EP1940742A1

EP1940742A1 - Synthesis of pure rutile structure titanium oxide nanostructures

Info

Publication number: EP1940742A1
Application number: EP06779296A
Authority: EP
Inventors: Dominik Eder; Ian A. Kinloch; Alan H. Windle
Original assignee: Cambridge University Technical Services Ltd CUTS; Cambridge Enterprise Ltd
Current assignee: Cambridge Enterprise Ltd
Priority date: 2005-09-06
Filing date: 2006-09-05
Publication date: 2008-07-09
Also published as: GB0518139D0; WO2007028972A1; JP2009506975A

Abstract

A method for synthesising a titanium oxide nanostructures consisting at least predominantly of rutile phase comprises the steps of providing a nanostructured template; coating the template with a titanium oxide containing colloidal phase, heat treating the titanium oxide coated template from to form a crystalline phase consisting at least predominantly of anatase phase; further heat treating the titanium oxide coated template to convert the anatase phase into rutile phase.

Description

SYNTHESIS OF PURE RUTILE STRUCTURE TITANIUM OXIDE

NANOSTRUCTURES

The present invention relates to a method of synthesis of titanium oxide nanostructures, and to nanostructures so synthesised, and to a colloidal phase which may be used in preparing such nanostructures.

Titania (titanium oxide, TiO₂) is one of the most industrially important transition metal oxides, being used as a colour pigment and as a photo-catalyst [1-3] . World production is 4 megatons a year.

Titania has a band gap of 3.2 eV and can thus absorb light with wavelengths of 320 run and below to activate water molecules to create strongly Oxidizing hydroxyl radicals and strongly reducing dioxygen ions (O₂ ^") .

Thus, titania is able to decompose toxic organic substances [4] and to reduce toxic environmental gases (NO_x, SO_x) as well as to oxidize ethylene and additionally to destroy killer spores and bacteria, as developed recently by NASA [5] .

Titania' s excellent photocatalytic activity is also used in the so called dye-sensitized TiO₂ solar cells [6,7] . In this application in particular, a high specific surface area of the titania particles is of great advantage, since more dye molecules (mostly ruthenium complexes) may then be adsorbed on the nanostructured surface increasing the photolytic efficiency of the solar cells . During the last few years titania has attracted much interest as a sensor material for H₂, O₂, CO_x and other gases [8-10] . Strongly reduced titania materials (Magneli phases [11,12]) may be also used as electrode materials, since they show a very high electronic conductivity, corrosion stability and high overpotentials for gas development reactions. In addition, titania has been successfully used as a catalyst and catalyst support for many years [13,14] . One of the key abilities in most of these applications is the great ability of titania to form substoichiometric phases and oxygen vacancies [15] . The formation of those species and their influence on the catalytic and electric performance has been studied intensively throughout the last years [16-22] . Considering the enormous importance of TiO₂ in the fields of photo-catalysis, environmental catalysis and electrochemistry, the fabrication of this material in a nanoscale anisotropic morphology is of interest.

Grimes [23] showed that the use of titania as nanotubes (TiO₂-NTs) coated with palladium particles leads to a 1000 fold higher sensitivity for sensing small hydrogen concentrations in the atmosphere, which favours applications in fuel cell technology. Additionally, TiO₂- NT based sensors can be operated at room temperature and thus exhibit a longer lifetime [24] . Another group studied the possible application of TiO₂-NTs in dye- sensitized solar cells and documented a significantly higher photolytic efficiency [25] . In all these studies a difference of the physical and chemical properties between TiO₂-NTs and Tiθ₂ particles was measured.

A suggestion has been made that TiO₂-NTs could also be used as a precursor for supra-conducting titanate NTs [23] Of the two main crystalline phases of TiO₂, anatase and rutile, anatase has hitherto been available in much more finely divided forms as well as being cheaper. It has thus has been the material of choice, despite possible performance advantages of rutile in areas such as photocatalysis and environmental catalysis. In particular, rutile absorbs light over a wider wavelength range than anatase, and thus appears whiter.

The first synthesis of titania nanotubes reported used a polymer mould, on which titanium oxide was deposited electrochemically [26] . The commonest and simplest synthesis is the hydrothermal treatment of TiO₂, first described by Kasuga et al . [27] and subsequently enhanced by various groups [28-31] . The resulting TiO₂ nanotubes, however, are of a tri-titanate structure rather than anatase or rutile.

Some groups have produced TiO₂-NTs using porous alumina [32-34] or polymer strains and fibres [35] as templates: after coating with titanium oxide by a sol-gel method [36] and removing the templates thermally, titania nanotubes predominantly consisting of anatase were obtained.

There is no evidence that nanotubes of anatase or an anatase/rutile mixture have been successfully converted to rutile while preserving their structural integrity as nanotubes. The studies of both Sun et al . [37] and Wang et al . [38] report the successful formation of TiO₂ coatings on carbon nanotube templates. Sun et al . [37] used multi-walled carbon nanotubes (MWCNTs) , which were coated with a modified sol-gel process to produce TiO₂ (anatase phase or anatase/rutile phase mixture) coated nanotubes with diameters between 300 and 800 run. Sun et al. also mentions that they successfully oxidised out the carbon nanotubes. However there is no suggestion in either of these publications that any attempt was made to convert the TiO₂ to rutile.

Varghese et al . produced anatase nanotubes by anodizing a titanium foil and studied their phase transformation during annealing in oxygen and argon [39] . They found that anatase transforms into rutile at temperatures as high as 620 ⁹C, at which the titania nanotube structure had already decomposed to leave a wormlike material at best. Additionally, the size of the crystals may be crucial for this phase transformation, as anatase crystals smaller than 14 nm are highly stable and will not be converted [40] .

So far, no pure rutile phase nanotubes have been produced.

Accordingly, there remains a need for a method of synthesising a rutile phase titania nanostructure.

In a first aspect, the present invention provides a method for synthesising a titanium oxide nanostructure which consists at least predominantly of rutile phase comprising the steps of: a) providing a nanostructured template; b) coating the template with a titanium oxide containing colloidal phase; c) first heat treatment of the titanium oxide coated template from b) to form a crystalline phase which consists at least predominantly of anatase phase; d) second heat treatment of the titanium oxide coated template from c) to convert the anatase phase into rutile phase.

Preferably, the product consists of at least 90 %, preferably at least 95 %, rutile phase. In a preferred embodiment, the product consists only of rutile phase.

The titanium nanostructure will have a structure determined by the structure of the template.

Preferably, the titanium nanostructure comprises nanotubes and/or nanoporous solid. Preferably, at least one dimension of the titanium nanostructure is between 0.5 nm and 1000 nm. Preferably, the template comprises high aspect ratio cylinders or tubes. Suitably, the aspect ratio is in the range of 100 to 500.

Preferably, the template is of optionally doped carbon. Suitably, the template is doped with nitrogen. However, the template may also be of another material, in particular a material which can be conveniently removed (for example by oxidation) . Possible materials include a nitride, a sulfide, a carbide or gallium arsenide. More preferably, the template comprises carbon nanotubes, carbon nanofibres, carbon herringbones, carbon onions or a mixture thereof. The carbon nanotubes may be as synthesised, or may have been functionalised by oxidation or acid treatment. The use of a template, which has been functionalised to improve adhesion of the titanium oxide coating (for example with aromatic groups) , is particularly preferred.

Highly preferably, the template comprises single walled carbon nanotubes, multi-walled carbon nanotubes or a mixture thereof.

Preferably, the template is of a defined architecture. For example, the template may be a carbon nanotube mat, wherein the nanotubes are aligned or unaligned. In this case, preferably the titanium oxide coating occupies substantially all of the space within the template, so that after removal of the template a titanium oxide nanoporous structure is formed.

However, the template may also not be of a defined architecture: for example it may be carbon nanotube powder .

Preferably, the template has a continuous or discontinuous coating of material . Suitably, the coating material comprises a Group VIB transition metal, a Group VIIIB transition metal, a lanthanide series metal, an actinide series metal, or a mixture thereof. Examples of coating materials are Ni, Pd, Pt and Au.

A template coated in this way is potentially useful in the preparation of a supported metal catalyst bearing metal on its inner surface. The metal can be deposited on the template by known techniques, for example sputtering chemical modification (such as self assembled monolayers (SAM) of thiols, alcohols, carboxylic acids etc.), wet impregnation, sputtering, atomic layer deposition or reductive adsorption of metal precursors . This modified template is coated with titanium oxide according to the invention. When the template is removed, the metal coating particles are deposited on the inner surface of the titania nanostructure. Preferably, the components used to form the colloidal phase comprise an inorganic titanium precursor, e.g. titanium chloride, or an organic titanium precursor, e.g. titanium isopropoxide (TTIP) and tetrabutyloxytitanate (TBOT), or a mixture thereof. Preferably, the components used to form the colloidal phase comprise water, an alcohol (for example ethanol or benzyl alcohol), acetone, a particle shaper (for example benzyl alcohol or acetylacetone) , a surfactant (for example sodium docecylbenzene sulfonate - SDBS) or a mixture thereof. In a preferred embodiment, when the components used to form the colloidal phase are mixed, a sol (colloidal mixture of solid and liquid) containing titanium oxide particles is formed and the template is introduced. A sol-gel transition is then caused. This may be done for example by adding water or acid (for example, acetic acid) or alkali (for example, ammonium hydroxide) to the colloidal phase. Neutralisation may be also used to cause the sol-gel transition. The nature of the gel gives good adhesion to the template.

Preferred sols are described in more detail below as sols 1, 2 and 3. Preferably, coating is carried out by dip coating (particularly suitable for a template of defined architecture such as a carbon nanotube mat) or by stirred coating (particularly suitable to achieve uniform coating of a template of non-defined architecture such as carbon nanotube powder) .

Preferably, the colloidal phase is applied to the template at a temperature in the range of 0⁰C to 90⁰C.

Suitably, the molar ratio of water to titanium (as element) is in the range of 0.1:1 to 60:1. Preferably, the ratio of CNT: TiO₂ is between 1% and 50%.

Preferably, the coating is dried either in an oxidising atmosphere or an inert atmosphere at a temperature of from ambient (20⁰C) to 15O⁰C, and preferably for a period of time between 10 minutes and 20 hours (leading to a xerogel) . Additionally, the drying may be conducted in vacuo or supercritical solvent (leading to an aerogel) , or via freeze-drying (leading to a cryogel) .

Preferably, the first heat treatment is carried out under an oxidising atmosphere (calcination) , for example an atmosphere of air or oxygen. Suitably, the first heat treatment is carried out at atmospheric pressure. Humidity should be controlled during the first heat treatment, as this could affect the gelation of dip- coated material .

Preferably, the first heat treatment is carried out at a temperature in the range of 200 ⁰C to 500 ⁰C. The first heat treatment temperature depends on the composition of the sol used for coating. Where the sol comprises TBOT and benzyl alcohol (as in sol 3) a first heat treatment temperature of 400⁰C to 500⁰C is necessary.

Preferably, the first heat treatment is carried out for a time of 1 minute to 10 hours. The first heat treatment time depends on the nature of the coating process . For dip coating, a first heat treatment time of 1 to 10 minutes is adequate. For stirred coating, a first heat treatment time of 1 to 10 hours is appropriate. Typically, after the first heat treatment the crystalline phase is 100% anatase phase.

Preferably, the second heat treatment is carried out at a temperature in the range of 550⁰C to 95O⁰C. This controls the surface morphology.

Preferably the second heat treatment is carried out under a non-oxidising atmosphere. More preferably, the non-oxidising atmosphere used for heat treatment is an inert atmosphere, a non-inert atmosphere or a mixed inert and non-inert atmosphere. Suitably, the atmosphere comprises nitrogen, inert gas or a mixture thereof. These are inert gases . Suitably, the atmosphere comprises hydrogen. This is a non-inert (reducing) gas.

The second heat treatment may be continuous or discontinuous .

Preferably, the second heat treatment time is from 30 minutes to 20 hours, more preferably from 30 minutes to 4 hours .

Preferably, the second heat treatment is carried out at atmospheric pressure. However, other pressures (for example partial vacuum) might be used. Preferably, the method further comprises the step of: e) removal of the template after step d) . Suitably, step e) is carried out by oxidation at high temperature or by the use of acid. For example, removal of the template may be effected by heating in air, oxygen or steam at 400 - 1000⁰C, more preferably at 450 - 700⁰C, most preferably at 520 - 56O⁰C. Additionally, the template may be removed in a suitable oxygen plasma at low temperatures (preferably room temperature) .

Preferred acids for removal of the template include hydrochloric acid, sulphuric acid, nitric acid or mixtures of two or more thereof. A 3 : 1 mixture of sulphuric acid to nitric acid at a temperature of 50 - 130⁰C is particularly preferred.

Optionally, the method further comprises the step of: f) Coating the surface of the titanium oxide with a continuous or discontinuous coating of material after step d) or step e) . Coating is preferably carried out before step e) . This is because if the template is removed before coating, the coating may be deposited on the inner surface of the nanostructure to a certain extent, and the amount and distribution cannot be easily controlled. Preferably, the coating material comprises a Group VIB transition metal, a Group VIIIB transition metal, a lanthanide series metal, an actinide series metal, or a mixture thereof.

A nanostructure coated in this way is potentially useful as a supported metal catalyst bearing metal on its outer surface. The metal can be deposited on the template by known techniques, for example wet impregnation, sputtering, atomic layer deposition, reductive adsorption of metal precursors, or the techniques discussed in connection with coating of the template above.

Coating of the nanostructure in this way can be combined with coating of the template as discussed above. In this way, a nanostructure coated on the outer and inner surfaces can be obtained. Optionally, the method further comprises the step of: g) removing anatase phase.

Preferably, the wall thickness of titanium dioxide is preferably between 1 to 20 crystal layers and most preferably between 1 and 10 crystal layers, particularly, between 1 and 6 crystal layers.

In a second aspect, the invention relates to a titanium nanostructure synthesised by the method described above. In a third aspect, the present invention relates to a titanium oxide containing colloidal phase formed from components comprising: tetrabutyloxytitanate and/or titanium isopropoxide; benzylalcohol and/or acetone; and solvent.

Preferably, the solvent is an alcohol, for example ethanol. In a preferred embodiment, the colloidal phase is formed from components comprising tetrabutyloxytitanate, benzyl alcohol and ethanol. The invention will be further described with reference to a preferred embodiment, as shown in the Figures, in which:

Fig. 1 shows SEM (scanning electron microscopy) images of the product rutile phase nanotubes after removing the template .

Fig. 2 shows XRD (x-ray diffraction) data for the product rutile phase nanotubes of Fig. 1 (a) before and (b) after removing the template.

In this embodiment pure rutile nanotubes are produced by coating a sacrificial carbon nanotube template with a sol and causing a sol-gel transition, which forms an anatase coating with a crystallite size of 18-19 πin (sol 3) . After calcination followed by suitable heat treatment the templates are removed thermally. The carbon nanotubes act as a support and prevent the anatase tubes from collapsing, providing a simple pathway to rutile nanotubes, whose dimensions can be controlled by controlling the dimensions of the carbon nanotube template.

Multi and single walled carbon nanotubes were used as templates. These nanotubes were either used as synthesised, were functionalised by oxidation or acid treatment, or were coated with metals (for example Ni, Pd, Pt or Au) .

The templates were treated with sols (colloidal phase) as described below at temperatures between 0 ⁰C and 90⁰C. The ratio of CNT : TiO₂ was between 1% and 50%.

A defined amount of water, acid or alkali was added to induce gelation and precipitation of TiO₂ on the nanotubes . The following sols were prepared for coating of the carbon nanotube templates .

Sol 1: This sol was reported by Sun et al . [37] for coating of carbon nanotubes. It uses an inorganic titanium salt as a precursor for the sol. TiCl₄ was diluted with water in an ice bath to 0.3 M. This aqueous solution was then mixed with (NH₄J₂SO₄ solution. The final molar ratio of Ti : SO₄ ^2-WaS 1:2. The mixture was stirred at 9O⁰C for 1 h and afterwards treated with 2.5 M NH₄OH until the pH value was 7 to cause precipitation. Subsequently, the precipitated product was washed with distilled water several times, and then dried at 110⁰C [41] .

Sol 2: For comparison, organic titanium precursors and a sol-gel process which Zhang et al . described for producing titania nanotubes on porous alumina templates [34] was used. Titanium isopropoxide (TI) was used with acetyl-acetone (ACAC) , ethanol and water in the following ratio 1:2:3:20 (TI : ACAC :H₂O : EtOH) . Sol 3: The sol mostly used in this work consists of mainly organic titanium precursors like tetrabutyloxytitanate (TBOT) or titanium isopropoxide (TI), which were dissolved in ethanol and mixed with benzyl alcohol (BA) or acetone. Benzyl alcohol is known to act as a surface shaper, keeping the particle size low [42]. After adding the suspension of CNTs, gelation was initiated by adding small drops of water or acetic acid. The reaction temperature was varied from O²C to 90²C and the mixture was stirred for a time between 10 min and 3 hours. The resulting gel was either desiccated in air at room temperature, dried at 100⁰C or filtered and afterwards calcined at various temperatures .

The resulting composite material in each case was collected by filtration, washed with ethanol or water and dried in air, and then calcined in air at 400⁰C for 4 hours, during which the coating crystallised as anatase phase.

Afterwards, the calcined materials were treated in flowing nitrogen with exclusion of oxygen at temperatures between 650⁰C and 95O⁰C for 2-4 hours, during which the anatase phase transformed into rutile phase while continuing to exist as a continuous layer on the carbon nanotubes .

A final oxidation in air or oxygen at 520 - 560⁰C removed the carbon nanotube templates leaving titania nanotubes consisting of the rutile phase, which was pure as determined separately by electron and X-ray diffraction. These were stable at least up to 700⁰C. The specific surface area, measured with nitrogen physisorption according to BET (Brunauer Emmett Teller) , is encouragingly large (between 20 and 120 m²/g) , depending on applied process parameters (e.g. concentration of particle shaper and titanium concentration, as well as conditions for second heat treatment) .

Fig. 1 shows SEM (scanning electron microscopy) images of the product rutile phase nanotubes after removing the template. It can be seen that the rutile product is well-defined hollow nanotubes. The product consists of two possible morphologies, rough/thick wall structure (upper SEM images) and smooth/thin wall structure (lower SEM images) . The morphology can be controlled by process parameters, e.g. concentration of titanium precursor and concentration of water.

Fig. 2 shows XRD (x-ray diffraction) data for the product rutile phase nanotubes of Fig. 1 (a) before and (b) after removing the template. It can be seen that after template removal no anatase was found.

The sequence of heating steps used in the preferred embodiment (calcination at 400⁰C under air or oxygen atmosphere, followed by heat treatment at 800⁰C under nitrogen or other non-oxidising atmosphere, followed by template removal at over 400⁰C under air or oxygen atmosphere) allows the desired rutile phase nanostructures to be obtained.

Calcination causes anatase phase to crystallise from an amorphous titania phase.

The heat treatment step converts the anatase phase to rutile phase. A non-oxidising atmosphere allows the template to be preserved so that the nanostructure does not collapse. The final template removal step is carried out after the rutile phase titanium oxide nanostructure has been formed.

Whilst the invention has been described with reference to a preferred embodiment, it will be appreciated that various modifications are possible within the scope of the invention.

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Claims

Claims :

1. A method for synthesising a titanium oxide nanostructure which consists at least predominantly of rutile phase comprising the steps of: a) providing a nanostructured template; b) coating the template with a titanium oxide containing colloidal phase; c) first heat treatment of the titanium oxide coated template from b) to form a crystalline phase which consists at least predominantly of anatase phase; d) second heat treatment of the titanium oxide coated template from c) to convert the anatase phase into rutile phase.

2. A method as claimed in Claim 1 , wherein the product consists only of rutile phase.

3. A method as claimed in either one of the preceding claims, wherein the template comprises high aspect ratio cylinders or tubes.

4. A method as claimed in any one of the preceding claims, wherein the template is of optionally doped carbon, a nitride, a sulfide, a carbide or gallium arsenide.

5. A method as claimed in any one of the preceding claims, wherein the template is doped with nitrogen.

6. A method as claimed in Claim 4 or Claim 5, wherein the template comprises carbon nanotubes, carbon nanofibres, carbon herringbones, carbon onions or a mixture thereof.

7. A method as claimed in Claim 6, wherein the template comprises single walled carbon nanotubes, multi-wall carbon nanotubes or a mixture thereof.

8. A method as claimed in any one of the preceding claims, wherein the template has a continuous or discontinuous coating of material .

9. A method as claimed in Claim 8 , wherein the coating material comprises a Group VIB transition metal, a Group VIIIB transition metal, a lanthanide series metal, an actinide series metal, or a mixture thereof .

10. A method as claimed in any one of the preceding claims, wherein the template is of a defined architecture.

11. A method as claimed in Claim 8, wherein the template is a carbon nanotube mat.

12. A method as claimed in any one of the preceding claims, wherein the components used to form the colloidal phase comprise titanium chloride, titanium isopropoxide, tetrabutyloxytitanate or a mixture thereof.

13. A method as claimed in any one of the preceding claims wherein the components used to form the colloidal phase comprise water, alcohol, acetone, an organic particle shaper, or a mixture thereof.

14. A method as claimed in any one of the preceding claims, wherein step b) comprises causing a sol-gel transition of the colloidal phase.

15. A method as claimed in any one of the preceding claims, wherein the first heat treatment is carried out at a temperature in the range of 200⁰C to 500⁰C.

16. A method as claimed in any one of the preceding claims, wherein the first heat treatment is carried out under an oxidising atmosphere.

17. A method as claimed in any one of the preceding claims, wherein the second heat treatment is carried out at a temperature in the range of 550⁰C to 95O⁰C.

18. A method as claimed in any one of the preceding claims, wherein the second heat treatment is carried out under a non-oxidising atmosphere.

19. A method as claimed in Claim 18, wherein the atmosphere comprises nitrogen, inert gas or a mixture thereof.

20. A method as claimed in Claim 18 or Claim 19, wherein the atmosphere comprises hydrogen.

21. A method as claimed in any one of the preceding claims, wherein the second heat treatment is continuous or discontinuous.

22. A method as claimed in any one of the preceding claims, further comprising the step of: e) removal of the template after step d) .

23. A method as claimed in Claim 22, wherein step e) is carried out by oxidation at high temperature or by the use of acid.

24. A method as claimed in any one of the preceding claims, further comprising the step of: f) Coating the surface of the titanium oxide with a continuous or discontinuous coating of material after step d) or step e) .

25. A method as claimed in Claim 24, wherein the coating material comprises a Group VIB transition metal, a Group VIIIB transition metal, a lanthanide series metal, an actinide series metal, or a mixture thereof .

26. A method as claimed in any one of the preceding claims, further comprising the step of: g) removing anatase phase.

27. A method as claimed in any one of the preceding claims, wherein the wall thickness of the titanium oxide nanostructure is between 1 to 20 crystal layers .

28. A titanium oxide nanostructure synthesised by a method as claimed in any one of the preceding claims .

29. A titanium oxide-containing colloidal phase formed from components comprising: tetrabutyloxytitanate and/or titanium isopropoxide; benzylalcohol and/or acetone; and a solvent.