Ni Nanocomposite Spheres and Their

Jul 26, 2007 - In this paper we present a novel and facile method to fabricate polystyrene/nickel (PS/Ni) nanocomposite spheres. In this approach, the...
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J. Phys. Chem. C 2007, 111, 11829-11835

11829

Facile Fabrication Method of PS/Ni Nanocomposite Spheres and Their Catalytic Property Min Chen, Juan Zhou, Lin Xie, Guangxin Gu, and Limin Wu* Department of Materials Science, AdVanced Materials Laboratory, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: February 8, 2007; In Final Form: April 25, 2007

In this paper we present a novel and facile method to fabricate polystyrene/nickel (PS/Ni) nanocomposite spheres. In this approach, the negatively charged PS colloids bearing -COOH groups on the surfaces were used as templates, and the Ni2+ ions and a small amount of Ag+ ions were then adsorbed onto the surfaces of the PS colloids via electrostatic interaction and reduced in situ by hydrazine hydrate. Transmission electron microscopy, scanning electron microscopy, and energy-dispersive X-ray spectrometry indicated that metal nanoparticles were successfully deposited onto the surfaces of the PS colloids. The influence of some parameters, such as the amount of -COOH and PVP groups on the PS colloid surface, Ni2+ ion concentration, etc., on the morphology of the nanocomposite spheres was investigated. UV-vis spectra showed these nanocomposite spheres had a very good catalytic property.

Introduction Considerable efforts have been devoted to the design and morphology-controlled fabrication of metallodielectric colloidal core-shell composite spheres due to their wide applications in various fields1-10 such as surface-enhanced Raman scattering (SERS),11-13 photonic crystals,14-17 catalysis,18-19 nanoelectronic devices,20 biochemical tagging reagents,21-22 information storage,23 and so on. Basically, the intrinsic properties of these core-shell composite spheres are mainly determined by the shape, size, composition, crystallinity, and structure of the metal nanoparticles.24 Although a variety of chemical and physicochemical routes, including the inverse micelle method,25 electroless deposition,26 the layer-by-layer (LBL) self-assembly method,27 and the solvent-assisted method,4 have been explored for coating the dielectric particles (silica or polymer particles) with metal, most of the coating materials were the noble metal nanoparticles, such as Au or Ag.28-31 Recently, another important transitional metal, nickel, has continued to be of great interest in terms of its expected applications in high-density magnetic recording or magnetic sensors, catalysis, and Ni-based batteries.32-35 For instance, Tierno et al.36 prepared uniform and stable core-shell microspheres composed of a poly(methyl methacrylate) (PMMA) core and a thin metallic shell of nickel-phosphorus or other metal alloys via dispersion polymerization of MMA followed by electroless plating. By changing the deposited metallic materials, various magnetic properties, from paramagnetic to ferromagnetic, were achieved. Jin et al.33 prepared SiO2/Ni composite spheres and hollow nickel spheres via a two-step method as follows: first, nickel hydroxide [Ni(OH)2] was coated onto the surfaces of sol-gel-derived silica particles by a homogeneous precipitation of slowly decomposing urea in nickel nitrate solution, and then the dried composite spheres were reduced in H2 at 450 °C for 1 h to obtain SiO2/Ni composite spheres. By immersing the core-shell particle powder in 0.5 mol/L NaOH, the silica cores were dissolved by the alkaline solution and * To whom correspondence should be addressed. E-mail: lxw@ fudan.ac.cn, [email protected].

Figure 1. Sketch of the coating procedure of the PS colloid with Ni nanoparticles.

hollow nickel spheres were obtained. The catalytic activity of these spheres and the activity as carriers for noble metal catalysts were studied by acetone hydrogenation reaction. Using the same procedures, Wang et al.34 also prepared PS/Ni composite particles. In this paper, we present a novel and facile approach to obtain monodisperse PS/Ni composite spheres with a PS core coated by Ni nanoparticles via a one-step process. In this approach, the negatively charged PS colloids bearing -COOH groups on the surfaces were used as templates, and the Ni2+ ions and a small amount of Ag+ ions were then adsorbed onto the surfaces of PS colloids via electrostatic interaction and directly reduced in situ by hydrazine hydrate to form PS/Ni nanocomposite spheres. The influence of some parameters, such as the amount of -COOH and PVP groups on the PS colloids surface, Ni2+ ion concentration, etc., on the morphology of the nanocomposite spheres was investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometry, and X-ray diffraction (XRD). The catalytic performance of the PS/Ni nanocomposite spheres was

10.1021/jp0711085 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/26/2007

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TABLE 1: Summary of the Formulations To Prepare PS Colloidsa sample code

AA amt (g)

PVP amt (g)

sample code

AA amt (g)

PVP amt (g)

A0/P3 A2/P3 A4/P3 A6/P3

0 0.2 0.4 0.6

3 3 3 3

A4/P1.5 A4/P4.5 A4/P6

0.4 0.4 0.4

1.5 4.5 6.0

a Other conditions: St amount, 10 g; AIBN amount, 0.2 g; EtOH amount, 45 g; H2O amount, 5 g.

TABLE 2: Summary of Formulations To Prepare PS/Ni Nanocomposite Spheres at Different Conditionsa sample code

PS colloid

Ni(NO3)2‚6H2O amt (g)

NaOH vol (mL)

run 1 run 2 run 3 run 4 run 5 run 6 run 7 run 8 run 9 run 10 run 11

A0/P3 A2/P3 A4/P3 A6/P3 A4/P1.5 A4/P4.5 A4/P6 A4/P3 A4/P3 A4/P3 A4/P3

1.16 1.16 1.16 1.16 1.16 1.16 1.16 1.61 3.22 1.16 1.16

3 3 3 3 3 3 3 3 3 2 4

Figure 2. TEM images: (a) bare PS colloids of sample A4/P3 in Table 1; (b) PS/Ni composite spheres obtained with AgNO3 as seeds; (c) PS/ Ni composite spheres without AgNO3 as seeds; (d) bare nickel particles without PS colloid and AgNO3. The scale bar is 1 µm.

a PS dispersion amount, 5 g; N2H4‚H2O amount, 4.71 g; AgNO3 amount, 5 mg; EG amount, 95 g.

investigated by studying the change of the optical density at the wavelength of the absorbance maximum of the dye with a UV-vis spectrometer. Experimental Section Materials. Styrene (St) was purchased from Shanghai Chemical Reagent Co. (China) and purified by treatment with a 5 wt % aqueous NaOH solution to remove the inhibitor. Poly(vinylpyrrolidone) (PVP; Mw ) 30 000), acrylic acid (AA), absolute ethanol, silver nitrate (AgNO3), sodium hydroxide (NaOH), hydrazine hydrate (N2H4‚H2O; 85%), ethylene glycol (EG), and nickel nitrate [Ni(NO3)2‚6H2O] were all purchased from Shanghai Chemical Reagent Co. and used as received. Methylene blue trihydrate (MBt; 373.9 g/mol) was supplied by Shanghai Jiaye Dye stuff Industry Co. (China) and used as received. Ultrapure water (>17 MΩ cm-1) from a Milli-Q water system was used throughout the experiment. Synthesis of Monodisperse PS Colloids. The monodisperse negatively charged PS colloids were prepared by dispersion polymerization described in our previous paper37 and as follows: all of the stabilizer (PVP), AIBN, and H2O and half of the St and ethanol were charged into a 250 mL three-neck flask equipped with a mechanical stirrer, a thermometer with a temperature controller, a N2 inlet, a Graham condenser, and a heating mantle. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for ca. 30 min and then heated to 70 °C under a stirring rate of 100 rpm for 1.5 h, followed by addition of the solution of the rest of the St and ethanol and the entire AA. The reaction was continued until the conversion of St reached 95% and then cooled to room temperature. The typical formulation in this study was as follows: St, 10 g; AA, 0.4 g; PVP, 3 g; AIBN, 0.2 g; EtOH, 45 g; H2O, 5 g. The obtained particles were separated from the reaction medium by centrifugation at ∼3000 rpm, washed twice with ethylene glycol, and redispersed in ethylene glycol for further use (the solid content was controlled at 10%). A series of PS colloids with different amounts of -COOH and PVP

Figure 3. XRD pattern of run 3 in Table 2 after removal of the template PS colloids.

groups on the surfaces were synthesized using the formulations as shown in Table 1. Preparation of PS/Ni Nanocomposite Spheres. The typical coating process was illustrated in Figure 1: 5 g of PS colloid dispersion, 3 mL of 1 mol/L NaOH aqueous solution, 4.71 g of N2H4‚H2O and 55 g of EG were charged into a 250 mL roundbottom flask equipped with mechanical stirrer, thermometer with a temperature controller, a Graham condenser and a heating mantle, and stirred at room temperature for 5 min. The reaction mixture was then heated to 60 °C slowly, followed by dropwise addition of a mixture solution containing 1.16 g of Ni(NO3)2‚ 6H2O, 5 mg of AgNO3 and 40 g of EG over a period of 40 min and then kept at 60 °C for 2 h. During the reaction, the color of the solution gradually turned black, indicating the reduction of Ni2+. The obtained composite spheres were separated from the reaction medium by centrifuged at ∼1500 rpm, washed for several times with ethanol and then redispersed in ethanol for further examination. (The solid content of the composite spheres was controlled at 5%.) The coating reactions were carried out at different conditions, as summarized in Table 2. Catalytic Property of the PS/Ni Composite Spheres. A 0.1 g portion of alcoholic PS/Ni solution and 0.017 g of dye, MBt, were dissolved in 90 g of deionized water and then rapidly

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Figure 4. TEM and SEM images of PS/Ni composite spheres obtained from the PS colloids with various AA amounts (a, 0 g; b, 0.2 g; c, 0.4 g; d, 0.6 g). The scale bars are 1 µm for the TEM images.

injected with 10 mL of aqueous solution containing 0.057 g of NaBH4 under stirring. The change in color of the system could be indicated by the variation in optical density at the wavelength of the absorbance maximum (λmax) of the dye. Characterization of PS/Ni Nanocomposite Spheres. Morphology of Nanocomposite Particles. A transmission electron microscope (TEM Hitachi H-600, Hitachi Corp., Japan) was used to observe the morphology of the obtained PS colloids and PS/Ni composite spheres. The particles dispersions were diluted, ultrasonicated at 25 °C for 10 min, and then dried onto carbon-coated copper grids before examination. SEM images were obtained using a scanning electron microscope (SEM Philips XL30 apparatus). The particle dispersions were diluted and dried on a cover glass prior to examination. Elemental analysis was characterized by an energy-dispersive X-ray spectrometer. X-ray Analyzer. The dried PS/Ni composite spheres were immersed in toluene for 1 h to remove the core PS particles, and the residue was separated by centrifugation at ∼3000 rpm for 15 min, dried in a vacuum oven at 40 °C overnight to yield dried powders, and examined with a Rigaku D/Max-RA X-ray diffraction meter using Cu KR radiation (λ ) 0.15418 nm). UV-Vis Spectrum. UV-vis adsorption spectra were recoded at room temperature on a UV-vis spectrometer (Mapada, UV1800PC spectrophotometer, China). A quartz cell with a 1 cm optical path length was used.

Results and Discussion Role of the AgNO3. The monodisperse negatively charged PS colloids were prepared via dispersion polymerization based on the procedures described in the Experimental Section. The typical TEM image, as demonstrated in Figure 2a, showed uniform spherical PS colloids with an average diameter of 1.1 µm were obtained for the sample A4/P3 in Table 1. In the subsequent coating process, Ni(NO3)2‚6H2O and a trace of AgNO3 dissolved in EG were dropwise added to the PS dispersion containing NaOH and N2H4‚H2O. The color of the system gradually turned black, suggesting the Ni2+ ions were reduced and the composite spheres with the PS core coated with Ni nanoparticles were obtained, as demonstrated in Figure 2b. However, a control experiment without AgNO3 displayed needle-like nickel particles coexisted with PS colloids randomly, and no typical core-shell composite spheres were found, as shown in Figure 2c, indicating that the Ag+ plays an important role in the formation of PS/Ni composite spheres with a coreshell structure. This could be explained as follows: Ag+ ions on the PS colloids were first reduced by N2H4‚H2O, and then as-formed Ag nanoparticles served as nucleation sites. Figure 2d further demonstrated the morphology of the nickel particles prepared in the absence of PS colloids and AgNO3; the aggregation of hedgehog-like nickel particles could be easily seen.

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Figure 5. TEM and SEM images of PS/Ni composite spheres obtained from the PS colloids with different PVP amounts (a, 1.5 g; b, 3.0 g; c, 4.5 g; d, 6.0 g). The scale bars are 250 nm for the TEM images.

The as-prepared PS/Ni core-shell composite spheres (after separation from the reaction medium by centrifugation) were immersed in toluene to remove the template PS particles and then dried to be examined with XRD, as shown in Figure 3. The diffraction peaks in the range 25° < 2θ < 85° can be indexed as face-centered cubic (fcc) Ni(111), Ni(200), and Ni(220), which was in good accordance with ASTM (American Society for Testing and Materials) standard 4-850. Effect of the -COOH Groups on the Surfaces of the PS Colloid. In our method, the monodisperse negatively charged PS colloids were obtained by using AA as a comonomer, which ensured the Ag+ and Ni2+ ions could be rapidly captured by PS colloids via electrostatic interaction; a similar strategy was used in our previous work.38,39 The metal ions were then reduced in situ by hydrazine hydrate directly to form PS/Ni nanocomposite spheres. To investigate the effect of the -COOH group amount of the PS colloid surfaces on the morphology of the composite spheres, a series of PS colloids with different AA contents were prepared and then coated with Ni nanoparticles (runs 1-4 in Table 2). From the TEM and SEM images in Figure 4, it can be seen there was still a very small amount of the metal particles deposited onto the surfaces for the PS colloid without -COOH groups on the surfaces (run 1, Figure 4a1,a2), which could be attributed to the good affinity between PVP with Agas reported before.40 When the AA amount increased to 0.2 and 0.4 g in the formulation (runs 2 and 3), the rougher surfaces of the spheres compared with the original PS colloids

indicated the formation of metal nanoparticles surrounding the core particles (Figure 4b1,b2,c1,c2), and the composite spheres displayed monodispersity. However, when the AA amount was further increased to 0.6 g, although the surface coverage degree obviously increased, the composite spheres tended to aggregate since many more Ni nanoparticles were deposited onto the surfaces of the PS colloid. Effect of the PVP Amount. The effect of the PVP charge on the TEM and SEM morphology of PS/Ni nanocomposite spheres is illustrated in Figure 5 (runs 3 and 5-7 in Table 2). When 1.5 g of PVP, which was the least amount to ensure the polymerization stability, was used to prepare the PS colloid, the corresponding PS/Ni nanocomposite spheres had a typical core-shell structure. As the PVP amount increased, the surface coverage degree of the PS/Ni composite spheres slightly increased, probably because of the good affinity between PVP and Ni. Effect of the Ni(NO3)2 Concentration. Figure 6 demonstrates the morphology of PS/Ni composite spheres as a function of the Ni(NO3)2‚6H2O concentration with the other parameters equal (runs 3, 8, and 9 in Table 2). When the Ni(NO3)2‚6H2O content was relatively low, e.g., 1.16 g, the PS colloid was coated by a small amount of Ni nanoparticles (see Figure 6a1,a2). As more Ni(NO3)2‚6H2O was used, more Ni nanoparticles were deposited onto the surfaces of the PS colloid (see Figure 6b1,b2). However, at higher Ni(NO3)2‚6H2O content, e.g., 3.22 g, some Ni nanoparticles aggregated, probably because of the

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Figure 6. TEM images of PS/Ni composite spheres obtained with various Ni(NO3)2‚6H2O contents (a1, a2, 1.16 g; b1, b2, 1.61 g; c1, c2, 3.22 g). The scale bars are 1 µm (a1-c1) and 250 nm (a2-c2).

TABLE 3: Elemental Analysis of the PS/Ni Nanocomposite Spheres Obtained at Various Ni(NO3)2‚6H2O Concentrations concn, wt % (at various Ni(NO3)2‚6H2O concentrations) element

1.16 g

1.61 g

3.22 g

C O Ni

83.52 9.34 7.14

64.29 2.41 33.3

61.92 3 35.08

fast self-nucleation of Ni nanoparticles at high Ni(NO3)2‚6H2O concentration (see Figure 6c1,c2). The change of the morphology of PS/Ni composite spheres with Ni(NO3)2‚6H2O concentration could be observed more intuitively from the SEM images of the composite spheres, as indicated in Figure 7. Effect of the NaOH Concentration. In this research, Ni2+ ions were reduced by N2H4‚H2O at basic condition as follows: 2Ni2+ + N2H4 + 4OH- f 2Ni + N2 + 4H2O. Thus the pH of the system should have a very crucial impact on the morphology of the composite spheres. To confirm this, the composite spheres were further prepared with 2 and 4 mL of NaOH (runs 10 and 11 in Table 2, respectively). In comparison with the PS/Ni nanocomposite spheres prepared with 3 mL of NaOH (see Figure 4c1), when 2 mL of NaOH was used, the needle-like Ni coexisted with the PS colloid and no discrete core-shell nanocomposite spheres were found, as seen in Figure 8a, which might be due to the incomplete reduction of Ni2+ under low basic concentration. However, when too much NaOH was used, e.g., 4 mL, Ni nanoparticles tended to aggregate first and then adsorbed onto the PS surfaces, as shown in Figure 8b, which was possibly because of the fast self-nucleation of Ni nanoparticles at high NaOH concentration. EDX Analysis. Figure 9presents the EDX elemental analyses of the PS/Ni nanocomposite spheres prepared from various Ni(NO3)2‚6H2O concentrations. As can be seen, the Ni element appeared in all the samples, which further indicates the successful deposition of Ni nanoparticles on the PS surfaces. Table 3 shows that the content of Ni increased from 7.14% to 35.08% distinctly with increasing Ni(NO3)2‚6H2O concentration, suggesting the surface coverage degree of the PS colloids by Ni nanoparticles increased, which was consistent with the TEM and SEM images in Figures 6 and 7. However, the content of Ni slightly increased from 33.3% to 35.08% when the Ni(NO3)2‚ 6H2O amount increased from 1.61 to 3.22 g, which was probably

Figure 7. SEM images of the PS/Ni composite spheres obtained at various Ni(NO3)2‚6H2O contents (a1, a2, original PS colloid; b1, b2, 1.16 g; c1, c2, 1.61 g; d1, d2, 3.22 g).

Figure 8. TEM images of PS/Ni composite spheres obtained with different NaOH amounts (a, 2 mL; b, 4 mL).

because of the self-nucleation of Ni particles, causing free Ni particles which were removed from the solution in the subsequent centrifugation process. Catalytic Property of the PS/Ni Composite Spheres. It has been experimentally demonstrated that metal nanoparticles have high catalytic activities for hydrogenation, hydroformylation, carbonylation, etc.,18,41,42 but in most cases, they would coalesce during the catalytic processes, because nanosized metal particles

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Figure 10. UV-vis spectra of MBt reduced by NaBH4 as a function of the reaction time and catalyst (a, 0.1 g of PS/Ni solution; b, 0.2 g of PS/Ni solution; c, 0.1 g of bare Ni solution) (other reaction conditions: dye MBt amount, 0.017 g; NaBH4 amount, 0.057 g; deionized water amount, 100 g).

a very slow reduction rate of the dye was observed, probably because of the big particle size of the bare Ni particles (600800 nm). This evidence confirmed that the as-prepared PS/Ni nanocomposite spheres had very good catalytic performance. Conclusions Figure 9. EDX spectra of the PS/Ni composite spheres obtained at various Ni(NO3)2‚6H2O contents (a, 1.16 g; b, 1.61 g; c, 3.22 g).

in the solution are active and tend to coalesce due to van der Waals forces and high surface energy unless they are protected. Here we also explored the catalytic property of the PS/Ni composite spheres preliminarily by studying the change of the absorbance density at the maximum absorbance wavelength (λmax) of the dye. Figure 10 illustrates the evolution of the UVvis spectra of the dye during the reaction of MBt reduced by NaBH4. When the system contained PS/Ni nanocomposite spheres, the absorbance at λmax of the dye quickly decreased with reaction time, and the greater the amount of PS/Ni composite spheres used, the lower the absorbance at λmax, with the other parameters equal (see Figure 10a,b). However, when the system contained bare Ni particles as shown in Figure 2d,

On the basis of this study, the nanocomposite spheres with PS as the core coated by Ni nanoparticles could be successfully prepared via a facile approach. To guarantee the formation of PS/Ni nanocomposite spheres, a small amount of Ag+ ions were employed as seeds. The surface coverage degree of PS with Ni nanoparticles could be controlled through altering the AA and PVP contents in the PS colloids and the Ni2+ ion concentration in the reaction system. As the AA and PVP contents and the Ni2+ ion concentration increased, more Ni nanoparticles were deposited onto the PS cores, but too many AA or Ni2+ ions would cause some aggregation of PS/Ni composite spheres, and a too low or too high NaOH content did not form nanocomposite spheres. A preliminary investigation indicated these nanocomposite spheres exhibited a very good catalytic performance. This method probably could be used for preparation of other composite spheres with dielectric core spheres (silica, polysty-

PS/Ni Nanocomposite Spheres rene, etc.) coated by different metal nanoparticles, especially transitional-metal nanoparticles. Consequently, a series of composite spheres possibly could be obtained. Acknowledgment. This work is financially supported by the Trans-century Outstanding Talented Scholar Foundation of China Educational Ministry, Shanghai Special Nano Foundation, and Shanghai Shuguang Scholar Foundation. References and Notes (1) Gittins, D. I.; Susha, A. S.; Schoeler, B.; Caruso, F. AdV. Mater. 2002, 14, 508. (2) Ji, H. T.; Lirtsman, V. G.; Avny, Y.; Davidov, D. AdV. Mater. 2001, 13, 1253. (3) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (4) Zhang, J. H.; Liu, J. B.; Wang, S. Z.; Zhan, P.; Wang, Z. L.; Ming, N. B. AdV. Funct. Mater. 2004, 14, 1089. (5) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Angew. Chem., Int. Ed. 2006, 45, 813. (6) Shi, W.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Langmuir 2005, 21, 1610. (7) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (8) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743. (9) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (10) Kim, K.; Kim, H. S.; Park, H. K. Langmuir 2006, 22, 8083. (11) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (12) Dick, L. A.; Mcfarland, A. D.; Haynes, C. L.; Vanduyne. J. Phys. Chem. B 2002, 106, 853. (13) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Phys. Chem. 1999, 111, 4729. (14) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (15) Liang, Z.; Susha, A. S.; Caruso, F. AdV. Mater. 2002, 14, 1160. (16) Zhang, W. Y.; Lei, X. Y.; Wang, Z. L.; Zheng, D. G.; Tam, W. Y.; Chan, C. T.; Sheng, P. Phys. ReV. Lett. 2000, 84, 2853. (17) Wang, Z. L.; Chan, C. T.; Zhang, W. Y.; Chen, Z.; Ming, N. B.; Sheng, P. Phys. ReV. B 2001, 64, 113 108. (18) Jiang, Z. J.; Liu, C.-Y.; Sun, L.-W. J. Phys. Chem. B 2005, 109 (5), 1730.

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