One-Step Aqueous-Phase Synthesis And Property Study Of Au-M(M=Ag,Pd)Nanomaterials

Because of excellent physicochemical properties（conductivity,stability,etc.）,noble metal nanomaterials were widely used in energy,electronic,optical fields,including SRES,biological imaging and catalysis.The properties of the noble metal nanomaterials are closely related to their morphology,structure,size and components.Creating simple and effective synthetic method is of great importance to the development of noble metal nanomaterials.Compared with the two step or multi-step seeded method,one-step method,which can avoid the complex procedure,is simple and convenient.When using water as solvent,which is friendly to environment,the reaction temperature would be limited by the boiling point of water at atmospheric pressure.Au and Ag are excellent plasmonic materials.When utilizing Au and Ag to prepare one-dimensional nanostructures,except for combining the properties of Au and Ag,the localized surface plasmon resonance（LSPR）peak could realize the full coverage of near ultraviolet to near-infrared/mid-infrared region.At present,one-dimensional segmented Au-Ag nanomaterials were usually synthesized by seeded method.In the existing one-step method,organic solvent was used to achieve higher reaction temperature.Femtosecond transient absorption spectroscopy could be used to study the electronic dynamics of materials after excitation.In previous research,the research of electronic dynamics in one dimensional nanomaterial were mainly focused on monometallic nanorods.Because of its 30 twin boundaries,20{111}facets and 12 vertexes,icosahedral nanoparticles possess unique advantages in applications.Au is excellent plasmonic material and Pd is excellent catalytic material.When utilizing Au and Pd to prepare icosahedral nanomaterials,the advantages of Au,Pd and icosahedral structure could be integrated.Based on the above problems,we developed pressure-assisted one-step aqueous-phase strategy,which use N₂ to increase the internal pressure of reaction vessel,to prepare Ag-Au-Ag heterojunction nanorods and Au-Pd icosahedral nanoparticles.Major results are as follows:1.Developing pressure-assisted one-step aqueous-phase strategy to prepare Ag-Au-Ag heterojunction nanorods with controllable aspect ratios.There are three key factors to realize the preparation of Ag-Au-Ag heterojunction nanorods by one-step aqueous-phase method:（1）the formation of Ag Cl precipitates,which greatly decreased the concentration of free Ag⁺;（2）using polyvinyl pyrrolidone（PVP）as reducing agent,which resulted in slow reaction rate;（3）the utilization of N₂,which increased the internal pressure of reaction vessel and allowed the reaction temperature to reach higher value.The length of as prepared Ag-Au-Ag heterojunction nanorods could vary from tens of nanometers to several microns,indicating that the LSPR peak could realize the full coverage from near-ultraviolet to near-infrared/mid-infrared region.In this research,the increase of internal pressure not only increased the temperature limit of reaction,but also could be used as a new parameter to modulate the aspect ratio of Ag-Au-Ag heterojunction nanorods.Compared with pure Ag nanorods with the same diameter,calculated results indicated that the d-band center of Ag-Ag-Ag heterojunction nanorods would move toward Fermi level,and the upshift value could be modulated by the content of heterojunction.It means that the existence of heterojunction structure could be used to adjust the surface reactivity of heterojunction nanorod.2.Using femtosecond transient absorption spectroscopy to study Ag-Au-Ag heterojunction nanorods.When the wavelength and frequency of pump laser were kept constant,the e-ph scattering time in one-dimensional direction would slightly increase firstly and then decrease quickly with the increase of length,which resulted in the decrease of heterojunction content.This phenomenon was different from that of normal solid nanoparticles with larger size（>10nm）,which would not change with the variation of size.The electron-phonon scattering time were non-linearly dependent on the length of Ag-Au-Ag heterojunction nanorods,indicating that the existence of heterojunction deeply influenced the energy transfer between electronic system and lattice in one-dimensional direction.Besides,the phonon-phonon scattering time also slightly increased firstly and then decreased quickly with the decrease of heterojunction content,illustrating that the energy transfer between heterojunction nanorods and surrounding environment were also influenced by heterojunction structure.When using the same Ag-Au-Ag heterojunction nanorods and simply changing the frequency of pump laser,the relaxation time after excitation would increase with the increase of frequency.In this situation,the relaxation time of transverse mode were more sensitive to the changes of pump laser.When simply changing the wavelength of pump laser,the relaxation time would decrease with the increase of wavelength.3.Au-Pd alloy and core-shell icosahedral nanoparticles were prepared by the same one-step aqueous-phase strategy.In this method,because of the complexation effect of Br^-（provided by cetyltrimethyl ammonium bromide）and the usage of weak reducing agent（PVP）,Au Br₄^-and Pd Br₄^2-were simultaneously reduced and formed Au/Pd alloy icosahedral nanoparticles firstly.After that,apart from the continuously deposition of Au and Pd atoms,the galvanic replacement and atomic diffusion would happen,resulting in the formation of Au@Pd icosahedral nanoparticles.With the increase of time,the shell thickness of Au@Pd icosahedral nanoparticles would increase.However,when the shell thickness increased to some extent,the further deposition of Pd atoms would happen on some specific facets,resulting in anisotropic growth and the formation of short nanorods.In this experiment,when simply changing the ratio of Br^-and Cl^-while keeping the concentration of CTA⁺to be constant,alloy Au/Pd icosahedral nanoparticles with different size,Au/Pd@Au and Au@Pd icosahedral could be prepared at 70 min,realizing the modulation of structure and atomic distribution.When additionally adding 20μL Ag NO₃（100.00 m M）to the reaction condition of Au-Pd icosahedral nanoparticles,alloy and half alloy half core-shell melon-seed shaped Au-Pd nanoparticles could be prepared by varying reaction time.When simply changing the amount of Ag NO₃,Au-Pd nanoparticles with different shape could be prepared,achieving the modulation of shape.Au-Pd nanorods could be prepared by additionally adding KI to the reaction conditions of Au-Pd icosahedral nanoparticles.The aspect ratio of Au-Pd nanorods could be manipulated by adjusting the addition amount of H₂Pd Cl₄ or the reaction time.