One-pot inorganic route to highly stable water-dispersible Ag2S quantum dots

Direct aqueous synthesis of Ag2S QDs is a challenge because hydrophobic Ag2S QDs tend to aggregate to form large polycrystals. In this article, the low temperature one-pot route for obtaining highly stable aqueous colloid solutions of Ag2S QDs, using NH3 molecules as monodentate ligands, is reported. Ionic forms of all the species in solution including free ligand, free and complex silver ions, and free sulfide ions at any given experimental pH were calculated. The obtained speciation diagrams were necessary to determine synthetic parameters such as concentration ratios [Ag+]/[HS−] and [NH3]/[Ag+], and pH. The comprehensive investigation of Ag2S QDs' colloid solutions was carried out by high-resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy (STEM), UV-vis-NIR spectroscopy, dynamic light scattering. The Ag2S QDs possessed monoclinic (acanthite) crystalline structure with an average size about 33 nm. Estimations of hydrodynamic diameter and degree of double layer charge in colloid solutions, i.e. the ς-potential measurements, proved high stability of colloidal system of Ag2S QDs on the order of several months under ambient conditions. Combination of both electrostatic and steric stabilization mechanisms kept Ag2S QDs uniformly dispersed in aqueous medium, with NH3 molecules playing a crucial role in avoiding aggregation of as-prepared Ag2S QDs. Colloid Ag2S QDs exhibited NIR emission at 1280 nm under 808 nm excitation.