WOx

Article pubs.acs.org/JPCC

Low Temperature Annealing Improves the Electrochromic and Degradation Behavior of Tungsten Oxide (WOx) Thin Films Kunyapat Thummavichai, Liam Trimby, Nannan Wang, C. David Wright, Yongde Xia, and Yanqiu Zhu* College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, United Kingdom

ABSTRACT: This research aims to understand the fundamental aspects of annealing on the electrochromic performance of tungsten oxides, using as-synthesized W18O49 substoichiometric bundled nanowires benchmarked against commercial WO3 nanoparticles. Linking detailed structural analyses with the electrochromic measurement results, we have investigated the electrochromic performance effects of low temperature annealing, up to 350 °C, on tungsten oxide (WOx) thin films, trying to establish the fundamental heat treatment−structure−performance loop. We have found that the annealing treatment at low temperature improved the optical modulation and long-term durability of the WOx thin films, without changing the structure and morphology of the as-synthesized samples. The 350 °C annealing was found to have the best stability improvement for the WO3 nanoparticle films during the electrochromic assessments, with a 4% improvement for Li+ intercalation and a 12% improvement for deintercalation, compared with the untreated WO3 samples. Further improvements have been achieved for the W18O49 nanowire thin films, with a stability improvement of 36% for Li+ intercalation and 60% for deintercalation against the as-prepared W18O49 nanowire samples during the electrochromic performance testing.

1. INTRODUCTION

where M is alkali metal, and the fractional number of sites that filled in the WO3 lattice is determined by the subscript x in the general formula of the M+ intercalated compound MxWO3. The stability and degradation of electrochromic materials are essential issues affecting the ultimate performance of chromic devices; however, these issues have not been fully dealt with in the past and fundamental understanding remains a huge challenge. According to Hashimoto et al., the cause of degradation in WOx electrochromic devices is a result of Li+ accumulation at the initial stage of coloring and bleaching cycles.7 Similar ion-trapping models were also proposed to explain the degradation of WOx thin films.8,9 To study the electrochromic performance and understand the degradation behavior of the alkali-metal-intercalated WOx compounds, the fundamental insertion parameters of the alkali metal ions such as the diffusion coefficient need to be investigated to establish links with the detailed kinetic behavior of the intercalation

Among many chromogenic materials, tungsten oxides WOx (2 ≤ x ≤ 3) have been intensively studied due to their potential for outstanding electrochromic performance.1 It has been reported that WOx materials have better electronic conductivity (10−10−6 S/cm) and faster lithium ion (Li+) insertion than many other oxides.2,3 Specifically, they exhibit excellent cyclic stability, high coloration efficiency (CE), good memory, and high contrast ratio, when compared with other transition metal oxides. Nanoscale WO x has been proposed to have extraordinary potential in future energy-related applications.4 The electrochromic characteristics of WOx, being reversible under double injection of electrons and metal cations (Li+, H+, etc.), depends heavily on the nature and structure of the materials.5 The mechanism of these coloration processes can be expressed as follows:6 x M+ + x e− + MO ⇌ MxWO3 3  bleaching

Received: June 27, 2017 Revised: August 25, 2017 Published: September 1, 2017

coloration © 2017 American Chemical Society

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DOI: 10.1021/acs.jpcc.7b06300 J. Phys. Chem. C 2017, 121, 20498−20506

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The Journal of Physical Chemistry C

Figure 1. XRD patterns. (A) Thin film of WO3 nanoparticles with and without heat treatment (reference: JCPDF No. 83-0951). (B) Thin films of bundled W18O49 nanowires with and without heat treatment (reference: JCPDS No. 01-073-2177). Squares (■) indicate the peaks from the ITO substrate following JCPDF No. 06-0416.

compounds,10 because the processing parameters have a strong influence on the crystalline structures of the compounds. Therefore, a systematic investigation endeavoring to relate specific WOx nanowire bundled structures to their stability in electrochromic devices is needed. Low temperature annealing treatment is one simple solution to deal with the degradation issue in both gas sensor and chromic applications. Recent reports have pointed out that the annealing heat treatment affected the structure homogeneity, eliminated the defects, and improved crystallinity of transition metal films,11−15 which in turn could affect the Li+ trapping behavior, therefore improving efficiency for chromic gas sensor applications.16,17 This study aims to investigate the structural changes of two types of different WOx thin films, when subjected to annealing at different temperatures ranging from 250 to 350 °C under rich Ar atmosphere, to understand the influence of resulting structural changes on Li+ insertion within the annealed WOx, and to establish links between the electrochromic performance and the processing features of these films.

rpm for a further 5 s. Each sample was recoated for 10 rounds to obtain a good and uniform coverage of desired film thickness, and then dried at 80 °C in an oven for several hours. Some as-dried thin films reported in the second part of this study were further heat-treated under a rich Ar atmosphere in a tube furnace, up to 350 °C for 2 h under 100 sccm Ar flow rate, in order to assess the effects of heat treatment. The thickness of the films was around 300 nm on average for all samples, measured using the profilometry technique. 2.3. Characterization. The structure and morphology of the films coated on ITO were measured by using X-ray diffraction (XRD) from 10 to 70° in the 2θ range, and scanning electron microscopy (SEM; a Philips XL-30 machine operated at a voltage of 20 kV). Moreover, Raman spectra were obtained using a Renishaw RM1000 Raman microscope (RENISHAW, Wooton-Under-Edge, U.K.), with 532 nm laser excitation and a 50× microscope objective lens, at room temperature. High resolution transmission electron microscopy (HR-TEM) images were taken using a JEOL-2100 machine operated at an acceleration voltage of 20 kV. A potentiostat and UV−vis spectroscopy were also utilized to assess the electrochromic performance of the film samples. For electrochemistry measurements, we used a standard three-electrode cell, with the Pt, WOx thin films, and the calomel Ag/AgCl as the counter, working, and reference electrodes, respectively. A 0.5 M LiClO4 in propylene carbonate solution was used as the electrolyte in our investigation for all samples.

2. EXPERIMENTAL SECTION 2.1. Materials. Two types of WOx were used in this study: bundled ultrathin W18O49 (WOx, x = 2.72) nanowires and commercial WO3 nanoparticles. The W18O49 nanowires were synthesized by a solvothermal method with tungsten hexachloride (WCl6) as the precursor.18 Briefly, 0.004 M well-dissolved WCl6 in cyclohexanol solution was transferred to a 100 mL Teflon-lined stainless steel autoclave for solvothermal synthesis at 200 °C for 6 h. The resulting final product was collected after being thoroughly rinsed with deionized water, ethanol, and acetone several times to remove the solvent residues, and dried at 80 °C for 12 h for later use. Meanwhile, WO3 nanoparticles with an average diameter of 40 nm, purchased from a commercial company in China, were also used to prepare the thin films, for comparison purposes against the bundled W18O49 nanowires. 2.2. Thin Film Preparation. The thin films were prepared using the spin-coating technique. A 0.1 g sample of each precursor was dispersed in 2 mL of ethanol, well mixed by an ultrasonic bath for 1 h prior to use. The suspension was then immediately dropped onto an ITO conducting glass substrate (2.5 × 2.5 cm) inside the spin coater. The machine was run at 1200 rpm for 5 s in the first step, and continued to spin at 1500

3. RESULTS AND DISCUSSION The WO3 nanoparticles and the W18O49 nanowires behaved differently when they were subjected to low temperature heat treatment, and their fine crystal structure changes were recognized in the XRD results. Parts A and B of Figure 1 show the crystalline structures of the as-prepared and heattreated WO3 nanoparticles and bundled W18O49 nanowires, respectively. The as-prepared W18O49 nanowires exhibited the strongest diffraction peaks at 23.1, 28.6, and 47.2°, which were assigned to the (010), (310), and (020) planes. These nanowires correspond to a monoclinic phase structure, with lattice constants of a = 18.32, b = 3.79, c = 14.04 Å, and β = 115.03° (reference: JCPDS No. 01-073-2177). The increased relative intensities and narrowed feature of the peaks of (010) and (310) showed that the crystal planes of the nanowires preferably grew along the [010] direction. For comparison, the 20499

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also showed no sign of morphological changes before (Figure 2B) and after the annealing (Figure 2D,F,H). The bundles kept their originally random distribution and orientation intact after the annealing. The only difference between the two types of thin films was that the WO3 nanoparticles exhibited isolated distributions while the bundled nanowires easily formed networks. In localized areas, the nanowire bundles appeared to be quasi-aligned, originating from the spin-coating process, which could be beneficial for the films to have better mechanical stability and higher conductivity than those of nanoparticle films. Prior to annealing, the Raman spectra of WO3 nanoparticles clearly show three main spectral regions which refer to the stretching and bending vibration modes. The two strongest Raman shifts at about 809 and 717.8 cm−1 can be assigned to the O−W−O stretching vibration modes, while 329 and 247.4 cm−1 belong to the O−W−O bending vibration modes of monoclinic WO3 (Figure 3A). Moreover, the peaks below 200 cm−1 are attributed to the lattice vibration modes.19 The W18O49 nanowires also show three similar Raman shift regions, except that the peaks are slightly shifted toward lower wavenumbers for the stretching and lattice modes (Figure 3B), and toward higher wavenumbers for the bending modes, compared with the spectra of WO3. The two main peaks in the highest region at 805.9 and 710.4 cm−1 can be assigned to the O−W−O stretching mode. The bands at 327.5 and 264.5 cm−1 belong to the O−W−O bending mode; 186 and 132 cm−1 belong to the lattice mode.20 After heat treatment, the Raman bands of both WO3 and W18O49 are shifted toward lower and higher wavenumbers in different regions, as shown in Figure 3, and the overall intensities of each region are decreased and some peaks disappear. All samples show no sign of the WO bond, which means any trace of water was eliminated. These Raman spectra have confirmed that the chemical bonds within WO3 and W18O49 have been reinforced via lattice relaxation (i.e., defect elimination) after the low temperature heat treatment. The slightly changed bonding within both WO3 and W18O49 after the annealing was also supported by our HR-TEM observations, as shown in Figures 4 and 5A,C,E,G. The lattice spacing of WO3 nanoparticles was affected by the heat treatment under Ar, and a small reduction of the (010) plane space was recognized with increased temperatures, from 0.39 to 0.37 nm from room temperature up to 350 °C. A similar trend was observed in the (010) plane space for all W18O49 nanowire samples, being slightly smaller than that of the as-prepared

strongest diffraction peaks at 23.1, 23.6, and 24.4° were assigned as the (002), (020), and (200) planes of the monoclinic phase for the WO3 nanoparticles. The measured lattice constants of the commercial WO3 were a = 7.33, b = 7.54, c = 7.68 Å, and β = 90.884° (reference: JCPDF No. 830951). It is noted that the intensities of the three main peaks for WO3 did not show any noticeable changes before and after the heat treatments at various conditions. In terms of morphology, the WO3 nanoparticle thin films did not show recognizable differences or signs of agglomeration before (Figure 2A) and after the annealing (Figure 2C,E,G).

Figure 2. SEM images of thin films before and after annealing at different temperatures of 250, 300, and 350 °C, for WO3 nanoparticles (A, C, E, and G) with scale bar = 100 μm, and bundled W18O49 nanowires (B, D, F, and H) with scan bar = 10 μm, respectively.

The particles appeared to have remained with their average diameter of 40 nm. Similarly, the bundled W18O49 nanowires

Figure 3. Raman spectra of different heat treatment samples of both WO3 nanoparticles (A) and W18O49 nanowires (B) irradiated at 532 nm laser excitation. 20500

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Figure 4. HR-TEM images of thin films of WO3 nanoparticles treated at different annealing temperatures, before (A, C, E and G) and after (B, D, F and H) electrochemical testing, The HR-TEM images are presented with a scan bar = 5 nm. (A and B) As-prepared; (C and D) 250 °C; (E and F) 300 °C; (G and H) 350 °C. All insets are the corresponding diffraction pattern of each sample.

Figure 5. HR-TEM image of thin films of bundled W18O49 nanowires heat-treated at different temperatures before (A, C, E, and G for room temperature, 250 °C, 300 °C, 350 °C, respectively) and after electrochemical testing (B, D, F, and H for room temperature, 250 °C, 300 °C, 350 °C, separately). All HR-TEM images are presented with a scan bar = 5 nm. Insets are the diffraction patterns of bundles.

sample, as shown in Figure 5A,C,E,G, from 0.38 nm down to 0.37 nm. A smaller plane spacing indeed originated from fewer structural defects and a stronger bonding between atoms. The ultrathin nanowires have a diameter of less than 5 nm, as described before, hence during the annealing process it is easier for their internal structural defects to move out of the structures and dissipate on the surface than larger particles. Therefore, prior to annealing at room temperature, the nanowires exhibited smaller (010) plane space than the 10 times larger WO3 nanoparticles that are more likely to contain lattice defects internally. After annealing, both oxides dramatically eliminated the plane defects and thus exhibited an almost identical (010) plane space of 0.37 nm. Figures 4 and 5 also show the lattice changes of both WO3 and W18O49 thin films, with and without annealing, after 1000 cycles of cyclic voltammetric (CV) testing, used to assess the annealing effects on the Li+ retaining characteristics for the two types of oxide. The WO3 did not exhibit any change in the lattice spaces between the as-purchased and the 250 °C annealed samples; however, a slightly increased (010) lattice space for the 300 and 350 °C annealed samples was observed, compared with those before the CV testing (Figure 4B,D,F,H). For the nanowires after the 1000th cycle of electrochemistry testing, the d space values of (010) were slightly increased for

all annealed samples (Figure 5B,D,F,H), while no changes occurred to the as-prepared thin films, possibly due to the apparent earlier reduction in the d values after annealing. The enlarged d values of the oxides after the CV tests could be attributed to Li+ ions being trapped (left behind) within the oxide lattice, which could not be detected using the TEM technique. It has been proposed that Li+ inserted into the oxide lattice during the coloration process cannot be completely extracted during the detrapping process.7 It has been argued that the Li+ would be located close to the W5+ color centers, binding to the O coordination shells of W5+ sites. The more Li+ remaining within the lattice, the larger the d value of the (010) would be observed after the CV testing. A highly efficient chromic process not only demands more Li+ to enter the oxide lattice to promote an intense coloration, but also requires the Li+ to be able to come out of the lattice smoothly during the bleaching stage, to achieve high stability and cyclic performance. It is interesting to find out the annealing effect on these properties. To further evaluate the effectiveness of the diffusion path in our variously annealed oxide structures, we have tried to obtain the effective diffusion coefficient for both the intercalation and deintercalation processes. The effective diffusion coefficient D for Li+ intercalation/deintercalation with units of cm2/s can be 20501

DOI: 10.1021/acs.jpcc.7b06300 J. Phys. Chem. C 2017, 121, 20498−20506

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The Journal of Physical Chemistry C

Figure 6. Cyclic voltammetric (CV) profiles of as-purchased WO3 nanoparticles without heat treatment, and annealed at 250, 300, and 350 °C (A, C, E, and G, respectively). CV curves of as-prepared W18O49 nanowires without heat treatment, and heat-treated at 250, 300, and 350 °C (B, D, F, and H, respectively). The CV experiment was conducted at room temperature in a 0.5 M LiClO4 gel electrolyte, scanned between −1.5 and 1.5 V at different high scan rates of 20, 40, 60, 80, and 100 mV/s, respectively. Insets show anodic (oxidation reaction) and cathodic (reduction reaction) peak current densities as a function of the square root of the scan rates.

literature.22,23 By using the slope from the linear relationship between the current and the v1/2 graph with the above equation, the values of the estimated Li+ diffusion for the intercalation of WO3 nanoparticles are 1.6 × 10−10, 1.5 × 10−10, 1.79 × 10−10, and 1.97 × 10−10 for room temperature, 250 °C, 300 °C, and 350 °C samples, respectively. Moreover, values of the the Li+ diffusion for deintercalation of WO3 are 7.7× 10−11, 5.86 × 10−11, 7.57 × 10−11, and 8.09 × 10−11 for room temperature, 250 °C, 300 °C, and 350 °C samples, respectively. Similarly, we obtained the intercalation/deintercalation values for W18O49 nanowires of 6.85 × 10−10/5.8 × 10−10, 1.72 × 10−9/4.3 × 10−10, 1.23 × 10−9/1.01 × 10−9, and 8.24 × 10−10/7.56 × 10−10 for room temperature, 250 °C, 300 °C, and 350 °C samples, respectively. The Li+ intercalation values from both WO3 and W18O49 samples are much higher than those of deintercalation. During the cathodic scan, the film became dark blue, corresponding to the coloration stage, which was directly related to the reduction of the W6+ state to the W5+ state, as

estimated from both anodic and cathodic peak current densities value as a function of the square root of the scan rate (v1/2), by assuming a simple solid state diffusion controlled process:1,21 D1/2 =

ip 5

(2.69 × 10 )n3/2AC0v1/2

(Randles−Sevick equation)

where ip is the peak current (A), n is the number of electrons transferred in unit reaction (electron stoichiometry) and assumed to be 1, A is the surface area of the WO3 and W18O49 electrode film (cm2), C0 is the concentration of the diffusion species Li+ (mol/cm3) and v is the scan rate, V/s. For the calculation at high scan rates due to the oxidation peaks overlap, we used the maximum and minimum values of current density in the positive region and the negative region of the CV profile to estimate to the DLi+ value, according to the 20502

DOI: 10.1021/acs.jpcc.7b06300 J. Phys. Chem. C 2017, 121, 20498−20506

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The Journal of Physical Chemistry C Table 1. Summary of Effective Diffusion Coefficient (DLi+) Values of Different Thin Films diffusion coefficient, DLi+ (cm2/s) intercalation (cathodic peak)

deintercalation (anodic peak)

sample

1st cycle

1000th cycle

diff (%)

1st cycle

1000th cycle

diff (%)

WO3, as-purchased WO3, 250 °C WO3, 300 °C WO3, 350 °C W18O49, as-prepared W18O49, 250 °C W18O49, 300 °C W18O49, 350 °C

1.2 × 10−10 1.3 × 10−10 1.55 × 10−10 1.78 × 10−10 8.14 × 10−10 1.21 × 10−9 9.8 × 10−10 6.18 × 10−10

5.9 × 10−11 6.23 × 10−11 7.31 × 10−11 9.43 × 10−11 3.89 × 10−10 7.46 × 10−10 6.97 × 10−10 5.1 × 10−10

51 52 52 47 52 38 28 16

3.33 × 10−11 3 × 10−11 3.69 × 10−11 4.1 × 10−11 5.05 × 10−10 4.01 × 10−10 5.53 × 10−10 3.7 × 10−10

1.54 × 10−11 1.55 × 10−11 1.81 × 10−11 2.36 × 10−11 1.5 × 10−10 1.97 × 10−10 2.99 × 10−10 3.32 × 10−10

54 48 51 42 70 50 47 10

well as the W4+ state to the W5+ state. Reversely, during the anodic scan the thin film switched from blue to colorless, defining a charge transfer from W5+ to W6+ and from W5+ to W4+ (oxidation). During these reversible processes, the intercalation and deintercalation of electrons from the electrode, as well as the Li+ from the electrolyte, resulted in the optically colored and bleached states of the thin films.24 The electrochromic mechanisms for WO3 and W18O49 in the Li+-based electrolyte used in this study can be expressed as

WO3. For all samples, the magnitude of the diffusion coefficient for Li+ intercalation was greater than that of the deintercalation (Dintercalation > Ddeintercalation), which clearly demonstrated the presence of the charge-trapping phenomenon, as well as the slower bleaching than coloration processes. The trapped Li+ must remain inside the WOx structures, therefore resulting in the increased lattice distance after 1000 cycles, as verified by the HR-TEM results shown in Figures 4 and 5. From Table 1, the annealed WO3 samples exhibited marginally better values than the original WO3 thin film, by 6, 3, and 12% of the deintercalation for the 250, 300, and 350 °C annealed samples, respectively. For the W18O49 samples, the 250 and 300 °C annealing resulted in a noticeable improvement in the diffusion, achieving the DLi+ values for intercalation (1.21 × 10−9 and 9.8 × 10−10 cm2/s for 250 and 300 °C samples, respectively) which are 49 and 20% improvements respectively, against the unannealed W18O49 sample (8.14 × 10−10 cm2/s for intercalation). However, slightly higher deintercalation of W18O49 annealed sample can be obtained at the 300 °C sample with a value of 5.53 × 10−10 cm2/s compared with 5.05 × 10−10 cm2/s from deintercalation of the original W18O49. Moreover, the stabilities in both intercalation and deintercalation of W18O49 annealed samples were clearly recognized, increased by 14, 24, and 36% for intercalation and 20, 23, and 60% for deintercalation of 250, 300, and 350 °C treatments, respectively, benchmarked against the original W18O49. The reasons for the nonlinear improvement trend of both DLi+ and the stability for the annealed samples at different temperatures remains unclear; nevertheless, the improvements due to annealing were pronounced. The switching response for coloration and bleaching of all thin films samples can be determined by the chronoamperometry scan (CA), at pulse potential ±1.5 V for 20 s. The response time for coloration and bleaching is defined as the time needed for excess current to reduce to 10% of its absolute maximum value.23,25 The as-purchased WO3 thin film showed a response time of 5 s/1 s for coloration/bleaching, compared with the 350 °C annealed sample of 8.4 s for the coloration and 1.5 s for bleaching. The as-prepared W18O49 obtained coloration and bleaching times of 11 and 3.3 s, respectively, against the 13.3 and 4.4 s for 300 °C annealed sample. These slower values for both the annealed samples and the nanowires against the pristine samples and nanoparticles respectively are rather surprising. However, these values did not reflect the total amounts of Li+ intercalated into or deintercalated out of the lattice, i.e., the intensity of the coloration/bleaching processes. To achieve higher intensity, even given the similar DLi+ values, more Li+ transportation would be involved during the process,

intercalation

WO3 + x Li+ + x e− XoooooooooooooY LixWO3 deintercalation

intercalation

W18O49 + x Li+ + x e− XoooooooooooooY LixW18O49 deintercalation

Generally, the bundled W18O49 nanowire thin films showed better charge-insertion density than that of the WO3 nanoparticle thin films (Figure 6), which means that the W18O49 nanowires provided faster Li+ intercalation/deintercalation kinetics, compared with the WO3 nanoparticles. The inset in Figure 6 displayed the anodic and cathodic peak current density values of different thin film samples. The approximate linear relationship indicated a diffusion-controlled process. An easy and smooth path for the Li+ to pass through for both ways was therefore imperative for the diffusion kinetics, while different levels of defects within the crystalline structures would therefore have an impact on such diffusion path, thus efficiency, when other parameters in question were kept constant, except annealing. We will also compare the coefficient changes between the first and the 1000th CV cycles, endeavoring to understand the stability of different annealed oxides in the chromic reaction, and to assess any improvement in the electrochromic behavior of these WOx thin films. These tests were run at a selected scan rate of 60 mV/s for each sample, and the resulting D values are presented in Table 1. The different DLi values between the first and the 1000th cycles for various samples could be used to appraise the stability, for both the coloration and bleaching stages. As summarized in Table 1, the 250, 300, and 350 °C annealed WO3 thin films exhibited slightly higher DLi+ values (1.3 × 10−10, 1.55 × 10−10, and 1.78 × 10−10 cm2/s, respectively) for the intercalation reaction and (3.69 × 10−11 cm2/s for 300 °C sample and 4.1 × 10−11 cm2/s for 350 °C sample) for deintercalation compared with the original as-purchased WO3 thin film sample (1.2 × 10−10 cm2/s for intercalation and 3.33 × 10−11 cm2/s deintercalation). However, the deintercalation of the 250 °C annealed WO3 sample showed a slightly lower DLi+ value (3 × 10−11 cm2/s deintercalation) than the as-purchased 20503

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temperature and the 250 °C, 300 °C, and 350 °C annealed W18O49 thin films, respectively. For the WO3 samples, the corresponding ΔOD values were much lower than the nanowire samples, being 0.2, 0.22, 0.2, and 0.25 for the unannealed and 250 °C, 300 °C, and 350 °C heat-treated samples, respectively. It was obvious that the W18O49 nanowire samples outperformed the WO3 nanoparticles by a significant margin. The 350 °C annealed WO3 sample slightly improved by 10%, whereas the 300 °C annealed W18O49 nanowire sample achieved a nearly 100% improvement and its coloration state almost absorbed all light. Obviously, this would demand more Li+ transfer as discussed earlier. Furthermore, due to the easy stacking feature of the nanoparticles against the randomly tangled nanowires in the preparation of thin films, given the similar film thicknesses in the device, the WO3 would be packed more densely than the nanowires, as evidenced by the lower bleaching transmittance shown in Figure 8, against that of W18O49 nanowires in Figure 9. Because of this, the ΔOD for WO3 is much smaller. As a result, this could also be the reason for less Li+ demand for the coloration, which helps to explain the fast response time as well.

hence a longer time would be required. We need to ascertain the intensity to verify this deduction. To validate the above analysis and validate the true annealing effects on these WOx materials, we have constructed a chromic device prototype using different samples, and carried out in situ transmission/absorption testing at room temperature. An assembly schematic and the complementary device are shown in Figure 7. A 300 nm thick WOx thin film and 0.6 nm of Pt

Figure 7. Schematic construction (left) and actual prototype (right) of a chromic device.

were coated on each side of the ITO using spin-coating and sputtering techniques, as the working and counter electrodes, respectively. A Gene frame was used as a spacer to store the electrolyte between the working and counter electrodes. A small potential between 1.5 and −3 V was applied to reach a colored state and a bleached state during the testing. The in situ transmittance measurements were carried out in the wavelength range 300−800 nm. The change in the optical density (ΔOD) is calculated from the measured spectral transmittance, according to the following equation:

4. CONCLUSION In summary, low temperature annealing under a rich Ar condition of both WO3 and W18O49 nanomaterials has been conducted in order to investigate the fundamental influence of structural features on their electrochromic properties. We have found that the annealing successfully increased the kinetics of Li+ ions and enhanced the electrochromic stability of in both cases. Combined XRD, Raman, SEM, and HR-TEM analyses on the two types of WOx, before and after annealing, as well as prior to the first and after the 1000th CV cycles, have confirmed the crystalline changes of the WOx. The reduced (010) plane spacing after annealing indicates an improved crystalline structure, which can be attributed to the variation of oxygen vacancy and relaxation/elimination of defects within the WOx. Li+ trapping has been indirectly confirmed by the enlarged (010) d values in samples after the 1000th CV cycle. Both the intercalation and deintercalation coefficient values DLi+ were increased by annealing, and the optical density

ΔOD(λ) = log(Tbleached(λ)/Tcolored(λ))

where Tbleached is the transmittance of the bleached state and Tcolored is the transmittance of the colored state. Figures 8 and 9 show the in situ UV−vis transmittance spectra of the WO3 and W18O49 devices during the coloration and bleaching process, measured between 1.5 V (bleached stage) and −3 V (colored stage). The changes in the optical density (ΔOD) of the W18O49 samples were estimated at 630 nm (because this wavelength is relatively sensitive to the human eye), resulting in 0.64, 0.53, 1.26, and 0.35 for room

Figure 8. Transmittance spectra of WO3 nanoparticles without heat treatment (A) and with heat treatment at different temperatures, 250 (B), 300 (C), and 350 °C (D), in their colored and bleached states on the in situ application of potential range of 1.5 and −3 V. 20504

DOI: 10.1021/acs.jpcc.7b06300 J. Phys. Chem. C 2017, 121, 20498−20506

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Figure 9. Transmittance spectra of W18O49 nanowires without heat treatment (A) and with heat treatment at 250 (B), 300 (C), and 350 °C (D), in their colored and bleached states by applying an in situ potential ranging between 1.5 and −3 V.

(ΔOD(λ), λ = 630 nm) was significantly improved by nearly 100% for the prototype made from the 300 °C annealed W18O49 nanowires. These results demonstrate that choosing an appropriate annealing strategy improves electrochromic performance for the WOx thin films. This study will help in understanding the relationship between crystalline structures and coloration/bleaching behavior of these nanowires, benefiting future chromic device development.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

C. David Wright: 0000-0003-4087-7467 Yanqiu Zhu: 0000-0003-3659-5643 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Qioptiq for some financial support. REFERENCES

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DOI: 10.1021/acs.jpcc.7b06300 J. Phys. Chem. C 2017, 121, 20498−20506