whose sizes and shapes are controlled by the dimensional restrictions induced by the
zeolite framework. Moreover, the negative charge of the zeolite framework and the
coordinating properties of the lattice oxygen atoms provide an additional stabilization of (partially charged) silver clusters.
Silver-exchanged zeolites were first reported in the early 60s by Ralek and
collaborators [62]. In this study, a visual color change in the zeolite samples with
respect to the silver loading and water content was observed. Nevertheless, at that
time no explanation for this phenomenon was proposed. After this report, two
groups started to study silver-exchanged zeolites independently. Seff and collaborators [63] initiated the detailed structural characterization of silver-loaded zeolites
using X-ray diffraction experiments and proposed their use as chemical sensors,
whereas Jacobs and co-workers [64] studied the silver clusters formation mechanisms in zeolites by analyzing their redox processes. One of the first studies on the
photoluminescence characterization of silver-exchanged zeolites was reported by
Ozin and collaborators [65]. In this study blue and green emission colors were
observed in silver-loaded faujasite zeolites having different silver contents. Later,
Calzaferri and co-workers [66] reported the formation of luminescent quantum-sized
silver sulfide clusters in Linde Type A (LTA) zeolites. In this report, the formation of
emissive species was achieved by the photosensitization of silver clusters by halides.
More recently, the characterization of fluorescence in heat-treated silver-exchanged
zeolites was carried out [19]; herein the emissive features of silver-exchanged
zeolites were attributed to the presence of partially reduced small oligoatomic silver
clusters. In Table 1 a compilation of photophysical properties of silver clusters
confined in zeolites is displayed, including steady-state and time-resolved
photoluminescence results.
2.2 Synthesis Strategy to Confine Luminescent Silver Clusters
in Zeolites
Space-confinement of silver clusters in zeolites is a versatile method to stabilize
highly luminescent silver clusters using a ship-in-a-bottle approach. Starting from a
variety of zeolite topologies with different silicon-to-aluminum ratios and alkali
metal ions (i.e., sodium, potassium, lithium), silver uptake is accomplished through
cation exchange by immersing zeolite precursors (in powder form) in silver nitrate
solutions of different concentrations (to study the effect of zeolite silver loading), the
cation exchange step is employed to replace the counter-balancing ions present in the
zeolites by silver ions. Once the zeolites have been exchanged with silver ions, a
subsequent activation step is required to promote the formation of luminescent silver
clusters; such activation can be achieved by heat treatment, X-ray irradiation, and
Photoactivation [18, 19, 68]. As a result of the activation, partially reduced silver
clusters are formed through different pathways where the required electrons needed
for metal ion reduction are provided by the zeolite framework oxygen (resulting in
Highly Luminescent Metal Clusters Confined in Zeolites
83
zeolite framework. Moreover, the negative charge of the zeolite framework and the
coordinating properties of the lattice oxygen atoms provide an additional stabilization of (partially charged) silver clusters.
Silver-exchanged zeolites were first reported in the early 60s by Ralek and
collaborators [62]. In this study, a visual color change in the zeolite samples with
respect to the silver loading and water content was observed. Nevertheless, at that
time no explanation for this phenomenon was proposed. After this report, two
groups started to study silver-exchanged zeolites independently. Seff and collaborators [63] initiated the detailed structural characterization of silver-loaded zeolites
using X-ray diffraction experiments and proposed their use as chemical sensors,
whereas Jacobs and co-workers [64] studied the silver clusters formation mechanisms in zeolites by analyzing their redox processes. One of the first studies on the
photoluminescence characterization of silver-exchanged zeolites was reported by
Ozin and collaborators [65]. In this study blue and green emission colors were
observed in silver-loaded faujasite zeolites having different silver contents. Later,
Calzaferri and co-workers [66] reported the formation of luminescent quantum-sized
silver sulfide clusters in Linde Type A (LTA) zeolites. In this report, the formation of
emissive species was achieved by the photosensitization of silver clusters by halides.
More recently, the characterization of fluorescence in heat-treated silver-exchanged
zeolites was carried out [19]; herein the emissive features of silver-exchanged
zeolites were attributed to the presence of partially reduced small oligoatomic silver
clusters. In Table 1 a compilation of photophysical properties of silver clusters
confined in zeolites is displayed, including steady-state and time-resolved
photoluminescence results.
2.2 Synthesis Strategy to Confine Luminescent Silver Clusters
in Zeolites
Space-confinement of silver clusters in zeolites is a versatile method to stabilize
highly luminescent silver clusters using a ship-in-a-bottle approach. Starting from a
variety of zeolite topologies with different silicon-to-aluminum ratios and alkali
metal ions (i.e., sodium, potassium, lithium), silver uptake is accomplished through
cation exchange by immersing zeolite precursors (in powder form) in silver nitrate
solutions of different concentrations (to study the effect of zeolite silver loading), the
cation exchange step is employed to replace the counter-balancing ions present in the
zeolites by silver ions. Once the zeolites have been exchanged with silver ions, a
subsequent activation step is required to promote the formation of luminescent silver
clusters; such activation can be achieved by heat treatment, X-ray irradiation, and
Photoactivation [18, 19, 68]. As a result of the activation, partially reduced silver
clusters are formed through different pathways where the required electrons needed
for metal ion reduction are provided by the zeolite framework oxygen (resulting in
Highly Luminescent Metal Clusters Confined in Zeolites
83