2.7e). The contact angle on the etched Al substrate is about 33°, due to the cooperation of the hydrophilic property of Al and the nanostructures.
By modifying with 1H,1H,2H,2H‐perfluorodecyltrichlorosilane/toluene solution, the surface energy of the etched Al substrate could be decreased, leading to the formation of a superhydrophobic surface on the Al substrate. Figure 2.7i shows that the contact angle on the prepared superhydrophobic surface is about 153°. Figure 2.7j shows the values for three different surfaces. Figure 2.7k shows a photograph of water drops with different sizes on the superhydrophobic surface. It is found that the superhydrophobic surface could form positive charges when it made contact with a polyamide film in the TENG.
2.3.2.3 Polymer
Polymers, such as PTFE, FEP, and PDMS, are ideal triboelectric materials for TENGs due to the flexibility and the excellent dielectric property. The surface roughness and environmental stability of the polymers in the TENGs can be enhanced with many advanced methods, such as plasma (ICP)‐reactive ion etching, template method, and 3D printed method. PDMS possesses a great deal of advantages, such as flexibility, transparency, and high electronegativity, and can be easily produced in special shapes. To study the influence of the surface morphology for the TENG, Dudem et al. developed functional polydimethylsiloxane (NpA‐PDMS) layers by soft imprint lithography [62]. The prepared layers possessed nanopillar‐like architectures, which could be used to increase the output voltage of the TENG.
PTFE is a common negative triboelectric material, exhibiting strong electron‐attracting ability and excellent flexibility. Guo et al. utilized inductively coupled plasma (ICP) ion etching to produce nanostructures on the PTFE film [63]. The fabricated PTFE film could form more charges when it made contact with the electrode, resulting in high output performance of the TENG. Wang et al. fabricated sponge‐like porous PTFE thin films by using deionized (DI) water as the soft template [64]. Compared with that based on flat PTFE film, the output performance based on porous PTFE thin film was obviously enhanced. Zhao et al. fabricated polytetrafluoroethylene/polyethene oxide (PTFE/PEO) membranes by using electrospinning method. In order to further enhance the charge density on the composite membranes, they introduced a high amount of stable static negative charges on the surface of the membranes [65].
Figure 2.7 Superhydrophobic surfaces on the Al substrates. (a) SEM image of the polished Al substrate. (b) High magnification SEM image of the polished Al substrate. (c) Image of the contact angle of water on the polished Al substrate. (d) SEM image of the etched Al substrate. (e) High‐magnification SEM image of the etched Al substrate. (f) Image of the contact angle of water on the etched Al substrate. (g) SEM image of the etched Al substrate modified with low surface energy material. (h) High‐magnification SEM image of the modified Al substrate. (i) Image of the contact angle of water on the modified Al substrate. (j) Measured contact angles of three different Al surfaces. (k) Photograph of the water drops on the superhydrophobic surface.
Source: Reproduced with permission from Zhao et al. [61]. Copyright 2016, American Chemical Society.
2.3.2.4 Nanoparticle and Nanowire
Nanoscale materials including nanoparticles and nanowires have attracted broad interest due to their unique chemical and physical characteristics. Unlike bulk materials, reducing the size of materials to the nanoscale could make them exhibit high reactivity. Nanoparticles (NPs) with high surface energy can be easily re‐formed into functional materials, and nanowires (NWs) offer high surface area and restrain the mechanical degradation [66]. These nanoscale materials have been widely used in applications of energy collecting and storing devices.
NWs with high surface area that restrain the mechanical degradation could be used to optimize the performance of TENGs. Jiang et al. developed a Ag nanowire‐based TENG, where Ag nanowires were used as both triboelectric layers and electrodes. Ag nanowires were fabricated through a polyol synthesis method [67]. Figure 2.8a,b shows that the diameter and length of the nanowire are about 70 nm and 10 μm, respectively. Photographic paper was used as the substrate to fix the Ag nanowire slurry, resulting in a metal electrode with excellent conductivity (Figure 2.8c). The Ag nanowires closely accumulate on the photographic paper to form the Ag nanowires membrane, as shown in Figure 2.8d. Cheon et al. fabricated PVDF–Ag NW composite by the electrospinning method. The electrostatic interactions between the Ag NWs and the dipoles of the PVDF chains could promote β‐phase crystal formation of PVDF, which can be further used to increase the output performance of the TENGs [68]. From a triboelectric perspective, NPs could increase the surface roughness and dielectric property of friction films, leading to enhanced performances of TENGs. Jiang et al. reported a Ag nanoparticle‐based TENG, where Ag nanoparticles were used as both triboelectric layers and electrodes [69]. Ag nanoparticles with a diameter of about 50 nm were fabricated through an ice−water bath method as shown in Figure 2.8e. The Ag nanoparticles were stuck on the photographic paper to form the Ag membrane, increasing the mechanical stability of nanoparticles, as shown in Figure 2.8f. It is found that the membrane could form positive charges when it made contact with a FEP film in the TENG.
Figure 2.8 Ag nanoparticles and Ag nanowires. (a) SEM image of the Ag nanowires. (b) High‐magnification SEM image of Ag nanowires. (c) A photograph of the electrode based on Ag nanowires. (d) The SEM image of the electrode.
Source: Reproduced with permission from Jiang et al. [67]. Copyright 2018, American Chemical Society.
(e) SEM image of the low‐density Ag nanoparticles on Al foils. (f) The membrane prepared by Ag nanoparticles on a photographic paper.
Source: Reproduced with permission from Jiang et al. [69]. Copyright 2017, American Chemical Society.
NPs can be used to increase the surface roughness of triboelectric layers. Lee et al. fabricated textile electrodes with nanostructured geometries, where Al NPs were grown by using thermal evaporation [70]. When the Al NPs were contacted with the PDMS, triboelectric charges could be generated due to their different triboelectric series. It is found that Al NPs could remarkably increase the output voltage of the TENG. Chun et al. developed an Au NP‐embedded mesoporous TENG, where Au NPs were embedded into the pores of the PDMS. It is found that the contact between Au NPs and PDMS could enhance the surface potential energy, resulting in high output performance of TNEG [71]. Zhang et al. explored the effect of the output performance of TENGs based on Cu NP‐embedded films and ZnO NP‐embedded films, respectively. The results showed that Cu NPs can effectively increase the output performance, but ZnO NPs hardly do that [72].
2.3.3 Performance
Currently, mechanical stability and high electrical output are two main performances considered by numerous researchers for TENGs. It is found that the favorable mechanical property could increase the electrical property of the devices, especially in WD‐TENGs.
