Electrochemical Synthesis Of Alkaline Earth Molybdates And Their Luminescence

Alkaline earth molybdates are important luminescent materials, which belongs to the scheelite family and have a tetragonal structure with I41/a space group. They have attracted a lot of interest because of their potential applications, such as scintillators, laser-host materials, cryogenic detectors for darkmatter, heterogeneous catalysts, or stimulated Raman scattering material. Thus, efforts on the synthesis of alkaline earth molybdates and improvement of their luminescent properties are significant.However, the preparation of these molybdates was usually carried out by solid-state reaction and self-propagating high-temperature sintering synthesis, which always need high reaction temperature and bring out many problems such as low photoluminescent property and inhomogeneous composition due to MoO3 tendency to vaporize at the synthetic condition. The recently developed wet chemical routes like hydrothermal, microemulsion route, molten salt and so on still need special equipment and consume a large amount of organic additives in some cases. Electrochemical synthesis is an important technique for functional materials in recent years. The advantages of electrochemical techniques over the other techniques are simple manipulation, mild conditions, short synthesis time and environmental friendliness. In this thesis, a serial of alkaline earth molybdates crystallites were successfully prepared by electrochemical technique using molybdenum plate as working and counter electrodes. The influences of processing conditions on the component, morphology and PL intensity were systemically studied. The preparation was carried out by constant current and the formation mechanism of molybdates by electrochemical process and room temperature photoluminescent property were also investigated. The main conclusions acquired from the present work are mainly as follows:1. CaMoO₄, SrMoO₄, and BaMoO₄ crystallites were prepared by electrochemical method using alkaline earth acetates as electrolytes, and the influences of current value and electrolytic solvent on their component, structure, morphology and PL intensity were also studied. Based on the characterization results of X-ray diffraction, scanning electron microscope and photoluminescence, it was observed that the sample prepared at high current possesses high crystallinity. The morphologies of products can be modified by changing the applied current value, but the change regularity is dependent on the species of the product. The PL property was codetermined by the crystallinity and morphology of the products, where high crystallinity and morphologic homogeneity is beneficial to the PL property.The addition of organic solvent into electrolytic solution, such as ethanol, ethyl glycol, glycerol, acetone, can change the crystallinity, morphology, and PL property of the products. With the increase of organic solvent content, the crystallinity of the products increased first and then decreased, but their particle size decreased gradually. For some special molybdates, oriented aggregation of the primary crystallite occurred in the process of electrochemical synthesis due to the effects of the organic molecular. The PL property also depended on the crystallinity and morphology of the products, and high crystallinity, morphologic homogeneity and respect ratio favor high PL intensiy in our e_xperiments. The possible effect mechanism of the solvent content and the crystallization models under electrochemical conditions were proposed from the viewpoint of crystal growth.2. Ca_xSr_1-xMoO₄, Sr_xBa_1-xMoO₄, and Ca_xBa_1-xMoO₄ were prepared by electrochemical method using mi_xed alkaline earth acetates as electrolytes, and the relations between composition ratio of electrolytic solution and the quality of the as-prepared crystallites were discussed. The results indicate that the composition and PL property of product can be easily modified by changing composition ratio of electrolytic solution.3. Rare earth ions and transition metal ions doped alkaline earth molybdates were successfully prepared by electrochemical method, which indicate that the electrochemical technique is an effective method to synthesize doped molybdate and provide a new route to improve their PL property. According to the e_xperimental results, we concluded that the BaMoO₄ doped with La³⁺ ions possess higher green emission intensity due to their increased defects. As for the system of SrMoO₄ doped with Mn²⁺ ions, the PL property depended on the doping concentration of Mn²⁺ ions and crystallinity of the product, which can be easily adjusted by changing the initial content of Mn²⁺ ions in electrolytic solution.