Synthesis And Structure Of ZrO₂-based Nanocrystals

Zirconia nanomaterials have attracted great interest owing to their unique properties such as acid sites and alkali sites as well as oxidoreduction ability and potential application in optoelectronic devices, solar cells, gas sensors and acid- or base-catalyzed reactions. However, aggregation or poor monodispersity and low specific surface area of the materials resulted from their high hydrolysis reaction and subsequent high temperature treatment processes greatly affect their properties and restrict their potential application.The specific surface area and crystal phase of the zirconia could be improved through the addition of transition metal or surfactant species in the synthesis process. But it is a key problem to carry out the controllable synthesis of zirconia nanomaterials with high specific surface area and good monodispersity.The zirconia nanocrystals with high surface area and good monodispersity were synthesized via vapor hydrolysis method without high temperature treatment processes through avoiding the direct contact of water and zirconium source. The structure and properties of as-synthesized zirconia nanocrystals were characterized with X-ray powder diffraction(XRD), nitrogen gas adsorption-desorption technique, scanning electron microscopy(SEM), transmission electron microscopy(TEM) and selected-area electron diffraction(SAED). The experiment results indicate that as-synthesized zirconia nanocrystals with tetragonal structure possessed high specific surface area ranging from 188 to 450 m2·g-1 and average sizes of less than 5 nm through the variation of the synthesis parameters such as precursor concentration, reaction temperature and time as well as solvents. In addition, the formation mechanism that the hydrolysis rate and crystallization rate cooperatively control the synthesis of zirconia nanocrystals was proposed.The tetragonal sulfated zirconia solid acid catalysts were prepared by directly impregnating ammonium persulphate on the crystalline zirconia following high temperature treatment. The calcined catalyst at 500 ℃ exhibited high catalytic activity in transesterification of soybean oil with methanol. The yield of fatty acid methyl esters achieved 84.6% because of the preferable superacid sites on the surface of the catalyst.The sulfated zirconia solid acid catalysts with abundant sulfur content were prepared by the vapor-controlled route using ammonium persulphate as sulfate species. The XRD analysis results confirmed that the samples calcined at 500 ℃ exhibited the tetragonal structure at any S/Zr molar ratio and possessed high sulfur content of 8.6 wt %-10.4 wt % on the surface of zirconia, indicating that the introduction of sulfur favored the stabilization of tetragonal phase. The phase transition from tetragonal to monoclinic phase occurred and the content of tetragonal phase decreaseed from 81 vol % to 63 vol % for the obtained samples calcined at 600 ℃, ascribed to the loss of sulfur species. Importantly, they showed excellent catalytic activity with 81.6%-100% yields of biodiesel for the transesterification of soybean oil with methanol.The ordered mesoporous zirconia nanocrystals were prepared by a vapor hydrolysis reaction using triblock copolymer(P123 and F127) and ionic type(CTAB) templates separately. The wide-angle XRD results confirmed that all the prepared samples exhibited tetragonal structure of zirconia. The samples showed reflection peaks at about 2θ=1°, which revealed that they possessed mesoporous structure. However, the ordering of the mesopores was related to the structure and content of the templates. The N2 adsorption-desorption results show the materials possess the pore volume of 0.09-0.23 cm3·g-1, the BET surface areas of ranging from 112.5 to 166.9 m2·g-1 and the average pore size of 3.3 nm. The SEM images clearly show that the mesopores were consistent with the intercrystalline mesopores from aggregation of the roughly spherical nanoparticles with the grain size of about 5 nm.On the base of the ordered mesoporous zirconia nanocrystals, hierarchically meso/microporous Al-Zr nanocrystals materials with tetragonal structure were explored through vapor hydrolysis reaction using aluminum isopropoxide as precursor. The properties of resultant materials were characterized by several kinds of means. The experimental results show that all the crystallinity and well-ordered mesoporous structure of the samples were correlated with the Al/Zr molar ratio. Compared with the mesoporous zriconia nanocrystals, the samples exhibited higher pore volume of 0.13-0.56 cm3·g-1 and larger BET surface areas ranging from 191.6 to 313.2 m2·g-1. It was also found that the introduction of Al into zirconia nanocrystals also increaseed the percentage of microporous surface areas ranging from 3.7% to 45.6% in the materials. This indicates that Al3+ could randomly substitute some of Zr4+ in the host lattice, which could further favor the formation of the micropores in the materials. The SEM and TEM images further confirm that the hierarchically porous structure of the materials was consistent with the intercrystalline mesopores from aggregation of the roughly spherical nanoparticles and the short-range ordering micropore channels within the crystals.