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Zinc oxide (ZnO) is a unique material that has prompted a vast amount of research. Various morphologies and sizes of ZnO materials have led to a wide range of promising applications. Although research related to the applications of ZnO is progressing rapidly, it has been an enormous challenge to produce uniform ZnO materials. A unique synthesis method to produce ZnO materials with various morphologies has been studied and is presented in this paper. Field-emission scanning-electron microscopy has been utilized to characterize ZnO materials in this study to show how the synthesis conditions control the morphologies of ZnO.

Zinc oxide, a wide-band-gap semiconducting oxide, has attracted tremendous attention due to potential applications such as transparent conductive films, high-efficiency vacuum fluorescent displays, field-emission displays, solar-cell windows, and acoustic wave devices. In order to realize all these applications, it is crucial to devise simple and efficient methods for preparing ZnO on a large scale at low cost.

A variety of ZnO nanostructures have been fabricated using different synthesis methods in recent years for many promising applications. There are many synthesis methods available to produce ZnO nanostructures, including thermal evaporation of zinc oxide powders (at 1,400°C),¹ vapor-phase transport processes,^2,3 and solution synthesis.^4-6 All these synthesis methods have some disadvantages. Z. Ren et al. have combined a general molten-salt process to develop a new synthesis approach: molten-salt-assisted thermal evaporation.⁷

With this method, they were able to produce aligned ultra-long ZnO nanobelts. An adaptation of Ren’s method to produce various ZnO nanostructures is presented in this paper as a combustion synthesis method with varying thermal processes. The obtained ZnO nanostructures have been characterized with x-ray diffraction (XRD) and cold field-emission scanning- electron microscopy (FESEM).

Materials used in this experiment include zinc metal (in dust form), a surfactant, sodium chloride, and platinum metal (both wire and sheet form). Zinc dust (~3-10 μm) was obtained from Fisher Scientific and sodium chloride was obtained from Fisher Chemicals. The surfactant used was IGEPAL CO- 660, which contains ethoxylated nonylphenol [C₉H₁₉C₆H₄(OCH₂CH₂)_nOH] and polyethylene glycol [H(OCH₂CH₂)_nOH].

In a typical synthesis, the materials were mixed by following Ren’s method: 1 g of zinc dust mixed with 8 g of NaCl and 4 mL of IGEPAL CO-660, and subsequently ground for one hour. The ground paste-like mixture was loaded into an alumina crucible and covered with a platinum sheet leaving an opening for vapor release. The crucible was then loaded into a box furnace and heated at 800°C. Some slightly different methods from Ren’s were applied at this point, using two heating routes for synthesis of ZnO.

In heating route 1 (R1), the crucible was loaded into a furnace preheated at 800°C and held at this temperature for two hours. In heating route 2 (R2), the crucible was loaded into a furnace at room temperature, then heated up to 800°C at a rate of 10°C/min. and held two hours at 800°C. Platinum sheets and wires were used for both cases on the top of crucibles. When the crucible was loaded into an 800°C preheated furnace (R1), the reactants in the crucible immediately ignited, creating fumes (which were observed from the window of the furnace).

The as-synthesized products were characterized and analyzed using a Scintag XDS2000 powder XRD with Cu Kα radiation, and a Hitachi S-4700 field-emission scanning-electron microscope.

All samples had similar XRD patterns with high crystallinity, even though the ZnO samples have various morphologies. Figure 1 shows a typical XRD pattern of all the resulting samples. The XRD diffraction peaks can be indexed as wurtzite (ZnO) having a hexagonal structure (space group C6mc) with lattice parameters a = 3.2496 Å and c = 5.2065 Å (JCPDS Card No. 36-1451). Only peaks from wurtzite and platinum (from platinum wire or sheet) were detected.

The morphology of the resulting ZnO samples from heating route 1 (R1) and heating route 2 (R2) was directly examined by FESEM without carbon or gold coating due to the semiconducting nature of ZnO.

A typical FESEM image for ZnO grown on platinum sheet from R1 is shown in Figure 2. This image indicates that a high yield of uniform ZnO nanorods was formed on the platinum surface. The diameters of the nanorods are about 300 nm and the lengths are 1.5–2 μm.

A typical FESEM image for ZnO grown on platinum sheet from R2 is shown in Figure 3. This image shows that a very high yield of uniform needle-like ZnO nanostructures (with sharp ends) was formed on the platinum substrate.

It is obvious that ZnO nanostructures obtained from both R1 and R2 in this paper are different from aligned ultralong ZnO nanobelts obtained by Ren’s group.⁷ This might be due to the different substrates used. Platinum was used in this project, while gold was used by Ren and his colleagues. The morphology difference of ZnO nanostructures obtained from R1 and R2 is mainly due to the difference of thermal processes of these two routes. For R1, the crucible was loaded into an 800°C preheated furnace and the reactants in the crucible were immediately ignited, producing fume.

In this case, the entire zinc source was consumed in a very short period of time, resulting in ZnO nanorods with uniform diameters and lengths. For R2, however, the zinc source was heated from room temperature and was gradually consumed while the temperature increased, which led to the formation of a needle-like nanostructure shown in Figure 3. During the R2 process, there is a critical temperature point (this temperature was not recorded, but is known to be less than 800°C). At this temperature point, the reactants in the crucible were ignited and started combustion.

After this critical point, the temperature in the crucible rose rapidly from burning the organic matter in the crucible. Zinc started to form zinc vapor, which reacted with platinum (whose temperature was still the same as the furnace chamber) and air to form a ZnO particle layer first (shown in Figure 3). As the temperature of the furnace chamber rose, crystalline ZnO began to grow on top of the previously formed ZnO particle layer. The concentration of zinc gradually decreased and this led to the formation of needle-shaped ZnO particles shown in Figure 3. This indicates that controlling the heating process will provide a means of controlling the final ZnO morphology.

A platinum wire was also used in R1. The resulting ZnO structures, however, were far more complicated than those on the platinum sheets. Figures 4–7 show some of the nanostructures obtained on platinum wire, which are all from the same run. Figure 8 shows an overall image of ZnO obtained on the platinum wire. Analyzing all the FESEM images of ZnO obtained on the platinum wire indicates that approximately 45% of the particles contain the morphological structure in Figure 4, approximately 45% are ZnO nanobelts, and ZnO superstructures account for 10% of the ZnO morphologies. The reason uniform ZnO nanostructures on platinum wire are not obtained is not completely clear. However, the geometry of the substrate is most likely one contributing factor.

Various ZnO nanostructures can be obtained by changing the thermal routes for combustion synthesis of ZnO and the geometries of substrates. These thermal changes and varying substrates might provide a means of controlling ZnO. Field-emission scanning-electron microscope imaging has been a necessary tool for determining morphologies of the ZnO structures, which is a key component to the usefulness of ZnO materials.

Typically, various ZnO structures have the same crystalline structure. The morphology of ZnO is a key factor in differences in their properties. Many applications of ZnO nanostructures have been identified due to their morphological dependence. For example, the luminescence property of ZnO is morphology-dependent. The relative intensity of luminescence is greatest for nanowire and least for nano-particle (nanowire > powder > nano-needle > nano-particle).⁸ Field-emission scanning- electron microscopy, therefore, as a powerful image analyzer, is very useful for studying various ZnO nanomaterials morphologies.

Support from the U.S. Department of Energy under the Hydrogen Storage Program Grant number DE-PS36- 03GO93013, is greatly appreciated.

REFERENCES

1. Z.W. Pan, Z.R. Dai, and Z.L. Wang, Science, 291 (2001), p. 1947.

2. M.H. Huang et al., Adv. Mater, 13 (2001), p. 113.

3. M.H. Huang et al., Science, 292 (2001), p. 1897.

4. L. Vayssieres et al., J. Phys. Chem. , 105 (2001), p. 3350.

5. L. Vayssieres et al., Chem. Mater., 13 (2001), p. 4395.

6. L. Vayssieres, Adv. Mater, 15 (2003), p. 464.

7. Z. Wang et al., Adv. Mater, 18 (2006), p. 3275.

8. X.H. Sun et al., J. Phys. Chem. B, 109 (2005), p. 3120.

Zhiyong Xu, Jiann-Yang Hwang, Bowen Li, and Xiaodi Huang are with the Institute of Materials Processing and Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931; Howard Wang is with the Department of Mechanical Engineering, Binghamton University, Binghamton, NY 13902-6000. Dr. Hwang can be reached at (906) 487-2600; e-mail jhwang@mtu.edu.