High-efficiency sandwich-like hierarchical AgBr-Ag@MIL-68(Fe) photocatalysts: Step-scheme photocatalytic mechanism for enhanced photoactivity

Abstract

• Heterojunctions formed by sandwiching Ag NPs between AgBr and MIL-68(Fe). • Novel impregnation–precipitation more effective than conventional synthesis. • Hybrid photocatalyst removed both aqueous RhB and Cr(IV) efficiently. • The reaction mechanism of bifunctional S-scheme AgBr-Ag@MFe was elucidated. Although both step-scheme (S-scheme) heterojunction and metal–organic framework (MOF) photocatalysts have been intensively studied, few efforts have been devoted to combining them to establish high-performance MOF-based S-scheme photocatalysts. In this work, binary Ag@MIL-68(Fe) (Ag@MFe) hybrids were first fabricated by decorating Ag nanoparticles (Ag NPs) onto MIL-68(Fe), an Fe-based MOF. Owing to the high specific surface area of MIL-68(Fe), the plasmonic Ag NPs were uniformly distributed on its surface. Then, AgBr particles were deposited on the Ag@MFe hybrids via precipitation to form the final ternary sandwich-like hierarchical AgBr-Ag@MFe photocatalyst. The 30 %AgBr-1.5 %Ag@MFe sample exhibited excellent photocatalytic performance for the degradation of rhodamine B under visible light irradiation ( λ ≥ 420 nm), with an effectiveness 9.2 and 2.1 times higher than those of the original MIL-68(Fe) and binary 1.5 %Ag@MFe hybrid, respectively. Furthermore, 30 %AgBr-1.5 %Ag@MFe performed as a high-efficiency bifunctional photocatalyst towards the detoxification of water contaminants (mixture of a dye and Cr(VI)). The significantly enhanced photoactivity was attributed to the effective spatial separation of photogenerated electron–hole pairs via S-scheme charge transfer. Moreover, the plasmonic Ag NPs assembled into the contact interface of AgBr and MIL-68(Fe) enhanced the vectorial interfacial electron transfer. The formation of an S-scheme heterojunction was supported by X-ray photoelectron spectroscopy and electron spin resonance studies. This novel bifunctional AgBr-Ag@MIL-68(Fe) hybrid photocatalyst system not only provides new opportunities for environmental remediation but also highlights a promising strategy to construct high-efficiency S-scheme photocatalysts.