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NMR, ESR and Luminescence Characterization of Bismuth Embedded Zeolites Y Hong-Tao Sun, Yoshio Sakka, Naoto Shirahata, Yoshitaka Matsushita, Kenzo Deguchi, and Tadashi Shimizu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp401861c • Publication Date (Web): 28 Feb 2013 Downloaded from http://pubs.acs.org on March 11, 2013

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The Journal of Physical Chemistry

NMR, ESR and Luminescence Characterization of Bismuth Embedded Zeolites Y Hong-Tao Sun,*,†,‡ Yoshio Sakka,§ Naoto Shirahata,||,┴ Yoshitaka Matsushita,# Kenzo Deguchi,□ and Tadashi Shimizu□ †

International Center for Young Scientists (ICYS), National Institute for Material Sciences (NIMS), 1-2-1 Sengen, Tsukuba-city, Ibaraki 305-0047, Japan

‡

Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan

§

Advanced Ceramics Group, Materials Processing Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba-city, Ibaraki 305-0047, Japan ||

World Premier International Research Center Initiative for Materials Nanoarchitronics (MANA), NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

┴

PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan #

Research Network and Facility Services Division, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

ACS Paragon Plus Environment

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National Institute for Material Science, 3-13 Sakura, Tsukuba 305-003, Japan

KEYWORDS: zeolites, bismuth, photoluminescence, NMR, ESR

ABSTRACT: Thermal treatment of bismuth-embedded zeolite Y yields luminescent Bi+ substructures without the formation of metallic nanoparticles. The structural and photophysical features of the resulting zeolite Y have been thoroughly characterized by using extensive experimental techniques including NMR, ESR, 2-dimentional excitation-emission and absorption spectra. NMR and ESR results indicated that some Al and oxygen are expelled from the zeolite Y framework after undergoing thermal treatment. The detailed analyses of luminescence and absorption spectra, coupled with TDDFT calculations, suggest that all Bi+ substructures (i.e., Bi44+, Bi33+, Bi22+, and Bi+) are optically active in the near-infrared (NIR) spectral range. It is found that Bi+, Bi22+, Bi33+ and Bi44+ units result in NIR emissions peaking at ca. 1050, 1135, 1145, 1240/1285 nm, respectively. The emission lineshapes under diverse excitation wavelengths greatly depend on the Bi concentration and annealing temperature, as a result of the change in the relative concentration, the spatial distribution, as well as local structural features of Bi active species. Specifically, the above analyses imply that the reducing agents for Bi3+ are water molecules as well as framework oxygen. These findings represent an important contribution to the understanding of the processes involved in the formation of Bi+ and of the luminescence mechanisms of Bi+ substructures in zeolite Y frameworks, which are not only helpful for the in-depth understanding of experimentally observed photophysical properties in other Bi-doped materials, but also important for the development of novel photonic material systems activated by other p-block elements.


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1. INTRODUCTION Zeolites are crystalline microporous aluminosilicates based on AlO4 and SiO4 tetrahedra that are connected by shared oxygen atom bridges. Combinations of these tetrahedral structures lead to over 176 unique zeolite framework types with diverse channels,1 making them the most important and versatile heterogeneous catalysts.2 In particular, zeolite-supported metal clusters (e.g., Ir and Pt clusters of several atoms), typically prepared by ion exchange followed by heating in air and reduction in hydrogen, have attracted increasing attention over the past years owing to their excellent catalytic selectivity;3 the catalytic properties of such zeolites depend strongly on the sizes and shapes of the clusters, and also on the location of the clusters within the pores. In addition to the studies mentioned above, using zeolites as templates for the formation of peculiar optically active centers such as nanocrystals, metal clusters, and dyes has been widely adopted in order to develop new hybrid material systems and to find novel functional applications.4-17 For instance, in an early research effort, Seff and coworkers found that silver polyhedra such as Ag6 can be obtained in zeolite frameworks by annealing the product under high vacuum conditions.4 Recently, Cremer et al. reported that this kind of Ag-related centers display characteristic luminescence colors, depending on the nature of the cocation, the amount of exchanged silver, and the host topology; the analysis of the fluorescent properties in combination with ESR spectroscopy suggests that a Ag6+ cluster with doublet electronic ground state is associated with the appearance of an emission at 690 nm, having a decay of a few hundred microseconds, while the nanosecond-decaying 550 nm emitter is tentatively assigned to the Ag3+ cluster.5 The principle advantage of this spatial confinement strategy exploiting nanoporous structures as templates lies in the facile control of metal species through simple adjustment of thermal treatment processes, resulting in a cheaper and smarter route to developing new optical materials.


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As is well known, zeolites usually contain more than 20 wt% water molecules owing to their porous structures. Thus, the efficiency of optical emitters becomes problematic in the nearinfrared (NIR) spectral region due to the fast relaxation of the excitation energy through nonradiative vibrational deactivation.14 Recently, we found that the combination of zeolites with bismuth compounds leads to strong, air-stable, long-lived, ultrabroadband and tunable NIR photoluminescence.14-17 Detailed experimental results suggested that bismuth related active centers contribute to the observed emissions, and these centers are sealed in a low-vibrational environment by non-active bismuth agglomerates even when the sample still contains a large amount of water.14-17 Although this approach leads to the sample with the high-efficiency NIR emission, the exact active center as well as the more detailed mechanism of isolating active centers from water molecules in this system is not clear, owing to the complexity resulting from the co-existence of multi-type bismuth species.14 Bismuth, with the electronic configuration of (Xe)4f145d106s26p3, is the heaviest stable element in the periodic table, and shows a variety of oxidation states such as 0, +1, +2, +3, and +5. Additionally, bismuth has a profound propensity to form cationic and anionic molecular clusters, characterized by multi-type electronic structures.1832

To examine the exact active centers located in the zeolite framework, in a subsequent study

Sun et al. developed a new way to obtain bismuth with an oxidation state of Bi+ and provided direct experimental evidence of the formation of Bi+ based on the analysis of high-resolution synchrotron powder X-ray diffraction (XRD) data.33 The experimental results as well as theoretical calculations suggest that the NIR emission is attributable to the substructures of Bi+. However, it is still hard to exactly establish the relationship between photoluminescence (PL) properties and such Bi+ species. A better understanding of the PL mechanism in this system is


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required to further rationally build high-efficiency emitting materials using such subvalent Bi centers or other p-block elements. Among Bi species, it is widely recognized that subvalent Bi (i.e., the average Bi valence is +1, or between 0 and +1) is the most unstable form, which was usually stabilized by molten salts, Lewis acids or negatively charged polyhedra.19-31 Although experimental evidence clearly indicated that Bi+ exists in the zeolite Y framework, to date, however, the formation mechanism of Bi+ remains unknown as a consequence of the complex physiochemical properties of bismuth. A deeper understanding of the redox process of bismuth is of vital importance to the establishment of growth mechanism of Bi+ substructures, which may greatly enrich the designability of luminescent porous structures and be helpful for the development of novel systems using a broad range of p-block elements. In zeolites, it is known that substitution of Si atoms by Al creates negative charges on the framework, compensated either by cations inside the channels and cages or by protons bonded to framework oxygens. To a certain extent, in all zeolitic frameworks a fraction of four-coordinate framework aluminum species shows particularly low stability, which leads to a tendency for them to migrate into extraframework positions when undergoing a high-temperature or moisture process.34-37 In general, the structures of zeolites containing extra ions or clusters could be evaluated through the Rietveld refinement of high-resolution XRD data.34-42 However, this standard approach is not suitable for the identification of extraframework Al species. Generally, 27

Al MAS (magic angle spinning) solid-state NMR is used to determine the Al coordination,

which can be done with high resolution and quantitatively.34-37 Until now, zeolites containing Bi usually were treated at temperatures over 400 oC to obtain NIR-active Bi.14-17,33 The distribution of Al species in these samples is rather poorly understood. Meanwhile, previous work revealed


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that some framework oxygen tends to be expelled from the structure if zeolites undergo hightemperature treatment.43-45 It is noted that this scientific issue is currently underexploited in Bi containing zeolites prepared at temperatures over 400 oC.14-17,33 An in-depth investigation of the coordination environment of Al as well as confirming whether oxygen vacancies exist will not only serve to the establishment of a much clearer structure-property relationship in these systems, but also is helpful to deepen the understanding of the formation mechanism of active Bi species. In this article, we aim to address three key issues: (1) identification of the distribution of Al and oxygen vacancies in Bi embedded zeolites; (2) the dominant formation processes of NIRactive Bi; and (3) NIR emission mechanisms of zeolites containing Bi. To these ends, we thoroughly investigate the structural characteristics of thermally-treated zeolites Y through NMR and ESR measurements, which helps us to gain insight into the formation processes of Bi active centers, oxygen vacancies as well as extraframework Al. In particular, for the first time, 2dimentional (2-D) excitation-emission (ex/em) graphs of subvalent Bi in zeolites are taken, which, coupled with the XRD analysis, turn out to be an effective way to clearly elucidate their PL mechanisms. In order to develop an in-depth understanding of the complex PL evolution in a computationally feasible manner, we carry out time-dependent density-functional theory (TDDFT) calculations for some sub-nanometer Bi clusters, which offer additional insight into PL mechanisms in these systems.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Bi Embedded Zeolite Y. Zeolites Y with a Si/Al ratio of 2.59 were purchased from Tosoh Co. Japan. Ultrapure water (18.2 MΩ/cm) was used throughout the work. Zeolites were stirred in 500 mL Bi3+ aqueous solutions prepared from Bi(NO3)3·5H2O (Wako


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Pure Chemical Industries, Ltd., 99.9%) at room temperature for 24 h to exchange the Na ions with Bi3+ ions (see Table S1 in Supporting Information). The products were removed by centrifugation, then thoroughly washed with deionized water, and dried in air at 120 oC. The obtained powders were first put into a 5 mL flask with an adapter, and then inserted into the chamber of a vacuum furnace. The sample was thermally treated from room temperature to 340 (or 400) oC at a rate of 10 oC/min, and kept at 340 (or 400) oC for 24 h under vacuum at ca. 6 x 10-5 Pa. The dehydrated powders were allowed to cool to room temperature and were stored in a glovebox (< 2 ppm H2O; < 0.1 ppm O2) for the subsequent measurements. Meanwhile, to compare the emission features of such dehydrated samples with hydrated counterparts, we also annealed zeolite Y with low Bi concentration at 750 and 875 oC for 30 min in an Ar atmosphere, and then these samples were fully exposed in air for over 24 h, which were used for the following NMR, ESR, and luminescence measurements. The heat-treated Bi-embedded zeolite samples are denoted Y-Bix-Ty, where x and y represent the amount of Bi atoms in one unit cell and the annealing temperature, respectively. The atomic ratios of Bi:Na:Al:Si were determined by the analysis of inductively coupled plasma-optical emission spectrometer (ICP-OES: IRIS Advantage, Nippon Jarrell-Ash, Yokohama, Japan). The measured atomic ratios of Bi:Na:Al:Si for the Y-Bi8.5 and Y-Bi13.3 samples are 8.5:26.6:53.5:138.5 and 13.3:15.2:53.4:138.6, respectively. 2.2. Powder XRD. All dehydrated samples for XRD measurements were sealed into Lindemann glass capillaries with an inner diameter of 0.3 mm. To overcome the influence of water and oxygen on bismuth active centers in the zeolite framework, the loading of annealed zeolites into capillaries was carried out in a high-purity N2 filled glovebox (< 2 ppm H2O; < 0.1 ppm O2). We employed high-resolution synchrotron XRD at the BL15XU NIMS beam line of


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SPring-8 to obtain high-quality diffraction patterns of such dehydrated samples. The capillary was rotated during the measurement to reduce the preferred orientation effect and to average the intensity. X-ray wavelength used is 0.65297 Å. For the samples annealed at 750 and 875 oC, XRD spectra were taken by X-ray diffractometer (Rigaku RINT 2000/PC, λ=1.54059 Å). All XRD spectra were taken at room temperature. 2.3. NMR Spectra. 27Al MAS NMR experiments were carried out with a JEOL ECA500 NMR spectrometer using a 4 mm double-resonance probe-head. The resonance frequency for 27Al was 130.3 MHz. All of the spectra were obtained at MAS spinning speed of 15 kHz, using 1us pulse width and a recycle delay of 2 s. The

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Al chemical shifts were referenced to the

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Al peak

position of 1M AlCl3 solution. All samples were rehydrated prior to recording. 2.4. ESR Spectra. For measuring ESR spectra, the dehydrated zeolites were loaded into quartz tubes in a glovebox (< 2 ppm H2O; < 0.1 ppm O2) and then the tubes were sealed by glue to separate them from the air. For the samples annealed at 750 and 875 oC, the powders were loaded into quartz tubes after being kept in air over 24 h. The spectra were taken by a JES-FA100 ESR spectrometer (JEOL, Tokyo, Japan) operating in the X-band and equipped with a cylindrical TE011 cavity. The following acquisition parameters were used: frequency 9.44 GHz, time constant 0.03 sec, sweep width 50 mT, and power 1.0 mW. All ESR spectra were taken at room temperature. 2.5. 2-D ex/em Graphs. In order to take 2-D ex/em spectra, the dehydrated powders were sandwiched between two fused quartz glasses and the two glasses were then sealed with glue. Note that fused glass used does not show any PL. To record PL spectra of the samples annealed at 750 and 875 oC, the powders were fully exposed in air and then loaded into a cell for the subsequent measurement. For all samples, 2-D ex/em graphs were taken at room temperature by


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a Horiba NanoLog spectrofluorometer equipped with a monochromated Xe lamp and a liquid N2 cooled photomultiplier tube (PMT) (Hamamatsu, R5509-72). Emission spectra ranging from 1000 to 1600 nm were taken at different excitation wavelengths from 400 to 900 nm with 10 nm intervals. It is noted that all spectra were corrected for the spectral response of the detection system. 2.6. Diffuse Reflectance Spectra. The dehydrated powders were sandwiched between two fused quartz glasses and the two glasses were then sealed with glue. Note that the glasses used do not show any absorption band. Diffuse reflectance spectra of the products were measured by a UV-vis-NIR spectroscope (V-570, JASCO, Japan) equipped with an integrating sphere. The absorbance was transformed by the Kubelka-Munk method.

3. RESULTS AND DISCUSSION XRD, NMR and ESR analyses: To examine the crystallinity and purity of the obtained samples, we first took representative XRD spectra (see Figure S1 and S2 in Supporting Information). It is obvious that the structures of zeolites Y keep well after undergoing high temperature processes. All diffraction peaks observed can be well assigned to the phase of zeolite Y. The absence of any new peaks in the XRD patterns indicates that no other crystalline phases develop in the final products. To elucidate the impact of thermal treatment on the Al speciation in Bi-embedded zeolites, next we performed

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Al MAS NMR studies of as-received and annealed zeolite powders. As is

seen, zeolites Y series exhibit well-known peaks of the tetrahedral framework Al (AlIV) at about 60 ppm (Figure 1). Notably, new bands located at around 0 ppm, commonly assigned to nonframework octahedral Al (AlVI), appear,34,43,46-49 when zeolites Y were thermally treated in


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vacuum or Ar. Interestingly, it is found that the relative amounts of octahedral Al in the samples obtained at the same temperature increase as the increase of Bi amount in a unit cell from 8.5 to 13.3. Additionally, the NMR spectra of Y-Bi8.5-T340 and Y-Bi8.5-T400 samples possess rather weak shoulders at ca. 30 ppm, and do not demonstrate remarkable difference at around 0 and 60 ppm, although the Y-Bi8.5-T400 sample has relatively broader line widths. In contrast, the linewidths corresponding to the bands peaking at 0 and 60 ppm for the Y-Bi13.3-T400 sample are much larger than those for the Y-Bi13.3-T340, and stronger shoulder at ca. 30 ppm develops. The signal with an isotropic shift at 30 ppm is most likely attributable to pentacoordinated Al (AlV) or distorted tetrahedral Al.46-47 Besides, it is observed that annealing Y-Bi8.5 sample at 750 o

C for as short as 0.5 h results in the formation of AlVI and AlV. The combined results described

above suggest that increasing Bi concentration as well as annealing temperature would influence the distribution of Al species in the zeolite Y framework, i.e., some Al exists as the extraframework Al. ESR is a proven powerful technique to study paramagnetic sites in diverse material systems including zeolites. To know more details of the structural feature of the obtained zeolites, we took a series of ESR spectra. As demonstrated in Figure 2, it is obvious that zeolite Y with Bi is ESR silent before thermal treatment, indicating that no oxygen vacancies exist. After calcination, clear signals at around g=2.002 and g=2.004 are observed for the samples annealed at 340 and 400 oC, respectively, evidencing that some paramagnetic sites occur. Additionally, for the YBi13.3-T400 sample, one very weak signal occurs at g=2.0195. There are two possibilities for the assignment of such signals. One plausible explanation is that the ESR signals result from some Bi related species with unpaired electron (e.g., Bi2+ or Bi0) owing to the reduction of Bi3+. The second possibility is that these signals are from electron centers created in the zeolite matrix.


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Generally, zeolites tend to lose partial framework oxygen when they were annealed at high temperatures in either air or inert atmosphere, followed by the capture of electrons at the sites of oxygen vacancies.43,44 To verify which scenario is correct, we further recorded the ESR spectra of the zeolites stabilized by air (Figure S3 in Supporting Information). In these samples, Birelated active centers can be totally destroyed, as confirmed by the PL measurement. Interestingly, these samples display nearly identical g values in comparison with those before exposure in air. This similarity leads us to infer that the signals as shown in Figure 2 originate from the intrinsic electron centers in the zeolitic framework, rather than from Bi-related species. We will discuss the detailed formation mechanism of such paramagnetic sites below.

Figure 1. Solid-state 27Al MAS NMR spectra of zeolites Y containing Bi. The NMR spectrum of zeolite Y as received was also taken, denoted by zeolite Y.


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Figure 2. Representative ESR spectra of thermally-treated zeolites Y containing Bi. Y-8.5 represents the sample without undergoing thermal treatment. It is necessary to note that here the intensity of the ESR signal cannot correlate with the defect concentration, since the loading levels of the samples are different when taking the measurements.

2-D ex/em graphs of zeolites Y: To examine the photophysical behavior, we took 2-D ex/em graphs of zeolites Y containing Bi (Figure 3). Representative emission spectra, together with the decomposed results, are shown in Figure 4 and Figure S4-S8. It is noteworthy that zeolites Y without Bi doping do not show any NIR PL signal when excited in the range of 400-900 nm, indicating that the emissions are attributable to Bi-related active centers. Owing to the complex ex/em features observed here, at the beginning we would like to focus on the discussion of PL behaviors of Y-Bi8.5-T340 and Y-Bi8.5-T400 samples (Figure 3a, 3b, 4a and 4b). As displayed in Figure 3a, Y-Bi8.5-T340 possesses four notable peaks appearing at the ex/em wavelengths of ca. 410/1150, 530/1150, 710/1150, and 850/1150 nm, among which the intensity of the third peak is the strongest. Further examination of the emission spectra under various excitation


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wavelengths reveal a slight difference of their lineshapes (Figure 4a); obviously, 710 nm excitation leads to the narrowest emission band relative to those at 410 and 53o nm excitation. Interestingly, when the treatment temperature is increased to 400 oC, corresponding to the YBi8.5-T400 sample, it is observed that the ex/em characteristics are remarkably different (Figure 3b); two obvious peaks occurring at the ex/em wavelengths of ca. 500/1150 and 710/1140 nm, among which the former peak is stronger. Furthermore, the Y-Bi8.5-T400 sample displays similar emission spectra under both 410 and 500 nm excitation (Figure 4b), while a featureless/noisy one appears at 850 nm excitation (Figure 3b). A further detailed examination of the results shown in Figure 3a and 3b helps us to obtain more interesting information of these centers. Firstly, both Y-Bi8.5-T340 and Y-Bi8.5-T400 samples have an excitation band located at 710 nm, which can be assigned to the characteristic band of one Bi active center. Clearly, this emitter exists in both samples. A basic practical question then arises: is it the only NIR emitter in these systems? To answer this, we compare the emission spectra of Y-Bi8.5-T340 and Y-Bi8.5-T400 samples at 710 nm excitation. It is found that the emission peaks of Y-Bi8.5-T340 and Y-Bi8.5-T400 samples are at 1150 and 1140 nm, respectively (Figure 4a, 4b and Figure S4 in Supporting Information). This leads us to infer that other Bi active centers should also contribute to the broad NIR PL. The occurrence of simultaneous emissions from more than one emitter under one excitation wavelength could become possible in the case that the effective energy migration of the excitation energy between emitters could take place owing to their overlapping energy levels. Clearly, the above result indicates that the emitter with an emission peak at ca. 1140 nm possesses an excitation band at 710 nm, although other active centers may also have absorption in this range. Secondly, a blue shift of the excitation band from 530 to 500 nm occurs when the annealing temperature of the Y-


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Bi8.5 sample increases to 400 oC from 340 oC, whereas the peak wavelength of the emission at these excitation wavelengths is at 1150 nm. This result suggests at least two emitters possessing absorption bands with peaks at ca. 530 and 500 nm, one or all of which show the NIR emission at ca. 1150 nm; the overlapping of these excitation bands makes it possible to effectively transfer the excitation energy between them, thus leading to slight shifts of emission peaks. In order to confirm above assignments, we further recorded the 2-D ex/em graph of the YBi13.3-T340 sample. As plotted in Figure 3c, this sample demonstrates three notable peaks occurring at the ex/em wavelengths of ca. 410/1140, 530/1140, and 710/1140 nm, among which the intensity of the second peak is the strongest. Compared with the photophysical behaviors of Y-Bi8.5-T340 and Y-Bi8.5-T400 samples, it is reasonable to infer that the emitters in Y-Bi8.5T340 and Y-Bi8.5-T400 also exist in Y-Bi13.3-T340 sample, although the relative concentration of these emitters may be different. Furthermore, it is interesting to note that the excitation bands are virtually identical for the samples annealed at 340 oC, while the emission intensities at these excitation wavelengths show a large difference (Figure 3a and 3c): the Y-Bi8.5-T340 sample demonstrates the strongest emission at 710 nm excitation, while Y-Bi13.3-T340 at 530 nm. This clearly suggests that the 530 and 710 nm excitation bands as found cannot be assigned to one emitter. All these evidences indicate that Bi emitters in zeolites Y can be easily tuned by simply changing the annealing temperature or Bi concentration. The above experimental facts clearly reveal that at least two emitters exist in the zeolite Y framework, but what are they and why do they have similar photophysical properties? As is well known, XRD analyses can provide a direct picture on the distribution of elements in a crystalline system. In an earlier effort, we successfully employed this technique to determine Bi species in a typical sample (note that the sample in ref. 33 corresponds to the Y-Bi8.5-T400 sample studied


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here), and obtained some key Bi-related features.33 The first is that Bi occupies a single type site in the sodalite cages, which is coordinated to three oxygen atoms of the base of the prism. Secondly, the radius of Bi was determined to be 1.465 Å, indicating that the valence of Bi is +1. It is also noteworthy that the experimental radius of Bi+ determined is nearly identical with the theoretically evaluated one (1.5 Å).50 Thirdly, depending on the occupancy probability of Bi+ in the zeolite framework, diverse substructures of Binn+ (n=1-4) could form in the sodalite cages, where n stands for the number of Bi+ per sodalite cage. Furthermore, the theoretical absorption spectra for Bi44+, Bi33+ and Bi22+ obtained from TDDFT calculations suggest that these substructures display some allowed and forbidden electronic transitions in the visible and NIR spectral regions (see Table S2). All these results evidence that the emitters in the zeolite Y framework are Bi+ substructures.

Figure 3. 2-D ex/em graphs of dehydrated zeolites Y thermally treated in vacuum.


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Figure 4. Representative emission spectra (upper) and decomposed curves (down) under the excitation of indicated wavelengths (unit: nm) for (a) Y-Bi8.5-T340, (b) Y-Bi8.5-T400, and (c) YBi13.3-T340 samples. The solid and dotted green lines are the fitted and decomposed curves, respectively. The normalized emission spectra of Y-Bi13.3-T400 are shown in (d); note that these spectra cannot be decomposed successfully by the Gaussian or Lorentzian functions because of their unsymmetrical lineshapes.

Generally, the emission intensity at a specific wavelength for a system containing more than one emitter can be determined by the following equation: n

n

i =1

i =1

I total = ∑ I i ,dir + ∑ I i ,tra

(1)


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where Itotal is the total emission intensity at a specific wavelength, Ii,dir the emission intensity in the case that the emitter has absorption at the excitation wavelength, and Ii,tra the intensity when the emission is dominated by an energy transfer process as a result that the emitter has no absorption at the excitation wavelength. As is known, in the low-excitation regime the luminescence intensity Ii,dir is proportional to σΦNτ/τrad, where σ is the excitation cross section, Φ the photon flux, N the content of optically active centres, τ the PL lifetime, and τrad the intrinsic radiative lifetime.51 In contrast, Ii,tra depends on more parameters; the energy transfer process in a system requires a good overlap between donor emission and acceptor absorption bands, and also depends strongly on the spatial distance between donor and acceptor. Combined this consideration with the experimental results shown above, it is obvious that the observed broad ex/em lineshapes result from the superimposition of the ex/em bands of multiple emitters in the zeolite framework. With all these structural and photophysical properties in mind, next we tried to assign the observed emission bands to specific Bi emitters, assuming that all emitters possess Gaussian emission lineshapes. In order to realize this, we make a detailed analysis of representative emission spectra for Y-Bi8.5-T340, Y-Bi8.5-T400 and Y-Bi13.3-T400 samples by Gaussian decomposition using Origin 8.0 software (Figure 4, and Figure S6-S8 in Supporting Information). Since the signals under the excitation of > 750 nm are rather noisy, we mainly focus on the emission spectra at shorter excitation wavelengths. The decomposition results are summarized in Table S3. It is noted that this approach merely allow us to roughly estimate emission peaks assigned to Bi emitters, given that these emitters might have comparable emission bands in some specific NIR spectral ranges. It is found that all spectra studied can be well decomposed into two or three Gaussian bands (Figure 4a-4c); interestingly, one rather weak peak is located in the


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range of 1020-1050 nm for the Y-Bi8.5-T340 sample at some excitation wavelengths. Although the study of material systems containing Bi+ can dates back to half a century ago,18,19 it is not until recently that the PL properties of Bi+ was experimentally evaluated.29,51-54 Such previous results suggested that Bi+ can show luminescence at < 1100 nm, which is quite similar to the emission range discussed above. This, in combination with the fact that Bi+ indeed exists in the zeolite Y framework, suggests that this weak band results from the electronic transition of Bi+. Aside from this band, two stronger Gaussian bands peaking at ca. 1147 and 1240 nm can be well resolved for the Y-Bi8.5-T340 sample. In comparison, the Y-Bi8.5-T400 sample demonstrates two bands peaking at ca. 1140 and 1241 nm; interestingly, under 710 nm excitation three bands can be obtained, with peaks at 1135, 1142 and 1225 nm. It is revealed that, although partial Al and O are expelled from the zeolite Y framework, the porous crystalline structure of Y-Bi8.5T400 keeps well, as evidenced by thermogravimetric analysis.33 However, it is possible that the defects of aluminum and oxygen may influence the characteristics of electronic transitions of Bi+ substructures, because of their slight structural deviation from ideal symmetry. Here, to make meaningful assignments of these bands, we referred to our previous calculated results on the electronic transitions of Bi+ substructures (Table S2 in Supporting Information). Obviously, Bi33+ and Bi44+ possess three and two electronic transitions, respectively, over 1100 nm; a distinct difference between Bi33+ and Bi44+ can be observed, i.e., the difference of the transition energies in the NIR for the former is much larger than that for the latter. The combination of the theoretical results and experimental facts observed here helps us to make the following assignments: Bi33+ results in the PL bands peaking at 1142-1148 nm, and Bi44+ at 1225-1242 nm. The annealing-temperature-dependent PL as shown in Figure 4a and 4b indicates that more than one emitter contributes to the emissions with peaks in the range of 1130-1150 nm. In such a case,


18

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it is rather challenging to accurately obtain the emission characteristics of these emitters in this narrow range (e.g., full-width at half maximum (FWHM) of the emission) by Gaussian decomposition because of the superimposition of the emission as well as complicated energy migrations between them under different excitation wavelengths. For all spectra studied here (Figure S6-S8 in Supporting Information), we found that only the Y-Bi8.5-T400 sample simultaneously shows decomposed emissions peaking at 1135 and 1142 nm when excited by 710 nm; note that the FWHM of the band at 1142 nm is just 55 nm, much smaller than the typical values of 94-138 nm as obtained from other decomposed curves with similar emission ranges (Table S3 in Supporting Information). We believe that this may be due to the competitive radiative relaxation of the excitation energy from the excited levels of Bi33+ and another emitter to their ground levels; given that Bi22+ has an absorption peak over 1000 nm, it is reasonable to assign the 1135 nm band to the Stokes emission from Bi22+. In addition to emission bands as found above, the Y-Bi13.3-T340 sample shows one more band peaking in the range of 1285-1295 nm; the appearance of this band is accompanied by the reduced FWHM of the band at ca. 1243 nm. For Bi-related emitters, it is possible that the emitters have multiple emission bands owing to the radiative electronic transitions from the first several excited levels to the ground level. This has been proved to be feasible in specific Birelated emitters.32 For instance, Sun et al. found that a single crystal of (K-crypt)2Bi2 containing [Bi2]2- displays ultra-broad NIR PL because of the electronic transitions from the first three excited levels to the ground level.32 In view of the characteristics of electronic transitions of Bi44+, the occurrence of 1285-1295 nm band may result from the electronic transition of Bi44+ from the lowest excited level to the ground level, because of notable aluminum and oxygen vacancies


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which make forbidden transition become allowed or partially allowed. Similarly, the bands at 1141 or 1143 nm can be assigned to Bi33+. In contrast, the emission spectra of the Y-Bi13.3-T400 sample cannot be decomposed successfully by the Gaussian or Lorentzian functions because of their rather unsymmetrical lineshapes. The emission peaks under various excitation wavelengths are in the range of 11601190 nm. This becomes understandable if we correlate the photophysical properties with the structural features of the Y-Bi13.3-T400 sample. As evidenced by

27

Al MAS NMR and ESR

measurements (Figure 1 and 2), the Y-Bi13.3-T400 sample should contain more extraframework and/or framework distorted Al species and oxygen vacancies relative to other samples. This may be one main reason for the unsuccessful solution of its crystalline structure using the same procedure as shown in ref. 33. Additionally, the TG analysis of the Y-Bi13.3-T400 sample reveals some ‘in-out’ windows for water molecules are blocked (Figure S9 in Supporting Information), which is in good accordance with NMR and ESR results. Taking into account all information described above, we conclude that the porous structure of the Y-Bi13.3-T400 sample was partially destroyed because of the loss of framework Al and O. We therefore propose that the NIR emission from the Y-Bi13.3-T400 is attributable to some ‘distorted Bi+ substructures’. This can be understood as being a result of a combination of two factors. First, the distortion of zeolite Y framework greatly influences the structure of Bi+ species (e.g., bond length or angle), making it deviate from regular structures as found in the defect-less zeolite Y.33 Then, the distorted Bi+ substructures possess new electronic structures with respect to the regular counterparts, leading to red shifts of the emission bands. Second, the structural distortion may also affect the characteristics of electronic transitions in the NIR region, e.g., making forbidden-like transition


20

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become allowed or partially allowed, resulting in long-wavelength emission tails (Figure 4d). More experimental evidence shown below will help us to identify which is the dominant. The thorough examination of the observed PL results allow us to assign the NIR emission to specific Bi+ species, as summarized in Table S3. However, a basic question still remains: what is the intrinsic absorption/excitation bands corresponding to each emitter? The above analyses, especially those linked to 2-D ex/em graphs as well as TDDFT calculations, lead us to make the following assignments: (1) Bi33+ should have two notable bands at 500 and 710 nm; (2) Bi44+ has a strong absorption band peaking at 530 nm, and possibly a relatively weak band at 850 nm; (3) Bi22+ possesses the strongest absorption at ca. 410 nm; (4) Besides the strong absorption bands described above, all emitters have some absorption bands in the range of 600-1000 nm, which act as ‘ladders’ for the transfer of the excitation energy from one emitter to the others, rendering it possible to show luminescence for one emitter without absorption at some excitation wavelengths.

Absorption spectra of zeolite Y: The above assignment was further assessed by the analyses of the experimentally determined absorption spectra for a series of samples. Interestingly, all samples show two notable absorption bands peaking at 414 and 603 nm, and a relatively weak band or shoulder at ca. 860 nm, among which 414 nm peak is the strongest. Additionally, the YBi8.5-T340 sample displays one weak absorption shoulder at 530 nm, whereas other samples do not display obvious absorption at this wavelength, possibly owing to the strong absorption background from 500 to 730 nm. Further detailed comparison between the absorption spectra and PL results gives rise to many interesting clues linked to Bi emitters in this system. First, the shape of the absorption spectra greatly deviates from the corresponding PLE spectra for all


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samples (see Figure 3 and 5, and Figure S5 in in Supporting Information), further evidencing that more than one active center exists in the samples. Second, from PLE spectra we can only find one weak excitation band at ca. 410 nm and excitation tails at ca. 600 nm for some samples (Figure S5 in in Supporting Information), which is considerably different from the phenomenon observed in the absorption spectra. This suggests that the active centers mainly contributing to the strong absorption at 414 and 603 nm are not efficient NIR emitters, possibly owing to their intrinsic forbidden transitions. On the other hand, the occurrence of the PLE band at ca. 410 nm indicates that this emitter can act as an energy antenna for other Bi+-related active centers. Thirdly, it can be seen clearly that the relative intensity for the absorption peaks at 414 and 603 nm strongly depends on Bi concentrations and annealing temperatures, leading us to infer that these two bands cannot simply be assigned to one emitter. Most possibly, other emitters also contribute to the absorption band at 603 nm, thus resulting in broad absorption ranging from 530 to 730 nm. This analysis, combined with TDDFT results (Table S2), supports well the assignment of 414 nm band to Bi22+. Fourthly, Bi44+ and Bi22+ have allowed absorption over 750 nm (Table S2), which should mainly contribute to the observed absorption bands peaking at ca. 860 nm. Furthermore, the absorption features as described above leads us to re-consider the characteristics of distorted Bi+ substructures. As theoretically revealed, the change of bond length of Bi+ substructures results in simultaneous red or blue shift of all absorption bands for one emitter (Figure S10 in Supporting Information). It is found that the Y-Bi13.3-T400 sample demonstrates similar absorption bands with respect to Y-Bi8.5-T340 and Y-Bi8.5-T400 samples. In particular, all above samples have virtually identical absorption peaks at 414 and 603 nm. Considering these facts, distorted Bi+ substructures in the Y-Bi13.3-T400 sample can be understood as a result of the slight change of electronic structures in the NIR spectral range for


22

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Bi+ species, because of the influence of framework distortion. This phenomenon was also observed when the samples were thermally treated in an Ar atmosphere (Figure S11 in Supporting Information).

Figure 5. Absorption spectra of Y-Bi8.5-T340, Y-Bi8.5-T400, Y-Bi13.3-T340, and Y-Bi13.3T400 samples. Note that the steps at 850 nm are due to the measurement artifacts.

Simplified energy-level diagrams of Bi+-related active centers: The above photophysical and structural characterizations allow us to obtain interesting and useful information of Bi+ substructures. In particular, the NIR PL mechanism in studied systems becomes clearer. To better understand the photophysical behaviors of three emitters determined, next we depict the simplified energy-level diagrams of Bi44+, Bi33+, Bi22+, and Bi+, based on the experimental and theoretical results. As demonstrated in Figure 6, all emitters have visible or NIR absorption bands, and possess electronic transitions over 1000 nm. Interestingly, the ex/em behaviors of Bi+ substructures display an interesting evolution: the larger the cluster, the longer the emission wavelength. The similarity of the emission ranges between Bi33+ and Bi22+ is one of the dominant


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reasons for the excitation-wavelength dependent shift of the emission peaks at 1140-1150 nm. Remarkably, the abundance of the excited levels for these emitters presents multiple routes for migration of the excited energy between them, which directly results in the inconsistence between the PLE and absorption spectra, since the ultimate emission at a specific wavelength is dominated by a series of parameters as shown in equation 1. In particular, the increase of Bi concentration and annealing temperature results in gradual collapse of the crystalline structure of zeolites Y. This seriously affects the emission behaviors of these emitters in the NIR: (1) it can make some forbidden transition become possible, e.g., the appearance of 1285 emission band in the Y-Bi13.3-T340 sample; (2) it may also result in a large change in the emission lineshapes when the structure was seriously destroyed, e.g., the emission from the Y-Bi13.3-T400 sample.

Figure 6. Simplified energy-level diagrams of Bi44+, Bi33+, Bi22+, and Bi+ emitters identified in zeolite Y. The black thick lines represent the energy levels determined by absorption or 2D ex/em graphs, while the thin ones are vibrational sub-levels. Additionally, the possible excited levels of these emitters as revealed by TDDFT calculations were shown as the thick green lines. Note that we did not show the energy levels over 1300 nm for Bi33+, since the transitions from these levels to ground level should be forbidden. The blue and green lines with arrows represent the possible


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absorption in visible and NIR spectral ranges. The dotted black and red thick lines with arrows represent the nonradiative and radiative relaxation of excitation energies, respectively. The possible migration routes of the excitation energies between emitters are shown by violet thick lines with double arrows.

Formation mechanism of Bi+-related active centers in zeolites Y: Since the emission from zeolite Y is from Bi+-related active centers, one basic question is reasonably proposed: what is the formation mechanism of Bi+ in zeolite systems? Obviously, the key to successfully answer this question is to identify the possible reducing agents for Bi3+. As is well known, zeolites contain lots of coordinated water molecules, which will be expelled from the pores of zeolites at high temperatures and has been found to be an excellent reducing agent for high-valence metal ions.6 For instance, it was observed that thermal treatment can induce the formation of Ag0 and Cu+ in Ag+ and Cu2+ doped zeolites, respectively.6,10 Although most of water molecules will be expelled quickly from zeolite pores, it is confirmed that the residual water in nearly dehydrated zeolites is responsible for above conversions.6,10 Interestingly, it was also observed that the reoxidation of Cu+ occurs when the obtained zeolites were fully exposed in an environment containing oxygen and water.10 This phenomenon is quite similar to that we observed for bismuth-embedded zeolites. We found that the color of the Y-Bi8.5-T400 sample can change from green to nearly white when it was exposed in air for less than 10 minutes. Based on this, it is speculated that the following reaction might take place when the samples undergo thermal treatment,

∆ [Bi3+ ]M + H2O  → [Bi+ ,2H + ]M +1/ 2O2 ↑

(2)


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where [Bi3+]M and [Bi+,2H+]M represent zeolites Y before and after treatment, respectively. Furthermore, in a control experiment, we found that the color of the obtained samples is closely related to the annealing time, and a longer time scale such as 24 hours is prerequisite to make the conversion of Bi3+ to Bi+ go to completion. Given this phenomenon as well as the fact that oxygen vacancies indeed exist in the final products, a second reduction process should exist, which is believed to be related to the zeolite structures themselves. As evidenced by ESR, oxygen vacancies develop in the annealed zeolites, thus leading us to propose the following reduction mechanism,

O |

O |

O |

| O

| O

| O

∆ o − Al − o − Si − o − Al − o + Bi 3+ →

O |

O |

O |

| O

| O

| O

(3)

o − Al KV K Si − o − Al − o + Bi + + 1 / 2O2 ↑

That is, after the supply of water molecules is depleted and the zeolite is fully dehydrated, additional Bi3+ ions can be progressively reduced to Bi+ by framework oxygen ions. Obviously, the above reaction leads to the formation of positively-charged oxygen vacancies (denoted ‘…V…’) in the frameworks. Such vacancies tend to capture one electron possibly based on the delocalization of electrons in the frameworks, making the final products ESR-active (Figure 2). The resulting ESR signals can be altered by spin-orbit coupling which changes the local magnetic field experienced by the electron. This may be one dominant reason for the appearance of signals at 2.0042 and 2.0195 for the Y-Bi13.3-T400 sample. Additionally, the absence of Bi0 evidenced by XRD as well as ESR analyses results from the weak reducing abilities of water


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molecules and framework oxygen. Although H2 can reduce Bi3+ to Bi0 in zeolite frameworks, the final product does not show any PL signal.14 The above analyses suggest that the reduction of Bi3+ to Bi+ may be a synergic effect of water molecules as well as framework oxygen.

4. CONCLUSION We have systematically investigated the structural and photophysical features of zeolites Y containing Bi using extensive experimental techniques including NMR, ESR, 2-D ex/em and absorption spectra. We find that the thermal treatment of zeolites Y containing Bi results in the change of the distribution of Al species in zeolite Y frameworks and leads to the formation of oxygen vacancies. Detailed analyses of the obtained luminescence and absorption spectra, coupled with TDDFT results, help us to conclude that all Bi+ substructures (i.e., Bi44+, Bi33+, Bi22+, and Bi+) are optically active in the NIR spectral range. Bi+, Bi22+, Bi33+ and Bi44+ units result in NIR emissions peaking at ca. 1050, 1135, 1145, 1240/1285 nm, respectively. Additionally, it is proposed that the partial collapse of the crystalline structure of zeolites Y could affect the emission behaviors of such emitters in the NIR. On the basis of the structural and photophysical analyses, we propose that the reducing agents for Bi3+ to Bi+ are water molecules and framework oxygen. These findings represent an important contribution to the understanding of the processes involved in the formation of Bi+ and of the luminescence mechanisms of Bi+ substructures in zeolite Y frameworks. These results are not only helpful for the in-depth understanding of experimentally observed photophysical properties in a number of Bi-doped materials,18,19,51-64 but also important for the development of novel photonic material systems activated by other p-block elements.


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ASSOCIATED CONTENT Supporting Information. XRD patterns, ESR spectrum, decomposed emission spectra, PLE spectra, TG curves, theoretical absorption spectra of Bi22+, 2-D ex/em graph, and tables of synthesis parameters and calculated electronic structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT H.-T. Sun gratefully acknowledges the start-up funding support for new staff from Hokkaido University, and support from the International Center for Young Scientists, National Institute for Materials Science, Japan. H.-T. Sun thanks the support from X. Shi in Hokkaido University for the XRD measurement.

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