Preperation Of Molybdate And Tungstate Films With Scheelite Structure By Galvanic Cell Method And Its Photoluminescence

Soft Solution Processing (abbreviated as SSP) is an environmentally friendly technique for advanced solid state materials, and it was presented at the end of 20th century. As an important part of SSP, electrochemical technique has been received much attention, because of its some obvious advantages in the preparation of functional films, such as low energy consumption, high yield and one-step formation of the thin films etc. On the basis of electrochemical preparation technique, a simpler, cheaper and more environmentally consciously electrochemical route, i.e. galvanic cell method has been proposed to prepared functional thin film materials.In this thesis, scheelite-type molybdate films (include BaMoO4, SrMoO4, CaMoO4 and Ba1-xSrxMoO4) and tungstate films (include BaWO4, SrWO4 and CaWO4) were prepared directly on molybdenum and tungsten substrates by galvanic cell method at room temperature, respectively. The crystal phase structure, surface morphology, film composition and photoluminescence properties were investigated by means of modern analyses techniques, such as XRD, Raman, SEM, AFM, XPS and PL. The influences of treatment conditions, such as reaction time, concentration of electrolyte, pH value of solution and surfactant etc. on the formation of films were investigated, and the photoluminescence spectra of molybdate and tungstate films were also studied. The main conclusions acquired from the present work as follows: (1) A series of molybdate and tungstate polycrystalline films with scheelite-type structure were prepared directly on metal molybdenum and tungsten substrates by using galvanic cell method at room temperature, indicating the feasibility of galvanic cell method to prepare films.(2) The effect of different treatment conditions on the formation of film have been studied in detail. The results indicate that:①The growth of crystals is faster at the initial stage, with increasing of treatment time, the tetragonal bipyramid structure of the grains is more perfect, and the as-prepared films are denser.②The oriented growth of grains can be controlled by adjusted the concentration of cation. The adequate high concentration of cation is favor to decrease the grain size, but over high concentration of cation may restrict crystal growth on account of lower pH value of the solution.③The surfactant can also control the surface morphology and the densification of films. For example, the grain sizes of BaMoO4 and BaWO4 films decreased after adding SDS, and the grain sizes of BaMoO4 films increased after adding CTAB.④The higher pH value can promote the dissolution of Mo and affect the concentration of ions. Considering the coordination of substrate dissolution and solution reaction, the feasible pH value of solution is about 11-13.⑤The dissolution of metal substrates can be accelerated after adding oxidant such as H2O2 and NaClO, and crystal growth can be promoted accordingly. However, excess oxidant can induce unfavorable crystal growth on substrate and decrease the adhesion of crystals on substrate.(3) Compared with conventional electrochemical method, the driving force of the galvanic cell electrochemical reaction is the difference between an inert counter electrode and the working electrode. Under the micro-current condition, mass transformation is easier, and the grain growth is freer.(4) It is more difficult to prepare CaMoO4 film than that of SrMoO4, BaMoO4, SrWO4 and BaWO4 films. The CaMoO4 film can not be obtained although the electrochemical reaction time is up to 500 h, due to the lower pH value and the lower concentration of Ca2+ ions. After adding adequate oxidants, the dense and well-crystallized CaMoO4 films can be prepared by the galvanic cell method.(5) The relation between the component x of solid solution Ba1-xSrxMoO4 film and /()Ba 2 +B2+S2+C Ca + Cr ratio in the electrolyte solution was investigated, indicating that the /()Ba 2 +B2+S2+C Ca + Cr ratio has an important influence on the crystal phase structure and morphology of the as-prepared films. As the increasing of Sr2+ content, the diffraction angle shifts to higher 2θ, the component x in the films can be controlled by adjusting the /()Ba 2 +B2+S2+C Ca + Cr ratio.(6) The room temperature photoluminescence properties of molybadate and tungstate films were investigated. The luminescence spectra of molybdate films mainly composed of two or three emission peaks; the maximum emission spectra of the films with various compositions are different and the similar results appeared in the films prepared under different treatment conditions. The results indicate that the emission peak of CaMoO4 film is smooth and sharp, while SrWO4 and CaWO4 show single luminescence peaks.