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Synthesis of FeS2 and FeS nanoparticles by high-energy mechanical milling and mechanochemical processing
Journal of Alloys and Compounds 390 (2005) 255–260
Synthesis of FeS2 and FeS nanoparticles by high-energy mechanical milling and mechanochemical processing P.P. Chin, J. Ding∗ , J.B. Yi, B.H. Liu Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore Received 30 January 2004; received in revised form 8 June 2004; accepted 28 July 2004
Abstract High-energy mechanical milling of the FeS2 pyrite led to the formation of sub-micron particles with nano-scaled grains. FeS2 nanoparticles were obtained, when NaCl was used as the dispersion medium. FeS2 pyrite and FeS troilite phases could be formed after high-energy mechanical milling. The formation of FeS2 pyrite required a long milling time. The resultant powders consisted of nano-scaled particles. The use of the solid dispersion medium (NaCl) can promote dispersion of nanoparticles. This work has shown that iron sulfide nanoparticles can be synthesized by mechanochemcial process with a solid dispersion medium. © 2004 Elsevier B.V. All rights reserved. Keywords: Pyrite; Troilite; Solid-state dispersion; Mechanical milling; Mechanochemical process
1. Introduction Iron sulfides are currently recognized as advanced inorganic materials with non-conventional applications, such as high-energy density batteries, precursors for the synthesis of superconductors, diagnostic materials, materials for photoelectrolysis, solar energy materials and chalcogenide glasses [1–4]. In addition, iron sulphide has been suggested to play a crucial role in life origin [5,6]. Therefore, iron sulphide nanoparticles may have a significant potential for applications in many areas. For example, iron sulphide nanoparticles may be used in electrochemical reaction in the investigation of life origin as suggested in [7]. Nanoparticles of sulfides have been synthesized recently by different chemical routes with the aim to prepare materials with controlled particle morphology and size distribution [8]. The routes of synthesis described in most of the papers have applied the solvothermal synthesis with the intervention of microwave, sonochemical and autoclave techniques. ∗
Corresponding author. Tel.: +65 68744317; fax: +65 67763604. E-mail address: [email protected] (J. Ding).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.07.053
Although sulfides can be synthesized from the elements by heating, the degree of homogeneity of the final product may depend on preparation processes, starting materials, and the size of powders. This shortage may be overcome by high-energy ball milling in which fine alloying particles may be formed from elemental coarse powders through mechanochemical reactions. Before a mechanical milling, powder(s) is loaded together with several heavy balls (steel or WC) in a container. By vigorously shaking or high speed rotation, a high mechanical energy will be applied on the powders because of collision of heavy balls. The milling process embraces a complex mixture of fracturing, grinding, high-speed plastic deformation, cold welding, thermal shock, intimate mixing. The milling process will promote the diffusion of the particles. Hence, an alloying phase may be formed at low temperature (mechanochemical process) [9–12]. Recently, research on the mechanochemical processes is of technological interest [13], as the technique of mechanical milling has emerged as a versatile technique for producing nanostructural and non-equilibrium materials. Nanostructural pyrite has been fabricated using mechanical milling [11]. Mechanochemical process has been success-
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fully employed to the synthesis of different nanopowder materials including sulfide [8,14,15]. In this study, the synthesis of iron sulfide was demonstrated through mechanochemical reactions in the Fe–S system (Pyrite FeS2 and troilite FeS). Microstructure evolution is also one of the main problems to be studied. Recently, reduction of particle size was demonstrated by mechanical milling together with a solid-state dispersion medium (here NaCl) [16]. In this work, mechanical milling of the mixture of micron-sized FeS2 and NaCl powders was carried out.
2. Experimental The mineral FeS2 , pure iron powder (99.9% and <200 m) and sulfur powder (99%) were used as the starting materials for mechanical milling. The FeS2 mineral was manually grounded into particles with a particle size below 100 m before mechanical milling. Mechanical milling was carried out using a vibrated mill (Spex 8000 Mixer/Mill). In order to investigate the influence of the starting materials, different starting materials were used for mechanochemical processing (FeS2 and 0.33Fe–0.67S for FeS2 pyrite, and 0.5Fe–0.5FeS2 and 0.5Fe–0.5S for the formation of FeS troilite). About 10 g of starting mixture were loaded together with several 15 mm steel balls in an air-tight hardened steel container before mechanical milling. The powder/steel ball ratio was 1/10. All the powder handling (loading and un-loading) was done in a pure argon filled glove box. The mechanical milling was performed up to 72 h. For some millings, NaCl was used as the solid dispersion medium. The Fe–S powder mixture and NaCl (with a weight ratio of 1:4) were loaded together before mechanical milling. After mechanical milling, NaCl was removed by a washing process with de-ionized water. Structures of mechanically milled powders were investigated using X-ray diffraction (XRD, Philips) with a radiation of Cu K␣. A field-emission scanning electron microscope (SEM, Philips XL30 FEG) was used for the study of the morphology. The particle size (or grain size) was studied using a transmission electron microscope (TEM, JEOL 100CX). Fe57 -M¨ossbsauer spectroscopy was used for the phase analysis. A vibrating sample magnetometer (VSM) was used for the room temperature magnetic measurements. In order to avoid oxidation, powders were mixed with vacuum grease before M¨ossbauer spectroscopy. For magnetic measurements, powder was sealed with aluminum foil.
Fig. 1. X-ray diffraction patterns of the initial FeS2 pyrite powder (a) and the powder after mechanical milling for 24 h (b).
phase. The small line broadening was probably due to reduction of grain/particle size (as shown in our TEM micrograph in Fig. 4). Fig. 2 shows the M¨ossbauer spectra of the initial FeS2 powder before mechanical milling and the powder after mechanical milling for 24 h. The spectra show paramagnetic doublet with chemical shift = 0.31 mm/s and quadrupole splitting = 0.62 mm/s, which are well expected for FeS2 with the pyrite structure [17–19]. This result indicates that mechanical milling does not cause change in crystallographic structure which is consistent with the result as shown in Fig. 1. Our magnetic measurements confirmed paramagnetism of the asmilled powders. Fig. 3 shows the SEM micrographs of the initial and subsequently milled FeS2 powders. The hand-grounded FeS2 powder consisted of micron-sized particles with a broad particle
3. Results and discussion Fig. 1 shows the XRD patterns of the FeS2 initial powder and the FeS2 powder after milling for 24 h. The handgrounded FeS2 powder was of single phase with the pyrite structure. As shown in Fig. 1, mechanical milling did not cause change in structure. The spectrum shows single pyrite
Fig. 2. M¨ossbauer spectra of the initial FeS2 powder and the FeS2 powder after mechanical milling for 24 h.
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Fig. 3. SEM micrographs of the initial FeS2 powder and FeS2 powder after milling for 24 h.
size distribution from a few to ∼100 m. After mechanical milling, the powder appeared as an agglomeration of fine particles. The morphology and particle size could not be clearly analyzed using SEM. It is to note that all the mechanically milled Fe–S powders in this work have a similar SEM micrograph as shown in Fig. 3b. Fig. 4 shows the transmission electron microscope (TEM) micrographs of mechanically milled FeS2 powders with and without a medium (NaCl). Under TEM, the mechanically milled FeS2 samples (Fig. 4a) appeared as sub-micron particles with a nanocrystalline structure. Such a microstructure is typical as observed in many materials after mechanical milling [20]. As reported previously [16], nanoparticles can be obtained after mechanical milling together with a solid-dispersion medium. Oxide nanoparticles have been successfully fabricated using the method [21]. This work is the first attempt to synthesize metal sulfide nanoparticles using a solid dispersion medium (here NaCl). FeS2 and NaCl with a weight ratio of 1:4 were mechanically milled for 48 h. Our M¨ossbauer and magnetic measurements showed that NaCl didn’t react with FeS2 and the crystallographic structure of FeS2 remained unchanged. The TEM micrograph of the resultant particles is shown in Fig. 4b. It can be seen that the particle size is much
Fig. 4. TEM micrographs of mechanically milled FeS2 powders without (a) and with (b) NaCl.
smaller than that after milling without NaCl (Fig. 4a). The typical particle size was found in the range of 20–30 nm. In this work, we have studied the synthesis of FeS2 pyrite nanoparticles by mechanochemical processing, i.e. high-energy mechanical milling of powder mixture of elemental powders (Fe and S with the ratio of 1:2, as denoted as 0.33Fe–0.67S). Fig. 5 shows the XRD patterns of 0.33Fe–0.67S after milling for different times. It can be seen that the formation of FeS2 pyrite requires a long milling time. After milling for 24 h, only the peak for ␣-Fe was visible. The intensity of the ␣-Fe was strongly reduced after milling for 48 h. However, the peaks of pyrite were not obvious in the spectrum (Fig. 5). After milling for 72 h, all the XRD peaks could be identified as the FeS2 pyrite phase. The result indicates that the formation of pyrite requires a long milling time from elemental powders. The formation of FeS2 pyrite could be clearly monitored by M¨ossbauer spectroscopy. Fig. 6 shows the M¨ossbauer spectra of 0.33Fe–0.67S before mechanical milling and after mechanical milling for 48 and 72 h. Before mechanical milling, the magnetic sextet is expected for iron atoms in ␣Fe. After mechanical milling for 48 h, the M¨ossbauer spec-
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Fig. 5. X-ray diffraction patterns of the 0.33Fe–0.67S powders after mechanical milling for (a) 24, (b) 48 and (c) 72 h, respectively.
trum could be considered as a mixture of ␣-Fe (magnetic sextet) and FeS2 pyrite (paramagnetic doublet). After mechanical milling, 90% of Fe atoms were found in the FeS2 pyrite phase. About 10% of iron atoms were found in another doublet with chemical shift of 1.26 mm/s and quadrupole splitting of 3.01 mm/s. The M¨ossbauer parameters indicate the formation of FeSO4 ·H2 O (szomolnokite). The formation of FeSO4 ·H2 O was probably due to oxidation during mechanical milling or oxidation during M¨ossbauer measurement. The as-milled powder was very reactive. It is to note, that the con-
Fig. 6. M¨ossbauer spectra of the 0.33Fe–0.67S in the starting state and after mechanical milling for 48 and 72 h, respectively.
tainer was opened in the pure argon gas filled glove box. The powder was mixed with grease in the glove box before taking out in air for M¨ossbauer examination. However, the mixing with vacuum grease might not avoid oxidation entirely. The requirement of a long milling time for the formation of FeS2 pyrite was also confirmed in our magnetic measurements, as shown in Fig. 7. After mechanical milling for 24 and 48 h, the as-milled powders still had a relatively high magnetization which may be due to the presence of ferromagnetic ␣-Fe. After milling for 72 h, the low magnetization value was expected by the paramagnetic FeS2 phase. The small step at very low magnetic field indicated the presence of a small amount of magnetic phases (here probably ␣-Fe). The small amount of magnetic phases was not detected in our XRD and M¨ossbauer studies (Fig. 6), probably due to its very small particle (or grain) size, which might lead to superparamagnetism. It is also possible that the amount is too small to be detected by XRD and M¨ossbauer examination. It can be seen in Fig. 8, that nanosized particles were resulted from the mechanical milling of Fe and S elemental powders. A typical particle size of 20–30 nm was observed. However, the particles were not well dispersed with a certain degree of agglomeration. In order to obtain well-dispersed particles, NaCl was used as a medium in another milling. Our M¨ossbauer and magnetic study showed the formation of FeS2 pyrite after milling for 72 h. After the removal of NaCl, particles with a size around 10 nm were well dispersed. This work has shown that mechanical milling of Fe and S elemental powders can lead to the formation of well dispersed FeS2 nanoparticles with a particle size around 10 nm when a solid-state dispersion medium is present. The second part of this work was the synthesis of FeS troilite nanoparticles by mechanochemical porcessing. Two different starting mixtures were used, namely 0.5Fe–0.5FeS2 (a mixture of Fe and FeS2 pyrite) and 0.5Fe–0.5S (elemental powders). Fig. 9 shows the XRD patterns of the 0.5Fe–0.5FeS2 and 0.5Fe–0.5S powders after mechanical milling for 24 h. The FeS troilite phase could be formed for
Fig. 7. Hysteresis loops of 0.33Fe–0.67S powders after mechanical milling for 24, 48 and 72 h.
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Fig. 10. M¨ossbauer spectrum of 0.5Fe–0.5FeS2 powder after milling for 24 h.
Fig. 8. TEM micrographs of mechanically milled 0.33Fe–0.67S without NaCl (a) and with NaCl (b).
the two samples (of different starting powder mixtures) after mechanical milling for a relatively short time (24 h). For the sample 0.5Fe–0.5S, using the Scherrer method, a particle size was calculated to be approximately 10 nm according to the line broadening in Fig. 9. For the sample 0.5Fe–0.5FeS2 , the particle size was estimated to be 15 nm, which is a little larger than that of the sample 0.5Fe–0.5S. Fig. 10 shows the M¨ossbauer spectrum of 0.5Fe–0.5FeS2 . The magnetic sextet with a hyperfine field of 260 kOe and
Fig. 9. X-ray diffraction patterns of 0.5Fe–0.5S (a) and 0.5Fe–0.5FeS2 (b) after milling for 24 h.
a chemical shift of 0.66 mm/s is well expected for the antiferromagnetic FeS troilite phase [22]. A magnetic hysteresis loop is shown in Fig. 11 of the 0.5Fe–0.5FeS2 after milling for 24 h. The sample had a low value of magnetization (approximately 4 emu/g at 90 kOe). The low magnetization indicated the anti-ferromagnetic nature with incomplete compensation [23]. The 0.5Fe–0.5S powder after mechanical milling for 24 h had nearly the same M¨ossbauer and magnetic results as shown in Figs. 10 and 11. Very small particles were observed under TEM, as shown in Fig. 12a For 0.5Fe–0.5FeS2 and 0.5Fe–0.5S powders after milling for 24 h, the sizes of most particles were found around 10 nm. Some agglomeration was observed in these powders as shown in Fig. 12a. In a separated experiment, NaCl was used as the dispersion medium. Fig. 12b shows the TEM micrograph of the FeS nanoparticles after the removal of NaCl. It can be seen that the particles were better dispersed. The particle size (approximately 6–8 nm) is smaller than that without NaCl medium.
Fig. 11. Hysteresis loop of 0.5Fe–0.5FeS2 powder after milling for 24 h.
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Fig. 12. TEM micrographs of mechanically milled 0.5Fe–0.5S without (a) and with (b) NaCl.
4. Conclusion In this study, nanocrystalline powders were obtained with the mineral (FeS2 pyrite) as the starting material by highenergy ball milling. The FeS2 pyrite phase could be formed after mechanochemical processing for a long time (72 h) when a mixture of elemental Fe and S powders was used as the starting material. The as-milled powder consisted of nanosized particles with a typical particle size around 10 nm. The use of the solid dispersion medium (NaCl) could result in well-dispersed FeS2 nanoparticles and the solid dispersion medium could promote the formation of fine particles. FeS troilite phase could be formed after mechanochemical processing for a relatively short milling time (24 h) when a mixture of Fe and FeS2 or Fe and S was used as the starting material. The resultant powder consisted of very small particles (approximately 10 nm). Again, the use of NaCl as the dispersion medium could promote the formation of nanoparticles.
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References [21] [1] P. Bal´az, Z. Bastl, T. Havl´ık, J. Lipka, I. Toth, Mat. Sci. Forum 235–238 (1997) 217. [2] P.R. Bonneau, R.R. Jarris Jr., R.B. Kaner, Nature 349 (1991) 510. [3] I.J. Ferrer, F. Caballero, H.C. Delas, C. Sarchez, Solid State Commun. 89 (1994) 349.
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V.K. Tikhomirov, J. Non-Cryst. Solids 256/257 (1999) 328. J.R.P. Williams, Nature 343 (1990) 213. M.J. Russell, A.J. Hall, A.P. Gize, Nature 344 (1990) 387. M.J. Russell, R.M. Daniel, A.J. Hall, J.A. Sherringham, J. Mol. Evol. 39 (1994) 231. P. Bal´az, E. Boldiz´arov´a, E. Godoc´ıkov´a, J. Briancin, Mater. Lett. 57 (2003) 1585. C. Suryanarayana, Progr. Mater. Sci. 46 (2000) 1. E. Gaffet, F. Bernard, J.C. Niepce, F. Charlot, C. Gras, G. Le Caer, J.L. Guichard, P. Delcroix, A. Mocellin, O. Tillement, J. Mater. Chem. 9 (1999) 305. G. Le Caer, P. Delcroix, S. Begin-Colin, T. Ziller, Hyperfine Interact. 141/142 (2002) 63. B.L. Huang, E.J. Lavernia, J. Mater. Synth. Process. 3 (1995) 1. C.J. Warris, P.G. McCormick, Miner. Eng. 10 (1997) 1119. J.Z. Jiang, R.K. Larsen, R. Lin, S. Mørup, I. Chorkendorff, K. Nielsen, K. Hansen, K. West, J. Solid State Chem. 138 (1998) 114. P.G. McCormick, T. Tsuzuki, J.S. Robinson, J. Ding, Adv. Mater. 13 (2001) 1008. J. Ding, Y. Shi, L.F. Chen, C.R. Deng, S.H. Fuh, Y. Li, J. Magn. Magn. Mater. 247 (2002) 249. V.K. Garg, Y.S. Liu, S.P. Puri, J. Appl. Phys. 45 (1974) 70. Y. Nishihara, S. Ogawa, J. Chem. Phys. 71 (1979) 3796. J.P. Eymery, Eur. Phys. J. 5 (1999) 115. J. Ding, Y. Li, L.F. Chen, C.R. Deng, Y. Shi, Y.S. Chow, T.B. Gang, J. Alloy Comp. 314 (2001) 262. H.M. Deng, J. Ding, Y. Shi, X.Y. Liu, J. Wang, J. Mater. Sci. 36 (2001) 3273. M. Matsuo, M. Kawakami, K. Sugimori, Hyperfine Interact 126 (2000) 53. N.N. Greenwood, T.C. Gibb, M¨ossbauer Spectroscopy, Chapman & Hall, London, 1971.
Synthesis of FeS2 and FeS nanoparticles by high-energy mechanical milling and mechanochemical processing P.P. Chin, J. Ding∗ , J.B. Yi, B.H. Liu Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore Received 30 January 2004; received in revised form 8 June 2004; accepted 28 July 2004
Abstract High-energy mechanical milling of the FeS2 pyrite led to the formation of sub-micron particles with nano-scaled grains. FeS2 nanoparticles were obtained, when NaCl was used as the dispersion medium. FeS2 pyrite and FeS troilite phases could be formed after high-energy mechanical milling. The formation of FeS2 pyrite required a long milling time. The resultant powders consisted of nano-scaled particles. The use of the solid dispersion medium (NaCl) can promote dispersion of nanoparticles. This work has shown that iron sulfide nanoparticles can be synthesized by mechanochemcial process with a solid dispersion medium. © 2004 Elsevier B.V. All rights reserved. Keywords: Pyrite; Troilite; Solid-state dispersion; Mechanical milling; Mechanochemical process
1. Introduction Iron sulfides are currently recognized as advanced inorganic materials with non-conventional applications, such as high-energy density batteries, precursors for the synthesis of superconductors, diagnostic materials, materials for photoelectrolysis, solar energy materials and chalcogenide glasses [1–4]. In addition, iron sulphide has been suggested to play a crucial role in life origin [5,6]. Therefore, iron sulphide nanoparticles may have a significant potential for applications in many areas. For example, iron sulphide nanoparticles may be used in electrochemical reaction in the investigation of life origin as suggested in [7]. Nanoparticles of sulfides have been synthesized recently by different chemical routes with the aim to prepare materials with controlled particle morphology and size distribution [8]. The routes of synthesis described in most of the papers have applied the solvothermal synthesis with the intervention of microwave, sonochemical and autoclave techniques. ∗
Corresponding author. Tel.: +65 68744317; fax: +65 67763604. E-mail address: [email protected] (J. Ding).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.07.053
Although sulfides can be synthesized from the elements by heating, the degree of homogeneity of the final product may depend on preparation processes, starting materials, and the size of powders. This shortage may be overcome by high-energy ball milling in which fine alloying particles may be formed from elemental coarse powders through mechanochemical reactions. Before a mechanical milling, powder(s) is loaded together with several heavy balls (steel or WC) in a container. By vigorously shaking or high speed rotation, a high mechanical energy will be applied on the powders because of collision of heavy balls. The milling process embraces a complex mixture of fracturing, grinding, high-speed plastic deformation, cold welding, thermal shock, intimate mixing. The milling process will promote the diffusion of the particles. Hence, an alloying phase may be formed at low temperature (mechanochemical process) [9–12]. Recently, research on the mechanochemical processes is of technological interest [13], as the technique of mechanical milling has emerged as a versatile technique for producing nanostructural and non-equilibrium materials. Nanostructural pyrite has been fabricated using mechanical milling [11]. Mechanochemical process has been success-
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fully employed to the synthesis of different nanopowder materials including sulfide [8,14,15]. In this study, the synthesis of iron sulfide was demonstrated through mechanochemical reactions in the Fe–S system (Pyrite FeS2 and troilite FeS). Microstructure evolution is also one of the main problems to be studied. Recently, reduction of particle size was demonstrated by mechanical milling together with a solid-state dispersion medium (here NaCl) [16]. In this work, mechanical milling of the mixture of micron-sized FeS2 and NaCl powders was carried out.
2. Experimental The mineral FeS2 , pure iron powder (99.9% and <200 m) and sulfur powder (99%) were used as the starting materials for mechanical milling. The FeS2 mineral was manually grounded into particles with a particle size below 100 m before mechanical milling. Mechanical milling was carried out using a vibrated mill (Spex 8000 Mixer/Mill). In order to investigate the influence of the starting materials, different starting materials were used for mechanochemical processing (FeS2 and 0.33Fe–0.67S for FeS2 pyrite, and 0.5Fe–0.5FeS2 and 0.5Fe–0.5S for the formation of FeS troilite). About 10 g of starting mixture were loaded together with several 15 mm steel balls in an air-tight hardened steel container before mechanical milling. The powder/steel ball ratio was 1/10. All the powder handling (loading and un-loading) was done in a pure argon filled glove box. The mechanical milling was performed up to 72 h. For some millings, NaCl was used as the solid dispersion medium. The Fe–S powder mixture and NaCl (with a weight ratio of 1:4) were loaded together before mechanical milling. After mechanical milling, NaCl was removed by a washing process with de-ionized water. Structures of mechanically milled powders were investigated using X-ray diffraction (XRD, Philips) with a radiation of Cu K␣. A field-emission scanning electron microscope (SEM, Philips XL30 FEG) was used for the study of the morphology. The particle size (or grain size) was studied using a transmission electron microscope (TEM, JEOL 100CX). Fe57 -M¨ossbsauer spectroscopy was used for the phase analysis. A vibrating sample magnetometer (VSM) was used for the room temperature magnetic measurements. In order to avoid oxidation, powders were mixed with vacuum grease before M¨ossbauer spectroscopy. For magnetic measurements, powder was sealed with aluminum foil.
Fig. 1. X-ray diffraction patterns of the initial FeS2 pyrite powder (a) and the powder after mechanical milling for 24 h (b).
phase. The small line broadening was probably due to reduction of grain/particle size (as shown in our TEM micrograph in Fig. 4). Fig. 2 shows the M¨ossbauer spectra of the initial FeS2 powder before mechanical milling and the powder after mechanical milling for 24 h. The spectra show paramagnetic doublet with chemical shift = 0.31 mm/s and quadrupole splitting = 0.62 mm/s, which are well expected for FeS2 with the pyrite structure [17–19]. This result indicates that mechanical milling does not cause change in crystallographic structure which is consistent with the result as shown in Fig. 1. Our magnetic measurements confirmed paramagnetism of the asmilled powders. Fig. 3 shows the SEM micrographs of the initial and subsequently milled FeS2 powders. The hand-grounded FeS2 powder consisted of micron-sized particles with a broad particle
3. Results and discussion Fig. 1 shows the XRD patterns of the FeS2 initial powder and the FeS2 powder after milling for 24 h. The handgrounded FeS2 powder was of single phase with the pyrite structure. As shown in Fig. 1, mechanical milling did not cause change in structure. The spectrum shows single pyrite
Fig. 2. M¨ossbauer spectra of the initial FeS2 powder and the FeS2 powder after mechanical milling for 24 h.
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Fig. 3. SEM micrographs of the initial FeS2 powder and FeS2 powder after milling for 24 h.
size distribution from a few to ∼100 m. After mechanical milling, the powder appeared as an agglomeration of fine particles. The morphology and particle size could not be clearly analyzed using SEM. It is to note that all the mechanically milled Fe–S powders in this work have a similar SEM micrograph as shown in Fig. 3b. Fig. 4 shows the transmission electron microscope (TEM) micrographs of mechanically milled FeS2 powders with and without a medium (NaCl). Under TEM, the mechanically milled FeS2 samples (Fig. 4a) appeared as sub-micron particles with a nanocrystalline structure. Such a microstructure is typical as observed in many materials after mechanical milling [20]. As reported previously [16], nanoparticles can be obtained after mechanical milling together with a solid-dispersion medium. Oxide nanoparticles have been successfully fabricated using the method [21]. This work is the first attempt to synthesize metal sulfide nanoparticles using a solid dispersion medium (here NaCl). FeS2 and NaCl with a weight ratio of 1:4 were mechanically milled for 48 h. Our M¨ossbauer and magnetic measurements showed that NaCl didn’t react with FeS2 and the crystallographic structure of FeS2 remained unchanged. The TEM micrograph of the resultant particles is shown in Fig. 4b. It can be seen that the particle size is much
Fig. 4. TEM micrographs of mechanically milled FeS2 powders without (a) and with (b) NaCl.
smaller than that after milling without NaCl (Fig. 4a). The typical particle size was found in the range of 20–30 nm. In this work, we have studied the synthesis of FeS2 pyrite nanoparticles by mechanochemical processing, i.e. high-energy mechanical milling of powder mixture of elemental powders (Fe and S with the ratio of 1:2, as denoted as 0.33Fe–0.67S). Fig. 5 shows the XRD patterns of 0.33Fe–0.67S after milling for different times. It can be seen that the formation of FeS2 pyrite requires a long milling time. After milling for 24 h, only the peak for ␣-Fe was visible. The intensity of the ␣-Fe was strongly reduced after milling for 48 h. However, the peaks of pyrite were not obvious in the spectrum (Fig. 5). After milling for 72 h, all the XRD peaks could be identified as the FeS2 pyrite phase. The result indicates that the formation of pyrite requires a long milling time from elemental powders. The formation of FeS2 pyrite could be clearly monitored by M¨ossbauer spectroscopy. Fig. 6 shows the M¨ossbauer spectra of 0.33Fe–0.67S before mechanical milling and after mechanical milling for 48 and 72 h. Before mechanical milling, the magnetic sextet is expected for iron atoms in ␣Fe. After mechanical milling for 48 h, the M¨ossbauer spec-
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Fig. 5. X-ray diffraction patterns of the 0.33Fe–0.67S powders after mechanical milling for (a) 24, (b) 48 and (c) 72 h, respectively.
trum could be considered as a mixture of ␣-Fe (magnetic sextet) and FeS2 pyrite (paramagnetic doublet). After mechanical milling, 90% of Fe atoms were found in the FeS2 pyrite phase. About 10% of iron atoms were found in another doublet with chemical shift of 1.26 mm/s and quadrupole splitting of 3.01 mm/s. The M¨ossbauer parameters indicate the formation of FeSO4 ·H2 O (szomolnokite). The formation of FeSO4 ·H2 O was probably due to oxidation during mechanical milling or oxidation during M¨ossbauer measurement. The as-milled powder was very reactive. It is to note, that the con-
Fig. 6. M¨ossbauer spectra of the 0.33Fe–0.67S in the starting state and after mechanical milling for 48 and 72 h, respectively.
tainer was opened in the pure argon gas filled glove box. The powder was mixed with grease in the glove box before taking out in air for M¨ossbauer examination. However, the mixing with vacuum grease might not avoid oxidation entirely. The requirement of a long milling time for the formation of FeS2 pyrite was also confirmed in our magnetic measurements, as shown in Fig. 7. After mechanical milling for 24 and 48 h, the as-milled powders still had a relatively high magnetization which may be due to the presence of ferromagnetic ␣-Fe. After milling for 72 h, the low magnetization value was expected by the paramagnetic FeS2 phase. The small step at very low magnetic field indicated the presence of a small amount of magnetic phases (here probably ␣-Fe). The small amount of magnetic phases was not detected in our XRD and M¨ossbauer studies (Fig. 6), probably due to its very small particle (or grain) size, which might lead to superparamagnetism. It is also possible that the amount is too small to be detected by XRD and M¨ossbauer examination. It can be seen in Fig. 8, that nanosized particles were resulted from the mechanical milling of Fe and S elemental powders. A typical particle size of 20–30 nm was observed. However, the particles were not well dispersed with a certain degree of agglomeration. In order to obtain well-dispersed particles, NaCl was used as a medium in another milling. Our M¨ossbauer and magnetic study showed the formation of FeS2 pyrite after milling for 72 h. After the removal of NaCl, particles with a size around 10 nm were well dispersed. This work has shown that mechanical milling of Fe and S elemental powders can lead to the formation of well dispersed FeS2 nanoparticles with a particle size around 10 nm when a solid-state dispersion medium is present. The second part of this work was the synthesis of FeS troilite nanoparticles by mechanochemical porcessing. Two different starting mixtures were used, namely 0.5Fe–0.5FeS2 (a mixture of Fe and FeS2 pyrite) and 0.5Fe–0.5S (elemental powders). Fig. 9 shows the XRD patterns of the 0.5Fe–0.5FeS2 and 0.5Fe–0.5S powders after mechanical milling for 24 h. The FeS troilite phase could be formed for
Fig. 7. Hysteresis loops of 0.33Fe–0.67S powders after mechanical milling for 24, 48 and 72 h.
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Fig. 10. M¨ossbauer spectrum of 0.5Fe–0.5FeS2 powder after milling for 24 h.
Fig. 8. TEM micrographs of mechanically milled 0.33Fe–0.67S without NaCl (a) and with NaCl (b).
the two samples (of different starting powder mixtures) after mechanical milling for a relatively short time (24 h). For the sample 0.5Fe–0.5S, using the Scherrer method, a particle size was calculated to be approximately 10 nm according to the line broadening in Fig. 9. For the sample 0.5Fe–0.5FeS2 , the particle size was estimated to be 15 nm, which is a little larger than that of the sample 0.5Fe–0.5S. Fig. 10 shows the M¨ossbauer spectrum of 0.5Fe–0.5FeS2 . The magnetic sextet with a hyperfine field of 260 kOe and
Fig. 9. X-ray diffraction patterns of 0.5Fe–0.5S (a) and 0.5Fe–0.5FeS2 (b) after milling for 24 h.
a chemical shift of 0.66 mm/s is well expected for the antiferromagnetic FeS troilite phase [22]. A magnetic hysteresis loop is shown in Fig. 11 of the 0.5Fe–0.5FeS2 after milling for 24 h. The sample had a low value of magnetization (approximately 4 emu/g at 90 kOe). The low magnetization indicated the anti-ferromagnetic nature with incomplete compensation [23]. The 0.5Fe–0.5S powder after mechanical milling for 24 h had nearly the same M¨ossbauer and magnetic results as shown in Figs. 10 and 11. Very small particles were observed under TEM, as shown in Fig. 12a For 0.5Fe–0.5FeS2 and 0.5Fe–0.5S powders after milling for 24 h, the sizes of most particles were found around 10 nm. Some agglomeration was observed in these powders as shown in Fig. 12a. In a separated experiment, NaCl was used as the dispersion medium. Fig. 12b shows the TEM micrograph of the FeS nanoparticles after the removal of NaCl. It can be seen that the particles were better dispersed. The particle size (approximately 6–8 nm) is smaller than that without NaCl medium.
Fig. 11. Hysteresis loop of 0.5Fe–0.5FeS2 powder after milling for 24 h.
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Fig. 12. TEM micrographs of mechanically milled 0.5Fe–0.5S without (a) and with (b) NaCl.
4. Conclusion In this study, nanocrystalline powders were obtained with the mineral (FeS2 pyrite) as the starting material by highenergy ball milling. The FeS2 pyrite phase could be formed after mechanochemical processing for a long time (72 h) when a mixture of elemental Fe and S powders was used as the starting material. The as-milled powder consisted of nanosized particles with a typical particle size around 10 nm. The use of the solid dispersion medium (NaCl) could result in well-dispersed FeS2 nanoparticles and the solid dispersion medium could promote the formation of fine particles. FeS troilite phase could be formed after mechanochemical processing for a relatively short milling time (24 h) when a mixture of Fe and FeS2 or Fe and S was used as the starting material. The resultant powder consisted of very small particles (approximately 10 nm). Again, the use of NaCl as the dispersion medium could promote the formation of nanoparticles.
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