An in situ fluorine and ex situ titanium two-step co-doping strategy for efficient solar water splitting by hematite...

A unique two-step co-doping strategy of in situ fluorine doping followed by ex situ titanium doping enhances the performance of the hematite photoanode in photoelectrochemical water splitting much more effectively than single-step co-doping strategies that are either all in situ or all ex situ. The optimized fluorine, titanium co-doped Fe₂O₃ photoanode without any cocatalyst achieves 1.61 mA cm^-2 at 1.23 V_RHE under 100 mW cm^-2 solar irradiation, which is ∼2 and 3 times those of titanium or fluorine singly-doped Fe₂O₃ photoanodes, respectively. The promotional effect is attributed to the synergy of the two dopants, in which the doped fluorine anion substitutes oxygen of Fe₂O₃ to increase the positive charges of iron sites, while the doped titanium cation substitutes iron to increase free electrons. Moreover, excess titanium on the surface suppresses the drain of in situ doped fluorine and agglomeration of hematite during the high-temperature annealing process, and passivates the surface trap states to further promote the synergy effects of the two dopants.