  The structural, electronic, and magnetic properties of pristine, defective,
and oxidized monolayer TiS3 are investigated using first-principles
calculations in the framework of density functional theory. We found that a
single layer of TiS3 is a direct band gap semiconductor, and the bonding nature
of the crystal is fundamentally different from other transition metal
chalcogenides. The negatively charged surfaces of single layer TiS3 makes this
crystal a promising material for lubrication applications. The formation
energies of possible vacancies, i.e. S, Ti, TiS, and double S, are investigated
via total energy optimization calculations. We found that the formation of a
single S vacancy was the most likely one among the considered vacancy types.
While a single S vacancy results in a nonmagnetic, semiconducting character
with an enhanced band gap, other vacancy types induce metallic behavior with
spin polarization of 0.3-0.8 {\mu}B. The reactivity of pristine and defective
TiS3 crystals against oxidation was investigated using conjugate gradient
calculations where we considered the interaction with atomic O, O2, and O3.
While O2 has the lowest binding energy with 0.05-0.07 eV, O3 forms strong bonds
stable even at moderate temperatures. The strong interaction (3.9-4.0 eV)
between atomic O and TiS3 results in dissociative adsorption of some
O-containing molecules. In addition, the presence of S-vacancies enhances the
reactivity of the surface with atomic O, whereas it had a negative effect on
the reactivity with O2 and O3 molecules.
