Materials Chemistry and Physics 112 (2008) 230–233
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Silicon monoxide assisted way to alpha-alumina nanostructures and their photoluminescence Sheng Wang, Mingwang Shao ∗ , Lei Lu, Hong Wang, Huazhong Gao, Jun Wang Anhui Key Laboratory of Functional Molecular Solids, and College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e
i n f o
Article history: Received 7 March 2007 Received in revised form 17 May 2008 Accepted 22 May 2008 Keywords: Nanostructure Chemical synthesis Luminescence
a b s t r a c t Alumina nanoribbons and nanorings were prepared via high temperature route using aluminum foil and silicon monoxide as raw materials. Here, silicon monoxide acted as a precursor and its slow thermal decomposition decreased the whole reaction rate, which is in favor of high quality of products. These products, grown in situ on the surface of aluminum, had strong photoluminescence (PL) which may be ascribed to the existence of F+ center. The formation of nanorings might be explained with the (1 0 1¯ 0) polar plane. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of nanomaterials with well-controlled size, morphology, and chemical composition may open new opportunities in exploring the chemical and physical properties of the substance. Since Wang’s discovery of semi-conducting oxide nanobelts [1], a variety of materials have been investigated including, ZnO [2], Zn [3], GaP [4], Ge3 N4 [5], and In2 O3 [1]. Recently, ring structures as building blocks have attracted intense researches because of their novel optical [6], photoelectrical [7], and magnetical [8] properties and promising applications. Nanorings of numerous materials have been fabricated based on a great deal of methods and growth models [6–8]. Owing to their brittleness, ceramics have been regarded as materials of modest performance, especially under tension or bending conditions. In contrast to metals or polymers, thermal stability of ceramics above 700 ◦ C makes them suitable materials for high-temperature applications. Microstructural design is critical in order to obtain reliable ceramic materials. Since the 1960s, a wide range of oxide, non-oxide, and composite materials has been developed so that they have excellent properties. Such materials are usually termed as advanced ceramics. The use of Al2 O3 polycrystalline fibers and whiskers as strengtheners in high-temperature composites is of great interest due to their high elastic modulus and their thermal and chemical stability [9]. Alumina has conventionally been used in making crucibles for fusion of metals, tubular
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[email protected] (M. Shao). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.05.051
reactors for high-temperature experiments, thin-walled fairings for radar antennas, and combustion-chamber linings for rocket motors [10]. Alumina macroscopic phase displays its distinguishable role as reinforcement for biomaterials, which has drawn much attention and interest in fabricating one-dimensional alumina nanostructures [11]. Recently, amorphous alumina nanostructures including nanotubes [12], nanowires [13], nanoplatelets [14], and nanoribbons [15] have been prepared by various methods. However, to the best of our knowledge, there has been no report on alumina nanorings up till now. Although the synthesis of diversified ␣-Al2 O3 morphologies has been previously reported, the preparation of ␣-Al2 O3 singlecrystalline nanostructures still demands further investigation. Herein, SiO was served as the precursor and the procreant SiO2 was used in the reaction with aluminum. The total reaction rate was controlled with the decomposition of SiO in high temperature, which resulted in products with high quality and good morphology. In this paper the growth mechanism of ␣-Al2 O3 nanoribbons and nanorings has been discussed. In addition, we found that the as-prepared products have an obvious violet photoluminescence (PL) with emission peak at 395 nm under excitation at 255 nm. 2. Experimental procedures The synthesis of ␣-Al2 O3 nanoribbons was carried out in a high-temperature tube furnace that was heated by SiMo heaters. An alumina tube was mounted horizontally inside the tube furnace. The details were as follows: the SiO powder (99.99%) was put in an alumina boat, which was tightly covered with a piece of aluminum foil (99.9%, 50 mm × 10 mm × 0.3 mm) placed in the center of the tube. After the tube had been evacuated to 10 Pa, the system was heated to 800 ◦ C at a heating rate of 5 ◦ C min−1 , held for 30 min, heated again up to 1300 ◦ C and held for another 4 h. Then, the furnace was cooled to the room temperature naturally.
S. Wang et al. / Materials Chemistry and Physics 112 (2008) 230–233
Fig. 1. The XRD pattern of as-prepared products.
A thick layer of white, fluffy products was formed on the surface of aluminum foil. The phase and the crystallography of the products were characterized with a Shimadzu XRD-6000 X-ray diffractometer (XRD) equipped with Cu K␣ radiation ( = 0.15406 nm); a scanning rate of 0.02◦ s−1 was applied to record the pattern in the 2 range of 10–100◦ . The morphology and microstructure of the samples were analyzed using a scanning electron microscope (SEM) (JEOL-6300F, 5 kV), transmission electron microscope (TEM) (Hitachi H-800), and a high-resolution transmission electron microscope (HRTEM) (JEOL-2010, 200 kV), respectively. PL spectrum was recorded using the solid products employing a Hitachi F-4500 fluorescence spectrophotometer at room temperature.
3. Results and discussions Fig. 1 shows the XRD pattern of the as-prepared products. All diffraction peaks can be indexed as ␣-Al2 O3 with lattice constants a = 0.4762 ± 0.0009 nm, c = 1.298 ± 0.004 nm, which is in accordance with the JCPDS data (a = 0.4758 nm, c = 1.299 nm, JCPDS card No. 10-0173). The SEM image (Fig. 2a) reveals that the products, with the morphology of ribbon, are in large scale and their length is up to tens of micrometers. Fig. 2b shows a single nanoribbon with width of 220 nm and thickness of 90 nm. On the basis of XRD and SEM results, the as-synthesized products basically present a high-purity material of ␣-Al2 O3 with ribbon-like morphology. Fig. 3a displays the TEM image of a typical single nanoribbon with the width of 300 nm further verifying the results from the SEM. The selected area electron diffraction (SAED) pattern (Fig. 3b, inset) illustrates the presence of bright diffraction spots signifying the crystalline nature. The spots can be indexed as (0 0 0 2) and (3 0 3¯ 0) planes which orient along the [1 2¯ 1 0] zone axis. Fig. 3b gives the HRTEM image taken from the nanoribbon. It can be observed that
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the lattice fringes are very clear and free of defects, revealing that the crystallinity of ␣-Al2 O3 nanoribbons is perfect. The interlayer space in the HRTEM image is about 0.65 nm, which is consistent with the distance between (0 0 0 2) planes of ␣-Al2 O3 , indicating that the ␣-Al2 O3 nanoribbons grow along the [0 0 0 1] direction. Combination of the SAED pattern and HRTEM image deduces that the nanoribbon has top and bottom planes of (1 2¯ 1 0) and (1¯ 2 1¯ 0), left and right planes of (1 0 1¯ 0) and (1¯ 0 1 0), and front ¯ It should be appointed that and back planes of (0 0 0 1) and (0 0 0 1). (1 0 1¯ 0) plane is a polar surface. As it is shown in Fig. 4, due to Al3+ terminated (1 0 1¯ 0) and O2− -terminated (1¯ 0 1 0), the surface polar changes result in a pair of positively and negatively charged polar surfaces on the inner and outer surfaces of the nanoribbon. A dipole moment exists between the two surfaces of (1 0 1¯ 0) and (1¯ 0 1 0). The dipole moment can be neutralized by symmetric charge distribution in the nanoring. As a result of bending, the electrostatic energy of the entire system is expected to decrease. On the other hand, in the process of nanoribbon rolling up to form a ring or spiral structure, the decrease of electrostatic energy and the increase of elastic energy should be taken into consideration. The elastic deformation from forming a ring will no doubt lead to the increase of the elastic energy. When the increase of the elastic energy is far below the decrease of the electrostatic energy, a nanoring may take shape by the bending of the nanoribbon. Therefore, the minimization of the total energy serves as the incentive for the formation of the nanoring. The above-proposed mechanism agrees with the models established for AlN [16]. Indeed, there exist ␣-Al2 O3 nanorings in our products, which grow on the edges of the aluminum foil. Fig. 5 depicts SEM image with several ␣-Al2 O3 nanorings. And a representative ␣-Al2 O3 nanoring (Fig. 5, inset) is clearly shown with diameter of 3.2 m and the width of about 100 nm. The growth of the ring can be understood on the basis of polar surfaces of the alumina-structured Al2 O3 . The operation may involve three reactions expressed as follows: 2SiO → Si + SiO2
(1)
4Al(g) + SiO2 → 2Al2 O(g) + Si(s, l)
(2)
Al2 O(g) + 2SiO(g) → Al2 O3 + 2Si(s, l)
(3)
First, SiO decomposes and produces silicon and silicon dioxide (Eq. (1)): the latter reacts with aluminum and obtained Al2 O (g) (Eq. (2)) [15]. The vapors of Al2 O and SiO react to produce alumina (Eq. (3)) [15], which grows on the surface of aluminum in situ. In this experiment, SiO is employed to serve as precursor to control the total reaction rate and get high quality of products. The He crystalline one-dimensional materials growth mechanism usually takes two routes: vapor–liquid–solid and vapor–solid mechanism. The former is based on catalysts, while the latter from the vapor of precursors deposits directly to nanowires and nanoribbons without a liquid state. As far as the current experiment is
Fig. 2. SEM images of (a) ␣-Al2 O3 nanoribbons in large scale and (b) a representative single nanoribbon.
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S. Wang et al. / Materials Chemistry and Physics 112 (2008) 230–233
Fig. 3. (a) TEM image of an individual Al2 O3 nanoribbon and (b) SAED pattern (inset) and HRTEM image of the Al2 O3 nanoribbon.
Fig. 5. SEM image of several Al2 O3 nanorings and a single nanoring in high magnification.
concerned, ␣-Al2 O3 has high melting point (2054 ◦ C) and there is no detection of catalysts particles at the ends of the products, which, as a result, indicate the features of vapor–solid mechanism. The growth process is mainly determined by growth kinetics combined with thermodynamics [17]. PL spectrum (Fig. 6) of the products is examined under the room temperature. The as-synthesized nanoribbons have a stable, strong
Fig. 4. The structure model of (a) ␣-Al2 O3 and (b) its (1 0 1¯ 0) polar plane.
Fig. 6. Emission spectrum of the as-synthesized Al2 O3 nanoribbons, excited at 255 nm, and its excitation spectrum, recorded at room temperature.
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violet emission band centered at 395 nm when they are excited at 255 nm. Its peak position hardly changes with excitation wavelength. The high aspect ratio and peculiar morphologies of ␣-Al2 O3 nanoribbons should favor the existence of oxygen vacancies [18]. As regards to the reason of the emission in ␣-Al2 O3 nanoribbons, Peng et al. [15] discussed effects of increasing or decreasing oxygen vacancies. It has been reported that F+ center (oxygen vacancies with one electron) in ␣-Al2 O3 causes ultraviolet or violet PL bands in nanostructured alumina [12,15,17]. The result in our experiment is ascribed to the existence of F+ center. 4. Conclusions In summary, ␣-Al2 O3 nanoribbons and nanorings were obtained using Al foil and SiO powder under high temperature. In addition, it has been investigated that the violet emission property of the sample under ultraviolet excitation can be attributed to F+ centers in the ␣-Al2 O3 nanostructures. Considering their optical properties, high specific surface area, and low cost, the ␣-Al2 O3 nanostructures might have potential application in various areas. Acknowledgements The project was supported by the National Natural Foundation of China (20571001), the Education Department (No. 2006KJ006TD)
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