Analysis and characterization of BiVO₄/FeOOH and BiVO₄/α-Fe₂O₃ nanostructures photoanodes for photoelectrochemical water splitting

In this work, BiVO₄/FeOOH and BiVO₄/Fe₂O₃ electrodes were prepared by electrodeposition process and studied as a photoanode for water splitting application. Firstly, Iron (III) oxide–hydroxide (FeOOH) was deposited on BiVO₄ thin films, varying the deposit charge. Secondly, FeOOH was converted to Fe₂O₃ by an annealing treatment at 500 °C in air. Structural, optical, morphological, and electrochemical characterization was performed by XRD, UV–Vis spectroscopy, FE-SEM, and electrochemical impedance. In both cases, the monoclinic phase of BiVO₄ was correctly coupled to each material. In all cases, a decrease in band gap was observed compared to pristine BiVO₄ which is indicative of an enhancement in photonic absorption, FE-SEM results revealed the coalescence of the BiVO₄ particles with particles in the form of nanosheets when FeOOH and Fe₂O₃ were coupled. This increase in the rough surface is beneficial for the increase in reactive sites since it increases the semiconductor/electrolyte contact area in addition to increasing the photon-material interaction through multiple reflections provided by these 2D structures, thus improving charge transport. In the case of BiVO₄/FeOOH, it is observed that as the deposit charge density (amount of electrical charge distributed per unit area) increases, there is an improvement in the photocurrent and less recombination effects due to the fact that no peaks are seen in the cathodic direction, which is characteristic of carrier recombination. The highest current density is observed in the BiVO₄–FeOOH-250 mC sample with a value close to 0.20 mA/cm². While BiVO₄/Fe₂O₃ films present higher photocurrents as the deposited charge increases reaching a value close to 0.35 mA/cm², which is related to the increase in thickness and the decrease in the band gap, promoting greater light absorption. In addition, a correct band alignment for the oxygen evolution reaction was observed for both types of electrodes.