Effect of annealing ambience on the formation of surface/bulk oxygen vacancies in TiO2 for photocatalytic hydrogen evolution

Highlights: • Surface and bulk oxygen vacancies were formed depending on the annealing conditions. • Surface oxygen vacancy can expand light absorption and improve charge separation. • SETOV led to visible light absorption, while played as the recombination center. • TiO₂ with two kinds of oxygen vacancies exhibited different photocatalytic mechanism. The surface and bulk oxygen vacancy have a prominent effect on the photocatalytic performance of TiO₂. In this study, TiO₂ possessing different types and concentration of oxygen vacancies were prepared by annealing nanotube titanic acid (NTA) at various temperatures in air or vacuum atmosphere. TiO₂ with the unitary bulk single-electron-trapped oxygen vacancies (SETOVs) formed when NTA were calcined in air. Whereas, TiO₂ with both bulk and surface oxygen vacancies were obtained when NTA were annealed in vacuum. The series of TiO₂ with different oxygen vacancies were systematically characterized by TEM, XRD, PL, XPS, ESR, and TGA. The PL and ESR analysis verified that surface oxygen vacancies and more bulk oxygen vacancies could form in vacuum atmosphere. Surface oxygen vacancies can trap electron and hinder the recombination of photo-generated charges, while bulk SETOVs act as the recombination center. The surface or bulk oxygen vacancies attributed different roles on the photo-absorbance and activity, leading that the sample of NTA-A400 displayed higher hydrogen evolution rate under UV light, whereas NTA-V400 displayed higher hydrogen evolution rate under visible light because bulk SETOVs can improve visible light absorption because sub-band formed by bulk SETOVs prompted the secondary transition of electron excited.