| Because of the special physical and chemical properties, noble metal nanoparticles (NPs) such as Au and Ag, are widely used in many fields including surface enhanced spectroscopy, chemical sensor or biosensor, surface plasmon enhanced photocatalysis and photovoltaic devices and photochromics. As the properties of noble metal NPs depend on their sizes, shapes, and structures, it receives the interests in the fundamental research and the applications to control the synthesis of noble metal NPs. Photocatalytic growth of noble NPs using semiconductors is a facile method preparing noble metal NPs-semiconductor compound substance. Up to now, though the photocatalytic growth has been used to prepare noble NPs-semiconductor compound films, the growth mechanism of noble metal NPs is still an open question, thus the controlled photocatalytic growth of NPs is not realized. In this thesis, the photocatalytic growth of Ag NPs has been systematically studied using TiO2 films. We investigated the factors influencing growth of Ag NPs, the growth mechanism, and the properties of Ag NPs-TiO2 films used as the substrate for surface enhanced Raman scattering (SERS). The main results of this thesis are summarized as follows:(1) Photocatalytic growth of Ag NPs using the fresh TiO2 films:Using scanning electron microscopy (SEM) and in-situ extinction spectroscopy, the evolution of photocatalytic Ag NP growth on the fresh TiO2 film were revealed. Ostwald ripening (OR) was found to be the mechanism dominating the heterogenous growth behavior of Ag NPs on the fresh TiO2 films. By quantitatively determining the growth rate of Ag NPs in the conditions of different irradiation flux and concentrations, we proposed a reduction model to explain the influences of photoinduced carriers and Ag+ ions on the growth rate of Ag NPs. The growth of Ag NPs is found depending on the AgNO3 concentration under a given irradiation flux. When the AgNO3 concentration is lower than a critical value, the Ag NP growth is controlled by the concentration of Ag+ions in the solution, thus the growth rate is increasing with the AgNO3 concentration. When the AgN03 concentration is higher than the critical value, the Ag NP growth is controlled by the irradiation flux, thus the growth rate keeps at a constant, which is determined by the irradiation flux. In addition, we discussed the other factors influencing the growth of Ag NPs., such as the thickness and calcination temperature of TiO2 films as well as the ITO/TiO2 films.(2) Photocatalytic growth of Ag NPs using the aging TiO2 films:The Ag-TiO2 films with the Ag nanoplates were fabricated using the aging TiO2 films and a critical AgNO3 concentration was found necessary for the formation of Ag nanoplates. Based on the study of Ag NP growth behavior, we suggested a phenomenological growth model that includes the heterogenous nucleation on the TiO2 films and the self-nucleation in the solution to explain the growth of Ag NPs on the TiO2 films and the formation of Ag nanoplates. Furthermore, a mechanism of oriented attachment assisted by plasmon resonance and a mechanism of charge accumulation in the special regions other than TiO2 phase were proposed to be responsible for the formation of Ag nanoplates. The phenomenological growth model not only describes the dependence of Ag NP formation on the irradiation flux and the AgNO3 concentration, but also is evidenced by the observation for Ag nanoplates and TiO2 films using SEM, transmission electron microscopy and atomic force microscopy. Based on the above study, we developed a new method to control the photocatalytic growth of Ag NPs using oil-decorated TiO2 films, thus the controlled formation of Ag nanoplates was achieved. The controlled formation of Ag nanoplates using the oil-decorated TiO2 films is not only a solid evidence supporting our phenomenological growth model, but also provides us a new way to control the growth behavior of Ag NPs.(3) Study of Ag-TiO2 films used as the SERS substrates:Using the R6G as the probe molecule, the SERS activity of Ag-TiO2 films, which were prepared using the fresh and the aging TiO2 films, was evaluated. The result indicated that the SERS activity increases with the increase in the growth time of Ag NPs. The enhancement factor was determined to be as high as 106 and found to be related to the roughness of Ag-TiO2 films, thus the enhancement is attributed to the "lightning rod" effect. Using Ag magnetron sputtering, the SERS activity of Ag-TiO2 films was further improved and the SERS enhancement factor was increased by-400. By the SEM and AFM observation, the SERS enhancement is ascribed to the increase in the "hot spots" that are the sub-10nm gaps between Ag NPs. In addition, we study the surface enhanced fluorescence of the Ag-TiO2 films using the RhB molecules as the probe, which was mixed in the TiO2 gel and coated on the surface of Ag-TiO2 films. It is found that the fluorescence intensity was enhanced as high as ?7 times. |
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