  We report on the ab initio discovery of a novel putative ground state for
quasi two-dimensional TiO$_2$ through a structural search using the minima
hopping method with an artificial neural network potential. The structure is
based on a honeycomb lattice and is energetically lower than the experimentally
reported lepidocrocite sheet by 7~meV/atom, and merely 13~meV/atom higher in
energy than the ground state rutile bulk structure. According to our
calculations, the hexagonal sheet is stable against mechanical stress, it is
chemically inert and can be deposited on various substrates without disrupting
the structure. Its properties differ significantly from all known TiO$_2$ bulk
phases with a large gap of 5.05~eV that can be tuned through strain
engineering.
