ELSEVIER Materials Science and Engineering A217/215 (1996) 54-57

MATERIALS SCIENCE &

EWCIWEERNWG

A

Encapsulation of nano particles by surfactant reduction

B. Jeyadevan’, Y. Suzuki, K. Tohji*, I. Matsuoka Depurtment Geoscience and Technology, Toi~olczl University, Aoba-ku, Sendai 980, Jnpcm

Abstract

Stabilization of ferromagnetic transition metal particles under an oxidizing atmosphere would lead to their use in potential applications where ferromagnetic oxide particles are in use now. It is believed that the above could be achieved through encapsulating the metal particles with a material that is stable in the oxidizing atmosphere. In this paper, we propose a novel method to encapsulate the particle with carbon and discuss the results. The particles are primarily coated with a surfactant and the coated particles are placed in an arc-discharge chamber filled with He gas and arc discharged for a specified time. In the above treatment, the He atoms are ionized and the bombardment of the He ions on the surface of the particle reduces the surfactant and leads to encapsulation of the particle. The particles treated using the above method have been characterized with XRD, FTIR, TEM and XPS.

Keyworcis: Ferromagnetic transition metal particles; Surfactant reduction; Encapsulation of nano particles

1. Introduction

The technological importance of fine ferromagnetic transition metal and metal oxide particles are felt very much in our day to day life and their use in magnetic data storage, magnetic toner in xerography, in magnetic fluid, in magnetic ink as contrast agents in magnetic resonance imaging etc. are on the increase. The fine ferromagnetic metal particles show greater performance over the oxides of the same in various fields, however, there are practical difficulties in preserving the metal particles in that form for prolonged periods because the metal particles transform into metal oxides. The trans- formation of the metal particles to metal oxides can only be prevented by encapsulating the particles with materials that are stable in the oxidizing atmosphere. In this direction, researches have been carried out to provide a protective coating for the particle with car- bon material. The carbon coating is achieved by arc discharging the electrode consisting of a mix of graphite and the metal particles [l-4]. This method has been reported to provide a complete coating of the particle, but, the percentage of coated particles is very small and

* Corresponding author. ’ Present address: Dept. of Geosciences, Mining Engineering and

Material Processing, Mining College, Akita, Japan 010.

furthermore the particles are associated with unwanted amounts of soot. Though the carbon coated nano particle size is roughly the same through out the reac- tor, the nano crystal phase of these particles cannot be controlled due to the difference in temperature of for- mation and cooling rate in different parts of the reac- tor. Here, we report an attempt made to encapsulate the particles with carbon using a novel method where the magnetic particles are coated with a surfactant, kept inside the arc chamber a few centimetres away from the arc and arc discharged under a He atmosphere for a specified period of time. The particles treated by the above method have been characterized with XRD, FTIR, TEM and XPS.

2. Experimental

In this series of experiments the ultra fine magnetite particles were used as the material to be encapsulated. The magnetite particles with an average diameter of 10 nm were prepared using the co-precipitation technique and coated with sodium oleate (CI,H,,COONa) (51. The surfactant coated particles were placed 3 cm below the electrode in the arc-discharge chamber as shown in Fig. 1. The arc discharge was performed under a He gas pressure of 100 Torr and a discharge current of 60 A using Tungsten electrodes. The particles treated above were characterized using FTIR, XRD, XPS and TEM.

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Fig. 1. The schematic diagram of the arc discharge equipment.

3. Results and discussion

The surfactant coated magnetite particles were placed on a flat plate in the arc chamber as shown in Fig. 1 and the arc-discharge treatment was performed. During arc-discharge, the He atoms in the chamber are ionized and expected to bombard the surface of the surfactant coated particles and reduce the smfactant to carbon and encapsulate the particle. The surfactant coated particles which look brown to the naked eye turn to black once they are treated in the arc discharge cham- ber. This change is visible even in particles not exposed to the surface. The above was true for the sample that was close to the arc but the sample away from the arc remained brown. As there is no other source of carbon in the chamber, it is natural to presume that the surfactant, the only source of carbon has been reduced.

The treated particles were examined using FTIR to ascertain whether the particle or the surfactant ad- sorbed on the surface of the particle underwent any

j /

ii

4000 3500 3000 2500 2000 1500 1000 500

WAVE LENGTH (cm”)

Fig. 2. The DRIFT spectroscopy of magnetite samples.

change. Qualitative analysis of the surfactant coated magnetite has been performed by means of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Fig. 2 shows the DRIFT spectroscopy of the (a) non-coated (b) coated and (c) coated and arc treated magnetite samples. As seen in the figure the magnetite peak is found around 600 cm -I and the sodium oleate peaks appears around wave lengths 1500 and 2900 cm-‘. The DRIFTS profile of the particles treated in the arc discharge chamber does not show the peaks related to oleate and also the magnetite peak is relatively weak. This confirms the fact that the ad- sorbed oleate has undergone transformation. The sod- ium oleate is unstable in an oxidizing atmosphere and shows an exothermic peak around 230 K [6,7]. But, in the above case the chamber is kept under an inert atmosphere and the transformation is believed to be due to the reduction of oleate and encapsulation of the particles with carbon.

Though the sample is kept a few centimetres away from the arc, there is a possibility for the sample to

M - Magnetite W . Tungsten M

(b) -Lk&-AA coated magnetite

w

,&a”dy”& ,

LO 30 40 50 GO 70

2 Theta

Fig. 3. The X-ray diffraction patterns of magnetite samples.

56 3. Jeyadevan et al. 1 Materials Science and Engineering A217/218 (1996) 54-57

Fig. 4. (a) Transmission Electron Micrograph of oleate coated magnetite particles before arc treatment. (b) Transmission Electron Micrograph of oleate coated magnetite particles after &c treatment.

have reached temperatures high enough for sintering to occur. To determine the state of the magnetite particles used in these experiments X-ray diffraction of powder specimens of (a) non-coated (b) coated and (c) coated and arc treated magnetite samples were carried out and the results are shown in Fig. 3. As can be seen, the diffraction profile of magnetite ‘was found in all three samples and in the case of arc treated sample the diffraction peaks of Tungsten was also found. This was due to the evaporation of the anode during arc dis-

L80 282 284 286 258 290 292

Binding Energy (eV)

Fig. 5. The X-ray Photoelectron Spectroscopy of magnetite samples.

charge. Once the basic data on the above process is gathered tungsten rods will be replaced with carbon. Further, the particle size of these samples were mea- sured from the FWHM of the X-ray diffraction profile using the Sherrer’s formula and was found that the treatment has not caused any increase in the average particle size. The Transmission Electron Micrograph (TEM) of sodium oleate coated magnetite and of treated magnetite are shown in Fig. 4 (a) and (b). As seen in these micrographs the particle size of these two samples were very similar and it satisfies the results predicted by the XRD data.

The results of the FTIR, XRD and TEM, experi- ments show that the change has only taken place on the surface of the particle and has not changed the metal phases. Therefore, we analyzed the surface of the parti- cle by X-ray Photoelectron Spectroscopy (XPS). The important feature of this technique is that the effective sample thickness is only I-10 nm or less and also the detennination of depth profiles by means of Ar ion sputtering. The C 1s spectra of the coated and non- coated samples, before and after sputtering for specified times is given in Fig. 5. The Is peak position of carbon is around 285.5 eV. But, the charge up voltage was determined to be 1 eV. Therefore, the effective peak position is 284.5 eV. The binding energy of carbon for carbides is between 281-283 eV and for carbon with Nitrogen, Sulphur, Oxygen (alcohol, ethers, etc.) the binding energies are above 286 eV. The binding energy for carbon as graphite is 284.5 eV. Therefore, the value of the binding energy of the carbon in the coated sample

B. Jeyadevan et al. / Materials Science and Eizgineering A21 7/218 (1996) 54-57

takes a value of carbon as graphite. Further, as seen in this figure, the carbon peak in the treated particles remained even after 6.1 min sputtering which is three times the time necessary to remove the surface adsorbed carbon. The above data also supports the fact that the oleate is reduced to carbon and the possibility of the encapsulation of the particles.

The characterization of the treated particles suggests that the possibility of the magnetite particles being encapsulated by carbon is quite high. In the next step, we propose to examine these particles under a high resolu- tion microscope to investigate the degree of encapsula- tion and then to extend this process to metal particles.

Acknowledgements

The Authors wish to thank the Japanese Ministry of Education, Science and Culture for the Grant-in-Aid under General Scientific Research (No. 07805090) for the present study.

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