J. Phys. Chem. C 2007, 111, 7319-7329
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Accessing Binary CdE [E ) S, Se, Te] and Ternary CdxZn1-xE [E ) S, Se] Materials in Mesoporous Architectures Using Silylated-Chalcogen Reagents Elizabeth A. Turner,† Harald Ro1 sner,‡ Yining Huang,*,† and John F. Corrigan*,† Department of Chemistry, The UniVersity of Western Ontario, London, Ontario N6A 5B7, Canada, and Institu¨t fu¨r Nanotechnologie, Forschungszentrum Karlsruhe GmbH, 76344 Eggenstein-Leopoldshafen, Germany ReceiVed: NoVember 25, 2006; In Final Form: March 5, 2007
Binary cadmium chalcogenide materials (CdS, CdSe, and CdTe) have been successfully synthesized at room temperature in the mesoporous environments of MCM-41 and MCF (mesocellular foam) by utilizing silylatedchalcogen reagents [E(SiMe3)2, E ) S, Se, Te] as an efficient delivery source of E2-. The encapsulated materials are easily prepared by the initial complexation of anhydrous cadmium acetate to ethylenediamine functionalized mesoporous material. The subsequent addition of E(SiMe3)2 leads to the preferential formation of CdE materials within the host. The observed blue shift in absorption maximum is in agreement with the expected quantum confinement of these materials given the nanometer dimensions of the mesoporous architecture. Mild thermal treatment of CdS and CdSe composites demonstrates the ability to control particle growth under specified thermal conditions, ultimately leading to a red shift in absorption maximum upon increasing thermolysis temperature. The utility of silylated-chalcogen reagents was further demonstrated in the formation of ternary CdxZn1-xE (E ) S, Se) encapsulated in MCF. The addition of the molecular precursor, (N,N′-TMEDA) Zn(ESiMe3)2 (TMEDA ) N,N,N′,N′-tetramethylethylenediamine), to Cd-MCF yields Cd0.33Zn0.76S- and Cd0.34Zn0.6Se-MCF materials, where the absorption maximum lies between that of the respective parent binary composites. All materials have been characterized by 13C CP-MAS NMR and UV-vis spectroscopy, highresolution transmission electron microscopy, energy-dispersive X-ray, and nitrogen sorption analysis.
Introduction In the past 2 decades there has been substantial growth in both the synthesis1 and characterization2 of nanomaterials, specifically those of the II-VI family.1d,3 Synthetically there are many challenges in obtaining metal-chalcogenide (ME, M ) Zn, Cd, Hg and E ) S, Se, Te) nanostructures which involve not only controlling particle size but also developing an efficient and easily handled delivery source of chalcogen (E2-) to the metal center. By combining the use of inorganic mesoporous frameworks to influence particle size while manipulating the chemistry of trimethylsilyl-chalcogenide reagents on the pore surface, both these synthetic obstacles can be effectively targeted. Trimethylsilyl-chalcogenolate and chalcogenide reagents (RESiMe3, E(SiMe3)2, R ) alkyl, aryl, ferrocenyl) have been shown to be valuable in the formation of structurally characterized nanoclusters4 and nanoparticles.5 The highly reactive nature of these reagents allows for the facile and homogeneous delivery of either chalcogenolate (RE-) or chalcogenide (E2-) bonding interactions to a metal salt. These silylated-chalcogen reagents react readily with both transition and main group metal salts (either halides or acetates), where M-E bonding arrangements are promoted by the generation and elimination of the corresponding trimethylsilane.6 It has recently been shown that the chemistry of silylated-chalcogen reagents can be exploited on a modified mesoporous surface to form ZnE nanomaterials within the host framework.7 * To whom correspondence should be addressed. Phone: (+1) 519-6612111 ext. 86384 (Y.H.); (+1) 519-661-2111 ext. 86387 (J.F.C.). Fax: (+1) 519-661-3022. E-mail:
[email protected] (Y.H.);
[email protected] (J.F.C). † The University of Western Ontario. ‡ Forschungszentrum Karlsruhe GmbH.
The advent of both MCM-418 and SBA-159 spawned a huge interest in the synthesis and encapsulation of II-VI materials.10 The regular hexagonal array of nearly uniform mesoporous channels is ideal for the control of particle dimensions whereby the pore walls prevent particle aggregation. Of the cited examples many have focused on the synthesis of CdS11 and CdSe,12 with a handful of examples investigating the synthesis of ZnS.13 Generally, when synthesizing ME nanoparticles within the architecture there are two major approaches for initially integrating the metal salt in the hexagonal array. In one method, the ionic nature of the host environment is exploited allowing desired metal cations (M2+) to be introduced into the framework structure via ion exchange.11a-e,l2a,13a-c Alternatively, the host material can undergo organic functionalization with an appropriate alkoxysilane, a procedure that has been thoroughly studied in the literature.14 Through the use of an organic thiol or ethylenediamine ligand, the preferred metal salt can be adsorbed into the framework.11f-k,13d-f Upon encapsulating the metal, binary chalcogenide materials are typically formed using H2E (E ) S, Se),11b-g,12a,13a-e although Na2S,11a,i-k,13f Na2SeO3,12-c and thiourea11l have also been successfully used as a chalcogen source. There are only a few reported examples in which synthesized cadmium chalcogenide materials are thermolyzed within the mesoporous/zeolitic framework.11f,15 In a typical procedure, Cd2+ is incorporated into the framework; the following chalcogen inclusion is then preformed at various elevated temperatures. In these examples, a red shift in absorption maximum is noted with an increase in reaction temperature, thereby inducing particle aggregation. A recent example investigated the pyrolysis of cadmium organochalcogenolates within SBA15, thus generating the desired cadmium chalcogenides.15b
10.1021/jp067833h CCC: $37.00 © 2007 American Chemical Society Published on Web 04/28/2007
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SCHEME 1: Proposed Synthetic Scheme for the Synthesis of CdE in MCM-41 and MCFa
a (a) Ethylenediamine functionalized mesoporous material (F-meso), (b) coordination of Cd(II) to pore surface via the chelate effect (Cd-meso), and (c) synthesis of CdE materials through the use of silylated-chalcogenide reagents (CdE-meso).
Pyrolysis of these molecular precursors results in the formation of the thermodynamically favored polymorph for each of the respective cadmium chalcogenide materials. We report herein the synthesis and thermolysis of CdE materials within both MCM-41 and MCF (mesocellular foam). Although cadmium chalcogenides enclosed in mesoporous frameworks have been extensively studied in the literature, this route using bis(trimethylsilyl)chalcogenide offers an alternative approach to CdE materials both in MCM-41 and MCF. In addition, this methodology offers relatively simple handling procedures while accessing the higher congeners (Se and Te) of the CdE series (Scheme 1). The use of both MCM-41 and MCF allows for the formation of larger sized particles in MCF while modulating the photophysical properties of the encapsulated particles. Furthermore, thermolysis of the initially synthesized particles under mild heating conditions gives rise to particle growth while the mesoporous architecture provides a confining environment. Energy-dispersive X-ray (EDX) analysis, nitrogen adsorption analysis, transmission electron microscopy (TEM) analysis, UV-vis, and solid-state NMR spectroscopy have been used to investigate the formation of binary complexes anchored to the mesoporous surface. In addition, the method using traditional trimethylsilylchalcogenide reagents can easily be extended to the formation of ternary metal-chalcogenide mesoporous materials using zinc-trimethylsilyl-chalcogenolate [(N,N′-TMEDA)Zn (ESiMe3)2] precursor complexes in the presence of Cd-MCF. The reactive pendent trimethylsilyl moieties liberate AcOSiMe3 while promoting the formation of Cd-E-Zn bonding interactions. As such, we report the synthesis and characterization of cadmium zinc chalcogenide [E ) S, Se] materials in MCF. Experimental Section Materials. All synthetic procedures and manipulations were performed under inert nitrogen atmosphere using standard Schlenk techniques and gloveboxes, unless otherwise noted. Toluene was purchased from Caledon and was dried and collected using an MBraun MB-SPS solvent purification system with tandem activated alumina/activated copper redox catalyst.16 Dichloromethane (EMD) and chloroform-d (Cambridge Isotope Laboratories) were dried and distilled over P2O5. Benzene was purchased from Fischer Scientific, and 100% ethanol was purchased from Commercial Alcohols. Cetyltrimethylammonium bromide, N-[3-trimethoxysilylpropyl]ethylenediamine, trin-butylphosphine, cadmium acetate dihydrate (98%), mesitylene (98%), tetraethylorthosilicate (98%), poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol) (P123), zinc acetate (99.99%) and N,N,N′,N′-tetramethylethylenediamine (TMEDA) were purchased from Aldrich. N-brand sodium silicate (28.7% SiO2) was purchased from the PQ Corp. Concentrated sulfuric acid was obtained from Caledon, and hydrochloric acid was obtained from ACP. MCM-418 (synthesized under hydrothermal conditions), F-MCM-417 and the reagents E(SiMe3)2 (E ) S, Se, Te)17 and (N,N′-TMEDA) Zn(ESiMe3)2 (E ) S, Se)18b were synthesized according to literature procedures. Synthesis of MCF. In an Erlenmeyer flask, P123 (8.0 g) was dissolved at 35 °C in 1.6 M HCl (300 mL). Mesitylene (4.6 mL) was added dropwise to the solution and allowed to stir at 35 °C for an hour, upon which the solution became cloudy. Tetraethylorthosilicate (17.0 g) was then added, and the reaction was allowed to proceed with stirring at 35 °C for 24 h. The solution was then transferred to a Teflon container within a steel bomb and further heated to 100 °C for 72 h. The white solid was filtered and washed with copious amounts of deionized water and allowed to dry overnight. The triblock copolymer was removed via calcination at 500 °C for 6 h. Functionalization of MCF (F-MCF). Calcined MCF (3.0 g) was degassed under vacuum at 10-3 Torr for 10 min and then suspended in dry toluene (60 mL). N-[3-Trimethoxysilylpropyl]ethylenediamine (TPED; 3.0 mL, 13.8 mmol) was added to the suspension such that the ratio between MCF and TPED was 1:1 (m:v). The resulting mixture was allowed to reflux for 48 h. The solid was then isolated via inert atmosphere filtration and washed repeatedly with CH2Cl2 (3 × 20 mL). The white solid was dried under vacuum (10-3 Torr) at 110 °C overnight. Elemental analysis: C, 13.09%; H, 2.80%; N, 5.21%. (For comparison F-MCM-41 elemental analysis: C, 12.12%; H, 2.68%; N, 4.98%). Synthesis of Cadmium-Meso (Cd-MCM-41/-MCF). Cadmium acetate [Cd(OAc)2] was coordinated to tri-n-butylphosphine [P(nBu3)] according to the literature procedure.19 Cadmium acetate dihydrate (0.839 g, 3.15 mmol) was suspended in benzene (10 mL) and ethanol (100%, 50 mL) and degassed under nitrogen for 10 min. A 2.5 equiv amount of P(nBu3) (1.96 mL, 7.88 mmol) was added to the solution and allowed to stir at room temperature (RT) for 1.5 h until fully dissolved. The solvent was removed under vacuum (10-3 Torr), producing a cloudy oil. The product was then dissolved in dry CH2Cl2 (20 mL). In a separate flask, F-MCM-41 (or F-MCF; 2.0 g) was suspended in 20 mL of CH2Cl2; the solution of solubilized
CdE and CdxZn1-xE in Mesoporous Materials Cd(OAc)2 was added to the suspension of F-MCM-41 (or F-MCF) such that the ratio between F-MCM-41 (or F-MCF) and the cadmium complex was 1:1 (m:m). The reaction was allowed to proceed with stirring at RT for 96 h upon which the white solid was isolated via filtration and washed repeatedly with CH2Cl2 (3 × 20 mL) and dried under vacuum (10-3 Torr) for several hours. Synthesis of Cadmium Sulfide-Meso (CdS-MCM-41/ MCF). Cd-MCM-41 (or Cd-MCF; 0.10 g) was suspended in CH2Cl2 (10 mL). S(SiMe3)2 (0.05 mL, 0.24 mmol) was added to the suspension and allowed to stir for 24 h at room temperature. The white solid was isolated via centrifugation, washed with CH2Cl2 (3 × 10 mL), and dried under vacuum. Synthesis of Cadmium Selenide-Meso (CdSe-MCM-41/ MCF). Cd-MCM-41 (or Cd-MCF; 0.10 g) was suspended in CH2Cl2 (10 mL). Se(SiMe3)2 (0.05 mL, 0.24 mmol) was added to the suspension, upon which the solution became yellow in color. The reaction was allowed to stir for 24 h at room temperature. The bright yellow solid was isolated via centrifugation, washed with CH2Cl2 (3 × 10 mL), and dried under vacuum. The solid was stored in a glovebox as long-term exposure to air resulted in decomposition of the sample, as evidenced by a darkening of the sample color. Synthesis of Cadmium Telluride-Meso (CdTe-MCM41/MCF). Cd-MCM-41 (or Cd-MCF; 0.10 g) was suspended in CH2Cl2 (10 mL) in a foil-wrapped Schlenk tube. In a separate foil-wrapped flask, Te(SiMe3)2 (0.03 mL, 0.14 mmol) was added to CH2Cl2 (10 mL). The solution of Te(SiMe3)2 was added to the suspension of Cd-MCM-41 (or Cd-MCF) upon which the solution immediately became yellow-orange. The reaction was allowed to stir for 5 min at room temperature. The orange solid was isolated via centrifugation, washed with CH2Cl2 (3 × 10 mL), and dried under vacuum. The solid was stored in a glovebox and protected from light as short-term exposure to air resulted in rapid decomposition of the sample, leading to a darkening of the sample color. Thermolysis of CdS- and CdSe-MCM-41/MCF. Using a Mettler Toledo TGA/SDTA 851e CdE-MCM-41/-MCF (E ) S, Se; 30 mg) was heated in a platinum crucible under nitrogen atmosphere from 25 to 75 °C at 10 °C min-1. The sample was annealed at 75 °C for 15 min upon which an aliquot of the sample was removed and stored in a glovebox. The remainder of the sample was further heated at 10 °C intervals up to 135 °C, where at each interval the sample was annealed for 15 min and a portion of the sample was removed and stored for further analysis. Further heating beyond 135 °C resulted in the leaching of CdE from the designated framework (vide infra). Synthesis of Zinc-MCF (Zn-MCF). The procedure for Zn-MCF is slightly modified from the synthesis of ZnMCM-41.7 Zinc acetate (0.92 g, 5 mmol) was dissolved in CH2Cl2 (20 mL) and 2.5 equiv of 3,5-lutidine (1.43 mL, 12.5 mmol) and allowed to stir for 30 min, producing a clear and colorless solution of the (3,5-lutidine)2Zn(OAc)2. The solution was then added to a suspension of F-MCF (1.0 g) in CH2Cl2 (20 mL), where the ratio of F-MCF to zinc complex was 1:2 (m:m). The resulting mixture was allowed to stir at room temperature for 96 h. The solid was then isolated via inert atmosphere filtration and washed repeatedly with CH2Cl2 (3 × 30 mL) and dried under vacuum (10-3 Torr) for several hours. Synthesis of Zinc Sulfide-MCF (ZnS-MCF). Zn-MCF (0.10 g) was suspended in CH2Cl2 (10 mL). S(SiMe3)2 (0.05 mL, 0.24 mmol) was added to the suspension and allowed to stir for 24 h at room temperature. The white solid was isolated
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7321 via centrifugation in a glovebox, washed with CH2Cl2 (2 × 5 mL), and dried under vacuum (10-3 Torr). Synthesis of Zinc Selenide-MCF (ZnSe-MCF). Zn-MCF (0.10 g) was suspended in CH2Cl2 (10 mL). Se(SiMe3)2 (0.05 mL, 0.23 mmol) was added to the suspension and allowed to stir for 24 h at room temperature. With continual stirring the color of the suspension changed from white to pale yellow. The yellow solid was isolated via centrifugation in a glovebox, washed with CH2Cl2 (2 × 5 mL), and dried under vacuum (10-3 Torr). The solid was stored in a glovebox as long-term exposure to air resulted in decomposition of the sample, leading to a darkening of the sample color. Synthesis of Cadmium Zinc Sulfide-MCF (CdSZnMCF). In a Schlenk tube (N,N′-TMEDA)Zn(SSiMe3)2 (0.09 g, 0.23 mmol) was dissolved in CH2Cl2 (20 mL). Cd-MCF (0.10 g) was added to the flask, and the reaction was allowed to stir at room temperature for 1 h. The ratio between MCF and the zinc thiolate precursor was 1:1 (m:m). The colorless solid was isolated via centrifugation, washed with CH2Cl2 (3 × 10 mL), and dried under vacuum. Synthesis of Cadmium Zinc Selenide-MCF (CdSeZnMCF). In a Schlenk tube (N,N′-TMEDA)Zn(SeSiMe3)2 (0.11 g, 0.23 mmol) was dissolved in CH2Cl2 (20 mL) at -78 °C. Cd-MCF (0.10 g) was added to the flask, and the reaction was allowed to stir at 0 °C for 45 min. The ratio between MCF and the zinc selenolate precursor was 1:1 (m:m). The pale yellow solid was isolated via centrifugation, washed with CH2Cl2 (3 × 10 mL) and dried under vacuum. Characterization. Thermogravimetric analyses (TGA) and sample thermolyses were carried out with a Mettler Toledo TGA/SDTA 851e. Surface area measurements were determined using a Micromeritics ASAP 2020 analyzer, in which the adsorption and desorption isotherms were obtained at 77 K after dehydrating each sample at 105 °C for 10 h. The cadmium chalcogenide-MCM-41/-MCF samples were dehydrated at 40 °C for 5 h prior to analysis. The surface area was calculated from the linear part of the BET plot, while pore size distributions were estimated using the 4V/A approximation. Ultravioletvisible (UV-vis) absorption spectra were recorded on an Ocean Optics SD2000 UV-vis fiber optic spectrometer equipped with a Mini-D2T light source and a UV2/OFLV-4 detector. The spectra were obtained as a mineral oil mull between two quartz plates, where mineral oil is dried with a sodium-potassium amalgam. The progress of each reaction was monitored using a Varian CP-3800 gas chromatograph coupled to a Varian Saturn 2000 GC/MS/MS mass spectrometer used as the detector. Solidstate 13C cross-polarization magic angle spinning (CP-MAS) NMR spectra were recorded at an operating frequency of 100.46 MHz, using a Varian/Chemagnetic Infinitypulse 400 wide bore spectrometer at a field strength of 9.4 T. Solution 1H and 31P{H} NMR spectra were recorded on a Varian Mercury 400 spectrometer with an operating frequency of 400.09 and 161.83 MHz, respectively, and referenced to tetramethylsilane (TMS) and H3PO4 respectively. Energy dispersive X-ray analyses were carried out by Dr. Brian Hart at Surface Science Western (UWO). A Quartz Xone EDX analysis system coupled to a Leo 440 scanning electron microscope (SEM) equipped with a Gresham light element detector was used to obtain semiquantitative analysis of Cd, Zn, S, Se, and Te. Analyses were carried out using a 20 kV electron beam rastered over 100 µm × 100 µm areas and repeated to ensure reproducibility. Elemental analysis was obtained from Guelph Chemical Laboratories Ltd. (Guelph, Canada). Samples for high-resolutiontransmission electron microscopy (HR-TEM) were obtained by
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dispersing the solid in CH2Cl2 and dropping these dilute suspensions on gold-plated grids, allowing the excess solvent to evaporate. TEM investigations were performed in a Philips Tecnai F20 ST operated at 200 kV. The TEM was equipped with an EDAX energy-dispersive X-ray SiLi detector with a S-UTW (super-ultrathin window). EDX analyses were carried out in the STEM mode with a HAADF (high-angle annular dark field) detector using a nanometer-sized probe (1 nm spot size for the presented measurements). Use of an analytic double tilt holder (Philips) was made. High-resolution micrographs were taken with a 1K × 1K CCD camera and analyzed with the software package Digital Micrographs (Version 3.7.4, Gatan Co.) to perform fast Fourier transformations (FFT). Results and Discussion Synthesis of Binary Cadmium Chalcogenide Materials. The synthesis and encapsulation of cadmium chalcogenides within a mesoporous architecture were investigated in both MCM-41 and MCF. Ideally, the larger pore size of MCF should yield larger particles contained within the framework. For the purposes of this study, MCM-41 with a pore size of 3.5 nm and MCF with a pore size of ∼20 nm was employed. Synthesis and Characterization of Functionalized MCF. Synthetic conditions used for the functionalization of MCM417 were similarly employed in the functionalization of MCF. As previously observed, using a ratio of 1:1 (MCM-41:TPED) ultimately leads to high surface coverage while maintaining an appreciable surface area, and it was assumed that these same conditions utilized in the functionalization of MCF would generate materials having similar surface coverage. In addition, employing identical reaction conditions between MCM-41 and MCF would allow for a direct comparison between the uptake of cadmium acetate and ultimately the formation of CdE particles within the mesoporous architecture. The functionalization of MCF was monitored both by TGA (to determine surface coverage) and by GC/MS (to observe the incorporation of TPED). Much like in the preparation of F-MCM-41, the first hour of refluxing leads to a significant portion of TPED grafted to the pore wall. According to the TGA of F-MCF (Figure S1, Supporting Information) after 1 h there is ∼22 wt % incorporation of TPED, which is slightly higher than the 17 wt % loading observed after 1 h with MCM-41. Further refluxing leads to increased grafting, where by 43 h maximum loading is achieved with 26 wt % incorporation into the MCF framework. The reaction can also be followed by GC/MS where an internal standard of hexadecane is added to the reaction mixture. The ratio of TPED to hexadecane can be used to monitor the loading of TPED, where a decrease in the ratio suggests loading of the precursor molecule. According to the GC/MS study the amount of TPED remaining in the reaction solution dramatically decreases within the first hour of reflux, coinciding with the results obtained from TGA (ratio at 0 h, 4.2; ratio at 1 h, 1.72). After 24 h there is no significant change in the ratio, but continued refluxing for 43 h gives rise to a slightly decreased ratio (1.39), which does not change after 68 h of reflux. This trend observed in the GC/MS study is also noted in the TGA analysis. In comparing F-MCM-41 with F-MCF it seems both materials have approximately the same weight percent loading of TPED within each of their frameworks; this has also been confirmed by C, H, and N elemental analysis, which shows nearly identical C, H, and N weight percentages. It should be pointed out, however, that the same percent ligand loading does not imply
Figure 1. 13C CP-MAS NMR spectra of (a) F-MCF, (b) Cd-MCM41, (c) Cd-MCF, (d) CdS-MCM-41, and (e) CdS-MCF. Silicon grease (/).
the same ligand distribution with respect to the two materials. In the case of F-MCM-41 the functionalization process to achieve maximum loading is found to take 96 h, while the functionalization of MCF takes only 42 h. The faster reaction time with MCF can perhaps be attributed to the larger pore size thus allowing greater diffusion in and out of the channel system in comparison to MCM-41, which after 1 h of reflux has a significant amount of TPED anchored to the pore wall, thereby decreasing the already smaller pore size. This smaller pore size of MCM-41 could ultimately hinder movement of reactant species through the channel structure. The anchoring of TPED within MCF is confirmed by 13C cross-polarization magic angle spinning (CP-MAS) NMR spectroscopy. Figure 1a shows the 13C NMR spectrum of F-MCF, in which the assigned peaks correspond to the carbon atoms of the propylethylenediamine fragment as previously reported in the literature20 and also observed for F-MCM-41.7 The chemical shift at 8.9 ppm corresponds to the carbon atom of the propyl chain directly bound to the silicon center, while the slightly downfield shift (22.4 ppm) in the 13C NMR spectrum is associated with the second carbon atom of the propyl chain. The shift at 39.0 ppm corresponds to the carbon directly bound to the primary amine, while the resonance at 50.9 ppm is associated with two overlapping chemical species, those being the two carbon atoms bound to the secondary amine of the propylethylenediamine fragment. The 13C NMR spectrum confirms the grafting of the organic ligand to the pore surface via the displacement of the methoxy groups from the silicon atom. 29Si CP-MAS NMR spectra for MCF and F-MCF (Figure S2, Supporting Information), demonstrate the Q2, Q3, and Q4
CdE and CdxZn1-xE in Mesoporous Materials silicon sites for MCF, where there is a greater propensity of Q3 silicon sites. Formation of F-MCF results in the conversion of Q3 silicon sites to Q4 silicon sites as noted in the 29Si CP-MAS NMR spectrum. In addition, the anchoring of TPED to the surface results in the appearance of two signals, corresponding to both terminal and cross-linked silicon environments.21 This behavior is as expected based on previous work with F-MCM-41.7 Incorporation of Cadmium Acetate into a Mesoporous Assembly. There are two major considerations when incorporating cadmium acetate [Cd(OAc)2] within the porous framework. First, due to the air and moisture reactivity of silylated-chalcogen reagents, all chemical species used in the preparation must be fully dehydrated. Second, cadmium acetate must be completely soluble, thus preventing its deposition on the mesoporous surface and ensuring that excess cadmium acetate is washed away after the reaction is complete. Anhydrous cadmium acetate ligated with 2 equiv of trialkylphosphine is only partially soluble in organic solvents. The issue of solubility can be circumvented if, however, the dihydrate form of cadmium acetate is solubilized in an ethanol:(20% benzene) solution with 2 equiv of tri-nbutylphosphine.19 Here the bound water is displaced, thus favoring the coordination with two phosphine ligands. Removal of the ethanol:benzene mixture under vacuum (10-3 Torr) yields a clear viscous oil void of water and freely soluble in organic solvents. Given the monodentate nature of the phosphine ligands, incorporation into the functionalized mesoporous surface should result in the displacement of tri-n-butylphosphine from the metal center with the concomitant coordination of cadmium to the anchored bidentate nitrogen ligand within the pore channel. The exchange process outside of the mesoporous framework can be monitored by solution 1H and 31P{1H} NMR spectroscopy in CDCl3 using TMEDA as a representative bidentate ligand. By monitoring the exchange process in solution, one can ensure the expected reactivity occurs. It was found that, upon coordination of TMEDA to Cd(OAc)2 there is an upfield shift in peak position in the 1H NMR spectrum for the protons associated with the phosphine alkyl chain (1.30 [12H], 1.14 [24H], and 0.64 [18H] ppm) versus when tri-n-butylphosphine is solely coordinated to Cd(OAc)2 (1.55, 1.23, and 0.74 ppm). In addition, there is an upfield shift in peak position for the protons associated with the acetate group (from 1.81 to 1.72 ppm) when TMEDA is coordinated to the metal center. The 31P{1H} NMR spectrum of [P(nBu3)]2Cd(OAc)2 displays a single broad peak at -8.4 ppm. Upon coordinating TMEDA to cadmium acetate, however, the broad peak at -8.4 ppm is replaced by a new broad peak at -19.2 ppm. Given the broadness of the phosphorus shift in the TMEDA-exchanged material in addition to the integration values for the phosphine protons found the 1H NMR spectrum, this suggests that tri-n-butylphosphine is not displaced from the metal center but rather cadmium adopts a six-coordinate geometry via coordination to TMEDA. Nonetheless, the results of this NMR spectroscopy study suggest that incorporation of the cadmium acetate precursor within the functionalized mesoporous material should favor the grafting of Cd(OAc)2 to the pore wall, even if the phosphine ligands are not displaced in the process. The coordination of Cd(OAc)2 to the grafted bidentate ligand within MCM-41 was monitored via TGA to determine the optimal loading conditions. Aliquots of the reaction solution were taken at specified time intervals, filtered, and washed several times with CH2Cl2, and then the resulting solid was dried under vacuum and analyzed by TGA. As is apparent in Figure
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Figure 2. Nitrogen adsorption (s) and desorption (--) isotherms for (a) MCF (offset by 30 cm3 g-1), (b) F-MCF (offset by 60 cm3 g-1), and (c) Cd-MCF.
S3 (Supporting Information), prolonged reaction times did not lead to a higher percent loading of Cd(OAc)2 within the MCM41 framework. Stirring at room temperature for 30 min resulted in a maximum overall loading of ∼36 wt % (percentage includes the percentage of TPED anchored to the pore), which was not altered with continual stirring. These same reaction conditions were thus employed in the coordination of Cd(OAc)2 in the MCF framework resulting in an identical overall percent loading (36 wt %). This result coincides well with the similar loading of TPED found for both MCM-41 and MCF (vide supra). 13C CP-MAS NMR spectroscopy provides the most direct evidence for the coordination of Cd(OAc)2 to the grafted ethylenediamine ligand. Shown in Figure 1b,c are the 13C NMR spectra for Cd-MCM-41 and Cd-MCF, respectively. In both cases it is apparent from the chemical shift at ∼178 ppm, associated with the carbonyl of cadmium acetate, that the metal salt is incorporated in the porous structures. It is interesting to note that in the solid state the chelate effect predominates such that there is displacement of the monodentate phosphine ligands from the metal center and coordination to the anchored bidentate nitrogen ligand. This is supported by the 13C NMR spectra in which there is no evidence for the presence of tri-n-butylphosphine, in addition, the 31P MAS NMR spectrum of Cd-MCM41 shows no evidence for a phosphorus species. Thus, unlike the solution state in which cadmium adopts a six-coordinate geometry thereby accommodating the bidentate TMEDA ligand while maintaining the monodentate phosphine ligands, in the solid state these monodentate ligands are displaced from the metal center. It is possible that the steric confinement of the mesoporous environment in addition to having a prepositioned ligand on the pore wall thus requires the displacement of trin-butylphosphine for the facile coordination of cadmium acetate to the tethered ethylenediamine ligand. The mesoporosity of both MCM-41 and MCF upon incorporating Cd(OAc)2 can be monitored using nitrogen adsorption analysis (Table S1, Supporting Information). Shown in Figure 2 are the nitrogen sorption isotherms for the MCF series, where the retention of a Type-IV isotherm is indicative of the presence of mesopores.22 The initial MCF material has a relatively high surface area of 715 m2 g-1. Upon functionalization the BET surface area diminishes significantly (250 m2 g-1) as a result of maximum loading of the organic ligand; this decrease in surface area is also accompanied by a decrease in pore volume. On the basis of pore size distribution plots (Figure S4, Supporting Information), the pore size only changes slightly upon incorporation of the organic moiety, as would be expected.
7324 J. Phys. Chem. C, Vol. 111, No. 20, 2007 The pores average 22 nm, which is consistent with what has been observed by HR-TEM (vide infra). Incorporation of Cd(OAc)2 into MCF results in a slightly lower surface area of 240 m2 g-1; however, there is still sufficient space within the architecture for the formation of CdE materials. The isotherms observed for the MCF series show a very broad hysteresis loop at high relative pressures associated with capillary condensation. This Type-H1 hysteresis is characteristic for MCF materials23 and is different than the typical isotherms observed for SBA-15.24 Similarly, the nitrogen adsorption and desorption isotherms were measured for Cd-MCM-41. As has been previously reported, the surface area of F-MCM-41 was found to be 480 m2 g-1.7 The coordination of Cd(OAc)2 within the porous structure reduced the surface area to 374 m2 g-1. In addition, the Type-IV isotherm associated with mesoporous materials is observed. On the basis of the nitrogen adsorption study of both MCM-41 and MCF, it appears that the resulting materials retain sufficiently high surface areas after the incorporation of Cd(OAc)2, thereby indicating that there is available pore space for further chemical manipulation within the framework. Synthesis and Characterization of CdE-Mesoporous Materials. With the advent of MCM-41 and MCF, the past decade has seen a tremendous growth in the reported examples of either forming or encapsulating II-VI nanomaterials within these architectures.10 Of these metal-chalcogenides, CdS and CdSe have been the most extensively studied materials given their interesting spectroscopic properties which appear in the visible region. Generally, the chalcogenide source in these syntheses is gaseous H2E (E ) S, Se)11b-g,12a,13a-e although Na2S,11a,i-k,13f thiourea,11l and Na2SeO212c have also found utility in these preparations. Previously it has been reported that silylated-chalcogen reagents can be used in the formation of ZnE materials in MCM-41.7 Such reagents have been proven to be very effective in the formation of binary II-VI5 and ternary II-II′-VI materials.25 It is well-known that bis(trimethylsilyl)chalcogenides [E(SiMe3)2] react readily with metal acetates through the liberation of acetoxytrimethylsilane (AcOSiMe3) to yield metal-chalcogenolate complexes with pendent trimethylsilyl moieties.18 The terminal trimethylsilyl group is quite reactive, and in the case of the higher congeners (Se and Te) the corresponding coordination complexes are only stable at relatively low temperature. This same reactivity is found to occur in the solid state when E(SiMe3)2 is added to zinc acetate-MCM-41.7 The propensity of surface silanol groups (even following functionalization) in addition to the unstable nature of the pendent trimethylsilyl group promotes the formation of binary zinc chalcogenide (ZnE) materials. Given the success of this method in forming ZnE materials in MCM-41, the same principal was thus applied in forming CdE materials in both MCM-41 and MCF. The solid-state reaction of Cd(OAc)2 with E(SiMe3)2 on a mesoporous surface is easily monitored via the formation of solution-state byproducts (AcOSiMe3). Both E(SiMe3)2 and AcOSiMe3 are observed using GC/MS; thus, by adding an internal standard (toluene) to the reaction, the consumption of E(SiMe3)2 can be monitored by a decrease in ratio between E(SiMe3)2 and toluene; likewise, the concomitant formation of AcOSiMe3 results in an increase in ratio between AcOSiMe3 and toluene. As was previously determined in the synthesis of ZnE-MCM-41 materials,7 a ratio of 1:0.5 (MCM-41:E(SiMe3)2; m:v) was ideal for the complete conversion of all acetate groups within the mesoporous framework; hence, the same stoichiom-
Turner et al. etry was used for the formation of CdE materials in both MCM-41 and MCF. The CdS materials are found to be colorless solids, while the CdSe materials are brightly yellow colored solids. It is interesting to note that the yellow color associated with CdSe-MCM-41 is slightly less intense in color than the CdSe-MCF material, indicative of smaller sized particles forming in the MCM-41 framework, thus coinciding with what is observed by UV-vis spectroscopy (vide infra). The CdTe materials are orange in color, where again CdTe-MCM-41 is a less intense orange in comparison to CdTe-MCF. Regardless, the CdTe materials are quite sensitive to air where even brief exposure leads to the immediate decomposition; thus, these materials are only handled under a nitrogen atmosphere. According to the GC/MS study, AcOSiMe3 is generated immediately following the addition of E(SiMe3)2 to the cadmium-mesoporous material. In the case of CdS and CdSe, complete reactivity is achieved within 30 min of stirring at room temperature, although a significant amount of E(SiMe3)2 remains in the reaction solution. Continual stirring results in the consumption of E(SiMe3)2 with the concomitant formation of O(SiMe3)2. The reactions were allowed to proceed for 24 h, and the resulting solids were isolated via centrifugation in a glovebox. In comparison, the complete formation of CdTe within the mesoporous frameworks occurs within 5 min of stirring at room temperature. According to EDX analysis, prolonged reaction times lead to the decomposition of excess Te(SiMe3)2, as observed by the deposition of elemental tellurium. Consequently, the CdTe materials were isolated via centrifugation after only 5 min of stirring at room temperature. The 13C CP-MAS NMR spectra were obtained for CdSMCM-41 and CdS-MCF (Figure 1d,e). The absence of the carbonyl chemical shift at ∼178 ppm clearly demonstrates the reactivity of S(SiMe3)2 with cadmium acetate. It is also important to note that the reaction occurs with complete conversion of all acetate groups incorporated into the mesoporous host. The 13C NMR spectra for both CdS-MCM-41 and CdS-MCF indicate the presence of a new chemical species according to the chemical shift found at ∼1 ppm in each spectrum. As was observed in the formation of ZnEMCM-41,7 excess surface silanol groups (which were not functionalized initially) react with the generated AcOSiMe3 to form a trimethylsilyl group on the pore surface,26 consistent with the chemical shift observed in the 13C NMR spectrum. This reactivity is further confirmed with 29Si CP-MAS NMR spectroscopy of CdSe-MCM-41 and CdSe-MCF (Figure S2, Supporting Information), where in both cases a chemical shift at ∼17 ppm is observed and coincides with the presence of a trimethylsilyl group on the host surface. On the basis of the GC/MS study and the 13C NMR spectra, a similar mechanism as proposed for the formation of ZnEMCM-41 is observed in the synthesis of CdE-MCM-41 and CdE-MCF. The reaction of E(SiMe3)2 with grafted Cd(OAc)2 occurs preferentially in the solid state. Upon complete conversion of all metal centers, excess E(SiMe3)2 reacts with chemisorbed water on the mesoporous surface, such that Me3SiOH is generated which undergoes self-condensation to form O(SiMe3)2.27 Chemisorbed water is present in the materials as the dehydration conditions of 110 °C at 10-3 Torr (for F-meso) are not sufficient for its removal. Cadmium chalcogenides not initially generated by the reaction of E(SiMe3)2 and Cd(OAc)2 can also be formed if terminal trimethylsilyl-chalcogenolate ligands are generated at the cadmium center. Given the reactivity and lability of these ligands especially in the presence of chemisorbed water, the cadmium complex would rapidly
CdE and CdxZn1-xE in Mesoporous Materials
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7325
Figure 3. Nitrogen adsorption (s) and desorption (--) isotherms for (a) CdS-MCF and (b) CdS-MCM-41.
decompose to CdE, particularly for the selenolate and tellurolate complexes given the room-temperature nature of the reaction. Ultimately, the reaction of E(SiMe3)2 with Cd(OAc)2 at the mesoporous surface results in the formation of anchored CdE and the generation of a trimethylsilyl group on the pore wall. Retention of the mesoporosity is examined using nitrogen adsorption analysis for the series CdS. The isotherms for CdSMCM-41 and CdS-MCF are shown in Figure 3, the data are tabulated in Table S1 (Supporting Information). Upon forming CdS in MCM-41, there is loss of the Type-IV isothermal shape, while CdS-MCF retains the Type-IV isotherm. ZnEMCM-41 materials were also found to display non-Type-IV isotherms upon the synthesis of zinc chalcogenides within the framework; thus, this result is not atypical of metal-chalcogenides encapsulated in MCM-41. The BET surface area decreases significantly upon forming CdS-MCM-41 (170 m2 g-1), while the surface area of CdS-MCF remains relatively unaltered (254 m2 g-1) from Cd-MCF. In both materials the pore volumes decrease (as to be expected), while there is no apparent change in pore size from cadmiummesoporous materials, which is also expected on the basis of previous work with ZnE-MCM-41 materials.7 The high-resolution transmission electron microscope (HRTEM) images of MCF and CdS-MCF are illustrated in Figure 4. From the HR-TEM images the pore size is calculated to be ∼17 nm upon formation of CdS-MCF, which has not changed significantly from the original pore size of MCF (∼20 nm). It is also apparent from the TEM image that the pores appear slightly irregular and disordered in addition to displaying the characteristic silica strut features typical of MCF materials.23 These HR-TEM images are further evidence suggesting the formation of MCF type materials. Nonetheless, on the basis of the observed Type-IV isotherm, in addition to the HR-TEM image of CdS-MCF (Figure 4c), it can be concluded that CdS encapsulated within MCF is confined to the mesoporous environment. Although a Type-IV isotherm is not observed for CdS-MCM-41, the low-angle powder X-ray diffraction (PXRD) pattern (Figure S5, Supporting Information) indicates retention of the d100 peak, indicating the presence of a mesostructure; however, since there is loss of a Type-IV isotherm, it is possible that there is potential blockage of the pores, although the particles have been confined to mesopore environment as observed by HR-TEM. The synthesis of cadmium chalcogenides within the mesoporous host is confirmed by EDX analysis and UV-vis spectroscopy. To ensure reproducibility, EDX analysis was
Figure 4. High-resolution transmission electron microscopy (HR-TEM) images of (a) MCF, (b) magnified view of MCF, and (c) CdS-MCF.
performed on three different areas of the sample and the average of the three analyses (using atomic percentages) was calculated in determining the overall ratio. In MCM-41 the CdS sample has a ratio of 1:0.95 (Cd:S), CdSe has a ratio of 1:1.13 (Cd:Se) and CdTe has a ratio of 1:1.17. Similarly in MCF the CdS sample has a ratio of 1:0.92 (Cd:S), CdSe has a ratio of 1:0.87 (Cd:Se), and CdTe has a ratio of 1:1.31 (Cd:Te). In all cases the reported ratios are in good agreement with the expected 1:1 ratio between cadmium and chalcogen, thereby substantiating the formation of binary cadmium chalcogenides within the respective frameworks. The UV-vis absorption spectra for CdE-MCM-41 and CdE-MCF are shown in Figure 5a,b, respectively. For the sulfide materials a clear absorption maximum is observed at 327 (3.79 eV) and 340 nm (3.65 eV) for CdS-MCM-41 and CdS-MCF, respectively. Similarly, CdSe-MCM-41 samples display a pronounced absorption maximum at 380 nm (3.26 eV),
7326 J. Phys. Chem. C, Vol. 111, No. 20, 2007
Figure 5. UV-vis absorption spectra for (a) CdE-MCM-41 and (b) CdE-MCF, where (i) is CdS, (ii) is CdSe, and (iii) is CdTe within the respective host material. The spectra have been normalized for clarity.
while that of CdSe-MCF is slightly red-shifted to 390 nm (3.18 eV). Also observed for CdSe-MCF is the second excitonic transition at higher energy. The red shift of the absorption maximum between CdE materials formed in MCM41 versus MCF is consistent with the formation of larger sized particles within the larger pore system of MCF. Nonetheless, the confinement of CdS and CdSe within a porous system (whether it be MCM-41 or MCF) results in a blue shift in the reported absorption maximum from that of the respective bulk materials, where bulk CdS absorbs at 499 nm (2.48 eV) and bulk CdSe absorbs at 716 nm (1.73 eV).28 As can be seen in Figure 5, the absorption profile of CdTe in both MCM-41 and MCF is much broader than the corresponding sulfide and selenide materials. This broadness is typical for telluride materials and results from a broad distribution of sizes within the sample. The assigned absorption maxima of CdTe-MCM-41 (387 nm; 3.20 eV) and CdTe-MCF (405 nm; 3.06 eV) are red-shifted versus the lighter chalcogenides, but blue-shifted relative to bulk CdTe (841 nm; 1.48 eV),28 again indicating the quantum confinement of CdTe within the host framework. Thermolysis of CdS and CdSe within the Framework. Thermolysis of CdS and CdSe within MCF and MCM-41 should result in particle growth while the siliceous framework controls the size of the particles. Ultimately, the larger pore dimensions of MCF should yield larger sized particles than those obtained in MCM-41. In the case of CdS, both in MCM-41 and MCF, the initially colorless materials developed a pale yellow hue upon thermolysis which darkened with increasing temperature. Similarly, the color of CdSe in both MCM-41 and MCF gradually darkened into a vibrant yellow with thermolysis. The obtained samples were examined by UV-vis spectroscopy, as shown in Figure 6. An increase in thermolysis temperature is found to lead to a red shift in the absorption maxima for both CdS and CdSe
Turner et al. encapsulated in either MCM-41 or MCF, where this red shift is consistent with particle growth. Thermolysis of both CdS and CdSe samples lead to slightly broader absorption spectra, likely resulting from a broader distribution of particle sizes obtained upon thermolysis. The absorption maximum of CdSe-MCM41 is found to shift from 380 (3.26 eV) to 425 nm (2.92 eV) upon heating from 85 to 135 °C (Figure 6c), while the absorption maximum for thermolyzed CdSe-MCF appears at lower energy, changing from 407 nm (3.05 eV) at 75 °C to 454 nm (2.73 eV) at 135 °C (Figure 6d). The slightly lower energy absorption associated with thermolyzed CdSe-MCF is indicative of larger sized particles being formed in the larger cavity of MCF. At the highest thermolysis temperature, however, UV-vis spectroscopy data indicated that the semiconductor particles are found to be no larger than ∼2 nm in size in both MCM-41 and MCF.29 Cadmium sulfide is also found to grow in size with increasing thermolysis temperature as noted by the red shift in absorption maximum. Shown in Figure 6a,b are the absorption profiles for the thermolysis of CdS in MCM-41 and MCF, respectively. In this case it is found that, regardless of the host framework, the absorption maximum shifts from 340 (3.65 eV) to 367 nm (3.38 eV) upon heating from 75 to 125 °C; thus, essentially the same sized particles of CdS are formed in both MCM-41 and MCF upon thermolysis with the particles generated at the highest thermolysis temperature being ∼3 nm in size.30 It is important to note that although MCF has a pore space of ∼20 nm, the thermolyzed particles (either CdS or CdSe) do not come close to achieving those dimensions. This is in part due to the dispersion of cadmium centers throughout the channel structure, where there is likely a nonuniform distribution of cadmium throughout the framework. Ultimately these particles can only grow if there are enough metal centers within the vicinity to allow the aggregation of encapsulated cadmium chalcogenides at the temperatures used. MCF materials are also known to have a secondary porosity associated with the framework walls, where such pores range in size from 1 to 6 nm. Thus, it is possible that the limited growth of CdE within MCF is a result of particles trapped within this secondary porosity.31 An increase in thermolysis temperature beyond 135 °C results in the formation of bulk cadmium chalcogenides. The initial grafting of TPED was monitored by TGA in which it was observed that at 150 °C the organic ligand begins to decompose within the pore. Thus, when the thermolysis is carried out upward of 135 °C, the organic ligand, which anchors the CdE material within the host, is cleaved, resulting in the mobility of these particles throughout the channel system and eventually resulting in the aggregation of CdE outside of the pores. Particles leached from the framework have been observed by HR-TEM (Figure S6, Supporting Information). On the basis of the thermolysis study, CdS and CdSe synthesized in the pores of MCM-41 and MCF are initially small cluster aggregates that upon thermolysis form into small sized particles. Regardless, these materials are anchored to the surface by the organic group as demonstrated by the thermolysis at 150 °C. Synthesis of Ternary Cadmium Zinc Chalcogenide-MCF Materials. Given the success in forming binary cadmium chalcogenide materials in both MCM-41 and MCF, it was proposed that ternary II-II′-VI materials could potentially be accessed in a similar manner (Scheme 2). In recent years there has been an increasing interest in the formation of mixed-metal-chalcogenide materials.32 Manipulation of particle size is one course of action that can be used in
CdE and CdxZn1-xE in Mesoporous Materials
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Figure 6. UV-vis absorption spectra for the thermolysis of (a) CdS-MCM-41, (b) CdS-MCF, (c) CdSe-MCM-41, and (d) CdSe-MCF. Reported temperatures correspond to the thermolysis temperature. The spectra have been normalized for clarity.
SCHEME 2: Proposed Synthetic Pathway for the Formation of CdxZn1-xE-MCF Materials Using (N,N′-TMEDA)Zn(ESiMe3)2 as the Chalcogen Precursor
dictating resultant physical properties; however, an additional degree of control can be introduced by substituting a portion of one metal atom (M) for another metal atom (M′) in a given ME material, ultimately giving rise to MM′E materials. As such, the ensuing physical properties lie in between those of ME and M′E, while these properties can further be influenced by change in particle size. Our laboratory has recently investigated the synthesis of ZnxCd1-xE nanoclusters25a-d and nanoparticles25e formed through the use of trimethylsilyl-chalcogenide reagents. A typical trimethylsilyl-chalcogenide reagent (SiMe3-E-R) reacts with a metal salt (M-X) through the elimination of X-SiMe3. The R group is generally an alkyl or aryl group, but as well could be another SiMe3 moiety. These reactions characteristically give rise to binary ME materials as by considering the R group as a metal (SiMe3-E-M′); then reaction with MX will yield M-E-M′ bonding contacts by a similar loss of X-SiMe3. A considerable amount of our research has focused on the formation of such metallic complexes with pendent trimethylsilyl groups,18 in particular, the synthesis of (N,N′-TMEDA) Zn(ESiMe3)218b has found utility in the formation of ternary ZnxM′1-xE materials.25 This same approach is demonstrated with anchored cadmium acetate found on the pore walls of MCF. Synthesis and Characterization of CdxZn1-xE-MCF. Although the precursor complexes [(N,N′-TMEDA) Zn(ESiMe3)2] can be isolated under inert conditions, in the solution state these complexes have a limited lifetime. In addition, the selenide derivative is generally stable in solution
a low temperature (
