Investigation Of The Carrier Density Effect Of Metal Oxide Nanomaterials Using Surface-enhanced Raman Scattering

Surface-enhanced Raman scattering(SERS)is a phenomenon that the Raman signal is greatly enhanced when molecules are adsorbed on or near the surface of the substrate Since SERS was discovered,SERS has attracted more and more research interests and has been widely applied in many fields due to its incomparable advantages.As is known to all,the enhanced substrate is particularly important for SERS,and every major breakthrough in SERS field is inseparable with the development of enhanced substrate;Because of the advantages of low consumption and stability of metal oxides,it has been developed as an important SERS substrate.Electromagnetic field enhancement(EM)mechanism and chemical enhancement(CM)mechanism are widely accepted to interpret the enhancement mechanism of SERS.The former is mainly related to the local surface plasmon resonance(LSPR)of the material,while the charge transfer(CT)process between the probe molecules and the substrate is required for the latter.When the density of free carriers of nanomaterials changes,there are some strong effect on its LSPR or the bandgap structure which is closely related to CT.Based on this theory,we proposed to study the carrier density effect of metal oxide nanomaterials using SERS.Around the idea,a series of probe molecule/metal oxide nanomaterials system was constructed,the free carrier density of nanomaterials is regulated by self-doping,foreign impurity doping and high pressure induction,the free carrier density effect of metal oxide nanomaterials was studied by observing and analyzing the spectral changes of probe molecules.This work is of great significance to further understand the free carrier density effect of metal oxide nanomaterials and to promote the modification design of non-noble metal SERS substrate.In this thesis,a series of probe molecular/metal oxide nanomaterials system was constructed,and the free carrier density of nanomaterials was regulated by self-doping,foreign impurity doping and high pressure induction.The carrier density effect was deeply studied by SERS technology,which was mainly composed of the following parts:1.SERS study on carrier density effect of self-doped molybdenum tungsten oxide hybrid Based on the method of self-doping,the surfactant free molybdenum tungsten oxide hybrid nanomaterials(MWO NMs)was prepared by solvothermal method.2 mmol of molybdenum metal powder and 2 mmol of tungsten metal powder were added to isopropanol solution and mixed evenly.Then,the H2O2 solution was injected,a yellow-green suspension was obtained after the reaction,and then transferred it to the reactor for solvent thermal reaction.In the solvothermal process,due to the shortage of oxygen elements,some metal atoms in the MWO NMs could not be completely oxidized into positive hexavalent cations,thus forming positive pentavalent cations in low valence state,the XPS analysis also confirmed this assumption.In addition,the Mo ions and the W ions enter each other's lattice during the doping process,disturbing the lattice structure and causing a large number of lattice defects in the MWO NMs.The generation of pentavalent cations and lattice defects resulted in a high concentration of holes which acting as free carriers,and increased the density of free carriers of the material.So that the LSPR absorption of the material blue shift to the visible region,coupled with the laser wavelength used in SERS experiment,and the strongest EM enhancement and CT enhancement were obtained.The carrier density effect of self-doped nanomaterials was studied by SERS,based on the spectral intensity change of the R6G probe molecules.This work provided a new perspective for improving the SERS performance of non-noble metal nanomaterials.2.Investigation of carrier density of Ga-doped ZnO NPs using SERS:doping induced band gap shrinkage The intrinsic semiconductor is a perfect semiconductor without impurities and lattice defects,doping impurity into intrinsic semiconductor is also an effective method to modify the characters of semiconductor materials.In this chapter,the Ga3+was doped into the ZnO NPs,the high valence state element(Ga3+)replaced the low valence state element(Zn2+),and the n-type doping was formed after the cations was doped into the material lattice.Unbonded electrons in the conduction band(CB)act as free carriers,which greatly increases the density of free carriers of the materials.Moreover,the doped Ga3+ions can induce a trailed electron energy states at the bottom of the CB,which will decrease the position of CB and inducing a band gap(Eg)shrinkage of semiconductor.A series of Ga-doped ZnO(GZO)NPs with different bandgap values were obtained through controlling the doping concentration of Ga3+,and used them as the enhancement substrate for SERS exploration.The energy band property of semiconductor substrate is closely related to the CT enhancement,the effect of impurity doping on carrier density effect of nanomaterials was explored by analyzing the changes of peak intensity of 4-MBA molecule in SERS spectra.The band gap of the GZO NPs shrank to the minimum value(3.16ev)when an appropriate doping concentration of Ga3+(the actual ratio of Ga3+/Zn2+is 3.8%),the molecular energy level of 4-MBA and the energy band position of GZO reached the optimal match,presenting the optimal SERS signal.In this experiment,SERS was used to explore the effect of foreign impurity doping on carrier density effect of nanomaterials,which provides a new insight into the controllable modification of impurity semiconductor,and deepened the understanding of CM mechanism in the field of semiconductor SERS substrate.3.SERS Study on carrier censity of TiO2 NPs tuned by high pressure induction It is the first attempt to adjust the carrier density of semiconductor NPs by high pressure induction to tune the change of semiconductor bandgap.In this work,the anatase phase TiO2 NPs with different sizes was synthesized under atmospheric pressure,the relaevant experisments shows that the TiO2 NPs with the size of 10.9 nm have the best SERS performance,then,it was chosen to carry out the high pressure experiments.The carrier density change of TiO2 NPs in the TiO2@4-MBA system was induced by the high-pressure method which produced by the diamond anvil cell(DAC),and the shrinkage of bandgap of TiO2 NPs was achieved.Since the absorption peak of TiO2 and 4-MBA in this system are far away from the wavelength of the laser used in the experiment,the enhancement contribution of EM in this system can be ignored.The SERS signal of this system mainly came from the enhancement of CT,because the CT process was very sensitive to the change of TiO2 bandgap,there is a unique advantage for SERS technology in exploring the bandgap change caused by the change of carrier density which induced by the high pressure.The experimental results show that the internal pressure in the DAC gradually increases with the increase of applied pressure,and the band gap of the TiO2 NPs gradually shrinks.When the pressure was 4.40 GPa and Eg=3.231 eV,bandgap structure of TiO2 NPs and molecular energy level structure reached the optimal matching degree,presenting the strongest SERS signal.The effect of high pressure induction on the carrier density effect of semiconductor NPs was explored by analyzing the change of peak intensity of 4-MBA molecules in the SERS spectrum.Meanwhile,this chapter also provided a new idea for exploring CT under high pressure conditions.4.Carrier density of TiO2@N719@Ag system tuned by hifh pressure induction Based on the work in the previous chapter,TiO2@N719@Ag NPs dye-sensitized solar cell system was constructed by using the Ag NPs obtained by Lee's method and the TiO2 NPs obtained by hydrothermal method,and high pressure induction of the system was investigated using SERS.It was found that the tuning of the semiconductor bandgap changes which derived from the high pressure induction tuned the carrier density of the semiconductor NPs were also applicable to this composite system.Moreover,the Ag NPs introduced into the system can provide additional SPR contribution,enhance the optical absorption of the system and improve the SERS signal;the presence of Ag NPs also provided additional CT paths to further enhance the SERS intensity.High pressure induction can adjust the carrier density of TiO2 NPs and cause band gap shrinkage,making the CT process in the system more likely to occur.When the pressure was 2.48 GPa,the SERS signal of the system reached the maximum value.According to the SERS spectrum of N719 molecule,the correctness of that the change of carriers density of semiconductor can be induced by high pressure was further verified via analyzing the changes of its spectral peak intensity and the degreeof CT,which broadened the idea of CT research in dye-sensitized solar cell system under high pressure.