Understanding the mechanism of saturated and mono-/tri-lacunary Keggin SiWx doped in Bi2WO6 and BiOBr for efficient photocatalytic CO2 reduction

Different from the doping of common simple ions, polyoxometalates (POMs) with the ability to regulate electron transfer as a large dopant and their doping mechanism is rarely studied. In this work, saturated and mono-/tri-lacunary Keggin SiWx (x = 9, 11, 12) have been successfully doped in Bi2WO6 and BiOBr through a two-step hydrothermal method, respectively, in which SiWx respectively replaced WO42− of Bi2WO6 and Br− of BiOBr, and are denoted as SiWx-Bi2WO6 and SiWx-BiOBr. As a shallow electron trap, SiWx inhibits photoinduced carrier recombination of Bi2WO6 and BiOBr. Besides, SiWx-Bi2WO6 and SiWx-BiOBr behave differently in photocatalytic CO2 reduction, which the CO yields of SiW9-Bi2WO6, SiW11-Bi2WO6 and SiW12-Bi2WO6 are 1.5, 3.3 and 6.2 times higher than Bi2WO6, while the CO yields of SiW9-BiOBr, SiW11-BiOBr and SiW12-BiOBr are 1.6, 2 and 2.6 times that of BiOBr. The corresponding reasons for the different performances can be attributed to the different hydrophobicity of the materials, reduction of the Bi2WO6 and BiOBr nanosheet stacking thickness and inhibition of carrier recombination. The deep insights into the influences of saturated and lacunary POMs on photocatalytic performances provide more ideas for the rational design of photocatalytic materials for CO2 reduction to high-valuable products.