Electrochemical Oxidation of Metal-Catechol Complexes as a New Synthesis Route to the High-Quality Ternary Photoelectrodes: A Case Study of FeTiO Photoanodes.

A new electrochemical, solution-based synthesis method to prepare uniform multinary oxide photoelectrodes was developed. This method involves solubilizing multiple metal ions as metal-catechol complexes in a pH condition where they are otherwise insoluble. When some of the catechol ligands are electrochemically oxidized, the remaining metal complexes become insoluble and are deposited as metal-catechol films on the working electrode. The resulting films are then annealed to form crystalline multinary oxide electrodes. Because catechol can serve as a complexing agent for a variety of metal ions, the newly developed method can be used to prepare a variety of multinary oxide films. In the present study, we used this method to prepare n-type FeTiO photoanodes and investigated their photoelectrochemical properties for use in a photoelectrochemical water-splitting cell. We also performed a computational investigation with two goals. The first goal was to investigate small electron polaron formation in FeTiO. Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in FeTiO has not been examined prior to this work. The second goal was to investigate the effect of substitutional Sn doping at the Fe site on the electronic band structure and the carrier concentration of FeTiO. The combined experimental and theoretical results presented in this study greatly improve our understanding of FeTiO for use as a photoanode.