APPLIED PHYSICS LETTERS 97, 131908 共2010兲

Spectroscopic characterization of bismuth embedded Y zeolites
Hong-Tao Sun,1,a兲 Yoshio Sakka,2 Yuji Miwa,3 Naoto Shirahata,2 Minoru Fujii,3 andHong Gao1
1International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS),
1-2-1 Sengen, Tsukuba-city, Ibaraki 305-0047, Japan
2
Nano Ceramics Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba-city,
Ibaraki 305-0047, Japan3
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,Rokkodai, Nada, Kobe 657-8501, Japan

共Received 11 August 2010; accepted 12 September 2010; published online 29 September 2010;
corrected 5 October 2010兲
Bismuth embedded Y zeolites were studied by using UV-vis- near infrared 共NIR兲 diffuse reflectance,
Raman, and steady-state NIR photoluminescence spectroscopy. The results suggest that Bi53+ andBi+ active centers coexist in the dehydrated and hydrated zeolite framework, both of which
contribute to NIR emission. Furthermore, it was revealed that the high-temperature annealing leads
to the formation of Bi2O3 clusters, which act as blocks for selectively closing down the “in-outwindows” of H2O and O2 molecules in the zeolites. It is believed that these materials can find a widearray of applications as active media of broadly tunable micro or nano-optical sources. © 2010
American Institute of Physics. 关doi:10.1063/1.3496460兴
Zeolite based materials represent a large family of aluminosilicates constituted by corner-linked TO4 tetrahedra
共T stands for either Si or Al兲 that adopt a remarkable varietyof structures containing channels and/or cavities of different
dimensions. The zeolite framework has a net negativecharge, arising from 关AlO4/2兴− units that replace neutral
关SiO4/2兴 units, which equals the number of aluminum atoms
in the framework.1–13 The electrostatic neutrality is ensured
by an equivalent number of extraframework cations. This
peculiar property, combined with the competitive costs of thezeolite raw materials and adjustable morphological properties of the matrix, makes them great potentials in areas suchas catalysis, ion exchange, molecular sieves, or drying
agents.1 Recent results also indicated the potential use of
optically functionalized zeolites as luminescent and lasermaterials.4–13 For instance, Sun et al.13 demonstrated that
Y-type zeolites exchanged with bismuth ions show strong
near infrared 共NIR兲 emission after thermally treatment in an
inert atmosphere, due to the formation of bismuth related
active centers. However, there still exist lots of unsolved
scientific difficulties in this area. First, there is not a clear
picture on the origin of bismuth active centers, although experimental results suggested that subvalent Bi contributes tothe NIR emission. Time-resolved and excitation-wavelengthdependent photoluminescence 共PL兲 results revealed that multitype, rather than single-type bismuth active centers coexist
in the Y structure, acting as charge compensators of theframework.13,14 One possibility is that Bi+ is the bismuth
related active centers, which locates in different coordination
environments in the structure. The other possibility is that
more than one type of subvalent Bi centers 共i.e., Bi+ andcationic Bi clusters兲 cocontribute to the emission. Up tillnow, it is unclear which scenario is correct. Second, the colorof the as-annealed products is green, which becomes shallow
pale white after full exposure to air. No results available canreveal what is the reason for this evolution. Third, to explain
highly efficient PL from Bi doped zeolites, Sun et al.13 proa兲

Electronic mail: timothyhsun@gmail.com.

0003-6951/2010/97共13兲/131908/3/$30.00

posed that there exist Bi related agglomerates in zeolites
annealed at high temperatures, which act as blocks forselectively closing down the “in-out windows” of water molecules. However, there is no evidence to support the existence of agglomerates in the samples.
In this letter, to gain a more detailed insight into NIR PL
mechanism and Bi distribution in the as-annealed and hydrated Bi embedded zeolites, we characterized the samples
by using extensive techniques including UV-vis-NIR diffuse
reflectance, Raman, and steady-state NIR PL spectroscopy.
The results suggest that Bi53+ and Bi+ coexist in the zeolite
framework, both of which contribute to NIR emission. Furthermore, the high-temperature annealing leads to the formation of Bi2O3 clusters, which act as blocks for selectively
closing down the “in-out windows” of H2O and O2 molecules in the zeolite framework.
The NH4 form of Y zeolites were purchased from Tosoh
Co. Japan 共Zeolite Y, SiO2 / Al2O3 = 7兲. The Bi3+-exchangedzeolites were prepared by a method reported in Ref. 11. The
exchanged zeolites were calcined at 750, 830, and 900 ° Cfor 30 min in an Ar atmospheric condition. After thermaltreatment, the samples were sealed into glass bottles for further measurements. Note that the samples for treatment at750 ° C were first loaded into glass bottles, then transferred
to high temperature region of a tube furnace, and finallyquickly sealed by caps when the annealed samples cooled
down to room temperature; while for the samples at 830 and900 ° C, the powders were loaded into Al2O3 ceramic boats,and quickly transferred and sealed into glass bottles after
annealing. Diffuse reflectance spectra of the products weremeasured by a UV-vis-NIR spectroscope 共V-570, JASCO,Japan兲 equipped with an integrating sphere. To measure theUV-vis-NIR spectra of the as-annealed samples, the powders
were quickly transferred into a cell with silica glass window,
and then sealed it tightly for measurements. Note that thecolors of the samples do not show a notable change aftermeasurements. These samples were denoted as “asannealed” 共also described as “dehydrated”兲 ones in Figs. 1and 3 while those exposed to air for 30 min and 24 h were as97, 131908-1© 2010 American Institute of Physics

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Appl. Phys. Lett. 97, 131908 共2010兲

FIG. 1. UV-vis-NIR diffuse reflectance spectra of the samples annealed at共a兲 750 ° C, 共b兲 830 ° C, and 共c兲 900 ° C. The as-annealed samples are dehydrated ones. 30 min and 24 h are the exposure times of the dehydrated
samples in air. Note that for samples exposed to air over 24 h, the color does
not show notable change. The steps at 850 nm are due to the measurement
artifacts.

“30 min” and “24 h,” respectively. The atomic ratio of Bi/Alin the final products was determined to be ⬃23% by energydispersive x-ray spectroscopy. Raman spectra were performed by using Jobin Yvon T-64 000 system under the excitation of 514.5 nm line from an Ar+ laser.
X-ray powder diffraction results revealed that the annealed zeolite structure keeps intact, and no new diffraction
peaks except those of zeolite were observed, indicating no
new crystalline phases develop in these samples. Interestingly, the as-annealed products show temperature dependence of color development; at 750 ° C, the powders aregreen while at higher temperatures, the samples become
shallow green. Figure 1 shows the effect of atmosphere on
the absorption property of the samples annealed at different
temperature. The as-annealed samples display two main absorption bands centered at 415 and 600 nm and a weak absorption tail over 850 nm; for those annealed at 830 and900 ° C, a new shoulder at ⬃700 nm appears. Exposure theas-annealed samples to air results in a fast color change fromgreen to shallow pale white. The hydrated powers demonstrate absorption bands at 535, 715, and 970 nm in addition
to a weak one ⬍500 nm. The analysis of atmosphere dependence of bismuth active centers leads us to conclude thatbismuth active centers occupy at least two kinds of sites in
the framework as follows: one kind of site is water and oxygen reachable, where the bismuth active centers can be destroyed by them and the other is separated from them.
Obviously, the absorption bands shown in Fig. 1 cannot
be assigned to the electronic transitions of single-type Bi
active center, such as Bi3+ or Bi2+.15,16 As is known, bismuth
has profound propensity to form clusters or to exist as Bi+ in
ionic liquids or crystals, which demonstrate strong visible
and NIR absorption bands.17,18 For instance, Bi53+ and Bi+
can coexist in Bi– GaCl3-benzene system at varioustemperatures;18 the Bi53+ displays main absorption bands at428, 520 and 875 nm while Bi+ at 609 and 694 nm. It isnoteworthy that the absorption characteristics of the asannealed samples are similar with those of liquids containing
Bi53+ and Bi+.18 This leads us to postulate that Bi53+ and Bi+
coexist in our as-annealed zeolites; that is, Bi53+ is the 415
nm related bismuth active centers, and Bi+ results in strong
absorption centered at 600 nm. Once the sample is hydrated,
Bi53+ or Bi+ ions at water/oxygen-reachable sites are destroyed.

FIG. 2. Raman spectra of the samples annealed at 750°, 830°, and 900 ° C.
The products were denoted as x-y, where x and y represent the annealing
temperature and exposure time in air, respectively. The mother zeolites 共1兲
and Bi doped zeolites 共2兲 annealed at 750 ° C in air were used as references.
Unfortunately, Raman spectra of the dehydrated samples suffer from strong
fluorescence and Raman bands cannot be identified.

The structure of Y zeolite is generated by connecting
sodalite units with hexagonal prisms to give a framework
characterized by big, empty cavities 共␣-cages兲 with a diameter of about 13 Å and a large pore opening of 7.4 Å.1 It is
recognized that the cations are mainly located in a few welldefined sites 共i.e., SI, SI⬘, SII, SII⬘, and SIII sites兲.19 Site SI
is located in the center of the hexagonal prism, closely surrounded by six oxygen atoms of the two bases of the prism.
Cations located in this site are almost totally inaccessible toguest molecules. Cations in site SI⬘ are located inside thesodalite cage. These cations are accessible only to molecules
able to penetrate through a six-membered ring from the supercages into the sodalite cavities. This penetration is notpossible for small molecules such as CO or N2 but occurs forH2.20,21 In contrast, SII and SIII sites are accessible to CO,
N2, O2, and H2O molecules. Given the peculiar structuralcharacteristics of Y zeolites and air-sensitivity of bismuth
active centers in the framework, it is suggested that some
Bi53+ and Bi+ ions locate inside of ␣-cages or on the surfaceof the zeolite, which can be destroyed by O2 and H2O. Inview of the kept absorption band ⬍500 nm for the hydrated
products, partial Bi53+ clusters should be inside the sodalite
cage considering the space available in the hexagonal prismand the sodalite cage, which has no chance to interact with
H2O molecules owing to the existence of bismuth agglomerates 共as revealed below兲. In the hydrated samples, most possibly, Bi+ mainly occupies the SI or SI⬘ site; the absorption
bands ranging from 400 to 550 nm can be attributable toBi53+ while those at 700 and 970 nm to Bi+.18,22–24
Since zeolites give rise to very weak Raman scattering,
Raman spectra always provide an excellent method for
detecting small amounts of certain inorganic ions or molecules in their structures.25,26 As shown in Fig. 2, Y zeolites
display characteristic scattering bands in the range of
200– 600 cm−1, which is consistent with the results reported

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Sun et al.

ters to NIR emission. This is under way in our group.
In summary, we studied Bi embedded Y zeolites throughUV-vis-NIR diffuse reflectance, Raman, and steady-stateNIR PL spectroscopy measurements. The results suggest thatBi53+ and Bi+ active centers coexist in the dehydrated andhydrated zeolite framework. Furthermore, the hightemperature annealing leads to the formation of Bi2O3 clusters, which act as blocks for closing down the “in-out windows” of O2 and H2O molecules. These results present amuch clearer picture on the active centers in Bi embedded Yzeolites, which may be helpful to further prepare NIR emitting zeolites containing high concentration of bismuth activecenters. It is believed that these materials can find a widearray of applications as active media of broadly tunable micro or nano-optical sources.

FIG. 3. 共Color online兲 NIR PL spectra of the as-annealed 共AA兲 and hydrated
共exposure to air for 24 h兲 samples under 403 nm excitation. The sample
annealed in air does not show NIR PL. Note that the long-wavelength
共⬎1600 nm兲 signal is suppressed owing to the drop of InGaAs sensitivity.
27,28

by Dutta et al. and Smirnov et al.
These bands can be
assigned to the T–O–T bending modes 共T is Si or Al兲. Interestingly, new bands develop at 138 and 160 cm−1 for Bi
doped zeolites thermally treated in air and Ar atmospheres. It
was reported that most cationic bismuth clusters show theircharacteristic scattering peaks.17,18,22–24 For instance, Bi53+
shows the strongest Raman scattering at ⬃134 cm−1. It isnoteworthy that bismuth compounds can also display bands
in the low-frequency range of 100– 200 cm−1. It was revealed by Denisov et al.29 that Bi2O3 shows strong Raman
scattering at ⬃138 and 150 cm−1. The comparison of Raman
spectra shown in Fig. 2, combined with the analysis of PL
property of Bi doped zeolites, leads us to further clarify theorigin of both bands at 138 and 160 cm−1. The samples annealed in Ar atmosphere can show broad NIR emission 共Fig.
3兲 while that annealed in air does not show any PL under theidentical excitation condition. This phenomenon is in accordance with the results in Ref. 11, suggesting that bismuth
active centers are sensitive to oxidation atmosphere and canbe destroyed by oxygen. However, both NIR active and inactive zeolites doped with Bi show Raman bands at nearly
identical frequencies. This indicates that the 138 and160 cm−1 bands are mainly attributable to Bi2O3 instead ofbismuth active centers such as Bi53+. The lack of notable
Raman scattering from Bi53+ in these samples might lie in
their low concentration, since Bi3+ dominates in the samples
annealed in Ar atmosphere.13 The above results clearly revealed that Bi2O3 clusters form in the zeolites, which can act
as blocks for separating active centers from O2 and H2O.
The dehydrated zeolites show a broad emission from930–1590 nm, consisting of a peak at ⬃1100 nm and aweak PL tail over 1250 nm; in contrast, all hydrated ones
display PL bands centered at ⬃1166 nm 共Fig. 3兲. The redshift in PL peak and the absence of long-wavelength emission tail for the hydrated samples, compared to those of dehydrated ones result from the different distribution of Bi53+
and Bi+ active centers 共i.e., the amount of Bi53+ and Bi+, therelative ratio of Bi53+ / Bi+, and the distance between Bi53+
and Bi+兲 in these samples. More detailed work is needed tofurther clarify the contribution of Bi53+ and Bi+ active cen-H. Sun gratefully acknowledges the funding support
from the International Center for Young Scientists andNational Institute for Materials Science, Japan 共Grant No.
215114兲.
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