JOURNAL OF APPLIED PHYSICS 115, 033507 (2014)

Enhanced near infrared emission from the partially vitrified Nd31
and silver co-doped zeolite Y
Hui Lin,a) Sa Chu Rong Gui, Kenji Imakita, and Minoru Fujiia)
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan(Received 12 September 2013; accepted 2 January 2014; published online 16 January 2014)
Near infrared (NIR) emission from the Nd3þ doped zeolite Y was strongly enhanced by partiallyvitrifying the zeolite structure via extra loading silver ions and post annealing. Under the low
annealing temperatures at 450  C and 650  C, the states of the loaded silver were determined to be
the co-existence of the isolated Ag0 atoms, the Agþ ions, and the Ag2þ dimers. However, there wasno enhancement in the NIR emission by the introduction of these small silver clusters. Underhigher annealing temperature at 900  C where the lattice of the zeolite Y was partially collapsed
into the amorphous phase, strong NIR emission enhancement at 1064 nm with a factor of 6.8 was
observed. The partial vitrification process by the co-loading of silver and post heat-treatment hadstrong effect on eliminating the H2O molecules, which can greatly enhance the NIR emission.
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862232]
V

I. INTRODUCTION

Bioluminescence imaging based on the micro/
nano-scale fluorescent materials has advantages like highsensitivity, low cost, high feasibility and flexibility, etc.1–3
Previously, tremendous amount of work has been done on
the visible and the near infrared (NIR) (I) (0.65–0.95 lm)“human optical window” up-conversion luminescence under
the NIR excitation. Among the above, Er3þ, Yb3þ co-doped
NaYF4 nano-particles were most intensively investigated forthe high up-conversion fluorescence quantum efficiency
enabled by the low phonon energy of the NaYF4 host.4–7
Lately, a secondary “human optical window” luminescence
located in the 1–1.35 lm NIR range under the NIR excitation
has been explored for its promisingly low scattering from thehuman tissues and the low background fluorescence from theorganic medicine molecules or the human tissues.8–11 Sincebioluminescence imaging often deals with the luminescencetraceable drug delivery, porous materials in various forms
are preferred for their strong ability of drug storage/release
within the pores or the channels. Recently, self-activated porous silica nano-fibers, Er3þ, Yb3þ:NaYF4 nano-crystals embedded in the porous silica nano-fibers and the porous
Nd3þ:YAG nano-fibers have been fabricated by electrospinning and exhibited excellent bio-luminescence imaging anddrug delivery properties.12–14 Intensity variation of the up- or
down-conversion emission has been observed due to thechange of the environmental conditions around the luminescent enters. Speaking of the porous materials, zeolite is oneof the most famous for its nature-born size, well-definedpores and channels, and actually has been put into practicaluse in catalysis, molecule sieving, and ion exchanging, etc.
Recent research shows that zeolites are excellent luminescence host materials owing to their strong ability of separating the luminescent centers from the aggregation induceda)Electronic addresses: linh8112@163.com and fujii@eedept.kobe-u.ac.jp

0021-8979/2014/115(3)/033507/7/$30.00

quenching.15–18 However, due to the strong and fast
non-radiative vibrational de-activation of the luminescentcenters from excitation state by water molecules, or -OH
bonds, reports on the NIR luminescence in the zeolite matrices were few. Tuning down the vibrational energy by extra
loading the low vibrational organic guests,19,20 or by the separation of the luminescent centers from the water molecules
or the detrimental -OH bonds by partially sealing the poreswith inorganic clusters21 have been demonstrated as efficientsolutions to ensure the strong NIR emission. Here, in thiswork, we report the enhanced NIR luminescence from partially vitrified Nd3þ doped zeolite Y by co-loading of silver
and post heat-treatment. Nd3þ was chosen as the luminescentcenters because the 1.06 lm emission from the Nd3þ:4
F3/2!4I11/2 transition falls well in the second “human optical window” (1–1.35 lm). The Stokes shift between the
Nd3þ 1064 nm emission and the 800 nm NIR excitation isbeneficial to accurate signal collection. Another advantageof the rare earth luminescence is that once the host matrix isdetermined, the emission wavelength of is fixed, independentof the excitation power or the excitation wavelength. This isfavorable to the stable signal collection or even to the fingerprint identification. It also should be noted that the quantum
yield of the rare earth down-conversion luminescence is usually considerably higher than that of the up-conversion
process.
II. EXPERIMENTAL
A. Sample preparation

First, Nd(NO3)36H2O was weighed and dissolved inde-ionized (DI) water to get a 50 mmol/L solution (40 mL).
Then 2 g zeolite Y (SiO2/Al2O3 ¼ 7, Tosoh, Japan) was
added into the solution and the suspension was stirred at
80  C for 36 h. The product was centrifuged and rinsed by DI
water and then dried at 80  C. The powders were annealed
from 900  C to 1200  C in air. Then, one half of the powders

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annealed at 900  C were stirred in a 250 mmol/L AgNO3 solution (40 mL) at 80  C for 8 h and then the aforementioned,rinsing and drying process was repeated. The dried powders
were further annealed at 450  C, 650  C, and 900  C in air.
The obtained samples were named as SNd- C (, Ag- C),
according to the doping conditions and the annealing
temperatures.
B. Characterization

The structure of the samples was investigated by X-ray
diffraction (XRD) (Rigaku, Ultima IV, Japan) and FTIR
(PerkinElmer, Spectrum GX, USA). Morphology of the samples was observed by transmission electron microscopy(TEM) (JSM-2100, JEOL, Japan). The valence states of thesilver dopant were examined by X-ray photoelectron spectroscopy (XPS) (Automated XPS Microprobe PHI X-toolULVAC-PHI, Japan). Diffuse reflection spectra of thesamples were measured by a UV/VIS/NIR spectrometer(Shimazu 3700, Japan). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured on aspectrofluorometer (Horiba Fluorog 3 Jovin, Japan). The
decay time of the Nd3þ 1064 nm emission under the 803 nm
excitation from an optical parametric oscillator (OPO)
pumped by a third harmonic Nd:YAG laser was measured by

J. Appl. Phys. 115, 033507 (2014)a photomultiplier tube (Hamamatsu, R5509-72), and then thesignal was analyzed with a photon-counting multichannel
scaler. For all the optical spectra measurement, the samples
were loosely packed into the sample holder without any
solvent.
III. RESULTS AND DISCUSSIONXRD h-2h scans of the Nd3þ doped zeolites with different annealing temperatures are presented in Fig. 1(a). The
diffraction patterns for the samples annealed at 900  C are
identical to zeolite Y (JCPDS43-0168), which indicated thatthe zeolite structure was not changed. However, the diffraction intensity became weaker for SNd-900  C compared withthat of the untreated pure zeolite Y. Annealing at 950  C
may be a critical point since the zeolite diffraction intensity
became further weaker and a broad amorphous band began
to turn out. For the samples annealed at 1000  C, the composition of the sample was separated into mullite Al6Si2O13(JCPDS15-0776) and amorphous phase. When annealed at1100  C, a new phase, cristobalite silica (JCPDS39-1425)
turned out, coexisting with the mullite Al6Si2O13 and theamorphous phase. When annealed at 1200  C, the amorphous
phase almost fully turned into the cristobalite phase silica
and the sample was of SiO2 and Al6Si2O13 mixed phases.

FIG. 1. XRD h-2h scans of the Nd3þ single doped (a) and the Nd3þ, silver co-doped (b) zeolites under different annealing temperatures; the diffraction peaks
were indentified: $ for zeolite Y, 䉱 for Al6Si2O13, and 䊉 for cristobalite phase silica.

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FIG. 2. Schematic diagrams showing the cation sites in the ideal zeolite Y
framework: the blue skeleton stands for the zeolite Y framework, and the
red balls stand for the cation sites.

XRD h–2h scans of the Nd3þ and silver co-loaded zeolite Y
are given in Fig. 1(b). It can be seen that compared with theuntreated pure zeolite Y, the diffraction intensity wasdecreased for SNd-900  C, Ag-450  C, and SNd-900  C, Ag-650  C.
While for SNd-900  C, Ag-900  C, the diffraction intensity fromzeolite Y became very weak and a characteristic amorphousbroad band turned up. It is interesting that unlike SNd-1000  C
and SNd-1100  C for which the zeolite Y phase was totally sacrificed and the new phase (mullite) turned up instead, while
the zeolite Y structure was still partially maintained for
SNd-900  C, Ag-900  C without showing extra crystalline phases.
Schematic diagram of the ideal zeolite Y structure ispresented in Fig. 2. There are a set of well defined sites forthe cations in the zeolite Y structure.22 SI is located in thecenter of the hexagonal prism which is inaccessible to thedetrimental -OH bonds or the water molecules. SI0 is locatedin sodalite cage close to the center of the prism-sodalite
shared 6-member ring. Ag2þ dimers can be formed with one
Ag0 atom occupying the SI site linked to one Agþ occupying
the SI0 site.23 SII0 is located in the sodalite cage close to the

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FIG. 3. XPS spectra for the Nd3þ and silver doped zeolite Y samples.

center of the sodalite self-own 6-member ring. SII is in the
super cage, next to SII0 and in line with SII and the center ofthe sodalite self-own 6-ring. Recent study based on PL spectra and high-resolution powder XRD shows that NIR luminescent centers such as Biþ located in the sodalite cage
mainly contribute to emission.24 SII* is located deeper in the
super cage and also in line with SII0 and SII. SU is in the center of the sodalite cage. Thus, the luminescent centers
located at the SI0 , SII0 , SII, S II*, and SU sites in the sodalite
cage and the super cage are vulnerable to be quenched by theexistence or the entrance of the water molecules or thehighly vibrational -OH bonds.
XPS spectra of silver for all the silver doped samples
and SNd-900  C were shown in Fig. 3. Except for SNd-900  C,
the two peaks for Ag 3d3/2 and Ag 3d5/2 located at 368.3 eV
and 374.3 eV were detected for all the silver doped samples
with post heat-treatment. There was a 0.3 eV negative shifteach for the Ag 3d3/2 and the Ag 3d5/2 peaks, compared withthe counterparts of SNd-900  C, Ag-untreat (Ag 3d3/2 @ 368.0 eV
and Ag 3d5/2 @ 374.0 eV, respectively). The negative shifts
indicated the partial reduction of Agþ ions into Ag0 atoms

FIG. 4. Absorption spectra of the samples derived from the Kubelka-Monk formula in the visible-NIR range (a) and the UV range (b); the inset in Fig. 3(b) is
a magnified view.

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FIG. 5. PL spectra of SNd-900  C, Ag-untreat, SNd-900  C, Ag-450  C, SNd-900  C, Ag-650  C, and SNd-900  C, Ag-900  C under the 265 nm (a) and the 306 nm (b) excitations in
the visible range.

during the post heat-treatment process.25 The formation of
Ag0 atom from Agþ ion can be through the negative chargetransfer from the framework O2 or water as the reducing
agent.26 However, it is not suitable to estimate the ratio of
Ag0/Agþ, since it is difficult to distinguish Ag0 atoms fromAgþ ions in the XPS spectra accurately for quantitative
calculation.
The absorption peaks due to the transitions from theNd3þ: 4I9/2 ground state to the higher energy states havebeen assigned in Fig. 4(a). Due to the Nd3þ-Agþ ion
exchanging during the loading of the Agþ, the absorption of
Nd3þ became lower than that of the Nd3þ single doped samples. The absorption spectra in the UV range were presented
in Fig. 4(b). For SNd-900  C, Ag-untreat, the absorption of silver
was almost negligible. For SNd-900  C, Ag-450  C and SNd-900  C,
Ag-650  C, the absorption peak located at 265 nm is assigned to
the Agþ ions.27 The absorption at 306 nm for SNd-900  C,
Ag-450  C and SNd-900  C, Ag-650  C was attributed to the 2S!2Ptransition of the isolated Ag0 atom located in the center ofthe hexagonal prism.23,28 For SNd-900  C, Ag-900  C, the 265 nm

and the 306 nm absorption peaks totally disappeared, and noother new absorption peaks were detected.
To further determine the states of the loaded silver, theemission spectra in the visible range under the 265 nm andthe 306 nm excitation were investigated. For SNd-900  C,
Ag-450  C and SNd-900  C, Ag-650  C, the strongest emission peaks
under the 265 nm excitation was both located at 544 nm (seeFig. 5), which was attributed to Ag0 atoms from theAg0(SI)-Agþ(SI0 ) dimers.25 While for SNd-900  C, Ag-900  C
under the 265 nm excitation, a new emission peak centeredat 394 nm turned up. This emission band may be attributed
to the oxygen vacancy defects, since for the untreated purezeolite Y, a similar emission band with a lower intensity has
also been observed. Under the 306 nm excitation, two strong
emission peaks located at 494 nm and 544 nm were observedfor SNd-900  C, Ag-450  C and SNd-900  C, Ag-650  C. The 494 nm
emission was attributed to the emission from the isolated
Ag0 atom at site SI in the hexagonal prism.29 One clear tendency that can be seen is that the intensity of these emissions
dropped drastically when annealed at 650  C and 900  C,

FIG. 6. NIR emission spectra of the Nd3þ (and silver) doped zeolite Y
samples.

FIG. 7. FTIR spectra for the pure zeolite Y, SNd-untreat, SNd-900  C, SNd-900  C,
Ag-untreat, SNd-900  C, Ag-450  C, SNd-900  C, Ag-650  C, and SNd-900  C, Ag-900  C.

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FIG. 8. TEM morphology of pure zeolite Y (a), SNd-900  C (b), and SNd-900  C,
Ag-900  C (c) individual particles.

which was in accordance with the absorption spectra andimplied that the small silver clusters may turn into large particles or may be incorporated into the amorphous phase athigh heat-treatment temperatures. For SNd-900  C, Ag-900  C,
there was no emission in the visible range under the 306 nm
excitation. The shapes of the emission peaks under the
265 nm and 306 nm excitations for SNd-900  C, Ag-untreat weresimilar to the counterparts of SNd-900  C, Ag-450  C. However,
the emission intensity decreased by 1 order of magnitude.
Seen from the PL spectra in the NIR range (shown in Fig.
6), there was no emission in the NIR range upon the 803 nm
excitation for the untreated pure zeolite Y. The NIR emission
for the sample without annealing was the weakest among the
Nd3þ doped samples. For SNd-900  C, Ag-450  C and SNd-900  C,
Ag-650  C, the integrated emission intensity for the 1064 nm
emission both dropped about 42.4%, compared with that of
SNd-900  C. The possible reason is that these samples got
hydrated again to some extent during the silver loading andthe second round low temperature thermal treatment at 450  C
and 650  C. In addition, Agþ ions, Ag0 atoms andAg0(SI)-Agþ(SI0 ) dimers were too small to isolate the Nd3þluminescent centers from the -OH bonds and the H2O molecules, thus made no positive contribution to the NIR emissionof Nd3þ. For SNd-900  C, Ag-900  C, there was a large enhancement with a factor of 6.8 for the integrated 1064 nm emission
compared with the counterpart of SNd-900  C. Since there wasno absorption around 803 nm from silver in the absorption

spectra, the enhanced NIR emission cannot be attributed tothe energy transfer from silver to Nd3þ. It was reasonable thatthe partially vitrification by annealing the silver co-loaded
sample at 900  C made the main contribution to the NIR
emission enhancement. In this case, the role of the low vibrational organic compounds (bis(perfluoromethylsulfonyl)amine,
PMS-H) in the zeolite framework was so obvious that strong

FIG. 9. Decay curves for the Nd3þ 1064 nm emission under the 803 nm
excitation.

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TABLE I. Decay time of the Nd3þ 1064 nm emission under the 803 nm excitation.
Sample nameSNd-untreat

SNd-900  C

SNd-900  C,Ag-untreat

SNd-900  C,Ag-450  C

SNd-900  C,Ag-650  C

S Nd-900  C, Ag-900  C

Decay time (ls)

11

23

13

25

21

237

NIR emission could be observed even for the sample directly
dried at 150  C without high temperature annealing, which
may be attributed to the strong combination between the
PMS-H and the -OH bonds.19
FTIR spectra were measured to take a further look at the
microstructure evaluation of the zeolite, as shown in Fig. 7.
The characteristic absorption bands of the -OH bonds locatedaround 3400 cm1 to 3600 cm1 were observed in the FTIR
spectra.22 For SNd-900  C, Ag-450  C and SNd-900  C, Ag-650  C, theamount of the -OH bonds did not seem to be reduced at all.
For SNd-900  C, Ag-900  C, the -OH absorption band becameslightly lower, which implied that at 900  C silver had positive effect on eliminating the high vibrational -OH bonds by
vitrifying the zeolite Y structure during the postannealing
dehydration process. The absorption band located at
1634 cm1 is attributed to the structural or absorbed H2O inthe silicates.30 For SNd-900  C, Ag-900  C, the absorption at
1634 cm1 was reduced at least 50% compared with othersamples. This was believed to be the main reason for thestrong enhancement for the NIR emission. For the pure zeolite Y, the sharp absorption peak located at 1400 cm1 is
attributed to NH4Br. Since the other samples were
ion-exchanged with Nd3þ and Agþ, the absorption by
NH4Br in the FTIR spectra has been greatly reduced. The
intense absorption band around 1100 cm1 was the characteristic asymmetric Si-O/Al-O stretching vibration.16
TEM morphology of pure zeolite Y clearly exhibits the
typical polyhedron profile (Fig. 8(a)). The morphology of
SNd-900  C shown in Fig. 8(b) is similar to that of pure zeolite
Y. While for SNd-900  C, Ag-900  C, collapse of the polyhedronmorphology can be clearly observed in Fig. 8(c). Large silver
nano-particles were not observed for SNd-900  C, Ag-900  C,
which may be attributed to the low doping concentration ofsilver in the zeolite Y framework. Based on thesemi-quantitative calculation from the XPS data, the atomic
concentration of silver is 1%. Small silver cluster like
Ag2þ dimer whose scale is less than 1 nm, is insensitive to
TEM or XRD h-2h detection but sensitive to optical spectra.
The decay curves of the Nd3þ 1064 nm emission were
given in Fig. 9. The decay curves of the Nd3þ 1064 nm emission for SNd-untreat, SNd-900  C, SNd-900  C, Ag-untreat, SSNd-900  C,
Ag-450  C and SNd-900  C, Ag-650  C all showed at least two exponential decay features. The meanÐdecay time (sm) values cal1
culated from the equation sm ¼ t0 ½IðtÞ=I0 dt, (where I(t) isluminescence decay Intensity at time t, I0 is the maximal
value of the decay intensity, and t0 corresponds to the initialtime when I0 occurs), as listed in Table I, were close to thedata (22 ls) reported in Ref. 19. The slight drop in the emission lifetime for SNd-900  C, Ag-650  C implied that the structure
collapse of the crystalline zeolite Y might start. While for
SNd-900  C, Ag-900  C, the emission decay time rocketed to237 ls, which was close to that of the Nd:YAG single crystal(254 ls for the 0.1 at. % Nd:YAG).31 The strong NIR

emission in the NIR (II) range, the large Stokes shift, and thelong emission decay time together make the Nd3þ doped zeolite Y with silver as a dehydration catalyst a very promisingmaterial for the luminescence traceable drug delivery. For
practical bio-application, water solubility is a key issue to besolved, so surface modifications, like ligands exchange,32 for
instance, will be conducted in order to achieve a hydrophilic
surface in our future work.
IV. CONCLUSIONS

The 1064 nm NIR emission from the Nd3þ doped zeolite
Y has been enhanced by partially collapsing the porous zeolite structure into the amorphous phase with the co-loadingof silver and the post heat-treatment. The emission decay
time at 1064 nm under the 803 nm excitation reached 237 ls,
which was close to that of the Nd:YAG laser crystals. The
reason for the enhanced emission intensity and the prolonged
emission decay time is mainly due to the promoted elimination of the H2O molecules from the zeolite Y from framework. The strong NIR emission from the Nd3þ doped zeolitematerials may have applications for the luminescence traceable drug delivery.
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