Nanostructured ZnO films in forms of rod, plate and flower: Electrodeposition mechanisms and characterization
Graphical abstract
Introduction
Most of the integrated nanodevice applications employing ZnO involve specifically designed nanostructures with their shape, size, orientation etc. For example, nanowire and nanorod structured ZnO films offer remarkably large surface area, which is very important for light harvesting efficiency in solar cell applications [1], [2], [3], [4], [5], [6], [7]. On the other hand, nanoplate structured ZnO films are interest of applications like sensors, photocatalytic nanoreactors and nanocontainers etc. Among all the other methods (molecular beam epitaxy [8], chemical vapor deposition [9], hydrothermal synthesis [10], sprays pyrolysis [11], pulsed laser deposition [12] etc.), electrochemical deposition is the most efficient route for tuning the morphology [13], [14], [15], [16], [17], [18], [19]. Since the deposition kinetics could be controlled precisely with adjusting the electrolyte solution composition and concentration, as well as the employed electrochemical deposition parameters [14], [20], [21], [22].
It has been well recognized that it is highly important to obtain well-defined distinct nanostructures (rod, plate etc.) with desired alignment, order, homogenous distribution and reproducibility. For this purpose, a seed layer (which is also ZnO) may be prepared on the surface, prior to ZnO film production. The seed layer could be performed via chemical or electrochemical routes, and it is expected to govern the uniformity and ordering of nanostructures.
In electrochemical synthesis of ZnO, deposition kinetics is controlled with OH− generation and Zn2+ transportation rate from bulk solution to substrate surface. Actually, the precursor (generally Zn(NO3)2) concentration is crucial for both events, as well as employed electrochemical synthesis parameters like deposition potential etc. Moreover, Tena-Zaere et al. [23] have demonstrated the critical role of the ratio between the electrochemical OH− generation rate and Zn2+ concentration gradient nearby the electrode surface.
Additionally, the capping effect of various ions (present in synthesis bath) has been reported frequently, regarding their impact on preferential growth dimensions of electrodeposited ZnO nanostructures. Moreover, Pradhan and Leung [24], [25] have reported the electrosynthesis of 2D nanostructured (wall like) ZnO films. In this study, they explained the mechanism with the capping effect of chloride ions on polar (002) surface, under fast ZnO deposition conditions (0.1 M Zn(NO3)2 + 0.1 M KCl and −1.1 V deposition potential). They have also reported that large fraction of these nanowalls tends to form local groups and decreasing the Zn(NO3)2 concentration (from 0.1 to 0.05 M) leads to formation of large quantity of stacked ZnO nanodisks.
In present work, it was emphasized that optimization between precipitation rate and capping effect could be used for fabrication of well-defined ZnO nanoplates as highly ordered and vertically aligned distinct constructions, with high reproducibility. For this purpose, we have slowed down the precipitation rate by reducing both precursor concentration and OH− generation rate (i.e. nitrate reduction, electrochemically). In the mean time we have increased chloride concentration for favoring the capping effect. Additionally, we have also investigated the mechanism of shifting the morphology from rod/plate to flower like structure of ZnO, for better understanding the reproducibility. For this reason, the influence of various supporting electrolytes (sodium/ammonium salts of acetate) has been investigated, for interpretation of OH− concentration nearby the surface.
Section snippets
Experimental
The electrochemical deposition of ZnO nanostructures was performed in a conventional three-electrode system. ITO coated glass substrate (surface resistivity 8–12 Ω/sqr) was used as a working electrode, where a platinum sheet was the counter and Ag/AgCl (3 M KCl) as the reference. Prior to deposition process, the ITO coated glass substrates were ultrasonically cleaned with 1 M (mol/L) NaOH solution, ethanol and distilled water for 3 min, respectively.
The aqueous deposition bath solution included 5 mM
Electrochemical deposition of ZnO nanostructures
In this present study, the ZnO deposition was carried out potentiostatically (−1.0 V) and chronoamperometric plots were recorded and given in Fig. 1. For the purpose of electrocrystallization, Zn(NO3)2 precursor and various supporting electrolytes (KCl, CH3COONH4 and CH3COONa) were studied. Basically, the three stages of deposition are electrochemical reduction of nitrate species to generate OH−, chemical precipitation of Zn2+ ions with OH− and dehydration of Zn(OH)2. Therefore the growth
Conclusions
The mechanisms for the growth of different nanostructures have been clarified with FE-SEM studies realized for various deposition periods, also, taking into account the electrolyte composition and electrode/solution interface structure. The precipitation rate of Zn(OH)2 was slowed down by decreasing the deposition potential (i.e. OH− generation rate) and Zn2+ diffusion rate from bulk solution to electrode surface. In the meantime, we have employed high chloride concentration for enhancing the
Acknowledgements
This research was supported by the Çukurova University Scientific Research Projects (BAP) Coordination Unit. Authors thank to BAP for financial support. We are also greatly thankful to the Scientific and Technological Research Council of Turkey (TUBITAK- BİDEB) 2211-National Ph.D. Fellowship Programme.
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