Patent Application
20180247722
The disclosure provides for transparent conductors comprised of metal-reduced graphene oxide or graphene core-shell nanowires, process of preparation thereof, and methods of use thereof.
Transparent conducting electrodes play an important role in many optoelectronic devices, such as displays (LCD & LED), photovoltaic devices, touch panels, and electrochromic windows. Although indium-tin-oxide (ITO) has been widely used in industry for a long time, several concerns are raised recently. For example, ITO is relatively expensive, brittle (not compatible with flexible substrates), and it shows strong absorption in the near-IR region, which is not ideal for solar cell and photodetector applications.
Disclosed herein is an innovative approach to wrap GO nanosheets around or attach to the surface of metal nanowires (NWs). In further embodiments, the obtained GO coated nanowires can be annealed under mild thermal conditions or by using plasma-based approaches, and then reduced using a graphene oxide reducing agent. Examples of graphene oxide reducing agents include, but are not limited to, chemical reducing agents (e.g., hydrazine), heating under a reducing atmosphere, and photo-thermal reduction. Using the methods disclosed herein, high quality metal-rGO core-shell NWs can be obtained. The disclosure also provides for high performance transparent conducting films comprising the core-shell nanowires disclosed herein. The conducting films described herein exhibited excellent optical and electric performance under tested conditions. Further, these conducting films were highly stable, and perform as well as ITO and silver NW thin films. The disclosure further provides for electrodes comprising the conducting films of the disclosure for a variety of electronic devices.
In a particular embodiment, the disclosure provides a method to synthesize nanowires comprising a metal nanowire core and a graphene or graphene oxide shell, comprising: adding a solution comprising metal nanowires in a first solvent to a solution comprising graphene oxide nanosheets or graphene nanoribbons in a second solvent in order to form a mixture; agitating the mixture (e.g., by using ultrasonfication) to form metal nanowires that comprise a shell or coating of graphene oxide or graphene, wherein the first solvent and the second solvent may be the same solvent or alternatively, different solvents. In a further embodiment, the first solvent is a nonpolar solvent. Examples of nonpolar solvents include, but are not limited to, toluene, pentane, cyclopentane, hexane, cyclohexane, heptane, ligroin, benzene, 1,4-dioxane, chloroform, carbon tetrachloride, diethyl ether, dichloromethane, xylene, methyl-tert-butyl ether, and any mixture thereof. In yet a further embodiment, the second solvent is a polar protic solvent and/or a polar aprotic solvent. Examples of polar aprotic solvents include, but are not limited to, tetrahydrofuran, ethyl acetate, acetone, dimethylformamide, acetonitrile, dimethyl sulfoxide, and any mixture thereof. Examples of polar protic solvents include, but are not limited to, ammonia, formic acid, n-butanol, t-butanol, n-propanol, isopropanol, nitromethane, ethanol, methanol, acetic acid, water, and any mixture thereof. In a particular embodiment, the polar protic solvent comprises an alcohol.
In a further embodiment, a method to synthesize nanowires comprising a metal nanowire core and a graphene or graphene oxide shell described herein further comprises: purifying the nanowires by: (i) dispersing the nanowires in a polar solvent; and (ii) collecting the nanowires by centrifugation; wherein steps (i) and (ii) can be repeated one or more times. Examples of polar solvents include, but are not limited to, tetrahydrofuran, ethyl acetate, acetone, dimethylformamide, acetonitrile, dimethyl sulfoxide, ammonia, formic acid, n-butanol, t-butanol, n-propanol, isopropanol, nitromethane, ethanol, methanol, acetic acid, water, and any mixture thereof. In a particular embodiment the polar solvent is isopropanol. In a further embodiment, the nanowires described herein can be collected by centrifugation or filtration.
In a certain embodiment, metal nanowires comprise diameters between 1 nm up to 1 μm are produced using the synthesis methods disclosed herein. In another embodiment, the metal nanowires disclosed herein can be comprised of silicon, germanium, copper, aluminum, tin, zinc, nickel, iron, titanium, chromium, vanadium, manganese, cobalt, silver, gold, and platinum. In a particular embodiment, the metal nanowires disclosed herein are comprised of copper. In a further embodiment, the copper nanowires have an average diameter between 2 nm to 30 nm.
In a certain embodiment, the disclosure also provides that the graphene oxide nanosheets or graphene nanoribbons disclosed herein have diameters between 2 nm to 50 nm. In a particular embodiment, the graphene oxide nanosheets have an average diameter of about 10 nm. In another embodiment, the ratio by weight of metal nanowires to graphene nanosheets or graphene nanoribbons is 1:20 to 20:1. In a further embodiment, the ratio by weight of metal nanowires to graphene nanosheets or graphene nanoribbons is 1:10 to 10:1. In yet a further embodiment, the ratio by weight of metal nanowires to graphene nanosheets or graphene nanoribbons is about 1:1.
In another embodiment, the disclosure provides a method to synthesize nanowires comprising a metal nanowire core and a graphene or graphene oxide shell described herein which further comprises: reducing the coating of graphene oxide on the nanowire to reduced graphene oxide by using a chemical, thermal, photothermal, or electrochemical reduction process.
In a particular embodiment, the disclosure further provides that the coated nanowires produced by the methods disclosed herein are characterized by having a diameter less than 50 nanometers and having a coating of graphene oxide, reduced graphene oxide, or graphene of around 1 to 10 nm, and wherein the nanowire has an aspect ratio greater than 1.
In a certain embodiment, the disclosure provides for a nanowire comprising: a core of copper that is 10 to 21 nm in diameter; and a shell of graphene oxide, reduced graphene oxide, or graphene that is 1 to 10 nm in thickness, wherein the shell is in contact along the length dimension of the copper core and wherein the nanowire has an aspect ratio greater than 1.
In another embodiment, the disclosure also provides a method to produce a conducting film of annealed nanowires, comprising: (A) forming a network of nanowires disclosed herein on a substrate; (B) annealing the network of nanowires by using plasma-based approach or by annealing at temperature between 200° C. to 300° C.; and if the coating is graphene oxide, (C) reducing the annealed network of nanowires in the presence of a graphene oxide reducing agent so as to form a conducting film comprising an annealed network of nanowires comprising a metal nanowire core and a reduced graphene oxide coating, wherein the graphene oxide reducing agent is selected from (i) a reducing atmosphere comprising hydrogen; (ii) one or more chemical agents selected from hydrazine, lithium naphthalenide, sodium naphthalenide, potassium naphthalenide, thiourea dioxide, NaHSO3, sodium borohydride, lithium aluminum hydride, thiophene, and/or ascorbic acid; and/or (iii) exposure to strong light, and wherein (B) and (C) can be performed as a single reaction step as opposed to two separate steps, when the annealing is done at temperature between 200 to 300° C. and the reducing agent is (i) or (iii). In a further embodiment, the network of nanowires is formed on a substrate by: filtering down a dispersion of nanowires onto a polytetrafluoroethylene porous membrane to from a network of nanowires; and transferring the network of nanowires from the membrane to a substrate by applying pressure to backside of the membrane and forcing intimate contact between the network of nanowires to the substrate. In another embodiment, the substrate is glass. In yet another embodiment, the network of nanowires are annealed at temperature of about 260° C. under an atmosphere comprising argon and hydrogen. In a particular embodiment, the disclosure provides for a conducting film produced by a method disclosed herein. In a further embodiment, the disclosure also provides for a transparent electrode comprising a conducting film disclosed herein. In yet a further embodiment, the disclosure further provides for an optoelectronic device comprising a transparent electrode disclosed herein. Examples of optoelectronic devices include, but are not limited to, LCD displays, a LED displays, photovoltaic devices, touch panels, solar panels, light emitting diodes (LEDs), organic light emitting diode (OLEDs), OLED displays, and electrochromic windows.
As used herein and in the appended claims, the singular forms “a,” “and,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a nanowire” includes a plurality of such nanowires and reference to “the conductor” includes reference to one or more conductors or equivalents thereof known to those skilled in the art, and so forth.
Also, the use of “or” means “and/or” unless stated otherwise. Similarly, “comprise,” “comprises,” “comprising” “include,” “includes,” and “including” are interchangeable and not intended to be limiting.
It is to be further understood that where descriptions of various embodiments use the term “comprising,” those skilled in the art would understand that in some specific instances, an embodiment can be alternatively described using language “consisting essentially of” or “consisting of.”
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although many methods and reagents similar to or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods and materials are now described.
All publications mentioned herein are incorporated herein by reference in their entirety for the purposes of describing and disclosing methodologies that might be used in connection with the description herein. With respect to any term that is presented in the publications that is similar to, or identical with, a term that has been expressly defined in this disclosure, the definition of the term as expressly provided in this disclosure will control in all respects.
Intense research efforts have been devoted to the development ITO replacements for next generation electronics. Among them, metal nanowire (NW) films hold great promise for low-cost transparent electrode applications because of their excellent electrical and optical properties, as well as their solution-processibility. As the most conductive material, silver is one of the best choices. Silver nanowires with an average diameter of ˜100 nm and an average length of ˜100 μm have been successfully synthesized. Highly transparent and conductive Ag NW based thin films have been fabricated and a small sheet resistance of ˜20 Ohms/sq with a transmittance of ˜90% (at 550 nm) were achieved, which is close to the commercial ITO substrates. Nevertheless, two problems remain. First, silver is an expensive metal and the material cost is high. Second, the diameter of the Ag NW is large (˜100 nm) and therefore, the light scattering effect is strong. The thick wires lead to large haze values and the pixels behind the transparent conductor become blurry, which is not ideal for display applications. Alternatively, copper nanowires can be good candidates for transparent conductors.
Copper has a conductivity value similar to silver; it is 20 times cheaper than silver; and the synthesis of ultra-thin Cu NWs with an average diameter as low as 17 nm has been demonstrated. Fairly good optical and electrical performance was also obtained for the Cu NW based transparent films by a number of groups. However, thin Cu NWs are intrinsically unstable under ambient conditions. Rapid surface oxidation of the Cu NWs reduces the conductivity dramatically, preventing the real application of such transparent electrodes. To improve the stability of the Cu NWs films, several approaches have been examined. For example, over-growth of a layer of Ni on Cu NWs, or coating the Cu NWs with a very thin layer of Al2O3 by atomic layer deposition, etc. Although the stability indeed improved, either the total transparency or the overall conductivity of the films decreased significantly. To maintain the high optical transparency and good electrical conductivity, coating or wrapping a very thin conformal layer of conductive and chemically stable material on the surface of the Cu NWs is desired. Recently, graphene coating on Cu NWs has been proposed to solve this issue. By studying a single wire model, it was found that the graphene coating could improve not only the stability, but also the electric and thermal conductivity of the Cu NWs. To achieve this goal, a plasma enhanced chemical vapor deposition method has been used to grow a thin layer of graphene on Cu NWs. These plasma-based procedures utilize high temperatures during the deposition (˜600° C.). Such procedures, however, are less than ideal for ultra-thin Cu NWs (e.g., see
In certain embodiments, the disclosure provides a solution-based method that is capable of producing high quality ultra-thin metal-reduced graphene oxide or graphene core-shell nanowires. By controlling the surface chemistry, graphene oxide (GO) nanosheets or graphene nanoribbons are wrapped around the Cu NWs surface to form a coating. In a particular embodiment, the GO coating is between 1 to 100 nm, 1 to 50 nm, 1 to 20 nm, 2 to 10 nm, or 3 to 5 nm in thickness.
Additionally, the disclosure also provides methods for fabricating transparent conducting films comprising the core-shell nanowires disclosed. The transparent conducting films exhibit excellent conductivity (e.g., sheet resistance=28 Ohms/sq; transparency=89% at 550 nm). Additionally, the transparent conducting films were found to be very stable, even when exposed to air.
The methods disclosed herein allow for the tuning of the core-shell nanowires to fit specific applications, e.g., the core-shell nanowire composition and/or dimensions can be varied so as to produce nanowires that are ideally suited for particular optoelectronic devices. Examples of such optoelectronic devices include but are not limited to, photovoltaics, LED displays, LED diodes, OLED displays, OLED diodes, touch displays, and electrochromatic windows. Metal nanowires for information displays and other like applications have to be very thin (<30 nm) to keep light scattering (haze) at a minimum, but not too thin to sacrifice conductivity. The very thin core-shell nanowire based films disclosed herein were unexpectedly found to have lower haze values compared to naked Cu nanowires despite having larger diameters from the reduced graphene oxide coating. Accordingly, films comprised of the ultra-thin core-shell nanowires disclosed herein are ideal for use in information display panels and other similar applications. Alternatively, films or conductors comprised of thicker core-shell nanowires are particularly suitable for photovoltaics, LED diodes and OLED diodes, due to the increased light scattering effects. The methods disclosed herein allow for the synthesis of varying sizes of nanowires, including very thin nanowires (e.g., <25 nm) to very thick nanowires (e.g., >900 nm).
It should be further understood, that the approaches described herein can not only be used to coat nanowires of varying thickness, but also can be used to coat various metal or semiconducting nanowires, such as nanowires made from transition metals (e.g., Cu, Ti, V, Cr, Mn, Fe, Co, Ni, and Zn); post transition metals (e.g., Al and Sn); precious metals (e.g., Au, Ag, Pt, and Pd); or semiconductors, such as pure elements like Si, Ge, or Ga; binary semiconducting compounds, such as compounds made from elements Groups III and V (e.g., GaAs), elements of groups II and VI, elements of groups IV and VI, and between different group IV elements (e.g, SiC); and ternary compounds, such as metal oxides and alloys.
The term “nano” in regards to a “nanowire”, “nanosheet”, or other structure, is in reference to the diameter dimension of the wire or structure, whereby a “nanowire” and “nanosheet” has a diameter from ≥1 nm to <1.0 μm.
The term “graphene” as used herein refers to a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons. Graphene is characterized by being an incredibly strong material, an excellent electrical conductor, an excellent heat conductor, very flexible, and transparent. In certain embodiments, the graphene material is in the form of graphene nanoribbons (GNRs). GNRs can be wrapped around the nanowire cores disclosed herein to form a shell. Additionally, “graphene” as used herein can be functionalized with heteroatoms (i.e., introducing impurities) to impart favorable physical and/or chemical characteristics for certain applications. Such functionalization methods, include, photoinduction (see Ju et al., Nature Nanotechnology 9:348-352 (2014)) and chemical modification (see Liu et al., J. Mater. Chem., 21:3335-3345 (2011); Georgakilas et al., Chemical Reviews 112(11):6156-6214 (2012)). Alternatively, graphene can be “doped” by altering the number of electrons surrounding atoms of graphene by using electrical signals (see Baeumer et al., Nature Communications 6:6136 (2015)).
The term “graphene oxide” or “GO” as used herein refers to a material comprised of carbon, oxygen, and hydrogen in variable ratios. In particular embodiments, for the GO material disclosed herein the C:O ratio is between 1.0 and 20.0, between 1.2 and 15.0, between 1.5 and 10.0, between 1.7 and 7.0, between 1.8 and 5.0, between 1.9 and 4.0, between 2.0 and 3.5, or between 2.1 and 2.9. Graphene oxide is obtained by treating graphite with strong oxidizers, and then exfoliating the layers of graphite oxide into flakes of graphene oxide using mechanical (e.g., sonication) or chemical means (e.g., treating with base). Graphene oxide in comparison to graphene and reduced graphene oxide is hydrophilic, and an electrical insulator. “Graphene oxide” as used herein may be functionalized with additional heteroatoms, nanoparticles, organic compounds, polymers, and biomaterials to impart favorable physical and/or chemical characteristics for certain applications. Such functionalization methods typically include chemical modification (see McGrail et al., Chem. Mater. 26(19):5806-5811 (2014)); Li et al., Chem. Mater. 27(12):4298-4310 (2015)); Navaee et al., RSC Adv. 5:59874-59880 (2015)); Zhai et al., Composites Science and Technology 77:87-94 (2013)); Chen et al., Chemical Reviews 112(11):6027-6053 (2012)).
The term “reduced graphene oxide” or “rGO” as used herein refers to reducing graphene oxide (GO) using reducing agents to produce reduced graphene oxide (rGO). “Reduced graphene oxide” generally comprises a material that can be similar or very similar to pristine graphene, depending on the graphene oxide material used and the way the reduction is achieved. In comparison to pristine graphene, rGO may comprise, depending on the reduction method, some ratio of C:O. Pristine graphene, by contrast, does not comprise oxygen atoms. In particular embodiments, for the rGO material disclosed herein the C:O ratio is between 20.0 and 90.0, between 21.0 and 80.0, between 22.0 and 70.0, between 23.0 and 60.0, between 24.0 and 50.0, between 25.0 and 40.0, or between 26.0 and 30.0. Typically, graphene oxide can be reduced to rGO using chemical (e.g., treatment with hydrazine hydrate at 100° C. for 24 hours), thermal (e.g., exposure hydrogen plasma for a few seconds; heating in distilled water at varying degrees for different lengths of time; heating in a furnace), photoreduction (e.g., exposure to strong pulse light) or electrochemical means (e.g., linear sweep voltammetry). Once rGO has been produced, it can be functionalized in a similar manner as described for graphene above.
In various embodiments, the diameter of, for example, a “rod” or “wire” is from 1 nm up to 1 μm, about 1.5-900 nm, about 2-800 nm, about 2.5-700 nm, about 3-600 nm, about 3.5-500 nm, about 4-400 nm, about 4.5-300 nm, about 5-200 nm, about 5.5-100, about 6-50 nm, about 6.5-40 nm, about 7-35 nm, about 7.5-30 nm, about 8-25 nm, about 9-24 nm, about 10-23 nm, about 12-22 nm, about 17-21 nm, about 1 nm, about 1.5 nm, about 2 nm, about 2.5 nm, about 3 nm, about 3.5 nm, about 4 nm, about 4.5 nm, about 5 nm, about 10 nm, about 15 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 21 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, or about 50 nm. The length of the “rod” or “wire” is about 50-100 nm, about 80-500 nm, about 100 nm to 1 μm, about 200 nm to 2 μm, about 300 nm to 3 μm, about 400 nm to 4 μm, about 500 to 5 μm, about 600 nm to 6 μm, about 700 nm to 7 μm, about 800 nm to 8 μm, about 900 nm to 9 μm, about 1 to 10 μm, about 2 to 15 μm, about 3 to 20 μm, or about 5 to 50 μm. Typically, for a metal nanowire disclosed herein (e.g., a Cu-nanowire), the length will be at least 100 nm.
The term “aspect ratio” refers to the ratio of a structure's length to its width. Hence, the aspect ratios of the elongated structures of the disclosure will be greater than one (i.e., length>diameter). In a particular embodiment the aspect ratio, for example, a “wire” is greater than 1, greater than 10, greater than 100, greater than 200, greater than 300, greater than 400, greater than 500, greater than 600, greater than 700, greater than 800, greater than 900, greater than 1,000, greater than 1,500, greater than 2,000, or greater than 5,000. Typically the aspect ratio for a Cu-nanowire of the disclosure will be greater than 100, greater than 200, greater than 300, greater than 400, greater than 500, greater than 600, or greater than 700.
The methods disclosed herein allow for the production of high-quality metal nanowires having various sized diameters in the nanometer range, including wires that have diameters below 50 nm. Nanowires with small diameters generate only a small scattering effect, which is beneficial for transparent conductor applications, while nanowires with larger diameters are more suited for photovoltaics, LEDs, OLEDS, etc., where a large light scattering effect is beneficial. Moreover, the metal nanowires produced by the methods disclosed herein as compared to other candidates for transparent electrodes can be comprised of relatively inexpensive materials. For example, the nanowires can be comprised of copper, which is one of the most earth-abundant metal elements with excellent electrical properties. Additionally, methods to produce the metal nanowires can be solution-based, which is readily scalable and does not require a specially designed reaction chamber with ultra-high vacuum, temperature or delicate plasma control. Moreover, the methods disclosed herein can be easily adapted to allow for size control and controlled growth for a variety of metal-based nanowires other than copper. For example, silver, gold, aluminum, zinc, nickel, tin, iron, vanadium, titanium, and platinum-based nanowires can be synthesized using the methods disclosed herein. Moreover, the methods disclosed herein are generally applicable and can also be used to produce rGO coated-semiconductor-based nanowires, such as rGO coated silicon and germanium nanowires.
For the solution-based methods disclosed herein, the synthesis reaction comprises a metal containing precursor compound, typically a metal containing salt. Any number of metal salts are compatible with the methods disclosed herein, including copper based salts, like Cu(I)I, Cu(I)Br, Cu(I)Cl, Cu(I)F, Cu(I)SCN, Cu(II)Cl2, Cu(II)Br2, Cu(II)F2, Cu(II)OH2, Cu(II)D-gluconate, Cu(II)MoO4, Cu(II) (NO3)2, Cu(II) (ClO4)2, Cu(II) P2O7, Cu(II)SeO3, Cu(II)SO4, Cu(II)tartrate, Cu(II)(BF4)2, Cu(II)(NH3)4SO4, and any hydrates of the foregoing; gold based salts, like Au(I)I, Au(I)Cl, Au(III)Cl3, HAu(III)Cl4, Au(III)Br3, HAu(III)Br4, Au(III)OH3, K(Au(III)CL4) and any hydrates of the foregoing; silver based salts, like Ag(I)BrO3, Ag2(I)CO3, Ag(I)ClO3, Ag(I)Cl, Ag2(I)CrO4, Ag(I)citrate, Ag(I)OCN, Ag(I)CN, Ag(I)cyclohexanebutyrate, Ag(I)F, Ag(II)F2, Ag(I)lactate, Ag(I)NO3, Ag(I)NO2, Ag(I)CLO4, Ag3(I)PO4, Ag(I)BF4, Ag2(I)SO4, Ag(I)SCN, and any hydrates of the foregoing; aluminum based salts, like AlI3, AlBr3, AlCl3, AlF3, Al(OH)3, Al-lactate, Al(PO3)3, AlO4P, AL2(SO4)3, and any hydrates of the foregoing; zinc based salts, like ZnI2, ZnBr2, ZnCl2, ZnF2, Zn(CN)2, ZnSiF6, ZnC2O4, Zn(ClO4)2, Zn3 (PO4)2, ZnSeO3, ZnSO4, Zn(BF4)2, and any hydrates of the foregoing; nickel based salts, like NiI2, NiBr2, NiCl2, NiF2, (NH4)2Ni(SO4)2, Ni(OCOCH3)2, NiCO3, NiSO4, NiC2O4, Ni(ClO4)2, Ni(SO3NH2)2, K2Ni(H2IO6)2, K2Ni(CN)4, and any hydrates of the foregoing; and platinum based salts, like Pt(II)Br2, Pt(II)Cl2, Pt(II)(CN)2, Pt(II)I2, Pt(II)(NH3)2Cl2, Pt(IV)Cl4, H2Pt(IV)(OH)6, H2Pt(IV)Br6, Pt(IV)(NH3)2CL4, and including any hydrates of the foregoing, (wherein (I) indicates a +1 oxidation state, (II) indicates a +2 oxidation state, (III) indicates a +3 oxidation state, and (IV) indicates a +4 oxidation state, respectively, for the metal ion).
In a particular embodiment, the solution-based methods of the disclosure utilize a reducing reagent and surface ligand(s) which selectively controls the morphology and size of the resulting metal nanowire products. In a further embodiment, the methods of the disclosure utilize a silane-based reducing agent. Examples of silane-based reducing agents include, but are not limited to, trietylsilane, trimethylsilane, triisopropylsilane, teiphenylsilane, tri-n-propylsilane, tri-n-hexylsilane, triethoxysilane, tris(trimethylsiloxy)silane, tris(trimethylsilyl)silane, di-tert-butylmethylsilane, diethylmethylsilane, diisopropylchlorosilane, dimethylchlorosilane, dimethylethoxysilane, diphenylmethylsilane, ethyldimethylsilane, ethyldichlorosilane, methyldichlorosilane, methyldiethoxysilane, octadecyldimethylsilane, phenyldimethylsilane, phenylmethylchlorosilane, 1,1,4,4-tetramethyl-1,4-disilabutane, trichlorosilane, dimethylsilane, di-tert-butylsilane, dichlorosilane, diethylsilane, diphenylsilane, phenylmethylsilane, n-hexylsilane, n-octadecylsilane, n-octylsilane, and phenylsilane. In yet a further embodiment, the methods disclosed herein utilizes a metal containing precursor compound and silane-based reducing agent at a defined molar ratio. For example, the molar ratio between the metal containing precursor compound to silane-based reducing agent is in the range of 1:100 to 100:1, 1:50 to 50:1, 1:30 to 30:1, 1:20 to 20:1, 1:10 to 10:1, 1:5 to 5:1, 1:4 to 4:1, 1:3 to 3:1, 1:2 to 2:1, 2:3 to 3:2, or about 1:1.
In another embodiment, the methods disclosed herein to produce metal nanowires comprise a surface ligand that also functions as a solvent for the synthesis reaction. Examples of surface ligands include, but are not limited to, oleylamine, trioctylphosphine oxide (TOPO), oleic acid, 1,2-hexadecanediol, trioctylphosphine (TOP), or any combination of the foregoing. Alternatively, the methods disclosed herein can comprise a surface ligand and one or more organic nonpolar solvents. Examples of organic nonpolar solvents include, but are not limited to, toluene, pentane, cyclopentane, hexane, cyclohexane, heptane, ligroin, benzene, 1,4-dioxane, chloroform, carbon tetrachloride, diethyl ether, dichloromethane, xylene, methyl-tert-butyl ether, or a mixture of any of the foregoing.
By varying the reaction conditions, such as the temperature at which the reaction takes place, the amount of starting metal precursor compound, choice of silane based reducing agent, additional solvents, etc. can all affect the structural properties, such as the diameter, length and shape, of the resulting nanowires. For example, it was found that by slowly heating and maintaining a reaction mixture at 160° C. generated Cu-nanowires that had diameters of 19±2 with an aspect ratio greater than one. By changing the reaction temperature, it could be expected that the diameters of the resulting Cu-nanowire may also change. Accordingly, the methods disclosed herein can be run at room temperature or at an elevated temperature, wherein the heating may be performed with a controlled ramp (e.g., 0.5° C., 1° C., 1.5° C., 2° C., 2.5° C., 3° C., 4° C., or 5° C. per minute). In a particular embodiment, the methods of the disclosure are performed at a temperature between about 20° C. to 360° C., about 30° C. to 300° C., about 50° C. to 250° C., about 80° C. to 220° C., about 100° C. to 200° C., about 120° C. to 180° C., or about 140° C. to 170° C. In another embodiment, the methods disclosed herein may be maintained at a set temperature or at various temperatures for a suitable period of time to allow for product formation. For example, depending upon the identity and/or concentration of starting materials, the reaction temperature, etc. the reactions may be maintained at temperature for as little as a few minutes to more than 24 hours. In a particular embodiment, the reaction may be maintained at a temperature(s) for at least 30 minutes, at least 1 hour, at least 2 hours, at least 3 hours, at least 4 hours, at least 5 hours, at least 6 hours, at least 7 hours, at least 8 hours, at least 9 hours, at least 10 hours, at least 12 hours, at least 16 hours, or at least 24 hours. In an alternate embodiment, the reaction may be maintained at a temperature(s) between 1 to 48 hours, between 1 to 24 hours, between 3 to 12 hours, between 4 to 9 hours, between 5 to 8 hours; or about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, or about 12 hours.
In certain embodiments herein, the disclosure further provides methods for the production of graphene oxide nanosheets that can then be used to coat or wrap around the nanowires in order to form a metal nanowire core and graphene oxide shell. The structure and properties of graphene oxide nanosheets depend on the particular synthesis method used and degree of oxidation. Graphene oxide is hydrophilic and easily hydrated when exposed to water vapor or immersed in liquid water or other polar solvents, e.g. alcohols. Typically, graphene oxide is made using Hummers method or Brodie's method, or variations thereof. Examples of such methods that can be used to make the graphene oxide nanosheets described herein, include those described in Sun et al. (“Large scale preparation of graphene quantum dots from graphite with tunable fluorescence properties,” Phys. Chem. Chem. Phys. 15:9907-9913 (2013)); Marcano et al. (“Improved Synthesis of Graphene Oxide”, ACS Nano, 4(8):4806-4814 (2010)); and Chen et al. (“An improved Hummers method for eco-friendly synthesis of graphene oxide”, Carbon 64:225-229 (2013)). In a particular embodiment, a method to produce graphene oxide nanosheets comprises first heating graphite at an elevated temperature (e.g., 40° C.) in the presence of strong oxidants (e.g., H2SO4 and KMnO4, and optionally NaNO3) under stirring, and then heating at much higher temperature (e.g., the reflux temperature of the solvent) under stirring, to yield graphene oxide.
In further embodiments, the disclosure provides methods to mix and wrap or coat the graphene oxide nanosheets around the metal nanowires of the disclosure. It was found that the mixing and wrapping processes can effectively occur using mild ultra-sonication in a solvent system that comprises a nonpolar organic solvent (e.g., toluene) and a polar solvent (e.g., an alcohol). Accordingly, the hydrophilic GO nanosheets can be diluted with a polar solvent and added to the metal nanowires in a nonpolar organic solvent. The ratio of the metal nanowires to graphene oxide nanosheets can be modified to tune the coverage and shell thickness of the resulting metal-GO core-shell NWs. In a particular embodiment, the ratio of graphene oxide nanosheets to metal nanowires (wt:wt) is 1:20 to 20:1, 1:15 to 15:1, 1:10 to 10:1, 1:5 to 5:1, 1:4 to 4:1, 1:3 to 3:1, 1:2 to 2:1, 1.5:1 to 1:1.5, or about 1:1. The coated nanowires can then be purified by washing (e.g., 2× isopropanol) and centrifugation.
In alternate embodiments, the disclosure also provides methods to wrap graphene nanoribbons (GNRs) around the metal nanowires of the disclosure to form a graphene shell. Examples of such methods include adding the metal nanowires to an aqueous dispersion of GNRs and agitating the mixture (e.g., sonication). The GNRs wrap around the metal nanowires via an electrostatic absorption process. The ratio of the metal nanowires to GNRs can be modified to tune the coverage and shell thickness of the resulting metal-graphene core-shell NWs. In a particular embodiment, the ratio of GNRs to metal nanowires (wt:wt) is 1:20 to 20:1, 1:15 to 15:1, 1:10 to 10:1, 1:5 to 5:1, 1:4 to 4:1, 1:3 to 3:1, 1:2 to 2:1, 1.5:1 to 1:1.5, or about 1:1. The coated nanowires can then be purified by washing and centrifugation.
In certain embodiments, the disclosure also provides for fabricating a conducting nanowire network film comprising the metal-GO or graphene core-shell NWs disclosed herein. The metal-GO or graphene core-shell NWs are diluted in a solvent and ultrasonicated to generate a homogenous suspension. The suspension is then dispersed onto a nonabsorbent porous membrane (e.g., polytetrafluoroethylene porous membrane) using vacuum filtration. The resulting GO-nanowire or graphene-nanowire network can then be transferred to a substrate, e.g., glass, an annealed at an elevated temperature (e.g., 200° C. to 260° C.). In a particular embodiment, the GO-nanowire or graphene-nanowire network is annealed at around 260° C. In a particular embodiment, the GO-nanowire network is annealed under a reducing atmosphere (e.g., 10% Hydrogen gas in argon) at temperature sufficient to thermally reduce the GO to rGO (e.g., 260° C. to 300° C.). In alternate embodiments, other methods can be employed to anneal and reduce GO, including the use of chemical agents, such as hydrazine, lithium naphthalenide, sodium naphthalenide, potassium naphthalenide, thiourea dioxide, NaHSO3, sodium borohydride, lithium aluminum hydride, thiophene, and ascorbic acid; and the use of light (i.e., photothermal reduction). In regards to the latter method, graphene oxide has been shown to be efficiently reduced using infrared irradiation, as well as light from other portions of the electromagnetic spectrum (e.g., near infrared region). The resulting annealed nanowire network film is characterized by having long term stability when exposed to air; having high percent light transmittance at 550 nm (e.g., between 80 to 99% transmittance); and having small sheet resistance (e.g., between 15 to 40 Ohms/sq).
The disclosure further provides a conducting electrode comprising the conducting nanowire network film disclosed herein. In a further embodiment, the conducting electrode is a transparent conducting electrode. In yet a further embodiment, the conducting electrode is used in optoelectronic devices, such as displays (e.g., LCD, LED and OLED), light sources (e.g., LED diodes and OLED diodes), photovoltaic devices, touch panels, and electrochromic windows. In a particular embodiment, solar cells and/or photodetectors comprise a conducting electrode disclosed herein.
The following examples are intended to illustrate but not limit the disclosure. While they are typical of those that might be used, other procedures known to those skilled in the art may alternatively be used.
Materials:
All the chemicals used herein were purchased from Sigma-Aldrich (Sigma-Aldrich, St. Louis Mo.) and used as received.
Ultra-Thin Copper Nanowire Synthesis:
CuCl2 (0.5 mmol; 85 mg) and oleylamine (5 g) were mixed in a reaction vessel. The mixture was sonicated at ambient temperature until it became clear blue solution. Upon addition of tris(trimethylsily)silane (2 mmol; 0.5 g) as a reducing regent, the reactor was slowly heated up to about 120-160° C. (2° C./min). The reaction was maintained at about 160-165° C. for 8-10 h under stirring. The color of the solution turned a light orange and further into reddish brown, indicating the formation of copper nanowires. The product was harvested by centrifugation at 6000 rpm for 5 min. The nanowires were then washed repeatedly with hexanes using centrifugation-redispersion cycles to remove excess oleylamine. The product was dispersed in toluene for further characterization and further fabrication.
Ultra-Thin Gold Nanowire Synthesis.
To synthesize a gold nanowire, HAuCl4 is used as gold source and oleylamine is added as a ligand. Triethylsilane is chosen as the reducing regent instead of tris(trimethylsily)silane given the higher activity of gold precursor. After performing the reaction at room temperature for 10 hours, Au nanowires with a mean diameter of around 3 nm are obtained
Graphene Oxide Nanosheet Synthesis:
Graphene oxide nanosheets with a diameter of ˜10 nm were synthesized using the method taught by Sun et al. (“Large scale preparation of graphene quantum dots from graphite with tunable fluorescence properties,” Phys. Chem. Chem. Phys. 15:9907-9913 (2013)).
Cu GO Core-Shell Nanowire Preparation:
A graphene oxide nanosheet aqueous solution (1 mg/mL, 0.5 mL) was diluted in 20 mL methanol. To this diluted GO solution, a Cu nanowire toluene suspension (2 mg/mL, 2.5 mL) was added under stirring. The mixture was ultrasonicated for 3 min to form the copper-graphene oxide core-shell nanowires. The nanowires were separated by centrifugation at 10000 rpm for 10 min. The nanowires were then washed twice with isopropanol using centrifugation-redispersion cycles to remove excess graphene oxide and impurities. The purified copper-graphene oxide core-shell nanowires were dispersed in 3 mL isopropanol for storage. To tune the coverage and shell thickness of the resulting core-shell nanowires, the ratio of the copper nanowires and graphene oxide can be modified.
Conducting Film Fabrication:
To make a conductive thin film, Cu nanowires were diluted using isopropanol by 100 times and ultrasonicated for 5 min to form a homogenous suspension. The thin film was constructed by filtering down the nanowires from the dispersion onto a polytetrafluoroethylene porous membrane (Sartorius Stedim Biotech, pore size 450 nm) via vacuum filtration. The nanowire network was transferred on to a piece of glass by applying pressure to the backside of the membrane and forcing an intimate contact with the substrate. Then, the copper nanowire thin film was annealed under forming gas at various temperatures for 30 min to improve junction contact.
Structural and Electrical Characterizations:
The structural properties of the core-shell nanowires were examined using transmission electron microscope (FEI TitanX 60-300), high-resolution transmission electron microscopy (FEI Tecnai G20), fourier transform infrared spectroscopy, and scanning electron microscope (SEM, JOEL JSM—6340F). Sheet resistance of nanowire thin film was measured using four-point probe method (CDE-RESMAP-270). The transmittance and haze measurement was carried out on a Shimadzu UV-2550 ultraviolet-visible near-infrared spectrophotometer with an integrating sphere.
Strategy for Graphene Oxide Wrapping Copper Nanowires and Film Fabrication.
An illustration of the overall strategy of GO wrapping and film fabrication is provided in
Structural Characterization of the Produced Copper-Graphene Oxide Core-Shell Nanowires by a Variety of Techniques.
Interestingly, after wrapping, the core-shell NWs form a very stable colloidal suspension in IPA for several days, whereas the as-synthesized Cu NWs aggregate after a few minutes in either toluene or IPA (see
Strategy to Fabricate the Core-Shell Nanowire Conducting Films on Glass Using a Filtration Method.
A dilute nanowire suspension in IPA was filtered onto a filter membrane by vacuum. The resulting film was transferred to a glass substrate by pressing the open side of the membrane onto the substrate. The films were then annealed under argon with 10% hydrogen (at 180 or 260° C.) to reduce GO and any residual native copper oxides, and to create a close contact junction between wires. A scheme demonstrating the whole process can be found in
The stability of the transparent films in air was studied to demonstrate the advantages of the GO wrapping approach. Three types of conducting film were recorded: Cu NWs (180° C. annealed), Cu GO core-shell NWs (180° C. annealed), and Cu r-GO core-shell NWs (260° C. annealed). The sheet resistance of 5 films for each type at both room temperature and 80° C. (humidity=40±10%) were recorded and the average values are summarized in
Haze is another important parameter that defines the quality of a transparent electrode. It is defined as the percentage of transmitted light that is scattered through a larger angle than a specified reference angle (e.g., 2.5°) with respect to the direction of the incident beam. It is useful for display applications, in which light scattering will reduce the sharpness of the image and result in a blurred image. The haze of the nanowire mesh conducting film highly depends on the diameter of the nanowire and a previous study showed that our ultrathin Cu NWs can be used to produce conducting thin films with very small haze. Here the Cu r-GO core-shell NW films show even lower haze values compared to the Cu NW films.
A method was used to model the transmission, haze, and sheet resistance of the core-shell nanowire meshes using Mie theory. The calculated haze versus transmission for Cu and Cu r-GO nanowires with different thicknesses of r-GO is shown in
A number of embodiments have been described herein. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. Accordingly, other embodiments are within the scope of the following claims.
This application claims priority under 35 U.S.C. § 119 from U.S. Provisional Application Ser. No. 62/219,358 filed Sep. 16, 2015, the disclosure of which is incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2016/051889 | 9/15/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62219358 | Sep 2015 | US |
