Tunable conduction band energy and metal-to-ligand charge transfer for wide-spectrum photocatalytic H2 evolution and stability from isostructural metal-organic frameworks

Abstract

Metal-organic framework (MOF) materials with their synthetic tunability and structural regularity offer an interesting platform to achieve photocatalytic H2 production. To determine whether metal ions in MOF materials affect the photocatalytic hydrogen (H2) evolution activity, very few studies investigated MOF materials having the same crystal structures and ligands. In this study, we describe the synthesis of isostructural MOF materials. Cadmium (II) or copper (II) were linked with the 4′-(2,4-disulfophenyl)-3,2′:6′,3″-terpyridine (H2DSPTP) organic ligand to form Cd-MOFs and Cu-MOFs, respectively. Although Cd-MOFs and Cu-MOFs had the same valence band (VB) energy, the conduction band (CB) level of Cu-MOFs was more negative than that of Cd-MOFs. Therefore, the reduction ability of Cu-MOFs was greater than that of Cd-MOFs. Moreover, Cu-MOFs showed excellent absorption of visible and even near-infrared (NIR) radiation, because it has strong metal-to-ligand charge transfer (MLCT) character. Interestingly, the rate of photocatalytic H2 generation was 18.96 μmol h−1 in the presence of Cu-MOFs, which were irradiated with NIR light; however, Cd-MOFs did not report any such activity. Moreover, the performance and stability of Cd-MOFs were different from that of Cu-MOFs. In this study, the proof of knowledge for rational photocatalyst design introduced here furnishs the notional framework for constructing high-activity MOF materials.