## 4 Discussion
The photographs for powder revealed that both Fe\({}^{2+}\) and Fe\({}^{3+}\) co-exist in the synthesized GR samples (Figure 3). As the oxidation of Fe\({}^{2+}\) to Fe\({}^{3+}\) is spontaneous in terms of thermodynamics, goethite (Fe(III)OOH) impurity was often obtained during GR synthesis. The greenish color for both samples indicated that oxidation of Fe\({}^{2+}\) was efficiently prohibited and the GR phase was obtained well. This result corresponded well to the XRD pattern (Figure 1) with only GR (hydrotalcite) peaks observed. From XAS spectroscopy, we could confirm that the local structure of GR was not significantly affected by the type of intercalated molecules. The XANES spectra (Figure 4a) of both GRs showed same main edge (1s \(\rightarrow\) 4p transition) absorption [27], and thus the average oxidation state of Fe was almost similar to each other. There existed a pre-edge for both GRs, implying the potential Jahn-Teller distortion around Fe. Those kinds of pre-edge could be interpreted as symmetry that allowed 1s \(\rightarrow\) 3d electron transition [18]. Although the Fe(OH)\({}_{6}\) in GR is considered to have octahedral geometry, the real symmetry is rather assigned to D\({}_{3\text{d}}\) due to the packing of octahedrons [28]. In terms of the edge positions (pre-edge and main edge) as well as the shape of the white line, both GRs would have same local symmetry around Fe. The R-space EXAFS patterns were also similar for both GR-SO\({}_{4}\) and GR-PEDOT. This result indicated that the inter-atomic distances of both Fe-O and Fe-Fe were not affected by different interlayer molecules. It is, therefore, concluded that both GRs had the same structure of layer framework and different interlayer and/or surface chemistry. The difference in interlayer anions is thought to influence the oxidation of Fe\({}^{2+}\) in GR. The partial O\({}_{2}\) pressure gradually decreased for both GR-SO\({}_{4}\) and GR-PEDOT until 30 min. After then, the pattern became different; GR-PEDOT kept gradual decreasing while GR-SO\({}_{4}\) decreased O\({}_{2}\) partial pressure abruptly. At time point 60 min, the difference in O\({}_{2}\) partial pressure between GR-SO\({}_{4}\) and the GR-PEDOT treated group was \(\sim\)30%. This result suggested that the oxidation of GR was retarded in the presence of the conducting polymer, PEDOT:PSS. Applying a time vs. O\({}_{2}\) pressure equation, \(p=99.4-0.725\text{ln}(t-6.93)\) (\(p\): partial pressure of O\({}_{2}\), t: time in min), we could extrapolate that O\({}_{2}\) scavenging would continuously occur by GR-PEDOT for a prolonged time. In general, the oxidation process of Fe\({}^{2+}\) (\(-0.506\) V vs. saturated calomel electrode (SCE)) is thermodynamically favored combined with the reduction of oxygen (0.57 V vs. SCE) so that the green rust could be easily transformed to goethite (Fe(III)OOH) under ambient air as we reported in the previous literature [22]. However, the oxidation potential of PEDOT:PSS is relatively higher than that of Fe\({}^{2+}\)[29; 30], and thus the oxidation of PEDOT by O\({}_{2}\) is less feasible than oxidation of Fe\({}^{2+}\). The action of PEDOT on the GR surface could be (i) protecting GR from oxidation, and (ii) retarding oxidation by atmospheric O\({}_{2}\) through adjusted reduction potential. Therefore, it could be suggested that Figure 5a showed the oxygen-scavenging profile by the transformation of green rust to goethite without PEDOT:PSS but PEDOT:PSS could retard the oxidation as shown in Figure 5b.