CRYSTAL GROWTH & DESIGN
Controllable Growth of ZnO Microcrystals by a Capping-Molecule-Assisted Hydrothermal Process Hui Zhang, Deren Yang,* Dongshen Li, Xiangyang Ma, Shenzhong Li, and Duanlin Que
2005 VOL. 5, NO. 2 547-550
State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China Received August 4, 2004;
Revised Manuscript Received December 1, 2004
ABSTRACT: Different shapes of ZnO microcrystals have been achieved controllably by a capping-molecule-assisted hydrothermal process. The flowerlike, disklike, and dumbbell-like ZnO microcrystals of hexagonal phase have been obtained respectively using ammonia, citric acid (CA), and poly(vinyl alcohol) (PVA) as the capping molecules. Only a very strong UV emission at ∼380 nm is observed in the photoluminescence (PL) spectra of the three kinds of ZnO microcrystals, indicative of their high crystal quality. The formation mechanisms for the hydrothermally synthesized microcrystals in different morphologies have been phenomenologically presented. Introduction Over the past decade, numerous efforts have been employed in controlling the sizes and shapes of inorganic nanocrystals because these parameters represent key elements that determine their electrical and optical properties.1-3 In the vapor synthesis, catalyst, temperature, and composition are often used to control the sizes and shapes of inorganic nanocrystals.4 The vaporliquid-solid (VLS) growth mechanism, in which the nanowires grow out of supersaturated liquid alloy droplets, has been extremely successful in preparing one-dimensional nanomaterials.5 The size of the liquid alloy droplets can easily control the diameter of onedimensional nanomaterials.6 While in the liquid synthesis, capping molecules, regular or inverse micelles and complexing agents are commonly used to regulate the sizes and shapes of inorganic nanocrystals.7-8 Previously, we have successfully employed this method to controllably grow nanostructures including Se nanotubes, flowerlike ZnO nanostructures, CdS nanorods, Bi2S3 nanowires, SnS2 nanobelts, and star-shaped PbS.9-14 ZnO is a key semiconductor for low-voltage and short wavelength electrooptical devices due to its wide band gap (3.2 eV) and large exciton binding energy of 60 meV.15 Quite a few interesting nanostructures of ZnO including nanobelts, nanobridges, nanonails, and nanoribbons have been fabricated by thermal evaporation of oxide powders.16-19 As we know, ZnO is a polar crystal with hexagonal phase, and the high anisotropy of ZnO leads to the oriented growth along the c axis.20 Therefore, one-dimensional ZnO nanostructure can self-assemble in the liquid medium. For example, we have prepared the sword-like ZnO nanorods by an organic free hydrothermal process.21 It is very difficult to controllably synthesize ZnO crystals of different morphologies by solution routes. On the contrary, by means of a vapor route, Wang et al. fabricated many interesting ZnO microcrystals such as nanorings, nanobows, and lord rings taking advantage of the action of electrostatic * To whom correspondence should be addressed. E-mail: mseyang@ zju.edu.cn.
polar charge by the addition of a few impurities in the growth process.22-23 As we know, in liquid medium, although the growth habit of ZnO microcrystals is mainly determined by its intrinsic structure, it is also affected by the external conditions such as temperature, pH value of solutions, and capping molecules. Among them, the capping molecules can tailor the surface energy according to capping the surface of nuclei and thus control the shapes of microcrystals. The shape evolution of PbS nanocrystals with a symmetric rock-salt structure provides a good example of such processes.24 Herein, we report the controllable growth of flowerlike, disklike, and dumbbell-like ZnO microcrystals fabricated via a hydrothermal process assisted by the capping molecule of ammonia, citric acid (CA), or poly(vinyl alcohol) (PVA). The formation mechanisms for the flowerlike, disklike, and dumbbell-like ZnO microcrystals have been phenomenologically presented. Experimental Section In a typical synthesis of ZnO microcrystals, 1 g of Zn(CH3COO)2 was put into 60 mL of water under stirring. After 10 min of stirring, 2 mL of ammonia, 0.5 g of CA and 1 g of PVA were introduced into above aqueous solutions, respectively. Then, 10 mL of 2 M NaOH aqueous solution was added into the two aqueous solutions with CA and PVA, respectively. The pH value of above three aqueous solutions was equal to 10, 11, and 10, respectively. And then those solutions, which were filled to 120 mL by water, were transferred into Teflon-lined stainless steel autoclaves, which were sealed and maintained at 200 °C for 20 h. Subsequently, the resulting solid product was centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final product, and finally dried at 60 °C in air. The obtained samples were characterized by transmission electron microscopy (TEM) and selected area electron diffraction (SAED), which was performed with a JEM 200 CX microscope operated at 160 kV. The field emission scan electron microscope (FESEM) images were obtained on a Fei Sirion. The photoluminescence spectrum (PL) was achieved on an Edinbergh instrument FLS 920 using a 325 nm excitation line.
Results and Discussion Figure 1 illustrates the FESEM images of ZnO microcrystals prepared by the hydrothermal processes
10.1021/cg049727f CCC: $30.25 © 2005 American Chemical Society Published on Web 03/02/2005
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Figure 1. FESEM images of ZnO microcrystals prepared by using different capping molecules: (a) and (b) ammonia; (c) and (d) CA; (e) and (f) PVA.
respectively using ammonia, CA, and PVA as the capping molecules. The right images are of high magnification, focusing on a region in the left images of lower magnification. As can be seen, different morphologies such as the flower, disk, and dumbbell are respectively hydrothermally vividly obtained by using capping molecules of ammonia, CA, and PVA. Concretely speaking: (1) the flowerlike structure consists of a center hexagonal rod and six clusters of sword-like rods laterally initiating from the center rod; (2) the disk is actually a cylinder with about micrometers in diameter and several hundreds nanometers in thickness; moreover, the surface of disk is microscopically rough, the reason for which is not thoroughly clear; (3) the dumbbell consists of two hexagonal ZnO microcrystals, namely, twinned crystal. Further structural characterization of the ZnO microcrystals is performed using TEM. Figure 2a shows the TEM image of the flowerlike ZnO microcrystals. Unfortunately, three-dimensional flowerlike morphology cannot be clearly observed under TEM. However, the sword-like petals with the same size as determined from the FESEM image can be observed. The inset selected area electron diffraction (SAED) pattern performed on the individual sword-like petal indicates that the swordlike petals are single crystalline in nature and can be indexed as the hexagonal ZnO phase. Figure 2b shows the TEM image of disklike ZnO microcrystals. The circular morphology with rough surface and uniform size can be observed. The inset SAED pattern composed of bright spots in 6-fold symmetry indicates that the disklike ZnO microcrystal is a single crystal that is of hexagonal phase. Figure 2c shows the TEM image of dumbbell-like ZnO microcrystals, which is actually twinned crystal. The inset SAED pattern also indicates that the dumbbell-like ZnO is also of high crystallinity and hexagonal phase.
Zhang et al.
As described above, the morphology of ZnO microcrystals can be manipulated by using different capping molecules, indicating that the anisotropic growth of ZnO is significantly affected by the capping molecules. Figure 3 illustrates a conceptual model to explain the formation of ZnO microcrystals of different morphologies by using different capping molecules and pH values. In the following, the formation mechanism addressing the specific capping molecules and pH value is respectively elucidated. In the case in which ammonia acted as the capping molecules and pH was around 10, in the precursor solution, the Zn source was primarily in the form of [Zn(NH3)4]2+, while the remaining Zn source existed in the form of Zn(OH)2 precipitates. In our previous study, it has been proved that the ratio of Zn(OH)2 to [Zn(NH3)4]2+ substantially determines the morphology of ZnO crystals. Concretely speaking, under the condition that the precursor solution contained a small quantity of Zn(OH)2 and a large quantity of [Zn(NH3)4]2+, the anisotropic growth of ZnO was favorable when the pH value was larger than 7.10,21 Therefore, in the present situation, a hexagonal cone was preferably formed along the 〈0001〉 orientation, as shown in Figure 3a(2). In the following hydrothermal process, according to the SEM pictures shown in Figure 1a,b, it is conceived that secondary nucleation of ZnO crystals occurred on the six planes of hexagonal cone. The remaining [Zn(NH3)4]2+ complexes decomposed into free Zn2+ ions and then transformed into growth units of ([Zn(OH)4]2-) at high pH value. These growth units facilitated the ZnO crystal growth initiating from the six planes of hexagonal cone, which was also oriented in the 〈0001〉 direction. Thus, we can see parallel ZnO “swords” rooted on each plane of hexagonal cone. It should be stated that essential understanding of the ZnO crystal growth revealed in Figure 1a,b demands more in-depth experimental and theoretical work. When using CA as the capping molecules and at pH of 11, in the precursor solution the Zn source was primarily in the form of Zn(OH)2 precipitates and [Zn(C6H5O7)4]-10 During the hydrothermal process, large quantity of ZnO nuclei were first formed due to the decomposition of a part of Zn(OH)2 precipitates, as shown in Figure 3b(1). On the other hand, certain Zn(OH)2 precipitates transformed into the growth units of [Zn(OH)4]2- under alkaline condition. It is well-known that the polar growth of ZnO crystal along 〈0001〉 direction proceeds through the adsorption of growth units of [Zn(OH)4]2- onto (0001) plane. However, in the present case, the negative charged [Zn(C6H5O7)4]-10 complexes preferably adsorbed on the (0001) plane, thus repelling the growth units of Zn(OH)4]2-, as illustrated in Figure 3b(2). Accordingly, the intrinsically anisotropic growth of ZnO along the 〈0001〉 direction was substantially suppressed. As a consequence, the disklike ZnO crystals were formed. It has been previously reported that dumbbell-like ZnO twinned crystals can be formed by the hydrothermal process using KBr or NaNO2 as the mineralizer.25 In such a hydrothermal process, K+ or Na+ may be taken as the bond bridge between the growth units to form a crystal nucleus of the dumbbell-like twinning crystal and take (0001) as the composition plane. In our
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Figure 2. TEM images of ZnO microcrystals prepared by using different capping molecules: (a) ammonia; (b) CA; (c) PVA. The upper-left insets of the TEM images correspond to the SAED pattern of the ZnO microcrystals.
Figure 4. PL spectra of the ZnO microcrystals prepared by different capping-molecule-assisted hydrothermal processes.
Figure 3. Growth schematic diagrams of the ZnO microcrystals prepared by the capping-molecule-assisted hydrothermal process.
experiment, when using PVA as the capping molecules and at pH of 11, most of Zn2+ ions were precipitated into Zn(OH)2. Under hydrothermal conditions, the Zn(OH)2 precipitates were transformed into growth units of [Zn(OH)4]2-. In the presence of PVA, as shown
in Figure 3c(2), the growth units can be bonded by the PVA at the composition plane (0001) to form the nucleus of dumbbell-like twinning crystal, which is similar to the case in which KBr or NaNO2 was used as the mineralizer for the hydrothermal process, as mentioned before. The growth of the individual crystallite in the twin crystal took place along the polar c-axis by means of the incorporation of growth units on the growth interfaces (0001) and thus dumbbell-like ZnO crystals were formed ultimately. The room temperature photoluminescence (PL) spectra of the above-mentioned three kinds of ZnO microcrystals were recorded as shown in Figure 4. The
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wavelength of the excitation beam is 325 nm. Strong peaks at ∼380 nm corresponding to near band-edge emission are observed for all the samples, and no other peaks such as green emission (∼520 nm) were found. It should be mentioned that the green emission is usually detected in the ZnO microcrystals prepared by the vapor phase process, which is attributed to the singly ionized oxygen vacancy in ZnO.26 It is commonly accepted that high-quality ZnO crystals only emit UV light. Consequently, it is reasonably believed that the three kinds of hydrothermally synthesized ZnO microcrystals are of good quality. Conclusion In summary, ZnO microcrystals in flowerlike, disklike, or dumbbell-like morphology have been controllably synthesized by the hydrothermal process using ammonia, CA, or PVA as capping molecules. The flowerlike and disklike ZnO microcrystals are hexagonal and single crystalline in nature, while the dumbbell-like microcrystal is hexagonal and twinning crystalline in nature. The ZnO crystal growth mechanism based on the growth units of [Zn(OH)4]2- is employed to explain the formation of the three different ZnO microcrystals. Only a very strong UV emission at ∼380 nm is observed for the hydrothermally synthesized ZnO microcrystals, indicative of their high crystal quality. It is anticipated that method presented in this work offers a solution to controlling the morphology of crystal in hexagonal structure. Acknowledgment. The authors would like to acknowledge the financial support of the Natural Science Foundation of China (60225010) and of the Key Project of Chinese Ministry of Education. We thank Prof. Youwen Wang for the TEM and FESEM measurements. References (1) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241.
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