Facet-selective electrostatic assembling of composite and its use in catalysis systems

Abstract

A method (100) for manufacturing a composite, the method comprises providing (101) crystal particles comprising semiconductor materials, the crystal particles having a point of zero charge, PZCp, being a pH at which a net charge of total particle surface is equal to zero, each crystal particle having at least a first and a second facet; providing (102) flakes comprising semiconductor materials or metallic materials, the flakes having a point of zero charge, PZCf, being a pH at which a net charge of total flake surface is equal to zero, the flakes having average thickness below 30 nm, wherein an absolute difference, ΔPZC, between the point of zero charge of the crystal particles and the point of zero charge of the flakes is at least 1; mixing (103) the crystal particles and the flakes in liquid to form a mixed solution; and adjusting (104) a pH value of the mixed solution to be between the point of zero charge of the crystal particles and the point of zero charge of the flakes, thereby forming a composite comprising flakes arranged on at least one of the first or the second facet of the crystal particle. A composite produced by said method and a use of said composite in catalysis systems are also disclosed.

Description

[0001] FACET-SELECTIVE ELECTROSTATIC ASSEMBLING OF COMPOSITE AND ITS USE IN CATALYSIS SYSTEMS

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a method for manufacturing a composite and a composite obtainable from said method. Further, the present invention relates to a use for said composite and said method.

[0004] BACKGROUND OF THE INVENTION

[0005] Solar energy, a sustainable and renewable free resource, is paramount for our modern society to combat enormous environmental and energy issues. As its solution, photocatalysis on semiconductors has attracted considerable attention: water splitting to produce hydrogen, leaving no carbon emissions behind when used in fuel cells; converting CO2 into valuable chemicals and fuels; and air and water purification by decomposing pollutants.

[0006] However, photocatalytic activities of the existing state of the art are as of yet far from an advanced level beyond practical needs despite continuous efforts. A bottleneck lurks in the recombination of photo-generated charge carriers of electrons and holes that contribute to photocatalytic reactions. Hence, charge separation plays a crucial role in enhancing photocatalytic activities. As a vital step in the photocatalysis process, electron-hole pairs generated by light irradiation must be separated with semiconductor photocatalysts.

[0007] In this regard, particulate titanium oxide (TiOz) has been found to be useful in photocatalysts and other applications. A problem with conventional TiOz is that some of the excited electrons and holes recombine at defects within the titanium oxide as the recombination centres, which thus lowers the photocatalytic activity.

[0008] Still, to date, there are no existing single-crystal photocatalysts that may provide long-term charge separation to be efficient enough. Further, photo-deposition or electro-deposition is to date only the general method that enables precise selective deposition of co-catalysts onto specific facets of photocatalyst crystals, where, however, the deposited co-catalysts have to be in the form of precursors. During the deposition process, the precursors are transformed into targeted materials of co- catalysts. Thus, pre-synthesized co-catalysts cannot be combined with multi-faceted single-crystal materials by the conventional photo-deposition method.

[0009] Thus, there is a need for improved methods that are easy to perform, and which uses affordable pre-synthesized materials in order to manufacture more efficient materials with well-defined assembling of co-catalysts and single-crystal semiconductors for various a photocatalytic and electrochemical applications.

[0010] SUMMARY OF THE INVENTION

[0011] In view of the above, it is an object of the present technology to provide improved production methods such that improved composite materials that may be used in catalysis systems may be realized, which altogether alleviate the above-mentioned drawbacks.

[0012] Another object is to provide a method for manufacturing a composite, such that the method is designed to be simple, i.e., easy to perform and cheap to use, with commercially available and affordable materials.

[0013] Another object is to provide a composite that is designed for providing efficient charge separation of photo-generated electrons and holes, i.e., to make the lifetime longer for the charge separated species before they recombine.

[0014] Another object is to provide a composite designed as heterostructure coupling semiconducting photocatalysts with co-catalysts of metallic materials to form a Schottky barrier or metal oxides to build a p-n junction or direct Z-scheme heterojunction at their interface.

[0015] Another object is to provide a composite designed for providing active reaction sites for charge carriers by assembling 2D materials co-catalysts, which reduce activation energy and overpotential of reactions and thus can consume charge carriers efficiently.

[0016] Another object is to provide methods that combine facet-engineering to produce multi-faceted single-crystal photocatalysts with 2D materials co-catalysts.

[0017] A promising strategy to promote the charge separation is to develop heterostructure coupling photocatalysts with co-catalysts of metals. The internal electric field built at the space charge layer gives rise to the vectorial separation of charge carriers, preventing the recombination. In addition, recent advances have revealed that photo-generated electrons and holes move toward different crystal facets of single-crystal semiconducting materials, preventing charge separation.

[0018] Thus, to achieve at least one of the above objects and also other objects that will be evident from the following description, a method for manufacturing a composite, a composite and a use of the composite and the method, are as defined in the independent claims. Preferred variations to the inventive concept will be evident from the dependent claims.

[0019] According to a first aspect there is provided a method for manufacturing a composite comprises providing crystal particles comprising semiconductor materials, the crystal particles having a point of zero charge, PZCp, being a pH at which a net charge of total particle surface is equal to zero, each crystal particle having at least a first and a second facet. The method further comprises providing flakes comprising semiconductor materials or metallic materials, the flakes having a point of zero charge, PZC / , being a pH at which a net charge of total flake surface is equal to zero, the flakes having average thickness below 30 nm. Preferred average thickness may be from 0.1 nm to 3 nm.

[0020] An absolute difference, APZC, between the point of zero charge of the crystal particles and the point of zero charge of the flakes is at least 1. The crystal particles and the flakes are mixed in liquid to form a mixed solution. A pH value of the mixed solution is adjusted to be between the point of zero charge of the crystal particles and the point of zero charge of the flakes, thereby forming a composite comprising flakes arranged on at least one of the first or the second facet of the crystal particle.

[0021] According to a second aspect there is provided a composite comprising crystal particles comprising semiconductor materials. The crystal particles are polyhedron shaped having at least a first and a second facet. The composite further comprises flakes comprising semiconductor materials or metallic materials, the flakes are having an average thickness below 30 nm. The flakes are selected from the group consisting of MXenes, graphene, graphene oxide, reduced graphene oxide, borophene, boron nitride, carbon nitride, borocarbonitrides, transition metal oxides (TMOs), transition metal dichalcogenides (TMDs), and metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and layered double hydroxides (LDHs). The flakes are arranged on at least one of the first or the second facet of crystal particles.

[0022] According to a third aspect there is provided a composite which is produced by the method according to the first aspect.

[0023] According to a fourth aspect there is provided a use of a composite according to the second or the third aspect in catalysis systems, such as water splitting, CO2 reduction, water purification, air purification, self-cleaning, energy storage, batteries, supercapacitors, sensors, medical treatments or photovoltaics.

[0024] Thus, it has advantageously been found that according to the method of the present inventive concept it is possible to selectively assemble flakes i.e., nanothin two-dimensional (2D) material on particular facets of a single-crystal exposed with different crystallographic facets (multi-faceted single-crystal) due to the difference in electrostatic interaction between the different facets of the crystal particles and also the 2D material.

[0025] For example, the composite according to the present inventive concept may increase the hydrogen production in when used in the photocatalytic hydrogen evolution reaction (HER) more than 150 times compared to conventional catalysts. Other advantages will be appreciated when reading the detailed description.

[0026] BRIEF DESCRIPTION OF THE FIGURES

[0027] The above and other aspects of the present inventive concept will now be described in more detail, with reference to appended drawings showing variants of the present inventive concept. The figures should not be considered limiting the inventive concept, instead they are used for explaining and understanding the inventive concept.

[0028] As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and are thus provided to illustrate the general structures of variants of the present inventive concept. Figure 1 schematically illustrates a block scheme of a method for manufacturing a composite material.

[0029] Figure 2 illustrates that 2D materials may be selectively deposited onto specific facets of multi-faceted semiconducting crystals, where facet A is intrinsically more positively charged than facet B.

[0030] Figure 3 is a schematic illustration of the assembling of flakes onto specific facets of crystal particles.

[0031] Figure 4 illustrates a surface Zeta potential of pH of M01.33C and BiVCh as a function of pH. PZC is derived from the intersection of the horizontal line of 0 mV with the zeta potential curves. PZC of M01.33C and BiVCh were determined to be 0.82 and 2.66, respectively.

[0032] Figure 5 illustrates SEM images of pure BiVCh (a) and Moi.ssC / BiVC assembling mixed in the solution with pH 1.5 (b), 2.0 (c), 2,5 (d), 3.0 (e), and 4.5 (f).

[0033] Figure 6 illustrates a SEM image of assembling of adventitious non-stoichio- metric Mo oxides and BiVC mixed in the solution with pH 7.0.

[0034] Figure 7 illustrates the zeta potential vs pH values for TiCh and M01.33C MXene.

[0035] Figure 8 illustrates SEM images of pristine TiCh and Moi.ssC / TiCh composite assembled at pH3, where M01.33C MXenes are selectively deposited on the (101) facet of TiOz.

[0036] Figure 9 illustrates photocatalytic hydrogen evolution of pristine TiO? vs Moi.33C / TiO2 composite assembled at different pH values.

[0037] Figure 10 is a schematic illustration of M01.33C MXene deposited on the (101) facet of TiO2 and the band alignment of the resulting Moi.33C / TiO2 composite.

[0038] Figure 11 illustrates STEM images of Moi.33C / TiO2 composite assembled at pH 3, where M01.33C MXene may be found selectively on the (101) facet of TiCh.

[0039] Figure 12 illustrates STEM-EDX images of Moi.33C / TiO2 composite assembled at 3, where M01.33C MXene may be found selectively on the (101) facet of TiCh.

[0040] Figure 13 illustrates SEM images of pristine CU2O and MO1.33C / CU2O composite assembled at 11, where M01.33C MXenes are selectively deposited on the squareshaped (100) facet of CU2O. DETAILED DESCRIPTION OF THE INVENTION

[0041] The present inventive concept will now be described more fully hereinafter with reference to the accompanying drawing, in which preferred variants of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the variants set forth herein; rather, these variants are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. Although individual features may be included in different variants, these may possibly be combined in other ways, and the inclusion in different variants does not imply that a combination of features is not feasible. In addition, singular references do not exclude a plurality. In the context of the present invention, the terms "a", "an" does not preclude a plurality.

[0042] Definitions

[0043] Prior to outlining the present disclosure in more details, a set of terms and conventions is first defined:

[0044] Composite

[0045] A composite or composite material is a material which is produced from or being a combination of two or more materials with different physical and / or chemical properties and by the combination the two or more materials create a composite or composite material with properties that are unlike the individual materials, e.g., to become stronger, lighter, improved photocata lytic activity, improved electrochemical activity, etc.

[0046] Crystal particles

[0047] Crystal particles or crystals or crystalline solid is a solid material whose constituents, such as atoms, molecules, or ions, are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. Crystal particles may be single crystals which may be identifiable by their geometrical shape comprising flat faces or facets with sharp angles, thus in specific orientations. The crystal particles may preferably be single-crystal. The crystal particles may be semiconducting materials. The crystal particles may be polyhedron shaped, such as cubic, tetragonal, hexagonal, octahedral, decahedral, and tetradecahedron. Said polyhedron may have 6 to 14 facets. Thus, the single-crystals may be multi-faceted.

[0048] The crystal particles may be selected from the group consisting of transition metal oxides, transition metal sulfides and perovskite. Examples of transition metal oxides may be TiCh, CU2O, or BiVCh. An example of a transition metal sulfide may be M0S2. An example of a perovskite may be Y2Ti20sS2.

[0049] The crystal particles may also have an average total particle surface area in a range of 1-500 nm. However, the size is not so important in view that is important that the crystal particles have at least two different facets.

[0050] TiO2 is the most studied semiconducting material amongst various metal oxides and known as the model material. Thus, phenomena, which take place on TiCh, and their mechanisms are applicable to other materials. Furthermore, due to its cheap cost and chemical and mechanical stabilities, TiCh is one of the most promising photocatalyst materials.

[0051] BiVC is specifically interesting for photocatalytic applications due to its optimal bandgap of 2.4 eV, enabling the usage of visible light, and its superior oxidation photocatalytic activity. Furthermore, single-crystal BiVC with large size and tailored exposed facets can be facilely synthesized. These advantageous properties make BiVC an excellent model photocatalyst system to validate the concept of the facet- selective electrostatic assembling.

[0052] CU2O is inexpensive, non-toxic, and abundantly available with useful optical, electronic, and catalytic properties. Moreover, the preparation of CU2O polyhedra with different facets is simple and scalable, making CU2O an attractive candidate for practical applications.

[0053] Facets

[0054] The first facet may have a first Miller index. Further, the second facet may have a second Miller index. According to the present inventive concept the first and the second Miller indices may be different. The flakes may be arranged at only one of the first or the second facet. Thus, facet-selective electrostatic assembling of the flakes on crystal particles is achieved. The facets of a single-crystal may have different shapes, for example triangle, square, rectangle, pentagon, hexagon, octagon, rhomb, kite, circle, oval or more irregular shapes. Thus, a single-crystal may have facets with different shapes in the same single-crystal, for example the crystal-particles may have facets that are square- and rectangle-shaped, or square- and triangle-shaped, or other combinations of shapes.

[0055] Flakes

[0056] A flake is considered to be a thin flattened piece or layer. Thus, the flakes may be seen as two-dimensional (2D) layer(s) or nanosheets. The flakes may have an average thickness less than 30 nm, preferably the average thickness may be in a range of 0.1- 25 nm, even more preferably the average thickness may be in the range 0.1 nm to 3 nm. The flakes may also be considered as co-catalyst which may be selectively deposited onto specific facets of a multi-faceted crystal particle.

[0057] The flakes may be selected from MXenes, graphene, graphene oxide, reduced graphene oxide, borophene, boron nitride, carbon nitride, borocarbonitrides, transition metal oxides (TMOs), transition metal dichalcogenides (TMDs), and metalorganic frameworks (MOFs), covalent organic frameworks (COFs) and layered double hydroxides (LDHs). An example of boron nitride is hexagonal boron nitride (h-BN).

[0058] Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. Graphene has become a valuable and useful nanomaterial due to its exceptionally high tensile strength, electrical conductivity, transparency, and being the thinnest two-dimensional material in the world. Graphene may also be provided as modified graphenes such as graphene oxide or reduced graphene oxide.

[0059] Transition metal dichalcogenides (TMDs) are a class of layered materials which are interesting due to their scalability and thickness-dependent electrical and optical properties. M0S2 may be an example of a TMD. Further, metal organic frameworks (MOFs) are a class of porous polymers consisting of metal clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures, one example may be 1,4-benzenedicarboxylic acid (BDC). Most MOFs are crystalline compounds, but there may also be amorphous MOFs, and other disordered phases. In catalysis, MOFs may hold properties for application in conducting solids and as supercapacitors. In contrast to MOFs, covalent organic frameworks (COFs) are made entirely from light elements (H, B, C, N, and O) with extended structures. Layered double hydroxides (LDHs) are a versatile group of materials, which may be used in many applications within catalysis, energy conversion and storage, drug delivery, and environmental decontamination. An example of a LDH may be Ni(OH)2-Co(OH)2.

[0060] MXenes may be transition metal carbides, transition metal nitrides, transition metal borides or transition metal carbonitrides. Further, MXenes may be compounds of formula: Mn+iXnTm, wherein M is selected from Ti, Nb, V, Mo, Ta, Zr, Cr, Sc and Y; X is selected from C, N and B; T is selected from F, OH, O and Cl; and n is an integer between 1-4; and m is an integer between 0-2. Examples of MXenes may be Ti3C3, Ti3CN, Zr3C2, Hf3C2, Ti2C, V2C, Nb2C, Mo2C, Mo2N, Ti2N, Ti4N3, Nb4C3, Ta4C3, V4C3, (MO,V)4C3WI.33C, Nbi.33C, MOI.33C, MOI.33YO.6?C, MO4VC4, Mo2TiC2, Cr2TiC2, M02SCC2, or Mo2Ti2C3. As a preferred example the MXene may be M01.33C.

[0061] Point of zero charge (PZC)

[0062] Crystal facets may have various surface zeta potentials. The surface potentials vary depending on the pH of the solution or liquid, and the pH where the surface is uncharged is a so-called point of zero charge (PZC).

[0063] The pH value of the mixed solution may be adjusted to be within a pH range of wherein k is a constant below 0.4.

[0064] Theoretically the entire APZC range may be used but in practice a smaller range is sufficient. While most metal oxides have PZC in a range between 2 and 10, MXenes have been found to possess extremely low PZC of around 1.

[0065] Point of zero charge (PZC) is connected to the isoelectric point (IEP), which is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. These values may be determined by various techniques. The pH where the zeta potential reverses is called IEP, whereas the pH where the surface potential reverses is called PZC. They are numerically similar or the same often, and thus, IEP could be referred as PZC. There are amongst 4 representative methods to determine PZC and IEP: salt addition method, mass titration method, fast titration method, and zeta-potential method.

[0066] Miller indices

[0067] Miller indices is a notation system in crystallography for lattice planes in crystal (Bravais) lattices. Herein, a family of lattice planes of a given (direct) Bravais lattice is determined by three integers h, k, and £, the Miller indices. They are written (hk£), and denote the family of (parallel) lattice planes (of the given Bravais lattice) orthogonal to ghke = hbi + kbi + bs, where bi are the basis or primitive translation vectors of the reciprocal lattice for the given Bravais lattice. pH

[0068] The pH may be in a range from 0.5 to 14, preferably within a pH range of 1 to 12, preferably within a pH range of 2 to 11. Which pH that may be used is dependent on the specific crystal particle and flakes that are to be mixed. The pH value of the mixed solution may be adjusted by adding an acid or a base to the solution. Examples of acids may be acetic acid, hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid or carbonic acid. Examples of bases may be sodium carbonate, potassium carbonate, sodium hydroxide or potassium hydroxide.

[0069] In the prior art, tuning pH has been used to combine two materials but randomly without the facet selectivity, or the assembling of two materials has also been performed by solvothermal method or electrodeposition (photo-deposition) method.

[0070] Thus, it has surprisingly been found in the present inventive concept that selective assembling of the flakes on a specific facet of the crystal particle by tuning pH. For example, for TiCh and MXenes, pH 2-3 is optimal, whereas for CU2O and MXenes, and for BiVO4 and MXenes, pH 11 and 1.5-2.0 are optimal, respectively.

[0071] Mixing

[0072] The step of mixing may be by sonicating, vortex mixing, using stirrer magnets or using baffles, preferably the step of mixing is by sonicating. The assembled materials need to be sonicated before and after being mixed in the liquid. This is to avoid formation of aggregates. As an alternative, the crystal particles and the flakes may be separately sonicated before the step of mixing, preferably said further step of separately sonicating the crystal particles and the flakes is performed in darkness.

[0073] The mixed solution may be in a mole ratio in a range of 100:0.1 to 100:20. This ratio is empirically expected to be efficient for various applications. The liquid for mixing the crystal particles and the flakes may be selected from aqueous solutions, such as water; and non-aqueous solutions, such as liquid ammonia. Non-aqueous solutions generally make it more difficult to measure pH but should not affect pH drastically.

[0074] The mixing may be performed at an ambient temperature in a range of 20-30 °C, preferably around 25 °C. Further, the mixing and / or the adjusting a pH value may be performed during a time period in a range of 1-60 minutes. Practically, a time period of 1-30 minutes is good enough. However, it could be longer as well, such as 60 minutes or more.

[0075] Darkness

[0076] The term darkness herein is to be interpreted as the light with wavelengths shorter than the wavelength, which corresponds to the band gap of the materials, should be cut, or minimized to be weaker than the room light intensity.

[0077] A more general definition of the term darkness may be that it has the meaning that the method step is taking place in the complete or partial absence of light. There may also be degrees of darkness, from the complete darkness of e.g., the woods on a moonless night to the darkness of e.g., a room lit by candlelight. Darkness may also be defined as a lack of light or illumination. The step of mixing and / or adjusting a pH value may be performed in darkness or complete darkness. Herein, the term complete darkness has the meaning of the complete lack of light or illumination.

[0078] MXenes do not have bandgap but still can be damaged by UV and visible light irradiation. For this, the light intensity should be weaker than that of the room light.

[0079] In the prior art, electro deposition (photo deposition) is a conventional method used to combine materials with specific facets of single-crystals. This deposition occurs under light irradiation. Meanwhile, the present method is conducted under darkness or minimum ambient light irradiation to avoid undesired reactions.

[0080] Thus, ideally, it is better to perform the method in the complete darkness, for example, the mixing and adjusting of pH may be in a container completely covered to shut out any light.

[0081] Assembling two materials using electrostatic forces has been conventionally employed but without the facet selectivity. Facet-selective assembling has been demonstrated by photo-deposition or electro-deposition methods, where, however, deposited co-catalysts are spherical or randomly shaped. Furthermore, presynthesized co-catalysts cannot be assembled, and precursors of the targeted cocatalysts have to be prepared for the photo-deposition or electro-deposition methods. According to the present inventive concept of namely facet-selective electrostatic assembling springboards from the inventor's hypothesis that various crystal facets have various surface zeta potentials. With this method, one can assemble multi-faceted single-crystal semiconducting materials with 2D materials as co-catalysts, where 2D materials are selectively deposited onto specific facets of the single-crystals.

[0082] Figure 1 illustrates a flow diagram of steps of an embodiment of a method 100 for manufacturing a composite from crystal particles and flakes. In step 101 crystal particles are provided comprising semiconductor materials, the crystal particles having a point of zero charge (PZCp) being a pH at which a net charge of total particle surface is equal to zero, each crystal particle having at least a first and a second facet and in step 102 flakes are provided comprising semiconductor materials or metallic materials, the flakes having a point of zero charge (PZC / ) being a pH at which a net charge of total flake surface is equal to zero, and also the flakes are having average thickness below 30 nm. Hence, an absolute difference (APZC) between the point of zero charge of the crystal particles and the point of zero charge of the flakes is at least 1.

[0083] In step 103 the crystal particles and the flakes are mixed in liquid, that may be selected from aqueous solutions, such as water and non-aqueous solutions, such as liquid ammonia, to form a mixed solution. The reaction temperature may be in a range of 20-30 °C, preferably around 25 °C. The method may be performed during a time period in a range of 1-30 minutes.

[0084] In step 104 the pH value of the mixed solution is adjusted to be between the point of zero charge of the crystal particles (PZCp) and the point of zero charge of the flakes (PZC ), thereby forming a composite comprising flakes arranged on at least one of the first or the second facet of the crystal particle.

[0085] The surface potentials vary depending on the pH of the liquid or solution, and the pH where the surface is uncharged is a so-called point of zero charge (PZC). While most metal oxides have PZC in a range between 2 and 10, MXenes have been found to possess extremely low PZC of around 1.

[0086] Further, the practical working region may be between pH 2 and 7 but the theoretical possible working region may be between 0.82 and 7.8. In other words, if generalized, the pH may be between the PZC of MXenes and the PZC of multi-faceted semiconducting crystals. In such region MXenes are negatively charged, while the surface charges of two facets of the multi-faceted crystals are large enough to have selectivity.

[0087] In figure 2 two different cases of forming the composite of the present inventive concept are presented to demonstrate that the adjusting of pH is highly dependent on the separate materials being used.

[0088] In easel) PZC / is smallerthan PZCp (see figure 2a). Since the 2D materials orflakes are negatively charged and facet A is more positively charged than facet B, the flake will selectively assemble on facet A of the crystal particle.

[0089] In case 2) PZCp is smallerthan PZC / (see figure 2a). Since facet B is more negatively charged than facet A, the positively charged flake will selectively assemble on facet B of the crystal particle.

[0090] Thus, in Casel): PZCZD (i.e., PZC / ) < PZCsemi (i.e., PZCp) and in Case2): PZCsemi (i.e., PZCp) < PZC2D (i.e., PZC ). The difference of | PZCZD - PZCsemi | should be larger than 1. If the difference is smaller than 1, it would be difficult to differentiate surface charges of 2D materials and semiconducting crystals. The theoretical optimal region for both cases is pH between PZC and PZCp. PZC / and PZCp values may be experimentally obtained by Dynamic light scattering (DLS) using aqueous liquid that contains the materials.

[0091] On the other hand, practical optimal region would be narrower than the theoretical region because of instability of the materials. For instance, some semiconducting crystals corrode at very low pH; such pH region is not suitable for assembling.

[0092] Figure 3 illustrates the method 100 of assembling flakes onto crystal particles by providing semiconducting crystal particles and flakes comprising semiconductor materials or metallic materials and mixing the crystal particles and the flakes in liquid to form a mixed solution. The crystal particles and the flakes in the mixed solution may be in a mole ratio in a range of 100:0.1 to 100:20 to have adequate material in the solution for forming the composite. The mixing may be by sonicating, vortex mixing, using stirrer magnets or using baffles, preferably the mixing is by sonicating the solution. Further, the step of mixing may be performed at an ambient temperature, such as a temperature in a range of 20-30 °C, preferably around 25 °C. The pH of the solution is adjusted to manipulate the surface potentials of the crystalline particles and the flakes, which renders electrostatic assembly during sonication and formation of composites. The step of adjusting the pH value of the mixed solution may be by adding an acid, such as acetic acid, hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid or carbonic acid. In another embodiment the step of adjusting the pH value of the mixed solution may be by adding a base, such as sodium carbonate, potassium carbonate, sodium hydroxide or potassium hydroxide, to the solution. The pH value of the mixed solution may be adjusted to be within a pH range of (PZCp+PZC / ) / 2±k*APZC, wherein k is a constant below 0.4. In other words, the pH value of the mixed solution may be adjusted to be within a broad pH range of 0.5 to 14, preferably within a pH range of 1 to 12, preferably within a pH range of 2 to 11. The pH is really dependent on the PZC of the crystal particles and the flakes to be combined or assembled.

[0093] Composites may form according to e.g., case 1) wherein the PZC / is lower than the PZCp, here facet A of the crystal particle is more positive than facet B and thus the negatively charged flake selectively assembles on facet A forming a composite that is designed for providing efficient charge separation, i.e., to make the lifetime longer for the charge separated species before they recombine. In another embodiment the composite may form according to case 2 (see figure 2), wherein the PZCp is lower than the PZC / , here facet B of the crystal particle is more negatively charged than facet A. Since the flake or 2D material is positively charged it selectively assembles on facet B when forming the composite. To avoid undesired side-reactions the steps of mixing and / or the adjusting the pH value may be performed in darkness. The method 100 may also comprise a further step of separately sonicating the crystal particles and the flakes before the step of mixing, preferably said further step of separately sonicating the crystal particles and the flakes is also performed in darkness. This is to avoid aggregation of the crystal particles and the flakes.

[0094] Further, according to the second aspect there is provided a composite that comprises crystal particles comprising semiconductor materials, the crystal particles are polyhedron shaped having at least a first and a second facet; and flakes comprising semiconductor materials or metallic materials, the flakes are having an average thickness below 30 nm, and being selected from the group consisting of MXenes, graphene, graphene oxide, reduced graphene oxide, borophene, boron nitride, carbon nitride, borocarbonitrides, transition metal oxides (TMOs), transition metal dichalcogenides (TMDs), metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and layered double hydroxides (LDHs), wherein the flakes are arranged on at least one of the first or the second facet of crystal particle.

[0095] The crystal particles may be polyhedron shaped having 6 to 14 facets. The facets of a crystal particle comprise different Miller indices. In one embodiment the first facet has a first Miller index and the second facet has a second Miller index, it follows that the first and the second Miller indices are being different from each other. Due to the different electrostatic forces imposed on the crystal particles, the flakes are arranged at only one of the first or the second facet (see e.g., figure 3). Further, the crystal particles may have an average total particle surface area in a range of 1-500 nm.

[0096] The crystal particles may for example be transition metal oxides, such as TiCh, CU2O, or BiVCh. In another embodiment the crystal particles may be transition metal sulfides, such as M0S2. In yet another embodiment the crystal particles may be a perovskite, such as Y 2^20562.

[0097] For the purpose of proving the present inventive concept MXenes have been studied. MXenes are transition metal carbides, transition metal nitrides, transition metal borides or transition metal carbonitrides. MXenes may also be described as compounds of formula: Mn+iXnTm, wherein M is selected from Ti, Nb, V, Mo, Ta, Zr, Cr, Sc and Y; X is selected from C,N, or B; T is selected from F, OH, O and Cl; and n is an integer between 1-4; and m is an integer between 0-2. MXenes may be selected from the group consisting of: TisC2, TisCN, ZrsC2, HfsC2, Ti2C, V2C, Nb2C, M02C, M02N, Ti2N, Ti4Ns, Nb4Cs, Ta4Cs, V4C3, (MO,V)4C3WI.33 , Nbi.ssC, M01.33C, M01.33Y0.67C, M04VC4, Mo2TiC2, Cr2TiC2, M02SCC2, and Mo2Ti2C3. In a preferred embodiment the flakes may be M01.33C. However, the flakes are not limited to MXenes, any semiconducting 2D material having an average thickness in a range of 0.1-25 nm is according to the present inventive concept.

[0098] A composite according to the present inventive concept may be produced by the method of the first aspect of the present invention.

[0099] Regarding applications of the composite, there is provided a use of a composite according to the second or the third aspect in catalysis systems, such as water splitting, CO2 reduction, water purification, air purification, self-cleaning, energy storage, batteries, supercapacitors, sensors, medical treatments or photovoltaics. Catalysis system may mean photocatalysis, photoelectrocatalysis, such as solar cell, and electro-chemistry.

[0100] It will be appreciated that the present inventive concept is not limited to the variants shown e.g., in the examples of the present application. Several modifications and variations are thus conceivable within the scope of the invention which thus is defined by the appended claims.

[0101] Examples

[0102] Materials

[0103] Decahedral single-crystal BiVCh and tetradecahedron single-crystal CU2O nanoparticles were synthesized by a hydrothermal method employing surfactants.

[0104] Decahedral single-crystal TiCh nanoparticles were synthesized by a gas-phase process entailing titanium chloride oxidation.

[0105] Flakes of M01.33C MXene were produced by etching a 3D atomic laminate of (Moz / sSci / shAIC and subsequent delamination according to a literature procedure.

[0106] Analysis

[0107] The surface Zeta potentials of the aqueous solutions of MXene (50 pg / mL) and BiVC (250 pg / mL) as a function of pH were measured by dynamic light scattering analysis using a Zetasizer Nano-ZS90 (Malvern Instruments) in capillary cells of DTS1070 (Malvern instruments). The zeta potential is calculated by electrophoretic mobility measured by the electrophoretic light scattering technique of the Zetasizer. Three measurements were conducted for the zeta potential at each pH value, and the mean value was plotted. The pH value of the prepared solutions was adjusted by HNO3 and KOH using a pH meter (SevenCompact pH / lon meter S220 Mettler Toledo). A three-point calibration of an electrode was performed before measurements using technical buffers at the pH values of 2, 7, and 11. The solution was used to determine Zeta potential by dynamic light scattering analysis.

[0108] Scanning electron microscopy (SEM) was used to image the surface topography of the prepared composites, by using a LEO 1550 scanning electron microscope.

[0109] High-angle annular dark field (HAADF) STEM imaging was carried out by Titan Cubed G2 60-300 (TEM / STEM, FEI Co., now Thermo Fisher Scientific) operated at 300 kV. This microscope has an aberration corrector for STEM (DCOR, CEOS), four- quadrant windowless super-X SDD (silicon drift detector) system. The probe current was ~60 pA for STEM observation. The convergence semi-angle of the electron probe was 18 mrad. The typical probe diameter was less than 0.1 nm. Forward scattered electrons of an angular range from 38 to 184 mrad were detected by a HAADF detector for STEM imaging.

[0110] The hydrogen evolution experiments were performed in a slurry-type, water- cooled reactor illuminated from a side with a monochromatic UV LED light source centered at 365±6 nm (SOLIS, Thorlabs, incident light intensity of 576 mW). The experiments were carried out in a batch-type mode with the reaction solution kept at 15 °C under constant stirring at 650 rpm throughout the experiment. Firstly, a stock solution of the catalyst was prepared by mixing 90 mL of aqueous HNO3solution at pH 3 with 10 mL of HPLC -grade methanol followed by a re-adjustment of the pH to 3 by using diluted HNO3. 80 mL of the obtained solution was mixed with 80 mg of faceted TiO2and ultrasonicated for 10 minutes to obtain a homogenous suspension. To this, 220 pL of the fresh MOI.33C MXene solution (approx, concentration of 9 mg mL-1) corresponding to approx. 2.5 wt.% MXene in TiO2was added to obtain a uniform deposition of the M01.33C flakes onto the photoactive TiCh via electrostatic attachment. Similar deposition experiments have been performed at lower (1.5) and higher (4.5) pH values to verify pH- and facet-dependent assembly mechanisms. In a single HER run, 2 mL of the TiCh / M01.33C solution mixture was added to the reactor and purged with Argon (flow rate of 10 mL min-1) to remove dissolved oxygen. During light irradiation, the reactor was kept airtight and its headspace was probed every 30 minutes using a gas-tight syringe and analyzed using gas chromatography (Shimadzu GC-2030 equipped with a barrier discharge ionization detector). A 5 / 6-point calibration profile was used to accurately quantify the amount of hydrogen evolved and translate it to mole values that are discussed in the main text. EXAMPLE 1: FACET-SELECTIVE ELECTROSTATIC ASSEMBLING OF M01.33C ON BiVO4

[0111] A composite according to the invention, comprising Moi.ssC / BiVC , was prepared according to the method of the present invention, by providing crystal particles of BiVC and providing flakes of M01.33C in 5 wt.% and mixing the crystal particles in distilled water at various pH values in darkness. The mixture was sonicated for 10 min in darkness.

[0112] SEM was performed with a LEO 1550 for imaging of the prepared samples.

[0113] Zeta potentials of M01.33C and BiVO4 were acquired at various pH values (see Figure 4). The obtained curve of zeta potential of M01.33C and BiVO4 as a function of pH shows the same trend that particles are positively charged at highly acidic pH while they are negatively charged at neutral and basic pH. PZC of M01.33C and BiVO4 were derived from the intersection of the zeta potential curves with the horizontal line at 0 mV, found to be 0.82 and 2.66, respectively. The PZC of M01.33C is well below than previously reported PZC of MXenes, such as around 0.97 for VzCTm. This suggests M01.33C forms a more stable colloidal solution and is negatively charged within the broad pH range. The obtained PZC of BiVC is well consistent with the experimental and theoretical range of 2.5-3.5.

[0114] Thus, facet-selective electrostatic assembling of M01.33C and BiVCh was performed by mixing them in the aqueous solutions with various pH. SEM images clearly show that the surface of BiVCh crystals is covered by sheet-like materials (see Figure 5). At pH 1.5 and 2.0, the BiVC (110) surface is preferentially coupled with the very thin flakes with the size of tens and hundreds nm peculiar to delaminated MXenes (see Figure 5b and Figure 5c), as compared to the reference of BiVC without Mxenes (see Figure 5a). At pH 2.5 and 3.0, although the attachment of the flakes was confirmed, they were randomly deposited on both BiVC (110) and (010) surfaces (Figure 5d and Figure 5e). At pH 4.5 and 7.0, both BiVCh (110) and (010) surfaces are covered by spherical nanoparticles, of Mo oxides, which would originate from the oxidation of M01.33C MXenes (see Figure 5f and Figure 6). EXAMPLE 2: FACET-SELECTIVE ELECTROSTATIC ASSEMBLING OF M01.33C ON TiO2

[0115] A composite according to the invention, comprising Moi.ssC / TiCh, was prepared according to the method of the present invention, by providing crystal particles of TiCh exposed with (101) and (001) facets and providing flakes of M01.33C in 5 wt.% and mixing the crystal particles in distilled water at various pH values. The mixture was sonicated for 10 min. The preparation of the solution was carried out under darkness.

[0116] Zeta potentials of M01.33C and TiCh were acquired at various pH values (see Figure 7). PZC of M01.33C and TiCh were found to be 0.82 and 6.65, respectively. SEM was performed with a LEO 1550 for imaging of the prepared samples. At pH 3.0, M01.33C MXene flakes were selectively deposited onto the (101) facets of TiO? (see Figure 8). Here, the (101) facet would be more highly positively charged than the (001) facet while the MXene flakes are negatively charged. Therefore, the MXene flakes are electro-statically attracted selectively to the (101) facet.

[0117] Photocatalytic hydrogen evolution reaction tests were carried out for pristine TiO? and Moi.33C / TiO2 assembly mixed at various pH values (see Figure 9). The assembly mixed at pH 3 showed 147 fold increase in the photocatalytic activities compared to the pristine TiCh.

[0118] In such an assembly system, the photo-generated electrons and holes are first separated to the (101) and (001) facets of TiCh, respectively and subsequently, electrons are further trapped by MXene due to the Schottky barrier at the interface (see Figure 10). Furthermore, MXene can make the redox reactions more efficient by consuming the accumulated electrons effectively by lowering the overpotential and activation energy of redox reactions. Here, by adding oxidation co-catalysts on the (101) facets, holes can be also trapped by the co-catalysts, which can consume holes effectively.

[0119] Figure 11 illustrates STEM images of Moi.33C / TiO2 composite assembled at pH 3, where M01.33C MXene may be found selectively on the (101) facet of TiCh.

[0120] Figure 12 illustrates STEM-EDX images of Moi.33C / TiO2 composite assembled at pH 3, where M01.33C MXene may be found selectively on the (101) facet of TiCh.

[0121] TiO2 is considered as the model material of metal oxides and semiconducting photocatalysts. Therefore, the phenomena demonstrated on TiCh are also considered universal for various metal oxides and photocatalysts. Therefore, the facet -selective electrostatic assembly method of the present invention is expected to be universally applicable for various combinations of semiconducting single-crystal materials and 2D co-catalyst materials.

[0122] EXAMPLE 3: FACET-SELECTIVE ELECTROSTATIC ASSEMBLING OF M01.33C ON Cu2O

[0123] A composite according to the invention, comprising MO1.33C / CU2O, was prepared according to the method of the present invention, by providing crystal particles of CU2O exposed with (111) and (100) facets and providing flakes of M01.33C in 5 wt.% and mixing the crystal particles in distilled water at various pH values. The mixture was sonicated for 10 min. The preparation of the solution was carried out under darkness.

[0124] SEM was performed with a LEO 1550 for imaging of the prepared samples. At pH 11, negatively charged MXene flakes were selectively deposited onto the (100) facet of CU2O (see Figure 13). At pH 11, MXenes are highly negatively charged whereas the square-shaped (100) facet of CU2O is more positively charged than the (111) facet, making MXenes electrostatically attracted selectively toward the (100) facet.

Claims

CLAIMS1. Method (100) for manufacturing a composite, the method comprising the steps of:- providing (101) crystal particles comprising semiconductor materials, the crystal particles having a point of zero charge, PZCp, being a pH at which a net charge of total particle surface is equal to zero, each crystal particle having at least a first and a second facet;- providing (102) flakes comprising semiconductor materials or metallic materials, the flakes having a point of zero charge, PZC / , being a pH at which a net charge of total flake surface is equal to zero, the flakes having average thickness below 30 nm, wherein an absolute difference, APZC, between the point of zero charge of the crystal particles and the point of zero charge of the flakes is at least 1;- mixing (103) the crystal particles and the flakes in liquid to form a mixed solution; and- adjusting (104) a pH value of the mixed solution to be between the point of zero charge of the crystal particles and the point of zero charge of the flakes, thereby forming a composite comprising flakes arranged on at least one of the first or the second facet of the crystal particle.

2. Method according to claim 1, wherein the pH value of the mixed solution isPZC +PZCf adjusted to be within a pH range of - - - - ± k * APZC, wherein k is a constant below 0.4.

3. Method according to claim 1 or 2, wherein the step of mixing is by sonicating, vortex mixing, using stirrer magnets or using baffles, preferably the step of mixing is by sonicating.

4. Method according to any of the proceeding claims, wherein the steps of mixing and / or the adjusting a pH value is performed in darkness.

5. Method according to any of the proceeding claims, wherein the method comprises a further step of separately sonicating the crystal particles and the flakes before the step of mixing, preferably said further step of separately sonicating the crystal particles and the flakes is performed in darkness.

6. Method according to any of the proceeding claims, wherein the crystal particles are selected from the group consisting of transition metal oxides, such as TiCh, CU2O, or BiVCh; transition metal sulfides, such as M0S2; and perovskite, such as Y2Ti20sS2.

7. Method according to any of the proceeding claims, wherein the crystal particles are polyhedron shaped having 6 to 14 facets.

8. Method according to any of the proceeding claims, wherein the first facet has a first Miller index and the second facet has a second Miller index, the first and the second Miller indices being different.

9. Method according to any of the proceeding claims, wherein the flakes are arranged at only one of the first or the second facet.

10. Method according to any of the proceeding claims, wherein the crystal particles are having an average total particle surface area in a range of 1-500 nm.

11. Method according to any of the proceeding claims, wherein the flakes are selected from the group consisting of MXenes, graphene, graphene oxide, reduced graphene oxide, borophene, boron nitride, carbon nitride, borocarbonitrides, transition metal oxides (TMOs), transition metal dichalcogenides (TMDs), and metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and layered double hydroxides (LDHs).

12. Method according to claim 11, wherein MXenes are transition metal carbides, transition metal nitrides, transition metal borides or transition metal carbonitrides.

13. Method according to claim 11 or 12, wherein MXenes are compounds of formula: Mn+iXnTm, wherein M is selected from Ti, Nb, V, Mo, Ta, Zr, Cr, Sc and Y; X is selected from C,N, or B; T is selected from F, OH, O and Cl; and n is an integer between 1-4; and m is an integer between 0-2.

14. Method according to any of claims 11 to 13, wherein MXenes are selected from the group consisting of: TisCz, TisCN, ZrsCz, HfsCz, TizC, V2C, NbzC, M02C, M02N, Ti2N, Ti4Ns, Nb4Cs, Ta4Cs, V4C3, (MO,V)4C3WI.33 , Nbi.ssC, M01.33C, M01.33Y0.67C, M04VC4, Mo2TiC2, Cr2TiC2, M02SCC2, and Mo2Ti2C3, preferably M01.33C.

15. Method according to any of the proceeding claims, wherein the flakes are having an average thickness in a range of 0.1-25 nm.

16. Method according to any of the proceeding claims, wherein the crystal particles and the flakes in the mixed solution are in a mole ratio in a range of 100:0.1 to 100:20.

17. Method according to any of the proceeding claims, wherein the step of mixing is performed at a temperature in a range of 20-30 °C, preferably around 25 °C.

18. Method according to any of the proceeding claims, wherein the steps of mixing and / or the adjusting a pH value is performed during a time period in a range of 1-30 minutes.

19. Method according to any of the proceeding claims, wherein liquid for mixing the crystal particles and the flakes is selected from aqueous solutions, such as water; and non-aqueous solutions, such as liquid ammonia.

20. Method according to any of the proceeding claims, wherein the step of adjusting a pH value of the mixed solution is by adding an acid, such as acetic acid, hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid or carbonic acid; or a base, such as sodium carbonate, potassium carbonate, sodium hydroxide or potassium hydroxide, to the solution.

21. Method according to any of the proceeding claims, wherein the pH value of the mixed solution is adjusted to be within a pH range of 0.5 to 14, preferably within a pH range of 1 to 12, preferably within a pH range of 2 to 11.

22. A composite comprising:- crystal particles comprising semiconductor materials, the crystal particles are polyhedron shaped having at least a first and a second facet; and- flakes comprising semiconductor materials or metallic materials, the flakes are having an average thickness below 30 nm, and being selected from the group consisting of MXenes, graphene, graphene oxide, reduced graphene oxide, borophene, boron nitride, carbon nitride, borocarbonitrides, transition metal oxides (TMOs), transition metal dichalcogenides (TMDs), and metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and layered double hydroxides (LDHs), wherein the flakes are arranged on at least one of the first or the second facet of crystal particle.

23. Composite according to claim 22, wherein the first facet has a first Miller index and the second facet has a second Miller index, the first and the second Miller indices being different.

24. Composite according to claim 22 or 23, wherein the flakes are arranged at only one of the first or the second facet.

25. Composite produced by the method according to any of claims 1 to 21.

26. Use of a composite according to any of claims 22 to 25 in catalysis systems, such as water splitting, CO2 reduction, water purification, air purification, selfcleaning, energy storage, batteries, supercapacitors, sensors, medical treatments or photovoltaics.

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