Evolution of Calcite Nanocrystals through Oriented Attachment and Fragmentation: Multistep Pathway Involving Bottom-Up and Break-Down Stages

A nonclassical multistep pathway involving bottom-up and break-down stages for the evolution of calcite nanograins ∼50 nm in size was demonstrated in a basic aqueous system. Calcite nanofibrils ∼10 nm wide were produced as the initial crystalline phase via amorphous calcium carbonate through ion-by-ion assembly by the carbonation of Ca(OH) ₂ at a high pH of ∼13. Bundles ∼50 nm in diameter were then formed by the subsequent oriented attachment of the nanofibrils. Monodispersed calcite nanograins were finally obtained through spontaneous fragmentation of the fibrous forms via a decrease in pH by further carbonation.