Solvent-directed and Anion-Modulated Self-Assemblies of Nanoparticles: a Case of ZnO

Self-assembly of nanoparticles is one of the most appealing fields in nanoscience. In the present article, we demonstrate solvent-directed and anion-modulated self-assemblies of ZnO recrystallized from layered basic zinc acetate nanoflakes by heat treatment in liquid media. Diverse controllable ZnO patterns, including nanodots, nanodisks, twinned mesocrystals, mesoporous nanospheres, unprecedented triangular nanoprisms and porous nanosheets, have been achieved from the same precursor, indicating a fascinating platform for studies of self-assembly. It is confirmed that polar protic solvents such as water, alcohol and methanol not only provide liquid media to disperse the precursor nanoflakes but also force their decomposition under mild conditions without basic additives and direct self-assemblies of the resulting ZnO. Both the physicochemical property and the structure of the solvent play important roles. Due to the anion exchange reactions of zinc hydroxide salts and face-selective electrostatic interactions between the added anions and wurtzite ZnO, the conversion and self-assembly process can be effectively modulated by the addition of anions. When the affinity of added anions to the zinc hydroxide layers is weak, the conversion process will be slackened and the preferential assembly direction of ZnO will be modified. However, when the affinity is strong, other stable zinc hydroxide salts instead of ZnO will be obtained. Our findings give new insights into the solid-phase transformation and assembly from the precursor nanoflakes to ZnO. In addition, the outstanding light scattering feature of the resulting ZnO mesocrystals confers application significance on them.