Concerted Growth and Ordering of Cobalt Nanorod Arrays as Revealed by Tandem in Situ SAXS-XAS Studies

Abstract

The molecular and ensemble dynamics for the growth of hierarchical supercrystals of cobalt nanorods have been studied by in situ tandem X-ray absorption spectroscopy-small-angle X-ray scattering (XAS-SAXS). The supercrystals were obtained by reducing a Co(II) precursor under H2 in the presence of a long-chain amine and a long-chain carboxylic acid. Complementary time-dependent ex situ TEM studies were also performed. The experimental data provide critical insights into the nanorod growth mechanism and unequivocal evidence for a concerted growth-organization process. Nanorod formation involves cobalt nucleation, a fast atom-by-atom anisotropic growth, and a slower oriented attachment process that continues well after cobalt reduction is complete. Smectic-like ordering of the nanorods appears very early in the process, as soon as nanoparticle elongation appears, and nanorod growth takes place inside organized superlattices, which can be regarded as mesocrystals.

Coupling Sub‐Nanometric Copper Clusters with Quasi‐Amorphous Cobalt Sulfide Yields Efficient and Robust Electrocatalysts for Water Splitting Reaction

“…To more specific, copper has a coordination number of 3.4 for Cu@CoS x , which is much lower than that (a coordination number of 12) for bulk copper (Table S2, Supporting Information). This result, together with several previously reported works, indicates copper species in Cu@CoS x are present in the form of sub‐nanometric metal clusters, and during the deposition of CoS x film on CF, some Cu atoms must self‐diffuse from the CF into the as‐formed CoS x film. Based on recent experimental and theoretical works on the surface properties of metallic copper, the occurrence of Cu atoms' self‐diffusion for the formation of Cu@CoS x is proposed to be mainly because (i) copper has a weak cohesive energy that makes copper surface reconstruction and atom transfer very easy and (ii) the copper surface reconstruction and atom transfer could be significantly promoted by some polar surface adsorbate molecules and ions.…”

A Seed‐Mediated Approach for the Preparation of Modified Heterogeneous Catalysts

“…Figure presents the modified catalysts obtained by heating four mixtures of [Co{N(SiMe 3 ) 2 } 2 (thf)], HDA, LA, and 15 % Co/Al 2 O 3 –SiO 2 in toluene at 110 °C under 3 bars of H 2 for 24 h. In these mixtures the Co/HDA/LA ratio has been varied, keeping the Co concentration constant. Although Co NRs can normally grow from a solution containing Co, HDA, and LA in a molar ratio of 1:1.7:1.2 (Co/HDA/LA) in the absence of any support, it can be seen from Figures a–d that in the presence of the 15 % Co/Al 2 O 3 –SiO 2 catalyst, it was necessary to increase the amount of ligands, and particularly that of LA, to obtain quantitatively anisotropic nano‐objects (NRs/NWs). This is attributed to the adsorption of part of the ligands on the 15 % Co/Al 2 O 3 –SiO 2 catalyst, which modifies the Co/HDA/LA ratio in the solution and affects the shape of the growing nano‐objects.…”

Shaping non-noble metal nanocrystals via colloidal chemistry

“…1c) and nanowires were successfully obtained by decomposition of a pre-synthesized olenic complex [Co(h 3 -C 8 H 13 )(h 4 -C 8 H 12 )] or by reduction of [Co{N(SiMe 3 ) 2 } 2 (thf)] in anisole under a pressure of 3 bar H 2 in the presence of alkylamines and carboxylic acids as ligands. 51,52 Cormary et al have conducted a beautiful time dependent in situ X-ray absorption (XAS) study to follow the evolution of both the oxidation state and coordination environment of these precursors over the course of the reaction. 52 These data suggested that the starting precursor rapidly exchanges ligands to the mixture of alkylamines before getting reduced by the hydrogen.…”

“…51,52 Cormary et al have conducted a beautiful time dependent in situ X-ray absorption (XAS) study to follow the evolution of both the oxidation state and coordination environment of these precursors over the course of the reaction. 52 These data suggested that the starting precursor rapidly exchanges ligands to the mixture of alkylamines before getting reduced by the hydrogen. Unfortunately, the dramatic differences in timescale between the laboratory and in situ XAS synthesis did not allow the correlation of the reduction with the nanorod growth.…”