TiO2 phtotocatalyst has been attracting great research attentions due to its potential in solar energy conversion/storage for various applications. When TiO2 is excited, both reduction and oxidation reactions could happen. Photocatalytic reduction could provide promising solutions to hydrogen production from water splitting, CO2 reduction for fuel production, and the removal of various environmental pollutants. To enhance the photocatalytic reduction efficiency, sacrificial agents are usually needed to deplete the photogenerated holes. However, it could increase the complexity and cost of the operation, and may not be appropriate for some applications (for example, drinking water treatment) due to the addition of substances with potential hazard.
For the enhancement of the photocatalytic efficiency, noble/transition metal modification is widely used as the electron trapping center to enhance the photogenerated electron-hole pair separation. However, this material design strategy could not solve problems associated with the addition of sacrificial agents for photocatalytic reduction. Thus, it would be most desirable to design a photocatalyst system for photocatalytic reduction in which the charge carrier recombination could be minimized by modifications with hole trapping and consumption capability. As a semimetal element, bismuth may provide the hole trapping and consumption capability. Unlike most metals, bulk Bi has a relatively low work function of ~ 4.22 eV, close to that of TiO2 at ~ 4.20 eV. With its size decrease into the nano range, a transition from metal to semiconductor could occur with the moving up of its conduction subbands and moving down of its valence subbands. So photogenerated electrons could not transfer from TiO2 to Bi quantum dots anymore, while photogenerated holes could transfer from TiO2 to Bi quantum dots and be consumed by oxidizing Bi0 to Bi3+. Thus, it could enhance the lifetime of photogenerated electrons for an efficient reduction process, while no sacrificial agents are needed to deplete holes.
In this study, Bi quantum dots were deposited on rutile TiO2 nanoparticle surface to create the Bi/TiO2 heterojunction photocatalyst. In this Bi/TiO2 photocatalyst, rutile TiO2 served as the main visible light absorber, while Bi quantum dots served as the hole trapping centers to enhance the charge carrier separation and eliminate the need of sacrificial agents to consume photogenerated holes in photocatalytic reduction process. Thus, an efficient photocatalytic bromate reduction under visible light illumination was achieved by this Bi/TiO2 photocatalyst without the addition of sacrificial agents in the reaction solution, and it demonstrated a good regeneration capability and reusability. This study demonstrated a novel strategy for the design of photocatalysts with strong photocatalytic reduction capabilities for a broad range of technical applications.