GB2489458A

GB2489458A - Liquid crystal templating

Info

Publication number: GB2489458A
Application number: GB1105245.3A
Authority: GB
Inventors: Adam Squires; Joanne Margaret Elliott; Samina Akbar
Original assignee: University of Reading
Current assignee: University of Reading
Priority date: 2011-03-29
Filing date: 2011-03-29
Publication date: 2012-10-03
Also published as: WO2012131296A3; WO2012131296A2; GB201105245D0

Abstract

A process for the preparation of porous, structured films of metals and other materials is described, through the use of liquid crystal templating. The process involves the steps of applying a thin layer of a structure-directing agent such as a non-ionic amphiphile in a solvent to a substrate and material deposited onto the substrate electrolytically. The structure directing agent forms an inverse lyotropic liquid crystalline phase. The structure directing agent may comprise phytantriol, and the inverse liquid crystal phase may be selected from Q229 (Q11P), Q224 (Q11D) and Q230 (Q11G). The porous metal film can comprise platinum or alloys thereof. The films thus produced may be used in sensors, catalysts, fuel cells, capacitors or solar cells.

Description

Electrodeposition Method and Product Obtained Thereby

Summary of the Invention

The present invention relates to a process for the preparation of a film having a high surface area by electrodeposition, to a porous film obtained by such a process, and to several applications of such films.

Background of the Invention

Solid films having high available surface area find a range of applications, including catalysis, fuel cells, batteries, capacitors and sensors.

Standard techniques for the production of high surface area materials, such as sintering, etching, and vapour-phase deposition, tend to give materials having a distribution of pore size. The mechanical integrity of these materials can be low. In addition, the techniques often require harsh conditions such as elevated temperatures that might not be compatible with certain substrates.

Another approach that has been adopted is templated electrodeposition. In this method, a material (for example, a metal) is electrodeposited from a solution onto the surface of an electrode in the presence of a bulk lyotropic liquid crystalline phase. The liquid crystalline phase directs the deposition of material and gives rise to a surface having a highly ordered arrangement of pores of a substantially uniform size.

Description of Prior Art

US6,503,382 discloses a method of preparing a porous film involving electrodeposition of a metal from a mixture of a solvent, a source of metal and a surfactant in a homogeneous mixture. The electrodeposited film obtained has a substantially uniform structure with regular pore size. The surfactant forms a lyotropic liquid crystalline phase; hexagonal phases are used to provide surfaces having a hexagonal arrangement of pores.

US2008/0096089 discloses a method for the fabrication of a mesoporous material comprising the electroreduction of a solution containing metal ions, solvent and a structure-directing agent which is not in a liquid crystalline phase. The material is stated to have a random structure of pores and a high roughness factor.

A problem associated with known electrodeposition methods is that they require a solvent phase with a high concentration of surfactants. This is not efficient in terms of the amount of surf actant used.

A further problem associated with known methods is the fact that all electrodes must be immersed in the liquid crystalline phase. This presents problems, especially for higher-viscosity (e.g. bicontinuous cubic) phases.

A further problem associated with known methods is that they do not allow for replenishment of material to be deposited during the course of electrodeposition.

A further problem associated with known methods is that they do not allow for the solution containing the precursor to the material to be deposited to be readily changed during the course of electrodeposition without replacing the liquid crystalline templating phase. This makes the deposition of a second material during the course of electrodeposition restricted to certain very specific combinations that are accessible only by changing the deposition voltage.

A further problem associated with known methods is that they do not allow for removal of by-products from the reaction mixture.

These and other problems associated with the prior art are addressed by the present invention.

Brief Description of the Invention

According to a first embodiment, the invention provides a process for the preparation of porous film comprising the steps of: i) applying a first composition comprising a first solvent and a structure directing agent to a substrate and optionally removing at least a portion of the first solvent to form a layer comprising structure directing agent on the substrate; ii) providing a second composition comprising a second solvent and a precursor of a material to be deposited; wherein the structure directing agent forms an inverse lyotropic liquid crystalline phase in the presence of said second composition; iii) contacting said layer with said second composition; and iv) electrochemically depositing said material onto said substrate.

According to a second aspect of the invention, there is provided a porous film obtainable by the process of the first aspect.

According to a third aspect of the invention, there is provided a chemical or biological sensor comprising a porous film of the invention.

According to a fourth aspect of the invention, there is provided a catalyst comprising a porous film of the invention.

According to a fifth aspect of the invention, there is provided a fuel cell comprising a porous film of the invention.

According to a sixth aspect of the invention, there is provided a capacitor comprising a porous film of the invention.

According to a seventh aspect of the invention, there is provided a solar cell comprising a porous film of the invention.

Figure 1 is a diagram of a prior art method of template electrodepostion, comprising a bulk liquid crystalline phase in which all the electrodes are immersed.

Figure 2 is a diagram of an electrodeposition method according to the invention.

Figure 3 is a representation of the tetrahedral network of water channels in a porous 0224 cubic phase of phytantriol. a) shows the two water channel network, and b) the structure of templated platinum deposited according to an embodiment of the invention.

Figure 4 shows the X-ray scattering pattern for a) phytantriol in excess water, b) phytantriol in excess hexachloroplatinic acid solution, c) nanostructured Ft-film deposited on gold electrode according to the invention, and d) Nanostructured Ft-film deposited on platinum electrode according to the invention. The inset figures represent the 2D-SAXS patterns.

Figure 5 shows cyclic voltammograms for platinum electrode in 0.5 M aqueous sulfuric acid at 100 mVs1 between +1.2 and -0.35 V vs Ag/AgCI. The unbroken line (-) represents mesoporous electrode, roughness factor was estimated to be 1212. The broken line (...) represents the polished electrode, roughness factor was estimated to be 2.1.

Figure 6 Left: SEM image of nanostructured platinum film deposited at platinum electrode. Right: TEM images of fragments taken from a nanostructured platinum film that had been deposited at platinum foil.

Figure 7 is a graph showing the variation of roughness factor with respect to charge density.

Figure 8 is a phase diagram showing the phase behavior of water/phytantriol mixtures.

Detailed Description of the Preferred Embodiments

According to the process of the invention, a layer of a composition comprising a structure-directing agent and a first solvent is applied to a substrate. Without wishing to be bound by such theory, the resultant film of structure-directing agent adopts the inverse liquid crystal phase on subsequent immersion in the second solvent and retains the structural integrity of said phase during electrodeposition. This was wholly contrary to the expectations of the inventors.

Structure-Directing Agent The structure-directing agent serves to form an inverse lyotropic liquid crystalline phase in the second solvent, and directs the electrochemical deposition of the material to be deposited, and so determines the structure of the porous film ultimately formed.

The skilled person will be aware that many substances and combinations of substances are capable of forming inverse lyotropic liquid crystalline phase with particular solvents under specific conditions. Preferred structure-directing agents are amphiphiles. More preferred structure-directing agents are i) anionic amphiphiles (including the salts of carboxylates, sulphonates, sulphates, sulphamates, sulphinates, phosphonates, phosphate esters, and phosphinates), ii) cationic amphiphiles (including the salts of primary, secondary and tertiary amines), iii) zwitterionic amphiphiles (including amino acids) and iv) polar non-ionic amphiphiles (including amines, amine oxides, amides, phosphines, phosphine oxides, phosphonate esters, phosphate esters, thiols, sulphoxides, sulphonates, polyoxyethylene block co-polymers, glycosides and alcohols), the latter class being particularly preferred.

A preferred class of structure directing agent has the formula RQ, wherein Q represents a polyoxyethylene group (CH2CH2O)-H where n is an integer between 2 and 60, preferably between 2 and 12, and R represents a hydrophobic group of the same families as those described for the alcohols, below. Alternative classes of structure directing agents are diblock copolymer combinations such as RQR or QRQ, wherein Q and R are as hereinbefore defined.

An alternative, more highly preferred class of structure directing agent are alcohols having a hydrophobic group. Such alcohols may be monohydric, dihydric, trihydric or polyhydric. Preferably, the alcohol comprises a lipophilic hydrocarbon group, which may optionally contain unsaturated or aromatic moieties. Suitable compounds include organic surfactant compounds of the formula RQ wherein R represents a linear or branched alkyl, aryl, aralkyl or alkylaryl group having from 6 to about 6000 carbon atoms, preferably from 6 to about 60 carbon atoms, more preferably from 12 to 20 carbon atoms, more preferably 18 carbon atoms. R may further comprise linking moieties such as ester, amide, and ether groups. Q represents a group CmHn(OH)p wherein m is an integer from 1 to 6, preferably 3, and n and p are integers such that the sum of n and p is 2m+1. Preferably, 0 is a group (CHOH)2CH2OH. A very highly preferred structure directing agent is 3,7,11,15-tetramethyl-1,2,3-hexadecanetriol, or phytantriol.

It is preferable that the structure-directing agent is stable to electrochemical conditions.

First Solvent The first solvent is selected such that a layer of the first composition comprising the structure-directing agent can be applied to the substrate, for example by painting, spin-coating, dip-coating or spraying. The first solvent is preferably sufficiently volatile for it to be removed from the substrate by evaporation to leave a layer of structure-directing agent. Suitable first solvents include chloroform, alcohols, hydrocarbons, ketones, cyclic ethers, aliphatic ethers, nitrites, alkanes, and mixtures thereof. A particularly preferred class of solvents is alcohols, particularly Cl-CS monohydric, dihydric, trihydric or polyhydric alcohols, such as methanol, ethanol, ethylene glycol, propan-1-ol, propan- 2-ol, butan-1-oI, 2-methylpropan-2-ol, etc. Mixtures of solvents are also contemplated.

Ethanol is very highly preferred.

The ratio of first solvent to structure-directing agent is selected based on the desired thickness of layer of structure directing agent required, and are typically in the range of between 1:100 w/w and 100:1 w!w structure-directing agent:solvent, preferably between 1:10 and 1:1 w/w, more preferably between 1:3 and 3:4 w/w, most preferably about 1:2 wlw.

The first composition is preferably a solution, suspension, gel, or emulsion, but is very preferably a solution.

Inverse Lyotropic Liquid Crystalline Phase The term inverse lyotropic liquid crystalline phase here refers to a composition having a continuous phase of structure-directing agent, and wherein the interface between the structure-directing agent and the water curves towards the water. The inverse lyotropic liquid crystalline phase may also have a continuous aqueous phase, that is, a bicontinuous phase. Examples of inverse lyotropic liquid crystalline phase include inverse cubic phases, the inverse hexagonal phase, and inverse micellar phases.

Preferred are the inverse hexagonal (Hi1) phase and the inverse bicontinuous cubic (Qii) phases. More preferred are inverse bicontinuous cubic phases.

The inverse hexagonal (H11) phase consists of parallel cylindrical water channels surrounded by the structure-directing agent, stacked into a 2-D hexagonal array.

More highly preferred are the inverse bicontinuous cubic phases (Q11). Inverse bicontinuous cubic phases can further be characterised as primitive (Q11P), double diamond (011D) and gyroid (Qii°), also known as 0229, 0224 and 0230 respectively. These phases each have a unique three dimensional structure comprising a bilayer of structure-directing agent separating two discrete aqueous domains. Of these, double diamond (011D) is preferred.

A number of experimental techniques may be used to determine the particular phase formed by a solvent/ structure-directing agent combination under particular conditions.

These include polarising optical microscopy, which can distinguish between the different classes of lyotropic liquid crystalline phase, and small-angle X-ray scattering, which gives information about the symmetry of the system, from which the phase may be determined.

With reference to the preferred structure-directing agent phytantriol, the phase behaviour is explained in Langmuir 2003, 19, 9562-9565. The phase diagram is reproduced in Figure 8.

The H11 phase shows a series of Bragg reflections where the first four follow the relationship 1: 13: V4: V7. In excess aqueous solvent, this phase appears at temperatures between 44-60 00 with a lattice parameter of approximately 40 A. The preferred 011D (0224) phase shows Bragg reflections where the first six follow the relationship 12: 13: 14: 16: 18: 19, indexed as the 110, 111, 200, 211, 220, and 221 reflections. This is in agreement with a cubic lattice of Pn3m crystallographic space group. With phytantriol, under excess water conditions, as in the layer in this invention, this phase has a lattice parameter of 66 A, and a water content of 28% (wlw). The lattice parameter of the 0224 decreases with increasing temperature down to 60 A at 44°C. The phase and lattice parameter in excess aqueous solution of 8 wt% hexachloroplatinic acid are the same as in water. These correspond to the structure adopted by the thin layer of structure directing agent in the second solvent, as described in this invention. According to a preferred embodiment, the structure-directing agent is phytantriol, and the inverse lyotropic liquid crystalline phase is the phase where the first observable Bragg peaks are in the ratio 12: 13: 14: 16: 18: 19.

Substrate Suitable substrates include those formed from materials with sufficient electrical conductivity to permit electrodeposition of the material to be deposited. Preferred substrate materials include metals, metalloids, semiconductors, organic conductive materials and carbon, and mixtures and alloys thereof, indium tin oxide coated substrates and conducting polymer coated substrates. More preferred substrate materials are metals, carbon and mixtures and alloys thereof. More preferred substrate materials are noble metals and mixtures and alloys thereof. The most preferred substrate materials are gold and platinum and mixtures and alloys thereof.

The physical form of the substrate is selected according to the form of the film which it is desired to obtain. Preferably, the substrate is in the form of a wire or plate.

To ensure good electrical contact, the substrate is optionally cleaned prior to application of the composition comprising a first solvent and a structure directing agent. Suitable cleaning techniques are apparent to the skilled person, and include mechanical (e.g. polishing), chemical (e.g. pickling) and electrochemical (e.g. by cycling in acidic solution).

Application of Layer The layer of first composition comprising first solvent and a structure directing agent may be applied to the substrate by any suitable means. Preferred means of application include spin coating, spray application, painting, and dip coating.

Following or during application of the composition to the substrate, optionally, and preferably, at least a portion of the first solvent is removed. Preferably, the first solvent is allowed to at least partially evaporate. This may be at ambient (i.e. 20 00) or elevated temperature, and may be assisted by reduced pressure or current of air or other gas.

Preferably, at least an amount of the solvent is removed. Preferably, most of the solvent is removed. Preferably, more than 50 % of the solvent is removed. More preferably, more than 75 % of the solvent is removed. More preferably, more than 90 % of the solvent is removed. More preferably, more than 95 % of the solvent is removed. More preferably, more than 99 % of the solvent is removed. The amount removed is preferably at least such that any residual solvent does not prevent either the formation of the inverse lyotropic liquid crystalline phase on addition of the second mixture, or the electrochemical deposition process Second Composition The second composition is selected such that electrochemical deposition of the material to be deposited onto the substrate may be carried out, and that the layer comprising structure directing agent adopts the desired inverse lyotropic liquid crystalline phase on the substrate in the presence of the second composition, and retains it during electrodeposition.

The second composition comprises the second solvent and the precursor of the material to be deposited. Preferred second compositions are electrically conductive.

This conductivity is usually conferred by the ionic nature of the precursor of the material to be deposited, as in the case of the most favoured embodiment, where the precursor is hexachloroplatinic acid.

Suitable second solvents include water, alcohols, amides, aliphatic ethers, and mixtures thereof. Preferred solvents are selected from water, formamide, ethylene glycol or glycerol. Mixtures of such solvents are also comprehended within the invention. A particularly preferred class of solvent is water.

Material to be Deposited Preferred materials to be deposited within the meaning of the invention in this context are selected from metals, binary inorganic compounds in particular metal chalcogenides such as metal oxides, and organic polymers. The materials are preferably semiconductors or conductors. Suitable precursors for such materials to be deposited will be apparent to the skilled person in by reference to conventional electrodeposition techniques.

Similarly, one or more source materials may be used in the second composition in order to deposit one or more materials selected from a particular species or combination of species, either simultaneously or sequentially. By appropriate selection of source material and electrodeposition conditions, the composition of the deposited film can be controlled as desired.

A preferred class of materials to be deposited is metals and combinations of metals.

Preferred metals include zinc, tantalum, niobium, cadmium, aluminium, gallium, indium, thallium, tin, lead, antimony bismuth, platinum, palladium, gold, rhodium, ruthenium, silver, nickel, cobalt, copper, iron, chromium and manganese, preferably platinum and gold, and most preferably platinum.

In some embodiments, combinations of metals may be deposited either sequentially or simultaneously Certain combinations of metals may be deposited simultaneously as a uniform manner (i.e., as a deposited alloy). These combinations include nickel/cobalt, tin/copper, copper/silver, copper/nickel, nickel/iron and lead/manganese. Tertiary, quaternary and higher combinations are also envisaged.

In other embodiments, two or more metals are electrodeposited in sequential fashion.

This may be achieved in several ways. In one embodiment, after deposition of one metal, the second composition is replaced with another composition containing a salt of a second metal, which is deposited to give a layer of the second metal. In another embodiment, the two or more metals are both present, for example as ions in solution, in the second composition. Where the two metals differ in terms of reduction potential, the electrodeposition potential may be chosen to reduce only one of the two metals.

This will form an initial layer comprising substantially only the more noble metal.

Subsequent increase of the potential will allow deposition of a layer comprising both metals. Thus, alternating layers of single metal and combination of metals may be achieved.

Preferred non-oxide semiconductors/conductors are selected from single elemental species, preferably selected from germanium, silicon and selenium, and binary species, preferably selected from such as gallium arsenide, indium stibnate, indium phosphide and cadmium sulphide.

Preferred metal oxide semiconductors include oxides of titanium, zinc and tin.

Suitable organic polymers include aromatic and olefinic polymers, for example conducting polymers such as polyaniline, polypyrrole and thiophene, or derivatives thereof. These will generally be associated with organic or inorganic counterions, for example chloride, bromide, sulphate, sulphonate, tetrafluoroborate, hexafluorophosphate, phosphate, phosphonate, or combinations thereof.

Other suitable organic materials include insulating polymers such as polyphenol, polyacrylonitrile and poly(ortho-phenylene diamine).

Precursor to the material to be deposited The material to be deposited is present, preferably as a solution, in the solvent in the form of the "precursor', that is converted into the solid material during the electrodeposition process. In embodiments wherein the material to be deposited is a metal, the precursor preferably consists of a salt containing metal ions, or complexes of metal ions. The skilled person will be aware that when metal ions are present in solution, there must also generally be a counterbalancing anion (counterion) also present. Suitable counterions are preferably selected from hydrogen (H), fluoride, chloride, bromide, iodide, sulphate, sulphonate, tetrafluoroborate, hexafluorophosphate, phosphate or phosphonate, or a combination thereof. A particularly preferred precursor is hexachloroplatinic acid (HCPA) in the embodiment wherein the material to be deposited is platinum metal. Another preferred precursor is titanium trichloride, in the embodiment wherein the material to be deposited is titanium (IV) oxide Further suitable precursors for materials to be deposited will be apparent to the skilled person in by reference to conventional electrodeposition techniques.

The concentration of the precursor to the material to be deposited in the second composition may be adjusted such that a reasonable rate of deposition is achieved using the chosen charge density. Preferred concentrations are from 1 to 60 % w/w, more preferably 5 to 10 % w/w.

In an optional, and preferred, embodiment, the concentration of the precursor of the material to be deposited may be maintained at a steady level. This is impractical using known templated electrodeposition techniques, which involve a viscous, bulk liquid crystal phase of structure-directing agent comprising material to be deposited, as illustrated in Figure 1. In the present invention, only a layer of structure-directing agent is present (Figure 2); the second composition can be replenished with further material to be deposited, maintaining the concentration at a constant level.

The second composition may additional comprise further additives usual in the field of electrodeposition. For example, the second composition may comprise further electrolytes to increase the ionic strength of the composition, and/or buffers to control the pH. Preferably, the pH of the second composition is adjusted to from 1 to 14, and more preferably from 2 to 6.

A particularly preferred aspect of the present invention is the electrodeposition of more than one layer of material to be deposited by subsequent contact of the layer of structure directing agent with a composition comprising a further material to be deposited and a further solvent. Thus, according to this aspect, there is provided a process of the invention having the further steps of v) providing a third composition comprising a third solvent and a precursor of the second material to be deposited; vi) optionally, washing the said layer; vii) contacting said layer with said third composition; and viii)electrochemically depositing said second material onto said first material.

Known techniques of templated electrodeposition of materials employ bulk liquid crystalline phases of structure-directing agents. These suffer from the disadvantage that the electrodes (including the substrate) must be immersed in the bulk liquid crystalline phase, which also comprises the material to be deposited. This makes it impractical or impossible to replace the material to be deposited with a subsequent, different material, without also replacing the liquid crystal phase of the structure-directing agentS Surprisingly, because the inverse lyotropic liquid crystalline phase used in the present invention forms a layer on the substrate which is stable in the excess solvent used for electrodeposition, the layer can be exposed to sequential electrodeposition treatments with different deposited materials simply by removing the layer from the first composition comprising the precursor of the first material to be deposited and exposing the layer to electrodeposition with a subsequent composition comprising a precursor of a further material to be deposited.

The skilled person will understand that this process can be repeated in an iterative fashion to build up subsequent layers of material on the porous film.

Electrodeposition Preferably, the substrate bearing the layer of structure-directing agent is brought into contact with the second composition for a period of time prior to the electrodeposition process. This is to allow sufficient solvation and permit the relevant inverse lyotropic liquid crystalline phase to form in the second composition. Preferably, this is conducted for at least 1 minute, more preferably, for at least five minutes, still more preferably for at least 10 minutes.

The apparatus for electrolysis preferably consists of a conventional three electrode system, comprising the working electrode (i.e. the substrate), a counter electrode, preferably a large surface area platinum flag, and a reference electrode, preferably calomel or AgfAgCl.

Electrodeposition is carried out in accordance with the desired characteristics of the porous film. The specific electrodeposition conditions of pH, temperature, potential, current density and deposition period will depend on the source material used and the thickness of film to be deposited.

The charge density for electrostatic deposition is preferably in the range from 0.1 to 50 C cm, more preferably from 0.5 to 20 C cm2, and still more preferably from 1 to 8 C cm2 and most preferably from 1 to 6.2 cm2.

Preferably, the potential applied has a value in the range +1OV to -by, preferably +5V to -5V, and more preferably +1V to -lv, relative to the standard Ag/AgCI electrode.

Preferably, for potentiostatic deposition at variable potential, the applied potential is stepped between fixed limits generally within the range from +1 OV to -1 OV, preferably + V to -5 V, still more preferably +1 V to -1 V, most preferably +0.6 V to -0.2 V relative to the standard Ag/AgCI electrode.

The temperature at which electrodeposition is conducted is preferably in the range from 15 to 80 °C., more preferably 20 to 40 °C.

The temperature at which electrodeposition is conducted is preferably in the range from to 80 00., more preferably 20 to 40 00. for the use of a 011D phase of phytantriol as a template. Also preferred is the range 45 to 60 00. in embodiments using the Hi1 phase of phytantriol as a template.

The electrodeposition is preferably carried out so as to deposit a film of a thickness from 1 nm to 200 pm, preferably from 5 nm to 100 pm, more preferably 10 nm to 50 pm.

Post-treatment of Films The electrodeposited porous films may be treated to remove the structure-directing agent. This is preferably achieved by treatment with a solvent. Suitably, this is the same as the first solvent as set out above. Ethanol, chloroform, and mixtures thereof are preferred. Films

The films obtained by templating from a 011D phase have a unique structure characterised by a three-dimensional tetrahedral network of microscopic wires of deposited material, and have a very high internal surface area and roughness factor.

Figure 6 (left hand image) shows a scanning electron microscope (SEM) image of nanostructured platinum film deposited at platinum electrode. The roughness factor was estimated to be 426 at 200 mVs1 in 0.5 M aqueous H2S04 vs Ag/AgOl. The right-hand image of figure 6 shows transmission electron microscopy (TEM) images of nanostructured platinum film deposited at platinum foil. The diameter of the intersecting channels is estimated to be 4.85 nm.

In those cases where the inverse lyotropic liquid crystalline phase of structure-directing agent is an inverse cubic phase, more preferably an inverse bicontinuous cubic phase having a three dimensional structure comprising a bilayer separating two discrete aqueous domains, surprisingly, only one of the aqueous domains appears to be replaced with metal. This is represented in Figure 3. The structure of the tetrahedral network of water channels in a porous 0224 cubic phase of phytantriol, is shown at a), and that of the templated platinum at b).

The deposited nanostructured Pt-films showed two Bragg peaks with the relative position of V3: V8. The lattice parameters are calculated to be 130.2 and 132.4 A respectively (Figure 4). Thus, in a further aspect, the invention relates to a nanostructured platinum film exhibiting a small angle X-Ray scattering pattern with peaks in a relative position of 13: J8.

On the basis of X-ray analysis, it is estimated that nanostructured Pt-films were developed through only one channel of 0224 phase of phytantriol exhibiting another cubic phase with the space group of 0227 (Figure 3). Uses

The films of the invention may have applications as follows: in sensors such as gas sensors, for example for carbon monoxide, methane, and hydrogen sulphide; chemical sensors, for example for process control in the chemicals industry, and biosensors, for example for glucose or therapeutic drugs; in energy storage cells, batteries and capacitors, for example as anode or cathode electrodes or solid electrolyte; in solar cells; in electrochromic devices such as display devices as electrodes or solid electrolytes or electroactive components; in field emitters, for example display devices or electronic devices; as nanoelectrodes, for example for electrochemical studies; in electrocatalysis; in magnetic devices, for example magnetic recording media or giant magnetoresistive media; in optical devices such as non-linear optical media, evanescent wave devices, surface plasmon polariton devices, or optical recording media; for scientific applications such as in surface enhanced optical processors, chemical reactions in confined geometries, or physical processes in confined geometries; for chemical separations, for example in gas separation, electrostatic precipitators, electrochemical separators or electrophoresis; in devices for the controlled delivery of therapeutic agents; in fuel cells as storage media for hydrogen and other gaseous fuels.

Examples

Electrode surface: The electrodeposition process has been carried out on two electrode materials -gold and platinum. Samples prepared for electrochemical surface area studies were deposited on platinum or gold wire electrodes of diameter 0.5 mm and 1 mm respectively, sealed directly into glass. Prior to use, each electrode was polished using alumina (Buehler) in three grades (25, 1.0 and 0.3 pm). Each electrode was then cleaned electrochemically by cycling in 0.5 M aqueous sulfuric acid between - 0.2 and +1.2 V vs. Ag/AgCI at 200 my s. Larger area samples prepared for small-angle x-ray scattering (SAXS) analysis were deposited onto -50 mm2 area gold plate electrodes (prepared from archival gold coated DVD5) without any pre-treatment, or 1-5 mm2 area platinum foil that was flamed in a Bunsen flame before use.

Electrode modification: The bare platinum and gold electrodes were coated with a thin film of phytantriol by dipping each electrode into a mixture of phytantriol and ethanol (w/w ratio of 1:2) followed by drying for not less than two hours, under ambient conditions. During this time the ethanol evaporated leaving a thin film of phytantriol on the electrode surface.

Electrodeposition of platinum films: The phytantriol-coated electrodes were soaked in hexachloroplatinic acid (HCPA) solution (8 wt% in water) for at least 10 mm to allow sufficient hydration to produce the cubic phase, Q224, of phytantriol at the surface of the electrode. Electrodeposition was then carried out in an electrochemical cell containing ml of HCPA solution (8 wt% in water). All electrochemical experiments were carried out on a purpose built electrochemical workstation, interfaced to a personal computer using a ClO-DASO8 data acquisition card (Measurement Computing), using software for data recording written in-house. Electrodeposition was achieved under potentiostatic control; the potential was stepped from +0.6 V to -0.2 V vs. AgIAgCI at room temperature. The voltage was then held at -0.2V for between 10 and 50 minutes.

Longer deposition times resulted in higher deposition charge densities, which produced thicker films with higher accessible surface areas due to the internally accessible 3D nanostructure; charge densities of between 1 and 8 C cm2 were passed at the different electrodes (1 -6.2 C cm2 for electrochemical surface area studies; 1-8 C cm2 for larger area samples for SAXS analysis).

Post-treatment of working electrodes: After deposition the working electrodes were removed from the templating cell and soaked in ethanol (10-15 ml) which was replaced after one hour; this process was then repeated. Further washing was carried out by soaking the electrodes in a mixture of ethanol and water (V/V ratio of 1:1) for 30 mm followed by rinsing with plenty of water and soaking in deionised distilled water for one hour. Prior to further use each electrode was then rinsed with fresh deionised distilled water.

Structural features of the nanostructured pt-films were studied by small angle X-Ray scattering. Figure 4 shows X-ray scattering pattern for a) Phytantriol in excess water; b) Phytantriol in excess hexachloroplatinic acid solution; c) Nanostructured Pt-film deposited on gold electrode and d) Nanostructured Pt-film deposited on platinum electrode. Inset figures represent the 2D-SAXS pattern. For 0224 in phytantriol, the Bragg peaks with the relative positions V2, i3, V4, J6 showed a lattice parameter of 65.0 A in excess H20 and 65.2 A in excess HCPA.

The deposited nanostructured Pt-films showed two Bragg peaks with the relative position of V3: i18. The lattice parameter was estimated to be 130.2 and 132.4 A respectively. On the basis of X-ray analysis, it was concluded that nanostructured Pt-films were developed through only one channel of 0224 phase of phytantriol exhibiting another cubic phase with the space group of 0227 (Figure 3).

The area of the nanostructured-Pt films was investigated by cyclic voltammetry in 0.5 M H2S04 vs Ag/AgCI at room temperature between the limits -0.35 and +1.2 V (Figure 5).

The current associated with the hydrogen adsorption! desorption process was integrated for determination of the surface area of the electrode.

Surface morphology of the nanostructured platinum films was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results are shown in Figure 6. The left hand figure is a SEM image of nanostructu red platinum film deposited at platinum electrode. The roughness factor was estimated to be 426 at mVs1 in 0.5 M aqueous H2S04 vs Ag!AgCI. The right figure is a TEM image of nanostructured platinum film deposited at platinum foil. The diameter of the intersecting channels was estimated to be 4.85 nm.

Finding the optimum: Different conditions used! effects on performance: Performance" was characterised by the electrochemically accessibly surface area increase. This is measured by Roughness Factor, defined as the electrochemically accessibly surface area (determined by the current during a redox process) divided by the geometrical surface area of the electrode (measured macroscopically) Metal Substrate: Both platinum and gold substrates gave high surface area platinum nanostructures, with a roughness factor that increased linearly with deposition charge density. Roughness factors were approximately 200 for every 1 C cm2 passed. SAXS confirmed that both had the same expected nanostructure.

Figure 7 shows the linear relationship was observed for the roughness factor verses deposition charge density. The highest roughness factor was estimated to be 1 212.

Soaking time: We found that the electrode coated with phytantriol using the procedure described above should be left to soak in HCPA solution for at least 10 minutes in order for the phytrantiol! HCFA solution to self-assemble into the required bicontinuous cubic structure. Soaking times of under 10 minutes gave slightly reduced roughness factor values (by 10%) on templating. Increasing the soaking times above 10 minutes up to 50 minutes gave the same high surface area with no further enhancement.

Deposition Voltage: Deposition voltages of -0.15, -0.2, -0.25 and -0.3V were tested.

We found that only -0.2V gave the required high surface area. Films deposited at -0.25V showed <30% of the roughness factor obtained at -0.2V; films deposited at -0.15V or - 0.3V gave roughness factor values that were smaller still (<10% of the value obtained at -0.2V).

Claims

CLAIMS1. A process for the preparation of porous film comprising the steps of: i) applying a first composition comprising a first solvent and a structure directing agent to a substrate and optionally removing at least a portion of the first solvent to form a layer comprising structure directing agent on the substrate; ii) providing a second composition comprising a second solvent and a precursor of a material to be deposited; wherein the structure directing agent forms an inverse lyotropic liquid crystalline phase in the presence of said second composition; iii) contacting said layer with said second composition; and iv) electrochemically depositing said material onto said substrate.
2. A process according to claim 1 wherein the structure directing agent is selected from the group of polar non-ionic amphiphiles.
3. A process according to claim 1 or 2 wherein the structure-directing agent is phytantriol.
4. A process according to any preceding claim wherein the inverse lyotropic liquid crystalline phase is selected from 0229 (QD), 0224 (0111D) and 0230 (QO).
5. A process according to any preceding claim wherein more than 95 % of the first solvent is removed.
6. A process according to any preceding claim wherein the substrate is a conductor or semi-conductor.
7. A process according to any preceding claim wherein the material to be deposited is a metal or an oxide thereof.
8. A process according to claim 7 wherein the metal is present as a solution of ions in the said second solvent.
9. A process according to claim 7 or 8 wherein the metal is platinum, preferably in the form of hexachloroplatinic acid.
10. A process according to any preceding claim comprising the further steps of v) providing a third composition comprising a third solvent and a precursor of a second material to be deposited; vi) optionally, washing the said layer; vii) contacting said layer with said third composition; and viii)electrochemically depositing said second material onto said first material.
11. A process according to any preceding claim comprising the further step of washing the porous film to remove structure-directing agent.
12. A porous film obtainable by the process of any one of claims 1 to 11.
13. A chemical or biological sensor comprising a film as claimed in claim 12.
14. A catalyst comprising a film as claimed in claim 12.
15. A fuel cell comprising a film as claimed in claim 12.
16. A capacitor comprising a film as claimed in claim 12.
17. A solar cell comprising a film as claimed in claim 12.
18. A process substantially as described herein with reference to any of the examples and/or figures.
19. A porous film substantially as described herein with reference to any of the examples and/or figures.