Condensing water vapor to droplets generates hydrogen peroxide.

It was previously shown [J. K. Lee , , 116, 19294-19298 (2019)] that hydrogen peroxide (HO) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that HO is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the HO production in condensate microdroplets showed that HO was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of HO was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern HO generation. We also found that the HO production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of HO occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form HO from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.