Patent Application: US-201114001845-A

Abstract:
a method of producing graphene comprises forming a composition comprising magnesium and carbon , and isolating graphene from the composition . the isolated graphene is crystalline .

Description:
the present invention is based on the discovery that carbon produced by burning magnesium in carbon dioxide , in contact with solid carbon dioxide ( also referred to as dry ice ), followed by washing with acid , produces crystalline graphene in high yield . although this chemical reaction had previously been carried out , it was never recognized that the major product produced was graphene , which could be isolated by filtering a suspension of the reaction products through a filter . unlike graphene produced by other methods , such as by exfoliation of graphite , the graphene is crystalline , and of a small particles size . by separating the products of the reaction by particle size , for example by filtration , the graphene could be isolated . thus , burning magnesium metal in a co 2 environment produces carbon materials as shown in equation 1 . although the metal - co 2 propulsion system for mars missions have been explored , 15 the conversion of co 2 into solid nanostructured carbon materials has not been reported . therefore , this approach involving combustion of magnesium metal in carbon dioxide to form few - layer graphene is unprecedented . the way the magnesium metal is introduced , in different shapes and forms , may be varied , as may be the experimental design . other metals , such as transition metals including zn , and alkaline and alkaline earth metals including li , may also be included with the magnesium . burning magnesium coils , spirals or ribbons inside a quartz - glass chamber filled with dry ice is another possible variation . during the burning process , the magnesium may be in liquid form , and may form a molten mixture with added metals , such as transition metals including zn , and alkaline and / or alkaline earth metals including li . the graphene is isolated from the solidified mixture containing magnesium metal , magnesium oxide and / or magnesium carbonate , using acids , preferably an aqueous solution of an acid . other carbon - containing gasses may be used to support the burning of the magnesium , such as carbon monoxide , or mixtures of carbon dioxide and carbon monoxide , and inert gases , such as argon , may be included . the presence of oxygen or oxygen - containing gases ( such as h 2 o ) is also possible . it may also be possible to form liquid magnesium and introduce the carbon dioxide ( or other carbon - containing gas ) to allow chemical reaction , without the characteristic flame or self - sustaining nature of burning . it may also be possible to include carbon in the form of graphite or carbonate ( such as magnesium carbonate ) with the magnesium metal . preferably , the magnesium contains at most 1 % impurity metals , such as calcium , more preferably at most 0 . 1 %, and most preferably at most 0 . 01 %. preferably , salts of magnesium , such as magnesium chloride are not present , or are present in an amount of at most 1 %, more preferably at most 0 . 1 %, and most preferably at most 0 . 01 %. various metals may be added to the magnesium prior to reaction with the carbon dioxide , for example alkali metals ( such as li ), alkaline earth metals ( such as be or ca ), or other elements from the periodic table including transition metals , post - transition metals , and rare - earth metals ( such as b , al , sc , ti , v , cr , mn , fe , co , ni , cu , zn , y , zr , nb , mo , ru , rh , pd , ag , cd , hf , ta , w , re , os , ir , pt , au , hg , th , ce , pr , ga , in , sn , tl , pb , la , nd , sm , eu , gd , tb , ho , dy , er , tm , yb , lu , bi , ge and / or si ). preferably , such added metals are present in an amount of at most 10 atomic percent , more preferably at most 5 atomic percent . not wishing to be bound by any particular theory , the preparation condition of the graphene may be described is different ways . for example the metal containing magnesium is in liquid form , and is therefore at a temperature above 650 ° c . and below a temperature of 1100 ° c ., for example 650 - 1000 ° c ., or 650 - 900 ° c ., or 700 - 800 ° c . carbon produced during the reaction may directly form graphene , or may dissolve in the liquid metal , and as the metal is consumed the carbon may precipitate as graphene , and / or the graphene may form upon cooling . since the liquid metal is in contact with solid carbon dioxide , which has a temperature of − 78 . 5 ° c ., the molten metal cools very quickly as the reaction reaches completion , and in less than 10 seconds , less than 5 seconds , or less than 1 second or even less , the molten metal cools at least 1000 ° c ., at least 800 ° c ., or at least 700 ° c . ; the graphene may form as the melt cools . the graphene may be isolated by removing by - products produced during formation of the graphene . magnesium metal , other metals which may be present , oxides and carbonates , may be removed by washing with water and / or aqueous acids . carbon containing by products may be removed by sorting the material by particle size , for example by filtration , centrifugation and / or pasteur separation . repeated isolation steps , such as washing , filtration and / or centrifugation , may be performed until the desired purity is obtained . different particle sizes of graphene may also be separated , for example by filtration with one or more filters of different pore sizes , for example 500 nm pores , 400 nm pores , 350 nm pores , 300 nm pores , 250 nm pores , 200 nm pores , 150 nm pores , 100 nm pores , and / or 50 nm pores . anodized aluminum oxide filters of various sizes are commercially available , and may be easily prepared in almost any pore size . preferably , the graphene is isolated by washing with acid followed by filtration to remove any graphite which may be present . preferably , the isolated graphene contains less than 10 % by weight of non - graphene material , more preferably less than 5 % by weight of non - graphene material , including less than 4 %, less than 3 %, less than 2 %, less than 1 %, less than 0 . 5 %, less than 0 . 1 %, or even less than 0 . 05 %, by weight , of non - graphene material . single - layer graphene may be prepared from the crystalline graphene . for example , the crystalline graphene may be exfoliated by placing the material between two pieces of sticky tape , and pulling apart the two pieces of tape ; one or more layers of the crystalline graphene may stick to each piece of tape . alternatively , the graphene may be suspended in a liquid , such as water or an organic solvent , and sonicated for an extended period of time 26 . the single - layer graphene may be selected by filtration or centrifugation , for example . the graphene may be used as a solid or suspended to form a graphene dispersion ( also referred to as a graphene ink ). for example , a dispersion or suspension may be formed by mixing the graphene with water or an organic solvent and subjecting the mixture to sonication , mixing and / or milling 27 . the dispersion or suspension may then be printed or applied to a surface to form conducting wires or layers 26 . transparent conductive films may be produced from the graphene by mixing the dispersion or suspension with a polymeric material . the procedures described in ref . 26 may be used to form graphene ink . the procedures described in ref . 28 may be used to form transparent conductive films , by substituting the crystalline graphene for carbon nanotubes . in this way , electronic device elements , such as electrical connection and touch screens may be formed . the electronic device elements may be incorporated into an electronic device such as an integrated circuit , a programmable logic device , a data communications device , a clock generation device , a display ( such as a flexible display or a conductive display ), a computer , an airplane , a mobile telephone or an automobile . raman spectroscopy is considered to be an effective tool for characterization of mono or few - layer graphenes , and several theoretical and experimental studies have been reported recently . 18 - 22 raman spectrum of the nanostructured carbon species obtained during our experiments is depicted in fig4 . the two major components of the spectrum consisted of peaks at 1570 cm − 1 and 2645 cm − 1 , which are commonly designated as the g - band and the g ′- band or 2d - band respectively . in a recent study on the structure of graphene , ferrari et al . demonstrated clearly that the number of layers in a graphene structure can be revealed from the raman peaks , and thus , graphite can be easily distinguished from graphenes . 18 , 20 the position and shape of the g ′ band in the raman spectrum identify the presence and number of layers of the graphene structures respectively . with a 633 nm raman spectrum , the g ′ band peak of graphene was found at about 2645 cm − 1 , 18 which closely matches our finding as shown in fig4 . in the case of monolayer graphene , the g ′ band is a sharp single peak ; while in the case of bi - or multi - layer graphenes , there are splittings generated either from the phonon branches or the electronic bands . the g ′ band shifted more towards 2700 cm − 1 in the case of graphenes with more than 7 - 10 layers , which is indistinguishable from graphite . 18 from the observed spectrum , splitting of the 2d band ( inset of fig4 ) and its position indicate that few - layer graphene is the majority of our product . moreover , the peak intensities of the g - band and the g ′- band are also related to the number of layers of the graphene structures . gupta et al . compared the peak intensities of different layer structures of graphene and found that with the number of layers of five or more the g - band grows higher with respect to the intensity than that of the g ′- band , 19 which again was a confirmation of our product to be few - layer graphene . the other band found in the spectrum is the d - band at 1325 cm − 1 , which is of significantly lower intensity and represents some lattice defects present in the structure . 23 fig5 ( a ) and ( b ) show tem images of the few - layer graphene , prepared by our new method described above , in which graphene sheets with varying length between 50 nm and 300 nm are observed . the high - resolution tem ( fig6 ) clearly exhibits the signature image of the few - layer graphene with the number of layers ranging from 3 - 7 . the measured lattice space of this material is about 3 . 5 å , which is in good agreement with the thickness of a mono - layered graphene ( 3 . 4 å ). the inset image in fig6 , corresponding to the diffraction pattern of few - layer graphene , is indicative of the crystallization . the x - ray diffraction pattern of our bulk product is shown in fig7 . the prominent ( 002 ) peak at 26 . 3 degrees is observed along with the ( 101 ) peak at 44 . 6 degree . the other characteristic peak for graphene structure is ( 100 ) peak located at 28 of 43 . 2 degree , which is overlapped with one of mgo peaks . while we examined the purity of the product via edx spectroscopy , the absence of any impurity other than a trace amount of mg and o in the product was confirmed ( fig8 ). the trace amount of mg ( 2 . 38 atomic weight percent ) and o ( 7 . 30 atomic weight percent ) is mainly due to the trapped mgo and some absorbed o 2 . therefore , the contribution of mgo to the peak at 2θ of 43 . 2 degree is small , and this peak can be readily assigned to ( 100 ) diffraction of graphenes . although the exact mechanism of the formation of graphene is still under investigation , the high temperature generated during the burning of magnesium metal likely plays a role . it is possible that the combustion of the solid magnesium in gaseous co 2 favors the rapid flee of the solid product from the reaction center . as such , the retention time of the sp 2 carbon atoms in the reaction core may not be long enough to form graphite . instead , only few - layer graphene is kinetically favored . in conclusion , the current methodology produces few - layer graphene captured directly by igniting mg in co 2 . the structure of few - layer graphene product was confirmed by tem , raman spectroscopy and xrd and they are all consistent with the data available in the literature . the synthetic process is cost effective and can be used to produce few - layer graphene in large quantities . furthermore , the use of non - toxic chemicals and recyclable materials during the synthesis constitutes this work as part of green chemistry . mg metal of different forms and shapes was ignited in dry ice chambers covered with blocks or slabs of dry ice . specifically , the following experiments were carried out . method 1 : several strips ( about an inch long ) of magnesium metal ( 5 . 0 g ) were ignited inside a square block of dry ice . the typical dimension of a cubic hole dug into the dry ice box is approximately 2 × 2 × 2 inches . the yield is 90 % ( 1 . 1 g ) graphene materials . method 2 : the mg turnings ( 5 . 0 g ) were ignited inside a dry ice block in a procedure identical to that described above , but it was covered with another block of dry ice . the yield of graphene materials , based on consumed mg turnings , was 85 % ( 1 . 05 g ). method 3 : the granular mg turnings ( 5 . 0 g ) were ignited inside a square block of dry ice with a 4 × 4 × 4 inches of deeper and wider hole . it has been observed that the granular mg metal burns much faster than its turnings or strips to yield 83 % ( 1 . 02 g ) of the graphene product . method 4 : this method involved the magnesium coils with an ending metal wick or fuse for ignition . while 5 . 0 g of mg was ignited in this fashion , the flame lasted much longer to produce the graphene materials in higher yields ( 92 %, 1 . 16 g ) than from the methods 1 - 3 described above . method 5 : another technique was employed to ignite the magnesium metal where mg strips ( 5 . 0 g ) were placed along with dry ice shavings and its powder inside the hole that had been dug out in a block / slab of dry ice . about 1 . 10 g ( 90 %) of the few - layer graphene was isolated from this method . method 6 : in this method , the cubic holes were replaced by a series of channels that were carved on a solid slab or a long block of dry ice in which the mg strips ( 5 . 0 g , 6 - inch long ) were ignited to obtain 1 . 11 g ( 90 %) of few - layer graphene product . method 7 : in a procedure identical to that described above in method 1 , the metal strips of zinc ( zn , 5 . 40 g ) and mg ( 2 . 0 g ), in a molar ratio of 1 : 1 , were ignited inside a dry ice cube . about 450 mg of few - layer graphene ( 91 % yield ) was obtained as the final product . method 8 : in a procedure , identical to that described in method 1 , an equimolar mixture of lithium metal ( li , 1 . 13 g ) and mg metal ( 4 . 0 g ) strips were ignited inside a dry ice cube . extreme care was exercised in handling the more reactive lithium metal strips . thus , the li metal strips were cut inside a glove - box and then brought outside using argon filled vials . about 850 mg ( 86 % yield ) of few - layer graphene material was isolated from this process . method 9 ( prophetic ): this process uses a dry ice chamber with holes at both ends in order for mg ribbons protrude out as in fig1 . this facilitates the ignition of the metal from both ends and hence faster redox reaction within the chamber containing dry ice . method 10 ( prophetic ): a large - size mg spiral will be placed in a quartz - glass reaction chamber with both ends of the metal protruding through holes located at the opposite ends as shown in fig2 . after filling the chamber with powdery dry ice , the mg metal will be ignited at both ends simultaneously . for the safety purpose , the entire apparatus will be placed in a large metal container . method 11 ( prophetic ): in order to investigate the structural influence of mg metal on the formation of graphene nanosheets , several forms of nanostructured mg metal will be ignited inside the dry ice block as in method 1 . the known nanowires , nanorods and / or nanospheres of mg metal will be prepared , 17 and then used subsequently . after completion of the reduction of co 2 to carbon with concomitant oxidation of mg to mgo , the black / white solid product mixture was slowly transferred into a large flask or beaker containing 100 ml of 3 m hcl that resulted in vigorous effervescence indicating that the residual and / or contaminated mg metal and the white product mgo were reacting with hcl to produce water - soluble mgcl 2 . this mixture was stirred over a period of 3 . 0 h at room temperature during which time no more effervescence was observed and that the white turbid mixture turned black indicating that the mgo has been completely converted into mgcl 2 . at this point , the heterogeneous mixture was filtered , washed many times with 3m hcl and , finally , with de - ionized water until the ph of the filtrate turned to 7 . 0 indicating that no more washings were needed to remove the residual by - product mgo and hcl . the black residue , collected on the filter , was dried in vacuo and characterized for the presence of few - layer graphene . a unique purification technique is employed to isolate graphene nanosheets from bulk product materials . the graphene nanosheets are typically within the dimension of 50 - 300 nm . an anodized aluminum oxide ( aao ) template with a pore size of 100 nm to 300 nm was used as a filter to allow only the graphene nanosheets , while the bulk materials of larger sizes , including graphites , will remain on the filter . the experimental set up is depicted in fig3 . the process involved dispersion of about 1 gram of graphene materials in 50 ml of deionized water via ultrasonication for 30 minutes . a specially designed filter funnel is used to mount the aao disc . the discs are typically of 25 mm in diameter and can be placed on the hollow filter funnel . the funnel is fitted to a filter flask that is connected to a vacuum - 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