[0001] This application claims priority to U.S. Provisional Patent Application No. 62/307,350, filed March 11, 2016, and to U.S. Provisional Patent Application No. 62/308,142, filed March 14, 2016, the entire content of each of which is incorporated by reference herein. BACKGROUND OF THE INVENTION

[0002] Metal oxide nanostructures have structural and physical properties that make the nanostructures suitable for many technological applications, such as solar fuel cell and battery technology, tissue engineering, and photocatalysis. Ilmenite is an abundant titanium-iron oxide mineral that can be used as a starting material for making metal oxide nanostructures. There remains a need for economical and scalable methods of forming metal oxide nanostructures from ilmenite and other metal oxide starting materials. SUMMARY OF THE INVENTION

[0003] Embodiments of the disclosure relate generally to metal oxide nanostructures/ nanomaterials and processes of preparing the same. In one aspect, methods of forming a metal oxide nanostructure are provided. In some embodiments, the method comprises: immersing a precursor metal oxide in an aqueous acid solution to form an acid-treated metal oxide;

separating the acid-treated metal oxide from the aqueous acid solution; immersing the acid-treated metal oxide in an aqueous base solution at a temperature of at least about 110°C, wherein the immersing step comprises stirring; and drying the acid- and base-treated metal oxide to yield the metal oxide nanostructure. [0004] In some embodiments, the precursor metal oxide comprises a titanate. In some embodiments, the precursor metal oxide comprises ilmenite. In some embodiments, the precursor metal oxide comprises a powder. In some embodiments, the powder comprises particles between 2 to 10 μm in size. In some embodiments, the powder comprises particles under 5 μm in size. In some embodiments, the method further comprises a step of milling a precursor metal oxide (e.g., ilmenite) to form oxide powders having an average size (e.g., a median particle size or a mean particle size) of 10 μm or less. [0005] In some embodiments, the aqueous acid solution comprises hydrochloric acid (HCl) or citric acid. In some embodiments, the aqueous acid solution comprises HCl at a concentration of about 2 M to about 10 M. In some embodiments, the step of immersing the precursor metal oxide in the aqueous acid solution is performed at a temperature and time sufficient to at least partially leach out a metal from the precursor metal oxide. In some embodiments, the precursor metal oxide comprises ilmenite and the step of immersing the precursor metal oxide in the aqueous acid solution is performed at a temperature and time sufficient to at least partially leach out iron from the precursor metal oxide. In some embodiments, the step of immersing the precursor metal oxide in the aqueous acid solution is performed at a temperature of at least about 80°C. In some embodiments, the step of immersing the precursor metal oxide in the aqueous acid solution is performed at a temperature of between 80°C and 100°C. In some embodiments, the step of immersing the precursor metal oxide in the aqueous acid solution is performed at for at least about 2 hours. In some embodiments, the step of immersing the precursor metal oxide in the aqueous acid solution is performed for about 2-4 hours. In some embodiments, the step of immersing the precursor metal oxide in the aqueous acid solution results in the formation of an acid-treated metal oxide that comprises at least about 90% titanium. [0006] In some embodiments, prior to the step of immersing the acid-treated metal oxide in an aqueous base solution, the acid-treated metal oxide is washed with an aqueous solution. In some embodiments, the washing comprises centrifugation and/or filtration. [0007] In some embodiments, the aqueous base solution comprises sodium hydroxide (NaOH) or potassium hydroxide (KOH). In some embodiments, the aqueous base solution comprises NaOH at a concentration of at least 2 M. In some embodiments, the step of immersing the acid-treated metal oxide in an aqueous base solution is performed in a high pressure vessel (> 100 bar) at a temperature of about 100°C to about 140°C. In some embodiments, the step of immersing the acid-treated metal oxide in an aqueous base solution is performed at a temperature of about 120°C. In some embodiments, the step of immersing the acid-treated metal oxide in an aqueous base solution comprises autoclaving. In some embodiments, the step of immersing the acid-treated metal oxide in an aqueous base solution comprises continuous stirring. In some embodiments, the step of immersing the acid-treated metal oxide in an aqueous base solution is performed for at least about 36 hours. [0008] In some embodiments, prior to the drying step, the method further comprises washing the acid- and base-treated metal oxide in an aqueous solution. In some embodiments, prior to the drying step, the method further comprises washing the acid- and base-treated metal oxide in HCl. In some embodiments, the drying step comprises oven drying the acid- and base-treated metal oxide at a temperature of at least about 100°C for at least about one hour. [0009] In some embodiments, the method of forming nanostructures (e.g., nanofibers) comprises: leaching a precursor metal oxide in an aqueous acid at a temperature and time sufficient to at least partially leach out iron;

autoclaving leached precursor metal oxide in an aqueous base at a temperature and time sufficient to form nanofibers; and

drying the nanofibers.

[0010] In some embodiments, the heating temperature is between 80°C and 95°C. [0011] In some embodiments, the method further comprises performing a sodium ion (Na+) exchange on the nanofibers after autoclaving. In some embodiments, the autoclaving temperature is above 120 °C. [0012] In some embodiments, the nanofibers are dried until all water is removed. In some embodiments, the drying is at a temperature of about 100°C for about 1 hour. [0013] In some embodiments, the precursor comprises powder between 2 to 10 μm in size. In some embodiments, the precursor comprises powder under 5 μm in size. In some embodiments, the precursor comprises metal oxide titanate powder or oxide mixtures of iron, copper, vanadium, and manganese. [0014] In some embodiments, the aqueous acid is HCl or citric acid. In some embodiments, the aqueous base is 2-10 M NaOH. [0015] In some embodiments, the nanostructure (e.g., nanocomposite) is a metal oxide nanofiber dominated by Ti and containing Na and a transition metal. [0016] In some embodiments, the nanocomposite is further doped with an element selected from the group consisting of d-block metals, lanthanoids, and actinoids. [0017] In another aspect, nanostructures produced according to the methods described herein are provided. In some embodiments, the nanostructure is a nanofiber. In some embodiments, the nanostructure is a nanofiber comprising titanium dioxide (TiO ₂ ). In some embodiments, the nanostructure is a nanocomposite. In some embodiments, the nanostructure is a nanocomposite comprising an oxide of titanium and at least one more transition metal (groups 8 to 12 that include Mn, Fe, Co, Ni, Cu, Zn, Ag, Pd, Pt, Ir, Au, Cd, Ru) and/or multiple element component alloys of transition metals (e.g. Au/Ag, Ni/Cd, Fe/Au, Ni/Pt, Pt/Ni), wherein titanium is the predominant metal in the nanocomposite. In some embodiments, the nanocomposite comprises an oxide of titanium and one or more of copper, iron, vanadium, or manganese. [0018] In another aspect, compositions comprising such nanostructures (e.g., nanofibers or nanocomposites) are provided. In some embodiments, scaffolding for tissue engineering comprising such nanostructures are provided. In some embodiments, antimicrobial films comprising such nanostructures are provided. In some embodiments, photovoltaic cells comprising such nanostructures are provided. In some embodiments, high capacity storage battery components comprising such nanostructures are provided. In some embodiments, photocatalytic environmental remediation assemblies comprising such nanostructures are provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG.1 illustrates a scanning electron microscopy image of Na _x Fe _y Ti _(l-z) O _z nanofibers synthesized via a two-step sequential process of acid (4M HCl) and base (10 M NaOH ~ 114 hrs) hydrothermal treatment of ball-milled ilmenite (FeTiO ₃ ) powder. [0020] FIGS. 2A-2B illustrate scanning electron microscope images of treated, ball-milled ilmenite powder after (A) acid (4 M HCl) treatment alone or (B) base (10 M NaOH) treatment alone. [0021] FIG.3 illustrates an embodiment of a method for forming nanofibers. [0022] FIGS. 4A-4B illustrate that similar nanofibers form after sequential acid treatment (4 M HCl) and base treatment (10 M NaOH) of naturally occurring ilmenite obtained from ores in Canada and Pakistan. Following base treatment, the samples were washed in aqueous HCl and dried at 100°C. (A) 4 hours acid treatment followed by 48 hours base treatment. (B) 4 hour acid treatment followed by 36 hours base treatment. DETAILED DESCRIPTION OF THE INVENTION

I. INTRODUCTION

[0023] Disclosed herein are various embodiments of nanostructures/nanomaterials (e.g., nanocomposites, nanofibers, etc.) having advantageous morphologies, and thus final properties, as well as advantageous methods of manufacturing such nanostructures. In some embodiments, the nanostructure is a metal oxide nanostructure, such as a nanostructure comprising a metal oxide of copper (Cu), iron (Fe), titanium (Ti), vanadium (V), or manganese (Mn). In some embodiments, the nanostructure is a metal oxide nanofiber, such as a nanofiber comprising a metal oxide of Cu, Fe, Ti, V, or Mn. [0024] As described herein, methods of synthesizing metal oxide nanostructures from a metal oxide starting material (e.g., ilmenite) have been developed that comprise treating the metal oxide starting material with an acid treatment, followed by base treatment at high temperature. Embodiments of the disclosed methods have the advantage of easily being scaled up for the mass production of nanofiber or tubular nanostructures (e.g., titanate nanostructures), and thus are suitable for industrial applications. Additionally, the methods described herein use readily available starting materials and reagents. For example, the starting materials can be obtained from naturally occurring mineral sources such as ilmenite (FeTiO ₃ ) which is cheap and abundant; large reserves of ilmenite of greater than 680 million tons exist. [0025] Various embodiments of the disclosure are an unexpected improvement over the published methodologies, in which either base treatment or acid treatment was used, such as Tao et al., "Expanding the applications of the ilmenite mineral to the preparation of nanostructures: TiO ₂ nanorods and their photocatalytic properties in the degradation of oxalic acid," Chemistry 2013, 19 (3), 1091-6; and Simpraditpan et al., A; "Effect of calcination temperature on structural and photocatalyst properties of nanofibers prepared from low- cost natural ilmenite mineral by simple hydrothermal method," Materials Research Bulletin 2013, 48 (9), 3211-3217, the entirety of each of which is hereby incorporated by reference. Simpraditpan and Tao describe protocols to fabricate nanofibers from low-cost ilmenite. However, as shown in Figures 2A-2B, base or acid treatments alone do not produce the advantageous nanofiber morphology as achieved by embodiments of the disclosed methods. Additionally, previously described methods are difficult to reproduce or are contradictory with one another. Thus, these approaches are not robust and reproducible as the nanofiber yield is strongly dependent upon the purity of the starting material. [0026] Other published methods to make nanofibers use electrospin technology. The electrospin method starts with salt precursors (e.g. TiC1 ₄ and FeCl ₃ ) that are suspended in a polymeric matrix that is electrospun onto a substrate and then heated to remove the organic matter. The problem with this approach, however, is that it is difficult to produce nanofibers on a large scale. II. DEFINITIONS

[0027] The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, because the scope of the present invention will be limited only by the appended claims. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent. In some cases, terms with commonly understood meanings are defined herein for clarity and/or for ready reference, and the inclusion of such definitions herein should not be construed as representing a substantial difference over the definition of the term as generally understood in the art. [0028] The term "comprising" is intended to mean that the compounds, compositions and methods include the recited elements, but does not exclude others. "Consisting essentially of" when used to define compounds, compositions and methods, shall mean excluding other elements that would materially affect the basic and novel characteristics of the claimed invention. Embodiments defined by each of these transition terms are within the scope of this invention. [0029] As used herein, the term "nanostructure" refers to a material that has a size in at least one dimension that is on the nanoscale. In some embodiments, a nanostructure has a size in at least one dimension of less than about 100 nm. In some embodiments, a nanostructure has a size in at least one dimension of between about 1 nm and about 100 nm. In some embodiments, a nanostructure is in the form of a nanofiber or a nanoplate (e.g., a fiber or a plate having a thickness of less than about 100 nm). In some embodiments, a nanostructure is in the form of a "nanocomposite," which as used herein, refers to a composite of two or more materials in which at least one of the materials has a size in at least one dimension that is on the nanoscale (e.g., less than about 100 nm). [0030] As used herein, the term "precursor metal oxide" refers to a composition comprising an oxide of one or more metals. In some embodiments, the precursor metal oxide is a mineral or ore comprising an oxide of one or more metals, e.g., a naturally occurring mineral or ore such as ilmenite. In some embodiments, the precursor metal oxide comprises one or more additional components (e.g., a metal or an impurity) that is removed or separated from the metal oxide according to a method as described herein. [0031] As used herein, "titanate" refers to a compound comprising an oxide of titanium and at least one additional metallic element (including but not limited to copper, iron, manganese, and vanadium). In some embodiments, a titanate comprises an oxide of titanium and iron (e.g., FeTiO ₃ ). [0032] Conditional language, such as "can," "could," "might," or "may," unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include or do not include, certain features, elements, and/or steps. Thus, such conditional language is not generally intended to imply that features, elements, and/or steps are in any way required for one or more embodiments. [0033] Conjunctive language such as the phrase "at least one of X, Y, and Z," unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z. [0034] Language of degree used herein, such as the terms "approximately," "about," "generally," and "substantially" as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms "approximately", "about", "generally," and "substantially" may refer to an amount that is within less than or equal to 10% of, within less than or equal to 5% of, within less than or equal to 1% of, within less than or equal to 0.1% of, and within less than or equal to 0.01% of the stated amount. If the stated amount is 0 (e.g., none), the above recited ranges can be specific ranges, and not within a particular % of the value. For example, within less than or equal to 10 wt./vol. % of, within less than or equal to 5 wt./vol. % of, within less than or equal to 1 wt./vol. % of, within less than or equal to 0.1 wt./vol. % of, and within less than or equal to 0.01 wt./vol. % of the stated amount. III. METAL OXIDE NANOSTRUCTURES

[0035] In one aspect, metal oxide nanostructures are provided. In some embodiments, a metal oxide nanostructure is prepared according to a method described herein. In some embodiments, nanostructures are materials that have sizes in at least one dimension of between 1 nm and 100 nm (or between about 1 nm and about 100 nm), e.g., between about 1 nm and about 80 nm, between about 1 nm and about 60 nm, between about 1 nm and about 40 nm, between about 5 nm and about 75 nm, or between about 5 nm and about 50 nm. In some embodiments, a nanostructure has a size in at least one dimension of less than 100 nm, e.g., less than 90 nm, less than 80 nm, less than 70 nm, less than 60 nm, less than 50 nm, less than 40 nm, less than 30 nm, or less than 20 nm. In some embodiments, a nanostructure has a diameter of between about 1 nm and about 100 nm, e.g., between about 1 nm and about 80 nm, between about 1 nm and about 60 nm, between about 1 nm and about 40 nm, between about 5 nm and about 75 nm, or between about 5 nm and about 50 nm. In some embodiments, a nanostructure has a diameter of less than 100 nm, e.g., less than 90 nm, less than 80 nm, less than 70 nm, less than 60 nm, less than 50 nm, less than 40 nm, less than 30 nm, or less than 20 nm. In some embodiments, a nanostructure has a width of between about 1 nm and about 100 nm, e.g., between about 1 nm and about 80 nm, between about 1 nm and about 60 nm, between about 1 nm and about 40 nm, between about 5 nm and about 75 nm, or between about 5 nm and about 50 nm. In some embodiments, a nanostructure has a width of less than 100 nm, e.g., less than 90 nm, less than 80 nm, less than 70 nm, less than 60 nm, less than 50 nm, less than 40 nm, less than 30 nm, or less than 20 nm. [0036] For example, in some embodiments, thermally stable mixed metal oxide nanocomposites are disclosed. In some embodiments, the nanocomposites can be nano- metal oxides that contain two or more metals. As another example, metal oxide nanofibers, such as titanate nanofibers, can be formed, though the disclosure is not so limiting and it will be understood that the particular methods disclosed can be used for the formation of other types of nanofibers. In some embodiments, metal oxide nanomaterials such as TiO ₂ /Fe ₂ O ₃ and FeTiO ₃ can be formed. Small percentages of other transition metals can be incorporated into the nanostructures. [0037] In some embodiments, disclosed herein are embodiments of a simple and robust method for the preparation of mixed metal oxide (MMO) nanostructures, such as those formed using mixed metal oxides of Cu, Fe, Ti, V, or Mn. As described herein, embodiments of the methodology can use two steps of sequential acid and base hydrothermal treatment of metal oxide mixtures (e.g., V, Mn, Fe, Cu titanates, and MO ₂ /M _x TiO ₃ mixtures where M = Mg, Cu, Fe, Ti, V, Mn) in order to consistently, predictably, and reproducibly form the nanofibers. [0038] In some embodiments, thermally stable nanomaterial mixed metal oxide nanostructures can be formed. An example of a nanofiber formed from an embodiment of the disclosed method is shown in Figure 1, which is a scanning electron microscopy image of metal oxide nanofibers dominated by Ti, but also containing Na and Fe by Energy Dispersive Spectroscopy (EDS) analysis. In some instances, the nanostructures (e.g., nanofibers) can further be doped/incorporated with d-block-metals that exhibit catalytic properties (e.g., to the right of group 7) such as, for example, Ag, Au, Pt, Pd, Ru, Os, Cu, Ni, lanthanoids, and actinoids, though other types of doping can be used as well. Doping nanostructures can refer to adding small amounts of other elements such as those listed above to the base MMO nanostructure. In various embodiments, doping with elements listed above can enhance any of the physical or electrical properties of the nanomaterial. In some embodiments of the disclosed method, incorporation of doping metals can be performed in conjunction with the formation of MMO nanofibers. [0039] In some embodiments, the produced nanofibers can be 1-100 nm thick or any ranges within this range, e.g., 2-75 nm (or about 2 to about 75 nm) thick or 2-50 nm (or about 2 to about 50 nm) thick and microns in length. In some embodiments, the nanofibers can be longer than typically formed nanofibers known in the art. However, other dimensions of nanofibers can be formed as well, and the particular dimensions do not limit the disclosure. [0040] In some embodiments, the nanostructure (e.g., nanofiber) that is formed comprises predominantly a single metal. For example, in some embodiment the nanostructure comprises predominantly titanium. As used herein, the term "predominantly" means that that metal makes up at least about 50% (e.g., at least 60%, at least 70%, at least 80%, at least 90%, or at least 95%) of the metals in the nanostructure; thus, as a non-limiting illustration, a nanostructure that comprises "predominantly titanium" means that titanium makes up at least about 50% of the metals in the nanostructure. In some embodiments, the nanostructure comprises at least 90% of a single metal (e.g., comprises at least 90% titanium). In some embodiments, the nanostructure comprises predominantly a single metal (e.g., titanium) and further comprises sodium. In some embodiments, the nanostructure further comprises residual or trace amounts of one or more other metals and/or minerals (e.g., which can be residual or trace amounts that remain after treating a precursor metal oxide such as ilmenite with acid treatment and base treatment). For example, in some embodiments the nanostructure comprises predominantly titanium, further comprises sodium, and further comprises residual or trace amounts of one or more of iron, manganese, zinc, or silicates, and/or is doped with one or more other metals (e.g., one or more transition metals, alkali metals, and/or alkaline earth metals). IV. METHODS OF PRODUCING METAL OXIDE NANOSTRUCTURES

[0041] In another aspect, methods of producing metal oxide nanostructures are provided. In some embodiments, a metal oxide nanostructure as described herein is formed from the sequential two-step process of acid and base hydrothermal treatment of metal oxide ("MO") mixtures (e.g., V, Mn, Fe, Cu titanates, and MO _y /M _x TiO ₃ mixtures where M = Cu, Fe, Ti, V, Mn). In some embodiments, a metal oxide nanostructure yield of greater than 50% can be achieved, though this can increase with greater time treatments as discussed herein. [0042] In some embodiments, the method comprises: immersing a precursor metal oxide in an aqueous acid solution to form an acid-treated metal oxide;

separating the acid-treated metal oxide from the aqueous acid solution; immersing the acid-treated metal oxide in an aqueous base solution at a temperature of at least about 100°C, wherein the immersing step comprises stirring; and drying the acid- and base-treated metal oxide to yield the metal oxide nanostructure.

[0043] In some embodiments, the precursor metal oxide comprises an oxide mixture of two or more (e.g., two, three, four, five, six or more) transition metals, such as but not limited to titanium, iron, copper, vanadium, manganese, scandium, chromium, nickel, and zinc. In some embodiments, the precursor metal oxide comprises a titanate. In some embodiments, the precursor metal oxide comprises ilmenite, which is an oxide mineral comprising titanium-iron oxide (FeTiO ₃ ). In some embodiments, the precursor metal oxide (e.g., ilmenite) is in a particulate form, e.g., a powder. In some embodiments, the powder comprises particles that have an average size (e.g., diameter) of less than about 5 μm. In some embodiments, the powder comprises particles that have an average size (e.g., diameter) of less than about 10 μm. In some embodiments, the powder comprises particles that have an average size (e.g., diameter) of greater than about 2 μm. In some embodiments, the powder comprises particles that have an average size (e.g., diameter) of between about 2 to about 10 μm. In some embodiments, the powder comprises a population of particles wherein at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the particles have an average size (e.g., diameter) of less than about 10 μm, e.g., less than about 5 μm. In some embodiments, the powder comprises a population of particles wherein at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the particles have an average size (e.g., diameter) of between about 2 to about 10 μm or of less than about 10 μm (e.g., less than less than about 5 μm). Methods of determining particle size and particle size distribution are known in the art. See, e.g., Tao et al., CrystEngComm, 2013, 13:1322- 1327. In some embodiments, the methods described herein comprise a step of milling a precursor metal oxide (e.g., ilmenite) to produce a powder. Methods of processing metal oxide ores such as ilmenite into powders are known in the art. See, e.g., WO 1995/008004. [0044] In some embodiments, the precursor metal oxide (e.g., ilmenite) is immersed in an aqueous acid solution that comprises a mineral acid or an organic acid. In some embodiments, the aqueous acid solution comprises a mineral acid, including but not limited to hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boric acid, hydrofluoric acid, hydrobromic acid, perchloric acid, or hydroiodic acid. In some embodiments, the aqueous acid solution comprises hydrochloric acid. In some embodiments, the aqueous acid solution comprises an organic acid, including but not limited to citric acid, formic acid, acetic acid, carbonic acid, lactic acid, malic acid, oxalic acid, or benzoic acid. In some embodiments, the aqueous acid solution comprises citric acid. In some embodiments, the aqueous acid solution has a pH of less than about 5. In some embodiments, the aqueous acid solution has a concentration of at least about 2 M, e.g., about 2 M, about 3 M, about 4 M, about 5 M, about 6 M, about 7 M, about 8 M, about 9 M, or about 10 M. In some embodiments, the aqueous acid solution has a concentration of about 2 M to about 10 M, e.g., about 4 M to about 10 M or about 6 M to about 10 M. In some embodiments, the aqueous acid solution comprises HCl at a concentration of about 2 M to about 10 M. [0045] In some embodiments, the step of immersing the precursor metal oxide in an aqueous acid solution comprises heating the solution. Without being bound to a particular theory, in some embodiments heating the aqueous acid solution may increase the amount of metals and/or impurities that are leached out of the precursor metal oxide into the aqueous acid solution. In some embodiments, the aqueous acid solution is heated to a temperature of at least about 80°C, e.g., at least 85°C, at least 90°C, or at least 95°C. In some embodiments, the aqueous acid solution is heated to a temperature of up to about 100°C. In some embodiments, the aqueous acid solution is heated to a temperature of about 80°C to about 100°C, e.g., about 80°C to about 95°C. [0046] In some embodiments, the precursor metal oxide is immersed in the aqueous acid solution for at least about 30 minutes, at least about 60 minutes, at least about 90 minutes, at least about 120 minutes, or longer. In some embodiments, the precursor metal oxide is immersed in the aqueous acid solution for at least about 1 hour, at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, at least about 6 hours, or longer. In some embodiments, the precursor metal oxide is immersed in the aqueous acid solution for about 1 hour to about 6 hours, e.g., about 2 hours to about 4 hours. [0047] In some embodiments, the step of immersing the precursor metal oxide in an aqueous acid solution comprises immersing the precursor metal oxide in the aqueous acid solution for a sufficient length of time to leach out at least some metal(s) and/or impurities from the precursor metal oxide such that at the end of the acid treatment step, the resulting product comprises predominantly a single metal. In some embodiments, at the end of the acid treatment step, the total metal composition of the resulting product comprises at least 60%, at least 70%, at least 80%, or at least 90% of a single metal. In some embodiments, at the end of the acid treatment step, the resulting product comprises predominantly titanium. For example, in some embodiments, the precursor metal oxide comprises ilmenite and at the end of the acid treatment step, the resulting product comprises predominantly titanium (i.e., at least some of the iron in the ilmenite leaches out). In some embodiments, the total metal composition of the resulting product after acid treatment comprises at least 70%, at least 80%, at least 90%, or at least about 95% titanium. [0048] In some embodiments, up to about 30%, up to about 25%, up to about 20%, about 15%, up to about 10%, or up to about 5% of the total metal composition of the resulting product after acid treatment comprises a second metal. In some embodiments, about 1% to about 30%, about 5% to about 25%, about 5% to about 20%, or about 10% to about 20% of the total metal composition of the resulting product after acid treatment comprises a second metal. As a non-limiting example, in some embodiments, the precursor metal oxide comprises ilmenite and at the end of the acid treatment step, the total metal composition of the resulting product comprises predominantly titanium (e.g., comprises at least 70%, at least 80%, at least 90%, or at least about 95% titanium) and further comprises up to about 30%, up to about 25%, up to about 20%, about 15%, up to about 10%, or up to about 5% of iron. Without being bound to a particular theory, in some embodiments the amount of a second metal that is present in the acid-treated metal oxide may promote or enhance the formation of particular structures. For example, in some embodiments, a lower amount of second metal (e.g., iron) present in the acid-treated metal oxide may promote or enhance the formation of fibrous structures (e.g., nanofibers) while a higher amount of second metal (e.g., iron) present in the acid-treated metal oxide may promote or enhance the formation of plate-like structures. A person of ordinary skill in the art will recognize that the conditions of the acid treatment step (e.g., concentration of acid, temperature, and length of time) can be adjusted to increase or decrease the amount of second metal (e.g., iron) that is present in the acid-treated metal oxide at the end of the acid treatment step. [0049] Subsequent to the acid treatment step, the acid-treated metal oxide is separated from the aqueous acid solution, and in some embodiments, the acid-treated metal oxide is washed to remove residual acid. In some embodiments, the acid-treated metal oxide is washed with an aqueous solution (e.g., deionized water or distilled water). In some embodiments, the washing is performed by filtration. In some embodiments, the washing is performed by centrifugation. [0050] The acid-treated metal oxide is then immersed in an aqueous base solution. In some embodiments, the aqueous base solution comprises sodium hydroxide or potassium hydroxide. In some embodiments, the aqueous base solution has a concentration of at least about 2 M, e.g., about 2 M, about 3 M, about 4 M, about 5 M, about 6 M, about 7 M, about 8 M, about 9 M, or about 10 M. In some embodiments, the aqueous base solution has a concentration of about 2 M to about 10 M, e.g., about 5 M to about 10 M. In some embodiments, the aqueous base solution comprises NaOH at a concentration of at least 5 M. [0051] In some embodiments, the step of immersing the acid-treated metal oxide in an aqueous base solution is performed at a temperature of about 100°C to about 140°C, e.g., about 100°C to about 120°C, about 110° to about 140°C, or about 120°C to about 140°C. In some embodiments, the step of immersing the acid-treated metal oxide in an aqueous base solution is performed at a temperature of at least about 110°C. In some embodiments, the acid-treated metal oxide in the aqueous base solution is heated under pressure, e.g., using an autoclave. [0052] In some embodiments, acid-treated metal oxide is then immersed in the aqueous base solution for at least about 12 hours, at least about 15 hours, at least about 18 hours, at least about 24 hours, at least about 30 hours, at least about 36 hours, at least about 42 hours, at least about 48 hours, at least about 54 hours, at least about 60 hours, at least about 72 hours, at least about 84 hours, or at least about 96 hours. In some embodiments, acid-treated metal oxide is then immersed in the aqueous base solution for about 12 hours to about 96 hours, e.g., for about 24 hours to about 60 hours, for about 36 hours to about 60 hours, or for about 36 hours to about 48 hours. [0053] In some embodiments, the acid-treated metal oxide in the aqueous base solution is stirred such that the acid-treated metal oxide sample remains suspended in the solution. For example, in some embodiments, the stirring is performed using paddles or a stir bar. In some embodiments, the acid-treated metal oxide in the aqueous base solution is stirred continuously throughout the length of the base treatment step. Without being bound to a particular theory, in some embodiments the step of stirring the acid-treated metal oxide sample in the aqueous base solution promotes or enhances the formation of fibers, e.g., longer fibers and/or a greater quantity of fibers. Also without being bound to a particular theory, the amount and/or size of a nanostructure (e.g., nanofiber) that is formed during the base treatment step may vary depending on conditions such as the concentration of base, the temperature, and the length of time of the base treatment step. A person of ordinary skill in the art will recognize that the conditions of the base treatment step (e.g., concentration of base, temperature, length of time, and amount or speed of stirring) can be adjusted to form the desired shape, size, and quantity of a particular nanostructure (e.g., nanofibers). [0054] After the base treatment step and prior to the drying step, in some embodiments, the method further comprises washing the acid- and base-treated metal oxide in an aqueous solution. In some embodiments, the acid- and base-treated metal oxide is washed with an aqueous solution (e.g., deionized water or distilled water). In some embodiments, the washing is performed by filtration. In some embodiments, the washing is performed by centrifugation. In some embodiments, prior to the drying step, the method further comprises washing the acid- and base-treated metal oxide in HCl to remove excess sodium ions that may be present in the sample. [0055] In some embodiments, the sample is dried for a length of time sufficient to remove substantially all liquids from the resulting nanostructures (e.g., nanofibers). In some embodiments, the sample is oven dried at a temperature of about 100°C or higher (e.g., at least about 100°C or at least about 110°C) for at least one hour (e.g., for at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 hours or more). A person of ordinary skill in the art will appreciate that other methods of drying the sample can also be used. [0056] Optionally, in some embodiments, one or more metals can be incorporated into the metal oxide nanostructure during the synthesis methods described herein. In some embodiments, the one or more metals are transition metals, alkali metals, and/or alkaline earth metals. In some embodiments, the one or more metals are transition metals that exhibit catalytic properties, such as but not limited to Ag, Au, Pt, Pd, Ru, Os, Cu, Ni, lanthanoids, and actinoids. In some embodiments, the one or more metals (e.g., transition metals, alkali metals, and/or alkaline earth metals) are incorporated into the nanostructure by adding the one or more metals to the aqueous base solution into which the acid-treated metal oxide is immersed. Thus, in some embodiments, the methods described herein comprise immersing the acid-treated metal oxide in an aqueous base solution at a temperature of at least about 100°C, wherein the aqueous base solution comprises one or more transition metals, alkali metals, and/or alkaline earth metals. In some embodiments, the one or more metals (e.g., transition metals, alkali metals, and/or alkaline earth metals, e.g., Ag or Au) are coated onto the nanostructure, e.g., after the nanostructure (e.g., nanofiber) is formed and dried. [0057] Disclosed below with respect to Figure 3 is an exemplary method 300 for forming the advantageous nanostructures. [0058] First, a metal oxide starting material can be provided 302. The metal oxide starting material can be, for example, metal oxide titanate powder, such as those formed from an ilmenite (FeTiO ₃ ) rock. In some embodiments, the starting material can be an oxide mixture of iron, copper, vanadium, and manganese, e.g., a powder. In some embodiments, the powder can be less than 5 um in size (or less than about 5 um in size). In some embodiments, the powder can be less than 10 um in size (or less than about 10 um in size). In some embodiments, the powder can be greater than 2 um in size (or greater than about 2 um in size). [0059] The powder can be heated at a range of 80-95°C (or about 80 to about 95°C) in an aqueous acid to leach out iron 304 in the powder to better expose any titanium to the reaction. The aqueous acid can be, for example, HCl or citric acid (e.g., 2-10 M HCl such as 4 M HCl or 10 M HCl), though other acids can be used as well. The powder in the acid can be heated for >30 minutes (or > about 30 minutes) per 0.5 g (or about 0.5 g) of powder. In some embodiments, the acid treatment can occur for up to 4 hours (or up to about 4 hours). The product formed from this acidic hydrothermal treatment can be known as the sample. The temperature and time in the aqueous acid can be selected based on the materials used (e.g., the metal oxide starting material and/or the aqueous acid). For example, in some embodiments, metals may leach out at a higher temperature for some metal oxides compared to other metal oxides. Also, in some embodiments, metals may leach out faster in a relatively stronger acid compared to a relatively weaker acid. As yet another example, in some embodiments, metals may leach out faster with increasing temperature. Accordingly, the temperature and time in the aqueous acid can be selected to at least partially leach out the metal based on the starting composition and structure that can vary in a naturally occurring ore. [0060] After the acid treatment, the acid-treated metal oxides can be autoclaved (e.g., heated under pressure) with an aqueous base 306. In some embodiments, the autoclave temperature can be above 120°C (or above about 120°C). In some embodiments, the autoclave temperature can be at least 150°C (or at least about 150°C). In some embodiments, the autoclave temperature can be 150°C or lower (or about 150°C or lower). Further, an example aqueous base can be a 2-10 M (or about 2– about 10 M) NaOH solution though other bases can be used as well. In some embodiments, the aqueous base can be greater than 2-10 M NaOH. In some embodiments, 20 mL (or about 20 mL) of base can be used per 0.5 g (or about 0.5 g) of sample. The autoclaving in the aqueous base can be performed until nanofiber formation. In some embodiments, this step can last for 12 hours to 96 hours (3-5 days) (or about 12 hours to about 96 hours). The temperature and time in the aqueous base can be selected based on the materials used (e.g., the metal oxide used and/or the aqueous base) and/or the desired shape, size, and quantity of the nanofibers to be produced. For example, in some embodiments, nanofibers may form at a higher temperature for some metal oxides compared to other metal oxides. Also, in some embodiments, nanofibers may form faster in a relatively stronger base compared to a relatively weaker base. As yet another example, in some embodiments, nanofibers may form faster with increasing temperature. Further, in some embodiments, longer and/or more nanofibers may form with longer time. Accordingly, the temperature and time in the aqueous base can be selected to form the desired shape, size, and quantity of the nanofibers based on a number of variables. For example, longer times can produce longer nanofibers and more complete conversion of the starting materials. [0061] After treatment with the base, the based-treated sample can be washed with water (e.g., deionized water, distilled water, or another inert solution) 308. In some embodiments, the washing can be done with a centrifuge. [0062] After washing, a sodium (Na ⁺ ) ion-exchange with HCl 310 can be performed which can remove sodium ions from the surface of the nanofibers to yield a more consistent final product. In some embodiments, the sample is suspended in 1 M HCl (or about 1 M HCL), although other acids may be used. After the ion-exchange, washing and vacuum filtering can be performed. [0063] The resulting residue can then be oven dried 312. In particular, the residue can be oven dried at 100°C (or about 100°C) for one hour (or about one hour). The drying time can be long enough to remove water from the amount of material being processed. Thus, larger amounts of material may use longer drying times. Further, in some embodiments, additional drying time does not harm the product. V. APPLICATIONS OF METAL OXIDE NANOSTRUCTURES

[0064] In another aspect, applications and uses of the metal oxide nanostructures (e.g., nanofibers and nanocomposites) described herein are provided. Embodiments of the disclosure address the need for large scale synthesis of metal oxide nanofibers. Non-limiting applications of metal oxide nanomaterials (e.g. TiO ₂ /Fe ₂ O ₃ , FeTiO ₃ ) include tissue engineering, wound dressings, solar fuel cell and battery technology, gas sensors, photocatalysis (e.g., for the destruction of harmful organic and biological contaminants in air), and water treatment systems. The methods described herein provide a sustainable, green approach to synthesizing metal oxide nanostructures that have application in these technologies. [0065] The flexibility of embodiments of the above described methodology can be extended to the correct proportions of oxides for commercial applications in high capacity storage batteries, fuel cells, and photocatalytic environmental remediation (such as through treatment in air and water cleanup of chemical and biological contaminates). Thus, in some embodiments, the disclosure provides fuel cells, solar cells, photovoltaic cells, and high capacity storage battery components comprising a metal oxide nanostructure as described herein. Moreover, embodiments of the nanostructures (e.g., nanofibers) can be used was scaffolding for tissue engineering or in biomedical devices. A non-limiting example is in stem cell research as 3-dimensional matrix containing nutrients for cellular and tissue growth. Another example is in orthopedic medicine where the implant of these nanofibers can aid in tissue regeneration after joint replacement. In other embodiments, the nanostructures described herein can be used in the preparation of an anti-microbial coating or film, e.g., for food packaging. VI. EXAMPLES

[0066] The following examples are offered to illustrate, but not to limit, the claimed invention. Example 1: Generation of Titanium Dioxide Nanofibers

[0067] Ilmenite ore was obtained and milled into powder form. The ilmenite powder was heated at a temperature of 80°C in an aqueous acid solution comprising 4 M HCl for four hours. After acid treatment, the resulting sample was washed by centrifugation and filtration to remove excess acid and dissolved impurities (e.g., metal ions such as Fe). The sample was then immersed in an aqueous base solution comprising 10 M NaOH and was heated to 120° to 140°C under pressure by autoclaving. The sample was treated for 48 hours (for generating the nanofibers shown in Figure 4A) or for 36 hours (for generating the nanofibers shown in Figure 4B). Following base treatment, the resulting sample was washed and a sodium ion exchange with HCl was performed to remove excess sodium ions. The sample was then oven dried for at least 12 hours at a temperature of 110°C. [0068] Figures 4A-B illustrate the nanofibers that were formed from a four hour acid treatment followed by a 48 hour base treatment (Figure 4A) or from a four hour acid treatment followed by a 36 hour base treatment (Figure 4B). This example illustrates that the method described herein is robust and that similar products are obtained even when starting with materials found in nature that exhibit variation in composition. For example, ilmenite ore used in this example was obtained from Pakistan and Canada. Ilmenite ore from Pakistan is known to contain more Si, Cr, Mn, V, and Fe than ilmenite ore from Canada, while the ore from Canada contains more Mg. [0069] While several components, techniques and aspects have been described in the foregoing description with a certain degree of particularity, various changes can be made in the specific designs, constructions and methodology herein above described without departing from the spirit and scope of this disclosure. Certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as any subcombination or variation of any subcombination. [0070] Moreover, while methods may be depicted in the drawings or described in the specification in a particular order, such methods need not be performed in the particular order shown or in sequential order, and that all methods need not be performed, to achieve desirable results. Other methods that are not depicted or described can be incorporated in the example methods and processes. For example, one or more additional methods can be performed before, after, simultaneously, or between any of the described methods. Further, the methods may be rearranged or reordered in other implementations. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products. Additionally, other implementations are within the scope of this disclosure. [0071] While a number of embodiments and variations thereof have been described in detail, other modifications and methods of using the same will be apparent to those of skill in the art. Accordingly, it should be understood that various applications, modifications, materials, and substitutions can be made of equivalents without departing from the unique and inventive disclosure herein or the scope of the claims. [0072] All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually intended to be incorporated by reference, and are incorporated by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.