Molybdenum disulfide (MoS2) has recently received great interest as a 2D material with a device-relevant direct bandgap of ~2eV. Potential applications include photocatalysis for solar fuel production. The dependence of its properties on chemical environment and flake size provides opportunities for optimization but also requires individual flakes to be studied in situ – and this is what Du et al report on here. Using PeakForce SECM on a Dimension Icon AFM, the authors were able to uncover and characterize the size dependence of MoS2 catalytic activity and identify flake recharging as a rate-limiting step for small monolayer flakes. These results may help guide optimization of 2D photocatalysts in future devices.