J. Phys. Chem. C 2007, 111, 12939-12943
12939
Insight into the Structures and Properties of Morphology-Controlled Legs of Tetrapod-Like ZnO Nanostructures Hua Zhang,† Li Shen,† and Shouwu Guo*,†,‡ Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science and Technology, Shanghai, 200237, P. R. China, and Research Institute of Micro/Nano Science and Technology, Shanghai Jiaotong UniVersity, Shanghai, 200240, P. R. China ReceiVed: May 27, 2007; In Final Form: July 13, 2007
The fine structure characterization of individual legs is essential for understanding the detailed formation mechanism and the origin of the unique properties of tetrapod-like ZnO nanostructures. We have synthesized tetrapod-like ZnO nanostructures with thin needle (TN-ZnO), uniform hexagonal prism (TU-ZnO), and hierarchical hexagonal prism (TH-ZnO) legs through oxidization of zinc vapor followed by ZnO condensation at relatively lower temperatures. The individual legs of as-synthesized ZnO nanotetrapods were characterized complementarily by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron dispersive spectrum (EDS), and cathodoluminescence (CL). We demonstrated that the legs have wurtzite structure and prefer to grow along the [0001] direction. We found that all legs grew from similar ZnOx nuclei, where x is about 0.3, and all of them showed a strong visible luminescent property. EDS and CL spectra obtained from different regions in an individual leg illustrated that the strong visible luminescence resulted from their surface states rather than the heavy oxygen vacancy. The possible nucleation and growth mechanisms of the legs with different morphologies are discussed.
Introduction Zinc oxide, an important II-VI semiconductor material, holds great promise for short-wavelength optoelectronic devices due to its wide direct band gap of 3.37 eV and large exciton binding energy of ∼60 meV. The nanoscaled ZnO materials, because of the quantum confined electron energy state and high surfaceto-volume ratio, have physical and chemical properties differing from that of the bulk materials and have attracted much attention during the past decade. ZnO nanostructures including zerodimensional (0D) nanoparticles, one-dimensional (1D) nanowires (nanorods), and two-dimensional (2D) nanoribbons have been fabricated routinely,1 and their applications in field emission devices,2 piezoelectric devices,3 solar cells,4 and lasers5 have been widely explored. Recently, three-dimensional (3D) tetrapod-like ZnO nanostructures, consisting of four tetrahedrally arranged legs connected at a common center, have also been synthesized by several research groups and prompted much more research on their unique structural character and potential applications.6-9 For instance, it has been reported that tetrapodlike ZnO nanostructures used as an ethanol sensor have high sensitivity and short response time.7 The enhanced photocatalytic activity in rhodamine B photodegradation, high transfection efficiency and low cytotoxicity,8 and unique dielectric properties9 of tetrapod-like ZnO nanostructures have also been demonstrated. However, to date, the composition determination, structure characterization, and property investigation on tetrapodlike ZnO have mainly been performed on a collection of the nanostructures, quite less on an individual structure. The study on a single leg of tetrapod-like ZnO has not been reported yet. * To whom correspondence should be addressed. E-mail: swguo@ ecust.edu.cn. † East China University of Science and Technology. ‡ Shanghai Jiaotong University.
This restricts not only the insight into the exact formation mechanisms of tetrapod-like ZnO but also the understanding of the fundamental physics of the novel complicated nanostructures. Here we describe a simple synthetic strategy for preparation of tetrapod-like ZnO nanostructures with controlled leg dimensions and morphologies. The crystalline structures, chemical composition, and luminescence properties of single legs of the tetrapod-like ZnO nanostructures were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM, including HRTEM), and X-ray powder diffraction (XRD). Energy dispersive spectrum (EDS) and cathodoluminescence (CL) were applied to investigate the chemical compositions and luminescent intensities of different regions of individual legs of tetrapod-like ZnO nanostructures. The correlation between the structure (composition) and property of the individual legs is discussed. The possible nucleation and growth mechanisms of the legs with different morphologies are proposed. Experimental Section All tetrapod-like ZnO nanostructures were fabricated in a horizontal quartz tube furnace with two thermal zones for Zn evaporation and ZnO condensation. Two quartz boats, one containing the raw zinc powder and the other holding a Si substrate onto which the ZnO nanostructures condensed, were placed into the evaporation and condensation zones, respectively. Typically, the evaporation zone temperature was set to 700 °C, and the condensation zone temperatures were set to 500 or 550 °C. To avoid the zinc (raw material) oxidization during the rise in furnace temperature, the quartz tube was first pumped down to 99.99%) nitrogen gas was introduced with flow rate of 400 sccm (sccm denotes standard cubic centimeter per minute). When the temperatures
10.1021/jp074086v CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007
12940 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Figure 1. SEM images of tetrapod-like ZnO nanostructures with different leg morphologies, (a) TN-ZnO, (b) TH-ZnO, and (c) TUZnO (inset is high-magnification image).
of the evaporation and condensation zones reached the designated values, oxygen was introduced with a flow rate of 100 sccm to react with the zinc vapor for 10 min. The morphology control on the legs of tetrapod-like ZnO nanostructures was achieved by varying the time that the Si substrates were being maintained thermostatically within the furnace and the subsequent cooling speed. X-ray powder diffraction (XRD) patterns were recorded on a D/MAX 2550 VB/PC diffractometer using Cu KR radiation, λ ) 1.54 Å. Scanning electron microscopy (SEM) images, complete CL images, and CL spectra were obtained on a FEI Sirion 200 field emission scanning electron microscope (FESEM) equipped with a Gatan MonoCL3 system. The tetrapodlike ZnO nanostructures deposited on the Si substrate were used directly as the SEM specimens. Transmission electron microscopy (TEM) images, electron diffraction patterns (ED), and EDS were taken on a JSM-2010 transmission electron microscope operated at 200 kV. The tetrapod-like ZnO nanostructures on the Si substrate were first suspended in ethanol and then transferred onto a copper TEM grid by dropping the ethanol suspension on the grid followed by evaporation. Results and Discussion As shown in Figure 1a, when the temperature for Zn evaporation and the time for Zn vapor oxidation (i.e., the oxygen gas flowing time) were set to 700 °C and 10 min, respectively, and the Si substrate was cooled down from 500 °C to room temperature (∼23 °C) rapidly by taking the quartz tube out of the furnace right after the Zn vapor oxidization, nanotetrapods with four needle-like legs, TN-ZnO, were obtained. The diameter of the needlelike legs changed from ∼90 to ∼30 nm gradually from the root to the tip. The average leg length is about 2 µm. With the other experimental conditions unchanged, except that the Si substrates were kept thermostatically at
Zhang et al. 500 °C for 20 min in the furnace before slowly cooling down to room-temperature (i.e., cooling the Si substrate together with the furnace), tetrapod-like ZnO nanostructures with four hierarchical hexagonal prism legs, TH-ZnO, were obtained, as shown in Figure 1b. The hierarchical hexagonal prism legs are composed of two parts. The thicker part has a uniform diameter of ∼70 nm and ∼1 µm in length. The thinner region is of ∼30 nm in diameter and ∼1.5 µm in length. A different diameter in one leg implies that the formation of the hierarchical hexagonal prism legs goes possibly through two steps. During the first step, at 500 °C for 20 min, the thick parts of the hierarchical legs formed; however, the ZnO vapor was not consumed completely. Thus, during the second step (the temperature decreased slowly), supercooled ZnO vapor was generated that might speed up the ZnO nanostructure growth along one direction, presumably, the [0001] direction, to form the thinner part of the hierarchical legs. This hypothesis was confirmed by the formation of TH-ZnO (as shown in Figure S1a) with a much longer thin part of the hierarchical legs when the Si substrate was cooled down to room temperature abruptly after the 20 min thermostatic stay in the furnace (taking the quartz tube out of the furnace). Interestingly, when we increased the ZnO condensation zone temperature from 500 °C to 550 °C, and the Si substrates were kept thermostatically for 20 min followed by slowly cooling down to room temperature, ZnO nanotetrapods with four uniform legs, TU-ZnO, were produced, shown in Figure 1c. With comparison of the TH-ZnO and TN-ZnO legs, the legs of TU-ZnO have uniform hexagonal prism morphology with dimensions of about 110 nm in diameter and 1.5 µm in length. In the comparison of TN-ZnO and TH-ZnO, the uniform hexagonal prism legs of TU-ZnO are broader but shorter. The detailed reason for uniform hexagonal prism legs formation is not clear for us at the moment, but we speculate that the elevated condensation temperature may accelerate the leg (ZnO nanocrystal) growing speeds along the (100) and (010) facets, which can increase the leg diameter notably, and consequently more ZnO was consumed. Therefore, when the Si substrate temperature starts to decrease, there is not enough ZnO to form a thinner leg region as in TN-ZnO and TH-ZnO. It has been reported that the flow rates of the oxygen and the carrier gas dramatically affect the morphologies of ZnO nanostructures prepared through the vapor oxidization route.10-12 For example, tetrapod-like ZnO nanostructures with different morphologies have been synthesized at 850 °C by varying the oxygen concentration in the carrier gas flow. ZnO dendrites and wires have been prepared by control of the carrier gas flow rate. With the use of different carrier gases, such as air, argon, and nitrogen, ZnO nanowires, nanorods, and nanotetrapods were produced at elevated temperature of 950 °C. To explore the effects of the oxygen flow rate on the morphologies of tetrapodlike ZnO nanostructures in this work, experiments were conducted to fabricate TH-ZnO nanostructure under the same conditions described above but with different oxygen flow rates of 40 and 80 sccm, respectively. As shown in Figure S1b,c, we found, unexpectedly, that most as-produced ZnO nanostructures have the TH-ZnO structural characteristic indicating that the oxygen flow rate almost does not influence the morphology of TH-ZnO legs. Similar phenomena were also observed from TNZnO and TU-ZnO which were prepared under different oxygen gas flow rates. The results make us believe that the temperature of ZnO condensation and the substrate cooling procedure play more important roles in controlling the leg morphologies of tetrapod-like ZnO nanostructures.
Tetrapod-Like ZnO Nanostructures
Figure 2. XRD pattern of TH-ZnO powders, showing the wurtzite crystal structure.
Figure 3. (a) TEM image of a typical TH-ZnO, (b) HRTEM image of the upper leg, and (c) down leg indicating the preferring growth direction of the tetrapod legs is [0001]. Insets are the corresponding ED patterns.
To further understand the structure and formation mechanism of tetrapod-like ZnO nanostructures, the legs of as-synthesized ZnO nanostructures were characterized by XRD, TEM, and HRTEM. XRD patterns, Figure 2, showed that the nanostructures we prepared had a wurtzite structure with lattice constants of a ) 0.324 nm and c ) 0.519 nm. A low-magnification TEM image of a typical TH-ZnO is shown in Figure 3a, which is consistent with the SEM imaging result aforementioned (Figure 1). To understand the growing behavior of the individual legs of tetrapod-like ZnO, ED patterns and HRTEM images were taken from the circled areas in Figure 3a on different legs and the detailed results are illustrated in Figure 3b and Figure 3c, respectively. The lattice spacing of 0.52 and 0.28 nm deduced from Figure 3b correspond to (0001) and (101h0) facets in the ED pattern, indicating that the leg grows along the [0001] direction. In Figure 3c, the ED pattern is not perfect due to the limitation of the tilt angle for the sample in the TEM chamber
J. Phys. Chem. C, Vol. 111, No. 35, 2007 12941 but (101h0) and (0002) diffraction spots can be seen clearly. The lattice spacing of 0.26 nm in the HRTEM image, Figure 3c, corresponding to the (0002) facet, shows that the preferred leg growth direction is [0001]. No defects such as stacking faults, dislocations, and amorphous layers can be seen in the legs. These results are consistent with those reported by other research groups.13 The local chemical compositions not only significantly affect the physical and chemical properties of nanostructured materials but also play important roles in ZnO tetrapod nucleation and growth behaviors. The latter is critical to control the nanoscaled materials’ morphology and size. Here, we closely investigated the compositions of different regions of single legs of tetrapodlike ZnO. The EDS results acquired from the different region on single legs of the tetrapod-like ZnO were depicted in Figure 4. The values labeled at different circles represent the local zinc contents (atomic percent in ZnO). As shown in Figure 4a, the thicker region of a TH-ZnO leg has Zn rich composition and the zinc content from the root to the joint of thicker to thinner changes slightly from 77.94% to 71.70%. At the thinner region, the zinc content decreases rapidly from 68.42% to 36.77%, switching from Zn rich to Zn deficient. In a TN-ZnO leg, Figure 4b, the zinc content from root to tip decreases gradually from 76.50% (Zn rich) to 49.51% (near stoichiometry). In a TU-ZnO leg, Figure 4c, the zinc content remains almost a constant but with a Zn rich regime through the entire leg. Interestingly, all the legs of the tetrapod-like ZnO nanostructures we synthesized contain almost the same Zn content of ∼77% at their roots. This unique feature suggested that they might grow from similar ZnOx nuclei, where x is about 0.3, far below the ZnO stoichiometry. The reason might be that at the beginning, an abundance of zinc vapor was generated but not enough oxygen has been introduced in the furnace yet; thus, only a few gaseous ZnO molecules were formed. However, as illustrated by another group,14 the lifetime of gaseous ZnO is very short, and in Zn vapor it could change into Zn/ZnO droplets (assuming the composition of ZnOx, x , 1), with the melting point it should be much lower than that of ZnO. With the introduction of more oxygen, the Zn/ZnO droplets were saturated by ZnO, and the ZnO nanocrystal nucleation and growth took place according to the VLS (vapor-liquid-solid) mechanism. However, the exact transition mechanism from droplet to tetrapod-like morphology is unclear for us right now and is under further investigation. To understand the correlation between the structure (composition) and luminescent property, complete CL images of tetrapod-like ZnO nanostructures and their individual legs were acquired on a scanning electron microscope. Figure 5 shows both complete CL and SEM images of a single TH-ZnO acquired with an acceleration voltage of 10 keV. The diameters of the thicker and thinner parts of a hierarchical leg are determined to be ∼45 and ∼15 nm, respectively. The complete CL image is almost indistinguishable from the SEM image, indicating that the CL emission took place throughout the whole leg. The CL spectra of a TH-ZnO leg at different regions (labeled in Figure 5a) were also recorded on the SEM and were normalized by r2 (here r is the radius of the specific region of the leg) to calibrate the leg dimension effects on CL emission intensity. As shown in Figure 5c,d, two emission peaks in the CL spectra centered at ∼378 and 498 nm were observed. The sharp UV peaks at ∼378 nm with a narrow full-width-halfmaximum (fwhm) may be attributed to the near band edge emission of ZnO. In accordance with previous reports,15 both surface state and oxygen vacancy may respond to the visible
12942 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Zhang et al.
Figure 4. EDS acquired from different regions of the legs of different ZnO nanostructures, (a) TH-ZnO, (b) TN-ZnO, and (c) TU-ZnO. The numbers represent the zinc content (atomic percent in ZnO).
ZnO, have been successfully fabricated through simple Zn vapor oxidization and ZnO condensation at relatively lower temperatures. This synthetic strategy is important because it widens the low cost controllable nanofabrication route and should have general applicability to nanostructured metal oxides preparations. The detailed structures, chemical compositions, and luminescence properties of single legs of different tetrapod-like ZnO nanostructures have been studied systematically. The present EDS data acquired at different regions on single legs of the tetrapods demonstrate that the thicker part of the legs have more oxygen vacancies than the thinner part; however, the stronger visible CL intensities emitted form the thinner part, having larger surface to volume ratios, show that the surface state may play a more important role in the visible emissions rather than the oxygen vacancy. The strong visible (blue to green) emission characteristic makes as-synthesized ZnO nanostructures possible for applications in short wavelength photoelectric devices, fluorescence labels, biological detectors. Attempts to characterize the common center of the tetrapod are under processing, which will be helpful in fully understanding the tetrapod-like structure formation mechanism.
Figure 5. (a) SEM and complete (b) CL images of a TH-ZnO, (c) CL spectrum of a single TH-ZnO leg, showing two peaks centered at 378 nm and 498 nm, and (d) CL spectra recorded from different regions of a single leg in the TH-ZnO. The emission intensities have been normalized by r2 (r is the radius of the specific region of the leg).
emission of ZnO nanostructures around 498 nm. In our case, as we described above, the EDS results demonstrated clearly that the thicker part of the TH-ZnO tetrapod legs have more oxygen vacancies than the thinner part; however, the absolute (after normalized by r2) visible CL intensity emitted from the thinner part is much stronger than that from the thicker part (Figure 5d). Thus, we believe that the great amount oxygen vacancies do contribute to the visible emission, but the surface state may play a more important role in the visible emission, because the thinner part has a larger surface to volume ratio. Nevertheless, the visible emission of all as-investigated ZnO nanostructures is so strong that it could be detected by a common digital camera under ultraviolet analyzer (2F-1 type) irradiation (λ ) 254 nm), and the pictures were shown in Figure S2. The strong visible (blue to green) emission character may find applications in short wavelength photoelectric devices, fluorescence labels, and biological detectors. Conclusions Tetrapod-like ZnO nanostructures with controlled leg dimensions and morphologies, including TH-ZnO, TU-ZnO, and TN-
Acknowledgment. Authors acknowledge gratefully the Shanghai Pujiang Scholarship Program (Grant No. 06PJ14025), National High Technology Research and Development Program (863 Program) of China (Grant No. 2006AA04Z309), and China Postdoctoral Science Foundation (Grant No. 20060400627) for financial support. Supporting Information Available: SEM and PL images of ZnO nanostructure prepared at different experimental conditions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Umetsu, M.; Mizuta, M.; Tsumoto, K.; Ohara, S.; Takami, S.; Watanabe, H.; Kumagai, I.; Adschiri, T. AdV. Mater. 2005, 17, 2571-2575. (b) Viswanatha, R.; Sarma, D. D. Chem.sEur. J. 2006, 12, 180-186. (c) Wang, X. D.; Song, J.; Wang, Z. L. Chem. Phys. Lett. 2006, 424, 86-90. (d) Shen, G. Z.; Bando, Y.; Chen, D.; Liu, B.; Zhi, C.; Golberg, D. J. Phys. Chem. B 2006, 110, 3973-3978. (e) Li, Q. C.; Kumar, V.; Li, Y.; Zhang, H. T.; Chang, R. P. H. Chem. Mater. 2005, 17, 1001-1005. (2) (a) Lee, C. Y.; Tseng, T. Y.; Li, S. Y.; Lin, P. Nanotechnology 2006, 17, 83-88. (b) Sasa, S.; Ozaki, M.; Koike, K.; Yano, M.; Inoue, M. Appl. Phys. Lett. 2006, 89, 053502. (3) (a) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625-1631. (b) Christman, J. A.; Woolcott, R. R.; Kingon, A. I.; Nemanicha, R. J. Appl. Phys. Lett. 2006, 73, 3851-3853. (4) (a) O’Regan, B.; Schwartz, D. T.; Zakeeruddin, S. M.; Gra¨tzel, M. AdV. Mater. 2000, 12, 1263-1267. (b) Zeng, L.; Dai, S.; Xu, W.; Wang, K. Plasma Sci. Technol. 2006, 8 (2), 172-175. (5) (a) Huang, M. H.; Mao, S.; Fick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897-1899. (b) Song, J. K.; Szarko, J. M.; Leone, S. R.; Li, S.; Zhao, Y. J. Phys. Chem. B
Tetrapod-Like ZnO Nanostructures 2005, 109, 15749-15753. (c) Szarko, J. M.; Song, J. K.; Blackledge, C. W.; Swart, I.; Leone, S. R.; Li, S.; Zhao, Y. Chem. Phys. Lett. 2005, 404, 171-176. (6) (a) Kwok, W. M.; Djurisic, A. B.; Leung, Y. H.; Xhan, W. K.; Phillips, D. L. Appl. Phys. Lett. 2005, 87, 223111. (b) Che, Z.; Shan, Z.; Cao, M. S.; Lu, L.; Mao, S. X. Nanotechnology 2004, 15, 365-369. (c) Djurisic, A. B.; Leung, Y. H.; Cheah, K. W.; Chan, W. K. Appl. Phys. Lett. 2004, 84, 2635-2637. (7) Gao, T.; Wang, T. H. Appl. Phys. A 2005, 80, 1451-1454. (8) Wan, Q.; Wang, T. H.; Zhao, J. C. Appl. Phys. Lett. 2005, 87, 083105. (9) Han, J. G.; Zhu, Z. Y.; Ray, S.; Azad, A. K.; Zhang, W.; He, M.; Li, S.; Zhao, Y. Appl. Phys. Lett. 2006, 89, 031107. (10) (a) Wang, F. Z.; Ye, Z. Z.; Ma, D. W.; Zhu, L. P.; Zhuge, F. J. Cryst. Growth 2005, 274, 447-452. (b) Wang, F. Z.; Ye, Z. Z.; Ma, D. W.; Zhu, L. P.; Zhuge, F. Mater. Lett. 2005, 59, 560-563. (c) Roy, V. A. L.; Djurisic, A. B.; Chan, W. K.; Gao, J.; Lui, H. F.; Surya, C. Appl. Phys. Lett. 2003, 83 (1), 141-143. (11) (a) Chen, Z.; Shan, Z. W.; Cao, M. S.; Lu, L.; Mao, S. X. Nanotechnology 2004, 15, 365-369. (b) Dai, Y.; Zhang, Y.; Li, Q. K.; Nan, C. W. Chem. Phys. Lett. 2002, 358, 83-86.
J. Phys. Chem. C, Vol. 111, No. 35, 2007 12943 (12) (a) Chen, Z. G.; Ni, A.; Li, F.; Cong, H.; Cheng, H. M.; Lu, G. Q. Chem. Phys. Lett. 2007, 434, 301-305. (b) Leung, C. Y.; Djurisic, A. B.; Leung, Y. H.; Ding, L.; Yang, C. L.; Ge, W. K. J. Cryst. Growth 2006, 290, 131-136. (13) (a) Shiojiri, M.; Kaito, C. J. Cryst. Growth 1981, 52, 173-177. (b) Nishio, K.; Isshiki, T.; Kitano, M.; Shiojiri, M. Philos. Mag. A 1997, 76, 889-892. (c) Iwanaga, H.; Fujii, M.; Takeuchi, S. J. Cryst. Growth 1993, 134, 275-280. (d) Ronning, C.; Shang, N. G.; Gerhards, I.; Hofsass, H. J. Appl. Phys. 2005, 98, 034307. (14) Dai, Y.; Zhang, Y.; Wang, Z. L. Solid State Commun. 2003, 126, 629-633 and references therein. (15) (a) Zhang, S. B.; Wei, S. H.; Zunger, A. Phys. ReV. B 2001, 63, 075205. (b) Cheng, W. D.; Wu, P.; Zou, X.; Xiao, T. J. Appl. Phys. 2006, 100, 054311. (c) Vanheusen, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Voigt, J. A. Appl. Phys. Lett. 1996, 68 (3), 15-18. (d) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A. J. Appl. Phys. 1996, 79, 7983-7985. (e) Yao, B. D.; Vhan, Y. F.; Wang, N. Appl. Phys. Lett. 2002, 81, 757-759.
