Synergistic effect of Sn doping and hydrogenation on hematite electrodes for photoelectrochemical water oxidation

Hematite (α-Fe2O3), which is abundant, chemically stable, and environmentally benign, is a promising photoanode material that can oxidize water in photoelectrochemical (PEC) water splitting. However, the poor electrical properties of hematite limit its intrinsic activity with regard to PEC water splitting. Herein, we report an innovative dual modification strategy that combines metal (Sn) doping and H2 treatment to prepare an efficient hematite photoanode for PEC water splitting. Sn doping and the subsequent H2 treatment generate different electron donors, Sn4+ and oxygen vacancies (Vo), in hematite. Electrochemical impedance measurements revealed that dual-modified hematite with a synchronous presence of Sn4+ and Vo exhibited significantly improved electrical properties with 111 times higher donor density and 10 times smaller charge transfer resistance than those of bare hematite. As a result, the photooxidation current was 4.61 mA cm-2 (at 1.6 VRHE), which is 55 times higher than that of bare hematite (83 μA cm-2) and substantially higher than the sum of the photooxidation current exhibited by the two single-modified hematites (298 μA cm-2 for Sn doping and 681 μA cm-2 for H2 treatment). A stable photocurrent was maintained under prolonged illumination over 12 h without showing any sign of deactivation. Additional cobalt treatment further increased the PEC water oxidation performance of the dual-modified hematite, achieving a superior Faraday efficiency (ca. 99%) and stability. Compared to the oxygen-deficient heat treatment (under Ar) of Sn-doped hematite, the combination of Sn doping and H2 treatment induces outstanding synergistic improvement of PEC activity (photocurrent twice that of Sn-doped and Ar-treated hematite).