Defect-state regulation in a fluorite high-entropy oxide for photocatalytic hydrogen evolution

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Abstract
Defect states in semiconductor photocatalysts can influence interfacial electron transfer and charge-carrier recombination, depending on their electronic properties and spatial distribution relative to reactive interfaces. Accordingly, a hexadecyltrimethylammonium bromide (CTAB)-assisted sol-gel method is employed to regulate oxygen-related defect environments in a single-phase cubic fluorite (Ce0.2Zr0.2Pr0.2Eu0.2Y0.2)O2-δ high-entropy oxide (HEO). Adjusting the CTAB amount during synthesis modifies lattice microstrain, nanocrystallite aggregation, specific surface area, surface cation oxidation-state distributions, and defect-associated oxygen environments. An intermediate CTAB amount yields the highest H2 evolution rate, coinciding with the highest relative contribution of the defect-associated species to the O 1s spectrum and comparatively low electron paramagnetic resonance (EPR) signal intensity. The optimized single-phase, cocatalyst-free fluorite HEO achieves H2 evolution rates of 483 and 1429 μmol g-1 h-1 at incident irradiances of 100 and 250 mW cm-2, respectively, and outperforms TiO2 P25 under identical conditions. These results identify CTAB-mediated modulation of oxygen-related defect environments as a viable strategy for enhancing photocatalytic H2 evolution in fluorite HEOs.

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