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J. Phys. Chem. C 2007, 111, 7200-7206
Electrochromic Stability of WO3 Thin Films with Nanometer-Scale Periodicity and Varying Degrees of Crystallinity Se´ bastien Sallard,† Torsten Brezesinski,*,‡ and Bernd M. Smarsly*,† Max-Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany, and Department of Chemistry and Biochemistry, UniVersity of California at Los Angeles, Los Angeles, California 90095-1569 ReceiVed: December 11, 2006; In Final Form: February 20, 2007
Electrochromic tungsten oxide (WO3) thin films with nanometer-scale porosity have been synthesized via a sol-gel procedure making use of evaporation-induced self-assembly. According to wide-angle X-ray scattering data combined with electrochemical analyses, the degree of crystallinity ranging from fully amorphous to 100% crystalline can be adjusted by straightforward annealing. The three-dimensional cubic pore structure is thereby almost not affected. Aside from the material characterization, in this work we specifically focus on the overall electrochemical and electrochromic behavior (coloration efficiency, charge capacity, etc.) upon changes in the operating temperature. As a main result, only the mesoporous highly crystalline WO3 films display long-term cycling stability under realistic environmental conditions. We further demonstrate that sufficient crystallinity is needed to ensure stability of the inherent electrochemical properties at high operating temperatures (up to 70 °C). Thus, only the WO3 films with a highly crystalline framework exhibit almost unchanged electrochemical/electrochromic characteristics after prolonged potentiostatic cycling and exposure to elevated operating temperatures. In contrast, amorphous and partially crystalline films suffer from irreversible performance degradation due to structural modifications.
Introduction For many years tungsten oxide (WO3) has been extensively studied as a promising electrochromic material.1-3 This transition metal oxide lacks the mechanical flexibility of conductive polymers but promises better electrochemical stability, thus facilitating long-lifetime applications. Meanwhile, WO3 is already integrated in low-voltage electrochromic devices for smart windows, which emphasizes the high technological relevance of this kind of material. However, currently available devices still suffer from several shortcomings, such as quite long electrochromic response times, etc. Thus, novel conceptual approaches improving the electrochromism of such systems can be expected to result in a direct technological impact. Even though not all details of the mechanism of electrochromism are fully understood, the overall coloration process can be described as the simultaneous insertion of cations and electrons into the inorganic matrix during the reduction step (see Scheme 1).4 Lithium ions predominantly act thereby as cation source in organic solutions, whereas protons are utilized in aqueous solutions.5-7 It is known that the kinetics of propagation of polarons (e-, M+) is mainly limited by cation diffusion in the inorganic matrix, which is strongly dependent on the overall material morphology8,9 and the size, shape, and valence of the respective cations. Recently, various strategies have been proposed to improve the electrochemical performance of sol-gel derived tungsten oxide films by optimizing the cation insertion. Fur* Address correspondence to either author. E-mail:
[email protected] (T.B.);
[email protected] (B.M.S.). Tel.: + 1-310-794-6618 (T.B.); + 49-331-567-9508 (B.S.). † Max Planck Institute of Colloids and Interfaces. ‡ University of California at Los Angeles.
thermore, it has been shown that a better accessibility of the WO3 matrix (for the electrolyte) provided by three-dimensional mesoporosity also has a positive impact on the electrochemical and electrochromic behavior. In particular, the response times can be shortened by down-scaling the WO3 matrix to the nanometer-scale and simultaneous shortening of the mean ion diffusion path lengths. Tungsten oxide coatings produced by physical techniques like sputtering or thermal evaporation are dense/nonporous and display relatively slow switching times (on the order of up to several minutes for both the coloration and bleaching step). However, not all kinds of WO3 nanoscale systems necessarily lead to improved electrochromic properties. As described recently, certain non-organized pore architectures can even impede the accessibility of the inner surface.10-11 Moreover, it is known that a prolonged diffusion of cations in the matrix (e.g., as a consequence of repetitive potentiostatic cycling) can alter or damage fragile pore morphologies. Thus, the pore structure should be mechanically/electrochemically stable, fully accessible, and well-defined in shape. Such self-assembled mesopore architectures can be expected to strongly influence important characteristics like the kinetics of coloration/decoloration, the charge capacity, and the coloration efficiency.5 Aside from the electrolyte accessible surface area, crystallinity and oxygen deficiency also affect the electrochromic properties of WO3. For instance, a change in oxygen stoichiometry12,13 leads to a different polaron propagation14 and thus to a modified electrochemical and electrochromic behavior.12,15 Consequently, the treatment of films has a direct impact on the overall performance. Up until now, the simultaneous control of both mesoporosity and crystallinity in WO3 coatings has been rarely described due to the fact that these architectures often undergo partial or even
10.1021/jp068499s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007
Electrochromic WO3 Thin Films
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7201
SCHEME 1: Schematic of the Basic Principles of Electrochromism in Mesoporous WO3 Thin Filmsa
a
The insertion/extraction of lithium during reduction/oxidation and the associated coloration/bleaching of the inorganic matrix can be seen.
complete collapse of the mesopore structure upon crystallization.5,7 However, very recently, it was demonstrated that certain block copolymer templates, such as “KLE” or “PIB-PEO”,16 allow for the generation of mesoporous highly crystalline thin films,17 inter alia, also WO3 films with nanometer-scale periodicity.18 The degree of crystallinity ranging from fully amorphous to 100% crystalline can thereby be adjusted by a straightforward annealing procedure. As a main result of our previous work on self-assembled WO3, the electrochromic response times for both the coloration and bleaching step were significantly shortened as compared to nonporous WO3 layers prepared by sol-gel methodology. Several important aspects, however, had not been investigated and remained unclear. Extending our previous work,18 the main objective of this study was to analyze the electrochemical longterm stability as a function of the crystallinity. Evidently, such measurements are crucial if ion insertion/extraction can alter the mesopore architecture as outlined above. Moreover, we investigated the evolution of various electrochromic characteristics depending on the operating temperature. Any potential application of electrochromic metal oxide films requires a stable performance upon temperature changes. One should also consider that the temperature of smart windows exposed to intense sunshine can reach 70 °C or more. The purpose of this work was therefore also to answer the basic question of whether the presence of a mesoporous ordered network interferes with the long-term and temperature stability or not. These parameters were studied in detail for mesoporous amorphous and partially and fully crystalline WO3 thin films. The obtained results provide novel insights into the electrochemical and electrochromic performance of self-assembled tungsten oxide films. Furthermore, for the first time, this work allows separating the influence of well-defined mesoporosity and crystallinity on physical parameters with direct practical relevance under realistic environmental conditions. The WO3 films used in our study are thereby particularly suited to serve as a model system. Experimental Section Synthesis. In a water-free container, 0.125 grams of H(CH2CH2CH2(CH)CH2CH3)89(OCH2CH2)79OH (referred to as “KLE”) dissolved in a mixture of 6 mL of EtOH and 2 mL of THF are combined with 0.5 g of WCl6. Once the solution is homogeneous, films can be produced via dip-coating on polar substrates in a controlled environment. Optimal conditions are given for 5-10% relative humidity and a constant withdrawal rate of 5-10 mm/s. After deposition, the films need to be stabilized at 100 °C for 1 h and annealed at 300 °C for 12 h to cross-link the inorganic network. Calcination to remove the structuredirecting agent and to induce crystallization is done in oxygen atmosphere using a 1.5 h ramp up to 500 °C.
Characterization. WAXS experiments in θ-2θ geometry were performed using a D8 diffractometer from Bruker instruments (Cu KR radiation) equipped with an energy-dispersive solid-state detector. TEM images were recorded with a Zeiss EM 912Ω at an acceleration voltage of 120 kV, whereas a Philips CM200 FEG microscope with a field emission gun was utilized for HRTEM. Tapping mode AFM images were acquired with a multimode AFM from Vecco Instruments employing Olympus microcantilevers (resonance frequency: 300 kHz; force constant: 42 N/m). Electrochemical measurements were performed in 1.0 M LiClO4 in propylene carbonate (PC) in a potential range of (1 V vs Ag using Autolab 12 potentiostat/ galvanostat (Eco Chemie). WO3 films on FTO-covered glass were utilized as working electrodes; Pt mesh was taken as an auxiliary electrode, and a silver wire served as a pseudoreference electrode. Results and Discussion Nanometer-Scale Structure and Composition. In this work, we specifically employed cubic mesoporous WO3, which was produced via dip-coating making use of evaporation-induced self-assembly (EISA)19 and a recently developed recipe.18 This recipe was slightly modified and optimized because a “KLE” block copolymer with different composition was utilized. The mesoscopic structure of the present thin films is consistent with previous results obtained for “KLE”-templated WO3 and other related systems. Figure 1a shows a lowmagnification transmission electron microscopy (TEM) image of a WO3 film with nanometer-scale periodicity after calcination at 550 °C. A well-defined cubic mesoporous matrix with 1213 nm diameter pores can be seen. This structure is further confirmed by small-angle X-ray scattering (data not shown). The electron diffraction (ED) pattern (Figure 1a inset) recorded from the same localized area shows diffraction rings indicative of a polycrystalline material. The d-spacings are in perfect agreement with monoclinic WO3. The high crystallinity of the framework indicated by ED is also observed with high-resolution TEM (HRTEM). Figure 1b presents HRTEM images, which show characteristic lattice planes of WO3 nanocrystals in the pore walls. Aside from electron microscopy, atomic force microscopy (AFM) is a well-established tool to study the macroscopic homogeneity of films. Figure 1c shows tapping mode AFM images, which emphasize both the flatness (the root-mean-square roughness is less than 1 nm and the maximum height variation about 4 nm) and the mesostructural order of the crack-free top surface. This texture is consistent with a system that has an interface of open pores. Moreover, the uniform arrangement of pores at the surface confirms the presence of a homogeneous cubic lattice of mesopores.
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Figure 1. Characterization of periodically ordered WO3 thin films after crystallization at 550 °C. Part a shows a TEM image, which demonstrates the homogeneity of the mesopore structure. The inset presents an ED pattern recorded from the same localized area. Part b shows two HRTEM images. Both ED and HRTEM confirm the high crystallinity of the pore walls. Part c presents tapping mode AFM images, which show hexagonally arranged open pores at the surface.
Figure 2. Temperature-dependent WAXS patterns of self-assembled WO3 thin films. The black lines represent a reference diffraction pattern [No.: 43-1035] according to the Joint Committee for Powder Diffraction Studies (JCPDS).
Figure 2 shows temperature-dependent wide-angle X-ray scattering (WAXS) patterns, which give clear evidence of the polycrystalline structure with the pore walls being composed of phase-pure monoclinic WO3. The crystallization takes place at about 500 °C within a narrow temperature range. However, calcination at 500 °C produces films with only partially crystalline WO3 matrix, whereas films treated at 550 °C can be considered as 100% crystalline (see the electrochemical part). On the basis of the line broadening of the (002) reflection (Scherrer equation), an average nanocrystal size of 13 nm is obtained. This size remains unchanged up to 600 °C. A calcination of films at even higher temperatures leads to diffuse sintering and is further associated with mesostructural collapse. Cyclic Voltammetry. In contrast to our previous study where the electrochemical measurements were performed in an inert environment, this time the film properties were investigated under ambient conditions, i.e., in an atmosphere containing oxygen and water (our experimental setup did not allow performing these in-situ measurements under variation of temperature in combination with a UV-vis spectrophotometer under an inert atmosphere). First, a basic electrochemical characterization was done to ensure a typical redox behavior upon Li insertion. Figure 3 shows cyclic voltammograms of mesoporous WO3 thin films with varying degrees of crystallinity. Amorphous films display broad featureless peaks of Li insertion/extraction, typical of amorphous WO3. By contrast, films calcined at 500 °C and 550 °C, respectively, show well-developed peaks at about -0.6 V vs Ag due to phase transitions (monoclinic/tetragonal and tetragonal/cubic) during multistep Li insertion into the WO3 lattice.18 For the first cycles the amount of inserted charge follows the expected trend, namely Qred/400 °C > Qred/500 °C >
Figure 3. Cyclic voltammograms of WO3 thin films with nanometerscale periodicity (second cycle, scan rate 10 mV/s). Curve A: amorphous film (400 °C). Curve B: partially crystalline film (500 °C). Curve C: fully crystalline film (550 °C).
Qred/550 °C. This shows that the presence of nanocrystals increases the energy barrier for Li insertion.12 Moreover, the shape of the voltammograms confirms the expected redox behavior of the different sol-gel derived WO3 thin films being a precondition for the subsequent electrochromic study. It should be noted that cyclic voltammetry provides a further independent technique (in addition to WAXS) to analyze the degree of crystallinity of such thin films, as shown recently for self-assembled TiO2 layers.20 Optical Absorption. The coloration/decoloration of the mesoporous ordered WO3 films was studied by monitoring the optical absorbance in the course of Li insertion/extraction using a UV-vis spectrophotometer. Figure 4 shows absorption spectra as a function of the electrochemical potential. The potential was varied in order to determine the optimum operating conditions for the electrochromic measurements. Since there was no significant change in absorption between 0 and 1 V vs Ag, we specifically focused on a potential range of 0 V to -1 V. The absorption curves clearly show that the mesoporous crystalline films are practically 100% transparent in the oxidized state. By contrast, the amorphous films are still slightly brown due to incomplete removal of the organic template leading to absorption at wavelengths of 350-450 nm. Owing to the higher energy barrier for Li insertion into the monoclinic WO3 lattice,12 an electrochemical potential of -0.4 V is needed to induce a coloration of the crystalline films. On the other hand, -0.2 V is already sufficient for the amorphous films. However, all spectra show a continuous increase in absorption by decreasing the electrochemical potential. We further find that the presence of nanocrystals shifts the absorption maximum to higher wavelengths, which is in agreement with previously reported data on sol-gel derived WO3 coatings.14 The absorption maximum of the present films is located in the near-infrared
Electrochromic WO3 Thin Films
Figure 4. Absorption spectra of mesoporous WO3 thin films. Part a: amorphous film (400 °C). Part b: partially crystalline film (500 °C). Part c: fully crystalline film (550 °C). The spectra were acquired at different electrochemical potentials in steps of 200 mV from 0 V to -1 V vs Ag. The films show an increase in absorption as the potential decreases (indicated by the arrow at a wavelength of 630 nm).
region albeit the films appear dark blue.21 The relatively low absorption values can be explained by the film thickness (150 nm, porosity of about 30%) and the actual quantity of WO3. On the basis of these measurements, we chose a potential range of (1 V vs Ag in order to achieve reasonable Li intercalation, which is in turn associated with the coloration. The overall coloration/decoloration was followed at a wavelength of 630 nm. Long-Term Stability. Aside from typical electrochromic characteristics like the coloration efficiency, etc., long-term stability is an important issue for any potential application of such coatings. Hence, repetitive potentiostatic oxidation/reduction experiments (square wave technique) were performed at different operating temperatures addressing particularly the charge capacity (Figure 5a), the optical density (Figure 5b), and the charge reversibility (Figure 5d). It turned out that both the amount of charge inserted during the reduction step and the charge reversibility at ambient operating conditions are significantly different than those obtained previously.18 Interestingly, the charge reversibility of the mesoporous amorphous WO3 thin films went up to 93%, whereas it was only about 40% in our previous study.18 This difference can be attributed to the fact that the previous measurements were performed under dry conditions in a glove box. Livage et al. described the phenom-
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7203 enon that already traces of water in propylene carbonate,22 which acts as a solvent for the electrolyte, strongly influence the electrochromic properties of sol-gel derived WO3 layers.23-25 Nonetheless, the overall charge reversibility during cycling at 20 °C follows the expected trend, namely highly crystalline WO3 > partially crystalline WO3 > completely amorphous WO3. The most relevant parameter describing the performance of electrochromic devices is the coloration efficiency, which is defined by η ) ∆OD/Qred; with the optical density ∆OD ) log(Γox/Γred) ) Absred - Absox,4,26-28 Γ as optical transmittance in the oxidized/reduced state, and Abs ) log(1/Γ). Qred stands for the charge inserted per square centimeter. Figure 5 presents different curves, inter alia, ∆OD and Qred depending on both time and operating temperature, which show a strong correlation between these parameters. The coloration efficiency (Figure 5c) of the mesoporous material described here is comparable to literature data on WO3 films produced by other techniques,27,28 showing a typical range of η ) 20-50 cm2/C.6,29 As a main result, the highly crystalline WO3 films display a better coloration efficiency than the amorphous ones. Cycling experiments performed over several hours clearly demonstrate that the long-term stability depends on the state of crystallinity. In general, amorphous WO3 exhibits the lowest charge reversibility likely because of Li trapping (irreversible insertion) within the mesoporous framework. Furthermore, we find that several electrochemical/electrochromic characteristics degrade at ambient operating conditions already. After 30 cycles, these films loose 45% (in relation to the maximum value) of the optical density and 20% of the inserted charge, and the coloration efficiency decreases by 35%. This significant drop in performance can be attributed to the instability of the amorphous network upon electrochemical stimuli.24,30 By contrast, the partially crystalline films exhibit a comparatively good long-term stability. They show a decrease of ∆OD by only 16%, Qred by 5%, and η by 15%. Obviously, the improved performance can be traced back to the stable crystalline parts, whereas the residual amorphous parts still suffer from Li insertion/extraction.24 However, the highly crystalline samples demonstrate the best long-term stability with respect to all different parameters, proving that excellent electrochromic stability can be achieved if the porous inorganic matrix is sufficiently inert. These films do not show performance degradation even after several hundred cycles, as already demonstrated in our previous study.18 This result, in turn, confirms the general paradigm that metal oxide coatings must be fully crystalline to achieve best performance. In conclusion, the data show the necessity of having a highly crystalline inorganic matrix between the mesopores, and thus the need for appropriate structure-directing agents that allow a controlled crystallization of the initially amorphous network. Influence of the Operating Temperature. The influence of the operating temperature on the electrochromic/electrochemical performance was investigated by controlling the temperature of the PC solution during the measurements. Marked changes in the overall behavior depending on the degree of crystallinity can be seen (Figure 5). It turned out that the electrochromic properties of the highly crystalline material are particularly stable with respect to prolonged exposure to elevated temperatures. By contrast, the mesoporous amorphous and partially crystalline films show an irreversible performance degradation due to structural changes.24,30,31 A closer look at the different parameters reveals further subtleties of their temperature dependence. For highly crystalline films, elevated operating temperatures (50 °C or 70 °C) even increase the charge capacity and the
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Figure 5. Electrochemical and electrochromic characteristics of WO3 thin films with nanometer-scale periodicity and varying degrees of crystallinity as a function of the operating temperature. Part a: amount of inserted charge Qred. Part b: optical density ∆OD. Part c: coloration efficiency η. Part d: charge reversibility Qox/Qred. The notation of films is as follows: amorphous (blue circuits, 400 °C), partially crystalline (red circuits, 500 °C), and fully crystalline (black circuits, 550 °C). A, B, C, and D correspond to the following operating temperatures: 20 °C, 50 °C, 70 °C, and 20 °C (after cooling from 70 °C).
absorbance at 630 nm (Figure 6). The charge reversibility, however, remains stable over several hours. More importantly, after cooling the electrolyte solution, the absolute values of all these parameters drop back to the initial ones, thus proving an excellent reversibility even upon drastic changes in the operating temperature. This intricate behavior can be explained by Arrhenius’s law regarding the activation energy needed for Li intercalation. Previous studies have shown that vacancies located deeply inside a close-packed (dense) highly crystalline WO3 film are usually inaccessible for the lithium ions.12,13 However, in the case of mesoporous materials, most of the nanocrystal surfaces are accessible, and thus an increase in the operating temperature allows more vacancies to be addressed by lowering the energy barrier for Li intercalation. Two effects are generally possible: (1) an increase in lattice vibrations or (2) an increase in the kinetic energy of the lithium ions. This is further supported by the finding that after each heating step the charge reversibility is slightly lower than before, which shows that a certain, small fraction of Li is irreversibly trapped at new binding sites. The facilitated Li insertion at higher temperatures also explains why for the first cycles after the cooling step the charge reversibility is larger than 1. After cooling the PC solution from 70 to 20 °C, we find almost identical coloration efficiency values (Figure 5c) for the highly crystalline WO3 films as outlined above. The only deterioration is a slight increase in absorption after the bleaching step. This means that either a certain minor fraction of the film is irreversibly modified or that some lithium ions cannot overcome the energy barrier for extraction (Figure 6). Thermochromic effects could theoretically also account for the optical changes but can be excluded based on the acquired absorption spectra. The electrochemical/electrochromic properties of the mesoporous amorphous and partially crystalline films at elevated operating temperatures are inferior to those of the highly crystalline samples. For instance, the amount of inserted charge strongly decreases, and the coloration efficiency is significantly
lowered. In conclusion, the experiments clearly show that the irreversible degradation of film properties can be attributed to structural modifications of the still flexible amorphous matrix. On the other hand, the fully crystalline WO3 network is sufficiently inert and able to withstand such modifications. The different behavior of the mesoporous ordered WO3 networks can be further illustrated by plotting the rate of reduction/oxidation and coloration/decoloration, respectively, against the operating temperature (Figure 7). Apparently, a positive slope is observable only for the highly crystalline samples, whereas the rates for the amorphous and partially crystalline films decline. The positive slope can be attributed to both a facilitated diffusion of polarons and a facilitated insertion of Li into the monoclinic lattice, as mentioned above. Switching Kinetics. The kinetics of the Li insertion/extraction process depending on the crystallinity of the tungsten oxide and the operating temperature was investigated using chronoamperometry. The current response and the coloration/bleaching at 630 nm were recorded simultaneously during repetitive oxidation/reduction of the mesoporous WO3 thin films. A quantitative comparison of characteristic response times (Table 1, corresponding switching curves are shown in the Supporting Information) is based on the time the system needs to reach 90% of the total insertion/extraction capacity and the maximum coloration/decoloration, respectively. This comparison reveals interesting results with respect to the influence of the operating temperature. In our previous study,18 we found that mesoporous amorphous films exhibit the fastest electrochromic response times.12,27 However, for films with moderate or no crystallinity, high operating temperatures massively slow down the switching kinetics.1 Moreover, the inconsistent values (Table 1) after the cooling step (from 70 °C to 20 °C) confirm the irreversibility of the underlying structural changes. As expected, the highly crystalline layers show superior coloration/bleaching kinetics with response times of only 20-40 s even at elevated operating temperatures. Furthermore, the switching times return
Electrochromic WO3 Thin Films
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7205
Figure 6. Absorption data of periodically ordered WO3 thin films depending on the operating temperature (right) recorded during repetitive potentiostatic oxidation/reduction of films (voltage steps of (1 V vs Ag, step duration 2 and 1 min, respectively). Part a: amorphous film (400 °C). Part b: partially crystalline film (500 °C). Part c: fully crystalline film (550 °C). The notation of A, B, C, and D is the same as in Figure 5.
Figure 7. Rate of reduction/oxidation (black squares) and coloration/ decoloration (blue circuits) of self-assembled WO3 networks as a function of the operating temperature. The solid symbols correspond to the reduction and coloration, respectively. Part a: amorphous film (400 °C). Part b: partially crystalline film (500 °C). Part c: fully crystalline film (550 °C).
to the initial values (recorded at ambient operating conditions) after exposure to elevated operating temperatures for prolonged time. This emphasizes the enormous stability of the crystalline framework. It is important to mention that for all samples the electrochromic response is strongly related to the current response. This final result is in agreement with the work of Choy et al.12 and gives evidence for the good homogeneity of the present periodically ordered networks. More detailed investigations of the coloration/bleaching kinetics go beyond the scope of this work and will be subjected in a separate study.
20 °C 20 °C 50 °C 70 °C 20 °C second cycle last cycle last cycle last cycle last cycle (s) (s) (s) (s) (s)
TABLE 1: Characteristic Response Times for the Li Insertion/Extraction into/from Mesoporous WO3 sample fully amorphous partially crystalline fully crystalline
25/69a 43/63b 26/70 45/65 22/50 39/50
17/7 39/28 21/13 38/28 20/37 31/30
15/67 43/67 17/45 38/56 23/34 30/16
33/76 47/77 25/74 45/76 20/15 29/16
51/88 48/78 56/81 51/84 20/34 30/38
a The upper set of values per column represents the electrochromic response times for the coloration and bleaching, respectively. b The lower set corresponds to the current response times (insertion/ extraction).
Conclusion In this work, the overall electrochemical and electrochromic behavior of self-assembled WO3 films with nanometer-scale periodicity is analyzed as a function of the degree of crystallinity ranging from fully amorphous to 100% crystalline. Tungsten oxide represents a suitable model system because the functionality can be well quantified in terms of several parameters like the coloration efficiency, the charge capacity, the optical density, etc. Recently, we have already reported the preparation of WO3
networks with a high degree of mesostructural order and varying degrees of crystallinity.18 In this previous study, kinetics measurements had revealed that for both amorphous and crystalline films the three-dimensional mesoporosity significantly improves the electrochromic response times due to shortening of the mean ion diffusion path lengths. However, several important aspects remained unclear, especially the impact of crystallinity on the long-term stability under various operating conditions.
7206 J. Phys. Chem. C, Vol. 111, No. 19, 2007 The present work demonstrates that only the mesoporous highly crystalline WO3 layers exhibit long-term stability under realistic environmental conditions. Furthermore, we find that sufficient crystallinity is needed to sustain a stable electrochemical performance at elevated operating temperatures. Amorphous films, on the other hand, suffer form irreversible performance degradation due to structural modifications. In conclusion, the presented data provide strong support of the general paradigm that three-dimensional mesoporosity must be combined with a fully crystalline inorganic matrix to achieve best performance. Moreover, the results emphasize both the homogeneity of the sol-gel derived thin films and the fact that only the use of suitable structure-directing agents, such as block copolymers of the “KLE” type, allow for the generation of films with the required degree of mesostructural order and crystallinity. Currently, experiments are performed under inert conditions to ascertain the impact of oxygen/water and the film thickness on the overall electrochemical/electrochromic behavior. Further work will be directed toward a more detailed study of the switching kinetics and the electrochromic performance of WO3 materials with hierarchical pore structures.32 Acknowledgment. Funding of this project was provided by the Max-Planck-Society. T.B. acknowledges the support of a DFG postdoctoral fellowship. Supporting Information Available: Switching kinetics of WO3 thin films with nanometer-scale periodicity and varying degrees of crystallinity. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201. (2) Livage, J.; Ganguli, D. Sol. Energy Mater. Sol. Cells 2001, 68, 365. (3) Patra, A.; Auddy, K.; Ganguli, D.; Livage, J.; Biswas, P. K. Mater. Lett. 2004, 58, 1059. (4) Deb, S. Philos. Mag. 1973, 27, 801. (5) Cheng, W.; Baudrin, E.; Dunn, B.; Zink, J. I. J. Mater. Chem. 2001, 11, 92. (6) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. J. Am. Chem. Soc. 2001, 123, 10639. (7) Alexander, B. D.; Augustynski, F. J. Solid State Electrochem. 2004, 8, 748. (8) Judeinstein, P.; Livage, J. J. Chim. Phys. 1993, 90, 1137. (9) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2001, 60, p. 217218 and ref 94, 240, 241, 307 and 309 therein. (10) Walcarius, A.; Etienne, M.; Lebeau, B. Chem. Mater. 2003, 15, 2161.
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