Eight semiconducting MOFs constructed with conjugated ligands and d-metals (Cd, Zn, Co and Ni) serve as functional materials for oxygen evolution reactions, photocatalytic degradation of dyes and photoluminescence

Abstract

MOFs with semiconducting behavior exhibit unique optoelectronic properties, and they are hailed as future low-bandgap materials, which are expected to serve as a new generation of functional materials to solve some thorny problems in the field of environment and energy. In this work, eight semiconducting MOFs with chemical formulae, {[Cd(m-bix)(ATP)]}_n (1), {[Zn₂(m-bix)(ATP)₂]·DMF}_n (2), {[Cd(m-bix)(BDC)(H₂O)]}_n (3), {[Co₂(m-bix)(BDC)₂(H₂O)₃]·2H₂O·CH₃OH}_n (4), {[Ni₂(m-bix)(BDC)₂(H₂O)₃]·2H₂O·CH₃OH}_n (5), {[Cd(m-bix)(Cl)]}_n (6), {[Zn(m-bix)(CO₃)]}_n (7), and {[Cd(FDC)(H₂O)₃]·2H₂O}_n (8) (m-bix = m-bis(imidazol-1-ylmethyl)benzene; H₂ATP = 2-aminoterephthalic acid; H₂BDC = p-phthalic acid; H₂FDC = 2,5-furandicarboxylic acid), were successfully synthesized by hydrothermal techniques. Comprehensive characterization of the as-synthesized MOFs showed that they display interesting topologies and fascinating structures (of 6 and 8 being 1D, 1, 2, 3, and 7 being 2D and 4 and 5 being 3D). The incorporation of π-conjugated ligands and abundant intermolecular hydrogen bonds endows these MOFs with higher carrier transport rates, resulting in unique optoelectronic properties. The optical band gaps of MOFs 1–8 obtained by UV reflection spectroscopy were 2.86, 2.59, 3.91, 3.01, 3.69, 3.19, 3.09, and 3.87 eV, respectively, proving that they are all potential semiconductor materials. Electrochemical oxygen evolution reaction (OER) catalysis studies revealed that MOFs 4–5 are efficient catalysts for the OER process with lower overpotentials of 381 and 397 mV at 10 mA cm⁻² and lower Tafel slopes of 75.1 and 74.4 mV dec⁻¹. Photoluminescence studies demonstrated that the Cd-based and Zn-based MOFs, especially MOF 2, exhibit strong violet light emission properties in the range of 405–446 nm, which are expected to be used in the development of new luminescent materials. In addition, the photocatalytic performance of these MOFs was evaluated. Among them, the best photocatalytic performance of 4 and 6 was attributed to the combination of the band gap value and luminescence performance, resulting in more photogenerated carriers and thus more catalytically active sites. This work not only provides a certain theoretical and experimental basis for further research on the optoelectronic properties of MOFs, but also affords an advanced strategy for the development of novel multifunctional materials to solve real-life environmental and energy problems.