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Apr 11, 2012 - An unusual designated-tailoring on zone-axis preferential growth of surfactant-free ZnO mesocrystals (spindle-like and peanut-like) wit...
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Unusual Designated-Tailoring on Zone-Axis Preferential Growth of Surfactant-Free ZnO Mesocrystals Shaodong Sun, Xiaozhe Zhang, Jie Zhang, Xiaoping Song, and Zhimao Yang* School of Science, State Key Laboratory for Mechanical Behavior of Materials, MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, ShaanXi, Xi’an 710049, People’s Republic of China S Supporting Information *

ABSTRACT: An unusual designated-tailoring on zone-axis preferential growth of surfactant-free ZnO mesocrystals with different features (shapes and sizes) was successfully achieved via an additive-free complex-precursor solution method. The formation of ZnO mesocrystals here is essentially determined by the characteristic of [Zn(OH)4]2− precursors, and an oriented nanoparicle aggregation with tailoring sizes and shapes can occur in different concentration of reactants at higher reaction temperature. Spindle-like ZnO mesocrystals with tunable sizes (along the c-axis direction) were synthesized by adjusting the concentration of hydroxyl ions, and peanutlike ZnO mesocrystals with controllable sizes (along the c-axis direction) and shapes (perpendicular c-axis direction) were prepared by tailoring the concentration of zinc ions. Structural and morphological evolutions were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and fieldemission scanning electron microscopy (FESEM). The study is of great significance in bottom-up assembly of controllable ordering architectures, and provides a good opportunity to understand the formation mechanism and fundamental significance of zone-axis preferential growth of ZnO mesocrystals. Significantly, it is believed that the precursor driven assembly of mesostructures reported here would provide a green way to design more and more surfactant-free metal oxide architectures with well-defined shapes.

1. INTRODUCTION For bottom-up nanotechnology, highly ordered superstructure has become fascinating in materials design and biomineralization because of their potential applications and significant scientific value.1 To precisely tailor the architecture and morphology of the products, a thorough understanding of the growth mechanism and formation process is imperative. As a new class of ordered superstructures, mesocrystals have been successfully elucidated by Cölfen and co-workers for the growth of minerals via a nonclassical particle-mediated process (oriented attachment and mesoscale transformation).1−4 It can be defined as a colloid crystal composed of individual nanocrystals as the building units that are aligned in a common crystallographic fashion with notable internal porosity and rough surface, as well as exhibiting elongated diffraction spots resembling that of a single crystal.1−6 The study of mesocrystals is not only helpful for us to design the crystal morphology by using anisotropic building units but also to provide a new possibility for constructing multifunctional materials with potentially unique and exciting properties. Currently, much attention has been directed toward the investigation in various mesocrystal systems, including minerals, metallic oxides, metallic sulfides, and alloys, even in pure metals.1−10 However, the underlying growth mechanism of the mesocrystals with unusual features still remains largely unexplored. As an important wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), ZnO (zinc oxide) has been actively investigated because of its potential in © 2012 American Chemical Society

ultraviolet laser, sensing, optoelectronics, piezoelectric nanogenerators, and photovoltaic applications and so forth.11,12 In the past decade, many efforts have been devoted to the shapeand size-controlled synthesis of ZnO nano- and microarchitectures. Further construction of ZnO mesocrystals from the nanoparticles has reported only recently.11−15 Because the wurzite ZnO structure possesses both a polar surface (0001) and a nonpolar surface (101̅0), which interact quite differently with surface protecting surfactants or organic additives,11 the formation of ZnO mesocrystals always involves the nanocrystal building blocks sharing the same crystallographic structure with an interspersed organic assembler. For example, one-dimensional (1D) ZnO mesocrystals were synthesized by a facile solution route under the assistance of poly(vinyl pyrrolidone) (PVP).13 Spherical ZnO mesocrystals were prepared by a solvent hydrothermal route in the presence of N,Ndimethylformamide (DMF) and PVP.14 Core/shell ZnO mesocrystal microspheres have been synthesized on a large scale using a facile one-pot hydrothermal method in the presence of the water-soluble polymer poly(sodium 4styrenesulfonate).15 Three-dimensional (3D) ZnO mesocrystal multipods can be grown from the hydrated ionic liquid precursor tetrabutylammonium hydroxide at room temperature.12 Although shape-controlled fabrication of ZnO mesoReceived: January 13, 2012 Revised: April 8, 2012 Published: April 11, 2012 2411

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Table 1. Sample Denotations and Their Corresponding Detailed Experimental Conditions and Final Shapes sample

concentration of Zn(NO3)2

concentration of NaOH

A B C D E F G

0.04 M 0.004 M 0.004 M 0.04 M 0.06 M 0.08 M 0.12 M

0.2 M 0.02 M 0.002 M 0.004 M 0.004 M 0.004 M 0.004 M

reaction temperature 80 80 80 80 80 80 80

°C °C °C °C °C °C °C

reaction time 60 60 60 60 60 60 60

min min min min min min min

shape characteristics spindle-like spindle-like spindle-like peanut-like peanut-like peanut-like peanut-like

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The synthetic strategy to prepare surfactant-free ZnO mesocrystals is based on the additive-free complex-precursor solution method. Structural analysis of the as-prepared spindle-like ZnO crystals with small sizes (sample A) was carried out by XRD, and the result is shown in Figure S1 (see Supporting Information). All the diffraction peaks can be indexed to the standard hexagonal structure of ZnO (JPCDS file No. 36-1451). No peaks of impurities such as zinc hydroxide were detected, indicating that pure hexagonal phase of ZnO crystal are obtained in a Zn(NO3)2/NaOH/H2O system. The morphology of the asprepared products (sample A) was observed by FESEM. Figure 1a shows the FESEM image of the as-prepared ZnO products, and it can be found that ZnO crystals are almost the welldefined spindle-like architectures (sample A). A typically individual spindle-like ZnO particle is shown in the inset of Figure 1a, which can be closely found that the formation of rough surface of the architectures with well-assembled nanoparticle building blocks can be achieved. The average sizes of length and width are about 266 ± 32 nm and 170 ± 19 nm, respectively. The size distribution diagrams are shown in Figure S2 (see Supporting Information). The morphology and structural characterization of the as-prepared ZnO crystals are also performed on TEM, HRTEM, and SAED investigations. The corresponding results are displayed in Figure 1b−e. Figure 1b gives the low-magnification TEM image of the as-prepared ZnO spindle-like architectures, suggesting that the particles are uniform and monodisperse. Figure 1c is a typical TEM image of an individual ZnO particle, which further demonstrates that the particle has a rough surface, and it can be obviously found that the architecture is composed of abundant nanoparticle building blocks. The corresponding SAED pattern is shown in Figure 1d; the diffraction pattern with elongated spots indicates that the spindle-like ZnO architecture has a relatively imperfect single-crystal-like characteristic, and it can be indexed to hexagonal phase of ZnO crystal viewed along the [1120̅ ] zone axis. This observation suggests that the spindle-like ZnO product has nearly the same crystallographic orientations among nanoparticle building blocks as single-crystal-like architecture, so the formation of ZnO mesocrystals is artificially synthesized, which might be attributed to an oriented attachment mechanism determined by the self-assembly of these nanoparticle building blocks.13 This result is similar to the SAED pattern of the reported ZnO mesocrystals,13 which has a single-crystal-like structure and preferential growth orientation along [0001]. The detailed microstructure of the ZnO mesocrystals is further investigated by HRTEM, as shown in Figure 1e. Two lattice fringes are marked by white lines and arrows, and the corresponding lattice spacing is about 0.52 nm, which is in good agreement with the d value of the (0001) facets of ZnO crystal, which is corresponding to the SAED

crystals has been achieved by the above solution synthesis route, the invariably residual surfactants or organic additives attached to the surfaces of building blocks may cause awful problems in their applications.6 Moreover, these additiveassisted synthesis routes not only increased the cost but also made it more difficult for larger-scale production. Therefore, it is still a challenge for material scientists to develop new facile methods to fabricate well-defined ZnO mesocrystlas with building blocks as surfactant-free as possible in the absence of capping agents. To date, the relevant experimental investigation is still unavailable. Herein, we present the significant evidence on the one-pot synthesis of surfactant-free nanoparticle-aggregated spindle-like and peanut-like ZnO mesocrystals by a facile additive-free solution method. To the best of our knowledge, there are less reports on the green solution-phase synthetic route for the selfassembly of surfactant-free ZnO nanocrystals into well-defined 3D mesocrystals. The study is of great significance in bottomup assembly of controllable ordering architectures, and provides a good opportunity to understand the mechanism of preferential growth of surfactant-free ZnO mesocrystals along different zone-axis directions.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals used in our experiment were of analytical grade and used without further purification. Aqueous solutions were prepared using ultrapure water. 2.2. Synthesis of Surfactant-Free ZnO Mesocrystals. In a typical procedure, Zn(NO3)2 was dissolved in ultrapure water using a beaker under a constant stirring at 80 °C for 2 min. A white precipitate was produced when a sodium hydroxide solution was added dropwise into the above solution. After being stirred for 60 min, the precipitations were allowed to cool to room temperature naturally. Afterward, the obtained products (samples A−G) were centrifuged at 8000 rpm for 2 min (XIANYI TG16-WS centrifuge). The precipitates were centrifuged twice more in ultrapure water and anhydrous ethanol, respectively, and finally were dried at 60 °C for 12 h in a vacuum oven. The shapes of the final products together with the corresponding detailed experimental conditions are listed in Table 1. 2.3. Characterization. Powder X-ray diffraction (XRD) pattern was recorded on a Bruker-AXS D8 Advance diffractometer operated at 40 kV voltage and 40 mA current using Cu Kα radiation (λ = 1.5406 Å) in the range (20−80°). The morphology of the products was investigated by field-emission scanning electron microscope (FESEM) using JEOL (JSM-7000F) at an accelerating voltage of 20 kV. The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) analysis images as well as selected-area electron diffraction (SAED) pattern analysis were performed on a JEOL JEM-2100 transmission electron microscope operating at an accelerating voltage of 200 kV. The sample for the TEM analysis was prepared by ultrasonic dispersion for 60 s with ethanol (4 mL) in a 5 mL centrifuge tube. Then, the products were dropped onto a carbon-coated copper grid and dried in air before TEM analysis. The crystal structure of ZnO was drawn by using the Diamond 3.2 program. 2412

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(eq 2). Finally, ZnO was synthesized by the decomposition of [Zn(OH)4]2− species (eq 3) at higher reaction temperature. Zn 2 + + 2OH− → Zn(OH)2 ↓

(1)

Zn(OH)2 + 2OH− → [Zn(OH)4 ]2 −

(2)

[Zn(OH)4 ]2 − → ZnO + 2OH− + H 2O

(3)

According to the above chemical reactions, the spindle-like ZnO mesocrystals reported here are essentially determined by the characteristic of [Zn(OH)4]2− precursors synthesized in different reaction conditions.16 The different [Zn(OH)4]2− precursors synthesized in different reaction conditions can modify the reaction process (eq 3), which might affect the competition between thermodynamics and kinetics during the reduction of precursors, nucleation, and growth of ZnO crystals.17,18 This process is similar to that occurring in reduction of Cu2O crystals in which the shape-tailored synthesis of polyhedral architectures can be successfully achieved by controlling the precursors.19,20 In our synthesis, it was found that the reaction temperature was important to the formation of spindle-like ZnO mesocrystals. Figure S3, Supporting Information, shows the FESEM images of the products obtained at low reaction temperature (see Supporting Information). It can be seen that the products were nanoplatelets (Supporting Information, Figure S3a, 25 °C) and irregular nanoparticles (Supporting Information, Figures S3b−S3d: 50, 60, and 70 °C), respectively. Therefore, the higher reaction temperature is in favor of the formation of ZnO mesocrystals with well-defined spindle-like morphologies (Figure 1). However, the formation of [Zn(OH)4]2− species not only depended on the precursor formation temperature but also the molar ratio between sodium hydroxide and zinc salt.17,18 It has been demonstrated that the formation of the spindle-like mesostructures in the alkali solution depends greatly on the nucleation frequency of ZnO. The predominant OH− ions play a crucial role in controlling the synthesis of the different crystalline facets because of the greater formation of the [Zn(OH)4]2− complex.17,18 The negative charged [Zn(OH)4]2− complexes can preferably adsorb on the surface of ZnO nuclei, which is in favor of the growth of ZnO nuclei along the [0001] direction.21,22 Figure S4, Supporting Information, is the FESEM results of the products obtained by changing the molar concentration ratio between sodium hydroxide and zinc salt (see Supporting Information), and it can be observed that the irregular plate-like ZnO crystals are formed as the ratio is 1:5 and 1:3 (Figures S4a and S4b, Supporting Information). While the ratio is increased to 1:1 and 3:1, spindle-like architectures are formed in the products (Figures S4c and S4d, Supporting Information), suggesting that the molar ratio between sodium hydroxide and zinc salt plays a significant role in the formation of spindle-like ZnO mesocrystals. Consequently, the ZnO mesostructures with well-defined shapes can only be obtained in the presence of appropriate alkali conditions. Although some explanations were given based upon the action of intrinsic electric fields,13,15 which direct the growth of dipole ZnO crystals, the exact mechanisms for the formation of spindle-like ZnO mesocrystals are yet uncertain. However, from the experimental results of the above investigation, a possible growth mechanism of these as-prepared ZnO mesocrystals is proposed. After the addition of excess NaOH solution into the reaction Zn(NO3)2 solution at high temperature, ZnO nuclei

Figure 1. (a) FESEM image of the as-prepared spindle-like ZnO crystals (sample A); the inset is an individual particle; (b) TEM image of the as-prepared spindle-like ZnO crystals; (c) individual spindle-like ZnO particle; (d) SAED pattern of the product as shown in panel c; (e) HRTEM image of the product as shown in panel c; the inset is the corresponding FFT image.

pattern.13 From the HRTEM image, it can also be clearly found that the many nanoparticles (marked by red arrows) have the same crystallographic orientations along the c-axis directions, thus indicating the formation of ZnO mesocrystals.13 The HRTEM image also indicates that the oriented aggregation and attachment of the nanoparticles occurs preferentially between the (0001) lattice planes. The corresponding fast Fourier transform (FFT) of the HRTEM image taken from the area of several nanoparticle building blocks is shown in the inset of Figure 1e, which is expressed as single-crystal-like reflections, corresponding to the formation of significant mesostructures in the as-prepared spindle-like ZnO architectures. The above observations convincingly support that the surfactant-free spindle-like ZnO mesocrystals can be successfully synthesized by our additive-free complex-precursor solution method. 3.2. Growth Mechanism. The detailed formation process of the as-synthesized surfactant-free ZnO mesocrystals can be described as follows: as appropriate molar ratio between Zn(NO3)2 and NaOH solution was used at relatively higher reaction temperature, ZnO crystals could be synthesized from the following reactions. Initially, when OH− ions were added into the zinc salt solution, Zn(OH)2 was first precipitated from the solution (eq 1). When the concentration of OH− ions was excessive enough, [Zn(OH)4]2− complexes would be formed 2413

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are adequately covered by the [Zn(OH)4]2− complexes, and the decomposition of unstable [Zn(OH)4]2− complexes played an important role on the formation of mesocrystals, which might change the crystallization process and result in the formation of mesostructures by self-assembly of new ZnO nanoparticles.13 Initially, small ZnO nanoparticles were generated immediately at higher reaction temperature, as evidenced by a fast change of the reaction system from transparent to white precipitations. Subsequently, in order to minimize the overall energy of the reaction system, small nanoparticles tended to aggregate rapidly along the c-axis direction. Figure S5, Supporting Information, shows the corresponding TEM and HRTEM images of the products obtained in a small amount of OH− ions, and it can be seen that many nanoparticles adsorbed onto the products. Figure S5b, Supporting Information, is a typical individual particle composed of many building blocks. The corresponding HRTEM image is displayed in Figure S5c, Supporting Information, and it can be found that the lattice fringes of these nanoparticles have been attached. The lattice fringes are marked by white lines and arrows, and the corresponding lattice spacings are 0.262 and 0.265 nm, which are in good agreement with the d value of the (0001) facets of ZnO. It indicates that nanoparticle building blocks have the same growth directions to form larger structures driven by the oriented attachment mechanism, which is similar to the previously reported mesocrystal examples, leading to the familiar rod-like structures due to their growth habit.13,17,23 In our experiment, the size scales of the as-prepared spindle-like ZnO architectures are fully different in three dimensions, which might depend on the number of the aggregated subunits.24 As shown in the HRTEM image (Figure 1e), it can be obviously observed that the aggregation and attachment occurs preferentially between the (0001) lattice planes of the nanoparticles. It suggests that the number of nanoparticles preferentially aggregated in the direction of the c-axis. Therefore, the preferential anisotropic growth of the as-prepared spindle-like ZnO mesocrystals might be attributed to the different nanoparticle-aggregation potentials, leading to the anisotropic growth rate are ordered as follows: c > b > a (Figure 1e), which is similar to the observations on the oriented aggregation of CuO mesocrystals.24 The corresponding growth process and formation mechanism is illustrated in Figure S6 (see Supporting Information). It is known that high surface energy facets tend to grow along their normal direction and eventually disappear from the final appearance according to the Gibbs−Wulff theorem,25 thus the crystalline facets are facile to evolve into the low-index planes during the growth process, unless the surface charge is compensated by capping agents (molecules or ions).26 The ZnO crystal can be described as a hexagonal close packing of zinc and oxygen atoms. The main crystal planes include a polar zinc (0001) facet, a polar oxygen (1001̅) facet, and six symmetric nonpolar (0001̅) planes parallel to the [0001] direction. Atomic stacking and morphology models of ZnO crystal is shown in Figure 2. The Zn-terminated (0001) plane is active in promoting one-dimensional growth. So the growth rate (V) of different crystal planes can be described as: V[0001] > V[101̅0] > V[0001̅].27 In our experiment, the opposite ions ([Zn(OH)4]2−) could be adsorbed onto the polar (0001) surface terminated by Zn2+ ions due to charge compensation, which resulted in the redistribution of the surface energy and the growth rate of different facets changed. On the basis of the reaction equations (eqs 1−3), the formation of [Zn(OH)4]2−

Figure 2. Crystallographic structures of (0001) and (0001̅) (a) and (101̅0) (b) facets of ZnO crystal; (c) corresponding morphology model.

precursors determines the final shapes of ZnO crystals, so it is believed that the intrinsic growth of ZnO along different zoneaxis directions can be substantially tailored to form these spindle-like ZnO mesocrystals by adjusting the concentration of reactants. 3.3. Synthesis of Spindle-Like ZnO Mesocrystals with Controllable Sizes (Along c-Axis Direction). In our present experiment, it can be seen that the concentration of OH− ions, which determines the formation of [Zn(OH)4]2− precursors, can control the growth of ZnO along the c-axis [0001] direction. As the concentrations of both sodium hydroxide and zinc salt decreased 10 times under otherwise same reaction conditions, the detailed characterization of the products is shown in Figure S7 (sample B, Supporting Information). Figure S7a, Supporting Information, is the typical FE-SEM image of these products, and we can find that the ZnO spindle-like architectures are uniform and monodisperse in this synthesis. The size distribution diagrams are shown in Figure S8 (see Supporting Information). The average sizes of length and width are about 752 ± 75 nm and 294 ± 30 nm, respectively. The inset of Figure S7a, Supporting Information, shows an individual spindle-like architecture, and it can be seen that the product has a rough surface with well-assembled nanoparticles building blocks. Figure S7b, Supporting Information, displays a representative TEM image of an individual particle, and the inset is the corresponding SAED pattern. The presence of somewhat elongated spots in this diffraction pattern implies that the product is a mesocrystal, and it can be also indexed to the hexagonal phase of ZnO crystal viewed along the [112̅0] zone axis, which implies that the preferential growth orientation of this spindle-like ZnO mesocrystal is along the c-axis [0001] direction. Figure S7c (see Supporting Information) shows the HRTEM image of the area marked by a red square in Figure S7b, Supporting Information. The two lattice fringes are marked by white lines and arrows, and the corresponding lattice spacing is about 0.52 nm, which is in good agreement with the d value of the (0001) facets of ZnO. An obvious oriented aggregation and attachment of the nanoparticles preferential 2414

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along the c-axis [0001] direction is observed in the HRTEM image, suggesting the synthesis of ZnO mesocrystals. The corresponding FFT image as shown in the inset of Figure S7c, Supporting Information, can further demonstrate the formation of ZnO mesocrystals. Furthermore, Na+ in sodium hydroxide and NO3− in zinc nitrate are not responsible for the formation of spindle-like morphologies. When potassium hydroxide and zinc chloride are used instead of sodium hydroxide and zinc nitrate in the synthesis of sample B, the spindle-like products still can be obtained (Figure S9, Supporting Information). On the basis of the synthesis process of sample B, when the concentration of sodium hydroxide further decreased ten times under otherwise same reaction conditions, the morphological and structural characterization of these products are shown in Figure S10 (sample C, Supporting Information). Figure S10a, Supporting Information, is the low-magnification FE-SEM image of these products, and it can be seen that the uniform and monodisperse ZnO spindle-like architectures have much larger sizes than those of the above products (Figures 1 and S7, Supporting Information). The average sizes of length and width are about 1443 ± 143 nm and 503 ± 50 nm, respectively. The size distribution diagrams are shown in Figure S11 (see Supporting Information). Figure S10b, Supporting Information, is a typical FESEM image of an individual spindle-like architecture, and the product also has a rough surface with well-assembled nanoparticle building blocks. Figure S10c, Supporting Information, shows a TEM image of an individual particle, and the inset is the corresponding SAED pattern, which displays typical elongated spots, suggesting the preferential growth orientation along the c-axis [0001] direction of the spindle-like ZnO mesocrystal. Figure S10d, Supporting Information, is the HRTEM image of the area marked by a red square in Figure S10c. The lattice spacing is about 0.52 nm, which is in good agreement with the d value of the (0001) facets of ZnO. Oriented aggregation and attachment of the nanoparticle building blocks (marked with red arrows) preferential along the c-axis [0001] direction are also seen in the HRTEM image. The corresponding FFT image (the inset of Figure S10d, Supporting Information) also displays a singlecrystal-like characteristic. The HRTEM and FFT results indicate that these larger spindle-like ZnO architectures are also mesocrystals. From the results as shown in Figures 1, S7, and S10, Supporting Information, we can find that the spindlelike ZnO mesocrystals with controllable sizes (along c-axis directions) can be successfully achieved by tailoring the concentration of hydroxyl ions. 3.4. Synthesis of Peanut-Like ZnO Mesocrystals with Controllable Sizes (Along c-Axis Direction) and Shapes (Along a- and b-Axis Directions). More interestingly, it can be found that the tailoring of the concentration of Zn2+ ions can also affect the growth features of ZnO along a- and b-axis directions (namely, the shapes and sizes of {0001} facets). Figure 3 is the FESEM image of the products obtained with different molar ratios R between Zn2+ and OH− ions, which demonstrates that the size- and morphology-evolution of the products along the a- and b-axis directions (the shapes of {0001} facets) with a tailoring of the R value. When the R value was 10/1 (sample D), the products transformed from spindlelike architectures (Figure S7, Supporting Information) into peanut-like ones (Figure 3a), and the average length of the cdirection is about 1108 ± 93 nm (Figure S12a, see Supporting Information). These peanut-like architectures were well-defined and monodisperse, and it can be seen that the formation of

Figure 3. FESEM image of the products obtained with different molar ratio R between Zn2+ and OH− ions based on the synthesis process of sample B, and the insets denote the schematic illustration of the corresponding products. (a) R = 10/1 (sample D); (b) R = 15/1 (sample E); (c) R = 20/1 (sample F); (d) R = 30/1 (sample G); (e) schematic illustration of shape-evolution of the growth features along the a- and b-axis directions.

rough surfaces of the particle is achieved. Importantly, it is worth noticing that the areas of {0001} facets become larger than those of the particles as shown in Figure S7, Supporting Information. When the R value was 15/1 (sample E), the products were still peanut-like particles with rough surfaces (Figure 3b). The average length of these products was approximately 932 ± 72 nm (Figure S12b, see Supporting Information). Figure 4 shows the corresponding TEM image, SAED pattern and HRTEM image. A typical TEM image of an individual particle is displayed in Figure 4a. The corresponding SAED pattern is shown in Figure 4b, which demonstrates that the particle has a single-crystalline feature. It can be also indexed to the hexagonal phase of a ZnO crystal viewed along the [112̅0] zone-axis, which suggests that the preferential growth orientation of this peanut-like ZnO particle is along the c-axis [0001] direction. Figure 4c is the HRTEM image taken from the area marked with a red circle in Figure 4a, and it can be seen that three sections (I, II, and III) have the same crystallographic orientation along the c-axis direction, also implying the formation of ZnO mesocrystals. In order to uncover the growth process and shape-evolution of the peanut particles of sample E, a control experiment was performed. Figure S13 (Supporting Information) shows the TEM images of the products obtained in different volumes of NaOH solution under otherwise the same conditions based on the synthesis of sample E. Figure S13A, Supporting Information, is the TEM image of the products obtained as the volume of 2415

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process can be further occurred, and the scales of particles were further increased. Along with the reaction time increase, all of the nanowire building blocks would be self-tailoring by the rearrangement process,29 finally resulting in the formation of peanut-like morphologies (see sample E). The corresponding growth process and formation mechanism (aggregation and ripening) is illustrated in Figure S14 (see Supporting Information). By increasing the R value to 20/1 (sample F), it can be interestingly found that the size of particles was developed into submicrometer spheres (Figure 3c). Moreover, other peanut-like particles with prism building blocks were successfully formed. The average length of these products was about 405 ± 63 nm (Figure S12c, see Supporting Information). The above results indicate that the length along the c-axis direction can be tailored with the changing of the R value. By further increasing the R value to 30/1 (Figure 3d, sample G), peanut-like architectures with shorter length (255 ± 40 nm, Figure S13d, see Supporting Information) were synthesized from the rounded shape of the twin-crystal surface, and the {0001} facets were hexagons. This process is similar to the twostep mechanism found by Goia and co-workers in the synthesis of hexagonal ZnO prisms.29 On the basis of the above results, the shape-evolution can be attributed to the presence of a greater amount of Zn2+ ions and can improve the protection of {0001} planes of ZnO, leading to the shape-evolution of the growth features along the a- and b-axis directions. The morphology transformation diagram is shown in Figure 3e, indicating that the structural features along the a- and b-axis directions can be tailored by adjusting the amount of zinc ions. 3.5. Schematic Illustration of Zone-Axis Preferential Growth of ZnO Mesocrystals. For hexagonal wurtzite ZnO crystals, it has a dipole moment along the c-axis direction, i.e., the (0001) facet terminated at the Zn atomic layer is positively charged and the opposite (0001̅) facet terminated at the O atomic layer has a negative charge (as shown in Figure 2). The negative OH− ions, selectively adsorbed onto the positive (0001) facets, could stabilize the (0001) facets, and the growth rates along the ⟨0001⟩ directions were confined. Hence, the positive (0001) facets were protected, and the outward preferential growth of ZnO has occurred along with the increase of OH− ions, thus the spindle-like architectures with sharp tips were finally synthesized (section I). Contrarily, the positive Zn2+ ions, easily adsorbed onto the opposite (0001)̅ facets, could stabilize the (0001̅) facets, and the growth rates along the directions were restrained. Thus, the (0001̅) facets were protected, while the (0001) facets were gradually disappearing along with the increase of Zn2+ ions; thus, the peanut-like architectures with flat tips were finally synthesized (section II). On the basis of the understanding of the size- and morphology-evolution and corresponding intrinsic lattice of ZnO crystal, a facile strategy to artificially design and fabricate surfactant-free ZnO mesocrystals with controllable sizes and morphologies has been demonstrated via our additive-free complex-precursors solution method. A schematic illustration of the zone axis preferential growth and reaction pathways of various ZnO mesocrystals in different concentration of reactants is listed in Figure 5. The c-axis preferential growth of spindle-like ZnO mesocrystals is mainly determined by the concentration of OH− ions (Figures 1, S7, and S10, Supporting Information). When the concentration of the OH− ions was decreased from 0.002 to 0.2 M, the size of spindle-like ZnO mesocrystals from microscale to nanoscale can be successfully

Figure 4. TEM image (a), SAED pattern (b), and HRTEM image (c) of sample E.

NaOH was 10 mL, and it can be seen that many nanoparticleaggregations and nanosheet precursors were formed. From the 15 mL sample (see Figure 13B-1, Supporting Information), a large amount of twin-crystals have been synthesized, and a typical twin-architecture with double-subunits is shown in Figure S13B-2, Supporting Information. The aggregation is in an off-centered position rather than fixed preferentially on top with the same ends, and it is similar to that occurred in the previous report.28 It is noted that the growth of the nanoparticle aggregations is very fast, so it is difficult to observe the evolution process of these twin crystals. As above mentioned (see Figure 2), ZnO has a dipole moment along the c-axis, namely, the (0001) surface terminated at Zn atomic layer is positively charged and the opposite (0001̅) surface terminated at O atomic layer has a negative charge. When two ZnO subunits are separated by negative [Zn(OH)4]2− precursors, the inner surfaces of the ZnO subunits would be positively charged, thus both outer surfaces of the twin crystals (double-subunits) would be negatively charged. However, the ZnO nanocrystals also have a polar property, thus a strong interaction will occur between the positively charged surface of nanocrystals and the negatively charged surface of the twincrystals. Deposition of nanocrstals on the double-subunits of ZnO twin crystals is possible as the amount of NaOH increased (as shown in Figure S13C, Supporting Information). When a single-layer of nanocrystals is deposited on the negative surface of a twin-core, the facet close to the core would be positively charged, and the top surface would become negatively charged, leading to the fact that the dipolar moment in each half of a twin-core would enhance with the stacking of nanowire building blocks. During this process, the stacking process can be accelerated until the amount of the free nanocrystals in the solution becomes low, so the wholly sizes of the products gradually increased. Consequently, for the 60 mL sample (see Figure S13C, Supporting Information), it can be observed that the size of the products was increased, and many nanowire building blocks composed of nanocrsytals were prepared. Upon further increasing the amount of NaOH (80 mL, see Figure S13D, Supporting Information), the nanoparticle aggregated 2416

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ASSOCIATED CONTENT

S Supporting Information *

XRD pattern, size distribution diagrams, and additional FESEM, TEM, HRTEM, and SAED images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation of China (NSFC No. 51071116) and the Natural Science Foundation of Shaanxi Province (No.2011JZ008) is acknowledged.

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REFERENCES

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Figure 5. Schematic illustration of the zone-axis preferential growth and reaction pathways of various ZnO mesocrystals in different concentrations of reactants.

synthesized (section I). The a- and b-axis preferential growth of peanut-like ZnO mesocrystals is determined by the concentration of Zn2+ ions (Figure 3). When the molar ratio R between Zn2+ and OH− ions increased from 10/1, 15/1, and 20/1 to 30/1, the various ZnO products transformed from spindle-like mesocrystals into peanut-like ones were achieved (section II). The shape- and size-evolution of these assynthesized surfactant-free ZnO mesocrystals could be attributed to tailor the aggregation behavior of nanoparticle building blocks in different concentrations of reagents.

4. CONCLUSIONS In conclusion, a designated-tailoring on zone-axis preferential growth of surfactant-free ZnO mesocrystals with different features (shapes and sizes) was successfully achieved via an additive-free complex-precursor solution method. The formation of these ZnO mesocrystals here is essentially determined by the characteristic of [Zn(OH)4]2− precursors, and an oriented nanoparicle aggregation with tailoring sizes can occur in different concentrations of reactants at higher temperature. Spindle-like ZnO mesocrystals with controllable sizes (along the c-axis direction) were prepared by adjusting the concentration of hydroxyl ions, and peanut-like ZnO mesocrystals with controllable sizes (along c-axis direction) and shapes (along the a- and b-axis directions, namely, perpendicular c-axis direction) were synthesized by tailoring the concentration of zinc ions. This study is of great importance in the synthesis of surfactant-free ZnO mesocrystals, and it not only enriches the family of ZnO architectures but also offers a good opportunity to understand the formation mechanism and fundamental significance of zone-axis preferential growth of ZnO mesocrystals. Significantly, it is believed that the precursor driven assembly of mesostructures reported here would provide a green way to design more and more surfactant-free metal oxide architectures with well-defined shapes. 2417

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Crystal Growth & Design

Article

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