synthesis of highly ordered sno2-fe2o3 composite nanowire arrays

ARTICLES 1044 Chinese Science Bulletin Vol. 50 No. 10 May 2005 Chinese Science Bulletin 2005 Vol. 50 No. 10 1044 — 1047 Synthesis of highly ordered SnO 2 /Fe 2 O 3 composite nanowire arrays by electrophoretic deposition method LI Jianjun 1 , ZHANG Xingtang 1 , CHEN Yanhui 1 , LI Yuncai 1 , HUANG Yabin 1 , DU Zuliang 1 & LI Tiejin 1,2 1. Key Lab for Special Functional Materials, Henan University, Kaifeng 475001, China; 2. Lab of Photochemistry, Jilin University, Changchun 130021, China Correspondence should be addressed to Du Zuliang (email: zld@ henu.edu.cn) Abstract Highly ordered SnO 2 /Fe 2 O 3 composite nano- wire arrays have been synthesized by electrophoretic deposi- tion method. The morphology and chemical composition of SnO 2 /Fe 2 O 3 composite nanowire arrays are characterized by SEM, TEM, EDX, XPS, and XRD. The results show that the SnO 2 /Fe 2 O 3 composite nanowires are about 180 nm in width and tens of microns in length, and they are composed of small nanoparticles of tetragonal SnO 2 and rhombohedral ɑ-Fe 2 O 3 with diameters of 10－15 nm. The SnO 2 /Fe 2 O 3 com- posite nanowires are formed by a series of chemical reac- tions. Keywords: SnO 2 /Fe 2 O 3 composite nanowire arrays, electrophoretic deposition, AAO template, sol particles. DOI: 10.1360/982004-792 Since the carbon nanotubes (CNTs) were found in 1991, the studies of one-dimensional nanomaterials, e.g. nano- tube, nanowire, especially highly ordered nanoarrays, have attracted much attention due to their particular func- tions and potential applications. One-dimensional (1D) nanostructures can be used as building blocks nanoelec- tronic devices, such as optical storage devices, single-ele- ctron transport devices, and electronic sensors [1 — 4] . Vari- ous methods have been advanced to synthesize 1D nanos- tructures, including electrochemical deposition, CVD, VLS, VS, L-L-S, Sol-Gel, and template method [5 — 8] . Sol-gel processing is a wet chemical route for the synthe- sis of 1D nanomaterials, motivated by electrostatic attrac- tion between oppositely charged sol particles and the tem- plate walls [9 — 11] . Recently, a number of groups have suc- ceeded in synthesizing nanowires using a method of com- bining sol-gel template processing with electrophoretic deposition [12,13] . In this manner, sol particles were driven to a certain electrode by the force of external electro-field and aggregated into the AAO template holes tightly, lead- ing to the formation of solid nanowire arrays. For a long time, composite materials of semi-con- ductors have been studied extensively owing to their unique optical and electric properties [14 — 17] . SnO 2 and Fe 2 O 3 are both important inorganic semiconductors and have potential applications in Li-ion batteries, gas sensors, chemical-catalyst, and magnetic storage devices [18 — 20] . Their composites also attracted great attention owing to their stable, outstanding gas-sensitive properties and po- tential application in Li-ion battery electrode [21 — 23] . Al- though some groups have synthesized and studied thin films and nanopowders of SnO 2 /Fe 2 O 3 composite [21,22] , the synthesis of SnO 2 /Fe 2 O 3 composite nanowires has not been reported to our knowledge. We have synthesized homogeneous morphology and highly ordered SnO 2 /Fe 2 O 3 composite nanowire arrays by combining sol-gel template processing with electrophoretic deposition. The chemical composition and morphology of the products were char- acterized by SEM, TEM, EDX, XRD and XPS. 1 Experimental (ⅰ) Reagent and apparatus. In this work, scanning electron microscopy (SEM, JEOL JSM-5600LV) and transmission electron microscopy (TEM, JEOL JEM- 100CX-Ⅱ and JEOL 2010 with EDX) were used to inves- tigate the morphology and chemical composition of the nanowires. Samples were sputter-coated with a thin Au layer prior to observation in the SEM. Through X-ray dif- fraction (XRD, Phillips X’ Pert Pro MPD) the phase and crystal structure were determined, and the X-ray photo- electric spectrum (XPS, KRATOS AXIS ULTR) was used to characterize the valence of the elements of the nanowires. The porous anodic aluminum oxide (AAO) template with a pore diameter of 200nm used in this work was from Whatman Co., England. All the reagents used in this experiment, including SnCl 2 . 2H 2 O, FeCl 3 . 6H 2 O, formaldehyde solution, hydrochloric acid, and dehydrated alcohol, were of analysis reagent grade. Water used in all process was tri-distilled water. (ⅱ) Synthesis of the colloid. The Fe 3+ colloid was synthesized using the method of ref. [24]. In a typical synthesis, 2.7 g FeCl 3 .6H 2 O was dissolved in 40 mL de- hydrated alcohol, with 5 mL formaldehyde added drop- wise. The solution was stirred electromagnetically for 2 h and became brown colloidal (PH value 2-3). Following ref. [19], 0.40 g SnCl 2 . 2H 2 O was dissolved in 40 mL de- hydrated alcohol, with 3 mL hydrochloric acid added dropwise. The solution became a yellowish sol after 2 h of circumfluence in water bath (80℃), and 24 h of aging at room temperature. Then, the two types of prepared colloid were mixed under magnetic stirring. A light brown stable sol was finally obtained. (ⅲ) Fabrication of nanowire arrays. Electrophoretic deposition occurred in a bi-electrodes system, with a working electrode (negative electrode) of aluminum and a ARTICLES Fig. 1. SEM images of SnO 2 /Fe 2 O 3 composite nanowire arrays. (a) Top view; (b) side view. Pt thread counter electrode. The electrodes paralleled to each other and were set approximately 2.0 cm apart. The alumina membrane with a surface covered with a thin film of gold, was fixed on the working electrode. For electro- phoretic deposition, a potential of 5 V was applied on the electrodes, and sustained for 2 h. At the end of electro- phoretic deposition, excess sol was blotted off the mem- brane with filter paper. Samples prepared in this manner were annealed at 630℃ for 6 h. By carefully wet chemi- cal etching with 1 mol/L NaOH part of the alumina mem- brane was removed before the characterization of SEM, XRD, and XPS. After all the alumina membrane had been removed, the samples were studied by TEM and EDX. 2 Results and discussion (ⅰ) Scanning electron microscope (SEM) analysis. Figure 1 shows the SEM images of the SnO 2 /Fe 2 O 3 com- posite nanowire arrays after removing part of the alumina membrane. Figs. 1 (a) and (b) are the top view and side view of the as-prepared samples. The nanowires became a little aggregate owing to lack of support from the mem- brane which had been dissolved prior to the characteriza- tion. Obviously, the obtained nanowires have smooth sur- face, unique morphology, and orderly distribution. (ⅱ ) Transmission electron microscope (TEM) an- alysis. Fig. 2 shows the TEM images of the SnO 2 / Fe 2 O 3 composite nanowires and the corresponding EDX spectrum. The nanowires are straight and long with diameters around 180 nm which are a little thinner than those of the AAO template pores because of the densifica- tion of the gel during annealing process [25] . The lengths of the nanowires are up to tens of microns, corresponding to the thickness of the AAO template. The surface of the nanowires looks smooth in large scale images, but, in fact, the nanowires are composed of lots of very small nanoparticles as shown in small scale images. Fig. 2(c) is the EDX spectrum of a single nanowire, which proves the existence of Sn, Fe, and O elements in the nanowire. The presence of the Cu peak is due to the application of copper grid during TEM observation. A very low peak of Na can also be observed in the spectrum and this is caused by the residual of NaOH used for dissolving AAO tem- plate. The EDX spectrum confirms the exclusive compo- sition of the as-prepared nanowires with Sn, Fe and O elements. (ⅲ) X-ray photoelectron spectrum (XPS) analysis. Figure 3 is the corresponding XPS spectrum of the sam- ples as shown in Fig. 1. Fig. 3(a) shows that Sn 3d has two peaks at 486.8 and 495.2 eV. According to the refer- ential spectrum, they correspond to 3d 5/2 and 3d 3/2 peaks of Sn in SnO 2 respectively. The two peaks separate clearly, which indicates that all Sn in tin oxide is Sn (Ⅳ). In Fig. 3(b), the binding energy of Fe 2p is 710.9 and 724.6 eV respectively, corresponding to the 2p 3/2 and 2p 1/2 peak of Fe in ɑ-Fe 2 O 3 . The binding energy of O 1s in Fig. 3(c) is 530.0 eV, corresponding to the binding energy of oxygen element in oxide. All the above proves that the as-pre- pared nanowires are composite of Fe 2 O 3 and SnO 2 . (ⅳ) X-ray diffraction analysis. Fig. 4 shows the XRD pattern of the sample after being annealed in air for 6 h. The peaks indexed with and without a box represent the diffraction peaks of Fe 2 O 3 and SnO 2 respectively. The broad peak between 15° and 35° is from the uncovered glass substrate for the little amount of samples. All the diffraction peaks of Fe 2 O 3 are well corresponding to the literature of rhombohedral ɑ-Fe 2 O 3 (JCPDF number 24-0072), and the three peaks of SnO 2 correspond to the diffraction peaks of (110), (200), and (301) crystal faces of tetragonal SnO 2 (JCPDF number 46-1088) respectively, suggesting that the obtained nanowires are composed of highly crystalline rhombohedral ɑ-Fe 2 O 3 and tetragonal SnO 2 . The widened diffraction peaks indicate that the SnO 2 /Fe 2 O 3 composite nanowires are composed of very small nanoparticles [26] . According to Sherrer formula, the even diameters of the nanoparticls are 10—15 nm, which is in agreement with the investigation of TEM. The mechanism of the formation of the nanowires can be explained as follows. FeCl 3 . 6H 2 O was dissolved in Chinese Science Bulletin Vol. 50 No. 10 May 2005 1045 ARTICLES Fig. 2. TEM images of SnO 2 /Fe 2 O 3 composite nanowire array (a) × 1500; (b) ×10000; (c) EDX spectrum. dehydrated alcohol followed by the addition of formalde- hyde solution, and stable Fe(OH) 3 sol was obtained after enough stirring. At the same time, Sn(OH) 2 sol was formed by 2 h of circumfluence of the alcohol solution of SnCl 2 . 2H 2 O. The colloidal particles in the sol are all typi- cal double-layer structure. Because the inner part of the colloidal particle (inside of the slip plane) is positively charged [12,27] , the colloidal particles moved to the cathode Fig. 3. XPS of SnO 2 /Fe 2 O 3 composite nanowire array in AAO template. (a) Sn 3d; (b) Fe 2p; (c) O 1s. when an external electric field was applied to the elec- trodes. They aggregated into the pores of AAO template and coagulated. While the as-prepared samples were an- nealed at 630℃ in air for 6 h, Sn(OH) 2 and Fe(OH) 3 in the gel were dehydrated and oxidized, leading to the for- mation of SnO 2 and Fe 2 O 3 respectively. The reaction equations are as follows: 2Sn(OH) 2 → 2SnO+2H 2 O↑ 1046 Chinese Science Bulletin Vol. 50 No. 10 May 2005 ARTICLES 2Fe(OH) 3 → Fe 2 O 3 +3H 2 O↑ 2SnO+O 2 → 2SnO 2 As a result, the gel was translated into SnO 2 /Fe 2 O 3 com- posite nanowires because of the confinement of the AAO template pores. The high annealing temperature caused high crystalline of tetragonal SnO 2 and rhombohedral ɑ-Fe 2 O 3 . The diameters of the prepared nanowires are a little smaller than those of the AAO template pores due to the densification reaction during sintering process [28] . Fig. 4. XRD spectrum of composite nanowire array in AAO template. The peak numbers in and out of panes represent diffraction peaks of Fe 2 O 3 and SnO 2 , respectively. 3 Conclusion Highly ordered SnO 2 /Fe 2 O 3 composite nanowire arrays have been synthesized by electrophoretic method using AAO as template. The results of characterization show that the obtained nanowires are composed of well-crystalline tetragonal SnO 2 and rhombohedral ɑ-Fe 2 O 3 . The diameters of the nanowires are around 180 nm, a little smaller than those of the pores of the AAO template, due to the densification reaction in the annealing process. SnO 2 /Fe 2 O 3 composite nanowires have better electron directional transport property than the thin films and powders of composite SnO 2 /Fe 2 O 3 , and are well ar- rayed, which makes it convenient to fabricate nanodevice with higher sensitivity. It is believed that the SnO 2 /Fe 2 O 3 composite nanowires will find extensive applications in gas sensors, etc. Acknowledgement This work was supported by the Prophase Project of “973” Plan (Grant No. 2002CCC02700) and the National Natural Science Foundation of China (Grants No. 20371015 and 90306010). References 1. Misewich, J. A., Martel, R., Avouris, P. et al., Electrically induced optical emission from a carbon nanotube FET, Science, 2003, 300: 783－786. 2. Bezryadin, A., Lau, C. N., Tinkham, M., Quantum suppression of superconductivity in ultrathin nanowires, Nature, 2000, 404: 971－ 974. 3. Collins, P. G,, Arnold, M. S., Avouris, P, Engineering carbon nanotubes and nanotube circuits using electrical breakdown, Sci- ence, 2001, 292: 706－709. 4. Collins, P. G., Bradley, K., Ishigami, M. et al., Extreme oxygen sensitivity of electronic properties of carbon nanotubes, Science, 2000, 287: 1801－1804. 5. Patzke, G. R., Krumeich, F., Nesper, R., Oxidic nanotubes and nanorods–anisotropic modules for a future nanotechnology, Angew. Chem. Int. Ed., 2002, 41: 2446－2461. 6. Murphy, C. J., Jana, N. R., Controlling the aspect ratio of inorganic nanorods and nanowires, Adv. Mater., 2002, 14: 80－82. 7. Dai, L., Patil, A., Gong, X., Aligned Nanotubes. ChemPhys. Chem., 2003, 4: 1150－1169. 8. Xia, Y., Yang, P., Sun, Y. et al., One-dementional nanostuctures: synthesis, characterization, and applications, Adv. Mater., 2003, 15: 353－398. 9. Brinda, B., Lakshmi, Peter, K. et al., Sol-Gel template synthesis of semiconductor nanostructures, Chem. Mater. 1997, 9: 857－862. 10. Lei, Y., Zhang, L. D., Meng, G. W. et al., Preparation and photo- luminescence of highly ordered TiO 2 nanowire arrays, Appl. Phys. Lett., 2001, 78: 1125－1127. 11. Zhou, Y. K., Huang, J., Li, H. L., Synthesis of highly ordered LiMnO 2 nanowire arrays (by AAO template) and their structural properties, Appl. Phys. A, 2003, 76: 53－57. 12. Limmer, S. J., Cao, G., Sol-Gol electrophoretic deposition for the growth of oxide nanorode, Adv. Mater., 2003, 15: 427－431. 13. Limmer, S. J., Cao, G., Electrophoretic growth of lead zirconate ti- tanate nanorods, Adv. Mater., 2001, 13: 1269－1272. 14. Gratzel, M., Photoelectrochemical cells, Nature, 2001, 414: 338－ 344. 15. Subramanian, V., Wolf, E., Kamat, P. V., Semiconductor-metal composite nanostructures, To what extent do metal nanoparticles improve the photocatalytic zctivity of TiO 2 films? J. Phys. Chem. B, 2001, 105: 11439－11446. 16. Wu, J. J., Wong, T. C., Yu, C. C., Growth and characterization of well-aligned nc-Si/SiO x composite nanowires, Adv. Mater., 2002, 14: 1643－1646. 17. Tan, O. K., Cao, W., Zhu, W. et al., Ethanol sensors based on nano-sized a-Fe 2 O 3 with SnO 2 , ZrO 2 , TiO 2 solid solutions, Sensors and Actuators B, 2003, 93: 396－401. 18. Kolmakov, A., Zhang, Y., Cheng, G. et al., Detection of CO and O 2 using tin oxide nanowire sensors, Adv. Mater., 2003, 15: 997－ 1000. 19. Li, M., Martin, C. R., Scrosati, B., Nanomaterial-based Li-ion bat- tery electrodes, Journal of Power Sources, 2001, 97-98: 240－243. 20. Vayssieres, L., Beermann, N., Lindquist, S. E. et al., Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: application to iron (Ⅲ) oxides, Chem. Mater., 2001, 13: 233－235. 21. Jiao, Z., Wang, S. Y., Bian, L. F. et al., Materials stability of SnO 2 /Fe 2 O 3 multilayer thin film gas sensor, Research Bulletin, 2000, 35(5): 741－745. 22. Reddy, C. V. G., Cao, W., Tan, O. K. et al., Preparation of Fe 2 O 3(0.9) -SnO 2(0.1) by hydrazine method: application as an alcohol sensor, Sensors and Actuators B, 2002, 81: 170－175. 23. Matsumuraa, T., Sonoyamaa, N., Kannoa, R. et al., Lithiation mechanism of new electrode material for lithium ion cells—the a-Fe 2 O 3 –SnO 2 binary system, Solid State Ionics, 2003, 158: 253－ 260. 24. Liang, Z., Zheng, S., Guo, S., Gas-sensing transmissive optical character of Fe 2 O 3 :SnO 2 thin film, Laser Journal, 1997, 18 (3): 23 －27. 25. Xiao, Z. L., Catherine, Y., Fabrication of alumina nanotubes and nanowires by etching porous alumina membranes, Nano Lett., 2002, 2: 1293－1297. 26. Cong, Q., Two dimensional X-ray diffraction of polycrystalline (in Chinese), Beijing: Science Press, 1997: 220－232. 27. Yang, H., Lu, W., Applied Electrochemistry (in Chinese), Beijing: Science Press, 2001: 10－17. 28. Limmer, S. J., Chou, T. P. Cao, G. Z., A study on the growth of TiO 2 nanorods using sol electrophoresis, Journal of Material Sci- ence, 2004, 39: 895－901. (Received December 22, 2004) Chinese Science Bulletin Vol. 50 No. 10 May 2005 1047 . photo- luminescence of highly ordered TiO 2 nanowire arrays, Appl. Phys. Lett., 2001, 78: 1125－1127. 11. Zhou, Y. K., Huang, J., Li, H. L., Synthesis of highly ordered. Bulletin 2005 Vol. 50 No. 10 1044 — 1047 Synthesis of highly ordered SnO 2 /Fe 2 O 3 composite nanowire arrays by electrophoretic deposition method