Fabrication Of Graphene-Like Two-Dimensional Nanomaterials With Surface-Enhanced Electrochemical Energy Storage Performances

New technique for material fabrication is of fundamental importance in the development of material science.The isolation of graphene by mechanical cleavage sets off a revolution in material science and technology that is coming into a two-dimensional（2D）age.The atomically thin 2D nanostructure opens up an emerging research field of new material and has attracted numerous scientific enthusiasms on various potential properties.Ultrathin 2D nanomaterials hold geometrically sheet-like structures with atomic or molecular-scale thickness（typically less than 5 nm）and large lateral size（over 100 nm or even up to tens of micrometres）,leading to an ultrahigh aspect ratio or 2D anisotropy.Beyond well-known graphene,a large amount of graphene-like ultrathin 2D nanomaterials,including transition metal dichalcogenides,layered metal oxides,transition metal carbides and layered-double hydroxides,have been fabricated via various methods.Due to their ultimate structural anisotropy,ultrahigh specific surface area and strong quantum confinement of electrons in two dimensions,these ultrathin 2D nanomaterials exhibit many fascinating physical,optical,chemical and electronic properties,which hold great potential in various applications such as electronic devices,catalysis,energy storage and conversion,sensing and biomedicine.Despite significant advances in the fabrication and applications of graphene and related 2D materials,it remains a challenge to develop novel synthetic technique for large-scale production of this kind of graphene-like ultrathin 2D nanomaterials.In this thesis,we have successfully developed novel synthetic strategies towards large-scale preparation of monolayer graphene,layered metal hydroxides and non-layered metal oxides nanosheets and investigated their surface-enhanced electrochemical energy storage performances.These new strategies are of great importance for the development of graphene and related 2D materials science and technology and can push the field a key step further ahead.Further research focusing on these as-synthesized2D nanomaterials might boost significant advances in physics,chemistry and material science.More importantly,these new synthetic strategies are favorable for industrial production,and the resultant 2D nanomaterials might find a large variety of practically commercial applications in batteries,supercapacitors,catalysts,sensors,and so on.More specifically the key points of the research achievement in this thesis are:We designed a general synthetic strategy for monolayer graphene preparation.The novel synthetic method is based on a direct solid-state pyrolytic conversion of sodium carboxylate,such as sodium gluconate and sodium citrate,assisted by Na₂CO₃ salt.Gram-scale quantity of monolayer graphene can be readily prepared in several minutes.The present pyrolytic conversion can overcome low monolayer content of exfoliation method and low production of chemical vapor deposition（CVD）.We developed a large-scale synthesis of ultrathinα-Ni（OH）₂ nanosheets via microwave-assisted liquid-phase growth.The well-defined nanosheets show a micron-sized planar area,1.52 nm thicknesses,and a high specific surface area(190.15 m² g^-1).The ultrathin 2D nanostructure can convert almost active materials into surfaces with high activity to facilitate the surface-dependent electrochemical processes and thus maximize the overall properties.When used as electrode materials for supercapacitors,the nanosheets exhibit a maximum specific capacitance of 4172.5 F g^-1 at a current density of 1 A g^-1.Even at higher rate of 16 A g^-1,the specific capacitance is still maintained at 2680 F g^-1with 98.5%retention after 2000 cycles.Electrochemical measurements also demonstrate a reversible conversion-type electrochemical behaviour with a high activity toward lithium in lithium ion battery.When cycled at 100 mA g^-1 current density,the ultrathinα-Ni（OH）₂nanosheets can deliver ultrahigh initial discharge and charge specific capacities of 1744.9and 1364.6 mAh g^-1.We fabricated high-quality ultrathin NiO nanosheets at large scale through a lamellar hydroxide intermediate strategy.The well-defined nanosheets show a graphene-like morphology with large planar area,ultrathin thickness（<2 nm）.In comparison with the bulk material,the NiO nanosheets exhibit unique surface and electronic structure with considerable under-coordinated surface nickel atoms and crystal lattice volume expansion.The detected local coordination geometry and the electronic states endow the ultrathin NiO nanosheets with the great potential in surface-dependent electrochemical reaction and catalytic processes.When used as anode materials for lithium ion batteries,the ultrathin NiO nanosheets exhibit a high reversible lithium storage capacity of 715.2 mAh g^-1 at 200mA g^-1 current density in 130 cycles with an excellent cycling stability and rate capability.In supercapacitors,the ultrathin NiO nanosheets deliver a maximum specific capacitance of2236 F g^-1 at 0.5 A g^-1.We developed a general microwave-assisted method to fabricate high-quality 2D ultrathin multicomponent transition-metal oxide nanosheets with a geometrically graphene-like architecture,including Co₃O₄,NiCo₂O₄,ZnCo₂O₄,and CuCo₂O₄ nanosheets.All the as-synthesized nanosheets are well-defined and freestanding with a micron-sized planar area and ultrathin thickness.When used as anode materials for lithium ion batteries,the ultrathin ZnCo2O4 nanosheets exhibit a high reversible lithium storage capacity of930-980 mAh g^-1 at 200 mA g^-1 current density in 200 cycles with an excellent cycling stability and good high-rate capability.The ultrathin NiCo₂O₄ nanosheets could deliver a high first discharge capacity(1287.1 mAh g^-1)at 200 mA g^-1 current density.The reversible lithium storage capacity still retains at 804.8 mAh g^-1 in the 100th cycle,suggesting a good cycling stability.We presented a microwave-assisted synthesis of ultrathin SnO₂ nanosheets at gram-scale.The two-dimensional（2D）anisotropic growth depends on microwave dielectric irradiating coupled with surfactant structural directing.The ultrathin 2D nanostructure holds great surface tin atom percentage with high activity,where the electrochemical reaction processes could be facilitated that highly dependent on surface.Compared with 1D SnO₂ nanorods,the ultrathin SnO₂ nanosheets exhibit remarkably improved electrochemical lithium storage properties with a high reversible capacity of 757.6 mAh g^-1 at a current density of 200 mA g^-1 up to 40 cycles as well as excellent rate capability and cycling stability.