WO2009040479A1

WO2009040479A1 - Novel particles and method of producing the same

Info

Publication number: WO2009040479A1
Application number: PCT/FI2008/050543
Authority: WO
Inventors: Juha Maijala; Juha Merta; Jun Shan; Heikki Tenhu
Original assignee: Oy Keskuslaboratorio - Centrallaboratorium Ab
Priority date: 2007-09-28
Filing date: 2008-09-29
Publication date: 2009-04-02
Also published as: FI20075676A0; FI20075676L

Abstract

Metal nanoparticles and methods for the production thereof. The novel nanoparticles having an average particle size of 1 to 10 nm and comprising at least one protective group bonded to metal, in particular copper particles, said group being derived from a monomeric thiol. They can be produced by reducing, in the presence of a reducing agent, dithioester monomers to the corresponding thiols while simultaneously reducing a copper precursor to the corresponding metal to yield amorphous or nanocrystalline copper nanoparticles. The novel metal nanoparticles can be used for producing semiconductive or conductive thin layers on a substrate.

Description

NOVEL PARTICLES AND METHOD OF PRODUCING THE SAME

Background of the Invention

Field of the Invention

The present invention relates to novel nanoparticles and to the manufacture thereof. In particular, the present invention concerns novel metal nanoparticles in which the metal is protected with organic ligands or groups. The invention also discloses methods of producing such particles and to uses of the new metal nanoparticles.

Description of Related Art

Metallic nanoparticles, such as gold, silver, and copper, have attracted extensive scientific and industrial interest due to their unique electronic, optical, and catalytic properties. They are the most promising nanomaterials and play an important role in fabrication of nanodevices as a result of their high electrical conductivity and chemical inertness.

Significant progress has been achieved in the past decades in the synthesis and characterization of monodisperse gold and silver nanoparticles. Thus, in our earlier studies, we have successfully synthesized a variety of gold nanoparticles protected with different polymers that were end-functionalized with a dithiobenzoate group and synthesized via reversible-addition- fragmentation chain transfer (RAFT) polymerization. The dithiobenzoate end-functionalized polymers can be reduced to a thiol simultaneously when reducing a metal precursor by adding a reductant.

Being less expensive than gold and silver, copper is a more attractive metal than these for use on an industrial scale and is interesting in nanoscience and nanotechnology because of its superior electrical conductivity and high cost performance. Attempts to chemically synthesize copper nanoparticles have been inadequate so far. The main reason is that copper is very prone to be oxidized on the nanoscale. Thus, for avoiding oxidation copper nanocrystals have been synthesized by chemical reduction in reverse micelles, the size and shape of copper nanocrystals being controlled by hydration of the reactants, the dynamic character of the micelles, surfactant, and the reducing agent concentration. Another method employing Schiffrin and Brust's reaction for preparing alkanethiol-protected gold nanoclusters has been extended to prepare copper nanoparticles. But this approach involves a complication when applied to the preparation of alkanethiol-protected copper nanoparticles because a bilayer complex of copper(I) thiolates is being formed. As examples of other protective materials for copper nanoparticles, the following can be mentioned: bis(ethylhexyl)hydrogen phosphate (HDEHP), sugar glucose, xanthates, poly(N-vinyl-2-pyrrolidone) (PVP), and dendrimer templates, such as poly(amidoamine) (PAMAM) and poly(propylene imine) (PPI).

None of the above approaches has provided copper particles which exhibit, in combination, good stability against oxidation and an average particle size in the lower nanometer range.

There is a interest in producing similar particles also of other (non-noble), conductive metals, such as aluminium, zinc, nickel, cobalt and indium and mixtures thereof.

Summary of the Invention

It is an aim of the present invention to provide novel metal nanoparticles.

It is another aim of the invention to provide a method of producing metal nanoparticles.

It is a third aim of the invention to provide uses of the novel metal nanoparticles.

These and other objects, together with the advantages thereof over known nanoparticles and methods for the preparation thereof, which shall become apparent from specification which follows, are accomplished by the invention as hereinafter described and claimed.

The present invention is based on the idea of providing nanoparticles with a core of a suitable conductive metal, such as copper, said nanoparticles having an average particle size of, generally about 1 to 10 nm, and comprising at least one protective group or ligand bonded to the metal particles. Preferably, the protective group is derived from a thiol.

The novel nanoparticles are amorphous or nanocrystalline and exhibit interesting properties. The present invention also provides a method of producing nanoparticles having an average particle size of about 1 to 10 nm, comprising the step of reducing, in the presence of a reducing agent, dithioester monomers to the corresponding thiols while simultaneously reducing a copper precursor to the corresponding metal to yield amorphous or nanocrystalline metal nanoparticles.

The reaction step and the step of recovering the product are advantageously carried out in an atmosphere which is essentially free from oxygen to protect the metal particles to which the organic ligands are bonded from oxidation during the preparation.

The novel nanoparticles can be used, e.g., for producing conductive and semiconductive structures and components.

More specifically, the present products are characterized by what is stated in the characterizing part of claim 1.

The method according to the invention is characterized by what is stated in the characterizing part of claim 16 and the novel uses are defined in claim 32.

Considerable advantages are obtained by means of the invention. Surprisingly, it has been found that in the novel nanoparticles the metal particle core is well protected against oxidation and the novel metal nanoparticles can be stored for extended periods of time. By heating the present nanoparticles, in particular copper nanoparticles, larger crystalline agglomerates can be formed. The nanoparticles as such are semiconductive and they can be used for producing semiconductive or conductive thin layers on a substrate for example by printing technologies or lithography.

Next, the invention will be examined more closely with the aid of a detailed description and working examples.

Brief Description of the Drawings

In the attached drawings, Figure 1 shows the FTIR spectra of the starting material of 4-Cyanopentanoic acid dithiobenzoate (CPAD) and the as-prepared CuNPs; Figure 2 shows the ¹H and ¹³C NMR spectra of the as-prepared CuNPs; Figure 3 shows TGA curves of CuNPs-I and -2; Figures 4a to 4d show HRTEM images, EDS spectra and SAED patterns (inset) of CuNP-I (top row) and CuNP -2 (bottom row);

Figures 5a and 5b shows in situ WAXS heating measurements of both CuNPs from 30 ⁰C up to ca. 240 ⁰C under helium flow; the numbers on the right hand side are temperatures in Celsius; and Figure 6 shows the WAXS pattersn for hexagonal Cu₂S phases of CuNP-I at moderate temperatures and the theoretical positions (dotted line) and intensities (solid line) of the reflections from hexagonal Cu₂S (JCPDS 26-1116).

Detailed Description of Preferred Embodiments

As discussed above, in the present invention novel nanoparticles having a core of a metal surrounded by protective organic ligands are provided along with a method of preparing such nanoparticles using as a starting material a monomeric thiol.

According to a preferred embodiment, in the present nanoparticles the protective group bonded to metal is derived from a monomeric thiol having the formula I

R¹R²SH

wherein R¹ stands for a cyclic group and R² stands for a bivalent group.

R¹ preferably stands for a cyclic group selected from aromatic and aliphatic groups. The cyclic aromatic group is preferabl a benzyl or phenyl which optionally bears 1 to 5 substituents on the ring, or naphthyl, which optionally bear 1 to 11 substituents on the ring structure. The bivalent group R² is, for example, a linear or branched aliphatic group selected from alkylene, alkenylene and alkynylene which optionally bears 1 to 3 substitutents. In substituents R¹ and R², the substitutents are preferably selected from the group of halogen, alkyl and alkenyl.

"Halogen" preferably designates chloro, bromo or iodo.

"Alkyl" stands for a hydrocarbon radical containing preferably 1 to 18, more preferably 1 to 14 and in particular preferred 1 to 12 carbon atoms. The alkyl can be linear or branched. In particular, the alkyl group is a lower alkyl containing 1 to 6 carbon atoms, which optionally bears 1 to 3 substituents selected from methyl and halogen. Methyl, ethyl, n- propyl, i-propyl, n-butyl, i-butyl and t-butyl are particularly preferred.

"Alkenyl" contains preferably 2 to 18, more preferably 2 to 14 and particularly preferred 2 to 12 carbon atoms. The alkenyl can be linear or branched. The branched alkenyl is preferably branched at the alpha or beta position with one and more, preferably two, Ci to C₆ alkyl, alkenyl or alkynyl groups.

"Alkylene" groups generally have the formula -(CH₂)I- in which r is an integer 1 to 10. One or both of the hydrogens of at least one unit -CH₂- can be substituted by any of the substituents mentioned below. The "alkenylene" groups correspond to alkylene residues, which contain at least one double bond in the hydrocarbon backbone. If there are several double bonds, they are preferably conjugated. "Alkynylene" groups, by contrast, contain at least one triple bond in the hydrocarbon backbone corresponding to the alkylene residues.

In the following description and in the examples, the invention will be disclosed with particular reference to copper and copper nanoparticles. It should be pointed out that copper is a preferred embodiment. The invention can, however, also be carried out using other conductive metals such as aluminium, zinc, nickel, cobalt and indium and similar metals which, in the present context can be characterized as being "non-noble" metals. Mixtures of two or more of copper, aluminium, zinc, nickel, cobalt and indium can also be employed. Thus, the details given below with respect to copper particles are mutatis mutandis applicable to said other metals and to mixtures of two or more metals. The obtained particles are essentially amorphous or nanocrystalline and have a valence of copper of +1. They have, on an average, 10 to 90 %, preferably 50 - 60 % organic protectant bonded to each metal particle based on molar equivalents.

The organic groups surround the metal core formed by metal particles and prevent oxidation. It would appear, although this is just one possibility, that the protective groups are bonded for example to the copper particles by coordination forces or by covalent bonds.

The sintered metal nanoparticles, depending on the metal used, generally have a resistivity of about 10^"8 to 10^"3 ohm*m, and when they are heated above about 100 ⁰C they will crystallize. In particular, copper nanoparticles will yield crystalline copper sulphide, viz. hexagonal Cu₂S at temperatures below 200 ⁰C and cubic Cui.sS at temperatures above about 200 ⁰C.

The method of producing the above described metal, as illustrated by the example of copper nanoparticles typically comprises the steps of reducing, in the presence of a reducing agent, dithioester monomers to the corresponding thiols while simultaneously reducing a copper precursor to the corresponding metal to yield amorphous or nanocrystalline copper nanoparticles, and recovering the nanoparticles. The reactants are typically mixed together in an aqueous medium to form a reaction mixture, a reducing agent is added, and the reaction mixture is vigorously agitated to promote intimate mixing of the components during the reduction reactions. The nanoparticles obtained from the reaction step are recovered by mechanical separation methods. Preferably the reaction product is recovered, and potentially filtered and washed, under inert gas protection.

According to one preferred embodiment, the method is carried out by reacting, in the presence of a reducing agent, a metal precursor with an organic compound having formula II

R³^)SSR¹ II

wherein R¹ has the same meaning as above, preferably it represents aryl,

R³ stands for an alkanoic acid which optionally bears a substituent, and R⁴ stands for an alkyl group to provide a compound having the formula I.

This reaction is preferably carried out in an atmosphere which contains less than 1 % oxygen. In particular, the reaction is carried out in an inert atmosphere essentially free from oxygen. Examples of suitable reaction gases include nitrogen, argon, helium, hydrogen and carbon dioxide and mixtures thereof. Nitrogen and argon are particularly preferred.

The reaction can be carried out at ambient temperature (i.e. about 20 to 25 ⁰C), generally the temperature is in the range of 10 to 40 ⁰C.

The reaction medium is preferably aqueous, in particular in water or in mixtures of water and other polar solvents, such as alkanols, are used.

In Formula II, R³ stands for an alkanoic acid having 2 to 6 carbon atoms and which is substituted with at least one cyano group at an optional position.

Ri preferably stands for an aryl group, such as phenyl. In one particularly preferred embodiment, the reactant of Formula II comprises a substituted dithiobenzoate, such as 4- cyanopentanoic acid dithiobenzoate.

When copper nanoparticles are produced, the copper precursor comprises an organic or inorganic copper (II) salt, preferably a copper (II) salt which is soluble in water. In particular, the copper salt is selected from the group of copper chloride, copper nitrate, copper sulphate and copper acetate and mixtures thereof.

Similar inorganic or organic salts can be employed for the other metals selected from the group of Al, Zn, Ni, Co and In and mixtures thereof.

According to an embodiment of the invention, at least an equimolar amount of a compound according to Formula II with respect to the metal/copper precursor, preferably the compound of Formula II is used in a molar amount of 1 : 1 to 5 : 1 , in particular 1 : 1 to 3 : 1 , in respect to the copper precursor. The reducing agent is a selective reducing agent. Preferably the reducing agent is selected from the group of sodium borohydride, sodium cyanoborohydride, sodium dithionite, sodium triacetoxyborohydride, litium aluminium hydride, diisobutylaluminum hydride, dimethylsulphide borane, hydrazine, phenyl silane and sodium bis(2-methoxyethoxy)- aluminumhy dride .

The novel nanoparticles are interesting materials. As will be discussed in more detail in the example below, two samples of copper nanoparticles with different compositions were prepared using different ratios between the starting material and copper precursor (CuCl₂). Both samples show a feature of the amorphous state or the nanocrystalline structure from the measurements of HRTEM and selected area electron diffraction (SAED) patterns. The copper nanoparticles were characterized by XPS and AES to reveal the valence of copper +1 and no oxidation. Moreover, it is very interesting to find that in the in situ WAXS heating experiments these copper nanoparticles crystallize upon heating up to 100 ⁰C and the crystalline size grows up from a few nanometer to a relatively large dimension much dependent upon temperature. The crystalline structures of both samples were attributed to a hexagonal Cu₂S at low temperatures and a cubic Cui.sS phase at high temperatures.

It has been known that copper sulfides (CuxS, 1 < x < 2) exist several solid phases such as Cu₂S (chalcocite), Cu₁^₆S (djurleite), Cui.sS (digenite), CU1.75S (anilite), Cu_1-12S (yarrowite), Cu_1-06S (talnakhite) and CuS (covellite). All of these phases have been identified as p-type semiconducting materials due to copper vacancies within the lattice.

The different Cu_xS phases show reportedly low band gap energies of 1.2 eV for the bulk, which makes them potentially ideal for application of photoinduced voltaics or catalysis. According to our interesting results revealed in the in situ WAXS measurements, the formation of large crystals of Cui.sS upon heating to moderate temperatures shows significant potential application in fabrication of nanodevices.

Based on the above, the present metal nanoparticles, in particular copper nanoparticels, can be used for making semiconductive or conductive thin layers on a substrate. Typically such substrates can be selected from group of webs and sheets of paper and cardboard and similar fibrous substrates, and various polymer materials present as films or sheets. On substrates of the indicated kind, thin layers can be formed by printing or lithography and similar techniques.

The following example illustrates the invention.

Example

Chemicals. Copper(II) chloride anhydrous (BDH Laboratory Supplies, England), sodium borohydride (power, 98%, Sigma-Aldrich), ethanol (99.5wt%, Altia Oyj, Finland), chloroform (HPLC, Rathburn Chemicals Ltd), and n-hexane (95%, LAB-SCAN) were used as received. 4-Cyanopentanoic acid dithiobenzoate (CPAD) was synthesized according to the synthetic procedure described earlier. Nitrogen (99.999%) (Oy AGA Ab, Finland) was used as protective gases in the synthesis of copper nanoparticles. The water used for all the measurements was purified and deionized in an Elgastat UHQ-PS purification system.

Particle synthesis. Two types of copper nanoparticles protected with organic ligands, CuNPs-I and -2, were synthesized in water-ethanol solutions with corresponding molar ratios of 2:1 and 1 :1 of CPAD to copper (II) chloride. The following reaction scheme was followed (Scheme A):

room iemperanura. N₁ {Cpβ-ltAFT agent)

Scheme A

For synthesizing CuNP-2 (CPAD:CuCl₂ = 1 :1) about 0.3 mmol of CPAD and 0.3 mmol of CuCl₂ were dissolved into a mixture of 8 mL of deionized water and 10 ml ethanol in a round-bottom flask, which was sealed up with a tight fitting septum and degassed under N₂ flow. To the vigorously stirred reaction solution was added dropwise a fresh solution of 150 mg OfNaBH₄ dissolved into 2 ml of deionized water under N₂. The reaction solution turned immediately dark brown. The reaction was maintained for 2 h at room temperature under N₂ flow to prevent oxidation of particles. The brown reaction mixture was first centrifuged to collect a brownish precipitate, followed by washing with deionized water and ethanol, respectively. The crude product was further purified by dissolving into a tiny amount on chloroform and then precipitating with addition of hexane. All the purification was performed under N₂ protection. Finally, the product was dissolved in chloroform again, subsequently filtered using syringe filter (0.45 μm, Millipore), and dried under N₂ flow. The brownish particles in the solid state kept under N₂ in freezer over six months seem stable against oxidation. Otherwise, the sample became black and was not at all soluble in chloroform.

The resulting copper nanoparticles were found to be hydrophobic, soluble in chloroform and THF, but not in water, ethanol, acetone, or hexane.

Analysis of the samples revealed that both CuNP samples showed very similar FTIR spectra, thus, Figure 1 only demonstrates one of them as well as the one of the starting material of CPAD. It can be seen that for CuNPs the aromatic C-H stretch bands in the range of 3100-3000 cm^"1, the aromatic C=C stretches between 1600 and 1450 cm^"1, the aromatic overtones from 1960 to 1660 cm^"1, and the C-H bending motions of aromatic ring with five adjacent H atoms at 760 and 699 cm^"1 are typically assignable to substituted benzene ring, while the strong absorption bands in the range of 3000-2850 cm^"1 are attributed to the aliphatic C-H stretch bands. A strong absorption peak at 1259 cm^"1 and a band at ca. 680 cm^"1 are attributed to the C-S stretches; a markedly broad and strong band at ca. 1020 cm^"1 to the C=S stretch; an absorption at 800 cm^"1 to the C-S-Cu bending mode. However, no evident absorption originates from either cyano group (CN) at ca. 2246 cm^"1 or carboxylic acid (COOH) at ca. 1700 cm^"1 as involved in the starting material of 4- cyanopentanoic acid dithiobenzoate (see Figure 1).

The spectral data suggests that the protective ligand bound to CuNPs is mainly in the form of the PhCH₂S- that is a reduced derivative of dithiobenzoate side in the starting material of 4-cyanopentanoic acid dithiobenzoate in the above preparative reaction, where the reductant of sodium borohydride was used excessively. Evidence for the above suggestion can also be found in the ¹H and ¹³C NMR spectra of the as-prepared CuNPs as shown in Figure 2. In the ¹³C NMR spectra, a broad peak in the range of chemical shift from 125-132 ppm centered at 128.3 ppm and a tiny peak centered at 29.5 ppm can be assigned to a benzene ring and a methylene, respectively, which correspond very well with the ¹³C NMR spectra of benzylthiol. Moreover, a broad absorption peak between 6-8 ppm (Ar-H) in the ¹H NMR spectrum of CuNPs is similar to that in the case of benzylthiol, except for a peak of -S-H at ca. 1.7 ppm. In addition, some other tiny peaks shown in the ¹H NMR spectrum of CuNPs may imply some unknown matter present in the sample.

Figure 3 shows the percentage weight losses of both CuNPs. For CuNP-I, the weight loss of the organic ligand when heating up to 800 ⁰C is 59.7 %, while the weight loss for CuNP -2 is 51.8 %. This means that CuNP-I contains the organic ligand ca. 8 % more than CuNP -2, most likely due to the fact that the higher molar ratio between CPAD and CuCl₂ was used in the preparation of CuNP-I, the smaller particle size was obtained. Smaller particles need more ligands to protect. The calculated molar ratios are PhS/Cu = 1/1.3 for CuNP-I and 1/1.8 for CuNP-2, respectively, and may be written Cui.₃(SPh) and Cu_1-8(SPh).

The morphologies of both CuNPs were observed by HRTEM (Figure 4). Both samples spread on the carbon films covered Ni grids in the form of the dark spots that are composed of many granular nanoparticles. The UV-vis spectra of both CuNP samples dissolved in chloroform show a continuous smooth absorption without any peaks in the range of wavelength from 400-800 nm (not shown). Although we cannot clearly see the CuNPs, the EDS analysis carried out on the dark spots for both samples shown in Figure 4 reveal evidently the elemental signals of Cu, S, and C that derive from the samples themselves. It is worth noting that there is no detectable O signal in the EDS analysis. This means that the resulting CuNPs mainly protected with benzylthiolate ligand are chemically stable against oxidation. Moreover, the SAED patterns of both CuNPs shown in Figure 4 (inset) reveal all diffusive rings, most likely due to the very small particles (in the amorphous state or nanocrystallites).

It can be concluded that the as-prepared CuNPs primarily behave as amorphous nanoparticles or nanocrystallites, and their phase may inferably be sensitive to heat; to some extent, quite few nanocrystalline copper nanoparticles (less than 3 nm) may be prepared and included.

Based on XPS data, in combination with the spectral data discussed above, it can be concluded that the as-prepared copper particles do not contain oxidized copper, that is, the copper particles are very stable against oxidation during preparation and storage.

XPS data, in combination with Cu LMM AES spectra, makes it possible to distinguish the copper state. The valence state of Cu in the as-prepared copper particles is +1.

To observe the phase transition to the polycrystalline structure from amorphous state, in situ WAXS heating experiments of both CuNPs were conducted from 30 ⁰C up to ca. 240 ⁰C under helium flow, and the WAXS patterns are shown in Figures 5a and 5b. It will appear that at low temperatures between 30 and ca. 100 ⁰C the particles display very broad scattering peaks. This feature is attributed to amorphous structure or nanocrystallites. For CuNP-I shown in Figures 5a and 6, when heating up to about 100 ⁰C, the broad peaks start to rise and are identified as reflections from a hexagonal Cu₂S phase as well. With continuously heating to over 130 ⁰C, the hexagonal Cu₂S phase disappears and another phase which is identified as a cubic high temperature Cu_1-8S (digenite) phase occurs. CuNP -2 shows a similar behavior as CuNP-I (see Figure 5b). These sharp scattering peaks shown in WAXS measurements clearly reveal that the as-prepared nanoparticles can crystallize upon heating to moderate temperatures and the size of crystallites formed is increasing with further heating. It is an interesting finding that both CuNP samples form the same crystalline structure of a cubic high temperature Cui.sS phase upon heating.

Further, at temperature below about 100 ⁰C the sizes of the nanocrystallites remain nearly constant. By heating over 100 ⁰C, the crystallite sizes start to increase. If taking into account the thermal decomposition behaviors of both CuNPs measured by TGA (Figure 3), it can be seen that both samples just start to decompose slowly when heating up to 100 ⁰C, and the crystallites do not grow until at about 100 ⁰C, due to the fact that the ligands protect the crystallites from growing. In contrast, when heating up to 150-160 ⁰C, from which the weight loss is going to a steep stage (see Figure 3), the calculated crystallite size is increasing markedly upon heating. It is clear to see that the decomposition of the protective ligands releases sulfur and copper atoms and enhances the crystalline growth. Relatively large crystallites are formed after sintering.

Based on the above, it can be concluded that the novel copper nanoparticles have a significant potential application in fabrication of nanodevices in future.

Claims

Claims:

1. Nanoparticles having an average particle size of 1 to 10 nm and comprising at least one protective group bonded to metal particles, said group being derived from a monomeric thiol.

2. The nanoparticles according to claim 1, wherein the protective group bonded to the metal particles is derived from a monomeric thiol having the formula I

R¹R²SH

wherein R¹ stands for a cyclic group and

R² stands for a bivalent group, said metal particles being essentially protected against oxidation.

3. The nanoparticles according to claim 1 or 2, wherein the monomeric thiol is obtainable by reducing the corresponding dithioester.

4. The nanoparticles according to any of claims 1 to 3, wherein R¹ stands for a cyclic group selected from aromatic and aliphatic groups.

5. The nanoparticles according to claim 4, wherein the cyclic aromatic group is a benzyl or phenyl which optionally bears 1 to 5 substituents on the ring, or naphthyl, which optionally bear 1 to 11 substituents on the ring structure.

6. The nanoparticles according to any of the preceding claims, wherein the bivalent group is a linear or branched aliphatic group selected from alkylene, alkenylene and alkynylene which optionally bears 1 to 3 substitutents.

7. The nanoparticles according to claim 5 or 6, wherein the substitutents are selected from the group of halogen, alkyl and alkenyl.

8. The nanoparticles according to any of the preceding claims, wherein said particles are essentially amorphous or nanocrystalline.

9. The nanoparticles according to any of the preceding claims, wherein the metal is selected from the group of copper, aluminium, zinc, nickel, cobalt and indium and mixtures thereof.

10. The nanoparticles according to any of the preceding claims, wherein said particles have a valence of copper of + 1.

11. The nanoparticles according to any of the preceding claims, wherein the nanoparticles are capable of crystallizing upon heating to yield hexagonal Cu₂S at temperatures below about 200 ⁰C and cubic Cu_1-8S at temperatures above about 200 ⁰C.

12. The nanoparticles according to any of the preceding claims, having on an average 50 - 60 % organic protectant bonded to each metal particle, in particular to each copper particle.

13. The nanoparticles according to any of the preceding claims, wherein the protective groups are bonded to the metal particle, in particular copper particle, by coordination forces or by covalent bonds.

14. The nanoparticles according to any of the preceding claims, wherein the nanoparticles have a resistivity of about 10^"8 to 10^"3 ohm*m.

15. The nanoparticles according to any of claims 1 to 14, having the schematic formula III

CH^

wherein the black sphere represents one or several copper atoms forming a particle.

16. A method of producing amorphous metal nanoparticles having an average particle size of about 1 to 10 nm, comprising the steps of reducing, in the presence of a reducing agent, dithioester monomers to the corresponding thiols while simultaneously reducing a metal precursor to the corresponding metal to yield amorphous or nanocrystalline metal nanoparticles.

17. The method according to claim 16, comprising

- reacting, in the presence of a reducing agent, a metal precursor with an organic compound having formula II

R³^)SSR¹ II

wherein R¹ has the same meaning as above, R³ stands for an alkanoic acid which optionally bears a substituent, and R⁴ stands for an alkyl group

to provide a compound having the formula I and - recovering the product.

18. The method according to claim 16 or 17, wherein the reaction is carried out in an atmosphere which contains less than 1 % oxygen, preferably the reaction is carried out in an inert atmosphere essentially free from oxygen.

19. The method according to claim 18, wherein the atmosphere comprises a gas selected from the group of nitrogen, argon, helium, hydrogen, carbon dioxide and mixtures thereof.

20. The method according to any of claims 16 to 19, wherein the reaction is carried out at a temperature of 10 to 40 ⁰C, preferably at ambient temperature.

21. The method according to any of claims 16 to 20, wherein the reaction is carried out in aqueous medium.

22. The method according to any of claims 16 to 21, wherein R³ stands for an alkanoic acid having 2 to 6 carbon atoms and which is substituted with at least one cyano group at an optional position.

23. The method according to any of claims 16 to 22, wherein R¹ stands for an aryl group, in particular a phenyl group.

24. The method according to any of claims 16 to 23, wherein the compound according to formula II comprises 4-cyanopentanoic acid dithiobenzoate.

25. The method according to any of claims 16 to 24, wherein the metal is selected from the group of copper, aluminium, zinc, nickel, cobalt and indium and mixtures thereof.

26. The method according to any of claims 16 to 25, wherein the copper precursor comprises an organic or inorganic copper (II) salt, preferably a copper (II) salt which is soluble in water.

27. The method according to claim 26, wherein the copper salt is selected from the group of copper chloride, copper nitrate, copper sulphate, copper acetate and mixtures thereof.

28. The method according to any of claims 16 to 27, comprising using at least an equimolar amount of a compound according to Formula II with respect to the metal precursor, preferably the compound of Formula II is used in a molar amount of 1 : 1 to 3: 1 in respect to the copper precursor.

29. The method according to any of claims 16 to 28, wherein the reducing agent is a selective reducing agent selected from the group of sodium borohydride, sodium cyanoborohydride, sodium dithionite, sodium triacetoxyborohydride, litium aluminium hydride, diisobutylaluminum hydride, dimethylsulphide borane, hydrazine, phenyl silane, sodium bis(2-methoxyethoxy)aluminumhydride and mixtures thereof.

30. The method according to any of claims 16 to 29, wherein the reaction is carried out under vigorous stirring.

31. The method according to any of claims 16 to 30, wherein the reaction product is recovered and potentially filtered and washed under inert gas protection.

32. The use of nanoparticles according to any of claims 1 to 15 for producing semiconductive or conductive thin layers on a substrate.

33. The use according to claim 32, wherein the substrate is selected from group of sheets and webs of paper, cardboard and various polymer materials.

34. The use according to claims 32 or 33, wherein the thin layers are formed by printing or lithography.

35. The use according to any of claims 32 to 34, wherein the thin layers are formed by heating the nanoparticles to form essentially crystalline agglomerates.

36. The use according to any of claims 32 to 35 in photoinduced voltaics or catalysis.

37. The use according to any of claims 32 to 36, comprising using copper nanoparticles.