J. Phys. Chem. C 2010, 114, 5867–5870
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Flame Synthesis of Tin Oxide Nanorods: A Continuous and Scalable Approach Jie Liu,† Feng Gu,†,‡ Yanjie Hu,† and Chunzhong Li*,† Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science & Technology, Shanghai, 200237, China, and Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians UniVersity (LMU), Munich 80799, Germany ReceiVed: December 8, 2009; ReVised Manuscript ReceiVed: February 27, 2010
Well-crystalline SnO2 nanorods were first synthesized via a continuous and scalable iron-assisted flame approach with production rate up to 50 g/h in laboratory-scale. The as-prepared SnO2 nanorods with uniform length up to 200 nm and diameter around 20 nm are smooth and single crystal rutile structures, growing along the [001] direction. Iron dopant is incorporated into the SnO2 lattice and selectively effects a specific SnO2 crystal plane, promoting the further crystal oriented growth into nanorods. Meanwhile, the photoluminescence (PL) spectrum of such SnO2 nanorods exhibits a broad, stronger orange-emission peak around 620 nm, suggesting potential applications in optoelectronics. It is noteworthy that this dopant-assisted flame approach provides a new strategy for sequentially engineering one-dimensional nanomaterials. The large-scale and low-cost approach for preparing nanostructures with different shapes is believed to be the prime topic in nanoscience and nanotechnology.1-5 Flame synthesis can amount to millions of tons annually with the advantages of scalable, continuous, without post-treatment and low cost becoming an established way to routinely produce a variety of commodities like SiO2, Al2O3, and TiO2.6,7 However, flamemade metal oxides are always spherical primary particles and chainlike agglomerates instead of nanostructures with different shapes, particularly, one-dimensional semiconductor nanostructures with unique electronic and optical properties for nanodevice applications. This point has promoted intensive research toward controlled synthesis of one-dimensional nanostructures via flame approach for large-scale applications in the future.8-10 Pratsinis et al.11 prepared ZnO nanorods via a flame spray pyrolysis by introducing indium and tin dopants induce preferential growth. Bakrania et al.12 prepared SnO2 nanorods via the flame approach by increasing the flame residence time. However, the yield was low and the morphology could not be well controlled. Therefore, it is still a challenge, via flame approach, to realize morphology-controlled synthesis of onedimensional nanostructures for further promoting their applications.13 During the past 15 years, our group adopted and investigated the flame approach systematically for synthesizing nanomaterials. A series of nanostructures with different morphologies have been obtained (e.g., TiO2 solid spheres,14 A12O3 hollow spheres,15 TiO2 ball-in-shell structures,16 and carbon tubes17). In this paper, we first report one rapid, continuous, and scalable synthesis of single-crystalline SnO2 nanorods via flame approach by introducing Fe dopant and adjusting flame residence time (equal to the flame height, FH) to facilitate the anisotropic growth of SnO2. The formation of these structures in the gas phase makes flame aerosol synthesis an appealing process for continuous and scalable synthesis of one-dimensional nanomaterials. * To whom correspondence should be addressed. Phone: +86 21 64250949. Fax: +86 21 64250624. E-mail:
[email protected]. † East China University of Science & Technology. ‡ Ludwig-Maximilians University.
Figure 1. (a) TEM image of 2.5 atom % Fe-doped SnO2 nanorods, and (b-d) EDS analysis, HRTEM image, and corresponding SAED pattern taken from the white box in panel a, showing the preferred [001] orientation.
SnO2 nanorods were prepared by feeding the volatile precursor vapor into a hydrogen-oxygen coflow diffusion flame. Dopant species (iron chloride anhydrous, lithium chloride, zinc chloride), concentrations (between 0 and 2.5 atom % with respect to the Sn metal), and the flame residence time (15, 30, and 50 cm) were systematically studied by fixing other flame parameters. The detailed flame experimental setup and parameters were shown in the Supporting Information. From the TEM, HRTEM, and SAED results shown in Figure 1, single crystalline SnO2 nanorods with a diameter of around 20 nm and length up to 200 nm can be obtained. The adjacent interplanar spacing perpendicular to the preferential growth direction is 0.239 nm, which corresponds to (200) planes. So, the clear lattice fringes confirm that the nanorod is single crystalline and that the preferential growth direction is [001].18 The EDS result reveals the presence of Sn, Fe, and O in the samples. The peaks of Cu and C may come from the TEM mesh.19 Figure 2a shows the XRD patterns of Fe-doped SnO2 samples (0-2.5 atom % Fe). All the diffraction peaks could be
10.1021/jp911628r 2010 American Chemical Society Published on Web 03/11/2010
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Liu et al.
Figure 3. Nanoparticles and nanorods images: (a) pure SnO2 with FH 50 cm, (b-e) 1.0-2.5 atom % Fe-doped SnO2 with FH 50 cm, showing that the morphologies of the Fe-doped SnO2 clearly change from cuboid to nanorod as Fe dopant concentration increases, (f, g) 2.5 atom % Fe-doped SnO2 with FH 30 and 15 cm, and (h, i) 2.5 atom % Li, Zndoped SnO2 with FH 50 cm.
Figure 2. (a) X-ray diffraction patterns for 0.0-2.5 atom % Fe-doped SnO2. Crystal plane assignments indicated for each diffraction signal and the SnO2 characteristic peaks from JCPDS 41-1445 are shown in the upper and lower portion of the frame, respectively. Inset in panel a shows SnO2 lattice and the two growth planes (101) and (200). (b) Average crystal sizes for (101) and (200) planes as a function of Fe dopant concentration. (c) Relationship between the XRD lattice aspect ratio and Fe doping concentration; the XRD lattice aspect ratio can be established by (size(200)/size(101)).
indexed to rutile SnO2 (JCPDS No. 41-1445).20 In comparison with pure SnO2 samples, the lattice parameters of doped SnO2, calculated according to the equation 1/d2 ) (h2 + k2)/a2 + l2/ c2, based on the (110) and (101) crystal planes, are smaller (for pure SnO2: a ) 4.739 Å, c ) 3.186 Å; for doped SnO2(2.5 atom % Fe): a ) 4.735 Å and c ) 3.181 Å), which can be attributed to the smaller ionic radii of Fe3+ (0.64 Å) (Sn4+ (0.69 Å)), indicating Fe is incorporated into the SnO2 lattice.21,22 Interestingly, the experimental results showed the Fe doping and flame residence time can influence the morphology (e.g., particle size, aspect ratio) of the SnO2 samples. With increasing Fe doping concentration from 0.0 to 2.5 atom %, the crystallite sizes would be varied from 22.9 to 28.3 nm when taking the (101) plane into account, shown in Figure 2b. Previously, Adhikari et al.21 reported similar results and they prepared Fedoped SnO2 by a chemical coprecipitation method and the crystallite size would increase with Fe doping concentration below 7.5%. Due to the tetragonal structure of rutile SnO2, the crystal aspect ratio can also be estimated from the ratio of particle sizes taking (200) and (101) planes.11 Figure 2c gives the dependence of doping concentration of Fe on the aspect ratio of SnO2 rods. The aspect ratio increased from 1 to 1.3 with increasing Fe concentration from 0.0 to 2.5 atom %, which indicates Fe doping can induce the preferential growth of the SnO2 nanorods along the [001] direction. Figure 3a-e showed the influence of Fe dopant concentration on the morphology of SnO2. When the Fe doping concentration was 1.0 atom %, only a small portion of nanorods formed. When the Fe concentration increased to 1.5 atom %, more nanorods with elongated
Figure 4. (a) Bright-field STEM image of a single 2.5 atom % Fedoped SnO2 nanorod, (b, c) the corresponding elemental mapping of Sn (b) and Fe (c), and (d) line-scanning analysis across the nanorod indicated by the line as shown in panel a.
morphology were formed. When the Fe concentration further increased to 2.5 atom %, the sample was mainly composed of well-defined nanorods with diameters of 20-40 nm and length up to 200 nm (the yield is estimated to be above 90% from TEM result). For the flame height shown in Figure 3e-g, when the height was low (15 cm), only nanoparticles of about 20 nm formed. When the flame height was further increased to 30 and 50 cm, nanoparticles disappear gradually while nanorods formed. The increase of flame height would prolong the residence time of the reagents in the high-temperature zone, which would benefit nanorod formation. To investigate the distribution of Fe in the doped samples, STEM elemental mapping and line-scanning measurements were performed. Figure 4 shows the STEM image of a single 2.5 atom % Fe-doped SnO2 nanorod (Figure 4a), and the corresponding elemental mapping (Figure 4b,c), revealing the homogeneous distribution of Fe. The uniform distribution of Fe into the SnO2 lattice was also confirmed by line-scanning analysis (Figure 4d).
Flame Synthesis of Tin Oxide Nanorods
Figure 5. Room temperature photoluminescence spectrum of (a) Fedoped SnO2 nanorods (2.5 atom % Fe, FH ) 50 cm) and (b) Fe-doped SnO2 nanoparticles (2.5 atom % Fe, FH ) 15 cm). The excitation wavelength is 325 nm.
In the flame environment, the formation of SnO2 nanorods is different from those of SiO2 and TiO2 nanocrystallites owing to the low melting point (1625 °C) of SnO2. In the absence of Fe, the SnCl4 precursor reacted to form SnO vapor, which coagulated and oxidized to stable SnO2 vapor in the initial part of the flame. Subsequently, the SnO2 vapor directly condensed to SnO2 nanoparticles and the particle sizes mainly depended on the precursor concentration and the residence time, not strongly on temperature.23,24 However, the particle formation would be influenced after introducing Fe into the system. In the initial flame stage, FeCl3 and SnCl4 reacted to form gasphase Fe2O3 and SnO2 monomers. For the similar melting point of Fe2O3 and SnO2, the two types of monomers will coagulate simultaneously and form amorphous particles of both metal oxides.25 During the first stage, the presence of Fe2O3 monomer would accelerate coagulation and growth of SnO2, which is equal to increasing the concentration of SnO2. So the primary particle sizes increased as the Fe doping concentration increased. In the late flame stage, the amorphous mixture crystallized into a single rutile SnO2 phase, and Fe is incorporated into its lattice for low doping concentration and high temperature.26 To reduce the system energy, Sn4+ ions in the (101) plane with higher surface energy are more likely substituted by Fe3+, which can disrupt the crystal growth in the (101) plane, so the [001] direction is the favored growth direction. The final SnO2 nanorods would be formed by further crystal oriented growth by particle annealing and rearrangement during the late flame process (enhanced by maintaining a prolonged high temperature and increasing Fe doping concentration). To better understand the effect of Fe doping on the SnO2 nanorod formation, some other kinds of ions (e.g., Zn2+, Li+) are chosen in this study shown in Figure 3h,i. Compared to SnO2 nanorods derived from Fe doping, only SnO2 nanoparticles would be obtained, which can be attributed to the higher valency of Fe. This conclusion agreed well with that reported by Pratsinis et al.11 They prepared ZnO nanorods with Sn and In dopants by flame pyrolysis synthesis. Figure 5 shows the photoluminescence (PL) spectra of the SnO2 nanorods. A broad, strong orange-emission peak located around 620 nm was observed. Previous paper reported a similar orange emission from SnO2 nanobelts synthesized by thermal evaporation.27 Herein, this strong PL peak might be related to the crystalline defects produced during the rapid flame process, which could generate oxygen vacancies for the partially incomplete oxidation and crystallization.28,29 Additionally, modi-
J. Phys. Chem. C, Vol. 114, No. 13, 2010 5869 fication of the SnO2 by introduction Fe leads to the substitution of Sn4+ ions by Fe3+ and generates oxygen vacancies, and the relative intensity of this peak increased with increasing Fe content (the detailed influence of Fe content on the photoluminescence of SnO2 is discussed in the Supporting Information).30,31 The high density of oxygen vacancies interacted with interfacial tin would lead to the formation of a considerable amount of trapped states within the bandgap, resulting in the enhanced photoluminescence. Therefore, the PL intensity was increased remarkably when increasing the flame height and Fe content due to the more defects formed into the host. A continuous and scalable flame synthesis approach has been successfully developed for the preparation of SnO2 nanorods, probably for the first time. Structural and morphological characterizations have shown that 2.5 atom % Fe-doped SnO2 nanorods with uniform length up to 200 nm and diameter around 20 nm are smooth, straight, and single crystal rutile structures, growing along the [001] direction. Interestingly, increasing the Fe doping concentration and flame resident time would benefit for nanorod formation, which is attributed to that iron dopant is incorporated into the SnO2 lattice and selectively disrupts a specific SnO2 crystal plane, promoting the further crystal oriented growth into nanorods. In contrast, Li and Zn, with lower valency than Fe, have no effect on the SnO2 texture. Furthermore, the photoluminescence (PL) spectrum of such SnO2 nanorod exhibits a broad, stronger orange-emission peak around 620 nm, which is attributed to the crystalline defects and rodlike morphology, suggesting potential applications in optoelectronics. More importantly, this dopant-assisted flame synthesis method provides a new pathway for continuous and scalable design and fabrication of other 1D metal oxides. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20925621, 20706015, 20906027), the Program of Shanghai Subject Chief Scientist (08XD1401500), the Shanghai Shuguang Scholars Tracking Program (08GG09), the Special Projects for Key Laboratories in Shanghai (09DZ2202000), the Special Projects for Nanotechnology of Shanghai (0852 nm02000, 0952 nm02100), and Alexander von Humboldt Foundation. Supporting Information Available: Detailed Experimental Section and a graph showing the influence of Fe content on the photoluminescence of SnO2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cheng, B.; Russell, J. M.; Shi, W. S.; Zhang, L.; Samulski, E. T. J. Am. Chem. Soc 2004, 126, 5972. (2) Wei, G. D.; Qin, W. P.; Han, W.; Yang, W. Y.; Gao, F. M.; Jing, G. Z.; Kim, R. J.; Zhang, D. S.; Zheng, K. Z.; Wang, L. L.; Liu, L. J. Phys. Chem. C 2009, 113, 19432. (3) Du, N.; Zhang, H.; Chen, B. d.; Ma, X. Y.; Yang, D. R. Chem. Commun. 2008, 3028. (4) Hu, J. Q.; Ma, X. L.; Shang, N. G.; Xie, Z. Y.; Wong, N. B.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2002, 106, 3823. (5) Athanassiou, E. K.; Grass, R. N.; Stark, W. J. Nanotechnology 2006, 17, 1668. (6) Paul, R. P. Combust. Inst. 2007, 31, 1773–1788. (7) Rosner, D. E. Ind. Eng. Chem. Res. 2005, 44, 6045. (8) Strobel, R.; Pratsinis, S. E. J. Mater. Chem. 2007, 17, 4743. (9) Sahma, T.; Ma¨dler, L.; Gurlo, A.; Barsan, N.; Pratsinis, S. E.; Weimar, U. Sens. Actuators. B 2004, 98, 148. (10) Bao, Q. L.; Zhang, J.; Pan, C. X.; Li, J.; Li, C. M.; Zang, J. F.; Tang, D. Y. J. Phys. Chem. C 2007, 111, 10347. (11) Height, M. J.; Ma¨dler, L.; Pratsinis, S. E. Chem. Mater. 2006, 18, 572. (12) Bakrania, S. D.; Perez, C.; Wooldridge, M. S. P. Combust. Inst. 2007, 31, 1797.
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