Reseach On Preparation And Application Of Nanoporous Structure On Low Activity Mo Surface

Nanoporous metal is a kind of material with nanometer-sized pores and ligaments.Recently,nanoporous materials have attracted great interest for their wide application in separation,catalysts,sensors and surface-enhanced Raman scattering(SERS),owing to their high surface area,high strength,low density and light weight.To date,they have played an important role in scientific research and national economic construction.Molybdenum has been applied in many application fields because its low coefficient of thermal expansion,high hot strength and good electrical conductivity.However,according to the previous research results,our group found out that the surface activity of Mo is very low,which would cause difficulties in preparing Mo-based laminar composites such as Mo/Ag and Mo/Pt/Ag laminar composites.To improve the surface activity of Mo,preparing a nanoporous layer on Mo substrate seems to be an appropriate way.This is mainly because the ligaments of nanoporous metal are composed of nanometer-sized grains.Compared with bulk coarse-grained metal,nanoprous metal with much higher surface area would perform better sur face activity.In this paper,nanoporous Mo film(NPMF)was prepared on Mo substrate by dealloying based on immiscible Mo-Zn system.The concrete process was as follows: first,a layer of nanocrystalline Zn was electrodeposited on Mo substrate.Then,the as-electrodeposited sample was annealed at 400?C.Finally,the sample was dealloyed in concentrated nitric acid.The obtained NPMF has an average pore size of ~ 200 nm and a film thickness of ~ 300 nm.The effects of dealloying temperature and time on surface morphology of obtained NPMF were discussed in detail.The dealloying mechanism of annealed Mo/Zn bilayered sample was investigated by using X-ray diffraction(XRD),grazing incidence X-ray diffraction(GIXRD),Auger electron spectroscopy(AES)and transmission electron microscopy(TEM).We consider the successful dealloying of annealed Mo/Zn bilayered sample could be attributed to the long-distance of Mo atoms in Zn layer,not Mo Zn22 phase which was formed during annealing.While the long-distance of Mo atoms in Zn layer owed to the nanocrystalline structure of Zn layer which could provide sufficient diffusion channels for Mo atoms.These diffusion channels involve those defective grain boundaries(GBs)and vacancies formed via the semi-solid state of some small Zn grains and the atomic thermal migration when annealing at the temperature near the melting point of Zn(400?C).In addition,taking the dealloying of Mo-Zn immiscible system as a sample,a new dealloying approach which is suit for immiscible laminar composites is proposed.The catalytic performance of NPMF in hydrogen evolution reaction(HER)was also studied.The obtained NPMF possessed superior catalytic performance than smooth Mo foil,the Tafel slope of NPMF is 85 mv dec-1 and the stability of NPMF in 0.5mol·L-1 H2SO4 electrolyte solution is good.Although the onset potential of NPMF is a little bit higher,it can be made up by depositing other catalysts.In this paper,the Pt electrodepositing process on NPMF was investigated.By applying 2.5A dm-2 as the current density,it will need at least 4h to cover the nanoporous layer completely.We can save the depositing time by increasing the current density in the future.