Exploring Electrochromics: A Series of Eye-Catching Experiments To

Article pubs.acs.org/jchemeduc

Exploring Electrochromics: A Series of Eye-Catching Experiments To Introduce Students to Multidisciplinary Research Leo J. Small, Steven Wolf, and Erik D. Spoerke* Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: Introducing students to a multidisciplinary research laboratory presents challenges in terms of learning specific technical skills and concepts but also with respect to integrating different technical elements to form a coherent picture of the research. Here we present a multidisciplinary series of experiments we have developed in the Electronic, Optical, and Nano Materials group at Sandia National Laboratories to introduce students to essential experimental methods and concepts spanning thin film synthesis, crystallography, electrochemistry, and optical spectroscopy. With minimal assistance from a qualified instructor, students apply a sol− gel method to synthesize electrochromic tungsten oxide (WO3) thin films and evaluate their performance with electrochemistry, UV−vis spectroscopy, and X-ray diffraction. We find that the color changing WO3 films capture the students’ attention, are technologically relevant, and make excellent materials platforms for multidisciplinary research as they invite investigation with a range of laboratory techniques. The variety of experimental methods combined here challenges the students to correlate the interplay between structure, processing, and properties central to materials science. The modular nature of this experiment set permits it to be tailored to the time constraints of individual students and also allows it to be applied to upper-level materials science or chemistry laboratories. KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Analytical Chemistry, Physical Chemistry, Inorganic Chemistry, Inquiry-Based/Discovery Learning, Crystals/Crystallography, Electrochemistry, Materials Science, UV-Vis Spectroscopy

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INTRODUCTION The experience of introducing students to a coherent, multidisciplinary research study provides a valuable opportunity for students not only to learn a number of new experimental techniques and explore new scientific instrumentation, but it also illustrates how these different research components come together to create a comprehensive research project. We have developed a series of experiments to introduce our students to methods integral to our laboratory: sol−gel thin film synthesis, electrochemistry, UV−vis spectroscopy, and X-ray diffraction. We specifically focus on studies of electrochromic tungsten oxide (WO3) thin films whose color changing properties stimulate students’ interest, while their real-world applications as smart windows, electrochromic devices, image storage, or gas sensors1−3 help the students recognize the relevance of the work. Meanwhile, the variety of experimental methods used to study these materials challenges the students to correlate results and understand the molecular processes at play. These experiments are best suited for upper-level students who are taking core materials science classes covering crystal structure, thermodynamics, kinetics, and electronic properties. We strive to use everyday laboratory research as a template for teaching students, an approach recognized as invaluable in academic environments.4−9 Here we demonstrate the successful exten© XXXX American Chemical Society and Division of Chemical Education, Inc.

sion of this process to a national laboratory environment, where traditional classrooms or teaching labs are replaced with handson experiences in a professional laboratory setting. In this paper, we present essential background information on electrochromics, followed by several experimental modules and data collected by student interns in our laboratory. As presented here, there are 5 experimental modules that emphasize WO3 thin film synthesis: thin film characterization, electrochemical testing in 1 M HCl, variation of electrolyte, and electrochemical testing of 300 and 500 °C-annealed WO3 in 1 M HCl. In Supporting Information, we have described how each module may be explored in approximately 4 h lab sessions using the tools and concepts described here. The experiments may be conducted either by individual students in a mentor/ intern environment or in groups of roughly 2−4. Each module is designed to develop a specific experimental skillset, further revealing how the processing of the materials system influences the observed structure and properties. In the first module, students use a standard procedure to synthesize the WO3 films onto optically transparent, electrically conductive substrates. In the second module, synthesized samples are characterized using processes such as UV−vis spectroscopy and X-ray diffraction.

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For x < 0.3, this process is reversible, though if x is considerably greater than 0.3, the film may become irreversibly converted to MxWO3.14 This color changing behavior can be explored by integrating an electrochromic WO3 thin film as the working electrode in an electrochemical cell. As shown in Figure 2, a WO3 thin film,

After reviewing the fundamentals of electrochemistry, these synthesized, characterized thin films are then probed electrochemically and spectroscopically in modules 3−5, exploring first the basic electrochromic behavior in 1 M HCl before examining how variations in electrolyte composition and thin film annealing temperatures influence the electrochromic behavior of the thin film. Examining these collective data, students are challenged to understand how different intercalating ions and film crystallinity impact electrochromic film performance. Depending on how these data are collected, we show that sufficiently large data sets can be generated such that basic computer science skills may be engaged/required for data analysis. By performing this analysis on a well-known system such as tungsten trioxide, rather than a new area of research, reasonably predictable outcomes are typically produced, and students can learn to compare their results with published literature. After completing these modules, our students10 are proficient in basic sol−gel thin film synthesis, electrochemistry, UV−vis spectroscopy, X-ray diffraction and analysis of large data sets. More importantly, the students understand the complementary knowledge gained by each additional technique and how this contributes to an overall understanding of the materials system.

Figure 2. Schematic of the electrochemical cell and the processes occurring during electrochromic operation of MxWO3. The WO3 thin film is shown grown on a conductive fluorine-doped tin oxide (FTO) electrode.

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BACKGROUND Electrochromism is a property attributed to materials which reversibly change color upon injection of charge. Among the best known electrochromic ceramic materials is tungsten trioxide, WO3, the crystal structure of which is shown in Figure 1.11,12 WO3 typically adopts a monoclinic crystal

grown on a transparent conductor such as fluorine-doped tin oxide (FTO), is immersed as the working electrode in 1 M aqueous HCl, LiCl, or NaCl opposite an inert counter electrode and a reference electrode. In this configuration, a voltage is applied across the cell producing a color change as a result of four atomistic processes at play.17 1. A cation M+, from the aqueous electrolyte crosses the electrolyte−WO3 interface. 2. M+ diffuses through the WO3 film, intercalating into the WO3 lattice. Simultaneously, 3. An electron from the bottom electrode (FTO) crosses the electrode−WO3 interface. 4. The electron moves through the WO3 film, reducing W6+ to W5+. Throughout the following modules, students gain insight into how processes 1−4 are affected by the choice of WO3 thin film processing conditions and intercalating ion. While current electrochromics research almost exclusively uses nonaqueous electrolytes under dry atmospheres, we utilize aqueous 1 M HCl, LiCl, and NaCl to simplify the experimental procedures while still providing the fundamental materials response.

Figure 1. Atomic structure of (left) WO3 and (right) MWO3, demonstrating how the M+ ion (yellow) intercalates into the WO3 lattice. Blue, red, and yellow spheres represent tungsten, oxygen, and sodium, respectively.

structure, readily visualized as the slightly distorted cubic structure shown in Figure 1 (left). The electrochromic behavior of the material is activated by the injection of electrons into the material to reduce W6+ to W5+, a process charge-balanced by the simultaneous intercalation of monovalent cations (e.g., H+, Li+, Na+) into the crystal lattice (Figure 1, right). These processes result in the formation of the tungsten bronze structure seen in Figure 1 (right) and produce a significant change in the optical absorption of the material, changing from clear to dark blue.13−15 More advanced students may be challenged to provide an alternative view of this crystal structure as a distorted cubic perovskite structure (MxWO3), where W6+ resides at the body center of the cube, and O2− sits on the center of the cube faces. Intercalation of monovalent cations M+, into the cube corners of WO3 balances the charge during the electroreduction of W6+ to W5+. In either case, the overall reaction may be written as follows:16 WO3 + x M+ + x e− ↔ MxWO3

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SYNTHESIS OF WO3 THIN FILMS A sol−gel method is used to spin-coat WO3 thin films onto commercially available glass slides coated with FTO (Pilkington, Perrysburg, OH). To perform all of the experiments described here (plus 1 extra), students should prepare 1 in. × 1 in. slides. Synthesis of the spin coating solution is straightforward but requires use of concentrated hydrogen peroxide (a strong oxidizer) and glacial acetic acid (combustible, corrosive), and should be prepared in advance by an experienced laboratory member.18 Briefly, 3.0 g of tungsten powder (>99.9%, Aldrich) is added to a solution 70 mL of H2O2 (60%, Fisher Scientific) and 10 mL deionized water prechilled to 0 °C. This reaction is exothermic, with much gas

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ELECTROCHEMICAL TESTING Here the basics of electrochemistry and cyclic voltammetry are taught to students. More detailed information can be found in Bard and Faulkner’s Electrochemical Methods.22 The electrochemical cell is constructed with the WO3/FTO film as the working electrode, a platinum wire counter electrode, and Ag/ AgCl reference electrode (BASi, West Lafayette, IN), as depicted in Figure 2. The region of FTO previously masked off is connected to the working electrode of the potentiostat via alligator clip. Cyclic voltammograms (CVs) are run between −900 and +1000 mV vs Ag/AgCl at 25 mV/s, repeating the cycles 10 times for each sample. Care must be taken not to expose the alligator clip to solution, else electrochemistry will also be performed on the alligator clip. To avoid contacting the alligator clip with solution, only the lower portion of the film is exposed to solution. This lower area is measured, and the current in the CV is scaled by this area. For the most reproducible results when comparing different film conditions or electrolyte conditions throughout this experimental program, the last (10th) CV should be used for sample comparison. During initial CVs, the films are “brokenin.” WO3 has a high electronic resistance, but HWO3 is quite conductive. Once a small amount of H+ (or Li+, Na+) intercalates into the lattice, the electronic resistance of the film becomes quite small, and the data become more consistent. Using the last (10th) CV minimizes this “break in” effect. Students are then asked to identify the electrochemical processes occurring in each part of the CV, depicted in Figure 3, and calculate the charge passed in the reduction and

being released; the reaction should not be performed in a sealed vessel and proper venting is required (chemical fume hood is strongly recommended). The solution is stirred for 2 h, then centrifuged at 3000 rpm for 15 min. The resulting clear supernatant is added to 70 mL of glacial acetic acid (99.9%, Fisher Scientific) and refluxed overnight at 70 °C. After refluxing, the solvents are removed by rotary evaporation, yielding a bright yellow powder of acetylated peroxotungstic acid. The dried powder is stored under air in an airtight, opaque container. Powders are stable in excess of four months, and compromised powders are easily identified by their white color and insolubility in ethanol. Students are given the acetylated peroxotungstic acid powder with which they make a solution of 1.25 g in 10 mL of ethanol (200 proof) to use as the spin-coating solution. Students may note that the powder does not dissolve immediately; complete dissolution may take up to an hour at room temperature. During this time, FTO-coated glass substrates (TEC 250, Pilkington) are rinsed in deionized water, sonicated in 2propanol for 20 min, and cleaned by UV-ozone for 30 min.19 Alternative solvent-based substrate cleaning procedures may be employed, with the emphasis on removal of oils, surfactants, or other contaminant residue from the manufacturer. (For example, samples may be sonicated for 15 min each in sequential methylene chloride, acetone, and deionized water before air-drying.) Once cleaned, students should not touch the cleaned surface (even with gloved hands), as resulting contamination will impact WO3 thin film quality. A corner of the FTO film is masked off with scotch tape (this untreated area will serve as an electrical contact during subsequent electrochemical testing). The conductive, FTO side of the substrate is then coated with 250−300 μL of the spin-coating solution, dispensed through a 0.2 μm syringe filter, and spun at 2000 rpm for 60 s. At this point any spin-coating solution that has reached the glass back of the substrate is removed with a lint-free wipe dampened with ethanol. The substrate is dried at 100 °C for 5 min on a hot plate, and cooled to room temperature. (If necessary, hot plate temperatures can be determined using a K-type thermocouple, mechanically adhered to the hot plate and then removed prior to film annealing.) Alternatively, substrates may be dip-coated20 if spincoating equipment is not readily accessible, though inexpensive spin coaters may be fabricated using PC cooling fans.21 If dipcoating is to be considered, the same precursor solution described above may be used, but the students will need to explore details such as precursor concentration, dip rate, dip number, and solvent evaporation time prior to drying the films on the hot plate. In addition, deposited films will have to be removed from the back of the dip-coated sample (prior to drying) to avoid confounding UV−vis measurements of the deposited films. Due to the potential complexity of dip-coating, spin-coating is recommended as the preferred thin film deposition technique. This process of spin-coating, cleaning, drying, and cooling is repeated for a total of four layers. Subsequently, substrates are left in open air on the hot plate at 100 °C for 3 h. We have explored variations of this ultimate annealing temperature up to 500 °C, but unless otherwise noted, the data shown below are for samples annealed at 100 °C. (The trends presented are consistent at other temperatures tested.) The deposited films should be of uniform, nearly clear color. Streaks are undesirable and may be the result of insufficient spinning solution or dust particulates.

Figure 3. Voltammogram of a WO3 thin film in 1 M HCl; 25 mV/s scan rate.

oxidation reactions (as related to the area of the oxidation and reduction peaks). Students may also be encouraged to use this information to determine the approximate composition of their films, using eq 1 to guide their calculations. On the basis of the conditions described here, for a film approximately 500 nm thick, roughly 10% of the film is reduced.

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UV−VIS SPECTROSCOPY Perhaps the most exciting part of this experiment is the rapid color change of the WO3 film from clear to dark blue. This visible change can be quantified using UV−vis spectroscopy to C

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VARIATION OF ELECTROLYTE Here CVs are repeated with fresh WO3 thin film samples, using 1 M LiCl or 1 M NaCl instead of HCl. Students are asked to overlay the CVs onto the same plot, scaling the current on each sample by the area exposed to solution. An example plot is provided in Figure 6. It is clearly seen that, to obtain the same

measure the percent change in the optical transmission of the WO3 thin film as a function of wavelength. Any basic optical spectrophotometer, such as clever homemade optical spectrometers,23 will suffice for this measurement; we use an Ocean Optics USB-2000 (Ocean Optics, Dundin, FL). Reference spectra of clean FTO-glass are recorded. Students then measure two films: one which has been cycled and stopped at +1000 mV (clear), and one which has been stopped at −900 mV (blue). The plots are overlaid and the wavelength of maximum change in optical transmission is identified, as shown in Figure 4.

Figure 6. Voltammograms of WO3 thin films in 1 M HCl, LiCl, or NaCl; 25 mV/s scan rate.

current density, the least cathodic potential is required for HCl, followed by LiCl and then NaCl. Students are asked to explain the reason for this trend. The simplest explanation for this phenomenon is that the smallest cation (H+) requires the least lattice strain, making intercalation energetically less unfavorable than for Li+ or Na+. Further, smaller cations diffuse faster, due to mass and strain considerations. More detailed theories are provided in the literature.24−26

Figure 4. Optical transmission of visible light through a WO3 thin film clear and darkened. Inset: image of clear and darkened WO3 thin films.

For more advanced students, we have written a custom LabView program to tailor real-time data collection of electrochemical and optical data. Using this approach, the text files generated are too large for typical spreadsheet programs, as voltage, current, and optical intensity at all wavelengths are recorded once per second. Students are challenged to dissect, analyze, and present the data in a meaningful way using Matlab (Mathworks, Natick, MA), or other similar data processing software. From these data, the relations between optical transmittance, wavelength, and applied voltage are evaluated, as shown in Figure 5.

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VARIATION OF FILM CRYSTALLINITY Correlating the electrochromic behavior of these materials to their crystallinity provides students with a deeper understanding of the materials chemistry behind their observations. Ideally, crytallographic data will be obtained by X-ray diffraction (XRD) on the students’ films directly, but in cases where XRD capabilities are not accessible, the data provided below or found elsewhere in the literature may be used to help guide the correlation between crystal structure and electrochromic behavior. In the present experiments, the degree of crystallinity was varied using three annealing temperatures: 100, 300, or 500 °C. The crystallinity of films annealed at these temperatures is determined by a qualified instructor using an X-ray diffractometer. Prior to these studies, basic principles of X-ray diffraction (XRD) are reviewed27,28 and a clean FTO-coated glass substrate is also analyzed as a reference. Students are asked to plot the data, identify FTO and WO3 peaks, and then index the WO3 peaks. If available XRD analysis software cannot readily identify these diffraction patterns, a brief literature search may be required for complete pattern indexing. For the FTO background sample, students are asked to explain why the recorded FTO peaks are shifted from that of the SnO2 powder pattern (possibly provided by the XRD analysis program). The most commonly observed room temperature phase of WO3 is monoclinic. This, combined with a possible crystal texture, makes phase identification more complicated than for

Figure 5. Relative optical transmittance of a WO3 thin film as a function of wavelength of light and applied voltage vs Ag/AgCl. D

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advanced experiments could further explore the relationship between ion transport and crystallography by examining the variation in the CV response as a function of scan rate. Differences in ionic resistance in the material, for example, will influence how rapidly the current in the film responds to changing voltage during CV experiments.22 Although these materials are robust and the qualitative electrochromic response is remarkably forgiving to “poor” film quality, instructors should be aware that poor quality (e.g., porous) films may result in variations in the electrical resistance of the films which may complicate the quantitative interpretation of the students’ results. Film defects such as cloudiness, visible pinholes in the film, or “speed boat” line defects from dust particles can be indicative of potentially poor film quality.

standard cubic structures often reviewed in undergraduate classes. Example data from our films is provided in Figure 7,

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CONCLUSIONS Above we have outlined a collection of multidisciplinary experimental approaches and technical themes that can be used to introduce students to the complex character of modern materials chemistry laboratories. We have developed the series of modules presented herein to provide students with a solid experimental footing in sol−gel thin film synthesis, electrochemical testing, UV−vis spectroscopy, analysis of large data sets, and X-ray diffraction. Students learn to appreciate how the complementary knowledge gained by each technique adds to the overall understanding of the materials system. For example, increasing the annealing temperature increases film crystallinity, which in turn alters the intercalation of ions into the film. The color changing WO3 films excel at capturing the students’ attention and promoting one of the greatest motivators in science: curiosity.

Figure 7. X-ray diffraction θ − 2θ scans (Cu Kα radiation) of WO3 thin films on FTO-glass directly after annealing at 100, 300, or 500 °C.

recorded using Cu Kα radiation. Here no appreciable crystallinity is observed in the films annealed at 100 or 300 °C. At 500 °C, however, monoclinic WO3 is observed with preferential orientation of the (200) planes parallel to the substrate surface.29,30 These films may also be electrochemically interrogated. CVs from WO3 films annealed at 300 or 500 °C are plotted in Figure 8. With the combined knowledge from XRD with

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ASSOCIATED CONTENT

S Supporting Information *

A description of specific equipment and reagents recommended for this series of experiments as well as a timeline for the varied experimental modules of the program; a series of suggested thought-provoking questions relevant to the technical concepts and experiments described here. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Erik D. Spoerke: Electronic, Optical, and Nano Materials, Sandia National Laboratories, P.O. Box 5800 MS 1411, Albuquerque, NM 87185-1411. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 8. Voltammograms in 1 M HCl of WO3 thin films annealed at 300 or 500 °C; 25 mV/s scan rate.

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ACKNOWLEDGMENTS

The authors gratefully acknowledge key laboratory support from Jill Wheeler. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC0494AL85000.

previous results, students are asked to explain the electrochemical results. In the more crystalline case (500 °C), a greater potential is required for oxidation of the HWO3, the process of removing H+ from the lattice. Just as in Figure 6, these data can be used to infer details about mobility of the ions in these thin film materials. In the present case, the data suggest that it is more difficult to remove a proton (H+) from the more ordered, crystalline material synthesized at 500 °C than from the relatively disordered material formed at 300 °C. More E

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(26) Sequeira, C.; Rodrigues, L.; Santos, D. Cation Diffusivity in Nonstoichiometric Tungsten Trioxide Films. J. Electrochem. Soc. 2012, 1, R126−139. (27) Cullity, E. Elements of X-ray Diffraction; 2nd ed.; AddisonWesley: Reading, MA, 1978. (28) Lifshin, E. X-ray Characterization of Materials; Wiley: New York, 2008. (29) Andersson, G. On the Crystal Structure of Tungsten Trioxide. Acta Chem. Scand. 1953, 7, 154−158. (30) Kobayashi, T. Antiparallel Dipole Arrangement in Tungsten Trioxide. Phys. Rev. 1953, 91, 1565.

REFERENCES

(1) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. Photoelectrochromic Windows and Displays. Nature 1996, 383, 608. (2) Deb, S. Electrochromic Properties of Ni Oxide Thin Films in Diluted Acidic Electrolytes and Their Stability. Sol. Energy Mater. Sol. Cells 2008, 92, 160−163. (3) Ito, K.; Ohgami, T. Hydrogen detection based on coloration of anodic tungsten oxide film. Appl. Phys. Lett. 1992, 60, 938. (4) Furtak, E. The Problem with Answers: An Exploration of Guided Scientific Inquiry Teaching. Sci. Educ. 2006, 90, 453−467. (5) Hurd, P. Scientific Literacy: New Minds for a Changing World. Sci. Educ. 1997, 82, 407−416. (6) Hurd, P. Science Education for the 21st Century. Sch. Sci. Math. 2000, 100, 282−288. (7) Knutson, C.; Jackson, M., Jr.; Beekman, M.; Carnes, M.; Johnson, D.; Johnson, D.; Keszler, D. Mentoring Graduate Students in Research and Teaching by Utilizing Research as a Template. J. Chem. Educ. 2014, 92 (2), 200−205. (8) Kurdziel, J.; Turner, J.; Luft, J.; Roehrig, G. Graduate Teaching Assistants and Inquiry-Based Instruction: Implications for Graduate Teaching Assistant Training. J. Chem. Educ. 2003, 80 (10), 1206. (9) Walker, J.; Sampson, V. Argument-Driven Inquiry: Using the Laboratory to Improve Undergraduates’ Science Writing Skills through Meaningful Science Writing, Peer-Review, and Revision. J. Chem. Educ. 2013, 90, 1269−1274. (10) Students have typically comprised round student interns from local universities or visiting student interns from across the U.S., though these modules are amenable to more traditional lab-based teaching. (11) Haegg, G. The Spinels and the Cubic Sodium-Tungsten Bronzes as New Examples of Structures with Vacant Lattice Points. Nature 1935, 135, 874. (12) Tanisaki, S. Crystal Structure of Monoclinic Tungsten Oxide at Room Temperature. J. Phys. Soc. Jpn. 1960, 15, 566−573. (13) Bange, K. Colouration of Tungsten Oxide Films: A Model for Optically Active Coatings. Sol. Energy Mater. Sol. Cells 1999, 58, 1− 131. (14) Bange, K.; Gambke, T. Electrochromic Materials for Optical Switching Devices. Adv. Mater. 1990, 2, 10−16. (15) Niklasseon, G.; Granqvist, C. Electrochromics for Smart Windows: Thin Films of Tungsten Oxide and Nickel Oxide, and Devices Based on These. J. Mater. Chem. 2007, 17, 127−156. (16) Agrawal, A.; Cronin, J.; Zhang, R. Review of Solid State Electrochromic Coatings Produced Using Sol-Gel Techniques. Sol. Energy Mater. Sol. Cells 1993, 31, 9−21. (17) Granqvist, C. Electrochromic Tungsten Oxide Films: Review of Progress 1993−1998. Sol. Energy Mater. Sol. Cells 2000, 60, 201−262. (18) Deepa, M.; Joshi, A.; Srivastava, A.; Shivaprasad, S.; Agnihotry, S. Electrochromic Nanostructured Tungsten Oxide Films by Sol-gel: Structure and Intercalation Properties. J. Electrochem. Soc. 2006, 153, C365−C376. (19) Vig, J. UV Ozone Cleaning of Surfaces. J. Vac. Sci. Technol., A 1985, 3, 1027−1034. (20) Brinker, C.; Frye, G.; Hurd, A.; Ashley, C. Fundamentals of SolGel Dip Coating. Thin Solid Films 1991, 201, 97−108. (21) Chakraboty, M.; Chowdhury, D.; Chattopadhyay, A. SpinCoating of Polystyrene Thin Films as an Advanced Undergraduate Experiment. J. Chem. Educ. 2003, 80, 806−809. (22) Bard, A.; Faulkner, L. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 2000. (23) Quagliano, J.; Marks, C. Demystifying Spectroscopy with Secondary Students: Designing and Using a Custom-Built Spectrometer. J. Chem. Educ. 2013, 90, 1409−1410. (24) Dini, D.; Decker, F.; Masetti, E. A Comparison of the Electrochromic Properties of WO3 Films Intercalated with H+, Li+, and Na+. J. Appl. Electrochem. 1996, 26, 647−653. (25) Granqvist, C. Progress in Electrochromics: Tungsten Oxide Revisited. Electrochim. Acta 1999, 44, 3005−3015. F

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