EXAMPLES

[0063] In the following examples OHP represents overhead projector, GV stands for Gentian Violet dye.

Example 1

[0064] A nanocrystalline titanium dioxide sol was applied to the surface of a previously cleaned glass microscope slide by spin coating 0.5 ml of the titanium dioxide sol at 1500 rpm for 30 seconds. The glass slide was then fired at 450° C. for 30 minutes. Once cool the process was repeated two further times to give 3 coats of the nanocrystalline titanium dioxide. The slide was then immersed in an aqueous 1×10 ⁻ 6M solution of tris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride ) for 30 minutes to allow adsorption of the sensitising agent to the titanium dioxide. The slide was removed, rinsed with water to remove any unbound ruthenium complex and then stained with a 0.3% Gentian Violet solution (N-4[Bis[4-dimethylamino)-phenyl]methylene]-2,5-cyclohexadie n-1-ylidene]-N-methylmethanaminium chloride) in 20% ethanol by immersing in the dye for 5 minutes. Once again the slide was washed with water to remove any unbound dye. The whole process was repeated once more for a second identical slide and then twice more, but omitting immersing these two slides in the tris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride ) to give two un-sensitized control slides.

[0065] One each of the sensitised and un-sensitised slides was kept in total darkness. The other two slides were placed on top of an overhead projector fitted with two 24V 250 W tungsten halogen bulbs. Decolourisation of the purple colour was monitored as the Gentian Violet was decomposed photocatalytically. Within 50 minutes there was a noticeable difference between the colour of the sensitised and un-sensitised slides particularly when compared to the controls stored in the dark. On the slide with the sensitised titanium dioxide the Gentian Violet was decomposing and hence the purple colour fading. This continued until there was no purple colour left. Decomposition of the Gentian Violet did also occur on the un-sensitised slide but at a significantly slower rate.

Example 2

[0066] A nanocrystalline titanium dioxide sol thickened with methylcellulose was screen printed on to a series of cleaned glass microscope slides. The printed titanium dioxide films were then fired at 450° C. for 30 minutes. Half of the slides were then immersed in an aqueous 1×10 ⁻⁶ M solution of tris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride ) for 30 minutes to allow adsorption of the sensitising agent to the titanium dioxide. The slides were then removed from the sensitising solution and washed with water to remove any unbound ruthenium complex. All the slides, both sensitised and unsensitised were then divided into two groups. One set was immersed in 0.3% Gentian Violet solution (N-4-[Bis[4-dimethylamino)-phenyl]methylene]-2,5-cyclohexadi en-1-ylidene]-N-methylmethanaminium chloride) in 20% ethanol for 5 minutes and the second set into a 0.3% aqueous solution of Acid Orange dye (4-[(2-hydroxy-1-napthalenyl)azo]-benzenesulfonic acid monsodium salt) for 5 minutes. A sensitised and unsensitised slide dyed with either the Gentian Violet or Acid Orange stains was placed in total darkness and used as a control for each treatment. A second equivalent set was left exposed to daylight next to the window of a south-facing window. A third and final set was also left exposed to the daylight through a south-facing window but these slides were covered with a 6 mm thick piece of Perspex which substantially absorbs the UV component of the light. Decolourisation of both the purple and orange colours was monitored as both the Gentian Violet and Acid Orange were decomposed photocatalytically. After 48 hours exposure to light the slides dyed with Gentian Violet and left directly on the open bench were partially decolourised. The slides stored under the Perspex had begun to decolourize but at a slower rate than those not under Perspex. By day 7 the dye on all the slides left just on the bench had either completely or almost completely disappeared. The slides under Perspex reached the same amount of decolourization on day 14. There was no change in the colour of the slides stored in the dark. All the light exposed slides were re-dyed with either Gentian Violet or Acid Orange and treated exactly as before. None of the Acid Orange stained slides would re-stain. The sensitised titanium dioxide slides stained with Gentian Violet and exposed directly to daylight decolourised completely within 24 hours. The unsensitised slide had still not completely decolourised 5 days after restaining. The slides under Perspex were still coloured 5 days after restaining, however the sensitised slide had faded to a greater extent than the unsensitised slide.

Example 3

[0067] To 1.0 ml of a nanocrystalline titanium dioxide solution, 4.0 ml of a 3.4×10 ⁻⁵ M aqueous solution of tris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride ) were added and the resulting solution mixed well using a vortex mixer. A film of this solution was prepared by spin coating 0.1 ml of this solution at 1500 rpm on a clean glass microscope slide for 30 seconds. The film was dried using a hand held hot air drier and the process repeated twice more to give a total of 3 coats on the microscope slide. A second slide was then prepared in exactly the same way. Both slides were then immersed into a solution of 0.3% Gentian Violet (N4-[Bis[4-dimethylamino)-phenyl]methylene]-2,5-cyclohexadie n-1-ylidene]-N-methylmethanaminium chloride) in 20% ethanol for 5 minutes. The slides were removed, rinsed with water to remove any excess stain and allowed to air dry. One slide was kept in total darkness and the second was placed on top of a piece of 6 mm thick Perspex (to remove any UV light) on an overhead projector fitted with two 24V 250 W tungsten halogen bulbs. The purple colour on the light exposed slide steadily decomposed and after 3 hours had completely faded. There was no change in the colour of the slide stored in darkness.

Example 4

Titania Sols Preparation

[0068] Kormann Method. (C. Kormann D. W. Bahnemann, M. R. Hoffmann, J. Phys. Chem., 1988, 92, 5196)

[0069] TiCl ₄ (3.5 ml) was slowly added to cold de-ionised water (900 ml) under vigorous stirring. The resulting clear solution was stirred at 0° C. for 3 hours then dialysed between 2 hours and 24 hours. The clear solution is then dried using a rotary-evaporator (Temperature of water bath=30° C.). The resulting white powder (TiO ₂ ) is then re-suspended into de-ionised water at the desired concentration. The dialysis membrane-Visking-Size1350/1 MWCO 1350 Daltons was treated prior to use. The membrane was left for 30 minutes at 80° C. in a solution containing EDTA (1 mM) and 2% NaHCO ₃ . The membrane was then washed thoroughly with de-ionised water.

[0070] Method According to GB 1 412 937

[0071] An aqueous TiCl ₄ solution (50 ml of TiCl ₄ diluted in 500 ml de-ionised water) was added into a beaker containing de-ionised water (3 L) and concentrated ammonia (40 ml) with continuous stirring. The white mixture was stirred for about 20 minutes then allowed to settle. The supernatant was removed using a peristaltic pump. The volume was completed again to 3 L with de-ionised water, stirred then allowed to settle. The supernatant was removed. This process was repeated twice. The volume was completed with de-ionised water to 3.5 L. The mixture was stirred, the pH was checked (pH 8.8) then a nitric acid solution (1M) was added slowly to get pH close to 3.3. The mixture was stirred for 30-45 minutes then allowed to settle. The supernatant was removed and the volume was made up with de-ionised water to 3-3.5 L. The mixture was washed until the conductivity was below 500 μS. The supernatant was removed then nitric acid (1M, 23.2 ml) was added to the white mixture. The mixture was stirred for about 20 minutes then was left to age for about a week. In order to increase the peptisation step, the mixture can be heated gently to 60-70° C. for 30 minutes then allowed to settle.

[0072] Isopropoxide Route.

[0073] Titanium-isopropoxide (Aldrich, 400 ml, 97%) was added rapidly to a beaker containing de-ionised water (1 L). The precipitated TiO ₂ was decanted and washed 4 times with de-ionised water (4×500 ml) then filtered. The wet filtered solid was digested at 70° C. with concentrated nitric acid (16.7 ml) and de-ionised water (volume total 800 ml) for 30 to 1 h 30 min to produce a sol.

Example 5

[0074] Synthesis of Tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride)

[0075] RuCl ₃ ,xH ₂ O (1.33 mmol Ru), 1-methyl-2-pyrrolidinone (15 ml) and 2,2′-dipyridyl-4,4′-dicarboxylate (4.1 mmol) were added into a round bottomed flask and then purged with Ar or N ₂ . The mixture was heated to reflux in the dark for 1 h30 min. 1-methyl-2-pyrrolidinone (25 ml) was added to the flask and the reflux was continued for a further 2 hours under Ar or N ₂ . The mixture was allowed to cool to room temperature and kept under Ar or N ₂ overnight. The dark mixture was filtered. The resulting reddish brown solid was washed with 1-methyl-2-pyrrolidinone (2×20 ml) and diethyl ether (3×2 ml) then dried under vacuum. Yield=0.61 g. Product contained 1.2 moles of 1-methyl-2-pyrrolidinone.

Example 6

[0076] Activity Test

[0077] A mixture of TiO ₂ sol (1 ml, 1 g/L) and tris(2,2′-dipyridyl4,4′-dicarboxylate) ruthenium(II)(dichloride) (4 ml, c=3.4×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. The pH was recorded then the target dye gentian violet (GV dissolved in 20% ethanol solution, 0.05 ml or 0.08 ml, 0.03 wt/v %) was added and the mixture was stirred again. The initial colour of the sensitised TiO ₂ sol was yellow. Addition of gentian violet produced a purple colour at pH 3 and higher. A UV/Visible spectrum was taken at this stage. The vial containing the mixture was placed onto an overhead projector (2 cm height from the glass, in order to reduce heat). A UV/Visible spectrum was used to observe the colour change over a period of time at lamda max of the Target dye (Gentian violet or crystal violet)=588 nm in the white light spectrum. (OHP used: Model Ensign. Lamp:24V-250 W-3860 lux)

[0078] In order to get quantitative data a UV/Visible spectrum was taken at different times. The results are shown in FIG. 1 .

Example 7

[0079] Effect of Different Steps of a Preparation, Effect of Peptisation and pH Effect.

[0080] The “activity” has been tested at different stages of the preparation of the TiO ₂ by hydrolysis of TiCl ₄ (Kormann Method-see example 4 for details). The effect of peptisation has also been looked at. From the results shown in Table 1, the peptisation as well as the particle size seemed to have little effect on the activity.

[0081] The details of the different preparations as well as the activity test are summarised in Example 4 and 6. 1

Example 7b

[0082] The pH of the sensitised sol (TiO ₂ sol prepared by Kormann method) was found to be different at each step of the process. The results are summarised in Table 2. The pH was measured using a pH meter (HANNA Instruments-HI8424 microcomputer). 2

[0083] The results indicate that the activity may be related to pH.

Example 8

[0084] The Effect of pH on Activity of Various Sols

[0085] Four different sols have been tested at pH ranging from 2 to 7. They are:

[0086] (Sol 1) Hydrolysis of TiCl ₄ followed by a dialysis, dried on rotary-evaporator then re-suspended. The pH of the sol was adjusted with HCl (1M) or NaOH (0.01M).

[0087] (Sol 2) Hydrolysis of TiCl ₄ followed by a dialysis only. The pH of the sol was adjusted with HCl (1M) or NaOH (0.01M).

[0088] (Sol 3) Precipitation of titanium-isopropoxide followed by peptisation with nitric acid. The pH of the sol was adjusted with HNO ₃ (0.1M) or NaOH (0.01M).

[0089] (Sol 4) Precipitation of TiCl ₄ followed by preptisation with nitric acid. The pH of the sol was adjusted with HNO ₃ (0.1M) or NaOH (0.01M).

[0090] This experiment was carried out over a 2 hour period. All sols were prepared at 1 g/L and the amount of Gentian Violet (0.08 ml, 0.03 wt/v %) was kept the same for each type of sol. Most of the sensitised sols had a precipitate or were precipitating. However, the systems were still working even in presence of a precipitate. The “activity” was reduced with an increase of pH. The results are summarised in Table 3.

[0091] A mixture of TiO ₂ sol (1 ml, 1 g/L) and tris(2,2′-dipyridyl-4,4′-dicarboxylate) ruthenium(II)(dichloride) (4 ml, c=3.4×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. The pH was adjusted then the target dye gentian violet (GV dissolved in 20% ethanol solution, 0.08 ml, 0.03 wt/v %) was added and the mixture was stirred again. The initial colour of the sensitised TiO ₂ sol was yellow. Addition of gentian violet produced a purple colour at pH 2.5 and higher. Below pH 2.5, addition of gentian violet produced a blue-green colour. A UV/Visible spectrum was taken at this stage. The vial containing the mixture was placed onto an overhead projector (2 cm height from the glass, in order to reduce heat). A UV/Visible spectrum was used to observe the colour change over a period of time. (OHP used: Model Ensign. Lamp:24V-250 W-3860 lux). In order to get quantitative data a UV/Visible spectrum was taken at different times. 3

[0092] The results are also shown in Figure, FIG. 3 , and FIG. 4 , and FIG. 5 .

Example 9

[0093] Addition of Stabilisers

[0094] Buffer solutions were obtained by diluting the powder buffer (BDH chemicals) into the required amount of de-ionised water. Attempts to stabilise TiO ₂ sols prepared from the hydrolysis of TiCl ₄ (Kormann method) were made using buffer solution pH 7 and pH 9.2. Addition of buffer solution pH 7 into a TiO ₂ sol (10 ml, 1 g/L) produced a precipitate at pH 7. Addition of a buffer solution pH 7 or pH 9.2, de-ionised water and solid TiO ₂ produced cloudy solutions at different pHs (see Table 4). 4

[0095] Solutions (1) and (4) were found to be cloudier than solutions (2) and (3). The particle size was higher for (1) and (4) this may correspond to the cloudiness of the solutions. After 24 hours, a slight precipitate was observed in solutions (1) and (4).

[0096] Attempts were made to increase pH with sodium hydroxide solutions failed. Addition of acetylacetonate to a TiO ₂ sol produced a stable sol with a pH of 2.3-2.4. Increasing the pH with a sodium hydroxide solution produced precipitation at pH around 7.

[0097] The activity was tested for the sensitised solution (1), sensitised TiO ₂ sols at pH 5.0 and 8.8 (note: the pH was increased by addition of NaOH (0.01M)). The target dye decolourised quicker at pH 5 although for all three samples there was some target dye left not decolourised after 2 hours. The results are summarised in FIG. 6 .

[0098] Poly(vinyl alcohol) (PVA) was tested as a potential stabiliser for TiO ₂ sols. It was found that addition of a large excess or too little caused precipitation of the sols when the pH was increased with sodium hydroxide. PVA can be dissolved by sonication or by gentle heating in water then can be added to a TiO ₂ sol. Addition of PVA directly to a TiO ₂ sol, produced a precipitate.

[0099] The activity of a sensitised sol containing PVA at pH around 3 and 7 was tested. The results show that PVA and an increase in pH slowed down the activity.

Example 10

[0100] TiO ₂ : Sensitiser Ratio Effect

[0101] The ratio TiO ₂ :Sensitiser or TiO ₂ :Ru has been looked at for a particular TiO ₂ sol (Kormann method, TiO ₂ sol dialysed only). The sol tested was obtained from hydrolysis of TiCl ₄ followed by a dialysis. The experiment involved variation of ruthenium and kept the TiO ₂ fixed. The target dye was decolourised quicker in 1:6 TiO ₂ :Ru ratio than in 1:2 TiO ₂ :Ru ratio

[0102] The target dye gentian violet (0.05 ml, 0.03 wt/v %) was decolourised within 3 hours in 1:6 TiO ₂ :Ru ratio whereas in 1:4 TiO ₂ :Ru ratio and 1:2 TiO ₂ :Ru ratio the gentian violet decolourised within 4 and 5 hours, respectively.

Example 11

[0103] Light Source Effect.

[0104] A mixture of TiO ₂ sol (1 ml, 1 g/L) and tris(2,2′-dipyridyl-4,4′-dicarboxylate) ruthenium(II)(dichloride) (3 ml, c=3.4×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. Addition of gentian violet (0.05 ml, 0.03 wt/v %) produced a purple colour. A UV/Visible spectrum was taken at this stage. The vial containing the mixture was placed onto an overhead projector (2 cm height from the glass, in order to reduce heat) or in a light box. UV/Visible spectra were used to observe the colour change over a period of time. Normally all the activity work was done using an overhead projector as a light source. Other light sources such as daylight bulbs (40 W and 100 W), tungsten filament tube (35 W), tungsten filament bulb (100 W) and fluorescent tube (8 W) have also been investigated during this study. The results showed that the process still works with all light sources tried albeit much slower using daylight or tungsten filament bulbs than the light from an overhead projector because of the higher intensity of the overhead projector.

[0105] The results are shown in FIG. 7

Example 12

[0106] A range of dyes have been tested as potential sensitising agent. They include: copper or iron complexes containing sulfonated phtalocyanine ligands, silicon complex containing phtalocyanine ligand and ruthenium complexes containing bipyridyl or functionalised bipyridyl complexes (e.g: carboxylate, phosphonate) ligands and anions (e.g.: Cl, NCS) and rose bengal.

[0107] A mixture of TiO ₂ sol (1 ml, 1 g/L) and sensitizing agent (3 ml, c=3.4×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. Addition of gentian violet (0.05 ml, 0.03 wt/v %) was added as a target dye. A UV/Visible spectrum was taken at this stage. The vial containing the mixture was placed onto an overhead projector (2 cm height from the glass, in order to reduce heat). UV/Visible spectra were used to observe the colour change over a period of time. All dyes tested decolourised the target dye gentian violet at different rates.

Example 13

[0108] Activity of Different TiO ₂ Sols.

[0109] Several TiO ₂ sols have been tested for their activity including commercially available types from the Millennium Performance Chemicals, 85 Avenue Victor Hugo, 92563 Rueil-Malmaison Cedex, France. A mixture of TiO ₂ sol (1 ml, 1 g/L) and tris(2,2′-dipyridyl -4,4 ′-dicarboxylate)ruthenium(II)(dichloride) (4 ml, c=3.4×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. Addition of gentian violet (0.08 ml, 0.03 wt/v %) produced a purple colour. A UV/Visible spectrum was taken at this stage. The vial containing the mixture was placed onto an overhead projector (2 cm height from the glass, in order to reduce heat). UV/Visible spectra were used to observe the colour change over a period of time. The results are summarised in Table 5. 5

Example 14

[0110] Activity of Different Sols in Basic Medium.

[0111] The TiO ₂ sol (made from isopropoxide route) containing PVA (MW: 15,000) was prepared as follows. PVA (0.10 g, MW:15,000) was diluted in hot de-ionised water (50 ml) then allowed to cool to room temperature. A known amount of concentrated TiO ₂ sol was added to the PVA solution under vigorous stirring. The volume was completed to 100 ml with de-ionised water. Final TiO2 concentration 1 g/L.

[0112] Basic System no PVA.

[0113] A mixture of TiO2 sol (isopropoxide route, 10 ml, 1 g/L) and tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride) (40 ml, 3.4×10 ⁻⁵ M) was stirred using a stirrer hotplate. The pH was adjusted by addition of a sodium hydroxide solution (0.1M) to pH 10. Gentian violet (0.08 ml, 0.03 wt/v %) was added to the mixture (volume used: 5 ml).

[0114] Basic System with PVA (1).

[0115] A mixture of TiO ₂ sol containing PVA at pH 10.03 (1 ml, 1 g/L) and tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride) also at pH 10.1 (4 ml, c=3.4×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. The pH (9.85) was adjusted with a sodium hydroxide solution (0.1M) in order to get pH 10. Gentian violet (0.08 ml, 0.03 wt/v %) was added to the mixture.

[0116] Basic System with PVA (2).

[0117] A mixture of TiO2 sol containing PVA (10 ml) and tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride) (40 ml, c=3.4×10 ⁻⁵ M) was stirred using a stirrer hotplate. The pH was adjusted with a sodium hydroxide solution (0.1M) to pH 10. Gentian violet (0.08 ml, 0.03 wt/v %) was added to the mixture (volume used: 5 ml).

[0118] Millennium Basic System.

[0119] A mixture of Millennium TiO ₂ sol in Basic medium(1 ml, 1 g/L) and tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)dich loride (4 ml, c=3.4×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. The pH was adjusted with a sodium hydroxide solution (0.1M) to pH 10.

[0120] Addition of gentian violet (0.05 ml, 0.03 wt/v %) produced a purple colour in all systems. A UV/Visible spectrum was taken at this stage. The vial containing the mixtures were placed onto an overhead projector (2 cm height from the glass, in order to reduce heat). UV/Visible spectra were used to observe the colour change over a period of time. The results are summarised in table 6. 6

Example 15

[0121] A solution containing a TiO2 sol (Millennium TiO2 sol in basic medium, 10 g/L, 5.0 ml), tris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride) (1.8 ml, 3.4×10 ⁻⁵ M), Igepal® CO-720 (0.18 g) and de-ionised water (3.2 ml) was stirred for few minutes using a rotamixer. The pH was adjusted to 10 by addition of a sodium hydroxide solution (0.1M). The mixture was wrapped into some aluminium foil and left standing overnight to equilibrate. A solution containing a TiO2 sol (Millennium TiO2 sol in basic medium, 10 g/L, 5.0 ml), Igepal® CO-720 (0.18 g) and de-ionised water (5.0 ml) was stirred for few minutes using a rotamixer. The pH was adjusted to 10 by addition of a sodium hydroxide solution (0.1M).

[0122] Thin films of these solutions were prepared by spin coating 0.1 ml of these solutions at 100 to 500 rpm on a clean glass microscope slide for 80 seconds. The film was dried using a hot air gun and the process repeated to give a total of 2 coats on the microscope slide. A second slide was then prepared in exactly the same way. All slides were then immersed into a solution of 0.3% Gentian Violet in 20% ethanol for 5 minutes. The slides were removed, rinsed with water to remove any excess stain and allowed to air dry. One slide was kept in total darkness and the second was placed onto an overhead projector (Model Ensign. Lamp: 24V-250 W-3860 lux). The purple colour on the films faded after 3 hours 30 min. There was no change in the colour of the slide stored in darkness.

Example 16

[0123] The titania sols have been characterised by TEM (transmission electron microscopy). The samples were prepared by pipetting a few drops of the sol onto holey carbon films. Gold grids were used to avoid support corrosion. The microscope used was a Philips CM20, operated at 200 kV.The results are summarised in Table 7. 7

Example 17

[0124] Activity of Stabilised Sensitised Titania Versus Titania Only.

[0125] TiO ₂ Only with PVA.

[0126] The TiO ₂ sol (made from isopropoxide route) containing PVA (MW:15,000) was prepared as follows. PVA (0.10 g, MW:15,000) was diluted in hot de-ionised water (50 ml) then allowed to cool to room temperature. A known amount of concentrated TiO ₂ sol was added to the PVA solution under vigorous stirring. The volume was completed to 100 ml with de-ionised water. Final TiO2 concentration 1 g/L. A mixture of TiO ₂ sol containing PVA (1 ml, 1 g/L) and de-ionised water (4 ml) was stirred for about 1 minute using a rotamixer. Gentian violet (0.08 ml, 0.03 wt/v %) was added to the mixture.

[0127] Sensitised TiO ₂ with PVA-Acidic Medium.

[0128] The TiO ₂ sol (made from isopropoxide route) containing PVA (MW15,000) was prepared as follows. PVA (0.10 g, MW 15,000) was diluted in hot de-ionised water (50 ml) then allowed to cool to room temperature. A known amount of concentrated TiO ₂ sol was added to the PVA solution under vigorous stirring. The volume was completed to 100 ml with de-ionised water. Final TiO2 concentration 1 g/L. A mixture of TiO ₂ sol containing PVA (1 ml, 1 g/L) and tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride) (4 ml, c=4.3×10 ⁻⁵ M) was stirred for about 1 minute using a rotamixer. Gentian violet (0.08 ml, 0.03 wt/v %) was added to the mixture.

[0129] Sensitised TiO ₂ with PVA-Basic Medium.

[0130] A mixture of TiO2 sol containing PVA (10 ml) and tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride) (40 ml, c=3.4×10 ⁻⁵ M) was stirred using a stirrer hotplate. The pH was adjusted with a sodium hydroxide solution (0.1M) to pH 10. Gentian violet (0.08 ml, 0.03 wt/v %) was added to the mixture (volume used: 5 ml).

[0131] A UV/Visible spectrum is taken at this stage. The vials containing the mixtures were placed onto an overhead projector (2 cm height from the glass, in order to reduce heat). UV/Visible spectra were used to observe the colour change over a period of time. TiO2 without dye has significantly slower photocatalytic activity when compared with sensitised TiO ₂

Example 18

[0132] Breakdown of 4-chlorophenol (halogenated pollutant) Using TiO2.

[0133] A microscope slide containing a thin film of sensitised TiO ₂ was added into a solution of 4-chlorophenol (99+%, Aldrich, 8 ml, 10 ⁻⁴ M). The vial containing the solution and the slide was placed onto an overhead projector. The degradation of 4-chlorophenol was monitored using UV/Visible analysis. A spectrum was taken over a period of time at max of the 4-chlorophenol (=280 nm). The absorbance at 280 nm was decreasing over time.

Example 19

[0134] Demonstration of the Photocatalytic Nature of TiO2 in Addition to Sensitiser

[0135] A mixture of TiO ₂ sol (5 ml, 10 g/L), tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dic hloride) (1.8 ml, 3.4×10 ⁻⁵ M) and de-ionised water (3.2 ml) was stirred for about 1 minute using a rotamixer. Half of the volume of the mixture was used for testing. A target dye, gentian violet (0.08 ml, 0.03 wt/v %) was added to the mixture producing a blue-purple colour. A UV/Visible spectrum was taken. The vial containing the mixture was placed onto an overhead projector (2 cm height from the glass in order to reduce heat). A UV/Visible spectrum was taken over a period of time in order to observe the colour change. Once the spectrum showed no trace of target dye, the same amount of gentian violet was added to the same mixture. The all process was repeated twice. The target dye gentian violet was still decolourising on the third addition but at a slower rate than on the first addition.

Example 20

[0136] Zinc oxide was prepared according to the method outlined by Bahnemann et al, J. Phys. Chem., (1987), 91, 3789. The oxide suspension (made by stirring the ZnO solid into a sodium hydroxide solution at pH9) was then sensitised with 4,4′-dicarboxa late,tris (2,2′bibyridyl) Ru (II) dichloride according to the method outlined in previous examples. Gentian violet dye was added to both the sensitised sample and the non-sensitised control sample, and the UV/visible spectrum was recorded as a function of time under illumination with white light (5,000 lux). The results demonstrate that the absorption peak associated with Gentian Violet decreases faster with the sensitised ZnO compared to the control.

Example 21

[0137] The methods described below offer general synthetic pathways to sensitising agents specifically designed to function in combination with semiconductors where the desired operating condition is such that the un-coated semiconductor surface presents a significant number of adsorption sites with a negative charge. Typical positively charged groups for use as binding sites include, but are not limited to R ₄ N+ groups and R ₄ P+ groups, where R is as hereinbefore described

[0138] Novel Dyes Based on Terpyridyl

[0139] Terpyridyl-based sensitisers with phosphonate chelating ligands (Graetzel at al, WO 95/29924) and with other ligand types (Graetzel et al, WO 94/0449) have been used in conjunction with titania in dye-sensitised solar cells. The terpyridyl group of general formula I can be synthesised with e.g. R1 as a positively charged unit. One example where R5-7 of formula II are methyl, is synthesised according to procedures where the intermediate is made by the method outlined in Recl.Trav. Chim. Pays. Bas, 1959, v78, 408. This nitrated aryl group is then changed into the terpyridyl unit by the method outlined by McWhinne et al (J Organoetallic chem.., 1968, v11, 499). The nitro group is then reduced to the amine by hydrazine hydrate under Pd/C catalysis followed by reaction with excess methyl iodide to form the quaternary nitrogen terpyridyl ligand desired.

[0140] Yet another variant of the positively charged terpyridyl molecule [described by general formula I] can be synthesised by reacting 2-acetylpyridine with 4-nitrobenzaldehyde in base followed by ring closure with ammonium acetate according to methods outlined by E Constable et al (J Chem Soc Dalton Trans, 1992, 2947), followed by reduction of the nitro group to the amine and quaternisation as described previously to form a compound described by formula I with R2 and R 3 as hydrogen and R1 as 6

[0141] Yet another general preparative method for a terpyridal group of general formula I is based on derivatising the structure below (Potts et al, JACS 1987, v109, 3961) by oxidising it to the carboxylic acid by e.g. the methodology outlined by Dodd et al (Synthesis (1993), V3, 295). Amination of the carboxylate is then carried out by standard procedures outlined in e.g. Chem Rev. (1981) V81, p589. 7

[0142] Bipyridyls

[0143] The synthesis of a compound of general formula III with R8,9=NH2 is described in (JACS 1958, V80, 2745) and (J Chem Soc Perkin Trans 2, 1996, 613) and a compound of formula III can be quarternised by the preceding methods outlined for terpyridyls.

[0144] Phthalocyanines

[0145] Phthalocyanine dyes can be synthesised with amine nitrogen groups by e.g. Buchwald ammination of halide precursors to produce outer-ring derivatives such as (J Org Chem 2000, V65, 1158), the amine groups of which are then quarternised.

[0146] Tetra-aza-annulenes (TADAs)

[0147] TADAs of the general formula V can be derivatised at R1, R2, R3, and R4 by the general methods outlined above.

[0148] Dimers

[0149] The bipyridyl compounds tris(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) dichloride and tris(2,2′-bipridyl) ruthenium (II) dichloride can be dimerised using pyrazine derivatives such as pyrazine, pyrimidine and 4,4′-bipyridyl linking ligands according to procedures detailed in (E A Seddon & K R Seddon, The Chemistry of Ruthenium, Elsevier, New York 1984, p 436).