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

In the present work, composite films consisting of Bi24Fe2O39 nanocrystals and an amorphous iron oxide (FeOx) 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 FeOx matrix. Electrochemical tests on such FeOx/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 FeOx 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.