Graphene supported heterogeneous catalysts: An overview

Owing to the fast development of wireless information technologies at the high-frequency range, the electromagnetic interference problem has been of increasing significance and attracting global attention. One key solution for this problem is to develop materials that are able to attenuate the unwanted electromagnetic waves. The desired properties of these materials include low reflection loss value, wide attenuation band, light weight, and low cost. This review gives a brief introduction to graphene-based composites and their electromagnetic absorption properties. The ultimate goal of these graphene absorbers is to achieve a broader effective absorption frequency bandwidth (fE) at a thin coating thickness (d). Representative and popular composite designs, synthesis methods, and electromagnetic energy attenuation mechanisms are summarized in detail. The two key factors, impedance matching behavior and attenuation ability, that determine the electromagnetic behavior of graphene-based materials are given particular attention in this article.

A brief introduction to the fabrication and synthesis of graphene based composites for the realization of electromagnetic absorbing materials

Direct observations of solution-phase nanoparticle growth using in situ liquid transmission electron microscopy (TEM) have demonstrated the importance of “non-classical” growth mechanisms, such as aggregation and coalescence, on the growth and final morphology of nanocrystals at the atomic and single nanoparticle scales. To date, groups have quantitatively interpreted the mean growth rate of nanoparticles in terms of the Lifshitz–Slyozov–Wagner (LSW) model for Ostwald ripening, but less attention has been paid to modeling the corresponding particle size distribution. Here we use in situ fluid stage scanning TEM to demonstrate that silver nanoparticles grow by a length-scale dependent mechanism, where individual nanoparticles grow by monomer attachment but ensemble-scale growth is dominated by aggregation. Although our observed mean nanoparticle growth rate is consistent with the LSW model, we show that the corresponding particle size distribution is broader and more symmetric than predicted by LSW. Follow...

Direct observation of aggregative nanoparticle growth: kinetic modeling of the size distribution and growth rate.

To design and fabricate rational surface architecture of individual particles is one of the key factors that affect their magnetic properties and microwave absorption capability, which is still a great challenge. Herein, a series of Co20Ni80 hierarchical structures with different surface morphologies, including flower-, urchin-, ball-, and chain-like morphologies, were obtained using structure-directing templates via a facile one-step solvothermal treatment. The microwave reflection loss (RL) of urchin-like Co20Ni80 hierarchical structures reaches as high as −33.5 dB at 3 GHz, with almost twice the RL intensity of the ball- and chain-like structures, and the absorption bandwidth (<−10 dB) is about 5.5 GHz for the flower-like morphology, indicating that the surface nanospikes and nanoflakes on the Co20Ni80 microsphere surfaces have great influences on their magnetic microwave absorption properties. Electron holography analysis reveals that the surface nanospikes and nanoflakes could generate a high density of stray magnetic flux lines and contribute a large saturation magnetization (105.62 emu g−1 for urchin-like and 96.41 emu g−1 for flower-like morphology), leading the urchin-like and flower-like Co20Ni80 to possess stronger microwave RL compared with the ball-like and chain-like Co20Ni80 alloys. The eddy-current absorption mechanism μ′′(μ′)−2(f)−1 is dominant in the frequency region above 8 GHz, implying that eddy-current loss is a vital factor for microwave RL in the high frequency range. It can be supposed from our findings that different surface morphologies of magnetic hierarchical structures might become an effective path to achieve high-performance microwave absorption for electromagnetic shielding and stealth camouflage applications.

Dependency of magnetic microwave absorption on surface architecture of Co20Ni80 hierarchical structures studied by electron holography.

Well-dispersed magnetically recyclable core–shell Ag@M (M = Co, Ni, Fe) nanoparticles (NPs) supported on graphene have been synthesized via a facile in situ one-step procedure, using methylamine borane (MeAB) as a reducing agent under ambient condition. Their catalytic activity toward hydrolysis of ammonia borane (AB) were studied. Although the Ag@Fe/graphene NPs are almost inactive, the as-prepared Ag@Co/graphene NPs are the most reactive catalysts, followed by Ag@Ni/graphene NPs. Compared with AB and NaBH4, the as-synthesized Ag@Co/graphene catalysts which reduced by MeAB exert the highest catalytic activity. Additionally, the Ag@Co NPs supported on graphene exhibit higher catalytic activity than the catalysts with other conventional supports, such as the SiO2, carbon black, and γ-Al2O3. The as-synthesized Ag@Co/graphene NPs exert satisfied catalytic activity, with the turnover frequency (TOF) value of 102.4 (mol H2 min–1 (mol Ag)−1), and the activation energy Ea value of 20.03 kJ/mol. Furthermore, the ...

Graphene-supported Ag-based core-shell nanoparticles for hydrogen generation in hydrolysis of ammonia borane and methylamine borane.

Double-shelled Ag/C/Ni nanocables have been synthesized through a deposition covering process of Ni nanoparticles (NPs) onto Ag/C pentagonal prism nanowires (NWs). The proposed synthesis mechanism is corroborated by scanning electron microscopy, transition electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and UV-vis absorption spectroscopy. The resulting Ag/C/Ni nanocables with an average diameter of ∼270 nm are made up of Ag NW core (∼200 nm diameter) with internal amorphous C layer (∼10 nm thickness) and outer Ni shell (∼25 nm thickness). The UV-vis absorption spectroscopy analysis indicates that the covering of the Ni shell on the Ag/C nanowire can dampen the surface plasmon resonance (SPR) of Ag wire core and lead to a red-shifted SPR absorption peak. In particular, compared with Ni NPs, the resultant double-shelled Ag/C/Ni magnetic nanocables exhibits higher catalytic activity for the dehydrogenation toward aqueous ammonia borane under ambient atmosphere, and its calculated activation energy is lower than those of many bimetallic catalysts.

Controllable assembly of Ag/C/Ni magnetic nanocables and its low activation energy dehydrogenation catalysis

Magnetic double-shelled Ag@C@Co pentagonalprism nanocables are fabricated using a synchronous growth and oriented assembly process, in which the second shell of Co is arranged along the edges of Ag@C pentagonalprism nanowires (NWs). The resulting Ag@C@Co pentagonalprism nanocables exhibit an average diameter of ≈400 nm and consist of Ag core NWs with diameter of ≈200 nm and C middle layers with a thickness of ≈10 nm as well as outer Co shells with a thickness of ≈100 nm. UV-vis absorption spectroscopy shows that the Co shell on Ag@C NWs can damp the surface plasmon resonance (SPR) of the Ag core wires and lead to a red-shifted SPR absorption peak. Additionally, the Ag@C@Co nanocables have the ferromagnetic behavior, which can be controlled by modulating the shell density. The resulting magnetic Ag@C@Co nanocables exert excellent selected catalytic activity along the edges toward the dehydrogenation of ammonia borane aqueous under ambient conditions at room temperature.

Oriented Growth and Assembly of Ag@C@Co Pentagonalprism Nanocables and their Highly Active Selected Catalysis Along the Edges for Dehydrogenation

A new magnetic Pt-loaded NiCo nanochain, with the diameter from 80 nm to 120 nm, has been prepared through microwave-induced assembly process followed by the galvanic displacement performance. Pt nanoparticles are distributed on the surface of NiCo nanochains. The products are investigated as hydrolytic dehydrogenation catalyst for potential hydrogen energy applications. Compared with NiCo nanochains, the Pt-loaded NiCo nanochains present exceedingly high catalytic activity toward the hydrolytic dehydrogenation of ammonia borane aqueous under ambient atmosphere at room temperature, where the Ni16Co80/Pt4 nanochains exhibit high catalytic activity with a lower activation energy of 45.72 kJ mol(-1) and a superior dehydrogenation rate of 1.17 × 10(4) mL min(-1) g(-1), suggesting the potential application in hydrogen fuel and chemical industry.

Fabrication of Pt-loaded NiCo nanochains with superior catalytic dehydrogenation activity.

A three-dimensional (3D) broom-like nanostructured magnetic FeNi catalyst was synthesized using inexpensive Fe and Ni as precursors in a controllable microwave-assisted route. In this 3D broom-like nanostructure, the length of an FeNi nanorod, which serves as a building block, depends on the molar content of Fe. With a decrease in the Fe content from 100 to 45% in the Fe(x)Ni(96-x) system, the length varies from 8 μm to 1 μm, which is corroborated by SEM, TEM, XRD, EDS and XPS. The magnetic behavior measurement results show that the magnetic saturation and coercivity are strongly influenced by the length of the nanorods and the Fe content. The sample of Fe nanorods gives a maximum magnetization saturation at 196 emu g(-1), and a maximum coercivity of 241.23 Oe is obtained for Fe78Ni22. These economical 3D FeNi broom-like nanostructures, with large surface areas and dispersed active sites, can dramatically facilitate the diffusion and transportation of a reactant to improve the reactivity. In particular, Fe89Ni11 broom-like nanostructures exert an excellent reactivity towards the reductive dechlorination of 1,1,2,2-tetrachloroethane.

Jin Peng

Papers

Controllable assembly of Ag/C/Ni magnetic nanocables and its low activation energy dehydrogenation catalysis

Oriented Growth and Assembly of Ag@C@Co Pentagonalprism Nanocables and their Highly Active Selected Catalysis Along the Edges for Dehydrogenation

Fabrication of Pt-loaded NiCo nanochains with superior catalytic dehydrogenation activity.

Microwave-assisted synthesis and high dechlorination activity of magnetic FeNi broom-like nanostructures