ZnO Doughnuts: Controlled Synthesis, Growth Mechanism, and

CRYSTAL GROWTH & DESIGN

ZnO Doughnuts: Controlled Synthesis, Growth Mechanism, and Optical Properties

2007 VOL. 7, NO. 1 136-141

Tandra Ghoshal, Soumitra Kar,* and Subhadra Chaudhuri Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata-700032, India ReceiVed May 17, 2006; ReVised Manuscript ReceiVed October 23, 2006

ABSTRACT: ZnO doughnutlike cluster particles were prepared by a simple solvothermal route using an ethylene glycol (EG)water solvent system. It was observed that ZnO doughnuts were obtained when an equal or higher volume fraction of EG was used in the solvent. Microstructural studies revealed that these doughnut-shaped particles were clusters of small hexagonal plates arranged in a regular fashion. Because of the capping property of EG, these constituent hexagonal plates arranged in an angular oriented attachment scheme with a view to minimize their surface energies. The resulting assemblies were concave from one side and convex from the reverse side. With the increasing percentage of EG, the concentration of the capping element increased, resulting in the reduction of the size of the constituent plates, which in turn helps the formation of more and more densely packed ZnO doughnuts. Increases in temperature and pressure also favor the formation of densely packed ZnO doughnuts. These ZnO samples exhibited a strong UV emission, along with a defect-related broad green emission peak. Introduction Because the novel properties of semiconductors depend on their size, shape, and crystalline structure,1 it is important to control the shape and size of semiconductors in a controllable way. Various semiconductor materials are always in a research focus in material science because of their unique electronic and optical properties and extensive applications. Among these materials, zinc oxide (ZnO) is an important n-type semiconductor with a wide direct band gap (3.37 eV) and large exciton binding energy of 60 meV. The strong exciton binding energy of ZnO compared to the thermal energy at room temperature (26 meV) ensures an efficient exciton emission at room temperature under low excitation energy.2,3 ZnO has a broad range of high technology applications, including surface acoustic wave filters,4 nanoscale lasers,5 light-emitting diodes,6 photo detectors,7 varistors,8 gas sensors,9 and solar cells.10 Different morphologies of ZnO, such as nanobelts,11 nanocombs,12 tetrapod,13 and nanonails14 have been synthesized by various physical methods such as thermal vapor transport,15 chemical vapor deposition,16 metalorganic vapor-phase epitaxy procedures,17 etc. However, these methods involve special equipment, complex process control, or high temperatures and are unfavorable for industrialization. Besides, wet chemical methods, which are appealing for their simple manipulation, low production cost, and easy upward scaling, have been applied to synthesize ZnO materials at the nano/microscale level.18-20 Various types of self-assembled two- and three-dimensional nano/microstructures of ZnO were also reported by wet chemical routes21-23. A simple synthesis route and systematic study of the self-assembled nano/microstructures of ZnO are still important to exploring different structural aspects of these structures. Here, doughnut-shaped ZnO microparticles have been synthesized by the solvothermal process with an ethylene glycolwater system as the medium. By controlling the volume ratio of ethylene glycol and water, we have varied the size and morphology of the ZnO products. Also, the effect of temperature * Corresponding author. Tel: 091-033-2473-4971. Fax: 91-033-24732805. E-mail: [email protected].

Table 1. Brief Summary of the Experimental Parameters and Corresponding Results sample no.

volume ratio EG:W

T (°C)

morphology

diameter (µm)

1 2 3 4 5 6 7 8 9 10

1:5 1:2 1:1 2:1 3:1 5:1 2:1 2:1 2:1 2:1

200 200 200 200 200 200 160 230 60% filled 90% filled

rodlike platelike doughnut doughnut doughnut doughnut doughnut doughnut doughnut doughnut

10-12 7-8 12-13 7-9 6-7 3-4 9-11 12-13 10-12 13-15

and pressure was studied for a fixed volume ratio of the above solvents. Room-temperature optical absorbance and photoluminescence properties of ZnO microparticles were investigated to explore the optical and crystal quality of the products. Experimental Section For the solvothermal synthesis of the ZnO microstructures, a closed cylindrical Teflon-lined stainless steel chamber with a 45 mL capacity was used. All the reagents and solvents were of analytical grade and used without any further purification. In a typical preparation process, zinc nitrate hexahydrate [Zn(NO3)2‚6H2O] was added to the Teflon chamber filled to a certain fill percentage by a mixed solvent of ethylene glycol (EG) and water (W). Different sets of experiments were carried out with different compositions of the solvent (EG:W volume ratio was varied between 1:5 and 5:1). The details of the experimental parameters used and corresponding brief results are listed in Table 1. After 10 min of stirring, the closed chamber was placed inside a preheated box furnace at 200 °C for 12 h. The precipitate was collected, washed with water and ethanol several times, and dried in air at an ambient temperature. The products were characterized by X-ray powder diffraction (XRD, Seifert 3000P) with Cu KR radiation, and the compositional analysis was done by energy dispersive analysis of X-ray (EDAX, Kevex, Delta Class I). Microstructures of the products were obtained by scanning electron microscopy (SEM, Hitachi S-2300). For absorption measurement, the powder samples were dispersed in ethanol with the help of ultrasonic treatment. Photoluminescence measurements were carried out at room temperature, using 300 nm as the excitation wavelength with a luminescence spectrometer (Hitachi F-2500).

10.1021/cg060289h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006

ZnO Doughnuts

Figure 1. (a) XRD patterns of the ZnO products and (b) EDAX spectrum of a representative ZnO sample.

Results and Discussion Structural Characterization. The dried products were characterized by XRD to determine the crystal structure of the products. Figure 1a shows the XRD pattern of the samples obtained at different compositions of the solvents at 200 °C. All the diffraction peaks were in good agreement with that of the hexagonal wurtzite structure of ZnO (JCPDS card 36-1451). The measured lattice constants c0 and a0 of this hexagonal phase were 5.22 and 3.25 Å, respectively (c0/a0 ) 1.60). The relative intensity of the peaks corresponding to the (101)/(002) plane is highest for sample 6, but gradually decreases from sample 5 to sample 1. For sample 1, the strong peak corresponds to the (100) plane. The relative intensity of the peaks corresponding to the (101)/(002) and (100)/(002) planes varied significantly from the literature values (JCPDS card 36-1451), which indicates the different tropism of the products. Actually, the preferred orientation of sample 1 along the (100) direction could be properly explained on the basis of the crystallographic nature and morphology of the ZnO crystals. In the case of onedimensional (1D) ZnO crystals, growth is in the longitudinal direction, i.e., the growth direction is (002), whereas the side facets of the ZnO hexagonal rods/whiskers were composed of the (100) family of planes. The SEM studies discussed in the following section reveal that sample 1 was composed of ZnO microcrystalline hexagonal rods. The side facets of the sample were more exposed to the XRD beam, resulting in the stronger (100) peak, whereas in sample 2, hexagonal disks or sheets with exposed (002) top and bottom planes were formed. Thus, the (002) peak is the most intense peak for sample 2. All the other samples were clusters of large number of small hexagonal sheets having no specific exposed surface. Thus, no such preferred crystallization was detected in the XRD pattern for the rest of the samples. The chemical purity and stoichiometry of the samples were tested by EDAX. One representative EDAX

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spectrum is shown in Figure 1b that indicates the presence of only Zn and O as the constituent elements. Morphological Study. The shape and size of the products obtained under different experimental conditions were determined from the SEM studies; the results are listed in Table 1. Figure 2a shows the SEM images of sample 1 synthesized with the lowest amount of EG in the mixed-solvent system. The image reveals that the products are microrods with diameters of 4-12 µm and lengths of 10-20 µm. Closer observations reveal that a few of these microrods are composed of a linear assembly of a number of hexagonal sheets. One such microrod is shown in the inset of Figure 2a. Figure 2b shows the morphology of sample 2 synthesized with a 1:2 EG:W solvent composition. The image reveals that the higher concentration of EG in the solvent system restricted the longitudinal growth of the ZnO microcrystals and only hexagonal plates having a diameter of about 5-12 µm and a thickness of ∼2-4 µm were obtained. Both sides of the plates are flat. The image also reveals the interpenetrating nature of a few plates. With a further increase in the percentage of EG in the solvent system, instead of the individual crystals, a regular assembly of the ZnO plates was observed. Figure 2c shows that sample 3 (prepared with a 1:1 EG:W condition) consists of doughnut-shaped hexagonal microparticles of sizes 5-12 µm. These microstructures possessed a concave central part on one side, whereas the reverse side is slightly convex in nature. Figure 2d shows a higher magnification image of one such single structure, revealing the stacking fashion of the individual hexagonal sheets within the doughnut-shaped microparticles. The image reveals that the doughnut-shaped microparticle is a layered assembly of numerous well-edged sheets. These sheets are arranged at progressively increasing angles to the radial axis and highly directed to form arrays in a regular fashion. Images e and f of Figure 2 show the SEM images of sample 4 (prepared with a 2:1 EG:W ratio). The products obtained under this condition are doughnut-shaped microparticles of size 5-8 µm, but the sheets are stacked together more tightly than in sample 3. Images g and h of Figure 2 show the SEM images of the products obtained with higher percentages of EG in the solvent (3:1 and 5:1 EG:W, respectively). The images reveal that relatively uniform doughnutshaped microparticles of sizes 2-6 and 3-4 µm, respectively, are formed under these conditions. It was observed that with the increasing percentage of EG in the solvent, the size of the doughnuts decreased systematically. In all the systems, the concave central part was observed on one side of the doughnuts and its depths increased with the percentage of EG, but the diameter of this concave part decreased simultaneously. Also, the shape of the doughnuts changes slowly from hexagonal to circular with increasing EG. The increase in the percentage of EG also reduced the size of the constituent sheets significantly, and they are much more tightly bounded with each other. The transformation from hexagonal shape to circular and the reduction of the diameter of the central concave part can be ascribed to the reduction in the size of the constituent sheet of the doughnuts. The angle between two sheets decreases consistently with an increase in the concentration of ethylene glycol because of the reduction in the size of the individual sheets inducing the formation of a more compact structure. It is observed that the doughnut shape of the microparticles remains intact and that the sheets are stacked together even after a long ultrasonication time, indicating that the doughnut structure of ZnO is quite stable. It is well-known that temperature and pressure can influence the morphology of the products under solvothermal synthesis.

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Figure 2. SEM images of the ZnO microstructures with different morphologies produced at (a) 1:5, (b) 1:2, (c) 1:1, (e) 2:1, (g) 3:1, and (h) 5:1 ethylene glycol:water volume ratios at 200 °C. Images (d) and (f) show a closer view of the doughnut-shaped microstructures produced with 1:1 and 2:1 ethylene glycol:water volume ratios, respectively.

A series of experiments show that the reaction conditions play a key role in the formation of ZnO doughnuts. With a goal of understanding the effect of temperature on the size, shape, and staking nature of the individual sheets of the doughnuts, we fixed the solvent composition at 2:1 EG:W for all the experiments. Figure 3a shows the morphology of the ZnO products obtained at 160 °C. The microparticles are almost spherical in shape and most of them have a small concave part at their center. The depth of the concave part is quite small compared to that of the structures prepared at 200 °C. The higher magnification image of one such structure is shown in Figure 3b, which reveals that the constituent sheets are arranged in a rather irregular and loosely bound fashion, giving it a mesoporous look. This observation indicates that a lower temperature is not favorable for the well-regulated stacking fashion, whereas high temper-

ature (230 °C) favors the formation of hexagonal doughnuts composed of densely packed small sheets, as revealed from the SEM image shown in Figure 3c. Interpenetrating growth between a few doughnuts is also observed in this case. The filling fractions of the solvent in the reaction vessel are also varied systematically from 60 to 90% in order to evaluate the effect of the pressure developing inside the reaction at a given temperature. It is a known phenomenon that the pressure generated inside the sealed vessel increases systematically with increasing temperature and percentage fill of the solvent. However, there are other critical factors, such as total volume of the container and the boiling point of the solvent, which play important roles in determining the pressure inside the container. In our case, the solvent composition (2:1 EG:W), total volume of the container, and temperature are fixed for all these studies.

ZnO Doughnuts

Figure 3. SEM images of the doughnut-shaped ZnO microstructures obtained at (a) 160 and (c) 230 °C for 2:1 ethylene glycol:water volume ratio. Image (b) shows the closer view of a doughnut-shaped, porous microstructure produced at 160 °C with the same volume ratio of the solvents.

Figure 4a shows the morphology of the products obtained under 90% filling condition, revealing the formation of doughnuts with a small concave part at the center of one of its faces. In this condition, the individual crystallites are too small and much too densely packed. Figure 4b shows the SEM image of ZnO products obtained under 60% filling, revealing the formation of the doughnut structures composed of well-defined sheets. Figure 4c shows a closer view of one broken doughnut, which reveals the staking fashion of the individual sheets inside the doughnuts. Growth Mechanism. Lee et al.24 reported that there are four different parameters, kinetic energy barrier, temperature, time, and capping molecules, that can influence the growth pattern of crystals under nonequilibrium kinetic growth conditions in the solution-based approach. In our experiments, temperature, pressure, and the amount of capping molecules are the key parameters. Ethylene glycol is a neutral solvent with two OH bonds and is generally referred to as a capping material. Capping materials are linked to the surface of crystallites via either covalent bonds or other bonds such as dative bonds.25 Also, Zn2+ ions could react with the EG molecules to form coordination complexes of EG(Zn2+) such as Zn(EG)2(2+) or Zn(EG)3(2+), presumably depending on the concentration of EG.26,27 The complex formation was confirmed by the fact that when the experiment was repeated in the presence of EG, i.e., without using water, a blackish-brown greasy product was formed. The XRD pattern of this product reveals the formation of a complex

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Figure 4. SEM images of the doughnut-shaped ZnO microstructures produced at filling fractions of (a) 90 and (b) 60% for a 2:1 ethylene glycol:water volume ratio at 200 °C. Image (c) clearly shows the nanoplates at the middle of the doughnut-shaped ZnO microstructures produced at a filling fraction of 60%.

material with no characteristic ZnO peak. Thus, the presence of water was essential in the solvent system for reducing the coordination ability of the EG molecule with the Zn2+ ions. At a higher temperature and pressure and in the presence of water molecules, these complexes break up to produce ZnO crystal. The dissolved oxygen in the solvent system acted as the oxygen source. Also, at high temperature and pressure, a few water molecules or EG molecules can dissociate to produce (OH)-1, which acted as the oxygen source. The lower the concentration of EG, the lower the possibility of complex formation. In the absence of ethylene glycol, i.e., when only water was used as the solvent, as there were no or very few (OH)-1 ions present to dissociate Zn(NO3)2‚6H2O, no product was obtained. The growth mechanism for the formation of ZnO nanorods at a lower concentration of EG can be understood on the basis of the following reactions. There is another possibility: that at higher temperature and pressure, the Zn(NO3)2‚6H2O might also dissociate to form Zn(OH)2 in the presence of (OH)-1 ions in the solvents. At high temperature, this Zn(OH)2 transformed to ZnO.

Zn(NO3)2‚6H2O + (OH)-1 f Zn(OH)2 f ZnO These ZnO species form a ZnO seed. These ZnO seeds guided

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by the crystal symmetry agglomerated together to form a hexagonal planer nucleus. In general, a relatively larger ZnO crystal is a polar crystal whose positive polar plane is rich in Zn and the negative polar plane is rich in O. Thus, the Zn-rich positive (0001) surface, being the more reactive surface, can attract new ZnO species or the opposite ionic species to its surface. The six side facets are generally bound by the (101h0) family of planes. In the ZnO crystal growth process, different families of planes follow the growth sequence (0001) > (101h1) > (101h0). Thus, normally, a ZnO rod with the (0001) growth direction bound by six facets is formed. With the increase in the amount of ethylene glycol, the complex formation probability increased, resulting in a decrease in the growth rate along the (0001) direction. Thus, hexagonal plates are formed. With the increase in ethylene glycol, the resulting plates experience an attachment process due to the capping ability of EG. The surface energy of an individual plate is quite high, with two exposed flat planes, and they tend to make an angle with each other to decrease the surface energy by reducing exposed areas. On the other hand, with the well-matched crystal lattice and active surface, the adjacent plates are prone to fuse to each other, driven by the gaining of free energy and lattice-free energy.28 This process occurring for the plates eventually leads to the formation of doughnut-shaped microparticles. In agglomerates, many structures are associated with one another through chemical bonds and physical attraction forces at interfaces. Once formed, agglomerates are very difficult to destroy even after a long ultrasonication time. With the increase in the amount of EG, the resulting plates are found to attach to each other more tightly because of the EG capping property. Thus, the angle between two adjacent plates decreases and the depth of the concave part at the middle also decreases. The concave part at the middle can be explained thermodynamically. For a given material, concave surfaces have a much lower surface energy than convex surfaces. The SEM images shown in Figures 3 and 4 reveal that an increase in temperature as well as in the filling fraction of the reaction chamber had identical effects on the morphology of the products. The microstructural studies indicated that with the increase in temperature and pressure in the chamber, the capping ability of the EG molecules increased significantly, resulting in the formation of more densely packed doughnut structures with small constituent sheets. Optical Properties. Optical properties of ZnO samples were determined through UV-vis absorbance spectroscopy and photoluminescence studies. The ZnO powder products were dispersed in spectroscopic grade ethanol for UV-vis absorption measurements. Figure 5a shows the optical absorbance spectra of ZnO powders produced at different volume ratios of the solvents. The spectra show a single absorption peak centered at 368 nm. The calculated band gap is 3.369 eV, which is in accordance with the bulk value of the band gap of ZnO. The study of photoluminescence spectra is an effective method that evaluates both ZnO defects and its optical properties available as a photonic material. The PL spectra of the ZnO products were measured with an excitation wavelength of 300 nm at room temperature to examine the quality of the product. Figure 5b shows the room-temperature PL spectra of the ZnO products prepared at different volume ratios of the solvents. The PL spectra show a strong UV and a broad green emission peak 393 and 493 nm, respectively. The strong UV emission corresponds to the near band edge emission of ZnO due to annihilation of excitons.29 The excellent room-temperature UV emission property should be attributed to the well-crystalline

Ghoshal et al.

Figure 5. (a) Optical absorbance spectra and (b) room-temperature PL spectra of ZnO microstructures produced at different ethylene glycol: water volume ratios.

nature of the as-synthesized ZnO products. The origin of green luminescence from the undoped ZnO is associated with the intrinsic defect centers like oxygen vacancy (VO), zinc vacancy (VZn), zinc interstitial (Zni), oxygen interstitial (Oi), or antisite oxygen (OZn). There have been many proposed models that explain the origin of green emission. Generally, the green emission corresponds to the singly ionized oxygen vacancy in ZnO and results from the recombination of a photogenerated hole with the single ionized charge state of this defect.30 Vanheusden et al.29 proposed that the singly ionized VO center is the main reason for green emission of the ZnO crystal, whereas Davolos31 suggested that it should be attributed to the lowest lattice microstrain. Fu et al.32 considered that the green emission of zinc oxide results from the electron transition from the conduction band to the antisite defect OZn level. Liu et al.33 and the group of Carter34 proposed that the Zni is responsible for the green emission. The PL spectra show a gradual red shift (from 379 to 394 nm) in the peak position of the UV emission with a decreasing volume of EG in the solvent. This red shift could be ascribed to the formation of surface states below the conduction band. This observation is also consistent with the microstructural studies. Compact doughnuts were produced with a higher volume fraction of EG, whereas individual microstructures were produced for the lower volume fractions. Thus the exposed surface area was higher in the products synthesized with lower volume fraction of EG. Another observation from the PL spectra is that the defect-related green emission decreases with an increasing percentage of EG. This could be attributed to the fact that the EG molecules at high temperature produced (OH)-1 ions, which reacted with the Zn2+ ions to produce Zn(OH)2 that finally decomposed to produce ZnO. Thus, as the volume fraction of EG increased, the possibility of oxygen vacancy site formation decreased, resulting in the decrease in the intensity of the green emission band. Figure 6a shows the PL spectra of ZnO prepared at different temperatured with a

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good crystal quality of the ZnO microstructures. The synthesis route is easily controllable and reproducible. Acknowledgment. We are thankful to the Department of Science and Technology (DST), Government of India, for financial assistance during the tenure of this work. T.G. expresses her sincere gratitude to the Council of Scientific and Industrial Research (CSIR, Government of India) for sanction of a research fellowship during the tenure of this work. References

Figure 6. Room-temperature PL spectra of ZnO microstructures produced at different (a) temperatures and (b) filling fractions for the 2:1 ethylene glycol:water volume ratio.

2:1 EG:water volume ratio. The PL spectrum of the sample prepared at 160 °C showd a broad blue-green emission band, revealing the presence of a large number of surface states and vacancy-related deep states. The PL spectrum of the sample prepared at 230 °C showed a strong green emission band along with a small red shift in the UV emission peak. This indicated the presence of large quantities of defect centers. This could be attributed to the fact that at higher temperature, the atomic species desorbed from the surface of the newly formed particle because of the increased mobility, resulting in the formation of large quantities of defect centers. Figure 6b shows PL spectra of ZnO doughnuts prepared at different filling fractions of the solvent while the EG:water volume ratio was kept fixed at 2:1. Almost identical emission spectra were obtained from all the samples. Conclusions ZnO microstructures with different morphology, such as rods, plates, and doughnut-shaped clusters, were prepared by a simple solvothermal approach. Different volume ratios of the solvents have been used as a shape modifier and proved to be efficient at controlling the shape of ZnO microstructures. The effects of temperature and pressure for a fixed volume ratio of the solvents have been discussed. The possible growth mechanisms for the formation of different microstructures have been proposed. The optical absorption and PL spectra of the samples indicated a

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