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12 Y. Yu, X. Wang / Extreme Mechanics Letters ( ) – Fig. 8(e) presents the Voc variation as a function of the removed thicknesses. ∼95% of Voc was remained after 2 μm-thick material was polished. Such a durable TENG performance was contributed by the deep penetration (∼3 μm) of AlOx dopants. Further polishing the film dramatically reduced Voc due to the expose of unmodified PDMS. A minimum value of ∼0.3 V was received after removing 5 μm polymer, which was comparable to the TENG output with the pristine PDMS pair. The up limit of this SIS doping process was revealed by varying the infiltration cycles and measuring the related TENGs output. As shown in Fig. 8(f), the peak Voc was improved from 0.3 to 1.9 V by only 2 cycles of SIS, suggesting the high effectiveness of this polymer modification approach. The Voc kept increasing with the increment of SIS cycles and eventually saturated at ∼2.4 V after 10 cycles of infiltration. Almost stable values were recorded when further increasing the cycle numbers. This Voc variation is believed to be a consequence of the unique reaction process of AlOx in the polymer material. During the first 10 cycles of infiltration, TMA diffused into PDMS and accumulated inside the polymer film, resulting in the rapid increase of Voc. After 10 cycles, more deposition might cover the entire PDMS surface and prevent further inside infiltration. Thus the growth mode switched back to regular ALD deposition of continuous Al2O3 film on the surface. Because additional cycles of infiltration contributed no internal doping, the electric property and TENG output would not exhibit any further change. The underlying mechanism of this SIS strategy shares certain similarity with the aforementioned ion injection approaches. Both of them control the surface charge density of triboelectric polymer by introducing extra charged ions or molecules. However, compared to other chemical modification techniques, the SIS doping process extends the modification region from the surface to bulk portion of triboelectric polymers. This improvement is important for developing high-performance TENGs with good resistance to the surface wearing issue. Moreover, the SIS doping is versatile in tailoring broad range of triboelectric polymers since a variety of metalorganic molecules possess high permittivity in most polymer chains [81]. With proper selection of doping precursor and precise control of infiltration condition, SIS doping could be a promising strategy to engineering electric and dielectric properties of many polymers, and improving the performance of functional polymer based devices, such as TENGs. 5. Molecular-targeting functionalization The chemical modification process through molecular- targeting functionalization is closely related to the chemi- cal sensing function of TENGs. TENG is a good platform for developing various sensors including motion and chemi- cal sensors [46]. By recording the output variation, TENG- based motion sensors can detect different types of me- chanical deformations such as human touch and sound. Meanwhile, TENGs can also be used as self-powered chem- ical sensors since the amplitude of the triboelectric sig- nal is proportional to the surface charge density that is largely influenced by the surface chemical state. The opera- tion principle of the TENGs-based chemical sensor relies on the surface charge density variation induced by targeting molecules. When the target molecules are attached on the surface of triboelectric polymers, possible charge transfer could emerge between the chemical species and triboelec- tric materials, leading to the change of TENG output [21, 82–84]. In this section, the TENG-based chemical sensors are discussed with an emphasis on surface functionaliza- tion for detecting specific molecules. Li et al. reported a self-powered phenol sensor based on the β-cyclodextrin (β-CD) enhanced triboelectrification process [85]. Fig. 9(a) presents the schematic of the TENG device with PTFE and β-CD decorated TiO2 NWs as two electrodes. TiO2 NWs were grown on Ti foil with an average diameter and length of 73.4 nm and 1. 92 μm. In this structure, β-CD was a bifunctional component that can recognize phenol molecule and enhance the TENG performance at the same time. On one hand, β-CD molecule is composed of a hydrophobic internal cavity and hydrophilic external side chains. This special structure allows it to capture guest molecules with appropriate polarity and dimension, and forming a host/guest inclusion complex. The diameter of β-CD cavity is particularly suitable for the selective absorption of phenol, which making it a particular molecular probe for phenol [86]. On the other hand, β-CD can alter the overall TENG performance by conducting charge transfer with TiO2 NWs. Specifically, β-CD is binding on TiO2 surface via hydroxyl groups. Electrons could thus be injected from the hydroxyl groups to TiO2 surface due to the coordination effect between the ligand and the metal under visible light (Fig. 9(b)) [87]. Consequently, the positive charges on TiO2 surface that produced from the triboelectrification with PTFE were partially neutralized. This means PFTE can extract extra electrons from TiO2 after β-CD functionalization, and further increase its surface charge density, and therefore improve the TENG output. The influences of β-CD concentration on the triboelectric output were studied with a 4 cm × 4 cm TENG (Fig. 9(c)). Both current and voltage signals were proportionally increased with the augment of β-CD concentration (Fig. 9(d)). When TENG was treated with 80 mM β-CD, the electric output reached ∼55 V, which was more than 8 times larger than the unmodified devices. A voltage plateau emerged at the region of β-CD concentration larger than 80 mM, which may be caused by the saturation of β-CD on TiO2 surface. This experiment strongly evidenced the TENG performance-gain function of the β-CD modification. The phenol detecting ability of this β-CD tailored triboelectrification was evaluated on TENGs modified by a fixed β-CD concentration of 80 mM. The constant volumes (20 mL) of phenol solution with varied concentrations were introduced to treat the β-CD decorated TiO2 NWs. As shown in Fig. 9(e), the current output was clearly dependent on the phenol concentration. At the region of 10–100 mM phenol, the current signal decreased almost linearly with the increase of the phenol concentration. The reduction of electric output is ascribed to the phenol absorption onto the TiO2 NWs surface via the β-CD cavity.PDF Image | Chemical modification of polymer surfaces for advanced triboelectric nanogenerator development
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