WO2017121792A1

WO2017121792A1 - Process for impregnating porous materials and process for preparing nanostructured product

Info

Publication number: WO2017121792A1
Application number: PCT/EP2017/050554
Authority: WO
Inventors: Koen VANDAELE; Klaartje DE BUYSSER; Pascal van der Voort
Original assignee: Universiteit Gent
Priority date: 2016-01-14
Filing date: 2017-01-12
Publication date: 2017-07-20

Abstract

The invention relates to a process for preparing a porous material comprising metal, metalloid, or non-metal precursor, said process comprising the steps of: - dispersing the porous material in a hydrophobic solvent; - adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, thereby forming a mixture; and - selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution; thereby preparing a porous material comprising metal, metalloid, or non-metal precursor. The process may additionally comprise a step of converting the precursor, and a step of removing the porous material. Also described is a nanostructured metal, metalloid, or non-metal decomposition product; or metal, metalloid, or non-metal; or metal, metalloid, or non-metal oxide. ln a preferred embodiment, nanostructured bismuth is prepared using mesoporous silica or mesoporous titania as the porous material, viz. template.

Description

PROCESS FOR I MPREGNATING POROUS MATERIALS AND PROCESS FOR PREPARING

NANOSTRUCTURED PRODUCT

Field of the invention

This invention concerns a process for impregnating porous materials with metal, metalloid, or non-metal precursors. The deposited precursors may be converted to various products such as metals, metalloids, or non-metals, or to metal, metalloid, or non-metal oxides.

Background to the invention

Porous materials have gained wide interest owing to their large surface area, tunable pore size, adjustable structure, and surface properties. They have found various industrial applications, ranging from catalysis, adsorption, sensing, molecular separation, ion exchange, optics, photovoltaics, etc.

Porous materials can be ordered or disordered and are usually classified by their pore size. According to lUPAC definitions, microporous materials have pore diameters of less than 2 nm, mesoporous materials have pore diameters between 2 nm and 50 nm, while macroporous materials have pore diameters of greater than 50 nm. Microporous materials include the widely used zeolites. Materials with large pores such as mesoporous materials have found interest as carriers of relatively larger molecular entities and of various materials.

Porous materials of great importance are silica and alumina based materials but also other porous oxides such as those of niobium, tantalum, titanium, zirconium, cerium, and tin have been used.

Silica is a material that can be synthesized easily as high quality ordered mesoporous material. The synthesis proceeds via a soft templating procedure in which a silica precursor condensates around an ordered liquid crystal lattice and forms amorphous mesoporous silica upon calcination. Usage of so-called templating agents, such as surfactants, leads to structured materials with narrow pore size distribution. In general, it is very difficult to synthesize other mesoporous materials in that way, due to the fact that their crystallization forces destroy the "soft" liquid crystal lattice leading to structural collapse.

One way to synthesize mesoporous metal oxides is by means of a hard template replication method. Herein a metal salt dissolved in ethanol or water is typically impregnated in a mesoporous silica template material and subsequently calcined. The replicated mesoporous metal oxide is obtained after template removal through chemical etching. Although this hard templating replication methodology in principle works, it leads to poor quality replicated structures if the template material is insufficiently filled or if external material is deposited on the template's surface. Mesoporous metal oxides therefore are difficult to manufacture due to the lack of robust and scalable preparation procedures.

Porous materials not only find use as templates in the manufacture of mesoporous metal oxide replica structures, but also in the manufacture of a variety of nanostructured materials such as nanocomposites, nanowires, nanowire networks, and nanoparticle loaded materials. In all these techniques, precursor material in liquid form, preferably as a solution, is impregnated in the pores of the template and subsequently converted into solid form. As in the manufacture of mesoporous metal oxide replica structures, insufficient filling of the pores and/or deposition of material on the external surface of the template may lead to poor quality products. These problems are particularly prominent in the production of high quality nanowires where insufficient filling in the pore channels leads to fragmentation of the nanowires rather than a continuous self-supporting network. The latter is of particular importance where electrically conducting nanowires or composite structures are desired.

Several impregnation methods are available to infiltrate metal containing precursors into a porous template and they can be classified in three general routes: wet chemistry impregnation, chemical/physical vapor impregnation, and electrochemical impregnation.

One procedure in wet chemistry impregnation is the conventional solvent impregnation method where a large amount of precursor solution is added to a template material. Only a small fraction of the metal precursor infiltrates into the pores through diffusion, while some will deposit on the outer surface of the template generating large aggregates after drying and/or thermal treatment. The major part of the precursor solution is filtered off. This typically leads to low loadings and many external aggregates.

Since many precursor solutions are ethanol based, surface modification has been used to make the pore walls more hydrophobic or hydrophilic, which enhances the interaction with the precursor. This may lead to a better impregnation, but does not significantly enhance the loading capacity. Moreover, only materials with a sufficiently high solubility in this solvent can be impregnated.

Enhancing the loading of the precursor has been attempted via high pressure impregnation or impregnation with supercritical C0₂ as solvent. Although higher loadings could be obtained, this technique is only applicable for specific materials and the deposition of external material could not be avoided. In addition, the smaller the pore size, the higher the pressure required to force the precursor into the pores.

In the incipient wetness or dry impregnation method an amount of precursor solution equal to the pore volume of the template material is added to the template so that no material is deposited on the external surface of the template. Typically, a metal salt is dissolved in water or ethanol as precursor. The dry powder is stirred to enable all of the precursor solution to draw into the template's pores. The precursor is believed to infiltrate into the pores through capillary action rather than through diffusion. After impregnation, the material is dried and calcined to form the metal oxide. A disadvantage of this method is that the maximum loading is limited by the solubility of the precursor in the solvent. Furthermore, this methodology leads to inhomogeneous impregnation due to the difficulty to homogenize the mixture of precursor solution and dry powder. As is also the case for the aforementioned techniques, multiple impregnation and calcination steps are required to enhance the loading of the metal oxide. A more advanced version of the incipient wetness technique, referred to as the "double solvents" method, was initially used to impregnate a water-based precursor solution into mesoporous silica and recently for the impregnation of nanoparticles inside metal-organic frameworks ("MOFs"). A non-polar (hydrophobic) solvent, such as hexane, is added in a large quantity to the template material followed by the addition of a volume of water-based precursor solution less or equal to the total pore volume of the template. In this way no external material is deposited and homogeneous filling of the template is obtained, but this methodology still faces the problem that many steps are required to obtain sufficiently high loading. Since a diluted solution of a metal salt in water is impregnated, the loading is typically low.

A precursor solution may also be impregnated under reduced pressure which is said to facilitate the infiltration of the precursor in the template's pores.

In the evaporation-induced impregnation the metal salt dissolved in ethanol is stirred until the solvent is completely evaporated while the metal salt crystallizes inside the pores. This method is only suitable when the salt is highly soluble in ethanol. It usually leads to a higher loading than other methods, but it does not exclude the deposition of external material. The method may be combined with the incipient wetness technique.

Chemical/physical vapor impregnation is more suitable for the impregnation of metals and is a commonly used technique to fabricate nanowires in anodized alumina. However, it is less suitable to impregnate mesoporous powders with extremely small pore diameters as it is difficult to impregnate the entire channel and pore blocking easily occurs.

Electrochemical impregnation of metals has been successfully performed for mesoporous thin films, but due to diffusion limitations this method cannot be used to impregnate bulk sized samples, nor can powders be impregnated as it is impossible to create electrical contacts. El Hassan et al., Ann. Chem. Sci. Mat., 2005, 30(3), pp.315-326 discloses SBA-15 silicas as hard templates for the nanocasting of oxide nanoparticles. A volume of aqueous solution set equal to the pore volume of the silica template. The amount of manganese precursor is too small to allow the crystallization of MnO_x particles everywhere in the porosity of the silica template. After impregnation, the material was calcined.

Huang et al., J. Colloid Interface Sci. 359 (201 1 ) 40-46 discloses synthesis of confined Ag nanowires within mesoporous silica via double solvent technique and their catalytic properties. After impregnation, the silica template is separated through filtration and subsequently dried in air, followed by a thermal treatment.

Liu et al., Materials Lett. 60 (2006) 154-158 discloses template synthesis of one-dimensional nanostructured spinel zinc ferrite. After impregnation, the total solvent was volatilized by exposure in air with stirring.

Jiao et al., Chem. Commun., 2005, 5618-5620 discloses growth of porous single-crystal Cr₂0₃ in a 3-D mesopore system. After impregnation, the silica template is separated through filtration and subsequently dried. Problems associated with the current impregnation methods of porous templates include incomplete and inhomogeneous filling of the pores, as well as leaching phenomena leading to deposits on the surface porous template. Because of the low loading in many cases, multiple loading steps are required to obtain an acceptable degree of loading. The resulting nanostructured materials are of poor quality, with a fragmented and irregular structure. Particularly when the nanostructured material is to be used as an electrically conducting material, the low loading and fractured material results in inadequate properties.

Both the current impregnation methods and the resulting products leave room for improvement. There is a need for preparation methods for nanostructured materials in which metal, metalloid, or non-metal is deposited in a porous material in high loading with no or negligible leaching, with no or limited amounts of externally deposited material. There is a need to processes to prepare impregnated materials without the need to perform multiple steps, in particular without one or more intermediate calcination steps.

Summary of the invention

This invention concerns new preparation procedures that enable the impregnation of various materials in porous materials, which are meant to meet one or more of the needs mentioned herein and further needs recognized in the prior art.

In particular, the invention concerns new preparation routes to prepare metals, metalloids, or non-metals, or their precursors deposited in porous materials. The invention further provides the manufacture of nanostructured materials such as structured mesoporous materials, nanocomposites, and metallic nanowire networks embedded in a porous template. The invention also provides efficient ways to load high amounts of nanoparticles in porous materials. In comparison with known impregnation methods, a very high loading efficiency with a negligible amount of external deposited material is obtained without the necessity to perform intermediate calcination steps. A diluted precursor solution can be impregnated while solidified precursor salt is deposited in the pores as the precursor solvent is removed. In addition, decomposition of the precursor salt to a denser intermediate during the impregnation process can yield to even further enhancement of the pore loading. Particularly when the nanostructured material is to be used as an electrically conducting material, the high loading results in compressed powders having an electrical percolation path. In addition, acid based precursor solutions, precursor solutions in methanol/formic acid, may be successfully impregnated in mesoporous silica. This enables the synthesis of mesoporous materials which were difficult to make before. Hydrolysis of the metal precursors may be avoided in this way. According to a first aspect, the present invention relates to a process for preparing a porous material comprising metal, metalloid, or non-metal precursor, said process comprising the steps of:

- dispersing the porous material in a hydrophobic solvent; - adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, forming a mixture; and

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution; preferably continuously removing the hydrophilic solvent from the mixture;

thereby preparing a porous material comprising metal, metalloid, or non-metal precursor. Preferably, the step of adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, forming a mixture, is performed in a controlled manner.

In some preferred embodiments, the hydrophilic solvent is removed from the mixture at a temperature that is higher than the boiling point of the hydrophilic solvent at the applied pressure.

In some preferred embodiments, the boiling point of the hydrophilic solvent is lower than the boiling point of the hydrophobic solvent at the applied pressure.

In some preferred embodiments, the volume of the metal, metalloid, or non-metal precursor solution is lower than the total pore volume of the porous material.

In some preferred embodiments, the porous material is an oxide or sulfide of a metal or of a metalloid, or a mixture thereof, preferably wherein the porous material is selected from the group comprising: silica, alumina, ceria, titania, cobalt(ll,lll) oxide, iron(lll) oxide, nickel oxide, manganese oxide, yttrium stabilized zirconia, or a mixture thereof; more preferably wherein the porous material is selected from silica, alumina, and mixtures thereof; most preferably wherein the porous material is silica.

In some preferred embodiments, the metal, metalloid, or non-metal precursor is a precursor of bismuth, antimony, bismuth-antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide; preferably a precursor of bismuth or bismuth selenide.

In some preferred embodiments, the porous material has an average pore diameter of at least 2.0 nm and at most 50.0 nm, preferably of at least 5.0 nm and at most 10.0 nm, preferably of at least 6.0 nm and at most 8.0 nm. Nitrogen adsorption at 77 K is used as it is recommended by the lUPAC for the determination of the surface area and mesopore size distribution. The pore diameter was determined using the BJH model on the adsorption branch of the N₂ sorption isotherm. Generally, small diameters are desired because they lead to a higher surface area.

In some embodiments, the porous material has an average pore diameter of at least 2.0 nm and at most 50.0 nm, preferably of at least 5.0 nm and at most 40.0 nm, preferably of at least 10.0 nm and at most 30.0 nm. For example, Bi_xSbi__x nanowire composites with dimensions between approximately 10 and 30 nm show enhanced thermoelectric properties compared to bulk material.

In most preferred embodiments, the porous material is a mesoporous material, having pore diameters between 2.0 nm and 50.0 nm, preferably mesoporous silica. Starting from mesoporous silica templates with these specifications, replicated structures with the same dimensions can be obtained.

In some preferred embodiments, the hydrophilic solvent is selected from the group comprising: water, a mineral acid (such as HCI), a carboxylic acid (such as formic acid), methanol, or a mixture thereof; and the hydrophobic solvent is selected from the group comprising: a C₆-C₁₀ arene, a C₁-C₈ alkyl C₆-C₁₀ arene, such as toluene or xylene, a C₆-C₁₂ alkane, such as hexane, heptane, octane, nonane, or a mixture thereof; wherein the alkane, arene, or alkylarene is optionally fluorated or chlorated.

In some preferred embodiments, the metal, metalloid, or non-metal is a metal of which the surface is passivated.

According to a second aspect, the present invention relates to a process for preparing a porous material comprising a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; said process comprising the steps of:

- preparing a porous material comprising nanostructured metal, metalloid, or non-metal precursor according to the first aspect of the invention, and preferred embodiments thereof; and

- converting the metal, metalloid, or non-metal precursor in the porous material to a nanostructured metal, metalloid, or non-metal decomposition product; or to nanostructured metal, metalloid, or non-metal; or to nanostructured metal, metalloid, or non-metal oxide.

In some preferred embodiments, the metal, metalloid, or non-metal precursor is converted through chemical reduction, decomposition, oxidation, nitridation, chemical vapor reaction, or a combination thereof.

According to a third aspect, the present invention relates to a process for preparing a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; said process comprising the steps of:

- preparing a porous material comprising a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; according to the second aspect of the invention, and preferred embodiments thereof; and

- removing the porous material.

According to a fourth aspect, the present invention relates to a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non- metal; or nanostructured metal, metalloid, or non-metal oxide; obtainable by the process according to the third aspect of the invention, and preferred embodiments thereof.

According to a fifth aspect, the present invention relates to a metal, metalloid, or non-metal precursor deposited in a porous material wherein the filling degree of the metal, metalloid, or non-metal precursor is at least 60% of the theoretical maximum, preferably at least 65%, preferably at least 70%, preferably at least 75%, preferably at least 80%, preferably at least 85%.

According to a sixth aspect, the present invention relates to a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; having a diameter of at most 50 nm, preferably of at most 40 nm, preferably of at most 30 nm, preferably of at most 20 nm, preferably of at most 15 nm, for example at most 10 nm, for example at most 8 nm; and an aspect ratio of at least 50, preferably of at least 75, preferably of at least 100; wherein the metal is selected from the group comprising: bismuth, antimony, bismuth-antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide, preferably wherein the metal is bismuth or bismuth selenide.

Brief description of the figures

Fig. 1 represents a schematic representation of an impregnation setup according to a preferred embodiment of the invention. The following numbering is adhered to: 10 - Dean Stark separator; 12 - PFA flask; 14 - heating plate; 16 - additional funnel or syringe pump; 20 - Graham condenser; 22 - cooling IN; 24 - cooling OUT; 1 10- Bismuth precursor solution; 120 - silica template and apolar (hydrophobic) solvent; 130 - apolar (hydrophobic) phase; 140 - aqueous phase or hydrophilic solvent.

Fig. 2 illustrates Vp,max internal (Fig. 2A), Vp,max external (Fig. 2B) and an actual combination of internal and external filling (Fig. 2C).

Fig. 3 illustrates bismuth nanowires according to a preferred embodiment of the invention. Fig. 4 illustrates nanowires according to preferred embodiments of the invention (Fig. 4A, Fig. 4B, Fig. 4C, and Fig. 4D).

FIG. 5A illustrates an isotherm linear plot for Ni templated from KIT-6 mesoporous silica with a BET surface area and pore volume of respectively 180 m²/g and 0,15 cm³/g. Fig. 5B illustrates the pore volume.

Detailed description of invention

Before the present system and method of the invention are described, it is to be understood that this invention is not limited to particular systems and methods or combinations described, since such systems and methods and combinations may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. As used herein, the singular forms "a", "an", and "the" include both singular and plural referents unless the context clearly dictates otherwise. The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of" as used herein comprise the terms "consisting of", "consists" and "consists of".

The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints. The term "about" or "approximately" as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of +/-10% or less, preferably +/-5% or less, more preferably or less, and still more preferably +/-0.1 % or less of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" or "approximately" refers is itself also specifically, and preferably, disclosed. Whereas the terms "one or more" or "at least one", such as one or more or at least one member(s) of a group of members, is clear per se, by means of further exemplification, the term encompasses inter alia a reference to any one of said members, or to any two or more of said members, such as, e.g., any >3, >4, >5, >6 or >7 etc. of said members, and up to all said members.

All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.

In the present description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration only of specific embodiments in which the invention may be practiced. Parenthesized or emboldened reference numerals affixed to respective elements merely exemplify the elements by way of example, with which it is not intended to limit the respective elements. It is to be understood that other embodiments may be utilised and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

According to a first aspect, the present invention relates to a process for preparing a porous material comprising metal, metalloid, or non-metal precursor, said process comprising the steps of:

- dispersing the porous material in a hydrophobic solvent;

- adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, forming a mixture; and

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution; preferably continuously removing the hydrophilic solvent from the mixture; preferably through a Dean-Stark set-up;

thereby preparing a porous material comprising metal, metalloid, or non-metal precursor. Preferably, the step of adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, forming a mixture, is performed in a controlled manner. The term "controlled manner" herein refers to an addition wherein the addition speed of the precursor solution to the hydrophobic solvent is controlled by a controlling means, for example wherein the controlling means is a syringe pump. The optimal addition speed may have been previously obtained for the system.

The precursor may be a multi-component precursor. The process according to the invention gives the ability to control the alloy composition and/or the doping level of the nanostructured target material.

The terms "hydrophilic solvent" and "polar solvent" may be used interchangeably. The terms "hydrophobic solvent", "apolar solvent", and "non-polar solvent" may be used interchangeably. As used herein, the term "dispersed" refers to a mixture in which fine particles of one substance are scattered throughout a solvent. The porous material is dispersed in the solvents (and in the mixture), therefore it is not dissolved. In some embodiments, the precursor solution is prepared by dissolving about the amount of precursor salt that the pores can contain, for example at least the amount of precursor salt that the pores can contain, in a solvent selected from the group comprising: water, HCI, MeOH, formic acid, or a combination thereof. The amount of precursor salt (g) may be calculated as pore volume V_p (cm³) x density salt (g/cm³).

Typically, the precursor solution is polar or hydrophilic (impregnates the silica template), while the hydrophobic solvent is a medium in which the impregnation is performed. For example, the hydrophobic solvent may be a 250 mL round bottom flask filled with approx. 100-150 mL n- octane. Silica template may be dispersed in the hydrophobic solvent. For example, the hydrophilic solvent may be 20 - 50 mL water based precursor, added in a dropwise manner (by use of syringe pump) to the hydrophobic solvent.

In some embodiments, during the impregnation, the ratio of hydrophilic precursor vs hydrophobic solvent is at most 1 :10, preferably at most 1 :20, preferably at most 1 :50, for example at most 1 :100, respectively.

Combinations wherein the chemical composition of the porous template material is identical to and/or indistinguishable from the chemical composition of any of the following:

- metal, metalloid, or non-metal precursor;

- nanostructured metal, metalloid, or non-metal decomposition product;

- nanostructured metal, metalloid, or non-metal; or,

- nanostructured metal, metalloid, or non-metal oxide;

do not fall under the scope of this invention.

The process comprises the step of:

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution.

This means that the hydrophilic solvent is removed during impregnation. The process of the invention differs from the prior art that the hydrophilic solvent is removed during impregnation, instead of after impregnation. Typically, the hydrophilic precursor solution is added at a rate of 4 mL/h to refluxing n-octane. In some embodiments, the precursor solution is added at a rate of at most 100 mL/h, preferably at most 50 mL/h, preferably at most 20 mL/h, preferably at most 10 mL/h, preferably at most 8 mL/h, preferably at most 6 mL/h, preferably about 4 mL/h. The addition of 40 mL precursor solution may take 10 h. The temperature of the heating plate may be set to 160 °C, gentle refluxing of both the hydrophilic and hydrophobic solvents. The precursor solution addition is preferably slow so that the amount of precursor solution present in the round bottom flask is lower than the pore volume of the template. In that case, deposition of material on the external surface of the template may be avoided. A higher addition speed may be used, for example when accompanied with a higher temperature of the heating plate (resulting in a faster evaporation rate), or when a higher boiling hydrophobic solvent is used.

In some preferred embodiments, the hydrophilic solvent is removed from the mixture at a rate of at least 0.1 mL/h, preferably at least 0.2 mL/h, preferably at least 0.5 mL/h, preferably at least 1 .0 mL/h, preferably at least 2.0 mL/h, preferably at least 3.0 mL/h, for example at least 4.0 mL/h, for example at least 5.0 mL/h, for example at least 6.0 mL/h, for example at least 8.0 mL/h, for example at least 10.0 mL/h. In some preferred embodiments, the hydrophilic solvent is removed from the mixture at a rate of at most 100.0 mL/h, preferably at most 50.0 mL/h, preferably at most 20.0 mL/h, preferably at most 15.0 mL/h, preferably at most 10.0 mL/h, for example at most 8.0 mL/h, for example at most 6.0 mL/h, for example at most 5.0 mL/h. In some preferred embodiments, the hydrophilic solvent is removed from the mixture at a rate of at least 0.1 mL/h and at most 100.0 mL/h, preferably at least 0.2 mL/h and at most 50.0 mL/h, preferably at least 0.5 mL/h and at most 20.0 mL/h, preferably at least 1 .0 mL/h and at most 10.0 mL/h, preferably at least 2.0 mL/h and at most 8.0 mL/h, preferably at least 3.0 mL/h and at most 6.0 mL/h, for example at least 4.0 mL/h and at most 5.0 mL/h.

In some preferred embodiments, the step of:

- adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, forming a mixture;

and the step of:

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution; are performed simultaneously.

In some preferred embodiments, the rate of the step of:

is at most equal to, preferably less than, the rate of the step of:

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution. For example, when the hydrophilic precursor solution is added at a rate of 4 mL/h, the hydrophilic precursor solvent is selectively removed from the system at a rate of 4 mL/h or faster. The rate of addition of the precursor solution may be at most 99% the rate of the removal of the hydrophilic solvent, for example at most 98%, for example at most 95%, for example at most 90%, for example at most 80%, for example at most 70%, for example at most 60%, for example at most 50%.

In some preferred embodiments, the step of:

and the step of: - removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution; are performed simultaneously; and

the rate of the step of:

is at most equal to, preferably less than, the rate of the step of:

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution. The rate of addition of the precursor solution may be at most 99% the rate of the removal of the hydrophilic solvent, for example at most 98%, for example at most 95%, for example at most 90%, for example at most 80%, for example at most 70%, for example at most 60%, for example at most 50%.

In some preferred embodiments, the hydrophilic solvent is selectively removed. The term "selective removal" as used herein refers to the removal of the hydrophilic solvent, without removal of the hydrophobic solvent (or only partial removal of the hydrophobic solvent). In some preferred embodiments, the hydrophobic solvent is not removed from the mixture during removal of the hydrophilic solvent. In some embodiments, both the hydrophobic solvent and the hydrophilic solvent are removed from the mixture simultaneously, but the hydrophobic solvent is entirely or partially (for example at least 99% by volume, or at least 98%, or at least 95%, or at least 90%, or at least 80%, or at least 70%, or at least 60%, or at least 50%) added back to the mixture, preferably entirely added back to the mixture. For example, the hydrophobic solvent may return to the reaction flask through a Dean Stark setup.

In some preferred embodiments, the hydrophilic solvent is removed, while preferably maintaining the amount of hydrophobic solvent in the reaction vessel in order to maintain a two-phase or also called "two-solvent" system. A flow of hydrophobic solvent in and out of the system may occur but no total removal of the hydrophobic solvent preferably occurs during the impregnation. In other words, in some embodiments, there is a net removal of hydrophilic solvent, with a rate at least equal to the addition rate of the hydrophilic solvent to the mixture, while there is preferably no net removal of the hydrophobic solvent.

In some preferred embodiments, at all times during the process, the mixture comprises at an amount of dissolved precursor that is at most equal to, preferably less than, the remaining free pore volume.

The inventors have surprisingly found that the process of the invention and preferred embodiments thereof result in efficient ways to load high amounts of nanoparticles in porous materials. In comparison with known impregnation methods, a very high loading efficiency with a negligible amount of external deposited material is obtained without the necessity to perform intermediate calcination steps. Particularly when the nanostructured material is to be used as an electrically conducting material, the high loading results in compressed powders having an electrical percolation path. It is possible to obtain an electrical conducting material when compressing the silica template with metal nanowires confined in the template's pores. Only when long interconnected paths are formed, percolation paths are present.

Since solidified precursor is deposited inside the pores in a continuous manner, dense and continuous nanowires can be formed inside the pores. No or limited amount of loading of material outside the pores is seen. Furthermore, specific alloy compositions and doping levels can be continuously controlled.

Preferably, the present invention also relates to a process for preparing a porous material comprising metal, metalloid, or non-metal precursor, said process comprising the steps of:

- dispersing the porous material in a hydrophobic solvent;

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution; at a temperature that is higher than the boiling point of the hydrophilic solvent at the applied pressure; thereby preparing a porous material comprising metal, metalloid, or non-metal precursor. Preferably, the step of adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, forming a mixture, is performed in a controlled manner. Preferably, the present invention also relates to a process for preparing a porous material comprising metal, metalloid, or non-metal precursor, said process comprising the steps of:

- dispersing the porous material in a hydrophobic solvent;

- removing the hydrophilic solvent from the mixture; preferably selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution; at a temperature that is higher than the boiling point of the hydrophilic solvent at the applied pressure and lower than the boiling point of the hydrophobic solvent at the applied pressure; thereby preparing a porous material comprising metal, metalloid, or non-metal precursor. Preferably, the step of adding a metal, metalloid, or non-metal precursor solution in a hydrophilic solvent to the dispersed porous material and the hydrophobic solvent, forming a mixture, is performed in a controlled manner.

As used herein the terms "porous material" and "porous template" are equivalent and may be used interchangeably. The porous material for use in the invention includes microporous, mesoporous and macroporous material, preferably the porous material is a mesoporous material. In some preferred embodiments, the porous material has an average pore diameter of at least 2 nm and at most 50 nm, preferably of at least 5 nm and at most 10 nm, preferably of at least 6 nm and at most 8 nm. Nitrogen adsorption at 77 K is used as it is recommended by the lUPAC for the determination of the surface area and mesopore size distribution. The pore diameter was determined using the BJH model on the adsorption branch of the N₂ sorption isotherm. The porous material for use in the invention includes porous material which may be ordered or amorphous, preferably the porous material is ordered. The porous materials can be inorganic or organic, preferably inorganic. In some preferred embodiments, the porous material for use in the invention inorganic ordered mesoporous material.

Porous materials are commercially available or can be prepared according to methodology extensively described in the prior art. They can be characterized by XRD, nitrogen adsorption and other techniques known in the art.

The quantum size effect of nanowires, such as bismuth nanowires has interesting properties. It is therefore desirable to have large mesoporous grains to limit imperfections between the grains. Monolithic mesoporous materials could be advantageous. The porous materials are preferably in particulate form. The average diameter of the particles of the porous material may be in the range of from about 500 nm to about 5 μm; or from about 5 μm; to 10 mm. The average mesoporous particle size can be measured by means of SEM measurements.

The porous material may be comprise materials such as carbon, silicon, metal, metalloid, or non-metal oxides, sulfides, hydroxides, carbonates, silicates, phosphates, etc. In some preferred embodiments, the porous material is an oxide or sulfide of a metal or of a metalloid, or a mixture thereof, preferably wherein the porous material is selected from the group comprising: silica (Si0₂), alumina (Al₂0₃), ceria (Ce0₂), titania (Ti0₂), cobalt(ll,lll) oxide (Co₃0₄), iron(lll) oxide (Fe₂0₃), nickel oxide (NiO), manganese oxide (MnO_x wherein x varies between 1 and 2), yttrium stabilized zirconia, or a mixture thereof; more preferably wherein the porous material is selected from silica, alumina, and mixtures thereof; most preferably wherein the porous material is silica. The inorganic oxides may also include mixed inorganic oxides such as Si0₂.AI₂0₃, MgO.Si0₂.AI₂0₃, and the like.

More preferred porous materials are silica and alumina as well as mixtures thereof. Most preferred is mesoporous silica, preferably ordered mesoporous silica. As used herein, "mesoporous silica" means silica having pores in the range of 2 nm to 50 nm and the terms "mesopore" or "mesoporous" and the like refer to porous structures having these pore sizes. No particular spatial organization or method of manufacture is implied by these terms. Preferred mesoporous silica have pore sizes in the range of 2 nm to 30 nm, or in the range of 2 nm to 20 nm, or in the range of 4 nm to 12 nm, or in the range of 4 nm to 10 nm. Preferred mesoporous silica have a pore volume that may be in the range from about 0.5 - 2 mL/g, or about 0.8 - 1 .5 mL/g as determined by N₂ sorption measurements. In some embodiments, the surface area of the porous silica may be in the range of from 400 to 1200 m2/g, in particular from 400 to 1200 m2/g. To obtain these parameters, nitrogen sorption experiments may be performed at 77K with a Micromeritics TriStar 3000 device. Samples are then vacuum dried at 120* for 12h prior to analysis. The surface area is calculated using the BET method while the pore size distribution is determined by analysis of the adsorption branch of the isotherms using the BJH method. The total pore volume is determined by the BJH desorption cumulative volume of pores.

The mesoporous silica is preferably ordered, the term "ordered" referring to ordered arrays of mesopores with regular pore size and morphology. The ordered porous material may have a 1 -D, 2-D, or 3-D channel structure, preferably a 2-D or 3-D channel structure, more preferably a 3-D channel structure. 3-D channel structures enhance the number of percolation paths, while 1 -D channel structures may possess inaccessible pores.

Examples of the 2-D mesostructured silica materials include MCM-41 (Mobil Composition of Matter), SBA-3 (Santa Barbara), SBA-15 (Santa Barbara No.15) and MSU-H (Michigan State University). Their structures consist of hexagonally close packed cylindrical pores, belonging to the p6mm space group. Of particular interest is SBA-15.

Typical 3-D hexagonal mesostructured silica include SBA-2, but also SBA-12 is known which has the same mesostructure with a P63/mmc space group. Of interest are the 3-D cubic mesostructured silica structures, which include, for example, MCM-48 (Mobil Composition of Matter), SBA-n (n = 1 , 6, 7, 1 1 , 12, 16) (Santa Barbara), KIT-n (n = 5, 6) (Korea Advanced Institute of Science and Technology), FDU-n (n = 2, 5) (Fudan University) and AMS-8 (Anionic Mesoporous Silica) materials. Of particular interest is KIT-6, more in particular KIT-6 having a pore volume of about 1 imL/g.

The term "metal, metalloid, or non-metal" is meant to cover individual metals, metalloids, or non-metals, as well as mixtures thereof. Suitable metals comprise non-transition and transition metals, lanthanides and actinides, and any mixtures or intermetal compounds thereof, e.g., Al, Ti, V, Mn, Fe, Co, Cu, Zn, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Ce, Hf, Ta, W, Re, Ir, Pt, Au, Bi. The term "metal" further includes alloys, which can be intermetallic but can also contain metalloids and/or non-metals. Examples of metalloids are B, Si, Ge, As, Se, Sb, Te, and mixtures thereof. Of interest are semiconductors such as Si, Ge, and GaAs. Further included are doped metals and metalloids. Also included are mixtures or compounds of metals and metalloids. Suitable non-metals are solid at room temperature and preferably are inorganic. They include metal oxides as well as metal sulfides. Particularly interesting metals, metalloids, or non-metals are Bi, Sb, bismuth selenide, bismuth telluride, bismuth antimony selenide, titanium dioxide, and titanium disulfide, preferably bismuth or bismuth selenide.

Metal precursors include metal salts, in particular metal salts soluble in water, methanol, formic acid, mineral acids, or in a combination thereof. These include nitrates and halides such as chlorides, e.g. titanium tetrachloride, bismuth nitrate or bismuth chloride, aluminium chloride. Also, alkoxides can be used, e.g. titanium tetraethoxide or titanium tetraisopropoxide. Metalloid precursors in particular include metalloid halides or alkoxides such as silicon tetrachloride, tetraethyl orthosilicate (TEOS), and tetraisopropyl orthosilicate. Typically, metal alkoxides hydrolyze in water, but when methanol or an acidified aqueous solution is used they could still be used. Preferably, the metal precursor is not miscible with the apolar (hydrophobic) solvent, or it will not be efficiently impregnated into the template's pores.

In some preferred embodiments, the metal, metalloid, or non-metal precursor is a precursor of bismuth, antimony, bismuth-antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide; preferably bismuth or bismuth selenide.

Hydrophobic solvents, also referred to herein as apolar solvents or non-polar solvents, for use in the process of the invention are preferably inert towards the metal, metalloid, or non-metal precursor and include aromatic hydrocarbons such as benzene, toluene and xylene, and non- aromatic hydrocarbons such as alkanes, e.g. pentane, hexane, heptane and octane; and mixtures thereof. In some preferred embodiments, the hydrophobic solvent is selected from the group comprising: a C₆-C₁₀ arene, a C₁-C₈ alkyl C₆-C₁₀ arene, such as toluene or xylene, a C₆-C₁₂ alkane, such as hexane, heptane, octane, nonane, or a mixture thereof; wherein the alkane, arene, or alkylarene is optionally fluorated or chlorated.

Hydrophilic solvents, also referred to herein as polar solvents, are preferably inert towards the metal, metalloid, or non-metal precursor and comprise water, alcohols, in particular alkanols such as methanol, and carboxylic acids, such as formic acid; including mixtures thereof such as aqueous alkanol mixtures or carboxylic acid mixtures. In some preferred embodiments, the hydrophilic solvent is selected from the group comprising: water, a carboxylic acid (such as formic acid), methanol, or a mixture thereof.

In some preferred embodiments, the hydrophilic solvent is selected from the group comprising: water, a carboxylic acid, methanol, or a mixture thereof; and the hydrophobic solvent is selected from the group comprising: a C₆-C₁₀ arene, a C₁-C₈ alkyl C₆-C₁₀ arene, such as toluene or xylene, a C₆-C₁₂ alkane, such as hexane, heptane, octane, nonane, or a mixture thereof; wherein the alkane, arene, or alkylarene is optionally fluorated or chlorated.

The hydrophilic and hydrophobic solvents are preferably not mixable (i.e. they do not form a homogeneous mixture) but rather form a two phase system of a hydrophilic and a hydrophobic phase. Hydrophilic and hydrophobic solvent systems that can be used comprise, for example water/hexane; water/xylene; methanol/heptane; methanol/octane; methanol/heptane-octane. Formic acid also forms an immiscible system with any of the apolar (hydrophobic) solvents mentioned.

In some embodiments, the porous material (for example mesoporous silica) is dispersed in the hydrophobic solvent, for example in toluene or xylene, thus forming a hydrophobic porous material dispersion.

In some embodiments, the hydrophilic solvent is removed from the porous silica dispersion while adding the precursor solution. The dispersion may for example be heated and the pressure reduced thereby causing hydrophilic solvent to be boiled off. The metal precursor salts are dissolved in the one or more hydrophilic solvents, forming the precursor solution. The one or more hydrophilic solvents may be acidified prior or after dissolving the metal precursor salts. The acid used preferably is that from which the metal precursor salt is derived. For example, if a nitrate salt is used, the acid will preferably be nitric acid, in case of a chloride the acid will preferably be hydrochloric acid.

The precursor solution is added to the hydrophobic porous material dispersion. The adding preferably takes place in an appropriate recipient, which preferably is made of a hydrophobic material such as a fluorinated polymer, e.g. Teflon or PFA. This avoids wetting of the precursor on the recipient's wall.

In some preferred embodiments, the hydrophilic solvent is removed from the mixture at a temperature that is higher than the boiling point of the hydrophilic solvent at the applied pressure. In some preferred embodiments, the hydrophilic solvent is removed from the mixture at a temperature that is lower than the boiling point of the hydrophobic solvent at the applied pressure. In some preferred embodiments, the boiling point of the hydrophilic solvent is lower than the boiling point of the hydrophobic solvent at the applied pressure.

In some preferred embodiments, the volume of the metal, metalloid, or non-metal precursor solution is lower than the total pore volume of the porous material. This may avoid deposition of the precursor on the outer surface of the porous material. This may avoids clogging of the pores on the outer surface of the porous material.

In some embodiments, the hydrophobic solvent comprises one or more hydrophobic solvent components. In some embodiments, the hydrophilic solvent comprises one or more hydrophilic solvent components. In some preferred embodiments, the boiling point of the lowest boiling component of the hydrophilic solvent is higher than the boiling point of the highest boiling component of the hydrophobic solvent at the applied pressure. In some preferred embodiments, the boiling point of the highest boiling component of the hydrophilic solvent is higher than the boiling point of the lowest boiling component of the hydrophobic solvent at the applied pressure.

The metal precursor solution is added to the dispersion at a temperature that is higher than the boiling point of the hydrophilic solvent component with the lowest boiling point at the applied pressure. In one embodiment, the boiling point of the lowest boiling component of the hydrophilic solvent is higher than the boiling point of the highest boiling component of the hydrophobic solvent at the applied pressure. In another embodiment, the boiling point of the highest boiling component of the hydrophilic solvent is higher than the boiling point of the lowest boiling component of the hydrophobic solvent at the applied pressure. The pressure may be reduced to below atmospheric pressure or otherwise the pressure may be increased to above atmospheric pressure. Temperature and pressure are selected such that the hydrophilic solvent boils off so that continuous removal of all the hydrophilic solvent from the biphasic mixture takes place. The removed hydrophilic solvent may be captured for example by using a Dean/Stark trap.

The pressure may be reduced below atmospheric pressure by a vacuum pump with a pressure controller. The pressure may be reduced or increased to range from 1 mbar to 1 bar, or from 100 to 500 mbar, for example the pressure may be set at about 150 mbar. The temperature may be above 1000ºC such as in the range of 25 ºC to 300 ºC, or from 125 ºC to 180ºC, e.g. at about 160ºC. In some embodiments, the metal is decomposed at elevated temperatures during impregnation.

The reduced pressure and the desired temperature preferably are set prior to the addition of the precursor solution, by heating the recipient holding the hydrophobic porous material dispersion and reducing pressure.

Alternatively, the impregnation procedure described above can also be executed at high pressure, for example at a pressure in the range of 1 bar to 50 bar. Higher pressures might be suitable to alter the boiling point of the apolar (hydrophobic) solvent. In such embodiments, the impregnation might take place in a high pressure vessel in which the dispersion of porous material in hydrophobic solvent is introduced and the vessel is pressurized. This causes the boiling temperature of the hydrophobic solvent to increase. The hydrophilic precursor solution is then added and hydrophilic solvent is removed. In this procedure the increased temperature may cause thermal decomposition of the precursor.

During the adding of the precursor solution to the hydrophobic porous material dispersion, the porous material becomes impregnated with the precursor solution by infiltration into the pores through capillary action. Due to hydrophilic solvent removal this eventually leads to solid precursor being deposited in the pores.

The precursor may decompose partly to various metal, metalloid, or non-metal compounds during the hydrophilic solvent removal. For example, BiCI₃ may decompose to BiOCI. Due to the systematic removal of the hydrophilic solvent, density and concentration of the precursor increases. In this way extremely high loading efficiencies may be obtained.

Once all hydrophilic solvent, such as water, is removed and the precursor solution is absorbed into the pores, the porous material loaded with precursor may be filtered off, and may be washed with hydrophobic solvent, such as the solvent used in the impregnation step, and dried.

If desired, after the filtration, washing and drying steps, the porous material loaded with precursor may be subjected to a calcination step, in particular when the porous material is a metal oxide such as silica or alumina. The calcined material may be processed further or may be subjected to a further impregnation step. In some instances, several calcination/impregnation steps may be desirable.

Although the impregnation process of the invention enables a high loading degree of material inside the pores, in some instances it may not be possible to obtain 100% filled pores for metals and most metal oxides. As a consequence, several impregnations may be necessary to obtain a self-sustaining replicated structure.

As used herein, the term "nanostructured" refers to an arrangement, structure, or part of something of molecular dimensions. The matrix itself has nanoscaled pores, the impregnated material has nanoscale dimensions and after removal of the porous materials, nanowires/nanopowders/nanoparticles are obtained. The term "nanostructured" is considered as an arrangement of one or more materials with dimensions below 50 nm, most preferably one of the constituents should be between 5 and 20 nm. The arrangement can consist of the material and voids or several dissimilar materials. The preferred structures are nanowires or mesoporous structures ("array of interconnected nanowires"). The obtained structures could be nanoparticles.

As used herein, the term "precursor" refers to a compound that dissolves in the hydrophilic solvent, thereby providing a precursor solution. The precursor is typically a salt, such as nitrates, acetates, chlorides with a good solubility in a hydrophilic solvent.

As used herein, the converted products; i.e. nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; do not dissolve in the hydrophilic solvent. They have been chemically converted. Typically, there is a volume change during the conversion. Examples of suitable conversion methods are provided below. The precursor typically differs from the end products in chemical composition, solubility in water, density, and/or molecular weight. One example is BiC|₃ as precursor, BiOCI as an intermediate, and Bi metal as and end product. There are various ways to convert the precursors into the end products. Depending on the synthesis parameters this can be detrimental for the porous material. Preferably, the conversion is performed without destroying the porous material. If the synthesis temperature is rather mild, the porous material will remain intact. For example, as long as the crystallization force of the target material does not destroy the pore walls of the template the porous material is not destroyed. If this does happen, the calcination temperature can be reduced.

The metal, metalloid, or non-metal precursor deposited in the porous material can be converted to a metal, metalloid, or non-metal decomposition product, such as BiCI₃ to BiOCI, TiCI₄ to TiOCI₂ or Ti0₂, Bi(N0₃)₃.5H₂Oto Bi(N0₃)₃ or BiON0₃ or derivative, etc., or to a metal, metalloid, or non-metal, or to a metal, metalloid, or non-metal oxide. The precursor that is impregnated may be decomposed by chemical treatment, which creates extra pore volume so that more precursor solution can be impregnated without the need to perform intermediate filtrations and calcinations steps. For example, metal salts may be treated with hydrazine to reduce to their metallic form.

Once deposited in the pores of the porous material, the metal, metalloid, or non-metal precursor may be converted to the corresponding metal, metalloid, or non-metal using art- known reduction methods. For example, in case of a metal salt by chemically reducing the metal salt with a hydride such as NaBH₄ in a suitable solvent such as THF, or with hydrogen gas. In the latter instance, hydrazine may be added. The precursor may be reduced in one step or in more reduction steps with the same or with different reductants. In this way, nanofilaments of metal, metalloid, or non-metal are formed inside the porous material. In case of metals, a nanowire network of metal is thus formed.

In some embodiments, the chemical vapor reaction may comprise the step of passivation through, preferably short, exposure to reactive vapor. In some embodiments, the chemical vapor reaction may comprise the step of complete altering of the composition of the impregnated material through, preferably extended, exposure to a chemical vapor. For example, in case Bi nanowires are passivated with H₂Se, one would obtain Bi+Bi₂Se₃, where Bi₂Se₃ is the passivation layer. Depending on the extent of the treatment one could convert all Bi to Bi₂Se₃. In that case the entire composition of the starting material is altered.

The metal, metalloid, or non-metal precursor deposited in the pores of the porous material may be converted to the corresponding metal, metalloid, or non-metal oxides using art-known oxidation methods. A suitable oxidant may be used, such as for example air, oxygen gas, optionally humidified or plasma treatment.

After the reduction or oxidation step, the material may optionally be subjected to a calcination step. The material may further be subjected to a densification step. After the reduction or oxidation step, the resulting nanocomposite material may be processed further, e.g. by one or more filtration, washing and drying steps. After isolation, the nanocomposite material may be subjected to one or more further reduction oxidation steps using the same or different reductants or oxidants. After these further reduction or oxidation steps, if desired, the nanocomposite material may be processed further, e.g. by one or more filtration, washing and drying steps.

The nanocomposite powder may then be further processed such as by peptization and subjected to thermoelectric measurements.

The process of the invention is particularly suited for the impregnation of bismuth salts into mesoporous silica. The loading efficiency of Bi may be further enhanced by adding hydrazine formate to the impregnated salt, which reduces the bismuth salt to metallic bismuth, while the silica remains unaffected. This allows more precursor solution to be impregnated in the pores. Where metal nanowires are formed the metal surface may be surface treated with various agents that are preferably are gaseous. Examples of such agents include, but are not limited

Surface treatment can be applied to passivate the material's surface to avoid oxidation when exposed to air, to dope the material, to alter the composition, or to make a new material. For example, in the case of bismuth nanowires, the latter can be treated with H₂Se gas to form a Bi₂Se₃ passivation layer at the nanowire surface.

The treatment of the impregnated material with a volatile reactive gas may also increase the amount of impregnated material, creating extra percolation paths, and/or strengthening the nanowire network. For example, when a mesoporous silica template is completely impregnated with BiCI₃, upon treatment with Me₃Bi, the volatile bismuth source can react with BiCI₃ and form 2 Bi_(s) and 3 MeCI_(g). Consequently, due to the reaction of the gaseous precursor with the metal salt inside the pores, the pores are extra filled.

The surface of the porous material may be modified e.g. to alter its affinity towards the precursor solution or reduce its reactivity. For example, in the case of silica or alumina, organic groups may be grafted on the surface of the silica. Examples of such groups are but not limited to aminoalkyl, hydroxyalkyl and mercaptoalkyl groups such as aminoethyl, aminopropyl, hydroxyethyl, hydroxypropyl, mercaptoethyl and mercaptopropyl. Such groups may be introduced by reacting the silica or alumina with a triethoxysilyl alkyl amine, hydroxide or sulhydryl. These groups are preferably present to control the affinity for specific precursor solutions in order to optimize the impregnation efficiency. Surface modification may also be desirable to avoid reaction of the impregnated material with the silica template during thermal treatment.

- removing the porous material.

The person skilled in the art has access to various methods to remove the porous material without removing the nanostructured material within, and will choose a suitable method depending on the combination of porous material and nanostructured material. Combinations wherein the porous material cannot be removed without removing the nanostructured material fall outside the scope of this aspect. Methods to remove the porous material without removing the nanostructured material within may be found in Chemistry of Mat., Deng, Protocol for the Nanocasting Method- Preparation of Ordered Mesoporous Metal Oxides, hereby incorporated in its entirety by reference.

HF and NaOH are the most used etching solutions. NaOH is often preferred over HF as it is safer to handle and it usually does not react with the replicated material unless we have an amphoteric material. Generally, mesoporous silica is used as template material. The amorphous Si0₂ can be chemically removed under basic conditions, such as with NaOH or KOH. Alternatively, HF can be used, but due to its high reactivity, it can also affect the target material. Therefore, NaOH was used in the described examples.

An example procedure may proceed as follows (values may vary depending on the target material):

- For example, 1 g composite powder (Si0₂ template + target product in the pores) is dispersed in 20 imL 1 mol/L NaOH solution.

- The dispersion is stirred for 3 h at room temperature enabling the Si0₂ to selectively dissolve.

- The dispersion is transferred to a falcon flask and centrifuged for 20 min at 5000 rpm, until all solid is separated (material dependent).

- The liquid phase is decanted while the solid powder is diluted in 20 imL water. The powder is washed by stirring the dispersion for 5 min at RT.

- The dispersion is centrifuged again for 30 min at 5000 rpm, until all solid is separated and repeated 1 more time with water and twice with ethanol.

- The final product should have less than 0.1 at% Si0₂.

- The final powder is dried at 60 °C.

The material deposited in the pores may be isolated by removing the porous template material, e.g., by extraction with a suitable etching solution. Removal of the template material leads to nanowires. The nanowires may have been surface treated as described above. Examples include metal, doped or alloyed metal, semiconductor, or metaloxide structures, such as nanowires or mesoporous structures. Removal of the template material in porous materials in which a metal or metalloid is deposited leads to porous (in particular mesoporous) metal or metalloid oxide materials. For example, porous silica template material may be removed with HF or with a strong basic solution such as an alkali metal hydroxide solution. In case the silica template is removed under such basic conditions, not any material can be replicated via this hard templating procedure. In case heat treatment is required, some materials may also react with the silica template.

According to a fifth aspect, the present invention relates to a metal, metalloid, or non-metal precursor deposited in a porous material wherein the filling degree of the metal, metalloid, or non-metal precursor is at least 60% of the theoretical maximum, preferably at least 65%, preferably at least 70%, preferably at least 75%, preferably at least 80%, preferably at least 85%. Combinations wherein the porous material cannot be distinguished from the precursor fall outside the scope of this aspect.

The theoretical maximum loading is calculated based on the pore volume of template and the density of the precursor salt. An example is provided below:

- 1 g mesoporous silica template with a pore volume of e.g. 1 cm³/g can contain 1 cm³ precursor salt in its pores.

- The maximum amount of precursor salt that can be impregnated in a single impregnation is: Umax (9) = pore volume (cm³) ^* density salt (g/cm³).

Depending on the impregnation technique, a fraction of that theoretical maximum can be impregnated.

Preferably, a water based precursor solution is used. Therefore, the solubility is generally not a problem as the solubility of metal salts is higher. If the solubility is low for all available metal salts, for example in ethanol, a diluted precursor solution can still be impregnated and still obtain a high filling degree due to the fact that solid is deposited into the pores. A longer impregnation time may be required. For example, if a 20 imL precursor solution with a concentration of 0.2 mol/L is impregnated at a rate of 4 imL/h, the impregnation will take 5 h. The impregnation of 80 imL 0.05 mol/l precursor solution will yield the same pore loading but will last 20 h instead.

The inventors have found that pores with a diameter of 6-7 nm could be successfully impregnated with but not limited to, water, water-methanol and water-HCI-methanol based precursor solutions. The precursor salts are typically chloride or nitrate salts, which are sufficiently small to enter the template's pores. According to a sixth aspect, the present invention relates to a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; having a diameter of at most 50 nm, preferably of at most 40 nm, preferably of at most 30 nm, preferably of at most 20 nm, preferably of at most 15 nm, for example at most 10 nm, for example at most 8 nm; and an aspect ratio of at least 50, preferably of at least 75, preferably of at least 100; wherein the metal is selected from the group comprising, but not limited to: bismuth, antimony, bismuth- antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide, preferably wherein the metal is bismuth, bismuth antimony or bismuth selenide. The diameter may be obtained by microscopy (SEM - TEM) analyses.

In some embodiments, the nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; has a diameter of at most 15 nm; and an aspect ratio of at least 50, preferably of at least 75, preferably of at least 100; wherein the metal is selected from the group comprising: bismuth, antimony, bismuth-antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide, preferably wherein the metal is bismuth or bismuth selenide.

In some embodiments, the nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; has a diameter of at most 50 nm, preferably of at most 40 nm, preferably of at most 30 nm, preferably of at most 20 nm, preferably of at most 15 nm, for example at most 10 nm, for example at most 8 nm; and an aspect ratio of at least 100; wherein the metal is selected from the group comprising: bismuth, antimony, bismuth-antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide, preferably wherein the metal is bismuth or bismuth selenide.

In some embodiments, the nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; has a diameter of at most 50 nm, preferably of at most 40 nm, preferably of at most 30 nm, preferably of at most 20 nm, preferably of at most 15 nm, for example at most 10 nm, for example at most 8 nm; and an aspect ratio of at least 50, preferably of at least 75, preferably of at least 100; wherein the metal is bismuth or bismuth selenide.

In some embodiments, the nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; has a diameter of at most 15 nm; and an aspect ratio of at least 100; wherein the metal is selected from the group comprising: bismuth, antimony, bismuth- antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide, preferably wherein the metal is bismuth or bismuth selenide. In some embodiments, the nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; has a diameter of at most 15 nm; and an aspect ratio of at least 50, preferably of at least 75, preferably of at least 100; wherein the metal is bismuth or bismuth selenide.

In some embodiments, the nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; has a diameter of at most 50 nm, preferably of at most 40 nm, preferably of at most 30 nm, preferably of at most 20 nm, preferably of at most 15 nm, for example at most 10 nm, for example at most 8 nm; and an aspect ratio of at least 100; wherein the metal is bismuth or bismuth selenide.

The processes of the invention or preferred embodiments thereof, offer a number of advantageous aspects. In particular, the processes according to the invention, or preferred embodiments thereof, may enable very high loading efficiency compared to conventional methods resulting in high quality structures.

Moreover, the processes according to the invention, or preferred embodiments thereof, may allow the manufacture of various new nanocomposite thermoelectrics with superior properties compared to existing materials. In addition, the processes of the invention, or preferred embodiments thereof, can be used for the manufacture of other mesoporous materials for solid-state applications, catalysis or other applications.

This invention extends the range of mesoporous materials that can be fabricated via the hard templated replication method. Namely, any water soluble metal salt can be impregnated in mesoporous silica in an efficient manner. Also metal precursor solutions that would normally hydrolyze in water can by impregnated as acid based precursor. The impregnation method of the invention is simple, easily scalable, cheap, and may result in high quality replicated structures for any water or acid based precursor solution. Due to the fact that extremely high loading efficiencies can be obtained, fully interconnected nanowire networks can be fabricated possessing unique physical properties compared to their bulk counterparts, for instance for thermoelectric heat to electricity conversion applications.

Limited or no leaching phenomena may be observed during impregnation resulting in no or negligible amounts of externally deposited material. No or limited intermediate filtration and calcination steps are required for a sufficiently high degree of filling, which may reduce the preparation time drastically. Due to the fact that the impregnation occurs through capillary impregnation no diffusion limitations occur for the impregnation of small pore mesoporous silica materials.

The nanowires may find application in various fields such as semiconductors, thermoelectrics, sensors, catalysis, etc. The following examples are illustrative and should not be interpreted in any way so as to limit the scope of the invention.

Example 1

Nanostructured bismuth was synthesized as a composite material by impregnating KIT-6 mesoporous silica template material with a solution of Bi(N0₃)3-5H₂0 salt dissolved in nitric acid or Bi₂0₃ dissolved in hydrochloric acid. 1 g KIT-6 template material was used, which had a pore volume of about 1 imL/g as determined by N₂ sorption measurements. The mesoporous silica template was dispersed in 100 imL toluene or xylene and brought into a 250 imL PFA round bottom flask equipped with a dean stark separator and reflux cooler. Next, 5.66 g Bi(N0₃)₃.5H₂0 was dissolved in 20 imL 65% nitric acid and brought into an addition funnel placed on top of the reaction vessel. The entire system was connected to a vacuum pump with a pressure controller. Prior to the addition of the acid based precursor, the recipient was heated to 160ºC while the pressure was reduced to 150 mbar causing the water to boil off. All water was removed from the system by means of the dean stark setup. Subsequently, the precursor was added dropwise into the suspension. Since the volume of the precursor solution was lower than the total pore volume at any single moment of the impregnation, the precursor infiltrates the pores through capillary action and decomposes partly upon solvent removal. Since the impregnation was performed at elevated temperatures and all water was systematically removed, the precursor decomposes partially and increases in density compared to the starting salt. In this way very high loading efficiencies were obtained. Once all the precursor was added, the composite powder was filtered off, washed with toluene and dried overnight at room temperature.

The impregnated bismuth salt in the silica template's pore channels was completely reduced to metallic bismuth between 220ºC and 265ºC for 10h in a Ar - 5% H₂ bubbled through hydrazine hydrate. To avoid oxidation when exposed to air, the sample was treated at 250 °C for 15 min with H₂Se gas to form Bi₂Se₃. After reaction, the nanocomposite powder was pelletized and used for thermoelectric measurements. Mesoporous bismuth selenide could be isolated by removing the silica by chemical etching in a 1 M sodium hydroxide solution for 3h. The mesoporous material was recovered through centrifugal separation and washed twice with water and ethanol and subsequently vacuum dried.

Fig. 1 represents a schematic representation of the impregnation setup used in example 1 . The following numbering is adhered to: 10 - Dean Stark separator; 12 - PFA flask; 14 - heating plate; 16 - additional funnel or syringe pump; 20 - Graham condenser; 22 - cooling IN; 24 - cooling OUT; 1 10- Bismuth precursor solution; 120 - silica template and apolar (hydrophobic) solvent; 130 - apolar (hydrophobic) phase; 140 - aqueous phase.

Example 2 TiOCI₂ dissolved in diluted chloric acid was used as precursor solution for the synthesis of mesoporous Ti0₂. The impregnation method was conducted as described above. The impregnated material was heat treated at 400 °C for 4h.

Example 3

The below example demonstrates how the filling degree may be calculated. The present inventors were able to obtain a filling degree of 27% for metallic Bi while the theoretical maximum is 31 % when the pores were completely filled with BiCI₃. The filling degree was calculated from data obtained by a Micromeritics TriStar 3000 device. The filling degree was obtained as following:

The filling degree of any impregnated salt is calculated as following (BiCI₃ as example):

Vp,max internal is illustrated in Fig. 2A. Vp,max external is illustrated in Fig. 2B.

In case of BiCI₃: pore filling degree is 27% for metallic Bi while the theoretical maximum is

31 %, which suggests 86% internal and 14% external, as illustrated in Fig. 2C.

The mass loading of the impregnated material can be calculated from the amount of material added in the precursor and can be verified by XRF analysis. This is optional.

Maximum loading of the metal salt in the template's pores (excluding decomposition upon impregnation), 1 g template as example:

Example 4

Typically, bismuth nanowires with an aspect ratio of 100-200 were obtained. The maximum aspect ratio is determined by the size of the mesoporous template particles which is in the 1 -2 μm; range, but not limited to this range. Fig. 3 illustrates bismuth nanowires according to a preferred embodiment of the invention.

Example 5

Fig. 4 illustrates nanowires according to preferred embodiments of the invention. Fig. 4A illustrates Bi₂Se₃ nanowires templated by SBA-15, with a BET surface area of 13 m²/g. The at% of Bi in the silica+Bi composite is typically 18-22%, which is the theoretical maximum. The aspect ratio is > 100 (ratio nanowire length/diameter). Fig. 4B illustrates mesoporous Bi templated by KIT-6, with a BET surface area of 60 m²/g. The at% of Bi in the silica+Bi composite is typically 18-22%, which is the theoretical maximum. The mesoporous grain size is >150 nm. Fig.4C illustrates Bi nanowires templated by SBA-15. The at% of Bi in the silica+Bi composite is typically 18-22%, which is the theoretical maximum. The aspect ratio is typically > 100. Fig.4D illustrates Bi nanowires templated by SBA-15. The at% of Bi in the silica+Bi composite is 8,5 %, which is the theoretical maximum when Bi(N0₃)3 is used as bismuth salt. The aspect ratio is typically > 20.

Example 6

KIT-6 mesoporous silica template material was loaded with nickel nitrate salt (Ni(N0₃)₂.6H₂0) by impregnating with a solution of Ni(N0₃)₂.6H₂0 dissolved in a mixture of H₂0, formic acid (FA) and methanol (MeOH) in a double solvent system. 2.5 g KIT-6 mesoporous silica template material with a pore volume of 0.92 cm³/g was dispersed in 100 mL n-octane in a 250 mL perfluoroalkoxy alkane (PFA) round bottom flask while stirring at 700 rpm. The recipient was equipped with a dean stark separator and heated to 165°C to enable gentle refluxing of n-octane. Subsequently, 7.73 g Ni(N0₃)₂.6H₂0 was dissolved in 10 mL H₂0, 10 mL FA and 70 mL MeOH and added to the reaction vessel using a syringe pump. The precursor solution was added in a continuous manner to the PFA flask at a rate of 4 imL/h until all Ni precursor solution was added to the suspended template solution. Meanwhile, the aqueous mixture was continuously eliminated from the system with the dean stark separator. After impregnating the silica template, the powder was collected through filtration, washed with n-hexane and dried overnight at 60°C.

The Ni(N0₃)₂ precursor confined in the silica template's pores was reduced to metallic Ni in a tubular flow furnace in a flow of Ar - 5% H₂ and FA vapor by passing the gas through a FA containing bubbler before entering the furnace. The samples were reduced at 350°C for 4 h, using a flow rate of 10 imL/min.

The silica template was removed in a 1 mol/L NaOH solution for 3 h and washed twice with water and twice with ethanol. The powder was collected through centrifugal separation.

Nitrogen sorption experiments were performed at 77 K with a Micromeritics TriStar device. Samples were vacuum dried at 120°C for 12 h prior to analysis. The surface area was calculated using the BET method while the pore size distribution was determined by analysis of the adsorption branch of the isotherms using the BJH method.

Both SBA-15 and KIT-6 were used as template for the synthesis of mesoporous Ni, and could be impregnated using the process according to the invention. KIT-6 templated Ni resulted in a mesoporous replicated structure. Mesoporous Ni with a BET surface area and pore volume of respectively 180 m²/g and 0,15 cm³/g were obtained. To the best of our knowledge, no reports of mesoporous Ni with such high surface area have been encountered. KIT-6 was preferred because its pore system consists of 3D interconnected pores. Consequently, the replicated structures are mechanically stronger than those obtained via SBA-15. SBA-15 yielded mainly nanowires, which have a very low pore volume. A surface area of 80m2/g is still very high. FIG. 5A illustrates an isotherm linear plot for Ni templated from KIT-6 mesoporous silica with a BET surface area and pore volume of respectively 180 m²/g and 0,15 cm³/g. Fig. 5B illustrates the pore volume.

Claims

1 . A process for preparing a porous material comprising metal, metalloid, or non-metal precursor, said process comprising the steps of:

- dispersing the porous material in a hydrophobic solvent;

- selectively removing the hydrophilic solvent from the mixture; thereby impregnating the porous material with the metal, metalloid, or non-metal precursor solution;

thereby preparing a porous material comprising metal, metalloid, or non-metal precursor.

2. The process according to claim 1 , wherein the hydrophilic solvent is removed from the mixture at a temperature that is higher than the boiling point of the hydrophilic solvent at the applied pressure.

3. The process according to any one of claims 1 or 2, wherein the boiling point of the hydrophilic solvent is lower than the boiling point of the hydrophobic solvent at the applied pressure.

4. The process according to any one of claims 1 to 3, wherein the volume of the metal, metalloid, or non-metal precursor solution is lower than the total pore volume of the porous material.

5. The process according to any of claims 1 to 4, wherein the porous material is an oxide or sulfide of a metal or of a metalloid, or a mixture thereof, preferably wherein the porous material is selected from the group comprising: silica, alumina, ceria, titania, cobalt(ll,lll) oxide, iron(lll) oxide, nickel oxide, manganese oxide, yttrium stabilized zirconia, or a mixture thereof.

6. The process according to any of claims 1 to 5, wherein the metal, metalloid, or non- metal precursor is a precursor of bismuth, antimony, bismuth-antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide.

7. The process according to any of claims 1 to 6, wherein the porous material has an average pore diameter of at least 2 nm and at most 50 nm, preferably of at least 5 nm and at most 10 nm, preferably of at least 6 nm and at most 8 nm, as determined by microscopy.

8. The process according to any of claims 1 to 7, wherein the hydrophilic solvent is selected from the group comprising: water, a mineral acid, a carboxylic acid, methanol, or a mixture thereof; and the hydrophobic solvent is selected from the group comprising: a C₆-C₁₀ arene, a C₁-C₈ alkyl C₆-C₁₀ arene, such as toluene or xylene, a C₆-C₁₂ alkane, such as hexane, heptane, octane, nonane, or a mixture thereof; wherein the alkane, arene, or alkylarene is optionally fluorated or chlorated.

9. The process according to any of claims 1 to 8, wherein the metal, metalloid, or non- metal is a metal of which the surface is passivated.

10. A process for preparing a porous material comprising a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; said process comprising the steps of:

- preparing a porous material comprising nanostructured metal, metalloid, or non-metal precursor according to any one of claims 1 to 9; and

1 1 . The process according to claim 10, wherein the metal, metalloid, or non-metal precursor is converted through chemical reduction, decomposition, oxidation, nitridation, chemical vapor reaction, or a combination thereof.

12. A process for preparing a nanostructured metal, metalloid, or non-metal decomposition product; or a nanostructured metal, metalloid, or non-metal; or a nanostructured metal, metalloid, or non-metal oxide; said process comprising the steps of:

- preparing a porous material comprising a nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non- metal; or nanostructured metal, metalloid, or non-metal oxide; according to any one of claims 10 or 1 1 ; and

- removing the porous material.

13. A nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; obtainable by the process of claim 12.

14. A metal, metalloid, or non-metal precursor deposited in a porous material wherein the filling degree of the metal, metalloid, or non-metal precursor is at least 60% of the theoretical maximum, preferably at least 65%, preferably at least 70%, preferably at least 75%, preferably at least 80%, preferably at least 85%.

15. A nanostructured metal, metalloid, or non-metal decomposition product; or nanostructured metal, metalloid, or non-metal; or nanostructured metal, metalloid, or non-metal oxide; having a diameter of at most 50 nm, preferably of at most 40 nm, preferably of at most 30 nm, preferably of at most 20 nm, preferably of at most 15 nm, for example at most 10 nm, for example at most 8 nm; and an aspect ratio of at least 50, preferably of at least 75, preferably of at least 100; wherein the metal is selected from the group comprising: bismuth, antimony, bismuth-antimony, bismuth selenide, bismuth telluride, bismuth-antimony telluride, titanium dioxide, or titanium disulfide, preferably wherein the metal is bismuth or bismuth selenide.