Phase stability of calcite nanocrystals synthesized by additive-free thermal chemical vapor deposition and precipitation

Calcium carbonate (CaCO₃) nanocrystal mostly occur naturally micro sized in unstable mixed phases making it unsuitable for critical biomedical application. Hence many attempts to synthesize them with the use of additives to gain stable single-phase calcite nanocrystal. However, the toxicity of most additives could cause it not suitable for a more sensitive application. Therefore, in this study, a few different methods were used to synthesize CaCO₃ in single-phase calcite without using any additives. A few different methods were attempted such as novel thermal chemical vapor deposition method (CVD), hydrothermal method, gas diffusion method and titration-precipitation method. The novel thermal CVD method was first time ever reported to synthesize single-phase nanocrystal optimally on fluorine tin oxide substrate in steady flow of 100 sccm CO₂ gas at 400 °C however in less stable CaCO₃ phase. The hydrothermal method at 150 °C produced a mixture of phases of less stable phase of CaCO₃, with the presence of impurities. Whereas gas diffusion method synthesized single phase calcite nanocrystal at 168 h in 25 °C nevertheless produced significantly lower yield. This work concludes that titration – precipitation method is most feasible to synthesize single-phase calcite nanocrystal. It was found that even at low concentration titration-precipitation method successfully synthesized single-phase calcite nanocrystal. The crystallite size was determined to be 86 nm under the optimal conditions of a 0.5 M concentration of calcium chloride and ammonium carbonate precursors, mixed in a 1:1 ratio at 25 °C. With each method, elemental analysis was done to confirm its chemical composition and phase whereas surface morphology analysis was done to describe its structure. It was explained how precursor concentration affects its structural morphology more specifically demonstrated by the presence of micro strain and change of lattice parameter in the crystal lattice. Other parameters sensitive to each different method affecting ion availability, and molecular interaction were also discussed. This study effectively outlined an innovative, additive-free technique for producing stable, singlephase CaCO₃ nanocrystals that may be used in sensitive biosensor or biomedical applications.