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Dithiol-mediated incorporation of CdS nanoparticles from reverse micellar system into Zn-doped SBA-15 mesoporous silica and their photocatalytic properties
Journal of Colloid and Interface Science 268 (2003) 394–399 www.elsevier.com/locate/jcis
Dithiol-mediated incorporation of CdS nanoparticles from reverse micellar system into Zn-doped SBA-15 mesoporous silica and their photocatalytic properties Takayuki Hirai,∗ Masanori Nanba, and Isao Komasawa Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan Received 14 February 2003; accepted 1 September 2003
Abstract CdS nanoparticles, as prepared in reverse micellar systems, were incorporated into alkanedithiol-modified Zn-doped SBA-15 mesoporous silica (dt–ZnSBA-15; pore diameter, ca. 4 nm), which were themselves prepared via hydrolysis of tetraethylorthosilicate (TEOS) in the presence of Zn(NO3 )2 and triblock copolymer, as a nonsurfactant template and pore-forming agent, followed by contact with dithiol molecules. A particle-sieving effect for the dt–ZnSBA-15 was observed, in that the incorporation of the nanoparticles was remarkably decreased with increasing the nanoparticle size. The resulting CdS–ZnSBA-15 composite was then used as photocatalysts for the generation of H2 from 2-propanol aqueous solution. Under UV irradiation (λ > 300 nm), a high photocatalytic activity was observed for this composite material. This is effected by electron transfer from the photoexcited ZnS (dithiol-bonded Zn on SBA-15) to CdS nanoparticles. The photocatalytic activity is increased with a decrease in the number of methylene groups in the dithiol molecules, according to the rank order 1,10-decanedithiol < 1,6-hexanedithiol < 1,2-ethanedithiol. 2003 Elsevier Inc. All rights reserved. Keywords: CdS nanoparticles; Mesoporous silica support; Size-selective incorporation; Photocatalyst
1. Introduction There has been much interest recently in the preparation and processing of nanoparticles formed from various materials such as metals [1,2], metal sulfides [3–9], and metal oxides [10], when based on reverse micellar systems, owing to their size-dependent properties. There have been several reports relating to the immobilization of nanoparticles from reverse micellar systems onto supports made from inorganic and organic materials. The direct recovery and immobilization of CdS nanoparticles from reverse micellar solutions using thiol-modified mesoporous silica [11,12] and polystyrene [13], in which the thiol groups were combined chemically on the supports, has been presented in previous papers. This immobilization method requires a simple addition of the supports together with mild stirring of the solution to produce resulting composites. The composites obtained demonstrate photocatalytic activity for H2 generation from * Corresponding author.
E-mail address: [email protected] (T. Hirai). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.09.011
2-propanol aqueous solution. In addition, a particle-sieving effect for the mesoporous silica (FM41) was observed, in that the incorporation of the CdS nanoparticles was decreased both by increasing the particle size and by decreasing the pore size for the FM41 [11,12]. Higher photocatalytic activity was obtained for CdS–FM41 with the larger pore sizes. Dithiol molecules can also be used for immobilization of CdS nanoparticles. Colvin et al. have demonstrated the immobilization of CdS nanoparticles, prepared in the reverse micelles, on to the self-assembled dithiol monolayers on an Au substrate [14]. The preparation of an Au–CdS nanoparticle multilayer [15] and a CdS nanoparticle multilayer on Au substrate [16] have also been investigated. Nakanishi et al. demonstrated the layer-by-layer self-assembly of CdS and ZnS nanoparticles on Au substrate, from a reverse micellar solution, using dithiol molecules [17–20]. The recovery and immobilization of CdS nanoparticles, preparared in reverse micelles, onto dithiol-modified Zn-doped silica particles was investigated [21]. The Zn-doped silica particles were easily modified by thiol groups (–SH) of dithiol by the formation of
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Zn–S bonding. The resulting composite, CdS–Zn–SiO2, prepared by immobilization of CdS nanoparticles from reverse micellar solution, demonstrated high photocatalytic activity for H2 generation from 2-propanol aqueous solution under UV irradiation, effected by electron transfer from the photoexcited ZnS (dithiol-bonded Zn on SiO2 ) to CdS nanoparticles. In the present work, Zn-doped mesoporous silica material has been newly synthesized, and the CdS nanoparticles, prepared in reverse micellar solution, have been incorporated into dithiol-modified mesopores. SBA-15 [22–24], having larger pore size and higher stability as compared with MCM-41, has been employed as a mesoporous silica material, to ensure the pore surface modification by dithiol molecules and to enhance the photocatalytic activity of the resulting composites. The resulting composite, CdS–ZnSBA-15, has been utilized as a photocatalyst for H2 generation from 2-propanol aqueous solution, where a high photocatalytic activity was expected under UV irradiation, effected by electron transfer from photoexcited ZnS site to CdS nanoparticles.
2. Experimental 2.1. Chemicals Sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT; AOT), 1,2-ethanedithiol, 1,6-hexanedithiol, 1,10-decanedithiol, tetraethylorthosilicate (TEOS), and 3-mercaptopropyltrimethoxysilane (MPTMS) were supplied by Tokyo Chemical Industry, Ltd. Triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (average molecular weight, 5800), was supplied by Aldrich. Isooctane (2,2,4-trimethylpentane) was obtained from Ishizu Seiyaku Ltd. Cd(NO3 )2 ·4H2 O, Na2 S·9H2 O, Zn(NO3 )2 ·6H2 O, and all other chemicals were obtained from Wako Pure Chemical Industries, Ltd. All reagents were used without further purification. 2.2. Preparation of dithiol-modified Zn-doped SBA-15 mesoporous silica (dt–ZnSBA-15) Zn-doped SBA-15 (ZnSBA-15) was prepared via an almost similar procedure for mesoporous silica SBA-15 [22–24]. Amphiphilic triblock copolymer (2 g) was dispersed in a mixture of 15 g of water and 60 g of 2 mol/l HCl under stirring, followed by the addition of 0.607 g of Zn(NO3 )2 ·6H2 O and 4.25 g of TEOS (molar ratio Si:Zn = 1:0.1). The resulting gel mixture was stirred continuously for 24 h at 313 K, and finally crystallized in a Teflonlined autoclave at 373 K for 48 h. The solid white product was separated by centrifugation, filtered off, washed with deionized water, and dried in vacuo at room temperature. This was calcined in static air at 823 K for 24 h to decompose the triblock copolymer to give a white powder (ZnSBA-15). The resulting ZnSBA-15 particles (0.25 g)
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were dispersed in water (10 ml)-acetone (25 ml) mixture containing 50 µl 1,2-ethanedithiol (or 1,6-hexanedithiol, 1,10-decanedithiol) under stirring at room temperature. Following 24 h stirring, the resulting dithiol-modified SBA-15, denoted as dt–ZnSBA-15 hereafter, were collected via centrifugation, washed thoroughly with acetone to remove the unreacted thiols, and then dried in vacuo. Thiol-modified SBA-15 (t–SBA-15) particles were also prepared, for comparison purposes, according to the procedure reported by Feng et al. [25]. SBA-15 (0.26 g), prepared via a similar procedure in the absence of Zn(NO3)2 , was stirred for 120 h with 1 ml of MPTMS in 20 ml chloroform. The resulting t–SBA-15 particles were washed thoroughly with chloroform and dried at 343 K in vacuo for 24 h. 2.3. Preparation and incorporation of CdS nanoparticles The CdS nanoparticles were prepared, according to the procedure reported previously [9], in 0.1 mol/l AOT/isooctane reverse micellar solution. The concentration of the reactants and the water content, Wo (= [H2 O]/[AOT]), of the reverse micellar solution were controlled by adding the required quantity of aqueous solution. A 10 ml AOT-isooctane reverse micellar solution of required Wo value (normally Wo = 4), containing 0.3 mmol/l Cd(NO3 )2 , was added rapidly to a second micellar solution (10 ml) of the same Wo value, containing 0.3 mmol/l Na2 S, and the mixture was stirred by magnetic stirrer at 298 K in a glass vessel. Three minutes following the mixing of the two solutions, 0.25 g of dt–ZnSBA-15 was added to 20 ml of the reverse micellar solution and stirred gently for 24 h, followed by a separation by centrifugation. The precipitate obtained was washed with hexane and acetone and then dried in vacuo. The resulting CdS-incorporated dt–ZnSBA-15 is abbreviated henceforth as CdS–ZnSBA-15. 2.4. Analysis The powder X-ray diffraction patterns of SBA-15 and ZnSBA-15 samples were recorded on a Philips PW-3050, where a Cu target Kα-ray (operating at 40 kV and 30 mA) was used as X-ray source. SEM and EDX measurements were carried out using an FE-SEM (Hitachi S-5000L) and a SEM equipped with EDX (Hitachi S-2250N and Philips EDAX DX-4). Transmission electron microscopy measurements were carried out by Hitachi HF-2200 (Hitachi HighTechnologies Co.). The pore properties of ZnSBA-15 and dt–ZnSBA-15 were analyzed by N2 physisorption at 77 K using a Belsorp 18Plus-SP (Bel Japan Inc.). Before N2 adsorption–desorption measurements, both ZnSBA-15 and dt–ZnSBA-15 samples were pretreated at 523 K under vacuum for 2 h. The pore size distribution was calculated from the adsorption branch of N2 adsorption–desorption isotherms using the conventional Dollimore–Heal (DH) method [26]. The water content of the reverse micellar solution (Wo ) was determined by a Karl Fischer moisture
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meter (Kyoto Electronics MKS-1s). The absorption spectra for the CdS nanoparticles in micellar solution were recorded on a diode-array UV–visible spectrophotometer (Hewlett–Packard 8452A). The diffuse reflectance spectra for the CdS nanoparticles, incorporated into dt–ZnSBA-15, were recorded using a UV–visible spectrophotometer (Japan Spectroscopy V-550) equipped with an integrating sphere attachment (ISV-469), following the dispersion of the composites in water. The band gap energy (Eg ) and diameter (dp ) of the CdS nanoparticles were estimated on the basis of their absorption onset wavelength, according to the previously reported equations [27,28], and using the reported parameters [29]. The quantity of Cd contained in the composites was determined by using an ICP-AES (Nippon Jarrell–Ash ICAP-575 Mark II), following dissolution of CdS in 2 mol/l HCl aqueous solution.
Fig. 1. Powder XRD patterns for SBA-15 and ZnSBA-15.
2.5. Photoirradiation experiment Photoirradiation experiment was carried out as reported previously [12,30]. A 0.02 g of sample of the composite was dispersed in 20 ml of a 15 vol% 2-propanol aqueous solution. In this, 2-propanol was employed as a sacrificial electron donor for the positive hole, photogenerated on the semiconductors. The solution in the test tube was purged with argon for 1 h, sealed with a septum, and then photoirradiated with a 2 kW xenon lamp (Ushio UXL-2003D-O). Irradiation light of wavelength λ < 300 nm and light in the IR range were cut off by the Pyrex glass of the tube and by the water filter, respectively. When wavelengths only greater than 400 nm were required, 15 wt% NaNO2 aqueous solution was used as a water filter. The quantity of H2 formed in the gas phase of the tube was measured by gas chromatography (Shimadzu GC-14B equipped with TCD), with a column packed with activated charcoal (2 m) and molecular sieve 5A (1 m) at a column temperature of 303 K.
3. Results and discussion 3.1. Characterization of dt–ZnSBA-15 The ZnSBA-15 particles prepared were 1–2 µm in size. Small-angle powder XRD patterns for SBA-15 and ZnSBA15 are shown in Fig. 1. ZnSBA-15 shows a well-resolved pattern with a prominent peak at 0.8◦ , and two weak peaks at 1.3◦ and 1.7◦ , which resembles well the patterns for Zn-free SBA-15 synthesized in this study (Fig. 1) and reported previously [22,23]. Thus, ZnSBA-15 has a high degree of hexagonal mesoscopic organization, as for SBA-15. The EDX analysis showed that the molar ratio Si/Zn = 1:0.024 for the ZnSBA-15. The dithiol modification of ZnSBA-15 was successfully carried out, and the resulting dt–ZnSBA-15 showed characteristic absorption attributable to ZnS, formed via dithiol bonding to Zn site on ZnSBA-15, as shown by a diffuse reflectance spectrum (Fig. 2). SEM and TEM images
Fig. 2. Diffuse reflectance spectrum for ZnS, formed via formation of Zn–S bonding, in ethanedithiol–ZnSBA-15.
for ethanedithiol–ZnSBA-15 are shown in Fig. 3. Periodic mesopores are observed by TEM measurement (Fig. 3b). Fig. 4 shows the N2 adsorption isotherm for ZnSBA-15 and ethanedithiol–ZnSBA-15. A typical irreversed type IV adsorption isotherm with H1 hysteresis loop is observed. The isotherm exhibits a sharp inflection in P /P0 range from 0.55 to 0.80 characteristic of capillary condensation within uniform pores. The P /P0 position of the inflection points is clearly related to a diameter in the mesopore range and the sharpness of the step indicates the uniformity of the mesopore size distribution. The pore size distribution can be calculated from the Kelvin equation and is presented as a Dollimore–Heal plot [26] in Fig. 5. The average pore diameter of ethanedithiol–ZnSBA-15 (ca. 4 nm) is smaller than that of ZnSBA-15 (ca. 6 nm), and thus indicating that the mesopore wall of ZnSBA-15 is modified by the dithiol molecules. 3.2. Incorporation of CdS nanoparticles into dt–ZnSBA-15 The absorption spectrum for CdS nanoparticles, prepared in reverse micellar solution (Wo = 4), as shown by the solid line in Fig. 6a, disappears from the supernatant solution, consequent to the addition of ethanedithiol–ZnSBA-15, as shown by the broken line in Fig. 6a. This indicates that
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Fig. 5. Pore diameter distribution for ZnSBA-15 and ethanedithiol– ZnSBA-15.
Fig. 3. (a) SEM and (b) TEM images for ethanedithiol–ZnSBA-15.
Fig. 6. Absorption spectra for CdS nanoparticles in reverse micellar solution ((a) Wo = 4 and (b) Wo = 8), 3 min after formation (solid lines), and after 24 h stirring with ethanedithiol–ZnSBA-15 (broken lines). (c) Diffuse reflectance spectrum for CdS nanoparticles prepared at Wo = 4 and incorporated in ethanedithiol–ZnSBA-15. Fig. 4. Nitrogen adsorption–desorption isotherms for ZnSBA-15 and ethanedithiol–ZnSBA-15.
almost all the CdS nanoparticles in reverse micellar solution are incorporated into ethanedithiol–ZnSBA-15. In contrast, no appreciable change in the solid line in Fig. 6a was observed when the ZnSBA-15, without dithiol modification, were used in place of dt–ZnSBA-15. The diffuse reflectance spectrum for CdS nanoparticles, incorporated into ethanedithiol–ZnSBA-15, is shown in Fig. 6c. The characteristic absorption for size-quantized CdS nanoparticles is
observed, as shown by the blue shift of the absorption onset, as compared with that for bulk CdS (ca. 500 nm). Thus, as in the case using thiol-modified MCM-41 (FM41) [11,12], the incorporation of CdS nanoparticles, formed in the reverse micellar solution, into dt–ZnSBA-15 was achieved, via chemical bonding between CdS and the –SH group of dithiol, only by the simple addition of the dt–ZnSBA-15 particles and mild stirring. The quantity of CdS incorporated was 3.67 µmol of CdS/g of CdS–ZnSBA-15. Similar results were obtained with hexanedithiol–ZnSBA-15
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and decanedithiol–ZnSBA-15, and the quantities of CdS incorporated were 3.72 and 3.87 µmol of CdS/g of CdS–ZnSBA-15, respectively. The incorporation of the CdS nanoparticles is affected remarkably by the Wo value of the reverse micellar systems, because the nanoparticle size increases with increasing Wo value [3–9]. As shown by the solid and broken lines in Fig. 6b, the characteristic absorption for CdS nanoparticles is hardly decreased by the addition of ethanedithiol– ZnSBA-15 into the reverse micellar solution of Wo = 8. The estimated diameter of CdS nanoparticles prepared in the Wo = 8 reverse micellar system is 3.9 nm, and is larger than that prepared in the Wo = 4 system (3.2 nm). Thus, the difference in the incorporation of CdS nanoparticles is attributable to a “particle-sieving effect” for the mesopores of dt–ZnSBA-15, in that the smaller nanoparticles are incorporated more easily into the mesopores. As suggested in the previous study by using FM41 [12], the incorporation of nanoparticles into mesopores likely to proceeds via adsorption of the water droplets in the micellar solution and immobilization of nanoparticles in the droplets by the –SH groups. This mechanism is consistent with the present result that the appearance of particle-sieving effect; the nanoparticles are immobilized mainly in the mesopores, not on the macroscopic surface of the dt–ZnSBA-15 particles.
the previous study [21], and suggesting that the high photocatalytic activity of CdS–ZnSBA-15 is effected by transfer of conduction band electron photoproduced in the ZnS to the conduction band of the incorporated CdS nanoparticles, and that the hydrogen generation occurs mainly on the CdS surface. The number of methylene groups in the dithiol molecules affects the photocatalytic activity of CdS– ZnSBA-15, as observed for CdS–Zn–SiO2 [21]. As shown in Table 1, the photocatalytic activity decreases with a ranking order of 1,10-decanedithiol (entry 5) < 1,6-hexanedithiol (entry 4) < 1,2-ethanedithiol (entry 2). This decrease in the photocatalytic activity is attributable to the bulky molecules of hexanedithiol and decandithiol, which thus reduces the transfer of conduction band electron photoproduced in the ZnS to the conduction band of the CdS. The quantity of H2 generated by 24-h photoirradiation of CdS–ZnSBA-15 (entry 2, 16.74 µmol of H2 /µmol of Cd) is smaller than that of CdS–Zn–SiO2 (157.8 µmol of H2 /µmol of Cd) [21]. This may occur since the CdS nanoparticles is incorporated in the mesopores in the CdS–ZnSBA-15, whereas the nanoparticles are completely exposed on the CdS–Zn–SiO2. The quantity of H2 obtained using CdS– ZnSBA-15 is, however, greater than that obtained using CdS–L-FM41 (1.43 µmol of H2 /µmol of Cd for 48-h photoirradiation) [11], because of the effective electron transfer from ZnS to CdS in CdS–ZnSBA-15.
3.3. Photocatalytic properties of CdS–ZnSBA-15 The results of the photocatalytic H2 generation is summarized in Table 1. Under UV irradiation (λ > 300 nm), good photocatalytic activity for CdS–ZnSBA-15 was observed (entry 2). The photocatalytic activities for dt–ZnSBA-15 (entry 1) and CdS–SBA-15 (entry 6) are much smaller than that for CdS–ZnSBA-15, thus indicating that both ZnS (dithiol-bonded Zn on the SBA-15, as shown by Fig. 2) and the CdS nanoparticles are essential for the high photocatalytic activity of CdS–ZnSBA-15. In addition, when the CdS–ZnSBA-15 is irradiated to visible light (λ > 400 nm), such that ZnS is not photoexcited, the quantity of hydrogen formed is much decreased (entry 3). These findings are consistent with the results obtained for CdS–Zn–SiO2 in
4. Conclusion The present paper thus describes the preparation and enhanced photocatalytic properties of the composite photocatalyst, mesoporous silica incorporating CdS nanoparticles via dithiol modification. Although the photocatalytic properties of the composite particles of coprecipitated and core–shell type bulk CdS–ZnS have been investigated [31,32], the enhanced activities observed, as compared to that of CdS alone, have been attributed mainly to the passivation of the surface state of CdS by ZnS deposition. We propose a novel mechanism for excellent photocatalytic reaction via electron transfer from photoexcited ZnS to CdS nanoparticles, based
Table 1 The results of photocatalytic H2 generationa Entry 1. dt–ZnSBA-15 2. CdS–ZnSBA-15 3.c CdS–ZnSBA-15 4. CdS–ZnSBA-15 5. CdS–ZnSBA-15 6. CdS–SBA-15d a b c d
Dithiol 1,2-Ethanedithiol 1,2-Ethanedithiol 1,2-Ethanedithiol 1,6-Hexanedithiol 1,10-Decanedithiol
Quantity of H2 formed (µmol/g composite)
(µmol/µmol Cd)
0.039 61.38 0.703 10.70 5.550 0.316
0.0016b 16.74 0.0379 2.876 1.433 0.0626
The CdS nanoparticles were prepared at Wo = 4. Photoirradiaton (λ > 300 nm) was carried out for 24 h in 15 vol% 2-propanol aqueous solution. In µmol/µmol Zn. Photoirradiation at λ > 400 nm. Prepared by using 3-mercaptopropyltrimetoxysilane-modified SBA-15 (t–SBA-15).
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on the nanostructure consist of ZnS site, dithiol molecules, and CdS nanoparticles. Zn-doped SBA-15 mesoporous silica is found to be a good support, because of its particle sieving ability, to immobilize the size-quantized CdS nanoparticles selectively and to achieve a well-defined nanostructure.
Acknowledgments The authors are grateful to Masao Kawashima of the Gas Hydrate Analyzing System (GHAS), Osaka University, for his help in the particle characterization and to the Division of Chemical Engineering, Osaka University for the Lend–Lease Laboratory System. The authors are also grateful to the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), for the financial support through Grants-in-Aid for Scientific Research (Nos. 12450311 and 13650813) and Scientific Research on Priority Areas “Fundamental Science and Technology of Photofunctional Interfaces” (No. 15033244), and to the New Energy and Industrial Technology Development Organization (NEDO) for financial support through the “Nanotechnology Materials Program—Nanotechnology Particle Project” based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI).
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Dithiol-mediated incorporation of CdS nanoparticles from reverse micellar system into Zn-doped SBA-15 mesoporous silica and their photocatalytic properties Takayuki Hirai,∗ Masanori Nanba, and Isao Komasawa Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan Received 14 February 2003; accepted 1 September 2003
Abstract CdS nanoparticles, as prepared in reverse micellar systems, were incorporated into alkanedithiol-modified Zn-doped SBA-15 mesoporous silica (dt–ZnSBA-15; pore diameter, ca. 4 nm), which were themselves prepared via hydrolysis of tetraethylorthosilicate (TEOS) in the presence of Zn(NO3 )2 and triblock copolymer, as a nonsurfactant template and pore-forming agent, followed by contact with dithiol molecules. A particle-sieving effect for the dt–ZnSBA-15 was observed, in that the incorporation of the nanoparticles was remarkably decreased with increasing the nanoparticle size. The resulting CdS–ZnSBA-15 composite was then used as photocatalysts for the generation of H2 from 2-propanol aqueous solution. Under UV irradiation (λ > 300 nm), a high photocatalytic activity was observed for this composite material. This is effected by electron transfer from the photoexcited ZnS (dithiol-bonded Zn on SBA-15) to CdS nanoparticles. The photocatalytic activity is increased with a decrease in the number of methylene groups in the dithiol molecules, according to the rank order 1,10-decanedithiol < 1,6-hexanedithiol < 1,2-ethanedithiol. 2003 Elsevier Inc. All rights reserved. Keywords: CdS nanoparticles; Mesoporous silica support; Size-selective incorporation; Photocatalyst
1. Introduction There has been much interest recently in the preparation and processing of nanoparticles formed from various materials such as metals [1,2], metal sulfides [3–9], and metal oxides [10], when based on reverse micellar systems, owing to their size-dependent properties. There have been several reports relating to the immobilization of nanoparticles from reverse micellar systems onto supports made from inorganic and organic materials. The direct recovery and immobilization of CdS nanoparticles from reverse micellar solutions using thiol-modified mesoporous silica [11,12] and polystyrene [13], in which the thiol groups were combined chemically on the supports, has been presented in previous papers. This immobilization method requires a simple addition of the supports together with mild stirring of the solution to produce resulting composites. The composites obtained demonstrate photocatalytic activity for H2 generation from * Corresponding author.
E-mail address: [email protected] (T. Hirai). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.09.011
2-propanol aqueous solution. In addition, a particle-sieving effect for the mesoporous silica (FM41) was observed, in that the incorporation of the CdS nanoparticles was decreased both by increasing the particle size and by decreasing the pore size for the FM41 [11,12]. Higher photocatalytic activity was obtained for CdS–FM41 with the larger pore sizes. Dithiol molecules can also be used for immobilization of CdS nanoparticles. Colvin et al. have demonstrated the immobilization of CdS nanoparticles, prepared in the reverse micelles, on to the self-assembled dithiol monolayers on an Au substrate [14]. The preparation of an Au–CdS nanoparticle multilayer [15] and a CdS nanoparticle multilayer on Au substrate [16] have also been investigated. Nakanishi et al. demonstrated the layer-by-layer self-assembly of CdS and ZnS nanoparticles on Au substrate, from a reverse micellar solution, using dithiol molecules [17–20]. The recovery and immobilization of CdS nanoparticles, preparared in reverse micelles, onto dithiol-modified Zn-doped silica particles was investigated [21]. The Zn-doped silica particles were easily modified by thiol groups (–SH) of dithiol by the formation of
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Zn–S bonding. The resulting composite, CdS–Zn–SiO2, prepared by immobilization of CdS nanoparticles from reverse micellar solution, demonstrated high photocatalytic activity for H2 generation from 2-propanol aqueous solution under UV irradiation, effected by electron transfer from the photoexcited ZnS (dithiol-bonded Zn on SiO2 ) to CdS nanoparticles. In the present work, Zn-doped mesoporous silica material has been newly synthesized, and the CdS nanoparticles, prepared in reverse micellar solution, have been incorporated into dithiol-modified mesopores. SBA-15 [22–24], having larger pore size and higher stability as compared with MCM-41, has been employed as a mesoporous silica material, to ensure the pore surface modification by dithiol molecules and to enhance the photocatalytic activity of the resulting composites. The resulting composite, CdS–ZnSBA-15, has been utilized as a photocatalyst for H2 generation from 2-propanol aqueous solution, where a high photocatalytic activity was expected under UV irradiation, effected by electron transfer from photoexcited ZnS site to CdS nanoparticles.
2. Experimental 2.1. Chemicals Sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT; AOT), 1,2-ethanedithiol, 1,6-hexanedithiol, 1,10-decanedithiol, tetraethylorthosilicate (TEOS), and 3-mercaptopropyltrimethoxysilane (MPTMS) were supplied by Tokyo Chemical Industry, Ltd. Triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (average molecular weight, 5800), was supplied by Aldrich. Isooctane (2,2,4-trimethylpentane) was obtained from Ishizu Seiyaku Ltd. Cd(NO3 )2 ·4H2 O, Na2 S·9H2 O, Zn(NO3 )2 ·6H2 O, and all other chemicals were obtained from Wako Pure Chemical Industries, Ltd. All reagents were used without further purification. 2.2. Preparation of dithiol-modified Zn-doped SBA-15 mesoporous silica (dt–ZnSBA-15) Zn-doped SBA-15 (ZnSBA-15) was prepared via an almost similar procedure for mesoporous silica SBA-15 [22–24]. Amphiphilic triblock copolymer (2 g) was dispersed in a mixture of 15 g of water and 60 g of 2 mol/l HCl under stirring, followed by the addition of 0.607 g of Zn(NO3 )2 ·6H2 O and 4.25 g of TEOS (molar ratio Si:Zn = 1:0.1). The resulting gel mixture was stirred continuously for 24 h at 313 K, and finally crystallized in a Teflonlined autoclave at 373 K for 48 h. The solid white product was separated by centrifugation, filtered off, washed with deionized water, and dried in vacuo at room temperature. This was calcined in static air at 823 K for 24 h to decompose the triblock copolymer to give a white powder (ZnSBA-15). The resulting ZnSBA-15 particles (0.25 g)
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were dispersed in water (10 ml)-acetone (25 ml) mixture containing 50 µl 1,2-ethanedithiol (or 1,6-hexanedithiol, 1,10-decanedithiol) under stirring at room temperature. Following 24 h stirring, the resulting dithiol-modified SBA-15, denoted as dt–ZnSBA-15 hereafter, were collected via centrifugation, washed thoroughly with acetone to remove the unreacted thiols, and then dried in vacuo. Thiol-modified SBA-15 (t–SBA-15) particles were also prepared, for comparison purposes, according to the procedure reported by Feng et al. [25]. SBA-15 (0.26 g), prepared via a similar procedure in the absence of Zn(NO3)2 , was stirred for 120 h with 1 ml of MPTMS in 20 ml chloroform. The resulting t–SBA-15 particles were washed thoroughly with chloroform and dried at 343 K in vacuo for 24 h. 2.3. Preparation and incorporation of CdS nanoparticles The CdS nanoparticles were prepared, according to the procedure reported previously [9], in 0.1 mol/l AOT/isooctane reverse micellar solution. The concentration of the reactants and the water content, Wo (= [H2 O]/[AOT]), of the reverse micellar solution were controlled by adding the required quantity of aqueous solution. A 10 ml AOT-isooctane reverse micellar solution of required Wo value (normally Wo = 4), containing 0.3 mmol/l Cd(NO3 )2 , was added rapidly to a second micellar solution (10 ml) of the same Wo value, containing 0.3 mmol/l Na2 S, and the mixture was stirred by magnetic stirrer at 298 K in a glass vessel. Three minutes following the mixing of the two solutions, 0.25 g of dt–ZnSBA-15 was added to 20 ml of the reverse micellar solution and stirred gently for 24 h, followed by a separation by centrifugation. The precipitate obtained was washed with hexane and acetone and then dried in vacuo. The resulting CdS-incorporated dt–ZnSBA-15 is abbreviated henceforth as CdS–ZnSBA-15. 2.4. Analysis The powder X-ray diffraction patterns of SBA-15 and ZnSBA-15 samples were recorded on a Philips PW-3050, where a Cu target Kα-ray (operating at 40 kV and 30 mA) was used as X-ray source. SEM and EDX measurements were carried out using an FE-SEM (Hitachi S-5000L) and a SEM equipped with EDX (Hitachi S-2250N and Philips EDAX DX-4). Transmission electron microscopy measurements were carried out by Hitachi HF-2200 (Hitachi HighTechnologies Co.). The pore properties of ZnSBA-15 and dt–ZnSBA-15 were analyzed by N2 physisorption at 77 K using a Belsorp 18Plus-SP (Bel Japan Inc.). Before N2 adsorption–desorption measurements, both ZnSBA-15 and dt–ZnSBA-15 samples were pretreated at 523 K under vacuum for 2 h. The pore size distribution was calculated from the adsorption branch of N2 adsorption–desorption isotherms using the conventional Dollimore–Heal (DH) method [26]. The water content of the reverse micellar solution (Wo ) was determined by a Karl Fischer moisture
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meter (Kyoto Electronics MKS-1s). The absorption spectra for the CdS nanoparticles in micellar solution were recorded on a diode-array UV–visible spectrophotometer (Hewlett–Packard 8452A). The diffuse reflectance spectra for the CdS nanoparticles, incorporated into dt–ZnSBA-15, were recorded using a UV–visible spectrophotometer (Japan Spectroscopy V-550) equipped with an integrating sphere attachment (ISV-469), following the dispersion of the composites in water. The band gap energy (Eg ) and diameter (dp ) of the CdS nanoparticles were estimated on the basis of their absorption onset wavelength, according to the previously reported equations [27,28], and using the reported parameters [29]. The quantity of Cd contained in the composites was determined by using an ICP-AES (Nippon Jarrell–Ash ICAP-575 Mark II), following dissolution of CdS in 2 mol/l HCl aqueous solution.
Fig. 1. Powder XRD patterns for SBA-15 and ZnSBA-15.
2.5. Photoirradiation experiment Photoirradiation experiment was carried out as reported previously [12,30]. A 0.02 g of sample of the composite was dispersed in 20 ml of a 15 vol% 2-propanol aqueous solution. In this, 2-propanol was employed as a sacrificial electron donor for the positive hole, photogenerated on the semiconductors. The solution in the test tube was purged with argon for 1 h, sealed with a septum, and then photoirradiated with a 2 kW xenon lamp (Ushio UXL-2003D-O). Irradiation light of wavelength λ < 300 nm and light in the IR range were cut off by the Pyrex glass of the tube and by the water filter, respectively. When wavelengths only greater than 400 nm were required, 15 wt% NaNO2 aqueous solution was used as a water filter. The quantity of H2 formed in the gas phase of the tube was measured by gas chromatography (Shimadzu GC-14B equipped with TCD), with a column packed with activated charcoal (2 m) and molecular sieve 5A (1 m) at a column temperature of 303 K.
3. Results and discussion 3.1. Characterization of dt–ZnSBA-15 The ZnSBA-15 particles prepared were 1–2 µm in size. Small-angle powder XRD patterns for SBA-15 and ZnSBA15 are shown in Fig. 1. ZnSBA-15 shows a well-resolved pattern with a prominent peak at 0.8◦ , and two weak peaks at 1.3◦ and 1.7◦ , which resembles well the patterns for Zn-free SBA-15 synthesized in this study (Fig. 1) and reported previously [22,23]. Thus, ZnSBA-15 has a high degree of hexagonal mesoscopic organization, as for SBA-15. The EDX analysis showed that the molar ratio Si/Zn = 1:0.024 for the ZnSBA-15. The dithiol modification of ZnSBA-15 was successfully carried out, and the resulting dt–ZnSBA-15 showed characteristic absorption attributable to ZnS, formed via dithiol bonding to Zn site on ZnSBA-15, as shown by a diffuse reflectance spectrum (Fig. 2). SEM and TEM images
Fig. 2. Diffuse reflectance spectrum for ZnS, formed via formation of Zn–S bonding, in ethanedithiol–ZnSBA-15.
for ethanedithiol–ZnSBA-15 are shown in Fig. 3. Periodic mesopores are observed by TEM measurement (Fig. 3b). Fig. 4 shows the N2 adsorption isotherm for ZnSBA-15 and ethanedithiol–ZnSBA-15. A typical irreversed type IV adsorption isotherm with H1 hysteresis loop is observed. The isotherm exhibits a sharp inflection in P /P0 range from 0.55 to 0.80 characteristic of capillary condensation within uniform pores. The P /P0 position of the inflection points is clearly related to a diameter in the mesopore range and the sharpness of the step indicates the uniformity of the mesopore size distribution. The pore size distribution can be calculated from the Kelvin equation and is presented as a Dollimore–Heal plot [26] in Fig. 5. The average pore diameter of ethanedithiol–ZnSBA-15 (ca. 4 nm) is smaller than that of ZnSBA-15 (ca. 6 nm), and thus indicating that the mesopore wall of ZnSBA-15 is modified by the dithiol molecules. 3.2. Incorporation of CdS nanoparticles into dt–ZnSBA-15 The absorption spectrum for CdS nanoparticles, prepared in reverse micellar solution (Wo = 4), as shown by the solid line in Fig. 6a, disappears from the supernatant solution, consequent to the addition of ethanedithiol–ZnSBA-15, as shown by the broken line in Fig. 6a. This indicates that
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Fig. 5. Pore diameter distribution for ZnSBA-15 and ethanedithiol– ZnSBA-15.
Fig. 3. (a) SEM and (b) TEM images for ethanedithiol–ZnSBA-15.
Fig. 6. Absorption spectra for CdS nanoparticles in reverse micellar solution ((a) Wo = 4 and (b) Wo = 8), 3 min after formation (solid lines), and after 24 h stirring with ethanedithiol–ZnSBA-15 (broken lines). (c) Diffuse reflectance spectrum for CdS nanoparticles prepared at Wo = 4 and incorporated in ethanedithiol–ZnSBA-15. Fig. 4. Nitrogen adsorption–desorption isotherms for ZnSBA-15 and ethanedithiol–ZnSBA-15.
almost all the CdS nanoparticles in reverse micellar solution are incorporated into ethanedithiol–ZnSBA-15. In contrast, no appreciable change in the solid line in Fig. 6a was observed when the ZnSBA-15, without dithiol modification, were used in place of dt–ZnSBA-15. The diffuse reflectance spectrum for CdS nanoparticles, incorporated into ethanedithiol–ZnSBA-15, is shown in Fig. 6c. The characteristic absorption for size-quantized CdS nanoparticles is
observed, as shown by the blue shift of the absorption onset, as compared with that for bulk CdS (ca. 500 nm). Thus, as in the case using thiol-modified MCM-41 (FM41) [11,12], the incorporation of CdS nanoparticles, formed in the reverse micellar solution, into dt–ZnSBA-15 was achieved, via chemical bonding between CdS and the –SH group of dithiol, only by the simple addition of the dt–ZnSBA-15 particles and mild stirring. The quantity of CdS incorporated was 3.67 µmol of CdS/g of CdS–ZnSBA-15. Similar results were obtained with hexanedithiol–ZnSBA-15
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and decanedithiol–ZnSBA-15, and the quantities of CdS incorporated were 3.72 and 3.87 µmol of CdS/g of CdS–ZnSBA-15, respectively. The incorporation of the CdS nanoparticles is affected remarkably by the Wo value of the reverse micellar systems, because the nanoparticle size increases with increasing Wo value [3–9]. As shown by the solid and broken lines in Fig. 6b, the characteristic absorption for CdS nanoparticles is hardly decreased by the addition of ethanedithiol– ZnSBA-15 into the reverse micellar solution of Wo = 8. The estimated diameter of CdS nanoparticles prepared in the Wo = 8 reverse micellar system is 3.9 nm, and is larger than that prepared in the Wo = 4 system (3.2 nm). Thus, the difference in the incorporation of CdS nanoparticles is attributable to a “particle-sieving effect” for the mesopores of dt–ZnSBA-15, in that the smaller nanoparticles are incorporated more easily into the mesopores. As suggested in the previous study by using FM41 [12], the incorporation of nanoparticles into mesopores likely to proceeds via adsorption of the water droplets in the micellar solution and immobilization of nanoparticles in the droplets by the –SH groups. This mechanism is consistent with the present result that the appearance of particle-sieving effect; the nanoparticles are immobilized mainly in the mesopores, not on the macroscopic surface of the dt–ZnSBA-15 particles.
the previous study [21], and suggesting that the high photocatalytic activity of CdS–ZnSBA-15 is effected by transfer of conduction band electron photoproduced in the ZnS to the conduction band of the incorporated CdS nanoparticles, and that the hydrogen generation occurs mainly on the CdS surface. The number of methylene groups in the dithiol molecules affects the photocatalytic activity of CdS– ZnSBA-15, as observed for CdS–Zn–SiO2 [21]. As shown in Table 1, the photocatalytic activity decreases with a ranking order of 1,10-decanedithiol (entry 5) < 1,6-hexanedithiol (entry 4) < 1,2-ethanedithiol (entry 2). This decrease in the photocatalytic activity is attributable to the bulky molecules of hexanedithiol and decandithiol, which thus reduces the transfer of conduction band electron photoproduced in the ZnS to the conduction band of the CdS. The quantity of H2 generated by 24-h photoirradiation of CdS–ZnSBA-15 (entry 2, 16.74 µmol of H2 /µmol of Cd) is smaller than that of CdS–Zn–SiO2 (157.8 µmol of H2 /µmol of Cd) [21]. This may occur since the CdS nanoparticles is incorporated in the mesopores in the CdS–ZnSBA-15, whereas the nanoparticles are completely exposed on the CdS–Zn–SiO2. The quantity of H2 obtained using CdS– ZnSBA-15 is, however, greater than that obtained using CdS–L-FM41 (1.43 µmol of H2 /µmol of Cd for 48-h photoirradiation) [11], because of the effective electron transfer from ZnS to CdS in CdS–ZnSBA-15.
3.3. Photocatalytic properties of CdS–ZnSBA-15 The results of the photocatalytic H2 generation is summarized in Table 1. Under UV irradiation (λ > 300 nm), good photocatalytic activity for CdS–ZnSBA-15 was observed (entry 2). The photocatalytic activities for dt–ZnSBA-15 (entry 1) and CdS–SBA-15 (entry 6) are much smaller than that for CdS–ZnSBA-15, thus indicating that both ZnS (dithiol-bonded Zn on the SBA-15, as shown by Fig. 2) and the CdS nanoparticles are essential for the high photocatalytic activity of CdS–ZnSBA-15. In addition, when the CdS–ZnSBA-15 is irradiated to visible light (λ > 400 nm), such that ZnS is not photoexcited, the quantity of hydrogen formed is much decreased (entry 3). These findings are consistent with the results obtained for CdS–Zn–SiO2 in
4. Conclusion The present paper thus describes the preparation and enhanced photocatalytic properties of the composite photocatalyst, mesoporous silica incorporating CdS nanoparticles via dithiol modification. Although the photocatalytic properties of the composite particles of coprecipitated and core–shell type bulk CdS–ZnS have been investigated [31,32], the enhanced activities observed, as compared to that of CdS alone, have been attributed mainly to the passivation of the surface state of CdS by ZnS deposition. We propose a novel mechanism for excellent photocatalytic reaction via electron transfer from photoexcited ZnS to CdS nanoparticles, based
Table 1 The results of photocatalytic H2 generationa Entry 1. dt–ZnSBA-15 2. CdS–ZnSBA-15 3.c CdS–ZnSBA-15 4. CdS–ZnSBA-15 5. CdS–ZnSBA-15 6. CdS–SBA-15d a b c d
Dithiol 1,2-Ethanedithiol 1,2-Ethanedithiol 1,2-Ethanedithiol 1,6-Hexanedithiol 1,10-Decanedithiol
Quantity of H2 formed (µmol/g composite)
(µmol/µmol Cd)
0.039 61.38 0.703 10.70 5.550 0.316
0.0016b 16.74 0.0379 2.876 1.433 0.0626
The CdS nanoparticles were prepared at Wo = 4. Photoirradiaton (λ > 300 nm) was carried out for 24 h in 15 vol% 2-propanol aqueous solution. In µmol/µmol Zn. Photoirradiation at λ > 400 nm. Prepared by using 3-mercaptopropyltrimetoxysilane-modified SBA-15 (t–SBA-15).
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on the nanostructure consist of ZnS site, dithiol molecules, and CdS nanoparticles. Zn-doped SBA-15 mesoporous silica is found to be a good support, because of its particle sieving ability, to immobilize the size-quantized CdS nanoparticles selectively and to achieve a well-defined nanostructure.
Acknowledgments The authors are grateful to Masao Kawashima of the Gas Hydrate Analyzing System (GHAS), Osaka University, for his help in the particle characterization and to the Division of Chemical Engineering, Osaka University for the Lend–Lease Laboratory System. The authors are also grateful to the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), for the financial support through Grants-in-Aid for Scientific Research (Nos. 12450311 and 13650813) and Scientific Research on Priority Areas “Fundamental Science and Technology of Photofunctional Interfaces” (No. 15033244), and to the New Energy and Industrial Technology Development Organization (NEDO) for financial support through the “Nanotechnology Materials Program—Nanotechnology Particle Project” based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI).
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