CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2254–2257
Communications Epitaxy of Single-Crystalline Zigzag Tin Dioxide Nanobelts Zhen Fang,†,‡ Kaibin Tang,*,† Jianmin Shen,† Guozhen Shen,§ and Qing Yang† Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, 230026, People’s Republic of China, College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu, 241000, People’s Republic of China, and AdVanced Materials Laboratory National Institute for Materials Science (NIMS) Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ReceiVed NoVember 2, 2006; ReVised Manuscript ReceiVed August 7, 2007
ABSTRACT: Ultra-long (several millimeters) single-crystalline zigzag tin dioxide nanobelts growing along the [101] crystal direction were prepared by a chemical vapor transport epitaxy (CVTE) method at 850 °C. The product was characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and an X-ray energy-dispersive spectrometer (EDS). Studies indicate that the zigzag nanobelts are single tetragonal SnO2 crystals with a width of hundreds of nanometers. The optical property was also investigated by the photoluminescence (PL) spectra. The possible formation mechanism of these ultra-long zigzag-like nanobelts was proposed on the basis of experiments. One-dimensional nanometer-sized semiconductor materials, such as nanobelts, nanowires, and nanorods, have attracted considerable attention because of their novel properties and potential applications in numerous areas, such as nanoscale electronics and photonics.1–3 Recently, large efforts have been devoted to investigate semiconducting oxide nanomaterials, such as ZnO, In2O3, and SnO2.4–6 Among them, SnO2 is attracting more and more attention. As a n-type semiconductor with a large bandgap (Eg ) 3.6 eV), SnO2 has been extensively used as optoelectronic devices, gas sensors for detecting leakages, transparent conducting electrodes, and catalyst supports.7–11 For instance, the sensor assembled by individual SnO2 nanobelts is sensitive to a parts per million (ppm) level of NO2 at room temperature under UV light.12 In addition, field-effect transistors made of SnO2 nanowires exhibit excellent n-type transistor characteristics with threshold voltages ∼ -50 V and on/off ratios of ∼103 at room temperature.13 The morphologies of inorganic nanomaterials have important influences on their various electrical and optical properties; therefore, the synthesis of nanomaterials with different morphologies is crucial. Up to now, well-defined SnO2 nanostructures with an abundant variety of shapes have been achieved through chemical or physical methods. For example, SnO2 nanoribbons, nanodiskettes, nanowires, and nanotubes have been synthesized by evaporation of SnO or SnO2 powder at high temperature, and SnO2 nanotubes with square-like cross-sections have been synthesized using a combustion chemical vapor deposition method.14,15 Furthermore, SnO2 single-crystalline nanorods and polycrystalline nanowires were obtained through the solution-phase route.16,17 Other synthetical * To whom correspondence should be addressed. Telephone: +86-5513601791. E-mail:
[email protected]. † University of Science and Technology of China. ‡ Anhui Normal University. § National Institute for Materials Science.
routes, such as laser ablation18 and the template-based method,19 were also reported. Recently, SnO2 nanobelts with interesting zigzag shapes have been reported by Duan and Huang.20,21 In their experiment, Sn or SnO is used as the tin source in a vapor process. However, the requirement for special designed reactant vapor confinements makes the procedure quite complicated. Thus, it is still a challenge to find simple synthetic methodology for the preparation of zigzag SnO2 nanobelts. Herein, we report a simple chemical vapor transport epitaxy route to synthesize ultra-long zigzag-like tin dioxide nanobelts on a copper substrate. In this process, commercial copper foil is used instead of the conventional gold-coated silicon substrate, which greatly lowers the synthetic cost and may be used as an efficient substrate for the preparation of other metal oxide nanostructures. The experimental setup used for the synthesis of SnO2 nanostructures consists of a horizontal cylindrical furnace, a quartz tube, a gas-supply system, and a temperature-control system. In a typical process, analytical-grade tin powder (3 g) was placed in an alumina boat, which was covered with a copper foil 5 mm above. The boat was placed in the center of the quartz tube. After that, the furnace was heated at the rate of 10 °C/min to 850 °C and kept at that temperature for 2 h with pure argon at a flow rate of about 200 sccm. After the reaction, the furnace was cooled to room temperature naturally and white fuzzy-like products several millimeters long were found on the surface of the copper substrate. The resulting products were characterized by X-ray powder diffraction (XRD) using a Philips X’Pert Pro diffractometer with Cu KR radiation (λ ) 1.541 87 Å). The morphologies of the sample were examined by scanning electron microscopy (SEM, JOEL JSM-6700F). Transmission electron microscopy (TEM) images of the samples were obtained from a Hitachi model H-800 transmission electron microscope using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images, electronic
10.1021/cg0607755 CCC: $37.00 2007 American Chemical Society Published on Web 10/16/2007
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Figure 1. XRD pattern of as-obtained SnO2 nanobelts.
diffraction (ED) patterns, and energy-dispersive spectrometry (EDS) of the samples were obtained from a JEOL-2011 transmission electron microscope. The room-temperature photoluminescence (PL) spectrum was obtained on a Hitachi 850 fluorescence spectrophotometer using a 325 nm excitation line of the He–Cd laser. Figure 1 shows the XRD pattern of the as-obtained products. All of the diffraction peaks can be readily indexed as a tetragonal rutile-phase SnO2 with lattice parameters a ) 4.73 Å and c ) 3.18 Å, which is in accordance with the literature [a ) 4.737 Å and c ) 3.186 Å, Joint Committee on Powder Diffraction Standards (JCPDS) card number 41-1445]. No crystalline impurity peaks were observed, indicating the high purity of the products. Optically, it appears white and covers the copper foil just like fuzz. The morphology of the product was checked using SEM. Figure 2a is the typical SEM image of the product, which shows that the product consists of interesting zigzag structures on a large scale. A typical zigzag structure has a length of about 40 µm, and some are even in the millimeter scale. Careful observation of the product depicts the uniformity of the periodicity along the whole nanobelt. Besides, zigzag nanobelts with larger periodicity are also found. The detailed structure of an individual zigzag nanobelt is characterized by TEM. As shown in Figure 2b, the periodicity and widths of the zigzag nanobelt are about 1 µm and 50 nm, respectively. The measured periodicity angle is about 67°. The zigzag SnO2 nanobelts were further observed by HRTEM. Parts c and d of Figure 2 are the HRTEM images taken on the corresponding parts marked in Figure 2b. No twin crystal boundary could be found in this zigzag-like nanobelt, which means the nanobelts are single-crystal. The clearly resolved interplanar distance in this figure is measured j crystal plane of to be 0.26 nm, corresponding to the (101) or (101) tetragonal SnO2. The separation angle between them is measured as 67°, which consists of the theoretical angle (67.86°) of these planes. Considering that the separation angle in Figure 2b is similar to j in tetragonal-phase Figure 2c and the (101) plane is equal to (101) SnO2, it is deemed that the zigzag-like nanobelts were formed by j or periodical changing of its growth directions from [101] to [101] vice versa. The drive force of changing the growth direction may result from the small change in growth kinetics. The same zigzag structures were also found in ZnO, which incline to growth along the (0001) polar plane.22–24 In our experiments, the copper substrate is believed to play critical roles in controlling the growth direction. Studies show that the zigzag-like SnO2 nanobelts were not observed when platinum was used as the substrate. Straight SnO2 nanowires are obtained instead when platinum flake was used as the substrate (Figure 3a). Figure 3b shows the TEM image of a single SnO2 nanowire, which has a diameter of about 100 nm. The inset of
Figure 2. (a and b) SEM and TEM images of the zigzag-like nanoblets. (c and d) HRTEM images marked in b.
Figure 3. (a) SEM and (b) TEM images and EDS spectrum of the single SnO2 nanowire. (c) HRTEM image marked in b.
Figure 3b is the EDS spectrum of this nanowire, which indicates that the straight nanowires are composed of Sn and O. The selected
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Figure 4. (a) XRD of the upside of the copper substrate. (b) XRD of the underside of the copper substrate.
area electron diffraction (SAED) pattern shown in Figure 3c can j zone axis of tetragonal SnO2. The HRTEM be indexed to the [113] image recorded from the edge of the nanowire is shown in Figure 3c. The regular spacing of the observed lattice planes was ca. 0.33 nm, consistent with the separation of the (110) plane of tetragonal SnO2. All of the above results demonstrate that the growth direction is vertical to the (332) plane. It is clear that the straight SnO2 nanowires have quite different growth habits than the zigzag nanobelts. Namely, the substrate is surely responsible for this diversification in the growth direction. The reason for this diversification of the growth direction is still under investigating, but this result made us believe that the properties of the substrate could control the growth direction of one-dimensional nanomaterials. A series of experiments were conducted to study the formation mechanism of these zigzag SnO2 nanobelts. If the reaction of the synthesis of SnO2 zigzag nanobelts was terminated as soon as the reaction temperature reached 850 °C, as the XRD shows in Figure 4, SnO2 was obtained at the underside of the copper substrate, which faces the reactant, while the Cu2O appeared at the opposite side of the substrate. These results indicate that Sn is oxidized prior to copper by the residual oxygen in the quartz tube with the tin deposit on the substrate. In other words, the growth direction of SnO2 nanobelts may depend upon the crystal structure of copper. Epitaxy had been used to grow oriented crystals for a long time.25–27 In our experiment, the size of the (101) plane of SnO2 is 4.73 × 5.7 Å; therefore, the diagonal length of SnO2 is 7.4 Å (Figure 5a), which is about 2 times that of the (100) plane of copper (d100 ) 3.6 Å). When the lattice match is taken into consideration, growth along the [101] direction on the copper substrate is reasonable. Our growth hypothesis is further verified by Figure 5b, which shows the zigzag nanobelts connecting the copper substrate directly. When platinum was used as the substrate, the growth direction of SnO2 nanowires is vertical to the (332) plane. The length of each sides in the (332) triangle plane of SnO2 is close to 2.24 Å, which is nearly equal to the d111 of platinum (d111 ) 2.23 Å) [match as shown in Figure 5c; for clarity, only one triangle plane was used to denote the Pt (111) and SnO2 (332) planes]. The growth direction of nanowires also relies on the crystal structure of the platinum substrate. Considering the polycrystalline substrate used in our experiment, epitaxial growth of SnO2 nanostructures needs to match special planes of the substrate; for example, SnO2 (101) matches Cu (100), because of the random arrangement of the (100) plane in the copper polycrystalline substrate. The SnO2 nanobelts are not a regular array. The fact that zigzag nanobelts and straight nanowires can be obtained on different substrates indicates that the substrate plays a critical role in the formation process. Once the processing temperature increased to the melting point of tin, tin was vaporized and deposited
Figure 5. (a) Scheme of the match between Cu (100) and SnO2 (101). (b) Zigzag nanobelt growth on the copper substrate. (c) Scheme of the match between Pt (111) and SnO2 (332).
Figure 6. PL spectra of (a) zigzag-like nanobelts and (b) nanowires.
on the copper substrate finally and then the oxidation happened at the liquid/solid interface. On the restriction of crystalline epitaxy, the different crystal plane was formed on the different substrate, and then with the constant evaporation of tin vapor, the length of SnO2 nanostructures increased. It is known that optical properties of SnO2 nanostructures are sensitive to the synthetic conditions, morphologies, etc. For example, SnO2 nanorods prepared in solution show a red emission at 580 nm,17 while fishbone-like SnO2 nanoribbons shows a strong green emission band at 500 nm.28 In the present work, PL properties of these SnO2 nanostructures were also investigated. Figure 6 shows a room-temperature PL spectrum of zigzag-like SnO2 nanobelts (line a in Figure 6) and straight SnO2 nanowires (line b in Figure 6). It can be seen that there is a strong emission band centered at ∼565 nm for zigzags. In comparison to the PL feature of the nanowires (broad emission centered at 585 nm), the peak intensity of zigzaglike SnO2 nanobelts increased. The stronger luminescence in zigzaglike nanobelts might be determined by the high density of oxygen and tin vacancies that have appeared during the growth.18 Further-
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more, the growth orientation of SnO2 nanowires may generate highquality crystallinity with low-defect densities.21 The oxygen vacancies, which are believed to be located on the surface of the nanobelts, interact with interfacial tin vacancies and lead to the formation of a considerable amount of trapped states within the bandgap,17,29 which results in a higher PL intensity. As a bandgap semiconductor with an energy gap of 3.62 eV, the band–band emission peak (∼360 nm) of the SnO2 nanobelts is not observed here because of PL detection limitations. In summary, ultra-long zigzag-like SnO2 nanobelts were obtained through the chemical vapor transport epitaxy (CVTE) mechanism under 1 argon of atmospheric pressure. Studies show that the substrate plays a key role in deciding the growth direction of SnO2 nanostructures. In comparison to the Au-catalyzed growth of onedimensional nanostructures, this method is cheap and easy to operate. We believe that such a synthetic route is versatile and can be adapted for the fabrication of nanobelts of other metal oxides.
Acknowledgment. Financial support from the National Natural Science Foundation of China and the Program for New Century Excellent Talents (NCET) at the university is gratefully acknowledged. We are thankful for funds from the Anhui province of the Natural Science Foundation.
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