Synthesis and Structures of Morphology-Controlled ZnO Nano- and

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Synthesis and Structures of Morphology-Controlled ZnO Nano- and Microcrystals Wenqin Peng,* Shengchun Qu, Guangwei Cong, and Zhanguo Wang

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1518-1522

Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China ReceiVed October 7, 2005

ABSTRACT: Zinc oxide flower-like bunches were directly synthesized on indium-doped tin oxide (ITO) glass substrates through a simple chemical bath deposition process. By adjusting precursor concentration, other morphologies (spindles and rods) were also obtained. All of them are hexagonal and single crystalline in nature and grow along the [0001] crystallographic direction. The possible growth mechanisms for these nano- and microcrystals were proposed. It was revealed that both the inherent highly anisotropic structure of ZnO and the precursor concentration play crucial roles in determining final morphologies of the products. In addition, vibrational properties of ZnO crystals with different morphologies were investigated by Raman spectroscopy. Introduction Zinc oxide (ZnO), an important semiconductor material with a band gap of 3.37 eV, has aroused great interest because of the strong commercial demand for optoelectronic devices operating at blue and ultraviolet regions.1 Compared with other wide-band-gap semiconductors (e.g., ZnSe, GaN), ZnO has higher exciton binding energy (60 meV) at room temperature, which is advantageous for exciton-related device applications. To date, room-temperature UV lasing behavior has been reported for ZnO.2 Thermal stability and irradiation resistance are two important factors that expedite its use and development in harsh environments, such as high-temperature, high-frequency electronics.1 Other applications include chemical sensors,3 solar cells,4 surface acoustic wave devices,5 varistors,6 and transparent conductors.7 The control over size and morphology of nanometer- and micrometer-sized semiconductor materials represents a great challenge to realize the design of novel functional devices. This is because optical and electronic properties of these materials, which finally determine practical applications, can be modulated by varying their size and morphology.8 Recently, special attention has been devoted to nano- and microscaled zinc oxide of one-dimensional (1D) structures, such as wires or rods,9 tubes,10 coaxial cables,11 and belts or ribbons,12 and of complex architectures based on 1D structures, such as branches,13 urchins,14 and networks.15 These materials may provide opportunities to exploit novel properties due to unique 1D structures and explore possible new phenomena arising from hierarchical structures. These special 1D structures are also expected to act as building blocks or interconnects in electronic and photonic devices.16 In the aspect of material synthesis, thermal evaporation,11,14 laser ablation,17 metallorganic chemical vapor deposition (MOCVD),18 and molecular beam epitaxy (MBE)19 have been successfully used to tune the morphology of ZnO 1D nano- and micromaterials. For these methods, vacuum technique, high temperature, catalysts or the use of noxious gas compounds may be required, which will increase the cost and limit the choice of substrates. In addition to the above methods, solution-based chemical approaches have * To whom correspondence should be [email protected]. Phone: 86-10-82304240.

addressed.

E-mail:

Figure 1. XRD pattern of the ZnO sample prepared from 0.03 M precursor. ITO substrate is labeled with asterisk (/). The vertical lines at the bottom correspond to the standard XRD pattern of wurtzite ZnO (JCPDS No. 361451).

become a promising choice because of their low growth temperature, low cost, and good potential for scale-up. Vayssieres et al. reported ZnO nano- and microrods and microtubes on various substrates produced by a template-less and surfactantfree hydrothermal process.9,10 Tian and co-workers proposed the use of controlled seeded growth and citrate anions to produce helical ZnO rods and columns.20 ZnO nanorod and prism arrays were synthesized by varying the solvent and the alkaline condition, with zinc foils used as both a reagent and a substrate.21 In this paper, we report one-step synthesis of morphologytunable ZnO 1D nano- and microstructures via a chemical bath deposition process. Indium-doped tin oxide (ITO) conducting glass was used as a substrate for the direct growth of ZnO. Three types of morphologies, that is, nanorods, nanoflowers, and microspindles, were successfully achieved by varying the precursor concentration. Detailed growth mechanisms for the three types of ZnO structures were discussed. Experimental Section Commercial indium tin oxide (ITO) glass was cut into 10 × 10 mm2 slides as substrates for the direct deposition of ZnO and then carefully

10.1021/cg0505261 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/16/2006

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Figure 2. XPS spectra of the ZnO sample as analyzed in Figure 1: (a) survey spectrum of the sample, (b) Zn 2p spectrum, and (c) O 1s spectrum.

Figure 3. (a,b) FE-SEM images of the flower-like ZnO sample prepared from 0.03 M precursor, (c) TEM image of a flower-like bunch, and (d) HRTEM image of a single rod and SAED pattern (inserted).

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Figure 4. (a,b) FE-SEM images of the rod-like ZnO sample prepared from 0.01 M precursor, (c) TEM image and SAED pattern of the rod-like ZnO sample, (d,e) FE-SEM images of the spindle-like sample prepared from 0.1 M precursor, and (f) TEM image and SAED pattern of the spindle-like sample. cleaned in deionized water, ethanol, and acetone. All chemical reagents in our experiments were of analytical grade (AR) and used without further purification. In a typical experiment, an equimolar (0.03 M) aqueous solution of zinc nitrate (Zn(NO3)2) and hexamethyltetramine (C6H12N4) was prepared. Pretreated ITO substrate was put into the bottle filled with the above solution. Then, the bottle was sealed and heated at a constant temperature of 90 °C for 4 h. After deposition, the ZnO film, which had the flower-like morphology, was cleaned with deionized water and then dried in an air atmosphere. Similarly, rod-like and spindle-like samples were also obtained when the concentration of precursor (zinc salt) was 0.01 and 0.1 M, respectively. To identify the crystalline phase and structure, X-ray diffraction (XRD) patterns were recorded on a multipurpose X-ray diffraction system (PANalytical) with a Cu KR radiation source at 40 kV and 30 mA. The morphology was observed using a JEM-2010 transmission electron microscope (TEM) and a JSM-6700F scanning electron microscope (SEM). X-ray photoelectron spectroscopy (XPS) was performed on a VG Escalab-MKII X-ray photoelectron spectrometer equipped with a dual X-ray anode (Mg and Al). Data were obtained with Al KR (1486.6 eV) as the excitation source. Room-temperature Raman spectra were collected on a Jobin-Yvon T64000 Raman

spectrometer using the 514.5 nm line of an argon ion laser as the excitation source.

Results and Discussion Figure 1 shows a typical XRD pattern of the ZnO nanostructures prepared from 0.03 M precursor. All diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO crystal (JCPDS No. 36-1451), except that the peaks marked with the asterisk are from ITO glass. No characteristic peaks of other impurities were detected in the pattern. The sharp diffraction peaks indicate the good crystallinity of the prepared crystals. It is noted that the relative intensities of the peaks differ from the standard pattern of the bulk material, which should be caused by preferred orientation and distribution of the ZnO crystals on the substrate surface.22 The surface element composition of the as-prepared sample was also studied by XPS analysis. Figure 2a shows the survey spectrum of the sample. No peaks of other elements except Zn, O, and C were observed. The presence of C comes mainly from atmospheric contamination due to the

Morphology-Controlled ZnO Nano- and Microcrystals

Figure 5. Raman spectra of the ZnO samples prepared from different precursor concentrations: (a) 0.01, (b) 0.03, and (c) 0.1 M.

exposure of the sample to air. The binding energies in all the XPS spectra were calibrated using that of C 1s (286.4 eV). Figure 2b,c also displays high-resolution spectra for Zn and O regions, respectively. The binding energies of Zn 2p3/2 and Zn 2p1/2 are 1021.7 and 1044.8 eV, respectively. And the O 1s peak is centered at 530.8 eV. These binding energies are very close to the standard values of bulk ZnO.23 Both XRD and XPS analysis indicate that the as-prepared products are pure ZnO. The morphology and microstructure of the sample analyzed in Figures 1 and 2 were investigated using SEM and TEM. Parts a and b of Figure 3 show the typical SEM images of the sample. These images reveal that a bulk quantity of flower-like bunches exist. Every bunch is composed of closely packed submicrometer-sized rods with diameters of 200-400 nm and lengths of 1.5-3 µm and forms radiating structures. Figure 3c shows the TEM image of a flower-like bunch. After long ultrasonic treatment during the preparation of the TEM specimens, the flower-like nanostructures were not destroyed. This indicates that the formation of the flower-like nanostructures is not due to aggregation. To further obtain structural information for the radiating rods, high-resolution TEM (HRTEM) images and the corresponding selected area electron diffraction (SAED) patterns were also recorded on single rods. The (0002) lattice plane of hexagonal ZnO, with lattice spacing of about 0.52 nm, can be clearly identified in the lattice-resolved HRTEM image (Figure 3d). This indicates that ZnO rods preferentially grow along the [0001] direction. The wurtzite structure of the radiating rods was further confirmed by the SAED pattern. As shown in the inset of Figure 3d, the two indexed bright diffraction spots in mutually orthogonal directions correspond to the (0002) and (01-10) planes, respectively. ZnO samples of rod-like or spindle-like structures were also synthesized by decreasing or increasing the precursor concentra-

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tions. When the precursor concentration is lowered to 0.01 M, only rod-like morphology can be observed. As shown in Figure 4a,b, these nanorods are in high density and randomly scattered on the substrate surface. The sizes in diameter and length of these scattered rods are similar to those of individual rods in flower-like structures in Figure 3. When the precursor concentration is increased to 0.1 M, micrometer-sized ZnO crystals in bulk quantity are obtained (Figure 4d). These ZnO microstructures have spindle-like shape, with center diameters of 2-3 µm and lengths of about 10 µm. From the high-resolution SEM image in Figure 4e, the surface of the spindle-like ZnO is rather rough. The SAED images (Figure 4c for the rod-like products and Figure 4f for the spindle-like products) confirm the hexagonal wurtize structure of the rod-like and spindle-like ZnO products obtained and suggest that they grow along the [0001] direction. Raman spectroscopy was carried out to study the vibrational properties of the ZnO crystals prepared at different precursor concentrations. Figure 5 presents the Raman spectra of these samples at the range of 250-650 cm-1. The dominant feature around 438 cm-1 is due to E2(high) mode, which is a typical Raman peak of bulk ZnO crystal. It is found that the E2(high) mode becomes stronger when the precursor concentration increases, which may mean improved crystallinity of the asprepared ZnO crystals. The peaks at 382 and 411 cm-1 correspond to A1(TO) and E1(TO) phonons of ZnO crystal, respectively. The E1(LO) peak (583 cm-1) was less pronounced, indicating good quality of the as-prepared samples since the E1(LO) mode is associated with defects such as oxygen vacancy, zinc interstitial, or their complexes.24 The peak located at 333 cm-1 may be attributed to a multiphonon scattering process (E2H-E2L).25 To understand the observed behaviors of ZnO, it is necessary to study its growth mechanism. As a wurtzite-structured metal oxide, zinc oxide belongs to the P63mc space group and has a highly anisotropic structure along the c-axis. The (0001) plane (terminated with zinc) of ZnO has the maximum surface energy, while the (000-1) plane (terminated with oxygen) has the minimum surface energy. As a result, the growth along the [0001] direction has a faster rate than that along other directions and is much more favorable. Thus, 1D wire-like or rod-like structures of ZnO are easily formed. The growth of ZnO crystals is also controlled by nucleation and growth processes in aqueous solution. Based on surface energy minimization, ZnO crystallites from zinc hydroxyl nucleate along the c-axis and then grow into 1D nanorods (Figure 4a-c). When the precursor concentration increases, the nucleation of ZnO is so rapid that more ZnO nuclei form in the initial stage. These nuclei may aggregate together due to excess saturation. Each of them individually grows along the c-axis into rod-like crystal, and thus flower-like architectures are finally formed (Figure 3). At a yet higher precursor concentration, many more nuclei are generated, and aggregate and grow into closely packed flower-like bunches. Every bunch is so dense that individual

Figure 6. Schematic illustration of the possible formation process for rod-like, flower-like, and spindle-like ZnO structures.

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rods cannot be distinguished. When two flower-like bunches dock against each other, a spindle-like microcrystal is formed by an oriented particle aggregation process (Figure 4d-f). This process is termed oriented attachment and has been reported by Banfield et al.26 The oriented attachment process is thermodynamically favorable, since the surface energy is significantly reduced due to the elimination of the interface. The rough surface may also suggest that the spindle-like ZnO crystals are formed from flower-like bunches of rods (Figure 4e). The possible growth routes of these as-prepared ZnO nano- and microcrystals can be schematically summarized in Figure 6. Based on the above experiments and analysis, the formation of the three types of ZnO structures is controlled by both inherent structural characteristics of ZnO and external experimental conditions (i.e., precursor concentrations in our case). Other external factors, such as reaction time, reaction temperature, surfactants, and solvents will also influence the size and morphology of ZnO crystals. More in-depth studies are necessary to further understand their growth process, which can provide important information for crystal design and morphology-controlled synthesis of ZnO and other oxides. Conclusion In summary, we have realized the high-yield fabrication of ZnO crystals of controllable morphologies (rods, flower-like bunches, and spindles) by changing the precursor concentration through a facile chemical bath deposition method. Possible formation mechanisms of these ZnO structures were discussed. It was found that final morphologies are controlled mainly by highly anisotropic structure of ZnO and the precursor concentration. These ZnO crystals not only can offer models to study physical properties of 1D structures but also may find potential applications in novel optoelectronic devices. In addition, this simple and low-cost method is expected to allow the large-scale production of other oxides with controllable morphologies, which is advantageous for practical application. Acknowledgment. This work was supported by National Natural Science Foundation of China (Contract Nos. 60276014 and 60006001).

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