One-pot synthesis of cobalt–rhenium nanoparticles taking the unusual β-Mn type structure

Using a facile one-pot colloidal method, it is now possible to obtain monodisperse Co1−xRex nanoparticles (NPs), with excellent control of Re stoichiometry for x < 0.15. Re-incorporation in terms of a solid solution stabilizes the β-Mn polymorph relative to the hcp/ccp variants of cobalt. The NPs are thermally stable up to 300 °C, which may make them attractive as model catalysts.

o-DCB/OA/Re2(CO)10 mixture reached the targeted temperature, the Co2(CO)8 precursor solution was rapidly injected into the hot mixture. The formed colloidal suspension was aged for 2-4 hours and subsequently quenched with 10 mL o-DCB. The NPs were flocculated using excess 2-propanol and isolated by centrifugation. After discarding the supernatant, the NP precipitate was washed with 2-propanol for at least three times before redispersion in hexane.
In order to tune the Co-Re metal composition, the relative amounts of Co and Re carbonyl precursors were systematically adjusted (Table S1). and dried under inert atmosphere.

Computational details
All calculations were performed within the periodic density functional theory framework, as it is implemented in the VASP code. 3,4 The interactions between the core (Co: [Ar], and Re:[Xe]) and the valence electrons were described using the projector-augmented wave (PAW) method. 5,6 We have used the Perdew, Burke, and Ernzerhof (PBE) 7 spin polarized gradient corrected functional for the exchange-correlation part of the potential for the structural optimization. Our previous calculations suggested that structural parameters in metals can be reliably predicted only by using large energy-cutoff to guarantee basis-set completeness.
Hence, we have used a cut-off of 700 eV. The atoms were deemed to be relaxed when all atomic forces were less than 0.02 eV Å -1 and the geometries were assumed to be optimized when the total energy converged to less than 1 meV between two consecutive geometric optimization steps. Only the FM states were considered for all phases studied.

4
Different sizes of the nanoparticles (NP) have been constructed from optimized bulk phase structures through different supercell sizes. The k-points were generated using the Monkhorst-Pack method with a grid size of 111, for structural optimization for NPs. For the 15% Re substituted Co initial structure generation, the structural data for Co in P63/mmc, Fd-3m, P4132 were taken from the ICSD database and we used the ab initio random searching structure (AIRSS) 8 method to generate possible model structures for the Re substitution in Co matrix, coupled with VASP calculations. Iterative relaxation of atomic positions was stopped when the change in total energy between successive steps was < 1 meV/cell.

Site Preferences in β-Mn-type
Co-Re phases.  8 For the Co0.97Re0.03 sample, SR-PXRD revealed some weak additional reflections (indicated by asterisk in Figure S3a). The origin of these reflections is not fully understood; however, possibly some can be related to hcp/ccp intergrowth particles. 9 With increasing Re content ( Figure S3b-c) the above mentioned reflections disappear, yielding single-phase particles.
Representative TEM images of the Co0.60Re0.40 NPs are presented in Figure S4. Particles are characterized by a tri-modal size distribution; the main population being spherical bimetallic Co-Re NPs with an average diameter of ca. 7 nm. Some ultra-small clusters and larger aggregates (with sizes of ca. 1 nm and 50-70 nm, respectively) were also identified. STEM-EDX mapping suggests that the latter are Re-rich ( Figure S5).

DFT simulations.
The lowest energy configurations of different Co polymorph derived NPs and their corresponding 15% Re substituted Co NPs are shown in Figure S6 along with their relative stability.