LIQUID CRYSTAL TEMPLATED DEPOSITION METHOD

The present invention relates to a method of depositing metals having a good mesostructure from salts or other compounds thereof using a liquid crystal templating technique.

The mesostructured materials produced in the present invention, which are generally porous in nature and so may be described as "mesoporous", are sometimes referred to as "nanostructured". However, since the prefix "nano" strictly means 10 , and the pores in such materials normally range in size from 10 ^" " to 10 ^" " m, it is better to refer to them, as we do here, as "mesostructured".

The preparation and use of liquid crystalline phases is disclosed in US Patents No 6,503,382 and 6,203,925, the disclosures of which are incorporated herein by reference.

In general terms, liquid crystal templating comprises forming a liquid crystal comprising at least two "soft matter" phases arranged in a generally crystal-like regular array. This soft matter is often loosely referred to as "liquid", hence the term "liquid crystal". A solid material is deposited from one of these phases, either chemically or electrochemically, and naturally assumes the configuration of the phase from which it is deposited. The soft matter is then removed. This permits the preparation of materials having more-or-less regular structures which could not be achieved in any other way.

The liquid crystal phases are usually prepared with the aid of a surfactant and many such surfactants have been proposed for use in the process, including both ionic and non-ionic surfactants. However, in practice, apart from a few academic exercises, only non-ionic surfactants have actually been used and, in general, these have given good results in laboratory tests.

In a laboratory experiment, it is acceptable to allow deposition to take place over a considerable period of time. Indeed, it may even be desirable that deposition should take place slowly, so that the progress of the reaction can be more closely observed. However, in industrial production, it is undesirable that production processes should be unduly prolonged and it is commonly desired that they should be completed as quickly as is consistent with obtaining the desired product in good yields and having the required properties. In an electrochemical liquid crystal templating deposition method in which a metal is deposited from a metal salt or similar compound, one way of achieving these desiderata is by increasing the concentration of the metal salt or other metal compound to as high a level as is achievable.

Unfortunately, we have found that, when high metal compound concentrations are used in conventional electrochemical processes, the liquid crystal becomes unstable and it becomes impossible to prepare metals having a good mesostructure. Surprisingly, this does not appear to be a problem when the mesoporous material is formed by a chemical deposition method. This is exacerbated with the use of 'impure' non-ionic surfactants such as the Brij ^® family which are commonly employed for their low cost. The concentration level at which the liquid crystal becomes unstable varies from metal to metal, but is easily determined by simple experiment. In the case of nickel, which is one of the metals for which liquid crystal templating deposition is of particular value, the concentration at which the liquid crystal phase becomes unstable is especially low, and this instability is a major problem when up-scaling laboratory processes for use in industry. However, the reason for the instability is not completely clear. Despite this, we have surprisingly found that the use of an ionic surfactant in place of the conventional non-ionic surfactant avoids the problem of instability.

Thus, in one aspect, the present invention consists in a process which comprises: forming a mixture comprising a metal compound from which the metal or a compound of the metal may be deposited, a solvent and a surfactant in amounts sufficient to form a liquid crystal phase in the mixture; and electrochemically depositing the metal or a compound of the metal from the metal compound, characterised in that the surfactant is an ionic surfactant and the metal compound is present in the aqueous component of the liquid crystal phase-containing mixture at a concentration which, in a comparative mixture identical to the liquid crystal phase-containing mixture except that the ionic

surfactant is replaced by a mixture of compounds of general formula CH ₃ -(CH ₂ )^- (CH ₂ CH ₂ CO _y -OH, where y is a number and the abundance of the compound having that value of y is approximately that shown in the following Table,

would cause the liquid crystal phase to be unstable or produce a deposit with a cathodic charge density less than half the value of that obtained using the ionic surfactant, with the same deposition charge density.

A commercially available mixture of compounds of formula CH ₃ -(CH ₂ )i ₅ -(CH ₂ CH ₂ O) _y -OH having the relative abundances of compounds of different values of y shown in the above Table is Brij 56, which is widely available, e.g. from Univar Ltd, United Kingdom.

The cathodic charge density referred to herein may be measured by the method described in detail in Example 5 hereafter.

In a further aspect, the present invention consists in a process which comprises: forming a mixture comprising a metal compound from which the metal or a compound of the metal may be deposited, a solvent and a surfactant in amounts sufficient to form a liquid crystal phase in the mixture; and electrochemically depositing the metal or a

compound of the metal from the metal compound, characterised in that the surfactant is an ionic surfactant and the metal compound is present in the aqueous component of the liquid crystal phase-containing mixture at a concentration of at least 0.4 M.

Formation of mixtures containing liquid crystal phases is now a well established technology and the details of the preparation of such mixtures is well known to those skilled in the art and so requires no explanation here.

The solvent is included in the mixture in order to dissolve the metal compound and to form a liquid crystalline phase in conjunction with the surfactant, thereby to provide a medium for the deposition reaction. Generally, water will be used as the preferred solvent. However, in certain cases it may be desirable or necessary to carry out the reaction in a non-aqueous environment. In these circumstances a suitable organic solvent may be used, for example formamide or ethylene glycol.

Any ionic surfactant capable of forming a liquid crystal phase in the mixture of the present invention may be used. Preferred surfactants are those having an ionic group attached, directly or indirectly, to one or more hydrocarbon chains having at least 8 carbon atoms, preferably from 8 to 30 carbon atoms. By "ionic group" we mean a group, such as an ammonium group, which already contains ions, or a group, such as an amine group, which can readily form ions. Examples of such compounds include amines and ammonium compounds e.g. of formula NR ¹ R ² R ³ Or N ⁺ R ¹ R ² R ³ R ⁴ X ^" , where at least one of R , R ² and R ³ or R ¹ , R ² , R ³ and R ⁴ represents a hydrocarbon group having at least 8, preferably at least 10, more preferably from 8 to 30 and most preferably from 10 to 20, carbon atoms, and X ^' represents an anion. Other examples include salts containing long chain fatty acid or hydrocarbon residues, said residues each having at least 8, preferably at least 10, more preferably from 8 to 30 and most preferably from 10 to 20, carbon atoms. Specific examples of preferred surfactants include cetyltrimethylammonium chloride (CTAC), cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS), hexadecyl amine (HDA), dodecyltrimethylammonium chloride (DTAC) and dioctyl sodium sulphosuccinate (also known as Aerosol OT - AOT). AOT and SDS are anionic surfactants while the others specified by the formulae NR ¹ R ² R ³ or N ⁺ R ¹ R ² R ³ R ⁴ X ^" are cationic. Of these, the

preferred surfactants are the ammonium compounds, especially cetyltrimethylammonium bromide.

The present invention may be used in connection with any metal or compound of a metal which it is desired to form into a mesostructure by deposition from a liquid crystal phase. Examples of such metals include: nickel, platinum, cobalt, iron, tin, lead, selenium, manganese, chromium, copper, zinc, niobium, molybdenum, titanium, palladium, gold, silver, cadmium, and mercury, or mixtures or alloys of any two or more thereof. The invention is of especial value in connection with nickel, cobalt, zinc, iron, tin, copper, lead, selenium, or cadmium, or a mixture or alloy of any two or more thereof, more preferably nickel or cobalt or a mixture or alloy thereof, especially nickel and mixtures of nickel with other metals, e.g. nickel/cobalt, since, in these cases, the instability of the liquid crystal system is manifest at relatively low concentration levels. The metal compounds employed to form the liquid crystal system are preferably metal salts. The salts used will, of course, depend on the metal or compound of the metal to be deposited and should be soluble in the solvent employed. Examples of such salts include the chlorides, acetates, sulphates, bromides, nitrates, sulphamates, and tetrafluoroborates, especially those of the above metals, and preferably nickel (II) chloride, nickel (II) acetate, nickel (II) sulphate, nickel (II) bromide, nickel (II) nitrate, nickel (II) sulphamate, and nickel (II) tetrafluoroborate.

Depending on the reaction conditions, the metal itself may be deposited or a compound of the metal may be deposited. Examples of such compounds of metals include the oxides and hydroxides.

These salts, or other metal compounds, are present in the aqueous component of the reaction mixture in relatively high concentrations, higher than would allow the formation of a stable liquid crystal phase were a non-ionic surfactant, such as decaethylene glycol monohexadecyl ether, used. In general, the concentration of the salt the aqueous component should be at least 0.4 M, more preferably at least 0.6 M. The maximum concentration is, of course, saturation and this varies from one salt to another, but the value for any salt is well known or can easily be determined. Still more preferably, the concentration is from 0.4 M to 4 M, more preferably from 0.6 M to 3 M and most preferably from 0.8 M to 2 M.

Where a mixture of two or more salts of different metals are employed, the minimum concentration of 0.4 M applies only to the salt having the highest concentration. The other salt or salts may be present in lower concentrations. For example, where a mixture of nickel and cobalt salts is employed, it will normally be the nickel salt that is at the higher concentration. Should a mixture of two or more salts of the same metal be employed, the total concentration of the two or more salts should be at least 0.4M.

The mixture of solvent, surfactant and metal salt, optionally with other components such as are well known in the art, will form a liquid crystal phase. The desired metal is then deposited from the mixture using conventional electrochemical means. Since mesostructured materials often lack structural strength, they are preferably deposited onto a substrate, e.g. a metal, such as gold, copper, silver, platinum, tin, aluminium, nickel, rhodium or cobalt, or an alloy containing any of these metals. The substrate may, if desired, be microporous, with pores of a size preferably in the range from 20 to 500 micrometres. Where the substrate is a metal foil, the substrate preferably has a thickness in the range from 2 to 50 micrometres. The substrate preferably is a nickel foil.

Suitable methods for depositing mesoporous materials as films onto a substrate by electrochemical deposition are known in the art. For example, suitable electrochemical deposition methods are disclosed in EP-A-993,512; Nelson, et al., "Mesoporous Nickel/Nickel Oxide Electrodes for High Power Applications' ', J. New Mat. Electrochem. Systems, 5, 63-65 (2002); Nelson, et al., "Mesoporous Nickel/Nickel Oxide - a Nanoarchitectured Electrode " , Chem. Mater., 2002, 14, 524-529.

Preferably, the mesoporous material is formed by electrochemical deposition from a lyotropic liquid crystalline phase. According to a general method, a template is formed by self-assembly from the long-chain surfactants described above and water into a desired liquid crystal phase. The mesoporous structure has a periodic arrangement of pores having a defined, recognisable topology or architecture, for example cubic, lamellar, oblique, centred rectangular, body-centred orthorhombic, body-centred tetragonal, rhombohedral, hexagonal. Preferably, the mesoporous structure has a periodic pore arrangement that is hexagonal, in which the mesoporous metal or

compound of the metal is perforated by a hexagonally oriented array of pores that are of uniform diameter and continuous through the thickness of the metal or compound of the metal.

The invention is further illustrated by the following non- limiting Examples.

EXAMPLE 1

A liquid crystal template was made by mixing 3O g of cetyltrimethylammonium bromide (CTAB) with 30 g of an aqueous solution consisting of 0.56 M nickel (II) chloride (NiCl ₂ ) and 0.24 M cobalt (II) chloride (CoCl ₂ ). An electrochemical cell using the mixed liquid crystal as electrolyte and nickel foil positive and negative electrodes was then assembled. A saturated calomel reference electrode (SCE) was also inserted to control the subsequent electrodeposition of nanoporous material. Electrodeposition of the mesoporous nickel/cobalt containing layer was carried out by applying a constant potential of -0.75 V versus the SCE reference to one of the nickel foils. Electrodeposition was carried out for 50 minutes, after which time a charge density of -2.0 ClcnP- had passed. The electrodeposited film was then washed in deionised water for 24 hours to remove the liquid crystal template.

Once washed, the charge storage capacity of the electrodeposited film was measured using cyclic voltammetry in 6 M potassium hydroxide (KOH) solution versus a mercury/mercury oxide reference electrode (Hg/HgO, with 6 M KOH). At a scan rate of 20 mV/s, the film was cycled continuously between 0 V and 0.55 V. On the third cycle, the film had a cathodic charge density of 416 mC/cm^.

EXAMPLE 2

A liquid crystal template was made by mixing 3O g of cetyltrimethylammonium bromide (CTAB) with 30 g of an aqueous solution consisting of 0.84 M nickel (II) chloride (NiCl ₂ ) and 0.36 M cobalt (II) chloride (CoCl ₂ ). An electrochemical cell using the mixed liquid crystal as electrolyte and nickel foil positive and negative electrodes was then assembled. A saturated calomel reference electrode (SCE) was also inserted to control the subsequent electrodeposition of nanoporous material.

Electrodeposition of the mesoporous nickel/cobalt containing layer was carried out by applying a constant potential of -0.75 V versus the SCE reference to one of the nickel foils. Electrodeposition was carried out for 25 minutes, after which time a charge density of -2.0 Clever had passed. The electrodeposited film was then washed in deionised water for 24 hours to remove the liquid crystal template.

Once washed, the charge storage capacity of the electrodeposited film was measured using cyclic voltammetry in 6 M potassium hydroxide (KOH) solution versus a mercury/mercury oxide reference electrode (Hg/HgO, with 6 M KOH). At a scan rate of 20 mV/s, the film was cycled continuously between 0 V and 0.55 V. On the third cycle, the film had a cathodic charge density of 324 mC/cm .

EXAMPLE 3

A liquid crystal template was made by mixing 30 g of cetyltrimethylammonium bromide (CTAB) with 30 g of an aqueous solution consisting of 0.84 M nickel (II) chloride (NiC^) and 0.36 M cobalt (II) chloride (C0CI2). An electrochemical cell using the mixed liquid crystal as electrolyte, nickel foil as negative electrode and a graphite sheet as positive electrode was then assembled. A saturated calomel reference electrode (SCE) was also inserted to control the subsequent electrodeposition of nanoporous material. Electrodeposition of the mesoporous nickel/cobalt containing layer was carried out by applying a constant potential of -0.75 V versus the SCE reference to one of the nickel foils. Electrodeposition was carried out for 40 minutes, after which time a charge density of -2.6 C/cm^ had passed. The electrodeposited film was then washed in deionised water for 24 hours to remove the liquid crystal template.

Once washed, the charge storage capacity of the electrodeposited film was measured using cyclic voltammetry in 6 M potassium hydroxide (KOH) solution versus a mercury/mercury oxide reference electrode (Hg/HgO, with 6 M KOH). At a scan rate of 20 mV/s, the film was cycled continuously between 0 V and 0.55 V. On the third cycle, the film had a cathodic charge density of 442 mC/cm .

EXAMPLE 4

A liquid crystal template was made by mixing 3O g of cetyltrimethylammonium bromide (CTAB) with 30 g of an aqueous solution consisting of 0.84 M nickel (II) chloride (NiC^) and 0.36 M cobalt (II) chloride (CoC^). An electrochemical cell using the mixed liquid crystal as electrolyte, nickel foil as negative electrode and a graphite sheet as positive electrode was then assembled. A saturated calomel reference electrode (SCE) was also inserted to control the subsequent electrodeposition of nanoporous material. Electrodeposition of the mesoporous nickel/cobalt containing layer was carried out by applying a constant potential of -0.75 V versus the SCE reference to one of the nickel foils. Electrodeposition was carried out for 75 minutes, after which time a charge density of -3.2 Clever had passed. The electrodeposited film was then washed in deionised water for 24 hours to remove the liquid crystal template.

Once washed, the charge storage capacity of the electrodeposited film was measured using cyclic voltammetry in 6 M potassium hydroxide (KOH) solution versus a mercury/mercury oxide reference electrode (Hg/HgO, with 6 M KOH). At a scan rate of 20 mV/s, the film was cycled continuously between 0 V and 0.55 V. On the third cycle, the film had a cathodic charge density of 614 mC/cm .

EXAMPLE 5 (Comparative)

Procedures similar to those described in the preceding Examples were repeated, but using nickel or cobalt salts or mixtures of these salts at various concentrations and replacing the cetyltrimethylammonium bromide by an equivalent amount of Brij ^® 56. It was found possible to electrodeposit nickel and nickel/cobalt containing films from hexagonal phase liquid crystal templates based on this Brij surfactant over a period of about twenty hours, which is, in practice, too long to be commercially attractive, when the metal ion (nickel and/or cobalt) concentration was about 0.2 M. However, using compositions of higher metal ion concentration of 0.5 M the electrodeposition process was faster but the deposits were cracked and patchy. At metal ion concentrations of 0.8 M the liquid crystal phase could be seen by eye to destabilise after only three minutes with a resulting poor quality electrodeposit. Here, a liquid crystal template was made

by mixing 30 g of Brij ^® 56 with 30 g of an aqueous solution consisting of 0.8 M nickel (II) chloride (NiCl ₂ ) and 0.36 M cobalt (II) chloride (CoCl ₂ ). An electrochemical cell using the mixed liquid crystal as electrolyte, nickel foil as negative electrode and a graphite sheet as positive electrode was then assembled. A saturated calomel reference electrode (SCE) was also inserted to control the subsequent electrodeposition of material. Electrodeposition of the nickel/cobalt containing layer was carried out by applying a constant potential of -0.75 V versus the SCE reference to one of the nickel foils. Electrodeposition was carried out for 75 minutes, after which time a charge density of -3.2 C/crn^ had passed. The electrodeposited film was then washed in deionised water for 24 hours to remove the liquid crystal template.

Once washed, the charge storage capacity of the electrodeposited film was measured using cyclic voltammetry in 6 M potassium hydroxide (KOH) solution versus a mercury/mercury oxide reference electrode (Hg/HgO, with 6 M KOH). At a scan rate of 20 mV/s, the film was cycled continuously between 0 V and 0.55 V. On the third cycle, the film had a cathodic charge density of 21 mC/cm .