Surfactant-Assisted in situ Chemical Etching for the General Synthesis of ZnO Nanotubes Array
In this paper, a general low-cost and substrate-independent chemical etching strategy is demonstrated for the synthesis of ZnO nanotubes array. During the chemical etching, the nanotubes array inherits many features from the preformed nanorods array, such as the diameter, size distribution, and alignment. The preferential etching along c axis and the surfactant protection to the lateral surfaces are considered responsible for the formation of ZnO nanotubes. This surfactant-assisted chemical etching strategy is highly expected to advance the research in the ZnO nanotube-based technology.
KeywordsZnO Nanotubes array Chemical etching Surfactant Substrate independent
One-dimensional (1D) nanostructures are of particularly interest due to their unique properties different from that of bulk and nanoparticles [1–5]. It is well known that the hollow nanostructures have more prominent advantages (such as the enhanced confinement effect and larger area–volume ratios) over the other 1D nanostructures, such as nanowires and nanorods, and thus are promising candidates in catalysts, gas sensors, phosphors, and solar cells [6–9]. Zinc oxide (ZnO), with a direct band gap of 3.37 eV and a high exciton binding energy of 60 meV at room temperature, is an important technological semiconductor due to its distinguished optical, electrical, and piezoelectrical properties [10, 11]. Various kinds of 1D ZnO nanostructures array, such as nanorods, nanobelts, nanoneedles, and nanotips, have recently been synthesized by a good number of techniques [10, 11]. In contrast, only limited studies have been reported on the fabrication of ZnO nanotubes array, which is probably because the tubular form is generally available in layered materials such as carbon nanotubes. Current synthesis of ZnO nanotubes array is mainly based either on the chemical vapor deposition or the electrochemical approach [12–17], which generally requires economically prohibitive temperatures and complex processes, and is unfavorable for mass production. Additionally, despite the good controllability realized by the electrochemical method, the growth of ZnO nanotubes array can only be achieved on the conductive substrates, which greatly confines the wide applications of the resulted nanotubes in many other fields (such as gas sensing, which generally utilize nonconductive quartz substrates). Exploring strategy for a general low-cost and substrate-independent fabrication of ZnO nanotubes array is thus highly desirable.
Recently, a controllable synthesis of 1D novel nanostructures with different morphologies has been realized in our group by using various chemical etching methods [18–20], in which the preferential etching along c axis was found to be an important etching behavior for ZnO polar crystal. In this paper, by using the surfactant as the protection layer to the nonpolar surfaces of ZnO nanorod, we demonstrate a surfactant-assisted chemical etching approach to synthesize the ZnO nanotubes array. This chemical etching strategy is substrate independent and facile in manipulation, thus provides an avenue of a general and low-cost fabrication of the ZnO nanotubes array, which is highly expected to advance the ZnO nanotube-based technology.
Typically, ZnO nanocrystals were spin coated onto silicon wafer to form a seed layer according to the literature , and hydrothermal growth was carried out by suspending the silicon wafer upside down in an autoclave filled with an aqueous solution of 6 ml ammonia (25 wt%) and 80 ml zinc chloride solution (0.1 M) at 95°C for 70 min (growth step). After growth, the wafer was thoroughly rinsed with de-ionized water and suspended once again into a 90 ml solution containing ammonia (0.5 wt%) and cetyltrimethyl ammonium bromide (CTAB; 0.5 wt%) at room temperature for 3.5 h (etching step) and then washed with de-ionized water again. The as-synthesized samples were characterized by X-ray diffraction (XRD, Philips X’pert PRO), field emission scanning electron microscopy (FE-SEM, Sirion 200), and transmission electron microscopy (TEM, JEOL 2010, 200 kV). A He–Cd laser system with the excitation wavelength of 325 nm was used to investigate the photoluminescence properties of the final products.
Results and Discussion
It was found that the using of the surfactant in the etching step is much critical for the formation of the tubular nanostructure. If CTAB is not used in the etching process, the ZnO nanorods will become shorter and shorter (always with shallow pits at the top end) and finally be etched into the broken pieces (Figure S1). Too much CTAB used in the etching step (over 2 wt%) will lead to the formation of a thin surfactant coat on the surface of ZnO nanorods array, and thus block the subsequent etching (Figure S2). In addition, the amount of ammonia used in the etching step is also important for the controllable fabrication of ZnO nanotubes array. In the etching step, too much (larger than 5 wt%) or too little (lower than 0.1 wt%) amount of ammonia will result in the fast dissolution of ZnO nanorods array or no effect on the morphology change of ZnO nanorods.
It is easily understood that another possibility in producing ZnO nanotubes array is to combine the growth and etching into one step, i.e. after the growth of the ZnO nanorods array, directly decreasing the temperature from 95 to below 75°C, without taking out the silicon wafer. However, it was found that the resulted product was nanorods array with the similar feature obtained in the above-mentioned growth step, which might be due to the low NH3 concentration dissolved in the reaction solution.
The surfactant-assisted in situ chemical etching strategy presented in this study is general, and other substrates (polymers, ITO glasses, ceramic tubes/plates, etc.) also can be used to fabricate the ZnO nanotubes array via the same chemical etching process, which allows for the applications of ZnO nanotubes array in many fields, such as electronics, photocatalysis, and gas sensing. Additionally, the easy mediation in the diameter of the preformed nanorod array [23, 24] in the growth step makes it possible to produce the ZnO nanotubes with different diameters.
In summary, a general low-cost and substrate-independent synthesis of ZnO nanotubes array via a surfactant-assisted chemical etching strategy is demonstrated. The as-obtained ZnO nanotubes array shows an intensive ultraviolet photoluminescence emission, indicating the nanotubes array is promising for application in future nanoscale optoelectronic devices. The preferential etching along c axis and the surfactant protection to the lateral surfaces are considered responsible for the formation of ZnO nanotubes. This surfactant-assisted chemical etching strategy is highly expected to advance the research in the ZnO nanotube-based technology.
This work was supported by the National Natural Science Foundation of China (No: 10904145 and No: 10704074), and the Anhui Natural Science Foundation (No: 090414188).
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