Overlooked Role of Photogenerated Holes in Persistent Free Radical Formation on Hematite

Persistent free radicals (PFRs) have garnered considerable attention due to their long lifetime and high reactivity. However, the roles of photogenerated carriers in PFR formation remain underexplored. We compared and analyzed the PFR formation on hematite-SiO₂ loaded catechol, combining experimental and theoretical investigations. Significant PFRs were observed only under ultraviolet light irradiation. The PFR concentration on hematite nanoplates (HP, 1.29 × 10¹⁷ spins/mg) was higher than those on hematite nanocubes (HC, 9.19 × 10¹⁶ spins/mg) and nanorods (HR, 7.02 × 10¹⁶ spins/mg). A stronger stability of PFRs on HR (183 h of t_1/e) was observed compared with HP (95.4 h of t_1/e) and HC (37.7 h of t_1/e). Photoelectrochemical analysis and quenching experiments indicated that photogenerated holes, rather than electrons, controlled the PFR formation. Photogenerated holes manipulate the asymmetric distribution of up-spin and down-spin electrons in the p orbital of catechol to regulate PFR formation. Hole quantity and exposed facets caused significant differences in the concentration and stability of PFRs. The high concentration of PFRs on HP is due to abundant holes, while the weak stability of PFRs on HC is due to the exposed {012} facet. This study introduces a novel mechanism for PFR formation regulated by photogenerated holes, contributing to a better understanding of their environmental function and associated risks.