SciELO - Scientific Electronic Library Online

 
vol.45 número3Intercalation of Lithium and Donor Species in Layered Transition Metal Oxides and Sulfides.: Environment Effects on Lithium Diffusivity índice de autoresíndice de assuntospesquisa de artigos
Home Pagelista alfabética de periódicos  

Serviços Personalizados

Journal

Artigo

Indicadores

Links relacionados

Compartilhar


Boletín de la Sociedad Chilena de Química

versão impressa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.45 n.3 Concepción set. 2000

http://dx.doi.org/10.4067/S0366-16442000000300022 

SYNTHESIS AND CHARACTERIZATION OF MANGANESE
COLLOIDS AND ACTIVE SOLIDS PREPARED WITH
ORGANICS SOLVENTS

GALO CARDENAS T.* AND JOSE ACUÑA E.

Departamento de Polímeros, Facultad de Ciencias Químicas,
Universidad de Concepción, Casilla 160-C, Concepción (CHILE)
(Received: March 28, 2000 - Accepted: July 31, 2000)

In memoriam of Doctor Guido S. Canessa C.

SUMMARY

Manganese colloids 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 load and zeta potential were achieved. lt was found that the colloids do not possess electrical charge, therefore it is postulated that their stability is by simple solvation. 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.

The presence of solvents can be observed and their incorporation in the finely divided solids can be observed by FT-IR. Characteristic bands for each solvent were measured. The TGA - FTIR shows the presence of the C = O and C - H bands indicating the acetone in the solid. By means of thermogravimetric analysis and DSC the thermal stability of the solids and the transition heat from the thermal analysis can be determined.

Keywords: Nanostructures; metal colloids; vapor deposition; electron microscopy; infrared spectroscopy.

RESUMEN

Se obtuvieron coloides de manganeso por cocondensación del metal a 77 K con etanol, 2-propanol, 1,2-dimetoxietano, 2-metoxietanol y acetona. La distribución de partículas se obtuvo para los coloides más estables mediante microscopia electrónica de transmisión.

Se obtuvieron medidas electroforéticas tales como concentración de coloides y potencial zeta. Se encontró que los coloides no tienen carga eléctrica., sin embargo se postula que su estabilidad es por simple solvatación. Los coloides muestran inestabilidad a temperatura ambiente.

En algunas coloides se observan bandas de absorción en la región del UV. En la región visible no se observa absorción de plasma.

Los sólidos activos obtenidos por evaporación del solvente contienen una cierta cantidad de solvente incorporado y debido a su reactividad producen una mezcla con óxido de manganeso.

La presencia de solventes puede ser observada por su incorporación en los sólidos activos mediante FT-IR. Se observan las bandas características de cada solvente.

Los estudios de TGA-FTIR muestran la presencia de las bandas correspondientes al C=O y C-H indicando la incorporación de acetona en el sólido. Mediante estudios de termogravimetría y DSC se pudo determinar la estabilidad térmica y los calores de transición de los sólidos.

Palabras claves: nanoestructuras, coloides metálicos, deposición por vapores, microscopía electrónica, espectroscopia infrarroja.

INTRODUCTION

Manganese metal due to its half – filled 3d shell, possess a full spherical symmetry. The stable form of the metal at room temperature, the a-form cannot be readily used into targets as a consequence of a very few manganese cluster studies.

The resistive heating of Manganese produces dimmer clusters due to their low promotion energy (2.14 eV)1) In fact, spectroscopic studies of small Mn clusters formed in rare gas matrices have been reported for dimmers, trimmers and pentamers2). The ground state of the dimmer was 1S with the spins on the two atoms antiferromagnetically coupled 2c). The trimmer could be an equilateral triangle subject to a small Jahn-Teller distortion 2a) and the pentamer should be a plane pentagon 2b).

There have been reports of Mn cluster ions 4), manganese cluster oxides 4b, 5) and alloy clusters containing manganese 6). Manganese and manganese oxide cluster ions have been produced by sputtering. On the other hand, alloy clusters, MnnCom and MnnTam were obtained by laser evaporation from two separate metal samples. This method give a broad size distribution of neutral manganese clusters produced at low temperature 7). Likely, Mn2 dimmer and small Mnn clusters are stabilized. Also species such as MnnO+ and MnnC+, together with some unidentified ions are observed.

In the present paper, we show the presence of small colloids in solid Mn clusters stabilized by organic solvents.

EXPERIMENTAL

Preparation of metal film

The metal atom reactor has been already describes 8,9); as a typical example, an alumina-tungsten crucible was charged with 0.1888 g. Mn metal (pieces). 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 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 0.5h to 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.

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

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.

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 them to dry.

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 potencial

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

RESULTS AND DISCUSSION

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

Scheme 1. Synthesis of manganese colloids

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

Table I. MN colloid stability with oxygenated solvent

The higher stability of the last two systems is probably due to the presence of –OH group, forming hydrogen bonds with metal cluster. The 2-methoxyethanol can

produce five or six member rings, which are thermodynamically very stable (i) and (ii).

Only for Mn-2-methoxyethanol it was possible to obtain particle size in the TEM.

They exhibited a mean size of 348.5 Å showing spheres. The speed of the evaporation produced a rapid growth of Mn density increasing the cluster size.

We were able to obtain only reproducible data from TEM using the Mn-2- methoxyethanol. The micrograph shows spherical particles mostly isolated constituted for Mn clusters with some organic residues from the solvent. An average particle size of 348.5Å with a standard deviation of 72.4Å was calculated. (See Figure l). The other unstable colloids, probably due to a rapid clustering, grow up very fast and flocculated rapidly.


Fig. 1 Electron micrograph of Mn-2-methoxyethanol colloid. Particle size average 348.5Â, s = 72.4Â

The electrophoresis measurements were applied to the more stable systems such as Mn-ethanol and Mn-2-methoxyethanol. Due to the absence of electrophoretic migration at low voltage, the stabilization should be a result of the solvation processes. Only slow migration towards the cathode was observed.

After solvent evaporation, active solids or films can be obtained.

The films of active solids contained organic solvent incorporated. The elemental analysis corroborated this affirmation, being Mn-1,2- dimethoxyethanol with the lowest C/H ratio and Mn-2-methoxyethanol the highest. (See Table 2). 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 II.

Table II.

The FTIR of the films are summarized in Table III. The spectra of Mn- acetone and Mn-2-methoxyethanol are shown in Figures 2 and 3. The most important observations are:

Table III.


Fig. 2. FT-IR spectrum of Mn-acetone film in KBr pellet.


Fig. 3. FT-IR spectrum of Mn-2-methoxyethanol in KBr pellet.

i) In the Mn-acetone film the absence of nC=O, at 1725-1700 cm-1 for ketones is a good indication for metal cluster interaction with the solvent through the oxygen. See Figure 2.

ii) The bands at 1620 and 1650 cm-’ correspond to the nC=C. This is probably due to the formation of an insaturation in the a-carbon, the process is summarized in the following structures: (iii), (iv), (v) and (vi).

iii) The nC=O corresponding to the alcoxi group appeared at 1062cm-1.

Similar values have been obtained for other films previously reported (10 - 13).

lt is quite interesting the studies of TGA-FTIR of the Mn-acetone films. The gas phase IR showed the typical spectrum of acetone where it is clear that the band at 1739 cm-1 corresponded to nC=O corroborating the interaction with the metal cluster in the solid. Also the band of alkene at 1647 cm-1 showed the small amount in the film containing the clusters.

Fig. 4. TGA-FTIR spectrum of Mn-acetone film obtained at 10ºC/min heated in N2 atmosphere and collected between 4 and 19 min.

lt has been reported the formation of Mn clusters ranging from Mn13 to Mn70, prepared by laser vaporization and flow-tube reactor (14). Then MnC clusters were determined by mass spectra at several temperatures. The Mnn CH2 small species are clearly stable. In our case, Mn clusters of similar size are involved in the film formation.

DSC Studies

The Mn-2-propanol DSC curve showed one endothermic and two exothermic peaks. Peak 1 should be 2-propanol adsorbed due to its proximity value with the boiling point. Peaks 2 and 3 are due to the different metal cleavage sites on the clusters. The intensity is dependent of the solvent amount.

The Mn-acetone DSC curve showed two endothermic peaks. The DH1 is much greater than DH2 mainly due to higher population of one of the cluster sites by the solvent.

The peak appeared in the same zone of 2-propanol already mentioned before. The higher value of the enthalpy ratios is due to the higher nucleophilicity of the -OH group. This group is more reactive with the steric cluster site.

Mn-2-methoxyethanol DSC curve showed four endothermic peaks. Peaks 2 and 4 in the same range of the systems describes above (acetone and 2-propanol). The presence of two coordination sites of the solvent with clusters should justify the presence of more peaks. Peaks 2 and 4 should be the interaction through the -OH group, and peaks 1 and 3 to the linkage of –OCH3 group in the solvent, with the clusters. Table IV summarizes the data.

Table IV.

TGA Studies

The Mn-acetone films showed two decomposition peaks, one around 536 K and other at 741 K. The activation energy (Ea) of the decomposition reaction TD1, is greater than TD2 (7.9 and 4.2 kJ/mol).

The n values are fractionary in both cases, the decomposition should be carried out in stages involving a radical product formation.

The Mn-2-propanol showed three important decompositions. The first peak corresponded to the solvent vaporization, the next two peaks were also at the same temperature than the above system. The Ea of the decomposition reaction ranged between 8.7 and 1.08 kJ/mol being much higher the first one. The values were very small, and in most of the cases the first decomposition peak was the more significant.

Most of the films exhibited a decomposition kinetic close to n = 0 within the experiment error and approximate calculations.

Most of the films were Mn-acetone and 2-methoxyethanol with a TD ranging between 500 and 700 K which corroborate the possible cluster structure shown in structures (iii) – (vi).

Table V.

CONCLUSIONS

Mn-2-methoxyethanol is the only stable colloid 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 solvents like ketones and alcohols are not able to stabilize the Mn clusters at room temperature.

The most relevant feature is to report the particle size of the Mn-2-methoxyethanol colloid with an average size of 348.5Å, similar to Ag-dimetoxymethane with 225 Å and Ag-2-methoxyethanol previously report (15, 16).

ACKNOWLEDGMENTS

The authors would like to thank Fondecyt Grant # 2970053 and J. Acuña acknowledges the Conicyt scholarship for Ph. D. studies.

To whom correspondence should be addressed
email:gcardena@udec.cl

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.Cheffl.Phys. 89,6 (1988);         [ Links ]

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

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.Chern.Phys. 99,140 (1993)        [ Links ]

8. G. Cárdenas and K. J.Klabunde, Bol.Soc.Chil.Quím. 33,163 (1988)        [ 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. Oliva, Eur. J.Solid State Inorg. Chem. 33, 1135 (1996)        [ Links ]

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

Creative Commons License Todo o conteúdo deste periódico, exceto onde está identificado, está licenciado sob uma Licença Creative Commons