Improving the Surface Oxygen Vacancy Concentration of Bi2O4 through the Pretreatment of the NaBiO3·2H2O Precursor as a High-Performance Visible Light Photocatalyst.

The oxygen-deficient bismuth oxide, Bi2O4, synthesized by a typical hydrothermal method using commercial NaBiO3·2H2O as a raw material only has a relatively low concentration of surface oxygen vacancies (OVs). How to improve the visible light photocatalytic performance of Bi2O4 via tuning its surface OV concentration is still a huge challenge. In this study, improving the surface OVs of Bi2O4 was successfully realized through the pretreatment of commercial NaBiO3·2H2O, including thermal treatment in air and hydrothermal treatment in 10 M NaOH solution, forming NaBiO3·xH2O intermediate products first, and then hydrothermal preparation of Bi2O4 target products using NaBiO3·xH2O instead of commercial NaBiO3·2H2O as the precursor. The enhanced surface OV content not only narrows the band gap of Bi2O4 and thus extends its optical response range but also captures more photoexcited electrons and thus increases the charge carriers' separation efficiency and prolongs the charge carriers' lifetime of Bi2O4. Among the above-mentioned two pretreatment methods, the effects of the hydrothermal pretreatment are superior to those of the thermal treatment, involving the increase of surface OVs, the optical harvesting capacity, and the charge carriers' separation efficiency. Accordingly, Bi2O4 prepared by the hydrothermal pretreatment route exhibits the optimal visible light catalytic performance toward the removal of methyl orange (MO) and phenol due to its most abundant surface OV concentration, which is 2.59 times and 4.26 times higher than that of Bi2O4 synthesized directly by the commercial NaBiO3·2H2O route, respectively. Holes (h+) and superoxide radicals (•O2-) are identified as the main active species, while singlet oxygen (1O2) and hydroxyl radicals (•OH) are verified as the second and third important active species for organic pollutant removal, respectively. This work has developed a novel strategy to promote the catalytic performance of single Bi2O4 induced by the enhanced surface OV concentration through the pretreatment of the precursor, commercial NaBiO3·2H2O.