Investigating the Carbonation of Ca(OH)2 using Cryo- and Liquid Cell TEM
- Abstract number
- 258
- Presentation Form
- Contributed Talk
- DOI
- 10.22443/rms.mmc2023.258
- Corresponding Email
- [email protected]
- Session
- EMAG - In-situ EM Techniques & Analysis
- Authors
- Dr Stephanie Foster (2), Dr Martha Ilett (2), Dr Yifei Xu (1), Dr Zabeada Aslam (2), Dr Helen Freeman (2), Professor Rik Drummond-Brydson (2), Professor Fiona Meldrum (2)
- Affiliations
-
1. Fudan University
2. University of Leeds
- Keywords
liquid cell, LCTEM, crystallisation, in-situ, materials science, TEM
- Abstract text
Calcium carbonate is a important mineral for the production of numerous materials, such as paper, plastic and rubber, in addition to being a ubiquitous component of many household products. The vast quantity required to satisfy demand is produced industrially via the carbonation of Ca(OH)2, however this reaction has been significantly under-studied compared with other methods of synthesising CaCO3 and the underlying mechanism is still debated. Therefore, there is a need to clearly observe and capture the dynamics of the reaction unfolding in-situ.
In this work, the reaction was studied using conventional, cryogenic (cryo) and liquid cell (LC) transmission electron microscopy (TEM) in a correlative approach, to determine whether the carbonation reaction occurs via a solid-state transformation or follows a dissolution-precipitation pathway. To effectively observe the reaction in TEM, thin hexagonal plates of Ca(OH)2 were prepared and dispersed in alcohol, before adding water to initiate the reaction with atmospheric CO2. Samples of the reaction were drawn at different time points before pausing the reaction using a Vitrobot, which were brought to room temperature under vacuum for study with conventional TEM or studied as prepared in cryoTEM. The reaction was also observed dynamically in a flow LC, where the Ca(OH)2 suspension was loaded first and water was flowed through the cell in-situ.
In the early stages of the reaction, the hexagonal plates of Ca(OH)2 were found to initially dissolve preferentially on the basal {0001} faces, contrary to previous AFM and modelling studies which suggest that the prismatic faces dissolve first. This was particularly evident in the LCTEM videos captured of the dissolution process, where EDX was used to confirm that water had entered the LC. Further reaction led to the formation of amorphous calcium carbonate (ACC) on the outer prismatic edges, yielding intricate skeletal structures that clearly retained a hexagonal morphology. This strongly indicated that the dissolution occurs via an interface coupled dissolution-precipitation reaction, that results in pseudomorphic replacement of the Ca(OH)2 plates with ACC. The amorphous species ultimately dissolved to precipitate the most stable crystalline phase of CaCO3, calcite. Calcite crystals observed at the earliest stages of growth using cryo-TEM had elliptical morphologies and possessed granular structures. Closer inspection showed that these particles consisted of many individual, aligned needle-like crystallites. At a later stage of growth, scalenohedral calcite crystals were formed and the individual crystallites could no longer be distinguished, indicative of an oriented attachment growth mechanism.
In summary, the reaction mechanism behind the carbonation of Ca(OH)2 was elucidated using LCTEM, where it was possible to directly observe and identify the various stages of the dissolution-precipitation reaction as they occurred. Cryo-TEM was also used to characterise the reaction and glean further mechanistic information on crystal growth, as the structure of beam-sensitive intermediate species and crystalline species at early growth stages could be examined in detail. Overall, these results demonstrate that a correlative approach employing both cryo- and LCTEM can provide broad insight into crystallisation reaction mechanisms.