Carbon-encapsulated Metal Nanoparticles Research Articles

Consolidated Co- and Fe-based Fischer-Tropsch catalysts supported on jellyfish-like graphene nanoflake framework

Different carbon nanomaterials are considered as promising supports for Fischer-Tropsch synthesis (FTS) catalysts of syngas transformation into hydrocarbons. At the same time, consolidation of carbon nanoparticles into a dense bulk material is one of the most important challenges faced by the catalytic industry. In this work consolidated Co and Fe catalysts supported on graphene nanoflakes (GNFs) were for the first time synthesized by spark plasma sintering (SPS). The synthesized catalysts contained reduced carbon-encapsulated metal nanoparticles embedded into the GNF framework, which ensured their activity in FTS without pre-activation. SPS significantly increased both the activity (from 77 to 91 µmolCO×gCo−1 ×s−1) and TOF (from 0.024 to 0.073 s−1) of the Co/GNF catalyst compared to the unsintered sample. In contrast, the activity of the Fe catalysts decreased after SPS, but their C5+ selectivity and olefins to paraffins ratio increased. These enhancements were attributed to the unique core-shell structure of the nanoparticles, highly active surface cobalt atoms, and close contact between carbon support and iron particles. SPS was proved to be a powerful approach to activate carbon-supported FTS catalysts, increase their density and modify their catalytic performance.

A target-customized carbon shell structure of carbon-encapsulated metal nanoparticles for fuel cell applications

We uncover the secrets to controlling the carbon shell structure on the surface of metal nanoparticles at the sub-nm scale to improve the performance and durability of fuel cells.

Peroxymonosulfate activation by carbon-encapsulated metal nanoparticles: Switching the primary reaction route and increasing chemical stability

This study explores the technical merits of carbon encapsulation via an electrical wire explosion method to enhance the peroxymonosulfate activation performance of metals. Reflecting the nature of core-shell structures, the outer carbon layers hampered the reductive conversion of peroxymonosulfate to sulfate radicals by the inner metal cores, whereas the metal cores increased the overall electrical conductivity as a pivotal factor in non-radical activation. Hence, the impact of carbon wrapping hinged on the peroxymonosulfate reduction capability of the metals, i.e., kinetic retardation in organic degradation with reactive Cu, Fe, and Ni-Fe, but acceleration with unreactive Ni. Further, composite fabrication switched the major degradative pathway from radical-induced oxidation to mediated electron transfer, as determined from the effects of methanol and chloride, formaldehyde and bromate formation yields, reactivity toward multiple organics, and electron paramagnetic resonance spectral features. The protective carbon shells enabled pH-insensitive peroxymonosulfate activation, prevented metal ion leaching, and alleviated catalyst deactivation.

Graphite carbon-encapsulated metal nanoparticles derived from Prussian blue analogs growing on natural loofa as cathode materials for rechargeable aluminum-ion batteries

Aluminum-ion batteries (AIBs) are attracting increasing attention as a potential energy storage system owing to the abundance of Al sources and high charge density of Al3+. However, suitable cathode materials to further advance high-performing AIBs are unavailable. Therefore, we demonstrated the compatibility of elemental metal nanoparticles (NPs) as cathode materials for AIBs. Three types of metal NPs (Co@C, Fe@C, CoFe@C) were formed by in-situ growing Prussian blue analogs (PBAs, Co[Co(CN)6], Fe[Fe(CN)6] and Co[Fe(CN)6]) on a natural loofa (L) by a room-temperature wet chemical method in aqueous bath, followed by a carbonization process. The employed L effectively formed graphite C-encapsulated metal NPs after heat treatment. The discharge capacity of CoFe@C was superior (372 mAh g−1) than others (103 mAh g−1 for Co@C and 75 mAh g−1 for Fe@C). The novel design results in CoFe@C with an outstanding long-term charge/discharge cycling performance (over 1,000 cycles) with a Coulombic efficiency of 94.1%. Ex-situ X-ray diffraction study indicates these metal NP capacities are achieved through a solid-state diffusion-limited Al storage process. This novel design for cathode materials is highly significant for the further development of advanced AIBs in the future.

Facile Preparation of Carbon Shells-Coated O-Doped Molybdenum Carbide Nanoparticles as High Selective Electrocatalysts for Nitrogen Reduction Reaction under Ambient Conditions.

Electrochemical nitrogen reduction reaction (NRR) has been considered as a promising alternative to the traditional Haber-Bosch process for the preparation of ammonia (NH3) under ambient conditions. The development of cost-effective electrocatalysts with suppressive activity for hydrogen evolution reaction is critical for improving the efficiency of NRR. Herein, oxygen-containing molybdenum carbides (O-MoC) embedded in nitrogen-doped carbon layers (N-doped carbon) can be easily fabricated by pyrolyzing the chelate of dopamine and molybdate. A rate of NH3 formation of 22.5 μg·h-1·mgcat-1 is obtained at -0.35 V versus reversible hydrogen electrode with a high faradaic efficiency of 25.1% in 0.1 mM HCl + 0.5 M Li2SO4. Notably, the synthesized O-MoC@NC-800 also exhibits high selectivity (no formation of hydrazine) and electrochemical stability. The moderate electron structure induced by the interaction between O-MoC and N-doped carbon shells can effectively weaken the activity of hydrogen evolution reaction and increase the faradaic efficiency of NRR. Additionally, by applying the in situ Fourier transform infrared spectroscopy, an associative reaction pathway is proposed on O-MoC@NC-800. This work provides new insights into the rational design of carbon-encapsulated metal nanoparticles as efficient catalysts for NRR at ambient conditions.

FeCo-N-C oxygen reduction electrocatalysts: Activity of the different compounds produced during the synthesis via pyrolysis

During the synthesis of earth-abundant oxygen reduction reaction (ORR) electrocatalysts formed by metal, nitrogen and carbon via pyrolysis, a variety of side products arise in addition to the intended structure of nitrogen-coordinated metal inserted on a carbon matrix (M-N-C). In this study, some of these side products, such as nitrogen-doped carbon and carbon-encapsulated metal nanoparticles were selectively synthesized, and their electrocatalytic activities for the ORR were measured in acid and alkaline electrolytes. The ORR polarization curves showed that carbon-encapsulated iron-cobalt nanoparticles do not present an increased activity in relation to pure carbon. When the surfaces of these encapsulated nanoparticles were doped with nitrogen using thermal treatment in N2 or in NH3, the resulting ORR curves presented a slight shift of the onset potentials to higher values in both electrolytes. Remarkably, when a nitrogen-rich molecule (imidazole) was used as the nitrogen precursor, the highest ORR activity was achieved in both media. Only for those materials thermally treated in N2 or in NH3 atmosphere or synthesized in the presence of imidazole, the EXAFS characterization revealed the occurrence of iron-nitrogen and cobalt-nitrogen bonds, this being an indicative of the formation of M-N-C species. The presence of imidazole may lead to Fe and Co complexation during the first step of the synthesis, permitting better dispersion of the metallic atoms on the carbon powder, favoring the formation of M-N-C to a higher extent. Therefore, the results disclosed herein pointed out that carbon-encapsulated metal nanoparticles or even nitrogen-doped carbon-encapsulated metal nanoparticle cannot be responsible for the high half-wave potentials encountered in polarization curves for the ORR in M-N-C materials synthesized via pyrolysis. Only M-N-C species, even being present at low amounts, can result in high ORR activity whether in acid or alkaline media.

Metal catalyzed preparation of carbon nanomaterials by hydrogen–oxygen detonation method

A hydrogen–oxygen gas detonation was direct initiated by using a 20 J electronic spark, and the pressure and temperature of which were measured by a pressure sensor and high-speed camera, respectively. The results showed the mixed gas was direct initiated in the propagation of detonation wave. The carbon nanomaterials were prepared by decomposition of ferrocene and cobalt (III) acetylacetonate (Co(acac)3), a the samples were characterized by X-ray diffractometer, transmission electron microscope, engergy dispersive X-ray detector and Raman spectrometer. The results indicated that carbon-encapsulated metal nanoparticles were fabricated by using ferrocene, ferrocene–Co(acac)3 as a precursor, and the core–shell nanostructures were carbon-encapsulated Fe/Fe3C nanoparticles (Fe@C) and carbon-encapsulated Co nanoparticles (Co@C). However, the Fe–Co alloy was absent in sample from ferrocene–Co(acac)3. It is interesting that the sample from Co(acac)3 were Co@C and multi-walled carbon nanotubes (MWCNTs), and the crystallization degrees of the carbon and Co nanoparticles in the MWCNTs were higher than that of in carbon-encapsulated metal nanoparticles, however, the degree of graphitization of the powders was low. The physical properties of precursors, hydrogen content and rapid reaction were the main factors which contributed to the different morphologies and the absence of Fe–Co alloy.

Carbon-encapsulated metal nanoparticles deposited by plasma enhanced magnetron sputtering

Carbon-encapsulated gold nanoparticles (CEGNs) were synthesized by plasma enhanced magnetron sputtering using C2H2 and Ar mixtures as the precursor. The morphology, microstructure and optical property of the CEGNs were characterized using TEM, SEM, and UV–Vis absorption spectrum. The results show that the gold NPs with narrow size distribution ranging from 7 nm to 20 nm were encapsulated by amorphous carbon nanospheres. The fraction of C2H2 in the gas mixture of C2H2 and Ar seemed to have a significant influence on the gold particle size and distribution of the as-made CEGNs. As the fraction of the C2H2 increased from 5% to 10%, the average particle size decreased from about 13 nm to 7 nm and the carbon nanospheres with single gold NPs core evolved to the carbon chain with multi-cores. In addition, carbon-encapsulated silver nanoparticles (CESNs) were also prepared by the same method. This indicates that the plasma enhanced magnetron sputtering may be one of the best ways for the deposition of the carbon-encapsulated metal nanoparticles (CEMNs). The UV–Vis absorption spectra of the CEGNs and CESNs reveal strong absorption bands due to the surface plasma resonance of the nanoparticles, implying that the as-made CEMNs could be applied in optical limiting devices.

Fabrication and Properties of Carbon-Encapsulated Cobalt Nanoparticles over NaCl by CVD.

Carbon-encapsulated cobalt (Co@C) nanoparticles, with a tunable structure, were synthesized by chemical vapor deposition using Co nanoparticles as the catalyst and supported on a water-soluble substrate (sodium chloride), which was easily removed by washing and centrifugation. The influences of growth temperature and time on the structure and magnetic properties of the Co@C nanoparticles were systematically investigated. For different growth temperatures, the magnetic Co nanoparticles were encapsulated by different types of carbon layers, including amorphous carbon layers, graphitic layers, and carbon nanofibers. This inferred a close relationship between the structure of the carbon-encapsulated metal nanoparticles and the growth temperature. At a fixed growth temperature of 400 °C, prolonged growth time caused an increase in thickness of the carbon layers. The magnetic characterization indicated that the magnetic properties of the obtained Co@C nanoparticles depend not only on the graphitization but also on the thickness of the encapsulated carbon layer, which were easily controlled by the growth temperatures and times. Optimization of the synthesis process allowed achieving relatively high coercivity of the synthesized Co@C nanoparticles and enhancement of its ferromagnetic properties, which make this system promising as a magnetic material, particularly for high-density magnetic recording applications.

Combustion synthesis: Novel routes to novel molecular nanomaterials

Combustion synthesis in a mode of electrothermal explosion was used to synthesize 1D carbon nanostructures by magnesium reduction of various carbonates [Li2CO3, Na2CO3, CaCO3, FeCO3, (NH4)2CO3] either in argon or in air, at an ambient pressure of 0.1 or 1.0 MPa. At the same time, several possible catalysts were tried, such as Fe, Co, Ni, Pd, Nd, Ta, Ti, Nb, W, and NiO powders. The Na2CO3 system proved to be the most promising for fabrication of fibrous products. On the other hand, our synthesis of carbon encapsulated metal nanoparticles opens up possibilities for new biomedical applications.

Morphologies and dispersion stabilities of carbon-encapsulated metal (Ag and Cu) nanoparticles synthesized by pulsed-wire evaporation (PWE)

Carbon-encapsulated Ag and Cu nanoparticles were synthesized using pulsed-wire evaporation (PWE). All as-made materials were composed of nanocapsules with a uniform particle size at and below 50 nm. The nanocapsules consisted of an outer carbon layer shaped multi-shell. The dispersion stability of the solvent depends on the measured zeta potential for the particles in water, ethanol, ethylene glycol (EG), and polyethylene glycol (PEG). In the case of Ag@C, a stable dispersion was observed in the PEG suspension only. The stable dispersions of Cu@C were observed in EG and PEG without showing any clarification or sedimentation. Flocculation by a coalescing reaction between the nanoparticles was insignificant due to carbon layers on the surfaces of the metal (Ag and Cu) particles.

Size Prediction of Carbon-Encapsulated Nickel Nanoparticles

A simple theoretical model to predict the size control of carbon-encapsulated metal nanoparticles is developed using heat transfer and carbon diffusion theories. Taking carbon-encapsulated nickel nanoparticles as an example, the minimum size of carbon-encapsulated structure that can be formed as a function of the ambient temperature is calculated and the effect of activation energies for carbon diffusion on the size of carbon-encapsulated nickel nanoparticles is examined. The theoretical results are in good agreement with the experiment, suggesting that our model can be used to guide the size-controlled synthesis of carbon-encapsulated metal nanoparticles.

Synthesis of carbon encapsulated non-ferromagnetic metal nanoparticles

Graphite (graphene) encapsulated metal (GEM) nanoparticles are a core-shell structured composite material. Contrary to the fast development of ferromagnetic GEM, the non-ferromagnetic core-shell structure was rarely studied, although these materials are potentially useful in many applications, such as thermal dissipation, electromagnetic wave absorption and radioactive waste disposal. In this study, non-ferromagnetic carbon encapsulated metal (CEM), i.e., core metals without graphite-catalyzing capability, were successfully synthesized with two new crucible designs. The new designs can effectively increase or decrease the thermal efficiency of the arc for high or low melting metals, respectively, thereby controlling the carbon-to-metal vapor ratio at an adequate value. Preliminary results showed that non-ferromagnetic metals, e.g. Mo and Cu, can be encapsulated by these two new setups. The percentage of well-encapsulated nanoparticles has been improved from negligible to about 4–6%. Additionally, a new centrifugal purification method was also developed to gather the non-ferromagnetic Mo- and Cu-CEM nanoparticles, which cannot be collected by a strong magnetic field. The new synthesis setups are proven to be feasible, and effective; they should be easily adapted to the synthesis of other CEM nanoparticles with various core metals.

Synthesis and Characterization of Graphite-Encapsulated Metal (Fe,Co,Ni) Nanoparticles by a Detonation Method

A method for synthesizing carbon-encapsulated metal nanoparticles(CEMNPs) is reported. In the proposed method, a composite precursors containing various nitrate dissolved in absolute ethanol is ignited by a nonelectric detonator in nitrogen gas in an explosion vessel. Upon the completion of detonation reaction, CEMNPs (Fe@C, Ni@C, Co@C) with diameters ranging from a few nanometers to about 20 nm are produced in the explosion vessel.The material characteristics of these nanoparticles are then examined with the XRD, TEM, EDX and VSM, which characterize the feature of morphology, components, phases and magnetism of nano-composite particles. The composite particles whose coating shell were graphite carbon could be dispersed finely. The core of nanoparticles were composed of iron,cobalt and nickel crystal to that of the above explosive precursors.The magnetic analysis indicated that the different composite nanoparticles have good ferromagnetism and superparamagetism in room temperature.

Synthesis, Properties, and Applications of Low-Dimensional Carbon-Related Nanomaterials

In recent years, many theoretical and experimental studies have been carried out to develop one of the most interesting aspects of the science and nanotechnology which is called carbon-related nanomaterials. The goal of this paper is to provide a review of some of the most exciting and important developments in the synthesis, properties, and applications of low-dimensional carbon nanomaterials. Carbon nanomaterials are formed in various structural features using several different processing methods. The synthesis techniques used to produce specific kinds of low-dimensional carbon nanomaterials such as zero-dimensional carbon nanomaterials (including fullerene, carbon-encapsulated metal nanoparticles, nanodiamond, and onion-like carbons), one-dimensional carbon nanomaterials (including carbon nanofibers and carbon nanotubes), and two-dimensional carbon nanomaterials (including graphene and carbon nanowalls) are discussed in this paper. Subsequently, the paper deals with an overview of the properties of the mainly important products as well as some important applications and the future outlooks of these advanced nanomaterials.

Synthesis of Carbon-Encapsulated Metal Nanoparticles from Wood Char

Carbon-encapsulated metal nanoparticles were synthesized by thermal treatment of wood char, with or without transition metal ions pre-impregnated, at 900°C to 1,100°C. Nanoparticles with concentric multilayer shells were observed. The nanoparticles were analyzed by scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive X-ray (EDX) spectroscopy. The EDX spectrum showed that carbon was the dominant element in the shells. TEM and XRD analysis indicated that the generated carbon shells had structures similar to that of graphite with an average interplanar distance of 0.34 nm. The effects of temperature and pre-impregnation of metal ions on the yield of carbon-encapsulated metal nanoparticles were studied.

Synthesis of carbon encapsulated metal (Ni and Cu) nano particles and applications for chiral catalysts

Carbon-encapsulated Ni and Cu nanoparticles with a core/shell structure were synthesized by levitational gas condensation (LGC). Methane (CH4) gas was used to coat the surface of the Ni and Cu nanoparticles. The Ni particles had a core diameter of 10 nm, and were covered by 2–3 nm thin carbon layers. In the case of Cu, the particles had a core diameter of 30 nm, and multi-shells with a thickness of 2–3 nm. Biginelli reaction in the presence of l-proline and Ni encapsulated nanoparticles was carried out to change the ratio between stereoisomers in favor of the S-enantiomer for 3,4–dihydropyrimidine (DHPM) with an excess of about 18%.

Synthesis of Carbon-Encapsulated Metal Nanoparticles by a Detonation Method

Carbon-encapsulated metal nanoparticles are synthesized by a detonation method using a home-made water-soluble composite explosive. The results of the study show that detonation products contain face-centered cubic cobalt/nickel nanocrystals approximately 10–25 nm in size, which are encapsulated by thin (3–5 nm) carbon layers. These spherical and ellipsoidal nanoparticles exhibit a good superparamagnetism state at 300 K.

Synthesis of Carbon-Encapsulated Metal (Fe,Co,Ni) Nanoparticles by Detonation Decomposition of Composite Explosives

A detonation method for synthesizing carbon-encapsulated metal nanoparticles (CEMNPs) is reported. The composite precursors containing various nitrate dissolved in absolute ethanol is ignited by a nonelectric detonator in nitrogen gas in an explosion vessel. The material characteristics of these nanoparticles are then examined with the XRD,TEM,EDX and RS. The results show that the composite particles whose coating shell were graphite carbon could be dispersed finely. The core of nanoparticles were composed of iron, cobalt and nickel crystal to that of the above explosive precursors.

Morphology and Structure of Aerosol Carbon-Encapsulated Metal Nanoparticles from Various Ambient Metal−Carbon Spark Discharges

The morphology and structure of aerosol carbon encapsulated metal nanoparticles (CEMNs) of various transition metals (anode; Ti, Cu, Zn, Mo, Pd, W, Pt, or Au) formed by ambient spark discharge at the same electrical operating specifications were analyzed. CEMNs were produced with aggregated carbon particles, and their yields and sizes varied according to the metal-to-carbon fraction of each discharge relating to the ionization potential of the electrode material. Each encapsulated metal had natural crystallinity for all discharges, but carbon graphitization for the Mo-C and W-C configurations, which have relatively small differences in melting temperature between the materials, was particularly weak. An empty zone in the carbon shell was also detected in the CEMNs because of the difference in density between the molten and solid phases of the core metal during encapsulation.