Thermal Stability of (ZnS)(Ag2S)x Heteronanostructures of Zinc and Silver Sulfides

Heteronanostructures (ZnS)(Ag₂S)_x, where x = 0.002–0.50, are synthesized by hydrochemical coprecipitation. The ZnS nanoparticle size in the resulting heteronanostructures is 2–4 nm. Annealing of the synthesized (ZnS)(Ag₂S)_x heteronanostructures in air at temperatures of 25–530°C and above leads to a change in their phase composition due to the oxidation of cubic zinc sulfide to hexagonal zinc oxide. Oxidation begins at a temperature of ~250°C; the zinc oxide content in the heteronanostructures after annealing at 530°C achieves ~26–30 wt %. The nanoparticle size of the resulting ZnO varies in a range of 12 to 17–25 nm. Studies of the oxidation of (ZnS)(Ag₂S)_x heteronanostructures in air show that the initial weight loss observed upon heating to ~120°C is attributed to the removal of adsorbed moisture. The subsequent weight loss that occurs upon heating from ~250 to ~430–450°C is associated with the onset of oxidation of the ZnS sulfide and the formation of the ZnO oxide. The most significant weight loss is observed upon heating from ~450 to ~580°C; it is attributed to an increase in the ZnO content, partial oxidation of sulfur, and the removal of sulfur in the form of SO₂. The occurrence of oxidation stages is confirmed by the presence of maxima in the temperature dependences of ion currents corresponding to H₂O, CO₂, and SO₂. The studied heteronanostructures are thermally stable under heating to ~200–250°C.