New Insights into CdS Quantum Dots in ZeoliteâY - The Journal of

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J. Phys. Chem. C 2007, 111, 10298-10312

New Insights into CdS Quantum Dots in Zeolite-Y Nak Cheon Jeong, Hyun Sung Kim, and Kyung Byung Yoon* Center for Microcrystal Assembly, Center for Nanoporous Materials, Department of Chemistry, and Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Korea ReceiVed: January 6, 2007; In Final Form: April 22, 2007

When dry Cd2+-exchanged zeolites Y are exposed to dry H2S under a rigorously anhydrous condition, CdS quantum dots (QDs) are formed in the supercages of zeolite-Y regardless of the loading levels of CdS from 0.01% to 32% and regardless of the Si/Al ratio of zeolite-Y between 1.8 and 2.5. The absorptions with the maximums (λmax) e 290 nm are assigned as those arising from isolated CdS QDs with the sizes smaller than or equal to the size of a supercage (1.3 nm); the absorptions with λmax between 290 and 380 nm are assigned as those arising from interconnected CdS QDs that were formed by the interconnection of isolated CdS QDs through the supercage windows; and the absorptions with λmax > 400 nm are assigned as those arising from mesosized (3-10 nm) CdS QDs residing in or on the surfaces of amorphous aluminosilicate. The H+ ions alone, which are generated during the formation of CdS, do not destruct the zeolite-Y framework causing the formation of amorphous aluminosilicate. Instead, the water-induced agglomeration of isolated and interconnected CdS QDs to mesosized CdS QDs in the presence of H+ ions leads to the destruction of the zeolite-Y framework. The size of the interconnected CdS QD which is formed by moisture adsorption increases as the loaded amount of CdS increases for a given zeolite and as the size of the zeolite host increases. The presence of a tetraethylammonium ion in each supercage not only gives rise to the formation of very small QDs within zeolites Y but also prevents the zeolite framework from destruction.

Introduction Preparation of quantum dots (QDs) in high monodispersity, organization of them into two- (2D) and three-dimensional (3D) arrays, and stabilization of the highly unstable species for longterm applications in the atmosphere are important issues in nanoscience. Zeolites have been regarded as the ideal hosts for the generation of highly uniform QDs with sizes smaller than 1.5 nm, the organization of them into regularly spaced 3D arrays, the prevention of them from aggregation into larger particles, and the protection of them from the attack by water and other reactive compounds on the basis of the following. While it is difficult to make QDs with the sizes smaller than 1.5 nm in solution, it is rather difficult to make QDs with the sizes larger than 1.5 nm in zeolites because of the pore size limitation. The zeolite pores are highly uniform in size and shape. The pores exist in regular intervals within the crystalline frameworks. The pores can be kept anhydrous by coating the surface with long hydrocarbon chains.1 In this respect, the works of Herron et al.,2-9 Stucky and Mac Dougall,10 Thomas and Liu,11 Mallouk et al.,12 Calzaferri et al.,13-18 Terasaki et al.,19-21 Fox and Pettit,22 Ozin et al.,23 and others24-31 on the generation of various QDs within various zeolites, their characterization, and the investigation of their physicochemical properties conducted during the 1980s and 1990s should be evaluated as the pioneering works that foresaw the future. Although Fox and Pettit reported the application of zeoliteencapsulated CdS as the photocatalysts for water splitting22 and our group32 and Sugimoto et al.31 investigated third-order nonlinear optical responses of QD-incorporating zeolites, the zeolite-encapsulated QDs have not yet been fully exploited for * Corresponding author. E-mail: [email protected].

practical applications despite their great potential. In this respect, efforts should be directed at exploring practical applications of QD-incorporating zeolites. For this, the deep understanding of the natures, locations within the zeolite frameworks, and the optical properties of the zeolite-encapsulated QDs will be an important guideline. Among various QDs encapsulated in zeolites, understanding the natures, locations, and the optical properties of the CdS QDs in zeolite-Y is most important because they are the first QDs that were generated within zeolites,2-5 and this work triggered many related researches afterward. In the case of CdS QDs in zeolite-Y, however, controversies still exist over the location of the QDs within the zeolite structure (Chart 1) and the origin of the ∼350 nm band. Wang and coworkers first proposed that CdS QDs exist in the form of (CdS)4 within each sodalite cage (denoted as [(CdS)4]sod) until when the loading level of CdS is smaller than 23% (Chart 2A).2-5 This model was reviewed by Herron7,8 and Stucky and Mac Dougall10 in several review articles as if it was firmly established. Jentys et al.24 even performed an ab initio calculation on the stability of [(CdS)4]sod by adopting its geometry. Wang and co-workers27 concluded that CdS QDs exist within the sodalite cages of zeolite-Y on the basis of a positron annihilation study. However, they produced CdS by stirring Cd2+-exchanged zeolite-Y in an aqueous solution of Na2S, despite the well-known fact that moisture sensitively affects the nature and size of CdS QD (vide infra), indicating that they prepared the samples inappropriately. Ozin and co-workers23 cited [(CdS)4]sod as such in their report on the preparation of II-VI QDs within zeolites by metal organic chemistry vapor deposition (MOCVD) technique. Sugimoto et al.31 also adopted [(CdS)4]sod as the CdS QDs during their measurements of the

10.1021/jp070107+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007

CdS Quantum Dots in Zeolite-Y CHART 1: Structures of Zeolite-Y (A) and Its Supercage (B).

CHART 2: Illustration of the Zeolite-Y Incorporating a (CdS)4 Unit in Each Sodalite Cage (A) and the Zeolite-Y Incorporating a Larger CdS QD in Each Supercage (B).

nonresonant third-order nonlinear optical properties of CdS incorporating zeolites A and X dispersed in a refractive-index matching fluid. Thomas and Liu,11 however, proposed that CdS QDs exist within supercages (Chart 2B), and when their sizes are larger than that of a supercage, they exist in the intracrystalline voids created by destruction of several supercages. Fox and Pettit22 proposed that CdS QDs exist as individual particles within supercages of zeolite-Y when the loading level is below 3-4% and as three-dimensionally interconnected agglomerates by interconnection of individual QDs through the supercage windows at higher loading levels. Wark et al.30 proposed that the produced CdS QDs exist within supercages and large CdS QDs are formed by coalescence of small QDs within the mesopores that were created by framework collapse during the formation of large QDs. Regarding the origin of ∼350-nm band, Wang et al. and Stucky and Mac Dougall ascribed the absorption band to the “through-zeolite-framework” interaction of the neighboring [(CdS)4]sod units.2-5,10 However, Thomas and Liu ascribed the absorption band merely to the CdS QDs residing within supercages.11 We now report that CdS QDs in zeolite-Y exist within supercages regardless of the loading levels of CdS from 0.01 to ∼35% and regardless of the Si/Al ratio between 1.8 and 2.5, and the ∼350 nm bands arise because of the interconnected CdS QDs through the supercage windows without breaking the framework. We also clarify the nature of CdS QDs and their absorption bands.

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10299 Experimental Section Materials. Tetraethyl orthosilicate (TEOS, Acros), aluminum isopropoxide [Al(iPrO)3, Aldrich], tetramethylammonium hydroxide (TMAOH, 25% aqueous solution, Aldrich), tetraethylammonium bromide (TEABr, Aldrich), sodium silicate (Na2SiO3, 17-19% of Na2O and 35-38% of SiO2, Kanto), sodium aluminate (NaAlO2, 31-35% of Na2O and 34-39% of Al2O3, Kanto), sodium hydroxide (NaOH, Samchun), cadmium nitrate tetrahydrate [Cd(NO3)2‚4H2O, Junsei], and hydrogen sulfide (H2S, Regas) were used as received. Octadecyltrichlorosilane (ODC) and octadecyltrimethoxysilane (ODM) were purchased from Aldrich and used after purification by vacuum distillation. Commercial zeolite-Y with the Si/Al ratio of 2.5 was the product of UOP (lot no. 1997060001). This zeolite was calcined at 550 °C for 24 h. The calcined zeolite was stirred in a 1 M NaCl solution overnight and washed with copious amounts of distilled deionized water until the wash was free from chloride. After washing with methanol, the zeolite was dried in vacuum. This zeolite is denoted as Y2.5 to emphasize its Si/Al ratio. Synthesis of Na+-Exchanged Zeolite-Y Having 0.7 TMA+ Ions in Each Supercage and 1.0 TMA+ Ion in Each Sodalite Cage. This zeolite is designated as [Na(TMA)sup(TMA)sod]Y1.8, where (TMA)sup and (TMA)sod represent TMA+ ions residing in a supercage and a sodalite cage, respectively, and the subscript 1.8 denotes the Si/Al ratio of the zeolite. The average size of the crystals was 80 nm (apex-to-apex length). Accordingly, this zeolite is also referred to as Y0.08µ when the zeolite size is concerned. The above zeolite was synthesized by modification of the literature procedures.33-35 The details of the procedure are described in Supporting Information (SI 1). The chemical composition of the above zeolite is shown in SI 2, and the scanning electron microscope (SEM) image is shown in Figure 1A. The numbers of (TMA)sup and (TMA)sod ions per unit cell (per eight supercages or eight sodalite cages) were determined by thermogravimetric analyses (TGA; vide infra). Preparation of Zeolites Y with Larger Sizes. We also synthesized zeolites Y with the average apex-to-apex lengths of 2, 4, 8, and 28 µm, respectively, which are denoted as Y2µ, Y4µ, Y8µ, and Y28µ, respectively. The fully Na+-exchanged forms are denoted as [Naf]-Yxµ, where x ) 2, 4, 8, and 28, respectively (see SI 2). The procedures for the preparation of [Naf]-Yxµ (x ) 2, 4, 8, and 28) are entirely different from that of [Naf]-Y0.08µ. The mole ratios for Al2O3:SiO2:Na2O:H2O employed for the syntheses of the above zeolites are listed in Table 1. The general procedure is described as follows. NaAlO2 (7 g) was dissolved in distilled deionized water (p g). To this transparent aqueous solution, a dilute silica solution consisting of Na2SiO3 (q g) and distilled deionized water (r g) was added with vigorous stirring. For the synthesis of [Naf]-Y8µ, the NaOH solution consisting of NaOH (16 g) and distilled deionized water (100 g) was additionally added to the initially prepared synthesis gel after stirring for 1 h. For the synthesis of [Naf]-Y28µ, the NaOH solution consisting of NaOH (4.0 g) and distilled deionized water (30 g) was additionally added to the corresponding initially prepared synthesis gel after stirring it for 1 h. The gel was aged for 10 h at room temperature with continuous stirring. Each synthesis gel was then transferred into a polypropylene bottle and heated at 100 °C for 24 h for [Naf]Yxµ(x ) 2, 4, and 8) and at 90 °C for 48 h for [Naf]-Y28µ. The zeolite crystals were collected by filtration and washed with copious amounts of distilled deionized water. Their chemical compositions are listed in SI 2 and their SEM images are shown in Figure 1B-E.

10300 J. Phys. Chem. C, Vol. 111, No. 28, 2007

Jeong et al.

Figure 1. SEM images of Yxµ, x ) 0.08 (A), 2 (B), 4 (C), 8 (D), and 28 (E).

TABLE 1: Mole Ratios of Al2O3:SiO2:Na2O:H2O and p, q, and r Values in the Synthesis Procedures 2µ

[Naf]-Y [Naf]-Y4µ [Naf]-Y8µ [Naf]-Y28µ a

mole ratio

pa

qa

ra

1:8:5.3:167 1:8:5.3:500 1:10:20:700 1:3.4:5.1:400

50 150 150 100

33 33 41 14.4

20 60 100 50

In grams.

Preparation of Na+-Exchanged Zeolite-Y1.8 Having 1.0 TMA Ion in Each Sodalite Cage. This zeolite is designated as [Na(TMA)sod]-Y1.8. This zeolite was prepared by ion exchange of (TMA)sup ions in [Na(TMA)sup(TMA)sod]-Y1.8 with Na+. For this, [Na(TMA)sup(TMA)sod]-Y1.8 (1.0 g) was introduced into a saturated aqueous NaCl solution (100 mL), and the mixture was stirred for 24 h. The above procedure was repeated three times. The chemical composition of [Na(TMA)sod]-Y1.8 is shown in SI 2. Preparation of Fully Na+-Exchanged Zeolite-Y1.8 ([Naf]Y1.8). [Na(TMA)sup(TMA)sod]-Y1.8 was calcined at 550 °C for 12 h to produce zeolite-Y1.8 having both Na+ and H+ as the charge-balancing cations. The above zeolite is designated as [Na53H14]-Y1.8 on the basis of the chemical composition listed in SI 2. After cooling to room temperature, the ion exchange of H+ in the zeolite with Na+ was carried out by repeating a standard ion-exchange procedure three times using [Na53H14]-Y1.8 (1.0 g) and saturated NaCl solution (100 mL). The chemical composition of the resulting [Naf]-Y1.8 is shown in SI 2.

Preparation of Cd2+-Exchanged Y1.8 Having 1.0 TMA Ion in Each Sodalite Cage {[Cdn(TMA)sod]-Y1.8, n ) 0.01, 0.1, 1, 6, 12, and 30}. The details of the synthetic procedures are described in SI 1. The chemical compositions of the resulting [Cdn(TMA)sod]-Y1.8 are shown in SI 2. Preparation of Cd2+-Exchanged, Organic Cation-Free Y1.8 {[Cdn]-Y1.8, n ) 0.01, 0.1, 1, 6, 12, and 34}. The preparation of [Cdn]-Y1.8 zeolites with six different degrees of Cd2+ exchange was similarly carried out from [Naf]-Y1.8. The chemical compositions of the above zeolites are listed in SI 2. Preparation of Cd2+-Exchanged Y2.5 {[Cdn]-Y2.5 n ) 0.01, 0.1, 1, 6, 12, and 27}. The preparation of [Cdn]-Y2.5 zeolites with six different degrees of Cd2+ exchange was similarly carried out from [Naf]-Y2.5. The chemical compositions of the above zeolites are listed in SI 2. Preparation of Cd2+-Exchanged Y1.8 Having 1.0 TEA Ion in Each Supercage. {[Cdn(TEA)sup]-Y1.8, n ) 0.01, 0.1, 1, 6, 12, and 30}. Six 250 mL flasks containing 20 mL of aqueous 0.3 M TEABr solution were prepared, and into each flask was added 0.3 g of each [Cdn]-Y1.8, n ) 0.01, 0.1, 1, 6, 12, and 34. Subsequently, the flasks were shaken for 24 h at room temperature. The Cd2+- and TEA+-exchanged zeolite-Y1.8 crystals were separated from the supernatant solutions by centrifugation at 9000 rpm and washed with copious amounts of distilled deionized water until the washes were free from bromide. The chemical compositions of resulting [Cdn(TEA)sup]-Y1.8 are shown in SI 2. Preparation of Cd2+-Exchanged Y2.5 Having 1.0 TEA Ion in Each Supercage. {[Cdn(TEA)sup]-Y2.5, n ) 0.01, 0.1, 1, 6, 12, and 23}. The preparation of [Cdn(TEA)sup]-Y2.5 with six different degrees of Cd2+ exchange was similarly carried out from [Cdn]-Y2.5, n ) 0.01, 0.1, 1, 6, 12, and 27. The chemical compositions of the above zeolites are listed in SI 2. Formation of CdS QDs Within Zeolites. For instance, each [Cdn(TMA)sod]-Y1.8 (0.3 g) was transferred into a tubular flask equipped with a greaseless stopcock, and the zeolite crystals were dehydrated by heating the flask at 250 °C for 5 h after connecting it to vacuum (10-5 Torr). H2S gas (760 Torr) was then introduced into the flask, and the interaction of H2S with Cd2+ ions was allowed for 1 h at room temperature. Excess H2S was removed by evacuation for 2 h at room temperature. The chemical compositions of the above zeolites are listed in SI 2. On the basis of their chemical compositions, they are designated as [(CdS)n(TMA)sod]-Y1.8. The preparation of [(CdS)n]-Y1.8, [(CdS)n]-Y2.5, [(CdS)n(TEA)sup]-Y1.8, and [(CdS)n(TEA)sup]-Y2.5 were carried out similarly. Their chemical compositions are listed in SI 2. Preparation of ODC-Treated Zeolites. To investigate the effect of an anhydrous environment on the 13C NMR chemical shift of the (TMA)sup and (TMA)sod ions, we prepared dry, ODCtreated [Na(TMA)sup(TMA)sod]-Y1.8, [Na(TMA)sod]-Y1.8, [Cd30(TMA)sod]-Y1.8, and [(CdS)30(TMA)sod]-Y1.8, respectively, by treating each dried zeolite with ODC in the glovebox as described in our previous report.1 They are designated as ODC[Na(TMA)sup(TMA)sod]-Y1.8, ODC-[Na(TMA)sod]-Y1.8, ODC[Cd30(TMA)sod]-Y1.8, and ODC-[(CdS)30(TMA)sod]-Y1.8, respectively. The preparation of ODC-treated, TEA-incorporating zeolites-Y, namely, [Cd30(TEA)sup]-Y1.8, [(CdS)30(TEA)sup]Y1.8, [Cd23(TEA)sup]-Y2.5, and [(CdS)23(TEA)sup]-Y2.5 was carried out similarly. They are denoted as ODC-[Cd30(TEA)sup]Y1.8, ODC-[(CdS)30(TEA)sup]-Y1.8, ODC-[Cd23(TEA)sup]-Y2.5, and ODC-[(CdS)23(TEA)sup]-Y2.5, respectively.

CdS Quantum Dots in Zeolite-Y

Figure 2. Thermogravimetric and differential thermogravimetric analyses of [Naf]-Y1.8, [Na(TMA)sup(TMA)sod]-Y1.8 and [Na(TMA)sod]-Y1.8 (A and B) and [Cd30(TEA)sup]-Y1.8 and [Cd23(TEA)sup]Y2.5 (C and D).

Measurements of the Amounts of Moisture Adsorbed into CdS-Incorporating Zeolites. Dry CdS-incorporating zeolite (∼50 mg) was spread on a light, shallow, plastic container designed for weight measurement (40 × 40 mm2), and the container was placed on top of a microbalance, then the weight was quickly measured after a desired period of time. The zeolite powder was then quickly transferred into a flat, round, fused silica cell (inner diameter ) 19 mm), and the spectrum of the moist sample was measured immediately on a UV-vis spectrophotometer. The time interval was about 3 min. Each measurement was conducted with fresh dry CdS-incorporating zeolite. Instrumentation. The diffuse-reflectance UV-vis spectra of the samples were recorded on a Varian Cary 5000 UV-visNIR spectrophotometer equipped with an integrating sphere. Barium sulfate was used as the reference. The diffuse reflectance spectra were converted into the Kubelka-Munk (K/M) formalism. The cross-polarized, magic angle spinning (CP-MAS) 13C NMR spectra of the TMA+- or TEA+-incorporating zeolites were recorded on a Bruker Analytische GmbH solid-state Fourier transform (FT)-NMR spectrometer (DSX 400 MHz) installed in the Korea Basic Science Institute located in Kyungpook National University. The chemical shifts were assigned with respect to a 13C NMR peak of adamantane, by fixing its left, more intense peak value at 38.30 ppm. The instrumentation for scanning electron microscope (SEM) images, elemental analyses, X-ray powder diffraction patterns of the samples, thermogravimetric analyses (TGA) of the zeolite-Y samples, and transmission electron microscope (TEM) images of zeolites are described in SI 1. Results and Discussion Analyses of TMA+ Ions in Zeolites-Y1.8 with TGA. The TGA thermograms of [Naf]-Y1.8, [Na(TMA)sup(TMA)sod]-Y1.8, and [Na(TMA)sod]-Y1.8 (see SI 2 for compositions) are shown in Figure 2. [Naf]-Y1.8 showed 19% weight loss due to dehydration in the 30-150 °C region (dotted curve in Figure 2A). The dehydration continued until the temperature reached 350 °C and the amount of water loss in the 150350 °C region corresponded to 1.5% of the total weight. This

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10301 indicates that the total dehydrated amount in the 30-350 °C region was 20.5%. The above phenomenon is more clearly demonstrated by the differential thermogram shown in Figure 2B (dotted curve). Thus, the differential thermogram shows a sharp peak in the 30-150 °C region and a tail which extends to 350 °C. This shows that the major water loss (∼93%) occurs in the 30-150 °C region and the residual water loss (∼7%) occurs in the 150-350 °C region. As noted, dehydration is the only weight loss event in the case of [Naf]-Y1.8 in the 30-670 °C region. [Na(TMA)sup(TMA)sod]-Y1.8 showed three clearly distinguished weight loss events at 30-300, 300-410, and 510670 °C regions (dashed curve in Figure 2A,B). They represent dehydration, combustion of (TMA)sup, and combustion of (TMA)sod ions, respectively, consistent with the literature reports.34-36 The corresponding amounts of weight losses were 17%, 2.7%, and 3.3%, respectively. The amount of water loss from [Na(TMA)sup(TMA)sod]-Y1.8 (17%) being lower than that from [Naf]-Y1.8 (20.5%) is ascribed to the presence of TMA+ ions in the supercages and sodalite cages, which gives rise to the diminution of the cage space available for water adsorption. The above thermograms thus unambiguously demonstrate the presence of both (TMA)sup and (TMA)sod ions within [Na(TMA)sup(TMA)sod]-Y1.8. The calculated numbers of (TMA)sup and (TMA)sod ions per supercage and sodalite cage, respectively, were 0.7 and 1.0, respectively. The above result confirms that the (TMA)sod-containing zeolites-Y1.8 prepared under our synthetic condition contain one TMA+ ion in each sodalite cage. The thermogram of [Na(TMA)sod]-Y1.8 showed only two weight losses at 30-350 and 550-670 °C regions (solid curve in Figure 2A,B), because of dehydration and combustion of (TMA)sod ions, respectively. This shows that (TMA)sup ions were readily and completely removed from the supercages by ionexchange with Na+ ions while (TMA)sod ions remain intact. The corresponding amounts of weight loss were 20% and 3%, respectively. This result also shows that [Na(TMA)sod]-Y1.8 contains one TMA+ ion in each sodalite cage. Analyses of TEA+-Incorporating Zeolites-Y1.8 and-Y2.5 with TGA. The TGA thermogram of [Cd30(TEA)sup]-Y1.8 (see SI 2 for composition) showed three clearly distinguished weight loss events at 30-140, 140-310, and 310-450 °C regions (solid curve in Figure 2C). The first 11.4% weight loss at 30140 °C is ascribed to dehydration. The dehydration continued until the temperature reached 310 °C and the amount of water loss in the 140-310 °C region corresponded to 6.1% of the total weight. Thus, the total dehydrated amount in the region of 30-310°C was 17.5%. The 3.9% weight loss in the 310430 °C region is ascribed to the combustion of (TEA)sup from the analogy of the combustion of (TMA)sup in the similar temperature region (300-410 °C). The above phenomenon is more clearly demonstrated by the differential thermogram shown in Figure 2D (solid curve). The thermograms thus unambiguously demonstrate the presence of (TEA)sup within [Cd30(TEA)sup]-Y1.8. The calculated number of (TEA)sup ions per supercage was 0.97. Thus, the above result unambiguously shows that ∼1.0 TEA+ ion exists in each supercage of [Cd30(TEA)sup]-Y1.8. Similarly, the thermogram (dashed curve in Figure 2C) and the differential thermogram (dashed curve in Figure 2D) of [Cd23(TEA)sup]-Y2.5 showed water loss events at 30-140 °C and 140-310 °C, respectively, and combustion of (TEA)sup ions at the 310-460 °C region. The calculated number of (TEA)sup ions per supercage was 0.98. Thus, [Cd23(TEA)sup]-Y2.5 contains ∼1.0 TEA+ ion in each supercage.


Jeong et al.

Figure 3. MAS CP solid state 13C NMR spectra of various zeolites (as indicated).

Analyses of TMA+ Ions in Hydrated and Dry TMA+Containing Zeolites by Solid-State 13C NMR: Evidence I That CdS QDs Exist in Supercages. We also investigated (TMA)sup and (TMA)sod ions with CP-MAS solid-state 13C NMR spectroscopy. The pristine, hydrated [Na(TMA)sup(TMA)sod]Y1.8 gave two 13C resonances at 58.3 and 56.3 ppm because of (TMA)sod and (TMA)sup ions, respectively (Figure 3A, bottom). Although the observed chemical shifts are slightly smaller than those observed by Hayashi et al. (58.7 and 56.7 ppm, respectively),37 their difference between the two peaks (2.0 ppm) coincides with ours. The above result further shows that the 13C NMR peak of TMA+ ion shifts to downfield upon increasing the degree of confinement onto the ion (from supercage to sodalite cage). The 13C NMR spectrum of the hydrated, Na+-exchanged [Na(TMA)sod]-Y1.8 (Figure 3A, second from the bottom) clearly showed the absence of the resonance due to the (TMA)sup ion (at 56.3 ppm), indicating that (TMA)sup ions were readily removed by ion-exchange with Na+, consistent with the TGA results described in the previous section (vide supra). Likewise, the hydrated, Cd2+-exchanged [Cd30(TMA)sod]-Y1.8 (Figure 3A, third from the bottom) also gave a 13C NMR spectrum that is essentially the same with that of hydrated [Na(TMA)sod]-Y1.8, demonstrating that the ion-exchange of (TMA)sup by Cd2+ also occurs readily. Interestingly, hydrated [(CdS)30(TMA)sod]-Y1.8 (Figure 3A, top) gave two peaks at 58.1 and 56.5 ppm. The ratio of their peak areas was 7:3. Coupled with the X-ray powder diffraction pattern of hydrated [(CdS)30(TMA)sod]-Y1.8 (disappearance of the diffraction lines of zeolite-Y) and the TEM images

(disappearance of the zeolite-Y lattice; vide infra), we conclude that the upfield shift of (TMA)sod from 58.3 to 58.1 indicates that ∼70% of the TMA+-incorporating sodalite cages underwent partial structural collapses leading to a slight increase in the space surrounding the TMA+ ion, and the upfield shift to 56.5 ppm indicates that ∼30% of the TMA+-incorporating sodalite cages underwent severe structural collapses leading to a large increase in the space surrounding the TMA+ ion. The size of the newly produced larger space seems to be larger than that of a sodalite cage but slightly smaller than that of a supercage. We showed that the coating of zeolites with ODC (octadecyltrichlorosilane) effectively protects the interiors of zeolites from water adsorption.1 Accordingly, as a means to obtain the chemical shifts of the 13C NMR spectra of (TMA)sup and (TMA)sod under the dry conditions, we coated the surfaces of [Na(TMA)sup(TMA)sod]-Y1.8, [Na(TMA)sod]-Y1.8, [Cd30(TMA)sod]-Y1.8, and [(CdS)30(TMA)sod]-Y1.8 with ODC in a drybox and obtained their 13C NMR spectra as shown in Figure 3B. There are additional resonances at ∼33, ∼23, and ∼14 ppm (Supporting Information, SI 3) arising from the surface-tethered OD groups (see ref 1) in addition to the peaks of TMA+ ions at ∼56-58 ppm. The chemical shifts of (TMA)sup and (TMA)sod ions of ODC[Na(TMA)sup(TMA)sod]-Y1.8 appeared at 56.9 and 58.1 ppm, respectively (Figure 3B, bottom). As noticed, the chemical shift of dry (TMA)sup (56.9 ppm) shifted downfield by 0.6 ppm with respect to that of hydrated (TMA)sup (56.3 ppm, Figure 3A, bottom). From the phenomenon that the chemical shift of TMA+ shifts downfield as the degree of confinement increases (vide

CdS Quantum Dots in Zeolite-Y SCHEME 1: Schematic Illustration of the Movements and Positions of H2O Molecules, Cd2+ Ions, and CdS QDs During Dehydration (A, B) and CdS Formation (C)

supra), we ascribe the large downfield shift to the attachment of (TMA)sup ions to the supercage wall, which is a kind of partial confinement, as a result of dehydration. Furthermore, the peak width of (TMA)sup in the anhydrous condition is much broader than when it is in the hydrated zeolite. This phenomenon is ascribed to the heterogeneity of the contact sites of the supercage wall (such as the S6R window to the sodalite cage, the S4R window to D6R, and the entrance to the supercage) for TMA+ ions and to the variation of the orientations of TMA+ ions on the same contact site. In contrast to the chemical shift of (TMA)sup, those of (TMA)sod in ODC-[Na(TMA)sup(TMA)sod]-Y1.8 and ODC-[Na(TMA)sod]-Y1.8 (58.1 ppm, Figure 3B, bottom and second from the bottom) repeatedly shifted upfield by 0.2 ppm with respect to those of (TMA)sod ions in the hydrated (TMA)sod (58.3 ppm). Although small, the upfield shift clearly indicates that the space within the sodalite cage increases to some extent upon removal of water from the zeolites. This indicates that either a few water molecules coexist in a sodalite cage with a TMA+ ion or a few water molecules partially intrude into the TMA+-incorporating sodalite cages, leading to a higher degree of confinement within sodalite cages (Scheme 1A, left). Therefore, we ascribe the upfield shift of dry (TMA)sod in ODC-[Na(TMA)sup(TMA)sod]Y1.8 and ODC-[Na(TMA)sod]-Y1.8 with respect to those in the hydrated counterparts to the removal of water from the TMA+occupying sodalite cages (Scheme 1A). The chemical shift of the (TMA)sod ion in ODC-[Cd30(TMA)sod]-Y1.8 (Figure 3B, third from the bottom, 58.4 ppm) shifted downfield by 0.3 ppm with respect to those in ODC[Na(TMA)sup(TMA)sod]-Y1.8 and ODC-[Na(TMA)sod]-Y1.8 (58.1 ppm). We ascribe the above phenomenon to the migration of Cd2+ ions into the sodalite cage during dehydration and to the occupation of II′ sites of the sodalite cage as illustrated in Scheme 1B, consistent with the result of McCusker and Seff.38

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10303 Upon formation of CdS, the chemical shift of (TMA)sod in ODC-[(CdS)30(TMA)sod]-Y1.8 (Figure 3B, top) shifted upfield by 0.3 ppm with respect to that of (TMA)sod in ODC-[Cd30(TMA)sod]-Y1.8. This serves as direct evidence that Cd2+ ions migrate from sodalite cages (II′ site) back to supercages in the dry state when H2S molecules are introduced into supercages as illustrated in Scheme 1C. In contrast to our observation, Thomas and Liu11 proposed that the Cd2+ ions located within sodalite cages do not migrate into supercages when the number of Cd2+ ions in the unit cell exceeds 19. However, our results clearly show that Cd2+ ions readily migrate from sodalite cages back to supercages even when the number of Cd2+ ions (30) exceed 19 by 11, in the presence of H2S molecules. Our report thus provides insights into the dynamics of CdS formation and provides strong evidence that CdS QDs are formed in supercages (Chart 2B) rather than in sodalite cages (Chart 2A). The nonappearance of another peak at ∼56.5 ppm in the case of ODC-[(CdS)30(TMA)sod]-Y1.8, unlike the hydrated [(CdS)30(TMA)sod]-Y1.8 (Figure 3A, top), indicates that the generated H+ ions alone during the formation of CdS QDs do not lead to the destruction of the framework. The fact that the chemical shift of (TMA)sod in ODC-[(CdS)30(TMA)sod]-Y1.8 (Figure 3B, top) is the same with those of ODC[Na(TMA)sup(TMA)sod]-Y1.8 (Figure 3B, bottom) and ODC[Na(TMA)sod]-Y1.8 (Figure 3B, second from the bottom) further confirms that CdS QDs do not form in the sodalite cages of Y1.8. Otherwise, the chemical shift should have shifted significantly downfield. We therefore conclude that the earlier conclusion made by Herron and co-workers2-5 which states that CdS QDs exist within each sodalite cage in the form of [(CdS)4]sod is no longer valid. Analyses of TEA+ Ions in Hydrated and Dry TEA+Containing Zeolites by CP-MAS Solid-State 13C NMR: Evidence II That CdS QDs Exist in Supercages. We also obtained the CP-MAS solid-state 13C NMR spectra of TEA+containing zeolites-Y1.8 and -Y2.5. Figure 3C shows the methylene carbon peaks of TEA+ in hydrated and ODC-coated [Cd30(TEA)sup]-Y1.8, hydrated and ODC-coated [(CdS)30(TEA)sup]-Y1.8, respectively, and Figure 3D shows the corresponding methylene carbon peaks in Y2.5 in the 50-55 ppm region. Their full spectra are shown in Supporting Information (SI 4). Whereas the chemical shift of the methylene carbon of TEA+ in hydrated [Cd30(TEA)sup]-Y1.8 is 52.5 ppm (Figure 3C, bottom), that of TEA+ ion in ODC-[Cd30(TEA)sup]-Y1.8 is 52.9 ppm (Figure 3C, second from the bottom), showing a 0.4 ppm downfield shift. Again, we ascribe this phenomenon to the tight attachment of the TEA+ ion to the supercage wall, which corresponds to the increase in the degree of confinement of the organic cation. The methylene peak of TEA+ further shifted downfield by 0.2 ppm (53.1 ppm) in hydrated [(CdS)30(TEA)sup]-Y1.8 (Figure 3C, third from the bottom) with respect to that of ODC-[Cd30(TEA)sup]-Y1.8. We ascribe this phenomenon to the increase in the degree of confinement of the TEA+ ions in supercages caused by the water-induced aggregation of the CdS QDs. The red shift of λmax from 282 to 305 nm in the UV-vis spectrum (see next sections) supports that CdS QDs indeed aggregate into larger QDs. This result again serves as evidence that CdS QDs are formed inside the supercages since the formation of the CdS QDs within the sodalite cages would not affect the chemical shift of the methylene carbon of (TEA)sup. Interestingly, unlike the case of hydrated [(CdS)30(TEA)sup]Y1.8 (Figure 3C, third from the bottom), the chemical shift of the methylene group of ODC-[(CdS)30(TEA)sup]-Y1.8 did not

10304 J. Phys. Chem. C, Vol. 111, No. 28, 2007 experience a downfield shift with respect to that of ODC-[Cd30(TEA)sup]-Y1.8. Coupled with the UV-vis spectral analyses of the zeolite (vide infra), we believe the absence of downfield shifts occurs because the sizes of the CdS QDs formed inside the supercages under the dry condition are not big enough to “push” the (TEA)sup ions onto the framework wall. This may allow us to estimate the size of the QDs. The 13C NMR spectra of the TEA+ methylene carbon in ODC-treated and hydrated zeolites-Y2.5 (Figure 3D) were identical with the corresponding spectra in ODC-treated and hydrated zeolites-Y1.8 (Figure 3C), despite the fact that their Si/Al ratios are higher, indicating that the difference in the Si/ Al ratio (1.8 vs 2.5) does not affect the behavior of CdS formation and the location of CdS (in supercages). Recall that in the case of hydrated [(CdS)30(TMA)sod]-Y1.8 (Figure 3A, top) the chemical shift of (TMA)sod shifted upfield with respect to that of hydrated [Cd30(TMA)sod]-Y1.8 (from 58.3 to 58.1 and 56.5 ppm) because of framework collapses (vide supra). Interestingly, however, in the cases of hydrated [(CdS)30(TEA)sup]-Y1.8 (Figure 3C, second from the top) and hydrated [(CdS)23(TEA)sup]-Y2.5 (Figure 3D, second from the top), the chemical shifts of (TEA)sup shifted downfield rather than upfield with respect to the corresponding hydrated [Cdn(TEA)sup]-Yx (n ) 30 and x ) 1.8 or n ) 23 and x ) 2.5) from 52.5 to 53.1ppm, indicating that the frameworks retained their structures even after moisture absorption. The analyses of the frameworks by X-ray diffraction patterns (vide infra) further confirm the above phenomenon. Thus, the presence of a TMA+ ion in each sodalite cage and the presence of a TEA+ ion in each supercage make a marked difference in terms of the framework stability in the presence of moisture. This phenomenon also serves as indirect evidence that CdS QDs are formed in the supercages from the beginning. UV-vis Spectra of Dry Samples: Evidence III That CdS QDs Exist in the Supercages. The diffuse-reflectance UVvis spectra of dry [(CdS)n]-Y1.8, [(CdS)n(TMA)sod]-Y1.8, and [(CdS)n]-Y2.5 are compared in Figure 4A-C. Their λmax are listed in Table 2. First of all, the spectrum of CdS QD gradually red shifted with increasing n. We ascribe this phenomenon to the increase in the size of the CdS QD in accordance with the general behavior of QDs. Interestingly, unlike the spectra of Wang and co-workers (λmax ) ∼ 350 nm, onset ) ∼450 nm)2-5 and Thomas and Liu (λmax ) ∼ 330 nm, onset ) ∼430 nm),11 the observed λmax of our dry samples were shorter than 300 nm, and the onsets were shorter than 380 nm even when the loading level reached 30% or beyond. The detailed discussions on these points are described in the later part of this report (vide infra). Furthermore, all the three sets of UV-vis spectra were essentially identical for a given n in the dry state. The above results thus unambiguously demonstrate that similar “sets” of CdS QDs are formed in all three types of zeolites for a given n, regardless of the presence of one TMA+ ion in each sodalite cage and regardless of the Si/Al ratio of the framework between 1.8 and 2.5. Such a phenomenon cannot occur if CdS QDs were formed in sodalite cages, since in the case of [(CdS)n(TMA)sod]Y1.8 a TMA+ ion already occupies most of the space of each sodalite cage. This result forces us to conclude that CdS QDs form in the supercages regardless of the loading level from 0.01 to ∼30 CdS per unit cell and regardless of the Si/Al ratio of the framework between 1.8 and 2.5. In the above, the reason we are describing the QDs produced in zeolites as sets is because the spectra are broad, in particular, when n > 1, indicating that there are different QDs with varying sizes and shapes.

Jeong et al.

Figure 4. Diffuse reflectance UV-vis spectra of various (CdS)nincorporating zeolites (as indicated) in the dry (left) and moist (right) conditions. The λmax of bulk CdS (473 nm, see SI 5) is indicated with an arrow in panel H.

Furthermore, the λmax of dry [(CdS)n(TEA)sup]-Y1.8 (Figure 4D) and dry [(CdS)n(TEA)sup]-Y2.5 (Figure 4E) for a given n blue shifted by 10-15 nm (by ∼0.2 eV) with respect to those observed from dry [(CdS)n]-Y1.8, [(CdS)n]-Y2.5, and [(CdS)n(TMA)sod]-Y1.8. Such a phenomenon was more pronounced at higher loading levels. We ascribe the above phenomenon to the size diminution of the CdS QDs caused by the decrease in the available space within a supercage due to the presence of a large organic cation in each supercage since the electronic spectrum of a QD blue shifts with decreasing the size.39 Thus, while the presence of a TMA+ ion in each sodalite cage does not affect the mode of spectral change of CdS QD with n, the presence of a TEA+ ion in each supercage sensitively affects it. The above result again emphasizes that CdS QDs exist within the supercages of zeolite-Y regardless of the Si/Al ratio from 1.8 to 2.5 and regardless of the loading level. The above result further shows that one can finely tune the size of QDs by incorporating inert cations with various sizes into supercages. UV-vis Spectra of Hydrated Samples: Evidence IV That CdS QDs Exist in the Supercages. The dry [(CdS)n]-Y1.8, [(CdS)n(TMA)sod]-Y1.8, and [(CdS)n]-Y2.5 were exposed to the ambient atmosphere for 2 h to fully hydrate the zeolites. The adsorbed amounts of moisture during the above period were ∼15-22%. The adsorbed amount of moisture decreased as the incorporated amount of CdS increased. The diffuse-reflectance UV-vis spectra of the fully hydrated samples are compared in Figure 4F-H. Upon hydration, all the λmax of the dry samples monotonously red shifted to the 330-338 nm region, and new shoulder bands


J. Phys. Chem. C, Vol. 111, No. 28, 2007 10305

TABLE 2: λmax of Some (CdS)n incorporating Y1.8 and Y2.5 Under Dry and Hydrated Conditions [(CdS)n]-Y1.8

[(CdS)n(TMA)sod]-Y1.8

[(CdS)n]-Y2.5

n

dry

hydrated

n

dry

hydrated

n

dry

0.01 0.1 1 6 12 34

215 215 232 240 275 292

245, 305 335, 416 335, 425 (sh) 335, 425 (sh) 335, 420 (sh) 338, 418 (sh)

0.01 0.1 1 6 12 30

215 215 232 240 275 297

217, 302 335, 414 330, 418 336, 415 330, 416 330, 421 (sh)

0.01 0.1 1 6 12 27

219 221 235 245 270 295

[(CdS)n(TEA)sup]-Y1.8

a

refa (dry)

280b 350c 350c

hydrated 307 333, 418 (sh) 322, 411 (sh) 327, 420 (sh) 328, 420 (sh) 331, 424 (sh)

[(CdS)n(TEA)sup]-Y2.5

n

dry

hydrated

n

dry

hydrated

0.01 0.1 1 6 12 30

219 221 234 250 268 282

221 222 237 294 300 305

0.01 0.1 1 6 12 23

220 222 234 244 263 284

221 223 236 288 296 308

Reference 4. b At 1.1%. c At 3-18%.

appeared with λmax at ∼430 nm, regardless of n for n g 1. The ∼430 nm shoulder bands newly appear because of the moistureinduce aggregation of smaller CdS QDs to the mesosized (310 nm sized) CdS QDs by destructing the framework as described in the later part of this report (vide infra). Furthermore, the hydration-induced modes of the spectral change for a given n are nearly the same in the above three zeolites, regardless of the presence of a TMA+ ion in each sodalite cage and regardless of the Si/Al ratio of the zeolite-Y between 1.8 and 2.5. However, the hydration-induced mode of spectral changes of (TEA)sup-containing zeolites were entirely different (Figure 4I,J). Thus, in the cases of [(CdS)n(TEA)sup]-Y1.8 and [(CdS)n(TEA)sup]-Y2.5, the λmax shifted from ∼244-284 nm to ∼288-308 nm for n > 6 (see Table 2), and the ∼430 nm shoulder bands (due to mesosized CdS QDs) did not appear. We ascribe the above phenomenon to the decrease in the adsorbed amount of water in the zeolites due to the presence of TEA+ ions in the supercages, and hence, due to the formation of smaller QDs than those formed in (TEA)sup-free zeolites. The increase in the hydrophobicity of the supercages, as a result of the presence of TEA+ ions, may also lead to the decrease in the water absorbing power of the supercages, which subsequently leads to the decrease in the driving force to aggregate CdS QDs into mesosized QDs, which causes the destruction of zeolite-Y frameworks. In any case, the above result not only serves as additional evidence that CdS QDs are formed in the supercages but also reveals an interesting fact that the presence of a large organic cation prevents the moisture-induced formation of mesosized CdS QDs within zeolite crystals. Intermediate Spectra during Moisture-Induced Spectral Changes of [(CdS)0.1]-Y1.8, [(CdS)6]-Y1.8, and [(CdS)34]Y1.8. In the previous sections, we compared the diffusereflectance UV-vis spectra of dry and fully hydrated (CdS)ncontaining zeolites-Y (Figure 4). As a possible means to gain insights into the relationship between the moisture content and the spectral change and into the relationship between the absorption bands and the nature of CdS species, we measured the diffuse-reflectance UV-vis spectra of [(CdS)n]-Y1.8 (n ) 0.1, 6, and 34) at different levels of moisture uptake. The three sets of spectral changes are shown in Figure 5 and the corresponding water contents and λmax are listed in Table 3. [(CdS)0.1]-Y1.8 (Figure 5A). The dry sample gave a sharp absorption band at ∼215 nm (spectrum 1), and this band is defined as the initial band. This band did not appear from moisture-adsorbed [Naf]-Y1.8, moisture-adsorbed [Cd0.1]-Y1.8, and H2S-adsorbed dry [Naf]-Y1.8. Thus, this band appeared only

from zeolite-Y1.8 containing a very small amount of CdS. Upon adsorption of moisture, the 215 nm band gradually disappeared, yielding new absorption bands at longer wavelength regions. At the water content of 5.0% (spectrum 2), new bands appeared at 250 and 290 nm, respectively, while the intensity of the 215 nm band diminished. As noted, they are all relatively sharp bands [bandwidths (fwhm) ) 0.44 eV]. As the water content increased to 9.3% (spectrum 3) and 13.0% (spectrum 4), the intensities of the 215 nm band further decreased, and the newly appeared 250 nm band also decreased while the intensity of 290 nm increased and subsequently shifted to 300 nm. At the water content of 17.5% (spectrum 5), the initial 215 nm band disappeared completely, and the 300 nm band further red shifted to 313 nm. Simultaneously, the new band at 400 nm, arising from mesosized CdS QDs (vide infra) appeared. At the water content of 20.7% (spectrum 6), the 313 nm band further red shifted to 328 nm. Simultaneously, the intensity of the 400 nm band increased considerably while the absorption in the 270-320 nm region decreased significantly. Finally, at the water content of 22.2% (spectrum 7), the 313 nm band shifted to 335 nm, and the 400 nm band shifted to 416 nm, accompanying a significant increase in intensity. When the fully hydrated sample was kept in the dark in the atmosphere for 2 d (spectrum 8), the 335 nm band disappeared more significantly, and the intensity of the 416 nm band increased further. [(CdS)6]-Y1.8 (Figure 5B). The dry zeolite gave a broad band with λmax at 240 nm (spectrum 1). Thus, unlike [(CdS)0.1]Y1.8, the narrow initial 215 nm band did not appear. At the water content of 4.0% (spectrum 2), both the intensity of the 240 nm band and its bandwidth decreased while a new band appeared at 288 nm. At the water content of 7.9% (spectrum 3), the intensity of the 240 nm band further decreased while the intensity of the ∼290 nm band increased. At the water contents of 11.5% and 14.9%, respectively (spectra 4 and 5), absorption bands began to appear at 304 and 322 nm, respectively, while the 240 nm band decreased more significantly. At the water content of 17.7% (spectrum 6), the 322 nm band further shifted to 331 nm, and an absorption band appeared at 415 nm, indicating the formation of mesosized CdS QDs. At the water content of 19.7%, the 322 nm band red shifted to 335 nm, and the 415 nm band red shifted to 420 nm accompanying an increase in intensity (spectrum 7). It is noted that when the loaded amount of CdS is high, the final spectrum (at the water content of 19.7%) is very broad, covering from 200 to 550 nm, indicating that there are a large variety of CdS species with different sizes in the fully hydrated zeolites when


Figure 5. Diffuse reflectance UV-vis spectra of [(CdS)0.1]-Y1.8 (A), [(CdS)6]-Y1.8 (B), and [(CdS)34]-Y1.8 (C) after exposure to the atmosphere for varying periods of time shown in the inset (as indicated). The corresponding adsorbed amounts of water are shown in each inset.

the loading level of CdS is high. Even after keeping the zeolite in the moist atmosphere for 2 d (spectrum 8), the intensity of the absorption bands in the 200-350 nm region decreased only a bit (in contrast to the case of n ) 0.01), and the intensity of the band due to mesosized CdS QDs increased also only a bit. [(CdS)34]-Y1.8 (Figure 5C). In the case when n ) 34, the intensity of the 290 nm band was highest even under the dry condition (spectrum 1). The presence of 215 nm band as a shoulder and many shoulder bands between 215 and 290 nm was also noticed. At the water content of 4.2% (spectrum 2), λmax shifted to 300 nm. At the water content of 8.6% (spectrum 3), λmax further shifted to 305 nm, and the band arising due to mesosized CdS QDs appeared as a tail band. At the water content of 12.7% (spectrum 4), λmax shifted to 331 nm, and the band arising from mesosized CdS QDs increased a bit more. At the water content of 15.2% (spectrum 5), λmax shifted to 335 nm, and the band due to mesosized CdS became more apparent.

Jeong et al. This sample did not pick up moisture any more from the atmosphere even after exposure to the moist atmosphere for 50 d. Despite this, the absorption intensities below 400 nm decreased significantly during the long aging time while the intensity of the band due to mesosized CdS QDs (spectrum 6) further increased. After aging for 100 d, the intensities of the bands due to mesosized CdS QDs became dominant (spectrum 7). The above three sets of results clearly demonstrate that a variety of absorption bands appear from the CdS QDs in zeolite-Y depending on the loading level of CdS and the adsorbed amount of moisture. This result directly opposes the reports of Wang et al.2-5 and Thomas and Liu11 which mentioned that the CdS QDs have absorptions either at ∼290 nm or at ∼350 nm and the transition from the ∼290 nm band to ∼350 nm band is abrupt upon increasing the loading level (from 1.1% to 3-4%). Since we did not observe bands with λmax > 300 nm in the dry samples (spectrum 1 in each case), the appearance of the ∼350 nm bands in the samples of Wang et al.2-5 and the 330 nm bands in the samples of Thomas and Liu11 clearly indicate that their samples already adsorbed significant amounts of water. In support of the above, they did not mention that their samples were handled in a drybox. It is also interesting to note that the 290 nm band is commonly observed in both cases, namely, after moisture adsorption when the CdS loading level is small (n e 6) and from the beginning in the dry state when the loading level is high (n ) 34). Effects of Zeolite Size on the Spectra of Dry and Hydrated CdS-Containing Zeolites-Y. The spectra of dry (dashed curve) and hydrated (solid curve) [(CdS)f]-Yxµ (x ) 0.08, 2, 4, 8, and 28) are shown in Figure 6A. The λmax of the dry zeolites appeared at 283-290 nm. This result shows that the λmax of dry zeolites remain nearly constant when the loading levels are the same, regardless of the size of the zeolite host. However, the λmax of the hydrated samples progressively red shifted with increasing the zeolite size, from 336 to 372 nm. In contrast with our result, Wang et al.2-5 observed only a ∼350 nm band and Thomas and Liu11 observed only a ∼330 nm band, indicating that Wang et al.2-5 used zeolite-Y with the sizes of 2-3 µm and Thomas and Liu11 used smaller zeolite-Y (or -X). Assignments of Absorption Bands. We assign the initial band that appeared from dry [(CdS)0.1]-Y1.8 (215 nm band, Figure 5A, spectrum 1) as the monomeric CdS existing in a supercage. We propose that the Cd2+ ion is coordinated in a tetrahedral geometry to the three oxygen atoms in a single sixmembered ring (S6R) of a sodalite cage and a sulfide ion, S2-, on the basis of the following reasons. First, the band appears only when the loaded amount of CdS is equal to or less than 0.1 (n e 0.1) per unit cell, which corresponds to 1 or less than 1 CdS per 80 supercages. Second, the band appears also from zeolite A (data not shown). Third, the species is stable for weeks in the dry state at room temperature, indicating that this species is not mobile at room temperature as long as they are kept dry. Fourth, the Cd2+ ion has a strong tendency to form tetrahedral complexes.40 Fifth, as a close analogy to this assignment, it has been known that a Cd2+ ion forms a tetrahedral complex with three oxygen atoms on a S6R and an ethylene molecule in supercages.41 In this regard, monomeric CdS can be more precisely designated as (ZO)3CdS, where ZO stands for a framework oxygen atom. On the basis of the fact that the λmax of 1.3 nm sized CdS QD is 290 nm,42 we assign the 290 nm band as the absorption arising from CdS QD that fills a supercage since a size of a supercage is 1.3 nm. Wang et al.2-5 assigned this band as the


J. Phys. Chem. C, Vol. 111, No. 28, 2007 10307

TABLE 3: λmax of Various Peaks of [(CdS)n]-Y1.8 (n ) 0.1, 6, and 34) at Various Water Contents zeolite

spectrum no.

t

water content (%)

[(CdS)0.1]-Y0.08

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7

0 min 6 min 12 min 17 min 26 min 37 min 60 min 2d 0 min 5 min 11 min 17 min 25 min 34 min 59 min 2d 0 min 7 min 17 min 30 min 59 min 50 d 100 d

0.0 5.0 9.3 13.0 17.5 20.7 22.2 22.7 0.0 4.0 7.9 11.5 14.9 17.7 19.7 19.9 0.0 4.2 8.6 12.7 15.2 15.4 15.4

[(CdS)6]-Y0.08

[(CdS)34]-Y0.08

λmax (nm) 215 215 215 215

absorption arising from isolated [(CdS)4]sod, which is no longer valid. It is also important to note that this band is most commonly observed from our and other group’s samples. It is therefore quite reasonable that this band is associated with the size of zeolite-Y supercage. On the basis of the above, the absorption bands with λmax < 290 nm as the absorptions arise from CdS QDs with the sizes e 1.3 nm. In a simpler way, we assign the absorption bands with λmax e 290 nm as the absorptions arising from the CdS QDs isolated within a supercage. These QDs are designated as

Figure 6. Diffuse reflectance UV-vis spectra of fully (CdS)-loaded Yxµ (A) in the dry (dashed curve) and hydrated (solid curve) states and those of [(CdS)34]-Y1.8 (B) in the dry (dashed curve) state and after ODM coating (solid curve).

250 250

292 292 300 313

400 (sh) 400 (sh)

328 240 240 240 240 (sh) 240 (sh) 240 (sh)

335

416 419

331 335 337

415 (sh) 420 (sh) 421(sh)

331 335 334 335

416 (sh) 416 (sh) 417 (sh) 418 (sh) 420

288 288 304 322

290 300 305

“isolated QDs” (Scheme 2A). Combined with the TEM and X-ray powder diffraction analyses (vide infra), we assign the absorption bands with λmax between 290 and 380 nm as the “CdS QDs that were formed by interconnection of isolated QDs through the supercage windows without destructing the framework”. These QDs are denoted as “interconnected QDs” (Scheme 2B). We ascribe the phenomenon that the absorption of interconnected QD red shifts with increasing the size of the zeolite host (Figure 6A) to the increase in the probability of forming more extensively interconnected QDs as the total amount of isolated QDs in a closed system (zeolite crystal) increases. Wang et al.,2-5 however, assigned this band as the [(CdS)4]sod superlattice that was formed by through-framework interaction between the isolated [(CdS)4]sod species, which is no longer valid. Finally, the absorptions with λmax > 400 nm as mesosized (3-10 nm) CdS QDs residing in the mesopores created within or on the surfaces of the amorphous aluminosilicate resulted from the destruction of the zeolite-Y framework. These are denoted as “mesosized QDs” (Scheme 2C). TEM Analyses of ODM-[(CdS)34]-Y1.8 and Hydrated [(CdS)34]-Y1.8. We prepared ODM-[(CdS)34]-Y1.8 and analyzed it with TEM on the basis of our previous discovery1 that the coating of the (CdS)-containing zeolite-Y surface with ODM (octadecyltrimethoxysilane) not only leads to the effective protection of the zeolite interiors from moisture adsorption but also leads to the selective agglomeration of isolated QDs into interconnected QDs without forming mesosized QDs. During TEM analyses, however, we could not increase the magnification higher than 150 000 despite the fact that it is not possible at this magnification to observe isolated and interconnected QDs because of the very high vulnerability of the framework structure to the electron beam. Nevertheless, we found that all of the zeolite crystals retained original lattice structures as the typical TEM image shown in Figure 7A. Since ODM coating of [(CdS)34]-Y1.8 leads to the red shift of the absorption from λmax e 290 nm to the absorption with 290 < λmax < 380 nm without showing any absorption with λmax > 400 nm (Figure 6B), indicating the formation of interconnected QDs but not mesosized QDs, the well preservation of the lattice structure of zeolite-Y strongly supports that the interconnected QDs are formed in the supercages without destructing the zeolite-Y framework. It is also important to note that although the sample was exposed to the atmosphere for 2 h, the framework structure of ODM-[(CdS)34]-Y1.8 is well-preserved, indicating that the


Jeong et al.

SCHEME 2: Illustration of Isolated (A), Interconnected (B), and Mesosized (C) CdS QDs and the Ranges of Their Absorption Maximums

ODM-coating is effective for keeping the interior from the moisture adsorption. In strong contrast, however, when uncoated [(CdS)34]-Y1.8 was analyzed by TEM after exposure of the sample to the atmosphere for 2 h, the lattice structure was not visible (Figure 7B), indicating that the framework already collapsed during the 2 h exposure period. Instead, CdS QDs with the sizes of 3-6 nm appeared, and most of them looked to be imbedded within the amorphous aluminosilicate, which was still preserving the parental octahedral crystal morphology. This result is consistent with the solid-state 13C NMR spectrum of hydrated [(CdS)30(TMA)sod]-Y1.8 (Figure 3A, top) described earlier (vide supra). The TEM images of 2 d exposed [(CdS)34]-Y1.8 revealed that the sizes of the CdS crystals have increased to 7-10 nm and a large portion of the crystals exist on the surface of the amorphous aluminosilicate material (Figure 7C). X-ray Diffraction Analyses. From the above result that CdScontaining zeolites undergo framework collapse upon exposure to the atmosphere, we conducted X-ray diffraction analyses of the zeolites to collect detailed information on the kinetics and the causes of the framework destruction. In addition, we also prepared dry [Naf]-Yn and fully H+-exchanged zeolites-Y ([Hf]-Yn) (n ) 1.8 or 2.5). [Hf]-Yn were prepared by evacuating fully NH4+-exchanged zeolites-Yn at 250 °C for 5 h. BaSO4 was used as the internal standard. In a glovebox, an aliquot of dried BaSO4 (100 mg) and each dry zeolite sample (200 mg) were introduced into a vial. The mixture was thoroughly mixed with the help of a spatula. The capped, sample-containing vial was then brought out of the glovebox, and each sample was quickly loaded onto a sample holder in the atmosphere. The X-ray diffraction pattern of each sample was then obtained immediately and after 0.5, 1, 2, and 24 h. The results are shown in Figure 8. The plots of the relative intensity of zeolite-Y diffraction at 6.2° with respect to that of BaSO4 diffraction at 28.7° for [Naf]Yn and [Hf]-Yn (n ) 1.8 or 2.5) are shown in Figure 9A, and those for CdS-containing zeolites-Y are shown in Figure 9B. Interestingly, the diffraction intensity of [Naf]-Yn sharply decreased by ∼40% during the initial 0.5 h and then stopped decreasing. Since dry [Naf]-Yn does not decompose by moisture adsorption, the initial sharp ∼40% decrease is ascribed to the natural decrease of the diffraction intensity caused by filling the pores with water. In the case of [Hf]-Yn, the diffraction intensity at 6.2° sharply decreased by ∼47% during the initial 0.5 h and by ∼53% after 24 h. The additional ∼13% decrease of the diffraction intensity may occur because of the filling of the zeolite pores with larger amounts of water since [Hf]-Yn have larger pore volumes than those of [Naf]-Yn. It may also occur in part because of the partial framework destruction. In the cases of [(CdS)30(TMA)sod]-Y1.8 and [(CdS)27]-Y2.5, the diffraction intensity of zeolite-Y at 6.2° decreased by 30%

and 38%, respectively, during the 0.5 h period and by 52% and 72%, respectively, during the 1 h period (Figure 9B). The decrease further continued to 85% and 87%, respectively, as the exposure time increased to 2 h. In the case of [(CdS)30(TMA)sod]-Y1.8, the intensity further decreased to 97% after 24 h. Thus, consistent with the TEM results, the above results confirm that the frameworks of [(CdS)30(TMA)sod]-Y1.8 and [(CdS)27]-Y2.5 undergo near-to-complete destruction during their 2 h exposure to the atmosphere. In the case of ODC-[(CdS)34]-Y1.8, however, the decrease is essentially nil during the 2 h period and is only 10% during the 24 h period. This result clearly shows the remarkable protecting effect of ODC coating against water ingress into the zeolite pores, which enables the analyses of the intrazeolite QDs under the dry condition even after exposure to the atmosphere. In the cases of [(CdS)30(TEA)sup]-Y1.8 and [(CdS)23(TEA)sup]Y2.5, the diffraction intensities decreased by less than 40% even after exposure of them to the atmosphere for 24 h, indicating that only the natural decreases due to water filling of the pores occur. This result demonstrates the marked phenomenon that [(CdS)30(TEA)sup]-Y1.8 and [(CdS)23(TEA)sup]-Y2.5 retain their framework structures even after exposure of them to the atmosphere without surface coating with ODC or ODM. The above results show that one has to be extremely careful with the handling of CdS-containing zeolites-Y produced by exposure of Cd2+-exchanged zeolites-Y to H2S if their surfaces are not coated afterward with ODC or if the zeolites do not simultaneously contain TEA+ ions or other large organic cations within the supercages, since the frameworks are highly prone to collapse upon adsorption of moisture. The above results can be summarized using the following equations: [nCd2+ + nH2S]sup f [(CdS)isos + 2nH+]sup

(1)

q[(CdS)isos + 2nH+]sup + mH2O f {[(CdS)intcnts + 2nH+ + mH2O]sup}q (2) [(CdS)isos + 2nH+]sup + pH2O f {[(CdS)intcnts + 2nH+ + pH2O]sup}q (3) {[(CdS)intcnts + 2nH+ + pH2O]sup}q f {(CdS)mesos + 2nqH+ + pqH2O}amor Al-Si (4) where, (CdS)isos, (CdS)intcnts, and (CdS)mesos represent isolated, interconnected, and mesosized CdS QDs, respectively, and [ ]sup, {[ ]sup}q, and { }amor Al-Si represent a zeolite-Y supercage, two or more adjacent supercages (q g 2), and a mesopore in an amorphous aluminosilicate material, respectively. The number n represents the number of Cd2+ and H2S in a supercage,


Figure 7. TEM images of ODM-[(CdS)34]-Y1.8 (A), uncoated [(CdS)34]-Y1.8 (B) after exposure to the atmosphere for 2 h, respectively, and uncoated [(CdS)34]-Y1.8 after exposure to the atmosphere for 2 d (C). Magnification ) 150 000.

and the arbitrary numbers m and p represent small and large numbers, respectively. Thus, isolated CdS QDs are produced in the supercage under the dry condition (eq 1). Upon adsorption of small amounts of water (mH2O), the isolated QDs aggregate only into interconnected QDs (eq 2). Upon adsorption of large amounts of water (pH2O), the isolated QDs aggregate into interconnected QDs (eq 3) and further to mesosized QDs accompanying the destruction of the framework (eq 4).

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10309 Additional Comparisons of Our Results with Those of Other Groups. In the previous sections, we have described the key differences of our results with those of Wang et al.2-5 regarding the location of CdS QDs and the nature of ∼350 nm. In addition to the aforementioned differences, the following differences were noted. First, while our 1%-loaded rigorously dry zeolite gave λmax at 232 nm, their 1.1%-loaded zeolite-Y gave λmax at 280 nm. Second, while the spectrum of our 1.3%loaded zeolite-Y sample was sharp and nearly single Gaussian with the λmax at 232 nm and the onset of the absorption tail appeared at 260 nm (Figure 4), that of the 1.1%-loaded zeolite-Y sample of Wang et al.2-5 was very broad with at least two λmax at ∼280 nm (stronger shoulder band) and ∼330 nm (weaker shoulder band), respectively. The onset of their absorption tail was 400 nm. Third, while the spectrum of our 6.1%-loaded sample was nearly single Gaussian and the λmax and the onset of the absorption were 240 and 300 nm, respectively, that of the 5.8%-loaded zeolite-Y sample of Wang et al.2-5 was very similar to their 1.1%-loaded sample (λmax at ∼280 nm) except that the intensity is higher and the onset of the absorption tail is red shifted to 425 nm. Again, the above differences arise because the CdS-incorporating zeolites of Wang et al.2-5 inadvertently contained significant amounts of water. Thomas and Liu11 reported that the 270 nm band in CdSincorporating zeolites A and X is ubiquitous regardless of the CdS loading from ∼4.2 to ∼19% and regardless of the zeolite type. They therefore concluded that the band does not represent a CdS species of a specific size which is formed in supercages, but the band rather arises because of an intrinsic nonbonding transition between the valence and the conduction bands of CdS regardless of the size. However, our results clearly show that it is a CdS QD with a specific size and that it is not a ubiquitous band because it is not observed when the loading level is less than 12% (Figures 4 and 5) in the dry state. The fact that they observed the 270 nm band even at the loading level of ∼4.2% strongly indicates that their samples inadvertently adsorbed certain amounts of water since we observed the band from lightly loaded samples after moisture adsorption as shown in Figures 4 and 5. Their 270 nm band is likely to be the absorption arising from the isolated CdS QD with the size of the supercage of zeolite A or X. Thomas and Liu assigned the 300 nm band to the absorption band of CdS QDs residing in the supercages of zeolite A and mentioned that the band is insensitive to the size of CdS QD.11 Wang et al.2-5 also reported that the related 350 nm band did not further red shift with increasing the loading level. However, our results clearly showed that the bands appear only when the CdS-containing zeolites-Y were exposed to the moist atmosphere and they red shift with increasing the adsorbed amount of water (Figure 5) for a given zeolite and with increasing the size of the zeolite host for a given loading level (Figure 6). Wark et al.30 also produced CdS QDs in zeolite X (Si/Al ) 1.2) by treating dry Cd2+-exchanged zeolite X with H2S. The UV-vis spectrum of their dry CdS-incorporating zeolite X also showed a strong ∼350 nm band and a weak tail band whose onset is 430 nm, in addition to ∼270 nm band, despite the fact that the loading level of CdS is only ∼9%. The presence of the strong ∼350 nm band and the long tail band strongly indicate that moisture was inadvertently introduced into the zeolite during the formation of CdS QDs. Fox and Pettit22 prepared CdS QDs within zeolite-Y by dispersing dried Cd2+-exchanged zeolite-Y in dry acetonitrile and subsequently bubbling dry H2S into the heterogeneous acetonitrile solution. Although they did not report the UV-vis spectra, they mentioned that “while the


Jeong et al.

Figure 8. X-ray diffraction patterns of various zeolites (as indicated) after exposure to the atmosphere for 0, 1, 2, and 24 h (as indicated). BaSO4 was added as the internal standard. The diffraction patterns of pure hydrated bare zeolite-Y and pure BaSO4 are shown at the bottom of each panel as references.

zeolites with the loading levels less than 3-4% gave highly blue-shifted spectra, the zeolite with the loading level of 11% gave a spectrum akin to that of 20 nm sized CdS particles”. The results of Fox and Pettit also indicate that moisture was introduced into the zeolite during their preparation of CdS QDs. Sugimoto et al.31 prepared CdS QDs within zeolites-A and -X by adopting the procedure of Wang et al.2-5 and reported that the colors of their dry samples were pale yellow, indicating that moisture was also introduced into the zeolites during their preparation of CdS QDs. Calzaferri and Bru¨hwiler16 mentioned that the preparation procedure and the source of zeolite sensitively affect the properties of the final CdS QDs. This is indeed true since, if extreme caution is not taken into the experimental procedure, moisture can be inadvertently adsorbed into the CdS-incorporating zeolite, causing irreproducibility of the spectra depending on the adsorbed amount of moisture. For reproducibility of the data, the sample preparation, transfer, and loading must be conducted in a rigorously dried environment if their surfaces are not coated with ODC. We emphasize here that our experimental results (including all of the diffuse reflectance spectra) are highly reproducible. Summary. We have investigated the natures and locations of CdS QDs in zeolite-Y which are formed upon exposure of dry Cd2+-exchanged zeolites-Y to dry H2S under a rigorously anhydrous condition. The Cd2+ ions in the supercages of zeolite-Y migrate into the sodalite cages upon drying but migrate back to the supercages to form CdS QDs upon adsorption of H2S. Thus, CdS QDs are formed in the supercages

of zeolite-Y, regardless of the loading levels of CdS from 0.01 to 32% and regardless of the Si/Al ratio of zeolite-Y between 1.8 and 2.5. The absorption bands of the CdS QDs in the supercages vary from 215 to ∼380 nm depending on the loaded amount of CdS, the adsorbed amount of water, and the size of the zeolite crystal. We assign the absorption bands with λmax e 290 nm as the absorptions arising from isolated CdS QDs with the sizes smaller than or equal to the size of a supercage, the absorption bands with 290 nm < λmax < 380 nm as those arising form the interconnected CdS QDs that were formed by interconnection of isolated QDs through the supercage windows, and the absorption bands with λmax > 400 nm as those arising from mesosized CdS QDs with the size of 3-10 nm residing in the mesopores created within or on the surfaces of the amorphous aluminosilicate, which were formed by the collapse of the zeolite-Y framework during the formation of mesosized QDs. Regardless of the nature of the QD (isolated, interconnected, and meso) the absorption band red shifts with increasing the size of the QD, consistent with the general property of QDs. During the formation of interconnected CdS QDs by moisture adsorption, the size of interconnected QD increases as the loaded amount of CdS increases for a given zeolite with the same size and increases as the size of the zeolite host increases for a given loaded amount of CdS. The mesosized CdS QDs are initially formed within the mesopores created within the amorphous aluminosilicate. However, as the exposure time to the moist atmosphere increases, they develop into larger mesosized QDs residing mostly on the surfaces of the amorphous aluminosilicate material. The destruction of the zeolite-Y framework does not


J. Phys. Chem. C, Vol. 111, No. 28, 2007 10311 [Na(TMA)sod]-Y1.8: Y1.8 incorporating (TMA)sod. [Cdn(TMA)sod]-Y1.8: [(TMA)sod]-Y1.8 exchanged with n Cd2+ ions per unit cell. [(CdS)n(TMA)sod]-Y1.8: [(TMA)sod]-Y1.8 incorporating n (CdS) per unit cell in supercages. [(CdS)n(TEA)sup]-Yx: Zeolite-Y1.8 or zeolite-Y2.5 incorporating both n CdS per unit cell and one TEA+ ion in each supercge. ODC: octadecyltrichlorosilane. ODM: octadecyltrimethoxysilane. ODC-coated [(CdS)30ODC-[(CdS)30(TMA)sod]-Y1.8: (TMA)sod]-Y1.8. ODM-[(CdS)n]-Yx: ODM-coated [(CdS)n]-Yx. TMA+: tetramethylammonium ion. TEA+: tetraethylammonium ion. Supporting Information Available: Additional experimental details, denotations and compositions of various zeolites used in this study, full range 13C NMR spectra of various samples of ODC-coated zeolites, and UV-vis spectrum of bulk CdS. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 9. Relative intensity of the diffraction peak of zeolite-Y at 2θ ) 6.2° after exposure to the atmosphere for 0, 0.5, 1, 2, and 25 h with respect to that of its 0 h exposed sample for [Naf]-Yx and [Hf]Yx (x ) 1.8 and 2.5) (A) and various CdS incorporating zeolites Y (as indicated) (B).

readily occur by the H+ ions alone which were generated during the formation of CdS from Cd2+ and H2S. Instead, the waterinduced agglomeration of isolated and interconnected CdS QDs to mesosized CdS QDs in the presence of H+ ions gives rise to the destruction of the zeolite-Y framework. The presence of a TEA+ ion in each supercage not only gives rise to the formation of very small QDs within zeolites-Y but also prevents the zeolite framework from destruction. Acknowledgment. We thank the Ministry of Science and Technology (MOST) and Sogang University for supporting this work through the Creative Research Initiatives (CRI) and the Internal Research Fund programs, respectively. Appendix Yxµ: zeolite-Y with apex-to-apex length of x µm. [Naf]-Yx: fully Na+-exchanged Yx (x represents the Si/Al ratio, 1.8 or 2.5). [Hf]-Yx: fully H+-exchanged Yx (x ) 1.8 or 2.5). [Cdn]-Yx: Yx exchanged with n Cd2+ ions per unit cell (x ) 1.8 or 2.5). [(CdS)n]-Yx: Yx incorporating n (CdS) per unit cell (x ) 1.8 or 2.5). (TMA)sup: the TMA+ ion located in a supercage. (TMA)sod: the TMA+ ion located in a sodalite cage. (TEA)sup: the TEA+ ion located in a supercage. [Na(TMA)sup(TMA)sod]-Y1.8: zeolite-Y1.8 incorporating 0.7 TMA+ ion in each supercage and 1.0 TMA+ ion in each sodalite cage.

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