Abstract
Cobalt nanoparticles with diameters of 8 nm have recently shown promising performance for biomedical applications. However, it is still unclear how the shape of cobalt clusters changes with size when reaching the nanoparticle range. In the present work, density functional theory calculations have been employed to compare the stabilities of two non-crystalline (icosahedron and decahedron) shapes, and three crystalline motifs (hcp, fcc, and bcc) for magic numbered cobalt clusters with up to 1500 atoms, based on the changes in the cohesive energies, coordination numbers, and nearest-neighbour distances arising from varying geometries. Obtained trends were extrapolated to a 104 size range, and an icosahedral shape was predicted for clusters up to 5500 atoms. Larger sized clusters adopt hcp stacking, in correspondence with the bulk phase. To explain the crystalline/non-crystalline crossovers, the contributions of the elastic strain density and twin boundary from the specimen surfaces to the cohesive energy of different motifs were evaluated. These results are expected to aid the design and synthesis of cobalt nanoparticles for applications ranging from catalysis to biomedical treatments.
Highlights
In the last few decades, cobalt nanoparticles have been investigated and used mainly in catalysis [1, 2] and magnetic data storage fields [3,4,5]
Density functional theory calculations have been employed to compare the stabilities of two non-crystalline shapes, and three crystalline motifs for magic numbered cobalt clusters with up to 1500 atoms, based on the changes in the cohesive energies, coordination numbers, and nearest-neighbour distances arising from varying geometries
As we expect the transition from icosahedron to hcp motif to occur somewhere in the mediumsize range, with different strain contributions acting on thestability of the non-crystalline structures, we have considered the following issues: (i) Can the chosen level of density functional theory (DFT)
Summary
In the last few decades, cobalt nanoparticles have been investigated and used mainly in catalysis [1, 2] and magnetic data storage fields [3,4,5]. Computer simulations have found that three-dimensional isomers are always more stable than their planar counterparts for the smallest clusters of various metals [22,23,24] Both theory and experiment have shown the preference of non-crystalline geometries (icosahedron and decahedron) over the crystalline shapes (fcc, hcp, or bcc) in clusters up to 50 or 100 atoms, with the crystalline/noncrystalline crossover point depending on the species. As we expect the transition from icosahedron to hcp motif to occur somewhere in the mediumsize range, with different strain contributions acting on the (in)stability of the non-crystalline structures, we have considered the following issues: (i) Can the chosen level of DFT provide enough accuracy for modelling cobalt clusters? As we expect the transition from icosahedron to hcp motif to occur somewhere in the mediumsize range, with different strain contributions acting on the (in)stability of the non-crystalline structures, we have considered the following issues: (i) Can the chosen level of DFT provide enough accuracy for modelling cobalt clusters? (ii) Given the size (8 nm) and composition (monometallic cobalt) of the system of interest, what is the most stable cluster structure from an energetic point of view? and (iii) What are the effects of the cluster size on the overall contribution of the elastic strain energy and twin boundary from the specimen surfaces that characterise the cluster motifs?
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