In situ work function measurements of W, WO₃ nanostructured surfaces

Surface nanostructuring enables the fabrication of materials with highly desirable properties. Nanostructured tungsten surfaces have potential applications in solar water splitting. Exposing a polished tungsten surface to helium plasma induces various surface morphological changes. Depending on the helium ion energy, temperature, and fluence, helium clusters, helium bubbles and foam-like nanostructures develop on the tungsten surface. In this study, tungsten foam-like nanostructures were formed and/or oxidised, and then examined using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) without breaking the vacuum. The chemical state of nanostructured W or WO3 was not modified in comparison to the pristine one. However, measuring the line width of the emitted electrons from the onset of the secondary electrons up to the Fermi edge and subtracting value from the incident photon energy, the work function acquired in situ by UPS for a nanostructured W surface increased by 0.9 eV in comparison to the pristine one. Helium ions effectively eliminated field emission sites via sputtering/implantation and thereby increased the work function. No change in work function was measured for WO3-pristine and its fuzz: the oxidation hindered the effect of helium. In contrast to the W-fuzz sample, no helium bubbles were identified in WO3-fuzz, as helium diffused out during oxidation.