Structural Adjusting,Physical Properties And Applications Of Magnetic Metal Oxides With Oxygen Octahedral Structure

Double perovskite A2NiMnO6 compounds,owning to its chemical composition and structural flexibility,show a lot of unique physicochemical properties and potential applications,including sensors,optional electronic devices,OER catalysis,magnetic capacitance,magnetoresistance devices and so on.Many physicochemical properties of A2NiMnO6,such as magnetoresistance,magnetocaloric and catalytic properties,are directly related to its degree of B-site cation order.In addition,when A site is magnetic rare earth ions,such as Dy,related to the 3d-4f exchange interactions between the FM network of Ni/Mn sublattice and the rare-earth ions spins,the rare-earth sublattice will have influence on the magnetic and electrical properties of A2NiMnO6 compounds.Meanwhile,there are a lot of shortcomings in experimental and theoretical understanding of certain properties and phenomena,such as magnetic phase transitions at low temperatures and short-range magnetic ordering in high temperature.Therefore,in order to further explore the physicochemical properties of such materials and well apply them to practical applications,such as OER catalysis,we have systematically investigated the physical properties and structural properties as well as the application of such typical magnetic metal oxides with oxygen-containing octahedral structures(including Dy2NiMnO6 and CoFe2O4).The regulation of the degree of B-site cation order,magnetoelectric and OER catalytic properties of Dy2NiMnO6 could be realized via the B-site nonmagnetic ions doping.Furthermore,considering that the reduction of size and the improvement of conductivity could improve the catalytic performance of OER,the Al-doped La/Dy2NiMnO6 nanofibers were synthesized and their OER activity were investigated.In addition,the spinel ferrites AFe2O4 with octahedral structure,due to its structure and magnetoelectric properties flexibility,low cost and high activity,enables a wide range of applications,such as electrocatalysis,photonics,energy storage devices and so on.In order to further explore the structure and catalytic properties of those materials,the CoFe2 alloy/CoFe2O4 nanofibers were synthesized and their OER activity were investigated.The details work we conducted is as following.In chapter one,we introduced the structure of double perovskite and spinel,the basic magnetic concepts related to the transition metal manganese oxide.In addition,recent advances in A2NiMnO6 and spinel ferrites AFe2O4 compounds,the recent development and research on the OER of the perovskite and spinel compounds have been reviewed and discussed.In chapter two,magnetic and structural properties of nonmagnetic Al ion doping in Dy2NiMnO6 compounds have been investigated systematically.A resonance effect of ionic valences has been observed while the A1 ion doping in Ni site(B site),i.e.not only the Mn valence decreasing,but also the Ni valence increasing,without destroying the long-range Ni-O-Mn orders.It is found that the optical band gap and the resistivities at a certain temperature(such as 285K)show a non-monotonous dependence on the Al ion doping content.In addition,the magnetic transition temperature Tci,TC2 and TN,which is attributed to the FM interactions of Mn4+-O-Ni2+,Mn3+-O-Ni3+,and AFM interactions of the Dy3+ with the Ni(A1)/Mn moments,change with the Al ion doping content increasing.In chapter three,the magnetic and structural properties of the nonmagnetic Zn ion doped Dy2Ni1-xZnxMnO6(0≤x≤0.3)compounds have been systematically investigated.It is found that the ferromagnetic transition temperature Tc continuously decreases,and the anti-ferromagnetic transition temperature TN continuously increases,with the Zn ion doping content increasing.The coercivity at low temperature(30 K)decreases with the Zn ion doping content increasing.Remarkable phonon softening for all the Raman spectra of the Dy2Ni1-xZnxMnO6 samples is observed in the symmetric stretching mode behavior due to the spin phonon coupling at temperature much above Tc.The temperature dependences of the ESR g-factor reveal that the content of the Ni2k(Zn2+)/Mn4+ configurations increases with the Zn ion doping content increasing.Combining the results of the XPS and magnetization measurement,it can be concluded that the short-range magnetic ordering above Tc is caused by the short range antiferromagnetic interactions of the anti-site disorders(Mrn4+-O-Mn4+,Ni2+-O-Ni2+)and the short range antiferromagnetic interactions between the Ni(Zn)/Mn sublattice and the Dy3+ sublattice.The Zn ion doping could suppress the formation of the Ni3+/Mn3+ ions and promote the formation of the single oxidation state of Ni2+/Mn4+in Dy2NiMnO6.In chapter four,a series of non-magnetic Al ion doped La/Dy2NiMnO6 nanoparticles have been synthesized.What’s more,a series of non-magnetic Al ion doped La/Dy2NiMnO6 nanofibers have been synthesized through a facile method of electrospinning.In view of their OER activity,it is found that the A1 ion doping in B-site(Mn site)is indeed,which could improve the OER activity.Reducing the synthesis temperature could promote the formation of the disordered Ni3+/Mn3+configuration,increasing the Al ion doping content could promote the formation of the Ni3+-O-Mn3+configuration,both of them could improve the OER activity of those compounds.In chapter five,the CoFe2 alloys/CoFe2O4 porous hollow complex nanofibers have been synthesized via a facile method of electrospinning combining with two-step heat treatment.The obtained CoFe2 alloys/CoFe2O4 porous hollow complex nanofibers show high electrical conductivity with charge transfer resistance of two orders of magnitude lower than that of the single CoFe2O4 porous hollow nanofibers and even much smaller than that of the commercial RuO2.When used as an electrocatalyst for OER,it demonstrates a superior electrochemical performance with low over-potential(300 mV at 10 mA cm-2),small Tafel slope(73.34 mV dec-1)and long-term durability in 1M KOH.More important,the direct growth of CoFe2 alloys on the CoFe2O4 base of the porous hollow nanofibers can be an effective way to control the coupling of the CoFe2 alloys and CoFe2O4,the electronic conductivity,the active sites and the electrochemical performance of this materials for OER.