Self-catalytic Synthesis of ZnO Tetrapods, Nanotetraspikes, and

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Self-catalytic Synthesis of ZnO Tetrapods, Nanotetraspikes, and Nanowires in Air at Atmospheric Pressure Yang Liu,†,‡ Zhenhua Chen,† Zhenhui Kang,†,‡,§ Igor Bello,*,† Xia Fan,†,| Ismathullakhan Shafiq,† Wenjun Zhang,† and Shuit-Tong Lee† Center Of Super-Diamond and AdVanced Films (COSDAF), Department of Physics and Materials Science, and Department of Biology and Chemistry, City UniVersity of Hong Kong, Hong Kong SAR, China and Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China ReceiVed: January 30, 2008; ReVised Manuscript ReceiVed: March 25, 2008

ZnO tetrapods uniformly distributed over the surface of zinc foils were synthesized in situ at 900 °C in air. Control experiments indicate that ZnO tetrapods are evolved from ZnO microspheres and their clustered complexes, which act as the centers advancing to further one-dimensional (1D) growth and branching structures in four characteristic directions. Using a specific alkali solution promotes tetrapods with pyramidal arms that further progress in growing ZnO tetraspikes and nanowires with increasing reaction time. It is believed that the pretreatment of zinc foils using alkali solution is an important step for this air, high-temperature synthesis. The evolution mechanism from ZnO microspheres and their clustered complexes to ZnO tetrapods and the evolution of ZnO nanowires through nanotetraspike morphology are illustrated. The synthesized ZnO tetrapod structure was found to exhibit strong photoluminescence in the UV light range. 1. Introduction Zinc oxide (ZnO) is an important electronic and photonic material due to its wide direct band gap of 3.37 eV. It has a fairly large excitation binding energy (60 meV) and exhibits near-UV emission and transparent conductivity at room temperature and above. The noncentral symmetrical crystallographic structure of ZnO can lead to piezoelectricity as well as pyroelectricity, which are certainly interesting properties for building electromechanically coupled sensors and transducers. The availability of a rich genre of nanostructures1–3 and their electronic and photonic properties suggest ZnO as an ideal material for nanoscale optoelectronics,4 electronics,5,6 and biotechnology.7 Functional devices such as piezoelectric nanogenerators,8,9 optically pumped nanolasers,4,10 solar cells,11 and field emission devices12 have already been demonstrated. Since the first report of ultraviolet lasing from ZnO nanorods,4 considerable effort has been devoted to the fabrication of welldefined one-dimensional (1D) ZnO nanostructures with various morphologies. Among many existing preparative techniques,4–19 the vapor-phase transport process assisted with noble metal catalysts is one of the major vapor methods for growing 1D ZnO nanostructures. Several articles report the synthesis of ZnO tetrapods and the evolution of nanowires by vapor transport processes and vaporization of Zn from Zn precursors, including ZnO powders or mixtures of ZnCO3 and graphite, in different dynamic flow gas atmospheres.20–28 Some of these syntheses were obtained with the aid of catalysts. However, complex manipulation, the necessity of carrier gases, and uncontrolled residue of catalysts * Corresponding Author. Email: [email protected]. † COSDAF and Department of Physics and Materials Science, City University of Hong Kong. ‡ On leave from Department of Chemistry, Northeast Normal University, Changchun, Jilin, China. § Department of Biology and Chemistry, City University of Hong Kong. | Chinese Academy of Sciences.

used in these methods may prevent some applications of ZnO nanoproducts. In this work, we use the vaporization of surface pretreated Zn foil in an air environment to obtain different nanostructures, including well-faceted tetrapods and nanowires that evolve from nanotetraspike morphology. The growth process was in the absence of metal catalysts. The evolution mechanism from ZnO microspheres and their small clustered complexes to tetrapods, and the evolution of nanowires through nanotetraspike phase, are illustrated. 2. Experimental Section In a typical procedure, a piece of zinc foil with thickness of 0.5 mm and size of 1 cm × 1 cm was cleaned by sonication in ethanol for 5 min and dried in air. Then, a drop of 3 M KOH aqueous solution was first dropped on the clean zinc foil. After the liquid drop spread, the zinc foil was placed in an alumina boat that was loaded to an air furnace (1 atm) and heated to 900 °C. This temperature was maintained constant during the growth process. As a result of this high temperature reaction, the zinc foil was coated with a thick layer of white powder. In alternative experiments, the surfaces of foils were treated by a drop of K2CO3 solution instead of KOH to obtain morphologically different ZnO structures. The morphology and structure of the samples were analyzed with a Philips XL30 FEG scanning electron microscopy (SEM) and a FEI/Philips Techal 12 BioTWIN transmission electron microscopy (TEM), and high-resolution transmission electron microscopic (HRTEM) images were obtained with a CM200 FEG TEM at 200 kV. The X-ray diffraction (XRD) patterns were recorded by a Siemens D500 diffractometer. The photoluminescence (PL) spectra were measured at room temperature employing a Perkin Elmer luminescence spectrometer LS50B.

10.1021/jp800907g CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008

Self-catalytic Synthesis of ZnO Tetrapods

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Figure 1. SEM images and structure of synthesized ZnO tetrapods; (a) ZnO tetrapods uniformly cover the Zn foil; (b, c) magnified SEM images of ZnO tetrapods; (d) XRD pattern of the ZnO tetrapods prepared on a zinc foil.

Figure 2. (a) TEM image of the tetrapods as-prepared. The inset is a SAED pattern collected from a tetrapod arm; (b) HRTEM image of a randomly selected tetrapod arm; (c) HRTEM image taken at the joint of two tetrapod arms.

3. Results and Discussion Morphological analysis of the white overlayer formed reveals a particle deposit evenly distributed over the surface of the zinc foil, as illustrated by the SEM image in Figure 1a. The SEM image, in Figure 1b, indicates that all particles have a tetrapod shape with smooth surfaces. The tetrapods are uniform in size and shape. The average diameter and length of a pod is 200 and 600 nm, respectively. The magnified image, in Figure 1c, clearly shows well-defined hexagonal-prism morphology of each arm of an arbitrary tetrapod with angles of about 109° between the axes of adjacent arms. The faceted morphology suggests a single crystalline structure of the arms. The XRD pattern collected from these ZnO tetrapods, shown in Figure 1d, can readily be indexed as the wurtzite ZnO crystalographic structure.

Figure 2a shows a typical bright field TEM image taken from ZnO particles dispersed on a TEM grid. The ZnO particles have characteristic tetrapod morphology that is similar to that observed by SEM. The selected area electron diffraction (SAED) pattern recorded on arms of the tetrapods (inset) shows that all four arms are single-crystalline, which is consistent with SEM observation of faceted surfaces. As shown in HRTEM (Figure 2b) recorded at the edge of an arm, the displayed lattice spacing of 0.52 nm perpendicular to the arm axis corresponds to the interplanar spacing of (001) planes of ZnO, indicating the preferential growth of tetrapod arms along the [001] direction. HRTEM analyses of randomly selected arms of arbitrary tetrapods indicate practically identical structure growing along the [001] direction. The HRTEM image collected at the joint of a tetrapod, in Figure 2c, shows the crystalographic planes

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Figure 3. SEM images of ZnO particles synthesized for different reaction times: (a) 6, (b) 10, and (c) 15 h.

between two arms, where the boundary between the arms with the same wurtzite structure can readily be seen. The visual discrepancy in spacing of the two arms arises from different angles between the electron beam and the two arms. To elucidate the formation process of ZnO tetrapods, their growth morphologies were studied in reaction time sequence. Obviously the ZnO product, in Figure 3a, which was evolved for a shorter reaction time of 6 h, is largely composed of microspherical particles clustered in small complexes. However, no tetrapods can be observed. The clustered spheroid particles are obviously deformed at their contacts. Their positions are optimized during the thermodynamic solid-vapor-solid mass transport. These particles may then grow in some characteristic directions at the detriment of their own mass and masses of other vanishing particles that are progressively consumed, as illustrated by the sample prepared with longer reaction time (10 h) in Figure 3b. The particles thus become cave structures that pass into a linked rod configuration. Further increasing the reaction time to 15 h leads to obvious development of tetrapods, as evident in Figure 3c. Thus, the illustrated evolution of tetrapods is primarily based on the formation of ZnO microspheres and their small complexes, which further lead to 1D growth in distinct directions to form regular branched ZnO tetrapod structures with angles of approximately 109° between the axes of adjacent arms. Explanation of this observation is certainly not trivial. However, the analysis of the time-sequential stages of the growth, in Figures 3a-c and 1a, is very suggestive. Morphologically different ZnO tetra-arm structures can be synthesized with assistance of a drop of K2CO3 solution to treat

Liu et al. the surface of the zinc foil before high-temperature reaction. The reaction in air leads to nanotetraspikes with a very high aspect ratio, as illustrated by SEM images in Figure 4. Well faceted spikes at their bases imply their crystalline nature. Each spike at its end, in Figure 4, panels a and b, is terminated with considerable contraction of its lateral size and a small segment of a nanowire. The junctions of the spikes at their roots are laterally supported with faceted morphological formations being mostly three side pyramids with three seams between adjacent sides of pyramidal planes as presented in Figure 4, panels c-e. These seams may result from growing three individual crystals until their reactive planes are enclosed. Alternatively, the seams can be residues of clustering microspherical particles since each side of the pyramid seems to be monolithic and crystalline with one of three neighboring spikes. Exception from the observed pyramidal regularity is the triangular joint in Figure 4e. Perhaps the junction of tetraspikes can be different, which was reported for tetrapods elsewhere.20 However, in the case of the presented tetraspikes, triangular pyramidal joints dominate, although this has not been statistically investigated. The longer reaction time yields the further lateral contraction of individual ZnO spikes and extension of the terminating segments of nanowires. For example, the nanotetraspikes prepared by high-temperature reaction in air for 25 h are characteristic with very long segments of nanowires at their ends, as shown in Figure 5a. Finally, increasing the reaction time to 30 h, all the tetrapods turn to nanowires with a mean diameter of about 100 nm, as seen in Figure 5b. This observation is quite different from the previous report by Djurisˇic´ et al.22 In their work the oxidation of zinc powder in air resulted in the formation of tetrapods, but no nanowires were found. The crystalline caved nanotetraspikes laterally contracted with increasing reaction time and finally formed nanowires, leaving little evidence on their evolution history and initial growth stage. The tetrapods and nanowires progressing from tetraspikes are evolved in synergy of mass transport involving transformation of morphological configurations via solid phase and solid-vaporsolid phase transformations. Some explanations have been presented for the growth mechanism of ZnO tetrapods. It has generally been accepted that the formation of ZnO tetrapods occurs via vapor–liquid– solid (VLS) growth mechanism. In this work, the formation of ZnO tetrapods can be understood via a modified VLS mechanism. Here, the pretreatment of zinc foils by alkali solutions prior to high temperature reactions results in the formation of Zn(OH)2 on the zinc foils. This oxidation process is described by the following reactions: Zn + 2OH-+ 2H2O f[Zn(OH)4]2+ H2v and [Zn(OH)4]2- f Zn(OH)2 + 2OH-. Then subsequent heating process turns Zn(OH)2 into ZnO clusters, which, in terms of a chemical equation, is Zn(OH)2 f ZnO + H2O. The ZnO clusters are formed on the melting Zn surface, which eases the formation of ZnO nanocomplexes acting as nucleaction sites of the nanostructures grown. At the high temperature of 900 °C, close to the boiling point of Zn (BP ) 907 °C at 1 atm), Zn vapor is oxidized in air to form ZnO, which preferentially condense on the surface of ZnO clusters, leading to the formation of ZnO microspherical centers. This self-catalytic growth28 plays an important role in the formation of ZnO nanostructures. With increasing reaction time from 6 to 20 h, the structure of ZnO crystals gradually transforms into its thermodynamically preferred tetrapod configuration. The very first step of ZnO nucleation is given by the treatment of alkali solution. Because the subsequent transformation is obvious in Figure 3, it is reasonable to believe that the pretreatment of

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Figure 4. SEM images of nanotetraspikes grown on oxidized zinc foil pretreated with a K2CO3 solution: (a) spatial distribution of nanotetraspikes; (b) nanotetraspikes laterally contracted and terminated by nanowires at their ends; (c-e) morphology of pyramidal joints at the bases of nanotetraspikes.

Figure 5. SEM images of evolution of ZnO nanowires from tetraspikes: (a) progressing conversion of tetraspikes to nanowires observed after the reaction time of 25 h; (b) final nanowire products after the reaction time of 30 h.

zinc foils by an alkali solution is an important step for in situ air, high-temperature synthesis of uniform ZnO tetrapods, tetraspikes, and nanowires on zinc foils. Nucleation and growth models of ZnO tetrapods were discussed by several groups.28–31 In our case, we observed that the center core of the ZnO tetrapods consists of four hexagonally faceted crystals fused in a twin configuration, as shown in Figures 2c and 4c. The nucleation stage of tetrapods is fairly affected by alkali treatment of the Zn foil. It leaves a thin oxide layer on zinc surface behind. Heating to high temperature mobilizes ZnO molecules on the liquefied Zn surface to form ZnO nanocomplexes that work as nuclei in subsequent growth. At high temperature, Zn is vaporized and forms ZnO in the gas phase, which is condensable at the given temperature of 900 °C. The zinc oxide molecules preferably condense on the ZnO nuclei, which grow to spheroid particles. In synergy with the gas phase process, Zn surface oxidizes and enhances growing spheroid particles as described. The formation of the spheroid particles is fairly consistent with the suggestion of Ronning et al.28 They predict that spheroid (ZnO)i clusters are stable for 11 e i e 15. They presume that the further grow of spheroids via condensation mechanism lead to the instability of the spheroid structure and collapsing the spheroid to tetrahedron,

which is said to be a more stable bulk crystalline structure. However, in our system, study of the evolutional stages suggests otherwise. Figure 3a indicates laterally enlarged spheroids that are in contact with neighboring spheroids. These ball particles are deformed in their contact areas. Their contact angles very much resemble those of the fused four arms of tetrapods and nanotetraspikes. More likely, the tetrapods are formed by aggregation of four deformed spheroids that are oriented in the most preferable thermodynamic orientation. Fusing the four deformed spheroid crystallites determines the twin angles between the crystallites and four distinctive growth directions for the formation of tetrapods or tetraspikes. Any combination of three neighboring arms is fused of three crystals, resulting in a symmetric triangular pyramid at the roots of the arms with an exception seen in Figure 4d. Thus, each root of the tetrapod (tetraspike) arm is at couple triangular pyramidal apexes. This model can explain the angle of about 109° between the axes of adjacent tetrapod arms. Hence, the high-temperature air-zinc reaction leads to the transformation into its thermodynamically preferred configuration with four ZnO arms due to the fast growth rate of hexagonal ZnO along the direction of the c-axis via the VLS process. The evolution of ZnO nanowires from nanotetrapikes obviously proceeds via lateral contractions of

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Liu et al. cence study reveals the UV emission band centered at 380 nm to be stronger than previously reported. Finally, it is reasonable to assume that the described process, absent of metallic catalysts and easily scalable, might be used for fabrication of various ZnO and other metal oxides nanostructures in bulk quantities. Acknowledgment. The work was supported by the NSFC/ RGC Joint Research Scheme (N_CityU125/05), the Strategic Research Grant of the City University of Hong Kong (7002121), the National Basic Research Program of China (973 Program) (2006CB933000, 2007CB936000), and an 863 project (2006AA03Z302). References and Notes

Figure 6. Room-temperature PL spectrum of ZnO tetrapods.

the arms and bases of the tetraspikes. However, the reaction causing this effect is unclear, unless Zn enriched tetraspike structures exist. Investigation of optical properties of synthesized materials shows that tetrapods, in particular, give strong UV photoluminescence (PL) at room temperature when excited by 244 nm laser. As demonstrated in Figure 6, a sharp and strong UV peak at 3.26 eV (λ ) 380 nm) dominates the PL spectrum, and a broad green band is found in the range of 2.07-2.95 eV (λ ) 420-600 nm). The UV band emission can be assigned to the emission from a free exciton under low excitation intensity.32 The relative intensity of the UV band is larger than previously reported.21,22 The higher relative intensity might be associated with the reduced defect density in ZnO tetrapods.25,33 It is reasonable to assume that crystallinity of well-faceted arms of ZnO tetrapods with hexagonal-prism morphology, as shown in Figure 1, is better and that these tetrapod structures confine less defects than those with needlelike and cylindrical shapes. The green band may originate in the electron transition from the level of the ionized oxygen vacancies to the valence band.34 The intensity of the deep-level emission is determined by the concentration of the oxygen vacancies in ZnO crystals.35 Therefore, the strong and intense UV emission, the weak emission related to the vacant ionized-oxygen levels, and the absence of the well-known stronger and broader emission in the yellow-green band in the PL spectrum illustrate the good crystallization quality and nearly stoichiometric nature of the ZnO tetrapods. 4. Conclusions In summary, we have synthesized ZnO nanostructures in situ on zinc foils in air with the absence of metallic catalysts. ZnO structures with tetrapod and nanotetraspike morphologies were obtained when zinc foils were pretreated by KOH and K2CO3 solutions, respectively. In the later case, ZnO nanotetraspikes with arms laterally contracted were formed first and finally turned to ZnO nanowires with increasing reaction time. The evolution of tetrapods and nanowires progressing from tetraspikes are elucidated in synergy of mass transport involving transformation of morphological configurations via solid phase and solid-vapor-solid phase transformations. Photolumines-

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