Single Crystalline Hematite Films for Solar Water Splitting: Ti-Doping and Thickness Effects

Undoped and Ti-doped (2 at. %) epitaxial hematite thin films, in the thickness range 5-50 nm, were grown by atomic oxygen assisted molecular beam epitaxy (AO-MBE) on Pt( 111) substrates in the framework of hydrogen harvesting from sunlight-induced water splitting. Such single crystalline samples are suitable model systems to study thickness and doping effects on the photoelectrochemical properties; we demonstrate that they also allow disentangling intrinsic transport properties from mingled overall properties due to the usually unknown contributions from morphology or crystalline structure defects. From their photoelectrochemical characteristics (I(V) curves, incident photon to current efficiency measurements, and electrochemical impedance spectroscopy), we evidence the existence of an optimum layer thickness, which is higher for Ti-doped samples (30 nm) as compared to undoped ones (20 nm). Our results suggest that this effect is due to an increase of the carrier concentration combined with higher carriers' diffusion lengths in the doped samples stressing intrinsic modifications of the hematite layer upon titanium doping that cannot be accounted for by simple structural or electronic structure changes.