Laser Synthesis Of Carbon Encapsulated Metal-based Materials And Their Electrocatalytic Properties

Carbon encapsulated metal-based electrocatalysts have attracted tremendous attentions because of their unique properties.First,the existence of carbon shell can prevent the inner metal from contacting with harsh electrolyte during electrocatalytic process,and hence boost the durability of catalysts.Second,the electron penetration between inner metal and carbon shell promote the adsorption and activization of reactants on the carbon shell,and give rise to an improved electrocatalytic activity.Whereas,the controllable synthesis of carbon encapsulated metal-based electrocatalysts is still a great challenge,and their catalytic performances are limited due to the insufficient of effective preparation technologies for structure and electronic property modulation.Laser processing and fabrication in liquids,including laser ablation in liquids(LAL)and laser irradiation in liquids(LIL),is a special synthetic method to fabricate various nanostructures under laser-induced extreme nonequilibrium conditions.Its unique photo-thermal effect and plasma/metal induced decomposition reactions of organic molecule provide a possibility for the controllable synthesis of carbon encapsulated metal-based electrocatalysts.Based on the above analysis,we have developed some facile and universal synthetic methods to fabricate kinds of carbon encapsulated metal-based nanomaterials by using laser processing and fabrication in liquids.The formation mechanisms of these carbon encapsulated metal-based nanomaterials are studied.Their component,structure and morphology are optimized by modulating laser parameters and solvents to acquire a better electrocatalytic performance towards different electrochemical reactions.The detailed research contents and results are given as follows:(1)A versatile synthetic strategy based on LAL technique is developed to prepare carbon encapsulated metal-based nanomaterials.Carbon encapsulated metal,metal carbide and metal oxide(M@C,MCX@C,or MOy@C)with core-shell structure were obtained by ablating metal targets(Cu,Ag,Au,Pd,Pt,Ti,V,Nb,Cr,Mo,W,Ni,Zr,Mn,Fe and Zn)in acetone.The formation mechanism of carbon encapsulated metal-based nanomaterials was studied by analyzing ablation process and reaction conditions.We conclude that the composition difference of "core" for carbon encapsulated metal-based nanomaterials is decided by the solubility of carbon atom in metals and the affinity of oxygen to metals.Besides,the metal-catalyzed carbonization and subsequent laser irradiation effect during LAL process show significant influence on the crystallinity of the external carbon shell.(2)The carbon encapsulated Au nanoparticles(Au@C)is fabricated by laser ablating Au target in acetone,and the application of Au@C in electrocatalysis is studied.The external carbon shell of Au@C is only 2-3 carbon layers,and contain abundant structural defects that provide an access for the reactants to reach the core Au nanoparticles.For this reason,Au@C nanoparticles exhibit excellent ORR and HER performance.Electrochemical tests show that Au@C possess excellent catalytic activity toward ORR with a half-wave potential of 0.87 V,better than that of commercial Pt/C(0.86 V).Meanwhile,Au@C is also a favorable HER catalyst which has an overpotential of 170 mV at a current density of 10 mA/cm2.More importantly,Au@C presented superior stability and keep their catalytic activity after 3000 CV cycles for ORR in 0.1 M KOH and HER in 0.5 M H2SO4 due to the protection of carbon shell.(3)A simple route is developed to fabricate carbon-encapsulated ultrafine FeNiOx NPs loaded on CNT(FeNiOx@C-CNT)based on LIL technique.The FeNiOx NPs display an average size of 2.3 nm with a thin carbon shell coated(about 1-3 carbon layer),which were uniformly dispersed on the surfaces of CNT.Benefit from the above features,FeNiOx@C-CNT exhibits excellent OER activity,which displays a low overpotential of 267 mV at 10 mA/cm2.In addition,FeNiOx@C-CNT shows almost unfading electrocatalytic activity after 20000 CV cycles due to the protection of carbon shell.Such synthetic strategy is also universal and could be extended to fabricate other carbon-encapsulated metal-based structure,such as FeOx@C-CNT,CoOx@C-CNT,NiOx@C-CNT,MnOx@C-CNT,Ru@C-CNT and Pt@C-CNT.(4)In this work,we employ a novel strategy to prepare amorphous FeNi alloy nanoparticles encapsulated in N doped carbon(A-FeNi@NC),which is obtained by using LIL technique to transform crystalline FeNi alloy nanoparticles encapsulated in N doped carbon(C-FeNi@NC)into A-FeNi@NC.The crystalline-amorphous transition of FeNi alloy was driven by the rapid heating and quenching process during LIL process.OER test results indicated that A-FeNi@NC exhibit enhanced electrocatalytic activity compared with C-FeNi@NC.We proposed that the electronic property of the catalysts has been optimized due to the variation in the crystallinity of FeNi alloy,and achieve an enhanced OER activity.Importantly,this strategy is versatile and could be used to prepare other carbon encapsulated amorphous transition metal nanoparticles and enhance their OER activity,such as A-FeCo@NC,A-CoNi@NC and A-Ni@NC.