Cobalt Nanocrystals Research Articles

Formation of ε-Co nanocrystals in borosilicate and aluminosilicate glasses

Formation of cobalt nanocrystals in durable borosilicate and aluminosilicate glass matrices is studied. Using a hydrogen gas flow treatment, ε-Co nanocrystals precipitate primarily in these glasses. This rare Co allotrope, which so far has mainly been synthetized by organometallic routes is found for the first time in a glass. XRD and TEM results indicate that ε-Co is present in glasses ≥ 500 °C and that it successively transforms into the more stable α- and β-Co allotropes. Primary formation of the ε-Co in glasses is in agreement with the Ostwald-Volmer rule of stage as it yields the lowest density change with respect to the parent glass among the three Co allotropes.

Hierarchical nitrogen-doped porous carbon incorporating cobalt nanocrystal sites for nitrophenol reduction

The direct pyrolysis of metal organic frameworks (MOFs) has recently provided an effective method to prepare various metal-incorporated composite carbon materials. In this work, hierarchical Co-embedded N-doped porous carbon microspheres were synthesized by controlled pyrolysis of bimetallic Zn-Co Prussian blue analogues (PBAs), in which the metallic Zn component served as thermally removable template and ligand [Co(CN)6]3− served as both Co and N sources. When devoted to catalyzing the reduction of 4-nitrophenol (4-NPh) into 4-aminophenol (4-APh), the optimized catalyst exhibited a large apparent rate constant of 1.35 min−1 and high turnover frequency (TOF) of 0.0204 s−1, which was superior to the majority of noble-metal-based catalysts. Besides, a home-made continuous flow catalysis system was readily constructed. Control experiments confirmed that both partially uncovered and totally in-built active metallic Co species acted as catalytically active sites. This work might provide a further understanding of the identification of active sites within metal-incorporated carbon-based catalysts.

Cobalt nanocrystals on carbon nanotubes in the Fischer-Tropsch synthesis: Impact of support oxidation

The effect of oxidation of a carbon nanotube (CNT) support on the structure and catalytic performance in the Fischer-Tropsch (FT) synthesis of pre-synthesized cobalt nanocrystals (NC) has been investigated. 6 nm CoO-NC were prepared using colloidal techniques and impregnated on pristine and surface-oxidized CNT. The CoO-NC on pristine CNT were reduced completely to Co, while ∼50 % CoO reduction was observed on oxidized CNT (oxCNT), indicating a stronger Co-support interaction on oxCNT. In FT, the turnover frequency (TOF) after 100 h was exceptionally high on pristine CNT (150·10−3 s-1) against 63·10−3 s-1 on oxCNT. In the initial stage of FT, the TOF was 35 % lower on oxCNT than on pristine CNT, which matched with 32 % lower crystallinity of cobalt determined using in situ XRD. Furthermore, deactivation was observed mainly on oxCNT. These results demonstrate the large impact of the support on the structure and performance of nanocrystal-based catalysts.

The amorphization of metal nanoparticles in graphitic shells under laser pulses

It is shown that graphitic shells encapsulating metal nanoparticles facilitate the amorphization of metals and stabilize the amorphous phase against recrystallization. In an in-situ electron microscopy experiment, where the objects are exposed to laser pulses during their observation, the amorphization of iron and cobalt nanocrystals in graphitic shells is demonstrated. The infrared nanosecond pulses lead to fast melting of the metal which then dissolves carbon atoms from the shell. Fast cooling of the liquid solution after the pulse results in the solidification of an amorphous metal-carbon phase. The amorphous phase is metastable and can be recrystallized by repeated laser pulses or slow thermal annealing. The recrystallization needs heterogeneous nucleation but is unfavorable at the metal-graphite interface and so stabilizes the amorphous phase against recrystallization. The analysis of the experiments explains the formation mechanisms of an amorphous metal-carbon phase as a metastable solution of carbon in a transition metal and shows how that the encapsulation by a graphitic shell can be a route towards the stabilization of otherwise unfavorable amorphous metal or metal-carbon phases.

Enhanced structural and magnetic properties of fcc colloidal crystals of cobalt nanoparticles.

We report the elaboration of supercrystals made up of dodecanoic acid-coated 8.1 nm-Co nanocrystals with controlled supercrystallinity, morphology and magnetic properties. Supercrystal growth is controlled using a solvent-mediated ligand-ligand interaction strategy. Either face-centered cubic supercrystalline films or single colloidal crystals composed of cobalt nanocrystals are obtained. The change in supercrystal morphology is explained by Flory-type solvation theory using Hansen solubility colloidal parameters. The use of the same batch of Co nanocrystals for the fabrication of supercrystalline films and colloidal crystals enables accurate comparative structural and magnetic studies using (high-resolution) transmission electron microscopy, field emission gun scanning electron microscopy, grazing incidence small-angle X-ray scattering and vibrating sample magnetometry. The nearest neighbor distance between nanoparticles is interpreted using theoretical models proposed in the literature. We evidence the increase in both geometric anisotropy and magnetic dipolar interactions for colloidal crystals compared to supercrystalline films.

Hierarchically structured Co3O4/SiO2 composites by Co nanocrystals transformation

Hierarchical structured mesoporous Co3O4/SiO2 composites have been fabricated by the in-situ transformation of cobalt nanocrystals in the mesoporous silica. The hierarchical porous composites show good structural stability during the thermal treating process and demonstrate high crystallinity after phase transformation. The highly crystalline Co3O4/SiO2 composite has uniform mesopores and exhibits good adsorption ability. This method offers a promising way to the design of hierarchical structured mesoporous materials with highly crystalline framework.

Preparation and characterization of cobalt and copper oxide nanocrystals

Abstract Copper oxide and cobalt oxide (Co3O4, CuO) nanocrystals (NCs) have been successfully prepared using microwave irradiation. The obtained powders of the nanocrystals (NCs) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric (TGA) analysis and Fourier-transform infrared spectroscopy. The obtained results confirm the presence of both nanooxides which have been produced during chemical precipitation using microwave irradiation. TEM micrographs have shown that the obtained nanocrystals are characterized by high dispersion and narrow size distribution. The results of X-ray diffraction confirmed those obtained from the transmission electron microscope. Optical absorption analysis indicated the direct band gap for both kinds of the nanocrystals.

Ultra-small cobalt nanocrystals embedded in 2D-MoS2 nano-sheets as efficient co-catalyst for solar-driven hydrogen production: Study of evolution rate dependence on cobalt nanocrystal size

2D-MoS2 nanostructures are attractive co-catalysts for photocatalytic hydrogen evolution due to their suitable water reduction potentials and high stability. However, the catalytic activity of MoS2 is greatly limited by the catalytically inert basal planes. Doping of transition-metal ions into MoS2 structure is an effective way to activating the basal planes. To this end, the formation of size-controlled metal nanocrystals with clean surface and mono dispersed nature is a current challenge. Here we utilized pulsed laser ablation in liquid approach to generate high purity size controlled cobalt nanocrystal by adjusting the laser fluences and systematically evaluate the effect of cobalt nanocrystals size and concentration on MoS2 to activate the basal planes. A set of Co-MoS2/CdS nanorods with different cobalt size were examined, and an optimal cobalt size of 3.1 nm was obtained. The optimized CdS/Co-MoS2 nanocomposite showed a very high H2 production rate (275 mmol h−1 g−1) with outstanding stability. To the best of our knowledge, this is the best performance reported for CdS/MoS2 based nanocomposites. The remarkable hydrogen evolution rate and stability may be due to reduced recombination rate and greatly increased density of catalytic active sites which is determined by photoluminescence and impedance spectroscopy. Finally, we believe that the strategies applied in the present study to form robust photocatalysts and its utilization in solar driven hydrogen production would inspire the development of other low-cost photocatalysts for renewable fuel production.

Dehydrogenation of N-Heterocycles with Cobalt Nanocrystals on Carbon

Dehydrogenation of N-Heterocycles with Cobalt Nanocrystals on Carbon

The Influence of Relative Content of a Metal Component in a Dielectric Matrix on the Formation and Dimensions of Cobalt Nanocrystallites in Cox(MgF2)100 – x Film Composites

The influence of relative content of a metal component on the phase composition and substructure of Cox(MgF2)100 – x in a wide range of x = 16–63 at % is studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. The layers of nanocomposite with a micron thickness have been obtained by the ion-beam sputtering of the target in the argon environment. The results reveal that the relative metal cobalt content in the MgF2 dielectric matrix strongly affects the phase composition and substructure of nanocomposites. With a lower content of cobalt, it is in the amorphous state in the form of clusters in the MgF2 nanocrystalline matrix. An increase of cobalt content to x = 29 at % on a sitall substrate and to x = 42 at % on a glass substrate in the X-ray amorphous MgF2 dielectric matrix leads to the formation of cobalt nanocrystals with a hexagonal crystallographic system, whose sizes are on the order of 10 nm. These are predominately oriented in the basis plane of the (001) hexagonal lattice of α-Со. A further increase of cobalt content to c = 59 at % enlarges the α-Со nanocrystals to ~20 nm with retaining the same orientation. In accordance with a fine structure analysis of XPS spectra of Со 2p and О 1s, cobalt is strongly oxidized on the surface of all the composites; only on the surface of samples with a low content of Co is X‑ray amorphous cobalt found in a metallic state along with cobalt oxide. The IR spectra of these samples with the lowest metal phase content exhibit the pronounced modes from the nanocrystalline MgF2 dielectric phase.

Role of Oleylamine Revisited: An Original Disproportionation Route to Monodispersed Cobalt and Nickel Nanocrystals

Herein is highlighted an original disproportionation mechanism for the formation of hexagonal closed packed cobalt nanoparticles starting from low-valent cobalt complexes. By coupling experimental and theoretical studies, we demonstrate that oleylamine promotes the disproportionation of the starting metal complexes and controls the growth of the nanocrystals, in contrast to the role of reducing reagents proposed elsewhere. We also decipher the sensitivity of the synthesis in regard to the nature of the phosphine and of the halogen ligands in the starting cobalt precursors. Understanding of the mechanism allows the development of a less expensive and less toxic synthesis of these nanoparticles by decreasing the amount of oleylamine in alkane solvents. Finally, an extension of this process is presented and allows reaching size- and shape-controlled nanosphere and nanorod nickel nanoparticles.

Nitrogen-coordinated cobalt nanocrystals for oxidative dehydrogenation and hydrogenation of N-heterocycles.

To endow non-noble metals with the high catalytic activity that is typically exhibited by noble metals is the central yet challenging aim for substituting noble metals. In this regard, by exploiting the coordination effect of nitrogen, we prepared cobalt nanocrystals stabilized by nitrogen-doped graphitized carbon (Co NCs/N-C). The obtained Co NC/N-C catalyst showed extraordinary performances toward both oxidative dehydrogenation of N-heterocycles and its reverse hydrogenation process under extremely mild conditions. A nearly quantitative conversion could be achieved for oxidative dehydrogenation even at room temperature (25 °C), for which the coordination effect of nitrogen is responsible: the interaction of Co-N induces a partial positive charge on the Co surface, thereby promoting the reaction. In contrast, cobalt nanocrystals supported by pristine carbon (Co NCs/C) proved to be inactive for oxidative dehydrogenation, owing to the lack of nitrogen. Moreover, in Co NCs/N-C, the N-doped graphitized carbon formed a protective layer for Co NCs, which preserved the active valence of Co species and prevented the catalyst from leaching. It was found that the catalyst still retained its excellent catalytic activity after five regeneration cycles; in comparison, its cobaltous oxide counterpart (CoO x /N-C) was barely active. As for the mechanism, electron paramagnetic resonance (EPR) analysis revealed the formation of superoxide anion radicals during the dehydrogenation process. Interestingly, the pressure of feed hydrogen had little effect on the hydrogenation process. Our Co NC/N-C catalyst is capable of activating molecular oxygen and hydrogen as effectively as noble metals; the coordination effect of nitrogen and the protection by the carbon layer in combination confer tremendous potential on the Co NCs/N-C for substituting noble-metal-based catalysts and soluble catalysts for homogeneous reactions.

ВЛИЯНИЕ КОНЦЕНТРАЦИИ ЖЕЛЕЗА НА КРИСТАЛЛИЗАЦИЮ АМОРФНОГО СПЛАВА СИСТЕМЫ Co-Fe-B-Nb

In the modern world, technical devices are being constantly improved, creation of which requires materials with the best functional properties. Amorphous and nanocrystalline iron- and cobalt-based alloys are some of such materials. They have proved to be good in the fields of radio- and microelectronics, due to the fact they have high magnetic characteristics. It is also known that these properties can be improved by the formation of a partially crystalline structure is such alloys. However, such a structure cannot always be formed using a standard method of isothermal annealing; therefore, alloying components are added to the alloy composition to slow down the crystallization process. Different content of added components also affects the sequence of phase transformations during crystallization. As most of the properties are structure-dependent, the formed structure also determines the material characteristics. Therefore, establishment of the dependence of the formed structure in amorphous alloys after heat treatment is an important task of condensed matter physics. The crystallization of the amorphous alloys of Co-Fe-B-Nb system was studied by X-ray diffraction. The samples were crystallized using isothermal annealing of the alloys with different content of components under the same conditions. The dependence of the formed structure on the content of an alloying component is determined. It is shown that the formed structure significantly depends on the concentration of iron. With the iron content of 10 at.% and 16 at.%, the structure consists of cubic cobalt nanocrystals and a solid solution of iron in cobalt. With a decrease in the concentration to 5 at.%, the crystallization mechanism changes: crystallization begins with the precipitation of Co 23 B 6 boride crystals. The reasons for the effect of iron concentration in the alloy composition on crystallization are discussed.

Влияние относительного содержания металлической компоненты в диэлектрической матрице на образование и размеры нанокристаллов кобальта в пленочных композитах Co-=SUB=-x-=/SUB=-(MgF-=SUB=-2-=/SUB=-)-=SUB=-100-x-=/SUB=-

AbstractThe influence of relative content of a metal component on the phase composition and substructure of Co_ x (MgF_2)_100 – _ x in a wide range of x = 16–63 at % is studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. The layers of nanocomposite with a micron thickness have been obtained by the ion-beam sputtering of the target in the argon environment. The results reveal that the relative metal cobalt content in the MgF_2 dielectric matrix strongly affects the phase composition and substructure of nanocomposites. With a lower content of cobalt, it is in the amorphous state in the form of clusters in the MgF_2 nanocrystalline matrix. An increase of cobalt content to x = 29 at % on a sitall substrate and to x = 42 at % on a glass substrate in the X-ray amorphous MgF_2 dielectric matrix leads to the formation of cobalt nanocrystals with a hexagonal crystallographic system, whose sizes are on the order of 10 nm. These are predominately oriented in the basis plane of the (001) hexagonal lattice of α-Со. A further increase of cobalt content to c = 59 at % enlarges the α-Со nanocrystals to ~20 nm with retaining the same orientation. In accordance with a fine structure analysis of XPS spectra of Со 2 p and О 1 s , cobalt is strongly oxidized on the surface of all the composites; only on the surface of samples with a low content of Co is X‑ray amorphous cobalt found in a metallic state along with cobalt oxide. The IR spectra of these samples with the lowest metal phase content exhibit the pronounced modes from the nanocrystalline MgF_2 dielectric phase.

Role of well-defined cobalt crystal facets in Fischer-Tropsch synthesis: a combination of experimental and theoretical studies.

Pure-phase cobalt nanocrystals with well-defined specific exposed facets were synthesized via controllable reduction of their CoO counterparts while retaining the same scale of particle size. Three different catalysts, i.e. hexagonal close-packed (hcp) Co pyramid with (10-11) and rodlike with (10-10) facets, as well as face-centred cubic (fcc) Co octahedron with (111) exposed were obtained and studied for Fischer-Tropsch synthesis (FTS) reaction. No noticeable changes of either the shape or the exposed facets were found under practical FTS reaction conditions. The hcp (10-11) facet exhibits the highest FTS activity and C5+ product selectivity with the lowest apparent activation energy and CH4 selectivity. Theoretical calculations of the energy barrier for CO dissociation and methanation of the reaction intermediate CHx rationalize the experimental results.

Preparation of Cobalt Nanocrystals Supported on Metal Oxides To Study Particle Growth in Fischer-Tropsch Catalysts.

Colloidal synthesis of nanocrystals (NC) followed by their attachment to a support and activation is a promising route to prepare model catalysts for research on structure-performance relationships. Here, we investigated the suitability of this method to prepare well-defined Co/TiO2 and Co/SiO2 catalysts for the Fischer–Tropsch (FT) synthesis with high control over the cobalt particle size. To this end, Co-NC of 3, 6, 9, and 12 nm with narrow size distributions were synthesized and attached uniformly on either TiO2 or SiO2 supports with comparable morphology and Co loadings of 2–10 wt %. After activation in H2, the FT activity of the TiO2-supported 6 and 12 nm Co-NC was similar to that of a Co/TiO2 catalyst prepared by impregnation, showing that full activation was achieved and relevant catalysts had been obtained; however, 3 nm Co-NC on TiO2 were less active than anticipated. Analysis after FT revealed that all Co-NC on TiO2 as well as 3 nm Co-NC on SiO2 had grown to ∼13 nm, while the sizes of the 6 and 9 nm Co-NC on SiO2 had remained stable. It was found that the 3 nm Co-NC on TiO2 already grew to 10 nm during activation in H2. Furthermore, substantial amounts of Co (up to 60%) migrated from the Co-NC to the support during activation on TiO2 against only 15% on SiO2. We showed that the stronger interaction between cobalt and TiO2 leads to enhanced catalyst restructuring as compared to SiO2. These findings demonstrate the potential of the NC-based method to produce relevant model catalysts to investigate phenomena that could not be studied using conventionally synthesized catalysts.

Water Splitting: Cobalt Nanocrystals Encapsulated in Heteroatom‐Rich Porous Carbons Derived from Conjugated Microporous Polymers for Efficient Electrocatalytic Hydrogen Evolution (Small 42/2018)

Water Splitting: Cobalt Nanocrystals Encapsulated in Heteroatom‐Rich Porous Carbons Derived from Conjugated Microporous Polymers for Efficient Electrocatalytic Hydrogen Evolution (Small 42/2018)

Cobalt Nanocrystals Encapsulated in Heteroatom-Rich Porous Carbons Derived from Conjugated Microporous Polymers for Efficient Electrocatalytic Hydrogen Evolution.

Cobalt nanocrystals encapsulated in N,O-dual-doped porous carbons as efficient and stable electrocatalysts for hydrogen evolution reaction (HER) are reported. A heteroatom-rich-conjugated microporous polymer is first chemically deposited on a carbon fiber cloth, and after addition of a cobalt salt, pyrolyzed to produce a heteroatom-doped C/Co nanocrystal composite. With this process, the use of additional binders for preparation of electrodes can be avoided. With a trace cobalt loading (0.46 wt%), the electrodes achieve a low Tafel slope of 46 mV dec-1 and overpotential of only 69 mV at a current density of 10 mA cm-2 in 0.5 m H2 SO4 . Experimental and computational studies reveal that the superior HER behavior is due to a decreased free energy of hydrogen adsorption, induced by i) electrons transferred from the cobalt nanocrystals to graphite layers and ii) N,O-dual doping reduced the Fermi level of neighboring C atoms.

Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer–Tropsch synthesis performance

Abstract Non-porous carbon sphere was used as support to synthesize supported cobalt Fischer-Tropsch catalysts with high activity and durability. Strong metal-support interaction was avoided and intrinsic activity of pristine cobalt nano-particles was studied. Thermal decomposition synthesis method was applied to obtain cobalt catalysts with high dispersion and narrow particle size distribution. Furthermore the cobalt size can be controlled by the molar ratio of o-dichlorobenzene/ benzylamine. Compared with supported cobalt catalysts prepared by incipient wetness impregnation method and ultrasonic impregnation method, the catalyst prepared by thermal decomposition method showed higher catalytic activity, higher long chain hydrocarbons selectivity and lower methane selectivity.

Confined small-sized cobalt catalysts stimulate carbon-chain growth reversely by modifying ASF law of Fischer\u2013Tropsch synthesis

Fischer–Tropsch synthesis (FTS) is a promising technology to convert syngas derived from non-petroleum-based resources to valuable chemicals or fuels. Selectively producing target products will bring great economic benefits, but unfortunately it is theoretically limited by Anderson–Schulz–Flory (ASF) law. Herein, we synthesize size-uniformed cobalt nanocrystals embedded into mesoporous SiO2 supports, which is likely the structure of water-melon seeds inside pulps. We successfully tune the selectivity of products from diesel-range hydrocarbons (66.2%) to gasoline-range hydrocarbons (62.4%) by controlling the crystallite sizes of confined cobalt from 7.2 to 11.4 nm, and modify the ASF law. Generally, larger Co crystallites increase carbon-chain growth, producing heavier hydrocarbons. But here, we interestingly observe a reverse phenomenon: the uniformly small-sized cobalt crystallites can strongly adsorb active C* species, and the confined structure will inhibit aggregation of cobalt crystallites and escape of reaction intermediates in FTS, inducing the higher selectivity towards heavier hydrocarbons.