Ytterbium modification of pristine and molybdenum-modified hematite electrodes as a strategy for efficient water splitting photoanodes

In recent years, the surface modification of photoanodes for photoelectrochemical water splitting with passivation overlayers has attracted considerable attention. In this respect, a novel, easy and simple methodology to introduce ytterbium oxide as an overlayer on hematite nanorod electrodes is reported in this work. The hematite electrodes were synthesized by means of a chemical bath method, while the ytterbium precursor was introduced through an impregnation method (drop-casting). FE-SEM, XRD, and XPS were employed to characterize the electrode both structurally and morphologically. The reported results reveal that the impregnation method did not cause apparent changes in the hematite structure and morphology, retaining the nanorod structure. Importantly, adding ytterbium yields a significant improvement in the photo-activity (14x at 1.23 V vs RHE) without altering significantly the photo-onset. The obtained results suggest that ytterbium induces the formation of a passivating layer, pointing to the fact that other lanthanide oxides would behave similarly. A study of a bifunctional modification of hematite employing ytterbium and molybdenum was also carried out. It reveals that the photocurrent obtained by employing both strategies increases with respect to that obtained with the application of only one of the procedures. Importantly, the order in which modification is done greatly affects the final electrode performance. Understandably, the best results are obtained when Mo is introduced prior to Yb, leading to a synergetic effect in the sense that the resulting photocurrent is larger than the sum of the photocurrents obtained through the application of one of the modifiers.Authors are grateful to MINECO of Spain for the financial support through project MAT2015-71727-R (FONDOS FEDER). A.C. wants to acknowledge the University of Alicante for a predoctoral grant (FPU-UA)