Mechanism of Improved Au Nanoparticle Size Distributions Using

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Mechanism of Improved Au Nanoparticle Size Distributions Using Simultaneous Spatial and Temporal Focusing for Femtosecond Laser Irradiation of Aqueous KAuCl4 Johanan H. Odhner, Katharine Moore Tibbetts, Behzad Tangeysh, Bradford B. Wayland, and Robert J. Levis* Department of Chemistry and the Center for Advanced Photonics Research, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *

ABSTRACT: The production of gold nanoparticles (AuNPs) by irradiation of aqueous [AuCl4]− with femtosecond laser pulses is investigated using simultaneous spatial and temporal focusing (SSTF) and compared to the results of conventional geometric focusing (GF). The effects of capping agent, laser power, reaction conditions in the cuvette, and laser chirp are studied, and we find that SSTF produces smaller particles with fewer irregular structures and fewer outlying large particles than GF in all cases except for one, in which the particle size distributions are the same. The difference is primarily ascribed to the intrinsic plasma properties of the two geometries: SSTF produces a plasma that is more homogeneous and spatially symmetric than that of GF, promoting efficient intrinsic mixing of the solution.

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Laser filamentation is an important aspect of femtosecond laser propagation that strongly influences the intensity in the sample and therefore the reagent concentration. Filamentation is characterized by numerous nonlinear propagation effects including self-focusing, intensity clamping, propagation at high intensity beyond the Rayleigh range expected from geometric optics, and white-light generation,31,32 all of which can have a significant impact on the formation of reactive water species by a laser pulse. For instance, clamping of the peak laser intensity at the focus due to filamentation33 serves to limit the production of reactive species and therefore leads to lower reagent concentrations. White light generation (a result of filamentation) has been reported to enhance AuNP fragmentation through linear absorption of the radiation via the surface plasmon resonance (SPR),34−37 thus reducing the average particle size distribution of AuNPs in solution. Also, stirring of the solution is typically omitted in femtosecond laser AuNP synthesis,21,23−26 and circulation of the solution occurs only by diffusion and cavitation bubble-induced currents. The dynamics of laser produced cavitation bubbles has been shown to be highly sensitive to the shape of the laser plasma,38 which in turn is strongly influenced by filamentation. To circumvent the detrimental effects of filamentation, we recently implemented simultaneous spatial and temporal focusing (SSTF)39,40 for controlled femtosecond laser-assisted synthesis of AuNPs.26 In that work, we demonstrated that the

INTRODUCTION Developing experimentally simple and “green” synthetic approaches for controlling the formation of metal and metal alloy nanoparticles has gained increasing interest because of emerging applications of these nanomaterials in technology1−4 and medicine.5−8 Gold nanoparticles are the most studied class of nanoparticles and thus serve as a benchmark for new approaches to synthesizing and stabilizing metal nanoparticles.9 Wet chemical techniques such as thermal decomposition, chemical reduction, and seed-mediated growth have been extensively applied to the synthesis of gold nanoparticles (AuNPs) with different sizes and morphologies.10−14 These methods rely on the application of surfactant molecules to direct particle growth, restrict particle size, and stabilize the colloidal suspensions to prevent aggregation and precipitation.10,15 Experimental parameters including the concentration of reactants, type of surfactant, presence of oxidizing/reducing agents, temperature, pH, etc., dictate the final size, dispersity, and shape of the particles.16−20 Femtosecond (fs) laser irradiation of aqueous [AuCl4]− in the presence of chemical surfactants has recently emerged as a promising “green” method for synthesizing size-tunable AuNPs, typically by varying the concentration of the added surfactant in solution during irradiation.21−26 Nanoparticle formation in these experiments is generally attributed to the reduction of [AuCl4]− to Au(0)24,27,28 by reactive species such as hydrogen radical (H•) and solvated free electrons (eaq−) generated through optical breakdown of water.29,30 Under these conditions, the quantity of reactive water species produced during laser irradiation determines the reagent concentration. © 2014 American Chemical Society

Received: August 4, 2014 Revised: September 11, 2014 Published: September 19, 2014 23986

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synthesis of AuNPs can occur through postirradiation reduction of [AuCl4]− by H2O2 generated in the strong laser field in the presence of laser-generated Au(0) seeds. Increased reduction rates and smaller, more narrowly dispersed spherical particles were synthesized with the addition of polyethylene glycol (PEG) as a capping agent. In this work we undertake a systematic investigation of the effect of focusing geometry on the size and dispersity of AuNPs produced by femtosecond laser irradiation of aqueous [AuCl4]− solutions using SSTF and geometric focusing (GF). We demonstrate that SSTF produces smaller particles with narrower size distributions than GF under almost all conditions studied, and we identify the mechanism that leads to the better control of particle size in SSTF as being the superior circulation of reactants through the focus in that geometry.

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METHODS Materials and Solutions. Potassium tetrachloroaurate(III) hydrate was purchased from Strem Chemicals. Methoxy poly(ethylene glycol) (PEG45, Mn = 2000, PDI = 1.06, Sigma-Aldrich) was purified by precipitation from petroleum ether prior to use. Stock solutions of KAuCl4 and PEG45 were prepared to the final concentrations of 0.2 mM and 0.09 mM with submicron-filtered high-performance liquid chromatography grade deionized water from Fisher chemicals. In all laser irradiation studies, 1.5 mL of aqueous [AuCl4]− solution (0.2 mM) was diluted to the total volume of 3.0 mL with another 1.5 mL of deionized water or PEG45 (0.09 mM) solution. Polymer-containing solutions were prepared to final [AuCl4]− and PEG45 concentrations of 0.1 mM and 0.045 mM, resulting in a molar ratio of [AuCl4]− to PEG45 chains of 1 to 0.45. The concentration of [AuCl4]− and molar ratio of [AuCl4]−:PEG45 were constant in all laser irradiation studies. Instrumentation. A titanium−sapphire-based chirpedpulse amplifier delivered 5 mJ, 35 fs pulses (full width at half-maximum (fwhm) intensity) with bandwidth centered at 790 nm at a 1 kHz repetition rate. Figure 1a illustrates the optical setup used to deliver pulses via GF and SSTF. In both cases, the pulses were focused through an f = 50 mm aspheric lens into a 10 × 10 × 40 mm3 quartz cuvette containing 3.0 mL of the liquid sample. For SSTF, pulses were spectrally dispersed using a grating pair (1200 l/mm) prior to focusing, as illustrated by the dispersed spectrum in Figure 1A. This dispersion of the pulse in space and time lowers the intensity of the beam away from the focus, thereby avoiding optically induced breakdown of the solvent before the focal point. The lens focuses the laser pulse simultaneously in space and time, restoring the temporal profile when all of the spectral components are recombined at the focal point.39,40 In this implementation of SSTF, the negative frequency chirp introduced by the grating pair is not precompensated, which results in a pulse duration of ∼36 ps fwhm at the focal point. For geometric focusing, the gratings were bypassed by two gold mirrors, as illustrated by the magenta path in Figure 1A. When the input pulse duration is minimized (Fourier transformlimited, TL) under geometric focusing conditions, elongation of the focus, white light generation, and heating of the cuvette were observed. The significant nonlinear optical effects introduced by GF with TL pulses are evident in the photograph of the sample cuvette in Figure 1B as compared to the photograph showing the cuvette under SSTF focusing conditions in Figure 1C. To produce temporally chirped pulses, the distance between the gratings in the internal

Figure 1. Femtosecond laser irradiation setup for GF and SSTF conditions (A). To implement GF, the initial laser beam (magenta) is directed via mirrors (a and b) to the f = 50 mm lens (e), which focuses the beam into a 10 × 10 × 40 mm3 cuvette (f). To implement SSTF, the initial laser beam (red) is spectrally dispersed on a pair of gratings (c and d) to produce a dispersed beam (red-yellow spectrum) prior to focusing through the lens (e) into the cuvette (f). Photographs of sample irradiation with 1.8 mJ pulses focused with GF (B) and SSTF (C). The digital camera exposure is 1/60 s with F6.3 in both photographs.

amplifier compressor was varied (approximately ±5000 fs2/ mm) to achieve the desired pulse duration. The pulse durations used in this investigation were verified using frequency-resolved optical gating.41 The pulse energy was varied using a half-wave plate and polarizer placed before the setup. The beam waist in the focus was measured to be 10 ± 2 μm in air at low power for both SSTF and GF conditions, giving a calculated maximum fluence range from 123 to 560 J/cm2 in the focus when losses due to reflections on the cuvette are accounted for. No particles are formed in the solution in the absence of laser irradiation under any of the conditions presented here. Electronic spectra of the samples were recorded on a Shimadzu UV-1800 spectrophotometer in the same cuvette used for laser irradiation. The size distributions of the particles were analyzed by using a JEOL JEM-1400 transmission electron microscope operating at an accelerating voltage of 120 kV. Aqueous dispersions of particles resulting from laser irradiation experiments were diluted by a factor of 5 with deionized water. One drop of each solution was deposited on the Formvar side of an ultrathin carbon type-A 400 mesh copper grid (Ted Pella Inc., Redding, CA), and the droplet was then blotted and allowed to evaporate under ambient conditions overnight. Statistical analysis and histograms were obtained by using Origin lab 7.5 software on a minimum of 200 particle counts. The photographs of laser irradiation experiments with GF and SSTF were taken using a Nikon D70 digital camera with F6.3 and an exposure of 1/60 s.

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RESULTS AND DISCUSSION Comparison of SSTF and GF with and without Polymer Surfactant (PEG45). Aqueous solutions of KAuCl4 23987

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with and without PEG45 were irradiated for times ranging from 2 to 10 min under GF and SSTF conditions to determine the irradiation time required to reduce all of the [AuCl4]− in solution to Au(0). Electronic spectra of the samples were recorded immediately after termination of the laser irradiation to assess the amount of [AuCl4]− consumed. Complete reduction of [AuCl4]− to Au(0) is characterized by the disappearance of the [AuCl4]− ligand-to-metal charge transfer (LMCT) band (∼210 nm) and the saturation of the SPR band centered at ∼520 nm, corresponding to spherical AuNPs (Figure S1 in the Supporting Information).42−44 Irradiation was terminated immediately after full reduction of [AuCl4]− to minimize melting and fragmentation of the AuNPs in the strong laser field.35,37,45,46 For each pulse energy and focusing condition, the reduction rate was ∼40% faster when PEG45 was added to the solution. This result has been observed previously and is attributed to reactive radicals generated by PEG45 fragmentation during laser irradiation.26 Figure 2 shows the electronic spectra of AuNPs recorded immediately following irradiation of surfactant-free [AuCl4]−

Figure 2. Electronic spectra of the gold nanoparticles produced from femtosecond laser irradiation of (A) [AuCl 4 ] − and (B) [AuCl4]−:PEG45 solutions using GF (red) and SSTF (blue) conditions with a laser pulse energy of 1.8 mJ. The laser irradiation time was 9 min for [AuCl4]− (A) and 5.5 min for [AuCl4]−:PEG45 solutions (B).

(Figure 2A) and [AuCl4]−:PEG45 (Figure 2B) solutions with GF (red) and SSTF (blue). For these experiments, the pulse energy was 1.8 mJ and GF irradiation was conducted with the TL pulse. The irradiation times necessary to achieve full reduction under these conditions were 5.5 and 9 min for the solutions with and without PEG45, respectively. For either surfactant-free or PEG45-containing solutions of [AuCl4]−, the plasmon resonance band of AuNPs produced with GF (red) shows an absorbance higher than that of the AuNP produced with SSTF (blue). Given the complete reduction of the [AuCl4]−, we conclude that the higher absorbance of the SPR feature for both precursor solutions using GF indicates the formation of larger AuNPs.16,17 No significant red shift of the SPR was observed, indicating that the synthesized AuNP have an average size of less than 30 nm for both GF and SSTF.42,44 To determine the size distribution of the Au nanoparticles produced using SSTF and GF conditions, transmission electron microscopy (TEM) was performed on the products. Representative TEM images were analyzed, and size distributions were calculated using >300 particles as presented in Figure 3. TEM analysis of the surfactant-free samples under GF conditions showed the formation of a broad distribution of AuNPs ranging from ∼2 to 50 nm with an average size of 13.6 ± 8.0 nm (Figure 3A). Both very small nanoparticles (20 nm), whereas the entire set of size distributions for SSTF as a function of pulse energy shifts to larger particle sizes with decreasing pulse energy. Both the TEM analysis and electronic spectra indicate that at any given laser pulse energy, SSTF produces smaller, more uniform AuNPs with significantly less evidence of fusing. The power study of [AuCl4]− irradiation using SSTF and GF suggests that intensity clamping plays a limiting role in the case of GF at the pulse energies used here, where the mean particle sizes at different powers are within one standard deviation of each other (Figure 5A−C) despite the 4.5-fold increase in laser power from lowest to highest. One would expect that increasing the laser power would increase the intensity correspondingly, leading to the formation of more reactive radicals and the generation of more seeds, which would in turn decrease the mean particle size. Such a trend is observed in SSTF, where an increase in the laser power is associated with a significant change in the nanoparticle distribution. Thus, SSTF affords some control over the particle size distribution with power, whereas this is not the case with GF in the intensity regime studied here. The observation of white light generation and elongation of the focus (evidenced by bubble tracks along the path of the laser) in the cuvette under GF conditions suggests that filamentation occurs in the case of GF. Filamentation is characterized by intensity clamping,33 which leads to a reduction in the maximum laser intensity achieved in the focus (based on calculations that assume focusing in a vacuum), and should therefore lead to a decrease in the number of reactive species formed by the laser pulse. Also, in the case of GF, we observe the formation of bubbles on the inside surface of the cuvette where the laser beam passes through and permanent refractive index changes in the glass of the cuvette for long irradiation times. These effects could attenuate the incident laser pulse energy by scattering and refraction and therefore also lead to a decrease in the intensity in the focus. Mechanism of SSTF Processing. To gain insight into the chemical differences between products formed with SSTF and

samples without PEG45. The size distribution of the sample was obtained from counting spherical and spheroidal particles, while the fused and irregularly shaped Au structures were excluded from size analysis. Including these particles would increase both the average size and standard deviation of the GF results. In comparison, laser irradiation of the PEG45-containing solution with SSTF resulted in the formation of uniform, narrowly dispersed AuNPs with an average size of 5.8 ± 1.1 nm (Figure 2D). The TEM images of the SSTF sample show no evidence of fused or irregularly shaped structures (Figure 2D and Figure S5 of the Supporting Information). The results of TEM analysis clearly show that smaller, more uniform AuNPs are obtained with SSTF compared with those obtained using GF under the current experimental conditions both with and without added PEG45. These results are consistent with the electronic spectra of the samples presented in Figure 2, in which the higher absorbance of the SPR in the GF samples suggests formation of larger nanoparticles. We note that the fractional reduction in the mean particle size due to the addition of PEG45 is comparable for SSTF and GF. Effects of Laser Pulse Energy on AuNP Size Distributions. To examine the effects of the laser pulse energy on AuNP size, aqueous [AuCl4]−:PEG45 solutions were irradiated with GF and SSTF at laser pulse energies of 1.8, 0.9, and 0.4 mJ, where complete reduction of the [AuCl4]− to Au(0) occurred after 5.5, 11.0, and 32.0 min of irradiation, respectively, for both GF and SSTF. Figure 4 shows the

Figure 4. Electronic spectra of the gold nanoparticles produced from femtosecond laser irradiation of [AuCl4]−:PEG45 solutions using (A) GF and (B) SSTF with laser pulse energies of 1.8 mJ (blue, solid line), 0.9 mJ (green, dotted line), and 0.4 mJ (red, dashed line). The laser irradiation times (GF and SSTF) were 5.5, 11, and 32 min for the solutions irradiated with laser pulse energies of 1.8, 0.9, and 0.4 mJ, respectively.

electronic spectra of AuNPs produced using GF (Figure 4A) and SSTF (Figure 4B) at laser pulse energies of 1.8 mJ (blue, solid line), 0.9 mJ (green, dotted line), and 0.4 mJ (red, dashed line). Complete disappearance of the [AuCl4]− LMCT band (∼210 nm) indicates the full reduction of [AuCl4]− to Au(0) has occurred during laser irradiation. Observation of a welldefined SPR band at ∼520 nm indicates dominant formation of spherical Au NPs at any given laser pulse energy and focusing conditions (Figure 4A,B). The electronic spectra of AuNPs produced with GF did not show any significant change in the intensity and position of the SPR band at different laser pulse energies (Figure 4A). In contrast, Figure 4B shows a systematic decrease in the intensity of the SPR band as the laser pulse energy is increased from 0.4 to 1.8 mJ under SSTF conditions. Because the intensity of the SPR band is proportional to the average size of AuNPs,16,17 this trend indicates formation of smaller AuNPs as the laser pulse energy is increased. The lack of change in the position of the SPR band indicates formation of AuNPs with average sizes less than 30 nm.42,44 Directly 23989

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Figure 5. Representative TEM images and corresponding size distribution histograms of the gold nanoparticles produced from femtosecond laser irradiation of [AuCl4]−:PEG45 solutions by using GF (A−C) and SSTF (D−F) with laser pulse energies of (A, D) 0.4 mJ, (B, E) 0.9 mJ, and (C, F) 1.8 mJ. All size distribution histograms are obtained from counting at least 200 particles.

GF, water was irradiated using GF and SSTF and titrated with KMnO4 to quantitate the formation of hydrogen peroxide, which is used as a proxy for the concentration of reactive species formed in the solution. Formation of hydrogen peroxide from femtosecond laser irradiation of water has been previously observed27 and was found to contribute to the formation of AuNP in femtosecond laser irradiation of KAuCl4−.26 The pulse duration under GF conditions was also varied up to ±7 ps using the internal grating compressor in the laser amplifier in order to assess the effect of self-focusing on the results presented above. The number of moles of hydrogen peroxide generated after 4 min of laser irradiation as a function of pulse duration is shown in Figure 6. Previous work has shown that the concentration of peroxide increases linearly with irradiation time at a constant pulse energy.26 An irradiation time of 4 min was chosen here to ensure sufficient generation of peroxide so as to accurately determine its concentration, while avoiding any uncontrolledfor variables such as cuvette heating and bubble formation that were observed to occur for long irradiation times. The decrease in the amount of hydrogen peroxide generated by the laser pulse around the minimum pulse duration under GF conditions is attributed to intensity clamping that arises because of filamentation. Even so, the TL pulse under GF conditions still produces ∼50% more peroxide than that under SSTF, which does not explain the difference in the size distributions of the AuNPs presented above. Therefore, we conclude that some other physical difference between the two focusing conditions causes this effect. Another significant difference between GF and SSTF is the observed behavior of the cavitation bubbles produced by the

Figure 6. Moles of peroxide formed upon irradiation of 3 mL of pure water under GF (red squares) and SSTF (blue circle) conditions using 1.8 mJ pulses for 4 min. The top axis indicates the magnitude of the linear chirp placed on the GF pulse, and the bottom axis indicates the resulting pulse duration. The minimum pulse duration (“0”) was ∼35 fs. The SSTF data point is centered at “0” for comparison; the pulse duration of the SSTF pulse was ∼36 ps.

two focusing conditions. Under femtosecond laser irradiation, optical breakdown and plasma generation in the water lead to the formation of cavitation bubbles.47−49 In GF, bubbles are formed along the length of the focal region with little or no translational kinetic energy and rise slowly from the interaction region toward the surface in a stream, similar to previous observations.50 Because the bubbles linger in the focal region, they can interfere with the focusing dynamics of subsequent laser pulses and thus reduce the intensity in the focus. More importantly, the bubbles cause circulation of the solution due to the currents they produce in the liquid, which mixes the laser23990

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Figure 7. Electronic spectra of AuNPs produced by irradiation of [AuCl4]−:PEG45 solution with 1.8 mJ pulses with SSTF (blue, solid lines), GF with a TL pulse (red, dashed lines), GF with a negatively chirped pulse (magenta, dashed-dot lines), and GF with a positively chirped pulse (green, dotted lines). The experimental conditions are (A) without moving the cuvette or stirring the solution, (B) moving the cuvette, and (C) moving the cuvette and stirring the solution.

vertically (Figure 7B), and (III) the cuvette is moved laterally and vertically and the solution is stirred with a magnetic stir bar during irradiation (Figure 7C) for each input pulse geometry (SSTF, GF TL, GF −20 000 fs2 and GF +20 000 fs2). Systematically reducing nonlinear and thermal effects in the cuvette wall (condition II) and increasing circulation (condition III) result in successively smaller average particle size under GF conditions, as can be clearly seen by the reduction of the SPR absorption band intensity from Figure 7A to Figure 7C. However, this trend is also accompanied by an increase in the SPR band intensity at longer wavelengths (>600 nm), suggesting the formation of a few larger particles under conditions II and III with GF. Irradiation with SSTF, on the other hand, does not follow a similar trend going from conditions I to III. When the cuvette is translated only (condition II), the SPR absorption feature is reduced in intensity and slightly blue-shifted, suggesting the formation of particles smaller than those formed under condition I. However, the SPR absorption peak actually increases in intensity when stirring is performed (condition III), which is consistent with the formation of particles larger than those formed under conditions I or II. Positively and negatively chirped pulses have very similar electronic spectra for each condition and have SPR absorption bands that are consistent with particles smaller than GF for all measurements. Despite the lower absorption of the SPR peak in the chirped pulse data for condition III, the peak intensity measured from the inflection point at ∼445 nm for SSTF is smaller than that for the chirped GF pulses, which also exhibit a broad absorption toward the long wavelength side of the spectrum. TEM images of the samples producing the spectra shown in Figure 7 are presented in Figures 8 (condition I), 9 (condition II), and 10 (condition III) for (A) SSTF; (B) GF, TL pulse; (C) GF, −20 000 fs2; and (D) GF, +20 000 fs2. The right-hand panel in each figure shows the particle size distribution taken from counting at least 300 individual particles (red bars) and the corresponding fractional mass distribution of Au(0) (blue bars). In Figure 8 (condition I), the SSTF size distribution (Figure 8A) is clearly superior to any of the GF conditions (Figure 8B−D), all of which contain large (>20 nm) particles that significantly skew the particle mass distributions. Figure 9 shows that condition II improves the size and mass distributions of particles produced by GF with the TL pulse (Figure 9B), but SSTF produces a narrower distribution (Figure 9A). The two GF chirped pulse conditions still contain large particles (up to 30 nm in diameter) that broaden the mass

produced reactants with the precursor solution. Under GF conditions the low translational energy of the bubbles leads to inefficient mixing of the solution. However, in the case of SSTF, bubbles are ejected from the focus radially with high translational kinetic energy. This effect has been observed previously using intense femtosecond laser pulses38 and can be attributed to the high degree of longitudinal symmetry of the SSTF plasma.51 As the cavitation bubble generated by optical breakdown of the water collapses after its initial expansion, the front and back walls of the bubble travel inward at the same rate, leading to ejection of bubbles outward. This is in contrast to the dynamics of a longitudinally asymmetric plasma, where jet formation and bubble ejection along the propagation axis of the laser dominates. As a result, the mixing of the solution under SSTF conditions is much more efficient than that under GF conditions. This difference between GF and SSTF conditions suggests that the better mixing in the case of the SSTF geometry facilitates more efficient reduction by distributing reactive species throughout the cuvette and replenishing the focal volume with fresh solution, while the GF case relies largely on diffusion to transport reactive photoproducts to unreduced Au species. To test whether or not the decrease in particle size in SSTF compared to that in GF is due solely to the intrinsic mixing occurring in SSTF, additional experiments were carried out. The effects of refractive index modification in the cuvette wall and bubble formation on the inside surface of the cuvette were investigated by translating the cuvette laterally and vertically during irradiation of [AuCl4]−:PEG45 solutions with 1.8 mJ pulses. Experiments were also carried out using a magnetic stir bar to mix the solution during irradiation in conjunction with translating the cuvette. In addition to SSTF and GF with TL pulses, the same experiments were performed with positively and negatively chirped pulses (1.6 ps fwhm, ± 20 000 fs2) in the GF geometry to control for filamentation effects. In order to compare the results of these experiments to those presented above, a third set of experiments was conducted under the original conditions (i.e., without a magnetic stir bar or lateral movement of the cuvette) for the same four pulse conditions. In all experiments, the [AuCl4]−:PEG45 solutions were irradiated until fully reduced, at which point irradiation was stopped immediately to minimize postreduction melting and fragmentation of the nanoparticles. Figure 7 shows the electronic spectra of the AuNPs produced under the experimental conditions where (I) the cuvette is stationary (Figure 7A), (II) the cuvette is moved laterally and 23991

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Figure 8. TEM images, size distributions (red bars), and mass distributions (blue bars) for irradiation with SSTF (A), GF TL (B), GF −20 000 fs2 (C), and GF +20 000 fs2 (D) under condition I (stationary cuvette, no stirring).

Figure 9. TEM images, size distributions (red bars), and mass distributions (blue bars) for irradiation with SSTF (A), GF TL (B), GF −20 000 fs2 (C), and GF +20 000 fs2 (D) under condition II (lateral and vertical motion of the cuvette).

distributions (Figure 9C,D). Figure 10 shows that condition III minimizes the differences in size distributions between SSTF (Figure 10A) and the GF conditions (Figure 10B−D), although some large particles in the latter samples broaden the mass distributions. Overall, the measured size distributions agree with the electronic spectra in Figure 7 and the interpretations discussed above. Additional TEM images of the products produced at conditions (I)−(III) are provided in Figures S11− S13 of the Supporting Information. It is evident from the data in Figures 7−10 that translating the cuvette laterally and vertically (condition II) facilitates better energy deposition, leading to faster reduction and smaller size distributions than when the cuvette is stationary. This effect is particularly significant for GF with the TL pulse, which shows a narrowed size distribution (Figure 9B) as compared to condition I (Figure 8B). Under conditions I and II the average particle size for SSTF is ∼25−27% smaller than that for GF in all cases, which we attribute to the better circulation of the

solution caused by the cavitation bubble dynamics resulting from the well-controlled intensity profile of the SSTF focus. The very large (>20 nm) particles observed in the GF distributions, which broaden the mass distributions of Au(0) to such an extent that they constitute ∼50% of the particle mass under condition I for chirped pulses (Figure 8C,D), are not observed at all in the SSTF case. These large particles are reduced in size with the introduction of cuvette translation (condition II, Figure 9), but still persist. Condition III (translating the cuvette and stirring with a magnetic stir bar), does not significantly reduce the average particle size for the GF focusing conditions compared to the results of irradiation under condition II. In contrast, the average particle size in the SSTF samples increases when the solution is stirred. These results for condition III indicate that the benefits of SSTF for generating smaller particles are lost when the solution is stirred, probably because of turbulent mixing caused by the interference of the natural SSTF-induced current with that of the orthogonally 23992

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CONCLUSIONS

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ASSOCIATED CONTENT

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We have undertaken a systematic study of the effect of focusing geometry on femtosecond laser-assisted reduction of [AuCl4]− to form AuNPs encompassing simultaneous spatial temporal focusing and geometric focusing as a function of laser chirp. We found that the shortest pulse under GF conditions is not optimal for the production of nanoparticles because of the strong nonlinear effects induced even in the tight focusing geometry investigated here. Furthermore, maximizing the formation of radical and reducing species by chirping the GF pulse to minimize nonlinear effects is not optimal for the formation of small, narrowly dispersed particles. Instead, the consistent improvement in the nanoparticles produced with SSTF suggests that the circulation dynamics of the solution appears to be the most critical factor controlling particle size and dispersity. The natural circulation imparted by the cavitation bubbles from the SSTF plasma appears to provide an optimal degree of mixing that effectively disperses the Au(0) nuclei formed in the plasma to the unreacted solution where the AuNP can grow. This circulation can promote the formation of a larger number of smaller AuNPs than other conditions in two ways. First, the dispersal of the Au(0) nuclei out of the laser focus can result in a greater number of nuclei forming over the time of irradiation. The synthesis of smaller AuNP as laser pulse energy is increased with SSTF suggests that a greater number of nuclei are formed to produce the smaller AuNP for a fixed amount of Au(III) in solution. However, the observation of more reactive water species formed using GF conditions suggests that more Au(0) nuclei should form than with SSTF. Thus, a second possible mechanism for the production of smaller AuNP formation with SSTF is that the lack of circulation in GF conditions causes Au(0) nuclei formed in the focal region to fuse together prior to growth, thus resulting in fewer, but larger AuNPs. This process appears to be particularly prominent when using chirped pulses in the GF geometry, where large AuNP of up to ∼80 nm were observed.

* Supporting Information S

Figure 10. TEM images, size distributions (red bars), and mass distributions (blue bars) for irradiation with SSTF (A), GF TL (B), GF −20 000 fs2 (C), and GF +20 000 fs2 (D) under condition III (lateral movement of the cuvette and stirring the solution).

Additional electronic spectra and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

oriented stir-bar current. This presumably creates local currents and reduces the mobility of reactants and precursors in solution, leading to less efficient mixing, reduction in the number of nuclei, and thus an overall increase in the average particle size. It is important to note that even under condition III no large particles are observed under SSTF irradiation, whereas there are still an appreciable number of large particles occurring in all GF experiments. Finally, we note that the best particle size distribution of 4.8 ± 1.5 nm obtained with SSTF (condition II, Figure 9A) is significantly smaller than the best size distribution of 6.1 ± 2.1 nm obtained using GF with a positively chirped pulse (condition III, Figure 10D), which is consistent with the bulk measurements in the electronic spectra of Figure 7B,C.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this research by the Army Research Laboratory through contract W911NF-10-2-009 is gratefully acknowledged. The authors are also grateful for a National Science Foundation instrumentation grant (CHE-0923077) for the JEOL JEM-1400 TEM used in this research.

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REFERENCES

(1) Roucoux, A.; Schulz, J.; Patin, H. Reduced Transition Metal Colloids: A Novel Family of Reusable Catalysts? Chem. Rev. (Washington, DC, U.S.) 2002, 102, 3757−3778.

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