The Structural Evolution and Diffusion During the Chemical Transformation From Cobalt to Cobalt Phosphide Nanocrystals

Nanoscale systems can display interesting and unpredictable transformation kinetics that increase the structural complexity of the original material. Characterization of the transformation routes to form the complex final structure is one of the major challenges in nanoscience. A more complete understanding of the transformation pathways would provides a means to improve synthesis techniques as well as an insight into the control of chemical and physical properties, in order to optimize the nanocrystals (NCs) for use in applications. We report the structural evolution and the diffusion processes which occur during the phase transformation of nanocrystal ɛ-Co to Co₂P to CoP, from a reaction with tri-n-octylphosphine (TOP). Extended X-ray absorption fine structure (EXAFS) investigations were used to elucidate the changes in the local structure of cobalt atoms which occur as the chemical transformation progresses. Results from EXAFS show both the Co₂P and CoP phases contain excess Co. Results from EXAFS, transmission electron microscopy, X-ray diffraction, and density functional theory calculations reveal that the inward diffusion of phosphorus is more favorable at the beginning of the transformation from ɛ-Co to Co₂P by forming of an amorphous Co-P shell, while retaining a crystalline cobalt core. When the major phase of the sample turns to Co₂P, the diffusion processes reverse and cobalt atom out-diffusion is favored, leaving a hollow void, characteristic of the nanoscale Kirkendall effect. For the transformation from Co₂P to CoP theory predicts an outward diffusion of cobalt while the anion lattice remains intact. In real samples, however, the Co-rich nanocrystals continue Kirkendall hollowing.