Abstract Integration of advanced nanogenerator technology with conventional textile processes fosters the emergence of textile‐based nanogenerators (NGs), which will inevitably promote the rapid development and widespread applications of next‐generation wearable electronics and multifaceted artificial intelligence systems. NGs endow smart textiles with mechanical energy harvesting and multifunctional self‐powered sensing capabilities, while textiles provide a versatile flexible design carrier and extensive wearable application platform for their development. However, due to the lack of an effective interactive platform and communication channel between researchers specializing in NGs and those good at textiles, it is rather difficult to achieve fiber/fabric‐based NGs with both excellent electrical output properties and outstanding textile‐related performances. To this end, a critical review is presented on the current state of the arts of wearable fiber/fabric‐based piezoelectric nanogenerators and triboelectric nanogenerators with respect to basic classifications, material selections, fabrication techniques, structural designs, and working principles, as well as potential applications. Furthermore, the potential difficulties and tough challenges that can impede their large‐scale commercial applications are summarized and discussed. It is hoped that this review will not only deepen the ties between smart textiles and wearable NGs, but also push forward further research and applications of future wearable fiber/fabric‐based NGs.
Although contact electrification (triboelectrification) (CE) has been documented since 2600 years ago, its scientific understanding remains inconclusive, unclear, and un-unified. This paper reviews the updated progress for studying the fundamental mechanism of CE using Kelvin probe force microscopy for solid–solid cases. Our conclusion is that electron transfer is the dominant mechanism for CE between solid–solid pairs. Electron transfer occurs only when the interatomic distance between the two materials is shorter than the normal bonding length (typically ∼0.2 nm) in the region of repulsive forces. A strong electron cloud overlap (or wave function overlap) between the two atoms/molecules in the repulsive region leads to electron transition between the atoms/molecules, owing to the reduced interatomic potential barrier. The role played by contact/friction force is to induce strong overlap between the electron clouds (or wave function in physics, bonding in chemistry). The electrostatic charges on the surfaces can be released from the surface by electron thermionic emission and/or photon excitation, so these electrostatic charges may not remain on the surface if sample temperature is higher than ∼300–400 °C. The electron transfer model could be extended to liquid–solid, liquid–gas and even liquid–liquid cases. As for the liquid–solid case, molecules in the liquid would have electron cloud overlap with the atoms on the solid surface at the very first contact with a virginal solid surface, and electron transfer is required in order to create the first layer of electrostatic charges on the solid surface. This step only occurs for the very first contact of the liquid with the solid. Then, ion transfer is the second step and is the dominant process thereafter, which is a redistribution of the ions in solution considering electrostatic interactions with the charged solid surface. This is proposed as a two-step formation process of the electric double layer (EDL) at the liquid–solid interface. Charge transfer in the liquid–gas case is believed to be due to electron transfer once a gas molecule strikes the liquid surface to induce the overlapping electron cloud under pressure. In general, electron transfer due to the overlapping electron cloud under mechanical force/pressure is proposed as the dominant mechanism for initiating CE between solids, liquids and gases. This study provides not only the first systematic understanding about the physics of CE, but also demonstrates that the triboelectric nanogenerator (TENG) is an effective method for studying the nature of CE between any materials.
The sculpting of crystalline materials from amorphous films by electron beam irradiation in transmission electron microscopy (TEM) offers an effective way for fabrication of nanostructure and devices. However, the synthesis of multifunctional complex oxide and related composites for possible device application is difficult to achieve. Here, we show that the crystallization pathways of amorphous bismuth ferrite films could be tuned by controlled electron beam irradiation and in situ heating in TEM. The results show that Bi segregates from amorphous films and then aggregates into crystalline nanoparticles (the particle size can be tuned by electron dose rates) under electron beam irradiation below 100 °C, while Bi2Fe4O9 nanocrystals are observed at boundary areas between quasi-liquid Bi nanoparticles at 300 °C due to the cooperative effect of electron beam irradiation and thermal heating. Moreover, the Bi/Bi2Fe4O9 metal/semiconductor solid state heterostructure with nearly atomically sharp interfaces emerges when cooling down to room temperature. This finding expands the variety of nanostructures synthesized by electron bombardment and offers a new way to fabricate complex architectures and possible functional devices at the nanometer scale with direct in situ TEM observation and monitoring.
