Electrochemical Gelation of Metal Chalcogenide Quantum Dots: Applications in Gas Sensing and Photocatalysis

Abstract

Conspectus Metal chalcogenide quantum dots (QDs) are prized for their unique and functional properties, associated with both intrinsic (quantum confinement) and extrinsic (high surface area) effects, as dictated by their size, shape, and surface characteristics. Thus, they have considerable promise for diverse applications, including energy conversion (thermoelectrics and photovoltaics), photocatalysis, and sensing. QD gels are macroscopic porous structures consisting of interconnected QDs and pore networks in which the pores may be filled with solvent (i.e., wet gels) or air (i.e., aerogels). QD gels are unique because they can be prepared as macroscale objects while fully retaining the size-specific quantum-confined properties of the initial QD building blocks. The extensive porosity of the gels also ensures that each QD in the gel network is accessible to the ambient, leading to high performance in applications that require high surface areas, such as (photo)catalysis and sensing. Metal chalcogenide QD gels are conventionally prepared by chemical approaches. We recently expanded the toolbox for QD gel synthesis by developing electrochemical gelation methods. Relative to conventional chemical oxidation approaches, electrochemical assembly of QDs (1) enables the use of two additional levers for tuning the QD assembly process and gel structure: electrode material and potential, and (2) allows direct gel formation on device substrates to simplify device fabrication and improve reproducibility. We have discovered two distinct electrochemical gelation methods, each of which enables the direct writing of gels on an active electrode surface or the formation of free-standing monoliths. Oxidative electrogelation of QDs leads to assemblies bridged by dichalcogenide (covalent) linkers, whereas metal-mediated electrogelation proceeds via electrodissolution of active metal electrodes to produce free ions that link QDs by binding to pendant carboxylate functionalities on surface ligands (non-covalent linkers). We further demonstrated that the electrogel composition produced from the covalent assembly could be modified by controlled ion exchange to form single-ion decorated bimetallic QD gels, a new category of materials. The QD gels exhibit unprecedented performance for NO2 gas sensing and unique photocatalytic reactivities (e.g., the “cyano dance” isomerization and the reductive ring-opening arylation). The chemistry unveiled during the development of electrochemical gelation pathways for QDs and their post-modification has broad implications for guiding the design of new nanoparticle assembly strategies and QD gel-based gas sensors and catalysts.

Colloidal Quantum Dots for Explosive Detection: Trends and Perspectives

“…131 The macroscale objects thus formed fully retain the size-dependent properties of the initial building blocks, while the threedimensionally connected pore network ensures electronic/ ionic communication and high accessibility to the ambient, being metal chalcogenide CQD gels a notable example. 132 Recently, Geng et al 133,134 proposed an electrochemical gelation method allowing direct gel formation on electrode substrates. This new strategy could offer the key to drastically reduce the time for gas diffusion, thus possibly enabling the realization of devices with fast response time (Figure 14).…”

Electrochemiluminescence of Semiconductor Quantum Dots and Its Biosensing Applications: A Comprehensive Review

“…Therefore, shortening the surface ligands is an effective strategy to accelerate charge transfer and enhance the ECL efficiency of QDs. Luo et al proposed an electrochemical gelation method for assembling metal chalcogenide QDs into a three-dimensional gel in which each QD was accessible to the ambient [49]. In addition, Cai et al developed a novel waterinduced gelation strategy in which CdSe QDs could be self-assembled into an all-inorganic aerogel [50].…”

Regulating electron transfer and orbital interaction within metalloporphyrin-MOFs for highly sensitive NO2 sensing

“…Specifically, charge transfer during the gas sensing process and the interactions between analytes and adsorption sites at the atomic-level are fundamental factors in causing signal changes in gas induction. 1,2 Although a variety of gas sensitive materials, such as semiconducting metal oxides, 3–5 metal dichalcogenides, 6,7 graphene-based hybrids, 8,9 and conducting polymers, 10,11 have attracted significant attention in the past few decades, their ambiguous structures or adsorption sites have limited the comprehension of the reaction processes between gas-sensitive materials and analytes at the atomic level, including electron transfer routes, electron spin states and orbital symmetry/overlap between adsorbates and adsorption sites, all of which have a microregulatory on gas-sensitive properties.…”