Double-Sided Comb-Like ZnO Nanostructures and Their Derivative Nanofern Arrays Grown by a Facile Metal Hydrothermal Oxidation Route Xiuyan Li,† Fenghua Zhao,† Junxiang Fu, Xianfeng Yang, Jing Wang, Chaolun Liang, and Mingmei Wu*
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 409–413
State Key Laboratory of Optoelectronic Materials and Technology/MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, and Instrumental Analysis and Research Center, Sun Yat-Sen (Zhongshan) UniVersity, Guangzhou, 510275, People’s Republic of China ReceiVed June 17, 2008; ReVised Manuscript ReceiVed September 16, 2008
ABSTRACT: Double-sided comb-like ZnO nanoferns have been successfully synthesized via a one-step hydrothermal synthesis without using any catalysts or templates. Their structure and morphology were characterized by scanning electron microscopy, transmission electron microscopy, and Fourier fast transformation technique. A possible growth mechanism is proposed that the nanobranches are grown epitaxially from both (0001) side surfaces of a primarily formed backbone nanobelt. Photoluminescence and photocatalysis properties have been investigated at room temperature. The ZnO nanoferns exhibit enhanced visible emission and photocatalysis as compared to as-prepared ZnO nanorods. Typically, their photocatalysis can be comparable to commercial P-25, even higher than P-25. With a change of growth condition, more complex ZnO nanostructure arrays with additional secondary grown branches could be fabricated. The structural evolution of the hierarchically derivative ZnO nanofern arrays has been mentioned.
1. Introduction Nanostructured materials have attracted extraordinary research interest in the last decades due to sizes and morphologies induced performance enhancement.1 The properties of nanocrystals depend not only on their chemical composition, phase, size, and shape, but their assemblies as well.2,3 The controlled growth of one-dimensional nanorods (nanowires) through chemical vapor transport and condensation has been successfully extended to sequentially hierarchical growth of dendritic nanostructures with the assistance of noble metal nanocrystals.4 However, the precise parameters of limited growth conditions such as reaction temperature, reaction time, metal particles, and their size distribution, gas flows, and the flow rates are required.5 In the chemical vapor deposition (CVD)6 routes to complex nanostructures, elevated temperature, and sophisticated equipment are generally indispensable. Thus, simple and facile growth approaches to complex nanostructures and even their arrays are remaining great challenge. Significant achievement has been made in esteemed Alivisatos’ group, in which II-IV and III-V semiconductor tetrapods have been synthesized from organic solvent in the presence of chelating ligands.7,8 As a typical wide bandgap II-IV semiconductor, ZnO nanostructures have been attracting much more attention.9,10 Both the growths and related structures of these nanomaterials have been considerably investigated due to their promising potentials in gas sensors,11 optoelectronics,12 field-emission devices,13 dye-sensitized solar cells,14 and photodegradation of harmful organic molecules.15,16 In addition to individual nanorod/nanowire and nanobelt,17 a complex nano- and microstructure such as a nanocomb,18 nanobridge,19 nanotetrapod,20 hollow spheres,21 and microphone-like one,22 integrated by onedimensional building blocks, has been described in a variety of reports. Typically, since the first report by Wang23 and Yang,6 ZnO nanocombs have stimulated much more research inter* To whom correspondence should be addressed. Tel.: +86-20-84111823. Fax: 86-20-84111038. E-mail:
[email protected]. † Equal contribution to this work.
ests.24-28 These research interests have been covering the growth and structures and their functions in nanolasers, nanosensors, and nanotweezers.6,29,30 To the best of our knowledge, the comblike hierarchical nanostructures, either one-sided or two-sided, have only been obtained at elevated temperatures in a furnace through chemical evaporation and depositions via VLS or VS approaches.31 In recent years, solution chemical growth of hierarchical ZnO nanostructures with increasing complexity have attracted special attention due to the facile route and low-cost expense without using sophisticated equipment, elevated temperature, and expensive chemical sources.32-34 To obtain these tree-like nanostructures, precursory solid ZnO nanoparticles or rods were in general adopted as seeds for in situ nucleation and growth.34,35 In a previous paper, we reported ZnO pine nanotree arrays through a simple hydrothermal oxidation of zinc metal under facile conditions.33 Herein, we extended our previous work and report two-sided comb-like ZnO nanoferns and their derivative arrays grown in large-scale via a one-step facile hydrothermal route. Compared with other methods, the hydrothermal process is quite simple, low-cost, and environmentally benign, and only metallic zinc foil was used, without use of any additional noble metal catalyst and templates. The ultralong ZnO nanoferns were characterized in detail, and the possible growth mechanism is proposed. Compared with assynthesized ZnO nanorods, the ZnO nanoferns exhibit a significantly improved efficiency in photocatalysis. Furthermore, increasing complexity of ZnO nanostructures can be achieved by modifying experiment condition and the interfaces between two one-dimensional building blocks, that is, first grown branches and second grown ones are suggested.
2. Experimental Section 2.1. Preparation. The hydrothermal growth of ZnO nanoferns is very simple. The source materials include commercially available and cheap zinc foil and aqueous solution of hydrazine. Details are as follows. The commercial Zn foil was washed with ethanol and distilled water respectively in ultrasonic pool. The square zinc foil with size of 1.0 ×
10.1021/cg8006348 CCC: $40.75 2009 American Chemical Society Published on Web 12/03/2008
410 Crystal Growth & Design, Vol. 9, No. 1, 2009 1.0 cm (width × width) was used as both Zn source and an in situ substrate. The growth procedures are as follow. The zinc foil was placed on the bottom of a Teflon cup (20 mL) in a stainless autoclave and then the cup was filled with 4 mL of mixed solution of hydrazine monohydrate (HH, 85%) and distilled water with different volume ratios. The autoclave was sealed and kept in an oven at an elevated temperature for a certain period of time. After the growth, the film was washed with ethanol and distilled water and then dried in a desiccator. 2.2. Characterization. Thermal FE (field emission) Environment Scanning Electron Microscope (Quanta 400, FEI Company, Philips) was used for SEM observation. Samples were gold-coated prior to the SEM analysis. JEOL JEM-2010HR equipped with Oxford EDS spectrometer and Gatan GIF Tridiem systems was used for TEM (transition electron microscopy) observation. TEM samples were prepared by scraping the surface product from the plate and dispersing the powders in a solvent and put on holey carbon film supported by a copper grid. Photoluminescence (PL) property was studied on a Fluorolog-3 fluorescence spectrophotometer at room temperature. An ozone-free, 450-W xenon lamp passing through the double-grating excitationandsingle-gratingemissionmonochromatorsinaCzerny-Turner configuration was used as the excitation light source. 2.3. Photocatalysis. Photocatalytic activities of the ZnO thin film were assessed by the photodegradation of Rhodamine B (RhB). The illumination was provided by a high-pressure Hg lamp (300W). All experiments about photocatalysis were carried out at ambient temperature. Before and after the irradiation experiments, the solution concentration of RhB was analyzed by a UV-vis spectrophotometer at its maximum absorption wavelength of 553 nm.36 P-25 (a brand name of TiO2 from Degussa) film was employed as the reference for comparison with the photocatalytic activity of the ZnO film, as prepared in a similar way as described in the reference.37 Its preparation procedure is briefly described as follows: A glass plate was pretreated by repeatedly dipping in the P-25 ethanol suspension (25 g/L) and then dried in air at 100 °C for 12 h.
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Figure 1. (a) Low and (b) high-magnification SEM images of doublesided fern-like ZnO nanostructures. (c) Low-magnification TEM image of ZnO nanostructures. (d) Enlarged images of the white rectangle area in c. (e) HRTEM images recorded from the area indicated by the white square in d. Inset is the FFT pattern of the corresponding HRTEM.
3. Results and Discussion 3.1. Synthesis and Structural Characterization of ZnO Nanoferns. The typical ultralong two-sided comb-like ZnO nanoferns were prepared at 140 °C for 4 h in a mixed solvent (4.0 mL) of water (2.4 mL) and hydrazine (1.6 mL; Figure 1). The powder XRD pattern (Figure S1) suggests its phase purity except for those from the substrate of zinc source. The general morphology of the as-prepared ZnO microsutructure observed under scanning electron microscopy (SEM) as shown in Figure 1a and Figure S2 reveals that ZnO nanoferns have been grown in large-scale. The typical ZnO nanoferns in general have a length range of tens of microns and a width of about 1.0 µm. With the increasing magnification from Figure 1a to b and Figure S2 (upper to lower), fern-like morphology appears to be much clearer. Each of such ZnO “nanoferns” is composed of a “backbone” and two rows of densely aligned “nanobranches” grown on both sides of the “backbone” (Figure 1b). Each nanofern appears as a “millipede”. To better understand the relationship between the backbone and the nanobranches of this fern-like nanostructure, further characterization by using transmission electron microscopy (TEM) and FFT pattern (Figure 1c-e) was performed. A low magnification TEM image of a nanofern is given in Figure 1c and a related higher magnification TEM image from the solidlined rectangle in Figure 1c are shown in Figure 1d. The HRTEM taken from the square part in Figure 1d illustrates a monolithic lattice plane feature, suggesting a possible single crystal nature of the nanoferns. The lattice spacing of 0.52 nm between adjacent lattice planes corresponds to the distance between two (0001) crystal planes of wurtzite ZnO, indicating [0001] as the preferred growth direction for all nanobranches and [101j0] as the one of the backbone. Distinct boundaries at
Figure 2. (a-c) SEM images and (d) schematic illustration of the ZnO nanostructures, showing the growth procedure from nanobelts to nanocombs. The white arrows in (d) indicating the growth directions of the nanobelt and the double sided nanoferns in which both sides of the branches are growing along the +[0001] direction.
the junction of the main backbone and the side branch could not be observed in the high resolution TEM image (Figure 1e). The related FFT pattern (Figure 1e, inset) confirms a monolithically single-crystalline nature of the part enclosed by the square in Figure 1d. Based on the above structural characterization, it can be concluded that the backbone is grown with [101j0] direction and enclosed by two large ((12j10) (up and down) surfaces, while the side branches are grown epitaxially along [0001] direction. 3.2. Growth Mechanism. To understand the growth procedure of the ZnO nanoferns, time-dependent experiments were carried out at different stages (Figure 2). At the initial growth stage, such as 1.0 h, ultralong ZnO nanobelts (Figure S3) were
Double-Sided Comb-Like ZnO Nanostructures
grown with quite a few branches (Figure 2a). As our previous ZnO nanotrees, these ZnO nanobelts were grown via a zinc metal chemical oxidation.33 With further growth to 2.0 h, premature but more nanobranches were grown on both sides (Figure 2b) and to 4.0 h, neatly and densely oriented nanobranches were yielded symmetrically on both backbone’s side surfaces (Figures 2c and S2). Due to the noncentral symmetric characteristic of wurtzite ZnO, the (0001) of ZnO is terminated with Zn2+ cations, and the (0001j) is terminated with O2- anions, which are the two typical polar surfaces of ZnO. The Znterminated (0001) surface is chemically active and its selfcatalysis can result in epitaxial growth of teeth on side surface. The O-terminated (0001j) surface is relatively chemically inert and generally produces no epitaxial growth.18 This is the reason why the one-side nanocombs are most frequently received and they can be grown with a purity of nearly 100%.18 Although symmetric double-sided nanocombs have also been produced previously by CVD,6,18,23 their formation cannot be explained by the polar surface model if the backbone is a single crystal with both sides of the combs terminated by Zn-(0001) and O-(0001j), respectively. It was proposed that there was an inversion domain boundary (IDB) parallel to the belt-like backbone.18 The evidence of a distinguished groove-like contrast at the middle of the backbone belt in Figure 1b implies that there is a boundary along the [101j0] oriented growth direction. The above evidence indicate that the backbone belt is not a single-crystal but composed of two nanoribbons with inner (0001j) and outer (0001) side surfaces. Thus, each rectangular backbone belt was formed by two larger up/down ((1j21j0) planes, and two smaller oppositely grown +(0001) side surfaces (Figure 2d). At this moment, we can obviously speculate that the growth of the ZnO nanofern structure follows a two-step process (Figure 2d). First, during the period of initial growth stage, Zn foil surface is oxidized and belt-like ZnO nanostructures are formed under the hydrothermal conditions with the assistant of hydrazine molecules which serve as both pH-value regulator and Znatom ligands. As strong coordination of the nitrogen atom at either head of the hydrazine molecules to Zn-atoms occurs, a possible lamellar structure composed of Zn-hydrazine appears in the growth system, and the growth along the Zn-terminated (0001) polar is significantly confined,33,38-40 resulting in the formation of the backbone with belt-like structures (Figure 2d1). Second, due to the active behavior of (0001) lattice planes,18 the nanobranches nucleate and grow by taking the advantage of epitaxial growth from both (0001) side surfaces (Figure 2d2) as those extensively documented nanocombs, either one-sided or two-sided, fabricated from thermal evaporation and deposition in furnace.18,23 Based on the crystallographically geometrical relationship between backbone and nanobranches (Figure 1e), it can be immediately concluded that each rectangle blade-like branch is enclosed by broad ((101j0) planes, and two small ((1j21j0) side surfaces (Figure 2d). The blade-like nanobranches with such a configuration have also been observed in our previous work for ZnO pine nanotrees,33 which were grown in aqueous solution of ethylenediamine. The present branches in the nanoferns and those in the nanotrees have identical configuration in crystallographically geometrical relationship for outer surfaces. However, herein, the aligned nanobranches are crystallographically perpendicular to the main “backbone”, which is quite different to those of a ZnO pine nanotree, in which the aligned nanobranches are pointing downward to the substrate with an angle of about 108° with the main “trunk”. In addition, the present quadrilateral belt-like nanostructure of the backbone
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Figure 3. Room-temperature photoluminescence of (a) double-sided fern-like ZnO nanostructures and (b) ZnO nanorods.
is different to previous hexagonally prismatic submicrotrunk in a ZnO pine tree. 3.3. Photoluminescence and Photocatalysis. The study of PL property can tell significant information about defects and illustrate potentials in optoelectronic and catalytic applications. The optical properties of the as-prepared ZnO ferns on the in situ substrate are investigated by PL measurements. The PL emission spectrum is obtained under an ultraviolet excitation at 330 nm. Two typical emissions, one narrow one at 380 nm and the other broad one at 550-570 nm were detected (Figure 3a). The UV emission at about 380 nm is typically from a free exciton recombination related to the band gap as well established,33,41 and the broad green emission represents the deep level defects resulted from the singly ionized oxygen vacancies and other defects,42,43 in which the surface oxygen deficiencies could be electron capture center, reduce the recombination rate of electrons and holes, and thus, enhance the photocatalysis. The boundary of two opposite (0001j) planes along the middle of each backbone surely makes contribution to the defects. In addition, the belt-like backbone and the bladelike branches certainly provide more possibility to induce more surface areas. The high-surface-area of these fern-like structures could be loaded with more organic dyes for dye-sensitized solar cells, achieve good light harvesting efficiency and excellent exciton dissociation, and thus improve the photocurrents. The green emission is relatively much enhanced as compared to previous nanotrees, presenting evidence with more surface deficiencies and consequently these two-sided comb-like ZnO nanoferns may have promising potentials in applications in catalysis and solar cells.14-16,33 To investigate the photocatalytic activity of the comb-like ZnO nanofern film, Rhodamine B (RhB) solution (initial concentration: 1.0 × 10-5M) was chosen as a model compound. Both P-25 film (see Figure S4 in the Supporting Information) and ZnO nanorods film (see Figure S5 in the Supporting Information) was employed for comparison (with identical film area). The results are shown in Figure 4 (C0 and C are the equilibrium concentration of RhB before and after UV light irradiation, respectively). Under the UV light, but without the use of any catalysts, the RhB photodegradation is small (Figure 4a). However, with the addition of either P-25 or ZnO films, the photodegradation is highly enhanced. Under identical conditions with exposure to UV light, the ZnO nanofern film exhibits much higher activity than either P-25 nanoparticle or ZnO nanorod film. Exposure of the solution of RhB to UV light for 7.0 h resulted in RhB photodegradation of about 97.1, 74.0, and 54.3% with the assistant of ZnO nanofern film, P-25 nanoparticle film and ZnO nanorod film as the photocatalyst, respectively. The maximum absorption peak of the RhB molecule diminishes gradually as the exposure time increases
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Figure 4. Photodegradation of RhB under UV with different catalysts: (a) without using any catalyst; (b) ZnO nanorod film; (c) P-25 nanoparticle film; (d) ZnO nanofern film.
and completely disappears at last when the ZnO nanofern film is used as photocatalyst. New absorption peak does not appear in visible region, which indicates the complete photodegradation of RhB. The degradation of RhB follows pseudofirst-order kinetics. The highest photocatalytic activity of ZnO nanoferns among them may result from their dendritic structures with high surfaces, unique growth orientation and defects, which is consequently relevant to the significant green emission (Figure 3). For the surface structures, one major issue related to improved photocatalysis might be due to that the branches of a ZnO nanofern grow along [0001] direction, namely, more quantity of active (0001) lattices expose on outer surfaces than other ZnO nanostructures.44 A clearer and more straightforward understanding of the improved photocatalysis of ZnO nanoferns is still under way. By the way, for practical application in solution, fern-like photocatalysts are much more favorably separated from solution as compared to powdery nanoparticles after use.45 3.4. Hierarchical Nanofern Arrays with Increasing Complexity. Up to now, a diversity of ZnO nanostructures has been developed from solution chemistry by simply modifying the growth conditions. Generally, reaction temperature is a key function to modify the chemical reaction in both thermodynamics and kinetics. Here, with the reaction temperature up to 160 °C, a few additional rod-like nanobranches were grown from the backbone site, that is, at the central part of the oppositely grown nanobranches as mentioned in Figure 2d, after the zinc plate (substrate) was kept at the bottom in the solution in the tightly closed vessel for 4.0 h (Figure 5). These ZnO nanostructures appear with increasing complexity from the central part to the edge part of each substrate, that is, as illustrated from Figure 5a-d. The form of the first grown branches (denoted as FGBs, Figure 5e) has a preference from rectangle blades to hexagonal-prism-based nanothorns (from Figure 5a-d, right panel, and as a model, please see Figure 5e3). It appears that each of the original {1j21j0} surfaces of a nanoblade (Figures 2d and 5e3) evolved into two {101j0} side surfaces on the prismbased nanothorns (from Figure 5a-d, right panel) as schematically illustrated in Figure 5e3. Following the evolution from Figure 5a-d, highly orientated and tightly packed nanobranch arrays tend to be larger and the individual nanofern consequently tends to be larger either. It can be clearly observed that several second grown branches (SGBs) have appeared from a FGB on each nanofern (Figure 5, denoted in Figure 5e2-top). The projection of these rod-like SGBs along the zone axis of [101j0] has an angle of about 69° with the projection of the FGBs along the identical zone axis (Figure 5b (inset) and e2-up). This angle is similar to that between trunk and branches in our previously reported ZnO pine nanotree.33 A close and detailed view in Figure 5b-right
Figure 5. SEM images of dendritic ZnO nanostructure prepared at 160 °C for 4.0 h at different sites of the film from center (a) to edge (d) gradually: e1-e2-up, zone axis [101j0]; e1-e2-down, zone axis [1j21j0]; e3, zone axis [101j0].
shows that the nanobranches are grown from the {1j21j0} side surfaces of a FGB, indicating that the growth of the SGBs from a FGB seems to be attributed to the negligible (
