One-Step Synthesis of Graphene-Based Hybrid Structures with Surface Microspheres with a Template-Free Method

The flexible assembly properties of GO nanosheets in a solution system enable them to be effectively constructed into novel and ordered macroscopic material structures. Among various 3D structures, graphene hollow spheres (GHSs) have attracted great attention due to their wide application potential such as compounds in catalyst carriers and electrode materials. In this paper, a novel template-free self-assembly method was proposed to produce GHSs with better spherical shape, thinner walls, and sizes at the microscale. A bubble generator was used to introduce uniform and numerous bubbles in the GO sheets, inducing curling and wrinkling around the GO/water interface. The structural evolution of the GO dispersion during hydrothermal and drying processes was investigated after different treatments. It was found that the acidification treatment promotes the hydrothermal reduction reaction of GO sheets, which leads to the reduction of their oxygen content and the decrease of their interlayer spacing (d-spacing). Furthermore, the micro- and nanobubble treatment contributes to the homogeneous dispersion of GO sheets and induces their curling behavior. When the hydrothermal reaction products were dried at 100 °C, the release of solvent and gas from the GO sheets was inhibited by the small d-spacing, leading to the expansion of the surface layers and the formation of graphene hollow microspheres. The detailed formation mechanism of graphene hollow microspheres was verified by molecular dynamics (MD) simulations of water molecule permeation behavior with different graphene layer spaces. The findings of this study will contribute to the structural design of graphene-based materials.