showing that Ag-FAU zeolites were the best performing materials with EQEs
reaching 69%, whereas Ag-LTA zeolites displayed a modest EQE of 16%
[27]. The quest for optimizing the luminescence performance of Ag-zeolites derived
in a recent study where by fine-tuning the synthesis parameters of silver-exchanged
FAU zeolites EQEs reaching 100% were reported [73]. As for Ag-LTA systems,
several attempts were performed to push EQEs toward industrial appealing values.
One approach that rendered promising results was the use of lithium as counterbalancing cations in Ag-LTA zeolites [70], where maximum EQEs values of about
62% were recorded together with a dynamical emission color response controlled by
water present in the zeolites. This report opened up the possibility of developing
switchable optoelectronic responses in Ag-LTA materials. This was later studied at
the atomic scale by EXAFS experiments, revealing that water and framework
oxygen ligands strongly affect the organization of silver clusters valence electrons
causing the peculiar on-off photoluminescence switching of the materials [83].
3 Characterization of Silver Clusters Confined in Zeolites
Deciphering the physicochemical properties of silver clusters confined in zeolites is
fundamental for the understanding of the different processes behind cluster formation and their functional properties. More importantly, the detailed structural characterization allows researchers to deeply understand the structure-to-properties
relationship, which provides the fundamental basis for silver clusters optimization
for different applications. Because the optical, electronic, and catalytic properties of
sub-nanometer silver clusters are strongly linked to their size, composition, morphology, and electronic state, several experimental techniques have been applied to
perform the characterization of silver clusters confined in zeolites. It is worth to
mention that due to the small size of silver clusters contained in zeolite matrices,
some techniques that are normally applied in the characterization of larger silver
nanoparticles are not suitable for the study of sub-nanometer-sized silver clusters. In
this section, we briefly revise common characterization techniques employed in the
analysis of silver-exchanged zeolites using representative examples. These include
UV-VIS-NIR and X-ray spectroscopy, electron microscopy, theoretical modeling,
fluorescence microscopy, EPR spectroscopy, and thermogravimetric analysis
(Fig. 3).
3.1 UV-VIS and FT-IR Spectroscopy
Sub-nanometer silver clusters present a molecular-like behavior due to their special
electronic structure which consists of discrete energy levels, resulting in the generation of optical transitions with absorbance bands that can be monitored by UV-Vis
spectroscopy. This technique has been successfully applied in the characterization of
Highly Luminescent Metal Clusters Confined in Zeolites
87
reaching 69%, whereas Ag-LTA zeolites displayed a modest EQE of 16%
[27]. The quest for optimizing the luminescence performance of Ag-zeolites derived
in a recent study where by fine-tuning the synthesis parameters of silver-exchanged
FAU zeolites EQEs reaching 100% were reported [73]. As for Ag-LTA systems,
several attempts were performed to push EQEs toward industrial appealing values.
One approach that rendered promising results was the use of lithium as counterbalancing cations in Ag-LTA zeolites [70], where maximum EQEs values of about
62% were recorded together with a dynamical emission color response controlled by
water present in the zeolites. This report opened up the possibility of developing
switchable optoelectronic responses in Ag-LTA materials. This was later studied at
the atomic scale by EXAFS experiments, revealing that water and framework
oxygen ligands strongly affect the organization of silver clusters valence electrons
causing the peculiar on-off photoluminescence switching of the materials [83].
3 Characterization of Silver Clusters Confined in Zeolites
Deciphering the physicochemical properties of silver clusters confined in zeolites is
fundamental for the understanding of the different processes behind cluster formation and their functional properties. More importantly, the detailed structural characterization allows researchers to deeply understand the structure-to-properties
relationship, which provides the fundamental basis for silver clusters optimization
for different applications. Because the optical, electronic, and catalytic properties of
sub-nanometer silver clusters are strongly linked to their size, composition, morphology, and electronic state, several experimental techniques have been applied to
perform the characterization of silver clusters confined in zeolites. It is worth to
mention that due to the small size of silver clusters contained in zeolite matrices,
some techniques that are normally applied in the characterization of larger silver
nanoparticles are not suitable for the study of sub-nanometer-sized silver clusters. In
this section, we briefly revise common characterization techniques employed in the
analysis of silver-exchanged zeolites using representative examples. These include
UV-VIS-NIR and X-ray spectroscopy, electron microscopy, theoretical modeling,
fluorescence microscopy, EPR spectroscopy, and thermogravimetric analysis
(Fig. 3).
3.1 UV-VIS and FT-IR Spectroscopy
Sub-nanometer silver clusters present a molecular-like behavior due to their special
electronic structure which consists of discrete energy levels, resulting in the generation of optical transitions with absorbance bands that can be monitored by UV-Vis
spectroscopy. This technique has been successfully applied in the characterization of
Highly Luminescent Metal Clusters Confined in Zeolites
87