In-Situ Nanoscale Investigation of Calcium Sulfate Nanoparticles Immersed in Deionized Water

It is very common to synthesize and use nanomaterials nowadays. Because most nanomaterials are prepared in the laboratory, especially for those synthesized using chemical methods in aqueous environments, it is difficult to observe and understand the reaction process accurately. Herein, employing transmission electron microscopy (TEM), aberration-corrected scanning transmission electron microscopy (STEM) and liquid holder techniques, we investigate the reaction of calcium sulfate with water. Using TEM, we obtained clear evidence that the morphology and microstructure of calcium sulfate hemihydrate (Bassanite, CaSO₄•0.5H₂O) and calcium sulfate dihydrate (Gypsum, CaSO₄•2H₂O) are both changed when encountering deionized (DI) water. Bassanite reacts with water to form gypsum, with the microstructure changing from powder to nanowires with needle or rod shape. When gypsum reacts with DI water, the microstructure changes to nanowires without any compositional or crystallographic variation, as confirmed by electron diffraction. Using a liquid holder, the structure transformation of the gypsum is tracked and recorded in real time. Some of the gypsum powders are observed to dissolve in DI water. The dissolution of nanowires that formed during reaction of gypsum powder and water starts when the particles are exposed to the electron beam.