Graphite Nanocrystals Research Articles

In situ observations of graphitic staples in crumpled graphene

Graphene is an exceptionally effective lubricant additive that performs well when crumpled rather than as flat sheets. In the literature, it has been suggested that crumpled graphene is analogous to wadding paper, and that this morphology significantly improves self-dispersion in lubricants and anti-wear behavior. We report here in situ transmission electron microscopy of crumpled graphene sliding, demonstrating that it possesses intra-sheet links and graphitic nanocrystals that provide additional mechanical stability and enable the material to reversibly deform. Crumpled graphene is closer to crumpled paper with additional staples, with the intra-sheet links and graphitic nanocrystals as graphitic staples.

A study on the preparation of pitch-based high-strength columnar activated carbon and mechanism of phenol adsorption from aqueous solution.

Coal tar pitch was ground into powder and hydroformed with high pressure. After pre-oxidation, the pitch was activated by CO2 at high temperature. The effects of different preparation conditions on the yield, pore structure and phenol adsorption capacity of activated carbon were investigated, and activated carbon prepared under suitable conditions had good adsorption performance. A pore volume of 1–10 nm is the main absorption structure according to the analysis of pore size distribution and phenol adsorption capacity. The activated carbon showed high mechanical strength through compressive strength tests. Graphite nanocrystals around 5 nm were observed in the TEM images, and it illustrates that grain refinement results in the high strength. These nanocrystal stacked structures are easier to make and enlarge pores by activation than graphite layer stacked structures. Surface functional groups are considered not to be the active sites of phenol adsorption as suggested by the results of FTIR and Boehm's titration, and acidic oxygen-containing functional groups are harmful to phenol adsorption, which happen to be removed in the reductive preparation atmosphere. The donor–acceptor complex mechanism can be ruled out, and the π–π interactions are considered the most likely mechanism. The Langmuir and Redlich–Peterson models are better fitted to the adsorption isotherms. Adsorption kinetics fit the intraparticle diffusion model best. Comparison of different activated carbons shows that suitable pore size is important for phenol adsorption. Thermodynamic parameters demonstrate that the adsorption process is spontaneous and exothermic, and the entropy increases. Pitch-based high-strength columnar activated carbon is an effective and low cost adsorbent for phenol wastewater treatment. This carbon nanocrystal material also provides a new direction for catalyst carriers.

Preparation and Capacity-Fading Investigation of Polymer-Derived Silicon Carbonitride Anode for Lithium-Ion Battery.

Polymer-derived silicon carbonitride (SiCN) materials have been synthesized via pyrolyzing from five poly(silylcarbondiimide)s with different contents of carbon (labeled as 1–5#). The morphological and structural measurements show that the SiCN materials are mixtures of nanocrystals of SiC, Si3N4, and graphite. The SiCN materials have been used as anodes for lithium-ion batteries. Among the five polymer-derived SiCN materials, 5#SiCN, derived from dichloromethylvinylsilane and di-n-octyldichlorosilane, has the best cycle stability and a high-rate performance at the low cutoff voltage of 0.01–1.0 V. In lithium-ion half-cells, the specific delithiation capacity of 5#SiCN anode still remains at 826.7 mA h g–1 after 100 charge/discharge cycles; it can even deliver the capacity above 550 mA h g–1 at high current densities of 1.6 and 2 A g–1. In lithium-ion full cells, 5#SiCN anode works well with LiNi0.6Co0.2Mn0.2O2 commercial cathode. The outstanding electrochemical performance of 5#SiCN anode is attributed to two factors: (1) the formation of a stable and compact solid electrolyte interface layer on the anode surface anode, which protects the electrode from cracking during the charge/discharge cycle; and (2) a large amount of carbon component and the less Si3N4 phase in the 5#SiCN structure, which provides an electrochemical reactive and conductive environment in the SiCN structure, benefit the lithiation/delithiation process. In addition, we explore the reason for the capacity fading of these SiCN anodes.

NANOSTRUCTURAL PROCESSES OF MELTING AND MOULDING OF CAST IRON WITH GLOBULAR GRAPHITE

It is shown that melting and molding of cast iron with globular graphite are complex physical and chemical nanostructural processes. The major role in these processes is played by the centers of crystallization of dendrites of graphite, graphite nanocrystals, the dissolved and adsorbed oxygen. The role of the modifying ligature of FeSiMg is reduced generally to essential decrease of concentration of the adsorbed oxygen and an enrichment of fusion by the centers of crystallization of graphite dendrites.

Synthesis of multicolor photoluminescent carbon quantum dots functionalized with hydrocarbons of different chain lengths

We report the synthesis of novel multicolor photoluminescent carbon quantum dots (CQDs) from multi-wall carbon nanotubes, and their covalent functionalization with amines by peptide bonds. The resulting CQDs consisted of quasi-spherical graphite nanocrystals around 10 nm diameter, which were capped by amines with different hydrocarbon chains such as propylamine, octylamine, dodecylamine and octadecylamine. The introduction of nitrogen atoms and the hydrocarbon chains in the surface of the CQDs dramatically affected their photoluminescence profiles, quantum yields and solubility. The photoluminescence emission wavelength of these novel organic-soluble CQDs depended on the excitation wavelength and their quantum yields varied with the chain length of the hydrocarbon chain attached to the surface of the carbon dots.

NANOSTRUCTURAL PROCESSES OF MELTING AND CASTING OF CAST IRON WITH FLAKY GRAPHITE

It is shown that melting and casting of cast iron with flaky graphite are the complex physical and chemical nanostructural processes. The majorrole in these processis played by the centers of crystallization of dendrites of graphite, graphite nanocrystals, the dissolved and adsorbed oxygen. The modifying ligature of FS75 is used to decrease concentration of the adsorbed oxygen and to fusion enrichment by the centers of crystallization of dendrites of graphite.

Influence of a magnetic field on the formation of carbon dust particles in very low-pressure high-density plasmas

Carbon dust particles with radius of a few hundreds of nanometers are formed in a steady-state plasma produced by an electromagnetic surface wave at 200 MHz in very low-pressure conditions. The influence of an applied magnetic field is evidenced by ex situ methods (scanning and transmission electron microscopy, Raman spectroscopy). The observed spherical dust particles are formed of aggregates of graphite nanocrystals embedded in amorphous carbon shells. In addition, the size of the dust particles increases with the magnetic field intensity. Growth occurs as the particles are confined near the sheath edge and their confinement is improved by applying a magnetic field as the corresponding electric field increases in this region. As a consequence, the average dust particle radius is directly related to the magnetic field intensity.

Desmin detection by facile prepared carbon quantum dots for early screening of colorectal cancer

Th aim of this study was to develop a new facile chemical method for early screening of colorectal cancer.The -C(O)OH groups modified Carbon Quantum Dots (CQDs) were prepared by an facile innovative route of acid attacking on carbon nanotubes (CNTs). The -C(O)OH groups were further transported into -C(O)Cl groups by SOCl2 treating. The obtained ClCQDs were conjugated onto the anti-Desmin, which were applied for testing the Desmin concentration in serum by using linearly fitted relationship with photoluminescence (PL) intensity.The obtained carbon quantum dots are quasispherical graphite nanocrystals with photoluminescence at about 455 nm. The Desmin with concentration of 1 ng/mL can lead to a decrease of PL intensity for anti-Desmin conjugated CQDs with good linearity. This assay had good specificity for Desmin with in interferential substances of immunoglobulin G (IgG), alpha fetoprotein (AFP), and carcinoembryoic antigen (CEA).A new facile acid attack method was developed to prepare ClCQDs, which could conjugate onto the anti-Desmin for detection of Desmin in serum with high sensitivity and specificity. As the detection limit is lower than 1 ng/ mL, this work provides a promising strategy for the evaluation of colorectal cancer risk with low cost and excellent sensing performance.

Preliminary investigation of electrical conductivity of monolithic biochar

Monolithic biochar is explored as electrode material in supercapacitor – a fast-charging, long-lasting energy storage device. Electrical conductivity of electrode is critical to supercapacitor's performance. Traditionally, biochar is used as a solid fuel for combustion where electrical conductivity is largely irrelevant and ignored. This work explores electrical conductivity of monolithic biochar and elucidates its dependence on micro and macro structures of biochar. Electrical conductivity of biochar was found to be highly dependent on its degree of carbonization. Bulk conductivity of biochar can increase by over six orders of magnitudes when its carbon content changes from 86.8 to 93.7 wt%. Transmission electronic microscope and x-ray diffraction analyses revealed the presence of graphite nanocrystals (∼3 nm) and the growth of biochar crystallinity after heat treatment at 950 °C. The highest skeletal conductivity of carbon was 343.2 S/m, found in a heat-treated sugar maple biochar with 96.2 wt% of carbon. It is higher than that of graphite single crystal in direction perpendicular to graphene plane (333.3 S/m). Moreover, it was observed conductivity of monolithic biochar increased with compressive loading and dropped to the pre-compression level when the loading was released - a new phenomenon termed “elastic behavior of electrical conductivity of biochar”.

Graphene-polymer coating for the realization of strain sensors.

In this work we present a novel route to produce a graphene-based film on a polymer substrate. A transparent graphite colloidal suspension was applied to a slat of poly(methyl methacrylate) (PMMA). The good adhesion to the PMMA surface, combined with the shear stress, allows a uniform and continuous spreading of the graphite nanocrystals, resulting in a very uniform graphene multilayer coating on the substrate surface. The fabrication process is simple and yields thin coatings characterized by high optical transparency and large electrical piezoresitivity. Such properties envisage potential applications of this polymer-supported coating for use in strain sensing. The electrical and mechanical properties of these PMMA/graphene coatings were characterized by bending tests. The electrical transport was investigated as a function of the applied stress. The structural and strain properties of the polymer composite material were studied under stress by infrared thermography and micro-Raman spectroscopy.

A favorable chromium coating electrodeposited from Cr(III) electrolyte reveals anti-wear performance similar to conventional hard chromium

Decades of application has proved that electrodeposited chromium (Cr) coating is a perfect coating material. Unfortunately, almost all of these excellent coatings for anti-wear, i.e., hard Cr coatings, are produced by traditional electrodepositing process based on highly toxic Cr(VI). In this research, we reported a promising Cr coating embedded with graphite nanocrystals, which was electrodeposited from a Cr(III) electrolyte and reveals favorable anti-wear performance similar to that of conventional hard Cr coating in a range of sliding velocities. In addition, the developing direction of researches on electrodepositing thick Cr coating from Cr(III) electrolyte was elucidated via comparative investigation of the friction and wear behaviors of three typical Cr coatings.

Annealing-Based Electrical Tuning of Cobalt-Carbon Deposits Grown by Focused-Electron-Beam-Induced Deposition.

An effective postgrowth electrical tuning, via an oxygen releasing method, to enhance the content of non-noble metals in deposits directly written with gas-assisted focused-electron-beam-induced deposition (FEBID) is presented. It represents a novel and reproducible method for improving the electrical transport properties of Co-C deposits. The metal content and electrical properties of Co-C-O nanodeposits obtained by electron-induced dissociation of volatile Co2(CO)8 precursor adsorbate molecules were reproducibly tuned by applying postgrowth annealing processes at 100 °C, 200 °C, and 300 °C under high-vacuum for 10 min. Advanced thin film EDX analysis showed that during the annealing process predominantly oxygen is released from the Co-C-O deposits, yielding an atomic ratio of Co:C:O = 100:16:1 (85:14:1) with respect to the atomic composition of as-written Co:C:O = 100:21:28 (67:14:19). In-depth Raman analysis suggests that the amorphous carbon contained in the as-written deposit turns into graphite nanocrystals with size of about 22.4 nm with annealing temperature. Remarkably, these microstructural changes allow for tuning of the electrical resistivity of the deposits over 3 orders of magnitude from 26 mΩ cm down to 26 μΩ cm, achieving a residual resistivity of ρ2K/ρ300K = 0.56, close to the value of 0.53 for pure Co films with similar dimensions, making it especially interesting and advantageous over the numerous works already published for applications such as advanced scanning-probe systems, magnetic memory, storage, and ferroelectric tunnel junction memristors, as the graphitic matrix protects the cobalt from being oxidized under an ambient atmosphere.

Characterization of piezoresistive properties of graphene-supported polymer coating for strain sensor applications

An optically transparent nanographite coating has been developed on a poly-methyl methacrylate (PMMA) substrates. The adhesion to the PMMA surface combined with the shear stress allowed an uniform and continuous spreading of the graphite nanocrystals on the substrate surface with formation of a very uniform graphene multilayer coating. This coating is characterized by high piezoresitivity and it is suitable to work as sensible and reliable local strain sensor. Its piezoresistive features have been characterized by bending tests yielding a gauge factor (GF) which is of the order of 50. The structural and strain properties of the compound were studied under stress by infra-red thermography (IRT) and micro-Raman spectroscopy. The strain strength has been estimated as a function of the bending load. The electrical transport was investigated as a function of the applied stress. Features are consistent with an intergrain electrical transport mechanism among graphene platelets.

Graphitic carbon nitride nanocrystals decorated AgVO3 nanowires with enhanced visible-light photocatalytic activity

In the present study, a novel graphitic carbon nitride (g-C3N4) nanocrystals/AgVO3 photocatalyst was successfully synthesized by a simple two-step method. The as-prepared g-C3N4/AgVO3 heterostructures included g-C3N4 nanocrystals assembling uniformly on the surface of AgVO3 nanowires. The successful modification of AgVO3 nanowires by g-C3N4 nanocrystals was found to improve the photocatalyst activity significantly because of enhanced light absorption and improved separation of photo-generated electron–hole pairs.

Pomegranate-like N,P-Doped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution.

Well-defined pomegranate-like N,P-doped Mo2C@C nanospheres were prepared by simply using phosphomolybdic acid (PMo12) to initiate the polymerization of polypyrrole (PPy) and as a single source for Mo and P to produce N,P-doped Mo2C nanocrystals. The existence of PMo12 at the molecular scale in the polymer network allows the formation of pomegranate-like Mo2C@C nanospheres with a porous carbon shell as peel and Mo2C nanocrystals well-dispersed in the N-doped carbon matrix as seeds. This nanostructure provides several favorable features for hydrogen evolution application: (1) the conductive carbon shell and matrix effectively prevent the aggregation of Mo2C nanocrystals and facilitate electron transportation; (2) the uniform N,P-doping in the carbon shell/matrix and plenty of Mo2C nanocrystals provide abundant catalytically highly active sites; and (3) nanoporous structure allows the effective exposure of active sites and mass transfer. Moreover, the uniform distribution of P and Mo from the single source of PMo12 and N from PPy in the polymeric PPy-PMo12 precursor guarantees the uniform N- and P-co-doping in both the graphitic carbon matrix and Mo2C nanocrystals, which contributes to the enhancement of electrocatalytic performance. As a result, the pomegranate-like Mo2C@C nanospheres exhibit extraordinary electrocatalytic activity for the hydrogen evolution reaction (HER) in terms of an extremely low overpotential of 47 mV at 10 mA cm(-2) in 1 M KOH, which is one of the best Mo-based HER catalysts. The strategy for preparing such nanostructures may open up opportunities for exploring low-cost high-performance electrocatalysts for various applications.

Ultra-thin carbon-based nanocomposite coatings for superior wear resistance under lubrication with nano-diamond additives

Ultra-thin carbon-based nanocomposite coatings comprised of graphite nanocrystals embedded in diamond-like carbon (DLC) matrix were deposited on stainless steel substrates by the C60ion beam method, as a wear protective coating.

Structural transformation of implanted diamond layers during high temperature annealing

In the recent years graphitization of ion-beam induced amorphous layers became the basic tool for device fabrication in diamond. The etchable graphitic layers can be removed to form free-standing membranes into which the desired structures can be sculpted using FIB milling. The optical properties of the devices fabricated using this method are assumed on the model of sharp diamond–air interface. The real quality of this interface could depend on degree of graphitization of the amorphous damage layers after annealing. In the present work the graphitization process was studied using conventional and analytical TEM. It was found that annealing at 550°C results in a partial graphitization of the implanted volume with formation of the nano-crystalline graphitic phase sandwiched between layers of tetrahedral amorphous carbon. Annealing at 1400°C resulted in complete graphitization of the amorphous layers. The average size of graphite nano-crystals did not exceed 5nm with predominant orientation of c-planes normal to the sample surface.

Low-cost formation of bulk and localized polymer-derived carbon nanodomains from polydimethylsiloxane.

We present two simple alternative methods to form polymer-derived carbon nanodomains in a controlled fashion and at low cost, using custom-made chemical vapour deposition and selective laser ablation with a commercial CD-DVD platform. Both processes presented shiny and dark residual materials after the polymer combustion and according to micro-Raman spectroscopy of the domains, graphitic nanocrystals and carbon nanotubes have successfully been produced by the combustion of polydimethylsiloxane layers. The fabrication processes and characterization of the byproduct materials are reported. We demonstrate that CVD led to bulk production of graphitic nanocrystals and single-walled carbon nanotubes while direct laser ablation may be employed for the formation of localized fluorescent nanodots. In the latter case, graphitic nanodomains and multi-wall carbon nanotubes are left inside microchannels and preliminary results seem to indicate that laser ablation could offer a tuning control of the nature and optical properties of the nanodomains that are left inside micropatterns with on-demand geometries. These low-cost methods look particularly promising for the formation of carbon nanoresidues with controlled properties and in applications where high integration is desired.

Raman and thermal desorption spectroscopy analyses of amorphous graphite-like carbon films with incorporated xenon

Structural and thermodynamic properties of xenon incorporated in amorphous carbon films deposited by means of a sputtering system free of vacuum pumping during the deposition were investigated by visible Raman (Vis–Raman) scattering and thermal desorption spectroscopy (TDS), respectively. Vis–Raman measurements, carried out before and after the xenon desorption, revealed a polycrystalline material rich in C-sp2 sites that form randomly dispersed nanosized graphite clusters (nanocrystals) of approximately 1 nm. After xenon desorption, a compensating mechanism, activated by the thermal heating, promotes a more ordered C-sp2 network as revealed by the linewidths of both D and G bands, as well as by the ID/IG ratio evolution. The TDS thermograms show that the xenon onset effusing temperature is approximately 120 °C. Besides, they also revealed two different regimes, at low and at high temperatures, associated with desorption of xenon atoms trapped either in an interconnecting void network or within the graphite nanocrystals. From the latter regime, the xenon diffusion free energy (activation energy) was determined to be 1.2 eV (115.7 kJ/mol) on the basis of the diffusion-limited desorption standard model.

Graphene /TiO2 Composite Electrode: Synthesis and Application towards the Oxygen Reduction Reaction

A graphene oxide (GO) was prepared from a commercial graphite nano-crystals using Hummers’ method. Later, GO powder was reduced either by placing it under a H2 gas flow in a controlled gas reactor at 450 oC or using a hydrazine hydrate solution (HH) in a 1000-Watt microwave oven. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction patterns (XRD) confirmed that both of the oxidation and reduction processes of graphite and GO powders, respectively, were incomplete. Also, the surface area of the H2 gas - reduced GO powder was found to be higher than the case where HH was used to reduce the same powder. Fourier transform infrared spectroscopy (FT-IR) and XPS have revealed that GO surface consists mainly of hydroxyl, epoxy, carbonyl and carboxylic groups. The electrocatalytic properties of (i) glassy carbon (GC), (ii) commercial TiO2 (P25-TiO2/GC), (iii) TiO2-supported GO (GO/TiO2/GC), (iv) TiO2 – supported HH – reduced GO (HHRGO/TiO2/GC) and (v) TiO2 – supported H2 gas – reduced GO (H2RGO/TiO2/GC) electrodes towards the oxygen reduction reaction (ORR) in acidic solution in presence and absence of UV radiation were examined. The results have shown that the H2RGO/TiO2/GC electrode has the best electrocatalytic activity in terms of current at a certain potential but glassy carbon electrode (GC) was found to be the best in terms of the onset potential of the ORR.