Spinel Type Complex Metal Oxide Functional Materials: Preparation, Characterization And Their Properties

Spinel-type complex metal oxides are an important class of inorganic functional materials.They have found wide applications in areas such as high-temperature material,semiconductor,optoelectronic,magnetic material, biology,medicine,sensor,catalyst and catalyst support,because of their important structural and physicochemical properties.At present,the most commonly used route is the conventional solid-state reaction method which requires calcination of physical mixtures of precursors at considerably elevated temperatures for a long time.Therefore,the particles agglomerate and sinter during the calcination process,resulting in the nonuniform of the product and low surface area,which seriously affected their applications.Hydrotalcite-like compounds or layered double hydroxides(LDHs) belong to a large class of nature and synthetic anionic clays,in which the metal ions are bonded to hydroxyl groups to form two-dimensional brucite-like layers that are stacked together through electrostatic interactions between interlayer anions and positively charged layers.Within the layers,the cations are uniformly distributed on an atomic level without segregation of 'lakes' of separate cations.Therefore,calcination of LDHs precursors at a lower temperature and shorter time can give spinel-type complex metal oxides with uniform composition and structure.However,these spinel-type complex metal oxides are always mixed with the oxide of the divalent metal.This reflects the fact that in LDHs,the ratio M^â…¡/M^â…¢is typically in the range 2～4 whereas in a spinel the required ratio is M^â…¡/M^â…¢=0.5.This thesis describes the synthesis of serious LDHs materials and uses them as precursors to obtain spinel-type complex metal oxides with special structures.Based on this thought,we prepared ZnAl₂O₄ and ZnCr₂O₄ spinels with mesopore networks and unusually high specific surface areas using ZnAl-LDHs and ZnCr-LDHs as precursors.Similarly,after changing M^â…¡to Mg and introducing Ti into the layer of LDHs precursor,we have prepared MgFe-LDHs and MgFeTi-LDHs,which are also used as precursors to obtain high specific surface area mesoporous MgFe₂O₄ and Ti-doped MgFe₂O₄ spinels.In above pathways,the large amount of highly dispersed M^â…¡O formed during the calcination of LDHs precursors,as self-generated sacrificial templates,have an obvious segregation and inhibition effect on the growth of spinel particles,and leaves porous structures after selective leaching.Besides, we investigated the influence of M^â…¡/M^â…¢molar ratio and calcination temperature on the structure and textural properties of products.Increasing the calcination temperature to 900℃,single-crystalline ZnGa₂O₄ spinel phosphor with average particle size of around 150 nm has been prepared from ZnGa-LDHs precursor.Compared with the traditional high-temperature solid-state reaction method,this synthetic approach involves a much lower calcination temperature and shorter calcination time,and the obtained ZnGa₂O₄ spinel particles are smaller and uniform.The X-ray crystal structure,composition,surface/near-surface chemical states,Ga³⁺coordination environment and Ga-O distance of ZnGa₂O₄ spinel products by two methods have been detailed investigated.It has been confirmed that that Ga³⁺ions locate not only on the octahedral sites but also on the tetrahedral sites in ZnGa₂O₄ spinel structure,and the Ga-O coordination environment has a great influence on the photoluminence of ZnGa₂O₄ phosphors.The increased band gap energy comes from the shortening of Ga-O bond length in the ZnGa₂O₄ spinel obtained by the precursor route.After further improvement of the LDHs precursor route with the introduction of pre-calcination and sintering process steps,complex metal oxide monoliths(MgAl₂O₄,MgFe₂O₄ and In_2-xMg_xO₃)with macropore frameworks have been obtained.The microstructures(morphologies,particle sizes,spacing among particles and compositions)of above macroporous monoliths have been finely tuned by changing precursor composition and sintering temperature of precursors.Further investigation shows that these as-prepared monoliths display promising surface super hydrophobicity for n-octadecanoic acid modified MgAl₂O₄ spinels,ferromagnetism for MgFe₂O₄ ferrites and semiconductor optical behaviors for Mg-doped In₂O₃,respectively. Hydrothermal and solvethermal syntheses are soft solution processes (SSP)and environment-friendly technology.They are commonly used for the preparation of various inorganic functional materials.Based on the urea hydrolysis homogeneous precipitation,high surface area mesoporous ZnAl₂O₄ and NiAl₂O₄ spinels with high thermal stability have been prepared via a facile template-free hydrothermal approach.After detailed characterization,it has been found that this strategy involves two-step reaction process,including the formation and structural transformation of precursor.By controlling the synthesis parameters such as reaction temperature,urea concentration,and calcination temperature,the structural and textural properties of mesoporous spinels can be regulated.However,after adding appropriate amount of different alcohols,crystallized micro/mesoporous Zn-Al complex oxides have been successfully prepared in a single step synthetic route without organic templates.In addition,the influence of alcohol types and adding amounts on the structure and textural properties of micro/mesoporous Zn-Al complex oxides have also been detailedly investigated.