Microscopic Kinetics Pathway of Salt Crystallization in Graphene Nanocapillaries

The fundamental understanding of crystallization, in terms of microscopic kinetic and thermodynamic details, remains a key challenge in the physical sciences. Here, by using in situ graphene liquid cell transmission electron microscopy, we reveal the atomistic mechanism of NaCl crystallization from solutions confined within graphene cells. We find that rock salt NaCl forms with a peculiar hexagonal morphology. We also see the emergence of a transitory graphitelike phase, which may act as an intermediate in a two-step pathway. With the aid of density functional theory calculations, we propose that these observations result from a delicate balance between the substrate-solute interaction and thermodynamics under confinement. Our results highlight the impact of confinement on both the kinetics and thermodynamics of crystallization, offering new insights into heterogeneous crystallization theory and a potential avenue for materials design.

Figure 1

Growth of the hexagonal-shaped B1-NaCl island. (a) Schematic of droplets encapsulated in a GLC, and the B1-NaCl crystal lattice viewed along the [001] and [110] crystallographic directions along with their corresponding diffraction patterns. (b) TEM image of a NaCl crystal grown in the GLC, showing low-index facets with 120° angles. (c) Corresponding diffractogram of the TEM image in (b), indicating NaCl oriented along [110]. (d),(e) Snapshots of the NaCl crystal growth process. While the (001) plane grows continuously, the $(\overline{1}11)$ surface shows a sawtoothed plane composed of (001) and $(\overline{1}10)$ facets, indicating that the layer-by-layer lateral growth of (001) and $(\overline{1}10)$ facets dominate. (f),(g) Corresponding schematics of the TEM images shown in (d),(e).

Figure 2

Formation of the $h\text{−}\mathrm{NaCl}$ phase. (a) TEM image of NaCl crystals grown in a GLC, showing an in-plane hexagonal lattice. Inset: diffractogram of a graphene-only area with a rotation angle of approximately 30°. (b)–(d) The corresponding diffractograms of selected regions (yellow boxes) in (a), clearly showing sixfold symmetry (inner yellow circles). The rotation angles between NaCl and graphene (outer white dashed circle) varies. (e) EELS of the pocket area, showing signals only from C, Cl, and Na. (f)–(h) Sequential TEM images of $h\text{−}\mathrm{NaCl}$ crystal growth. The two predominant surfaces have an angle of 30°.

Figure 3

Transformation from $h\text{−}\mathrm{NaCl}$ to $\{110\}\text{−}\mathrm{B}1\text{−}\mathrm{NaCl}$. (a)–(e) Sequential TEM images of a graphene pocket. Dark contrast in the middle of the image indicates the area of the trapped solution. Initially there is no crystal signal from the pocket [(a), inset]. After 5 s nuclei with the hexagonal structure fill the whole graphene pocket with a uniform lattice (b). Inset of (b) shows spots with sixfold symmetry. In the next stage the hexagonal structure shrinks (c), followed by transformation to B1-NaCl nuclei (d). Dashed red and yellow lines highlight the $h\text{−}\mathrm{NaCl}$ and B1-NaCl regions, respectively. Eventually, a large $\{110\}\text{−}\mathrm{B}1\text{−}\mathrm{NaCl}$ crystal with a hexagonal shape is observed (e). (f)–(j) Schematics of the corresponding processes in (a)–(e).

Figure 4

Energetics of NaCl clusters. (a)–(c) Schematic representations of $\{001\}$ B1, $\{110\}\text{−}\mathrm{B}1$, and $h$ clusters bound to graphene, as indicated at the bottom of each panel. (d) Formation energy per formula unit ($e_f$) vs $1/N$. In vacuum, $\{001\}\text{−}\mathrm{B}1$ clusters (squares) are more stable than $\{110\}\text{−}\mathrm{B}1$ clusters (hexagons) for finite $N$. In solvent, the two structures are energetically similar. While the bulk energy of the hexagonal structure (dotted line) is significantly higher than that of the B1 structure (dashed line), the energy difference is far less pronounced for $h$ clusters (circles). (e) Interaction energies with graphene vs $N$. The $\{110\}\text{−}\mathrm{B}1$ clusters interact much more favorably with graphene than do the $\{001\}\text{−}\mathrm{B}1$ clusters. The vertical dashed line separates small clusters (all clusters interact with graphene similarly) and large clusters ($\{110\}\text{−}\mathrm{B}1$ cluster interacts more strongly with graphene).