AN ELECTROCHEMICAL PROCESS FOR PRODUCING NANOPARTICLESOF CUPRATE HYDROXYCHLORIDES

Abstract

 The present invention relates to an electrochemical process for producing nanoparticles of mixed copper hydroxide-chloride compounds responding to the chemical formula M_xCu_4-x(OH)_yCl_z wherein M is one or more metal cations from the group comprising a divalent earth alkali metal cation, a divalent transition metal cation or a trivalent transition metal cation, and wherein 0 ≤ x ≤ 1, 5.5 ≤ y ≤ 6.5 and 1.5 ≤ z ≤ 3, the method comprising the steps of
(1) supplying to a cathode compartment of an electrochemical cell, wherein the cathode compartment comprises a catholyte and is equipped with a cathode comprising a gas diffusion electrode with a porous electrochemically active material, a liquid water based mixture containing dissolved therein Cl^- ions, at least one precursor salt containing the one or more metal cations M, and at least one Cu²⁺ precursor salt, wherein the ratio of the concentration of Cu²⁺ to M is smaller than 10:1,
(2) adjusting the pH of the reaction mixture to a value between 2.0 and 6.0,
(3) supplying an O₂ containing oxidant gas to the gas diffusion electrode,
(4) subjecting the cathode to an electrochemical potential which is below the thermodynamic limit of O₂ reduction at the pH of the reaction mixture,
(5) applying a potential to the gas diffusion electrode to cause reduction of the O₂ contained in the oxidant gas to one or more of the corresponding peroxide, OH^-, ionic and/or radical reactive O containing species,
and isolating nanoparticles of M_xCu_4-x(OH)_yCl_z.

Description

[0001] The present invention relates to an electrochemical process for producing nanoparticles of mixed copper hydroxide-chloride compounds of the type M_xCu_4-x(OH)_yCl_z, according to the preamble of the first claim.

[0002] The present invention also relates to nanoparticles of mixed copper hydroxide-chloride compounds of the type M_xCu_4-x(OH)_yCl_z, and to applications for these nanoparticles.

Background art.

[0003] The synthesis of materials exhibiting spintronic properties is at the forefront pursuit of novel magnetic ground states (i.e., quantum magnets), which were first realized by herbertsmithite¹ ZnCu₃(OH)₆Cl₂. Due to its characteristics as a spin-liquid, herbertsmithite may have applications in quantum computing devices, i.e. for storage and memory purposes^2,3. However, before these materials can be integrated into such technologies, the congruent synthesis of herbertsmithite remains a major challenge to be overcome.

[0004] Materials such as herbertsmithite, clinoatacamite, and paratacamite show interesting properties for quantum magnets, due to their effects of frustration. It has been experimentally proven for many materials that micro- and nano-scale dimensioned particles have superior or different properties over their macro-scale counterparts, which is also expected for Zn_xCu_4-x(OH)₆Cl₂. However, nanoparticles with these compositions are not realized until today, although the magnetic properties of spin transition materials may vary with size, and the effects at the nanoscale are particularly unknown.

[0005] A major synthesis problem is the production of sizeable amounts at relevant rates, with controllable physicochemical features. Up to now, only a limited number of synthesis routes of herbertsmithite have been identified. The main shortcomings of these methods are that they provide macroscale herbertsmithite particles (e.g., mm-range), and only pertaining condensed materials, whereas stable dispersions and nanoparticles may also display unique properties. The first method reported for synthesizing crystalline Zn_xCu_4-x(OH)₆Cl₂ (in which x=1 for herbertsmithite, x=0 for clinoatacamite and 0.3<x<1 for paratacamite) was disclosed in 2012. Herbertsmithite was originally found in nature only in 2004⁴.

[0006] Polymorphs of ZnCu₃(OH)₆Cl₂ are typically synthesized by hydrothermal, or solvothermal methods (e.g., 458 K - 473 K). Related cuprate compounds of the form MCu₃(OH)₆Cl₂ with divalent cations (i.e., M= Mg²⁺, Co²⁺, Fe²⁺, Mn²⁺ and Ni²⁺)⁵ and with trivalent cations, MCu₃(OH)₆Cl₃ (M= Y³⁺ and lanthanides (Nd³⁺ and Sm³⁺)^6,7, have been achieved by unconventional solid-state reactions at higher temperature (e.g., 463 K). To date, these methods have been limited to very low production rates. The motivation to achieve these synthesis routes was that in herbertsmithite, copper ions are arranged on triangular grids known as the kagomé lattice where, despite the existence of strong exchange interactions, spins do not order down to the lowest measured temperature⁸.

[0007] Besides the aforementioned synthesis approaches, an ionothermal method, originally designed for the fabrication of new zeolitic solids, has been described for the preparation of materials with kagomé lattices¹². However, the synthesis rates are rather slow (weeks- to months-scale). As an example, it took 10 months to grow a 1 mm monocrystal of this material in a complicated hydrothermal reactor^10,13.

[0008] There is thus a need to an economically feasible process for synthesizing nano particles of herbertsmithite. Although there is one work presenting the formation of nanoscale clinoatacamite by hydrothermal synthesis at 363 K - 368 K, this process requires the addition of a toxic organic buffer i.e., 2-(N-morpholino) ethanesulfonic acid¹⁴ and it does not evidence the control of the material properties (e.g., crystallite size, stoichiometry, layer spacing, etc.). Moreover, no equivalent methods are available for producing nanoscale herbertsmithite. Green synthesis routes are preferred instead.

[0009] The present invention therefore aims at providing an economically feasible process for the production of nano particles of crystalline M_xCu_4-x(OH)_yCl_z, with controllable physicochemical properties, wherein 0 ≤ x ≤ 1, 5.5 ≤ y ≤ 6.5 and 1.5 ≤ z ≤ 3.

[0010] This is achieved according to the present invention with a process which shows the technical features of the characterizing portion of the first claim.

[0011] Thereto the present invention relates to electrochemical process for producing nanoparticles of mixed copper hydroxide-chloride compounds responding to the chemical formula M_xCu_4-x(OH)_yCl_z wherein M is one or more metal cations from the group comprising a divalent earth alkali metal cation, a divalent transition metal cation or a trivalent transition metal cation, and wherein 0 ≤ x ≤ 1, 5.5 ≤ y ≤ 6.5 and 1.5 ≤ z ≤ 3, the method comprising the steps of

(1) supplying to a cathode compartment of an electrochemical cell, wherein the cathode compartment comprises a catholyte and is equipped with a cathode comprising a gas diffusion electrode with a porous electrochemically active material, a liquid water based mixture containing dissolved therein Cl^- ions, at least one precursor salt containing the one or more metal cations M, and at least one Cu²⁺ precursor salt, wherein the ratio of the concentration of Cu²⁺ to M is smaller than 10:1,

(2) adjusting the pH of the reaction mixture to a value between 2.0 and 6.0,

(3) supplying an O₂ containing oxidant gas to the gas diffusion electrode,

(4) subjecting the cathode to an electrochemical potential which is below the thermodynamic limit of O₂ reduction at the pH of the reaction mixture,

(5) applying a potential to the gas diffusion electrode to cause reduction of the O₂ contained in the oxidant gas to one or more of the corresponding peroxide, OH^-, ionic and/or radical reactive O containing species,

and isolating nanoparticles of M_xCu_4-x(OH)_yCl_z

[0012] Up to now, no electrochemical methods for the synthesis of such compounds have been described.

[0013] According to a preferred embodiment the concentration of the at least one M containing precursor salt in the catholyte is maximum 10 mmole/l, preferably maximum 2mmole/l.

[0014] According to a further preferred embodiment, the Cu²⁺ precursor salt in the catholyte is maximum 8mmole/l.

[0015] According to a still further preferred embodiment, the ratio of the concentration of Cu²⁺ to M is maximum 7:1, more preferably maximum 5:1, most preferably maximum 4:1. Most preferably however, where M is Zn²⁺ or Ln²⁺ the ratio of the concentration of Cu²⁺ to Zn²⁺ or Cu²⁺ to Ln²⁺ is maximum 4:1 and minimum 3:1.

[0016] According to a first preferred embodiment, M is M²⁺ and is one or more divalent metal cations selected from the group comprising Zn²⁺, Mg²⁺, Co²⁺, Fe²⁺, Mn²⁺ , Ni²⁺, Pd²⁺, Sm²⁺, Eu²⁺ or mixtures thereof, preferably one or more divalent metal cations selected from the group comprising Mg²⁺, Co²⁺, Zn²⁺ or mixtures thereof. According to another preferred embodiment, M is one or more trivalent metal cations selected from the group comprising Y³⁺, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺, Rh³⁺ or a mixture thereof, preferably one or more trivalent metal cations selected from the group comprising Y³⁺, La³⁺, Nd³⁺ , Sm³⁺ or a mixture thereof. It is remarked that suitable trivalent metal cations include other metal cations which may either be di- or trivalent such as for example Co³⁺ and Mn³⁺, etc..

[0017] Where use is made of Zn²⁺, the end product may preferably comprise crystalline nanoparticles which respond to chemical formula Zn_xCu_4-x(OH)₆Cl₂ wherein 0 ≤ x ≤ 1. In a further preferred embodiment, the end product may preferably contain monocrystalline nanoparticles which respond to chemical formula Zn_xCu_4-x(OH)₆Cl₂.

[0018] The process of this invention permits producing nano particles of a spin transition material, which may be crystalline and which respond to the general formula M_xCu_4-x(OH)_yCl_z, wherein x may be made to vary between 0 and 1, y may be made to vary between 5.5 and 6.5 and z may be made to vary between 1.5 and 3, by varying the reaction conditions, a.o. by varying the concentration of the metal cation containing precursor salt, the Cu²⁺ precursor salt and the [Cu²⁺]/[M²⁺] or the the [Cu²⁺]/[M³⁺] ratio in the catholyte. In a preferred embodiment where M²⁺ is Zn²⁺, Zn_xCu_4-x(OH)₆Cl₂ may be obtained. Expressed differently, the process of the present invention permits M_xCu_4-x(OH)_yCl_z, in particular Zn_xCu_4-x(OH)₆Cl₂ with a desired stoichiometry. This is important as the stoichiometry determines the saturation magnetization of the product as well as other magnetic properties such as the spin-liquid character. For example, by varying the concentration of M and Cu²⁺ and the [Cu²⁺]/[M] ratio, in particular the concentration of Zn²⁺ and Cu²⁺ and the [Cu²⁺]/[Zn²⁺] ratio supplied to the catholyte, M_xCu_4-x(OH)_yCl_z compounds in particular Zn_xCu_4-x(OH)₆Cl₂ compounds with a desired value of x, y and z can be produced, wherein x may range between 0 and 1 and may take any value between 0 and 1, y may be take any valye between 5.5 and 6.5 and z may take any value between 1.5 and 3. For example, Zn_xCu_4-x(OH)₆Cl₂ with x = 0.3 can be typically produced by supplying to the reaction mixture a relatively low concentration of [Cu²⁺] of 7.3 mM and [Zn²⁺] of 2.3 mM. The skilled person will be capable of identifying the individual concentrations of [Cu²⁺] and [M], in particular of [Cu²⁺] and [Zn²⁺] and the ratio in which they are used to tailor the chemical composition and stoichiometry of the end product to be obtained.

[0019] The nano particles size that may be obtained with the process of this invention may be crystalline and have been found to have an average size of between 1.0 and 30 nm, often between 10 and 16 nm. With nano particles is meant within the scope of this invention, particles with an average particle size below 100 nm, and more specifically between 1.0 and 30 nm. To determine the average particle size, use is made of transmission electron microscopy (TEP) using a JEOL JEM 2100FII operated at 200 keV. For the observation of the sample in the microscope, the particles were dispersed in butanol and a drop of the suspension was placed onto a copper grid covered by a carbon film. The mean particle size and distribution were evaluated by counting more than 100 particles by means of Digital Micrograph™. After that, data were fitted to a Normal distribution to obtain the mean particle size (dNP) and standard deviation (σ). As the particles are monocrystalline, the mean particle size was confirmed by XRD, which was also used to confirm the crystallographic structure of the nanoparticles.

[0020] The process of this invention may be carried out in one single reactor, i.e., an electrochemical cell, and is assumed to proceed in one step at the three phase junction between the porous gas-diffusion cathode, the electrolyte and the oxidant gas. In the process of this invention gas-diffusion electro crystallization (GDEx) of soluble, in particular water soluble salts of copper and other metal ions as precursor salts or reactants is carried out, by subjecting the reactants to reactive precipitation with intermediaries issued from the oxygen reduction reaction (ORR) at the gas-diffusion cathode.

[0021] The process of this invention presents the advantage that it may be carried out at moderate temperature, in particular at room temperature and at atmospheric pressure, and presents the advantage that formation of economically feasible amounts of nano particles may be observed within a relatively short period of time i.e., within a few minutes up to a few hours to obtain completion of the process, relative to the amount targeted.

[0022] In the process of this invention the pH of the catholyte is preferably adjusted to a value between 2.0 and 6.0, preferably between 2.0 and 5.0, more preferably between 2.5 and 3.5 to achieve a sufficiently high yield of precipitated particles. If so desired, to keep the pH within the desired limits as described above, an amount of a weak protonic electrolyte may be supplied to the catholyte. Addition of the weak protonic electrolyte may not only increase the conductivity of the catholyte, but that it may also increase the current density over the cathode. Moreover, the presence of the weak protonic electrolyte has the effect that variations in the pH of the catholyte in the course of the oxidation reaction may be reduced to a minimum, to minimize the risk to the occurrence of unwanted side reactions. Within the indicated pH ranges, the pH is smaller than the pH range within which a relative predominance exists of the precursor salts in the ionic form. Within the indicated pH ranges, the pH is namely smaller than the pKa of the precursor salts.

[0023] Without wanting to be bound by this theory, the inventors believe that the oxygen present in the oxidant gas, is electrochemically reduced at the active porous carbon layer of the gas-diffusion cathode to form a.o. OH^- (see Figure IE). The products obtained by the reduction of oxygen, profusely available at the electrochemical interface, react with the metals ions in solution (e.g. Cu²⁺, Zn²⁺), which are transported to the hydrophilic porous carbon on the cathode, via the electrolyte. When these metal ions meet the oxygen reduction reaction products or the highly reactive intermediaries, supersaturation is reached, which in turn leads to nucleation of e.g., hydroxides or oxides. Additive OH- concentration and supersaturation keep ongoing, thus secondary nucleation and crystal growth proceed during the transient period of residence of the primary nuclei formed within the cathodic interface.

[0024] The method of this invention may be carried out in a water based catholyte which only contains water as the liquid phase or water in combination with a solvent. Although the use of water is preferred in view of minimizing toxicity of the end product, the method of this invention may however also be carried out in a catholyte which contains a mixture of water and one or more organic solvents, or in an aprotic organic solvent or a mixture of two or more aprotic organic solvents. Suitable organic solvents include dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate, and acetonitrile, or their equivalents known to the skilled person. When use is made of such organic solvents, to ensure a sufficient conductivity, the solvent may contain a supporting electrolyte, for example tetrabutylammonium chloride (TBAC), or tetrabutylammonium bromide (TBAB). The skilled person will be capable of selecting the most appropriate solvent and the amount of solvent used, taking into account a.o. the solubility of the Cu²⁺ precursor salt and the precursor salt comprising the di- and/or trivalent metal cation therein, the ability of the end product to precipitate therein, the particle size to be achieved and the envisaged application of the nanoparticles. An appropriate selection of the solvent will permit controlling the dimensions of the average particle size of the nanoparticles and their dispersibility. Therefore, water, a polar solvent or a mixture hereof may generally be used when the formation of larger nanoparticles is envisaged.

[0025] In order to ensure that the ionic conductivity of the catholyte is sufficiently high in the course of the process and that the electrochemical conversion proceeds sufficiently fast and/or if it is envisaged to permit keeping the average size of the particles small and limit particle aggregation, the catholyte may contain a supporting electrolyte. The supporting electrolyte is preferably added in a concentration of between 5.0 and 150.0 g/l of catholyte, preferably between 10.0 and 100 g/l, more preferably between 10.0 and 50.0 g/l. The presence of the supporting electrolyte will permit to control variations in the conductivity of the reaction mixture as a result of the conversion of the reactant precursor salts into the desired end product, and therewith limit the risk to slowing down of the reaction or the formation of end products with an unwanted stoichiometry. The use of these concentrations of supporting electrolyte will in general result in a catholyte with and an ionic conductivity of at least 1.0 mS/cm, preferably at least 10 mS/cm. The inventors have moreover observed that by varying the amount of supporting electrolyte supplied to the reaction mixture, not only the size of the nano particles formed may be varied and controlled but also their dimensions, with lower concentrations of supporting electrolyte giving rise to the formation of nano particles with a larger average particle size.

[0026] Supporting electrolytes suitable for use with this invention are generally known to the skilled person and they include aqueous solutions of one or more soluble salts, for example soluble Na, K or Mg salts, but many other salts may be used as well as long as the cations do not interfere in the reactions involved in the process of this invention. The supporting electrolyte contains a halogenide salt, but may contain a supporting electrolyte for example a carbonate, a sulfate, a nitrate, a perchlorate or phosphate, or be based on any other suitable anion, and mixtures of the afore mentions supporting electrolytes may be used as well. Particularly suitable supporting electrolytes are aqueous chloride salt based solutions, more in particular an aqueous solution comprising NaCl as such to minimize the risk that the reaction product would contain other counter ions than OH^- and Cl^-. In the method of this invention namely, the chloride ion is incorporated in the M_xCu_4-x(OH)_yCl_z nanoparticles of this invention, and incorporation of other anions is to be minimized or even avoided.

[0027] The presence of the supporting electrolyte will permit to maintain the ionic conductivity of the catholyte at a sufficiently high level in the course of the reaction, to have the electrochemical conversion proceeding sufficiently fast, to keep the averageparticle size small and limit aggregation to larger particles within a desied extent. To that end, preferably use is made of a catholyte with and an ionic conductivity of at least 1.0 mS/cm, preferably at least 10 mS/cm. Maintaining of the conductivity at a sufficiently high level may be of particular importance when the process of this invention is operated in a continuous manner, and continuous supply of precursor salts and withdrawal of end product takes place. By the presence of the binary electrolyte, the electrolytic conductivity may be increased to at least 5 mS.cm^-1, more preferably between 20 and 80 mS.cm^-1 and even more preferably between 20 and 50 mS.cm^-1 and the risk to a varying conductivity in the course of the process may be minimised.

[0028] Many metal salts are suitable for use as a precursor salt in the process of this invention to provide the metal cations M, such as chloride, carbonate, nitrate, sulfate, perchlorate or phosphate, or mixtures comprising two or more of the afore-mentioned salts, although chloride salts are preferred as the chloride ion is incorporated in the end product and contamination with other anions is to minimized. In a particularly preferred embodiment use is made of zinc chloride. An appropriate choice of the M²⁺ and Ln³⁺ salts, in particular Zn²⁺ precursor salt not only permits tailoring the geometry of the crystalline nano particles produced from triangular to spherical or any other desired geometry, and therewith the degree of magnetization and spin-liquid or spin-glass nature, but also permits controlling the crystallinity. The inventors have namely observed that the reactivity of the precursor salt in the process of this invention may vary with the nature of the anion, and that the use of smaller anions give rise to the formation of nano particles with a higher degree of crystallinity. Preferably however the chloride salt, in particular the zinc chloride is used as it may simultaneously act as chloride donor for the end product. Similarly, many Cu²⁺ precursor salts are suitable for use as a precursor salt in the process of this invention, such as Cu²⁺ chloride, Cu²⁺ nitrate, Cu²⁺ sulfate, Cu²⁺ perchlorate, Cu²⁺ carbonate or Cu²⁺ phosphate or mixtures comprising two or more hereof. Preferably however use is made of Cu²⁺ chloride used as it may simultaneously act as chloride donor for the end product.

[0029] The oxidant gas used in the process of this invention may consist of pure O₂ or a mixture of O₂ with one or more other gases, which are preferably inert to the electrochemical reaction. Examples of such inert gases include N₂, or a noble gas, more particularly Ar. When using a mixture of gases (e.g., O₂ and N₂) the skilled person will be capable of adjusting the molar fraction of the oxidant gas in such a way that it is sufficiently high to enable its electrochemical reduction, as low oxygen molar fractions may limit the extent of reaction due to production of O₂ containing species with low reactivity or not enough of them to reach the conditions to form the nanocrystals intended. Thereby, preferably the O₂ mole fraction in the O₂ containing oxidant gas is at least 0.10, preferably at least 0.15, although the O₂ mole fraction in the O₂ containing oxidant gas may be as high as 1. As O₂ is an essential element of the oxidation process and a source for OH^- production, varying the O₂ mole fraction in the oxygen containing oxidant gas will permit to control the stoichiometry of the end product, and favour either formation of ZnCu₃(OH)₆Cl₂ + ZnO at lower O₂ mole fractions, or formation of an end product with increasing x at higher O₂ mole fractions.

[0030] The rate with which the O₂ containing oxidant gas may be supplied to the cathode is preferably variable. In a preferred embodiment a supply rate with which the oxidant gas is supplied to the gas diffusion electrode ranges between 5.0 and 300.0 ml/min, preferably between 5.0 and 250.0 ml/min, more preferably between 5.0 and 150.0 ml/min.

[0031] In order to ensure a sufficiently high reaction rate, the current applied to the gas diffusion electrode ranges between 10 and 1000 Am-², preferably between 10 and 500 Am-², more preferably between 25 and 250 Am-². The current density namely determines the rate of production of OH^- and H₂O₂.

[0032] In the process of this invention, usually the working potential of the cathode is set at a value between -50.0 and -750 mV vs. Ag/AgCl, preferably at a value between -100.0 and -650 mV, more preferably between -250 and -500 mV. An appropriate selection of the working potential will assist in obtaining an end product with a desired stoichiometry and nano particles with a desired average particle size. In general, more negative potentials approaching respectively -750 mV, - 650 mV or -500mV are expected to increase the reaction rate. Potentials more negative than -750 mV could also lead to the product desired, however the hydrogen evolution reaction would be a competing process, reducing the current efficiency.

[0033] The process of this invention permits producing crystalline nanoparticles of a spin transition material, which respond to the general formula M_xCu_4-x(OH)_yCl_z as described above. The process of this invention permits producing crystalline nanoparticles of a spin transition material, which respond to the general formula M_xCu_4-x(OH)_yCl_z wherein 0 ≤ x ≤ 1, 5.5 ≤ y ≤ 6.5 and 1.5 ≤ z ≤ 3, using gas-diffusion electro crystallization (GDEx), starting from a reaction mixture containing soluble, in particular water soluble salts of copper and soluble, in particular water soluble salts of other metal ions as described above, by the reactive precipitation of intermediaries issued from the oxygen reduction reaction (ORR) at the gas-diffusion cathode. By varying the reaction conditions as disclosed above, either mono disperse nano particles of the desired reaction product may be obtained or particles with a broader average particle size distribution.

[0034] The process of this invention permits providing optimal reaction conditions for the production of M_xCu_4-x(OH)_yCl_z, so that short nucleation and growth periods and high reaction rates may be obtained, while minimising the risk to immediate agglomeration of precipitated particles. The flowing conditions of the electrolyte impose only a transient contact of metal ion precursors with the reactive species at the saturated electrochemical interface-rendering transient nucleation conditions. The possibilities of growth are feeble, as encountering other metal ions is restricted by their high dilution. The rate of interfacial processes typically scales with available surface area (i.e., surface active sites) which is provided by the highly-porous GDE (i.e., >800 m² g^-1), facilitating a high rate of production of HO₂^-, OH- and their radicals, which quickly react with the metal ion precursors available at the interface. In analogy to growth, aggregation is restricted, as the nanoparticles dispersed in the aqueous solution are few and then collisions between them are not likely; furthermore, the particles can be easily transferred to pure water, where they can set apart even better, as the repulsive forces become stronger (i.e., linked to double layer expansion due to the lower ionic strength-and sometimes pH-vs. those of the synthesis medium). Particularly, the synthesis of ZnCu₃(OH)₆Cl₂ is fast under the operational conditions here employed, with rates approaching 40 mg/min. The process is highly reproducible and it involves mild synthetic conditions (e.g., 291 K and atmospheric pressure), in contrast to all previous options reported thus far, such as hydrothermal or solvothermal methods (e.g., ∼458-473 K) ^4,13

[0035] The nanoparticles M_xCu_4-x(OH)_yCl_z as described above, in particular nanoparticles of Zn_xCu_4-x(OH)₆Cl₂ wherein 0 ≤ x ≤ are suitable for use as catalysts, active materials for batteries, medical applications for example drug delivery, in semiconductors, in quantum computing devices (i.e., for storage and memory purposes).

[0036] The present invention also relates to nanoparticles of M_xCu_4-x(OH)_yCl_z, wherein wherein 0 ≤ x ≤ 1, 5.5 ≤ y ≤ 6.5 and 1.5 ≤ z ≤ 3, and wherein M is a divalent or a trivalent metal cation. Preferably M is a metal cation selected from the group comprising _yCl_z wherein M is one or more metal cations selected from the group comprising Zn²⁺, Mg²⁺, Co²⁺, Fe²⁺, Mn²⁺ , Ni²⁺, Pd²⁺, Co³⁺, Mn³⁺, Y³⁺, La³⁺, Ce³⁺, Pr³⁺ , Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺ and Rh³⁺, and mixtures of two or more hereof. In a further preferred embodiment, M²⁺is one or more of M= Zn²⁺, Mg²⁺, Co²⁺, Fe²⁺, Mn^2+, in particular Zn_xCu_4-x(OH)₆Cl₂ wherein 0 ≤ x ≤ 1, having an average particle size of between 5.0 and 20.0 nm. Where M²⁺ is Zn²⁺, nanoparticles may be produced with the following properties depending on the average particle size :

Composition	Clinoatacamite	Paratacamite	Herbertsmithite	Herbertsmithite
Stochiometry	Cu2(OH)3Cl,	Zn0.3Cu3.7(OH)6Cl2	ZnCu3(OH)6Cl2 + ZnO	ZnCu3(OH)6Cl2 (single phase)
Average particle size (nm)	16 ± 0.4 nm	14 ± 0.2 nm	10 ± 0.1	5 - 20 nm
Crystallite size (nm)	16 nm	14 nm	10 nm	5 to 20 nm
Thermal dependence of Magnetization (ZFC) (A m2 kg-1) (from 2 to 300 K) at 8 kA m-1	-0.05	-0.01	0.05	0.05
Thermal dependence of Magnetization (FC) (A m2 kg- 1) (from 2to 300 K) at 8 kA m-1	0.5	0.65	0.2	0.05
Curie Temperature (K)	6.5	6	4.5	-
Field dependence of Magnetization (A m2 kg-1) (from -4000 to 4000 8 kA m-1) at 2 K	6.5	11	11	11

[0037] The present invention further relates to nanoparticles of of M_xCu_4-x(OH)_yCl_z,, wherein M is a trivalent metal cation selected from one or more of La³⁺ Nd³⁺ and Sm³⁺ and mixtures hereof.

[0038] The present invention is further elucidated in the examples below, including the following figures :

Figure 1 shows a schematic layout of the electrocrystallization reactor suitable for use with the present invention, and the reactions occurring therein.

Figure 2 shows a suggested mechanism for the synthesis of spin transition nanoparticles : Effect of charge consumption on structural control of (a) Cu₂(OH)₃Cl vs (b) CuO and stoichiometric composition control of spin transition compounds (Zn_xCu_4-x(OH)₆Cl₂), (c) x = 0.3 (Paratacamite) and (d) x = 1 (Herbertsmithite). (e) control experiment without metals in solution.

Figure 3 shows the temperature dependence of mass magnetization (M) for Zn_xCu_4-x(OH)₆Cl₂ and CuO as measured under ZFC-FC conditions; (a1-a2) for clinoatacamite;
(b1-b2) for tenorite; (c1-c2) for paratacamite and (d1-d2) for herbertsmithite, along with TEM imaging for Cu_4-xZn_x (OH)₆Cl₂ products made by GDEx using 6.3 mM Cu²⁺ and 2 mM Zn²⁺ as the metal precursor.

Figure 4 shows

(A). XRD patterns Co-Kα (λ=1.7928 Å) of solid products obtained using 6.3 mM Cu²⁺ and 2 mM Zn²⁺ as metal precursor. From top to bottom: Spin transition compounds with formula *Zn_xCu_4-x(OH)₆Cl₂ at x = 0, XRD patterns of (a) Clinoatacamite (Cu₂(OH)₃Cl) was produced using a charge consumption of 987 C L^-1 (green line), while only (b) copper oxide (CuO) at a charge consumption of 1876 C L^-1. (black line). c) x = 0.3 (red line); (d) x = 1 + impurity of Zincite (○ZnO) (blue line). The asterisk and circular markers show the most prominent peak positions of Zn_xCu_4-x(OH)₆Cl₂ and ZnO phases respectively. The square marker represent the polymorph Cu₂(OH)₃Cl.

(B). FTIR absorption spectra of the synthesized Zn_xCu_4-x(OH)₆Cl₂ with x = 0 (a) and Cu₂(OH)₃Cl and (b) CuO; c) x = 0.3 and (d) x = 1;. The spectra graph is shown in two regions of the hydroxyl stretching (3500-3200 cm^-1) and deformation (1000 - 8000 cm^-1). Characteristic frequencies are indicated with vertical dashed lines).

(C). M against field over temperature sweep at 2.0 K. Stoichiometric coefficient on the interlayer site ranges of x between 0 to 1.

Figure 5 shows (a) Photograph of liquid samples taken at different pH using 9 mM of Cu²⁺ according to increasing charge density (0 to 3100 C L^-1), (b) the Effect of single Cu²⁺ concentration on charge density consumption using CuCl₂.

Figure 6 shows the effect of mixed Cu²⁺ and Zn²⁺ concentration on charge consumption during the synthesis of herberthsmithite, using high (a) respectively low (b) initial Cu²⁺ concentration.

Figure 7 shows the mean particle size and distribution of Zn_xCu_4-x(OH)₆Cl₂ products with a desired value for x, a) x = 0 for clinoatacamite ; b) x = 0.3 for paratacamite and c) x = 1 for herbertsmithite.

Figure 8 shows (a) ZFC-FC thermal magnetization of a duplicates sample with stoichiometric coefficient (x =1 and x= 0.3). (b) Magnetization against field over temperature sweep at 2.0 K in duplicate samples.

Figure 9 shows XRD patterns Co-Kα (λ=1.7928 Å) of solid products obtained using 6.3 mM Cu²⁺ and 2 mM M²⁺ (Zn, Mg, Co) or Ln³⁺ (Y, Sm, La, Nd) as metal precursor using (A) low volumetric charge density (end pH 6) and (B) high volumetric charge density (end pH 11).

Gas-diffusion electrocrystallization reactor.

[0039] Use was made of the electrochemical reactor design described in Gallego et al., [^20] (i.e., for half-cell studies focused on the cathode). As a working electrode for the GDEx process, use was made of 10 cm² of VITO CoRE® multilayered carbon-based gas-diffusion cathodes. VITO CoRE® electrodes consisted of a current collector (stainless steel gauze), covered with an active layer (i.e. porous electrically-conducting matrix) of 20% PTFE and 80% active carbon, and a hydrophobic gas diffusion outer layer (porous PTFE). Norit®SX1G (878 m² g^-1, Norit Americas Inc., USA) was employed as the active carbon source.

[0040] The counter electrode (anode) consisted of a 10 cm² of platinum sheet laser-welded to a titanium (Ti) plate current collector.

[0041] The anode and cathode compartments were separated by an ion-permeable separator (Zirfon® Perl UTP 500, Agfa, Belgium)¹⁵. A 3 M KCl saturated Ag/AgCl reference electrode (+200 mV vs. SHE) (REF 321, Radiometer Analytical, Hach, USA) was inserted in proximity of the working electrode, via an external connector chamber, filled with 3 M KCl. A long cotton thread (e.g., 30 cm, packed within the external connector chamber) verged the reference and the working electrodes, through a small channel, with the purpose of establishing a microchannel enabling a continuous capillary-suction of electrolyte. The thread contained an external hydrophilic wax layer¹⁶. All potentials here reported are referred versus the Standard Hydrogen Electrode (SHE).

[0042] Experiments were conducted on a multichannel potentiostat (VMP-3, Bio-Logic SAS, France). The GDEX process was controlled chronoamperometrically at - 0.15 VSHE and room temperature (18 °C) of synthesis. The pH in the catholyte was monitored using a sensor for continuous measurement of pH in liquid media (SE555X/1-NMSN pH sensor, Knick, Germany). A pH data logger was connected to the Bio-Logic potentiostat in order to simultaneously follow up the pH vs charge (Q) evolution. The data logger consisted of a pH transmitter (A1491N-P1-10-0000, Knick, Germany) with a converter (P15000 H1, Knick, Germany) which transforms the 4-20 mA signal delivered by the transmitter into a 0-10 V signal.

[0043] All experiments were performed in batch mode with recirculation. The catholyte was recirculated at 15 L h-1 throughout the cathode compartment, where gas-diffusion electro crystallization of Zn_xCu_4-x(OH)₆Cl₂ occurred. The total liquid volume of catholyte and anolyte was 500 mL, which was continuously stirred in the recirculation reservoir (i.e., a borosilicate glass bottle) using a polygonal, PTFE-coated rotating magnetic stirring bar (i.e., 200 rpm). The air was fed through the cathodic gas compartment at a flow rate of 200 ml min-1. A mass gas flow meter and controller (GF40 Bronkhorst hi-tech B.V, Netherlands) was set in place. An overpressure of 15 mbarg over a water column (see Figure 1 for a schematic layout) was applied at the gas exhaust.

[0044] The electrochemically-driven synthesis was carried out using a mixture of Cu²⁺ and Zn²⁺ ions as the metal precursors, and O₂ (in air) as the oxidant gas through the gas-diffusion cathode. An acidified supporting electrolyte was employed as an anolyte and catholyte, composed by a NaCl aqueous solution (0.5M), adjusted at pH 3 by adding HCl (1M). In order to tune the formation of stable Zn_xCu_4-x(OH)₆Cl₂ products with the desired value for x, different [Cu²⁺]₀/[Zn²⁺]₀ ratios were evaluated. CuCl₂·2H₂O 99.999% (Sigma-Aldrich, Germany) and ZnCl₂ 99.999% (Sigma-Aldrich, Germany) were supplemented, respectively.

[0045] A solid precipitate was formed and collected in the electrolyte solution and left to sediment under stagnant conditions. The precipitate was washed with deionized water with a pH set at that of the final synthesis solution, and recentrifuged (Jouan CR422, France) at 3000 rpm, washed three times, and dried afterwards under nitrogen atmosphere, resulting in a particulate powder.

Particle size, morphology and structure characterization

[0046] Particle size and morphology were measured by transmission electron microscopy (TEM) using a JEOL JEM 2100FII operated at 200 keV. For the observation of the sample in the microscope, the particles were dispersed in butanol and a drop of the suspension was placed onto a copper grid covered by a carbon film. The mean particle size and distribution were evaluated by counting more than 100 particles by means of Digital Micrograph™. After that, data were fitted to a Normal distribution to obtain the mean particle size (dNP) and standard deviation (σ). The X-ray powder diffraction (XRD) patterns were obtained with a diffractometer (Empyrean, Malvern Panalytical, United Kingdom) using CoKα radiation (λ=1.7928 Å) with 40 mA-45 kV and a finer step size of 0.013° in the same scan range. Quantitative phase analysis (QPA) by Rietveld refinement method with the HighScore Plus software (Empyrean, Malvern Panalytical, United Kingdom) was carried out for the quantitative analysis of the phase distrubutions (%), using the measured diffraction profile and a calculated profile crystal from the inorganic crystal structure database (ICSD)^18,19 (see table 2)

[0047] To determine the stoichiometry of the end product, an amount of 50 mg of the end product powder sample was transferred into a digestion vessel. Then, 6 ml HCl and 2 ml HNO₃ were added. The screw cap of the digestion tube was gently turned and placed on a heating block. The temperature was slowly increased to (105 ± 5) °C and kept during 120 min. The tube was cooled down to room temperature and water is added to the volume mark. The copper and zinc content in the digested solution was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent, 5100, USA) in an axial view using ICP-OES equipped with a baffled cyclonic spray chamber and a conical nebulizer. The atomic emission line of 324.754 nm and 213.857 nm were used for copper and zinc, respectively.

[0048] The metal content in the precipitate sample and the phase percentage calculated by QPA were used to determine the stoichiometric coefficient x at the Cu_4-xZn_x(OH)₆Cl₂ interlayer.

[0049] Infrared spectroscopy (IR) measurements were performed on a Thermo Scientific, Nicolet iS5 (Waltham, MA, USA), with a diamond plate was used to distinguish between clinoatacamite and related Zn-polymorphs. The spectra were recorded in the range of 4,000 - 400 cm-1 with a resolution of 2 cm-1.

Magnetic characterization analysis.

[0050] The magnetic characterization was carried out on powder samples in a glycerin capsule and performed in a SQUID magnetometer Quantum Design MPMS-5S, with 5 T maximum applied field and temperature range from 2 to 400 K. The ZFC-FC curves were measured at 8 kA/m (100 Oe) from 2 to 300 K, with a step of 0.25 K from 2 to 6 K, 0.5 K from 6 to 12 K and more than 1 above 12 K. The hysteresis loops up to 5 T were measured at 2, 3, 4, 6 and 15 K.

Examples.

[0051] GDEx allows the selective preparation of Zn_xCu_4-x(OH)₆Cl₂ materials. By varying the [Cu²⁺]0/[Zn²⁺]0 ratio in the electrolyte (wherein [Cu²⁺]0 and [Zn²⁺]₀ correspond to the initial concentration of copper and zinc ionic precursors, respectively), together with a systematic control of the electric charge, the stoichiometric composition of the Zn_xCu_4-x(OH)₆Cl₂ products and their structural control, between Cu₂(OH)₃Cl and CuO, can be customized (Figure 2).

[0052] The formation of Cu₂(OH)₃Cl and CuO, for x = 0, and Zn_xCu_4-x(OH)₆Cl₂ within the range of 0.3 < x <1 was individually targeted, by controlling the operational conditions of the gas diffusion controlled electrochemical reaction (GDEx).

[0053] To produce an end product with x = 0, use was made of an initially-colorless solution of copper chloride (i.e., at pH 3), which turned into an opalescent and greenish-colored dispersion upon reaching pH ∼5. The color change (see Figure 5) can be attributed to the precipitation of Cu₂(OH)₃Cl (see green line (a) in Figure 2), by following Reaction 1:

A plateau of precipitation could be distinguished at pH 4.7 - 5.5, where 1630 C L^-1 were consumed from a starting pH of 3. The pH was left to evolve and upon reaching a pH of 5.5, the Cu²⁺ ions were fully removed from the solution (see Figure 5), followed by a sharp increase of pH, up to 11 which was reached at a charge consumption of 1875 C/L. Tenorite (CuO) formed rapidly under basic conditions (black line), according to reaction 2 :

[0054] The formation of Zn_xCu_4-x(OH)₆Cl₂ with x = 0.3 could be individually targeted with a combination of low charge consumption (778 C L^-1), and a relatively-low concentration of Cu²⁺ (7.3 mM) and Zn²⁺ (2.3 mM) as the metal precursors as shown in the red line (c) of Figure 2. The pH vs charge evolution of sample x = 1, represented by the blue line (d) in Figure 2, is distinct from that with x = 0, represented by the black line (b) in Figure 2. As zinc ions are substituted onto the interlayer site, a shorter plateau of precipitation is distinguished at pH 4.7 - 5.5, wherein 1000 C L^-1 had been consumed from a starting pH of 3. This is attributed to the insertion of chloride ions as part of the chemical structure in the product form, which-with respect to CuO implies a reduced consumption of hydroxide ions, as apparent from comparing Reaction (2) vs Reaction (3).

The extent of copper and zinc removal from the liquid phase is shown in Figure 6. At a higher Cu²⁺ concentration of 574 mg L^-1 and a Zn²⁺ concentration of 91 mg L^-1, 100 % of the Cu²⁺ and 53 % of the Zn²⁺ ions are removed from the liquid phase at a charge consumption of 2460 C L^-1, while 300 mg L^-1 of Cu²⁺ and 106 mg L^-1 achieved 100 % of Cu²⁺ and 44 % removal of Zn ions at a charge consumption of 1810 C L^-1.

[0055] In a control experiment (see orange line in Figure 2) was carried out without metals in solution. The charge needed to raise the pH from 3 to 11 was 440 C L^-1. The generation of peroxides and free radicals are transient intermediaries, formed together with OH^-ions-as per the established mechanism of O₂ reduction at non-catalyzed carbon electrodes^20,21. After oxygen diffuses to the electrocatalytic surface (i.e., activated carbon) of the gas diffusion electrode (GDE), O₂ is reduced (see Figure 1B). The imposed cathodic polarization conditions (e.g., -0.15 VSHE) drive this electrolysis mainly to OH- ions via a 4 electron (4 e^-) transfer (Eq. 4):

On the basis of the charge consumed, it is estimated that a profuse amount of OH^- is produced (e.g., >1300 mol m^-3) resulting from the oxygen reduction reaction, from early stages of the GDEx process. This would immediately result in a pH of 14 within the porosity of the gas-diffusion cathode²¹, facilitating the onset for hydroxide supersaturation and hence for reactive precipitation (i.e., crystallization) at the electrochemical interface (see Figure 2).

Particle size and magnetic properties of spin transition nanoparticles

[0056] The thermal and field dependence of magnetization (M) over temperature sweep at 2.0 K are shown in Figure 3 (a1,b1,c1,d1) for in which (a1) x=0 for clinoatacamite, (b1) for tenorite (CuO), (c1) 0.3<x<1 for paratacamite and (d1) x=1 for herbertsmithite.

[0057] ZFC thermal dependence of magnetization provides information about the magnetic properties of these spin transition nanoparticles at H= 8 · 10⁶ A m^-1 (see Fig 3 (a1,b1,c1,d1). On cooling below T∼8K, Zn_xCu_4-x(OH)₆Cl₂ compounds showed a ferromagnetic transition, which involves also a significant antiferromagnetic component down to 2K, indicating the formation of spin-spin correlations; such behavior is a common feature of quantum kagomé systems^1,3-5,7,22. Negative magnetization is shown for the first time in Cu₂(OH)₃Cl, which allows us to differentiate between Zn-paratacamite compounds.

[0058] FC thermal magnetic susceptibility measurements were employed to further characterize the magnetic properties down to 2K. In spin transition compounds with stoichiometric coefficient of x =1, 2 times less long-range antiferromagnetic (AFM) ordering or spin freezing was found, with respect to that in paratacamite nanoparticles with a stoichiometric coefficient of x = 0.3 down to 2 K, suggesting that the candidate spin liquid behavior is sustained at the nanoscale. On the contrary, compounds with x = 0 showed three times higher magnetic transition at T_Curie = ∼ 6 K than compounds with x = 1 (T_Curie = ∼ 4.5 K), which supports a long-range AFM ordering, spin freezing or spin-glass-behavior in compounds without Zn in the interlayer site.

[0059] The morphologies of the Zn_xCu_4-x(OH)₆Cl₂ particles made by GDEx, at different charge consumption extents, in which x=0 for clinoatacamite and 0.3<x<1 for paratacamite and x=1 for herbertsmithite, are shown in Figure 3 (b-c). The experimental evidence for x = 0, rendered (a3) clinoatacamite (Cu₂(OH)₃Cl), which was produced as a mixture of faceted nanocrystals and large spiky nanowhiskers (16 ± 0.4 nm), when using a charge consumption of 987 C L^-1, while only (b3) tenorite (CuO) microwhiskers of 1.5 µm with needle-structures, were found at a high charge consumption of 1876 C/L.
The frequency of the spin transition nanoparticles sizes and the corresponding Normal distribution fits are shown in Figure 7. The addition of Zn²⁺ had a significant effect on the particle size and polydispersity degree. When the content of zinc was increased from x = 0.3 to x = 1, particle sizes reduced from 14 to 10 nm, even at higher alkaline pH (i.e., 11). The absence of Zn in the structure, under basic conditions, allows the rapid decomposition of Cu₂(OH)₃Cl to CuO microwhiskers of 1.5 µm with needle-structures (see Figure 3).

[0060] The XRD patterns and IR spectra of 4 representative samples are shown in Figure 4. The diffraction patterns of Figure 4A are indexed with the following compounds; Zn_0.25Cu_3.75(OH)₆Cl₂ (ICSD-n° 192076), Zn_0.85Cu_3.15(OH)₆Cl₂ (ICSD-n° 424325), ZnO (ICSD-n° 26170), Cu₂(OH)₃Cl (ICSD-n° 64956) and CuO (ICSD-n° 67850). Table S1 compares the results obtained from quantitative phase analysis (QPA) by Rietveld refinement method and ICP-OES of powder samples. The synthesis of Zn_xCu_4-x(OH)₆Cl₂ compounds using less than 1000 C L^-1, resulted in a product with stoichiometric coefficient x = 0.3 and the lowest amount of impurities, such as ZnO and Cu(OH)₂ (Table S1). At higher charge consumption (2000 C L^-1), the synthesis of compounds with higher Zn content (x = 1) is differentiated, but a lower purity is obtained, due to the final pH of synthesis of 11 (see Figure 1 and duplicate experiment data in Table S1).

[0061] The XRD traces of Cu₂(OH)₃Cl and Zn_xCu_4-x(OH)₆Cl₂ are indistinguishable by X-ray analysis (see Figure 4A (a) vs 4A (c)) due to the difficulty to differentiate Cu and Zn. The crystal structure of Cu₂(OH)₃Cl has been determined by several authors^11,14,24, which reported that pure Cu₂(OH)₃Cl occurs in the form of three different polymorphs: atacamite, clinoatacamite, and paratacamite. While the first two polymorphs constitute the thermodynamically-stable phase of Cu₂(OH)₃Cl at ambient temperatures (orthorhombic and monoclinic structure type), paratacamite (rhombohedral structure) has to be stabilized by partial substitution of Zn or Ni for Cu^24,25. According to the experimental work of Malcherek and Schlueter²⁴, the XRD diffraction pattern of Zn_xCu_4-x(OH)₆Cl₂ is weak and therefore easily mistaken by that of Cu₂(OH)₃Cl. Differences in the hydroxyl groups of clinoatacamite and herbertsmithite can be used for appropriate identification. Thus, infrared spectroscopic (IR) studies of CuO and Zn_xCu_4-x(OH)₆Cl₂ were undertaken to overcome the barrier of crystal structure identification encountered with XRD (Figure 4B). Our data indicate that Cu₂(OH)₃Cl and ZnCu₃(OH)₆Cl₂ have close lying bands in the hydroxyl stretching region. However, a buried peak at 3,400 cm^-1 is unique for the Cu₂(OH)₃Cl structure and can be used as a fingerprint for its detection. This, together with slight shifts in the three major bands of the hydroxyl stretching region, allow us to confidently identify our synthesized Cu₂(OH)₃Cl as clinoatacamite. The IR spectrum of the synthesized CuO compounds supports the phase purity observed by XRD, with no indication of Cu₂(OH)₃Cl, and the stability of ZnCu₃(OH)₆Cl₂. Similar observations are reported using Raman spectroscopy and IR^14,26.

[0062] Figure 9 shows XRD patterns Co-Kα (λ=1.7928 Å) of solid products obtained using 6.3 mM Cu²⁺ and 2 mM of M²⁺ such as (Zn, Mg, Co) or M³⁺ including (Y, Sm, La, Nd) as metal precursor using (A) at low volumetric charge density (end pH 6) and (B) high volumetric charge density (end pH 11).

[0063] In an effort to determine the nature of the ferromagnetic component, magnetization against field over temperature sweep at 2.0 K was recorded for each sample (Fig 4C). The hysteresis loops at 2.0 K of Cu₂(OH)₃Cl shows a kick at low fields suggesting the presence of two magnetic phases, one ferromagnetic and the other antiferromagnetic (the majority one). The negative magnetization can be the result of the coupling of the two phases with resulting net magnetization opposite to the applied field. As temperature increases, the coupling vanishes. Compounds with Zn_xCu_4-x(OH)₆Cl₂ structure exhibit a common paramagnetic-like behavior, where Cu₂(OH)₃Cl developed a higher ferromagnetic hysteresis loop than compounds with x=1.These results are consistent with those of Colman et al⁵ who indicate the presence of a small amount of ferromagnetic impurity (see Figure 4).

[0064] The less ordering temperature in herbertsmithite is a clear indication of spin frustration, which inhibits the tendency of a spin to order under cooling conditions. However, the Curie temperature has not been suppressed as described in the synthesis of a single crystal of herbertsmithite¹. This result may be explained by the fact that below a critical size, magnetic particles become single domain in contrast with the usual multidomain structure of microcrystalline samples, which can exhibit unique phenomena such as unusual coercivities²⁷, quantum tunneling of the magnetization^28,29, and superparamagnetism^30,31. Another possible explanation for this is that the size reduction of herbertsmithite could affect the novel magnetic properties, due to the small volume or the high surface/volume ratio (spin canting effect)^32,33, a phenomenon through which spins show a lack of full alignment at the surface.

[0065] The properties of the products prepared are summarized in table 1 below.

Table 1.

Composition	Clinoatacamite	Paratacamite	Herbertsmithite	Herbertsmithite
Stochiometry	Cu2(OH)3Cl,	Zn0.3Cu3.7(OH)6Cl2	ZnCu3(OH)6Cl2 + ZnO	ZnCu3(OH)6Cl2 (single phase)
Particle size (nm)	16 ± 0.4 nm	14 ± 0.2 nm	10 ± 0.1	5 - 20 nm
Crystallite size (nm)	16 nm	14 nm	10 nm	5 to 20 nm
Thermal dependence of Magnetization (ZFC) (A m2 kg-1) (from 2to 300 K) at 8 kA m-1	-0.05	-0.01	0.05	0.05
Thermal dependence of Magnetization (FC) (A m2 kg-1) (from 2to 300 K) at 8 kA m-1	0.5	0.65	0.2	0.05
Curie Temperature (K)	6.5	6	4.5	-
Field dependence of Magnetization (A m2 kg-1) (from -4000 to 4000 8 kA m-1) at 2 K	6.5	11	11	11

Table 2 Quantitative phase analysis (QPA) by Rietveld method and ICP-OES of powder samples.

Sample code	Charge consumed (C L-1)	ICP powder mg g-1 solid		(mmol)		Cu/Zn ratio	Phase percentage (%)				Cu and Zn in Cu4-xZnx(OH)6Cl2		Stoichiometric coefficient
		Cu	Zn	Cu	Zn		Cu4-xZnx(OH)6Cl2	ZnO	Cu(OH)2	CuO	Cu (mM)	Zn (mM)	(x)
Sample (a1)	778	469	64	7.4	1.0	7.3	82	3.2	15	0	7.1	0.8	0.3
Sample (a2)	850	524	72	8.3	1.1	7.3	98	1.6	1.8	0	8.2	1.1	0.4
Sample (b1)	2000	462	150	7.3	2.3	3.1	77	15	8	0	6.2	2.1	1.0
Sample (b2)	2018	467	144	7.4	2.2	3.2	80	16	4	0	6.2	2.1	1.0
Sample (c)	987	546		8.6			100	0	0	0	8.6		0
Sample (d)	1876	783		12.3						100	12.3		0

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Claims

1. An electrochemical process for producing nanoparticles of mixed copper hydroxide-chloride compounds responding to the chemical formula M_xCu_4-x(OH)_yCl_z wherein M is one or more metal cations from the group comprising a divalent earth alkali metal cation, a divalent transition metal cation or a trivalent transition metal cation, and wherein 0 ≤ x ≤ 1, 5.5 ≤ y ≤ 6.5 and 1.5 ≤ z ≤ 3, the method comprising the steps of

(6) supplying to a cathode compartment of an electrochemical cell, wherein the cathode compartment comprises a catholyte and is equipped with a cathode comprising a gas diffusion electrode with a porous electrochemically active material, a liquid water based mixture containing dissolved therein Cl^- ions, at least one precursor salt containing the one or more metal cations M, and at least one Cu²⁺ precursor salt, wherein the ratio of the concentration of Cu²⁺ to M is smaller than 10:1,

(7) adjusting the pH of the reaction mixture to a value between 2.0 and 6.0,

(8) supplying an O₂ containing oxidant gas to the gas diffusion electrode,

(9) subjecting the cathode to an electrochemical potential which is below the thermodynamic limit of O₂ reduction at the pH of the reaction mixture,

(10) applying a potential to the gas diffusion electrode to cause reduction of the O₂ contained in the oxidant gas to one or more of the corresponding peroxide, OH^-, ionic and/or radical reactive O containing species,

and isolating nanoparticles of M_xCu_4-x(OH)_yCl_z

2. A process as claimed in claim 1, wherein M is one or more divalent metal cations selected from the group comprising Zn²⁺, Mg²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pd²⁺, Sm²⁺, Eu²⁺, preferably one or more divalent metal cations selected from the group comprising Mg²⁺, Co²⁺, Zn²⁺.

3. A process as claimed in claim 1 or 2, wherein M is one or more trivalent metal cations selected from the group comprising Y³⁺, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺, Rh³⁺, Co³⁺, Mn³⁺, preferably one or more trivalent metal cations selected from the group comprising Y³⁺, La³⁺, Nd³⁺, Sm³⁺.

4. A process according to any one of the previous claims, wherein the concentration of the at least one M containing precursor salt in the catholyte is maximum 10 mmole/l, preferably maximum 2mmole/l.

5. A process according to any one of the previous claims, wherein the concentration of the Cu²⁺ precursor salt in the catholyte is maximum 8 mmole/l.

6. A process according to any one of the previous claims, wherein the ratio of the concentration of Cu²⁺ to M is maximum 7:1, more preferably maximum 5:1, most preferably maximum 4:1.

7. A process as claimed in any one of the previous claims, wherein the catholyte contains a supporting electrolyte, in a concentration of between 5.0 and 150.0 g/l of catholyte, preferably between 10.0 and 100 g/l, more preferably between 10.0 and 50.0 g/l.

8. A process as claimed in claim 7, wherein the supporting electrolyte contains a chloride salt, in particular NaCl.

9. A process as claimed in any one of the previous claims, wherein the at least one M containing precursor salt is a chloride precursor salt.

10. A process as claimed in any one of the previous claims, wherein the at least one Cu²⁺ precursor salt is a chloride precursor salt.

11. A process as claimed in any one of the previous claims wherein the the O₂ mole fraction in the oxidant gas ranges between 0.05 and 1.0, preferably between 0.05 and 0.75, more preferably between 0.10 and 0.30.

12. A process as claimed in any one of the previous claims, wherein the current applied to the gas diffusion electrode ranges between 10 and 1000 Am^-2, preferably between between 10 and 500 Am^-2, more preferably between 25 and 250 Am^-2.

13. A method as claimed in any one of the previous claims, wherein the catholyte is a catholyte selected from one or more of an aqueous solution, an organic solvent, a mixture of two or more organic solvents, a mixture of water with one or more organic solvents.

14. A method as claimed in claim 13, wherein the organic solvent is selected from the group comprising one or more of dimethylformamide (DMF), dimethyl sulfoxide (DMSO), propylene carbonate, and acetonitrile as solvents and tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide (TBAB) as supporting electrolyte.

15. A nanoparticle of M_xCu_4-x(OH)_yCl_z wherein M is one or more metal cations selected from the group comprising Zn²⁺, Mg²⁺, Co²⁺, Fe²⁺, Mn²⁺ , Ni²⁺, Pd²⁺, Co³⁺, Mn³⁺, Y³⁺, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺ and Rh³⁺ and mixtures of two or more hereof, wherein 0 ≤ x ≤ 1, 5.5 ≤ y ≤ 6.5 and 1.5 ≤ z ≤ 3, the nanoparticle having an average particle size of between 5.0 and 20.0 nm.

16. A product selected from one or more of a catalyst, an active material for batteries, drug delivery, a semiconductor, a quantum computing device, a data storage device, a memory device comprising nanoparticles obtained with the method according to any one of claims 1-14, or the nanoparticles according to claim 15.

Drawing