Self-catalytic Synthesis, Structures, and Properties of High-Quality

Mar 5, 2009 - ABSTRACT: High-quality tetrapod-shaped ZnO (T-ZnO) nanostructures were synthesized through the direct reaction of Zn and zinc acetate ...
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Self-catalytic Synthesis, Structures, and Properties of High-Quality Tetrapod-Shaped ZnO Nanostructures Huifeng Li,† Yunhua Huang,‡ Yue Zhang,*,†,‡ Junjie Qi,‡ Xiaoqin Yan,‡ Qi Zhang,‡ and Jian Wang†

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1863–1868

State Key Laboratory for AdVanced Metals and Materials and Department of Materials Physics and Chemistry, UniVersity of Science and Technology Beijing, Beijing 100083, China ReceiVed September 19, 2008; ReVised Manuscript ReceiVed January 12, 2009

ABSTRACT: High-quality tetrapod-shaped ZnO (T-ZnO) nanostructures were synthesized through the direct reaction of Zn and zinc acetate (ZAc) via a thermal evaporation method in Ar at 650 °C without any catalyst. Controlling the experimental parameters showed that ZAc precusor plays an important self-catalytic role in the vapor transport process. The individual legs of as-synthesized ZnO nanotetrapods were characterized using field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction. The photoluminescence (PL) measurements showed the different PL features of T-ZnO nanostructures. Two typical emission peaks at ∼387 nm and ∼495 nm were observed. Especially, the emission peak at 455-495 nm includes four subordinate peaks. Field emission investigation revealed that the T-ZnO nanostructures with different morphologies possess good field emission property with the largest turn-on field of 1.85 V/µm, and the field emission density reaches 1.92 × 10-4 A/cm2 when the field is 5.5 V/µm. The growth mechanism was discussed in detail. A self-catalysis vapor-liquid-solid growth mechanism was proposed for the formation of the ZnO nanotetrapods.

1. Introduction Zinc oxide (ZnO) is an important electronic and photonic material due to its wide direct band gap of 3.37 eV. It has a fairly large excitation binding energy (60 meV) and exhibits near-UV emission and transparent conductivity at room temperature and above. Many types of ZnO nanostructures, such as nanowires,1 nanorod arrays,2 nanocombs,3 nanobelts,4 nanorings,5 nanocables,6 and other nanostructures,7 have been synthesized by various processes, such as thermal evaporation deposition, template-mediated growth, metal-organic vaporphase epitaxy, and carbothermic method. Tetrapod-shaped ZnO (T-ZnO) nanostructures, as a unique morphology, are expected to exhibit some special properties because each leg of T-ZnO is a single crystalline structure and with controllable morphology.8 The T-ZnO nanostructures are reported to have good response to ethanol and methane at different levels and temperatures, which make them a good choice for gas sensors.9 Several studies reported the synthesis of T-ZnO nanostructures by the vapor transport process and vaporization of Zn from Zn precursors. For example, Dai et al. first reported the T-ZnO nanocrystals were successfully synthesized by oxidation of Zn powder.10 Yu et al. also prepared the T-ZnO nanocrystals by heating the mixture of ZnCO3 and graphite in different dynamic flow gas atmospheres with NiO nanocrystals as a catalyst.11 Liu et al. synthesized the T-ZnO nanocrystals using the KOH or K2CO3 aqueous solution to obtain morphologically different ZnO structures by a self-catalytic process,12 and so on. Among the above processes, some were achieved with the aid of catalysts and others were acquired at a higher temperature. Complex manipulation and uncontrolled residue of catalysts used in these methods, however, may prevent some applications of ZnO nanoproducts. In this work, we reported the synthesis of tetrapod-shaped ZnO nanostructures by vaporing Zn and zinc acetate (ZAc) first in Ar and then in O2 atmosphere at 650 °C in the absence of any catalysts. We investigated here deterministic growth of as* Corresponding author. E-mail: [email protected]. † State Key Laboratory for Advanced Metals and Materials. ‡ Department of Materials Physics and Chemistry.

synthesized tetrapod-shaped ZnO nanostructures from the nanometer to the micrometer scale. Different tetrapods have been synthesized by simply adjusting the partial pressure of oxygen within the system. Compared with previous reports, where only Zn powder was used and no ZAc was added,13-15 the use of ZAc evidently decreased the reaction temperature, and a selfcatalytic mechanism for the formation of T-ZnO nanostructures was proposed.

2. Experimental Details T-ZnO nanostructures were synthesized through the direct reaction of a mixture of Zn powder and ZAc powder in a furnace with a horizontal quartz tube. First, Zn powder and ZAc powder with a molar ratio of 10:1 were ground fully into a mixture before being loaded into a quartz boat. Second, the Si substrate with the polished side facing the powder was fixed upon the boat, and the boat with the mixture was placed at the center of the furnace. The process was carried out at 650 °C for 15 min under a flow of Ar at a constant rate of 80-100 sccm (standard cubic centimeters per minute). Then, the flow of Ar is turned off and the flow of O2 turned on at a constant rate of 5-40 sccm for 15 min. After the reaction, a white product was collected from the Si substrate. The morphologies and structures of the synthesized product were characterized using X-ray powder diffraction (XRD) (Rigaku DMAXRB, Japan), field emission scanning electron microscopy (Zeiss, SUPRA-55, Germany), high-resolution transmission electron microscopy (HRTEM) (JEOL-2010, Japan), and selected area electron diffraction (SAED). Photoluminescence (PL) measurements (Hitachi 4500, Japan) were performed at room temperature using the Xe lamp line of wavelength 310 nm as the excitation. Field emission measurements were carried out by using a diode configuration in a vacuum chamber, which was pumped down to ∼10-5 Pa. The cathode was the ZnO nanostructures on a silicon substrate. The anode was a stainless steel plate, which was separated 200 nm from the sample. The data were recorded automatically by the field emission detection system. All the measurements were carried out at room temperature.

3. Results and Discussion Figure 1a shows a typical SEM image of an as-synthesized sample when a flow of Ar at a constant rate of 80 sccm was first used for 15 min and then it was changed to a flow of O2 at

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Figure 1. (a) Low-magnified SEM image of T-ZnO; (b) high-magnified SEM image of T-ZnO and (inset: single T-ZnO nanocrystal image); (c) EDX spectrum.

a constant rate of 40 sccm for 15 min. It is found that the sample entirely consists of highly uniform tetrapod-shaped nanostructures, which have four legs with an average diameter of 200 nm and a length of 2-3 µm. The high-magnification image in Figure 1b shows that the four legs of T-ZnO are cylindrical nanorods with the same size. The cross-connection of the four legs was responsible for the nucleation and growth of the tetrapod structures (inset in Figure 1b).16 The EDX spectrum (Figure 1c) shows that the sample is mainly composed of zinc and oxygen. The obvious copper peaks are from the copper microgrid for HRTEM research. The XRD pattern of the sample (corresponding Figure 1a) is shown in Figure 2. The diffraction peaks were identified to the hexagonal ZnO crystalline phase with a wurtzite structure (JCPDS card No. 36-1451), and no diffraction peaks from other impurities are detected. Further structural characterization of individual T-ZnO nanostructure is performed by using high-resolution TEM (HRTEM), as shown in Figure 3. Figure 3a is a low-magnification TEM image of a T-ZnO nanostructure, from which we can see that ZnO nanostructures have characteristic tetrapod morphology, same as the observation made by SEM (Figure 1b). Figure 3b is a high-magnification TEM image of a T-ZnO. Obviously, each of the T-ZnO nanorods is a high-quality nanocrystal, and there are a few smaller ZnO particles on the surfaces of the

Figure 2. XRD pattern of the T-ZnO nanocrystals.

nanorods by adsorption. Figure 3c shows a HRTEM image taken of the edge of the nanorod leg in Figure 3b, which displays clear lattice fringes. The lattice spacing of 0.32 nm between adjacent lattice planes corresponds to the distance between two

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Figure 3. (a) Low-magnified TEM image of T-ZnO, (b) high-magnified TEM image of T-ZnO, and (c) HRTEM image of T-ZnO (inset: SEAD image of T-ZnO).

(21j1j0) crystal planes. In addition, the [21j1j0] direction is perpendicular to the leg axis, indicating the preferential growth of the legs along the [0001] direction. Figures 3b and 3c show that there are many small particles of ZnO on the edge of the nanorod leg, and corresponding HRTEM images could be clearly observed, such as the lattice spacing of 0.28 nm, corresponding to the interplanar spacing of (101j0) planes of ZnO. The selected area electron diffraction (SAED) pattern recorded from one leg of the T-ZnO (inset in Figure 3c) shows that the leg is single-crystalline, which is consistent with SEM observation of faceted surfaces. Large numbers of experiments indicated that the shapes of T-ZnO nanocrystals depend strongly on the experimental parameters. When we changed the constant of Ar from 80 to 100 sccm and the O2 from 40 to 20 sccm, the morphology of T-ZnO was changed too, as shown in Figure 4a. The four legs of nanocrystals are baseball-bat-shaped nanorods with the same size. From the high-magnification SEM image (Figure 4b) we can clearly see that the each leg of T-ZnO was shaped like a baseball bat with an average diameter of 200 nm and a length of 1-2 µm. In addition, from Figure 4b we also can observe that the top of the baseball bat is hexagonal. When the parameter of the flow of Ar is maintained and the flow of O2 is changed from 20 to 10 sccm, the SEM images of as-synthesized T-ZnO sample are shown in Figures 4c and 4d, which show that the legs of the T-ZnO are of rather perfect symmetry and have needle-shaped tips. These legs are about 350 nm in diameter and the size of the tips is about 40 nm. They are usually several micrometers in length. With reduction of the flow of O2 to 5 sccm, and with the other experimental parameters kept constant, the morphology of needle-shaped ZnO was changed to condyle-like, as shown in Figure 4e. When the partial pressure of oxygen within the system was adjusted, the morphology of T-ZnO was changed obviously. Figure 4f is a single condyle-shaped T-ZnO, which has an obvious interface; the angle of the two arms is 109°. The typical growth mechanisms of ZnO nanomaterials reported in the references mainly include vapor-solid (VS),17

vapor-liquid-solid (VLS),18 and polar growth mechanism.19 The presence of solidified spherical droplets at the tips of nanomaterials was commonly considered to be evidence of the VLS mechanism. Therefore, in our case, a modified VLS mechanism is proposed for the formation of T-ZnO nanostructures, as shown in Figure 5. At the beginning of the reaction, ZAc was heated to decompose into Zn vapor under an Ar atmosphere, and small ZnO droplets were generated because of the presence of O atoms resulting from the decomposition of ZAc. Then it became the core of ZnO, and the evaporation of the Zn powder encountered certain restrictions because the atmosphere was Ar at that time. When the Ar atmosphere was changed to an O2 atmosphere, the vapor of Zn fully integrated with the O2, and a solid-liquid interface was formed. Thereafter, the core of ZnO begins to growth further and eventually forms T-ZnO nanostructures. As described above, by changing the gas flow, i.e., the partial pressure of oxygen in the reaction system, different morphological ZnO nanostructures were formed. The reason may be that gas flows have an important effect on the growth of different crystal planes of ZnO nanostructures. It is known that the gaseous ZnO exists only as highly activated species with an extremely short lifetime. The process of the initial nucleation includes diffusion, collisions of atoms, and reaction between the vapor molecules (including vapor Zn and O2). When the supersaturation increases to a level at which nuclei formed, the ZnO nuclei grow to sizes larger than the critical size. With collisions of atoms affecting the growth rate of each crystal plane at different partial pressures of oxygen, ZnO nanostructures formed into different morphologies.20,21 Furthermore, ZAc used here served as not only a precursor but also a catalyst, which obviously lowers the reaction temperature. Figure 6 shows the PL spectra of different morphologies of T-ZnO nanostructures. The excited wavelength was 325 nm. Two typical emission peaks at ∼387 nm and at 455-495 nm were observed, which was assigned to the ultraviolet (UV) emission and green emission. ZnO is a direct-gap semiconductor with high exciton binding energy (60 meV), leading to the observation of

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Figure 4. Different morphologies of T-ZnO: (a) low-magnified SEM image of club-shaped, (b) high-magnified club-shaped, (c) low-magnified needle-shaped, (d) high-magnified needle-shaped, (e) low-magnified condyle-shaped, and (f) high-magnified condyle-shaped.

UV emission at room temperature. The green emission is generally referred to as the deep level or trapped state emission.22 Figure 6 line (a), corresponding cylindrical shape (Figure 1a), demonstrates a strong green emission band and a weak UV emission band centered at 495 and 387 nm, respectively. It is well-known that the UV emission was attributed to the near band edge emission of the wide band gap ZnO. Vanheusden attributed the green transition to the single ionized oxygen vacancy in ZnO and suggested that the emission comes from the radiative recombination of a photon generated hole with an electron occupying the oxygen vacancy.23 The strong green light emission in ZnO nanorods might be related to a high level of oxygen vacancies concentration in them compared to the ZnO nanowires.22 Therefore, it is reasonable to believe that there exist some oxygen vacancies in the T-ZnO nanorods and the green light emission from the T-ZnO nanorods could be attributed to the above-mentioned single ionized oxygen vacancy. Specially, the inset is an enlarged image of an emission peak at 455-495 nm which includes four subordinate peaks corresponding to the area labeled in Figure 6. We consider that it results from the combination of PL spectra with different peak values. PL peak position can be changed along with the type or quantity of oxygen vacancies which are affected by the dimension of ZnO nanostruc-

tures.24 Wang et al.25 reported the shift of PL peaks of ZnO nanobelts with different dimensions and confirmed such size effect. Therefore, subordinate peaks are produced essentially by size effect of ZnO nanostructures. The field emission (FE) properties of the T-ZnO were characterized by using a Keithley 2410 SourceMeter as analyzer. An area of 0.24 cm2 was chosen as the tested cathode. The samples were connected to the cathode, while another parallel stainless-steel plate served as the anode at a distance of 200 nm away from the cathode. The current data were acquired by varying the applied dc voltage between the cathode and anode from 0 to 1100 V with a step of 10 V. The FE current-voltage characteristics were further analyzed by a simplified Fowler-Nordheim (F-N) equation,

J)A

(

β2E2 Bφ3/2 exp φ βE

)

(1)

where J is the emission current density, E is the macroscopic field, φ is the work function of the emitter, A and B are the F-N constants with values of 1.56 × 10-10 AV-2 eV and 6.83 × 103 V eV-3/2 µm-1, respectively.26,27 The β is the FE enhancement factor that represents the true value of the electric

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Figure 5. Schematic drawing illustrating the mechanism for the ZnO nanostructures.

Figure 6. Photoluminescence spectra of different morphologies of T-ZnO nanocrystals at room temperature: (a) cylindrical, (b) needle-shaped, (c) condyle-shaped, and (d) baseball-bat-shaped.

Figure 7. Field emission characteristics in J-E plot of the different morphologies of T-ZnO nanostructure. The insets are Fowler-Nordheim (F-N) plots corresponding to the field emission from the T-ZnO: (a) cylindrical, (b) needle-shaped, (c) baseball-bat-shaped, and (d) condyleshaped.

field at the tip compared to its average macroscopic value. E ) (V/d) is the applied field, d is a distance between the anode and the cathode, and V is the applied voltage. Figure 7 illustrates the emission current density corresponding to different morphologies of T-ZnO nanostructures. From the J versus E curves, the turn-on fields (defined as the E corresponding to the J of 0.1 µA/cm2)28 of samples a, b, and c are 1.85, 2.56, and 5.3 V/µm, Sample d, corresponding to the condly shaped T-ZnO, did not emit at this electric field. Meanwhile, the emission current densities of samples a, b, and c reached 1.78 × 10-4, 1.92 × 10-4, and 1.21 × 10-5 A /cm2 at a bias field of 5.5 V/µm, respectively. It is evident that the field emission properties of T-ZnO are related to the morphology, the cylindrical T-ZnO nanostructures possess a good field emission property with the lowest turn-on field of 1.85 V/µm, and the needle-shaped T-ZnO nanostructures possess a good field emission reaching the highest density of 1.92 × 10-4 A/cm2 when the field is 5.5 V/µm. To further understand the emission behavior, the J-V data are analyzed in light of the classical

Fowler-Nordheim law. The corresponding F-N plots are shown in the inset of Figure 7, which exhibits linear relation, confirming that the emitting behavior of samples a and c follows F-N theory well. On the other hand, the emission of sample b has a lower enhancement rate and departs from the F-N relation in the high field. The β can be calculated from the slope of the F-N plot [ln(J/V2) versus 1/V plot] if the work function was known. Assuming that the work function of ZnO was 5.3 eV,29 β values were calculated to be 1667, 3586, and 463 corresponding to samples a, b, and c at the distance 200 µm. The excellent field emission property and the good β values are related to the fact that the local electric field is greatly enhanced due to the better crystalline geometry. Furthermore, since the polyhedral shape gives them the advantage of having many sharp tips and edges, the electrons can be emitted easily from the T-ZnO even if a small voltage was applied. So the electrons can be emitted easily in the applied field.30

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4. Conclusions In summary, high-quality T-ZnO nanostructures were synthesized via a self-catalytic method by the vaporization of Zn and ZAc powder. With adjustment of the experimental parameters, different morphological T-ZnO nanostructures were synthesized. HRTEM indicated that the growth of each leg of the T-ZnO is along the [0001] direction. A self-catalysis VLS growth mechanism was responsible for the formation of the ZnO nanotetralegs. The PL measurements showed the different spectra features of T-ZnO nanostructures, and two typical emission peaks at ∼387 nm and at ∼495 nm were observed. Specially, the emission peak at 455-495 nm includes four subordinate peaks. Field emission investigation revealed that the T-ZnO nanostructures with different morphologies possess good field emission property with the largest turn-on field of 1.85 V/µm, and the field emission density reaches 1.92 × 10-4 A/cm2 when the field is 5.5 V/µm. Acknowledgment. This work was supported by the National Basic Research Program of China (Grant No. 2007CB936201), the Funds for International Cooperation and Exchange (Grant Nos. 50620120439, 2006DFB51000), and the Science Foundation (Grant Nos. 50772011, NCET-07-0066).

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