Synthesis And Property Studies Of Iron-based Oxide Graphene-like Structures And Polycompound Hollow Nanostructures

Due to the unique physical and chemical properties,nanomaterials have great application potentials in many fields such as energy,information,environment,catalysis and medicine,which have attracted great research interests.The chemical and physical properties of materials are mainly dependent on their chemical composition,structure and morphology.The multiple metal compounds,with complex chemical composition,various metal ions,variable chemical valence states,and many metal active sites,have been widely used in many fields.It is believed that controllable synthesis of nanomaterials with different compositions can further tune their chemical and physical properties,and improve their performances in lithium ion battery,supercapacitor,catalysis and so on.2D graphene-like materials,with atomic thickness and the infinite plane expanding ultrathin two-dimensional features,owning novel electronic properties and high specific surface areas,have been widely used in many fields ranging from electronics to energy storage.Hollow micro-nano structural materials with large specific surface area,low density,as well as good surface permeability,have aroused considerable interests in a growing range of research fields,including medicine,catalysis,optoelectronic,clean energy conversion and storage systems,and many others.Therefore,synthesis of iron-based oxides graphene-like structures and polycompound hollow nanostructures are of great importance in scientific researches and practical applications.This thesis is mainly divided into two parts:In the first part,we firstly synthesized graphene-like ?-Fe2O3 ultrathin nanosheets.On this basis,by controlling experimental conditions,we successfully synthesized a series of graphene-like iron oxide and iron-based composite structures,such as ?-Fe2O3 ultrathin nanosheets,Fe3O4 ultrathin nanosheets,LiFesO8 ultrathin nanosheets,iron oxide/ferrite composites,a-Fe2O3@graphene composites and ?-Fe2O3@carbon nanotube composites.In the second part,we successfully synthesized a series of hollow polycompound nanostructures through facile methods basing on Ostwald ripening and microemulsion method.These hollow polycompound nanostructures include hollow ZnxCd1-xS nanospheres,hollow ZnxMn1-xS nanospheres and Mn-doped Co3O4 hollow microprisms.The properties of the obtained nanomaterials were studied and the relationship between the properties and the morphologies of the nanomaterials were discussed.The main contents are summarized as follows:1.Synthesis and property studies of unit-cell-thick iron oxide nanosheetsGraphene-like iron oxide ultrathin nanosheets could bring brand-new properties.Graphene-like hematite(?-Fe2O3)ultrathin nanosheets with unit-cell thickness of 1.37 nm were one-step synthesized in the presence of Al3+cations using a metal-ion-intervened hydrothermal method.The ?-Fe2O3 nanosheets were then converted to magnetic maghemite(?-Fe2O3)and Fe3O4 with thickness of 0.84 nm by subsequent simple calcination processes,which is the first synthesis of unit-cell-thick magnetic iron oxide ultrathin nanosheets.The obtained ?-Fe2O3 ultrathin nanosheets manifest excellent electrochemical properties as potential anode material for lithium-ion batteries in terms of high specific capacity,remarkable cycling stability,and superior rate capability.The graphene-like structures of ?-Fe2O3 would provide large effective surface areas that facilitate penetration of the electrolyte and transport of lithium ions in the electrode,and the ?-Fe2O3 nanosheets with high crystallinity and structural stability would also help improving their performances in lithium ion battery.The iron oxide ultrathin nanosheets with unit-cell thickness are expected to have a wide range of applications in energy,magnetism and catalysis fields.2.Synthesis of graphene-like ferrite ultrathin nanosheets and iron-based compositesThe complex iron-based compounds have good properties of magnetism and catalysis and have been widely used in many fields such as information storage,catalysis and electrode materials.The synthesis of complex iron-based ultrathin nanosheets would effectively improve their properties and get more extensive applications.On the basis of the previous chapter,we used ?-Fe2O3 ultrathin nanosheets as hard template,and by ion impregnation method and high-temperature calcination,the graphene-like lithium ferrite ultrathin nanosheets and iron oxide/ferrite composites ultrathin nanosheets including LiFe5O8,Li2Fe2.4O4.6 and ?-Fe2O3@MFe2O4(M=Ni,Co,Zn)ultrathin nanosheets were synthesized.Novel carbon materials(graphene and carbon nanotubes,etc,)show many advantages including low density,high strength,high conductivity and low thermal expansion.The preparation of iron oxide/carbon composite can effectively improve their performances and become extremely important.In this chapter,by using in situ synthesis method,?-Fe2O3/graphene composites were synthesized,and the composition of the products can be controlled by adjusting the ratio of Fe3+ and graphene.By C2H2 pyrolysis vapor deposition method,the carbon nanotubes were grown on the surface of nanosheets of iron oxide,and the ?-Fe2O3/carbon nanotube composites were obtained.The lithium ferrite ultrathin nanosheets and iron-based composites are uniform morphology and size,and expected to have a wide range of applications in the energy and catalysis fields.3.Synthesis and photocatalysis studies of hollow ZnxCd1-xS nanospheresHollow structured materials show many advantages including porous structure,large surface area,high light-harvesting efficiency and fast mobility of charge carriers,and have promising application in the field of photocatalytic processes.In this study,hollow ZnxCd1-xS nanospheres with different molar ratios were synthesized via a facile hydrothermal process.In the reaction process,carboxymethyl cellulose(CMC)acts as a soft template,by dividing the solutiou into numerous "channels".At the same time,the precipitation-dissolving balance of ZnxCd1-xS in ammonia solution coupling with Ostwald ripening process,resulted in the formation of hollow ZnxCd1-xS nanospheres under hydrothermal conditions.It was found that the photocatalysis performance of the as-prepared samples could be enhanced by formation of ZnxCd1-xS solid solutions.In addition,their photocatalytic activities are dependent on the Zn/Cd molar ratios and nanostructures of ZnxCd1-xS solid solutions.The unique hollow structure and the formation of the metal solid solution are the keys to the photochemical performance improvement of the samples.4.Self-assembly synthesis of hollow Zn1-xMnxS nanostructures and their pseudocapacitive property studiesControl over the chemical composition and morphology of metal sulfides can improve the the electrochemical performance.In this chapter,mixed-metal sulfide Zn1-xMnxS nanorod-assembled hierarchical hollow spheres were successfully synthesized via a template-free solvothermal process basing on Ostwald ripening process.In the reaction system the glycerol would play a key role for the formation of ZnxMn1-xS hierarchical hollow nanostructures by forming a "quasi-microemulsion-template".When applied as pseudocapacitive electrode material,the Zn1-xMxS mixed metal sulfide hollow spheres with different Zn/Mn ratio showed different pseudocapacitive performances,and the Zn0.25Mn0.75S hollow spheres showed the best pseudocapacitive property.Hollow structure and the introduction of a small amount of zinc ions are beneficial to the improvement of pseudocapacitive property of ZnxMn1-xS hollow spheres.5.Synthesis and property studies of Mn-doped Co3O4 hollow microboxesElement doping can improve the electrochemical performance of transition metal oxide.In this chapter,a series of columnar Mn doped cobalt-based precursors were synthesized via a solvothermal process by regulating the Mn content.By calcining the precursors in the air,Mn-doped Co3O4 hollow microboxes were obtained.When Mn doped Co3O4 hollow microboxes were used as lithium ion battery cathode material,it was found that Mn doping ratio can significantly affect the discharge capacity of the samples,and 5%Mn-doped Co3O4 hollow microboxes display the superior electrochemical performances.