Plasma Synthesis of Large Quantities of Zinc Oxide Nanorods

Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China, and .... synthesis of ZnO nanorods in a radio frequency (RF) pla...
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J. Phys. Chem. C 2007, 111, 194-200

Plasma Synthesis of Large Quantities of Zinc Oxide Nanorods Hu Peng,†,‡ Yuan Fangli,*,† Bai Liuyang,†,‡ Li Jinlin,† and Chen Yunfa† Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: August 20, 2006; In Final Form: October 8, 2006

In this paper, ZnO nanorods with high yields were successfully synthesized using a radio frequency (RF) thermal plasma system in a one-step, continuous, and scalable way by the thermal evaporation of different zinc compounds, and the aspect ratio of synthesized nanorods could be well controlled by adjusting the feeding rates of starting materials and the oxygen flow rate in the process. The experimental results reveal that the growth mechanism of ZnO nanorods in the RF plasma reactor is governed by a wall-free and two-directional growth mechanism that is proposed for the first time in this paper. The plasma synthesis method provides a facile way to synthesize one-dimensional nanostructures of ZnO and other materials.

Introduction In the past several years, the preparation of one-dimensional (1D) nanoscale materials has stimulated great interest due to their attractive prospects in fundamental physical science and potential nanodevice applications.1-4 A variety of 1D nanostructures (nanorods, nanobelts, nanotubes, nanocables, etc.) have been prepared, including element,5-8 oxide,9-12 nitride,13,14 sulfide,15-17 and others.18-21 Since the first report of nanobelts of semiconducting oxides in 2001, semiconducting materials have been the focus of 1D nanostructure studies because of their exceptional properties ranging from electric, chemical, and optical to magnetic, and these properties can find potential applications in conducting interconnectors and nanoscale electronic, optoelectronic, and sensing devices.2,22-24 For example, GaAs and InAs nanowires have found applications in developing a high-speed field effect transistor, or a laser working at lowthreshold current density and high gain. Gas sensors fabricated by a single SnO2 nanobelt have been proven to be sensitive to environmental polluting species like CO and NO2 as well as ethanol for breath analyzers and food control applications.25 ZnO is a wide band gap (3.37 eV) semiconductor with exceptional semiconducting, piezoelectric, and pyroelectric multiple properties. Its unique properties are due to its large exciton binding energy (60 meV) and characteristic bond strength, which have potential applications in optoelectronics, varistors, detectors, and nanoresonators.26-29 For example, ZnO has been effectively used as a gas sensor material based on its near-surface modification of charge distribution with certain surface-adsorbed species, and ZnO nanorods would provide significant enhancement in sensitivity due to their high surfaceto-volume ratio.30 Up to now, many researchers have studied the synthesis and fabrication of ZnO nanorods, and different synthesis techniques have been developed.31-36 Predominantly, ZnO nanorods are synthesized using vapor deposition process and wet-chemistry routes, and well-controlled 1D nanostructures could be obtained by these methods. However, the main drawback of these methods is the small-scale quantities. The * Corresponding author. Phone: +86-10-82627058. Fax: +86-1062561822. E-mail: [email protected]. † Institute of Process Engineering. ‡ Graduate School of the Chinese Academy of Sciences.

Figure 1. Schematic illustration of the plasma processing setup

TABLE 1. Typical Operating Parameters for Plasma Processing parameters

values

center gas, argon sheath gas, nitrogen carrier gas, nitrogen oxidative gas, oxygen powder feeding rate

1.0 m3/h 5.0 m3/h 150 L/h 0.5-50 L/min 10-60 g/min

synthesis of 1D nanostructures in bulk quantities is essential for them to be used in various applications. There have been some attempts to synthesize ZnO 1D structures in a large scale, and Height et al.37 reported the large-scale synthesis of ZnO nanorods by flame spray pyrolysis and proposed a wall-free growth mechanism. In their case, trace amounts of dopants were introduced to induce the preferential growth of ZnO along some specific crystal planes, and nanorod formation was attributed

10.1021/jp065390b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/23/2006

Plasma Synthesis of ZnO Nanorods

Figure 2. (a) SEM image of as-obtained ZnO nanorods. (b) Magnified SEM image, displaying the diameter of nanorods. (c) XRD pattern of the products.

to the selective effect of dopants on the specific crystal plane. Using dopants presents a restriction to pure ZnO nanorod production. The plasma synthesis process, including both physical vapor deposition and chemical vapor deposition, is one of the most efficient methods for producing nanostructured metal, ceramic, and mixed ceramic nanoparticles. The plasma process can also be used to synthesize 1D nanostructures, such as Nb2O5. Nb2O5 nanowires deposited on the surface of a substrate were synthesized by exposure of Nb foil in the low-temperature oxygen plasma.38 The use of substrates on the microscale level still restricts large-scale production. Thermal plasma is a powerful tool for synthesizing well-dispersed nanoparticles in a continuous and scalable process. With the high processing

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Figure 3. (a) Low-magnification TEM image of the synthesized ZnO nanorods. (b) TEM image of a single nanorod. (c) HRTEM image of a synthesized ZnO nanorod.

temperature (up to 1.0 × 104 K) in the flame zone and fast quenching rate (105-106 K/s) at the flame tail, it is easy to obtain nanostructured powders with high melting points, but the products generated are typically dominated by spherical morphology. ZnO rodlike particles were synthesized in plasma before;39 however, their formation was not well-controlled, and the products were not uniformly rodlike crystals. In the present paper, we report the rapid and continuous synthesis of ZnO nanorods in a radio frequency (RF) plasma system. The raw materials were injected into the plasma system and subsequently underwent vaporization, oxidation, and growth processes that occurred exclusively in the flowing gas without

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Figure 4. The predominant ZnO nanostructures synthesized at different oxygen flow rates: (a) 0.5 L/min, (b) 2.5 L/min, (c) 5.0 L/min, and (d) 45 L/min.

the use of any substrates and catalysts. The well-controlled ZnO nanorods were obtained in a flowing manner that had not been achieved previously, and the influences of the process parameters on the morphology of the final products were systemically investigated, which give a way to understand the wall-free and two-directional growth mechanism. Experimental The experiments were carried out in an RF thermal plasma system under atmospheric pressure. The experimental setup consists of an RF generator (30 kW, 4 MHz), a plasma generator, a reactor, and a powder-collecting filter. Argon and nitrogen were used as the plasma-forming gas and sheath gas, respectively. Specially, the reactor was covered with heat preservation material to decrease the temperature gradient. The schematic illustration of the setup is shown in Figure 1. For nanorods synthesis, the starting powders of zinc, zinc oxide (purchased from Beijing Chemical Reagents Company and used without further purification), and basic carbonate of zinc (BCZ; synthesized in our laboratory) were supplied into the plasma flame by the carrying gas in a continuous way and then underwent vaporization, crystallization, and growth processes following the flowing gas in the reactor; finally, rodlike crystals were obtained. Specially, when zinc powder was used as the starting material, oxygen gas was injected into the system together with the sheath gas to react with the zinc powder. The final products were collected at the bottom of the collector, and the details of the processing parameters are given in Table 1. The phases of the products were characterized by an X-ray diffractometer (Philips X’Pert PRO MPD) through the 2θ range

from 10 to 90° at a scan rate of 0.02 deg s-1 using Cu KR radiation and operating at 40 kV and 30 mA. The morphology of the particles was investigated by both a field-emission scanning electron microscope (SEM, JSM-6700F) and transmission electron microscope (TEM, Hitachi H-800). The detailed morphology and structural characterization were investigated by a high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) in the same TEM (JEOL JEM-2010). Raman spectrometer analysis was taken on a LABRAM-HR laser Raman spectrometer at room temperature using an Ar-ion laser with a 514 nm emission wavelength as the excitation laser. Results and Discussion After the experiment, large-scale, white, wool-like ZnO products were obtained in the collector, and the morphology and structure of the synthesized products are shown in Figure 2. Figure 2a illustrates the typical SEM image of obtained ZnO nanocrystals using zinc powder as the starting material. From Figure 2a we can see that uniformly rodlike products with a length of several microns were obtained in the experiment, and no particles were found. A magnified SEM image reveals the detailed morphology, as shown in Figure 2b, which indicates that the as-synthesized products were composed of rodlike nanostructures, and the obtained ZnO nanorods display a uniform diameter of about 50 nm along the nanorod stem. All the peaks in the X-ray diffraction (XRD) pattern of the sample (Figure 2c) can be well indexed to the hexagonal ZnO wurtzite structure with lattice constants of a ) 0.324 nm and c ) 0.521 nm. No peaks due to zinc powder or other impurities were detected, indicating the high purity of the products.

Plasma Synthesis of ZnO Nanorods

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Figure 6. The product obtained using (a) ZnO and (b) BCZ as the starting materials.

Figure 5. The predominant ZnO nanostructures synthesized at different feedstock rates: (a) 20 g/min, (b) 37 g/min, and (c) 53 g/min.

Further morphology and structural analyses of the ZnO nanorods were performed using a TEM. From the TEM image shown in Figure 3a, we can see that uniform nanorods were obtained in the experiment, and the typical length of the products is about 2 µm, as shown in Figure 3b. In addition, the ZnO nanorods display a uniform diameter of about 50 nm, which is also proved by the SEM image, as mentioned above, and the tips of the two ends exhibit a round shape. The HRTEM image in Figure 3c measured along the stem shows a clean and perfect structure without dislocation and stacking faults observed. The lattice spacing of adjacent lattice planes is 0.52 nm, corresponding to the (0001) plane of ZnO crystal, and the inset SAED pattern was taken from the same nanorod. It, together with the

HRTEM, confirms that the synthesized ZnO nanorods are single crystalline and grow along the [0001] direction. Additionally, the surfaces of the ZnO nanorod are clean, and no amorphous layer is observed from the image. In the plasma synthesis process, oxygen partial pressure and supersaturation of zinc vapor in the reactor play key roles in the growth of the nanorods. Figure 4 shows the typical SEM images of nanorods obtained at different oxygen partial pressures, which were controlled by the flow rate of oxygen gas. As shown in Figure 4a, irregular nanorods were dominant when zinc was oxidized at a very low oxygen rate of about 0.5 L/min, and the XRD analysis showed that the products were mixtures of Zn and ZnO (see Supporting Information). With the oxygen flow rate increasing, for example, to 2.5 L/min, the morphology of the corresponding products became more and more uniform (showed in Figure 4b). High yields of the uniform nanorods were obtained while the oxygen rate was about 5.0 L/min, as shown in Figure 4c, and a single ZnO structure was confirmed by XRD analysis. With the oxygen rate continuously increasing, for example, at 45 L/min, short ZnO nanorods were obtained (showed in Figure 4d). The reason is that a large amount of ZnO nuclei were formed simultaneously at a high oxygen flow rate, which consumes the great mass of zinc vapor. Additionally, the excessively oxidative gas acts as a cooling gas simultaneously, which all restrict the growth of ZnO nanorods. The supersaturation of zinc vapor was controlled by the feedstock rate of the starting material. Figure 5 reveals the typical SEM images of nanorods synthesized at different feeding rates, from which we can conclude that the aspect ratio of the

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Figure 7. TEM images of synthesized (a) bipod, (b) quasi-tripod, (c) T-shaped, and (d) tetrapod ZnO nanocrystals.

final products could be closely controlled by adjusting the supersaturation. When Zn powder was injected at a rate of 20 g/min, uniform nanorods with an aspect ratio of 14 were obtained, as shown in Figure 5a. However, the aspect ratio decreases to 8 at a feeding rate of 37 g/min (showed in Figure 5b). When the feeding rate was increased to 53 g/min, short and thick rods with an aspect ratio of 2 were dominant in the final products, as shown in Figure 5c. The mechanism is similar to that of increasing the oxygen flow rate. As the supersaturation of zinc vapor increased with the Zn feeding rate, the number of nuclei was increased in the following oxidation process, and some nuclei collided together and induced the growth of crystals along the non-C axis from the thermodynamic point of view, thus some rods with a large diameter formed. But all of the products were composed of well-developed nanorods. In the experiments, different starting materials with zinc were fed into the plasma flame to investigate their morphology and structural characteristics in a plasma system. Figure 6 shows the product morphology using ZnO and BCZ as precursors. From the images we can see that the 1D nanostructures are the dominant features of these products, but are different from the nanorods synthesized using zinc powder as the starting material. The nanowhiskers are almost curly, and there are some sheetlike nanocrystals using ZnO as the starting material, as shown in Figure 6a. XRD analysis proves that both of the products are hexagonal ZnO. Thermodynamic analysis shows that, when the source materials are heated to temperatures higher than 1400 °C, they are entirely decomposed into stoichiometric molecular species, such as Zn(g) and O2(g) (while using ZnO as the starting material)

or Zn(g), H2O(g), CO2(g), and O2(g) (while using BCZ as the starting material). In thermal plasma, the high processing temperature provided by the flame adequately drove the starting materials to react as mentioned above, and the oxygen gas was then diluted by the working gas and formed definite oxygen partial pressure. Subsequently, the evaporated zinc reacted with the diluted oxygen to form ZnO 1D structures at a lower temperature. The same result could also be obtained using other materials, such as Zn(OH)2, thus this method provides a facile way to synthesize ZnO nanorods by different starting materials. In addition, the time of ZnO nanorod growth in the reactor is approximately several seconds, and yields of the products are over 20 g/min. Comparing the products with those of conventionally synthesized methods, the yields in the plasma synthesis are overwhelming. Accordingly, plasma synthesis is a rapid and scalable way to synthesize ZnO nanorods. This method can also be used to synthesize other 1D nanostructures such as ZnS (see Supporting Information). In the above process, the products mostly consist of 1D, rodlike whiskers. There is also a certain amount of interesting ZnO nanocrystals obtained, as shown in Figure 7, including bipod, quasi-tripod, T-shaped, and tetrapod ZnO nanocrystals. Isomorphous crystal structures such as bipod and T-shaped have been reported in SnO2, TiO2, and PbS obtained by solutionbased routes;25,44,45 the solvent molecules play a key role in the shape evolution of the nanocrystals. The bipod shape was proven to be a twinned crystal structure, and the two legs are related by twinning on the special crystal plane with each other. These nanostructures obtained in a gas reaction, especially for ZnO, were seldom reported before.39 In the plasma process, no solvent

Plasma Synthesis of ZnO Nanorods

Figure 8. (a) Schematic illustration of the growth mechanism and (b) TEM image of synthesized double-direction needlelike nanostructures.

Figure 9. Raman spectra of ZnO nanorods synthesized at different oxygen flow rates: (a) 45 L/min, (b) 2.5 L/min, and (c) 5.0 L/min.

molecules existed to affect the growth habit of the crystals, but the gas molecules or the ionized gas collided with the crystals, which may function as solvent molecules in solution to accelerate the shape evolution of nanocrystals. Consequently, bipod, quasi-tripod, T-shaped, and tetrapod ZnO nanocrystals were synthesized in the plasma process. The growth mechanism of nanorods via a gas-phase reaction typically involves the vapor-liquid-solid (VLS) process and the vapor-solid (VS) process. In our experiment, no metal catalysts were used in the process, and the round tips at the end of the synthesized nanorods typically confirmed that the growth mechanism of nanorods is governed by the VS process, which is different from those in the references reported

J. Phys. Chem. C, Vol. 111, No. 1, 2007 199 previously. In the conventional process, the Zn vapor was first generated by evaporation, chemical reduction, or gaseous reaction, and then the transportation and condensation occurred in order to form 1D nanostructures. In this process, the templates are involved to serve as energetic favorable sites for the adsorption of reactant molecules or to confine the growth direction of nanorods. The growth direction of as-synthesized nanorods is perpendicular to the substrate; consequently, only one sharp tip on their top ends forms. However, in our experiment, no substrate existed to confine the growth direction of nanorods, and all the processes occurred exclusively in space. The nanorods had two directions to grow, and the round tips at the two ends proved that the nanorods grew in two directions along the [0001] axis. The schematic illustration of the growth mechanism is shown in Figure 8a. RF thermal plasma processes have a high processing temperature (up to 1.0 × 104 K) in the flame zone and a fast quenching rate (105-106 K/s) in the flame tail. Meanwhile, the thermal nonequilibrium effects of plasma provide a unique reaction field for solid solution formation.40,41 Zinc powder was vaporized in the plasma flame and then quickly condensed to obtain high-level supersaturating vapor when they were transported away from the flame. The supersaturating vaporized molecules then collided with oxygen to form ZnO nuclei in the plasma reactor. High-level supersaturation ensures the intensive growth driver of ZnO nuclei, and the nuclei continuously adsorb zinc and oxygen atoms in the falling process with the hot gas flow. Due to the high growth temperature and the wall-free environment, the nuclei grow, conforming to the growth habit of a hexagonal crystal structure in which the low-energy surface tends to be flat, thus the nuclei grow up gradually into a rod shape along the two ends of the stem. In order to investigate the growth mechanism more clearly, a contrasting experiment was carried out under the same conditions without the heat preservation material covered on the reactor, for the purpose of increasing the temperature gradient. As we all know, the growth rate of a crystal is strongly influenced by the system temperature, and the growth rate would be continuously slowed by decreasing the temperature under a high temperature gradient. Figure 8b shows the TEM image of as-synthesized products, which reveals a double-direction needlelike nanostructure. The diameter of the nanorods gradually tapers down from the center (about 50 nm) to the two ends of the rod (about 10 nm), and the tips exhibit extreme sharp morphology. The tapered stem is a result of the decrease in the temperature, and all these characteristics reveal that the nanorods are governed by a wall-free and two-directional growth mechanism. The sharp nanotips are suitable to serve as devices such as field emitters, probes of AFM, and so forth, while the existence of double-direction needle tips could provide two discharge points. When charge carriers migrate through a single double-direction nanoneedle, carriers go from one point discharge to another point discharge. We envisage that this property may stimulate the design of interesting electronic devices. Figure 9 shows the Raman spectra of ZnO synthesized at different oxygen flow rates. From the image we can see that the three spectra have similar shapes. It is well-known that single-crystalline ZnO has eight sets of optical phonon modes at the Γ point of the Brillouin zone, in which the A1+E1+2E2 modes show the Raman activity.42 The figure shows a typical Raman spectrum of the products. The peak at 435 cm-1 corresponds to the E2 mode of ZnO crystal, and the peak at 378 cm-1 should originate from the vibration modes of A1 (TO). Additionally, from the figure we can also see that there is no

200 J. Phys. Chem. C, Vol. 111, No. 1, 2007 distinct peak existing around 559 cm-1, which is contributed to the E1 (LO) mode of ZnO associated with the oxygen deficiency in the crystal.43 Such a strong intensity of the E2 mode and weak E1 mode indicate that the synthesized ZnO nanorods have very low oxygen vacancy. In addition, the peak at 328 cm-1, which is assigned to the second-order Raman spectrum arising from zone-boundary phonons 3E2H-E2L could also be observed in the spectra. Conclusions Uniform ZnO nanorods with high yields were obtained by plasma synthesis in a one-step and continuous manner. The obtained nanorods display perfect single-crystalline structures free from dislocation and defects, and the aspect ratio could be closely controlled. The experimental results reveal that the oxygen partial pressure and the supersaturating level of the material vapor play key roles in the morphology of the final products. Uniform ZnO nanorods with high yields of 24 g/min were obtained at the oxygen flow rate and zinc powder feedstock of 5.0 L/min and 20 g/min, respectively. The nanorod growth was along the [0001] axis and governed by a wall-free and twodirectional growth mechanism. Supporting Information Available: Details of the experimental method for synthesizing BCZ are provided, and corresponding TEM image and XRD pattern are shown in Figures S1 and S2. Figure S3 reveals the XRD pattern of products shown in Figure 4a of the manuscript. Figures S4 and S5 show the morphology and structure of synthesized ZnS products using this system. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (2) Patolsky, F.; Lieber, C. M. Mater. Today 2005, 20. (3) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H. J.; Yang, P. Nature 2003, 422, 599. (4) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235. (5) Lu, X. M.; Fanfair, D. D.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2005, 127, 15718. (6) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (7) Wirtz, M.; Martin, C. R. AdV. Mater. 2003, 15, 455. (8) Vivekchand, S. R. C.; Gundiah, G.; Govindaraj, A.; Rao, C. N. R. AdV. Mater. 2004, 16, 1842. (9) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9. (10) Ohgi, H.; Maeda, T.; Hosono, E.; Fujihara, F.; Imai, H. Cryst. Growth Des. 2005, 3, 1079. (11) Zheng, B.; Wu, Y. Y.; Yang, P. D.; Liu, J. AdV. Mater. 2002, 14, 122.

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