Graphene-wrapped pine needle-like cobalt nanocrystals constructed by cobalt nanorods for efficient microwave absorption performance

Magnetic metal nanocrystals tend to be advanced microwave absorption substances as they possess simultaneous dielectric and magnetic losses.

The interfacial energy of metallic nanoparticles on the boundary with polyatomic alcohols

The interface energy is general factor, which has determined the critical size and equilibrium shape of nanoparticles, velocity of its growth and stability. The electron-statistical method, based on the Thomas-Fermi theory taking into account its current state, allows one to calculate the interfacial energy of metallic objects of different dimensions at the boundary with various media. Based on this method, we have developed a variant for calculating the interfacial energy of the system « low-dimensional metallic phase - polar dielectric film». The polyatomic alcohols (diols), used, for example, as non-aqueous media for the synthesis of metal nanoparticles, were chosen as the external medium. Also we have chosen cobalt nanocrystals as a low-dimensional metallic phase. Expressions are obtained for the external and internal contributions to the interfacial energy of the system, including the polarization correction, due to the presence of a dielectric fluid in the external region of the system. The effect of a limited dielectric fluid layer on the Gibbs boundary coordinate is analyzed. The presence of a dielectric leads to a shift of the Gibbs boundary to the external region of the system, that is, the so-called effect of «pulling the tail» of the electron density is observed. It is shown that with increasing dielectric constant, the magnitude of the polarization contribution increases rapidly in magnitude. The interface energy of a cobalt nanocrystal at the interface with polar polyatomic alcohols is calculated. The interfacial energy of the faces of cobalt nanocrystals decreases nonlinearly with an increase in the linear dimensions of the metal phase. It is shown that the dielectric coating changes the character of the dimensional and orientational dependence in comparison with the interfacial energy of macrocrystals and thin films at the interface with vacuum. With a constant size of the metal phase and an increase in the thickness of the dielectric coating, the interface energy of faces and anisotropy increase. It has been established that 1,2-ethanediol is the most effective surfactant for cobalt particles of the polyatomic alcohols considered in this paper. The dependencies obtained in this work are consistent with the literature data for the thin films of alkaline metals and other system.

Disk-Shaped Cobalt Nanocrystals as Fischer–Tropsch Synthesis Catalysts Under Industrially Relevant Conditions

Colloidal synthesis of metal nanocrystals (NC) offers control over size, crystal structure and shape of nanoparticles, making it a promising method to synthesize model catalysts to investigate structure-performance relationships. Here, we investigated the synthesis of disk-shaped Co-NC, their deposition on a support and performance in the Fischer–Tropsch (FT) synthesis under industrially relevant conditions. From the NC synthesis, either spheres only or a mixture of disk-shaped and spherical Co-NC was obtained. The disks had an average diameter of 15 nm, a thickness of 4 nm and consisted of hcp Co exposing (0001) on the base planes. The spheres were 11 nm on average and consisted of ε-Co. After mild oxidation, the CoO-NC were deposited on SiO2 with numerically 66% of the NC being disk-shaped. After reduction, the catalyst with spherical plus disk-shaped Co-NC had 50% lower intrinsic activity for FT synthesis (20 bar, 220 °C, H2/CO = 2 v/v) than the catalyst with spherical NC only, while C5+-selectivity was similar. Surprisingly, the Co-NC morphology was unchanged after catalysis. Using XPS it was established that nitrogen-containing ligands were largely removed and in situ XRD revealed that both catalysts consisted of 65% hcp Co and 21 or 32% fcc Co during FT. Furthermore, 3–5 nm polycrystalline domains were observed. Through exclusion of several phenomena, we tentatively conclude that the high fraction of (0001) facets in disk-shaped Co-NC decrease FT activity and, although very challenging to pursue, that metal nanoparticle shape effects can be studied at industrially relevant conditions.