Construction And Electrochemical Properties Of Hierarchical Architectures Of Transition Metal Compounds

2D nanomaterials would greatly enhance the electrochemical properties because of their special structures and surface properties.However,their easily stacking feature in use would seriously hinder the fully ustilization of 2D advantages.Constructing 3D hierarchical nanostructures from 2D nanosheets/nanoplates was widely considered as an effective strategy to maximize structural advantages of 2D nanomaterials because those structures not only inherit unique merits from the building nanosheets/nanoplates but also offer many collective additional characteristics,such as excellent structural stability.In this work,transition metal ion-organic complex precursors with 3D hierarchical structures assembled by thin nanosheets were first synthesized through facile solution methods.The transition metal compounds?metal oxides/molybdenum carbide?were subsequently obtained from calcination of as-prepared precursors.Since those 3D hierarchical structures constructed by 2D nanosheets/nanoplates possess high surface area,structural merits of 2D nanomaterials as well as structural stability,they could be used as electrode materials of energy storage and conversion including lithium storage,pseudocapacitance and hydrogen evolution reaction?HER?.Fe-glycolate precursor with two typical morphologies,including microflowers composed of nanosheets and sheaf-like microrods organized by nanowires,were fabricated through cheating Fe ion with ethylene glycol using simple solution method.After annealing the precursor in the inert atmosphere,Fe₃O₄ micro-/nanostructures with well-retained morphologies would be obtained.Furthermore,a simple metal ion exchange strategy was adopted to prepare hierarchical structure of Cu doped Fe-glycolate,which was used to fabricate self-stacked Cu Fe₂O₄-Fe₂O₃ porous nanosheets after calcination in air.Notably,the method for synthesis of iron oxide based 3D hierarchical structures could be extended to other transition metal oxides,such as Mn,Ni and Co.For example,stacked Mn₃O₄ porous nanoplates were successfully prepared through annealing Mn-glycolate precursor.Because of several structural advantages including large surface area,broad porosity distribution and ultrafine primary nanocrystallites etc.the as-prepared 3D hierarchical micro-/nanostructures converted from glycolate precursor manifest high-performance lithium storage in term of high capacity,excellent rate capability and remarkable cyclability.For example,Fe₃O₄ microflowers exhibit excellent Li-ion storage properties with a high reversible and stable capacity as high as 964.3 m A h g^-1at 500 m A g^-1after 100 cycles,and superior rate capability of 487 m A h g^-1at 8 A g^-1.In order to further boost Li-ion storage properties of Fe₃O₄ electrode materials,hierarchical hollow structures were formed.A facile solvothermal method has been developed to fabricate hierarchical Fe–glycerate hollow spheres composed of nanosheets.A novel one-pot self-templated mechanism is observed for the formation of hollow spheres,which involves the simultaneous chemical transformation of initially formed Fe-IPA to Fe–glycerate.These Fe–glycerate hollow spheres can be easily converted to hierarchical Fe₃O₄ hollow spheres without any structural deterioration by annealing in N₂ gas.In virtue of the structural advantages,the obtained Fe₃O₄ hollow spheres manifest excellent electrochemical properties as potential anode materials for lithium-ion batteries in terms of high specific capacity(1063 m Ah g^-1at 500 m Ag^-1),remarkable cyclability?capacity retention of about94%after 100 cycles?,and superior rate capability(457 m Ah g^-1at 10 Ag^-1).The above solvothermal method could be extended for the synthesis of hierarchical NiCo₂O₄ hollow structures using mixed solvents of IPA and 1,3-propanediol.We have developed a one-step solvothermal method to fabricate hierarchical microtubes constructed by ultrathin NiCo-LDH nanosheets.Prism-like precursor particles are initially formed during the solvothermal reaction,which are then chemically transformed to NiCo-LDH microtubes.The as-synthesized NiCo-LDH microtubes can be easily converted to NiCo₂O₄ microtubes by annealing in air without an apparent structural change.Benefiting from the unique structural features,these hierarchical NiCo₂O₄ microtubes exhibit enhanced pseudocapacitance with excellent rate performance and remarkable cycle life over 12000 cycles.Furthermore,hierarchical metal carbide hollow structures constructed from porous nanosheets were fabricated and investigated as a high-performance and low-cost electrocatalyst for HER.An unusual template-engaged strategy has been utilized to controllably synthesize Mo-polydopamine nanotubes,which are further converted into hierarchical?-Mo₂C nanotubes by direct carburization at high temperature.Benefitting from several structural advantages including ultrafine primary nanocrystallites,large exposed surface,fast charge transfer,and unique tubular structure,the as-prepared hierarchical?-Mo₂C nanotubes exhibit excellent electrocatalytic performance for HER with small overpotential in both acidic and basic conditions,as well as remarkable stability.