Study On The Preparation And Properties Of Electrochromic WO₃-MoO₃ Films On Flexible Substrates By Sol-gel Method

Electrochromic materials could be applied widely in the field of the military camouflage and solor energy usage due to its changing of colour and heating absorbtion with the applied voltage. Presently, the study on the electrochromic devices based on plate glass is active, and some products are on sale. However, it is difficult for these devices to use in the weapons equipments with complex surfaces because of their frangibility and poor toughness. Therefore, it is an important significance of developing the plastic electrochromic divice. In this paper, the preparation technologies about the corresponding materials for the plastic electrochromic divice were devoped, and the electrochromic cell sample with the laminated structure of PET/ITO/WO₃-MoO₃/PMMA-PC-LiClO4/TiO₂/ITO/PET was fabricated. The electrochromic mechanism and dynamics of the WO₃-MoO₃ hybrid oxide films were discussed.Tungsten powder, molybdium powder and hydrogen peroxide were used to prepare the aquous WO₃, MoO₃ and WO₃-MoO₃ sols. The stability and film formability of the sols on the PET/ITO substrates were studied. It is shown that the optimizing ratio to prepare WO₃ sol is W(g):H₂O₂(mL):C₂H₅OH(mL):HAc(mL)= 1:5.5:5:1.3. It is better for the hybrid sol to be saved at low temperature and the film to be fabricated at a moderate temperature in a dry atmosphere. The stable time of the WO3 sols with the addition of MoO₃ sols becomes longer because the potencial wall of the hybrid sols increases with the repelling potential increasing and the suction potential decreasing.The compositions and structures of the WO₃-MoO₃ films were analysed by the means of FT-IR, TG-DTA, XRD, SEM et al.The results show that the deviating temperature of the bonded water and dividing temperature of the peroxide bond of the hybrid gels are equal to those of the MoO₃ gels, while their crystalized temperature is same as the WO₃ gels. The WO₃-MoO₃ films annealed under 250℃is amorphous. The bcc crystal of WO₃·0.5H₂O is formed when the heat treatment temperature is 350℃.When the heat treatment temperature increases to 450℃,several kinds of crystal separated out from the hybrid oxide films, such as WO₃·0.5H₂O,W_0.53Mo_0.47O₃,H_0.56Mo_0.25W_0.75O₃,Mo₉O₂₆ and so on.Effects of the heat treatment temperature, thickness, composition and substrate materials of the hybrid oxide films on the electrochromic properties were studied by the spectrophotometer and the electrochemical workstation. The electrochromic mechanism and the dynamic process were also analysed by XPS. It is shown that the substrates have little effect on the properties of modulating the visual light, but the threshold potential of the film based on PET/ITO is higher than that of Glass/ITO slightly. With increasing of thickness of the hybrid films, both the transmittance of the prepared films and the threshold potential decrease, while the magnitude of the modulating light and the inserted charge density incease. At the same time, the changing of light transmittance between the coloured film and the bleached film increases at first, and then decreases with the increasing of the molybdenum, and gets to maximum with 30atm% Mo atoms.Only W⁶⁺ appears in the as-prepared WO₃ films made by the sol-gel method. With the addition of molybdenum, the five valences of W and Mo come forth from the as-prepared hybrid oxide film because of the increasing of oxygen cavity. At the coloured state, some W⁶⁺ deoxidize to W⁵⁺ and W⁴⁺,and partial Mo⁶⁺ and Mo⁵⁺ deoxidize to Mo⁴⁺.Different energy absorbance is caused when the inserted electrons hop between the border atoms with border valence, and as a result, the hybrid oxide films become grey. When bleaching, both W and Mo atoms revert to the as-prepared state, and the hybrid oxide film becomes transparent again. The results of the electrochromic dynamic reveal that the control step during the colouring process is the diffusing speed of the inserted lithium ions. The colouring speed is slower than the bleaching speed. The hybrid oxide film remains the colouring state for 48 hours under open circuit, but its electrochromism becomes irreversible.Titanium oxide sol was synthesized with Ti(OBu)₄, ethanol, acetic acid and deion water, which could remain stable for over two months. Nanotitanium oxide films were prepared on the PET/ITO and Glass/ITO substrates by the sol-gel method. The structure and electrochromic properties of the films were characterised and compared. It is shown that the titanium dioxide particles of the TiO₂/ITO/PET film are bigger than those of the TiO₂/ITO/Glass film. With the annealing temperature increasing, the K value of the TiO₂/ITO/PET film increases from 0.56 to 0.97 and the lifetime from less than 50 cycles to more than 10000 cycles, while its intercalation charge density decreases from 14.12mC/cm² to 6.61 mC/cm².Both the TiO₂/ITO/PET film and the TiO₂/ITO/Glass film reveal weak cathodic colouration and strong capability of saving lithium ions. The electrochemical response time and the saving ions capability of the TiO₂/ITO/PET film are matching to those of the WO₃-MoO₃/ITO/PET film. Furthermore, the TiO₂/ITO/PET film remains high transmittance at the state of colouring and bleaching, so it can be used as a count electrode of the electrochromic WO₃-MoO₃ film.The gel electrolyte of the PMMA-PC-LiClO₄ system was prepared by the in-suit polymerization method. Effects of concentration of the lithium salt and content of the MMA monomer on the room conductance of the gel electrolyte were discussed. The results reveal that the room conductance gets to maximum of 1.43×10^-3S/cm when the concentration of LiClO₄/PC is 1mol/L and the content of MMA is 50%.The transmittance of the gel electrolyte is over 85% and the electrochemical property remains stable under 4.5V, so it can meet the requirement of the electrochromic device.The all-solid flexible electrochromic cell sample with the laminated structure of PET/ITO/WO₃-MoO₃/PMMA-PC-LiClO₄/TiO₂/ITO/PET was fabricated in this paper. It is 5×2.5cm² in size,and 0.4mm in thickness. The electrochromic properties of the cell were tested. The transmittance variation between the coloured state and the bleached state is 31% at the wavelength of 600nm and the K value is 0.58. The colouring time and the bleaching time are about 180s and 120s, respectively.