Contact-Electrification Between Two Identical Materials: Curvature Effect.

It is known that contact-electrification (or triboelectrification) usually occurs between two different materials, which could be explained by several models for different materials systems (Adv. Mater. 2018, 30, 1706790; Adv. Mater. 2018, 30, 1803968). But contact between two pieces of the chemically same material could also result in electrostatic charges although the charge density is rather low, which is hardly to be understood from the physics point of view. In this paper, by preparing a contact-separation mode triboelectric nanogenerator (TENG) using two pieces of an identical material, the direction of charge transfer during contact-electrification is studied regarding to its dependence on curvatures of the sample surfaces. For materials such as polytetrafluoroethylene, fluorinated ethylene propylene, Kapton, polyester and Nylon, the positive curvatured surfaces are net negatively charged, while the negative curvatured surfaces are inclined to be net positively charged. Further verification on the abovementioned trends is conducted under vacuum (~ 1 Pa) and higher temperature (≤ 358 K) conditions. Based on the received data acquired for gentle contacting cases, we propose a curvature dependent charge transfer model by introducing the curvature induced energy shifts of the surface states. However, this model is subject to be revised if the mutual contact mode turns into sliding mode or more complicated hard-pressed contact mode, in which a rigorous contact between the two pieces of the same material could result in nano-scale damage/fracture and possible species transfer. Our study provides a primitive step toward understanding the basics of contact-electrification.