Methods of oxidizing multiwalled carbon nanotubes

Abstract

Methods of oxidizing multiwalled carbon nanotubes are provided. The multiwalled carbon nanotubes are oxidized by contacting the carbon nanotubes with gas-phase oxidizing agents such as CO 2 , O 2 , steam, N 2 O, NO, NO 2 , O 3 , and ClO 2 . Near critical and supercritical water can also be used as oxidizing agents. The multiwalled carbon nanotubes oxidized according to methods of the invention can be used to prepare rigid porous structures which can be utilized to form electrodes for fabrication of improved electrochemical capacitors.

1 . A method of oxidizing multiwalled carbon nanotubes having a diameter no greater than 1 micron, said method comprising contacting said multiwalled carbon nanotubes with a gas-phase oxidizing agent under conditions sufficient to form oxidized nanotubes.  
     
     
         2 . The method of  claim 1 , wherein said oxidation occurs on the exterior side walls of said multiwalled carbon nanotubes.  
     
     
         3 . The method of  claim 1 , wherein said oxidation occurs on the surface of said multiwalled carbon nanotubes.  
     
     
         4 . The method of  claim 1 , wherein said diameter of said carbon nanotubes is from 2 to 100 nanometers.  
     
     
         5 . The method of  claim 1 , wherein said diameter of said carbon nanotubes is from 3.5 to 75 nanometers.  
     
     
         6 . The method of  claim 1 , wherein said multiwalled carbon nanotubes include at least a plurality of graphitic layers that are substantially parallel to the axis of said nanotubes.  
     
     
         7 . The method of  claim 1 , wherein said multiwalled carbon nanotubes are substantially cylindrical, graphitic nanotubes having a length to diameter ratio of greater than 5 and a diameter of less than 0.1 micron.  
     
     
         8 . The method of  claim 1 , wherein said multiwalled carbon nanotubes are substantially cylindrical, free of a continuous pyrolitically deposited carbon overcoat, the projection of the graphite layers on said nanotubes extending for a distance of at least two nanotube diameters.  
     
     
         9 . The method of  claim 1 , wherein said multiwalled carbon nanotube is a fishbone fibril.  
     
     
         10 . The method of  claim 1 , wherein said multiwalled carbon nanotubes are grown on supported catalysts.  
     
     
         11 . The method of  claim 1 , wherein said oxidized nanotubes comprise moieties selected from the group consisting of carbonyl, carboxyl, aldehyde, phenolic, hydroxy, esters, lactones and derivatives thereof.  
     
     
         12 . The method of  claim 1 , wherein said oxidized nanotubes exhibit upon titration an acid titer of from 0.05 to about 0.6 meq/g.  
     
     
         13 . The method of  claim 1 , wherein said oxidized nanotubes exhibit upon titration an acid titer from 0.1 to 0.4 meq/g.  
     
     
         14 . The method of  claim 1 , wherein said oxidized carbon nanotubes exhibit a weight loss of from 1% to 60% by comparison to unoxidized carbon nanotubes.  
     
     
         15 . The method of  claim 1 , wherein said oxidized nanotubes exhibit a weight loss from 2% to 15% by comparison with said unoxidized carbon nanotube.  
     
     
         16 . The method of  claim 1 , wherein said gas-phase oxidizing agent is selected from the group consisting of CO 2 , O 2 , steam, N 2 O, NO, NO 2  ,O 3 , ClO 2  and mixtures thereof.  
     
     
         17 . The method of  claim 1 , wherein said gas-phase oxidizing agent is diluted with an inert diluant selected from the group consisting of nitrogen, noble gases and mixtures thereof.  
     
     
         18 . The method of  claim 1 , wherein said gas-phase oxidizing agent is near critical or supercritical water.  
     
     
         19 . The method of  claim 1 , wherein said oxidizing of said multiwalled carbon nanotubes with said gas-phase oxidizing agent is performed for a period of time from about 0.1 hours to about 24 hours.  
     
     
         20 . The method of  claim 1 , wherein said oxidizing of said multiwalled carbon nanotubes with said gas-phase oxidizing agent is performed for a period of time from about 1 hour to about 8 hours.  
     
     
         21 . The method of  claim 1 , wherein said oxidizing of said multiwalled carbon nanotubes with said gas-phase oxidizing agent is performed in a temperature range from about 200° C. to about 600° C. and in a range of partial pressure of said oxidizing agent from about 1 torr to about 7600 torr whenever said gas-phase oxidizing agent is selected from the group consisting of O 2 ,O 3 , N 2 O, NO, NO 2 , ClO 2  and mixtures thereof.  
     
     
         22 . The method of  claim 21 , wherein the partial pressure range of the gas-phase oxidizing agent is from 5 torr to 760 torr.  
     
     
         23 . The method of  claim 1 , wherein said oxidizing of said multiwalled carbon nanotubes with said gas-phase oxidizing agent is performed in a temperature range from about 400° C. to about 900° C. and in a range of partial pressure of the oxidizing agent from about 1 torr to about 7600 torr whenever said gas-phase oxidizing agent is CO 2  or steam.  
     
     
         24 . The method of  claim 23 , wherein the partial pressure range of the gas-phase oxidizing agent is from 5 torr to 760 torr.  
     
     
         25 . The method of  claim 1 , further comprising a secondary treatment step of said oxidized nanotubes with a reactant suitable to react with moieties of said oxidized nanotubes thereby adding at least a secondary group onto the surface of said oxidized nanotubes.  
     
     
         26 . The method of  claim 25 , wherein said additional secondary group is selected from the group consisting of an alkyl or aryl silane wherein said alkyl has C 1  to C 18 , said aryl has C 1  to C 18,  an alkyl of C 1  to C 18  or an aralkyl group of C 1  to C 18 , a hydroxyl group of C 1  to C 18  and an amine group of C 1  to C 18 .  
     
     
         27 . The method of  claim 25 , wherein said additional secondary group is a fluorocarbon.  
     
     
         28 . The method of  claim 1  further comprising dispersing said surface-oxidized nanotubes into a liquid medium.  
     
     
         29 . The method of  claim 28 , wherein after being dispersed in said liquid medium, said oxidized nanotubes are filtered and dried to form a mat.  
     
     
         30 . The method of  claim 29 , further comprising heating said mat from 200° C. to 900° C.  
     
     
         31 . The method of  claim 29 , further comprising forming said mat into an electrode.  
     
     
         32 . A method for producing a network of carbon nanotubes comprising the steps of: 
 (a) oxidizing said carbon nanotubes with a gas-phase oxidizing agent under conditions sufficient to form oxidized nanotubes;    (b) subjecting said oxidized nanotubes to conditions sufficient to cause crosslinking.    
     
     
         33 . The method of  claim 32 , wherein said conditions include heating said oxidized nanotubes in air in a temperature range from 200° C. to 600° C.  
     
     
         34 . The method of  claim 29 , wherein said conditions include heating said oxidized nanotubes in an inert atmosphere in a temperature range from 200° C. to 2000° C.  
     
     
         35 . A method for producing a network of oxidized carbon nanotubes comprising the steps of: 
 (a) oxidizing said carbon nanotubes with a gas-phase oxidizing agent under conditions sufficient to form oxidized nanotubes;    (b) treating said oxidized nanotubes with a reactant suitable to react with moieties of said oxidized nanotubes thereby adding at least a secondary group onto the surface of said oxidized nanotubes;    (c) further contacting said nanotubes bearing secondary groups with an effective amount of crosslinking agent.    
     
     
         36 . The method of  claim 32 , wherein said gas phase oxidizing agent is selected from the group consisting of CO 2 , O 2 , steam, N 2 O, NO, NO 2 , O 3 , ClO 2  and mixtures thereof.  
     
     
         37 . The method of  claim 35 , wherein said crosslinking agent is selected from the group consisting of a polyol or polyamine.  
     
     
         38 . The method of  claim 37 , wherein said polyol is a diol and said polyamine is a diamine.  
     
     
         39 . The method of  claim 32 , wherein said oxidized nanotubes comprise moieties selected from the group consisting of carbonyl, carboxyl, aldehyde, ketone, hydroxy, phenolic, esters, lactones and derivatives thereof.  
     
     
         40 . A method of treating aggregates of carbon nanotubes which comprises contacting said aggregates with an gas-phase oxidizing agent under conditions sufficient to oxidize said carbon nanotubes.  
     
     
         41 . The method of  claim 40 , wherein said aggregates have a macromorphology resembling a shape selected from the group consisting of bird nests, combed yarn and open net aggregates.  
     
     
         42 . The method of  claim 40 , wherein said aggregate particles have an average diameter of less than 50 microns.  
     
     
         43 . The method of  claim 40 , wherein said carbon nanotubes are substantially cylindrical with a substantially constant diameter are multiwalled having graphitic layers concentric with the nanotube axis and are substantially free of pyrolitically deposited carbon.  
     
     
         44 . The method of  claim 40 , wherein said carbon nanotubes are a fishbone fibril.  
     
     
         45 . The method of  claim 40 , wherein said treated aggregates resemble a weathered rope.  
     
     
         46 . A method for preparing a rigid porous structure comprising the steps of: 
 (a) oxidizing a multiplicity of multiwalled carbon nanotubes according to the method of  claim 1  to oxidized nanotubes;    (b) dispersing said oxidized nanotubes in a medium to form a suspension;    (c) separating said medium from said suspension to form a porous structure of entangled oxidized nanotubes wherein said nanotubes are interconnected to form a rigid porous structures.    
     
     
         47 . The method of  claim 46 , wherein said multiwalled carbon nanotubes are uniformly and evenly distributed throughout said structure.  
     
     
         48 . The method of  claim 46 , wherein said carbon nanotubes are in the form of aggregate particles selected from the groups consisting of aggregate particles resembling a shape selected from the group consisting of bird nest, combed yarn and open net.  
     
     
         49 . The method of  claim 46 , wherein said oxidized nanotubes are in the form of aggregate particles resembling a weathered rope.  
     
     
         50 . The method of  claim 46 , further comprising heating said suspension in air to a temperature in a range from about 200° C. to about 600° C. thereby forming said rigid porous structure.  
     
     
         51 . The method of  claim 46 , further comprising heating said suspension in an inert gas to a temperature in a range from about 200° C. to about 2000° C. therebyforming said rigid porous structure.  
     
     
         52 . The method of  claim 46 , wherein said medium is water or organic solvents.  
     
     
         53 . The method of  claim 46 , wherein said medium comprises a dispersant selected from the group consisting of alcohols, glycerin, surfactants, polyethylene glycol, polyethylene imines and polypropylene glycol.  
     
     
         54 . The method of  claim 46 , wherein said suspension further comprises gluing agents selected from the group consisting of cellulose, carbohydrate, polyethylene, polystyrene, nylon, polyurethane, polyester, polyamides and phenolic resins.  
     
     
         55 . The method of  claim 46 , further comprising the steps of: 
 (a) forming said rigid porous structure into a mat; and    (b) forming said mat into an electrode.    
     
     
         56 . An electrochemical capacitor having at least one electrode comprising the oxidized carbon nanotubes prepared by the method of  claim 1 .  
     
     
         57 . An electrochemical capacitor having at least one electrode prepared by a method which comprises the following steps: 
 (a) contacting aggregates of carbon nanotubes with a gas-phase oxidizing agent under conditions sufficient to oxidize said carbon nanotubes;    (b) dispersing said aggregates of oxidized nanotubes prepared in step (a) in a liquid medium to form a slurry;    (c) filtering and drying said slurry to form a mat of oxidized carbon nanotubes;    (d) subjecting said mat to conditions sufficient to cause the crosslinking of said oxidized carbon nanotubes.    
     
     
         58 . The electrochemical capacitor of  claim 57 , wherein said conditions of step (d) include heating said mat from 180° C. to 350° C.  
     
     
         59 . An electrochemical capacitor having at least one electrode formed by a method comprising the following steps: 
 (a) dispersing aggregates of carbon nanotubes in a liquid medium to form a slurry;    (b) filtering and drying said slurry to form a mat of carbon nanotubes;    (c) treating said mat according to the method of  claim 1  under conditions sufficient to oxidize said carbon nanotubes.    
     
     
         60 . The capacitor of  claim 55 , wherein said gas-phase oxidizing agent is selected from the group consisting of CO 2 , O 2 , steam, N 2 O, NO, NO 2 , O 3 , ClO 2  and mixtures thereof.