The process of free radical generation in contact electrification at solid-liquid interface

Solid-liquid interfaces are ubiquitous in nature and widely used in various applications such as catalysis, sensing, batteries, etc. Recently, we found the existence of electron transfer at the interface in solid-liquid contacting, and proposed a “two-step” theory for the formation of solid-liquid electric double layer (EDL). In this work, we found that hydroxyl radical (·OH) was generated when the droplet contact with the tube inner wall without external interference, and the concentration of·OH was increased with increasing pH, demonstrating that the·OH was generated from hydroxide ions (OH-) by electron transfer during contact electrification. This can be further supported by the fact that capillaries made of materials with stronger electron-withdrawing ability could generate more·OH. Interestingly, superoxide anion radical (·O2-) was not generated at the solid-liquid interface if no external energy is provided. However, if ultrasonic was applied,·O2- could be generated at the solid-liquid interface, because ultrasonication provides transition energy for the electron to transfer from the solid to O2 at the interface. Based on the generation of reactive oxygen species (·OH and·O2-), we demonstrated a proof-of-concept for degradation of tetracycline pollution. This work provides an excellent strategy to study the solid-liquid interface and also provides a new insight into the free radical generation at the interface in solid-liquid contacting.