Low-Temperature Thermal Dissociation of Ag Quantum Clusters in Solution and Formation of Monodisperse Ag₂S Nanoparticles

Abstract

We report the effect of temperature on the stability of glutathione-protected Ag 25 clusters. The clusters are stable up to 50 °C. Interestingly, above this temperature, they decompose to yield Ag 2 S nanoparticles with an average diameter of 3 ± 1 nm, crystallizing in monoclinic acanthite polymorph. Unlike conventional methods of syntheses of Ag 2 S, where a temperature of ∼200 °C is needed, our study shows the possibility of synthesis of Ag 2 S nanoparticles at much lower temperatures. This is in contrast with silver nanoparticles protected with thiolates, which typically give silver and alkyl/aryl disulfide upon thermal activation. The mechanism of cluster decomposition and formation of silver sulphide nanoparticles was investigated using various analytical techniques such as ultraviolet–visible spectroscopy, X-ray diffraction scanning electron microscopy, energy-dispersive analysis of X-rays, transmission electron microscopy, and electrospray ionization mass spectrometry. The monolayer of the cluster undergoes S–C bond cleavage, as revealed by mass spectrometry. This is somewhat unusual because Ag–S cleavage is expected in view of its lower bond energy.