Single-metal-atom-incorporated quantum dots and covalent organic frameworks: a comparison of quantum effects and electronics

Abstract

Quantum dots (QDs) and covalent-organic frameworks (COFs) are two potential structurally and electronically rich units for incorporating single metal atoms in isolated positions. The incorporation of single metal atoms in QDs results in several intrinsic structural features favouring the electronics of electrochemical processes at the solid–liquid interface with favourable electron transfer as a result of unique zero-dimensionality with sp² domains on the basal plane and functional groups at the edges, along with graphitic domains to prevent structure collapse. Preeminent achievements and outstanding progress of metal-atom-incorporated QDs, as atomically thin graphitic structures with lateral dimensions <10 nm, offer unique potentials for their bulk form for energy conversion and storage. Single metal atoms with variable oxidation states are harnessed inside an N-rich-functionalized carbon quantum dot (CQD) matrix via a low-temperature method that offers dynamic electronic properties in single-atom-incorporated M-SA@QDs with rich redox chemistry of metal cations. Single-metal-atom sites act as electron antennas on the QDs that attract charges and photocarriers to the solid–liquid interface to activate protons to initiate an electrochemical process such as the hydrogen evolution reaction (HER). Downsizing MOF particles into QDs confines individual atoms, shortening the mass transfer route and amplifying interfacial electron transfer and electron regulation. Such M-SA@QDs exhibit greater electron accumulation on the metal atom and an up-shifted d-band center for adsorption strength and atom utilization efficiency. Herein, the current strategies for confining metal atoms in QDs in M-SA@QDs and the spatial confinement strategy are discussed. The emerging feature of M-SA@QDs is then compared with a COF-based M-SA@COF material with focus on the metal-loading and electronics of COFs for hosting metal atoms. Promising electronic and optical properties with their comparative advantages between QDs and COFs are highlighted. This article further links the electronic features and the electrochemical performance of QD and COF-based single atoms. Finally, this article attempts to outline future directions for exploring the role of QDs and COFs in creating heterointerfaces with M-SA and their links to advanced electronic features.