Significantly accelerated photochemical and photocatalytic reactions in microdroplets

Photochemistry and photocatalysis are promising approaches to derive solar energy for solving the global energy and environment crisis. However, some practical aspects remain daunting, mainly due to the slow reaction kinetics, especially for multiphase reactions involving gaseous reactants. Here, we report that the oxidation of methyl orange, golden orange II, and levoglucosan by Fe(III)-oxalate photochemistry and by g-C3N4 photocatalysis can be significantly increased (up to two orders of magnitude) with decreasing microdroplet diameter, from around 2,000 to 100 μm. Using simulated Raman scattering microscopy, we observe strong interface enrichment and pH alternation within the microdroplets. Experimental and theoretical results indicate that the rate increase is mainly caused by the size-dependent air-water interface properties of microdroplets. Furthermore, we demonstrate the scalability of microdroplet photoreactions. This work not only provides an efficient pathway for improving the photochemistry and photocatalysis efficiencies but also shows important implications for atmospheric photochemistry.