Controlled synthesis of prismatic calcite crystals mediated by polyethylene glycol

Non-ionic polymer polyethylene glycol (PEG) with varying molecular weights has been applied as an additive for the crystallization of calcium carbonate. However, it is not known how PEG influences the polymorphs and morphologies of calcium carbonate via a typical slow gas diffusion strategy using ammonium carbonate as the carbonate source while the slow introduction of CO₂ to the solution might induce the exquisite control of additive to the polymorphs and the crystallization process of calcium carbonate. Herein, dispersed one-dimensional calcite microprisms exposing {1 0 0}, {1 1 0} side planes and the {1 0 4} ending planes were obtained for the first time via the gas diffusion method in the presence of PEG with varying molecular weights ranging from 3350 to 8000 g mol⁻¹. It shows that PEG with molecular weights in between 3350 g mol⁻¹ and 8000 g mol⁻¹ can stabilize amorphous calcium carbonate precursors in solution and significantly inhibit nucleation according to the titration analysis and the early stage intermediate phases analysis. It is proposed that PEG selectively adsorbs onto calcite crystal planes (e.g., {1 1 0} and {1 0 0}), suppressing their growth rates, and induces the formation of prismatic calcite crystals elongated along the c-axis via a non-classical crystallization pathway involving nanoparticle aggregation and oriented attachment. Particularly, PEG4000 exhibits the strongest nucleation inhibition and stabilization effect on the {1 0 0}, {1 1 0} side planes, yielding prismatic calcite crystals with the largest aspect ratio of length and diameter (≈2.6). We also realized the formation of oriented one-dimensional prismatic calcite arrays by using PEG6000 via epitaxial growth and secondary nucleation on geological calcite substrates. This study elucidates the molecular-weight-dependent crystallization regulation mechanism of PEG. By tuning the molecular weight of PEG, precise control over the morphology of calcite crystals and the growth of micrometer-scale prismatic arrays on geological calcite is achieved, providing a new strategy for the biomimetic construction of inorganic materials with specific crystal orientations and morphologies.