How Should Iron and Titanium be Combined in Oxides to Improve Photoelectrochemical Properties?

We discuss here for the first time how to combine iron and titanium metal ions to achieve a high photoelectrochemical activity for TiO2-based photoanodes in water splitting devices. To do so, a wide range of photo electrode materials with tailored Ti/Fe ratio and element vicinity were synthesized by using the versatility of aqueous sol gel chemistry in combination with a microwave-assisted crystallization process. At low ferric concentrations, single phase TiO2 anatase doped with various Fe amounts were prepared. Strikingly, at higher ferric concentrations, we observed the concomitant crystallization of two polymorphs of Fe2TiO5. The as-synthesized compounds were tested as photoelectrodes and compared with pure nanoparticles of TiO2, Fe2TiO5, and alpha- or gamma-Fe2O3 and with corresponding nanocomposites. When TiO2 is slightly doped by Fe, the performance of this photoelectrode improves particularly in the low-bias region (<1.0 V vs reversible hydrogen electrode.) The photoanode exhibits a higher photocurrent than nanocomposite with TiO2/Fe2O3 and FeTi2O5 and more cathodic onset potential. The former can be partly explained by a lower bandgap and a hole with a longer lifetime. For the latter, we propose that the nature of the heterojunction impacts charge carrier recombination. The results presented herein not only answer whether iron and titanium should be combined in the same structure or into heterostructured systems but also on the importance of the arrangement of ions in the structure to improve the performances of the photoanode.