Please cite this article in press as: P.G. Li, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.10.130 ARTICLE IN PRESS G Model JALCOM-18671; No. of Pages4 Journal of Alloys and Compounds xxx (2008) xxx–xxx Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Facile route to straight SnO 2 nanowires and their optical properties P.G. Li a,∗ ,M.Lei a , W.H. Tang a ,X.Guo a ,X.Wang b a Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018, China b Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China article info Article history: Received 19 September 2008 Received in revised form 8 October 2008 Accepted 15 October 2008 Available online xxx Keywords: Nanostructured materials Gas–solid reactions Transmission electron microscopy Optical property abstract Rutile SnO 2 nanowires were fabricated by a simple chemical vapor method using as-synthesized SnO 2 nanoparticles as starting material. These nanowires with unusual [12-1] growth direction are very straight and uniform in diameter and length. Self-catalytic vapor–liquid–solid (VLS) mechanism should be responsible for the growth of the nanowires. The photoluminescence (PL) spectrum exhibits a wide yellow emission centered at 576 nm with a relatively small orange emission at 629 nm. The Raman spectrum exhibits four additional modes that are not allowed by rutile-type structure in first-order Raman-scattering at the zone center. The possible reasons for the unusual PL and Raman spectrum are proposed. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In the past decade, one-dimensional nanostructures, such as nanotubes, nanowires and nanobelts, have attracted consider- able attentions because of their peculiar structure characteristics and excellent physical properties [1–3]. As an important n- type semiconductor with wide bandgap (E g = 3.6 eV at 300 K), SnO 2 have been widely used for transparent conductors, gas sensor solar cells, lithium-ion batteries, and electronic devices [4–10].Recently,one-dimensional nanowire structure has attracted increasing attentions, owing to its enhanced surface to volume ratio and promising applications for gas sensors [11,12] and electronic nanodevices [13] Up to now, various methods includ- ing direct-oxidized growth [14–16], molten-salt synthesis [17,18], hydrothermal method [19,20], laser-ablation synthesis [21], car- bothermal reduction [22,23], and template method [24], etc. have been developed to fabricate SnO 2 nanowires. However, the as- synthesized nanowires easily bend and length and diameter is not uniform, which limits their promising applications. So, fabri- cation of straight SnO 2 nanowires with uniform size and smooth surface is still a challenge up to now. In this work, we devel- oped a novel chemical vapor method to synthesize large-scale SnO 2 nanowires with uniform size using SnO 2 nanoparticles as starting materials. The structure property and growth mecha- nism were investigated in detail. In addition, some interesting ∗ Corresponding author. Tel.: +86 571 86843468; fax: +86 571 86843222. E-mail address: peigangiphy@yahoo.com.cn (P.G. Li). optical features of the SnO 2 nanowires were presented in the paper. 2. Experimental The starting material is SnO 2 nanoparticles synthesized by a hydrothermal method reported by literature [25]. In the experiment, an alumina boat contain- ing 5 g SnO 2 nanoparticles was loaded into the center of a horizontal alumina tube and 10 mm × 10 mm-sized 6H-SiC substrate for growth of SnO 2 nanowires was placed on the downstream end of the alumina tube. Direct thermal evaporation of SnO 2 nanoparticles was performed at 1550 ◦ C for 90 min with an Ar flow rate of 300 SCCM. Finally, the furnace was cooled to room-temperature and white products were deposited on 6H-SiC substrate. Powder X-ray diffraction (XRD) of the product was characterized by PaNalytical X’Pert Pro MPD X-ray diffractometer with Cu K␣ radiation. The morphology of the as-synthesized product was examined by field-emission scanning electron micro- scope (FEI XL30 S-FEG). The transition electronic microscopy (TEM) images and high-resolution TEM (HRTEM) of samples were collected on the JEOL 2010F trans- mission electron microscope. The X-ray photoelectron spectra (XPS) are recorded on a VGESCALAB MKII X-ray photoelectron spectrometer, using nonmonochroma- tized Mg K␣ X-ray as the excitation source. Raman measurement was performed on a multichannel modular triple Raman system (JY-T64000) using a 532 nm laser as excitation source. Photoluminescence (PL) spectrum of the nanowires was collected at RT using the 325 nm line of a He–Cd laser as the excitation source. 3. Results and discussion Fig. 1a shows the SEM image of the SnO 2 nanoparticles syn- thesized by a hydrothermal method. The average size of these nanoparticles is about 5 nm, indicating the SnO 2 nanoparticles can be decomposed at relatively low temperature comparing with micropowders. SEM images of the product deposited on 6H-SiC substrate are shown Fig. 1b and c. Fig. 1b shows that straight 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.130 Please cite this article in press as: P.G. Li, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.10.130 ARTICLE IN PRESS G Model JALCOM-18671; No. of Pages4 2 P.G. Li et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx Fig. 1. (a) SEM image of the SnO 2 nanoparticles synthesized by hydrothermal method. (b and c) SEM images of the SnO 2 nanowires. (d) EDS analysis of the SnO 2 nanowires. nanowires with high density are distributed over the entire sur- face of the substrate. SEM image with higher magnification (Fig. 1c) clearly indicates that these nanowires are of uniform size and smooth surface, and the average diameter and length of these nanowires are 80 nm and 5 m, respectively. The energy disper- sive X-ray spectroscopy (EDS) spectrum (Fig. 1d) indicates that the sample only consists of Sn and O element. The average O/Sn ratio is 1.92:1, close to the chemical composition of SnO 2 . Fig. 2 shows a typical XRD pattern of the nanowires deposited on 6H-SiC sub- strate. All the diffraction peaks can be well indexed as rutile SnO 2 (ICDD-PDF No. 41-1445). The diffraction peaks are sharp and no other impure peaks are detected, confirming the good crystallinity of the nanowires. The composition of the Product can be further determined by XPS spectra (Fig. 3). The binding energy centered at 530.75, 486.88, 495.38 eV for O1s, Sn3d 5/2 and Sn3d 3/2 , respec- tively, are in good agreement with the value of the bulk SnO 2 . Quantification of the Sn3d and O1s peaks gives an average Sn/O atomic ratio of 1:1.95, indicating the O-deficient formation of the SnO 2 nanowires. Fig. 2. XRD pattern of the SnO 2 nanowires. Fig. 3. XPS spectra of the obtaine d SnO 2 nanowires: (a) O1s region, (b) Sn3d region. Please cite this article in press as: P.G. Li, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.10.130 ARTICLE IN PRESS G Model JALCOM-18671; No. of Pages4 P.G. Li et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx 3 Fig. 4. (a) TEM image of the SnO 2 nanowires. (b) TEM image of a single nanowire. The corresponding FFT pattern is shown in the inset. (c) TEM-based EDS spectrum of the single nanowire. (d) HRTEM image of the nanowire. TEM image (Fig. 4a) clearly indicates that nanowires are of smooth surface and rather uniform size along the growth direction. Fig. 4b shows a typical nanowire with a clear surface. TEM- based EDS analysis of the nanowire (Fig. 4c) confirms that the nanowire mainly consists of Sn and O element, and average Sn/O atomic ratio of 1:1.94, which exhibits the O-deficient condition of the nanowire. The corresponding FFT pattern and HRTEM image clearly show that the nanowire is single crystalline and grows along [12-1] direction, which is different from common [1 0 1] and [1 1 0] growth direction [26,27]. No obvious defects and dis- locations are observed, and the interplanar space is 0.356 nm and 0.236 nm, which corresponds to the (101) and (200) plane of the rutile crystalline SnO 2 (Fig. 4d), further confirming rutile structure of the nanowire. Based on the experimental results, con- ventional vapor–liquid–solid (VLS) cannot dominate the growth of the nanowires due to no metal catalyst such as tin particle attached on the tip of nanowire. Vapor–solid (VS) mechanism also cannot explain the growth process because SnO 2 powder unavoid- ably decomposes at high temperature. We deduce that the growth of the nanowires follows a self-catalytic VLS process. First, SnO 2 nanoparticles decomposed into Sn and SnO vapor. Sn and SnO vapor subsequently are transported to low temperature zone and form liquid droplets. The liquid droplets gradually absorb oxygen and are further oxidized into SnO 2 droplets. The enhanced absorption and diffusion of tin oxides occurred at SnO 2 liquid tip will finally form SnO 2 nanowires. A typical room-temperature photoluminescence (PL) spectrum is shown in Fig. 5. The spectrum is dominated by a strong yellow emission centered at 576 nm with a small orange emission shoul- der at 629 nm. Near band edge (NBE) emission (centered at around 320 nm) is not detected, which is ascribed to strong surface effects due to the larger surface-aspect ratio and the more surface defects [28]. In this work, the deep-level (DL) emission such as yellow and orange emission may be induced by bulk defects such as oxygen vacancy (V o ), and tin interstitial (Sn i ), etc. It is interesting to observe that the orange emission disappears after annealing in air at 850 ◦ C for 3 h, whereas no change happens as annealing at Ar and N 2 atmo- sphere. These results indicate that the yellow emission is related to the V o , whereas orange emission originates from Sn i [29,30]. Due to the synthesis process is in the O-deficient condition, V o is unavoid- able exist. As a native defect of the n-type SnO 2 , The V o cannot be eliminated during the above annealing process. Nevertheless, Sn i can be oxidized and thus removed by annealing in air or oxy- gen atmosphere. So, orange emission is not commonly detected in Fig. 5. Room-temperature photoluminescence spectrum of the SnO 2 nanowires. Please cite this article in press as: P.G. Li, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.10.130 ARTICLE IN PRESS G Model JALCOM-18671; No. of Pages4 4 P.G. Li et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx Fig. 6. Room-temperature Raman scattering spectrum of the SnO 2 nanowires. the O-rich synthesis condition [28,31,32]. However, details on these bulk defects such as their distributing forms need to be further investigated. Fig. 6 shows the R aman spectra of the as-synthesized SnO 2 nanowires. Rutile SnO 2 belongs to the point group D 14 4h and space group P4 n /mnm. According to the group theory, the active Raman modes B 1g ,E g ,A 1g , and B 2g can be observed in first-order spectrum. A 1g and B 2g modes vibrate in the plane perpendicular to the c-axis while the E g mode vibrates in the direction of the c-axis [33].As shown in Fig. 5, three active Raman scattering peaks at 482.3, 638.3 and 779.1cm −1 can be assigned to E g ,A 1g and B 2g mode, respec- tively, in good agreement with those of rutile SnO 2 single crystal [34]. Nevertheless, the four additional modes are also observed, all of which are not allowed by rutile-type structure in first-order Raman-scattering at the zone center. Among them, the mode peak at 699.7 cm −1 reported in previous article [27] is caused by the finite-size effects of SnO 2 . The other additional peaks at 548.7, 591.1 and 733.8 cm −1 have not been reported up to now. We deduce that these additional modes may be in line with the defect-induced phonon modes due to the surface disorder and large amount of O vacancies and Sn interstitial. However, the exact mechanism needs to be further investigated. 4. Conclusions Large-scale rutile SnO 2 nanowires were fabricated by a simple chemical vapor method using SnO 2 nanoparticles as starting mate- rials. These straight nanowires are uniform in diameter and length, and grow along the unusual [12-1] direction. The PL spectrum exhibits a wide yellow emission centered at 576 nm with a small orange emission shoulder at 629 nm. The yellow emission is related to the oxygen vacancies (V o ), whereas orange emission originates from tin interstitial (Sn i ). The Raman spectrum presents some new features of the nanowires. The additional mode at 699.7 cm −1 is attributed to finite-size effects of SnO 2 . The peaks at 548.7, 591.1, and 733.8 cm −1 , respectively, were related to the defect-induced phonon modes due to the surface disorder and large amount of O vacancies and Sn interstitial. 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