J. Phys. Chem. C 2007, 111, 7671-7675
7671
Preferential Growth of SnO2 Triangular Nanoparticles on ZnO Nanobelts J. X. Wang,† X. W. Sun,*,†,§ S. S. Xie,‡ Y. Yang,† H. Y. Chen,‡ G. Q. Lo,§ and D. L. Kwong§ School of Electrical and Electronic Engineering, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798, Institute of Physics, Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing, China 100080, and Institute of Microelectronics, 11 Science Park Road, Singapore Science Park II, Singapore 117685 ReceiVed: February 3, 2007; In Final Form: March 29, 2007
ZnO-SnO2 binary nanostructure was synthesized by a simple vapor-phase transport method. Pyramid-like single-crystalline SnO2 nanoparticles were found grown on the surface of the ZnO nanobelts with definite orientations, aligning into a line along two preferential directions [011h0] and [011h1]. The ZnO-SnO2 binary nanostructures were studied and their growth mechanism was discussed. The ZnO-SnO2 binary nanostructures were used to fabricate CO gas sensor with sensitive response. The photoluminescence of the binary oxide nanostructure was also measured.
1. Introduction One-dimensional (1D) oxide nanostructures have attracted great interest recently. Oxide nanostructures with various morphologies have been synthesized, such as nanobelts, nanowires, nanotubes, nanocombs, and nanorings.1,2 Fabrication of complex functional nanostructures is a crucial step toward the realization of functional nanodevices.3-5 Though fabrication of simple oxide nanostructures has progressed significantly during the past few years, synthesis of binary oxide 1D nanostructures remains a challenge. Neither the fundamental growth behaviors nor the properties are well understood. 1D binary oxide nanostructures consisting of different materials may combine the different physical properties into one nanostructure.6 Moreover, due to the formation of the heterojunction between different materials with various band gaps, the binary oxide nanostructures are expected to possess some novel electric and optical properties. Therefore, fabrication of binary oxide nanostructures and investigation of their growth behaviors are necessary. Presently, few binary oxide nanostructures have been synthesized due to the complicate relationship in the binary system.7-9 ZnO and SnO2 are two key semiconducting oxides. Their nanostructures have been extensively studied and demonstrated to have unique properties compared to their bulk materials.1 Many nanodevices based on these nanostructures have been fabricated, for example, sensors,10 transistors,11 UV lasers,12 photodetectors,13 etc. In this paper, we shall present a ZnOSnO2 binary nanostructure using the vapor-phase transport method. 2. Experimental Section The synthesis of the ZnO-SnO2 binary nanostructure was carried out in a quartz tube mounted on a horizontal furnace. About 1 g of SnO and 4 g of ZnO/C mixed powder (weight ratio 3:1) were placed separately along the down stream direction * Address correspondence to this author. Phone: +65-67905369. E-mail:
[email protected]. † School of Electrical and Electronic Engineering. § Institute of Microelectronics. ‡ Institute of Physics.
Figure 1. XRD pattern of the as-prepared binary nanobelts. The star symbols indicate the peaks originating from the rutile SnO2 phase.
in a quartz boat. The quartz boat was paced in the center of the quartz tube. A (100) silicon slice was used as the substrate placed about 3-4 cm downstream from the quartz boat. The quartz tube was sealed and heated to 900 °C. Then the tube was pumped down to 100 Torr. Meanwhile, N2 was introduced as a carried gas with a flow rate of 100 sccm. The temperature was maintained for 2 h and then cooled to room temperature. After the reaction, a white wool-like material was observed on the silicon substrate. The resulting products were collected for characterization by using scanning electron microscopy (SEM; Hitachi S-5200), X-ray powder diffractometer (XRD; Semesi), and high-resolution transmission electron microscopy (HRTEM; JEM-2010F). The gas-sensing properties of the binary oxide nanobelts were measured by using a computer-controlled gas sensing characterization system reported elsewhere.14 The binary oxide nanobelts were moved to a sapphire substrate and gold pastes were used to form two bonding pads. The photoluminescence (PL) was measured with a He-Cd laser operating at 325 nm as the excitation source. 3. Results and Discussion The XRD pattern of as-prepared products is shown in Figure 1, where most of the diffraction peaks can be indexed to the hexagonal wurtzite phase of ZnO with lattice constants of a )
10.1021/jp070963l CCC: $37.00 © 2007 American Chemical Society Published on Web 05/09/2007
7672 J. Phys. Chem. C, Vol. 111, No. 21, 2007
Wang et al.
Figure 2. (a-d) SEM images of binary nanobelts with (a) low, (b) medium, and (c) (d) high magnifications, respectively. Insets of part d are SEM images of two kinds (triangle and arrowhead) of nanoparticles. (e) SEM image of a nanobelt showing nanoparticles preferentially grown in two directions. (f) Nanoparticles grown on the edge of the nanobelts. (g and h) SEM images of randomly distributed nanoparticles on nanobelts.
0.325 nm and c ) 0.520 nm. Two weak diffraction peaks, as indicated in the figure, were indexed to be rutile SnO2. No other composite phase was found, which indicates the product consisted of ZnO and SnO2. Parts a-d of Figure 2 show the SEM images of the as-grown nanostructures with various magnifications. Many belt-like structures can be observed in the product. The widths of the nanobelts range from several hundred nanometers to several micrometers, and the lengths are about several tens of micrometers. Some of the nanobelts have a saw-like edge. The nanobelts show unique morphologies compared to traditional nanobelts. Pyramid-like particles with sizes of about 100-200 nm were found grown in the center of the nanobelts and aligned in a line through the whole length of the nanobelts. All the particles have definite orientations. For most of the particles, their edges are parallel to each other. From Figure 2, two kinds of particles can be identified on the nanobelts: arrowhead shaped and triangular shaped, as shown in the down-left and up-right insets of Figure 2d, respectively. In the SEM images, most of the particles align along the growth
direction forming a line in the center of the nanobelts. Figure 2e shows the nanoparticles aligned along two directions in a nanobelt and forming an interesting pattern on the surface. The included angle of the two directions is measured to be about 28°. In addition, nanoparticles aligned on the edge of the nanobelts were also found (Figure 2f). Moreover, we found when the growth temperature increased to 1100 °C, the nanoparticles distributed randomly on the surface of the nanobelts (Figure 2g,h). However, for each nanoparticle, they still have the same shape and orientation, indicating the definite crystallographic orientation relationship between the nanoparticles and nanobelts. Figure 3a shows a bright field TEM image of the binary nanobelts. A line of triangle nanoparticles was observed in the center of the nanobelts. The up inset of Figure 3a shows the SAED pattern taken from area A without the nanoparticles. The bright spots indicate the single-crystal feature of the nanobelts. The SAED is indexed to be hexagonal ZnO with a zone axis of [21h1h0]. The growth direction of the ZnO nanobelts, a prefer-
Growth of SnO2 Triangular Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 21, 2007 7673
Figure 3. (a) TEM image of the binary nanobelts and SAED patterns taken from the none-nanoparticle region (up) and the nanoparticle region (down). (b and c) EDS spectrum taken from the none-nanoparticle region and the nanoparticle region, respectively. (d) HRTEM image of a SnO2 nanoparticles grown on ZnO nanobelt.
entially aligned direction, is determined to be [011h0]. On the basis of the included angle between the two preferential aligned directions (Figure 2 e), another aligned direction is determined to be [011h1]. Parts b and c of Figure 3 show the EDS pattern of the nanobelts and nanoparticles. The EDS pattern taken on the nanobelts area shows only two elements Zn and O with the atomic ratio about 2:1 (the Cu peak in the EDS pattern originates from the copper grid). However, from the EDS taken on the nanoparticles part of the nanobelts, Sn was found besides Zn and O, which indicates the different constituent between the nanoparticles and the nanobelts. Figure 3d shows the HRTEM image of a triangle nanoparticle with a size of about 20 nm. Clear edges of the nanoparticles can be observed in the HRTEM image. Three groups of parallel fringes with d spacing of 0.26, 0.26, and 0.34 nm were observed, which corresponds to the (101), (011h), and (110) planes of the rutile SnO2, respectively. In addition, another parallel fringe in the area without nanoparticles is also observed, which can be indexed to the (0001) plane of ZnO with the lattice distance d ) 0.52 nm. The TEM results indicate the nanobelts are binary oxide composite nanobelts with single crystalline SnO2 nanoparticles grown on the single-crystal ZnO nanobelts. It is worth mentioning that the SAED taken in area B (with the nanoparticles) shows a complicated pattern, as shown in the bottom inset of Figure 3a. The SAED pattern indicates two sets of diffraction spots. One can be indexed to be the [21h1h0] ZnO pattern, which is the same as the SAED pattern taken from area A without nanoparticles. Another set of SAED patterns belongs to rutile SnO2 along the [11h1h] zone axis. In addition,
many weak satellite spots are observed in the SAED pattern, which may be produced by the double diffraction from the overlapping area of the SnO2 nanoparticles and the ZnO nanobelts. A similar situation has been previously observed in Zn/ZnO nanobelts.15 As we presented above, the nanoparticles have two kinds of morphologies: triangle and arrowhead-like nanoparticles. Figure 4a shows an HRTEM image of an arrowhead-like nanoparticle. Different from Figure 3d, Morie fringes with large d spacing of 0.96 nm were observed. Figure 4b shows the corresponding Fourier filtered transform (FFT) pattern of the HRTEM image, which is similar to the SAED pattern shown in Figure 3c. Therefore, the complicated SAED pattern should come form the electron diffraction of the arrowhead-like nanoparticle. HRTEM images of the left and right sides of the arrowhead-like nanoparticle are shown in Figure 4, parts c and e, respectively, and the corresponding FFT image are shown in parts d and f, respectively. Both the FFT patterns of the left and right branches are indexed to be [21h1h0] ZnO and [11h1h] SnO2. However, the two FFT patterns are different due to the various orientations of SnO2 nanoparticles on the surface of ZnO nanobelts. The FFT pattern of the whole nanoparticle shown in Figure 4b can be considered as the overlapping of the two FFT patterns from left and right parts. Moreover, in Figure 4c,d, Morie fringes distributed along [110] of SnO2 and [011h1] of ZnO can be observed. The formation of the morie fringes is due to the interference between the Bragg reflections from the two crystals of SnO2 and ZnO. The d spacing of the Morie fringes is given by dM ) dbeltdparticle/[dbelts - dparticle],16 where dbelt and dparticle are the corresponding d
7674 J. Phys. Chem. C, Vol. 111, No. 21, 2007
Wang et al.
Figure 5. Schematic diagram of the SnO2 nanoparticles grown on the surface of the ZnO nanobelts.
Figure 4. (a and b) HRTEM image of an arrowhead-like nanoparticle and the corresponding FFT pattern. (c and d) HRTEM image taken form the left part of a arrowhead-like nanoparticle and the corresponding FFT pattern. (e and f) HRTEM image taken form the right part of a arrowhead-like nanoparticle and the corresponding FFT pattern.
spacings of the overlapping planes for ZnO nanobelts and SnO2 nanoparticles, respectively. In this case, the d spacing of Morie fringes is calculated to be 0.955 nm, which agrees with our experiment observation well (Figure 2e). We speculate that the growth of the binary oxide composite nanobelts involves a two-step process (Supporting information, Figure 1S). Because the substrate is placed near the ZnO/C powder, the metastable ZnOx and Zn vapors produced by the precursors can be easily transported to the substrate. At the same time, the quantity of the ZnO precursor is more than that of the SnO precursor. The bond enthalpies in gaseous diameter species of Sn-O and Zn-O bonds are 531.8 ( 12.6 and 150 ( 4 kJ/ mol,17 respectively, and the lattice energies in a thermochemical cycle of SnO2 and ZnO are 11807 and 3971 kJ/mol, respectively.18 It is more difficult to break the Sn-O band than the Zn-O band and thus it is easier to form ZnO than SnO2 from the metal or their suboxides. Higher vapor pressure of Zn and ZnOx could be created and thus ZnO nanobelts grew quickly at first. With the gradual exhaust of the ZnO precursor, Sn and SnOx vapor becomes significant, and SnO2 nanoparticles eptaxially grew on the ZnO nanobelts matrix and finally formed SnO2-ZnO binary oxide nanobelts. The growth mechanism of SnO2 nanoparticles is similar to that of SnO2 nanowires, which was discussed in detail in our previous report.19 From the TEM and HRTEM analysis, we can see that SnO2 nanoparticles have an epitaxial relationship with the ZnO nanobelts as follows: [11h1h]SnO2//[21h1h0]ZnO, (011h)SnO2//(0002)ZnO.
Meanwhile, lattice constants in (011h)SnO2 and(0002)ZnO planes are close, which can explain why all the nanoparticles have the same orientation with the nanobelts. On the other hand, the similar orientation and close planar distance facilitate the epitaxial growth of the heterostructures. The triangle SnO2 nanoparticles grown on the (211h0) of ZnO nanobelts are confined by three planes (101), (110), and (011h). On the basis of the TEM results shown in Figure 4, the formation of the arrowhead-like nanoparticles is most likely due to simultaneous growth along [011h 1] and [011h 1h ] of the ZnO nanobelts, respectively. Structurally, [011h1] and [011h1h] are two equivalent crystallographic directions of ZnO. A small SnO2 crystal has the same chance to grow up along these two directions and finally develops into an arrowhead-like nanoparticle. Figure 5 presents a schematic picture of their structure relationship. As for the two preferentially aligned directions [011h0] and [011h1], we speculate this may be due to the nanoparticles preferentially growing along the planar defect (e.g., stacking default) exist in the nanobelts. From the energy point of view, planar defects have higher energy than the defect-free surface of the ZnO nanobelts. The defect regions thus are the favorable sites for SnO2 nanoparticle growth (Supporting Information, Figure 2S). In addition, this point is further supported by the fact that SnO2 nanoparticles grew on the edge of the nanobelts. Moreover, with the increasing reaction temperature, more nucleated sites exist on the surface of the ZnO nanobelts, and result in the random distribution of the SnO2 nanoparticles on ZnO nanobelts. The PL spectrum of the binary oxide nanobelts was shown in Figure 6a. The spectrum shows a weak UV peak at 375 nm and a broad peak at 600 nm. In addition, two weak shoulder peaks at 660 and 740 nm can also be observed. The UV peak can be attributed to the near band edge emission of ZnO.20 No UV emission related to SnO2 is observed. The emission at 600, 660, and 740 nm may be due to the existence of other luminescent centers, such as impurity, vacancy, or doping the Sn atom in ZnO lattices. The composite ZnO-SnO2 nanobelts were used to fabricate the CO gas sensor. In comparison, ZnO film and ZnO nanowire array based gas sensors were also constructed and tested. The fabrication details were reported elsewhere.21,22 Figure 6b shows the CO gas sensing responses of the corresponding ZnO-SnO2 composite nanobelt, the ZnO nanowire, and the ZnO film gas sensors. It can be seen from Figure 6b that the resistance of the composite nanobelts reduces with the presence of 300 ppm CO at 350 °C. The reversible cycles of the response curve of the ZnO-SnO2 nanobelt gas sensor indicate a stable and repeatable operation, with a sensitivity S calculated to be 4.6 (S ) Rair/ Rgas, where Rair and Rgas are the resistance of the gas sensor in air and in testing gas, respectively). The response and recover
Growth of SnO2 Triangular Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 21, 2007 7675 definite orientations. The nanoparticles preferentially aligned along the [011h0] and [011h1] of ZnO nanobelts. In addition, the nanoblets show good response to the 300 ppm CO gas at 350 °C. This method can be applied in other binary systems to synthesize many other functional binary oxide nanostructures. Supporting Information Available: Figure 1S shows SEM images of the ZnO-SnO2 nanostructures obtained at 900 °C for 20 min (a), 1 h (b), and 3 h (c); Figure 2S shows the SnO2 nanoparticles grown along the planar defects of the ZnO nanobeltss(a) HRTEM image of two SnO2 nanoparticles grown along the planar defects of ZnO nanobelt (the planar defect extends along the [011h0] direction) and (b) SEM image of the SnO2 nanoparticles grown along the planar defects of ZnO nanobelts. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 6. (a) PL spectrum of the binary ZnO-SnO2 nanobelts. (b) Response curves of the binary ZnO-SnO2 nanobelts, the ZnO nanowire arrays, and the ZnO film when exposed to 300 ppm CO at 350 °C.
time of the ZnO-SnO2 nanobelt gas sensor are 52 and 550 s, respectively. The diameters of the ZnO nanowires used to construct the gas sensor are about 200-300 nm, and the thickness of the ZnO film is about 1 µm. At the same condition, the sensitivity of the ZnO nanowire arrays based gas sensor is 2.4, which is lower than that of the ZnO-SnO2 nanostructures. For ZnO film gas sensor, the sensitivity is only 1.5, which is significantly lower compared to both nanostructures. These results indicate that nanostructure (ZnO) performs better than its thin film counterpart, and even better performance can be achieved with heteronanostructures (ZnO-SnO2) compared to single phase nanostructures (ZnO). 4. Conclusion We have synthesized the ZnO-SnO2 binary oxide nanobelts using the VPT method. Pyramidal SnO2 nanoparticles epitaxially grew on the surface of the single crystalline ZnO nanobelts with
(1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, D. F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (2) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (3) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (4) Keating, C. D.; Natan, M. J. AdV. Mater. 2003, 15, 451. (5) Park, W. I.; Yi, G. C.; Kim, M.; Pennycook, S. J. AdV. Mater. 2003, 15, 526. (6) He, R. R.; Law, M.; Fan, R.; Kim, F.; Yang, P. D. Nano Lett. 2002, 21, 109. (7) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304,1787. (8) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (9) Wang, J. X.; Chen, H. Y.; Gao, Y.; Liu, D. F.; Song, L.; Zhang, Z. X.; Zhao, X. W.; Dou, X. Y.; Luo, S. D.; Zhou, W. Y.; Wang, G.; Li, J. Q.; Xie, S. S. J. Cryst. Growth 2005, 284, 73. (10) Wang, J. X.; Sun, X. W.; Wei, A.; Lei, Y.; Cai, X. P.; Li, C. M.; Dong, Z. L. Appl. Phys. Lett. 2006, 88, 233106. (11) Heo, Y. W.; Tien, L. C.; Kwon, Y.; Norton, D. P.; Pearton, S. J.; Kang, B. S.; Ren, F. Appl. Phys. Lett. 2004, 85, 2274. (12) Zhang, C. F.; Dong, Z. W.; You, G. J.; Qian, S. X.; Deng, H. Opt. Lett. 2006, 31, 3345. (13) Kind, H.; Yan, H.; Law, M.; Messer, B.; Yang, P. AdV. Mater. 2002, 14, 158. (14) Wang, J. X.; Sun, X. W.; Yang, Y.; Huang, H.; Li, Y. C.; Tan, O. K.; Vayssieres, L. Nanotechnology 2006, 17,4995. (15) Kong, X. Y.; Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 570. (16) Ding, Y.; Kong, X. Y.; Wang, Z. L. J. Appl. Phys. 2004, 95, 306. (17) Kerr, J. A. CRC Handbook of Chemistry and Physics, 81st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2000. (18) Jenkins, H. D. B. CRC Handbook of Chemistry and Physics, 79th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1998. (19) Wang, J. X.; Liu, D. F.; Yan, X. Q.; Yuan, H. J.; Ci, L. J.; Zhou, Z. P.; Gao, Y.; Song, L.; Liu, L. F.; Zhou, W. Y.; Wang, G.; Xie, S. S. Solid State Commun. 2004, 130, 89. (20) Li, Y.; Meng, G. W.; Zhang, L. D.; Phillipp, F. Appl. Phys. Lett. 2000, 76, 2011. (21) Yang, Y.; Sun, X. W.; Chen, B. J.; Xu, C. X.; Chen, T. P.; Sun, C. Q.; Tay, B. K.; Sun, Z. Thin Solid Films 2006, 510, 95. (22) Wang, J. X.; Sun, X. W.; Yang, Y.; Huang, H.; Lee, Y. C.; Tan, O. K.; Vayssieres, L. Nanotechnology 2006, 17, 4995.
