Direct imaging of construction of carbon onions by curling few-layer graphene flakes.

The violent reaction processing and required high-temperature environment involved in growing carbon onions make it difficult to obtain an insight into their evolution mechanism. By using deionized water as the medium of arc discharge, we successfully froze the synthetic reaction at intermediary stages and observed detailed structures of the obtained intermediates of carbon onions. Here we present the atomic-scale scanning transmission electron microscopy investigation of carbon onions produced by arc discharge in water. We directly observed that carbon onions at intermediary growth stage are characterized by unclosed few-layer graphene shells. Meanwhile, a kind of graphene flakes composed of 3 layers or less were also observed in the sample. The kindred evolution linkage was induced to exist among these few-layer graphene flakes and carbon onions in the arc discharge synthetic process. On the basis of microscopy observations, we propose that carbon onions are constructed by curling few-layer graphene flakes, which is beneficial for structural designs and controls of related carbon materials used in different fields.