Directly assembling ligand-free ZnO nanocrystals into three-dimensional mesoporous structures by oriented attachment.

Mesoporous materials have found a great number of important utilities due to their well-defined pore structure and high internal surface area, which are routinely synthesized with the assistance of block copolymers or templates. So far, a key challenge is how to assemble directly ligand-free inorganic nanocrystals into mesoporous structures, so that the high surface activity of ligand-free nanocrystals is not destroyed by further treatment to remove organic species or templates. In this paper, we report the direct assembly of highly uniform ZnO mesoporous ellipsoids from ligand-free ZnO nanocrystals of ∼5 nm. The size of the synthesized uniform ZnO mesoporous ellipsoids can be efficiently tuned from 132 × 75 to 190 × 111 nm (length × width), by varying the size and concentration of primary ZnO nanocrystal building blocks and the composition of the designed assembling solvent. The BET detection indicates that these ZnO mesoporous ellipsoids have high specific surface areas reaching to 136.57 m(2)/g, while their average BJH pore diameters are located at 8.8 nm. Especially, the high-resolution TEM images and XRD analysis revealed the occurrence of an oriented attachment mechanism in the assembly of uniform ZnO mesoporous ellipsoids, which supplied an important proof for the possibility of constructing stable three-dimensional structures by oriented attachment. The benefits of these ZnO mesoporous ellipsoids were demonstrated by their excellent photocatalytic activity under weak UV irradiation.