J. Phys. Chem. C 2007, 111, 10273-10277
10273
Zn-Assisted Synthesis and Photoluminescence Properties of MgO Nanotubes H. B. Lu,† L. Liao,† H. Li,‡ Y. Tian,† J. C. Li,*,† D. F. Wang,† and B. P. Zhu† Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices, Ministry of Education, Wuhan UniVersity, Wuhan 430072, and State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Changning Road, Shanghai 200050, People’s Republic of China ReceiVed: April 1, 2007; In Final Form: May 14, 2007
A newly developed one-step Zn-assisted route has been used to produce MgO nanotubes (MNTs) by simple thermal evaporation of the mixed Zn and Mg powders without catalysts. The MNTs have average outer diameters of 80 nm and wall thicknesses of about 15 nm. The growth mechanism of MNTs is discussed in detail. The Zn/ZnO nanocables formed at the initial growth stage are crucial, act as interim templates for the formation of MNTs, and disappear automatically after formation of MNTs. Room-temperature photoluminescence measurement reveals structural-defect-related visible emissions of the synthesized MNTs. This kind of MNT with a high surface area may have promising applications as the sorbent for chemisorption and destructive adsorption of various pollutants and as a protective template by filling the MNTs with lowmelting-point metals or semiconductor nanoparticles, due to their excellent refractory and thermal properties.
Introduction Ever since the discovery of carbon nanotubes, the synthesis of tubular nanomaterials has aroused worldwide interest in both fundamental studies and their potential application, such as chemical sensors, catalysts, and storage or release systems.1-4 A well-established way to prepare tubular oxide nanostructures is a template-originated process, i.e., encapsulating oxides between tubular assemblies of an organogelator5 or forming tubular oxides in the nanopores of anodic alumina.6 However, the application of the existing well-known template methods is rather limited due to the complex preparation procedure of the templates and the extreme difficulty of removing the templates. Recently, magnesium oxide (MgO) has been attracting great attention because of its important use as a passivation layer in high-electron-mobility transistors,7 a substrate for thin film growth,8 and additives in refractory, paint, and superconductor products.9-11 Nanoporous MgO with a high specific surface area has been used as a coating layer onto a TiO2 electrode, which promotes the adsorption of dye molecules and enhances the energy conversion efficiency by as much as 45% compared to that of the uncoated TiO2 electrode.12 MgO is also a potential catalyst and catalyst support for various reactions and is a promising sorbent for chemisorption and destructive adsorption of various pollutants. Studies of MgO on the adsorption of H2,13 CO,14,15 NH3,16 pyridine,17 and nitrobenzene15 have been reported. Tubular MgO nanostructures with a high surface area can play even more important roles in the application areas where high efficiency and high activity properties are required, such as the aforementioned areas. Up to now, many studies have been reported on the preparation of different nanostructures of MgO, such as nanorods,18 nanobelts,19,20 nanowires,21 nanoflowers,22 and fishbone fractal nanostructures.23 However, only several papers on the synthesis of tubular MgO nanostructures * To whom correspondence should be addressed. Phone: +86-276875-2567. Fax: +86-27-6875-2569. E-mail:
[email protected]. † Wuhan University. ‡ Chinese Academy of Science.
have been published. Zhan et al.24 obtained tubular MgO nanostructures by heating a mixture of MgO, C, and Ga2O3 powders at a high temperature of 1200 °C. Ga was used as a catalyst in the growth process, and Ga droplets were capped on the mouths of the MgO nanotubes. Hao et al.25 prepared tubular MgO nanostructures by thermal evaporation of metal Mg powders at 800 °C to form Mg/MgO core/shell nanostructures along with subsequent sublimation of Mg to remove the Mg cores from the Mg/MgO nanostructures. It is therefore highly necessary to further explore a simple, low-cost, catalyst-free, and low-temperature synthesis route in which the template automatically disappears to manufacture MgO nanotubes (MNTs) with controllable diameter. In this paper, we report a newly developed Zn-assisted catalyst-free way for synthesizing MNTs in bulk quantities by simple thermal evaporation at lower temperature (650 °C) of mixed Zn and Mg powders. Our approach is effective and facile. The growth mechanism of MNTs is discussed in detail. In this work, the Zn/ZnO core/shell nanostructures formed at the initial growth stage are crucial, act as interim templates for the formation of MNTs, and disappear automatically after formation of MNTs. The high surface area of MNTs makes them a new type of adsorbent as well as a nearly stoichiometric chemical reagent. Experimental Section The synthesis of MNTs was performed in a conventional horizontal tube furnace. The raw material was a mixture of metallic Zn and Mg powders at a weight ratio of 5:1. The powders were placed on a quartz boat that was inserted into the center of the horizontal tube furnace. Si(100) p-type substrates were etched by hydrofluoric acid and cleaned by deionized water and ethanol. The cleaned Si substrates were located at a downstream position that was about 2 cm away from the raw material. The system was pumped to a base vapor pressure of about 3 Pa, and then argon was introduced into the system with a flow rate of 80 sccm as a carrier gas. Afterward,
10.1021/jp0725432 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
10274 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Lu et al.
Figure 1. Typical SEM images of the synthesized MNTs: (a) lowmagnification SEM image of MNTs; (b) high-magnification SEM image of MNTs revealing hollow tubular structures.
Figure 2. XRD pattern recorded from the MNTs on Si(100) substrates.
the tube was heated to 650 °C at a rate of 15 °C/min. When the temperature reached 500 °C, an air flow of 40 sccm was then added to the quartz tube. The synthesis process was carried out under a pressure of 20 Pa at 650 °C and lasted for about 30 min. The gas flows were cut off when the heating power of the furnace was turned off, and the samples were then naturally cooled at 3 Pa. The morphology and crystalline structure of the as-synthesized products were characterized by a Sirion FEG scanning electron microscopy (SEM) instrument equipped with an energydispersive X-ray (EDX) spectrometer, a D8 advanced X-ray diffraction (XRD) instrument, and a JEOL JEM 2010 transmission electron microscopy (TEM) instrument. The high-resolution atom lattice images were observed by a JEOL JEM 2010 FEF high-resolution transmission electron microscopy (HRTEM) instrument with a point resolution of 0.19 nm. The roomtemperature photoluminescence (PL) spectra were measured using a He-Cd laser as the excitation source (325 nm). Results and Discussion Parts a and b of Figure 1 show typical SEM images at different magnifications of the as-prepared products deposited on the Si substrates. It can be found that the products are distributed on the substrate in a large quantity. They cover the whole area of the 4 cm × 10 cm Si substrates uniformly. Larger scale production can be achieved by modifying the preparation equipment. Figure 1b shows clearly that the products possess hollow tubular structures. The nanotubes have lengths of several micrometers. Some of the nanotubes exhibit a straight morphology, while many of them are usually twisted with several straight parts. It is worth noting that the tubes are transparent, as revealed by the higher resolution SEM image in Figure 1b, suggesting that the walls of the tubes are very thin. The XRD pattern of the obtained nanotubes is displayed in Figure 2. The diffraction peaks can be well-indexed to the cubic MgO with a lattice constant of a ) 4.211 Å. To obtain more structural information on MNTs, TEM and HRTEM were used to characterize the tubular configuration and
Figure 3. TEM and HRTEM images of MNTs: (a) typical bright field TEM image of the MNTs exhibiting tubular structures; (b) corresponding SAED pattern of MNTS in (a); (c) a high-magnification TEM image of a single MNT; (d) corresponding HRTEM pattern of the MNT.
crystalline structures of MNTs. Figure 3a shows a typical bright field TEM image of the thin MNTs, exhibiting a tubular structure with a uniform wall thickness for most of the MgO nanostructures. The TEM measured result from more than 20 MNTs indicates that the outer diameters and wall thicknesses are mainly in the ranges of 77-83 nm (50 and 90 nm for the thinnest and thickest ones, respectively) and 13-18 nm, respectively. The average outer diameter is about 80 nm. The selected area electron diffraction (SAED) pattern taken from the MNTs in Figure 3a is depicted in Figure 3b. The diffraction circles correspond to the (111), (200), (220), (222), (400), (420), and (422) planes of the cubic MgO, providing further confirmation that the nanotubes are cubic MgO. Also, in the SAED pattern, besides the diffraction rings from the cubic MgO structure, no additional ring was clearly observed, which are in coincidence with the result of XRD (Figure 2). Figure 3c presents a representative high-magnification TEM image of a single MgO MNT, where the contrast between the sidewall and the inside hollow region can be clearly identified. The diameter of the MNT is about 80 nm, and the wall thickness is about 14 nm. The corresponding HRTEM pattern of the MNT is exhibited in Figure 3d, showing the polycrystalline nature of MNTs. Careful observations reveal that the outer shells of MNTs are indeed composed of a large number of small MgO nanocrystals. Different nanocrystals can be clearly seen in the HRTEM image, and boundaries of these nanocrystals are outlined with dashed lines in Figure 3d. The diameters of the nanocrystals are evaluated to be around 5-25 nm, with a mean value around 8 nm. The measured fringe spacing between adjacent lattice planes of every nanocrystal agrees well with the lattice spacing of the (200) planes (0.21 nm) of cubic MgO, illustrating the singlecrystal nature of each single nanocrystal. The preparation of this kind of MNT has been repeated and proved to be reproducible. To explore the detailed formation mechanism of MNTs, other growth experiments were performed. That is, we grew MgO nanostructures using pure Mg powder instead of a mixture of Mg and Zn powders as the raw
Synthesis and Photoluminescence of MgO Nanotubes
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10275
Figure 4. (a) SEM image of MgO nanowires synthesized without Zn powder in the raw material. (b) Corresponding XRD pattern of MgO nanowires.
Figure 5. (a) SEM image of Zn/ZnO nanocables grown at the initial stage when the reaction temperature rose to 550 °C. The inset shows the corresponding EDX spectrum. (b) XRD pattern of Zn/ZnO nanocables. (c) TEM image of an individual Zn/ZnO nanocable. (d) HRTEM image of the ZnO shell of the Zn/ZnO nanocable.
material and keeping the other experimental conditions unchanged. In this case, the obtained products are long solid wirelike nanostructures rather than nanotubes (see the SEM image in Figure 4a), and they can be identified as phase-pure cubic MgO structures by XRD (Figure 4b). This reveals that the Zn powders in the original source materials played a crucial role in the formation process of the MNTs. In addition, in another experiment, we ceased the synthesis process when the temperature rose to 550 °C (using a mixture of Mg and Zn powders and keeping the other conditions unchanged). The obtained products were then examined by SEM, XRD, EDX spectroscopy, and HRTEM. The SEM result (Figure 5a) shows that the products are slightly curved wirelike nanostructures with lengths of several micrometers. The TEM measurement result from more than 20 Zn/ZnO nanocables reveals that the diameters and shell thicknesses are mainly in the ranges of 76-80 and 10-18 nm, respectively. They are close to those (77-83 and 13-18 nm, respectively) of the MNTs. The XRD pattern exhibited in Figure 5b illuminates that the wirelike nanostructures are composed of Zn and ZnO and that the intensities of the diffraction peaks from Zn are much stronger than those from ZnO, indicating that the content of Zn crystal is much more than that of ZnO in the sample. In addition, only Zn and O signals can be observed in the EDX spectrum (the inset of Figure 5a); this further demonstrates that the wirelike nanostructures
are made of Zn and O without Mg (out of the detecting sensitivity of EDX spectroscopy). The detected Zn:O ratio is about 3:1, which is much higher than that (1:1) of bulk ZnO. The TEM image (Figure 5c) of a representative single wirelike nanostructure shows clearly a distinct core/shell configuration of this product. From the TEM image, we can further find that the shell is actually composed of nanocrystals that can be indexed to hexagonal wurtzite ZnO, according to the results of the corresponding HRTEM pattern (Figure 5d) and XRD (Figure 5b); i.e., the measured fringe spacing of 0.28 nm between adjacent lattice planes well agrees with the lattice spacing of the (100) planes of the hexagonal structure ZnO and the diffraction peaks of the ZnO crystal present in the nanostructural sample. These results combined with the EDX result of a much larger ratio of Zn to O, namely, a high Zn:ZnO ratio, indicate that the cores can therefore be considered to be made of Zn crystals. That is, the wirelike nanostructures are Zn core/ZnO shell nanocables. This experiment reveals that the formed wirelike nanostructures at the initial stage of the preparation process of MgO nanotubes are Zn core/ZnO shell nanocables without MgO. It is well understood that Zn nanowires are first formed on Si substrates due to the vaporization of Zn (the melting point is 419 °C) and the lack of oxygen, since no air (only Ar) is introduced before the temperature reaches 500 °C. After introduction of air (starting at 500 °C), ZnO layers are
10276 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Lu et al.
formed on the surfaces of Zn nanowires owing to both the oxidization of the surfaces of Zn nanowires and the deposition of the oxidized Zn vapor coming from the Zn source, resulting in thin ZnO shells covering the surfaces of the Zn cores. This kind of formation mechanism of the Zn core/ZnO shell has been reported in our previous paper,26 and it is similar to the formation mechanism of the Mg/MgO and the ZnOx/ZnO core/shell nanostructures reported by Hao et al.25 and Zhang et al.,27 respectively. It is worth noting that the Zn/ZnO nanocables in our exprement are used as interim templates for forming MNTs and also for controlling the dimensions and wall thicknesses of the MNTs in the growth design of MNTs. This has been confirmed by the facts that only MgO (without the coexistence of Zn and ZnO) appears in MNTs and the outer diameters and wall thicknesses of MNTs are close to the outer diameters and shell thicknesses of Zn/ZnO nanocables (77-83 and 13-18 nm for MNTs, 76-80 and 10-18 nm for Zn/ZnO nanocables). The detailed formation mechanism of MNTs is described as follows. With rising temperature, sublimated or vaporized Mg (the melting point is 649 °C) was deposited on the surfaces of Zn/ ZnO nanocables and substituted for ZnO, resulting in the formation of MgO shells and the disappearance of the ZnO shells. The reaction equations are
Mg + ZnO f MgO + Zn
(1)
2Mg + O2 f 2MgO
(2)
At the same time, along with the rising temperature, Zn cores with a low melting point were gradually sublimated and removed from the Zn/MgO nanocables, while high-melting-point (2830 °C) MgO shells remained intact. As a result, MgO nanotubes were formed after the Zn cores disappeared completely from the Zn/MgO nanocables due to sublimation. Such a formation model of the hollow oxide nanostructures caused by the sublimation of the cores of the core/shell configurations has been reported in our previous paper,26 and it is similar to the formation mechanism reported by Hao et al.25 and Zhang et al.27 where Mg (ZnOx) cores disappear due to sublimation from the Mg/MgO (ZnOx/ZnO) core/shell structures. However, for our synthesis route, the formation mechanism of MgO nanotubes is essentially dissimilar to that of Hao; that is, the Zn/ZnO nanocables act as interim templates to form MNTs via a substitution reaction between Mg and ZnO. This strategy may lead us to use the Zn/ZnO core/shell to control the diameters and wall thicknesses of the MgO nanotubes and also develop new ways to manufacture and modulate different nanostructures of MgO and other oxides. These experiments will be investigated in more detail in our successive work. Structurally, MgO has a cubic structure with a ) 4.211 Å and ZnO has a hexagonal crystal structure with lattice constants a ) 3.249 Å and c ) 5.206 Å; they are mutually incompatible in structure. Therefore, it is possible to form polycrystalline MNTs by the substitution reaction. The PL spectra were also examined to study the optical properties of the samples. Figure 6 shows the room-temperature PL spectrum of MNTs. The spectrum is broad and asymmetric with a strong blue emission centered at about 470 nm and a yellow-green emission band at 565 nm. It has been well Gaussian fitted to three bands. The peak positions of the three Gaussian bands are located at about 463, 520, and 575 nm. Hao et al.28 have reported a similar blue band with a peak position around 470 nm of branched MgO nanostructures and assigned the band to structural defects. Rosenblatt et al.29 have obtained 390 and 530 nm bands in time-resolved spectra of bulk MgO
Figure 6. Room-temperature PL spectrum of MNTs.
with different defect densities. They have attributed these bands to F+ and F centers, respectively. In our experiment, rapid evaporation, incomplete crystallization, and a trace of Zn impurity may induce various structural defects, such as oxygen vacancies and Mg interstitials. Moreover, the high-surface-tovolume-ratio MNTs composed of small nanocrystals should also favor the existence of large quantities of structural defects. Therefore, the three bands in the PL of MNTs can be ascribed to various defects and impurities with different densities. A large quantity of defects and impurities in MNTs can provide new energy levels in the band gap of MgO which can contribute to visible luminescence centers. Conclusion In conclusion, we report on the bulk synthesis of MNTs via a newly developed one-step Zn-assisted thermal evaporation method using mixed Zn and Mg powders as the source material. The Zn/ZnO nanocables formed at the initial growth stage are crucial, act as interim templates for forming MNTs and controlling the dimensions of MNTs, and disappear automatically after the growth of MNTs. The PL measurement with a Gaussian fitting shows three emission bands centered at 463, 520, and 575 nm which are attributed to various structural defects with different densities in MNTs. This type of MNT with a high surface-to-volume ratio may be used as an effective adsorbent, chemical reagent, and protective template by filling the MNTs with low-melting-point metals or semiconductor nanoparticles due to its excellent refractory and thermal properties. The proposed synthetic pathway is facile and reproducible with no catalysis droplets capped on the mouths of the MgO nanotubes. It may be extended to the synthesis of other kinds of nanotubes or nanocables. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 10575078). References and Notes (1) Li, J.; Lu, Y. J.; Ye, Q.; Cinke, M.; Han, J.; Meyyappan, M. Nano Lett. 2003, 3, 929. (2) Mitchell, D. T.; Lee, S. B.; Trofin, L. T.; Li, N.; Nevanen, T. K.; Soderlund, H. C.; Martin, R. J. Am. Chem. Soc. 2002, 124, 11864. (3) Lawrence, J.; Xu, G. Appl. Phys. Lett. 2004, 84, 918. (4) Kam, N. W. S.; Jessop, T. C.; Wender, P. A.; Dai, H. J. J. Am. Chem. Soc. 2004, 26, 6850. (5) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (6) Cheng, B.; Samulski, E. T. J. Mater. Chem. 2001, 11, 2901. (7) Luo, B.; Johnson, J. W.; Kim, J.; Mehandru, R. M.; Ren, F.; Gila, B. P.; Onstine, A. H.; Abernathy, C. R.; Pearton, S. J.; Baca, A. G.; Briggs, R. D.; Shul, R. J.; Monier, C.; Han, J. Appl. Phys. Lett. 2002, 80, 1661. (8) Jiang, J. C.; Meletis, E. I.; Yuan, Z.; Chen, C. L. Appl. Phys. Lett. 2007, 90, 051904. (9) Bhargava, A.; Alarco, J. A.; Mackinnon, I. D. R.; Page, D.; Iiyushechkin, A. Mater. Lett. 1998, 34, 133. (10) Yuan, Y. S.; Wong, M. S.; Wang, S. S. J. Mater. Res. 1996, 11, 8. (11) Wagner, G. W.; Bartram, P. W.; Kopper, O.; Klabunde, K. J. J. Phys. Chem. B 1999, 103, 3225.
Synthesis and Photoluminescence of MgO Nanotubes (12) Jung, H. S.; Lee, J. K.; Nastasi, M.; Lee, S. W.; Kim, J. Y.; Park, J. S.; Hong, K. S.; Shin, H. Langmuir 2005, 21, 10332. (13) Colbourn, E. A. Surf. Sci. Rep. 1992, 15, 281. (14) Nieves, I.; Klabunde, K. J. Mater. Chem. Phys. 1988, 18, 485. (15) Pacchioni, G.; Minerva, T.; Bagus, R. S. Surf. Sci. 1992, 275, 450. (16) Echterhoff, R.; Knozinger, E. Surf. Sci. 1990, 230, 237. (17) Itoh, H.; Utamapanya, S.; Stark, J. V.; Klabunde, K. J.; Schlup, J. R. Chem. Mater. 1993, 5, 71. (18) Yang, P.; Lieber, C. M. J. Mater. Res. 1997, 12, 2981. (19) Bando, L. Y.; Sato, Y. T. Chem. Phys. Lett. 2002, 81, 757. (20) Ma, R.; Bando, Y. Chem. Phys. Lett. 2003, 370, 770. (21) Yin, Y.; Zhang, G.; Xia, Y. AdV. Funct. Mater. 2002, 12, 293. (22) Fang, X. S.; Ye, C. H.; Xie, T.; Wang, Z. Y.; Zhao, J. W.; Zhang, L. D. Appl. Phys. Lett. 2006, 88, 013101.
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10277 (23) Zhu, Y. Q.; Hsu, W. K.; Zhou, W. Z.; Terrones, M.; Kroto, H. W.; Walton, D. R. M. Chem. Phys. Lett. 2001, 347, 337. (24) Zhan, J. H.; Bando, Y. S.; Hu, J. Q.; Golberg, D. Inorg. Chem. 2004, 43, 2462. (25) Hao, Y. F.; Meng, G. W.; Zhou, Y.; Kong, M. G.; Wei, Q.; Ye, M.; Zhang, L. D. Nanotechnology 2006, 17, 5006. (26) Lu, H. B.; Liao, L.; Li, J. C.; Wang, D. F.; He, H.; Fu, Q.; Xu, L.; Tian, Y. J. Phys. Chem. B 2006, 110, 23211. (27) Zhang, Y. G.; Wang, Z. Y.; Lu, F.; Zhang, Y.; Xiao, Y. H.; Zhang, L. D. Appl. Phys. Lett. 2006, 89, 113110. (28) Hao, Y. F.; Meng, G. W.; Ye, C. H.; Zhang, X. R.; Zhang, L. D. J. Phys. Chem. B 2005, 109, 11204. (29) Rosenblatt, G. H.; Rowe, M. W.; Williams, G. P., Jr.; Williams, R. T.; Chen, Y. Phys. ReV. B 1989, 39, 10309.
