Abstract
Investigation on the formation mechanism of crystals via amorphous precursors has attracted a lot of interests in the last years. The formation mechanism of thermodynamically meta-stable vaterite in pure alcohols in the absence of any additive is less known. Herein, the crystallization process of vaterite microdisc mesocrystals via proto-vaterite amorphous calcium carbonate (ACC) in isopropanol was tracked by using Ca K-edge X-ray absorption spectroscopy (XAS) characterization under cryo-condition. Ca K-edge X-ray absorption near edge structure (XANES) spectra show that the absorption edges of the Ca ions of the vaterite samples with different crystallization times shift to lower photoelectron energy while increasing the crystallization times from 0.5 to 20 d, indicating the increase of crystallinity degree of calcium carbonate. Ca K-edge extended X-ray absorption fine structure (EXAFS) spectra exhibit that the coordination number of the nearest neighbor atom O around Ca increases slowly with the increase of crystallization time and tends to be stable as 4.3 (±1.4). Crystallization time dependent XANES and EXAFS analyses indicate that short-range ordered structure in proto-vaterite ACC gradually transform to long-range ordered structure in vaterite microdisc mesocrystals via a non-classical crystallization mechanism.
Highlights
Calcium carbonate is one of the most abundant biominerals existing in nature, which is often applied as a crystallization model for the investigation of nucleation and crystal growth mechanism [1,2].The polymorphs of calcium carbonate crystals include three anhydrous crystals: calcite [3], aragonite [4], and vaterite [5], and three hydrated crystal forms: calcium carbonate monohydrate, ikaite, and calcium carbonate hemihydrate [6]
We term the calcium carbonate samples with different crystallization times, e.g., vaterite-0.5d means the sample was crystallized for 0.5 d while it may still contain a certain amount of proto-vaterite amorphous calcium carbonate (ACC)
X-ray absorption near edge structure (XANES) spectra show that the absorption edges of the Ca ions of the vaterite samples with different crystallization times shift from 4052.90 to 4052.29 eV while increasing the crystallization times from 0.5 to 20 d, indicating the increase of crystallinity degree of calcium carbonate
Summary
Calcium carbonate is one of the most abundant biominerals existing in nature, which is often applied as a crystallization model for the investigation of nucleation and crystal growth mechanism [1,2].The polymorphs of calcium carbonate crystals include three anhydrous crystals: calcite [3], aragonite [4], and vaterite [5], and three hydrated crystal forms: calcium carbonate monohydrate, ikaite, and calcium carbonate hemihydrate [6]. Calcium carbonate is one of the most abundant biominerals existing in nature, which is often applied as a crystallization model for the investigation of nucleation and crystal growth mechanism [1,2]. There are a few amorphous calcium carbonate (ACC). Phases with different water contents, which are important precursors in both biomineralization and bio-inspired crystallization systems [7,8,9,10,11,12]. Vaterite is the thermodynamically least stable phase of the three anhydrous crystalline phases. Different solvents and additives can induce the nucleation and crystallization of thermodynamically meta-stable vaterite. Vaterite crystals have been synthesized in aqueous solutions [13,14,15], alcohol–water mixed solvents [16,17], dimethyl formamide–water mixed
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