Patent Application: US-85663307-A

Abstract:
this invention is directed to compositions and methods of incorporating metal particles into carbon nanotube films , sheets , and networks . metal salts that are soluble in water , alcohol , polar organic solvents , and mixtures thereof are used to deposit metal particles onto carbon nanotube films , sheets , and networks . metal salts increase conductance of nanotube films by spontaneously depositing gold on the nanotube . the concentration and time of exposure to metal salt solution allows the tuning of conductivity and transparency for a transparent carbon nanotube network . metal salts added to nanotube ink add functional properties of the metal to nanotube conductors .

Description:
it was surprisingly discovered that electrochemical deposition of metals onto conductive transparent cnt films and networks is a beneficial way to improve conductivity of the cnt film while minimally affecting transparency . electrochemical deposition is controlled , at least in part , by applying a potential to the cnt network exposed to a metal salt solution . additionally , by the process of the invention , metal spontaneously deposits onto the cnt network when the electrochemical potential of the cnt network causes spontaneous precipitation or deposition from the metal salts . two specific aspects of this invention are novel and unexpected . first , prior art has demonstrated that removal of metal particle improves the conductivity of nanotube networks . metal particles typically are present in as - produced carbon nanotubes as residual catalyst . these particles are considered impurities and hinder the formation of conductive nanotube networks . additionally , these particles are composed of a high weight percent of metal oxide , which are typically non - conductive materials . therefore it was unexpected that formatting metal particles on nanotube networks would improve conductivity . it was surprisingly discovered that depositing metal particles by precipitating metal from a metal salt onto carbon nanotubes through a redux reaction significantly increased conductivity of a carbon nanotube - containing film while preserving transparency . second , it was surprising and unexpected to observe a large increase in conductivity of a cnt network with the deposition of metal particles significantly below the percolation threshold . if a greater number of particles were deposited , it is obvious that transparency would suffer significantly . it was surprisingly discovered that small particles ( less than 200 nm ) well below the percolation threshold do not significantly decrease optical transparency , but do significantly increase electrical conductivity because of a beneficial nanotube - metal interaction at the nanoscale . ref : mikhail e . itkis , ferenc borondics , aiping yu , and robert c . haddon , nano lett ., 7 ( 4 ), 900 - 904 , 2007 preferably , the metal particles deposit from salt in such a matter that their optical cross - section is about less than 200 nm on average or at least in part to not interact with visible light and thus to not affect transparency . without wishing to be bound by theory , these metal particles increase the volume conductivity of the cnt network several ways . first , the particles act to withdraw electronic density from semiconducting carbon nanotubes , making them more doped . second , metals deposit at defects and provide alternative low - energy conductive pathways for charges to pass through the film . third , metal particles bridge tube - tube junctions and reduce the resistance between tubes . fourth , the presence of metals fills the pores of the cnt network with a material that has high intrinsic volume conductivity . the volume conductivities of silver , copper , and gold are 6 . 301 × 10 5 s / cm , 5 . 96 × 10 5 s / cm , and 4 . 521 × 10 5 s / cm , respectively , which are higher than the conductivity of the cnt network . if the total amount of metal remains low , then the effect on transmission is low . this method enables cnt sheet resistance to be useful as an electrode for applications like lcd , plasma , emi shielding , solar , and oled . in a preferred embodiment , metal is deposited from solution of a metal salt onto a carbon nanotube network . examples of metal salts include transition metal salts , precious metal salts , lanthanide salts , metal salts containing chloride , metal salts containing nitrates , k 2 cr 2 o 7 , agno 3 , vcl 3 , sbcl 3 , cucl 2 , cocl 2 , nicl 2 , zncl 2 , cr ( no 3 ) 3 , fecl 3 , ticl 4 , and ai ( no 3 ) 3 , agno 3 , haucl 4 , kaucl 4 , k 2 ptcl 4 , k 2 pdcl 4 , combinations thereof . in another preferred embodiment , gold is deposited onto a nanotube network from gold salt solution . in another preferred embodiment , silver is deposited onto a nanotube network from silver salt solution . deposition of the metal is driven by the electrochemical potential of the cnt film . preferably , this electrochemical potential spontaneously drives the deposition of the metal ; more preferably , an electrochemical potential is applied to the cnt film to cause deposition of metal . preferably , the solvent is , for example , a polar solvent , a polar protic solvent , an alcohol , methanol , water , a mixture of alcohol and water , or combinations thereof . in a preferred embodiment , the metal salt solution is deposited onto the cnt film by dipping the cnt film in the metal salt solution . in another preferred embodiment , the metal salt solution is deposited onto the cnt film by wet - coating the metal salt solution onto the cnt film . in another preferred embodiment , the metal salt solution is deposited onto the cnt film by gravure printing of the metal salt solution onto the cnt film . in another preferred embodiment , the metal salt solution is deposited onto the cnt film by inkjet , bubble jet , or spraying the metal salt solution onto the cnt film . in a preferred embodiment , the metal salt solution is deposited onto the cnt film in a pattern . in another preferred embodiment , the metal salt solution is deposited onto the cnt film in a pattern to selectively improve the conductivity of the cnt network . in a preferred embodiment , metal salts are added to carbon nanotube film forming dispersions , or carbon nanotube inks . in a further preferred embodiment , metal salts are added to nanotube inks in water and alcohol mixtures . in a further preferred embodiment , the metal salt is soluble in the solvent in which the carbon nanotubes are dispersed . in a most preferred embodiment , the metal salt converts to metal in the presence of cnt , and , preferably , the metal deposits on the dispersed cnt . in a preferred embodiment , about equal weight percentages of cnt and metal from the metal salt are present in the film forming liquid . in another preferred embodiment , between about a ratio of 2 . 5 : 1 and 1 : 1 metal to nanotube is present in the film forming liquid . in another preferred embodiment , between about a ratio of 1 : 5 and 1 : 10 metal to nanotube is present in the film forming liquid . in another preferred embodiment , between about a ratio of 1 : 10 and 1 : 100 metal to nanotube is present in the film forming liquid . in one embodiment , metal salts are added to film former containing cnt and impurities , such as amorphous carbon , graphite , catalyst , damaged cnt , organic contamination , ionic contamination , or combinations thereof . in this embodiment , the metal salts selectively precipitate onto impurities and cause the density of the impurities to increase . in a further preferred embodiment , the dense impurity is removed by centrifugation and decantation of the supernatant , by filtration , by magnetic separation , or by a selective further reaction . in a preferred embodiment , the deposition of metal particles onto the cnt increases functional properties of the cnt film . preferably , the deposition of metal particles onto the cnt increases the conductivity of the cnt network , preferably by between 10 % and 50 %. preferably , the deposition of metal particles onto the cnt decreases sheet resistance of the cnt network by between 10 % and 20 %, more preferably by between 10 % and 50 %, more preferably by between 30 % and 50 %, or most preferably by between 50 % and 90 %. in preferred embodiments , the deposition of metal particles onto the cnt does not decrease broad spectrum transmittance . preferably , transmittance in the visible region is not reduced . in certain embodiments , deposition of metal particles onto the cnt decreases transmittance in the visible region by between about 5 % and 10 %, or more preferably by between 0 . 05 % and 5 %. in a preferred embodiment , the deposition of metal particles onto the cnt improves the mechanical robustness of the cnt film , such as abrasion resistance of the cnt film . in preferred embodiments , the metal particles bridge cnts and / or cnt ropes or bundles ( ropes or bundles being the preferred configuration of cnts on films in preferred embodiments of this invention so as to optimize conductivity ), to improve mechanical and electrical connectivity between the nanotubes . in one embodiment , the metal particles precipitated onto the cnt network facilitate a chemical reaction . in a preferred embodiment , the metal particles release ions that are toxic to pathogens , bacteria , or viruses . in another preferred embodiment , the metal particle surface is toxic to pathogens , bacteria , or viruses . in a further preferred embodiment , the metal particles comprise silver . in another embodiment , the metal particle acts as a catalyst to increase the rate of a chemical reaction or create a product that is not favorable without the presence of a catalyst . preferably , the metal comprises platinum or palladium . in a preferred extension of these embodiments , the catalyst or antimicrobial particles are deactivated by applying a bias to the cnt network . the particles are activated reversibly and repeatably by applying a reverse bias . in a further preferred embodiment , the application of a bias to the cnt network cleans the particles and increases their surface activity after cleaning . in a further preferred embodiment the progress of the chemical reaction is monitored through the transparent conductive cnt film by the use of spectroscopic methods , for example by infrared spectroscopy . in preferred embodiments , the deposition of metal particles onto the cnt improves sheet resistance stability , heat stability , uv stability , humidity stability , or combinations thereof , of the cnt - metal particle film . the following examples illustrate embodiments of the invention , but should not be viewed as limiting the scope of the invention . nanoparticle precipitation from salt solutions of gold , platinum , palladium , and 2 nm colloidal gold were examined in this example . the chemical reactions are : generally , cnts were spray coated from water and alcohol onto glass slides to make transparent conductive coatings . for this example and other examples , the cnts used were single walled carbon nanotubes . cnt coated slides were dipped for 0 to 60 minutes in a metal salt solution ( fig1 ). more typically , dip times were kept to less than five minutes . occasionally , the samples were rinsed after dip coating , but the rinsing process was observed to cause visible tears in the cnt film . since macroscopic tears inevitably affect sheet resistance and do not accurately portray the microscopic changes of the sample , rinsing samples was avoided in order to decrease loss of samples . un - rinsed samples retained some metal salt , which can adversely affect transparency , especially if the colored salt solution does not dry uniformly . literature reports on procedures use 1 : 1 water to ethanol mixtures , but this solvent mixture did not dry uniformly , leaving colored blotches of metal salt on the coating , thereby reducing transmittance . to avoid rinsing , dip solvents such as methanol , water and isopropanol were chosen that would not leave metal salt spots . it was surprisingly discovered that haucl 4 was soluble in a range of alcohols up to high concentrations (& gt ; 20 mm ), making it one of the more versatile metal salts . aurate salt solutions were bright yellow in all solvents . a 5 mm solution of haucl 4 was prepared in 1 : 1 water to ethanol . several cnt samples were immersed in the solution for a varying length of time , from & lt ; 1 second to 10 minutes . once dried , it was observed that the sheet resistance dropped from 15 % to 42 %. in initial experiments , the change in sheet resistance could not be correlated to changes in transparency , since the 1 : 1 water to ethanol solution did not dry evenly on the sample , leaving streaks and salt marks . also , the solution caused the cnt film to delaminate and tear for some samples upon drying . this effect clearly muted some of the r s , improvement from the aurate salt solution . additionally , the solution appeared to have an adverse effect on the mechanical properties of the silver electrodes . frequently , the electrodes would peel off the glass substrate , requiring the test to be repeated . to improve on the observed water spots , the dip solvent was changed to methanol , which dries quickly and wets evenly the cnt coated glass . also , the effect of the concentration of the salt solution on optical uniformity of the coated sample was studied . high aurate salt concentrations had shorter dip times , but caused a high degree of nonuniformity in the sample ; more gold precipitated in close proximity to the electrodes due to a different electrochemical potential close to the electrodes . lower concentration solutions gave a more uniform coating across the length of the sample . the uniformity is improved at low concentrations due to less residual metal salt remaining on the sample upon removal from the dip bath . a concentration dependence was found on the range of r s , improvement for dip times less than 5 minutes ( fig2 ). specifically , low concentrations resulted in a smaller improvement in rt performance . this result is unsurprising , since lower concentration solutions have slower reaction kinetics than high concentration solutions . the dependence of dip time on r s , improvement and on optical transparency for samples dipped in 0 . 75 mm haucl 4 solutions ( fig3 ) was evaluated . as expected , the transmittance decreased when samples were left in the dip solution for longer times . rapidly dipped samples showed an increase in transparency due to gentle cleaning or removal of contaminant from the sample , while the multiple minute dips showed a drop of up to absolute 1 . 9 % t . it appeared that the sheet resistance dropped over a fairly tight range for samples ( 15 . 6 - 17 . 7 %) dipped less than two minutes . however , longer sample dips showed r s , drops of up to 36 % at a penalty of ca . 3 . 5 % t . for longer dip times (& gt ; 5 minutes ), gold precipitation was visible to the naked eye on the film . higher solution concentrations ( 2 mm , 5 mm and 20 mm ) and longer dips did not appear to offer any significant advantage in conductivity , but considerable loss in transparency ( fig4 ) due to film damage . a 2 mm solution of aurate salt was used as a bath solution for freshly sprayed cnt samples around 500 ohms / sq . this experiment differed from the one above in two ways : the solution concentration was 2⅔ times that of the previous experiment , and the dip times were significantly increased . the data presented above shows that sheet resistance does not continue to improve after initial decreases of ca . 30 %. small tears in the film were observed upon close examination of the film after dipping . these tears undoubtedly caused an increase in sheet resistance , even if the local conductivity of the cnt film increased . as seen from the transparency measurements , gold continued to deposit onto the cnt film for an hour . the samples with heavy gold deposition appeared to be yellowish in color and hazy . the lowest obtained sheet resistance was measure to be about 160 ohms / square with a transparency of about 85 %. cnt films dipped in 5 mm haucl 4 solution in meoh for 10 seconds and for 5 minutes were examined . the two films had visibly different appearances : the 10 second dip film showed only a 0 . 6 % decrease in transparency , whereas the 5 minute dip film showed a 6 . 6 % decrease in transparency with visible non - uniformity in gold deposition ; the areas near the electrodes appeared to preferentially deposit gold . the sheet resistance of the 10 second dip fell by 26 %, whereas the 5 minute dip dropped 33 %. in terms of rt performance improvement , clearly the 10 second dip is favorable ; the 5 minute sample renormalized to a 92 . 2 % t at 500 ohms , whereas the 10 second dip renormalized to 94 . 1 % t at 500 ohms . the samples were examined using optical microscopy to determine morphology differences ( fig5 ). the morphology of the gold coatings is similar for different dip times , but presents notable differences . in both cases , the cnt film shows up as grey , with the gold particles appearing as yellow metallic , reflective quasi - circles . the arrangement of the gold particles appeared random near the center of the sample . up to several millimeters from the electrodes , some slides showed gold particles precipitating in high concentration and along preferred directions . these sites served as nucleation points for the gold particles due to incomplete cleaning of the slide surface or due to a change in the electronic structure of the cnt close to the electrode . also , examination of basked silver electrodes shows that silver sloughs off the electrode and deposit on the slide near the electrode . based on our optical microscopy work , we found that the size of particle deposited onto the nanotube increased with increasing time . as seen in fig5 b , the 10 second dip has small particles that are close to 200 nm in diameter , which is the resolution limit of the optical microscope . particles smaller than 200 nm were not seen with an optical microscope . longer dips resulted in larger particles that were found to be approximately 1 micron in diameter ( fig5 a ). these particle sizes are surprisingly much larger than those observed in the literature , despite using similar dip times and concentrations . if concentration is invariant , then larger particles arise from cnt film of embodiments of this invention having a greater electrochemical potential . the presence of submicron contaminants also serves as nucleation sites . submicron particles have been observed in our nanotube films previously . these sites serve as the dominant mechanism for loss of transmittance , whereas much smaller particles that do not interfere with light are contributing to conductivity . thus , when the particles are removed , conductivity improves dramatically , whereas transmittance will not change . frequency of particles is high but varies over a large range . qualitative analysis shows the loss of transmission of the film is not due to deposition of new particles with time . rather , the loss of transmission is more likely due to the original gold particles growing progressively larger with time . this growth mechanism has implications for the change in sheet resistance and transparency ( fig6 ). there is a three step mechanism for changes in resistance and transparency of cnt films with gold particle precipitation . the first step involves the formation of “ nanoparticle seeds ” on favorable sites on the cnt film . these sites are sidewall defects , cnt ends , amorphous carbon , nanotube sidewalls , or combinations thereof . transparency changes very little during this phase , but conductivity shows marked improvements because of local deposition of conductive additives with a diameter less than the wavelength of light . once these sites have been occupied by a gold nanoparticle , they serve as nucleation sites for further gold precipitation , leading to increased nanoparticle size . resistivity continues to drop , but not as precipitously as in step one . transparency decreases substantially , since the particle size begins to be large enough to scatter light and the optical cross section of the particles begins to be significant . the nanoparticles continue to grow until they reach the next step , where the particles become sufficiently large to create conduction pathways directly through the gold ; this stage is effectively the percolation threshold for conductivity through the colloidal particles . once the percolation threshold is met , the resistance drops substantially . these films maintain some transparency due to nonuniform particle coverage and the thin layer of deposited particles . as the particles grow , the transparency continues to decrease until the gold film is effectively a polycrystalline film . an electrochemical potential will bring the final stage to completion , depending on the metal . optical and scanning electron microscope examinations of the films showed the absence of bacteria , fungus , pathogens , microbes , or other forms of living microscopic biological matter . it was attempted to precipitate platinum and palladium nanoparticles from their respective salts . the platinate salt was difficult to work with , since it has a low solubility in water and practically no solubility in alcohol . additionally , platinate solutions with a fraction of alcohol spontaneously precipitated as platinum colloid several hours after preparation of the solution , limiting the length and number of experiments for a batch of solution . platinate salt forms dark red solutions in water , but turn to grey , metallic suspensions when left in the dark under ambient conditions . cnt slides dipped into 5 mm aqueous palatinate solution showed no change in sheet resistance or in transparency . the lack of change in conductivity and transparency indicated that no platinum precipitated onto the cnt network . longer dipping times resulted in some film peel - off . the palladate salts were soluble in methanol and stable for up to several days , forming a clear light brown solution . a cnt sample on glass was dipped in the palladate salt solution for ten seconds , which resulted in a 3 % drop in r s , and a 1 % drop in transparency . a five minute dip resulted in a 7 % drop in r s , and a 22 % drop in transparency . the resulting film was grey and metallic , indicating that palladium readily deposited onto the cnt film , but did not dramatically improve the conductivity of the film . 2 nm of colloidal gold were deposited onto cnt networks to determine if any benefit could be derived . 2 nm gold ( 1 . 5 × 10 14 particles / ml ) was purchased from ted pella and was used as an aqueous suspension or in 3 : 1 isopropanol : water . 2 nm gold differs from larger gold colloids in that it is a completely colorless suspension . it was attempted to dip coat cnt slides with 2 nm gold , but this resulted in an increase in sheet resistance of tens of ohms without a change in transparency . a drop of 2 nm gold was dried on a cnt coated slide , which also resulted in an increased sheet resistance . silver metal was deposited onto cnt networks from silver nitrate solution . in the first instance , silver nitrate in water was used as a dip solution . in one experiment , a saturated solution of agno 3 in meoh was used as a dip solution . a slide was sprayed to 285 ohms / sq , then dipped for one minute in the solution . after drying , the sheet resistance measured 236 . 7 ohms / sq , a 17 % drop . after rinsing , the sheet resistance measured 241 . 5 ohms / sq . transparency did not change during processing . like gold , silver nanoparticles precipitated spontaneously onto the nanotube network and improved conductivity . optical and scanning electron microscope examinations of the films showed the absence of bacteria , fungus , pathogens , microbes , or other forms of living microscopic biological matter . aurate salts were added to nanotube inks to observe their effects on rt performance and ink stability of stable nanotube dispersion . a 10 mm solution of haucl 4 in isopropanol and water was prepared and serially diluted to 1 mm and 0 . 1 mm haucl 4 solutions in isopropanol and water . nanotubes were purified with an acid reflux . a nanotube concentration of 30 mg / l of ink at absorbance of 1 . 0 is assumed . the specified ratio of gold to nanotube is the weight ratio of gold metal . a sufficient amount of aurate solution was added to not heavily dilute the ink . thus all samples had less than 500 ml of aurate solution added to 10 ml of ink with the exception of the 10 : 1 au : swnt sample , which had about 1 . 3 ml of aurate solution added . ink was prepared in a large batch and divided into 10 ml aliquots . each aliquot was sonicated for approximately a half a minute before spraying to disperse the ink . the samples were spray deposited on glass slides to about 500 ohms / sq . the sprayed samples had % t and rs data measured , then renormalized to 500 ohms / sq . the data is listed in the table below . all samples were examined for flocculation prior to and during spraying . all samples showed slight flocculation , which can be stabilized by controlling ph with ammonia , trimethyl amine , or by using a non - acidic gold salt . as can be seen from the table and from fig9 , the rt performance of the ink does not change from the control up to the tested value of equal weight percent of gold to nanotube . this change is in contrast to many additives , such as water soluble polymers , which cause a dramatic increase in sheet resistance at less than one percent of the nanotube weight . adding aurate salts to inks will cause gold particles to precipitate onto the nanotubes , as taught by kim et al . ( angewandte chemie interantional edition , 2006 , 45 , 104 - 107 ) and choi et al . ( journal of the american chemical society , 2002 , 124 , 9058 - 9059 ). these results show that gold precipitates on dispersed nanotubes coated in surfactant and on a surface . in the case of a dispersion without surfactant , gold deposition will occur , as well . thus , in the cases described above in this section , gold particles precipitate onto the nanotube sidewalls . if the deposition is sufficiently aggressive , then packing of nanotubes will be hindered substantially upon drying the dispersion . however , this was not observed , so the gold particles do not significantly hinder network formation below equal mass of gold and nanotube . other embodiments and uses of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein . all references cited herein , including all publications , u . s . and foreign patents and patent applications , are specifically and entirely incorporated by reference . the term comprising as used throughout this application includes the more limiting terms and phrases “ consisting essentially of ” and “ consisting .” it is intended that the specification and examples be considered exemplary only with the true scope and spirit of the invention indicated by the following claims . choi et al . 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