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Self-catalyzed growth of magnesium oxide nanorods
Materials Letters 60 (2006) 2017 – 2019 www.elsevier.com/locate/matlet
Self-catalyzed growth of magnesium oxide nanorods M. Zhao, X.L. Chen ⁎, W.J. Wang, Y.J. Ma, Y.P. Xu, H.Z. Zhao Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China Received 8 July 2005; accepted 20 December 2005 Available online 19 January 2006
Abstract MgO nanorods with spherical particles at the tips were fabricated through direct reaction of Mg and oxygen. The nanorods have diameters ranging from 20 to 50 nm and their lengths are up to 10 μm. The diameters of the spherical particles are about 3–4 times larger than that of the nanorods. X-ray diffraction and energy-dispersive X-ray fluorescence results prove that the nanorods are composed of face-centered cubic (fcc) MgO. The transmission electron microscopy observations show that the nanorods are single crystalline. The growth direction of the nanorod is along [111]. A self-catalyzed vapor–liquid–solid growth model is adopted to account for the formation of the MgO nanorods. © 2005 Elsevier B.V. All rights reserved. PACS: 81.10.Bk; 79.60.Jv
1. Introduction MgO is a promising material with such potential applications as catalysis, additives in refractory, paint and superconductor products, and as substrates for thin film growth [1,2]. Recently, one-dimensional (1D) solid MgO nanomaterials (nanorods and nanowires) have attracted considerable interest for scientific research because of their application as a component in hightemperature superconductors (HTSCs) [3–6]. It was reported that MgO nanorods with the diameter of 5–50 nm and length of 1–3 μm were grown and incorporated into HTSCs to form nanorod–HTSC composites [3–6]. Examples include: BSCCO2212, BSCCO-2223, (Bi, Pb)-2223, TBCCO-2223 superconductors [3–6]. Measurements demonstrated that the critical current densities (Jc's) of the nanorod–HTSC composites were enhanced dramatically at high temperatures and magnetic fields as compared with the undoped samples. Investigations revealed that a proper introduction of MgO nanorods as the effective pinning centers significantly enhanced the flux pinning in the nanorod–HTSC composites. Different approaches have been used for the synthesis of MgO nanorods, for example, the vapor–liquid–solid growth method using MnCl2 as the precursor [3], the modified vapor–solid growth process using
⁎ Corresponding author. Fax: +86 10 82649646. E-mail address: [email protected] (X.L. Chen). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.121
a mixture of MgO powder and carbon powder as the starting materials [6], and the technique involving a hydrothermal hydrolysis to produce Mg(OH)2 nanorods from MgSO4·7H2O and a subsequent thermal decomposition to obtain MgO nanorods [7]. However, from the viewpoint of actual application, exploration of a suitable technique to grow large-scale MgO nanorods is still a challenging research area. Herein, we describe an efficient route to the synthesis of MgO nanorods using Mg as the starting material. Comparing with those methods using MnCl2 as precursor or using a mixture of MgO powder and carbon powder as the starting materials, Mg is not only reactant but also catalyst in this technique, which is more direct and easy. Furthermore, at the tips of the nanorods, spherical-shaped particles are frequently found, which is morphologically different with that synthesized by other researchers. The reported method is relatively simple, low cost and more effective, so it is very practical for large scale applications. 2. Experimental The system that we use to synthesize MgO nanorods with spherical tips is basically similar to that designed to form networked rectangular MgO nanostructures [8]. The chemical reaction involved can be expressed as follows: 2 Mg(vapor) + O2(gas) → 2 MgO(solid). The reaction was carried out in a conventional tube furnace with a horizontal quartz glass tube.
2018
M. Zhao et al. / Materials Letters 60 (2006) 2017–2019
The length of the quartz glass tube is 60 mm. In a typical run, the commercial Mg ribbons or chips (99.5% purity) and the Si substrate, separated by 5 mm in a quartz boat, were loaded into the center of the horizontal quartz tube. The tube was placed in a conventional tube furnace. After that, the quartz tube was first pumped, then filled with argon gas, and then repumped. This operation was repeated more than thrice to ensure there is little. After that, the furnace was heated under an argon flow of 20 ml/min. When the temperature reached 900 °C, a mixture of argon / oxygen (Ar / O2 = 10) gas was introduced into the quartz tube. The furnace temperature was kept constant for 10 min. Finally the power was switched off and the furnace was allowed to cool naturally under an argon flow. After cooling, a white layer was found on the silicon substrate. The deposited products were characterized and analyzed by X-ray diffraction (XRD, Rigaku D/max-2400 diffractometer with Cu K α radiation), scanning electron microscopy (SEM, Hitachi 4200 FE-SEM), energy-dispersive X-ray fluorescence (EDX, attached to the Hitachi 4200) and transmission electron microscopy (TEM, using a Philips CM200 FEG and JEOL 2010, 200 kV). 3. Results and discussion Fig. 1 gives general and high-magnification SEM images of the deposition on the Si substrate. Fig. 1a shows that the Si substrate surface was covered by randomly distributed rod-like nanomaterials.
Fig. 2. (a) EDS pattern of the as-grown nanorods indicates that the nanostructures are composed of Mg and O and the molecular ratio of Mg / O calculated from the EDX data is approximately 1 : 1. The signal of Si in the spectrum should be the contribution of Si substrate. (b) XRD pattern of the nanostructures. Two crystal phases, cubic MgO and cubic Si (Si substrate), are indexed.
Fig. 1. (a) SEM image shows a general view of the deposition. The nanorods dominate the nanostructure. The left and right insets display the detailed characteristics of the nanorods. (b) SEM image shows a closer view of the MgO nanorod.
The diameters of the nanorods are in the range of 20–50 nm. The lengths are within 10 μm. The closer view in Fig. 1b reveals that most of the nanorods have spherical-particles at their tips. The diameters of the nanoparticles are about 3–4 times larger than that of the nanorods. In addition, it can be observed that the particle only can be found at one tip of the nanorod and there is no particle at another end, as shown in the two insets in Fig. 1a. EDX analysis is carried out to determine the chemical composition of the as-grown products (Fig. 2a). The result indicates that these nanorods are composed of magnesium and oxygen with an atomic ratio of approximately 1 : 1, which is in a good agreement with the stoichiometric ratio of MgO. The signal of Si in the image should be the contribution of the Si substrate. Furthermore, the XRD measurement is carried out at room temperature to examine the lattice structure and phase purity of the obtained nanostructures. As shown in Fig. 2b, the XRD pattern can be well indexed to the fcc structured MgO phase with the cell parameter of a = 4.211 Å, which is in good agreement with the known data [9]. The first strong peak is due to the reflection (111) of the cubic Si substrate [10].
M. Zhao et al. / Materials Letters 60 (2006) 2017–2019
2019
not only functions as the reactant but also plays a crucial catalytic role. The growth of the MgO nanorods may involve the following steps. Firstly, the Mg vapor from the high temperature region is carried by argon flow to the low-temperature zone and deposits in the form of liquid droplet on the substrate. Once 900 °C was reached, a little amount oxygen was introduced in the quartz tube. Then, the Mg droplets on the substrate react with oxygen and forms MgO nuclei, which further serve as seeds for MgO nanorods growth. Finally, the MgO nuclei grow to MgO nanorods when continuous introduction of Mg vapor and oxygen.
4. Conclusion To conclude, we have described the synthesis of MgO nanorods with spherical nanoparticles at the tips under chemical vapor deposition conditions. SEM observations reveal that the diameters of the nanorods range from 20 to 50 nm. The lengths are within 10 μm. The diameter of the spherical particle at the end of a nanorod is about 3–4 times of the diameter of the nanorod. The XRD and TEM analysis indicates that the assynthesized nanorods are single-crystalline MgO with facecentered-cubic structure. The growth mechanism of the MgO nanorods is proposed briefly. Acknowledgement
Fig. 3. (a) SAED pattern of a MgO nanorod taken along [110]. (b) HRTEM of a MgO nanorod.
Fig. 3a is the selected area electron diffraction (SAED) pattern of a typical nanorod taken along [110] direction. It can be indexed based on an fcc MgO cell with lattice parameter of a = 4.211 Å [9], which is consistent with the above XRD result. Fig. 3b shows a high-resolution transmission electron microscopy (HRTEM) image of a single MgO nanorod. It reveals that the spacing of 0.24 nm between adjacent lattice planes corresponds to the distance between (111) crystal planes, indicating [111] as the growth direction for the MgO nanorods. The SAED and HRTEM results prove that the nanorods are singlecrystalline MgO nanorods with [111] growth direction. The vapor–liquid–solid (VLS) growth mechanism has been widely used for the syntheses of nanomaterials. Examples include: In2O3, ZnO, Si, InN and GaN nanowires [11–15]. Such transition metals as Ag, Ni, Fe, Cu, Au and Ti are often employed as catalysts. The most remarkable sign of the VLS mechanism is the presence of particles at the ends of the nanowires, nanorods and nanotube, which serve as the catalyst under chemical vapor deposition (CVD) conditions. In our study, spherical nanoparticles are frequently found at the tips of the MgO nanorods. Since no additional metal was involved as the catalyst and no new phase appeared except MgO in the synthesis process, we propose that the spherical particles are composed of MgO. Therefore, we believe that the formation of the MgO nanorods was controlled by a self-catalyzed VLS growth mechanism. In the formation process, Mg
This work is financially supported by the Chinese Academy of Sciences and the National Natural Science Foundation of China. References [1] R.Z. Ma, Y. Bando, Chem. Phys. Lett. 370 (2003) 770. [2] Y.Q. Zhu, W.K. Hsu, W.Z. Zhou, M. Terrones, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 347 (2001) 337. [3] Z. Cui, G.W. Meng, W.D. Huang, G.Z. Wang, L.D. Zhang, Mater. Res. Bull. 35 (2000) 1653. [4] W.D. Huang, W.H. Song, Z. Cui, B. Zhao, M.H. Pu, X.C. Wu, T. Hu, Y.P. Sun, J.J. Du, Supercond. Sci. Technol. 13 (2000) 1499. [5] P.D. Yang, C.M. Lieber, Science 273 (1996) 1836. [6] P.D. Yang, C.M. Lieber, J. Mater. Res. 12 (1997) 2981. [7] L. Yan, J. Zhuang, X.M. Sun, Z.X. Deng, Y.D. Li, Mater. Chem. Phys. 76 (2002) 119. [8] M. Zhao, X.L. Chen, X.N. Zhang, H. Li, H.Q. Li, L. Wu, Chem. Phys. Lett. 388 (2004) 7. [9] ICDD-PDF: 45-0946. [10] ICDD-PDF: 27-1402. [11] J. Zhang, X. Qing, F.H. Jiang, Z.H. Dai, Chem. Phys. Lett. 371 (2003) 311. [12] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [13] T.L. Kamins, R.S. Williams, D.P. Basile, T. Hesjedal, J.S. Harris, J. Appl. Phys. 89 (2001) 1008. [14] C.H. Liang, L.C. Chen, J.S. Hwang, K.H. Chen, Y.T. Hung, Y.F. Chen, Appl. Phys. Lett. 81 (2002) 22. [15] J. Zhang, L. Zhang, F.H. Jiang, Z.H. Dai, Chem. Phys. Lett. 383 (2004) 423.
Self-catalyzed growth of magnesium oxide nanorods M. Zhao, X.L. Chen ⁎, W.J. Wang, Y.J. Ma, Y.P. Xu, H.Z. Zhao Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China Received 8 July 2005; accepted 20 December 2005 Available online 19 January 2006
Abstract MgO nanorods with spherical particles at the tips were fabricated through direct reaction of Mg and oxygen. The nanorods have diameters ranging from 20 to 50 nm and their lengths are up to 10 μm. The diameters of the spherical particles are about 3–4 times larger than that of the nanorods. X-ray diffraction and energy-dispersive X-ray fluorescence results prove that the nanorods are composed of face-centered cubic (fcc) MgO. The transmission electron microscopy observations show that the nanorods are single crystalline. The growth direction of the nanorod is along [111]. A self-catalyzed vapor–liquid–solid growth model is adopted to account for the formation of the MgO nanorods. © 2005 Elsevier B.V. All rights reserved. PACS: 81.10.Bk; 79.60.Jv
1. Introduction MgO is a promising material with such potential applications as catalysis, additives in refractory, paint and superconductor products, and as substrates for thin film growth [1,2]. Recently, one-dimensional (1D) solid MgO nanomaterials (nanorods and nanowires) have attracted considerable interest for scientific research because of their application as a component in hightemperature superconductors (HTSCs) [3–6]. It was reported that MgO nanorods with the diameter of 5–50 nm and length of 1–3 μm were grown and incorporated into HTSCs to form nanorod–HTSC composites [3–6]. Examples include: BSCCO2212, BSCCO-2223, (Bi, Pb)-2223, TBCCO-2223 superconductors [3–6]. Measurements demonstrated that the critical current densities (Jc's) of the nanorod–HTSC composites were enhanced dramatically at high temperatures and magnetic fields as compared with the undoped samples. Investigations revealed that a proper introduction of MgO nanorods as the effective pinning centers significantly enhanced the flux pinning in the nanorod–HTSC composites. Different approaches have been used for the synthesis of MgO nanorods, for example, the vapor–liquid–solid growth method using MnCl2 as the precursor [3], the modified vapor–solid growth process using
⁎ Corresponding author. Fax: +86 10 82649646. E-mail address: [email protected] (X.L. Chen). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.121
a mixture of MgO powder and carbon powder as the starting materials [6], and the technique involving a hydrothermal hydrolysis to produce Mg(OH)2 nanorods from MgSO4·7H2O and a subsequent thermal decomposition to obtain MgO nanorods [7]. However, from the viewpoint of actual application, exploration of a suitable technique to grow large-scale MgO nanorods is still a challenging research area. Herein, we describe an efficient route to the synthesis of MgO nanorods using Mg as the starting material. Comparing with those methods using MnCl2 as precursor or using a mixture of MgO powder and carbon powder as the starting materials, Mg is not only reactant but also catalyst in this technique, which is more direct and easy. Furthermore, at the tips of the nanorods, spherical-shaped particles are frequently found, which is morphologically different with that synthesized by other researchers. The reported method is relatively simple, low cost and more effective, so it is very practical for large scale applications. 2. Experimental The system that we use to synthesize MgO nanorods with spherical tips is basically similar to that designed to form networked rectangular MgO nanostructures [8]. The chemical reaction involved can be expressed as follows: 2 Mg(vapor) + O2(gas) → 2 MgO(solid). The reaction was carried out in a conventional tube furnace with a horizontal quartz glass tube.
2018
M. Zhao et al. / Materials Letters 60 (2006) 2017–2019
The length of the quartz glass tube is 60 mm. In a typical run, the commercial Mg ribbons or chips (99.5% purity) and the Si substrate, separated by 5 mm in a quartz boat, were loaded into the center of the horizontal quartz tube. The tube was placed in a conventional tube furnace. After that, the quartz tube was first pumped, then filled with argon gas, and then repumped. This operation was repeated more than thrice to ensure there is little. After that, the furnace was heated under an argon flow of 20 ml/min. When the temperature reached 900 °C, a mixture of argon / oxygen (Ar / O2 = 10) gas was introduced into the quartz tube. The furnace temperature was kept constant for 10 min. Finally the power was switched off and the furnace was allowed to cool naturally under an argon flow. After cooling, a white layer was found on the silicon substrate. The deposited products were characterized and analyzed by X-ray diffraction (XRD, Rigaku D/max-2400 diffractometer with Cu K α radiation), scanning electron microscopy (SEM, Hitachi 4200 FE-SEM), energy-dispersive X-ray fluorescence (EDX, attached to the Hitachi 4200) and transmission electron microscopy (TEM, using a Philips CM200 FEG and JEOL 2010, 200 kV). 3. Results and discussion Fig. 1 gives general and high-magnification SEM images of the deposition on the Si substrate. Fig. 1a shows that the Si substrate surface was covered by randomly distributed rod-like nanomaterials.
Fig. 2. (a) EDS pattern of the as-grown nanorods indicates that the nanostructures are composed of Mg and O and the molecular ratio of Mg / O calculated from the EDX data is approximately 1 : 1. The signal of Si in the spectrum should be the contribution of Si substrate. (b) XRD pattern of the nanostructures. Two crystal phases, cubic MgO and cubic Si (Si substrate), are indexed.
Fig. 1. (a) SEM image shows a general view of the deposition. The nanorods dominate the nanostructure. The left and right insets display the detailed characteristics of the nanorods. (b) SEM image shows a closer view of the MgO nanorod.
The diameters of the nanorods are in the range of 20–50 nm. The lengths are within 10 μm. The closer view in Fig. 1b reveals that most of the nanorods have spherical-particles at their tips. The diameters of the nanoparticles are about 3–4 times larger than that of the nanorods. In addition, it can be observed that the particle only can be found at one tip of the nanorod and there is no particle at another end, as shown in the two insets in Fig. 1a. EDX analysis is carried out to determine the chemical composition of the as-grown products (Fig. 2a). The result indicates that these nanorods are composed of magnesium and oxygen with an atomic ratio of approximately 1 : 1, which is in a good agreement with the stoichiometric ratio of MgO. The signal of Si in the image should be the contribution of the Si substrate. Furthermore, the XRD measurement is carried out at room temperature to examine the lattice structure and phase purity of the obtained nanostructures. As shown in Fig. 2b, the XRD pattern can be well indexed to the fcc structured MgO phase with the cell parameter of a = 4.211 Å, which is in good agreement with the known data [9]. The first strong peak is due to the reflection (111) of the cubic Si substrate [10].
M. Zhao et al. / Materials Letters 60 (2006) 2017–2019
2019
not only functions as the reactant but also plays a crucial catalytic role. The growth of the MgO nanorods may involve the following steps. Firstly, the Mg vapor from the high temperature region is carried by argon flow to the low-temperature zone and deposits in the form of liquid droplet on the substrate. Once 900 °C was reached, a little amount oxygen was introduced in the quartz tube. Then, the Mg droplets on the substrate react with oxygen and forms MgO nuclei, which further serve as seeds for MgO nanorods growth. Finally, the MgO nuclei grow to MgO nanorods when continuous introduction of Mg vapor and oxygen.
4. Conclusion To conclude, we have described the synthesis of MgO nanorods with spherical nanoparticles at the tips under chemical vapor deposition conditions. SEM observations reveal that the diameters of the nanorods range from 20 to 50 nm. The lengths are within 10 μm. The diameter of the spherical particle at the end of a nanorod is about 3–4 times of the diameter of the nanorod. The XRD and TEM analysis indicates that the assynthesized nanorods are single-crystalline MgO with facecentered-cubic structure. The growth mechanism of the MgO nanorods is proposed briefly. Acknowledgement
Fig. 3. (a) SAED pattern of a MgO nanorod taken along [110]. (b) HRTEM of a MgO nanorod.
Fig. 3a is the selected area electron diffraction (SAED) pattern of a typical nanorod taken along [110] direction. It can be indexed based on an fcc MgO cell with lattice parameter of a = 4.211 Å [9], which is consistent with the above XRD result. Fig. 3b shows a high-resolution transmission electron microscopy (HRTEM) image of a single MgO nanorod. It reveals that the spacing of 0.24 nm between adjacent lattice planes corresponds to the distance between (111) crystal planes, indicating [111] as the growth direction for the MgO nanorods. The SAED and HRTEM results prove that the nanorods are singlecrystalline MgO nanorods with [111] growth direction. The vapor–liquid–solid (VLS) growth mechanism has been widely used for the syntheses of nanomaterials. Examples include: In2O3, ZnO, Si, InN and GaN nanowires [11–15]. Such transition metals as Ag, Ni, Fe, Cu, Au and Ti are often employed as catalysts. The most remarkable sign of the VLS mechanism is the presence of particles at the ends of the nanowires, nanorods and nanotube, which serve as the catalyst under chemical vapor deposition (CVD) conditions. In our study, spherical nanoparticles are frequently found at the tips of the MgO nanorods. Since no additional metal was involved as the catalyst and no new phase appeared except MgO in the synthesis process, we propose that the spherical particles are composed of MgO. Therefore, we believe that the formation of the MgO nanorods was controlled by a self-catalyzed VLS growth mechanism. In the formation process, Mg
This work is financially supported by the Chinese Academy of Sciences and the National Natural Science Foundation of China. References [1] R.Z. Ma, Y. Bando, Chem. Phys. Lett. 370 (2003) 770. [2] Y.Q. Zhu, W.K. Hsu, W.Z. Zhou, M. Terrones, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 347 (2001) 337. [3] Z. Cui, G.W. Meng, W.D. Huang, G.Z. Wang, L.D. Zhang, Mater. Res. Bull. 35 (2000) 1653. [4] W.D. Huang, W.H. Song, Z. Cui, B. Zhao, M.H. Pu, X.C. Wu, T. Hu, Y.P. Sun, J.J. Du, Supercond. Sci. Technol. 13 (2000) 1499. [5] P.D. Yang, C.M. Lieber, Science 273 (1996) 1836. [6] P.D. Yang, C.M. Lieber, J. Mater. Res. 12 (1997) 2981. [7] L. Yan, J. Zhuang, X.M. Sun, Z.X. Deng, Y.D. Li, Mater. Chem. Phys. 76 (2002) 119. [8] M. Zhao, X.L. Chen, X.N. Zhang, H. Li, H.Q. Li, L. Wu, Chem. Phys. Lett. 388 (2004) 7. [9] ICDD-PDF: 45-0946. [10] ICDD-PDF: 27-1402. [11] J. Zhang, X. Qing, F.H. Jiang, Z.H. Dai, Chem. Phys. Lett. 371 (2003) 311. [12] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [13] T.L. Kamins, R.S. Williams, D.P. Basile, T. Hesjedal, J.S. Harris, J. Appl. Phys. 89 (2001) 1008. [14] C.H. Liang, L.C. Chen, J.S. Hwang, K.H. Chen, Y.T. Hung, Y.F. Chen, Appl. Phys. Lett. 81 (2002) 22. [15] J. Zhang, L. Zhang, F.H. Jiang, Z.H. Dai, Chem. Phys. Lett. 383 (2004) 423.