High-speed AFM tracks mystery dissolution

Editorial

Rebecca Pool

Wednesday, August 2, 2017 - 20:30
Image: Investigating dissolution at step edges of calcite in water with high-speed frequency modulation AFM [Kazuki Miyata et al, NanoLetters,  2017, 17 (7), pp 4083–4089]
 
Using atomic force microscopy and claiming a world first, researchers from Japan-based Kanazawa and Tohoku Universities, and Aalto University, Finland, have imaged how a calcite surface dissolves in water with atomic-scale resolution.
 
Dissolution of calcite is key step in geologic carbon sequestration, so understanding this mechanism is crucial to safely capturing carbon dioxide from the air and storing it underground.
 
But while a microscopic understanding of this process has been advanced by imaging nanoscale step flows via AFM, optical interferometry, and X-ray microscopy, atom dynamics at the step edges, have not been well understood.
 
Given this, the researchers developed high-speed frequency modulation AFM, enabling true atomic-resolution imaging in liquid.
 
(a) Atomistic model of calcite surface. (b) The dissolution processes of calcite surface in water observed with high-speed FM-AFM. It is observed that the step is moving from lower-right to upper-left. Along the step is also seen the transition region. (c) Averaged height profile measured along the line PQ indicated in (b). The height of a monolayer step is ~0.3 nm, but that of the transition region is smaller. A terrace described in the Figure indicates a flat area at the atomic level on the crystal surface. The upper terrace is higher by one monolayer of CaCO3 than the lower terrace. [Kanazawa University]
 
Imaging speeds were improved from ~1 frame/min to ~1 frame/sec without losing high spatial resolution.
 
And as the researchers highlight, this frame rate is around 50 times faster than that for conventional FM-AFM.
 
Imaging revealed that a transition region of a few nm width along a step is formed as an intermediate state in the dissolution processes.
 
According to the researchers, the formation of this transition region was not foreseen by previous studies, so that without high-speed FM-AFM, it would not have been discovered.
 
In addition, in order to elucidate the origin of the transition region and dissolution mechanism, the team examined the validity of various transition region models by density functional theory calculations and by molecular dynamics simulations.
 
Analyses indicated that the transition region would most likely be a Ca(OH)2 monolayer formed as an intermediate state in the dissolution processes of calcite.
 
With a model that places a monolayer of Ca(OH)2 in proximity of a step at the boundary of upper terrace and lower terrace, molecular dynamics simulation was performed for about 7.5 ns to confirm that the monolayer of Ca(OH)2 existed stably adjacent to the step without being separated from the crystal surface. [Kanazawa University]
 
To the team's knowledge, this is the very first proposal for the dissolution processes at atomistic level based on such direct experimental evidences.
 
Research is published in NanoLetters.
 
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