Gabriel Ceriotti

Coal as an abundant source of graphene quantum dots

Coal is the most abundant and readily combustible energy resource being used worldwide. However, its structural characteristic creates a perception that coal is only useful for producing energy via burning. Here we report a facile approach to synthesize tunable graphene quantum dots from various types of coal, and establish that the unique coal structure has an advantage over pure sp2-carbon allotropes for producing quantum dots. The crystalline carbon within the coal structure is easier to oxidatively displace than when pure sp2-carbon structures are used, resulting in nanometre-sized graphene quantum dots with amorphous carbon addends on the edges. The synthesized graphene quantum dots, produced in up to 20% isolated yield from coal, are soluble and fluorescent in aqueous solution, providing promise for applications in areas such as bioimaging, biomedicine, photovoltaics and optoelectronics, in addition to being inexpensive additives for structural composites.

Graphene Oxide as a High-Performance Fluid-Loss-Control Additive in Water-Based Drilling Fluids

Graphene oxide (GO) performs well as a filtration additive in water-based drilling fluids at concentrations as low as 0.2 % (w/w) by carbon content. Standard American Petroleum Institute (API) filtration tests were conducted on pH-adjusted, aqueous dispersions of GO and xanthan gum. It was found that a combination of large-flake GO and powdered GO in a 3:1 ratio performed best in the API tests, allowing an average fluid loss of 6.1 mL over 30 min and leaving a filter cake ~20 μm thick. In comparison, a standard suspension (~12 g/L) of clays and polymers used in the oil industry gave an average fluid loss of 7.2 mL and a filter cake ~280 μm thick. Scanning electron microscopy imaging revealed the extreme pliability of well-exfoliated GO, as the pressure due to filtration crumpled single GO sheets, forcing them to slide through pores with diameters much smaller than the flakes flattened size. GO solutions also exhibited greater shear thinning and higher temperature stability compared to clay-based fluid-loss additives, demonstrating potential for high-temperature well applications.

Direct Real-Time Monitoring of Stage Transitions in Graphite Intercalation Compounds

Graphite intercalation compounds (GIC) possess a broad range of unique properties that are not specific to the parent materials. While the stage transition, changing the number of graphene layers sandwiched between the two layers of intercalant, is fundamentally important and has been theoretically addressed, experimental studies revealed only macroscopic parameters. On the microscale, the phenomenon remains elusive up to the present day. Here we monitor directly in real time the stage transitions using a combination of optical microscopy and Raman spectroscopy. These direct observations yield several mechanistic conclusions. While we obtained strong experimental evidence in support of the Daumas-Herold theory, we find that the conventional interpretation of stage transitions as sliding of the existing intercalant domains does not sufficiently capture the actual phenomena. The entire GIC structure transforms considerably during the stage transition. Among other observations, massive wavefront-like perturbations occur on the graphite surface, which we term the tidal wave effect.

Chemical Mass Production of Graphene Nanoplatelets in ∼100% Yield

Successful application of graphene is hampered by the lack of cost-effective methods for its production. Here, we demonstrate a method of mass production of graphene nanoplatelets (GNPs) by exfoliation of flake graphite in the tricomponent system made by a combination of ammonium persulfate ((NH4)2S2O8), concentrated sulfuric acid, and fuming sulfuric acid. The resulting GNPs are tens of microns in diameter and 10-35 nm in thickness. When in the liquid phase of the tricomponent media, graphite completely loses its interlayer registry. This provides a ∼100% yield of GNPs from graphite in 3-4 h at room temperature or in 10 min at 120 °C.

Nonlinear Photoluminescence Imaging of Isotropic and Liquid Crystalline Dispersions of Graphene Oxide

We report a visible-range nonlinear photoluminescence (PL) from graphene oxide (GO) flakes excited by near-infrared femtosecond laser light. PL intensity has nonlinear dependence on the laser power, implying a multiphoton excitation process, and also strongly depends on a linear polarization orientation of excitation light, being at maximum when it is parallel to flakes. We show that PL can be used for a fully three-dimensional label-free imaging of isotropic, nematic, and lamellar liquid crystalline dispersions of GO flakes in water. This nonlinear PL is of interest for applications in direct label-free imaging of composite materials and study of orientational ordering in mesomorphic phases formed by these flakes, as well as in biomedical and sensing applications utilizing GO.

Rapid method for the purification of graphene oxide

A rapid and facile purification method for graphene oxide (GO) is important for its production above the gram scale. Such a method would allow for the development of GOs large-scale industrial applications. Out of several protocols in this study, including centrifugation, filtration, precipitation and decantation, filtration using a gas-press proved to be the most effective. Gas-press filtration using filter beds of Celite, perlite, glass wool, ceramic tape, or woven glass fibre allowed for adequate purification of 1 g of crude large-flake (∼30 μm flake diameter) GO in less than 60 min using a lab-scale set-up. The present technique could be easily scaled-up, it generates minimal waste, and can be tuned by changing the dimensions of the equipment, pressure, and filter bed. This would allow a user to obtain a higher work-up efficiency. The quickly purified product is called efficiently purified GO or EGO.

Corrigendum: Coal as an abundant source of graphene quantum dots

Nature Communications 4: Article number: 2943 (2013); Published: 6 December 2013; Updated: 23 April 2015. In this Article, the bituminous coal graphene quantum dots (b-GQD) are described throughout this paper as having a crystalline hexagonal structure. Following further careful study of the high-resolution transmission electron microscopy data, this claim is too rigorous, and the b-GQDs instead should be referred to as crystalline.