Enhanced Photoreduction of Chromium(VI) Intercalated Ion Exchange in BiOBr0.75I0.25 Layers Structure by Bulk Charge Transfer

Because of its low separating and utilizing efficiency of photogenerated charges in the bulk, the application of photocatalytic technique has been restricted for decades. In this paper, based on interlayer ion-exchange between BiOBr0.75I0.25 layers, we demonstrated a specific interfacial process of photocatalytic Cr(VI) reduction by a direct bulk-charge transfer. The results showed that Cr(VI) was effectively converted to nontoxic Cr(III), even under neutral conditions (pH 7.0). According to ultraviolet visible (UV-vis) spectroscopy, powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis, the CrO42- anion was readily intercalated into the anion lattice layer of BiOBr0.75I0.25 by ion exchange with OH- in the interlayer, forming a = Bi-O-CrO3- complex. Electron paramagnetic resonance (EPR) and photoelectron-chemistry measurements further revealed that excitation of =Bi-O-CrO3- by irradiation was crucial for photocatalytic detoxification of Cr(VI) under pH 7.0. The excitation of =Bi-O-CrO3- in BiOBr0.75I0.25 strengthened the bulk-charge transfer as follows: (i) electron transfer from O-II to Cr-VI produced Cr-V and O-1, respectively, via ligand-to-metal charge transfer excitation; and (ii) electron injection from BiOBr0.75I0.25 to [=Bi-O--(CrO3-)-O-V]* (or leaving holes in valence band) resulted in the reduction of Cr(V) to Cr(III). After that, the newly formed Cr(III) in the interlayer of BiOBr0.75I0.25 was deintercalated into solution due to the space charge repulsion between Cr3+ and Bi2O22+ slab, resulting in stable Cr(VI) reduction performance in a wide pH range from 2.0 to 7.0.