J. Phys. Chem. B 2002, 106, 3823-3826
3823
Large-Scale Rapid Oxidation Synthesis of SnO2 Nanoribbons J. Q. Hu,† X. L. Ma, N. G. Shang, Z. Y. Xie, N. B. Wong, C. S. Lee, and S. T. Lee* Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong, SAR, P. R. China ReceiVed: July 5, 2001; In Final Form: January 8, 2002
One-dimensional nanostructures of SnO2 with a ribbonlike morphology have been prepared in large scale via rapid oxidation of elemental tin at 1080 °C. The products were characterized with scanning electron microscopy, X-ray powder diffraction, transmission electron microscopy, Raman scattering, and photoluminescence spectroscopy. The as-synthesized SnO2 nanoribbons appeared to be single crystals and had preferred [110] and [203] growth directions. The lengths of the nanoribbons were up to several hundreds of micrometers, and the typical width and thickness were in the range of 30-150 nm and 10-30 nm, respectively. The strong photoluminescence of the nanoribbons in the visible region suggested possible applications in nanoscaled optoelectronic devices. A possible growth mechanism for the SnO2 nanoribbons was proposed.
Introduction
Experimental Section
Tin dioxide, SnO2, is an n-type semiconductor with a wide band gap (Eg ) 3.6 eV, at 300 K) and well-known for its potential applications in gas sensors,1 dye-based solar cells,2 transparent conducting electrodes,3 and catalyst supports.4 In recent years, nanocrystalline SnO2 has been reported to have some different characteristics from the bulk crystal; much attention has been focused on the synthesis of SnO2 thin films or nanoparticles and exploration of their novel properties. A variety of methods, such as sol-gel,5 chemical vapor deposition (CVD),6 magnetron sputtering,7 sonochemical,8 and thermal evaporation,9 are available to prepare SnO2 thin films or nanoparticles. In contrast, there are few reports on the synthesis of SnO2 with one-dimensional (1D) nanostructures. Since the discovery of carbon nanotubes in 1991,10 the 1D nanostructures have stimulated great interest among materials scientists because of their peculiar properties and potential uses.11 Considerable efforts have been made to fabricate some important inorganic materials with 1D nanostructures (nanotubes or nanowires), for examples WS2,12 MoS2,13 CdS,14 GaN,15 BN,16 SiC,17 TiC,18 Ga2O3,19 and Si.20 These nanostructures have the geometrical characteristic of round cross section. Recently, the newly distinct 1D nanostructures with beltlike (or ribbonlike) morphology, which has a rectangular cross section, have been successfully prepared for semiconducting metal oxides by evaporating at high temperature.21 It is expected that these beltlike nanostructures may represent important building blocks for nanodevices and may offer exciting opportunities for both fundamental research and technological applications. In the present study, a simple rapid oxidation of elemental tin was proposed for large-scale production of SnO2 nanoribbons. The synthetic reaction was carried out in an alumina tube at 1080 °C for 30 min, using Fe(NO3)3 as oxidant and Ar mixed with 5% H2 as the carrier gas.
A horizontal alumina tube (outer diameter, 42 mm; length, 80 cm) with a copper coldfinger was mounted inside a hightemperature tube furnace. A mixture of elemental tin (3 g, 100 mesh, Goodfellow, 99.5%) and Fe(NO3)3 (5 g, Goodfellow, 98+%) powders was placed on an alumina wafer (40 mm × 25 mm × 1 mm). After transferring the wafer to the center of the alumina tube, the tube was evacuated by a mechanical rotary pump to a pressure of 6 × 10-2 Torr. During the experiment, a constant flow of Ar mixed with 5% H2 was maintained at a flow rate of 50 sccm, and the pump continually evacuated the system so that the pressure inside the tube was kept at 350 Torr. The temperature of the furnace was increased to 800 °C from room temperature and kept at 800 °C for 30 min and then further increased to 1080 °C for 30 min. After the furnace was cooled to room temperature, a white woollike product was formed in a high yield on the inner wall of the tube near the cooling finger. The morphologies and crystal structure of the product were characterized using scanning electron microscopy (SEM; Philips XL 30 FEG) and X-ray diffraction (XRD; Siemens D-500 with Cu KR radiation and a normal θ-2θ scan). Further structural analysis of individual nanoribbons was performed using transmission electron microscopy (TEM; Philips, CM20 and CM200 FEG, at 200 kV). Raman scattering spectra were measured with a Renishaw micro-Raman sectrometer at room temperature. The 514 nm line of an Ar+ laser was used for the excitation, and the Raman signals were measured in a backscattering geometry with a spectral resolution of 1.0 cm-1. Room-temperature photoluminescence (PL) spectra, which were excited with an argon ion laser operating at 280 nm wavelength, were recorded with a PERKIN ELMER luminescence spectrometer (LS50B).
* To whom correspondence should be addressed. E-mail: apannale@ cityu.edu.hk. † On leave from Department of Chemistry, University of Science and Technology of China, Anhui, Hefei 230026, P. R. China.
Results and Discussion A typical SEM image (Figure 1a) shows that the assynthesized products consist of a large quantity of nanometer wirelike structures. The diameters of these nanostructures ranged from 30 to 150 nm, and lengths were up to several hundreds of micrometers. The yield of the products was so high (20-30%,
10.1021/jp0125552 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002
3824 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Figure 1. SEM images of the SnO2 samples synthesized at (a) 1080 °C and (b) 1350 °C.
Figure 2. XRD pattern of the SnO2 sample as in Figure 1a.
according to the amount of tin used) that the alumina tube was clogged with the woollike materials after a growth period of 30 min. In our experiment, we also observed that the width of the nanostructure was sensitive to the reaction temperature. When the growth temperature was increased to 1350 °C, the diameter range of these nanostructures increased markedly to 50-350 nm (Figure 1b). The XRD pattern (Figure 2) reveals the overall crystal structure and phase purity of the as-synthesized products. All of the diffraction peaks can be indexed to the tetragonal rutile structure of SnO2 with lattice constants of a ) 4.740 and c ) 3.190 Å, which agrees well with the reported values (a ) 4.738 and c ) 3.187 Å) from JCPDS card (41-1445). No characteristic peaks of impurities, such as elemental Sn or other tin oxides, were observed.
Hu et al.
Figure 3. (a) TEM image of SnO2 nanoribbons; (b) TEM image of a single SnO2 nanoribbon with [110] growth direction, inset showing the SAED pattern along the [001] axis; (c) HRTEM image of the nanoribbon as in panel b, the arrow showing its growth direction; (d) TEM image of a single SnO2 nanoribbon with [203] growth direction, inset showing the SAED pattern along the [010] axis.
TEM and selected area electron diffraction (SAED) studies of the as-synthesized products provide further insight into the wirelike structures of these SnO2 materials. It is of interest that we observed very long smooth straight and curved ribbonlike morphology (Figure 3a,b,d). The lengths of the ribbons were up to several hundreds of micrometers, even to the scale of millimeter. Each ribbon has a uniform width and thickness along its entire length, and the typical widths and thickness of the ribbons are in the range of 30-160 nm and 10-30 nm, respectively. These structures, called nanoribbons, are distinct from the previously reported nanotubes and nanowires.10-20 From the TEM images, it can be been that the curved nanoribbons have many streak (or striation) contrasts on their observed faces and these are bending contours because of lattice bending in thin TEM samples. These streak lines changed in position as the samples were tilted in the TEM observation. The growth directions for the nanoribbons were determined from the SAED patterns and high-resolution TEM images of individual ribbons (in our experiment, we examined more than 10 individual ribbons). The inseting SAED pattern of the ribbon shown in Figure 3b was recorded with the electron beam along the [001] zone axis (perpendicular to the straight section of the curved ribbon). It demonstrated that this particular ribbon was a single crystal with a growth direction of [110]. An HRTEM image (Figure 3c) taken from the ribbon as in Figure 3b further confirmed that the single-crystal ribbon grew along the [110] direction (indicated with an arrow). It also showed that the ribbon was structurally uniform and no dislocations or other planar defects were observed in the examined area of this ribbon. Similar studies of the SAED pattern (Figure 3d) recorded with
Synthesis of SnO2 Nanoribbons
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3825 forms at the initial stage of oxidation of tin.26 At the processing temperature, the SnO vapor can be transported to the deposition zone by the carrying gas. As we have known, SnO is metastable and will decompose to SnO2 and Sn. Moreover, the higher the reaction temperature the faster is the rate of the phase transformation from SnO to SnO2, as shown in the following reaction:27
2SnO f SnO2 + Sn
Figure 4. Room-temperature Raman spectrum from SnO2 nanoribbons.
Figure 5. Room-temperature photoluminescence spectrum of SnO2 nanoribbons.
electron beam along the [010] zone axis exhibited the lattice constants and the growth direction for this ribbon of [203]. Figure 4 shows the room-temperature Raman spectrum of the as-synthesized SnO2 nanoribbons. The Raman shift peaks are 425, 462, 620, and 762 cm-1, which are attributed to the A2g, Eg, A1g, and B2g vibration modes of SnO2, respectively.22 These peaks further confirm that the SnO2 nanoribbons possess the characteristics of the tetragonal rutile structure.23 However, by contrast with the previously reported spectra,22,23,24 we noted that not only the positions of these peaks shifted about 10 cm-1 toward lower frequencies but also the widths of these peaks broadened. This may be because the SnO2 nanoribbons have some growth defects such as vacancies of oxygen and vacancy clusters. Figure 5 shows a room-temperature PL spectrum of the assynthesized SnO2 nanoribbons. It is clear that there are two strong peaks at 392 and 439 nm and two weak peaks at 486 and 496 nm. Earlier reports indicated that SnO thin films exhibited a broad dominant peak near 396 nm (3.13 eV).25 The newly observed peaks at 439, 486, and 496 might be due to other luminescence centers, such as nanocrystals and defects in the SnO2 nanoribbons, but that is not yet clear. It is well-known that at temperatures higher than 125 °C, Fe(NO3)3 decomposes stepwise to Fe2O3, NO2, and O2. Considering the low melting point of tin (231.9 °C), the melting of metallic tin will take place. The liquid tin could provide an energetically favored site for the absorption of oxygen or react with oxygen to yield tin oxides. As the temperature further increases, pure liquid tin has been shown to oxidize rapidly especially at 700 and 800 °C. It is generally agreed that SnO
The decomposition of SnO will result in the precipitation of SnO2 nanoparticles, which are the nuclei of the SnO2 nanoribbons. Once initial nucleation is formed, the nanoribbons growth will tend to continue by aggregation of molecular SnO2 on the inner wall of the tube near the cooling finger. This is due to the temperature gradient along the tube axis, which will provide the external driving force for the nanoribbons growth. It is noted that the nucleation and growth of the SnO2 nanoribbons always occur at the appropriate position of the inner wall of the tube. Because the position with the same temperature was normal to the axial of the tube, only the particular position with the suitable temperature will be needed for the nucleation of the SnO2 nanoribbons. In the formation of the nanoribbons, the nucleation and growth of SnO2 are attributed to the decomposition of the formed SnO. Therefore, the formation process of the SnO2 nanoribbons is somewhat similar to the oxide-assisted growth for the semiconductor (silicon or germanium) nanowires,28 except that SnO2 nanoribbons are grown instead of Sn nanowires. In both cases, volatile oxide SnO or SiO (or GeO), which is served as a source material, is transported to the deposition zone by the carrier gas and then undergoes a decomposition (disproportionation) reaction to induce the nucleation and growth of SnO2 nanoribbons or Si (or Ge) nanowires. Our current understanding on the formation process of SnO2 nanoribbons is still limited. More in-depth studying on the growth mechanism of the SnO2 nanoribbons and controlling of the reaction kinetics are clearly needed. Conclusions In summary, 1D nanostructures of SnO2 with a ribbonlike morphology have been prepared in large scale via rapid oxidation of elemental tin at 1080 °C. The as-synthesized SnO2 nanoribbons were single crystals and had preferred [110] and [203] growth directions. The strong photoluminescence of the nanoribbons in the visible region suggests possible applications in nanoscale optoelectronic devices. Although the investigation of the properties of this material is in progress, it is believed that the SnO2 nanoribbons may be used to enhance the performance of gas sensor devices. By choosing suitable synthetic parameters, such as reaction temperature and time, it is reasonable to expect that the present route can be extended to obtain other semiconducting metal oxides with this nanostructure. Acknowledgment. The authors are grateful to the City University of Hong Kong for the award of a Strategic Research Grant (Account Number 7001112). References and Notes (1) Ansari, G.; Boroojerdian, P.; Sainkar, S. R.; Karekar, R. N.; Alyer, R. C.; Kulkarni, S. K. Thin Films 1997, 295, 271. Varghese, O. K.; Malhotra, L. K. Sens. Actuators, B 1998, 53, 19. (2) Ferrere, S.; Zaban, A.; Gsegg, B. A. J. Phys. Chem. B 1997, 101, 4490.
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