Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/182—Graphene
  • C01B32/198—Graphene oxide
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/20—Graphite
  • C01B32/21—After-treatment
  • C01B32/23—Oxidation
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/20—Graphite
  • C01B32/21—After-treatment
  • C01B32/22—Intercalation
  • C01B32/225—Expansion; Exfoliation

Definitions

• This invention relates to the chemical synthesis of graphene oxide. Specifically, as compared to prior art methods, the invention disclosed herein provides a simple, cost-effective method of providing relatively large and high quality graphene oxide materials while preventing the creation of toxic gasses and avoiding the use of H 3 PO 4 .
• graphene consists of a single layer of graphite (i.e. , sp 2 hybridized carbon atoms).
• Graphene is approximately two hundred times stronger than steel, nearly one million times thinner than a human hair, and more conductive than copper.
• graphene, and in particular, high quality graphene is desirable for use in various industries. For example, obtaining high quality graphene is of significant importance for electronic and photonic based applications.
• chemical vapor deposition method is the preferred route of manufacturing high quality graphene for these applications. Chemical vapor deposition, however, is expensive and cannot currently produce the quantities of graphene demanded for large-scale industrial applications at a reasonable cost.
• graphite e.g. , bulk graphite
• graphite e.g. , bulk graphite
• One source of excellent, high quality pure bulk graphite is Sri Lankan vein graphite.
• Sri Lanka has a longstanding reputation for its high quality crystalline vein graphite with purity levels ranging from 80-99% carbon.
• Sri Lankan vein graphite is mined as lumps and is considered to have a high degree of crystalline perfection, excellent electrical and thermal conductivities, and superior cohesive energy as compared to other natural graphite materials.
• Hummers (1958) further improved upon this method (see Hummers et al, 1958, herein "Hummer”).
• Hummers's method which is commonly used today, graphite is oxidized by treatment with KMn04 and NaN03 in concentrated H2SO4.
• the vicinal diol may be oxidized to diketone, which leads to the formation of holes in the graphene basal plane.
• Such chemical defects in the resulting chemically converted graphene diminish the highly sought after electrical and mechanical properties as compared with pristine, high quality graphene.
• each of these prior art methods involves the generation of one or more toxic gases, such as
• H 3 PO 4 is undesirable due to its cost and the increased complexity of the reaction method.
• KMnC is one of the strongest oxidants, especially in acidic media.
• Complete intercalation of graphite with concentrated H 2 SO 4 can be achieved with the assistance of KMnCu by forming graphite bisulfate ⁇ see Sorokina et al, 2005). Accordingly, the formation of graphite bisulfate gives reaction stability, so the role of NaNC and/or H 3 PO 4 is unnecessary for the synthesis of graphene oxide (herein "GO") using Hummers method.
• GO graphene oxide
• the method disclosed herein is scalable, cheaper, and safer than prior art methods.
• the chemically exfoliated graphene oxide created by the method disclosed herein has high solubility in both aqueous and polar organic solvents and can be casted into thin membranes as well as exfoliated into single to few layer graphene oxide structures with relatively large lateral dimensions as compared to structures created by prior art methods.
• this application discloses a modified chemical oxidation method that synthesizes graphene oxide from graphite using only of H2SO4, KMnC and quenching with H2O2 and/or H 2 O or ice.
• the method of the present invention uses no H 3 PO 4 , the central protecting reagent used in the method disclosed in Tour. It was surprisingly and unexpectedly discovered that, in the correct proportions, H2SO4, KMnC , H2O2, and/or H2O alone could be used as reagents without t PC o create high quality graphene oxide from graphite. Before Applicants' invention it was believed that the use of H 3 PO 4 was essential in the creation of high quality graphene oxide from graphite in a toxic-fume free method.
• graphite is placed into a vessel where H 2 SO 4 is added.
• KMnCu is added to this tbSCVgraphite mixture, while stirring.
• the stirring is then continued for several hours and the reaction is quenched with ice, H 2 O, and/or ice and H 2 O 2 .
• the supernatant is then discarded, leaving a graphene oxide slurry.
• the remains are then washed several times starting with deionized water followed by a 1 :2 watenHCl mixture to remove Mn 2+ ions and other impurities. Washing is then carried out one last time with ethanol and diethylether in order to obtain graphene oxide powder.
• the brown color solid material obtained after this step is then dried at room temperature under vacuum.
• the pilot scale process is also performed in this embodiment, in order to understand scalability of the reaction.
• the graphene oxide slurry can be exfoliated by adding a portion of the graphene oxide slurry dropwise to an aqueous solution and then ultra-sonicating the aqueous solution/graphene oxide slurry. The ultra-sonicated mixture can be transferred to an appropriate substrate if desired.
• the graphene oxide Once the graphene oxide has been dispersed in an aqueous solution, it yields monomolecular or substantially monomolecular sheets of graphene oxide. These sheets can then be reduced to obtain reduced graphene oxide, the graphene form.
• FIG. 1 is an X-ray Diffraction pattern obtained for a graphene oxide powder according to an example embodiment of the present invention
• FIG. 2 is a Thermogravimetric Analysis spectrum obtained for a graphene oxide according to an example embodiment of the present invention
• FIG. 3 is a Fourier Transform IR spectrum for a graphene oxide powder according to an example embodiment of the present invention
• FIG. 4 are Raman spectra for graphene oxide
• FIG. 5 is a Nuclear Magnetic Resonance (NMR) spectra for graphene oxide
• FIG. 6 is an Atomic Force Microscopy image of a graphene oxide flake on a mica substrate
• FIG. 7 shows TEM images for graphene oxide obtained on a lacey-carbon TEM grid and SAED pattern
• FIG. 8 is an UV/VIS spectrum for highly-oxidized graphene oxide
• GK Bogala Graphite
• a pilot scale process is also performed by using approximately 100 g of natural high purity (>99%) vein graphite with the same process.
• the reaction time is increased up to 20 hours.
• the graphene oxide slurries were then exfoliated by adding about 5 mg of the viscous graphene oxide slurries dropwise into about 200 ml of deionized water. These slurry/water mixtures were then placed into an ultra-sonication device (Grant, USA, 120 W, 150 Hz) for 20 minutes. The ultra-sonicated, graphene oxide slurry/water mixtures were then transferred dropwise onto a freshly cleaved mica sheet to obtain Atomic Force Microscopy image.
• an ultra-sonication device Grant, USA, 120 W, 150 Hz
• X-Ray Diffractometric data were measured on a D8-Bruker AXS Diffractometer equipped with MBraun PSD position sensitive detector and the X-axis was restricted within a range (of 2 ⁇ ) from 5° to 55°.
• Fig. 1 at (a) shows a representative XRD spectrum obtained from graphene oxide created according to an embodiment method of the present invention.
• Fig. 1 at (a) shows an interlayer spacing of 9.48 ⁇ 0.12 A.
• the XRD interlayer spacing is proportional to the degree of oxidation. This in turn is related to the facility to exfoliate the GO into monolayer sheets, which on reduction can lead to monolayer graphene.
• Fig. 1 at (b) illustrate XRD spectrum obtained from graphene oxide from pilot scale process, with an interlayer spacing of 9.59 ⁇ 0.12 A confirming an extremely high degree of oxidation. A value that is this high has never been reported in the literature to date.
• Thermogravimetric analysis was carried out on SDT Q600 analyzer equipped with a temperature compensated thermobalance under a high purity N 2 purged environment with a gas flow rate of 100 ml/min. The sample was heated from 35°C to 1000°C with a rate of 5°C/min.
• Fig. 2 shows a TGA spectrum obtained for a graphene oxide created according to an embodiment method of the present invention.
• the TGA spectrum of Fig. 2 shows a significant weight loss between 130°C to 220°C. This corresponds to the release of CO and CO 2 release from the most labile functional groups. The slower weight loss beyond that to 1000°C can be attributed to the removal of more stable oxygen functionalities.
• FTIR Fourier Transform Infrared Spectroscopy
• FTIR spectrum is shown in Fig. 3.
• the following functional groups were identified.
• the hydroxyl stretching band (3000-4000 cm “1 ),
• the peak at 1732 cm “1 was assigned as carbonyl
• C 0 double bonds stretching vibration, the sharp and strong absorption at 1624 cm “1 assigned as the stretching mode of intercalated water molecules.
• C C from unoxidized sp 2 CC bonds (1590- 1620 cm 1 ), C-0 vibrations and C-O-C (-epoxy-) vibration at 1200 cm _1 and below.
• the observed spectral peak positions are in very good agreement with published data on graphene using the method disclosed in Tour.
• Raman spectroscopy of samples, lab and pilot scale process was performed by a Renishaw In Via Raman Spectrometer using a 514.5nm wavelength laser. The data were collected with an objective of 50x, scanning the spectrometer from 100 cm 1 to 3500 cm 1 . Raman spectra of the two samples lab process and pilot scale process are shown in Fig. 4 at (a) and (b) respectively.
• Usually graphene oxide has two prominent peaks called D and G and lesser intense higher order peaks 2D and S3.
• the G peak corresponds to the E2G phonon at the Brillouin zone centre and is observed at 1580 cm 1 for graphite.
• the G peak of lab processed sample is wider and blue-shifted to 1587 cm 1 confirming the higher order oxidation which is similar to the method disclosed in Tour.
• the D peak which requires a defect for its activation, arises due to the breathing modes of sp 2 rings, is centered at 1352 cm 1 .
• the G peak position of pilot scale process sample remains at 1580 cm 1 and
• the D peak is centered at 1347 cm 1 due to extremely high oxidation which already observed as in XRD.
• the ratio I(D)/I(G) for these GO derived from other methods is normally around 1 or more, compared to 0.95 for the lab process and 0.94 for pilot scale process.
• the lower I(D)/I(g) ratio indicates that the relative number of defects in the sp 2 bonded graphene structure which arises in the current oxidation method is lower.
• the values of L a for samples is around 1.3 nm.
• Fig. 5 illustrates solid state direct 13 C pulse NMR spectra for highly-oxidized graphene oxide.
• the 13 C NMR spectra were obtained at 50.3 MHz, with 10 kHz magic angle spinning, a 90° 13 C pulse, 40 ms FID and 20 second relaxation delay.
• six peaks were observed at 62, 73, 87, 130, 159 and around 173 ppm are assigned to epoxides, alcohols, lactols, graphitic carbons, carboxylates, and ketones respectively.
• the NMR results also well exhibits the oxidation process and good agreement with the other methods reported.
• Fig. 6 shows an atomic force microscopy image (“AFM") that confirms the creation of relatively large (approximately 5 microns x 7.5 microns) sheet single to few layers of high quality graphene oxide.
• AFM atomic force microscopy image
• the lateral size of this sheet is much larger than the reported values obtained using Hummers's or Tour's methods.
• the AFM image confirms that the graphene oxide sheets created by the methods of the present invention are high quality and, similar to the graphene oxide created by Tour's method, do not contain substantial defects.
• Fig. 7 at (a) shows TEM image for mono/few layer highly-oxidized graphene oxide obtained on a lacey-carbon grid.
• the corresponding shows Selective Area Electron Diffraction (SAED) patterns for graphene oxide is shown in Fig. 7 at (b).
• SAED Selective Area Electron Diffraction
• Fig. 8 shows the UV-Vis absorption spectrum for graphene oxide, at 0.1 mg ml 1 concentration, value of the present disclosure is 231.6 nm, resulting from ⁇ — 71* transitions of the aryl rings. This implies the presence of the largest undamaged conjugated graphitic domains within the graphene layers. Additionally, a small shoulder peak at around 300 nm is due to the normalized absorbance of n— K * transitions implying an increase in the relative population of

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