Synthesis,Microstructure,Physiochemical Properties Of NiFe₂O₄ And Tantalum-containing Oxides

Nanomaterials are defined as crystalline and amorphous material of nanometer,which are in size of 1-100 nm.Crystalline and amorphous metal oxide nanoparticles have recently attracted numerous researchers’attention because of the unique physiochemical properties with the change of particle size.It is important scientific significance to study the particle size and crystallinity for metal oxides.In previous report,researchers focus on the controlled synthesis of simple oxides with given grain sizes or crystallinity,and exploring the determination of grain growth mechanismto achieve new functional materials,optimize materials performance,and broaden the relevant technological applications.Nevertheless,the mechanism of grain growth of multi-metal oxide nanomaterials and the transformation fromamorphous to crystallinity of metal oxidesin the hydrothermal condition are still hardly accessible.Herein,NiFe₂O₄ of prototype multi-metal oxide and tantalum-containing oxides with peculiar electronic structures are chosen as the research subject.And we study the effect of particle size and crystalline structure on optical and magnetic properties.The main content of this paper includes the following parts:（1）The relation of growth kinetic,microstructure and magnetic properties for NiFe₂O₄ nanoparticle were studied.NiFe₂O₄ nanoparticles were synthesized to show grain sizes ranging from 4.3 to 780 nm by a sol-gel auto-combustion method.The results demonstrate that the grain growth mechanism for NiFe₂O₄ nanoparticles is uncovered to follow an Ostwald ripening model（400-700^oC）and grain boundary diffusionmodel（700-1000^oC）.Andanequation D⁵=(1.37×10¹⁵)t exp?（-150.75 RT）was shown in an Ostwald Ripening model.The structure of characterization and analysis indicated that Fe³⁺ions of surface/interface might migrate to the octahedral site of NiFe₂O₄ lattice in Ostwald ripening process,and more Fe³⁺ions at surface/interface of NiFe₂O₄ nanoparticles would occupy the tetrahedral sites during the grain-boundary diffusion process.Further,the cell volume exhibits the decrease and then slight increase and further increase to a constant of 580.2 A³ with the nanoparticle growing,which were attributed to a reduced strong repulsive interaction of the parallel surface defect dipoles and the migration process of magnetic ions.Increasing grain size brought about an increase of saturation magnetization（M_r）of the NiFe₂O₄ particles.Crossover variation of H_c as well as M_r andχ_p were also found to depend on grain size for NiFe₂O₄ nanoparticles.The magnetic behavior could be attributed to core with the normal spin arrangement and a surface layer with the non-collinear spin arrangement,mixed canted structure from both of core and surface spins,ferrimagnetically aligned core spins and a spin-glass-like surface layer for NiFe₂O₄ particles.（2）Regulated crystalline structure of tantalum-containing oxides and a superior performance in unassisted photocatalytic water splitting were studied.The crystalline-to-amorphous transformation of tantalum-containing oxides was synthesized by varying the pH values in different base species（N-N solution,NaOH and NH₃·H₂O solution）under hydrothermal condition.We proposed a transformation mechanism based on nucleation-dissolution-recrystallization and further intensified the influence of base concentration on crystalline Na₂Ta₂O₆ transformed into a mixture of crystalline Ta₂O₅ and amorphous Na_xTa_yO_z,then turned into crystalline Ta₂O₅ and finally to amorphous TaO_x as confirmed by XRD,TEM,EDS,N₂-physisorption,Raman,IR,and XPS analysis.Photocatalytic results showed that the hydrogen production rate of amorphous TaO_x prepared at pH=3 is close to the crystallization of Ta₂O₅ five times underλ>280 nm light irradiation without any cocatalysts and electron acceptors.These findings develop a novel and efficient pathway towards synthesizing the different composition,crystallinity,morphology of tantalum-containing compounds under hydrothermal conditions and could open opportunities for further investigating the photocatalytic property of Tantalum-containing materials.