Thermally Reversible Transformation between Two Isomers of Silver Sulfide Magic-Size Clusters

Isomers of semiconductor magic-size clusters (MSCs) share the same molecular formula but can exhibit distinct optical and electronic properties. Most of the reported MSC isomers have been so far limited to cadmium chalcogenides. Herein, we report the reversible transformation between two Ag2S MSC isomers synthesized in solution under mild reaction conditions. These MSC isomers, which we denote as MSC-356 and MSC-306 according to their characteristic excitonic peaks in their absorption spectra (356 and 306 nm, respectively), share the same inorganic core and organic ligand shell composition confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, inductively coupled plasma optical emission spectrometry, powder X-ray diffraction, and Raman spectrometry. Reversible transformation between these two isomers is thermally induced. The isomerization from MSC-356 to MSC-306 follows the first-order kinetics with an activation energy of 233 kJ mol-1. It occurs within a narrow temperature window of 54-66 °C, with lower temperatures resulting in significant delays in the transition, while the reverse conversion from MSC-306 back to MSC-356 requires a significantly higher temperature of 110 °C. These two isomers also display different energy band structures and electronic properties, with MSC-306 possessing a slightly smaller work function (3.85 eV) than MSC-356 (3.92 eV); the former also shows better catalytic performance in the hydrogen evolution reaction. The synthetic strategy reported here enables the synthesis of environmentally benign silver chalcogenide MSC isomers and provides important insights into their growth mechanisms and nanoscale structure-property relationships.