Novel Method for High-Yield Synthesis of Rutile SnO2 Nanorods by Oriented Aggregation Jin Q. Sun,*,† Ji S. Wang,†,‡ Xiu C. Wu,† Guo S. Zhang,† Jun Y. Wei,† Shu Q. Zhang,† H. Li,§ and Dai R. Chen‡
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1584-1587
The Laboratory of Micronano Tribology and Modern Design, Shandong UniVersity of Science and Technology, Qingdao 266510, People’s Republic of China, China Department of Chemistry, Shandong UniVersity, Jinan 250100, People’s Republic of China, and Physics Department, Ocean UniVersity of China, Qingdao 266032, People’s Republic of China ReceiVed October 29, 2005; ReVised Manuscript ReceiVed April 4, 2006
ABSTRACT: Primary SnO nanoparticles prepared via a solid-state reaction have been used to form oriented chainlike aggregation nanostructures, which can subsequently recrystallize into effective SnO2 nanorods by means of heat treatment in the presence of NaCl. NaCl is critical to not only prevent a drastic increase in the size of the precursors but also provide aqueous and kinetic conditions which are helpful for the oriented growth of the primary nanoparticles. The new method is convenient, inexpensive, and efficient for the preparation of SnO2 nanorods with fairly satisfying uniformity. The investigation of a scalable and economic method for the fabrication of one-dimensional (1D) systems has attracted much attention in materials synthesis. Especially, 1D materials have been enthusiastically pursued for their application in the study of electrical transport, of optical phenomena, and as functional units in nanocircuitry. SnO2 with a large band gap (Eg ) 3.6 eV at 300 K1) used as an important n-type semiconductor is quite attractive, thus making it ideal to work with as transparent conducting electrodes for organic light-emitting diodes and solar cells.2-5 In addition, SnO2-based nanomaterials have been extensively studied and have served as chemical sensors for environmental and industrial applications.6-9 Several methods have been employed to prepare SnO2 nanocrystals, such as hydrothermal methods,10 evaporating metal oxide powders,11 microemulsion technology,12 chemical deposition,13,14 solid-state reactions,15 and laser ablation.16 Although each method developed for the fabrication of 1D nanostructures has succeeded in achieving high-quality materials, complex process control, high reaction temperatures, or high cost has restricted its wide application in the production of SnO2 crystalline nanorods. Oriented attachment is a special kind of aggregation that provides an important route to form new single crystals, twins, and intergrowths. Penn and Banfield have shown that inorganic nanocrystals, made up of hundreds or even thousands of atoms, can be the basic building blocks for the fabrication of highly ordered extended solids under hydrothermal conditions.17 Shen and co-workers reported some results of the growth of nanosize rutile condensates by oriented attachment in solution.18,19 Moldovan and co-workers studied the grain growth process of nanocrystalline materials and proposed a new growth mechanism: grain-rotation-induced grain coalescence in colloidal systems.20 However, there have been few reports about the midterm stage of oriented aggregation during the initial nanoparticle growth, which is critical to understand the 1D nanostructure growth mechanism. Computer simulation has been found to be a powerful tool for understanding the growth * To whom correspondence should be addressed. Tel: +86 532 605 4512. Fax: +86 532 605 7987. E-mail:
[email protected]. † Shandong University of Science and Technology. ‡ Shandong University. § Ocean University of China.
mechanism of nanostructures. Hui et al. used the genetic algorithm method to study the growth behavior of magnesium nanowire. They found that magnesium nanowire has a helical multishell structure.21 By means of molecular dynamics simulations, they also studied the melting properties of metallic nanowires and found that the melting of the nanowire starts from the interior of the nanowire.22,23 All of the aforementioned results are helpful for understanding the growth mechanism of 1D nanostructured materials. In this paper, we will report a new and interesting growth mechanism of one-dimensional materials. We found that the chainlike aggregation of primary nanoparticles can be further transformed into nanorods at high temperature in NaCl, which is a very useful process. This new process reported in this paper meets the demands of industrial production satisfactorily without using any organic solvent, template, or applied electric or magnetic field.24 Experimental Section The following is a typical experimental procedure for the high-yield synthesis of SnO2 nanorods by the oriented aggregation of single SnO nanoparticles. First, fine SnO particles are synthesized by a solid-state reaction. Solid powders of SnCl2‚2H2O (2.26 g, 10 mmol, AR grade) and NaOH (0.08 g, 20 mmol, AR grade) were ground with a mortar and pestle for 15 min and mixed together with NaCl (AR grade) in a molar ratio of 1:2, respectively, and then this mixture was ground for another 30 min. Second, the precursors were oxidized and grown into SnO2 nanorods by annealing for 2 h at 400 °C. The products were washed with water (3 × 500 mL) and dried for 2 h at 60 °C. The yield of SnO2 nanorods is over 98%. XRD experiments were performed on a Rigaku (Japan) Dmax γA rotating-anode X-ray diffractometer equipped with graphite-monochromated Cu KR radition (λ ) 1.542 Å) employing a scanning rate of 0.02° s-1 in the 2θ range from 10 to 80°. Transmission electron microscopy (TEM) tests were carried out with an H-800 instrument using an accelerating voltage of 150 kV. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEOL-2010 TEM using an accelerating voltage of 200 kV.
Results and Discussion The morphology of the precursors of SnO2 nanoparticles and nanorods is shown in Figure 1a. The precursors consist of sphere-like nanocrystals. The average diameters of the precursors
10.1021/cg050574l CCC: $33.50 © 2006 American Chemical Society Published on Web 06/02/2006
Rutile SnO2 Nanorods
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Figure 1. Nanorods synthesized via oriented aggregation of single nanoparticles: (a) TEM image of the precursors of nanoparticles prepared by solid-state reactions with inorganic salts such as NaCl at ambient temperature; (b) TEM image of the oriented aggregation of single SnO2 nanoparticles into nanorods with a ratio of nanoparticles to nanorods of up to 98%; (c) magnified TEM of the oriented nanorods.
Figure 2. (a) Indexed X-ray diffraction pattern and (b) EDS analysis of the highly oriented rutile structure of SnO2 nanorods.
are about 8 nm, and the primary nanocrystals were almost monodisperse. TEM images of prepared nanorods are illustrated in Figure 1b,c. It can be seen that the sample consists of uniform nanorods with average diameters of 30 nm and lengths up to several micrometers. Figure 1b suggests that the nanorods seem to grow homocentrically and their yield is very high. The X-ray diffraction (XRD) results of the synthesized SnO2 nanorods are given in Figure 2a. All diffraction peaks can be perfectly indexed to the rutile SnO2 structure. The SnO2 lattice constants obtained by refinement of the XRD data of the nanorods are a ) 4.742 Å and c ) 3.182 Å, which are consistent with those of bulk SnO2 (JCPDS file No. 77-0450). However, the diffraction peaks are broadened because of the small particle size. An energy disperse spectroscopic (EDS) analysis of the nanorods, performed at 30 kV coupled to a field-emission scanning electron microscope (Figure 1b), as well as an individual investigation of the nanorods at 200 kV with a transmission electron microscope, showed that tin and oxygen are the only detected elements, without any sodium or chlorine contamination. To further study the fine structure of SnO2 nanorods, HRTEM combined with a fast Fourier transform (FFT) analysis technique is employed (Figure 3b,c). The clear lattice boundary in HRTEM images illustrates the high crystallinity of the nanorods. The interplanar spacings are about 0.336 and 0.263 nm, respectively. The results of an FFT (insert of Figure 3c) analysis further show
that they correspond to the (1h10) and (1h01) planes of rutile SnO2, respectively. These results also reveal that the growth plane of the nanorods is (111) and their growth direction is [112h],16 which is different from the results of previous reports.25,26 The growth of the 1D nanostructure is dependent on two factors: the surface energy determines the preferential growth surfaces,25 whereas the growth kinetics determines the final structure. The important advantages of this synthetic method are its control of precursor (crystalline) size and its transformation of nanoparticles into nanorods. We note that our synthetic procedure closely resembles a previous report except for the addition of inorganic salts (NaCl) used as a salt-assisted additive in our experiments.8 However, we have obtained remarkably different results, owing to chlorine salts playing an important role in the entire procedure. The crystal morphology and its size can be further altered with inorganic salts such as NaCl and KNO3. In self-propagating high-temperature synthesis (SHS), the particle sizes of the products from a metathesis reaction are easily controlled by the addition of an inert additive such as an alkalimetal salt (NaCl) because it can potentially assist in the reaction when it melts at high temperature and can be easily removed after the reaction.27 Salt-assisted aerosol decomposition can be used to obtain nanoparticles with adjustable size, narrow size distribution, high crystallinity, and good stoichiometry.28 On the other hand, in hydrothermal synthesis studies, nitrate salts may cause some negative effects on the growth under similar reaction conditions.29 Thus, it is thought that adding inorganic salts to the hydrothermal synthesis will reduce the overall reaction rate and broaden the distribution of product. In our experiment, NaCl is used as a salt-assisted additive to control the size of precursors in the solid-state reaction.30 It is known that the structure of products of solid-state reactions depends on the rate of nucleation and growth of the reaction products. In the process of the reaction of SnCl2‚2H2O and NaOH and to produce SnO nanocrystals, salt-assisted additives, including the byproduct NaCl, are expected to cause cage-like shells surrounding the SnO particles, preventing their growth. Simultaneously, the diffusion of solid particles at ambient temperature is often short range, thus helping to yield nanosized products. For comparison, we did a experiment without salt under the same conditions; unfortunately, only a few large crystals with the ordinary rutile structure morphology were observed. NaCl can be used as a functional group and adsorbs at the surface of particles, restricting the advance of grain boundaries during annealing.31 On the basis of the above discussion, it can be concluded that salt assistance is important for not only controlling the size of precursors in the solid-state reaction but also inhibiting crystallite growth in the oriented aggregation course.
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Figure 3. (a) Typical TEM image of 30 nm SnO2 nanorods prepared by oriented growth during annealing at 400 °C and (b, c) HRTEM images of the corresponding individual nanorods.
Acknowledgment. We acknowledge support from both the Program of Science and Technology Bureau of Qingdao (Nos. 05-1-NS-47 and 05-2-HY-46) and the National Nature Science Foundation of China (No. 50571093). Supporting Information Available: Text and a figure giving additional details on the transformation of nanoparticles into nanorods. This material is available free of charge via the Internet at http:// pubs.acs.org. Figure 4. TEM image of the typical intermediate step of the SnO2 nanorod growth process.
The growth mechanism of the nanorods can be understood on the basis of oriented aggregation by polar forces.30-36 To reveal intermediate details in the nanoparticle to nanorod transition, parts of the primary nanoparticles in the early stages of annealing were examined by TEM. A great abundance of chainlike aggregations in the sample are shown in Figure 4. The primary nanoparticles can transform into SnO2 nanorods by oriented aggregation. Jiggling of nanoparticles by the driving force may allow adjacent particles to construct the low-energy structures, represented by a coherent particle-particle interface.35 Tang et al. also reported that the intermediate step of the formation of the nanowire was found to be pearl-necklace aggregates.32 They thought that a strong dipole-dipole interaction is the driving force of nanoparticle self-organization. Wang et al. believed that the self-coiling of nanorings is likely to be driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation.33 For inorganic nanostructures that expose a charge-polarized surface, electrostatic forces can drive oriented aggregation or self-assembly, especially in environments where these forces are uncontrolled by solvents. Because the melting point for a given nanostructure can be onethird of its bulk melting point, and the required annealing temperature is usually one-third of the melting temperature, it is thus possible to chemically join the particles and finish recrystallization at 400 °C. That is, the particles can be driven to an oriented aggregation by polar forces and then the pearl-necklace aggregation can be further recrystallized to form nanorods. In summary, SnO2 nanorods have been successfully synthesized by oriented aggregation of single nanoparticles. A saltassisted additive replacing the traditional organic surfactant is a new and effective technology to control the size of precursors in the solid-state reaction under ambient conditions and to inhibit crystal growth during annealing. The intermediate step of pearlnecklace aggregation has been studied, which is helpful for understanding the growth mechanism of the nanorods. This synthesis procedure can also be extended to other types of semiconductors: Mn3O4 nanowires and nanobelts,37,38 Cr2O3 nanorods,39 and TiO2 nanobelts.40 Importantly, this method is very simple, its yields are high, and the morphology of the nanorods can be easily controlled.
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