A method for preparing metal nanoparticles

This invention relates to a method for preparing of particles of nanometer size, especially to the preparation of metal nanoparticles.

It is very likely that the use of nanoparticles will find increasing use in the near future. The trend of using particles significantly smaller than the common 0,5 micrometer particles seems evident.

Cobalt metal powder is commonly used as a powder as a binder of tungsten carbide in hard metal parts. Another significant application is anodes in lithium ion batteries. In addition to traditional applications, cobalt nanoparticles will be used in antennas and memory materials of mobile equipment.

Currently, commercially available nanoparticles are very expensive. Due to the high price their industrial use is rather limited.

One way of producing nickel and cobalt metal powders with 0,5 micrometer size is reducing their corresponding metal salts with hydrogen. The method is, however, limited and particle sizes below 100 nm has not been achieved.

Another known method to prepare a powder involves thermal decomposition of their corresponding metal, e.g. cobalt oxalate, salts. Particles prepared by this method are smaller than those obtained by the method described above, but they are polydisperse. Additionally, the price of the starting materials is considered too high for the metal industry. Thermal decomposition of metal carbonyls have also been used to prepare nanoparticles. These methods produce usually vry pure particles. The size and size distibution depend highly on the use stabilizators and the production method.

Numerous other methods for the preparation of nanopartices have been reported, including chemical reduction, inverse emulsions, "polyol process" and hot "injenction". One example is given in WO 2006/095002 A1. Control of both nucleation and growth is important in the production. These steps are poorly

controlled in many of the above mentioned methods, which leads to a broad size distribution and difficulty unpredictable particle size. The nucleation and growth processes can be affected by the used reactants, physical conditions and surface active agents.

The hot injection method, where the nuclei are formed by injecting reagents which decompose in higher temperatures into a hot solvent, is particularly interesting. The problem of this method is, however, that after the good supersaturation of the starting point the solution is cooled rapidly due to the endothermic nature of the phase formation reaction whereby nucleation stops instantly. A problem in the hot injection method is the small scale volume of the same. The method can be used for production in laboratory scale only. One especially good group of precursors in the hot injection method is metal carbonyls. The only decomposition products of this group are metal and carbon monoxide gas, and thus the final product is very pure.

The purpose of this invention is to show a method by which the preparation of nanoparticles is considerably cheaper than by methods used up to date. Additional purpose is to achieve a method which is well controlled whereby the final product is of the desired quality. The low price would naturally favor a marked increase of the use of the powder in nano scale.

The afore mentioned and other advantages of the invention have been achieved as shown characteristic in the annexed claims.

The principles of the invention are given in the description below.

Contrary to hot injection, in which the decomposition of the metal carbonyl is achieved by injecting the same into a hot solvent, nucleation is according to this invention achieved by rapidly dropping the carbon monoxide gas pressure which causes the metal carbonyl to decompose. Hence, all nuclei are formed simultaneously. The number of the formed nuclei, and hence the particle size when all starting material has been totally consumed, can be controlled by the pressure.

A metal carbonyl in solvent is initially heated with a stabilizator under high carbon monoxide pressure to the desired temperature. Then the pressure is rapidly dropped, which results in instant decomposition of the metal carbonyl and the formation of nuclei. Cobalt carbohyl that has not reacted is slowly decomposed onto the nuclei through heating. In the method described above the particle size is primarily determined by the number of nuclei and the cobalt carbonyl remaining the solution after the decomposition step. These properties can be easily controlled with the pressure drop and the corresponding temperature.

The solvent used in the heating step is chosen for the purpose so that the used materials are suitably soluble in the solvent and that, on the other hand, the boiling point of the solvent is high enough so that the solvent does not, under the used temperatures, boil. One exemplary and useful solvent is dodecane.

One advantage of the method according to the invention is that pressure can be changed more rapidly than temperature, and hence, the method is well suited for higher scale industrial producing.

Additional advantage of the invention is that metal carbonyl can be prepared by standard procedures in the same reaction vessel. This greatly increases the security of of the use of metal carbonyls. Furthermore, instantaneous nucleation is achieved through the rapid pressure drop. This is clearly more rapid and better way than heating or mixing of chemicals.

The invention can also be used fro the preparation of composite materials, e.g. coating of tungsten carbide with cobalt.

Experimental results relating to the invention are presented in the Figures, in which some features of the of invention is presented.

Figure 1 presents the decomposition of cobalt carbonyl Co2(CO)8 in dodecane as a function of temperature. The carbon dioxide pressures have been set at 40 ⁰ C, after which the reactor was sealed.

The dependence on the particle size and the decomposition pressure for the preparation given in Example 2 is presented in Figure 2.

The transmission electron micrographs of the particles described above are shown in Figure 3.

Example 1

Figure 1 shows the decomposition of Co ₂ (CO) ₈ in dodecane as function of temperature when the parameter was the initial pressure carbon monoxide, at 40 ⁰ C. The test was effected in a closed reactor. In the test carbon monoxide was assumed to follow the ideal gas law. The reactor was heated at 1 °C/min. From the curves it can be seen that at lower initial pressures Co ₂ (CO) ₈ forms Co ₄ (CO) ₁₂ + 4 CO. In higher initial pressures the decomposition leads directly to a pure metal and carbon monoxide. Increasing the pressure of carbon monoxide will lead the reaction towards higher temperatures as can be concluded starting from simple thermodynamic principles. In this way it can be concluded that carbon monoxide can be used to stabilize cobalt carbonyl at elevated temperatures. The temperature of the pressure drop is to be chosen so that the metal is principally in carbonyl form.

Example 2

0,5 g dicobalt carbonyl Co ₂ (CO) ₈ , 1 ml trioctylamine and 20 ml dodecane was placed in a 100 ml reactor. The reactor was first flushed with carbon monoxide and the pressure was then increased to 50 bar. The reactor was heated at 3 ⁰ C /min to 170 ⁰ C. The pressure was rapidly dropped to 1 bar (p1). The mixture was kept at 170 ⁰ C for 20 minutes and cooled to room temperature. The product was collected as a black colloid that contained cobalt nanoparticles with a diameter of approximately 6 nm.

The above described procedure was also repeated so that the decomposition pressure was 2.5, 3.2, 4.5 and 5.3 bar. The particle size increased accordingly to 20, 31 , 81 , and 139 nm

The dependence of the particle size on the decomposition pressure is shown in Figure 2. Controlling of the particles size is thus quite easy by adjusting the end pressure.

The transmission electron micrographs of the particles described above are shown in Figure 3. The decomposition pressure is given in the upper corner of each figure.

The principles of the invention applied to the preparation of cobalt nano particles have been described above. It is, however, clear that cobalt is not the only metal the particles of which in nano scale can be prepared by analogous methods.

Example 3

0,5 ml iron penta carbonyl Fe(CO) ₅ , 1.65 ml tridodecyl amine and 20 ml paraffin was placed in a 100 ml reactor. The reactor was first flushed with carbon monoxide and the pressure was raised to 50 bar. The reactor was heated at 3 ⁰ C /min to 250 ⁰ C. The pressure was rapidly dropped to 1 bar (p1). The mixture was kept at 250 ⁰ C for 20 minutes and cooled to room temperature. The product was collected as a black colloid that contained iron nanoparticles with a diameter of approximately 3 nm.

The procedure described above was also repeated so that the decomposition pressure was 4 bar. The particle size increased to 8 nm.