Study On Liquit-junction Nanocrystalline Titanium Dioxide Film Ultraviolet Detector And Its Photoelectric Properties

Nanocrystalline TiO₂ film, with advantages of simple process, stable physical and chemical properties, large surface area, high photoelectric activity, is a ideal material for making low cost ultraviolet detector with high sensitivity. According to its characteristic of selective absorption for UV light, a nanocrystalline TiO₂ film liquid-junction UV detector was prepared using sol-gel method in this work, and its photoelectric properties were also studied together with theory analysis. The major works are shown as follows:Nanocrystalline TiO₂ was prepared by sol-gel method. Thermodynamics research indicated that the relative stability of the nanocrystals with different crystal phase is related to their grain size. When the grain size was smaller than 21 nm, the nanocrystals in anatase with higher photoelectric activity was more stable than the one in rutile. Therefore, in order to prepare nanocrystalline TiO₂ film electrode with high photoelectric activity, a grain size less than 21 nm must be guaranteed. In addition, phase transition siganificantly improved the grain boundary energy and increased the difference between adjacent grains, which leaded to a remarkable increase on grain growth rate.The TiO₂ film electrodes were prepared by sol-gel method on ITO conductive glass using roll-coating, spin-coating and dip-coating methods, respectively. It can be seen that dip-coating method is the only one that can obtain uniform, firm and completely transparent TiO₂ film. The prepared film was composed with approximately spherical grains with about 50 nm in size, and the primary particle size of the grains is about 15 nm. A strong UV absorption from 200 to 378 nm for the TiO₂ film could be observed, but no absorption is observed at wavelengths longer than 378 nm. When illuminated with 320 nm UV light, the electrode showed the biggest photocurrent, and its intensity was proportional to electrode potential. The study on relationship of calcine temperature and film thickness with the electrode photoelectric activity indicated that anatase film electrode with high photoelectric activity could be obtained at 450℃. But when the temperature was higher than 450℃, anatase to rutile phase transition took place, which resulted in a gradually decrease on electrode photoelectric activity. The increase on film thickness could enhance the UV absorption of electrode and produce more photogenerated carrier. However, after the times of coating film was more than 20, the increase on recombination rate of the photogenerated electron-hole pairs leaded to the decrease on photoelectric activity of electrode.The TiO₂-Zn film electrode was prepared by Zn²⁺ doping using sol-gel method. An increase of 40 percent on photoelectric properties of the electrode was observed compared with undoped one. This can be attributed to the inhibited effect of electron traps produced by Zn²⁺ to the recombination process of photogenerated electron-hole pares. In addition, it was observed for the first time that 0.1 mol% Zn²⁺ doping could make the grains grow in a preferred orientation on the film, and formed cylindrical grains perpendicular to the surface of the film.In basic electrolyte, the photoelectric properties of nanocrystalline TiO₂ film electrode prepared by the sol-gel method were investigated under different electrode potential. Two kinds of surface states, deep surface state O_L^- (S_c) formed bynon-protonic lattice oxygen ion and shallow surface state Ti-O(-|¨)O(-|¨)O^- (S₀) formed by chemisorbed O₂, were formed on the hydroxylated surface of TiO₂ particles. The remarkably change of the relative position between Fermi level of TiO₂ (E_Fn) andsurface state level caused by the variation of electrode potential could significantly affected the electron-trapping probability of surface states. According to this result, a simple model, the transfer property of photogenerated carriers in alkaline electrolyte varied with electrode potential, was built up, which would be favorable for analyzing the transfer and recombination properties of photogenerated carriers in nanocrystalline TiO₂ film electrodes.By comparing with various electrolyte, a mixture aqueous solution of 0.4 M phthalic acid monopotassium salt (C₈H₅KO₄) and 0.4 M sodium hydroxide (NaOH) was used as electrolyte, and its pH value is 12.0. Under the conditions of 0.6 V bias potential and 320 nm UV illumination with power density of 0.56 mW/cm², the as prepared UV detector presented a maximum photocurrent density of 44.85μA/cm², and the intensity rate of photocurrent and dark current is 85. The produced photocurrent is so stable that no obvious change could be observed during the 200 s measure time, and its response time is less than 1 second, showing excellent UV photoelectric property.