Exposed Crystal Face Controlled Synthesis of 3D ZnO Superstructures

Jul 27, 2010 - Before exposure to UV light, the suspension was aged in the dark for 30 ...... Hydrothermal growth of wheatear-shaped ZnO microstructur...
0 downloads 0 Views 7MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Exposed Crystal Face Controlled Synthesis of 3D ZnO Superstructures Seungho Cho,† Ji-Wook Jang,† Jae Sung Lee, and Kun-Hong Lee* Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, Korea 790-784. †These authors contributed equally to this work Received May 26, 2010. Revised Manuscript Received June 29, 2010 We report a method for synthesizing exposed crystal face controlled 3D ZnO superstructures under mild conditions (at room temperature or 90 C under 1 atm) without organic additives. The exposed crystal faces of the building blocks of the 3D structures were controlled by varying the reactant concentrations and the reaction temperatures. On the basis of the experimental results, we speculated a possible mechanism for the formation of the four distinct 3D ZnO superstructures (structures I, II, III, and IV) under the different growth conditions. The optical properties of the 3D ZnO superstructures were probed by UV-vis diffuse reflectance spectroscopy. The spectra were shifted depending on the dimensions and sizes of the building blocks of the 3D superstructures. The photocatalytic activities of the 3D superstructures varied according to the exposed crystal faces, which could be controlled by this method (structure I > structure IV > structure III > structure II).

1. Introduction The self-assembly of nanoscale building blocks into complex superstructures, inspired by and simulating natural phenomena, has been a research hotspot.1 Material scientists and engineers have paid increasing attention to the synthesis of complex micro/ nanoarchitectures, especially to three-dimensional (3D) hierarchical architectures that are assembled using 1D and 2D nanoscale building units. Such hierarchical architectures combine the features of micrometer and nanometer scale building blocks and have unique properties that are distinct from the properties of monomorphological structures.2 Zinc oxide (space group = P63mc; a = 0.324 95 nm, c = 0.520 69 nm) is an important II-VI semiconductor that has a wide direct band gap of 3.37 eV at room temperature and a large exciton binding energy of ∼60 meV. Zinc oxide (ZnO) has useful characteristics, such as a large piezoelectric constant, and its electrical conductivity can be easily modified. ZnO has received considerable attention over the past few years because of these unique properties and has been used in a variety of applications.3-10 In particular, hierarchically self-assembled 3D ZnO superstructures constructed using 1D and 2D ZnO nanoscale building blocks have been extensively *Corresponding author. E-mail: [email protected]. (1) C€olfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (2) Gao, X. F.; Jiang, L. Nature 2004, 432, 36. (3) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (4) Rao, G. S. T.; Rao, D. T. Sens. Actuators, B 1999, 55, 166. (5) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2003, 81, 3648. (6) Tominaga, K.; Umezu, N.; Mori, I.; Ushiro, T.; Moriga, T.; Nakabayashi, I. Thin Solid Films 1998, 334, 35. (7) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (8) Zeng, H.; Cai, W.; Liu, P.; Xu, X.; Zhou, H.; Klingshirn, C.; Kalt, H. ACS Nano 2008, 2, 1661. (9) Zeng, H.; Xu, X.; Bando, Y.; Gautam, U. K.; Zhai, T.; Fang, X.; Liu, B.; Golberg, D. Adv. Funct. Mater. 2009, 19, 3165. (10) Hara, K. Sol. Energy Mater. Sol. Cells 2000, 64, 115. (11) Liu, B.; Yu, S. H.; Zhang, F.; Li, L. J.; Zhang, Q.; Ren, L.; Jiang, K. J. Phys. Chem. B 2004, 104, 4338. (12) Sounart, T. L.; Liu, J.; Voigt, J. A.; Hsu, J. W. P.; Spoerke, E. D.; Tian, Z.; Jiang, Y. B. Adv. Funct. Mater. 2006, 16, 335. (13) Zhou, X. F.; Zhang, D. Y.; Zhu, Y.; Shen, Y. Q.; Guo, X. F.; Ding, W. P.; Chen, Y. J. Phys. Chem. B 2006, 110, 25734. (14) Zhang, D. F.; Sun, L. D.; Zhang, J.; Yan, Z. G.; Yan, C. H. Cryst. Growth Des. 2008, 8, 3609.

Langmuir 2010, 26(17), 14255–14262

investigated11,12 on account of their outstanding electronic, optical, and catalytic properties13-15 and due to their potentially wide-ranging applications in nanolasers, solar cells, and other functional devices.16-19 Crystalline materials have different properties and activities depending on their exposed crystal faces. For example, anatase TiO2 crystals with a large percentage of {001} surfaces have high catalytic reactivity.20 The {110} surfaces of rutile TiO2 nanorods provide reductive sites, and {011} surfaces provide oxidative sites in a mixture of anatase and rutile particles.21 Pt {111} faces show high methanol oxidation reactivity in fuel cell tests.22 ZnO (001) surfaces have been found to be chemically active whereas (001) surfaces were found to be inert.23,24 Therefore, the fine control over exposed crystal faces is indispensable for exploring the potential uses of crystalline structures as a source of smart and efficient materials. ZnO structures can be synthesized via vapor phase methods or wet chemical processes.25 Among these methods, solution-based approaches have been successfully used to control ZnO crystal growth through the use of organic agents, such as ethylenediamine,26 citrates,27 amino acids,28 polyacrylamide,29 (15) Zhou, X. F.; Hu, Z. L.; Fan, Y. Q.; Chen, S.; Ding, W. P.; Xu, N. P. J. Phys. Chem. C 2008, 112, 11722. (16) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (17) Tammy, P. C.; Zhang, Q. F.; Glen, E. F.; Cao, G. Z. Adv. Mater. 2007, 19, 2588. (18) Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (19) Yang, J. L.; An, S. J.; Park, W. I.; Yi, G. C.; Choi, W. Adv. Mater. 2004, 16, 1661. (20) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638. (21) Ohno, T.; Sarukawa, K.; Matsumura, M. New J. Chem. 2002, 26, 1167. (22) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967. (23) Jang, E. S.; Won, J.-H.; Hwang, S.-J.; Choy, J.-H. Adv. Mater. 2006, 18, 3309. (24) Wang, Z. :: L.; Kong, X. Y.; Zuo, J. M. Phys. Rev. Lett. 2003, 91, 185502. € ur, U.; (25) Ozg€ Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S.-J.; Morkoc-, H. J. Appl. Phys. 2005, 98, 041301. (26) (a) Liu, B.; Zeng, H. C. Langmuir 2004, 20, 4196. (b) Xu, L. F.; Guo, Y.; Liao, Q.; Zhang, J. P.; Xu, D. S. J. Phys. Chem. B 2005, 109, 13519. (27) (a) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. F. Nature Mater. 2003, 2, 821. (b) Kuo, C. L.; Kuo, T. J.; Huang, M. H. J. Phys. Chem. B 2005, 109, 20115. (c) Cho, S.; Jung, S.-H.; Lee, K.-H. J. Phys. Chem. C 2008, 112, 12769. (d) Cho, S.; Jang, J.-W.; Jung, S.-H.; Lee, B. R.; Oh, E.; Lee, K.-H. Langmuir 2009, 25, 3825. (28) Gerstel, P.; Hoffmann, R. C.; Lipowsky, P.; Jeurgens, L. P. H.; Bill, J.; Aldinger, F. Chem. Mater. 2006, 18, 179. (29) Peng, Y.; Xu, A. W.; Deng, B.; Antonietti, M.; C€olfen, H. J. Phys. Chem. B 2006, 110, 2988.

Published on Web 07/27/2010

DOI: 10.1021/la102126m

14255

Article

Cho et al.

sodium poly(styrenesulfonate),30 poly(diallyldimethylammonium chloride),30 vitamin C,31 diblock copolymers,32 and gelatin.33 Numerous opportunities are present during synthesis for the absorption of organic molecules onto the surfaces of ZnO, which may influence the nucleation and growth of ZnO crystals. Thus, adsorbed organic additives may decrease the efficient surface areas, resulting in decreased catalytic and sensing efficiencies. In this article, we describe a solution-based method for synthesizing exposed crystal face controlled 3D ZnO superstructures under mild conditions (at room temperature or 90 C under 1 atm) without organic additives. The exposed crystal faces of the building blocks for the 3D superstructures were controlled simply by varying the reactant concentrations and the reaction temperatures. On the basis of these results, we discuss a possible mechanism for the formation of the four distinct 3D ZnO superstructures. The optical properties and photocatalytic activities of the 3D ZnO superstructures are also examined.

2. Experimental Section Synthesis of Structure I. All chemicals used in this study were of analytical grade and were used without further purification. An aqueous solution containing 0.03 M zinc acetate dihydrate (Zn(CH3COO)2 3 2H2O, 99%, Samchun, 0.025 M) and 0.1 M sodium peroxide (Na2O2, 28.0-30.0 wt %, Samchun) was prepared (pH 12.9) and maintained at room temperature for 5 h. After the reaction, the solution was filtered through a polycarbonate membrane filter (ISOPORE). The filtered powders were washed several times with deionized (DI) water and dried in an oven at 60 C for 12 h. Synthesis of Structure II. An aqueous solution containing 0.02 M zinc acetate dihydrate and 0.2 M sodium peroxide was prepared at room temperature (pH 13.8). The solution was maintained at 90 C for 5 h followed by filtration. The filtered powders were washed several times with DI water and dried in an oven at 60 C for 12 h. Synthesis of Structure III. An aqueous solution containing 0.01 M zinc acetate dihydrate and 0.2 M sodium peroxide was prepared at room temperature (pH 13.8). The solution was maintained at 90 C for 5 h followed by filtration. The filtered powders were washed several times with DI water and dried in an oven at 60 C for 12 h. Synthesis of Structure IV. An aqueous solution containing 0.02 M zinc acetate dihydrate and 0.03 M sodium peroxide was prepared at room temperature (pH 12.2). The solution was maintained at 90 C for 5 h followed by filtration. The filtered powders were washed several times with DI water and dried in an oven at 60 C for 12 h. Characterization. The morphology, crystallinity, crystalline nature, chemical composition, and optical properties of the samples were characterized by field-emission scanning electron microscopy (FESEM, JEOL JMS-7401F, operated at 10 keV), X-ray diffraction spectrometry (XRD, Mac Science, M18XHF using Cu KR (λ = 0.154 06 nm) radiation), high-resolution scanning transmission electron microscopy (HR-STEM, Cs-corrected, JEOL JEM-2200FS with an energy-dispersive X-ray spectrometer, operated at 200 kV), and UV-vis diffuse reflectance spectroscopy (Shimadzu, UV2501PC). Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption was measured using a Micromeritics analyzer (ASAP 2020 V3.01 H analyzer). (30) Garcia, S. P.; Semancik, S. Chem. Mater. 2007, 19, 4016. (31) Cho, S.; Jeong, H.; Park, D.-H.; Jung, S.-H.; Kim, H.-J.; Lee, K.-H. CrystEngComm 2010, 12, 968. € (32) (a) Oner, M.; Norwig, J.; Meyer, W. H.; Wegner, G. Chem. Mater. 1998, 10, 460. (b) Wegner, G.; Baum, P.; M€uller, M.; Norwig, J.; Landfester, K. Macromol. Symp. 2001, 175, 349. (c) Taubert, A.; Palms, D.; Weiss, O.; Piccini, M. T.; Batchelder, D. N. Chem. Mater. 2002, 14, 2594. (d) Mun~oz-Espí, R.; Jeschke, G.; Lieberwirth, I.; Gomez, C. M.; Wegner, G. J. Phys. Chem. B 2007, 111, 697. (33) Bauermann, L. P.; del Campo, A.; Bill, J.; Aldinger, F. Chem. Mater. 2006, 18, 2016.

14256 DOI: 10.1021/la102126m

Photocatalytic Activity Measurements. The photodecomposition of Orange II dye (4-(2-hydroxy-1-naphthylazo)benzenesulfonic acid, Aldrich) was used to study the catalytic properties of the four ZnO superstructures. The photocatalytic reactions in aqueous solution were carried out at room temperature in a closed system using a mercury lamp (1 W cm-2, model 66905, Newport Co.). The as-synthesized ZnO powders were transferred to 100 mL of a 5.0  10-5 M Orange II solution. Before exposure to UV light, the suspension was aged in the dark for 30 min to equilibrate the adsorption and desorption of dye molecules. With stirring, the suspensions were placed under UV light. The quantity of dye in solution was determined by measuring the absorption intensity at 486 nm, the main absorption peak of the Orange II dye, using a UV2501PC (Shimadzu) spectrometer.

3. Results and Discussion Synthesizing 3D ZnO superstructures usually requires the formation of aggregated ZnO nuclei in an initial homogeneous nucleation process. A small quantity of ZnO growth units led to the formation of initial ZnO single crystalline crystals with a relatively small number of defects during the initial homogeneous nucleation process. These initial single crystalline structures passed though a further growth process to form single crystalline structures (separated nanoscale building blocks). In contrast, a large number of ZnO growth units in an initial homogeneous nucleation process led to a burst in homogeneous nucleation, which formed aggregated ZnO nuclei. The ZnO crystals produced during the initial growth stage had crystalline grains and boundaries. These boundaries contained more defects than other regions and were not thermodynamically stable. Each grain of the initial ZnO crystals may have grown preferentially along one growth direction, and secondary growth from the defects may also have occurred due to the large number of growth units. The crystal surfaces containing defects tended to further decrease their energy through surface reconstruction, which provided active sites for secondary nucleation.34 Thus, complex 3D structures formed in the presence of abundant growth units. Zn(II) exists as Zn2þ, Zn(OH)þ, Zn(OH)2, Zn(OH)3-, and Zn(OH)42- in aqueous solution, with species ratios that depend on the pH.35 A relatively large quantity of Zn(OH)42-, which acts as a ZnO growth unit,36 can exist under alkaline conditions. Sodium peroxide was used to increase the alkalinity of the reaction solution for the synthesis of 3D ZnO superstructures. Sodium peroxide hydrolyzes to give sodium hydroxide and hydrogen peroxide according to the reaction Na2 O2 þ 2H2 O f 2Naþ þ 2OH - þ H2 O2

ð1Þ

Hydroxyl anions (OH-) are provided by the rapid hydration of sodium peroxide, which increases the pH of the reaction solution. Without sodium peroxide, we could not obtain 3D ZnO structures. ZnO microrods were synthesized without sodium peroxide (Supporting Information, Figure S1). Under alkaline conditions, the following reactions produce the nucleation and growth of ZnO crystals: Zn2þ þ 4OH - f ZnðOHÞ4 2 -

ð2Þ

ZnðOHÞ4 2 - f ZnO þ H2 O þ 2OH -

ð3Þ

We synthesized structure I from the reaction of 0.03 M zinc acetate dihydrate and 0.1 M sodium peroxide aqueous solution at (34) Gao, Y.; Koumoto, K. Cryst. Growth Des. 2005, 5, 1983. (35) Reichle, R. A.; McCurdy, K. G.; Hepler, L. G. Can. J. Chem. 1975, 53, 3841. (36) Li, W. J.; Shi, E. W.; Zhong, W. Z.; Yin, Z. W. J. Cryst. Growth 1999, 203, 186.

Langmuir 2010, 26(17), 14255–14262

Cho et al.

Article

Figure 1. (a, b) SEM images of structure I synthesized from the reaction of an aqueous solution containing 0.03 M zinc acetate dihydrate and 0.1 M sodium peroxide at room temperature for 5 h. (c) XRD pattern of structure I powders. (d) TEM image of a 3D structure. (e) HR-TEM image of the area indicated by the black circle in (d). (f) SAED pattern of a building block of the 3D structure. (g) HAADF image of the 3D structures. (h) Elemental mapping of Zn. (i) Elemental mapping of O.

room temperature over 5 h. Figure 1a,b shows SEM images of the powders (structure I). These 3D structures consisted of 2D plate building blocks. Figure 1c displays the X-ray diffraction (XRD) pattern of the 3D structures. The XRD pattern could be indexed as a pure wurtzite ZnO structure (JCPDS No. 36-1451) with calculated lattice constants of a = 0.3248 nm and c = 0.5211 nm. These lattice constants were consistent with previously reported data. To investigate the crystalline nature of the as-prepared sample, magnified images of the synthesized structures were obtained by using a high-resolution scanning transmission electron microscope (HR-STEM). Figure 1d shows a bright-field TEM image of a 3D superstructure (structure I). TEM observations confirmed that the building blocks were thin sheet structures (with an average thickness of ∼10 nm). Dark regions in Figure 1d indicate the 2D plates, the planar surfaces of which are nearly parallel to the direction of electron beam irradiation. The HR-TEM image (Figure 1e) shows 2D lattice fringes with spacings of 2.8 and 5.2 A˚, in agreement with the interspacings of the {1010} and (0001) planes, respectively. These observations identified the building blocks of structure I as 2D structures with {2110} planar surfaces. The selected area electron diffraction (SAED) pattern (Figure 1f) confirmed that each building block was single-crystalline in nature. The compositions of the 3D structures were investigated by energy dispersive X-ray spectroscopy (EDS). Figure 1g shows a highangle annular dark-field (HAADF) image of the 3D structures. The EDS elemental maps demonstrated that Zn (Figure 1h) and Langmuir 2010, 26(17), 14255–14262

O (Figure 1i) were distributed homogeneously within the 3D structures. Quantitative analysis disclosed that the mean atomic ratio (Zn:O) in the ZnO structures was 0.507:0.493. No evidence of other impurities was found. These data also confirmed the high purity of the ZnO 3D structures. Before discussing the formation mechanism of these 3D ZnO structures, it should be noted that ZnO crystal structures consist of hexagonally close-packed oxygen and zinc atoms. The crystals exhibit several main crystal planes: a top tetrahedral cornerexposed polar zinc (0001) face, six symmetric nonpolar {1010} planes parallel to the [0001] direction, and a basal polar oxygen (0001) face.37 Each plane has a different polarity. The nonpolar {1010} planes, which have relatively low surface energies, are more stable than the polar (0001) and (0001) planes, which have high surface energies.38,39 Because the system tends to minimize the total surface energy, ZnO crystals usually grow preferentially along the [0001] direction in the absence of surfactant. It is known that the growth of ZnO in an aqueous solution is affected by external conditions such as solution pH value, solution concentration, temperature, and the atomic arrangement at the growth surface.40 (37) Xu, L.; Guo, Y.; Liao, Q.; Zhang, J.; Xu, D. J. Phys. Chem. B 2005, 109, 13519. (38) Wander, A.; Harrison, N. M. Surf. Sci. 2000, 457, L342. (39) Meyer, B.; Marx, D. Phys. Rev. B 2003, 67, 035403. (40) Li, J.; Srinivasan, S.; He, G. N.; Kang, J. Y.; Wu, S. T.; Ponce, F. A. J. Cryst. Growth 2008, 310, 599.

DOI: 10.1021/la102126m

14257

Article

Cho et al.

Figure 2. (a, b) SEM images of structure II synthesized from the reaction of an aqueous solution containing 0.02 M zinc acetate dihydrate and 0.2 M sodium peroxide at 90 C for 5 h. (c) SEM image of the tip of a building block of structure II. (d) XRD pattern of structure II powders. (e) TEM image of a 3D structure. (f) HR-TEM image of the area indicated by the black circle in (e). (g) SAED pattern of a building block of the 3D structure. (h) HAADF image of the 3D structures. (i) Elemental mapping of Zn. (j) Elemental mapping of O.

We speculated that the reason for the formation of ZnO nanosheet building blocks with {2110} planar surfaces was as follows. In the case of 0.03 M zinc acetate dihydrate at room temperature (pH 12.9), the adsorption of anions such as acetate anions and Zn(OH)3- in the solution is expected to compete with Zn(OH)42-, ZnO growth units for occupation of the positively charged (0001) plane. Thus, the abundant nongrowth unit anions stabilized the surface charge and structure of the (0001) surfaces to some extent, allowing fast growth along Æ1010æ. Thus, ZnO crystals preferentially grew along the Æ0001æ and Æ1010æ directions (both are fast growth directions), which led to the formation of ZnO nanosheets with {2110} planar surfaces. A more detailed and deeper understanding of the formation of {2110} surface-dominant building blocks warrants further investigation. Structure II was synthesized from the reaction of 0.02 M zinc acetate dihydrate and 0.2 M sodium peroxide aqueous solutions at 90 C for 5 h. Figure 2a,b shows SEM images of structure II powders. The structures had urchin-like shapes that were composed of 1D nanoneedle building blocks. The nanoneedle tips are shown in Figure 2c. The building blocks tapered to sharp points. Figure 2d displays the XRD pattern of structure II. The XRD pattern could be indexed as a pure wurtzite ZnO structure with calculated lattice constants of a = 0.3249 nm and c = 0.5212 nm. Figure 2e shows a bright-field TEM image of a 3D superstructure (structure II). TEM analysis indicated that the building blocks had a [0001] growth direction and {1010} side walls. Figure 2f 14258 DOI: 10.1021/la102126m

shows an HR-TEM image of the tip of the building block marked by the black circle in Figure 2e. The HR-TEM image shows that the main exposed planes of the tip were {1011} and {1010} surfaces, and some exposed (0001) planes also existed. The building blocks were highly crystalline with a lattice spacing of 0.26 nm, corresponding to the distance between (0002) planes in the ZnO crystal lattice. The SAED pattern (Figure 2g) confirmed that each building block was single crystalline in nature. Figure 2h shows a HAADF image of the 3D structures. The EDS elemental maps demonstrated that Zn (Figure 2i) and O (Figure 2j) were distributed homogeneously within the structures. Quantitative analysis found that the mean atomic ratio (Zn:O) in the ZnO structures was 0.505:0.495. No evidence of other impurities was found. The concentration of sodium peroxide in the solution increased to 0.2 M and the concentration of acetate anions decreased (0.02 M zinc acetate dihydrate), which caused the solution pH to increase (pH 13.8). Under these conditions, the concentration ratio of Zn(OH)42- anions to nongrowth unit anions was higher than in the structure I case.35 The reaction temperature also increased to 90 C. Thus, a large quantity of ZnO growth units existed in the reaction solution. The plethora of growth units led to the aggregation of ZnO nuclei during the initial homogeneous nucleation process. The ZnO crystals formed in the initial growth stage had crystalline grains and boundaries. From these aggregated initial crystals, ZnO crystals grew preferentially along the [0001] growth directions to form 3D superstructures composed of single crystalline nanoneedles. Langmuir 2010, 26(17), 14255–14262

Cho et al.

Structure III was obtained from the reaction of 0.01 M zinc acetate dihydrate and 0.2 M sodium peroxide aqueous solutions at 90 C for 5 h. Panels a and b of Figure 3 show that the synthesized structures had 3D urchin shapes composed of 1D building blocks, which were similar to the structures found in structure II. However, the building blocks had flat ends. The XRD pattern of the powders could be indexed as a pure wurtzite ZnO structure with calculated lattice constants of a = 0.3248 nm and c = 0.5212 nm. The crystal growth directions were analyzed by TEM. Figure 3d shows a TEM image of a typical structure. HR-TEM (Figure 3e) shows that the nanorods (building blocks) were highly crystalline with a lattice spacing of 0.52 nm, which corresponded to the distance between the (0001) planes in the ZnO crystal lattice. SAED patterns showed that each building block was single crystalline in nature and confirmed that the growth direction was [0001] (Figure 3f). The EDS elemental maps demonstrated that Zn (Figure 3g) and O (Figure 3h) were distributed homogeneously within the 3D structures. Quantitative analysis found that the mean atomic ratio (Zn:O) in the ZnO structures was 0.504:0.496. These data also confirmed the high purity of the 3D ZnO structures composed of nanorods. When the 0.01 M zinc acetate dihydrate and 0.2 M sodium peroxide aqueous solutions were used for synthesis, 3D structures formed in the early stages of the reaction as in the case of structure II. However, as the reaction proceeded, the concentration of ZnO growth units dropped below a given threshold earlier than did the concentration of growth units during synthesis of structure II. The low-concentration growth units may have preferentially incorporated into more active sites such as edges and corners rather than into the flat terraces of the crystals. Thus, the 3D superstructures (structure III), which were composed of flat (0001) surface-terminated 1D building blocks, were synthesized. We synthesized structure IV from the reaction of 0.02 M zinc acetate dihydrate and 0.03 M sodium peroxide aqueous solutions at 90 C for 5 h. Figure 4a,b shows SEM images of the synthesized structure IV powder. 3D assembled structures were formed by conelike building blocks. Figure 4c displays the XRD pattern of structure IV. The XRD pattern could be indexed as a pure wurtzite ZnO structure with calculated lattice constants of a = 0.3248 nm and c = 0.5209 nm. Figure 4d shows a TEM image of the 3D structures. HR-TEM (Figure 4e) and the corresponding SAED image (Figure 4f) showed that the direction of the cone axis was [0001], and the conelike building blocks were single crystalline. As shown in Figure 4e, each crystalline cone of structure IV had a lattice spacing of 0.26 nm, corresponding to the interspacings of the (0002) planes. Exposed crystal faces were mainly composed of {1011}, {1010}, and (0001) planes. Figure 4g shows a HAADF image of the 3D structures. The EDS elemental maps demonstrated that Zn (Figure 4h) and O (Figure 4i) were distributed homogeneously within the 3D superstructures. Quantitative analysis found that the mean atomic ratio (Zn:O) in the ZnO structures was 0.505:0.495. These data also confirmed that high-purity 3D ZnO structures were synthesized. If the concentration of sodium peroxide was reduced to 0.03 M, the pH of the solution decreased to 12.2. At this pH, a significant quantity of Zn(OH)3- existed as the Zn species as well as the growth units, Zn(OH)42-.35 Thus, the concentration ratio of the ZnO growth unit, Zn(OH)42-, to the other anions, such as acetate anions and Zn(OH)3-, was lower than that present in the reaction solution for the preparation of structures II or III. The relatively larger concentration of anions in the solution and on the positively charged (0001) plane effectively blocked contact between the growth units, Zn(OH)42-, and the (0001) crystal surface. Under these conditions, ZnO crystal growth along the six symmetric Langmuir 2010, 26(17), 14255–14262

Article

Figure 3. (a, b) SEM images of structure III synthesized from the reaction of an aqueous solution containing 0.01 M zinc acetate dihydrate and 0.2 M sodium peroxide at 90 C for 5 h. (c) XRD pattern of structure III powders. (d) TEM image of a 3D structure. (e) HR-TEM image of the area indicated by the black circle in (d). (f) SAED pattern of a building block of the 3D structure. (g) Elemental mapping of Zn. (h) Elemental mapping of O.

directions Æ1010æ was relatively enhanced, and the aspect ratio of the building blocks decreased relative to the aspect ratios of building blocks for structures II and III. Thus, the 3D structures were synthesized from cone structures with a [0001] axis. Figure 5 depicts the as-prepared structures, which are summarized in terms of the building blocks and assembled structures (structures I, II, III, and IV). We synthesized the 3D ZnO superstructures with different exposed crystal faces simply by varying the reactant concentrations and reaction temperatures. The UV-vis diffuse reflectance spectra of the as-prepared 3D ZnO nanostructures are presented in Figure 6. They show broad and strong absorptions in the ultraviolet region near the visiblelight region, which is characteristic of ZnO wide-band-gap semiconductor materials. However, the spectra were shifted depending on the dimensions and sizes of the nanoscale building blocks for the 3D structures. Structure I had 2D building blocks, the average thickness of which was ∼10 nm, and the building blocks of structure IV were smaller than those of structures II and III. DOI: 10.1021/la102126m

14259

Article

Cho et al.

Figure 4. (a, b) SEM images of structure IV synthesized from the reaction of an aqueous solution containing 0.02 M zinc acetate dihydrate and 0.03 M sodium peroxide at 90 C for 5 h. (c) XRD pattern of structure IV powders. (d) TEM image of a 3D structure. (e) HR-TEM image of the area indicated by the black circle in (d). (f) SAED pattern of a building block of the 3D structure. (g) HAADF image of the 3D structures. (h) Elemental mapping of Zn. (i) Elemental mapping of O.

Figure 5. Summary of the synthesized 3D ZnO superstructures and their building blocks.

Structures I and IV exhibited absorption bands that were blueshifted relative to the absorption bands of structures II and III. The photocatalytic activities of the as-prepared 3D ZnO superstructures were evaluated by measuring the degradation rate of 14260 DOI: 10.1021/la102126m

aqueous Orange II solutions in the presence of UV light radiation. The concentration of Orange II in solutions under exposure to UV light was monitored as a function of time. The initial concentration of Orange II was 5.0  10-5 M (Ci). Under conditions in which the Langmuir 2010, 26(17), 14255–14262

Cho et al.

Article

Figure 6. UV-vis diffuse reflectance spectra of 3D ZnO superstructures.

Lambert-Beer law is applicable, the concentration c of the absorbing component is proportional to the absorbance A as follows:41 A ¼ εcl

ð4Þ

where l is the length of the light path through the absorbing layer and ε is the molar absorption coefficient. Conduction band electrons (ecb-) and valence band holes (hvbþ) are generated on the surfaces of the ZnO crystals when the ZnO structures are illuminated by UV light with an energy greater than the band gap energy. Holes can react with water adhering to the surfaces of the ZnO nanostructures to form highly reactive hydroxyl radicals (OH•), whereas oxygen acts as an electron acceptor by forming a superoxide radical anion (O2•). These superoxide radical anions further form hydroxyl radicals, the powerful oxidation ability of which can degrade the organic dye.42 Figure 7a shows the normalized concentration (with respect to the optical absorbance measurements at 486 nm) of the Orange II solution containing 50 mg of catalyst (structures I, II, III, or IV) under UV light irradiation (initial concentration of Orange II dye (Ci): 5.0  10-5 mol/L; 100 mL). First, in the absence of catalyst, the Orange II concentration was almost constant during UV irradiation of the solution, which confirmed the photostability of the dye. A comparison was made with commercial ZnO nanopowders. In each experiment that included catalyst, the suspension was aged in the dark for 30 min to equilibrate the adsorption and desorption of dye molecules before exposure to UV light. Orange II was fully decomposed after UV light irradiation for 90 min in the presence of 50 mg of structure I. In the case of structures II, III, and IV, the dye concentrations decreased to 0.226Ci, 0.135Ci, and 0.036Ci, respectively, after UV irradiation for 120 min. The specific surface areas of the 3D ZnO structures were calculated using the BET equation.43 The specific areas of structures I, II, III, and IV were 24.7707, 5.6791, 6.2818, and 10.2588 m2/g. The surface photocatalytic dye degradation occurred in the following sequence of steps: dispersion, diffusion, adsorption on a surface, surface reaction, and desorption from the surface. The large surface area of the ZnO catalysts offered more opportunities for degradation of Orange II molecules. Thus, we conducted the photocatalytic experiments with different weights of catalysts inversely proportional to the specific surface areas. Figure 7b shows the photocatalytic activities of 3D structures with the same surface areas. The trend was similar to that shown in Figure 7a with 50 mg of catalyst:  (41) Houskova, V.; Stengl, V.; Bakardjieva, S.; Murafa, N.; Kalendova, A.; Oplustil, F. J. Phys. Chem. A 2007, 111, 4215. (42) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (43) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.

Langmuir 2010, 26(17), 14255–14262

Figure 7. (a) Normalized concentrations (with respect to the optical absorbance at 486 nm) of the 100 mL Orange II solution (initial concentration (Ci): 5.0  10-5 M) without catalyst; with 50 mg of structure I; with 50 mg of structure II; with 50 mg of structure III; with 50.0 mg of structure IV; with 50.0 mg of commercial ZnO nanopowders as a function of the UV irradiation time. (b) Normalized concentration of the 100 mL Orange II solution (initial concentration (Ci): 5.0  10-5 M) with 11.4 mg of structure I; with 50 mg of structure II; with 45.2 mg of structure III; with 27.7 mg of structure IV as a function of the UV irradiation time. In each experiment containing catalyst, the suspension was aged in the dark for 30 min to equilibrate the adsorption and desorption of dye molecules before exposure to UV light.

structure I>structure IV>structure III>structure II. The (0001) planes previously showed a higher photocatalytic activity than the {1010} planes.23 Powders of structure III had a higher areal proportion of exposed (0001) faces than did structure II, which led to a higher photocatalytic activity for structure III than for structure II. Structures II and III had six symmetric side walls enclosed by {1010} surfaces. On the other hand, structure IV was composed of conelike building blocks, of which the exposed crystal surfaces were {1011}, (0001), and {1010} faces. Because structure IV had a reduced areal proportion of {1010} surfaces with relatively low photocatalytic activity, structure IV showed better photocatalytic activity than structures II or III. Structure I showed superior photocatalytic activity over the other 3D ZnO structures (structures II, III, and IV). The average thickness (∼10 nm) of the building blocks of structure I approached the regime in which quantum size effects are prominent. Therefore, photogenerated electron/hole pair recombination was reduced, which in turn enhanced the charge-transfer rates in the materials.44 This was essential for enhancing the photocatalytic efficiency of the degradation of dye molecules. Another possibility is that the main (44) Ye, C. H.; Bando, Y.; Shen, G. Z.; Golberg, D. J. Phys. Chem. B 2006, 110, 15146.

DOI: 10.1021/la102126m

14261

Article

Cho et al.

exposed plane, {2110}, of structure I had a high photocatalytic activity. Wander and Harrison found that the surface energy of the {2110} surface was higher than that of the {1010} surface and was very close to that observed for the polar (0001) surface.45 Thus, structure I, in which almost all surfaces were {2110} surfaces, may have yielded an enhanced photocatalytic efficiency. The photocatalytic activities of the exposed crystal face controlled 3D ZnO superstructures were examined. From these experimental results, we determined that the photocatalytic activities of the 3D superstructures varied and could be optimized as a function of their exposed crystal faces, which were controlled by the method described here. The durability of photocatalytic activity was also studied by reuse of the catalysts in fresh Orange II solution under UV light irradiation. Figure S2 (in the Supporting Information) shows the photocatalytic results for five cycles using structure I, II, III, or IV (120 min irradiation for each cycle). There are no significant changes in the photocatalytic activities even after five cycles. Hence, these results imply that the as-synthesized 3D ZnO superstructures are exhibiting good stability and recyclability.

4. Conclusions In summary, we report a method for synthesizing exposed crystal face controlled 3D ZnO superstructures, summarized in Figure 6, under mild conditions (at room temperature or 90 C under 1 atm). This method permitted control over the structure of the exposed crystal faces in the 3D structures without the use of (45) Wander, A.; Harrison, N. M. Surf. Sci. 2000, 468, L851.

14262 DOI: 10.1021/la102126m

organic additives or complex experimental procedures. The exposed crystal faces of the building blocks were controlled simply by varying the reactant concentrations and the reaction temperatures. On the basis of these results, we speculated a possible mechanism for the formation of the four distinct 3D ZnO superstructures under different growth conditions. The 3D ZnO superstructures exhibited broad and strong absorptions in the ultraviolet region near the visible-light region. The spectra were shifted depending on the dimensions and sizes of the building blocks of the 3D superstructures. The photocatalytic activities of the exposed crystal face controlled 3D ZnO superstructures were examined. The photocatalytic activities of the 3D superstructures varied as a function of their exposed crystal faces, which could be controlled by the method described here. We expect that this method may be exploited for other applications, such as catalytic reactions and chemical sensors. Acknowledgment. This work was supported by grants from the second phase BK21 program of the Ministry of Education of Korea and the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (Grant 2010-0000797). Supporting Information Available: SEM images of the structures synthesized from the reaction without sodium peroxide (Figure S1); cyclic photodegradation of Orange II dye under UV light irradiation (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(17), 14255–14262