Color Centers on Hydrogenated TiO₂ Facets Unlock Fluorescence Imaging

Hydrogenation of TiO₂ provides a promising strategy to realize fluorescence imaging. The fluorescence of hydrogenated TiO₂ arises from photoluminescence (PL) from the color centers. Color centers changed the surface electronic states to shorten fluorescence lifetimes, to unlock the intrinsic fluorescence of hydrogenated TiO₂. Specifically, the formation of color centers and their role in determining electronic states are highly facet-dependent. Color centers corresponding to surface oxygen vacancies (Vo) on {201} and {101} facets, surface Ti³⁺ on {001} facets, and subsurface Vo on {100} facets were discerned, following distinct Vo formation pathways and diffusion behaviors, as well as electron localization. The electronic states in the color centers are contributed by Ti 3d orbitals with different energy levels. Distinct electronic states on each facet give rise to TiO₂ coloration from white to dark gray, and the energy levels in color centers trigger unique PL emissions, enabling dark-gray hydrogenated {201} TiO₂ to emit bright intrinsic fluorescence.