J. Phys. Chem. B 2006, 110, 11751-11756
11751
Mechanism of Laser-induced Size-reduction of Gold Nanoparticles as Studied by Nanosecond Transient Absorption Spectroscopy Kunihiro Yamada,† Yuki Tokumoto, Takashi Nagata, and Fumitaka Mafune´ * Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed: February 17, 2006; In Final Form: April 15, 2006
Gold nanoparticles with an average diameter of ≈8 nm (Au ≈ 15 000) were irradiated with a tightly focused pulse laser at 355 nm in an aqueous solution of sodium dodecyl sulfate (SDS). Transient absorption spectra of the solution were measured at 25-100 ns after the laser irradiation. The observed transient absorption around 720 nm is assignable to the 2p r 1s transition of solvated electrons produced via multiple ionization of the gold nanoparticles. The nascent charge state of the gold nanoparticles was estimated from the transient absorbance. The dependence of the charge state on the SDS concentration shows a gradual increase from ≈ +60 to ≈ +70 in the 2 × 10-4 to 3 × 10-4 M range and an abrupt increase up to ≈ +710 at the critical micelle concentration (CMC) of SDS, 8 × 10-3 M. TEM measurements after laser irradiation reveal that the gold nanoparticles fragment into Au≈1000 at a SDS concentration of 3 × 10-4 M, whereas they are significantly dissociated into Au≈100 above the CMC. The observed correlation between the nascent charge states and the extent of size reduction of the gold nanoparticles after the laser treatment indicates that the size reduction is caused by the Coulomb explosion of the highly charged gold nanoparticles. The mechanism of laser-induced size reduction is quantitatively discussed based on the liquid drop model.
1. Introduction Gold nanoparticles have been attracting much attention because of their size-dependent physical and chemical properties.1-7 Considerable effort has been directed toward preparing nanoparticles with a particular diameter.8-12 More recently, there has been a growing interest in controlling the shape and size of the nanoparticles by laser irradiation.13-16 For instance, surfactant-free gold nanoparticles in propanol were found to aggregate under the irradiation of a CW Ar ion laser to form networked structures.17 Gold nanorods were reshaped into the spherical nanoparticles under the irradiation of a pulse Nd:YAG laser.18 The aggregation and the reshaping of the gold nanoparticles are considered to be induced by photoexcitation followed by melting of the nanoparticles. On the other hand, thiol-stabilized or surfactant-stabilized gold nanoparticles are known to be pulverized into the small particles by pulse-laser irradiation.19-22 The mechanism of the sizereduction has been proposed by several groups: Kamat and coworkers concluded from picosecond photoabsorption spectroscopic measurements that Coulomb explosion of the photoionized metal nanoparticles was responsible for the fragmentation.19 On the contrary, Koda and co-workers proposed a possible mechanism suggesting that fragmentation of the photoexcited gold nanoparticles proceeds through melting and vaporization caused by multiphoton absorption.20 Recently, Plech et al. observed the structural changes of nanoparticles and the water molecules in the vicinity of the nanoparticles by timeresolved X-ray scattering.21 They found that the particles undergo a melting transition within a time scale of 1 ns, and * To whom correspondence should be addressed. E-mail: mafune@ cluster.c.u-tokyo.ac.jp. † Permanent affiliation: Department of Applied Chemistry, Graduate School of Science and Engineering, Chuo University.
hence, they are fragmented into small particles by the thermal process. Henglein et al. observed that silver ions together with solvated electrons were produced when an aqueous solution of nanoparticles were irradiated with a pulse laser.23,24 The ions and electrons that were produced are then dissolved in the solution. This laser dissolution is possibly the cause of the sizereduction of gold nanoparticles. Although several mechanisms have been proposed, the mechanism of the size reduction is not yet well understood. In a previous study, we have demonstrated that gold nanoparticles can be either aggregated or fragmented by photoexcitation in a controlled manner. The studies involved irradiation of gold nanoparticles in an aqueous SDS solution with a 532nm or 355-nm laser: The results showed that in dilute solutions the nanoparticles were found to aggregate, whereas in the concentrated solutions they were found to be spherical and sizereduced.25,26 The gold nanoparticles were even further sizereduced when the SDS solution was above its CMC. On the basis of these experimental findings, the issues addressed in the present study consist of (1) the dynamics of photoexcited gold nanoparticles leading to fragmentation and (2) the role of SDS in the photoinduced aggregation/fragmentation processes. In this article, we observe solvated electrons formed by laser irradiation of gold nanoparticles in an aqueous solution of SDS by using nanosecond transient absorption spectroscopy. The abundance of the solvated electrons is measured as a function of the SDS concentration, and the correlation between the nascent charge state of the photoexcited gold nanoparticles and the size distribution of the nanoparticles after the laser treatment is examined. 2. Experimental Section Gold nanoparticles were prepared by laser ablation of a gold metal plate in an aqueous solution.27 Here, the gold metal plate
10.1021/jp061020b CCC: $33.50 © 2006 American Chemical Society Published on Web 05/27/2006
11752 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Figure 1. Schematic diagram of the apparatus for nanosecond transient absorption spectroscopy: (laser) Nd:YAG pulse laser, (Xe) Xe lamp, (pm) photomultiplier, (sm) spectrometer, (mcp) linear diode allay, (m) mirror, (l) lens, (i) iris, (fl) focusing lens, (bs) beam splitter, (ms) magnetic stirrer, (s) sample, and (osc) oscilloscope.
was placed on the bottom of a glass vessel filled with 10 mL of a 10-5 M SDS aqueous solution. The fundamental (1064 nm) of a Nd:YAG pulse laser operating at 10 Hz with a pulse energy of 90 mJ was focused by a 250-mm focal length lens onto the metal plate. The concentration of the gold atoms dispersed in the solution as gold nanoparticles was typically 1.2 mM after 36 000 laser shots. Hereafter, when the concentration of gold nanoparticles is given in this paper, it refers to the concentration of gold atoms dispersed as gold nanoparticles. The average size of the gold nanoparticles produced was determined to be ≈8 nm (Au≈15000) with a standard deviation of 5 nm. The gold nanoparticles that are produced are negatively charged, because they are covered with DS- and OH-.28 In the transient absorption measurements, a 0.2-mM solution of gold nanoparticles in a 2 × 10-4 to 5 × 10-2 M SDS solution was used. As shown in Figure 1, the solution in an optical SiO2 cell was irradiated with the output of the third harmonic (355 nm) of a Nd:YAG pulse laser (pulse energy, 50 mJ‚pulse-1; pulse width, 10 ns; repetition rate, 10 Hz). The pulse laser was focused by the lens onto the solution through the opening of the optical cell. The laser fluence was set to be 94.3 MW‚cm-2 at the surface of the aqueous solution. A magnetic stirrer was used to keep the solution homogeneous during the measurements. Transient absorption spectra of the solution were obtained at 25 and 100 ns after the laser irradiation by using a Xe lamp as a probe light, which was introduced into the cell perpendicular to the laser pulse. The broad-band probe light was dispersed by a spectrometer (Hamamatsu C5094), and the light intensities at each wavelength were measured by a MOS linear image sensor equipped with an image intensifier having a +630 above 10-2 M. Size-reduction of Gold Nanoparticles by Coulomb Explosion. Highly charged gold nanoparticles are so electronically unstable that they can undergo Coulomb explosion. According to the liquid drop model for Coulomb explosion,38-40 a multiply charged particle becomes unstable as soon as the disruptive Coulombic force exceeds the attractive cohesive force. The criterion for the Coulomb explosion is expressed quantitatively by “fissility” defined as X ) Ec/2Es, where Ec and Es are the Coulombic energy and the surface energy of the particle of interest, respectively. In the gas phase, multiply charged cluster ions are expected to readily dissociate into the small ones (multifragmentation) when X g 1 (Rayleigh limit), both the evaporation and fission competitively occur in the range 0.3 < X 8 × 10-3 M), the nascent charge state of the photoionized nanoparticles exceeds +550. The fissility becomes much larger than unity and, as a result, the nanoparticles are severely fragmented by the disruptive Coulombic force. This inference is consistent with the observation that the average diameter of the gold nanoparticles is reduced down to ≈1.5 nm by the laser irradiation. In the size-reduction scheme discussed above, we assume a two-step mechanism where the nanoparticles are initially prepared in a highly charged state via the multiple ionization and subsequently undergo a Coulomb explosion. It is, however, also possible that the ionization and fragmentation occur competitively within the time duration of a single laser pulse: in the meantime of a 10-ns laser pulse, multiply ionized nanoparticles dissociate into smaller ones and the resultant particles further absorb multiple photons to reach highly charged states. As we measured the number density of solvated electrons right after the 10-ns laser irradiation, the “charge state” discussed above corresponds to the nominal charge that the nanoparticles which survive the Coulomb explosion during the laser pulse would finally possess after the 10-ns laser irradiation. In any case, we can conclude that gold nanoparticles are fragmented into small pieces by the Coulomb explosion. SDS Concentration Dependence of the Charge State. As shown in Figure 7, the average charge state of gold nanoparticles after the 10-ns laser irradiation depends characteristically on the SDS concentration. We propose here a possible scheme which reasonably explains the observed nanoparticle charge state dependence on the SDS concentration. Although the observed quantity c0 is free from the decay processes occurring in the
Laser-induced Size-reduction of Gold Nanoparticles nanosecond time scale, it is probably subjected to ultrafast decay processes in the realm of
