Water–solid contact electrification causes hydrogen peroxide production from hydroxyl radical recombination in sprayed microdroplets

Contact electrification between water and a solid surface is crucial for physicochemical processes at water–solid interfaces. However, the nature of the involved processes remains poorly understood, especially in the initial stage of the interface formation. Here we report that H(2)O(2) is spontaneously produced from the hydroxyl groups on the solid surface when contact occurred. The density of hydroxyl groups affects the H(2)O(2) yield. The participation of hydroxyl groups in H(2)O(2) generation is confirmed by mass spectrometric detection of (18)O in the product of the reaction between 4-carboxyphenylboronic acid and (18)O–labeled H(2)O(2) resulting from (18)O(2) plasma treatment of the surface. We propose a model for H(2)O(2) generation based on recombination of the hydroxyl radicals produced from the surface hydroxyl groups in the water–solid contact process. Our observations show that the spontaneous generation of H(2)O(2) is universal on the surfaces of soil and atmospheric fine particles in a humid environment.