<bold>Synthesis of heterojunction photocatalysts composed of Ag2S quantum dots combined with Bi4Ti3O12nanosheets for the degradation of dyes</bold>

Facilitating the separation of photogenerated electron/hole pairs and widening the light-responsive region are crucial to enhance the overall photocatalytic performance of photocatalysts. To achieve this aim, here we have prepared Ag2S/Bi4Ti3O12 heterojunction composite photocatalysts by assembling Ag2S quantum dots onto the surface of Bi4Ti3O12 nanosheets. Transmission electron microscopy observation demonstrates that two types of Ag2S quantum dots separately with size of 40-70 and 7-17nm are uniformly assembled onto the surface of large-sized Bi4Ti3O12 thin nanosheets. The as-prepared Ag2S/Bi4Ti3O12 heterojunction composites exhibit much enhanced light absorption (particularly in the visible and near-infrared region) and highly efficient separation of electrons and holes photogenerated in Bi4Ti3O12. Rhodamine B (RhB) aqueous solution was chosen as the target organic pollutant to evaluate the photocatalytic performance of the samples under simulated sunlight irradiation. It is found that the Ag2S/Bi4Ti3O12 heterojunction composites manifest significantly enhanced photocatalytic activity toward the RhB degradaton. In particular, the 15wt% Ag2S/Bi4Ti3O12 composite exhibits the highest photocatalytic activity, which is ca. 2.8 and 4.0 times higher than bare Bi4Ti3O12 and Ag2S, respectively. The enhanced photocatalytic activity of the composites can be explained as a result of the Z-scheme electron transfer from the conduction band of Bi4Ti3O12 to the valence band of Ag2S, and thus more photogenerated holes in the valence band of Bi4Ti3O12 and electrons in the conduction band of Ag2S are able to participate in the photocatalytic reactions. Active species trapping experiments were carried out, from which it is concluded that photogenerated holes and center dot O-2(-) radicals play the dominant and secondary role in the photocatalysis, respectively.