Atomic-Scale Processes at the Fluorite–Water Interface Visualized by Frequency Modulation Atomic Force Microscopy

The crystal growth and dissolution processes of a fluorite (CaF 2 ) crystal have attracted much attention due to the importance in the industrial, environmental, and medical applications. While previous studies clarified nanoscale processes at the fluorite–water interface, atomic-scale origins of the processes have yet to be understood. In this study, we have investigated atomic-scale processes at the fluorite–water interface by frequency modulation atomic force microscopy (FM-AFM). We performed atomic-resolution imaging of a fluorite(111) surface in water (pH = 2 and 6.5), saturated solution (pH = 2 and 6), and supersaturated solution (pH = 6, σ = 10 and 100). Based on the results, we present three major findings. First, atomic-scale roughening of the crystal surface takes place at low pH due to the proton adsorption. Second, surface adsorbates with a subnanometer-scale height are formed on the crystal surface at high pH. They are most likely to be calcium hydroxo complexes physisorbed on the crystal surface. Finally, the formation of these complexes can be suppressed by increasing fluorite concentration owing to the increased proportion of Ca 2+ and F – in the electric double layer. These findings mark an important step toward the full understanding of the physicochemical processes at the fluorite–water interface.