Study On The Welding Technique And Application Of Nanomaterials

Nanomaterials have many physicochemical properties superior to those of bulk materials composed of the same composition due to the special quantum confinement and surface state effects,thus the nano composite components constructed with the low-dimensional nanomaterial units are widely used in the fields of electronics,photonics,sensing,catalysis,surface enhanced Raman spectroscopy,energy,and environmental monitoring.However,the integration of these low-dimensional nanomaterial units faces many challenges in practical applications,such as poor contact between the nanomaterial units and poor adhesion of the dispersed independent nanomaterial units to the supporting substrate.The nanowelding of dispersed independent nanomaterial units into an interconnected network structure can effectively overcome the above problems.However,the existing nanowelding technologies generally face the problems of poor universality,high energy consumption and damage to brittle nanomaterials or substrates,etc.Furthermore,the common fabrication strategies of three-dimensional nanoporous gold（NPG）,as a three-dimensional nanoporous material with great application potential,generally suffer from high energy consumption,resource waste,use of corrosive solvents,pollution caused by the residual sacrificial components,etc.Based on above problems,this thesis used the citrate-coated gold（Au）nanoparticles（NPs）sol prepared by the Frens’citrate reduction method,the dense Au NP film from the self-assembly of Au NPs at the water-oil interface,and the Polyvinylpyrrolidone（PVP）-coated silver（Ag）nanowires（NWs）synthesized by polyol reduction method as the research objects to study the efficient low-temperature nanowelding and application of monolayer Au NP film and Ag NW film,and the efficient fabrication and application of high-purity three-dimensional NPG.The main research contents and results of this thesis are listed as follow:（1）External field-strengthened Ostwald nanowelding.An external field（e.g.,light,direct current,and alternating current）-strengthened Ostwald nanowelding strategy enabling the low-temperature nanowelding of Au NP film composed of Au nanoparticles（NPs）with nanoscale spacing in solution was exploited and an electron localization mechanism was proposed to understand it.The external field-derived local electrons not only greatly can strengthen the dissolution of Au surface atoms and the reduction of the dissolved Au³⁺ions,but also can confine the transport of Au³⁺ions within the nanogap between the Au NPs together with ordered water molecules.Consequently,the electrochemical Ostwald ripening process of the Au NPs is fully strengthened,which,along with the local electrons-strengthened surface atom diffusion（induced by the strengthened electrostatic repulsion）,enables the external field-strengthened Ostwald nanowelding of Au NP film.The interdigital electrodes from Au NPs and transparent conductor from Ag NWs fabricated by the external field-strengthened Ostwald nanowelding show excellent performances.（2）Electrochemical nanowelding of Au nanoparticles in sulfuric acid solution.An electrochemical strategy was developed for the efficient low-temperature nanowelding of Au NP film supported on the non-conductive substrate in 0.5 M H₂SO₄ solution.The electrochemical properties,morphologies and crystal facet distributions of the Au NP films during electrochemical treatment processes were further studied.During electrochemical Cyclic voltammetry（CV）process（i.e.,interplay of the electrochemical Ostwald ripening and local electron-strengthened surface atom diffusion）,the electrochemical surface area of Au NP thin films increased with the cycling number,which was attributed to the gradual filling of the nanogaps between Au NPs and the gradual transformation of the Au NP film from isolated Au NPs to an interconnected network structure.Furthermore,the Au NP film also can be nanowelded into interconnected network structure through the electrochemical potentiostatic polarization（i.e.,only local electron-strengthened surface atom diffusion）of Au NP film.The interconnected network-structured Au NP film prepared by the electrochemical CV of initial Au NP film shows good electrocatalytic activity for the electrooxidation reaction of ethanol.（3）Bottom-up fabrication of three-dimensional nanoporous gold from Au nanoparticles using nanowelding.A general bottom-up direct current（i.e.,DC）nanowelding strategy to fabricate high-purity three-dimensional NPG from Au NPs was exploited.The DC nanowelding process can gradually transfer the multilayer Au NP film into three-dimensional NPG at low temperature within 10 s,while not damaging the spherical structures of the basic building units（i.e.,Au NPs）.This is because during the DC nanowelding,electrons are mainly localized at the high-resistance junctions between Au NPs,and the electrostatic repulsion induced by the localized electrons at the junctions in turn strengthens the Au surface atomic diffusion to initiate a mild solid-state diffusion nanowelding.Furthermore,during the DC nanowelding of the multilayer Au NP film,the nanostructure（e.g.,thickness,sizes of ligaments and pores）of the fabricated three-dimensional NPG can be easily tuned by adjusting the DC intensity and treating time,as well as the layers of the initial multilayer Au NP film and size of initial Au NPs,thereby providing great operational flexibility for the fabrication of functional nanoporous materials.The three-dimensional NPG fabricated by DC nanowelding has good electrocatalytic activity for methanol electrooxidation reaction.（4）Fabrication of three-dimensional nanoporous gold by halogen-assisted high-efficiency electrochemical nanowelding.An electrochemical nanowelding technique was developed to realize the rapid fabrication process of three-dimensional NPG from Au NPs in solution.Three-dimensional NPG can be rapidly fabricated by electrochemical CV of multilayer Au NP film in 0.5 M H₂SO₄ solution containing a small amount of halide ions.The nanostructure of the fabricated three-dimensional NPG can be well tuned based on the type and concentration of halide ion,as well as the scan rate and cycling number of the electrochemical CV,in addition to the layers of the initial multilayer Au NP film and the size of initial Au NPs.The three-dimensional NPG fabricated by halogen-assisted high-efficiency electrochemical nanowelding exhibited good electrocatalytic activity for the ethanol electrooxidation reaction.（5）Fabrication of black gold by electrochemical nanowelding and its application in photoelectric catalysis.An electrochemical technique was exploited to realize the ultrafast fabrication process of black gold from Au NPs in solution.Black gold can be efficiently fabricated by the electrochemical potentiostatic polarization of multilayer Au NP film in 0.5 M H₂SO₄+1.0 M KCl solution.The black gold fabricated by the electrochemical potentiostatic polarization of multilayer Au NP film at optimized potential for 5 s has the best nanostructure.Furthermore,the black gold fabricated under the optimal conditions shows good photoelectric catalytic performance for methanol electrooxidation reaction,light absorption,photothermal conversion and photo-response,and signal enhancement effect for the surface enhanced Raman spectroscopy of rhodamine 6G.