Reversible emission evolution from Ag activated zeolite Na-A upon
dehydration/hydration
Hui Lin, Kenji Imakita, and Minoru Fujii
Citation: Applied Physics Letters 105, 211903 (2014); doi: 10.1063/1.4902530
View online: http://dx.doi.org/10.1063/1.4902530
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APPLIED PHYSICS LETTERS 105, 211903 (2014)

Reversible emission evolution from Ag activated zeolite Na-A upon
dehydration/hydration
Hui Lin,a) Kenji Imakita, and Minoru Fujiia)
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan(Received 27 October 2014; accepted 12 November 2014; published online 25 November 2014)
Reversible emission evolution of thermally treated Ag activated zeolite Na-A upon dehydration/
hydration in vacuum/water vapor was observed. The phenomenon was observed even for the
sample with low Agþ-Naþ exchanging (8.3%), indicating that the emission from Ag activatedzeolites may not come from Ag clusters while from the surrounding coordinated Agþ ions or Ag0
atoms. It was disclosed that the characteristic yellow-green emission at 560 6 15 nm is strongly
associated with the coordinating water molecules to the Agþ ions or Ag0 atoms, which is clear
evidence for that the efficient emission from Ag activated zeolites may not originate from theC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4902530]
quantum confinement effect. V

Silver (Ag) activated guest-host systems in various forms
including DNAs,1 proteins,2 or polymers3 encapsulated Ag
clusters, Ag activated zeolites,4 and glasses5,6 have been demonstrated as highly efficient luminophores, which may havepromising application in bio-labeling,7 white light emitting
diodes (LEDs),8 down-converter for solar cells,6 probes forsome specific hazardous chemicals or ions,2,9 optical recording/encoding,10,11 dosimetry,12 etc. However, till now, theexact luminescence mechanism from Ag activated materials is
still not sufficiently clear. One of the most proposed explanations is that the emission is from the sub-nm scale moleculelike Ag clusters complying with the quantum confinement
effect, that is, when the size of the metal clusters is comparable to the electron Fermi wavelength (0.5 nm for Ag), discrete energy levels can be formed and fluorescent moleculelike emission properties will be exhibited.13,14 The concept ofmetal clusters is amazing since it bridges the gap between
individual metal ions/atoms and metal nanoparticles in the luminescence and photonics community. Concerning the hostmaterials to stabilize the highly agglomerable Ag clusters,zeolites is known to be one of the few solid matrices owing to
its spatially well confined cages and channels. Among the various types of zeolite matrices, Ag activated zeolite A is intensively investigated, from which the strong and broad yellowgreen emission peaked at 530–590 nm is the most often
observed. Previously, this emission was tentatively attributed
to the linear Ag3þ (Agþ6-ring coordinated-Ag04-ring coordinatedAgþ6-ring coordinated) trimer cluster formed in the sodalite cage(also called b-cage).4 Formation of Ag cluster in zeolite A canbe achieved via various activations such as thermal treatment,15,16 UV illumination,17 and X-ray irradiation.18 The existence of Ag clusters in zeolite A has been demonstrated by
X-ray absorption fine structure (XAFS),19,20 far-infrared spectroscopy,21 electron spin resonance (ESR),4 scanning tunneling electron microscopy (STEM),22 etc. However, recently it
has been proposed by Miyanaga et al. that based on the in situ
XAFS and photoluminescence (PL) spectra in different gasa)Authors to whom correspondence should be addressed. Electronic
addresses: linh8112@163.com and fujii@eedept.kobe-u.ac.jp0003-6951/2014/105(21)/211903/4/$30.00

environmental conditions, the emission from the Agþ fully
exchanged zeolite Na-A (denoted as Ag12, Na0-A) may be
due to structural change of the zeolite framework induced by
the formation and breakdown of Ag clusters other than the Ag
clusters itself. Their conclusion was supported by the fact thatemission from the sample exposed in air was of the highest intensity, while in this situation Ag clusters were actually“broken up.”23 Indeed, the existence of Ag clusters does not
ensure that the emission comes from clusters. Furthermore,theoretically, the quantum confinement effect applies only for
those Ag clusters with the diameter less than 2 nm. In fact, efficient emission from large Ag dots with the average diameterof 18 6 3 nm was still observed. From the high resolution
transmission electron microscopy images of the luminescentand the non-luminescent Ag nano dots of the same size, theorigin of emission from such large nano dots was explained in
the way of a multi-domain, polycrystalline structure with anaverage (single crystalline) domain scale of 0.9 nm.24 Still,this reminds that the luminescence mechanism of Ag species
may not simply be the quantum confinement effect, since noobvious gaps or “walls” were observed between the domains(the single crystalline sub-nm grains) to prevent the electron
migration. It has been shown that two key factors influence
the emission properties of Ag species: one is the quantum confinement effect, specifically, the size and the configuration(including the shape and the electronic charges);13,14 the otheris the coordinating environment.25,26 However, it seems thatsize and configuration were too much emphasized as the main
origin of the emission, with the influence of coordinating environment left behind. Very recently, it was proposed by Chenet al. that the emission mechanism of Ag clusters conjugatedby carboxylate ligands with the nanocomposite size in therange of 2–5 nm is the ligand (oxygen) to-metal (Agþ)-metal(Ag0) charge transfer transition.27 Meantime, the strong dependence of emission wavelength on excitation was assigned
to the surface plasmon resonance (SPR) absorption from theAg metal nano core. This is an attempt for the exploring themechanisms of emission from metal clusters in another way.
However, since the total size of the nanocomposite is 2–5 nm(Ag0 nano core is below 5 nm), efficient SPR cannot be

105, 211903-1

C 2014 AIP Publishing LLC
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Lin, Imakita, and Fujii

expected generally. Compared with the complicated cationsites in organic and glass matrices, situations for zeolites may
be simplified due to the almost definite cation sites located in
the cages and channels, thanks to the long-distance periodical
structure due to its inorganic crystalline nature. Furthermore,
the definite sizes of the pores, cages, and channels of zeolites
are selectively permeable to certain kinds of molecules, so
environmental change for the Ag species is feasible toachieve. Here, in this work, we report the reversible emission
evolution of Ag activated zeolite A via the hydration/dehydration process in water vapor/vacuum. It is disclosed that thebright yellow-green emission at 560 6 15 nm and the nearinfrared (NIR) emission at 855 nm are possibly originatedfrom the charge transfer transitions from the framework oxygen to the Ag0 atoms/Agþ ions with the assistance of H2O
molecules, rather than the previously most proposed confinement effect.
Ag activated zeolite Na-A samples were prepared by ion
exchanging and thermal treatment. Details for the sample
preparation were described elsewhere.28 3g zeolite 4A powders (Tosoh, Japan) were stirred at room temperature for
24 h in AgNO3 solutions, in which nominal Ag1, Na11-A(Agþ-Naþ exchanging is 8.3%) and Ag4.4, Na7.6-A (AgþNaþ exchanging is 36.7%) samples can be achieved,
respectively. The actual loading content of Agþ ions into thezeolite’s framework is somehow lower than the nominal
value since Agþ can still be examined in the filtrate after the
Agþ-Naþ exchanging process. The Agþ loaded zeolite powders were washed by centrifuging, dried at 50  C and thenfinally annealed at 550  C for 2 h in air. To get a clear viewof the emission characteristics of Ag species confined in zeolites, it is necessary to know the luminescence properties ofpure zeolites upon heat-treatment first. So pure zeolite 4A
powders without any treatment and thermally treated at
350  C, 550  C, and 650  C in air for 2 h were also prepared.
The samples were loaded into a cryostat for measuring the
PL spectra in vacuum/water vapor and the temperature dependent PL. The first spectrum at room temperature in vacuum for the temperature dependent PL measurement was
taken when the emission evolution in vacuum almost
stopped in terms of the converting rate within our experimental conditions. Excitation-emission 3D contours and PL
spectra taken at room temperature in air were directly measured on a fluorospectrometer (Jobin Yvon, Fluorog 3,
Horiba, Japan).
From the excitation-emission 3D contours of the purezeolite 4A annealed at different temperatures presented inFig. S1,29 it is shown that similar emission features to that of
the thermally treated Ag activated zeolite Na-A appear.4,28
This phenomenon leads us to infer that the emission frompure zeolite Na-A upon thermal treatment might come fromthe Naþ-zeolite guest-host interaction. Compared with theAgþ activated zeolite Na-A, the emission intensity is of 2orders of magnitude weaker, which can be attributed to theweak coordination between the Naþ ions and the framework
oxygen.30
For zeolite 4A, the critical size of the a-cage is 4.2 Å,and the critical size of the b-cage is 3.0 Å.31 The dynamic
diameter of H2O and O2 molecules is 2.7 Å and 3.6 Å,respectively. So H2O molecules can enter and stay at bothAppl. Phys. Lett. 105, 211903 (2014)

the a-cages and the b-cages, while only the a-cages are available for oxygen molecules. PL spectra evolution of theAg4.4, Na7.6-A sample from air to vacuum and then to water
vapor or oxygen are presented in Fig. 1. As the evacuatingtime goes by, the intensity of the 560 6 15 nm and the855 nm emissions simultaneously decreased, while a new
broad emission peaked at 678 nm started to appear, grow,and finally became the dominant emission. When water
vapor was introduced into the cryostat’s chamber, the560 6 15 nm emission was immediately (<30 s) recovered.
Though it was proposed that dehydration is a common
method for forming Ag clusters;4 here, it is clearly shownthat involvement of a proper amount of H2O molecules isvital to the bright 560 6 15 nm emission. The NIR emission peaked at 855 nm was partially quenched, which may
be caused by over-adsorption of the highly vibrational H2O
molecules. The H2O-sensitive emission feature might enable
Ag activated zeolites to be a sensor or gauge for humidity. In
Ref. 23, the visible emission was totally quenched after
the Ag12, Na0-A sample being exposed in oxygen for 24 h.
On the contrary, after being exposed in oxygen for 24 h, intensity of the 560 6 15 nm emission remained unchanged;

FIG. 1. PL evolution of Ag4.4, Na7.6-A from air to vacuum then to water
vapor (a) or oxygen (b).

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Lin, Imakita, and Fujii

while the intensity of the 678 nm emission was increasedby almost 100% in our experiment (see Fig. 1(b)). Here, it
should be noted that the above emission evolution is reversible and does not show obvious emission quenching after fivecycles of repetition. The enhancement of the 678 nm emission in oxygen may be attributed to the further dehydrationof the zeolite A framework by the extremely dry oxygen
introduced from the gas cylinder, since the same phenomenon was observed when nitrogen was introduced instead.
The inner surface of zeolite A has strong ability of capturing
small polar molecules like H2O due to the strong electricfield between the cations and the framework O2–. While fornon-polar molecules such as O2 and N2, the poor affinity
with the pore walls32 makes the coordination with the zeolite
A framework (including the cations) negligible.
In Ref. 4, good homogeneity of the emission spectra
was exhibited for different individual Ag6, Na6-A zeolite
microcrystals, indicating that the broad 560 6 15 nm emission is mainly not caused by inhomogeneous broadening.
Seen from the temperature dependent PL spectra presented
in Fig. 2, the emission peaks are still quite broad even at
10 K, which indicates that the broad emission feature is not
only attributed to the phonon-electron coupling either. The
broad emission feature of Ag activated zeolite at 10 K also
seems to be against the explanation based on quantum confinement effect as the emission origin. Namely, since theconfiguration, especially, the ion and atom number and theshape of the Ag clusters, are perfectly confined in the cages
and channels of zeolites, sharp emission line(s) is expectedto the observed at cryogenic temperatures. However, this iscompletely not the case for Ag activated zeolites.
Excitation-emission 3D contour in the visible range for
Ag1, Na11-A exposed in air for 30 min after annealing at
550  C is shown in Fig. S2(a), from which the 560 6 15 nm
emission still can be well observed. Besides the emission
evolution upon dehydration/hydration in vacuum/water
vapor, the same phenomenon was also observed upon dehydration/hydration by thermal annealing/(being exposed in air
for 30 min), as shown in Fig. 2(b). It should be noted that
according to the X-ray diffraction data of the vacuum dehydrated silver hydrogen zeolite A, it is shown that the ratio of
the possibility for the Agþ ions or Ag0 atoms to occupy the

FIG. 2. Temperature dependent PL spectra for Ag4.4, Na7.6-A under the
355 nm excitation.

Appl. Phys. Lett. 105, 211903 (2014)

6-ring (a-cage), 6-ring (b-cage), 8-ring (a-cage), and 4-ring(b-cage) co-cation sites is 44:9:7:6.33 While in Refs. 34 and35, it is disclosed that the yellow color appearance of thedehydrated Agþ exchanged zeolite Na-A is only associatedwith the Agþ ions sitting on the 4-ring coordinated sites in
the a-cages and the 4-ring coordinated sites in the b-cages
are not available for Agþ ions. In this case, formation of Ag
clusters in the b-cages for Ag1, Na11-A and Ag4.4, Na7.6-A isalmost impossible. Furthermore, from the results reported inRef. 23 where it is shown that Ag clusters cannot be formed
even for the Agþ fully exchanged zeolite A when the samples are exposed in air or in water vapor, it seems that thepossibility of forming Ag clusters for Ag1, Na11-A andAg4.4, Na7.6-A in water vapor or air is negligible. Further
investigation of the specific configuration of Ag activatedzeolite A with low Agþ loading concentration by XAFS will
be conducted in our future work.
Noteworthy, similar to the reversible emission color
change, reversible appearance color changes of Agexchanged zeolite A upon dehydration was reported early inthe 1960s.36 During the following four decades, the yellow
and brick-red appearance was assigned to the formation of
Ag32þ clusters and the interaction of two (or more) Ag32þ
clusters, respectively.37 Calzaferri et al. proposed that thecolor appearance of Ag activated zeolite A is due to the optical absorption from the ligand to metal charge transfer transition (specifically, from the framework oxygen to the empty
5s obit of Agþ ions).38 In 2003, the same group further proposed that the electronic structure of the d10 ions-zeolite
guest-host materials is dominated by the charge transfer transitions from the framework oxygen to the d10 ions, and thesetransitions can be influenced a lot by specific coordination
conditions.30 Meantime, it was also pointed out that Cuþ
exchanged and Agþ exchanged zeolites showed dynamic
luminescence characteristics associated with the hydration
state. The optimal amount of water for achieving the maximal emission intensity depends on the co-cations and on thetypes of zeolites, due to the different electrostatic interaction
between the co-cations and the H2O molecules.30,34,35
Interestingly, the co-cations in the zeolite framework alsostrongly influences the luminescence property of semiconductor clusters such as Ag2S and PbS.39 In Ref. 23, the insitu XAFS results in various gas environment show thatthe nearest Ag-O distance (with coordination number >4) isalways below 2.4 Å (the Ag-O bond length in Ag2O
2.087 Å), making strong coordination of Agþ ions or Ag0
atoms by the framework oxygen possible. Though the emission of Agþ ions is generally reported to be located at highenergies,40,41 it is shown that emission at lower energies ispossible via the ligand-to-Agþ ion charge transfer transition,42 especially, when reducing ligands are present.43
Amazingly, zeolite framework is famous for its abundanceof negative charge centers hence it can be a hotbed for the
charge transfer process. Combined with the previous reports
from which it is shown that partial reduction of the Agþ ions
into Ag0 atoms is essential for emission either in the case of
Ag activated zeolites or the DNA (or protein) encapsulated
Ag clusters, we infer that Ag0 atoms coordinated by H2O
molecules and the zeolite A’s framework oxygen may also
participate in the emission process. Here, we tentatively

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Lin, Imakita, and Fujii

assign the 560 6 15 nm and the 855 nm emissions to thecharge transfer transitions from the framework oxygen toAgþ-Ag0 involving the participation of H2O molecules.
Though, at present, we are incapable of assigning the emission peaks to exact coordination conditions, it is clear thatorigin of the emission from Ag activated zeolites is not
mainly influenced by the existence of clusters while by thevariation of coordinating environment. The almost invariable
emission wavelength within a considerably wide excitation
wavelength range for Ag activated zeolite Na-A4,28 indicatesthe homogeneous coordinating environment for Ag0 atoms
or Agþ ions, due to the long-distance periodical lattice
structure of the zeolite A matrix. Meantime, the invariable
emission wavelength under different excitation wavelengths
also excludes the existence of SPR absorption due to the large
Ag nanoparticles on the surface of the zeolite A microcrystals.
For glass matrices in Refs. 44 and 45, large red shifts are usually observed under longer excitation wavelengths, whichwere assigned to the wide size distribution of Ag clusters.
Now, it seems that the non-periodical structure of the glass
matrix, in results that the inhomogeneous coordination of Agþions and Ag0 atoms may be an alternative explanation.
Based on the reversible emission evolution of thermallyannealed Ag activated zeolite Na-A with low Ag loading content upon dehydration/hydration in vacuum/water vapor, it isproved that origin of the emission from Ag activated zeolite A
may not be the quantum confinement effect, but may be thecharge transfer transition from the framework oxygen to AgþAg0 with the participation of H2O molecules. The H2O-sensitive emission feature might enable Ag activated zeolites to bea sensor or gauge for humidity. This work also demonstrated
that zeolites is a group of excellent matrices for investigating
the luminescence physics of sub-nm scale Ag species activated materials for its inorganic crystalline nature.
Hui Lin would like to express his sincere thanks to Prof.
Dr. Kanna Aoki for her generous and valuable discussion on
the gas permeation properties of zeolite A.
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