MULTIPODAL NANOTUBES AND PROCESS FOR MAKING SAME

Abstract

Nanostructures, nanostructure arrays and a method of forming same are provided, wherein the nanostructures comprise ordered, self-organized, anodically formed single nanotubes, multipodal nanotubes or a combination thereof.

Description

PRIORITY

This application claims priority of U.S. Provisional Patent Application Ser. No. 61/417,733, entitled “Multipodal Nanotubes and Process for Making Same”, filed Nov. 29, 2010.

TECHNICAL FIELD

The present disclosure relates to the field of nanostructure arrays. More specifically, the present disclosure relates to nanostructures having increased hierarchical structures comprising single nanotubes, multipodal nanotubes and/or a combination thereof. The present disclosure also relates to an electrochemical anodization technique for controllably producing the foregoing nanostructure arrays.

BACKGROUND

The process of electrochemical anodization to form oriented nanotubes and nanostructure arrays is known. For example, anodization techniques are commonly applied to form n-type semiconducting, near-vertically oriented, self-organized titania (TiO₂) nanotube arrays. Such TiO₂ nanotube arrays constitute a mechanically robust, often functionalized architecture having a high surface area with vectorial electron percolation pathways. Very similar anodization techniques have also been used to form near-vertically oriented, self-organized nanotube and nanostructure arrays in other valve metal oxides such as, for examples, hafnium oxide, zirconium oxide and iron oxide.

The particular structure of nanotube and nanostructure arrays allows for their use in a variety of applications, including, without limitation, gas sensors, photocatalysts and scaffolds for excitonic solar cells. Other known applications, where the tubular structure and modifiable pore-size of the nanotubes array are the properties of interest, include drug eluting coatings for medical implants, solid-phase microextraction (SPME) fibers and stem cell differentiation.

Despite significant progress in the field of nanotube and nanostructure array production (i.e. the ability to tune the length, wall thickness and diameter of nanotubes), a number of applications, including the aforementioned techniques, would benefit from the production of nanostructures having a more complex topology than known titania or other nanotube arrays. For instance, applications and techniques that may rely upon volumetric filling or surface functionalization of nanotubes might benefit from the production of multipodal nanotubes.

It is known that modifying the anodization parameters, such as, for example, the voltage or temperature applied to the structures as they form may produce more complex nanostructures. However, the impact of modifying anodization parameters remains relatively unknown. Attempts have been made to increase the degree of complexity of nanostructures through multi-step sonoelectrochemical anodization methods. The resulting number and frequency of nanostructures may be increased by causing the nanostructures to “branch”, i.e. to cause the nanostructure to divide, such that the parent nanostructure branches into a “Y-shaped” nanostructure having two identically-sized daughter nanostructures. Despite the foregoing process producing hierarchically branched nanostructures, however, the resulting structures are still limited because the division of the parent nanostructure necessitates that the two or three daughter structures be identical in size. This is true even where the nanostructure is “multi-branched”, such as having two, three, four, or even more branches.

There is a need for nanotubes and nanostructure arrays having a more complex topology, such as, for example, multipodal nanostructures. There is further a need for a controllable and reproducible method of producing nanotubes and nanostructure arrays having more complex topology.

SUMMARY

An electrochemical anodization method of producing nanostructure arrays having single nanotubes, multipodal nanotubes, or a combination thereof, is provided. A nanostructure array comprising a plurality of oriented, tapered nanostructures, wherein some or all of the nanostructures may have combined to be at least bipodal (i.e., having at least two “legs”), is further provided.

An electrochemical anodization method for producing a nanostructure array having single nanotubes, multipodal nanotubes, or a combination thereof, is provided, wherein the method comprises the steps of:

• • a. providing a substrate capable of undergoing anodization,
  • b. providing an electrolytic solution for receiving the substrate,
  • c. providing means for restricting the mobility of ions in the electrolytic solution, and
  • d. anodizing the substrate to produce single nanotubes, multipodal nanotubes or a combination thereof.

In one embodiment, the means for restricting, reducing or slowing the mobility of ions may comprise providing an electrolytic solution having a viscosity sufficient to restrict the mobility of ions within the solution. In another embodiment, the means for restricting or slowing the mobility of ions may comprise providing a substrate having pre-existing nanostructures. In a further embodiment, the means for restricting the mobility of ions may comprise providing an electrolytic solution having a viscosity to restricting the mobility of ions, a substrate having pre-existing nanostructures, or a combination thereof. It is contemplated that other means for restricting or slowing the mobility of ions may be provided and/or combined with the means described herein.

In one embodiment, the present method may comprise providing an electrolytic solution comprising a mixture of:

a. a solvent,

b. a halide-bearing species, and

c. de-ionized water.

A nanostructure array comprising a plurality of oriented, tapered nanostructures, wherein some or all of the nanostructures have combined to form multipodal (e.g. at least bipodal) nanotubes, is further provided.

In one embodiment, the present nanostructures may comprise a pore size (e.g. diameter) of at least 150 nm.

Broadly stated, in some embodiments, a method of producing a nanostructure array is provided, comprising: providing a substrate capable of undergoing electrochemical anodization to form at least one nanostructure, providing an electrolytic solution for receiving the substrate, providing means for restricting the mobility of ions in the electrolytic solution and anodizing the substrate to form single nanotubes, multipodal nanotubes and/or a combination thereof.

Broadly stated, in some embodiments, a nanostructure array is provided, wherein the array may comprise a plurality of oriented, combined nanostructures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is SEM images of multipodal titania nanotubes anodized in a diethylene glycol (DEG) solution with 0.25% HF and 2% water (a) at 120V for 44 h, (b) at 120V for 47 h, and (c) and (d) at 150V after 47 h. Arrows in FIG. 1(a) point to multipodal nanotubes not obscured by the topology or tilt angle;

FIG. 2(a) is an SEM image of the cross-section of titania nanotubes formed by anodization at 120V in a DEG electrolytic solution with 0.25% HF and 1% water showing a clear taper from the base to the mouth; FIG. 2(b) shows anodic current density as a function of anodization time for 120V anodization with identical DEG electrolytic solution (0.25% HF and 1% water);

FIG. 3 is an SEM image of the surface of a Ti foil anodized in a DEG-based electrolytic solution containing 0.25% HF and 1% H₂O for 43 hours at 120V. Two distinct regions consisting of close-packed and widely separated nanotubes are demarcated by the delineated border;

FIG. 4 is SEM images of titania nanotubes anodized at 120V in a DEG-based electrolytic solution with 0.25% HF and 1% water after (a) 40 h, (b) 43 h, (c) 45 h, and (d) 47 h;

FIGS. 5(a) and (b) are SEM images of titania nanotubes anodized at 120V in a DEG-based electrolytic solution with 0.25% HF and 1% water (a) after 45 h of anodization and 1 hour in the same bath without electric field (four consecutive bipodal nanotubes can be seen), (b) after 45 h of anodization; FIG. 5(c) is a graph showing pore size of the individual and combined nanotubes vs. anodization time; and FIGS. 5(d) and (e) are schematic images of the pore size increment in individual (d) and combined nanotubes (e);

FIG. 6 is an SEM image of the surface of a Ti foil anodized in a DEG-based electrolytic solution containing 0.25% HF and 1% H₂O for 45 hours at 120V. Two distinct regions consisting of close-packed and widely-separated nanotubes are seen (showing more advanced chemical dissolution of the widely-separated nanotubes than that observed in FIG. 3). Many of the surviving nanotubes in the chemically etched region are multipodal (circled);

FIG. 7 is an SEM image of closely compact TiO₂ nanotubes after 22 hours of anodization;

FIG. 8 shows a cross-sectional view of the grown individual nanotubes as of 22 hours of anodization in which their tapered conical shape can be seen. This image demonstrates that none of the nanotubes are combined during or following the first step of anodization;

FIG. 9 is an SEM image of nanotubes produced by two-step anodization, wherein the first step consisted of a 22 hr anodization followed by a subsequent 19.5 hr anodization (no nanotube combination is observed);

FIG. 10 shows nanotubes produced by two-step anodization, wherein the first step consisted of a 22 hr anodization followed by a subsequent 23.5 hr anodization. Nanotube combination is observed to be in the initial stages as the randomly oriented straight lines are locations where nanotube combination is occurring or has already occurred;

FIG. 11 shows a line of combining nanotubes;

FIG. 12 shows several lines of combining nanotubes;

FIG. 13 shows lateral increment of the pore size due to the nanotube combination process;

FIG. 14(a) shows an SEM image (top view) of titania nanotubes subjected to 22 hours of anodization in the first step followed by a 19.5 hour anodization in the second step (the typical outer diameter of the nanotube is ˜150 nm); FIG. 14(b) shows an SEM image (top view) of titania nanotubes subjected to 22 hours of anodization in the first step followed by a 23.5 hour anodization in the second step (the typical outer diameter of the nanotube is still ˜150 nm);

FIG. 15 shows an SEM (top view) of nanotubes combining following anodization in formamide,

FIG. 16 shows an SEM (top view) of close-packed multipodal nanotubes following anodization in formamide;

FIG. 17 shows an SEM (top view) of separated nanotubes following anodization in formamide (enlarged view); and

FIG. 18 shows an SEM (top view) of separated nanotubes following anodization in formamide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Nanotubes and nanostructure arrays having increased hierarchical structure are provided. More specifically, nanotubes having more complex topology, nanostructure arrays having increased hierarchical structure and a controllable process of making same are provided.

According to one aspect of the invention, the present anodization techniques may used to controllably fabricate a nanostructure array having an oriented, complex hierarchical structure and topology. One-step and/or multi-step techniques described herein may be used to form nanotubes having increased hierarchical topology, such as, for example, multipodal nanotubes, as well as nanostructure arrays having more complex structures, such as, for example, nanotube arrays having single nanotubes, multipodal nanotubes, or a combination thereof.

Anodization Process

Preferred methods of the present anodization techniques will now be described with reference to FIGS. 1-18. It is contemplated that any anodization technique that is capable of slowing or inhibiting mass transport and/or reducing the mobility of ions, may be provided. In other words, without limiting the present technology in anyway, it may be desirable to provide any anodization technique that is capable of restricting or constraining the transport/movement of ions through the electrolytic solution (i.e. more onerous ion travel). The foregoing hindrance of mass ion transport may further prevent possible dissipation of ion gradients within the electrolytic solution, thereby preserving the scarcity of certain ions at different areas along the nanostructure during formation thereof.

It should be understood that known anodization one-step or multi-step techniques may be utilized by way of the present method and that a skilled person in the art would know and understand how such techniques may be modified to result in the present nanotubes having more complex structure and/or nanostructure having increased hierarchical topology. One can further appreciate that there may be many features that distinguish the instant technology from known nanotubes, nanostructure arrays and methods of producing same. Indeed, the present embodiments have been included to communicate the features of the design, structure and associated method of the techniques and are by way of example only, and in no way intended to duly limit the disclosure thereof.

The present anodization techniques may comprise providing a substrate capable of undergoing anodization. In one embodiment, the present techniques may comprise the use of a titanium foil. It should be appreciated that the material undergoing anodization need not be titanium, but may be a variety of other substrates, such as, for example, silicon wafers coated with vacuum deposited titanium or glass substrates coated with titanium, or valve metals, including but not limited to zirconium, iron, tantalum, niobium or hafnium. Further, it is contemplated that the substrate may or may not include existing nanostructures thereon.

For example, the present techniques may comprise the use of a 0.25 mm thick titanium foil (99.7%, Sigma Aldrich). The titanium may be anodized utilizing known two-electrode anodization methods. The two-electrode anodization may be used having titanium foils as both the anode and the cathode. Alternatively, the cathode may comprise any number of metals, including, for example, platinum and gold, as would be known to a person skilled in the art.

The titanium foils may be dimensioned such that the anode is larger than the cathode. For example, the anode and the cathode may be dimensioned such that the anode is 1.25 cm×3.8 cm and the cathode is 0.6 cm×3.8 cm. It would be known to a person skilled in the art that the anode and cathode foils might only have half of their length immersed in the electrolytic solution.

Prior to anodization, the titanium foils may be cleaned ultrasonically with soap, de-ionized water and isopropyl alcohol and then dried with nitrogen gas. Anodization may be carried out at room temperature.

The present anodization techniques may comprise an electrolytic solution, for receiving the substrate.

In one embodiment, the electrolytic solution may comprise a solvent having a viscosity sufficient to slow or inhibit the movement of ions through the solution. For example, the present anodization technique may comprise a solvent having a viscosity greater than water (1 cP) and at least greater than 3 cP. In a preferred embodiment, the electrolytic solution may comprise diethylene glycol (DEG; Fisher Chemical), ethylene glycol or any other suitable organic solvent of viscosity greater than 10 cP.

The electrolytic solution may further comprise a halide-bearing species. In one embodiment, the halide-bearing species may be selected from the group consisting of a fluoride-bearing species or a chloride-bearing species. In a preferred embodiment, the halide-bearing species may be a fluoride-bearing species. For example, the fluoride-bearing species may be HF (48% solution, Sigma Aldrich). The HF concentration of the electrolytic solution may be less than 0.5%. In a preferred embodiment, the HF concentration may be between 0.25% and 0.35%. In another embodiment, the fluoride-bearing species may be NH₄F.

The electrolytic solution may further comprise de-ionized water. In one embodiment, the concentration of de-ionized water may be at least 2%.

It is contemplated that the electrolytic solution may comprise any solvent capable of dissolving a halide-bearing species. In one embodiment, the electrolytic solution may comprise, for example, formamide as the solvent. Having regard to FIGS. 15-18, the use of different solvents, such as formamide, may result in different inter-tubular spacings between the nanostructures, and further the production of regular, non-multipodal nanotubes of smaller diameter in the “background” of the combined, multipodal nanostructures, thereby demonstrating a nanostructure array having a further increased hierarchical structure.

The present anodization technique may comprise the application of voltage sufficient to create an electric field. In one embodiment, the technique may comprise the application of voltage of at least 70V. In a preferred embodiment, the voltage applied to the anodization may be between 120V and 150V. It should be understood that a skilled person would know and understand the appropriate voltage level to be applied for the particular anodization technique being used.

It is contemplated that a multi-step anodization process may also be used to create the present nanostructure array. For example, in one embodiment, the anodization process may occur by way of a first step, wherein the voltage applied is at least 10 V, and at least one subsequent step(s), wherein the voltage applied is at least 35V. In another embodiment, the at least one subsequent step(s) may comprise increasing the voltage during the course of the anodization. For example, the voltage applied may begin at at least 35V and increase to at least 50V (at a predetermined rate such as, for example, 1V every 5 minutes).

The anodization process may be applied to the electrolytic solution for a duration sufficient to create nanotubes having increased hierarchical structure, and, more particularly, combined nanotubes. In one embodiment, the anodization process may be applied to the electrolytic solution for a duration of at least 20 hours. In a preferred embodiment, the anodization process may be applied for a duration of at least 40 hours.

Where a multi-step anodization process may be used, the anodization process may occur by way of a first step, wherein the duration of the anodization is at least 45 hours, and at least one subsequent step(s), wherein the duration of the anodization is at least 3 hours.

Following anodization, the substrate foils containing the nanotubes may be cleaned by rinsing the foils with isopropyl alcohol and drying them in air. Subsequently, the foils may be placed into 0.1 M HCl acid for an hour and then dried in the oven for one hour at 100° C.

Morphology of the nanotubes including their length, diameter, wall thickness and separation may be investigated using a scanning electron microscope (SEM, ZEISS) as well as a field-emission scanning electron microscope (FESEM, JEOL 6301F).

Formation of Multipodal Nanotubes

EXAMPLE 1

One-Step Electrochemical Anodization, DEG Solvent, Anodization Voltages of 120 V or Greater, Anodization Durations>40 Hours and HF Concentrations Lower than 0.5%

The present example demonstrates the use of the present anodization techniques to controllably produce oriented, complex nanotubes having increased hierarchical structure and topology. More specifically, the present example demonstrates a process referred to as “nanotube combination”, whereby the present anodization results in “parent” nanotubes leaning towards each other and combining or conjoining to form multipodal “daughter” nanotubes having at least two “legs” (see FIGS. 1-14). According to one aspect of the invention, nanotube combination may result from the conjoining of two or more parent nanotubes to form one multipodal daughter nanotube having a larger pore size than each of the individual parent nanotubes.

The present example shows that the parent nanotubes need not be the same diameter, size or fabricated at the same time in order for nanotube combination to occur. For example, two parent nanotubes having different original pore size and diameter that are fabricated during two separate anodization processes may combine to form a multipodal daughter nanotube. It should be understood that the multipodal daughter nanotube may be combined to comprise at least two “legs”, and indeed, may comprise more than two “legs”.

FIG. 1 depicts the present anodically-formed multipodal titania (TiO₂) nanotube arrays, wherein a number of the visible nanostructures are bipodal (FIGS. 1(a),(b)), or even tetrapodal where two bipodal nanotubes have combined (FIG. 1(c)). Although the nanotube in FIG. 1(d) appears bipodal at first glance, it is actually tetrapodal and only appears bipodal because the process of nanotube combination for the constituent bipodal nanotubes is complete.

As a result of nanotube combination, the present anodization process may be utilized to fabricate a nanotube array having larger pore sizes than known anodization processes. Anodization in DEG-based solvents, for example. appears to exhibit certain unpredictable and unusual features such as the formation of hollow nanotubes with very large pore sizes (up to ˜900 nm) and discretization of nanotubes by large inter-tubular spacing.

Having regard to FIG. 2, a plurality of the nanostructures formed with the present method, whether or not multipodal structures, may exhibit a tapered conical or pyramidal appearance, having a wider base and narrower mouth portion. The present nanotube combination process may thus also have a decisive role in the simultaneous increment of both pore size and inter-tubular spacing of nanotube arrays formed by the present techniques, thereby providing a means of monitoring and modifying inter-tubular spacing. Such control over the nanostructure may potentially provide an advantage over, and deviation from, the close-packed architecture resulting from known anodization techniques.

It is hypothesized that the “leaning” or “angling” of the nanostructures towards each other may be as a result of the forces produced during the present “time-dependent” anodization. Indeed, the current nanotube behaviour during the present anodization process appears to result in nanotube combination (see FIGS. 5a-5e).

By way of background, field-assisted oxide dissolution and cation migration, field-assisted oxidation of Ti and chemical etching are the competing reactions responsible for the growth of TiO₂ nanotube arrays.

The field-assisted reactions occur on either side of the barrier layer at the base of the nanotubes and are responsible for driving the Ti/TiO₂ interface deeper into the Ti foil, a process that increases the length of the nanotubes. Chemical etching shortens the length of the nanotubes. The relevant chemical equations are as follows:

Field assisted oxidation: Ti+2H₂O→TiO₂+4H⁺+4e⁻  (1)

Field assisted migration: Ti⁴⁺+6F⁻→[TiF₆]²⁻  (2)

Field assisted dissolution: TiO₂+6F⁻+4H⁺→[TiF₆]²⁻+2H₂O   (3)

Chemical dissolution: TiO₂+6HF→[TiF₆]²⁻+2H₂O+2H⁺  (4)

Having regard to FIG. 2(a), nanotubes formed in HF bearing DEG-based electrolytes at large anodization potentials appear to exhibit a taper, wherein the nanostructure comprises a wider base and a narrower mouth. This taper may occur as a consequence of the significant variation in the conductivity of the electrolyte resulting over the course of the anodization process.

The low conductivity of the DEG-based electrolyte has been remarked upon by others and occurs due to a combination of three factors:

• • a) The high viscosity of DEG and the concomitant low ionic mobilities
  • b) Low concentration of ionic charge carriers due to low dissociation of the weak acid (HF) and
  • c) Large hydrodynamic radius of dissociated ions due to solvation by water and DEG molecules.

As the anodization of Ti proceeds, the concentration of [TiF₆]²⁻ ions increases with time due to the chemical reactions represented by equations (2), (3) and (4). Due to a more delocalized distribution of charge in the complex, [TiF₆]²⁻ ions are also less solvated and therefore more mobile. Consequently, the conductivity of the electrolyte increases with anodization duration, which manifests itself in a higher anodization current density at the same potential, an effect clearly seen in the anodization current transient plot of FIG. 2(b), during the first 20 hours of anodization. The increase in the conductivity of the electrolyte may result in a large proportion of the applied anodization potential available for the anodization process since the potential drop across the electrolyte (anodization current i x electrolyte resistance R) reduces with time. Therefore, the base of the nanotubes, which forms later in the process, may experience a higher effective anodization voltage than the top (mouth) of the nanotubes, which are formed relatively early in the process. Due to the well-known dependence of the diameter of the nanotubes on the anodization voltage, a tapered nanotube morphology wider at the base than at the top is produced.

The field-assisted oxidation process generates ions according to equation (1) and results in local acidification at the pore bottom. On the other hand, F ion starvation occurs where F ions are consumed by dissolution reactions. Therefore, while F concentration is maximum at the mouth of the tubes (nearly equal to concentration in the bulk electrolyte) and drops to a minimum at the pore bottom, H⁺ ion concentration is maximum at the pore bottom and decreases towards the mouth of the tubes. Such a fluoride ion concentration gradient along the length of the nanotubes has been confirmed by compositional analysis using X-ray photoelectron spectroscopy (XPS). It is known that less fluoride results in a thick oxide layer which suppresses the transport of titanium, oxygen and fluoride ions, and excess fluoride results in a thin oxide layer which enhances the transport of titanium, oxygen and fluoride ions, thus inducing inward growth faster. Nanotube length increases so long as the rate of movement of the Ti/TiO₂ interface is faster than the rate of loss of TiO₂ nanotubes by chemical etching. The anodization current is roughly proportional to the strength of the field-assisted reactions and is thus indicative of the rate at which the Ti/TiO₂ interface is moving into the Ti foil.

As shown in FIG. 2(b), the anodization current in the present anodization technique increases for the first ˜20 hours of anodization and then decreases nearly monotonically. The increase in anodization current may occur due to an increase in electrolyte conductivity over time. Thus, the rate of movement of the interface peaks at ˜20 hours into the anodization process and appears to decline thereafter due to a paucity of fluoride ions at the pore bottom. At this point in the anodization process, field-assisted dissolution weakens relative to field-assisted oxidation, resulting in an increase in the thickness of the barrier layer. The thicker barrier layer retards the solid state ionic transport of reactants through the barrier layer and causes a decrease in the anodization current density. If purely high field ionic conduction was involved, then the current would be expected to continuously decrease with time. If purely mass transport control was involved, the anodization current would be expected to level off instead of decreasing. In our scheme, it is believed that the anodization reaction may be under mixed control of the high field solid state ionic transport and mass transport. Chemical etching, in contrast, is relatively constant and becomes more dominant as the anodization current decreases.

A mechanism is proposed that explains the foregoing observations and accounts for the unique formation of multipodal TiO₂ nanotubes in HF-DEG-water electrolytes in the foregoing example. A salient feature of the DEG-based bulk electrolyte is its high viscosity (η_DEG=32 cP at 298 K) which prevents its penetration into the inter-tubular spaces of close-packed nanotube arrays. Further, a gradient in fluoride-bearing species exists along the length of the growing nanotube, with the highest concentration corresponding to that of the bulk existing at the mouth of the tube and decreasing toward the barrier layer. Thus, in the first 20 hours of the anodization process, when nanotubes are increasing in length, chemical etching, even though isotropic, only shortens the height of the nanotubes by etching from the top. The solid state transport of reactant ions through the barrier layer occurs through a high-field process exponentially dependent on the electric field across the barrier layer and therefore sensitive to barrier layer thickness. When the anodization current begins to decrease after 20 hours, there is increased competition for the lower current from all the nanotubes and minor variations in barrier layer thickness play a significant role in allocating current among nanotubes. As chemical etching becomes more dominant, nanotubes in regions where the barrier layer is slightly thicker grow into the Ti foil more slowly but experience the same rate of chemical etching, thus gradually becoming shorter than nanotubes in regions where the barrier layer is slightly thinner.

Due to the tapered structure of the nanotubes, a decrease in the height of such nanotubes also increases inter-tubular spaces where the viscous electrolyte can now penetrate-thus the same nanotubes experience more accelerated rates of dissolution due to chemical attack from the sides in addition to etching from the top. Soon, these nanotubes are completely consumed. Also, since the Ti/TiO₂ interface in the regions of thicker barrier layer moves into the metal more slowly, these regions are gradually more elevated with respect to adjacent regions with a thinner barrier layer (see FIG. 3, which shows two such regions adjacent to each other). The region enclosed by the delineated border in FIG. 3 has relatively close-packed nanotubes as well as dark regions indicative of depth and greater topographic contrast. The barrier layer is visible in the region outside the delineated border, which is lighter on account of being at a higher elevation. In this elevated region, several nanotubes have been consumed by chemical etching resulting in a wider separation. Several of the still-remaining nanotubes in this region have experienced severe sidewall etching (some of these are pointed out by the arrows in FIG. 3).

FIG. 4 shows SEM images of the obtained titania nanotube arrays at different anodization times. After 40 hours of anodization (FIG. 4a), the nanotubes are still fairly close-packed but from this point onward, chemical etching becomes dominant. FIGS. 4(b), 4(c) and 4(d) show that the nanotube structures become successively less close-packed in the course of the next few hours resulting in a dramatic decrease in the areal density of nanotubes on the substrate. The absence of sidewall chemical etching in the regions of closely compacted nanotubes where highly viscous electrolyte cannot penetrate into the intertubular spaces preserves those nanotubes. However, as can be seen in FIG. 4(a), despite the closely packed structure, there are separated regions throughout the sample facilitating their sidewall etching due to the electrolyte penetration which increases the bare area in those regions at longer times as shown in FIGS. 4(b), 4(c) and 4(d).

Nanotubes of very large diameter (extending to optical and near-infrared wavelengths) may be obtained in DEG-electrolytes as seen in FIGS. 5(a) and 5(b). Two concurrent processes may be responsible. Although closely packed nanotubes do not seem to undergo chemical etching of their sidewalls, they do appear to experience etching from the top, which shortens them because of the presence of the electrolyte at their mouth. Type I nanotubes of large pore-size are formed by the top etching process, which increases their diameter due to their tapered conical shape. Type II nanotubes of large pore-size form by the combination of nanotubes having smaller pore size. As shown in the diagram of FIG. 5(c), the pore size of both Type I (individual nanotubes) and Type II (multipodal nanotubes) was increased for longer anodization times subsequent to the formation of the self-organized nanotubular structures on the surface growing from just over than 300 nm after 40 h of anodization to about 900 nm after 47 h for combined nanotubes. The reason for the pore size increment of the Type I nanotubes is shown schematically in FIG. 5(d).

The nanotube combination process is schematically shown in FIG. 5(e) in which FIG. 5(e) (step I) represents the common surface area of the two adjacent nanotubes that are “leaning” towards one another. As with, for example, ethylene glycol (EG) and water, DEG is a highly structured solvent with a three-dimensional spatial network of hydrogen bonds. It is known that H⁺ ions, OH⁻ (hydroxide) ions and glycoxide ions have anomalously high conductance and mobility in these electrolytes due to the proton jump mechanism. Halide ions, on the other hand, have much lower conductance and mobility in EG and DEG. A consequence of this asymmetry is that hydroxide ions and glycoxide ions consumed at the Ti/TiO₂ interface during oxidation are replenished from the bulk electrolyte more quickly than fluoride ions consumed in the electrochemical dissolution of the barrier layer and this asymmetry becomes more pronounced as nanotube length increases. Also, the bulkier [TiF₆]²⁻ ions produced at the pore bottom do not disperse quickly into the bulk electrolyte due to their low mobility in the viscous electrolyte and their coulombic attraction to the anode.

It should be mentioned that the chemical etching at the mouth of the nanotube occurs continuously and the high viscosity of the electrolyte limits the long distance movement of the dissolved material which increases the concentration of the dissolved material in the electrolyte/nanotube interface region. As depicted in FIG. 5(e)(II), this highly saturated electrolyte etches both the side wall and interface between the leaning nanotubes and the small amount of saturated electrolyte at the mouth becomes super-saturated and additional dissolved nanotube material becomes deposited onto the inner surface of the nanotube (the electrolyte/nanotube interface). Hence, the side wall becomes thicker which results in the reduction of in chemical etching rate relative to that of the nanotube inter-wall. The SEM images in FIGS. 5(a) and 5(b) clearly show this stage after 45 h of anodization with the same electrolyte mentioned before. According to FIG. 5(e)(III), the difference in the chemical etch rate dissolves the inter-wall deeper into the nanotube which results in the nanotube combination as seen in the SEM image in FIG. 1(d) for 47 h of anodization.

As can be seen in FIGS. 5(a) and 5(b), if the leaning nanotubes possess a large interface extending from the top to the bottom, the resulting “combined nanotube” looks like a single large pore size nanotube. Where, for example, the interface does not extend all the way down, then the combined nanotube may appear like a typical “branched” large diameter nanotube possessing multiple “legs”. The number of such distinct “legs” may depend upon the number of nanotubes which combined to produce the larger diameter nanotube.

FIG. 5(a) shows that during the first hour after voltage removal, the nanotube combination process was at its initial stage and the inter-wall between the nanotubes did not proceed deeply into the nanotubes. Nevertheless, at longer times, the nanotubes are completely etched and an irregular film is redeposited from the supersaturated electrolyte.

Bending and bunching of high-aspect ratio (>150) TiO₂ nanotubes grown in fluoride ion bearing glycerol-water electrolytes has been previously observed due to surface tension effects during the drying process. By supercritical drying in CO₂, such bending has been minimized or even eliminated. The SEM images in FIGS. 1(c), 1(d), 4(a), 4(b) and 6 demonstrate that the present nanotubes may “bend” or lean towards one another before combining. The nanotubes formed in the present study have much lower aspect ratios of ˜5-25, therefore implying that the forces causing the bending may be much larger.

Nanotube combination efficiently allocates scarce fluoride bearing species since two or more nanotubes can obtain fluoride-bearing species from the same puddle of bulk electrolyte after nanotube combination. Consequently, it is possible that multipodal nanotubes that obtain access to fluoride-bearing species in the bulk electrolyte by the process of pore combination may continue to grow (the Ti/TiO₂ interface below them keeps moving deeper into the Ti metal). This is supported by FIG. 6, where multipodal nanotubes appear to survive longer than other nanotubes subsequent to the first 40 hours of anodization, when field-assisted processes weaken leaving chemical etching dominant. This may result in the weaker dissociation of HF, which results in fluoride ion scarcity which may be the critical factor for nanotube combination. This is further supported by the observation that when ammonium fluoride (which has higher dissociation) and tetrabutyl ammonium fluoride (which dissociates completely) are used instead of HF as the fluoride-bearing species in the diethylene glycol anodization electrolyte, F— ions are not scarce and the formed nanotubes remain close-packed for very long anodization durations and the diameters are capped at ˜300 nm. As mentioned previously, a gradient in fluoride ion concentrations exists along the length of the nanotube with the maximum concentration in the bulk electrolyte close to the mouth of the nanotube. Viscous electrolytes inhibit the rapid mass transport required for equalization of the fluoride ion concentrations due to the low diffusion coefficients of ions in them. Mass transport is also inhibited by the presence of a pre-existing layer of vertically oriented nanotubes since such nanotubes present narrow channels for flow of the relevant species and once again prevent dissipation of fluoride ion gradients generated due to the anodization process. Accordingly, it is contemplated that even in less viscous electrolytes such as formamide (η=3.3 cP), nanotube combination to form multipodal nanotubes can still occur in a controllable and reproducible fashion, provided a layer of nanotubes of diameter <150 nm was already present to significantly slow down transport processes. The resulting multipodal nanotubes are shown in FIGS. 16-18.

In summary, the formation of multipodal nanotubes in the foregoing example is demonstrated using the following process parameters: DEG electrolyte, anodization voltages of 120 V or greater, anodization durations >40 hours and HF concentrations lower than 0.5%. It is known and understood that multipodal nanostructures may still be achieved as a result of nanostructure combination with any number of these process parameters being modified.

One advantage of the multipodal nanostructure is that the differential chemical functionalization of each individual “leg” or “pod” may allow for multiplexed sensing and the loading of multiple drugs. Providing more than one “leg” per nanostructure may result in a more robust attachment of the nanostructure onto a desired substrate, which could also render the nanostructures to be good load bearing elements for mounting heavier structures. Further, the multipodal topology of the present nanostructures may also lend itself to the use of three-terminal devices, electrical interconnect networks and nanoelectromechanical systems. The syntheses and applications of multipodal quantum dots—mainly tetrapodal nanocrystals of II-VI semiconductors such as ZnO, CdS and CdTe—are a focus of intense research activity. It follows that the present multipodal structure may provide certain advantages in applications such as photocatalysis and photovoltaics due to the larger surface-to-volume ratio and more facile charge separation at the core-leg interfaces. The hierarchical topology of multipodal nanotubes, consisting of multiple discrete nanotubes of smaller diameter combining to form a single nanotube of larger diameter, could be applied for phase separation in a fluid comprising several ingredients and for microfluidic and optofluidic applications.

EXAMPLE 2

Multi-Step Electrochemical Anodization, DEG Solvent, Anodization Voltages of 120V-150V or Greater, Anodization Durations>40 Hours and HF Concentrations Lower than 0.5%

This experiment was done to explore the controllability of the nanotube combination process by changing the time sequence of anodization and by more closely observing the time period during which nanotube combination is initiated. The main characteristic of this experiment was that contrary to the previous works the anodization was not performed in one continuous step, but rather performed in a sequence of steps such as, for example, two discrete steps.

The same electrolytic solution was used in this Example as in the previous Example 1. For clarity, the present electrolytic solution comprised of 0.25% HF and 1% water in DEG. The voltage applied was 120 V. The first step was identical to that used above up to the point that the combination process begins.

Previously, it was observed that the first 22 hours of anodization in the electrolyte mentioned above resulted in closely compacted nanotubes as shown in FIG. 7. At this point in the process (when no sign of nanotube combination was observed; see FIG. 8), the samples were taken out of the solution, rinsed in water, then in isopropanol, and then dried by a stream of nitrogen. The samples then underwent a cleaning process in 0.2 M HCl followed once again by rinsing and drying.

At this point, the samples which underwent the first anodization step were separated into two groups. One group of samples underwent a subsequent step of the anodization for 19.5 hours in a fresh electrolyte of the same recipe used before, whereas another group of samples underwent a subsequent step for 23.5 hours.

Where the anodization was performed continuously for >40 hours in the first step (and in Example 1), multipodal nanotubes were produced due to the process of nanotube combination. However, where a first step (22 hours) was combined with a second step (19.5 hours), no combination occurred (see FIG. 9).

Where, however, the anodization in the subsequent step was conducted for 23.5 hours, the nanotube combination process appeared to be initiating. The alignment shown in FIG. 10 suggests that the nanotubes are likely in the process of combining. Closer views of such combining lines are depicted in FIGS. 11 and 12. Combination of adjacent nanotubes results in pore size increment in one dimension as shown in FIG. 13.

FIGS. 14(a) and 14(b) show the top views of nanotubes which were subjected to anodization for 19.5 hours and 23.5 hours, respectively, in the subsequent anodization step. The pore diameter of the nanotubes remains approximately the same in both cases (˜150 nm). It is likely that, during the four hours of anodization separating the two samples, the process of chemical etching may have dissolved a significant amount of material near the mouth of the nanotubes. Since the nanotubes are tapered with a narrow diameter at the top and a wide diameter at the base, the four additional hours of etching experienced by the 23.5 hour anodized samples may have resulted in a larger tube-diameter. However, FIGS. 12 and 13 show that that this is not the case. Instead, despite of the four additional hours of etching, the 23.5 hour anodized sample has nearly the same average nanotube diameter as the 19.5 hour anodized sample. As such, it is contemplated that a multi-step sequential anodization process may either arrested or severely slow the process of chemical etching, thereby allowing greater control of the process of chemical etching and the process of nanotube combination separately. This results in a substantial and previously unknown level of control over the anodization process in general and over the nanotube combination process in particular. It also shows a general electrochemical path towards the controllable combination of oriented nanostructures on a substrate (where the said nanostructure might be formed by a variety of methods including but not limited to chemical vapor deposition, solvothermal synthesis, vapor-liquid-solid growth, templated synthesis, electrochemical synthesis and photolithography).

EXAMPLE 3

Multi-Step Anodization, Titanium Substrate Having Pre-Existing Nanotubes, Formamide Solvent, Anodization Voltages of at Least 10V (First Step) and at Least 35V (Second Step), and Increasing to at Least 50V at a Predetermined Rate, Anodization Durations of at Least 45 Hours (First Step) and 3 Hours (Second Step) and NH4F Concentrations Lower than 0.5%.

Having regard to FIGS. 15-18, the present Example was done to determine whether nanotube combination could occur on a substrate having pre-existing nanostructures, via a one-step or multi-step process. The present Example supports the importance of restricting/constraining mass transport in the present process, particularly where a solvent having relatively low viscosity is used (e.g. formamide). It is contemplated that the slowing of mass transport occurred in the present Example as a result of the pre-existing nanostructures on the array.

Step 1:

The first step of this Example 3 comprised anodizing a titanium substrate at 10 V for 45 hours in a fresh formamide electrolytic solution containing 0.3385% NH₄F and 5% H₂O. This first step resulted in the formation of self-organized vertically oriented nanotubes with an average diameter of 50 nm and an average wall-thickness of 19 nm.

Step 2:

The second step of this Example 3 comprised taking the sample produced during step 1 out of the electrolytic solution, and rinsing same with isopropanol and dried in a stream of nitrogen. The same sample was then further anodized in a fresh formamide electrolytic solution containing 0.3385% NH4F and 3% H2O in formamide. The anodization commenced at 35 V and was increased at a predetermined rate of 1 V/5 minutes to 50V over an entire anodization duration of 3 hours and 20 minutes.

As is shown in FIGS. 15-18, the foregoing multi-step anodization process resulted in the formation of multipodal nanotubes with an average diameter of ˜160 nm.

Although preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention. The terms and expressions used in the preceding specification have been used herein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.

Claims

1. An electrochemical anodization method for producing a nanostructure array having single nanotubes, multipodal nanotubes, or a combination thereof, the method comprising:

a. providing a substrate capable of undergoing anodization,

b. providing an electrolytic solution for receiving the substrate,

c. providing means for restricting the mobility of ions in the electrolytic solution, and

d. anodizing the substrate to produce single nanotubes, multipodal nanotubes or a combination thereof.

2. The method of claim 1, wherein the means for restricting mass transport comprises providing an electrolytic solution having a mixture of:

e. a solvent having a viscosity sufficient to restrict the mobility of ions,

f. a halide-bearing species, and

g. de-ionized water.

3. The method of claim 2, wherein the solvent viscosity is between 3 and 1000 cP.

4. The method of claim 3, wherein the solvent is selected from a group consisting of diethylene glycol (DEG) and ethylene glycol.

5. The method of claim 4, wherein the electrolyte is DEG.

6. The method of claim 1, wherein the concentration of the halide-bearing species is less than 0.5%.

7. The method of claim 6, wherein the concentration of the halide-bearing species is between 0.25% and 0.3%.

8. The method of claim 6, wherein the halide-bearing species is a fluoride-bearing or a chloride-bearing species.

9. The method of claim 8, wherein the halide-bearing species is HF.

10. The method of claim 1, wherein the anodization occurs at a voltage of at least 70V.

11. The method of claim 10, wherein the anodization occurs at a voltage of between 120V to 150V.

12. The method of claim 1, wherein the anodization occurs for a duration of at least 40 hours.

13. The method of claim 12, wherein the anodization occurs for a duration of 45 to 47 hours.

14. The method of claim 1, wherein the substrate is titanium.

15. The method of claim 1, wherein the substrate may contain pre-existing nanostructures.

16. The method of claim 1, wherein the means for restricting the mobility of ions comprises providing a substrate having pre-existing nanostructures.

17. The method of claim 16, wherein the electrolytic solution comprises a mixture of:

h. a solvent,

i. a halide-bearing species, and

j. de-ionized water.

18. The method of claim 17, wherein the solvent has a viscosity between 3 and 1000 cP.

19. The method of claim 18, wherein the solvent is formamide.

20. The method of claim 17, wherein the concentration of the halide-bearing species is less than 0.5%.

21. The method of claim 20, wherein the halide-bearing species is a fluoride-bearing species or a chloride-bearing species.

22. The method of claim 21, wherein the fluoride-bearing species is NH4F.

23. The method of claim 17, wherein the anodization occurs at a voltage of at least 10V.

24. The method of claim 17, wherein the anodization occurs for a duration of at least 40 hours.

25. The method of claim 24, wherein the anodization occurs for a duration of 45 to 47 hours.

26. The method of claim 17, wherein the method further comprises:

k. Rinsing the nanostructure array,

l. Performing at least one subsequent anodization.

27. The method of claim 26, wherein the subsequent anodization comprises providing a second electrolytic solution comprising a mixture of:

i. A solvent,

ii. A halide-bearing species, and

iii. De-ionized water.

28. The method of claim 27, wherein the solvent viscosity is between 3-1000 cP.

29. The method of claim 28, wherein the solvent is formamide.

30. The method of claim 27, wherein the concentration of the halide-bearing species is less than 0.5%.

31. The method of claim 30, wherein the halide-bearing species is a fluoride-bearing or chloride-bearing species.

32. The method of claim 31, wherein the fluoride-bearing species is NH4F.

33. The method of claim 26, wherein the subsequent anodization occurs at a voltage of at least 10V.

34. The method of claim 33, wherein the subsequent anodization occurs at a voltage of 35V.

35. The method of claim 26, wherein the voltage is increased to a voltage of at least 50V at a predetermined rate.

36. The method of claim 26, wherein the subsequent anodization occurs for a duration of at least 3 hours.

37. An electrochemical anodization method for producing a nanostructure array having complex hierarchical structure, comprising:

a. providing a titanium substrate capable of undergoing anodization,

b. providing an electrolytic solution for receiving the substrate comprising a mixture of: i. a solvent having a viscosity of at least 32 cP, ii. a fluoride-bearing species, wherein the concentration of the fluoride-bearing species is less than 0.5%, and de-ionized water, and

c. anodizing the substrate at a voltage of between 120V-150 V for at least 40 hours.

38. An electrochemical anodization method for producing a nanostructure array having a complex hierarchical structure, comprising:

a. Providing a titanium substrate capable of undergoing anodization, wherein the substrate comprises pre-existing nanostructures,

b. Providing an electrolytic solution for receiving the substrate comprising a mixture of: i. A solvent having a viscosity of between 3-1000 cP, ii. A fluoride-bearing species, wherein the concentration of the fluoride bearing species is less than 0.5%, and iii. De-ionized water,

c. Anodizing the substrate in a first step at a voltage of at least 10V,

d. Rinsing the nanostructure array, and

e. Anodizing the substrate in at least one subsequent step at a voltage of at least 35V.

39. A nanostructure array comprising:

a plurality of oriented, tapered nanostructures, wherein some or all of the nanostructures may be at least bipodal.

40. The nanostructure array of claim 39, wherein the plurality of nanostructures may comprise a pore size of at least 150 nm.