Study On The Preparation, Photochromism, Electrochemiluminescence And Photocatalytic Activity Of Prevoskite-type Composite Oxides

Perovskite-type oxides of ABO₃ have been extensively studied in recent years because of their unique physical and chemical properties. In this thesis, perovskite-type oxides of ABO₃ with different compositions and adjustable properties were taken as the subject and a series of ABO₃ and their doped composite oxides were successfully designed and synthesized by solid state reaction or soft-chemistry method. The obtained products showed good photocatalytic and photochromic properties. The influence of crystal structures, compositions and surface configuration of ABO₃ composite oxides on their photocatalytic activity was studied, and the electrochemiluminanscent property of electrode modified with silver-doped Perovskite-type composite oxides the was also explored. The thesis mainly focused on:Perovskite-type composite oxides of AFeO₃(A=Ce,Pr,Nd,Sm,Gd,Dy,Er,Y) and ANiO₃ (A=Nd,Gd) were successfully synthesized by the Stearic acid sol-gel method at a relatively low temperature (500℃-800℃).The obtained products showed good crystallinity, high BET surface area and small average size. The photocatalytic property of ABO₃ and doped Nd_xFe_1-xNiO₃ composite oxides was evaluated, using photodecomposition of methylene blue as the model system and an XPA photochemical reaction meter. The results indicated that ABO₃ composite oxides have good photocatalytic activity and the inverse proportion of Fe/Ni has quite effect on its photoactivity in Nd_xFe_1-xNiO₃ composite oxides. In comparison with Nd_xFe_1-xNiO₃,NdFeO₃ showed better photocatalytic activity, and the decomposition rate of methylene blue was up to 99% within 90 min.The ceramic powder of perovskite type,La_1-xAg_xTiO_3+λ,has been synthesized by solid state reaction, sol-gel and solvothermal methods. The influence of synthetic methods and doping content of Ag on the size, surface composition and shape of the samples were investigated. It was found that the doping of Ag^(â… ) played a key role in the formation of perovskite-type structure because Ag^(â… ) was helpful to achieve the charge equilibrium for La₂O₃-TiO₂ system. Moreover, the doping of Ag^(â… ) [x = 0.08-0.1 (molar ratio)] can lead to the formation of Ag-La_1-xAg_xTiO_3-λ with unique perovskite-type structure at lower temperature and shorter reaction time. However, small amounts of Ag was found to co-exist with La_1-xAg_xTiO_3-λ when the doping content of Ag^(â… ) is more than 0.1, while reactants were found when the doping content of Ag^(â… ) is less than 0.08. So the amount of doping Ag is limited. The UV-Vis spectra of the obtained Ag-La_1-xAg_xTiO_3+λ.powders were studied and the band gaps of each sample were estimated based on Kubelka-Munk Function. The results showed that Eg decreased in the sequences of Eg_(Ag0.1-LT3) > Eg_(Ag0.1-LT2) > Eg_(Ag0.1-LT1)Eg_(Ag0.08-LT3)> Eg_(Ag0.06-LT2) > Eg_(Ag0.6-LT1);Eg_(Ag0.1-LT3) > Eg_(Ag0.08-LT3).It was found that Ag-La_1-xAg_xTiO_3+λ powders showed good photochromic property under ultraviolet light irradiation in air at room temperature. The effect of synthetic methods and doping content of Ag on the photochromic property of Ag-La_1-xAg_xTiO_3+λ powders was investigated. The results indicated that, for the samples prepared with the method, the more amount of Ag was doped, the deeper color of Ag-La_1-xAg_xTiO_3+λ was observed under UV radiation. The photochromic phenomenon of Ag-La_1-xAg_xTiO_3+λ is presumably due to the interconversion between Ag⁺ and Ag.The Cyclic voltamrnograms (CVs) behavior and electrochemiluminescent property of La_(2-x)/3Ag_xTiO_3+λ were explored, using La_(2-x)/3Ag_xTiO_3+λ modified glass carbon electrode. The data showed that La_(2-x)/3Ag_xTiO_3+λ obtained with different synthetic methods and different doping contents of Ag had different Cyclic voltammograms behaviour, indicating that the synthetic methods and doping contents of Ag have sonsiderable influence on the shape and content of Ag on the surface.La_(2-x)/3Ag_xTiO_3+λ modified glass carbon electrode also displayed electrochemi- luminescent property. This phenomenon may be caused by the substitution of Ag on the surface, particle size, surface composition and shape, etc. The electrochemi- luminescent mechanism of this system needs to be further investigated.The nanocrystalline titania with regular channel structure was synthesized, using a cetyltrimethylammonium bromide and nonsurfactant glycerol as a mixed structure-directing agent, by solvothermal method. Combination of the mixed structure-directing agent and solvothermal method resulted in the formation of nanocrystalline titania (HT2) with smaller size, higher crystallinity, better dispersibility, narrower particle distribution and much larger surface area. The experiment results showed that the average size of HT2 is 5-10 nm,the pore diameter is 4.2-6.2 nm and BET surface area is ca.285.3 m²Â·g^-1.Possible reason is that the combined structure-directing agents prevent the near crystals from congregating to each other effectively during the crystal growth and calcinations process, this is very helpful to obtain regular channel structure and small sized TiO₂ nanocrystals.