Biomimetic Synthesis of Nacrelike Faceted Mesocrystals of ZnO

Sep 24, 2009 - 1, Section 4, Roosevelt Road, Taipei, Taiwan 10617, Republic of China ...... Wang , D.; Seo , H. W.; Tin , C. C.; Bozack , M. J.; Willi...
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J. Phys. Chem. C 2009, 113, 18053–18061

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Biomimetic Synthesis of Nacrelike Faceted Mesocrystals of ZnO-Gelatin Composite Yao-Hung Tseng,† Hsia-Yu Lin,‡ Ming-Han Liu,† Yang-Fang Chen,‡ and Chung-Yuan Mou*,† Departments of Chemistry and Physics and Center of Condensed Matter Science, National Taiwan UniVersity, No. 1, Section 4, RooseVelt Road, Taipei, Taiwan 10617, Republic of China ReceiVed: June 1, 2009; ReVised Manuscript ReceiVed: September 1, 2009

A fabrication of ZnO hierarchical mesocrystal was achieved by a biomimetic method using gelatin as structuredirecting agent. It was found that the ZnO-gelatin microcrystal with well-defined hexagonal twin plate shape is built by the stacking of nanoplates. The irregularly edged nanoplates can adjust themselves to each other throughout the microcrystal, resulting in a roughly hexagonal edge. Selected area electron diffraction (SAED) analysis of the ZnO-gelatin microcrystal demonstrates that all the stacked nanoplates are aligned and oriented to form a single-crystal structure with hexagonal symmetry. The hierarchical structure of ZnO was found to resemble that of naturally occurring nacre. Similar to nacreous architecture, the nanoplate of ZnO was constructed from the oriented attachment of ZnO nanoparticles. More importantly, the lattices of the stacked nanoplates are aligned through mineral bridges between neighboring plates. A mechanism scheme is proposed for the formation of the gelatin-ZnO hybrid hierarchical structure. The preserved hexagonal shape of the mesocrystal structure consequently results in a whispering gallery mode (WGM) of optical emission where light was confined in the hexagons by total internal reflection. The observation of WGM mode emission in the ZnO hexagon structure shows promises for nanoscale fabrication of optoelectronic devices. Introduction Biominerals have attracted the attention of many researchers because of their sophisticated hierarchical structures and wellcontrolled morphologies.1 The formation of hierarchical nanostructures in biominerals, such as nacreous layers (mother-ofpearl), is associated with an intimate interaction between the inorganic solid and biomolecules under mild conditions in solution. Inspired by nature, biomimetic syntheses of inorganic materials have been intensively studied to mimic the effects of biopolymers on the crystallization of inorganic materials.2,3 These biological crystals are usually not formed through the classical ion-mediated crystallization. They, coined as mesocrystals, are constructed or transformed from larger units of nanoparticles instead of ions.4,5 Mesocrystals, although of singlecrystal construction, are exquisitely interspersed in nanoscale with biopolymers. Much effort has been focused on extending the biomimetic strategy to synthesize functional inorganic materials recently.6 Control of the shape, size, and interface organization of the nanocrystal is of great fundamental and practical interest. A key step in building the superstructure in mesocrystal is through the nonclassical self-oriented attachment of nanocrystals,7 in which elimination of high-energy surfaces by epitaxial attachment of nanocrystals is the driving force of crystal growth. This has been demonstrated in many inorganic materials such as CaCO3,3 hydroxyapatite,8 PbSe,9 SnO2,10 and TiO2.11,12 In some rare cases, one can even obtain a faceted single crystal from oriented attachment of nanoparticle building blocks.3 ZnO is an important wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV). It has been actively investigated because of its potential in optoelectronics,13 sensing,14 piezoelectric nanogenerators,15 and photovoltaic ap* Corresponding author: fax 886-2-23660954; e-mail [email protected]. † Department of Chemistry and Center of Condensed Matter Science. ‡ Department of Physics.

plications.16 Although high-temperature fabrication techniques have been extensively employed to produce high-quality ZnO nanostrucures,17 biomimetic soft solution fabrication approaches offer advantages in providing diversity in morphology and interfaces. Judicious choices of the protecting biopolymers would modulate the shape and size of the nanoparticles of ZnO obtained. Further construction of mesocrystals of ZnO from the nanoparticles has appeared only recently.18,19 However, up to the present a detailed picture of the formation process of the mesocrystals is still lacking. The wurzite structure of ZnO is a rather interesting crystal structure to study via biomimetic approach because it has a quite simple crystal structure of single morphism. Moreover, the ZnO structure possesses both a polar surface (0001) and a nonpolar surface {101j0}, which interact quite differently with surfaceprotecting surfactants or polymers. Previously, Pacholski et al.20 made nanorods of ZnO by self-assembly of nanoparticles through the oriented attachment of its (0001) polar surfaces. The attachment along the c-direction is due to elimination of the unprotected high-energy (0001) surface.21 It is then interesting to investigate oriented attachment of ZnO nanoparticles with their nonpolar surfaces by protecting the polar surfaces. In this case, one would expect to obtain nanoplates of ZnO instead of nanorods. Further stacking of the nanoplates then would lead to the next tier of hierarchical structure of ZnO. In this paper, we report a synthesis of a nacrelike hierarchical crystal structure of ZnO under mild conditions. A ZnO hierarchical structure with uniformly sized hexagons is found to be built from stacking of nanoplates. The nanoplates are obtained from attachment of self-organized ZnO nanoparticles. We choose to employ the biopolymer gelatin, which is known to contain many polar amino acids, as surface protecting agent for the polar surfaces of ZnO. After a detailed study of the internal structure of the porous ZnO-gelatin single crystal, we find the resulting micrometer-sized ZnO of hexagonal shape made from stacking of nanoplates. We will discuss the formation

10.1021/jp905145y CCC: $40.75  2009 American Chemical Society Published on Web 09/24/2009

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mechanism of these mesocrystal hexagons. Moreover, the hexagons are so well-faceted that we can observe total internal reflection of light confined in the hexagonal mesocrystals, that is,the so-called whispering-gallery mode (WGM). Recently, Kim et al.22 reported an observation of WGM-enhanced emission from hexagonal ZnO nanodisks in which luminescence was spatially localized near the boundary of the nanodisk. This kind of light localization within micrometer-sized materials is potentially useful for further integration of optoelectronic devices. Experimental Section All chemicals, hexamethylenetetramine (HMT) (99%), Zn (NO3)2 · 6H2O (95%) (from Acro¨s), and granular gelatin (type B, 225 Bloom, from Sigma) were used without purification. To prepare the ZnO mesocrystals, typically a proper amount of gelatin (0.03, 0.15, or 0.3 g) was dissolved in doubly distilled water (30 mL) to form a gel with various concentrations of gelatin (1, 5, or 10 g/L). Then a mixture of Zn (NO3)2 · 6H2O (3 mmol) and HMT (3 mmol) was added to the gelatin solution. The aqueous solution was sealed in the autoclave at 80 °C for 21 h. The precipitates were centrifuged and washed three times with deionized water at 40 °C. Then the obtained powder was dried at 60 °C for 1 day. X-ray diffraction (XRD) analysis was performed on a Philips X’Pert diffractometer, using Cu KR radiation (λ ) 1.5418 Å). A thin cut of ZnO crystal was prepared by a focused ion beam (FIB) from a JEOL-JIB-4500 dual-beam scanning electron microscope (SEM). The detailed procedure for preparation of the FIB samples is described in Supporting Information. For observing a cross section of the FIB-cut samples, a JEOL-JSM7600F field emission scanning electron microscope operated at 2 kV was used. Also another JEOL-JSM-6700F field emission scanning electron microscope operated at 10 kV was employed. The transmission electron microscopy (TEM) image and selective area electron diffraction (SAED) patterns were taken on a Hitachi S-7100 instrument operating at 75 kV. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-2010 instrument operating at 200 kV. The cathodoluminescence (CL) spectra were carried out on a JEOL-JSM6500 scanning electron microscope equipped with Gatan-MonoCL3 operating at 15 kV. Results First we present the SEM micrographs of ZnO samples produced from the reaction between Zn(NO3)2 · 6H2O and hexamethylenetetramine (HMT) at 80 °C for 21 h with various amount of gelatin. Figure 1a shows the SEM image of the ZnO hexagonal ZnO microcrystals produced in the presence of 5 g/L gelatin. The hexagon disk-shaped twin crystal clearly exhibits a central grain boundary that was due to twinning.19 Both the side view (Figure 1b) and top view (Figure 1c) of the crystals illustrate that hexagonal microcrystals were formed from the stacking of nanoplates, at least near the external surface. The nanoplates, of thickness 20 nm, were found to be stacked along the c-direction in a parallel fashion as shown in Figure 1d. On top of the mesocrystals, one can often find dimples at the center consisting of flowerlike structures. Also, one top view of a crystal (Figure 1e) caught a growing ZnO mesocrystal clearly showing that the nanoplates grow from the center outward and stack into multilayers to form the hierarchical structure. The vertical alignment of the nanoplates seems to be nearly perfect to give a roughly faceted single crystal. For the ZnO-gelatin microcrystals prepared in the presence of 10 g/L gelatin, the

Tseng et al. same hexagonal microcrystals made of nanoplates were also obtained (Figure S1c-e in Supporting Information). In addition, the average size (diameter and thickness) of the ZnO crystals does not change significantly as the concentration of gelatin is increased from 1 to 10 g/L (Table S1 in Supporting Information). The faceted organization of ZnO from nanoplates has never been reported before. The side view in Figure 1d shows roughedged sides of the nanoplates, giving us little hint why all the nanoplates cooperate to form a microcrystal of hexagon shape. On the other hand, with a lesser amount of gelatin (1 g/L) the ZnO crystal shows less substructure of nanoplates (Figure 1f, Figure S1b in Supporting Information). They appear to be a solid mass of ZnO with more smooth faces. Also, we did not observe dimples on the top of this sample in Figure 1f. Figure 2a shows the XRD patterns for the as-synthesized powders fabricated at various gelatin concentrations. All diffraction peaks can be indexed to the wurtzite phase of ZnO (JCPDS 36-1451) (Figure S2 in Supporting Information). The diffraction peaks became broadened as the amount of gelatin used in the reaction was increased. The broadening effect is ascribed to the existence of ZnO nanoparticles for the samples prepared with 5 and 10 g/L gelatin. In addition to the XRD study, the ZnO samples were characterized by TEM and selective area electron diffraction (SAED). Figure 2b shows the TEM image of the ZnO microcrystal formed in the presence of 1 g/L gelatin. The SAED analysis (Figure 2b, inset) confirms the single-crystal quality of ZnO microcrystal. The same analysis was performed on samples prepared with 5 and 10 g/L gelatin, respectively. The single-crystal SAED pattern (for the 5 g/L gelatin sample) shown in Figure 2c demonstrates that all the stacked nanoplates are aligned and oriented into a single crystal structure with hexagonal symmetry. For the sample prepared with 10 g/L gelatin, similarly hexagonally shaped ZnO particles with oriented stacking of nanoplates were found. (Figure S3 in Supporting Information). To further understand the internal structure of the ZnO microcrystals, the ZnO crystals prepared with 5 g/L gelatin were microtomed to slices and examined by HRTEM. The cross-sectional view of the slice of the ZnO microcrystal (Figure 3a) reveals that the microcrystal was composed of interconnected and aligned nanoplates. By magnifying the view of the nanoplates, it was found that the plates were built from an aggregation of ZnO nanoparticles of rather uniform size, ca. 10-20 nm (Figure 3b). Moreover, the nanoplates were connected to each other through some “mineral bridges” (Figure 3c). A HRTEM image shows that the lattices of two neighboring nanoplates were aligned through the mineral bridge (Figure 3d). This result indicates that the mineral bridge plays an important role in the formation of well-aligned nanoplates so that single-crystal quality of the microcrystal was found. For more images of the inner structure of the ZnO hexagons, high-resolution SEM was used to analyze microtomed ZnO sample (10 g/L gelatin). As shown in Figure 4a, clearly the entire ZnO microcrystal is constructed from stacking of the nanoplates. Then the extension of nanoplates contributes to the outer feature of microcrystals. Interestingly, we notice that there is a dimplelike depression near the center of the whole aggregate (Figure 4a), in agreement with the flowerlike structure observed from the top view. In addition, high amplification of the SEM image (Figure 4b) reveals that the interconnected nanoplates are composed of nanoparticles of sizes of 24 ( 6.2 nm (104 particles were counted). In order to clearly observe the overall internal structure of the ZnO microcrystal, we take some thin-cut samples for SEM observation. ZnO samples prepared with 1 and 5 g/L gelatin

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Figure 1. SEM images of (a) ZnO crystals prepared with 5 g/L gelatin, (b) the hexagonal ZnO crystal with platelike features around its surface, (c) a top view of one ZnO crystal, (d) a side view of the ZnO crystal in panel b, (e) a top view catching a growing mesocrystal of ZnO, and (f) a side view of the crystal prepared with 1 g/L gelatin.

were also thin-cut and analyzed. The ZnO microcrystals were thin-cut by focused ion beam (FIB) (see Figure S4 in Supporting Information). The slice plane was located near the center of the crystal. For ZnO crystal prepared with 1 g/L gelatin, only a few rifts and pores in the inner structure of the crystal are revealed (Figure 5b). When the concentration of gelatin is increased to 5 g/L, not only are rifts increased in the inner structure of ZnO but also the plate feature begins to appear at the crystal edge (Figure 5d). We note that the sample in Figure 5c,d was repeatedly washed before the FIB thin-cut was made. The heavy washing procedure may have reduced the amount of the gelatin on the nanoplates, especially near the top and basal surface of the crystal. Thus the nanoplates at the top and basal surface of the crystal begin to dissolve and recrystallize to produce a smoother surface. The smoother top surface of this crystal seems to be produced via classical recrystallization, while the appearance of the rifts and pores in this sample indicates that the crystal was originally formed by nonclassical

crystallization. This result illustrates that when more gelatin within the crystal was available to protect the (0001) and (0001j) planes of ZnO crystals from recrystallization, the plate feature is more clearly observed at the crystal edge. Also, the parallel arrangement of the nanoplates in this ZnO crystal is consistent with the previous observation in Figure 4a. To investigate the optical properties of the ZnO hierarchical structure, cathodoluminescence (CL) measurements were carried out under electron radiation in SEM. The CL spectrum of ZnO (Figure 6a) shows a strong UV peak at about 382 nm due to band edge emission and a green emission around 570 nm due to surface oxygen defects.23 In the synthesis solution, zinc ion is in oversupply, which may favor oxygen defect formation. The appearance of green emission is in agreement with previous reports of ZnO with large surface area.24 Under CL mode, one can see that the edge areas around the hexagons are bright. The SEM image (Figure 6b) and its corresponding CL mapping image (Figure 6c) of the microcrystals shows that the emission

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Figure 2. (a) XRD pattern. From top to bottom: ZnO prepared with 0, 1, 5, and 10 g/L gelatin. (b, c) TEM images and the corresponding SAED patterns of ZnO crystals prepared with (b) 1 g/L gelatin and (c) 5 g/L gelatin.

at 382 nm localizes only along the edge of hexagons. The magnified CL image (Figure 6d) clearly shows an enhancement of emission near the boundary of the hexagonal cavity. An apparent enhancement of light in a localized mode is shown. The hexagonally shaped ZnO structure may sustain two different kinds of optical modes: a Fabry-Pe´rot-type mode,25,26 where the light bounces back and forth between the two opposing edge faces, and a whispering-gallery mode (WGM), where the light strikes the ZnO-air boundary at 60° relative to the normal to the boundary surface (Figure 6d, inset). This kind of spatial localization of luminescence is typically attributed to WGM emission in dielectric resonant cavities.22,27-29 In a WGM mode, the light wave can be considered to circulate around the resonant cavity due to multiple total internal reflections at the cavity’s boundary. Discussion In biomineralization, the organic molecules or ions act to control nucleation or to terminate crystal growth by reducing the surface energy of the specific crystal plane. In the crystallography of ZnO, a crystal possesses a polar surface (0001) enriched in Zn cations, a polar basal plane (0001j) enriched in O anions, and nonpolar planes {101j0}. In general, the polar plane is energetically less favorable when ZnO grows in the solution without surface protection. Therefore, rod morphology is normally observed in many of the previous reports because the growth velocity of the polar plane (c-plane) is faster than that of the nonpolar plane.24,30 In our synthesis, indeed rodshaped and irregularly shaped ZnO polycrystals (Figure S1a in Supporting Information) were obtained in the absence of gelatin additive. It is apparent that gelatin plays an important role in the formation of nanoplates. The isoelectric point of gelatin in

Tseng et al. aqueous solution is around 5.0. Hence, the gelatin molecules are expected to possess negative charges in the reaction bath at the pH value of 6.4. The negative carboxylate charge groups in gelatin are supposed to strongly interact with the (0001) polar plane of ZnO. A large number of carboxylic groups in gelatin are able to coordinate with Zn2+ to decrease the surface energy of (0001) and suppress its growth. According to TEM and SAED (Figure 2c) analyses, indeed, the formation of nanoplate is due to the inhibited growth of ZnO (0001) plane. In thermal gravimetric analysis (TGA) of the ZnO sample prepared with 5 g/L gelatin, a 12% weight loss from 200 to 500 °C reveals a strong binding of gelatin on ZnO nanoplates (data not shown). In addition to TGA, adsorption of gelatin on the ZnO crystals was confirmed by the IR spectra of a series of ZnO samples (Figure S5 in Supporting Information). Thus, the formation of nanoplates may be understood as a consequence of the surface adsorption of gelatin on ZnO (0001) plane. Although several other ZnO nanoplate structures have been reported in the aqueous solution previously, such as one fabricated by adding citrate ions,31-33 a unique well-faceted ZnO single crystal of hexagonal shape assembled from nanoplates is found in this work. There are many factors in forming this highly ordered hierarchical structure of ZnO. First, in synthesis the HMT molecules decompose slowly in the heated solutions to yield ammonia and formaldehyde that initiate the reaction. The homogeneous release of OH- ions by HMT molecules in aqueous solution, with the help of the encapsulating gelatin, would lead to the production of uniformly sized ZnO nanocrystals. Surface protection also slows down the Ostwald ripening process. This phenomenon can be observed in the crosssectioned samples (Figure 5). It demonstrates that when the more concentrated gelatin was used in the reaction, one has less recrystallization process. After the solution ion sources are exhausted, we have the situation of no growth and little ripening. Then the aggregations of ZnO nanoparticles undergo oriented attachment to lower the interfacial energy. As the polar carboxylate groups in gelatin mostly interact with the polar surfaces of ZnO, the less protected nonpolar surface would be the attachment surface. Thus the fusions of {101j0} surfaces of the nanoparticles gradually lead to nanoplates. The stability of polar oxide surfaces in ZnO is one of the puzzling problems in surface science.35 According to classical electrostatic criteria, the polar surfaces (0001) have a divergent electrostatic surface energy due to the presence of a nonzero dipole moment (Figure 7). Without surface reconstruction, the adsorption of charged species on the polar surface is an alternative process to stabilize the nanoplate structure.35 Nonetheless; the nanoplates still possess a substantial dipole moment along the c-direction. The long-range dipole-dipole interaction between the nanoplates could be the driving force for stacking of the plates. One line of evidence for the dipolar field is the central dimple often observed on the mesocrystal (Figures 1 and 4a). Previously, Wang et al.36 had observed such a dimple structure in calcite mesocrystals and ascribed it to the dipole field. For formation of the complex ZnO mesostructure, the mineral bridge is proposed to be the dominant mechanism for building the nanoplates into the hierarchical structure with single-crystal character. When gelatin molecules are strongly adsorbed on the ZnO (0001) planes and presumably separate them, some mineral bridges could still exist between neighboring nanoplates, which is similar to the scenario observed in nacre.37 From TEM, HRTEM, and HRSEM analyses, three levels of hierarchies in the ZnO structure were present in this report. In the beginning

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Figure 3. (a) TEM image of a cross section of ZnO microcrystals prepared with 5 g/L gelatin. (b) Magnification view of ZnO nanoplates composed of ZnO nanoparticles. (c) Dashed circles indicate the mineral bridges between two neighboring nanoplates. (d) HRTEM image of the lattice alignment of two neighboring nanoplates through the mineral bridge. The arrow indicates the position of the mineral bridge. Dashed lines indicate the lattice fringe of the mineral bridge, and solid lines represent the lattice fringe of the two neighboring nanoplates.

of the reaction, ZnO nanoparticles stabilized by gelatin molecules were produced (level 1). We have found from the peak width of XRD that the more gelatin is used, the smaller the nanoparticles are. This is reasonable since more surface protection of ZnO would limit its growth. Once the ZnO nanoparticles are formed, they stick to each other via orientation alignment to lower the surface energy. Since the oriented attachment of the as-produced ZnO nanoparticles along their c axes was inhibited, the nanoplates are formed by aggregation of the ZnO nanoparticles along their a or b axes (level 2). A heterogeneous nucleation of twinned hexagonal plates was then formed. The twinned nanoplates have been previously reported by Bauermann et al.38 At the final stage, the regular hexagonal “single crystal” was formed by the stacking of well-aligned nanoplates (level 3). As Figure 3c,d shows, apparently the mineral bridges between neighboring nanoplates act as communicators for the lattice alignment of the nanoplates. Although the negatively charged gelatin strongly adsorbs mostly on the (0001) planes of ZnO, the mineral bridges may have been formed due to a slight weakening of the gelatin adsorption ability as the pH value of the solution decreased from 6.4 to 6.2. The aggregation of ZnO nanoplates in this way results in the occlusion of gelatin molecules in the ZnO single crystals, which was confirmed by

thermal gravimetric analysis (TGA), giving a content of ca. 12% gelatin in the final product. The ZnO product with single-crystal character constructed from the smaller building blocks may be classified as a mesocrystal. Mesocrystals with well-defined shapes had been reported to be constructed by the aggregation of nearly isotropic nanoparticles.3,39 More recently, an alternative “self-similar” process was reported for the formation of mesocrystals.40,41 The superstructure associated with self-similar growth is formed by mesoscale assembly of nanoparticle subunits giving the shape of the final mesocrystal. Here, the hierarchically structured faceted hexagon built from the nanoplates, which is in turn formed from nanoparticles, is investigated in great detail for the first time. Recently, the fabrication of stacked ZnO nanoplates with different morphologies has been reported.42-45 For example, Tang et al.42 showed that various ZnO superstructures with sheet, platelet, and ring shapes may be fabricated by selforganization of ZnO nanoparticles by use of the surfactant cetyltrimethylammonium bromide (CTAB) as template. Tian et al.31,32 prepared complex structures from stacking nanoplates. Taubert et al.19 considered two possible mechanisms for the complex ZnO mesocrystals: “stack of pancakes” and “corn-onthe-cob”(with a solid crystalline cob). They favored the “corn-

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Figure 4. (a) SEM image of a cross section of ZnO microcrystals prepared with 10 g/L gelatin; the arrow indicates the position of the dimple. (b) Interconnected ZnO nanoplates composed of ZnO nanoparticles.

on-the-cob” model on the basis of TEM evidence. However, based on clear observations of the interior structures, the ZnO structure fabricated here is definitely formedby the “stacking of nanoplates ” (Figures 3a and 4a), which is different from the ”corn-on-the-cob” structure. Also, Zhang et al.34 obtained ZnO multilayer structures by controlling the growth temperature and the amount of polyvinylpyrrolidone (PVP) in the reaction. The multilayer structure was proposed to form via layer-by -layer deposition. Very recently, Yang et al.45 reported that the growth of oriented columnlike structure of ZnO sheets on the substrate. They found that the structure of ZnO sheets is similar to the layered structure of Ostrea riVularis shells. Interestingly, the ZnO hierarchical structure produced here possesses three levels of hierarchy (nanoparticles, nanoplates, and the oriented superstructure), which is individually corresponding to each level of structure of nacre.37,48 In this case, our ZnO microcrystal is really quite similar to the architecture of the biomineral nacre. For the fabrication of ZnO hierarchical structure, Mo et al.46 demonstrated the ZnO nanoparticles were self-assembled into hierarchical mesocrystals via the organization of the colloids mediated by the polyelectrolyte. Li et al.47 showed that the ionic liquid precursor acts not only as a solvent for the reaction but

Tseng et al. also as an inhibitor to stabilize the primary particles for constructing the mesocrystals by oriented aggregation. Our proposed mechanism for the formation of mesocrystal ZnO may also apply to some of these cases, although details of the nanostructures were not examined in these previous reports. The formation of single-crystal structure may be ascribed to the lattice epitaxial attachment. However, lattice matching alone cannot account for formation of the overall hexagonal shape in the ZnO crystals. The side view of Figure 1d is most striking in that all the stacked nanoplates terminate roughly at the same vertical alignment to form a hexagon outer shape. In order to understand the relative attachment of the nanoplates, we make a simple model calculation of the specific surface area of ZnO with two extreme configurations assumed. If we assume the ZnO hexagons are thick solids (without interior surface) 3.1 µm wide and 2 × 1 µm thick (twin disk), their surface area would be 0.22 m2/g. On the other hand, if the ZnO consists of separated hexagonal nanoplates 3.1 µm wide and 20 nm thick, its surface area would have been 17.8 m2/g. The specific area of the calcined ZnO sample (5 g/L gelatin), as measured by nitrogen adsorption, is 2.2 m2/g. This value is just between the above two extreme estimates, which is consistent with our TEM result (Figure 3a) that many of the stacked nanoplates are interconnected. Furthermore, it is believed that the appearance of abundant mineral bridges (Figure 3c) is important for preserving the single-crystal character of mesocrystals. The formation mechanism of the ZnO hierarchical hexagonal plate structure is summarized in Scheme 1. First the ZnO nanoparticles (ca. 20 nm) are nucleated and stabilized by the gelatin molecules (step 1). Due to the strong adsorption of gelatin on the (0001) and (0001j) planes of ZnO crystalline, the growth of nanoparticles along c direction is restricted. To reduce the surface energy, the ZnO nanoparticles stick to each other along the a or b direction by oriented attachment of the nonpolar surfaces, resulting in the formation of thin plates with thickness 20 nm (step 2). At the pH value of 6.4 in our reaction, the negative charges of gelatin may induce twin crystal formation as reported by Bauermann et al.38 (step 3). Once the nanoplates are formed, the dipole field between the nanoplates may lead to the lattice matching of ZnO crystals, resulting in the orientation alignment of the nanoplates (step 4). The positive and negative charges of the different side chains of gelatin would adsorb on the (0001j) and (0001) planes of ZnO, respectively. Presumably, the positive charges such as in arginine residues would adsorb on the oxide-rich (0001j) surface while the negatively charged glutamic acid residues would adsorb on the Zn2+-rich surface. In addition to the dipole field, the mineral bridges grown between the ZnO surface would help the alignment of ZnO lattice between two nanoplates (step 5). After the stacking of multilayer nanoplates, the generated dipole field is strong enough to influence the alignment of nanoplates located at the top and basal surface of ZnO microcrystal, which causes the formation of the dimple (step 6). Theoretically, the dipole field could be nearly canceled in a perfect ZnO twin crystal. However, for samples of finite thickness, the residue quadrupolar field influence is apparently strong during the growth of the crystals. In these samples, the nanoplates are produced, which are then guided by the quadrupole field to build the ZnO microcrystals. Although the dipole field in a twin crystal would be mostly canceled, the nanoplates distributed at the top and basal surface of the ZnO twin crystal would suffer a dipole field from neighboring stacked nanoplates, resulting in the alignment of nanoplates in three dimensions along the dipole field.(Figure 4a) Eventually, the growth of the nanoplates along the a or b

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Figure 5. SEM image of FIB thin-cut ZnO mesocrystals. The thin cuts were performed on a JEOL-JIB-4500 FIB-SEM. (a, b) Sample prepared with 1 g/L gelatin; (c, d) sample prepared with 5 g/L gelatin. The samples were repeatedly washed before FIB thin cuts were made.

Figure 6. (a) CL spectrum of ZnO microdisks prepared with 5 g/L gelatin. (b) SEM image and (c) corresponding monochromatic CL image monitored at 382 nm. (d) Magnified monochromatic CL image of the hexagonal ZnO crystals showing WGM emission. (Inset) Illustration of the light pathway in WGM.

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Figure 7. Polar surface of ZnO.

SCHEME 1: Formation Mechanism of the Hierarchical Structure of ZnO Hexagons

direction could be terminated at the same edge due to the precise alignment of dipolar interaction between neighboring plates and the energy reduction in adsorption of gelatin on the opposite sides of two nanoplates. It is worth mentioning that classical crystallization could also play some role in this reaction. In particular, the Ostwald ripening process could occur when the amount of the gelatin is not enough to stabilize the nanoplates. To illustrate it, different washing procedures were applied for the sample synthesized with the same condition (5 g/L gelatin-ZnO). The abundant water molecules may penetrate easily into the loosely packing nanoplates located at the top and basal surface of the crystal so that the concentration of gelatin on the nanoplates is lowered.

Tseng et al. When the gelatin is not sufficient to stabilize the nanoplates, the nanoplates begin to dissolve and recrystallize into the crystal with a flatter surface. Consequently, it is found that the top flowerlike structures of the ZnO crystals were smoothed out by a repeated washing action (Figure 5c,d). In contrast, the ZnO sample washed with a normal procedure shows the dimple on the top of the crystal (Figure 1e). The nacrelike structure of ZnO reported here is not unlike the structure in natural nacre of aragonites.37,48 Recently, the macroscopic assembly and orientation of aragonite plates (200-600 nm thick) in nacreous layer has been investigated in great detail.48-50 Three levels of hierarchy of the nacreous layer were reported by Oaki and Imai.49 The aragonite plates composed of nanoparticles were found to assemble into the wellorganized nacreous architecture. Interestingly, the “mineral bridge” was observed and proposed to act as the communicator for alignment of the plates in the nacre. Also, nacrelike structures of hierarchical organized K2SO4-poly(acrylic acid) composite was successfully fabricated under mild conditions in their lab. Here, we also have three tiers of hierarchical structures: the nonpolar surfaces of nanoparticles are fused into nanoplates and then the nanoplates are stacked to single crystals through the mineral bridges. Finally, we discuss the luminescence properties of the ZnO hexagons. In addition to the band edge emission at 382 nm, we observed a strong broad green emission between 500 and 650 nm. The green emission of ZnO nanostructures is due to the recombination of a photogenerated hole with electron of the singly ionized oxygen vacancies in the surface lattices of the ZnO. As the interface is much increased in our ZnO-gelatin composite, one would expect a stronger green emission. The cathodoluminescence (CL) spectra of ZnO prepared with and without gelatin are shown in the Supporting Information (Figure S6). For the ZnO synthesized without gelatin, the green luminescence (500-650 nm) is indeed very low. The high intensity of the green luminescence of ZnO/gelatin hexagons can be rationalized by the large surface-to-volume ratio of the ZnO-gelatin composite. The observation of WGM in our ZnO hexagons is interesting for it leads to higher luminescence efficiency. However, because the sizes of the hexagons are in the micrometer range (Table S1 in Supporting Information), the number of nodes is very large (50-100).51 Wavelength selection is thus not apparent. The ZnO microcrystal with WGM emission is a promising candidate for an optically resonant cavity. Because the multiple total internal reflections occurring at the resonator’s boundary, the loss of electromagnetic waves in the cavity is small. Accordingly, the WGM emission generally leads to a high Q-factor, where light is confined in the circumference of a dielectric disk.28 Recently, Zhang et al.52 reported thin films composed of random aggregations of ZnO nanocrystallites that exhibited high conversion efficiency in dye-sensitized solar cells. The generation of light localization was due to the light scattering effect in the highly disordered structure. The increased interaction between photons and dye molecules results in high conversion efficiency in the solar cell. In our ZnO materials, the integration of nanostorage property (dye incorporation) and the faced hexagonal shape (light trapping) of the microcrystal may show promise for producing a photovoltaic device with high performance. Besides, the hexagonal microcrystal made of nanoplates is believed to have advantages in the photocatalytic reaction due to the exposure of a large amount of the photocatalytically active (0001) planes of nanoplates.33,53 Herein, the hierarchical ZnO structure with integrated properties of WGM

Nacrelike Faceted ZnO Mesocrystals emission and abundantly exposed polar (0001) planes implies that it may find many applications in integrated low-threshold nanoscale optoelectronics, photovoltaic devices, and photocatalytic materials. The large surface area in nanoplates of ZnO may also be useful in sensor application.54 In nature there are many examples of complex fine nanostructures for optical functions, such as the intricate “eyes” (lenses) of the brittlestar Ophiocoma wendti55 and the basalia spicules from the glass sponge Euplectella aspergillum as multimode optical fibers.56 Our ZnO material is one of the first examples of laboratory self-assembly of an inorganic mesocrystal with optical function. Conclusion In summary, we have successfully fabricated ZnO structure with three levels of hierarchy. The self-assembled ZnO hierarchical structure reported in this study was found to resemble the structure of naturally occurring nacre. The gelatin molecules used in this reaction not only stabilize the ZnO nanoparticles but also show a strong inhibiting effect on the growth of (0001) surfaces of ZnO. Interestingly, the ZnO nanoplates formed from the aggregation of nanoparticles are stacked into a hierarchical structure of a single crystal of hexagonal external shape through mineral bridges. The preserved hexagonal shape of the hierarchical structure consequently results in the WGM optical emission. The presence of WGM emission in the extended ZnO hierarchical nanostructure in this work not only sheds light on biomimetic materials but also provides a route for nanoscale biosensing.57 The synthesis of the hierarchical ZnO structure in this paper could lead to new approaches to control the crystal size, orientation, and spatial patterning with various optical properties that are critical for optoelectronic applications. Acknowledgment. This work was supported by a grant from the National Science Council of Taiwan. Supporting Information Available: Six figures showing SEM images, XRD patterns, TEM image, and SAED pattern for ZnO; preparation of FIB samples; and IR and CL spectra of ZnO; and one table comparing different ZnO crystals. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Mann, S. Biomineralization; Oxford University Press: Oxford, U.K., 2001. (2) Bauermann, L. P.; Bill, J.; Aldinger, F. J. Phys. Chem. B 2006, 110, 5182. (3) Zhan, J.; Lin, H.-P.; Mou, C.-Y. AdV. Mater. 2003, 15, 621. (4) Co¨lfen, H.; Antonietti, M. Mesocrystals nd Nonclassical Crystallization; John Wiley & Sons Ltd.: New York, 2008. (5) Zhou, L.; O’Brien, P. Small 2008, 4, 1566. (6) Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (7) Zhang, Q.; Liu, S. J.; Yu, S. H. J. Mater. Chem. 2009, 19, 191. (8) Onuma, K.; Ito, A. Chem. Mater. 1998, 10, 3346. (9) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (10) Leite, E. R.; Giraldi, T. R.; Pontes, F. M.; Longo, E.; Beltran, A.; Andres, J. Appl. Phys. Lett. 2003, 83, 1566. (11) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549. (12) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. J. Am. Chem. Soc. 2004, 126, 14943. (13) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. Nano Lett. 2007, 7, 1003. (14) Fan, Z.; Wang, D.; Chang, P.-C.; Tseng, W.-Y.; Lu, J. G. Appl. Phys. Lett. 2004, 85, 5923. (15) Wang, Z. L.; Song, J. Science 2006, 312, 242.

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