Enhanced Cr(VI) reduction by direct transfer of photo-generated electrons to Cr 3d orbitals in CrO42--intercalated BiOBr with exposed (110) facets

The adsorptive behavior and photoreduction processes of CrO42- were investigated in 110-BiOBr and 001-BiOBr suspensions without/with visible-light irradiation. On the basis of X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) measurements, CrO42- ions were adsorbed on the surface of 001-BiOBr in the form of = Bi-O-Cr-O-3- complexes, and their photoreduction depended on the pH of the solution. By contrast, CrO42- were predominantly intercalated into the bulk of 110-BiOBr, forming [OBi](+)-(O-CrO2-O)(2-)-[BiO](+) complexes. It was found that 110-BiOBr showed higher efficiency for photoreduction of CrO42- at pH 2 and 7. Density functional theory (DFT) calculation further revealed that both the acceptor Cr 3d and donor O 2p of CrO42- impurity levels were introduced into 110-BiOBr bulk, enlarging the visible absorption range of BiOBr. Therefore, the photoreduction of Cr(VI) occurred by direct photo-generated electron transfer from the improved valence band of 110-BiOBr to the Cr 3d orbitals for reduction of Cr(VI) to Cr(III) without the involvement of H+.