SODIUM BISMUTH TITANATE CATALYST Related Application [0001] This application claims priority from Australian Patent Application No. AU2021903326, filed on 18 October 2021, the entire contents of which is incorporated herein by reference. Field [0002] The present invention relates to the field of catalysts. In particular, the present invention is directed to a catalyst for splitting water into hydrogen plus hydrogen peroxide. In one particularly preferred embodiment, the catalyst of the invention can be used to split seawater into hydrogen plus oxygen or hydrogen plus hydrogen peroxide. In another embodiment, the catalyst of the invention can be used to split seawater into hydrogen alone (where the oxygen remains dissolved in the water). However, it will be appreciated that the invention is not limited to this particular field of use. Background [0003] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field. [0004] In the transition away from a carbon-based fossil fuel economy, hydrogen gas is considered to be a potential fuel of the future as its only combustion product is water. Hydrogen gas can be produced in a number of ways, but current methods fall into either “blue” (reformed fossil fuels) or “green” (splitting of water using renewable sources power or biomass) categories. [0005] In order avoid the use of “blue” carbon-based hydrogen sources, the efficient electrolysis of water powered by a “green” renewable energy source is perhaps the most sought-after technology at present. In particular, the ability to use solar power to split water and produce hydrogen and oxygen has been the focus of much research in recent times, with the recognition that catalysts are required to lower the activation barrier to split water and hence harvest more energy than is required to split water. However, current technologies have been hampered due to inefficient catalyst design, with large electrolytic cell overpotentials required to overcome these inefficiencies. Indeed, many current technologies require the application of external power in addition to split water and create hydrogen gas. [0006] In another example, hydrogen peroxide (H ₂ O ₂ ) is a useful oxidant that is widely used in mining, electronics, pulp, and textile bleaching industries and has seen demand rise as a disinfectant following the emergence of COVID-19. However, the production of liquid hydrogen peroxide is currently limited by the high energy consumption and complex preservation and separation technologies required, as a “green” method is not widely commercially available. [0007] Accordingly, there is a need for a catalyst that is more efficient in splitting water, preferably seawater. The catalyst may be a photocatalyst, piezocatalyst, or photo-piezocatalyst that splits water to produce hydrogen plus hydrogen peroxide, or hydrogen plus oxygen, when illuminated with actinic radiation, such as sunlight, and/or mechanical or acoustic vibration. [0008] It is an object of the present invention to overcome or ameliorate one or more the disadvantages of the prior art, or at least to provide a useful alternative. Summary of Invention [0009] In a first aspect of the present invention, there is provided an NBT-based catalyst adapted to have band energies comprising a conduction band (CB) of less than about -0.18 eV and a valence band (VB) of greater than about 1.60 eV, wherein the NBT is defined by the approximate formula Na _0.5±x Bi _0.5±y Ti _1.0± zO ₃ , wherein 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5, x + y ≤ 0.5, and z < 0.2. [00010] As the skilled person would appreciate, NBT is a crystalline solid solution with an average stoichiometric ratio of Na _0.5 Bi _0.5 TiO ₃ . However, NBT is known to exhibit variable concentrations of Na and Bi, and may exhibit variable Ti content as well. It is also known that these stoichiometric ratios can vary with temperature or the presence of dopant and/or impurity ions. Accordingly, the NBT-based catalyst of the present invention may comprise any solid solution of NBT. [00011] The skilled person will appreciate that when an NBT-based catalyst has band energies whereby the CB is about -0.18 eV and the VB is about 1.60 eV, these band energies are about, or approximately, or substantially align with, or coincide energetically with, or are slightly larger in absolute scale than, the potentials required to drive the hydrogen evolution reaction (HER) and/or the oxygen reduction reaction (ORR) to generate hydrogen peroxide, which are 0 eV and 0.68 eV, respectively. It will be appreciated that, in combination, the HER and the ORR may be referred to as “splitting water”, referring to the electrolysis of water into hydrogen plus oxygen or hydrogen plus hydrogen peroxide. The skilled person will also appreciate that, when the NBT-based catalyst is adapted to have band energies whereby the CB and/or VB energies are in excess of the potentials required to drive the HER and/or ORR (whereby “in excess”, it is meant that the CB is more negative than -0.18 eV and the VB is more positive than 0.68 eV), the NBT-based catalyst will still generate hydrogen plus oxygen or hydrogen plus hydrogen peroxide from water (that is, “split water”), although the efficiency of the catalyst drops as the CB band energy of the NBT-based catalyst increases above the theoretical potential for the HER reaction. Similarly, the efficiency of the catalyst drops as the VB band energy of the NBT-based catalyst becomes more positive while remaining below the theoretical potential for the ORR reaction. [00012] As is commonly known, the band gap is defined as the difference between the conduction band (CB) and the valence band (VB), also referred to as the band edge energies. In other words, the band edge energies (conduction and valence bands) straddle the band gap. In the NBT-based catalyst of the present invention, the CB is close to and slightly more negative than the potential of the relevant HER half-reaction, which is 0 eV, and the VB is significantly more positive than the potential of the relevant ORR half-reaction, which is 0.68 eV. In the NBT-based catalyst of the present invention, preferably, the CB of the catalyst may be about, or more negative than, 0 eV, such as more negative than about -0.05 eV, or more negative than about -0.1 eV, or more negative than about -0.15 eV, or more negative than about -0.18 eV, or more negative than about -0.2 eV, or more negative than about -0.3 eV, or more negative than about -0.5 eV. In one embodiment, the CB may be about -0.18 eV. The NBT-based catalyst of the present invention may have a VB of about, or more than, a potential required by the ORR half-reaction, which is 0.68 eV. Preferably, the VB of the catalyst may be more positive than 0.68 eV, or more positive than about 0.7 eV, or more positive than about 0.8 eV, or more positive than about 0.9 eV, or more positive than about 1.0 eV, or more positive than about 1.1 eV, or more positive than about 1.2 eV, or more positive than about 1.4 eV, or more than about 2.4 eV. In one embodiment, the VB may be about 1.60 eV or more positive. It is preferred that the band gap is about 1.78 eV, or slightly greater or slightly lower than about 1.78 eV. Preferably, the band gap of the NBT-based catalyst of the present invention is less than the band gap of highly crystalline NBT, which is known to be between about 2.9 eV and about 3 eV. By “slightly greater” or “slightly lower”, it is anticipated that the band gap of the NBT-based catalyst of the present invention may vary by only about ±5%, or about ±10%, or about ±15%, or about ±20%, or about ±25%. In other words, the band gap may be about 1.78 eV, or it may be about 5% greater (i.e., about 1.87 eV), or about 10% greater (i.e., 1.96 eV), or about 15% greater (i.e., about 2.05 eV), or about 20% greater (i.e., about 2.14 eV), or about 25% greater (i.e., about 2.23 eV), or it may be about 5% lower (i.e., about 1.69 eV), or about 10% lower (i.e., 1.60 eV), or about 15% lower (i.e., about 1.51 eV), or about 20% lower (i.e., about 1.42 eV), or about 25% lower (i.e., about 1.34 eV). The variation in band gap energy may be due to an NBT-based catalyst being adapted to have a more negative CB, or a more positive VB, or a combination of both. By way of example, when there is a +10% variation in the band gap (i.e., when the band gap is about 1.96 eV), the band energies may be -0.18 eV and 1.78 eV (that is, when the VB contributes 10% more to the band gap), or they may be -0.36 eV and 1.60 eV (that is, when the CB contributes 10% more to the band gap), or they may be -0.27 eV and 1.69 eV (that is, when the CB and VB both contribute 5% to the band gap), or they may be any range therein. Put differently, when there is a 10% variation in the band gap, the CB may be between about -0.18 eV and about -0.36 eV, and the VB may be between about 1.6 eV and 1.78 eV. As the skilled person would appreciate, the lower the band gap energy (so long as the band energies align with, or substantially align with, the CB and VB energies defined above), the broader the wavelength of actinic radiation and/or frequency of vibration that can be absorbed, the more efficient the catalyst will be in splitting water. With a lower band gap, a lower amount of the absorbed energy is dissipated into the water as heat (as a greater portion of the CB or VB energy is used to drive the respective reactions). [00013] As the skilled person would appreciate, the band energies of the NBT-based catalyst described herein, particularly where the CB is less than about -0.18 eV, or between about -0.18 eV and -0.63 eV (i.e., up to 25% variation, as described above), the conduction band energy does not align with, or overlap with, a potential required to produce chlorine gas from chloride ions, which is -1.36 eV. It will be appreciated that this aspect of the invention is advantageous, as having the decomposition potential for chloride outside of the band gap allows seawater to be split into hydrogen plus oxygen or hydrogen plus hydrogen peroxide without producing chlorine gas. [00014] The NBT-based catalyst of the present invention may further comprise NBT ₄ (Na _0.5 Bi _4.5 Ti4O ₁₅ ). In such embodiments, the NBT may be the major phase, and the NBT ₄ may be the minor phase. The NBT-based catalyst of the present invention may also comprise BTO (Bi ₄ Ti ₃ O ₁₂ ). In such embodiments, the NBT may be the major phase, the NBT ₄ may be a minor phase and the BTO may be a minor phase or a trace phase. In other embodiments, the NBT- based catalyst of the present invention may comprise NBT as the major phase, NBT ₄ as a minor phase, and BTO as a trace phase. In some embodiments, the NBT-based catalyst may also comprise TiO ₂ in a trace phase. [00015] The NBT-based catalyst may be a heterojunction structure. When present, heterojunctions may form between any phase present in the catalyst. For example, heterojunctions may form between the major NBT and minor NBT ₄ phases, or between the major NBT and minor BTO phases, or between the major NBT and tract BTO phases, or between the major NBT and trace TiO ₂ phases, or between the minor NBT ₄ and minor BTO phases, or between the minor NBT ₄ and minor TiO ₂ phases, or between any other equilibrium or non-equilibrium phases present in the material. Without wishing to be bound by any theory, it is understood that heterojunctions allow for charge transfer to occur between phases with different band gaps. It may be possible for the skilled person to tune the properties of the NBT-based catalyst by varying the amount and types of phases that result in heterojunction formation. [00016] Without wishing to be bound by theory, it is contemplated that it may be possible to tune, or adapt, or adjust the band gap, and/or the band edge energies, of the NBT-based catalyst of the invention by controlling the interface between the NBT-NBT ₄ interface, or the NBT-BTO interface, or the NBT-TiO ₂ interface, or the NBT ₄ -BTO interface or the NBT-NBT ₄ -BTO triple point interface, or the NBT-NBT ₄ -TiO ₂ triple point interface, or the NBT-NBT ₄ -BTO-TiO ₂ quadruple point interface. As the skilled person would appreciate, such control may be exerted by the extent of the interfacial area, or secondary effects such as (1) crystallographic alignment between phases (i.e., coherent, semi-coherent, or incoherent interface), (2) nature of bonding (covalent vs ionic), (3) intervalence charge transfer (electron exchange between ions of different or even the same phases), and (4) defect type and concentration. [00017] The NBT-based catalyst of the invention may be in any suitable physical form. For example, it may be in the form of solid nanoparticles, which in one embodiment may be spherical, also referred to as nanospheres. As used herein, terms such as “nanoparticles”, “nanospheres” and the like refer to particles or spheres that have a diameter that are measured in nanometres or may be slightly larger. The diameter of the nanospheres may be in the nanoscale, i.e., between about 1 nm and 999 nm or they may be slightly larger, up to about 1.5 μm (that is, 1500 nm). The average diameter of the nanospheres may be between about 1 nm and about 1200 nm, or between about 5 nm and 900 nm, or between about 10 nm and about 800 nm, or between about 50 nm and about 750 nm, or between about 100 nm and about 500 nm, or between about 200 nm and about 700 nm, or between about 500 nm and about 900 nm. In one embodiment, the average diameter is between about 200 nm and about 700 nm. [00018] In one embodiment, the NBT-based catalyst of the invention may have at least one of the sodium atoms in the NBT lattice substituted by another element. The element may be any element capable of forming a 1+ cation. For example, the NBT may have at least a portion of the sodium atoms substituted by another alkali metal. The alkali metal may be lithium, or it may be potassium, or it may be rubidium (Rb), or it may be cesium (Cs), or it may be any combination thereof. In another example, the NBT may have at least a portion of the sodium atoms substituted by a transition metal, such as Cu ⁺ or Ag ⁺ , or a post-transition metal such as Pb ⁺ or Tl ⁺ , or a cationic halogen, such as Br ³⁺ . [00019] In a second aspect of the present invention, there is provided a method for making an NBT-based catalyst, comprising reacting bismuth and titanium, in approximately stoichiometric ratios, in the presence of excess sodium (viz., above the stoichiometric ratio). [00020] The NBT-based catalyst of the second aspect may be the catalyst of the first aspect. [00021] The proportions of the different phases (that is, NBT, NBT ₄ , BTO, TiO ₂ ) that can be engineered is dependent on the use of excess sodium (that is, greater than that required by stoichiometry). The excess sodium may be an amount that is greater than required to match the molar amounts of bismuth and titanium. The excess sodium may be at least 5 times in excess of that required by stoichiometry, or at least 7.5 times in excess, at least 10 times in excess, at least 12.5 times in excess, or at least 15 times in excess, or at least 20 times in excess. In one embodiment, the sodium excess is at least 12.5 times in excess compared to the matching molar amount of bismuth and titanium. [00022] The method of the second aspect may utilise any known process for forming an NBT- based catalyst. The NBT-based catalyst may be prepared by a process selected from any one of: solid-state reaction, solution decomposition, precipitation, sol-gel processing, or hydrothermal synthesis, or any other suitable process. In one embodiment, the process may be hydrothermal synthesis. The hydrothermal synthesis may be carried out in an autoclave, or any other suitable apparatus. [00023] The method of the second aspect may further comprise the optional step of contacting the catalyst with water so as to leach sodium ions from the NBT. The water may be substantially free of sodium ions, or it may be entirely free of sodium ions (i.e., it may be deionised water). The optional leaching step may be carried out before use, particularly if the leaching of sodium ions is not anticipated during use. As the skilled person will appreciate, it is expected that the leaching of sodium ions can be reduced or eliminated upon immersion in seawater or an Na- containing solution with an Na concentration equal to or greater than of the equilibrium solubility of Na derived from NBT. [00024] In a third aspect of the present invention, there is provided a catalyst produced from the method of the second aspect. [00025] In a fourth aspect of the present invention, there is provided a method for producing hydrogen from water or an aqueous solution or suspension, the method comprising the steps of: contacting the catalyst according to the first or third aspects with water or an aqueous solution or suspension and exposing the catalyst to actinic radiation, such as a source of visible and/or UV light, and/or mechanical or acoustic vibrations. Preferably a co-catalyst is used, such as Pt, in order to facilitate charger carrier (electrons and holes) separation through plasmon resonance. [00026] In the case when hydrogen gas and the oxygen gas are generated together, they may be collected together, or they may be collected separately. If collected together, the gasses may be separated, or they may be used together to form a new product. A method to produce the gases separately comprises the use of a pair of electrodes, wherein one electrode comprises the catalyst of the invention and the other electrode is, for example, Pt. [00027] The light source may be any suitable light source. It may be a naturally occurring light source, such as solar light, or it may be an artificially produced light source, such as an LED light or an incandescent bulb, a fluorescent source, or a halogen bulb, for example. In a preferred embodiment, the light source is solar light. [00028] The method of the fourth aspect may also comprise exposing the catalyst to vibrations as well as actinic radiation. This process preferably is simultaneous but it also may be sequential. The catalyst may be photo-piezocatalytic. The vibrations may be mechanical vibrations, or acoustic vibrations, or the result of pressure fluctuations, or the result of thermal fluctuations, or they may be any combination of these. [00029] In a fifth aspect of the present invention, there is provided a method for producing hydrogen gas from water or an aqueous solution or suspension, the method comprising the steps of: contacting the catalyst according to the first or third aspects with the water or aqueous solution or suspension, and exposing the catalyst to vibrations. [00030] The method of the fourth or fifth aspects may also result in oxygen gas being produced. When oxygen and hydrogen gases are produced, they may be collected together, or they may be collected separately. The method of the fourth or fifth aspects may also result in hydrogen peroxide being produced. [00031] The water used in the method of the fourth or fifth aspects may be any suitable water source. The water may be used directly from a natural water source, or it may be partially treated before use (e.g., filtered to remove suspended solids), or it may be extensively treated before use (e.g., desalinated), or it may be potable water or it may be deionised water. In one preferred embodiment, the water may be seawater. The seawater may be used as collected, or it may be seawater that is partially treated. The seawater may be treated to improve turbidity (i.e., filtered to remove suspended solids) or it may be treated to alter the chemical composition (i.e., pH adjusted, or degassed to remove dissolved carbon dioxide, or partially deionised to remove ions that may also be involved in the catalysis by the NBT-based catalyst of the present invention, such as, for example, Mg ²⁺ or Br-). In this preferred embodiment, partial or substantial desalination is not required before being used in the method. When seawater is used with the NBT-based catalyst of the present invention, it is an advantage that no chlorine gas is produced, even when chloride ions are present in the water being split. It is also anticipated by the inventors that the use of seawater, which is relatively high in dissolved sodium ions, also beneficially reduces degradation of the catalyst during use, as the relatively high dissolved sodium concentration limits further sodium leaching from the NBT-based catalyst. However, the skilled person would appreciate that some water sources, such as seawater, may not be suitable for the production of hydrogen peroxide, as the dissolved solids (such as ions and organic matter) may react with the hydrogen peroxide being produced. [00032] In a sixth aspect of the present invention, the catalyst of the first or third aspects may be used to split water. The water being split may be seawater. Brief Description of Drawings [00033] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures. [00034] Figure 1: X-ray diffraction (XRD) pattern of nanoparticles fabricated with different NaOH concentrations at constant Bi/Ti ratio (peak intensities at identical scales; 1 M = amorphous, 2.5 -12.5 M = NBT + NBT ₄ , 5-15-17.5 M = NBT + NBT ₄ + BTO, showing that the major peaks are indicative of NBT, the minor peaks are indicative of NBT ₄ , and the trace peaks are indicative of BTO. However, BTO is present below the level of detection at lower molarities, as evidenced by electron microscopy; it is not detectable by X-ray diffraction at these levels, however, [00035] Figure 2A: X-ray photoelectron spectroscopy (XPS) spectra for NBT-based nanoparticle catalyst (5 M) showing regions for Bi 4f and Ti 2p (with Bi 4d). [00036] Figure 2B: shows the XPS data for the samples produced with varying concentrations of NaOH. The peaks for the V _O ^•• are the most indicative, so they are quantified numerically. The theoretical maximal [V _O ^•• ] (viz., V _O ^•• concentration) are 13.33% for NBT ₄ and 16.66% for NBT. [00037] Figure 3A: SEM and TEM images of nanospheres (5 M) at different magnifications. [00038] Figures 3B and 3C: SEM images of nanoparticles fabricated with different NaOH concentration at constant Bi/Ti ratio (1 M = irregular amorphous, 3-13 M = spherical NBT + NBT ₄ , 15-17.5 M = roughened spherical NBT + NBT ₄ + BTO. The nanospherical NBT + NBT ₄ (3-13 M) is able to form a homogeneous spherical nanoparticle owing to the structural similarities of the two phases, which are isostructural and with similar a-axis and b-axis lattice parameters (the c-axis lattice parameter of NBT ₄ is ~10 times that of NBT). These allow the two phases to exhibit limited but extensive solid solubility as well as extensive coherent or semicoherent interfacial bonding. The roughened spherical appearance of the nanoparticles (15- 17.5 M) suggests that it derives from the precipitation of BTO, which also is isostructural with NBT and NBT ₄ , owing to exceeding of the solubility limit of BTO. This precipitation has a significant morphological effect on the nanospheres, even at trace BTO level. [00039] Figure 4: (a) reusability (cycle index) testing after UV irradiation of nanoparticles (5 M) for four cycles (1 h/cycle), and (b) photodegradation efficiency of methylene blue (MB) solution with different quenchers after UV irradiation for 1 h (TEOA = triethanolamine, BQ = P-benzoquinone, IPA = isopropyl alcohol). [00040] Figure 5A: (a) High-resolution transmission electron microscopy (HRTEM) image, (b) selected area electron diffraction (SAED) patterns, and (c) EDS mapping of samples of the NBT-based catalyst nanoparticles (5 M) showing NBT and BTO phases. [00041] Figure 5B: High-resolution transmission electron microscopy (HRTEM) image, and selected area electron diffraction (SAED) patterns of samples of the NBT-based catalyst nanoparticles showing (12.5 M) NBT and NBT ₄ phases; BTO was not detected in this area. [00042] Figure 6A: These data plot the oxygen vacancy concentration ([V _O ^•• ]) as a function of the relative amounts of NBT (in the combined NBT ₄ + NBT and [NaOH]; BTO is not considered as it is a trace phase) and [NaOH]. [00043] Figure 6B: These data plot the H ₂ evolution rate at 5 h of piezocatalysis as a function of the relative amounts of NBT and [NaOH]. [00044] Figure 7: These data plot H ₂ evolution amount from 1-5 h photocatalysis from water of difference sources (deionised water, low-[Na] aqueous solution, and simulated seawater) with an NBT + NBT ₄ catalyst (5 M). [00045] Figure 8A: H ₂ evolution amount and rate during piezocatalysis as a function of increasing [NaOH] (variable time (left) and at 5 h (right)). [00046] Figure 8B: H ₂ evolution amount and rate during piezocatalysis (12.5 M) as a function of water type (deionised (DI), simulated seawater, natural seawater) (variable time (left) and at 5 h (right)). [00047] Figure 8C: H ₂ evolution amount and rate during catalysis as a function of catalyst type (12.5 M). Definitions [00048] The following definitions are provided to enable the skilled person to understand better the invention disclosed herein. These are intended to be general and are not intended to limit the scope of the invention to these terms or definitions alone. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains. [00049] As used herein, the term “catalyst” means a material or substance that increases the rate of a chemical reaction without itself undergoing a permanent chemical change. The term “catalyst” is intended to encompass related terms such as “photocatalyst” (whereby light is absorbed by the catalyst to initiate and/or increase the rates of chemical reactions), “piezocatalyst” (whereby vibrations are absorbed by the catalyst to initiate and/or increase the rates of chemical reactions), and “photo-piezocatalyst” (whereby light and vibrations are absorbed by the catalyst to initiate and/or increase the rates of chemical reactions) and the like. [00050] As used herein, the term “comprising” means “including”. Variations of the word “comprising”, such as “comprise” and “comprises”, have correspondingly varied meanings. As used herein, the terms “including” and “comprising” are non-exclusive. As used herein, the terms “including” and “comprising” do not imply that the specified integer(s) represent a major part of the whole. [00051] The transitional phrase “consisting of” excludes any element, step, or ingredient not specified. If in the claim, such would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase “consisting of” appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. [00052] The transitional phrase “consisting essentially of” is used to define a composition, process or method that includes materials, steps, features, components, or elements, in addition to those literally disclosed, provided that these additional materials, steps, features, components, or elements do not materially affect the basic and novel characteristic(s) of the claimed invention. The term “consisting essentially of” occupies a middle ground between “comprising” and “consisting of”. [00053] Where applicants have defined an invention or a portion thereof with an open-ended term such as “comprising”, it should be readily understood that (unless otherwise stated) the description should be interpreted to also describe such an invention using the terms “consisting essentially of” or “consisting of”. In other words, with respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of”. [00054] Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). [00055] Also, the indefinite articles “a” and “an” preceding an element or component of the invention are intended to be non-restrictive regarding the number of instances (i.e., occurrences) of the element or component. Therefore “a” or “an” should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular. [00056] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %”, “ratio” will mean “weight ratio” and “parts” will mean “weight parts”. [00057] The terms “predominantly” and “substantially” as used herein shall mean comprising more than 50% by weight, unless otherwise indicated. [00058] As used herein, with reference to numbers in a range of numerals, the terms “about”, “approximately” and “substantially” are understood to refer to the range of -10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1 % to + 1 % of the referenced number, most preferably -0.1 % to +0.1 % of the referenced number. Moreover, with reference to numerical ranges, these terms should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, from 8 to 10, and so forth. [00059] As used herein, wt% refers to the weight of a particular component relative to total weight of the referenced composition. [00060] The complete disclosures of the patents, patent documents and publications cited herein are incorporated by reference in their entirety as if each were individually incorporated. Description of Embodiments [00061] The following description conveys exemplary embodiments of the present invention in sufficient detail to enable those of ordinary skill in the art to practice the present invention. Features or limitations of the various embodiments described do not necessarily limit other embodiments of the present invention or the present invention as a whole. Hence, the following detailed description does not limit the scope of the present invention, which is defined only by the claims. [00062] The present invention relates to an improved catalyst for the catalysis of water and seawater, as well as a method for producing the catalyst and the use of the catalyst. [00063] In particular, the inventors have developed a catalyst that, upon illumination with a light source, preferably solar light, and/or exposure to vibrations, preferably ultrasound, produces hydrogen plus oxygen or hydrogen plus hydrogen peroxide, without an external power source required to supplement the solar light; the catalyst also can be piezoactivated without external power, such as tidal, wind, industrial, transport, etc. Advantageously, as will be described in more detail below and with reference to the examples, the catalyst of the present invention has been engineered so that the band gap is relatively low (compared to prior art catalysts), and the band positions are closely aligned with either or both the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) half-reaction potentials (for conventional water splitting processes) or the hydrogen evolution reaction (HER) and hydrogen peroxide evolution reaction (for intermediate water splitting processes (IWS)), thereby reducing or eliminating the overpotential required to electrolyse water. Catalysis [00064] Catalysed photolysis refers to a process whereby light is absorbed by a catalyst, which leads to the breaking of a covalent bond and hence a chemical reaction. Catalysed piezolysis refers to a process whereby mechanical energy excites a catalyst, which leads to the breaking of a covalent bond and hence a chemical reaction. Catalysed electrolysis refers to a process whereby electricity excites a catalyst, which leads to the breaking of a covalent bond and hence a chemical reaction. Catalysed thermolysis refers to a process whereby heat is absorbed to excite a catalyst, which leads to the breaking of a covalent bond and hence a chemical reaction. In some embodiments, more than one means of excitation may be applicable to the catalyst. For example, the catalyst may be excited by both light and mechanical energy (that is, catalysed photo-piezolysis), or the catalyst may be excited by both light and electrical energy (that is, photo-electrolysis), or the catalyst may be excited by both mechanical energy and heat (that is, catalysed piezo-thermolysis), or the substrate may be excited by light and heat (that is, thermo- photolysis). [00065] In the catalyst of the present invention, the catalyst is in the form of an NBT-based catalyst, and the reaction includes splitting water to produce hydrogen plus oxygen, or intermediate water splitting to produce hydrogen plus hydrogen peroxide. By “NBT-based catalyst”, it is meant that the catalyst contains or comprises, as a major phase, NBT. [00066] In particular, the catalyst of the present invention is a semiconductor. Unlike materials such as metals, which have a continuum of energy states, semiconductors have defined energy states, which include an energy “void” region that extends from the top of the filled valence band edge (VB) (i.e., the highest occupied electron state) to the bottom of the vacant conduction band edge (CB) (i.e., the lowest unoccupied electron state). This void region is referred to as the “band gap” and is calculated as the difference between the CB and VB edge energies. It is understood that a semiconductor catalyses a reaction by absorbing a quantum of energy (for example, a photon if the catalyst is a photocatalyst) that is at least, if not more, energetic than the band gap of the semiconductor material. This energy promotes an electron into the conduction band and leaves a positive hole in the valence band. Electrons and holes are charge carriers. The larger the band gap, the less likely it is for the electron and the hole to recombine, annihilate, and release the energy as heat, although this recombination in the bulk of the material competes with the diffusion of the electrons and the holes to the surface, whereby both the excited negative electron and the positive hole can be used to drive reactions; the reduction reaction’s being mediated by the excited electron, and the oxidation reaction’s being mediated by the hole. [00067] In the catalysed photolysis of water, which is one embodiment of the present invention, the steps required to split water are: (1) light is absorbed by the photocatalyst; (2) an electron is excited from the valence band to the conduction band, simultaneously producing a positive hole in the valence band; (3) the recombination of the excited electrons and holes compete with the diffusion of these charge carriers to the surface, where they can participate in reactions; (4) when an excited electron does diffuse to the surface, it can participate in a reduction reaction, such as the HER (4H ⁺ + 4e- → 2H ₂ ) and, when a hole does diffuse to the surface, it can participate in an oxidation reaction, such as the OER (2H ₂ O + 4h ⁺ → O ₂ + 4H ⁺ ), or hydrogen peroxide evolution reaction (2H ₂ O + 2h ⁺ → H ₂ O ₂ + 2H ⁺ ). [00068] As the skilled person will appreciate, nanoparticulate photocatalysts conventionally utilise solar energy to split water into hydrogen plus oxygen, where mixtures of gases result when a powder is irradiated. A preferred alternative is the generation of hydrogen gas while the residual oxygen remains dissolved in the water. Such photocatalysts can be problematic due to (1) poor absorption of visible light (mostly UV absorption) and (2) rapid charge carrier recombination times. Although these problems may be ameliorated by careful design of the catalyst, another approach is to utilise a piezocatalyst (that is, a catalyst that is excited by vibrations, such as mechanical or acoustic vibrations). Piezocatalysis has the three advantages of (1) it does not involve light absorption, (2) vibrations travel through a fluid more efficiently than light), and (3) charge carrier separation is facilitated by the establishment of a large-scale electrical field through the polarisation induced by mechanical force, such as ultrasound. Accordingly, in one embodiment of the present invention, the catalyst is a piezocatalyst. The piezocatalyst may be capable of catalysing water to produce hydrogen plus oxygen, or it may be capable of producing hydrogen plus hydrogen peroxide. The piezocatalyst may be excited by any suitable source of vibrational energy, such as for example, an ultrasound generator (ultrasonicator), waste mechanical vibrations from an industrial source (such as a motor or dynamo) that provides a consistent or substantially consistent source of vibrational energy, or they may be provided by a renewable environmental source, such as by harnessing an environmental source, such as wave action or wind. When a piezocatalyst is excited by mechanical energy, the same mechanism occurs as the photocatalyst above (that is, an excited electron can diffuse to the surface, where it can participate in a surface reaction, such as the HER and when a hole diffuses to the surface, it can participate in an oxidation reaction, such as the OER or hydrogen peroxide evolution reaction (ORR). [00069] Whether the catalyst is excited by a photon or vibrational energy, the precise reduction and oxidation reactions that occur depend on the energies of the valence and conductive bands, and the reactants in contact with the catalyst. For instance, in order for a catalyst to split water according to the HER and ORR reactions above, the band gap must be at least 0.68 eV (ORR) whereby the valence band must be at least 0.68 eV (or higher, i.e., more positive), and the conduction band should be at least 0 eV (or lower, i.e., more negative), which are the potentials required to drive each of these half-reactions. If a material has a band gap that is less than 0.68 eV, or the VB is less than 0.68 eV, or the CB is greater than 0 eV, then the material will be unable to split water, at least via the HER and ORR pathways discussed above. On the other hand, if the CB is significantly in excess of the minimum voltage requirements to drive a reaction (e.g., more than about 0.5 eV in excess), the energy used to promote each electron into the conduction band will be essentially wasted as heat, significantly reducing the efficiency of the catalyst. As the skilled person would appreciate, while it is desirable for the CB to be above the HER potential, and it is essential for the VB to be below the ORR potential in order to split water, the further the VB is below the ORR potential, the more advantageous the NBT-based catalyst. That being said, the skilled person will also appreciate that there will be some compromise between the location of the band edges relative to the HER and ORR potentials and the overall size of the band gap. For example, if the band gap is too large, this may negatively affect the efficiency of the catalyst, or if the CB and HER potential are too close together, this may negatively affect the recombination time (i.e., the time taken for electrons and holes to recombine and annihilate each other) and hence efficiency of the catalyst. The skilled person would also appreciate that the same applies for the intermediate water splitting reaction, whereby preferably the band gap is at least 1.77 eV, whereby the valence band must be at least 1.77 eV (or higher, i.e., more positive), and the conduction band should be at least 0 eV (or lower, i.e., more negative), which are the potentials required to drive each of these half- reactions. [00070] As will be evident to the skilled person, in one embodiment of the present invention, the catalyst of the present invention is suitable for the catalysis of water to produce hydrogen plus oxygen, and so preferably has a band gap greater than 1.23 eV. In another embodiment of the present invention, the catalyst of the present invention is suitable for the catalysis of water to produce hydrogen plus hydrogen peroxide, and so preferably has a band gap greater than 1.77 eV. In either embodiment, the band gap may be between 1.23 eV and about 3.2 eV, for instance it may be between about 1.25 eV and about 1.8 eV, or between about 1.5 eV and about 2 eV, or between about 1.4 eV and about 2.2 eV, or it may be about 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.3, 1.31, 1.32, 1.33, 1.34, 1.35, 1.36, 1.37, 1.38, 1.39, 1.4, 1.41, 1.42, 1.43, 1.44, 1.45, 1.46, 1.47, 1.48, 1.49, 1.5, 1.51, 1.52, 1.53, 1.54, 1.55, 1.56, 1.57, 1.58, 1.59, 1.6, 1.61, 1.62, 1.63, 1.64, 1.65, 1.66, 1.67, 1.68, 1.69, 1.7, 1.71, 1.72, 1.73, 1.74, 1.75, 1.76, 1.77, 1.78, 1.79, 1.8, 1.81, 1.82, 1.83, 1.84, 1.85, 1.86, 1.87, 1.88, 1.89, 1.9, 1.91, 1.92, 1.93, 1.94, 1.95, 1.96, 1.97, 1.98, 1.99, 2, 2.05, 2.1, 2.15, 2.2, 2.25, 2.3, 2.35, 2.4, 2.45, 2.5, 2.55, 2.6, 2.65, 2.7, 2.75, 2.8, 2.85, 2.9, 2.95, 3, 3.05, 3.1, 3.15, or 3.2 eV, or any range therein. As the skilled person would appreciate, the measured band gap may be affected by the measurement environment. For example, the band gap measured for a catalyst under dry conditions may be about 2.5 eV, but may be 3.2 eV when measured under wet conditions. In a preferred embodiment, the catalyst may have a CB that is equal to, or may substantially align with, or may be more negative than, the potential required by the HER, which is about 0 eV, and a VB that is equal to, or may substantially align with, or may be more positive than, the potential required by the OER, which is about 1.23 eV, or the potential required by the ORR, which is about 0.68 eV. By “substantially align”, it is meant that the variation from the minimum CB or VB values may be no more than about 1.15 eV. Accordingly, the catalyst may have a CB less than 0 eV, or between 0 eV and about -1.15 eV, and the VB may be greater than 1.23 eV, or between 1.23 eV and about 2.38 eV, or it may be greater than 0.68 eV, or between 0.68 eV and 1.83 eV (depending on the product being produced). For example, the CB may be between about 0 eV and about -0.75 eV, or between about -0.1 eV and about -0.6 eV, or between about -0.25 eV and about -0.5 eV, or it may be about 0, -0.01, -0.02, -0.03, -0.04, -0.05, -0.06, -0.07, -0.08, -0.09, - 0.1, -0.11, -0.12, -0.13, -0.14, -0.15, -0.16, -0.17, -0.18, -0.19, -0.2, -0.21, -0.22, -0.23, -0.24, - 0.25, -0.26, -0.27, -0.28, -0.29, -0.3, -0.31, -0.32, -0.33, -0.34, -0.35, -0.36, -0.37, -0.38, -0.39, - 0.4, -0.41, -0.42, -0.43, -0.44, -0.45, -0.46, -0.47, -0.48, -0.49, -0.5, -0.51, -0.52, -0.53, -0.54, - 0.55, -0.56, -0.57, -0.58, -0.59, -0.60, -0.61, -0.62, -0.63, -0.64, -0.65, -0.66, -0.67, -0.68, -0.69, -0.7, -0.71, -0.72, -0.73, -0.74, -0.75, -0.76, -0.77, -0.78, -0.79, -0.8, -0.81, -0.82, -0.83, -0.84, - 0.85, -0.86, -0.87, -0.88, -0.89, -0.9, -0.91, -0.92, -0.93, -0.94, -0.95, -0.96, -0.97, -0.98, -0.99, - 1, -1.01, -1.02, -1.03, -1.04, -1.05, -1.06, -1.07, -1.08, -1.09, -1.1, -1.11, -1.12, -1.13, -1.14, or - 1.15 eV, or any range therein. Likewise, the VB may be between 1.23 eV and about 2.05 eV, or between about 1.3 eV and about 1.8 eV, or between about 1.5 eV and about 2 eV, or it may be about 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 0.1, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.3, 1.31, 1.32, 1.33, 1.34, 1.35, 1.36, 1.37, 1.38, 1.39, 1.4, 1.41, 1.42, 1.43, 1.44, 1.45, 1.46, 1.47, 1.48, 1.49, 1.5, 1.51, 1.52, 1.53, 1.54, 1.55, 1.56, 1.57, 1.58, 1.59, 1.6, 1.61, 1.62, 1.63, 1.64, 1.65, 1.66, 1.67, 1.68, 1.69, 1.7, 1.71, 1.72, 1.73, 1.74, 1.75, 1.76, 1.77, 1.78, 1.79, 1.8, 1.81, 1.82, 1.83, 1.84, 1.85, 1.86, 1.87, 1.88, 1.89, 1.9, 1.91, 1.92, 1.93, 1.94, 1.95, 1.96, 1.97, 1.98, 1.99, 2, 2.01, 2.02, 2.03, 2.04, 2.05, 2.06, 2.07, 2.08, 2.09, 2.1, 2.11, 2.12, 2.13, 2.14, 2.15, 2.16, 2.17, 2.18, 2.19, 2.2, 2.21, 2.22, 2.23, 2.24, 2.25, 2.26, 2.27, 2.28, 2.29, 2.3, 2.31, 2.32, 2.33, 2.34, 2.35, 2.36, 2.37, or 2.38 eV, or any range therein. In other embodiments, one or both of the CB or VB values may not substantially align with the potential required for the HER or ORR/OER, respectively (i.e., may be more than 1.15 eV different compared to the potential required), but may still provide suitable catalytic behaviour. In one embodiment, the catalyst of the present invention has a band gap or about 1.78 eV, a CB of about -0.18 eV and a VB of about 1.60 eV. However, other particular combinations of band edge locations (VB and CB) and overall band gap energies may be more efficient than others, and it may be possible to optimize the location of the band edges and the band gap in order to split water efficiently or carry out some other catalysed reactions. As the skilled person would appreciate, in some embodiments the catalyst may have a band gap, and/or VB edge energy, and/or CB edge energy that makes the catalyst capable of catalysing either water splitting reactions (that is, HER and OER reactions) and/or intermediate water splitting reactions (that is, HER and ORR reactions). In some embodiments, the reaction scheme catalysed by the catalyst may be selectable (for example, light irradiation may produce one product, and vibrations may produce a different product). [00071] In one embodiment, the band gap of the NBT-based catalyst of the present invention may vary by only about 5%, or about 10 %, or about 15%, or about 20%, or about 25%, but no more than about 25%. In other words, the band gap may be about 1.78 eV, or it may be about 5% greater (i.e., about 1.87 eV), or about 10 % greater (i.e., 1.96 eV), or about 15% greater (i.e., about 2.05 eV), or about 20% greater (i.e., about 2.14 eV), or about 25% (i.e., about 2.23 eV). The variation in band gap energy may be due to an NBT-based catalyst being adapted to have a more negative CB, or a more positive VB, or a combination of both. By way of example, when there is a 10% variation in the band gap (i.e., when the band gap is about 1.96 eV), the band energies may be -0.18 eV and 1.78 eV (that is, when the VB contributes 10% more to the band gap), or they may be -0.36 eV and 1.60 eV (that is, when the CB contributes 10% more to the band gap), or they may be -0.27 eV and 1.69 eV (that is, when the CB and VB both contribute 5% to the band gap), or they may be any range therein. Put differently, when there is a 10% variation in the band gap, the CB may be between about -0.18 eV and about -0.36 eV, and the VB may be between about 1.6 eV and 1.78 eV. It follows that the CB of the NBT-based catalyst may be adapted to be between -0.18 eV and -0.63 eV, or between about -0.18 eV and about - 0.54 eV, or between about -0.36 eV and about -0.63 eV, or about -0.18, -0.19, -0.2, -0.21, -0.22, -0.23, -0.24, -0.25, -0.26, -0.27, -0.28, -0.29, -0.3, -0.31, -0.32, -0.33, -0.34, -0.35, -0.36, -0.37, - 0.38, -0.39, -0.4, -0.41, -0.42, -0.43, -0.44, -0.45, -0.46, -0.47, -0.48, -0.49, -0.5, -0.51, -0.52, - 0.53, -0.54, -0.55, -0.56, -0.57, -0.58, -0.59, -0.60, -0.61, -0.62, -0.63 eV or any range therein, and that the VB of the NBT-based catalyst may be adapted to be between about 1.6 eV and about 2.05 eV, or between about 1.60 eV and about 1.96 eV, or between about 1.78 eV and about 2.05 eV, or about 1.6, 1.61, 1.62, 1.63, 1.64, 1.65, 1.66, 1.67, 1.68, 1.69, 1.7, 1.71, 1.72, 1.73, 1.74, 1.75, 1.76, 1.77, 1.78, 1.79, 1.8, 1.81, 1.82, 1.83, 1.84, 1.85, 1.86, 1.87, 1.88, 1.89, 1.9, 1.91, 1.92, 1.93, 1.94, 1.95, 1.96, 1.97, 1.98, 1.99, 2, 2.01, 2.02, 2.03, 2.04 or 2.05 eV or any range therein, and the band gap may be the difference between any CB and any VB provided herein. [00072] Another advantage of a catalyst with band energies that substantially align with the HER and OER half-reaction potentials, or the HER and ORR half-reaction potentials, or are between about -0.75 eV and about 2 eV, is that unwanted side reactions may be avoided, especially if ions are present in the electrolyte that may interfere with the HER, OER, and/or ORR reaction. For instance, the conversion of chloride ions into chlorine gas is known to occur in some catalysis systems, where chloride salts are used as an electrolyte, or where a chloride- containing water, such as seawater, is used. However, the oxidation of chloride ions to chlorine requires a potential of about -1.36 eV, which is a significantly more negative voltage than the CB of the catalyst of the present invention can provide. Accordingly, the catalyst of the present invention is not capable of producing chlorine gas from chloride ions. In this regard, the present invention is particularly suitable for use in producing hydrogen plus oxygen or hydrogen plus hydrogen peroxide from water that contains, or comprises, significant amounts of chloride ions, such as seawater. Catalyst [00073] As described above, photo-piezocatalytic processes require a catalyst that is capable of absorbing light (i.e., a photocatalyst) or mechanical vibrations (i.e., a piezocatalyst), and both light and mechanical vibrations. The photo-piezocatalyst of the present invention comprises NBT, which is a covalent solid that comprises sodium, bismuth, titanium, and oxygen at stoichiometric ratios of about Na _0.5 Bi _0.5 TiO ₃ , which is used to catalyse water splitting reactions, so that hydrogen plus oxygen or hydrogen plus hydrogen peroxide are produced. [00074] As the skilled person would be aware, NBT is a semiconducting material that has been previously investigated as a photocatalyst for use in catalysing water splitting processes. Usually, NBT is produced and used for this purpose in a polycrystalline solid form. By “polycrystalline”, it is meant that the constituents of the material (in this case, the four elements) are arranged in a randomly oriented array of nanoscale grains, each of which is comprised of an ordered lattice. [00075] The NBT-based catalyst of the present invention may comprise, or consist of, or consist essentially of, NBT. The catalyst of the present invention may comprise crystalline NBT as the major phase and an additional semiconductor as the minor or trace phase in a heterojunction arrangement. Accordingly, the additional semiconductor may also comprise other phases that are chemically compatible with (i.e., under equilibrium conditions) or nonreactive but incompatible with NBT (i.e., under nonequilibrium conditions). These phases included, but are not limited to, NBT ₄ , BTO, and/or TiO ₂ . In some embodiments, the catalyst of the present invention comprises trace amounts of BTO. By “trace amount”, it is meant that the catalyst comprises less than about 1 wt% of BTO compared to the mass of the NBT. In some embodiments, the catalyst comprises NBT as a major phase, and NBT ₄ as the minor phase, whereby the minor phase may comprise at least 1% wt%, or at least 5% wt%, or at least 10% wt%, of the catalyst. For example the catalyst may be about 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10% wt% or within any range therein. Without being bound to theory, the inventors understand that NBT and NBT ₄ are isostructural (that is, have similar crystal structures). When the lattice parameters are similar, this can produce “shoulders” in an X-ray diffraction (XRD) pattern of catalysts produced with NaOH as the sodium source (these shoulders can mistakenly be attributed to partial amorphization or low crystallinity of the material). This feature can be seen in Figure 1, whereby the NBT ₄ is believed to be responsible for the shoulders at lower angles of the NBT major phase peaks. Trace amounts of BTO can be detected at high concentrations of NaOH (15- 17.5 M) (although the inventors surmise that BTO is present at many, if not all, concentrations of NaOH at trace amounts that are below the detection limit of the X-ray diffraction instrument used). [00076] As shown in the Examples and discussed in more detail below, the inventors of the present invention understand that the NBT-based catalyst of the present invention advantageously comprises a heterojunction that has electronic properties that are more suitable for catalysing water splitting reactions, particularly in terms of the band gap and band energies. In other words, an NBT-based catalyst of the present invention may be adapted to have a beneficial band gap and band energies more effectively to produce hydrogen gas from water by formation of a heterojunction as part of the catalyst material. As the skilled person would appreciate, a heterojunction is an interface that occurs between two layers or regions of different crystalline semiconductors in contact. As the skilled person would appreciate, an advantage of a heterojunction is that this arrangement can facilitate the transfer of charge carriers, where the electrons and holes generated by both semiconductors diffuse in opposite directions, thereby avoiding recombination and hence mutual annihilation. Heterojunctions thus have the capacity for significant improvement of the utilisation rates of electrons and holes to levels greater than those of the individual semiconductors. [00077] Advantageously, semiconductors, and particularly heterojunctions, are known to be photocatalytic, piezocatalytic, photo-piezocatalytic, electrocatalytic, or thermocatalytic. Consequently, such materials have the potential to exhibit combined catalytic activities through the simultaneous application of light, mechanical force, electricity, and/or heat. The heterojunction formed in nanoparticles comprising NBT and lower amounts of NBT ₄ and/or BTO may be a straddling gap (type I) heterojunction, a staggered gap (type II) heterojunction, or a broken gap (type III) heterojunction. In all three cases, the heterojunction structure, when present, may shift both the CB and VB edges to more favourable positions. The skilled person may understand that this may be referred to more broadly as superior band alignment. [00078] As the skilled person would appreciate, a piezocatalyst is capable of using mechanical energy (such as vibrations) to produce electrical energy (such as the generation of charge carriers). When a piezoelectric material is used in a catalyst, it can be activated catalytically by the application of mechanical force (such as ultrasound). When two (or more) piezoactive phases are present and in contact, the performance of such materials is optimised at these interfaces, known as morphotropic phase boundaries. Without being bound to theory, the inventors understand that, in the catalyst described herein, the major, minor, and trace phases of the catalyst (that is, NBT, NBT ₄ , and BTO, or any combination of NBT (major), NBT ₄ minor, and other phase (trace)) are all piezoactive phases that comprise morphotropic phase boundaries at their interfaces. Consequently, in addition to their potential effects as heterojunctions, these interface regions also have the potential to provide the basis to optimise the piezocatalytic or photo-piezocatalytic performance at different morphotropic phase boundaries, where two phases are in equilibrium (NBT/NBT ₄ , NBT ₄ /BTO, NBT/BTO), as well as at a triple point, where three phases are in equilibrium (NBT/NBT ₄ /BTO). There also is the possibility of the suggested existence of the quadruple point (NBT/NBT ₄ /BTO/TiO ₂ (or another trace phase)). [00079] The NBT-based catalyst of the present invention may be in any suitable physical form. In one embodiment of the present invention, the NBT-based catalyst may be in the form of solid nanospheres, meaning that they are spherical nanoparticles. The nanospheres may be entirely, or at least substantially, spherical in shape. The nanospheres may have a circular, or at least substantially circular, cross-section. The size of each of the nanospheres may be on the nanoscale, for instance each of the nanospheres may have a diameter of between about 1 nm and 999 nm. The nanospheres, when present as a plurality, may have an average size of between 1 nm and 999 nm, or between about 100 nm and about 900 nm, or between about 150 nm and about 800 nm, or between about 200 nm and about 700 nm, or between about 250 nm and about 750 nm, or between about 300 nm and about 600 nm, or they may have an average diameter of about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 600, 610, 620, 630, 640, 650, 660, 670, 680, 690, 700, 710, 720, 730, 740, 750, 760, 770, 780, 790, 800, 810, 820, 830, 840, 850, 860, 870, 880, 890, 900, 910, 920, 930, 940, 950, 960, 970, 980, 990, 995 or 999 nm, or any range therein. The distribution of the size of each of the nanoparticles may follow a normal distribution around the average diameter, or it may approximate a normal distribution. The distribution may be narrow, whereby two standard deviations from the mean encompasses a variation of about 10 nm, or about 20 nm, or about 50 nm, or about 10% of the average size, or about 20% of the average size, or the distribution may be broad, whereby two standard deviations from the mean encompasses a variation of about 200 nm, or about 250 nm, or about 300 nm, or about 400 nm, or about 50% of the average size, or about 75% of the average size, or about 100% of the average size. For example, if the average diameter of the nanoparticles was 450 nm, a narrow distribution would be defined as two standard deviations encompassing the range of between about 400 nm and about 500 nm (i.e., +/- 50 nm, or about 11%), or a broad distribution would be defined as two standard deviations encompassing the range of between about 200 nm and about 700 nm (i.e., +/- 250 nm, or about 62%). In a preferred embodiment, the average diameter of the nanospheres is between about 200 nm and about 700 nm, or optionally each of the nanoparticles is between about 200 nm and about 700 nm in diameter. [00080] The nanospheres of the present invention may also be porous, in that they comprise one or more pores, which are voids that extend into the nanoparticle from the surface. The average pore size of the nanospheres may be between about 4 and 10 nm, or they may be between about 5 nm and about 9 nm, or they may be between about 6 nm and about 7.5 nm, or they may be between about 6.5 nm and 7 nm, or they may be about 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8, 8.18.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9 or about 10 nm, or any range therein. In some embodiments, the average pore size of the nanoparticles may be between about 6.4 and about 6.5 nm. The pore volume of the nanoparticles, which refers to the total volume of the pores per gram of nanospherical catalyst, may be at least 0.01 cm ³ /g, at least 0.02 cm ³ /g, or at least 0.03 cm ³ /g, or at least 0.04 cm ³ /g, or at least 0.05 cm ³ /g, or at least 0.06 cm ³ /g, or at least 0.07 cm ³ /g, or at least 0.08 cm ³ /g, or at least 0.09 cm ³ /g, or it may be between about 0.01 cm ³ /g and about 0.08 cm ³ /g, or between about 0.02 cm ³ /g and about 0.07 cm ³ /g, or between about 0.03 cm ³ /g and about 0.06 cm ³ /g, or it may be about 0.01, 0.012, 0.014, 0.016, 0.018, 0.02, 0.022, 0.024, 0.025, 0.026, 0.027, 0.028, 0.029, 0.03, 0.031, 0.032, 0.033, 0.034, 0.035, 0.036, 0.037, 0.038, 0.039, 0.04, 0.041, 0.042, 0.043, 0.044, 0.045, 0.046, 0.047, 0.048, 0.049, 0.05, 0.051, 0.052, 0.053, 0.054, 0.055, 0.056, 0.057, 0.058, 0.059, 0.06, 0.061, 0.062, 0.063, 0.064, 0.065, 0.066, 0.067, 0.068, 0.069, 0.07, 0.071, 0.072, 0.073, 0.074, 0.075, 0.076, 0.077, 0.078, 0.079, 0.080, 0.082, 0.084, 0.086, 0.088, 0.09, 0.092, 0.094, 0.096, 0.098 or 0.01, or any range therein. In some embodiments, the pore volume may be about 0.063 cm ³ /g. The specific surface area of the nanoparticles, as measured and calculated according to the Brunauer, Emmett and Teller method (i.e., the BET specific surface area) may be at least 0.5 m ² /g, or it may be at least 1 m ² /g, or it may be at least 2 m ² /g, or it may be at least 5 m ² /g, or it may be at least 10 m ² /g, or it may be at least 20 m ² /g, or it may be between about 1 and about 25 m ² /g, or it may be between about 5 and about 22 m ² /g, or it may be between about 10 and about 21 m ² /g, or it may be about 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8, 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, 13, 13.1, 13.2, 13.3, 13.4, 13.5, 13.6, 13.7, 13.8, 13.9, 14, 14.1, 14.2, 14.3, 14.4, 14.5, 14.6, 14.7, 14.8, 14.9, 15, 15.1, 15.2, 15.3, 15.4, 15.5, 15.6, 15.7, 15.8, 15.9, 16, 16.1, 16.2, 16.3, 16.4, 16.5, 16.6, 16.7, 16.8, 16.9, 17, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6, 17.7, 17.8, 17.9, 18, 18.1, 18.2, 18.3, 18.4, 18.5, 18.6, 18.7, 18.8, 18.9, 19, 19.1, 19.2.19.3, 19.4, 19.5, 19.6, 19.7, 19.8, 19.9, 20, 20.1, 20.2, 20.3, 20.4, 20.5, 20.6, 20.7, 20.8, 20.9, 21, 21.1, 21.2, 21.3, 21.4, 21.5, 21.6, 21.7, 21.8, 21.9, 22, 22.1, 22.2, 22.3, 22.4, 22.5, 22.6, 22.7, 22.8, 22.9, 23, 23.1, 23.2, 23.3, 23.4, 23.5, 23.6, 23.7, 23.8, 23.9, 24, 24.1, 24.2, 24.3, 24.4, 24.5, 24.6, 24.7, 24.8, 24.9 or 25 m ² /g or any range therein. In some embodiments, the BET specific surface area may be about 20.36 m ² /g, or between about 20.3 and 2.4 m ² /g. The BET method reports what is considered to be a pore size. However, these pores may be surface artefacts rather than volumetric pores. In this sense, the pore size provides an estimate of the extent of the surface roughness profile, as distinguished from the surface area. [00081] As discussed above, the NBT-based catalyst of the present invention comprises sodium, bismuth, titanium, and oxygen. In some embodiments, at least a portion of the ions may be substituted with other ions of the same charge. For example: at least a portion of the sodium ions may be replaced with cations of the same charge, such as for example, alkali metals, such as lithium (Li ⁺ ) and/or potassium (K ⁺ ) ions; and/or a transition metals, such as Cu ⁺ or Ag ⁺ ; and/or post-transition metals, such as thallium (Tl ⁺ ). Substitutions on the metal sublattice also may be used to create defects by introducing ions of valence different from those of sodium, bismuth, titanium, such as rare earths, such as Mg ²⁺ and/or Ca ³⁺ ; transition metals, such as Cr ³⁺ and/or Fe ³⁺ ; or post-transition metals, such as Sn ⁴⁺ and/or Pb ²⁺ . Further, almost all cations potentially may be included in the lattice in the form of interstitial solutes. Further, the same processes but for anions can be used to alter the oxygen sublattice of the NBT structure. Further, the intrinsic composition of NBT itself may be altered through the introduction of intentional deficiency below the stoichiometric amount of any or all of the ions of the structure. In all such embodiments, the structure may be distorted due to irregularities in and/or differential size and charge effects. The substitutional or interstitial solid solubility of dopants and impurities may occur during production of the NBT-based catalyst (i.e., the element may be added (dopant) or present (impurity) as a reactant during the production process and incorporated into the crystalline NBT during crystal. Without being bound to theory, it is understood that the introduction of foreign cations and/or anions may introduce crystal defects into the NBT. When the substituting cation of higher valence or when any interstitial cation is introduced, these cations are donor dopants that add electrons to the system for electronic charge compensation. Conversely, when the substituting cation is of lower valence, these cations are acceptor dopants that remove electrons from the system for electronic charge compensation. However, ionic charge compensation is a common alternative to electronic charge compensation. In such cases, a donor dopant would annihilate intrinsic oxygen vacancies whereas an acceptor dopant would increase their concentration. In almost all cases, such introductions have the potential to reduce the crystallinity through structural distortion. [00082] The NBT-based catalyst of the present invention may optionally comprise a co-catalyst. The co-catalyst may be applied to at least a portion of the surface of the NBT-based catalyst. The co-catalyst may also be catalytic, although this is not necessary. The co-catalyst may be applied to increase the electrical conductivity, thereby enhancing the charge transfer between the co-catalyst and the NBT; this serves to enhance the charge separation of the charge carriers, thus hindering and delaying recombination. The co-catalyst may be used to suppress the HER, OER, and/or ORR reactions. It may be advantageous to suppress the OER so as to generate hydrogen gas alone in order to suppress the formation of hydrogen + oxygen gas mixtures; that is, the oxygen remains dissolved in the water. The co-catalyst may be metallic. It may any suitable material, such as, for example, platinum, palladium, cobalt, copper, conductive carbon (e.g., graphite sheets or nanotubes) and the like. Production Method [00083] The NBT-based catalyst of the present invention is produced using a modified hydrothermal method. The method generally comprises combining a bismuth-containing species with a titanium-containing species and a sodium-containing species in a liquid reaction mixture, and in specific molar ratios, before heating the reaction mixture and then washing steps of the produced solid are carried out. [00084] To obtain the NBT-based catalyst of the present invention, and particularly the heterojunction-containing nanoparticulate catalyst described herein, the molar ratio of the bismuth to titanium in the reaction mixture should in approximately stoichiometric ratio, or at about 1:1, and the sodium should be in a significant molar excess in order to produce NBT (rather than another solid form, such as BTO or Bi ₂ O ₃ as the major phase). The sodium should be present in excess of at least 100 mol% of the stoichiometric amount, or at least 150 mol%, or at least 200 mol%, or at least 250 mol%, or at least 300 mol%, or at least 500 mol%, or between about 100 mol% and about 750 mol%, or between about 200 mol% and about 500 mol%, or between about 250 mol% and about 400 mol%, or it may be about 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 550, 600, 650, 700, or 750 mol% or any range therein. In some embodiments, the sodium is present at about 250 mol% of the stoichiometric amount. The molar excess of sodium may be achieved by conducting the reaction by adding NaOH of a molarity of at least 2.5 M, or between about 3 M and about 28 M, or between about 5 M and about 15 M, or between about 7 M and about 15 M, or at a molarity of about 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 21, 22, 23, 24, 25, 26, 27 or 28 M. As a result of this stoichiometric excess of sodium, the resulting molar ratio of sodium to bismuth to titanium (Na : Bi: Ti) is at least 5:1:1, or at least 7.5:1:1, or at least 10:1:1, or at least 12.5:1:1, or at least 15:1:1, or 33:1:1, or between about 5:1:1 and about 37:1:1, or between about 10:1:1 and about 25:1:1, or between about 12.5:1:1 and about 20:1:1, or it may be about 5:1:1, 5.5:1:1, 6:1:1, 6.5:1:1, 7:1:1, 7.5:1:1, 8:1:1, 8.5:1:1, 9:1:1, 9.5:1:1, 10:1:1, 10.5:1:1, 11:1:1, 11.5:1:1, 12:1:1, 12.5:1:1, 13:1:1, 13.5:1:1, 14:1:1, 14.5:1:1, 15:1:1, 15.5:1:1, 16:1:1, 16.5:1:1, 17:1:1, 17.5:1:1, 18:1:1, 18.5:1:1, 19:1:1, 19.5:1:1, 20:1:1, 20.5:1:1, 21:1:1, 21.5:1:1, 22:1:1, 22.5:1:1, 23:1:1, 23.5:1:1, 24:1:1, 24.5:1:1, 25:1:1, 27.5:1:1, 30:1:1, 32.5:1:1, 35:1:1, or 37.5:1:1 or any range therein. [00085] The process used to produce the NBT-based catalyst of the present invention may utilise any suitable method known in the art. For instance, the NBT may be prepared by a process selected from a solid-state reaction, solution decomposition, sol-gel processing, or hydrothermal synthesis. In one particularly preferred method, hydrothermal synthesis is used to produce the NBT-based catalyst described herein. [00086] In an example of a hydrothermal method, the bismuth-containing species, such as bismuth nitrate pentahydrate (Bi(NO ₃ ) ₃ ·5H ₂ O) or any other suitable bismuth salt, is combined with an equimolar amount of a titanium-containing species, such as titanium tetra-isopropoxide (TTIP; Ti[OCH(CH ₃ )] ₄ ) or any other suitable titanium complex or salt, in a liquid, such as ethanol or any other liquid suitable for dissolving the bismuth-containing species and the titanium-containing species. To this solution an aqueous solution of sodium hydroxide (NaOH) is added dropwise until a molar excess amount is achieved. Advantageously, sodium hydroxide acts in two ways: to add sodium ions to the reaction mixture, as well as acting as an oxidant to convert the bismuth nitrate pentahydrate into bismuth hydroxide (Bi(OH) ₃ ) for formation into NBT. The resulting suspension is then heated in an autoclave for a period of time. The mixture may be heated to a temperature of between about 100°C and about 600°C, or between about 150°C and about 200°C, or between about 160°C and about 180°C, or between about 250°C and about 400°C , or between about 300°C and about 600°C , or it may be heated to a temperature of about 100°C, 105°C, 110°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 230°C, 235°C, 240°C, 245°C, 250°C, 255°C, 260°C, 265°C, 270°C, 275°C, 280°C, 285°C, 290°C, 295°C, 300°C, 305°C, 310°C, 315°C, 320°C, 325°C, 330°C, 335°C, 340°C, 345°C, 350°C, 355°C, 360°C, 365°C, 370°C, 375°C, 380°C, 385°C, 390°C, 395°C, 400°C, 405°C, 410°C, 415°C, 420°C, 425°C, 430°C, 435°C, 440°C, 445°C, 450°C, 455°C, 460°C, 465°C, 470°C, 475°C, 480°C, 485°C, 490°C, 495°C, 500°C, 505°C, 510°C, 515°C, 520°C, 525°C, 530°C, 535°C, 540°C, 545°C, 550°C, 555°C, 560°C, 565°C, 570°C, 575°C, 580°C, 585°C, 590°C, 595°C, 600°C, or any range therein. The reaction time, being the time that the autoclave is held at the temperature described above and optionally being allowed to cool, may be between about 6 hours and about 3 months, or between about 12 hours and about 36 hours, or between about 18 hours and about 30 hours, or between about 1 day and 7 days, or between about 1 month and 3 months, or for about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47 or 48 hours, or 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89 or 90 days (i.e., 3 months), or any range therein. In one embodiment, the autoclave is held at a temperature between about 140°C and about 200°C for a time of between about 12 hours and 48 hours. In one particularly suitable method, the autoclave is held at about 180°C for about 24 hours. After heating, the suspension is separated from the liquid, which may occur using any suitable method, such as by centrifuging and decanting the liquid, or by filtering the suspension and collecting the filtrate. The resulting particulate solid may then be washed with new aliquots of the reactant liquid, and/or water, and/or any other suitable liquid. In one embodiment, the particulate solid may be washed up to three times with each ethanol and water. The washed particulate solid is then dried and ground in order to minimise aggregation of the particulate solid before use. [00087] Before use as a catalyst, the method above may comprise a further optional step of contacting the particulate solid with water so as to leach sodium ions from the NBT-based catalyst. The water used to leach the sodium ions from the NBT-based catalyst will preferably be low in dissolved sodium ions, or substantially free of dissolved sodium ions. It may be deionised water. The optional step of leaching the sodium ions from the NBT-based catalyst of the present invention may be particularly required when the water to photolyse is low in, or free of, sodium ions in order to minimise or preclude leaching may during use of the NBT-based photocatalyst. Use [00088] As discussed above, the NBT-based catalyst of the present invention may be adapted for use in generating from water or seawater (1) hydrogen gas alone via HER; (2) hydrogen gas plus oxygen gas via HER and OER, respectively; and (3) hydrogen gas plus hydrogen peroxide liquid via HER and ORR, respectively. The first two are referred to as “water splitting” and the latter is referred to as “intermediate water splitting”. [00089] In one embodiment, the present invention provides a method for producing hydrogen gas plus hydrogen peroxide liquid from water. The method requires that the catalyst of the present invention, as described herein, is in contact with water. The catalyst may be partially in contact with the water (e.g., the catalyst may be floating on a surface, or the water may be applied to top surface of a solid catalyst), or the catalyst may be entirely submerged in the water, or a combination of the two arrangements. The water may be obtained from any suitable water source. As the catalyst does not form an electrolysis cell, the water does not require a dissolved electrolyte in order to carry charge between the cathode and the anode. Accordingly, the water may be distilled water or deionised water. It is a further advantage of the present invention that the catalyst has band energies located close to the potentials required to carry out the HER and the ORR, which are less than, and do not substantially overlap, with the potentials required to oxidize anions that may be present in the water. For instance, chloride ions are particularly prevalent in natural water sources. The oxidation of chloride ions to chlorine requires a potential of about -1.36 eV, which is a significantly more negative voltage then the CB of the catalyst of the present invention can provide (-0.18 eV). Accordingly, water sources of a natural origin may be used in this method, as chloride gas cannot be produced by the NBT-based catalyst of the present invention. The natural water source may be river water, seawater, brackish water, bore water, or any other suitable natural water source. However, as the skilled person would be aware, hydrogen peroxide can react with dissolved ions or organic solids found in natural waters such as seawater. Therefore, a natural water source may not be suitable for the production of hydrogen gas plus hydrogen peroxide unless specific preliminary treatment is carried out (including, for example, microfiltration, and/or ultrafiltration, and/or reverse osmosis) before use. [00090] With the catalyst at least partially in contact with the water, a method that relies on use of the catalyst as a photocatalyst also requires illumination of the catalyst with a light source. As would be evident to the skilled person from the discussion above, the NBT-based catalyst of the present invention may be used as a photocatalyst, in that it absorbs photons to produce excited negative electrons and positive holes, both of which can facilitate chemical reactions, such as the HER, OER, and ORR required to split water. Accordingly, in this method, the catalyst that is in at least partial contact with the water, must be arranged so as to be able to absorb photons from the light source. The light source may be any suitable source of photons at a visible wavelength (i.e., between about 400 nm and about 700 nm) and/or at an ultraviolet wavelength (i.e., between about 100 nm and about 400 nm). The light source may be naturally occurring (such as solar energy) or it may be from an artificial source (such as an LED light, an incandescent bulb, fluorescence envelope, or the like). As the band gap for the NBT-based catalyst of the present invention is adapted to be about 1.78 eV, it is capable of absorbing all, or substantially all, wavelengths of visible light (as photons of visible light with frequencies between about 400 nm and about 700 nm have energies of between about 3.1 eV and 1.75 eV, respectively) and/or ultraviolent light (as photons of ultraviolet light with frequencies between about 100 nm and about 400 nm have energies of between about 12.4 eV and 3.15 eV, respectively). However, when the catalyst is entirely, or substantially, submerged in the water, the water cannot be of a depth or turbidity that would significantly reduce the number of photons accessible to the catalyst. It is anticipated that the skilled person will be able to design and implement a device or apparatus that would allow for the catalyst to absorb photons, even when entirely, or substantially, submerged in the water. As the band gap for the NBT-based catalyst of the present invention is adapted to be about 1.78 eV, it is capable of absorbing all, or substantially all, wavelengths of visible light (as photons of visible light with frequencies between about 400 nm and about 700 nm have energies of between about 3.1 eV and 1.75 eV, respectively) and/or ultraviolent light (as photons of ultraviolet light with frequencies between about 100 nm and about 400 nm have energies of between about 12.4 eV and 3.15 eV, respectively). However, when the catalyst is entirely, or substantially, submerged in the water, the water cannot be of a depth or turbidity that would significantly reduce the number of photons accessible to the catalyst. It is anticipated that the skilled person will be able to design and implement a device or apparatus that would allow for the catalyst to absorb photons, even when entirely, or substantially, submerged in the water. [00091] With the catalyst at least partially in contact with the water, a method that relies on use of the catalyst as a piezocatalyst also requires exposure to vibrational or mechanical force. As would be evident to the skilled person from the discussion above, the NBT-based catalyst of the present invention may be used as a piezocatalyst, in that it absorbs vibrations or mechanical force to produce excited negative electrons and positive holes, both of which can facilitate chemical reactions, such as the HER, OER, and ORR required to split water. Accordingly, in this method, the catalyst that is in at least partial contact with the water, must be arranged so as to be able to absorb vibrations or mechanical force. The vibrations may be produced by any suitable source. For example, acoustic vibrations can be provided by an ultrasonicator. The ultrasonicator may be operated at a power of greater than about 300 W, for instance it may be between about 300 W and 600 W, or between about 350 W and 550 W, or between about 400 W and 500 W, such as about 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 600 W or higher, or any range therein. Irrespective of the frequency of the vibrations that are applied to the catalyst, which may be between about 10 and about 500 Hz, such as between about 10 and 50 Hz, or between about 50 and 100 Hz, or between 100 and 200 Hz, or between 200 and 500 Hz, for example it may be 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 19, 200, 220, 240, 260, 280, 300, 320, 340, 360, 380, 400, 420, 440, 460, 480, or 500 Hz or any range therein. In one embodiment, the frequency of the vibrations may be about 40 Hz. However, when the catalyst is entirely, or substantially, submerged in the water, there may be some attenuation of energy and frequency that occurs as the vibrations pass through the suspension, which must be taken into account by the skilled person during design of a suitable apparatus. [00092] It is also an advantage of the present disclosure that the band energies can be “bent” by the use of vibrational or mechanical force. As discussed above, the catalyst of the present disclosure is not only a photocatalyst but is also piezocatalyst. Without being bound to theory, it is understood by the inventors that the piezopotential of the NBT-based catalyst of the present invention exerts an effect known as “band bending”, whereby both the CB and VB energy bands are altered in parallel upon exposure to, and absorption of, vibrational or mechanical energy. As the skilled person would appreciate, “band bending” allows the catalyst to facilitate different redox half reactions, as the potentials of the excited electrons and the holes are altered. In addition, the application of both light and vibrational energy together allows the catalyst to produce different product(s) than if just light, or just vibrational or mechanical energy, is provided. In this regard, the catalyst of the present invention is preferably photo-piezocatalytic. [00093] When undertaking the photocatalytic, piezocatalytic or photo-piezocatalytic method described above, the excited catalyst will produce (1) hydrogen gas alone, (2) both hydrogen gas and oxygen gas, or (3) hydrogen gas and hydrogen peroxide from the water or seawater. In a method of producing hydrogen gas and oxygen gas, the HER and OER occur at the same site (i.e., the surface of the NBT-based catalyst), and so the hydrogen gas and oxygen gas will be produced as a mixture (as opposed to a traditional electrolysis cell, whereby the cathode and anode are physically separated and so the two gases can be physically separated). Whilst mixtures of hydrogen gas and oxygen gas may be preferred for some particular uses, such as in syngas or the production of certain chemicals when combined with a carbon source, such mixtures are generally avoided as they can be combustible, and pure products are usually more sought after. Accordingly, in one embodiment, the hydrogen gas and oxygen gas are separated. The gases may be separated shortly after leaving the water. Such separation may be carried out by any suitable method known in the art. For example, they may be separated by a specialised gas-permeable membrane. Optionally, a scavenger additive may be added to the water to prevent the formation of either the hydrogen gas or the oxygen gas in order to produce a pure, or at least more pure, product. For instance, as the skilled person would be aware, an alcohol such as methanol or ethanol can be added to the water as a scavenger in order to prevent oxygen production. In one alternative arrangement, the NBT-based catalyst of the present invention may be used to form an electrode which forms an electrical circuit with another electrode material; for example, the NBT-based catalyst of the present invention may be used as a photocatalytic anode that produces oxygen, which is in a circuit with a cathode, such as platinum, for example, at which hydrogen gas is produced. Alternatively, in a preferred embodiment, the NBT-based catalyst of the present invention may be used to produce hydrogen gas plus hydrogen peroxide liquid. As the skilled person would appreciate, in this mode the products are a gas (hydrogen gas) and a liquid (hydrogen peroxide), therefore the products remain largely separated. However, hydrogen peroxide is reactive, and so maintaining active hydrogen peroxide in solution may require optimisation of the water used as well as and components and operating conditions of the apparatus. Examples [00094] The present invention will now be described with referred to specific examples, which should not be construed as in any way limiting. Example 1: Material Synthesis and Characterization [00095] To produce the NBT-based catalyst of the present invention, the following procedure was followed.2.910 g of bismuth nitrate pentahydrate (Bi(NO ₃ ) ₃ ·5H ₂ O; reagent grade, 98 wt%, Sigma-Aldrich) was added to 15 mL absolute ethanol (C ₂ H ₅ OH; reagent grade, 96 wt%, Chem- Supply) and then magnetically stirred at 300 rpm for 2 hours.1.755 mL of tetra-isopropoxide (TTIP; reagent grade, 97 wt%, Sigma-Aldrich) was added dropwise during stirring over about 15 min to achieve 1:1 molar ratios of Bi:Ti, followed by stirring at 300 rpm for about 1 hour.15 mL of 5 M sodium hydroxide solution (NaOH; regent grade, 98 wt%, Sigma-Aldrich) then were added dropwise over about 15 min in order to increase the pH to approximately 14.2-14.8, as determined by a pH meter (FG2, Mettler-Toledo, USA), followed by stirring at 650 rpm for around 1 hour. The amount of NaOH was 250 mol% excess of the stoichiometric amount, which fixed the Na:Bi ratio at 12.5:1 and was necessary to ensure NBT formation. The resulting suspension was transferred into a Teflon-lined (55 mm ID x 80 mm H) stainless steel autoclave and heated in an electric oven at a rate of 2°C/min to 180°C and treated for 24 hours, followed by natural cooling. The precipitate was separated from the liquid by centrifugation at 5000 rpm for 10 minutes then decanted, before the addition of a 15 mL aliquot of liquid and hand shaking for about 10 min, then centrifugation (under the same conditions), and further decanting. The liquids used were 15 mL of either absolute alcohol or deionised water. These washing processes were done alternatively between ethanol and water a total of three times each, starting with absolute alcohol, followed by water and so on, giving a total of six washings. The washed powder was dried in an electric oven at a rate of 2°C/min to 100°C and treated for 24 hours. The dried powder was deagglomerated by hand-grinding for about 15 min using an agate mortar. [00096] A further range of samples was produced to investigate the effects of varying NaOH concentrations (and therefore sodium ions) in the nanoparticles. Samples were produced according to the method above, whereby the volume of NaOH was maintained at 15 mL, but the concentration used was 2.5, 5, 7.5, 10, 12.5, or 15 M. [00097] The powders produced from this method were characterized by X-ray diffraction (XRD; MPD-Scherrer PANalytical Xpert Multipurpose X-ray diffractometer; CuKα, 45 kV, 40 mA, 0.026261° 2θ step size), see Figure 1. The surface chemistry was determined using X-ray photoelectron spectroscopy (XPS; Thermo Scientific ESCALAB 250Xi X-ray Photoelectron Spectrometer Microprobe, 13 kV, 12 mA, 500 μm spot size), see Figure 2A (5M NaOH) and Figure 2B (variable NaOH concentrations). The peaks for the V _O ^•• are the most important, so they are indicated numerically. The theoretical maximal [ V _O ^•• ] are 13.33% for NBT ₄ and 16.66% for NBT. High-resolution images of the particles (5 M) were obtained by Field Emission Scanning Electron Microscopy (FESEM; FEI Nova NanoSEM 230, 3 kV) and Field Emission Transmission Electron Microscopy (FETEM; Philips CM200, 200 kV and JEOL JEM- F200 cold field emission gun, 200kV), see Figures 3A and 3B. The specific surface area, pore volume, and average pore size were evaluated by the Brunauer-Emmett-Taller (BET) nitrogen adsorption method (Quantachrome Autosorp 1MP, 0.1 g sample; 1 h at 150°C degassing), see Table 1. Table 1: Characterisation of NBT-based nanoparticles (5 M). [00098] The optical indirect band gap (Eg) was determined by optical absorption using UV-Vis spectrophotometry (UV-Vis; PerkinElmer Lambda 35 UV-visible spectrometer), the valence band maximum was determined by XPS, and the conduction band minimum was determined by the difference, see Table 2. For the samples for which the Eg was determined using aqueous suspensions (i.e., method 1 of Table 2), the Tauc method was used, which for a crystalline semiconductor, the optical indirect band gap is calculated by the equation: where α = absorption coefficient, h = Planck’s constant, v = frequency, and A = constant. For the samples for which the E _g was determined using dry powders (i.e., methods 2 and 3 of Table 2), the Kubelka-Munk method was used, in which the optical indirect band gap is calculated by the equation: where R _∞ = relative diffuse reflectance, h = Planck’s constant, v = frequency, and A = constant. Table 2: Energy band structures of nanoparticles (5 M). * Method 1 = UV-Vis absorption of suspension; Method 2 = UV-Vis absorption of dry powder; Method 3 = UV-Vis reflection of dry powder [00099] The data for the samples measured using Method 1 (aqueous suspensions) are considered to be the most indicative because dye degradation requires immersion of the powders in an aqueous solution and repeat measurements using two different UV-Vis units were completely consistent. Example 2: Photocatalytic Testing [000100] The photocatalytic performance was examined by the photodegradation of methylene blue (MB) solution. The previously centrifuged powder was washed by addition of 15 mL of deionised water, hand shaking for about 10 minutes, centrifuged at 10,000 rpm for 10 minutes, and decanted. These washing processes were done a total of three times. The washed powder was dried in an electric oven at a rate of 2°C/min to 100°C and treated for 24 hours. The dried powder was deagglomerated by hand-grinding for around 10 minutes using an agate mortar. The MB solution was prepared by dissolving methylene blue (M9140; dye content ≥82 wt%, Sigma- Aldrich) in deionised water at 10 ^-5 M concentration (0.0064 g MB was diluted in 2 L deionised water). Surface saturation of the powder was achieved by adding 8 mg of the sample to 8 mL MB solution in a 50 mL Pyrex beaker, followed by magnetic stirring at 300 rpm for 15 minutes while being held in the dark in a light-obstructed black box. The sample was exposed to 365 nm UV irradiation from a UV lamp (8 W, 3UV-38, UVP) while being magnetically stirred at 300 rpm for different times up to 2 hours in the black box. The distance between the UV lamp and the MB solution was maintained at about 10 cm. The exposed solution was separated from the powder by centrifuging at 5,000 rpm for 10 minutes, followed by decanting and UV-Vis testing. The extent of MB degradation was determined from the peak intensity at 664 nm. [000101] The cyclic stability of the powder was assessed by undertaking the MB degradation testing for photocatalytic performance four times, with the described washing procedure’s being done after each test. For the MBT-based catalyst, the degradation increases with increasing irradiation in time and photoactivity was recorded in the presence of UV light. MB degradation curves can be fit to the generally observed pseudo-first-order kinetics which are described by Equation 3: (Equation 3) where c ₀ = Initial concentration of MB solution; c = Final concentration of MB solution; k = rate constant; and t = Time. When in contact with the NBT-based catalyst, about 83% of MB was degraded after 1 hour, and about 85% of MB was degraded after 2 hours. [000102] The types of active sites were assessed through MB degradation testing for photocatalytic performance applied to solutions containing trapping agent additions that affect charge carriers and reactive species. These included 1.0 mM concentrations of triethanolamine (TEOA), a quencher of holes (h ⁺ ); P-benzoquinone (BQ), a quencher of •O ₂ - (superoxide radical); and isopropyl alcohol (IPA), a quencher of •OH (hydroxyl radical), see Figure 4. In particular, Figure 4(a) shows the stability and recyclability of NBT (5 M) in terms four-cycle testing, where the photodegradation extent is exponential. This is consistent with the observed poor crystallinity of the NBT structure, the solubilities of materials often are inversely proportional to their crystallinities, which results from the greater stability of the well crystallised analogue in contrast with the lower stability of the poorly crystallised analogue, and Figure 4(b) shows the roles of active species for NBT (5 M), as revealed by the effects of the scavengers triethanolamine (TEOA), P-benzoquinone (BQ), isopropyl alcohol (IPA). These three scavengers are known to quench active holes (h+), the superoxide radical (•O ₂ -), and the hydroxyl radical (•OH), respectively. These data show that the order of effectiveness of these active species in photodegradation is IPA < BQ < TEOA. In other words, the photodegradation can be increased by >60% with the generation of active holes, which is facilitated by lowering of the E _g and/or the introduction of shallow midgap states. Further, the photodegradation was increased by >40% with the generation of •O ₂ -, which can be generated readily through a range of chemical, photochemical, and photocatalytic methods. Example 3: Heterojunction Analysis [000103] The phases NBT, NBT ₄ , and BTO are part of a homologous series of chemically and structurally similar phases that exhibit different stacking sequences. This series is described by the formula (1-x) Bi ₄ Ti ₃ O ₁₂ – xNa _0.5 Bi _0.5 TiO ₃ (BTO – NBT). These are better known as Aurivillius phases, which are intergrowths of pseudo-perovskite blocks (A _m-1 B _m O _3m-1 ) ^2- and fluorite-like (Bi ₂ O ₂ ) ^2- layers. In the former, m is an integer describing the number of perovskite blocks layered between the fluorite layers. For BTO, m = 3, for NBT ₄ , m = 4, and, for NBT, m = ∞. Owing to this nature, BTO, NBT ₄ , and NBT can be expected to occur together. [000104] With this in mind, a closer investigation of the high-resolution transmission electron microscopy (HRTEM) images taken of a single specific location of the nanoparticles (5 M) described above indicate that heterojunctions of NBT/NBT ₄ and NBT/BTO, although it is almost inevitable NBT ₄ /BTO heterojunctions form. As can be seen in Figure 5A(a), there are two different and relatively clear lattice fringes in the observed areas at 0.26 nm and 0.30 nm, which are in good agreement with those expected for the respective (101) plane (32.1° 2θ) of NBT and the (117) plane (30.1° 2θ) of BTO, which are the major peaks for the respective phases, as shown in Figure 1. The HRTEM image also shows a clear planar grain boundary, where the lattice spacings at the interfacial angle closely correspond, with a lattice mismatch of ~5.6%. This indicates a stable semicoherent (<16% mismatch) interface as the basis for a heterojunction. The selected area electron diffraction (SAED) pattern in Figure 5A(b) supports the presence of these two phases through the (101) and (202) planes of NBT and the (117) plane of BTO. The EDS mapping in Figure 5A(c) shows that the elements Na, Bi, Ti, and O are distributed homogeneously in the nanoparticles. The generation of NBT/BTO heterojunctions would facilitate the transfer photogenerated electrons, resulting in efficient electron-hole pair separation. As can be seen in Figure 5B (12.5 M), heterojunctions of NBT and NBT ₄ have also been identified in the HRTEM images of a single specific location of samples produced with relatively low amounts of sodium. Since all of the samples contain both NBT and NBT ₄ , it is clear that they also contain heterojunctions of these two phases. Concerning the NBT/BTO heterojunctions, although X-ray diffraction required 15 M NaOH to generate detectable BTO, the SAED data for 5 M NaOH also revealed BTO. It is possible that BTO also is present in all of the compositions. If so, then all of the samples exhibit triple point heterojunctions with variable amounts of the three different phases. The simultaneous presence of these three phases represents a nonequilibrium phase assemblage (i.e., NBT is not compatible with BTO). Since the compatible phases NBT and NBT ₄ are the two main phases and hence in equilibrium, this suggests that the use of excessive [NaOH] is a driving force to precipitate the nonequilibrium phase BTO. This also explains why BTO is a trace phase. This also suggests that the other nine phases that are compatible with NBT and were suggested to act, as such, as potential trace heterojunctions, may not be the case. In all such cases, these phases must be piezocatalysts; this is unknown. Example 4 – Oxygen Vacancies [000105] It is widely expected that the performance of the catalyst of the present invention may depend on, or be influenced by, the amount of oxygen vacancies in the material. Figure 6A shows the oxygen vacancy concentration [V _O ^•• ] as a function of the relative amount of NBT (in the mixture of NBT ₄ + NBT); BTO is not considered as it is a trace phase) and [NaOH]. This strongly logarithmic plot makes clear that the [V _O ^•• ] is maximised at ~67 vol% NBT (33 vol% NBT ₄ ; as the respective true densities of NBT ₄ and NBT are 7650 kg/m ³ and 5540 kg/m ³ , the weight fractions are ~73 wt% NBT and ~27 wt% NBT ₄ ). Further, Figure 6B shows the H ₂ evolution at 5 h as a function of the relative amount of NBT and [NaOH]. This plot shows that the performance does not track with the [V _O ^•• ]. In fact, it suggests that the performance is strongly dependent on the phase proportions, where both NBT ₄ and NBT contribute principally through their piezocatalytic activities but that NBT is significantly more effective than NBT ₄ . As with the [V _O ^•• ] in Figure 6A, there appears to be a transition between NBT ₄ -dominant behaviour and NBT-dominant behaviour at ~67 vol% (~73 wt%) NBT. Example 5 – Water Splitting [000106] In an initial demonstration of the water-splitting capacity of a catalyst of the present invention, a range of nanoparticles was fabricated with different NaOH concentrations at constant Bi/Ti ratios. Platinum (Pt, 3 wt%) was used a co-catalyst and ethanol was used as a scavenger to block oxygen (O ₂ ) generation to demonstrate hydrogen gas production (since gaseous products formed using a catalytic powder are difficult to separate). Hydrogen gas evolution of these nanoparticles for a range of water sources is shown in Figure 7. Notably, chlorine gas was not detected in any of these experiments. [000107] Further experiments have also been carried out, again with Pt as co-catalyst. In contrast, only triethanolamine (TEOA, 15 vol%) was used as a hole scavenger. This change was affected because TEOA contributes to enhancement of charge carrier separation through hole annihilation and no oxygen is produced (hydrogen peroxide is produced), regardless of the presence or absence of an O ₂ scavenger. As can be seen in Figure 8A, the H ₂ evolution during piezocatalysis as a function of increasing NaOH concentration is shown; the trends are approximately linear. In Figure 8B, the H ₂ evolution during piezocatalysis as a function of water type (deionised (DI), simulated seawater, and natural seawater) is shown; the latter two exhibit half the H ₂ evolution of that of DI water. In Figure 8C, the H ₂ evolution as a function of catalysis type is shown; photocatalysis shows low H ₂ evolution but piezocatalysis and photo- piezocatalysis are significantly higher. These figures suggest that photocatalysis and piezocatalysis may be additive mechanisms. Most significantly, when compared to the initial data in Figure 7, whereby maximal H ₂ evolution at 5 hours of light irradiation was ~12.5 μmol/g, the data of show that, while the outcome for photocatalysis was comparable (Figure 8C), piezocatalysis and photo-piezocatalysis (Figure 8C) exhibit average H ₂ evolutions at 5 h irradiation of ~665 μmol/g, which is greater by a factor of >50. Example 6 – H2O2 Generation [000108] As mentioned above, the catalysis of water can produce either (1) hydrogen gas alone, (2) hydrogen gas plus oxygen gas, or (3) hydrogen gas plus hydrogen peroxide liquid. The production of hydrogen peroxide has advantages, requiring only two electrons to split water (instead of four electrons for conventional water splitting processes relying on HER and OER). When the catalyst is a nanoparticle (such as that of the present invention) and the anodic and cathodic reactions cannot be physically separated, the production of hydrogen peroxide may be preferred, since the products are a gas (hydrogen gas) and a liquid (hydrogen peroxide) that do not need to be separated (as compared to conventional water splitting, which produces hydrogen gas plus oxygen gas). A further advantage is that hydrogen peroxide is an environmentally friendly commercial product of considerable industrial importance. However, the ORR reaction exhibits a strong material selectivity and so does not result from all catalytic reactions. [000109] To demonstrate hydrogen peroxide production by the catalyst of the present invention, 100 mg of the 12.5 M NaOH nanopowders were suspended in 100 mL of deionised water in a vial. The vial was sealed with a septum and sonicated for 5 h, during which the gas was sampled by hypodermic syringe every hour (with H ₂ evolution subsequently measured). Ice was added to the water bath in order to maintain the temperature below 25°C in order to minimise H ₂ O ₂ decomposition.10 mL aliquots of solution were isolated from the nanopowders by filtration (since centrifuging also can decompose H ₂ O ₂ ) to which 0, 2, 4, or 6 mL of a 5 mM potassium permanganate (KmnO ₄ ) solution were added by pipetting. The colour of the solution was measured after ~10 min, whereby, if excess KmnO4 was present (that is, not degraded by hydrogen peroxide), the solution would retain a pink colour, or would be colourless if excess hydrogen peroxide. A control sample of pure DI water also was examined as a baseline pink colour. [000110] The results showed that the 0, 2, and 4 mL samples were colourless, and the 6mL and DI control samples were pink. An interpolation of the results indicated that a mixture containing about 5 mL of KmnO ₄ solution represents the conversion from coloured to colourless; therefore the calculated concentration of H ₂ O ₂ was 3 mg/L. Since the H ₂ evolution after 5 h was ~5 mg/L and the H ₂ /H ₂ O ₂ ratio should be 1/1, then only ~60% of the H ₂ O ₂ was detected. This could have arisen from (1) H ₂ O ₂ decomposition, (2) H ₂ escape across the septum, (3) inaccuracy in colour assessment, and/or (4) low precision of gas chromatography mass spectrometry (GC-MS) technique. [000111] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms in particular features of any one of the various described examples and it may be provided in any combination in any of the other described examples. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.