Solar-powered photoelectrochemical (PEC) water splitting has been a promising candidate for producing hydrogen in a clean and renewable way. Photoelectrodes are key components in PEC cells for efficient and stable hydrogen generation because they play crucial roles in absorption of photons, the separation and transportation of photo-generated charge carriers, as well as the chemical reactions with water. A variety of metal oxides for efficient photoelectrode have been intensively explored, but it is still challenging to find desirable materials to satisfy lots of requirements for PEC water splitting.

Iron oxide (hematite, Fe₂O₃) has recently attracted much attention due to its earth abundance, low cost as well as desirable material properties for PEC water oxidation including narrow band gap energy of 2.0~2.2eV for visible light absorption and proper energy band alignment, etc. However, Fe₂O₃ has very short hole diffusion length and low carrier mobility, which causes considerable recombination of photo-generated electrons and holes. A lot of approaches such as nanostructures, heterojunction with other materials, surface modification, etc. have been reported to prevent the recombination of charge carriers and improve electrical properties of Fe₂O₃; however, these require complex manufacturing processes.

In the present work, we found a much simpler way to improve the electrical properties of Fe₂O₃ film, namely defect-pairs due to co-doping. Titanium (Ti) and carbon (C) co-doped thin Fe₂O₃ film (i.e. (Ti,C)-Fe₂O₃) has been realized via a combination of simple solution-based spin-coating and tube furnace annealing process. This film turns out to lead significantly enhanced PEC performance when used as a photoanode: an impressively high photocurrent density of more than 4.5mAcm^-2 was achieved at 1.23V_RHE under AM1.5G solar spectrum and 1 sun illumination. This is compared to the value of Ti-doped Fe₂O₃ film, which is only about 2.6mAcm^-2 photocurrent density at 1.23V_RHE even though the optical properties of each film are similar. The origin for such substantial enhancement was revealed using a series of experimental and computational spectroscopies. X-ray absorption spectroscopy, electrochemical impedance measurements and density-functional-theory calculations both indicate that C atoms can be more deeply and heavily doped under the existence of Ti dopants in Fe₂O₃ film and then the defect-pairs of Ti and C increase not only charge carrier density but also electron’s mobility. An emphasis should be placed on the fact that this achievement was not assisted by co-catalysts and complex nanostructuring methods; hence even higher performance is expected when the film is further treated with extra-cares.