Identifying reactive sites and measuring their activities is crucial for enhancing the efficiency of every catalyst. Reactivity maps can guide the development of next-generation photocatalysts like 2D transition metal dichalcogenides, which suffer from low conversion rates. While their electrocatalytic sites are well-studied, their photocatalytic sites remain poorly understood. Using scanning photoelectrochemical microscopy, we spatially resolve the photoreactivity of MoS2 monolayers, a prototypical 2D transition metal dichalcogenide, for redox reactions, including H2 production from water. Aligned-unaligned excitation-detection measurements reveal that photogenerated holes and electrons exhibit distinct behaviors. Oxidation products localize at the excitation spot, indicating stationary holes, while photoreduction occurs up to at least 80 microns away, showing exceptional electron mobility. We also elucidate the photochemical reactivity according to the nature of the electronic excitation, showing that the internal quantum efficiency of strongly-bound A-excitons outperforms weakly-bound (free-carrier like) C-excitons across the flake. These findings offer novel guidance to rationally design 2D photocatalysts via engineering their optical and charge extraction abilities for efficient solar energy conversion.

Department of Chemistry and NIS Centre University of Turin Turin Italy

Department of Chemistry Rice University Houston TX USA

Department of Electrical and Computer Engineering Rice University Houston TX USA

Department of Physics and Astronomy Rice University Houston TX USA

Fakultät für Physik Munich Quantum Center and Center for NanoScience Ludwig Maximilians Universität München Nanoinstitut München München Germany

Munich Center for Quantum Science and Technology München Germany

Nanoinstitute Munich Fakultät für Physik Ludwig Maximilians Universität München München Germany

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Olomouc Olomouc Czech Republic

Technical University of Munich Garching Germany

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