US20100173070A1 - Porous Substrates, Articles, Systems and Compositions Comprising Nanofibers and Methods of Their Use and Production - Google Patents

Porous Substrates, Articles, Systems and Compositions Comprising Nanofibers and Methods of Their Use and Production Download PDF

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US20100173070A1
US20100173070A1 US12/715,126 US71512610A US2010173070A1 US 20100173070 A1 US20100173070 A1 US 20100173070A1 US 71512610 A US71512610 A US 71512610A US 2010173070 A1 US2010173070 A1 US 2010173070A1
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Prior art keywords
nanofibers
substrate
method
nanowires
porous
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US12/715,126
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Chunming Niu
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Nanosys Inc
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Nanosys Inc
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Priority to US54146304P priority Critical
Priority to US10/941,746 priority patent/US8025960B2/en
Priority to US11/331,445 priority patent/US7553371B2/en
Priority to US11/511,886 priority patent/US20110039690A1/en
Application filed by Nanosys Inc filed Critical Nanosys Inc
Priority to US12/715,126 priority patent/US20100173070A1/en
Publication of US20100173070A1 publication Critical patent/US20100173070A1/en
Assigned to NANOSYS, INC. reassignment NANOSYS, INC. SECURITY AGREEMENT Assignors: PRVP HOLDINGS, LLC
Assigned to NANOSYS, INC. reassignment NANOSYS, INC. RELEASE BY SECURED PARTY (SEE DOCUMENT FOR DETAILS). Assignors: NANOSYS, INC.
Application status is Abandoned legal-status Critical

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    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2239/00Aspects relating to filtering material for liquid or gaseous fluids
    • B01D2239/12Special parameters characterising the filtering material
    • B01D2239/1216Pore size
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D2239/00Aspects relating to filtering material for liquid or gaseous fluids
    • B01D2239/12Special parameters characterising the filtering material
    • B01D2239/1225Fibre length
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G01N30/02Column chromatography
    • G01N30/50Conditioning of the sorbent material or stationary liquid
    • G01N30/52Physical parameters
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10TTECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00Stock material or miscellaneous articles
    • Y10T428/29Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
    • Y10T428/2982Particulate matter [e.g., sphere, flake, etc.]
    • Y10T428/2991Coated

Abstract

Porous and/or curved nanofiber bearing substrate materials are provided having enhanced surface area for a variety of applications including as electrical substrates, semipermeable membranes and barriers, structural lattices for tissue culturing and for composite materials, production of long unbranched nanofibers, and the like. A method of producing nanofibers is disclosed including providing a plurality of microparticles or nanoparticles such as carbon black particles having a catalyst material deposited thereon, and synthesizing a plurality of nanofibers from the catalyst material on the microparticles or nanoparticles. Compositions including carbon black particles having nanowires deposited thereon are further disclosed.

Description

    CROSS REFERENCE TO RELATED APPLICATIONS
  • This application is a divisional of U.S. patent application Ser. No. 11/511,886 filed Aug. 29, 2006, which is a continuation in-part of U.S. patent application Ser. No. 11/331,445 filed Jan. 11, 2006, now U.S. Pat. No. 7,553,371 which is a continuation-in-part of U.S. patent application Ser. No. 10/941,746, filed Sep. 15, 2004, entitled “POROUS SUBSTRATES, ARTICLES, SYSTEMS AND COMPOSITIONS COMPRISING NANOFIBERS AND METHODS OF THEIR USE AND PRODUCTION” by Dubrow and Niu, which claims priority to and benefit of provisional U.S. Patent Application Ser. No. 60/541,463, filed Feb. 2, 2004, the full disclosures of which are hereby incorporated by reference in their entirety for all purposes.
  • FIELD OF THE INVENTION
  • The invention relates primarily to the field of nanotechnology. More specifically, the invention pertains to nanofibers, including methods of synthesizing or stabilizing nanofibers, articles comprising nanofibers, and use of nanofibers in various applications.
  • BACKGROUND OF THE INVENTION
  • Nanotechnology has been simultaneously heralded as the next technological evolution that will pave the way for the next societal evolution, and lambasted as merely the latest batch of snake oil peddled by the technically overzealous. Fundamentally, both sides of the argument have a number of valid points to support their position. For example, it is absolutely clear that nanomaterials possess very unique and highly desirable properties in terms of their chemical, structural and electrical capabilities. However, it is also clear that, to date, there is very little technology available for integrating nanoscale materials into the macroscale world in a reasonable commercial fashion and/or for assembling these nanomaterials into more complex systems for the more complex prospective applications, e.g., nanocomputers, nanoscale machines, etc. A variety of researchers have proposed a number of different ways to address the integration and assembly questions by waving their hands and speaking of molecular self assembly, electromagnetic assembly techniques and the like. However, there has been either little published success or little published effort in these areas.
  • In certain cases, uses of nanomaterials have been proposed that exploit the unique and interesting properties of these materials more as a bulk material than as individual elements requiring individual assembly. For example, Duan et al., Nature 425:274-278 (September 2003), describes a nanowire based transistor for use in large area electronic substrates, e.g., for displays, antennas, etc., that employs a bulk processed, oriented semiconductor nanowire film or layer in place of a rigid semiconductor wafer. The result is an electronic substrate that performs on par with a single crystal wafer substrate, but that is manufacturable using conventional and less expensive processes that are used in the poorer performing amorphous semiconductor processes. In accordance with this technology, the only new process requirement is the ability to provide a film of nanowires that are substantially oriented along a given axis. The technology for such orientation has already been described in detail in, e.g., International Patent Application Publications. WO 03/085700, WO 03/085701, and WO 2004/032191, as well as U.S. Pat. No. 7,067,328, (the full disclosures of each of which are hereby incorporated by reference herein, in their entirety for all purposes) and is readily scalable to manufacturing processes.
  • In another exemplary case, bulk processed nanocrystals have been described for use as a flexible and efficient active layer for photoelectric devices. In particular, the ability to provide a quantum confined semiconductor crystal in a hole conducting matrix (to provide type-II bandgap offset), allows the production of a photoactive layer that can be exploited either as a photovoltaic device or photoelectric detector. When disposed in an active composite, these nanomaterials are simply processed using standard film coating processes that are available in the industry. See, e.g., U.S. Pat. No. 6,878,871, and incorporated herein by reference in its entirety for all purposes.
  • In accordance with the expectation that the near term value of nanotechnology requires the use of these materials in more of a bulk or bulk-like process, certain aspects of the present invention use nanomaterials not as nanomaterials per se, but as a modification to larger materials, compositions and articles to yield fundamentally novel and valuable materials compositions and articles.
  • BRIEF SUMMARY OF THE INVENTION
  • The present invention is directed, in one aspect, to a novel presentation of nanomaterials that enables a broader use and application of those materials while imparting ease of handling, fabrication, and integration that is lacking in previously reported nanomaterials. In particular, one aspect of the present invention provides a porous substrate upon which is attached a plurality of nanofibers. The nanofibers may be attached to any portion or over the entire overall surface of the substrate or may be localized primarily or substantially upon the interior wall surfaces of the apertures that define the pores that are disposed through the porous substrate.
  • The articles of the invention may be employed as filtration media to filter gas, fluids or the like, or they may be employed as semipermeable barriers, e.g., breathable moisture barriers for outerwear, bandages, or the like. The articles of the invention may also be employed to integrate nanomaterials into electronic devices, in which the nanomaterials impart useful characteristics, e.g., as electrodes and or other active elements in photovoltaic devices and the like, or they may be used to integrate these nanomaterials into physical structures, e.g., composites, or biological structures, e.g., tissue. Synthesis of nanofibers on a porous or curved substrate can facilitate production of large numbers and/or a high density of long, unbranched nanofibers for use in any of a variety of applications.
  • Thus, a first general class of embodiments provides methods of producing nanofibers. In the methods, a substrate comprising a) a plurality of apertures disposed therethrough, the substrate comprising an overall surface area that includes an interior wall surface area of the plurality of apertures, or b) a curved surface is provided. A plurality of nanofibers is synthesized on the substrate, wherein the resulting nanofibers are attached to at least a portion of the overall surface area of the substrate of a) or to at least a portion of the curved surface of b).
  • The substrate can comprise a solid substrate with a plurality of pores disposed through it, a mesh (e.g., a metallic mesh, e.g., a mesh comprising a metal selected from the group consisting of: nickel, titanium, platinum, aluminum, gold, and iron), a woven fabric (e.g., an activated carbon fabric), or a fibrous mat (e.g., comprising glass, quartz, silicon, metallic, or polymer fibers). As other examples, the substrate can comprise a plurality of microspheres (e.g., glass or quartz microspheres), a plurality of fibers, e.g., glass or quartz fibers (e.g., microfibers, fiberglass, glass or quartz fiber filters), or a foam. In certain embodiments, the plurality of apertures in the substrate of a) have an effective pore size of less than 10 μm, less than 1 μm, less than 0.5 μm, or even less than 0.2 μm. In other embodiments, the plurality of apertures in the substrate of a) have an effective pore size of at least 25 μm, at least 50 μm, at least 100 μm, or more.
  • The nanofibers can comprise essentially any type of nanofibers. In certain embodiments, the nanofibers comprise nanowires, and the methods can include synthesizing the plurality of nanowires by depositing a gold colloid on at least a portion of the overall surface area of the substrate of a) or on at least a portion of the curved surface of b) and growing the nanowires from the gold colloid, e.g., with a VLS synthesis technique. The plurality of nanofibers optionally comprises a semiconductor material selected from group IV, group II-VI and group III-V semiconductors (e.g., silicon).
  • The methods optionally include surrounding or at least partially encapsulating the substrate and the resulting attached nanofibers with a matrix material; dissolving a soluble substrate following synthesis of the nanofibers on the substrate; forming a coating on the resulting nanofibers, wherein the coating is contiguous between adjacent nanofibers; disposing a layer of porous material on the resulting nanofibers (and optionally disposing the substrate on a second layer of porous material, sandwiching the nanofiber-bearing substrate); and/or functionalizing the nanofibers (e.g., by attaching a chemical moiety or nanocrystal to their surface).
  • In one class of embodiments, yield of the resulting nanofibers having a length greater than 10 μm (e.g., greater than 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm) is at least 10% greater than yield of nanofibers of that length synthesized on a planar non-porous substrate of the same surface area using substantially the same growth process. The yield from the methods is optionally at least 25%, 50%, 75%, or even 100% greater than the yield from growth on the planar non-porous substrate.
  • The nanofibers are optionally removed from the surface area of the substrate of a) or the curved surface of b) following synthesis of the nanofibers, e.g., by sonicating the substrate, to produce a population of detached nanofibers. In one class of embodiments, at least 10% of the nanofibers in the population of detached nanofibers have a length greater than 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, while at most 50% of the nanofibers have a length less than 10 μm.
  • Articles or populations of nanofibers produced by the methods form another feature of the invention. Thus, one exemplary class of embodiments provides an article comprising a substrate having a curved surface, and a plurality of nanofibers (e.g., nanowires) attached to at least a portion of the curved surface of the substrate. The substrate can comprise, e.g., a plurality of microspheres or one or more glass fiber, quartz fiber, metallic fiber, polymer fiber, or other fiber.
  • As for the embodiments above, the plurality of nanofibers optionally comprises a semiconductor material selected from group IV, group II-VI and group III-V semiconductors (e.g., silicon). Optionally, at least 10% of the nanofibers present on the curved surface have a length greater than 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, while at most 50% of the nanofibers present on the curved surface have a length less than 10 μm. The nanofibers can be preformed and deposited on the substrate to produce the article, or the plurality of nanofibers can be attached to the portion of the curved surface by having been grown on the portion of the curved surface. The article optionally includes a matrix material surrounding at least a portion of the curved surface and plurality of nanofibers. Devices or compositions including the article form another feature of the invention, for example, an implantable medical device comprising an article of the invention, e.g., attached to and covering at least a portion of the surface of the implantable medical device.
  • Another general class of embodiments provides methods of stabilizing nanofibers (e.g., nanowires). In the methods, a population of nanofibers is provided, and a coating is formed on the nanofibers. The coating is contiguous between adjacent nanofibers in the population. A first material comprising the nanofibers is optionally different from a second material comprising the coating. In one class of embodiments, the coating comprises a carbide, a nitride, or an oxide, e.g., an oxide of silicon, titanium, aluminum, magnesium, iron, tungsten, tantalum, iridium, or ruthenium, or an oxide of the material comprising the nanofibers. In another class of embodiments, the nanofibers are comprised of silicon and the coating is comprised of polysilicon.
  • The population of nanofibers is optionally provided by synthesizing the nanofibers on a surface of a substrate. The methods can include functionalizing the coating with a chemical binding moiety, a hydrophobic chemical moiety, a hydrophilic chemical moiety, or the like.
  • Populations of nanofibers formed by the methods are another feature of the invention. One general class of embodiments provides a population of nanofibers that includes nanofibers (e.g., nanowires) and a coating on the nanofibers, wherein the coating is contiguous between adjacent nanofibers in the population. As for the methods described above, a first material comprising the nanofibers is optionally different from a second material comprising the coating. In one class of embodiments, the coating comprises a carbide, a nitride, or an oxide, e.g., an oxide of silicon, titanium, aluminum, magnesium, iron, tungsten, tantalum, iridium, or ruthenium, or an oxide of the material comprising the nanofibers. In another class of embodiments, the nanofibers are comprised of silicon and the coating is comprised of polysilicon. The coating can be functionalized with a chemical binding moiety, a hydrophobic chemical moiety, a hydrophilic chemical moiety, or the like. The nanofibers are optionally attached to a substrate. In one embodiment, the nanofibers are attached to and cover at least a portion of a surface of an implantable medical device.
  • Yet another general class of embodiments provides an article comprising a substrate having a plurality of apertures disposed therethrough, the substrate comprising an overall surface area that includes an interior wall surface area of the plurality of apertures, and a plurality of nanofibers attached to at least a portion of the overall surface area of the substrate. The substrate can comprise, for example, a solid substrate (e.g., a silica based wafer, a metallic plate, or a ceramic sheet or plate) while the plurality of apertures comprises a plurality of pores disposed through the solid substrate. As another example, the substrate can comprise a mesh, e.g., a polymer mesh or a metallic mesh (comprising, e.g., nickel, titanium, platinum, aluminum, gold, or iron). As yet another example, the substrate can comprise a woven fabric, e.g., a fabric comprising fiberglass, carbon fiber, or a polymer (e.g., polyimide, polyetherketone, or polyaramid). As yet another example, the substrate can comprise a fibrous mat, e.g., a fibrous mat comprising silica based fibers (e.g., glass and silicon), metallic fibers, or polymer fibers.
  • In certain embodiments, the plurality of apertures have an effective pore size of less than 10 μm, for example, less than 1 μm, less than 0.5 μm, or less than 0.2 μm. In other embodiments, for example, embodiments in which synthesis of long unbranched nanofibers are desired, the plurality of apertures have an effective pore size of at least 25 μm, at least 50 μm, at least 100 μm, or more.
  • The nanofibers (e.g., nanowires) can comprise essentially any suitable material. For example, the plurality of nanofibers can comprise a semiconductor material selected from group IV, group II-VI and group III-V semiconductors, e.g., silicon. The nanofibers can be pre-formed and deposited on the substrate, or they can be attached to the portion of the overall surface area of the substrate by having been grown on the portion of the surface area. The plurality of nanofibers is optionally electrically coupled to the substrate. The plurality of nanofibers can be functionalized with a chemical binding moiety, e.g., a hydrophobic chemical moiety.
  • In one class of embodiments, a matrix material surrounds or at least partially encapsulates the substrate and plurality of nanofibers. The matrix material can at least partially intercalate into the apertures. In one embodiment, the matrix material and the plurality of nanofibers have a type-II energy band-gap offset with respect to each other. The matrix material optionally comprises a polymer, for example, a polyester, an epoxy, a urethane resin, an acrylate resin, polyethylene, polypropylene, nylon, or PFA. In one aspect, the invention provides implantable medical devices. For example, an implantable medical device can include an article of the invention attached to and covering at least a portion of a surface of the implantable medical device.
  • In one class of embodiments, the substrate comprises activated carbon, e.g., an activated carbon fabric. At least a first population of nanocrystals can be attached to the nanofibers, for example, nanocrystals comprising a material selected from the group consisting of: Ag, ZnO, CuO, Cu2O, Al2O3, TiO2, MgO, FeO, and MnO2. At least a second population of nanocrystals is optionally also attached to the nanofibers, where the nanocrystals of the second population comprise a different material than do the nanocrystals of the first population. In certain embodiments, the nanofibers are functionalized with a chemical moiety, e.g., a chemical moiety that absorbs or decomposes a non-organic gas. Preferred nanofibers in these embodiments include carbon nanotubes and silicon nanowires. An article of clothing can comprise the nanofiber-enhanced substrate of the invention.
  • Various techniques can be used to protect the nanofiber bearing substrate. For example, in one class of embodiments, the substrate (e.g., a woven fabric) comprises a first surface, and the article further comprises a first layer of porous material disposed on the first surface of the substrate. Optionally, the substrate comprises a second surface, and the article also includes a second layer of porous material disposed on the second surface of the substrate, whereby the substrate is sandwiched between the first and second layers of porous material. As another example, the article can include a coating on the nanofibers, which coating is contiguous between adjacent nanofibers.
  • Yet another general class of embodiments provides methods of producing a vapor absorbing fabric. In the methods, a porous fabric substrate that comprises a plurality of apertures disposed therethrough is provided. The substrate comprises an overall surface area that includes an interior wall surface area of the plurality of apertures. A plurality of nanofibers attached to at least a portion of the overall surface area of the fabric substrate is also provided, and the nanofibers are functionalized with a moiety that absorbs or decomposes at least one organic or non-organic gas, thereby producing a vapor absorbing fabric.
  • The fabric is preferably an activated carbon fabric. The nanofibers can be functionalized with a chemical moiety that absorbs or decomposes at least one non-organic gas. Preferably, the nanofibers are functionalized by attaching at least a first population of nanocrystals to the nanofibers, which first population of nanocrystals comprises a first material that absorbs or decomposes at least one non-organic gas. The vapor absorbing fabric can be incorporated into an article of clothing or other protective apparatus.
  • A related class of embodiments also provides methods of producing a vapor absorbing fabric. In the methods, a porous fabric substrate that comprises a plurality of apertures disposed therethrough is provided (e.g., a mesoporous carbon fabric). The substrate comprises an overall surface area that includes an interior wall surface area of the plurality of apertures. A plurality of nanocrystals is attached to at least a portion of the overall surface area of the fabric substrate, which nanocrystals absorb or decompose at least one non-organic gas, thereby producing the vapor absorbing fabric.
  • BRIEF DESCRIPTION OF THE FIGURES
  • FIG. 1 Panels A and B show a schematic illustration of a porous substrate having nanowires attached to its surfaces.
  • FIG. 2 Panels A and B show a schematic illustration of nanowires attached to the interior wall portions of a porous substrate material.
  • FIG. 3 Panels A and B show a schematic illustration of the articles of the invention incorporated in a filtration cartridge.
  • FIG. 4 Panels A and B show a schematic illustration of a layered textile that incorporates a substrate of the invention as a semi-permeable moisture barrier for use in, e.g., outdoor clothing.
  • FIG. 5 Panels A and B show a schematic illustration of the articles of the invention incorporated in a self adhesive, moisture repellant bandage.
  • FIG. 6 Panels A and B are schematic illustrations of the substrate material of the invention incorporated into a photovoltaic device.
  • FIG. 7 is a schematic illustration of an article of the invention used as a lattice for incorporation into a composite matrix for use as, e.g., a dielectric layer.
  • FIG. 8 Panels A and B schematically illustrate separation media incorporating the substrates of the invention in conjunction with column apparatus for performing chromatographic separations.
  • FIG. 9 Panels A and B show electron micrographs of cross-fused or linked nanowires creating an independent mesh network as used in certain aspects of the present invention.
  • FIG. 10 schematically illustrates a process for producing a cross-linked nanowire mesh network for use either in conjunction with or independent from an underlying porous, e.g., macroporous, substrate.
  • FIG. 11 illustrates a composite material that employs the porous substrates of the invention disposed within a matrix material.
  • FIG. 12 Panels A and B illustrate an example of the nanofiber bearing, porous substrates of the invention.
  • FIG. 13 schematically illustrates a nanofiber-enhanced fabric for use, e.g., in protective clothing or apparatus.
  • FIG. 14 schematically illustrates protection of a nanofiber-bearing substrate by disposing it between layers of porous material.
  • FIG. 15 Panel A depicts an electron micrograph of reticulated aluminum. Panel B depicts an electron micrograph of nanowires grown on a reticulated aluminum substrate.
  • FIG. 16 depicts silicon nanowires grown on quartz fiber filters (Panels A-B), grown on quartz fiber filters and removed from the substrate by sonication (Panels C-D), and grown on a glass fiber (Panel E).
  • FIG. 17 Panel A depicts simulated nanowires growing on a 5 μm diameter fiber. Panel B depicts a graph of the collisions per nanowire as a function of fiber radius for simulated nanowire growth on a fiber.
  • FIG. 18 Panel A depicts an electron micrograph of carbon black powdered particles having gold colloid particles deposited thereon.
  • FIG. 18 Panels B and C depict an electron micrograph of silicon nanowires grown from the carbon black supported gold colloid particles of FIG. 18 Panel A.
  • DETAILED DESCRIPTION I. General Description of the Invention
  • The present invention generally provides, inter alia, novel articles and compositions that employ nanowire surfaces or surface portions to impart unique physical, chemical and electrical properties. In particular, the present invention is directed, in part, to porous substrates that have nanowires attached to at least a portion of the overall surfaces of the porous substrates in order to provide materials that have a wide range of unique and valuable properties for a wide range of different applications.
  • The application of nanowires to the various surfaces of porous substrates not only improves the performance of porous substrates in applications where they are already used, but also improves performance of substrate materials in a number of other different applications, where such porous substrates may or may not conventionally be employed.
  • By way of example, incorporation of nanowire enhanced surfaces in membranes or other semi-permeable barriers can enhance filtration efficiencies. In particular, by providing nanowires within the pores of existing membranes or other permeated layers, one can provide higher filtration efficiencies without the expected increase in pressure drop across the filter (see Grafe et al., Nanowovens in Filtration-Fifth International Conference, Stuttgart, Germany, March 2003). Relatedly, such nanofibers may be used to impart alternate properties to such barriers, e.g., breathable moisture repellant barriers, antibacterial/antiseptic barriers. Such barriers would be widely applicable in the outdoor clothing industry but would also be particularly useful as bandages or surgical dressings due to their permeability to oxygen but impermeability to moisture or particles including bacteria, as well as the use of antimicrobial nanofibers. This latter application is particularly interesting in light of the dry adhesive characteristics of nanowire/nanofiber enhanced surfaces (see, e.g., U.S. Pat. No. 7,056,409, incorporated herein by reference in its entirety for all purposes). Relatedly, such nanofiber enhanced surfaces can also be used in the construction of chemical and/or biological protective barriers, e.g., clothing, optionally permeable to moisture but absorbing chemical vapors.
  • While some researchers have proposed depositing nanofibers onto membranes to achieve higher surface areas, the ability to attach fibers to the surface, and particularly to grow such fibers in situ, provides numerous advantages over simple deposition of fibers. In particular, in merely depositing fibers on membranes, it is difficult to get uniform or complete, e.g., penetrating, coverage of the fibers over the total surface area of the membrane, whereas in situ growth methods give far better coverage of interior surfaces, and thus provide much greater surface area for the membrane or barrier. Additionally, such methods provide for varied orientations of such fibers from the surfaces to which they are attached, i.e., having fibers extend from the surface as opposed to laying flat against the surface.
  • In addition to improving the function of porous substrates, the use of porous substrates in conjunction with nanofibers/nanowires also provides a unique, ultra high surface area material that can be used in a wide variety of applications that may have little to do with the use of porous substrates, per se. For example, ultra high surface area electrical components may have a variety of applications as electrodes for interfacing with, e.g., biological tissue (e.g., in pacemakers), coverings for other biological implants as tissue lattice or anti-infective barriers for catheters, or the like.
  • In still other applications, porous substrates provide a unique synthesis lattice for providing dense populations of nanofibers/nanowires for use in a variety of different applications, e.g., for use in composite films, etc. Such films may generally be applied as semiconductive composites, dielectric films, active layers for electronic or photoelectric devices, etc.
  • In still other applications, porous substrates provide a unique synthesis lattice for synthesizing nanofibers, particularly long, unbranched nanowires at high yield and/or density.
  • A broad range of potential applications exists for these techniques, materials, and articles and will be apparent to one of ordinary skill in the art upon reading the instant disclosure.
  • II. Articles of the Invention, Structure and Architecture
  • As noted above, in one aspect, the articles of the invention incorporate porous substrates as a foundation of the article. The porous substrates used in accordance with the present invention typically include any of a variety of solid or semisolid materials upon which the nanowires may be attached, but through which apertures exist. As such, these substrates may include solid contiguous substrates, e.g., plates, films, or wafers, that may be flexible or rigid, that have apertures disposed through them, e.g., stamped or etched metal or inorganic perforated plates, wafers, etc., porated or perforated films, or the substrate may include aggregates of solid or semisolid components e.g., fibrous mats, mesh screens, amorphous matrices, composite materials, woven fabrics, e.g., fiberglass, carbon fiber, polyaramid or polyester fabrics, or the like. As will be apparent, any of a wide variety of different types of materials may comprise the substrates, including organic materials, e.g., polymers, carbon sheets, etc., ceramics, inorganic materials, e.g., semiconductors, insulators, glasses, including silica based materials (e.g., silicon, SiO2), etc., metals, semimetals, as well as composites of any or all of these.
  • Additionally, substrates, e.g., rigid or solid substrates, may be engineered to have additional topographies, e.g., three dimensional shapes, such as wells, pyramids, posts, etc. on their surface to further enhance their effectiveness, e.g., provide higher surface areas, channel fluids or gases over them, provide prefiltration in advance of the filtration provided by the porous substrate, per se, etc. Additionally, although referred to as including a porous substrate, it will be appreciated that in application, multiple substrates may be provided together in a single article, device or system. Further, although described and exemplified primarily as planar porous substrates, it will be appreciated that the porous substrates may be fabricated into any of a variety of shapes depending upon the application, including non-planar three dimensional shapes, spheres, cylinders, disks, cubes, blocks, domes, polyhedrons, etc. that may be more easily integrated into their desired application. Substrates, e.g., planar sheet substrates, are optionally rigid or flexible.
  • Examples of metal substrates include steel/iron, nickel, aluminum, titanium, silver, gold, platinum, palladium, or virtually any metal substrate that imparts a desirable property to the finished article, e.g., conductivity, flexibility, malleability, cost, processibility, etc. In certain preferred aspects, a metal wire mesh or screen is used as the substrate. Such meshes provide relatively consistent surfaces in a ready available commercial format with well defined screen/pore and wire sizes. A wide variety of metal meshes are readily commercially available in a variety of such screen/pore and wire sizes. Alternatively, metal substrates may be provided as perforated plates, e.g., solid metal sheets through which apertures have been fabricated. Fabricating apertures in metal plates may be accomplished by any of a number of means. For example, relatively small apertures, e.g., less than 100 μm in diameter, as are used in certain aspects of the invention, may be fabricated using lithographic and preferably photolithographic techniques. Similarly, such apertures may be fabricated using laser based techniques, e.g., ablation, laser drilling, etc. For larger apertures, e.g., greater than 50-100 μm, more conventional metal fabrication techniques may be employed, e.g., stamping, drilling or the like.
  • Polymeric and inorganic substrates may be similarly structured to the metal substrates described above, including mesh or screen structures, fibrous mats or aggregates, e.g., wools, or solid substrates having apertures disposed through them. In terms of polymeric substrates, again, the primary selection criteria is that the substrate operate in the desired application, e.g., is resistant to chemical, thermal or radiation or other conditions to which it will be exposed. In preferred aspects the polymeric substrate will also impart other additional useful characteristics to the overall article, such as flexibility, manufacturability or processibility, chemical compatibility or inertness, transparency, light weight, low cost, hydrophobicity or hydrophilicity, or any of a variety of other useful characteristics. Particularly preferred polymeric substrates will be able to withstand certain elevated environmental conditions that may be used in their manufacturing and/or application, e.g., high temperatures, e.g., in excess of 300 or 400° C., high salt, acid or alkaline conditions, etc. In particular, polymers that tolerate elevated temperatures may be particularly preferred where the nanowires are actually grown in situ on the surface of the substrate, as such synthetic processes often employ higher temperature synthetic processes, e.g., as high as 450° C. Polyimide polymers, polyetherketone, polyaramid polymers and the like are particularly preferred for such applications. Those of skill in the art will recognize a wide range of other polymers that are particularly suitable for such applications. Alternatively, lower temperature fiber synthesis methods may also be employed with a broader range of other polymers. Such methods include that described by Greene et al. (“Low-temperature wafer scale production of ZnO nanowire arrays”, L. Greene, M. Law, J. Goldberger, F. Kim, J. Johnson, Y. Zhang, R. Saykally, P. Yang, Angew. Chem. Int. Ed. 42, 3031-3034, 2003), or through the use of PECVD, which employs synthesis temperatures of approximately 200° C. In the case where the porous substrate is merely the recipient of nanofibers already synthesized, e.g., where the substrate is either to be coupled to the nanowires or is to act as a macroporous support for the nanowires, a much wider variety of porous substrates may be employed, including organic materials, e.g., organic polymers, metals, ceramics, porous inorganics, e.g., sintered glass, which would include a variety of conventionally available membrane materials, including cellulosic membranes, e.g., nitrocellulose, polyvinyl difluoride membranes (PVDF), polysulfone membranes, and the like.
  • In some cases, the porous substrate may comprise a soluble material, e.g., cellulose, or the like. Following attachment of the nanofibers, and optionally placement of the overall substrate into its ultimate device configuration, the supporting porous substrate may be dissolved away, leaving behind an interwoven mat or collection of nanofibers. For example, a soluble mesh may be provided with nanofibers attached to its overall surfaces or interior wall surfaces as described herein. The mesh may then be rolled into a cylindrical form and inserted into a cylindrical housing, e.g., a column for separations applications. The supporting mesh is then dissolved away to yield the column packed with nanofibers. Further, as described above, the porous matrix may comprise any of a number of shapes, and be soluble as well, so as to yield any of a variety of shapes of aggregations of fibers, once the substrate is dissolved.
  • As noted above, the apertures of the substrates used herein typically are defined in terms of their effective pore size or “effective porosity”. Although described as apertures or pores, it will be appreciated that the term “aperture” or “pore” when used in the context that it is disposed through a substrate, refers simply to a contiguous pathway or passage through a substrate material, whether that material be a single solid piece of substrate material or a mesh or mat of aggregated pieces of substrate material. Thus, such “apertures” or pores do not need to represent a single passage, but may constitute multiple passages strung together to form the contiguous path. Likewise, an aperture or pore may simply represent the space between adjacent portions of substrate material, e.g., fibers, etc. such that the spaces provide a contiguous path through the material. For purposes of the invention, pore or aperture size, in the absence of any nanofibers disposed thereon, will typically vary depending upon the nature of the application to which the material is to be put.
  • For example, filtration applications will typically vary pore size depending upon the nature of the particles or other material to be filtered, ranging from tens to hundreds of microns or larger for coarser filtration operations to submicron scale for much finer filtration applications, e.g., bacterial sterilizing filters. Similarly for semi-permeable barrier applications, such pores will typically vary depending upon the type of permissible permeability is sought. For example, breathable moisture barriers may have pore sizes from tens of microns to the submicron range, e.g., 0.2 μm, or smaller. In some cases, it may be desirable to have an effective pore size that is less than 100 nm, and even less than 20 nm, so as to block passage of biological agents, e.g., bacteria and viruses.
  • The articles and substrates described herein may include nanowires substantially on any and all surfaces of the substrate material including both exterior surfaces and the surfaces that are within the pores. Together, these surfaces upon which nanowires may be disposed are referred to herein as the “overall surface” of the substrate material, while the wall surfaces that are disposed upon the interior walls of the pores are generally referred to herein as the “interior wall surfaces” of the substrate material or pores. As will be clear to one of ordinary skill in reading the instant disclosure, a reference to a surface as an interior wall surface for certain embodiments, e.g., in the case of a fibrous mat or wool like substrate does not necessarily denote a permanent status of that surface as being in the interior portion of a pore or aperture as the basic flexibility and/or malleability of certain substrate materials may provide the ability to shift or move the various portions of the substrate material's overall surface around.
  • As noted above, the substrates of the invention gain significant unique properties by incorporating nanofibers or nanowires on their surfaces. For most applications, the terms “nanowire” and “nanofiber” are used interchangeably. However, for conductive applications, e.g., where the nanofibers' conductive or semiconductive properties are of interest, the term “nanowire” is generally favored. In either instance, the nanowire or nanofiber generally denotes an elongated structure having an aspect ratio (length:width) of greater than 10, preferably greater than 100 and in many cases 1000 or higher. These nanofibers typically have a cross sectional dimension, e.g., a diameter that is less than 500 nm and preferably less than 100 nm and in many cases, less than 50 nm or 20 nm.
  • The composition of the nanofibers employed in the invention typically varies widely depending upon the application to which the resulting substrate material is to be put. By way of example, nanofibers may be comprised of organic polymers, ceramics, inorganic semiconductors and oxides, carbon nanotubes, biologically derived compounds, e.g., fibrillar proteins, etc. or the like. For example, in certain embodiments, inorganic nanofibers are employed, such as semiconductor nanofibers. Semiconductor nanofibers can be comprised of a number of Group IV, Group III-V or Group II-VI semiconductors or their oxides. Particularly preferred nanofibers include semiconductor nanowires or semiconductor oxide nanofibers.
  • Typically, the nanofibers or nanowires employed are produced by growing or synthesizing these elongated structures on substrate surfaces. By way of example, U.S. Pat. No. 7,301,199 discloses methods of growing uniform populations of semiconductor nanowires from gold colloids adhered to a solid substrate using vapor phase epitaxy. Greene et al. (“Low-temperature wafer scale production of ZnO nanowire arrays”, L. Greene, M. Law, J. Goldberger, F. Kim, J. Johnson, Y. Zhang, R. Saykally, P. Yang, Angew. Chem. Int. Ed. 42, 3031-3034, 2003) discloses an alternate method of synthesizing nanowires using a solution based, lower temperature wire growth process. A variety of other methods are used to synthesize other elongated nanomaterials, including the surfactant based synthetic methods disclosed in U.S. Pat. Nos. 5,505,928, 6225,198 and 6,306,736, for producing shorter nanomaterials, and the known methods for producing carbon nanotubes, see, e.g., U.S. Pat. No. 7,416,699 to Dai et al. As noted herein, any or all of these different materials may be employed in producing the nanofibers for use in the invention. For some applications, a wide variety of group III-V, II-VI and group IV semiconductors may be utilized, depending upon the ultimate application of the substrate or article produced. In general, such semiconductor nanowires have been described in, e.g., U.S. Pat. No. 7,301,199, incorporated herein above. In certain preferred embodiments, the nanowires are selected from a group consisting of: Si, Ge, Sn, Se, Te, B, diamond, P, B—C, B—P(BP6), B—Si, Si—C, Si—Ge, Si—Sn, Ge—Sn, BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, BeSiN2, CaCN2, ZnGeP2, CdSnAs2, ZnSnSb2, CuGeP3, CuSi2P3, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)2, Si3N4, Ge3N4, Al2O3, (Al, Ga, In)2(S, Se, Te)3, Al2CO, and an appropriate combination of two or more such semiconductors. The nanofibers optionally comprise a gold tip.
  • In the cases of semiconductor nanofibers, and particularly those for use in electrical or electronic applications, the nanofibers may optionally comprise a dopant from a group consisting of: a p-type dopant from Group III of the periodic table; an n-type dopant from Group V of the periodic table; a p-type dopant selected from a group consisting of: B, Al and In; an n-type dopant selected from a group consisting of: P, As and Sb; a p-type dopant from Group II of the periodic table; a p-type dopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of the periodic table; a p-type dopant selected from a group consisting of: C and Si; or an n-type dopant is selected from a group consisting of: Si, Ge, Sn, S, Se and Te.
  • In some cases, it may be desirable to utilize nanofibers that have a self sterilizing capability, e.g., in semipermeable bandage, clothing, filtration or other applications. In such cases, the nanofibers may be fabricated from, e.g., TiO2, which upon exposure to UV light oxidizes organic materials to provide a self cleaning functionality (See, e.g., U.S. Pat. No. 7,579,077, and incorporated herein by reference in its entirety for all purposes).
  • Additionally, such nanofibers may be homogeneous in their composition, including single crystal structures, or they may be comprised of heterostructures of different materials, e.g., longitudinal heterostructures that change composition over their length, or coaxial heterostructures that change composition over their cross section or diameter. Such coaxial and longitudinal heterostructured nanowires are described in detail in, e.g., Published International Patent Application No. WO 02/080280, which is incorporated herein by reference for all purposes.
  • The nanowire portion of the articles of the invention are preferably synthesized in situ, e.g., on the desired surface of the porous substrate. For example, in preferred aspects, inorganic semiconductor or semiconductor oxide nanofibers are grown directly on the surface of the porous substrate using a colloidal catalyst based VLS (vapor-liquid-solid) synthesis method such as those described above. In accordance with this synthesis technique, the colloidal catalyst is deposited upon the desired surface of the porous substrate (which in some cases may include the overall surface of the porous substrate). The porous substrate including the colloidal catalyst is then subjected to the synthesis process which generates nanofibers attached to the surface of the porous substrate. Other synthetic methods include the use of thin catalyst films, e.g., 50 nm, deposited over the surface of the porous substrate. The heat of the VLS process then melts the film to form small droplets of catalyst that form the nanofibers. Typically, this latter method may be employed where fiber diameter homogeneity is less critical to the ultimate application. Typically, catalysts comprise metals, e.g., gold, and may be electroplated or evaporated onto the surface of the substrate or deposited in any of a number of other well known metal deposition techniques, e.g., sputtering etc. In the case of colloid deposition, the colloids are typically deposited by first treating the surface of the substrate so that the colloids adhere to the surface. Such treatments include those that have been described in detail previously, e.g., polylysine treatment, etc. The substrate with the treated surface is then immersed in a suspension of colloid.
  • Alternatively, the nanofibers may be synthesized in another location and deposited upon the desired surface of the porous substrate using previously described deposition methods. For example, nanofibers may be prepared using any of the known methods, e.g., those described above, and harvested from their synthesis location. The free standing nanofibers are then deposited upon the relevant surface of the porous substrate. Such deposition may simply involve immersing the porous substrate into a suspension of such nanofibers, or may additionally involve pretreating all or portions of the porous substrate to functionalize the surface or surface portions for fiber attachment. A variety of other deposition methods are known, e.g., as described in U.S. Pat. Nos. 7,067,328, and 6,962,823, the full disclosures of which are incorporated herein by reference in their entirety for all purposes.
  • Where nanofibers are desired to be attached primarily to the interior wall portions of the surface of the porous substrate, such deposition may be accomplished by growing the nanofibers in such locations or by selectively depositing the nanofibers in such locations. In the case of in situ grown nanofibers, this may be accomplished by depositing a layer of another material on all of the exterior surfaces of the substrate, e.g., a resist, before depositing the colloids. Following immersion in colloid, the resist layer may be developed and removed to yield substrate having colloid substantially only deposited on the interior wall surfaces of the substrate.
  • FIGS. 1 and 2 schematically illustrate substrates according to the present invention. In particular, FIG. 1 shows a schematic illustration of a porous nanowire carrying substrate of the invention. As shown in FIGS. 1A and 1B, a porous substrate 102 is provided. For purposes of exemplification, a mesh or screen is employed as the porous substrate, although fibrous mats are also useful in such applications. As shown in FIG. 1B, nanofibers 104 are provided that are, at least in part, disposed on the internal wall portions 106 of the apertures or pores, and which extend into the void area 108 of the pores, yielding openings or passages through the overall material that are somewhat more restrictive or narrow than those provided by the underlying substrate, itself. As shown in FIG. 1, the nanofibers 104 are also disposed on other surface portions of the mesh (the overall surface).
  • FIGS. 2A and 2B schematically illustrate the case where nanofibers are primarily disposed only on the interior wall portions of the apertures that define the pores. As shown, a perforated substrate 202 forms the underlying porous substrate. A plurality of apertures 208 are fabricated through the substrate 202, e.g., by punching etching or other known fabrication methods. As shown in FIG. 2B, an expanded view of the aperture 208 is provided that details the presence of nanofibers 204 attached to the interior wall portions 206 of the aperture. As shown, the nanofibers generally protrude away from the interior wall surface 206. This is typically accomplished by growing the nanofibers, in situ, using a catalytic growth CVD process, whereupon the fibers grow away from the surface upon which the catalyst is initially deposited. Other methods may also be employed to deposit nanofibers on these interior wall portions that may or may not result in the fibers protruding into the void space of the apertures, including immersing the porous substrate in a suspension of nanofibers that are chemically able to attach to the surfaces of interest.
  • FIG. 12 shows a photograph of a silicon substrate that has pores or apertures disposed through it. Silicon nanofibers were grown over the surface of the substrate, including within the pores. The substrate was a 0.1 mm thick silicon wafer with regularly spaced 100 μm holes disposed through it. FIG. 12A shows a view of a larger area of the substrate, while FIG. 12 B shows a closer up view of the pore and substrate surface, as well as the nanofibers on those surfaces.
  • In alternative arrangements, the porous substrates may be employed in steps that are discrete from the synthesis process, and that employ the porous substrate as a capture surface for the nanofibers. In particular, nanofibers may be produced as suspensions or other collections or populations of free-standing, e.g., a population of discrete and individual members, nanofibers. Such free standing nanofibers are generally produced from any of the aforementioned processes, but including a harvest step following synthesis whereby the nanofibers are removed from a growth substrate and deposited into a suspending fluid or other medium or deposited upon a receiving substrate, or otherwise moved from a growth or synthesis environment into a manipulable environment, e.g., a fluid suspension. The population of nanowires is then deposited over a porous substrate to yield a mat of deposited nanofibers that form a micro or nanoporous network over the underlying porous substrate. In accordance with this aspect of the invention, the pores in the porous substrate are typically selected so that they are smaller than the largest dimensions of the nanofibers to be deposited thereon, e.g., the length of the nanofiber. For example, where nanofibers in a particular population have an average length of approximately 10 μm, the pores in the substrate will typically be smaller in cross section than 10 μm, e.g., less than 5 μm, less than 2 μm, or smaller. To ensure sufficient capture of nanofibers, the largest cross section of the pore in the porous substrate will typically be less than 50% of the average largest dimension of the nanofiber population, generally the length, in some cases, less than 20% of such dimension, and in many cases, less than 10% of such dimension.
  • The nanofiber mat is then optionally fused or cross-linked at the points where the various fibers contact each other, to create a more stable, robust and potentially rigid fibrous membrane. The void spaces be