Fabrication of Active Z-Scheme Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺/COF Photocatalyst for Round-the-Clock Efficient Removal of Total Cr

Photoreduction is recognized as a desirable treatment method for hexavalent chromium (Cr(VI)). However, it has been limited by the intermittent solar flux and limited light absorption. In this work, a novel Z-scheme photocatalyst combining a covalent organic framework (COF) with Eu²⁺, Dy³⁺ co-doped Sr₂MgSi₂O₇ (Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺) is synthesized, which shows the high spectral conversion efficiency and works efficiently in both light irradiation and dark for Cr(VI) reduction. Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺ serves as both an electron transfer station and active sites for COF molecule activation, thus resulting in 100% photoreduction of Cr(VI) (50 mL, 10 mg/L) with high light stability and over 1 h dark activity. Moreover, the XPS and FT-IR analyses reveal the existence of functional groups (Si-OH on Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺, and -NH- on COF_TP-TTA) on the composited catalyst as active sites to adsorb the resultant Cr(III) species, demonstrating a synergistic effect for total Cr removal. This work provides an alternative method for the design of a round-the-clock photocatalyst for Cr(VI) reduction, allowing a versatile solid surface activation for establishing a more energy efficient and robust photocatalysis process for Cr pollution cleaning.