Curvature effects on liquid–solid contact electrification

Abstract

Abstract The generation of triboelectric charges between two pieces of chemically identical materials has been researched systematically, implying that the charges were a consequence of the surface curvature at the macroscale level. However, there is still a lack of research on how nanoscale curvature affects triboelectric charges, which is of great importance due to the universal existence of nano-curvatures on a rough surface. Besides, it is also unclear whether such a curvature effect is valid for different materials in contact, such as for a liquid-solid triboelectric nanogenerator (TENG). In this paper, the magnitude of charge transfer during liquid-solid contact electrification (CE) is quantified to investigate its dependency on the curvature of sample surfaces. Electrostatic force microscope (EFM) experiments show that convex surface (negative curvature) is more conducive to the charge transfer with liquid drops than that of a concave surface (positive curvature). A statistical trend is found in which a smaller curvature would typically lead to a higher charge rate of negative charges after CE. In addition, the charge transfer from the surface to atmosphere follows an exponential decay at a fixed temperature after separation from contact. Differently from the curvature effect on triboelectric charge generation, the charges on the concave surface are more likely to be emitted into the atmosphere than those on the convex surface. Based on these results, we propose a curvature-dependent charge transfer model for CE by introducing the curvature-induced energy shifts of the surface states. Finally, it is demonstrated that the short-circuit current output performance of a liquid-solid TENG could be greatly improved by designing the insulating surface with a more nano-layer structure, which increases surface coverage of convex areas. The best performed TENG used the surfaces with more negatively curved regions, suggesting that a convex surface leads to more electrons negatively charging the surface. In general, this study provides not only new insights for understanding CE, but also a novel pathway to improve the performance of the liquid-solid TENG.