Journal of the Chilean Chemical Society
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.2 Concepción jun. 2007
J. Chil. Chem. Soc, 52, Nº 2 (2007) págs.: 1182-1185
SYNTHESIS AND CHARACTERIZATION OF Co-Ni COLLOIDS PREPARED IN NONAQUEOUS SOLVENTS
GALO CÁRDENAS TRIVIÑO* AND VIVIANA LILLO GARRIDO
Laboratorio de Materiales Avanzados, Departamento de Polímeros , Facultad de Ciencias Químicas, Edmundo Larenas 129, Casilla 160-C, Concepción-Chile
The synthesis of Co-Ni bimetallic colloids nanoparticles by the CLD method were obtained. The metals were melted and evaporated in a metal atom reactor and cocondensed with ethanol, 2-propanol and 2-methoxyethanol at 77 K.
The colloids were characterized by transmission electron microscopy (TEM), ultraviolet spectroscopy (UV-VIS), EDX and electrophoresis. The stability at room temperature was also measured.
The higher stability dispersion (> 40 days) was Co-Ni/ ethanol, Co-Ni/ 2-methoxyethanol and Co-Ni/ 2-propanol (< 10 days). The electrophoresis showed that particles are positively charged for the alcohols (62 mV for ethanol and 1.4x10-3 mV for 2-propanol) and 9.64 mV negatively charged for 2-methoxyethanol. The particles size of the bimetal colloids ranges from 5.7 to 9.8 nm.
The active solids were obtained by solvent evaporation from the colloids dispersion. The microanalysis showed the presence of C, H between 5 to 13% which is indicative that the solvent is incorporated in the solid.
The thermogravimetric analysis (TGA) reveals the thermal stability of the active solids. The Co-Ni/ ethanol and Co-Ni/ 2-propanol exhibit a weight loss of 20% w/w, while the Co-Ni/ 2- methoxyethanol shows a weight loss of 42% being the less stable solid.
The FTIR of the active solids in KBr pellets exhibit the presence of the solvents incorporated in the active solid.
Keywords: Nano particles, colloids, cobalt, nickel.
The bimetallic particles have been widely studied from a point of view of catalytic properties. This is due that in most cases the bimetal show higher catalytic properties than monometallic particles in some industrial reactions (1-7).
The codeposition of metals and organic solvents is carried out at 77K with a matrix formation. After warm up the metal- organic matrix produces the nucleation and particles growing of nanometric dimensions . Finally, a colloid dispersion of nanoparticles is obtained. The method has been studied by Klabunde (5-7) and Cárdenas (8-13).
The advantages and disadvantages of the technique in the preparation of nanostructured materials such as Au, Pd, Cu, Ni, Pt, Co, Fe, Zn, Cd, Sn, Pr, Yb, Er have been studied (5,6,15,16).
The stability of colloids dispersions depends on two fundamental aspects:
I. The oxidation potential of the metal dispersed (14) being the most stables the dispersion of stable metals ( Au, Pd ) and less stable the Zn, Ge and Sn metals.
II. The solvatation, there are organics molecules capable to solvate particles such as 2-propanol, 2-methoxyethanol and acetone of higher dielectric constant.
The main goal of this work is the synthesis and characterization of colloidal bimetallic system Co-Ni in oxygenated organic solvents with potential magnetic and electric properties.
Transmission electron microscopy ( TEM)
The electron micrograph were obtained through the equipment Jeol JEM EX- 1200 II whit 4Å of resolution. A drop of the colloid dispersion of Co-Ni/ solvent was deposited on a Cu grid of 150 mesh covered with a thin carbon film.
The particle size was determined by optical measurements , the diameters of a specific particle population were measured randomly. The data set was represented in frequency histograms, to which distributions of the Gaussians and normal type were adjusted to obtain the respective measurements.
The absorption spectra, from 200 until 500 nm, of the colloids were measured at 25° C in a Spectronic Genesis 2 model spectrophotometer, using quartz cell. The background was set up with the proper solvent and then each colloid sample was examined.
The electrophoresis experiments were carried out using a Zeta meter system 3.0+, a quartz cell was used then rinsed with ionized water and later with the corresponding solvent. The cell was filled with the colloid avoiding the absence of bubbles. The platinum electrodes were in contact and connected to the power supply in order to generate an electric field with the colloid.
The C, H were determined in an automatic analyzer Fisons Instrument EA 1108 using sulfamilamide as standard. The Co and Ni were determined by atomic absorption in a Perkin Elmer 3100 with Co and Ni lamps, respectively.
Infrared spectra were measured by using a FT-IR Nicolet Magna 5 PC spectrophotometer coupled to a PC with Omnic software analysis. The film were placed into holder directly in the IR laser beam. Spectra were recorded at a resolution of 4 cm-1 and 128 scans were accumulated between 400 and 4000 cm-1 .
A Perkin Elmer model TGA-7 thermogravimetric analysis ( TGA ) system with a microprocessor driven temperature control unit and a TA data station was used. The mass of sample was generally in the range 2- 3 mg. The sample was placed in the balance systems equipment and the temperature was raised from 25 until 550° C at a heating rate of a 10°C/ min. The mass sample pan was continuously recorded as a function of the temperature.
The electrical conductivity measurements, were carried out using 120 mg of sample and pressed up to 16600 psi making a disc in which was measured electrical resistivity using a RCL Bridge Fluke PM 6304. The electric resistivity R, is related with the geometry of the section, in which a current flows with a proportionality constant , ρ, called electrical resistivity, a parameter characteristic of each material. The equation that represent this relation is:
RESULTS AND DISCUSSION
The colloid dispersions of this bimetallic system in organic solvents are quite stable. The following scheme shows the formation of bimetallic colloids and active powders.
The stability of the bimetallic colloids was measured until particle precipitation was observed in the dispersions at room temperature. Table I summarizes the stability and average particle size obtained by TEM.
SD: standard deviation
The higher stability of ethanol colloids is due to their higher dielectric constant than 2-methoxyethanol and 2- propanol which is corroborated by a charge stabilization instead of solvation process. Also, the lower concentration increases the stability being in 2-propanol the smaller particle size around 5.7 nm and higher around 9.8 nm for ethanol. The size increases with the colloid concentration due to the agglomeration process. Figure 1 show the electron-micrograph of 2-methoxyethanol and figure 2 the electron micrograph of Co-Ni/ ethanol.
The particle sizes of this bimetallic colloids are quite similar to those obtained for NiSn (17) due to their similarities in the alloyed.
On the other hand, the zeta potential (ξ) are very low in the range of 1.4 for 2-propanol to 62 mV for ethanol. This values are quite smaller compared with the bimetal NiSn that ranges from 24 to 67 mV . The electrophoresis show a very low value for 2-propanol ( 1.4x10-3 mV ) and higher for ethanol ( 62 mV ). These values are related with their stability being 2-propanol the less stable and ethanol the more stable dispersion due to their higher ξ at room temperature. The same behavior was observed previously for NiSn colloids. But in this case due to partial flocculation during the electrophoretic mobility measurement, the value obtained is very low. Both bimetallic colloids containing alcohols exhibit positive charge.
The EDX analysis of the particles exhibit the presence of Co and Ni being the intensity of Co twice than Ni approximately which is in agreement with their metal ratio (18).
The UV-vis measurements exhibit absorption bands for Co-Ni dispersions in ethanol and 2-propanol at 206 nm. This value it the same that Creighton theoretical calculation for aqueous colloids assuming a particle size of 10 nm. The methoxyethanol bimetallic colloid exhibit bands at 230 nm and 270 nm due to different cluster size.
When the colloids are exposed to high vacuum the solvent evaporates and active powders with some organic solvent trapped in the solid are observed.
The elemental analysis shows the presence of C and H between 7 and 16%. The solid with higher percentage of organic material is the solid obtained from 2-methoxyethanol, this fact explain their lower thermal stability. On the other hand, the solid from 2-methoxyethanol is the more thermal stable due to their lower carbonaceous content. See table II.
The results are according to the ratio that metals were combined (65/35 = 1.86) for metals synthesized in 2-propanol with a ratio 1.58, and 1.66 for ethanol and 1.64 for 2-methoxyethanol. The higher amount of oxygen is due to the oxidation during the handling in the elemental analysis.
The FT-IR show bands characteristics of the solvents with small shift in the 1400 cm-1 region (see fig 6). A disappearance of the 1329 cm-1 band for ethanol in the bimetal colloid and the bands at 1088 and 1046 of C-O stretching, which might indicate the interaction of the oxygen with the metals. The same phenomena is observed for the C-O stretching at 1124 and 1067 cm-1 in the pure solvent, both band collapse in one at 1074 cm-1 probably due to the cyclic structure between the solvent and the bimetals (19). The most important signals are summarized in table III.
The TGA shows the thermal stability of the solids Co-Ni/2-propanol solid is the most stable with 19.3% weight loss at 550° C. The less stable system is Co-Ni/2-methoxyethanol with 42.2% weight loss due to the higher solvent incorporated, which is corroborated by their higher carbon content.
The TGA exhibit a first weight loss around 100°C with near 10%. Around 250°C a second weight loss of around 20% was observed. It is important to observe that around 300°C a weight loss around 20% was measured for Co-Ni/ethanol and propanol solids while Co-Ni/2-methoxyethanol exhibit a higher around here to their higher carbon content.
The bimetallic Co-Ni system can produce stable colloids in organics solvent and also after solvent evaporation active solids. This active solids can be used as catalyst for olefin reduction (propenes, butanes, carbonyl, amines).
The reduction of acetophenone using Ni(5.0)-Sn(1.0) /MgO at 100 psi with 60 % conversion was tested (20). Also the reduction of nitrobenzene to aniline under similar conditions was carried out.
The electrical conductivity of the active powders from the three studied systems of Co-Ni were carried. The results were not reproducible but all were in the range of 10-7-10-8 Ω-1cm-1. The values are in the range of semiconductors but can be classified such as, insulators or bad semiconductors.
The authors would like to thank the financial support of grant FONDECYT 1040456.
1. Clarke J., Chem. Rev., 75, 291 (1975). [ Links ]
2. Kugler E. and Boudart M.,. Catal J, 59, 201 (1979). [ Links ]
3. Mehrotra P. and Verykios X., J. Catal., 88, 409 (1984). [ Links ]
4. Toolenaar F., Reinalda D. and Ponec V., Catal J., 64, 110 (1980). [ Links ]
5. Klabunde, K.J. Chemistry of Free Atoms and Particles Academic Press Inc.1980. [ Links ]
6. Klabunde, K.; Youngers, G.; Zuckerman, E.; Tan, B. J. Solid State Inorg. Chem.1992, 29, 227. [ Links ]
7. Stoeva, S. I. ; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc.2002, 124, 2305. [ Links ]
8. Cárdenas, G.; Klabunde, K. J.; Dale, E. B. Langmuir 1987, 3, 986. [ Links ]
9. Cárdenas, G.; Alvial M.; Klabunde, K. J. Bol. Soc. Chil. Quím. 1990, 35, 277. [ Links ]
10. Cárdenas, G.; Oliva, R. Bol. Soc. Chil. Quím. 1993, 38, 301. [ Links ]
11. Cárdenas, G.; Ponce, A. Colloid Polym. Sci. 1996, 74, 788. [ Links ]
12. Cárdenas, G.; Vera, V.; Muñoz, C. Mat. Res. Bull. 1998, 33, 645. [ Links ]
13. Cárdenas, G.; Oliva, R. Colloid Polym. Sci. 1999, 277, 164. [ Links ]
14. Creighton J.A, J. Chem. Soc. Faraday. Trans, 87,3881(1991). [ Links ]
15. Kimura K.; Bandow, S. Bull. Chem. Soc. Jpn. 1983, 56, 3578. [ Links ]
16. Ozin, G. Acc. Chem. Res. 1977, 10, 21. [ Links ]
17. Cárdenas G., León Y., Moreno Y., Peña O., Colloid Polym Sci (2006) 284: 644-653. [ Links ]
18. Cárdenas G., J. Chil.Chem. Soc. (2005), 50, 603. [ Links ]
19. Cárdenas G., Salgado, E., J. Chil. Chem. Soc. (2006), 51, 1036. [ Links ]
20. León, Y., Ph.D. thesis Univ. de Concepción (2005). [ Links ]