Fabrication of Hierarchical ZnO Nanostructures via a Surfactant

Apr 29, 2009 - ARC Centre of Excellence for Electromaterials Science, Monash University, Wellington Road, Clayton 3800, Australia, Department of Chemi...
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CRYSTAL GROWTH & DESIGN

Fabrication of Hierarchical ZnO Nanostructures via a Surfactant-Directed Process

2009 VOL. 9, NO. 6 2906–2910

Xiao Li Zhang,† Ru Qiao,‡ Ri Qiu,§ Ju Chang Kim,§ and Young Soo Kang*,| ARC Centre of Excellence for Electromaterials Science, Monash UniVersity, Wellington Road, Clayton 3800, Australia, Department of Chemistry, Zhejiang Normal UniVersity, Jinhua 321004, China, Department of Chemistry, Pukyong National UniVersity, 599-1 Daeyeon-3-dong, Namgu, Busan 608-737, Korea, and Department of Chemistry, Sogang UniVersity, #1 Shinsu-dong, Mapo-gu, Seoul 121-742, Korea ReceiVed February 23, 2009; ReVised Manuscript ReceiVed March 29, 2009

ABSTRACT: In this work, a facile surfactant-directed process is developed for controlling the morphology of ZnO with distinctive shapes ranging from mushrooms, bihemispheres, dumbbells, bilayer hexagonal disks and prisms to flower-like nanosheet aggregations. The flower-like aggregated nanosheets were formed by oriented attached 4 nm nanoparticles. The hexagonal disks and prisms 1-2 µm in diameter and 50 nm to approximately 4 µm in thickness (or length) were enclosed by {101j0} facets. The growth mechanism of these bilayer hexagonal structures was studied by microscopy and spectroscopy. In addition, by replacing the conventional oil-water system, the simple alcohol-water surfactant system employed here provides a more environmentally benign and biosafe approach. Introduction The optical, electronic, and catalytic responses of a material depend on its particular size, shape, composition, and local dielectric environment.1 In modern materials science, controlling the size and shape of materials is one of the most important techniques for controlling their chemical and physical properties.2 Zinc oxide, a direct band gap semiconductor with a large bandgap of 3.37 eV and a large exciton binding energy of 60 meV, is a candidate for applications such as room-temperature UV lasers,3 light-emitting diodes,4 solar cells,5 photoelectronics,6 and sensors.7 As a polar crystal, ZnO can be described as a number of positively charged (0001) planes rich in Zn2+ ions, alternating with negatively charged (0001j) planes rich in O2ions, stacked along the c axis. The positive (0001) surfaces and the negative (0001j) surfaces can therefore have different selfcatalysis properties.8 However, spontaneous polarization along the c axis leads to the common formation of one-dimensional structures due to electrostatic interaction, and the large polar surface is generally energetically unfavorable.9 Among the synthetic methods developed, solid vapor phase (SVP) approaches have been used for fabricating a diversity of nano- or microscale nanostructures at high temperature. Although SVP processes are simple and produce high-quality products,10 they also suffer from low yields, a major limitation. As an alternative, solution phase synthetic approaches using thermal treatment of the reactants in different solvents may be the simplest and most effective way to prepare sufficiently crystallized materials at relatively low temperatures. As an additional benefit, solution-based methods also allow considerable influence of reaction species on the final size and morphology of the as-synthesized samples on a large scale.11 Recent reports have demonstrated that the large polar surface of ZnO can be generated when the surface charge is compensated by using polymers and surfactants as passivation agents.12 * To whom correspondence should be addressed. E-mail: yskang@ sogang.ac.kr. † Monash University. ‡ Zhejiang Normal University. § Pukyong National University. | Sogang University.

Micelles formed by surfactants can have various shapes, such as spheres, rods, ellipsoids, and disks, as well as much more complex structures, created by adjusting experimental parameters. Therefore, size- and shape-controlled nano- or microstructures can be fabricated through solution phase synthesis.13 In this work, we demonstrate facile, rational surfactant-directed morphology control on hybrid ZnO materials in an environmentally benign alcohol-water reaction medium. ZnO materials with diverse shapes ranging from mushroom-like, double hemispheres, dumbbell, to hexagonal bilayer disks and flowerlike aggregation of sheets can be easily prepared on a large scale. Experimental Section Materials. Zinc nitrate hexahydrate [Zn(NO3)2 · 6H2O, analytic reagent, 95%, Junsei], sodium dodecylbenzenesulfonate [CH3(CH2)11C6H4SO3Na, SDBS, technical grade, Aldrich], sodium hydroxide (NaOH, analytic reagent, >93%, Junsei), and ethanol (CH3CH2OH, >99.9%, Hayman) were used as received without further purification. Synthesis. In a typical synthesis, specific amounts of zinc nitrate, SDBS, and sodium hydroxide were added into a 45 mL ethanol-water solution with vigorous stirring over 2 h. Then the reaction mixture was transferred into a Teflon-lined autoclave and kept at a constant temperature for several hours. The precipitates were collected and washed with deionized water and then dried under vacuum. Characterization. The crystal structure was determined with powder X-ray diffraction (XRD, Philips X’Pert-MPD system, Cu KR radiation, λ ) 1.54056 Å) at a scanning rate of 0.02° per second for 2θ in the range from 10° to 80°. The morphology and composition of the product materials were evaluated using transmission electron microscopy (TEM, HITACHI, H-7500), high-resolution TEM and selected area electron diffraction (HR-TEM and SAED, JEOL, JEM-2010), field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy (FE-SEM/EDS, JEOL, JSM-6700FSEM).

Results and Discussion The ZnO hexagonal bilayer disks were synthesized by a solution phase approach using a two-phase system consisting of water and ethanol. This approach differs from the conventional water/oil systems due to its facile production and avoidance of environmental pollution.12,14 Figure 1a shows a typical FE-SEM image of ZnO samples synthesized in an ethanol solution containing 9.6 mmol of

10.1021/cg900226h CCC: $40.75  2009 American Chemical Society Published on Web 04/29/2009

Fabrication of Hierarchical ZnO Nanostructures

Figure 1. (a) FE-SEM image of as-synthesized ZnO hexagonal disks. (b, c) Enlarged FE-SEM image and TEM image display detailed bilayer structures of hexagonal disks. (d) HR-TEM image and SAED pattern (inset) of ZnO hexagonal disks. (e) A corresponding XRD pattern of the same sample.

SDBS, 2.4 mmol of Zn(NO3)2, 3.6 of mmol NaOH, and 3 mL of deionized water with a reaction temperature of 90 °C for the first 4 h and 180 °C for another 4 h. The as-synthesized product is uniform, dominated by hexagonal bilayer disks with uniform size and well-defined shape. The disks are 1-1.5 µm in diameter and 50-200 nm thick. Enlarged FE-SEM and TEM images show single disks with a bilayer structure in Figure 1b,c. The well-resolved smooth edges and flat hexagonal plane indicate very good crystalline quality of the disk. The contrast in the TEM image Figure 1c reveals a second layer with a smaller diameter behind the first hexagonal disk. Furthermore, highresolution TEM images and the corresponding SAED pattern Figure 1d were obtained with the electron beam perpendicular to the hexagonal facet. The observed lattice spacing of 0.28 nm indexes to the lattice fringe of d100 of wurtzite zinc oxide. The diffraction spots of the SAED pattern can be indexed to the diffraction spots of the [0001j] zone of the wurtzite structure, indicating that the top and bottom surfaces of the bilayer disks are the ( (0001) plane and the side surfaces parallel to the c-axis

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are the ( {011j0} planes. In Figure 1e, the corresponding XRD patterns match well with a hexagonal wurtzite structure (P63mc, a ) 3.2501 Å, c ) 5.2071 Å). The ethanol solution reaction system was studied by monitoring the influences of surfactant amount, water volume, NaOH amount, and reaction temperature on the morphology evolution of products as recorded by microscopy. Without water addition and with a constant amount of zinc nitrate, the morphology of products changes slightly from hazelnut-like to mushroom-like structures as more SDBS is used. With a SDBS to Zn2+ ratio of 1.0, the products have a hazelnut-like morphology shown in Figure S1a, Supporting Information. The clearer structure of the hazelnut-like particle is revealed by the magnified inset FESEM image. There is a joint boundary between the two hemiellipsoidal parts. It is found that the particles have a longer hemipart with a smooth particle surface and a short rough hemipart. The asymmetry of the particles implies that the nucleation and growth of each half of the particles are not identical. Increasing the SDBS to Zn2+ ratio to 2.0, in Figure S1b, Supporting Information, mushroom-like particles were obtained. Further increase of SDBS causes a decrease in the size of the bottom part of product as shown in Figure S1c,d, Supporting Information. Interestingly, with a SDBS to Zn2+ ratio of 4.0, the products (Figure S1e,f, Supporting Information) present a regular hemispherical portion and a hexagonal step (or a hexagonal step with hexagonal concavity around it). The corresponding XRD of the products is shown in Figure S1g, Supporting Information. All peaks can be assigned to those of wurzite zinc oxide. Compared with the XRD pattern in Figure 1, the broader peaks here also suggest that these mushroomlike structures consist of aggregated small particles. By keeping a constant SDBS concentration and decreasing the Zn2+ concentration, the mushroom-like morphology of the products can be maintained. However, with an SDBS to Zn2+ ratio of 40, only irregular shaped structures are produced, as shown in Figure S1h, Supporting Information. The products in Figure S1d, Supporting Information have the most uniform morphology among the tested conditions; therefore, we carried out the subsequent experiments using those conditions 9.6 mmol of SDBS and 2.4 mmol of Zn2+. Keeping the SDBS and Zn2+ concentrations constant, the influence of different amounts of water on the products morphology was also studied. The anionic surfactant can be made to form micelles with various shapes by adjusting the experimental parameters. The self-assembled DBS2- layers at the interface of water and ethanol likely act as a template for growing ZnO. The hydrophilic heads of DBS2- form an anionic surface when exposed to water: thus Zn2+ cations can directly attach to the negatively charged DBS2- template to initiate the crystal growth. With water introduced into the reaction system, diverse morphological products can be produced by simply adjusting the water volume, as shown in Figure 2. With 1.0 mL of water as shown in Figure 2a, spherical products containing two hemispheres are obtained. With 2.0 mL of water, dumbbell-like structures are produced (Figure 2b). Increasing the water volume to 4.0 mL (Figure 2c), the products have uniform and regular shapes with two hexagonal prisms. The corresponding SAED rings in the inset of Figure 2c indicate that the hexagonal prisms are multicrystalline structures. Further increasing the water volume leads to polydisperse, nonuniformly shaped samples. In Figure 2d, for example, some particles are multilayered while others remain hexagonal prisms bilayers. When the water volume is increased to 24 mL (Figure 2e), flower-like nanosheet aggregations are obtained. The inset shows

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Figure 2. Microscopy studies of the water effect on product morphology. FE-SEM images with (a) 1 mL, (b) 2 mL, (c) 4 mL, (d) 8 mL, (e) 24 mL of water added. Inset in (c) is the corresponding ED pattern of the product. Inset in (e) is high resolution SEM image of products. (f) TEM image of a piece of sheet from sample in (e). Insets in (f): top, ED pattern; bottom, high resolution TEM image from the sheet.

a high-resolution FE-SEM image taken from a single flower, clearly showing the aggregation structure of the flower. Highresolution TEM observation was also used to study the structural details of the flower-like sheets; an example is shown in Figure 2f. The sample was prepared by sonicating the products in ethanol for several minutes before dripping them on the grid. A single sheet is found to be 0.75 µm in width and 10 µm in length. The insets of Figure 2f show the corresponding SAED spots (top) and a high magnification TEM image (bottom) suggests that the sheet is formed by oriented attachment of approximately 4 nm sized particles. The aggregative particulate growth involves self-alignment of nanocrystallites along their common crystallographic orientations. Given that their external surfaces are not smooth and no crystal facets are observed in these sheets, it is believed that the formation of these nanosheets is based on the aggregative growth mechanism. FT-IR analyses were carried out to complement the microscopy studies. Figure 3 shows FT-IR spectra of pure SDBS (e) and of the products obtained from the reactions with different amounts of water added. Compared to the pure SDBS, all of the products show a new absorption peak at approximately 450 cm-1 assigned to ZnO and confirming its formation. A sharp peak at 3530 cm-1 in Figure 3c,d is characteristic of a nonassociating H-O stretching vibration, suggesting the presence of free hydroxyls.15 In spectrum 4(b), a weak absorption peak can also be observed at the same position. The absorptions centered at 2900 cm-1 are assigned to C-H stretching vibrations. Absorptions attributed to S-O vibrations in SDBS (ca. 1189 cm-1) can also be detected in the products. Finally, a shift of the absorption from 1130 cm-1 to 1120 cm-1 was found. On the basis of the above IR results, we proposed that Zn2+ cations interact strongly with the sulfonate radicals of SDBS with increased water addition. Therefore, we propose the following explanation for the formation of the bilayer hexagonal disks shown in Figure 1. As far as we know, the anionic surfactant usually forms selfassembled double layers and multilayers in lamellar micelles. The Zn2+ (0001) surface bonds strongly to DBS2- due to electrostatic interaction, and the densely packed DBS2- will protect the surface from further “etching” or reaction, resulting

Figure 3. FT-IR spectra of (a) pure SDBS and products synthesized with (b) 0 mL, (c) 4 mL, (d) 12 mL, and (e) 24 mL of water.

in the formation of the flat side of the disks. The DBS2- template stabilizes the surface charge and the structure, allowing the fast growth along , leading to the formation of hexagonal disks enclosed by {101j0} facets. The growth of ZnO disks can take place simultaneously on both sides of the template and thus result in back-to-back growth of bilayer hexagonal disks. In Figure 4, the influence of surfactant amount on the bilayer structures was studied using FE-SEM images. The ethanol-water solution contains 3 mL of water, 2.4 mmol of zinc nitrate and 3.6 mmol of NaOH. The surfactant SDBS is increased incrementally from 1.2 to 19.2 mmol. As the SDBS increases, the diameter of the products is approximately constant at 1.7 µm, while the thickness decreases from ca. 650 to 100 nm. At the

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Figure 4. FE-SEM images of ZnO samples synthesized with different amounts of SDBS: (a) and (b) 1.2 mmol, (c, d) 2.4 mmol, (e, f) 4.8 mmol, (g, h) 19.2 mmol.

Figure 5. FE-SEM images of ZnO samples synthesized with different reaction parameters. 1.2 mmol of SDBS with 2.4 mmol of NaOH at 140 °C 4 h: (a) increasing NaOH to 3.0 mmol and (b) prolonging the reaction time to 10 h.

Figure 6. By prolonging the reaction time to 8 h, bidisks in Figure 4a can change to biprisms.

highest SDBS concentration of 19.2 mmol, the prepared products show irregular rough edges.

Figure 7. Illustration of the formation process of biprisms.

Thus, the following explanation for the observed effects of SDBS on disk morphology was raised. Free surface sites are available for growth once Zn2+ ions have reached the surface. Crystal growth is probably not limited by incorporation of Zn2+ to the crystal but rather by transport to the crystal surface through the thick hydrophobic alkyl chain of SDBS. A high concentration of SDBS generates thicker alkyl chains on the crystal surface but shorter alkyl chain blocks, which shield the growth sites on the crystal surface against the solution. Thus, the high concentration of SDBS slows down crystal growth along the c-axis. And also, the short solution block reduces the number of undisturbed growth sites available and forces the formation of crystals with various morphologies as observed in Figure 4c,e,g. Contrarily, a lower concentration of SDBS leads to a faster Zn2+ delivery to the growth sites and generates faster crystal growth as shown in Figure 4a,b. The morphologies of the products in Figure 4 can also be obtained at a lower reaction temperature (140 °C) by (a) increasing the NaOH concentration or (b) prolonging the reaction time (as shown in Figure 5). A small amount of NaOH leads to the formation of wafers and large plates (Figure 5 top): no bilayer hexagonal disks were seen in this case. By increasing the NaOH amount, bilayer hexagonal disks and wafers coexist. The additional NaOH and high reaction temperature can accelerate the hydrolyzing of Zn2+ and produce more nuclei simultaneously at the beginning of the reaction which leads to the faster growth. Prolonging the reaction time, the products also transform into bilayer hexagonal disks. With a prolonged reaction time, and the aspect ratio (defined as plate thickness vs plate diameter) of the sample in Figure 4a can gradually increase from 0.34 to 2.4. The long biprisms can be found in Figure 6. Without any surfactant to counterbalance the charge, crystal growth preferentially occurs on the (0001j) facet, thus prolonging the reaction time and leading to the products with a high aspect ratio. EDS analyses were performed around the hexagonal surfaces and the interfaces of individual biprism structures. As expected, the interface region is bound

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by the (0001j) plane rich in O2- anions (atomic ratio of Zn/O ) 0.95), whereas the hexagonal surfaces are bound by the (0001) plane rich in Zn2+ cations (atomic ratio of Zn/O ) 0.99). The same phenomenon can be observed in the experiment without using NaOH, as shown in Figure S2, Supporting Information. By prolonging the reaction time to 8 h, the aspect ratio increases from 0.5 to 1.0. And the atomic ratio of Zn/O at the interface region is 0.85, while at the hexagonal surfaces it is 0.86. The above results and discussions illustrate the morphology evolution as a function of the reaction time under the same starting conditions which are illustrated in Figure 7. At the early stage of reaction, the supersaturated nuclei will first form floccules and wafers with the direction of surfactant. Because of the stabilized surface charge and the structure by the surfactant, the hexagonal disks will form with the preferable growth along . And densely packed surfactant on Zn2+ (0001) surface results in flat (0001) surfaces. Following an increases in ZnO22- ions, the competing absorptions of ZnO22ions and DBS2- will lead to growth on both length and diameter of the disks. The small amount of surfactant on O2- (0001j) surface will also absorb ZnO22- with a very slow rate and produce a bilayer on the (0001j) side. Once the bilayer forms, the new (0001) side will grow fast, and thus the biprisms will be obtained.

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A facial surfactant-directed solvothermal synthesis approach has been demonstrated for fabricating ZnO hybrid materials on a large scale with diverse morphologies in an aqueous ethanol solution. By adjusting the reaction parameters, such as reactant concentration, water amounts, and reaction temperature, the morphology of the products can be easily controlled resulting in structures that are mushroom-like, double hemispheres, dumbbells, bilayer hexagonal prisms, or flower-like nanosheet aggregations and bilayer hexagonal disks. Because of the charge compensation of the anionic DBS2- template at the Zn2+ (0001) surface of ZnO, the fast growth along forms hexagonal disks enclosed by {101j0} facets. Because of the polar structure of ZnO crystals, bilayer morphologies are the most common structures. In addition, the synthetic flexibility of the present surfactant-directed ethanol solution approach creates an environmentally benign route for morphology controlled synthesis of inorganic hybrid materials on a large scale.

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Acknowledgment. This work is financially supported by the Brain Korea 21 program and the nano R&D program (Korea Science & Engineering Foundation, Grant 2007-02628). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

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