Nontopological transformation of hierarchical TiO2 by self-regulated etching and capping roles of F− for photocatalytic H2 evolution

In the past decade, crystal facet engineering of TiO2 though various controllable strategies has attracted a lot of attention due to its strong facet dependent photoactivities. In this work, we report on nontopological transformation of titanium oxydifluoride (TiOF2) to hierarchical TiO2 structures by hydrothermal tailoring. The fluorine-containing species in TiOF2 crystal perform the roles as either an etching agent or a capping agent, which can conveniently modulate morphologies and facets of TiO2 crystals via dissolution or oriented collapse under hydrothermal condition. The Wulff construction and different surface energies of shape and facet lead to the controlled formation of hierarchical TiO2, respectively with (1 0 1) faceted nanowalls, (0 0 1) and (1 1 1) faceted nanosheets, or their multi-facets. All TiO2 with single exposed facet exhibit higher photocatalytic H2 evolution performance than the commercial P25 TiO2. Moreover, the (1 0 1) faceted nanowalls demonstrate superior photocatalytic H2 evolution to (1 1 1) and (0 0 1) faceted TiO2 nanosheets in presence of hole scavenger (methanol) due to favoured hole transfer at (1 0 1) facet/methanol interface. This work presents novel methodology for shape and facet control of TiO2 towards solar to fuel conversion.