Influence of surface iron content on the photoelectrochemical properties of BiFeO films deposited via SCBD

Abstract

In the present work, composite films consisting of Bi₂₄Fe₂O₃₉ nanocrystals and an amorphous iron oxide (FeO_x) phase have been fabricated by a pulsed micro-plasma cluster source (PMCS) coupled with supersonic cluster beam deposition (SCBD) on a fluorine-doped tin oxide glass substrate, followed by air annealing. The use of BiFe targets with different composition allowed to tailor the Fe/Bi relative concentration from 6 % to 71 % in the resulting electrode materials, that were tested as photoanodes for the oxygen evolution reaction (OER), both in KOH aqueous solutions and in the presence of Na₂SO₃ as holes scavenger. The morphological characterization shows the presence of a crystalline Bi₂₄Fe₂O₃₉ phase surrounded by a dominant amorphous FeO_x matrix. Electrochemical tests on such FeO_x/Bi₂₄Fe₂O₃₉ phases reveals that the improvement of the charge transfer and charge injection constants, the shift of the flat band potential from 0.39 to 0.75 V vs. RHE and the increased photocurrent take place for an increased relative Fe content. The presence of FeO_x amorphous phase is indeed responsible for the band gap reduction from 2.57 to 2.36 eV, enabling enhanced visible light absorption, and of the significantly improved charge injection efficiency in the electrolyte solution at high biases, confirming that Fe sites are better for hole injection compared to Bi centers.