Spontaneous stepwise formation of polar-facet-dominant ZnO crystals for enhanced catalytic H2O2 generation

Abstract

Noncentrosymmetric polar materials having polar facets exhibit unique and often useful physicochemical properties. However, the fraction of polar facets having a high specific surface energy is, generally, low because of free energy minimization during crystal growth. In this study, a mechanism for the spontaneous formation of polar ZnO crystals with high sphericity and a high fraction of polar ZnO facets, as supported by the results of material characterization, is proposed. Specifically, in a supersaturated solution of l-ascorbic acid, complexes of ZnO and organic species form nanoparticles, which then aggregate and merge to form large spherical structures. In these spherical structures, crystalline domains of polar ZnO oriented along [0001] dominate in the radial direction. In addition, during the growth period, the central part of the structure, which has a high curvature, releases strain by outward mass transfer and dissolution, and the spheres becomes hollow. The resulting hollow spherical particles having a high fraction of polar facets show higher activity for H2O2 generation via O2 reduction than rod-like ZnO structures, which have a dominant nonpolar surface, prepared in the absence of l-ascorbic acid.