Preparation, Structure And Properties Study On Phosphotungstate By Vanadium Doping

Heteropoly acid and its salts are a new type of electrochromic materials. The research about heteropoly acid and its salts is mainly about selective catalysis, photocatalysis, photochromic and electrochromic properties, etc. But electrochromic property is rarely studied. Such material can be written as Polyoxometalate, also can be abbreviated as POM. Take H3PW12O40·xH2O as example, It is Keggin structure which has TD symmetry. Keggin structure can be described as an intermediate XO4(X=P, Si) tetrahedral surrounded by4octahedrons of M3O10(M=W, Mo).Phosphotungstic acid films are prepared by two key aspects including sol-gol preparation and spain-coating process. The particles of phosphotungstic acid relatively is large which can disperse in many organic solution to form sols, The common methods of preparing thin films are spin coating method and spin coating method. Phosphotungstic acid film has excellent physical and chemical properties, such as the chemical catalysis, electronics, electrochromistry. Its redox property is extremely strong which belongs to Bruce acid. The existence of atoms W expands its use of electrochromism.The main content of this paper is preparing a mixture sol with phosphotungstic acid as raw material doped with ammonium metavanadate which provides vanadium source in ethanol and preparing the film with the method of spin coating. With the test methods of SEM, XRD, IR, TG and so on the structure, electrochemical and electrochromic properties of thin films are tested. The influence of different doping, different hydrothermal temperature, different heat treatment temperature on film slao are researched in order to find the optimum doping ratio, hydrothermal temperature and heat treatment temperature and explain its electrochromic mechanism. The film has excellent electrochemical properties such as strong reservoir, strong cyclic stability and better response time.The films made in the method of spin-coating possess better uniformity and smoother surface. Under the annealing treatment at the respective temperature of250℃,350℃,450℃,550℃it illustrates the film after being annealed at350℃has stronger diffractive peaks and few decomposed component as showed in the XRD chart. SEM images of doped phosphotungstic acid showed that globular particles with diameter0.5μm spread the surface of the film while the pure phosphotungstic acid film has obviously large globular particles with diameter about3μm, the electrochemical experiments demonstrate that the smaller diameter has some positive effect. IR spectra showed vibration absorption peaks of W-Oc-W move to low wave markedly which proof that the position of some W atoms in the crystal lattice are replaced by V atoms, XPS results showed before hydrothermal treatment there has been redox reaction while V-Oc-W was formed in the restructuring process during the hydrothermal treatment.The doped sol-gel methods improved the easiness of filming, increased the adhesion strength between the film and the substrate and greatly improved the capacity, the stability and the electrochromic properties of the film. Under the doping rate(nv:nw=3:10), hydrothermal treatment temperature of80℃, the capacity of the film can reach a maximum of120mC/cm2, and has good circle reversibility with response time in about8s. The light modulation(AT) of the film at400nm reach30%which is better than the pure phosphotungstic acid film.With the doping rate nv:nw=3:10, hydrothermal treatment temperature of120℃, the capacity of film is90mC/cm2which is lower than the temperature of80℃, however, the circle stability greatly improved as demonstrated by50times circles, The results showed that the capacity almost kept stable and response time is6s. But the circle stability of the film under160℃can't kept the stablility. In a word, the film of phosphotungstic acid doped by ammonium metavanadate with the doping rate nv:nw=3:10, the hydrothermal treatment temperature at120℃made in the method of spin-coating then annealed at350℃showed better circle stability, high electrical capacity and excellent response time. The whole electrical doule-layer capacitor simulated by Nova software is equivalent to the circuit diagram R(RctQ1)Q2, The Faraday impedance Rct is70Ω under the voltage+1V.