Disentangling Photochromism and ... - ACS Publications

Oct 13, 2016 - promising light and thermal management technology to reduce energy ... and air conditioning (HVAC) and lighting, which together account...
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Disentangling Photochromism and Electrochromism by Blocking Hole Transfer at the Electrolyte Interface Yang Wang,† Jongwook Kim,† Zhengning Gao,‡ Omid Zandi,† Sungyeon Heo,† Parag Banerjee,‡ and Delia J. Milliron*,† †

McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1589, United States Department of Mechanical Engineering and Materials Science, Washington University, St. Louis, Missouri 63130, United States

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S Supporting Information *

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photochromism, which can be undesirable in electrochromic devices. Photochromism is an optical change in response to illumination, and tungsten oxide is well-known to exhibit efficient darkening in response to ultraviolet (UV) radiation. As a semiconductor with a bandgap of 2.6 eV, WO3 can absorb wavelengths less than 475 nm in the solar spectrum, thus generating electron−hole pairs.15 Light absorption can be used favorably as the first step in solar energy conversion,16 or the excited electronic state can drive photochemistry at the tungsten oxide surface. Both crystalline and amorphous WO3 have been shown to exhibit a photochromic response. Although the exact mechanism may differ in specific cases, a protoncoupled electron transfer process has been deduced, especially in case organic molecules are present at the tungsten oxide surface.17,18 Many oxygen-containing organic molecules, including alcohols, ethers, esters, etc., can adsorb to the WO3 surface by donating the unshared electron pair on their oxygen atoms into the vacant d orbital of WO3, thus forming a W−O coordination bond. In the proposed photochromic process, these molecules become hydrogen atom donors, resulting in proton intercalation in WO3 and a reduced electronic state with a characteristic absorption spectrum similar to that which can be achieved by electrochemical reduction. Nonetheless, this photochromic process is detrimental to the long-term stable operation of electrochromic devices since it continuously perturbs the charge state of the device and degrades the material in contact with the WO3 and can even lead to irreversible coloration due to ion trapping in strongly reduced WO3.19 Polymer gel electrolytes commonly used in electrochromic devices incorporate various oxygen-containing molecules, such as dimethoxyethane, tetraethylene glycol dimethyl ether (tetraglyme), propylene carbonate, poly(ethylene oxide), etc.20−22 Under UV irradiation, photogenerated holes in the valence band of WO3 have sufficient energy to oxidize these molecules that may adsorb to the WO3 surface inducing neighboring CH and CO bonds to break, thereby produce protons and small molecular fragments, including smaller alcohols and ethers in the case of tetraglyme (Figure 1a).23 As a result, the electrons and protons can transfer to WO3, the

lectrochromic dynamic windows are emerging as a promising light and thermal management technology to reduce energy consumption in buildings by heating, ventilation, and air conditioning (HVAC) and lighting, which together account for approximately 25% of the total U.S. energy demand.1−3 Previous simulation studies have shown that smart windows can reduce a building’s peak energy needs by up to 40% relative to static windows.4 Several types of electrochromic devices exist for window applications, among which a thin-film battery-like configuration has attracted a great deal of interest.5 This configuration consists of a thin layer of electrochromic material (e.g., WO3 or conjugated polymers) coated on a transparent conductive oxide (TCO) working electrode and a charge storage layer (e.g., Prussian blue, NiO, CeO2) coated onto a TCO counter electrode. These two electrodes are joined by a layer of ion-conducting electrolyte (such as a polymer gel). In typical operation, the device darkens when electrons and ions are inserted into the working electrode, and it bleaches when the charges are extracted to be stored in the counter electrode. Tungsten oxide (WO3) is certainly the most studied and commercially relevant electrochromic material in the batterytype device configuration,6 because of (1) its strong polaronic and plasmonic absorption of visible and near-infrared (NIR) light when reduced,7 (2) high coloration efficiency,2 and (3) good electrochemical reversibility and fast switching kinetics.8 In crystalline WO3, edge- and corner-sharing WO6 octahedra form periodic structures with open tunnels of interstitial sites, which facilitate ion intercalation and diffusion. When WO3 is cathodically charged, injected electrons are compensated by intercalated cations (H+ or Li+), which reduce tungsten cations from W6+ to W5+ or W4+ and concurrently change the material from a clear, transparent state to dark blue. Bleaching, the opposite process, occurs upon electrochemical oxidation. Nanostructuring of tungsten oxide has been pursued to maximize switching kinetics,8−11 including the recent report by Kim et al.,8 that a nanostructured mesoporous electrode, constructed from tungsten oxide nanocrystals via self-assembly, showed deep, fast, and stable electrochromic switching of both visible and NIR light. Furthermore, oxygen vacancies can be introduced deliberately in WO3−x to enhance the electronic conductivity and coloration efficiency,8,12 which is the change in optical density per unit charge. However, higher oxygen vacancy concentrations and the large specific surface area of nanostructured tungsten oxide are also associated with an enhancement of its photocatalytic response,13,14 including © XXXX American Chemical Society

Received: September 7, 2016 Revised: October 7, 2016

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DOI: 10.1021/acs.chemmater.6b03793 Chem. Mater. XXXX, XXX, XXX−XXX

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electrolyte, so it would not impede the electrochemical kinetics of WO3−x during electrochromic switching. For example, Frenning et al. reported the use of amorphous Ta2O5 as the Li+ conducting electrolyte layer in an amorphous WO3−x based electrochromic device, and they found that the Li+ diffusion coefficient in the Ta2O5 layer (3 × 10−11 cm2/s) was about 3 times higher than that in the WO3−x film.28,29 We note that an analogous approach was previously utilized by Lewis and coworkers using ALD of defective TiO2 conformal layers to protect Si, GaAs, and GaP photoelectrodes in contact with KOH during photoelectrochemical water oxidation.30 By using ALD, a thin conformal layer of Ta2O5 with variable thickness (0−2.5 nm) was deposited on mesoporous WO3−x nanocrystal films (see Methods and Figure S1). Ta2O5 ALD was carried out using pentakis(dimethylamino)tantalum(V) (PDMAT) as the Ta precursor and H2O as the oxidant at 250 °C using a modified version of previously reported methods.31−33 Briefly, one ALD cycle consisted of a 10 s PDMAT exposure followed by purging and subsequent exposure to a 0.5 s H2O pulse. A planar growth rate of 1.05 Å/cycle on a silicon substrate was measured by spectroscopic ellipsometry. Variable thicknesses of Ta2O5 were deposited on WO3−x films simply by adjusting the number of ALD cycles. Scanning electron microscopy (Figure 2a,b) was used to compare the film morphology for a mesoporous WO3−x nanocrystal film before and after the deposition of a 2.5 nm Ta 2 O 5 layer. This comparison shows that the WO 3−x nanocrystallite domains were uniformly coated with a conformal layer of Ta2O5 (Figure 2b). Mapping the elemental distributions with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy (Figure 2c) confirms the conformal coating of the amorphous Ta2O5 layer on the nanocrystalline WO3−x. EDX spectra were also collected on a film cross section (Figure 2d), demonstrating that the Ta2O5 penetrates the full film thickness resulting in a uniform distribution of both tungsten and tantalum (Figure 2e). Such a uniform conformal coating, despite the small pore size and high surface area, is indeed a distinctive advantage of ALD. The average pore size before and after ALD was analyzed by ellipsometric porosimetry (Figure S2). An initial pore radius of 6.4 nm is reduced to ∼1−2 nm after depositing a nominal 2 nm film of Ta2O5. On the basis of the results of Kim et al.,8 this

Figure 1. Schematic representation of WO3 photochromism under UV irradiation. (a) The proton-coupled electron transfer exchange reaction at the WO3−electrolyte interface when tetraglyme is the surface adsorbate. (b) The approach adopted in this work to block the hole transfer process by using a conformal Ta2O5 hole-blocking layer. The band edges are indicated, referenced to vacuum.15,27

electrons filling the valence band holes and stabilizing the photogenerated electrons in the conduction band, thus forming reduced and colored HxWO3. This type of photochromism has also been observed for many other transitional metal oxides, including TiO2, ZnO, MoO3, etc.24−26 As electrochromic windows are typically under strong solar irradiation and must meet 30-year lifetime requirements,2 it is important for electrochromic windows to avoid this photochromic side effect that can decompose the electrolyte components and also cause uncontrolled charging and coloration. This work reports a novel, facile, and effective method to mitigate the photochromic side effect of WO3 by using atomic layer deposition (ALD) to coat a conformal thin Ta2O5 layer onto an oxygen-vacancy doped tungsten oxide (WO3−x) nanocrystal film. With a wide bandgap of 3.9 eV (Figure 1b), Ta2O5 is not itself susceptible to photochromism. It also presents an exceptionally deep valence band edge, leading to a large hole transfer barrier (0.7 eV)27 that blocks holes at the WO3−x valence band edge from transferring to surface bound adsorbates. Such valence band offset in addition to excellent lithium-ion (Li+) conductivity of Ta2O5 was the basis of our hypothesis to prevent photochromism in WO3−x. Ta2O5 is an effective ion conductor that has even been used as a solid state

Figure 2. Changes in morphology and elemental composition for a mesoporous WO3−x nanocrystal film after Ta2O5 deposition. (a, b) SEM images for a bare (a) and coated (b) WO3−x nanocrystal film with a 2 nm Ta2O5 layer. (c) High-resolution TEM image and EDX mapping for a WO3−x nanocrystal film with a 2 nm Ta2O5 layer, in which green and red colors represent tungsten and tantalum, respectively. (d, e) A cross-section SEM image and EDX scan for the Ta2O5 coated WO3−x film, in which the orange line indicates where the EDX line scan (e) takes place. B

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Figure 3. Hole-blocking properties of Ta2O5. (a) Transmittance spectra and corresponding photograph for the WO3−x films with different Ta2O5 coating thicknesses after 3 h of UV irradiation. The dark blue spectrum and bottom photograph corresponds to the bare WO3−x film, the orange spectrum and middle photograph is for a similar WO3−x film with a 2 nm Ta2O5 coating, and the gray spectrum and top photograph corresponds to a control WO3−x film that was not exposed to UV light. The spectrum for WO3−x with 1 nm Ta2O5 is also shown (light blue). (b) Chronoamperometry photocurrent measurement for the WO3−x films with 0 nm (dark blue), 1 nm (light blue), and 2 nm (orange) Ta2O5 coatings. The light on and off was controlled by a mechanical shutter with an interval of 10 s.

current can be expected as it is energetically unfavorable for the holes to escape from the valence band of WO3−x. Under an applied voltage of 0.7 V (vs Ag/AgCl) and simulated AM 1.5 sunlight, Figure 3b shows that photocurrent density decreases dramatically with the presence of the Ta2O5 coating. Compared to that for the bare WO3−x film (dark blue line), the photocurrent density for the WO3−x film coated with a 2 nm Ta2O5 layer is 10 times lower (orange line), which supports the fact that the suppression of photochromism can be ascribed to effective blocking of hole transfer by the ultrathin conformal Ta2O5 coating. To investigate if the presence of a Ta2O5 coating hinders the Li+ transport kinetics at the electrode−electrolyte interface, in situ spectroelectrochemical (SEC) and electrochemical measurements were performed to evaluate the effect of Ta2O5 layers on the electrochromic switching of nanocrystalline WO3−x electrodes. The SEC experiments were conducted in a cuvette with Li metal foil as the counter and reference electrodes, and with 1.0 M LiTFSI in tetraglyme as the supporting electrolyte (see Methods). Figure 4a shows the SEC performance of the WO3−x film with a 2 nm Ta2O5 coating under different applied voltages (vs Li/Li+). This film exhibited strong, fast, and selective optical contrast in both the visible and the NIR range, which is comparable with the data obtained for bare WO3−x films (Figure S4). It shows dual-band electrochromic behavior with three different optical modes, namely, a bright mode (at 4.0 V) where the film is transparent to both visible and NIR light, a cool state (at 2.3 V) where the film blocks the NIR light but still transmits most of the visible light, and a dark mode (at 1.5 V) where both visible and NIR light are significantly blocked.8,34 In fact, the Ta2O5, which is itself weakly electrochromic at 1.5 V (vs Li/Li+), contributes to the visible coloration in the dark mode (solid vs dashed dark blue lines in Figure 4a), thus leading to a more neutral color than the typical pronounced blue of WO3−x alone (Figure S4). Note that this film was previously irradiated by UV light for 3 h (Figure 2a), and the small amount of photochromic darkening can be completely bleached at 4.0 V for 2 min (Figure 4a, gray line). Thus, this Ta2O5 coated WO3−x film displayed great photochemical stability with no irreversible coloration or compromise of electrochromic switching properties.

pore size is sufficient for penetration of electrolyte to facilitate rapid electrochromic switching. To test the efficacy of the Ta2O5 coating in preventing photochromism, WO3−x nanocrystal films with different Ta2O5 coating thicknesses (0, 1, and 2 nm) were exposed to UV irradiation, and their transmittance spectra were recorded before and after UV exposure. A drop of liquid electrolyte (0.1 M LiTFSI in tetraglyme) was cast onto the films to recapitulate the electrode−electrolyte interface in an electrochromic device, and the film was directly exposed to UV irradiance equivalent to 1 sun (see Methods). After 3 h of UV irradiation, Figure 3a shows that the bare WO3−x nanocrystal film was visibly darkened (bottom photograph) due to the photochromic effect, and its transmittance was significantly reduced (dark blue line), especially in the NIR region, compared to the control sample that was not UV-irradiated (gray line and top photograph). However, the photochromic effect weakens as the Ta2O5 coating thickness increases; for instance, with a 2 nm Ta2O5 coating, 3 h of UV exposure did not significantly darken the WO3−x film or lower its visible transmittance though the NIR transmittance was reduced somewhat (orange line and the middle photograph), compared to the control. This result is clear evidence that a 2 nm conformal layer of Ta2O5 is sufficient to dramatically suppress the photoinduced hole transfer process at the WO3−x electrode−electrolyte interface, thus attenuating the photochromic side effect. To ascertain whether the reduced photochromic effect is in fact due to hole blocking, we measured electron transfer directly via photocurrent measurements, which were conducted in a standard 3-electrode photoelectrochemical cell (see Methods). WO 3−x films with different Ta 2 O 5 coating thicknesses were used as the working electrode (photoelectrode), and a Pt foil and Ag/AgCl wire were used as the counter and reference electrodes, respectively. The cell was filled with an aqueous electrolyte of 50 mM KI as hole scavenger with 0.1 M H2SO4. Without Ta2O5, the photogenerated holes in WO3−x quickly transfer to the electrolyte, oxidizing I− to I3− (−4.8 eV vs vacuum), while the photogenerated electrons move to the Pt counter electrode and reduce I3− and H+ (Figure S3). As Ta2O5 presents an energetic barrier for hole transfer (0.7 eV), a lower photoC

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photochemical stability. This method is also applicable to other electrochromic applications based on WO3 or other transition metal oxides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03793. Experimental and characterization details, XPS, ellipsometric porosimetry, SEM, and optical data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(D.J.M.) E-mail: [email protected]. Author Contributions

Figure 4. Electrochemical performance of Ta2O5 coated WO3−x films. (a) Transmittance spectra of a WO3−x film with a 2 nm Ta2O5 coating on ITO-coated glass at different switching potentials (vs Li/Li+ in 1.0 M LiTFSI/tetraglyme) with a corresponding photograph of this film in the dark state (at 1.5 V). The dark-state transmittance spectrum for a similar WO3−x film without Ta2O5 is shown for comparison (dashed line). (b) Normalized charging and discharging profiles following potentiostatic steps for three WO3−x films with different Ta2O5 coating thicknesses (as labeled). (c) Charge and discharge profile for the same film cycled at 1.5 and 4.0 V for 200 cycles, with each voltage step lasting for 2.5 min.

Y.W. and J.K. contributed equally to this work. Notes

The authors declare the following competing financial interest(s): D.J.M. has a financial interest in Heliotrope Technologies, a company pursuing commercial development of electrochromic devices.

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ACKNOWLEDGMENTS This work was performed at the University of Texas at Austin and Washington University in St. Louis. J.K., Y.W., O.Z., and D.J.M. acknowledge the funding from the U.S. Department of Energy ARPA-E and the Welch Foundation (F-1848). Z.G. and P.B. acknowledge support from U.S. Army RDECOM, acquisition Grant W911NF-15-1-0178, under Subgrant RSC15032. ALD precursors were kindly obtained from Dr. Ravindra Kanjolia at EMD Performance Materials.

A control study was performed with a similar WO3−x nanocrystal film with a 2 nm ZnO layer, which was also prepared by ALD (see Methods). ZnO, with a bandgap of 3.2 eV, can also provide a valence band offset (0.2 eV) to block the hole transfer from WO3−x to the electrolyte. However, the lithium ion conductivity of ZnO is much lower than that of Ta2O5, and thus it may negatively affect the electrochromic switching of WO3−x. The SEC result for this film (Figure S5) shows that the ZnO coating significantly reduced the electrochromic optical contrast of WO3−x, compared to the counterpart coated with Ta2O5. This difference clearly reveals the advantages of choosing Ta2O5 as the hole blocking layer, as it is strongly hole blocking while being ion conductive. Besides high optical contrast under electrochemical switching, the Ta2O5 coated WO3−x film exhibits rapid switching between different optical modes. Figure 4b compares the charging and discharging kinetics for the WO3−x films with different Ta2O5 coating thicknesses; the presence of a Ta2O5 layer only slightly slows the electrochemical kinetics. Thus, a trade-off between effective hole blocking and fast kinetics is present but favorable. Systematic studies were performed over a range of Ta2O5 layer thickness (0−2.5 nm) to find a good balance for this trade-off, and the best overall performance was achieved with a 2 nm Ta2O5 coating. Finally, electrochemical cycling tests (Figures 4c and S6) conducted at 1.5 V (charging) and 4.0 V (discharging) for the WO3−x film with a 2 nm Ta2O5 coating showed no charge capacity or optical contrast fade after 200 charge/discharge cycles, thus indicating excellent electrochemical durability. In conclusion, the technique developed here is a straightforward and effective strategy to reduce the photochromic side effect in WO3-based electrochromic devices. As environmental durability is a major challenge for electrochromic dynamic windows to achieve broad market viability, this approach represents a meaningful new avenue for improving their

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REFERENCES

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