Simple Synthesis of Single-crystalline Nanoplates of Magnesium Oxide

Transcription

1 CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 19, NUMBER 5 OCTOBER 27, 2006 ARTICLE Simple Synthesis of Single-crystalline Nanoplates of Magnesium Oxide Hai-xia Niu a,b, Qing Yang a,b, Fei Yu c, Kai-bin Tang a,b, Wei Zhou b a. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei , China; b. Department of Chemistry, University of Science and Technology of China, Hefei , China; c. Department of Modern Mechanics, University of Science and Technology of China, Hefei , China (Dated: Received on August 12, 2005; Accepted on August 22, 2005) Single-crystalline nanoplates of magnesium oxide were successfully synthesized through a calcinations route from the newly produced Mg(OH) 2 precursors, which were directly prepared from the commercial bulk magnesium powders under a hydrothermal process in the absence of any additions. Scanning electron microscope (SEM) analysis indicated that the nanoplates were 2-6 µm in average width and about 80 nm in thickness. Transmission electron microscopy (TEM) images revealed that there was large quantity of nanopores with diameters ranging from 5 to 40 nm distributed in these nanoplates. The room-temperature photoluminescence (PL) spectrum of the nanoplates illustrated a strong blue emission band at 416 nm and a weak green emission band at 559 nm. Brunauer-Emmett-Teller (BET) analysis exhibited a feature of high surface of m 2 /g for the products. The fabrication mechanism of the product was also discussed. Key words: MgO, Nanoplate, Hydrothermal process I. INTRODUCTION The chemical synthesis of inorganic materials with unusual and complex nanostrucutres are of great interest in materials fields because the properties of materials mainly depend on their shape, size, and structures [1-5]. The two-dimensional (2D) structures are believed to have marvelous ability to control optical properties due to their anisotropic structures [6,7]. Specially, the fabrication of nanopaltes has obtained much attention [6-8]. Being a very important wide bandgap insulator, MgO has extensive application in the areas of toxic waste remediation, refractory, paint and superconductor products [9-11]. In addition, it is reinforced agents owing to its shows excellent adsorbtion properties in both enhanced surface areas and intrinsically higher surface reactivity [7,12]. In the field of catalysis, MgO can be exploited for base catalysis in many organic reactions and catalyst support as well as in the application of optical transmitters and substrates for thin film growth. Thus the employed MgO must have sufficiently high surface area to allow diffusion of active species [13,14]. So it is very important to develop alternative synthetic methods to produce MgO nanocrystals with high specific surface area in a controllable manner. In this work, we have successfully fabricated pure single-crystalline nanoplates of MgO with a high surface area, which were obtained through the calcination of newlyproduced Mg(OH) 2 precursors. The precursors were directly synthesized using commercial Mg powders Author to whom correspondence should be addressed. qyoung@ustc.edu.cn, Tel: , Fax: as the starting material under a hydrothermal condition. The as-prepared MgO nanocrystals with highly specific area and good optical property would provide potential value for the uses in many fields. This simple and easily conducted route without any other additions could provide a potential advantage for the large-scale production with high purity and relatively low cost. II. EXPERIMENTS In a typical synthesis, a total of 10 mmol of Mg ( 99.0%) powders with 36 ml of distilled water was put into a Teflon-lined autoclave of 50 ml capacity. The autoclave was sealed and maintained at 180 C for 36 h and then cooled to the room temperature naturally. The resulting product was washed with distilled water and absolute ethanol for several times. After being dried in air at 60 C for 12 h, white Mg(OH) 2 powders were obtained. MgO products could be achieved after the following calcinations of these Mg(OH) 2 precursors at 400 C for 4 h in air. The X-ray powder diffraction (XRD) patterns of the products were recorded on a Philips X pert X-ray diffractometer with Cu Kα radiation (λ= Å) employing a scanning rate of 0.02 /s in the 2θ ranges from 10 to 70. Scanning electron microscope (SEM) images were performed on a Hitachi X-650 scanning electron microanalysizer. Transmission electron microscopy (TEM) investigations were conducted on a Hitachi H-800 transmission electron microscope. Highresolution transmission electron microscopy (HRTEM) image was carried out on a JEOL-2010 TEM at an acceleration voltage of 200 kv. The room-temperature photoluminescence (PL) spectrum was performed on a Fluorolog-3-Tau Steady-state/Lifetime Spectrofluorom- 438 c 2006 Chinese Physical Society

2 Chin. J. Chem. Phys., Vol. 19, No. 5 eter. The specific surface area of the product was explored by the Brunauer-Emmett-Teller (BET) method on a Micromeritics ASAP 2000 apparatus. Thermogravimetric (TG) analysis was conducted on a TG 50H thermogravimeter. Synthesis of Single-crystalline Nanoplates of MgO 439 highly ordered diffraction spots of the ED pattern (inset of Fig.2(D)) also suggested that the sample was wellcrystallized single crystals, which was in agreement with the results of XRD. III. RESULTS AND DISCUSSION Figure 1 showed a typical XRD pattern of the asprepared MgO sample. All the reflection peaks could be readily indexed to a pure cubic phase MgO [space group Fm3m] with calculated cell parameters of a=4.213 A, which was in agreement with that in JCPDS card (No.4829). No characteristic peaks for the other impurities such as Mg(OH)2 were observed. These relatively intensive peaks suggested that the products were well crystallized, and the broad peaks indicated that the products had small size dimensions. FIG. 2 (A) The SEM image of the MgO nanoplates. (B) A corresponding magnified SEM image for the nanoplates. (C) The typical TEM image of a porous MgO nanoplates and its magnified TEM image inserted. (D) The HRTEM image of the nanoplates recorded on the edges, the inset is the corresponding ED pattern for the nanoplates. FIG. 1 The XRD pattern of the MgO sample. The SEM images of the as-prepared MgO sample were shown in Fig.2(A) and (B), which clearly showed that the products consisted of large quantity of nanoplates. The thickness of these nanoplates was about 80 nm with typical lateral dimensions between 2 and 6 µm. The morphology and structure of the nanoplates were also examined by TEM. Figure 2(C) revealed that there were many pores distributed in the nanoplates. Estimated from the magnified TEM image (inset of Fig.2(C)), the main pore size was about 5-40 nm. The proportion of the porous nanoplates is above 99% and their yield is about 97% based on the original reagent of Mg. A typical HRTEM image of the nanoplates (observed on the edge) revealed fringe spacing was nm (Fig.2(D)), which corresponded to the separation between the (200) planes. The growth direction could be determined as along [100] direction. Through careful observation from Fig.2(D), some white spots due to the defects or pores formed in the calcination process had been found. The intensity and The PL measurement was carried out using an exaction wavelength of 230 nm and a filter of 370 nm at room temperature. Two emission bands could be observed in the PL spectrum (Fig.3) of the MgO nanoplates: one blue was centered at 416 nm and the other green was at 559 nm. Both bands were attributed to F+ and F center, respectively [15]. Compared with those of the bulk MgO [15], the PL bands were relatively broadened, which was due to the small dimension of the nanoparticles in the nanoplates [16]. Meanwhile, there was a red-shifted phenomenon of the PL peaks compared with the bulk MgO [15] owing to the quantum confinement effect from the small dimensions. The observed rags in the profile were relevant to the presence of the defects in the nanoplates, which was consistent with the HRTEM investigation. The surface area of the MgO sample determined by the BET technology was found to be m2 /g and it was much larger than the data previously reported [7]. For potential application in basic heterogeneous catalysis, it was necessary that the MgO sample possessed a high thermal stability [17]. Our products did not show c 2006 Chinese Physical Society

3 440 Chin. J. Chem. Phys., Vol. 19, No. 5 Hai-xia Niu et al. FIG. 3 The room-temperature PL spectrum of the MgO nanoplates. tained to MgO products after calcination processes even though the MgO nanoplates were less reported to date [7,19,20]. In the present route, the nanoplates of MgO were topochemically produced from the early produced Mg(OH) 2 nanoplates, which could be deduced from the comparison between a typical SEM image (Fig.5(A)) of the Mg(OH) 2 precursors and the one of MgO nanoplates (Fig.2(B)) obtained in the present route. The size of Mg(OH) 2 nanoplates was larger than that of MgO, because the latter had a higher density [7]. Through further observation from TEM image (Fig.5(B)) for the precursors, it could be found that the surfaces of the nanoplates were clean without any porous structure or amorphous layer, different from the MgO sample calcined from the precursors. The corresponding ED pattern (insert of Fig.5(B)) for the precursors indicated that they also possessed single-crystalline feature. any significant structural change after being heated at 800 C for 3 h, which could be convinced by TEM investigation (unshown). With so large surface area and high thermal stability, the as-prepared MgO nanoplates were expected to have excellent performance in catalysis, catalysts carriers and hydrogen storage, etc [18]. As reported, finding a correlation between the synthesis method and the final structure was a key point to the design of materials [1]. In the present route, the nanoplates of MgO could be topochemically obtained from the early produced precursors of Mg(OH) 2. The XRD pattern of these early obtained Mg(OH) 2 precursors was shown in Fig.4. All diffraction peaks in the pattern could be perfectly indexed to a pure hexagonal phase of Mg(OH) 2 with calculated lattice constants a=3.147 Å, c=4.769 Å. The results were in agreement with the reported data as shown in JCPDS Card No The sharp and intensive characteristic diffraction peaks of the precursors showed that the Mg(OH) 2 were also well crystallized. The general morphology of the Mg(OH) 2 precursors was often re- FIG. 4 The XRD pattern of the Mg(OH) 2 precursors FIG. 5 The SEM image (A) and the TEM image (B) for the Mg(OH) 2 precursors. The insert of (B) is the corresponding ED pattern for the precursors naonplates. The TG measurement was carried out in N 2 to analyze the thermal behavior of the Mg(OH) 2 precursors. There were two weight losses in the TG profile (Fig.6). One was a small mass loss from 30 to 100 C due to the evaporation of water molecules absorbed in air by the sample. The other was a pronounced weight loss in the temperature range of C ascribed to the dehydration of Mg(OH) 2. However, the theoretical weight loss for the Mg(OH) 2 transfabrication to MgO was 30.8%, which was slightly higher than the observed 28.4%. This result was probably owing to the incompleteness of the decomposition reaction in such temperature range [7,20]. Generally, the single-crystalline MgO nanoplates were produced from the subsequent process of calcination from the early produced Mg(OH) 2, which were directly obtained by using commercial Mg powders (Fig.7) as the starting material under a hydrothermal condition in the first stage. The reactions involved in the process could be simply described as follows: Mg + 2H 2 O Mg(OH) 2 +H 2 (1) Mg(OH) 2 MgO + H 2 O (2) It was found that the final morphology of the MgO c 2006 Chinese Physical Society

4 Chin. J. Chem. Phys., Vol. 19, No. 5 Synthesis of Single-crystalline Nanoplates of MgO 441 ner part was the unreacted Mg while the outer one was Mg(OH)2. These gray products also demonstrated plate-like structures (Fig.8(A)) although the core part of the Mg granules were not reacted completely in a relative short duration (24 h). Interestingly, the plate-like morphologies of the Mg(OH)2 nanocrystals could also retained to the final MgO nanocrystals (Fig.8(B)) after the subsequent calcinations process. FIG. 6 A typical TGA result of the produced Mg(OH)2 precursors. nanoplates were grown and retained by the template effects from the plate-like Mg(OH)2 precursors, which were often influenced by the synthetic conditions employed. For the Mg(OH)2 precursors, the Mg granules ranging from several tens microns to hundreds microns (Fig.7) were used in the route. It appears that the size of these solid Mg granules was much larger than that of the produced Mg(OH)2 nanoplates (Fig.5). Under such hydrothermal conditions in the first step, the solid Mg would react heterogeneously with water on the interfaces between Mg and water to produce Mg(OH)2. Obviously, the growth of the Mg(OH)2 nanocrystals from the heterogeneous reaction in the hydrothermal process was mostly controlled kinetically, which deduced that the reaction time would play an important role in the fabrication of Mg(OH)2 with plate-like structures. When the reaction time was less than 30 h, such Mg granules could not react completely. Take an experiment of 24 h for example, the products obtained in the solution were in gray color, which suggested the products contained some unreacted Mg under such conditions. Careful observations would find that the in- FIG. 8 (A) The SEM image for the precursors obtained under hydrothermal conditions for 24 h, (B) the SEM image for the product after calcinations of that precursors shown in (A). Meanwhile, reaction temperature could also affect on the fabrication of the Mg(OH)2 precursors. When the reaction was carried out from 160 to 200 C under the same hydrothermal conditions, the Mg granules could be reacted completely and the produced Mg(OH)2 nanocrystals were the similar nanoplates. When the temperature was below 150 C, the reaction could not be carried out completely within 30 h. From above analysis, it was believed that the fabrication mechanism of our products was similar with previous reports [20]. IV. CONCLUSION FIG. 7 The SEM image for the bulk Mg starting materials. In summary, the single-crystalline nanoplates of MgO have been successfully fabricated on a large scale through the controlled removal of water molecules from the pre-synthesized Mg(OH)2 precursors, which were prepared directly from Mg powders via a hydrothermal method in the absence of any additions. The high surface feature and good optical property make the product a good candidate in many application fields. The advantages of our method lie in the high yield, mild reaction conditions and permitting scaled-up industrial manufacturing. The simple synthetic route has the potential to be developed as a general method for the preparation of other nanostructured metal oxides. c 2006 Chinese Physical Society

5 442 Chin. J. Chem. Phys., Vol. 19, No. 5 Hai-xia Niu et al. V. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No , No , No , No ), the Anhui Provincial Natural Science Foundation (No ) and the Chinese Ministry of Education. [1] X. G. Peng, L. Manna, W. Yang, J. Wickham, E. K. Scher, Andreas and A. P. Alivisatos, Nature 404, 59 (2000). [2] N. E. Kelly, S. O. Lee and K. D. M. Harris, J. Am. Chem. Soc. 123, (2001). [3] L. Manna, E. C. Scher and A. P. Alivisatos, J. Am. Chem. Soc. 122, (2000). [4] W. Lu, P. Gao, W. B. Jian, Z. L. Wang and J. Fang, J. Am. Chem. Soc. 126, (2004). [5] A. Ghezelbash, M. B. Sigman and B. A. Jr. Korgel, Nano Lett. 4, 537 (2004). [6] L. P. Jiang, S. Xu, J. M. Zhu, J. R. Zhang, J. J. Zhu and H. Y. Chen, Inorg. Chem. 43, 5877 (2004). [7] H. T. Zhang, G. Wu and X. H. Chen, Langmuir 21, 4281 (2005). [8] Y. C. Cao, J. Am. Chem. Soc. 126, 7456 (2004). [9] Y. S. Yuan, M. S. Wong and S. S. Wang, J. Mater. Res. 11, 8 (1996) [10] A. Bhargava, J. A. Alarco, I. D. R. Mackinnon, D. Page and A. Ilyushechkin, Mater. Lett. 34, 133 (1998). [11] R. Ma and Y. Bando, Chem. Phys. Lett. 370, 770 (2003). [12] J. Jiu, K. I. Kurumada, M. Tanigaki, M. Adachi and S. Yoshikawa, Mater. Lett. 58, 44 (2004). [13] W. C. Li, A. H. Lu, C. Weidenthaler and F. S. Th, Chem. Mater. 16, 5676 (2004). [14] Y. Chen, J. H. Li, Y. Yang, Xiao Z. and J. Dai, Ceramics International 29, 663 (2003). [15] G. H. Rosenblatt, M. W. Rowe, G. P. Jr. Williams and R. T. Williams, Phys. Rev. B 39, (1989). [16] Z. Li, Y. Xie, Y. Xiong and R. Zhang, New J. Chem. 27, 1518 (2003). [17] J. Roggenbuck and M. Tiemann, J. Am. Chem. Soc. 127, 1096 (2005). [18] X. L. Li and Y. D. Li, Chem. Eur. J. 9, 2726 (2003). [19] Y. Li, M. Sui, Y. Ding, G. Zhang, J. Zhuang and C. Wang, Adv. Mater. 12, 818 (2000). [20] Y. Ding, G. Zhang, H. Wu, B. Hai, L. Wang and Y. Qian, Chem. Mater. 13, 435 (2001). c 2006 Chinese Physical Society