Water adsorption on vanadium oxide thin films in ambient relative humidity

Abstract

In this work, ambient pressure X-ray photoelectron spectroscopy (APXPS) is used to study the initial stages of water adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s and valence band XPS spectra were collected as a function of relative humidity in a series of isotherm and isobar experiments. Experiments were carried out on two VO 2 thin films on TiO 2 (100) substrates, prepared with different surface cleaning procedures. Hydroxyl and molecular water surface species were identified, with up to 0.5 ML hydroxide present at the minimum relative humidity, and a consistent molecular water adsorption onset occurring around 0.01 % relative humidity. The work function was found to increase with increasing relative humidity, suggesting that surface water and hydroxyl species are oriented with the hydrogen atoms directed away from the surface. Changes in the valence band were also observed as a function of relative humidity. Results were similar to those observed in APXPS experiments on other transition metal oxide surfaces, suggesting H 2 O-OH and H 2 O-H 2 O surface complex formation plays an important role in the oxide wetting process and water dissociation. Compared to polycrystalline vanadium metal, these vanadium oxide films generate less hydroxide and appear to be more favorable for molecular water adsorption.

Methanol Adsorption on Vanadium Oxide Surfaces Observed by Ambient Pressure X-ray Photoelectron Spectroscopy

“…Initially, the C–Ox peak 1: O peak 1 ratio <1:2. This suggests that hydroxide is present on the surface at low relative pressure regardless of methanol being present, as was also shown in the water adsorption experiments conducted by this group . The continued growth of O peak 1 suggests that some dissociative adsorption is occurring.…”

“…Additional methanol molecules adsorbed between the molecularly bound methanol molecules and adjacent vanadyl groups can facilitate methanol dissociation to methoxy and hydroxy through a mediated transfer of hydrogen atoms . More methoxy formation leads to more hydroxy formation, and more water formation frees up additional sites for methoxy formation. , Interactions between adsorbed methanol and OH may result in a threshold coverage above which surface reactions are enhanced, similar to the role of H 2 O–OH interactions in enhanced water adsorption. ,− The hysteresis-like behavior in these experiments also suggests that this is the case.…”

“…Methanol in contact with surfaces has an O 1s binding energy of 533.3–533.9 eV, while surface methoxy groups have a binding energy of 531.4–531.7 eV, so the growth of O peak 1 and 2 suggests that the number of both species on the surface increases as the saturation conditions for methanol are approached . However, O peak 1 could also be due to the surface −OH groups, and O peak 2 also matches the O 1s binding energy of molecular water. − Spectra taken on this sample surface prior to methanol introduction, and previous experiments conducted by this group on vanadium oxides, have shown that even in UHV there is −OH present, so it is likely that at least some of the O peak 1 intensity can be attributed to −OH . Previous studies of methanol adsorption on vanadium oxides have shown that the degree to which methoxy formation dominates the surface reaction depends on the amount of −OH on the surface and water formation from these −OH groups .…”

“…Changes in the isotherm experiments were much more gradual. The temperature dependence is notable when these experiments are compared to water adsorption experiments on similar films, in which adsorption onsets occurred at a constant relative pressure, independent of the temperature . This observation indicates that there is a different reaction mechanism at play in methanol adsorption.…”

Electrospun polyacrylonitrile nanofiber membranes for air filtration application

“…The survey XPS spectrum and The survey spectrum exhibits three intense peaks at 284.9, 398.5 and 532.6 eV corresponding to C 1s, N 1s, and O 1s core levels respectively. The oxygen peak at 532 eV is due to water molecules adsorbed on iber surface (Tas et al 2016;Goodacre et al 2020). The high-resolution XPS spectra N 1s spectra shown in Fig.…”