Elucidating Strong Field Photochemical Reduction Mechanisms of

May 9, 2016 - Behzad Tangeysh , Katharine Moore Tibbetts , Johanan H. Odhner , Bradford B. Wayland , and Robert J. Levis. Langmuir 2017 33 (1), 243-25...
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Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl4]−: Kinetics of Multiphoton Photolysis and RadicalMediated Reduction Katharine Moore Tibbetts,*,†,‡ Behzad Tangeysh,† Johanan H. Odhner,† and Robert J. Levis† †

Department of Chemistry and Center for Advanced Photonics Research, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *

ABSTRACT: Direct, multiphoton photolysis of aqueous metal complexes is found to play an important role in the formation of nanoparticles in solution by ultrafast laser irradiation. In situ absorption spectroscopy of aqueous [AuCl4]− reveals two mechanisms of Au(0) nucleation: (1) direct multiphoton photolysis of [AuCl4]− and (2) radicalmediated reduction of [AuCl4]− upon multiphoton photolysis of water. Measurement of the reaction kinetics as a function of solution pH reveals zeroth-, first-, and second-order components. The radical-mediated process is found to be zeroth-order in [AuCl4]− under acidic conditions, where the reaction rate is limited by the production of reactive radical species from water during each laser shot. Multiphoton photolysis is found to be first order in [AuCl4]− at all pHs, whereas the autocatalytic reaction with H2O2, the photolytic reaction product of water, is second order.

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and long-lived reactive species (e.g., solvated e−, H•, H2O2) produced upon strong field photolysis of water35 act as the primary reducing agents that drive conversion of [AuCl4]−, as for other synthesis methods involving no chemical reducing agents such as γ radiolysis36,37 and plasma discharge.38,39 However, in the case of strong field irradiation, only H2O2 has been definitively observed to promote reduction during and after the laser irradiation30,31 and no formal kinetics or mechanisms involving H2O2 or any other putative reducing agent have been determined. To expand the mechanistic understanding of strong field solution phase [AuCl4]− photoinduced chemistry and to explore methods of exercising control over the size of the AuNP products, we investigate the effects of solution pH on the reaction rate and AuNP size under fixed laser irradiation conditions (i.e., fixed pulse energy and duration). The solution pH is known to be an important reaction parameter in chemical reduction methods, determining both the reduction kinetics of [AuCl4]− and the AuNP size.40−43 These pH effects arise from the speciation of the aqueous Au(III) complex, where [AuCl4]− dominates under acidic conditions and [Au(OH)4]− under basic conditions, with mixtures of [AuClx(OH)4−x]−, x = 1−3, species present under neutral conditions.44−46 Theoretical47 and experimental44−46 studies indicate that [AuCl4]− is less stable and thus more reactive than [Au(OH)4]−, consistent with observed slower reaction rates for AuNP synthesis.41−43

INTRODUCTION The size- and shape-dependent optical and electronic properties of plasmonic Au nanoparticles (AuNP) make these materials desirable for applications including photothermal therapy and drug delivery,1,2 spectroscopy and sensing,3,4 photocatalysis,5,6 and photovoltaics.7,8 Of particular importance for these applications is the ability to selectively tailor the size and shape of the AuNP during synthesis to obtain the desired properties, making control over the kinetics of nucleation and nanoparticle growth critical.9,10 Ever since LaMer’s development of classical nucleation theory in 1950,11 this nucleation− growth model has commonly been used to describe the kinetics of AuNP formation up to the present day.9,12,13 However, as early as Turkevich’s 1951 study of HAuCl4 reduction by sodium citrate,14 alternative AuNP formation mechanisms including autocatalysis14−16 and aggregative growth17,18 have proven to better account for the observed kinetics of Au precursor reduction and AuNP growth. In particular, the quantitative Finke−Watsky rate law describing the autocatalytic nucleation and growth of Ir nanoclusters,19 along with subsequent modifications to account for particle aggregation,20 has recently been applied to HAuCl4 reduction and AuNP growth under a variety of reaction conditions.21−25 Recently, strong field ultrashort pulsed laser irradiation has been explored as a simple synthetic route to form AuNP from aqueous [AuCl 4 ] − without added chemical reducing agents.26−32 Here, the term “strong field” refers to conditions under which the peak intensity of the laser radiation at the focus is sufficiently high to induce optical breakdown of the solvent.33 Previous studies26−28,34 have proposed that short© XXXX American Chemical Society

Received: March 28, 2016 Revised: May 4, 2016

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[AuCl4]− solution (S). The gratings act to spectrally disperse the pulse in space and time. The focal lens refocuses the pulse in space and time as its spectral components are recombined, and the temporal profile is restored when all of the spectral components are spatially overlapped at the focal point. The negative frequency chirp introduced by the grating pair results in a 36 ps pulse duration at the focal point in the sample. All samples of aqueous [AuCl4]− were irradiated with 2.5 mJ pulses for times between 60 and 660 s, depending on the irradiation time required for complete conversion of [AuCl4]− to AuNP. UV−vis spectra were measured in situ during femtosecond laser irradiation with a home-built instrument shown schematically in Figure 1. The output of a stabilized deuterium− tungsten light source (Ocean Optics) was delivered via a 600 μm diameter optical fiber (Ocean Optics, F1 in Figure 1) to a pair of Al-coated off-axis parabolic mirrors (Thorlabs, M1 and M2). These focus the beam in the sample (S), with the propagation perpendicular to that of the SSTF laser beam. The defocusing UV−vis beam is recollimated with a second pair of off-axis parabolic mirrors (M3 and M4 and delivered via a second optical fiber F2 to a spectrometer (Ocean Optics, HR4000). Spectra were acquired once per second during laser irradiation and for 60 s following the termination of irradiation. Characterization. A JEOL JEM-1400 transmission electron microscope (TEM) operating at an accelerating voltage of 120 kV was used for imaging. The TEM grids were prepared by depositing 2 μL of the aqueous dispersions on the Formvar side of an ultrathin carbon type-A 400 mesh copper grid (Ted Pella Inc., Redding, CA) and the droplet was blotted and allowed to evaporate under ambient conditions overnight. Particle size distributions were determined by counting 300 particles from each sample on three different areas of the TEM grid.

The lower reactivity of Au species with increasing numbers of OH ligands has also been attributed to the decreasing reduction potential of [AuClx(OH)4−x]− as x decreases.42 In this work, we find that upon strong field irradiation, the reduction rate of aqueous [AuClx(OH)4−x]− increases with initial solution pH, in contrast to the chemical reduction experiments discussed above. Using in situ UV−vis measurements during laser irradiation to obtain an empirical rate equation describing reduction of aqueous [AuClx(OH)4−x]−, we identify two distinct mechanisms of [AuClx(OH)4−x]− reduction to Au(0): (1) direct multiphoton photolysis of [AuClx(OH)4−x]− and (2) the previously proposed radicalmediated nucleation by photolysis of water.26−28,34 The higher reduction rate of [AuClx(OH)4−x]− under basic conditions is attributed to an increase in the yield of reactive reducing species upon water photolysis, which overcomes the lower reactivity of [Au(OH)4]− as compared to [AuCl4]−. As a result, mechanism 1 dominates under acidic conditions, whereas mechanism 2 dominates under basic conditions in the case of strong field laser processing.

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EXPERIMENTAL METHODS Materials. Potassium tetrachloroaurate(III) was used as obtained from Strem Chemicals. A stock solution of aqueous [AuCl4]− was prepared from a weighed sample of KAuCl4, which was then diluted with HPLC-grade deionized water (Fisher Scientific) to a concentration of 0.1 mM. The solution pH was adjusted by adding an appropriate amount of a 2 M stock solution of HCl or KOH (Fisher Scientific). The pH was measured with a microprocessor pH meter (pH 212, Hanna Instruments). The pH of the working [AuCl4]− solution was 3.4 ± 0.2 and was adjusted to values between 2.4 ± 0.1 and 9.8 ± 0.3 by addition small amounts of HCl or KOH stock solution (Supporting Information, Table S1). Instrumentation. Laser irradiation was performed using a titanium−sapphire-based chirped-pulse amplifier (Coherent, Inc.) delivering 5 mJ, 35 fs pulses with bandwidth centered at 790 nm at a 1 kHz repetition rate. Irradiation of the aqueous [AuCl4]− sample was carried out using a simultaneous spatial and temporal focusing (SSTF) geometry.48,49 The use of SSTF minimizes detrimental nonlinear optical effects such as filamentation, self-focusing, and intensity clamping that occur upon interaction between intense femtosecond laser radiation and a condensed phase sample33 by lowering the intensity of the radiation away from the focal spot. The latter effects have been shown to adversely affect the size distributions of AuNPs synthesized using fs laser irradiation.31 A schematic illustration of the apparatus is shown in Figure 1. The laser pulses were spectrally dispersed using a grating pair (1200 l/mm, G1 and G2 in Figure 1), then focused with an f = 50 mm aspheric lens (L) into a quartz fluorimeter cuvette containing 3.0 mL of aqueous

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RESULTS AND DISCUSSION In situ UV−vis measurements are used to monitor the conversion of [AuCl4]− to AuNP during laser irradiation. The spectra obtained in a typical experiment conducted at a solution pH of 3.4 (0.1 mM [AuCl4]− with no HCl or KOH added) are shown in Figure 2. Figure 2A shows the UV−vis spectra of the solution at selected times after commencement of irradiation. The arrows indicate the decay of the UV absorption feature corresponding to the LMCT band of [AuCl4]− and growth of the surface plasmon feature at ∼520 nm, corresponding to conversion of [AuCl4]− to AuNP. Magnification of the plasmon feature over the spectral range 450−650 nm (Figure 2B) shows that the maximum plasmon absorbance blue shifts as the reaction nears completion (i.e., when the absorbance stops growing), and that the absorbance decreases upon continued irradiation (dashed line arrow). This behavior may be attributed to AuNP fragmentation by the laser once all [AuCl4]− is consumed.29 The maximum plasmon absorbance as a function of irradiation time is fit to a cubic spline function (Figure 2C) to determine the completion time of the reaction τ, defined as the time when the absorbance of the plasmon feature reaches a maximum, indicated by the arrow at τ = 247 s in Figure 2C. The associated cessation in growth of the absorbance at 450 nm indicates that Au(0) has reached a constant concentration.50 The plasmon absorbance vs irradiation time as a function of initial solution pH (taken from spectra shown in the Supporting Information, Figure S1) is shown in Figure 3A. The plots show that the conversion of [AuCl4]− to AuNP proceeds significantly faster with increasing pH. The final

Figure 1. Schematic diagram of the SSTF and UV−vis setups. B

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Figure 2. (A) UV−vis spectra of [AuCl4]− solution (pH 3.4) at selected irradiation times indicated in the legend. (B) Magnification of the AuNP plasmon feature in panel A recorded at the indicated times showing experimental data (dots) and spline fits (solid lines). The dashed line tracks the progress of plasmon absorbance with time. (C) Plasmon absorbance versus irradiation time with experimental data (dots) and a spline fit (solid line). τ is indicated by the dashed line. The experimental absorbance at 450 nm as a function of time is shown for reference.

Figure 3. (A) Plasmon absorbance as a function of irradiation time (dots, experimental data; black curves, spline fits to determine τ). (B) Spectra of the final AuNP products at each initial solution pH. (C) Reduction time τ (red) and plasmon absorbance (blue) as a function of initial solution pH. The error bars on the ordinate and abscissa represent the standard deviations in reported τ/absorbance and the measured initial solution pH, respectively, over four independent experiments.

Figure 4. (Left) representative TEM images of particles synthesized at the indicated solution pH. (Right) statistics of particle size distributions vs pH. The inset depicts the particle size distributions for pH 2.5 (red) and 5.4 (green). The red arrows indicate fusion and/or melting of the particles. The blue arrows indicate particles that are significantly larger than the average particle size (see text for details).

of Figure 4. The right panel shows the mean and standard deviation of the size distributions as a function of pH, with the inset showing the size distributions obtained for pH 2.5 and 5.4. Additional TEM images and all size distributions are provided in the Supporting Information, Figure S2. In agreement with the UV−vis spectra reported in Figure 3B, Figure 4 shows that the mean AuNP diameter decreases as pH rises, reaching a minimum of 4.8 ± 1.9 nm at pH 5.4. Although large particles and some melted or fused particles (red arrow) are visible at

AuNP spectra (Figure 3B) show a trend toward lower plasmon absorbance as pH increases up to pH ∼ 7, consistent with production of smaller AuNP.50 These trends of faster reduction time τ and lower plasmon absorbance as solution pH increases are plotted in Figure 3C. TEM analysis was performed on selected samples to confirm the AuNP size distribution trends expected from spectra in Figure 3. Representative TEM images for samples synthesized at pH 2.5, 2.7, 3.4, 5.4, 7.9, and 8.4 are shown in the left panel C

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Figure 5. Concentration of [AuCl4]− vs irradiation time at different initial solution pH (dots) with fits to eqs 2 and 3 (black dashed and solid curves, respectively): (A) initial pH 2.4−5.4; (B) initial pH 6.7−9.8.

Table 1. Rate Constants from Fitting Experimental Data in Figure 5A to Eqs 2 and 3 eq 2 k1 (×10

pH

−3

−1

s )

0.11 ± 0.01 0.12 ± 0.07 0.4 ± 0.1 0.5 ± 0.1 1.4 ± 0.2 2.2 ± 0.5 2.4 ± 0.4 7±2 29 ± 2 29 ± 2

2.4 2.5 2.7 3.4 5.4 6.7 7.9 8.4 9.3 9.8

eq 3 k2 (M 117 140 170 270 330 420 600 600 700 600

−1

± ± ± ± ± ± ± ± ± ±

−1

k1 (×10

s )

d[A] d[B] = = k1[A] + k 2[A][B] dt dt

[A(t )] = 1+

s )

k2 (M−1 s−1)

k3 (×10−7 M s−1)

± ± ± ± ± ± ± ± ± ±

0.31 ± 0.02 0.4 ± 0.1 0.9 ± 0.2 1.1 ± 0.4 1.6 ± 0.3 0.7 ± 0.2 0.7 ± 0.2 0.5 ± 0.1 0.12 ± 0.06 0.3 ± 0.1

110 140 170 230 260 430 550 600 700 600

6 20 10 20 10 80 30 100 100 100

nonlinear least-squares regression results in the dotted black curves. At pH ≥ 6.7 (Figure 5B), eq 2 fits the experimental data well. In contrast, for the experiments at pH ≤ 5.4 (Figure 5A), significant deviation of the fitted curve from the experimental data is apparent. This discrepancy between the data and the model can be addressed empirically by adding a linear term k3 (corresponding to a zeroth-order rate constant) to eq 2, [A(t )] = 1+

k1 + [A(0)] k2 k1 e(k1+ k 2[A(0)])t k 2[A(0)]

− k 3t (3)

which improves the fitting quality for the experiments conducted at pH ≤ 5.4 (Figure 5, solid curves). Comparing the errors in fitting the data to eqs 2 and 3 with the statistical F test51 shows that the improvement in fitting with eq 3 is statistically significant for pH ≤ 5.4 (Supporting Information, Figures S7 and S8). The rate constants obtained from fitting the experimental data in Figure 5A to eqs 2 and 3 at all initial solution pH values are given in Table 1. Reported errors are the standard deviations from four experiments performed at each initial solution pH. To validate the need for a linear term in the model to correctly describe the reduction kinetics at an initial solution pH ≤ 5.4, we conducted a series of experiments at an initial solution pH of 3.4 in which laser irradiation was terminated before all of the [AuCl4]− was consumed. Under these conditions, [AuCl4]− remaining in solution after the laser is turned off is consumed by reaction with the existing AuNP and H2O2 formed during irradiation, 30 whereas no further nucleation is expected in the absence of laser irradiation.32 Reduction of [AuCl4]− by H2O2 in the presence of AuNPs is

(1)

where [A] is the concentration of the precursor and [B] is the concentration of the metal nanoparticles. The rate constants k1 and k2 correspond to the rates of nucleation of metal clusters (assumed to be slow) and autocatalytic growth into nanoparticles (assumed to be fast), respectively. Integration of eq 1 gives the time-dependent precursor concentration19 k1 + [A(0)] k2 k1 e(k1+ k 2[A(0)])t k 2[A(0)]

−1

0.08 ± 0.01 0.06 ± 0.03 0.14 ± 0.06 0.4 ± 0.1 1.0 ± 0.3 2.0 ± 0.8 2.6 ± 0.4 7±2 29 ± 2 29 ± 6

5 20 20 10 10 20 30 100 100 100

pH 2.5, at pH 5.4, the vast majority of the particles are smaller than 10 nm. However, a few large particles are present at pH 5.4 and become more common at higher pH (blue arrows). Fusion and melting of the small particles is also observed at high pH (red arrow). These features result in higher mean particle diameters with larger particle dispersity as the pH increases above 5.4. To probe the mechanism of the photochemical reduction reaction, the time-dependent concentration of [AuCl4]− was obtained from the in situ UV−vis spectra22,23 (details provided in the Supporting Information, Figure S3−S6). Based on previous studies of metal nanoparticle synthesis reactions,19,21−23 the reduction kinetics may be expected to follow the Finke−Watsky autocatalytic nucleation−growth rate law,19 −

−3

(2)

where [A(0)] is the initial precursor concentration. Figure 5 shows the time-dependent concentration of [AuCl4]− during irradiation for experiments conducted at each initial solution pH. Fitting these data sets to eq 2 via D

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Figure 6. (A) Time-dependent concentrations of [AuCl4]− after termination of laser irradiation (dots) with fits to eq 4 (black solid lines). (B) Values of k2 as a function of the fraction of [AuCl4]− left following irradiation (red triangles). Reference values of k2 from Table 1 from eq 2 (blue triangle) and eq 3 (green triangle) as well as a least-squares regression of the data (black curve) are shown.

well-known in the literature52−54 and in our experiments was found to follow an autocatalytic rate law, i.e., the autocatalytic component of eqs 1−3, −

d[A] d[B] = = k 2[A][B] = k 2[A](1 − [A]) dt dt 1 [A(t )] = (1 − [A(0)]) k 2t 1 + [A(0)] e

(4)

where [A] is the concentration of [AuCl4]−, [B] = 1 − [A] is the concentration of Au(0), and [A(0)] is the concentration of [AuCl4]− present upon termination of the laser irradiation, which we take as the initial condition for the autocatalytic kinetics. This set of experiments thus isolates the autocatalytic rate constant k2. The extracted value of k2 from fitting the timedependent [AuCl4]− concentration (after termination of laser irradiation) to eq 4 is therefore expected to converge to the “true” value of k2 in Table 1 as the irradiation time approaches that for full consumption of [AuCl4]−. The time-dependent concentrations of [AuCl4]− after irradiation for times ranging from 85 to 190 s were fit to eq 4 (Figure 6A), and the resulting k2 values are plotted versus the fraction of [AuCl4]− present when the laser is turned off in Figure 6B. The error bars on the ordinate and abscissa, respectively, are the standard deviations in k2 values and concentration of [AuCl4]− upon termination of laser irradiation over four experiments conducted at each irradiation time. For comparison, the k2 values listed in Table 1 for eqs 2 and 3 are shown. The extracted k2 values clearly converge to the k2 value in eq 3, as shown by the least-squares regression of the data for incomplete [AuCl4]− consumption to an exponential function. This result strongly suggests that eq 3 captures the true kinetics of the reduction process, indicating that the zeroth-order term is both real and significant. To interpret the changes in the rate constants as a function of solution pH, their values from eq 3 at pH ≤ 5.4 and from eq 2 at pH ≥ 6.7 are plotted versus pH in Figure 7. Note that according to Table 1, at pH ≥ 6.7 eqs 2 and 3 produce statistically identical k1 and k2 values, whereas k3 in eq 3 becomes negligibly small. For reference, the pH range where each [AuClx(OH)4−x]− species is observed42 is indicated by colored arrows. Under acidic conditions, all of the rate constants in eq 3 are strongly suppressed and increase slowly until pH 5.4. At higher pH where eq 2 describes the kinetics, k1 increases exponentially and k2 increases linearly until pH ∼ 9, at

Figure 7. Values of k1 (red solid squares), k2 (green solid triangles), and k3 (blue solid diamonds) in eq 3 and values of k1 (dark red open squares) and k2 (dark green open triangles) in eq 2 as a function of initial solution pH. The ordinate and abscissa error bars denote the standard deviations in the k value and initial solution pH over four experiments, respectively. The pH ranges where each [AuClx(OH)4−x]− species is present42 are indicated with the colored arrows.

which point they saturate and do not change appreciably with increasing pH. Under acidic conditions (pH ≤ 5.4), we propose that the rate constants in eq 3 describe direct multiphoton photolysis of [AuClx(OH)4−x]− to form Au(0) (k1), autocatalytic reduction of [AuClx(OH)4−x]− by H2O2 (formed from the products of water photolysis) in the presence of an Au surface (k2), and reduction of [AuClx(OH)4−x]− to Au(0) by reactive radicals formed from photolysis of water (k3). Under basic conditions (pH ≥ 6.7), the transition from eq 3 to eq 2 to describe the reaction kinetics suggests that both direct multiphoton photolysis of [AuClx(OH)4−x]− and its reduction by reactive radicals upon water photolysis correspond to k1, whereas k2 corresponds to autocatalytic reduction of [AuClx(OH)4−x]− by H2O2. Direct photolysis of [AuClx(OH)4−x]− by n laser photons may proceed as E

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The Journal of Physical Chemistry A [AuClx (OH)4 − x ]− nhν x 4−x H 2O2 + eaq − ⎯→ ⎯ Au(0) + Cl 2 + 2 2

may proceed less efficiently under acidic due to destabilization of putative dissociation intermediates,55 or because other photolysis products (e.g., Cl•) can rapidly back-oxidize Au(0). The low value of k2 is attributed to the ability of H2O2 to oxidize AuNPs under acidic conditions,56 rendering the autocatalytic reduction reaction less favorable. Suppression of radical-mediated photolysis (k3) is expected due to the limited formation of reducing radical species in water at pH 2.5, based on the lower yield of H2O2 upon water irradiation (Supporting Information, Figure S3). The significant increase in the overall reaction rate at pH ≥ 5.4 is in distinct contrast to the observed slower rates of [AuClx(OH)4−x]− reduction with chemical reducing agents under basic conditions.40−42 In our experiments, the increase in k2 with rising pH is expected due to the higher oxidation potential of H2O2 under basic conditions,57

(5)

Under the present experimental conditions, the number of photons in the laser focus greatly exceeds the number of [AuClx(OH)4−x]− molecules, and the photolysis rate k1 will only depend on the number of [AuClx(OH)4−x]− molecules present in the focus. Thus, direct photolysis must be first order under all pH conditions, regardless of which rate law applies. The autocatalytic reaction with rate constant k2 follows the mechanism52 3 H 2O2 + Au m 2 3 → Au m + 1 + O2 + 3HCl + Cl− 2

[AuCl4]− +

(6)

H 2O2 ⇌ 2e− + 2H+ + O2

and depends on the formation of H2O2 during irradiation. The formation of HCl as a byproduct is consistent with the observed decrease in pH of the solution following irradiation (Supporting Information, Table S1). Under acidic conditions, reactions with rate constant k3 arise from the generation of reactive radical species by multiphoton photolysis of water35 and have previously been proposed as the dominant mechanism for photochemical reduction of aqueous [AuCl4]− to form AuNPs.26,27 Water can decompose upon multiphoton absorption by the reactions

E° = +0.146 V

nhν

(8)

2OH• → H 2O2

(9) −

•

The short-lived hydrated electrons eaq and/or H can go on to reduce [AuClx(OH)4−x]− via the reactions [AuClx (OH)4 − x ]− + 3eaq − → Au(0) + xCl− + (4 − x)OH−

(10)

[AuClx (OH)4 − x ]− + 3H• → Au(0) + xCl− + (4 − x)OH− + 3H+

(13)

which should accelerate the autocatalytic process in eq 6. However, the transition in rate law from eq 3 to eq 2 (i.e., disappearance of k3), along with the exponential increase in k1, requires further analysis. The higher stability of [Au(OH)4]− compared to [AuCl4]− 44,45,47 would be expected to impede dissociation and thus lower k1, as in previous studies that have reported a lower nucleation rate under basic conditions for chemical reduction of [AuCl4]−.41−43 Additionally, assuming that eqs 7 and 8 form the rate limiting step in the radicalmediated reduction of [AuClx(OH)4−x]−, the increased concentration of reducing radical species (measured by H2O2 yield, Supporting Information, Figure S3) should result in increasing k3, instead of its observed disappearance. This conundrum can be resolved if the rates of eqs 7/8 increase sufficiently such that eqs 10/11 become the rate-limiting step in the radical-mediated reduction process. This change in ratelimiting step would result in a concomitant change in the reaction kinetics from zeroth to first order in [AuClx(OH)4−x]−. Thus, the disappearance of k3 and fast increase in the value of the first-order rate constant k1 may be attributed to the change in reaction order for the radicalmediated process. Furthermore, the transition in reaction order is consistent with the observation that eq 2 (i.e., without k3) fits the experimental data well at pH ≥ 6.7, suggesting that the observed k1 at these pH values represents the combination of direct photolysis and radical-mediated reduction. From this analysis, it can be seen that direct multiphoton photolysis of [AuCl4]− is predominantly responsible for the generation of Au nuclei under acidic conditions, whereas the participation of radical-mediated nucleation increases under basic conditions due to the enhanced production of reactive radical species from multiphoton photolysis of water.

(7)

H 2O ⎯→ ⎯ H• + OH•

(12)

H 2O2 + 2OH− ⇌ 2e− + O2 + 2H 2O

nhν

H 2O ⎯→ ⎯ eaq − + H+ + OH•

E° = −0.695 V

(11)

For the kinetics of these reactions to be zeroth order in [AuClx(OH)4−x]−, the rate of the overall reaction leading to Au(0) must be limited by the formation of reactive species in eqs 7 and 8. This equates to the generation of a limited number of reactive radicals from water in the focal volume that diffuse through the solution (aided by cavitation-induced mixing31) and react on a time scale that is short compared to the total reaction time. In this way it can be seen that the rate of reaction is limited by the number of radicals generated and does not depend on the concentration of [AuClx(OH)4−x]− in solution and, therefore, is zeroth order in [AuClx(OH)4−x]−. We can now rationalize the pH dependence of the observed rate constants in Table 1 and Figure 7 with the reactions proposed in eqs 5−11, the known speciation trends of [AuClx(OH)4−x]− with pH,42 and the observed increased production of H2O2 in our experiments via water photolysis with pH (Supporting Information, Figure S3). Under highly acidic conditions (pH ≤ 2.5), the overall reduction rate of [AuCl4]− is very slow. Direct photolysis (k1)

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CONCLUSION We have, for the first time, demonstrated that two mechanisms of Au(0) nucleation occur during strong field photochemical reduction of aqueous [AuCl4]− to form AuNP. The previously proposed mechanism of radical-mediated nucleation26−28,34 has been shown to exhibit zeroth-order kinetics with respect to [AuCl4]− under acidic conditions due to the limited production of reactive radical species from water photolysis. The additional F

DOI: 10.1021/acs.jpca.6b03163 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A mechanism of direct multiphoton photolysis of [AuCl4]− has not been previously observed and was found to exhibit firstorder kinetics with respect to [AuCl4]−, consistent with the assumption of an excess of photons in the laser focus. The relative importance of these nucleation mechanisms was controlled by varying the pH of the initial solution, which also resulted in significant control over the AuNP size from ∼5−20 nm without the use of surfactants. These mechanistic insights are expected to enable greater control over the properties of uncapped AuNP using strong field photochemical reduction, making it possible to tailor these properties for a variety of applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03163. Additional UV−vis and TEM data, details of UV−vis spectral analysis to determine [AuCl4]− concentration, statistical analysis of nonlinear regression fitting to rate equations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*K. Moore Tibbetts. E-mail: [email protected]. Phone: (804)-828-7515. Present Address ‡

Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, United States. 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 an NSF instrumentation grant (CHE-0923077) for the JEOL JEM-1400 TEM used in this research.

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