Field of the invention

This invention relates generally to the electrodeposition of metals and alloys without the use of an external electrical current. The method uses the electrical energy created within the cell arrangement and therefore will be called self-fuelled electrodeposition (SFED). The invention has particular application to the coating of flexible elongate elements and may be especially useful for the coating of carbon nanotube (CNT) arrays and assemblies, including spun yarns, with noble metals such as gold, platinum and palladium. It also has applications for electrocatalysis where nanoparticles of platinum and its alloys are deposited onto suitable high-surface-area carbon containing substrates or onto other suitable substrates. Further envisaged applications involve deposition of membranes of these metals on mesoporous conducting or semiconducting polymers or similar materials.

Background of the invention Carbon nanotubes, particularly of the single-walled variety, have a range of spectacular properties such as high elastic modulus and high mechanical strength that are of great technological interest. Single-walled nanotubes also display excellent properties such as high electrical conductivity (10-30 kS cm ^"1 ) and high thermal conductivity. The practical difficulty has been in maintaining these properties in assemblies of the carbon nanotubes.

In a surprising development, it was shown (Science 306, 1358 (2004) and international patent publication WO 2007/015710) that twist could be inserted into a carbon nanotube assembly that is drawn laterally from a forest of carbon nanotubes on a substrate, thereby producing a yarn of reasonable strength and tenacity. This twisted CNT structure exhibited significantly improved properties compared to an earlier yarn described in Nature 419, 801 (2002) and in US patent application US 2004/0053780, in which the yarn drawn out from the forest of nanotubes maintains the nanotubes as a substantially parallel array essentially bound together by van der Waals forces. Measured electrical conductivities for these CNT yarns has been somewhat disappointing given the conductivities that would be required in applications for which the yarns may otherwise be of considerable interest. For example, typical measured conductivities of twisted carbon nanotube yarns have been in the region of 500 to 600 S cm ^'1 at around 300 K. This is two to three orders of magnitude less than the better noble metal conductors.

US patent publication 2006/0135030 discloses a method for metalising carbon nanotubes using an electroless plating technique in which the carbon nanotubes are dispersed in a solution containing a metal salt and a soluble reducing agent. The technique described is essentially conventional electroless plating and the context is an overall process in which the metalised carbon nanotubes are dispensed onto a field emission cathode. In this disclosure, the carbon nanotubes are individualised and thus there is no consideration of the issue of treating carbon nanotube assemblies such as yarns. A similar limitation applies to international patent publication WO 2005/021845, where there is disclosed a process for the metal coating of nanofibres made by electrospinning: in this case the nanofibres are not carbon nanotubes but polymer nanofibres.

Li et al, at Advanced Materials (2007), 19, 3358, have described chemical and physical treatments of spun carbon nanotube fibres with a view to improving their electrical properties. These authors reported incorporation of gold particles onto the walls of multi-walled carbon nanotube fibres after treatments including annealing in air, oxidation in concentrated nitric acid, and immersion in an electroless coating solution comprising HAuCI ₄ in ethanol. Electrical conductivity was found to have increased by a factor of two, mostly due to the oxidation in concentrated nitric acid, and gold particles were only sparsely distributed on the carbon nanotube yarn. The contribution to the increasing conductivity from the gold nanoparticle incorporation on the acid treated fibres amounted to only about 20%. Reported conductivity was in the region of 1150 S cm ^'1 for gold-incorporating fibres. International patent publication WO 2007/033188 discloses a substrate-enhanced electroless deposition (SEED) method of depositing metal nanoparticles onto a substrate. In this technique, the reducing metal is used as a metal support for the substrate to be coated. The disclosed method requires the entire substrate to be physically mounted on or in direct contact with the reducing metal support which is not a viable arrangement for certain applications including the coating of carbon nanotube yarns and carbon nanotube arrays or assemblies.

It is an object of the invention to provide a method by which, in a preferred application, metal may be deposited on a carbon nanotube array or assembly to an extent that electrical conductivity of the resultant composite structure is substantially greater than the untreated carbon nanotube array or assembly. The resulting metal-coated yarn can be soldered onto a circuit board.

It is also an object of this invention in one or more embodiments to provide a method where metal nanoparticles can be deposited on a substrate which can be a suitable carbon-based material, a polymer or a similar material preferably with a high BET surface area. Such substrates may be coated solely with platinum nanoparticles or solely with palladium nanoparticles or solely with gold nanoparticles or as a mixture or an alloy of these metals with a suitable composition for the application in hand. Such electrodes of metal-coated carbon have applications for example, as electrocatalysts such as those employed in fuel cells.

It is also an object of this invention in one or more embodiments to provide a method where a metal such as palladium can be deposited as a membrane of a suitable thickness on a suitable substrate such as a mesoporous conducting or a semiconducting polymer. Such membranes have applications, for example, in hydrogen purification and other technologies.

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

Summary of the invention

The invention describes a novel self-fuelled electrodeposition (SFED) method in which the immersed substrate to be coated is in electrical contact with a source of electrons also exposed to the coating electrolyte solution.

In a first aspect, the invention provides a method of depositing metal on a substrate, including at least partly immersing the substrate in an electrolyte solution containing ions of the metal, while the substrate is in electrical contact with a body of a material selected as a source for a supply to said substrate via said electrical contact of sufficient electrons to there promote spontaneous reduction of said metal ions whereby to deposit the metal as one or more of particles, a coating and a film on the substrate.

In its first aspect, the invention also provides apparatus for depositing metal on a substrate, including means for retaining an electrolyte solution containing ions of the metal, a body of a material selected as a source for a supply via electrical contact to a substrate when it is immersed in said electrolyte solution of sufficient electrons to promote spontaneous reduction of said metal ions whereby to deposit the metal as one or more of particles, a coating and a film on the substrate.

The body of material selected as a source for a supply of sufficient free electrons may be any of a range of suitable non-noble metals including copper, silver, aluminium, nickel and zinc. Of these, most preferred are those with the highest electrical conductivity such as copper and silver.

Preferably, the solution does not contain a soluble reducing agent, in contrast to conventional electroless deposition of metals. In some applications, however, a soluble reducing agent may be required and combined with the currently proposed method for more complex metal alloy deposition. Preferably an external power source is not used. However, applications can be envisaged where an external source of power or other electrical elements are used in combination with the self-fuelled electrolysis for particular applications in hand as apparent to a person skilled in the art. Advantageously the apparatus further includes barrier means for dividing the electrolyte solution into respective chambers containing the substrate and the body of material.

The deposited metal, whether a coating, a film or otherwise, may initially be deposited as multiple nanoparticles, (i.e. particles of nano rather than micro or larger dimensions). Deposited particles, whether or nano dimensions or larger, may be crystalline particles. The method is especially suitable but not limited to the deposition of noble metals including gold, platinum and palladium. For deposition of noble metals, the body of material selected as a source for a supply of sufficient free electrons is preferably copper for reasons that include low cost, high electrical conductivity and the formation of insoluble oxidation product which easily detaches from the metal surface, thus facilitating continuous contact between the metal and the solution. Other suitable metals for the free electron source body, when depositing noble metals, include silver (especially when depositing gold and palladium) or zinc (especially when depositing platinum).

The method has also been found suitable for depositing high electrical conductivity metals including copper, silver and nickel. Zinc has proven a suitable material for the free electron source body with these metals, as has copper when depositing silver.

The deposited metal may be an alloy containing two or more metals.

The substrate may be a surface already coated with a layer in accordance with the invention. By successive passes in this manner, bimetallic (e.g. Pt-Au or Pd-Au) or tri- metric nanocrystalline depositions may be achieved.

In one arrangement, the electrical contact is external to the solution, for example via one or more conducting wires or bars. In another arrangement, the electrical contact is by direct attachment of the electron supply body of material to the object having the substrate to be coated. The electrical contact and the electron source body in the latter case are one and the same, and may comprise the same material in other embodiments. In a particularly useful application, the substrate coated may be the surfaces of nanofibres in a nanofibre array or assembly, especially carbon nanotubes in a carbon nanotube array or assembly such as a carbon nanotube yarn. The preferred form of such yarn is one in which the carbon nanotubes are spun or twisted together, such as disclosed in the aforementioned international patent publication WO 2007/015710. In a second aspect, the invention provides a carbon nanofibre array or assembly, for example a carbon nanotube array or assembly and most preferably a carbon nanotube yarn, on which metal is deposited, e.g. as a coating, by a method according to the invention, such an array or assembly having an electrical conductivity substantially greater than the uncoated array or assembly. In a third aspect, the invention provides a carbon nanofibre array or assembly, for example a carbon nanotube array or assembly and most preferably a carbon nanotube yarn, having thereon a metal coating of substantially uniform thickness and exhibiting an electrical conductivity substantially greater than the uncoated array or assembly.

Preferably, the electrical conductivity of the coated array or assembly of the second or third aspect of the invention is at least an order of magnitude greater, and preferably two to three orders of magnitude greater, than the uncoated array or assembly. The coating metal may be a noble metal, eg gold, platinum or palladium, or a high electrical conductivity metal such as copper, silver or nickel. The metal layer may have plural layers of different metals, e.g. Pt-Au or Pd-Au. In another aspect the substrate can be a suitable carbon based or similar material preferably with a high BET surface area. Such substrates may be coated solely with platinum nanoparticles or solely with palladium nanoparticles or solely with gold nano- particles or as a mixture or an alloy of these metals with a suitable composition for the application in hand. Such electrodes of metal-coated carbon have applications in electrocatalysis such as those employed in fuel cells.

In another aspect a metal such as palladium can be deposited as a membrane of a suitable thickness on a suitable substrate such as a mesoporous conducting or a semi- conducting polymer. Such membranes have applications in hydrogen purification and other technologies.

Brief description of the drawings

The invention will now be further described, by way of example only, with reference to accompanying drawings, in which: Figure 1 is a diagram of an arrangement for depositing a film of nanocrystalline gold on a carbon nanotube yarn by a method according to an embodiment of the invention;

Figure 2 is a diagram of an alternative arrangement for electrically connecting the carbon nanotube yarn to an electron source body;

Figure 3 is a diagram of an arrangement for depositing a coating of gold on an array of carbon nanotubes in a forest configuration;

Figure 4 is a diagram of an arrangement for depositing a coating of platinum on a plate- like substrate such as carbon, conducting glass, polymer or a mesoporous material;

Figure 5 presents optical microscope images of a carbon nanotube yarn prior to coating, and after coating with Au, Pd and Ag respectively; Figure 6 depicts three optical microscope images of a carbon nanotube yarn prior to and after coating with gold to a composite diameter of 20 and 40 micron respectively;

Figure 7 presents SEM images for (a) Au-CNT yarn, (b) Cu-CNT yarn and (c) Pt-CNT yam; Figure 8 is a plot of the conductivity of a carbon nanotube yarn as a function of temperature before and after it is coated with gold in accordance with the method of the invention;

Figure 9 is a plot of the variation of the of the conductivity as a function of metal loading (measured by the diameter) for metal-CNT composite yarns (average diameter of the pure CNT yarn is 13 μm); and

Figure 10 illustrates temperature dependencies of the resistivity for (a) Au-, Cu-, Pd- and Pt-CNT composite yarns and original CNT yarn (b) Pd-CNT and Pt-CNT yarns in linear scale Embodiments of the invention

Figure 1 shows one possible arrangement for depositing a nanocrystalline gold film on a carbon nanotube yarn 12. The carbon nanotube yarn is immersed in a suitable electrolyte coating solution 10 containing ions for the metal to be deposited, eg an aqueous metal salt solution of HAuCI ₄ having 2.5 x 10 ^"3 mol/l Au. Ethanol solutions of the salt can also be used, but the deposition occurs at a lower rate. Both ends 14, 15 of the yarn 12 are connected to respective electron source bodies 16, 17 comprising copper wires also immersed in the solution. In this context, the electron source bodies 16, 17 can be viewed as wire anodes, the substance constituting the anodes as a reducing agent, and the salt solution 10 as an electrolyte. The respective connections between the carbon nanotube yarn and the electron source bodies 16, 17 are electrical connections externally of the solution, typically electrically conducting wires 18, 19. A barrier in the form of a ceramic frit 20 divides the solution 10 into respective chambers containing the carbon nanotube yarn 12 and the electron source bodies 16, 17. The frit 20 is useful for preventing contamination of the carbon nanotube yarn 12, by preventing insoluble particles formed at the wire anodes moving to the chamber containing the yarn. Frit may be replaced by a salt bridge or a similar arrangement. The concentration of the HAuCI ₄ solution can be varied depending on the requirements for the deposition. The carbon nanotube yarn can be conveniently loosely wrapped around a tubular glass stent, which allows all surfaces of the yarn to be in direct contact with the salt solution/electrolyte 10. It was observed that gold is steadily deposited, as a film of substantially uniform thickness, onto the surface of the carbon nanotube yarn without any external driving potential. The process can thus be described as a self-fuelled electro-deposition technique or self-fuelled electrolysis. The deposition process commences when the electrical connection 18, 19 is short circuited and stops when the concentration of either the reducing agent provided by the electron source bodies or wire anodes 16, 17, or the metal ion, reaches zero. Stirring of the electrolyte is not required but is useful for faster and uniform deposition. Heating of the electrolyte can be useful, for example, for faster deposition. Cooling of the electrolyte can be useful for example for producing smaller particle sizes. A desired composition for the resulting Au-CNT yarn can be achieved by adjustment of the concentration of the HAuCI ₄ solution and exposure time.

Most metals when immersed in HAuCI ₄ solution react to release gold. In the present process as illustrated in Figure 1 , if copper is used as the electron source reducing anode for deposition of gold nanoparticles, the electrochemical processes can be written as follows:

3Cu(s) + 3Cr(aq)→ 3CuCl(S) + 3e E° = -0.\2l V

HAuCl ₄ (aq) + 3e→ Au(sub) + 3Cr(aq) + HCl E° = 1.002 V

3Cu + HAuCl, (aq)→ Au(sub) + HCl(aq) + 3CuCl(s)

The Gibbs free energy for the above reaction is estimated to be -255 kJ mol ^"1 and the reaction therefore proceeds spontaneously. The competing reaction to SFED is the well known displacement deposition where the reduction of HAuCI ₄ occurs on the surface of the Cu anode. For this reason, the copper preferably should only be in contact with the solution during the deposition period. The impact of displacement deposition can be minimised by the use of sufficiently thin wires of copper and exposing smaller surface areas of the copper to the electrolyte. The use of good electrical conductors such as Cu and Ag for the electron source or reducing agent also reduces displacement deposition.

Platinum and palladium deposition on carbon nanotube yarn can also be achieved using similar arrangements. For example, the overall reaction for Pd deposition can be written as:

2Cw + K ₂ PdCl ₄ (aq)→ Pd (sub) + 2KCl(aq) + 2CuCl (s)

The incorporation of other metals including copper, silver and nickel can also be achieved using arrangements similar to that shown in Figure 1. The following table 1 sets out metal salt solutions employed for depositing palladium, platinum, copper, silver and zinc (M _D ), and in each instance suitable reducing metals (M _R ) for the electron source bodies 16, 17.

The coating of smaller lengths of carbon nanotube yarn can be achieved by the arrangement of Figure 2. The two ends of the yarn 112 are secured on a glass slide 25, immersed in the salt solution/electrolyte. One or both ends of the yarn are in electrical contact 118 with a deposit of Ag epoxy providing the electron source body 116. The other end of the yarn can be secured with a suitable non-conducting epoxy 30. In this case, the electron source body 116 and the electrical contact are one and the same, and comprise the same material.

Figure 3 illustrates an arrangement for coating the surfaces of carbon nanotubes while in a substrate-supported forest. The forest 212 is transferred still supported on its formation substrate 213 and immersed in the HAuCI ₄ solution 210. In this case, the electrical contact with the electron source body or anode 216 is via a conducting glass plate 218 that abuts the tips of the carbon nanotubes of the forest, and an electrically conducting link 219 from the glass to the electron source body. The conducting glass plate may be replaced for example by a gold-coated plate to provide even better electrical contact with CNT forest. Figure 4 illustrates a simple arrangement for depositing a coating or platinum on a plate- like substrate such as carbon, conducting glass, polymer or a mesoporous material, using an K ₂ PtCI ₄ solution.

All carbon nanotube yarn assemblies treated showed a change in colour due to metallic appearance. Examination under an optical microscope showed metal layers on top of the yarn. By selecting appropriate conditions the width of the metal coated carbon nanotube yarn can be changed from a very thin film or coating of a few microns to a thick coating which is at least twice that of the original diameter of the uncoated yarn. To determine whether the metal particles were firmly attached, samples of the metal coated yarn were immersed in 5M NaOH and 5M HNO ₃ overnight. Conductivity was measured as a function of temperature and the immersion in HNO ₃ or NaOH overnight did not change the conductivity.

The Au-CNT and Cu-CNT composite wires withstood repeated 'tape tests' without a change to resistivity or surface. In these tape tests a general utility tape was firmly attached to the composite wire and then removed, resistivity was re-measured and the surface examined under an optical microscope for any damage.

The Au-CNT and Cu-CNT wires could also be soldered onto circuit boards. Metal-CNT yarns containing Pt and Pd proved sufficiently robust for general handling and ultrasonic cleaning. However, surface damage occurred when tape tests were performed. Figure 5 shows optical microscope images of (a) pure CNT, (b) Au-CNT, (c) Pd-CNT and (d) Ag-CNT yarns.

Figure 6 depicts optical microscope images of an uncoated twisted carbon nanotube yarn of 13 micron diameter, and the same yarn after coating to respective composite diameters of 20 and 40 micrometres. The SEM image in Figure 7(a) shows smooth distribution of gold in the Au-CNT yarn. This morphology is not affected by heating the sample up to 400 ⁰ C. Heat treatment of Au-CNT yarns using temperatures greater than 400 ⁰ C leads to gradual oxidation of the CNTs. The surface of Cu-CNT yarn which is shown in Figure 7(b) shows a distribution of particles that are larger in comparison to that of Au-CNT. Although the surface of Pt- incorporated CNT yarn is much smoother (consisting of smaller nanocrystallites, some discontinuities (micron-size cracks) can be observed on the surface (Figure 7(c)). These cracks become wider following heating at 400 ⁰ C. For Pd-CNT yarns such cracks start to appear only following heating in air at 400 ⁰ C. The heating of Cu-CNT yarn to 400 ⁰ C lead to oxidation of Cu.

Table 2 sets out some representative measured conductivities for uncoated and coated yarns employing different noble metals, and Figure 8 is a plot of the conductivity of a specific carbon nanotube yarn as a function of temperature before and after it was coated with gold. It will be seen that conductivity is improved by one to three orders of magnitude, depending on the metal of the coating. Conductivities were approximately a factor of two lower than the bulk metal. For the coated-yarn, the conductivities were calculated using the total diameter of the coated yarn and can be called an 'effective conductivity'. In the case of coating the yarn with platinum at room temperature, the increase in conductivity was much smaller as platinum deposits as very small nanoparticles (< 5 nm in comparison to particle sizes of 30 - 40 nm for gold and palladium) and the presence of much larger quantities of grain boundaries appear to inhibit electron movement.

During the synthesis phase, the diameter of metal-CNT yarns increased as the loading of the metal was increased. In parallel, the measured electrical conductivity rose rapidly at first and then reached a plateau (Figure 9). For Cu-CNT yarns, the upper limit of the conductivity obtained in this study was 3 x 10 ⁵ Scm ^"1 which is a factor of 600 larger than that of pristine CNT yarn. For Au-CNT yarns, the upper limit of the conductivity obtained was 2 x 10 ⁵ Scm ^"1 . These values may be compared with pure copper and gold which have very-high room-temperature electrical conductivities of 5.9 x 10 ⁵ S cm ^"1 and 4.6 x

10 ⁵ S cm ^"1 . The limiting electrical conductivities for Pd-CNT and Pt-CNT yarns were found to be 2 x 10 ⁴ S cm ^"1 and 5 x 10 ³ S cm ^"1 , respectively. For Pt-CNT composite, the limiting conductivity is a factor of 40 smaller than that of pure platinum metal. This large discrepancy in the limiting electrical conductivity between pure metal and the Pt-CNT composite could be a result of the presence of microcracks on the surface of the composite wire (see Figure 9b). The impact of microcracks on the electrical conductivity is confirmed by heating experiments, in which a decrease in conductivity was observed for Pt-CNT and Pd-CNT yarns due to the widening of these features.

The particle sizes of the deposited coatings on the carbon nanotube yarns were estimated using X-ray diffraction (XRD) measurements and the Scherrer equation. The particle sizes in general were found to depend on the concentration and the temperature of the electrolyte. In place of water, other solvents such as ethanol can also be used in certain cases where the hydrophobic nature of the substrate may prevent sufficient contact with the electrolyte for uniform deposition. The process is slower due to the lower electrical conductivity of ethanol compared to water. The adherence of the metallic coating to the carbon nanotube yarn can be enhanced by suitable pre-treatment of the yarn using a method such as heating (annealing) at 400 ⁰ C, exposure to ozone, plasma or exposure to nitric acid or mixtures of other suitable acids for short periods of time. The carbon nanotube yarn may be also be functionalised prior to treatment by methods such as exposure to plasma or wet chemical oxidation with hydrogen peroxide or other suitable oxidants.

Figure 10 shows the relationship between the temperature and resistivity of the pure CNT yarn and metal-CNT yarns (four-point measurement, Quantum Design Materials Property Measurements System). The pure CNT yarn behaves as a typical semiconductor, with the resistivity increasing as the temperature decreases. The fitting of conductivity data to an electron-tunnelling conduction model and to a variable-range electron-hopping model indicated that neither mechanism can explain the charge conduction in the pristine CNT yarns for the entire temperature range. As observed by Li et al. (cited earlier), data taken at temperatures above 50 K are more consistent with the model for three-dimensional electron hopping than the model for tunnelling conduction.

The Au-CNT and Cu-CNT yarns show a typical metal-like dependence of electrical resistivity on temperature. The estimated temperature coefficient of resistivity is 3.2 x 10 ^"3 K ^"1 for Au-CNT and is 3.9 x 10 ^'3 K ^"1 for Cu-CNT. The corresponding values for pure gold and pure copper are 3.2 x 10 ^"3 K ^"1 and 3.9 x 10 ^"3 K ^"1 respectively. This confirms the prominent role of nanocrystalline gold and copper in conducting electricity through the corresponding composite yarns. The results clearly show that Au-CNT and Cu-CNT yarns are preferred embodiments of the present invention because they have unique properties in comparison to other metal-CNT yarns. The mechanical robustness and the nature of the electrical conductivity are particularly remarkable. The Au and Cu particles are more strongly attached to the CNT fibres in comparison to other metals studied. The high ductility of Au and Cu minimises the development of structural defects and provides the continuity of particle interconnectedness through these yarns.

As seen in Figure 10, Pt-CNT yarn also shows a general trend in the temperature dependence of the resistivity similar to that of pure CNT yarn; however the influence of temperature is smaller for the case of Pt-CNT yarn. The Pd-CNT yarns show behaviour of a semiconductor with distinct temperature regimes where the resistivity is relatively constant.

The carbon nanotube yarns may be twisted or untwisted and may be formed from single wall or multi-wall carbon nanotubes or from doped or otherwise modified forms of carbon nanotubes.

The technique of the invention can also be used as a convenient means for the deposition of metal nanoparticles on solid electrode supports such as conducting glass or carbon black for catalytic applications. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention. Table 1

Table 2