Does the Catalyst Always Assist the Methylene Blue Photodegradation or Does It Sometimes Hinder It?

Lattice defects, particularly oxygen vacancies, are commonly considered beneficial to photocatalytic performance, as they can reduce the bandgap, broaden the light absorption range, and suppress electron/hole recombination. In this perspective, high-entropy oxides (HEOs) could be considered “ideal” materials for photocatalysis because the random distribution of multiple metal cations within the crystalline lattice generates intrinsic structural disorder, which can be modulated by varying their composition and synthesis conditions. This work studies the photocatalytic activity of electrospun Cr–Mn–Fe–Co–Ni-based HEOs for the degradation of methylene blue in aqueous solution. The selected preparation conditions (calcination in air at 400 °C for 2 h) yield nanofibers consisting of ultra-small grains (≤9 nm) with copious oxygen vacancies on their surface. The presented results raise an important question: does the catalyst always promote the dye photodegradation or does it sometimes hinder it? A critical discussion is proposed, highlighting the case in which defects can induce competitive recombination pathways, negatively impacting the material’s photocatalytic performance.