DOI: 10.1021/cg9008448
A Novel Method to Synthesize a Large Area of Single Crystalline LiAl5O8 Nanorods
2010, Vol. 10 191–194
Mitch M. C. Chou,* Cheng Chien Hsu, Chun-Yu Lee, and Chenlong Chen Department of Materials & Opto-electronic Science National Sun Yat-Sen University, 80424, Kaohsiung, Taiwan, R.O.C. Received July 21, 2009; Revised Manuscript Received September 13, 2009
ABSTRACT: A large area of single crystalline LiAl5O8 nanorods was synthesized by a vapor phase transport process via Au-catalyzed crystal growth on LiAlO2 substrate. The nanorods were grown at high density, with the [111] axis maintained perpendicular to the [100] substrate surface by the direct reaction of CO gas and LiAlO2 substrate. The dependence of the nanorods’ diameters and lengths on the growth conditions was systematically investigated. The nanorods were characterized by X-ray diffraction, scanning electron microscopy, and high resolution transmission electron microscopy. These results indicated that the crystallized LiAl5O8 nanorods are free of segregations of a second phase and extended defects. The vapor-liquid-solid (VLS) crystal growth mechanism of the single crystalline LiAl5O8 is also discussed.
1. Introduction LiAl5O8 has a spinel structure. In normal and inverse spinels, there are two types of sites for the metal ions: the A site, which takes the tetrahedron site of four oxygens, and the B site, which lies in the center of distorted octahedron surrounded by six oxygen ions.1 LiAl5O8 is exceptional in that the ionic arrangement is intermediate between the normal and inverse spinel form.2 Glynn et al. proposed that LiAl5O8 has a long-range 1:3 ordering in the occupancy of the B sites by Li and Al ions.3 In this ordered phase, each Li ion is surrounded by six nearest-neighbor Al ions, whereas each Al ion has four Al and two Li ions. The primitive cell is cubic. By heating up LiAl5O8 above 1300 °C, the long-range order of Li ions is destroyed, and by rapid quenching to room temperature, a disordered phase, or the high temperature phase with a standard spinel structure can be obtained.4 There are three different oxides in the Li2O-Al2O3 system, LiAlO2, Li5AlO4, and LiAl5O8, with the melting points of 1750, 1116, and 1915 °C.5 The thermodynamically most stable compound is γ-LiAlO2. LiAl5O8 is stable over a substantial range of stoichiometry; however, the Li5AlO4 phase can usually be identified when synthesizing LiAl5O8. These mixed phases will influence its physical properties such as luminescent properties. At present, the common ways to synthesize LiAl5O8 materials include the wet-chemical method,6 the sol-gel method,7 the self-flux method by heating a powder mixture of Al2O3/Li2SiO4 = 1:2,8 and the solid-state reaction of Al2O3 and Li2CO3.9 Although LiAl5O8 has been prepared as either powder or crystalline forms, to our knowledge, no report of large-scale preparation of single crystalline LiAl5O8 nanorods has been made so far. In this manuscript, a chemical vapor deposition (CVD) method is adopted to grow a large area of LiAl5O8 single crystalline nanorods. This method is based on the structural relationship of [100] γ-LiAlO2 substrate and LiAl5O8 as well as the physical and chemical properties of [100] γ-LiAlO2 single crystal grown by the Czochralski pulling
technique.10 γ-LiAlO2 crystal has a tetragonal structure in which each Li and Al atom is coordinated at the center of a tetrahedron, with four oxygen atoms.11 The lattice parameters of γ-LiAlO2 are a=b=5.1687 A˚ and c=6.2679 A˚. LiAlO2 is commonly used as a substrate of nonpolar GaN and ZnO due to the small lattice mismatch.12-14 From the viewpoint of lattice mismatch between [111] LiAl5O8 and [100] γ-LiAlO2, [001]LiAlO2 // [112]LiAl5O8 has 3.22% lattice mismatch and [010]LiAlO2 // [110]LiAl5O8 has 8.26% lattice mismatch. It can be clearly seen that [111] LiAl5O8 will be parallel to [100] LiAlO2 substrate, Figure 1. There are two main novelties in this study. First of all, we adopted a vapor-liquid-solid (VLS) crystal growth method.15 Au is the catalyst. Carbon powder and ZnO are used as the source materials to form the CO reduction atmosphere. LiAl5O8 nanorods were synthesized by the direct reaction of CO gas and LiAlO2 substrate and grown from the bottom of the Au droplets. This growth mechanism was not proposed in the literature before. On the other hand, the growth direction of LiAl5O8 nanorods is not random but able to be controlled by the LiAlO2 substrate because of the good symmetry between the nanorods and substrate structures. This new synthetic technique opens up the possibility of LiAl5O8 nanorods for high efficiency luminescent applications. The luminescent properties of LiAl5O8 nanorods have been studied extensively by our group; these results will be discussed in a future publication. 2. Experimental Section
*Author to whom correspondence should be addressed. E-mail: mitch@ faculty.nsysu.edu.tw.
The LiAlO2 single-crystal substrates were obtained from 2 in. high-quality (100) LiAlO2 crystals grown using the Czochralski pulling technique. LiAlO2 substrates with rms roughness of 0.32-0.48 nm were used for all of the CVD experiments. More details about the crystal growth and polishing methods of LiAlO2 can be found in another article.10 The (100) LiAlO2 single crystal substrates were cleaned in an ultrasonic bath of acetone for 20 min following by being dried by N2 gas before being loaded into the quartz tube. An Au thin film used as a catalyst was deposited on a LiAl5O8 substrate by a Pelco SC-6 sputter coater before being loaded into the quartz tube placed in a furnace. The coating time of Au varies from 30-90 s. The thickness of Au film is between 15 and
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Figure 1. Ball-and-stick model of the [111] LiAl5O8 and LiAlO2 substrate. The triangles indicate nucleation sites for [111] LiAl5O8 nanorods.
Figure 3. (a) X-ray diffraction pattern of LiAl5O8 nanorods; (b) X-ray analysis of the material deposited at the cold end of the CVD furnaces. It was identified as Zn metal. JEOL3010 at 200 kV), selected-area electron diffraction, and energy dispersive spectrometry (EDS, OXFORD INCA x-stream). Figure 2. SEM images of LiAl5O8 nanorods grown on LiAlO2 substrate for different Au coating times of (a) 35 s, (b) 60 s, and (c) 90 s and for different Ar flow rates of (d) 15 sccm, (e) 20 sccm, (f) 25 sccm at the growth temperature of 900 °C. 60 nm. The quartz tube was degassed under a vacuum and purged with Argon gas which is used to dilute and carry the residual gas. Carbon and ZnO powders mixed uniformly at a weight ratio of 1:1 were used as the material sources and put in a sapphire boat. The distance between the substrate and crucible was around 2-4 cm. The temperature of the source was increased to permit a carbon thermal reaction and ZnO was reacted with carbon to form CO gas. The source and substrate were heated up to 900-950 °C from room temperature at a rate of 30 °C/min. The growth time was 60 min. The diameters and lengths of LiAl5O8 nanorods can be controlled by changing the Ar gas flow rate, growth temperature, and the size of Au droplet. By optimization of the growth temperature and gas flow rate, LiAl5O8 nanorods were synthesized on the (100) LiAlO2 single-crystal substrates by CVD in a threetemperature-zone furnace. The morphologies and crystal structures of the resulting LiAl5O8 nanorods were characterized using scanning electron microscopy (SEM, JEOL JSM6330TF) and X-ray diffraction (XRD, Siemens D5000 X-ray, Cu KR λ = 1.541 A˚ at 40 kV and 30 mA). Further structural and elemental analyses of a single nanorod were performed using high resolution transmission electron microscopy (HRTEM,
3. Results and Discussion 3.1. SEM Analysis of LiAl5O8 Nanorods. Figure 2a,b is the SEM images of LiAl5O8 nanorods grown on LiAlO2 substrate for the Au coating time of 35 s, and 60 s. There are several important features for these nanorods. First, most of the nanorods grow perpendicular to the substrate. Second, as a result of the VLS growth, the nanorods grow only in the areas with Au thin-film coating. Third, the nanorods with the larger diameters have been achieved by using the longer coating time of Au film. Figure 2c shows the Au coating time of 90 s. The average diameter of the nanorod is much larger because more small Au droplets accumulate to form a large one. LiAl5O8 nanorods grow under these big droplets. Some droplets even can let two nanorods grow simultaneously. The lengths of the LiAl5O8 nanorods were found to depend strongly on the argon flow rates. Figure 2d is the tilted 30° SEM image of LiAl5O8 nanorods at a flow rate of 15 sccm. The average length of nanorods is around 380 nm. Once the flow rate is increased to 20 sccm, and 25 sccm, the average length becomes 420 nm and 520 nm, Figure 2e,f. This is because more Ar gas will carry more CO to permit the LiAlO2 substrates to decompose into LiAl5O8 nanorods.
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Table 1. Chemical Compositions of LiAl5O8 Nanorods Examined by the Energy Dispersive Spectrometry (EDS, OXFORD INCA x-stream) element C O Al Fe Cu Zn
weight %
atomic %
16.75 33.41 22.63 6.07 14.64 6.05
29.30 43.87 17.62 2.82 4.84 2.09
Table 2. The Chemical Compositions of Au Droplet Examined by the Energy Dispersive Spectrometry (EDS, OXFORD INCA x-stream) element C Cr Fe Cu Au
Figure 4. (a) TEM image and the corresponding diffraction pattern of LiAl5O8 nanorod; (b) high-resolution TEM image.
3.2. Structural Analysis of LiAl5O8 Nanorods. The crystal structure of the nanorods was examined by XRD. Figure 3a shows a typical XRD pattern of LiAl5O8 grown on LiAlO2 substrate. The main peaks are indexed as (222), (311), and (400). The calculated lattice parameter a is 7.922 nm which is consistent with ref 2. Although the LiAl5O8 nanorods grow preferentially perpendicular to the substrate, some nanorods still tilt a little bit. Hence, some peaks, such as (311) and (400), are also found. The main peak (222) indicated that LiAl5O8 nanorods grew along the (111) directions on the (100) LiAlO2 substrate; this is because of the good symmetry between the nanorods and substrate crystal structures. After the furnace was cooled to room temperature, materials with silver color were coated at the end of the furnace. It was identified as Zn metal by XRD analysis, Figure 3b. Some white powder was spread on the inner wall of the growth chamber. It was identified as Li2O. Figure 4a is the transmission electron microscopy (TEM) image and its corresponding selected area electron diffraction patterns of a single LiAl5O8 nanorod, where an Au droplet can be clearly seen on its tip. In this image, the [111] direction is parallel to the long axis of the rod. The TEM analysis proves the high-quality crystallized LiAl5O8 nanorod to be free of segregations of a second phase and extended defects. Figure 4b shows a high-resolution TEM
weight %
atomic %
2.27 0.66 0.57 38.04 58.47
17.06 1.15 0.92 54.06 26.81
image of a single LiAl5O8 nanorod. The clear lattice fringes in this image indicated a single crystal structure of the nanorod. Energy dispersive spectrometry (EDS) of the nanorod showed that the main compositions of LiAl5O8 nanorods are Al and O, listed in Table 1. A small amount of Zn (6.05% at weight ratio) was found in the nanorods. This is because Zn vapor generated by carbon thermal reaction dissolved into the LiAl5O8. Most of the Zn metal is coated at the end of the CVD furnace and confirmed by XRD analysis. Cu and C from the copper grid which is coated with carbon were found with weight ratios of 14.64% and 16.75%, respectively. No Au element was found in the nanorod. The compositional analysis of droplet is also done by EDS compositional analysis, listed in Table 2. Au (58.47% at weight ratio) is the main element. Some small amount of Fe (0.57%) and Cr (0.66%) elements are from the impurity of source material or Au target. No Zn content was found in the Au droplet. 3.3. Proposed Formation Mechanism of LiAl5O8 Nanorods. The VLS crystal growth mechanism was proposed by R. S. Wagner.15 The main feature of this mechanism is the presence of intermediates that serve as catalysts between the vapor feed and the solid growth at elevated temperatures under CVD conditions. VLS growth should be a practical route for the syntheses of one-dimensional nanometer-sized structures (i.e., nanowires and nanorods) provided that the diameters of the resulting fiber-like structures can be controlled within the nanometer range. The most important parameter in VLS growth is the catalyst added in the reaction. In our work, we found that gold was an ideal catalyst in the reaction of CO and LiAlO2 to form LiAl5O8 nanorods. Although the detailed mechanism of phase diagrams of Li-Au-Al-O-Zn is still not fully understood, we suggest that very small miscible droplets of Au may be generated rapidly during the heating process of the reaction and hence act as nucleation sites in the VLS growth of LiAl5O8 nanorods. Figure 5 illustrated the proposed VLS growth mechanism of LiAl5O8 on [100] LiAlO2 substrate. At first, Au thin film was coated on the LiAlO2 substrate. During the growth process, the Au film as a catalyst agglomerated to form the small dots on heating. CO reduction atmosphere was formed by the reaction between ZnO and carbon. From our previous experiences of growing LiAlO2 single crystal by the Czochralski
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4. Conclusions
Figure 5. Proposed formation mechanism of LiAl5O8 nanorods.
method, it is difficult for LiAlO2 to get decomposed at a lower temperature due to its high melting point (1750 °C). But with exposure of CO atmosphere at a temperature of 900-950 °C, it becomes possible that LiAlO2 can be decomposed into LiAl5O8 and Li2O. The generated Li2O vapor was spread and coated on the inner wall of the growth chamber. The LiAl5O8 crystallites are very likely precipitated from the bottom of the Au droplets during heating and are subsequently trapped at the solid/liquid interface. Indeed, the occurrence of this Li-poor phase could be explained by Li2O loss according to the reaction: 5LiAlO2 sf LiAl5 O8 þ 2Li2 Ov CO
The EDS analysis showed that Au was only found on the tip of the nanorod, not the body of the nanorod. This result fulfills the requirement of the VLS growth mechanism. On the other hand, EDS revealed that a little amount of Zn exists in the nanorod. This indicated that the growth mechanism might involve a more complicated composition, such as a system of Li-Au-Al-O-Zn. As a result of the VLS growth, the nanorods grow only in the areas with Au thinfilm coating. Under the same growth condition but without Au catalyst, the LiAl2O8 nanorods will not grow. It was proposed that the intermediate product (a complicated compound of Li-Au-Al-O-Zn) is miscible into a Au droplet at high temperature. The Au nucleation sites formed at the first step ensure the VLS growth mechanism of LiAl5O8 nanorods.
In summary, a large area of LiAl5O8 single crystalline nanorods was synthesized for the first time. A CVD method is adopted to grow a large area of LiAl5O8 nanorods. This method is mainly based on the structural relationship of [100] γ-LiAlO2 substrate and LiAl5O8. The orientational relationship between the nanorods and substrate is [111] LiAl5O8 // [100] LiAlO2. Because of the carbon thermal reaction, LiAlO2 substrate can react with CO gas directly to form LiAl5O8 nanorods. Gold thin film is used as the catalyst. LiAl5O8 nanorods only grow in the area with Au film coating. This gives direct evidence of the VLS growth mechanism. The diameters and lengths of the nanorods strongly depend on the coating time of Au film as well as on the flow rate of Ar carrier gas. The growth of LiAl5O8 nanorods needs to be in an environment of Li2O deficiency. Acknowledgment. This work is supported by NSC 973114-M-110-002 of Taiwan, ACORC, Center for Nanoscience & Nanotechnology of National Sun Yat-Sen University, Taiwan.
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