## 3 Results and Discussion  
### Electrochemical Reduction Behavior of ZnTe from an Aqueous Solution  
Figure 1a shows a cathodic polarization curve during the electrochemical growth of zinc telluride from an aqueous solvent containing Zn\({}^{2+}\) and HTeO\({}_{2}\)\({}^{+}\) ions. The polarization curve was obtained by plotting the cathode potentials, which were determined during the galvanostatic electrodeposition of 30 s each (total of 19 points). Based on the Nernst equation, the equilibrium potential of \(\text{E}_{\text{F}}\text{{}^{eq}}\) (HTeO\({}_{2}\)\({}^{+}\) + 3H\({}^{+}\) + 4e\({}^{-}\)\(\rightarrow\) Te + 2H\({}_{2}\)O) and \(\text{E}_{\text{H}}\text{{}^{eq}}\) (2H\({}^{+}\) + 2e\({}^{-}\)\(\rightarrow\) H\({}_{2}\)) was calculated to be approximately +0.33 V and \(-\)0.44 V versus Ag/AgCl, respectively, while that of \(\text{E}_{\text{Zn}}\text{{}^{eq}}\) (Zn\({}^{2+}\) + 2e\({}^{-}\)\(\rightarrow\) Zn) was estimated to be approximately -0.99 V. As shown in Figure 1a, the cathodic current began to rise at the potential of approximately \(-\)0.4 V. [PERSON] et al. revealed that the electrochemical reduction current of Te from an aqueous solvent containing HTeO\({}_{2}\)\({}^{+}\) ions emerged at approximately \(-\)0.2 V vs. Ag/AgCl [24]. In the present study, H\({}_{2}\)Cit\({}^{-}\) (or Hcit\({}^{2-}\)) ions were included in the bath to act as a ligand for HTeO\({}_{2}\)\({}^{+}\) ions. Hence, HTeO\({}_{2}\)\({}^{+}\) ions existed as complex ions with H\({}_{2}\)Cit\({}^{-}\) (or Hcit\({}^{2-}\)) ions. In our previous report, we revealed that Cit\({}^{3-}\) ions worked as a complex agent for MoO\({}_{4}\)\({}^{2-}\) ions and WO\({}_{4}\)\({}^{2-}\) ions in the synthesis of Ni-Mo alloys [25] and Ni-W alloys [26], using an induced co-deposition technique. Recently, we also found that H\({}_{2}\)Cit\({}^{-}\) (or Hcit\({}^{2-}\)) ions can act as a polarizer for the electrodeposition of metals [27]. Therefore, the electrochemical deposition potential of Te from the Te complex ions seemed to appear in a less noble (electrochemical) negative direction) region than that of Te from HTeO\({}_{2}\)\({}^{+}\). Hence, the cathodic current, which emerged at around \(-\)0.40 V, seemed to be induced by the deposition current of Te from the Te complex ions. As shown in Figure 1a, with an increase in the cathodic current density up to approximately 20 A/m\({}^{2}\), the cathode potential shifted down to approximately \(-\)0.8 V because the Te complex ions and H\({}^{+}\) ions can reach the diffusion limitation. In the cathode potential region, the pH near the cathode surface will increase up to approximately 6 and Zn\({}^{2+}\) ions  
Figure 1: Cathodic polarization curve for the electrodeposition of ZnTe from an aqueous solution containing ZnSO\({}_{4}\) -7H\({}_{2}\)O (0.1 M), TeO\({}_{2}\) (0.002 M), sodium sulfate (0.5 M), citric acid (0.05 M), and sodium citrate (0.05 M) (**a**). The solution pH and temperature were set to ca. 4.0 and 313 K, respectively. The effect of the cathode potential on Zn content in the electrodeposits was also shown in Figure 1 (**b**). will form Zn(Cit)2\({}^{4-}\) ions on the cathode surface, as shown in Equation (1) [28]. Consequently, the electrochemical growth of zinc telluride starts via the formation of Zn(Cit)2\({}^{4-}\) ions, according to Equation (2):  
\[\mathrm{Zn^{2+}+2 Cit^{3-}\rightleftharpoons Zn(Cit)_{2}^{4-}} \tag{1}\]  
\[\mathrm{Zn(Cit)_{2}^{4-}+Te+2e^{-}\to ZnTe+2 Cit^{3-}} \tag{2}\]  
With the shifting of the cathode potential down to less than \(-0.9\) V, the cathodic current density increased, again, more than \(20\) A/m\({}^{2}\) because the metallic zinc deposition proceeded, according to Equation (3):  
\[\mathrm{Zn(Cit)_{2}^{4-}+2e^{-}\to Zn+2 Cit^{3-}} \tag{3}\]  
Moreover, with an increase in the cathodic current density of more than \(100\) A/m\({}^{2}\), the cathode potential shifted down to less than \(-1.5\) V because Zn(OH)2 formed on the electrode.