Microdroplet assisted hollow ZnCdS@PDA nanocages' synergistic confinement effect for promoting photocatalytic H 2 O 2 production.

Solar-driven photocatalytic H 2 O 2 production is greatly impeded by the slow mass transfer and rapid recombination of photogenerated charge carriers for multiphase reactions. Polydopamine (PDA)-coated hollow ZnCdS (ZnCdS@PDA) octahedral cages with sulfur vacancies were constructed as micro-reactors to provide a delimited micro-environment for highly efficient paired H 2 O 2 production through water oxidation coupled with oxygen reduction. At neutral pH, hollow ZnCdS@PDA cages exhibited a high H 2 O 2 production yield of 45.5 mM g -1 h -1 without the assistance of sacrificial agents in bulk solution, which can be attributed to the distinguished space constraint in hollow nanocages and a surprisingly adjusted band structure. Compared to the bulk water system, H 2 O and O 2 inside the hollow nanocage can form an ideal system for boosting such nanoconfined H 2 O or O 2 molecules' adsorption/enrichment on the interior of the ZnCdS active sites. More importantly, the photocatalytic yield of H 2 O 2 generation (H 2 O 2 concentrations of 190-65.6 mM g -1 h -1 ) obtained in the abundant gas-liquid interface of microdroplets is dramatically higher than that obtained in an aqueous bulk environment under visible light conditions without using sacrificial agents. This enhancement can be attributed to the synergistic effect of the hollow ZnCdS@PDA nanocage reactor and the microdroplet confinement photocatalysis reaction. Particularly, the improved/confined enhancement of O 2 availability and enhanced charge separation, along with high catalytic durability are the main reasons leading to significant H 2 O 2 production due to an ultrahigh interfacial electric field and an extremely large specific surface area in microdroplets. In addition to producing a highly concentrated liquid of hydrogen peroxide during the microdroplet photoreaction, we also obtained white solid hydrogen peroxide powder with strong oxidizing properties reducing costs and increasing safety in storage and transportation. This study highlights that nano-liquid catalysis (using microdroplets) provides a very efficient pathway for accelerating semiconductor photocatalysis limited by gas diffusion in a liquid.