Optical Tailoring Of Plasmonic Heterostructures And Photocatalytic SERS Study

Recently,metal nanomaterials have received great attention due to their localized surface plasmon resonance(LSPR).So far various innovative structures have been created to meet the increasing requirement for practical applications of plasmonic nanomaterials.The LSPRs of synthetic metal materials are closely related to their size,morphology,and elemental composition,as well as interparticle spacing and conductive junctions,resulting in a great tailorability toward customizable LSPR properties.In particular,strongly coupled nanoparticle assemblies obtained by a bottom-up process can produce highly localized plasmonic nearfield(sub-diffractionally focused light field),which can further generate greatly improved optical,electrical,and magnetic properties after suitable physical/chemical modifications.However,due to electrostatic and steric repulsions from charged surface ligands,it is very challenging to achieve nano-and subnano-scale coupling of solution-borne nanoparticles,which makes interparticle charge transport hard to happen.It is highly desirable to fine-tune the conductivity of an interparticle region toward fully controlled charge transfer LSPR based on simple and efficient chemical strategies,which is also critical toward realworld applications of nanoplasmonic materials.In addition,it is worth noting that the LSPR-induced strongly localized electromagnetic field can generate high-energy hot carriers(electrons and holes)resulting from plasmon decay,which opens a whole new field of research for catalysis.In the process of light-to-chemical energy conversion,the efficiency of energy coupling is the key.In order to promote carrier utilization,a common strategy is to combine plasmonic materials with metal oxides to form heterostructures.To this end,one core task is to achieve designable interfaces involving plasmonic metal,oxide,and a surface reactant layer.The resulting nano-platform with tailored spatial relationships of different functional interfaces will be especially suitable for highly sensitive,in-situ,and real-time surface enhanced Raman spectroscopy(SERS)monitoring of a catalytic reaction with excellent spectral resolution.This provides an important chance to analyze catalytic mechanisms toward significantly improved catalyst performance.Based on the above background and the urgent demands,this doctoral dissertation focuses on the following aspects:1.DNA-programmable,AgAuE(E=S,Se)-primed conductive bridging of gold nanoassemblies toward highly tunable charge transfer plasmon resonance.Starting from Ag+-soldered,strongly coupled gold nanodimers,silver sulfide or selenide is deposited into their nanomeric/subnanomeric gaps with high regioselectivity.This process immediately induces a directional gold diffusion into the as-formed interstitial Ag2E phase,forming a ternary Ag,Au,and S(or Se)chalcogenide.Upon treating with a coordinating/reducing agent of tributylphosphine(TBP),the AgAuE solid residing in the gap position of the gold nanodimer is converted into a metallic AgAu alloy such that conductive bridging of the gold nanoparticles is realized.In this process,the ternary AgAuE primer has a good "wettability" on the gold surface,which can act as an interparticle"adhesive" layer to achieve a nearly 100%bridging yield.Accordingly,a facile while highly efficient chemical welding process working under mild solution-based conditions is developed,representing an innovative bridging mechanism based on Ag2E-driven gold diffusion and site-specific chemical reduction.This method is adaptable to gold nanomaterials with different sizes and geometrical shapes toward precisely controllable interparticle ohmic contact and continuously tunable charge transfer plasmon.Very importantly,this method is fully compatible with DNA-programmable assembly,which allows for the structural and optical reshaping of more complicated nanoassemblies.Benefiting from the above advantages,a variety of nanoparticle "molecules" and "polymers" can be produced with LSPR frequencies covering near-infrared I,II,and III regions.This method is of particular interests for research in nanoelectronics,enhanced magnetic dipolar transition,and optical sensing based on chemically gated charge transport properties.2.Plasmonic nanounits based on Au-copper oxide/sulfide heterostructures with visible and near infrared optical extinctions.Gold nanoparticles bearing surface-passivating fish sperm DNA(FSDNA)molecules are employed as nucleation seeds for the growth of plasmonic heterostructures.The FSDNA capping layer provides rich adsorptive sites for copper ions through electrostatic and coordinative interactions.Further coating of the FSDNA-protected gold nanoparticles with silica shells provides nanoreactors that spatially confine the nucleation reactions,and help to carry more copper ions "dissolving" in the silica matrices.During a calcination of the above structures in air,atomic copper species spontaneously diffuse to the gold seeds and form strongly interfaced Au-copper oxide heterostructures due to crystal-lattice mismatch.The method has a good adaptability to different metal nanoparticles with easily altered size of the copper oxide domains,promising an easy and highly controllable preparation of versatile plasmonic heterostructures toward plasmonic sensing applications.Based on a clear understanding of the interactions among FSDNA,metal nanoparticles,and copper ions,heterogeneous nucleation control of copper sulfides on gold nanoparticles can be realized in a solution phase.The resulting gold-copper sulfide heterostructures exhibit strong optical absorbance in a near-infrared wavelength region,critically important for bioimaging applications in a tissue-transparent optical window.3.In-situ,real time SERS monitoring of a plasmon-medicated photocatalytic reaction promoted by a dimeric nanoassembly of heterostructured Au-CeO2 Janus nanoparticles.Inspired by the findings in the previous chapters,a semi-synthetic strategy is developed to prepare SERS substrates with improved photocatalytic activities.By controlling the affinity of surface capping ligands(on gold nanoparticle seeds)toward a CeO2 product formed in a hydrolysis-oxidization reaction under hydrothermal conditions,two distinct CeO2 deposition formats are achieved.Strong affinity leads to uniformly coated core-shell structures,while weak affinity is helpful to generate heterostructured Au-CeO2 products.With the latter as initial building blocks,strongly coupled dimeric assemblies of the heterostructured Au-CeO2 nanoparticles stably dispersed in an aqueous phase are obtained with high purify,benefitting from a previously developed Ag ion soldering technique and agarose gel electrophoretic separation.The resulting dimeric structures feature intensive plasmonic light focus,making them a novel type of solution-phase SERS substrates.Importantly,the existence of a Schottky-type Au-oxide interface is useful in promoting charge separation during a plasmon decay process.The resulting structures with confined adsorption of reactant species and significantly enhanced hot-carrier generation in the Raman hotspots are ideal for a photocatalytic SERS study,aiming at demonstrating the oxidative property of spatially well-separated high-energy hot holes.Under the irradiation of a Raman laser,the in-situ oxidation of p-aminothiophenol(p-ATP)into p,p’-dimercaptoazobenzene(DMAB)is quantitatively monitored in real-time.The boosted reaction rate is investigated and attributed to the efficiently separated hot carriers at the heterogeneous Au-CeO2-reactant interfaces.Such a solution-based SERS analytical platform is highly reliable and reproducible,with a great promise to be applied to more catalytic reactions in terms of plasmon-mediated photocatalysis.