Electrochromism of Nanocrystal-in-Glass Tungsten Oxide Thin Films

Jan 15, 2019 - LTD, Shenzhen 518126 , People's Republic of China ... evaporated WO3 thin films present a fast colored/bleached time of 16/11, 16/14, a...
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Electrochromism of Nanocrystal-in-Glass Tungsten Oxide Thin Films under Various Conduction Cations Dong Qiu,†,‡ Hao Ji,†,§ Xinlei Zhang,†,§ Hongliang Zhang,*,† Hongtao Cao,*,† Guoxin Chen,† Tian Tian,† Zhiyong Chen,∥ Xing Guo,∥ Lingyan Liang,† Junhua Gao,† and Fei Zhuge† Inorg. Chem. Downloaded from pubs.acs.org by ICAHN SCHOOL MEDICINE MOUNT SINAI on 02/03/19. For personal use only.

†

Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, People’s Republic of China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § School of Microelectronics, Shandong University, Jinan 250100, People’s Republic of China ∥ Jisheng Photoelectric (SHENZHEN) Co. LTD, Shenzhen 518126, People’s Republic of China S Supporting Information *

ABSTRACT: The nanocrystal-in-glass (nanocrystals embedded amorphous matrix) tungsten oxide (WO3) thin films with a nanoporous characteristic were prepared via an electron beam evaporation technique. The e-beam evaporated WO3 thin films present a fast colored/bleached time of 16/11, 16/ 14, and 12/12 s, a large optical modulation of 92, 91, and 87% at 633 nm, and a high coloration efficiency of 61.78, 62.04, and 67.59 cm2 C−1 in Li+, Na+, and Al3+ electrolytes, respectively. On one hand, the improved electrochromic performance is mainly attributed to the short diffusion distance and buffering effect in the host matrix, which facilitates the ion insertion/ extraction and alleviates the structural collapse of the framework. On the other, owing to the strong electrostatic interactions between the trivalent cations and the host, the WO3 thin films in Al3+ possess a shallow diffusion depth and long cycle life. The individual contribution from the capacitance- or diffusion-controlled process is comprehensively demonstrated. Pseudocapacitive behavior in the nanocrystal-in-glass WO3 thin films is in favor of fast kinetics response and sound cycling stability. Our work offers an in-depth insight of the electrochromic mechanism for nanocrystal-in-glass WO3 thin films in various electrolytes and sheds light on the fundamental principle in the electrochromic devices. expansion of the WO3 during H+ (proton) insertion.8 It is demonstrated that the EC performance (such as coloration efficiency and optical modulation) in well-ordered material structure exhibits inferior properties, as compared to the amorphous EC materials such as WO3,7,9 V2O5,10,11 and NbOx.12 As stated previously,12 electrochromic performance of indium tin oxide (ITO)-in-NbOx composites with nanocrystalin-glass is better than the undoped amorphous NbOx. As demonstrated above, the superior optical properties make nanocrytal-in-glass TMO material with nanoporous in nature an ideal candidate for EC applications.13−15 Although proton or ion conductive materials using lithium proton (H+) or ion (Li+) electrolyte have been well established, the development of novel electrolytes for electrochromism is still a great challenge. To the best of our knowledge, reports on electrochromism of TMO materials under various conduction cations are very limited. Recently, it

1. INTRODUCTION An interesting feature of proton-conducting or ion-conducting electrolytes is their ability to modulate optical properties of transition metal oxides (TMOs) under low applied voltages, so-called electrochromism, because of intercalation/extraction of foreign protons or ions.1,2 Reversible intercalation/ extraction of foreign protons or ions into/from a host lattice constitutes the fundamental operating principle of electrochromic (EC) materials.3 Therefore, it is highly desirable to improve the host crystalline structures and the channel front for intercalation/extraction of foreign protons or ions,4−6 which eventually results in improved electrochromic performance. For example, Lee et al.7 have demonstrated that the crystalline WO3 nanoparticle films, as compared to the conventional amorphous WO3 films, exhibit greater charge density for proton intercalation and comparable coloration efficiency (CE), since these nanoparticle thin films have a larger active surface area and lower packing density. Another example is that the porous structure of WO3 film is helpful for promoting the electrolyte infiltration and alleviating the © XXXX American Chemical Society

Received: November 13, 2018

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DOI: 10.1021/acs.inorgchem.8b03178 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns of the WO3 thin films at different substrate temperatures. (b) SEM cross-section image, (c) TEM image, selected-area electron diffraction (SAED) patterns (inset of Figure 1c), and (d) HRTEM image of ultrathin WO3 films on Cu grid.

is demonstrated that the trivalent ion ions (Al3+) can be used as ions to bring about fast switch, high contrast, and high stability in the electrochromic devices.16 Consequently, electrochromism of nanocrystal-in-glass TMO EC material under various conduction cations is essential for understanding the working mechanism and promoting the EC performance.17,18 Herein, the EC performance of the WO3 thin films with nanocrystals in an amorphous matrix under various organic electrolytes was characterized by in situ transmittance spectra and electrochemical measurements. The pseudocapacitive characteristic in the WO3/electrolytes system is demonstrated via cyclic voltammetry (CV) profiles and electrochemical impedance spectra (EIS).

er Lambda 950) and an electrochemical workstation (CHI660D, Chenhua, Shanghai). Electrochemical measurements were carried out in a three-electrode cell. A platinum sheet and KCl saturated Hg/ HgCl2 were used as counter electrode and reference electrode, respectively. 0.1 M LiClO4, NaClO4, and Al(ClO4)3 dissolved into propylene carbonate (PC) were used as electrolytes, respectively. The chronoamperometry, chronocoulometry, and cyclic voltammetry measurements were conducted by applying voltage between −1.0 V and +1.0 V. The electrochemical impedance spectra were measured on an electrochemical workstation (Zennium, IM6) in the frequency range from 100 mHz to 100 kHz.

3. RESULT AND DISCUSSION 3.1. Characterization. Figure 1a shows the XRD patterns of the WO3 thin films deposited at different substrate temperatures (RT, 150 °C, 200 °C, and 250 °C), indicating that the WO3 thin films possess amorphous characteristics, similar to the results reported by Cai et al.8 The cross-sectional SEM image of the 435 nm thick WO3 thin film on the Si(100) substrate (the substrate temperature of 200 °C) is depicted in Figure 1b. The WO3 thin film exhibits a homogeneous, uniform, and nanogranular morphology, as seen that compact nanoparticles with a mean size of less than 10 nm were distributed all over the film (Figure 1b and the Supporting Information in Figure S1). Figure 1c,d shows the TEM and HRTEM images of the ultrathin nanogranular WO3 thin film deposited on the Cu grid, respectively. The selected area electron diffraction (SAED) patterns are also displayed in the insets to Figure 1c,d. A wormlike structure is exhibited, as seen in Figure 1c. The SAED pattern shown in the inset to Figure 1c presents diffused halo rings, doubly confirming the amorphous crystal structure. The HRTEM image (Figure 1 d) of the WO3 thin film reveals clear fringes with a fringe spacing of 0.392 nm, corresponding to the interplanar distance (d-spacing) of the (002) lattice plane of monoclinic WO3 (PDF # 43-1035), in line with the previous report.19 On the

2. EXPERIMENTAL METHODS 2.1. Synthesis. The WO3 thin films were deposited on the ITOcoated glass at various substrate temperatures of room temperature (RT), 150 °C, 200 °C, and 250 °C by an electron beam evaporation technique (MUE-ECO made in ULVAC, Japan). The background pressure was reduced to less than 2 × 10−3 Pa. Several pure WO3 particles with the diameter of ∼3 mm in a tungsten crucible were bombarded by an electron beam of 10 kV in the vacuum of 2 × 10−3 Pa. The deposition rate and thickness of thin films were 0.1 nm/s and 450 nm, respectively. 2.2. Measurements. The structure was analyzed by X-ray diffraction (XRD, Bruker D8 Advance using Cu−Kα (λ = 0.154178 nm) radiation and a θ−2θ configuration), X-ray photoelectron spectra (XPS) (AXIS UTLTRA DLD) using Al Kα (1486.6 eV) radiation as an X-ray source with a voltage of 15 kV and a power of 120 W at a pressure of ∼5 × 10−9 Torr, and high resolution transmission electron microscopy (HRTEM, JEOL2100). The chemical compositions, valence states, and depth profile of Al3+ distribution were analyzed by XPS. The morphology was investigated by atomic force microscope (AFM; Vecco Dimension 3100) and field-emission scanning electron microscopy (FESEM, S4800). In situ transmittance spectroscopy was obtained via UV−vis−IR spectroscopy (PerkinElmB

DOI: 10.1021/acs.inorgchem.8b03178 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) (αhν)1/2 versus photon energy (hν) plots, (b) refractive index, packing density (inset of Figure 2b), and (c−f) AFM images for WO3 thin films at various substrate temperatures.

amorphous surface nature, consistent with the surface morphological results by SEM analysis. The gradual decrease in surface roughness with the substrate temperatures is due to the keeping improvement of crystallinity.19,23 3.2. Kinetic Analysis of the Electrochromic and Pseudocapacitive Behavior. In order to investigate the electrochromic properties of the fabricated WO3 films interaction with different ions (Li+, Na+, and Al3+), typical WO3 films deposited at 200 °C were selected, owing to that these films possess moderate packing density, nanocrystal-inglass structure, and sound adhesion strength between the thin film and substrate to ensure electrochemical cycle stability (as we observed in the real measurements). Figure 3a shows the transmittance spectra and photographs in colored (dashed line) and bleached (solid line) state of the WO3 thin films deposited at 200 °C in various electrolytes of 0.1 M PCLiClO4, PC-NaClO4, and PC-Al(ClO4)3. A high optical transmittance modulation (ΔT) at λ = 633 nm (92, 91, and 87% for Li+, Na+, and Al3+ electrolytes, respectively) and in the near-infrared spectral regions between the applied voltage of −1.0 V (colored state) and +1.0 V (bleached state) is obtained. The charge density for the colored process can be obtained via a chronocoulometry method, which is conducted under a square-wave voltage of −1.0 V and +1.0 V with a pulse width of 45 s. The charge density is approximately −30.9, −26.0, and −40.0 mC cm−2 for Li+, Na+, and Al3+ electrolytes, respectively, as shown in Figure 3b. It is indicated that the WO3 thin films have a higher inserted charge density when using Al3+ electrolyte, which is attributed to Al3+ with a trivalent state as compared to Li+ and Na+. In order to investigate the chemical state evolution of the WO3 thin films, the valence states of W are quantitatively analyzed via XPS spectra, as illustrated in Figure 3c,d. Two peaks centered at

basis of the XRD, SEM, HRTEM, and SAED results, one can conclude that the WO3 thin film possesses a nanocrystal-inglass structure (nanocrystals in amorphous matrix), which is in favor of proton or ion conduction.13 As reported previously, the formation of the amorphous structure of the as-deposited WO3 film is due to the low kinetic energy of the evaporated atoms or molecules, which do not have enough energy to move on the ITO substrates for crystallization.20 Besides, the formation of the WO3 nanocrystals can be attributed to atomic rearrangement (atoms move faster on the surface of the films at higher substrate temperature). Also, the grain size increase with increasing annealing temperature is attributed to atomic rearrangement.21 The optical band gap of the WO3 thin films is determined by fitting the absorption spectra (see the Supporting Information S2). As seen in Figure 2a, the optical band gap of the WO3 thin films deposited at the temperature of RT, 150 °C, 200 °C, and 250 °C is calculated to be 3.08, 3.15, 3.11, and 3.08 eV, respectively. Figure 2b shows the refractive index (n) dispersion curves of the WO3 thin films deposited at different substrate temperatures. According to the calculation (see S3 in the Supporting Information), the packing density (P) of the WO3 thin films deposited at RT, 150 °C, 200 °C, and 250 °C is 0.79, 0.81, 0.82, and 0.85, as shown in the inset of Figure 2b, respectively. The low refractive indexes and pretty low packing density give a hint of the formation of nanopores/channels in the WO3 thin films, which facilitates shortening the diffusion length and accommodating large volume changes caused by ion intercalation/extraction, as reported previously.22 The height root-mean-square roughness (Rq) of the WO3 thin films deposited at RT, 150 °C, 200 °C, and 250 °C is 1.81, 1.46, 1.50, and 1.44 nm over an area of 5 × 5 μm, respectively, as displayed in Figure 2c−f. These results reveal the uniform C

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Figure 3. (a) Transmittance spectra, photographs, and (b) charge density for WO3 thin films in 0.1 M PC-LiClO4, PC-NaClO4, and PC-Al(ClO4)3. XPS spectra of W4f for the (c) colored and (d) bleached WO3 thin films in Li+ electrolyte. (e) XPS depth profile of Al3+ distribution of the cycled WO3 thin film for Al3+ electrolyte. (f) Electrochromic mechanism diagram of the nanocrystal-in-glass WO3 thin films in Li+, Na+, and Al3+ electrolytes.

35.95 and 38.05 eV are ascribed to W6+, while the peaks located at 35.61 and 37.71 eV are assigned to W5+ for WO3 in the colored state (Figure 3c). After Shirley background subtraction, the relative atomic concentration ratio of W6+ to W5+ is derived from the individual peak areas after taking the associated relative sensitivity factor (RSFW4f: 3.523) into consideration. According to the calculation (see S4 in the Supporting Information), the atomic concentration ratio W6+:W5+ ((IW6+/RSFW4f):(IW5+/RSFW4f)) is approximately 52.0% and 100% for the colored state and bleached state, respectively. At the bleached state, the resolved W4f peaks related to W5+ are not observed, as shown in Figure 3d, indicating that W6+ is dominated in the bleached state.24 When the negative bias is applied, W6+ ions are reduced to W5+ ones owing to the injection of foreign ions and electrons, resulting in the formation of MxWO3 (M = Li, Na, Al) in the colored state. While the potential is switched to the positive bias, W5+

are oxidized back to W6+ due to the extraction of ions and electrons from the host, bringing out the transparent WO3 in the bleached state. The reversible variation of valence states originates from the reversible redox reaction, which occurs in the vicinity of the WO3 thin film surface with nanogranular characteristic.25 Besides, the calculated atomic concentration ratio of O:W is approximately 4.6:1 (see Figure S2 in the Supporting Information), more than the theoretical stoichiometric value of 3:1, probably due to the chemisorption oxygen species on the nanogranular WO3 thin film surface. Figure 3e depicts XPS depth profile of Al3+ distribution of the cycled (after 30 injection cycles) WO3 thin film for Al3+ electrolyte. The Al3+ atomic concentration ratios (mol/mol) relative to tungsten decrease obviously from 18% at 0 s to 1.8% at 720 s with the increase of etching time, and are close to 0% after 720 s, similar to the results of Li+ in the previous report.26 The injected depth of Al3+ is estimated to be approximately 40.2 D

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Figure 4. (a) Dependence of the peak current densities (Ip) versus the square root of scan rate (v1/2). (b) Power law dependence of the peak current versus the scan rate. (c) Capacity and (d) separation of contributions from capacitive- and diffusion-controlled processes as a function of processes at different scan rates for the WO3 thin films in Li+, Na+, and Al3+ electrolytes.

nm at the etching rate of ∼3.35 nm/min. In fact, a depthdependent distribution is found in the different thickness films. The electrochromic mechanism diagram of the nanocrystal-inglass WO3 thin films in Li+, Na+, and Al3+ electrolytes is given in Figure 3f. The optical modulation of the WO3 thin films is generally believed to be associated with the double intercalation/extraction of electrons and guest cations, as the following reversible reaction: WO3 + xMn+ + nxe− ↔ MxWO3 (M = Li, Na, Al. n = 1 or 3), consistent with the previous report.17 The switching speed of electrochromic devices is essentially dependent on the ion diffusion coefficient and ion diffusion distance. As illustrated in Figure 4a (S5 in the Supporting Information), the diffusion coefficient (D) of Li+, Na+, and Al3+ electrolytes is, respectively, 9.4 × 10−10, 7.79 × 10−11, and 5.92 × 10−11 cm2 s−1, larger than the corresponding values (1.29 × 10−10, 5.28 × 10−11, and 4.85 × 10−13 cm2 s−1) in the previous report.27 Such large values is attributed to the structural characteristics with the nanoporous and the nanocrystal embedded amorphous phase that are in favor of ions diffusion.14,28 In general, the diffusion coefficient is influenced by steric resistance associated with ionic radius and electrostatic forces coupled with the charge of guest ions.29 For the ions solvated in propylene carbonate (PC), the ionic radii show the tendency of Na+ > Li+ > Al3+. Compared to Na+ and Al3+, diffusion coefficient of Li+ has the largest value due to its small radius and monovalent in nature. In addition, the b value can be calculated from a relationship between the peak current (i) and scan rate (ν) according to the following equation30,31 i = avb

where both “a” and “b” are the adjustable parameters. The peak current (i) obeys a law power in response to the scan rate (v), and the slope of log(i) versus log(v) represents “b”. The “b” value of 0.5 and 1 indicates semi-infinite linear diffusion and surface-controlled (capacitive) electrochemical reaction, respectively.32 Figure 4b displays the power law dependence of the peak current versus the scan rate. The “b” value of the WO3 thin films in Li+, Na+, and Al3+ electrolyte is calculated to be 0.66, 0.78, and 0.67, respectively. All of them are between 0.5 and 1, suggesting that the current is limited by both semiinfinite linear diffusion and capacitive-controlled one. In addition to the “b” value, the total charge and capacitive contributed charge, which helps us to gain further insight with respect to the kinetics of charge storage in the WO3, can be derived based on the following equation33 Q (v) = Q c + k × v−1/2

(2)

where k is a constant and k × v−1/2 represents the diffusioncontrolled charge. The capacitive contributed charge (Qc, mC cm−2) can be obtained from the total charge (Q, mC cm−2) versus v−1/2 (scan rate, mV s−1) plots. Qc is the y-intercept when scan rate reaches infinite. As presented in Figure 4c, Qc is calculated to be approximately 0.90, 0.67, and 1.50 mC cm−2 for the WO3 thin films in Li+, Na+, and Al3+ electrolyte (see S6 in the Supporting Information), respectively. On the basis of the calculated Qc, the separation of capacitive-controlled contribution from the diffusion-controlled process at different scan rates is quantitatively depicted in Figure 4d. The capacitive contributed occupancy is 29, 49, and 33% at 100 mV s−1 in Li+, Na+, and Al3+ electrolyte, respectively. Among

(1) E

DOI: 10.1021/acs.inorgchem.8b03178 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a, b) In situ time-dependent UV−vis transmittance spectra at λ633 nm (−1.0 V/+1.0 V, 90 s per cycle), (c) plots of in situ optical density variation as a function of charge density, and (d) time-dependent current density monitored at λ633 nm for the WO3 thin films in Li+, Na+, and Al3+ electrolytes.

them, the capacitive contributed occupancy in Na+ electrolyte is the biggest, on account of that Na+ with the biggest ionic radius shows the strongest steric effect with the host crystal framework, resultantly leading to the biggest surface controlled ratio.29 Correspondingly, the diffusion of Li+ and Al3+ into the shallow surface of the host WO3 is easier than that of Na+ because of their smaller radii.34 It should be noted that, as the scan rate increases, both the total charge and the diffusioncontrolled ratio are gradually decreased, similar to the previous report.33 The separation of contribution from capacitive- and diffusion-controlled processes is in line with the b values mentioned above. The cyclic voltammetry profiles of the WO3 thin films in Li+, Na+, and Al3+ electrolytes are illustrated in Figure S3a (see the Supporting Information). No obvious change in the shape of CV curves is observed with a duration of 4000 s, indicating relatively stable electrochemical behaviors that result from the buffering effect of nanocrystals in the amorphous matrix.13 The pseudocapacitance in nanocrystal4 and nanostructured35 WO3 is closely coupling with its electrochemical kinetics. The CV curves deviate from an ideal rectangular shape with broad redox peaks, indicating the presence of pseudocapacitance in the fast and reversible Faradaic reactions.6,36 As investigated previously, the thermally evaporated WO3 thin films possess a fast EC response and a highly pseudocapacitive performance.37 Such fast and reversible redox performance is associated with the similar W−O bond length in WO3 in different oxidation states.5 A low current density in Na+ electrolyte is in line with a low charge density (Figure 3b) during the redox processes.38 Figure S3b (see the Supporting Information) presents the CV curves at different scan rates from 20 to 90 mV s−1 with the applied potential between −1.0 V and +1.0 V. The cathode

current density is higher than the anode one because of the transition from the colored conductor MxWO3 (M = Li, Na, Al) to the bleached semiconductor WO3.39 Moreover, the anodic peaks are gradually shifted to more positive voltages with increasing the scan rates, since the inner diffusion resistance is increased in the so-called pseudocapacitive material.13 3.3. In Situ Transmittance Spectra. Figure 5a,b presents in situ time-dependent visible transmittance spectra at λ633 nm. When applying a −1.0 V/+1.0 V voltage for 45 s, the maximum transmittance modulation (ΔT) of coloration/ bleaching is observed to be 92, 91, and 87% for the WO3 thin films in Li+, Na+, and Al3+ electrolyte, respectively. The change of the optical transmittance spectra is ascribed to the redox reactions accompanied by double injection/extraction of ions/electrons in response to the applying voltages. In addition, intercalation is mostly governed by the interface between the electrode and electrolyte (charge transfer resistance), whereas extraction is mainly influenced by ionic diffusion in the electrochromic layers.4 In our study, the transmittance modulation becomes degraded after 25 cycles and 15 cycles for Li+ and Na+, respectively. In contrast, however, the maximum optical transmittance modulation is almost unchanged (∼87%) even after 30 cycles for Al3+. The ionic radius and volume of Na+ are larger than those of Li+ and Al3+.16,34 As stated previously,40 compared with Li+ and Al3+, superficial insertion of Na+ into the host matrix is much harder. Similarly, intercalation/extraction of Na+ into/from the WO3 matrix is more sluggish in our case, resulting in the greater degradation of the maximum optical transmittance modulation. Generally, traditional monovalent ions (Li+, Na+) usually bring about significant volume expansion in the host matrix during F

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WO3 thin films at a peak signal of 10 mV in the frequency region ranging from 100 mHz to 100 kHz. Also, the electrochemical impedance spectra of the bleached state are presented in Figure S4a (see the Supporting Information). The real resistance of the colored WO3 thin films is estimated to be in the order of magnitude of ∼102 ohm/cm2, lower than that of the bleached state (∼103 ohm/cm2), in a good agreement with the transition from conductor (MxWO3) to semiconductor (WO3).39 Accordingly, a high cathode current density in the colored state is exhibited (detailed data can be observed in Figure S3b of the Supporting Information). Generally, Randles equivalent circuits (inset in Figure 6a) are used to quantitatively analyze the impedance. The intercept of the real axis (x-intercept) and diameter of the semicircle in the high frequency region represents the resistance of electrolyte (Rs) and charge-transfer (Rct), respectively. CPE is a constant phase element and “W” represents the Warburg impedance of ionic diffusion kinetic in the low frequency zone.36,45 The Rs of the colored WO3 thin films is estimated to be 25.71, 48.26, and 19.35 Ω cm−2 when using Li+, Na+, and Al3+ electrolyte, respectively, while the Rct is determined to be 59.08, 85.10, and 156.74 Ω cm−2 for Li+, Na+, and Al3+ electrolytes, respectively. These fitted values are consist with the intercept of the real axis and diameter of the semicircle in Nyquist plots. The pretty lower value of Rs for the colored WO3 thin films in the Al3+ electrolyte is ascribed to the smaller ion radius as well as a stronger Coulombic electrostatic force of multivalent Al3+. Similarly, the Al3+ system exhibited lower Rs and higher Rct than Li+, Na+, and Mg2+ in the previous report.36 In addition, for multivalent Al3+, the volume expansion-induced structure collapse of the framework is suppressed by the strong Coulombic force between the inserted multivalent cations and the host anions, which facilitates switching reversibility.16 CPE-n (the exponent of CPE) of 0.5 and 1 is corresponding to a semi-infinite diffusion and an ideal capacitor (surfacecontrolled),35,46 respectively. In this work, CPE-n of the bleached WO 3 thin films (see S8 in the Supporting Information) in Li+, Na+, and Al3+ electrolytes is, respectively, to be 0.855, 0.813, and 0.845, all of them in between 0.5 and 1, indicating that both semi-infinite diffusion and surfacecontrolled process contribute to the electrochemical reaction. The Nyquist plots of the bleached WO3 thin films in Li+, Na+, and Al3+ electrolytes exhibit similar profiles with a slope close to 90°, revealing that a pseudocapacitive behavior is present in the reversible redox reactions.47 These further confirm the results described by the “b” values mentioned above. As a rule of thumb, CPE-n is linked to the porosity of the WO3 thin films.45,48 CPE-n is close to 1, suggesting that the porosity is approaching zero. In our case, CPE-n is in the vicinity of 0.85, manifesting the nanoporous characteristic of the WO3 thin films, in a good agreement with the packing density discussed above. Figure 6b shows the admittance plots for the colored WO3 thin films. Accordingly, the admittance plots are provided for the bleached WO3 thin films in Figure S8 in the Supporting Information. The lower limit of the high frequency region can be characterized by a knee frequency. The knee frequency of the colored WO3 thin films is 6.04, 4.87, and 10.37 kHz in Li+, Na+, and Al3+ electrolyte, respectively. As we know, a high knee frequency corresponds to an improved electrochemical response,49 further demonstrating that the EC response in Al3+ is quicker than that in Li+ and Na+. Figure 6c displays the plots of the real capacitance (solid line) and phase angle (dashed line) versus frequency for the bleached WO3 thin films

the intercalation process,16 giving rise to the degradation of the cycle life of electrochromic devices. Nevertheless, Al3+ can alleviate these drawbacks.16 Fast switching, high contrast, and highly stable electrochromic behaviors were observed by using the trivalent Al3+ as the insertion ions, which is attributed to the strong electrostatic force between the host framework and multivalent Al3+. Besides, the WO3 thin films with nanoporous structures offer many active sites for charge rearrangement, accommodating volume distortion deduced by foreign ions.3 As stated in the previous reports,41,42 the high ΔT is beneficial from a synergistic effect inside the nanocrystal-in-glass structure. Commonly, the response time in electrochromic devices is defined as the time for 90% change of overall transmittance modulation. The colored/bleached time of the WO3 thin films in Li+, Na+, and Al3+ electrolyte is 16/11, 16/14, and 12/12 s, respectively. The nanocrystal-in-glass structures of the WO3 thin film can shorten the EC response due to the facilitation of ion transferring.14 The coloring kinetics is slower than the bleaching one, stemming from the varied resistance from semiconductor WO3 to conductor MxWO3 (M = Li, Na, Al).39 In particular, a pretty quicker response for Al3+ is exhibited, due to the shallower intercalation/extraction depth in the nearsurface region, consistent with the previous report.27,43 Figure 5c shows the plots of optical density variation (ΔOD) at 633 nm as a function of charge density (Q) of the WO3 thin films in Li+, Na+, and Al3+ electrolytes. The coloration efficiency (η) (see S7 in the Supporting Information) of the WO3 thin films is, respectively, determined to be 61.78, 62.04, and 67.59 cm2 C−1 for Li+, Na+, and Al3+ electrolytes, comparable to the other report.16 The aforementioned results demonstrate that nanocrystal-in-glass and nanoporous structures of the WO3 thin films are highly desirable for EC applications. That is to say, the EC performance such as big optical modulation, fast coloration and bleaching response speed, and high coloration efficiency can be attributed to synergistic effect of structure uniform distribution of nanocrystals in an amorphous matrix, which is expected in TMO-based EC devices.8,13,14 For instance, Richard et al. have demonstrated that the disordered porous semicrystalline WO3 thin films exhibit a good performance with fast switching kinetics and excellent durability.44 Figure 5d illustrates the chronoamperometry plots monitored at λ633 nm for the WO3 thin films in Li+, Na+, and Al3+ electrolytes under the applied voltage ranging from −1.0 V to +1.0 V for 45 s, respectively. The reversibly switching current is ascribed to double injection/extraction of cations/electrons, accompanying the reversible optical modulation (shown in Figure 5a,b). The EC properties of the WO3 thin films in Li+, Na+, and Al3+ electrolytes are summarized in Table 1. 3.4. Electrochemical Impedance Spectra. Figure 6a displays the electrochemical impedance spectra of the colored Table 1. Summary of Electrochromic Characteristics for WO3 Thin Films in 0.1 M PC-LiClO4, PC-NaClO4, and PCAl(ClO4)3 WO3 +

Li Na+ Al3+

ΔT (%)

tb/tc (s)

η (cm2 C−1)

D (cm2 s−1)

92 91 87

11/16 14/16 12/12

61.78 62.04 67.59

9.40 × 10−10 7.79 × 10−11 5.92 × 10−11 G

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Figure 6. (a) Nyquist and (b) admittance plots for the colored WO3 thin films. (c) Real capacitance and phase angle versus frequency plots. (d) The real impedance (Z′) versus angular frequency (ω = 2πf) dependency plots in the low-frequency region.

4. CONLUSIONS In summary, the nanocrystal-in-glass WO3 thin films (nanocrystals embedded amorphous matrix) have a nanoporous structure, which effectively reduces the diffusion path and promotes the electrolyte infiltration. The enhanced electrochromic properties are mostly ascribed to a buffering effect of the nanocrystals embedded amorphous phase that can accommodate the volume change induced by the ion insertion/extraction. The WO3 thin films exhibit a high coloration efficiency of 61.78, 62.04, and 67.59 cm2 C−1 and wide optical modulation of 92, 91, and 87% at 633 nm in Li+, Na+, and Al3+ electrolytes, respectively. Due to the strong electrostatic forces, the Al3+ electrolyte presents a shallow insertion/extraction depth in the near-surface regions so as to stabilize the host framework, contributing to a fast kinetics response and long-term cycling stability. Both the surfacecontrolled and diffusion-controlled processes are discussed in detail in this report. As a reversible Faradaic reaction, the psedocapacitance behavior in the WO3 thin films represents promoted electrochemical reaction dynamics and cycle life, which is indispensable for high performance electrochromic thin films and devices.

in different electrolytes. In principle, at low frequency, a 90° phase angle (Φ) in the phase-angle plot presents an ideal capacitor (surface-controlled), corresponding to a vertical line (slope) in the Nyquist plot. Actually, the deviation from 90° to 45° phase angle represents a pseudocapacitive behavior, which indicates a transition from an ideal capacitor (surfacecontrolled) to the semi-infinite diffusion. 31 The real capacitance (C′) can be calculated by the following formula50 C′ =

− Z″ ωZ2

(3)

where Z″, ω, and Z are the imaginary impedance, angular frequency (ω = 2πf), and impedance, respectively. At 1 Hz, the C′ value is estimated to be 12.67, 9.59, and 15.20 μF cm−2 for the WO3 thin films in Li+, Na+, and Al3+ electrolyte, corresponding to the phase angle of 81°, 65°, and 77°, respectively. At the low frequency region, higher C′ is observed in Al3+ electrolyte, corresponding to a faster response rate, as reported previously.47 The capacitor response frequency at Φ = 45° (fΦ=45°) is 58.29, 24.58, and 64.94 Hz for the WO3 thin films in Li+, Na+, and Al3+ electrolyte, respectively. As reported previously,49 when the fΦ=45° value is shifted from 1 to 6 Hz, the response time is reduced from 1 to 0.2 s accordingly, meaning that the higher the fΦ=45°, the faster the response time. In our case, the fastest response time of 12/12 s for Al3+ electrolyte is observed. Furthermore, the redox reaction of the surface-controlled process is more reversible than that of the semi-infinite diffusion process.30 Additionally, the porous host materials with short diffusion length are good for facile penetration of guest ions. Figure 6d presents angular frequency (ω = 2πf) dependent real impedance (Z′) plots in the lowfrequency region. The smallest slope is exhibited for the bleached WO3 thin films in Al3+ electrolyte, indicating the fastest electrochemical kinetics.51 Therefore, the improved performance with faster switching speed, more desirable cycle stability and reversibility is obtained for the Al3+ electrolyte. As demonstrated similarly, Al3+ can be efficiently inserted into/ extracted from W18O49 nanowires and the volume expansion of the host framework is suppressed via strong Coulomb forces.16

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03178. Calculation equation, cycle voltammetry, XPS, and electrochemical impedance spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86 574 86688153. E-mail: [email protected] (H.Z.). *Tel./Fax: +86 574 86688153. E-mail: [email protected] (H.C). H

DOI: 10.1021/acs.inorgchem.8b03178 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ORCID

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Hongliang Zhang: 0000-0002-9295-8683 Hongtao Cao: 0000-0002-4458-4621 Lingyan Liang: 0000-0002-6285-6600 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the Applied Research Funds for Public Welfare Project of Zhejiang Province (2015C31114), the Program for Ningbo Municipal Science and Technology Innovative Research Team (Grant No. 2016B10005), and the National Natural Science Foundation of China (61604085).

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