Parametric Study on Dimensional Control of ZnO Nanowalls and Nanowires by Electrochemical Deposition
© The Author(s) 2010
Received: 30 May 2010
Accepted: 13 July 2010
Published: 28 July 2010
Skip to main content
© The Author(s) 2010
Received: 30 May 2010
Accepted: 13 July 2010
Published: 28 July 2010
A simple electrochemical deposition technique is used to synthesize both two-dimensional (nanowall) and one-dimensional (nanowire) ZnO nanostructures on indium-tin-oxide-coated glass substrates at 70°C. By fine-tuning the deposition conditions, particularly the initial Zn(NO3)2·6H2O electrolyte concentration, the mean ledge thickness of the nanowalls (50–100 nm) and the average diameter of the nanowires (50–120 nm) can be easily varied. The KCl supporting electrolyte used in the electrodeposition also has a pronounced effect on the formation of the nanowalls, due to the adsorption of Cl− ions on the preferred (0001) growth plane of ZnO and thereby redirecting growth on the (10 0) and (2 0) planes. Furthermore, evolution from the formation of ZnO nanowalls to formation of nanowires is observed as the KCl concentration is reduced in the electrolyte. The crystalline properties and growth directions of the as-synthesized ZnO nanostructures are studied in details by glancing-incidence X-ray diffraction and transmission electron microscopy.
ZnO-nanostructured films are one of the promising wide-band-gap (3.37 eV) semiconducting materials with a wide range of potential applications, including UV lasers, light emitting diodes, nanogenerators, transistors, sensors, catalysts, electron emitters, and solar cells [1, 2]. The efficiency and performance of any optical and electrical nanodevices are directly determined by the properties of underlying nanostructures, which are in turn greatly dependent on the crystallographic orientation, size, shape, and morphology. The deposition techniques and their corresponding deposition parameters play an important role in controlling the morphology and physical properties of the nanostructures. Both physical deposition, including thermal evaporation, metalorganic chemical vapor deposition, pulsed laser deposition [3–5], and chemical synthetic routes, including hydrothermal, solvothermal, sol–gel, electrochemical, chemical bath deposition [6–15], have been successfully employed to prepare a wide variety of ZnO nanostructures. The physical deposition routes have the advantages of producing high-quality materials, but also the disadvantage of the need for high temperature and catalysts such as Sn , Au [17, 18], Co , and NiO . Although catalyst-free, single-step [21, 22], and multi-step [23–26] physical deposition methods have been achieved recently to synthesize ZnO nanostructures, the required high growth temperature necessitates the use of expensive substrates such as sapphire and silicon. Unlike the physical deposition routes, wet-chemistry or solution-based approach has attracted renewed attention due to their low temperature, catalyst-free growth processes, which lead to the successful deposition of ZnO on inexpensive substrates, including glass [13, 27–29] and plastics [30–33]. In addition, the wet-chemistry approach has strong potential in mass-scale production and deposition on large-area substrates. In the wet-chemistry approach, the concentrations and the components of the solutions play a major role in controlling the shape and size of ZnO nanostructures [34, 35].
Of the several wet-chemistry methods, including hydrothermal, sol–gel techniques, and chemical bath deposition, that have been used to synthesize ZnO nanostructures, electrodeposition represents a versatile technique for producing nanostructures with easily controllable morphologies. Unlike other wet-chemistry approaches, electrodeposition requires the use of a conducting substrate, and therefore, it can be easily adapted for selected-area deposition of nanostructures by creating conducting patterns on the substrate. Moreover, electrodeposition offers other advantages, including a lower deposition temperature, a relatively short growth time, and more environment-friendly chemicals [13, 29]. While electrodeposition of ZnO is normally carried out below 90°C, with a deposition time less than 2 h and in simple aqueous salt solutions , the hydrothermal method, for example, often requires a temperature of 60–200°C, a deposition time of 1 h to a few days [31, 36], and less environment-friendly chemicals such as methenamine or diethylenetriamine [30, 31, 36]. Peulon et al.  and Izaki et al.  have pioneered the use of electrodeposition for growing ZnO films and other nanostructures. More recently, several studies have been conducted on the electrodeposition of ZnO thin films and nanostructures on conducting glass substrates [13, 27–29]. However, there are only a limited number of detailed reports on the dimensional control of the length and diameter of ZnO nanowires [28, 38], and no study is available on controlling the ledge thickness of nanowalls and indeed other similar kinds of two-dimensional (2D) ZnO nanostructures (such as nanoplatelets, nanosheets, and nanodisks).
In the past two decades, most of the nanomaterials research have primarily focused on zero-dimensional materials, particularly nanoparticles and quantum dots, and one-dimensional (1D) materials, such as nanowires, nanorods, and nanotubes. To date, only a limited number of studies have been carried out on 2D nanostructures [17, 18, 39–43]. In the present work, we demonstrate that electrodeposition can be used effectively to control the shape and size of the 2D (nanowalls) and 1D (nanowires) ZnO nanostructures. By carefully changing the electrolyte concentration, it is possible to produce nanowalls and nanowires with controllable ledge thicknesses and diameters, respectively. Furthermore, it has been recently confirmed that adsorption of Cl− ions on the preferred (0001) growth plane of ZnO is the underlying mechanism that drives the formation of 2D nanostructures [44–46] and of nanowires with increasing diameters . In the present work, we also provide a detailed study on the effect of the supporting electrolyte (KCl) concentration on the structural transition from the formation of nanowalls to that of nanowires. The electrodeposited ZnO nanostructures are extensively characterized by scanning electron microscopy (SEM), glancing-incidence X-ray diffraction (GIXRD), and transmission electron microscopy (TEM).
All the electrodeposition experiments were carried out in a three-electrode glass cell immersed in a water bath held at 70°C. A CH Instruments 660A electrochemical workstation was used for the nanostructure growth by amperometry potentiostatically at −1.1 V with respect to a Ag/AgCl reference electrode. An indium-tin-oxide (ITO)-coated glass substrate (with a sheet resistance of 4–8 Ω) was used as the working electrode with an exposed area of 10 × 5 mm2, while a Pt spiral wire served as the counter electrode. For the growth of ZnO nanowalls, a higher Zn(NO3)2·6H2O electrolyte concentration regime (0.1–0.2 M) was used, whereas nanowire growth employed a lower-concentration regime (0.0005–0.001 M). A KCl solution with a fixed concentration of 0.1 M was added as the supporting electrolyte to increase the conductivity. The deposition time was varied to control the film thickness of ZnO nanowalls and the length of ZnO nanowires. In another set of experiments, the Zn(NO3)2·6H2O concentration was kept constant at 0.1 M, while the KCl concentration was varied from 0.1 to 0.001 M in order to study the effect of Cl− ions on the growth evolution of ZnO nanowalls to nanowires. This result was also compared with that obtained with a second supporting electrolyte KNO3 (i.e., without Cl− ions). It should be noted that the concentrations mentioned above and used throughout the manuscript are the initial concentrations of the electrolyte. The actual concentration could change differently with deposition conditions as the electrodeposition proceeds with time. The morphology of resulting ZnO nanodeposits on the ITO–glass substrates and their corresponding film thickness were characterized using a LEO FESEM 1530 field-emission SEM. The microstructural properties of these ZnO-nanostructured films were analyzed using a PANalytical X’Pert Pro MRD XRD in glancing-incidence mode and a JEOL 2010 TEM operated at 200 kV.
Figure 3 shows more detailed SEM images of nanowalls deposited on ITO–glass in a 0.1 M Zn(NO3)2·6H2O (with 0.1 M KCl) solution at 70°C with different deposition times. Figure 3a shows a magnified SEM image of nanowalls depicting their smooth ledge surfaces, while Fig. 3b illustrates the termination of nanowall growth on the side by the physical obstruction of other nanowalls. It should be noted that the formation of compartment-like structure formed by nanowalls, as shown in Fig. 3b, could occasionally occur on parts of the substrate. Figure 3c shows the early stage of nanowall growth with a deposition time of just 1 min. At the initial stage, ZnO is found to grow as hexagonal disks directly on the ITO surface. Although there appears to be a few nanoparticles on the ITO surface, growth of these particles does not extend over the entire surface prior to the nucleation of nanodisks. Different crystal planes of the hexagonal disks are assigned in Fig. 3c. The assignment of crystal planes is based on the previous reports [3, 9] and the TEM investigation of the present work (discussed later). Furthermore, layer-by-layer growth on the (0001) plane (with the new layers marked by arrows) of these hexagonal disks is also evident. With increasing deposition time, the growth on the (0001) plane is eventually stopped by adsorption of Cl− ions (discussed later) and the growth is redirected on the (10 0) and (2 0) planes, forming nanowalls. The growth evolution of nanowalls with increasing deposition time has been discussed in more details elsewhere . Recently, Yu et al. observed ultraviolet lasing characteristics from ZnO disks synthesized by thermal evaporation at a temperature of 850°C . The present work demonstrates that similar type of hexagonal ZnO disks can also be prepared at a considerably lower deposition temperature and with very short deposition time. Figure 3d shows a cross-sectional SEM view of nanowalls (obtained with 5-min deposition time), confirming the absence of a seeding layer on the ITO surface.
The change in the shape and size of ZnO nanostructures can be mainly attributed to the rate of reaction, which depends directly on the initial concentration of the electrolyte used in the electrodeposition. As the current process is a bottom-up approach, a greater number of Zn2+ ions are available at a higher initial concentration, and therefore, a larger number of ZnO nanoparticles can be produced at a faster rate. The self-arrangement of these nanoparticles leads to the formation of different nanostructures, which is directly related to the rate of ZnO nanoparticle formation. In the higher-initial-concentration regime (0.1–0.2 M), we observed the formation of nanowalls with growth occurring in the [10 0] direction, whereas in the lower-concentration regime (0.0005–0.001 M), we obtained nanowires with growth occurring in the  direction (discussed later). At an extremely low electrolyte concentration (0.00025 M), formation of ZnO nanoparticles is expected to be even slower. These nanoparticles are found to self-assemble into spherical hollow nanospheres . In addition to the kinetic effect, other factors such as the nature of the electrolyte also play an important role in generating different nanostructures. The role of Cl− ions in the nanowalls formation is discussed in the next section.
We have demonstrated a simple electrochemical deposition technique for growing 2D (nanowalls) and 1D (nanowires) ZnO nanostructures on ITO–glass substrates at 70°C in an aqueous Zn(NO3)2·6H2O (mixed with KCl) solution. By judiciously manipulating the deposition conditions, the mean ledge thickness of the nanowalls and the diameter of the nanowires can be controlled over the ranges of 50–100 and 50–120 nm, respectively. The KCl supporting electrolyte concentration can be used to control the morphology of ZnO nanostructures, i.e., 2D and 1D nanostructure growth. The Cl− ions have been found to be an effective capping agent for stopping the growth on the (0001) plane of ZnO and redirecting the growth on the (10 0) plane to produce the nanowalls. In the absence or at a lower concentration (<0.001 M) of KCl, ZnO growth occurs primarily on the (0001) plane, producing molehill-like 1D nanostructures. The crystalline properties and growth direction of the as-synthesized ZnO nanostructures are studied by GIXRD and TEM, which confirm the different growth directions of the nanowalls (10 0) and nanowires (0001).
This work was supported by the Natural Sciences and Engineering Research Council of Canada.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.