Study On SERS Substrate Based On Nanowire/Cavity Structure And Piezoelectric Effects

With the development of human society,more and more pollutants are present in the environment in many forms,which would pose a serious risk to the survival of the human race.Therefore,it is crucial to study an accurate,sensitive and low-cost detection method.Raman spectroscopy has gained the attention of researchers because of its non-destructive nature in detecting the substances to be detected and its ability to reflect the fingerprint spectra of different molecules.However,as Raman scattering is the inelastic form of scattering,it has much less energy than elastic scattering,so the Raman spectral signal is also weaker,which makes it difficult to meet the needs of practical use.Through continuous research on Raman spectroscopy,it has been found that the Raman signal of pyridine molecules on the surface of rough cathode is enhanced,and this phenomenon is called Surface enhanced Raman scattering（SERS）,which can achieve the trace detection of substances to be detected by enhancing the Raman signal of substances to be detected,and it has been widely used in the fields of environmental monitoring,drug safety and chemical industry.Although existing Raman detection methods can meet most of the detection needs,SERS substrates with higher sensitivity,stability,detection of specific contaminants and multifunctional composites are still to be developed in the face of variable detection environments and substances.There are two main enhancement mechanisms for SERS,both chemical and electromagnetic enhancement mechanisms.In this paper’s work based on the electromagnetic enhancement mechanism for Raman enhancement,the Raman signal is enhanced by constructing a noble metal micro-nano array structure on the substrate surface to form a large and uniform“electromagnetic hot spot”on the substrate surface,forming strong surface plasmon resonance for enhancement.And by selecting piezoelectric materials with piezoelectric properties in the preparation materials of the substrate,the substrate can modulate the surface plasmon resonance（SPR）by piezoelectric effect.Secondly,the piezoelectric material is selected among the materials used for the preparation of the substrate to give the substrate a piezoelectric effect.By pressing the substrate,the piezoelectric material inside the substrate is deformed so that the charge inside the substrate is redistributed due to polarization,forming a piezoelectric potential on the surface of the substrate,thus regulating the electromagnetic field on the surface of the substrate to finally achieve the purpose of Raman enhancement.This thesis focuses on the preparation of Ag NPs/PVA cavity-surrounded bowl structured SERS substrate with rich nanostructures,as well as Ag/CuO NW/BaTiO₃@PVDF Bowl-shaped SERS substrate with enhanced Raman based on the synergistic effect of piezoelectric and SPR,and Au/Ag/CuO NW/PVDF SERS substrates with piezoelectric modulated Raman and photocatalytic properties.The specific studies are as follows.1.Ag NPs/PVA cavity surrounding bowl SERS substrateIn this work,we have prepared Ag NPs/PVA cavity surrounded bowl SERS substrates by combining the cavity surrounded bowl structure with nanoparticle structure.In the preparation process,we combined the gas-water self-assembly method and spin coating method to prepare a tightly arranged double-layer PS sphere array template,and used the template to prepare the cavity surrounded bowl structure by turning the mold,which has more outstanding optical focusing performance compared with the single bowl structure,and the smaller cavities around the bowl structure is more conducive to the enrichment of probe molecules.In addition,due to the rotational symmetry of the cavity surrounding bowl structure,accurate Raman detection can be carried out with incident light of different angles on the base.Meanwhile,COMSOL Multiphysics software was used to simulate the distribution of electromagnetic field on the surface of the base when incident laser irradiated at different angles.Both theoretical simulation and experiment proved that this substrate realized Raman detection in the case of incident laser with varying angles,which broadened the application scene of this SERS substrate.2.Ag/CuO NW/BaTiO₃@PVDF Bowl-shaped substrateThrough the study and application of bowl-shaped structure in the previous work,we found that the light focusing performance and large specific surface area of bowl-shaped structure are incomparable to other nanostructures.However,SPR formed in bowl-shaped structure can only be distributed on its surface,which leads to the low spatial resolution of bowl-shaped structure.In order to solve this problem,bowl-shaped structure was combined with nanowire structure in this work,Ag/CuO NW/BaTiO₃@PVDF Bowl-shaped SERS bases were prepared based on SPR and piezoelectric effect.The BaTiO₃@PVDF layer in the substrate can cause changes in surface charge distribution when subjected to external forces.The CuO NW/BaTiO₃@PVDF Bowl-shaped structure was synthesized,which has the following advantages:1.The large specific surface area and light focusing properties of the Bowl-shaped structure will enhance the SPR intensity around the noble metal NPs;2.The dense CuO NW in the Bowl-shaped structure can not only transfer the local electric field from the surface to the space,but also form the multi-dimensional structure to form multi-dimensional SERS“hot spot”and can grab nanoplastics,giving the substrate the ability to detect nanoplastics in a directional manner;3.The piezoelectric effect of the BaTiO₃@PVDF layer causes the substrate to generate polarized charges when subjected to external forces,inducing the enhancement of SPR.Both experimental and theoretical simulations demonstrate that this SERS substrate enables the targeted quantitative detection of nanoplastics with diameters of less than 50 nm.The availability of this substrate may facilitate the application of SERS for microplastic detection.3.Au/Ag/CuO NW/PVDF SERS substrateIn the second work,the substrate can be based on the piezoelectric effect to regulate the Raman signal,through further investigation,the piezoelectric effect can also improve the efficiency of photocatalysis.A multifunctional Au/Ag/CuO NW/PVDF SERS substrate was proposed by combining piezoelectric-regulated Raman and photocatalytic functions.The PVDF fiber prepared by electrospinning in the substrate has high piezoelectric output and improves the relative specific surface area of the substrate.While realizing piezoelectric Raman enhancement,it can degrade organic pollutant molecules by piezoelectric photocatalysis.The piezoelectric modulation of the SERS performance of the Au/Ag/CuO NW/PVDF SERS substrate was demonstrated experimentally using R6G and CV.The piezoelectric photocatalytic performance was investigated using MG and MB.Simulation of the distribution of electrostatic potential in Au/Ag/CuO structures using the Vienna Ab-initio Simulation Package（VASP）to explain the mechanism of piezoelectric photocatalysis.The enhanced SERS and catalytic performance can be attributed to the coupling of the piezoelectric and plasmonic effects of the CuO NW/PVDF and Au/Ag bimetals.The proposed Au/Ag/CuO NW/PVDF SERS substrate provides a robust method that employs plasmonic and piezoelectric materials for high-performance SERS detection and piezoelectric photocatalysis.This thesis consists of the following six parts: Chapter 1,Introduction,provides a detailed introduction to the discovery and research process of SERS.As well as the development process of Raman and SERS,two enhancement mechanisms for enhancing SERS,and the progress of the study of SERS substrates.Chapter 2 presents methods for the preparation of SERS substrates with rich surface nanostructures and piezoelectrically modulated SERS substrates,and provides a brief overview of the characterisation apparatus and simulation software used in the article.It also provides an overview of the content of the article.Chapter 3,Ag NPs/PVA cavity-surrounded bowl SERS substrates are prepared,which have abundant cavity-surrounded bowl nanostructures on the substrate surface,enabling the incident light variable angle detection of SERS substrate and broadening the use scenarios and environmental limitations of SERS substrate.Chapter 4 prepared Ag/CuO NW/BaTiO₃@PVDF Bowl-shaped SERS substrate.By combining the nanowire structure with the Bowl-shaped structure,the nanowire structure in the Bowl-shaped was successfully prepared,which realized the directional detection of nano-plastics less than 50 nm.At the same time,the base could generate regulation on SERS based on piezoelectric effect,which greatly improved the sensitivity of the base in detection.In Chapter 5,a multifunctional Au/Ag/CuO NW/PVDF SERS substrate was prepared.The PVDF fiber membrane prepared by electrospinning improves the piezoelectric output ability and specific surface area of the substrate.The SERS and photocatalytic properties of the substrate were also improved by means of piezoelectric regulation,thus achieving accurate detection and efficient treatment of organic dye molecules.Chapter 6 summarises the above work and provides an outlook on future research directions.