Real-Time Observation of CaCO3 Mineralization in Highly Supersaturated Graphene Liquid Cells

Abstract

The mineralization dynamics of calcium carbonate is investigated under highly supersaturated conditions using graphene liquid cell transmission electron microscopy. We demonstrate that the mineralization process has three steps: nucleation, diffusion-limited growth, and Ostwald ripening/coalescence. In addition, we show that the polymorphs precipitate in a specific order, from metastable aragonite to stable calcite, thus proving Ostwald's rule of stages. In highly supersaturated solutions, the aragonite phase crystallizes in a stable manner, in addition to the calcite phase.

Figure 1

(a) Schematic of the GLC structure containing the calcium bicarbonate solution. The precursor solution is prepared by bubbling carbon dioxide gas into the calcite-suspended water. The precursor solution is then trapped with multilayer graphene membranes. When the GLC was exposed to the electron beam, particles are precipitated as water evaporates. (b) Filtered HRTEM images of the precipitates and the corresponding FFTs with each crystal structure of calcite and aragonite. Simulated images are presented in each of the HRTEM images, surrounded by the dotted yellow line. Z.A. is the zone axis.

Figure 2

In situ dark-field TEM analysis of CaCO₃ crystallization. (a) Time series images corresponding to each mineralization stage. (b) Each black and blue dot gives the number of particles and total projected area of all precipitates as a function of irradiation time t. Based on the dynamics of mineralization, the whole process is divided into three stages: nucleation, diffusion-limited growth, and Ostwald ripening/coalescence. Red solid lines show the trends in each stage. (c) Logarithm relationship between the radius r of the particles and t. (d) Increase in the ratio of the number of particles larger than r during the growth step (Stage 2), ΔR_60,160(r). The peak indicated by the red dotted line shows the critical radius for the nucleation, r_c(nucleation).

Figure 3

Time series HRTEM images of mineralization are acquired. Nanocrystals are color-coded using IFFTs, where magenta and green represent aragonite and calcite polymorphs, respectively. FFT patterns from the white dotted box are indexed below the corresponding image. The sequential growth of CaCO₃ polymorphs, from aragonite to calcite, shows that the mineralization follows Ostwald’s rule of stages. (Z.A. is the zone axis.)

Figure 4

Schematic describing the whole mineralization process in the GLC. Calcium carbonate minerals are precipitated in the highly supersaturated solution by the evaporation of water and the degassing of carbon dioxide. The mineralization process occurs in three distinct stages. Under the highly supersaturated conditions, mineralization of the metastable aragonite phase is preceded by the stable calcite phase.