Fabrication Of Functional Material Assembled By Semiconductor Nanocrystal And Their Photoresponse Under Visible Light

Photocatalytic reaction over semiconductor photocatalyst is an advantage process that employing photogenerated hole in valance band and excited electrons in conduction band in semiconductor, which could degrade organic pollutants via oxidation/reduction reaction and generate hydrogen via water splitting. The effectiveness of sunlight-driven photocatalytic processes is mainly dependent on the semiconductor's absorption capacity toward visible light and the quantum yield of photoconversion. However, the traditional photocatalyst, such as TiO2, can not absorb visible light as well as low quantum yield, which limit the development of photocatalysis. Nanostructure architectures assembled by semiconductor nanocrystal with well-defined geometrical shapes would facilitate multilevel absorption toward visible light. The heterojunction in the assembled system can provide a driving force for the seperation of photogenerated charge carriers. In this dissertation, several works have been done as follows:(1) "Mulberry-like" CdSe nanoclusters with well-defined crystallinity have been assembled into vertically aligned TiO2 nanotubes by photo-assisted electrodeposition method to form new architecture for fabricating semiconductor nanocrystal sensitized photoelectrodes. The multiple staked "mulberry-like" CdSe nanoclusters, caused by the synergistic effect of photoetching and electrodeposition, broaden the absorption spectra to entire visible region with a promising behavior of photoinduced charge separation. A photocurrent density of 16 mA cm-2 was achieved under 100 mW cm-2 visible light illumination with 0 V bias potential (vs. SCE). The remarkable photoresponse should be ascribed to the high-quality of 3-D multij unction structure and the driving force for electron transfer in "mulberry-like" nanoclusters.(2) Ultrafine metal nanodots with a size of -2.0 nm in a narrow size distribution are uniformly decorated on graphene sheet through a sonolytic followed by hydrothermal reduction route without extra reducing agent. X-rays photoelectron spectroscopy (XPS) spectra suggest the size-dependent alternation of electronic structure of metal nanodot on graphene sheet, giving rise to unusual electronic and catalytic properties. Graphene sheet grafted Ag@AgCl composite is fabricated by photoreducing AgCl/graphene oxide (GO) hybrids prepared by deposition-precipitation method. UV-vis absorption spectra of Ag@AgCl/reduced GO (RGO) hybrids exhibit strong absorbance in the visible region due to the surface plasmon resonance (SPR) absorption of Ag nanocrystal. Compared with bare Ag@AgCl nanoparticle, a four-fold enhancement in the photodegradation rate toward rhodamine B is observed over Ag@AgCl/RGO hybrids under visible light irradiation.(3) Ag3PO4/graphene oxide (GO) hybrid microsphere was synthesized by a facile deposition-precipitation method. The morphology of the prepared hybrids microspheres could be modulated by the concentration of GO in the starting solution and the degree of oxidation of graphene sheet. In situ assembled Ag3PO4/GO microspheres exhibited almost four-fold enhancement in the photodegradation rate toward rodamine B under visible light compared with that of bare Ag3PO4. The specified photocatalytic experiments revealed that direct hole oxidation and O2·- radicals oxidation governed the photodegradation process over Ag3PO4/GO heteroj unction. This finding indicates that the heterojunction in the hybrids system facilitates charge separation and transfer from photo-excited Ag3PO4, keeping the high reactivity of the photogenerated charge carriers.(4) Novel three-dimensionally nanostructure derived from CdSe/CdS quantum dot andβ-cyclodextrin was synthesized. Supramolecular interactions between the adjacentβ-cyclodextrins anchored on quantum dots are believed to drive the formation of nanocage structure in the frameworks. The novel nanostructure possesses both the optic properties of quantum dots and the molecule recognition ability of cyclodextrins, which allow a highly sensitive determination of bisphenol A at a dynamic range from 0.04 ppm to 2 ppm via significant fluorescence enhancement.The above results illuminated that the well-defined nanostructure architectures assembled by semiconductor nanocrystal would efficiently expand the optical absorption of photocatalyst toward visible light and faciliate the photogenerated charge carriers transfer in the photocatalyst system, as well as keep the photostability of photocatalyst. These studies provide a feasible approach to design photocatalyst with high activity, which would promot the application of photocatalytic technology in pollution controlling.