Generation of environmentally persistent free radicals on faceted TiO2 in an ambient environment: roles of crystalline surface structures

The formation of environmentally persistent free radicals (EPFRs) is usually associated with the valence changes of transition metals. The crystalline structures of metal oxides and the role of degradation byproducts in the formation and stabilization of EPFRs have not been fully addressed. In this study, three types of anatase TiO2 with predominantly exposed {001}, {010}, and {101} facets were prepared as model crystal facet structures. Catechol (CT) was loaded onto the faceted TiO2 at room temperature, and EPFRs were only generated on {010}-TiO2 with valence unchanged. The p-d conjugation between Ti and the O from CT was a prerequisite for EPFR formation, and the presence of high-energy electrons involved in p-d conjugation was essential for EPFR stabilization based on quantum chemical calculations. The higher density of unsaturated titanium atoms and more favorable atomic arrangement for electron transfer are the reasons why only {010}-TiO2 stabilized EPFRs. The stability of EPFRs under light and dark conditions was also investigated. The intermediates from CT formed secondary EPFRs at the early stages of the photocatalytic process. Continuous illumination accelerated intermediate degradation and was accompanied by a decrease in EPFR types. This study provides an in-depth understanding of the crystalline structure and degradation byproducts involved in the formation of EPFRs.

Automated summary

This study investigates the influence of metal oxide structures and degradation byproducts on the formation of EPFRs. The results show that only certain facet structures of anatase TiO2 can stabilize the formation of EPFRs. The stability of EPFRs is affected by light conditions and the presence of degradation byproducts.