Spontaneous doping of the basal plane of MoS single layers through oxygen substitution under ambient conditions.

The chemical inertness of the defect-free basal plane confers environmental stability to MoS single layers, but it also limits their chemical versatility and catalytic activity. The stability of pristine MoS basal plane against oxidation under ambient conditions is a widely accepted assumption however, here we report single-atom-level structural investigations that reveal that oxygen atoms spontaneously incorporate into the basal plane of MoS single layers during ambient exposure. The use of scanning tunnelling microscopy reveals a slow oxygen-substitution reaction, during which individual sulfur atoms are replaced one by one by oxygen, giving rise to solid-solution-type 2D MoSO crystals. Oxygen substitution sites present all over the basal plane act as single-atom reaction centres, substantially increasing the catalytic activity of the entire MoS basal plane for the electrochemical H evolution reaction.