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

Tungsten disulfide, a transition metal dichalcogenide,has numerousapplications 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 whoserole has not been well-understood. To address this problem, the in situ probing of H2O molecule adsorption onWS(2) films supported on solid substrates has been performedin 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 interfaceof two immiscible liquids, water and toluene, transferred onto a solidsubstrate by the Langmuir-Schaefer technique. This experimentshows that edge defects in nanoflakes, tungsten dangling bond ensuingthe exfoliation in the liquid phase, represent active sites for theWO(3), WO3-x , and WO3 center dot nH(2)O formation under ambientconditions. These oxides interact with water molecules when the WS2 films are exposed to water vapor in the NAP-XPS reactioncell. However, water molecules do not influence the W-S chemicalbond, thus indicating the physisorption of H2O moleculesat the WS2 film surface.