| Title of Invention | GROWTH OF METAL/COMPOUND NANOTUBE ARRAYS |
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| Abstract | A method of growing metal/compound nanotube arrays uses a nanoporous template with cylindrical pores. The metal is electrodeposited on the pore surface by programmed motion of the ions and controlled electrodeposition. The ions in the electrolyte used in electrodeposition are always forced to graze the surface of the walls of the pores by the application of a rotating electric field in addition to the electrodeposition field. Thus ions only get deposited on the pore walls where they get discharged resulting in the formation of nanotubes. |
| Full Text | DESCRIPTION BACKGROUND OF THE INVENTION Field of the Invention This invention is related to the growth of metal/compound nanotube arrays that are aligned in a particular direction. More specifically, the invention is a method of growing metal/compound nanotube arrays in a nanoporous template to provide for uniformly-dimensioned metal nanotube arrays of high aspect ratio. Description of the Related Art The discovery of carbon nanotubes in 1991 initiated the interest on tubular nanostructures because of their immense fundamental importance as well as their potential applications in the nanoscale devices, sensors, catalysis, thermal materials, structural composites, field emission, biomedicine and energy storage/conversion. Porous membranes such as anodic alumina, polycarbonate membranes, block copolymers etc. have opened up the possibilities of the synthesis of arrays of ordered nanowires of a number of materials. Electrodeposition is one the most convenient methods of filling the pores of these membranes to get arrays of nanowires of various metals. Use of electrodeposition for the synthesis of ordered arrays of nanowires in such templates is well studied. However till date there are very few reports of the synthesis of dense arrays of aligned metal nanotubes using nanoporous templates. Most of the earlier work in this area involve chemical modification of the pore surface of porous templates to enhance the deposition of the metal on the pores. Such chemical modifications often add impurities to the nanotubes. Other methods consist of using special additives or using low deposition current densities. These methods are often too specific for a particular kind of metal to be deposited and cannot be used for a variety of materials. Often it is also specific to an application. The processes are time consuming because of the complexities involved. Thus a general and efficient method for the synthesis of metal nanotubes (with controlled length, diameter and wall thickness) still remains a challenge particularly in templates which allow synthesis of large ordered arrays. We present here a novel, versatile and general approach for preparing metal and alloy/compound nanotube (NT) arrays. The method uses a template like anodic alumina and has full control on length, diameter and wall thickness of the nanotube. The method is general enough and in principle can be applied to prepare single metal nanotubes of all metals or alloys or multilayers which can be deposited by electrodeposition technique. This method in principle can also be used to deposit nanotubes of many semiconductors which can be prepared by electrodeposition. The method of synthesis of metal nanotube arrays described here exploits the basic principle of electrodeposition in a rotating electric field, which we believe has never been exploited in such 2 synthesis using electrodeposition. The novelty of this method lies in its generality, simplicity and efficiency. The method also opens up an avenue for structure formation by electrodeposition by controlling the ion dynamics. As a generic example, the synthesis of single crystalline copper nanotube arrays by electrodepositing copper into the pores of porous anodic alumina template is demonstrated. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method of growing metal nanotube (MNT) arrays that are aligned in one direction. Another object of the present invention is to provide a template method of growing MNT arrays that yields substantially uniform length and diameter and wall thickness of the MNT arrays. Another object of the present invention is to have a control on wall thickness of the MNT by easily controllable external parameters. Still another object of the present invention is to provide a template method of growing MNT arrays that yields pure MNT arrays. A still further object of the present invention is to provide a template method of growing MNT arrays that is cost effective and easily adaptive to growth of such MNT arrays of wide variety of materials. Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. In accordance with the present invention, a method of growing metal/compound nanotube arrays is based on the principle of 'programmed motion of the ions' and electrodeposition on the pore surface of cylindrical pores of nanoporous templates such as anodic aluminum oxide and polycarbonate porous membranes. As a generic example we report the synthesis of single crystalline copper nanotube arrays by electrodepositing copper into the pores of porous anodic alumina template and polycarbonate membranes in presence of a lateral rotating electric field which is applied in addition to the longitudinal DC electrodeposition current. The applied rotating electric field makes the ions graze the surface of the tube in a helical path and thus makes the deposition selectively occurring in the region near the wall of the nanopores. 3 BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: FIG. 1A, 1B is a view of any portion of a typical porous membrane used in the method of the present invention; FIG. 1C, 1D show the a 200nm thick layer of silver/gold coated on one of the surfaces of the porous membrane. This coating being the working electrode in the three electrode potentiostatic configuration; FIG. 2A shows a typical saturated calomel electrode used as the reference electrode; FIG. 2B shows the construction of the counter electrode made of platinum wire and tip; FIG. 2C shows the combination of the SCE reference electrode and the counter electrode; FIG. 2D represents the various other remaining parts namely, the glass/plastic cylinder of the electrolytic cell in the order of their respective positions; FIG. 2D represents the various other remaining parts of the electrolytic cell in the order of their respective positions; FIG. 3A represents a complete assembly of all parts of the cell in the order of their respective positions; FIG. 3B represents a complete assembly of all parts of the cell in their respective positions along with connections of the electrodes to the potentiostat; FIG. 4A shows an arrangement of producing a rotating electric field; FIG. 4B shows electrodeposition setup placed in the rotating electric field setup; FIG. 4C shows a copper ring enclosing the four electrodes to ensure uniformity at edges; FIG. 5A, FIG. 5B FIG. 5C, FIG. 5D, FIG. 5E, FIG. 5F are the Simulation images; FIG. 6A Electrodeposition scheme for tube growth; FIG. 7A SEM (Scanning electron microscope) image of copper nanotube arrays; 4 FIG. 7B SEM (Scanning electron microscope) image of copper nanotube arrays after the removal of template; FIG. 7C SEM (Scanning electron microscope) image of copper nanotube arrays for low rotating electric field amplitude; FIG. 7D SEM (Scanning electron microscope) image of copper nanotube arrays for low rotating electric field amplitude; FIG. 7E Electrical resistance vs temperature of 200nm diameter nanotube arrays; FIG. 7F TEM (Transmission electron microscope) image of a single nanotube; DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, FIG. 1A, 1B represent nanoporous membranes used as templates in the present invention. Most widely used nanoporous templates are Anodic Aluminum Oxide (AAO) and polycarbonate membranes. The AAO templates are usually prepared by a two step anodization process and their pore diameters within the range of l0nm to 200nm depending on the anodization voltage and other condition. Unlike the polycarbonate membranes, the AAO are very much ordered. AAO templates are used in the present invention for the synthesis of metal nanotube arrays. However, the invention is general enough that it can be adapted to a large class of nanoporous membranes that have arrays aligned pores with axial symmetry. Metal nanotube arrays are synthesized in pores of AAO template by controlling the ionic trajectories inside the pores in such a way that they are electrodeposited on the pore walls. This is achieved by electrodepositing the ions inside the pores in presence of an additional lateral rotating electric field. Thus the detailed experimental set up consists of mainly two parts. (1) The basic electrodeposition set-up for nanowire growth. The nanowire growth set-up consists of the following parts (a) SCE: Saturated Calomel Electrode as the reference electrode. (b) Platinum counter electrode (c) Basic electrodeposition cell. (2) Rotating electric field set-up for changing the nanowire growth mechanism to nanotube growth. The first step in the synthesis of arrays of metal nanotubes in this invention consists of coating a silver/gold layer of 200nm thickness on one side of the porous Aluminum Oxide template. This 5 is shown in FIG. 1C, 1D. This silver or gold coating serves as the working electrode in the three electrode potentiostatic configuration of electrodeposition. The other two electrodes being a Saturated Calomel Electrode (SCE) as the reference electrode and a platinum electrode as the counter electrode. The typical designs of a Saturated Calomel Electrode (SCE) and the platinum counter electrode are shown in FIG. 2A and FIG. 2B respectively. FIG. 2C shows the combined arrangement of SCE reference electrode and the platinum counter electrode used in the present invention. FIG. 2D refers to the remaining parts of the electrodeposition cell. These consist of the glass or plastic vessel opened at the both ends, rubber O-ring, the porous Anodic alumina template with its silver/gold coated side facing the copper disc beneath it. The copper base serves as a good electrical contact to the silver/gold coating (working electrode) of AAO template. Electrical connections are made to the copper disc. The copper disc also gives good mechanical support to the AAO template. The rubber O-ring is placed on the top of the uncoated bare surface of the AAO template. The glass or plastic vessel is placed on the rubber O-ring in the same order as shown in FIG. 2D. The copper base disc and the glass vessel are now pressed using Teflon clamps (not shown in figure). A complete order of various parts of the electrodeposition cell are shown in FIG. 3A. A compact view of the electrodeposition cell with various parts at their respective meant positions is shown in FIG. 3B. FIG. 3C shows the electrode connections of the cell with the potentiostat for the electrodeposition. The glass/ plastic vessel remains pressed to the copper base disc with the help of a Teflon clamp in such a way that the O-ring takes care that no electrolyte comes out of the vessel and reaches the working electrode from outside. Thus the electrolyte has its only way to the working electrode through the nanopores of the AAO and hence the deposition takes place inside the pores. This is very efficient set up for making nanowires of materials which can be electrodeposited. The experimental scheme for producing rotating electric field is shown in FIG. 4A, which basically consists of two pairs of copper electrodes making the four sides of a square. A sinusoidally varying voltage (E0Sint) from a signal generator was applied to one of the pair of parallel copper electrodes facing each other. A signal with the same frequency and amplitude, phase shifted by /2 (E0Cost) was applied to the other pair of copper electrodes. The electrodeposition cell is placed in the middle of the four electrodes as shown in FIG. 4B. The electroeposition technique that is widely used to make metal nanowires in templates uses the deposition field which is essentially longitudinal so that they are along the axis of the pore. Present invention is based on the programmed motion of ions, i.e., to control the motion of ions during electrodeposition and restricting the ions to the walls of the pores. This is achieved by the rotating electric field which is always perpendicular to the electrodeposition field and thus the additional rotating field forces the ions to graze along the surface of the walls. The rotating electric field is produced by perpendicular superposition of two sinusoidal electric fields (of identical amplitude and frequency) to each other, differing by a phase of /2. This is in accordance with the Lissajous figures where two sinusoidal signals with identical amplitude and frequency give rise to a circle when superposed perpendicularly with a phase difference of /2. Since the pores to be filled have circular cross section, we used a phase difference of /2 between the lateral two sinusoidal fields. The various electric fields applied to achieve the desired deposition are shown in FIG. 6A. 6 The mechanism of the formation of nanotubes can be understood in the following way. Taking , the mobility of ions, the velocity V of an ion in an electric field E is proportional to the velocity with , the mobility of the ion being the proportionality constant These equations essentially imply So the motion of the ion due to the superposition of the electric fields follows a circular orbit with a radius R When the process of electrodeposition is started, an additional electric field EZ acts along the z- axis making the trajectory of each ion a helix. The radius of such a helix is given by R which can be easily be tailored by the field amplitude E0 and frequency (). A typical path is shown in FIG. 5A. FIG.5B show the paths for 10 ions together. Thus each ion gets deposited following such a trajectory. The trajectory will also depend on the initial position, which we take as random to start with. Within the pores of the templates the motion of the ions gets constrained by the pore walls if the radius R is more than or equal to the pore radius. The trajectory of a single ion getting constrained inside a nanopore is shown in FIG. 5C. This way all ions, irrespective of their initial positions, upon reaching the pore walls will move stay close to the surface wall of the pores. FIG. 5E shows the trajectories of 10 ions getting constrained by the walls of the cylindrical nanopores of the template. Thus inside the template pores the ions always move close to the pore walls and result in the formation of a tube when they get discharged on the moving deposition front. In reality the actual electric field experienced by the ions is not constant but it varies being maximum at the pore surface and minimum at the axis of the pore and this is better understood by the Debye Screening length which is ~ nm at low ionic concentrations. Nevertheless, the ions still are able to reach the pore surface after revolving with gradually increasing radius and once they reach the surface they persist to move on the surface because of the high field near pore surface because of the high dielectric constant of the liquid and the relatively less dielectric constant of alumina. 7 A computer simulation of the experiment using the above model makes the mechanism further clear. Each ion, irrespective of its initial position (which is randomly chosen) traverses a helix grazing the surface of the pore before getting deposited when it comes in contact with the atoms of the working electrode or the growing deposition front. Formation of a nanotube with diameter ~ 10nm, with 100000 atoms (whose initial positions and velocities are randomized) is shown in FIG. 5F. The nanotube so formed in the simulation has a wall thickness~2nm. This is a typical example of a metal nanotube formation. As a generic example, copper nanotube arrays formed are shown in FIG. 7A. These nanotubes are formed inside the nanoporous anodic alumina templates having pore diameters ~200nm. Since radius depends on the amplitude of individual electric fields used to create the lateral rotating electric field, a lower amplitude implies, a lower radius of the helix which an ion follows. Thus when the radius of the helix of an ion becomes lower than the radius of pores in the template, it results in many possible paths touching the pore walls as shown if FIG. 6C (a cross-sectional view of the paths). This result in increasing the wall thickness of the nanotube formed inside a nanopore. Thus the amplitude gives a control on the wall thickness of the nanotubes. Thick nanotubes formed with lower amplitudes of the rotating electric field are shown in FIG. 7C and FIG. 7D. These copper nanotube arrays are single crystalline in nature and they are electrically conducting. The resistance vs. temperature data of such an array of copper nanotube arrays with tube diameter ~200nm with wall thickness of 15-20nm is shown in FIG. 7E. The principal benefit of this approach is that it is general, and it does not need any chemical modification or partial coating of the pores for synthesizing the nanotubes. The method also does not alter the chemistry of the standard electrodeposition that is used for a given material. It only changes the external electric field configuration, Thus it can be applied to the synthesis of any metal/compound nanotubes. The simulation of the method gives a physical insight of how metal nanotubes can be formed inside porous templates. To our knowledge this is the first method which is not only simple and fast but also based on designing the shape of an electrodeposited metal by controlling the ionic dynamics inside an electrolyte. The MNT have a constant wall thickness throughout the length as seen in TEM image FIG. 7F. Thus they can act as hollow nanoelectrodes and give options of filling them with other materials like semiconductors and high dielectric constant materials for such applications in fields like nanoelectronics, solar cells, supercapacitors etc. The advantages of the present invention are numerous. By constraining NT growth to pre-sized tubes (cylindrical pores); NT having uniform length, diameter and thickness can be grown easily and simply. The process yields arrays of NT arrays embedded inside the nanopores of the template, which can be technologically exploited for applications such as catalysis, nanoelectronics, solar cells, supercapacitors etc. This inexpensive, simple, efficient method of growing NT arrays is fastest method of synthesizing NT arrays ever reported. 8 CLAIMS We Claim 1. A method of synthesis of metal and alloy/compound nanotube arrays comprising the steps of (a) coating one side of a nanoporous template with 200nm of silver or gold layer, which acts as the working electrode in the potentiostatic electrodeposition cell, (b) The template is placed in an electrodeposition cell comprising of a vertical glass vertical column opened at the bottom with the template at the bottom and an O-ring in between. The glass column holds the electrolyte being closed at the bottom with the template supported by a thick copper disc for mechanical strength. The O-ring between the glass column base and the bare side of the template makes sure that the template does not break when the glass column is pressed tight to the base plate with Teflon clips. Thus the bare side is in contact with the electrolyte and the coated side with the copper base plate for electrical connection. A Saturated Calomel Electrode (SCE) and the counter electrode consisting of platinum helix surrounding the reference Electrode is inserted from the top of the glass column, (c) The above electrodeposition cell is placed in a rotating electric field setup and electrodeposition is carried inside the pores of the nanoporous template. 2. The method of claim 1, wherein the nanostructures are nanotubes and nanotube arrays. 3. The method of claim 1, wherein the nanopore channels have diameters at least about l0nm to at most about 450nm. 4. The method of claim 1, wherein the nanoporous template comprises nanoporous anodic aluminium oxide or polycarbonate membrane or block copolymer membranes, porous silica or any such membranes of organic, inorganic or hybrid material that has ordered arrays of continuous linear nanopores. 5. The method of claim 1, wherein the electrodeposition is extended to the nanopore channel surfaces to achieve tubular nanostructures. 6. The method of claim 1, wherein the electrodeposition is extended to the nanopore channel surfaces by guiding the motion of ions of interest. 7. The method of claim 5, wherein the motion of ions being guided by the application of additional electric fields in addition to the longitudinal electrodeposition field. 8. The method of claim 7, wherein the additional electric fields are two sinusoidal electric fields, perpendicular to one another with the same amplitude and frequency and constant phase difference are superposed. The phase difference between the two sinusoidal electric fields is 90 degrees for the cross-section the nanotubes formed to be circular inside the cicurlarly cross- sectioned nanopores. 9 9. The method of claim 1, wherein the nanostructures are formed inside the ordered cylindrical pores of nanoporous templates by electrodeposition in presence of additional electric fields. 10. The method of claim 8, wherein the additional sinusoidal fields are applied by a couple of pairs of parallel plate copper electrodes such that they are placed on the four sides of a square. A sinusoidal voltage is applied between each couple of copper plates facing each other to get the corresponding sinusoidal field. The nanoporous template lies in the plane of the four copper plates with the electrodeposition field along the axis of the pores of the template. The three electric fields comprising of the two sinusoidal superposed fields and the electrodeposition field are mutually perpendical to each other. 11. The method of claim 10, wherein the area enclosed by the four copper plates constitutes the rotating electric field region. An additional copper ring of the same thickness and height is placed surrounding the four plates to make the field lines uniform near the ends of the copper electrodes. 12. The method of claim 1, wherein the nanotube arrays are metal or semiconductor or compound or alloys. 13. The method of claim 1, wherein the nanotube array is multilayerd tubes when deposition of different materials is carried out successively forming different concentric nanotubes with the same axis. 14. The method of claim 10, wherein the amplitude and frequency of the fields constituting the rotating electric field is a factor to tune the thickness of the nanotubes. 15. The method of claim 1, wherein the time of deposition is a factor of thickness of the nanotubes. 16. The method of claim 1, wherein the method has application for the synthesis of nanocpacitors with metal-dielectric-metal coaxial cylindrical layer arrays and also for the synthesis of helical nanoscale coils for nanoinductors. 17. The method of claim 1, wherein dense ordered arrays with uniform tubes aligned in one direction are formed at room temperature. 10 A method of growing metal/compound nanotube arrays uses a nanoporous template with cylindrical pores. The metal is electrodeposited on the pore surface by programmed motion of the ions and controlled electrodeposition. The ions in the electrolyte used in electrodeposition are always forced to graze the surface of the walls of the pores by the application of a rotating electric field in addition to the electrodeposition field. Thus ions only get deposited on the pore walls where they get discharged resulting in the formation of nanotubes. |
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| Patent Number | 269712 | |||||||||
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| Indian Patent Application Number | 1204/KOL/2007 | |||||||||
| PG Journal Number | 45/2015 | |||||||||
| Publication Date | 06-Nov-2015 | |||||||||
| Grant Date | 03-Nov-2015 | |||||||||
| Date of Filing | 30-Aug-2007 | |||||||||
| Name of Patentee | S N BOSE NATIONAL CENTRE FOR BASIC SCIENCES, KOLKATA | |||||||||
| Applicant Address | S N BOSE NATIONAL CENTRE FOR BASIC SCIENCES, BLOCK-JD, SECTOR-3, SALT LAKE, KOLKATA | |||||||||
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| PCT International Classification Number | C01B31/00 | |||||||||
| PCT International Application Number | N/A | |||||||||
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