients, the diffusion depth was much shallower for Al3+ ions, thus resulting in less damage to the ion diffusion channels. It is noteworthy that the ion-trapping-induced
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6 degradation in a-WO3 films can be successfully eliminated by a galvanostatic treatment as recently reported by Wen et al.,4 but the host framework damage would result in irremediable erosion of the electrochromic effect.18,31−33
IV. CONCLUSION
As a conclusion, we demonstrated that trivalent Al3+ ions, which are commonly believed to be immovable in solid-state electrodes, can be used as efficient coloring ions for sputter-deposited a-WO3 films. In situ analysis results show that the film cycled in Al3+ containing electrolyte exhibits desired wide optical modulation, high coloration efficiency and good long-term stability. By comparison with conventional Li+ and Na+ ions, the use of Al3+ has advantages of cost efficiency with earth-abundant aluminum reserves, environment-friendliness and high security, which may open up new paths to resolve the existing issues in device preparation and operation such as high production cost, electrodes corrosion and residual stress-induced host framework collapse etc. Our findings are of great importance in deeper understanding electrochromism and dismiss the generally accepted conception that only monovalent species can be competent for efficient coloring ions in electrochromic smart window field.
V. SUPPORTING INFORMATION
Supporting Information Available: Figure S1, the charge transfer rates for Li+ , Na+ , Al3+ ; Figure S2, the in situ coloration efficiencies at ν=0.14 V/s, 0.21 V/s, 0.28 V/s, 0.35 V/s; Figure S3, the ratios of capacitive current to diffusion current in different scan rate ranges, and Table S1, a summary of the electrochromic behaviors at different scan rates in Li+ , Na+ , Al3+ . This material is available free of charge via the Internet at http://pubs.acs.org.
VI. ACKNOWLEDGMENTS
This work was financially supported by the National Program on Key Research Project (2016YFB0303901), the Beijing Natural Science Foundation (2161001), the Fundamental Research Funds for the Central Universities (Grant No. YWF-16-JCTD-B03) and the financial support from the Swedish Research Council (No. 621-2014-5596).
VII. REFERENCES [1] Paul MS Monk, Roger J Mortimer, David R Rosseinsky Electrochromism and Electrochromic Devices, Cambridge University Press, 2007. [2] Paul MS Monk, Roger J Mortimer, David R Rosseinsky Electrochromism: Fundamentals and Applications, Wiley & Sons, 2008. [3] Claes G Granqvist Electrochromic Tungsten Oxide Films: Review of Progress 1993–1998, Sol. Energ. Mat. Sol. C. 2000, 60, 201–262.
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AlxWO3
Li
+ +
Na
Al
-1
Al
Coloration efficiency
3+
3+
WO3
40
ITO/Glass 0.07
0.14
0.21
0.28
Scan rate (V/sec) Figure. 5: TOC Graphic.
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-ŋ ave(t)/cm2A
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