Photocharging of Europium(III) Tellurium Oxide as a Photoelectrocatalyst

This work reveals that Eu₂TeO₆, as a photoelectrocatalyst, exhibits a sustained photocharging effect that results in an enhanced and "sustained" dark current (subsequent to the termination of illumination), which is greater than the current obtained through applied electrical potential alone. Samples of europium tellurium oxide, prepared through the employment of a Pechini sol-gel route and calcined at 400 and 900 °C, were characterized for selective physicochemical properties and comparatively evaluated as an electrocatalyst, photoelectrocatalyst, and photocharged electrocatalyst by employing, as a suitable case study, the oxygen evolution reaction (OER) in alkaline medium. The sample calcined at 400 °C exhibited amorphous characteristics while the sample calcined at 900 °C proved to be Eu₂TeO₆. The evaluation was conducted in a modified electrochemical cell housing a lamp, with the material deposited onto a glassy carbon electrode support rather than on fluorine- or indium-doped tin oxide glass plates. With a band gap of 4.31 eV, Eu₂TeO₆ (calcined at 900 °C) exhibited improved OER activity upon continuous UV-light irradiation, with the unexpected result of sustained improved activity for up to 40 min subsequent to the termination of the UV irradiation. At 1.8 V (versus RHE, reversible hydrogen electrode), the photocurrent, after 150 min of irradiation, increased by 107% from 0.44 to 0.91 mA cm^-2. Surprisingly, after 150 min of irradiation, the dark current was sustained at 0.86 mA cm^-2 for 40 min (at which point measurement was terminated), which represents a sustained increase of 95% compared to the initial unirradiated sample. The phenomenon observed here could contribute toward an improved understanding of photocharging and ultimately to the exploitation of more efficient solar energy conversion and storage.