Study On The Preparation And Visible Light Photochromism Of Nanocomposite Thin Films Based On Heteropolyoxometalates

So far, basic and applied researches on polyoxometallates have attracted widespreadattention; studies on polyoxometallate-oriented functional composites and components havebeen developed increasingly; and new preparation methods of polyoxometallate functionalcomposites are also constantly emerging. Thin film technology, allowing materials ofdifferent types and functions to be synthesized as required, will certainly give thin-filmmaterials newer features. Hence, it is always an interesting and exploring research subject forthe preparation and performance study of heteropolyometalates photochromic compositefilm.For all we know, there is less research about the visible-light photochromic system basedon heteropolyometalates （HPOM） at home and abroad. HPOM-based composite thin filmswith visible-light photochromism could be synthesized through compositing between HPOMand polymer or visible-light induced inorganic semiconductor nanoparticles. The influence ofthe component, content and preparation method on the structure and the visible-lightphotochromic behaviors of composite thin films have been investigated, and revealed theinteractions between the constitute, structure of composite films and the visible-lightphotochromic properties. According to the change transfer theory and energy level transitiontheory, the photo-production electron transfer mechamism in the POM-based compositesystem have been discussed, which provide fundamental basis for the development ofvisible-light photochromic system.I. Reversibly photochromic phosphomolybdic acid-ethyl cellulose （EC） composite filmwas prepared by hydrogen-bond interaction of dipping method, which was of wellphotochromic responsiveness and reversibility. Its structure, photochromism and mechanismwere studied by fourier transform infrared spectroscopy （FT-IR）, UV-visible spectroscopy,atomic force microscope （AFM） and X-ray photoelectron spectroscopy. FT-IR and AFMfindings showed that the phosphomolybdic acid-ethyl cellulose composite film was founddifferences in morphology and micro-structure before and after visible light radiation. FT-IRanalysis showed that Keggin phosphoaluminate molecular structure remained in this film.AFM findings indicated changes in surface topography before and after the phosphomolybdic acid-ethyl cellulose composite film was synthesized and illuminated. In visible light, thiscomposite film became from colorless to blue; UV-visible absorption spectroscopy showedthat electrons transferred from EC to heteropoly anions in the composite film and thephosphomolybdic anions scattered in EC macromolecules were able to be reduced toheteropoly blue under visible light radiation.II. Reversibly photochromic phosphomolybdic acid-polyvinylpyrrolidone （PVP）composite film was prepared by hydrogen-bond interaction of dripping method, which was ofwell photochromic responsiveness and reversibility. Its structure, photochromism andmechanism were studied by FT-IR, UV-visible spectroscopy, AFM and X-ray photoelectronspectroscopy. FT-IR and AFM findings showed that the phosphomolybdicacid-polyvinylpyrrolidone composite film was found differences in morphology andmicro-structure before and after visible light radiation. FT-IR analysis showed that Kegginphosphoaluminate molecular structure remained in this film. AFM findings indicated changesin surface topography before and after the phosphomolybdic acid-polyvinylpyrrolidonecomposite film was synthesized and illuminated. In visible light, this composite film becamefrom colorless to blue; UV-visible absorption spectroscopy showed that electrons transferredfrom PVP to heteropoly anions in the composite film and the phosphomolybdic anionsscattered in PVP macromolecules were able to be reduced to heteropoly blue under visiblelight radiation.III. Reversibly photochromic PMoA/SiO₂/ZnFe₂O₄composite films were prepared bysol-gel method, with PMoA molecules evenly distributed in these composite films. Thesefilms were of well photochromic responsiveness and reversibility. The structures,photochromisms and mechanisms of these films were studied by means of a series of trials.The results showed that PMoA/SiO₂/ZnFe₂O₄composite films were found differences inmorphology and micro-structure before and after synthesization. FT-IR analyses showed thatKeggin phosphomolybdic molecular structure remained in these composite films, which builta charge-transfer bridge between PMoA and ZnFe₂O₄. AFM findings indicated significantchanges in surface topography before and after PMoA/SiO₂/ZnFe₂O₄composite films weresynthesized and illuminated. In visible light, composite films became from colorless to blueand the heteropoly acid was reduced to the heteropoly blue. XPS findings indicated that thevisible light excited the ZnFe₂O₄to generate photoelectrons to lead to a photoreduction ofphosphomolybdic acid to heteropoly blue. The photochromic process in the system wasfollowed by the mechanism of photoelectron transfer.IV. A multilayer hybrid film fabricated from phosphomolybdic acid （denoted PMoA） and polyacrylamide （denoted PAM） by LBL technology shows photochromism under UVlight and visible light irradiation. With atomic force microscopy （AFM） and infrared spectra（IR）, the microstructure and composition of the hybrid films were studied. By means ofultraviolet–visible spectra （UV–vis）, X-ray photoelectron spectra （XPS） and electronresonance spectra （ESR）, the photochromic properties and mechanism of the multilayer filmswere investigated. It suggested that the multilayer films are uniform, dense and highlyadherent to the substrate with UV light and visible light phtochromism.