"Method of aligning carbon nanotubes in metal nanowires and applications thereof which include a fuel cell catalyst"

INTRODUCTION

Field of the Invention

The invention relates to composite materials of carbon nanotubes ("CNT") and metals or semiconductors.

Prior Art Discussion

Carbon nanotubes (CNTs) are described as single-walled nanotube ("SWNT") constituting a single layer of graphite folded into a tubular structure, or multi-walled nanotube ("MWNT") composed of multiple layers of graphite. The diameter spans from 0.4 to 3 nm for SWNTs and 1.4 to 100 nm for MWNTs while the length can be up to 1 mm. MWCNTs have multiple 1 D conducting shells and their conductance will thus be higher depending on the number of conducting shells. The properties of CNTs include high electrical conductivity, high thermal conductivity, large surface-to- volume ratio, chemical stability, superior mechanical strength, high aspect ratio, negative coefficient of resistance and a wide potential window of electrochemical stability. Despite their unique properties, CNTs are limited in applications in many fields because of their size. Thus, advantage of their properties has been taken in building CNT-composite materials based on ceramics, resins and metals with applications as sensors, conductive and high strength composites, energy conversion devices, radiation sources, catalysts, field emission displays, thermal management materials, batteries and nanoscale interconnects. CNTs are intrinsically tangled into bundles owing to the strong Van der Waals attractive forces existing between them. Their dispersion and separation can be aided by the use of surfactants. CNT composites wherein the CNTs are randomly dispersed in an un-oriented manner are structurally isotropic hence its properties are isotropic. However, when CNTs are

directionally aligned in a composite then the structure and properties of the composite are expected to be anisotropic - these include electrical conductivity, thermal conductivity, mechanical strength and coefficient of thermal expansion.

The one-step co-electrodeposition of CNTs and metal to afford planar CNT-metal composite films with the CNT embedded in random directions in the metal has been disclosed in the art as follows. CNTs were co-electrodeposited with nickel in Surface and Coatings Technology 191 (2005) 351-356 X.H. Chen et. al.; with copper in Electrochem Commun. 7, 19 (2005) S. Arai et. al.; with nickel-cobalt alloy in Surface and Coatings Technology 200 (2006) 4870-4875, L. Shei et. al.; with aluminium in Electrochem. 74 (3) (2006) 233 T. Yatsushiro et. al.; and with gold in ECS 210 ^th meeting, Mexico, abstract 0109 (2006) A. Vincenzo et. al. US20070036978A1 describes the reinforcement of CNTs in a thin copper film by electrodeposition. A problem with this approach is that the reinforcement of tangled, random, CNTs in a material produces an isotropic structure.

Fuel cells are efficient and environmentally friendly energy conversion devices. In a Direct Methanol Fuel Cell (DMFC) electric current is generated by the direct electrochemical oxidation of methanol. DMFCs have been attracting great attention for their potential applications in clean and mobile power sources owing to their high energy density, ease with which methanol can be manufactured and handled, stability of methanol in acidic medium and low operating temperature. However, the commercialization of DMFCs is inhibited by three main drawbacks: (i) cost of the precious metals employed , (ii) sluggish kinetics of methanol oxidation at the anode that leads to high

Over-potentials (hence low power density of methanol), and

(iii) catalyst poisoning by CO species. In most published works, the Pt loading of anode is above 1.0 mg cm ^"2 to ensure high methanol reactivity and to prevent methanol crossover to the cathode, this high cost restricts commercialization of the DMFC.

Clearly the lack of an efficient and inexpensive electrocatalyst for methanol oxidation is a challenge for the large scale utility of direct methanol fuel cells. This has

prompted attempts to fabricate an efficient CNT-based electrode tailored for DMFC, which minimizes electrocatalyst loading and maximizes electrocatalyst utilization.

The applications of CNTs in fuel cells as a catalyst support and electrode materials has been shown. The expectation is that the enhanced electrocatalytic properties of CNTs would reduce the amount of platinum and therefore substantially increase the commercial viability of fuel cells. In Applied Phys. Lett. 90 (2007) 063112 J. Choi et. al showed a tenfold catalytic activity enhancement for methanol oxidation by doping

Pt-Carbon black catalyst ink with SWNTs. They attributed this to the increase of catalyst utilization by improving interconnectivity among carbon black particles.

Recently, most of the investigations demonstrated that Pt electrocatalysts in the form of nanoparticles can be decorated on the external walls of the CNTs. Pt nanoparticles have been decorated on the surface of CNTs by chemical reduction, supercritical CO ₂ . pyrolysis and electrochemical reduction. With the exception of the electrochemical v method these methods are time-consuming and impurities can be involved. -.'

The fabrication of aligned CNT-Pt nanoparticle composite catalysts has been ~\ disclosed in the art. WO 2006/08702 Al describes the growth of a vertically aligned CNT array on carbon paper using chemical vapour deposition on which Pt nanoparticles were deposited by supercritical CO ₂ fluid deposition or electrochemical deposition. In Microchim. Acta 152 (2006) 267 F-S. Sheu et. al. describe the growth of an aligned CNT array on a tantalum substrate carbon paper using chemical vapour deposition on which Pt nanoparticles were deposited by electrochemical deposition. In the above two cases there are two major steps, initially growing an aligned array of CNTs and then decorating them with a metal. The alignment may be done after Pt decoration, as described in WO 2006/099593.

The invention is directed towards providing a simpler process for production of composite materials including CNTs. Another objective is that there is more versatility in configuration of the end product. Another objective is to provide a simpler route to the fabrication of CNT composite materials with anisotropic properties. A further objective is to provide a simpler, more cost-effective and better

controlled production of Pt/CNT composite fuel cell electrodes. More specifically an objective is to achieve an optimum Pt/CNT configuration with minimum Pt loading and guaranteed electronic continuity to improve efficiency of fuel cell electrodes.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method of depositing a composite material onto a substrate, the method comprising the steps of:

providing an electrically conductive cathode substrate; and

electro-depositing a composite material on the substrate from a bath of metal ions and dispersed nanoscale elongate carbon particles;

wherein the electro-deposition is within elongate volumes defined by a three- dimensional template (1), and the elongate volumes have dimensions so that the elongate carbon particles are aligned within them.

The invention therefore allows in a very simple manner deposition of a composite material on a very small scale with aligned nanoscale particles, using the benefit of the well-known electro-deposition technology.

In one embodiment, the carbon particles are carbon nanotubes. In another embodiment, the carbon particles are in the form of carbon nanohorns, or fullerenes, or carbon nanofibers. Such forms of nanoscale carbon particles are readily available and have many advantageous properties for applications of the composite material.

In one embodiment, the metal is platinum or an alloy thereof. This is particularly advantageous for catalytic and sensing applications.

In one embodiment, the volumes are of generally cylindrical configuration.

In one embodiment, the template has a honeycomb configuration.

In one embodiment, the template is removed to reveal exposed composite material wires. The template may be dissolved, in one example using sodium hydroxide. In one embodiment, residual composite material is separated from the sodium hydroxide solution by centrifuging and supernatant sodium hydroxide solution is removed and the composite material is re-dispersed in deionised water to clean it, and the composite material is then dispersed in ethanol.

In another embodiment, the template is of alumina material. This is a particularly good material to work with, and may be readily dissolved.

In one embodiment, the template material is selected from the group consisting of anodic titanium oxide, porous silicon, and porous anodic zirconia.

In one embodiment, the carbon nanotubes are multi-walled, and they may for example be of 0.7 microns in length and 9.5 to 10 ran in diameter, and the composite material may be c. 1 micron in length.

In one embodiment, the ratio of metal to carbon is controlled by choice of template volume dimensions and carbon particle dimensions. This is a particularly simple way of controlling the important aspects of relative concentrations of the composite material, even though the scale is very small.

In a further embodiment, the concentration of carbon particles in the bath is in the range of 8 to 12 μg carbon particles per ml of bath and surfactant of 0.1 to 0. 3 %w/v where the metal is Pt.

The deposition time is preferably in the range of 30 to 40 mins, the template volume diameter is preferably 15 nm to 400 ran, and the template pore density is preferably 10 ⁸ to 10 ¹⁰ per cm ² .

In one embodiment, the template is inserted between upper and lower plates, the upper plate having an aperture to expose the template to the bath solution, and a conducting

plate is disposed between the lower plate and the template, the template being located so that a lower platinum-titanium coated base is face down on the conducting plate thereby protecting it from the solution.

In one embodiment, the length of the deposited composite material is controlled by control of the deposition time. This is another example of how an important aspect of the composite material can be easily controlled in the invention.

In a further embodiment, different composite materials are deposited into the volumes to provide a segmented composite material, by using different baths at different stages of deposition. This allows considerable versatility.

In one embodiment, the template is aligned with the volumes normal to the substrate, or alternatively if may be aligned with the volumes parallel to the substrate.

In one embodiment, the metal is Cu or an alloy thereof.

In another aspect, the invention provides a fuel cell electrode, and a fuel cell comprising such an electrode, in which the electrode comprises a composite material comprising an array of nanowires of a composite material comprising Pt and embedded aligned CNTs, the nanowires having a width dimension in the range of 15 to 400 nm.

In another aspect, the invention provides an electrical interconnect comprising a composite material comprising an array of nanowires of a composite material comprising Cu and embedded aligned CNTs, the nanowires having a width dimension in the range of 15 to 400 nm.

In a still further aspect, the invention provides a sensing probe comprising a composite material comprising an array of nanowires of a composite material comprising Pt and embedded aligned CNTs, the nanowires having a width dimension in the range of 15 to 400 nm

DETAILED DESCRIPTION OF THE INVENTION

Brief Description of the Drawings

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:-

Fig. 1 shows an alumina template at the start of a deposition process;

Fig. 2 shows a conducting contact layer at the base of the template;

Fig. 3 shows diagrammatically co-deposition of CNTs and platinum;

Fig. 4 shows a holder set up used for co-deposition;

Figs. 5(a) to 5(c) are transmission electron micrograph (TEM) images of aligned CNTs in platinum nanowires following alumina dissolution;

Fig. 6 is a scanning electron micrograph (SEM) of a CNT-Pt nanowire array following alumina dissolution;

Figs. 7 (a) and 7(b) show a horizontal alumina template and horizontal array of CNT- metal nanowires, respectively;

Fig. 8 shows a cyclic voltammogram for methanol oxidation at the CNT-Pt nanowire array; and

Fig. 9 shows the stability of the CNT-Pt nanowire array as a catalyst for methanol oxidation.

Description of the Embodiments

Composite material production, overview

In our invention we develop an optimised Pt/CNT catalyst configuration where the CNTs are aligned in Pt nanowires through the co-electrodeposition of Pt and dispersed CNTs in cylindrical cavities of a porous template.

Referring to Figs. 1 to 4 an alumina template 1 is produced by anodisation of aluminium. An underlying conductive base 2 is provided by deposition of a metallic layer. In this case 100 nm Pt was e-beam evaporated with an underlying 10 nm titanium adhesion layer. Pt/CNT composite nanowires were electrodeposited in the pores of the alumina template 1 from a Pt bath with Nafion™-dispersed CNTs under constant current conditions using a two-electrode cell. The conductive base 2 served as a cathode and a platinum oxygen-evolving anode was used. The Pt and CNTs co- deposit into the cylindrical cavities 3 normal to the base in the template to provide plugs 10 of CNTs 11 vertically aligned in Pt 12. Because of the relative dimensions of the CNTs 11 and the cavities 3 the CNTs align vertically in the configuration shown in Fig. 3. Actual examples are shown in Figs. 5(a) to 5 (c) and 6. The alumina template is dissolved in sodium hydroxide, leaving the aligned composite nanowires 10 as shown in Fig. 6.

Production method, more detail

In more detail, the process involves developing a stable dispersion of carbon nanotubes in a commercial platinum bath. There is electrodeposition of a Pt/CNT composite in a platinum-backed porous alumina template 1 having 200 nm pores. The CNTs 11 are multi-walled, 9.5-10 nm in diameter and typically 0.7 micron in length, however following sonication the length is shortened.

The TEM images of Figs 5(a) to 5(c) and the SEM image of Fig. 6 show aligned CNTs in the Pt host metal 12 formed by electrodeposition in the template.

The invention may allow for deposition in smaller pore openings for aligned CNT deposition where the template and the CNT are of similar dimension (e.g. 10 nm CNT in 15-20 nm template), thereby decreasing the Pt content of the catalyst. Choice of

physical dimensions of the template, particularly the pore diameter and density in the template allow tuning of the Pt/CNT ratio. This is very important for optimising the material for the desired application.

Example- deposition method

MWCNTs (Nanocyl®-3150) approximately 10 run in diameter and 700 nm in length were supplied by Nanocyl™ (www.nanocyl.com). A platinum bath (Platinum-DNS bath) based on 10 g platinum/litre was supplied by Metakem™ (www.metakem.com). Nafion™ perfluorinated ion-exchange resin (5 wt. %) was supplied by Sigma Aldrich.

Alumina circular membranes (Anodisc® 25) with 2.5 cm diameter, 60 micron thickness, 200 nm pore size and 10 ⁹ pore openings per cm ² of membrane were supplied by Whatman™.

The MWCNTs were dispersed in the platinum bath with the aid of the Nafion™ surfactant. A solution of Pt/MWCNT/surfactant composing of 10 μg of carbon nanotubes per ml and 0.25 wt % surfactant was prepared and sonicated for 10 min to afford a cloudy suspension. Sonication leads to shortening the length of the MWCNTs. Co-deposition of Pt and the MWCNTs was effected in a two-electrode cell with a target substrate designated as cathode and a platinum wire as anode. The bath was operated at 50 ⁰ C and 0.01 A/cm ² .

In order to induce deposition in the pores of the alumina template the template was firstly made conducting by depositing 100 nm platinum with an underlying 10 nm titanium adhesion layer using e-beam evaporation. The template was soaked in the bath for 5 minutes to allow the solution to penetrate and sufficiently wet the platinum- plugged pores prior to deposition. In order to ensure that composite deposition is restricted to the inside of the pores of the template, the exposed platinum on the non- template side is masked from the bath. For this the holder setup 20 shown in Fig. 4 was employed. The alumina template (27) was inserted between two 1 cm thick PTFE plates (25, 28). The lower plate 28 was solid while the upper plate 26 had a hole of 2.5 cm diameter which served to expose the template (or "membrane") 27 to the plating

bath solution. A conducting copper plate 29 was sandwiched between the lower PTFE plate 28 and the template 27. The template 27 was inserted in the holder 20 so that the lower platinum-titanium coated side is face down (on the copper plate) thereby protecting it from the solution. An O-ring 26 sits on the upper side of the alumina template for added support before the two plates are secured together with four 0.5 cm diameter screws.

The Pt/MWCNT composites deposited therein were 3D wires, the length of which was controlled by the deposition time (preferred 35 to 40 mins) and was measured using SEM and TEM.

SEM images of the composite array were recorded using a Hitachi S4000™ SEM at an acceleration voltage of 20 kV. Having deposited the composite in the pores, the membrane was attached to a SEM conducting stub with an adhesive carbon sticker.

The side of the template 27 which had the 100 ran platinum -10 nra titanium backing layer was adhered to the adhesive carbon sticker. A few drops of 1 M sodium hydroxide were applied to the template for 40 minutes to allow for dissolution of the alumina. The solution was then removed and a few drops of water were applied to clean any residue from the template before recording a scanning electron micrograph (SEM). As a final step the template was dried under a stream of Nitrogen for 1 minute. SEM images were recorded at 20 kV using Hitachi S4000™ to obtain the image of Fig. 6. It is evident that the nanowires have collapsed somewhat together, presumably to reduce the free energy of the material.

The template 27 was immersed in 1 M sodium hydroxide and sonicated for 5 minutes to release the MWCNT-Pt nanowires from the template. The resulting residue was allowed to remain for 1 hour in the 1 M sodium hydroxide solution to dissolve the alumina porous network. The residual nanowires are separated from the 1 M sodium hydroxide solution by centrifuging at 13,500 rpm for 10 min. The supernatant sodium hydroxide solution was removed and the nanowires were re-dispersed in deionised water to clean them. The aqueous dispersion was gently agitated before separating the nanowires by centrifuging at 13,500 rpm for 10 min. This cleaning step was repeated

three times in total. Finally, the nanowires were dispersed in ethanol. A drop of the ethanolic solution was dropped on a carbon-coated copper mesh TEM grid. The nanowires were imaged using JEOL 2000FX operating at accelerating voltage of 80 kV to obtain the images of Figs. 5(a) to (c).

Templates

The template used can include porous Si, porous anodic titania, polycarbonate and porous anodic zirconia. Alumina and similar metal oxide templates can be integrated on silicon wafers via thin film processing. Such a template can be lithographically patterned to define an active area for the construction of a micro fuel cell electrode.

The template volume diameter is preferably 15 nm to 400 nm and the template pore density is preferably 10 ⁸ to 10 ¹⁰ per cm ² .

One-Dimensional Template

Our invention is not limited to using "vertically" aligned pores, normal to the base. It is possible to use a template having horizontally-aligned pores parallel to the base. This enables the fabrication of molecular and electronic devices compatible with planar processing technology. Despite the potential for nanofabrication of alumina templates it is still difficult to use them to fabricate nanoelectronic devices and nano- electromechanical systems.

It was shown by Chen et.al J. Electrochem. Soc. (2005) 152 no. 12 D227 that horizontally aligned 1 D alumina can be fabricated through anodisation from the cross section of the sandwiched structure of SiO ₂ /Al/SiO ₂ /Si. This technique can be used to provide a template with horizontally aligned pores.

Referring to Fig. 7(a) a template 30 with a conducting layer 35 in contact with the alumina can be fabricated. There is an underlying semiconductor layer 33, which remains when the template 35 is dissolved. An aligned CNT-metal nanowire composite is electrodeposited in the pores 32 of the template 30. The alumina can then be dissolved in sodium hydroxide to yield a material having a ID array 40 of aligned

CNT-metal nanowires 41 integrated on the layer 33. In this array the nanowires are aligned horizontally on the substrate 33. The ID array structure should permit easy access to nanowire composites from the top surface to form gated devices that is not feasible using 2D arrays. The nanowires may be used as components in nanoscale molecular electronics, and in sensing and actuating devices which are focused on their electrical properties.

Alumina templates with smaller or larger pore sizes than that indicated here (200 nm) may be used. Multi walled CNTs with smaller or larger diameter than indicated here (10 nm) or single walled CNTs may be used. The method may be used for alignment of CNTs in a semiconductor or any metal alloy that can be electrodeposited from solution. The invention may be applied to co-deposition of carbon nanohorns, fullerenes or carbon nanofibers with platinum or another metal.

Also, the nanowires may be segmented into different materials, simply by using different baths at different stages of co-deposition.

Electrochemical Analysis of the Material (Figs. 8 and 9)

Optimised electrocatalytic activity of Pt/CNT composite for methanol oxidation as a fuel cell catalyst may be achieved by tailoring the content of Nafion-dispersed CNT in the plating bath. Fig. 8 shows a cyclic voltammogram for oxidation of 1 M methanol in 1 M sulphuric acid at a clean 10 micron Pt microdisk ( — line in plot) where the current is multiplied xlO, and at a CNT-Pt nanowire array ( — line in plot). The CNT- Pt nanowire array was deposited in 200 nm alumina pores from a bath with 0.25% w/v Nafion and 10 μg CNT/ml to afford 1 micron long nanowires. The CNTs were vertically aligned in the composite Pt/CNT nanowires, as evidenced by TEM images shown in Figs. 5(a) - 5(c). The potential was scanned between -1.2V and 0.9 V at a sweep rate of 20 mV s ^"1 at 25 ⁰ C vs. saturated calomel electrode (SCE). A three- electrode cell was used, the working electrode is the CNT-Pt nanowire array, the counter electrode is Pt and the reference electrode was saturated calomel electrode.

Onset potential of the methanol oxidation of the CNT-Pt nanowire array is lower than that of the Pt microdisk in the anodic sweep, which indicates that the CNT-Pt nanowire array reduces the activation over-potential for methanol oxidation. An anodic peak during the forward sweep corresponds to methanol oxidation and another anodic peak during the reverse sweep is due to the removal of incompletely oxidized carbonaceous species formed in the forward sweep.

The oxidation peaks in anodic sweep, Efwd, are observed at 0.56 V and 0.63 V for the CNT-Pt nanowire array and the Pt microdisc, respectively. The oxidation peak at the CNT-Pt nanowire array is more negative, which indicates that the methanol oxidation becomes energetically more favourable. The oxidation current for 1 M methanol at the Pt microdisk at room temp is 2.5 mA/cm ² . The oxidation current for 1 M methanol at the Pt-CNT composite at room temp is 73 mA/cm ²

Table 1 below shows influence of CNT concentration on methanol oxidation at Pt/CNT composite. CNT-Pt nanowire array was codeposited in the absence of Nation. A Pt nanowire array without CNTs embedded gives a limiting methanol oxidation current of 39 mA cm ^'2 It is evident that the addition of CNTs without Nation™ dispersing agent to a Pt bath gives Pt/CNT composite deposits with lower electrocatalytic activity for methanol than Pt deposits.

Table 1

Table 2 below shows influence of CNT concentration on methanol oxidation at Pt/CNT composite. CNT-Pt was co-deposited in with the addition of 0.5% w/v

Nafion™ in the Pt-CNT bath. It is clear from Table 2 that the optimum CNT concentration in the bath to yield a composite with highest efficiency for methanol oxidation is 10 μg /ml.

Table 2

Table 3 shows the influence of Nafion™ concentration on methanol oxidation at

Pt/CNT composite. The CNT concentration in the bath was 10 μg CNT/ml. If excess amount of dispersing agent is added it will aggregate thus interfering with dispersion of CNTs.

The optimum concentration of CNT and Nation™ in a Pt bath to afford a composite which shows greatest efficiency for methanol oxidation is 10 μg CNT/ml and 0.25 %

Nafion™.

Table 3

Level of Pt loading

For a 1 micron high Pt deposit which is equated in shape to a cylinder with a radius of 100 nm, the volume of a Pt cylindrical deposit is 3.142 x 10 ^"14 cm ³ . Based on the density of Pt 21.45 g cm ^"3 , the corresponding mass is 6.739 10 ^"14 g. There are 10 ⁹ pore openings per cm ² of membrane, if Pt deposition was 100 % efficient the associated Pt loading is 0.674 mg cm ^"2 , or 6.74 g m ^"2 which is an order of magnitude lower than recommended by a commercial supplier E-TEK USA. E-TEK provide 50-80 wt % Pt on high surface area carbon support Vulcan XC-72 (250 g m ^"2 ) for DMFC, with R loading (8.6 - 5.7 mg cm ^"2 ). In most published works, the Pt loading of the anode is above 1.0 mg cm ² to ensure high methanol reactivity and to prevent methanol crossover to the cathode. This leads to high cost and restricts commercialization of the DMFC.

The mass activity of the 1 micron long Pt/CNT nanowire array for methanol oxidation is 108 mA/mg, this value was determined by dividing the limiting current density for methanol oxidation (73 mA cm ^"2 ) by the Pt mass loading (0.67 mg cm ^"2 ).

CO tolerance

In Fig. 8 the anodic peak in the reverse scan for methanol oxidation on Pt is attributable to the removal of incompletely oxidized carbonaceous species formed in the forward scan. Hence, the ratio of the forward anodic peak current density ilβvd) to the reverse anodic peak current density {Irev),— — , can be used to describe the

Irev catalyst tolerance to carbonaceous species accumulation, in particular CO. A low Ifivdllrev ratio indicates poor oxidation of methanol to CO ₂ during the anodic scan and excessive accumulation of carbonaceous residues on Pt. A high Ifwd/Irev ratio represents the reverse case as more intermediate — — carbonaceous species are

Irev oxidized to CO ₂ in the forward scan thus decreasing the extent of CO poisoning on Pt sites. Essentially, a high Ifwd/Irev ratio, indicative of improved CO tolerance is desired. In Tables 2 and 3 the values for Iβvd, Irev and — — are given for Pt/CNT

Irev composites as a function of CNT concentration in the CNT-Pt bath. In our work — —

Irev

at a 10 micron diameter Pt microdisk is 1.14, this was raised to 1.85 at the Pt/CNT composite material, deposited from a bath with preferred CNT and Nafion™ concentration of 10 μg /ml and 0.25% w/v, respectively. It is widely accepted that adsorbed H ₂ O or small amounts of adsorbed OH at lower potential may be necessary to remove CO intermediates. This is usually accomplished by alloying Pt with Ru as Ru adsorbs water at lower potentials. However, the incorporation of CNTs could prove beneficial in adsorbing H ₂ O to some extent. To further the case for the incorporation of CNTs as the support material it was in the use of MWNTs as a sensor for CO (Sensors and Actuators B 99 (2004) 1 C-L.Chen et. al) the overpotential for CO oxidation is lowered by a decrease in the CO adsorption energy on MWNTs and the oxidation current increased.

Stability of Catalyst

Referring to Fig. 9, it is seen that there is only a slow decay in catalyst activity. The long-term stability of aligned CNT-Pt nanowire array electrode was investigated in 1 M methanol in 1 M sulphuric acid. The potential was scanned at 20 mV/s at 25 ⁰ C for 60 cycles between -1.2 and 0.89 V. It can be observed that the peak current density decreases gradually with successive scans. The loss of the catalytic activity may result from the consumption of methanol during the CV scan. It also may result from a structural change in the electrode due to a perturbation of the potentials during scanning in aqueous solutions, especially in the presence of the organic compound.

Influence of CNTs

The influence of CNTs on the oxidation of methanol at Pt should originate from (a) CNTs protruding from the surface of the Pt nanowires and (b) embedded subsurface CNTs. For comparison, the oxidation of 1 M methanol at 1 micron Pt-CNT nanowire array and 1 micron Pt nanowire array fabricated using the current invention should be considered.

Table 5 shows influence of aligned CNT embedded in Pt nanowire array on the oxidation of methanol. From Table 5 it can be seen that the addition of Nafion- dispersed CNTs to the Pt nanowire array:

1. lowers the over-potential for methanol oxidation, as indicated by the lower peak potential for the reaction, Efwd,

2. increases catalytic efficiency for methanol oxidation, as indicated by the higher methanol oxidation current density, I _fwd , and

3. increases CO tolerance, as indicated by the higher — — .

Irev

Both surface and subsurface CNTs promote enhanced kinetics of methanol oxidation owing to their rapid rate of electron transfer. The CNTs protruding from the surface of Pt nanowires have oxide functional groups on their surface which serve as active oxygen for the formation of CO ₂ . CNTs in the immediate subsurface layer will influence the properties of Pt by changing the electronic environment of Pt atoms. The energy levels of Pt could be altered which may weaken the bond between adsorbed CO and Pt, thereby decreasing the extent of CO poisoning.

Table 5

Advantages

Anode performance appears to be related to many factors, such as electrocatalysts utilization, the thickness of the electrocatalyst layer, and the porosity. The thickness of the Pt/CNT composite can be adjusted. The thinner the Pt/CNT electrocatalyst layer, the greater the efficiency of electron and proton transport. A thinner electrocatalyst layer and direct electronic path contribute to the reduced Ohmic losses. Reducing the thickness of the electrocatalyst layer can efficiently reduce mass transport resistance.

Silicon-based microfabrication technology provides a means of fabricating compact micro fuel cells. Hence there is a need for improved methods of preparation of fuel cell catalysts. Alumina templates can be integrated on silicon wafers via thin film

processing. Such a template can be lithographically patterned to define an active area for the construction of a micro fuel cell electrode.

There are several advantages associated with our method of producing Pt/CNT composites for fuel cell electrodes:

(i) it allows for facile application of Pt to CNT surfaces (unlike the difficulties encountered in attaching platinum nanoparticles to CNT surfaces owing to the inertness of the CNT walls).

(ii) it affords composites with good interfacial bonding and good adhesion at the CNT-metal interface.

(iii) it ensures an electronic pathway between Pt and CNTs

(iv) it permits integration of the Pt/CNT composite catalyst in a manner that avoids the introduction of a high contact resistance known to exist between Pt nanoparticle/CNT composites and current collectors. (v) it provides a high purity composite that has no binders added

(vi) it provides a material of high surface area that will remain constant over time.

This is an improvement on the method of electrochemically depositing Pt nanoparticles on an aligned array of CNTs, wherein the movement and agglomeration of the CNTs in the wet processing environment misaligns the CNT array and lowers the available surface area. Also as Pt nanoparticles agglomerate there is a decrease in available catalytic surface area with time.

Also the composite nanowires of the invention eliminate the use of chemical vapour deposition (CVD) as a necessary step in the production of an aligned CNT-Pt array. The invention described here is lower cost than those employing typical vacuum based CVD systems.

The material may be used for any catalytic application where electrooxidation is a key step in the device operation, such as a fuel cell electrode. The material has been shown to function as an improved methanol oxidation material with improved carbon monoxide by-product tolerance.

Thus, the invention provides an enhanced reaction active region of platinum for methanol oxidation as one example. The composite material has good wettability on the material on which it is deposited. Also, the method provides a composite whose structure and size can be as diverse as that dictated by the template into which it deposits. The size can be tuned by controlling the deposition charge passed in the bath. Because the CNTs are aligned within the template pores, and because of the relative dimensions of the CNTs and the pores, the CNTs tend to populate the immediate sub-surface in the end product composite material. This is advantageous for several applications such as catalysts.

'Another advantage is that the method affords a deposition route that does not require pre-activation of the conducting target deposition substrate. Further, the electrodeposited component can comprise any suitable material such as metal or combination of metals that is/are derived from a precursor metal salt plating bath which shows compatibility with the solubilised CNT solution.

The deposited material is easily handled and can also be used in various fields such as a catalyst in electrochemical oxidation of technologically significant species such and glucose in sensor applications, electrochemical reduction of technologically significant species such as oxygen, or in phosphoric acid fuel cells and solid polymer electrolyte fuel cells. The invention has applications as field-emission displays and electrodes where aligned CNTs with controlled density are desired.

Interconnect Applications The application of CNTs in interconnects is compelling owing to their extraordinary properties of high current-carrying capacity, mechanical stability, and thermal conductivity. Another advantage of CNT is their negative temperature coefficient of resistance that makes them more suitably disposed for the high temperature environment of local vias. The composite material has the ability to sustain high current stress without the electromigration problems. MWCNTs have been reported to carry current of densities up to 10 ⁹ -10 ⁿ A cm ^"2 and remain stable for an extended period of time at high temperature in air. In electro-migration the ease of migration is linked to atomic binding strength. The carbon atoms are tightly bound in CNT

sidewalls by sp2 bonds leading to a much higher threshold for electromigration, 7 eV is necessary to move an atom in CNTs while just 1-3 eV is required for metals. During electromigration the gradual displacement of metal atoms in a conductor from one point to another results in an open or short circuit. Electromigration is a function of current density and is exacerbated at high temperatures. Increasing device density in microelectronic applications is severely challenged by electromigration, the effect is further amplified by increasing electric fields as individual device dimensions shrink. It is shown that the current carrying capacity of copper vias/contacts fails to meet ITRS current density requirements beyond the 45 nm technology node. By 2013 the ITRS predicts 3.3 * 10 ⁶ A cm ^"2 which can only be supported by CNTs where 10 ⁹ A cm ^{" 2} in CNTs without heat sinks have been reported.

The template may be left in place to function as a low dielectric constant insulator material surrounding the electrically conductive metal composite for IC interconnect applications when the composite contains Cu and CNT's.

Imperfect metal-CNT contacts will give rise to undesirable contact resistance that raises the overall interconnect resistance. Our invention offers a solution by providing a well-adhered junction between metal and CNT with lower contact resistance. Recent process solutions have been able to reduce metal-CNT contact resistances below 1 Kω per CNT-metal contact. However, it is important to note that although a single CNT is capable of carrying the maximum current flowing through a via (for < 45 nm nodes), the resistance of a single CNT is high (6.5><10 ³ ω).

Interconnect resistance needs to be minimised due to performance and thermal concerns, hence the need for an array of numerous CNTs in parallel. It must also be ensured that the number of CNTs in an array is such that the current distribution does not exceed the range of a few μA per CNT as the resistance of CNTs increases substantially beyond this range. An optimum packing density of CNTs in an array is vital for technology to scale beyond the ITRS roadmap. It is evident that process technology needs to achieve densities of the order of 10 ⁶ CNTs per square-micron (or 1 CNT per nm-square area) in order to keep the resistance of local vias and contacts under control. Published works today have demonstrated densities of less than 100

CNTs per square-micron. The growth of an array of several CNTs forming parallel connections inside via holes, suitable for interconnect applications, has been recently demonstrated by Kreupl et.al. in Microelectronic Engineering 64 (2002) 399. Our method allows for the vertical alignment of CNTs in a copper nanowire array composite which should give lower electrical resistivity than copper. Our method can be scaled to control the packaging density of CNTs in a copper nanowire.

For a Cu/CNT composite it was shown in US2007/0036978 patent that the electrical resistivity was 20% lower for the composite than pure copper (1.22 μω-cm and 1.678 μω-cm, respectively). Our method allows for the vertical alignment of CNTs in a copper nanowire array which should give even higher electrical conductivity (lower electrical resistivity) than their composite due to the directional alignment of CNTs. The known anisotropy of the electrical conductivity for free-standing aligned CNT films is indicated by a higher electrical conductivity in a direction parallel to the vertically aligned CNTs.

Furthermore, in interconnects there is difficulty in contacting metal pins to CNTs. Our composite gives a solution to creating nanoscale metal contact to CNTs with a well- adhered junction/interface. Our method with patterning will allow for contact to a single CNT, unlike the bulk formation of CNT arrays (by growing CNT on a metal substrate). Our aligned CNT/metal nanowire composite ensures that the contacted CNTs are in the nanoscale, this is critical for nanoscale interconnects. It is also feasible to deposit planar and vertical Cu CNT composite interconnect by selection and electrochemical formation of template pore alignment as described earlier.

Probes

CNTs are potentially useful as mechanical, electrical and chemical probes, however, the main difficulty with their application lies in finding an efficient method for handling them. The invention offers a solution as it provides a CNT probe with a handle, by the method of aligning CNT in a metal or semiconductor nanowire. In such a CNT probe configuration the size of the outer metal/semiconductor 'collar' can be increased to a size which is large enough (ca. l μm) to be handled using an optical manipulator. Also the outer collar provides protection and strength for the CNT. The

composite obtained in this invention can be affixed to a suitable macroscopic sized manipulable mounting element to create a probe device that permits information to be obtained from a nanoscale environment. The probe could sense, measure, analyse and modify objects with nanometre resolution.

The probe tip would have a number of advantages over conventional microscopy probes used in STM and AFM. The probe consisting of a single or a small number of CNTs exuding from a metal nanowire has all its component atoms covalently bonded and are unlikely to move, even under extreme stress for example, when the tip crashes into the object being imaged. CNTs are very compliant, buckling in a gentle, controlled manner. Our composite of metal or semiconductor nanowire with exuding CNTs of variable length, diameter and number offer very high aspect ratio probes that can image trenches and pits that are not identified by currently used pyramidal probe tips. By coating electrically conducting CNT probe tips with an insulating material it is possible to confine electrical activity at the end of the probe. This enables probing biological and electrochemical environments.

The invention is not limited to the embodiments described, but may be varied in construction and detail.