WO2005062785A2

WO2005062785A2 - Solution-phase synthesis of metal oxide nanostructures

Info

Publication number: WO2005062785A2
Application number: PCT/US2004/042394
Authority: WO
Inventors: Edward T. Samulski; Bin Cheng
Original assignee: The University Of North Carolina At Chapel Hill
Priority date: 2003-12-17
Filing date: 2004-12-17
Publication date: 2005-07-14
Also published as: WO2005062785A3

Abstract

A method of preparing a metal oxide nanostructure, the method including mixing a metal precursor solution with a basic solution to form a reaction mixture; and heating the reaction mixture in a closed container to provide a pressure greater than ambient pressure. Metal oxide nanostructures having a mean aspect ratio of at least about 4:1 and a cross-sectional width of about 100 nanometers or less are also disclosed.

Description

TITLE SOLUTION-PHASE SYNTHESIS OF METAL OXIDE NANOSTRUCTURES CROSS REFERENCE TO RELATED APPLICATIONS This application is based on and claims priority to United States Provisional Application Serial No. 60/530,200, filed December 17, 2003, herein incorporated by reference in its entirety. GOVERNMENT INTEREST This invention was made with U.S. Government support under NASA University Research, Engineering and Technology Institute on Bio-Inspired Materials (BIMat) award No. NCC-1-02037. The U.S. Government has certain rights in the invention.

TECHNICAL FIELD The presently disclosed subject matter pertains to metal oxide nanostructures, and to methods for preparing metal oxide nanostructures.

In some embodiments, the presently disclosed subject matter provides a large-scale, single-step, direct hydrothermal method for preparing metal oxide nanostructures.

TABLE OF ABBREVIATIONS

1-D one-dimensional 2-D two-dimensional aA Zinc oxide sample A lattice constant a value as Zinc oxide sample B lattice constant a value c_A Zinc oxide sample A lattice constant c value

CB Zinc oxide sample B lattice constant c value eV electron volts

FET field-effect transistor

FPD flat panel display HRTEM high-resolution transmission electron microscropy

JCPDS Joint Committee on Powder Diffraction Standards kV kilovolts

LED light emitting diode mA milliamperes meV milli-electron volts nm nanometer(s)

TBAOH tetrabutyl ammonium hydroxide

TEAOH tetraethyl ammonium hydroxide

TEM transmission electron microscopy

TMAOH tetramethyl ammonium hydroxide

TPAOH tetrapentyl ammonium hydroxide

TPAOH tetrapropyl ammonium hydroxide

XRD x-ray diffraction micrometer(s) or micron(s)

BACKGROUND Studies on one-dimensional (1-D) nanostructures - such as but not limited to nanowires, nanorods or nanotubes - have received increasing attention due to their potential use as active components or interconnects in fabricating nanoscale electronic, optical, optoelectronic, electrochemical, and electromechanical devices. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 2001 , 291, 1947; J. Hu, T. W. Odom, C. Lieber, Ace. Chem. Res. 1999, 32, 435; H. M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 2001 , 292, 1897. Notable examples of 1-D nanostructure applications include light-emitting diodes (LEDs) (X. Duan, Y. Huang, Y. Cui, J. Wang, C. M. Lieber, Nature 2001, 409, 66), single-electron transistors (S. J. Tans, M. H. Devoret, H. Dai, A. Thess, R. E. Smalley, L. J. Gerrligs, C. Decker, Nature 1997, 386, 474; S. J. Tans, A. R. M. Verschueren, C. Dekker, Nature 1998, 393, 49), field-effect transistors (FETs) (S.-W. Chung, J.-Y. Yu, J. R. Heath, Appl. Phys. Lett. 2000, 76, 2068), biological and chemical sensors (Y. Cui, Q. Wei, H. Park, C. M. Lieber, Science 2001 , 293, 1289; S. R. Nicewarner-Pefia, R. G. Freeman, B.

D. Reiss, L He, D. J. Peria, I. D. Walton, R. Cramer, C. D. Keating, M. J.

Natan, Science 2001 , 294, 137), photodetectors (J. Wang, M. S. Gudiksen,

X. Duan, Y. Cui, C. M. Lieber, Science 2001 , 293, 1455), electron emitters (D. N. Davydov, P. A. Sattari, D. AIMawlawi, A. Osika, T. L. Haslett, M.

Moskovits, J. Appl. Phys. 1999, 86, 3983), and ultraviolet nanolasers (Y.

Wu, H. Yan, M. Huang, B. Messer, J. H. Song, P. Yang, Chem. Eur. J. 2002,

8, 1260). A number of methods have been developed to fabricate and assemble one-dimensional (1-D) nanostructures, including nanolithographic techniques (e.g., electron beam lithography, proximal probe patterning, and

X-ray patterning) (R. D. Piner, J. Zhu, F. Xu, S. Hong, C. A. Mirkin, Science

1999, 283, 661 ; Y. Xia, K. Paul, J. A. Rogers, G. M. Whitesides, Chem. Rev.

1999, 99, 1823), and many chemical methods (e.g., vapor-solid, vapor- liquid-solid, solution-solid). However, current approaches in the art typically employ high temperatures and/or special equipment. Thus, an ongoing need persists in the art for improved nanostructures and for improved methods of making nanostructures. SUMMARY A method of preparing a metal oxide nanostructure is disclosed herein. In some embodiments, the method comprises mixing a metal precursor solution with a basic solution to form a reaction mixture; and heating the reaction mixture in a closed container to provide a pressure greater than ambient pressure. In some embodiments, the method comprises collecting a metal oxide nanostructure from the reaction mixture. Metal oxide nanostructures having a mean aspect ratio of at least about 4:1 and a cross-sectional width of about 100 nanometers or less are also disclosed. Accordingly, it is an object of the presently disclosed subject matter to provide a novel method of preparing metal oxide nanostructures. An object of the presently disclosed subject matter having been stated hereinabove, which is addressed in whole or in part by the presently disclosed subject matter, other aspects and objects will become evident as the description proceeds when taken in connection with the accompanying Examples and Figures as best described herein below.

BRIEF DESCRIPTION OF THE FIGURES Figures 1A and 1 B are transmission electron microscopy (TEM) images of ZnO nanorods prepared in aqueous methanol (Fig. 1A) and aqueous ethanol (Fig. 1 B) at 150°C for 24 hrs as disclosed in the Examples.

The insets show the corresponding selected area electron diffraction patterns (scale bar, 100 nm). Figures 2A and 2B are plots showing X-ray diffraction (XRD) patterns of final ZnO nanorod products prepared in aqueous methanol (Fig. 2A) and aqueous ethanol (Fig. 2B) at 150°C as disclosed in the Examples. Figures 3A and 3B are high-resolution transmission electron microscopy (HRTEM) images of ZnO nanorods prepared as disclosed in the Examples (scale bar, 5nm). In Fig. 3A the edge of a ZnO nanorod prepared in aqueous methanol is shown. In Fig. 3B the end of one of the ZnO nanorods prepared in aqueous ethanol is shown. The results show that the ZnO nanorods grow along [0001] direction; the lattice spacing (2.56 ± 0.05A) corresponds to the distance between two (0001 ) planes. Figures 4A-4C are TEM images of 1-D ZnO nanostructures prepared as disclosed in the Examples, with different aspect ratios: (Fig. 4A) 5:1 ; (Fig. 4B) 20:1 ; (Fig. 4C) 100:1 (scale bars: 100nm, 250nm, and 1 m, respectively). Figures 5A and 5B are TEM images of Sn0₂ nanorods prepared as disclosed in the Examples, with different aspect ratios: (Fig. 5A) 17nm ± 3.4nm; (Fig. 5B) 30nm ± 3nm (scale bars: 100nm and 50nm, respectively). Figures 6A and 6B are 1-D and two-dimensional (2-D) lattices, respectively, of Sn0₂ nanorods prepared as disclosed in the Examples, with the [001] direction along the nanorod major axis (scale bars: 5nm in each). Figure 7 is a plot showing XRD patterns of Sn0₂ nanorods prepared as disclosed in the Examples. Figure 8 is a room temperature photo-luminescence spectrum of Sn0₂ nanorods prepared as disclosed in the Examples. Figure 9 shows TEM images of 1-D Ti0 nanorods prepared as disclosed in the Examples (scale bar, 100nm). Figure 10 is a plot depicting photocurrent-voltage characteristic of a solar cell made from Ti0₂ nanorods prepared as disclosed in the Examples. Figure 1 1 is a schematic diagram of a photovoltaic device.

DETAILED DESCRIPTION The presently disclosed subject matter will now be described more fully hereinafter with reference to the accompanying Examples, in which representative embodiments are shown. The presently disclosed subject matter can, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art. Unless otherwise defined, 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 presently described subject matter belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. I. Definitions As used herein, the term "about", when referring to a value or to an amount of mass, weight, time, temperature, volume, concentration, and/or percentage, is meant to encompass variations of in one embodiment ±20%, in another embodiment ±10%, in another embodiment ±5%, in another embodiment ±1 %, in another embodiment ±0.5%, and in still another embodiment ±0.1 % from the specified amount, as such variations are appropriate to perform the disclosed methods. The term "aspect ratio" refers to a ratio of the length of a nanostructure of the presently disclosed subject matter over the width of the nanostructure. The term "basic solution" refers to a solution having a pH above 7. In some embodiments, the basic solution has a pH ranging from at least about 7.5 to at least about 14. In some embodiments, the basic solution has a pH of about 12. The term "metal precursor solution" is meant to refer to a solution in which a metal is dissolved. Representative metal compounds that can be dissolved in a metal precursor solution include metal salts, such as acetate salts, chloride salts, nitrate salts, isopropoxide salts and any other suitable salt that has desirable solubility characteristics. The terms "one-dimensional" and "1-D" are used interchangeably and are meant to refer to a pattern of growth of a crystal structure that grows in a single direction from a starting point on or face of the crystal. The term "amorphous layer" is meant to refer to a layer of non- crystalline material that forms on metal oxide crystal structures, including but not limited to on surfaces of metal oxide crystal structures, that is non- uniform as compared to the bulk crystal structure. Such layers may comprise catalyst residues used in typical processes currently available in the art for the production of metal oxide crystals. The term "nanostructure" is meant to refer to any structure wherein at least one dimension of the structure is at the sub-micron, i.e., nanoscale range. The term "aprotic solvent" refers to a solvent molecule which can neither accept nor donate a proton. Typical aprotic solvents include, but are not limited to, acetone, acetonitrile, benzene, butanone, butyronitrile, carbon tetrachloride, chlorobenzene, chloroform, 1 ,2-dichloroethane, dichloromethane, diethyl ether, dimethylacetamide, A/,Λ/-dimethylformamide (DMF), dimethylsulfoxide (DMSO), 1 ,4-dioxane, ethyl acetate, ethylene glycol dimethyl ether, hexane, Λ/-methylpyrrolidone, pyridine, tetrahydrofuran (THF), and toluene. Certain aprotic solvents are polar solvents. Examples of polar aprotic solvents include, but are not limited to, acetone, acetonitrile, butanone, Λ/,Λ/-dimethylformamide, and dimethylsulfoxide. Certain aprotic solvents are non-polar solvents. Examples of nonpolar, aprotic solvents include, but are not limited to, diethyl ether, aliphatic hydrocarbons, such as hexane, aromatic hydrocarbons, such as benzene and toluene, and symmetrical halogenated hydrocarbons, such as carbon tetrachloride. The term "protic solvent" refers to a solvent molecule that contains a hydrogen atom bonded to an electronegative atom, such as an oxygen atom or a nitrogen atom. Typical protic solvents include, but are not limited to, carboxylic acids, such as acetic acid, alcohols, such as methanol and ethanol, amines, amides, and water. II. General Considerations Zinc oxide (ZnO) is of interest in many applications, including transparent conductive coatings (T. Minami, J. Vac. Sci. Techno!. A 1999, 17, 1765) electrodes for dye-sensitized solar cells (H. Rensmo, K. Keis, H. Lindstrόm, S. Sόdergren, A. Solbrand, A. Hagfeldt, S. E. Lindquist, L. N. Wang, M. Muhammed, J. Phys. Chem. B 1997, 101, 2598), gas sensors (K. S. Weiβenrieder and J. Mϋller, Thin Solid Films 1997, 300, 30), and electro- and photo-luminescent materials (C. M. Mo, Y. H. Li, Y. S. Lin, Y. Zhang, L. P. Zhang, J. Appl. Phys. 1998, 83, 4389; S. Sakahara, M. Ishida, M. A. Anderson, J. Phys. Chem. B 1998, 102, 10169). ZnO exhibits a direct bandgap of 3.37 eV at room temperature with a large exciton binding energy of 60 meV. The strong exciton energy can ensure an efficient exciton emission at room temperature under low excitation energy (Y. Chen, D. M. Bagnall, H. Koh, K. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 1998, 84, 3912). The synthesis of 1-D ZnO nanostructures has attracted considerable interest because of their promising applications in nanoscale optoelectronic devices. Other metal oxide nanostructures that can exhibit tunable electrical, optical, magnetic and chemical properties are also of interest. The wide band gap (E_g = 3.6 eV) semiconductor tin (IV) oxide (Sn0₂) is a case in point, with potential technological applicability in gas sensors (Law, M., Kind, H., Messer, B., Kim, F., Yang, P. D. Angew. Chem. Int. Ed. 2002, 41 , 2405; Wang, Y., Jiang, X., Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176), transparent conducting electrodes (He, Y. S., Campbell, J. C, Murphy, R. C, Arendt, M. F., Swinnea, J. S. J. Mater. Res. 1993, 8, 3131 ), and transistors and solar cells (Harrison, P. G., Willet, M. J. Nature 1988, 332, 337; Ferrere, S., Zaban, A., Gregg, B. A. J. Phys. Chem. B 1997, 101, 4490). Titanium (IV) oxide (Ti0₂) is of similar interest. III. Preparation Methods and Nanostructures Prepared by the Methods Small-diameter, single crystalline, one-dimensional (1 D) metal oxide nanostructures with different aspect ratios are synthesized by a solution phase method in accordance with the presently disclosed subject matter. Surprisingly, the nanostructures prepared by the presently disclosed subject matter exhibit a diameter ranging from about 3 nm to about 100 nm, in some embodiments, from about 10 nm to about 40 nm; and a length ranging from about 50 nm to several hundred nm, including up to about 10 μm long nanowires. The metal oxide nanostructures, in some embodiments, are characterized as pure hexagonal phase, c-axis grown single crystals. In a representative method of preparing small-diameter, single crystalline, 1 D metal oxide nanostructures, stock solutions of a metal precursor, e.g., a soluble salt such as zinc acetate dihydrate (Zn(Ac)₂-2H₂0), in a protic solvent, e.g., methanol or ethanol, and solutions of a basic solution, e.g., a hydroxide solution, in a protic solvent, e.g., water, methanol and/or ethanol, are prepared. A measured volume of the metal precursor solution is mixed with a measured volume of the basic solution to form a reaction mixture. The reaction mixture is transferred to a reaction vessel lined with a corrosion-resistant material, the reaction vessel is closed to provide an elevated pressure, and the reaction mixture heated for a period of time to form a precipitate. The precipitate can then be collected, washed with a protic solvent, e.g., water and ethanol, and dried. . In some embodiments, the method is free of an active concentrating step, such as but not limited to a rotating evaporator step. The metal precursor compound is typically provided at a concentration ranging from about 0.001 mol/L to 2 mol/L, including any desirable value there between, such as 0.01 mol/L, 0.1 mol/L, 0.5 mol/L, 1 mol/L, 1.5 mol/L and 2 mol/L. Representative metals in the metal precursor solutions include but are not limited to zinc, tin, titanium, aluminum, barium, strontium, lead, zirconium, and combinations thereof. A metal of any desired valence can be provided, such as, but not limited to, zinc (II), tin (II), tin (IV), and titanium (IV). Representative combinations of metals for the metal oxide nanostructures include barium titanium oxide (BaTi0₃), strontium titanium oxide (SrTi0₃) and lead zirconium oxide (PbZrTi0₃), also referred to in the art as PZT. The metal precursor solution, in some embodiments, comprises a metal precursor compound dissolved in a protic solvent. Representative protic solvents include water, alcohols, and combinations thereof. Representative alcohols include but are not limited to methanol, ethanol, propanol, and other lower alkyl alcohols, e.g., Ci to Cβ alcohols. Further, the protic solvent can optionally comprise alcohol and water at a ratio ranging from about 0.01 :99.99 water to alcohol through about 100:0 water to alcohol, including any and all desirable values falling within this range. Representative water to alcohol ratios thus including the following: about 1 :99, about 10:90, about 20:80, about 30:70, about 40:60, about 50:50, about 60:40, about 70:30, about 80:20, about 90:10, about 99:1 , about 99.99:0.01 , and about 100:0. Representative ratios for water and alcohol in the metal precursor solution are also disclosed in Examples provided herein below. In some embodiments the basic solution comprises a hydroxide solution. Particularly, the hydroxide can be provided from a hydroxide compound selected from the group consisting of an organic hydroxide compound, an inorganic hydroxide compound, and combinations thereof. Representative inorganic hydroxide compounds include, but are not limited to, sodium hydroxide and potassium hydroxide. Representative organic hydroxide compounds include, but are not limited it, tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPAOH), tetrabutyl ammonium hydroxide (TBAOH), tetrapentyl ammonium hydroxide (TPAOH). Combinations of any of the foregoing hydroxide compounds can also be employed. Optionally, the basic solution, including, but not limited to, the hydroxide solution, can be prepared by dissolving the hydroxide compound in a protic solvent. In some embodiments the protic solvent comprises water, alcohol, and combinations thereof. Representative alcohols include but are not limited to methanol, ethanol, propanol, and other lower alkyl alcohols, e.g., Ci to Cβ alcohols. Further, the protic solvent can optionally comprise alcohol and water at a ratio ranging from about 0.01 :99.99 water to alcohol through about 100:0 water to alcohol, including any and all desirable values falling within this range. Representative water to alcohol ratios thus including the following: about 1 :99, about 10:90, about 20:80, about 30:70, about 40:60, about 50:50, about 60:40, about 70:30, about 80:20, about 90:10, about 99:1 , about 99.99:0.01 , and about 100:0. Representative ratios for water and alcohol in the basic solution are also disclosed in the Examples presented herein below. As noted hereinabove, a typical basic solution has a pH ranging from about 7.5 to about 14, and in some embodiments the basic solution has a pH of about 8, 9, 10, 11 , 12, or 13. In a representative method of preparing a metal oxide nanostructure in accordance with the presently disclosed subject matter, the reaction mixture is heated in a closed container to provide a pressure greater than ambient pressure. In some embodiments, the heating provides a reaction temperature ranging from about 20°C to about 150°C, including but not limited to any desirable temperature therebetween. Representative temperatures include but are not limited to 50°C, 60°C, 70°C, 75°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, and 150°C. Representative heating profiles are disclosed in the Examples presented herein below. It is noted that these temperatures fall well below those temperatures observed in solid-state methodology for preparation of metal oxide crystals that is currently available in the art. Typically, solid-state methodologies have reaction temperatures ranging on the order of about 800°C to about 1000°C. In some embodiments of the presently disclosed subject matter the heating occurs over a time period ranging from about 3 hours to about 24 hours, including any desired time therebetween. For example, the heating can occur for about 3 hours, for about 4 hours, for about 5 hours, for about 6 hours, for about 7 hours, for about 8 hours, for about 9 hours, for about 10 hours, for about 11 hours, for about 12 hours, for about 13 hours, for about 14 hours, for about 15 hours, for about 16 hours, for about 17 hours, for about 18 hours, for about 19 hours, for about 20 hours, for about 21 hours, for about 22 hours, for about 23 hours, and for about 24 hours. In some embodiments, the temperature is kept constant for the time period of the reaction. In some embodiments the reaction container comprises a corrosion- resistant material. A representative reaction container is lined with a corrosion- resistant material such as polytetrafluoroethylene (PTFE) sold under the registered trademark TEFLON® by E.I. DuPont deNemours & Co., Wilmington, Delaware, United States of America. Heating in a closed container provides for a pressure in the container above ambient pressure. Thus, any pressure elevation associated with heating in a closed container is meant to be encompassed by the phrase "pressure greater than ambient pressure". The pressure developed in the closed container is related to the solvents used, the percentage of the container filled with solvents, and the employed temperature (Richard I. Walton, Chem. Soc. Rev. 2002, 31, 230- 238). In some embodiments, the pressure can range from 0 to about 4000 bar, including any desirable pressure therebetween. The metal oxide nanostructure is collected from the reaction mixture. Typically, yields on the order of 100% have been observed in accordance with the presently disclosed subject matter. In some embodiments collecting the metal oxide nanostructure comprises collecting a precipitate. Optionally, the precipitate is collected, washed with a protic solvent and dried. Representative protic solvents include but are not limited to water, and alcohol and combinations thereof. Representative alcohols include but are not limited to methanol, ethanol, propanol, and other lower alkyl alcohols, e.g., Ci to Cβ alcohols. As noted herein above, the term "nanostructure" is meant to refer to any structure wherein at least one dimension of the structure is at the sub- micron, i.e., nanoscale range. In some embodiments at least one dimension of the nanostructure is less than one micrometer, or 1 ,000 nanometers. In some embodiments at least one dimension of the nanostructure is less than 500 nanometers. In some embodiments at least one dimension of the nanostructure is less than 100 nanometers. In some embodiments the at least one dimension is less than 25 nanometers. In some embodiments the at least one dimension of the nanostructure is less than 10 nanometers. In some embodiments the at least one dimension of the nanostructure is less than 5 nanometers. In some embodiments, at least one dimension of a metal oxide nanostructure of the presently disclosed subject matter comprises a cross- sectional width. A metal oxide nanostructure of the presently disclosed subject matter can have a cross-sectional width of about 100 nanometers or less, about 50 nanometers or less, about 25 nanometers or less, about 10 nanometers or less, or even about 5 nanometers or less. In some embodiments the cross-sectional width comprises a circular or polygonal cross-section and the cross-sectional width is measured by a nominal diameter. Representative embodiments of the presently disclosed subject matter can have a cross-section with a nominal diameter ranging from about 3 nanometers to about 100 nanometers, including any desirable width therebetween, such as but not limited to about 3 nanometers, about 5 nanometers, about 10 nanometers, about 15 nanometers, about 20 nanometers, about 25 nanometers, about 30 nanometers, about 35 nanometers, about 40 nanometers, about 45 nanometers, about 50 nanometers, about 55 nanometers, about 60 nanometers, about 65 nanometers, about 70 nanometers, about 75 nanometers, about 80 nanometers, about 85 nanometers, about 90 nanometers, about 95 nanometers, and about 100 nanometers. A metal oxide nanostructure of the presently disclosed subject matter can have an overall length dimension of about 50 nanometers or less, including but not limited to 45, 40, 35, 30, 25, 20, 15, 10, or 5 nanometers or less. In some such embodiments, the nanostructure can be referred to as a nanorod. In some embodiments, a representative metal oxide nanostructure of the presently disclosed subject matter can have a length of 50 nanometers or greater, including about 100 nanometers, about 250 nanometers, about 500 nanometers, about 750 nanometers, and even over 1 ,000 nanometers into the micron length range. In this latter case, the nanostructure can be referred to as a nanowire. While the length of the nanostructure can exceed 1 micron, another dimension, such as cross- sectional width, can be maintained in the nanoscale range, as discussed hereinabove. Indeed, in some embodiments the aspect ratio of a metal oxide nanostructure of the presently disclosed subject matter can be controlled depending on the combination of solvents employed in the reaction. A metal oxide nanostructure of the presently disclosed subject matter can have a mean aspect ratio of at least 4.1 , and indeed can have a mean aspect ratio ranging from at least about 4:1 to at least 100:1. Any desirable aspect ratio between this range is also provided, including 5:1 , 10:1 , 20:1 , 50:1 , and any other aspect ratio that might be desirable and as would be apparent to one of ordinary skill in the art after a review of the present disclosure. In some embodiments, the presently disclosed methods do not employ a solid substrate, wherein the substrate would be seeded or not seeded, and/or do not employ a solid substrate and a blocking agent such as but not limited to citrate. In some embodiments, the nanostructures prepared in accordance with the presently disclosed subject matter are individual, unattached and suspended in the reaction mixture. As disclosed herein, in some embodiments a metal oxide nanostructure of the presently disclosed subject matter can have a single crystalline structure. The single crystalline structure while inherently three- dimensional can be further described as one-dimensional as one growth direction is preferred and leads to an anisometric nanostructure. Additionally, the metal oxide nanostructures of the presently disclosed subject matter are substantially pure. Particularly, in some embodiments, a surface of a metal oxide nanostructure of the presently disclosed subject matter is substantially free of an amorphous layer, such as but not limited to an amorphous surface layer. By the terms "substantially free of an amorphous layer" it is meant that an amorphous layer is not detectable by standard detection approaches, such as observation by transmission electron microscopy, including high-resolution transmission electron microscopy. Correspondingly, then, a substantially pure metal oxide nanostructure does not include an amorphous layer that is detectable by a standard observation technique. Some advantages of the presently disclosed methods of synthesizing metal oxide nanostructures include low cost, high purity, and increased yield. Accordingly, the presently disclosed methods do not require high temperatures, catalysts, or special and expensive equipment, which other synthesis methods in the art, e.g., chemical vapor deposition (CVD), thermal evaporation, molecular beam epitaxy (MBE), and high-temperature vapor transport process, require. The presently disclosed methods also address, in some embodiments, aggregation of the nanostructure after evaporation of the solvent, by using capping agents to form a passivation layer on the surface of the as-prepared nanostructures to obtain a single nanorod or nanowire when the nanostructures are redispersed in solution. IV. Applications The metal oxide nanostructures provided by the presently disclosed subject matter can be used as active components or interconnects in fabricating nanoscale electronic, optical, optoelectronic, electrochemical, and electromechanical devices. These nanostructures can be applied to light-emitting diodes (LEDs), single-electron transistors, field-effect transistors (FETs), flat-panel displays (FPDs), biological and chemical sensors, photodetectors, electron emitters, and ultraviolet nanolasers. The metal oxide nanostructures can also be used as transparent conductive coatings, electrodes for dye-sensitized solar cells, gas sensors, and electro- and photo-luminescent materials. Indeed, any metal having a desirable characteristic for one or more of these applications, and any other application as would be apparent to one of ordinary skill in the art after reviewing the present disclosure, can be chosen and employed in the methods and nanostructures of the presently disclosed subject matter. In some embodiments provided herein is a photovoltaic device comprising a metal oxide nanostructure of the presently disclosed subject matter. The nanostructure may be a nanorod or a nanowire, depending on the structural features of the photovoltaic device. Structures that can be used in a photovoltaic device are known in the art and can be employed in conjunction with a metal oxide nanostructure of the presently disclosed subject matter upon a review of the disclosure herein by one of ordinary skill in the art. Referring now to Figure 11 , a schematic of a photovoltaic cell 10 employing titanium dioxide nanostructures is presented. Cell 10 comprises, in operative orientation, glass 1 ; transparent Indium-doped tin oxide film 2; platinum catalyst film 3; seal 4; electrolyte 5; and dye-impregnated Ti0₂ nanostructure film 6; and load 8. A system or device comprising cell 10 can comprise light source 7, which can be used to direct light to cell 10.

Examples The following Examples have been included to illustrate modes of the presently disclosed subject matter. In light of the present disclosure and the general level of skill in the art, those of skill can appreciate that the following Examples are intended to be exemplary only and that numerous changes, modifications, and alterations can be employed without departing from the scope of the presently disclosed subject matter. Examples 1 and 2 Growing ZnO Nanorods And Nanowires In Solutions Solutions of the following zinc precursor compounds were employed: zinc chloride (ZnCl2-xH₂0), zinc nitrate (Zn(N0₃)₂-6H₂0), and zinc acetate (Zn(Ac)₂-2H₂0). 0.001 mol/L to 2 mol/L Zn²⁺ solutions. The precursor was dissolved in a mixture of water and alcohols (methanol, ethanol, propanol). The ratio of water/alcohol can be in the range of (about 0.01 to about 50)/(about 99.9 to about 50) on a volume/volume basis. The prepared precursor solution was mixed with a basic solution with a high pH value (e.g. a pH of about 12). The latter basic solution can be prepared using a hydroxide compound, such as but not limited to a hydroxide compound selected from the group consisting of sodium hydroxide (NaOH), tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPAOH), tetrabutyl ammonium hydroxide (TBAOH), tetrapentyl ammonium hydroxide (TPAOH), and combinations thereof. Reaction. The combined solution with both precursor and basic compounds was put in a PTFE-lined (such as that available from E.I. DuPont deNemours & Co., Wilmington, Delaware, USA, under the registered trademark TEFLON®) reactor, sealed to withstand an increase in pressure, and maintained at constant temperature (which might be any value in the range of 20^°C to 150°C) for 3 to 24 hours, (for example, any of amount of time within this range). Product. A white precipitate product was obtained and purified by washing with water and/or alcohol several times. The separation of the white precipitate from water can be done by centrifugation. The aspect ratios of the one-dimensional (1-D) ZnO nanostructures can be controlled by changing the alcohol and/or the ratio of water to alcohol. Examples 1A and 1B. Figure 1 shows transmission electron microscopy (TEM) images of 1-D ZnO nanorods with controllable aspect ratios. 10 mL of 0.1 M of zinc acetate stock solution in methanol was mixed with 20 mL of 0.5 M NaOH solution in methanol to get a clear solution which was transferred to a PTFE-lined (such as that available from E.I. DuPont deNemours & Co., Wilmington, Delaware, United States of America, under the registered trademark TEFLON®) stainless steel autoclave and heated at 150^°C for 24 hours. The reaction procedure in ethanol medium was the same. Example 1A. Aspect ratio = 4:1 ; length = 100nm; diameter = 25nm. The water to methanol ratio was 0.01 to 99.9. The reaction was conducted at 150^°C for 24 hrs. Example 1B. Aspect ratio = 20:1 ; length = 500nm, diameter = 25nm.

The water to ethanol ratio was 0.01 to 99.9. The reaction was conducted at 150^°C for 24 hrs. Characterization. The synthesized product was characterized by X- ray powder diffraction (XRD) (Rigaku Multiflex X-ray diffractometer, with Cu- K_α radiation, λ = 0.154178 nm at 40 kV and 40 mA), transmission electron microscopy (TEM; Philips CM-12, with a accelerating voltage of 100 kV), high-resolution transmission electron microscopy (HRTEM; Topcon EM-

002B, with a accelerating voltage of 200 kV). Fig. 2 shows X-ray diffraction patterns from final products prepared in methanol (Example 1A) and ethanol (Example (1B). The diffraction peaks are quite similar to those of bulk ZnO, which can be indexed as the hexagonal wurtzite structure of ZnO.

The calculated lattice constants are aA = 3.2529 A, c_A = 5.2099 A and as =

3.2522 A, c_B = 5.2084 A, respectively, for the methanol and ethanol products. Both lattice parameters are in good agreement with the JCPDS file of ZnO (JCPDS 36-1451 ). Figure 3 shows high-resolution TEM results indicating that all of these nanorods with different aspect ratios are single-crystalline. Examples 2A, 2B and 2C. Figure 4 shows transmission electron microscopy (TEM) images of 1-D ZnO nanostructures with controllable aspect ratios (from nanorods to nanowires). Example 2A. Aspect ratio = 5:1 ; length = 100nm; diameter = 20nm. The water to methanol ratio was 0.5 to 99.5. Reaction conducted at 150^°C for 24 hrs. Example 2B. Aspect ratio = 20:1; length = 800nm; diameter = 40nm. The water to methanol ratio was 10 to 90. Reaction conducted at 150^°C for 24 hrs. Example 2C. Aspect ratio = 100:1; length = 10μm; diameter = 100nm. The water to methanol ratio is 30 to 70. Reaction conducted at 150^°C for 24 hrs.

Discussion of Examples 1A, 1B, 2A, 2B and 2C. No evidence for oriented attachment of particles was found even at the primary stage of growth. This observation suggests that 1-D ZnO growth along the c-axis under hydrothermal condition is related to both its intrinsic crystal structure and external factors. ZnO is a polar crystal. The overall shape and aspect ratio of crystals are determined by the relative rates of growth of its various faces. In general the growth rate of a face will be controlled by a combination of internal, structurally-related factors (intermolecular bonding preferences or dislocations), and external factors (supersaturation, temperature, solvents and impurities) (N. Kubota, Cryst. Res. TechnoL, 2001 , 36, 749). Among the external factors, the effect of solvents on the nucleation, and growth of crystals has been studied (E. A. Meulenkamp, J. Phys. Chem. B, 1998, 102, 5566; E. M. Wong, J. E. Bonevich, P. C. Searson, J. Phys. Chem. B, 1998, 102, 7770; J. H. ter Horst, R. M. Geertman, G. M. van Rosmalen, J. Crystal Growth, 2001 , 230, 277; E. M. Wong, J. E. Bonevich, P. C. Searson, J. Phys. Chem. B, 1998, 102, 7770). While not desiring to be bound by any particular theory of operation, it is believed that the different aspect ratios of ZnO nanorods result from the different growth rates along c-axis in different reaction media, i.e., the growth rate along c-axis in ethanol is higher than that in methanol. Hence, again, while not desiring to be bound by any particular theory of operation, it is believed that morphologies of polar inorganic nanocrystals can be controlled by the interface-solvent interactions, which in turn, can be specified by choosing a suitable solvent.

EXAMPLE 3 Synthesis Of SnO₂ Nanorods In Solution This Example discloses a solution route to single-crystalline Sn0₂ nanorods with dimensions approaching those of molecules (diameter ~ 3.4 nm). In a typical procedure, a Sn⁴⁺ precursor (SnCI₄ ^■ 5H₂0, 0.001 mol) was dissolved in a basic mixture of alcohol and water (a pH of about 12). The clear solution with dissolved precursor was transferred to a PTFE (e.g., TEFLON®-brand, DuPont)-lined stainless steel autoclave and heated at 150^°C for 24 hrs. A white-gray precipitate was collected, purified and dried in air at ambient temperature with a yield of approximately 100%. Tin precursors that were used included: tin chloride (SnCI -5H₂0) and tin isopropoxide. 0.001 mol/L to 2 mol/L Sn⁴⁺ solutions. The precursor was dissolved in mixture of water and alcohols (methanol, ethanol, propanol). The ratio water/alcohol can be in the range of (about 0.01 to about 10)/(about 99.9 to about 90) on a volume/volume basis. The prepared precursor solution was mixed with a basic solution with high pH value (e.g., pH of about 12). The latter basic solution can be prepared using a hydroxide compounds, such as but not limited to a hydroxide compound selected from the group consisting of sodium hydroxide (NaOH), tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPAOH), tetrabutyl ammonium hydroxide (TBAOH), and combinations thereof. Reaction. The combined solution with both precursor and basic compounds was put in a PTFE (TEFLON® brand, DuPont)-lined reactor, sealed to withstand an increase in pressure, and maintained at constant temperature (which might be any value in the range of 20^°C to 150^°C) for 3 to 24 hours (for example, any amount of time within this range). Product. The white-grey precipitate product was obtained and purified by washing with water several times. The separation of the white precipitate from water can be done by centrifugation. The aspect ratios of the one-dimensional (1-D) Sn0₂ nanostructures can be controlled by changing the alcohol and/or the ratio of water to alcohol. Examples 3A and 3B. Figures 5A and 5B show transmission electron microscopy (TEM) images of 1-D Snθ₂ nanorods with controllable aspect ratios. Example 3A. Aspect ratio = 5:1 ; length = 17nm; diameter = 3.4nm. The water to methanol ratio is 0.5 to 99.5. Reaction conducted at 150^°C for 24 hrs. Example 3B. Aspect ratio = 10:1 ; length = 30nm, diameter = 3.0nm. The water to methanol ratio is 2 to 98. Reaction conducted at 150^°C for 24 hrs. Characterization. Figures 6A and 6B show high-resolution TEM results indicating that all of these nanorods with different aspect ratios are single-crystalline. The morphology, structure and size of the Sn0₂ nanorods were characterized with TEM and HRTEM. Figure 6A is a wide-field TEM image of the Sn0₂ nanorods that clearly indicates that the product is entirely comprised of a relatively uniform, rod-like morphology (15 - 20 nm rod lengths and 2.5 - 5 nm rod diameters). The lattice fringes in the HRTEM image (Figure 6B) further confirm the single-crystal nature of the Sn0₂ nanorods. The spacing between adjacent lattice planes is 0.337 nm, corresponding to (110) planes of rutile Sn0₂, which indicates that the preferential growth direction is [001]. The mean rod length (17 + 4 nm) and diameter (3.4 ± 0.6 nm) were extracted from TEM image of more than 300 nanorods. The synthesized Sn0₂ nanorods were characterized by X-ray powder diffraction (XRD) (Rigaku Multiflex X-ray diffractometer, with Cu-K_α radiation (λ = 0.154178 nm at 40 kV and 40 mA)), transmission electron microscopy (TEM; JEOL JEM-100CXII, with a accelerating voltage of 100 kV) and high- resolution transmission electron microscopy (HRTEM; JEOL JEM-2010F, with a accelerating voltage of 200 kV). Figure 7 shows the powder XRD- pattern from the as-prepared product. The diffraction peaks are quite similar to those of bulk Sn0₂, which can be indexed as the tetragonal rutile structure of Sn0₂ with lattice constants of a = 4.74 A and c = 3.18 A. This observation is in good agreement with the JCPDS file of Sn0₂ (JCPDS 41- 1445). No impurity peaks were observed, indicating the high purity of the final products. However, the peaks were relatively broad compared with the bulk material, corroborating the very small crystal size. Discussion of Examples 3A and 3B An analysis of the surface energies in different crystallographic orientations gives information about relative growth rates of the different crystal facets. The present findings for Sn0₂ single-crystalline nanorods show a mean aspect ratio of ~ 4:1 with the [001] direction along the major axis. Raman spectroscopy is also sensitive to crystal surface area. The room temperature Raman spectrum of the Sn0₂ nanorods had bands at 576 cm^"1 and 356 cm^"1 in addition to the Aι_g vibration mode at 631 cm^"1. However, the phonon modes of the first two bands were not observed in Raman spectra of bulk single crystal and polycrystalline Sn0₂. These modes, being related to the facet surface area of a crystal, arise from nanoscale Sn0₂ with small grain size. This finding agrees very well with the Matossi force constant model (X. M. Sun, X. Chen, Z. X. Deng, Y. D. Li, Mater. Chem. Phys. 2002, 78, 99; C. Xu, G. Xu, Y. Liu, G. Wang, So//αf State Commun. 2002, 122, 175). The Raman bands confirm the characteristics of the tetragonal rutile structure as well as the very small size of the Sn0₂ nanorods. Figure 8 shows the room temperature photoluminescence spectrum of the as-prepared Sn0₂ nanorods. A red emission at 580 nm was observed from the Sn0₂ nanorods using a He-Cd laser (~325 nm) as the excitation source. The low energy emission might be related to crystal defects or defect levels associated with oxygen vacancies, or tin interstitials that have formed during growth. However, this does not impact overall purity. Some defects of this nature (e.g. vacancies) can arise during crystal growth prepared by various methods, such as solution phase growth, high- temperature thermal evaporation, chemical vapor deposition or laser ablation. In summary, small diameter, single-crystalline Sn0₂ nanorods were prepared in solution at low temperature without using catalysts. The single- crystalline Sn0₂ nanorods show a mean aspect ratio of 4:1 with the [001] direction along the major axis. The optical measurements further show that the Sn0₂ nanorods possess surface characteristics that generate a red emission band that can be exploited in gas sensors or other optoelectronic devices.

Example 4 Growing TiO₂ Nanorods in Solutions Titanium precursors that were used included: Titanium (IV) chloride

(TiCU), Titanium (IV) isopropoxide. 0.001 mol/L to 2 mol/L Ti⁴⁺ solutions. The precursor was dissolved in mixture of water and alcohols (methanol, ethanol, propanol). The ratio water/alcohol can be in the range of (about 100 to about 50)/(about 0 to about 50) on a volume/volume basis. The prepared precursor solution was mixed with a basic solution with high pH value (a pH of about 12). The latter basic solution can be prepared using a hydroxide compound, such as a hydroxide compound selected from the group consisting of sodium hydroxide (NaOH), tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPAOH), tetrabutyl ammonium hydroxide (TBAOH), tetrapentyl ammonium hydroxide (TPAOH), and combinations thereof. In this Example, a NaOH solution was used. Reaction. The combined solution with both precursor and basic compounds was put in a PTFE (TEFLON® brand, DuPont)-lined reactor, sealed to withstand an increase in pressure, and maintained at constant temperature (which might be any value in the range of 20^°C to 150^°C) for 3 to 24 hours (optionally any time point within this range). Product. A white precipitate product was obtained and purified by washing with water several times. The separation of the white precipitate from water can be done by centrifugation. Example 4. Figure 9 shows a transmission electron microscopy (TEM) image of 1-D Ti0₂ nanorods: aspect ratio = 5:1 ; length = 100nm; diameter = 20nm. Reaction conducted in aqueous solution (without alcohol) at 150^°C for 24 hrs. Example 5 Application of 1-D Ti0₂: Photovoltaic Cell ("Solar cell") Figure 10 shows current versus voltage (I - V) characteristic of a dye- sensitized solar cell made from the Tiθ₂ nanorods. A schematic of a dye- sensitized solar cell is presented in Figure 11. The findings were comparable to reported values for powdered Ti0₂.

It will be understood that various details of the presently disclosed subject matter can be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.

Claims

CLAIMS What is claimed is: 1. A method of preparing a metal oxide nanostructure, the method comprising: (a) mixing a metal precursor solution with a basic solution to form a reaction mixture; and (b) heating the reaction mixture in a closed container to provide a pressure greater than ambient pressure.

2. The method of claim 1 , wherein a metal in the metal precursor solution comprises a metal selected from the group consisting of zinc, tin, titanium, aluminum, barium, strontium, lead, zirconium, and combinations thereof.

3. The method of claim 1 , wherein the metal precursor solution comprises a metal precursor compound dissolved in a protic solvent.

4. The method of claim 2, wherein the protic solvent comprises a component selected from the group consisting of water, an alcohol, and combinations thereof.

5. The method of claim 4, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol and combinations thereof.

6. The method of claim 4, wherein the protic solvent comprises alcohol and water at a ratio ranging from about 0.01 :99.99 water to alcohol through about 100.0:0.00 water to alcohol.

7. The method of claim 1 , wherein the basic solution comprises a hydroxide solution.

8. The method of claim 7, wherein the hydroxide solution comprises a solution selected from the group consisting of an inorganic hydroxide solution and an organic hydroxide solution.

9. The method of claim 8, wherein the hydroxide solution comprises a compound selected from the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPAOH), tetrabutyl ammonium hydroxide (TBAOH), tetrapentyl ammonium hydroxide (TPAOH), and combinations thereof.

10. The method of claim 7, wherein the hydroxide solution comprises hydroxide ion in a protic solvent.

11. The method of claim 10, wherein the protic solvent comprises a component selected from the group consisting of water, an alcohol, and combinations thereof.

12. The method of claim 11 , wherein the alcohol is selected from the group consisting of methanol, ethanol, isopropanol and combinations thereof.

13. The method of claim 11 , wherein the protic solvent comprises alcohol and water at a ratio ranging from about 0.01 :99.99 water to alcohol through about 100.0:0.00 water to alcohol.

14. The method of claim 1 wherein the basic solution has a pH of about 12.

15. The method of claim 1 , wherein the heating provides a reaction temperature ranging from 20°C to 150°C.

16. The method of claim 1 , wherein the heating occurs over a period of time ranging from about 3 hours to about 24 hours.

17. The method of claim 1 , wherein the closed container comprises a corrosion-resistant material.

18. The method of claim 1 , comprising collecting a metal oxide nanostructure from the reaction mixture.

19. The method of claim 18, wherein collecting the metal oxide nanostructure comprises collecting a precipitate.

20. The method of claim 19, wherein the precipitate is collected and washed with a protic solvent, and dried.

21. The method of claim 20, wherein the protic solvent comprises a component selected from the group consisting of water, an alcohol, and combinations thereof.

22. The method of claim 21, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol and combinations thereof.

23. A product made b'y the method of claim 1.

24. A metal oxide nanostructure having a mean aspect ratio of at least about 4:1 and a cross-sectional width of about 100 nanometers or less.

25. The metal oxide nanostructure of claim 24, wherein the metal oxide is selected from the group consisting of ZnO, Sn0₂, and Ti0₂.

26. A metal oxide nanostructure of claim 24, having a mean aspect ratio ranging from at least about 4:1 to at least about 100:1.

27. A metal oxide nanostructure of claim 24, having one of a circular and a polygonal cross-section.

28. A metal oxide nanostructure of claim 27, wherein the cross- section has a nominal diameter ranging from about 3 nanometers to about 100 nanometers.

29. The metal oxide nanostructure of claim 24, having a length of about 50 nanometers or less.

30. The metal oxide nanostructure of claim 24, having a length of about 50 nanometers or greater.

31. The metal oxide nanostructure of claim 24, having a single crystalline structure.

32. The metal oxide nanostructure of claim 24, wherein a surface of the nanostructure is substantially free of an amorphous layer.

33. A photovoltaic device comprising a metal oxide nanostructure of claim 24.