Investigation On Fabrication,Formation Mechanisms And Properties Of Nanoporous Metal Films

Nanoporous metals made by dealloying show promising applications in catalysis,sensing,actuation,energy storage and conversion owing to their bi-continuous ligament/pore structures.However,residual stresses from volume contraction,preferred etching along grain boundaries and stress-corrosion cracking during dealloying often lead to the formation of cracks on the surface of samples,making it difficult to maintain a continuous and self-supporting morphology.Meanwhile,the low fraction of the more noble element in the precursor alloy can result in poor mechanical integrity of the dealloyed sample.Thus,the preparation of self-supporting nanoporous metal films still face enormous challenges.Advanced in-situ(operando)techniques could offer deeper insights on corrosion mechanisms and porosity evolution during dealloying,which could provide the guidelines for regulating the microstructures and properties of nanoporous metals.However,the in-situ studies for dealloying mechanisms are hindered by their expensive apparatuses and high testing costs.Based on the above research background,in this dissertation,a facile in-situ X-ray diffraction(XRD)set-up was firstly designed to study the dealloying mechanism,and the Cu90Au10 and Al80Ag20 precursor alloys were chosen as examples to verify the feasibility of the device.Then,utilizing the liquid metal induced alloying-dealloying and melting-rolling-annealing-dealloying strategies,the continuous and self-supporting nanoporous metal films were successfully prepared.The dealloying mechanisms were investigated by ex-situ SEM and in-situ XRD set-up.Their properties and potential applications related to the microstructure have been further explored.Firstly,the phase evolution of the Cu90Au10 and Al80Ag20 precursors during dealloying was studied.The in-situ XRD results show that no intermediate phase appears throughout the dealloying process,implying a direct phase transformation from Cu(Au)solid solution to pure Au.The phase transformation of α-Al(Ag)→Ag(Al)→Ag was observed and no intermediate intermetallic compound was detected during the dealloying of Al80Ag20 precursor.In addition,despite its inert nature in NaOH,the Ag2Al phase can be completely dealloyed in HCl,which is attributed to the catalytic effect of the formed Ag and the size effect of Ag2Al.The two phases(α-Al(Ag)and Ag2Al)in Al80Ag20 exhibit sequential dealloying in HCl,suggesting the phase constitution of as-dealloyed products can be modulated by controlling dealloying time.Secondly,by liquid Ga assisted alloying-dealloying strategy,a nanoporous Cu film supported on a pure Cu substrate was successfully fabricated and the thickness of nanoporous Cu film can be precisely controlled by tuning the mass loading of Ga.The in-situ XRD results reveal that no intermediate phase appears during the dealloying of CuGa2.By comparing the surface morphology of samples before and after dealloying,the "inheritance effect" during dealloying was confirmed.The tensile and nanoindentation tests reveal that under the support of pure Cu substrate,the nanoporous Cu film exhibits obvious plastic deformation stage,and the minimal elongation still reaches 13.5%,which is about half of that of pure Cu.The excellent tensile mechanical properties are mainly attributed to the good interface bonding and certain modulus matching between the nanoporous layer and Cu substrate.The ductile Cu substrate enables the brittle nanoporous Cu layer to withstand large tensile stress.Thirdly,based on the extremely dilute precursor alloy of Cu99Au1,two kinds of selfsupporting nanoporous Au films with different structures were prepared by regulating the phase,composition,and microstructure evolutions during dealloying.The NPG-6 film(dealloying for 6 h)shows the unique hierarchically porous structure,while the NPG-30 film(dealloying for 30 h)reveals atypical three-dimensional(3D)bicontinuous ligament/pore structure.The in-situ XRD and ex-situ SEM results reveal the phase transformation of Cu(Au)→ Au(Cu)→ Au during dealloying,accompanied by dramatic volume shrinkage(up to 87%).The NPG-6 film has ultrahigh porosity(86.2%)and low density(2.43 g cm-3).In addition,the solar water evaporation test shows that the NPG-6 film shows a better water evaporation performance with a photothermal conversion efficiency of 94.5%under the light intensity of 1 kW m-2 compared with the NPG-30 film.The excellent broadband light absorption ability and hydrophilicity,a large number of nano/micro channels and the designed thermal insulation structure with onedimensional channel jointly contribute to the highly efficient solar water evaporation of the NPG-6 film.Moreover,the seawater distillation test confirms the NPG-6 film has a broad application potential in seawater desalination.Finally,on the basis of the above study,an extremely dilute ternary precursor alloy of Cu99Ag0.75Au0.25 was prepared by introducing Ag.The self-supporting nanoporous CuAgAu film with ultrafine ligament/pore structure was prepared by one-step dealloying.Afterwards,by the annealing-second dealloying process,the self-supporting nanoporous Au film with hierarchically porous structure was fabricated.The phase evolution of Cu99Ag0.75Au0.25 precursor during dealloying can be summarized as Cu(AgAu)→ Cu(Au)+Ag(Au)→ Ag(Au)→Au based on the in-situ XRD and ex-situ SEM results.The two self-supporting films both possess ultrahigh porosity and low density.Compared with nanoporous Au film,the nanoporous CuAgAu film exhibits a higher water evaporation rate(1.49 kg m-2 h-1)and photothermal conversion efficiency(93.6%)under the light intensity of 1 kW m-2.The enhancement of solar water evaporation performances for nanoporous CuAgAu film mainly originates from its stronger broadband light absorption caused by the ultrafine ligament size and resonance absorption of Cu,Ag and Au.