## 4 Results and Discussion
### Triboelectric Charging of PES Films Doped with ANA  
Figure 5 shows the relationship between \(V_{\text{sat}}\) and the humidity for the PES films prepared using different ANA doping levels. At all the tested doping levels, \(V_{\text{sat}}\) shifted negatively with increasing humidity according to the CWP model, as was the case for the RUT-doped samples. However, in contrast to the behavior observed with RUT, the humidity dependence increased with the increasing doping level. To clearly demonstrate this, the relationship between the humidity dependence and the TiO\({}_{2}\) doping level is illustrated in Figure 6. This relationship clearly shows the opposite trends of the effect of the RUT \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Doping Level (wt\%)**} & \multicolumn{4}{c}{**95\% CI**} \\ \cline{2-5}  & **RUT** & **ANA** & **AMO** & **P25** \\ \hline
0 & 0.50 & 0.50 & 0.50 & 0.50 \\
1 & 0.30 & 0.48 & 0.36 & 0.32 \\
5 & 0.30 & 0.44 & 0.44 & 0.30 \\
10 & 0.12 & 0.42 & 0.36 & 0.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Values of 95% CI for PES films doped with 0, 1, 5, and 10 wt% TiO\({}_{2}\). Figure 4: Plots of \(V_{\text{sat}}\) versus relative humidity for PES films doped with 1 wt% (**a**), 5 wt% (**b**), and 10 wt% (**c**) RUT nanoparticles. and ANA doping levels on the humidity dependence. To consider this contrasting doping level dependence, here we will only consider the major crystal facets of RUT and ANA. TiO\({}_{2}\) is often used in aqueous environments, and, thus, there are numerous examples of studies on the water-TiO\({}_{2}\) interactions (molecular adsorption or dissociative adsorption of water molecules) on each facet of the TiO\({}_{2}\) surface [14; 15]. As the TiO\({}_{2}\) XRD patterns in the Supplementary Materials (Figure S2) show, the major facets of RUT and ANA are the (110) and (101) surfaces, respectively. With respect to the water adsorption states on the former facet, dissociative adsorption [38; 39; 40] and a mixed dissociated-molecular configuration [41] have been considered; however, the possibility of molecular adsorption [41; 42] cannot be ruled out. It has also been reported that dissociative adsorption [43; 44; 45] occurs on the major surface defect of RUT (110). Because of the high active surface area, more oxygen defects are expected to form on the surface and quasi-surface of nanostructured TiO\({}_{2}\) compared to its bulk [46]. Moreover, the formation of oxygen vacancies requires less energy in the TiO\({}_{2}\) nanocrystals than in bulk TiO\({}_{2}\)[47]. Based on these previous reports, it is likely that dissociative adsorption of water occurs on the RUT (110) surface of the RUT-doped films in this study, resulting in a pair of hydroxyl species. This would reduce the amount of molecular water, which is the source of positively charged water, on the film surface, thereby resulting in a decrease in humidity dependence. On the other hand, molecular adsorption is generally favored on the ANA (101) surface, although dissociative adsorption is reported to be possible on the oxygen-defective ANA (101) surface [37; 48; 49; 50]. This molecularly adsorbed water may contribute to the increased production of positively charged water by friction, and, therefore, a trend of increasing humidity dependence with increasing doping level is observed, as shown in Figure 6.  
The 95% CI values in Figures 4 and 5 (Table 1) are smaller than those in Figure 3b. Such a decrease in the data variation associated with TiO\({}_{2}\) doping is likely related to charge stabilization. These experimental results can be well explained when assuming that the electrons transferred from the iron powder to the film surface during friction are trapped and stabilized by the TiO\({}_{2}\) nanoparticles, thereby reducing the variation caused by charge back flow during the separation of the film from the iron powder. Potential candidates for such trapping sites include surface defect sites [51; 52; 53; 54].