Rapid room-temperature synthesis of nanosheet-assembled ZnO mesocrystals with excellent photocatalytic activity

Abstract

We have investigated the growth of ZnO in a simple alkaline solution by chemical precipitation. ZnO nano(micro)structures with well-defined morphologies have been realized by tuning the Zn2+ : OH− ratio and the amount of the complexing agent NH4F. Importantly, we have demonstrated a fast spontaneous room-temperature formation of ZnO mesocrystals constructed with nanosheet subunits. The growth of ZnO involves the phase transformation from two intermediate compounds ZnF(OH) and Zn(OH)2. The nanosheets are observed to be ZnO (100) facets from the side- and top-view high-resolution TEM measurements. By comparing the arrangement of the (100) nanosheets in the mesocrystal with the organization of the {100} planes in the wurtzite crystal structure of ZnO, we identify for the first time the assembly of 2D nanosheets into 3D architectures by epitaxy. This unique self-assembly by epitaxy effectively separates the nanosheets in an ordered manner, producing a robust, geometrically ideal photocatalyst that is easily separable and recyclable. Being constructed of reactive (100) facets, the microstructured ZnO exhibits much higher photocatalytic efficiency than its nanoparticle counterpart. The defective nature of the (100) facets confirmed by the photoluminescence study explains the visible photocatalysis of the nanosheet assemblies.