Discrete Supertetrahedral Tn Chalcogenido Clusters Synthesized in Ionic Liquids: Crystal Structures and Photocatalytic Activity

Abstract

Discrete supertetrahedral Tn chalcogenido clusters, which can be regarded as the smallest semiconductor quantum dots with precise chemical composition, have attracted considerable attention due to their outstanding photoluminescent, photoelectric, and photo/electrocatalytic properties. Such discrete molecular clusters are suitable for solution processing towards functional materials and can be used as precursors for constructing open‐framework chalcogenides. Traditionally they were synthesized hydro(solvo)thermally with molecular solvents (e. g. water or organic amines), while until recently imidazolium‐based ionic liquids (ILs) were found suitable for their preparation acting as reactive solvent and stabilizer for molecular clusters. We discuss herein recent advances in the syntheses, crystal structures, and selected properties of discrete supertetrahedral Tn chalcogenido clusters obtained in ILs. In particular, the enhanced photocatalytic properties of monodispersed Tn clusters in solvents are highlighted.

A Discrete Ligand-Free T3 Supertetrahedral Cluster of Gallium Sulfide

“…Traditionally, chalcogenides containing supertetrahedral clusters have been synthesized by high-temperature methods (for purely inorganic solids) or by solvothermal synthesis, in which water or amines are often used as solvents. More recently, the ionothermal method, in which an ionic liquid acts as both the solvent and the structure-directing agent [ 20 ], has been successfully exploited for the preparation of chalcogenides [ 4 , 21 ]. Benefits of the ionothermal approach include the ability to carry the synthesis at ambient pressures and the absence of competition between the solvent and the structure directing agent.…”

“…Benefits of the ionothermal approach include the ability to carry the synthesis at ambient pressures and the absence of competition between the solvent and the structure directing agent. Ionothermal synthesis has been found to be advantageous for the synthesis of discrete supertetrahedral clusters [ 4 ]. It has been suggested that anion-π interactions between chalcogen ions and the organic cations in the ionic liquid may increase the stability of discrete clusters and, under ionothermal conditions, the chalcogens normally at the four corners of the clusters may be easily replaced by the halogens or organic ligands, which reduce the local negative charge and prevent the formation of extended structures [ 4 ].…”

“…Ionothermal synthesis has been found to be advantageous for the synthesis of discrete supertetrahedral clusters [ 4 ]. It has been suggested that anion-π interactions between chalcogen ions and the organic cations in the ionic liquid may increase the stability of discrete clusters and, under ionothermal conditions, the chalcogens normally at the four corners of the clusters may be easily replaced by the halogens or organic ligands, which reduce the local negative charge and prevent the formation of extended structures [ 4 ]. The use of surfactants in the “surfactant-thermal” method [ 22 ] offers a potential alternative for the synthesis of supertetrahedra-based clusters, which we have explored in the work presented here.…”

“…We speculate that the significantly higher viscosity of PEG-400, when compared to commonly used solvents in solvothermal reactions, might hinder the linkage of discrete supertetrahedral clusters into larger building blocks, and hence favour the crystallisation of discrete units. The cluster described here is the first example of a T3 gallium sulfide supertetrahedral cluster that exists as a discrete unit, with no organic ligands coordinated to the corners, in contrast to many of those produced using ionothermal methods [ 4 ]. Previous work on ligand-free supertetrahedral clusters, including [Ga 4 S 8 ] 8− , [In 4 S 8 ] 8− and [Ge 4 S 10 ] 4− [ 8 , 24 ], suggests that the cluster reported here is likely to be soluble in highly polar solvents such as water or formamide.…”

“…Tetrahedral chalcogenide nanoclusters are attractive as building blocks for multifunctional open-framework materials, in which porosity is integrated with semiconducting behavior [ 1 , 2 , 3 ]. Moreover, as the structures of these nanoclusters are closely related to those of II−VI or I−III−VI semiconductor nanocrystals, chalcogenide nanoclusters can be considered as ultrasmall quantum dots [ 2 , 4 ]. The uniform sizes and well-defined structures of tetrahedral chalcogenide nanoclusters can be advantageous for the investigation of the site- or size-dependent properties of quantum dots [ 5 ].…”