Condensing water vapor to droplets generates hydrogen peroxide

Abstract

It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A, 116, 19294-19298 (2019)] that hydrogen peroxide (H₂O₂) 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 H₂O₂ 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 H₂O₂ production in condensate microdroplets showed that H₂O₂ was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H₂O₂ 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 H₂O₂ generation. We also found that the H₂O₂ production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H₂O₂ occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H₂O₂ 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.

Keywords: green Chemistry; hydrogen peroxide; microdroplet; vapor condensation; water–air interface.

Fig. 1.

Schematic of the experiment setup. An environmental control chamber was equipped with an automatic humidity control loop. The temperature of the substrate on which water vapor was condensed to form water microdroplets was controlled using a Peltier cooler and a resistive electric heater that were controlled by a feedback temperature controller. (Inset) Processes involved in the condensation of a microdroplet.

Fig. 2.

Generation of hydrogen peroxide in water microdroplets formed by water-vapor condensation. (A) Bright-field image of microdroplets formed on the cooled Si wafer surface. (B) Mass spectrum showing reaction products from reacting carboxyphenylboronic acid 1 with H₂O₂ generated in condensed water microdroplets to form 4-hydroxybenzoic acid 2 and boric acid 3. (C) The absorption spectrum of aqueous PTO solution with added H₂O₂ and a condensed microdroplet (red circle). (D) Calibration curve of the absorption at 400 nm acquired from spectra shown in C. The red circle represents the concentration of H₂O₂ generated from the condensed water microdroplet. (Scale bar, 50 µm.)

Fig. 3.

Time course of H₂O₂ formation in condensed water microdroplets. Time-lapse bright-field images (Left) and size distribution (Right) of microdroplets formed on a Si wafer after (A) 0.5 min, (B) 2 min, and (C) 5 min of cooling with a Peltier cooler. (D) Time course of H₂O₂ concentration. Error bars represent 1 SD from three independent measurements. (Scale bar, 50 µm.)

Fig. 4.

Dependence of H₂O₂ production on surface treatment conditions. Si wafers were treated to be hydrophobic (A and D), hydrophilic (B and E), and etched for increased surface roughness (C and F). (A–C) Bright-field images taken at 2 min after the initiation of cooling. (D–F) Time courses of H₂O₂ concentration on each treated surface. Error bars represent 1 SD from three independent measurements. (Scale bar, 50 µm.)

Fig. 5.

Dependence of H₂O₂ generation from condensed water microdroplets on the relative humidity and the substrate temperature. (A) Time course of H₂O₂ concentrations on a bare Si wafer under different relative humidities. (B) Maximum H₂O₂ concentrations at different relative humidities. (C) Time course of H₂O₂ concentrations at different temperatures of a bare Si wafer (D) Maximum H₂O₂ concentrations at different surface temperatures of a bare Si wafer.

Fig. 6.

Schematic of the time course of H₂O₂ formation on cold surfaces by condensation of water vapor: (A) capture of water vapor on cold substrate, (B) nucleation formation, (C) growth of microdroplets, and (D) formation of thin water film and the dilution of H₂O₂.