P. V. Ashrit

Study of sol-gel prepared nanostructured WO3 thin films and composites for electrochromic applications

Abstract Nanostructured tungsten oxide (WO 3 ) thin films have been prepared by the sol–gel method following the inorganic route in which alcoholic solutions containing tungsten salts have been used as precursors. The WO 3 films were deposited by dipcoating on glass substrates. In order to create nanoporosity in some of these films, the films were deposited on glass substrates coated with a layer of polystyrene (PS) microspheres. This polymer was then removed either by thermal treatment or by solvent extraction, generating a high density of pores within the tungsten framework. The characterization of these films has been carried out by atomic force microscopy (AFM), optical studies and ATR–FTIR spectroscopy. The lithium intercalation of these WO 3 films has been carried out using a dry (in vacuo) technique. Results on the comparative study of the optical and electrochromic properties of these different types of WO 3 are presented here.

Electrolyte thickness dependence of the electrochromic behavior of ‘‘a‐WO3’’ films

The electrochromic behavior of all‐solid‐state device, Au/a‐WO3/LiF/Au has been studied as a function of the thickness of LiF as electrolyte. Thickness‐dependent coloration and response time have been observed. The electrochromic coloration takes place at different stages, and a significant change in the transmittance of the tandem over a wide wavelength range (500–1000 nm) occurs. Hence, the performance of this configuration suggests the possible application of the tandem in all‐solid‐state ‘‘switchable windows’’.

Study of Anatase to Rutile Phase Transition in Nanocrystalline Titania Films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel dip-coating process. The effect of acetylacetone, diethanolamine and polyethylene glycol on the structure of the heat-treated titania films was examined by Raman and FTIR spectroscopy and X-ray diffraction. The effect the complexing agents have on the anatase to rutile phase transition during the heat treatment process is studied. The understanding of this effect is expected to enhance our capacity to tailor the composition and morphology of films and thus their properties. The Raman and the infrared spectra of nanocrystalline titania films and the changes induced by the heat treatment were also investigated. We have also studied the size of the crystallites in TiO2 films and its dependence on the type of complexing agent used.

Low Temperature Sol-Gel Preparation of Nanocrystalline TiO2 Thin Films

TiO2 nanocrystalline thin films with varying degree of porosity have been prepared using a low temperature method. TiO2 films of the anatase form have been obtained by using a polyethylene glycol (PEG) modified sol-gel method. Densification and crystallization of the films was found to result from the thermal treatment of the dip coated films in boiling water. The films have been characterized by Raman, XRD, FTIR, AFM and optical methods. Highly transparent films with transmission in excess of 85% and porosity as high as 58% are formed predominantly of anatase crystallites of dimensions of the order of 5 nm. Initial results on lithium intercalation into these films resulting in an efficient optical modulation in the visible and near infrared regions demonstrate a good potential of these films for electrochromic applications.

Novel electrochromic devices based on complementary nanocrystalline TiO2 and WO3 thin films

Electrochromic devices were elaborated based on two complementary electrodes made of a nanocrystalline metal oxide thin film deposited on conducting glass. The first electrode holds a 5 μm thick nanocrystalline TiO2 film derivatized by a monolayer of a phosphonated triarylamine which can be rapidly oxidized by electron transfer to the conducting support followed by charge percolation inside the monolayer. The oxidation in accompanied by a blue coloration due to the absorption band at 730 nm of the stable triarylamminum radical cation. The second electrode bears a 0.2 μm thick nanocrystalline WO3 film which turns from colorless to blue by reduction and lithium ion insertion. The former electrode reaches an absorbance of at least 3 between 700 and 730 nm after full oxidation (16 mC/cm2) at 1.0 V vs. NHE while for the second, complete reduction at −1.3 V (74 mC/cm2) leads to A=2.4 at 774 nm. An electrochromic device comprising both electrodes separated by an electrolytic solution of 0.1 Li+ in 4,7-dioxaoctanitrile reaches an absorbance of 2.2 at 700 nm, 4 s after a voltage step to 1.5 V. The system was shown to sustain at least 14400 coloration-discoloration cycles without degradation.

Tunable electrochromic photonic crystals

Photonic crystals based on the electrochromic phenomenon have been fabricated and proposed for band gap tuning. Electrochromic tungsten trioxide (WO3) inverse opals have been fabricated by polystyrene colloidal crystal templating. The WO3 matrix was obtained through a dip-infiltrating sol-gel process, with subsequent removal of the polymer microspheres by calcination. Scanning electron micrographs confirm the ordering of the hexagonal macroporous structure. The reflection spectra show two pronounced Bragg diffraction peaks. By inserting lithium into the crystals, the first reflection peak shifts gradually toward shorter wavelength for 36 nm, while the second reflection peak shifts toward longer wavelength for about 28 nm. This should be of great interest for photonic device applications.

Near-zero IR transmission in the metal-insulator transition of VO2 thin films

Vanadium dioxide films have been prepared with different thicknesses using radio-frequency magnetron sputtering technique followed by postdeposition annealing in oxygen ambient. Films with a thickness of 300nm show a switching efficiency of ∼74% and most importantly with a near-zero infrared transmission in the switched state. As the film thickness decreases, the inherent transmission in the switched state increases along with reduced switching efficiencies.

Sol-gel-prepared ITO films for electrochromic systems

Abstract Indium tin oxide (ITO) films have been prepared by the sol-gel method using both organic and inorganic precursors. A computer-controlled dip-coating unit is designed and fabricated in our laboratory for a precise control of the parameters during the dip-coating process. These films have been characterized by X-ray diffraction optical and electrical study and also by atomic absorption spectroscopy. The optimized coatings exhibit a sheet resistance of ara und 100 Ω/□ and an average visible solar transmission of around 85%. A five-layer electrochromic system using these ITO layers as transparent electrodes was fabricated and tested. The performance of the electrochromic system indicates the high potential of these films for such applications, especially for large area coaling.

Synthesis and Characterization of Macroporous Tungsten Oxide Films for Electrochromic Application

Macroporous tungsten oxide films have been prepared by combining a nonhydrolytic sol-gel method with a molecular assembly templating strategy. The material has been prepared by hydrolysis of an ethanolic solution of tungsten ethoxide in the presence of polyethylene glycol (PEG), followed by calcination of the dip-coated films. AFM images indicated that an important morphological diversity can be obtained by simply varying the amount of PEG in the coating solution and the conditions of the heat-treatment. The formation of nanostructures of controlled shapes and patterns (fibrils or striped phases) with relatively uniform channel spacings is accounted for by strong hydrogen bonding interactions between the PEG and the partially hydrolyzed tungsten oxide oligomers. XRD and FTIR data showed that PEG delays the crystallization of WO3. When compared to sol-gel prepared tungsten oxide fims prepared without PEG, the coloration efficiency of the macroporous films appears to be significantly improved especially in the near-infrared region.

Dry lithiation study of nanocrystalline, polycrystalline and amorphous tungsten trioxide thin-films

Abstract The electrochromic (EC) performance, reversible coloration of certain materials under double injection of ions and electrons, of transition metal oxides (TMO) such as tungsten trioxide (WO3) depends strongly on the nature and structure of these materials. In this work, the electrochromic performance of tungsten trioxide films deposited under different conditions leading to nanocrystalline, polycrystalline and amorphous films has been studied by dry lithiation method. A comparative study of the EC coloration of these three types of films has been carried out. The techniques of spectrophotometry and atomic force microscopy have been employed for this study. Each type of tungsten trioxide (WO3) film exhibits a special nature of coloration indicating the potential for its specific application. The nanocrystalline films seem to exhibit a higher overall selective optical modulation, more efficient in the near infrared region and less in the visible, compared to the polycrystalline or amorphous films. These films also exhibit a superior normal state transmission in the clear state. Hence, these films may be of more interest for smart windows from the point of view of energy efficiency due to their highly selective optical modulation. The amorphous films and even the polycrystalline films under high degree of lithiation, may be more suited for large and small area display device application due to their efficient coloration in the visible region of the spectra.