Direct Probing of Water Adsorption on Liquid-Phase Exfoliated WS₂ Films Formed by the Langmuir-Schaefer Technique

Tungsten disulfide, a transition metal dichalcogenide, has numerous applications as active components in gas- and chemical-sensing devices, photovoltaic sources, photocatalyst substrates, etc. In such devices, the presence of water in the sensing environment is a factor whose role has not been well-understood. To address this problem, the in situ probing of H₂O molecule adsorption on WS₂ films supported on solid substrates has been performed in a near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) setup. Instead, on the individual nanoflakes or spray-coated samples, the measurements were performed on highly transparent, homogeneous, thin films of WS₂ nanosheets self-assembled at the interface of two immiscible liquids, water and toluene, transferred onto a solid substrate by the Langmuir-Schaefer technique. This experiment shows that edge defects in nanoflakes, tungsten dangling bond ensuing the exfoliation in the liquid phase, represent active sites for the WO₃, WO_3-, and WO₃·nH₂O formation under ambient conditions. These oxides interact with water molecules when the WS₂ films are exposed to water vapor in the NAP-XPS reaction cell. However, water molecules do not influence the W-S chemical bond, thus indicating the physisorption of H₂O molecules at the WS₂ film surface.