Hydrogen induced interface engineering in Fe 2 O 3 -TiO 2 heterostructures for efficient charge separation for solar-driven water oxidation in photoelectrochemical cells.

Semiconductor heterostructure junctions are known to improve the water oxidation performance in photoelectrochemical (PEC) cells. Depending on the semiconductor materials involved, different kinds of junctions can appear, for instance, type II band alignment where the conduction and valence bands of the semiconductor materials are staggered with respect to each other. This band alignment allows for a charge separation of the photogenerated electron-hole pairs, where the holes will go from low-to-high valance band levels and vice versa for the electrons. For this reason, interface engineering has attracted intensive attention in recent years. In this work, a simplified model of the Fe 2 O 3 -TiO 2 heterostructure was investigated via first-principles calculations. The results show that Fe 2 O 3 -TiO 2 produces a type I band alignment in the heterojunction, which is detrimental to the water oxidation reaction. However, the results also show that interstitial hydrogens are energetically allowed in TiO 2 and that they introduce states above the valance band, which can assist in the transfer of holes through the TiO 2 layer. In response, well-defined planar Fe 2 O 3 -TiO 2 heterostructures were manufactured, and measurements confirm the formation of a type I band alignment in the case of Fe 2 O 3 -TiO 2 , with very low photocurrent density as a result. However, once TiO 2 was subjected to hydrogen treatment, there was a nine times higher photocurrent density at 1.50 V vs. the reversible hydrogen electrode under 1 sun illumination as compared to the original heterostructured photoanode. Via optical absorption, XPS analysis, and (photo)electrochemical measurements, it is clear that hydrogen treated TiO 2 results in a type II band alignment in the Fe 2 O 3 -H:TiO 2 heterostructure. This work is an example of how hydrogen doping in TiO 2 can tailor the band alignment in TiO 2 -Fe 2 O 3 heterostructures. As such, it provides valuable insights for the further development of similar material combinations.