First Experimental Validation of the Revised Bandgap and Exciton-Limited Photocatalytic H2 Evolution over CeNbO4

Abstract The true bandgap of CeNbO4, a rare-earth niobate, has long been underestimated by conventional Tauc plot analysis. Recent Elliott–Toyozawa exciton modeling revealed an actual bandgap of 3.25 eV and an exciton binding energy of ∼100 meV, shifting the expected absorption from visible to ultraviolet. However, the photocatalytic H2 evolution performance of CeNbO4 has remained unexplored, and the photocatalytic consequences of such a strong excitonic effect have never been experimentally tested. Herein, we report the first systematic study of photocatalytic H2 production over CeNbO4 under UV irradiation. Pristine CeNbO4 exhibits a low H2 evolution rate of 5.6 μmol/h, consistent with the high exciton binding energy (≫kT at room temperature) that severely limits free carrier generation. Among four noble metal cocatalysts (Pt, Au, Ag, Ru), Pt shows the best promotion effect. Optimising Pt loading gives a maximum H2 evolution rate of 25.7 μmol/h at 4 wt % Pt, four times that of the pristine sample. Photoelectrochemical and photoluminescence measurements confirm that Pt nanoparticles act as electron traps, partially mitigating carrier recombination. This work provides the first baseline data for photocatalytic H2 evolution over CeNbO4 and delivers the first experimental validation, from a photocatalytic perspective, of the revised bandgap and exciton-limited behavior of this material.