Graphene Balls Research Articles

Graphene Balls Reduce Friction

A suspension of crumpled graphene balls in oil can reduce friction and wear between metal surfaces sliding over one another better than commercial lubricants, according to a study (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1520994113). The investigation, which was conducted by Northwestern University materials scientists Yip-Wah Chung and Jiaxing Huang and coworkers, was inspired by work carried out by Huang’s group in 2011 showing that crumpled graphene balls resist agglomeration, much like wadded paper balls. Many lubricants are formulated with microscopic particles of graphite and various inorganic compounds to increase slipperiness and reduce wear, for example, between moving engine parts. But repeated frictional forces can cause these particles to agglomerate, reducing a lubricant’s effectiveness. Having discovered that crumpled graphene balls don’t stick together, the Northwestern team tried using them as lubricant additives. Results of tribology tests, in which a steel pin was dragged acro...

Self-dispersed crumpled graphene balls in oil for friction and wear reduction

Ultrafine particles are often used as lubricant additives because they are capable of entering tribological contacts to reduce friction and protect surfaces from wear. They tend to be more stable than molecular additives under high thermal and mechanical stresses during rubbing. It is highly desirable for these particles to remain well dispersed in oil without relying on molecular ligands. Borrowing from the analogy that pieces of paper that are crumpled do not readily stick to each other (unlike flat sheets), we expect that ultrafine particles resembling miniaturized crumpled paper balls should self-disperse in oil and could act like nanoscale ball bearings to reduce friction and wear. Here we report the use of crumpled graphene balls as a high-performance additive that can significantly improve the lubrication properties of polyalphaolefin base oil. The tribological performance of crumpled graphene balls is only weakly dependent on their concentration in oil and readily exceeds that of other carbon additives such as graphite, reduced graphene oxide, and carbon black. Notably, polyalphaolefin base oil with only 0.01-0.1 wt % of crumpled graphene balls outperforms a fully formulated commercial lubricant in terms of friction and wear reduction.

Sea‐Urchin‐Inspired 3D Crumpled Graphene Balls Using Simultaneous Etching and Reduction Process for High‐Density Capacitive Energy Storage

A crumpled configuration of graphene is desirable for preventing irreversible stacking between individual nanosheets, which can be a major hurdle toward its widespread application. Herein a sea‐urchin‐shaped template approach is introduced for fabricating highly crumpled graphene balls in bulk quantities with a simple process. Simultaneous chemical etching and reduction process of graphene oxide (GO)‐encapsulated iron oxide particles results in dissolution of the core template with spiky morphology and conversion of the outer GO layers into reduced GO layers with increased hydrophobicity which remain in contact with the spiky surface of the template. After completely etching, the outer graphene layers are fully compressed into the crumpled form along with decrease in total volume by etching. The crumpled balls exhibit significantly larger surface area and good water‐dispersion stability than those of stacked reduced GO or other crumple approaches, even though they also show comparable electrical conductivity. Furthermore, they are easily assembled into 3D macroporous networks without any binders through typical processes such as solvent casting or compression molding. The graphene networks with less pore volume still have the crumpled morphology without sacrificing the properties regardless of the assembly processes, producing a promising active electrode material with high gravimetric and volumetric energy density for capacitive energy storage.

Fe3O4-decorated hollow graphene balls prepared by spray pyrolysis process for ultrafast and long cycle-life lithium ion batteries

Fe3O4-decorated graphene balls were prepared by a spray pyrolysis process. Analysis by Raman spectroscopy and X-ray photoelectron spectroscopy indicated that the spray pyrolysis at 800°C resulted in the complete reduction of graphene oxide sheets containing oxygen functional groups into graphene sheets, leading to the formation of Fe3O4-decorated graphene balls. The graphene content in the composite ball was 27 wt%. The Brunauer–Emmett–Teller surface area of the Fe3O4-decorated graphene balls was as high as 130m2g−1. The initial discharge and charge capacities of the Fe3O4-decorated graphene balls at a high current density of 7Ag−1 were 1210 and 843mAhg−1, respectively, and the discharge capacity was as high as 690mAhg−1 even after 1000 cycles. The stable reversible discharge capacities of the Fe3O4-decorated graphene balls decreased from 1040 to 540mAhg−1 with the increase in current density from 1 to 30Ag−1. The Fe3O4-decorated graphene balls with a uniform distribution of ultrafine Fe3O4 nanocrystals below 15nm showed superior electrochemical properties as anode materials for lithium ion batteries. The overall structure of the Fe3O4-decorated graphene balls was maintained even after long-term cycling.

Hierarchical nanohybrids with porous CNT-networks decorated crumpled graphene balls for supercapacitors.

One of the most challenging issues in developing supercapacitor technology is the rational design and synthesis of active electrode materials, at the nanoscale, with favorable morphologies, reasonable porous structure, and excellent conductivity. By transforming a two-dimensional (2D) graphene sheet into a crumpled ball shape, a novel three-dimensional (3D) graphene structure with a large surface area and aggregation-resistant properties has been proposed as an active material in supercapacitors to address the issues associated with the restacking of 2D graphene sheets. To further improve the mass transport/electron transfer and address the issue of limited contact spots between the crumpled graphene balls (CGBs) or between the CGBs and the current collector, we report here a unique hierarchical nanohybrid with porous carbon nanotube (CNT)-networks decorated CGBs (p-CNTn/CGBs), which not only greatly improves the affinity for bridging the active material and the current collector but also maintains favorable features for supercapacitor applications, such as a large surface area, 3D hierarchical nanostructure, excellent electrical conductivity, and outstanding aggregation-resistance. The performance established on the p-CNTn/CGBs far exceeded the bare CGB and reduced graphene oxide (RGO) counterparts in terms of specific capacitance and rate capabilities.

Uniform Decoration of Vanadium Oxide Nanocrystals on Reduced Graphene‐Oxide Balls by an Aerosol Process for Lithium‐Ion Battery Cathode Material

VO2-decorated reduced graphene balls were prepared by a one-pot spray-pyrolysis process from a colloidal spray solution of well-dispersed graphene oxide and ammonium vanadate. The graphene-VO2 composite powders prepared directly by spray pyrolysis had poor electrochemical properties. Therefore, the graphene-VO2 composite powders were transformed into a reduced graphene ball (RGB)-V2O5 (RGB) composite by post-treatment at 300 °C in an air atmosphere. The TEM and dot-mapping images showed a uniform distribution of V and C components, originating from V2O5 and graphene, consisting the composite. The graphene content of the RGB-V2O5 composite, measured by thermogravimetric analysis, was approximately 5 wt %. The initial discharge and charge capacities of RGB-V2O5 composite were 282 and 280 mA h g(-1), respectively, and the corresponding Coulombic efficiency was approximately 100 %. On the other hand, the initial discharge and charge capacities of macroporous V2O5 powders were 205 and 221 mA h g(-1), respectively, and the corresponding Coulombic efficiency was approximately 93 %. The RGB-V2O5 composite showed a better rate performance than the macroporous V2O5 powders.

Aerosol Processing of Graphene and Its Application to Oil Absorbent and Glucose Biosensor

Graphene (GR) is a flat monolayer of sp 2 -bonded single carbon atoms densely packed into a honeycomb crystal lattice. Due to its unique characteristics, GR is expected to contribute to enhanced nano-electronic, bio-electronic devices, etc. in the near future. However, the single-layered GR sheets have a tendency to form irreversible aggregates or even to restack easily due to strong attraction between sheets. Preventing of aggregation and restacking of GR is achieved by the dimensional transition from flat sheets to fractal-dimensional, nearly spherical crumpled balls using aerosol spray pyrolysis. This review first introduces the fabrication of crumpled GR and GR-composite by aerosol spray pyrolysis, and then discusses the material properties of GR: strain-hardening, compression and aggregation-resistant behavior. Finally, we introduce effective applications of hollow crumpled GR balls for oil absorbent and crumpled GR-composites for glucose biosensor, respectively.

Direct synthesis of 3D hollow porous graphene balls from coal tar pitch for high performance supercapacitors

A simple and effective method for direct synthesis of hollow porous graphene balls (HPGBs) for high performance supercapacitors.

High quality reduced graphene oxide through repairing with multi-layered graphene ball nanostructures

We present a simple and up-scalable method to produce highly repaired graphene oxide with a large surface area, by introducing spherical multi-layered graphene balls with empty interiors. These graphene balls are prepared via chemical vapor deposition (CVD) of Ni particles on the surface of the graphene oxides (GO). Transmission electron microscopy and Raman spectroscopy results reveal that defects in the GO surfaces are well repaired during the CVD process, with the help of nickel nanoparticles attached to the functional groups of the GO surface, further resulting in a high electrical conductivity of 18,620 S/m. In addition, the graphene balls on the GO surface effectively prevent restacking of the GO layers, thus providing a large surface area of 527 m2/g. Two electrode supercapacitor cells using this highly conductive graphene material demonstrate ideal electrical double layer capacitive behavior, due to the effective use of the outstanding electric conductivity and the large surface area.

Synthesis of multilayer graphene balls on mesoporous Co-MCM-41 molecular sieves by chemical vapour deposition method

The virtuous structure of mesoporous Co-MCM-41 molecular sieve was synthesized hydrothermally, and was characterized by various physico-chemical techniques such as ICP-AES, XRD, N2 physisorption isotherms, FT-IR, TGA, SEM, and TEM. They were used as catalytic templates for the growth of graphene balls (GBs) by CVD method at different temperatures (850, 900, and 950°C) using acetylene as a carbon precursor. The reaction temperature was optimized for the better formation of GBs. The synthesized novel carbon material was purified, and then systematically studied by XRD, TGA, SEM, HR-TEM and Raman spectroscopy techniques. The graphitic nature of material was confirmed by XRD analysis and Raman spectroscopy techniques. Thermal stability of GBs was above 600°C under static condition and confirmed by TGA/DTG studies. The analytical results show that synthesized carbon materials had spherical structures with high degree of graphitization. The SEM and TEM image shows that the carbon materials exists in spherical morphology, and the diameter of these GBs are quite uniform and in a narrow range of 45–55nm.

Effect of Sheet Morphology on the Scalability of Graphene-Based Ultracapacitors

Graphene is considered a promising ultracapacitor material toward high power and energy density because of its high conductivity and high surface area without pore tortuosity. However, the two-dimensional (2D) sheets tend to aggregate during the electrode fabrication process and align perpendicular to the flow direction of electrons and ions, which can reduce the available surface area and limit the electron and ion transport. This makes it hard to achieve scalable device performance as the loading level of the active material increases. Here, we report a strategy to solve these problems by transforming the 2D graphene sheet into a crumpled paper ball structure. Compared to flat or wrinkled sheets, the crumpled graphene balls can deliver much higher specific capacitance and better rate performance. More importantly, devices made with crumpled graphene balls are significantly less dependent on the electrode mass loading. Performance of graphene-based ultracapacitors can be further enhanced by using flat graphene sheets as the binder for the crumpled graphene balls, thus eliminating the need for less active binder materials.

Synthesis of Multilayer Graphene Balls by Carbon Segregation from Nickel Nanoparticles

Three-dimensional (3D) structured graphene is a material of great interest due to its diverse applications in electronics, catalytic electrodes, and sensors. However, the preparation of 3D structured graphene is still challenging. Here, we report the fabrication of multilayer graphene balls (GBs) by template-directed carbon segregation using nickel nanoparticles (Ni-NPs) as template materials. To maintain the ball shape of the template Ni-NPs, we used a carburization process using polyol solution as the carbon source and a thermal annealing process to synthesize graphene layers via carbon segregation on the outer surface of the Ni-NPs. The resulting GBs were hollow structures composed of multilayer graphene after the removal of core Ni-NPs, and the thickness of the graphene layers and the size of GBs were tunable by controlling the graphene synthesis conditions. X-ray diffraction analysis and in situ transmission electron microscope characterization revealed that carbon atoms diffused effectively into the Ni-NPs during the carburization step, and that the diffused carbon atoms in Ni-NPs segregated and successfully formed a graphene layer on the surface of the Ni-NPs during thermal annealing. We also performed further heat treatment at high temperature to improve the quality of the graphene layer, resulting in highly crystalline GBs. The unique hollow GBs synthesized here will be useful as excellent high-rate electrode materials for electrochemical lithium storage devices.

Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets

Unlike flat sheets, crumpled paper balls have both high free volume and high compressive strength, and can tightly pack without significantly reducing the area of accessible surface. Such properties would be highly desirable for sheet-like materials such as graphene, since they tend to aggregate in solution and restack in the solid state, making their properties highly dependent on the material processing history. Here we report the synthesis of crumpled graphene balls by capillary compression in rapidly evaporating aerosol droplets. The crumpled particles are stabilized by locally folded, π-π stacked ridges as a result of plastic deformation, and do not unfold or collapse during common processing steps. In addition, they are remarkably aggregation-resistant in either solution or solid state, and remain largely intact and redispersible after chemical treatments, wet processing, annealing, and even pelletizing at high pressure. For example, upon compression at 55 MPa, the regular flat graphene sheets turn into nondispersible chunks with drastically reduced surface area by 84%, while the crumpled graphene particles can still maintain 45% of their original surface area and remain readily dispersible in common solvents. Therefore, crumpled particles could help to standardize graphene-based materials by delivering more stable properties such as high surface area and solution processability regardless of material processing history. This should greatly benefit applications using bulk quantities of graphene, such as in energy storage or conversion devices. As a proof of concept, we demonstrate that microbial fuel electrodes modified by the crumpled particles indeed outperform those modified with their flat counterparts.