Dual modification of hematite photoanode by Sn-doping and Nb₂O₅ layer for water oxidation

Porous hematite (α-Fe₂O₃) films doped with Sn(IV) and coated with an ultrathin (∼2 nm thick) Nb₂O₅ passivation layer were synthesized, and the photoelectrochemical (PEC) water oxidation performance and durability of the hematite were examined in detail. As compared to hematite samples modified by either doping or passivation, dual-modified hematite exhibited a promising PEC water oxidation performance under AM 1.5 irradiation. A stable photocurrent was maintained under prolonged irradiation over 24 h, while O₂ was produced from water with a Faradaic efficiency of over 80% without showing any sign of deactivation. This performance and durability could be decoupled into separate effects of Sn doping and Nb₂O₅ layer via in-depth surface characterization and electrochemical analyses. Sn doping increased the donor density (N_d) of bare hematite by a factor of 20 and significantly improved its conductivity, leading to enhanced charge transfer efficiency. The Nb₂O₅ layer exerted an effect similar to Sn doping because of the diffusion of a fraction of Nb(V) into the hematite lattice during the annealing process at 700 °C. The primary effect of the Nb₂O₅ layer is to passivate the hematite surface and make the surface more reactive toward the oxygen evolution through water oxidation. These effects are synergistically combined in the dual-modified hematite electrode.