J. Chil. Chem. Soc., 50, N 2 (2005), págs.: 455-460

SYNTHESIS OF IRON AND IRON-MANGANESE COLLOIDS AND NANOPARTICLES USING ORGANIC SOLVENTS

GALO CARDENAS T*¹, VIVIANA DELGADO G. AND JOSE ACUÑA E¹, CARLOS A.S. LIMA²

¹Departamento de Polímeros, Facultad de Ciencias Químicas Universidad de Concepción, Casilla, 160-C, Concepción (Chile).
²Top Lab., Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Casilla, 160-C, Concepción (Chile).

SUMMARY

Manganese, iron colloids and iron-manganese bicolloids were prepared by cocondensation of the metal at 77K with ethanol, 2-propanol, 1,2-dimethoxyethane, 2-methoxyethanol and acetone. The distribution of particle sizes was determined by transmission electron microscopy of the most stable dispersions.

Electrophoretic measurements such as colloid charge and zeta potential were achieved. lt was found that the colloids possess electrical charge, therefore it is postulated that their stability is by simple solvatation. The colloids showed instability at room temperature.

In some colloids absorption bands in the UV region were observed. In the visible region no plasma absorption was found.

Active solids obtained by evaporation of the solvent contain certain amount of the solvent incorporated, and due to their reactivity they produce a mixture with manganese oxide and iron oxide.

The X-ray of the powders of Fe-Mn-2-methoxyethanol does not allow to conclude about the alloyed composition.

The presence of solvents can be observed by FT-IR incorporation in the finely divided solids. Characteristic bands for each solvent were measured. By means of thermogravimetric analysis and DSC the thermal stability of the solids and the transition heat give us the carbonaceous residues in the films. The elemental analysis of the powders was carried out.

Keywords: A. Nanostructures; A. colloids; microscopy; C. infrared microscopy. B. vapor deposition; C . electron microscopy; D. infrared microscopy.

INTRODUCTION

The magnetic and electronic properties of small iron particles is an area of strong interest mainly due to the technological applications.

The main source of data has been obtained by the technique EMLTV (1,2). The beginning iron clusters between 2 and 25 atoms were studied. The experimental data of ionization potential obtained for the dimer and trimer were correlated with theoretical studies considering their electronic structure. The results show good correlation with the electronic structure that was carried out to magnetically "active clusters". In other work, for iron clusters with sizes between 6 and 90 atoms it was found that the ionization potential decreases with the increase of the cluster size.

The geometry of the iron clusters for several particle sizes has not been determined, but apparently it is strongly dependent of the preparation methods.

Some clusters with icosahedral structure could be possible for small clusters (N<100), but this structure is probably in competition with other structure not well defined. The predominant structure over the others depends on the condition of the vaporization source obtained by the technique MSTF (time of flying mass spectrometry) (3,4,5).

Kinetic and equilibrium experiments, which use ligands bonded to the cluster to obtain information about the strength site have demonstrated the existence of several structures (6)

There have been reports of Mn cluster ions (7), manganese cluster oxides (8,9) and alloy clusters containing manganese (10). Manganese and manganese oxide cluster ions have been produced by sputtering. On the other hand, alloy clusters, Mn_nCo_m and Mn_nTa_m were obtained by laser evaporation from two separate metal samples. This method gives a broad size distribution of neutral manganese clusters produced at low temperature (11) . Likely, Mn₂ dimmer and small Mn_n clusters are stabilized. Also, species such as Mn_nO⁺ and Mn_nC⁺, together with some unidentified ions are observed.

In the present paper, we show the presence of small colloids of Fe, Mn and Fe-Mn bicolloids stabilized by organic solvents. Also a study of the amorphous solids was carried out.

EXPERIMENTAL

PREPARATION OF METAL COLLOIDS

The metal atom reactor has been already described (12,13); as a typical example, an alumina-tungsten crucible was charged with 0.879 g. Mn metal (pieces) and 0.260g of Fe metal (lumps). Dry acetone was placed in a ligand inlet tube and freeze-pump-thaw degassed with several

cycles. The reactor was pumped down to 0.008 mbar while the crucible was warmed to red heat. A liquid nitrogen filled Dewar was placed around the vessel and Mn with Fe and acetone (52 mL) were deposited over 1 h using 40A. The matrix was a blue/purple color at the end of the deposition. The matrix was allowed to warm slowly under vacuum by removal of the liquid nitrogen Dewar for 1 h, upon meltdown a brown colloid was obtained. After addition of nitrogen up to 1 atm, the colloid was allowed to warm for another 0.5 h at room temperature. The solution was siphoned out under nitrogen into a flask ware. Based on metal evaporated and acetone inlet the molarity in metal could be calculated. Several concentrations were prepared under the same conditions. No presence of hydrogen evolved during the metal evaporation was observed. The vacuum remains constant during the evaporation.

The film was obtained by stripping the solvent under vacuum. The solvent evaporation on a substrate can be speeded by a N₂ flow or by using a warm substrate.

ELECTRON MICROSCOPY STUDIES

Transmission electron micrograph was obtained on a JEOL JEM 1200 EX 11 with 4 Å resolution by using copper grids coated with carbon foil. A drop of the colloid was placed on a copper grid and allowed it to dry.

THERMOGRAVIMETRIC ANALYSIS

A Perkin-Elmer Model TGA-7 thermogravimetric system, with a microprocessor driven temperature control unit and TA data station was used. The sample weight was recorded and generally ranged between 5-10 mg. The sample was placed in the balance system and the temperature was raised from 25 to 550°C at a heating rate of 10°C/min. The sample weight was continuously recorded as a function of temperature.

INFRARED STUDIES

Infrared Spectra was obtained using a Nicolet 5PC Spectrometer. KBr pellets were made for all the films. Spectra were recorded at a resolution of 2 cm^-1 and a minimum of 128 scans accumulation.

DIFFERENTIAL SCANNING CALORIMETER (DSC)

A Polymer Laboratory Simultaneous Thermal Analyzer STA 625 (TGA- DSC) was used. The transition energy was proportional to the area under the peak and the DH for the film were obtained.

UV-VIS SPECTROSCOPY

A Perkin-Elmer 2100 Spectrophotometer was used. The solvent was employed as a reference, the sample spectra between 200 and 800 nm were recorded.

ZETA POTENTIAL

A Laser Zee Meter Model 501, Pen Kem was used. The charge and zeta potential of the colloid were measured.

STATISTICAL ANALYSIS

An average of one hundred particles were counted in each micrograph. The histogram were obtained using the Origin 6 program.

RESULTS AND DISCUSSION

Iron-Manganese bicolloids were obtained by cocondensation of the metal with several solvents such as ethanol, 2-propanol, 1,2-dimethoxyethane, 2- methoxyethanol, and acetone.

The stability of the colloid was defined as the time that metal particles remain as sols at room temperature. The stability of the systems like Mn and Fe-1,2-dimethoxyethane, Mn and Fe-2-propanol and Mn-acetone are stable just for a few minutes. The stability increased for Fe-acetone; and in Mn and Fe-ethanol a gel formation was observed. On the other hand, Mn, Fe and Fe-Mn-2-methoxyethanol showed stability for several months even at several concentrations at room temperature (See Table 1).

Table 1. Stability of metallic and bimetallic colloids.

On the other hand, Fe-2-methoxyethanol and Fe-Mn-2-methoxyethanol are also very stable for several months at room temperature(see Table 1). The higher stability of the last two systems is probably due to the presence of ­OH group, forming hydrogen bonds with metal clusters. The 2-methoxyethanol can produce five or six member rings, which are thermodynamically very stable (i) and (ii).

The higher zeta potential was found for the Mn-2-methoxyethanol colloid indicative of a greater stability, followed by the bimetallic (Fe-Mn)-2-methoxyethanol, which exhibits an intermediate potential between both monometallic colloids. This is a way to probe the independent behavior of each metal in the bimetallic particle, since the atomic size of iron and manganese are similar. It has been previously reported the Ni-2-methoxyethanol which exhibits a lower zeta potential of 44 V(8), see Table 2. The positive charge is most probably due to the presence of iron and manganese oxide colloids.

Table 2.- Electrokinetic parameters of metal-2methoxyethanol colloids.

The Fe-2- methoxyethanol spectra of the colloid (fig.1) taken at time zero, show a continuous absorption in the UV region with no quantum size effect behavior.

Fig. 1. UV Spectrum of Fe-2-methoxyethanol at t = 0 , lmáx = 306 nm.

The bimetallic colloid spectrum (Fe-Mn)2-methoxyethanol (fig. 2) taken at zero time, shows a greater similarity to that of the monometallic Fe-2-methoxyethanol colloid than the Mn-2-methoxyethanol colloid under the same conditions.

Fig. 2 UV Spectrum of (Fe-Mn)-2-methoxyethanol bicolloid at t=0, lmax= 310 nm.

The electron micrograph for Fe, Mn and Fe-Mn with 2-methoxyethanol at zero time, shows the presence of metal clusters dispersed in the metal grid. The size of the particles are represented in a histogram.

Figure 3 shows the electron micrograph colloid clusters of Fe-2-methoxyethanol. The histogram shows a tendency of an asymmetric growth where the particles exhibit smaller sizes than the average size of 27 nm.

Fig.3 TEM of Fe-2-methoxyethanol colloid. The average particle size is 27 nm, s = 3nm.

On figure 4, the diffractogram obtained for the particles of Fe-2-methoxyethanol shows in the inside and outside part two diffuse circles corresponding to particles of amorphous nature, most probably due to the particles of smaller size, in their intermediate zone exist a series of concentric ring which correspond to polycrystalline particles. The compression of the spacing obtained by this method and the obtained by x-ray (table 3), indicate a hexagonal structure of e-Fe₂O₃.

Fig.4 Electron diffraction of Fe-2-methoxyethanol particles.

Table 3. Red Parameters of metal-2-metoxyethanol particles.

The X-ray powder of the Fe-Mn-2-methoxyethanol due to the amorphous material is impossible to conclude about the alloys composition, a wide peak was observed.

Figure 5 shows the electron micrograph of the colloidal clusters of Mn-2-methoxyethanol. The histogram shows an asymmetric tendency of asymmetric growth in which the particles exhibit small size than the average size of 32.4 nm.

Fig.5: TEM of Mn-2-methoxyethanol colloid. The average particle size is 32.4 nm, s = 4 nm.

The diffractogram taken to the particles in fig.5, shows in the inside a diffuse zone corresponding to the existence of polycrystalline and crystalline particles, respectively. The comparison of the spacing obtained by this method and that obtained by x-ray, indicates that there exist systems of MnO and Mn.

Figure 6 shows the electron micrograph of the colloidal clusters of (Fe-Mn)-2-methoxyethanol. The histogram shows a tendency to bimodal growth. This is due to the existence of three types of particles, two monometallic and one bimetallic colloids which are of different growing kinetics, the average particle sizes are 13.5 and 19.8 nm, respectively.

Fig.6: TEM of (Fe-Mn)-methoxyethanol. The average particle size is 19.8nm , s = 2 nm.

The diffractogram (fig. 7), taken to the particles of figure 6, showed in the inside a diffuse zone corresponding to the existence of particles of amorphous nature probably due to smaller particles. In the outside there are a few dots corresponding to crystalline particles. The comparison of the spacing obtained by this method and that obtained by X-Ray diffraction, table 3, indicates the presence of a cubic system corresponding to the intermetallic b-FeMn₄C but, this is a possibility since the presence of other monometallic systems like e-Fe₂O₃ and tetragonal g-Mn are observed.

Fig.7: Electron diffraction of (Fe-Mn)-2-methoxyethanol particles.

After solvent evaporation, active solids or films can be obtained. The films of active solids contained organic solvent incorporated. The elemental analysis corroborate this affirmation, being Mn-1,2-dimethoxyethanol with the lowest C/H ratio and Mn-2-methoxyethanol the highest (See Table 4). A more careful analysis of these data showed that Mn solids exhibited a mixture of metal and metal oxides with solvent incorporation. This observation was obtained from the elemental analysis, which is summarized in Table 4. It is difficult to establish a molecular formula due to the random incorporation of the solvent in these powders. Most probably are mixtures of Mn-Fe, Mn₂Fe and (MnFe)₂ but the amount of oxide incorporation is difficult to calculate.

Table 4. Elemental analysis of the particles the metal - organic solvent.

The IR spectra of Fe-2-methoxyethanol are shown in Figure 8. For Fe-2-methoxyethanol the bands at 1639 and 1533 cm^-1 correspond to the nC=C. This is probably due to the formation of an unsaturation in the a-carbon, the product is probably the following structure: (iii).

Fig.8: FTIR of Fe-2-methoxyethanol solids.

In the Fe-acetone film the absence of n_C=_O, at 1725-1700 cm^-1 for ketones is a good indication for metal cluster interaction with the solvent through the oxygen. See Figure 9.

Fig.9: FTIR of Fe-acetone solids.

The thermogram in figure 10 shows the TGA-DSC for Fe-2-methoxyethanol. In the TGA it can be observed a decomposition at 267°C. The order of reaction for this normal thermal decomposition is zero (n=0). In the DSC curve it observed two exothermic transition which are overlapped and their decomposition heat is 562 cal/g.m. The maximum in the first signal is obtained at 331°C corresponding to a decomposition process; the maximum in the 2^nd is at 391°C and corresponds to an amorphous phase transition to crystalline and or a particle growing process.

Fig. 10: TGA-DSC of Fe-2-methoxyethanol at 10°C/min.

CONCLUSIONS

Mn, Fe and (Fe-Mn)-2-methoxyethanol are the only stable colloids possible to obtain by using this methodology without any stabilizer. Also, it is possible to obtain a product with high incorporation of organic material giving an amorphous solid.

Under these conditions, 2-propanol is not able to stabilize the Fe and Mn clusters at room temperature. The ethanol is able to form gels with Fe and Mn .

The most relevant feature was to obtain the particle size of (Fe-Mn)-2-methoxyethanol colloid with an average size of 16.5 nm, smaller than Fe with 27 nm and Mn with 32.4 nm. Similar values have been obtained for Zn-Cd-2-butanol with 14.8 nm previously reported (15).

A coordinatively insaturated structure between iron and dehydrogenated 2-methoxyethanol was formed. The electron difracction patterns exhibit the presence of manganese and iron oxide and the presence of an intermetallic FeMn₄C specie.

ACKNOWLEDGMENTS

The authors would like to thank the financial aid from Fondecyt Grant # 1040456 and Dirección de Investigación from Universidad de Concepción.

REFERENCES

1. C.E. Moore, Natt. Bur. Stand U.S., Circ.467,27(1992)         [ Links ]

2. (a) K.D. Bier, T.L.Hasiett, A.D. Kirkwood and M. Moskovits, J. Chem. Phys. 89, 6 (1988);         [ Links ]

(b) C.A. Baremann, R.J. Van Zee, S.W. Bhat and W. Weitmeir, Jr. ibid 78,190 (1983)

3. J. Lignieres, B. D'Humeres and J.C. Rivod, Z. Phys. Dlg,207 (1991)         [ Links ]

4. (a) Y. Saito, H. lto and I. Katabuse, Z. Phys. D19,189 (1991;         [ Links ] (b) L. Hanley and S. Anderson, Chem. Phys. Lett. 122,410 (1985)         [ Links ]

5. (a) P.J. Ziemann and A.W. Castiemann, Jr. Phys. Rev. B.46, 13480 (1992);         [ Links ] (b) T.C. Devore, J.R. Woodward and J.L. Gole, J. Phys. Chem. 93,4920 (1989)         [ Links ]

6. Y. Sone, H. Hoshino, T. Hagamuma, A. Hakajina and K. Kaya, J .Phys. Chem. 95, 6830 (1991)         [ Links ]

7. J. Ho, L .Zhu, E.K. Parks and S.J. Riley, J .Chem. Phys. 99,140 (1993)         [ Links ]

8. G. Cárdenas and J.H. Acuña, Colloid Polymer Sci. 279, 442 (2001)         [ Links ]

9. G. Cárdenas and P.B. Shevlin, Bol. Soc. Chil. Quím. 32,111 (1987)         [ Links ]

10. G. Cárdenas and A. Ponce, Colloid Polymer Sci 274,788 (1996)         [ Links ]

11. G. Cárdenas and V. Vera, Mat. Res. Bull. 32, 97 (1997)         [ Links ]

12. G. Cárdenas, C. Muñoz and M .Rodríguez, Eur .Polymer J. 35,1017 (1999)         [ Links ]

13. G. Cárdenas, V. Vera, C. Muñoz, Mat. Res. Bull. 33,645 (1998)         [ Links ]

14. E.K. Parks, G.C. Niemen and S.R. Riley, J. Chem. Phys. 104, 3531 (1996)         [ Links ]

15. G. Cárdenas, R. Segura, J. Morales, H. Soto and C.A.S. Lima, Mat. Res. Bull. 35, 1251(2000).         [ Links ]

*Corresponding author, email: gcardena@udec.cl Phone: 56-41-204256, Fax: 56-41-245974