Water Adsorption and Dissociation on Polycrystalline Copper Oxides: Effects of Environmental Contamination and Experimental Protocol

Abstract

We use ambient-pressure X-ray photoelectron spectroscopy (APXPS) to study chemical changes, including hydroxylation and water adsorption, at copper oxide surfaces from ultrahigh vacuum to ambient relative humidities of ∼5%. Polycrystalline CuO and CuO surfaces were prepared by selective oxidation of metallic copper foils. For both oxides, hydroxylation occurs readily, even at high-vacuum conditions. Hydroxylation on both oxides plateaus near ∼0.01% relative humidity (RH) at a coverage of ∼1 monolayer. In contrast to previous studies, neither oxide shows significant accumulation of molecular water; rather, both surfaces show a high affinity for adventitious carbon contaminants. Results of isobaric and isothermic experiments are compared, and the strengths and potential drawbacks of each method are discussed. We also provide critical evaluations of the effects of the hot filament of the ion pressure gauge on the reactivity of gas-phase species, the peak fitting procedure on the quantitative analysis of spectra, and rigorous accounting of carbon contamination on data analysis and interpretation. This work underscores the importance of considering experimental design and data analysis protocols during APXPS experiments with water vapor in order to minimize misinterpretations arising from these factors.

Water adsorption on vanadium oxide thin films in ambient relative humidity

“…40,41 A RH of 0.01% has been identified as the critical onset value for the formation of these complexes. 42,43 On the VO 2 surface presented in this work, the presence of -OH precedes H 2 O adsorption and in most cases coverage of both species begins to increase more rapidly around 0.01% RH, so VO 2 is consistent with the proposed model. The difference in the kinetic barrier for -OH formation and the number and nature of -OH groups on surfaces can be related to defect concentration.…”

Neutron spectroscopy study of the diffusivity of hydrogen in MoS2

“…The O 1s line splitting that appears at 500 K indicates that a large fraction of the water molecules transfer from a strongly adsorbed state to almost free H 2 O molecules at this temperature. 44,45 To conclude, the XPS study shows that chemisorbed hydrogen shows a thermally activated mobility in MoS 2 when the temperature of the sample is raised to 400 K or above. This leads to a chemical transformation of the sample as reduction and oxidation of MoS 2 can be observed.…”

Water (Non-)Interaction with MoO3

“…Overall, the experimental results show that the introduction of defects (oxygen vacancies) through sputtering increases the number of hydroxyl groups at the surface, which, in turn, results in an increase in the adsorption of molecular water. However, in stark contrast to other metal oxide surfaces, [6][7][8][9]11,13,14,[16][17][18][19]22 the amount of hydroxyl groups and water at the surface at elevated RH is still small, pointing to a weak interaction of water with MoO 3 . To understand this phenomenon better, we next employ DFT to model the reaction.…”

“…We also tested the influence of a hot ion gauge filament on the results; as was demonstrated in other systems, a hot filament often causes cracking of gaseous species, including molecular water, resulting in an increase of surface hydroxylation of some metal oxides. 17 However, the O 1s spectra recorded during water dosing with the ion gauge on ( Figures S6 and S7 ) are nearly identical, highlighting the hydrophobicity and inertness of MoO 3 . Since the ion gauge was not found to influence hydroxylation and water adsorption in this case, we show the O 1s spectrum for OH–MoO 3 at 6% RH with the ion gauge on in Figure 3 b so that the MoO 3 and the OH–MoO 3 surfaces are compared under the same humidity conditions.…”