Electrochemical route to the synthesis of ZnO microstructures: its nestlike structure and holding of Ag particles
© Ding et al.; licensee Springer. 2013
Received: 20 November 2012
Accepted: 6 January 2013
Published: 15 February 2013
A simple and facile electrochemical route was developed for the shape-selective synthesis of large-scaled series of ZnO microstructures, including petal, flower, sphere, nest and clew aggregates of ZnO laminas at room temperature. This route is based on sodium citrate-directed crystallization. In the system, sodium citrate can greatly promote ZnO to nucleate and directly grow by selectively capping the specific ZnO facets because of its excellent adsorption ability. The morphology of ZnO is tuned by readily adjusting the concentration of sodium citrate and the electrodeposition time. Among the series structures, the remarkable ZnO nestlike structure can be used as a container to hold not only the interlaced ZnO laminas but also Ag nanoparticles in the center. The special heterostructures of nestlike ZnO holding Ag nanoparticles were found to display the superior properties on the surface-enhanced Raman scattering. This work has signified an important methodology to produce a wide assortment of desired microstructures of ZnO.
81 Materials science 81.07.-b nanoscale materials and structures Fabrication Characterization 81.15.-z Methods of deposition of films Coatings Film growth and epitaxy.
KeywordsZnO microstructures Nestlike structure Ag-ZnO nestlike heterostructure Electrodeposition
Construction of micro- and nanoscale semiconductor materials with special size, morphology, and hierarchy has attracted considerable attention for potential application due to their distinctive functions, novel properties, and potential applications in advanced devices and biotechnologies[1, 2]. Rational control over the experimental condition has become a hot topic in recent material research fields. ZnO is currently one of the most attractive semiconducting materials for optical and electronic applications because of its direct wide band gap (3.37 eV) and high exciton binding energy (60 meV). Since Yang observed the room temperature UV lasing from ZnO nanorod arrays, much effort has been devoted to tailor the morphology and size to optimize the optical properties. As a result, various ZnO nanostructures, including nanowires[5–7], nanotubes[8, 9], nanobelts, nanoflowers, nanospheres, nanobowls, dandelions, cages, and shells[16, 17] have been obtained by solid-vapor phase growth, microemulation, and hydrothermal methods[20, 21]. Hereunto, nanobowls, nanocups, or nanodishes have attracted much interest because they have been envisaged to further contain nanoparticles and immobilize biomolecules[23, 24]. Although conventional methods can produce various ZnO micro-/nanostructures, these different synthesis methods often greatly suffer from problems of high temperature, need for high vacuum, lack of control, and high cost. To develop a simple, fast, and controllable synthetic route that can not only control the ZnO micro-/nanostructures with series of shapes under ambient condition, but also produce the hierarchical structure, remains an important topic of investigation.
The electrochemical deposition technique has been recently developed as a promising alternative means for the fabrication of nanomaterials under ambient condition due to the low cost, mild condition, and accurate process control. Recently, Yang and co-workers reported the synthesis of ultrathin ZnO nanorods/nanobelts arrays on Zn substrates by electrochemical deposition. Our group reported an electrochemical route for the fabrication of highly dispersed composites of ZnO/carbon nanotubes. Herein, we report a tunable self-assemble strategy to selectively fabricate a series of ZnO with unique, pure, and larger quantity morphologies including petal-, flower-, sphere-, nest- and clew-shaped structures by electrochemical deposition. The size and morphology of the ZnO are systematically controlled by judiciously adjusting the concentration of the sodium citrate and the electrodepositing time in the self-assembly process. Significantly, the nestlike structure dominates the further formation of hierarchical superstructure. The ZnO nestlike structure can be used as a container not only to hold several interlaced ZnO laminas, but also to fabricate Ag-ZnO heterostructures by growing silver nanoparticles or clusters in the center of nests by electrochemical deposition method. The multiphonon Raman scattering of as-fabricated Ag-ZnO nestlike heterostructures is also largely enhanced by the strongly localized electromagnetic field of the Ag surface plasmon.
Synthesis of ZnO microstructures
Zinc foils (99.9%, Sigma-Aldrich Corporation, St. Louis, MO, USA) with a thickness of 0.25 mm were polished by sand paper then ultrasonically washed in absolute ethanol and dried in air before use. Electrochemical experiments with a CHI workstation were performed at room temperature in a two-electrode (Zn-Zn) system. For the production of nestlike ZnO, 0.01 mmol of sodium citrate and 14 μl of 30% H2O2 were added to 7 ml of deionized water under stirring at room temperature, adjusting the pH to 12. The two Zn foils (5 × 5 × 0.25 mm3) were put into the reaction solution in a parallel configuration with an interelectrode separation of 1 cm to apply a fixed electric potential of 3 V between the two Zn electrodes by using the electrochemical analyzer for the electrochemical deposition of ZnO nanostructures at room temperature. After being electrodeposited for 1 min, a whitish gray film was generated on the surface of Zn cathode. The Zn cathode with the deposited products was washed with distilled water for several times, dried at room temperature, and examined in terms of their structural, chemical, and optical properties. The ZnO petals, flowers, clews, and microspheres were fabricated by varying the molar quantity of sodium citrate and the deposition time synchronously, while keeping the other experimental conditions identical.
Preparation of Ag/ZnO heterostructures
A conventional cell with a three-electrode configuration was used throughout this work. The Zn cathode with the deposited nestlike ZnO structures was employed as the working electrode. A Pt wire served as the counter electrode, and the Ag/AgCl electrode was used as the reference electrode. The working electrode was biased at −0.6 V in 0.001 M AgNO3 solution for 1 min. Then the Ag clusters which were conglomerated by Ag nanoparticles were held in the center of ZnO nestlike structures on the surface of Zn cathode.
The as-prepared multiform ZnO microstructures or nanostructures and Ag/ZnO heterostructures on Zn foils were directly subjected to characterizations by the Hitachi S4800 scanning electron microscope (SEM; Hitachi High-Technologies Corporation, Tokyo, Japan) and the JEOL 2010F transmission electron microscope (TEM; JEOL Ltd., Tokyo, Japan) with high-resolution TEM imaging and energy dispersive X-ray. The samples used for TEM measurement were prepared by dispersing some products scraped from the Zn cathode in ethanol, then placing a drop of the solution onto a copper grid and letting the ethanol evaporate slowly in air. X-ray powder diffraction (XRD) measurement was performed on a Shimadzu XRD-6000 (Shimadzu Co. Ltd., Beijing, China) using Cu Kα radiation (1.5406 À) of 40 kV and 20 mA. Photoluminescence spectra were measured at room temperature using a Xe laser as an excitation source with a LS50 steady-state fluorescence spectrometer (Shimadzu, RF-5301PC). The resonant Raman spectra were performed using a Jobin Yvon LabRAM HR 800 UV micro-Raman spectrophotometer (Horiba Instruments, Kyoto, Japan) at room temperature. The 325-nm line of the He-Ne laser served as excitation light source.
Results and discussion
A serials of experiments showed that the existence of citrate ions played a key role in the formation of the ZnO complex microstructures. For the control experiment in the absence of citrate as we previously reported, the products were mainly nanoflowers which were composed of nanorods. When citrate was introduced in the solution, the period for the formation of ZnO nuclei (induction and latent periods in the crystal growth process) was remarkably prolonged, which suggested that citrate could slow down the nucleation and subsequent crystal growth of ZnO. On the basis of the previous analysis, we proposed a reasonable mechanism for the formation of ZnO structures. It is believed that sodium citrate is extensively used as the stabilizer and structure-directing agent because of its excellent adsorption ability[28, 29]. The additive citrate can form strong complexes [Zn(C6H5O7)4]10− with Zn2+ and owing to the stability of [Zn(C6H5O7)4]10− which is larger than [Zn(OH)4]2− in the present situation, there exists a large quantity of [Zn(C6H5O7)4]10− with negative charge and a small quantity of [Zn(OH) 4]2− in the precursor solution. It has been previously reported that citrate anions have been known to act as a capping agent of the (0001) surface of the ZnO crystal by adsorbing on the positive polar face of the (0001) surface[30, 31]. Thus, these [Zn(C6H5O7)4]10− ions are preferred to absorb positive polar plane (0001) surface through the -COO− and -OH functions, and decrease the growth rate of (0001) ZnO crystal surface by competing with growth units [Zn(OH)4]2−, which limits the anisotropy growth of ZnO at experimental pH value and leads to the formation of lamina-like ZnO nanostructures, as shown in Figure 1a,b. The stacking of the laminas is not completely ordered, and the laminas’ self-assembly at a later time is progressively more tilted leading to the formation of petal-like, flower-like, nestlike, clew-like, and spherical aggregates for adjusting the electrodeposition time and the concentration of sodium citrate.
It is worth mentioning that the morphologies of the products varied remarkably with the concentration of citrate. On the basis of the experiment results, we found that when the concentration of citrate was lower than 0.05 mmol (0.01 mmol in Figure 1e,f), the nascent square nanolaminas would self-assemble from bottom to top to form nestlike structures. On the other way around, when the concentration of citrate was higher than 0.05 mmol (0.1 mmol in Figure 1d,l,n), the nascent nanolaminas would self-assemble from center outwards to generate flower-like or microsphere structures. It has been reported that high citrate concentration (higher than 0.05 mmol) will attain [Zn(C6H5O7)4]10− supersaturated solution and Ostwald ripening controls structure growth by the diffusion of [Zn(C6H5O7)4]10− ions along the matrix-particle boundary tending to form spherical/hemispherical shapes from the center[32, 33]. In contrary to this, the lower citrate concentrations will not form [Zn(C6H5O7)4]10− supersaturated solution, which tend to self-assemble from bottom to top.
The room-temperature synthesis of ZnO with controllable morphologies, such as petal-, flower-, sphere-, nest-, and clew-shaped structures with adjusting the concentration of sodium citrate, and the deposition time by electrochemical route, has been realized for the first time. Only one or a few kinds of shapes within a narrow size range can be achieved from one of the previous methods . This result should facilitate the development of an effective and low-cost fabrication process for high-quality ZnO.
The product morphologies and sizes were highly controllable and modifiable and evolved from several micro-compressed laminas to flowerlike structures assembled by laminas and to the nestlike microstructure and microsphere in last.
The nest-shaped ZnO microstructures consisting of nanolaminas have been successfully synthesized by using sodium citrate. Our experimental results indicate that the ZnO nestlike structures can be used as a container not only to hold lamina-like ZnO, but also to be used to grow silver nanoparticles in the center of ZnO nests by electrochemical method.
The optical properties (PL and SERS) of the ZnO nests holding nanoparticles of Ag exhibit strong coupling between the metal and semiconductor.
scanning electron microscope
surface-enhanced Raman scattering
transmission electron microscope
X-ray powder diffraction
This work is supported by the Major Research Plan of NSFC (21233003), NSFC (21073018), Beijing Municipal Commission of Education, and the Fundamental Research Funds for the Central University.
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