Cerium titanates: An unexplored class of UV-visible light active photoanodes for water splitting
Résumé
Photoelectrochemical water splitting is hoped to become a competitive source of green hydrogen, but there is still a need to discover novel photoelectrode materials [1]. Cerium titanates have been known for more than 50 years but only a handful of studies have investigated their potential applications [2]. Recent reports have shown that the CeTi2O6 brannerite phase was a promising photocatalyst under UV-visible light illumination [3]. To date, no study has used cerium titanates for photoelectrochemical water splitting and described their photo-electronic properties. In this study, cerium titanates thin films were prepared by spin-coating on fluorine-doped tin oxide (FTO). The as-prepared photo-electrodes were characterized by SEM, XRD, and Raman spectroscopy. Photovoltammetry, DRS-UV-Vis, XPS, and UPS were combined to describe the band structure of this new material. The phase was successfully prepared at 700°C as confirmed by grazing incidence X-ray diffraction and Raman spectroscopy. Under an Xe arc lamp illumination (approx. 1 sun), a promising photocurrent of 0.3 mA.cm-2 at 1.23 V vs. NHE and a high photovoltage of 1.74 V were observed. The band gap of the films was estimated to be around 3.1 eV but was much lower for powders crystallized at the same temperature (2.6 eV). XPS analysis of the Ce3d core level and the Ce4f valence states demonstrated a surprisingly high density of Ce3+ (up to 50%) in the surface layers of the film. This behavior was correlated to a reducing agent during the synthesis, namely, ethylene glycol.
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