Electrodeposition of SnO₂-doped ZnO Films onto FTO Glass

Abstract

Well aligned $SnO_2$-doped ZnO nanorods were prepared by single step or 2-step electrochemical depositions in a mixture solution of zinc nitrate hexahydrate, ammonium hydroxide solution and 0.1 M tin chloride pentahydrate. The morphologies of electrochemically deposited $SnO_2$-doped ZnO were transformed from plain (or network) structures at low reduction potential to needles on hills at high reduction potential. Well aligned ZnO was prepared at intermediate potential ranges. Reduction reagent and a high concentration of Zn precursor were required to fabricate $SnO_2$ doped ZnO nanorods. When compared to results obtained by single step electrochemical deposition, 2-step electrochemical deposition produced a much higher density of nanorods, which was ascribed to less potential being required for nucleation of nanorods by the second-step electrochemical deposition because the surface was activated in the first-step. Mechanisms of $SnO_2$ doped ZnO nanorods prepared at single step or 2-step was described in terms of applied potential ranges and mass-/charge- limited transfer.

Fig. 1. FE-SEM images of ZnO films prepared by electrodeposition at -1.1 V at 70°C in different concentrations of zinc nitrate hexahydrate with/without ammonium hydroxide: (a) sample number 1, (b) sample number 2, (c) sample number 3, (d) sample number 4, (e) sample number 13 and (f) sample number 14 listed in Table 1. The inserted images are the cross-sectional images.

Fig. 2. FE-SEM images of SnO₂-doped ZnO films prepared by electrochemical deposition at -1.1 V at 70°C in 10 mM zinc nitrate hexahydrate containing ammonium hydroxide with the addition of different amounts of 0.1 M tin chloride pentahydrate: (a) 200 μl (sample number 5), (b) 400 μl (sample number 10), (c) 600 μl (sample number 11) and (d) 800 μl (sample number 12). The inserted images are the cross-sectional images.

Fig. 3. FE-SEM images of SnO₂-doped ZnO films prepared by electrochemical deposition at 70°C in 10 mM zinc nitrate hexahydrate containing 800 μl of 0.1 M tin chloride pentahydrate at different potentials: (a) -0.8 V (sample number 7), (b) -0.9 V (sample number 8), and (c) -1.0 V (sample number 9). EDX analysis of Fig. 3 (b) by TEM is shown in Fig. 3 (d).

Fig. 4. FE-SEM images of SnO₂ doped ZnO, which were prepared by 2-step electrochemical deposition. The first step was conducted at -1.1 V at 70°C for 1 h in Solution Class 1 consisting of 10 mM zinc nitrate hexahydrate and ammonium hydroxide solution. 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 at different potentials: (a) -0.8 V (sample number 17), (b) -0.9 V (sample number18), (c) -1.0 V (sample number 19) and (d) -1.1 V (sample number 20). EDX element mapping data of Fig.4 (a) was shown in Fig. 4 (e). The inserted images are the cross-sectional images.

Fig. 5. Cyclic voltammogram in the range of -1.2 – 0 V (vs. Ag/AgCl sat’d KCl) with a scan rate of 1 mV/s in solution of (a) 10 mM zinc nitrate hexahydrate, (b) 10 mM zinc nitrate hexahydrate + 9 ml ammonium hydroxide and (c) 10 mM zinc nitrate hexahydrate + 9 ml ammonium hydroxide + 400 μl of 0.1 M tin chloride pentahydrate.

Fig. 6. XRD patterns of FTO glass substrate and electrochemically deposited films.

Fig. 7. Schematic diagram of morphological changes in terms of preparation method.

Table 1. Experimental conditions for electrochemical deposition of samples. Solution Class 1 consists with zinc nitrate and ammonium hydroxide. Solution Class 2 is the electrolyte in which tin chloride pentahydrate was added from Solution Class 1.