Controlled Synthesis and Assembly of Nanostructured ZnO Architectures by a Solvothermal Soft Chemistry Process Liming Shen,† Ningzhong Bao,*,† Kazumichi Yanagisawa,‡ Arunava Gupta,† Kazunari Domen,§ and Craig A. Grimes|
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2742–2748
Center for Materials for Information Technology, UniVersity of Alabama, Tuscaloosa, Alabama 35487, Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi UniVersity, 2-5-1 Akebono-cho, Kochi 780-8520, Japan, Department of Chemical System Engineering, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan, and 217 Materials Research Laboratory, Department of Electrical Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed June 14, 2007; ReVised Manuscript ReceiVed September 28, 2007
ABSTRACT: Controlled synthesis and organization of nanostructured materials using simple and low-cost methods is important in order to exploit their unique properties for practical applications. We describe the results of a systematic study of the solvothermal soft chemical synthesis of novel nanostructured ZnO architectures. Various ZnO nanostructured microspheres and microrod aggregates have been synthesized using Zn(CH3COO)2 · 2H2O in ethylenediamine in the presence of HAuCl4 · 4H2O. In the absence of HAuCl4 · 4H2O, fibrous and coral-like ZnO rod aggregates are obtained. The morphologies and structures of the obtained ZnO architectures can be tuned by manipulating the relative amounts and the addition sequence of Zn(CH3COO)2 · 2H2O and HAuCl4 · 4H2O. The Au(ethylenediamine)23+ complex, formed from Au3+ by combining with two ethylenediamine molecules, plays a key structuredirecting role in controlling the morphology and structure of the obtained ZnO architectures. All of the nanostructured ZnO architectures can be indexed to the hexagonal wurtzite stricture, and they show variations in the optical absorption properties depending on the distribution and interaction of Au nanoparticles on the ZnO surface. The formation mechanism of the various nanostructured microspheres is also discussed. Introduction Noble metal (Au, Ag, or Pt)-loaded ZnO is important for photoelectron transfer in the bulk and interface of ZnO semiconductors.1–4 The noble metal-ZnO nanostructures can be considered as a type of Schottky photochemical diode. When Au (Ag or Pt) is deposited on the surface of ZnO to form a metal nanostructure, it changes the Fermi level equilibration and the band structure of ZnO through storing and shuttling photogenerated electrons from the ZnO to acceptors in a photocatalytic process.5–7 Under illumination of light, the exciton absorption bands of ZnO are strongly bleached by the presence of accumulated conduction band electrons.8,9 Thus, the efficiency of both the photocatalysis and photoelectric energy conversion can be greatly enhanced by depositing noble metals on the surface of ZnO.10–12 The properties and applications of noble metal-ZnO nanostructured materials are also determined by the morphology, the structure, and, in particular, the organization of nanostructured ZnO architectures. These include ZnO nanoribbons that have been used as waveguides for subwavelength photonics integration;13 highly ordered ZnO nanowire arrays used for room-temperature ultraviolet nanolaser,14,15 surface wettability control,16 and solar cells;17 and ZnO-based varistor ceramics prepared by liquid-phase sintering of ZnO powders with different sizes and morphologies.18 Further, ferromagnetism above room temperature has been observed in the bulk, in transparent micrometer-thickness films, and in the powder form of ZnO dilute magnetic semiconductors.19 Thus, noble metal-ZnO * To whom correspondence should be addressed. E-mail: nzhbao@ mint.ua.edu. † University of Alabama. ‡ Kochi University. § The University of Tokyo. | The Pennsylvania State University.
architectures are expected to exhibit novel properties with potential photophysical and photochemical applications. Generally, the synthesis procedure of noble metal-loaded ZnO consists of an initial preparation of ZnO nanomaterials and subsequent deposition of noble metal nanoparticles. ZnO nanomaterials, in the form of nanoparticles,20,21 nanorods,22 nanowires,23 nanobelts,24 nanotubes,,25,26 etc., have been prepared using various synthetic approaches, including solution chemistry, chemical vapor deposition using the VLS mechanism, mechanochemical reaction, etc.20–29 More complicated ZnO nanowire arrays and hollow ZnO urchins have been fabricated by Au-catalyzed VLS growth and thermal evaporation of metallic zinc powder, respectively.30 Subsequently, noble metal nanoparticles can be deposited on the surface of ZnO by chemical or photoreduction processes, sputtering, plasma, high vacuum thermal evaporation, etc.31,32 The development of a simple, effective, and economical process for directly creating novel noble metal-deposited ZnO materials with desired architectures is of importance but remains a key research challenge.33–35 The hydro/solvothermal soft chemical process is a simple, versatile, and low-cost synthetic approach for preparing materials under mild growth conditions that can potentially be scaled-up for practical applications.36 Microspheres of ZnO nanorods have been prepared using polymer-directed hydrothermal growth.37 Other architectures, such as flower-like and star-like ZnO, have been prepared by hydrothermal synthesis using structuredirecting agents such as surfactants and molecular amines.38,39 Moreover, both ZnO and Au with various morphologies have been prepared by amine-aided solvothermal reaction in ethylenediamine or other amines.40–45 We recently found that both Au and ZnO can be simultaneously prepared in a solvothermal reaction in ethylenediamine, and the existence of Au has a key effect on the formation of special comet-like ZnO architectures.46 In the present study, novel ZnO architectures of various
10.1021/cg0705409 CCC: $37.00 2007 American Chemical Society Published on Web 11/15/2007
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nanostructured microspheres and microrod aggregates have been prepared via solvothermal reactions in ethylenediamine in the presence of the Au(ethylenediamine)23+complex under different conditions. The relative amounts and the addition sequence of the reactants are varied in order to modify the structure of the ZnO architectures. The formation mechanism of these unique Audeposited ZnO architectures is proposed by studying the morphological and structural evolution of the intermediate products. UV-visible spectra have been obtained to characterize the optical absorption properties of the various ZnO microstructures. Experimental Section Materials. All chemicals, zinc acetate dihydrate (Zn(CH3COO)2 · 2H2O, >99.9% purity), hydrogen tetrachloroaurate tetrahydrate (HAuCl4 · 4H2O, >99.9% purity), ethylenediamine (H2N(CH2)2NH2, >99% purity), and ethanol (C2H5OH, dehydrated, >99.5% purity), were used as received with no further purification. For specific reactions, a 0.2 wt % HAuCl4 ethanol solution prepared by dissolving HAuCl4 · 4H2O in ethanol was used. Synthesis. In a typical solvothermal synthesis of ZnO in the absence of Au, a certain quantity of Zn(CH3COO)2 · 2H2O was dissolved in 20 mL of ethylenediamine in a Teflonlined stainless steel autoclave by stirring for 1 h. After sealing, the autoclave was heated at 160 °C for 24 h and then cooled to room temperature. The precipitate was collected, washed several times with distilled water and ethanol, and then dried in a vacuum oven at 70 °C for 12 h. For a typical solvothermal synthesis of ZnO in the presence of Au, a specific amount of 2 wt % HAuCl4 ethanol solution was first mixed with 20 mL of ethylenediamine, keeping all other reaction conditions the same. Further investigations on the effect of the concentration and the addition sequence of Zn(CH3COO)2 · 2H2O and the HAuCl4 ethanol solution were carried out. All the samples were synthesized in 20 mL of ethylenediamine. Characterization. Powder X-ray diffraction (XRD) patterns were obtained using a Rigaku Rotaflex type X-ray diffractometer. Cu KR radiation with a nickel filter, operating at 40 kV and 100 mA, was used. All the samples were measured in the continuous scan mode in the range of 30–60° (2θ) at a scan rate of 0.02° (2θ)/s. The peak positions and the relative intensities of the product peaks were characterized by comparing with the Joint Committee for Powder Diffraction Standards (JCPDS) data. The morphologies and structures of the products were observed using scanning electron microscopy (SEM, S-350, Hitachi S-4700, Hitachi Ltd., Japan) and transmission electron microscopy coupled with high resolution (TEM and HRTEM, Tecnai F-20). The optical absorption spectra were obtained using an ultraviolet-visible diffuse reflectance spectrometer (UV-vis DRS, V-560, JASCO).
Results and Discussion 1. Crystal Phase and Phase Composition of ZnO Architectures. Ethylenediamine has been previously used for the controlled synthesis of one-dimensional nanomaterials, such as sulfide nanorods, through the solvent coordinating molecular template mechanism.21 In the present study, the Au(ethylenediamine)23+ complex acts as a type of structure-directing agent for controlling the structure and growth of ZnO products. In the process, the Au(ethylenediamine)23+ complex converts to Au through the amine-aided solvothermal reduction, together with the formation and growth of novel ZnO architectures. The morphologies and structures of the obtained ZnO architectures have been controlled by changing the concentrations and the addition sequence of the reactants. The crystal phase and composition of all products have been identified from the respective XRD patterns. As shown in Figure 1, all the diffraction peaks of the products are well indexed to the hexagonal phase of wurtzite ZnO (space group P63mc; a ) 3.2498 Å; c ) 5.2066 Å; JSPDS card no.36–1451). The a and c lattice parameters of the as-synthesized samples are 3.2541 and 5.2125 Å, respectively, which are very close to the standard
Figure 1. XRD patterns of (a) droplet-like microspheres of ZnO nanorods (see Figure 2) prepared from 0.3 g of Zn(CH3COO)2 · 2H2O added to ethylenediamine containing 0.03 g of HAuCl4 ethanol solution; (b) mandarin-like microspheres of ZnO nanorods (see Figure 4) prepared from 0.5 g of Zn(CH3COO)2 · 2H2O added to ethylenediamine containing 1.0 g of HAuCl4 ethanol solution; (c) ZnO microrod aggregates (see Figure 5a1-3) prepared from 0.03 g of HAuCl4 ethanol solution added to 20 mL of ethylenediamine containing 0.3 g of Zn(CH3COO)2 · 2H2O; (d) ZnO microrod aggregates (see Figure 5b1-3) prepared from 0.03 g of HAuCl4 ethanol solution added to ethylenediamine containing 0.4 g of Zn(CH3COO)2 · 2H2O; (e) ZnO microrod aggregates (see Figure 5c1-3) from 0.09 g of HAuCl4 ethanol solution added to ethylenediamine containing 0.9 g of Zn(CH3COO)2 · 2H2O; (f) ZnO fibers and coral-like ZnO aggregates (see Figure 7), respectively, prepared from 0.3 and 0.6 g of Zn(CH3COO)2 · 2H2O in ethylenediamine. XRD patterns of standard ZnO and Au are also shown.
values. Moreover, characteristic diffraction peaks corresponding to Au(111) and Au(200) are also observed in the XRD patterns of the ZnO products synthesized in the presence of Au. 2. Morphology and Structure of Novel ZnO Architectures. 2.1. Microspheres of ZnO Nanorods. Figure 2 shows the morphologies and structures of the ZnO products synthesized using ethylenediamine in the presence of HAuCl4. Essentially the entire product consists of spherical droplets with an average diameter of about 40 µm with a concave to flat part (marked with arrows in Figure 2a), with over 90% of the product consisting of intact microspheres with a perfectly uniform surface (see Figure 2b). The surface of these intact droplets consists of an outer shell of approximately 3.5 µm in thickness and a denser inner ball (see Figure 2c). The outer shell consists of a number of small particles with irregular shapes. Parts d-f of Figure 2 show the detailed features of the dense inner ball. As shown in Figure 2d, the inner ball is composed of packed nanorods, with lengths of about 15 µm and diameters of about 60 nm, surrounding a hollow core. The ends of the nanorods, pointing toward the center, are tubular (see Figure 2e), and thus we hypothesize that the rods have a hollow structure. These tube-like ends aggregate closely to form a spherical shape (see Figure 2f) and grow around a central core which tends to easily
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Figure 2. SEM images of droplet-like microspheres of ZnO nanorods showing (a) the panorama of products; (b) a typical single droplet-like microsphere; (c) the surface shell structure of the typical microsphere; (d) the hollow core and the outer tightly and radially aligned ZnO nanorods within a broken droplet; and (e) the side-view and (f) topview of the ends, pointing toward the core, of the radially aligned ZnO nanorods within the broken droplet.
Figure 4. SEM images of mandarin-like ZnO microspheres prepared with increased starting concentration of the reactants, showing (a) the panorama of products, (b) front view of an intact mandarin-like microsphere, (c) back view of an intact mandarin-like microsphere, and (d) a mandarin-like microsphere without a core. SEM images of ZnO microspheres with special morphologies and structures: (e) a broken microsphere with the whole inner structure; (f) a broken microsphere with double shells; and (g) the outer shell of a hollow broken microsphere.
Figure 3. SEM images of droplet-like microspheres of ZnO nanorods prepared with shorter stirring time: (a) the surface and inner structure of the product, and (b) a twin droplet.
separate from the broken droplets because of the weak interconnection. Alternatively, all of these ZnO rods might grow from a central spherical intermediate which will dissolve during the subsequent growth of the ZnO nanorods located in the outer shell. The morphology of the microspheres can be controlled by controlling the stirring time of the starting solution. By reducing the stirring time to 1/4 the original amount, the typical product is still microspheres but has a different structural unit (see Figure 3a). These microspheres consist of regular conical rod aggregates, with thicker flat ends connected together to form a rough but continuous surface, and a thinner end growing around the hollow center of the microspheres. The core of the sphere is removed or dissolved during the growth of the surrounding conical rods. Some twin droplets (see Figure 3b) that may be
formed from the special intermediate of two connected particles are also observed. The size, morphology, and structure of the ZnO architectures can also be adjusted by varying the starting concentration of the reactants. Half-mandarin-like ZnO microspheres (see Figure 4) have been prepared by increasing the concentration of Zn(CH3COO)2 · 2H2O by a small amount and the amount of HAuCl4 to a larger extent. They have an average diameter of about 8 µm, which is smaller than that of the droplets shown in Figure 2a. Parts b and c of Figure 4 show the morphology and structure of the products viewed from different directions. An intact microsphere is composed of half-mandarin-shaped nanorod aggregates and a weakly connected central core. One end of all the conical rods grows around a central porous core, but the core appears to be weakly connected with the surrounding rods (see Figure 4d). The other ends of the conical rods have the largest width, which is about 700 nm. In addition to the main products shown in Figure 4b, we also observed several kinds of coexisting ZnO microspheres (see Figure 4e-g). Figure 4e shows a microsphere segment consisting of a small porous central core and an outer thin shell of particle aggregates, with a net-like binding connector. The porous central core is formed because a part of the spherical intermediate formed in the initial reaction stage is dissolved. A double-shell structure (see Figure 4f) forms while the net-like binding connector is of relatively high density. If both the porous central core and the net-like binding connector are lost or
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Figure 5. SEM images of ZnO microrod aggregates synthesized from (a1-a3) 0.03 g of 0.2 wt % HAuCl4 ethanol solution added to ethylenediamine containing 0.3 g of Zn(CH3COO)2 · 2H2O; (b1-b3) 0.03 g of 0.2 wt % HAuCl4 ethanol solution added to ethylenediamine containing 0.4 g of Zn(CH3COO)2 · 2H2O; and (c1-c3) 0.09 g of 0.2 wt % HAuCl4 ethanol solution added to ethylenediamine containing 0.9 g of Zn(CH3COO)2 · 2H2O.
dissolved, the broken segments show only hollow shell structure (see Figure 4g). Although ZnO microspheres show various morphologies and structures, these special architectures are developed through self-splitting and growth of the same original ZnO microsphere intermediates. Further, the reason for the difference of the inner structure between the droplet-like ZnO microspheres (see Figure 2 and 3) and mandarin-like ZnO microspheres (see Figure 4) may be due to the amount of ethanol added to the ethylenediamine. 2.2. Various ZnO Microrod Aggregates. The addition sequence of Zn(CH3COO)2 · 2H2O and HAuCl4 in ethylenediamine also has a strong influence on the morphologies and structures of the ZnO products. Different from the microspheres described above, microrod aggregates (see Figure 5) are obtained by adding Zn(CH3COO)2 · 2H2O to ethylenediamine prior to the addition of HAuCl4. Figure 5a1 is a high magnification SEM image of a typical rod aggregate with a size of about 80 µm. All the rods are very distinct, with an average diameter of about 10 µm, and are covered by some small Au particles. A sideview SEM image (see Figure 5a2) of this rod aggregate shows that each rod has a flat pointed end on which there are many regular homocentric spheres (see Figure 5a3), indicating the growth trace of the rod. The morphology and structure of rod aggregates can also be manipulated by changing the starting concentration of the reactants. With slightly increasing the concentration of Zn(CH3COO)2 · 2H2O, the products are ZnO rod aggregates with size of about 75 µm (see Figure 5b1). All the rods grow from their flat ends and become thicker during the growth process, as shown in Figure 5b2. Figure 5b3 shows that these rods stick together and have irregular surfaces that are influenced by the neighboring rods. The growing end possesses a conical shape, similar to that shown in Figure 5a3. The average length and
thickness of these rods are about 25 µm and 6 µm, respectively. When continually increasing the concentrations of both Zn(CH3COO)2 · 2H2O and HAuCl4, most of the obtained products are ZnO rod aggregates with size of up to 100 µm, as shown in Figure 5c1. A top-view SEM image (see Figure 5c2) of a hexagonal rod aggregate shows that the product consists of a central core with a large number of ZnO rods growing regularly around the central core, which indicates that all the rods grow on the six faces of a hexagonal core. Figure 5c3 is a side view SEM image of a rod aggregate. All the ZnO rods exhibit oblongtype shapes that stick closely to the neighboring rods. 2.3. Fibrous and Coral-like ZnO. Figure 6 shows the morphology and size of the ZnO products prepared by solvothermal reactions in ethylenediamine in the absence of HAuCl4. At low concentrations of Zn(CH3COO)2•2H2O, most of the products are fiber-like with an average length of about 100 µm (see Figure 6a1), with diameters ranging from 0.5 to 1.8 µm (see Figure 6a2), having sharp ends (see Figure 6a1) and a smooth surface (see Figure 6a2). If the starting concentration of Zn(CH3COO)2 · 2H2O is increased, most of the products are coral-like ZnO rod aggregates (see Figure 1). A magnified SEM image of a rod aggregate in Figure 2 shows that one end of all the ZnO rods grows together and another end protrudes in different directions, forming a coral-like ZnO rod aggregate. These rods have an average length of approximately 50 µm and an average diameter of about 6 µm. The length and the diameter of the ZnO rods are, respectively, half-and three times the dimension of those obtained at low Zn(CH3COO)2 · 2H2O concentrations. The above results indicate that nanostructured microspheres (see Figures 2-34) and microrod aggregates (see Figure 4) can be synthesized in the presence of Au. ZnO rods tend to grow tightly around a central core, while, in the absence of Au, either
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Figure 6. SEM images of (a1 and a2) ZnO fibers and (b1 and b2) coral-like ZnO rod aggregates (b1 and b2), respectively, synthesized from 0.3 g of Zn(CH3COO)2 · 2H2O and 0.6 g of Zn(CH3COO)2 · 2H2O in ethylenediamine.
ZnO fibers or ZnO rods grow separately. These nanostructured ZnO architectures contain both wurtzite ZnO and Au (see Figure 1a-e). The growth of these ZnO architectures is envisioned as the initial formation of intermediates, which then supply suitable surfaces for the nucleation and growth of the ZnO clusters. These ZnO clusters grow into rods which become thicker and stick closely to each other; the growth of all the rods end up with flat ends, which tightly stick to each other to form a continuous surface with many voids. The spherical intermediate can also be dissolved; resulting in the formation of microspheres with various inner structures (see Figure 4). The existence of Au can influence both the nucleation and the growth of ZnO. 3. Possible Formation Mechanism of Various ZnO Architectures. The formation of novel ZnO architectures has been investigated by studying the morphological and structural evolution of some intermediates coexisting with the architectures. Hollow ZnO nanostructures with similar structures have been reported by various methods, in which microsphere intermediates of metallic Zn, 30 zinc carbonate, 34 and, in particular, the Zn2+-polymer complex37 have been used as sacrificial templates to grow hollow ZnO architectures. The final products tend to have a spherical morphology or form aggregates in order to minimize the surface energy. Figure 7 shows the intermediate product of spherical particles coexisting with the novel ZnO architectures (see Figures 2-45). These smooth spherical particles (see Figure 7c and d), formed at the initial reaction stage, are 0.2–1 µm in diameter, very close to the size of the hollow core of most ZnO architectures. Almost all the sphere-like particles have an unpenetrable hole (see Figure 7e). Some microspheres (see Figure 7f) have points protruding on their surface. These points can grow longer to form short rods protruding out of the surface, as shown in Figure 7g. The spherical particles can grow together to form twin microspheres (see Figure 7h) that are the growth basis of the various ZnO architectures shown in Figure 2-45. In this study, no ZnO architecture with a closed shell and perfect spherical morphology has been obtained. We either observe a concave part or a flat part for each droplet (see Figure 2a), an unclosed shell for a mandarin-like ZnO microsphere (see Figure 4), an irregular microsphere, or part of a microsphere consisting of microrod aggregates (see Figure 5). All of these structural and morphologic features result from the unpenetrable hole of spherical intermediates shown in Figure 7e. A twin droplet shown in Figure 3b could grow from a twin microsphere intermediate shown in
Figure 7. (a and b) SEM images of intermediates coexisting with ZnO architectures. SEM images of (c) the intermediates formed at the initial stage; (d) several types of intermediate microspheres with a smooth surface or surface protruding points; (e) typical main intermediates of smooth microspheres each having an unpenetrable hole; (f) a small microsphere with surface protruding points as growth bases of nanorods; (g) a small microsphere with short rods protruding out of the surface; and (h) a smooth twin microsphere.
Figure 7h. The final structure is determined by the diameter and the packing density of the rods growing on the spherical intermediate (see Figure 7g). The zinc cation is not easily hydrolyzed with the precipitation of the hydroxide, except by raising the pH. In this work, because the Lewis base ethylenediamine is a good ligand that is often used to form the Zn(ethylenediamine)32+ complex with the zinc cation, and amines such as ethylenediamine will undergo slow chemical decomposition to generate OH- ions at elevated temperatures under hydro/solvothermal conditions, the hydrolysis of Zn2+ is controlled by a slow release of hydroxide from the Zn(ethylenediamine)32+ complex, avoiding the fast nucleation of ZnO. Therefore, we estimate that the spherical intermediates consist of ZnO hydrates and the Zn(ethylenediamine)32+ complex that will crystallize to form the spherical byproducts (see Figure 7a and b). They also can act as precursors for the growth of various ZnO architectures (see Figures 2-5) in which they are maintained (see Figure 5c2 and c3), convert to porous cores (see Figure 4d and e), or totally dissolve (Figures 2d-f and 3a), with a possible reason being that a high concentration of ethanol, added to ethylenediamine together with HAuCl4, increases the pressure of the reaction system and correspondingly increases the solubility of ZnO. We further investigated the growth of ZnO rod using HRTEM and SAED. As shown in Figure 8a, a typical ZnO synthesized in ethylenediamine exhibits a hemispherical end. The inset
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Figure 9. UV-visible diffuse reflectance spectra of the (a) fibrous and coral-like ZnO rod aggregates (see Figure 6); (b) droplet-like microspheres of ZnO nanorods (see Figure 2); (c) mandarin-like microspheres of ZnO nanorods (see Figure 4); and (d) ZnO microrod aggregates shown in Figure 5a1-a3, respectively.
Figure 8. (a) TEM image and (b) HRTEM image of a typical ZnO rod synthesized in ethylenediamine. The inset to part a is an SAED pattern.
selected area diffraction (SAED) pattern indicates that the ZnO nanowire grows along the [001] direction. The lattice fringes in the HRTEM image (Figure 8b) with the d spacing of 0.52 nm match the interspacing of (001) planes of the wurtzite ZnO. These results demonstrate that the ZnO nanorods grew along the [001] direction, as well as the c-axis direction. Consistent with the XRD pattern, the clear lattice image and SAED pattern indicate good crystalline quality of the ZnO nanorods. No dislocations or stacking faults are observed in the area examined. In our previous study,27,46 we prepared comet-like Au-ZnO superstructures of ZnO nanorod by solvothermal reaction in ethylenediamine and ZnO nanotubes by hydrothermal reaction. Both showed the growth direction to be along the [001] direction, as confirmed by SAED and HRETM characterization. The thermodynamically stable crystal structure of ZnO is wurtzite with the ionic and polar structure described as hexagonal close packing of oxygen and zinc atoms in point group 3m and space group P63mc with zinc atoms in tetrahedral sites. The ZnO crystal consists of the stacking order of coordination tetrahedron by sharing elements (corner, edge, or face of the coordination tetrahedron). The terminal vertex of a corner of the coordination tetrahedron can still bond with three growth units; the terminal vertex of the edge of the coordination tetrahedron can still bond with two growth units; the terminal vertex of the face of the coordination tetrahedron can only bond with one growth unit. So, along the [001] direction, the growth rate for which every coordination tetrahedron has a corner present at the interface is the fastest, and the growth rate for which every coordination tetrahedron has a face present at the interface is the slowest; the growth rates in other directions are intermediate between those for these two directions. Thus, we estimate that the interior structural character of ZnO determines the anisotropic growth of the ZnO rods along the [001] direction, and only the c-axis-oriented radially standing hexagonal ZnO rod arrays tend to form finally on the spherical intermediates. 4. Optical Absorption Properties of Novel ZnO Architectures. Figure 9 shows the UV-vis absorption spectra collected from various Au-ZnO superstructures. The first strong absorption band between 365 and 390 nm originates from ZnO and is characteristic of the ZnO wide-band semiconductor material. There is no difference in this characteristic adsorption
Figure 10. SEM images of (a) Au particle agglomerates prepared in the absence of Zn(ethylenediamine)32+; (b) droplet-like microspheres of ZnO nanorods (see Figure 2), showing surface Au nanoparticles; (c) mandarin-like microspheres of ZnO nanorods (see Figure 4); and (d) the surface of ZnO microrod aggregates (see Figure 5a1-a3), showing surface net-like Au nanoparticles.
among the various ZnO architectures. However, in contrast to the pure ZnO prepared in the absence of Au (Figure 9a), the spectra of all ZnO architectures prepared in the presence of Au show another broad absorption in the spectral range of 480–600 nm, most likely due to the surface plasmon resonance of the Au particles. The apparent onsets of the absorption for the droplet-like microshperes of ZnO nanorods (see Figure 2) and the mandarin-like microspheres of ZnO nanorods (see Figure 4) are at about 540 nm (see Figure 9b) and 500 nm (see Figure 9c), respectively. The onset of the absorption for the ZnO microrod aggregates (see Figure 5a1–a3) is at about 490 nm, but this is not obvious. The differences in the onset of the absorption for the various Au-ZnO architectures might be due to the different structures and the size of Au particles and ZnO architectures. Additionally, they are dependent on the distribution and interaction between Au and ZnO. Although the Au product synthesized in the absence of Zn(ethylenediamine)32+ generally leads to particle agglomerates (see Figure 10a), the Au products exhibit different organization pathways, leading to various ZnO architectures. Au nanoparticles with an average size about 150 nm have been separately deposited on the surface of the droplet-like microspheres of ZnO nanorods, as shown in Figure 10b. The Au nanoparticles adhere strongly on the surface of ZnO, indicating the strong interaction between Au and Zn, which is beneficial for accelerating the transfer of photoactivated electrons from
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ZnO to Au nanoparticles, resulting in the change of the band gap and band structure. On the contrary, Au nanoparticles with similar size stick together to form net-like agglomerates. They weakly attach on the surface of ZnO microrod aggregates (see Figure 10d). Au nanoparticles are not observed on the surface of mandarin-like microspheres of ZnO nanorods (see Figure 10c), probably since the Au product separates from the ZnO product because of the weak interaction between Au and ZnO. On the other hand, the basic units consisting of the droplet-like microspheres, the mandarin-like microspheres, and the microrod aggregates are ZnO nanorods (about 60 nm in diameter, see Figure 2e), nanostructured conical rods (
