880
J. Phys. Chem. B 2004, 108, 880-882
Photolysis Dynamics of Benzyl Phenyl Sulfide Adsorbed on Silver Nanoparticles Seol Ji Kim, Taeg Gyum Kim,† Chil Seong Ah,‡ Kwan Kim, and Du-Jeon Jang* School of Chemistry (NS60), Seoul National UniVersity, Seoul 151-742, Korea ReceiVed: July 16, 2003
The photolysis dynamics of benzyl phenyl sulfide adsorbed on silver nanoparticles suspended in water has been studied by using time-resolved transient-absorption spectroscopy. The adsorbed molecule desorbs from the metallic surface with a photoejected electron within 2 ns after excitation. The anionic species dissociates into benzenethiolate and benzyl radical on the time scale of 8 ns. The benzenethiolate combines to the metallic surface of a silver nanoparticle in 30 ns while the benzyl radical abstracts atomic hydrogen from water in 130 ns to form toluene.
Introduction Nanoparticles have attracted a great deal of interests because they have a variety of unique spectroscopic, electronic, and chemical properties arising from their small sizes and high surface/volume ratios.1-7 In particular, the quantum size effect, the nonlinear optical effect, and the electron dynamics of noblemetal nanoparticles are being extensively investigated as they can be applied in catalysts, sensors, drug delivery systems, photoelectronics, and magnetic devices.8-11 Noble-metal nanoparticles show new optical properties1,2,12,13 that are neither observed in atoms nor in bulk metals. One particular example is the presence of strong absorption bands in the visible region due to the surface-plasmon oscillation modes of electrons, which are coupled through the metallic surface to external electromagnetic fields. Because of these plasmon bands, the optical properties of platinum, palladium, silver, and gold colloidal nanoparticles have received considerable attention.4,14-26 Nanoparticles are usually derivatized or stabilized with organic molecules to modify their physical and chemical properties such as solubility, stability, and luminescence.10,27,28 The derivatization of a metallic nanoparticle with an alkanethiol is expected to occur by forming a covalent bond between the sulfur atom and the metallic surface, or by chemisorbing to the metallic surface with the sulfur atom. The adsorption of molecular monolayers on metal surfaces has also attracted enormous research interests.29-32 In addition to the fundamental interest in metal-adsorbate systems, practical considerations such as the modification of metal surfaces and the preparation of organic thin films have increased research activities in related research areas as well. The most widely studied systems include alkanethiols on gold and silver.29 Aromatic sulfides adsorbed on the surfaces of colloidal silver nanoparticles are known to undergo surface reactions involving the facile cleavage of a C-S bond with irradiation.30,31 Benzyl phenyl sulfide (BPS) absorbed on the surfaces of silver and gold has been reported to photodecompose exclusively into benzenethiolate and toluene as the final products.30,32,33 The * Author to whom correspondence should be addressed. E-mail: djjang@ plaza.snu.ac.kr. † Present address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA. ‡ Present address: Korea Research Institute of Standardsand Science, Daejeon 305-600, Korea.
Figure 1. UV/vis absorption spectra of silver nanoparticles without (dotted) and with BPS (solid) in water.
photoreaction of BPS is applicable for the nanopatterning of binary monolayers on silver.34 In this paper, we report the results obtained with time-resolved transient-absorption spectroscopy to show the detailed photoreaction mechanism of BPS adsorbed on silver nanoparticles suspended in water. Experimental Section Materials. All the reagents such as BPS and AgNO3 were used as purchased from Sigma and all the aqueous solutions were prepared using triply distilled water. The aqueous colloidal solutions of silver nanoparticles were prepared by using sodium citrate as the reductant.35 The measured typical diameters of silver nanoparticles were 10 nm. BPS-adsorbed silver colloidal solutions were prepared by adding 0.1 mL of 1-mM aqueous solutions of BPS slowly to 10 mL of silver colloidal solutions. Measurements. Absorption spectra were obtained using a UV/vis spectrophotometer (Scinco, S-2040). Pulses of 266 nm, 1 mJ, and 6 ns from a Q-switched Nd:YAG laser (Quanta System, HYL101) were directed to samples with the spot diameter of 2 mm to excite samples for the time-resolved spectrum and all the kinetic profiles of transient absorption except the profile of Figure 3a. Probe pulses emitted from an organic dye excited with the laser pulses split from sampleexcitation pulses were monitored by using an intensified CCD (Princeton Instruments, ICCD576G) having the gate time of 2 ns, which was attached to a 0.5-m spectrometer (Acton Research, Spectrapro 500). The laser and the intensified CCD were triggered with variable delays using a pulse/delay generator (Stanford Research Systems, DG535). The picosecond transientabsorption kinetic profile of Figure 3a, excited by 266-nm pulses
10.1021/jp030858l CCC: $27.50 © 2004 American Chemical Society Published on Web 12/23/2003
Photolysis of BPS Adsorbed on Ag Nanoparticles
J. Phys. Chem. B, Vol. 108, No. 3, 2004 881 Results and Discussion
Figure 2. Transient-absorption spectrum of BPS-adsorbed silver nanoparticles suspended in water, measured at the time delay of 2 ns after excitation with 6-ns pulses. A BPS aqueous solution without silver nanoparticles was used as the reference sample.
Figure 3. Transient-absorption kinetic profiles of BPS-adsorbed silver nanoparticles suspended in water. The probe wavelength (λpr) of each spectrum is indicated inside. The solid lines are the best fitted curves using the time constants given in Table 1.
of 25 ps and 1 mJ from a mode-locked Nd:YAG laser (Quantel, YG701), was detected with a 10-ps streak camera (Hamamatsu, C2830) attached with a CCD (Princeton Instruments, RTE128H).36 Kinetic constants were extracted by fitting measured kinetic profiles to computer-simulated curves convoluted with instrumental response functions.
The absorption spectrum of silver nanoparticles changes very significantly with addition of BPS (Figure 1), as the surfaceplasmon absorption spectrum of metal nanoparticles is reported to depend strongly on the nature and the concentration of adsorbed species.25,40,41 The absorption spectrum of the surfaceplasmon resonances shifts to the red and becomes broader with binding to BPS. In particular, the maximum absorbance decreases drastically and the tail extends to the red extremely. Very similar trends have been reported for colloidal silver having chemisorbed SH-.25,40 Considering the absorption changes in Figure 1, we assert that BPS-chemisorbed silver nanoparticles suspended in water have been well prepared. The transient-absorption spectrum of BPS-adsorbed silver nanoparticles measured at 2 ns after excitation shows three distinguishable transient absorption bands at 290, 335, and 450 nm together with a transient-bleach band at 400 nm (Figure 2). The transient absorption at 290 nm is attributed to BPS having a photoejected electron (BPS-) desorbed from the metallic surface while that at 335 nm to benzyl radical37 formed following the dissociation of BPS- (vide infra). As suggested with surface-enhanced Raman scattering results,30,31,33 BPS on silver dissociates to form benzyl radical. The photoreaction efficiencies of BPS adsorbed on silver and gold surfaces depend on the work functions and the chemisorption energies of metal clusters.34 Photoejection of electrons15,23 or thiolate anions28,38 is reported to lead size and shape transformations for nanoparticles of both gold and silver. The desorption of BPS- also modifies the extinction coefficients of metallic surface-plasmon resonances to give both the transient bleach at 400 nm and the transient absorption at 450 nm, as the photodesorption of thiolate anions is reported to change surface-plasmon resonances for silver and gold.28,38 The dielectric constant around silver nanoparticles also changes with heating38 to modify the extinction coefficient of the nanoparticles, contributing to the transientabsorption signal of surface-plasmon resonances. The transient absorption at 450 nm rises within 2 ns (Figure 3a) and decays on the time scale of 30 ns (Figure 3b). This indicates that the absorption increment of surface-plasmon resonances at 450 nm owing to the desorption of BPS- decays with the adsorption of benzenethiolate formed by the dissociation of the desorbed BPS- species. The adsorption and the desorption of thiolate anions from noble metals are reported to change the surface-plasmon resonances.25,28,41 It is reported that n-dodecanethiolate desorbs from silver nanoparticles dispersed in cyclohexane in 3.6 ns and readsorbs to their surfaces on the time scale of 40 ns.38 Compared with n-dodecanethiolate, BPSdesorbs from silver nanoparticles in a shorter time of 2 ns due to the weaker adsorption of BPS and benzenethiolate adsorbs to the metallic surfaces in a shorter time of 30 ns because of its simpler structure. It is noteworthy that the dissociation of a thiol from the silver surface or the reduction of a thiol is energetically less unfavorable than the oxidation of silver.42 Thus, if the silver surface ejects an electron during the desorption of BPS, BPS accepts the electron to become BPS-. The desorption of BPS is a process of electron charge-density shift from the metallic surface of a silver nanoparticle to the sulfur atom of BPS. Figure 3c shows that the transient absorption of BPS- at 290 nm rises in 2 ns, as suggested with the transient absorption rise of silver plasmon resonances in Figure 3a, and decays on the time scale of 8 ns. On the other hand, Figure 3d indicates that the transient absorption of benzyl radical37 at 335 nm rises in 8 ns, as the decay of BPS- absorption in Figure 3c, and decays on the time scale of 130 ns. These, with the fact that BPS adsorbed on silver
882 J. Phys. Chem. B, Vol. 108, No. 3, 2004
Kim et al. transient absorption at 335 nm abstracts atomic hydrogen from water in 130 ns to form toluene. Acknowledgment. The Korea Research Foundation (KRF2000-015-DP0193) has supported this work. D.J.J. thanks the Strategic National R & D Program for the grant of M1-021400-0108 while K.K. and S.J.K. also acknowledge the Center for Molecular Catalysis and the Brain Korea 21 Program, respectively. References and Notes
Figure 4. Schematic for the photoreaction mechanism of BPS adsorbed on noble-metal nanoparticles suspended in water.
TABLE 1: Time Constants Extracted from the Transient-Absorption Kinetic Profiles of Figure 3 profile
λpr (nm)
rise time (ns)
decay time (ns)
a b c d
450 450 290 335
2 2 2 8
30 30 8 130
and gold surfaces photodecomposes exclusively into benzene thiolate and toluene as the final products,30,32,33 suggest that BPS- formed in 2 ns decomposes into benzenethiolate and benzyl radical on the time scale of 8 ns. The former anionic photoproduct adsorbs to the silver surface in 30 ns. This shifts electron charge density from the sulfur atom of benzenethiolate to the metallic surface and restores the surface-plasmon resonances of a silver nanoparticle caused by the desorption of BPS-. The transient-absorption decay of benzyl radical in Figure 3d and the reports on the formation of toluene as a final photoproduct of BPS30,32,33 imply that the benzyl radical abstracts atomic hydrogen from water on the time scale of 130 ns to form toluene. It is intriguing that only the C-S bond of the benzyl moiety of BPS- is exclusively broken. As reported that the C-S bond dissociation energy of HS-C6H5 is larger by 60.7 kJ/mol than that of HS-C2H5,39 the cleavage of a C-S bond will be more likely to occur at the benzyl moiety of BPS- that at the phenyl moiety of BPS-. To explore the decomposition energetics of BPS-, we have performed ab initio semiempirical calculations as well. The results reveal that negatively charged BPS- is energetically destined to fragment into two species of benzenethiolate and benzyl radical, supporting our experimental observation. The potential energy surface displays that the potential energy function is repulsive for the C-S bond of the benzyl moiety whereas the energy minimum appears at 170 pm for the C-S bond of the phenyl moiety. Figure 4 summarizes schematically the photoreaction mechanism of BPS adsorbed on silver nanoparticles suspended in water. The BPS molecule desorbs from the metallic surface with a photoejected electron within 2 ns after excitation to give transient absorption of BPS- at 290 nm and transient absorption of plasmon resonances at 400 and 450 nm. The anionic species of BPS- dissociates into benzenethiolate and benzyl radical on the time scale of 8 ns. The benzenethiolate combines to the metallic surfaces of silver nanoparticles in 30 ns to restore surface-plasmon resonances, while the benzyl radical having
(1) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (2) Hodak, J. H.; Martini, I.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 6958. (3) Mafune, F.; Kohno, J.-Y.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2002, 106, 7575. (4) Logunov, S. L.; Ahmadi, T. S.; El-Sayed, M. A.; Khoury, J. T.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3713. (5) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (6) Hasnaoui, A.; Swygenhoven, H. V.; Derlet, P. M. Science 2003, 300, 1550. (7) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (8) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (9) Mafune, F.; Kohno, J.-Y.; Takeda, Y.; Kondow, T. J. Am. Chem. Soc. 2003, 125, 1686. (10) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (11) Raymo, F. M. AdV. Mater. 2002, 14, 401. (12) Hostetler, M. J.; Zhong, C.-J.; Yen, B. K. H.; Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 9396. (13) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (14) Fujiwara, H.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2589. (15) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (16) Hodak, J.; Martini, I.; Hartland, G. V. Chem. Phys. Lett. 1998, 284, 135. (17) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (18) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (19) Lee, D.; Lee, S.; Kim, H.; Jang, D.-J. Eur. Phys. J. D 2003, 24, 303. (20) Palpant, B.; Pre´vel, B.; Lerme´, J.; Cottancin, E.; Pellarin, M.; Treilleux, M.; Perez, A.; Vialle, J. L.; Broyer, M. Phys. ReV. B 1998, 57, 1963. (21) Hutter, E.; Fendler, J. H. Chem. Commun. 2002, 378. (22) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995, 267, 367. (23) Kamat, P. V.; Flumiani, M.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 3123. (24) Ah, C. S.; Hong, S. D.; Jang, D.-J. J. Phys. Chem. B 2001, 105, 7871. (25) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679. (26) Strelow, F.; Henglein, A. J. Phys. Chem. 1995, 99, 11834. (27) Kang, S. Y.; Kim, K. Langmuir 1998, 14, 226. (28) Ah, C. S.; Han, H. S.; Kim, K.; Jang, D.-J. Pure Appl. Chem. 2000, 72, 91. (29) Harnisch, J. A.; Pris, A. D.; Porter, M. D. J. Am. Chem. Soc. 2001, 123, 5829. (30) Joo, T. H.; Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1989, 93, 1422. (31) Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1990, 94, 2552. (32) Sandroff, C. J.; Herschbach, D. R. J. Phys. Chem. 1982, 86, 3277. (33) Joo, S. W.; Han, S. W.; Kim, K. Appl. Spectrosc. 2000, 54, 378. (34) Lee, I.; Han, S. W.; Kim, C. H.; Kim, T. G.; Joo, S. W.; Jang, D.-J.; Kim, K. Langmuir 2000, 16, 9963. (35) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (36) Jang, D.-J.; Kelley, D. F. ReV. Sci. Instrum. 1985, 56, 2205. (37) Kim, H.; Kim, T. G.; Hahn, J.; Jang, D.-J.; Chang, D. J.; Park, B. S. J. Phys. Chem. A 2001, 105, 3555. (38) Ah, C. S.; Han, H. S.; Kim, K.; Jang, D.-J. J. Phys. Chem. B 2000, 104, 8153. (39) Benson, S. W. Chem. ReV. 1978, 78, 23. (40) Strelow, F.; Henglein, A. J. Phys. Chem. B 1995, 99, 11834. (41) Henglein, A.; Meisel, D. J. Phys. Chem. B 1998, 102, 8364. (42) Lee, Y. J.; Jeon, I. C.; Paik, W.-K.; Kim, K. Langmuir 1996, 12, 5830.
