CRYSTAL GROWTH & DESIGN
Multilayer Quasi-Aligned Nanowire Webs of Aluminum Borate Haisheng Song, Junjie Luo, Miaodan Zhou, Elawadmihammed Elssfah, Jun Zhang, Jing Lin, Sujing Liu, Yang Huang, Xiaoxia Ding, Jianming Gao, and Chengcun Tang* Department of Physics, Central China Normal UniVersity, Wuhan, 430079, P. R. China
2007 VOL. 7, NO. 3 576-579
ReceiVed August 4, 2006; ReVised Manuscript ReceiVed December 10, 2006
ABSTRACT: Multilayer nanowire webs of aluminum borate (Al18B4O33) have been synthesized by a solid-state reaction of boron, boron oxide, and aluminum oxide at high temperature without using any template or substrate. Alumina powder on a micron scale provides an in-situ reaction container resulting in self-oriented growth of the aluminum borate nanowires. The orientation could be maintained by a quenching treatment. The addition of boron should be responsible for the generation of the web morphology. Without any further purification, a large amount of nanowire webs with high purity was obtained. Further experiments were carried out to investigate the morphological dependence on the reactant compositions. The possible growth mechanism for the nanowebs was detailed based on extensive analysis and examination. 1. Introduction Because of their unique properties and promising applications in electronics, photonics, biochemistry, etc., one-dimensional (1D) nanostructural materials have attracted great interest.1 So far, considerable research has been done to examine the synthesis and properties of nanostructures. Some nanostructures are expected to exhibit interesting properties superior to their bulk counterparts.2-4 The challenge at this stage is to organize nanostructural materials into functional nanodevices, such as nanotransistors and nanowebs. Network-like structures fabricated with nanowires, nanorods, and nanotubes as building blocks can function as both a device and an interconnection, playing a key role in the production of the next generation of nanoscale mechanical, electronic, and optoelectronic devices.5-7 Aluminum borate ceramics are of particular interest due to their high elastic modulus and tensile strength, excellent resistance to oxidation/corrosion, and high thermal conductivity, which can find applications in high-temperature structural components and nonlinear optical and tribological materials.8,9 The mechanical properties of aluminum borate are competitive with those of silicon carbide but at a price one-tenth to onetwentieth of SiC.10 However, nanowire/microtube-structured aluminum borate reported so far is always of freestanding appearance, and little attention has been paid to the preparation of ordered aluminum borate nanowires.11 No experimental results on aluminum borate has been reported in the literature with respect to the organization of nanowires into a weblike nanostructure, indicating an area of challenge. It is important to explore a facile route to the fabrication of network-like structures of aluminum borate nanowires in a large scale to expand the application field and further enhance their mechanical properties. However, because of the extremely small scale, it is rather difficult to organize nanowires, nanorods, or nanotubes into an ordered pattern by direct manipulation. Therefore, great effort has been made to investigate the vital growth process involving self-assembly, which is an economical and convenient approach to fabricate nanosructural networks. Alternative self-assembling approaches include diffusioncontrolled aggregation,12 nanoclusters,13 and template assisted growth,14 atomic adsorption along stress lines on flat surfaces,15 dealloying a chemically etched thin film,16 and surfactant* To whom correspondence should be addressed. Fax: + 86-2767861185. Tel: +86-27-67861185. E-mail:
[email protected].
induced mesoscopic organization.17 Recently, Graham et al. reported rapid self-assembly of two-dimensional networks of gallium oxide nanowires and nanotubes using hydrogen/oxygen plasma exposure of a gallium-droplet-covered substrate.18 Lieber et al. also developed a fluidic alignment technology to assemble 1D nanostructures.19 Electron beam lithography20 and scanning probe microscopy21 have also been used to prepare the nanowebs through the so-called top-down approach. For systems with a strong tendency of 1D growth, a manufacturing approach based on a self-organization phenomenon provides an attractive alternative. Aluminum borate is such a system suitable for a self-organization process under appropriate conditions, because aluminum borate crystallizes preferentially along its c-axis.22 This makes it possible to control the 1D growth by a self-assembly approach.23 In this paper, we report on the self-organization route to prepare dense nanowebs by a sequence of techniques: in-situ self-oriented reaction, boron-adjusted growth, and quenching. 2. Experimental Section Multilayer nanowire webs of aluminum borate were prepared using an Al-B-O reaction system. The starting materials used in the preparation were commercially available alumina powder (Al2O3, 99.0% purity), boron oxide powder (B2O3, 98.0% purity), and boron (B, 99.0% purity) with a molar ratio of 3:2:2, used as received. The starting materials (200 mg weight in total) were homogenized in an agate pestle and mortar and used as the initial reaction constituents. The reactants were placed in an alumina crucible located in an alumina tube, which was mounted in a traditional resistance-heating furnace. The system was first purged with a highly purity of argon for 30 min. The argon flow was then kept at 40 standard cubic centimeters per minute (sccm) when the furnace was heated to 1100 °C in 1 h. After this temperature was held for 2 h, the sample was quenched to room temperature in air, and the quenching time was usually 3-5 min. After the reaction, a white powder was obtained. The product was identified by means of X-ray diffraction (XRD, D/max-rB, Cu KR radiation) analysis. Infrared spectra (IR) were measured on a Nicolet NEXUS470 spectrophotometer. The overview of the sample morphology was checked by field emission scanning electron microscopy (SEM, JSM-6700F, JEOL). A powder sample was also ultrasonically dispersed in ethanol solution and was then transferred onto a copper grid covered with carbon film for transmission electron microscopy (TEM, JEM-2010F, JEOL) examinations.
3. Results and Discussion An SEM image of the product is shown in Figure 1a, displaying a surface made of high yield and highly dense
10.1021/cg0605281 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007
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Figure 1. Morphologies and EDS spectrum of the aluminum borate nanowire webs: (a) SEM images of the overview of the web products; (b) the cross section of the nanowebs which were constructed by multilayer quasi-arrays. (c) The magnified images showing the web consisted of the nanowires with the ordered orientation; (d) EDS spectrum taken from the web.
nanowire webs. Figure 1b is the cross section of the nanowebs, which were constructed by multilayer quasi-arrays. The magnified SEM image of the detailed morphology of web is shown in Figure 1c, indicating that the web consists of nanowires with ordered orientations. Adjacent layers with different orientations allow knitting together of the web, which is repeated to construct the whole multilayer nanoweb. Energy dispersive spectroscopy (EDS) (Figure 1d) taken from the nanowebs confirms the product element composition of Al, B, and O with a molar ratio approaching 3:1:5.9, deviating from the stoichiometric composition of Al18B4O33 and implying the existence of boron oxides in the nanowebs. An XRD pattern of the as-synthesized sample was shown in Figure 2a. All the diffraction peaks can be well indexed to the orthorhombic phase of Al18B4O33 with lattice parameters of a ) 0.768 nm, b ) 1.501 nm, c ) 0.566 nm, which are in good agreement with the standard parameters (JCPDS 32-0003). No diffraction peaks from any impurity were observed in the XRD pattern, indicating high purity and crystallinity of the final sample. Information on infrared spectroscopy of the samples would be further helpful in understanding the structure of the compounds (Figure 2b). The high-yield single-crystal Al18B4O33 nanowebs are similar to those results in the generally accepted literature.24 The vibration absorption region 1200-1460 cm-1 is due to B-O bond asymmetric stretching of BO3 units, while B-O bond stretching of BO4 units appears in the range 10001100 cm-1.25 From the spectra, the bands in the region 10001100 cm-1 are very weak. Therefore, it is indicated that there are minor BO4 units in all the samples. In addition, absorption peaks in the range of 900-810 cm-1 originate from vibrations mainly involving AlO4 units or BO3 units, bands in the region of 810-650 cm-1 are attributed to the vibrations of AlO5 units or BO3 units, and bands in the range of 600-500 cm-1 are due
Figure 2. XRD and IR patterns of the product. The peaks of the pattern can be well indexed as the orthorhombic-structured aluminum borate Al18B4O33.
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Figure 3. TEM images of the sample: (a) The image still keeps the web morphology after long time sonication; the arrow shows the nanowire taken SAED; (b) SAED pattern recorded from the direction perpendicular to the axis. The pattern can be indexed to the [2h10] zone axis of orthorhombic Al18B4O33; (c) TEM figure indicates two intersectant nanowires and the ellipse-like area is the joint point.
to the vibrations of AlO6 units or BO3 units.24 Therefore, Al18B4O33 nanowebs mainly consist of BO3, AlO4, AlO5, and AlO6 units. TEM was used to check the detail of the nanowebs. The ultrasonic time for the preparation of TEM specimens was 1 h to examine the contact degree with each other. After strong and long-time sonication, the nanowebs became smaller and thinner, but the multilayer structure and the highly aligned nanowires of the monolayer still came from the as-synthesized products (Figure 3a). At the edge of the webs, there were a few broken nanorods and particles. The broken nanorods and the breakages of the nanowires suggest the firm contact with nearby nanowires. Figure 3b is the selected area electron diffraction (SAED) pattern taken from one individual nanowire, which is shown by the arrow. The pattern can be indexed as an orthorhombic Al18B4O33 single crystal recorded from the [2h10] zone axis with the same crystalline parameters as the calculated results from XRD measurements. Combining high-resolution TEM with a defocus SAED technique, the axis direction of Al18B4O33 nanowires could be determined as [001] c-axis growth. Figure 3c is a TEM figure showing two contact nanowires, and the ellipse-like area indicates the joint point. From the joint point, the edge of the bottom nanowire (smaller diameter one) can be clearly identified, which may suggest that they contact each other physically. The images of SEM and TEM show the web morphology, but what results in the growth of the nanowebs? To understand the growth mechanism of the nanowebs, further experiments were systematically carried out. First, a duration time from 20 min to 4 h at 1100 °C was examined, while other conditions were kept the same as mentioned in the experimental section. Larger and longer nanowires were synthesized when the duration time was prolonged, but all the samples had a weblike morphology, and the time only affected the size of the nanowires. The starting materials were also tested for their effects onthe nanoweb products; the literature shows that starting
Song et al.
Figure 4. (a) The original micron scale alumina particles; (b) the cross section of one alumina particle; (c) the original fumed alumina; (d) the products when using fumed alumina as the reactant; (e) the experiment results without the addition of boron.
materials largely affect the morphology of aluminum borate.22 Experiments using fumed alumina (JCPDS 75-921) replacing the alumina particles (JCPDS 1-1308) and experiments without the addition of boron were performed, while other experimental conditions were kept the same as mentioned above. Figure 4 shows the morphologies of the starting materials and their corresponding products. Figure 4a displays the SEM image of the original alumina particles, which indicates a blocklike morphology. Figure 4b is the magnified image of one individual particle of Figure 4a, and the particle is present as a multilayer structure. Freestanding nanowires (Figure 4d) were obtained when alumina particles were replaced by fumed alumina (Figure 4c). The diameter of the fumed alumina is ∼30 nm, and the results are disordered nanowires the same as described in ref 26. Without the addition of boron after quenching, the products are quasi-aligned nanowires (Figure 4e) rather than multilayer aligned nanowire webs.27 According to previous reports on the synthesis of aluminum borate nanomaterials22,23,26,28 and the analysis of our experimental results, we believe that micron-scale alumina particles and the addition of boron play important roles in the formation of the nanowebs. We attempt to propose a formation mechanism, involving an in-situ solid liquid solid (SLS) growth mechanism discussed as follows.23,26,28 To every micron-scale alumina particle, it provides a special reaction container, and the reactions take place on the surface of the framework of the alumina particles. On the framework, two kinds of reactions took place at the same time to produce the liquid phase. The first one is that B2O3 melted and gradually reacted in situ with Al2O3 to form a Al4B2O9 liquid aluminum borate phase at temperatures between 450 and 1035 °C (eq 1).29 The other reaction is eq 2.30 Al droplets adsorbed B2O3 and Al2O3 to form a liquid phase. Both kinds of liquid droplets adhered to the framework of the alumina29 and competed to
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adsorb the reactive materials. When the liquids became supersaturated, aluminum borate nanowires grew and precipitated in the environment of the alumina framework. So the alumina particles provided an in situ reaction container resulting in a self-oriented growth process, and the two different liquid phases resulted in the nanowires depositing in different directions. The aligned nanowires in different directions intersect with each other and knit together the weblike morphology. At a higher temperature of ∼1100°C, the Al18B4O33 became more stable than Al4B2O929 and can be formed through reaction eq 3. While Al4B2O9 formed at lower temperatures, peritectic decomposition took place, and the compound decomposed into Al18B4O33 (eq 3). The chemical reactions involved are expressed below:23
2Al2O3 + B2O3 f Al4B2O9
(1)
2B + Al2O3 f 2Al + B2O3
(2)
9Al4 B2O9 f 2Al18 B4O33 + 5B2O3
(3)
Acknowledgment. This work was supported by the Fok Ying Tong Education Foundation (Grant No. 91050) and the National Natural Science Foundation of China (Grant No. 50202007).
References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
After the reaction, the sample was quenched to ambient temperature to maintain the aligned orientation, and nanowebs of the products were generated. Considering our further experimental results with our growth mechanism, they are also reasonable. When alumina particles (micron scale) are replaced by fumed alumina (nanometer scale) on which the nanowires grow freely in the open environment without the competing and excluding growth processes, the product contains freestanding nanowires (Figure 4d). Another experiment without the addition of boron showed that all the nanowires grew in the same reaction without competitive and excluding growth processes, and after the quench the products indicated quasi-aligned nanowires (Figure 4e) rather than aligned nanowire webs. It was the addition of boron that resulted in another reaction leading to nanowires being arranged in another direction.
(12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
4. Conclusions
(22)
In summary, multilayer nanowebs of aluminum borate were knitted together by quasi-arrays. The reactants microscale alumina and boron are very important for formation of the products. Further quenching treatment also maintains the alignment of the nanowires. The nanowires of the web have an ordered orientation and an accordant intersecting angle. Because of the tight contact, the nanowebs are actually independent units. We expect that our report provides an alternative to the further development of nanoscience and nanotechnology to organize nanomaterials into functional devices. For the aluminum borate nanowebs, large-scale ordered orientation and a firmly knitted arrangement will expand their application field and further enhance their mechanical properties.
(23) (24) (25) (26) (27) (28) (29) (30)
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