Novel Protective Coatings for Efficient Photoanodes with Tunable Intermediate Bands Induced by Transition-Metal Cations in TiO₂

The energetics alignment between the TiO₂-based protective coating and the photoanode is significant for efficient water splitting and other fuel conversion devices. Since different semiconductors have different valence band maximum energy supporting various oxidative reactions with different potentials, appropriate intermediate band (IB) positions are required to accomplish better charge transfer for efficient devices. This work found transition-metal cations doping can be used to fabricate TiMO_x (M standing for manganese, chromium, etc.), tuning the IBs by the atomic layer deposition method. Valence band X-ray photoelectron spectroscopy was conducted to measure the IB position and width, which illustrates that the IB is dependent on the doping element, concentration, and cluster size. To illustrate the photoelectrochemistry (PEC) performance, leaky TiO₂, TiO₂:Mn, and TiO₂:Cr coated gallium phosphide electrodes are tested in Fe(CN)₆^3-/4- redox solution under light illumination, which shows a large electron transfer rate compared to traditional leaky TiO₂ coating. With iridium oxide nanoparticles as cocatalyst deposited on the protective coating, the electrode conducts water oxidation reaction in pH=9 phosphate buffer solution and reached the light-limited current at 1-sun illumination, which shows high efficiency and stability. This success of achieving tunable IB protective coating expands the generality of the application of TiO₂-based protective coating for semiconductors and catalysts with different energy levels, thus providing a new pathway for designing stable and efficient PEC devices.