Investigations On Semiconductor-based TiO₂Surface-enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has attracted increasing attention dueto its broad application prospects in various fields such as electrochemistry, surfacescience, catalysis, chemical and biomolecular sensing, since its discovery in1974.SERS has become a widely used tool for the investigation of properties of variousnanoscale structures.At present, SERS-active substrate has been extended into semiconductor materialfield, which previously has been restricted to studies on noble metals and thetransition metals. But right now, SERS research based on semiconductor is still in itsprimary stage, the further deep explores on the diversiform style substrates and theirSERS enhancement mechanism need yet to be developed urgently. Therefore, thefurther deep SERS studies on semiconductors are still necessary. This undertaking notonly can develop SERS theory and novel SERS-active substrates, but also canbroaden the application area of Raman spectroscopy as a SERS technology.In this work, several investigations based on semiconductor TiO2serving asSERS-active substrates were mostly carried out, and some innovative results wereobtained. The main topics and results of this thesis are as follows:1.Influence of noble metal coupling on SERS performances of nanometre TiO2 In this work, traditional noble metal SERS-active substrate (Ag or Au) and novelsemiconductor SERS-active substrate (TiO2) were intentionally combined into thecomposite system for designing a novel SERS-active substrate with their synergeticcontribution (interaction) to SERS effect. The influences of the interaction betweennoble metal and semiconductor on improving SERS performances of nanometre TiO2were mostly discussed. A series of assemblies with4-mercaptobenzoic acid (4-MBA)molecule, TiO2and/or Ag (Au) nanoparticles (NPs) have been fabricated by aself-assembly method. In the sandwich-structure assemblies (TiO2/MBA/Ag(Au) andAg(Au)/MBA/TiO2), the SERS signals of4-MBA molecule exhibit obviousdifference in not only the intensity but also Raman frequency as compared with thatSERS enhancement in the TiO2/MBA, which can be attributed to the introduce ofnoble metal and its interaction/synergistic action with TiO2NPs. SERS enhancementbehaviors of4-MBA in the sandwich-structure assemblies strongly depend on thenatures of noble metal and4-MBA molecule, which can result in an influence on theTiO2-to-molecule charge transfer (CT) and consequent additional electromagneticfield effect. It is expected that this work will be helpful and valuable not only fordesigning semiconductor-based SERS substrates and deeply understandingTiO2-to-molecule CT mechanism, but also for practical application of SERStechnology in the nanomaterial field.2.Improving SERS performance of semiconductor TiO2by transition metal iondoping——a strategy of energy level regulation and contronlSERS performance of semiconductor TiO2nanoparticles (NPs) can be improved bya strategy of transition metal ion doping (energy level regulation and contronl). SERSperformance of semiconductor TiO2NPs can be effectively regulated and controlledby selecting an appropriate transition metal ion (Fe3+, Co2+and Ni2+) and varying itsdoping amount. The improvement of SERS performance of TiO2NPs can be ascribedto the introduce of metal ion dopant, which can come into the crystal lattice of TiO2to replace some Ti4+ions and embed (implant) abundant metal doping energy levels atthe position near by the conduction band bottom in the energy gap of TiO2. Theappropriate metal doping energy levels are favorable to the TiO2-to-molecule CT andsubsequent SERS enhancement of the adsorbed molecules. The intrinsic nature ofmetal ion dopant and its doping amount in TiO2NPs have an important influence onthe SERS enhancement of semiconductor TiO2NPs. For TiO2NPs SERS substrate, anappropriate doping amount of Fe3+, Co2+and Ni2+is0.5%,1%,3%mol, respectively,which can enormously improve SERS properties of TiO2substrate. This work isexpected to be valuable for improving SERS performances and practical applicationof TiO2SERS-active substrate, and also to be helpful for establishing a universalcharge-transfer SERS enhancement mechanism model to explain SERS phenomenaon broad-band-gap semiconductors.3.Detection Study for ciprofloxacin on TiO2NPs by Surface-Enhanced RamanSpectroscopyIn this part, for further exploiting advantages of TiO2NPs served as a new,nontoxic, biocompatible SERS-active substrate and expanding actual applicationscope and detectability of SERS technology in the pharmaceutical analysis field, TiO2NPs with different particle size were synthesized by the sol-hydrothermal process andserved as active substrate for SERS detection of drug molecules (ciprofloxacin, CIP).SERS behavior of drug molecules adsorbed on substrates as well as the interactionbehavior between molecules and substrates was mostly investigated. SERSenhancement of CIP molecules on TiO2NPs is ascribed to the contribution ofTiO2-to-molecule charge transfer (CT). This TiO2-to-molecule CT also can result in aSERS enhancement effect of CIP molecules on the optical vibrational modes of TiO2NPs.The calcination temperature (particle size) of TiO2substrates, adsorption time and pHvalue of solution have an important effect on the interaction between TiO2and CIPmolecules, and their SERS enhancement behaviors. When the calcination temperature of TiO2substrates is450°C, adsorption time is9h and pH value of solution is6, CIPmolecules exhibit largest SERS enhancement. This work not only expands actualapplication scope of TiO2SERS-active substrates, but also establishes foundation fordrugs (especially quinolone antibiotics) detection by SERS technology (providestheoretical and experimental foundations).