CRYSTAL GROWTH & DESIGN
Engineering of Nanotips in ZnO Submicrorods and Patterned Arrays
2009 VOL. 9, NO. 2 797–802
Mao-Song Mo,*,†,‡ Debao Wang,† Xusheng Du,‡ Jun Ma,‡ Xuefeng Qian,§ Dapeng Chen,*,| and Yitai Qian† Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, China, School of Aerospace, Mechanical and Mechatronic Engineering J07, The UniVersity of Sydney, Sydney, New South Wales 2006, Australia, School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, China, and Institute of Microelectronics, Chinese Academy of Science, Beijing 100029, China ReceiVed April 8, 2008; ReVised Manuscript ReceiVed October 20, 2008
ABSTRACT: Unusual nanotips have been selectively achieved in the synthesis of hexagonal-faceted ZnO submicrorods along with two types of its patterned arrays based on a mild precursor-hydrothermolysis reaction. In the synthesis, ethylenediamine (En) with two anchor atoms was introduced not only as a ligand and an alkali medium but also as a crystal-growth modifier for studying the crystal growth kinetics. It has been demonstrated that as-fabricated arrays are composed of ZnO submicrorods with exotic nanotips; the morphology of the nanotips, as well as the preferred spatial orientation and alignment of the rods, strongly depends on the adopted substrates. The growth mechanisms for the ZnO submicrorods and the patterned arrays have been discussed, respectively, on the basis of the growth model for the polar ZnO crystals.
1. Introduction Over the past decade, driven by the development of newgeneration solar cells and advanced micro- and/or nanodevices at low costs, researchers have been exploring new inexpensive nanostructured materials for designing photovoltaic systems that convert sunlight into electricity.1 The engineering and size-/ shape-controlled growth of high-quality nanostructures, especially the creation of oriented micro-/nanorod films with novel patterns (e.g., spatial orientation and ordered arrangement) on solid substrates, is a tremendous challenge and a key step toward the realization of advanced opto-electronic micro-/nanodevices.2 As one of the most promising candidates,2d,3 the nanostructured zinc oxide has received intensive attention because of its wide direct band gap (∼3.4 eV, 2p levels of O2- to 4s levels of Zn2+) and large exciton binding energy (∼60 meV), high mechanical strength and thermal/chemical stability, relatively low cost, as well as unique shape-/size-dependent opto-/pyro-/piezoelectric properties.4 Several strategies have been developed for the size-/ shape-controlled synthesis of the nanostructured ZnO,4-7 such as direct thermal physical evaporation (VS growth),5 catalysisassisted vapor-liquid-solid (VLS) growth,6 and solution-based chemical routes.7 Recent efforts also result in the realization of the self-assembly of nanoscale ZnO building blocks into larger, potentially applicable architectures.8 Among these strategies, the solution-based routes are very facile and promising, especially for the production of complex nanostructures that are difficult to access by VS and VLS strategies. Further, to date, almost all the previously reported solution-based and hightemperature VS/VLS syntheses of one-dimensional (1D) nanostructured ZnO crystals involve only a point-initiated monodirectional growth mode.7a-c,9 Herein, we report the finding of unusual crystal growth modes for selective engineering of thin nanotips in ZnO submicrorods and two types of its patterned * To whom correspondence should be addressed. E-mail:
[email protected] (M.-S.M.),
[email protected] (D.C.). Phone/Fax: +86-551-3607402. † University of Science and Technology of China. ‡ The University of Sydney. § Shanghai Jiao Tong University. | Chinese Academy of Science.
Figure 1. XRD pattern of ZnO submicrorods fabricated with 0.0075 mol of En.
arrays in aqueous solution, wherein bidentate ligand ethylenediamine (abbreviated as En) is employed not only as a ligand and an alkali medium but also as a crystal-growth modifier for studying growth kinetics. The novelty of the present protocol can be characterized by involving a point-initiated oppositely bidirectional growth mode for 1D ZnO micro-/nanoparticles and a repulsion field-driven aggregation-based growth mode for shape control and pattern design. The resulting nanostructured ZnO films can be envisaged as potential candidates for developing low-cost alternatives to crystalline silicon for use in photovoltaic cells.
2. Experimental Section All chemicals were purchased from Shanghai Chemical Reagent Co., Ltd., and used as received without further purification. The fabrication was carried out in a 60 mL Teflon-lined stainless steel autoclave. Asfabricated rod arrays were deposited directly onto the adopted substrates. For the growth of ZnO nanorods, typically, 0.005 mol of analyticalgrade ZnCl2 powders were dissolved in 45 mL of distilled water in a
10.1021/cg800362z CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
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Figure 2. Typical FE-SEM images of ZnO rods prepared with 0.0075 mol of En. (a, b) FE-SEM images of the as-obtained ZnO rods. (c) Highmagnification FE-SEM image of ZnO rods.
Figure 3. (a-c) Typical TEM images and SAED pattern (3c, inset) of individual ZnO rods with tapered nanorod tips. (d) HRTEM image of the individual rod shown in Figure 3c, with the growth directions of ([001], interplanar spacing d(0002) ) 2.58 Å. Teflon liner, then 0.0075-0.01 mol of analytical-grade En was introduced into ZnCl2 solution under magnetic stirring to form either a colloidal precursor [Zn(OH)2 + Zn(En)22+] suspension or a homogeneous transparent Zn(En)22+ complex solution. Afterwards, the liner was sealed in a stainless steel autoclave. The autoclave was then placed in a preheated oven and maintained at ∼180 °C for 12 h, and then allowed to cool to room temperature naturally. The resulting grayishwhite solid products were collected, washed several times with distilled water and absolute ethanol (to remove any possible residual impurities), and then vacuum-dried at 60 °C for 2 h. Patterned ZnO arrays were fabricated through the above-described experimental procedure, but a piece of clean zinc foil (Purity: 99.9%, Size: 60 ×10 × 0.127 mm) and a piece of clean thin glass slide (18 × 18 × 0.17 mm) were immersed into the precursor solution, separately in two liners. The structure and purity of the products were identified by powder X-ray diffraction (XRD), using a Philips X’Pert PRO SUPER X-ray diffractometer (λCu-KR ) 1.54187 Å, operating at 40 kV and 40 mA). The morphologies, micro-/nanostructure, and chemical composition of the products were investigated by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F, operating at 10 kV), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) (performed on a Hitachi (Tokyo, Japan) H-800 transmission electron microscope, and a JEOL-2010 high resolution transmission electron microscope, both operating at 200 kV). TEM and HRTEM samples were prepared by ultrasonically dispersing the as-obtained ZnO powders into absolute ethanol, then placing a drop of this suspension onto a copper grid coated with amorphous carbon film, followed by air-drying.
3. Results and Discussion 3.1. Growth and Structure Characterization of Thin Nanotips in ZnO Submicrorods. ZnO submicrorods with thin nanotips were obtained by the hydrothermal thermolysis of precursor suspension containing Zn(OH)2 and Zn(En)22+ with 0.0075 mol of En. Figure 1 shows a typical XRD pattern of the ZnO rods. All of the diffraction peaks in the XRD pattern can be readily indexed to the wurtzite-type ZnO (S.G.: P63mc, with a polar c-axis), with calculated cell parameters a ) 3.25 Å and
c ) 5.21 Å, consistent with the standard literature values for bulk ZnO (JCPDS Card file No. 36-1451). The sharp and strong characteristic peaks suggest that the samples are highly crystalline. The abnormally weak (002) diffraction peak, compared with the expected one, suggests that the ZnO rods might lie on the surface of the silicon wafer and have a [001]-elongated orientation, as further revealed by HRTEM and electron diffraction (ED) studies below. Energy dispersive X-ray spectroscopy (EDS, SI) microanalysis shows that only Zn and O elements can be detected within the experimental error, shedding light on the removal of adsorbing organic species in the resulting products after crystallization and washing-up. The XRD pattern and EDS microanalysis indicate that well-crystallized ZnO rods have been obtained through the present solution-phase synthetic route. The morphologies of the products were examined by FESEM. Low-magnification FE-SEM views clearly reveal that the crystallized ZnO consists of a large quantity of uniform submicrorods with tapered thin nanotips, and, in some regions, displays flower-like assemblies (Figures 2a and 2b). Highmagnification FE-SEM examinations indicate that these individual rods display a smooth hexagonal-faceted prismatic morphology with thin nanotips (Figure 2c). An enlarged top view of the thin tips, as shown in the inset of Figure 2c, reveals their hexagonal pyramidal morphology. Close analysis of these images indicates that the thin tips are in the range of 30-50 nm in diameter, and the rod stems are ∼200 nm in diameter and 2.5-3 µm in length, which affords a typical aspect ratio of about 14. Local microstructural features of the resulting ZnO submicrorods were further investigated by TEM. Figures 3a and 3c show TEM images of typical individual ZnO rods. It can be seen that these rods display an unusual four-section structure: one section of uniform long stem, two sections of short tapered
Nanotips in ZnO Submicrorods and Patterned Arrays
cones, and a thin nanorod tip. Two opposite tapered ends present in each rod suggest two opposite growth fronts of the rod. Interestingly, a clear characteristic boundary between the intermediate tapered cone and the thin nanorod tip has been observed (Figure 3a, inset). Such an interface implies a possible two-step temperature- or pressure-controlled process for the “confined” growth of the thin nanorod tip during the crystallization of the rods. Occasionally, some porous thin tips, as shown in Figure 3b, are also observed in some rods. The formation mechanism of porous tips is unclear for the time being and possibly assigned to oriented aggregation of nanoparticles.8b These special features are likely caused by an abrupt variation of the local growth conditions around the growing fronts of the rods as discussed below. A typical selected-area electron diffraction (SAED) pattern (Figure 3c, inset) taken from the individual rod in Figure 3c, exhibits sharp diffraction spots, and can be indexed to the [12j10]-zone-axis diffraction pattern of the wurtzite-type single-crystal ZnO, which is compatible with the above analytical result from XRD. In addition, the SAED patterns taken from different sections along the entire length of the rod are found to be identical within experimental accuracy. This indicates that both the thick long stem and the thin nanorod tip are iso-oriented along the positive and negative [001] directions. A typical HRTEM image (Figure 3d) recorded at the thin tip of the same individual rod in Figure 3c reveals a structural perfection, and unambiguously resolves the (0002) atomic planes of the wurtzite-type ZnO with an interplanar spacing of about 2.58 Å, confirming the single-crystalline nature of the thin nanorod tips and the preferential [001] growth orientation. 3.2. Formation Mechanism of Shaping ZnO Rods with Thin Nanotips. The nucleation and growth of a crystal is a very sensitive and complex process, depending not only on the intrinsic structure of the crystal but also on its surface defect structure and external factors (supersaturation, structure of the mother phase, temperature or pressure, and impurities). In particular, impurities like solvents and additives can play a crucial effect on the growth rates of different crystal faces and thus provide a useful tool for obtaining crystals with unusual shapes. The wurtzite-type ZnO crystal consists of alternately [001]stacked planes of tetrahedrally coordinated Zn2+ and O2- ions. O2- ions are arranged in a hexagonal close-packed (hcp) structure, and each Zn2+ is surrounded tetrahedrally by four oxygen ions, and vice versa. This implies that a permanent dipole moment exists in an ideal crystal along the c-axis of the crystal. This inherent asymmetry along the c-axis results in the natural anisotropic growth tendency in ZnO crystallites. Having taken into account the above experimental observations and the constraints of the crystal polarity, a two-stage growth process and a difference between the growth rates of different crystal facets are proposed to be responsible for the formation of the four-section, hexagonal-structured ZnO rods under the bidentate-ligand En medium in the mother solution. The first stage involved the growth of the hexagonal end-tapered rod with the rod stem bounded with six crystallographically equivalent {101j0} rectangular faces and two oppositely tapered pyramids bounded with six {101j1} and six {101j1j} trapezoidal faces along the ([001] directions. Specifically, in the present hydrothermal systems, the main chemical reactions involved can be formulated as follows:
Crystal Growth & Design, Vol. 9, No. 2, 2009 799 ∆
[Zn(En)2]2+(aq) + 2OH- 98 ZnO V +2En + H2O
(1)
∆
Zn(OH)2 98 ZnO V +H2O
(2)
ZnO + H2O T Zn(OH)2
(3)
-
Zn(OH)2+2OH T [Zn(OH)4] (aq) 2-
Zn(En)22+
(4)
Once the thermolysis of Zn(OH)2 and into ZnO clusters or nuclei takes place, two types of charged precursor units, [Zn(En)2]2+(aq) and [Zn(OH)4]2-(aq), will be responsible for the subsequent crystal growth. On the one hand, the negativecharged [Zn(OH)4]2-(aq) complex ions preferentially adsorb onto the positive-charged (0001)-Zn faces, and subsequently dehydrate and enter into the crystal lattices. This is similar to the previously reported point-initiated monodirectional growth mode for ZnO crystalline rods under hydrothermal conditions.7c On the other hand, the positive-charged Zn(En)22+(aq) complex ions, the primary charged precursor unit in present cases, preferentially attach on the opposite (0001j)-O faces and enter into the lattices by a thermolysis reaction [eq (1)]. As a result, the two types of charged precursor growth units afford the much faster growth rates of the opposite-charged ((0001) top faces than those of the different crystalline arrangement of atoms on the sides, resulting in the crystallite particles to elongate into a rod. According to the hypothetical sequence: V(0001j) > V(0001) > V(101j0) > V(101j1) > V(101j1j), one can explain the disappearance of the (0001) and (0001j) faces from the competitively growing morphology, as well as the consequent acicular habit of the rods. As we have known, the faster a crystallographical face grows, the faster it disappears. Accordingly, in our case, as crystal growth proceeds, the fast-growing ((0001) faces tend to disappear leaving the relatively slow-growing other faces behind, and instead the six crystallographically equivalent {101j1} faces and six {101j1j} faces emerge on the tips and bound the two opposite growth fronts of each rod along the ([001] directions, respectively. This growth mode results in the formation of hexagonal tapered cone ends in each rod. The growth rates of the {101j0} faces are relatively slow, and therefore, they remain to bound the hexagonal prism surfaces. Such a opposite bidirectional kinetic growth mode for 1D ZnO crystals is quite different from the previously observed point-initiated monodirectional growth of 1D ZnO micro-/nanocrystals.7a-c,9 The growth of thin nanorod tips took place at the second growth stage, most likely during the period of cooling the reacting system to room temperature. When the heating of the reacting system was stopped, the delicate growth equilibrium of the rod was upset momentarily and shifted toward the side of Zn(En)22+ all at once. This sudden switch of hydrothermal conditions evoked an instantaneous “shortage” of Zn-O growth units, and instead a secondary nucleation process accompanied by oriented aggregation of tiny nanoparticles might occur at energetically favorable sites on the surfaces of the rods. It has been reported that the top growing face of the rods is the preferential site for nucleation and growth.9 Thus, the (0001j) faces of the wellfaceted short pyramids left behind at the first stage might serve as energy-favorable special sites for further adsorbing of metatable Zn(En)22+(aq) complex ions and budding of ZnO nuclei accompanied by oriented attachment of tiny nanoparticles from the solution under the present conditions since the polar, oppositely charged ((0001) surfaces are electrically unstable and need to be reconstructed to neutralize both the net charge and the dipole repulsive interactions. As a result, a secondary
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Scheme 1. Illustration of the Two-Stage Growth Process of ZnO Rods with Thin Nanorod Tipsa
(a) Structural model of ZnO crystals, showing the opposite ((0001) polar faces for adsorbing [Zn(OH)4]2-(aq) and [Zn(En)2]2+(aq), respectively. (b) Growth of ZnO crystal into a prism crystal with opposite tapered ends via a point-initiated bidirectional growth mode (Stage 1). (c) Growth units diffusing and migrating onto and nucleating at the (0001j) top faces of ZnO pyramids, accompanied by oriented attachment of tiny nanoparticles from the solution, and then growing into a hexagonal-faceted thin nanorod tip (Stage 2). a
growth of a hexagonal-faceted thin nanorod tip took place at the (0001j) end of the ZnO rods, as driven by the natural polar growth behavior in ZnO crystal. The whole formation process is shown schematically in Scheme 1. These shaping ZnO rods with special thin nanorod tips can be envisaged as efficient field emitters to improve the emission efficiency of the host material since electrons are more easily emitted from the sharp tips of rods than from 1D rods/wires/tubes with uniform diameter.10 In the present reacting systems, En plays a crucial effect on the crystal morphology and microstructure of the final ZnO products. It functions not only as a transporter of Zn2+ ions but also as a crystal-growth modifier. It is noted that the ((0001) top/end faces with either all zinc or all oxygen atoms are highly charged but the other side faces of ZnO rods with alternating zinc and oxygen atoms are relatively nonpolar. This remarkable difference appears to favor the adsorbing and binding of neutral electron pair-donating bidentate-ligand En molecules onto the relatively nonpolar faces on the sides of a budding ZnO nucleus via the “bridging” complexing with exposed Zn2+, leaving the polar ((0001) faces relatively exposed to add new atoms from the solution. As a result, the lateral overgrowth of the rods with respect to the longitudinal growth is effectively impeded but charged species such as Zn(OH)42- and Zn(En)22+ complex ions selectively adsorb onto the exposed polar ((0001) faces, respectively. This presumption is substantiated by the results of our control experiments: the size of the lateral dimension and aspect ratio of the as-obtained ZnO rods strongly depend on the concentration of starting En. In the synthesis, upon slightly increasing the dosage of En from 0.0075 mol to 0.01 mol, with other conditions remaining fixed, the average size of the lateral dimension of the final ZnO rods could be dramatically modulated from ∼200 nm to ∼80 nm with the length of the particles being affected weakly. As shown in Figure 4a, a typical FE-SEM image of ZnO sample prepared with 0.01 mol of En clearly displays a much slenderer nanorod morphology with diameter of about 80 nm and length of up to 3 µm, which yields high aspect ratios in the range of 30-40. Additionally, when using monodentate-ligand aqueous ammonia (NH3 · H2O, 25 wt %) instead of En, with the other conditions held the same, only larger microrods and no nanorods were observed (Figure 4b). These results indicate that En may selectively adsorb onto the six neutral {011j0} side faces, and effectively decrease their growth rates.
Figure 4. (a) FE-SEM image of the thin ZnO nanorods obtained with 0.01 mol of En. (b) TEM image of ZnO microrods obtained with 1 mL of aqueous ammonia (25-28%, wt %).
Figure 5. XRD patterns of ZnO rod array films deposited (a) on zinc foil and (b) glass slide by the hydrothermal thermolysis of Zn(En)22+ with 0.01 mol of En.
3.3. Patterned Growth and Mechanisms of Ordered, Tip-Nanotructured ZnO Rod Arrays. Similarly, two types of patterning, tip-nanostructured ZnO rod array films can be selectively fabricated on crystalline zinc foil and amorphous glass slide, respectively, by the hydrothermal thermolysis of the precursor Zn(En)22+ with 0.01 mol of En. Powder XRD measurements (Figures 5a and 5b) and EDS microanalysis (not shown) indicate that rod array samples on two type of substrates are both the hexagonal wurtzite-type ZnO. Figure 6 shows representative FE-SEM images of two types of ZnO rod array films. From Figures 6a and 6b, it can be seen that the as-achieved ZnO products on zinc foil consist of many uniform, densely packed submicrorods with tapered tips, forming an ordered lawn-like array. Top-view and side-view of the samples reveal that the lawn-like arrays are built of a single layer of nearly vertically aligned rods ranging from 2-4 µm in length and 80-300 nm in diameter. In contrast, ZnO products on glass slide, as shown in Figure 6c, are composed of a large number of uniform, snowflake-like hyperbranch architectures ranging from 10-15 µm in size. High-magnification FE-SEM examinations reveal that the entire structure of these snowflakelike hyperbranch architectures is built of many radially oriented cones originating from the same root or core and linking to separate neighbors, with a thin tip and a thick root in each cone (Figures 6d and 6e). The diameters of the cone-shaped stem range from about ∼700 nm at the root to ∼100 nm at the top. It is interesting to note that all the constituent cones have rough surfaces with an unusual, thin circular nanocorona on the thin top faces in each cone and many nanoscale protuberances
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Figure 6. Representative FE-SEM images of two types of ZnO rod array films. (a, b) Nearly vertically aligned rod arrays on zinc foil. (c, f) 3D radially oriented snowflake-like cone arrays on glass slide.
sticking out from the side surface of each cone (see Figures 6e and 6f). The thin circular coronas are about ∼200 nm in diameter and several nanometers in thickness. The protuberances are in the range of several nanometers to several tens of nanometers. The rough surfaces of the cones with many nanoprotuberances are strongly indicative of a nonclassical aggregation-based crystallization mechanism.8a,b,11 The exact mechanism for the formation of the thin circular corona is unknown at this moment but might be caused by the abrupt transition of the growth mode from dipole-induced layer-bylayer 1D particle stack to side-by-side 2D ledge-flow9d particle attachment. That is, with a deep decrease in the hydrothermal temperature, Zn(En)22+(aq) will adsorb and partially thermolyse into Zn(En)X2+ (0 < X < 2) on the top (0001j)-O face, which will effectively block this face from further crystal growth/ particle stack together with Na+ from the glass substrate and consequently lead to relatively favorable side-by-side attachment of nanoclusters (presumably nanoplatelets) to minimize their Gibbs free energy at the last crystallization stage during the cooling of the reaction system. This side-by-side aggregationbased growth mode involves the in-plane epitaxial attachment of tiny ZnO nanoslabs onto the high-surface-energy crystalgrowing fronts of the cones that “fused” gradually into a “big” corona. Such thin coronas and nanoscale protuberances expose more facets and produce new surface structures with high surface-to-volume ratios, which unambiguously leads to an increase in the number of dangling bonds on the surfaces of the cones and therefore, induces the relaxation or reconstruction of the surfaces. These distinctive structural/morphological features originating from the aggregation-based crystal growth for multipod-type ZnO arrays quite differ from those of previously reported smooth multipod-type nanostructures of ZnO and chalcogenides.12 The formation of distinctive tip-nanostructured rod arrays on different substrates may be attributed to the various interfacial interactions between the newly formed ZnO clusters and the substrates. The metal Zn crystal possesses a hexagonal wurtzite structure with a strong chemical and structural affinity for the hexagonal ZnO crystal lattice. Statistically, the Zn foil harbors a great number of surface crystal grains with the ((0001) planes parallel or approximatively parallel to the foil surface. Hence,
with the metal Zn as the substrate, solution-derived ZnO rod array films with the c-axis orientation normal to the substrate surface are highly favored besides the fact that the nucleation and crystal growth are greatly promoted by colliding and trapping processes on such a “foreign” seeding hcp-structured substrate. With the glass as the substrate, however, no specific crystal faces within the noncrystalline glass can be available for the nucleation and subsequent oriented growth of ZnO, and instead there exists a difference in surface stress between ZnO and the glass. This may generate a strongly elastic interfacial interaction13 and hence a big repulsion field between the newly formed ZnO clusters and the glass substrate under present hydrothermal conditions. Such an elastic repulsion field affords a big thermodynamic driving force to impel the attached ZnO growth units to move fast randomly on the glass surface and cluster into fast-moving nuclei. The nuclei then spontaneously aggregate into “large” spherical particles to minimize the interfacial free energy when they meet and adhere to each other in the motion. The subsequent coarsening crystal growth most likely originates from polar faces of the budding ZnO nanocluster/nucleus on the side surfaces of the spherical aggregates, and the growth fronts rapidly radiate outwards and evolve into a large complex 3D “snowflake” driven by the dipole-dipole interaction between ZnO nanoclusters, as well as the physicogeometrical limit.8b The observed microscale thick stems and their rough surfaces are, to some extent, strong indications for the fast nanoclustering of ZnO growth units and subsequent aggregation-based coarsening crystal growth. We note that a similar immiscibility-driven cluster aggregation and subsequent radial-growth mechanism has been proposed for VS synthesis of hyperbranched ZnO nanowire arrays without corona tips on amorphous carbon.14 The present experimental results reveal that both the morphology and the microstructure of the final ZnO products are sensitive to the adopted substrate. The reason for this and the growth mechanism still need additional investigations.
4. Conclusions In summary, we have demonstrated a solution-phase crystalgrowth kinetics and thermodynamics regime for the design and
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selective fabrication of distinctive nanotips in ZnO submicrorods and two types of its patterned arrays by introducing bidentateligand En as the alkali source and the kinetic crystal-growth modifier. The lateral dimension control of the as-fabricated ZnO rods can be achieved by slightly altering the concentration of starting En in solution. Different substrates can lead to various repulsive fields between ZnO and the adopted substrates, and hence result in distinctive patterned rod arrays. The unusual hyperbranched particles with nanocoronas extend the available 3D shapes in nanostructured particle synthesis and provide a paradigm to study the growth kinetics by carefully observing and modeling the morphology of particles. The achieved ZnO rods and patterned rod arrays with special nanotips can be useful in various fields of nanoscale science and technology; for example, wiring up neighboring electrical devices in future micro-/nanoelectronics and helping solar cells efficiently channel current from one electrode to another in intricate multichannel optoelectronic devices. The facile solution strategy presented here and in our early work8a,b may afford a promising route for the patterned growth of novel shaping particles. Further investigations may lead to an extension of this crystal-growth kinetics and thermodynamics regime to the fabrication of the shaping nanostructures of other materials. Acknowledgment. This work was partially supported by a start-up grant (Project No. U2141) from the University of Sydney. Supporting Information Available: Energy dispersive X-ray spectrum (EDS) of the resulting ZnO rods with thin nanotips. This material is available free of charge via the Internet at http://pubs.acs.org.
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