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Article

Triboelectric Charging Behaviors of Polyester Films Doped with Titanium Dioxide Nanoparticles of Various Crystal Structures

by
Yudai Teramoto
,
Keita Ando
,
Satoru Tsukada
and
Katsuyoshi Hoshino
*
Department of Materials Science, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(3), 1468; https://doi.org/10.3390/app13031468
Submission received: 5 January 2023 / Revised: 19 January 2023 / Accepted: 19 January 2023 / Published: 22 January 2023
(This article belongs to the Special Issue Feature Papers in Surface Sciences and Technology Section)
Browse Figures
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Abstract

:

Featured Application

Toner materials for electrophotography.

Abstract

It is empirically known that titanium dioxide nanoparticles stabilize the contact and frictional charge of the host polymers to which they are added. However, the mechanism for the stabilization process has not yet been elucidated. In this study, polyester films doped with titanium dioxide nanoparticles of different crystalline forms were triboelectrically charged and the effect of humidity on their charging characteristics was subsequently investigated to elucidate the charge stabilization mechanism. Our first finding was that the rutile-, rutile–anatase mixed crystal (P25)-, and amorphous-dominant-type titanium dioxide nanoparticles reduced the sensitivity of the films to humidity (humidity dependence), while the anatase-type titanium dioxide enhanced the humidity dependence. This difference in action was explained by associating it with the different water adsorption forms on the major crystalline surface of each titanium dioxide type. The second finding was that doping with titanium dioxide nanoparticles, particularly rutile and P25 nanoparticles, reduced fluctuations in the amount of tribocharges of the polyester film. This crystalline-form-dependent difference in action was considered to be based on the depth of the electron traps involved in each titanium dioxide type. The above two findings have allowed us to propose the first mechanism of tribocharge stabilization by titanium dioxide.

1. Introduction

Titanium dioxide (TiO2) nanoparticles are multifunctional materials used in a wide range of applications, including white pigments [1], UV-shielding materials [2], photocatalysts [3,4,5], self-cleaning materials [6], dye-sensitized solar cells [7], electrochromic displays [8,9], electronic papers [10], and charge control agents for electrophotographic toners [11,12]. Thus, TiO2, along with SiO2, nanoparticles are among the most globally mass-produced nanomaterials [13]. TiO2 nanoparticles are mostly used under a moisture atmosphere, and, thus, the interactions between TiO2 and water play a crucial role in the above-mentioned applications. Consequently, these interactions have been studied extensively, mainly through theoretical/computational methods [14,15]. In such studies, anatase- and rutile-type crystals have been the primary research focus and it has been shown that the interactions depend on the crystal form and face of TiO2.
On the other hand, titanium dioxide nanoparticles that are unmodified or surface-modified with organic substances have been externally added to toners used in electrophotography as charge control agents [11,12], which contribute toward reducing toner aggregation, flowability fluctuations, and charge fluctuations [16,17]. The reduction effects of the first two phenomena have been clearly attributed to surface modifiers. In contrast, few studies have reported on how TiO2 contributes to charge stabilization, despite this being a crucial research issue. Hence, if the mechanism of this TiO2-induced charge stabilization can be clarified, it will contribute not only to toner design in the huge market for electrophotographic toner but also to our understanding of frictional charging phenomena for insulating and semiconducting materials [18,19,20], for which no consensus has been reached.
We have been conducting frictional charging experiments between insulating polymer films and iron powder beads (spherical iron particles with an average diameter of 80 μm) to investigate the effect of moisture (humidity) in the ambient atmosphere on the charging of polymer films [21,22,23,24,25,26,27]. These studies have shown that ambient moisture is actively involved in the charging behavior of the films and that moisture-induced charging has an equal or greater effect than the inherent charging of the film (proposal of the charged-water penetration (CWP) model; see Section 2 for more detail).
In this study, we hypothesized that the CWP model can be used to quantify the above-mentioned effect of TiO2 on toner charging and to better understand the relationship between the physical properties of TiO2 and the charging amount. Specifically, anatase, rutile, anatase–rutile mixed crystal, and amorphous TiO2 nanoparticles were dispersed in polyester films and their frictional electrification experiments were conducted in contact with iron particle powder. The results showed that the effects of humidity on the charging behavior of each film are markedly different from each other, and this can be explained by the difference in the water adsorption forms on the major crystal faces of TiO2. In addition, an unexpected synergistic charging effect was found when anatase–rutile mixed TiO2 was used, which is also reported herein along with the above results. The results of this study have led to the proposal of the first mechanism for the stabilization of tribocharges by titanium dioxide nanoparticles, which will contribute to the engineering and science of triboelectrification.

2. CWP Model

The details of the CWP model (Figure 1) have been described in detail in a previous paper [24]. Briefly, in this model, adsorbed water is present on the surface of the metal particle [28], and, during friction between the film and particle powder, this adsorbed water is also subject to friction. The adsorbed water is then assumed to dissociate into positively charged water and negatively charged species by gaining frictional energy (Step 1). A reasonable assumption is that the positively charged water contains H+ ions and the negatively charged species contain OH− ions; however, the identification of these species is a subject for future work. The positively charged water overcomes electrostatic attraction and penetrates the film with the lapse of time (Step 2). Mizuguchi et al. reported that during frictional charging of the toner and metal particle powder the local temperature at their contact point can reach or exceed 100 °C [29]. Such an increase in temperature increases the mobility of the polymer chains, which may draw the positively charged water into the film. The penetrating positively charged water then creates a film bulk charge that reduces the film Fermi level. On the other hand, the negatively charged species are assumed to remain on the film surface. Assuming that in the frictional charging of insulating polymers the charge transfer carrier is an electron and the electron transfer driving force is the difference between the metal particle and the film Fermi levels (electron transfer model in frictional electrification) [30], then the driving force increases more when positively charged water species penetrate into the film than when they do not. Electron transfer occurs from the metal particle powder to the film in such a way as to equalize the Fermi levels (Step 3), and the amount of electron transfer is greater when the charged water penetrates into the film. Because the amount of adsorbed water on the metal powder is expected to increase with increasing ambient humidity [28], the amount of positively charged water generated by friction also increases with increasing humidity. This induces further enhancement of the Fermi level difference and increases the amount of electron transfer from the metal particles. Thus, the negative charge of the film surface should increase with increasing humidity. However, when the humidity exceeds approximately 60%RH, the effect of water-induced charge leakage may prevail, and the amount of negative charge may decrease.

3. Materials and Methods

3.1. Tribocharging Apparatus

The in-house-made stainless steel apparatus used for the triboelectric charging experiments consisted of a cylindrical holder (~76 mm ø) to which the sample film was attached, a magnet roll (~16 mm ø) with a sleeve covered with iron powder (~80 µm in diameter, no resin coating), and a cam for adjusting the gap formed between the sample surface and sleeve (Figure 2a) [21,22,24,27]. Aluminum foil (50 µm thick, >99.30%, Takeuchi Metal Foil & Powder), which served as the substrate for the sample films, was fixed to the holder with adhesive tape and its edge was fitted into a narrow groove on the holder. The sample holder and magnet roll were grounded, and, thus, the substrate was also grounded. Tribocharging experiments were carried out by independently rotating the sample holder (15 rpm) and magnet roll (30 rpm). The apparatus was set in a closed glass chamber and the air inside was replaced by nitrogen with controlled humidity (Figure 2b). Fixed-humidity nitrogen gas was prepared by varying the amounts of wet nitrogen produced by passing distilled water and dry nitrogen produced by passing desiccants (calcium chloride and silica gel) through the mixture. The tribocharge amount was measured as the surface potential, Vs, using a potential probe (Model 555p-1, Trek Japan, Tokyo, japan). The relative humidity inside the chamber was monitored using a hygrometer (Model HMP134Y, VAISALA, Helsinki, Finland) at 20 ± 2 °C.

3.2. Materials

The host polymer used in this study was polyester (Vylon200, abbreviated as PES, Toyobo, Mn = 1.7 × 104). For titanium dioxide nanoparticles, rutile- (RUT, STR-100N; 16 nm; Sakai Chemical Industry: Osaka, Japan), anatase- (ANA, grade name SSP-M; average particle diameter 15 nm; Sakai Chemical Industry, Osaka, Japan), anatase–rutile mixed crystal- (P25; 21 nm; Nippon Aerosil, Tokyo, Japan), and amorphous-dominant (AMO, Idemitsu titania IT-S; 17 nm; Idemitsu Kosan, Tokyo, Japan [31])-type structures were used. Transmission electron microscopy (TEM) images and X-ray diffraction (XRD) patterns of the titanium dioxide nanoparticles are shown in Figures S1 and S2, respectively, in the Supplementary Materials. The AMO used in this study contained 10 wt% ANA, as determined using the differential scanning calorimetry (DSC) method [32,33,34].
Films with a thickness of 10 μm were prepared on aluminum foil (25 mm × 50 mm × 50 µm) by the commonly used spin-coating method (1400 rpm, 30 s). The coating solution for the PES film was prepared by dissolving 5.4 g of PES in a mixed solvent consisting of 9.0 mL toluene and 7.2 mL tetrahydrofuran (THF). For the preparation of the titanium-oxide-doped films, the coating solutions were prepared using the above-described method, except that the total amount of TiO2 and PES was adjusted to 5.4 g. The titanium dioxide nanoparticles were dispersed in the solutions using a mix rotor (VMR-5, AS ONE) for 3 h, followed by stirring using a magnetic stirrer for at least 12 h. The employed TiO2 doping levels were 1, 5, 7.5, and 10 wt%. After spin coating of the coating solutions, the resulting films were dried at 20 °C for 1 h, followed by drying in a dryer (DN64, Yamato) at 60 °C for 3 h. The aluminum foil samples were degreased by sonication in acetone for 30 min and then dried in the dryer at 60 °C.
Spherical iron particle powder (T80, Powdertech, Kashiwa, Japan, average particle size: 80 μm), which was not polymer-coated and reduced with hydrogen gas, was used as the counterpart to rub the films. The morphology of the iron particles was determined using scanning electron micrography as shown in the Supplementary Materials (Figure S3).

3.3. Preparation of a Layered Film

The layered film was prepared as follows: A solution consisting of 5.1 g PES, 9 mL toluene, and 7.2 mL THF was spin-coated (1400 rpm, 30 s) onto the aluminum foil and subsequently dried at 20 °C for 1 h. Next, the film was spin-coated with a coating solution consisting of 2.0 g PES, 0.222 g P25, 9 mL toluene, and 7.2 mL THF, followed by drying in the dryer at 60 °C for 3 h to obtain the layered film.

3.4. Preparation of Films for the Measurement of Water Content

A 50 mm × 50 mm × 10 μm film was prepared on a release paper by the method described in Section 3.2. The film was cut into two films of the same size, one of which was placed in a chamber with a nitrogen atmosphere of 40–45%RH and allowed to stand for 80 min. The film was then immediately taken out of the chamber (reference sample) and subjected to thermogravimetric analysis (TGA). The other film was set in the tribocharging apparatus installed in the chamber with a nitrogen atmosphere of 40–45%RH, allowed to stand for 20 min, and then frictionally charged with iron particle powder for 60 min (charged sample). Subsequently, the film sample was immediately removed from the chamber and subjected to TGA.

3.5. Measurements

Film thicknesses were measured using a surface profile measuring system (model Dektak 3030, Sloan, Santa Barbara, CA, USA). The electrical resistivity (surface electrical resistivity), ρ, and dielectric constant, ε, of the films were measured using a resistivity meter (Hiresta-GX MCP-T700, Mitsubishi Chemical Analytech, Yamato, Japan) equipped with a UR-SS ring probe (MCP-HTP15) and a quasistatic CV meter (model 595, Keithley, Cleveland, OH, USA). The amount of adsorbed water in the films was measured using a thermogravimetric analyzer (TGA-50, Shimadzu, Kyoto, Japan) at a heating rate of 5 K min−1 under nitrogen atmosphere. DSC analysis of the AMO powder was performed using a Shimadzu DSC-60 Plus calorimeter at a heating rate of 10 K min−1. Observations of the TiO2 nanoparticles and iron particles were made using a transmission electron microscope (H-7650, Hitachi, Tokyo, Japan) and a scanning electron microscope (ABT-32, Topcon, Tokyo, Japan), respectively.

4. Results and Discussion

4.1. Triboelectric Charging Behaviors of a PES Single Film

Figure 3a shows typical frictional charging behaviors of the PES single film under various humidity conditions (relationship between the film surface potential, Vs, and measurement time, t). The Vs showed typical behavior of increasing in the negative direction with increasing t and eventually becoming saturated. This behavior implies that the area of the contact point increases as the film is rubbed [35]. In addition, Vs or the charge amount of the film increased in the negative direction with increasing humidity. This can be explained using the CWP model described in Section 2. The value of Vs at 3600 s was defined as the saturation potential, Vsat, and a plot of Vsat versus humidity is shown in Figure 3b. In accordance with the CWP model, Vsat showed a tendency to shift negatively with increasing humidity. The straight line in the figure is a regression line calculated by the least-squares method using Origin 2022b software (OriginLab, Northampton, MA, USA), while the absolute value of its slope indicates the film sensitivity toward humidity. This slope will hereafter be referred to as the humidity dependence. Single regression analysis of the relationship between Vsat and the humidity afforded a value of 0.82 ± 0.25 V/%RH as the humidity dependence. The error range of 0.50 V/%RH was calculated as twice the standard error of the slope and corresponds to a 95% confidence interval (95% CI). The value of the 95% CI is a measure of the variability of the data relative to the regression line, whereby the larger the value, the greater the variability of the data is likely to be. Possible reasons for this variation in data can be the diversity of the surfaces (surface variability, variations due to contamination, roughness, sample history, fluctuation, etc.), as noted by Castle [36] and Grzybowski et al. [20], and the effect of charge back flow caused by electron tunneling or air breakdown during the separation of the two surfaces [20,37].

4.2. Triboelectric Charging of PES Films Doped with RUT

Figure 4 shows the relationship between Vsat and the relative humidity for the PES films prepared using different RUT doping levels. At all the tested doping levels, Vsat shifted negatively with increasing humidity, a behavior that follows the CWP model. In addition, at a constant relative humidity, the absolute value of Vsat decreased with the increasing RUT doping level. This was attributed to the increase in the relative permittivity, ε, of the film with increasing TiO2 doping level, as shown by its measured values in the Supplementary Materials (Table S1). Another possible factor for the decrease in the absolute value of Vsat is the decrease in the film electrical resistivity, ρ, due to TiO2 doping; however, as shown in the Supplementary Materials (Table S2), ρ was near-identical for all the tested films and was thus not considered a factor in this study. This may be attributed to the fact that continuous conductive channels of TiO2 nanoparticles are not formed at doping levels of 10 wt% or less. More important findings are that the 95% CI values in Figure 4 (0.12–0.30, see Table 1) are smaller than those in Figure 3b (0.50, corresponding to 0 wt% doping level in Table 1) and that the humidity dependence decreased as the RUT doping level increased. A discussion of these findings is provided in Section 4.3.

4.3. Triboelectric Charging of PES Films Doped with ANA

Figure 5 shows the relationship between Vsat and the humidity for the PES films prepared using different ANA doping levels. At all the tested doping levels, Vsat 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 TiO2 doping level is illustrated in Figure 6. This relationship clearly shows the opposite trends of the effect of the RUT 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. TiO2 is often used in aqueous environments, and, thus, there are numerous examples of studies on the water–TiO2 interactions (molecular adsorption or dissociative adsorption of water molecules) on each facet of the TiO2 surface [14,15]. As the TiO2 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 TiO2 compared to its bulk [46]. Moreover, the formation of oxygen vacancies requires less energy in the TiO2 nanocrystals than in bulk TiO2 [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 Figure 4 and Figure 5 (Table 1) are smaller than those in Figure 3b. Such a decrease in the data variation associated with TiO2 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 TiO2 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]. In addition, the variation in Figure 4 is smaller than that in Figure 5. This suggests that the depth of the trap on the RUT form is deeper than that on the ANA form [55] and the charge is more stabilized.

4.4. Triboelectric Charging of PES Films Doped with AMO

Figure 7 shows the relationship between Vsat and the humidity for the PES films prepared using different AMO doping levels. As was observed with RUT, the humidity dependence decreased with the increasing doping level. Few studies on the adsorption configuration of water on amorphous TiO2 have been reported in the literature [56]. In general, amorphous materials tend to comprise more catalytically active defect structures than crystalline materials do [57,58]; therefore, it is highly likely that water is dissociatively adsorbed at the defect sites. Consequently, AMO likely reduced the humidity dependence. In addition, as illustrated in Figure 6, the reduced humidity dependence despite the presence of anatase TiO2 implies that the decreasing effect of amorphous TiO2 exceeds the increasing effect of anatase TiO2. Although smaller 95% CI values were obtained for the AMO-doped PES films compared to that of the PES single film (Table 1), they were larger than those attained with RUT doping and similar to those obtained with ANA. This indicates that the charge stabilization achieved with AMO is comparable to that with ANA, allowing us to speculate that the depth of charge trapping in the amorphous part of AMO is comparable to that of ANA.

4.5. Triboelectric Charging of PES Films Doped with P25

Figure 8 shows the relationship between Vsat 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 TiO2 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 (P25mix) to investigate the cause of this high ability, and P25mix-doped PES films were then prepared. Figure 9 shows the relationship between the Vsat and humidity for the PES films prepared at different P25mix 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.
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.
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 Figure 4 and Figure 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 TiO2 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 °C [64] and the local temperature at the contact point between the iron powder and film exceeds 100 °C (see Section 2). To verify this assumption, a 1 μm thick PES layer doped with 10 wt% P25 was deposited on top of a 9 μm 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. The synthetic details of the layered film are provided in Section 3.3.
Figure 10 shows the relationship between Vsat and the humidity for the layered film (a) and single-layer film (b, reproduced from Figure 8a). Clearly, the humidity dependence of the former (−0.23) was significantly smaller than that of the latter (−0.85). This implies that P25 nanoparticles localized near the surface reduce humidity dependence more than those present throughout the film. Specifically, the P25 nanoparticles present in the subsurface layer contribute to the reduction. Although Figure 10a has few data points and cannot be simply compared with Figure 10b, its 95% CI value (0.14) is smaller than that of Figure 10b (0.30), again suggesting a contribution of the P25 nanoparticles in the subsurface. The position of the deepest P25 nanoparticles from the film surface, which can contribute to reducing the humidity dependence and data variability, will be determined by varying the film thicknesses in the future.

4.6. Measurements of the Water Contents in Films before and after Frictional Electrification

The results discussed in the previous sections indicate that the crystal form of the TiO2 nanoparticle has a significant effect on the humidity dependence for the charging of TiO2-doped PES films. Subsequent results revealed that doping with P25, RUT, and AMO nanoparticles reduced the humidity dependence of the film, while ANA doping increased it. To reconfirm this from an analytical perspective, we measured the amount of water in the films before and after frictional electrification, as described in Section 3.4.
Figure 11 shows the measured water contents in the reference and charged samples for the PES single film and TiO2-nanoparticle-doped films. The TiO2 doping level was 10 wt% and the humidity during the measurements was in the range of 40–45%RH. The water content was normalized per gram of film. The water content of the reference sample (6.90 mg g-film−1) of the PES single film was less than those of the TiO2-doped samples (14.8 ± 3.5 mg g-film−1). This is related to the water adsorbed by TiO2 described in Section 4.3 and Section 4.4 and may be attributed to the PES single film being relatively more hydrophobic than the TiO2-doped films. The water content of the charged sample of the PES single film was considerably higher than that of the corresponding reference sample. Based on the CWP model, the difference in water content between the two samples is estimated to be the amount of charged water produced by frictional electrification. A relatively large increase in the water content due to frictional charging was also observed for the ANA-doped film. Although not as pronounced as the PES single film and ANA-doped film, an increase in the water content due to frictional charging was also observed in the P25mix-doped film. These cases correspond to the TiO2 samples with large humidity dependences illustrated in Figure 6, indicating that the increase in the water content due to frictional electrification is greater for TiO2 samples where molecular adsorption of water is more likely to occur. On the other hand, in the case of the other TiO2 samples (RUT, AMO, and P25), the change in the water content due to frictional charging was small. This corresponds to the cases with small humidity dependence illustrated in Figure 6, indicating a relatively small or almost no increase in the water content of films doped with TiO2 where the dissociative adsorption of water is more likely to occur.

5. Conclusions

In this study, TiO2 nanoparticles of various crystal forms were doped into a polyester film and frictional charging experiments between the films and iron particle powder were subsequently conducted—measurements of the film surface potential versus time and the saturated surface potential versus relative humidity. The results revealed that TiO2-nanoparticle doping (1) changes the film sensitivity to ambient humidity (i.e., changes the humidity dependence) and (2) stabilizes the film charge (i.e., reduces the variation in the charge amount). With regard to Finding (1), the humidity dependence was reduced with rutile, rutile–anatase mixed crystal (P25), and amorphous-dominated TiO2 doping and increased with anatase TiO2. Finding (1) was successfully explained by combining our previously proposed CWP model with previous reports showing the dissociative adsorption and associative/molecular adsorption of water on TiO2.
Finding (2) may be explained by assuming that the electrons transferred from the iron powder to the film are trapped on the TiO2 nanoparticles during contact, whereby the trapping reduces the charge back flow from the film to the iron powder during separation. The variation was significantly reduced, especially when rutile and P25 TiO2 nanoparticles were used. This was interpreted, with reference to the previous reports, as the electron traps in the rutile and P25 TiO2 nanoparticles being deeper than those in anatase and amorphous TiO2, resulting in a more stabilized charge in the former case and more reduced charge back flow during separation.
The importance and novelty of this study lies in the fact that we were able to propose the first mechanism for the stabilization of tribocharges on polymer films based on Findings (1) and (2) above. TiO2 nanoparticles are used as an external additive for electrophotographic toners and are known to empirically reduce the variation in the amount of toner charge. The results of this study provide the academic background to the above rule of thumb and should provide useful guidelines for the doping amount and crystal form for future toner design. Moreover, this study significantly contributes to the industrial electrostatic field (electrophotographic toners [65], electrostatic hazard assessment [66], electret air filters [67], etc.), as it demonstrates that TiO2 doping influences the humidity dependence of the host polymers and that the selection of the doping amount and the crystal form of TiO2 is important for reducing this humidity dependence.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/app13031468/s1, Figure S1: TEM images of the TiO2 nanoparticles; Figure S2: X-ray diffraction patterns of the TiO2 nanoparticles; Figure S3: SEM image of an iron particle; Table S1: Relative permittivity of the TiO2-doped PES films; Table S2: Electrical resistivity of the TiO2-doped PES films.

Author Contributions

Y.T.: investigation and writing; K.A.: investigation and data curation; S.T.: validation and writing—review and editing; K.H.: supervision, conceptualization, writing, reviewing, editing, funding acquisition, and project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article [and/or] its Supplementary Materials.

Acknowledgments

The authors thank Hyuma MASU (Center for Analytical Instrumentation), Fumiyuki Shiba (Graduate School of Engineering), and Hideaki Shirota (Graduate School of Science) of Chiba University for their assistance with the XRD, TEM, and DSC measurements, respectively. Our thanks are extended to Powdertech Co., Ltd., Nippon Aerosil Co., Ltd., and Toyobo Co., Ltd. for providing the iron powder beads, P25, and polyester resin, respectively.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 01468 g001 550
Figure 1. Charged-water penetration (CWP) model for the triboelectric charging of polymer films.
Figure 1. Charged-water penetration (CWP) model for the triboelectric charging of polymer films.
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Figure 2. Apparatus (a) and measuring system (b) for the triboelectric charging of polymer films.
Figure 2. Apparatus (a) and measuring system (b) for the triboelectric charging of polymer films.
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Figure 3. (a) Triboelectric charging behavior of the polyester (PES) single film when charged with iron powder beads under various humidity conditions. (b) Plot of saturated surface potential (Vsat) versus relative humidity.
Figure 3. (a) Triboelectric charging behavior of the polyester (PES) single film when charged with iron powder beads under various humidity conditions. (b) Plot of saturated surface potential (Vsat) versus relative humidity.
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Figure 4. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) RUT nanoparticles.
Figure 4. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) RUT nanoparticles.
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Figure 5. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) ANA nanoparticles.
Figure 5. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) ANA nanoparticles.
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Figure 6. Plot of humidity dependence against doping level for the PES films doped with RUT, ANA, AMO, P25, and P25mix nanoparticles.
Figure 6. Plot of humidity dependence against doping level for the PES films doped with RUT, ANA, AMO, P25, and P25mix nanoparticles.
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Figure 7. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) AMO nanoparticles.
Figure 7. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) AMO nanoparticles.
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Figure 8. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) P25 nanoparticles.
Figure 8. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) P25 nanoparticles.
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Figure 9. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) P25mix nanoparticles.
Figure 9. Plots of Vsat versus relative humidity for PES films doped with 1 wt% (a), 5 wt% (b), and 10 wt% (c) P25mix nanoparticles.
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Figure 10. Plot of Vsat versus relative humidity for layered (a) and single-layer (b) films with an average doping level of 1 wt% P25.
Figure 10. Plot of Vsat versus relative humidity for layered (a) and single-layer (b) films with an average doping level of 1 wt% P25.
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Figure 11. Water content in the reference (gray bars) and charged (light-blue bars) samples for the PES single film and PES films doped with 10 wt% TiO2 nanoparticles at 40–50%RH.
Figure 11. Water content in the reference (gray bars) and charged (light-blue bars) samples for the PES single film and PES films doped with 10 wt% TiO2 nanoparticles at 40–50%RH.
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Table
Table 1. Values of 95% CI for PES films doped with 0, 1, 5, and 10 wt% TiO2.
Table 1. Values of 95% CI for PES films doped with 0, 1, 5, and 10 wt% TiO2.
Doping Level (wt%)95% CI
RUTANAAMOP25
00.500.500.500.50
10.300.480.360.32
50.300.440.440.30
100.120.420.360.12
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Teramoto, Y.; Ando, K.; Tsukada, S.; Hoshino, K. Triboelectric Charging Behaviors of Polyester Films Doped with Titanium Dioxide Nanoparticles of Various Crystal Structures. Appl. Sci. 2023, 13, 1468. https://doi.org/10.3390/app13031468

AMA Style

Teramoto Y, Ando K, Tsukada S, Hoshino K. Triboelectric Charging Behaviors of Polyester Films Doped with Titanium Dioxide Nanoparticles of Various Crystal Structures. Applied Sciences. 2023; 13(3):1468. https://doi.org/10.3390/app13031468

Chicago/Turabian Style

Teramoto, Yudai, Keita Ando, Satoru Tsukada, and Katsuyoshi Hoshino. 2023. "Triboelectric Charging Behaviors of Polyester Films Doped with Titanium Dioxide Nanoparticles of Various Crystal Structures" Applied Sciences 13, no. 3: 1468. https://doi.org/10.3390/app13031468

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