US20030167878A1

US20030167878A1 - Titanium-containing materials

Info

Publication number: US20030167878A1
Application number: US10/312,577
Authority: US
Inventors: Najeh Al-Salim; Timothy Kemmitt
Original assignee: Industrial Research Ltd
Current assignee: Industrial Research Ltd
Priority date: 2000-07-17
Filing date: 2001-07-17
Publication date: 2003-09-11
Also published as: AU8271101A; EP1324950A4; CA2415951A1; TW588017B; WO2002006159A1; JP2004507421A; NZ505774A; AU2001282711B2; EP1324950A1

Abstract

The invention relates to a method of preparing a solution containing colloidal particles which contain crystalline titanium dioxide wherein one or more hydrolysable titanium-containing compound(s) is stabilised by oxalic acid in a reaction medium.

The reaction further relates to the preparation of titania materials (including particulate materials, coating solutions and films) which comprise or include anatase phase titania, and so are suitable in photocatalytic applications.

The invention also deals with a method of preparing B-phase titania.

Description

FIELD OF THE INVENTION

This invention relates to processes for preparing titanium-containing materials. More particularly, it relates to processes of preparing solutions containing colloidal particles containing titanium ions, and to processes for preparing titanium dioxide containing materials from such solutions. This invention also relates to application of these titanium-containing materials.

BACKGROUND OF THE INVENTION

Titanium-containing composite materials have a range of applications. These include applications that depend on the ability of these materials to function as photocatalysts, constituents of film electrodes, hydrophilic surfaces, and to prevent the growth of micro-organisms.

Photocatalysts

Photocatalysis using titania and titania-based materials have been widely described in recent years for water treatment and air purification (Hoffmann, Martin, Choi and Bahnemann, Chem. Rev. 1995, 95, 69). Anatase was reported to be the most active phase of TiO ₂with a band gap of 3.2 eV (387 nm). Trace organic contaminants can be photodegraded on the surface of anatase particles under UV light to carbon dioxide, water, and possibly mineral ions.

Different factors have been reported to affect the photoactivity of anatase. High surface area and large pore diameter are extremely important to enhance the adsorption of contaminants and increase the light interaction with the material.

The crystallite size can also determine the extent of photoactivity of TiO ₂. Smaller crystallite size is considered essential to repress charge recombination of the photogenerated electrons and holes and render them more available for redox reactions at the surface of the photocatalyst particles.

Another way of increasing the photoactivity of anatase is by doping with metals or metal oxides. Of the metals, platinum doping has been found to be very effective in improving the photocatalytic process. The extent of doping and the dopant particle size are important factors. Doping a photocatalyst with more than 5% by weight platinum should be avoided as this could produce large platinum particle size and prevent the light from reaching the photocatalyst surface. Deposition of platinum metal on titania has been reported using a number of different methods. One of the methods is by photodeposition of platinum by irradiating hexachloroplatinic acid that is adsorbed on anatase powder under nitrogen first described by Kraeutler and Bard (J. Am. Chem. Soc. 1978, 100, 4317). Photodeposition of platinum was also achieved by illuminating a deaerated suspension of hexachloroplatinic acid and TiO ₂using ethanol as a sacrificial electron donor (Yamaguchi and Sato, J. Chem. Soc. Faraday Trans. 1, 1985, 81, 1237). High temperature reduction (480° C.) of hexachloroplatinic acid adsorbed on TiO₂in hydrogen gas was also reported (Pichat et al Nouv. J. Chim 1981, 5, 627636). Anderson and Xianzhi (Patent number WO9640430, 1996) described a method of reducing hexachloroplatinic acid adsorbed on TiO₂pellets using NaBH₄in dilute NaOH.

Doping of TiO ₂with silver by photodeposition is known (Schwarz et al, Chem. Rev. 1995, 95, 477) where TiO₂was dispersed in a dilute solution of silver nitrate and a sacrificial agent. The suspension was then irradiated with a mercury lamp. However copper 2+ was reported to be reduced to Cu₂O under irradiation of TiO₂(Sakata et al, Chem. Letters, 1998, 1253).

The formation of photocatalyst thin films has been reported to be useful in maintaining clean surfaces. Building surfaces like windows and tiles, automobile screens and mirrors, etc. may attract all sorts of contaminants such as oil deposits, smoke and dust. When a surface is coated with a photocatalyst film, this coating can degrade the deposited contaminants by the aid of natural or artificial UV light. This process of phenomenon is usually referred to as a “self-cleaning surface”.

Hydrophilic Surfaces

Windows, mirrors and other surfaces are fogged when cold due to the condensation of moisture droplets. One of the ways to avoid this phenomenon is to make the surface hydrophilic by applying a transparent hydrophilic coating, such as TiO ₂. Hydrophilicity can be expressed by measuring the contact angle of a water drop on the surface. In order to maintain the hydrophilicity of such a surface, it needs to be periodically exposed to a UV light. Exposure to even a low intensity of UV radiation from sunlight or fluorescent light can maintain the hydrophilicity. In this way the contact angle can be reduced to 2-5°, which is enough to prevent fogging of the surface. A recent Patent by TOTO Ltd (EP 0816466 A1, 1998) described a method of applying TiO₂-containing thin film that is capable of maintaining a contact angle of 3° when subjected to irradiation using a white fluorescent lamp having a UV intensity of 0.004 mW/cm².

Scrubbing Technology

One of the desired applications of photocatalysts is their use to degrade volatile organic compounds (VOCs) under UV irradiation. Many VOCs such as formaldehyde, toluene trichloroethylene (TCE), ethylene, etc. are considered toxic, carcinogenic, irritant or harmful to a different extent. Some of these are considered easy to photodegrade such as formaldehyde, while others, such as toluene and ethylene, are considered difficult to decompose. Photocatalysts can also be used in purification of water to degrade any traces of pollutants such as organic compounds, dyes, etc., and even microorganisms by allowing water to be in contact with the UV irradiated photocatalyst.

Of particular interest is the effect of ethylene gas on stored fruit, vegetables and cut flowers. These produce need to be consumed, exported or delivered to supermarkets and food stores while still being fresh. Ageing of produce can be caused by exposure to ethylene gas. This is known to be released by produce, such as apples and bananas and will affect other things that are stored in the same place. Ethylene is a plant hormone that causes ripening and ageing of fruit, vegetable and flowers even in low concentrations. The ripening speed may vary from one type of produce to another depending on the concentration of ethylene. For example, tomatoes ripen within hours if exposed to ethylene in a concentration of more than 100 ppm. Kiwi fruit may ripen even at lower concentration of few parts per million of ethylene. This can create problems in storing or transporting different produce in the same cold store or container.

Nelson et al (U.S. Pat. No. 6,240,767, 2001) have described a system for accurately monitoring part per million level of ethylene gas in atmosphere and fruit containers. Having an efficient way to control and degrade high level of ethylene is as important as monitoring the ethylene concentration during storage or transport.

Patterned Films

The development of new methods for depositing patterned ceramic films onto surfaces is especially important in microcircuit fabrication such as field-effect transistors that need to be down-sized to a submicrometer thickness. Titanium dioxide has a favourable dielectric and refractory properties to be utilised as part of a micropatterned film. Koumoto, Sugiyama and Seo (Chem. Mater. 1999, 11, 2305) have described a low temperature patterning process for TiO ₂deposition that utilises phenyltrichlorosilane as a patterning template which was irradiated with a Hg lamp through a photomask before deposition of TiO₂from acidic (NH₄)₂TiF₆solution. These patterns exhibit significant line edge roughness of ˜7.3 micrometer that corresponds to 28% variation according to the authors, which exceeds the usual 5% variation allowed by current electronics design rules. Kikuta, Tkagi and Hirano (J. Am. Ceram. Soc. 1999, 82, 1569) have shown that finer patterns of TiO₂can be produced from solutions of titanium alkoxide modified with alkanolamines by photolithography. The precursors were coated on a substrate and decomposed by ultra-high-pressure mercury lamp. The films were then heat-treated to convert it to anatase phase.

It is an object of the present invention to provide an alternative process for preparing titanium-containing materials, and/or to provide an alternative means of employing titanium containing compounds, and/or to at least provide the public with a useful alternative.

STATEMENTS OF THE INVENTION

In a first aspect of the invention there is provided a method of preparing a solution containing colloidal particles which contain titanium ions comprising or including the step of:

A. reacting or otherwise stablilising one or more hydrolysable titanium-containing compound(s) with oxalic acid in a reaction medium under conditions such that a colloidal solution is obtained.

Preferably A occurs under conditions such that peptization of the colloidal solution is substantially obtained and substantially maintained.

Preferably the conditions include stirring or agitation of the one or more hydrolysable titanium-containing compound(s) with oxalic acid in the reaction medium, more preferably at a temperature between ambient temperature to near the boiling point of the reaction mixture, even more preferably at a temperature between about 40° C. and about 80° C. Preferably the stirring or agitation occurs over a reaction time ranging from substantially 15 minutes up to substantially 3 hours.

Preferably the reaction medium comprises water or a water/alcohol mixture and wherein the titanium-containing compound is hydrolysable in water and/or in base.

Preferably the titanium containing compound is water-hydrolysable and the titanium-containing compound is of the formula Ti(OR) ₄, where R is a C₂-C₆linear or branched chain alkyl group; more preferably the titanium containing compound is titanium tetraisopropoxide and/or titanium tetrabutoxide.

Preferably the water-hydrolysable titanium containing compound is:

first combined with a solution of oxalic acid in alcohol, followed by addition of water, or

added directly to water, or to a mixture of water and an alcohol, to form a slurry, followed by addition of oxalic acid, or

added to a solution of oxalic acid in water or in a mixture of water and alcohol.

Preferably the water-hydrolysable titanium-containing compound is hydrolysed using water prior to reaction with or stabilisation by oxalic acid, to give a hydrolysis product.

Alternatively or additionally the titanium-containing compound is base-hydrolysable and the titanium-containing compound is selected from, but not restricted to, TiCl ₄and/or TiOSO₄.

Preferably the base-hydrolysable titanium-containing compound is hydrolysed to a hydrolysis product, using a base prior to reaction with or stabilisation by oxalic acid, the hydrolysis product being filtered and/or washed, to form a slurry before reaction with or stabilisation by, the oxalic acid.

Preferably the oxalic acid is either anhydrous oxalic acid, or hydrated oxalic acid, and preferably the amount of oxalic acid is such as to provide a mole ratio of oxalic acid:titanium in the range of about 0.2:1 to about 1:1.

Preferably the water content of the reaction medium is such as to provide a mole ratio of water:titanium in the range of from about 200:1 to about 800:1; more preferably in the range of from about 400:1 to about 600:1.

Preferably, when alcohol is present in the reaction medium, the alcohol is a mono hydroxyl aliphatic alcohol having the formula ROH, where R is a C ₁to C₄linear or branched alkyl group, such as ethanol or t-butanol, and the preferred amount of alcohol present is such as to provide a mole ratio of alcohol:titanium of from zero to 100:1, more preferably 10:1 to 50:1.

Preferably the solution may be stored at any concentration level prior to further use, preferably at up to about 32% by weight TiO ₂, between 0° C. and 20° C.

Preferably the oxalate concentration of the solution is at any stage reduced by irradiating the solution with UV light.

According to a second aspect of the invention there is provided a solution containing colloidal particles which contain titanium ions prepared substantially according to the preceding method.

In a third aspect of the invention there is provided a method of preparing a solution containing colloidal particles which contain crystalline titanium dioxide comprising or including the step of:

A. reacting or otherwise stabilising one or more hydrolysable titanium-containing compound(s) with oxalic acid in a reaction medium under conditions such that a colloidal solution is obtained.

Preferably the water-hydrolysable titanium containing compound is:

According to a fourth aspect of the invention there is provided a solution containing colloidal particles which contain crystalline titanium dioxide prepared substantially as herein described with reference to any one of more of the accompanying examples.

According to a fifth aspect of the invention there is provided a method of preparing a TiO ₂-Containing Product comprising or including the steps of:

I. preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid, or stabilised by reaction with oxalic acid, and

II. further processing of the solution to obtain the product.

Preferably I occurs under conditions such that peptization of the colloidal solution is substantially obtained and substantially maintained.

Preferably the conditions include stirring or agitation of one or more hydrolysable titanium-containing compound(s) with oxalic acid in a water or water/alcohol reaction medium, more preferably at a temperature between ambient temperature to near the boiling point of the reaction mixture, even more preferably at a temperature between about 40° C. and about 80° C. Preferably the stirring or agitation occurs over a reaction time ranging from substantially 15 minutes up to substantially 3 hours.

Preferably the TiO ₂phase in the product, at least initially, includes, is predominantly or is substantially anatase.

Preferably the method comprises or includes the steps of:

1) preparation of a solution containing colloidal particles which contain titanium ions wherein the particles are stabilised by oxalic acid or stabilised by having been reacted with oxalic acid, and

2) preparation of a colloidal mixture by addition of, or mixing with, one or more additives to the solution, and

3) further processing of the solution to obtain the product.

Preferably step 1) comprises or includes the method as previously described in one or more of the first-fourth aspects of the invention.

Preferably the additives of step 2) include one or more of:

a) silica or a silica precursor, (preferably when added it is as colloidal silica, and preferably added in an amount to yield a ratio substantially from 1 to 99 wt % relative to titanium in the product, more preferably from 30-60 wt %, and preferably the concentration of the colloidal silica is such as to provide between about 1 and 50% by weight in the product),

b) water, or alcohol, soluble ketone(s)

c) organic acid(s),

d) water soluble aliphatic or aromatic alcohol(s), diol(s) or polyol(s),

e) elhanolamine(s),

f) metal precursor(s),

g) surfactant(s) (and preferably when added the surfactant(s) is or includes one or more of the Brij series, Triton series, Tergitol series, Pluronic series, potassium dodecyl sulphate, or any other surfactant that does not cause gelling),

h) silane(s) (and preferably when added, it is is added neat or as solution in an aqueous or organic solvent that is miscible with water, preferably it is a hydrolysable or partially hydrolysable silane compound of a formula RSiX ₃, R₂SiX₂and SiX₄(where R is a simple or functionalised organic group and X could be a halide or an alkoxide group)).

Preferably if added, the metal precursor is a metal salt or metal complex, more preferably a soluble metal salt or complex, preferably of Pd, Pt, Ag and Cu.

When the metal is Pd or Pt preferably the precursor is (but not restricted to) one of the hexachloro-complexes of Pd or Pt, and preferably the Pd or Pt hexachloro-complex is mixed with a low carbon organic compound (such as formaldehyde, formic acid, methanol or ethanol) as a sacrificial compound, which is preferably added in excess relative to the precursor metal, more preferably at a etal:sacrificial compound mole ratio of approximately.1:5.

Alternatively or additionally, the metal is Ag and the precursor is one or more of (but not restricted to) silver acetate or silver nitrate.

Alternatively or additionally the metal is Cu and the precursor is one or more of (but not restricted to) copper acetate, copper sulphate and copper nitrate.

Preferably step 3) includes removal of the solvent, more preferably by one or both of the steps of:

i) causing the solution to gel (a gelling step),

ii) curing of the gel (a curing step) to remove or reduce the quantity of the oxalic acid and/or any one or more additives.

Preferably the gelling step is effected by one or more of:

evaporating the solvent; at room temperature or above; and/or

evaporating the solvent under a vacuum, with or without heating; and/or

addition of a gelling agent including (but not restricted to) a dilute mineral acid solution such as of HCl, HNO ₃or H₂SO₄, or an alkaline solution such as KOH, ammonia, sodium carbonate or tetraalkylammonium hydroxide.

Preferably a xerogel may be produced from the gelling step.

Preferably the curing of the gel is effected by exposure to UV radiation and/or by heat.

Preferably the wavelength of the UV radiation substantially or partially coincides with the band gap of the TiO ₂in the anatase phase.

Preferably the curing time is determined by the amount of oxalic acid to be decomposed and/or the wavelength of the radiation and/or the intensity of the radiation.

Preferably there is an additional step 4), which includes one or both the steps of:

i) impregnation of the titanium containing-product with a metal precursor, (an impregnation step) and/or

ii) transformation to a metal or metal oxide of any metal precursor added within step 2) and/or step 4) (a transformation step).

Preferably the metal of the precursor of i) may be one or more of Pd, Pt, Cu or Ag.

Preferably when the metal precursor includes Pd or Pt the metal precursor may be mixed with a sacrificial compound of formaldehyde, formic acid, methanol or ethanol, in excess relative to the metal recursor.

Preferably the transformation step occurs by one or more of:

i) heating at a suitable temperature to transform the metal precursor to the metal or metal oxide, and/or

ii) exposing the metal precursor or the TiO ₂-Containing Product containing the metal precursor, to a dilute hydrazine hydrate solution for a sufficient time to allow the complete reduction of the metal to zero valency, and/or

iii) UV irradiation to form metal particles and/or metal oxides within the titanium dioxide.

Preferably when:

the metal of the metal precursor is Ag, UV irradiation is employed and UV irradiation is stopped substantially when the colour of the TiO ₂-product changes to light grey-black, and/or

the metal of the metal precursor is Cu, UV irradiation is employed and UV irradiation is stopped substantially when the colour of the TiO ₂-Containing Product changes from light green to bronze, and/or

the metal of the metal precursor is Pd, UV irradiation is employed and UV irradiation is stopped substantially when the colour of the TiO ₂-Containing Product changes from grey to black and/or

the metal of the metal precursor is Pt, UV irradiation is employed and UV irradiation is stopped substantially when the colour of the TiO ₂-Containing Product changes from grey to black.

Preferably when hydrazine hydrate solution exposure is employed in the transformation step the TiO ₂-product is then washed with water to remove the excess hydrazine.

Preferably the final metal content in the TiO ₂of the TiO₂-product is less than 2% by weight, more preferably it is between 0.2 to 0.5% by weight.

Preferably sometime prior to step 3) the oxalate concentration of the solution is reduced by irradiating the solution with UV light.

Preferably the product is particulate in nature; more preferably the product is a powder; alternatively it is granular.

According to a sixth aspect of the invention there is provided a method of preparing a TiO ₂coating solution comprising or including the steps of:

I. preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid, or are stabilised by having been reacted with oxalic acid, and

II. further processing of the solution to obtain the coating solution.

Preferably I. occurs under conditions such that peptization of the colloidal solution is substantially obtained and substantially maintained.

Preferably the method comprises or includes the steps of:

1) preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid, or are stabilised by having been reacted with oxalic acid,

2) preparation of a colloidal mixture by addition of one or more additives to the solution.

Preferably step 1) includes or comprises the method as claimed in any one of the first to fourth aspects of the invention.

Preferably step 2) includes any one or more of the following:

i) Addition of or mixing with silica, or a silica precursor (preferably, when added or mixed, it is colloidal silica, and it is added in an amount to give a ratio substantially between 1 and 99 weight percent relative to titanium, more preferably from 30 to 60 wt % relative to titanium; and preferably the concentration of the colloidal silica is between about 1 and 50% by weight),

ii) Addition of or mixing with any proportion of water-soluble or alcohol-soluble ketone(s) (preferably when added or mixed, it is acetone and/or acetylacetone),

iii) Addition of or mixing with any proportion of organic acid(s) (preferably when added or mixed, and it is mono-, di- or multi-functional, with or without hydroxyl groups, more preferably it/they may be one or more of acetic, lactic, tartaric, citric, maleic, malic, malonic, diglycolic, benzoic, 1,2,4,5-C ₆H₂(COOH)₄, EDTA, and/or mixtures thereof),

iv) Addition of or mixing with any proportion of water-soluble aliphatic or aromatic alcohol(s), diol(s) or polyol(s) (preferably when added it/they may be one or more of ethanol, propanol, ethylene glycol, glycerol, polyethylene glycol, polyvinylalcohol, phenol, catechol, polysaccharides),

v) Addition of or mixing with any proportion of ethanolamine(s), (such as monoethanolamine, diethanolamine and triethanolamine, or a mixture thereof),

vi) Addition of or mixing with any proportion of surfactant(s) (preferably when added or mixed, it/they may be selected from one or more of the Brij series, Triton series, Tergitol series, Pluronic series, potassium dodecyl sulphate, or any other surfactant that does not cause gelling, and preferably at a concentration is between 0.01 to 5% by weight relative to TiO ₂),

vii) Addition of or mixing with one or more metal precursor(s),

viii) Addition of or mixing with one or more silane(s) (preferably when added or mixed, and it is a hydrolysable or partially hydrolysable silane compound(s) of a formula RSiX ₃, R₂SiX₂and SiX₄(where R is a simple or functionalised organic group and X could be a halide or an alkoxide group) and preferably added neat or as a water-miscible solution, preferably such that n the silane concentration is between 1-50%, more preferably 10-35% by total weight.).

Preferably when one or more metal precursor(s) is added or mixed it is a soluble metal salt or complex one or more of Pd, Pt, Ag and Cu.

Preferably the metal is one or more of:

Ag and the precursor is one or more of (but not restricted to) silver acetate or silver nitrate, and/or

Cu and the precursor is one or more of (but not restricted to) copper acetate, copper sulphate and copper nitrate, and/or

Pd or Pt and precursor is (but not restricted to) one of the hexachloro-complexes of Pd or Pt.

Preferably when the metal is Pd or Pt, the Pd or Pt hexachloro-complex is mixed with a low carbon organic compound as a sacrificial compound, of formaldehyde, formic acid, methanol or ethanol, which added in excess relative to the metal precursor.

Preferably here is the further step 3) of storing the coating solution at any concentration, preferably between 0-20° C., more preferably between 4-15° C.

According to an seventh aspect of the invention there is provided a method of preparing a TiO ₂-coated substrate comprising or including the steps of:

I. preparation of a coating solution substantially as previously described, and

II. further processing of the solution to obtain the coated substrate.

Preferably the TiO ₂phase in the coated substrate, at least initially, includes, is predominantly or is substantially anatase.

Preferably the substrate is one or more of (but not restricted to) glass, quartz, glass fibre, woven glass fibre, ceramics, silicon wafers, metals, polymer surfaces (such as polyethylene or polyester), wood, or building materials such as mortar, brick, tiles, or concrete.

Preferably step II includes

i) application of the coating solution to a substrate, and

ii) a gelling step, and

iii) a curing step.

Preferably application of the coating solution is effected by techniques such as (but not restricted to) spin-coating, dip-coating or spraying.

Preferably prior to application of the coating solution a protective layer of amorphous silica and/or alumina (and/or precursors thereof) is applied to the substrate.

Preferably the precursor(s) for amorphous silica may be selected from (but not limited to) the series tetraalkoxysilanes, alkoxychlorosilanes and the precursors for amorphous alumina may be selected from (but not limited to) the series aluminium trialkoxides, and the precursors are prepared by hydrolysing the silica and/or the alumina precursor(s) in acid solution.

Preferably curing of the gel is effected by exposure to UV radiation and/or by heat.

Preferably the wavelength of the UV radiation substantially or partially coincides with the band gap of anatase TiO ₂.

Preferably the curing time is determined by the amount of oxalic acid to be decomposed and/or the amount of silane (if present) and/or the amount of surfactant (if present) and/or the wavelength of the radiation and/or the intensity of the radiation.

Preferably the gelling step is effected by one or more of:

evaporating the solvent; at room temperature or above;

evaporating the solvent under a vacuum, with or without heating;

addition of a gelling agent including (but not restricted to) a dilute mineral acid solution such as HCl, HNO ₃or H₂SO₄, or an alkaline solution such as KOH, ammonia, sodium carbonate or tetraalkylammonium hydroxide.

Preferably there is a further step III which includes one or both the steps of:

ii) transformation of any metal precursor added within step I) and/or step III) (a transformation step).

Preferably the metal of the precursors of step i) is one or more of Pd, Pt, Cu or Ag, and preferably when the metal precursor is Pd or Pt the metal precursors is mixed with a sacrificial compound of formaldehyde, formic acid, methanol or ethanol, in excess relative to the metal precursor.

Preferably curing of the gel is effected by exposure to UV radiation and/or by heat and/or by evaporation of the solvent, and preferably a xerogel is produced either as an intermediate, or as a product. Preferably, when UV radiation is employed the wavelength of the UV radiation substantially or partially coincides with the photocatalytically active band gap of the TiO ₂in the anatase phase.

According to an eighth aspect of the invention there is provided a method of preparing a hardened TiO ₂-coated substrate comprising or including the steps of:

i) preparation of a coating solution substantially as described previously and wherein at least both an acid and an alcohol are added to the solution in a certain quantity,

ii) application of the coating solution to a substrate,

iii) heating to around 150° C.,

iv) further heating to around 450° C. to decompose the organic materials and/or to effect sintering of the coating.

According to a ninth aspect of the invention there is provided a method of preparing a patterned TiO ₂-coated substrate comprising or including the steps of:

i) preparation of a coated substrate substantially as described previously prior to any gelling or curing steps (if any),

ii) masking one or more regions of the coating,

iii) curing of the unmasked region(s) of coating by exposing the unmarked region(s) to an ultraviolet light to photocatalytically destroy the oxalic acid and other organic materials present in the titanium oxide of the unmasked region(s),

iv) development of the film.

Preferably the coating solution produces a film which contains 50 to 100% by weight titania after curing.

Preferably only one layer of coating solution is applied to obtain sharper and clearer patterns.

Preferably development of the film is by one or more of:

i) application of an acid solution wherein the acid solution is any dilute mineral acid such as sulphuric and/or an organic acid solution such as oxalic, lactic, citric, tartaric or sulphuric acid and/or an acidic salt solution where the salt is, ammonium sulphate or aluminium sulphate and/or;

ii) application of other materials such as hydrogen peroxide and/or;

iii) a radicational or mechanical method including ultrasonication, and/or

iv) any other method for redissolution of the UV—unexposed gel.

Preferably the development occurs at room temperature, or under conditions of heating, and preferably is followed by a final step of sintering the coating.

Preferably there may be one or more additional prior step to sintering including one or both of the steps of

evaporating the solvent; at room temperature or above and/or

evaporating the solvent under a vacuum, with or without heating.

Preferably curing of the gel is effected by exposure to UV radiation and/or by heat, and preferably a xerogel may be produced as an intermediate.

Preferably when UV radiation is employed the wavelength of the UV radiation substantially or partially coincides with the photo catalytically active band gap of the TiO ₂in the anatase phase.

According to a tenth aspect of the invention there is provided a method of increasing the content of rutile and/or TiO ₂—B phases in a TiO₂product including or comprising:

preparing a TiO ₂-containing product as substantially as previously described wherein the TiO₂phase is predominantly or at least partially anatase or is predominantly or at least partially TiO₂—B phase, and

heating to increase the TiO ₂—B and/or rutile content.

Preferably heating to substantially between 200° C. to 400° causes or initiates phase change of TiO ₂—B to anatase phase to titanium dioxide-B and/or rutile phase in the product. Additionally further heating to substantially above 400° C. will increase the content of the rutile phase in the product.

Preferably the TiO ₂undergoes a phase change substantially entirely to rutile at temperatures substantially higher than 500° C.

Alternatively addition of substantially up to 50% by weight silica results in stabilisation of the anatase and/or TiO ₂—B phase thereby requiring heating to over 600° C. to initiate and/or complete the transformation to rutile phase.

According to an eleventh aspect of the invention there is provided a TiO ₂-containing product substantially prepared according to the method previously described.

According to a twelfth aspect of the invention there is provided a TiO ₂-containing product prepared substantially as herein described with reference to any one or more of the accompanying examples.

According to a thirteenth aspect of the invention there is provided a TiO ₂-containing coating solution substantially prepared according to the method as previously described.

According to a fourteenth aspect of the invention there is provided a TiO ₂-containing coating solution prepared substantially as herein described with reference to any one or more of the accompanying examples.

According to a fifteenth aspect of the invention there is provided a TiO ₂-containing coated substrate substantially prepared according to the method as previously described.

According to a sixteenth aspect of the invention there is provided a TiO ₂-containing coated substrate prepared substantially as herein described with reference to any one or more of the accompanying examples.

According to a seventeenth aspect of the invention there is provided a TiO ₂-containing hardened film substantially prepared according to the method as previously described.

According to an eighteenth aspect of the invention there is provided a TiO ₂-containing hardened film prepared substantially as herein described with reference to any one or more of the accompanying examples.

According to a ninetheenth aspect of the invention there is provided a TiO ₂-containing patterned film substantially prepared according to the method as previously described.

According to a twentieth aspect of the invention there is provided a TiO ₂-containing patterned film prepared substantially as herein described with reference to any one or more of the accompanying examples.

According to a twenty first aspect of the invention there is provided a method of preparing a TiO ₂-based photocatalyst including or comprising the following steps:

1) preparation of a solution containing colloidal particles which contain titanium ions wherein the particles are stabilised by oxalic acid, or stabilised by reaction with oxalic acid,

2) further processing of the solution to obtain the photocatalyst.

Preferably the TiO ₂-based photocatalyst is a TiO₂particulate material, and the further processing includes a gelling and a curing step.

Preferably the TiO ₂-based photocatalyst is a TiO₂coating or film on a substrate, and the further processing includes preparation of a coating solution and application of the coating solution to the substrate, and a gelling and a curing step.

Preferably the TiO ₂phase in the particulate material, coating or film, at least initially, includes, is predominantly or is substantially anatase.

Preferably the TiO ₂-based photocatalyst acts as a photocatalyst upon irradiation of or exposure to UV light.

Preferably the TiO ₂-based photocatalyst is metal or metal-oxide doped, preferably the metal is selected from Pt, Pd, Cu or Ag.

Preferably the TiO ₂-based photocatalyst can be used to photocatalytically degrade organic compounds and wherein the degradation occurs via application of or exposure to UV radiation, and/or the TiO₂-based photocatalyst can act as a hydrophilic surface when coated on a substrate.

According to a twenty second aspect of the invention there is provided a TiO ₂-based photocatalyst prepared substantially according to the method as previously described.

According to a twenty third aspect of the invention there is provided a TiO ₂-based photocatalyst prepared substantially as herein described with reference to any one or more of the examples.

According to a twenty fourth aspect of the invention there is provided a method of preparing B phase TiO ₂, including or comprising the following steps:

1) preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid, or stabilised by reaction with oxalic acid, and

2) further processing of the solution to obtain TiO ₂predominantly or substantially in the TiO₂—B phase, and

3) heating of the TiO ₂to substantially between 200-300° C.

Preferably the further processing step 2) includes removal of the solvent and/or a gelling step and/or a curing step.

Preferably step 1) is substantially according to one or more of the methods described previously in the first-fourth aspects of the invention.

Preferably there is a further step 4) of heating beyond 450° C. provide TiO ₂in the rutile phase.

According to a twenty fifth aspect of the invention there is provided B phase TiO ₂, prepared substantially according to the method previously described.

According to a twenty sixth aspect of the invention there is provided B phase TiO ₂, prepared substantially as herein described with reference to any one or more of the examples.

Although the present invention is broadly as defined above, it is not limited thereto and also includes embodiments of which the following description provides examples.

BRIEF DESCRIPTION OF THE DRAWINGS

In particular, a better understanding of the invention will be gained with reference to the accompanying drawings and figures in which: [0220]
FIG. 1 shows a thermal analysis (TGA and DTA) of the powder prepared from gelling the sol in Example 5; [0221]
FIG. 2 shows X-ray diffraction (XRD) patterns of the powder prepared from gelling the sol in Example 9 with no silica added; [0222]
FIG. 3 shows the infrared spectra of the film prepared as in Example 10 and cured by UV light; [0223]
FIG. 4 shows the effect of added SiO[0224] ₂on the photoactivity and surface area using a 2M sol of the invention;
FIG. 5 shows the change in photoactivity with a catalyst loading using 2M sol of the invention +50% SiO[0225] ₂coated on woven glass fibre;
FIG. 6 shows the effect of humidity on the photoactivity using two coatings of 2M sol of the invention +50% SiO[0226] ₂coated on woven glass fibre;
FIG. 7 shows the UV degradation of some surfactants on TiO2 films followed by I.R spectroscopy; [0227]
FIG. 8 shows the x-ray diffraction spectrum of TiO2-B containing material; [0228]
FIG. 9 shows the surface profile of a TiO[0229] ₂patterned film;
FIG. 10 shows the scanning electron micrograph (SEM) of a TiO[0230] ₂patterned film;
FIG. 11 shows the photodecomposition of Rhodamine B dye on TiO[0231] ₂film;
FIG. 12 shows the photodecomposition of ethylene gas on platinised TiO[0232] ₂photocatalyst cloth;
FIG. 13 shows the photothermal decomposition of ethylene gas on a platinised TiO[0233] ₂photocatalyst cloth; and
FIG. 14 shows the photothermal decomposition of toluene gas on a platinised TiO[0234] ₂photocatalyst cloth.

DESCRIPTION OF THE INVENTION

As defined above, the invention relates to processes of preparing solutions containing colloidal particles which contain titanium (ie titanium-containing sols), and to processes for preparing titanium dioxide containing materials. In particular xerogels (a gel in which the solvent has been removed by evaporation at an ambient temperature), powders and coated films can be prepared from the solutions. Further, the invention also relates to the application of the titanium dioxide containing materials. [0235]
A. Preparation of Titanium-Containing Materials [0236]
A1—General [0237]
The applicants have found that titanium-containing colloidal solutions stabilised by oxalic acid can be prepared by stablisation or peptization. Peptization is the process by which colloidal sols are stabilised usually by addition of electrolytes. In the case of TiO[0238] ₂a pH of approximately <4 is required, thus acidic conditions are generally employed. These colloidal solutions have certain advantageous properties that render them highly suitable for use in preparing titanium dioxide-containing materials.
In particular, the applicants have found that titanium dioxide-containing materials prepared from such colloidal solutions do not require firing at high temperatures in order to have properties that make them suitable for use in such application as photocatalysts. This is because titanium dioxide prepared by removal of the solvent from such colloidal solutions has been found to contain or to comprise titanium dioxide directly in the anatase crystalline form. This form is primarily required for titanium dioxide to have photocatalytic properties in the UV range, or is at least the most convenient photocatalytic form of titanium dioxide. [0239]
Typically, conversion of titanium dioxide from the amorphous form, which is the typical phase produced initially in other processes, to the anatase crystalline form has generally required heating to temperatures of at least 300° C. to 400° C. In contrast, titanium dioxide materials prepared from the colloidal solutions of the present invention can be cured simply by exposure to solar light or an ultraviolet light source. Importantly, irradiation of the materials is effective to remove the oxalic acid present, as the titanium dioxide, which is already in the anatase form, photocatalyses decomposition of the oxalic acid. [0240]
The ability of the titanium dioxide materials to be cured without the need for high temperatures enables the preparation of composite materials comprising a substrate coated with a titanium dioxide film in which the substrate used does not need to be heat resistant. Thus, composite materials in which the substrate is, for example, a thermoplastic material or wood, can be prepared. [0241]
Alternatively, the titanium-dioxide containing materials of the invention can be cured by heating to appropriate temperatures. Generally, a temperature of as low as about 200° C. will be sufficient to decompose the oxalic acid present in the material and yield a product in which the titanium dioxide is in the anatase form. Further, anatase, TiO[0242] ₂—B, rutile phase or a mixture can be obtained, depending on the amount of oxalic acid used in the preparation and on the firing temperature. Thus, if crystalline TiO₂as the anatase phase or a mixture of anatase and titanium dioxide B is desired, the titanium dioxide material would generally be cured at a temperature of from about 200° C. to 400° C. Above 400° C. a mixture of anatase and rutile will generally be obtained, which will transform completely to rutile at high temperatures, as is known in the art.
The production of the titanium dioxide materials is carried out in two stages—firstly preparation of titanium containing colloidal solutions or sols, and then the preparation of the titanium-containing materials. [0243]
A2. Production of Titanium Sols [0244]
The solutions containing colloidal particles containing crystalline titanium dioxide of the present invention (the titanium sols) may be prepared by reacting a mixture containing a hydrolysable titanium-containing compound and oxalic acid, in a reaction medium which comprises either water or a mixture of water and an alcohol. [0245]
The titanium-containing compound used may be a titanium-containing compound that is hydrolysable in water or base. Also, the use of mixtures of two or more hydrolysable titanium-containing compounds is within the scope of the present invention. [0246]
It is particularly preferred that the water hydrolysable titanium-containing compound is a compound of the formula Ti(OR)[0247] ₄, where R is a C₂-C₆linear or branched chain alkyl group. Two preferred titanium-containing compounds are titanium tetraisopropoxide and titanium tetrabutoxide, which are hydrolysable in water.
Alternative titanium-containing compounds that may be used include TiCl[0248] ₄and TiOSO₄, which can be hydrolysed using a base prior to reaction with oxalic acid. In this case the hydrolysed products (hydrated titania or titanic acid) would preferably be filtered and washed, preferably with deionised water, before reaction with the oxalic acid. The hydrolysed product is more preferably used as slurry without drying.
The hydrolysable titanium containing compound may, for example in the case where the compound is of the formula Ti(OR)[0249] ₄, be first combined with a solution of oxalic acid in alcohol followed by addition of water. Alternatively, the titanium-containing compound may just be added directly to water (or to a mixture of water and an alcohol), then oxalic acid is added to the so formed slurry. Otherwise the titanium-containing compound may be added to a solution of oxalic acid in water or in water/alcohol solution.
The oxalic acid may be either anhydrous oxalic acid, or hydrated oxalic acid, such as H[0250] ₂C₂O₄. 2H₂O. It is preferred that the amount of oxalic acid is such as to provide a mole ratio of oxalic acid:titanium in the range of about 0.2:1 to about 1:1 to get the sols. Below 0.2 ratio either very white colloids or unstable colloids are obtained.
As indicated above, the reaction medium can comprise either water or a mixture of water and an alcohol. It is preferred that the water content is such as to provide a mole ratio of water:titanium in the range of from about 200:1 to about 800:1, more preferably from about 400:1 to about 600:1. Below 200:1 ratio peptization becomes difficult and may produce unstable colloids. The preferred amount of alcohol present is such as to provide a mole ratio of alcohol:titanium of from zero to 100:1, more preferably 10:1 to 50:1. [0251]
It is preferred that the alcohol, when present, is a mono hydroxyl aliphatic alcohol having the formula ROH, where R is a C[0252] ₁to C₄linear or branched alkyl group, such as ethanol or t-butanol.
In order to form a sol, ie a solution containing colloidal particles containing titanium ions, the reaction mixture is preferably stirred or agitated, at a temperature between ambient temperature to near the boiling point of the mixture, more preferably at a temperature between about 40° C. and about 80° C. [0253]
The sol can be stored at any desired concentration level, preferably up to about 32% by weight TiO[0254] ₂. It is however preferred that if a concentrated sol is prepared, that this be then diluted with water to give a concentration of about 2-20% by weight as TiO₂for preparation of thin films.
The reaction time required to form a colloidal solution will depend on the composition and concentration of the reaction mixture. However, in general, the required reaction time will range from about 15 minutes up to about 3 hours. [0255]
The sols thus prepared will contain colloidal particles of a submicrometer to a few nanometer sizes, or less and containing titanium ions, with the particles being stabilised by oxalic acid. [0256]
Without wishing to be bound by any theory, it is thought that the structure of the colloidal particles is likely to be nTiO[0257] ₂.H₂C₂O₄or similar, where n is a number greater than or equal to 1, ie TiO₂particles stabilised by oxalic acid or any of its dissociated forms, and that the oxalic acid prevents the titanium dioxide precipitating out of the solution.
A3—Preparation of the Titanium Dioxide Solid Materials [0258]
Once prepared, the sols may be irradiated by UV light to reduce the concentration of oxalic acid in the sol, especially when the oxalate concentration is about 0.5 mole ratio or higher relative to Ti. The UV light may conveniently be provided by a mercury lamp or a xenon lamp or any other intense UV source with a wavelength less than 400 nm. [0259]
The titanium-containing colloidal solutions may, by removal of the solvent, be used to prepare titanium dioxide or titanium dioxide-containing materials, including titanium dioxide powders and composite materials comprising a substrate coated with a film of titanium dioxide. [0260]
In one embodiment of this invention, if a titanium dioxide powder or film of higher surface area is to be prepared, it is usually preferred that the sol is first mixed with colloidal silica, in an amount of between about 1 and 99 weight percent relative to titanium, preferably from 30 to 60 wt % relative to titanium. The concentration of colloidal silica used can have any desired concentration, but is preferably between about 1 and 50% by weight. [0261]
A further option within the invention is to use a titania sol prepared according to the above, and further mix it with a hydrolysable or partially hydrolysable silane compound of a formula RSiX[0262] ₃and SiX₄(where R is a simple or functionalised organic group and X could be a halide or an alkoxide groups that may exist together) before causing the sol to gel. Such a silane can be added as neat or as solution in an aqueous or organic solvent that is miscible with water, such as ethanol, acetone, etc. Such a silane material can form a linkage between titania particles through —Ti—O—Si—O—Ti— bonding. This can suppress crystal growth of the titania particles as well as add better abrasion properties to the photocatalyst.
To prepare a xerogel or a powder, the sol is then preferably caused to gel. This may be achieved by evaporating the solvent at room temperature or above, or under a vacuum with or without heating. Alternatively, the sols can be caused to gel by adding a dilute mineral acid solution, such as HCl, HNO[0263] ₃or H₂SO₄, or an alkaline solution, such as KOH, ammonia, sodium carbonate or tetraalkylammonium hydroxide. These gels can if desired be redissolved in acidic solutions, such as sulphuric, nitric, oxalic, citric, lactic and tartaric acids, etc. at room temperature or by heating, depending on the type and concentration of the acid. The process of redissolving the gelled material is particularly important in film patterning, as it will be described later.
The gels can be cured to remove the oxalic acid and form titanium dioxide containing materials or films, by exposure to solar light or a UV light source, or by heating at appropriate temperatures. If a UV source is used, this can be a mercury lamp, a xenon lamp, a black light lamp, or other UV source. The wavelength of the light can be below 400 nm to coincide with the band gap of the titania photocatalyst. The required curing time will depend on the film thickness, the amount of oxalic acid to be decomposed, the wavelength of the radiation and its intensity. The UV cured material will contain TiO[0264] ₂in the anatase form. The curing process of a film prepared as in Example 10 below was monitored by infrared spectroscopy (FIG. 3).
Alternatively, the gels can be cured by heating at appropriate temperatures as will be known in the art, to form anatase, TiO[0265] ₂—B or rutile. At temperatures as low as 200° C. to 400° C., crystalline TiO₂, as the anatase phase or a mixture of anatase and titanium dioxide-B, is obtained. Above 400° C. a mixture of anatase and rutile will be obtained, which will transform completely to rutile at high temperature. This was monitored using thermal analysis (TGA, DTA) (FIG. 1) and powder X-ray diffraction (XRD) (FIG. 2). In this case the crystallite size gradually increased from 37 Å in the xerogel to 58 Å after heating at 300° C. for 1 hour, then rapidly increased above 100 Å accompanied with the formation of rutile. When silica (Nalcogel colloidal silica) was present with the titanium dioxide, the anatase form could be stabilised at higher temperature with a small change in the anatase crystallite size. As one can see from the examples given below, if the material contains 50% by weight silica the anatase phase can be stabilised up to 600° C. and the anatase crystallite size will change from 37 Å in the xerogel to only 56 Å after heating at 600° C. for 1 hour. Therefore inclusion of silica into the photocatalyst can stabilise the anatase phase and increase the surface area of the material, thus improves its photocatalytic activity (FIG. 4). The highest surface area and photoactivity obtained was with the 50% by weight SiO2 for which the specific surface area was 246.3 m²/g according to BET method for nitrogen sorption. The pore size distribution according to BJH analysis of the nitrogen desorption isotherm showed a maximum at 35 Å pore diameter and cumulative pore volume of pores between 17 and 100 Å of 0.1125 cm³/g with the average pore diameter of 28.2 Å. However, for the pure titania powder the surface area was 153 m²/g according to BET method. The pore size distribution according to BJH analysis showed a smaller maximum at 34.3 Å pore diameter and cumulative pore volume of pores between 17 and 100 Å of 0.02254 cm³/g with the average pore diameter of 24.5 Å. It is clear that blending amorphous silica can change the surface properties and increase the surface area of the material described in this invention, which is important for adsorption of pollutants.
Film Production [0266]
In order to prepare titanium dioxide films, the titanium-containing colloidal solutions may initially be mixed with one or more compounds that enhance the film-casting process to produce thicker and more abrasion resistant films. Water soluble alcohol, as much as 50%, has been found to enhance the film-forming process. In particular methanol and ethanol gave better results in spray coating. The sols may also be mixed with any proportion of water-soluble or alcohol-soluble ketones, such as acetone and acetylacetone. [0267]
Further, the sols may be mixed with any proportion of organic acids. The organic acid can be mono-, di- or multi-functional and it may also contain hydroxyl groups. Such acids include for example acetic, lactic, tartaric, citric, maleic, malic, malonic, diglycolic, benzoic, 1,2,4,5-C[0268] ₆H₂(COOH)₄, EDTA, and mixtures thereof.
The sols may also be mixed with any proportion of water-soluble aliphatic or aromatic alcohols, diols or polyols. Examples of such compounds include ethanol, propanol, ethylene glycol, glycerol, polyethylene glycol, polyvinylalcohol, phenol, catechol, polysaccharides and other polyols as will be known by persons skilled in the art, or a mixture of the same. [0269]
The sols may also be mixed with any proportion of ethanolamines, such as monoethanolamine, diethanolamine and triethanolamine, or a mixture thereof. [0270]
When such acids and alcohols are added together to the sol in a certain quantity, the sol can be coated on a substrate and heated to around 150° C. to give a polymeric hard film which allows the coated substrate to be easily handled. This characteristic has many applications, especially for example, in the robotic industry. This can be heated further to decompose the organic material and produce the anatase coating. [0271]
The applicants also prefer blending of amorphous silica by addition of silica colloid and/or silane compounds as mentioned earlier to the titanium dioxide colloid prepared in this invention. Such silane compounds may act as particle couplers to help improving the film thickness and getting more abrasion resistant films, which then either be cured under UV irradiation or heating to decompose any organic residues. [0272]
In addition, and especially if the film is to be formed on a hydrophobic surface such as some polymer surfaces, the sols may be mixed with any proportion of surfactant. The surfactant can be chosen from but not limited to the Brij series, Triton series, Tergitol series, Pluronic series, potassium dodecyl sulphate, or any other surfactant that does not cause gelling. Preferably, the surfactant concentration is between 0.01 to 5% by weight relative to TiO[0273] ₂.
Coated Substrates [0274]
The sols thus prepared can be coated on a variety of substrates. Examples of such substrates include glass, quartz, glass fibre, woven glass fibre, ceramics, silicon wafers, metals, polymer surfaces (such as polyethylene or polyester), wood, or building materials such as mortar, brick, tiles, or concrete. The coating method used may be any suitable method known in the art, such as spin-coating, dip-coating or spraying. [0275]
It is also preferred to apply a protective layer of amorphous silica or alumina on the substrate before coating with the photocatalyst, especially if the substrate consists of organic materials such as a polymer that could be deteriorated by the photocatalyst coating. The same method can be done with glass and other supported films that are prepared by heating. The precursors for amorphous silica and alumina can be prepared by hydrolysing a silicon alkoxide or an aluminium alkoxide in acidic solutions as is known to those who are skilled in the art. For examples precursors for amorphous silica and alumina can be chosen from the series tetraalkoxysilanes, alkoxychlorosilanes and aluminium trialkoxides but are not limited to these. It is more preferred that the alkoxy radicals would have low carbon backbones of C1-C5. [0276]
The films that are coated with the photocatalyst can be cured as described above, ie by exposure to solar light or a UV light source, or by heating at appropriate temperatures. [0277]
Fine Patterned Films [0278]
In the present invention, the applicants have been able to produce fine line patterns of less than 4 micrometer wide with very sharp edges using a low intensity black light lamp. This will enable the formation of TiO[0279] ₂patterns on polymeric substrates.
In one form of this invention it is possible, by utilising the photocatalytic properties of the titanium dioxide containing material as prepared by the present method, to produce a substrate coated with films having different patterns. [0280]
The coating solution may be chosen from any combination of the titania colloid with a silica colloid, a hydrolysable or partially hydrolysable silane and a surfactant. However it is preferred that the coating solution will produce a film which contains 50 to 100% by weight titania after curing. The titania content of the coating solution will be chosen so that it produces the required thickness of the patterned film. It is also preferred to apply only one layer to get sharper and clearer patterns. The patterning process comprises two steps. In the first step, parts of the film may be masked and the unmasked parts are exposed to an ultraviolet light. This will photocatalytically destroy the oxalic acid and other organic materials present in the titanium oxide of the unmasked portions. [0281]
The next step is to develop the film by acid treatment. The destruction of the oxalic acid in the unmasked parts renders those parts of the film insoluble, whereas the parts of the film that have not been exposed to UV light can be dissolved using a suitable acid solution or any material that reacts with or dissolves oxalic acid and or the titanium dioxide-containing gel, since such parts of the film are not cured. The acid solution may be any dilute mineral acid, organic acid solution or an acidic salt solution. Preferably, the acid is oxalic, lactic, citric, tartaric or sulphuric acid. The acidic salt can be, for example, ammonium sulphate or aluminium sulphate. Other materials such as hydrogen peroxide may also be used to develop the patterns. Additionally or alternatively a radiational or mechanical method such as ultrasonication can be used to develop the patterned film by removing the uncured parts. [0282]
In this way titanium dioxide films having very fine patterns of few micrometers wide or less and submicrometer to few nanometer thick can be produced on a variety of substrates, including plastic and polymeric surfaces, since it is done at room temperature and low level UV light (FIGS. 9, 10). This process may find a particular industrial applicability in the electronics industry such as field-effect transistors and other microdevices. [0283]
Doping [0284]
Here we describe at least two methods of dispersing platinum and other metal particles by photo- and chemical reduction in TiO[0285] ₂films and powders that are prepared in this invention.
The properties of the titanium dioxide-containing powders and films, for example the photocatalytic properties, may be enhanced by doping with a metal salt or complex. Suitable dopants will be known to those persons skilled in the art. [0286]
The composite may be either heated at a suitable temperature to form the metal oxide or UV irradiated to form metal particles inside the titanium dioxide or some cases can produce metal oxides. The addition of a soluble metal salt or complex may either be directly to the sols or by impregnation of the powders and films themselves with an aqueous or alcoholic solution of the metal salt for a sufficient time to allow adsorption. When the metal is chosen from the group Pd, Pt, Ag and Cu, the adsorbed metal salt or complex can be either thermally decomposed or photocatalytically decomposed under UV light. The photocatalytic doping process may take between few minutes to several hours. The photocatalytic process will involve photooxidation of the organic radical of the metal precursor and the photoreduction of the metal ion to zero oxidation state whereby the metal particles are dispersed uniformly on the surface of the catalyst particles. The final metal content in the photocatalyst is preferably less than 2% by weight and more preferably between 0.2 to 0.5% by weight. Above 0.5% by weight doping the activity of the photocatalyst may marginally increase. [0287]
In this method the preferred precursor for Pd and Pt are the hexachloro-complexes. It is preferred to mix these precursors with a sacrificial compound, more preferably the sacrificial compound is a low carbon organic compound such as formaldehyde, formic acid, methanol or ethanol that is added in excess relative to the doping metal. For example, if platinum and formaldehyde are to be used, the more preferable molar ratio is about 1:5 of Pt:formaldehyde. [0288]
For silver deposition the applicants prefer to use either silver acetate or silver nitrate, although other soluble silver compounds can be used. The precursor solutions can be added either to the colloid or impregnated in the catalyst particles or films after curing. No sacrificial agent is needed in this case. The colour of the silvered catalyst ranges between light grey to black. [0289]
In this invention the preferred copper precursors are copper acetate and copper nitrate, although other salts such as the sulphate can be used. The copper precursor can be added to the colloid before gelling or impregnated in the photocatalyst particles or films after gelling and curing. The colour of the catalyst will change from light green to bronze due to the reduction of Cu(II) to Cu(0) after exposure to UV light. [0290]
In another embodiment reduction of the metal precursor that is adsorbed on the titanium-containing catalyst can be performed by exposing such a catalyst to a dilute hydrazine hydrate solution for a sufficient time to allow the complete reduction of the metal to zero valency. In this case it is preferred to wash the catalyst with water to remove the excess of hydrazine. In this method hydrazine can penetrate into the catalyst particles or grains and reduce the metal precursor that is adsorbed inside the grains, whether the particles are of pure titania, a titanium-containing material, or silica particles. [0291]
B. Application of Titanium Dioxide Materials [0292]
The titanium dioxide powders, grains and composite materials containing titanium dioxide materials have a number of applications. [0293]
For example, the materials will perform as photocatalysts. The material is able to decompose organic compounds or pollutants in air and water under solar radiation or UV light. In particular the undesirably carbon structures are broken down into relatively harmless CO[0294] ₂and H₂O. Dyes can be photocatalytically discoloured or bleached using the materials prepared in this invention when irradiated with sunlight or artificial UV light. In this invention we present for example, a method for preparing and supporting a photocatalyst that is capable of reducing ethylene concentration rapidly and efficiently.
A particular example includes the filtering of ethylene (ethene) gases from horticultural storage facilities. This is a colourless gas produced by some fruits as they ripen. However, this gas also causes premature ripening of other fruits stored in the same facility. An example is the storage of apples and kiwifruit, where the former produces ethene that prematurely ripens the latter. This is a significant issue for horticultural exporters, as it may damage their product before it reaches the market. [0295]
In addition, the titanium dioxide films will have superhydrophilic properties when they are first prepared. This hydrophilicity is maintained by exposure to solar light or UV light, with a contact angle close to zero. This hydrophilicity is particularly useful for such applications as anti-fogging mirrors and glass windows, where the hydrophilic surface will prevent the formation of small water droplets, that cause the fogging. [0296]
Furthermore, the titanium dioxide-containing materials have anti-microbial activity, including activity against bacteria, viruses and fungi. These properties may be utilised for example in forming a surface such as a bench top, or tiles coated with a film of titanium dioxide, to reduce or eliminate the growth of micro-organisms. [0297]
The advantages of the applicant's invention include the following: [0298]
the ability to coat a wide range of surfaces including plastics and other thermally unstable substances [0299]
curing within 30 minutes under sunlight and les under UV light source [0300]
reduced costs related to equipment, energy savings and plant operation [0301]
ability to produce an abrasion-resistant finish at higher temperatures [0302]
ability to produce large sample sizes easily [0303]
coatings have high porosity and surface area ensuring high photocatalytic activity [0304]
although high temperatures are needed to ensure that the material or coating produced is abrasion resistant, relatively low temperatures can be used where abrasion resistance is not an issue—for example in an air filter where the material does not come into physical contact with anything other than air or gases [0305]
extensive testing has demonstrated that the TiO[0306] ₂material is highly effective at breaking up ethylene and other workplace compounds. Decomposition of these materials has improved from 30% to 95% per hour, illustrating the effectiveness of this invention as a scrubber.
The invention will now be described in more detail with reference to the following non-limiting examples. [0307]

C EXAMPLES

Example 1

Preparation of Titania Sol Using Titanium Tetraisopropoxide (TTIP) and UV Light [0308]
14.2 g of TTIP was added to 3.15 g of oxalic acid dihydrate solution in 20 ml absolute ethanol. The mixture was stirred for 10 minutes then solvent was evaporated under vacuum. Water was added to the white solid and the mixture was stirred at 75° C. to peptize it into a clear sol. The solution then was irradiated by a UV light using a mercury lamp for 3 hours. During that the amount of oxalic acid dropped to half its original concentration as determined by permanganate titration. [0309]

Example 2

Preparation of Titania Sol Using in situ α-Titanic Acid and 0.5 Mole Ratio of Oxalic Acid [0310]
α-Titanic acid was obtained by hydrolysing 14.2 g TTIP in 100 ml water. 3.15 g of oxalic acid dihydrate was added and the mixture was stirred at 70° C. to form a colourless sol. [0311]

Example 3

Preparation of Titania Sol Using α-Titanic Acid Prepared from TTIP and 0.5 Mole Ratio of Oxalic Acid [0312]
14.2 g of TTIP was added to oxalic acid solution in water (150 ml). The mixture was stirred at 70° C. to form the sol as described in Example 2. [0313]

Example 4

Preparation of Titania Sol Using α-Titanic Acid from Titanyl Sulfate [0314]
Titanyl sulphate solution was hydrolysed with dilute ammonia solution to get a white precipitate, which was filtered and washed with distilled water until it became free of sulphate. The resulting slurry was kept in a closed container and analysed for the content of TiO[0315] ₂by heating a specimen in a furnace to 500° C. 21 g of α-Titanic acid slurry (19% by weight TiO₂) was added to 200 ml of water containing 3.15 g of oxalic acid dihydrate. The mixture was stirred at 70° C. to form a bluish-white colloid after 1 hour. Powder XRD measurement after gelling and heating to 200° C. showed the presence of anatase phase. Anatase diffraction lines became sharper after heating to 300° C. for 30 minutes.

Example 5

Preparation of Titania Sol Using α-Titanic Acid and 0.3 Mole Ratio of Oxalic Acid [0316]
1.42 g of TTIP was hydrolysed in 40 ml water to form α-titanic acid. 0.19 g of oxalic acid solution in water was added and the mixture was stirred at 65-70° C. to get a colloidal solution. Powder XRD showed the presence of anatase phase. [0317]

Example 6

Preparation of Titania Sol Using α-Titanic Acid and 0.25 Mole Ratio of Oxalic Acid [0318]
14.2 g of TTIP was added to oxalic acid (1.57 g) solution in water (400 ml). The mixture was stirred at 65° C. to get a bluish-white colloid. Anatase phase was detected in the xerogel prepared once the solvent was removed. [0319]

Example 7

Preparation of Titania Sol Using Titanium Tetraisopropoxide (TTIP) and 0.5 Mole Ratio of Oxalic Acid. [0320]
14.2 g of TTIP was added to 3.15 g oxalic acid dihydrate solution in 50 ml absolute ethanol. The so formed complex was hydrolysed with 200 ml water and the mixture was stirred at 58° C. until a clear sol was obtained. [0321]
Powder XRD after gelling and heating the powder to 200° C. showed the presence of a relative 74% TiO[0322] ₂—B phase and 26% anatase phase. The powder heated to 250° C. showed that some of TiO₂—B was transferred to anatase with ratios of 57% TiO₂—B and 43% anatase. Between 300° C. to 410° C. only well crystalline anatase was obtained. Transfer to rutile phase started to occur after heating at 450° C. (FIG. 8).

Example 8

Preparation of Titania Sol Using Titaniumtetrabutoxide (TTB) and 0.25 Mole Ratio of Oxalic Acid. [0323]
17 g of TTB was added to 1.57 g oxalic acid solution in 40 ml t-butanol. The complex was hydrolysed with 500 ml of water and the mixture was stirred at 65° C. to get a clear sol. The sol was gelled then heated to 350° C. to get an off-white anatase powder. [0324]

Example 9

Preparation of Titania Sol Containing 30% Silica [0325]
A mixture was prepared as in Example 6. 5.7 g of [0326] Nalcogel brand 30% Silica colloid was added to the slurry and the resultant mixture was heated at 60° C. until a clear colloid was obtained.

Example 10

Titania Sol from TTIP [0327]
14.2 g of TTIP was added to 1.57 g hydrated oxalic acid in 40 ml absolute ethanol and stirred for five minutes at 50° C. The solution was hydrolysed with 400 ml of warm water and the mixture was stirred vigorously at 65° C. for two hours. The volume of the resulting clear sol was reduced to 50 ml under vacuum to get 8% by weight or to 25 ml to form 16% by weight colloid. The colloid was filtered using 0.4 micrometer Sartorius Minisart filter. [0328]

Example 11

Preparation of Titanium Sol Containing 50% Silica [0329]
A sol was prepared as in Example 10, then the required amount of [0330] Nalcogel brand 30% silica colloid was added.

Example 12

Low Temperature Preparation of Photocatalyst Grains and Powders [0331]
16-20% pure titania colloids or titania-silica colloids were prepared according to the examples above then were caused to gel. If grains are desired it was preferred to gel the colloid at 70-80° C., and if powders were desired it was preferred that the solvent is evaporated under vacuum at 50° C. Heating of such grains or powders at 200° C. for one hour produced anatase photocatalysts. The heating time can be made longer than one hour to increase the crystallite size. [0332]

Example 13

High Temperature Formation of Photocatalyst Grains and Powders [0333]
A 16% titania/silica colloid was prepared as in example 11. The colloid would preferably be gelled at 80° C. if grains were desired, but when a powder was desired the solvent was evaporated under vacuum first. The xerogels were heated at 500° C. for one hour to get a nanocrystalline anatase containing material, with crystallite size of 8.1 nm. [0334]

Example 14

Preparation of Thin Film and UV Curing at Room Temperature [0335]
The sol in Example 10 was coated on a 2×1 cm silicon plate. The film was exposed to mercury UV light at a distance of 5 cm for 30 minutes. Decomposition of the oxalate was monitored by infra red spectroscopy. (FIG. 3). [0336]

Example 15

Degradation of Surfactants on a TiO[0337] ₂Film
One drop of each of 2% by weight of surfactant solutions (Brij 98, Brij 97, Brij 78, Brij 58, Brij 35, Triton X-100, Tergitol 15-S-12) was spread on the film of example 14, each at a time and left to dry at room temperature. The films were subjected to irradiation from mercury UV lamp at a distance of 5 cm. The concentration of the surfactants was monitored using infrared spectroscopy. The absorption peaks for C—H stretching at around 2900 cm[0338] ⁻¹and for C—O—C bending at around 1100 cm⁻¹were found to disappear after 30 minutes of irradiation in the Brij series and after 50 minutes for Triton and Tergitol surfactants (FIGS. 7a and 7 b). This indicates that the TiO₂film cured at room temperature can photodegrade these surfactants under UV irradiation.

Example 16

TiO[0339] ₂Film on a Polymer Sheet
50×50×3-mm polyacrylic sheet was spin coated with TiO[0340] ₂using a 4% sol of Example 10 containing 0.2% Brij 97 surfactant. The film was irradiated under black light lamp for 2 hours. Coating was repeated in the same way to get a hydrophilic coating.

Example 17

Preparation of Thin Film and UV Patterning [0341]
The 4% sol in Example 10 containing 0.1% Brij 97 was spin coated on a 5×5×0.1 cm glass plate. A black and white image printed on a transparent thin cellulose acetate sheet was placed on the film surface. The plate was exposed to a black light lamp for 6 hours. The film was soaked in a dilute warm lactic acid solution to dissolve the unexposed area of the film and leaving the parts that were exposed to the UV radiation. [0342]

Example 18

TiO[0343] ₂/25%SiO₂Finely Patterned Film
10 ml of the 8% titania sol of Example 10 was well mixed with 0.1 g of 2% Brij 97 surfactant and 1 g of glycidoxypropyltrimethoxysilane. The mixture was spin coated on a pre-cleaned 50×50×1 mm glass plate. A finely patterned mask was securely placed on the film and was irradiated from the top at 10 cm distance under a black light lamp for 15 hours. The mask was removed and the film was soaked in a dilute warm lactic acid for 15 minutes. The so formed pattern was traced by a DETAK surface profiler (Sloan Technology Corporation) (FIG. 9) and by SEM (FIG. 10). [0344]

Example 19

A Hydrophilic Photocatalyst Thin Film at Room Temperature [0345]
A spin coating mixture, which contains 55% by weight TiO[0346] ₂and 45% by weight SiO₂was prepared by mixing 12 ml of 8% titania colloid from Example 10, 1.164 g of 30% Nacogel silica colloid, 1.716 g of glycidoxypropyl-trimethoxysilane and 0.1 g of 2% Brij 78 solution. A clean glass plate was spin coated with this mixture at 600 rpm for 2 minutes. The so coated plate was irradiated under black light lamp for 15 hours, after this the film became hydrophilic. Film thickness was 0.5 micrometer and the film did not scratch when tested by H9 pencil.

Example 20

Preparation of Heat Cured Thin Film and Testing for Hydrophilicity [0347]
The sol in Example 5 was spin coated on 50×50×1 mm glass plate. The plate was heated to 200° C. After cooling, the plate was coated again and heated. Five coatings were applied in this way. The film was stained with 0.1% oleic acid solution in acetone and left under the sun light. Decomposition of the oleic acid was estimated by the reduction of the contact angle of water on the surface, which was reduced to its original angle of <2 after 6 hours. [0348]

Example 21

Preparation of Heat Cured Film on Woven Glass Cloth [0349]
A piece of woven glass cloth (6×6 cm) was dip coated with the sol as prepared in Example 11. After drying in warm air, the cloth was heated at 210° C. for 15 minutes. The coating was repeated to produce multiple layers of the anatase catalyst. [0350]

Example 22

Testing the Photoactivity of Coated Glass Cloths (Degradation of Acetaldehyde) [0351]
A piece of woven glass cloth prepared in Example 21 was placed in a one litre gas tight reactor with a quartz window at the top. The reactor is provided with a small fan, and a thermohygrometer. The humidity was adjusted to 25±1% at 20° C. Acetaldehyde gas was injected in the reactor to give a concentration between 40-60 ppm. After 30 minutes equilibrium in the dark the coated glass cloth was irradiated by a black light lamp at 4 cm distance. The concentration of acetaldehyde was monitored using gas detectors. [0352]
The effect of added silica on the activity is shown in FIG. 4. The decomposition rate of acetaldehyde versus the number of coatings (thickness of the film) is shown in FIG. 5. [0353]

Example 23

The Effect of Humidity on the Photodegradation of Acetaldehyde [0354]
The humidity effect on the photodecomposition of acetaldehyde was also tested. The humidity range used for testing was between about 10 and about 90% relative humidity at 20° C. The humidity inside the reactor was changed either by circulating the air through a desiccant or by adding water vapour prior to the injection of acetaldehyde. The effect of humidity on the photoactivity is shown in FIG. 6. [0355]

Example 24

Photo-Platinised Photocatalyst Glass Cloth [0356]
A 6×6 cm woven glass cloth was prepared as in Example 21, then it was heated to 500° C. for 1 hour. Three millilitres of a doping solution containing 0.1% by weight platinic acid and 0.5% by weight formaldehyde was sprayed on the surface of the catalyst and allowed 5 minutes equilibrium time. The wet glass cloth was irradiated under UV light from a 20-Watt black light lamp for 5 minutes during which the colour of the catalyst changed to grey-black. The glass cloth was washed with distilled water and dried at 80° C. to get a platinised photocatalyst. [0357]

Example 25

Photo-Silvering a Photocatalyst Glass Cloth [0358]
A 25 ml of 16% by weight titania colloid was prepared as in example 10 and was well mixed with 1.26 ml of 1% silver nitrate solution producing a colloid with 0.2% by weight silver relative to TiO[0359] ₂. A 6×6 cm glass cloth was coated with this solution and dried at 70° C., then heated at 210° C. for 15 minutes. The photocatalyst cloth was irradiated under black light lamp for 30 minutes, during which its colour changed to grey.

Example 26

Silvering a Photocatalyst Glass Cloth Using Hydrazine [0360]
A photocatalyst glass cloth was prepared as in Example 25. After heating to 210° C. the cloth was cooled to room temperature then sprayed with a 1% hydrazine hydrate solution. After 5 minutes the cloth was washed with distilled water to remove excess hydrazine, then dried in the oven. Before using this cloth in photocatalysis experiments, it is preferred to irradiate it under UV light to photo-degrade any traces of adsorbed hydrazine. [0361]

Example 27

Copper Doped Titania Thin Film [0362]
20 ml of 4% titania containing colloid from Example 10 was mixed with 0.5 ml of 1% copper acetate solution to give 0.2% by weight Cu relative to TiO[0363] ₂. After addition of 0.1% Brij surfactant the solution was spin coated on a clean glass plate. The coated plate was irradiated under black light lamp for 5 hours, then another layer was applied by the same way. This film can be used as antibacterial coating.

Example 28

Photodegradation of a Dye [0364]
The film that was prepared as in Example 20 by heating to 200° C. was stained with 0.5% alcoholic solution of Rhodamine B base. The stained film was placed on a laboratory bench facing sunlight that was coming through a window. The fading of the dye colour was monitored using a Hewlett Packard diode array UV-visible spectrophotometer at wavelength 540 nm. It took 15 min for the dye on the film to completely disappear and become hydrophilic again (FIG. 11). [0365]

Example 29

Photodegradation of Ethylene Gas [0366]
A 6×6 cm platinised photocatalyst cloth was prepared as in Example 24 loaded with 0.09 g of the photocatalyst. The photocatalyst cloth was placed inside a 150 ml gas tight reactor. The humidity inside the reactor was adjusted to 40% at 20° C. 1 ml of 1% ethylene gas in air was injected into the reactor to produce 70 ppmv of ethylene inside the reactor. After 30 minutes equilibrium, the black light lamp was turned on and the ethylene concentration was monitored using a Hewlett Packard 6890 gas chromatograph equipped with an Innowax column and FID detector. After 30 minutes the temperature inside the reactor became 40° C. and 97.1% of the ethylene was decomposed (FIG. 12). [0367]

Example 30

Photothermal Degradation of Ethylene Gas [0368]
A 11×17 cm platinised photocatalyst cloth was prepared as in Example 24 which was loaded with 0.6 g photocatalyst. Humidity in the photoreactor was adjusted to 41% at 20° C. 0.6 ml of pure ethylene gas (26.78 micromole) was injected. After 30 minutes equilibrium, the UV lamp (black light lamp) and heat were turned on with the temperature inside the photoreactor was raised to 85° C. The concentration of ethylene was monitored by GC as in Example 29 and was found to be reduced by 98.9% after 30 minutes (FIG. 13) [0369]

Example 31

Photothermal Degradation of Toluene [0370]
The same platinised photocatalyst cloth of Example 30 was used and the humidity inside the reactor was 47% at 20° C. A 2.5 microlitre of pure liquid toluene (20 micromoles) was injected inside the reactor and left to evaporate and adsorbed on the photocatalyst for 40 minutes. The black light lamp and the heat were turned on to raise the temperature inside the reactor to 85° C. The concentration of toluene gas was monitored using the same technique as in Example 29. After 30 minutes the concentration of toluene was reduced by 93.8% and in one hour in was reduced by 99.4% (FIG. 14) [0371]
While the invention has been described with reference to particular embodiments and examples, those persons skilled in the art will appreciate that variations and modifications may be made without departing from the scope of the invention. [0372]

Claims

1. A method of preparing a solution containing colloidal particles which contain titanium ions comprising or including the step of:

A. reacting or otherwise stabilising one or more hydrolysable titanium-containing compound(s) with oxalic acid in a reaction medium.

2. A method of preparing a solution as claimed in claim 1 wherein A occurs under conditions such that peptization of the colloidal solution is substantially obtained and substantially maintained.

3. A method of preparing a solution as claimed in claim 2 wherein the conditions include stirring or agitation of the one or more hydrolysable titanium-containing compound(s) with oxalic acid in the reaction medium, at a temperature between ambient temperature to near the boiling point of the reaction mixture.

4. A method of preparing a solution as claimed in claim 3 wherein the reaction medium comprises water or a water/alcohol mixture and wherein the titanium-containing compound is hydrolysable in water and/or in base.

5. A method of preparing a solution as claimed in claim 4 wherein, the titanium containing compound is water-hydrolysable and is of the formula Ti(OR)₄, where R is a C₂-C₆linear or branched chain alkyl group.

6. A method of preparing a solution as claimed in claim 5 wherein the titanium-containing compound is titanium tetraisopropoxide and/or titanium tetrabutoxide.

7. A method of preparing a solution as claimed in claim 6 wherein the titanium containing compound is:

8. A method of preparing a solution as claimed in claim 4 wherein the titanium-containing compound is base-hydrolysable and the titanium-containing compound is selected from, but not restricted to, TiCl₄and/or TiOSO₄.

9. A method of preparing a solution as claimed in claim 8 wherein the base-hydrolysable titanium-containing compound is hydrolysed to a hydrolysis product, using a base prior to reaction with or stabilisation by oxalic acid, the hydrolysis product being filtered and/or washed, to form a slurry before reaction with or stabilisation by, the oxalic acid.

10. A method of preparing a solution as claimed in any one of the preceding claims wherein the amount of oxalic acid is such as to provide a mole ratio of oxalic acid:titanium in the range of about 0.2:1 to about 1:1.

11. A method of preparing a solution as claimed in claim 10 wherein the water content of the reaction medium is such as to provide a mole ratio of water:titanium in the range of from about 400:1 to about 600:1.

12. A method of preparing a solution as claimed in any one of claims 4 to 11 wherein, when alcohol is present in the reaction medium the alcohol is a mono hydroxyl aliphatic alcohol having the formula ROH, where R is a C₁to C₄linear or branched alkyl group, such as ethanol or t-butanol, and the preferred amount of alcohol present is such as to provide a mole ratio of alcohol:titanium of between 10:1 to 50:1 in the solution.

13. A method of preparing a solution as claimed in any one of the preceding claims wherein the oxalate concentration of the solution is at any stage reduced by irradiating the solution with UV light.

14. A solution containing colloidal particles which contain titanium ions prepared substantially according to the method as claimed in anyone of claims 1 to 13.

15. A method of preparing a solution containing colloidal particles which contain crystalline titanium dioxide comprising or including the step of:

16. A method of preparing a solution as claimed in claim 15 wherein A occurs under conditions such that peptization of the colloidal solution is substantially obtained and substantially maintained.

17. A method of preparing a solution as claimed in claim 16 wherein the conditions include stirring or agitation of the one or more hydrolysable titanium-containing compound(s) with oxalic acid in the reaction medium, at a temperature between ambient temperature to near the boiling point of the reaction mixture.

18. A method of preparing a solution as claimed in claim 17 wherein the reaction medium comprises water or a water/alcohol mixture and wherein the titanium-containing compound is hydrolysable in water and/or in base.

19. A method of preparing a solution as claimed in claim 18 wherein, the titanium containing compound is water-hydrolysable and is of the formula Ti(OR)₄, where R is a C₂-C₆linear or branched chain alkyl group.

20. A method of preparing a solution as claimed in claim 19 wherein the titanium-containing compound is titanium tetraisopropoxide and/or titanium tetrabutoxide.

21. A method of preparing a solution as claimed in claim 20 wherein the titanium containing compound is:

22. A method of preparing a solution as claimed in claim 18 wherein the titanium-containing compound is base-hydrolysable and the titanium-containing compound is selected from, but not restricted to, TiCl₄and/or TiOSO₄.

23. A method of preparing a solution as claimed in claim 22 wherein the base-hydrolysable titanium-containing compound is hydrolysed to a hydrolysis product, using a base prior to reaction with or stabilisation by oxalic acid, the hydrolysis product being filtered and/or washed, to form a slurry before reaction with or stabilisation by, the oxalic acid.

24. A method of preparing a solution as claimed in any one of claims 15 to 23 wherein the amount of oxalic acid is such as to provide a mole ratio of oxalic acid:titanium in the range of about 0.2:1 to about 1:1.

25. A method of preparing a solution as claimed in claim 24 wherein the water content of the reaction medium is such as to provide a mole ratio of water:titanium in the range of from about 400:1 to about 600:1.

26. A method of preparing a solution as claimed in any one of claims 15 to 25 wherein, when alcohol is present in the reaction medium the alcohol is a mono hydroxyl aliphatic alcohol having the formula ROH, where R is a C₁to C₄linear or branched alkyl group, such as ethanol or t-butanol, and the preferred amount of alcohol present is such as to provide a mole ratio of alcohol:titanium of between 10:1 to 50:1 in the solution.

27. A method of preparing a solution as claimed in any one of claims 15 to 26 wherein the oxalate concentration of the solution is at any stage reduced by irradiating the solution with UV light.

28. A solution containing colloidal particles which contain crystalline titanium dioxide prepared substantially according to the method as claimed in anyone of claims 15 to 27.

29. A solution containing colloidal particles which contain crystalline titanium dioxide prepared substantially as herein described with reference to any one or more of the accompanying examples.

30. A method of preparing a TiO₂-Containing Product comprising or including the steps of:

1) preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid or stabilised by having been reacted with oxalic acid as claimed in any one of claims 15 to 27, and

3) further processing of the solution to obtain the product.

31. A method of preparing a TiO₂-Containing Product as claimed in claim 30 wherein the TiO₂phase in the product, at least initially, includes, is predominantly or is substantially anatase.

32. A method of preparing a TiO₂-Containing Product as claimed in claims 30 or 31 wherein the additives of step 2) include one or more of:

a) silica or a silica precursor,

b) water, or alcohol, soluble ketone(s),

c) metal precursor(s),

d) surfactant(s),

e) silane(s).

33. A method of preparing a TiO₂-Containing Product as claimed in claim 32 wherein when silica is added it is as colloidal silica, it is added in an amount to yield a ratio substantially from 30-60 wt % relative to titanium in the product, and the concentration of the colloidal silica is such as to provide between about 1 and 50% by weight.

34. A method of preparing a TiO₂-Containing Product as claimed in claim 32 wherein when a metal precursor is added, it is a metal salt or metal complex of one or more of Pd, Pt, Ag and Cu.

35. A method of preparing a TiO₂-Containing Product as claimed in claim 34 wherein the metal is Pd or Pt and the precursor is one of the hexachloro-complexes of Pd or Pt.

36. A method of preparing a TiO₂-Containing Product as claimed in claim 35 wherein the Pd or Pt hexachloro-complex is mixed with a low carbon organic compound as a sacrificial compound, such as formaldehyde, formic acid, methanol or ethanol, the sacrificial compound added in excess relative to the precursor metal.

37. A method of preparing a TiO₂-Containing Product as claimed in claim 34 wherein the metal is Ag and the precursor is one or both of silver acetate or silver nitrate.

38. A method of preparing a TiO₂-Containing Product as claimed in claim 34 wherein the metal is Cu and the precursor is one or more of copper acetate, copper sulphate and copper nitrate.

39. A method of preparing a TiO₂-Containing Product as claimed in claim 32 wherein the silane(s) is added neat or as solution in an aqueous or organic solvent that is miscible with water, and is a hydrolysable or partially hydrolysable silane compound of a formula RSiX₃, R₂SiX₂and SiX₄(where R is a simple or functionalised organic group and X could be a halide or an alkoxide group).

40. A method of preparing a TiO₂-Containing Product as claimed in any one of claims 30 to 39 wherein step 3) includes one or both the steps of:

i) causing the solution to gel (a gelling step),

41. A method of preparing a TiO₂-Containing Product as claimed in claim 40 wherein curing of the gel is effected by exposure to UV radiation and/or by heat, and the wavelength of the UV radiation substantially or partially coincides with the photo catalytically active band gap of the TiO₂in the anatase phase.

42. A method of preparing a TiO₂-Containing Product as claimed in any one of claims 30 to 41 wherein there is an additional step 4), which includes one or both the steps of:

ii) transformation of any metal precursor to a metal or metal oxide added within step 2) and/or step 4) (a transformation step).

43. A method of preparing a TiO₂-Containing Product as claimed in claim 42 wherein the metal of the precursor of step i) is one or more of Pd, Pt, Cu or Ag.

44. A method of preparing a TiO₂-Containing Product as claimed in claim 42 or 43 wherein the transformation step is employed and occurs by one or more of:

ii) exposing the metal precursor or the TiO₂-Containing Product containing the metal precursor, to a dilute hydrazine hydrate solution for a sufficient time to allow the complete reduction of the metal to zero valency, and/or

45. A method of preparing a TiO₂-Containing Product as claimed in claim 44 wherein the final metal content in the TiO₂of the TiO₂-product is between 0.2 to 0.5% by weight.

46. A method of preparing a TiO₂-Containing Product as claimed in any one of claims 30 to 45 wherein sometime prior to step 3) the oxalate concentration of the solution is reduced by irradiating the solution with UV light.

47. A method as claimed in claim 46 wherein the product is a particulate product.

48. A method of preparing a TiO₂coating solution comprising or including the steps of:

1) preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid, or are stabilised by having been reacted with oxalic acid as prepared in any one of claims 15 to 27, and

49. A method of preparing a coating solution as claimed in claim 48 wherein step 2) includes any one or more of the following:

i) Addition of or mixing with silica, or a silica precursor,

ii) Addition of or mixing with any proportion of water-soluble or alcohol-soluble ketone(s),

iii) Addition of or mixing with any proportion of surfactant(s),

iv) Addition of or mixing with one or more metal precursor(s),

v) Addition of or mixing with one or more silane(s).

50. A method of preparing a coating solution as claimed in claim 49 wherein one or more metal precursor(s) is added or mixed and is a soluble metal salt or complex of the group of Pd, Pt, Ag and Cu.

51. A method of preparing a TiO₂-coated substrate comprising or including the steps of:

I. preparation of a coating solution as claimed in any one of claims 48 to 50, and

II. further processing of the solution to obtain the coated substrate.

52. A method of preparing a coated substrate as claimed in claim 51 wherein the TiO₂phase in the coated substrate, at least initially, includes, is predominantly or is substantially anatase.

53. A method of preparing a coated substrate as claimed in claim 52 wherein the substrate is one or more of (but not restricted to) glass, quartz, glass fibre, woven glass fibre, ceramics, silicon wafers, metals, polymer surfaces (such as polyethylene or polyester), wood, or building materials such as mortar, brick, tiles, or concrete.

54. A method of preparing a coated substrate as claimed in claim 53 wherein step II includes:

i) application of the coating solution to a substrate, and

ii) a gelling step, and

iii) a curing step.

55. A method of preparing a coated substrate as claimed in claim 54 wherein prior to application of the coating solution a protective layer of amorphous silica and/or alumina is applied to the substrate, or a precursor of amorphous silica selected from (but not limited to) the series tetraalkoxysilanes, alkoxychlorosilanes; or a precursor of amorphous aluminia selected from (but not limited to) the series aluminium trialkoxides and wherein the precursors may convert to the silica or alumina by hydrolysis in acid solution.

56. A method of preparing a coated substrate as claimed in any one of claims 54 to 55 wherein curing of the gel is effected by exposure to UV radiation and/or by heat and the wavelength of the radiation substantially or partially coincides with the photocatalytic band gap of anatase TiO₂.

57. A method of preparing a coated substrate as claimed in any one of claims 51 to 56 wherein there is a further step III which includes one or both the steps of:

ii) transformation to a metal or metal oxide of any metal precursor added within step I) and/or step III) (a transformation step).

58. A method of preparing a coated substrate as claimed in claim 57 wherein the metal of the precursors of step i) is one or more of Pd, Pt, Cu or Ag.

59. A method of preparing a patterned TiO₂-coated substrate comprising or including the steps of:

i) preparation of a coated substrate as claimed in any one of claims 51 to 58, but prior to any gelling or curing steps (if any),

ii) masking one or more regions of the coating,

iv) development of the film.

60. A method of preparing a patterned TiO₂-coated substrate as claimed in claim 59 wherein the coating solution produces a film which contains 50 to 100% by weight titania after curing.

61. A method of preparing a patterned TiO₂-coated substrate as claimed in claim 60 wherein development of the film is by one or more of:

i) application of an acid solution wherein the acid solution is any dilute mineral acid such as sulphuric and/or an organic acid solution such as oxalic, lactic, citric, tartaric and/or an acidic salt solution where the salt is, ammonium sulphate or aluminium sulphate, and/or,

ii) application of other materials such as hydrogen peroxide, and/or,

iii) a radiational or mechanical method including ultrasonication, and/or

iv) any other method for redissolution of the UV—unexposed gel.

62. A method of preparing a patterned TiO₂-coated substrate as claimed in claim 61 wherein development is at room temperature.

63. A method of preparing a patterned TiO₂-coated substrate as claimed in claim 62 wherein curing of the coating is by exposure to UV radiation, the wavelength of the UV radiation substantially or partially coinciding with the photocatalytically active band gap of the TiO₂in the anatase phase.

64. A method of preparing a patterned TiO₂-coated substrate as claimed in claim 63 wherein there is a final step of sintering the coating.

65. A method of increasing the content of rutile and/or TiO₂—B phases in a TiO₂product including or comprising:

preparing a TiO₂-containing product as claimed in any one of claims 30-47 wherein the TiO₂phase is predominantly or at least partially anatase, or is predominantly or at least partially TiO₂—B phase and

heating to increase the anatase and/or rutile content.

66. A method as claimed in claim 65 wherein heating to substantially between 200° C. to 400° causes or initiates phase change of TiO₂—B to anatase phase to titanium dioxide-B and/or rutile phase in the product.

67. A method as claimed in claim 65 wherein further heating to substantially above 400° C. will increase the content of the rutile phase in the product.

68. A method as claimed in claim 65 wherein the TiO₂undergoes a phase change substantially entirely to rutile at temperatures substantially higher than 500° C.

69. A method as claimed in any one of claims 65 to 68 wherein addition of substantially up to 50% by weight silica results in stabilisation of the anatase phase and/or TiO₂—B phase thereby requiring heating to over 600° C. to initiate and/or complete the transformation to rutile phase.

70. A TiO₂-containing product substantially prepared according to the method as claimed in any one of claims 30-47.

71. A TiO₂-containing product prepared substantially as herein described with reference to any one or more of the accompanying examples.

72. A TiO₂-containing coating solution substantially prepared according to the method as claimed in any one of claims 48 to 50.

73. A TiO₂-containing coating solution prepared substantially as herein described with reference to any one or more of the accompanying examples.

74. A TiO₂-containing coated substrate substantially prepared according to the method as claimed in any one of claims 51 to 64.

75. A TiO₂-containing coated substrate prepared substantially as herein described with reference to any one or more of the accompanying examples.

76. A method of preparing a TiO₂-based photocatalyst including or comprising the following steps:

1) preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid, or stabilised by reaction with oxalic acid as claimed in any one of claims 15 to 27,

2) further processing of the solution to obtain the photocatalyst.

77. A method of preparing a TiO₂-based photocatalyst as claimed in claim 76 wherein the TiO₂-based photocatalyst is a TiO₂particulate material, and the further processing includes a gelling and a curing step.

78. A method of preparing a TiO₂-based photocatalyst as claimed in claim 77 wherein the TiO₂-based photocatalyst is a TiO₂coating or film on a substrate, and the further processing includes preparation of a coating solution, application of the coating solution to the substrate, and a gelling and a curing step.

79. A method of preparing a TiO₂-based photocatalyst as claimed in claim 77 or 78 wherein the TiO₂phase in the particulate material, coating or film, at least initially, includes, is predominantly or is substantially anatase and/or TiO₂—B.

80. A method of preparing a TiO₂-based photocatalyst as claimed in claim 79 wherein the TiO₂-based photocatalyst acts as a photocatalyst upon irradiation of or exposure to UV light.

81. A method of preparing a TiO₂-based photocatalyst as claimed in claim 80 wherein the TiO₂-based photocatalyst is metal or metal-oxide doped, and the metal is selected from Pt, Pd, Cu or Ag.

82. A method of preparing a TiO₂-based photocatalyst as claimed in claim 79 or 80 wherein the TiO₂-based photocatalyst can be used to photocatalytically degrade organic compounds and wherein the degradation occurs via application of or exposure to UV radiation.

83. A method of preparing a TiO₂-based photocatalyst as claimed in claim 79 or 80 wherein the TiO₂-based photocatalyst can act as a hydrophilic surface when coated on a substrate.

84. A TiO₂-based photocatalyst prepared substantially according to the method as claimed in any one of claims 76-83.

85. A TiO₂-based photocatalyst prepared substantially as herein described with reference to any one or more of the examples.

86. A method of preparing B phase TiO₂including or comprising the following steps:

1) preparation of a solution containing colloidal particles which contain crystalline titanium dioxide wherein the particles are stabilised by oxalic acid, or stabilised by reaction with oxalic acid, as claimed in any one of claims 15 to 27

2) further processing of the solution to obtain TiO₂predominantly or substantially in the TiO₂—B phase,

3) heating of the TiO₂to substantially between 200-300° C.

87. A method of preparing B phase TiO₂as claimed in claim 86 wherein the further processing step 2) includes removal of the solvent and/or a gelling step and/or a curing step.

88. A method of preparing B phase TiO₂as claimed in claim 87 there is a further step 4) of heating beyond 450° C. provide TiO₂in the rutile phase.

89. B phase TiO₂, prepared substantially according to the method of claims 86 to 89.

90. B phase TiO₂, prepared substantially as herein described with reference to any one or more of the examples.