Patent Abstract:
A carbon composite material, including a plurality of spaced graphene sheets, each respective sheet having opposed generally planar surfaces, and a plurality of functionalized carbonaceous particles. At least some functionalized carbonaceous particles are disposed between any two adjacent graphene sheets, and each respective at least some functionalized carbonaceous particle is attached to both respective any two adjacent graphene sheets. Each respective graphene sheet comprises at least one layer of graphene and at least portions of respective any two adjacent graphene sheets are oriented substantially parallel with one another.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims priority to co-pending U.S. patent application Ser. No. 13/372,187, filed on Feb. 13, 2012, which claimed priority to then co-pending U.S. Provisional Patent Application Ser. No. 61/442,281, filed on Feb. 13, 2011. 
     
    
     TECHNICAL FIELD 
       [0002]    The novel technology relates generally to materials science, and, more particularly, to a high surface area graphene composite material. 
       BACKGROUND 
       [0003]    Graphene, a single-atom-thick sheet consisting of sp 2  hybridized carbon atoms arrayed in a honeycomb pattern, is the building block of graphitic carbons. Graphene may be viewed as an individual atomic plane of the graphite structure. Graphene as a two-dimensional nanosheet has attracted increasing interest due to its unique properties of high in-plane electronic conductivity, high tensile modulus, and high surface area, which make graphene an attractive candidate for applications in electronic devices and composite materials. Moreover, with its high surface area and good chemical stability, graphene may be used as a gas adsorbant, ultracapacitor material, or a supporting material for developing novel heterogeneous catalysts with enhanced catalytic activity. 
         [0004]    Graphene may be produced by any one of several methods, including the straightforward exfoliation technique of manually peeling off of the top surface of small mesas of pyrolytic graphite, chemical vapor deposition on metal surfaces, epitaxial growth on electrically insulating surfaces, such as SiC, and the like. Although multiple production methods do exist, large-scale applications of graphene require simple and cost effective methods of production. Hence, the primary route in making graphene is still the exfoliation of graphite oxides followed by a chemical reduction. 
         [0005]    In aqueous solvent dispersions of graphene prepared by chemical reduction, graphene sheets are separated by solvents stabilized by electrostatic forces associated with ionizable groups introduced during the exfoliation. However, like other dispersions of nanomaterials with high aspect ratios, after the solvent is removed from the dispersion, the dried graphene sheets (GSs) usually aggregate and form an irreversibly interconnected or tangled precipitated agglomerate. This agglomeration is driven by the van der Waals interactions between the neighboring graphene sheets, urging the graphene sheets to stack back together in a disorganized and typically haphazard fashion. This agglomeration also leads to a considerable loss of the effective surface area of graphene, which affects the graphene applications in, for example, supercapacitors, batteries, and catalyst supports, where a high surface area of active materials is desired for performance. Therefore, how to achieve the intrinsically ultra-high surface area of graphene in its solid state is of interest in advancing the applications of graphene materials. 
         [0006]    Anchoring nanoparticles on the graphene surface before the GS&#39;s aggregation is one effective way to keep the GS&#39;s high surface area. The deposition of Pt nanoparticles on a graphene surface before drying has been shown to increase the surface area of the composite from 44 m 2 /g to 862 m 2 /g with the anchoring of the Pt nanoparticles on the surface. Graphene polyoxometalate nanoparticle composites have been observed to yield a graphene surface area of about 680 m 2 /g. Graphene sheet/RuO 2  composites have been observed with increased surface area increases from 108 m 2 /g to 281 m 2 /g. These composites also exhibited a high specific capacitance 570 F/g and an enhanced rate capability. Although the surface area of GSs have been increased with the addition of the nanoparticles, the resulting specific surface area was still much lower than the theoretical surface area of 2630 m 2 /g of the isolated GSs. 
         [0007]    Thus, there is a need for graphene materials having effective surfaces areas approaching the theoretical maximum of 2630 m 2 /g. Further, there remains a need for a method of reliably producing the same. The present novel technology addresses these needs. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic illustration of the graphene sheet (GS) and the graphene sheet nanocarbon composites (GSNC) preparation process. 
           [0009]      FIG. 2  graphically illustrates nitrogen adsorption and desorption of the as-prepared GNCs with different nanocarbon content. 
           [0010]      FIG. 3A  illustrates TEM images of the as-prepared GSNCs from pure GSs. 
           [0011]      FIG. 3B  illustrates TEM images of the as-prepared GSNCs with 1% nanocarbon content and a surface area of 1256 m 2 /g. 
           [0012]      FIG. 3C  illustrates TEM images of the as-prepared GSNCs with functionalized nanocarbons. 
           [0013]      FIG. 3D  illustrates TEM images of the as-prepared GSNCs with 1% nanocarbon content and a surface area of 1256 m 2 /g. 
           [0014]      FIG. 4A  presents SEM images of the pure GSs. 
           [0015]      FIG. 4B  presents SEM images of GSNCs with a 1% nanocarbon content and a surface area of 1256 m 2 /g after drying. 
           [0016]      FIG. 5A  graphically illustrates CV curves of the as-prepared GSNCs with a surface area of 1256 m 2 /g, measured at potential intervals from −0.2 to 0.8 V (vs. SHE) in 1 M H 2 SO 4 . 
           [0017]      FIG. 5B  graphically illustrates voltage curves of the GSNCs with different nanocarbon content as the function of time. 
           [0018]      FIG. 5C  graphically illustrates the capacitance of the GSNCs with different nanocarbon content as the function of current density. 
           [0019]      FIG. 6  schematically illustrates Pt nanoparticle etching process on the surface of graphene sheets, according to another embodiment of the present novel technology. 
           [0020]      FIG. 7A  is a first atomic resolution electron micrographs showing the dynamic etching of graphene sheets by Pt nanoparticles and the resulting trenches left behind in the graphene according to the embodiment of  FIG. 6 . 
           [0021]      FIG. 7B  is a second atomic resolution electron micrographs showing the dynamic etching of graphene sheets by Pt nanoparticles and the resulting trenches left behind in the graphene according to the embodiment of  FIG. 7A . 
           [0022]      FIG. 7C  is a third atomic resolution electron micrographs showing the dynamic etching of graphene sheets by Pt nanoparticles and the resulting trenches left behind in the graphene according to the embodiment of  FIG. 7A . 
           [0023]      FIG. 7D  is a fourth atomic resolution electron micrographs showing the dynamic etching of graphene sheets by Pt nanoparticles and the resulting tortured path left behind in the graphene according to the embodiment of  FIG. 6 . 
           [0024]      FIG. 7E  is a fifth atomic resolution electron micrographs showing the dynamic etching of graphene sheets by Pt nanoparticles and the resulting etch path left behind in the graphene according to the embodiment of  FIG. 7D . 
           [0025]      FIG. 7F  is a sixth atomic resolution electron micrographs showing the dynamic etching of graphene sheets by Pt nanoparticles and the resulting etch path left behind in the graphene according to the embodiment of  FIG. 7D . 
           [0026]      FIG. 8A  is an electron micrograph of pristine graphene. 
           [0027]      FIG. 8B  is an electron micrograph of Pt nanoparticles etched graphene according to the embodiment of  FIG. 6 . 
           [0028]      FIG. 9  graphically illustrates the XPS spectra of graphene before and after Pt nanoparticulate etching, according to the embodiment of  FIG. 6 . 
           [0029]      FIG. 10A  graphically illustrates the N 2  adsorption isotherms and CO 2  capture properties of graphene composites for graphene, Pt/Graphene, and Pt/Graphene 800° C. at 77 K. P/P°, relative pressure; STP, standard temperature and pressure. 
           [0030]      FIG. 10B  graphically illustrates the N 2  adsorption isotherms and CO 2  capture properties of graphene composites for graphene, Pt/Graphene, and Pt/Graphene 800° C. at 273 K; filled and open symbols represent adsorption and desorption branches, respectively. 
           [0031]      FIG. 11  is a schematic illustration of a supercapacitor using electrodes made from the embodiment of  FIG. 1 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    For the purposes of promoting an understanding of the principles of the novel technology, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates. 
         [0033]    According to a first embodiment of the present novel technology, as illustrated in  FIGS. 1-5B , graphene sheets  10  were prepared by the exfoliation of graphite oxide (a layered material consisting of hydrophilic oxygenated graphene sheets with oxygen functional groups on their basal planes and edges), such as in water to yield a colloidal suspension of almost entirely individual graphene sheets  10 . Nanosized carbon particles  15 , typically carbon black particles  15 , were functionalized with hydrophilic groups, such as —SO 3 H (i.e., bisulfate or hydrogen sulfite), and the GSNCs  20  were prepared with different loadings of the functionalized carbon black particles  25  by simultaneous chemical reduction of both the graphene oxides  30  and the functionalized carbon black particles  25  while in solution. Typically, functionalized carbon black particles  25  are present in amounts of between about one weight percent and about ten weight percent; but the carbon black particle loading may typically vary from less than about 1 weight percent to as much as 50 weight percent, or more. Functionalization is the addition of functional groups onto the surface of a material by chemical synthesis methods or the like, and the functional group added can be subjected to ordinary synthesis methods to attach virtually any kind of compound onto the material&#39;s surface. The nanosized functionalized carbon black particles  25  attached to the surface of the GSs  10  and served as spacers to separate/support the neighboring GSs  10 , which prevented the haphazard restacking of the graphene sheets  10  into a randomly oriented solid particulate mass, and consequently, resulted in the generation of increased surface area. The specific surface area of the composites  20  was 1256 m 2 /g, and a maximum specific capacitance of 240 F/g was observed at a current density of 1 A/g. In addition, graphene sheet composite-based capacitors using this composite material  20  for the electrodes exhibited enhanced rate capability, the maximum sustainable continuous or pulsed current output. The above improved electrochemical performance of the GSNCs  20  is a product of their high surface area and high electronic conductivity of the GSs  10 . 
         [0034]    While the carbon nanoparticles  15  discussed herein are specifically carbon black, other allotropes of carbon may be selected. Amorphous carbon, glass carbon, coke, carbon graphitized to various degrees of graphitization, diamondlike carbon, and diamond may also be selected, with the electrical and physical properties of the resulting composite material  20  varying as a result. 
         [0035]    In the synthesis of the GSNCs  20 , the GSs  10  were obtained by in situ chemical reduction of exfoliated graphene oxides  30 . As shown in  FIG. 1 , the construction of the GSNCs involved the following steps: first, exfoliation  40  of graphite oxides, then, mixing  45  the graphene oxide sheets  30  and functionalized nanocarbons  25 , and finally, chemical reduction  50  of the mixture. The nanocarbons  15  were functionalized  55  by the dizonium reaction, and the nanocarbons  25  are highly hydrophilic after functionalization  55 . Graphene oxide sheets  30  exist in the liquid dispersion  60 . After reduction  50  of the compound  20  in its solid state, the graphene sheets  10  aggregate  65  and stack back into a layer structure like graphite. Graphene oxide sheets  30  and carbon nanoparticles  25  exist together in dispersion  60 ; in the solid state the nanocarbons  25  serve as spacers, preventing the graphene sheets  10  from restacking back to the graphite structure, and thus make the graphene sheet  10  accessible on both sides and allowing access to the high surface area graphene composite  20 . In the reduction process  50 , the well-dispersed graphene oxide sheets  30  and the functionalized nanocarbons  25  were reduced simultaneously and the functionalized nanocarbon particles  25  became anchored  75  to the graphene sheets  10 . The solid composites  20  float on the surface of the transparent liquid phase of the dispersion  60 . The resultant graphene sheets  10  with attached functionalized nanocarbons  25  aggregated together to yield the GSNCs  20  upon drying. 
         [0036]    Graphene oxides  30 , possessing a considerable amount of hydroxyl and epoxide functional groups on both surfaces of each sheet  30 , and carboxyl groups, mostly at the sheet edges, are strongly hydrophilic and can easily disperse in water. The nanocarbons  15  were functionalized  55  by diazonium reactions as shown in  FIG. 1A . In this process, the hydrophilic —SO 3 H functional group was grafted onto the surface of the nanocarbons  15 . As shown in  FIG. 1 , after functionalization  55  the nanocarbons  25  can disperse well in the water even if left for several months. After adding the functionalized nanocarbons  25  into the graphene sheet dispersion  60 , the two materials were able to be easily mixed and formed uniform dispersion  60 . 
         [0037]    In order to explore the effects of the nanocarbon content on the composite surface areas, a series of controlled experiments were conducted by varying the content of the nanocarbon in the GSNCs  20  to 0, 0.5, 0.8 and 1 wt. %. The addition of the nanocarbons  25  into the dispersion  60  of the graphene sheets  30  led to the formation of well-dispersed nanocarbon particles  25  on the surface of the graphene sheets  30 . The in situ formed nanocarbon particles  25  can serve as spacers to prevent aggregation/restacking of the individual graphene sheets  30  in the dispersion during the drying process and form a particle-sheet structured GSNC  20  in the solid state. It is reasonably expected that the in-situ-formed composites  20  have more of a rich porous structure and large available surface area for the charge-storage process than those obtained by drying the pure graphene sheets  10 , in which the restacking of the graphene sheets  10  inevitably occurs. Typically, the sheets  10  are freeze dried, although other convenient drying techniques may be employed. 
         [0038]    The nitrogen-adsorption and -desorption isotherms of the as-prepared GSs  10  with different nanocarbon content exhibited type IV characteristics ( FIG. 2 ), which are indicative of the presence of relatively large pores in the composites  20 . It is worth noting that the Brunauer-Emmett-Teller (BET) specific surface area of the graphene sheets without the addition of nanocarbons (77 m 2 /g) was much lower than the theoretical predictions for the isolated graphene sheets (2630 m 2 /g). With the increase in nanocarbon content in the composites  20 , the specific surface area also increased. The BET-specific surface area of the composites  20  with nanocarbon content of 1 wt. % reached as high as 1256 m 2 /g, which is much higher than that of the nanocarbons  25  (790 m 2 /g) and the pure GS  10  (77 m 2 /g). In further trials, the BET-specific surface area of the composites  20  with additional nanocarbon material  25  was observed to be up to 1875 m 2 /g, and values as high as 2000, 2100 and approaching the theoretical maximum are expected. 
         [0039]    The large specific surface area suggests that the introduction of nanocarbon particles  25  between 2D graphene sheets  10  effectively limits the face-to-face stacking from about forty layers of graphene sheets  10  per stack to about two layers of graphene sheets  10  per stack when compared with that of dried pure GS  80 . 
         [0040]    To further characterize the structure of the GSNCs  20 , the samples were examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) ( FIGS. 3 and 4 ). For comparison, the TEM images of the reduced GSs  10  without nanocarbons  15  and the functionalized nanocarbons  25  ( FIGS. 3A and 3D ) are also presented.  FIG. 3A  shows that the pure GSs  80  prepared by chemical reduction  50  were transparent with some wrinkles visible under TEM. The morphology of functionalized nanocarbons  25  can be seen in  FIG. 3B , which shows that the functionalized nanocarbon particles  25  were in the range of 5-30 nm, and that they tended to spontaneously agglomerate together to form large particles. The structure of the GSNCs  20  is shown in  FIG. 3C , which clearly shows that the functionalized nanocarbons  25  were homogeneously anchored  75  onto the surface of the graphene sheets  30  ( FIGS. 3C and 3D ). Through further comparisons of FIGS.  3 A &amp;  3 B with  FIGS. 3C &amp; 3D , it is clear that the graphene sheets  30  served as substrates to anchor  75  the hydrophilic nanocarbon particles  25 . Without the addition of the nanocarbons, it can be seen that the pure GS  80  was less transparent than the as-prepared GSNCs  20 , because the pure graphene sheet  10  spontaneously agglomerated/restacked back and formed a thick graphene sheet stack  80  (which has many more layers of graphene sheet than that of the GSNC  20 ) after drying. The GSs  10  in the composites  20  were almost transparent, which suggests that the GSs  10  were well separated by the nanocarbon particles  25 . The number of layers of graphene sheets  10  in the composites  20  was lower (typically about two layers of GS  10 , as suggested by BET data). Considering that sonication was used during the preparation of TEM specimens, the above observation also demonstrates the strong interactions/bonding between the nanocarbon carbon particles  25  and the graphene sheet  10  surface. The SEM images also clearly show the difference between the pure GS agglomerations  80  and the GSNCs  20 . The pure GSs  10  after drying tended to restack and form solid particles  80  ( FIG. 4A ). However, the layered structure can be seen clearly for GSNCs  20 , as the small nanocarbon particles  25  are highly dispersed on the graphene sheet  10  surfaces and served as spacers to prevent the graphene sheets  10  from restacking, which is consistent with the observed increased surface area of the graphene sheet/nanocarbon composites  20 . 
         [0041]    Recently, GS agglomerates  80  have been used as electrodes for supercapacitors; for example, chemically modified GSs electrode active materials in supercapacitors have been found to exhibit a specific capacitance of 135 F/g and  99 F/g in aqueous KOH and organic electrolytes, respectively. GS specimens  80  having a measured surface area of 534 m 2 /g have exhibited a capacitance of 150 F/g under the specific current 0.1 A/g. Based on the structure of the GSNCs material  20 , the composite  20  likewise is expected to have good electron conductivity, low diffusion resistance to protons/cations, easy electrolyte penetration, and high electroactive areas. Such composites  20  are promising candidates for electrode active materials for supercapacitors  100 , yielding high performance energy storage devices. 
         [0042]    The properties of these GSNCs  20  were measured using cyclic voltammetry (CV) and galvanostatic charge/discharge. The galvanostatic charge/discharge was used to calculate the specific capacitance of the GSNCs  20 . The CV curves ( FIG. 5A ) were nearly rectangular in shape, indicating a good charge propagation within the electrode. For the supercapacitor  100  using the activated carbon-based electrodes  105 , the CV curve shape and the specific capacitance significantly degraded as the voltage scan rate increased. In contrast, as the scan rate increased, the GSNCAs  100  base electrode  105  remained a rectangular shape with little variance, even at a scan rate of 200 mv/s ( FIG. 5 ). Another indication of good charge propagation is the low variation of specific capacitance with the increase of the charge/discharge current density as shown in  FIGS. 5B and 5C . The capacitance of the GSNCs  20  was 256 F/g at a discharge current density of 1 A/g, and the capacitance was 218 F/g when the discharge current density increased to 5 A/g, leaving only a 14.8% loss with the 400% increase on the discharge current density. Therefore, the added nanocarbons played a very important role in the electrochemical performance of the composites. The high performance of the GSNC electrode materials  105  in the supercapacitor  100  from the high surface area of this composite  20  is quite beneficial. 
         [0043]    The specific capacitance of the GSNCs  20  with different amounts of nanocarbons  25  at various current densities is shown in  FIG. 5C  for comparison. It is worth noting that the specific capacitance of the high surface area GS composites  20  was much higher than that of the pure GSs  80 , and the capacitance increased with the increase of the surface area. Hence, the increased surface area was responsible for the increase of the capacitance. The incorporation of nanocarbon particles  25  into the GSs  20  not only increased the surface area, but also acted as spacers between the graphene sheets  10  to create diffusion paths for the liquid electrolytes, which facilitated the rapid transport of the electrolyte ions, consequently resulting in the improved electrochemical properties of the GSNCs. Therefore, GSNCs  20  with a surface area of 1256 m 2 /g exhibited the maximum capacitance of 218 F/g at a current density of 5 A/g compared with pure graphene materials  80  with a capacitance of 46 F/g at the same current density, indicating that the unique structure of the novel GSNCs  20  facilitated the rapid transport of the electrolyte ions and electrons throughout the electrode  105 . 
         [0044]    The simple process for preparing high surface area GSs  20  by simultaneously reducing the graphene oxide sheets  30  and the functionalized nanocarbons  25  is more particularly described below. This method is easily scaled up for the mass-production of high surface area graphenes  20 . The nanocarbon particles  25  are generally dispersed uniformly on the surface of the graphene sheets  10 , serving as spacers between graphene sheets  10 , and preventing the restacking of the GSs  10  after drying or removal of the solvent. Consequently, the GSNC surface area has been observed as high as 1875 m 2 /g. The unique structure of the GSNCs  20  facilitated the high-rate transportation of electrolyte ions and electrons throughout the electrode  105 , resulting in the excellent electrochemical properties. The supercapacitor  100  based on the GSNCs  20  exhibited a specific capacitance of nearly 400 F/g at a current density of 1 A/g in a 1M H 2 SO 4  solution. The specific capacitance increased with the increase of the composite surface areas. The new high surface area GS material  20  is also useful as a sorbent for hydrogen storage, as a catalyst support for fuel cells, and as a component for other clean energy devices. 
       Example 1 
       [0045]    Synthesis of the graphene oxides (GO) and the functionalized nanocarbons: GO  30  was synthesized from natural graphite powder (325 mesh) by the modified Hummer method. The GO  30  was then suspended  110  in water to yield an opaque dispersion  60 , which was subjected to separation by centrifuge (five times) to completely remove residual salts and acids. The purified GO  30  was then dispersed  120  in purified water (0.5 mg/mL). Exfoliation  40  of the GO  30  was achieved by ultrasonication of the dispersion  60  using an ultrasonic bath. During the composite preparation process, the number of single layers in the GSs  30  as a precursor are typically controlled to be as small as possible. Graphite oxide is a layered material consisting of hydrophilic oxygenated GSs (graphene oxides)  30  bearing oxygen functional groups in their basal planes and edges. Under appropriate conditions, graphite oxides can undergo complete exfoliation in water, yielding colloidal suspensions  60  wherein the suspended material is composed almost entirely of individual graphene oxide sheets  30 . 
         [0046]    For the preparation of the nanosized carbon particles  25 , the EC300 carbon blacks  15  were modified with an —SO 3 H grafted layer in an aqueous medium by spontaneous reduction  50  of the corresponding in situ generated diazonium cation. The modification of EC300 carbon blacks  15  was prepared with a large excess of in situ-generated diazonium cations. In this experiment, 2 g of EC300 carbon blacks  15  were placed in a 0.5 M HCl solution  125  containing 3.5 g of sulfonic acid. The solution  125  was vigorously stirred for thirty minutes before sodium nitrite was added. Next, 3.6 g NaNO 2  was added to the solution  125  in order to ensure a total transformation of the amine into diazonium in spite of the nitrogen oxide gas release. For the reaction to be finished completely, the mixture was stirred for four hours and then heated up to 70° C. for another three hours. Finally, the mixture was filtrated, washed with water, and re-filtrated three times. 
         [0047]    Synthesis of the GSNCs: GSNCs  20  with different nanocarbon content were prepared by simultaneously reducing 50 the mixture of the graphene oxide sheets  30  and the highly hydrophilic nanocarbons  25 . Graphene oxide sheets  30  dispersed in water were mixed with the nanocarbons  25 . The mixture was stirred for thirty minutes and then subjected to ultrasonication for one hour at room temperature. Subsequently, a hydrazine solution was added into the mixture and the mixture was stirred and heat treated at 100° C. for 24 hours. Then the mixture was filtered and washed with purified water several times and dried at 60° C. for 24 hours in a vacuum. 
         [0048]    Characterization of the composites: the morphology of the graphene sheets  10 , the nanocarbons  25 , and the GSNCs  20  were characterized by a transmission electron microscope. The morphology of the composites  20  was also examined by a scanning electron microscope. The specific surface areas of the graphene sheet  10 , the nanocarbons  25 , and the GSNCs  20  were measured by the Brunauer-Emmett-Teller (BET) method of nitrogen sorption at the liquid nitrogen temperature (77 K). Further, the composite materials  20  are stable at elevated temperatures and exhibit degradation or etching at the nanocarbon particle  25  sites. 
         [0049]    Preparation and characterization of the supercapacitor electrode: A three-electrode-cell system was used to evaluate electrochemical performance using both cyclic voltammetry and galvanostatic charge/discharge techniques using an electrochemical workstation. A 1M H 2 SO 4  aqueous solution was used as the electrolyte. A platinum sheet and a saturated Ag/AgCl electrode were used as the counter and the reference electrodes, respectively. The working electrode  105  was prepared by casting a Nafion-impregnated sample onto a glassy carbon electrode with a diameter of 5 mm. Next, 17.5 mg of composite material  20  was dispersed by sonication for ten minutes in a 10 mL water solution containing 5 μL of a Nafion solution (5 wt. % in water). This sample (10 μL) was then dropped onto the glassy carbon electrode and dried overnight before electrochemical testing. The specific gravimetric capacitance was obtained from the discharge process according to the following equation: 
         [0000]    
       
         
           
             C 
             = 
             
               
                 I 
                  
                 
                     
                 
                  
                 Δ 
                  
                 
                     
                 
                  
                 t 
               
               
                 Δ 
                  
                 
                     
                 
                  
                 Vm 
               
             
           
         
       
     
         [0050]    where I is the current load (A), Δt is the discharge time (s), ΔV is the potential change during the discharge process, and m is the mass of active material in a single electrode (g). 
         [0051]    Graphene  10  is generally quite inert when exposed to gases such as oxygen and hydrogen at room temperature. At high temperatures, oxygen exposure can cause preferential etching at defects and edges because the carbon atoms at the defects and edges are extremely reactive (this is because the p z  electrons of these carbon atoms may not be involved in the conjugated electron system). In a hydrogen atmosphere, the carbon atoms in the graphene bulk remain inert even at high temperatures. However, carbon atoms both at defects and at the edges of a graphene sheet become very active when a reactive metal is positioned proximate to these atoms. At high temperatures, Pt nanoparticles  150  may be used to etch graphene  10  through the catalytic hydrogenation of carbon, where carbon atoms on the graphene edges dissociate on the surface of Pt nanoparticle  150  and then react with H 2  at the Pt nanoparticle  150  surface to form methane. This process is shown schematically in  FIG. 6 . In contrast, such etching does not occur on graphene materials at carbon black or like carbonaceous particle sites. 
         [0052]    The mechanism of etching of graphene  10  by Pt nanoparticles  150  at elevated temperature was observed in-situ using high-resolution environmental transmission electron microscopy. Graphene sheets  10  were loaded with 20 weight percent of Pt nanoparticles  150 , and subsequently placed onto a lacey carbon TEM grid. The Pt nanoparticles  150  are typically sized between a few nanometers up to ten microns across, and may even be larger. More typically, the Pt nanoparticles are between about 5 and about 80 nanometers in diameter, although the Pt nanoparticles  150  may more typically range from about 10 nanometers to about 50 nanometers in diameter. The Pt nanoparticles  150  are typically generally spherical, but may exhibit other morphologies. Further, the nanoparticles  150  may be made of PT-like materials, such as PT, Pd, Ni, combinations thereof, and the like. Likewise, in this example, the graphene sheets  10  were loaded with 20 weight percent Pt nanoparticles  150 , but the nanoparticle loading may typically vary from less than about 1 weight percent to as much as 50 weight percent, or more. The graphene samples  10  were heated to 800° C. and hydrogen gas was slowly introduced into the TEM objective lens, and equilibrated at a pressure of approximately 50 mTorr. As the graphene  10  began to etch adjacent the Pt nanoparticles  150 , the process was imaged continuously through the use of a high-frame rate camera. Image sequences extracted therefrom are presented as  FIG. 7A-7F . Initially, the Pt nanoparticles  150  were static after the hydrogen gas was introduced. Eventually, as shown in  FIGS. 7A-7C , the Pt nanoparticles  150  began to react with the graphene  10  at defect sites and the hydrogen gas to produce methane. Only those carbon atoms making up the graphene sheet  10  that were in direct contact with these Pt nanoparticles  150  were able to participate in this Pt-catalyzed hydrogenation reaction  155 . Once the process was initiated, the conversion process was able to continue, as there are an abundance of defects sites created continuously following the onset of the etching process  155 , leading to a self-sustaining reaction. In this case a straight trench was etched through the graphene sheet  10  ( FIG. 7C ). In other cases, the etching process  155  did not follow a straight line, but rather followed a more tortuous pathway ( FIGS. 7D-7F ). During the graphene etching process  155 , the Pt nanoparticles  150  were observed to maintain a crystallographic relationship with the graphene sheet  10 . After etching  155 , the Pt nanoparticles  150  are typically reclaimed and saved for future use. These observations indicate that the interaction between the Pt  150  and the graphene  10  at elevated temperature can create a variety of in-plane nanostructures  160  in the graphene  10 . The result of these interactions is the formation of nanoscale trenches, ribbons and islands  160 —and thus a dense network of edge sites  165 . 
         [0053]    Typically, the graphene sheets  10  are heated to a temperature sufficient for the etching process  155  to occur at a desired rate. The graphene sheets  10  carrying dispersed Pt nanoparticles  150  are typically heated to at least about 700 degrees Celsius, and are more typically heated to a temperature in the range from 750 degrees Celsius to 900 degrees Celsius. Likewise, a hydrogen gas environment supports the Pt-catalyzed hydrogenation reaction  155 , although other reducing environments may also be selected. 
         [0054]    In graphene  10 , each carbon atom uses 3 of its 4 valance band (2s, 2p) electrons (which occupy the sp 2  orbits) to form covalent bonds with the neighboring carbon atoms in the same plane. Each carbon atom in the graphene  10  contributes its fourth lone electron (occupying the p z  orbit) to form a delocalized electron system. Thus, the carbon atoms in the graphene plane  10  (excluding the carbon atoms on the defect sites such as the edges and holes) are saturated carbon atoms, with the three sp 2  electrons forming three covalent bonds and the fourth p z  electron forming a π bond. Real time observations indicate that the heat treatment process creates an abundance of defective edges  165 , in the form of embedded nanostructures of trenches, ribbons and islands  160  in the multilayer graphene sheets  10  (see  FIGS. 7 and 8B ). The resulting materials are anisotropic, having different properties in-plane and out-plane. Importantly, the carbon atoms along the edges of the resulting trenches, ribbons, and the islands  160  are likely to be unsaturated, with one of the electrons in the sp 2  orbitals not forming a covalent bond with the other carbon atoms. These unsaturated carbon atoms were observed not only from the nano-scale in situ TEM images/video ( FIG. 7 ), but also from the macro-scale XPS results in  FIG. 9 , which shows an 42.4% increase of the shake-up peak for the graphene sheet  10  after etching  155  (the shake-up peaks correspond to the unsaturated carbon atoms/dangling carbons). 
         [0055]    The resulting material provides an important platform for a wide variety of applications, including in catalysis, biomedical science, polymer science and energy science. This is because these unsaturated carbon atoms allow graphene  10  to be functionalized by chemically grafting other compounds or groups thereonto. Thus, these functionalized graphene  170  can be used, for example, sensors, catalysts, sorbents, and the like. Without such features, it is difficult to chemically graft compounds or groups onto graphene  10 . These unsaturated carbons also promote the establishment of weak bonding between graphene and other species. One such application is gas physisorption. Of particular interest is the physisorption of carbon dioxide. The p z  electrons and one sp 2  electron of these unsaturated carbon atoms at the defects sites will be available for bonding and will more readily form bonds with CO 2  molecules, which could in turn result in a significant improvement in CO 2  adsorption. The adsorbed CO 2  molecules (or other gas molecules) may be stored for later removal or reaction. 
         [0056]    While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.

Technology Classification (CPC): 8