Confinement of Highly Luminescent Lead Clusters in Zeolite A - The

Subscriber access provided by - Access paid by the | UCSB Libraries

Confinement of Highly Luminescent Lead Clusters in Zeolite A Wouter Baekelant, Saleh Aghakhani, Eduardo Coutiño-Gonzalez, Didier Grandjean, Koen Kennes, Dries Jonckheere, Eduard Fron, Francesco D'Acapito, Alessandro Longo, Peter Lievens, Maarten B.J. Roeffaers, and Johan Hofkens J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01107 • Publication Date (Web): 10 Mar 2018 Downloaded from http://pubs.acs.org on March 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Confinement of Highly Luminescent Lead Clusters in Zeolite A Wouter Baekelant,δ, ‡ Saleh Aghakhani,† Eduardo Coutino-Gonzalez,δ,¥ Didier Grandjean, † Koen Kennes, δ Dries Jonckheere,‡ Eduard Fron,δ Francesco d’Acapito,# Alessandro Longo,§,ζ Peter Lievens,† Maarten B. J. Roeffaers,‡,* Johan Hofkensδ,* δ

Chem&Tech - Molecular Imaging and Photonics, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven,

Belgium. ‡

Chem&Tech - Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, B-3001

Leuven, Belgium. †

Department of Physics and Astronomy, Laboratory of Solid State Physics and Magnetism, KU Leuven,

Celestijnenlaan 200D, B-3001 Leuven, Belgium. ¥

CONACYT - Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Parque Industrial

Querétaro, Sanfandila s/n, Pedro Escobedo 76703, Querétaro, México. #

CNR-IOM-OGG, c/o ESRF – LISA CRG - The European Synchrotron, CS 40220, 38043 Grenoble Cedex 9,

France. §

Netherlands Organization for Scientific Research (NWO), Dutch-Belgian Beamline, ESRF - The Europe-

an Synchrotron, CS40220, 38043, 71 Avenue des Martyrs, 38000 Grenoble, France. ζ

Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, UOS Palermo, Via Ugo La Malfa, 153,

90146 Palermo, Italy.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 36

ABSTRACT: Metal clusters confined inside zeolite frameworks display unique electronic, catalytic and optical properties. However, so far only confined silver clusters have shown peculiar luminescent properties, displaying high photoluminescent quantum efficiencies reaching almost unity. In this study, we demonstrate the self-assembly and confinement of highly luminescent lead (Pb) clusters into the molecular-sized cavities of Linde Type A (LTA) zeolites. These Pb-LTA samples display an intense deep-blue emission with external quantum efficiencies up to 69 % in their partially dehydrated state. A tetrahedral lead cluster (Pb4) with unusually short Pb-Pb distances and hydroxyl ligands was identified as the responsible of the luminescence as determined by X-ray absorption fine structure (XAFS) analysis. The in-depth characterization of the Pb-zeolites, reported here, sets the stage for elucidating the structureto-luminescent relationship of other zeolite embedded clusters.

2


Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Small oligoatomic metal clusters confined in solid matrices have recently emerged as innovative materials with remarkable physicochemical properties and potential applications in different fields such as catalysis1 and photonics.2 Their unique properties originate from the molecular-like discrete energy levels in the cluster’s electronic structure.3 However, stabilization of these oligoatomic metal clusters is required as they tend to aggregate and form larger nanoparticles, resulting in the disruption of such discrete energy levels and thus losing their peculiar optical properties. Such stabilization is often performed by confining metal clusters inside a host-scaffold. The properties of the guest-clusters, such as cluster nuclearity, size, shape, and electronic properties, are strongly influenced by the confinement scaffold.

A widely studied metal for the formation of luminescent clusters is silver for which several host-guest synthesis approaches have been developed, a representative example is the use of organic templates such as DNA and peptides.4–6 On the other hand, more durable approaches to stabilize luminescent silver clusters at room temperature comprise the use of inorganic scaffolds, such as glass7 and zeolite matrices.8 Confinement of silver clusters within glass renders excellent stabilization, both chemically and structurally, however the optimal photoluminescence efficiency of such systems has been reported not to exceed 20%.7 Besides, zeolites are more adaptable host systems compared to glass, therefore the luminescence of silver-zeolite composites are more prone to respond to external stimuli, such as moisture and adsorbates.9 This high adaptability of the Ag-zeolite samples combined with wellcontrollable synthesis parameters of the host-guest systems resulted into materials with broad emission bands (spanning the whole visible region), variable excitation wavelengths and large Stokes shifts.9–14 These peculiar properties in combination with high external quantum efficiencies (EQEs) of 3



Page 4 of 36

up to 97% make them excellent alternatives for phosphors in lighting applications and wavelength converters for solar cells.15–17

Although several studies dealing with the luminescence characterization of silver-exchanged zeolites have been published, reports on other luminescent metal zeolites are scarce. Even though a study by Claffy and Schulman from 195118 reported on the weak cathodoluminescence originating from other metals embedded in natural zeolite materials. One of these reported metals was lead (Pb), which in its ionic form like silver displays very good ion exchange properties in Linde-type A (LTA) zeolites,19–21 facilitating the incorporation of the metal guests into the host system. Luminescent oligoatomic Pb clusters confined in zeolites, to the best of our knowledge, have never been described as luminescent active clusters with the exception of the abovementioned observation of weak cathodoluminescence in heattreated natural zeolites without further explaining the luminescence origin.18 It has however been reported that lead-based species confined in other inorganic scaffolds display luminescent properties. Zatsepin and collaborators reported the luminescence properties of lead oxide species confined in silicate glass at low temperatures.22 Furthermore, quantum confined sub-nanometer PbS and PbSe semiconductor nanoparticles with emission spanning from 450 to 850 nm, at room temperature, when confined in glass substrates were reported elsewhere,23 whereas stabilization of sub-nanometer PbS species in LTA zeolites rendered yellow-orange emission at cryogenic temperatures.24,25 The optical properties observed for these sub-nanometer PbS species are in strong contrast with the near-infrared emission displayed by 3 to 10 nm sized PbS quantum dots.26

In this study, the self-assembly of highly luminescent Pb clusters confined in LTA zeolites display a deep-blue emission and EQEs of up to 69 %. This EQE value is achieved at remarkably low Pb content (3 wt%), making them cost-effective while minimizing the risk associated to Pb exposure. The samples 4


Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were investigated by means of steady-state and time-resolved luminescence techniques. Changes in the host-guest system, such as variations in the lead loading and water content have a profound impact in the optical properties displayed by Pb-LTA materials. The latter parameter was further studied by analyzing the cluster structure and electronic properties at the atomic level using X-ray absorption fine spectroscopy (XAFS). This XAFS investigation showed that the luminescence properties originate from tetrahedral Pb clusters with unusually short Pb-Pb distances (2.89 Å).

2. Experimental Section Sample preparation. The Pb-LTA zeolites were synthesized using a two-step protocol similar to the procedure described for luminescent silver-zeolites in earlier reports,15 starting from Na-LTA with an elemental composition of the normalized unit cell (N.U.C.) equal to Na12Si12Al12O48 (Figure S1). In the first step, 250 mg commercial Na-LTA zeolites (UOP, Figure S2) were exchanged with a 100 ml stoichiometric solution of Pb(NO3)2 (≥99.0 %, Sigma Aldrich) in an end-over-end shaker oven at room temperature for 16 h. The second step or activation step, was performed after filtration and comprises an overnight heat-treatment of the powders in a porcelain cup in a muffle oven at 450 °C, starting from room temperature with a heating rate of 5 degrees per minute. In order to avoid framework destruction by fast water removal, two 30 minute intervals at 80 °C and 110 °C were included. The samples were cooled down in ambient conditions to room temperature. In order to fully hydrate the Pb-LTA samples, they were placed in a closed container for 1 week with 99% relative humidity, originating from a saturated K2SO4 (≥99.0%, Sigma Aldrich) solution.16 Dehydration, on the other hand, was performed in a (muffle) oven at the specified treatment temperature. Subsequently the samples where cooled down to room temperature inside a desiccator to keep them dehydrated.

5



Page 6 of 36

Elemental analysis. The exchange efficiency of the Pb2+ ions was determined using 2 techniques, namely Inductive Coupled Plasma Mass Spectroscopy (ICP-MS) to probe the Pb content and Atomic Absorption Spectroscopy (AAS) to determine the Na content. Prior to this analysis, 50 mg of the zeolite samples were digested using a mixture 0.5 ml HNO3 and 3 ml of 40% HF. The ICP-MS was performed on an Agilent ICP-MS 7700X instrument and AAS on a Varian SpectrAA 20 plus. After analysis of the wt%, the atomic ratios of the Pb and Na were calculated by normalizing their combined charge to +12.

Photoluminescence characterization. The steady-state luminescent properties, such as 2D excitation/emission plots and EQEs, were recorded on an Edinburgh FLS980 fluorimeter. To measure the absolute EQE, the fluorimeter was equipped with an integrating sphere from Labsphere connected to the fluorimeter via optical fibers. A barium sulfate sample was used as reference during EQE measurements.

The temperature dependent experiments were performed by attaching an in-house spectroscopycompatible cell to the integrating sphere, as previously described by Coutino-Gonzalez et al.9 (Figure S3). Dehydration of the samples at different treatment temperatures was performed starting from a fully hydrated sample. Next, the sample was heated for 1 hour at different temperatures, followed by 10 minutes under high vacuum, with a pressure of below 1 mbar, to remove evaporated water and prevent reabsorption. Finally, after closing the sample chamber, the sample was cooled down to RT inside the in-house heating cell, before measuring the luminescent properties. The different treatment temperatures used here were 50°C, 100°C, 200°C, 300°C, 400°C and 450°C.

Time-resolved luminescence characterization. Microsecond time-resolved luminescence measurements were performed on an in-house built setup using a nitrogen laser (265 nm, 10 Hz, 7ns FWHM, Quanta-Ray INDI-40-10, Spectra-Physics) as excitation source. The 265 nm laser pulses are collimated 6


Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and focused on the sample by a 150 mm focal length lens, a 4 % of this light was sent to a fast photodiode to generate a trigger signal. A right angle configuration between excitation and light collection paths was used. A SpectroPro-300i monochromator/spectrograph was utilized to disperse the emitted light and select the desired wavelength. The luminescence signal was detected by a photomultiplier tube (PMT) (Hamamatsu, R928). The transient electrical signal was amplified and sent to a computer controlled oscilloscope. Homemade Labview based software was used to control and trigger the instruments as well as to read, average, and store the transient data.

The fluorescence decay times at the nanosecond time scale were determined by time-correlated single photon counting (TC-SPC). The frequency-tripled output (266 nm, 8.18 MHz, 2 ps FWHM) of a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics) was used as excitation light. The linearly polarized excitation light was rotated to a vertical direction by the use of a Berek compensator (New Focus) in combination with a polarization filter and directed onto the sample. The sample in powder form was placed in a quartz cuvette (1 mm path length, under 45°) and sealed by a Teflon stopper, and then mounted on the device. The emission was collected under 90° with respect to the incident light and guided through a polarization filter that was set at the magic angle (54.7°) with respect to the polarization of the excitation beam. The fluorescence was spectrally resolved by a monochromator (Sciencetech 9030, 100 nm focal length, wavelength accuracy 0.3 nm), and detected by a microchannel plate photomultiplier tube (MCP-PMT, R3809U-51, Hamamatsu). A time-correlated single photon timing PC module (SPC 830, Becker & Hickl) was used to obtain the fluorescence decay histogram in 4096 channels. The decays were recorded with 10000 counts in the peak channel, in time windows of 10 ns corresponding to 2.4 ps per channel and analyzed individually with a time-resolved fluorescence analysis (TRFA) software. The full width at half- maximum (FWHM) of the IRF was typically in the order of 42 ps.

7



Page 8 of 36

Thermogravimetric analysis. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500 thermogravimetric analyzer. The zeolite samples (15-25 mg) were treated under an oxygen flow of 90 ml min-1 with a linear heating ramp of 2.5 °C min-1 to 600 °C. TA Universal Analysis software was used for post-measurement data handling and analysis.

XAFS analysis. Initial XAFS data were collected on the Dutch Belgian DUBBLE Beamline (BM26A) at The European Synchrotron (ESRF, Grenoble, France), operating under 6 GeV, 200mA, (7/8 + 1) filling mode. The beam line is equipped with a Si (111) double-crystal monochromator and a vertically focusing Si mirror, which suppresses higher harmonics.

XAFS data were collected at LISA beamline (BM08) at the European synchrotron (ESRF, Grenoble, France) operating also under 7/8+1 filling mode on a dedicated experimental station. The monochromator was equipped with a pair of Si (311) crystals and run in Dynamically Focusing mode. The harmonic rejection was carried out by using a pair of Pt-coated mirrors (Ecut off = 31 keV). The XAS was measured in transmission mode at the Pb L3-edge (13035 eV) on hydrated or dehydrated samples sealed inside glass capillaries at room temperature. The ionization chambers were filled with Ar/He gas mixtures. The datasets were collected up to 13 Å-1 with acquisition times of about 20 min. Three spectra were averaged to improve the signal to noise ratio to an optimal level.

Electron paramagnetic resonance (EPR) analysis. Prior to the EPR measurement, the samples were dehydrated at 200°C and sealed inside a quartz tube. A Bruker 200D-SRC device in X-ban with a sweep of 2000 G centered around 3500 G was used to record the spectra. In which the samples were inserted in a double rectangular TE104 cavity and cooled down to a temperature of 150 K prior to the measurement.

8


Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and discussion Effect of the lead loading on the luminescent properties

In this study, commercial Na-LTA zeolites were exchanged with different Pb loadings ranging from 0.5 to 46.5 wt% or 0.05 to 6 Pb2+ cations per normalized unit cell (N.U.C.), which has a dry chemical composition of PbxNa12-2xSi12Al12O48. The cation exchange efficiency was determined by analyzing the Pb to Na ratios for the different Pb exchange amount, using a combination of atomic absorption spectroscopy (AAS) for Na+ and inductive coupled plasma mass spectroscopy (ICP-MS) for Pb2+. The calculated ratios and elemental compositions are shown in Table S1 in the supporting information (SI). The results show good agreement between theoretical (assuming 100% ion-exchange efficiency) and the obtained Pb contents. For simplicity, we will denote the samples using their theoretical Pb content, namely PbxLTA with x ranging from 0.05 to 6.

The excitation and emission profiles of Pb-LTA samples dehydrated at 200 °C, with different Pb loadings are depicted in Figure 1A and Figures S4 and S5. The effect of dehydration and the choice for 200 °C will be explained in more detail later on this article. For the samples with Pb content equal to or lower than Pb3-LTA (or 50% of the total ion exchange capacity), the emission profile (λem(max) = 391 ± 1 nm and FWHM 45 ± 5 nm) is independent of the Pb loading. Only at high Pb contents, Pb3-LTA and higher, a second emission peak (λem(max) = 327 ± 3 nm and FWHM 37 ± 5 nm) was observed upon deconvolution of the emission spectra (Figure S6). This second 327 nm peak gradually intensifies with the Pb content at the expense of the 391 nm emission peak. For all samples the excitation maximum was found between 250 and 260 nm and only for the fully Pb exchanged LTA zeolites, Pb6-LTA, a second minor excitation peak at 280 nm was observed. Although the emission and excitation maxima remain sta-

9



Page 10 of 36

ble with the Pb content of the LTA zeolites, the EQE and luminescence intensity are highly dependent on the Pb content (Figure 1B). The luminescence intensity and EQE strongly increases at very low Pb loadings reaching a maximum at Pb0.25-LTA, displaying an EQE of 69 ± 2 %. Upon further increasing the Pb content the EQE rapidly decreases to below 15% for Pb-LTA samples with Pb loadings equal to or higher than Pb2-LTA. This trend demonstrates that changes to the host-guest system, such as variations in Pb content, can profoundly influence the luminescent properties, such as the EQE in this case.

Figure 1. A) Excitation (broken lines) and emission (solid lines) spectra, and B) EQE values of heattreated (200 °C) Pb-LTA samples with different theoretical Pb contents based on the Pb concentration 10


Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in the ion-exchange solution assuming a 100% efficient ion-exchange. EQE values were determined by averaging the values of 3 measurements in 3 different batches (9 values in total) of the same sample composition, as also described in reference 16.

Effect of hydration level on the luminescent properties

Although the spectral features of the luminescence are relatively stable with loading for samples with Pb contents below Pb3-LTA, the optical properties of the Pb-LTA samples are significantly influenced by their hydration level, as depicted in Figure 2 for the Pb0.25-LTA sample. Upon hydration of the Pb0.25-LTA sample treated at 200 °C the EQE drops from 69% to below 1% and the recorded emission contains 2 bands (λem,1(max) = 330 ± 1 nm, FWHM1 = 41 ± 1 nm and λem,2(max) = 368 ± 7 nm, FWHM2 = 61 ± 1 nm, see also Figure S7) with an excitation maximum at 235 nm. A similar trend for hydration/dehydration cycles was observed for the samples with low Pb content (below Pb3-LTA) as depicted in Figure S8.

To study the effect of water on the luminescent properties of Pb-LTA samples in more detail, information from thermogravimetric analysis (TGA) was correlated to the luminescence properties of this sample measured at RT after a certain heat-treatment. The temperature dependent luminescence experiment was performed using an in-house spectroscopy-compatible cell (Figure S3), as described in a previous report9 and the experimental section. The humidity dependent experiments were performed on a Pb0.25-LTA sample, which was hydrated by placing it for 1 week in a closed container with a 99% relative humidity prior to the experiments.

11



Page 12 of 36

Figure 2. Effect of hydration level on the luminescent properties of Pb0.25-LTA sample. Picture under daylight (upper right) and 254 nm UV light illumination (lower right). The 22 x 22 mm2 quartz plate on top partially prevents water uptake by the sample. Their respective 2D emission/excitation plots and EQEs are shown on the left.

12


Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. A) TGA graph and EQE of the Pb0.25-LTA sample at different treatment temperatures. B) Emission spectra at 250 nm excitation for Pb0.25-LTA sample at different treatment temperatures. EQE values were determined by averaging the values of 3 measurements in 3 different batches (9 values in total) of the same sample composition, as also described elsewhere.16

At this point the water content of the zeolite amounted to 19.3 wt% or approximately 23 H2O molecules per N.U.C.. The TGA result, depicted in Figure 3A (Figure S9, Table S2), shows three areas of weight loss, which are in agreement with previous reports.27,28 The first water loss starts immediately when heating from RT and continues up to 40°C reaching 17.2 wt% of water content at this tempera13



Page 14 of 36

ture. This weight loss can be ascribed to (weakly) adsorbed water into the zeolite cavities.27 The second step of water loss starts at 40°C and ends around 130°C at this temperature the sample presents only 2.7 wt% of water content, the maximal weight loss for this area is located around 100 °C. The last temperature range for which dehydration was observed is between 200 °C and 300 °C, with a maximal loss at 260°C. During this step, the amount of water inside the zeolite drops from 2.7 to only 0.3 wt%. Above 300 °C, only water attached to the zeolite framework remains. This water is removed by heating the sample above 300 °C and after its complete removal at 800 °C the LTA zeolite framework collapses as reported in literature by XRD analysis.27

From the results of the temperature dependence of the luminescence properties, depicted in Figure 3B, three thermal regions were identified in which the spectral features remained relatively unaltered. However, the spectral changes between these three thermal regions are significant. The first region (region I) comprises RT and 50 °C, for which a relatively broad emission is observed when excited at 235 nm with maxima located at 330 and 368 nm, respectively. The next region (region II) corresponds to the temperature range between 100 °C and 300 °C. The excitation maximum for the Pb0.25-LTA is observed at 250 nm and the emission band sharpens significantly with a maximum at 391 nm. The third region (region III) was observed for samples treated above 300 °C. Here the emission maximum is located around 410 nm when excited at 255 nm.

The trends observed in the EQE values for the samples with different treatment temperatures are also related to the three above mentioned temperature regions (Figure 3A). The fully hydrated Pb0.25-LTA sample has an EQE below 1 % at RT, which increases to 7 % after heat-treating the sample at 100 °C. After sample treatment at 200 °C a pronounced increase of the EQE is observed, reaching the maximum EQE value of 69 %. Further dehydration of the sample decreases the EQE to 51 % for treatment at 14


Page 15 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 °C and reaching a plateau of about 35 % when treated at 400 and 450 °C. From TGA and temperature dependent luminescent studies, it can be observed that the deep-blue, highly luminescent species are formed in region II, after the removal of the adsorbed water in the first and second weight loss steps (below 200 °C) of the LTA zeolite. Although further removal of water (temperatures above 200 °C), does not significantly alter the excitation and emission properties, it has a negative influence on the EQE of Pb0.25-LTA. Time-resolved properties of Pb-LTA samples

Pb0.25-LTA sample dehydrated at 200 °C displays three different decay times, namely one short (3.7 ns) and two long lived species with lifetimes of 476 ns and 895 ns when excited at 265 nm (Figure S10), showing a large similarity with luminescent Ag-zeolites.12 Based on the amplitudes of the fluorescence decay trace, the 3.7 ns component can be calculated to only contribute for less than 1% of the emission, whereas the 476 ns (32 %) and 895 ns (67%) components have significant contributions. This indicates that the observed stationary emission occurs from the two excited states with long decay times. The 3.7 ns component can be attributed to a transition between states of the same spin multiplicity. While, the unusual long lifetimes suggest that these states have either a different spin multiplicity (forbidden transitions) compared to the ground state or are related to delayed fluorescence due to electron transfer. The nature of these states is subject of further investigation which is ongoing and will be reported separately.

15



Page 16 of 36

Figure 4. A) Pb L3-edge k3-weighted and B) their respective phase corrected FTs of Pb0.5-LTA with various hydration levels based on different treatment temperatures.

Structural and electronic properties of luminescent Pb-LTA samples

To get a better understanding of the structure and electronic properties of Pb species at the origin of these remarkable luminescence properties and their strong dependency upon the hydration level of 16


Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the Pb-LTA zeolites, fully hydrated and dehydrated (200 and 450 °C) Pb-LTA samples were characterized with X-ray absorption spectroscopy (XAS), such as extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES). EXAFS is a powerful technique for obtaining information on the atomic-scale bonding in clusters including metal-metal and metal-ligand bonding, and for estimating metal cluster sizes.29–31 To guarantee an optimal signal-to-noise ratio (S/N) of the EXAFS data, the investigation was carried out on Pb0.5-LTA zeolite that possesses in all hydration states nearly identical optical properties as the Pb0.25-LTA sample, while having twice the amount of Pb (Table S1 and Figure S11 and S12). EXAFS Pb L3-edge k3-weighted data of Pb0.5-LTA having different hydration states and their phasecorrected Fourier transforms (FTs), depicted in Figures 4A and 4B, show two main peaks in the range of 1.5 to 4 Å in the FT profiles (Fig. 4B). The first peak (1.5-2.9 Å) can mainly be attributed to several oxygen neighbours whereas the second multipeak (3.0-4.0 Å) corresponds to a mixed contribution of Si /Al and Pb neighbour atoms.24,32–37 Whereas the intensities of the two peaks remain rather constant up to 200°C, a sharp inversion of their relative intensities is observed at 450°C suggesting that significant structural changes are occurring upon dehydration. This is corroborated by the XANES analysis (Pb-L3) (Figure S13), which shows that only the Pb0.5-LTA dehydrated at 450°C has a small shoulder at 13027 eV on top of the 13037 eV present in the sample at all hydration states (hydrated and dehydrated at 200 and 450 °C). This small 13027 eV peak can be attributed to 2p to 6s electronic transitions,31,38 which is also observed in the PbO reference (13030 eV) and originates from a partial de-occupation of the 6s atomic orbitals resulting from the strong bonding interaction between the Pb 6s and O 2p molecular orbitals and the contribution of the antibonding interaction of Pb 6p and O 2p molecular orbitals. These interactions between the different orbitals can directly be linked to a decrease in the Pb-O distance.39 More details on the XANES analysis can be found in section VI of the SI. 17



Page 18 of 36

Details of the EXAFS analysis and the list of structural parameters obtained by fitting the data (Table S3) are also given in the supporting information. This analysis demonstrates the formation, in all different hydration states of Pb0.5-LTA material, of a new type of tetrahedral cluster (Pb4) with unusually short distance of ca. 2.90 Å as compared to Pb-Pb typical distance in metal state (3.49 Å).40 This short interatomic distance between the Pb of the cluster (Pbc) has, to the best of our knowledge, not been reported before. However a large resemblance between this Pb-Pb distance and the reported Ag-Ag distances in highly luminescent Ag-clusters can be observed.15 The tetrahedral Pb4 clusters formed inside the sodalite cages of the Pb-exchanged LTA zeolites are all coordinated at their faces to 2-to-4 extra-framework O-based ligands (Figure 5). Even though the occurrence of stable cubane-like Pb clusters have been claimed in computational studies on Pb clusters in gas41,42 and liquid phase,43 and experimentally characterized in similar zeolite systems33–36 their interatomic distances significantly differ from the Pb clusters described in this work. In particular, larger Pb-Pb distances in the range of 3.3 Å were always observed and the photoluminescence properties of such Pb species were not addressed. We can thus assume that these shorter Pb-Pb distances and enhanced Pb-Pb interactions, are necessary to create luminescent clusters as observed for the tetrahedral Pb4 clusters within this report. The Pb4 clusters represent a fraction of around 20% of all Pb-exchanged cations whereas the remaining fraction consists mostly of single Pb cations positioned in the six-membered rings (S6Rs) of both cluster and non-cluster containing sodalite cages. Additionally, another minor fraction of 10 to 15% of Pb atoms inside the sodalite cages are interacting directly with the oxygen of the S6R rings. Due to this significant percentage of Pb cations (around 80 %) which are not participating in the formation of the luminescent clusters, we can speculate that these are responsible for the rapid quenching of the photoluminescent EQE upon increasing the Pb loading. Moreover, the nature of the ligands as well as their distances to Pbc atoms change dramatically as a function of the water content leading to a large variety 18


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of cluster compositions and geometries that can be easily tuned by controlling the dehydration temperature of the materials.

Figure 5. 3D model of the unit cell of an LTA zeolite containing tetrahedral lead clusters inside their sodalite cages. The zoom in on the sodalite cage shows the encapsulated Pb4 cluster surrounded by oxygen-based ligands, such as -OH2, -OH or -O ligands depending on the hydration level of the zeolite. The black atoms represent Pb-atoms, whereas the red atoms represent the oxygen ligands.

In fully hydrated Pb0.5-LTA at RT, 20% of the exchanged Pb forms a Pb4(OH2)x (x= 2 to 4) tetrahedral cluster. Each Pbc is coordinated to ca. 2.5 water molecules at a distance of 2.64 Å, which is in agreement with the Pb-OH2 distances reported elsewhere.33,38 Upon dehydration at 200 °C, a part of the Pb4(OH2)x clusters swap their water ligands for hydroxyl groups, since we found a shorter average Pbcoxygen distance (Pbc-Ox) of 2.59 Å. This is in line with previously reported Pb-OH distances (2.50 Å).33,38 The alteration from water to hydroxyl ligands may result from a hydrolysis reaction of the zeolitic water in the presence of metal ions, as described in literature.44 In Pb0.5-LTA samples heat-treated at 200 °C, 17% of Pb atoms are now forming a combination of the aforementioned Pb4(OH2)x (x= 2 to 4) and Pb4(OH)y (y= 2 to 4) tetrahedral clusters, with each Pb atom coordinated to 2 OH-groups. Finally upon dehydration at 450 °C, the hydroxyl ligands are replaced by extra-framework oxygens, which can be 19



Page 20 of 36

formed during a hydrolytic reaction step involving two lead ions, forming oxidic bonds.44 The oxide is positioned at a much shorter distance of ca. 2.31 Å from the lead atoms. This distance is also in agreement with Pb-O distances reported in lead oxide (Pb-O = 2.31 Å).31 In this sample, 23% of Pbexchanged atoms form Pb4Oz (z= 2 to 4) tetrahedral clusters with each Pbc coordinated up to 2 extraframework oxygens. These oxidic bonds found in the fully dehydrated sample are in agreement with the XANES analysis, where the 13027 eV shoulder is a fingerprint for a Pb-O bond. Correlation of Pb4(OH2)x , Pb4(OH)y and Pb4Oz cluster structures obtained by EXAFS and XANES at different steps of the dehydration process with the TGA results (Figure 3A), suggests that the peak observed around 130°C removes the water from Pb4(OH2)x clusters whereas the second peak around 260°C may correspond to the transformation of the hydroxyl ligands of Pb4(OH)y clusters into extra-framework oxide bonds. Furthermore, these results suggest that there is a strong influence of the nature of the O-based ligands on the luminescent properties of the sample (Table 1). Pb4Oz and Pb4(OH)y have similar spectral features and display the highest EQEs of 37 and 69 %, respectively. The reason we attribute the high luminescent properties to Pb4(OH)y in the 200 °C-treated sample, can be deduced from the absence of luminescence in the hydrated sample. This sample possesses a very modest EQE below 1 % and has substantially different excitation and emission spectra compared to the dehydrated samples.

In addition, the Pb clusters identified in this work share large structural similarities with their Ag counterparts formed in Ag-exchanged FAU zeolites that were identified as the species at the origin of a strong luminescence with EQE up to nearly unity.15 This strongly suggests that the Pb4 tetrahedral clusters (Figure 5) unravelled by EXAFS and XANES analysis are likely at the origin of the peculiar luminescence properties observed in Pb-exchanged LTA zeolites. Comparing our experimental observations to those found by Liu and collaborators,40 we observe that these authors reported a HOMO-LUMO gap of 4.7 eV (264 nm) for cubane-like Pb4O4 clusters, which is in very good agreement with the excitation en20


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ergies of 4.96 and 4.86 eV (250 and 255 nm) measured in our Pb-LTA materials. Despite some significant discrepancies, such as that the abovementioned DFT study was conducted in gas phase and a different Pb-Pb distance was used instead of the distance observed in this work (2.89 Å), both cluster structures are sufficiently close to suggest that the origin of the optical excitation in Pb-LTA materials may be attributed to electronic transitions from similar types of HOMO and LUMO levels in the confined Pb4Ox2-4 cubane-like clusters.

Table 1. Pbc-Pbc distance, Pbc-Ox distance, lead oxygen bond type, excitation and emission maxima and EQE of Pb0.5-LTA sample.

Treatment

Pbc-Pbc

Pbc-Ox

Pbc-Ox

λexc(max)

λem(max)

EQE

(Å)

(Å)

bond type

(nm)

(nm)

(%)

Hydrated

2.90(1)

2.64(4)

-OH2

235 ± 3

350 ± 4

Confinement of Highly Luminescent Lead Clusters in Zeolite A - The

Recommend Documents