International Journal of Nanoscience Vol.5,Nos.2&3(2006)219224 World Scie O World Scientific Publishing Company CHEMICAL PREPARATION OF MgO WHISKERS XIAOLI WANGt CHENGLIN YAN, LONGJIANG zoU and DONGFENG XUE".E State Key Laboratory of Fine Chemicals and School of Chemical Engineering Dalian University of Technology, 158 Zhong shan Road, Dalian, 116012, P. R. China College of Petrochemical Engi Shenyang University of Technology Liaoyang, 111003, P. R. China Center of Material Testing and Analysis, Dalian University of Technology Dalian./16023. P R. china due@chem.dlut.edu.cn A useful method is described for the chemical synthesis of high length-radius ratio magnesium oxide (MgO) whiskers at ambient temperature. The diameter of Mgo whiskers 2-5 um with a length of 30-50 Am. The length-radius ratio generally falls Energy-dispersive X-ray spectrometer analysis reveals that the whiskers are O elements. Microscopic morphologies and structures are well studied b microscopy and X-ray diffraction. The growth mechanism of MgO whiskers is also proposed on the basis of crystallographic characteristic Keywords: Whiskers; magnesium oxide; one-dimensional; chemical preparation. 1. Introduction One-dimensional(1D) materials with a uniform diameter have attracted much attention due to their potential applications in mesoscopic physics and nanodevice technique. A variety of ID structures with different morphologies and compositions have been successfully fabricated by many research groups, and notable examples include Zns TiO2, and ZnO. Recently, much attention has been paid to ID oxide structures for their nteresting properties and applications such as optics, magnetism, superconductivity, and ferroelectricity Among the ID oxide materials, MgO is a typical wide-band-gap insulator. Many important applications have been found for its usage in catalysis, refractory material industry, paint, and superconductors. -10 ID MgO can display a unique capability to pin the magnetic flux lines within a high-temperature superconductor. MgO whiskers are well appropriate for insulation applications due to their low heat capacity and high melting point. Recently, it is found that they could improve the superconducting properties in addition to improve the mechanical properties when they are added into superconducting composites Until now, different morphologies of nano-MgO crystals such as MgO nanorods, 2 MgO fishbone fractal nanostructures and nanobelts have been obtained, respectively Particular interest is the recently developed method, such as the vapor-solid process
International Journal of Nanoscience Vol. 5, Nos. 2 & 3 (2006) 219—224 c World Scientific Publishing Company 219 CHEMICAL PREPARATION OF MgO WHISKERS XIAOLI WANG*,†, CHENGLIN YAN* , LONGJIANG ZOU‡ and DONGFENG XUE*,§ * State Key Laboratory of Fine Chemicals and School of Chemical Engineering Dalian University of Technology, 158 Zhongshan Road, Dalian, 116012, P. R. China † College of Petrochemical Engineering, Shenyang University of Technology Liaoyang, 111003, P. R. China ‡ Center of Material Testing and Analysis, Dalian University of Technology Dalian, 116023, P. R. China § dfxue@chem.dlut.edu.cn A useful method is described for the chemical synthesis of high length-radius ratio magnesium oxide (MgO) whiskers at ambient temperature. The diameter of MgO whiskers falls in the range of 2−5 µm with a length of 30−50 µm. The length-radius ratio generally falls in the range of 10−30. Energy-dispersive X-ray spectrometer analysis reveals that the whiskers are composed of Mg and O elements. Microscopic morphologies and structures are well studied by scanning electron microscopy and X-ray diffraction. The growth mechanism of MgO whiskers is also proposed on the basis of crystallographic characteristics. Keywords: Whiskers; magnesium oxide; one-dimensional; chemical preparation. 1. Introduction One-dimensional (1D) materials with a uniform diameter have attracted much attention due to their potential applications in mesoscopic physics and nanodevice technique.1–3 A variety of 1D structures with different morphologies and compositions have been successfully fabricated by many research groups, and notable examples include ZnS, TiO2, and ZnO.4 Recently, much attention has been paid to 1D oxide structures for their interesting properties and applications such as optics, magnetism, superconductivity, and ferroelectricity. Among the 1D oxide materials, MgO is a typical wide-band-gap insulator. Many important applications have been found for its usage in catalysis,5,6 refractory material industry, paint, and superconductors.7–10 1D MgO can display a unique capability to pin the magnetic flux lines within a high-temperature superconductor. MgO whiskers are well appropriate for insulation applications due to their low heat capacity and high melting point.11 Recently, it is found that they could improve the superconducting properties in addition to improve the mechanical properties when they are added into superconducting composites.9 Until now, different morphologies of nano-MgO crystals such as MgO nanorods,12 MgO fishbone fractal nanostructures13 and nanobelts14 have been obtained, respectively. Particular interest is the recently developed method, such as the vapor–solid process § Corresponding author
and catalytic growth, etc, by which nano-MgO crystals can be readily obtained. However, the synthesis of high length-radius ratio materials is still a big challenge in this field. The present paper reports a useful synthesis route for high length-radius ratio Mgo whiskers through a simple chemical process. At the same time, a possib mechanism of these whisker-like samples is also proposed 2. Experimental The investigated MgO whiskers are prepared from analytical reagent-grade powers of MgCl2 6H2O and Na2CO3. Firstly, MgCl2 6H,O dissolved in distilled water is stirred with Na2CO3 solution. The combined slurry is stirred for several minutes. After the slurry is filtrated, the white powders can be collected, washed with distilled water and absolute ethanol several times, and then dried in air at ambient temperature. The obtained sample are subsequently calcined in air in a muffle furnace at around 650C for 4-5 h. The temperature is increased very slowly to avoid the sudden collapse of precursor structure (i. e, to preserve the morphological features of precursors in the final MgO samples) In our experiment, MgO whiskers are fabricated by a simple chemical route mainly involving the following reactions CO3 +H,O=HCO3+Oh Mgt+ 20H= mg(oh) Mg2++C032-=MgCO CO3+Mg(OH)2=MgCO3 20H MgCO3= MgO+CO2 Mg(Oh)2= Mgo +H,O The obtained samples are then characterized using X-ray diffraction(XRD, XD-3A) with Cu Ka radiation and scanning electron microscopy(SEM, JSM-5600LV, JEOL equipped with an energy-dispersive X-ray spectrometer(EDS) 3. Results and discussion A typical XRD pattern is shown in Fig. 1. All diffraction peaks can be indexed as pure cubic MgO (JCPDS, 45-0946). The significant peak broadening indicates that our samples have a small grain size with a well crystallinity SEM images show that the samples consist of a large quantity of whiskers with diameters in the range of 2-5 um. It is clear that all whiskers are very uniform in both shape and size. Furthermore, although the maximum length is up to several hundreds of micrometers, most of them only have a typical length of several tens of micrometers (Fig. 2(a)). Neighboring whiskers may form an aggregation, as shown in Fig. 2(b). The high-magnification image e general morphology of these whiskers is shown in Fig. 2(c)
220 X. Wang et al. and catalytic growth, etc., by which nano-MgO crystals can be readily obtained. However, the synthesis of high length-radius ratio materials is still a big challenge in this field. The present paper reports a useful synthesis route for high length-radius ratio MgO whiskers through a simple chemical process. At the same time, a possible growth mechanism of these whisker-like samples is also proposed. 2. Experimental The investigated MgO whiskers are prepared from analytical reagent-grade powers of MgCl2⋅6H2O and Na2CO3. Firstly, MgCl2⋅6H2O dissolved in distilled water is stirred with Na2CO3 solution. The combined slurry is stirred for several minutes. After the slurry is filtrated, the white powders can be collected, washed with distilled water and absolute ethanol several times, and then dried in air at ambient temperature. The obtained samples are subsequently calcined in air in a muffle furnace at around 650°C for 4–5 h. The temperature is increased very slowly to avoid the sudden collapse of precursor structure (i.e., to preserve the morphological features of precursors in the final MgO samples). In our experiment, MgO whiskers are fabricated by a simple chemical route mainly involving the following reactions: CO3 2− + H2O = HCO3 − + OH− , (1) Mg2+ + 2OH− = Mg (OH)2 , (2) Mg2+ + CO3 2− = MgCO3 , (3) CO3 2− + Mg (OH)2 = MgCO3 + 2OH− , (4) MgCO3 = MgO + CO2 , (5) Mg (OH)2 = MgO + H2O . (6) The obtained samples are then characterized using X-ray diffraction (XRD, XD-3A) with Cu Kα radiation and scanning electron microscopy (SEM, JSM-5600LV, JEOL) equipped with an energy-dispersive X-ray spectrometer (EDS). 3. Results and Discussion A typical XRD pattern is shown in Fig. 1. All diffraction peaks can be indexed as pure cubic MgO (JCPDS, 45-0946). The significant peak broadening indicates that our samples have a small grain size with a well crystallinity. SEM images show that the samples consist of a large quantity of whiskers with diameters in the range of 2–5 µm. It is clear that all whiskers are very uniform in both shape and size. Furthermore, although the maximum length is up to several hundreds of micrometers, most of them only have a typical length of several tens of micrometers (Fig. 2(a)). Neighboring whiskers may form an aggregation, as shown in Fig. 2(b). The high-magnification image of the general morphology of these whiskers is shown in Fig. 2(c)
Chemical Preparation of MgO Whiskers 221 4000 2000 20 (degree) Fig 1 XRD pattern showing the single-phase cubic MgO. 10kV X580 SeuM 18kU X3, 800 ISkU >5,000 SM Fig. 2.(a)SEM image of MgO samples. (b) Neighboring MgO whiskers form an aggregation with the radiolarian morphology. (c)A high-magnification image of Mgo whiskers in(a
Chemical Preparation of MgO Whiskers 221 30 40 50 60 70 80 0 1000 2000 3000 4000 5000 6000 222 311 220 200 111 Intensity 2θ (degree) Fig. 1. XRD pattern showing the single-phase cubic MgO. Fig. 2. (a) SEM image of MgO samples. (b) Neighboring MgO whiskers form an aggregation with the radiolarian morphology. (c) A high-magnification image of MgO whiskers in (a). (a) (b) (c)
222X Fig 3. EDS pattern of Mgo whiskers obtained in the present work. The elementary analysis from the selected area energy-dispersive X-ray spectrometer in Fig. 3 confirms the ideal constitution of the Mgo whiskers obtained in our present work, which reveals that all whiskers are composed of MgO, which consists well with the XRD results as shown in Fig. 2. The formation of Mgo whiskers indicates that the nucleation and growth stage MgO whiskers are well controlled in our present process. On the basis of experimental facts, we may propose a formation mechanism of Mgo whiskers to deeply understand the whiskers formation process. The formation mechanism of whiskers is schematically illustrated in Fig. 4. At the beginning stage, a great deal of nucleus like Fig. 4(a)are formed. with the transformation of solvents, the nucleus begin to grow up(Fig. 4(b)).At the end of growth stage, many whiskers can be obtained, while some whiskers may be self-assembled as shown in Fig. 4(c) From the viewpoint of chemical reaction dynamics, the selective interaction between the solvents and crystal surface ions is thought to have the ability to decrease the growth rate of specific lattice planes, which thus causes the expression of these crystal planes in the final crystal form. In principle, the crystal growth and morphology are governed by the degree of supersaturation, the diffusion of reactants, the species at the crystal surface, the surface and interfacial energy, and the crystal structure. Additionally, the crystal structure is accounted for the final morphology. Structurally speaking, as for different crystallographic faces, the growth rate is inversely proportional to the diffraction index (i.e, Miller indices), those crystallographic faces with low diffraction (i.e, those lowest Miller indices)are always kept in the final samples. In this regard, the crystallographic characteristics can support our present morphology along [100]
222 X. Wang et al. Fig. 3. EDS pattern of MgO whiskers obtained in the present work. The elementary analysis from the selected area energy-dispersive X-ray spectrometer in Fig. 3 confirms the ideal constitution of the MgO whiskers obtained in our present work, which reveals that all whiskers are composed of MgO, which consists well with the XRD results as shown in Fig. 2. The formation of MgO whiskers indicates that the nucleation and growth stage of MgO whiskers are well controlled in our present process. On the basis of experimental facts, we may propose a formation mechanism of MgO whiskers to deeply understand the whiskers formation process. The formation mechanism of whiskers is schematically illustrated in Fig. 4. At the beginning stage, a great deal of nucleus like Fig. 4(a) are formed. With the transformation of solvents, the nucleus begin to grow up (Fig. 4(b)). At the end of growth stage, many whiskers can be obtained, while some whiskers may be self-assembled as shown in Fig. 4(c). From the viewpoint of chemical reaction dynamics, the selective interaction between the solvents and crystal surface ions is thought to have the ability to decrease the growth rate of specific lattice planes, which thus causes the expression of these crystal planes in the final crystal form. In principle, the crystal growth and morphology are governed by the degree of supersaturation, the diffusion of reactants, the species at the crystal surface, the surface and interfacial energy, and the crystal structure. Additionally, the crystal structure is accounted for the final morphology. Structurally speaking, as for different crystallographic faces, the growth rate is inversely proportional to the diffraction index (i.e., Miller indices), those crystallographic faces with low diffraction (i.e., those lowest Miller indices) are always kept in the final samples. In this regard, the crystallographic characteristics can support our present morphology along [100]
Chemical Preparation of MgO Whiskers 223 (010) Fig 4 Sketch drawing of the growth mechanism of MgO whiskers. (a) A nucleation stage.(b)A growth stage at the specific direction.(c) Formation of self-assembled whisker 5,g 5从r of Mgo whiskers synthesized by the hydrothermal reaction. A. In our experiments, MgO whiskers with other morphologies like those shown in Fig. 5 are also fabricated by changing some experimental conditions. The detailed synthetic procedure is described as follows: 0.02 mol Mg powders and 10 ml H2O2(30%) are added into a Teflon-lined autoclave and filled with distilled water up to 70% of the total volume. The hydrothermal synthesis is conducted at 150oC for 4 h in an electric oven. After cooling down to room temperature naturally, the white precursors are collected by filtration, and then washed with deionized water and absolute ethanol several times. Finally, the precursors are dried in air at 80.C for 4 h. The MgO whiskers are prepared by calcining precursors in air at 450C for 4 h. The SEM images of samples clearly show that the corresponding morphology may be effectively modified
Chemical Preparation of MgO Whiskers 223 (a) (b) (c) Fig. 4. Sketch drawing of the growth mechanism of MgO whiskers. (a) A nucleation stage. (b) A growth stage at the specific direction. (c) Formation of self-assembled whiskers. In our experiments, MgO whiskers with other morphologies like those shown in Fig. 5 are also fabricated by changing some experimental conditions. The detailed synthetic procedure is described as follows: 0.02 mol Mg powders and 10 ml H2O2 (30%) are added into a Teflon-lined autoclave and filled with distilled water up to 70% of the total volume. The hydrothermal synthesis is conducted at 150°C for 4 h in an electric oven. After cooling down to room temperature naturally, the white precursors are collected by filtration, and then washed with deionized water and absolute ethanol several times. Finally, the precursors are dried in air at 80°C for 4 h. The MgO whiskers are prepared by calcining precursors in air at 450°C for 4 h. The SEM images of samples clearly show that the corresponding morphology may be effectively modified. (a) (b) Fig. 5. (a) and (b) SEM images of MgO whiskers synthesized by the hydrothermal reaction. [100] （010）
224X. 4. Conclusion The high length-radius ratio MgO micro-whiskers may be successfully prepared in our present experiment. The diameter of the obtained whiskers falls in the range of 2-5 um while the length of these whiskers varies from 30 to 50 um. The length-radius ratio mainly falls in the range of 10-30. SEM images reveal that all whiskers are quite uniform in both shape and size. Further studies are needed to better define the growth mechanisms responsible for the currently observed structures, and to achieve the advanced control of morphologies of Mgo by adopting appropriate experimental routes Financial supports from FANEDD( Grant No. 200322), NSFC(Grant No. 20471012)and RFDP (Grant No. 20040141004)are greatly acknowledged References 1. A.P. Alivisatos, Science 271, 933(1996) 2. X. Duan, Y Huang, J. Wang and C M. Lieber, Nature 409, 66(2001). 3. w.Ade Heer, A Chatelain and D. Ugate, Science 277, 1179(1997) 4. Y C. Zhu, Y. Bando and D. F. Xue, Appl. Phys. Leff. 82, 1769(2003):Y. C. Zhu, Y. Bando, D F. Xue and D Golberg, J. Am. Che. Soc. 125, 16196(2003) 5. S H C Liang and I D. Gay, J Catal 101, 293(1986) 6. H. Tsuji, F. Yagi and H Kita, J Catal. 148, 759(1994). 7. Y Kawaguchi, Solid State Commun. 117, 17(2000) 8. A. Bhargava, J. A. Alarco, I. D. R. Mackinnon, D. Page and A. llyushechkin, Mater. Leff. 34 133(1998 9. Y.S. Yuan, M.S. Wong and sS wang, J Mater. Res 11,8(1996) 10. P D. Y ang and C. M. Lieber, Science 273, 1836(1996) 11. R. Z Ma and Y Bando, Chem. Phys. Lett. 370, 770(2003) 12. A. Cui. G. W. Meng. W.D. Huang, G. Z. Wang and L. D. Zhang, Mater. Res. Bull. 35, 1653 (2000) 13. Y. Q. Zhu, w.K.Hsu, w.Z. Zhou, M. Terrones, H. w. Kroto and D. R. M. Walton, Cher Phys.Le.347,337(2001) 14. J. Zhang, L Zhang, X Peng and X Wang, Appl. Phys. A 73, 773(2001)
224 X. Wang et al. 4. Conclusion The high length-radius ratio MgO micro-whiskers may be successfully prepared in our present experiment. The diameter of the obtained whiskers falls in the range of 2–5 µm, while the length of these whiskers varies from 30 to 50 µm. The length-radius ratio mainly falls in the range of 10–30. SEM images reveal that all whiskers are quite uniform in both shape and size. Further studies are needed to better define the growth mechanisms responsible for the currently observed structures, and to achieve the advanced control of morphologies of MgO by adopting appropriate experimental routes. Acknowledgments Financial supports from FANEDD (Grant No. 200322), NSFC (Grant No. 20471012) and RFDP (Grant No. 20040141004) are greatly acknowledged. References 1. A. P. Alivisatos, Science 271, 933 (1996). 2. X. Duan, Y. Huang, J. Wang and C. M. Lieber, Nature 409, 66 (2001). 3. W. A. de Heer, A. Chatelain and D. Ugate, Science 277, 1179 (1997). 4. Y. C. Zhu, Y. Bando and D. F. Xue, Appl. Phys. Lett. 82, 1769 (2003); Y. C. Zhu, Y. Bando, D. F. Xue and D. Golberg, J. Am. Chem. Soc. 125, 16196 (2003). 5. S. H. C. Liang and I. D. Gay, J. Catal. 101, 293 (1986). 6. H. Tsuji, F. Yagi and H. Kita, J. Catal. 148, 759 (1994). 7. Y. Kawaguchi, Solid State Commun. 117, 17 (2000). 8. A. Bhargava, J. A. Alarco, I. D. R. Mackinnon, D. Page and A. Ilyushechkin, Mater. Lett. 34, 133 (1998). 9. Y. S. Yuan, M. S. Wong and S. S. Wang, J. Mater. Res. 11, 8 (1996). 10. P. D. Yang and C. M. Lieber, Science 273, 1836 (1996). 11. R. Z. Ma and Y. Bando, Chem. Phys. Lett. 370, 770 (2003). 12. A. Cui, G. W. Meng, W. D. Huang, G. Z. Wang and L. D. Zhang, Mater. Res. Bull. 35, 1653 (2000). 13. Y. Q. Zhu, W. K. Hsu, W. Z. Zhou, M. Terrones, H. W. Kroto and D. R. M. Walton, Chem. Phys. Lett. 347, 337 (2001). 14. J. Zhang, L. Zhang, X. Peng and X. Wang, Appl. Phys. A 73, 773 (2001)
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