## 4 Results and Discussion
### Triboelectric Charging of PES Films Doped with P25  
Figure 8 shows the relationship between \(V_{\rm sat}\) and the humidity for the PES films prepared at different P25 doping levels. The humidity dependence decreased significantly with the increasing P25 doping level. P25, which is composed of 78% anatase, 14% rutile, and 8% amorphous forms [59], displayed the highest ability to reduce the humidity dependence of all the TiO\({}_{2}\) samples used in our study (Figure 6) despite its high anatase fraction. Next, RUT, ANA, and AMO were mixed in the same ratio as that in P25 (P25 mix) to investigate the cause of this high ability, and P25 mix-doped PES films were then prepared. Figure 9 shows the relationship between the \(V_{\rm sat}\) and humidity for the PES films prepared at different P25 mix doping levels. The humidity dependence increased slightly with increasing doping level (Figure 6). This result indicates that the humidity-dependence increasing effect of the more abundant ANA exceeded the humidity-dependence decreasing effect of RUT and AMO and is in contrast with the large humidity-dependence decreasing effect of P25. Figure 7: Plots of \(V_{\rm sat}\) versus relative humidity for PES films doped with 1 wt% (**a**), 5 wt% (**b**), and 10 wt% (**c**) AMO nanoparticles. P25 is known to have extremely high photocatalytic activity [60]. In P25, some RUT-ANA heterojunction particles have been reported to be present along with RUT, ANA, and AMO nanoparticles [61]. Thus, it has been reported that one factor for the high photocatalytic activity is the suppression of carrier recombination due to heterojunction formation [62]. On the other hand, there is also a report that the catalytic activity of RUT and ANA in P25 is exceptionally high [60]. Hence, the reason for the high catalytic activity of P25 is not yet understood. In the above humidity-dependence decreasing effect of P25, the explanation based on heterojunction formation is not valid because no photocatalytic reaction was involved in the present study. It can be speculated that the water dissociative adsorption capability of RUT and/or AMO-comprising P25 is very high, i.e., the reduction in humidity dependence is extremely effective; however, its verification is a subject for future work. Figure 8: Plots of \(V_{\rm sat}\) versus relative humidity for PES films doped with 1 wt% (**a**), 5 wt% (**b**), and 10 wt% (**c**) P25 nanoparticles. For P25, small 95% CI values similar to those observed with RUT were obtained at all tested doping levels (Table 1). The similarity between the 95% CI values of Figures 4 and 8 suggests that the RUT in P25 is responsible for charge trapping. This may be due to the fact that the trap levels of RUT are deeper than those of ANA and AMO, as discussed in Section 4.3, and the charges are more stabilized. Overall, it was found that RUT and P25 were responsible for significantly reducing the humidity dependence and 95% CI. However, the area ratio of P25 on the topmost film surface, calculated using the TiO\({}_{2}\) and PES densities [63,64], approximated 3% even at a doping level of 10 wt%. It is unlikely that this small portion (3%) of P25 controls the humidity dependence and 95% CI of the entire film. Thus, we may assume that the P25 present in the subsurface of the film also plays a role in these effects. This would be a reasonable assumption considering that the glass transition temperature of PES is 67 \({}^{\circ}\)C [64] and the local temperature at the contact point between the iron powder and film exceeds 100 \({}^{\circ}\)C (see Section 2). To verify this assumption, a 1 \(\upmu\)m thick PES layer doped with 10 wt%  
Figure 9: Plots of \(V_{\rm sat}\) versus relative humidity for PES films doped with 1 wt% (**a**), 5 wt% (**b**), and 10 wt% (**c**) P25 mix nanoparticles. P25 was deposited on top of a 9 \(\upmu\)m thick PES layer to produce a layered film containing 1 wt% P25 as an average of the entire film. Its charging properties were then compared with those of a single-layer film doped with 1 wt% P25.