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, WO3–x, 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.