Confinement Matters: Stabilization of CdS Nanoparticles inside a Postmodified MOF toward Photocatalytic Hydrogen Evolution

Abstract

Insights into developing innovative routes for the stabilization of photogenerated charge-separated states by suppressing charge recombination in photocatalysts is a topic of immense importance. Herein, we report the synthesis of a metal−organic framework (MOF)-based composite where CdS nanoparticles (NPs) are confined inside the nanosized pores of Zr 4+ -based MOF-808, namely, CdS@MOF-808. Anchoring L-cysteine into the nanospace of MOF-808 via postsynthetic ligand exchange allows the capture of Cd 2+ ions from their aqueous solution, which are further utilized for in situ growth of CdS NPs inside the nanosized MOF pores. The formation of CdS@MOF-808 opens up a possibility for visible-light photocatalysis as CdS NPs (1−2 nm) are a well-studied semiconductor system with a band gap of ∼2.6 eV. The confinement of the CdS NPs inside the MOF pores, close to the Zr 4+ cluster, opens up a shorter electron transfer route from CdS to the catalytic Zr 4+ cluster and shows a high rate of H 2 evolution (10.41 mmol g −1 h −1 ) from water with a loading of 3.56 wt % CdS. In contrast, a similar composite in which CdS NPs are stabilized on the external surface of MOF-808 reveals poor activity (0.15 mmol g −1 h −1 ). CdS NPs stabilized on the MOF-808 surface show slower and inefficient electron transfer kinetics compared to CdS stabilized inside the nanospace of the MOF, as realized by the transient absorption measurements. Therefore, this work unveils the critical role of stabilizing the photosensitizer NPs in close proximity of the catalytic sites in MOF systems towards developing highly efficient H 2 evolution photocatalysts.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel

“…Semiconductor quantum dots (QDs) have attracted significant attention over the last few decades, due to their excellent optical properties that arise due to quantum confinement effect, − and pave the way for a myriad of potential applications. − Cadmium chalcogenide QDs constitute an important class of these materials, with facile size, shape, and capping group-dependent wavelength tunability of emission across the visible region of the spectrum. ,− Besides, high photoluminescence quantum yields (PLQYs) and ease of chemical processability makes this particular class of QDs attractive. ,,− Optical amplification and lasing, demonstrated in these materials, − form one class of potential applications. Competition from efficient nonradiative processes of Auger recombination (AR) and surface trapping are important issues that need to be addressed in this context. , While surface trapping can be reduced effectively by growing a passivating shell of a higher band gap material around the emitting dot, AR poses a more formidable challenge.…”

Charge transfer in metal–organic frameworks

“…Ti IV , Ce IV and Zr IVbased MOFs have been commonly reported as photocatalysts attributed to their high chemical stability, low toxicity, and the variable redox states of the metal center (Ti IV /Ti III , Ce IV /Ce III , Zr IV /Zr III ). 52,[88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105][106] The presence of variable redox states of these metals and the inherent presence of metal-ligand coordination bonds allows LMCT to take place from the excited ligand to the metal-oxo cluster (Fig. 12a).…”

Pristine Metal–Organic Frameworks and their Composites for Renewable Hydrogen Energy Applications

“…Moreover, QDs and semiconduc tive materials have also been widely encapsulated in MOFs to enhance the photoresponse and photogenerated charge carrier efficiency of MOF composites and shorten the electron transfer pathway for highly efficient photocatalytic H 2 production. [191] For H 2 storage aspect, the insertion of functional materials like MNPs, alkali metal into MOFs can enhance the inter action between the surface of MOFs and H 2 molecules and thus improve H 2 uptake capacity. [192,193] For example, Kumar et al incorporated Sc/Li metal ions into metalBN framework (MBF) to prepare composites for reversible H 2 storage applica tion.…”