Catalyst faceting during graphene layer crystallization in the course of carbon nanofiber growth

The low temperature catalytic growth of multiwall carbon nanotubes (MWCNTs) rests on the continuous nucleation and growth of graphene layers at the surface of crystalline catalyst particles. Here, we study the atomic mechanisms at work in this phenomenon, by observing the growth of such layers in situ in the transmission electron microscope, in the case of iron-based catalysts. Graphene layers, parallel to the catalyst surface, appear by a mechanism of step flow, where the atomic layers of catalyst are “replaced” by graphene planes. Quite remarkably, catalyst facets systematically develop while this mechanism is at work. We discuss the origin of faceting in terms of equilibrium particle shape and graphene layer nucleation. Step bunching due to impeded step migration, in certain growth conditions, yields characteristic catalyst nail-head shapes. Mastering the mechanisms of faceting and step bunching could open up the way to tailoring the structure of low temperature-grown MWCNTs, e.g. with highly parallel carbon walls and, ultimately, with controlled structure and chirality.     

An advanced electrocatalyst with exceptional eletrocatalytic activity via ultrafine Pt-based trimetallic nanoparticles on pristine graphene

It has been a great challenge to directly deposit uniform metal particles onto pristine graphene due to its low surface energy and chemical inertness. Without any surfactant or functionalization, we have developed a unique synthesis of high-quality PtRuNi trimetallic nanoparticles supported on pristine graphene via a simple but effective supercritical route. Due to excellent wettability between supercritical carbon dioxide and the carbon surface, ultrafine metal particles are uniformly and firmly anchored on the graphene sheets. While well retaining its intrinsic structure and outstanding electronic conductivity, the pristine graphene with well-dispersed PtRuNi trimetallic nanoparticles shows significantly improved catalytic activity towards methanol oxidation, which is at least ten times higher than those of the commercial Pt/C and homemade Pt/XC-72 catalysts. The resulting trimetallic hybrid also exhibits high stability as compared to Pt and PtRu/pristine graphene composites and the reduced graphene oxide counterparts. In principle, the supercritical method can be applied to other metal nanoparticles in fabrication of high-performance graphene-based nano-catalysts.     

SiC-assisted growth of tubular graphenic cones with carbon nanotube tips

SiC-assisted growth of tubular graphenic cones (TGCs) with carbon nanotube tip is achieved with high yield in the microwave plasma chemical vapor deposition process. No pre-existing metal or semiconductor catalyst particles are required on the substrate prior to the deposition process. Instead, the in situ grown SiC crystallites serve as the catalyst for the growth of TGCs. Thanks to the easy formation of SiC on various substrates, the process is compatible with a wide range of substrates, i.e. Si, diamond, 2H–SiC, GaN, and SiO₂, but not only limited to them. In addition to developing the approach, the mechanism for the formation of TGCs is also proposed. When Si is used as the substrate, the 3C–SiC crystallites grow epitaxially on the surface, which further initiate the epitaxial growth of graphene with its basal plane parallel to the {111} planes of 3C–SiC. The further expansion of graphene is constrained by the 3C–SiC crystallites, leading to the formation of curved graphene layers, i.e. onion-like or bowl-shaped graphene-based carbon. These curved graphene layers are believed to be the possible nucleation sites for the growth of TGCs. Inclusion of boron in the gas phase promotes the growth rates and the final yields of TGCs.     

Anchoring of Pt and PtRu to carbon nanofibers studied by density functional theory calculations

PtRu particles supported on carbon nanofibers have been reported to have higher activity as anode catalysts in proton exchange membrane fuel cells than conventional catalysts. In the present work, density functional theory calculations are used to investigate the metal–carbon interface for different crystal facets of mono-metallic Pt and the PtRu alloy. The carbon side is modeled by graphene sheets with either zigzag or armchair termination. The strongest metal–carbon interaction is predicted for a (1 1 1) facet attached to a zigzag edge. The anchoring of the PtRu metal is found to have pronounced effects on the surface composition of the alloy. Whereas the bare surface is rich in Pt, the interface with carbon favors the stoichiometric bulk composition. Core level binding energies of carbon, platinum and ruthenium are found to provide valuable signatures of the interface and give means to interpret future high resolution photoemission core level spectroscopy experiments.     

A general strategy for the synthesis of reduced graphene oxide-based composites

Well-exfoliated graphene oxide sheets were initially fabricated through a modified pressurized oxidation method with powdered flake graphite as raw material. A variety of inorganic-reduced graphene oxide composites have been then successfully synthesized through a general solvothermal strategy with the graphene oxide sheets as supports, ethanol as solvent, and metal salts as precursors. After the solvothermal reactions, Ni(OH)₂ nanoparticles, Fe₂O₃ nanorods, W₁₈O₄₉ nanowires, ZnO nanoparticles, and Ag nanoparticles were in situ grown on the surfaces of the graphene oxide sheets, accompanied by effective reduction of graphene oxide to reduced graphene oxide. The as-prepared products have been systematically characterized by electron microscopy, X-ray diffraction, X-ray photoelectron spectrometry, and Raman spectroscopy. The present work opens up a versatile route for preparing the reduced graphene oxide-based composites.     

How the shape of catalyst nanoparticles determines their crystallographic orientation during carbon nanofiber growth

A theoretical model is presented that explains spontaneous changes in the crystalline orientation of nanoparticles. The spontaneous changes in crystalline orientation are attributed to the crystal anisotropy of the surface energy of nanocrystalline particles. We consider an important specific case of the chemical vapor deposition growth of carbon nanofibers, where previous studies have shown that both the catalyst nanoparticle shape and the nanofiber growth rate change with changes in the chemical potential of diluted carbon. Energetic considerations of the nanoparticle’s free surface and its interfacial energy with the nanofiber during these shape changes are shown to force a reorientation of the nanoparticle crystallographic axes at a critical growth rate. The model therefore reveals the mechanism by which the shape and crystallographic orientation of the catalyst nanoparticle are linked to the nanofiber growth rate. The model suggests a new way, based upon measurable geometry of nanoparticles during in situ growth experiments, to estimate the role of chemisorption in the attraction of the graphene film to the curved catalyst surface and the anisotropy energy of this interface.     

Exfoliated graphene sheets decorated with metal/metal oxide nanoparticles: Simple preparation from cation exchanged graphite oxide

We produced carbon hybrid materials of graphene sheets decorated with metal or metal oxide nanoparticles of gold, silver, copper, cobalt, or nickel from cation exchanged graphite oxide. Measurements using powder X-ray diffraction, transmission electron microscopy, and X-ray absorption spectra revealed that the Au and Ag in the materials (Au–Gr and Ag–Gr) existed on graphene sheets as metal nanoparticles, whereas Cu and Co in the materials (Cu–Gr and Co–Gr) existed as a metal oxide. Most Ni particles in Ni–Gr were metal, but the surfaces of large particles were partly oxidized, producing a core–shell structure. The Ag–Gr sample showed a catalytic activity for the oxygen reduction reaction in 1.0 M KOH aq. under an oxygen atmosphere. Ag–Gr is superior as a cathode in alkaline fuel cells, which should not be disturbed by the methanol cross-over problem from the anode. We established an effective approach to prepare a series of graphene-nanoparticle composite materials using heat treatment.     

Monolayer graphene from graphite oxide

Graphene, a new carbon material, is attracting presently an increasing research interest. It stems from the unique electrical and mechanical properties of graphene predicted by theory. Experimental studies of graphene are, however, severely curtailed by a lack of an appropriate technique for its preparation. Mechanical cleavage of graphite proved to be ineffective, since it yields only very small (a few microns in size) particles of monolayer graphene. The rapidly developing approach based on chemical exfoliation of graphite produces large-area coatings composed primarily of arbitrarily oriented multilayer graphene particles. We have developed a technique for preparation of monolayer graphene sheets involving liquid exfoliation of crystalline graphite, which includes synthesis of graphite oxide by deep oxidation as an intermediate stage. Electron diffraction traces, as well as the variation of diffracted intensities with local orientation of graphene sheets, AFM, and HRTEM images testify to a remarkably good monolayer structure of the graphite oxide particles obtained by our technique. These results open a way to setting up high-efficiency production of monolayer graphene sheets appropriate for electrical and optical measurements and fabrication of structures for use in the field of applications.     

Co-Mo catalyzed growth of multi-wall carbon nanotubes from CO decomposition

We investigated the growth of multi-wall carbon nanotubes (CNTs) catalyzed by SiO₂-supported Co-Mo bi-metallic catalyst in flowing CO at 700 °C. We found that both Co and Mo are present in catalytic particles at the tips of CNTs, but their compositions vary from one catalytic particle to another and significantly deviate from the initial mixing composition. The Co concentration and distribution in the catalytic particle of a CNT largely determines the length of the CNT. The CNT growth process is carbon adsorption on exposed area of a catalytic particle and subsequent precipitation at the CNT-catalyst interface or open CNT wall edges. The encapsulation of a catalytic particle was found to occur by the growth of the open-edged graphene walls around the particle. Two types of long CNTs were observed: one with their CNT walls ended at the CNT-particle interface, and the other with their CNT walls open to the environment. The former have diameters similar to their catalytic particle size while the latter have larger diameters.     

Growth of nano-textured graphene coatings across highly porous stainless steel supports towards corrosion resistant coatings

In this paper, we demonstrated for the first time the growth of 3D networks of graphene nano-flakes across porous stainless steel substrates of micron sized metal fibres, and its anti-corrosion properties. The controlled formation of graphene-grade coatings in the form of single sheets to complex and homogeneously distributed 2–4 μm long nano-pillars is demonstrated by Scanning Electron Microscopy. The morphology and stability of these structures are shown to be particularly related to the temperature and feed gas flow rate during the growth. The number of layers across the graphene materials was calculated from the Raman spectra and is shown to range between 3 and more than 15 depending on the growth conditions and to be particularly related to the time and flow rate of the experiment. The presence of the graphene was shown to massively enhance the specific surface area of the material and to contribute to the increased corrosion resistance and electrical conductivity of the material without compromising the properties or structure of the native stainless steel materials. This new approach opens up a new route to the facile fabrication of advanced surface coatings with potential applications in developing new thermal exchangers, separation and bio-compatible materials.     

Electrochemically exfoliated graphene oxide/iron oxide composite foams for lithium storage,produced by simultaneous graphene reduction and Fe(OH)3 condensation

We describe the production of graphene-based composites for energy storage, obtained by a combination of electrochemical and solution processing techniques. Electrochemically exfoliated graphene oxide sheets (EGO) are produced using an original setup that allows fast expansion of graphite flakes and efficient exfoliation of expanded graphite via an electrochemical route. The sheets are deposited on a sacrificial nickel foam together with an iron hydroxide colloidal precursor. Calcination treatment simultaneously renders the EGO foam conductive and transforms Fe(OH)₃ into hematite (α-Fe₂O₃), yielding a nanoporous Fe₂O₃ layer on the surface of the mesoporous EGO foam, creating an ideal structure for lithium storage. The obtained graphene/metal oxide hybrid is a continuous, electrically conductive three-dimensional (3D) composite featuring a hierarchical meso–nano porous structure. A systematic study of these composites, varying the Fe₂O₃:EGO ratio, is then performed to maximize their performance as nanostructured electrodes in standard coin cell batteries.     

Folding of graphene into elastic nanobelts

Here we report the finding of a new crumpled graphene structure – folded graphene belts (FGBs) – generated by means of shock cooling of an aqueous chemically converted graphene (CCG) dispersion. Unlike the traditional tubular hollow structures such as CNTs or CNSs, the as-made FGBs feature an accordion-like geometry in which the 2D graphene sheets were folded along multiple parallel axes. In situ scanning electron microscope (SEM) measurements revealed that the prepared FGBs were highly elastic and can keep their shape under repeated large strains. The formation and growth of ice crystals during the shock cooling step in liquid nitrogen are believed to be the driving force for the formation of such unique folded graphene structures.     

Catalytic chemical vapor deposition of methane on graphite to produce graphene structures

Graphene structures, obtained by catalytic chemical vapor deposition of methane on highly oriented pyrolitic graphite (HOPG), were examined using scanning tunneling microscopy. Depending on the Fe catalyst coverage and localization on the substrate steps and terraces, different graphene structures were obtained: curved graphene sheets at the edges of topmost stacked graphene bilayers, laterally grown terraces at the edges of individual graphene layers parallel to the HOPG basal plane and planar graphene islands on the terraces. A growth mechanism is proposed that takes into account the specific features of the spatial distribution of Fe catalytic nanoparticles on the substrate surface, driven by metal film-substrate interaction. The present synthesis approach is promising for the controlled growth and modification of graphene layers, as well as for engineering the edge characteristics of graphene systems at the atomic scales.     

Wear behavior of Fe3Al-TiN-TiB2 HVOF coatings: A comparative study between in situ and ex situ powder processing routes

In the present study, the tribological properties of High Velocity Oxy-Fuel (HVOF) coatings prepared from Fe₃Al-based composite powders were investigated. The iron aluminide matrix of the composite powders was reinforced with TiN and TiB₂ particles made using two different processing routes: a) an in situ method where fine ceramic particles were formed in the matrix by the reaction between Ti and BN, and b) an ex situ method where preformed coarse TiN and TiB₂ particles were added to the matrix. The tribomechanical performance of the coatings was assessed using indentations and pin-on-disc wear tests. Compared to ex situ samples, the Fe₃Al-based coatings strengthened with in situ ceramic particles exhibit higher microhardness and wear resistance regardless of the sliding velocity. The presence of voids, cracks and scratches/grooves in the wear track of the in situ coatings and the coating material transferred to the corresponding counterpart suggest that coatings with fine reinforcing particles fail predominantly via delamination and adhesive wear mechanisms. In the case of the ex situ coatings, the presence of a significant amount of hard ceramic particles within the wear track indicates that abrasive wear plays a dominant role in the degradation mechanism. Oxidation wear also contributed to material removal at high sliding velocity since transfer materials inside the wear track contain a high oxygen content compared to the unworn region regardless of the coating type.     

Facile synthesis of flexible and free-standing cotton covered by graphene/MoO2 for lithium-ions batteries

It is necessary to build flexible and free-standing materials for flexible/wearable electronics in high-performance lithium-ions batteries. Herein, we design and fabricate a flexible and free-standing 3 D carbon/MoO₂ composite through a facile immersing method followed by an annealing process. The carbon framework is supported by non-woven cotton totally covered by graphene sheets. The nanosized MoO₂ particles were uniformly anchored on cotton fibers and graphene sheets. The structure has several advantages, such as an interconnected 3D electronically conductive network, plenty of channels for electrolyte solution cross, and more active points for the electrode reaction. Compared with cotton/MoO₂ (C/MoO₂) without graphene sheets, the CGN/MoO₂ composite (cotton covered by graphene/MoO₂) showed much better thermal stability and excellent cycling performance. The proposed synthesis process paves a new way as promising electrode materials for high power battery applications such as roll-up displays and wearable devices.     

Raman spectroscopy and band structure of Pd-hybridized multilayer graphene

Graphene sheets prepared through liquid exfoliation of expanded graphite were hybridized with Pd nanoparticles. The impact of these particles on the electronic and physical structure of the graphene is determined through transmission electron microscopy and Raman spectroscopy using 532 and 325 nm excitation wavelengths. Based on the changes to the Raman D and G peaks, insights are provided concerning the deposition mechanism at pristine and defective lattice sites, as well as electronic scattering. These data are compared to ab initio band structure computations. For purposes of the model, the graphene/Pd hybrid was approximated by a charged graphene sheet. The resulting structure exhibited π–π¹ expansion approaching the Γ point of the Brillouin zone which was validated by tracking the Raman D band dispersion.     

The situ preparation of silica nanoparticles on the surface of functionalized graphene nanoplatelets

A method for situ preparing a hybrid material consisting of silica nanoparticles (SiO₂) attached onto the surface of functionalized graphene nanoplatelets (f-GNPs) is proposed. Firstly, polyacrylic acid (PAA) was grafted to the surface of f-GNPs to increase reacting sites, and then 3-aminopropyltriethoxysilane (APTES) KH550 reacted with abovementioned product PAA-GNPs to obtain siloxane-GNPs, thus providing reaction sites for the growth of SiO₂ on the surface of GNPs. Finally, the SiO₂/graphene nanoplatelets (SiO₂/GNPs) hybrid material is obtained through introducing siloxane-GNPs into a solution of tetraethyl orthosilicate, ammonia and ethanol for hours'' reaction. The results from Fourier transform infrared spectroscopy (FTIR) showed that SiO₂ particles have situ grown on the surface of GNPs through chemical bonds as Si-O-Si. And the nanostructure of hybrid materials was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All the images indicated that SiO₂ particles with similar sizes were grafted on the surface of graphene nanoplatelets successfully. And TEM images also showed the whole growth process of SiO₂ particles on the surface of graphene as time grows. Moreover, TGA traces suggested the SiO₂/GNPs hybrid material had stable thermal stability. And at 900°C, the residual weight fraction of polymer on siloxane-GNPs was about 94.2% and that of SiO₂ particles on hybrid materials was about 75.0%. However, the result of Raman spectroscopy showed that carbon atoms of graphene nanoplatelets became much more disordered, due to the destroyed carbon domains during the process of chemical drafting. Through orthogonal experiments, hybrid materials with various sizes of SiO₂ particles were prepared, thus achieving the particle sizes controllable. And the factors’ level of significance is as follows: the quantity of ammonia > the quantity of tetraethyl orthosilicate (TEOS) > the reaction time.     

Self-assembled high aspect ratio gold nanostar/graphene oxide hybrid nanorolls

Owing to their unique properties and potential applications in nanoelectronics, graphene and its derivatives have received extensive attention over the last decade. Noble metal nanostructures, on the other hand, enable the confinement and manipulation of light at the nanoscale. Integration of nanocarbons and plasmonic nanostructures is expected to result in synergistic optoelectronic properties that can potentially revolutionize the design and fabrication of optoelectronic devices. In this letter, we demonstrate a simple self-assembly approach to achieve synergistic ensemble of plasmonic gold nanostars and graphene oxide. Gold nanostars are directly nucleated and grown on the surface of graphene oxide by in situ reduction method producing differential surface charged hybrid macroanionic sheets, which are then kinetically rolled and simultaneously assembled into high aspect ratio hybrid nanorolls by means of the interplay of kinetics and graphene–gold interactions.     

A dilatometric and small-angle X-ray scattering study of the electrochemical activation of mesophase pitch-derived carbon in non-aqueous electrolyte solution

The electrochemical activation of certain pitch-derived carbons has been proposed as a promising route towards obtaining high-capacitance electrodes for electrochemical double-layer capacitors. In the present work, the mechanism of electrochemical activation of a graphitizable carbon after calcination and KOH-activation was studied by nitrogen adsorption, electrochemical dilatometry and in situ small-angle X-ray scattering (SAXS). During electrochemical activation, a large capacitance gain from 1 to 121 F/g (at 0 V in a 1 mol/L solution of Et₄NBF₄ in propylene carbonate) was accompanied by a significant irreversible swelling of the electrode by 24% (6%) for activation in the negative (positive) potential range, respectively. In situ SAXS provided clear evidence for the insertion of ions into the latent microporosity of the carbon during electrochemical activation. Thus, the mechanism of electrochemical activation of weakly activated graphitizable carbon is not strictly due to ion intercalation between parallel graphene sheets.