On the origin of contact-electrification

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.