Guiding Synthesis of Polymorphs of Materials Using Nanometric Phase Diagrams

Conventionally, phase diagrams serve as road maps for the design and synthesis of materials. However, bulk phase diagrams are often not as predictive for the synthesis of nanometric materials, mainly due to the increased significance of surface energy. The change of surface energy can drastically alter the total energy of the nanocrystals and thus yields a polymorph or metastable phase different from the stable phase in bulk, providing a means for controlling the synthesis of metastable phases. To achieve a theoretical and systematical understanding on the polymorphism of nanomaterials, metallic cobalt was chosen as a model system, where the two polymorphs, fcc and hcp phases, can be tuned with 100% selectivity in a solvothermal reaction. Advanced in situ synchrotron X-ray diffraction (XRD) technique and density functional theory (DFT) calculations were complementarily employed to reveal the size- and surface-dependent polymorphism at nanometer scale. The nanometric phase diagram provides a general predictive approach to guide the synthesis of metastable materials.