The tribovoltaic effect and electron transfer at a liquid-semiconductor interface

Contact electrification (CE) is a ubiquitous phenomenon, which occurs at almost any interfaces between any phases, but its mechanism still remains ambiguous, especially for the liquid-solid case. Here, the tribo-voltage and the DC tribo-current are generated by sliding a water droplet over a silicon surface. By varying the doping types and concentrations, the direction of the tribo-voltage and the tribo-current is found highly depending on the built-in electric field at the liquid-solid junction. It is suggested that the electrical output is induced by the triboelectric effect at the water-silicon interface, in which electron-hole pairs are excited at the silicon surface by the energy released at interface during sliding, and further separated by the built-in electric field, which is analogous to that occurs in the photovoltaic effect. The only difference is that the energy in tribovoltaic effect is provided by electron transfer across the sliding interface owing to CE and bonding interaction between liquid molecules and semiconductor surface, rather than by photon. It is demonstrated that the electron transfer exists at the liquid-solid interface, and the two-step model for the formation of the electric double-layer is further verified, in which the electron transfer is considered to occur in the first step when the liquid contacts a solid in the very first time, which is then followed by ion chemical/physical adsorption as the second step. The tribovoltaic effect provides a new approach for studying of the mechanism at liquid-solid CE.