Homogeneously Distributed CdS Nanoparticles in Nafion Membranes

mapping with energy-dispersive X-ray (EDAX) analysis. Results from the characterizations on the physical properties, the chemical stability, and the p...
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Langmuir 2005, 21, 11969-11973

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Homogeneously Distributed CdS Nanoparticles in Nafion Membranes: Preparation, Characterization, and Photocatalytic Properties Shiming Wang, Ping Liu, Xuxu Wang, and Xianzhi Fu* Research Institute of Photocatalysis, Chemistry & Chemical Engineering College, Fuzhou University, Fuzhou 350002, P. R. China Received April 22, 2005. In Final Form: August 22, 2005 Stable crystalline CdS nanoparticles were synthesized in Nafion ionomer membranes by using thioacetamide (TAA) as a nonionic precursor. Unlike the ionic precursors such as Na2S, TAA could diffuse into the cationic-exchangeable ionomer membranes much more uniformly. This led to the formation of homogeneously distributed CdS nanoparticles in the Nafion membranes, which was confirmed by elemental mapping with energy-dispersive X-ray (EDAX) analysis. Results from the characterizations on the physical properties, the chemical stability, and the photocatalytic properties of these CdS nanoparticles embedded in Nafion membranes are presented and discussed. The parallel data from the CdS nanoparticles in Nafion membranes prepared from the ionic Na2S precursor are also shown for comparison.

1. Introduction Crystalline cadmium sulfide (CdS) is an important visible-light-sensitive semiconductor with a band gap energy of 2.42 eV. In the past decade, nanocrystalline CdS has attracted much attention due to its unique physicochemical properties such as quantum size effect, nonlinear optical behavior, and photocatalytic function.1 It is known that the physical and chemical properties of semiconductor nanoparticles are strongly related to their size, assemble mode, and dimension.2 Therefore, it is of great interest to prepare crystalline CdS nanoparticles with controllable size, size distribution, and microstructure. Among the various synthetic methods, template synthesis is one of the most efficient methods for the preparation of homogeneously distributed CdS nanoparticles.3-5 Nafion is a well-known ionomer membrane as a solid proton-conducting electrolyte in electrochemical technology. In the structure of Nafion, there is a hydrophobic poly(tetrafluoroethylene) component with regularly spaced short perfluorovinyl ether side-chains, each terminated with a highly hydrophilic sulfonate group. The Nafion membranes possess many excellent properties, including high thermal stability (up to 200 °C in air), high mechanical strength, chemical inertness, and nanoporous structure. The ion cluster model (Figure 1) has been proposed to describe the porosity of Nafion.6 In this model, the Nafion structure is composed of numerous hydrophilic ionic clusters (pores) with diameters on the order of 40-50 Å that are interconnected by channels within the hydro* To whom correspondence should be addressed. Fax: +86-59183738608. E-mail: [email protected]. (1) (a) Henglein, A. Chem. Rev. 1989, 89, 1861-1873. (b)Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41-53. (2) Barnet, R. N.; Landman, U. Nature 1997, 387, 788-791. (3) (a)Hilinski, E. F.; Lucas, P. A.; Wang, Y. J. Chem. Phys. 1988, 89, 3435-3441. (b) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927-6931. (4) a) Rollins H. W.; Whiteside T.; Shafer G. J.; Ma J. J.; Tu M. H.; Desmarteau D. D.; Sun Y. P. J. Mater. Chem. 2000, 10, 2081-2084. (b) Rollins, H. W.; Lin, F.; Johnson, J.; Ma, J. J.; Tu, M. H.; Desmarteau, D. D.; Sun, Y. P. Langmuir 2000, 16, 8031-8036. (5) Duxin, N.; Liu, F. T.; Vali, H.; Eisenberg, A. J. Am. Chem. Soc. 2005, 127, 10063-10069, and references therein. (6) Hsu, W. Y.; Gierke, T. D. J. Membrane Sci. 1983, 13, 307-326.

Figure 1. Cluster-network model for Nafion proposed by Gierke et al. to show the dimensions of the clusters and channels.6

phobic perfluorocarbon matrix. Such hydrophilic pores in Nafion make it quite suitable as a host matrix for the encapsulation of semiconductor nanocrystals.7 Moreover, the exchangeability nature of H+ ions on the sulfonate group in the polymeric structure allows for the syntheses of nanoparticles through simple ion-exchange reactions.3a A variety of semiconductor nanoparticles, including CdS, PbS, Ag2S, Fe2O3, Fe3O4, and TiO2, have been synthesized in the hydrophilic nanocavities of Nafion membranes.3,4,8-13 (7) Murry, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (8) (a) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002-7003. (b) Mau, A. W. H.; Huang, C.-B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537-6542. (c) Kakuta, N.; White, J. M.; Campion, A.; Bard, A. J.; Fox, M. A.; Webber, S. E. J. Phys. Chem. 1985, 89, 48-52. (d) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 732-734. (e) Smotkin, E. S.; Brown, R. M.; Radenburg, L. K.; Salomon, K.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1990, 94, 7543-7549. (9) Albu-Yaron, A.; Arcan, I. Thin Solid Films 1990, 185, 181-185. (10) Inoue, H.; Urquhart, R. S.; Nagamura, T.; Grieser, F.; Sakaguchi, H.; Furlong, D. N. Colloids Surf. A 1997, 126, 197-208. (11) (a) Liu, P.; Bandara, J.; Lin, Y.; Elgin, D.; Allard, L. F.; Sun Y.-P. Langmuir 2002, 18, 10398-10401. (b) Liu, P.; Sun, Y.-P.; Fu, X. Z.; Wang, X. X.; Li, D. Z. Chin. J. Inorg. Chem. 2003, 19, 350-354. (12) Nandakumar, P.; Vijayan, C.; Dhanalakshmi, K.; Sundararajan, G.; Kesavan, N. P.; Murti, Y. V. G. S. Mater. Sci. Eng. B 2001, 83, 61-65.

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The Nafion-templating approach has several distinct advantages. First, the Nafion membrane provides a stable matrix to prevent the agglomeration and corrosion of the nanoparticles included.4b,11,14 Second, the optical, electrical, and catalytic properties of the nanoparticles embedded in the template may be remarkably modified by the hydrophilic regions in the Nafion membrane. Third, Nafion membranes have low absorbance in the UV-vis region, and their hydrophilic cavities and channels possess strong polarity and excellent ion-exchange properties. These characteristics may enhance the adsorptive capacity of the materials, leading to the enrichment of pollutants on the surface of catalytic nanoparticles in water. These adsorbed substrates may accelerate the separation of electron-hole pairs upon photoexcitation and, thus, improving the photocatalytic efficiency of semiconductor nanoparticles. Another advantage is that the nanoparticles embedded in Nafion membrane are easy to handle and recycle for catalytic purposes.15 Here we report the synthesis of stable crystalline CdS nanoparticles homogeneously distributed in Nafion membranes by using thioacetamide (TAA) as the precursor. Energy-dispersive X-ray analysis (EDAX) was employed to determine the line and surface element distribution in the cross-sections of the Nafion membrane. The chemical stability and the photocatalytic properties of the membranes with embedded CdS nanoparticles are also discussed, in comparison to those prepared from a conventional precursor of Na2S. 2. Experimental Section Materials. Cadmium nitrate tetrahydrate (Cd(NO3)2‚4H2O, analytic reagent grade), thioacetamide (CH3CSNH2, analytic reagent grade), sodium sulfide nonahydrate (Na2S‚9H2O, analytic reagent grade), and rhodamine B (tetraethyl-rhodamine, RhB, laboratory reagent grade) were purchased from Shanghai Chemical Reagents Company, China, and used without further purification. Deionized water was used throughout this study. Nafion-117 membrane with an equivalent mass of 1100 mg/ mmol and a thickness of 0.178 mm was provided by the DuPont Company. The membrane samples were cut into a suitable size and purified to remove colored impurities by means of a common procedure.16 In the purification, membrane samples were immersed in a concentrated nitric acid solution with stirring at 60 °C for 24 h. The acid was then decanted, and the samples were immersed sequentially in aqueous solutions of 60, 40, and 20% nitric acid with stirring, each for 1 h. They were then washed thoroughly with deionized water until the pH was neutral. The purified membrane samples were colorless and optically transparent down to 230 nm, and were kept fully hydrated before and during the preparation of nanoparticles. Characterizations. Powder X-ray diffraction (XRD) measurements were carried out on a Bruker D8 Advance X-ray Diffractometer. UV-vis absorption spectra were recorded on a Cary-500 UV-vis-NIR spectrophotometer. Scanning electron microscopy (SEM) and energy-dispersive X-ray analyses (EDAX) were conducted on a Philips LEO-1530 SEM system. Preparation of CdS-Nafion Membranes. In a typical procedure, a purified Nafion membrane was soaked in a Cd(NO3)2 (0.5 mol‚L-1) aqueous solution overnight, followed by rinsing with water and immersing in deionized water for 24 h. The film was then soaked in a TAA solution (0.2 mol‚L-1) for 20 min at room temperature and 30 min at 70 °C with stirring. After thorough washing and immersing in deionized water for 24 h, (13) (a) Mauritz,K. A. Mater. Sci. Eng. C 1998, 6, 121-133. (b) Mauritz, K. A.; Payne, J. T. J. Membrane Sci. 2000, 168, 39-51. (14) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 5028-5031. (15) Pathak, P.; Meziani, M. J.; Li, Y.; Cureton, L. T.; Sun, Y. P. Chem. Commun. 2004, 1234-1235. (16) Bunker, C. E.; Rollins, H. W.; Ma, B. J. Photochem. Photobiol. A 1999, 126, 71-76.

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Figure 2. X-ray diffraction patterns of (a) CdS(Na2S)-Nafion, (b) CdS(TAA)-Nafion, and (c) β-CdS from the JCPDS database. a light-yellow CdS-encapsulated Nafion sample (denoted as CdS(TAA)-Nafion) was obtained. As a comparison, a conventional precursor of Na2S was also used to synthesize another set of CdS-encapsulated Nafion membranes, as described by Sun et al.4 In a typical procedure, a purified Nafion membrane presoaked with Cd(NO3)2 was immersed in an aqueous solution of Na2S (0.2 mol‚L-1) for 30 min with vigorous stirring at room temperature. After thorough washing and immersing in deionized water for 24 h, a yellow CdS-encapsulated Nafion sample (denoted as CdS(Na2S)-Nafion) was obtained. Photocatalytic Activity Measurements. A 500 W highpressure xenon lamp (CHF-XM500W) was used as a visible light source with two cutoff filters to cut off the light below 450 nm and above 850 nm. The intensity of light on the surface of the reactor is 80 mW/cm2. RhB was used as the probe to study the photocatalytic activity of the samples. The photocatalytic degradation experiments were carried out in a commonly used batch photochemical reactor. The reactor, a cubic quartz cell, was cooled by a fan to keep the reaction temperature around 30 °C. In a typical reaction, 5 mL of 1.0 × 10-5 mol‚L-1 RhB solution and CdS-Nafion membrane with CdS-equivalent concentration of 1 g‚L-1 were used. The reaction was monitored at 554 nm via UVvis spectroscopy at the given irradiation time intervals.

3. Results and Discussion XRD and Optical Absorption. The XRD patterns of CdS(Na2S)-Nafion and CdS(TAA)-Nafion samples are shown in Figure 2. Both samples display diffraction peaks at 2θ of 26.5, 43.9 and 52.1°, which correspond to the diffraction of (111), (220), and (311) lattice plane of β-CdS crystal, respectively. The results reveal that the obtained crystalline CdS is cubic phase CdS with a zinc blend structure.12 Compared with the diffraction peaks for bulk CdS, all of the characteristic peaks in Figure 2 are broadened, in agreement with the nanoscopic nature of the crystalline CdS particles. It is noted that the fwhm (full width at half-maximum) of the XRD pattern of CdS(TAA)-Nafion is wider than that of CdS(Na2S)-Nafion. In addition, the intensity of the peaks for the former is weaker than the later. These might be due to the lower crystallinity of CdS nanoparticles in the CdS(TAA)-Nafion membrane than in the CdS(Na2S)-Nafion membrane. According to the Scherrer equation17 using the peak corresponding to the (111) lattice plane, the average diameters of the CdS nanocrystals in the CdS(Na2S)-Nafion and CdS(TAA)Nafion membranes are estimated to be 4.1 and 2.9 nm, respectively. The UV-vis absorption spectra of the CdS nanoparticles in the Nafion membranes are shown in Figure 3. For the CdS(Na2S)-Nafion and CdS(TAA)-Nafion samples, the absorption edges are 508 and 501 nm, respectively, (17) Klug, H. P.; Alexander, L. E., X-ray Diffraction Procedures; John Wiley and Sons: New York, 1959.

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Figure 3. UV/vis absorption spectra of the CdS(TAA)-Nafion (s) and CdS(Na2S)-Nafion (- - -).

Figure 5. UV-vis spectra of (a) CdS(TAA)-Nafion and (b) CdS(Na2S)-Nafion membranes dipped in 1mol‚L-1 HCl solution.

Figure 4. Line and surface elemental distribution (both Cd and S) from EDAX analyses of the cross-sections of the CdSembedded Nafion membrane: (a) the line and (b) the surface distribution pattern for the CdS(TAA)-Nafion sample and (c) the line and (d) the surface distribution pattern for the CdS(Na2S)-Nafion sample.

whereas for the bulk CdS crystal, it is 520 nm.18,19 The blue-shift of the absorption edge relative to that of bulk CdS suggests the presence of a quantum size effect. In addition, the blue-shift for the CdS(TAA)-Nafion sample is larger than that for the CdS(Na2S)-Nafion sample, (18) Ludolph, B.; Malik, M. A.; O’Brien P.; Revaprasadu, N. Chem. Commun. 1998, 1849-1850. (19) Johnson, B. J. S.; Wolf, J. H.; Zalusky, A. S.; Hillmyer, M. A. Chem. Mater. 2004, 16, 2909-2917.

which is consistent with the result from XRD analysis that the CdS nanoparticles in the former have a smaller particle size. Cross-Sectional Elemental Mapping. In the literature, sulfide salts, such as Na2S, were preferred to be used as a precursor4a to H2S gas3,8,20 in the template synthesis of CdS nanoparticles in ionomer membranes, because the former could be carried out much more efficiently. However, when such an ionic precursor was used in water, the precursor can undergo hydrolysis and thereafter, producing anions (e.g., S2- and HS-). The anions typically suffer from the diffusion limitation and therefore are difficult to penetrate deep into the membranes. This diffusion limitation is caused by the excluding effects of the sulfonate anions on the surface of cavities and channels in the membrane (Figure 1) considering the cationexchange characteristics of the Nafion membranes. Thus, it is expected that there is preferential accumulation of anions near the membrane surface, which might result in the preferential formation of CdS at the same locations. This effect is clearly confirmed by a cross-sectional EDAX elemental mapping of Cd and S across the CdS(Na2S)Nafion membrane sample. As shown in Figure 4, panels c and d, it is obvious that the amounts of both Cd and S near the surface of the membranes are much larger than that inside the bulk of the CdS(Na2S)-Nafion membrane. This result reveals that CdS nanoparticles are preferentially formed near the surface of the membrane. To obtain a homogeneous distribution of CdS nanoparticles in the Nafion membrane, a nonionic precursor, TAA, was chosen as the precursor for the synthesis of CdS. TAA decomposes upon heating and releases H2S gas, which is the precursor in the traditional template synthesis of CdS nanoparticles.3,8,20 Since TAA is nonionic, it can (20) Nandakumar, P.; Vijayan, C.; Murti, Y. V. G. S. Bull. Mater. Sci. 1997, 20, 579-582.

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Figure 6. UV-vis spectral changes of RhB aqueous solution in the presence of CdS(TAA)-Nafion membrane (third recycling degradation) as a function of irradiation time.

Figure 7. Effect of illumination time on the degradation of RhB when using (a) CdS(TAA)-Nafion and (b) CdS(Na2S)Nafion membrane samples.

uniformly diffuse into the Nafion membrane and remain stable until reaching equilibrium. This is supported by the cross-sectional EDAX elemental mapping of the CdS(TAA)-Nafion membrane. As shown in Figure 4, panels a and b, both line and surface distributions of Cd and S in the cross-sections of the CdS(TAA)-Nafion membrane were rather homogeneous, in sharp contrast to those of the CdS(Na2S)-Nafion sample. The results confirm that CdS nanoparticles are homogeneously distributed throughout the Nafion membrane, providing direct evidence that supports the advantage of using TAA as a precursor for the synthesis of highly distributed CdS nanoparticles in Nafion membranes. Chemical Stability. Unlike the naked CdS nanoparticles, those embedded in Nafion membranes are in fact quite stable in either air or acidic media. For example, there is negligible change of the absorption spectra and the color of both CdS(TAA)-Nafion and CdS(Na2S)Nafion membranes prepared in this work, even after they were exposed to air for nine months.

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These membranes are also stable toward acid treatment. As shown in Figure 5, there is little change in the absorption edges in the UV-vis spectra for the CdSembedded Nafion samples treated with a 1 mol‚L-1 hydrochloric acid solution for 130 min, indicating the excellent chemical stability of these nanoparticles. In addition, the nanoparticles in the CdS(TAA)-Nafion sample seem to be more stable than those in the CdS(Na2S)-Nafion sample in acid since there is less blue shift in the absorption edge upon immersion into the acid in the former (Figure 5a). This is consistent with the more homogeneous dispersion of the CdS nanoparticles in that sample. These stability testing results reveal that the Nafion membranes play an important role in the protection of the CdS nanoparticles from attack of either oxygen (in air) or acid (in aqueous solution). It should be noted that there is an initial red shift in the absorption followed by a smaller blue shift on further exposure to the acid for both samples. The initial red shift might be attributed to the different chemical environments in which the membranes are immersed. For the 0 min spectrum, the membranes are immersed in a neutral medium, whereas for the other spectra, it is an acidic medium. The continuously smaller blue shift on further exposure to the acid may be due to the partial etching of CdS nanoparticles in acidic solution.21 Photocatalytic Activity for Repeated Uses. One of the advantages of CdS nanoparticles synthesized in the Nafion membranes is that they may be conveniently recycled for catalytic purposes with little reduction in their catalytic activity after repeated uses. The photocatalytic activity of the CdS-embedded Nafion membrane was evaluated in terms of the CdS-catalyzed photodegradation of RhB in an aqueous solution upon which its optical absorption features (e.g., 554 nm) diminish. As shown in Figure 6, even after the third recycling of the CdS(TAA)Nafion membrane, there was still appreciable degradation of RhB in the aqueous solution upon irradiation. In fact, there was no evident change in the photocatalytic activity for the second and third repetitive uses of the same CdSembedded Nafion membranes (Figure 7). This suggests that these CdS nanoparticles in Nafion membranes indeed have favorable photochemical stability. In addition, the photocatalytic degradation of RhB obeys pseudo-first-order kinetics. The rate constants for the three runs were respectively 0.005, 0.009, and 0.009 min-1 over the CdS(TAA)-Nafion membrane and 0.004, 0.005, and 0.005 min-1 over the CdS(Na2S)-Nafion membrane. These results indicate that the photocatalytic efficiency of the CdS(TAA)-Nafion membrane is much higher than that of the CdS(Na2S)-Nafion membrane, likely due to the much more homogeneous distribution of CdS nanocrystals in the former. 4. Conclusions Crystalline CdS nanoparticles were prepared in Nafion ionomer membranes by using both nonionic and ionic precursors. It was found that the CdS nanoparticles were homogeneously dispersed across the membrane when using the nonionic TAA as a precursor. This is in sharp contrast to the particle distribution in the membrane when ionic Na2S was used as a precursor, in which CdS was predominantly formed near the surface. Unlike the unprotected CdS nanocrystals, those embedded in Nafion membranes prepared here were in fact quite stable in (21) Guo, W. Z.; Li, J. J.; Wang, Y. A.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 3091-3909.

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either air or acidic media. In addition, these nanoparticles are recyclable for photocatalytic purposes in that they remained similarly active even after repeated uses. Acknowledgment. This work was financially supported by the National Natural Science Foundation of

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China (Nos. 20373010, 20473017, and 20133010). The authors are indebted to Dr. Yi Lin at Clemson University for his instructive comment and kind help on preparing this paper. LA051072C