Preparation, Characterization, and Catalytic Effect of CS2-Stabilized

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Articles Preparation, Characterization, and Catalytic Effect of CS2-Stabilized Silver Nanoparticles in Aqueous Solution Xuchuan Jiang, Yi Xie,* Jun Lu, Liying Zhu, Wei He, and Yitai Qian Structure Research Laboratory and Laboratory of Nanochemistry & Nanomaterials, University of Science and Technology of China, Hefei Anhui 230026, People’s Republic of China Received September 25, 2000. In Final Form: March 9, 2001 CS2-stabilized silver nanoparticles of ∼6 nm in diameter are prepared in aqueous solution via chemical reduction of Ag+ ions by KBH4 in the presence of CS2. The product is proven to be Ag by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron diffraction (ED) patterns. In the UV-visible spectra, an intense surface plasmon is built up at 350-500 nm, and it is centered at 395 nm after the reduction of Ag+ ions. This can be assigned to a reaction product of Ag with CS2 because of the strong affinity of S on the surface of Ag particles. The silver nanoparticles in the presence of CS2 remain stable for months at room temperature. It is also found that the absorption band in the range 300-350 nm increases, is centered at 325 nm, and depends on the concentration of CS2, which maybe a derivative (CS or CnSm of CS2). Again the catalytic reduction of a dye, 2,7-dichlorofluorescein (DCF), in the presence of CS2-stabilized silver particles is investigated.

Introduction Nanoparticles of silver and other noble metals, in a more general sense, exhibit a variety of useful optical, electrical, and catalytic properties.1-4 All these properties strongly depend on the careful choice of preparative strategies, which could provide long-term stability and ease in processability.5 Various reduction processes have been used to prepare stable silver particles.6-10 However, the passivated thiols prepared in organic solvents may interfere with the surface functionalization of silver cores, with a surface for gold,11-17 and the alkanethiols with long alkyl chains are not appropriate for dispersion in aqueous solution, which limits their applications in the field of materials. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer Series in Material Science, No. 25; Springer-Verlag: Berlin, 1995; pp 187-201. (2) Meisel, D. J. Phys. Chem. B 1998, 102 (43), 8364. (3) Leggett, G. J. J. Phys. Chem. B 1998, 102 (1), 174. (4) Jana, N. R.; Sau, T. K.; Pal, T. J. Phys. Chem. B 1999, 103 (1), 115. (5) Schmid, G. Chem. Rev. 1992, 92, 1709. (6) Sato, T.; Ruch, R. Stabilitzation of Colloidal Dispersions by Polymer Adsorptions; Surfactant Science Series, No. 9; Marcel Dekker: New York, 1980; pp 65-119. (7) Aikens, P. A. J. Phys. Chem. B 2000, 104 (6), 1176. (8) Giersig, M. J. Phys. Chem. B 1999, 103 (44), 9533. (9) Sviridov, V. V. J. Phys. Chem. B 1997, 101 (41), 8129. (10) Quaroni, L.; Chumanov, G. J. Am. Chem. Soc. 1999, 121 (45), 10642. (11) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storheff, J. J. Nature 1996, 382 (6592), 607. (12) Watson, K. J.; Zhu, J.; Nguyen, S. B. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462. (13) Ulman, A. J. Mater. Educ. 1989, 11, 205. (14) (a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (b) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (15) Ihs, A.; Liedberg, B. Langmuir 1994, 10, 734. (16) Rao, C. N. R. J. Phys. Chem. B 1997, 101 (48), 9876. (17) Harfenist, S. A.; Wang, Z. L. J. Phys. Chem. B 1999, 103, 4342.

Here, we reported carbon disulfide (CS2)-stabilized Ag colloid preparation without adding any other stabilizers. CS2 was added to the aqueous solution of silver nitrate before the addition of the reducing agent KBH4. A notable decrease in the particle size to ∼6 nm could be obtained. Henglein18a reported the adsorption of CS2 onto the Ag particles synthesized by UV irradiation, and Kanjire19 reported the UV-visible spectra of CS2-derivatized gold nanoparticles. In our experiments, we have not only observed the 395-nm surface plasmon absorption band of Ag nanoparticles but also discussed the 325-nm absorption band resulting from the derivatives of CS2 ([CS2] g 0.20 mmol‚dm-3) reacting with Ag colloids. In addition, the CS2-stabilized Ag nanoparticles were used as catalyst to reduce 2,7-dichlorofluorescein (DCF), and it has been shown that the growing Ag particles have higher catalytic activity than the full-grown particles.4 Experimental Section Materials. Carbon disulfide (CS2)20 was analytical grade from Wako Chemical, and trace impurities were removed, and the final purity was verified by a single peak in the gas chromatograph. KBH4 and AgNO3 were reagent grade. Deionized water was used in our experiments. Preparation. Ag particles could be prepared from AgNO3 solution by adding reducing agents such as NaBH4,21a sodium ascrobate,21b or N2H4.21c In this work we used KBH4 as a reducing (18) (a) Henglein, A.; Meisel, D. J. Phys. Chem. B 1998, 102, 8364. (b) Henglein, A. Chem. Mater. 1998, 10, 444. (c) Mulvaney, P. Henglein, A. J. Phys. Chem. 1990, 94, 4187. (d) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (e) Mulvaney, P. Langmuir 1996, 12, 788. (f) Mulvaney, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991, 95, 7843. (19) Kanjire Torigoe, Kunio Esumi. J. Phys. Chem. B 1999, 103 (15), 2862. (20) The solubility of CS2 in water is 20.4 mmol‚dm-3 at 30 °C. See: Gmelin Hanbook of Inorganic Chemistry, 8th ed.; Springer: Berlin, 1977; Kohlenstoff, Vol. D4, p 193. (21) (a) Pal, T.; Sau, T.; Jana, N. R. Langmuir 1997, 13, 1481. (b) Pal, T.; Maity, D. S. Anal. Lett. 1985, 18, 1131. (c) Pal, T.; Maity, D. S.; Ganguly, A. Talanta 1986, 33, 255.

10.1021/la001361v CCC: $20.00 © 2001 American Chemical Society Published on Web 05/30/2001

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Figure 1. XRD pattern of products obtained from centrifuging the supersaturated suspensions of CS2-stabilized silver colloids. agent that was similar to NaBH4 in most cases. To 19.6 mL of AgNO3 aqueous solution (0.05-0.25 mmol‚dm-3) was added 0.02-2.0 mL of CS2 (20 mmol‚dm-3), and the mixture was stirred until CS2 was completely dissolved. Then 0.4 mL of ice-cooled freshly prepared KBH4 aqueous solution (0.3 mol‚dm-3) was quickly added to the above mixed solution under vigorous stirring. Upon addition of the KBH4, the solution gradually changed to a yellowish color for lower Ag+ ions concentrations (0.10 mmol‚dm-3), the solution became brown. Immediately a 2.5 mL portion was transferred to a quartz cell to measure the dependent UV-visible absorption spectra at 25 °C using a Spectrophotometer Specord 200 (Analytik Jena GmbH, Germany). Full grown and growing Ag colloids, which were produced after the complete reduction of Ag+ ions, were used as catalysts for the reduction of the anionic dye, 2,7-dichlorofluorescein.4

Results and Discussion This section is broken into several subsections. In the first subsection, the composition of the product was identified. Subsequently, the influence of several parameters on the UV-visible spectra of CS2 adsorbed onto the surface of Ag particles was investigated, and a possible reaction route was proposed during the reduction process. Finally, the catalytic effect of silver nanoparticles for the reduction of the dye of DCF is discussed. A. Identification of Ag Nanoparticles. To identify the components, the product was obtained through centrifuging the supersaturated suspensions of CS2stabilized silver nanoparticles carefully.22 The product was washed with deionized water and ethanol, respectively. The composition of the product was examined on a Japan Rigaku D/max-γ rotation anode X-ray diffractometer, using Ni-filtered Cu KR radiation. A scanning rate of 0.05° s-1 was applied to record the patterns in the 2θ range 10-70°. The temperature of the collection data is 25 °C. The XRD pattern of the product is shown in Figure 1, and the reflection peaks are indexed as 111, 200, and 220 planes, respectively, indicating that the product is silver with cubic symmetry (JCPDS, No. 4-783). In the meantime, X-ray photoelectron spectroscopy (XPS), model ESCALAB Mg KR (hυ ) 1253.6 eV) with a resolution of 1.0 eV, under a pressure of 1.0 × 10-4 Pa, was used to identify the composition of the product. From the XPS spectra (Figure 2) comes the XPS survey spectrum of Ag atoms. The O2, H2O, and CO2 molecules adsorb onto the Ag atoms (a), and the binding energy 368.1 eV for Ag3d (b) and the kinetic energy 895.4 eV for AgMNN (c) are authenticated from the XPS (Figure 2) spectra, respectively. This is consistent with that of Ag and not with that of Ag2S or Ag2O.23 In addition, the S atom is not detected (22) Korgel, B. A.; Zaccheroni, N.; Fitzmaurice, D. J. Am. Chem. Soc. 1999, 121, 3533.

Figure 2. XPS spectra of the products obtained from centrifuging supersaturated suspensions prepared in aqueous solution: (a) XPS survey spectrum of silver; (b) binding energy spectrum for Ag3d; (c) kinetic energy spectrum for AgMNN.

in the XPS survey spectrum, indicating that no Ag-S bond formed. The identification of product was also confirmed by the indexed electron diffraction (ED) patterns (shown in Figure 3). Transmission electron microscopy (TEM) and electron diffraction (ED) of CS2-stabilized Ag colloids are performed on a model Hitachi 800 transmission electron microscope, under the accelerating voltage of 200 kV, and the images are shown in Figure 3a and c. In contrast, the image of Ag nanoparticles reduced by KBH4 without any stabilizers is shown in Figure 3d. It is evident from the histograms (23) Wagner, C. D.; Riggs, W. W.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corporation, Physical Electronics Division: Printed in USA, 1979.

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Figure 3. TEM images, size distribution, and electron diffraction patterns of CS2-adsorbed Ag nanoparticles (a-c) and without addition of CS2 (d-f).

(Figure 3b and e) that the average diameter of CS2adsorbed Ag colloids is ∼6 nm, much smaller than that of those (∼28 nm) produced in the absence of CS2. In the latter case, the coagulation of silver particles can be seen in Figure 3e. The size distributions were obtained by measuring the diameters of 200 particles in an arbitrarily chosen area of a TEM image. The monodispersive colloids showed that the chemical adsorption of CS2 onto the surface of silver colloids prevented coagulation, although the CS2 or CnSm molecules on the surface cannot be observed due to their lower electron density. The ED patterns (Figure 3c and f) of Ag colloids show concentric circles due to random orientation of crystal planes. The radii of diffraction of polycrystalline rings are consistent with those data of XRD diffraction of silver. The reflection rings for CS2-Ag are indexed as 111, 200, 220, 311, and 420 planes in the ED pattern (Figure 3c), and those for Ag are indexed as 100, 110, 111, 200, 220, 311, and 420 planes in the ED pattern (Figure 3f), respectively. This difference is considered as caused by the silver particles unprotected by stabilizers crystallizing better than those stabilized by CS2 in aqueous solution. From the indexed

ED pattern (Figure 3c), the CS2-stabilized ones are identified to be Ag and not Ag2S, which is consistent with the XRD pattern (Figure 1). The statement is also verified by UV-visible spectra in the subsections that follow. B. UV-Visible Absorption Spectroscopy. The absorption spectra of the component solutions were measured prior to the Ag colloids’ preparation. As shown in Figure 4, 0.20 mmol‚dm-3 AgNO3 (solid line) shows an intense absorption band at ∼217 nm and a weak shoulder at ∼265 nm. On the other hand, 2.0 mmol‚dm-3 CS2 shows an absorption band at 206.6 nm (dot line), which is consistent with the literature.19 Moreover, Figure 4 shows the absorption spectra (short dash line) of the mixture immediately after KBH4 addition. It is found that the absorption at 265 nm for AgNO3 solution vanished and an intense surface plasmon band at around 395 nm appeared with a long tail extending to longer wavelength (500-800 nm). This suggests that it is a Ag24 and not a Ag2S absorption.25 In addition, it can also be found that (24) Yoko, M.; Hirotomo, H.; Kaoru, M.; Mitsumasa, T.; Mikio, H.; Shigeyoshi, A. J. Phys. Chem. B 1998, 102, 8389.

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Figure 4. UV-visible spectra for 0.20 mmol‚dm-3 AgNO3 (solid line), for 2.0 mmol‚dm-3 CS2 (dot line), and for Ag colloids prepared from the system of 0.2 mmol‚dm-3 AgNO3, 2.0 mmol‚dm-3 CS2, and 6.0 mmol‚dm-3 KBH4 (short dash line).

Figure 5. UV-visible spectra for silver colloids when [CS2] ) 2.0 mmol‚dm-3 [AgNO3] ) 0.15 mmol‚dm-3 and [KBH4] ) 6.0 mmol‚dm-3. The absorption bands centered at 325 nm are for CnSm or (CS), and those at 395 nm are for Ag colloids, respectively.

all the three systems do not absorb the light (λ > 600 nm); therefore, the UV-visible absorption range from 200 to 600 nm is employed in further investigations. Figure 5 shows the UV-visible spectra of the 395-nm band for Ag colloids and the 325-nm band for the products CnSm or (CS), after KBH4 is added to the mixture of AgNO3 and CS2 solutions. The 395-nm absorption band is assigned to be CS2-stabilized Ag colloids, although only parts (∼25%)18b of the Ag colloidal surface are occupied. Here we do not obtain the absorbance maximum at 275 nm for Ag42+ clusters,18c due to the Ag42+ clusters coagulating to be colloids or particles within a very short period of time. Van Hyning26 observed that the absorption peak associated with surface plasmon resonance of larger colloidal silver particles shifted from 375 to 385 nm within 0.5-1.5 s through the short time-evolution UV-visible spectra. The growth of Ag particles may be described as follows:27

Ag+ + Ag0 f Ag2+ f (Ag42+) f Agn clusters or colloids (1) Moreover, the absorption bands at 395 nm with similar morphology indicate that the Ag colloids have a similar (25) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (26) Van Hyning, D. L.; Zukoski, C. F. Langmuir 1998, 14 (24), 7034. (27) Zhu, Y. J.; Qian, Y. T.; Zhang, M. W.; Chen, Z. Y.; Lu, B.; Wang, C. S. Mater. Lett. 1993, 17, 314.

Jiang et al.

packing state,28 and they have slight blue-shiftings from 400 to 390 nm in Figure 5, which is caused by the nucleophile BH4- ions increasing the surface electron density of Ag colloids.29 From Figure 5, we also find that the 395-nm band develops with time, indicating that some reaction is taking place. After 700 s, the absorption maximum does not change again, which shows that the reduction reaction attains an equilibrium condition or all the Ag+ ions have been reduced to be Ag0. There are several possible reasons for this phenomenon, the most probable being an increase in the number density of particles that affects the surface plasmon band. According to the Drude model for dielectric properties, the Mie theory predicts that in the 6-10-nm particles size range the location of the surface plasmon peak does not change significantly but the absorbance maximum grows. The size of colloids can be estimated by the Mie theory and the calculation by Creighton and Eadon for spherical, monodispersive silver particles.18d,30 In addition, the surface plasmon peak is wide with a halfwidth of 100 nm for a particle diameter of 1 nm. While the particle is up to 10 nm in diameter, the half-width narrows and approaches a constant value of 25 nm. In our systems, the half-width of particles is about 65 nm, and the size of the 6-nm diameter particle is reasonable. The average size of particles is consistent with the observations in the TEM image (Figure 3a). Again the shift in the absorbance maximum may result from coagulation, or nonspherical particles, or particles involving density of states.18d,e Here an important factor is probably that the CS2-adsorbed molecules lead to the surface electron density of Ag colloids decreasing according to eq 4, and this results in the shifting of maximum to longer wavelengths, 395 nm relative to 385 nm.18d The effect of CS2 on the absorption spectra for silver colloids is also shown in Figure 5. A series of absorption bands centered at 325 nm are observed, and their absorbance maximum increases with time. The 325-nm absorption band may result from two factors, one is the products CnSm or (CS)n of CS2 reacting with Ag particles.18b,31 Another is speculated as the products of CS2 reacting with BH4- ions.30 It is worth noting that the change of absorption bands at 325 nm is consistent with that of Ag colloids at 395 nm. C. Reduction Process of Ag+ by KBH4. In this system for all practical purposes, KBH4 reduces Ag+ ions due to the BH4- itself and its oxide B(OH2)2- ions.32 Considering the reaction products of KBH4 and CS2 in aqueous solution, the reaction proceeded by two steps.19

BH4- + CS2 f -H3BSCHdS

(2)

5BH4- + 4CS2 f [B(SCH2S)4]5- + 2B2H6

(3)

The main factor for stabilizing Ag particles in water is considered as the adsorption of CS2 molecules onto the surface of Ag particles. This adsorption is irreversible, which suggests that the CS2 molecules may undergo a drastic change on the silver surface. CS2 is known to react with many metals at high temperature; with Raney-Ni (28) Quinten, M.; Schonauer, D.; Kreibig, U. Z. Phys. D 1989, 12, 521. (29) Blatchford, C. G.; Siimon, O.; Kerker, M. J. Phys. Chem. 1983, 87 (7), 2503. (30) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (31) Wang, C.; Zhang, X.; Qian, X.; Wang, W.; Qian, Y. T. Mater. Res. Bull. 1998, 33 (7), 1083. (32) Khain, V. Zh. Neorg. Khim. 1983, 28, 2482.

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even at room temperature.33 In fact, the silver colloidal particles sometimes behave like a base metal. For example, carbon tetrachloride is decomposed via electron transfer.18b Similarly, the reaction

Agn + CS2 f Agnδ+ + Sδ-(ads) + CS (or CnSm)

(4)

may occur. More detailed information about the effect of the nature of the adsorbed species upon the interaction of CS2 with silver might be obtained in a SERS experiment. The CS2 molecule is active and easily forms a long CnSm or (CS)n chain.31 The absorption band at 325 nm assigned to CnSm or (CS)n gives an explanation for the UV-visible spectra recorded in Figure 5. D. Catalytic Effect of Silver Colloids. The color of the dye remains unaltered for 1-2 h, if the dye solution is mixed with KBH4 solution, indicating that the dye reduction is insignificant. However, reduction of the dye by borohydride occurs very fast in the presence of AgNO3. Spiro34 and Miller35 reported the stable particles (where no Ag+ ions) could catalyze the dye reduction. Jana et al.4 reported the Ag colloids catalyzed DCF reduction in aqueous surfactant solutions. Stable and yellow CS2stabilized silver hydrosols, which were produced after the complete reduction of Ag+ ions, were used as catalyst. The hydrosols contain full grown silver particles (ca. 6 nm), which are the final products of growing colloids. For studying the catalytic effect of silver particles, the dye of DCF was selected and used here for its special properties.4 DCF (1.0 mL; 6.0 × 10-4 mol‚dm-3) was added to 20.0 mL of an aqueous AgNO3-CS2-KBH4 solution after the complete reduction of Ag+ ions assessed by the stable plasmon band of silver that does not change with time. For the catalytic reduction of the dye, the reducing agent used (KBH4) was 10-200-fold in excess. The presence of CS2 stabilized silver colloids from coagulating even in the presence of dye. In the reduction process of DCF, the induction period was not observed. Presumably, BH4- ions and the anionic dye of DCF not only adsorbed individually but also simultaneously on the particle surface.18f,29 As a consequence, a partial electronic charge transfer can occur from BH4- ions to the dye via the Ag particles. Here the comparative catalytic effect of CS2-adsorbed silver particles was discussed. The same initial concentrations of Ag+ ions (0.1 mmol‚dm-3) and CS2 (0.20 mmol‚dm-3) dissolved in aqueous solution were used to prepare growing and full grown silver particles for the catalytic rate comparison shown in Figure 6. From the successive spectra, the absorbance at 395 nm for growing Ag colloids increases from 0.12 to 0.98 while that for full(33) Bougault, J.; Cattelain, E.; Chabrier, P. Compte Rendus 1939, 208, 657. (34) Spiro, M. J. Chem. Soc., Faraday Trans. 1 1979, 75, 1507. (35) Miller, D. S.; Bard, A. J.; Mclendon, G.; Ferguson, J. J. Am. Chem. Soc. 1981, 103, 5336.

Figure 6. Successive UV-visible spectra of 2,7-dichlorofluorescein (DCF) during its catalytic reduction by growing (solid line) and full grown silver particles (hollow circle line) in aqueous CS2 solution with [AgNO3] ) 0.20 mmol‚dm-3, [CS2] ) 0.20 mmol‚dm-3, [KBH4] ) 6.0 mmol‚dm-3, and [DCF] ) 0.03 mmol‚dm-3.

grown ones increases from 0.3 to 0.4. On the other hand, the absorbance at 500 nm for DCF catalyzed by a growing Ag particle decreases from 1.23 to 0.81, as compared to the decrease from 0.40 to 0.30 for full grown ones within the same period of time. In the reaction process, the Ag particle is a media for the charge transfer from BH4- to anionic DCF. In fact, the growing silver particles were found to be a superior catalyst in most cases.4,36 The addition of CS2 prevents the silver colloids’ fast coagulation, which causes the growing silver particles to exhibit a high catalytic activity. The effect of CS2 on the reduction of DCF would be investigated in our further work. Summary The CS2-stabilized silver nanoparticles were prepared via chemical reduction by KBH4 in aqueous solution under ambient conditions. The 395-nm absorption band for Ag nanoparticles and the 325-nm absorption band for the derivatives of CS2 were observed. The catalytic reaction with CS2-stabilized Ag particles for reduction of DCF was studied via UV-visible spectroscopy, indicating that the growing Ag nanoparticles have higher catalytic activity compared to that of the full grown ones. The CS2-stabilized Ag particles may be useful as catalyst for other dye reduction reactions. Acknowledgment. Financial supports from the Chinese National Foundation of Natural Science Research and the Chinese Ministry of Education are gratefully acknowledged. LA001361V (36) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; John Wiley & Sons: New York, 1980.