WO2012051641A1

WO2012051641A1 - Metal oxide particles

Info

Publication number: WO2012051641A1
Application number: PCT/AU2011/000163
Authority: WO
Inventors: Lianzhou Wang; Xia WU; Gao Qing Lu
Original assignee: University Of Queensland
Priority date: 2010-10-22
Filing date: 2011-02-16
Publication date: 2012-04-26

Abstract

Disclosed herein are methods for producing metal oxide (e.g. TiO₂) particles comprising an inner metal oxide species within an outer metal oxide shell. The methods comprise heating an aqueous mixture comprising a metal oxide precursor (e.g. a titanium compound) and a carbonaceous substance capable of forming a carbonaceous template to a temperature at which the metal oxide precursor, water and carbonaceous substance react to produce composite particles comprising a carbonaceous template layer interposed between an inner metal oxide species and outer metal oxide shell. Also disclosed are TiO₂ particles comprising an inner TiO₂ species within an outer TiO₂ shell.

Description

_±

- 1 -

METAL OXIDE PARTICLES FIELD OF THE INVENTION

The present invention relates to metal oxide particles and to uses of those metal oxide particles.

BACKGROUND

Hollow metal oxides with well-defined architectures have recently drawn growing attention in a variety of research areas including catalysis, adsorption, micro-reactors and drug delivery due to their low density, high specific surface area, light scattering and harvesting properties and nanoporous structure. Hollow Ti0₂ microspheres, in particular, have a low density, high surface area, good surface permeability as well as a high light-harvesting efficiency. These properties may enable such particles to be useful in a variety of applications such as in water treatments, photocatalysis, photovoltaic devices and organic pollutant degradations.

SUMMARY OF THE INVENTION

The inventors have discovered that it is possible to produce metal oxide particles having a structure in which an outer metal oxide shell surrounds an inner metal oxide species (which may itself be a metal oxide shell). The inventors have produced multi-layered Ti0₂ particles having such a structure. To the best of the inventors' knowledge, Ti0₂ particles having such a structure are unique. The inventors' initial investigations indicate that these particles have a number of advantageous properties. The inventors have discovered that it is possible to synthesise these multi-layered metal oxide particles using a facile "one-pot" hydrothermal method. As will be appreciated, synthesis of any product via a "one-pot" method is highly desirable because of its relative simplicity. Accordingly, in a first aspect, the present invention provides a method for producing metal oxide particles comprising an inner metal oxide species within an outer metal oxide shell. In the method, an aqueous mixture comprising a metal oxide precursor and a carbonaceous substance capable of forming a carbonaceous template is heated to a temperature at which the metal oxide precursor, water and carbonaceous substance react to produce composite particles comprising a carbonaceous template layer interposed between an inner metal oxide species and outer metal oxide shell. _X

- 2 -

The method can be used to produce multi-layered Ti0₂ particles. The inventors believe that this method could also be used to produce multi-layered particles of metal oxides such as vanadium oxide, chromium oxide, manganese oxide, silicon dioxide, iron oxide, copper oxide and nickel oxide.

In some embodiments, the composite particles are subsequently calcined to remove the carbonaceous template.

The inner metal oxide species may be a particle of any shape. In some embodiments, the inner metal oxide species is also a metal oxide shell. Such metal oxide particles have a "shell in shell" structure and, if calcined, comprise a hollow metal oxide shell within an outer metal oxide shell.

In some embodiments, at least the outer metal oxide shell is a microsphere.

,

In a second aspect, the present invention provides a metal oxide particle produced by the method of the first aspect.

In a third aspect, the present invention provides a method for producing Ti0₂ particles comprising an inner Ti0₂ species within an outer Ti0₂ shell. In the method, an aqueous mixture comprising a titanium compound and a carbonaceous substance capable of forming a carbonaceous template is heated to a temperature at which the titanium compound, water and carbonaceous substance react to produce composite particles comprising a carbonaceous template layer interposed between an inner Ti0₂ species and outer Ti0₂ shell.

In some embodiments, the composite particles are subsequently calcined to remove the carbonaceous template. The titanium compound may, for example, be a titanium halide such as TiBr₄

TiCl4,TiCl3, TiF₃, TiF₄ or T1I4, titanium nitride (TiN), titanium carbide (TiC), titanium cyanide (TiCN), titanium diboride (TiB₂), titanium sulphide (TiS₂), a titanium oxide or hydroxide such as TiO, Ti₂0₃, Ti₃0₅, Ti0₂ or Ti(OH)₄.xH₂0, TiO_xN_y, TiO_xC_y, titanium phosphide (TiP), a titanium sulphate such as Ti₂S0₄.zH₂0, Ti₂(S0₄)₃ or

TiOS0₄.zH₂S0₄, titanium silicide (TiSi₂) or an organic titanium compound such as Ti(OCH(CH₃)₂)₄, Ti[0(CH2)₃CH₃]₄, Ti(OCH₃)₄.(CH₃OH)_z (where x and y are ,

- 3 - independently integers from 0 to 2 and z is an integer from 0 to 8) or a combination thereof.

The carbonaceous substance may, for example, be C₆Hi₂N₄ (hexamine), C0(NH₂)₂ (urea), CS(NH₂)₂ (thiourea), triethylamine, (NH₄)₂C0₃ (ammonium carbonate)_,'

C₂₅H₃iN₃ (4,4',4"-methylidynetris(N,N-dimethylaniline), Ci₂H₂₂0n (sucrose), C₆Hi₂0₆ (glucose), Q5H30O5 (2,7-di-tert-butyl-9,9-dimethyl-4,5-xanthenedicarboxylic acid), C₆H_l2 (methylcyclopentane), C₆Hi₂0₂, C₆Hi₂BN0₃ (boric acid), C₇H₅BF₄0₂, C₇H₇B0₄ (3-carboxyphenylboronic acid), C₇H₇S0_2S C₇H_i20₂S, C6H4S, C₄C1₂F₆ (2,3- dichlorohexafluoro-2-butene), C₄H₂F₂N₂, C₄H₈BrF, C H₉I, C₅H₃I0₂, C₅H₃FI, C₆H,₃I (1- iodohexane) or a combination thereof.

In some embodiments, the inner Ti0₂ species in the Ti0₂ particle is also a shell. Such Ti0₂ particles have a "shell in shell" structure and, if calcined, have a hollow Ti0₂ shell within an outer Ti0₂ shell.

In some embodiments, at least the outer Ti0₂ shell is a microsphere.

In a fourth aspect, the present invention provides a Ti0₂ particle produced by the method of the third aspect.

In a fifth aspect, the present invention provides a Ti0₂ particle comprising an inner Ti0₂ species (e.g. a Ti0₂ shell) within an outer Ti0₂ shell (e.g. a Ti0₂ microsphere). The inventors have found that Ti0₂ particles produced by the method of the present invention have excellent light scattering and harvesting properties, which makes them promising candidates for use in the photoanode of a dye-sensitized solar cell.

Accordingly, in a sixth aspect, the present invention provides the use of Ti0₂ particles produced by the method of the third aspect or Ti0₂ particles of the fourth or fifth aspect (which have been calcined) in a dye sensitised solar cell.

In a seventh aspect, the present invention provides a dye sensitised solar cell comprising a conducting layer comprising Ti0₂ particles produced by the method of the third aspect or Ti0₂ particles of the fourth or fifth aspect (which have been calcined). J

- 4 -

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a schematic of a proposed reaction mechanism for the formation of hollow shell-in-shell Ti0₂ spheres in accordance with an embodiment of the method of the present invention;

Figure 2 shows SEM (a-d) and TEM (e-h) images of the samples prepared with the following sucrose:TiF₄ molar ratios: (a) and (e) 1 : 1; (b) and (f) 5:1 ; (c) and (g) 10:1; (d) and (h) 15: 1; and

Figure 3 shows UV-visible spectra of various Ti0₂ particles in accordance with embodiments of the present invention and commercially available P25 Ti0₂ particles, as well as a schematic illustration of (a) light pathway through P25 Ti0₂ nanoparticles and (b) multi-reflections within Ti0₂ particles in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, in one broad form, the present invention provides a method for producing metal oxide particles (e.g. titanium dioxide, vanadium oxide, chromium oxide, manganese oxide, silicon dioxide, iron oxide, copper oxide and nickel oxide) comprising an inner metal oxide species (e.g. a metal oxide shell) within an outer metal oxide shell. In the method, an aqueous mixture comprising a metal oxide precursor and · a carbonaceous substance capable of forming a carbonaceous template is heated to a temperature at which the metal oxide precursor, water and carbonaceous substance react to produce composite particles comprising a carbonaceous template layer interposed between the inner metal oxide species and outer metal oxide shell. The metal oxide precursor may be any compound containing the metal that is capable of forming a metal oxide when heated in the presence of water.

The invention will be discussed below in the context of a method for producing Ti0₂ particles comprising an inner Ti0₂ species (e.g. a Ti0₂ shell) within an outer Ti0₂ shell. As those skilled in the art will appreciate, many other metal compounds will behave in a similar manner to titanium compounds under hydrothermal conditions. It is within the ability of a person skilled in the art to adapt the methods discussed below in order to produce particles of other metal oxides comprising an inner metal oxide species within an outer metal oxide shell.

In the method for producing Ti0₂ particles comprising an inner Ti0₂ species (e.g. a Ti0₂ shell) within an outer Ti0₂ shell, an aqueous mixture comprising a titanium compound and a carbonaceous substance capable of forming a carbonaceous template is heated to a temperature at which the titanium compound, water and carbonaceous substance react to produce composite particles comprising a carbonaceous template layer interposed between an inner Ti0₂ species and outer Ti0₂ shell.

The Ti0₂ particles produced by the method of the present invention are potentially useful in photocatalysis, solar cells and water purification processes, amongst other uses. Depending on the intended use of the Ti0₂ particles produced by the method of the present invention, the composite particles may subsequently be calcined to remove the carbonaceous template (e.g. if the intended use of the particles is in a dye sensitised solar cell). However, it may not be necessary to calcine the composite particles in some circumstances, for example, if the particles are intended for use as a carbon-based absorbent.

The titanium compound may be any titanium compound that will hydrolyse to form Ti0₂. Suitable titanium compounds include titanium halides such as TiBr₄ TiCU,TiCl₃, TiF₃, TiF₄ and T1I4, titanium nitride (TiN), titanium carbide (TiC), titanium cyanide (TiCN), titanium diboride (TiB₂), titanium sulphide (TiS₂), titanium oxides or hydroxides such as TiO, Ti₂0₃, Ti₃0₅, Ti0₂ and Ti(OH)₄.xH₂0, TiO_xN_y, TiO_xC_y, titanium phosphide (TiP), titanium sulphates such as i₂S0₄.zH₂0, Ti₂(S0₄)3 and TiOS0₄.zH₂S0₄, titanium silicides (TiSi₂) and organic titanium compounds such as Ti(OCH(CH₃)₂)4, Ti[0(CH₂)₃CH₃] and Ti(OCH₃)₄.(CH₃OH)_z (where x and y are independently integers from 0 to 2 and z is an integer from 0 to 8).

TiF₄ and TiOS0 are preferred titanium compounds because they have a relatively slow hydrolysis rate, which enables the reaction process to be more easily controlled, as well as being readily available and relatively cheap.

The carbonaceous substance may be any carbon containing substance that is capable of forming a carbonaceous template when heated in the presence of water. The

( ..

- 6 - cafbonaceous substance may, for example, undergo condensation, polymerisation and carbonization reactions in order to self assemble into the carbonaceous template.

Carbonaceous substances that are "carbon-rich" (i.e. contain a relatively high proportion of carbon) are preferred. Preferred carbonaceous substances are capable of undergoing condensation and hydrolysis reactions when heated in the presence of water. It is also preferable that the carbonaceous substance is capable of undergoing reactions to form a polymeric material when heated in the presence of water. Examples of specific carbonaceous substances suitable for use in the present invention include the compounds C6H12N4 (hexamine), CO(NH₂)2 (urea), CS(NH₂)₂ (thiourea), triethyl amine, (NH₄)₂C0₃ (ammonium carbonate), 5H31N3 (4,4',4"-methylidynetris(N,N- dimethylaniline), C₁₂H₂₂0ii (sucrose), C₆Hi₂06 (glucose), 5H30O5 (2,7-di-tert-butyl- 9,9-dimethyl-4,5-xanthenedicarboxylic acid), C₆H]₂ (methylcyclopentane), C6H_t20₂, C₆Hi₂BN0₃ (boric acid), C7H₅BF₄0₂, C₇H₇B0₄ (3-carboxyphenylboronic acid), C₇H₇S0₂, C₇H]₂0₂S, C6H4S, C₄C1₂F₆ (2,3-dichlorohexafluoro-2-butene), C₄H₂F₂N₂, C₄HgBrF, C4H9I, CsH₃l0₂, C5H3FI, C₆Hj3l (1-iodohexane). In some embodiments, two or more carbonaceous substances (e .g. two or more of the specific compounds listed above) could be used. Sucrose and glucose are preferred carbonaceous substances because they are readily available and relatively cheap.

Without wishing to be bound by theory, the inventors believe that under the applied hydrothermal conditions, the specific carbonaceous substances listed above undergo multi-step processes involving the dehydration, polymerization and carbonization of the carbonaceous substance, leading to the chemically induced self-assembly of carbonaceous templates. The surfaces of these carbonaceous templates in aqueous solution can be hydrophilic, having a distribution of -OH and -C-0 groups formed from non- or partially-dehydrated carbonaceous substances.

The hydrolysis of the titanium compound to form Ti(OH)_x and subsequently Ti0₂ is thought to proceed at a different rate to the reactions involving the carbonaceous substance, and to be highly dependent on the conditions in the aqueous mixture or solution (e.g. the pH and presence of other species). It is thought that as the relative proportions of the carbonaceous substance and titanium compound in the aqueous mixture changes, the rates of the dehydration, polymerization and carbonization reactions of the carbonaceous substance become comparable to the rates of hydrolysis _X

- 7 - of the titanium compound, at which time reactions where the titanium compound hydrolyses to form Ti(OH)_x become favoured over reactions involving the

carbonaceous substance. If carbonaceous templates are not present it is thought that the hydrolysis of the titanium compound will produce particles of Ti(OH)_x (and subsequently Ti0₂). However, if the carbonaceous templates have been formed before the reactions where the titanium compound forms Ti(OH)_x become favoured, it is thought that the hydrophilic groups on the surface of the carbonaceous templates act as "nuclei sites", resulting in the condensation and formation of a layer of Ti(OH)_x on the outside wall of the

carbonaceous templates.

As the Ti(OH)_x forms on the surface of the carbonaceous templates, the relative proportions of the carbonaceous substance and titanium compound in the aqueous mixture changes and reactions of the carbonaceous substance again become favoured. The -OH groups of substances formed via the dehydration, polymerization or other reactions of the carbonaceous substance present in the aqueous mixture can then react with and form a layer of the carbon template on the Ti(OH)_x layer. Subsequent further hydrolysis of the titanium compound can then cause another Ti(OH)_x layer to form.

The inventors believe that this process may be repeated a number of times under the applied hydrothermal conditions in order to form Ti0₂-carbon composites having numerous interposed Ti0₂/Ti(OH)_x and carbonaceous template layers. The method can therefore be used to produce Ti0₂ particles having more than 2 layers of Ti0₂. The inventors believe that Ti0₂ particles having variable structures can be prepared simply by adjusting the concentrations, relative proportions and types of the titanium compound and carbonaceous substance in the aqueous mixture. Adjusting the hydrothermal conditions to which the aqueous mixture is exposed will also affect the structure of the resultant Ti0₂ particles.

A schematic of a proposed reaction mechanism for the formation of hollow shell-in- shell Ti0₂ spheres in accordance with an embodiment of the present invention is shown in Figure 1. Typically, the inner Ti0₂ species is a shell, resulting in a hollow "shell-in-shell" structure if calcined. Such multi-shell Ti0₂ particles have significantly improved light harvesting efficiency over known Ti0₂ particles because of the light confinement within the hollow structure. As such, these multi-shell Ti0₂ particles may be used in applications which depend on the light-harvesting ability of the particles, such as dye sensitised solar cells and photocatalysts. The inner Ti0₂ species/shell and the outer Ti0₂ shell are typically roughly spherically shaped. Spherical Ti0₂ particles are useful because they are easy to process. However, by using specific carbonaceous substances and/or altering the reaction conditions it may be possible to produce Ti0₂ particles in which the inner Ti0₂ species and the outer Ti0₂ shell have non-spherical shapes, such as rods, cubes, plates, fibres or irregular shapes.

At least the outer Ti0₂ shell of the Ti0₂ particles may be a microsphere (i.e. it has a diameter in the micrometre range). Such particles are useful because the internal diameters of the microspheres allow multiple reflections of UV-visible light (especially visible light) within the interior cavity, which potentially enhances the light harvesting ability of the particles. The light weight of such particles is also useful in water treatment applications. The inner Ti0₂ species/shell may also have a diameter in the micrometre range or a diameter in the nanometre range.

The molar ratio of the titanium compound to the carbonaceous substance in the aqueous mixture can affect the structure of the resultant Ti0₂ particles (e.g. the thickness of the shell(s) in the Ti0₂ particles and the diameter or shape of the inner and outer Ti0₂ shells/species). The molar ratio of the titanium compound to the carbonaceous substance may be any molar ratio which results in the production of Ti0₂ particles comprising an inner Ti0₂ species within an outer Ti0₂ shell. The effective molar ratio will depend on the types of titanium compound and carbonaceous substance, as well as the hydrothermal conditions to which they are exposed. Exemplary molar ratios of the titanium compound to the carbonaceous substance in the aqueous mixture are from about 1 :45 to about 10:1 (e.g. from about 1 :5 to about 1 :20). Specific molar ratios of titanium compound to carbonaceous substance are 1 :5, 1 :10, 1 :15, 1:20 and 1 :30.

The molar ratio of the titanium compound to the water in the aqueous^' mixture can also affect the structure of the resultant Ti0₂ particles. The inventors believe this is due, at least in part, to the effect the concentration has on the pH of the aqueous mixture. The molar ratio of the titanium compound to the water may be any ratio effective to result in the production of Ti0₂ particles comprising an inner Ti0₂ species within an outer Ti0₂ shell. Molar ratios of titanium compound to water may, for example, be from about 1 : 1,500 to about 1 :400, which equates to the concentration of the titanium compound in water being from about 0.041mol/L to about 0.125mol/L. Such concentrations are believed to provide a pH suitable for the competing reactions that result in the formation of the carbonaceous template and Ti(OH)_x/Ti0₂ layers. The aqueous mixture can be formed by mixing the titanium compound, carbonaceous substance and water in any order. In embodiments where the titanium compound and carbonaceous substance are water soluble, then an aqueous solution will be formed on mixing. The aqueous mixture/solution may be formed my adding an aqueous mixture/solution including the titanium compound to an aqueous mixture/solution including the carbonaceous substance. Alternatively, the titanium compound and carbonaceous substance can be added to a desired volume of water either sequentially or at the same time.

The aqueous mixture comprising a titanium compound and a carbonaceous substance may, for example, be heated to a temperature of between about 100°C and about 250°C (e.g. between about 150°C and about 200°C, or to about 190°C) in order to cause the titanium compound, water and carbonaceous substance to react. Typically, the mixture will be pressurised so that the water remains in a liquid form. The inventors have found that heating the aqueous mixture to a temperature of about 190°C results in the formation of particles having an outer Ti0₂ shell with a diameter of about 1 -2 microns. Particles having such a size can have better light scattering effects than particles having other sizes, and are therefore advantageous in certain applications. If the aqueous mixture was heated to a temperature below 100°C, the inventors expect that the particle size of any particles formed would be much smaller. If the aqueous mixture was heated to a temperature above 250°C, the inventors expect that the particle size of any particles formed would be much bigger. Particles that are much bigger or smaller than about 1 -2 microns may not be suitable for some applications (e.g. in dye sensitised solar cells).

Furthermore, if the aqueous mixture was heated at a temperature of less than about 100°C, the reaction time would probably need to be significantly extended in order to produce a sufficient amount of appropriately sized particles.

The aqueous mixture comprising a titanium compound and a carbonaceous substance may, for example, be heated for a time of between about 12 hours and about 36 hours (e.g. about 24 hours). These times have been found to be sufficient to enable the titanium compound, water and carbonaceous substance to react to produce the desired composite particles. . U2011/000163

10

Typically, the aqueous mixture comprising a titanium compound and a carbonaceous substance will be poured into an autoclave, which is subsequently heated to the desired temperature for the desired length of time. The autoclave would typically then be allowed to slowly cool to room temperature.

As discussed above, in some embodiments, the composite Ti0₂-carbon particles are calcined to remove the carbonaceous templates, thus leaving just the Ti0₂ components of the particles. The composite Ti0₂-carbon particles may, for example, be calcined by heating them in air to a temperature in the range of about 400°C to about 600°C for about 3 to about 5 hours (e.g. to a temperature of about 550°C for about 4 hours).

The composite particles may be heated up to the final temperature using a ramping rate of about 5°C per minute. The particles would then typically be allowed to cool to room temperature at a natural rate (i.e. simply by removing the source of heat).

As discussed above, the inventors have found that the Ti0₂ particles of the present invention have excellent light scattering properties, which makes them promising candidates for use in the photoanode of a dye-sensitized solar cell. A dye-sensitized solar cell utilising the Ti0₂ particles of the present invention was prepared in the manner described below.

To prepare working electrodes for the dye sensitised solar cell, fluorine-doped tin oxide (FTO) glass (2.3 mm thickness, δΩ sq, Dyesol) as current collector was first cleaned with 2-propanol using an ultrasonic bath for 30min, and then thoroughly rinsed with water.

Ti0₂ paste for coating the FTO glass was prepared in the following manner. 5 g of a suspension of 10 wt. % ethyl cellulose in ethanol was added to a round bottomed flask containing 1 g of pure Ti0₂ (either P25 - the benchmark commercial Ti0₂ nanoparticles routinely used in dye sensitised solar cells, or Ti0₂ particles of the present invention, prepared as discussed below) and 4g of terpineol, and diluted with 20 ml of ethanol. This mixture was sonicated using an ultrasonic probe and then stirred, and the sonication and stirring steps were then repeated three times. The ethanol was removed by rotary-evaporator. The resulting slurry was casted on the FTO glass plates by a doctor-blade method, and kept in a clean box for 30min so that the paste can relax to reduce the surface irregularity and mechanical stress, and then dried at 100°C for 6min. The above procedure (i.e. coating, storing and drying) was repeated a number of times in order to increase the thickness of the Ti0₂ working electrode. In the earlier repetitions of this procedure, P25 Ti0₂ particles were used, but in the later repetitions, Ti0₂ particles in accordance with the invention were used. The resultant electrode has a bilayer structure, in which a layer of Ti0₂ particles in accordance with the invention are overlaid over a layer of the P25 Ti0₂ particles.

As will be appreciated, the dye-sensitised solar cell need not comprise such a bilayer structure. Indeed, Ti0₂ particles in accordance with the invention could be used on their own to coat the electrode. Alternatively, multiple layers of the Ti0₂ particles of the invention could be interposed with layers of the P25 Ti0₂ particles (or indeed, any other commercially available substances that have light scattering properties which can be used in dye sensitised solar cells). The Ti0₂ films were cut into 4x4mm squares and then heated at 550°C for 30min. When the films had cooled to 80°C, they were immersed into a 0.5mM N719 dye solution (Dyesol) in a mixture of acetonitrile and tert-butanol (volume ratio: 1 : 1) and kept at room temperature for 24 h to complete the sensitizer uptake. Subsequently, the dye-covered Ti0₂ electrode and a Pt-counter electrode (Dyesol) were assembled into a sandwich type cell and sealed with a thermoplastic membrane having a thickness of

30um (Surlyn, DuPont). In the sealed cell, a drop of electrolyte solution was introduced via vacuum backfilling. Finally, the hole was sealed using an Al-backed thermoplastic membrane (Bynel, DuPont). A preferred embodiment of the present invention will now be described in which the titanium compound is TiF₄ and the carbonaceous substance is sucrose.

For the samples discussed below, an aqueous solution (Milli-Q water, 18.2 ΜΩ cm) containing TiF₄ (Aldrich) was added to an aqueous solution containing sucrose (Chem- Supply) in a Teflon-lined stainless steel autoclave. The combined solutions were hydrothermally treated by heating the autoclave to 190°C for 24 hours and the autoclave was then allowed to naturally cool to room temperature. The resulting black precipitate was collected and washed several times with deionised water and ethanol, and finally dried in an oven at 50°C overnight. The hydrothermal reaction produced a spherical Ti0₂-carbon composite through the condensation, polymerization and carbonization of sucrose and simultaneous hydrolysis of the TiF₄ precursor. The spherical Ti0₂-carbon composite was subsequently calcined in air by heating it up to a temperature of about 550°C at a ramp rate of about 5°C for about 4 hours, which caused the removal of the carbon from the composite particles and the formation of pure Ti0₂ hollow microspheres with the unique "shell-in-shell" architecture discussed above.

Various aqueous solutions were prepared in which the molar ratio of TiF₄: sucrose :H₂0 was varied in the range of (0.33-l):(0.5-15):438.5. These solutions were subjected to the conditions described above to produce hollow shell-in-shell Ti0₂ microspheres. The morphologies of these particles were observed by using scanning electron microscopy (SEM, JEOL 6610) and transmission electron microscopy (TEM, JEOL 1010 at 100 kV).

Figure 2 presents the morphologies of Ti0₂-carbon composite particles and Ti0₂ hollow shell-in-shell spheres prepared as discussed above. As can clearly be seen, the resultant Ti0₂ particles are roughly spherically shaped under all the synthesis conditions. TEM images show that the Ti0₂ particles produced using aqueous mixtures comprising a low sucrose:TiF₄ ratio (1 :1) were solid particles (Figure 2e). Similar results were also observed for sucrose:TiF ratios of 1 :0.5 and 1 :2 (results not shown). However, when the sucrose:TiF₄ ratio was increased to 5:1 , as shown in Figure 2f, the TEM image shows noticeable hollow structures with a distinguishable smaller shell inside. Further increasing the sucrose:TiF ratio (10:1 and 15:1) led to similar final products that also had shell-in-shell hollow structures (Figure 2g and h, respectively). Similar results are observed when the Ti concentration in the solution is decreased with a constant concentration of sucrose (results not shown).

The inset in Figure 2 (h) is a magnified image of the shell and indicates the crystalline structure of the hollow outer sphere. The Ti0₂ hollow microspheres can be seen to have a nanocrystalline structure, which makes it suitable for use in a charge carrying material.

In.the following discussion, the abbreviations SI, S2 and S3 will be used to refer to Ti0₂ particles formed in accordance with the method of the present invention from an aqueous mixture comprising sucrose and TiF₄ in the ratios of ratio of 5:1, 10:1 and 15: 1 (sucrose :TiF₄), respectively. As can be seen in Figures 2b, 2c and 2d, the Ti0₂ microspheres exhibit quite uniform spherical shapes with diameters in the range of 1 - 2μπι for SI and S2 and 1.5-2.5 μπι for S3. The diameters of the inner shells of SI, S2 _J

- 13 - and S3 were similar, in the range of -0.5 μηι. Comparing Figures 2b and 2d, broken hollow spheres with a thinner shell can be seen in S3. The thickness of the outer Ti0₂ shells observed in the TEM images is around 140 nm for SI, 110 nm for S2 and 50 run for S3. This suggests that a lower TiF₄ to sucrose ratio leads to a lighter packing of the Ti0₂ particles, resulting in a thinner shell. An increase in the TiF₄ ratio results in a much denser packing and the formation of a robust and thicker shell.

The above results indicate that the ratio of sucrose to TiF₄ in the reactants plays a significant role in controlling the architecture of the resultant Ti0₂ particles. When the ratio of sucrose is relatively low, hollow structures are less likely to be formed.

However, by increasing the ratio of sucrose to five fold or more of TiF₄, Ti0₂ particles having a "shell-in-shell" hollow architecture can be prepared. Moreover, by varying the sucrose to TiF₄ ratio or by tuning the Ti concentration in the solution, the shell thickness of the hollow structured spheres can be easily tailored.

In order to determine whether the Ti0₂ particles of the present invention would be useful in a dye sensitised solar cell, the following experiments were performed.

The light absorption and scattering properties of the SI, S2 and S3 Ti0₂ particles were investigated by UV-visible light absorption/diffused reflectance spectrometer

(Shimadzu UV-2450), and compared to those of the commercially available P25 Ti0₂ nanoparticles (the benchmark Ti0₂ nanoparticles routinely used in dye sensitised solar cells). The UV-visible spectra are depicted in Figure 3. The estimated band gaps of S 1 , S2 and S3 are 3.14, 3.13 and 3.08eV according to Tauc plot, respectively, while the band gap of P25 is around 3.14eV. The slight decrease in the band gap energy of the Ti0₂ samples of the present invention might be due to the increased ratio of rutile content. The absorption spectra of all the Ti0₂ samples of the present invention exhibit a stronger absorption in the UV-visible range (350-700nm) than that of P25. As is schematically illustrated in the inset, the shell-in-shell hollow structure with an appropriate inner sphere diameter (insert b) would allow multiple reflections of UV- visible light within the interior cavity, thus enhancing the light harvesting and therefore offering an improved absorption property than P25 (insert a).

-

In dye sensitised solar cells, the amount of dye that is able to be loaded on the Ti0₂ electrode affects the amount of photogenerated electrons, and eventually the overall conversion efficiency. The saturation adsorption capacity of N719 dye (cis- bis(isothiocyanato)bis(2, 2'-bipyridyl-4, 4'-dicarboxylato)ruthenium (II) bis- tetrabutylammonium, Dyesol) on Ti0₂ particles of the present invention was therefore compared with that on P25 particles. The dye uptake ability was measured by soaking the Ti0₂ particles in 0.5mM N719 solution at room temperature for 24h. The solid inorganic materials and dye were separated and the concentration of the residual dye in solution was measured by UV-Vis spectrophotometer (Shimadzu UV-2450).

For the P25 Ti0₂ particles, the saturation adsorption capacity was estimated to be very high: 4.3 x 10^'5mol/g. The saturation adsorption capacity of SI , S2 and S3 particles was found to be 3.0x ] 0^"s, 3.1 *10^'5, and 3.3 ^χ 10^"5 mol/g, respectively. The relatively lower dye loading for Ti0₂ particles of the present invention compared to the P25 particles may be attributable to the considerably lower surface areas of the Ti0₂ particles of the present invention than that of P25. However, even though the dye loading ability of the Ti0₂ particles of the present invention are lower than that of P25, these values are more than 3 times higher than that of commonly used 400nm-diameter scattering particles with flat surfaces (CCIC Japan).

In order to investigate the anticipated light scattering effect of the Ti0₂ particles of the present invention on the photovoltaic properties of dye sensitised solar cells, a bilayer photoelectrode was prepared in which a layer of P25 film was covered with a layer of Ti0₂ particles of the present invention (SI). The photovoltaic characteristics of the bilayer photoelectrode (having thickness of ca. 15.0 μιη) were compared with those of photoelectrodes having a layer of only a P25 film, with thicknesses of 7.4 μπι and 14.6 μπι.

The photocurrent density-voltage (J-V) curves of the three photoelectrodes (data not shown) show that the short-circuit current density (Jsc) of the photoelectrode having a 7.4 μιη thick P25 film was around 13.0 mA cm^'2, while the photoelectrode having a 14.6 μπι thick P25 film was around 16.8 mA cm^"2 (probably because of the larger quantity of dye adsorption as a result of the doubled thickness in the photoelectrode). The photocurrent of the photoelectrode having the thicker P25 coating was less than twice that of the other due to the increased electrolyte diffusion distance and the film resistance. The bilayer photoelectrode with the P25 film overlaid with a layer of T1O2 particles of the present invention exhibited a remarkable further increase in the Jsc of 19.4 mA cm^"2, which results in an even higher efficiency than either of the other coatings. Considering the lower dye loading in the bilayer photoelectrode compared to the P25 only photoelectrodes, it can be reasonably argued that the presence of a top layer of Ti0₂ particles of the present invention may play a very important role in increasing the overall conversion efficiency. By employing the Ti0₂ microspheres of the present invention as a light scattering layer over a P25 film and carefully tuning the thickness of each layer, the inventors were able to further improve the overall conversion efficiency to 9.10% under AM 1.5G one sun intensity. Thus, the use of Ti0₂ particles of the present invention offers new opportunities for the development of high-efficiency dye sensitised solar cells.

It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. tb specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS:

1. A method for producing Ti0₂ particles comprising an inner Ti0₂ species within an outer Ti0₂ shell, the method comprising:

- heating an aqueous mixture comprising a titanium compound and a

carbonaceous substance capable of forming a carbonaceous template to a temperature at which the titanium compound, water and carbonaceous substance react to produce composite particles comprising a carbonaceous template layer interposed between an inner T1O₂ species and outer Ti0₂ shell.

2. The method of claim 1 , wherein the titanium compound is selected from the group consisting of: titanium halides, titanium nitride (TiN), titanium carbide (TiC), titanium cyanide (TiCN), titanium diboride (TiB₂), titanium sulphide (TiS₂), titanium oxides or hydroxides, TiO_xN_y, TiO_xC_y, titanium phosphide (TiP), titanium sulphates, titanium silicides (TiSi₂), organic titanium compounds and combinations thereof, wherein x and y are independently integers from 0 to 2.

3. The method of claim 1 or claim 2, wherein the titanium compound is TiF₄, TiOS0₄ or a mixture thereof.

4. The method of any one of claims 1 to 3, wherein the carbonaceous substance is selected from the group consisting of: C₆Hi₂N₄ (hexamine), CO(NH₂)₂ (urea), CS(NH₂)₂ (thiourea), triethylamine,

(4,4',4"-methylidynetris( ,N-dimethylaniline)), Ci₂H₂₂0n (sucrose), C₆H]₂0₆

(glucose), C25H30O5 (2,7-di-tert-butyl-9,9-dimethyl-4,5-xanthenedicarboxylic acid), C₆Hi₂ (methylcyclopentane), C₆Hi₂0₂, C₆Hi₂BN0₃ (boric acid), C₇H₅BF₄0₂, C7H7BO₄ (3-carboxyphenylboronic acid), C₇H₇S0₂, C₇Hi₂0₂S, C₆I S, C₄C1₂F₆ (2,3-dichlorohexafluoro-2-butene), C₄H₂F₂N₂, C₄H₈BrF, C₄H₉I, CsH₃I0₂, C5H3FI, C₆Hi3l (1-iodohexane) and combinations thereof.

5. The method of any one of claims 1 to 4, wherein the carbonaceous substance is sucrose, glucose or a mixture thereof.

The method of any one of claims 1 to 5, wherein the molar ratio of the titanium compound to the carbonaceous substance in the aqueous mixture is from about 1 :45 to about 10:1.

7. The method of any one of claims 1 to 6, wherein the molar ratio of the titanium

compound to the carbonaceous substance in the aqueous mixture is from about 1 :5 to about 1 :20.

8. The method of any one of claims 1 to 7, wherein the molar ratio of the titanium

compound to the water in the aqueous mixture is from about 1 : 1 ,500 to about 1 :400.

9. The method of any one of claims 1 ^'to 8, wherein the aqueous mixture comprising a titanium compound and a carbonaceous substance is heated to a temperature of between about 100°C and about 250°C.

10. The method of any one of claims 1 to 9, wherein the aqueous mixture comprising a titanium compound and a carbonaceous substance is heated to a temperature of between about 150°C and about 200°C.

1 . The method of any one of claims 1 to 10, wherein the aqueous mixture comprising a titanium compound and a carbonaceous substance is heated to a temperature of about 190°C.

12. The method of any one of claims 1 to 11, wherein the aqueous mixture comprising a titanium compound and a carbonaceous substance is heated for a time of between about 12 hours and about 36 hours.

13. The method of any one of claims 1 to 12, wherein the aqueous mixture comprising a titanium compound and a carbonaceous substance is heated for a time of about 24 hours.

14. The method of any one of claims 1 to 13, wherein the aqueous mixture comprising a titanium compound and a carbonaceous substance is heated in an autoclave.

15. The method of claim 14, wherein the autoclave is allowed to slowly cool to room temperature after heating.

16. The method of any one of claims 1 to 15, wherein the composite particles are

calcined to remove the carbonaceous template.

17. The method of claim 16, wherein the composite particles are calcined by heating in air to a temperature in the range of about 400°C to about 600°C for about 3 to about 5 hours.

18. The method of claim 16 or 17, wherein the composite particles are calcined by heating in air to a temperature of about 550°C for about 4 hours.

19. The method of any one of claims 16 to 18, wherein the composite particles are calcined by heating with a ramping rate of about 5°C per minute to a final temperature, and then cooled to room temperature.

20. The method of any one of claims 1 to 19, wherein the inner Ti0₂ species is a Ti0₂ shell.

21. The method of any one of claims 1 to 20, wherein at least the outer Ti0₂ shell is a microsphere.

22. A Ti0₂ particle produced by the method of any one of claims 1 to 21.

23. A Ti0₂ particle comprising an inner Ti0₂ species within an outer Ti0₂ shell.

24. The particle of claim 23, wherein the inner Ti0₂ species is a hollow Ti0₂ shell.

25. The particle of claim 23 or claim 24, wherein at least the outer Ti0₂ shell is a microsphere.

26. The use of Ti0₂ particles produced by the method of any one of claims 16 to 19 or Ti0₂ particles of any one of claims 23 to 25, in a dye sensitised solar cell.

27. A dye sensitised solar cell comprising a conducting layer comprising Ti0₂ particles produced by the method of any one of claims 16 to 19 or Ti0₂ particles of any one of claims 23 to 25.

28. A method for producing metal oxide particles comprising an inner metal oxide species within an outer metal oxide shell, the method comprising:

- heating an aqueous mixture comprising a metal oxide precursor and a

carbonaceous substance capable of forming a carbonaceous template to a temperature at which the metal oxide precursor, water and carbonaceous substance react to produce composite particles comprising a carbonaceous template layer interposed between an inner metal oxide species and outer metal oxide shell.

29. The method of claim 28, wherein the composite particles are calcined to remove the carbonaceous template.

30. The method of claim 28 or claim 29, wherein the metal oxide is titanium dioxide, vanadium oxide, chromium oxide, manganese oxide, silicon dioxide, iron oxide, copper oxide or nickel oxide.

31. The methqd of any one of claims 28 to 30, wherein the carbonaceous substance is selected from the group consisting of: C₆Hi₂N4 (hexamine), CO(NH₂)₂ (urea), CS(NH₂)₂ (thiourea), triethylamine, (NH4)₂C0₃ (ammonium carbonate), C₂5H₃iN₃ (4,4',4"-methylidynetris(N,N-dimethylaniline)), Ci₂H₂₂0i ] (sucrose), C₆Hi₂0₆

(glucose), C₂5H₃o0₅ (2,7-di-tert-butyl-9,9-dimethyl-4,5-xanthenedicarboxylic acid), C₆H₁₂ (methylcyclopentane), C₆H_]20₂, C₆H_]2BN0₃ (boric acid), C₇.H₅BF₄0₂, C₇H₇B0₄ (3-carboxyphenylboronic acid), C₇H₇S0₂, C₇Hi₂0₂S, C6H4S, C C1₂F₆ (2,3-dichlorohexafluoro-2-butene), C4H2F2N2, C₄¾BrF, C4H9I, C₅H₃I0₂, C₅H₃FI, C HnI (1-iodohexane) and combinations thereof.

32. The method of any one of claims 28 to 31, wherein the aqueous mixture comprising a metal oxide precursor and a carbonaceous substance is heated to a temperature of between about 100°C and about 250°C.

33. The method of any one of claims 28 to 32, wherein the aqueous mixture comprising a metal oxide precursor and a carbonaceous substance is heated for a time of between about 12 hours and about 36 hours.

34. The method of any one of claims 28 to 33, wherein the composite particles are calcined to remove the carbonaceous template by heating in air to a temperature in the range of about 400°C to about 600°C for about 3 to about 5 hours.

35. The method of any one of claims 28 to 34, wherein the inner metal oxide species is a shell.

36. The method of any one of claims 28 to 35, wherein at least the outer metal oxide shell is a microsphere.

37. A metal oxide particle produced by the method of any one of claims 28 to 36.