Controlled Crystal And Morphology Synthesis Of Layered Double Hydroxides

Layered Double Hydroxides(LDHs)or hydrotalcite-like compounds(HTLc)are a class of anionic clays materials whose structure is based on brucite(Mg(OH)₂)-like layers in which some of the divalent cations have been replaced by trivalent metallic cations giving positively-charged sheets and hydrated interlayer domains containing anionic species.This characteristic results in much more difficulty in controlling its structure and morphology than single metal hydroxides.The expected hexagonal plate- or flake-like crystallites with diameters from nanometers to microns were grown by various preparation methods such as co-precipitation,urea hydrolysis,separate nucleation and aging, ion-exchange,metal oxide precursor method,reflecting the rigidity and hardness of LDHs' host layer.In light of confined growth of nanoparticles,the inorganic/organic self-assembly and template chemistry,three kinds of templates have been used to control the crystal growth and morphology of LDHs.Using microemulsion as template,floccule or fiber-like LDH nanoparticles have been obtained.The microemulsion formed by octane-dodecylsulfonate-water has been used as microreactor and controls the crystal size of LDHs particles.The adsorption of dodecylsulfonate on LDH's layers leads to much larger specific suface area and pore volume than conventional method.In the chitosan(CS)aqueous solution,Mg/Al-CO₃^2-LDHs,Zn/Al-CO₃^2-LDHs, Ni/Al-CO₃^2-LDHs and Ni/Fe-CO₃^2-LDHs have been synthesized by co-precipitation method and urea hydrolysis method.Sheet-curved Ni/Al-CO₃^2-LDH particles have been obtained successfully.The complexation of chitosan molecule with Ni²⁺controls the precipitation speed of Ni²⁺and sheet-growth orientation,which result in smaller crystal size and sheet-curved morphology.For the curved or bended layer-growth,the crystal structure parameters have also been changed,such as lattice parameter.For the complexation of chitosan molecule with Ni²and Fe3+,Ni/Fe-CO₃2-LDHs show larger crystal size but planar layer morphology.For the weak complexation of chitosan molecule with Mg2+and Zn2+,Mg/Al-CO₃2-LDHs and Zn/Al-CO₃2-LDHs cannot be bended by chitosan molecule.Using vesicles as template,layer-disordered or bedned Mg/Al-LDHs,Co/Al-LDHs and Ni/Al-LDHs intercalated with organic anions have been obtained in three kinds of anionic surfactants(sodium dodecylsulfonate(SAS),sodium dodeculsulfate(SDS)and bis(2-ethylhexyl)sulfosuccinate(AOT))aqueous solution,experimentally indicating the general flexibility of this class of anionic clays.In a certain anion surfactant and metal ion concentration,surfactant anions form vesicle structures,which control the nucleation and nuclei growth of LDHs crystallites occuring along the curved and bended inorganic-organic interface.The formation process of special aggregate morphology has also been investigated.Using the in situ decomposition of interlayer DS to SO₄2-and ion-exchange of interlayer anion by CO₃2-,layer-disordered Mg/Al-LDHs with SO₄2-and CO₃2-inorganic anions intercalated have been obtained.During the great change of interlayer spacing,the bended layer morphology is retained,further showing the flexibility stability of LDHs' layer.</sup>