Synthesis And Magnetic Properties Of 3d Transition Metal (V?Fe) Selenides

3d transition metal elements have unique energy valley and spin because of its rich d-electronic structure,which makes 3d transition metal selenides exhibit many novel physical and chemical properties.However,the researches on 3d transition metal selenides are mostly focused on catalysis,photoelectric conversion,energy storage,and so on.The research on magnetism is relatively less.In this thesis,a series of transition metal selenides were prepared by high-temperature organic liquid-phase reaction,such as VSe₂,V_1-x-x Fe_x Se₂,and Fe₃Se₄.The effects of dopants and surfactant additions on the structure,morphology and magnetic properties of nanostructures were studied.The main conclusions are as follows:?1?Two-dimensional magnetic VSe₂ nanomaterials were synthesized by high temperature organic liquid-phase reaction using VO?acac?₂ and Se powder as reaction sources.The thickness of VSe₂ nanosheets can be adjusted by the concentration of precursors.The average size of the single VSe₂ nanosheets is more than 300 nm and the thickness is only 5 layers.The coercivity is about 240 Oe at room temperature.When the magnetic field is up to 0.5 T,the magnetization is about 0.036 emu/g.However,the coercivity reaches its maximum at 100 K?747 Oe?,which may be due to the characteristic of the charge-density-wave.In addition,we optimized the morphology by adding 1-dodecanethiol?1-DDT?.At high temperature,some sulfur atoms are doped into VSe₂ lattice to form VS_x Se_2-x nanomaterials,resulting in a slight decrease of their magnetic properties.?2?In order to optimize its magnetic properties,we used Fe?acac?₃ as Fe dopant and synthesized V_1-x-x Fe_x Se₂ magnetic nanomaterials.When x?0.1,the charge-density-wave behavior still exist in V_1-xFe_x Se₂ nanomaterials.However,with the amount of dopants continues to increase,the charge density wave characteristic disappears,and the coercivity and magnetization increase significantly.The increase of magnetization is due to the coupling between the 3d orbits of V and Fe,which causes some spin antiparallel arrangement flip into spin parallel arrangement.The enhancement of coercivity is closely related to the contribution of the second phase of Fe₃Se₄.?3?By changing the experimental conditions and adding surfactants,Fe₃Se₄ with different morphologies were obtained,and the magnetic properties of them also changed.For the sample without adding the surfactant,the Fe₃Se₄ nanomaterials was nanosheets,the coercivity is 3.37 kOe and its saturation magnetization is 2.84 emu/g at room temperature.When oleic acid?OA?was added,the morphology of Fe₃Se₄ is evolved to spherical,and their room temperature coercivity and magnetization are 3.36 kOe and 4.13emu/g respectively.When cetyltrimethyl ammonium bromide?CTAB?was added,the coercivity of 4.06 kOe can be obtained,while the saturation magnetization can reach to5.01 emu/g.If the experimental conditions were changed,the hexagonal Fe₃Se₄nanostructures can be synthesized.The coercivity is 22.46 kOe and the saturation magnetization is about 10 emu/g at 5 K.In conclusion,a series of 3d transition metal selenides of VSe₂,V_1-xFe_x Se₂ and Fe₃Se₄ were prepared by high-temperature organic liquid-phase method.The magnetic properties can be regulated from soft to hard by adding surfactant and replacing V atoms with Fe atoms in VSe₂ lattice.This work will play an important role for the research on the other transition metal selenides nanomaterials.