WO2009065180A1

WO2009065180A1 - Non-metal doped metal oxide nanosheets and method of production thereof

Info

Publication number: WO2009065180A1
Application number: PCT/AU2008/001728
Authority: WO
Inventors: Gao Qing Lu; Lianzhou Wang
Original assignee: University of Queensland UQ
Current assignee: University of Queensland UQ
Priority date: 2007-11-23
Filing date: 2008-11-21
Publication date: 2009-05-28
Anticipated expiration: 2010-05-23

Abstract

Non-metal doped metal oxide nanosheets having a formula (IV): MyOz-xDx (IV) wherein: M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a non-metal dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value greater than 0 and less than 8; and z-x is always a value greater than 0, and method of production thereof.

Description

NON-METAL DOPED METAL OXIDE NANOSHEETS AND METHOD OF PRODUCTION THEREOF

Field of the Invention

The present invention generally relates to nanosheets and a method of production thereof. More particularly, the invention relates to non-metal doped metal oxide nanosheets, their use and methods of production thereof.

Background Art

Increasing attention is being directed to nanoparticles and nanosheets and their potential use in photocatalytic reactions. One notable example is titania nanosheets. Exfoliated titania (Ti₀.₉iθ₂) nanosheets have been extensively investigated due to their unique physicochemical properties and potential functionalities (Sasaki, T., et al. J. Am. Chem. Soc. 1996, 118, 8329-8335; Sasaki, T., et al. Chem. Mater. 1997, 9, 602-608; Sasaki, T. Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682-4689; and Tanaka, T.; et al. Chem. Mater. 2003, 15, 3564-3568). Exfoliated Ti_0.9iO₂ nanosheets generally have a lateral length of about several hundred nanometres and thickness of about 0.75 nm. These nearly two-dimensional nanosheets can be thought of as paper-like building blocks for the fabrication of a variety of nanostructures. Tι^'o_.9iO₂ nanosheets have been studied for applications in photocatalysis, photoelectrochemical water splitting, photodegradation and superhydrophilicity (Choy, J-H., et al. J. Mater. Chem. 2001 , 11 , 2232-2234; Choy, J-H., et al. Chem. Mater. 2002, 14, 2486-2491 ; Wang, L.Z., et al., J. Phys. Chem. B 2004, 108, 4283-4288, Tanaka, T., et al Adv. Mater. 2004, 16, 872-875; Kim, T. W., et al. Adv. Fund Mater. 2007, 17, 307-314). All of these potential applications of Tio.9iO2 nanosheets involve a photoexcitation process, which is highly dependent upon the electronic structure of the Ti_0.9iθ₂ nanosheets.

One of the most important characteristics of the exfoliated Ti_0.9iO₂ nanosheets is their sharp optical absorption peak at about 260 nm. This absorption peak is believed to be associated with a larger band gap of 3.8 eV in the nanosheets structure, compared to the band gap of about 3.2 eV for bulk TiO₂ (Sakai, N. et al. J. Am. Chem. Soc. 2004, 126, 5851-5858). The wider band gap found in nanosheets means that only shorter wavelength UV light is absorbed. These nanosheets also have reduced conductivity. The wider band gap found in exfoliated Tio_.9iO₂ nanosheets limits their potential uses as they require external sources of UV light to irradiate the Tio.giO₂ in order to utilise any photocatalytic activity the Ti₀₉iθ₂ nanosheets may have. The need to use external irradiation sources adds to the expense of using Ti₀.₉iO₂ nanosheets in photocatalytic applications.

Known metal oxide nanosheets are ineffective as photocatalysts in the visible light range and/or are expensive to produce.

Summary of the Invention

In one aspect the invention provides a method of producing non-metal doped metal oxide nanosheets.

In a non-limiting example, there is provided a method of producing non-metal doped metal oxide nanosheets including the steps of: a) doping a metal oxide precursor with a non-metal dopant to form a non- metal doped metal oxide; b) protonating the non-metal doped metal oxide to form a protonated non- metal doped metal oxide; and c) exfoliating the protonated non-metal doped metal oxide to form non- metal doped metal oxide nanosheets.

In a particular example, the metal oxide precursor is preferably a compound having the formula (I):

A_nMyO₂ (I) wherein: A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, calcium, magnesium, caesium and francium; M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; n is a value greater than or equal to 0 and equal to or less than 8; and y and z are independently a value greater than 0 and equal to or less than 8.

Preferably, though not necessarily, the metal oxide precursor of formula (I) is Cs_nTJyO₂ wherein n is a value greater than 0 and equal to or less than 8; y is a value greater than 0 and equal to or less than 8; and z is a value greater than 0 and equal to or less than 8.

Most preferably the metal oxide precursor of formula (I) is Cso_.68Ti1_.83O4.

The dopant may be any chemical, compound or composition which is capable of donating the appropriate dopant atoms to form a non-metal doped metal oxide. The non-metal dopant may be an inorganic or organic compound, in solid, liquid or gaseous form.

Preferably, when the non-metal dopant is a gas it may be selected from one or more of the following: nitrogen, ammonia, methane, ethane, propane, butane, gas comprising B_xH_y, carbon monoxide, carbon dioxide, hydrogen sulphide or fluorine.

When the non-metal dopant is a gas it is preferably supplied with an inert or non-reactive gas, such as air, argon, helium or hydrogen. Preferably, though not necessarily, the dopant gas and the non-reactive gas are present in a 1 :1 volume ratio. When the non-metal dopant is an organic compound it may comprise one or more of the following: C₆H₁₂N₄, CO(NH₂)₂, CS(NH₂)₂, triethylamine, (NH₄)₂CO₃, C₂₅H₃iN₃, Ci₂H₂₂On, C^HsoOs, CeHi₂, CeHi₂O₂, CeHi₂BNOa, C₇HsBF₄O₂, C₇H₇BO_4, H₃N BH₃, C₆H₅N(C₂Hg)₂ BH₃, CS(NH₂)₂, C₇H₇SO₂, C₇Hi₂O₂S, C₆H₄S, C₄CI₂F₆, C₄H₂F₂N₂, C₄H₈BrF, C₄H₉I, C₅H₃IO₂, C₅H₃FI, C₆H₁₃I.

The dopant also may be selected from one or more inorganic compounds or solutions including carbon, boron, H₃BO₃, sulphur, (NH₄)₂S, iodine, HIO₃, HIO₄, NH₄I₁ Or NH₄IO₃.

Preferably the metal oxide precursor is doped with the non-metal dopant by calcining the metal oxide precursor in contact with the non-metal dopant. The doping step is preferably carried out in the presence of one or more non- reactive or inert gases, selected from the group comprising, for example, oxygen, hydrogen, argon, helium, and air.

Preferably the metal oxide precursor is calcined with the non-metal dopant at a temperature of between 200⁰C to 1800⁰C for a period of between 30 minutes and 5 days.

More preferably the metal oxide precursor is calcined in contact with the non- metal dopant at a temperature of between 600⁰C and 1000⁰C for a period of between 30 minutes and 3 days.

In a particular example, the metal oxide precursor may be calcined in contact with the non-metal dopant at a temperature of about 700⁰C for a period of about 60 minutes.

The non-metal doped metal oxide formed using the method preferably has a formula (II):

A_nM_yO₂._xDχ (II) wherein: A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, calcium, magnesium, caesium and francium;

M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a non-metal dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; n is a value greater than or equal to 0 and equal to or less than 8; x is a value greater than 0 and less than 8; y and z are independently a value greater than 0 and equal to or less than 8; and z-x is a value greater than 0.

According to a particular example, the non-metal doped metal oxide of formula (II) is Cs_nTiyO_z-xD_x, wherein n is a value greater than 0 and equal to or less than 8; x is a value greater than 0 and less than 8; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; and z-x is a value greater than 0.

According to a more specific example, the non-metal doped metal oxide of formula (II) is Cso.₆βTii .₈₃O_4-XN_x, wherein x is a value greater than 0 and less than 4.

The protonated non-metal doped metal oxide produced in the method preferably has a formula (III):

H_mA_n-mM_yO_z-xD_x (III)

wherein:

A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, caesium and francium; M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a non-metal dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; m, y, and z are independently a value greater than 0 and equal to or less than 8; n is a value either equal to m or greater than m and a value which is greater than 0 and less than or equal to 8; x is a value greater than 0 and less and 8; and z-x is a value greater than 0 and less than 8.

According to a particular example, the protonated non-metal doped metal oxide of formula (III) is H_mA_n-mTiyO_z-χD_x, wherein m is a value greater than 0 and less than or equal to 8; n=m; y is a value greater than 0 and less than or equal to 8; z is a value greater than 0 and less than or equal to 8; x is a value greater than 0 and less than or equal to 8; and z-x is a value greater than 0.

According to a more specific example, the protonated non-metal doped metal oxide of formula (III) is HoβsTii ₈3θ_4-xN_x wherein x is a value greater than 0 and less than 4.

The protonating step of the method is preferably carried out by mixing the non- metal doped metal oxide with an acidic solution. For example, the acidic solution may be selected from the group comprising hydrochloric acid, nitric acid, sulphuric acid, phosphoric acid, hydroflouric acid, hydroiodic acid, hydrobromic acid, acetic acid (HAC), perchloric acid, iodic acid (HIO₃) and periodic acid (HIO₄).

More preferably the acidic solution is hydrochloric acid. In a particular example, the acidic solution is 0.001 M to 15M hydrochloric acid. The exfoliating step of the method is preferably carried out by mixing the protonated non-metal doped metal oxide with an exfoliating agent. The exfoliating agent is preferably an organic compound selected from the group comprising tetraalkylammonium hydroxides including tetrabutylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetramethylammonium hydroxide.

The protonated non-metal doped metal oxide of formula (III) is preferably in contact with the exfoliating agent for a period of between 1 hour and two weeks, more preferably for 1 to 10 days, and most preferably for approximately 7 days.

The exfoliation step is preferably carried out at a temperature of between room temperature and 6O⁰C. Most preferably the exfoliation step is carried out at room temperature.

Preferably the non-metal doped metal oxide nanosheets are formed in a suspension.

The method may further include the step of re-ordering the non-metal doped metal oxide nanosheets to form a layered or pillared non-metal doped metal oxide.

The re-ordering step may be achieved by drying the nanosheets or adding a cationic solution.

Preferably the re-ordering step is carried out by evaporating any solvent present in the non-metal doped metal oxide nanosheet suspension, and drying the solid non-metal doped metal oxide nanosheets.

The cationic solution may be an acidic solution, a solution comprising inorganic or organic salts which may provide cations such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺ and the like; nanoclusters, such as Keggin type ions; inorganic nanoparticles; organic macromolecules such as dyes and the like, which are capable of providing cations to the non-metal doped metal oxide nanosheets or capable of inducing the re-ordering of the nanosheets.

When the cationic solution provides protons it is preferable that the cationic solution is an acidic solution selected from the group comprising hydrochloric acid, nitric acid, sulphuric acid, phosphoric acid, hydrofluoric acid, hydroiodic acid, hydrobromic acid, acetic acid (HAC), perchloric acid, iodic acid (HIO₃) and periodic acid (HIO₄). Preferably the cationic solution is selected from hydrochloric acid, nitric acid, sulphuric acid and phosphoric acid. More preferably the cationic solution is hydrochloric acid.

The method may further include the step of separating and drying the layered or pillared non-metal doped metal oxide.

The separation may require the addition of one or more suitable flocculant. By way of example the flocculant may be selected from the group comprising: inorganic acids, bases and salts; organic natural products, such as starch, guar and the like; or synthetic organic compounds such as polymers.

In another aspect there is provided at least one non-metal doped metal oxide nanosheet having a formula (IV):

M_yO_z-xD_x (IV) wherein:

M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tungsten, tantalum and vanadium;

D is a non-metal dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; x is a value greater than 0 and less than 8; y and z are independently a value greater than 0 and equal to or less than 8; and z-x is a value greater than 0.

Preferably the nanosheets have a formula (IV) wherein M is titanium; D is nitrogen; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value greater than 0 and less than 8; and z-x is a value greater than 0.

More preferably the nanosheets have a formula (IV) wherein M is titanium; D is nitrogen; y is a value greater than 0 and equal to or less than 1 ; z is 2; and x is a value greater than 0 and less than 2.

Most preferably the nanosheets have a formula Tio_.91O₂.xN_x, wherein x is a value greater than 0 and less than 2.

In a yet another aspect the invention provides for the use of non-metal doped metal oxide nanosheets of formula (IV) as photocatalysts.

In a still further aspect there is provided a method of coating substrates with non-metal doped metal oxide nanosheets of formula (IV), including the step of applying one or more layers of the non-metal doped metal oxide nanosheets of formula (IV) onto a substrate.

The substrate may be any substrate suitable for supporting a photocatalytic film. Preferably the substrate is glass, quartz glass, silicon wafer or ITO glass.

The method of coating substrates with non-metal doped metal oxide nanosheets of formula (IV) may further include the step of adding one or more layers of binder.

The layers of binder are preferably applied onto the substrate prior to coating with a layer of non-metal doped metal oxide nanosheets of formula (IV) and/or between layers of non-metal doped metal oxide nanosheets of formula (IV). The binder preferably includes an organic polyelectrolyte or charged inorganic nanoclusters.

The polyelectrolyte may be selected from the group comprising polyethylenimine (PEI), poly(allylamine hydrochloride), and poly(diallyldimethylammonium) chloride.

The charged inorganic nanoclusters may be selected from the group comprising partially hydrolysed nanoclusters of metal hydroxides and oxides including aluminium oxides and hydroxides, chromium oxides and hydroxides, cobalt oxides and hydroxides, ferric oxides and hydroxides, ferrous oxides and hydroxides, nickel oxides and hydroxides, tungsten oxides and hydroxides manganese oxides and hydroxides, titanium oxides and hydroxides, zirconium oxides and hydroxides and vanadium oxides and hydroxides.

The binder may be applied to the substrate using a dip-coating method, in which the substrate is placed in a solution of the polyelectrolyte or inorganic nanoclusters for period of between 1 minute and 2 hours. Preferably the substrate is placed in the solution of polyelectrolyte or inorganic nanoclusters for a period of between 15 to 60 minutes. More preferably the substrate remains in the solution of polyelectrolyte or inorganic nanoclusters for approximately 20 minutes before being rinsed and dried.

The non-metal doped metal oxide nanosheets of formula (IV) are preferably applied to the substrate by providing the nanosheets in suspension and coating the substrate with the nanosheets. Preferably the substrate remains in the nanosheet suspension for a period of between 1 minute to 2 hours. More preferably the substrate is placed in the suspension for a period of between 15 to 60 minutes. Most preferably the substrate remains in the suspension for approximately 20 minutes before being rinsed and dried.

In yet a further aspect the invention provides a substrate coated with non-metal doped metal oxide nanosheets of formula (IV). Brief Details of the Drawings

In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings provided by way of example to illustrate preferred embodiments of the invention, and wherein:

Fig. 1 is a schematic representation of an embodiment of the invention;

Fig. 2A is a tapping-mode AFM image of a PEI/Tio_.9iθ_2-XN_X bilayer deposited on a silicon wafer chip;

Fig. 2B is an XRD pattern of (a) an intermediate, layered Ho_.βsTh_.83O_4-XN_x and (b) restacked Tϊo_.9iO_2-xN_x nanosheets of the invention formed by drying the colloidal suspension of nanosheets at 5O⁰C;

Fig. 3 is a zeta potential graph for a) non-doped TΪ_O.91O₂ nanosheets and b) Tio_.9iθ_2-XN_X nanosheets;

Fig. 4λ is a UV-visible light absorbance spectra for the colloidal suspension of (a) Tϊ_0.9iO₂; and (b) Ti_0.9iO2_-xN_x nanosheets a concentration of 0.014g dm ³;

Fig 4B is a plot of transformed Kubelka-Munk function versus the energy of the light absorbed of (a) Tϊ₀.9iO₂; and (b) Ti₀.₉iO_2-xNχ.

Fig. 4C is a high resolution XPS spectra of (a) (PEI/Tϊo.₉iO₂)io and (b)

(PEI/Ti₀.9iO_2-xN_x)io films on quartz glass substrates;

Fig. 5A is a UV-visible absorption spectra for multilayer films of

(PEI/Tio.9iθ₂-χN_x)_n on a quartz glass substrate, in which "n" represents the bi-layer number; Fig. 5B is a graph showing the dependence of peak absorbance at 262nm on the number of deposition cycles;

Fig. 5C is XRD patterns for multilayer films of (PEI/ 11₀.91O₂-XNx)₁O (a) as grown sample and (b) sample kept at room temperature in air for one week;

Fig. 6 is a graph of photocurrent-applied potential curves of photoanodes on ITO substrates vs. Ag/AgCI reference electrode: (a) T1O_.91O₂ nanosheet film in dark; (b) Ti_Og₁O₂ nanosheet film under the irradiation of visible light from 420 nm to 770 nm; and (c) Tio_.9iθ2_-XN_X nanosheet film under the irradiation of visible light from 420 nm to 770 nm; and

Fig. 7 illustrates a comparative hydrophilicity test of thin films fabricated with undoped Tio_.9iO₂ and doped Ti_09IO_2-XN_x nanosheets.

Description of the Preferred Embodiments

The non-metal doped metal oxide nanosheets of formula (IV) may be formed by mixing and calcining at least one metal oxide precursor with a non-metal dopant, conducting a protonation or ion exchange step followed by exfoliating the protonated non-metal doped metal oxide to form non-metal doped metal oxide nanosheets.

The reaction mechanism can be summarised as follows:

Step i A_nM_yO_z ^doping » A_nM_yO_z-xD_x

Ion exchange

Step 2 A_nM_yO_z._xD_x ► H_nA_WnMyO^_xD_x Exfoliating agent

Step 3 H_nAw_nMyOz-_XD_{x ►} MyO_z-xD_x

The doping reaction of step 1 is preferably carried out by contacting the metal oxide precursor (A_nM_yO_z) with a non-metal dopant (D) and calcining at a temperature between 200⁰C to 1800⁰C for a period of between 30 minutes to 5 days.

The non-metal doped metal oxide (A_nM_yO_z-xD_x) produced by step 1 , above, has a layered structure in which metal oxide layers may be intercalated with a cation (A). The non-metal dopant (D) stoichiometrically replaces oxygen from the metal oxide layers to form the non-metal doped metal oxide of formula (II).

It will be appreciated that the conditions for the doping of step 1 will vary depending on the type of dopant used to exchange with oxygen in the metal oxide precursor.

By way of example if the dopant is nitrogen the metal oxide precursor may be calcined in a gaseous atmosphere containing ammonia gas or nitrogen, or in contact with a nitrogen containing organic substance such as CeH₁₂N₄, CO(NH₂)₂, CS(NH₂)₂, triethylamine, (NH₄)₂CO₃, and C₂₅H₃₁N₃.

When the dopant is carbon the gaseous atmosphere may be a gas comprising lower straight chain alkanes (C_xH_y) such as methane, ethane, propane and butane; carbon monoxide (CO); and/or carbon dioxide (CO₂). Alternatively, any simple organic substance, such as an alkane, an alkene, C₁₂H₂₂O₁₁, C₂sH₃oθ₅, CeHi₂, or CeHi₂O₂, may be in contact with the metal oxide precursor during calcination.

When the dopant is boron the gaseous atmosphere may be a gas comprising B_xH_y Alternatively, boron, H₃BO₃, or an organic substance containing boron, such as C₆H₁₂BNO₃, C₇H₅BF₄O₂, C₇H₇BO₄, H₃N BH₃ or C₆H₅N(C₂Hs)₂ BH₃ may be in contact with the metal oxide precursor during calcination.

When the dopant is sulphur, the gaseous atmosphere may comprise H₂S. Alternatively, sulphur, (NH₄)₂S, or sulphur containing organic substances, such as CS(NH₂)₂, C₇H₇SO₂, C₇Hi₂O₂S or C₆H₄S may be in contact with the metal oxide precursor.

If the dopant is fluorine, the cation intercalated titanate may be in contact with NH₄F or fluorine containing organic substances, such as C₄CI₂F₆, C₄H₂F₂N₂ or C₄H₈BrF.

When the dopant is iodine, the metal oxide precursor may be contacted with HIO₃, HIO₄, NH₄I, NH₄IO₃, or organic substances containing iodine, such as C₄H₉I, C₅H₃IO₂, C₅H₃FI, or C₆H₁₃I.

It is believed that the ion exchange or protonation of step 2 leads to the complete or partial replacement of the intercalated cations (A) by protons in the layered metal oxide precursor to form protonated non-metal doped metal oxide of formula (III).

If required the protonated non-metal doped metal oxide nanosheets may be separated and dried.

The addition of the exfoliating agent in step 3 to the protonated non-metal doped metal oxide of formula (III) is preferably, though not necessarily, carried out in suspension. This step results in the exfoliation of the protonated non- metal doped metal oxide through the exchange of protons with cations from the exfoliating agent which reduces and/or eliminates the attractive forces between the layers of non-metal doped metal oxide to form non-metal doped metal oxide nanosheets of formula (IV). The exfoliated non-metal doped metal oxide nanosheets may be used in suspension to form coatings and films. Alternatively, they may be reordered or restacked to form a layered non-metal doped metal oxide structure which can be separated and dried prior to use.

Preferably the layered non-metal doped metal oxides are dried at temperatures below approximately 100⁰C.

Step 3 of the method as described above and subsequent use of the non-metal doped metal oxide nanosheets is schematically represented in Figure 1.

The non-metal doped metal oxide nanosheets may be used in photocatalytic applications, such as self cleaning coatings, decomposition of organic compounds, and hydrogen production from the photocatalytic splitting of water.

If the metal oxide precursor of formula (I) contains intercalated cations within the layered structure of the metal oxide they may be formed by heating a cation donor precursor with a metal oxide. The reaction can be summarised as follows: calcination Step 1a Cation donor precursor + Metal oxide donor ► A_nM_yO_z

Preferably the cation donor precursor is an alkali earth metal salt or an alkali metal salt selected from the group comprising alkali metal halides; alkali earth metal halides; alkali metal sulphides; alkali earth metal sulphides; alkali metal sulphates; alkali earth metal sulphates; alkali metal carbonates; alkali earth metal carbonates; alkali metal nitrates; alkali earth metal nitrates; alkali metal hydroxides; alkali earth metal hydroxides; alkali metal acetates; alkali earth metal acetates; alkali metal dimethenylamine (AN(CH₂)2); alkali earth metal dimethenylamine (AN(CH₂^); alkali metal oxide; alkali earth metal oxides; alkali metal chlorate; alkali earth metal chlorate; alkali metal phosphate and/or alkali earth metal phosphate. Preferably, the metal oxide donor is selected from the group comprising metal oxides or hydroxides, including TiO, Ti₂θ₃, Ti₃O₅, Tiθ₂, TiO_xNy, TiO_xCy, Ti(OH)₄.xH₂O; mixed oxides of titanium, such as lanthanum titanium oxide; niobium oxides and mixed oxides thereof, such as calcium niobium oxide; nickel oxides, cobalt oxides, ferric and ferrous oxides, tantalum oxides; vanadium oxides; and tungsten oxides; metal nitride compounds, such as titanium nitride (TiN), niobium nitride, tantalum nitride and vanadium nitride; metal carbide compounds, including titanium carbide (TiC); metal cyanamide compounds, including titanium cyanamide (TiC_xN_y); metal boride compounds including titanium diboride (TiB₂); metal sulphide compounds, including titanium sulphide (TiS₂); metal halides compounds such as titanium halide, such as TiBr₄ TiCU₁TiCI₃, TiF₃, TiF₄, TiI₄; metal phosphide or phosphate compounds, such as titanium phosphide (TiP); metal sulphate compounds, including titanium sulphates such as Ti₂SO₄ XH₂O, Ti₂(SO₄)₃, and TiOSO₄ XH₂SO₄; metal suicides compounds including titanium suicides (TiSi₂); and/or organometallic compounds such as organic titanium compounds, including Ti(OCH(CH₃)₂)₄, Ti[O(CHz)₃CH₃J₄ and Ti(OCH₃)₄- (CH₃OH)_x.

The cation donor precursor and the at least one metal oxide donor may be calcined at a temperature of between 500⁰C to 1200⁰C for a period between of 0.5 and 40 hours.

More preferably the at least one cation donor precursor and said at least one metal oxide donor are calcined at a temperature of between 600⁰C to 1000⁰C for a period of between 2 hours to 30 hours.

The cation donor precursor and the metal oxide donor are more suitably calcined at a temperature of about 75O⁰C for a period of about 20 hours.

The following example describes an embodiment with reference to the formation of nitrogen doped titania nanosheets. It will be appreciated that other forms of the non-metal doped metal oxide nanosheets of formula (IV) may be formed using similar methods and without departing from the scope of the invention.

Example 1

A metal oxide precursor of Cso.6₈Ti₁.8₃O₄ was prepared by mixing caesium carbonate (CS2CO₃), with titania (TΪO2), and calcined at 75O⁰C in air for 20 hours. Approximately 60 - 70 grams of white crystalline caesium titanate (Cso_.68Ti_1.83O₄) was collected.

A nitrogen doped caesium titanate was formed by calcining the white Cso.68Ti1.₈3O4 powder at about 75O⁰C in an ammonia atmosphere for 2 hrs. The resultant bright yellow powder of nitrogen doped metal oxide has a formula Cso_.68Tii_.83θ_4-xNχ, wherein x is a value greater than 0 and less than 4.

The protonated form of the nitrogen doped titanate (H_0.68Ti_1.e₃O₄.xN_x) was prepared by the ion-exchange of the caesium in Cso_.68Tii_.83θ4_-xN_x with protons by placing the Cso.6₈Ti1.₈₃O₄.xN_x in 1M HCI solution for three days. The H₀.₆₈Ti_1.83O4.xNx was separated and dried.

The resultant yellow H_06STi_{1 8S}O₄-_XN_x (1.2g) was dispersed in an exfoliating agent, tetrabutylammonium hydroxide (TBA⁺OH^') solution (300 cm³, 0.2M), and was shaken for more than 7 days at room temperature to exfoliate the protonated nitrogen doped titanate into a yellow colloidal suspension of Tio_.9iθ_2-xNχ nanosheets.

The exfoliated nitrogen doped titania nanosheets (Tio.₉₁O₂.xNx) were used to create a thin film by dip coating onto a silicon wafer substrate, via an electrostatic layer-by-layer (LBL) self-assembly method. The silicon wafer was first prepared or pre-coated with a polyelectrolyte by dipping the silicon wafer into a polyethylenimine solution (PEI, 0.125 wt%, pH = 9) for 20 min and then rinsing thoroughly with water. Afterwards, the PEI coated silicon wafer was immersed into the colloidal suspension of Tio.₉iO_2-xN_x nanosheets (0.08 g dm^"3, pH = 9) for another 20 min, subsequently removed and rinsed with water and dried with nitrogen gas flow. Each layer of PEI and corresponding layer of Tio_.9i0_2-xNχ nanosheets will hereafter be referred to as the (PEI/Ti_0.9iθ₂-χN_x) bilayer.

Deposition of the (PEI/Ti_o.₉i O_2-XN_x) bilayer was repeated to reach desirable layer numbers for the fabrication of multilayer thin films.

Fig 2A shows an atomic force microscopy (AFM) image of the first layer of Ti_0.9iθ₂-χNχ nanosheets deposited onto a silicon wafer. Sectional analysis of the coated silicon wafer indicates the layer of nanosheets coating is a unilamellar single layer with an ultrathin thickness of around 1 nm and lateral dimensions of sub-micrometer size. This strongly indicates the method used results in the exfoliation of nitrogen doped layered titanate into Tio.giθ2_-XN_X nanosheets. The exfoliated nitrogen doped titania nanosheets were re-ordered or restacked into a layered structure by evaporating the solvent at 5O⁰C until solid Tio_.9iO_2-xN_x nanosheets were obtained.

Fig 2B is an XRD pattern of the restacked Tio_.9iθ₂-_XN_X nanosheets compared to the HTiON intermediate. It can be deduced from Fig 2B that the restacked nanosheets are well-defined layered structures with an interlayer distance of 1.81 nm. This interlayer distance is quite different from that of the parent layered precursor (Ho_.68Tii_.83θ_4-xN_x) shown by in the comparison of traces (a) and (b) in Fig 2B.

Zeta potential measurements demonstrated that the surface charge of Tio.9iθ2_-XN_X nanosheets is around -39 mV, which is very close to that of non- doped Tio.₉iO₂ nanosheets. The respective zeta potentials are illustrated in Fig 3. This clearly indicates that nitrogen doping in the nanosheets has negligible influence on the surface charge of the nanosheets. It is believed that it is important for the non-metal doped metal oxide nanosheets to have a similar surface charge as that of the metal oxide nanosheets counterpart to allow for successful LBL self-assembly of the doped nanosheets via an electrostatic attraction driving force.

Fig 4A is a UV-visible absorption spectra for a colloidal suspension of exfoliated nitrogen doped titania nanosheets that revealed the intrinsic absorption edge of titania nanosheets had a distinct red-shift, in addition to a shoulder absorption up to 450 nm after nitrogen doping. The band gap of the titania nanosheets determined from the corresponding transformed Kubelka-Munk function (K-M) plots (shown in Fig 4B) shifted from 4.38 eV to 4.25 eV after nitrogen doping, which differs from the reported 3.8 eV of Ti_{0 9I}O₂ nanosheets derived from photoelectrochemical measurements. (Sasaki, N., et al., J. Am. Chem. Soc. 2004, 126, 5851-5858). This variation, however, can be derived from the different method employed for band-gap calculation. Similar absorption spectrum for Ti_{0 9}iO₂ nanosheets suspension was also investigated by Sasaki et al., J. Phys. Chem. B 1997, 101 , 10159-10161. Nevertheless, the narrowing trend of the band gap after nitrogen doping is consistent with other evidence including a colour change from white to yellow.

The state of dopant N in the nanosheets was investigated by XPS spectra (see Fig. 4C). The N 1s spectra of both films fabricated from doped and un-doped nanosheets exhibited two peaks (1 and 2), which are assignable to the molecularly chemisorbed Y-N₂ and N species in polyelectrolyte PEI₁ respectively. Very importantly, an addition peak at about 396 eV was observed in the N 1s spectrum for Tio_.9iθ_2-XN_X nanosheets. This peak can be assignable to the atomic β-N in the network of Ti-O-Ti, namely the substitution of lattice O with N, strongly supporting the nitrogen doping in the architecture of nanosheets.

The ratio of N to O in Ti₀₉iO_2-xN_x nanosheets was estimated to be about 4.5 atom%. First-principle calculations further confirmed that the mixture of O 2p with N 2p states instead of isolated states in the band gap is responsible for a decreased band gap of nitrogen doped Tio_.9iO₂ nanosheets by elevating the top of the valence band. As for the shoulder absorption in the longer wavelength range, the role of concomitant oxygen vacancies with the replacement of lattice O with N atoms should be considered. In nitrogen doped bulk titania, oxygen vacancies can be formed to keep the charge balance and the generated localized states are located between 0.75 and 1.18 eV below the conduction band bottom, which can be contributed to the additional visible light absorbance. In the case of nitrogen doped titania nanosheets with macromolecular-sized thickness, the generation of oxygen vacancies is inevitable and hence renders such an additional absorption shoulder.

Figure 5A presents the UV-visible absorbance spectra of multilayer films of (PEI/Tio_.9i0₂-χNχ nanosheets). The absorption measurements conducted immediately after each cycle showed nearly linear increments (see Fig. 5B) of peak-top absorbance at ca. 262 nm, which strongly indicates the success of multilayer thin film growth of Tio_.91O₂.xN_x nanosheets. The peak absorbance of the films based on Tio_.9iθ_2-XN_X nanosheets is comparable to that from un-doped T1₀.91O₂ nanosheets, indicating Tio.₉iθ₂-χN_x nanosheets are also an excellent 2D building block. It is important to note that, in addition to the strong absorption peak of around 262 nm, an evident absorption in the range of 300nm to 450 nm was observed for the films fabricated with Ti_0.9iθ₂-χN_x nanosheets, which is of great importance to induce visible light activity.

Fig 5C illustrates the XRD pattern for a multilayered film of (PEI/Ti₀.₉i0₂-χN_x)io. The as-prepared multilayer film of (PEI/Ti₀.₉iθ₂-χNχ)io presented a broad XRD diffraction peak centring at around 6.5° (2Θ), which is associated with an interlayer spacing of -1.4 nm, a similar value to that for the multilayer films of Tio.9i0₂ nanosheets using both organic and inorganic binders, such as those reported in Wang, L.Z., et al., J. Phys. Chem. B 2004, 108, 4283-4288; Sasaki, T.; et al. Chem. Mater. 2001, 13, 4661-4667; and Wang, L., et al. M. Chem. Mater. 2002, 14, 4827-4832. This XRD peak shifted significantly to higher angle with an interlayer spacing of ~1.1 nm and was observed upon exposure of the film to indoor light in air for about one week. This shrinkage in interlayer spacing may be attributable to the dehydration of the film. It may also be due to the photo-induced decomposition of organic PEI under the irradiation of indoor light.

The photoresponse of the films with 10 layers on ITO glass as photoanodes to visible light in a standard three-electrode photoelectrochemical quartz cells was investigated. Fig. 6 presents the applied potential dependence of photocurrent densities for the photoanode. The measurements were taken with the electrodes in a 0.1 mol-dm^"3 NaOH electrolyte solution and a scan rate of 5 mVs^"1. The illuminated photoanode surface area with the deposition of 10 layers of nanosheets was 1 cm². The turn-on potential of the photocurrent for the photoanode with Ti_{0 9}iθ2 nanosheets was about 0.45 V, while the turn-on potential was about 0.15 V for Ti_{0 9}iθ₂-χN_x nanosheets using a Ag/AgCI reference electrode. The different turn-on potential, determined by the flat- potential of a semiconductor anode (Bard, A. J. J. Phys. Chem. 1982, 86, 172- 177) can be attributed to the derivation of the Fermi level in the Ti_{0 9}iO_2-χN_x nanosheets with the elevated valence band top. After the turn-on potential, the photocurrent of the photoanode with Tio₉iO_2-χN_x nanosheets significantly increased, while only a slight increment for the un-doped photoanode was observed. The limited localized states in the band gap of n-type Ti_{0 9}iO₂ nanosheets should be responsible for the observed slightly enhanced photoresponse to the visible light in the film of un-doped Ti₀₉iθ₂ nanosheets. In contrast, the significant photocurrent enhancement in the film of Ti₀₉iθ₂-χN_x nanosheets can be assignable to the visible light absorption of nitrogen doped nanosheets and possibly together with a small proportion of O vacancies related states photoexcitation.

Fig 7 shows the hydrophilicity property test of (PEI/Tio ₉iθ₂-χN_x)₁₀ film compared to a (PEI/Ti₀9iθ2)io multilayer film. Quartz glass slides were prepared with multilayer films of (PEI/Ti₀9iθ2_-xN_x)io and (PEI/Tiogiθ2)io using the methods described above. The hydrophilicity of these films was assessed by placing a drop of water on each film and irradiating for 30 minutes with visible light. Over a 30 minute period the contact angle of the drop on the (PEI/Tio₉iO_2-xN_x)i₀ reduced significantly, indicating that the hydrophilicity properties of the (PEI/Tio.9iθ2-χN_x)₁₀was better than the non-doped film (PEI/Tio.9i0₂)io .

The improved hydrophilicity properties of films made from the non-metal doped metal oxide nanosheets indicate they may be suitable as defogging or anti- fogging coatings.

In summary, non-metal doped metal oxide nanosheets have been prepared for the first time by the inventors. The optical absorption of the non-metal doped metal oxide nanosheets has been extended to a longer wavelength up into the visible light range. Multilayer films formed with these nanosheets show remarkable enhancement of photocurrent compared to that of undoped metal oxide nanosheets under visible light irradiation. Non-metal doped metal oxide nanosheets may be used as visible light photocatalysis in photoelectric areas. They may also be useful as antifogging and anti-reflective coatings.

It is to be understood that the above embodiments have been provided only by way of exemplification of the invention, and that further modifications and improvements thereto, as would be apparent to persons skilled in the relevant art, are deemed to fall within the broad scope and ambit of the present invention described herein.

In the specification and the claims the term "comprising" shall be understood to have a broad meaning similar to the term "including" and will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the term "comprising" such as "comprise" and "comprises".

Claims

Claims:

1. A method of producing non-metal doped metal oxide nanosheets including the steps of: doping a metal oxide precursor with a non-metal dopant to form a non-metal doped metal oxide; protonating the non-metal doped metal oxide to form a protonated non-metal doped metal oxide; and exfoliating the protonated non-metal doped metal oxide to form non-metal doped metal oxide nanosheets.

2. The method of claim 1 , wherein the metal oxide precursor is a compound having the formula (I):

A_nMyOz (I) wherein:

A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, calcium, magnesium, caesium and francium;

M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; n is a value greater than 0 and equal to or less than 8; and y and z are independently a value greater than 0 and equal to or less than 8.

3. The method of claim 2, wherein the metal oxide precursor of formula (I)

4. The method of claim 1 , wherein the non-metal dopant is selected from the group comprising nitrogen, ammonia, methane, ethane, propane, butane, gas comprising B_xH_y, carbon monoxide, carbon dioxide, hydrogen sulphide, fluorine, carbon, C₆H^N₄, CO(NH₂)₂, CS(NH₂)2, triethylamine, (NH₄J₂CO₃₁ C₂₅H₃₁N₃, alkanes, alkenes, C₁₂H₂₂On, C₂₅H₃₀O₅, C₆H₁₂, C₆H₁₂O₂, C₆H₁₂BNO₃, C₇H₅BF₄O₂, C₇H₇BO₄, H₃N-BH₃, C₆H₅N(C₂Hs)₂-BH₃, CS(NH₂)₂, C₇H₇SO₂, C₇H₁₂O₂S, C₆H₄S, C₄CI₂F₆, C₄H₂F₂N₂, C₄H₈BrF, C₄H₉I, C₅H₃IO₂, C₅H₃FI, C₆H₁₃I, boron, H₃BO₃, C₆H₁₂BNO₃, C₇H₅BF₄O₂, C₇H₇BO₄, H₃N BH₃, C₆H₅N(C₂Hs)₂ BH_3, sulphur,

H₂S, (NH₄)₂S, CS(NH₂)₂, C₇H₇SO₂, C₇H₁₂O₂S, C₆H₄S, NH₄F, C₄CI₂F₆, C₄H₂F₂N₂, C₄H₈BrF, iodine, HIO₃, HIO₄, NH₄I, or NH₄IO₃, C₄H₉I, C₅H₃IO₂, C₅H₃FI₁ Or C₆H₁₃I.

5. The method of claim 1 , wherein the metal oxide precursor is calcined with the non-metal dopant at a temperature of between 200⁰C to 1800⁰C for a period of between 30 minutes and 5 days.

6. The method of claim 5, wherein the metal oxide precursor is calcined in contact with the non-metal dopant at a temperature of about 700⁰C for a period of about 60 minutes.

7. The method of claim 1 , wherein the non-metal doped metal oxide formed has a formula (II): A_nM_yO_z-χD_x (II) wherein: A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, calcium, magnesium, caesium and francium; M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a non-metal dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; n is a value equal to or greater than 0 and less than or equal to 8; x is a value greater than 0 and less than 8; y and z are independently a value greater than 0 and equal to or less than 8, and z-x is a value greater than 0.

8. The method of claim 7, wherein the non-metal doped metal oxide of formula (II) is Cs₀.₆₈Tii.₈₃θ_4-xN_x.

9. The method of claim 1 , wherein the protonation step is carried out by mixing the non-metal doped metal oxide with an acidic solution selected from the group comprising hydrochloric acid, nitric acid, sulphuric acid, phosphoric acid, hydrofluoric acid, hydroiodic acid, hydrobromic acid, acetic acid (HAC), perchloric acid, iodic acid (HIO₃) and periodic acid (HIO₄).

10. The method of claim 1 , wherein the protonated non-metal doped metal oxide produced has a formula (III):

H_mA_n-mM_yO_z-xD_x (III)

wherein:

A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, caesium and francium; M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a non-metal dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; n is a value either equal to m or greater than m and a greater than 0 and equal to or less than 8; y is a value greater than 0 and equal to or less than or 8; x is a value greater than 0 and less than 8; z are independently a value greater than 0 and equal to or less than 8; m is a value between 0 and 8; and z-x is a value greater than 0.

11. The method of claim 10, wherein the protonated non-metal doped metal oxide of formula (III) is H_0.68Tii.₈₃θ_4-xN_x, wherein x is a value greater than 0 and less than 4.

12. The method of claim 1 , wherein the exfoliating step is carried out by mixing the protonated non-metal doped metal oxide with an exfoliating agent selected from the group comprising tetramethylammonium hydroxide; tetraethylammonium hydroxide; tetrapropylammonium hydroxide and tetrabutylammonium hydroxide.

13. The method of claim 1 , wherein the non-metal doped metal oxide nanosheets have the formula (IV):

M_y0_2-xD_x (IV) wherein: M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium;

D is a non-metal dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value greater than 0 and less than 8; and z-x is always a value greater than 0.

14. The method of claim 1 , wherein the method further includes the step of reordering the non-metal doped metal oxide nanosheets to form a layered or pillared non-metal doped metal oxide.

15. The method of claim 14, wherein the non-metal doped metal oxide nanosheets are initially in a suspension and are reordered by evaporating solvent from the suspension to obtain dried layered non- metal doped metal oxide.

16. Non-metal doped metal oxide nanosheets having a formula (IV):

M_yOz-xDx (IV) wherein: M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium;

17. The non-metal doped metal oxide nanosheets of claim 16, wherein the non-metal doped metal oxide nanosheets of formula (IV) are Tio₉i0₂-χN_x, wherein x is a value greater than 0 and less than 2.

18. Use of the non-metal doped metal oxide nanosheets of claim 16 as a photocatalyst.

19. A method of coating substrates with non-metal doped metal oxide nanosheets, including the step of: coating a substrate with non-metal doped metal oxide nanosheets of formula (IV):

M_yO_z-xD_x (IV) wherein: M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a non-metal dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value greater than 0 and less than 8; and z-x is always a value greater than 0.

20. The method of claim 19, wherein the substrate is glass, quartz glass, silicon wafer or ITO glass.

21. The method of claim 19, the method further including the step of adding one or more layers of binder.

22. The method of claim 21 , wherein the binder is selected from an organic polyelectrolyte or charged inorganic nanocluster.

23. The method of claim 21 , wherein the binder is an organic polyelectrolyte selected from polyethylenimine, poly(allylamine hydrochloride) or poly(diallyldimethylammonium) chloride.

24. The method of claim 21 , wherein the binder is charged inorganic nanoclusters selected from the group comprising partially hydrolysed nanoclusters of metal hydroxides including aluminium oxides and hydroxides, chromium oxides and hydroxides, cobalt oxides and hydroxides, ferric oxides and hydroxides, ferrous oxides and hydroxides, nickel oxides and hydroxides, tungsten oxides and hydroxides manganese oxides and hydroxides, titanium oxides and hydroxides, zirconium oxides and hydroxides and vanadium oxides and hydroxides.

25. The method of claim 21 , wherein the binder is applied onto the substrate prior to coating with a layer of the non-metal doped metal oxide nanosheets of formula (IV) and/or between layers of the non-metal doped metal oxide nanosheets of formula (IV).

26. A substrate coated with non-metal doped metal oxide nanosheets of formula (IV):

M_yO_z-xD_x (IV) wherein:

M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a non-metal dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value greater than 0 and less than 8; and z-x is always a value greater than 0.