3.3 Fluorescence Spectroscopy and Microscopy
Silver-exchanged zeolites have been extensively studied because of their promising
catalytic properties [90]. Colored ionic silver complexes were reported as responsible for their catalytic properties [85]; however, these complexes presented poor
luminescent properties at room temperature. Kanan and collaborators reported the
luminescence properties of silver-containing zeolites under liquid nitrogen conditions [101]. They attributed the photoluminescence activity to the presence of Ag
dimers, trimers, and tetramers species in different zeolite topologies (LTA, FAU,
ZSM-5) and established a correlation between different emission bands and silver
cluster size. One of the first photoluminescence studies on silver-exchanged zeolites
at room temperature was presented by De Cremer and co-workers [18], in which
individual silver-containing zeolite crystals were photo-activated using a fluorescence microscope set-up. The formation of highly luminescent green emitting
species in Ag-LTA was demonstrated. Subsequently, the same group reported the
characterization of fluorescence in heat-treated silver-exchanged zeolites [19]. In this
study, different types of emitters with characteristic luminescence colors were
observed, depending on the nature of the counter-balancing ions present in the
zeolite framework, the silver loading and the zeolite topology. Green, yellow, and
red emission colors were tentatively assigned to partially reduced Ag 3
n+ and Ag 6
m+
clusters. The first evidence of the influence of metal-guest and zeolite-host features
on the luminescence properties of these types of materials was presented in
this work.
The spectral evolution of the emission bands observed in silver-exchanged
zeolites during thermal treatment under different gas atmospheres was studied in
detail in a different report [102]. By using an in-situ heating stage mounted on a
fluorescence microscope, the process of thermally induced silver cluster formation
could be followed in detail, as well as the effect of hydration/rehydration on the
emission color of the formed clusters. In a latter study [27], the different factors that
could affect the luminescent performance of silver-loaded materials were assessed in
a quantitative way by measuring external quantum efficiencies (EQE); in the same
report a reduction-oxidation procedure to improve their luminescence performance
and water stability was described.
3.4 Electron Microscopy
Transmission electron microscopy (TEM) is a very powerful technique to characterize the structure and composition of materials at the atomic scale. However, the
use of electron microscopy is limited in aluminosilicate mesoporous matrices due to
the extreme sensitivity of these materials toward electron beam irradiation under
high vacuum conditions [103]. In the case of silver-exchanged zeolites, it is of
Highly Luminescent Metal Clusters Confined in Zeolites
91
Silver-exchanged zeolites have been extensively studied because of their promising
catalytic properties [90]. Colored ionic silver complexes were reported as responsible for their catalytic properties [85]; however, these complexes presented poor
luminescent properties at room temperature. Kanan and collaborators reported the
luminescence properties of silver-containing zeolites under liquid nitrogen conditions [101]. They attributed the photoluminescence activity to the presence of Ag
dimers, trimers, and tetramers species in different zeolite topologies (LTA, FAU,
ZSM-5) and established a correlation between different emission bands and silver
cluster size. One of the first photoluminescence studies on silver-exchanged zeolites
at room temperature was presented by De Cremer and co-workers [18], in which
individual silver-containing zeolite crystals were photo-activated using a fluorescence microscope set-up. The formation of highly luminescent green emitting
species in Ag-LTA was demonstrated. Subsequently, the same group reported the
characterization of fluorescence in heat-treated silver-exchanged zeolites [19]. In this
study, different types of emitters with characteristic luminescence colors were
observed, depending on the nature of the counter-balancing ions present in the
zeolite framework, the silver loading and the zeolite topology. Green, yellow, and
red emission colors were tentatively assigned to partially reduced Ag 3
n+ and Ag 6
m+
clusters. The first evidence of the influence of metal-guest and zeolite-host features
on the luminescence properties of these types of materials was presented in
this work.
The spectral evolution of the emission bands observed in silver-exchanged
zeolites during thermal treatment under different gas atmospheres was studied in
detail in a different report [102]. By using an in-situ heating stage mounted on a
fluorescence microscope, the process of thermally induced silver cluster formation
could be followed in detail, as well as the effect of hydration/rehydration on the
emission color of the formed clusters. In a latter study [27], the different factors that
could affect the luminescent performance of silver-loaded materials were assessed in
a quantitative way by measuring external quantum efficiencies (EQE); in the same
report a reduction-oxidation procedure to improve their luminescence performance
and water stability was described.
3.4 Electron Microscopy
Transmission electron microscopy (TEM) is a very powerful technique to characterize the structure and composition of materials at the atomic scale. However, the
use of electron microscopy is limited in aluminosilicate mesoporous matrices due to
the extreme sensitivity of these materials toward electron beam irradiation under
high vacuum conditions [103]. In the case of silver-exchanged zeolites, it is of
Highly Luminescent Metal Clusters Confined in Zeolites
91