WO2007033815A2 - Electrochemical method for producing nanoscalar metal compounds - Google Patents

Abstract

The invention relates to an electrochemical method for producing nanoscalar particles from a metal compound. In said method, metals are anodically dissolved in an electrolytic cell, the electrolyte solution forming a precipitating component, which produces a poorly soluble compound with the metal ions that are formed and comprising a surface modification agent, which permits surface-modified particles to be formed from the metal compound or a precursor of the latter. The method optionally comprises the isolation of the particles thus formed from the electrolyte solution and the optional calcination of said particles. The method is devoid of waste water and is therefore environmentally friendly and cost-effective. The nanoscalar particles that are produced can be easily redispersed in organic and aqueous media and have a high degree of purity. The particle size can lie significantly below 100 nm. The method is particularly suitable for producing simple metal oxides such as iron oxide and binary metal oxides such as ITO and ATO.

Description

Electrochemical process for the production of nanoscale metal compounds

The present invention relates to an electrochemical process for producing nanoscale metal compounds in an electrolytic cell, wherein the electrolyte solution comprises a surface modifier and a precipitating component.

Due to their special optoelectronic properties, indium tin oxide (ITO) and antimony tin oxide (ATO) offer a wide range of possible applications. In most cases, these mixed oxides are produced by (reactive) sputtering in the form of thin layers which are transparent in the visible wavelength range. More recent approaches have been made to prepare these layers by sol / gel process coating compositions. This requires nanoscale powders which are readily redispersible in the coating compositions.

These layers are capable of reflecting infrared light (K-wafers) and, after appropriate reduction, they show a comparatively high electrical conductivity. The combination of conductivity and transparency has in particular ITO to an indispensable structural element of display elements, such. As liquid crystal displays, let. As a result, there is an increasing demand for ITO and an increasing interest in processes for producing this mixed oxide. With regard to transparency and dispersibility, the production of nanoscale particles is of particular interest.

Over time, different methods for producing ITO have been developed. Important processes are aerosol processes, sol-gel processes and electrochemical processes. Aerosol processes, which are described, for example, in EP-A-0873791, lead to process-related hard agglomerates and moreover have the disadvantage of relatively low space-time yields. In combination with the high investment costs, these methods are uneconomical. Substantially more cost-effective, these mixed oxides are accessible via a precipitation reaction from the corresponding salts. In D EA-19849048, DE-T2-69818404; DE-T2-69303126, JP-A-06293517 and JP-A-06080422 describe the preparation of ITO powders by co-precipitation of indium and tin salts, eg chlorides or nitrates. For precipitation, a solution of the salts is made basic, for example by the addition of NH ₃ , as by-products, the NH ₄ salts of the indium and tin salts used, such as NH ₄ CI incurred. Since these salts adversely affect the properties of the formed ITO powder even in very low concentrations, the precipitated crude products must be washed consuming before calcination. Significant amounts of chloride or nitrate-containing wastewater accumulate. Since these are also contaminated with the heavy metals (In ³⁺ , Sn ⁴⁺ ), they can not be introduced without appropriate work-up in the sewage treatment plant.

The mixed oxides (eg ITO) prepared by the precipitation process are tempered after calcination in a reducing atmosphere (eg at 34O ⁰ C) in order to improve the conductivity of the powder. By crushing or dispersing the ITO powder can then be transferred into a suspension for coatings or pressed into shaped bodies.

In addition to the mentioned problem of by-product formation and the associated wastewater problem, it is disadvantageous in all precipitation processes that the salts to be used, such as e.g. InCb, need to be very pure. However, such pure salts are much more expensive than the underlying metals.

US-A-5417816 describes an electrochemical process for producing ITO in which indium and tin electrodes or prealloyed indium / tin electrodes are anodically dissolved. The electrolyte used is an NH ₄ NO ₃ solution whose concentration is specified as 0.2 to 5 mol / l. It is stated that the current efficiency of the process breaks down as soon as the concentration of NH ₄ NO ₃ falls below the value of 0.2 mol / l liter (16 g / l). The precipitated primary products of the electrolysis must be washed consuming due to the high salt content, so that even in this process a heavy metal Wastewater accumulates. After calcination, particles having a particle size of about 0.5 to 18 μm are obtained. Thus, they are not suitable for direct incorporation into coating compositions and thin films because of their size.

JP-A-63195101 discloses an electrochemical process for producing metal oxides by anodic dissolution of the corresponding metal in a divided cell. The electrolyte used are organic acids and / or ammonium salts of these acids. The electrolysis gives a metal ion solution. The metal ion solution is then concentrated to yield organometallic salts. Calcination yields indium oxide from these indium salts. This method requires a membrane to separate the cathode and anode compartments since the anolyte has a high concentration of dissolved In ³⁺ ions that can easily be cathodically deposited. This would lead to dendrite formation at the cathode.

The aim of the present invention is an environmentally friendly, economical process for the preparation of metal compounds, especially metal oxides and especially binary or doped metal oxides, preferably in the particle size range of not more than 100 nm. These nanoscale particles are said to have a high re-dispersibility in organic and aqueous media and have a high purity.

This object is achieved by an electrochemical process for the production of nanoscale particles from a metal compound, comprising the anodic dissolution of one or more metals in an electrolytic cell, the electrolytic solution of the electrolytic cell having a precipitating component which forms a sparingly soluble compound with the forming metal ions, which is the metal compound or a precursor thereof, and comprises a surface modifier for particles of the metal compound or a precursor thereof to form surface-modified particles of the metal compound or a precursor thereof, optionally separating the formed particles from the electrolyte solution and optionally the calcination of the particles formed. The particles produced may optionally be surface-modified.

In particular, the invention relates to an electrochemical process for producing spherical and / or crystalline metal oxides or doped metal oxides, in particular indium tin oxide (ITO) and antimony tin oxide (ATO). With the method according to the invention particles with a particle size of not more than 100 nm can be obtained. The powders produced in this way are distinguished by a very good redispersibility and a reactivity which is significantly higher than the known prior art in the subsequent reduction. The developed process works completely wastewater free. In addition, it has been found that the new powders in the production are not only much cheaper, but also surprisingly can be reduced at significantly lower temperatures.

In Fig. 1, an arrangement for the inventive method is shown. Fig. 2 shows the relationship between particle size and conductivity of the electrolyte solution again.

By the method according to the invention nanoscale particles are produced, ie, the particle size is less than 1 micron. By particle size, unless otherwise stated, the mean particle diameter, based on the surface agent, is determined by the BET method by nitrogen adsorption, it being possible to use Quantachrome Autosorb 6B as the measuring device for the measurement. The particle size is obtained by simple geometric conversion assuming spherical geometry and determined by standard methods density (at 20 ⁰ C). The average particle diameter is preferably not more than 200 nm, particularly preferably not more than 100 nm, for example 1 to 200, preferably 2 to 100 nm, for example 2 to 50 nm. Preference is given to obtaining particles which are substantially spherical.

The nanoscale particles produced are of a metal compound. The metal compound may contain one, two or more than two metals, where metal Compounds containing two metals are preferred. They may be stoichiometric or non-stoichiometric compounds. In the latter compounds, the atomic ratio of the metals in the compounds is variable, at least within certain limits. In particular, it may also be doped metal compounds. The doping element (s) may be any element, eg, As or P, but are preferably also a metal. Metals in this application also include semimetals such as germanium.

For the metal or metals of the metal compounds all metals of the Periodic Table of the Elements in question. The metal compound preferably contains at least one metal selected from Mg, Ca, Sr, Ba, Al, Ga, In ₁ Tl, Ge, Sn, Pb, Sb, Bi, Sc, Y, Ti, Zr, V, Nb, Ta, Mo. , W ₁ Fe, Co, Ni, Cu, Ag, Zn and Cd, as well as the lanthanides and actinides, such as Ce and La, or mixtures thereof. The metals can take on any oxidation state in the compounds that are common to the metals. The compounds may be present in any modification (eg, crystal form) that are conventional.

The metal compound is a compound of at least one metal having at least one non-metal. It can be e.g. to act ionic or covalent compounds. Anions can consist of one element or be complex. Examples are metal chalcogenides, such as oxides, sulfides, selenides and tellurides, metal halides, such as fluorides, chlorides, bromides and iodides, metal carbides, metal silicides, metal antimonides, metal nitrides, metal phosphides, metal hydroxides, metal carbonates, metal phosphates and metal phosphates. The metal compounds may contain more than one metal.

Metal oxides, metal hydroxides and metal oxide hydroxides, which are optionally hydrated, are preferred metal compounds. Particularly preferred metal compounds are metal oxides. In this case, binary metal oxides, ie oxides of 2 metals, are particularly preferred.

Examples of oxides, oxide hydroxides and oxides are BaO ₁ ZnO, CdO, GeO ₂ , TiO ₂ , ZrO ₂ , CeO ₂ , SnO ₂ , Al ₂ O ₃ , AIO (OH), Al (OH) ₃ , manganese oxides, In ₂ O ₃ , Y ₂ O ₃ , La ₂ O ₃ , Iron oxides such as Fe ₂ O ₃ (maghemite, hematite), Fe ₃ O ₄ (magnetite), FeO (OH), Cu ₂ O, Ta ₂ O ₅ , Nb ₂ O ₅ , V ₂ O ₅ , MoO ₃ or WO ₃ , corresponding mixed oxides, eg indium tin oxide (ITO), antimony tin oxide (ATO), fluorine doped tin oxide (FTO) ₁ calcium tungstate, and those with perovskite structure, such as BaTiO ₃ , BaSnO ₃ and PbTiO ₃ , others Examples of suitable particles are also spinels (for example MgO Al ₂ O ₃ ). ZnO, SnO ₂ , Al ₂ O ₃ , Al ₂ O ₃ -doped ZnO and In ₂ O ₃ , ITO, ATO and iron oxides are particularly preferred, with ITO, ATO and iron oxides being particularly preferred.

The production of the particles from a metal compound is carried out by an electrochemical process in which the one or more metals are connected in an electrolytic cell as an anode. In this case, the ore metals are dissolved anodically, whereby they go as cations in solution. As the metal (s), of course, the one or more metals are used which correspond to those of the metal compound to be produced. If more than one metal, preferably 2 metals, are dissolved anodically, they may be presented individually as separate anodes, or they may be presented together as a master alloy or amalgam as an anode.

It is a conventional electrolysis cell, in which an electrolyte solution is presented in a container, in which one or more anodes and one or more cathodes are immersed. The anode (s) and cathode (s) may be arranged in any conventional manner. A voltage is applied to the anode and cathode by means of a current source. The appropriate voltage and current will depend on the materials used and may be readily selected by one skilled in the art for the selected arrangement.

There are divided and undivided electrolysis cells. In divided electrolysis cells, the anode and cathode compartments are separated by ion permeance, e.g. through an ion exchange membrane or a diaphragm. In particular, an undivided electrolytic cell is used in the invention.

As the electrolyte solution, an aqueous solution of one or more electrolytes is preferably used. An electrolyte is contained in the water to provide the necessary conductivity of the electrolyte solution. The electrolyte solution also includes a surface modifier. The electrolytic solution may contain a surface modifier and an electrolyte. In the preferred embodiment, however, the surface modification agent simultaneously serves as an electrolyte in the electrolyte solution, so that no additional electrolyte is required. By electrolyte is meant herein any added matter other than water that forms the necessary ions for conductivity in the electrolyte solution, and includes the surface modifier as long as it contributes to conductivity by ion formation. For example, acids, bases and preferably salts (conductive salts) are suitable for the electrolytes, but these are preferably added in relatively small amounts. The ion concentration in the electrolytic solution is preferably less than 0.4 mol / l. Accordingly, for example, conductive salts are preferably used in a concentration of less than 0.2 mol / l. Conducting salts are preferably substantially 100% dissociated in the electrolyte solution. For compounds that do not dissociate completely, correspondingly higher concentrations are used.

The electrolytic solution preferably has a low ionic strength and hence a low conductivity, which is preferably not more than 30 mS / cm and more preferably not more than 10 mS / cm. It can e.g. not larger than 4 mS / cm.

The surface modifier contained in the electrolytic solution modifies the surface of the forming particles by interacting with the surface groups. As a result, the anodically formed primary products (nanoparticles) can be stabilized in situ without particle growth. The addition or adsorption onto the surface of the resulting primary particles can be carried out, for example, via a chemical bond, such as a covalent, coordinative or ionic bond, and / or polar (dipole-dipole) interaction or van der Waals forces. Preferred are chemical bonds. Such surface modifiers for nanoscale particles are familiar to those skilled in the art.

On the forming particles are surface groups or functional groups which are generally relatively reactive. For example, on particles of metal hydroxides or metal oxides as surface groups residual valences, such as hydroxyl groups and oxy groups. The surface modifier has at least one functional group that can interact with these surface groups of the forming particles in the above-mentioned manner.

The functional group of the surface modifier for attachment to the forming particles depends primarily on the surface groups of the particles and upon the desired interaction. Examples of preferred functional groups are carboxylic acid groups, carboxylate, acid amide groups (primary, secondary and tertiary) amino groups or the corresponding salts, hydroxyl groups and C-H-acidic groups, as in β-dicarbonyl compounds, and -SiOH or -SiOR groups. Also, several of these groups may be present simultaneously in one molecule (e.g., betaines, amino acids, EDTA).

The surface modifiers, which are preferably organic, are preferably selected from carboxylic acids or their salts, oxacarboxylic acids or their salts, hydroxycarboxylic acids and their salts, amino acids and their salts, other carboxylic acid derivatives, e.g. Acid amides or acid anhydrides, amines, β-dicarbonyl compounds and hydrolyzable silanes having at least one nonhydrolyzable group.

Preferred are organic carboxylic acids or carboxylates, wherein the groups COOH or COO ^' are bonded to alkyl or aryl radicals. Suitable counterions for the carboxylate are all customary, preference being given to NH ₄ ⁺ . Examples of such surface modifiers are optionally substituted (eg with hydroxy) saturated or unsaturated mono- and polycarboxylic acids (preferably monocarboxylic acids) having 1 to 24 carbon atoms, such as formic acid, acetic acid, propionic acid, butyric acid, pentanoic acid, hexanoic acid, acrylic acid, methacrylic acid, crotonic acid , Adipic acid, succinic acid, glutaric acid, oxalic acid, maleic acid, fumaric acid, benzoic acid, phenylacetic acid, itaconic acid and stearic acid, and the corresponding derivatives, such as acid anhydrides, chlorides, esters (preferably C 1 -C ₄ -alkyl esters) and -amides, for example caprolactam. The hydroxycarboxylic acids may be mono-, di- or polyhydroxycarboxylic acids. Examples are lactic acid, tartaric acid, citric acid and other fruit acids, salicylic acid and gallic acid. Other preferred surface modifiers are oxacarboxylic acids having 1 to 24 carbon atoms. These are carboxylic acids which contain oxa groups, ie ether bridges (-O-), in the alkyl chain. The oxacarboxylic acid may contain 1, 2, 3 or more oxa groups. Examples are methoxyacetic acid, 3,6-dioxaheptanoic acid and 3,6,9-trioxadecanoic acid (TODS) and their homologs.

Also suitable are amine compounds, such as ammonium salts and mono- or polyamines. Examples of these surface modifiers are quaternary ammonium salts of the formula NR ¹ R ² R ³ R ^{4 +} X ^" , where R ¹ to R ^{4 are} optionally different aliphatic, aromatic or cycloaliphatic groups having preferably 1 to 12, especially 1 to 8 carbon atoms, such as for example, alkyl groups having 1 to 12, in particular 1 to 8 and particularly preferably 1 to 6 carbon atoms (eg methyl, ethyl, n- and i-propyl, butyl or hexyl), and X ^"is an inorganic or organic anion, eg acetate, OH ^" , Cl ^" , Br ^" or I ^" ; Mono- and polyamines, in particular those of the general formula R _3-0 NH _n , where n = 0, 1 or 2 and the radicals R independently of one another represent alkyl groups having 1 to 12, in particular 1 to 8 and particularly preferably 1 to 6 carbon atoms, and ethylene polyamines. Specific examples are methylamine, dimethylamine. Trimethylamine, ethylamine, aniline, N-methylaniline, diphenylamine, triphenylamine, toluidine, ethylenediamine and diethylenetriamine.

Examples of β-dicarbonyl compounds are those having 4 to 12 carbon atoms, such as β-diketones and β-ketocarboxylic acids, and derivatives thereof. Examples are 2,4-hexanedione, 3,5-heptanedione, acetylacetone, diacetyl, acetonylacetone acetoacetic acid and acetoacetic acid-CrC ₄ -alkyl esters (acetoacetates), such as ethyl acetoacetate, and acetoacetamides. Further examples of surface modifiers are amino acids such as β-alanine, proteins and imines.

For surface modification of the particles it is also possible to use hydrolyzable silanes having at least one nonhydrolyzable group, for example silanes of the general formula R _n SiX _(4-n) (I), where the radicals X are the same or different and are hydrolyzable groups or hydroxy groups Radicals R are the same or are different and are not hydrolyzable groups and n is 1, 2 or 3, preferably 1 or 2.

In the general formula (I), the hydrolysable groups X, which may be the same or different, for example, hydrogen or halogen (F, Cl, Br or I), alkoxy (preferably Ci _-6 alkoxy, such as methoxy, ethoxy, n-propoxy, i-propoxy and butoxy), aryloxy (preferably C ₆ io aryloxy, such as phenoxy), acyloxy (preferably _C1-6 acyloxy, such as acetoxy or propionyloxy), alkylcarboxylic carbonyl (preferably C ₂ - ₇ alkyl-carbonyl such as acetyl), amino, monoalkylamino or dialkylamino. Preferred hydrolyzable radicals are halogen, alkoxy and acyloxy groups. Particularly preferred hydrolysable radicals are C _1-4 -alkoxy groups, in particular methoxy and ethoxy.

Examples of the non-hydrolysable radicals R, which may be the same or different, are alkyl (eg C _M o-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl and tert-butyl , pentyl, hexyl, octyl or cyclohexyl), alkenyl (for example, C _2- io-alkenyl, such as vinyl, 1-propenyl, 2-propenyl and butenyl), alkynyl (for example, C _2- io alkynyl such as acetylenyl and propargyl) , Aryl (preferably C _6-10 aryl such as phenyl and naphthyl), and corresponding alkylaryls and arylalkyls. The radicals R and X may optionally have one or more customary substituents, such as halogen or alkoxy.

Furthermore, the radical R may have a functional group. Specific examples of functional groups include the epoxide, hydroxy, ether, amino, monoalkylamino, dialkylamino, optionally substituted anilino, amide, carboxy, acrylic, acryloyloxy, methacrylic, methacryloyloxy, mercapto , Cyano, alkoxy, isocyanato, aldehyde, alkylcarbonyl, acid anhydride and phosphoric acid groups. These functional groups are bonded to the silicon atom via alkylene, alkenylene or arylene bridging groups which may be interrupted by oxygen or -NH groups. The abovementioned bridging groups and any substituents present, as in the case of the alkylamino groups, are derived, for example, from the alkyl, alkenyl or aryl radicals mentioned above or below. Of course, the radical R may also have more than one functional group. Concrete examples of not Hydrolyzable R radicals having functional groups are glycidyloxypropyl, (meth) acryloyloxypropyl, aminopropyl and 3-isocyanatopropyl.

Preferably, the surface modifier has a relatively low molecular weight. For example, the molecular weight may be less than 1,500, more preferably less than 1,000, and preferably less than 500, or less than 400 or even less than 300. Of course, this does not preclude a significantly higher molecular weight of the compounds (e.g., up to 2,000 and more).

Another group of useful surface modifiers are surfactants, e.g. cationic, anionic, nonionic and amphoteric surfactants. Nonionic surfactants are preferred, with polyethylene oxide derivatives being particularly preferred. It may be z. B. derivatives with saturated or unsaturated (mono) carboxylic acids, in particular with carboxylic acids having more than 7, preferably more than 11 carbon atoms, for. Polyethylene oxide derivatives with stearic, palmitic or oleic acid, such as those available under the trademark "Emulsogen." They may also be derivatives with sorbitan esters (sorbitan + carboxylic acid), such as those mentioned above being suitable as the carboxylic acid These products are commercially available under the trademark "Tween". Further, polyethylene oxide (mono) alkyl ethers, for example, with alcohols having more than 7, preferably more than 11 carbon atoms may be used, for. B. the products available under the brand "Brij".

Particularly preferred surface modifiers are the abovementioned carboxylic acids and carboxylic acid derivatives and their salts, in particular monocarboxylic acids and their salts, and oxacarboxylic acids and salts thereof. As counterions for the salts (carboxylates), all customary cations can be used, for example Na ⁺ , K ⁺ or NH ₄ ⁺ . NH ₄ salts are particularly preferred because the ammonium cation is pyrolyzable. Preferred surface modifiers are, for example, acetic acid and 3,6,9-trioxadecanoic acid and salts thereof, in particular their ammonium salts. These ammonium salts are pyrolyzable. As the surface modifier and / or the electrolyte, it is generally preferred to use pyrolyzable compounds. These can be removed during the calcination. Both Salts, the carboxylates act as a functional group for attachment to the surface of the particles.

Both carboxylic acids and the salts can act as an electrolyte. The carboxylic acids and the salts differ in particular in that different pH values result in the electrolyte solution. As explained below, adjusting the pH for the process may be important. It is preferred to use surface modifiers having a carboxylate group, i. Salts of carboxylic acids.

As precipitation component, it is possible to use all molecules or anions which form sparingly soluble compounds with the metal cations forming in the anodic dissolution. As a precipitating component, e.g. Hydrogen sulfides, sulfides, carbonates or phosphates, e.g. in the form of the ammonium, sodium or potassium salts. In particular, however, come as a precipitating component, water or hydroxide ions of the aqueous electrolyte solution into consideration. As known to those skilled in the art, hydroxide ions are present in water because of the dissociation equilibrium of water, the concentration of hydroxide ions depending on the pH of the solution.

In the process of the invention, the dissolving metal ions with the precipitating component, preferably H ₂ O or OH ^" , form sparingly soluble compounds, thereby forming in the solution the solid particles of the metal compound or a precursor thereof, which may be dispersed in the solution As is known to those skilled in the art, the sparingly soluble compounds are compounds which dissolve poorly in the respective medium, in this case the electrolyte solution, so that they precipitate out as a solid or form a dispersion For example compounds which do not dissolve more than about 10 g / l of the respective solvent (electrolyte solution) in the solvent at 20 ^° C. are referred to as sparingly soluble, but it is preferred for the present invention that the compounds which form are sparingly soluble are that the concentration of the forming metal ions in the electrolyte is very low and even preferably close to zero, because this with the precipitating component the form sparingly soluble particles. The concentration of dissolved metal ions in the electrolytic solution is preferably less than 10 g / L, more preferably less than 1 g / L, and even more preferably less than 10 mg / L. In the examples, the concentration of dissolved metal ions is less than 2 mg / l.

The forming particles of the metal compound or a precursor thereof may be e.g. to trade the metal sulfides, carbonates or phosphates. Preferably, however, it is the oxides and in particular the hydroxides or hydroxide oxides of the one or more metals. For this purpose, the pH of the electrolyte solution is adjusted so that precipitate the sparingly soluble or insoluble hydroxides or hydroxide oxides as particles or form a dispersion. The pH adjustment may conveniently be adjusted by the choice and amount of electrolyte and / or surface modifier.

Of course, those skilled in the art will be aware of the pH ranges required for the precipitation of the particular metal hydroxides or hydroxide oxides or may readily refer to general textbooks. In particular, the pH of the electrolyte solution is adjusted such that the metal compound or precursor of the target compounds precipitates as insoluble hydroxides and the free concentration of the corresponding metal cations in the electrolyte solution is very low (near zero).

If desired, the process can be used to prepare these hydroxides or hydroxide oxides. In a preferred embodiment, the hydroxides or hydroxide oxides obtained are used as precursors for the preparation of the corresponding metal oxides, which can be readily obtained therefrom by calcining.

The electrolyte solution contains surface modifiers having a high surface adsorption tendency on the formed particles. In order to achieve a sufficient conductivity, electrolyte salts are preferably added to the electrolyte solution as conductive salts of pyrolyzable ions which have sorption tendency and thus simultaneously serve as a surface modifier.

In the method of the present invention, the electrolytic solution contains a surface modifier, and unlike the prior art, the electrolyte is preferably used in a small amount, so that the ionic strength of the electrolytic solution is relatively small. Furthermore, the electrolyte solution is adjusted so that forming metal cations are to a certain extent intercepted and precipitated in situ. Surprisingly, this can result in surface rich, i. nanoscale metal compounds, preferably metal oxide particles (preferably after calcination) and in particular binary mixed metal oxides are obtained in high purity in a direct electrochemical manner, without causing wastewater. Without wishing to be bound by theory, the following is an attempt to explain the causes of these surprising results. This is exemplified by the formation of the hydroxides, but it applies analogously to the formation of other metal compounds.

The process produces the desired target products in particular via the anodic dissolution of correspondingly alloyed anodes in electrolyte solutions with low ionic strength in the presence of surface-modifying substances. In the anodic dissolution of the corresponding metals of the metal compounds, particles of the sparingly soluble hydroxides are formed in the electrolyte solution. The heavy or insoluble hydroxides precipitate directly in the Nernst diffusion layer in front of the anode, so that the free concentration of the corresponding metal cations in the electrolyte solution is extremely low. Since the electrolyte solution contains no free metal cations or only very low concentrations, the deposition of metal sludge or dendrites on the cathode is effectively suppressed. In contrast to other methods, therefore, a highly pure precipitate is obtained, which is free of metal sludge.

The presence of the surface modifiers results in the insoluble hydroxides formed directly in the Nernst's diffusion layer of the anode being modified in situ with the surface modifier and thus with respect to one uncontrolled particle growth can be stabilized. The stabilization of the nanoparticles thermodynamically creates the prerequisite that the products formed do not remain adsorbed on the surface of the anode and block them after a short time.

For this stabilization between the dispersed particles occurs according to the DLVO theory as a repulsive force, the electrostatic repulsion between the usually similarly charged coagulation partners in appearance. These effects are known from the literature.

This results from the potential of the electric double layer between the sol particles and the surrounding electrolytic solution and the thickness of the electric double layer. The higher the surface charge density, the greater the electrical potential at the colloid surface. The level of surface charge density of water-dispersed oxide and / or hydroxide particles is determined by the pH. It is the greater, the farther the pH is from the PZC (charge zero point).

Within the bilayer, a potential drop occurs as the distance from the colloid surface increases. The thickness of the diffused bilayer is determined by the ionic strength of the aqueous phase. It is the smaller, the larger the ionic strength, because with increasing ionic strength, the surface charges are always better shielded. Because the bilayer is effectively the protective layer of the sol particle against coagulation, the stability of the dispersion is: the greater the ionic strength within the dispersant, the smaller the thickness of the bilayer, the greater the coagulation tendency.

As a consequence of the above considerations, it is found that the concentration of the electrolytes, preferably of the surface-modifying electrolyte additives, is to be kept low in order to prevent a collapse of the stabilizing effect on the precipitation products formed in the Nernst diffusion layer due to high ionic strength. A high ionic strength in the electrolyte thus leads to agglomeration and coagulation of the particles and thus to low BET surface areas. This is confirmed experimentally in the following examples.

Therefore, if the electrolyte, e.g. Ammonium acetate, only at a low concentration (e.g., at most about 0.2 mol / l), the conductivity of the electrolyte is below that customary in engineering electrolysis. This approach differs from the usual approach to optimizing electrochemical processes, which generally seeks high conductivity to minimize IR drop of the electrolyte solution. As a consequence, in the case of the method concept of> 10 volts discussed here, a relatively high cell voltage is required, the effect of which is, however, economically insignificant.

The anodic dissolution can be carried out at any temperature. Preference is given to working at ambient temperature (eg 23 ° C) or slightly elevated temperature, such as 25 to 100 ⁰ C. If necessary, it is also possible to work at temperatures below ambient temperature. Preference is given to slightly elevated temperatures of, for example, from 25 to 80 ^° C., with particular preference being carried out at temperatures of from about 30 to 65 ^° C. in the electrolysis cell. In particular, an undivided cell is used.

The cathode may be an inert material, e.g. Stainless steel act. The anode and cathode are preferably made of identical material, i. both anode and cathode are of the metal (s) of the metal compound.

The formed surface-stabilized particles of the metal compound or a precursor thereof, in particular the metal hydroxide particles, carry surface charges via the adhering groups (eg acetate groups) and accordingly participate in the internal (ionic) current flow in the cell. This has the consequence that these amorphous particles precipitate electrophoretically on the anode and form a mucilaginous coating (see Example 3). To prevent a cover layer formation, the current flow is therefore preferably reversed cyclically. After the polarity reversal, the current flow increases rapidly in the short term and the particles migrate to the counterelectrode. In a preferred embodiment, as explained, metal compounds having at least two metals are prepared, with metal compounds comprising two metals, such as binary metal oxides, being particularly preferred. In order to obtain the desired atomic ratio between the metals in the metal compound in these binary metal compounds, there are several possibilities in the inventive method. The following explanations apply mutatis mutandis to the production of metal compounds with three or more metals.

In the electrochemical process of the invention, separate anodes of the respective metals may be provided for the preparation of metal compounds having two metals. In this case, the desired atomic ratio of the metals in the metal compound can be adjusted via the ratio of the anode currents. Another possibility is to introduce the two metals as separate electrodes and to periodically umupupolen, wherein the atomic ratio of the metals in the metal compound over the ratio of the poling period is set anodic / cathodic. Another possibility is that the two metals are presented as pre-alloyed anodes, wherein the atomic ratio of the metals in the metal compound is adjusted by the ratio of the metals in the alloy of the anode.

In the process according to the invention, particles of the metal compound or a precursor thereof are obtained. A precursor of the metal compound are those metal compounds which are not the desired target product and can be converted by calcination into the desired metal compound. In the preferred embodiment, particles of known metal hydroxides or metal hydroxide oxides, optionally hydrated, are obtained. Preferably, these compounds are used as precursors for the preparation of the desired target compound, namely the corresponding metal oxides, by calcination. The conversion of the hydroxides into the metal oxides by calcination is well known to those skilled in the art. In the process of the invention, surface-modified particles of the metal compounds or a precursor thereof are formed, which are often amorphous. These are present in the electrolyte solution as a dispersion or precipitate in particular as a precipitate. The particles are preferably separated off from the electrolyte solution by customary separation techniques (eg sedimentation, centrifuging and / or filtration / sieving) and dried. After the drying process, the surface modifier usually remains on the surface of the particles, but it may also be removed. If the resulting particles are not the desired target compound, they can be converted into them by calcination.

The person skilled in the art is well aware of the calcination technique. In this case, a substance is subjected to a heat treatment, wherein a conversion reaction takes place. This may be, for example, the removal of water of hydration, the partial or complete crystallization, the conversion to another modification (eg another crystal form) or a decomposition reaction. The calcination may optionally be carried out in an inert, an oxidizing and / or preferably a reducing atmosphere. A reduction or oxidation can also be carried out after a calcination. With the help of the reducing or oxidizing atmosphere, for example, the oxidation states of the metals can be adjusted as needed. Suitable temperatures for the calcination depend on the starting material and the desired conversion and are familiar to the person skilled in the art. Frequently, temperatures above 100, above 150 or above 200 ^° C. or even above 250 ^° C. are used. The calcination of the indium tin oxide / hydroxides to ITO is known and can be carried out, for example, at temperatures between 200 and 450 ⁰ C, preferably at least partially in a reducing atmosphere. Further details on the calcination of ITO and the properties of ITO are described, for example, in WO 00/14017.

The obtained particles are preferably subjected to calcination. In the preferred embodiment, the resulting optionally hydrated hydroxide or oxide hydroxide particles are converted into the corresponding metal oxide Particles transferred. Typically, calcination also decomposes or pyrolyzes the surface modifiers on the surface so that non-surface modified particles are obtained after calcination.

Since the surface modification is applied in the electrolysis with the same, the BET surface area of the particles is very high and is retained even during subsequent calcination. Nanoscale metal compounds can therefore unexpectedly be obtained by the process according to the invention, in particular particles having a particle size of not more than 100 nm and even significantly less, eg not more than 50 nm. The products calcined at 400 ^° C. show, for example, 80 to 150 m ² / g BET surface (nitrogen) a significantly higher surface than comparable particles according to the previously known methods. TEM images show that the particle size of the particles produced in the examples is even at 5 to 10 nm.

The electrolysis can be carried out batchwise or preferably continuously. In Fig. 1, an example of an arrangement is given. 1 is an undivided electrolytic cell with an inlet and a drain. The electrolyte reservoir and settling tank 4 contains the electrolyte solution. The electrolyte solution is passed through the pump 2 in the inlet of the electrolytic cell 1, wherein it is brought by means of a heat exchanger to the desired temperature. From the drain of the electrolytic cell 1, the electrolyte solution, which is charged by the electrolysis with the produced particles, flows back into the container 4, in which precipitate the particles. In this way, the electrolyte solution of the cell 1 is pumped via the settling vessel 4 in the circuit. With suitable flow guidance, the formed amorphous precipitation products settle in a simple manner at the bottom of the buffer vessel (hydro-cyclone principle) and can thus be continuously removed from the circulation. Fresh electrolyte solution or replenishers may be introduced at any location, e.g. in the container 4.

In the preferred method according to the invention, the metals, for example indium and tin metal, are separated or connected as an alloy as an anode. They can also be used as a cathode or it becomes an inert cathode used. In the preferably undivided electrolysis cell comprising the electrolyte solution, the anodic dissolution of the one or more metals then takes place. The forming surface-modified particles, which are often amorphous, are mechanically separated batchwise or preferably continuously, for example as a precipitate. The separated electrolyte solution is optionally recycled after filling in the electrolysis cell. The precipitate is dried and then preferably calcined, preferably in an oxidizing and / or reducing atmosphere, to obtain the desired product, eg a metal oxide, in particular an ITO powder.

A divided electrolysis cell has a negative effect on the cell potential and also requires an increased design effort. The comparison of the methods described in Examples 1 and 2 shows the disadvantages of the divided cell. The current density goes back there when using an anion exchange membrane under otherwise the same conditions by more than an order of magnitude. A reversed polarity of the divided cell is not possible if the cathode and anode compartment must remain separate. This results in the formation of layers on the anode with the consequences of contamination of the product by coarse particulate detachments and passivation of the anode. This results in a decrease in the current or increase in the cell potential. Therefore, undivided electrolysis cells are preferred.

The manufactured particles are u.a. used to produce transparent conductive films (displays and EMI applications) as well as refractive index modification in films.

The invention is explained in more detail by the following examples, without limiting them.

Example 1: (Comparison) Experiment with organic acid in undivided cell

In an aqueous solution containing 5 g / l of acetic acid, an In-Sn alloy alloy having a tin content of 7% by weight and an area of 3 cm ^{2 was} electrolytically dissolved. For this purpose, a voltage of 40 V was applied to the anode and a cathode made of stainless steel. The resulting current was about 100 mA, which was one Current density of about 30 mA / cm ² corresponds. The electrolysis cell consisted of an undivided, stirred vessel. The pH of the solution was 3. After 60 minutes of the test, a clear metallic dendrite growth at the cathode as well as a gray turbidity of the solution due to metallic particles which had separated from the cathode could be recognized.

Example 2: (Comparison) Experiment with organic acid in a divided cell

In an aqueous solution (pH 3) containing 5 g / l of acetic acid, an In-Sn alloy alloy having a tin content of 7 wt% and an area of 47 cm ^{2 was} electrolytically dissolved. The electrolysis cell was divided by an anion exchange membrane (thickness about 60 μm, type fumasep FAP) into the anode and cathode compartments. A voltage of 40 V was applied between the anode and the stainless steel cathode. The resulting current was initially 160 mA and dropped over the test period from 100 min to about 70 mA. On average, this corresponds to a current density of 2 mA / cm ² . The solution in the anode compartment remained clear, dendrite growth at the cathode remained off. However, despite the high cell voltage of 40 V, the current density was still significantly below that of a technically relevant method.

Example 3 Experiment with Neutral Conducting Salt in Undivided Cell

In an aqueous solution containing 0.375 g / L of ammonium acetate, an In-Sn alloy alloy having a tin content of 7% by weight and an area of 40 cm ^{2 was} electrolytically dissolved. For this purpose, a voltage of 40 V was applied between the described anode and the cathode (stainless steel cathode). The resulting current was initially about 1,000 mA and thus corresponded to a current density of about 25 mA / cm ² . The electrolysis cell consisted of an undivided, stirred vessel. The pH of the solution was 7, the temperature at 50 ⁰ C. After about half an hour of experimentation, the growth of a gray, mechanically easily peelable layer was observed on the anode. At the same time, the current dropped to about half the initial value. The conductivity was 0.7 mS / cm at the beginning and dropped to 0.3 mS / cm in the course of the experiment. Example 4 Experiment with neutral conducting salt in undivided cell and change of polarity

In an aqueous solution containing 0.75 g / l of ammonium acetate, two identical In-Sn alloy plate electrodes having a tin content of 5% by weight were electrolytically dissolved. The conductivity of the solution was comparatively low and was 1 mS / cm at 35 ° C at a pH of 7. At an applied voltage of 40 V, the polarity of the cell was reversed every 2 minutes. The electrode area was 125 cm ² and the mean flowing current 2.2 A with an electrode spacing of 18 mm. The structural design of the experiment is shown in Fig. 1. The electrolyte was pumped continuously and the electrolytically formed primary product was separated in a settling vessel designed as a hydrocyclone. The temperature of the electrolysis bath was kept constant via a heat exchanger. The conductivity remained constant over the duration of the experiment.

After a test period of 5.5 hours (Q = 12.6 Ah), the system was shut down. The electrolyte solution in the circulation of the cell was drained into the settling tank. After about 5 minutes the supernatant solution was clear and visually solids free. In the decanted electrolyte solution, only a very low concentration of indium and tin ions was found analytically (1.2 mg / l In and 0.04 mg / l Sn).

The separated by decantation, snow-white, flaky primary product was removed from the settling tank and dried at a temperature of 95 ⁰ C. The resulting white solid was then calcined at 400 ^° C. for 2 hours. BET analysis of the yellow ITO powder thus obtained revealed a specific surface area of 107 m ² / g. The chemical analysis of the ITO powder (with ICP-AES) showed that the tin content of the powder at 4.8% by weight within the measurement accuracy corresponded to that of the electrodes used. Obviously, in the case of the anodic dissolution of the prealloyed electrode, the two metals are dissolved out in the ratio present there and incorporated into the primary product which forms directly in the phase boundary in front of the electrode. Crystallographic analysis of the calcined solid by XRD revealed a bixbyite-type crystal structure. The current efficiency was 95% based on the anodically formed metal cations (In ³⁺ + Sn ⁴⁺ ).

Example 5: (Comparison) Experiment with alkaline electrolyte in undivided cell

In an aqueous solution containing 50 g / l of ammonium hydroxide, an In-Sn alloy alloy having a tin content of 7% by weight and an area of 3 cm ^{2 was} electrolytically dissolved. For this purpose, a voltage of 40 V was applied between the described anode and the stainless steel cathode used. The initially measured current of 400 mA collapsed after a few minutes to a few μA. This current drop was accompanied by a blackening of the anode surface. Particulate formation in the solution was not observed.

Example 6:

In aqueous solutions containing 2 and 5 and 8 g / L of ammonium acetate, respectively, two identical In-Sn alloyed plate electrodes, with a 5 wt.% Tin content, were electrolytically dissolved. The pH was 7. At an applied voltage of 40 V, the polarity of the cell was reversed every 2 minutes. The electrode area was 25 cm ² with an electrode spacing of 18 mm. The experimental setup was similar to that of Example 4. The conductivities of the solutions were 4.5, 6.6 and 11 mS / cm and the resulting average current densities 120, 270 and 370 mA / cm ² . The ITO powders obtained after calcination had specific surface areas of about 100 m ² / g, but differed in part qualitatively from those obtained in example 4. It was found that the color of the powders at high ammonium acetate concentrations deviates from the yellow color obtained in Example 4 and a transition to greyish ocher dyeing occurs.

Example 7:

The procedure was as in Example 4, but with reduced current density. In an aqueous solution containing 4 g / L of ammonium acetate, two identical In-Sn alloy plate electrodes having a tin content of 5% by weight were electrolytically dissolved. The pH was 7. At an applied voltage of 15V the polarity of the cell is reversed every 2 minutes. The electrode area was 125 cm ² with an electrode spacing of 18 mm. The experimental setup was the same as in Example 4. The conductivity of the solutions was 4 mS / cm and the resulting average current densities were 24 imA / cm ² . The ITO powder obtained after calcination had a BET specific surface area of 141 m ² / g. The color of the powder was ocher with a gray background.

Example 8: Reduction of the product obtained according to Example 4

The calcined product obtained according to Example 4 was reduced at 210 ° C in a stream of hydrogen (2 l / min). The color of the Festoffes changed within 15 min from yellow to green to green / blue. This corresponds to the known reduction reaction of the doped ITO in which free electrons are generated in the solid. A comparison product obtained from the conventional precipitation via InCb with subsequent washing and calcining shows no color change under these conditions. Its reduction started only at 25O ⁰ C and thus needed a 4O ⁰ C higher reduction temperature. This demonstrates that in the process according to the invention the incorporation of the Sn ⁴⁺ obviously takes place very homogeneously in the bixbyite lattice.

Example 9: ATO, experiment with neutral conducting salt in undivided cell

In analogy to Example 3, an antimony-doped tin oxide was prepared by the novel process.

In an aqueous solution containing 0.75 g / l of ammonium acetate, an alloyed Sb-Sn anode having an antimony content of 5 wt% and an area of 40 cm ^{2 was} electrolytically dissolved. For this purpose, a voltage of 40 V was applied between the described anode and a cathode made of stainless steel. The resulting current was initially about 1,230 mA, which corresponded to a current density of about 30 mA / cm ² . The electrolysis cell consisted of an undivided, stirred vessel. The pH of the solution was 7, the temperature at 60 ⁰ C. The electrolyte solution formed a gray, flaky primary product which, after drying and calcination, was an ocher-colored solid. The BET surface area of the calcined product was 100 mm ² / g. Crystallographic analysis of the calcined solid by XRD revealed a cassiterite-type crystal structure as expected for ATO.

Example 10: ATO

In an aqueous solution containing 0.75 g / l of ammonium acetate, two identical antimony tin alloy plate electrodes having an antimony content of 5% by weight were electrolytically dissolved. The conductivity of the solution was comparatively low and was 1 mS / cm at 35 ° C and a pH of 7. At an applied voltage of 40 V, the polarity of the cell was reversed every 2 minutes. The electrode area was 125 cm ² and the mean flowing current 3.4 A with an electrode spacing of 18 mm. The structural design of the experiment is shown in Fig. 1. The electrolyte was pumped continuously and the electrolytically formed primary product separated in a settling tank designed as a hydrocyclone. The temperature of the electrolysis bath was kept constant via a heat exchanger. The conductivity remained constant.

After a test period of 6.2 h (Q = 21 Ah), the system was shut down. The electrolyte in the circulation of the cell was drained into the settling tank. After about 60 minutes, the supernatant solution was clear and visually solids-free. The separated by decantation gray flaky primary product was removed from the settling tank and dried at a temperature of 95 ° C. The resulting gray solid was then calcined at 400 ^° C. for 2 hours. The calcined end product was gray.

Example 11: Iron oxide, experiment with neutral conducting salt in undivided cell

In analogy to Example 3 nanoscale iron oxide was prepared by the novel process. In an aqueous solution containing 0.375 g / l of ammonium acetate, a pure iron anode having an area of 60 cm ^{2 was} electrolytically dissolved. For this purpose, a voltage of 40 V was applied to the described anode and a cathode made of stainless steel. The resulting current was about 1100 mA, which corresponded to a current density of about 18 mA / cm ² . The electrolysis cell consisted of an undivided, stirred vessel. The pH of the solution was 7, the temperature at 60 ⁰ C.

A red-brown flaky primary product formed in the electrolyte after drying and calcining as a red-brown solid (maghemite and hematite phases).

Example 12

By way of example, the relationship between ionic strength and particle size of the particles obtained was investigated, with ammonium acetate being used as conductive salt in accordance with the above examples. The results are shown in FIG. Accordingly, the BET surface area increases with decreasing ion concentration in the electrolyte. With a high conductivity (to minimize the cell voltage) coarser powders are therefore obtained.

Claims

An electrochemical process for producing nanoscale particles from a metal compound, comprising the anodic dissolution of one or more metals in an electrolytic cell, wherein the electrolytic cell of the electrolytic cell has a precipitating component which forms a sparingly soluble compound with the forming metal ions, the metal compound or a precursor thereof, and a surface modifier for particles of the metal compound or a precursor thereof, to form surface-modified particles of the metal compound or a precursor thereof, optionally separating the formed particles from the electrolytic solution and optionally calcining the formed particles. 2. An electrochemical process according to claim 1, characterized in that the metal compound is an oxide, hydroxide or oxide hydroxide or a hydrate thereof. 3. An electrochemical process according to claim 1 or 2, characterized in that the metal compound comprises one, two or more metals, which is preferably a metal compound with 2 metals or a doped metal compound. 4. An electrochemical process according to claim 2 or 3, characterized in that the metal compound is a metal oxide which is optionally doped, in particular iron oxide, indium tin oxide (ITO) or antimony tin oxide (ATO), ZnO, SnO ₂ , Al ₂ O ₃ , Al ₂ O ₃ -doped ZnO and In ₂ O ₃ . 5. An electrochemical process according to any one of claims 1 to 4, characterized in that the surface modifier is a mono- or polycarboxylic acid or a salt thereof, an oxacarboxylic acid or a salt thereof, a hydroxycarboxylic acid or a salt thereof, a diketone, a An amino acid or a salt thereof, an amine or a salt thereof or an acid amide or a mixture of two or more of these components. 6. An electrochemical process according to claim 5, characterized in that the surface modifier is a monocarboxylic acid, the salt of a monocarboxylic acid, an oxacarboxylic acid, the salt of an oxacarbonic acid, a mono-, di- or polyhydroxycarboxylic acid or a salt thereof. 7. An electrochemical process according to any one of claims 1 to 6, characterized in that the electrolyte solution is an aqueous electrolyte solution. 8. An electrochemical process according to any one of claims 1 to 7, characterized in that the formed sparingly soluble compound is an optionally hydrated metal hydroxide or metal oxide hydroxide. 9. An electrochemical process according to any one of claims 1 to 8, characterized in that water or hydroxide ions serve as a precipitating component. 10. An electrochemical process according to any one of claims 1 to 9, characterized in that the concentration of the precipitating component is so large that the concentration of dissolved metal ions in the electrolyte solution is not greater than 10 g / l and preferably less than 1 g / l , 11. An electrochemical process according to any one of claims 1 to 10, characterized in that the concentration of the precipitating component is adjusted by adjusting the pH. 12. An electrochemical process according to any one of claims 1 to 11, characterized in that the electrolyte solution has an electrolyte conductivity of not more than 30 mS / cm and preferably less than 10 mS / cm. 13. An electrochemical process according to any one of claims 1 to 12, characterized in that the ion concentration in the electrolyte solution is less than 0.4 mol / l. 14. An electrochemical process according to any one of claims 1 to 13, characterized in that the electrolysis cell is an undivided electrolysis cell. 15. An electrochemical process according to any one of claims 1 to 14, characterized in that the surface modifier is simultaneously the conductive salt. 16. An electrochemical process according to any one of claims 1 to 15, characterized in that in the electrolysis periodic polarity reversal of anode and cathode is carried out, wherein the anode and cathode preferably consist of the same material. 17. An electrochemical process according to any one of claims 1 to 16, characterized in that the metal compound comprises two or more metals, which is preferably a binary metal oxide, and the two or more underlying metals are presented as separate anodes. 18. An electrochemical process according to claim 17, characterized in that the atomic ratio of the metals in the metal compound is adjusted via the ratio of the anode currents. 19. An electrochemical process according to any one of claims 1 to 16, characterized in that the metal compound comprises two metals, which is preferably a binary metal oxide, and the two metals are presented as separate electrodes and periodically reversed, the atomic ratio of the metals is adjusted in the metal compound via the ratio of the poling period anodic / cathodic. 20. An electrochemical process according to any one of claims 1 to 16, characterized in that the metal compound comprises two or more metals, it is preferably a binary metal oxide, and the two or more metals are presented as pre-alloyed anodes, wherein the atomic ratio of Metals in the metal compound on the ratio of metals in the alloy of the anode is set. Electrochemical process according to any one of Claims 1 to 18 and 20, characterized in that the cathode is an inert material, e.g. Stainless steel, is used. 22. An electrochemical process according to any one of claims 1 to 21, characterized in that the particles formed are separated from the electrolyte solution and then calcined. 23. An electrochemical process according to any one of claims 1 to 22, characterized in that the calcination is carried out for the conversion of hydroxides or oxide hydroxides into oxides, for the removal of water of hydration and / or for the partial or complete crystallization of the metal compound. 24. An electrochemical process according to any one of claims 1 to 23, characterized in that the calcination is carried out in an inert, oxidizing or reducing atmosphere. 25. An electrochemical process according to any one of claims 1 to 24, characterized in that the particles formed in the electrolyte solution give a dispersion or a precipitate.