Direct Imaging of Atomic-Scale Surface Structures of Brookite TiO2 Nanoparticles by Frequency Modulation Atomic Force Microscopy in Liquid

Abstract

Direct imaging of atomic-scale surface structures of TiO2 nanoparticles is a promising method for understanding the detailed mechanism of their photocatalytic activities. Atomic force microscopy (AFM) is one of the analytical tools used for direct imaging of surfaces, but atomic-scale AFM studies of TiO2 nanoparticles in liquid have not been reported. Here, we report two methods for fixing of TiO2 nanocrystals: an electrostatic method and a cross-linking method. Both methods enabled visualization of subnanoscale surface structures of brookite TiO2 nanoparticles in liquid by frequency modulation AFM (FM-AFM). The FM-AFM imaging results and density functional theory molecular dynamics calculations suggest that the subnanoscale structures in the FM-AFM images can be explained by the arrangement of the surface topmost atoms or their hydration structures on the brookite TiO2(210) surface. Our results open up various possibilities for studying atomic-scale surface structures of nanoparticles in liquids by AFM.