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.