LetterOne-step electrodeposition of single-crystal ZnO nanotube arrays and their optical properties
Introduction
One-dimensional (1D) ZnO nanostructure has been one of the most popular topics in material, chemistry and physics due to its great potential applications in piezoelectrics [1], optoelectronics [2], [3], solar cells [4], [5], [6], field emission [7], [8] and sensing devices [9], [10], [11] in recent years. Though lots of the research efforts are still directed toward preparing randomly oriented 1D ZnO such as nanowire, nanorod and nanotube, it has been realized that the construction of nanostructures with well-ordered alignment and morphology, is critical for scientific and technological applications [12]. ZnO nanotube arrays (ZNTAs) which have special tubular structure, porosity and large specific surface area, would lead to enhanced and novel functionality in nano electronics and nano photoelectronics in the future. Till now, many technologies have been utilized in preparation of ZNTAs, including chemical vapor deposition (CVD) [13], sol–gel method [14], template method [15], hydrothermal method [16], [17] and electrochemical deposition method [18], [19]. Among these various approaches, CVD needs complicated equipment, high reaction temperature and particular substrates. Although the template method is a simple and common adopted way to synthesize well aligned ZnO nanostructures with uniform size and large aspect ratio, when the template is removed, the arrays would be polluted to some extent, and the dispersed nanorods or nanotubes would agglomerate together, destroying the intrinsic ordered structures. For the hydrothermal method, the substrate has to be dipped inside the requisite solution for days to form tubular ZnO nanostructures and the rate of repetition is too low [20], [21]. Compared with these preparation methods, electrochemical deposition method is widely adopted because of its low reaction temperature, the controllability of the thickness and shape, high deposition rate, low-cost equipment and eco-friendly feature, more importantly, the thin films deposited are crystalline structure, so there is no need to further calcinate it to be crystallized. In recent years, studies on electrodeposition of ZNTAs have been reported by several research groups [22], [23], [24], [25], [26]. Generally, two-step method [22], [23], [24], namely, electrodeposition of ZnO nanorods on conducting substrates firstly and then etching the top of nanorods by chemical or electrochemical approach, were used to synthesize ZNTAs. Therefore, how to fabricate the single-crystalline ZNTAs by one-step electrodeposition approach attracted more attention. Tang et al. [25] firstly synthesized ZNATs with larger diameters directly on the FTO substrate by one-step method and they proposed that the surface condition of substrate played a key role in the nanotube formation, while Hwang [26] put forward a viewpoint that the formation of the ZnO nanotubes was governed by the self-etching process with H+ generated during the growth process by direct electrodeposition method. So, it is still a challenge to prepare ZNATs with small diameter in large scale by one-step electrodeposition method and illuminate the nanotube formation mechanism.
In this paper, ZNTAs with average diameter less than 200 nm were synthesized on FTO substrate by using a one-step electrodeposition method. The growth mechanism of the ZNTAs was preliminarily discussed, and it is assumed that the critical pH value of electrolyte solution played an important role in determining the formation of the ZNTAs. Furthermore, photoluminescence properties of the ZNTAs were also studied.
Section snippets
Experimental
Electrodeposition of ZNTAs was carried out in a standard three electrodes system comprising FTO, Pt wire and the saturated calomel electrode (SCE) as working, counter and reference electrodes, respectively. The electrolyte was 0.005 mol dm−3 ZnCl2 aqueous solution bubbling with saturated oxygen, and 0.1 mol dm−3 KCl aqueous solution was introduced as the supporting solution insuring the good conductivity of the electrolyte. The electrolyte pH value was controlled at 3.70 adjusted by 0.01 mol dm−3
Results and discussion
In general, ZNTAs on conducting substrates were prepared by a two-step electrodeposition/chemical reaction approach. In the first step, ZnO nanorod arrays were usually obtained according to the following equations:
The total equation can be written as follows:In the second step, as an amphoteric oxide, the obtained ZnO nanorods whose top surface ((0 0 1) crystal face) was more unstable than other surfaces, could be
Conclusion
In summary, single-crystalline ZNTAs with average diameter of less than 200 nm were prepared on FTO substrate by a one-step electrodeposition approach. It is shown that the electrodeposition time played an important role in determining the growth density, average diameter, crystallinity and PL properties of as-prepared ZNTAs when the pH value of electrolyte was controlled at 3.7. The PL spectra of the formed ZNTAs exhibited the ultraviolet emission band centered at 383 nm resulted from the
Acknowledgements
The work is supported by the National Nature Science Foundation of China (No. 51272025 and 50872011), and 973 Program of China (No. 2014CB643401).
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