TRANSPARENT CONDUCTING FILMS CONTAINING SINGLE-WALLED CARBON NANOTUBES DISPERSED IN AN AZO DYE
Described are carbon nanotube dispersions containing single-walled carbon nanotubes dispersed in a dispersant solution comprising a solvent (water, organic polar protic solvents, and/or organic polar aprotic solvents), and an azo compound. The single-walled carbon nanotubes are not cross-linked with covalent bonds. The dispersions are useful for fabricating transparent conductive thin films on flexible and inflexible substrates. Methods for making the transparent conductive thin films are also described.
This invention was made with government support under 14-CRHF-0-6055 awarded by the USDA/NIFA. The government has certain rights in the invention.
FIELD OF THE INVENTIONDescribed herein are flexible, transparent conducting thin film comprising carbon nanotubes dispersed in an azo compound such as Congo red. Also described herein are methods to make these flexible thin films.
BACKGROUNDThe demand for high quality transparent conductive films (TCFs) is very high because of their use in flat panel displays, electrochromic windows, photovoltaic solar cells, hand-held devices, energy technologies, and biosensors [1]. Indium tin oxide (ITO) film-coated transparent conducting materials (TCM) are commonly employed in industrial applications where TCFs are needed. Typically, however, ITO thin films are applied to glass substrates. A glass substrate, of course, is inflexible and easily broken. Additionally, casting an ITO film of uniform thickness onto a curved glass substrate is quite difficult and expensive. ITO-based TCMs are widely used, however, because they afford relatively low sheet resistance (˜20Ω/□) and fairly high transmittance (>80%) in the visible region of the solar spectrum[2]. In other words, TCFs having high conductivity and high transmittance in the visible spectrum are very desirable. However, due to increasing demand and limited availability, the cost of indium is on the rise [2]. ITO thin films can be formed on flexible substrates. However, ITO-based materials become brittle after only a few bending cycles; thus they are unsuitable for high flexibility applications.[3] As a consequence, many other materials, such as single-walled carbon nanotubes (SWNTs) [4-6] and their metal hybrids have been investigated as potential alternatives to ITO [7-9]. See also, for example, U.S. Pat. No. 8,697,180, issued Apr. 15, 2014, to Veerasamy; U.S. Pat. No. 8,048,490, issued Nov. 1, 2011, to Watanabe et al.; and U.S. Pat. No. 7,411,085, issued Aug. 12, 2008, to Hirakata et al.
As-synthesized SWNTs are highly hydrophobic bundles, which must be dispersed in a surfactant solution to take advantage of their many unique properties [10]. Methods to fabricate carbon SWNTs are well known in the art and will not be discussed herein. Carbon SWNTs can also be purchased from a large number of international commercial suppliers, such as Nanocyl s.a. (Sabreville, Belgium) and Nanostructured & Amorphous Materials, Inc. (Houston, Tex.). Dispersions of SWNTs can be formed into TCFs by spin-coating [11], dip-coating [12, 13], spray-coating [14], bar-coating [15] and by vacuum filtration [16, 17]. However, commonly known surfactants tend to stick to the SWNTs firmly, which significantly increases the inter-tube contact resistance. Removing any adhering surfactant by thermal annealing at high temperature is a prerequisite to reducing the sheet resistivity of SWNT films. However, annealing of SWNT films prepared on flexible substrates such as polyethylene terephthalate (PET) is not feasible. Thus, in conventional methods, the SWNT films are first prepared on a solid substrate that can withstand the heat off annealing. The film is then annealed on the solid substrate. After annealing, the film is then transferred from the solid substrate onto a flexible substrate. In short, preparing a flexible SWNT film with low resistivity and high transparency via conventional means is a challenging and laborious task.
There is thus a long-felt and unmet need for new methods or technologies to facilitate using thin flexible films SWNTs in various applications without the aforementioned laborious fabrication techniques.
SUMMARYSingle-walled carbon nanotubes (SWNTs) are known for their high conductivity, mechanical strength, transparency, and flexibility. These superior properties make SWNTs a potential candidate for use in next generation transparent electrodes. These types of electrodes are highly sought after for visual displays, touch screens, and energy-harvesting applications. However, to exploit the potential of SWNTs, they must be dispersed, i.e., debundled, in a surfactant solution for further processing and use. Disclosed and claimed herein is a novel method to prepare transparent conducting films (TCF) comprising SWNTs using an azo dye in water and/or an organic polar protic or polar aprotic solvent as the dispersant. The resulting TCF is not covalently cross-linked. The preferred azo dispersant is 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid), commonly known as Congo red (CR). It was found that SWNTs can be effectively dispersed in water by adding CR, which forms a stable dispersion (presumably by non-covalent adsorption). Using CR-SWNTs dispersion, SWNTs-based TCFs of uniform thickness were prepared (˜20 nm) on rigid glass substrates and flexible polyethylene terephthalate (PET), and polydimethylsiloxane (PDMS) substrates by drop-coating. Spin-coating and other methods can also be used. UV-VIS-NIR spectroscopy (UV-VIS-NIR), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and dynamic light scattering (DLS) measurements were employed to characterize the CR-SWNTs dispersion and the TCFs. The sheet resistance of the TCFs thus prepared was about 300 ohms per square (i.e., 300“Ω/” or 300 “Ω/sq”) on all of glass, flexible PET and PDMS substrates. Transmittance of the films on all substrates tested was about 82%, which is better than the transmittance of TCFs formed by dispersing SWNTs in the common surfactants sodium dodecyl sulfate (SDS), sodium cholate (SC), and polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether-type nonionic surfactants (e.g., Triton™ X-100-brand surfactant). (“Triton” is a common law trademark of the Dow Chemical Company.) It is notable that TCFs are not formed using SWNTs dispersed in any of SDS, SC, or Triton X-100-brand surfactant by drop-coating.
The SWNTs-CR TCFs disclosed herein are useful for various indications such as in flexible electronics, indium-free electrodes, bio-sensing substrates, photovoltaic panels, and the like. In addition, the color and conductivity of SWNTs-CR film are dependent on the pH of the contact solution. This novel property can be exploited for rapid and visible detection of certain pathogens, toxins, and other analytes in biological, pharmaceutical, environmental, and food samples by integrating SWNT-CR TCFs with field-effect transistors.
Thus, disclosed herein is a method for preparing SWNT-based TCFs by using an azo dye such as 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid) (Congo red, CR) as a dispersant in aqueous solution and/or a solution comprising an organic polar protic or polar aprotic solvent. CR, a symmetrical linear molecule, is both soluble in water as well as in organic solvents (dimethylformamide (DMF) and ethanol). See
Further disclosed herein is a carbon nanotube dispersion comprising single-walled carbon nanotubes dispersed in a dispersant solution. The dispersant solution comprises a solvent comprising water, organic polar protic solvents, and/or organic polar aprotic solvents, and an azo compound comprising at least one azo moiety. The single-walled carbon nanotubes are not covalently cross-linked. The azo compound may optionally comprise at least two azo moieties. The azo compound may also optionally comprise at least one sulfonic acid moiety or at least two sulfonic acid moieties. Preferred azo compounds are selected from the group consisting of:
wherein each R is identical or different and each R is independently selected from the group consisting of amino-substituted phenyl and amino-substituted naphthyl, Optionally, at least one of the R substituents is further substituted with at least one sulfonic acid moiety. The most preferred azo compound is 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid (Congo red).
Also disclosed herein are flexible, transparent, conductive film fabricated from a carbon nanotube dispersion as disclosed herein.
Additionally disclosed herein is a method of making a flexible, transparent, conductive film. The method comprises providing a dispersion of single-walled carbon nanotubes in a solution comprising a solvent selected from the group consisting of water, organic polar protic solvents, and/or organic polar aprotic solvents, and an azo compound as described in the preceding paragraphs. The dispersion is then contacted on a substrate, and the solution is removed, typically by gentle heating. This yields a flexible, transparent, conductive film formed on the substrate. The single-walled carbon nanotubes are not cross-linked with covalent bonds.
Numerical ranges as used herein are intended to include every number and subset of numbers contained within that range, whether specifically disclosed or not. Further, these numerical ranges should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 2 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and so forth.
All references to singular characteristics or limitations disclosed herein shall include the corresponding plural characteristic or limitation, and vice-versa, unless otherwise specified or clearly implied to the contrary by the context in which the reference is made.
All combinations of method or process steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.
The methods disclosed herein can comprise, consist of, or consist essentially of the essential elements and limitations of the method described herein, as well as any additional or optional ingredients, components, or limitations described herein or otherwise useful in synthetic organic chemistry.
CR=Congo red, i.e., 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid. DLS=dynamic light scattering. DMF=dimethylformamide. DMSO=dimethylsulfoxide. FT-IR=Fourier transform infrared spectroscopy. HMPA=hexamethylphosphoramide. ITO=indium tin oxide. PDMS=polydimethylsiloxane. PET=polyethylene terephthalate. SC=sodium cholate hydrate. SDS=sodium dodecyl sulfate. SEM=scanning electron microscopy, SWNT=single-walled carbon nanotube. TCF=transparent conductive film. TCM=transparent conducting material. TEM=transmission electron microscopy. THF=tetrahydrofuran. Triton X-100-brand surfactants=polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether-type surfactants, CAS No. 9002-93-1. UV-VIS-NIR=Ultra violet-visible-near infrared spectroscopy. “Ω/□”=ohms per square unit, a measure of sheet resistance (as contrasted to bulk resistance).
“Polar protic solvent,” explicitly includes, but is not limited to: methanol, ethanol, isopropanol, n-butanol, nitromethane, formic acid, acetic acid, and the like.
“Polar aprotic solvent,” explicitly includes, but is not limited to: acetone, acetonitrile, DMF, DMSO, ethyl acetate, HMPA, THF, and the like. Polar aprotic solvents lack an acidic hydrogen group and generally have a dipole moment of about 1.8 D or larger. The preferred polar aprotic solvents for use in the present disclosure have a dipole moment greater than about 2.8 D.
“Substrate” as used herein is to be interpreted broadly to include any suitably robust, flexible or inflexible panel, sheet, rod, bead, particle, etc. Glass substrates (planar slides or curved surface, bulk or particulate) are included in the definition, along with polymeric substrates (flexible or inflexible).
Flexible, Single-Walled Carbon Nanotube Films:The steps to prepare CR-SWNT films as disclosed herein are depicted schematically in
An attempt was made to disperse SWNTs in DMF and CR/DMF solution because CR is soluble in DMF (as well as in other organic polar protic and polar aprotic solvents). A stable dispersion of CR-SWNT in DMF was obtained with high concentration of SWNTs. However, SWNT were also dispersable in DMF alone with low amount of SWNTs. An attempt was also made to prepare SWNT film on polydimethylsiloxane (PDMS) film by drop-coating. CR-SWNT-DMF dispersion was uniformly dried without any aggregation on the PDMS film after the drying process at 60° C. See
The consistencies of the CR solution and CR-SWNTs dispersion were gel-like compared to the SWNTs dispersions in SDS and Triton-X-100-brand surfactant, which were liquid-like. This behavior was clearly observed by inverting the centrifuge tubes containing those dispersions. See
The SWNTs films were further characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM), which shows dense and uniform network of SWNTs forming 20-nm thick film on the substrate. Representative SEM photomicrographs at different resolutions are shown in
To elucidate why CR-SWNTs dispersions yield a uniform nanotube network film on glass, PET and PDMS substrates without any surface treatment, the surface tension of the dispersions was analyzed. The meniscuses of water and CR-SWNTs were similar to each other (compare
CR molecules could react or interact with the nanotubes by means of π-π-stacking interactions between aromatic moieties of CR and graphene. See the resonance structures of CR shown in
The distribution of the hydrodynamic radius (size) distribution of nanotubes in different surfactants (see
The interaction between CR and SWNTs was also confirmed by FTIR. The IR spectra of pristine SWNTs are depicted in
SWNTs films were prepared on PET substrates by spin-coating of CR-SWNTs dispersion and washing the treated substrates with DMF and ethanol. The films were transparent and conductive with a sheet resistance of 600Ω/□ at 82% transmittance. See
These results reveal that the sheet resistivity and transparency properties of the SWNTs films disclosed herein are as good as or better than those obtained via other reported methods. Moreover, the disclosed method using CR as surfactant is easily implemented, inexpensive, and the film can be cast on any rigid or flexible substrate without surface treatment by either drop-coating or spin-coating. Superhydrophobicity, high flexibility, and low cost of production of the CR-SWNTs films make them well-suited for electronic and indium-free electrode applications.
ExamplesThe following examples are included solely to provide a more complete description of the films and methods disclosed herein. The examples are not intended to limit the scope of the claims in any fashion.
P2-SWNTs were purchased from Carbon Solutions, Inc (Riverside, Calif., USA). CR, sodium dodecyl sulfate (SDS), sodium cholate hydrate (SC), Triton™ X-100-brand surfactant, ethanol and dimethylformamide (DMF) were received from Sigma-Aldrich (St. Louis, Mo., USA). All other reagents were of analytical grade and used without further purification. Deionized water with resistivity of 18 MΩcm was used.
Preparation of CR-SWNT Dispersion:P2-SWNTs (5 mg) were mixed in CR solution (10 mL, 1 mM) and then bath sonicated for 10 min. Subsequently, CR-SWNT mixture was ultrasonicated with a 130-W ultrasonic processor for one hour. Finally, CR-SWNTs dispersion was transferred into centrifuge tubes and centrifuged at 12,000 rpm for one hour. After centrifugation, the top supernatant solution was collected and stored for characterization and applications studies. For comparison studies, pristine P2-SWNTs were similarly dispersed in SDS, SC and Triton X-100-brand surfactant dispersions and supernatant SWNTs were collected after centrifugation.
SWNTs-Based TCF Preparation:SWNTs films were prepared on glass substrate or polyethylene terephthalate (PET) film by spin-coating or drop-coating of CR-SWNTs dispersion. SWNT-coated substrate was dried at 60° C. for one hour and then washed with DMF and ethanol multiple times. Finally, SWNT-coated substrates were dried at 60° C. for several hours. For drop-coating, CR-SWNTs dispersion or SDS-SWNTs or Triton X-100-brand-SWNTs dispersion was uniformly spread onto the substrate and dried at 60° C. in an air oven. All other steps were similar to that followed for spin-coating method.
Characterization of CR-SWNTs:SWNTs films were characterized using a scanning electron microscope (SEM) (Leo 1530 Field Emission SEM), LabRAMAramis Horiba JobinYvon Confocal Raman Microscope, and Bruker AFM microscopes. Dynamic light scattering (DLS) analysis were performed using 90 Plus Particle size analyzer (Brookhaven Instruments). PerkinElmer UV/vis spectrophotometer (Lambda 25) and PerkinElmer Spectrum 100 FT-IR spectrometer (with Universal ATR Sampling accessory) were used for characterizing SWNTs. SCS Speciality Coating Systems (6800 Spin Coater Series) was used to spin-coat nanotubes. CR-SWNTs dispersion was centrifuged using Eppendorf centrifuge model no. 5415C. Contact angle measurements were done using a Dataphysics OCA 15 Optical Contact Angle Measuring System. The van der Pauw method was employed to measure resistivity of the SWNTs films using four point-probe measurement system. (van der Pauw, L. J. (1958) “A method of measuring the resistivity and Hall coefficient on lamellae of arbitrary shape,” Philips Technical Review 20:220-224.)
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Claims
1. A carbon nanotube dispersion comprising:
- single-walled carbon nanotubes dispersed in a dispersant solution comprising a solvent selected from the group consisting of water, organic polar protic solvents, and organic polar aprotic solvents, and an azo compound comprising at least one azo moiety, wherein the single-walled carbon nanotubes are not covalently cross-linked.
2. The carbon nanotube dispersion of claim 1, wherein the azo compound comprises at least two azo moieties.
3. The carbon nanotube dispersion of claim 1, wherein the azo compound comprises at least one sulfonic acid moiety.
4. The carbon nanotube dispersion of claim 3, wherein the azo compound comprises at least two azo moieties
5. The carbon nanotube dispersion of claim 1, wherein the azo compound comprises at least two sulfonic acid moieties.
6. The carbon nanotube dispersion of claim 5, wherein the azo compound comprises at least two azo moieties
7. The carbon nanotube dispersion of claim 1, wherein the azo compound is selected from the group consisting of: wherein each R is identical or different and each R is independently selected from the group consisting of amino-substituted phenyl and amino-substituted naphthyl,
8. The carbon nanotube dispersion of claim 7, wherein at least one R is further substituted with at least one sulfonic acid moiety.
9. The carbon nanotube dispersion of claim 1, wherein the azo compound is 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid (Congo red).
10. A flexible, transparent, conductive film comprising a carbon nanotube dispersion as recited in claim 1.
11. A method of making a flexible, transparent, conductive film, the method comprising:
- providing a dispersion of single-walled carbon nanotubes in a solution comprising a solvent selected from the group consisting of water, organic polar protic solvents, and organic polar aprotic solvents, and an azo compound comprising at least one azo moiety;
- contacting the dispersion on a substrate, and then
- removing the solution;
- wherein a flexible, transparent, conductive film is formed on the substrate, and wherein wherein the single-walled carbon nanotubes are not covalently cross-linked.
12. The method of claim 11, comprising contacting the dispersion on a flexible substrate.
13. The method of claim 11, comprising contacting the dispersion on a flexible substrate selected from the group consisting of PET and PDMS.
14. The method of claim 11, comprising contacting the dispersion on a glass substrate.
15. The method of claim 11, wherein the azo compound comprises at least two azo moieties.
16. The method of claim 11, wherein the azo compound comprises at least one sulfonic acid moiety.
17. The method of claim 11, wherein the azo compound comprises at least two sulfonic acid moieties.
18. The method of claim 11, wherein the azo compound is selected from the group consisting of: wherein each R is identical or different and each R is independently selected from the group consisting of amino-substituted phenyl and amino-substituted naphthyl,
19. The method of claim 16, wherein at least one R is further substituted with at least one sulfonic acid moiety.
20. The method of claim 11, wherein the azo compound is 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid (Congo red).
Type: Application
Filed: May 1, 2015
Publication Date: Nov 5, 2015
Inventors: Sundaram Gunasekaran (Madison, WI), Ashok Kumar Sundramoorthy (Madison, WI)
Application Number: 14/701,599