Preparation And Visible-light Photochromism Of Hybrid Films Based On Polyoxometalates

Polyoxometalates were entrapped into various inorganic or organic networks to fabricate photochromic materials due to their excellent photoelectric properties and special structure. However, most studies on photochromism of polyoxometalates were based on ultraviolet light irradiation at home and abroad. Visible light occupies most of the energy spectrum of the solar light compared with ultraviolet light, thus it has theoretical and practical significance to extend the light response of these materials to the visible light region.Based on the current situation of little study on visible light photochromism of polyoxometalates, polyoxometalates-based composite thin films with visible-light photochromism could be synthesized through compositing between polyoxometalates and visible-light induced inorganic semiconductor nanoparticles. The interactions between the constitute and structure of composite films and the visible-light photochromic properties were investigated with X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, ultraviolet-visible spectra and transmission electron microscope analysis. According to the charge transfer theory and energy level transition theory, the photo-production electron transfer mechanism in the polyoxometalates-based composite system will be discussed, which provide fundamental basis for the development of visible-light photochromic system. The main research contents were outlined as following:1. BiVO₄/PMo A/SiO₂ composite film was synthesized by introducing the complex of BiVO₄ and PMo A into SiO₂ sol. The basis structure of BiVO₄ was obtained during film-forming and irradiation process. Composite film changed from colorless to blue and the photochromic process was fitted to be first-order kinetics. Electrons were generated by visible light excitation of BiVO₄. PMo A obtained electrons and photoreduction reaction occurred. It can be concluded that the photo-reduction process of BiVO₄/PMo A/SiO₂ composite film occurred according to electron transfer mechanism.2. According to the reversibility of photochromic materials, PWA was used to synthesize BiVO₄/PWA/SiO₂ composite film due to its lower reduction potential. Upon visible light irradiation, the composite films changed from colorless to blue and showed reversible photochromism in the presence of oxygen. Moreover, the bleaching process can finish in short time when heating the colored film and the coloration-decoloration process can be repeated many times. It can be concluded that the photo-reduction process of BiVO₄/PWA/SiO₂ composite film occurred according to electron transfer mechanism.3. Novel photochromic hybrid film was successfully synthesized by introducing Bi₂O₃ into phosphomolybdic acid（PMo A）/Ti O2 system as visible light sensitizer. Bi₂O₃ particles exhibited spherical shape and well dispersed in the composite. The basic structure of Bi₂O₃ was preserved in the obtained film, and the interaction between PMo A and Ti O2 was strengthened after combining Bi₂O₃. Upon visible light irradiation, the composite films changed from colorless to blue and showed reversible photochromism in the presence of oxygen. Moreover, photochromic performance of Bi₂O₃/PMo A/Ti O2 film was better than that of PMo A/Ti O2 film. The amount of PMo A participating in photo-reductive reaction increased after adding Bi₂O₃ into PMo A/Ti O2 composite, which resulted in photochromic efficiencies enhancing.