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Radiolytic preparation of nanophase cubic cobalt metal particles
Materials Research Bulletin, Vol. 33, No. 10, pp. 1555–1562, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/98 $19.00 1 .00
PII S0025-5408(98)00136-6
RADIOLYTIC PREPARATION OF NANOPHASE CUBIC COBALT METAL PARTICLES
S. Kapoor1, H.G. Salunke2, B.M. Pande1, S.K. Kulshreshtha1*, and J.P. Mittal1 1 Chemistry Group and 2Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India (Refereed) (Received December 8, 1997; Accepted January 19, 1998)
ABSTRACT Cobalt nanoparticles have been prepared by g-radiolysis in three different forms, namely, aqueous sol, self-supporting powder, and dispersed on the surface of Al2O3, and characterized for their particle size distribution and metallic characteristics, using a variety of techniques. Air-stable ferromagnetic cobalt particles were found to possess the face-centered cubic structure with an average crystallite size of 5– 6 nm. Magnetization studies carried out on frozen cobalt sol at 5 K also showed ferromagnetic behavior. The agglomeration of these crystallites produced particles in the range of >100 nm. © 1998 Elsevier Science Ltd
KEYWORDS: A. nanostructures, C. electron microscopy, C. X-ray diffraction INTRODUCTION Metal nanoparticles and the composite materials prepared from these particles have attracted the attention of a large number of investigators because of their potential applications in catalysis, microelectronics, optics, etc. Several investigations have been carried out on both the gas phase and the sol form to monitor the relationship between cluster size and chemical properties [1– 6]. Whereas a number of investigations [6 –9] have been reported for the noble metal particles in sol form, because of their inert nature, very few investigations are available
*To whom correspondence should be addressed: Dr. S.K. Kulshreshtha, Chemistry Division, B.A.R.C., Mumbai 400 085, India, FAX: 91 (22) 5560750 or 5560534, E-mail: [email protected] 1555
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[5,10 –14] for transition metal elements such as Fe, Co, and Ni, because of their highly reactive nature. The occurrence of quantum-size effects has created a new dimension in the applications of nanoparticles. For example, the gas phase reactivity of Fe, Co, and Ni metal clusters with different types of reactive gases such as H2, D2, CO, and NH3. has been found to show large size-dependent fluctuations [15–17]. For a specific size of clusters, the observed change in reactivity by two or three orders of magnitude has been ascribed to the change in electronic structure and the cluster packing which affects the nature of atomic sites available on the surface of the clusters. Furthermore, because of the large value of the surface-to-bulk-atom ratio, small clusters show anomalous electronic properties. It is of interest to know at what size these clusters develop metallic character which is reflected in the value of ionization potential of these clusters and their spectroscopic properties. For transition metal clusters such as Fe, Co, and Ni, study of their magnetic characteristics is a unique technique to test their metallic character. Self-supported metal nanoparticles of Fe, Co, Ni, etc. and their agglomerates are known to exhibit either ferromagnetic or superparamagnetic behavior, depending on their size. These properties show drastic changes if the metal particles are supported on inert substrates such as Al2O3 and SiO2, possibly due to the existence of significant metal support interactions [13,14]. Thus it is of interest to study such nanoparticles in free state as well as supported on an inert matrix, to determine whether metal support interactions occur. Successful preparation of very small aggregates with desired chemical characteristics is a difficult task and various methods, such as electrolysis, chemical reduction [8,18,19], gas evaporation [1– 4, 20,21], sol-gel technique [22,23], and radiolytic reduction [24 –30], have been used to prepare metal clusters in different forms. The reduction of metal ions in aqueous solutions leads to the formation of oligomeric clusters, which eventually leads to the formation of metallic particles. To stabilize these small metal particles in sol form, various stabilizers, such as polyvinyl alcohol, polyphosphate, and polyacrylic acid, have been used. Clusters in sol form have generally been characterized by their UV-vis absorption spectrum [31] arising due to their surface plasmons. Conventional techniques such as light absorption and scattering have been used to ascertain the metallic character and size of metal clusters, and the results have been compared with those obtained by electron microscopy. Although a few studies regarding the separation of metal particles using magnetic field have been reported, no information regarding the magnetic characteristics of metal particles in sol is available. In the present communication, we report the preparation and characterization of Co metal sol by g-radiolysis of cobalt solution. The characterization of the agglomerates obtained by hydrothermal treatment of this sol is also discussed.
EXPERIMENTAL All chemicals were of the highest purity commercially available and were used as received. Nanopure water was used throughout this study. CoSO4 (BDH), sodium formate (Sigma), and gelatin (BDH) were used. The gelatin was allowed to swell by soaking in water for 10 –15 min at ambient temperature. The solution became clear by warming for 2–3 min at 40 –50°C in a water bath, with continuous stirring. All solutions were freshly prepared and deaerated by bubbling high-purity N2. Equimolar concentrations of phosphate buffer were
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used for studying the reactions at pH 7. The g-radiolysis experiments were performed using 60 Co source at dose rate of 20 Gy/min. The metal sol was produced by irradiating nitrogen bubbled aqueous solution containing 1.0 3 1023 to 1.0 3 1022 mol dm23 CoSO4, 0.1% (wt/v) gelatin and 5.0 3 1022 mol dm23 formate at pH 7. The solutions were protected from light to avoid any photochemical reactions. Self-supported cobalt metal particles were obtained from the sol by heating it at >100°C for 2 h under inert atmosphere, followed by a centrifuging process. For the preparation of cobalt particles on the Al2O3 surface, 0.7633 g of Al metal powder was dissolved in concentrated HCl by heating. The aluminum hydroxide gel, prepared by adding NaOH, was mixed with a CoSO4 solution, formate, and pH 7 buffer and deaerated using N2 before receiving irradiation up to a g-dose of 36 kGy. The Al2O3 supported cobalt metal particles were obtained from the gel by heating it at >100°C for 2 h in an inert atmosphere. Absorption spectra were obtained on a Shimadzu spectrophotometer. Powder X-ray diffraction (XRD) measurements were carried out for the centrifuged samples of cobalt sol, using monochromatized Cu Ka radiation. Variable temperature magnetization measurements were carried out on the cobalt particles dispersed in aqueous phase, by freezing the cobalt sol down to 5 K in a sealed quartz tube. The measurements were taken with a SQUID magnetometer operating up to a magnetic field of 1.5 Kgauss. The data were corrected for the contribution due to blank solution used under identical conditions. The size of the cobalt metal particles obtained by centrifuging the sol or supported on inert Al2O3 powder was measured by a scanning electron microscope (Jeol JSMT 330-A) with an attached Kevex energy dispersive X-ray (EDAX) analyzer, for the compositional characterization of the samples. Transmission electron microscopy (TEM) measurements were carried out on the cobalt particles, using a Jeol JEM-2000FX electron microscope operated from 80 kV to 200 kV at different accelerating voltages. TEM samples were prepared by ultrasonically dispersing the hydrothermally treated Co metal agglomerates in acetone. A drop of each sample was placed on a graphite-coated Cu grid. RESULTS AND DISCUSSION On irradiation of water, the following radicals are produced H 2O 3 e 2aq, H •, OH •, H 2O 2, H 3O 1 OH• radicals are scavenged in the presence of formate ions. The e2aq reacts with metal ions to produce Co1, which in turn transforms to Co either by reacting with another e2aq or by dimerization and disproportionation reaction. Figure 1 shows the absorption spectrum of a g-irradiated CoSO4 solution along with that of the unirradiated CoSO4 solution. On irradiation, the solution turned blackish-brown and the spectrum showed a continuous increase in absorbance below 700 nm, with a sharp increase below 350 nm. This behavior is typical of colloidal cobalt metal particles [31]. The onset wavelength of the absorption spectrum was nearly independent of the cobalt ion concentration. This cobalt sol was stable under N2 atmosphere and was oxidized on exposure to air. On maintaining it for a long time (5 days) under N2 atmosphere, some flocculation of the particles was observed. The relevant region of the powder X-ray diffraction pattern of the cobalt nanoparticles obtained by hydrothermal treatment of Co sol, followed by centrifuging, is shown in Figure
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FIG. 1 Absorption spectra of 1.0 3 1023 mol dm23 CoSO4 solution: (a) before g-irradiation and (b) after g-irradiation up to a dose of 4.8 kGy. 2. The broad peak at 2u 5 44.3° is characteristic of cubic cobalt metal with a 5 3.541 Å. The normal form of bulk cobalt metal at room temperature is hexagonal, for which the most intense peak is supposed to be at 2u 5 47.6°. The excessive broadening of diffraction peaks observed was due to the very fine size of the cobalt crystallites. The average crystallite size
FIG. 2 Selective region of the X-ray diffraction pattern of cobalt particles having fcc structure.
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FIG. 3 SEM (a) and TEM (b) pictures of Co metal agglomerates.
estimated by the Debye-Scherrer method was in the range of 5– 6 nm. When the percentage of gelatin was increased from 0.1 to 1.0% (wt/v), the Co particles could not be precipitated after hydrothermal treatment, showing that they were bound to the polymeric chains of the stabilizer. Scanning electron microscopy (SEM) studies carried out by dispersing the cobalt sol on an Al or Cu sample holder did not give a clear indication of the existence of cobalt particles, possibly because of their very small size, which was beyond the detection limit (> 6 nm) of our instrument. However, the SEM patterns of the cobalt particles obtained by centrifuging the sol show a broad size distribution, with the size of the smallest particles in the range of >100 nm, as can be seen from Figure 3. The identification of these particles as cobalt was ascertained by recording the EDAX pattern after suitably adjusting the magnification of the system. Similarly, the SEM pictures of cobalt particles deposited on an Al2O3 matrix
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FIG. 4 Magnetization values (in arbitrary units) for frozen cobalt sol as a function of temperature (a) for Hext 5 500 gauss ■ zero-field-cooled and F field-cooled samples and (b) for Hext 5 1000 gauss Œ zero-field-cooled and field-cooled samples. Inset shows the hysteresis curve for frozen cobalt sol recorded at 5 K. suggested that the size of these particles was in the range of $50 nm. Typical results shown in Figure 3 demonstrate that the deposited particles were not spherical. A representative TEM image of the cobalt particles obtained by the centrifugal method is shown in Figure 4. This image suggests a wide distribution of particle size in the range of 10 –50 nm. It was not possible to record an electron diffraction pattern of these particles because of their magnetic nature. An important observation from this study is the large difference in the size of the Co particles as measured by the different techniques. The width measurement of X-ray diffraction peaks gives the average crystallite size produced by agglomeration of Co particles present in the sol and not the physical size of the particles. The presence of stabilizer inhibits the growth of crystallites in the sol and also on the surface of the cobalt particles during hydrothermal treatment. In SEM images, however, the physical size of the particles, consisting of a number of crystallites, is directly observed. Hence, the size of the particles in sol must be comparable to that observed from XRD results.
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Magnetic measurements carried out in the 5 # T # 255 K range showed the features characteristic of superparamagnetic cobalt particles. The magnetic moment values for the zero-field-cooled and field-cooled samples showed a significant difference, particularly at lower temperatures. The branching temperature for the zero-field-cooled and field-cooled magnetization values was found to increase with increase in an externally applied magnetic field. Further, for higher values of external magnetic fields, there was a sharp increase in magnetization values, indicating the possibility of magnetic ordering in these particles. The hysteresis loop recorded at 5 K for the same sample of Co sol clearly established the ferromagnetic ordering in the Co particles. Such features have been reported for a variety of magnetic materials consisting of fine particles [10,32]. The Co particles obtained by the hydrothermal treatment were found to be ferromagnetically ordered even at room temperature, because of their increased size due to agglomeration. CONCLUSION In conclusion, we have prepared nanosized cubic Co metal particles in the sol form, using g-radiolysis. These particles showed superparamagnetic behavior at room temperature because of their small size and underwent ferromagnetic ordering at lower temperatures. ACKNOWLEDGMENTS The authors are extremely thankful to Shri J. B. Singh of the Material Science Division for providing the TEM pictures of these samples. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
S.J. Riley and E.K. Parks, in Gas Phase Chemical Reactions of Transition Metal Clusters With Simple Molecules, eds. P. Jena, B.K. Rao, and S.N. Khanna, p. 277, Plenum, New York (1987). A. Kaldor, D.M. Cox, and M.R. Zakin, Adv. Chem. Phys. 70, 211 (1988). (a) E.K. Parks, G.C. Nieman, L.G. Pobo, and S.J. Riley, J. Chem. Phys. 88, 6260 (1988); (b) E.K. Parks, G.C. Nieman, L.G. Pobo, and S.J. Riley, J. Chem. Phys. 88, 1622 (1988). (a) E.K. Parks, G.C. Nieman, L.G. Pobo, and S.J. Riley, J. Chem. Phys. 86, 1066 (1987); (b) E.K. Parks, G.C. Nieman, L.G. Pobo, and S.J. Riley, J. Phys. Chem. 91, 2671 (1987). J.L. Marignier, J. Belloni, M.O. Delcourt, and J.P. Chevalier, Nature 703, 344 (1985). A. Henglein, J. Phys. Chem. 97, 5457 (1993). S. Kapoor, D. Lawless, P. Kennepohl, D. Meisel, and N. Serponne, Langmuir 10, 3018 (1994). S. Ayyappan, R.S. Gopalan, G.N. Subbanna, and C.N.R. Rao, J. Mater. Res. 12, 398 (1997). R. Seshadri, G.N. Subbanna, V. Vijayakrishnan, G.U. Kulkarni, G. Ananthakrishna, and C.N.R. Rao, J. Phys. Chem. 99, 5639 (1995). J.L. Marignier and J. Belloni, J. Chim. Phys. 85, 21 (1988). Y. Liu, Y. Zhu, Y. Zhang, Y. Qian, M. Zhang, L. Yang, and C. Wang, J. Mater. Chem. 7, 787 (1997). A.K. Santra, S. Ghosh, and C.N.R. Rao, Langmuir 10, 3937 (1994). S. Ayyappan and C.N.R. Rao, Eur. J. Solid State Inorg. Chem. t. 33, 737 (1996). R. Seshadri, R. Sen, G.N. Subbanna, K.R. Kannan, and C.N.R. Rao, Chem. Phys. Lett. 231, 308 (1994). S.C. Richtsmeier, E.K. Parks, K. Liu, L.G. Pobo, and S.J. Riley, J. Chem. Phys. 82, 3569 (1985).
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B.H. Weiller, P.S. Bechthold, E.K. Parks, L.G. Pobo, and S.J. Riley, J. Chem. Phys. 91, 4714 (1986). S.J. Riley, in Clusters of Atoms and Molecules 11, ed. H. Haberland, Springer Series in Chemical Physics, Vol. 56, p. 221 (1994). A.L. Oppegard, F.J. Darnell, and H.C. Miller, J. Appl. Phys. 32, 184S (1961). H. Schlesinger, J. Am. Chem. Soc. 75, 215 (1953). C.G. Granquist and O. Handeri, Phys. Rev. B 16, 3513 (1977). G.A. Niklasson, J. Appl. Phys. 62, 258 (1987). S. Roy, A. Chatterjee, and D. Chakraborty, J. Mater. Res. 8, 689 (1993). A. Chatterjee and D. Chakravorty, J. Phys. D 22, 1386 (1986). (a) Y. Zhu, Y. Qian, M. Zhang, Z. Chen, and G. Zhou, J. Mater. Chem. 4, 1619 (1994); (b). Y. Zhu, Y. Qian, M. Zhang, Z. Chen, L. Bin, and W. Changsui, Mater. Lett. 17, 314 (1993). Y. Zhu, Y. Qian, M. Zhang, Z. Chen, L. Binand, and Z. Guien, Mater. Sci. Eng. B 23, 116 (1994). P. Mulvaney, T. Linnert, and A. Henglein, J. Phys. Chem. 95, 7843 (1991). M. Mostafavi, M.O. Delcourt, N. Kegkhouche, and G. Picq, Radiat. Phys. Chem. 40, 445 (1992). (a) B.G. Ershov, E. Janata, M. Michaelis, and A. Henglein, J. Phys. Chem. 95, 8996 (1991); (b) B.G. Ershov, E. Janata, and A. Henglein, Radiat. Phys. Chem. 39, 123 (1992); (c) A. Henglein, Ber. Bunsen-Ges. Phys. Chem. 811, 556 (1977). (a) A. Henglein, M. Gutierrez, E. Janata, and B.G. Ershov, J. Phys. Chem. 96, 4598 (1992); (b) M. Kelm, J. Lilie, and A. Henglein, J. Chem. Soc., Faraday Trans. 1 71, 1132 (1975). M. Michaelis and A. Henglein, J. Phys. Chem. 96, 4719 (1992). J.A. Creighton and D.G. Eadon, J. Chem. Soc., Faraday Trans. 1 87 3881 (1991). I.M.L. Billas, A. Chatelain, and W.A. de Heer, J. Magn. Magn. Mater. 168, 64 (1997).
PII S0025-5408(98)00136-6
RADIOLYTIC PREPARATION OF NANOPHASE CUBIC COBALT METAL PARTICLES
S. Kapoor1, H.G. Salunke2, B.M. Pande1, S.K. Kulshreshtha1*, and J.P. Mittal1 1 Chemistry Group and 2Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India (Refereed) (Received December 8, 1997; Accepted January 19, 1998)
ABSTRACT Cobalt nanoparticles have been prepared by g-radiolysis in three different forms, namely, aqueous sol, self-supporting powder, and dispersed on the surface of Al2O3, and characterized for their particle size distribution and metallic characteristics, using a variety of techniques. Air-stable ferromagnetic cobalt particles were found to possess the face-centered cubic structure with an average crystallite size of 5– 6 nm. Magnetization studies carried out on frozen cobalt sol at 5 K also showed ferromagnetic behavior. The agglomeration of these crystallites produced particles in the range of >100 nm. © 1998 Elsevier Science Ltd
KEYWORDS: A. nanostructures, C. electron microscopy, C. X-ray diffraction INTRODUCTION Metal nanoparticles and the composite materials prepared from these particles have attracted the attention of a large number of investigators because of their potential applications in catalysis, microelectronics, optics, etc. Several investigations have been carried out on both the gas phase and the sol form to monitor the relationship between cluster size and chemical properties [1– 6]. Whereas a number of investigations [6 –9] have been reported for the noble metal particles in sol form, because of their inert nature, very few investigations are available
*To whom correspondence should be addressed: Dr. S.K. Kulshreshtha, Chemistry Division, B.A.R.C., Mumbai 400 085, India, FAX: 91 (22) 5560750 or 5560534, E-mail: [email protected] 1555
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[5,10 –14] for transition metal elements such as Fe, Co, and Ni, because of their highly reactive nature. The occurrence of quantum-size effects has created a new dimension in the applications of nanoparticles. For example, the gas phase reactivity of Fe, Co, and Ni metal clusters with different types of reactive gases such as H2, D2, CO, and NH3. has been found to show large size-dependent fluctuations [15–17]. For a specific size of clusters, the observed change in reactivity by two or three orders of magnitude has been ascribed to the change in electronic structure and the cluster packing which affects the nature of atomic sites available on the surface of the clusters. Furthermore, because of the large value of the surface-to-bulk-atom ratio, small clusters show anomalous electronic properties. It is of interest to know at what size these clusters develop metallic character which is reflected in the value of ionization potential of these clusters and their spectroscopic properties. For transition metal clusters such as Fe, Co, and Ni, study of their magnetic characteristics is a unique technique to test their metallic character. Self-supported metal nanoparticles of Fe, Co, Ni, etc. and their agglomerates are known to exhibit either ferromagnetic or superparamagnetic behavior, depending on their size. These properties show drastic changes if the metal particles are supported on inert substrates such as Al2O3 and SiO2, possibly due to the existence of significant metal support interactions [13,14]. Thus it is of interest to study such nanoparticles in free state as well as supported on an inert matrix, to determine whether metal support interactions occur. Successful preparation of very small aggregates with desired chemical characteristics is a difficult task and various methods, such as electrolysis, chemical reduction [8,18,19], gas evaporation [1– 4, 20,21], sol-gel technique [22,23], and radiolytic reduction [24 –30], have been used to prepare metal clusters in different forms. The reduction of metal ions in aqueous solutions leads to the formation of oligomeric clusters, which eventually leads to the formation of metallic particles. To stabilize these small metal particles in sol form, various stabilizers, such as polyvinyl alcohol, polyphosphate, and polyacrylic acid, have been used. Clusters in sol form have generally been characterized by their UV-vis absorption spectrum [31] arising due to their surface plasmons. Conventional techniques such as light absorption and scattering have been used to ascertain the metallic character and size of metal clusters, and the results have been compared with those obtained by electron microscopy. Although a few studies regarding the separation of metal particles using magnetic field have been reported, no information regarding the magnetic characteristics of metal particles in sol is available. In the present communication, we report the preparation and characterization of Co metal sol by g-radiolysis of cobalt solution. The characterization of the agglomerates obtained by hydrothermal treatment of this sol is also discussed.
EXPERIMENTAL All chemicals were of the highest purity commercially available and were used as received. Nanopure water was used throughout this study. CoSO4 (BDH), sodium formate (Sigma), and gelatin (BDH) were used. The gelatin was allowed to swell by soaking in water for 10 –15 min at ambient temperature. The solution became clear by warming for 2–3 min at 40 –50°C in a water bath, with continuous stirring. All solutions were freshly prepared and deaerated by bubbling high-purity N2. Equimolar concentrations of phosphate buffer were
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used for studying the reactions at pH 7. The g-radiolysis experiments were performed using 60 Co source at dose rate of 20 Gy/min. The metal sol was produced by irradiating nitrogen bubbled aqueous solution containing 1.0 3 1023 to 1.0 3 1022 mol dm23 CoSO4, 0.1% (wt/v) gelatin and 5.0 3 1022 mol dm23 formate at pH 7. The solutions were protected from light to avoid any photochemical reactions. Self-supported cobalt metal particles were obtained from the sol by heating it at >100°C for 2 h under inert atmosphere, followed by a centrifuging process. For the preparation of cobalt particles on the Al2O3 surface, 0.7633 g of Al metal powder was dissolved in concentrated HCl by heating. The aluminum hydroxide gel, prepared by adding NaOH, was mixed with a CoSO4 solution, formate, and pH 7 buffer and deaerated using N2 before receiving irradiation up to a g-dose of 36 kGy. The Al2O3 supported cobalt metal particles were obtained from the gel by heating it at >100°C for 2 h in an inert atmosphere. Absorption spectra were obtained on a Shimadzu spectrophotometer. Powder X-ray diffraction (XRD) measurements were carried out for the centrifuged samples of cobalt sol, using monochromatized Cu Ka radiation. Variable temperature magnetization measurements were carried out on the cobalt particles dispersed in aqueous phase, by freezing the cobalt sol down to 5 K in a sealed quartz tube. The measurements were taken with a SQUID magnetometer operating up to a magnetic field of 1.5 Kgauss. The data were corrected for the contribution due to blank solution used under identical conditions. The size of the cobalt metal particles obtained by centrifuging the sol or supported on inert Al2O3 powder was measured by a scanning electron microscope (Jeol JSMT 330-A) with an attached Kevex energy dispersive X-ray (EDAX) analyzer, for the compositional characterization of the samples. Transmission electron microscopy (TEM) measurements were carried out on the cobalt particles, using a Jeol JEM-2000FX electron microscope operated from 80 kV to 200 kV at different accelerating voltages. TEM samples were prepared by ultrasonically dispersing the hydrothermally treated Co metal agglomerates in acetone. A drop of each sample was placed on a graphite-coated Cu grid. RESULTS AND DISCUSSION On irradiation of water, the following radicals are produced H 2O 3 e 2aq, H •, OH •, H 2O 2, H 3O 1 OH• radicals are scavenged in the presence of formate ions. The e2aq reacts with metal ions to produce Co1, which in turn transforms to Co either by reacting with another e2aq or by dimerization and disproportionation reaction. Figure 1 shows the absorption spectrum of a g-irradiated CoSO4 solution along with that of the unirradiated CoSO4 solution. On irradiation, the solution turned blackish-brown and the spectrum showed a continuous increase in absorbance below 700 nm, with a sharp increase below 350 nm. This behavior is typical of colloidal cobalt metal particles [31]. The onset wavelength of the absorption spectrum was nearly independent of the cobalt ion concentration. This cobalt sol was stable under N2 atmosphere and was oxidized on exposure to air. On maintaining it for a long time (5 days) under N2 atmosphere, some flocculation of the particles was observed. The relevant region of the powder X-ray diffraction pattern of the cobalt nanoparticles obtained by hydrothermal treatment of Co sol, followed by centrifuging, is shown in Figure
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FIG. 1 Absorption spectra of 1.0 3 1023 mol dm23 CoSO4 solution: (a) before g-irradiation and (b) after g-irradiation up to a dose of 4.8 kGy. 2. The broad peak at 2u 5 44.3° is characteristic of cubic cobalt metal with a 5 3.541 Å. The normal form of bulk cobalt metal at room temperature is hexagonal, for which the most intense peak is supposed to be at 2u 5 47.6°. The excessive broadening of diffraction peaks observed was due to the very fine size of the cobalt crystallites. The average crystallite size
FIG. 2 Selective region of the X-ray diffraction pattern of cobalt particles having fcc structure.
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FIG. 3 SEM (a) and TEM (b) pictures of Co metal agglomerates.
estimated by the Debye-Scherrer method was in the range of 5– 6 nm. When the percentage of gelatin was increased from 0.1 to 1.0% (wt/v), the Co particles could not be precipitated after hydrothermal treatment, showing that they were bound to the polymeric chains of the stabilizer. Scanning electron microscopy (SEM) studies carried out by dispersing the cobalt sol on an Al or Cu sample holder did not give a clear indication of the existence of cobalt particles, possibly because of their very small size, which was beyond the detection limit (> 6 nm) of our instrument. However, the SEM patterns of the cobalt particles obtained by centrifuging the sol show a broad size distribution, with the size of the smallest particles in the range of >100 nm, as can be seen from Figure 3. The identification of these particles as cobalt was ascertained by recording the EDAX pattern after suitably adjusting the magnification of the system. Similarly, the SEM pictures of cobalt particles deposited on an Al2O3 matrix
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FIG. 4 Magnetization values (in arbitrary units) for frozen cobalt sol as a function of temperature (a) for Hext 5 500 gauss ■ zero-field-cooled and F field-cooled samples and (b) for Hext 5 1000 gauss Œ zero-field-cooled and field-cooled samples. Inset shows the hysteresis curve for frozen cobalt sol recorded at 5 K. suggested that the size of these particles was in the range of $50 nm. Typical results shown in Figure 3 demonstrate that the deposited particles were not spherical. A representative TEM image of the cobalt particles obtained by the centrifugal method is shown in Figure 4. This image suggests a wide distribution of particle size in the range of 10 –50 nm. It was not possible to record an electron diffraction pattern of these particles because of their magnetic nature. An important observation from this study is the large difference in the size of the Co particles as measured by the different techniques. The width measurement of X-ray diffraction peaks gives the average crystallite size produced by agglomeration of Co particles present in the sol and not the physical size of the particles. The presence of stabilizer inhibits the growth of crystallites in the sol and also on the surface of the cobalt particles during hydrothermal treatment. In SEM images, however, the physical size of the particles, consisting of a number of crystallites, is directly observed. Hence, the size of the particles in sol must be comparable to that observed from XRD results.
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Magnetic measurements carried out in the 5 # T # 255 K range showed the features characteristic of superparamagnetic cobalt particles. The magnetic moment values for the zero-field-cooled and field-cooled samples showed a significant difference, particularly at lower temperatures. The branching temperature for the zero-field-cooled and field-cooled magnetization values was found to increase with increase in an externally applied magnetic field. Further, for higher values of external magnetic fields, there was a sharp increase in magnetization values, indicating the possibility of magnetic ordering in these particles. The hysteresis loop recorded at 5 K for the same sample of Co sol clearly established the ferromagnetic ordering in the Co particles. Such features have been reported for a variety of magnetic materials consisting of fine particles [10,32]. The Co particles obtained by the hydrothermal treatment were found to be ferromagnetically ordered even at room temperature, because of their increased size due to agglomeration. CONCLUSION In conclusion, we have prepared nanosized cubic Co metal particles in the sol form, using g-radiolysis. These particles showed superparamagnetic behavior at room temperature because of their small size and underwent ferromagnetic ordering at lower temperatures. ACKNOWLEDGMENTS The authors are extremely thankful to Shri J. B. Singh of the Material Science Division for providing the TEM pictures of these samples. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
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