Synthesis of semiconductive ZnO oxide and tailing of its physical properties by doping [1] and surface treatment [2] have been widely investigated for potential usefulness in various novel applications, including ultraviolet laser emission, [3,4] piezoelectricity, [5] photo-catalysis, [6] solar cells, [7,8] gas sensing [9] and biological applications [10]. It is well known that ZnO shows a direct wide bandgap (3.37 eV), strong exciton binding energy (60meV), and non-toxicity [5,7].

Many reports have focused on the fabrication of ZnO through various facile methods such as thermal evaporation, [11] sputtering, [12] chemical vapor deposition, [13] sol-gel processes, [14] and electrochemical methods [15–18]. Among these, electrochemical methods provide well-defined nanostructured ZnO with a controllable crystallinity without catalysts at low cost. One-dimensional ZnO nanorods on several substrates (ITO, SnO₂, Zn foils, semiconductors) have been successfully prepared by template-free electrochemical deposition [19,20].

It was recently reported that the physical properties of ZnO can be greatly enhanced by doping of Sn (or SnO₂) into nanostructured ZnO [21]. Modification of the grain size, crystalline orientation, [22] and vibrational structures [23] by doping could effectively influence the chemical, optical and electrical properties of ZnO [24].

Sheini et al. demonstrated that Sn-doped ZnO nanowires can be produced by co-electrochemical deposition, showing that field emission current on the ZnO nanostructures was enhanced by 2.0% doping of Sn into ZnO [25]. Ahmad et. al. showed that lithium storage capacity in battery applications could be improved when a SnO₂/ZnO film is employed instead of pure ZnO [26]. Improvement of energy conversion efficiency of dye-sensitized solar cells in Sn-doped ZnO nanoparticles has been reported by Ye et al. [27]. However, the simultaneous doping of Sn/SnO₂ into ZnO by electrochemical methods has not been thoroughly investigated.

This study was conducted to systematically investigate the fabrication of well-defined nanorods consisting of SnO₂-doped ZnO by simple electrochemical methods. One-step and two-step electrodeposition methods were introduced to control the composition and morphology of ZnO nanorods. Morphological changes depending on electrochemical conditions were interpreted in terms of mass- or charge- transfer controlled reactions.

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, Sigma-Aldrich), ammonium hydroxide solution (NH₄OH, Sigma-Aldrich) and tin chloride pentahydrate (SnCl₄·5H₂O, Sigma-Aldrich) were used for the preparation of electrolytes. Experiments were carried out in a three-electrode system consisting of FTO glass with an exposure area of 1 cm² as the working electrode, Pt wire as the counter electrode and a reference electrode (Ag/AgCl sat’d KCl). The distance between the working electrode and counter electrode was 3.5 cm. Before deposition, the FTO glass was cleaned by immersion in 30 vol% HCl aqueous solution for 30 min, followed by ultra-sonication in acetone, de-ionized water and ethanol for 10 min.

Metal oxides were deposited on FTO glass via potentiostatic electrodeposition at applied voltages ranging from −0.7 to − 1.1 V (vs. Ag/AgCl sat’d KCl). Solution Class 1 was established for depositing only ZnO on FTO, in which zinc nitrate hexahydrate solution (5, 10 and 20 mM) and ammonium hydroxide solution (complexing agent) were mixed together. Note that ammonium hydroxide solution as added until the solution became clear. The electrolyte added 0.1 M tin chloride (200–800 μl) from Solution Class 1 was classified as Solution Class 2 which was used for one-pot preparation of SnO₂-doped ZnO films. The deposition using Solution Class 1 and 2 was carried out for an hour.

For comparison, two-step electrochemical deposition was conducted. Briefly, ZnO films were deposited in a solution of 10 mM zinc nitrate hexahydrate with ammonium hydroxide (Solution Class 1) at − 1.1 V (vs. Ag/AgCl sat’d KCl) for an hour and then annealed at 450°C for 2 h 30 min in air. The subsequent electrodeposition on as-prepared oxide films was performed in Solution Class 2 for 30 min at voltages ranging from −0.7 to − 1.1 V. The deposition bath temperature and stirring speed were maintained as 70°C and 360 rpm, respectively. All experimental conditions are listed in Table 1.

The structural features of samples were characterized by field emission scanning electron microscopy (FE-SEM, JSM 6700F, JEOL). The element composition was confirmed via in-situ energy-disperse Xray spectroscope (EDX) of HR-TEM (JEM-2100F, JEOL). X-ray diffractometer (XRD, DMAX 2500, Rigaku Co.) was employed to check crystallinity of SnO₂-doped ZnO films. Cyclic voltammetry was conducted to clarify electrochemical behavior of Solution Class 1 and 2 during electrodeposition.

Fig. 1 shows the SEM images of ZnO, which were prepared by electrochemical deposition at −1.1 V at 70°C in different concentrations of zinc nitrate hexahydrate with/without ammonium hydroxide (Solution Class 1). Plain (Figs. 1(a) and (e)) or network (Fig. 1(c)) structures were formed without ammonium hydroxide, whereas needles on hills (Fig. 1(b)) or nanorods (Fig. 1(d) and (f)) were prepared with ammonium hydroxide. Nanorods with greater density and more perpendicular orientation were deposited onto FTO glass when a higher concentration of zinc nitrate hexahydrate was employed (see insets of Fig. 1(d) and (f)). Fig. 1 clearly demonstrated that ammonium hydroxide, which played a role as a reduction reagent, and a large amount of zinc nitrate hexahydrate, which acted as a ZnO source, were essential to production of 2-D nanorods. If reduction reagent was not included in the electrolyte, electrochemical deposition occurred in a charge-transfer-limited manner, which usually led to plain and dense surface structures [28].

Conversely, when the concentration of precursor (herein, zinc nitrate hexahydrate) was too low, electrochemical deposition occurred in a mass-transfer-limited manner, which usually led to dendritic structures [26]. Thus, the above results indicate that an intermediate state was required for fabrication of 2-D nanorods. The key electrochemical formation mechanism can be explained by the following reactions [29]:

Hydroxyl ions, produced in Reaction 1, can be further provided by the dissociation of reduction reaction through:

The basic environment which is induced by hydroxyl ions is necessary to make metal oxide on electrodeposition process. However, note that hydroxyl ions generally form the metal hydroxide precipitating and inhibiting electrodeposition reaction. The excess of ammonium ions then forms the metal chelates which are well-dissolved in aqueous solution so that finally NH₄OH can play an important role as complexing agent.

Fig. 2. shows the SEM images of SnO₂-doped ZnO via one-pot electrochemical deposition at −1.1 V at 70°C in different amounts of 0.1 M tin chloride pentahydrate (Solution Class 2). Interestingly, the nanorod structures disappeared when very small amounts of tin chloride pentahydrate were added. Like the results shown in Fig. 1(b), all structures showed a low density of needles on a hill-like structure consisting of tiny granules, indicating that potentials for electrochemical deposition should be adjusted for the nanorods preparation. As shown in the cross-sectional view of Fig. 2 (see inserted image), hollow-type hills were present, demonstrating that the structure was not prepared by layer-by-layer, and that dendritic depositions instead occurred via mass-transfer-limited deposition. Thus, positively-shifted electrochemical potentials were applied (Fig. 3). As expected, nanorods were observed at − 0.9 V. When less potential (−0.8 V) was applied, plain-like (or network) structures were prepared (Fig. 3(a)). These results clearly demonstrated that proper positive shift on deposition voltage was essential to control the morphology of nanorods. Zn and Sn were detected upon EDX of TEM analysis (Fig. 3(d)), indicating that co-deposition was effectively performed. The co-deposition of SnO₂ in ZnO and can be explained by the following reactions:

Fig. 4 shows the SEM images of SnO₂-doped ZnO, which were prepared by 2-step electrochemical deposition. The first step was conducted at −1.1 V and 70°C for 1 h in Solution Class 1 consisting of 10 mM zinc nitrate hexahydrate and ammonium hydroxide solution, while the second step was carried out in Solution Class 2 consisting of 10mM zinc nitrate hexahydrate, ammonium hydroxide solution and 0.1 M tin chloride pentahydrate. We found that all structures prepared in the first step were dissolved when the single tin chloride pentahydrate was used in the second step instead of Solution Class 2. This was likely due to substitutive dissolution of zinc by Sn occurring since the standard electrode potential of Zn (Zn²⁺ + 2e → Zn, −0.76 V) was more negative than that of Sn (Sn²⁺ + 2e → Sn, −0.13 V).

Overall, nanorods with greater density and larger size distribution were prepared during 2-step electrochemical deposition than that of single step electrochemical deposition, indicating that 2-step electrochemical deposition led to easy nucleation of nanorods.

These findings were confirmed by the fact that the optimum condition for preparation of distinct nanorods in 2-step electrochemical deposition was −0.8 V, which is less potential than that involved in the single step electrochemical deposition (−0.9 V). Fig. 4(e) demonstrated that SnO₂-doped ZnO film was formed.

Cyclic voltammetry (CV) showed different reduction potentials as a function of various electrolytes (Fig. 5). In the case of no reduction reagent (Fig. 5(a)), there was no distinct reduction peak. Conversely, reduction peaks of −0.99 V and −0.97 V were observed in Solution Class 1 and Solution Class 2, respectively (Figs. 5(b) and (c)), indicating that less reduction potential was required in Solution Class 1 than when electrochemical deposition was conducted without reduction reagent. Reduction potential was further reduced in Solution Class 2. These results are in good agreement with those of Fig. 1, – Fig. 4. However, the difference from other one-pot deposition studies was the use of complexing agent. As the complexing agent was added, the intensity of the oxidation of Zn and both oxidation and reduction of tin tended to be weak. Other studies conducted in an acidic environment showed both deposition and oxidation current density, but the oxidation peaks were drastically decreased with complexing agent. At this time, however, it provided identical result that the deposition contents of Zn exhibited an overwhelming than that of Sn, as previously suggested Fig. 3(d) and 4(e) [30,31].

Although the formation of oxides through CV could not be confirmed clearly, the XRD results showed that the deposited materials were oxides. Fig. 6 shows the XRD data, which confirms that ZnO was prepared by all electrochemical depositions. Because FTO contains SnO₂ with very small concentration, it was not clear whether SnO₂ was electrochemically deposited onto FTO based on the XRD data. However, we indirectly concluded that the SnO₂ phase was prepared by our methods based on the fact that metallic Sn was not detected by XRD, even though the Sn component was measured by EDX data of TEM (Fig. 3(d) and Fig. 4(d)). Concludingly, SnO₂-doped ZnO films with diverse morphologies were effectively prepared on FTO glass via one-pot and 2- step electrodeposition method at constant voltages. The simplified schemes were illustrated in Fig. 7.

We demonstrated that a level of reduction of Zn ions in terms of potential and composition of electrolytes could lead to the formation of different morphologies of ZnO with a doping of SnO₂. In particular, as reduction potential increased, morphological changes were observed in the order of plain/network structures, nanorods, and needles on hills. Thus, for the preparation of 2-D nanorods, an intermediate reduction potential was required. The addition of tin source in the electrolyte led to the requirement for less reduction potential, which was confirmed by the results of cyclic voltammetry. In addition, XRD data and EDX data of TEM clearly confirmed that the formed nanorod structure mainly consisted of ZnO containing a small amount of SnO₂.

Compared to single step electrochemical deposition, 2-step electrochemical deposition provided a much higher density of nanorods due to less reduction potential being required for the nucleation of nanorods during 2-step electrochemical deposition.