Kinetically Synchronized Aqueous Co-precipitation and Hydrothermal Crystallisation of (Mg1−xCox)CO3 Rhombohedral Solid-Solution Nanoparticles: Thermodynamic Feasibility, Ethylene Glycol Kinetic Control, and Prospective Functional Applications
Year: 2026
Type: preprint
Abstract: Cobaltoan magnesite, (Mg1−xCox)CO3 (R3c; x = 0–1), has previously been accessible only under extreme solid-state conditions (≥600 °C, ≥15 kbar; Goldsmith & Northrop, 1965) or over geological timescales in supergene ore environments (Barton et al., 2014). We report the first deliberate laboratory synthesis of phase-pure (Mg,Co)CO3 solid-solution nanoparticles at 250 °C under autogenous pressure (~40 bar) via a modified polyol-assisted aqueous coprecipitation route. Thermodynamic feasibility is established through saturation-index analysis (SI(CoCO3) = +10.6, ΔG° = −60.5 kJ mol−1; SI(MgCO3) = +2.6, ΔG° = −14.8 kJ mol−1), confirming that both carbonate end-members are thermodynamically driven to precipitate from solution at ambient conditions and that the hydrothermal step supplies kinetic — not thermodynamic — activation energy. The principal kinetic challenge, the seven-order-ofmagnitude disparity in solubility products (Ksp(CoCO3) = 10−13 vs. 6.8×10−6 for MgCO3), is surmounted by ethylene glycol (EG) coordination of Co2+, confirmed in situ by a spontaneous pink→blue colour transition upon EG addition, which elevates the desolvation activation barrier and synchronises Co2+ and Mg2+ precipitation rates. Convergent evidence from five independent characterisation techniques confirms the R3c solid-solution product: (i) PXRD with Vegard's Law analysis of the composition-sensitive (113) reflection yields x(Co) ≈ 0.73 (Sample 3, pre-hydrothermal) and x(Co) ≈ 0.44 (Sample 4, post-hydrothermal); (ii) FTIR reveals a single ν2(CO32−) band at 870 cm−1, intermediate between MgCO3 (880) and CoCO3 (860 cm−1) endmembers — a necessary and sufficient criterion for isomorphic solid-solution identification by infrared spectroscopy; (iii) SEM-EDX shows a continuous unimodal composition distribution (x(Co) = 0.27–0.73) with no bimodal clustering indicative of phase separation; (iv) UV–Vis spectroscopy identifies characteristic Co2+ ligand-field and charge-transfer transitions with a direct optical band gap of ~3.1–3.4 eV; (v) DLS confirms near-monodisperse product (PDI = 0.053, Dh = 306 nm) following hydrothermal treatment, representing an 83% reduction in polydispersity relative to the amorphous precursor (PDI = 0.314). Prospective applications spanning heterogeneous catalysis, Li-ion battery cathode precursors, CO2 mineralisation, photocatalysis, and pH-responsive drug delivery are evaluated on the basis of the crystal chemistry and mixed-metal composition of the product phase.
Source: ChemRxiv
Language: English
Subfield: Biomaterials
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220940232