| Facing on the problem of the increasing environmental pollution and energy consumption, great effort should be made imminently to degrade pollutants and exploit new clean energy. Semiconducting materials, in particular, TiO2, are widely used in the field of environmental applications, like photodegradation of pollutants, and H2, O2production, owing to their stability, nontoxicity, and cheap availability, which is also a hot topic in the field of material science. The excitation of TiO2by photons with light energy greater than its band gap is the primary process underlying its vast area of photochemistry and photoelectrochemistry, for its suitable flat band potential that can induce the desired redox reactions without biased potential. However, due to the large band gap of titania(-3.2eV for anatase, and-3.0eV for rutile), it can only absorb the ultraviolet light, leading to low utilization of natural solar light. Besides, massive recombination of photogenerated charge carriers limit its overall photocatalytic efficiency. Therefore, to overcome the limitations of TiO2, in the last few years, significant study has been devoted to the search for modifications, such as noble metal deposition, coupling with other semiconductor, surface sensitization of TiO2by organic dyes or metal complexes, metal ion/nonmetal ion doping etc.Among them, noble metal nanoparticles (such as gold and silver), with special electronic structure, exhibit strong responses to electromagnetic fields because of their strong localized surface plasmon resonance (LSPR). Composite materials through directly depositing metal nanoparticles on semiconductors may represent a promising class of functional materials since strong coupling between the metal nanoparticles and the semiconductor.Based on the design and fabrication of the semiconductor-noble metal (or noble bimetals) nanoparticles, the dissertation has discussed the interaction mechanism of plasmonic noble metal nanoparticles with TiO2in the enhanced photocatalytic activity. The details are as following.(1) Polystyrene (PS)-gold (Au) core-shell nanocomposites with tunable size, high stability and excellent catalytic activity have been synthesized using a facile method that combines the ionic self-assembly with the in-situ reduction. In the method, the size and the coverage of gold nanoparticles (NPs) can be simply tailored by changing the amount of3-aminopropyltrimethoxysilane (APTES), the functionalization time, the protonation time and the amount of chloroauric acid (HAuCl4). Hence, the localized surface plasmon resonance absorption of the Au NPs on the PS spheres can be modulated. Importantly, the method has provided a technical route for design and preparation of supported catalysts.Moreover, the obtained Au NPs with controllable and uniform size on the surfaces of amino-functionalized PS spheres exhibit excellent size-dependent catalytic properties for the reduction of4-nitrophenol (4-NP) by NaBH4.(2) Novel SiO22-Ag-SiO2-TiO2multi-shell photocatalysts with wide-spectral-response were systematically designed and controllably synthesized, where the SiO2spheres were used as the cores, and the SiO2interlayers coated on the Ag nanoparticles (NPs) shells were used to separate the Ag from the TiO2shell to regulate the interaction between the localized surface plasmon resonance (LSPR) of Ag NPs and TiO2. The structures of the SiO2-Ag-SiO2-TiO2multi-shell photocatalysts can be tailored by changing the thickness of SiO2interlayers from1to2,5,8,12, and20nm.The photocatalytic activity tests show the enhanced photocatalytic efficiency under both ultraviolet (UV) and visible lights irradiation is related with the existence of Ag NP shells and the thickness of SiO2interlayers. The complicated coupling mechanisms between TiO2and plasmon are systematically discussed, and a clear physical picture for the complicated coupling processes is presented. The main reasons for the enhancement of photocatalytic activity of the SiO2-Ag-SiO2-TiO2multi-shell structures are the LSPR effect and scattering effect induced by Ag NPs.(3) N-doped anatase Titanium oxide (TiO2) with small Ag core-AgAu alloy shell nanoparticles (NPs), were successfully prepared by a modified seed growth method. According to the visible-light photocatalytic activity, the optimal N-doped TiO2-Ag seeds with the highest photocatalytic effeciency was chosen to prepare TiO2-Ag@AgAu nanocomposites. Changing the added HAuCl4volume, the obtained TiO2-Ag@AgAu nanocomposites with different molar ratio of Ag/Au exhibit prominently different optical properties. The photodegradation results show that the N-doped anatase TiO2has visible light response, and the TiO2-Ag@AgAu with Ag/Au=1:0.2exhibits the highest photocatalytic efficiency. The main mechanisms are relative to (a) the height of Schottky barrier,(b) the interaction between the Ag and Au atoms,(c) the electron inject to N-doped TiO2by the LSPR,(d) the plasmon resonance energy transfer (PRET) to TiO2,(e) the charge transfer to the Ag@AgAu NPs, and (f) the Forster resonant energy transfer (FRET) to Ag@AgAu NPs.In this study, the improved photocatalytic performance of TiO2can be achieved by doping N, and deposition of noble metal(or noble bimetals), for they can expand the light response to visible light, and suppress the recombination of electron and hole pairs. The investigation on the role of plasmonic noble metals in the enhanced photocatalytic activity, has offered experimental and theoretical basis for further improved semiconductors with excellent photocatalytic efficiency. |
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