Preparation And Property Study Of Zn,Fe,Co Oxides And Their Composites

Benefiting from the rich valence and electron configuration,the Zn,Fe,Co oxides are widely used in the fields of electrochemistry,sensors and catalysis,etc.Based on these advantages,this thesis takes the MOFs first as template,combining hydrothermal method and the subsequent thermal treatment process,a variety of Zn,Fe,Co oxides and its compounds were synthesized.Their supercapacitor and photocatalytic properties were studied.A series of different morphologies of?-Fe₂O₃ nanostructures were synthesized under the control of H₂PO₄^-ion.Their formation mechanisms,CO catalytic oxidation and electrochemical performances were investigated.The?-Fe₂O₃/rGO composites with various morphologies were prepared and the effects of morphology on the electrochemical and photocatalytic properties of composites were conducted.The specific research contents are as follows:1.The synthesis of MOFs derived Zn,Fe,Co oxides and their properties.Through the two-step calcination process,different MOFs derived transition metal oxides were successfully fabricated using MOFs as templates,including the ZnO,Co₃O₄,ZnO/Co₃O₄ and Co₃O₄/NiO composites.It is found that the metal oxides can retain the original shape of their precursors and also present the porous shell and hollow core properties.The properties of as-synthesized metal oxides were studied.As gas sensor,the ZnO showed higher response value and better selectivity for ethanol.The reason for the can be ascribed to the n-type semiconductor properties of the ZnO which performs high carrier mobility and quick response to a signal.The Co₃O₄ showed excellent CO catalytic oxidation performance:with the increase of calcination temperature,the active site decreased due to the reduced specific surface area and thus decreasing the catalytic activity.When evaluated as electrodes for supercapacitors,the ZnO/Co₃O₄ exhibit high specific capacitance of 455 F g^-1at a current density of 0.5 A g^-1,which is higher than that of the pure ZnO and Co₃O₄ electrodes.Moreover,the designed ZnO/Co₃O₄ exhibit excellent photocatalytic activities compared with the corresponding bare counterparts.As demonstrated,the formation of p-n heterojunction between ZnO and Co₃O₄ contributes to the fast separation of electron and holes.In addition,the core-shell Co₃O₄/NiO show remarkable specific capacitance of 435 F g^-1at 5 A g^-1and excellent stability with 86.3%capacitance retention after 2000 cycles,superior to that of the pure Co₃O₄.The introduction of NiO improves the overall specific capacity of electrode materials.At the same time,the core-shell structure contributes to the improvement of the cycle stability and maintenance of a structure.2.A series of different morphologies of?-Fe₂O₃ were synthesized under the control of H₂PO₄^-ion.The supercapacitor and CO catalytic oxidation properties were systemically studied.By controlling the H₂PO₄^-concentration and reaction time,the?-Fe₂O₃ with various morphologies?spindle,tubular,sheet and ring?were obtained.It was found that the key factor leading to the morphological evolution of?-Fe₂O₃ was that the specific adsorption of H₂PO₄^-on the specific plane of?-Fe₂O₃.The surface hydroxyl with unique doubly coordinated configuration on?001?plane is the most important factor that causes the morphology evolution of the products.The final morphologies of the products mainly depends on the growth control of H₂PO₄^-to the unique?001?plane.The electrochemical test shows that the sheet-like product performs excellent specific capacitance and rate capacity and ideal cyclic stability,which result from the relative higher specific surface area and the opened pore structure.For the CO catalytic oxidation properties,the order of the catalytic activity is as follows:tube<sheet<ring<spindle.The specific planes on a-Fe₂O₃ are indeed factors influencing the catalytic performances.3.Preparation of?-Fe₂O₃/rGO composites with different morphologies toward the supercapacitor and photocatalytic properties.The different morphology of?-Fe₂O₃/rGO composites can be successfully obtained through the in-situ synthesis and mechanical agitation methods.It was found that the?-Fe₂O₃ was densely and freely dispersed on the rGO layer.By comparing electrochemical properties,the sheet-like?-Fe₂O₃/rGO composites demonstrate excellent electrochemical performance:the highest specific capacitance,excellent cycling stability and rate capacity.The specific capacitance is 970 F g^-1at a current density of 1 A g^-1and the capacitance retention is 75%after 2000 cycles when the current density reaching to 5 A g^-1.It is mainly due to the synergistic effect between the?-Fe₂O₃ and rGO and the high conductivity of the rGO offers fast channel for the movement of electrons.In addition,the photocatalytic performance results are shown that the spindle?-Fe₂O₃/rGO composites performed the best catalytic properties,which is closely related to the specific exposed planes.Moreover,the introduction of rGO increases the specific surface area of the material,speeds up the transfer of photogenerated electrons and improves the separation efficiency of photogenerated electron-hole,thus showing excellent photocatalytic activity.