Enhanced Colouration Efficiency of Pulsed DC Magnetron Sputtered WO3 Films Cycled in H2SO4 Electrolyte Solution
In the present investigation, we report on DC power and pulsing frequency induced changes in electrochromic properties of pulsed DC magnetron sputtered WO3 films by intercalating/deintercalating H+ ions from 0.1 M H2SO4 electrolyte solution. The observed efficient colouration bleaching mechanism of WO3 films confirms the effective electrochromic nature of the films associated with the electrochemical intercalation/deintercalation of H+ ions and electrons into WO3 lattice. The higher optical modulation was observed in the visible region of the optical transmittance spectra of colored and bleached WO3 films. The maximum coloration efficiency of 79 cm2/C was observed the first time for the film deposited at a DC power of 150 W and a pulsing frequency of 25 kHz.
Nowadays, electrochromism is much exploited in commercial automotive markets for regulating the amount of radiation passing through the electrochromic mirrors which are electronically tinted or darkened to reduce the headlight glare . Electrochromism is nothing but exhibiting a change in transmittance when a small potential is applied. It is well known that transition metal oxides exhibit excellent metal-insulator transition behavior and possess electrochromic property. Among the transition metal oxides, tungsten oxide (WO3), an n-type cathodic electrochromic material having empty perovskite type of structure, has been recognized and studied as a promising candidate for electrochromic device and smart window applications due to its high coloration efficiency and better electrochemical stability . Thin WO3 film is visibly colorless or transparent in its oxidized state, that is, in W6+ state, and turns into blue color when it is reduced to W5+ state by applying a negative potential and this property can be reversed back by applying positive potential. This phenomenon can be facilitated when electrons and metal ions M+ (M = H, Li, Na, or K) are intercalated or deintercalated and the electronic structure of WO3 is modified by the upward shift of the Fermi level. Thus, the optical property of WO3 films transforms from transparent to an absorbing nature due to the filling of t2g band of perovskite structure by the excess electrons . This can be explained by the following reaction:
In general, the colouration of electrochromic WO3 films can be explained by the following aspects: electronic transitions between W5+ and W6+ ions , colour center formation at the oxygen vacancies , small polaron absorption , intraband transitions , and conduction band splitting in the presence of injected electrons and electronic transitions between the formed subbands . It is well recognized that the performance of an electrochromic device depends strongly on the overall material morphology, microstructure, and crystallinity, which are in turn related to the technique used for the preparation of active electrochromic electrode thin film. Hence, establishing optimum parameters or a special technique for WO3 thin film preparation may permit getting specific microstructures that are suitable for electrochromic device application .
WO3 thin film can be prepared by a wide number of techniques such as electrodeposition , spray pyrolysis , conventional DC sputtering , electron beam evaporation , and thermal evaporation . In addition, WO3 thin films were grown by chemical vapor deposition (CVD) method using tungsten hexacarbonyl and WF6 precursor solutions [14, 15]. Nagata et al.  have also prepared WO3 films by rf magnetron sputtering using a tungsten metal target. Among them, conventional DC sputtering has been considered a well-established technique and largely used for the deposition of thin films. However, during WO3 film deposition by using DC magnetron sputtering an electrically insulating layer is built up on the surface of the target which leads to arcing due to charge accumulation. In this case, the WO3 target is fused and only sputtered within a small region of the target surface; hence defects in the deposited layer are likely to be created . This problem can be overcome by using pulsed DC sputtering since the accumulated charge of insulating layers is neutralized easily during the change in polarity of a pulse and thereby the reduction in the arcing event. It consequently prevents deterioration of film properties and the deposition process also remains stable . Thus, the defect-free WO3 thin films with superior uniformity, quality, and specific microstructure can be deposited by pulsed DC magnetron sputtering. However, to date, very few reports are available on the preparation of WO3 thin films by pulsed DC magnetron sputtering technique . In the present investigation, we report the effects of pulsing frequency and DC power on electrochromic property of WO3 thin films prepared by pulsed DC magnetron sputtering. Various electrochemical parameters of WO3 films were evaluated and discussed in light of available reports.
Thin films of tungsten oxide (WO3) were deposited by pulsed DC magnetron sputtering technique (Advanced Energy Pinnacle Plus Pulsed DC Power Supply) using WO3 target. Powder of tungsten oxide (Aldrich; 99.999% purity) was uniaxially pressed at 20 MPa (2 inch dia.; 5 mm thick) and sintered at 1000°C for 8 hrs. The sintered WO3 pellet was used as a target to deposit thin WO3 films on precleaned fluorine doped tin oxide (SnO2:F) coated glass substrate. The distance between the target and the substrate was kept fixed at 9 cm. After achieving a base pressure of mbar, ultrapure (99.999%) argon gas was introduced with a flow rate of 27.4 sccm and the work pressure was maintained at mbar during sputtering. The films were deposited at room temperature (RT) by varying the DC power such as 50, 100, and 150 W and by altering the pulsing frequency, namely, 25, 50, and 100 kHz.
The electrochromic properties of WO3 films were studied by cyclic voltammetry technique using electrochemical analyzer/workstation (CH Instruments Inc., USA; Model: 604D) with a standard three-electrode configuration consisting of the sample (WO3/SnO2:F/glass) as the working electrode, Ag/AgCl as a reference electrode, and Pt counter electrode. Pulsing frequency and DC power induced changes in optical property of coloured and bleached WO3 films were measured by using a UV-Visible-NIR spectrophotometer (Ocean Optics HR 2000) in the wavelength range of 300–1000 nm.
3. Results and Discussion
The electrochromic behaviour of pulsed DC magnetron sputtered WO3 film was tested by intercalating/deintercalating H+ ions through three-electrode electrochemical cell using an electrolyte containing 0.1 M H2SO4 electrolyte solutions. The ion intercalation (i.e., coloration) and deintercalation (i.e., bleaching) processes were noted during the cycling of different scan rates, like 50, 100, and 150 mV/s. The current resulting from these scan rates is cathodic spike current (), which is associated with the coloring process of the film, and the anodic peak current () is associated with the bleaching process. The cyclic voltammograms of the films were recorded in the potential range from −1.0 to +1.0 V. During the intercalation of the ion, that is, in negative potential of the scan, the films have changed their colour into dark blue (at −1.0 V) and returned to their original colour in the positive potential, that is, while the deintercalation of ions (at +1.0 V). This is attributed mainly to the electrochemical process involved in the reaction represented by the formation of “tungsten bronze” according to (1). The clearly observed colouration bleaching mechanism of pulsed DC magnetron sputtered WO3 films confirmed the perfect electrochromic nature of the films associated with the electrochemical intercalation and deintercalation of H+ ions and electrons into WO3 lattice, which emphasized its suitability in electrochromic devices. Figures 1, 2, and 3 show the pulsing frequency induced variation in cyclic voltammograms of WO3 films cycled in 0.1 M H2SO4 electrolyte solution deposited at various DC powers of 50, 100, and 150 W, respectively. Each sample was cycled at different scan rates such as 50, 100, and 150 mV/s. It is observed from Figures 1–3 that the magnitudes of both anodic peak current and cathodic spike current increase with the increasing scan rates and DC power (during film preparation), which revealed that the intercalation/deintercalation of H+ ions is enhanced with the increasing DC power and scan rates. Thus, the WO3 films can be reversibly made transparent by electrochemical oxidation and colored by reduction in a proton containing solution according to (1).
The extent of the intercalated and deintercalated H+ ions can be studied by calculating the effective diffusion coefficient () by Randle-Sevcik equation : where is the peak current ( and ), is the diffusion coefficient, is the concentration of active metal ions in the electrolyte, is the scan rate, and is the number of electrons involved in the process. Table 1 shows the DC power, pulsing frequency, and scan rate induced variations in peak current and diffusion coefficient values of WO3 films cycled in 0.1 M H2SO4 electrolyte solution. It is observed that the diffusion coefficient varies from to cm2/s. It can be mentioned that Patil et al. reported that the diffusion coefficient of WO3 films cycled in H2SO4 electrolyte is of the order of 10−10 cm2/s . In addition, they have suggested that the values of WO3 films vary in the range from to cm2/s, depending on the preparation technique of the films . Hence, the evaluated diffusion coefficient values in the present work are in accordance with reported values [19, 20].
The changes in optical transmittances of colored and bleached WO3 films cycled in 0.1 M H2SO4 electrolyte solution were studied by UV-Vis-NIR spectrophotometer and the corresponding transmittance spectra are shown in Figures 4, 5, and 6 for the films deposited at various DC powers such as 50, 100, and 150 W, respectively. The insertion of H+ ions changes the transmittance from near ultraviolet up to the near infrared range and the reversible colour of the film from transparent to blue. In addition, the observed optical transmittance spectra possess higher optical modulation in the visible region and lower optical modulation in the infrared region. This can be attributed to the maximum optical absorption of WO3 films in their intercalation and deintercalation process in the visible range. From these marked variations between the transmittance spectra of coloured and bleached states, it is observed that all the films show good electrochromic colouration. The transmittance in the visible range has been found to be significantly different for the films in bleached and coloured states though the shape has no apparent change. This effect could make them useful in electrochromic device and smart window applications. In order to have better insight into the role of pulsed DC magnetron sputtered WO3 films in electrochromic device applications, we have attempted to evaluate the optical density (OD) and colouration efficiency (CE) of the films from transmittance spectrophotometry using the formula [21, 22] where and are the bleached and coloured transmittance, respectively. The coloration efficiency is given by where (mC/cm2) is the charge injected during the colouration cycle. Generally, tungsten oxide is known to have optical absorption maximum closer to the human eye sensitivity maximum. Hence, we would like to study spectral transmittance of WO3 films in the visible region ( nm). Table 2 shows the evaluated OD and CE values of pulsed DC sputtered WO3 films as a function of DC power and pulsing frequency. It is seen that the evaluated optical density varies between 0.056 and 0.211 and the colouration efficiency lies in the range of 37 to 79 cm2/C. The colouration efficiency of the films increased with increasing DC power during film preparation. However, for a particular DC power, the CE was reduced with increasing pulsing frequency and the maximum colouration efficiency of 79 cm2/C was observed for the film deposited at DC power of 150 W and pulsing frequency of 25 kHz. As mentioned above, the electrochromic performance of WO3 film depends on the crystal structure and morphology. If the film is less dense, the metal ions are easily injected and extracted from the film surface and these films can possess a better performance in the electrochromic studies . Hence, in the present work, the electrochromic response of pulsed DC sputtered WO3 films is strongly influenced by the pulsing frequency and DC power which in turn decide the structure and morphology of the deposited films. Several authors have reported the electrochromic properties WO3 films deposited by different techniques and cycled in various electrolyte solutions. For instance, Patil et al. reported that the colouration efficiency of spray deposited WO3 films varies between 40 and 56 cm2/C . In our earlier work, we have observed the colouration efficiency of 8 to 32 cm2/C for the electron beam evaporated WO3 films cycled in 0.1 M H2SO4 electrolyte solution . In addition, we have reported the maximum colouration efficiencies of 12 cm2/C and 15 cm2/C (at 633 nm) for the electron beam evaporated WO3 films cycled in 0.1 M KCl and 0.1 M LiClO4·PC electrolyte solutions, respectively . Further, Sun et al.  observed the maximum colouration efficiency of 42 cm2/C at 633 nm for the Li+ ion (LiClO4·PC electrolyte solution) intercalated reactive dc pulse sputtered WO3 films. Kitao et al.  reported the highest colouration efficiency of 60 cm2/C at 600 nm for the film prepared at the substrate temperature of 60°C. Based on the literature, to the best of our knowledge, the colouration efficiency of 79 cm2/C observed in the present work is the highest one for any WO3 film cycled in H2SO4 electrolyte solution.
In summary, this paper describes the pulsing frequency and DC power induced changes in electrochromic properties of pulsed DC magnetron sputtered WO3 films by intercalating/deintercalating H+ ions using three-electrode electrochemical cell configuration. The colouration bleaching mechanism of WO3 films confirm the perfect electrochromic nature of the films associated with the electrochemical insertion and extraction of H+ ions and electrons into WO3 lattice. The optical transmittance in the visible range has been found to be significantly different for the WO3 films in bleached and coloured states, which could be useful in electrochromic device and smart window applications. The maximum coloration efficiency of 79 cm2/C was obtained for the film deposited at 150 W and pulsing frequency of 25 kHz. Hence, it may be concluded that the pulsed DC magnetron sputtering can be used as a potential technique to grow device quality WO3 films with superior electrochromic performance.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
One of the authors (R. Sivakumar) gratefully acknowledges the Department of Science and Technology (DST), Government of India, New Delhi, for the financial support through Science and Engineering Research Council (SERC)—Fast Track Scheme for Young Scientists (Ref.: SR/FTP/PS-32/2009, dt. 11.10.2010). This work has been carried out under this Fast Track Project Scheme in Physical Sciences.
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