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A simple synthetic method for MSe2 (M=Fe, Co or Ni) nanocrystallites at low temperature
Materials Chemistry and Physics 69 (2001) 278–280
Materials Science Communication
A simple synthetic method for MSe2 (M=Fe, Co or Ni) nanocrystallites at low temperature Qingyi Lu, Junqing Hu, Kaibin Tang∗ , Bin Deng, Yitai Qian, Guien Zhou, Xianming Liu Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 28 September 1999; received in revised form 18 March 2000; accepted 14 April 2000
Abstract A simple solventothermal method has been employed for nanocrystalline MSe2 (M=Fe, Co or Ni) at 160◦ C. The final product phases were confirmed by X-ray diffraction (XRD). Transmission electron microscopy (TEM) revealed that the obtained crystalline MSe2 consist of nano-particles and a small amount of irregular plate-like products with relatively large size. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical synthesis; Nanocrystallites; Selenides
1. Introduction
2. Experimental
Nanoscale chalcogenides have attracted considerable attention due to their remarkable properties and brilliant application prospects [1]. Various interesting magnetic properties and crystallographic studies have been reported on transition metal dichalcogenides MSe2 (M=Fe, Co, or Ni ) with a pyrite structure [2–5]. These compounds are regarded as intermediate substances between ionic and covalent crystals, and the metal atoms seem to be divalent ionic configuration from the data of magnetic measurement [2]. MSe2 are important direct band gap materials as MS2 that have been widely and deeply investigated since nanosized MS2 can be easily prepared [6]. Nevertheless, few reports of the preparation of nanocrystalline selenides could be found, generally, because not so many Se-sources could be found, and most of them, such as H2 Se, are toxic and hazardous [6]. Conventionally, the crystalline MSe2 were prepared by solid state reactions between the elements at an elevated temperature, typically 600–1200◦ C [7]. Herein, we report a simple and mild method for MSe2 nanocrystallites at 160◦ C with FeCl3 , CoCl2 or NiCl2 and Se powders as reagents through a solventothermal process. In this route, the coordinating solvent plays an important role in the formation of MSe2 .
In a standard experimental procedure, appropriate amounts of FeCl3 ·6H2 O, CoCl2 ·6H2 O or NiCl2 ·6H2 O and Se powders were placed in an autoclave, which was filled with ethylenediamine up to 90% of its volume. The autoclave was then sealed and maintained at 160◦ C for 12 h. Cooled the autoclave at room temperature, and then filtered and washed the precipitate with distilled water to obtain the final product.
∗ Corresponding author. Fax: +86-0551-363-1760. E-mail address: [email protected] (K. Tang).
3. Results and discussion X-ray powder diffraction (XRD) was used to characterize the product. XRD patterns were collected on a Rigaku D/max ␥A rotation anode X-ray diffractometer using Ni-filtered Cu K␣ radiation. A typical XRD pattern of the as-prepared FeSe2 sample is shown in Fig. 1(a), in which all peaks correspond to the reflections of orthorhombic FeSe2 . After refinement the cell constants a=4.81 Å, b=5.77 Å, and c=3.59 Å are consistent with the reported values (JCPDS, No. 21–432). Fig. 1(b) shows the XRD pattern of the as-prepared CoSe2 sample. All peaks could be indexed as orthorhombic CoSe2 . After refinement, the cell constants a=3.61 Å, b=4.84 Å, and c=5.73 Å are in agreement with the reported values (JCPDS, No.10–408). Fig. 1(c) is the XRD pattern of the as-prepared NiSe2 sam-
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 2 9 8 - 4
Q. Lu et al. / Materials Chemistry and Physics 69 (2001) 278–280
279
Fig. 1. The XRD patterns of the obtained samples: (a) FeSe2 ; (b) CoSe2 ; (c) NiSe2 .
ple. All reflections can be indexed to cubic NiSe2 with a lattice constant a=5.97 Å (equal to the value of JCPDS, No. 11–552). According to Scherrer formula, the calculated grain size is about 100 nm. However, with the aid of a Hitachi H-800 transmission electron microscope we found that the main products have particle-like morphology for FeSe2 and NiSe2 with average sizes of 15 and 20 nm, respectively, and feather-like morphology for CoSe2 with an average diameter of 20 nm. Their transmission electron microscopy (TEM) images are shown in Fig. 2. From the TEM observation, a small amount of irregular plate-like products with a relatively large size (shown in Fig. 3) are also detected, which maybe due to the solvent’s characteristics. The strong coordinating ability and chelation of ethylenediamine might cause the non-homogeneous growth and finally lead to the formation of irregular plate-like products during the formation of the particle-like products. In our experiments, the solvent affects the reaction pathway greatly. To study the reaction mechanism, different solvents were tested. The results reveal that the coordinating ability of the solvent plays an important role in the formation of nanocrystalline MSe2 . Ethylenediamine (en), a bidentate compound, was selected as solvent due to its strong
Fig. 2. The TEM images of the main morphologies of the obtained samples: (a) FeSe2 ; (b) CoSe2 ; (c) NiSe2 .
Fig. 3. The TEM image of the produced FeSe2 .
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Q. Lu et al. / Materials Chemistry and Physics 69 (2001) 278–280
N-chelation ability. It can react with the reactants to form relatively stable complexes. This suggestion is supported by our experimental results that with the addition of en, Se and FeCl3 , CoCl2 or NiCl2 powders dissolved slowly. The stability of the complex is expected to decrease with the increase of the processing temperature. Under our experimental conditions, the complex decomposed and coordinated with Se2− to form MSe2 . The dispersion of the reactants in en enlarges the surface area greatly, which makes the activity of the reactants increase and enables the reaction to occur at low temperature. This method, thus, differs from the solid state reactions that need a high temperature to overcome the large activation energy barriers. Inert solvents, benzene and toluene, were also tested as solvent, respectively. With the addition of the solvents, the reactants did not dissolve and after the autoclave was maintained at l60◦ C even for 48 h, the reactants remained unreacted and no MSe2 were detected from XRD patterns. For comparison, different processing temperatures were tried for the preparation of MSe2 nanocrystallites. Keeping the autoclave at temperatures lower than l30◦ C even for 48 h did not enable crystalline MSe2 to form. With the increase of the processing temperature, the crystallinity of the products became better. At about 150◦ C, the products were well crystallized. Processing temperatures higher than 250◦ C led to the products with large grain size. So, it is appropriate to choose 160◦ C as the processing temperature for nanocrystalline MSe2 .
4. Conclusions In summary, we have reported a simple method for crystalline MSe2 through a solventothermal process by using FeCl3 , CoCl2 or NiCl2 and Se powders as reagents. The solvent is an important factor to the formation of MSe2 , and a strong donor ligand solvent, en, makes the synthetic reactions occur at low temperature and, thus, differ from the solid state reactions that take place at high temperature.
Acknowledgements This work was supported by Chinese National Natural Science Research Foundation. References [1] W. Wang, Y. Geng, Y. Qian, M. Ji, X. Liu, Adv. Mater. 10 (1998) 1479. [2] K. Adachi, K. Sato, M. Takeda, J. Phys. Soc. Jpn. 26 (1969) 631. [3] K. Sato, T. Sekiguchi, T. Miyadai, J. Magn. Magn. Mater. 54-57 (1986) 1033. [4] S. Waki, N. Kasai, S. Ogawa, Solid State Commun. 41 (1982) 835. [5] N. Inoue, H. Yasuoka, S. Ogawa, J. Phys. Soc. Jpn. 48 (1980) 850. [6] Y. Li, Y. Ding, Y. Qian, Y. Zhang, Li. Yang, Inorg. Chem. 37 (1998) 2844. [7] T.A. Bither, R.J. Bouchard, W.H. Cloud, P.C. Donohue, W.J. Siemons, Inorg. Chem. 7 (1968) 2208.
Materials Science Communication
A simple synthetic method for MSe2 (M=Fe, Co or Ni) nanocrystallites at low temperature Qingyi Lu, Junqing Hu, Kaibin Tang∗ , Bin Deng, Yitai Qian, Guien Zhou, Xianming Liu Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 28 September 1999; received in revised form 18 March 2000; accepted 14 April 2000
Abstract A simple solventothermal method has been employed for nanocrystalline MSe2 (M=Fe, Co or Ni) at 160◦ C. The final product phases were confirmed by X-ray diffraction (XRD). Transmission electron microscopy (TEM) revealed that the obtained crystalline MSe2 consist of nano-particles and a small amount of irregular plate-like products with relatively large size. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical synthesis; Nanocrystallites; Selenides
1. Introduction
2. Experimental
Nanoscale chalcogenides have attracted considerable attention due to their remarkable properties and brilliant application prospects [1]. Various interesting magnetic properties and crystallographic studies have been reported on transition metal dichalcogenides MSe2 (M=Fe, Co, or Ni ) with a pyrite structure [2–5]. These compounds are regarded as intermediate substances between ionic and covalent crystals, and the metal atoms seem to be divalent ionic configuration from the data of magnetic measurement [2]. MSe2 are important direct band gap materials as MS2 that have been widely and deeply investigated since nanosized MS2 can be easily prepared [6]. Nevertheless, few reports of the preparation of nanocrystalline selenides could be found, generally, because not so many Se-sources could be found, and most of them, such as H2 Se, are toxic and hazardous [6]. Conventionally, the crystalline MSe2 were prepared by solid state reactions between the elements at an elevated temperature, typically 600–1200◦ C [7]. Herein, we report a simple and mild method for MSe2 nanocrystallites at 160◦ C with FeCl3 , CoCl2 or NiCl2 and Se powders as reagents through a solventothermal process. In this route, the coordinating solvent plays an important role in the formation of MSe2 .
In a standard experimental procedure, appropriate amounts of FeCl3 ·6H2 O, CoCl2 ·6H2 O or NiCl2 ·6H2 O and Se powders were placed in an autoclave, which was filled with ethylenediamine up to 90% of its volume. The autoclave was then sealed and maintained at 160◦ C for 12 h. Cooled the autoclave at room temperature, and then filtered and washed the precipitate with distilled water to obtain the final product.
∗ Corresponding author. Fax: +86-0551-363-1760. E-mail address: [email protected] (K. Tang).
3. Results and discussion X-ray powder diffraction (XRD) was used to characterize the product. XRD patterns were collected on a Rigaku D/max ␥A rotation anode X-ray diffractometer using Ni-filtered Cu K␣ radiation. A typical XRD pattern of the as-prepared FeSe2 sample is shown in Fig. 1(a), in which all peaks correspond to the reflections of orthorhombic FeSe2 . After refinement the cell constants a=4.81 Å, b=5.77 Å, and c=3.59 Å are consistent with the reported values (JCPDS, No. 21–432). Fig. 1(b) shows the XRD pattern of the as-prepared CoSe2 sample. All peaks could be indexed as orthorhombic CoSe2 . After refinement, the cell constants a=3.61 Å, b=4.84 Å, and c=5.73 Å are in agreement with the reported values (JCPDS, No.10–408). Fig. 1(c) is the XRD pattern of the as-prepared NiSe2 sam-
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 2 9 8 - 4
Q. Lu et al. / Materials Chemistry and Physics 69 (2001) 278–280
279
Fig. 1. The XRD patterns of the obtained samples: (a) FeSe2 ; (b) CoSe2 ; (c) NiSe2 .
ple. All reflections can be indexed to cubic NiSe2 with a lattice constant a=5.97 Å (equal to the value of JCPDS, No. 11–552). According to Scherrer formula, the calculated grain size is about 100 nm. However, with the aid of a Hitachi H-800 transmission electron microscope we found that the main products have particle-like morphology for FeSe2 and NiSe2 with average sizes of 15 and 20 nm, respectively, and feather-like morphology for CoSe2 with an average diameter of 20 nm. Their transmission electron microscopy (TEM) images are shown in Fig. 2. From the TEM observation, a small amount of irregular plate-like products with a relatively large size (shown in Fig. 3) are also detected, which maybe due to the solvent’s characteristics. The strong coordinating ability and chelation of ethylenediamine might cause the non-homogeneous growth and finally lead to the formation of irregular plate-like products during the formation of the particle-like products. In our experiments, the solvent affects the reaction pathway greatly. To study the reaction mechanism, different solvents were tested. The results reveal that the coordinating ability of the solvent plays an important role in the formation of nanocrystalline MSe2 . Ethylenediamine (en), a bidentate compound, was selected as solvent due to its strong
Fig. 2. The TEM images of the main morphologies of the obtained samples: (a) FeSe2 ; (b) CoSe2 ; (c) NiSe2 .
Fig. 3. The TEM image of the produced FeSe2 .
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Q. Lu et al. / Materials Chemistry and Physics 69 (2001) 278–280
N-chelation ability. It can react with the reactants to form relatively stable complexes. This suggestion is supported by our experimental results that with the addition of en, Se and FeCl3 , CoCl2 or NiCl2 powders dissolved slowly. The stability of the complex is expected to decrease with the increase of the processing temperature. Under our experimental conditions, the complex decomposed and coordinated with Se2− to form MSe2 . The dispersion of the reactants in en enlarges the surface area greatly, which makes the activity of the reactants increase and enables the reaction to occur at low temperature. This method, thus, differs from the solid state reactions that need a high temperature to overcome the large activation energy barriers. Inert solvents, benzene and toluene, were also tested as solvent, respectively. With the addition of the solvents, the reactants did not dissolve and after the autoclave was maintained at l60◦ C even for 48 h, the reactants remained unreacted and no MSe2 were detected from XRD patterns. For comparison, different processing temperatures were tried for the preparation of MSe2 nanocrystallites. Keeping the autoclave at temperatures lower than l30◦ C even for 48 h did not enable crystalline MSe2 to form. With the increase of the processing temperature, the crystallinity of the products became better. At about 150◦ C, the products were well crystallized. Processing temperatures higher than 250◦ C led to the products with large grain size. So, it is appropriate to choose 160◦ C as the processing temperature for nanocrystalline MSe2 .
4. Conclusions In summary, we have reported a simple method for crystalline MSe2 through a solventothermal process by using FeCl3 , CoCl2 or NiCl2 and Se powders as reagents. The solvent is an important factor to the formation of MSe2 , and a strong donor ligand solvent, en, makes the synthetic reactions occur at low temperature and, thus, differ from the solid state reactions that take place at high temperature.
Acknowledgements This work was supported by Chinese National Natural Science Research Foundation. References [1] W. Wang, Y. Geng, Y. Qian, M. Ji, X. Liu, Adv. Mater. 10 (1998) 1479. [2] K. Adachi, K. Sato, M. Takeda, J. Phys. Soc. Jpn. 26 (1969) 631. [3] K. Sato, T. Sekiguchi, T. Miyadai, J. Magn. Magn. Mater. 54-57 (1986) 1033. [4] S. Waki, N. Kasai, S. Ogawa, Solid State Commun. 41 (1982) 835. [5] N. Inoue, H. Yasuoka, S. Ogawa, J. Phys. Soc. Jpn. 48 (1980) 850. [6] Y. Li, Y. Ding, Y. Qian, Y. Zhang, Li. Yang, Inorg. Chem. 37 (1998) 2844. [7] T.A. Bither, R.J. Bouchard, W.H. Cloud, P.C. Donohue, W.J. Siemons, Inorg. Chem. 7 (1968) 2208.