Nanostructured FTO/Zr-hematite interfaces for solar water-splitting applications

Production of green hydrogen via photoelectrochemical (PEC) processes is strategic to store and distribute solar-harvested energy. Hematite (alpha-Fe2O3) is a promising photoactive photoanode material candidate for PEC applications, but its synthesis as a photoanode thin film needs improvements to optimize the PEC device's per-formance. We have previously explored a new strategy consisting of the use of Zr4+ as a surfactant (Zr-hematite) and Ni2+ as a co-catalyst element in the synthesis of hematite photoanode thin films deposited onto a fluorine-doped tin oxide (FTO) on a glass substrate, obtaining a significant enhancement of device performance. In this work, we demonstrated how the interface and surface conditions are affected by the incorporation of Zr4+ and Ni2+. Chemical mapping performed during scanning transmission electron microscopy (STEM) analysis indicated that Zr4+ preferentially segregated at the FTO/Zr-hematite interface and grain boundaries, while Ni2+ was only present on hematite-free surfaces. The Zr4+ between the hematite and substrate interface was associated with improved back contact, facilitating electron collection and favoring electron flow through the external circuit. Structural analysis of this interface region revealed lattice distortions and strain field accumulation at the FTO/ Zr-hematite interface, which differed from the expected crystal structures of the database. Additionally, local FTO/Zr-hematite energy maps showed a 2.5-nm thick interface area containing a solid mixture of SnFeOx. Moreover, a change in the electronic state of Sn and Fe on the surface of the FTO substrate and Zr-hematite grains caused by the loss of oxygen atoms was observed. NiFeOx chemical bonding was observed with Ni addition by photoelectrodeposition on the Zr-hematite-free surface. This strong interaction explains the long-term stability test without traces of photocorrosion (ion release into the solution), as demonstrated in a previous paper. These findings introduce a novel perspective for developing strategies that enable synergistic photoanode modifications toward efficient solar-to-energy conversion.