Ball Milling of La₂O₃ Tailors the Crystal Structure, Reactive Oxygen Species, and Free Radical and Non-Free Radical Photocatalytic Pathways

Abstract

Non-free radical photocatalysis with metal oxide catalysts is an important advanced oxidation process that enables the removal of various emerging environmental pollutants, such as tetracycline. Here, four hexagonal La 2 O 3 photocatalysts with different densities of oxygen vacancy and crystalline features are synthesized and then further treated by ball milling. Ball milling of these La 2 O 3 photocatalysts is found to increase the amount of oxygen vacancies on their surfaces and thereby the amount of 1 O 2 species produced by them. The photocatalytic degradation of TC by these La 2 O 3 photocatalysts depends on the oxygen vacancies present on them. Furthermore, the ones with a strong (101) diffraction peak remove tetracycline from water systems largely with 1 O 2 and • OH species, whereas those with a weak (101) diffraction peak do so mainly via 1 O 2 and direct electron transfer (DET) process. Their overall catalytic properties are also studied by density functional theory calculations. Moreover, the organic products produced from tetracycline by La 2 O 3 photocatalysts containing a strong (101) diffraction peak are found to be less toxic than those produced by La 2 O 3 photocatalysts containing a weak (101) diffraction peak. This study also provides convincing evidence that the structures of La 2 O 3 determine the species that is produced by it and that end up mediating photocatalytic reaction pathways (i.e., free radical versus non-free radical) to degrade an emerging environment pollutant.