Photostability of Monolayer Transition-Metal Dichalcogenides in Ambient Air and Acidic/Basic Aqueous Solutions

We report on the photostability of monolayer (1L) transition-metal dichalcogenides (TMDCs) in air and in aqueous solutions, as probed using photoluminescence spectroscopy. 1L-WSe₂ was readily degraded under continuous irradiation of visible light in aqueous solutions, whereas 1L-MoS₂ was relatively stable in both ambient air and aqueous solutions. The stability difference between these two materials was mainly ascribed to the oxidization reaction at the interface of 1L-TMDCs and the O₂/H₂O redox system induced by both band alignment and photogenerated holes. This interpretation was strongly supported by the observation of the lower degradation rate of 1L-WSe₂ in the dark and in degassed water with a lower concentration of dissolved oxygen compared with the degradation rate of 1L-WSe₂ in distilled water. Furthermore, the degradation rate was also nearly proportional to the number of photogenerated carriers. The degradation rate under acidic conditions was smaller than that under the basic conditions. The results are attributed to the oxidation/reduction potential of 1L-WSe₂ and to the dissolution reaction of degraded species, both of which are strongly pH-dependent.