Variations in Atomic-Scale Step Edge Structures and Dynamics of Dissolving Calcite in Water Revealed by High-Speed Frequency Modulation Atomic Force Microscopy
K Miyata and Y Kawagoe and J Tracey and K Miyazawa and AS Foster and T Fukuma, JOURNAL OF PHYSICAL CHEMISTRY C, 123, 19786-19793 (2019).
Calcite dissolution plays critical roles in the global carbon cycle in the Earth, biomineralizations, and weathering of buildings. However, in spite of the importance, the atomistic events at the step edges during the dissolution have remained elusive because of the difficulties in their direct visualization. In this study, we used high-speed frequency modulation atomic force microscopy (FM-AFM) for visualizing various atomic-scale step edge structures and dynamics during the calcite dissolution in water. The obtained images reveal the coexistence of the steps with and without a transition region (TR); a Ca(OH)(2) monolayer formed along the step edge as an intermediate state in the dissolution. The TRs are more frequently observed along the slowly dissolving acute steps than along the rapidly dissolving obtuse steps. This finding and our imaging of the TR formation process suggest that the free energy to form a TR is comparable to that for removing ions directly from the step edge and an increase in the local ionic strength may alter this balance to facilitate the TR formation. Once the TRs are formed, their width or height shows no significant dependence on the crystallographic orientation or the step velocity. We also found that the FM-AFM images of the TRs show several variations. However, the hydration structures obtained by our molecular dynamics simulation demonstrate that the observed variations can be explained by the different tip trajectories on the different hydration layers. These findings should improve our understanding on both the calcite dissolution mechanism and the FM-AFM measurement principle.
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