WO2006095002A1

WO2006095002A1 - Method for the one-pot preparation of stable nano-sized particles under mild conditions

Info

Publication number: WO2006095002A1
Application number: PCT/EP2006/060575
Authority: WO
Inventors: Miquel A. PERICÀS-BRONDO; Ciril Jimeno Mollet; Anna Lagunas Targarona; Antoni MAIRATÀ PAYERÀS
Original assignee: Institut Català D'investigació Química
Priority date: 2005-03-11
Filing date: 2006-03-09
Publication date: 2006-09-14

Abstract

A method for the one-pot preparation of stable metallic nano-size particles with a narrow size distribution, comprising the decarbonylation of a metallic carbonyl precursor at a mild temperature in the presence of at least one coordinating compound selected from the group consisting of surfactants and coordinating solvents and at least one decarbonylation promotor. The method can further comprise an aging period and/or the isolation of the nano-size particles. Furthermore, a method for coating a substrate with metallic nano-size particles.

Description

Title of the invention

METHOD FOR THE ONE-POT PREPARATION OF STABLE NANO-SIZE PARTICLES UNDER MILD CONDITIONS

Field of the invention

The present invention relates to a method for preparing stable metallic nano-size particles. More specifically, the present invention is directed to a method for the preparation of stable metallic nano-size particles starting from a metallic carbonyl precursor at mild condicions in the presence of a surfactant and a decarbonylation promoter.

Background art

Metallic and metal oxide nanoparticles have become an area of growing interest and importance in a wide range of technological applications, due to their unique optical, electronic, magnetic, chemical and mechanical properties. Furthermore, nanoparticles, often metal oxides, are increasingly being associated with important environmental processes occurring in soils and the atmosphere. Hence, synthetic routes to produce nanoparticles for technological purposes are needed. Research has shown that the properties of the nanoparticles are often different from their bulk counterparts. With regard to metallic nanoparticles, for example, the conduction band, which is present in bulk metal, is absent in nanometal, where instead there are discrete states at the band edge. At the nanoscale level, transition metals show magnetic behaviour that is dependent on size. Other nanosized materials show specific enhancements in redox properties or exhibit a catalytic activity that cannot be reproduced by bulk metal particles. Many examples have been described which show that unique catalytic activity can be obtained if the special dimension of the catalytic particle is brought into the nanoscale regime. Nano-size substances have been found to be superior to bulk ones in battery applications, improving the toughness of ceramics, increasing the density of magnetic recording, enhancing the properties of light emitting diodes (LEDs) and as labels for detection assays. These precedents emphasize the need to synthesize nanoparticles with well-defined size, so that surface reactivities and properties can be optimized.

Currently, many techniques are being used for the preparation of submicrometer to nanometer sized particles. These include vapour-phase techniques such as vapour deposition, plasma-gas condensation (for example U.S. patent application US 2004009118), laser pyrolysis (for example U.S. patent application US 2003203205) or electron beam lithography as well as by liquid-phase techniques that employ microemulsions, precipitation, sol-gel or hydrothermal synthesis. The vapour-phase techniques usually require sophisticated methods and equipment. On the other hand, the preparation of uniform nanosized, monodispersed transition metal based particles still remains a significant challenge. The creation of monodispersed nanoparticles with controlled size is of great interest, as it gurantees identical physical and chemical properties.

In precipitation methods, the metal salt is dissolved in water and added to a high pH aqueous solution yielding a precipitate hat must then be digested for a lengthy period of time to form crystals. The size and morphology of the product are therefore very sensitive to variables such as the pH and temperature. Microemulsion synthesis involves the formation of a stable microemulsion by the addition of a metal salt to a solution containing a surfactant, organic solvent and water. Metal cations attach to the hydrophilic ends of surfactants molecules and, upon addition of a strong base, react with hydroxide ions to precipitate the oxide. As in the case of precipitation, a period of digestion is required to obtain a crystalline product. Also, the growth of the particles is limited by the local environment in the individual micelles and results in very little product being formed.

The sol-gel methods imply the addition of a gel to a solution of the precursor followed by drying the solution and calcinating the dry gel powders at high temperatures. The patent applications ES2190300 and ES2188343 describe some of such methods. The main drawback of the sol-gel methods are the high calcination temperatures required.

Microemulsion and sol-gel techniques are difficult to scale-up and require significant post-treatment to obtain crystalline products.

Hydrothermal techniques also require prolonged processing times and often result in particles that are too large and not very uniform. Near and supercritical water have been used with some success in continuous hydrothermal crystallization processes for the synthesis of oxide nanoparticles (via hydrolysis and rehydration; Yalin Hao and Amyn S. Teja, J. Mater. Res., 2003, Vol. 18, No. 2, 415-422) . Some drawbacks of the method are the high temperatures required (500-700 K) , the growth of particles in the reactor (leading to morphology changes) or agglomeration .

Recently, polymeric matrixes or surfactants have been used in colloidal syntheses of nano-size particles. Said compounds have the ability to control the size and shape of the growing particles. In addition, these compounds prevent the agglomeration of particles giving rise to agglomerates with a broad size distribution. The main disadvantage of the polymeric matrixes is that they mask the nanoparticles and thus attenuate their properties with regard to catalysis, etc.

Puntes, V. F.; Krishnan, K. M.; and Alivisatos, A. P., Science, 2001, 291, 2115-2117, describe a method for the preparation of nano-scale particles of cobalt starting from Co₂(CO)₈- This method implies the injection of a solution of the precursor to a refluxing solution of a mixture of two surfactants (oleic acid and trioctylphosphine oxide) . The main disadvantage of the method is that it involves the decomposition of the precursor and thus high temperatures are needed for the formation of the nano-size particles.

It is an object of the present invention to provide a method for the preparation of stable metallic nano-size particles with a narrow size distribution, which involves mild conditions and which overcomes the deficiencies of the prior art.

Summary of the invention

It is an object of the present invention to provide a method for the preparation of stable metallic nano-size particles with a narrow size distribution, which involves mild conditions and which overcomes the deficiencies of the prior art. This object is achieved by the method of the present invention.

The present invention relates to a method for the preparation of stable metallic nano-size particles with a narrow size distribution. Advantageously, the method of the present invention is a one-pot method. More specifically, the present invention refers to a method for the preparation of stable metallic nano-size particles starting from a metallic carbonyl precursor of formula M_x(C0)_y, wherein M is a metal and x and y are integers from 1 to 20. Advantageously, most carbonyl precursors are commercially available products.

The method of the invention comprises the decarbonylation of said carbonyl precursor at a mild decarbonylation temperature in the presence of at least one coordinating compound selected from the group consisting of surfactants and coordinating solvents and at least one decarbonylation promoter.

The term "mild decarbonylation temperature" as defined in the invention refers to a decarbonylation temperature not higher than the decomposition temperature of the metallic carbonyl precursor.

The term "coordinating compound" as defined herein refers to a compound which has the ability to coordinate to the nano-size particles. According to the method of the present invention, said coordinating compound is selected from the group consisting of surfactants and coordinating solvents. The term "coordinating solvent" as defined herein refers to a solvent which has the ability to coordinate to the nano-size particles. In a preferred embodiment of the present invention, the coordinating compound is a surfactant, preferably a long chain carboxylic acid or fatty acid. For example, the surfactant is selected from the group consisting of oleic acid, linoleic acid, alpha-linoleic acid, estearic acid, palmitic acid and mirystic acid.

The method according to the present invention takes place in solution, preferably in an organic solvent. In a preferred embodiment of the present invention, said organic solvent is a coordinating solvent, for example 0- dichlorobenzene or THF. In a further preferred embodiment, the method for the preparation of stable metallic nano-size particles according to the present invention is performed in the presence of at least one surfactant and at least one coordinating solvent.

The term "decarbonylation promoter" as defined in the invention refers to a compound which promotes the decarbonylation of the metallic carbonyl precursor. In a preferred embodiment of the invention, said decarbonylation promoter is a stable free radical and said decarbonylation takes place through a radical mechanism. Preferably, said stable free radical is a nitroxyle, for example 2, 2, 6, 6-tetramethylpiperidinyloxy free radical. In another preferred embodiment of the invention, said decarbonylation promoter is a nitrogen-based oxidant and said decarbonylation takes place through the oxidation of the carbonyl to carbon dioxide. Preferably, said nitrogen- based oxidant is an amine oxide.

Surprisingly, the addition of a decarbonylation promoter to the reaction mixture allows the decarbonylation of the precursor at mild conditions and the subsequent formation of the nano-size particles at mild conditions. Therefore, it is not necessary to apply high temperatures for the thermal decomposition of said precursor in order to successfully form nano-size particles .

An advantage of the method of the present invention is that it allows the preparation of nano-size particles under mild conditions. In contrast to the prior art, the method of the invention comprises the promoted decarbonylation of the metallic carbonyl precursor rather than the thermal decomposition of said metallic carbonyl precursor, so that milder temperatures are allowed. Therefore, the scale-up is facilitated, the method of the present invention being very susceptible of industrial application, with the subsequent economic and industrial advantages .

According to the method of the present invention, the ratio between the decarbonylation promoter and the metallic carbonyl precursor is from 1:1 to y:l, the value y corresponding to the formula M_x(C0)_y.

Advantageously, the present invention allows the preparation of stable metallic nano-size particles with a narrow size distribution. In a preferred embodiment of the invention, the diameter of a single nano-size particle differs from the average diameter no more than 10 nm, preferably no more than 5 nm.

The nano-size particles prepared by the method of the present invention are stabilized by a plurality of molecules of surfactant bound to them.

The nano-size particles prepared by the method of the present invention can be, but are not limited to, nanocrystals . The nano-size particles prepared by the method of the present invention can be, without limitation to the scope of the present invention, magnetic.

In a preferred embodiment, the stable nano-size particles prepared by the method of the present invention are associated into stable aggregates of nano-size particles. For example, the method of the present invention leads to the formation of stable nano-size particles associated into spherical or quasi-spherical stable aggregates of particles.

In a further preferred embodiment of the method according to the present invention, said stable aggregates are nano-size aggregates.

Some examples of nano-size particles that can be obtainable by the method of the present invention are homogeneous metal particles, homogeneous metal oxide particles or core-shell particles. The formation of metallic nano-size particles, metal oxide particles or core-shell particles depends on the particular metal. In a preferred embodiment of the method of the present invention, said nano-size particles are metallic oxide particles. In another preferred embodiment, said nano-size particles are core-shell particles wherein the core is metal and the shell is metal oxide.

The invention also refers to a method for the preparation of stable nano-size particles further comprising an aging period at an aging temperature not lower than the decarbonylation temperature. Advantageously, the aging temperature is a mild temperature. Advantageously, the method of the invention further comprising the aging step is still a one-pot method.

Advantageously, by controlling the nature of the coordinating compound, the ratio between the coordinating compound and the metallic carbonyl precursor, and the aging period, it is possible to control the average size of the aggregates of nano-size particles.

The present invention also refers to a method further comprising the isolation of the stable nano-size particles from the reaction media, preferably by precipitation. In a preferred embodiment, this precipitation is performed by adding a second solvent to the solution of nano-size particles, followed by centrifugation . Advantageously, the method of the invention further comprising the precipitation step is still a one-pot method. The nano-size particles obtained according to this preferred embodiment are in solid state.

Advantageously, the method of the present invention can be successfully applied to a variety of metallic carbonyl precursors, giving rise to a diversity of stable metallic nano-size particles.

The present invention also refers to a collection of stable nano-size particles comprising a plurality of molecules of the at least one coordinating compound bound to the nano-size particles, wherein said stable nano-size particles are obtainable by the method of the present invention, the stable nano-size particles having a narrow size distribution, the diameter of a single nano-size particle differing from the average diameter no more than 10 nm, preferably no more than 5 nm.

The collection of stable nano-size particles according to the present invention can be in solution or in solid state. In a preferred embodiment of the present invention, said collection of stable nano-size particles are associated into stable aggregates of nano-size particles. In a further preferred embodiment, said stable aggregates are nano-size aggregates.

The present invention also refers to a method for coating a substrate with metallic nano-size particles comprising the decarbonylation of a carbonyl precursor of formula M_x(C0)_y, M being a metal and x and y being integers from 1 to 20, at a temperature not higher than the decomposition temperature of the metallic carbonyl precursor, in the presence of the substrate, at least one decarbonylation promoter and at least one coordinating compound selected from the group consisting of surfactants and coordinating solvents.

For the purposes of the method of the present invention, said substrate can be a microporous material, for example a microporous polymeric material or a zeolite. An example of microporous polymeric material that can be used according to the present invention is a polystyrene / divinylbenzene resin. Alternatively, the substrate can be a non-porous material, for example a non-porous polymeric material. An example of non-porous polymeric material that can be used according to the present invention is polianiline .

In a preferred embodiment of the coating method according to the present invention, said substrate is a microporous material and the coating takes place inside the pores of said microporous material. In a further preferred embodiment, said substrate is a coordinating microporous material and the coating takes place both inside the pores and on the surface of the material. In another embodiment of the coating method according to the present invention, said substrate is a non-porous material and the coating takes place on the surface.

Advantageously, the coating method according to the present invention is a one-pot method. A further advantage of the method is that it allows the coating of a substrate with nano-size particles under mild conditions. Moreover, the method of the invention allows the coating of a substrate with stable nano-size particles which have a narrow size distribution, the diameter of a single nano- size particle differing from the average diameter no more than 10 nm, preferably no more than 5 nm. The invention also refers to a method for coating a substrate with nano-size particles further comprising an aging period at an aging temperature not lower than the decarbonylation temperature. Advantageously, the aging temperature is a mild temperature. Advantageously, the coating method of the invention further comprising the aging step is still a one-pot method.

The present invention also refers to a method for coating a substrate with metallic nano-size particles comprising the following steps: (a) decarbonylation of a carbonyl precursor of formula M_x(C0)_y, M being a metal and x and y being integers from 1 to 20, at a temperature not higher than the decomposition temperature of the metallic carbonyl precursor, in the presence of at least one decarbonylation promoter and at least one coordinating compound selected from the group consisting of surfactants and coordinating solvents;

(b) an aging period in the presence of the substrate.

The present invention also refers to a composite material comprising a collection of nano-size particles, said nano-size particles comprising a plurality of molecules of surfactant bound to them, wherein said composite material is obtainable by the coating method of the present invention and wherein the diameter of a single nano-size particle differs from the average diameter no more than 10 nm, preferably no more than 5 nm.

Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration and are not intended to limit the scope of the invention. It is intended that the scope of the present invention be defined by the claims appended hereto.

Brief description of the drawings

Fig. 1 is a TEM micrograph of the cobalt oxide nano-size particles of Example 1, associated into nano-size aggregates.

Fig. 2 is a diagram showing the profile of the residual carbonyl band of Co₂(CO)₈ at -2022 cm-1 corresponding to Example 1.

Fig. 3 is a TEM micrograph of the chromium oxide nano-size particles of Example 2, associated into nano-size aggregates .

Fig. 4 is a TEM micrograph of the iron oxide nano-size particles of Example 3, associated into nano-size aggregates. The micrograph has been enlarged in order to observe the nano-size particles inside the aggregates.

Fig. 5 is a TEM micrograph of the tungsten oxide nano-size particles of Example 4, associated into nano-size aggregates. The micrograph has been enlarged in order to observe the nano-size particles inside the aggregates.

Fig. 6 is the histogram corresponding to the tungsten oxide nano-size particles of Example 4.

Detailed description of particular examples

Example 1

0.1052 g of cobalt carbonyl are weighed in a 10 ml

IR flask and solved under argon atmosphere in 2 ml of degassed o-dichlorobenzene . 0,1 ml of oleic acid is added. Then, a solution of 0,3352 g of TEMPO in 2 ml of degassed o-dichlorobenzene is injected using a pumping syringe

(rate= 0,2 ml/min; diameter= 12, 8 mm) . The decarbonylation process is followed by IR-monitoring the different carbonyl bands profiles. Figure 2 shows the profiles of the residual carbonyl band of Co₂ (CO) ₈ at

-2022 cm-1. The decarbonylation is completed at r.t. After the injection, the solution is heated in a silicon bath at

50⁰C during 3 h. After this period, the experiment is aborted and the dark purple solution transferred to a 25 ml round bottomed flask and stored under argon in the freezer. TEM images (Figure 1) are directly registered over the reaction mixture without any size selective precipitation procedure. The HRTEM image showed CoO nano- size aggregates of nanoparticles with an average diameter of 88 nm. The average diameter and standard deviation of the nano-size particles inside the aggregates is 3+2 nm.

If toluene is used instead of o-dichlorobenzene, cobalt oxide nano-size aggregates of nanoparticles with an average diameter of 186 nm are obtained. If dioctyl ether is used instead of o-dichlorobenzene, cobalt oxide nano- size aggregates of nanoparticles with an average diameter of 625 nm are obtained.

Example 2

Chromium hexacarbonyl (100 mg, 0.45 mmol, white solid) in 8 mL of THF is placed under inert atmosphere

(colorless solution) . In another round-bottom flask, TEMPO (179 mg, 2.5 equiv., orange solid) is dissolved in approx.

2 mL of THF and added to the solution containing the rest of the components at reflux. The reaction was stirred for

4 h at reflux. During that time the solution slowly turns green and CO is liberated. Once the solution has cooled down to rt, the solid was separated from the solution by centrifugation . The HRTEM image (see figure 3) showed Cr₂O₃ nano-size aggregates of nanoparticles with an average diameter of 130 nm. The average diameter and standard deviation of the nano-size particles inside the aggregates is 6+2,5 nm.

Example 3

0,40 g (0,715 mmol) of triiron dodecacarbonyl are weighed in a 100ml two-necked flask and solved under argon atmosphere in 8ml of anhydrous o-dichlorobenzene (4A molecular sieves). 0,35ml (1,07 mmol) of oleic acid are added to this solution and the mixture is introduced in a silicon bath at 200⁰C. After atemperation, a solution of l,3673g (8,58 mmol) of TEMPO in 8ml anhydrous o- dichlorobenzene (4A molecular sieves) is canuled in. The reaction mixture is allowed to mechanically stir at 200⁰C over Ih. After this time and cooling to r.t., 5ml of anhydrous acetone (4A molecular sieves) are added. The suspension is centrifuged (90' 4,4rpm) and the black precipitate washed 2 times with dichloromethane

(centrifugation: 30' 4,4rpm each time). The solvent is evaporated under vacuum and the residual black powder dried with nitrogen current. The HRTEM images (see figure 4) showed Fe₂O₃ nano-size aggregates of nanoparticles with an average diameter of 61 nm. The average diameter and standard deviation of the nano-size particles inside the aggregates is 29+0,5 nm. Example 4

0,403 g (1,12 mmol) of tungsten carbonyl are weighed in a 100ml two-necked flask and solved under argon atmosphere in 8ml of anhydrous o-dichlorobenzene (4A molecular sieves). 0,18ml (0,56 mmol) of oleic acid are added to this solution and the mixture is introduced in a silicon bath at 200⁰C. After atemperation, a solution of l,0776g (6,75 mmol) of TEMPO in 8ml anhydrous o- dichlorobenzene (4A molecular sieves) is canuled in. The reaction mixture is allowed to stir at 200⁰C over Ih. After this time and cooling to r.t., 5ml of anhydrous acetone (4A molecular sieves) are added. The suspension is centrifuged (90' 4,4rpm) and the black precipitate washed 2 times with dichloromethane (centrifugation : 30' 4,4rpm each time) . The solvent is evaporated under vacuum and the residual black powder dried with nitrogen current. The HRTEM images (see figure 5) showed WO3 nano-size aggregates of nanoparticles with an average diameter of 167 nm. The average diameter and standard deviation of the nano-size particles inside the aggregates is 5,3+0,6 nm (see figure 6) .

Claims

1. A method for the preparation of stable metallic nano- size particles starting from a metallic carbonyl precursor of formula M_x(C0)_y, M being a metal and x and y being integers from 1 to 20, in the presence of at least one coordinating compound selected from the group consisting of surfactants and coordinating solvents, wherein the method comprises the decarbonylation of the carbonyl precursor at a temperature not higher than the decomposition temperature of the metallic carbonyl precursor in the presence of at least one decarbonylation promoter .

2. The method according to claim 1, wherein the decarbonylation promoter is selected from the group consisting of stable free radicals and nitroxyle radicals.

3. The method according to claim 1 or 2, wherein the stable nano-size particles comprise a plurality of molecules of the at least one coordinating compound bound to them.

4. The method according to any of claims 1 to 3, wherein the diameter of a single nano-size particle differs from the average diameter no more than 10 nm.

5. The method according to any of claims 1 to 4, wherein the nano-size particles are metal oxide nanoparticles .

6. The method according to any of claims 1 to 5, wherein the nano-size particles are associated into nano-size aggregates of nanoparticles.

7. The method according to any of claims 1 to 6, further comprising an aging step.

8. The method according to any of claims 1 to 7, further comprising the isolation of the stable nano-size

5 particles .

9. A collection of stable nano-size particles comprising a plurality of molecules of the at least one coordinating compound bound to the nano-size particles, wherein said

10 stable nano-size particles are obtainable by the method of any of claims 1 to 8, the diameter of a single nano-size particle differing from the average diameter no more than 10 nm.

15 10. A method for coating a substrate with metallic nano- size particles comprising the decarbonylation of a carbonyl precursor of formula M_x(C0)_y, M being a metal and x and y being integers from 1 to 20, at a temperature not higher than the decomposition temperature of the metallic

20 carbonyl precursor, in the presence of the substrate, at least one decarbonylation promoter and at least one coordinating compound selected from the group consisting of surfactants and coordinating solvents.

25 11. The method according to claim 10 further comprising an aging step.

12. A method for coating a substrate with metallic nano- size particles comprising the following steps:

30 (a) decarbonylation of a carbonyl precursor of formula M_x(CO)_y, M being a metal and x and y being integers from 1 to 20, at a temperature not higher than the decomposition temperature of the metallic carbonyl precursor, in the presence of at least one decarbonylation promoter and at

35 least one coordinating compound selected from the group consisting of surfactants and coordinating solvents; (b) an aging step in the presence of the substrate.

13. A composite material comprising a collection of nano- size particles, said nano-size particles comprising a plurality of molecules of surfactant bound to them, wherein said composite material is obtainable by the method of any of claims 10 to 12, the diameter of a single nano-size particle differing from the average diameter no more than 10 nm.