in an aqueous solution*

Physical and chemical properties of transition-metal dichalcogenides (TMDCs) are expected to be strongly affected by various surface interactions because of their extremely large specific surface area. In particular, variety of phenomena occurring at the interfaces of water and atomically thin TMDCs may be critical for their applications. However, knowledge on the optical properties of these materials in water has still been limited. Here we report photoluminescence (PL) properties of mechanically exfoliated monolayer (1L) MoS₂ in aqueous solutions with various pH. After the immersion into distilled water, considerable modifications in the PL intensity and spectral line shape were observed compared to those in ambient air condition. The results are mainly attributed to the electrochemical reactions at the interfaces between the 1L-MoS₂ and the aqueous solution that cause electron extraction from or injection into the 1L-MoS₂ depending on the redox potentials in the O₂/H₂O redox system that are determined by the solution's pH.