Crystal-Growth Process of Single-Crystal-like Mesoporous ZnO

Article pubs.acs.org/crystal

Crystal-Growth Process of Single-Crystal-like Mesoporous ZnO through a Competitive Reaction in Solution Eiji Hosono,† Takashi Tokunaga,‡ Shintaro Ueno,‡ Yuya Oaki,‡ Hiroaki Imai,‡ Haoshen Zhou,*,† and Shinobu Fujihara*,‡ †

National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan Department of Applied Chemistry, Keio University, Yokohama 223-8522, Japan

‡

S Supporting Information *

ABSTRACT: A novel crystal-growth system and morphology control of ZnO are achieved by using cetyltrimethylammonium chloride (CTAC), a general surfactant, in a new role. We synthesize ZnO via the layered hydroxide zinc acetate (LHZA) in ethanolic-aqueous zinc acetate solution. The CTAC surfactant plays three roles in the nucleation and crystal growth. First, it suppresses the formation of a strong concentration gradient in the solution. Second, it reduces the rates of nucleation and crystal growth of LHZA through diffusion-rate control in the solution. Finally, it expedites the formation of ZnO by increasing the solution pH. The competitive reaction, controlled by CTAC, between the nucleation and crystal growth of LHZA and the formation and crystal growth of ZnO results in the formation of a novel ZnO morphology. This morphology is composed of layered, nearly spherical particles, which are formed by the stacking of nanosheets with single-crystal-like, mesoporous structures.

1. INTRODUCTION The morphology control of functional inorganic materials on the nanoscale level has been studied widely. Solution synthetic methods are popular processes for the nanostructure control of functional materials. ZnO, a low-cost material, has great potential for many applications in areas such as transparent conductive oxide films,1 sensors,2 luminescence,3 and solar cells.4 In particular, applications in energy and environmental fields are now anticipated. Moreover, ZnO is a representative material in the research field of crystal growth for the control of nanostructured metal oxides in solution using processes such as the chemical-bath deposition method.5,6 Many researchers have been studying the nucleation and crystal growth of ZnO for a long time,7−16 and many reports describe one-dimensional morphologies such as the rod-like morphology,8−10,14−16 since ZnO grows preferentially in the [001] direction because of the thermodynamic stability of the crystal. A special feature of the solution method for crystal growth is that single-crystalline materials with high crystallinity are easily obtained. On the other hand, taking into account the applications in energy and environmental studies and basic studies on the nanostructure control of metal-oxide materials, the ideal © 2012 American Chemical Society

morphologies are considered to be single crystals and porous structures with high surface areas, because such materials can exhibit high electroconductivities and large interfacial reaction surface areas, respectively. Well-known highly porous materials include ordered mesoporous materials17−20 such as MCM-41,17,18 although many such materials are amorphous. In the processes, surfactants play interesting roles because they create nanoscale templates based on specific micelle structures. Recently, the formation of powders of crystalline mesoporous materials by methods, such as using amorphous phase21 or templates of amorphous mesoporous materials, has been reported.22,23 First, these powders of mesoporous materials are not singlecrystalline but polycrystalline materials. It is difficult to fabricate powders of single-crystalline mesoporous materials through surfactant template processes, although thin films with isooriented layered nanocrystalline domains by using diblock copolymer were reported.24 Self-template methods based on Received: January 26, 2012 Revised: April 16, 2012 Published: April 26, 2012 2923

dx.doi.org/10.1021/cg300116h | Cryst. Growth Des. 2012, 12, 2923−2931

Crystal Growth & Design

Article

analysis (TG-DTA) using a Mac Science 2020S analyzer with a heating rate of 2 °C/min in air.

the pyrolysis reaction of layered metal hydroxide nanostructres25−29 or the ZnC2O4·2H2O precursor30 formed in the solution are other fabrication processes for highly porous metal oxides. These methods give rise to new morphologies of the metal oxides compared with those formed directly in the solution. This is because the morphologies of directly formed metal oxides, such as hexagonal rods of ZnO, are based on the crystal structures and the surface energies of the facet. Of course, the self-template process is not purely a crystal-growth system because it includes the pyrolysis reaction. This is similar to the fabrication processes of ordered mesoporous materials such as MCM-41, which include pyrolysis reactions of the surfactants used as templates. The study of oriented porous ZnO films formed by the electrodeposition method4 has been reported, in which the cathodic formation of the ZnO/dye hybrid leads to a specific morphology. The ZnO film shows a high conversion efficiency4 for dye-sensitized solar cells using ZnO. This report exemplifies the advantages of porous materials with a single-crystalline nature. Moreover, the materials with the structure of the ordered nanoparticles superstructure are reported in these days.31−35 The specific crystal growth mechanism such as oriented attachment results in the formation of mesocrystals. The study of crystal growth for the polycrystalline materials with single crystalline nature is active. Here, we study a novel crystal-growth system, using a chemical reaction including hydrolysis, dehydration, and polycondensation, without the assistance of an electrochemical reaction in the solution. The formation of mesoporous ZnO with single-crystal-like nature is also investigated. We report a novel solution synthetic process for ZnO involving the application of surfactants in new roles. These roles do not include micelle formation for template provision. In the new reaction system, the surfactant is used to suppress the creation of a concentration gradient and to control the growth rate of the layered hydroxide zinc acetate (LHZA) crystal and its rate of transformation into ZnO. This is possible because the surfactant suppresses the nucleation and growth of LHZA by diffusion-rate control and also controls the pH in the solution system. The competitive reaction between the nucleation and crystal growth of LHZA and the formation and crystal growth of ZnO by hydrolysis and polycondensation gives rise to a specific novel ZnO morphology. Moreover, the internal parts of the ZnO particles show a mesoporous single-crystal-like nature. Therefore, this new crystal-growth system results in the formation of a new structure.

3. RESULTS AND DISCUSSION 3.1. Formation of ZnO through LHZA. The XRD patterns of the products formed using various CTAC

Figure 1. XRD patterns of products obtained in the solution with various CTAC concentrations. (a) CTAC: 0 M, (b) CTAC: 0.133 M, (c) CTAC: 0.267 M, (d) CTAC: 0.400 M.

Table 1. The pH of the Ethanolic-Aqueous Zinc Acetate Solutions at Various Concentrations of CTAC CTAC conc [M]

pH

0 0.133 0.267 0.400

5.48 6.53 6.90 7.25

concentrations and aging times at 90 °C are shown in Figure 1. In the solution without CTAC, almost pure ZnO is formed even at an early stage in the aging process (after 5 h), as shown in Figure 1a. On the other hand, the XRD pattern of the product obtained from the solution with 0.133 M CTAC confirms ZnO and two phases of LHZA, namely, Zn5(OH)8(CH3COO)2·2H2O36 and Zn5(OH)7.5(CH3COO)2.5,36 after 5 h of aging, as shown in Figure 1b, although unidentified peaks are also observed. After 10 h of aging, ZnO dominates, but the two phases of LHZA are still observed. In addition, a few unidentified peaks are observed. As the aging time is increased up to 10, 24, and 72 h, the fraction of ZnO increases, whereas that of Zn5(OH)7.5(CH3COO)2.5 decreases. Then, after 24 h of aging, a large amount of ZnO is seen, and very little Zn5(OH)8(CH3COO)2·2H2O is formed. Finally, after 72 h, almost pure ZnO is produced. When the CTAC concentration is increased to 0.267 M, as in Figure 1c, almost pure ZnO is formed at an early stage in the aging process (after 5 h). In the case of 0.400 M CTAC (Figure 1d), the XRD pattern of the products after 5 h of aging is not shown because no crystalline

2. EXPERIMENTAL SECTION Zn(CH3COO)2·2H2O (0.3 M) and cetyltrimethylammonium chloride (CTAC, 0−0.400 M) were dissolved in a mixed solution of ethanol/ H2O (96/4 vol%) by ultrasonic treatment for 30 min. The precursor solutions were placed into glass bottles with tight-fitting screw caps. The bottles were placed in an oven at a constant temperature of 90 °C for various durations. Subsequently, the products were filtered, washed with ethanol, and dried at room temperature. Crystal structure identification was performed by X-ray diffraction (XRD) with an AXS D8-02 diffractometer (Bruker) using Cu Kα radiation. The morphologies were observed by field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) using a Hitachi S-4700 and a Philips TECNAI F20 microscope, respectively. The thermal behaviors of the products were examined by thermogravimetry−differential thermal 2924



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Figure 2. TG-DTA curves of regents. (a) Zinc acetate dihydrate, (b) CTAC. TG-DTA curves of products obtained in the solution with various CTAC concentrations at 90 °C for 24 h. (c) CTAC: 0 M, (d) CTAC: 0.133 M, (e) CTAC: 0.267 M, (f) CTAC: 0.400 M.

5Zn(CH3COO)2 ·2H 2O → Zn5(OH)8 (CH3COO)2 ·2H 2O + 8CH3COOH

(1)

Zn5(OH)8 (CH3COO)2 ·2H 2O → 5ZnO + 2CH3COOH + 5H 2O

(2)

The observed LHZA phase Zn5(OH)7.5(CH3COO)2.5 in Figure 1 appears in eq 1 as one of the intermediate compounds. Almost pure ZnO is formed after 5 h of aging from the ethanolic-aqueous zinc acetate solution without CTAC. In contrast, LHZA is obtained predominantly from the ethanolicaqueous zinc acetate solution with 0.133 M CTAC, with just a small amount of ZnO formed in the early stages of the aging process. The reason for this difference in the products formed is considered as the impurity effect of the CTAC; that is, the longchain molecules in the solution suppress the diffusion of ions. This diffusion-control effect causes a delay in the formation of LHZA. On the other hand, in the case of higher concentrations of CTAC (0.267 and 0.400 M rather than 0.133 M), the rate of ZnO formation is faster. This is because of the pH of the solution. Table 1 shows the pH values of the solutions with various concentrations of CTAC. The autoprotolysis constant of ethanol is 18.9, and hence the pH is 9.45 at a neutral condition. Therefore, these solutions are relatively acidic in nature. From the table, it is seen that an increase in CTAC concentration increases the pH value. This leads to an increase in the ZnO formation rate, because the increasing OH− concentration accelerates ZnO formation.37 Therefore, through both diffusion control and pH control, the surfactant determines both (1) the nucleation and growth rate of the LHZA crystal, and (2) its rate of transformation into ZnO. In the TG-DTA curves of zinc acetate dihydrate shown in Figure 2a, the endothermic peak and weight loss at around 100 °C are attributed to the dehydration reaction (H2O removal). The weight loss at 200−300 °C corresponds to the pyrolysis reaction of the acetate group. In Figure 2b, the TG-DTA curves of CTAC show rapid weight loss and an endothermic peak at around 220 °C due to the pyrolysis of CTAC. Figure 2c−f

Figure 3. FT-IR spectra of products obtained in solution with various CTAC concentrations at 90 °C for 24 h. (a) The low magnification spectra, (b) the high magnification spectra.

Figure 4. FESEM images of products obtained in the solution without CTAC at 90 °C for 5 h. (a) The high magnification image, (b) the low magnification image.

phase was observed. Although a small amount of LHZA is present after 8 h, almost pure ZnO is obtained after 16 h. Therefore, it can be speculated that the rate of formation of ZnO may be higher in the solution with 0.400 M CTAC than in that with 0.133 M CTAC. The formation mechanism of ZnO from the ethanolicaqueous zinc acetate solution has been reported previously as a hydrolysis and condensation reaction.36 This series of reactions was described as follows: 2925



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Figure 5. FESEM images of products obtained in the solution with CTAC concentration of 0.133 M at 90 °C for various durations. (a1−a3) 5 h, (b1−b3) 16 h, (c1−c4) 24 h, (d) 168 h, (e) 336 h.

weight losses at around 220 °C are quite small in the DTA curves, the included weight loss from CTAC is small in all the samples, so it is concluded that almost no CTAC remains in the inner parts or surfaces. The weight loss at around 100−200 °C is not observed in (b) but is present in (d−f) in the products obtained from the solutions with CTAC concentrations of 0.133−0.400 M. In previous reports on LHZA,27 weight loss based on the dehydration reaction was observed in this temperature region. Therefore, a small amount of LHZA will be present in the products. One possible reason for this could be that a small amount of LHZA is formed during filtration and washing because of the change in the degree of supersaturation. However, the ratio is very small because even the total weight loss is very small. The FT-IR spectra of the products obtained by aging for 24 h from the precursor solutions with CTAC concentrations of 0− 0.400 M are shown in Figure 3. The peaks at around 3440 cm−1 and 1635 cm−1 are due to water,38,39 while those at around 2919 and 2850 cm−1 in Figure 3(b) are attributed to CH2vibrations40 of the acetate group or CTAC. The peaks at

shows the TG-DTA curves after aging for 24 h of the products obtained from precursor solutions with CTAC concentrations of 0, 0.133, 0.267, and 0.400 M, respectively. The weight loss in the curve of Figure 2c (from the precursor solution with 0 M CTAC) is around 1 wt %. According to the TG curve of zinc acetate (shown in Figure 2a), the weight loss at around 200− 320 °C is due to the pyrolysis reaction of the acetate group. The weight loss in Figure 2d−f (CTAC concentrations of 0.133−0.400 M) is around 2−5 wt %. From the TG curve in Figure 2d (0.133 M CTAC), it is considered that the weight loss at 120−300 °C corresponds to the pyrolysis of the residual acetate group and a small amount of LHZA. The pyrolysis of CTAC, which gives rise to a large weight loss at around 220 °C with a large endothermic peak according to the TG curve of CTAC (as confirmed in Figure 2b), takes a small part in the products from 0.133 M. The TG curve shown in Figure 2e (0.267 M CTAC) shows a shoulder followed by a small weight loss at around 220 °C, whereas no such shoulder is observed in the TG curve of Figure 2f (0.400 M CTAC). Considering that the endothermic peaks accompanying the above-mentioned 2926



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early stage of the aging process (after 5 h) (Figure 5(a1−3)). It is considered that the two-hemisphere-like particles are ZnO, because the plate-like morphology is assumed to be zinc hydroxide such as LHZA, which possesses a layered crystal structure.27,43 Although unidentified peaks are observed at early stage of 5 h and 10 h in XRD patterns, the growth of ZnO particles are focused in this paper. As the aging time is increased, the LHZA plates are not observed and the twohemisphere-like ZnO particles grow. The ZnO morphology then changes into a sharp reverse-hexagonal-bipyramidal morphology (Figure 5d), and a sharp hexagonal plate-stacking morphology (Figure 5e), through the nearly spherical morphology constructed from the nanosheet structure, as shown in Figure 5(b1−3) and (c1−4). In Figure 5(c1−4), the images of the products after 24 h of aging show the crystalgrowth system by the stacking of hexagonal sheets with thicknesses of several tens of nanometers. From these images, it is thought that each hexagonal sheet is likely to be constructed through the oriented stacking of nanosheets. As shown in the cross-sectional images of Figure 6a−d, cylindrical cores are observed in the inner parts of the layer-built spherical particles obtained by aging for 16 h. The core parts of the cylinders have the most porous structure. In Figure 6a, two different parts are observed. One part (surrounded by blue line) has a radial pattern. The other parts (surrounded by red lines) have no radial pattern. Moreover, the inner parts include many pores, the numbers of which decrease with increasing proximity to the edge regions of the particles. 3.3. Crystal-Growth Mechanism for ZnO. The formation of the two-hemisphere-like ZnO particles with the multilayered zinc oxide at an early stage is different from that reported in other works.7 In other works,7,27 the crystal growth is reported to occur in the radial direction from the core site (ref 7: ZnO, ref 27: LHZA). This type of growth is considered to be caused by the concentration gradient and crystal structure in the case of ref 27. In ref 7, it is suggested that the electric fields of the core and the dipole−dipole interactions give rise to nanoplatelet-based, core−shell mesocrystal microspheres. The scheme of general crystal growth is shown in Scheme 1. On the other hand, a crystal-growth mechanism for ZnO in the solution with a low CTAC concentration is suggested in Scheme 2. After just 5 h of aging, the two-hemisphere-like morphology is constructed by multilayers that are stacked in parallel. Subsequently, ZnO particles with a layered spherical structure formed from the stacking of this layered morphology are deposited after 16−24 h. This formation process consists of three steps after nucleation. The first step is the formation of

Figure 6. Cross section FESEM images of the products obtained in the solution with CTAC concentration of 0.133 M at 90 °C for 16 h. The cross section of vertical planes (a, c) and horizontal planes (b, d) to the stacked nanosheets.

around 1586 and 1418 cm−1 are due to the COO- groups of the acetate group,41,42 while those at around 1341 and 1045 cm−1 come from the CH3- groups of the acetate group.42 It is considered that these peaks are almost all due to the acetate group, because the spectrum of the product obtained from the solution without CTAC is similar to the spectra of the other products obtained from solutions containing CTAC, which does not include a carboxyl group. Judging from the TG-DTA and FT-IR results (3000−2900 cm−1), the product from the solution with 0.267 M CTAC includes a CTAC component. On the other hand, the FT-IR curve of the product from the precursor solution with 0.400 M CTAC does not indicate this CTAC component, so the presence of CTAC may be caused by inadequate washing. All products adsorb acetate group regardless of the addition of CTAC, but from the TG-DTA data, it is found that the amounts of adsorbed acetate group are small. 3.2. Morphology Change of ZnO. In Figure 4, FESEM images of the products obtained by aging for 5 h from the solution without CTAC are shown. When CTAC is not added to the solution, ZnO nanoparticles are obtained as shown in Figure 4a,b. On the other hand, a specific morphology is obtained from the solutions with CTAC, as shown in Figure 5. In the case of the solution with 0.133 M CTAC, particles with a two-hemisphere-like morphology with multilayered zinc oxide nanoparticles and plate-like particles are distinguished at an

Scheme 1. Nucleation and Crystal Growth Process in a General System

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Scheme 2. Crystal Growth Mechanism for ZnO with the Nearly Spherical Morphology Constructed from the Nanosheet Structure in the Solution with a Low CTAC Concentration and Formation Mechanism of Single Crystal-like Mesoporous ZnO via LHZA

We also observed the TEM images of the ZnO particles. The TEM images of (1) the nearly spherical morphology constructed by the nanosheets obtained by aging for 24 h in the solution with 0.133 M CTAC, and (2) the round reversehexagonal-bipyramidal morphology obtained by aging for 24 h in the solution with 0.400 M CTAC are shown in Figure 7a and b−d, respectively. We can see the porous structure in the lowmagnification images of both particles. However, the electrondiffraction (ED) patterns of both particles do not show polycrystalline ring patterns but single-crystal-like spot patterns. In Figure 7c, we can see an oriented lattice image in the mesoporous structure. This is caused by the competitive reaction between the nucleation and crystal growth of LHZA and the formation and crystal growth of ZnO, as shown in Scheme 2. At first, single-crystalline LHZA is formed. Subsequently, nucleation of ZnO is caused in the singlecrystalline LHZA by the dehydration and condensation reactions. At this time, the nucleation of ZnO is caused in the solid LHZA in a topotactic-like reaction, because the change in thermodynamic energy should be minimized. The

LHZA as an intermediate compound. This does not grow in the radial direction but forms stacked parallel sheets because the CTAC suppresses the creation of a radial concentration gradient. LHZA grows two-dimensionally based on the layered structure and resultant morphology indicates sheet-like shapes. The second step is the transformation of LHZA into ZnO through dehydration and polycondensation. During the polycondensation process, new layered compounds are stacked in a third step. In these reaction systems, CTAC is used to control the growth rate of the LHZA crystals and the rate of their transformation into ZnO. This is achieved because this surfactant suppresses the nucleation and growth of LHZA by diffusion-rate control, and determines the pH of the solution. The competitive reaction between the nucleation and crystal growth of LHZA and the formation and crystal growth of ZnO by hydrolysis and polycondensation results in the formation of layered spherical particles. The porous structure of the inner parts (as shown in Figure 6) is caused by gaps constructed during crystal growth and volume contraction upon transformation from LHZA to ZnO. 2928



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3.4. Morphologies and Crystal Growth for ZnO with High CTAC Concentration. In Figure S1a, Supporting Information, the products obtained from the solution with 0.267 M CTAC after 5 h of aging show the round reversehexagonal-bipyramidal morphology. This changes into the sharp reverse-hexagonal-bipyramidal or sheet-built morphology at 48 h as the aging time increases, as shown in Figure S1b,c. Images of the products from the solution with 0.400 M CTAC are shown in Figure S2, Supporting Information. The round reverse-hexagonal-bipyramidal morphology formed in the early stages (Figure S2a) is similar to that from the solution with 0.267 M CTAC, but it grows larger in size at an aging time of 24 h, as shown in Figure S2b. Upon further aging, the round morphology changes into that with sharp edges and flat surfaces, as shown in Figure S2c at 168 h and Figure S2d at 240 h. It is considered that the formation of the single hexagonalpyramidal structure is due to the dissolution and redeposition from the reverse-hexagonal-bipyramidal morphology, as shown in Figure S2d and Scheme 3. Upon aging in the solution for a long time, a more specific morphology is formed. As shown in Figure 8, nanorods are grown on each of the facets. The side planes of the nanorods form new planes and lines on the hexagonal particles, as shown in Scheme 3. The secondary nucleation and growth of ZnO have been studied in another report.8 The random arrangement changes into an oriented arrangement because the interface energy between the secondary and primary crystals is minimized. As a result, crystals with a disadvantageous arrangement (high interface energy) dissolve, and crystals with an advantageous arrangement (low interface energy) are redeposited. This process results in a specific morphology, as shown in Figure 8 and Scheme 3. In Figure 8f, we can see the morphology in the middle of formation of nanorods, such as Figure 8a−e.

Figure 7. TEM images of products obtained in the solution with CTAC concentrations of (a) 0.133 M for 24 h, (b−d) 0.400 M for 24 h. Electron diffraction patterns are inserted.

crystal growth of ZnO in the LHZA leads continuously to the single-crystal-like mesoporous structure because of this topotactic-like nucleation and crystal growth plus the volume change during the transformation. Generally, single crystallinelike porous materials are called as mesocrystals.31−35 It is considered that the formation of single crystalline-like ZnO porous materials via LHZA could be related to these formation mechanism of mesocrystals. According to BET surface-area measurements, the constructed pores are closed pores. However, the single-crystal-like mesoporous structure of the metal oxide in this solution method is a very interesting system in the field of crystal growth. The fabrication of ZnO by using flakelike Zn5(OH)8Cl2·H2O is reported.44 In this report, ring-like nanosheets standing on the spindle-like rods were formed and the special structure was not obtained by using other zinc salts. Thus, the unique precursor materials have a potential to form the special morphology.

4. CONCLUSIONS ZnO with a specific morphology is obtained through a novel crystal-growth system based on a new role played by the surfactant. The LHZA is formed in the ethanolic-aqueous zinc acetate solution and is then transformed into ZnO. The addition of CTAC, as a surfactant that suppresses the formation of a strong concentration gradient, results in novel crystal growth at an early stage of the process. Moreover, CTAC slows down the nucleation and growth rate of LHZA by diffusion-rate control and increases the formation rate of ZnO by increasing the pH of the solution. The competitive reaction between the nucleation and crystal growth of LHZA and the formation and

Scheme 3. Crystal Growth of ZnO Nanorods through the Secondary Nucleation on the Facet of ZnO

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Figure 8. FESEM images of the products obtained in the solution with CTAC concentration of 0.267 M at 90 °C for 336 h. (a−e) The nanorods grown on each of the facets. (f) The morphology in the middle of formation of nanorods. Wöhrle, D.; Funabiki, K.; Matsui, M.; Miura, H.; Yanagi, H. Adv. Funct. Mater. 2009, 19, 17−43. (5) O’Brien, P.; Saeed, T.; Knowles, J. J. Mater. Chem. 1996, 6, 1135− 1139. (6) Boyle, D. S.; Govender, K.; O’Brien, P. Chem. Commun. 2002, 80−81. (7) Liu, Z.; Wen, X. D.; Wu, X. L.; Gao, Y. J.; Chen, H. T.; Zhu, J.; Chu, P. K. J. Am. Chem. Soc. 2009, 131, 9405−9412. (8) Sounart, T. L.; Liu, J.; Voigt, J. A.; Huo, M.; Spoerke, E. D.; McKenzie, B. J. Am. Chem. Soc. 2007, 129, 15786−15793. (9) Elias, J.; Lévy-Clément, C.; Bechelany, M.; Michler, J.; Wang, G. Y.; Wang, Z.; Philippe, L. Adv. Mater. 2010, 22, 1607−1612. (10) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744−16746. (11) Li, C.; Li, G.; Shen, C. M.; Hui, C.; Tian, J. F.; Du, S. X.; Zhang, Z. Y.; Gao, H. J. Nanoscale 2010, 2, 2557−2560. (12) Janet, C. M.; Navaladian, S.; Viswanathan, B.; Varadarajan, T. K.; Viswanath, R. P. J. Phys. Chem. C 2010, 114, 2622−2632. (13) Yang, M.; Sun, K.; Kotov, N. A. J. Am. Chem. Soc. 2010, 132, 1860−1872. (14) Shi, L.; Bao, K.; Cao, J.; Qian, Y. CrystEngComm 2009, 11, 2009−2014. (15) Panigrahy, B.; Aslam, M.; Misra, D. S.; Bahadur, D. CrystEngComm 2009, 11, 1920−1925. (16) Iwanaga, H.; Shinata, N.; Nittono, O.; Kasuga, M. J. Cryst. Growth 1978, 45, 228−232. (17) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710−712. (18) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834−10843. (19) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chemelka, B. F.; Stucky, G. D. Science 1998, 279, 548−552.

crystal growth of ZnO by hydrolysis and polycondensation causes the creation of a specific novel ZnO morphology of layer-built spherical particles through the stacking of nanosheets. Moreover, these particles exhibit a mesoporous singlecrystal-like nature. The development of this crystal-growth system will contribute to the development of new and high functional materials.

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ASSOCIATED CONTENT

S Supporting Information *

FESEM images of products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+81) 29-861-3489 (H.Z.); (+81) 45-566-1551 (S.F.). Email: [email protected] (H.Z.); [email protected] (S.F.). Notes

The authors declare no competing financial interest.

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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published to the Web on April 26, 2012, with errors in the Introduction, Results and Discussion, the title of Scheme 3, caption of Figure 8, and Table 1. The corrected version was reposted on April 30, 2012.

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