Dual-Ligand Ti-MOFs with Push-Pull Effect for Photocatalytic H2 Production.

Enhancing the photogenerated electrons transfer efficiency is crucial for photocatalytic reactions. Herein, a dual-ligand-induced push-pull effect was manipulated to intensify the transfer of photogenerated electrons between organic ligands and metal clusters using NH2-MIL-125(Ti), a kind of Ti-based metal-organic framework (MOF), as the model system. The dual-ligand MOF, NH2/Cl-MIL-125, was designed and synthesized based on the Hammett constant (σm), in which -NH2 (σm = -0.16) and -Cl (σm = 0.37) were selected as the electron-pushing group and the electron-pulling group, respectively. Meanwhile, -CH3 (σm = -0.07, electron-pushing) and -H (σm = 0, neither electron-pushing nor electron-pulling) were selected as the reference groups to prepare NH2/CH3-MIL-125 and NH2/H-MIL-125, respectively, to validate the electron push-pull effect. NH2/Cl-MIL-125 (5.32 mmol g-1 h-1) exhibits a higher photocatalytic H2 evolution activity than single-ligand NH2-MIL-125 (1.93 mmol g-1 h-1), NH2/CH3-MIL-125 (4.45 mmol g-1 h-1), and NH2/H-MIL-125 (4.73 mmol g-1 h-1) under full-spectrum irradiation. The result can be attributed to the electron push-pull effect between -NH2 and -Cl, which boosts the electron transfer along the ligand-metal-ligand direction. Our dual-ligand-induced push-pull strategy for enhancing the electron transfer may offer some novel insights into the rational design and synthesis of photocatalysts for many other reactions.

On-Surface Bottom-Up Construction of COF Nanoshells towards Photocatalytic H2 Production.

The rational design of an outer shell is of great significance to promote the photocatalytic efficiency of core-shell structured photocatalysts. Herein, a covalent organic framework (COF) nanoshell was designed and deposited on the cadmium sulfide (CdS) core surface. A typical COF material, TPPA, featuring exceptional stability, was synthesized through interfacial polymerization using 1, 3, 5-triformylphloroglucinol (TP) and p-phenylenediamine (PA) as monomers. The nanoshell endows the CdS@TPPA nanosphere with ordered channels for unimpeded light-harvesting and fast diffusion of reactants/products and well-defined modular building blocks for spatially charge separation. Moreover, the heterojunction formed between CdS and TPPA can further facilitate the effective charge separation at the interface via lower exciton binding energy compared with that of pristine TPPA. By modulating the thickness of TPPA nanoshell, the CdS@TPPA nanosphere photocatalyst with the nanoshell thickness of about 8 ± 1 nm exhibits the highest photocatalytic H2 evolution of 194.1 µmol h-1 (24.3 mmol g-1 h-1, 8 mg), which is superior to most of the reported COF-based photocatalysts. The framework nanoshell in this work may stimulate the thinking about how to design advanced shell architecture in the core-shell structured photocatalysts to achieve coordinated charge and molecule transport.