Design And Synthesis Of Cobalt-based Oxide Nanostructures With Highly(111)-preferential Orientation For Electrocatalytic Oxygen Evolution

Metal organic frameworks are a class of advanced porous crystalline materials with various morphologies and adjustable composition,which could be also used as one of the most precursors for synthesizing other derived nanostructures with unique mesostructures and specific surface.In this thesis,a series of Co-based zeolitic imidazole frameworks(ZIF-67)with different morhphologies such as dodecahedron,microdisc,ultrathin nanosheet,have been synthesized and sequentially transformed into the correponding Co-based oxide nanostructures with highly(111)orientation and controlled surface defects by calcination in adjustable atmoshpere,where their electronic structures have been futher adjusted by doing other heterogeneous metal ions.The resultant Co-based oxide nanostructures have been used as electrocatalysts for oxygen evolution reaction(OER)and also exhibited signifantly enhanced electrocatalytic activities,where the relationship between the structure,chemical composition and the OER performance has been comprehensively investigated.ZIF-67 with dodecahedron-like morphology have been synthesized and used as the precursor,for obtaining hollow Co3O4 dodecahedron with highly(111)orientation by being calcined at a lower oxygen partial pressure(Ar:O2=9:1),where the intensity ratio of the diffration peaks for(111)and(311)crystal plane is about 1.2(I(111))/I(311)=1.20);and 6 times higher than that for the common Co3O4 standard sample(I(111)/I(311)=0.19)The as-synthesized hollow Co3O4 dodecahedron exhibits excellent OER performance with an overpotential of 328 mV and a Tafel slope of 60 mV·dec-1 at a current density of 10 mA·cm-2,which could be further improved by doping other heterogeneous metal ions such as Zn,Mg,Ni.For example,hollow Co3O4 dodecahedron with doing 10%of Zn ions exhibits a I(111)/I(311)of 0.68 and better OER performance with an overpotential η10 of 324 mV and the Tafel slope of 48 mV dec-1 Similarly,hollow Co3O4 dodecahedron with doping 50%of Mg ions has an OER performance with an overpotential η10 of 284 mV and the Tafel slope of 45 mV dec-1,whereas hollow Co3O4 dodecahedron with doing 50%of Ni ions shows an OER performance with overpotential η10 of as low as 277 mV,and the Tafel slope of 56 mV·dec-1.ZIF-67 with microdisc-like and ultrathin nano network-like morphology have been also synthesized and used as the precursor for obtaining the correponding Co3O4 nanostructures with highly(111)orientation by being calcined at a lower oxygen partial pressure(Ar:O2=9:1),which possess rich oxygen vacancy of 17.0%and 27.1%high atomic ratio of Co2+/Co3+(1.56 and 2.03)The microdisc-like Co3O4 nanostructure exhibites an OER performance with overpotential η10 of 267 mV and the Tafel slope of 51 mV·dec-1,whereas ultrathin nano network-like Co3O4 nanostructure shows an OER performance with overpotential η10 of 196 mV and the Tafel slope of 81 mV·dec-1.The morphology of ZIF-67 has been manipulated from dodecahedron to ultrathin nanosheet by simply changing the molar ratio of metal salt to organic ligand.Ultrathin Co3O4 nanosheets with highly(111)orientation could be also obtained by calcining the as-synthesized ZIF-67 ultrathin nanosheets under at lower oxygen partial pressures and exhibit the best OER performance with an overpotential η10 of 187 mV and the Tafel slope of 67 mV·dec-1.Furthermore,the OER performance of ultrathin Co3O4 nanosheets could be improved after doping 40%of Ni ions,whose η10 and Tafel slope is 61 mV·dec-1 respectively.These results suggest that the OER performance of cobalt-based oxide nanostructure could be improved by combinning crystal plane and morphology manipulation as well as electronic structure adjustion from doping other heterogeneous metal ions.