Luminescent silver sulfide clusters

Silver sulfide clusters are synthesized in the cavities of sodium and calcium zeolite A microcrystals by exposing the activated Ag+-loaded zeolite to H2S. The growth of the silver sulfide clusters during rehydration of the samples is investigated by means of diffuse reflectance spectroscopy. Monomers of Ag2S are formed at low Ag+-loading through reaction of AgSH molecules, yielding colorless composites, which exhibit a characteristic blue-green photoluminescence. Yellow colored samples showing an orange-red luminescence with an average decay time of 81 mus (-160 degreesC) are obtained at medium silver sulfide content. Further increasing the silver sulfide loading and therefore the cluster size causes a bathochromic shift of this emission which is accompanied by a shortening of the luminescence lifetime. The samples generally exhibit large Stokes-shifts, which can be attributed to HOMO-LUMO transitions with small oscillator strengths. The Ag2S-zeolite host-guest system constitutes a three-dimensional array of silver sulfide clusters. The experimental data and results from quantum chemical calculations suggest that the optical absorption and luminescence properties of this material are mainly due to the presence of isolated silver sulfide clusters inside the zeolite cavities. The characteristics of the composites are thereby to a certain extent influenced by the cocations. Most remarkably, stronger luminescence, visible at room temperature, could be observed for silver sulfide clusters in calcium zeolite A compared to clusters in sodium zeolite A.