The Emergence of 2D Building Units in Metal‐Organic Frameworks for Photocatalytic Hydrogen Evolution: A Case Study with COK‐47

Abstract

Metal‐organic frameworks (MOFs) are promising materials for photocatalytic water splitting reactions, but examples of visible light‐responsive, catalytically active, and stable MOFs are still rare. A detailed investigation is conducted for COK‐47 – a recently described MOF comprising 2D Ti‐O6 secondary building units (SBUs) – toward a photocatalytic hydrogen evolution reaction (HER), showing how overall particle morphology, surface area, and missing ligand defects are central parameters governing the material's ultimate performance. The newly synthesized COK‐47ISO is among the most active MOFs to date, yielding HER‐rates of 8.6 µmol h−1, and an apparent quantum yield (AQY) of 0.5% under visible light illumination. Optoelectronic and photoluminescence investigations, supported by theoretical calculations, enable the unraveling of its electronic structure along with charge transfer and recombination kinetics. A wavelength‐dependent reaction mechanism is proposed involving ligand to metal charge transfer (LMCT) and the main challenges for visible or UV photoexcitation are identified, demonstrating that the unique 2D layered structure aids charge separation and is key to the high performance. This work introduces COK‐47 as a promising alternative to the well‐known MIL‐125 family and offers directions for future studies

Harnessing a Ti-based MOF for selective adsorption and visible-light-driven water remediation

“…COK-47 was prepared based on the microwave-assisted synthesis recently reported by our group. 18 Its structure, composition, and optoelectronic properties were characterized using a variety of techniques including X-ray diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscopy (TEM), and diffuse reflectance spectroscopy (DRS).…”

“…We further demonstrated excellent charge carrier lifetimes by means of time-resolved emission spectroscopy (TRES) and showed COK-47 to be one of the best MOF-based photosystems for visible-light-driven hydrogen production. 18 Following these results, herein we unravel the potential of, COK-47, towards the visible-light-driven photocatalytic oxidative degradation of organic dyes. We test its degradation capabilities over a range of cationic and anionic dyes such as methylene blue (MB), crystal violet (CV), toluidine blue (TB), and methyl orange (MO) (Fig.…”

“…[13][14][15] In contrast, MOFs based on Ti-and Zr-containing SBUssuch as UiO-66 and MIL-125 familieshave demonstrated some of the highest resistance towards hydrolysis, which facilitates their implementation for wastewater remediation technologies including those based on photocatalysis. 9,16,17 Most recently, an 18 advanced class of 1D and 2D SBU based MOFs exhibiting superior ability to separate and conduct photogenerated charge carriers has come into the light owing to the chain-like and planar SBUs, respectively. [19][20][21][22]31 In 2019, Smolders et al reported the rst synthesis of a 2D-SBU based MOF, called COK-47, built of Ti-oxo SBUs connected via the biphenyl dicarboxylic acid (H 2 bpdc) ligand.…”

“…In addition to a complete set of characterizations for the MOF structure, the authors also demonstrated its potential as a photocatalyst for rhodamine 6G degradation under UV illumination. 23 Building on this, our group recently reported an optimized microwave-assisted synthesis of COK-47 resulting in isotropic nanometer-sized particle while preserving ideal ligand to metal stoichiometry, 18 which afforded lower semiconducting bandgap enabling efficient utilization of visible light. We further demonstrated excellent charge carrier lifetimes by means of time-resolved emission spectroscopy (TRES) and showed COK-47 to be one of the best MOF-based photosystems for visible-light-driven hydrogen production.…”

Iridium complex modified MOFs for enhancing photocatalytic hydrogen evolution

“…Metal–organic framework (MOF) materials, especially the classical UiO-66-NH 2 , are considered promising photocatalysts due to their large specific surface area, tunable pore structure and functionality. 3,13–23 The ordered structure and high crystallinity of MOFs give them a unique advantage in charge transfer. 24,25 However, the wider energy bands and shorter photogenerated carrier lifetimes have hindered their further development and application; for example, the photocatalytic activity of the classical UiO-66-NH 2 is extremely low (only 1.72 μmol g −1 h −1 ).…”