Preparation And Growth Mechanism Of Magnesium Alcohol Oxides And Oxide Nanowires

One dimensional(1D)nanostructures offer prospects for enhancing the electrical,thermal,and mechanical properties of a broad range of functional materials and composites,but their synthesis methods are typically elaborate and expensive.Therefore,it is very important to find a new way for large batch of namowires fabrication with reduced cost.Recently,our group demonstrate a direct transformation of bulk materials(like Al-Li alloys)into metal alkoxide and oxide nanowires under ambient conditions without the use of catalysts or any external stimuli.This method can realize scalable synthesis of ultra-low-cost one-dimensional materials and membranes.For many applications,the properties of MgO may be substantially more attractive than alumina.In fact,MgO offers higher thermal stability,better thermal conductivity,and betterer electrochemical stability,excellent biocompatibility,and so on.Therefore,for various fields,it is important to explore the low-cost synthesis of magnesium alkoxides and oxide nanowires and the corresponding formation mechanisms.The main contents and results are as follows:(1)Magnesium isopropoxide nanowires suspension with a high aspect ratio was succesfully produced at a moderate temperature of ?65? though dealloying of bimetal Mg_0.5Li_0.5 alloy with iso-propanol.According to SEM analysis,The mechanism of the alloy grain surface conversion to forest of namowires(which grows until the initial alloy grains completely disappear)involes dissolution of Li component of the alloy followed by greatly enhanced reactivity of Mgthat react with iso-propanol forming polymeric propoxides,which take shape of nanowires in order to minimize the Gibbs free energy of tehe system(via minimization of strain energy at front interface).Additionally,nanowires formation may be kinetically favorable because it enables rapid diffusion of the propanols(reaction species)and out-diffusion of hydrogen and lithium propoxides via aligned pores between the NWs.During synthesis,the NWs are initial produced as the suspension of small bundles,which then split into the separated NWs in the course of several hours.EDS and NMR studies suggested low steric effects in isopropoxide NWs,which enabled its increased toughness to sustain the surface stresses during the conversion of alloys to nanowires and higher length.(2)We proposed a new method for nanowires fabrication,which is,transformation of magnesium ethoxide particles into magnesium n-propoxide nanowires through alkoxy ligand exchange and metal alkoxide condensation at low temperature and pressure.The chemical fomulae of Magnesium ethoxide and Magnesium n-propoxide were figured out according to their 1H,13C NMR and TGA analysis.Further structure simulation uncovers the reason of deformation.Mgethoxides are fragile for the weak Van der Waals's force between the groups in two directions.While continuous-Mg-O(H)-Mg-bonds with strong mechanical properties of magnesium n-propoxide help to form 1D structures.The conversion mechanism of nanoparticles to magnesium n-propoxide nanowires was further revealed though time-indepenent SEM photos.We noticed the diameter of alkoxide nanowires varies with the shape,size,porosity,and ligand groups of magnesium alkoxides.Swelling temperature is a decisive factor for the successful ligand exchange of Mgethoxide with n-propanol.This method can provide a new idea for fabrication of metal alkoxides,metal oxides,fluorides,or nitrides with various morphologies.(3)Mg_0.9Li_0.1 alloy with lower lithium content was also successfully converted to magnesium alkoxide and magnesium oxide nanowires at low temperature and low pressure.This method can be highly extended to large industrial scale.We discussed the effects of lithium contents of Mg-Li alloy and types of alkoxy groups of alcohols on the morphology of metal alkoxides formed by dealloying process.It was confirmed that the transformation of nanowires from bulk metals require favorable conditions including advantages of thermodynamics and kinetics.Based on EDS spectrum and 1H,13C NMR analysis,we find that magnesium n-propoxide has a lower polymerization degree compared with the analysis magnesium isopropoxide nanowires.So,the final nanowire length is slightly shorter than the grain size of the magnesium-lithium alloy.The structure,steric hindrance and electronegativity of alkoxy wouuld also affect the coordination number of magnesium and polymerization degree of magnesium alkoxides.The type of alkoxy group also has an important influence on the overall morphology.MgO nanowires with similar morphology are formed through thermal decomposition/oxidation of magnesium n-propoxide nanowires.