In the Laboratory
Electrochromic WO3 Films: Nanotechnology Experiments in Instrumental Analysis and Physical Chemistry Laboratories Maria Hepel Department of Chemistry, State University of New York at Potsdam, Potsdam, NY 13676 and Department of Chemistry, University of Buffalo, Buffalo, NY 14260;
[email protected] The notion that solids, unlike liquids and gases, are impenetrable is well-justified on the basis of everyday experiences. Thus, students may perhaps accept the idea of doping solid semiconductors under extreme conditions, for instance, by an ion implantation process carried out at high electric fields or by bombardment with high-energy α particles, but to accept a view that ions would easily intercalate into a crystalline solid matter would require more elaborate elucidation. In an undergraduate laboratory, students also want to have clear evidence that the intercalation really takes place. This article describes how students can control and monitor an intercalation process. Of many important ion intercalation phenomena, we have selected electrochromism of WO3 films, extensively investigated as a material for digital displays and smart windows applications. In an electrochemical setting, driving ions into a WO3 film deposited on a working electrode results in a sharp color change of the film from colorless to dark blue. Since students do not associate a color change with ions entering the solid film, we carry out the intercalation experiments with simultaneous monitoring of the film mass using ultrasensitive nanogravimetry, able to detect nanomoles of ions inserted in the electrodic film, which is typically 200–350 nm thick. It is also important for students to be able to prepare WO3 films in the lab. This is done by electrodeposition, yielding films of well-controlled thickness, with WO3 nanoparticles that can be readily visualized and measured by atomic force microscopy (AFM). The film performance is then tested using potential pulses. The color change on intercalation can be observed with naked eyes but can also be recorded using a photodiode or spectrophotometer measuring the intensity of the reflected beam. Background Interactions of hydrogen with tungsten trioxide, and some other non-stoichiometric transition-metal oxides, lead to interesting electrochromic (1–7) and photochromic phenomena (8), both associated with cation intercalation into the metal oxide lattice. The intercalation process, which can be represented by the reaction equation, WO3 xH xe HxWO3 (1) has been found to cause lattice deformation (1, 9, 10). If the size of intercalating cation is small enough (e.g., H+, Li+) and the degree of intercalation is not too high, the lattice expansions and contractions on ingress or egress of ions will be reversible (9–13), thereby preserving the original film structure. The ingress of H+ ions into a WO3 film is accompanied with the formation of color centers (small polarons) and formal reduction of some W6+ sites to W5+ and W4+. The WO3 crystal structure consists of WO6 octahedra joined at their corners, which may be considered as a perovskite structure of CaTiO3 with all the Ca2+ sites vacant. During the intercalation process, the H+ ions enter into these vacant sites.
Overview of the Experiment In this experiment, electrochromic films of WO3 are synthesized by electrochemical deposition. The film deposition processes are characterized and elucidated using the electrochemical quartz crystal nanobalance (EQCN) technique (14, 15). The EQCN technique, comprising simultaneous voltammetric and nanogravimetric measurements, is easily implemented on modern electrochemical instrumentation (15). The inexpensive Elchema instrument, model EQL-520, combines an electrochemical workstation with EQCN and light reflectance measurement functionalities and is well-suited for the purpose of this experiment. The measurements are controlled and data are collected and analyzed by the Voltscan Data Logger model DAQ-20e. For AFM imaging, a Veeco nanoscope model IIIa is used in the experiments. Tungsten trioxide films are synthesized by the electrochemical deposition procedure. The electroplating solution for WO3 deposition is prepared by a modified pertungstic acid method (16, 17), in which a small quantity of metallic tungsten powder (1 g) is dissolved in conc H2O2 (5 mL). The exothermic reaction takes 15 minutes to complete. The obtained solution of pertungstic acid, H2WO5⋅H2O, is diluted and used for WO3 plating. The WO3 films are prepared by electrodeposition at constant potential (Edep = ‒0.5 V vs Ag∙AgCl reference electrode) on Aucoated quartz crystal (QC) piezoresonator wafers (QC-10AuPB from Elchema). The deposition time is 10 minutes. Testing of the film’s electrochromic behavior is done in 10 mM H2SO4 solution by applying repetitive potential pulses between E1 = +0.5 V and E2 = ‒0.5 V with pulse duration of 5–10 s. The film coloration to dark blue appears during the film reduction step (E2) and bleaching occurs during the film oxidation (E1). The color changes are fast (2, 17) and can be observed with naked eyes. During these potential pulse experiments, the mass change of the film is also monitored. Thus, during the film reduction, the mass increases concomitant with the film coloration process. During the film oxidation, mass decreases and the film is bleached (it becomes completely invisible). These invisible films are further characterized using AFM imaging showing WO3 nanoparticles, approximately 30 nm in diameter. Hazards Fine tungsten powder is flammable, H 2O2 is a strong oxidant causing burns on skin, is harmful when swallowed or inhaled, and may cause fire in contact with organic compounds. The reaction of W with H2O2 is exothermic and must be carried out under the hood in a beaker covered with a Petri dish. Do not use more than 2 g of W, unless reaction beaker is cooled. H2SO4 is corrosive and should be handled with care. Students should practice caution while performing this experiment and wastes should be collected for disposal by a suitable authority.
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Results and Discussion Electrodeposition of WO3 films Typical chronoamperometric and nanogravimetric characteristics for WO3 deposition process are presented in Figure 1. After application of a cathodic potential pulse (Figure 1A), a spike and decay of cathodic current are observed (Figure 1B). The formation of WO3 is manifested by the apparent mass increase Δm, measured as the resonant frequency shift Δ f of the QC|Au piezoresonator (Figure 1B), employed as the working electrode, (2) % m k % f where k = constant (14). The reductive deposition of WO3 films on a noble-metal electrode from pertungstic acid solution can be described by the reaction equation,
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where the major contribution to the mass increase during film deposition is due to the formation of hexagonal phase with composition WO3⋅(1∙3)H2O. The comparison of the theoretical value of (∂m∙∂Q)theor = 1,233 ng∙mC for eq 2 with the experimentally determined slope (∂m∙∂Q)exp gives students a wealth of information about the processes taking place during film deposition, as analyzed in the online supplement. The observed WO3 film growth rate under potentiostatic conditions (at Edep = ‒0.5 V) is typically (∂m∙∂t) = 65 ng∙s, which corresponds to the thickness growth rate, ∂h∙∂t (17), of 0.61 nm∙s, uh 1 um (4) ut R A ut where A is the surface area and ρ is the film density (ρ = 5.42 g∙cm3 for electrochemically grown WO3 films). 126
Figure 2. Tapping-mode AFM images of WO3 nanoparticles formed during initial stages of film deposition: (A, B) nucleation of WO3 nanowires on edges of atomic steps on highly oriented pyrolytic graphite surface, deposition potential Edep: (A) ‒0.3 V, (B) ‒0.4 V versus Ag/AgCl, image size: 10 × 10 μm2; (C, D) large WO3 particles deposited at Edep: (C) ‒0.3 V, (D) ‒0.4 V, and annealed at 300 °C, image size: 1 × 1 μm2; (E, F) small WO3 particles obtained by deposition at Edep = ‒0.5 V, image size: (E) 1 × 1 μm2, (F) 450 × 450 nm2, average nanoparticle diameter: 27 ± 4 nm.
The morphology of WO3 films is examined by scanning probe microscopy (18, 19) using tapping-mode AFM imaging (Figure 2). The film morphology depends on the deposition conditions. In this experiment, the size of WO3 nanoparticles is controlled by the electrode potential (in the range from Edep = ‒0.1 to ‒0.9 V), concentration of pertungstic acid, pH, annealing, and so forth. The average diameter of nanoparticles shown in Figure 2 is 27 ± 4 nm, determined by cross-section analysis of AFM images. Intercalation of H+ into WO3 films The intercalation experiments are carried out in 10 mM H2SO4 supporting electrolyte. It is instructive to determine the potential region of H+ intercalation by nanogravimetric measurements under linear potential scan (LSV) conditions. Such an experiment with a QC|Au|WO3 piezoelectrode is described in the online supplement and shows that negative frequency shift occurs simultaneously with the rise of cathodic current during the potential scan below E = 0.2 V. The electrochromic switching of WO3 films, between the oxidized (bleached) and reduced (colored) states, is tested by performing repetitive potential step experiments. The current and frequency shift responses of a WO3 film to a potential waveform with coloration potential Ecol = ‒0.5 V, bleaching potential Ebl = +0.5 V, τcol = 5 s, τbl = 5 s, are presented in Figure 3.
Journal of Chemical Education • Vol. 85 No. 1 January 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
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project driving student curiosity and enthusiasm to solve problems of this modern nanotechnology topic with perspectives of applications in electronic devices.
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This work was supported by the National Science Foundation Grant No. CCLI-0126402 and by FUSR Grant of SUNY Potsdam.
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Figure 3. (A) Pulsed potential program waveform for testing electrochromic switching in a WO3 film; (B) voltammetric (current) vs time and (C) EQCN frequency shift vs time responses to waveform in A: (D) film reflectance vs time, λ = 650 nm, during coloration (col) and bleaching (bl) processes; WO3 film was etched in Et4NCl solution.
The degree of H+ intercalation, x, is determined by integrating cathodic current (which is a one-step operation with Voltscan Data Logger). This procedure provides the charge, Q, consumed during intercalation. The x value is given by Q MWO3 (5) x F mf where mf stands for the mass of film, MWO3 is the molar mass of WO3⋅(1∙3)H2O (MWO3 = 237.8 g∙mol), and F is Faraday's constant. Values of x ranging from 0.05 to 0.3 are obtained. The color center density, c, is then calculated using formula x M WO3 NA (6) c R where NA is the Avogadro number. Values of c from 1 × 1021 to 5 × 1021 cm‒3 are obtained. Conclusions EQCN experiments with simultaneous recording of voltammetric and nanogravimetric characteristics, augmented with reflectance measurements and AFM imaging of electrochromic WO3 films, are an excellent instructional experience for instrumental analysis laboratory and physical chemistry laboratory students. The experiments are organized as a small research
1. Deb, S. K. Philos. Mag. 1973, 27, 801. 2. Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1998, 145, 1023. 3. Shiyanovskaya, I.; Hepel, M.; Tewksbury, E. J. New Mater. Electrochem. Systems 2000, 3, 241. 4. Kulesza, P. J.; Faulkner, L. R. J. Electroanal. Chem. 1989, 259, 81. 5. Borzenko, M. I.; Chojak, M.; Kulesza, P. J.; Tsirlina, G. A.; Petrii, O. A. Electrochim. Acta 2003, 48, 3797. 6. Kim, J.; Lee, L.; Niece, B. K.; Wang, J. X.; Gewirth, A. A. J. Phys. Chem. B 2004, 108, 7927. 7. Guerfi, A.; Nguyen, M. T.; Dao, L. H. J. Electrochem. Soc. 1989, 136, 2435. 8. Granqvist, C. G.; Azens, A.; Hjelm, A.; Kullman, L.; Niklasson, G. A.; Ronnow, D.; Stromme, Mattson M.; Veszelei, M.; Vaivars, G. Sol. Energy 1998, 63, 199. 9. Schrimer, O. F.; Wittwer, V.; Baur, B.; Kang, K. S. Electrochim. Acta 1977, 22, 751. 10. Gavrilyuk, A. L. Electrochim. Acta 1999, 44, 3027. 11. Cordoba de Torresi, S. I.; Gorenstein, A.; Torresim R. M.; Vazquez, M. V. J. Electroanal. Chem. 1991, 318, 131. 12. Dini, D.; Passerini, S.; Scrosati, B.; Decker, F. Sol. Energy Mater. Sol. Cells 1999, 56, 213. 13. Scarminio, J. Sol. Energy Mater. Sol. Cells 2003, 79, 357. 14. Bruckenstein, S.; Shay, M. Electrochim. Acta 1985, 30, 1295. 15. Hepel, M. Electrode-Solution Interface Studied with Electrochemical Quartz Crystal Nanobalance. In Interfacial Electrochemistry. Theory, Experiment and Applications;Wieckowski, A., Ed.; Marcel Dekker, Inc.: New York, 1999; pp 599– 630. 16. Hepel, M.; Hazelton, S. Electrochim. Acta 2005, 50, 5278. 17. Hepel, M.; Redmond, H.; Dela, I. Electrochim. Acta 2007, 52, 3541–3549. 18. Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129–1162. 19. Zhong, C. J.; Han, L.; Maye, M. M.; Luo, J.; Kariuki, N. N.; Jones, W. E. J. Chem. Educ. 2003, 80, 194.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jan/abs125.html Abstract and keywords Full text (PDF) Links to cited JCE articles
Color figures
Supplement An expanded version of the article
Additional tests for a research project
Handouts for the students
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