Metal-directed interfacial self-assembly of well-defined coordination polymer (CP) ultrathin films can control the metal complex arrangement and distribution at the molecular level, providing a convenient route for the design and fabrication of novel opto-electrical devices and heterogeneous catalysts. Here, we report the assembly of two series of CP multilayers with the transition-metal ions of Fe2+, Co2+, Zn2+ and Tb3+ as connectors and tripodal terpyridyl ligands of 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)tris(1-(4-([2,2′:6′,2″-terpyridin]-4′-yl)benzyl)pyridin-1-ium) (TerPyTa) and 4,4′,4″-(benzene-1,3,5-triyl)tris(1-(4-([2,2′:6′,2″-terpyridin]-4′-yl)benzyl)pyridin-1-ium) (TerPyBen) as linkers at the air–water interface. The as-prepared Langmuir–Blodgett (LB) films display strong luminescence, with the emission wavelength and relative intensity dependent on both the metal ions and linkers; among them, the Zn-TerPyTa and Zn-TerPyBen CPs give off the strongest luminescent emission centered at about 370 nm with an emission lifetime of approximately 0.2–0.3 ns. The Tb-TerPyTa CPs can give off emission at approximately 490, 546, 586, and 622 nm, attributed to the 5D4 to 7F3–6 electron transitions of typical Tb3+ ions. Finally, these CP LB films can act as efficient heterogeneous photocatalysts for the CO2 reduction to selectively produce CO. The catalytic efficiency can be optimized by adjusting the experimental conditions (light sensitizer, electron donor, and water content) and CP composition (metal ion and ligand) with an excellent yield of up to 248.1 mmol g–1. In particular, it is revealed that, under the same conditions, the catalytic efficiency of the Fe-TerPyTa CP LB film is nearly 2 to 3 orders of magnitude higher than that of the other metalated complexes investigated in the homogeneous system. UV–vis spectroscopy and cyclic voltammetry studies demonstrated that the dual active sites of Fe-terpyridine and TerPyTa units contribute to the enhanced catalytic activity. This work provides an effective method to introduce the earth-abundant metal complexes into CP films to construct efficient noble-metal-free photocatalysts for the CO2 reduction.
Understanding the structural dynamics of ligands and its interplay with the turnover rate of catalytic centers under reaction conditions are challenging but crucial regarding catalyst design. Here, an in-situ phase conversion of Ni-Fe hydroxide to a stable superoxo-hydroxide, with the formation of lattice O–O (Olatt–Olatt) ligands, was unveiled via operando 18O-labeling spectroelectrochemistry and machine-learning global optimization. By correlating the intrinsic activity of Fe with the ability of Olatt–Olatt formation in a series of Fe-incorporated transition-metal oxides/hydroxides, we disclose an Olatt–Olatt triggered Fe activation for oxygen-evolving electrocatalysis, which is further proved by first-principles calculations. The oxygen production follows an adsorbate evolution mechanism and the fast kinetics is attributed to diminished Fe–OH bonds in the presence of Olatt–Olatt. This work guides the rational design of high-performance Fe-incorporated electrocatalysts, and highlights the paramount role of ligands dynamic restructuring in awakening catalytic centers.
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