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

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.