Vapor diffusion method: Dependence of polymorphs and morphologies of calcium carbonate crystals on the depth of an aqueous solution

• The crystal polymorphs and morphologies highly depend on the solution depth.
• This is the first study relating the type of polymorph with the depth of solution.
• The concentration gradients are the main reasons to affect the polymorph formation.

The polymorph control of calcium carbonate by the vapor diffusion method is still a challenging issue because the resultant crystal polymorphs and morphologies highly depend on the experimental setup. In this communication, we demonstrated that the concentration gradients accompanied by the vapor diffusion method (ammonia concentration, pH and the ratio of CO32− to Ca2+ are changed with the solution depth and with time) are probably the main reasons to significantly affect the formation of crystal polymorphs. Raman, SEM and XRD data showed that calcite and vaterite crystals were preferred to nucleate and grow in the upper or the lower areas of aqueous solution respectively. The above results can be explained by the gradient effect.