Publication date: 15th December 2014
It is known that Ti(IV) in substitutional sites in the Fe2O3 lattice modifies its doping level as well as its surface photoelectrochemical features, which results in a significant enhancement of the photocurrent levels. In this contribution, a detailed characterization about the role of titanium is presented. A set of hematite/titania samples [x.TiO2-(1-x).Fe2O3, 0 < x < 0.2] have been prepared by co-dispersion and deposition of Fe2O3 and TiO2 nanoparticles. The films doping level and surface state profiles have been respectively determined from Mott-Schottky plots and electrochemical impedance spectroscopy. As titania concentration increases, surface states density increases too and its profile shows a maximum value for nominal mixing of 10% TiO2. Higher values of surface titanium involved different phases, as corroborated by HR-TEM and EELS that confirmed the presence of pseudobrookite (Fe2TiO5) and TiO2 phases, pointing out a limited concentration of titanium and a maximum density of surface states. An astonishing increase of charge transfer rate of more of two orders of magnitude has experimentally been determined at 1.23 V, attributed to the presence of titanium as additive in the Fe2O3 lattice outer layers, as evidenced with XPS measurements. The presence of Fe2TiO5 and TiO2 phases modify these charge transfer rates according to the expected band gap coupling for these heterostructures, correlating with the behavior determined from (internal and external) quantum efficiency measurements (IPCE and APCE). The presence of titania up to 10-15% gives rise to an improvement of more than a factor 15 in the photocurrent and more than a factor 30 for the APCE values at 320 nm for the 10% TiO2 sample in comparison with that shown by the simple Fe2O3.
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