Spontaneous doping of the basal plane of MoS₂ single layers through oxygen substitution under ambient conditions

The chemical inertness of the defect-free basal plane confers environmental stability to MoS₂ single layers, but it also limits their chemical versatility and catalytic activity. The stability of pristine MoS₂ 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 MoS₂ 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 MoS_2−xO_x crystals. Oxygen substitution sites present all over the basal plane act as single-atom reaction centres, substantially increasing the catalytic activity of the entire MoS₂ basal plane for the electrochemical H₂ evolution reaction.