Fabrication of Hierarchical Zinc Oxide Nanostructures through Multistage Gas-Phase Reaction Yue Wu, Zhonghe Xi, Gengmin Zhang,* Julan Zhang, and Dengzhu Guo Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2646–2651
ReceiVed March 16, 2007; ReVised Manuscript ReceiVed September 28, 2007
ABSTRACT: A hierarchical nanostructure of wurtzite zinc oxide (ZnO) was synthesized by the thermal evaporation of Zn/ZnO powders. It had a hexagonal prism as the center and six clusters of nanorods as the branches. The formation of this ZnO nanostructure is attributed to three sequential catalyst-free reactions. First, a Zn prism was deposited on the substrate as the result of the condensation of Zn vapor; then, oxygen was introduced in an appropriate temperature range and the Zn prism was oxidized; finally, secondary ZnO nanorods grew on the ZnO surfaces of the central prism and the snowflake-shaped nanostructure was attained. This work has revealed the perspective of achieving hierarchical ZnO nanostructures through simple and controllable gas-phase reactions.
1. Introduction So far, a very large number of nanometer-scale structures of zinc oxide (ZnO) have been attained.1–9 Among them, starlike ZnO nanostructures with a 6-fold symmetry are of particular interest.10–14 On the one hand, they are expected to find applications in such fields as sensors, electromechanical coupled devices and optical components in microelectronic mechanic systems;10,11,15 on the other hand, their hierarchical structure has provided a unique opportunity for better understanding the growth mechanism of ZnO nanostructures and further improving the controllability in their fabrication. By the thermal evaporation of a mixture of ZnO and SnO2, Gao et al. synthesized ZnO nanostructures that consisted of sets of central axial nanowires surrounded by radially oriented nanoribbons.Thegrowthwasdominatedbythevapor-liquid-solid (VLS) mechanism, and the Sn particles reduced from SnO2 served as the catalyst for the growth of the ZnO nanowires and nanoribbons.11,15 Lao et al. realized a heteroepitaxial growth of ZnO nanorods on In2O3 nanowires. The ZnO-In2O3 hierarchical heteronanostructures demonstrated 6-, 4- and 2-fold symmetries. Probably InOx vaporized first to form the core nanowires and ZnOx vaporized later to grow on the facets of these nanowires.10 Xu et al. fabricated the ZnO hexagram whiskers by adding additional In2O3 into the typical mixture source of ZnO and graphite powders. A hexagonal disk core was formed at high vapor Zn pressure and subsequently surrounded by the side branches that grew along the directions of 〈112j0〉. The use of In was intended to generate a higher pressure of Zn vapor, so that the growth of ZnO along the [0001] direction was suppressed.12 As reported in this article, we have obtained an asteroidal ZnO nanostructure through a catalyst-free multistage reaction in a thermal evaporation process. Unlike other works based on gas-phase reaction, foreign catalysts, such as Sn and In, were not necessary in our fabrication. Also, our products have demonstrated a different appearance from those reported previously.
2. Experimental Section Our fabrication approach was based on a simple thermal evaporation method, in which the frequently used metallic catalyst was not * To whom correspondence should be addressed. Phone: 86-10-62751773. Fax: 86-10-62762999. E-mail:
[email protected].
required.16 The synthesis was conducted in a horizontal tube furnace that resembled the one described in ref 17. It mainly consisted of a horizontal furnace, an alumina tube, a rotary vane vacuum pump and a gas supply system. A quartz boat that contained the source materials, i.e., powders of Zn and ZnO mixed in a mass ratio of 2:1, was pushed to the middle of the tube. The substrates, which were several 10 mm × 10 mm silicon wafers, were placed within 20 cm away from the middle at the downstream of the tube. After the tube cavity was evacuated, argon was introduced into the tube from one end and pumped out from the other end. The area of the inlet of the rotary pump was adjusted so that the pressure in the tube rose to 4 × 104 Pa under an argon flow of 60 sccm. In the first stage of the reaction, the furnace was switched on and the system temperature was elevated. When the temperature at the middle of the tube reached 950 °C, oxygen was also introduced into the tube with a flow rate of 6 sccm. In the second stage, the relay kept the system temperature unchanged for about 40 min. In this process, the temperature was not uniform along the tube and the substrates were approximately under 800 °C. Finally, the furnace was switched off. Argon and oxygen continued flowing until the whole system cooled down to room temperature spontaneously. As a result of this synthesis process, the Si substrates were covered with a layer of yellowish product. The sample was observed with a scanning electron microscope (SEM), and its crystal structure was analyzed with X-ray diffraction (XRD). During the SEM observation, the chemical composition of the sample was also characterized with energy-dispersive X-ray spectroscopy (EDS). More details of the sample structures were further revealed with a high-resolution transmission electron microscope (HRTEM). Raman spectra were also acquired as auxiliary data for the final determination of the chemical and crystalline composition of the products. The excitation source was a He-Ne laser with a 632.8 nm wavelength. The resolution of the analysis system was 1 cm-1.
3. Results and Discussion Figure 1 shows the SEM images of the products. As shown in Figures 1(a), (b) and (c), the core of this nanostructure was a hexagonal prism. Seven branches grew on this core prism, six from the side facets and one from the top, respectively. Hereafter this delicate nanostructure with 6-fold symmetry is referred to as a “snowflake”. The diagonal size of these snowflakes was on the order of 1–10 µm. Interestingly, as shown in Figure 1(b), sometimes two snowflakes were coupled up and reminiscent of a pair of gears, indicating their potential application in a nanoelectromechanical system. Figures 1(d) and (e) further reveal that each branch, growing either from the side facets or from the top one, was composed of an array of nanorods. Their end facets were also hexagons, which had
10.1021/cg070261l CCC: $40.75 2008 American Chemical Society Published on Web 06/21/2008
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Figure 1. SEM observation of the ZnO snowflakes: (a) ZnO snowflakes on the Si substrate; (b) a pair of ZnO snowflakes; (c) an individual ZnO snowflake; (d) a branch from a side facet of the core prism; (e) the branch on the top. (The scale bars are 50, 2, 5, 0.5 and 1 µm, respectively).
unequal edges but were very smooth. Such smooth facets could serve as mirror planes of nanolaser cavities.18 The XRD pattern is given in Figure 2. The peak positions in the XRD spectrum are consistent with the standard values for bulk wurtzite ZnO (a ) 0.3249 nm and c ) 0.5206 nm). Hence, the product was determined to be wurtzite ZnO. The (0002) peak, which is 5-6 times as high as the {101j0} and {101j1} peaks, is much stronger than the standard value for the bulk ZnO, suggesting a preferential [0001] growth direction of the ZnO nanorods.19 The single-crystalline nature of the ZnO snowflakes was revealed by the HRTEM observation. Figure 3(a) shows a TEM image of a side branch, which contained an array of ZnO nanorods. Figure 3(b) provides an HRTEM image of one nanorod from the array, and the lattice parameter was measured to be c ) 0.5207 nm, in good agreement with the lattice constant of the wurtzite ZnO. Hence, it is evident from both the HRTEM
observation and the XRD measurement that the ZnO nanorods in the branches grew along the c-axis, i.e., the [0001] direction. The Fourier analysis of the HRTEM pattern is given in the inset of Figure 3(b), further confirming that the nanorod was a wurtzite ZnO. One of the Raman spectra is given in Figure 4. A comparison between Figure 4 and Table 5 in ref 20 shows that all the peaks, with the exception of the Si-substrate-induced 520 cm-1 peak, are attributable to the wurtzite ZnO, further confirming the chemical component and the crystal structure of our samples. The 438 cm-1 peak has the largest intensity among all the ZnO peaks and is assigned to the E2-high mode. The 413 and 380 cm-1 peaks are assigned to the E1-TO and A1-TO modes, respectively. The peak around 591 cm-1 that corresponds to the E1-LO mode approached the short wavelength limit of our monochromator and was thus hardly visible. Also, the E2-low mode should have resulted in a peak around 100 cm-1, which
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Figure 2. XRD of the ZnO snowflake.
Figure 4. A Raman spectrum of the ZnO snowflake.
Figure 3. TEM observation of the branches of the ZnO snowflakes: (a) the nanorod array in a side branch; (b) the HRTEM image of a nanorod in the branch (the inset is the Fourier transformation).
went beyond the long wavelength limit of our monochromator and was therefore not observable. The A1-LO mode was not detected.21 The 330 cm-1 peak is not included in Table 5 of ¨ zgu¨r et al., previous researchers ref 20. As summarized by O have already studied similar peaks at 331 and 332 cm-1 under resonance condition, respectively.22,23 In both studies, the peaks were assigned to the second-order Raman scattering that resulted from the 2-E2(M) mode of the ZnO. When the frequency of the exciting light source approaches that of electronic transitions, intensity of certain vibrational modes, say the 2-E2(M) mode, can be enhanced.24 Interestingly, the resonance effect already manifests itself when the energy of the exciting photon is still much lower than the direct band gap of ZnO, Eg ∼ 3.3 eV, and the 330 cm-1 peak is even available in a nonresonant Raman scattering.20,23,25 Thus, though the exciting wavelength in our experiment was as long as 632.8 nm, this second-order vibration mode was still detectable. Since the secondary nanorods were terminated by mirrorlike smooth planes instead of particles, the VLS process is not considered in the probing of the growth mechanism of this snowflake-shaped hierarchical ZnO nanostructure.11 With a view to understanding its growth mechanism, the products obtained at different stages of the reaction were studied respectively and the results are given in Figure 5. On the basis of these observations, the hierarchical ZnO snowflake is tentatively considered to have grown in three steps: (1) the growth of a Zn hexagonal prism; (2) the oxidation of the prism; and (3) the secondary growth of ZnO nanorods on the prism surface. At the beginning, the furnace was turned on and the temperature of the alumina tube was raised. At this stage oxygen was still not introduced into the tube. The melting point and the boiling point of zinc are 420 and 907 °C, respectively.26 Hence the Zn vapor concentration in the tube must have been high in the absence of a large amount of oxygen. The Zn vapor condensed into hexagonal Zn prisms on the substrate,25,27 where the temperature was lower than that in the middle of the tube. Some such prisms are shown in Figure 5(a). The EDS given in Figure 5(b) shows that their major component was Zn instead of ZnO. After a period of time, when the temperature of the middle of the tube reached 950 °C, oxygen began to be introduced into the tube. Since the substrate temperature also rose accordingly, the Zn prisms that had been deposited on the substrate could undergo two changes, namely, evaporation and oxidation. Depending on whether the evaporation or the oxidation dominated, two kinds of prisms emerged. If the oxidation was
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Figure 5. Products at sequential stages of the reaction: (a) Zn prisms (scale bar: 50 µm); (b) EDS of the prisms; (c) ZnO nanostructure with hollow prisms (scale bar: 5 µm); (d) another ZnO nanostructure with a more visible hollow prism (scale bar: 2 µm); (e) secondary growth of ZnO nanorods on the side facets of a prism (scale bar: 2 µm); (f) the existence of transition between the prism facet and the nanorod (scale bar: 500 nm).
not fast enough, the original Zn prism just evaporated appreciably and finally became hollow, as shown in Figures 5(c) and (d). Yugang Zhang et al. used to obtain “nanocavities” in a similar process.25 When the prism was oxidized in time before considerable Zn evaporation occurred, the Zn prism was transformed into a ZnO one. The melting point of ZnO is 1975 °C,28 thus the evaporation would stop after this transformation. The XRD pattern shown in Figure 2 contains no Zn peaks, indicating that the oxidation was quite thorough. The structures shown in Figure 1 all evolved from this kind of ZnO prisms. Since ZnO has the tendency of growing along the [0001] direction preferentially,17 it is assumed that most part of the prism surface was turned into (0001) plane of ZnO. Therefore, when the oxidization of the Zn prism concluded, the continuous ZnO surface, most part of which was (0001) plane, served as the substrate for further deposition of secondary nanostructure. Owing to the mismatch
in the lattice between ZnO and Zn, the prism was unlikely a single crystal of ZnO. More probably, its hexagonal shape originated from the pristine Zn prism. With the successive arrival of Zn and O2 from the cavity of the alumina tube, the secondary ZnO nanostructure began to grow on the prism surface. The conditions of the epitaxial growth are very stringent, hence it was impossible for the ZnO to grow uniformly layer by layer over the whole prism. Actually, the growth process resembled the initial stage of the most common case in film growth, which follows the Volmer-Weber model.29–31 That is, discrete polycrystalline islands of ZnO, each of which contained a number of crystalline grains, formed on the prism surface first. Nonetheless, due to the considerable anisotropy in the speed of the crystal growth along different directions of ZnO, these islands did not subsequently evolve into continuous film. Among all the grains in one island, only the one whose [0001] direction happened to be perpendicular
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Figure 6. The schematic growth mechanism of the ZnO snowflakes. (a) A Zn prism was deposited on the substrate. (b) The prism was oxidized under oxygen atmosphere and transformed into a ZnO prism. (c) Nanorods grew from all the facets of the prism. (d) A ZnO snowflake with a solid core prism resulted.
to the prism surface could further grow epitaxially. The grains with other orientations did not have robust support from the underlying surface owing to the lattice mismatch between different planes of ZnO. It is well-known that the growth speed of the [0001] direction of ZnO is faster than that of other directions.17 Consequently, the grain with the right orientation, i.e., with the [0001] direction perpendicular to the surface, could grow much faster than other grains. Finally, clusters of secondary [0001]-oriented nanorods resulted on the whole surface of the core prism. This selected-growth-in-an-island model relies on several assumptions that still lack direct experimental or observational
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evidence. However, at least two facts can be readily interpreted with it. First, as shown in Figure 5(f), the secondary nanorods did not grow directly from the surface. Instead, they grew from some mound-shaped transition zones, which are believed to be the islands that formed during the initial nucleation of the ZnO on the surface of the hexagonal prism. In each island, only the grain with the right orientation was allowed to grow sufficiently, thus the diameter of the nanorod was obviously smaller than that of the underlying island. Second, all the secondary nanorods were along the direction perpendicular to the surface. Actually, this is the right direction for the lattice match. A schematic illustration of the growth process of the ZnO snowflake is given in Figure 6. The role of the occasion of the oxygen introduction into the tube was also studied. In different experiments, oxygen was introduced when the temperature at the middle of the tube was in the range of (1) 400-700 °C, (2) 700-1100 °C and (3) 1100-1300 °C, respectively. In the first case, large amounts of residual ZnO, in the form of white grains, were found in the quartz boat at the middle of the tube. Only sparse nanostructures emerged on the substrates at the downstream. The result indicated that in this temperature range the major reaction was the oxidation of Zn and no remarkable Zn evaporation occurred. In the second and third cases, neither Zn nor ZnO residue was left in the quartz boat. A variety of snowflake-like hierarchical nanostructures, as shown in Figure 1, were obtained when the oxygen was introduced at a temperature in range (2). In range (3), as shown in Figure 7, the major products were different ZnO nanostructures, including nanowires, nanorods and nanocombs, etc. At high temperature, the Zn vapor did not have a chance to condense in the tube, thus the Zn prism could not have grown on the substrate. Without the central prism, the
Figure 7. Products obtained when oxygen was introduced in the temperature range 1100-1300 °C: (a) nanowires; (b) nanorods; (c) a nanocomb. (The scale bars are 5 µm, 1 µm and 1 µm, respectively.)
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hierarchical structures were not available and only simpler configurations of ZnO were obtained. These comparison experiments on the oxygen introduction have demonstrated the controllability of our fabrication approach.
4. Conclusion A snowflake-shaped hierarchical nanostructure of wurtzite ZnO was synthesized in a multistage reaction during the thermal evaporation of Zn/ZnO powders. The reaction required no foreign catalyst and fell into three sequential stages: (1) Zn vapor condensed into a hexagonal Zn prism; (2) the prism was oxidized as oxygen was introduced; (3) secondary ZnO nanorods grew on the surfaces of the central prism and the hierarchical nanostructure resulted. Our fabrication has yielded a product that is different from those of the previously reported works based on gas-phase reactions, indicating the feasibility of attaining more ZnO hierarchical nanostructures in a simpler and more controllable manner. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 90606023 and 60771004), the MOST of China (No. 2006CB932402) and the National High Technology Research and Development Program of China (No.2006AA05Z107). The sample characterization was supported by the Instrumental Analysis Fund of Peking University.
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