Heterojunction of n-type Sr2TiO4 with p-type Bi5O7I with enhanced photocatalytic activity under irradiation of simulated sunlight

In this study, n-type Sr2TiO4 and p-type Bi5O7I, which have identical layered structures, were coupled and loaded with Ag to form a heterojunction photocatalyst. The synthesized Ag-loaded Bi5O7I/Sr2TiO4 heterojunction composite showed higher photocatalytic activity toward the degradation of methyl orange (MO) dye under irradiation of simulated sunlight than did a bare Sr2TiO4 sample. X-ray diffraction, UV-vis diffuse reflectance spectroscopy, and scanning electron microscopy analyses revealed that under alkaline conditions, bismuth ions penetrate into the Bi5O7I/Sr2TiO4 heterojunction and replace titanium ions owing to pH changes and the identical layered structures of Bi5O7I and Sr2TiO4, thus causing a reduction in the band-gap energies. Loading of Ag nanoparticles as a co-catalyst onto the Sr2TiO4 surface did not yield a p-n heterojunction or cause any improvement in the photocatalytic degradation activity. However, owing to the p-n junction, the internal built-in electric field within the space charge region at the Bi5O7I/Sr2TiO4 interface, and the electron-capture ability of Ag and localized surface plasmon resonance effect, the photo-excited electrons and holes separated and migrated effectively and the photocatalytic activity increased significantly. These results reveal that the Ag-loaded Bi5O7I/Sr2TiO4 heterojunction can serve as a promising photocatalyst.