Effects of MoS₂ Layer Thickness on Its Photochemically Driven Oxidative Dissolution

Area of Science:

• Materials Science
• Environmental Science
• Nanotechnology

Background:

• Two-dimensional (2D) molybdenum disulfide (MoS2) possesses unique optical and electronic properties, making it suitable for photocatalysis and photothermal applications.
• While generally considered insoluble, 2D MoS2 nanosheets can dissolve due to large surface areas and reactive sites, especially under illumination.
• Photochemical reactivity can accelerate the dissolution of 2D MoS2 nanosheets in aqueous environments.

Purpose of the Study:

• To investigate the influence of thickness on MoS2 dissolution under light illumination.
• To elucidate the mechanisms driving MoS2 reactivity and dissolution in environmental conditions.
• To assess the environmental stability of 2D MoS2 nanosheets based on their thickness.

Main Methods:

• Exfoliation of bulk MoS2 into nanosheets using ultrasonication.
• Controlled synthesis of MoS2 nanosheets with average thicknesses of ~18 nm and ~46 nm via iterative cascade centrifugation.
• Investigation of MoS2 dissolution under simulated sunlight and in dark conditions, analyzing Mo6+ concentration and solution pH.

Main Results:

• MoS2 dissolution was accelerated under simulated sunlight compared to dark conditions, indicated by increased Mo6+ composition and decreased pH.
• Light exposure promotes MoS2 oxidation, leading to faster dissolution.
• 18 nm thick MoS2 nanosheets exhibited significantly faster dissolution than 46 nm or bulk MoS2, attributed to enhanced superoxide radical (O2•−) generation.

Conclusions:

• The thickness of MoS2 nanosheets critically influences their photochemistry and dissolution rates.
• Light exposure accelerates MoS2 dissolution, impacting its environmental stability.
• Understanding thickness-dependent photochemistry is essential for predicting the environmental behavior of MoS2 nanomaterials.