Ultrafine metal species confined in metal–organic frameworks: Fabrication, characterization and photocatalytic applications

[Display omitted] •Fabrication strategies of ultrafine metal-based NPs confined in the pores of MOFs are elaborated.•Advanced characterizations on characterizing the relative location between metal-based NPs and MOFs are outlined.•Photocatalytic applications of metal-based NPs@MOFs in recent years are reviewed.•The synergistic effects between the two compounds are emphasized.•The current challenges and outlooks in this topic are presented. Photocatalysis, which can be utilized for H2 production, CO2 reduction, and water remediation, promising a bright future in addressing energy and environmental challenges. Metal-organic frameworks (MOFs) with highly uniform crystal structures provide internal conditions to restrain the aggregation of metal species and generate the composite with superb stability. Such confinement strategy offers advantages in designing of photocatalysts with abundant active sites, increased mass-transfer efficiency, enhanced light adsorption and high photo-generated carriers utilization rate due to the synergistic effects. Here, the recent advances in fabricating metal species confined in pores of MOFs are concluded. Advanced characterizations for characterizing the relative location between metal species and MOFs are displayed. Moreover, the photocatalytic applications of metal species@MOFs in energy and environment and the corresponding photocatalytic mechanisms are discussed. Challenges and perspectives are given, with good hope to promote the development of fabricating advanced and efficient photocatalyst.