Sonochemical Preparation of Shape-Selective ZnO Nanostructures Seung-Ho Jung,† Eugene Oh,‡ Kun-Hong Lee,*,† Yosep Yang,§ Chan Gyung Park,§ Wanjun Park,# and Soo-Hwan Jeong*,‡ Department of Chemical Engineering and Department of Material Science and Engineering, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea, Department of Chemical Engineering, Kyungpook National UniVersity, Daegu 702-701, Korea, and Semiconductor DeVice and Material Lab, Samsung AdVanced Institute of Technology, Yongin 449-712, Korea
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 1 265–269
ReceiVed March 27, 2007; ReVised Manuscript ReceiVed September 17, 2007
ABSTRACT: A simple and facile sonochemical route has been demonstrated for the shape-selective preparation of highly crystalline ZnO nanostructures, such as nanorods, nanocups, nanodisks, nanoflowers, and nanospheres. The concentration of precursor chemicals, the kind of hydroxide anion-generating agents, the ultrasonication time, and the use of a capping agent are key factors in the morphological control of ZnO nanostructures. This method is fast, simple, convenient, economical, and environmentally benign. On the basis of our shape-control of the ZnO nanostructures by the sonochemical technique, growth mechanisms of ZnO nanostructures were also proposed. We believe this technique will be readily adopted in realizing other forms of various nanostructured materials.
1. Introduction The shape control of semiconductor nanostructures has attracted considerable attention for potential applications due to their physical and chemical properties which are determined by morphology, size, and dimensions.1–4 Thus, the shape control of semiconductor nanostructures has been the topic of intensive investigation in recent materials chemistry. ZnO is one of the most important multifunctional semiconductors with its wide direct energy band gap of 3.37 eV and its large exciton binding energy (about 60 meV). As a result, various ZnO nanostructures, including nanowires,5 nanotubes,6 nanobelts,7 and nanodisks,8 have been reported for potential applications.9–13 In addition to the conventional vapor-phase methods of vapor transport and condensation,5,8–11 thermal evaporation,7,12 and metal-organic chemical vapor deposition,6 solution-phase methods have been developed as alternative ways to synthesize ZnO nanostructures with different shapes and dimensions. A hydrothermal method13,14 is a widely used technique that can control the shape and dimension of ZnO nanostructures among all solution-based approaches. Unlike conventional vapor-phase methods, the hydrothermal method can produce various ZnO nanostructures at a relatively low temperature (below 200 °C) using simple equipments; however, the reaction time required for the growth of ZnO nanostructures is too long (usually from a few hours to several days).3,13,14 Therefore, the development of a simple and fast synthetic route that can control the shape of ZnO nanostructures under ambient conditions remained an important topic of investigation. A sonochemical method has been recently investigated as a promising alternative technique for the fabrication of ZnO nanomaterials under ambient conditions.15,16 The reason is that this method is fast, simple, convenient, economical, and environmentally benign.17–19 Even though there is a study on aspect-ratio control of ZnO nanorods via sonochemical pretreatment,20 sonochemical synthesis of ZnO nanostructures with * To whom correspondence should be addressed. E-mail:
[email protected] (K.-H.L.),
[email protected] (S.-H.J.). † Department of Chemical Engineering, Pohang University of Science and Technology. ‡ Department of Chemical Engineering, Kyungpook National University. § Department of Material Science and Engineering, Pohang University of Science and Technology. # Samsung Advanced Institute of Technology.
different shapes has not yet been reported. Herein, we report a facile sonochemical fabrication of highly crystalline ZnO nanostructures with different shapes, such as nanorods, nanocups, nanodisks, nanoflowers, and nanospheres.
2. Experimental Procedures ZnO nanorods, nanocups, nanodisks, nanoflowers, and nanospheres were fabricated in a horn-type reaction vessel using an ultrasonic technique. All chemical reagents were used without further purification. Zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 98%, Aldrich) and hexamethylenetetramine (HMT, (CH2)6N4, 99%, Junsei) were used as zinc cation and hydroxide anion precursors, respectively, for the synthesis of ZnO nanorods, nanocups, and nanodisks. For the production of ZnO nanorods, a mixture of 0.02 M zinc nitrate hexahydrate aqueous solution (50 mL) and 0.02 M HMT aqueous solution (50 mL) was prepared at room temperature. Ultrasonication was performed by a sonochemical apparatus (frequency of 20 kHz) under ambient conditions, in order to synthesize ZnO nanorods. An ultrasonic wave was introduced at a power of 50 W (intensity of 39.5 W/cm2) for 30 min by a 1/2 in. diameter titanium tip. The ZnO nanorod-containing solution was filtered with a polycarbonate membrane that had pores of 100 nm in diameter. ZnO nanorods were washed with deionized (DI) water after filtration, and then dried in an oven. For the synthesis of ZnO nanocups, a mixture of 0.2 M zinc nitrate hexahydrate solution (50 mL) and 0.2 M HMT solution (50 mL) was prepared at room temperature. Ultrasonication was performed at an intensity of 39.5 W/cm2 for 2 h. The ZnO nanocupcontaining solution was filtered, and then the resulting powder particles were washed and dried. Unlike the synthesis of ZnO nanorods and nanocups, triethyl citrate (HOC(COOC2H5)(CH2COOC2H5)2, 99%, Aldrich) was used as an additional chemical to synthesize ZnO nanodisks. An aqueous solution of 100 mL, which contained zinc nitrate hexahydrate (0.01 M), HMT (0.01 M), and triethyl citrate (0.1 M), was prepared at room temperature. Ultrasonication was performed at an intensity of 39.5 W/cm2 for 30 min. The ZnO nanodisk-containing solution was filtered, and then the resulting powder particles were washed and dried. Zinc acetate dihydrate (Zn(CH3COO)2 · 2H2O, 98%, Junsei) and ammonia–water (28–30 wt%, Kanto) were used as zinc cation and hydroxide anion precursors, respectively, for the synthesis of ZnO nanoflowers and nanospheres. In case of ZnO nanoflowers, a mixture of zinc acetate dihydrate solution (90 mL) and ammonia–water (10 mL) was prepared at room temperature. Concentrations of zinc acetate dihydrate and ammonia were 0.01 and 1.57 M, respectively. In the case of ZnO nanospheres, triethyl citrate was added to the above mixture of zinc acetate dihydrate solution (90 mL) and ammonia–water (10 mL). Concentrations of zinc acetate dihydrate, triethyl citrate, and ammonia were 0.01, 0.01, and 1.57 M, respectively. Ultrasonication
10.1021/cg070296l CCC: $40.75 2008 American Chemical Society Published on Web 11/30/2007
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Figure 1. Schematic of the shape-selective synthesis of ZnO nanorods, nanocups, nanodisks, nanoflowers, and nanospheres via a sonochemical route. was performed at an intensity of 39.5 W/cm2 for 30 min. Each ZnO nanoflower- and nanosphere-containing solution was filtered, and then the resulting powder particles were washed and dried. The morphology of the as-prepared ZnO nanostructures was observed by field emission scanning electron microscope (FESEM, Hitachi S-4300SE). The crystallinity and crystal structure were investigated by transmission electron microscope (TEM, JEOL JEM-2010). The crystal structure of the ZnO nanostructures was also examined by X-ray diffraction (XRD, Max Science, M18XHF).
3. Results and Discussion The key idea of the shape-selective formation of ZnO nanostructures in our sonochemical research is to control the crystal growth in a specific direction. Figure 1 shows a condensed illustration of our strategies in the morphology control of ZnO nanostructures. First, ZnO nuclei generally evolve into nanorods by preferential c-axis ([0001] direction) oriented growth. Second, nanorods can be converted into nanocups by a decrease in the crystal growth rate along the [0001] direction and simultaneous local dissolution of the polar (0001) surfaces. Third, when the crystal growth along the [0001] direction is suppressed, nanodisks can be obtained due to the growth along the 6-fold symmetric directions, which are normal to [0001] direction. Fourth, multiple nanorods growth from center nuclei results in nanoflowers. Finally, when multiple nanorods growth from center nuclei is suppressed, nanospheres can be formed by the isotropic growth from center nuclei. Figure 2 shows both FESEM and TEM images of the asprepared ZnO nanostructures. Hexagon-shaped ZnO nanorods, with terraces and steps at the end of nanorods, could be clearly observed in the inset of Figure 2a. The average diameter and length are about 180 nm and 1.5 µm, respectively. Terraces and steps on the hexagon-shaped end of the nanorods, without metal nanoparticles, suggest that the as-prepared ZnO nanorods were synthesized by noncatalysis and layer-by-layer growth mechanisms.21 Figure 2c shows hexagon-shaped ZnO nanocups, very short nanorods with an open cavity at the end. The average outer-diameter and length of the ZnO nanocups are about 250 and 300 nm, respectively. From the TEM images in Figure 2b,d, ZnO nanorods and nanocups are highly crystalline with a lattice spacing of about 0.26 nm, which corresponds to an interlayer spacing of the (0002) planes in the ZnO crystal lattice. With the TEM observation, the electron diffraction patterns of ZnO nanorods and nanocups confirm that both of them had a [0001]
Figure 2. SEM (left) and TEM (right) images of ZnO nanostructures. (a, b) Nanorods. (c, d) Nanocups. (e, f) Nanodisks. (g, h) Nanoflowers. (i, j) Nanospheres. A corresponding electron diffraction pattern and a HRTEM image were inserted as an upper and a lower inset in TEM images, respectively. HRTEM images of five ZnO nanostructures were obtained at the point indicated by dotted circles in low-magnification TEM images.
direction, the polar c-axis of the ZnO crystal lattice (insets of Figure 2, panels b and d). Besides one-dimensional (1D) ZnO nanostructures, nanodisks, nanoflowers, and nanospheres were selectively synthesized as shown in Figure 2e-j. Hexagon-shaped ZnO nanodisks, with an average diameter of 500 nm, could be clearly observed in Figure 2e. The average thickness of the ZnO nanodisks was about 140 nm. Hexagonal ZnO nanodisks are highly crystalline with a lattice spacing of about 0.28 nm, which corresponds to j planes in ZnO crystal lattice, an interlayer spacing of the (1010) as shown in Figure 2f. While ZnO nanorods and nanocups have a single growth direction of [0001], ZnO nanodisks have six j j symmetric growth directions of ( [1010], ( [0110], and ( j [1100]. In Figure 2g, ZnO nanoflowers had many nanorods that correspond to petals grown from the same center. Most
Preparation of Shape-Selective ZnO Nanostructures
nanorods, which compose nanoflowers, showed sharp tip morphology rather than hexagon-shaped morphology of previous nanorods in Figure 2a. The length of nanorods varies from several hundred nanometers to 1 µm, and the average size of whole nanoflowers is about 1.5 µm. From the Figure 2h, highly crystalline nanorods grew radially from the same center as petals of ZnO nanoflowers. Each nanorod petal shows a lattice spacing of about 0.26 nm, which corresponds to an interlayer spacing of the (0002) planes in the ZnO crystal lattice. Figure 2i shows spherical ZnO crystals with diameters in the range of 200 nm to 1.4 µm. Although ZnO spheres showed wide variations in diameter, submicron ZnO nanospheres were dominant in our products. From the TEM images in Figure 2j, ZnO nanospheres are nonhollow and highly crystalline with a lattice spacing of about 0.28 nm, which corresponds to an j planes in the ZnO crystal lattice. interlayer spacing of the (1010) With the nonhollow feature and the 6-fold symmetry of the electron diffraction pattern in Figure 2j, we believe that ZnO nanospheres are formed by the isotropic growth from center nuclei. Control experiments were carried out to determine if ultrasonication is essential for producing shape-controlled ZnO nanocrystals (see Table S1, Supporting Information). Instead of the temperature increase due to ultrasonication, control experiments were carried out under rigorous stirring at 70 °C. From control experiments, we could know that ZnO nanorods with a small amount of atypical particles were produced without ultrasonication under nanorod growth conditions (see Figure S1, Supporting Information). However, under nanocup growth conditions, short nanorods and atypical particles were mainly obtained without ultrasonication (see Figure S2, Supporting Information). In addition, a small amount of nanorods with an open cavity at the end were fabricated. A very small amount of flake-like particles were synthesized under nanodisk growth conditions without ultrasonication (see Figure S3, Supporting Information). Finally, no ZnO nanocrystals were synthesized under both nanoflower and nanosphere growth conditions. As mentioned above, the morphology of ZnO nanostructures cannot be controlled without ultrasonication in our research. Cavitation phenomena, which are generated during ultrasonication, play a key role in the sonochemical synthesis of various ZnO nanocrystals. A large amount of energy, temperatures of 5000 K and pressures of up to 1800 atm,22–24 is known to be released from the collapse of microbubbles during ultrasonic cavitation. This energy is high enough to overcome the energy barriers which are needed for the growth of various ZnO nanocrystals at each condition. Therefore, we could conclude that ultrasonication is essential to producing shape-controlled ZnO nanostructures. Ultrasonic power intensity and ultrasonication time are important variables in ultrasonic cavitation (see Figures S4 and S5, Supporting Information). We set the ultrasonic power intensity at 39.5 W/cm2, which is considered as optimum power intensity in our research. To our knowledge, detail morphologies, such as rods, cups, disks, flowers, and spheres, were determined by the combination of the concentration of precursor chemicals, the kind of hydroxide aniongenerating agents, the ultrasonication time, and the use of a capping agent. In our research, zinc cations and hydroxide anions are provided by hydration of zinc nitrate hexahydrate (or zinc acetate dihydrate) and HMT (or ammonia–water), respectively. Here, zinc cations are known to readily react with hydroxide anions to form stable Zn(OH)42- complexes, which act as the growth unit of ZnO nanostructures.25 In addition, among the radicals,
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Figure 3. TEM images of the conversion process from ZnO nanorods to nanocups. (a-c) Low-magnification TEM images of ZnO nanorods synthesized in equimolar 0.01, 0.05, and 0.1 M zinc nitrate hexahydrate/ HMT aqueous solutions, respectively. (d) A low-magnification TEM image of ZnO nanocups synthesized in an equimolar 0.1 M zinc nitrate hexahydrate/HMT solution.
such as •H, •OH, •O2- and •HO2, generated in the sonolysis of water under the air atmosphere,26–28 •O2- radicals are known to participate in the synthesis of ZnO nanostructures.29 Therefore, the sonochemical growth mechanism of ZnO nanostructures was considered as follows:3,16,25–29 (CH2)6N4 + 6H2O f 4NH3 + 6HCHO
(1)
NH3 + H2O f NH+ 4 + OH
(2)
Zn2+ + 4OH- f Zn(OH)24
(3)
)))
Zn(OH)24 98 ZnO + H2O + 2OH
(4)
))) 3 Zn2+ + 2·O2 98 ZnO + O2 2
(5)
where the symbol ))) denotes ultrasonic irradiation. Since ZnO is a polar crystal with a polar c-axis ([0001] direction), when nuclei are formed in the initial growth stage of ZnO crystals, ZnO nanocrystals grow preferentially along the [0001] direction to form nanorods. This is due to the higher growth rate along the [0001] direction.25 To investigate the growth mechanism of ZnO nanocups, several additional experiments were conducted. Figures 3a-c show TEM images of ZnO nanorods synthesized in equimolar 0.01, 0.05, and 0.1 M zinc nitrate hexahydrate/HMT aqueous solutions, respectively. Ultrasonication was performed at an intensity of 39.5 W/cm2 for 30 min. The length of the ZnO nanorods decreased as the concentration of zinc nitrate hexahydrate/HMT aqueous solution increased. As the concentration of this aqueous solution increases, the amount of NH4+ ions produced from the hydration of HMT also increases. Therefore, Zn(OH)4-x(ONH4)x2- complexes are thought to be formed as the NH4+ ions become to bond with Zn(OH)42-, and these Zn(OH)4-x(ONH4)x2- complexes should be converted to Zn(OH)42- before ZnO crystals growth. Because of the endothermic conversion process which hinders the growth of ZnO nanorods, short ZnO nanorods formed as shown in Figure 3c.25 From very short ZnO nanorods in Figure 3c, ZnO nanocups were synthesized by the increase of the ultrasonication time up
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nanospheres have similar XRD patterns, except for relative peak intensity levels which were due to the random orientation. All diffraction peaks can be indexed as hexagonal wurtzite ZnO structure with calculated lattice constants of about a ) 3.25 Å and c ) 5.22 Å. These calculated lattice constant values are consistent with reported data. Pure hexagonal-phase ZnO nanorods, nanocups, nanodisks, nanoflowers, and nanospheres were synthesized under the current sonochemical method. This was because there was no diffraction peaks observed from other impurities in the XRD patterns.
4. Conclusions Figure 4. XRD patterns of ZnO nanorods, nanocups, nanodisks, nanoflowers, and nanospheres. Pure hexagonal-phase ZnO nanostructures were formed under the current sonochemical method.
to 2 h, as shown in Figure 3d. The increase in ultrasonication time indicates that energy is continuously added to the reaction system, and this hinders the ZnO nanorod growth. Therefore, the growth of ZnO nanorods may not be energetically favorable; instead, the dissolution of (0002) metastable surfaces of the assynthesized ZnO nanorods may be energetically favorable.30 Localized dissolution of (0002) surfaces under appropriate conditions was also reported in the formation of volcano-like ZnO nanostructures via a hydrothermal route.31 We believed that ZnO nanocups, very short nanorods with an open cavity at the end, were formed by the dissolution process of (0002) metastable surfaces of the as-prepared ZnO nanorods. In the case of nanodisks, the growth rate of the ZnO crystal along the [0001] direction decreases dramatically due to the addition of triethyl citrate. Citrate anions have been known to act as a capping agent of the (0001) surface of the ZnO crystal by adsorbing on the positive polar face of the (0001) surface.32,33 When citrate anions are adsorbed on the (0001) surface, these citrate anions prevent the contact between Zn(OH)42- and the (0001) ZnO crystal surface. As the growth along the [0001] direction is suppressed by citrate anions, ZnO crystal growth is mainly proceeded along the six symmetric directions to form hexagonal disks. Hydroxide anion-generating agent is the key factor in the synthesis of flower morphology. While HMT slowly generates hydroxide anions,34 ammonia–water provides hydroxide anions very fast due to its strong basicity. So, when ammonia–water is used as a hydroxide anion source material, a large amount of growth units (Zn(OH)42-) can be generated. In addition to the precursor of Zn(OH)42-, the precursor Zn(NH3)42+, which is also known to act as growth units of ZnO crystals, can be generated by the use of ammonia–water.3 Thus, a flower shape could be formed rather than individual nanorods due to many growth units around ZnO nuclei.35 In our research, the significant difference between the synthesis of ZnO nanoflowers and nanospheres is the addition of triethyl citrate. By the use of triethyl citrate as a capping agent, we already showed the growth of ZnO nanodisks instead of nanorods in the preceding part of this paper. In this context, when triethyl citrate is added as a capping agent to the same starting solution that is used for the synthesis of nanoflowers, multiple nanorods growth along the radial directions from center nuclei can be suppressed, and then isotropic growth from center nuclei can be mainly proceed to form sphere-like ZnO crystals rather than flower-like crystals. Figure 4 shows XRD patterns of ZnO nanorods, nanocups, nanodisks, nanoflowers, and nanospheres. As shown in Figure 4, ZnO nanorods, nanocups, nanodisks, nanoflowers, and
We have presented a simple sonochemical route that can control the shape of ZnO nanostructures. Highly crystalline ZnO nanorods, nanocups, nanodisks, nanoflowers, and nanospheres were selectively synthesized without any metal catalysts under ambient conditions. This method is fast, simple, convenient, economical, and environmentally benign. The concentration of precursor chemicals, the kind of hydroxide anion-generating agents, the ultrasonication time, and the use of a capping agent are key factors in the morphology control of ZnO nanostructures. We expect that this sonochemical technique can be readily adopted in realizing other forms of various nanostructured materials. Supporting Information Available: Table of conditions for control experiments; SEM images of ZnO nanostructures formed under various experimental conditions. This material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by both the Institutional and Academic Cooperative Research Program of Korea Research Institute of Standards and Science (KRISS) and Tera-level Nanodevices (TND) Program of the Korean Ministry of Science and Technology.
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