Investigation of porosity and heterojunction effects of a mesoporous hematite electrode on photoelectrochemical water splitting

In this paper, we report the porosity and heterojunction effects of hematite (α-Fe₂O₃) on the photoelectrochemical (PEC) water splitting properties. The worm-like mesoporous hematite thin films (MHFs) with a pore size of ∼9 nm and a wall thickness of ∼5 nm were successfully obtained through the self-assembly process. MHFs formed on FTO showed much better PEC properties than those of nonporous hematite thin films (NP-HF) owing to the suppression of charge recombination. The PEC data of MHFs under front and back illumination conditions indicated that the porous structure allows the diffusion of electrolyte deep inside the MHF increasing the number of holes to be utilized in the water oxidation reaction. A heterojunction structure was formed by introducing a thin layer of SnO₂ (∼15 nm in thickness) between the MHF and FTO for a dramatically enhanced PEC response, which is attributed to the efficient electron transfer. Our spectroscopic and electrochemical data show that the SnO₂ layer functions as an efficient electron transmitter, but does not affect the recombination kinetics of MHFs.