  Semiconducting oxides with reducible cations are ideal platforms for various
functional applications in nanoelectronics and catalysts. Here we report an
ultrathin monolayer alpha-MoO3 where tunable electronic properties and
different gas adsorbing behaviors upon introducing the oxygen vacancies (VO).
The unique property of alpha-MoO3 is that it contains three different types of
oxygen atoms occupying three Wyckoff sites that are absent in other
low-dimensional oxides and provides rich electronic hybridized states. The
presence of VO triggers intermediate state in the gap at ~0.59 eV below the
conduction band minimum and reduces the work function dramatically, together
with new excitations at near infrared. The realigned Fermi level associated
with the dangling state of VO reduces the neighboring Mo atoms and affects the
gas adsorption thereafter. The binding energy of H2O molecules above VO is 2.5
times up to -0.75 eV compared with that of perfect lattice site and trends of
transfer of electrons also reverse. The latter is related with the shallow
localized state in the band gap due to H2O adsorbed above perfect MoO3 which
becomes quenched upon adsorbing at the VO site. Those rich in-gap defective
states in oxygen deficient MoO3, broadening the light absorption and promoting
the uptake of water, are conductive to the application of alpha-MoO3 for
optoelectronics, photothermal therapy, and sensor of moisture.
