Determination and Optimization of the Luminescence External

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Determination and Optimization of the Luminescence External Quantum Efficiency of Silver-Clusters Zeolite Composites Eduardo Coutino-Gonzalez,†,# Maarten B. J. Roeffaers,‡,# Bjorn Dieu,† Gert De Cremer,‡,▽ Sven Leyre,§,∥,# Peter Hanselaer,§,∥,# Wim Fyen,⊥,# Bert Sels,*,‡,# and Johan Hofkens*,†,# †

Department of Chemistry, KULeuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, KULeuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium § Light & Lighting Laboratory, Catholic University College Ghent, Gebroeders Desmetstraat 1, B-9000 Gent, Belgium ∥ ESAT/ELECTA, KULeuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium ⊥ KULeuven Research & Development, KULeuven, Waaistraat 6, B-3000 Leuven, Belgium # SIM (Flemish Strategic Initiative on Materials), SOPPOM Program, Technologiepark 935, B-9052 Zwijnaarde, Belgium ‡

S Supporting Information *

ABSTRACT: We have measured for the first time the external quantum efficiency (EQE) of silver clusters containing zeolites (henceforth referred to as silver-clusters zeolite composites). These materials, fabricated by silver cation exchange followed by a thermal autoreduction process, have EQEs up to 69%. Because of their unique spectral features such as large Stokes shift and high EQE, these materials could be potentially used as phosphors for the fabrication of fluorescent lamps and as wavelength convertors in solar cells. An EQE comparison between less pure commercial silverloaded zeolites and self-synthesized silver-zeolites showed the importance of the chemical and optical purity of the starting host material. Besides this, the zeolite topology and silver content play an important role on the luminescent performance of such materials. The ability to reliably measure the EQE enabled us to further optimize the synthesis of silver-zeolite composites. A new reduction−oxidation cycle is demonstrated not only to improve the luminescent performance of the silver-zeolite composites but also to enhance their water stability.

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INTRODUCTION Silver-zeolite composites are a versatile family of materials. Their applications range from catalysts,1 to antibacterial materials,2 information storage,3−5 and pressure or chemical sensors.6 The use of zeolites as molecular scaffolds for the fabrication of luminescent materials by incorporating transition metals has been explored.7−10 The creation of such oligoatomic metal clusters in zeolite voids is based on a ship-in-a-bottle approach, taking advantage of the high cation exchange capacity of zeolites. One of the most popular methods to produce metal clusters in zeolitic matrices is by exchanging the original chargebalancing cations present in the zeolites with the desired metal ions, followed by a thermal treatment. Small clusters are thus formed, whose size is ideally limited by the cage/pore dimensions of the zeolite topology. The mechanism of cluster formation has been proposed as an “autoreduction” mechanism, in which the electrons needed for metal ion reduction are provided by zeolite framework oxygen (resulting in local lattice damages) or by oxygen of the hydration water in the zeolite.1,11−13 Alternatively, chemical reduction14 and photoactivation,15 have been used for the creation of metal clusters and nanoparticles in zeolite matrices. © 2013 American Chemical Society

We recently reported the production of luminescent silverclusters in zeolites.16 In this study, the effect of zeolite topology, silver loading, and counterion on the luminescence color was evaluated systematically; green and red emitters were mainly found in LTA zeolite, whereas for FAU zeolite, green and yellow emitters were observed. The photo-, chemo-, and hydrostability of the materials were investigated in detail.15−17 The study revealed dramatic differences among the different types of silver clusters; most of the samples presented a high photo- and chemostability. However, the luminescence performance of certain samples, as qualitatively observed, was highly affected by the level of hydration in the environment.17 Despite their water sensitivity, these materials have serious potential to be used as phosphor substitutes for the fabrication of fluorescent lamps or as wavelength converters in solar cells15−19 because of their large Stokes shift and luminescence performance. Next to price, the major decisive factor for such application is the external quantum efficiency (EQE). The EQE Received: January 16, 2013 Revised: March 14, 2013 Published: March 19, 2013 6998

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Figure 1. Schematic representation of the fluorimeter and the integrating sphere used in this study. The configurations “in” and “out” of the beam are indicated on the left side of the diagram.

of a luminescent material is defined as the ratio of the number of photons emitted to the amount of photons absorbed by a material;20 this measure takes into account losses associated with the absorption of photons by nonemissive species, like, for instance, in this case silver ions and impurities associated with the starting material. The EQE parameter is of paramount importance to carefully select the luminescent materials for different applications.21 To the best of our knowledge, there are no data available concerning the measurement of EQE of silverclusters zeolite composites. One of the reasons for this lack of information on the luminescent of silver-clusters zeolite composites is the difficulty of correctly measuring the EQE on highly scattering solid materials, especially for excitation wavelengths in the UV region.22−24 Since the report published by de Mello and collaborators,20 in which an integrating sphere was used to measure the absolute EQE of thin dye-doped films, many other studies were carried out to obtain the EQE of a wide range of solid samples showing the versatility of the method. For instance, the EQE of fluorescent dye-loaded zeolites,25 fluorescent nanopigments incorporated in solid matrices,26 and polymer light-emitting films27 has been reported using the integrating sphere method. Unlike the silver zeolites, most of the reported materials present small Stokes shifts for which reabsorption problems are obviously expected. Furthermore, their optimal excitation wavelengths fall in the visible area, whereas the silver-clusters zeolite composites are mainly excitable by ultraviolet light. Remarkably, whereas all reports agreed on the usage of the integrating sphere, the setup configuration and the methodology of the EQE experiments were sometimes different, often depending on the sample requirements. To obtain reliable results, it is critical to perform appropriate calibrations, especially for UV excitation, taking into account the nature of the sample, thin film, loose powder, powder incorporated in a

polymer, and so on, and corrections for reabsorption processes.21−29 In this study, we developed a robust methodology to measure the EQE of silver-zeolite composites using an integrating sphere. On the basis of this EQE methodology, we report the EQE of silver-loaded zeolites, the influence of impurities, silver loading, and zeolite-host related issues on the EQE of the oligoatomic silver clusters in zeolites and the further optimization of their luminescence performance.

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EXPERIMENTS AND METHODS Silver nitrate (AgNO3, Sigma-Aldrich, 99.9% purity) was used as received and zeolite LTA-K, LTA-Na, LTA-Ca, FAUX-Na, and FAUY-Na (all samples from Union Carbide) were calcined at 723 K for 1 day (with 15 min stops at 323, 353, 393, and 433 K to avoid zeolite structure damage) to remove organic impurities before utilization. Dry air, hydrogen, helium, and oxygen were purchased from Praxair. An aqueous silver nitrate solution was used to perform the cation exchange of the original counterion. Different degrees of silver loadings were achieved by varying the concentration of the silver nitrate solution (0.02 to 0.5% w/w), taking into account the silicon to aluminum ratio of the different zeolites. In the case of LTA topology, 12 counterions can be exchanged per unit cell (S1 Supporting Information). For FAU topology, 6.5 (FAUY) or 10.4 (FAUX) counterions can be replaced per unit cell. The silver-loaded zeolites are named according to their silver content as follows: LTA-MnAgq, FAUX-MnAgq, or FAUYMnAgq, where “M” stands for the counterion and “n” and “q” are the number of counterions and silver ions per unit cell respectively. The synthesis is fully described elsewhere.16 In brief, the zeolites were suspended in a solution containing the adequate amount of silver nitrate and then stirred for 2 h in the dark. After recovery of the silver-loaded zeolites by filtration, the powder was preheated at 353 K for 1 h and then transferred 6999

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to a muffle for its further calcination, first at 393 K for 1 h and then until 723 K overnight, using a temperature ramp of 5 K/ min. After calcination, the sample was cooled and stored in the dark. For the emission−excitation characterization, the heattreated sample was placed in a modified quartz cuvette (1 mm path length) and sealed by a Teflon stopper. Emission and excitation spectra were recorded using a Horiba Jobin Yvon fluorolog FL3-22 fluorimeter. Long-pass filters were used, when appropriate, to avoid influence from scattering. The emission was collected in front-face mode for detection. Absolute external quantum efficiencies were measured by using an integrating sphere coupled to a commercial spectrofluorimeter (Figure 1). The device is based on a Labsphere optical Spectralon integrating sphere (100 mm diameter), which provides high reflectance in the UV−visible and near-infrared region.27 The sphere accessories were made from Teflon (rod and sample holders) or Spectralon (baffle). Excitation was provided by a xenon lamp with a monochromator. Emitted fluorescence was collected in right angle and sent to a photon multiplier tube (PMT) through the side port of the sphere after passing a monochromator. In the sphere, a baffle between the sample holder and the detection port was placed to avoid direct observation of the scattered light and the fluorescence from the sample. To measure the absolute EQE appropriately and reliably, three experiments were performed. First, an empty 1 mm path length quartz cuvette was positioned inside the integrating sphere; this measurement yields the exact incident photon flux at the excitation wavelength (La) and the background signal at the emission wavelengths. Afterward, the cuvette was filled with the silver-clusters zeolite composites, and the excitation and emission intensity, in the presence of the sample, was measured “out” and “in” the beam (Figure 1), yielding the photon flux not absorbed by the specimen at excitation wavelengths due to indirect (Lb) and direct absorption (Lc) and the photon flux emitted by the specimen at emission wavelengths (Pb and Pc) (Figure 2), and the EQE is

Where A (direct absorption)

A=

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RESULTS AND DISCUSSION De Cremer et al. reported16 in a systematic way the luminescence properties of silver-loaded zeolites. In that study, several zeolite-host silver-guest factors were modified to obtain emissive clusters with different properties. The present study aims to show in a quantitative way the impact that such factors (zeolite-host, silver-guest) can have on the luminescence performance of silver-clusters zeolite composites. The results obtained are discussed in the following part. EQE of Silver-Loaded Zeolites - Commercially Produced Zeolites. Two commercial zeolites, LTA and FAU, with different cocations and Si to Al ratio were loaded with silver and heat-treated, and their corresponding EQE values are reported in Table 1. EQE values, on the order of 1 to 4% were found for the commercial LTA samples, whereas the observed EQE values are significantly higher for the FAU type zeolites. For instance, the EQE value of FAUX is 33%, whereas that of FAUY corresponds to 41%. The low EQE values observed for Ag-LTA can be due to the major formation of photonabsorbing nonemissive silver species,30,31 thereby lowering the overall EQE of the material. Such nonemissive silver species have been identified in different cavities of LTA.32 At present, the structure, nuclearity, and sometimes even the location of the silver clusters in LTA and FAU zeolites are not always wellcharacterized. A recent report33 demonstrated a Ag 6 n+ octahedron silver cluster in the center of the sodalite cages of LTA, but the formation of other silver structures has also been observed.6,34 In the case of FAU topology, the presence of a linear triatomic cluster located at the hexagonal prisms of the FAU framework has been suggested.6,17,30,31 However, there is also evidence of the formation of Ag4n+, Ag8n+ clusters in fully exchanged FAU topology.35 Despite the many efforts that researchers have put to elucidate and correlate the structure to the optical properties/function of oligoatomic metal clusters in zeolitic matrices, only few reports33,36 were able to visualize in a direct way the formation of metal clusters into zeolite cavities.

calculated using eq 1. Because part of the photons is reflected by the sample, the “out” of the beam approach allows us to account for the reabsorption phenomena that might be involved during the EQE measurements. Pc − Pb(1 − A) LaA

(2)

Contrary to the EQE reported for fluorescent dye-loaded zeolites, in which an overlapping of excitation−emission was observed,22 no reabsorption issues due to the emissive clusters are expected in the silver-loaded zeolites because their Stokes shift is large (Figure 3). However, some reabsorption by other (e.g., nonemissive) clusters present in the zeolites cannot be fully excluded. Verification of the EQE of silver-clusters zeolite composites and powder references was done in an alternative setup (S2 Supporting Information). The results obtained for the analyzed samples were in agreement for both setups. For the reduction−oxidation post-treatment, the heat-treated silver-containing zeolites were transferred to a quartz u-tube with gas flow adaptors. The tubes were tightly attached to a gas flow line inside a programmable oven. The reduction was performed at room temperature by passing a hydrogen flow (5 mL/min) during 10 min over the sample, followed by a helium flush for 1 min. The reduced samples were reoxidized under dry air at 723 K for 2 h (using a temperature ramp of 5 K/min with 15 min stops at 323, 353, 393, and 433 K to avoid zeolite structure damage). After this period, the sample was cooled and stored in the dark for further analysis.

Figure 2. Typical spectra of a EQE measurement. (A) Scatter (excitation) range and (B) fluorescence (emission) range. Sample, FAUX-Na4.7Ag5.7. Excitation wavelength 340 nm.

EQE =

Lb − Lc Lb

(1) 7000

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Figure 3. Excitation (A)−emission (B) spectra of (a) FAUX-Na4.7Ag5.7, (b) FAUY-Na3.5Ag3, (c) LTA-CaAg10, and (d) LTA-K6Ag6.

Table 1. External Quantum Efficiency of the Silver-Loaded Zeolite Materials under Ambient Conditions and High Humidity Environmentsa EQE high humidity environment sample

excitation (nm)

LTAb-K6Ag6 LTAc-K6Ag6 LTAb-Na6Ag6 LTAc-Na6Ag6 LTAb-Ca3Ag6 LTAc-Ca3Ag6 FAUXb-Na4.7Ag5.7 FAUXc-Na4.7Ag5.7 FAUYb-Na3.5Ag3 FAUYc-Na3.5Ag3

340 340 340 340 340 340 340 340 340 340

EQE fresh and dry sample 0.03 0.16 0.04 0.12 0.01 0.10 0.33 0.43 0.41 0.56

± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.01 0.01 0.01 0.02 0.03 0.02 0.02 0.03

1 day 0.01 0.13 0.01 0.11 0.01 0.05 0.27 0.41 0.09 0.26

± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.01 0.02 0.01 0.01 0.04 0.01 0.01 0.03

1 week

1 month

0.08 ± 0.02

0.04 ± 0.01

0.05 ± 0.01

0.02 ± 0.01

0.04 0.25 0.41 0.09 0.23

± ± ± ± ±

0.01 0.02 0.02 0.02 0.02

0.02 0.25 0.40 0.04 0.18

± ± ± ± ±

0.01 0.02 0.02 0.02 0.03

a Intermediate silver loadings (green and yellow emitters). bSamples prepared using the conventional procedure. cUsing the reduction−oxidation procedure. Standard deviations were calculated based on an average of three measurements in three different batches (nine values in total) of the same sample composition. The range of luminescence collection was fixed between 380 and 720 nm.

Several factors, such as the heterogeneity of silver clusters and chemical environments typically encountered in zeolites, hamper a direct structure to function correlation. Furthermore, the sensitivity (in situ restructuring, increased mobility) of the clusters to radiation damage imposed by the different characterization techniques37 complicates establishing such relationship. When high silver loadings were employed for LTA zeolites, the EQE did not change dramatically compared with the low and intermediate loadings (Figure 4); however, a red shift in the emission could be observed when higher silver loadings were used. For FAU topology, the effect of silver content is more remarkable; at low and intermediate silver loadings, similar high EQE values were found, whereas at high loadings the EQE decreased ∼50% (Figure 4). This decrease in EQE at high loadings could be explained by the fact that the extra silver ions included in the zeolite cavities are not participating in the formation of the proper luminescent clusters but rather form species that absorb UV light without showing luminescence.32 The studied counterions in LTA zeolite and silicon to aluminum ratio in FAU topology do not seem to have a large impact on the EQE of the silver-loaded zeolites. In the case of the LTA topology, the presence of a divalent ion such as calcium triggers the preferential formation of red-emitting clusters (Figure 5), with EQE ranging from 1 to 4% (Table 1). When small monovalent ions are used (K and Na), similar EQEs were found; however, a blue shift is observed in the emission. For the FAU topology, the high aluminum content zeolite (FAU-X) resulted in a reduction of the EQE of 8%

Figure 4. External quantum efficiency of silver-clusters zeolite composites as a function of silver loading.

compared with the lower aluminum content FAU-Y. The aluminum content is an indication of the cation exchange capacity of the material; the higher the silicon to aluminum ratio the fewer counterions that can be accommodated in the framework of the zeolite. A difference of about twice the silicon to aluminum ratio is reflected here in a reduction of the EQE by 10%. This could indicate that smaller numbers of silver ions can lead to a more efficient formation of luminescent clusters, whereas for the case of FAUX, the extra silver ions added had a 7001

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reorganization of the silver clusters distribution inside the zeolite cavities takes place,30,31,43 favoring the selective formation or stabilization of the highly emissive cluster types. It has also been suggested that the environment during the heat treatment of silver-zeolite composites may affect both the degree of silver reduction and the proportion of silver ions remaining, thereby dictating the final silver nanoparticle size.44 It is known that water plays an important role in the stabilization of such silver clusters in zeolite cavities.16 However, depending on the zeolite topology and cocation, the effect can be positive or negative.17 To test the stability toward water of the optimized samples, the samples were left for a certain period in a high humidity environment (S4 Supporting Information). It was seen that independent of the exposure time the silver-clusters zeolite composites still kept their original luminescence with little variation in EQE compared with the heat-treated samples (Table 1). Comparison of the Luminescence Performance of the Silver-Loaded Zeolites with Related Materials. External quantum efficiencies reported in literature for luminescent silver clusters embedded in organic and inorganic matrices range between 3 and 64%.45 The higher values were obtained when DNA scaffolds were used as stabilizing agents. When polymers were employed to entrap the silver clusters, a maximum EQE value of 19% was found.45 The silver-clusters zeolite composites presented in this study showed EQE values in a similar range of 1−69%. DNA-stabilized silver clusters display high EQE, but their main application is focused to biological topics; on the other hand, polymer-embedded silver clusters that could be applicable as phosphors display a much lower EQE compared with the silver-clusters zeolite composites described in this report. EQE values reported for commercial phosphors used in lighting applications exceed 90%.21,46 Because it is shown that large EQE improvements can be achieved by controlling the purity of the zeolite host or by posttreatments such as reduction−oxidation cycles, the EQE values reported here (up to 69%) prove the high potential of these silver-clusters zeolite composites for competing with the commercial phosphors currently used.

Figure 5. Pictures of the heat-treated silver-loaded zeolites under day light and 366 nm illumination.

negative effect on the EQE, in which the creation of nonemitting silver species can be involved. Purity of the Starting Material and External Quantum Efficiency of Silver-Loaded Zeolites. The quality of the starting materials is of paramount importance for the production of oligoatomic silver clusters in the cavities of zeolites. Trace metals, such as iron, can be present (up to 0.05%) in some precursors of the zeolite synthesis, and this metal can compete with the silver ions and might favor the creation of nonluminescent centers. Iron impurities were already described in zeolites using electron paramagnetic resonance;16 two different peaks around g = 2 and 4.3 were assigned to iron impurities, which are typically present in commercial zeolites. The peak at g = 2 was tentatively assigned to iron in a spherical environment, whereas the signal at g = 4.3 probably represents iron in an asymmetric environment.38−40 As a preliminary test, we compared two different batches of the same silver-loaded zeolite sample with different optical purities (FAUY-Na3.5Ag3 and LTA-Na6Ag6). When using FAU and LTA with high optical purity (S3 Supporting Information), EQEs of 69 and 31% were found, whereas in the case of the less pure commercial starting material the EQEs observed were 41 and 4%, respectively. This small amount of impurities already accounts for a EQE difference of ∼30% for both topologies. This indicates that the “intrinsic” QE of the emissive species is much higher than the measured EQE values, which are heavily influenced by various nonemissive light-absorbing/quenching impurities. Optimization Procedure to Increase the Luminescence Performance of the Silver-Clusters Zeolite Composites. The highly luminescent silver-clusters zeolite composites showed a high photo and thermal stability; however, upon exposure to a high humidity environment, the emission efficiency decreased dramatically for certain samples.17 We performed a post-treatment procedure of the calcined samples to optimize the selective production of luminescent silver clusters or increase their stability toward water. The posttreatment is based on a reduction−oxidation procedure, which has been used in the catalysis field;41,42 in such treatment, the formation of large silver nanoparticles or small silver clusters was observed depending on the environment of gas and temperature.30,41−43 This strategy seems attractive for the manipulation of the formation of a preferential emitting silver cluster and therefore the optimization of silver clusters production in zeolite materials. The post-treated silver-loaded zeolites presented higher EQE and a better stability toward water, as can be seen in Table 1. This might be explained by the fact that during the reduction−oxidation treatment, a

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CONCLUSIONS In conclusion, we provide for the first time luminescence EQE values of silver-clusters zeolite composites. Such materials are promising candidates to use as phosphors in fluorescent lamps, as wavelength convertors for solar cells, and in information storage devices. The different factors that could affect the luminescent performance of these materials were also analyzed. A comparison of EQE between less pure commercial silverzeolites and self-synthesized silver-zeolites revealed the influence of the purity of the starting material on the EQE of oligoatomic luminescent silver clusters in zeolites. Knowing the luminescence performance of these materials allows us to objectively judge their potential in various applications and perform appropriate optimization to enhance their luminescent features. These new luminescent materials can be produced in a low-cost process, and their environmental impact could be reduced by recovering and recycling the silver included in the zeolites. However, further optimization of their luminescence performance is still necessary to bring these alternative and cheap phosphors to the commercial market. Such phosphors based on silver-clusters zeolite composites could thus have a high and realistic potential for applications in different fields. Further research is underway to reveal mechanisms of cluster 7002

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(7) Kanan, S. A.; Tripp, C. P.; Austin, R. N.; Patterson, H. H. Photoluminescence and Raman Spectroscopy as Probes to Investigate Silver and Gold Dycianide Clusters Doped in A-Zeolite and their Photoassisted Degradation of Carbaryl. J. Phys. Chem. B 2001, 105, 9441−9448. (8) Calzaferri, G.; Bruhwiler, D.; Glaus, S. Quantum-Sized Silver, Silver Chloride and Silver Sulfide Clusters. J. Imaging Sci. Technol. 2001, 45, 331−339. (9) Guerrero-Martinez, A.; Fibikar, S.; Pastoriza-Santos, I.; LizMarzan, L. M.; De Cola, L. Microcontainers with Fluorescent Anisotropic Zeolite L Cores and Isotropic Silica Shells. Angew. Chem., Int. Ed. 2009, 48, 1266−1270. (10) Tsotsalas, M.; Busby, M.; Gianolio, E.; Aime, S.; De Cola, L. Functionalized Nanocontainers as Dual Magnetic and Optical Probes for Molecular Imaging Applications. Chem. Mater. 2008, 20, 5888− 5893. (11) Jacobs, P. A.; Uytterhoeven, J. B.; Beyer, H. K. Cleavage of Water Over Zeolites. J. Chem. Soc. Comm. 1977, 128−129. (12) Kim, Y.; Seff, K. Structure of a Very Small Piece of Silver Metal. The Octahedral Ag6 Molecule. Two Crystal Structures of Partially Decomposed Vacuum-Dehydrated Fully Ag+-Exchanged Zeolite A. J. Am. Chem. Soc. 1977, 99, 7055−7057. (13) Kim, Y.; Seff, K. The Octahedral Hexasilver Molecule. Seven Crystal Structures of Variously Vacuum-Dehydrated Fully Ag+Exchanged Zeolite A. J. Am. Chem. Soc. 1978, 100, 6989−6997. (14) Liu, N.; Gong, M. Z.; Zhang, P.; Li, L. X.; Li, W. B.; Lee, R. Silver-Embedded Zeolite Crystals as Substrates for Surface Enhanced Raman Scattering. J. Mater. Sci. 2011, 46, 3162−3168. (15) De Cremer, G.; Antoku, Y.; Roeffaers, M. B. J.; Sliwa, M.; Van Noyen, J.; Smout, S.; Hofkens, J.; De Vos, D. E.; Sels, B. F.; Vosch, T. Photoactivation of Silver-Exchanged Zeolite A. Angew. Chem., Int. Ed. 2008, 47, 2813−2816. (16) De Cremer, G.; Coutino-Gonzalez, E.; Roeffaers, M. B. J.; Moens, B.; Ollevier, J.; Auweraer, M.; Schoonheydt, R.; Jacobs, P. A.; De Schryver, F.C..; Hofkens, J.; et al. Characterization of Fluorescence in Heat-Treated Silver-Exchanged Zeolites. J. Am. Chem. Soc. 2009, 131, 3049−3056. (17) De Cremer, G.; Coutino-Gonzalez, E.; Roeffaers, M. B. J.; De Vos, D. E.; Hofkens, J.; Vosch, T.; Sels, B. F. In Situ Observation of the Emission Characteristics of Zeolite-Hosted Silver Species During Heat-Treatment. ChemPhysChem 2010, 11, 1627−1631. (18) Vosch, T.; Sels, B. F.; Roeffaers, M. B. J.; Hofkens, J.; De Vos, D. E.; De Cremer, G. Emissive Lamps Comprising Metal Clusters Confined in Molecular Sieves. Patent Application BE2008/00050 (WO 2009006707), 2009. (19) Vosch, T.; Sels, B. F.; Roeffaers, M. B. J.; Hofkens, J.; De Vos, D. E.; De Cremer, G. Solar Cells. Patent Application BE2008/00051 (WO 2009006708), 2009. (20) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. An Improved Experimental Determination of External Photoluminescence Quantum Efficiency. Adv. Mater. 1997, 9, 230−232. (21) Rohwer, L. S.; Martin, J. E. Measuring the Absolute Quantum Efficiency of Luminescent Materials. J. Lumin. 2005, 115, 77−90. (22) Johnson, A. R.; Lee, S.-J.; Klein, J.; Kanicki, J. Absolute Photoluminescence Quantum Efficiency Measurement of LightEmitting Thin Films. Rev. Sci. Instrum. 2007, 78, 096101−1−3. (23) Katoh, R.; Suzuki, K.; Furube, A.; Kotani, M.; Tokumaru, K. Fluorescence Quantum Yield of Aromatic Hydrocarbons. J. Phys. Chem. C 2009, 113, 2961−2965. (24) Murase, N.; Li, C. L. Consistent Determination of Photoluminescence Quantum Efficiency for Phosphors in the Form of Solution, Plate, Thin Film, and Powder. J. Lumin. 2008, 128, 1896− 1903. (25) Nicolet, O.; Huber, S.; Lovey, C.; Chapellet, S.; Perrenoud, J.; Pauchard, M.; Ferrini, R.; Zuppiroli, L. Quantum Yield Measurement of Fluorescent Zeolite Nanopigments. Adv. Funct. Mater. 2009, 19, 1877−1883.

formation and to correlate the structure characterization of luminescent silver clusters in zeolite matrices to their emissive properties.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images, structural formula, and structures of the starting zeolite materials; description and results of the alternative validation EQE setup; DRS spectra; and water stability tests. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.S.) and johan.hofkens@ chem.kuleuven.be (J.H.). Present Address ▽

Gert De Cremer: DSM Ahead, Urmonderbaan 22, 6167-RD Geleen, The Netherlands. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The majority of the experiments were carried out by Eduardo Coutino-Gonzalez. The experiments were conceived by Eduardo Coutino-Gonzalez, Maarten Roeffaers, Gert De Cremer, Bert Sels, and Johan Hofkens. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed within the framework of the IAP-7 program of the Belgian Federal government (Belspo). The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/ 2007-2013 under grant agreement n° 310651), from the Flemish government in the form of long-term structural funding “Methusalem” grant METH/08/04 CASAS, from the ‘Fonds voor Wetenschapplijk Onderzoek Vlaanderen’ (FWO grants G.0402.09, G0603.10. and G.0990.11), from the Hercules Foundation (HER/08/021), and from the Flemish ‘Strategisch Initiatief Materialen’ SoPPoM program.

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