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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.4 Concepción dic. 2001

http://dx.doi.org/10.4067/S0366-16442001000400008 

SYNTHESIS AND TEM STUDIES OF NICKEL COLLOIDS
PREPARED IN NONAQUEOUS SOLVENTS

GALO CÁRDENAS*, ALEJANDRA TELLO AND RODRIGO SEGURA

Departamento de Polímeros, Facultad de Ciencias Químicas,
Universidad de Concepción, Casilla 160-C, Concepción, Chile.
(Received: June 9, 2001 - Accepted: September 5, 2001)

SUMMARY

In this work metallic colloidal dispersions were obtained by simultaneous cocondensation of nickel atoms with organic solvents at 77 K. The atoms were produced by resistive heating and were reacted with 2-propanol, 2-methoxyethanol, 2-ethoxyethanol and acetone to produce colloids.

The kinetic stability of colloid dispersions was related to the solvation effect of organic molecules, e.g. low stability for ketones and higher stability for 2-propanol colloids. The colloidal particles were characterized by UV-Vis measurements showing an absorption band around 212 nm. The electrophoretic measurements reveal that particles are weakly positively charged.

The transmission electron microscopy studies reveal an average particles size distribution ranging from 4 to 23 nm depending on the solvent. Most of the colloids exhibit a spherical shape with some degree of agglomeration.

Keyword: metal colloids, nanostructures, vapor deposition, absorption band, electron microscopy.

RESUMEN

En este trabajo se preparan dispersiones coloidales por condensación simultánea de átomos de níquel con solventes orgánicos a 77K. Los átomos fueron producidos por calentamiento resistivo y se hacen reaccionar con 2.propanol, 2- metoxietanol, 2-etoxietanol y acetona para producir coloides de níquel.

La estabilidad cinética de las dispersiones coloidales fueron relacionadas con el efecto de solvatación de moléculas orgánicas, p. Ej., una baja estabilidad para cetonas y mayor estabilidad para los coloides de 2-propanol. Las partículas coloidales fueron caracterizadas mediante medidas de UV-Vis mostrando una banda de absorción alrededor de 212 nm. Las medidas de electroforesis revelan que las partículas están débilmente cargadas positivamente.

La microscopía electrónica de transmisión revela una distribución de tamaño promedio de partículas en el rango de 4 a 23 nm dependiendo del solvente. La mayoría de los coloides muestran una forma esférica con algún grado de aglomeración.

Palabras claves: coloides metálicos, nanoestructuras, depositación de vapores, bandas de absorción, microscopía electrónica.

INTRODUCTION

The nanometric particles are systems of great interest in material science nowadays. Their properties are different to the bulk material and at the same time different of the atomic state, with intermediate electronic properties. Another important property is that nanometric particles possess a great fraction of atoms localized on the surface, producing unique and unusual properties (1).

The codeposition of metal atoms with organic solvents at 77 K produces several interesting reactions (2). More lately have been the solvated metal atoms (3) in which the atoms are weakly stabilized, this solvation has probed to be a precursor of new metallic colloid (4,5) sometimes very stable at room temperature.

The most successful approach for the preparation of stable metal colloids in organic solvents are based on the clustering of metal atoms at low temperature (5). From another point of view, the solvated metal atoms are interesting when the solvate is decomposed; the atoms begin to grow in the organic solvent. This clustering is controlled by the organic solvent, metal, concentration and temperature. Colloids, films and supported catalysts or solids can be obtained.

The stabilization of colloidal particles can be explained by two methods i) steric, by bonding with the solvent and ii) electrostatic, the colloidal particles are charged due to the absorption of ions generated in solution (6).

EXPERIMENTAL

Metal colloids

The techniques used to evaporate metals in a static reactor and perform solvation using an excess of solvent are described elsewhere (7,8). 50 to 100 mg of nickel were placed in alumina-tungsten crucibles and cocondensed with 100 mL of solvent that had been previously dried and degassed by freeze-pump-down cycles at 77 K. After the cocondensation the matrix was kept under vacuum and remained stable. No vacuum changes were observed during the warm up.

Transmission electron microscopy

A drop of colloidal dispersion is placed on the copper grid of 100 mesh with carbon support. A JEOL JEM 1200 EXII was used to obtain the micrographs at several magnifications. The particle size was determined by measuring the diameter of 100 particles and then the frequency histograms were obtained using the Origin 6.0 program.

Electrophoresis

The electrophoresis to determine the electrophoretic mobility was obtained by using 310 Volts. The charge of the colloids was corroborated by using a Laser Zee Meter Model 501, Pen Kem.

UV-Vis

The absorption spectra of the colloids were measured at 25° C in a Spectronic Genesis 2, with dilution to avoid high absorption.

RESULTS AND DISCUSSION

The Ni colloids were obtained by cocondensation of the metal with several solvents such as: 2-propanol, acetone, 2-methoxyethanol and 2-ethoxyethanol.

The following scheme shows the colloid/solid synthesis.

The solvents are always added in excess to maintain the solvation of metal clusters. Table 1 summarizes the average particle size and stability of the colloids. It can be observed that sizes ranges from 4 to 23 nm. The most stable dispersion is in 2-propanol and the less stable in acetone.


The higher stability in Ni-2-propanol is due to the better solvation of the particles and to the formation of surface charges, which have been probed by electrophoretic migration measurement giving a zeta potential of 22.41mV. The other solvents presents lower polarity and the colloids stability gives low reproducibility values in electrophoretic mobility. The stabilization was the result of the particles acquiring a negative charge, creating a zeta potential between the Ni core and the surrounding solvent. The other systems under study are unstable and independent of their particle sizes since for similar concentrations Ni-2-methoxyethanol shows a particle size of 5.5 nm, Ni-acetone 6.7 nm and Ni-2-methoxyethanol exhibits a higher particle size of 23 nm. The last solvent allows weakly agglomeration of spherical partciles, producing an increase in the particle size.

Another way to obtain nickel nanoparticles has been reported in a rapid expansion of a nickel chloride solution in near critical ethanol into a room temperature solution of NaBH4 in DMF also containing poly(N-vinyl-2-pyrrolidone) for particle stabilization (9). Also, this nickel nanoparticles can be precipitated from the suspension using a magnet under the flask. After removal of the solvent, a black solid was obtained; these nanoparticles are amorphous with a broad diffraction pattern.

The TEM analysis of these particles deposited on a colloid film yields an average size of 5.8 nm in diameter, similar to our Ni-2-methoxyetanol colloids.

The transmission electron micrographs of some of the colloids are shown in figures 1 to 4, they were obtained either by clear or dark field technique. Along with the micrograph the frequency histogram can give information about the distribution of the particle sizes.

In Figure 1 we can observe the Ni-2-propanol colloid mostly spherical with a distribution between 5-8 nm in dark field and a mean size of 6.5 nm. These values are similar to those of the study carried out on Bi-2-propanol in which the particle size value ranged between 5-6 nm (10). The Ni-2-propanol suspension is stable with respect to gravity, showing no signs of precipitation after 30 days.


Fig. 1. The Electron MIcrograph and Histogram of Ni-2-propanol colloid 2,74 ¥ 103M, dark Field, M = 240K; Mean Size = 64,7Å .

Figure 2 shows a Ni-2-ethoxyethanol colloid with a distribution between 4.5-7.5 nm, the particles are well distributed with some isolated particles exhibiting a mean size of 5.5 nm. Similar values have been obtained for Au and Cu-dimethoxymethane (11).


Fig. 2. The Electron MIcrograph and Histogram of Ni-2-propanol colloid 2,74 ¥ 103M, dark Field, M = 150K; Mean Size = 55,5 Å.

Figure 3 shows the Ni-2-methoxyethanol colloid with a distribution between 20-25 nm most probably these particles are clusters with several atoms since the shapes are irregular and the size is bigger than the others described previously. The Mn-2-methoxyethanol also exhibits similar size and shape with 34.8 nm for the nickel using the same solvent (12).


Fig. 3. The Electron MIcrograph and Histogram of Ni-2-propanol colloid 2,74 ¥ 103M, dark Field, M = 100K; Mean Size = 22,9Å .

Finally, fig. 4 shows a Ni-acetone colloid with a distribution between 8.0-11 nm, the particles are spherical in shape and randomly distributed with a mean size of 9.4 nm. This value is bigger than that reported for Ag-acetone showing similar stability at room temperature (13).


Fig. 4. The Electron MIcrograph and Histogram of Ni-acetone

The UV-Vis were obtained for the dispersion of 2-propanol (see fig 5) mainly because they are the most stable system. The spectrum at t = 0 shows absorption bands at 212 nm and a shoulder at 270 nm, this is probably due to the effect of quantum size typical for this size of particle (4.46 nm). The bands disappear gradually by time with particle growing process. These results are in agreement with the values reported by Creighton (14) in which particles with size of 10 nm show a band near 210 nm. Curtis et al. found that Ni bulk absorption presents a band centered near 230 nm (15).


Fig. 5. UV Spectrum of Ni-2-propanol.

Previous studies with metal clusters indicate that metallic properties decrease while the size decreases (16,17) which makes difficult that the particle may exhibit those bands if the bulk does not.

By direct evaporation from the colloids, it was possible to obtain films and/or active solids. In fact, Riecke and coworkers reported a general approach for preparing highly reactive metal powders by reducing metal salts in ethereal or hydrocarbon solvents using alkali metals (18-20). This technique has the advantage of requiring minimal specialized equipment and the ability to produce large quantities of materials at a single time.

Riecke (18) method and the solvated metal atom dispersion method (21) produce highly active macroscopic metal particles with nanometer grain sizes. As a consequence the inclusion of hydrocarbon fragments during synthesis is observed (19).

It is important to point out that it is possible to obtain nickel colloids particles dispersed in non-aqueous solvent with high dielectric constants.

ACKNOWLEDGEMENTS

The authors would like to thank financial support from FONDECYT (Grant 1000527) and the Dirección de Investigación, Universidad de Concepción.

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*Corresponding author
Phone : (56-41)-204256, Fax: (56-41)-245974
E-mail: gcardena@udec.cl