AND DEVICES INCORPORATING THE SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of co-pending U.S. Provisional

Patent Application No. 61/281,915, filed on 24 November 2009, which is hereby incorporated herein.

BACKGROUND OF THE INVENTION

[0002] The disclosure relates generally to graphene dispersions, graphene membranes, methods of forming the same, and devices incorporating graphene membranes, and more particularly functionalized graphene.

[0003] There is an interest in the benefits of graphene materials and an interest in applications of graphene materials in the electronics, gas, and energy storage industries.

SUMMARY OF THE INVENTION

[0004] An aspect of the present invention relates to a graphene membrane comprising a porous membrane and a functionalized graphene film thereon.

[0005] A second aspect of the present invention relates to a graphene dispersion comprising functionalized graphene and distilled water.

[0006] A third aspect of method for forming a graphene dispersion, the method comprising: mixing graphite, a polycyclic aromatic hydrocarbon having attached a polar functional group, and an alcohol to form a first solution; agitating the first solution; adding distilled water to the first solution to form a second solution; agitating the second solution; adding distilled water to the second solution to form a third solution; and agitating the third solution to form the graphene dispersion.

[0007] A fourth aspect of the present invention relates to a chemical sensor comprising: at least one graphene membrane, the graphene membrane comprising a porous membrane and a functionalized graphene film thereon; two electrodes attached to opposite ends of the at least one graphene membrane; and an external lead having two wires, wherein each wire is attached to one of the two electrodes and to a device capable of measuring an electrical resistance.

[0008] A fifth aspect of the present invention relates to an electrochemical double layer capacitor (EDLC) comprising: at least two graphene membranes; an electrolyte solution that occupies a space between the at least two graphene membranes; two current collectors having the at least two graphene membranes therebetween; and a housing that encases the two current collectors, the at least two graphene membranes, and the electrolyte solution therein.

[0009] The illustrative aspects of the present invention are designed to solve the problems herein described and/or other problems not discussed. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which:

[0011] FIG. 1 shows transmission electron microscopy (TEM) images of an embodiment of functionalized graphene, in accordance with the present invention;

[0012] FIG. 2 shows a Raman spectrum of an embodiment of a functionalized graphene, in accordance with the present invention;

[0013] FIG. 3 shows an atomic force microscopy (AFM) image of an embodiment of functionalized graphene, in accordance with the present invention;

[0014] FIG. 4 shows a UV-visible spectrum of an embodiment of a graphene dispersion, in accordance with the present invention;

[0015] FIG. 5 shows a Raman spectrum of an embodiment of a graphene membrane, in accordance with the present invention;

[0016] FIG. 6 shows a schematic view of an embodiment of a chemical sensor, in accordance with the present invention;

[0017] FIG. 7 shows an electrical resistance plot of an embodiment of a chemical sensor, in accordance with the present invention;

[0018] FIG. 8 shows a partial cross-sectional schematic view of an embodiment an electrochemical double layer capacitor (EDLC), in accordance with the present invention; [0019] FIG. 9 shows a constant-current charge-discharge plot of an embodiment of an EDLC, in accordance with present invention; and

[0020] FIG. 10 shows cyclic voltammograms of an embodiment of an EDLC, in accordance with present invention.

[0021] It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Graphene is a single-atom-thick, perfectly two-dimensional lattice of pure sp -bonded carbon atoms that possesses a number of interesting quantum mechanical, optical, and molecular interaction phenomena. Graphene may be fabricated by micro-mechanical cleavage of graphite. It has been discovered that the micro-mechanical cleavage method may not be suitable for producing large quantities of graphene directly from graphite.

[0023] A graphene dispersion is presented in accordance with an embodiment of the present invention. The graphene dispersion may comprise functionalized graphene and distilled water. The functionalized graphene may be uniformly distributed throughout the distilled water and may include a polycyclic aromatic hydrocarbon having attached a polar functional group. In an embodiment, the polycyclic aromatic hydrocarbon may be pyrene. The polar functional group may include but is not limited to a hydroxyl, a carbonyl, a carboxyl, and an amino. The polar functional group may also be selected from one of 1-pyrenecarboxylic acid (PC A), pyrene-boronic acid; bromoacetyl pyrene; 1 -pyrene- 10-ketodecanoic acid; 1- pyrene-10-ketodecanoylcholesterol; 4,6-dichloro-2-(l-pyrenyl)-l,3,5-triazine; pyrene- 4,5-dione; pyrene-l,3,6,8-tetraone; aminopyrene; pyrenebutyric acid; and pyrene sulfonic acid.

[0024] The functionalized graphene may be distributed in the form of flakes and may be mono-, bi-, and/or multilayered. In an embodiment, a flake of monolayer functionalized graphene may have a thickness from approximately 0.7 nm to approximately 1.3 nm. In another embodiment, multilayer functionalized graphene may have a thickness from approximately 1.4 nm to approximately 9 nm. The functionalized graphene may have lateral dimensions, i.e., length/breadth that are in a range from approximately 100 nm to approximately 2 μιη.

[0025] In an embodiment, the graphene dispersion may contain approximately

8 to approximately 12 micrograms of functionalized graphene per 1 ml of distilled water. In another embodiment, the functionalized graphene may be distributed throughout the distilled water as monolayer PC A functionalized graphene flakes. In another embodiment, the functionalized graphene may be distributed throughout the distilled water as a combination of monolayer, bilayer, and/or multilayer PCA functionalized graphene flakes.

[0026] Transmissions electron microscopy (TEM), Raman spectroscopy, atomic force microscopy (AFM), and UV -visible (vis) spectroscopy were used to characterize an embodiment of the graphene dispersion of the present invention. Referring to FIG. 1, transmission electron microscopy (TEM) images of an embodiment of functionalized graphene flakes of the graphene dispersion are shown. The TEM images show graphene flakes that have been transferred onto a carbon film TEM grid and possess typical size, shape, and morphology such as having a size in a range from approximately 200 nm to approximately 2 μιη, having random shapes, and being flat/folded monolayer and multilayer attributed to the functionalized graphene flakes of the present invention.

[0027] Referring to FIG. 2, a Raman spectrum of an embodiment of the graphene dispersion is shown. The Raman spectrum shows bands that are

characteristic of monolayer graphene flakes present in the graphene dispersion. A G- band around 1583 cm ^"1 , corresponding to the zone center E _2g phonon (in-plane optical mode) of sp hybridized carbon close to the Γ point can be seen. Other peaks, observed at around 1349 cm ^"1 corresponding to the D-peak or the first order edge or defect-induced zone boundary phonons, and 2693 cm ^"1 corresponding to the D'-band or second-order zone boundary phonons can be seen.

[0028] The Raman spectrum of monolayer graphene has at least two unique signatures: a D'-band peak at around 2693 cm ^"1 and a G-peak at around 1583 cm ^"1 . First, the D'-band peak intensity is larger than the G peak (a feature that reverses itself in graphene with a number of layers >1 and in graphite). Second, the D'-band peak can be fitted with a single Lorentzian function, which is not the case for graphitic flakes with a higher number of layers, where two or more Lorentzian functions are required to fit the Raman data. The inset of FIG. 2 shows a single Lorentzian fit to the D'-band peak and shows the spatial Raman map (image width ~ 1.5μιη) of the monolayer graphene flake obtained from the integrated D'- peak. [0029] Referring to FIG. 3, an AFM image of an embodiment of

functionalized graphene flakes of the graphene dispersion is shown. Graphene flakes of the present invention were transferred from the graphene dispersion onto Si0 ₂ /Si substrates, which is typically required for building electronic devices. The step height at the edge of the graphene flake was measured after averaging out the surface roughness of the graphene flake and its neighbouring substrate. The measured sub- nanometer step height was approximately 0.7 nm which is comparable to known values for monolayer graphene flakes. The step-height reading of approximately 0.7 nm confirmed the presence of monolayer graphene flakes in the graphene dispersion.

[0030] Referring to FIG. 4, a UV-vis spectrum of an embodiment of the graphene dispersion is shown. The graphene dispersion exhibits a UV-vis absorption from approximately 250 nm to approximately 375 nm. The measured absorption range is characteristic of 1-pyrenecarboxylic acid (PCA) and confirms the presence of PCA on the graphene flakes, and thus, the functionalization of the graphene.

[0031] A method of forming a graphene dispersion is presented in accordance with an embodiment of the present invention. The graphene dispersion may be prepared by mixing graphite, a polycyclic aromatic hydrocarbon such as 1- pyrenecarboxylic acid (PCA), and an alcohol to form a first solution. The graphite may be in the form of a powder and the alcohol may be selected from one of methanol, ethanol, and isopropanol. For example, approximately 100 mg of graphite powder (Aldrich, particle size < 45 μιη) and approximately 16.5 mg PCA (Aldrich, 98%) may combined with approximately 50 ml of methanol and agitated for approximately 45 min using, for example, a Branson ^® 5510 bath sonicator. Agitation may alternately be performed by boiling the methanol containing the graphite powder and PCA.

[0032] To this solution, an additional and approximately 200 ml of distilled water may be added so as to form a second solution. The second solution may be processed via agitating and then decanting. For example, the second solution may be agitated via sonication for approximately 24 hrs and then allowed to settle for approximately 8 hrs to approximately 12 hrs. After settling, a clear supernatant liquid of the second solution may be decanted and an additional 100 ml to 200 ml of distilled water may be added to form a third solution. The third solution may then be agitated via sonication for approximately 2 hrs resulting in the formation of the graphene dispersion being stable, having a purple-grey color, and lacking any precipitation even after standing for days to 2-4 weeks.

[0033] An embodiment of the method of forming a graphene dispersion may further comprise washing the third solution to remove any unbound PCA and agitating it again to form the graphene dispersion. For example, the third solution may be centrifuged at approximately 10,000 rpm for approximately 20 minutes and the supernatant decanted. Additional distilled water may be added to the remaining third solution and it may be agitated via sonication for approximately 3 minutes. The aforementioned steps may be referred to as a single wash. The third solution may be washed from approximately three to approximately seven times. With each wash, any remaining unbound PCA may be removed further. One having ordinary skill in the art will recognize without any undue experimentation how many washes to apply to the third solution by monitoring the UV absorbance spectrum of the third solution at each wash step until the PCA absorption peaks remain unchanged. A final wash may be passed through a filter membrane with a standard pore size of approximately 10 μιη to remove any graphite microparticles.

[0034] The formed graphene dispersion may comprise PCA functionalized graphene in distilled water. In an embodiment, the graphene dispersion may comprise a mixture of monolayer, bilayer, and multilayer PCA functionalized graphene flakes in distilled water. Various embodiments and characterization data of the graphene dispersion have been previously described herein.

[0035] It has been discovered that an advantage that may be realized in the practice of some embodiments of a method of forming a graphene dispersion described herein is that the method does not require the chemical conversion of graphene into another form to enable the formation of the graphene dispersion.

[0036] It has been discovered that another advantage that may be realized in the practice of some embodiments of a method of forming a graphene dispersion described herein is that the method may be easily scaled up for large-scale

manufacture of the graphene dispersion as harsh chemicals or conditions such as strong acids or reducing agents are not used and thus, do not need to be disposed of afterward.

[0037] It has been discovered that another advantage that may be realized in the practice of some embodiments of a method of forming a graphene dispersion described herein is that the method uses inexpensive graphite as a starting material.

[0038] A graphene membrane is presented in accordance with an embodiment of the present invention. The graphene membrane may comprise a non-conducting porous supporting membrane and a functionalized graphene film thereon. The size of the graphene membrane has no limitation but may be dependent on the size of the non-conducting porous supporting membrane. The functionalized graphene film may be coated on one side of the porous membrane or alternatively, on both sides of the porous membrane

[0039] The functionalized graphene film may be a mono-, bi-, or multilayer film having a functional group comprising a polycyclic aromatic hydrocarbon having attached a polar functional group. The polar functional group may include but is not limited to a hydroxyl, a carbonyl, a carboxyl, and an amino group. In an embodiment, the polycyclic aromatic hydrocarbon having attached a polar functional group may include 1-pyrenecarboxylic acid, pyrene-boronic acid; bromoacetyl pyrene; 1 -pyrene - 10-ketodecanoic acid; 1 -pyrene- 10-ketodecanoylcholesterol; 4,6-dichloro-2-(l- pyrenyl)-l,3,5-triazine; pyrene-4,5-dione; pyrene- 1,3, 6, 8-tetraone; aminopyrene; pyrenebutyric acid; and pyrene sulfonic acid.

[0040] The functionalized graphene film may include functionalized graphene flakes and in particular, a mixture of monolayer and multilayer functionalized graphene flakes. Embodiments and characterization data of the functionalized graphene flakes have been previously described herein. The functionalized graphene film may have a thickness from approximately 50 nm to approximately 500 nm.

[0041] The porous membrane may include a material selected from one of poly(tetrafluoroethylene) Teflon ^® , cellulose acetate, nanoporous anodized alumina. In particular, the pore sizes may range from approximately 20 nm to approximately 200 nm. In an embodiment, the pore size may be approximately 100 nm. [0042] In an embodiment of the present invention, the graphene membrane may be incorporated in a chemical sensor such as a breath alcohol analyzer or an industrial leakage sensor. In another embodiment of the present invention, the graphene membrane may be incorporated in an electrochemical double layer capacitor.

[0043] The graphene membrane of the present invention may be fabricated by vacuum filtration of a graphene dispersion onto a porous membrane. Embodiments of the graphene dispersion have been previously described herein. The porous membrane may be a nanoporous Teflon ^® membrane having a pore size of

approximately 100 nm. Approximately 10-15 ml of the graphene suspension may be transferred onto the nanoporous Teflon ^® membrane resulting in a grey graphene film coated on one side of the nanoporous Teflon ^® membrane.

[0044] One having ordinary skill in the art will recognize without undue experimentation that other methods of transferring a dispersion having primary particles, aggregates, and the like onto a membrane not currently described herein are applicable to transferring embodiments of the dispersion of the present invention described herein. One having ordinary skill in the art will also recognize without undue experimentation that other methods of isolating primary particles, aggregates, and the like from a dispersion that has been transferred onto a membrane not currently described herein are applicable to isolating embodiments of the dispersion of the present invention described herein.

[0045] Raman spectroscopy was used to characterize the graphene membrane of the present invention. Referring to FIG. 5, a Raman spectrum of an embodiment of the graphene membrane is shown. Peaks at around 1590 cm ^" and around 2713 cm ^" correspond to signature graphene peaks of the graphene film and confirm its presence on the Teflon ^® membrane.

[0046] A chemical sensor 10 is presented in accordance with an embodiment of the present invention. Referring to FIG. 6, an embodiment of chemical sensor 10 is shown. Chemical sensor 10 may comprise at least one graphene membrane 15.

Embodiments and characterization data of graphene membrane 15 have been previously described herein. One having ordinary skill in the art would recognize without any undue experimentation that a functionalized graphene film capable of being dissolved by a target analyte/vapor would not be used in embodiments of chemical sensor described herein. Chemical sensor 10 may also include two electrodes 17 attached to opposite ends of at least one graphite membrane 15.

Electrodes 17 may include a material selected from one of Au, Ag, Pd, Pt, and Cu.

[0047] Electrodes 17 may be in contact with at least one external lead 20 via metallic paint (not shown) such as Ag paint. The external lead 20 may include two wires, wherein each wire may be connected one of the two electrodes 17 and to a device capable of measuring an electrical resistance and in particular, high resistances on the level of giga-Ohms. For example, a multimeter such as a Keithley Model 21400 Source meter may be used. An electrode-lead contact area may be coated with any non-conducting, soft, flexible polymer, for example, poly(dimethylsiloxane) (PDMS), so as to provide mechanical and electrical stability to the contact area. In an embodiment, the flexible polymer may be PDMS. One having ordinary skill in the art chemical sensor technology will recognize without undue experimentation how the elements of chemical sensor 10 may be operatively integrated with each other so as to function as a chemical sensor.

[0048] Chemical sensor 10 may be sensitive to a polar chemical vapor including but not limited to ethanol, methanol, isopropanol, acetone, ethylacetate, and etc. In an embodiment, the electrical resistance of chemical sensor 10 may be sensitive to the presence of vapors of alcohols selected from ethanol, methanol, and isopropanol, and in particular, ethanol, and hence, function as a conductometric sensor. Referring to FIG. 7, an electrical resistance plot of an embodiment of chemical sensor 10 is shown. Chemical sensor 10 was periodically exposed to saturated water vapor in air, saturated ethanol in air, and pure C0 ₂ gas. The ordinate of the plot is plotted in logarithmic scale due to the extremely large response in ethanol. A relative change in resistance to the water vapor and C0 ₂ was found to be approximately 80% and approximately -6% respectively while the relative change to resistance to the ethanol vapor was found to be greater than approximately 10,000%. Insets B, C, and D are expanded regions from representative areas of the data in the main plot, with the response sensitivity Δ resistance/resistance indicated in each case. As shown in FIG. 7, chemical sensor 10 is sensitive to an alcohol such as ethanol.

[0049] In an embodiment of the present invention, chemical sensor 10 may be incorporated in a breath alcohol analyzer. In another embodiment of the present invention, chemical sensor 10 may be incorporated in a industrial leakage sensor.

[0050] It has been discovered that an advantage that may be realized in the practice of some embodiments of a chemical sensor described herein is that the graphene membrane of the chemical sensor has pores on a lower side of a porous membrane so that more of a target analyte may be adhered to the graphene film through the non-conducting porous membrane in addition the target analyte that adheres to a graphene film, and thus the chemical sensor is more sensitive to the target analyte.

[0051] An electrochemical double layer capacitor 50 (EDLC) is presented in accordance with an embodiment of the present invention. Referring to FIG. 8, an embodiment of EDLC 50 is shown. EDLC 50 may comprise at least two graphene membranes 70 and 75. Various embodiments of graphene membranes 70 and 75 have been previously described herein. One having ordinary skill in the art would recognize without any undue experimentation that a functionalized graphene film of graphene membranes 70 and 75 capable of being dissolved by an electrolyte solution would not be used in embodiments of EDLC 50 described herein. EDLC 50 may also include two current collectors 80 and 85 having the graphene membranes 70 and 75 therebetween. Current collectors 80 may be any flat sheet of metal, for example, steel. Current collectors used in EDLCs are known in the art. In an embodiment, graphene membranes 70 and 75 may be configured such that they may be back to back on top of each other resulting in a layered geometry of a graphene film 71, a porous membrane 72, porous membrane 73, and graphene film 74. Porous membranes 72 and 73 may include Teflon ^® , and graphene films 71 and 74 may be a mixture of monolayer, bilayer, and multilayer 1 -pyrenecarboxylic acid functionalized graphene flakes.

[0052] EDLC 50 may also include a liquid electrolyte (not shown), for example, a 6 molar potassium hydroxide solution that may occupy a space between graphene membranes 70 and 75. The space may be the unoccupied space existing in the pores of porous membranes 72 and 73. Current collectors 80 and 85, graphene membranes 70 and 75, and the electrolyte solution may be sealed in a housing (not shown) such as an aluminum housing. Methods of manufacturing an EDLC 50 are known in the art.

[0053] Referring to FIG. 9, a constant-current charge-discharge plot for an embodiment of EDLC 50 is shown. EDLC 50 was operated at a constant current of approximately 2 mA. The plot shows the presence of a linear galvanostatic discharge which confirms the capacitive behavior of EDLC 50.

[0054] Referring to FIG. 10, cyclic voltammograms (CV) for an embodiment of EDLC 50 are shown. The CVs were obtained at various scan rates of EDLC 50 fabricated with two back to back graphene membranes having a graphene film comprising a mixture monolayer and multilayer PCA functionalized graphene flakes and a Teflon® porous membrane (see FIG. 8). The undistorted symmetric nature of the CVs obtained at higher scan rate is indicative of fast charge transfer response of EDLC 50. The slope of the discharge curve was used to calculate the specific capacitance of EDLC 50. A specific capacitance (capacitance/mass) for EDLC 50 was calculated from the CVs using the formula

I

^{Csp ~} (dV/ dt)m

where, / is discharge current, dV I dt is slope of a discharge curve, and m is mass of each electrode of EDLC 50 (in an embodiment, the mass was less than 10 μg, but 10 μg as a maximum upper limit was used for the present calculations). [0055] Similarly, an energy density of EDLC 50 was calculated using equation

W _Sp = (C x V ² )/2m ' and a power density of EDLC 50 was calculated using equation P _Sp = IV/ m ' where C is capacitance, V is the operating voltage of the capacitor, / is discharge current, and m ' is mass of both electrodes. The operating voltage (V) was approximately 1.05 V, the mass (m) of each electrode was approximately 10 μg, and the mass of both electrodes (m ') was approximately 20 μg.

[0056] In an embodiment, EDLC 50 may have a specific capacitance in a range from approximately 83 Faraday (F)/g to approximately 120 F/g; a power density in a range from approximately 44 kilowatt (kW)/kg to approximately 105 kW/kg, and an energy density in a range from approximately 4 Watt hr (Wh)/kg to approximately 9 Wh/kg. The minimum range values were calculated at a constant discharge current of approximately 1 mA and the maximum range values were calculated at a constant current of approximately 2 mA. In another embodiment, EDLC 50 may independently have a specific capacitance of approximately 120 F/g; a power density of approximately 105 kW/kg, and an energy density of approximately 9.2 Wh/kg.

[0057] It has been discovered that an advantage that may be realized in the practice of some embodiments of an EDLC described herein is that the EDLC has one of the highest specific capacitance reported for graphitic nanostructures.

[0058] The terms "first," "second," and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms "a" and "an" herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The modifier "approximately" used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context, (e.g., includes the degree of error associated with measurement of the particular quantity). The suffix "(s)" as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the metal(s) includes one or more metals). Ranges disclosed herein are inclusive and independently combinable (e.g., ranges of "up to approximately 25 wt %, or, more specifically, approximately 5 wt % to approximately 20 wt %", is inclusive of the endpoints and all intermediate values of the ranges of "approximately 5 wt % to approximately 25 wt %," etc).

[0059] While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the art, and are within the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.