Solvent-directed And Ion-modulated Self-assembly Of Nanoparticles: A Case Of ZnO

Self-assembly of nanoparticles is one of the most appealing fields in nanoscience and has been the focus of immense scientific interest due to its importance in building complex superstructures with new collective properties.However,the underlying mechanisms regulating the self-assembly are currently not well-understood due to the complexity of the processes that depend on the morphologies of the building blocks and many cooperative driving forces,such as electrostatic fields,magnetic fields,van der Waals forces,hydrogen bonds.In wet chemical synthesis of nanoparticles,solvents and ions are important substances.Taken their various interactions with nanocrystals into consideration,it can be inferred that solvents and ions have significant influence on the self-assembly of nanocrystals.In the present article,we demonstrate solvent-directed and ion-modulated self-assemblies of ZnO recrystallized from layered basic zinc acetate(LBZA)nanoflakes by heat treatment in liquid media.To exclude the complexity of the coexistence of many chemical species,we adopt a two-step method to examine the effect of solvents.Namely,the LBZA nanoflakes are first synthesized in methanolic solution at room temperature and then undergo heat treatment in various pure solvents or aqueous solutions.We first investigate the influence of water,methanol and alcohol,as well as the mixture solvents of water and alcohol.Then,we further study the influence of various ions including COOH",NO3-,CO32-,Cl-and SO42-.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.For example,the polarity of the solvent not only influence the electrostatic force,but also determine the solubility of acetate anions.If the solubility of acetate anions in solvent is too low,the main acetate anions would like to graft on the surface of nanocrystals and subsequently prevent them from self-assembling.On the other hand,the local structures of the mixture solvents may result in the similar distributions of the acetate anions released from the precursor and serve as templates for the self-assembly of nanocrystals.We find that the conversion and self-assembly process can be effectively modulated by the addition of ions,which depends on the affinity of ions to the nanocrystals.When the affinity of added anions to the zinc hydroxide layers is strong,chemical reactions may occur between them.However,when the affinity is weak,the preferential assembly direction of ZnO will just be modified.Diverse controllable ZnO patterns have been achieved from the same precursor,benefiting from the influence of various solvents and ions.In addition to the fundamental studies,the outstanding properties of the resulting ZnO nanostructures confer application significance on them.For example,the quantum dots with excellent luminescent property are promising in the field of bioimaging,while the mesocrystals with significant light scattering feature are potential to be used on solar cells.