Langmuir 2006, 22, 6361-6366
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Acceleration of Laser-Induced Formation of Gold Nanoparticles in a Poly(vinyl alcohol) Film Masanori Sakamoto, Takashi Tachikawa, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed February 1, 2006. In Final Form: May 1, 2006 Gold nanoparticles (AuNps) were fabricated in a poly(vinyl alcohol) (PVA) film using the photochemically generated benzophenone ketyl radical and PVA radical by laser irradiation as a reducing agent. The measurements of the surface plasmon band of AuNps indicated that AuNps continued growing in the PVA film for several hours or days after the laser irradiation. The formation process of AuNps in the PVA film was investigated by using laser flash photolysis and UV-vis absorption spectroscopy. Additive doping (formic acid or sodium 2-mercaptoethanesulfonate) in the PVA film dramatically accelerated or inhibited the formation rate of the AuNps, respectively. The doping of formic acid accelerated the formation rate of the AuNps by a factor of 10-20. On the contrary, doping of 2-mercaptoethanesulfonate inhibited the formation of AuNps. The mechanisms of the acceleration and inhibition were investigated by using laser flash photolysis. The effects of additives on the formation process of AuNps are discussed.
Introduction Since nanoparticles (Nps) are used in various fields such as biodetection, catalysis, and electronics, Np research had attracted a broad array of interests.1 A variety of methods for synthesizing gold nanoparticles (AuNps) have been developed under several conditions such as aqueous solution, sol, gel, micelle, polymer matrix, gas, and so on.1 Particularly, two-dimensional and threedimensional metal Np arrays in polymer matrixes are attractive for applications in nonlinear optics,2a photoimaging and patterning,2b magnetic devices,2c,d and sensing devices.2e,f The fabrication of AuNps in a polymer matrix by laser irradiation is quite attractive, because it can ideally produce them in a spatially local region with a resolution of about half the wavelength of the irradiation light. However, there have been only a limited number of reports about the fabrication method of AuNps in polymer matrixes using lasers.3 The potential problem of this method is that the direct excitation of AuCl4- requires a high-energy photon that possibly causes some undesired damage to the matrix. Thus, the fabrication method using a low laser power and low frequency is desirable. The fabrication method of AuNps using photochemically generated radicals by laser irradiation will possibly resolve these problems. A number of fabrication methods for the metal Nps using a radical as a reducing agent have been reported.4 Furthermore, many parent molecules * To whom correspondence should be addressed. E-mail: majima@ sanken.osaka-u.ac.jp. (1) For example, see: (a) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (c) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (d) Thomas, G. K.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888. (2) (a) Jose´-Yacama´n, M.; Pe´rez, R.; Santiago, P.; Benaissa, M.; Gonsalves, K.; Carlson, G. Appl. Phys. Lett. 1996, 69, 913. (b) Stellacci, F.; Bauer, C. A.; Meyer-Friedrischen, T.; Wenseleers, W.; Alain, V.; Kuebler, S. M.; Pond, S. J. K.; Zhang, Y.; Marder, S. R.; Perry, J. W. AdV. Mater. 2002, 14, 194. (c) Smith, G. B.; Deller, C. A.; Swift, P. D.; Gentle, A.; Garrett, P. D.; Fisher, W. K. J. Nanopart. Res. 2002, 4, 157. (d) Park, I.-W.; Yoon, M.; Kim, Y. M.; Kim, Y.; Yoon, H.; Song, H. J.; Volkov, V.; Avilov, A.; Park, Y. J. Solid State Commun. 2003, 126, 385. (e) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958. (f) Walter, E. C.; Ng, K.; Zach, M. P.; Penner, R. M.; Favier, F. Microelectron. Eng. 2002, 61-62, 555. (3) (a) Kaneko, K.; Sun, H.-B.; Duan, X.-M.; Kawata, S. Appl. Phys. Lett. 2003, 83, 1426. (b) Hirose, T.; Omatsu, T.; Sugiyama, M.; Inasawa, S.; Koda, S. Chem. Phys. Lett. 2004, 390, 166.
of radicals have absorptions at a the longer wavelength compared with the metal complex, and a variety of radicals can be conveniently generated by laser irradiation with a relatively low power. In the present paper, we fabricated AuNps in a poly(vinyl alcohol) (PVA) film using the photochemically generated benzophenone (BP) ketyl radical (BPH•) by laser irradiation as the reducing agent. The primary processes of the AuNp formation in the PVA film were investigated using laser flash photolysis. The measurements of the surface plasmon band of AuNps indicated that AuNps continued growing in the PVA film for several hours or days after the laser irradiation. The doping of additives (formic acid or sodium 2-mercaptoethanesulfonate) in the PVA film dramatically accelerated or inhibited the formation of the AuNps, respectively. The mechanisms of the acceleration and inhibition of the AuNp formation were investigated. The transient absorption measurement revealed that the reduction of AuCl4- by BPH• was not significantly enhanced by doping of formic acid. The doping of formic acid would affect the processes following the reduction of AuCl4-. Experimental Section PVA (Mw ) (8.9-9.8) × 104), BP, 2-mercaptoethanesulfonate, and HAuCl4 were purchased from Aldrich and used as received. The formic acid (guaranteed reagent) was purchased from Nacalai tesque and used as received. The PVA films containing BP and HAuCl4 (film 1) were made by the following method. First, a formic acid solution of PVA (5 (4) (a) Korchev, A. S.; Shulyak, T. S.; Slaten, B. L.; Gale, W. F.; Mills, G. J. Phys. Chem. B 2005, 109, 7733. (b) Yonezawa, Y.; Sato, T.; Kuroda, S.; Kuge, K. J. Chem. Soc., Faraday Trans. 1991, 87, 1905. (c) Henglein, A. Chem. Mater. 1998, 10, 444. (d) Sato, T.; Kuroda, S.; Yonezawa, Y. J. Photochem. Photobiol., A 1999, 127, 83. (e) Kometani, N.; Doi, H.; Asami, K.; Yonezawa, Y. Phys. Chem. Chem. Phys. 2002, 4, 5142. (f) Sato, T.; Maeda, N.; Ohkoshi, H.; Yonezawa, Y. Bull Chem. Soc. Jpn. 1994, 67, 3165. (g) Kapoor, S.; Mukherjee, T. Chem. Phys. Lett. 2003, 370, 83. (h) Kapoor, S.; Palit, D. K.; Mukherjee, T. Chem. Phys. Lett. 2002, 355, 383. (i) Kapoor, S. Langmuir 1998, 14, 1021. (j) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (k) Korchev, A. S.; Konovalova, T.; Cammarata, V.; Kispert, L.; Slaten, L.; Mills, G. Langmuir 2006, 22, 375. (l) Korchev, A. S.; Bozack, M. J.; Slaten, B. L.; Mills, G. J. Am. Chem. Soc. 2004, 126, 10. (m) Eustis, S. Krylova, G.; Eremenko, A.; Smirnova, N.; Schill, W. A.; El-Sayed, M. A. Photochem. Photobiol. Sci. 2005, 4, 154. (n) Finter, J.; Lohse, F.; Zweifel, H. J. Photochem. 1985, 28, 175. (o) Sakamoto, M.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Chem. Phys. Lett. 2006, 420, 90.
10.1021/la060304k CCC: $33.50 © 2006 American Chemical Society Published on Web 06/10/2006
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Figure 1. (A) Pictures of dried film 1 20, 40, and 60 min after the 355 nm laser irradiation. (B) Picture of formic acid-doped film 1 (degree of doping 6 wt %) containing BP and HAuCl4 5 min after the 355 nm laser irradiation. wt %) containing HAuCl4 (1 mM) and BP (15 mM) was cast on a quartz plate. Second, the sample was set in a glovebox to maintain the humidity (20%) and temperature (20 °C) and kept for 48 h. Under these conditions, an about 50-70 µm thick film containing BP and HAuCl4 was obtained. The color of the HAuCl4-doped PVA film was slightly yellow due to the absorption of AuCl4-. We monitored the change of the weight of the PVA film after the casting of the formic acid solution of PVA (see the Supporting Information). After 48 h, the weight of the PVA film was almost constant. The weight of the PVA film dried for 48 h did not change even when the film was set under the vacuum for 1 h. Therefore, we concluded that the formic acid was evaporated completely after the film was dried for 48 h. The PVA films containing BP (film 2) were made by casting a formic acid solution of PVA (5 wt %) containing BP (15 mM) on a quartz plate followed by drying under the same conditions as those for film 1 for 48 h. Under these conditions, an about 50-70 µm thick film containing BP was obtained. The 2-mercaptoethanesulfonate- and HAuCl4-doped PVA films (film 3) were made by casting a formic acid solution of PVA (5 wt %) containing HAuCl4 (1 mM), BP (15 mM), 2-mercaptoethanesulfonate (2 mM) on a quartz plate followed by drying under the same conditions as those for film 1 for 48 h. Under these conditions, an about 50-70 µm thick film containing BP, HAuCl4, and 2-mercaptoethanesulfonate was obtained. The 2-mercaptoethanesulfonate-doped PVA films (film 4) were made by casting a formic acid solution of PVA (5 wt %), BP (15 mM), and 2-mercaptoethanesulfonate (1 mM) on a quartz plate followed by drying under the same conditions as those for film 1 for 48 h. Under these conditions, an about 50-70 µm thick film containing BP and 2-mercaptoethanesulfonate was obtained. The degree of doping of films 1 and 2 by formic acid was adjusted by changing the drying time. The degree of doping is expressed by eq 1, where Wp and Wd are the mass of the dried PVA film and the mass of formic acid, respectively. degree of doping ) (Wd/Wp) × 100
(1)
Finally, the film was placed on a quartz plate/metal frame holder and irradiated by a 355 nm laser (3-20 mJ pulse-1, 1 or 50 shots, 10 Hz, 5 ns fwhm). To suppress any effects of byproducts formed by the laser irradiation, all absorption spectra and kinetic trace data were obtained by only the single laser excitation of the PVA films. The laser experiment was carried out using the third harmonic oscillation (355 nm) of Surelite II-10 (Continuum). The UV spectra were measured using a UV-3100PC (Shimazu). Transmission electron microscopy (TEM) was performed using a JEM-3000F (JEOL). All experiments were carried out 3-9 times. The experimental errors were estimated by the repetition of experiments.
Results and Discussion Formation of AuNps in a PVA Film. A 355 nm laser (3 mJ pulse-1, 50 shots, 10 Hz) irradiated the PVA film containing HAuCl4 and BP (film 1). Upon the 355 nm laser irradiation, the color change of the film 1 due to the formation of AuNps was not confirmed immediately. As shown in Figure 1A, the color change was barely observed at 40 min after the 355 nm laser
Figure 2. Time-resolved UV-vis absorption spectra of dried film 1 after the 355 nm laser irradiation.
Figure 3. Absorption spectra after the 355 nm laser irradiation of dried film 2. The inset shows the kinetic trace at 350 nm.
irradiation. After 60 min, the color change was evident in a photograph, indicating a slow growth. Although HAuCl4 is reduced thermally by PVA, the process was quite slow. No formation of AuNps was observed in the present experimental period and under the present conditions without laser irradiation. The result indicates that laser irradiation triggered the formation of AuNps. The time evolution of the absorption spectra of film 1 is shown in Figure 2. The absorption peaks at 350 and 531 nm were observed after the laser irradiation. The band at 531 nm was a typical surface plasmon band that originated from AuNps, indicating the generation of AuNps. The intensity of the surface plasmon band increased for several hours after the laser excitation. It is suggested that the broad absorption centered at 350 nm, which slowly decayed, was the quasi-stable product formed by the reaction between BPH• and the PVA radical since the band was also observed by excitation of film 2 (Figure 3). The generation of a similar absorption band by the photoexcitation of BP in the PVA film was reported by Horie et al.5 Transient Absorption Spectra of the PVA Film. To investigate the formation mechanism of AuNps in PVA, the transient absorption spectra of film 2 after the 355 nm laser irradiation were measured (Figure 4). Upon the excitation of film 2 by the 355 nm laser (20 mJ pulse-1, one shot), a strong absorption peak at 540 nm was observed. The absorption peak is assigned to BP in the triplet excited state (3BP*). Five microseconds after the laser irradiation, the strong absorption disappeared and a broad absorption peak at around 550 nm was observed. The transient absorption spectrum was similar to that of BPH• in the PVA film reported by Horie et al.5 Therefore, we concluded that the origin of the absorption peak at around 550 nm was BPH•. On the basis of these results, the initial stage of the photochemical reaction in the PVA was as follows. First, 3BP* (5) Horie, K.; Ando, H.; Mita, I. Macromolecules 1987, 20, 54.
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Figure 4. Transient absorption spectra of dried film 1 (A), film 2 (B), and film 4 (C) 200 ns, 5 µs, and 500 µs (black, red, and green lines, respectively) after the 355 nm laser irradiation.
Figure 5. Kinetic traces of dried film 1 (a), film 2 (b), and film 4 (c) at 590 nm after the 355 nm laser irradiation.
was formed by the laser irradiation (eq 2). Second, the formed hν
BP 98 3BP*
(2)
abstracted the hydrogen from PVA to generate BPH• and the PVA radical (eq 3).
3BP*
BP* + [-CH2CH(OH)-]n f BPH• + [-CH2C•(OH)-]n (3)
Figure 6. Kinetic traces of dried film 1 (O), film 2 (2), and film 4 (9) at 540 nm after the 355 nm laser irradiation.
The lifetimes (τ) of 3BP* and BPH• were measured at 590 and 540 nm (Figures 5 and 6, respectively), respectively, since BPH• does not show an absorption at 590 nm. Both decay curves were fitted with a double-exponential decay function. Since the fast decay component was the main one, we discuss the fast decay component in the present paper. The τ values of the fast decay components of 3BP* and BPH• (τ(3BP*) and τ(BPH•), respectively) in film 2 were estimated to be 2.5 ( 0.2 µs and 0.13 ( 0.02 s, respectively, under the present experimental conditions. To investigate the reactivity of 3BP* and BPH• toward AuCl4-, the transient absorption spectra of film 1 and the PVA films containing BP (21 M)6 and several concentrations of AuCl4were measured. In the case of film 1, the formation of 3BP* and BPH• was also observed. τ(3BP*) decreased with increasing
concentration of AuCl4- (Figure 7).6 The Au ions quench 3BP*.4a The kinetic traces of BPH• are shown in Figure 6. τ(BPH•) in film 1 was estimated to be 0.02 ( 0.01 s. τ(BPH•) decreased with increasing concentration of AuCl4- (Figure 8). Since the oxidation potential of BPH• was quite small (-0.25 V vs SCE), electron transfer from BPH• to AuCl4- did occur.
3
(6) The concentrations of BP and AuCl4- in the PVA film (cBP and cAuCl4-, respectively), which was made from the formic acid solution of PVA (5 wt %) containing HAuCl4 (1 mM) and BP (15 mM), were estimated by Lambert-Beer’s law as expressed by the equation OD ) cl, where l is the optical path length (l ) 70 µm) and is the extinction coefficient of BP and AuCl4- ( of BP was 127 cm-1 M-1 at 355 nm, and of AuCl4- was 4900 cm-1 M-1 at 322 nm).7 of BP was estimated in acetonitrile. From above equation, cBP and cAuCl4- were estimated to be 21 and 2.5 M, respectively. The concentration of 2-mercaptoethanesulfonate in film 3 was determined using the volume change following the drying of the solution, which was estimated by the concentration change of BP.
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Figure 7. τ(3BP*) vs concentration of additives ([additives]): dried PVA film containing BP (21 M) and several concentrations of 2-mercaptoethanesulfonate (b); dried PVA film containing BP (21 M) and several concentrations of AuCl4- (0).
Figure 9. Time-resolved UV-vis absorption spectra of formic aciddoped film 1 (degree of doping 5.3 ( 1 wt %) after the 355 nm laser irradiation.
film 1 was expressed as follows. BPH• and the PVA radical generated by the laser irradiation of film 2 reduce AuCl4- to form AuCl2- during the first step (eqs 5-7). Figure 8. τ(BPH•) vs concentration of additives ([additives]): dried PVA film containing BP (21 M) and several concentrations of 2-mercaptoethanesulfonate (b); dried PVA film containing BP (21 M) and several concentrations of AuCl4- (2); HCOOH-doped PVA film (degree of doping 10 ( 2 wt %) containing BP (21 M) and several concentrations of AuCl4- (0). Table 1. Electron-Transfer Rate Constants (kELT) in the Dried and Formic Acid-Doped Films (degree of doping 10 ( 2 wt %) Containing BP (21 M) and Several Concentrations of AuCl4kELT (M-1 s-1) [AuCl4-] (M)
dried film
formic acid-doped film
1.25 2.5 3.75
4(1 24 ( 19 71 ( 55
12 ( 7 13 ( 3 89 ( 40
(4)
the kELT value for each of the films since the estimated kELT values in the films changed depending on the concentration of AuCl4- (Table 1). The result indicates that it is impossible to treat the bimolecular reaction in the PVA film with the same method as that in the solution. The kELT value of film 1 was estimated to be 24 ( 19 M-1 s-1. Since the evolution of a surface plasmon band of AuNps proceeds for several hours, the electron transfer should not be the rate-determining step. A similar reduction process of HAuCl4 by sulfonated poly(ether ether) ketone ketyl radical in the aqueous solution was reported by Mills and co-workers.4a They concluded that the AuNps were formed by the following two-step mechanism. The first step is the formation of the intermediate gold species, which was formed from the reduction of AuCl4- by the ketyl radical. The second step is the transformation of the intermediate gold species to AuNps by a dark reaction. They proposed that Au(I) was formed as the final product of the first step and Au(I) very slowly disproportionates to form AuNps in the second step.8 According to their proposal, the formation process of AuNps in
(5)
AuCl4- + [-CH2C•(OH)-]n f AuCl42- + [-CH2CdO-]n + H+ (6) AuCl42- + [-CH2CH(OH)-]n f AuCl2- + [-CH2C•(OH)-]n + 2Cl- + H+ (7) Unfortunately, the formation of AuCl2- with the absorption peak at 240 nm9 was not detected due to the absorption of BP. The formed AuCl2- very slowly disproportionates to form AuNps during the second step (eq 8).8
3AuCl2- h 2Au + AuCl4- + 2Cl-
The electron-transfer rate (kELT) constant was estimated by eq 4, where τ0(BPH•) is the lifetime of BPH• in film 2 and [AuCl4-] is the concentration of AuCl4- in the PVA film. We estimated
kELT ) (1/τ(BPH•) - 1/τ0(BPH•))/[AuCl4-]
AuCl4- + BPH• f AuCl42- + BP + H+
(8)
Doping of Formic Acid. The laser-induced fabrication of AuNps in film 1 doped with formic acid was examined. It should be noted that the color change of film 1 was dramatically accelerated in the presence of formic acid. For the formic aciddoped PVA film, an obvious color change was observed only 5 min after the 355 nm laser (3 mJ pulse-1, 50 shots, 10 Hz) irradiation (Figure 1B). The absorption spectrum of formic aciddoped film 1 after the laser irradiation was similar to that of the undoped one (Figure 9). The kinetic traces of the plasmon band in the absence and presence of several concentrations of formic acid are shown in Figure 10. It seems that the formation process of the plasmon band was multicomponent, indicating the complicated formation process of AuNps in film 1. Because of the slow formation (i.e., second step) of AuNps caused by the disproportionation of Au(I) according to the proposed mechanism by Mills et al., the apparent formation rate should depend on the concentration of Au(I) in film 1. Immediately after the laser (7) Kartuzahanskii, A. L.; Studzhinskii, O. P.; Plachenov, B. T.; Sokolova, I. V. J. Appl. Chem. USSR 1986, 59, 2265. (8) Malone, K.; Weaver, S.; Taylor, D.; Cheng, H.; Sarathy, K. P.; Mills, G. J. Phys. Chem. B 2002, 106, 7422. (9) Kunkely, H.; Vogler, A. Inorg. Chem. 1992, 31, 4539. (10) (a) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2004, 108, 17269. (b) Perissinotti, L. L.; Brusa, M. A.; Grela, M. A. Langmuir 2001, 17, 8422. (c) de Tacconi, N. R.; Wenren, H.; McChesney, D.; Rajeshwar K. Langmuir 1998, 14, 2933.
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Figure 12. UV-vis absorption spectra of film 1 with several degrees of doping (wt %) (black line, 5.3 ( 1; red line, 2.5 ( 0.7; green line, 1.1 ( 0.8; blue line, 0.6 ( 0.3) by formic acid 5 days after the 355 nm laser irradiation.
Figure 10. Kinetic traces of the surface plasmon band of film 1 with several degrees of doping (wt %) (black circles, 5.3 ( 1; red circles, 2.5 ( 0.7; green triangles, 1.1 ( 0.8; blue inverted triangles, 0.6 ( 0.3) by formic acid.
Figure 11. Rate of growth of the surface plasmon band (k) vs degree of doping.
irradiation, the formation rate of AuNps should be high due to the high concentration of Au(I). The concentration of Au(I) then decreased due to disproportionation, resulting in the decreased formation rate of AuNps. The plots of the apparent initial rates of growth of the surface plasmon band (k) (within 2500 s after the laser irradiation) vs the degree of doping of film 1 are shown in Figure 11. The formation rate of AuNps in the formic acid-doped PVA film was 10-20 times faster than that in the undoped one. The doping of formic acid dramatically enhances the growing rate of the surface plasmon band. The UV-vis absorption spectra 5 days after the laser irradiation are shown in Figure 12. The intensity of the surface plasmon band of the formic acid-doped PVA film was greater than that of the undoped one. The peak positions of the surface plasmon bands were similar to each other, indicating that the sizes and shapes of AuNps were not significantly different. The AuNps in film 1 were observed by using TEM (Figure 13). The shape of AuNps in both films was spherical with about 8-12 nm
Figure 13. TEM image of AuNps formed in formic acid-doped film 1 (A) and the undoped film (B).
diameter. The doping of formic acid in film 1 did not significantly change the size and shape of AuNps. It is suggested that the size and shape of AuNps were made uniform by the stabilization of the PVA. These results indicate that the doping of formic acid increased the amount of the formed AuCl2-. τ(BPH•) values in the formic acid-doped PVA film containing BP (21 M) and several concentrations of AuCl4- were measured (Figure 8). It should be noted that τ(BPH•) values in the formic acid-doped PVA film were similar to those in the dried PVA film. The result indicates that the reduction of AuCl4- by BPH• was not enhanced by the doping of the formic acid. Additionally, the diffusion of BPH• and AuCl4- seems to be not significantly enhanced by the doping of formic acid. Three possible mechanisms can be proposed for the acceleration of the AuNp formation. The first mechanism is the increase in the concentrations of the reducing reagents after the laser irradiation. An increase in the concentration of the reducing reagents increases the initial concentration of the intermediate, that is, the initial rate of the AuNp formation. We propose the following mechanisms as the possible acceleration process. When 3BP* abstracts a hydrogen atom from PVA, the hydrogen atom of formic acid should be competitively abstracted; i.e., HCOO• should be generated as well as BPH• (see the Supporting Information). Because HCOO• was deprotonated to form H+ and CO2•-,10 CO2•- with a high reducing power reduces AuCl4- and enhances the formation of the intermediate gold species. Although the PVA radical is also expected to have a reducing ability, the mobility of the PVA radical in the PVA film should be low. The PVA radical cannot efficiently reduce AuCl4- compared with CO2•-. (11) Rogach, A. L.; Shevchenko, G. P.; Afanas'eva, Z. M.; Sviridov. V. V. J. Phys. Chem. B 1997, 101, 8129.
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The second mechanism is related to the mechanism proposed by Shevchenco et al.11 They found that the Ag nanoparticle was formed in the PVA matrix containing Ag+ and formic acid by UV irradiation and a dark reaction. According to their proposal, the Ag nanoparticle works as a microelectrode that collects electrons from formic acid in the dark. In our samples, no AuNp formation was observed without BP and laser irradiation even in the presence of formic acid. Thus, it is suggested that the small AuNp was formed due to the reduction by BPH• during the initial step. The formed AuNp would work as a microelectrode. The proposed mechanism of acceleration is shown below. First, the formed AuNps collect the electrons from the formic acid by the reaction shown in eq 9. Second, the Au(I) reduction occurs hν
Aun + xHCOOH 98 Aunx- + xCO2 + 2xH+
(9)
at the surface of the cathodically polarized Au particles, resulting in particle growth (eq 10). Since the standard potential of the gold microelectrode was expected to decrease with increasing size of the nanoparticle,12 the formic acid may reduce the formed AuNps although formic acid cannot reduce AuCl4-.
Aunx- + AuCl2- f Aun(x-1)- + 2Cl-
(10)
The third mechanism is the increase in the diffusion rate of Au(I) in the swollen PVA film. According to the free volume theory modified by Yasuda et al., the diffusion constant of the solute was enhanced by swelling of the polymer films.13 Since the disproportionation of Au(I) was assumed to be one of the rate-determining steps of the AuNp formation, the doping of formic acid to the PVA film may also accelerate the AuNp formation in the PVA film. Although the reaction rate between BPH• and AuCl4- was not significantly changed by the doping of formic acid, the evolution of the surface plasmon band of AuNps occurs for several minutes. The doping of formic acid may affect such slow diffusion of Au(I). It is suggested that the first or second mechanism was predominant rather than the third. Although the doping of formic acid increased the amount of formed AuNps, the amount of AuNps would not be increased by the third process. Additionally, the reaction rate between BPH• and AuCl4- was not significantly changed by the doping of formic acid, suggesting that the dramatic enhancement of diffusion of Au(I) was not expected. Therefore, we concluded that the formation of highly reductive CO2•- via the photochemical reaction between 3BP* and formic acid (first (12) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (b) Mosseri, S.; Henglein, A.; Eberhard, J. J. Phys. Chem. 1989, 93, 6791. (c) Mallik, K.; Madhuri, M.; Pradhan, N.; Tarasankar, P. Nano Lett. 2001, 6, 319. (13) (a) Yasuda, H.; Lamaze, C. E.; Ikenberry, L. D. Makromol. Chem. 1968, 118, 19. (b) Yasuda, H.; Peterlin, A.; Colton, C. K.; Smith, K. A.; Merill, E. W. Makromol. Chem. 1969, 126, 177. (c) Matsuyama, H.; Teramoto, M.; Urano, H. J. Membr. Sci. 1997, 126, 151.
mechanism) and microelectrode-like electron collection of AuNps from formic acid (second mechanism) would enhance the formation of AuNps predominantly. Doping of 2-Mercaptoethanesulfonate. In contrast with doping of formic acid, the doping of sodium 2-mercaptoethanesulfonate in the PVA film (film 3) completely inhibited the formation of AuNps. Even 3 weeks after the laser irradiation, no surface plasmon band was observed in the presence of the sodium 2-mercaptoethanesulfonate. The transient absorption of film 4 after the laser irradiation (20 mJ pulse-1, one shot) is shown in Figure 4C. Even in the presence of a high concentration of 2-mercaptoethanesulfonate (2.5 M), BPH• and 3BP* were observed. The kinetic traces of 3BP* and BPH• are shown in Figures 5c and 6, respectively. The change of τ(3BP*) by adding 2-mercaptoethanesulfonate was within the experimental error (Figure 7). τ(BPH•) was decreased with increasing concentration of 2-mercaptoethanesulfonate. The quenching rate constant of BPH• by 2-mercaptoethanesulfonate (kq) is expressed by eq 11, where [2-mercaptoethanesulfonate]
kq ) (1/τ(BPH•) - 1/τ0(BPH•))/ [2-mercaptoethanesulfonate] (11) is the concentration of 2-mercaptoethanesulfonate in the PVA film. Although BPH• was quenched by 2-mercaptoethanesulfonate, the quenching rate constant in film 4 (3.8 ( 3.3 M-1 s-1) was 7 times slower than that by HAuCl4 in film 1 (vide supra). The reduction of HAuCl4 by BPH• also occurs in the presence of 2-mercaptoethanesulfonate. The thiol could inhibit the nanoparticle formation via complexation with the intermediate gold species, or with small gold clusters.14
Conclusions In summary, AuNps were fabricated in a PVA film using photochemically generated BPH• and PVA radical as a reducing agent. We demonstrated the acceleration and inhibition of the AuNp formation by the doping of additives. The mechanisms of the acceleration and inhibition were investigated by using the transient absorption measurement. It indicated that the formation rate of AuNps was controllable by the doping of additives even in the PVA film. Acknowledgment. This work has been partly supported by a Grant-in-Aid for Scientific Research (Project 17105005, Priority Area (417), 21st Century COE Research, and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Supporting Information Available: Hydrogen abstraction of BP* from formic acid. This material is available free of charge via the Internet at http://pubs.acs.org.
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LA060304K (14) Warner, M. G.; Reed, S. M.; Hutchison, J. E. Chem. Mater. 2000, 12, 3316.
