Perovskite-type lanthanum iron oxide, LaFeO3, is a p-type semiconductor that can achieve overall water splitting using visible light while maintaining photostability. These features make LaFeO3 a promising photocathode candidate for various photoelectrochemical cells. Currently, the photoelectrochemical performance of a LaFeO3 photocathode is mainly limited by considerable bulk electron–hole recombination. This study reports a combined theoretical and experimental investigation on the atomic doping of LaFeO3, in particular, substitutional doping of La3+ with K+, to increase its charge-transport properties and decrease electron–hole recombination. The computational results show that K-doping enhances not only the charge-transport properties but also photon absorption below the bandgap energy of the pristine LaFeO3. The effect of K-doping was systematically investigated by comparing the electronic and atomic structures, majority carrier density, hole-polaron formation, and optical properties of pristine and K-doped LaFeO3. The computational results were then verified by experimentally characterizing the crystal structures, compositions, optical properties, and photoelectrochemical properties of LaFeO3 and K-doped LaFeO3 electrodes. For this purpose, pristine LaFeO3 and K-doped LaFeO3 were prepared as high-surface-area, high-purity photoelectrodes having the same morphology to accurately and unambiguously evaluate the effect of K-doping. The combined computational and experimental investigations presented in this study provide useful insights into the effect of composition tuning of LaFeO3 and other p-type oxides with a perovskite structure.

Highly effective doping in transition metal oxides is critical to fundamentally overcome low carrier conductivity due to small polaron formation and reach their ideal efficiency for energy conversion applications. However, the optimal doping concentration in polaronic oxides such as hematite has been extremely low, for example, less than a percent, which hinders the benefits of doping for practical applications. In this work, we investigate the underlying mechanism of low optimal doping concentration with group IV (Ti, Zr, and Hf) and XIV (Si, Ge, Sn, and Pb) dopants from first-principles calculations. We find that novel dopant-polaron clustering occurs even at very low dopant concentrations and resembles electric multipoles. These multipoles can be very stable at room temperature and are difficult to fully ionize compared to separate dopants, and thus they are detrimental to carrier concentration improvement. This allows us to uncover mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic oxides toward highly efficient energy conversion applications.

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 Fe 2 TiO 5 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 Fe 2 TiO 5 . Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in Fe 2 TiO 5 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 Fe 2 TiO 5 . The combined experimental and theoretical results presented in this study greatly improve our understanding of Fe 2 TiO 5 for use as a photoanode.

Achieving highly efficient energy conversion with transition metal oxides necessitates overcoming conductivity limitations due to the formation of small polarons. Detailed understanding of the interplay among intrinsic defects, dopants, and electron polarons can help devise strategies for achieving higher carrier concentrations, therefore improving carrier conductivity. This work employs first-principles calculations to reliably predict electron polaron concentrations in a prominent polaronic oxide, hematite (Fe2O3), by resolving interactions between charged defects and electron polarons and keeping charge neutrality condition among all charged species. This work addresses that both VO and Fei can be primary donors in undoped hematite depending on the synthesis conditions, such as synthesis temperature and oxygen partial pressure, despite the fact that VO owns an extremely high ionization energy compared to kBT. Furthermore, from calculations of a plethora of n-type dopants (group IV and V elements), we find that Ti, Ge, Sb, and Nb are able to raise electron polaron concentrations in hematite significantly without considering dopant clustering. However, the magnitude of electron polaron concentration increase would be smaller if the dopant has a high tendency of clustering, such as Ti. We reveal the critical role of synthesis conditions on tuning electron polaron concentrations of both undoped and doped hematite. Our theoretical analysis provides important insights and general design principles for engineering more conductive polaronic oxides.