Investigations On Semiconductor-based SERS Involved In Charge Transfer And Their Enhanced Mechanism

Surface-enhanced Raman Scattering (SERS) spectroscopy has become a widely used tool for explorations of interfacial properties and molecular interaction and for characterizations of adsorption behaviors and molecular structure. At present, SERS active substrates have been restricted to some noble metals and transitional metals. Very few studies of SERS on nonmetallic surfaces have been reported. A lack of SERS substrate generally limits the breadth of practical applications for SERS in various materials, especially in semiconductor materials, widely used in electrochemistry, catalysis and other industries. Furthermore, the nature of the enhancement mechanism of semiconductors-based SERS substrate is still ambiguous. Therefore, SERS studies on semiconductors are still necessary for the application of SERS on non-metallic surfaces. This undertaking not only develops novel SERS-active substrates, but also broadens the applicability of Raman spectroscopy, as a SERS technology to investigate the adsorption problems on semiconductor surfaces.In this work, several semiconductor-based SERS and their enhanced mechanism were mostly investigated, and some innovative results were achieved. The main topics and results of this thesis are as follows:1. Contribution of ZnO to Charge-Transfer Induced Surface-Enhanced Raman Scattering in Au/ZnO/PATP AssemblyThe assembly of Au/ZnO/PATP was fabricated through the layer-by-layer self-assembly method, and contribution of ZnO to charge-transfer (CT) induced SERS was mostly investigated. The Au/ZnO assembly has distinct Raman scattering enhancement to the adsorbed PATP molecule, which was attributed to the dominant contribution of the CT from the metal to molecule. The CT mechanism was assumed to involve the direct CT from gold to PATP as well as the indirect CT process assisted by ZnO from gold to PATP. The introduced ZnO results in the elevation of the gold Fermi level and also serves the function of transmitting the electron by its conduction band simultaneously, which augments the CT from the gold nanoparticles to the PATP molecules in the Au/ZnO/PATP system. These were responsible for a larger enhancement of PATP on the Au/ZnO than on the single gold nanoparticles substrate. It is expected that this work will be helpful to design SERS substrates based on semiconductor and to understand chemisorption and reaction mechanisms of molecules on semiconductor materials by SERS.2. Research on TiO₂ Nanoparticles Serving as SERS-active Substrate and Its Enhance MechanismDifferent TiO₂ nanoparticles (NPs) were prepared by a sol-hydrothermal process. In this work, we observed SERS for the molecules adsorbed on TiO₂ NPs for the first time, and proposed the TiO₂-to-molecule CT enhanced mechanism. The results show that TiO₂-to-molecule CT process and the enhancement of Raman signals for the molecules on TiO₂ is largely depend on the intrinsic nature of the adsorbed molecules and the surface property of semiconductor, both the stronger electron attracting ability of para groups of mercapto group bonded with TiO₂ surface and the plentiful surface states of TiO₂ nanoparticles are favourable to TiO₂-to-molecule CT and SERS for the molecules adsorbed on TiO₂. Moreover, the excited line-dependent SERS measurements show that the effective TiO₂-to-molecule CT can be excited only by the excited wavelength with appropriate energy. Based on this work (mechanism), the following study (Part 3, 4 and 5) were continued and deepened.3. Adsorption Study for 4-MBA on TiO₂ NPs by Surface-Enhanced Raman SpectroscopyIn this part, adsorption behaviors of 4-MBA on TiO₂ NPs were investigated by SERS. The binding sites of 4-MBA molecules on TiO₂ NPs are strongly depended on the pH value of the solution used for adsorption. The 4-MBA molecules are bonded to the TiO₂ surface simultaneously through the sulfur atoms and COO? groups at neutral or alkaline pH. However, they are bonded simply through the sulfur atom at acidic pH, which leads to the higher the capability of TiO₂-to-molecule charge transfer and the stronger SERS signals at natural case (pH=6). The 4-MBA molecules still possess high adsorptive stability on TiO₂ at comparatively high temperature (150°C). Moreover, the concentration-dependent experiments show that the saturated concentration for 4-MBA adsorbed on TiO₂ is about 10-3 M at natural case.4. Improved Surface-Enhanced Raman Scattering Properties of TiO₂ Nanoparticles by Zn dopantThe enhancement capacity of semiconductors-based SERS substrate is still considerably weaker than that of metals. Accordingly, researches on improving SERS properties of semiconductor are necessary and should be developed. However, as a compellent, nontoxic and biologically compatible SERS-active substrate, high SERS activity of TiO₂ is needed. To further improve SERS properties of TiO₂ and verify the validity and universality of the observed TiO₂-to-molecule CT mechanism, Zn doped TiO₂ NPs with various content of Zn were prepared and employed as SERS-active substrates. The results show that SERS performances of TiO₂ NPs can be significantly improved by doping with an appropriate amount of Zn. The SERS of 4-MBA adsorbed on 3% Zn-TiO₂, exhibits the highest intensity by a factor of six, as compared with the native enhancement of 4-MBA adsorbed on undoped TiO₂ NPs. Moreover, the higher SERS activity was still observed on the 3% Zn doped TiO₂ NPs at the temperature even up to 125°C. The improved performances of SERS-active TiO₂ NPs can be attributed to the introduction of Zn dopant, they can enrich the surface states (defects) of TiO₂ and improved separation efficiency of photogenerated charge carriers related to surface states in TiO₂, which all are favorable to the TiO₂-to-molecule CT and SERS for the adsorbed molecules.5. Charge-Transfer Induced Surface-Enhanced Raman Scattering on Ag-TiO₂ NanocompositesIn this part, a series of silver-deposited TiO₂ (Ag-TiO₂) NPs with varying content of Ag were prepared by a photoreduction method and were attempted to serve as SERS-active substrates for the first time. We proposed a new CT induced SERS mechanism involved to the synergetic contribution of incorporated Ag and TiO₂ in Ag-TiO₂ nanocomposites. The results show that the surface-deposited Ag on TiO₂ can inject additional electrons into molecules adsorbed on TiO₂ surface through the conduction band of TiO₂ NPs because of plasmon resonance absorption of Ag under incident visible laser, besides the intrinsic TiO₂-to-molecule CT contribution. The two contributions mentioned are responsible for whole SERS intensity of the molecules adsorbed on Ag-TiO₂ NPs. This work is valuable in developing nanosized TiO₂ used as a promising, nontoxic and biologically compatible SERS-active substrate as well as in studying the CT mechanism between Ag and TiO₂ for potential photoelectrochemical applications.