(703d) 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.