WO2018039701A1

WO2018039701A1 - Photocatalysts

Info

Publication number: WO2018039701A1
Application number: PCT/AU2017/000179
Authority: WO
Inventors: Yun Liu; Qingbo Sun; Wensheng Shi; Ray WITHERS
Original assignee: The Australian National University; Chinese Academy Of Sciences
Priority date: 2016-08-30
Filing date: 2017-08-30
Publication date: 2018-03-08

Abstract

The invention relates to a material of chemical formula (A⁵⁺ _xTi⁴⁺ _1-x-aTi³⁺ _a)(O_2-y-bN_y) in which A⁵⁺ is a pentavalent metal ion and the preparation of same. In this formula, if x= y, a=b=0; if x>y, b=0 and a=x-y; if x<y, a=0 and b=(y-x)/2. Also, x and y are within the following ranges: 0<x<0.25; and 0<y<0.10. The materials of the invention are useful as photocatalysts.

Description

Photocatalysts

Field

[0001 ] The application relates to a photocatalyst and to a process for making the same. Background

[0002] Ti0₂-based photocatalysts have attracted extensive interest in the fields of photochemistry and photo-physics due to their low cost, low toxicity, chemical stability, natural abundance and ease of preparation. They are now widely used in photo-degradation, dye- sensitized solar cells, hydrogen generation, fillers for paints, organic synthesis, chemical assays etc. making use of their specific physicochemical properties.

[0003] Initially, pure Ti0₂ was extensively studied in respect of its synthesis, properties, applications and theoretical calculations. Different routes have been explored in order to improve the photocatalytic performance of pure Ti0₂, including the control of crystal facets, biphase or multi-phase mixing, size and morphological tuning, core-shell structural design as well as noble-metal decoration. The intrinsic broad band-gap of Ti0₂ (about 3.2eV for anatase and about 3.0eV for rutile), however, determines that it can primarily only absorb ultraviolent light. This means that less than 5% of incident solar energy can be directly used by pure Ti0₂.

[0004] To utilize sunlight more efficiently, metal ions or non-metal ions are often mono-doped into Ti0₂ so that its bandgap can be narrowed and its optical absorption can be extended into the visible light range. Almost all elements in the periodic table have been mono-doped into Ti0₂ to date. However, the incorporation of mono-dopants tends to generate recombination centres in Ti0₂ host materials. These recombination centres are attributed to impurity levels or defect sites caused by extrinsic ion doping, and actually deteriorate the separation of photo-generated electron-hole pairs. Therefore, mono-doped Ti0₂ materials may on one hand narrow the bandgap, but on the other hand greatly lower the quantum efficiency of photo-generated charge carriers. The photocatalytic effect is therefore overall often not significantly improved, although some moderate enhancement has been observed. This has been considered to be a major disadvantage of mono-doping approach. [0005] The strategy of co-doping has been proposed to overcome the disadvantages of mono- doping. The simultaneous incorporation of dopant cations and anions might be capable of balancing the charges of co-doped Ti0₂ system, effectively diminishing recombination, enhancing the separation of electron-hole pairs and maintaining their visible light absorption ability. One of the co-dopants is usually a metal ion and the other is a non-metal ion. The atomic orbitals of doped metal ions mix with that of Ti 3d to induce a change in their conduction bands, and the orbitals of O 2p hybridise with those of non-metal ions to alter their valence bands. Hence, the electronic configuration and energy band structures of Ti0₂ systems are modified by co-doping. However, the location of co-dopants in Ti0₂ is not well understood. Non-metal ions are thought to be located at oxygen lattice sites or in interstitial positions of Ti0₂ or in both. In addition, the synergistic relationship of co-dopants, for instance, whether the cation-anion dopants bind together to form defect pairs or defect clusters or stay as isolated point defects, is also unclear.

[0006] Furthermore, it is technologically difficult to successfully introduce both cation and anion dopants into Ti0₂ in a predictable defect form. For instance, to prepare M and N^{' ~} (where Μ^{5 τ} is a pentavalent metal element and N ^~ is nitrogen anion) co-doped Ti0₂ materials, pre-prepared M⁵⁺ doped Ti0₂ is commonly post-treated with ammonia so as to incorporate N^3" ions through high temperature ammonolysis. Using this process, the resultant doping concentration of nitrogen achieved in final co-doped Ti0₂ samples is very low and generally less than 2 atom%. Moreover, the distribution of co-dopants in Ti0₂ is generally inhomogeneous, since the dopants tend to stay on the surface rather than diffuse into the interior. In the sol-gel synthesis of M and N ^" co-doped Ti0₂, ammonium chloride, ammonia solution, hexamethylenetetramine, urea, ethanediamine and ammonium niobate oxalate hydrate have all been used as nitrogen sources. Post-calcination treatment (commonly above about 400°C) is required for this approach to induce crystallisation, which can potentially lead to loss of nitrogen

5 ^! 3

and consequently change the actual doping ratios of M and N ^" ions. As a result of the presence of recombination centres from such unbalanced dopant ratios and consequent extra defects, the photocatalytic performance of co-doped Ti0₂ has not been greatly improved to date by means of co-doping.

[0007] Therefore, there is a need for a new process to make a highly efficient co-doped titanium dioxide with special defect pairs. Summary of Invention

[0008] In a first aspect of the invention, there is provided a material of chemical formula (A _xTi i_-x-a Ti a)(0₂-y-_{b y}) wherein: A is a pentavalent metal ion; if x=y, a=b=0; if x>y, b=0 and a=x-y; if x<y, a=0 and b=(y-x)/2; 0<x<0.25; and 0<y<0.10. More generally, if x approximately equals y, a and b are both approximately 0. In this context, "approximately equal" may indicate that the absolute value of x-y is less than about 0.005, and "approximately 0" may mean less than about 0.005.

[0009] The following options may be used in conjunction with the first aspect, either individually or in any suitable combination.

[00010] A may be niobium, vanadium or tantalum, or may be a mixture of any two or all of these. It may be niobium.

[0001 1 ] In one option, x=y. In another option, x>y. In a further option, x<y.

[00012] The material may have crystal structure of anatase, rutile, brookite or a mixed crystal of any two or all of these. The structure may be determined by X-ray diffraction.

[00013] The material may contain no phases which are pure oxides or nitrides of Ti or of A. It may contain no phases which are pure Ti0₂. It may contain no phases which are pure Ti₃N₄. It may contain no phases which are pure A₂0₅, e.g. no Nb₂0₅. It may contain no phases which are pure A3N5, e.g. no phases which are pure Nb₃N₅.

[00014] In one embodiment x=0.056 and y=0.079, whereby a=0 and b=0.01 15. In another embodiment x=0.056 and y=0.059, whereby a=0 and b=0.0015 (i.e. b is approximately 0). In a further embodiment x=0.056 and y=0.039, whereby b=0 and a=0.017.

[00015] In a second aspect of the invention, there is a provided process for making a material of chemical formula (A⁵ _xTi⁴⁺ _1-x-a Ti ⁺ _a)(0_2-y-bN_y) wherein A⁵ ' is a pentavalent metal ion and if x=y then a=b=0, if x>y then b=0 and if x<y then a=0; and 0<x<0.25; and 0<y<0.10; said process comprising: a) preparing a solution comprising a titanium salt or complex and a salt or complex of A^{5 r}; b) acidifying the solution and adding a nitrogen ion source to form a reaction mixture; and c) heating the reaction mixture to a temperature of from about 50 to about 300°C for sufficient time to form the material.

[00016] The following options may be used in conjunction with the second aspect, either individually or in any suitable combination.

[00017] The reaction solvent may comprise, or may consist essentially of, an alcohol, e.g. a C I to C6 alcohol.

[00018] Step a) may comprise combining a solution of the titanium salt or complex in a first solvent with a solution of the salt or complex of A⁵ ' in a second solvent, whereby the reaction solvent comprises, or consists essentially of, the first and second solvents. Alternatively it may comprise preparing a solution of the titanium salt or complex in the reaction solvent and adding the salt or complex of A^5" to the resulting solution. As a further alternative step a) may comprise preparing a solution of the salt or complex of A⁵ in the reaction solvent and adding the titani um salt or complex to the resulting solution.

Step b) may comprise adding nitric acid, e.g. concentrated or about 69%w/v nitric acid, to the solution so as to both acidify the solution and add a source of nitrogen ions. The nitric acid may be added to the solution at a volume ratio to the solution of less than about 20%. In this instance, the nitric acid serves to acidify the solution and also acts as the source of nitrogen ions. It is therefore not necessary in this case to separately acidify and add a nitrogen ion source. In this context i t should be noted that nitrogen is commonly added to the reaction in the form of nitrate ions which then convert during the reaction to nitrogen ions (i.e. N^{~ ~}). Accordingly the source of nitrogen ions is commonly a source of nitrate ions. It is thought that the nitrogen atoms in the formula of the materi al of the invention are bonded to the titanium atom(s). This bond may have partial covalent character. Therefore reference to nitride ions (or N^3") throughout this

specification does not necessarily indicate that the nitrogen is fully ionised and may refer to a nitrogen bonded to another atom (e.g. titanium) through a bond that has at least partial ionic character, whilst possibly also having some covalent character.

[00019] Step c) may be conducted in a sealed container. [00020] The titanium salt or complex and the salt or complex of A⁵ may, independently, be a nitrate, a chloride, a sulfate, a bisulfate, a carbonate, a bicarbonate, a metal-organic complex or a hydrate of any one or more of these.

[00021 ] In one embodiment, there is provided a process for making a material of chemical formula (A _xTi ^' _1-x-a Ti^{" '} a)(0_2-y-bN_y) wherein A is a pentavalent metal ion and if x=y then a=b=0, if x>y then b=0 and if x<y then a=0; and 0<x<0.25; and 0<y<0.10; said process comprising: preparing a solution comprising a titanium salt or complex and a salt or complex of A^{5 r} in a reaction solvent; acidifying the solution using concentrated nitric acid to form a reaction mixture; and heating the reaction mixture to a temperature of from about 50 to about 300°C, optionally about 200°C, for sufficient time, e.g. about 12 hours, to form the material. The step of heating may be in a sealed container.

[00022] The material of the first aspect may be made, or may be makeable, by the process of the second aspect. The process of the second aspect may make, or may be suitable for making, the material of the first aspect.

[00023] In a third aspect of the invention, there is provided use of a material according to the first aspect, or made by the process of the second aspect, as a photocatalyst. The use may comprise exposing the material to visible light.

[00024] In a fourth aspect of the invention there is provided a method for degrading an organic compound comprising exposing the compound to visible light in the presence of a material according to the first aspect, or made by the process of the second aspect. The exposing may be conducted in a solvent, e.g. an organic solvent. The solvent may be one that is not photodegradable by visible light (above 400 nm) in the presence of the compound. The exposing may be in the absence of UV radiation, i.e. the method may comprise exposing the compound to visible light having no UV component in the presence of the material. In some instances the exposing may be in the presence of both UV radiation and visible light. It may be in the presence of radiation covering, or within, the range of 200 to 800nm.

Brief Description of Drawings

[00023] Figure 1 : Schematic drawing of a typical defect pair, Nb-N-2Ti, formed in materials of

5 1 4 3 the invention: (A _xTi _1-x)(0_2-yN_y). In this figure, the large central ball is N^{' "}, the bottom ball is Nb⁵ and the two balls on either side of N ^~ represent 2 Ti^4' ions. Nb⁵ and N^3~ exist as a defect pair either directly bonded together or in close proximity to each other in the co-doped Ti0₂.

[00024] Figure 2: XRD patterns of the (A^{5 :} _xTi⁴"i_-x-aTi^{3 :} _a)(0_2-y_-bN_y) materials with fixed Nb^{5 '} ⁱ 3

doping concentration (5.6 at.% Nb^" ) and varied N^{' ~} doping levels (3.9at.%, 5.9at.% and 7.9at.%). This illustrates that the synthesized materials have an anatase structure and do not contain an impure phase. The numbers in Figure 2 represent the atomic percentage relevant to Ti ' (e.g. 7.9N^3"+5.6Nb^5÷ represents 7.9 at.% N^3"+5.6 at.% Nb⁵").

[00025] Figure 3: XPS data of the synthesized (A⁵ Yri^{4 '} _{1 -x-}aTi³"a)(0_2-y-bNy) materials with fixed

5 ^! 5 ^L 3

Nb doping concentration (5.6 at.% Nb ) and varied N^{' ~} doping levels (3.9at.%, 5.9at.% and 7.9at.%). The presence of dopants (A and N ions) and Tr is confirmed by XPS analysis. It

3 5 4 3 ' also shows that the generation of Ti^' ions can be controlled by x and y in (A _xTi i_-x-aTi _a)(0_2- v-bNy). (a), (b) and (c) show XPS data for the three materials over different ranges of binding energy.

[00026] Figure 4: UV-Vis absorption spectra of the synthesised (A^{5 r} _xTi⁴ i_-x-aTi³ _a)(0_2-y-bN_y) materials with fixed Nb^" doping concentration (5.6 at.% Nb ) and varied N^{" ~} doping levels (3.9at.%), 5.9at.%) and 7.9at.%). This demonstrates that the materials can absorb light from ultraviolet to visible light wavelengths.

[00027] Figure 5: Photo-degradation rates of rhodamine B over time under visible light (λ>400 nm) using the (A^5"r _xTi^4"'^" _1-x-aTi^3* _a)(0_2-y-bN_y) material with Nb^5, doping concentration (5.6 at.%

5 " 3

Nb ) and tunable N ^" doping levels (3.9at.%, 5.9at.% and 7.9at.%>). Results using a commercial P25 (Ti0₂ with anatase to rutile phase ratio of 75:25) are also shown.

Description of Embodiments

[00028] The invention relates to the design of defect chemistry for highly efficient Ti0₂-based photocatalysts and to a method for producing them. Extrinsic pentavalent cations and nitrogen anions are cooperatively introduced into Ti0₂. Co-doped cation and anion may be directly bonded together or exist in close proximity to each other, fonning defect pairs in the co-doped Ti0₂ materials. A proposed structure for defect pairs in the compounds of the invention is shown in Figl . The materials of the invention may be synthesised by means of a solvothermal route involving reactions of an alcohol with nitric acid. The resulting Ti0₂-based photocatalyst is highly efficient for photo -degradation, hydrogen generation and organic synthesis and may also be used in dye-sensitized solar cells.

[00029] The invention utilises a novel design strategy to produce a novel Ti0₂-based material

5 i^~ 4^J 3^~ _· with a unique defect structure. The material has chemical formula (A _xTi i_-x-aTi ^' _a)(0_2-y-bNy), in which 0<x<0.25 and 0<y<0.10, and A⁵ is a pentavalent cation, for example, V⁵ lr . Ta⁵ ions or a mixture thereof. In the synthesis of this material, nitrogen ions may be provided by reaction of the solvent and nitric acid (which is either added discretely or generated in situ). Post-calcination treatment is not required in the present synthetic process in order to crystallize the materials and in embodiments of the invention is not employed. During the preparation of the materials of the invention, A^{5 r} and nitrogen ions are simultaneously incorporated into the synthesized co-doped Ti0₂. It should be noted that the process does not always result in complete incorporation of all starting materials and hence the ratio of starting materials does not necessarily reflect the ratio of atoms in the product. Accordingly, the formula set out above for the material of the invention does not necessarily reflect a ratio of starting materials used in its production.

[00030] The materials of the present invention may be capable of absorbing electromagnetic radiation from ultraviolet through the entire visible light range, i.e. from 200 nm to 800 nm. This makes them extremely efficient as harvesters of electromagnetic radiation.

5 1 4 3 -'■

[00031 ] The invention relates to materials of chemical formula (A _xTi _1-x-aTi _a)(0_2-y-bN_y).

5 3

This therefore represents a titanium dioxide which has been co-doped with A and N^{~ "} ions. To the extent that there is an excess of A^{5 r} ions over N ^~ ions, some Ti⁴' ions are reduced to Ti³ ' ions to maintain electrical neutrality (dictated by parameter a). Similarly, to the extent that there are excess ^~ ions, there are corresponding vacancies (represented by a deficiency of oxygen atoms dictated by parameter b) in order to maintain electrical neutrality. These vacancies may be viewed as holes in the structure of the material

[00032] In certain embodiments of the invention, there are no vacancy or Ti^' ions, i.e. a=b=0 in

5 4 ^

the above formula, whereby the material has a formula (A _xTi i_-x)(0_2-yN_y). In this instance, x=y, i.e. there are equal numbers of A^5" ions and N ^~ ions. [00033] Parameter x is commonly in the range of 0<x<0.25. In particular, it is not 0. It may be between about 0 and 0.2, 0 and 0.15 0 and 0.1, 0 and 0.05, 0.1 and 0.25, 0.15 and 0.25, 0.2 and 0.25, 0.1 and 0.15, 0.15 and 0.2 or 0.1 and 0.2, e.g. about 0.05, 0.1 , 0.15, 0.2 or 0.24. Parameter y is commonly in the range 0<y<0.10. It also is not 0. It may be between about 0.01 and 0.1, 0.05 and 0.1 , 0 and 0.09, 0 and 0.05 or 0.02 and 0.05, e.g. about 0.01 , 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1. These ranges may or may not include the end values, provided that in no case is x=0 or less, or 0.25 or more, and in no case is y=0 or less, or 0.1 or more.

[00034] Parameter x may be adjusted during synthesis by adjusting the proportion of A^" to Ti in the solution prior to acidification. Parameter y may be adjusted during synthesis by adjusting the amount of nitrogen ion source used during the acidification step. Parameters a and b may be adjusted during synthesis by appropriate adjustment of x and y as discussed above. Thus the greater the proportion of A⁵ to Ti⁴ in the solution prior to acidification, the larger x will be and the greater the ratio of nitrogen to the solution of A^5" and Ti^4", the larger y will be. Since the values of a and b are a direct consequence of the values of x and y, appropriate adjustment of x and y can be used to achieve desired values for a and b.

[00035] Commonly a and b, independently, are less than about 0.05, or less than about 0.04, 0.03 or 0.02. They may, independently, be about 0.005 to 0.05, 0.005 to 0.02, 0.005 to 0.01 or 0.01 to 0.02. They may, independently, be about 0, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.01 1, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045 or 0.05. It will be appreciated that in general either a or b or both is 0.

[00036] A may be any atom, commonly a metal atom, capable of being in the form of a stable pentavalent positive ion. It may be a transition metal atom. It may, for example, be vanadium, niobium or tantalum.

[00037] The resulting material may have a crystal structure which is anatase, or rutile, or brookite or it may have a structure which is a mixture of any two or more of these. Typically, the material is a homogeneous material, i.e. the distribution of dopant ions is even throughout. There may be no discrete regions, optionally no crystallites or no phases, of an oxide or nitride of Ti or of A, i.e. there may be no pure Ti0₂, Ti₂0₃, Ti N₄, A₂0₅ or A₃N₅. Determination of crystal structure is typically conducted using x-ray diffraction. [00038] In order to prepare the material of the invention, a solution of a titanium salt or complex and a salt or complex of A⁵ is prepared in a reaction solvent. Commonly the titanium salt or complex is a Ti salt or complex. Each salt or complex, independently, may be a nitrate, a carbonate, a bicarbonate, a sulfate, a bisulfate, a chloride, a bromide, an alkoxide or some other suitable salt. Mixtures of any two or more of these may also be used. Suitable complexes include titanium complexes of dicarboxylic acids or hydrates thereof, e.g. C₈H₂o0₄Ti, C₁₂H₂₈0₄Ti, C₁₆H₃₆0₄Ti, or ammonium niobate oxalate hydrate. In some embodiments the components are simply dissolved in the solvent. In other embodiments separate solutions are prepared, i.e. a solution of a titanium salt or complex in a first solvent and a solution of a salt or complex of A⁵⁺ in a second solvent, and the two solutions combined. In this case the reaction solvent will be a combination of the first and second solvents, which is formed when the two solutions are combined. In some instances the first and second solvents are the same, however they may be different. If they are different, they should be miscible. Commonly at least one, optionally both, of the solvents (or the reaction solvent) is an alcohol. Suitable alcohols include CI to C6 straight chain, branched chain and cyclic alcohols. Examples include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, isopentanol, cyclopentanol, n-hexanol, cyclopentylmethanol, 3-methylcyclopentanol etc.

[00039] The concentration of titanium in the solution may be from about 5 to about 200mM, or from about 5 to 100, 5 to 50, 5 to 20, 10 to 200, 50 to 200, 100 to 200, 20 to 100, 20 to 50, 50 to 100 or 30 to 70mmol, e.g. about 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180 or 200mM. A suitable concentration is about 50mM. The concentration of A⁵ in the solution may be from about 0.1 to about 30mM, or about 0.1 to 20, 0.1 to 10, 0.1 to 5, 0.1 to 1, 1 to 20, 5 to 20, 10 to 20, 1 to 10, 1 to 5 or 5 to lOmM, e.g. about 0.1, 0.5, 1 , 5, 10, 15, 20, 25 or 30mM. The ratio of A to Ti in the solution may be about 0.005 to about 0.25 (i.e. 1 :200 to 1 :4) on a mole basis, or about 0.005 to 0.1 , 0.005 to 0.05, 0.005 to 0.01 , 0.01 to 0.25, 0.05 to 0.25, 0.1 to 0.25, 0.01 to 0.10 0.01 to 0.05 or 0.05 to 0.1, e.g. about 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 , 0.15, 0.2 or 0.25. If the solution is prepared by combining separate solutions of Ti^{4 r} and A⁵ ' , the skilled person will be readily able to determine, based on the above concentrations, suitable concentrations of these separate solutions and a suitable ratio in which to mix them.

[00040] Prior to acidification, the solution may be maintained for a period at moderate, e.g. ambient temperature. The moderate temperature in this instance may be from about 15 to about 30°C, or from 15 to 25, 15 to 20, 20 to 30, 25 to 30 or 20 to 25°C, e.g. about 15, 20, 25 or 30°C. The period may be from about 10 minutes to about 60 hours, or from about 10 minutes to about 20 hours, 10 minutes to 1 hour, 10 to 30 minutes, 30 minutes to 60 hours, 1 to 60 hours, 5 to 60 hours, 10 to 60 hours, 30 to 60 hours, 1 to 20 hours, 1 to 10 hours, 30 minutes to 12 hours, 30 minutes to 6 hours or 1 to 6 hours, e.g. about 10, 20, 30, 40 or 50 minutes or about 1 , 2, 3, 4, 5, 6, 12, 18, 24, 30, 36, 42, 48, 54 or 60 hours. During this period, the solution may be stirred, swirled, shaken or otherwise agitated for part or all of the period, or may be left unagitated.

[00041 ] The resulting solution is then acidified with addition of a source of nitrogen ions. This may for example be nitrate or a source thereof. A convenient way to achieve this is to add nitric acid. The nitric acid may be concentrated. It may have a w/v concentration of about 69%. Alternatively, it may have a concentration either above or below this value. It may, for example, have a concentration of from about 10 to about 80% w/v, or about 10 to 70, 10 to 60, 10 to 50, 10 to 40, 20 to 80, 40 to 80, 60 to 80 or 30 to 70%, e.g. about 10, 20, 30, 40, 50, 55, 60, 65, 70, 75 or 80%.

[00042] The acidification may be achieved by addition of from about 5 to about 200ml concentrated nitric acid per litre of Ti/A solution, or about 5 to 150, 5 to 100, 5 to 50, 5 to 10, 10 to 200, 20 to 200, 50 to 200, 100 to 200, 10 to 100, 10 to 50 or 50 to l OOml/litre, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180 or 200ml/litre. It may be achieved by adding about 5 to about 200mmol nitric acid per litre of solution, or about 5 to 100, 5 to 50, 5 to 20, 10 to 100, 50 to 100, 100 to 200 or 100 to 150mmol nitric acid per litre of solution, e.g. about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180 or 200mmol nitric acid per liter of solution. The molar ratio of nitrate to Ti may be from about 0.5 to about 80 (i.e. 1 :2 to 80: 1), or about 0.5 to 50, 0.5 to 20, 0.5 to 10, 0.5 to 5, 0.5 to 1 , 1 to 80, 10 to 80, 20 to 80, 50 to 80, 1 to 50, 1 to 20, 1 to 10 or 10 to 50, e.g. about 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 or 80. The molar ratio of nitrate to A⁵⁺ may be from about 10 to about 1600 (i.e. about 10: 1 to about 1600: 1), or about 10 to 1000, 10 to 500, 2 to 100, 100 to 50, 50 to 1600, 100 to 1600, 500 to 1600, 1000 to 1600, 10 to 1000, 10 to 500, 10 to 100, 100 to 100 or 100 to 500, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1000, 1 100, 1200, 1300, 1400, 1500 or 1600. The molar ratio of nitrate to Ti plus A may be from about 0.5 to about 80 (i.e. 1 :2 to 80: 1 ), or about 1 to 80, 50 to 80, 10 to 80, 20 to 80, 50 to 80, 0.5 to 50, 0.5 to 20, 0.5 to 10, 0.5 to 2, 1 to 50, 10 to 10, 10 to 50 or 30 to 60, e.g. about 0.5, 1 , 1 .5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 or 80.

[00043] The acidified solution is then heated to promote reaction. The heating may be to a reaction temperature of from about 50 to about 300°C, or from about 50 to 200, 50 to 100, 100 to 300, 200 to 300 or 100 to 200°C, e.g. about 50, 100, 150, 200, 250 or 300°C. It will be understood that many of the solvents described above have boiling points below or within the above ranges. If the reaction temperature is above the normal boiling point of the solvent, or of any of the solvents, or of an azeotrope of the solvents, then it will be necessary to conduct the heating under increased pressure. The heating may be conducted under increased pressure even if this is not the case. In the present context, "increased pressure" refers to a pressure above normal atmospheric pressure. It may be, for example, between about 1 and about 120 atmospheres, or about 1 and 100, 1 and 80, 1 and 50, 1 and 10, 1 and 5, 2 and 10, 5 and 120, 10 and 120, 50 and 120, 100 and 120, 5 and 100, 5 and 50, 5 and 20, 20 and 50 or 20 and 100 atmospheres, e.g. about 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 1 10 or 120 atmospheres. The increased pressure may be achieved by applying the desired pressure, or it may be achieved by conducting the heating in a sealed container. Commonly the reaction is conducted in a sealed container. This may assist in achieving the required pressure as described above. It may also assist in preventing loss of volatile reagents.

[00044] The heating may be conducted for sufficient time for formation of the material of the invention. The sufficient time may be from about 10 minutes to about 24 hours, or from about 1 to 24 hours, 12 to 24 hours, 10 to 60 minutes, 10 to 30 minutes, 30 to 60 minutes, 1 minutes to 12 hours, 30 minutes to 6 hours, or 20 minutes to 2 hours, e.g. about 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 minutes, or about 1, 2, 3, 4, 5, 6, 12, 18 or 24 hours.

[00045] In some instances, the acidified solution may be maintained for a period at moderate, e.g. ambient temperature prior to heating. The moderate temperature in this instance may be from about 15 to about 30°C, or from 15 to 25, 15 to 20, 20 to 30, 25 to 30 or 20 to 25°C, e.g. about 15, 20, 25 or 30°C. The period may be from about 10 minutes to about 60 hours, or from about 10 minutes to about 20 hours, 10 minutes to 1 hour, 10 to 30 minutes, 30 minutes to 60 hours, 1 to 60 hours, 5 to 60 hours, 10 to 60 hours, 30 to 60 hours, 1 to 20 hours, 1 to 10 hours, 30 minutes to 12 hours, 30 minutes to 6 hours or 1 to 6 hours, e.g. about 10, 20, 30, 40 or 50 minutes or about 1 , 2, 3, 4, 5, 6, 12, 18, 24, 30, 36, 42, 48, 54 or 60 hours. During this period, the acidified solution may be stirred, swirled, shaken or otherwise agitated for part or all of the period, or may be left unagitated.

[00046] The materials of the present invention may be used as photocatalysts. They may be used to catalyse a photochemical reaction. The photochemical reaction may be a photodegradation reaction. Thus the materials may catalyse, or promote, or accelerate, a photochemical, optionally photodegradation, reaction. This reaction may be a reaction of an organic compound or of an organometallic compound. It may be conducted at room temperature or below room temperature or above room temperature. It may, for example, be conducted at about 0 to about 100°C, or about 0 to 50, 0 to 20, 0 to 10, 10 to 100, 20 to 100, 50 to 100, 15 to 30 or 20 to 50°C, e.g. about 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100°C. It may be conducted in a solvent, e.g. in an aqueous solvent. The material of the invention may be present in the solvent at a concentration of from about 0.1 to about l Og/L, or about 0.1 to 5, 0.1 to 2, 0.1 to 1 , 0.1 to 0.5, 0.5 to 10, 1 to 10, 2 to 10, 5 to 10, 1 to 5, 1 to 2 or 0.5 to 2g/L, e.g. about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 or lOg/L.

[00047] The design strategy for the present invention involves the cooperative introduction of extrinsic pentavalent cations and nitrogen anions into Ti0₂. These are chemically bonded together, which induces large distorted defect pairs in Ti0₂ materials to form the novel compounds of the present invention. These may be viewed as having the simplified chemical formula (Α^{5 '} _χΤί⁴ V_x)(0_2-yN_y),where 0<x<0.25 and 0<y<0.10, A⁵ =V^{5 '} , lr and Ί <V . The value of x and y can be independently tuned within these ranges. As noted elsewhere, if x and y are different, adjustments to this formula must be made, either by reducing a small amount of Ti⁴⁺ to Ti³ to account for an excess of x over y, or by introducing vacancies into the structure (representing a deficiency in oxygen relative to the above formula) to account for an excess of y over x.

[00048] A suitable example of the synthesis of the material of the invention is described below. This is intended for illustration purposes and is not to be taken as in any way limiting on the invention.

[00049] In the suitable example, synthesis of the materials of the invention involves reaction of an organic solvent with nitric acid and a solvothermal step. Thus the organic solvents react with the nitric acid, which provides nitrogen ions for formation of the materials. Solvothermal reactions are used to form defect pairs in Ti0₂ and to synthesize the materials of the invention. Suitable organic solvents include all types of alcohol, such as methanol, ethanol, isopropanol and mixtures thereof. Nitrogen is therefore cooperatively introduced into Ti0₂ with A⁵ ions

5¹ 4 ⁺

under solvothermal conditions. The concentration of nitrogen ions in (A _xTi ^' _1-x)(0_2-yN_y) is commonly between 0 and 10 at.% (0<y<0.10). The volume ratio of nitric acid to reaction solvent may be from 0 to 0.2.

[00050] Suitable A⁵ ions in the materials of the invention include V⁵ , Nb³ , Ta⁵ and other pentavalent cations or a mixture of any two or more thereof. A^{5 '} ions are commonly introduced into the reaction in the form of their nitrate, chloride, sulphate or carbonate, a hydrated salt of any of these or an organometallic compounds, such as ammonium niobate oxalate hydrate, of the A⁵⁺ ion. A⁵ ions are cooperatively introduced into Ti0₂ together with nitrogen under solvothermal conditions. The concentrations of A^{5^} ions in the materials of the invention are between 0 and 25 atom% (0<x<0.25). The ionic concentration of A⁵ ions in the reaction solvent may be from O.OOOIM up to its saturated solubility, or from 0.0005, 0.001 , 0.005, 0.01 , 0.05 or 0.1M up to its saturated solubility. The saturated solubility will depend on the nature of the solvent and of A^{5 r}, but is readily determinable by routine methods or by references to the open scientific literature.

[00051 ] Ti⁴ ions in the materials of the invention are commonly introduced into the reaction in the form of a nitrate, chloride, sulphate or carbonate, the hydrated salts of these or an organometallic compound of Ti. The concentration of Ti ions in reaction solvent may be from O.OOOIM up to its saturated solubility, or from 0.0005, 0.001, 0.005, 0.01, 0.05 or 0.1M up to its saturated solubility. The saturated solubility will depend on the nature of the solvent, but is readily determinable by routine methods or by references to the open scientific literature.

[00052] The solvothermal step for synthesizing the materials of the invention is commonly conducted at a reaction temperature from 50°C to 300°C and reaction time of at least 0.5h in an autoclave. Thus initially the starting materials which provide A⁵ ' and Ti⁴ cations (as described earlier) are added into their respective reaction solvents at around room temperature, and the resulting solutions combined to form a single transparent solution after stirring. The molar ratios of A^{5 r} and Ti⁴ cations can be adjusted in the reaction solvents to achieve a desired value of x in the formula (A⁵ _xTi⁴¹ i_-x-aTi^{3 T} _a)(0_2-y-bN_y) of the final product. Then, nitric acid is added into the solution to form another transparent solution. The proportion of N in the final product may be controlled by suitable adjustment of the amount of nitric acid added. After stirring at room temperature for some time, the resulting solution is transferred into an autoclave and maintained at a temperature of between about 50°C to 300°C for at least about 0.5h to fonn the material of the invention. Once the autoclave is cooled down, the solid product is separated and washed with ethanol and water. The final product is then obtained by centrifuge and drying.

[00053] The concentration of Ti ions in the (Λ' , Γι ¹ _l-x-a rr _a)(()_:-v._b\_v) materials of the invention may be tuned by adjusting the ratios of A⁵ and N ions (i.e. by adjusting the values of x and y). This may be accomplished as described earlier. Thus if x>y, there is Ti^{3 '} ions in materials in order to obtain a charge balance. If x=y, there is no Ti^' ions in the claimed compounds since the charge is fully balanced by the co-doped A^' and N^{~ ~} ions. If x<y, there are no Ti' ions and there are oxygen vacancies to compensate for the extra positive charge.

[00054] A⁵ and nitrogen ions are both homogeneously distributed in the materials of the invention. The defect pairs produced by them are similarly homogeneously distributed. The materials of the invention may have crystal structure of anatase, rutile, brookite or of a mixed crystal structure of these. Commonly the materials of the invention do not contain any impurity of Nb₂0₅, Ti0₂, TiN or Nb₃N₅. They may contain no Ti₂0 .

[00055] The materials of the invention may be capable of absorbing electromagnetic radiation from ultraviolet to visible wavelengths (200 nm to 800 nm). They may be capable of absorbing electromagnetic radiation and/or of performing as photocatalysts, across the full range of from about 200 to about 800nm, or about 200 to 750, 200 to 700, 200 to 650, 200 to 600, 400 to 800, 400 to 750, 400 to 700, 400 to 650 or 400 to 600nm. They may be capable of absorbing electromagnetic radiation and/or of performing as photocatalysts at some wavelength(s) above about 400nm, or above about 450, 500, 550, 600, 650 or 700nm. They may also be capable of absorbing electromagnetic radiation and/or of performing as photocatalysts outside these ranges. They may be used in the field of photo-degradation, hydrogen generation, organic synthesis and dye-sensitized solar cells. They may be usable in any one or more, optionally all, of these applications in the wavelength ranges described above.

Example

[00056] The synthesis of (A⁵⁺ _xTi⁴ i_-x-aTi³⁺ _a)(0_2-y-_bN_y), where x=0.056 and y=0.059, is described as follows. 1) 0.68 ml TiCl₄ was injected into 120 ml ethanol to form a first transparent solution (A);

2) 0.092g NbCls was then dissolved in the above solution (A) to form transparent solution (B);

3) Solution (B) was stirred at room temperature for 1 h;

4) 4ml HN0₃ solution (69% w/v) was then added into stirred solution (B) and stirred for a further 0.5h to form transparent solution (C);

5) Solution (C) was transferred into an autoclave, which was then sealed;

6) The autoclave was kept in an oven at 200 °C for 10 h;

7) The reaction products were washed with ethanol and water and then centrifuged to obtain the final product. Table 1 shows the compositions of three different samples obtained using different volumes of nitric aci d.

[00057] Table 1. Chemical compositions of materials prepared at three typical conditions. The composition is calculated by their related XPS data.

[00058] Figure 2 to 4 show data for the above products. Thus Figure 2 shows XRD data to clarify crystal structure of the products, Figure 3 shows XPS data to determine elemental composition (see Table 1 above) and Figure 4 shows UV-Vis absorption, confirming that the compounds absorb through the entire visible range (400-700nm) of the spectrum.

[00059] The products were then tested for photochemical catalytic activity as follows. [00060] Rhodamine B (RhB) was chosen as a model organic compound to evaluate the photocatalytic properties of the synthesized (N,Nb) co-doped anatase Ti0₂ nanocrystals. 20 mg/L RhB solution was firstly prepared by mixing 20 mg RhB with 1 L distilled water. Then, the (N,Nb) co-doped Ti0₂ photocatalyst was added into a reactor filled with the RhB solution. The mass concentration of photocatalyst was fixed at l g/L (i.e. lg photocatalyst was added into 1L RhB solution). A 500 W Xe lamp with a cut-off wavelength of 400 nm was used to activate the photocatalytic reaction so that the entire experiment was conducted using only visible light. The distance between the Xe lamp and the reactor was maintained constant at 10 cm. Before visible light illumination, the RhB solution containing the photocatalyst was stirred for 30 minutes in the dark in order to achi eve the adsorption equil ibrium of RhB on the surfaces of the (N,Nb) co-doped anatase Ti0₂ nanocrystals. The Xe lamp was then switched on. A small amount of the reaction solution was removed and analyzed to determine degradation rate of RhB. The upper clear solution, which was obtained by centrifugation (12000 r/min), was measured by UV-Vis spectrometer. The peak of RhB at 552nm was used to characterise its decomposition. After the measurement, the clear solutions were returned to the reactor. This process was repeated until the RhB was completely decomposed. Less than 20 minutes were required for nearly complete RhB decomposition (see Figure 5).

[00061 ] High efficient photo-catalysts such as those of the present invention are useful in many applications, including photo-degradation, hydrogen generation, organic synthesis and dye- sensitized solar cells. In particular, they become more useful if they can efficiently make use of sunlight as the driving force. The present invention provides a novel Ti0₂-based photocatalyst designed according to the theory of defect chemistry. Nb-N-2Ti defect pairs are constructed in a co-doped titanium dioxide by methods described herein. The resulting materials absorb light across the entire visible light range and can convert the light energy to chemical energy with high efficiency. In respect of photo-degradation (e.g. Rhodamine B), they show very high photo- degradation rates. This provides excellent potential for environmental clean-up applications. Fig. 5 shows the reduction in Rhodamine B under irradiation at wavelength over 400nm in the presence of the materials detailed in Table 1. It can be seen that these result in far higher degradation rates than a comparable catalyst P25.

Claims

CLAI MS

1 . A material of chemical formula (A⁵ _xTi⁴ i_-x-aTi^3† _a)(0_2-y-bNy) wherein:

- A^{5 1} is a pentavalent metal ion;

- if x= y, a=b=0;

- if x>y, b=0 and a=x-y;

- if x<y, a=0 and b=(y-x)/2;

0<x<0.25; and

0<y<0.10

2. The material of claim 1 wherein A is selected from the group consisting of niobium, vanadium and tantalum.

3. The material of claim 1 or claim 2 wherein A is niobium.

4. The material of any one of claims 1 to 3 wherein x=y.

5. The material of any one of claims 1 to 3 wherein x>y.

6. The material of any one of claims 1 to 3 wherein x<y.

7. The material of any one of claims 1 to 6 having a crystal structure which is selected from the group consisting of anatase, rutile, brookite or a mixed ciystal of any two or all of these.

8. The material of any one of claims 1 to 7 which contains no pure oxide or nitride of Ti or of A.

9. The material of claim 1 wherein x=0.056 and y=0.079, whereby a=0 and b=0.01 15.

10. The material of claim 1 wherein x=0.056 and y=0.059, whereby a=0 and b=0.0015.

1 1 . The material of claim 1 wherein x=0.056 and y=0.039, whereby b=0 and a=0.017.

12. A process for making a material of chemical formula (A^{5 r} _xTi⁴ Y_x-aTi³ )(0_2-y-bN_y) wherein A⁵ ' is a pentavalent metal ion and if x=y then a=b=0, if x>y then b=0 and if x<y then a=0; and 0<x<0.25; and 0<y<0.10;

said process comprising:

a) preparing a solution comprising a titanium salt or complex and a salt or complex of Α^5τ in a reaction solvent;

b) acidifyin g the solution and adding a source of nitrogen ions to form a reaction mixture;

c) heating the reaction mixture to a temperature of from about 50 to about 300°C for sufficient time to form the material.

13. The process of claim 12 wherein the reaction solvent comprises an alcohol.

14. The process of claim 13 wherein the alcohol is a C I to C6 alcohol.

15. The process of any one of claims 12 to 14 wherein step a) comprises combining a solution of the titanium salt or complex in a first sol vent with a solution of the sal t or complex of A⁵ in a second solvent, whereby the reaction solvent comprises the first and second solvents.

16. The process of any one of claims 12 to 15 wherein step b) comprises adding nitric acid to the solution so as to both acidify the solution and add a nitrogen ion source.

17. The process of claim 16 wherein the nitric acid is about 69%w/v.

18. The process of claim 17 wherein the nitric acid is added to the solution at a volume ratio to the solution of less than about 20%.

19. The process of any one of claims 12 to 18 wherein the titanium salt or complex and the salt or complex of A⁵ are, independently, selected from the group consisting of a nitrate, a chloride, a sulfate, a bisulfate, a carbonate, a bicarbonate, a metal-organic complex or a hydrate of any one or more of these.

20. The process of any one of claims 12 to 19 wherein step c) is conducted in a sealed container.

21. Use of a material according to any one of claims 1 to 1 1 as a photocatalyst.

22. Use according to claim 21 comprising exposing the photocatalyst to visible light.

23. A method of degrading an organic compound comprising exposing said compound to visible light in the presence of a material according to any one of claims 1 to 1 1.