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Solvothermal preparation and characterization of nanocrystalline Bi2Te3 powder with different morphology
Journal of Physics and Chemistry of Solids 63 (2002) 2119–2121 www.elsevier.com/locate/jpcs
Solvothermal preparation and characterization of nanocrystalline Bi2Te3 powder with different morphology Yuan Denga, Xi-song Zhoua, Guo-dan Weia, Jing Liua, Ce-Wen Nana,*, Shu-jing Zhaob a
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China b School of Materials and Engineering, Jiamusi University, Heilongjiang 154000, People’s Republic of China Received 12 October 2001; received in revised form 5 January 2002; accepted 5 January 2002
Abstract Bi2Te3-based alloys are currently best-known, technological important thermoelectric materials near room temperature. In this paper, nanocrystalline Bi2Te3 with different morphologies was synthesized by a solvothermal process based on the reaction between BiCl3, Te, and KBH4 in N,N-dimethylformamide at 100– 180 8C. KBH4 was used as a reducing agent. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM). The particle morphologies and size are dependent on the reaction temperature and time. A possible formation mechanism is proposed. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Nanostructures; B. Chemical synthesis
1. Introduction The bismuth telluride based alloys are known as the best thermoelectric materials currently available for thermoelectric application near room temperature [1]. They have many important applications, for example, used as thermopiles, and thermal sensors, thermoelectric cooler for laser diodes [2 – 5]. Because of technological importance of Bi2Te3, a few methods have been developed to prepare bismuth telluride, the most straightforward of which is to melt the metal elements at elevated temperatures [6]. This process requires elevated temperature and a relatively long duration under special protection against oxidation, and it is not easy to get materials of chemical homogeneity. This is an important point as thermoelectric properties are greatly correlated with chemical homogeneity of the materials. Lowering the reaction temperature has become the goal of solid-state chemists. A low-energy-cost approach is the coprecipitation of bismuth and telluride as oxides from aqueous solution, and the coprecipitated oxides are converted directly to fine Bi2Te3 powder through hydrogen reduction [7]. Groshens and co-workers [8] reported a * Corresponding author. Tel./fax: þ 86-10-62773587. E-mail address: [email protected] (C.W. Nan).
different approach to the synthesis of Bi2Te3 by means of elimination reactions conducted in hexane at 2 30 8C. Using organometallic precursors is another route to Bi2Te3 [9]. Qian and co-workers [10] reported a solvothermal reaction of metal oxalates Bi2(C2O4)3 with Te in organic solvents at relatively low temperature (120 – 160 8C) to produce crystalline Bi2Te3. The solvothermal method is a promising approach to obtain many kinds of nanocrystalline non-oxide materials under comparatively low temperature. This method does not need organometallic precursors. In this study, we synthesized nanocrystalline Bi2Te3 with different morphology under mild conditions by the solvothermal method.
2. Experimental All chemicals are analytical grade and used without further purification. A mixture of BiCl3·2H2O (10 mmol), tellurium powder (15 mmol), KOH (80 mmol) and KBH4 (30 mmol) were put into a Telfon-lined autoclave of 100 ml capacity. The autoclave was filled with N,N-dimethylformamide (DMF) up to 90% of its capacity, maintained at 100– 180 8C for 10 – 50 h, and then cooled to room temperature naturally. The products were filtered and
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 2 6 1 - 5
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Y. Deng et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2119–2121
Fig. 1. X-ray diffraction patterns of Bi2Te3 obtained under (a) 100 8C for 24 h, (b) 100 8C for 48 h, (c) 180 8C for 48 h.
washed with distilled water and absolute ethanol in sequence. Finally, the dark products were dried at 80 8C. Powder X-ray diffraction (XRD) was used to characterize the samples. Data were collected on a Bruker D8 Advance X-ray diffractometer with Cu Ka radiation ðl ¼ The grain morphology and size were examined 1:54178 AÞ: by transmission electron microscopy (TEM). The TEM images were recorded on a Hitachi H-800 TEM, using an accelerating voltage of 200 kV.
3. Results and discussion The XRD pattern of the products is shown in Fig. 1. All peaks in the patterns correspond to the reflections of rhombohedral phase R3¯m, with cell constants a ¼ 4:38 A; c ¼ 30:50 A; which are in agreement with the reported c ¼ 30:48 A (JCPDS 15-0863). There values a ¼ 4:385 A; is no difference in the XRD patterns of the products prepared in different temperature and reaction time. X-ray energy dispersive analysis patterns for the prepared Bi2Te3 show the presence of only Bi and Te peaks. Though there is no difference in the XRD patterns of the products, Fig. 2 shows the TEM images of as-prepared Bi2Te3 with different morphologies. Fig. 2a shows that the Bi2Te3 powder obtained at 100 8C for 24 h consist of mainly nanorods with length up to 150 nm and average width of 10 nm. With increasing reaction temperature and time, the shape of those nanoparticles is nearly spherical, as shown in Fig. 2b. The particle sizes are 20 –40 nm. Bi2Te3 particles with square-plate (Fig. 2c) and spherical morphologies were obtained at 180 8C for 15 h. In general the resultant particles became bigger with increasing temperature and time. In the solvothermal process, the solvent plays an important role in the nucleation of nanocrystalline Bi2Te3. DMF was selected as the solvent because the starting materials can dissolve in it, and in alkaline medium DMF
Fig. 2. TEM micrographs of as-prepared bismuth telluride obtained under (a) 100 8C for 24 h, (b) 130 8C for 17 h, (c) 180 8C for 48 h.
can partialy decompose into dimethylamine, which is a good coordination ligand and has N-chelation. The critical temperature of dimethylamine (164.5 8C) is lower than that of water (374 8C), so the diffusion of ions in DMF– dimethylamine will be more rapid due to its lower viscosity, which leads to acceleration in the crystal growth [11]. In this solvothermal process, the formation mechanism of Bi2Te3 nanoparticles could be a combination of two independent pathways. The pathway (a) may be expressed as follows: 2 2 22 BH2 4 þ 4Te þ 8OH ! H2 BO3 þ 5H2 O þ 4Te
ð1Þ
3Te þ 5OH2 ! 2Te22 þ HTeO2 3 þ 2H2 O
ð2Þ
2
HCONðCH3 Þ2 þ OH ! HNðCH3 Þ2 þ HCOO
2
Bi3þ þ HNðCH3 Þ2 ! ½BiðHNðCH3 Þ2 Þ3þ 3þ
½BiðHNðCH3 Þ2 Þ
þ 3Te
22
! Bi2 Te3 þ HNðCH3 Þ2
ð3Þ ð4Þ ð5Þ
Firstly, Te is reduced to Te22, and then Te22 reacts with Bi3þ to form Bi2Te3 nanoparticles. In this pathway, KBH4 was used to reduce Te to Te22. The present of alkaline medium in this solvothermal process was found to be essential, and Bi2Te3 does not form without the alkaline medium. The alkaline medium could make the tellurium
Y. Deng et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2119–2121
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disperse in the liquid medium as telluride, which is beneficial for the formation of nanosized products. As known, tellurium, transition metals and some donor solvents can form organometallic complexes [12,13]. In addition, the dimethylamine has excellent donor solubility for many metals. The formation process of Bi2Te3 nanorods may be visualized as follows: bismuth chloride dissolves in dimethylamine to form the complex ion [Bi(HN(CH3)2)]3þ. The complex ions can be linked by hydrogen bonds, which exist in the amine donor solvents, and they self-assemble to a chain form. Then, by the reaction of the complex ions with tellurium, a chain structural intermediate forms. With increasing temperature and pressure in the system, the intermediate decomposes into Bi2Te3 nanorods. The pathway (b) is a direct combination of metal, which is also found in the preparation of PbTe [14]. It could be expressed as:
been synthesized via a solvothermal method based on the reaction between BiCl3, Te, and KBH4 in DMF at 100– 180 8C. Morphology and size of synthesized Bi2Te3 nanoparticles are dependent on the reaction temperature and time, which is probably due to different reaction processes.
3þ 3BH2 þ 24OH2 ! 3H2 BO2 4 þ Bi 3 þ 15H2 O þ Bi
ð6Þ
References
2Bi þ 3Te ! Bi2 Te3
ð7Þ
3þ
Bismuth ions (Bi ) could be reduced to metal bismuth readily by KBH4, according to the redox potential, i.e. 0 0 EBi 3þ =Bi ¼ 0:168 V; EH O=BH2 ¼ 21:12 V: In the early stage 2 4 of the reaction, metal bismuth was often found in our product. When the reaction time was prolonged, the metal bismuth disappeared. These two reaction processes dominate the formation of Bi2Te3 nanocrystals at different reaction temperature and time, which results in different morphologies of Bi2Te3 nanocrystals as shown in Fig. 2. When reaction temperature is low and reaction time is short, the pathway (a) is the dominant formation process; and it is easy to form rod-like nanoparticles. If the reaction temperature increases or reaction time is prolonged, the reaction (b) can occur, the morphology of Bi2Te3 nanocrystals tends to be sphereshaped. When reaction temperature is above 180 8C, square -plate crystals form.
4. Conclusion Nanocrystalline Bi2Te3 with different morphologies has
Acknowledgments The authors thank Professor Y. Li for his helpful discussion. This work was supported by the National Natural Science Foundation of China under grant No. 50072010 and 59825102.
[1] A. Boyer, E. Cisse´, Mater. Sci. Engng B 113 (1992) 103. [2] D.M. Rowe, C.M. Bhandari, Modern Thermoelectrics, Holt, Rinehart and Winston, London, 1983, p. 103. [3] B.M. Goltsman, V.A. Kudinov, I.A. Smirnov, Semiconducting Thermoelectric Materials Based on BiTe, Nauka, Moscow, 1972, p. 320. [4] F.J. DiSalvo, Science 285 (1999) 703. [5] D.M. Rowe, Handbook of Thermoelectroics, CRC Press, Boca Raton, FL, 1995. [6] F.D. Rosi, B. Ables, R.V. Jensen, J. Phys. Chem. Solids 10 (1959) 191. [7] J. Ritter, J. Inorg. Chem. 33 (1994) 6419. [8] T.J. Groshens, R.W. Gedridge, C. Lowe-Ma, Chem. Mater. 6 (1994) 727. [9] J.J. Ritter, P. Maruthamuthu, Inorg. Chem. 34 (1995) 4278. [10] S.H. Yu, J. Yang, Y.S. Wu, Z.H. Han, J. Lu, Y. Xie, Y.T. Qian, J. Mater. Chem. 8 (1998) 1949. [11] B. Li, Y. Xie, J.Z. Huang, H.L. Su, Y.T. Qian, Nanosruct. Mater. 11 (1999) 1067. [12] M.L. Steigerwald, T. Siegrist, S.M. Stuczynski, Inorg. Chem. 30 (1991) 4940. [13] J.G. Bernnan, M.L. Steigerwald, J. Am. Chem. Soc. 111 (1989) 9240. [14] C. Wang, G. Zhang, S. Fan, Y. Li, J. Phys. Chem. Solids 62 (2001) 1957.
Solvothermal preparation and characterization of nanocrystalline Bi2Te3 powder with different morphology Yuan Denga, Xi-song Zhoua, Guo-dan Weia, Jing Liua, Ce-Wen Nana,*, Shu-jing Zhaob a
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China b School of Materials and Engineering, Jiamusi University, Heilongjiang 154000, People’s Republic of China Received 12 October 2001; received in revised form 5 January 2002; accepted 5 January 2002
Abstract Bi2Te3-based alloys are currently best-known, technological important thermoelectric materials near room temperature. In this paper, nanocrystalline Bi2Te3 with different morphologies was synthesized by a solvothermal process based on the reaction between BiCl3, Te, and KBH4 in N,N-dimethylformamide at 100– 180 8C. KBH4 was used as a reducing agent. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM). The particle morphologies and size are dependent on the reaction temperature and time. A possible formation mechanism is proposed. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Chalcogenides; A. Nanostructures; B. Chemical synthesis
1. Introduction The bismuth telluride based alloys are known as the best thermoelectric materials currently available for thermoelectric application near room temperature [1]. They have many important applications, for example, used as thermopiles, and thermal sensors, thermoelectric cooler for laser diodes [2 – 5]. Because of technological importance of Bi2Te3, a few methods have been developed to prepare bismuth telluride, the most straightforward of which is to melt the metal elements at elevated temperatures [6]. This process requires elevated temperature and a relatively long duration under special protection against oxidation, and it is not easy to get materials of chemical homogeneity. This is an important point as thermoelectric properties are greatly correlated with chemical homogeneity of the materials. Lowering the reaction temperature has become the goal of solid-state chemists. A low-energy-cost approach is the coprecipitation of bismuth and telluride as oxides from aqueous solution, and the coprecipitated oxides are converted directly to fine Bi2Te3 powder through hydrogen reduction [7]. Groshens and co-workers [8] reported a * Corresponding author. Tel./fax: þ 86-10-62773587. E-mail address: [email protected] (C.W. Nan).
different approach to the synthesis of Bi2Te3 by means of elimination reactions conducted in hexane at 2 30 8C. Using organometallic precursors is another route to Bi2Te3 [9]. Qian and co-workers [10] reported a solvothermal reaction of metal oxalates Bi2(C2O4)3 with Te in organic solvents at relatively low temperature (120 – 160 8C) to produce crystalline Bi2Te3. The solvothermal method is a promising approach to obtain many kinds of nanocrystalline non-oxide materials under comparatively low temperature. This method does not need organometallic precursors. In this study, we synthesized nanocrystalline Bi2Te3 with different morphology under mild conditions by the solvothermal method.
2. Experimental All chemicals are analytical grade and used without further purification. A mixture of BiCl3·2H2O (10 mmol), tellurium powder (15 mmol), KOH (80 mmol) and KBH4 (30 mmol) were put into a Telfon-lined autoclave of 100 ml capacity. The autoclave was filled with N,N-dimethylformamide (DMF) up to 90% of its capacity, maintained at 100– 180 8C for 10 – 50 h, and then cooled to room temperature naturally. The products were filtered and
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 2 6 1 - 5
2120
Y. Deng et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2119–2121
Fig. 1. X-ray diffraction patterns of Bi2Te3 obtained under (a) 100 8C for 24 h, (b) 100 8C for 48 h, (c) 180 8C for 48 h.
washed with distilled water and absolute ethanol in sequence. Finally, the dark products were dried at 80 8C. Powder X-ray diffraction (XRD) was used to characterize the samples. Data were collected on a Bruker D8 Advance X-ray diffractometer with Cu Ka radiation ðl ¼ The grain morphology and size were examined 1:54178 AÞ: by transmission electron microscopy (TEM). The TEM images were recorded on a Hitachi H-800 TEM, using an accelerating voltage of 200 kV.
3. Results and discussion The XRD pattern of the products is shown in Fig. 1. All peaks in the patterns correspond to the reflections of rhombohedral phase R3¯m, with cell constants a ¼ 4:38 A; c ¼ 30:50 A; which are in agreement with the reported c ¼ 30:48 A (JCPDS 15-0863). There values a ¼ 4:385 A; is no difference in the XRD patterns of the products prepared in different temperature and reaction time. X-ray energy dispersive analysis patterns for the prepared Bi2Te3 show the presence of only Bi and Te peaks. Though there is no difference in the XRD patterns of the products, Fig. 2 shows the TEM images of as-prepared Bi2Te3 with different morphologies. Fig. 2a shows that the Bi2Te3 powder obtained at 100 8C for 24 h consist of mainly nanorods with length up to 150 nm and average width of 10 nm. With increasing reaction temperature and time, the shape of those nanoparticles is nearly spherical, as shown in Fig. 2b. The particle sizes are 20 –40 nm. Bi2Te3 particles with square-plate (Fig. 2c) and spherical morphologies were obtained at 180 8C for 15 h. In general the resultant particles became bigger with increasing temperature and time. In the solvothermal process, the solvent plays an important role in the nucleation of nanocrystalline Bi2Te3. DMF was selected as the solvent because the starting materials can dissolve in it, and in alkaline medium DMF
Fig. 2. TEM micrographs of as-prepared bismuth telluride obtained under (a) 100 8C for 24 h, (b) 130 8C for 17 h, (c) 180 8C for 48 h.
can partialy decompose into dimethylamine, which is a good coordination ligand and has N-chelation. The critical temperature of dimethylamine (164.5 8C) is lower than that of water (374 8C), so the diffusion of ions in DMF– dimethylamine will be more rapid due to its lower viscosity, which leads to acceleration in the crystal growth [11]. In this solvothermal process, the formation mechanism of Bi2Te3 nanoparticles could be a combination of two independent pathways. The pathway (a) may be expressed as follows: 2 2 22 BH2 4 þ 4Te þ 8OH ! H2 BO3 þ 5H2 O þ 4Te
ð1Þ
3Te þ 5OH2 ! 2Te22 þ HTeO2 3 þ 2H2 O
ð2Þ
2
HCONðCH3 Þ2 þ OH ! HNðCH3 Þ2 þ HCOO
2
Bi3þ þ HNðCH3 Þ2 ! ½BiðHNðCH3 Þ2 Þ3þ 3þ
½BiðHNðCH3 Þ2 Þ
þ 3Te
22
! Bi2 Te3 þ HNðCH3 Þ2
ð3Þ ð4Þ ð5Þ
Firstly, Te is reduced to Te22, and then Te22 reacts with Bi3þ to form Bi2Te3 nanoparticles. In this pathway, KBH4 was used to reduce Te to Te22. The present of alkaline medium in this solvothermal process was found to be essential, and Bi2Te3 does not form without the alkaline medium. The alkaline medium could make the tellurium
Y. Deng et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2119–2121
2121
disperse in the liquid medium as telluride, which is beneficial for the formation of nanosized products. As known, tellurium, transition metals and some donor solvents can form organometallic complexes [12,13]. In addition, the dimethylamine has excellent donor solubility for many metals. The formation process of Bi2Te3 nanorods may be visualized as follows: bismuth chloride dissolves in dimethylamine to form the complex ion [Bi(HN(CH3)2)]3þ. The complex ions can be linked by hydrogen bonds, which exist in the amine donor solvents, and they self-assemble to a chain form. Then, by the reaction of the complex ions with tellurium, a chain structural intermediate forms. With increasing temperature and pressure in the system, the intermediate decomposes into Bi2Te3 nanorods. The pathway (b) is a direct combination of metal, which is also found in the preparation of PbTe [14]. It could be expressed as:
been synthesized via a solvothermal method based on the reaction between BiCl3, Te, and KBH4 in DMF at 100– 180 8C. Morphology and size of synthesized Bi2Te3 nanoparticles are dependent on the reaction temperature and time, which is probably due to different reaction processes.
3þ 3BH2 þ 24OH2 ! 3H2 BO2 4 þ Bi 3 þ 15H2 O þ Bi
ð6Þ
References
2Bi þ 3Te ! Bi2 Te3
ð7Þ
3þ
Bismuth ions (Bi ) could be reduced to metal bismuth readily by KBH4, according to the redox potential, i.e. 0 0 EBi 3þ =Bi ¼ 0:168 V; EH O=BH2 ¼ 21:12 V: In the early stage 2 4 of the reaction, metal bismuth was often found in our product. When the reaction time was prolonged, the metal bismuth disappeared. These two reaction processes dominate the formation of Bi2Te3 nanocrystals at different reaction temperature and time, which results in different morphologies of Bi2Te3 nanocrystals as shown in Fig. 2. When reaction temperature is low and reaction time is short, the pathway (a) is the dominant formation process; and it is easy to form rod-like nanoparticles. If the reaction temperature increases or reaction time is prolonged, the reaction (b) can occur, the morphology of Bi2Te3 nanocrystals tends to be sphereshaped. When reaction temperature is above 180 8C, square -plate crystals form.
4. Conclusion Nanocrystalline Bi2Te3 with different morphologies has
Acknowledgments The authors thank Professor Y. Li for his helpful discussion. This work was supported by the National Natural Science Foundation of China under grant No. 50072010 and 59825102.
[1] A. Boyer, E. Cisse´, Mater. Sci. Engng B 113 (1992) 103. [2] D.M. Rowe, C.M. Bhandari, Modern Thermoelectrics, Holt, Rinehart and Winston, London, 1983, p. 103. [3] B.M. Goltsman, V.A. Kudinov, I.A. Smirnov, Semiconducting Thermoelectric Materials Based on BiTe, Nauka, Moscow, 1972, p. 320. [4] F.J. DiSalvo, Science 285 (1999) 703. [5] D.M. Rowe, Handbook of Thermoelectroics, CRC Press, Boca Raton, FL, 1995. [6] F.D. Rosi, B. Ables, R.V. Jensen, J. Phys. Chem. Solids 10 (1959) 191. [7] J. Ritter, J. Inorg. Chem. 33 (1994) 6419. [8] T.J. Groshens, R.W. Gedridge, C. Lowe-Ma, Chem. Mater. 6 (1994) 727. [9] J.J. Ritter, P. Maruthamuthu, Inorg. Chem. 34 (1995) 4278. [10] S.H. Yu, J. Yang, Y.S. Wu, Z.H. Han, J. Lu, Y. Xie, Y.T. Qian, J. Mater. Chem. 8 (1998) 1949. [11] B. Li, Y. Xie, J.Z. Huang, H.L. Su, Y.T. Qian, Nanosruct. Mater. 11 (1999) 1067. [12] M.L. Steigerwald, T. Siegrist, S.M. Stuczynski, Inorg. Chem. 30 (1991) 4940. [13] J.G. Bernnan, M.L. Steigerwald, J. Am. Chem. Soc. 111 (1989) 9240. [14] C. Wang, G. Zhang, S. Fan, Y. Li, J. Phys. Chem. Solids 62 (2001) 1957.