Electrochemical Behavior of TiO2 Nanoparticle Doped WO3 Thin Films

Nanoparticle TiO 2 doped WO 3 thin films by pulsed spray pyrolysis technique have been studied on fluorine tin doped (FTO) and glass substrate. XRD shows amorphous nature for undoped and anatase phase of TiO 2 having (101) plane for nanoparticle TiO 2 doped WO 3 thin film. SEM shows microfibrous reticulated porous network for WO 3 with 600 nm fiber diameter and nanocrystalline having size 40 nm for TiO 2 nanoparticle doped WO 3 thin film. TiO 2 nanoparticle doped WO 3 thin film shows ∼95% reversibility due tomay be attributed to nanocrystalline nature of the film, which helpful for charge insertion and deinsertion process. The diffusion coefficient for TiO 2 nanoparticle doped WO 3 film is less than undoped WO 3 .


Introduction
The electrochromism is key to green technology as it controls the energy of solar flux entering in building, utilizes solar energy, and increases comfort in buildings. Electrochromic materials are able to sustain reversible and persistent changes of their optical properties under the action of a voltage. The electrochromic phenomenon was discovered in tungsten oxide [1] and this material remains the most promising candidate for large-scale uses of electrochromic devices. Their potential applications include several technological areas, such as smart windows, self-dimming rear mirror, electrochromic display, and antiglare rear view mirrors of automobiles [2]. Among a variety of the transition metal oxides, tungsten oxide (WO 3 -x) is found to be the most efficient candidate for electrochromic applications. Granqvist [3] explains the theoretical explanation of electrochromic mechanism and reviews about different nanostructured oxides for electrochromic device and describes the mass fabrication by electrochromic foil technology. The electrochromic performance of tungsten oxide, reversible coloration under double injection of ions and electrons, strongly depends on its nature and microstructure. Nanoscale microstructure could improve the electrochromic performances of tungsten oxide thin film because it is easy for ion to intercalate and deintercalate in thin film. Mesoporous tungsten oxides exhibit superior high-rate ion-insertion performance for their short-diffusion length of lithium and hydrogen ions into electrochromic films [4]. Several researchers have attempted to employ the concept of nanotechnology to improve the drawbacks of thin-film-based electrochromic devices (ECDs) such as contrast, cycling life stability, and slow switching time between the colored and bleached states. The researcher developed transition metal oxide nanowires based ECDs that show a great improvement in coloration efficiency, contrast, and durability [5,6]. These ECDs show promising switching rate, life stability, and contrast. Grätzel [7] used TiO 2 nanoparticles and absorbed organic monomers to improve the switching speed. The coloration and bleaching rates of WO 3 film increase with the addition of the porous conducting metallic cover layer in a liquid electrolyte cell [8]. Park [9] has studied electrochromic properties of cosputtered Au-WO 3 nanocomposite thin films containing high gold concentration (60 mol% Au) and obtained a shorter response time relative to the pure WO 3 thin film. The doping of Au nanocrystals in WO 3 thin films shows fast coloration time which was ten times shorter as compared with the coloration time of the pure WO 3 thin film [10]. Some literature is available on WO 3 films doped with Co, Cr, and Ni for electrochromic properties for ECD [11,12]. He et al. [13] showed that the addition of gold over layer on WO 3 thin film formed by physical vapor deposition resulted in a better change of optical density in electrochromic process. Previously our lab studied [14,15] gold nanoparticles, multiwalled carbon nanotube doped in the WO 3 thin films by pulsed spray for improving EC properties. The small doping of Ni (0.3 at %) in WO 3 thin films improves electrochromic property, but EC property vanishes due to the limited ion diffusivity for higher concentration [16]. The effects of molybdenum (Mo) doping on the electrochromic behavior of spray pyrolyzed tungsten trioxide [WO 3 ] thin films have been studied [17]. Our earlier work of Ti and Nb doping in WO 3 thin films by pulsed spray pyrolysis shows with improved EC properties, but coloration efficiency decreases [18,19]. Very little literature is available on TiO 2 nanoparticle WO 3 thin films. The main goal is to increase coloration efficiency by TiO 2 nanoparticle doping in WO 3 matrix.

Experimental Details
The undoped and TiO 2 nanoparticle doped WO 3 thin films were grown on fluorine doped tin oxide (SnO 2 : F, FTO) coated glass and microscopic glass substrates using pulsed spray pyrolysis deposition technique. The precursor solution used for deposition of the films was prepared by dissolving pure WO 3 powder in ammonia and distilled water and then heated to 90 ∘ C (prior to spray) at which the formation of ammonium tungstate ((NH 4 ) 2 WO 4 ) is completed. Commercial TiO 2 nanoparticle (average particle size ∼25 nm) 4 mg is dissolved in 5 mL trioctylphosphine oxide (TOPO) and stirred for 15 minutes; then, ammonium tungstate was added slowly to this solution. Details of the films synthesis are described elsewhere [20]. In brief, the substrate was held at 250 ∘ C, while an airflow rate of 15 L/min was maintained during deposition. The solution spraying on and off time interval was kept 5 s (50% duty cycle) for both cases. The thickness of all deposited films was about 200 nm. After deposition, the films were annealed in air at 325 ∘ C for 0.5 h.
The structural properties were studied using an X-ray diffractometer (XRD) with Cu K radiation of wavelength 1.542Å. X-ray diffraction data were used to identify the phases present in the films and to find out changes in the lattice parameters, if any. The surface morphology of the films was studied using scanning electron microscopy. The optical measurements were carried out at room temperature using a UV-VIS-NIR spectrophotometer. The transmittance of the films was measured in a wavelength range 200-3000 nm. The electrochemical injection or extraction of electrons and H + ions was carried out using a solution of 0.001 M of H 2 SO 4 electrolyte solution with platinum as a counter electrode and saturated calomel as a reference electrode (SCE). It was confirmed that our data for the undoped films compare very well with the best results reported by other groups [ Undoped WO 3 Figure 1: XRD of undoped and TiO 2 nanoparticle doped WO 3 thin films. [10][11][12]21]. This ensures the quality of our films processing conditions and procedures.

Results and Discussion
The X-ray diffraction patterns of the undoped and TiO 2 nanoparticle doped WO 3 thin films deposited on glass substrate are shown in Figure 1. The XRD pattern for undoped WO 3 films shows no distinct diffraction peaks, indicating the amorphous nature. The boards hump at diffraction angle ∼20-30 degrees is originating from glass substrate. In case of TiO 2 nanoparticle doped WO 3 films, one peak at = 3.346Å is observed, which indexed as (101) of anatase phase of TiO 2 in accordance with JCPDS-ICDD card number 75-1537. The morphology for the undoped and TiO 2 nanoparticle doped WO 3 films was examined using SEM. Figure 2(a) shows SEM image for undoped WO 3 film deposited at 250 ∘ C and annealed at 325 ∘ C on glass substrate. It clearly shows microfibrous reticulated surface morphology throughout the film. The film appears to be of a multilayered fibrous nature, with mean fiber diameters of about 600 nm. This morphology is helpful for ion intercalation and deintercalation of the film. Figure 2(b) shows the SEM image of TiO 2 nanoparticle doped WO 3 thin films. TiO 2 nanoparticle doping ruptures WO 3 fibrous reticulate network and TiO 2 nanoparticles were segregated in the surface of the film to form large clusters with particle size ∼40 nm.
Cyclic voltammograms (CV) were recorded for all the undoped and TiO 2 nanoparticle doped films with linear potential sweep between −500 and +500 mV at a scan rate of 50 mV/s and shown in Figure 3. The arrow indicates the scan direction. When a negative potential was applied, a blue color was observed, indicating the oxide reduction followed by H + ions intercalation. After the reversal of the potential, the anodic current started to flow, corresponding to the deintercalation process. There is a well-defined anodic peak for the film, indicating that the H + insertion-removal process is highly reversible with little hysteresis. The area of the hysteresis curves and the height and position of anodic The diffusion coefficient ( ) of H + ions during intercalation and deintercalation is calculated by using Randles-Sevcik equation [22]: where is the peak current density (anodic peak current density ( pa ) and cathodic peak current density ( pc )), is number of electron, 0 is the concentration of active ions in the solution, ] is the scan rate, and A is the film area. By substituting all the values, the diffusion coefficient at 50 mV/s scan rate is calculated for intercalation and deintercalation processes for the undoped and TiO 2 nanoparticle doped films. The diffusion coefficient is 1.86 × 10 −9 and 1.38 × 10 −11 cm 2 /s for the undoped and TiO 2 nanoparticle doped films, respectively. This indicates that the charge insertion of TiO 2 nanoparticle doped WO 3 films is less than undoped thin film.
The amount of H + ions intercalation and deintercalation with respect to time was carried by using chronocoulometry (CC) studies, which can further be used to estimate reversibility and coloration efficiency. Figure 4 shows charges intercalated/deintercalated versus time transients for the films at ±0.5 V for the step of 20 seconds. In the forward scan, the charges are intercalated into the film by diffusion resulting process in coloration due to reduction of W +6 to W +5 states. In the reverse scan, the intercalated charge is removed from the film, resulting in bleaching due to oxidation of W +5 to W +6 states. The reversibility of the films is calculated by the ratio of charge deintercalated ( di ) from the film into  electrolyte to the charge intercalated ( ) into the film from the electrolyte. The electrochromic reversibility was found to be ∼75% for undoped WO 3 and 95% for TiO 2 nanoparticle doped WO 3 films. The electrochromic properties calculated from CV and CC were listed in Table 1.
The dependence of optical transmittance on wavelength in the 200-3000 nm for the undoped and TiO 2 nanoparticle doped WO 3 films in bleached and colored condition for 10 sec is shown in Figure 5. The maximum transmittance observed was ∼80% at 860 nm and ∼75% at 910 nm for undoped and TiO 2 nanoparticle doped WO 3 films in bleached state, respectively. All the colored films show similar trend as in bleached films; the only difference is that the maximum transmittance value decreases as compared to the respective bleached films. The undoped and TiO 2 nanoparticle doped WO 3 films exhibit above ∼50% optical transmission in the ∼400-1300 nm range and drop abruptly beyond ∼1600 nm. The drop in transmission in near infrared (NIR) region can be attributed to an increased reflectance of the films in NIR region [21,23]. The coloration efficiency ( ) at a particular wavelength correlates with the optical contrast, that is, the change in optical density (ΔOD) with charges intercalated per unit electrode area ( ), and can be expressed as where and are the transmittances of the film in the bleached and colored states, respectively. The coloration efficiency decreases in the case of TiO 2 nanoparticle doped WO 3 films, as expected, indicating that Ti does not take part in the electrochromism mechanism. The values of coloration efficiency at 630 nm for 10 sec are calculated using the above equation for the undoped and TiO 2 nanoparticle doped WO 3 films and listed in Table 1.

Conclusion
In conclusion, TiO 2 nanoparticles doping effects on electrochromic WO 3 films synthesized using pulsed spray pyrolysis deposition technique have been investigated. The fibrous reticulated morphology is converted into granular structure at TiO 2 nanoparticle doping. The TiO 2 particulates were segregated on the surface of the film, for doped film. It is seen that TiO 2 nanoparticles doping can lead to significant surface morphology changes, which plays an important role in electrochromic properties of WO 3 thin films. It was found that the coloration efficiency decreased in case of TiO 2 nanoparticles doped films. This indicates that the charge inserted into TiO 2 nanoparticle doped WO 3 thin film is not sufficient for reducing tungsten species and also observed diffusion coefficient for this film is about ten times less than undoped WO 3 films.