posted on 2021-07-27, 19:07authored byBurcu Karagoz, Roman Tsyshevsky, Lena Trotochaud, Yi Yu, Osman Karslıoğlu, Monika Blum, Bryan Eichhorn, Hendrik Bluhm, Maija M. Kuklja, Ashley R. Head
NOx concentrations in some geographic
regions are harmful to human health. Gas filters to trap NOx and other toxic chemicals contain metal oxides,
including MoO3 and CuO. These materials are also being
investigated for NOx gas sensors. In a
step to understand the fundamental adsorption mechanism in sensors
and the effect on binding site availability in gas filters, ambient-pressure
X-ray photoelectron spectroscopy (APXPS) was used to study the interaction
of NO2 with polycrystalline MoO3 and CuO surfaces
under pressures up to 0.01 Torr (14 parts per million volume (ppmv)).
Density functional theory-based computational modeling was performed
to reveal the mechanisms of NO2 interactions with the MoO3(010) and CuO(111) surfaces to aid interpretation of the experimental
results. With pressure dependence, NO2 interacts with reduced
Mo5+ atoms generated by oxygen vacancies and abstracts
hydrogen atoms from hydroxyl groups on MoO3 without accumulating
N-containing species on the surface; vacancy-induced electronic states
in the band gap are also removed, hinting toward an increase in the
resistivity of the material. N-containing species begin accumulating
on the CuO surface at atmospherically relevant pressures of 140 ppbv.
NO2 only decomposes at oxygen vacancy sites of CuO. The
nitrogen species leave the CuO surface upon evacuation, highlighting
the importance of in situ surface characterization when studying gas
sensing and adsorption mechanisms. These results imply that NO2 removes hydroxyl and Ovac binding sties on these
materials when used in gas filtration and sensing applications. Furthermore,
the results show the key role of Ovac sites in the gas
sensing mechanism of MoO3 and highlight the potential of
APXPS for further studies of gas sensors.