Water Adsorption on α-Fe₂O₃(0001) at near Ambient Conditions

Abstract

We have investigated hydroxylation and water adsorption on R-Fe 2 O 3 (0001) at water vapor pressures up to 2 Torr and temperatures ranging from 277 to 647 K (relative humidity (RH) e 34%) using ambient-pressure X-ray photoelectron spectroscopy (XPS). Hydroxylation occurs at the very low RH of 1 × 10 -7 % and precedes the adsorption of molecular water. With increasing RH, the OH coverage increases up to one monolayer (ML) without any distinct threshold pressure. Depth profiling measurements showed that hydroxylation occurs only at the topmost surface under our experimental conditions. The onset of molecular water adsorption varies from ∼2 × 10 -5 to ∼4 × 10 -2 % RH depending on sample temperature and water vapor pressure. The coverage of water reaches 1 ML at ∼15% RH and increases to 1.5 ML at 34% RH.

Concentration and chemical-state profiles at heterogeneous interfaces with sub-nm accuracy from standing-wave ambient-pressure photoemission

“…The modulation of the high binding energy (HBE) component, probably reflecting the presence of carboxylic and/or bicarbonate groups, shows a much smaller amplitude, and this in turn suggests a much broader depth profile for this species that thus averages over the phase of the SW. The presence of carbon in different oxidation states in the adsorbed layer on our sample under in situ measurement conditions at this high a relative humidity is due to both the sample preparation protocol as well as the inherent background contamination in experiments at elevated pressures, and has been observed in a number of experiments under similar conditions 21 .…”

“…1b phase above the sample, liquid-like water adsorbed on and above the Fe 2 O 3 surface, oxygen in the alkali hydroxides and possibly also in carboxyl-or carbonate species that we cannot resolve in this peak and that we designate simply as OH À and oxide oxygen from the Fe 2 O 3 . The assignment of these peaks is based on previous APXPS studies 21 . Gas-phase peaks are seen in all APXPS spectra, as the X-ray beam is wide enough to pass through the gas above the sample in a region also seen by the spectrometer 6,7 .…”

Comparative study of GeO2/Ge and SiO2/Si structures on anomalous charging of oxide films upon water adsorption revealed by ambient-pressure X-ray photoelectron spectroscopy

“…This is based on reports by other groups on the interaction of water vapor with the surfaces of various metal oxides including SiO 2 . 28,[32][33][34][35] In these studies, it was argued that the hydroxylation precedes the growth of a molecular water film, and the former starts at an RH of much lower than 1%. After the saturation of hydroxyl formation at a RH of around 1%, the adsorption of molecular water begins as a result of the attractive interaction between H 2 O molecules and hydroxyls.…”

How Indium Nitride Senses Water

“…at 533.6 eV on the other hand represents molecular water at the InN surface due to previous exposure to humidity. 21,22 A decrease in the spectral weight of peak 'iii' results from continuous desorption of water by the X-ray beam. This resulted in a shift of the main O 1s core level (peak 'i'), as well as the main peaks in the In 4d ( Fig.…”