Synthesis And Magnetic Properties Of Carbon-Coated Fe₃O₄ And ZnFe₂O₄ Nanorods

One-dimensional magnetic nanomaterials have unique magnetic anisotropy,i.e.,shape anisotropy,and are an important component for building new magnetic functional materials.It is widely used in magnetic storage,sensitive devices,microwave absorption,biocatalysis,and other fields.This paper focuses on the synthesis of one-dimensional magnetic nanomaterials,with emphasis on the microscopic regulation of the morphology,crystal structure and elemental valence of one-dimensional magnetic nanomaterials and the effect of structure on magnetic properties.The main contents are as follows:1.Using hydrothermal carbonization process（HTC）,Fe₃O₄@C nanorods were synthesized by a two-step heated hydrothermal method using Fe（NO₃）₃·9H₂O,FeSO₄·7H₂O and cold water soluble starch（straight chain starch,（C6H10O5）n）as precursors.The effect of hydrothermal conditions on their growth was investigated,and the formation mechanism of Fe₃O₄@C nanorods was discussed.The results show that the length of Fe₃O₄@C nanorods is about 44-54 nm,the diameter is about 8-9 nm,and the thickness of carbon layer is 7-12 nm.Refinement of the X-ray spectrum revealed that the Fe₃O₄ sample has a face-centered cubic anti-spinel structure with a lattice constant of 8.3963?.The carbon shell of the nanorods consists of amorphous state and partially graphitized carbon.The graphitization of the carbon layer decreases sequentially with the increase of iron salt content in the precursors.The Fe₃O₄@C nanorods have good stability at room temperature,and the decomposition temperature of its carbon layer is 388.33°C.The mass fraction of Fe₃O₄ in the sample is 78.09%.Combining the effects of different hydrothermal conditions on the nanorod morphology,it is found that the nanorod growth actually follows a synergistic soft and hard template mechanism.The straight chain starch molecules dehydrate and polymerize among themselves,acting as soft templates,and Fe³⁺ions and Fe²⁺ions attach to the starch molecule chains to nucleate and grow into Fe₃O₄ nanorods.The Fe₃O₄ nanorods in turn act as a supporting framework and catalyst,and the straight-chain starch continues to cross-link and carbonize to form carbon layers.2.The magnetic properties of Fe₃O₄@C nanorods at low and room temperatures were investigated.The Fe₃O₄@C nanorods were found to exhibit ferromagnetism in the range of 5 K-300 K.It reaches a saturation magnetization intensity of 77.32 emu/g at room temperature,which is relatively close to the corresponding value for bulk materials（92emu/g）.As the temperature decreases,the coercivity increases from 4.54Oe at 300 K to 743.84 Oe at 5 K.No peak is found in the ZFC-FC curve,indicating a blocking temperature above 300 K.The ZFC curve shows a hump-like feature at 112k,corresponding to the Verwey transition.3.Using hydrothermal carbonization process（HTC）,ZnFe₂O₄@C nanorods were successfully synthesized by using Fe（NO₃）₃·9H₂O,FeSO₄·7H₂O,Zn（NO₃）₂·6H₂O,ZnSO₄·7H₂O and cold water soluble starch as raw materials.By investigating the effects of starch content on the phase purity,stability,grain size,carbon layer thickness and magnetic properties of nanorods,it was found that the phase purity of ZnFe₂O₄@C samples gradually decreased with the decrease of starch content,the length of nanorods increased from 48 nm to 78 nm,the diameter increased from 12 nm to 18 nm,the thickness of carbon layer gradually became thinner,and the stability of samples gradually deteriorated.The lattice constant was found to be 8.441?by refinement of the X-ray spectrum.4.The magnetic properties of ZnFe₂O₄@C nanorods were investigated.The reasons for the gradual decrease of the magnetization intensity after increasing to the highest value of 8.29 emu/g with the increase of the nanorod grain size are also discussed.The ZnFe₂O₄@C nanorods are close to superparamagnetic at room temperature.XPS analysis showed that the ZnFe₂O₄@C sample is a mixed spinel structure,with Zn²⁺present not only in the tetrahedral A sites but also in the octahedral B sites of the spinel structure,and Fe³⁺present not only in the octahedral B sites but also in the tetrahedral A sites.The magnetization behavior of the samples at room temperature was explained by the ratio of Zn²⁺ions in the octahedral B-site and tetrahedral A-site,and the content of oxygen in the lattice.It was found that the valence state of Znin the sample was positive divalent and that of Fe was positive trivalent.As the starch content decreased,the percentage of Zn²⁺ions in the octahedral sites B sites versus tetrahedral A sites in the samples first increased and then decreased,which was the same trend as the magnetization of the samples.In the sample with the lowest starch content,the percentage of Zn²⁺ions in the octahedral B site versus the tetrahedral A site increased slightly,but the magnetization intensity remained the lowest.On the one hand,this is due to the increase of the impurity Fe OOH content.On the other hand,the small amount of oxygen vacancies in its lattice also leads to a decrease in the magnetization intensity.5.The magnetic properties of ZnFe₂O₄@C nanorods at low temperatures were investigated.Hysteresis loops and M-T curves of the samples were tested from 5k-300k.ZnFe₂O₄@C nanorods were found to be ferromagnetic at 5k and superparamagnetic when the temperature was increased to 50-300k.When the starch content is reduced to the lowest,the sample grain size increases,ZnFe₂O₄@C nanorods are ferromagnetic from 5k-300k,and the coercivity value increases.The coercivity,saturation magnetization strength and effective anisotropy constant increase with decreasing temperature.The M-T curve shows that the magnetization intensity of FC decreases continuously with increasing temperature,and the ZFC curve shows a wider maximum centered at 88k,with an irreversible temperature of 257k.When the starch content is the lowest,the magnetization intensity of the M-T curve decreases and the ZFC curve shows a relatively sharp peak at 18 k,similar to the behavior of bulk ZnFe₂O₄.This change in magnetic properties also reflects the gradual decrease in cation disorder.