Influence of Cationic Surfactants on the Formation and Surface

Sep 10, 2013 - Australia, GPO Box 2100, Adelaide SA 5001, South Australia. §. Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australi...
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Influence of Cationic Surfactants on the Formation and Surface Oxidation States of Gold Nanoparticles Produced via Laser Ablation Yuen-Yan Fong,† Jason R. Gascooke,‡ Bradley R. Visser,† Hugh H. Harris,† Bruce C. C. Cowie,§ Lars Thomsen,§ Gregory F. Metha,† and Mark A. Buntine*,∥ †

Department of Chemistry, The University of Adelaide, Adelaide SA 5005, Australia Flinders Centre for Nanoscale Science and Technology, School of Chemical and Physical Sciences, The Flinders University of South Australia, GPO Box 2100, Adelaide SA 5001, South Australia § Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia ∥ Department of Chemistry, Curtin University, GPO Box U1987, Perth, WA 6845, Australia ‡

ABSTRACT: We report on the time evolution of gold nanoparticles produced by laser ablation in the presence of the cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) in aqueous solution. The broader applicability of a laser-induced nanoparticle formation kinetic model previously developed by us for the case of anionic surfactants in aqueous solution [J. Phys. Chem. C 2010, 114, 15931−15940] is shown to also apply in the presence of cationic surfactants. We explore the surface properties of the nanoparticles produced in the presence of the cationic surfactants via synchrotron X-ray photoelectron spectroscopy (XPS). The XPS data indicate that at CTA+ concentrations approximating the aqueous critical micelle concentration AuIII is present on the nanoparticle surface. Such oxidation is not observed at (i) lower CTA+ concentrations, (ii) in the presence of an anionic surfactant, or (iii) in the case of pure water as a solvent. copy (TEM),18,19 X-ray photoemission spectroscopy (XPS),15 and zeta potential measurements23 to provide valuable insight into interactions between the nanoparticle and any coexisting surfactant or ligands present in solution. Chemical synthesis is the most common technique to generate various sizes and shapes of gold nanoparticles. This involves the chemical reduction of a suitable starting material, such as HAuCl4 or NaAuCl4, in aqueous solution along with organic molecules that act as stabilizers.3,16,24 During the reduction process, different shapes of gold nanoparticles, including nanorods,25 nanotriangles,26 and nanospheres,3,16 have been generated and their properties analyzed using various spectroscopic techniques. One of the most common stabilizers used for synthesizing spherical AuNPs are compounds containing thiol ligands.27 These molecules bind to the AuNP via strong sulfur−gold interactions. Johnson and co-workers24 have specifically reported on the effects that different functional groups have on the AuNPs and, importantly, found that changing functional groups in the thiol ligand leads to differing oxidation states of gold in the AuNPs. In this paper, we explore the role of surfactants on the surface oxidation states of AuNPs.

1. INTRODUCTION Gold (Au) is a prototypical transition metal used to study the changing properties of clusters as their size increases from several atoms up to the bulk. In particular, for small gold nanoparticles (AuNP), it has been found that differing numbers of gold atoms result in specific physical and chemical properties.1−4 Physical properties that change with the particle size include the peak wavelength and intensity of the surface plasmon resonance in metallic nanoparticles5,6 and quantum confinement,7,8 while changes to chemical properties such as chemical oxidations9,10 and bonding interactions with a variety of molecular species11−14 have also been observed. It has been widely commented that these unique properties may result in the potential use of AuNPs in diverse applications such as catalysts15 and biochemical sensors.13,14 To help realize these aspirations, developing a firmer understanding of the surface chemistry of AuNPs is of central importance since many of these proposed applications require functionalization of the nanoparticle surface. AuNPs can be generated in various sizes and shapes via chemical reduction from gold salts,3,16 size-selected deposition in ultrahigh vacuum,12,17 from chemically synthesized, ligandstabilized clusters,2 or via in situ laser ablation of a metallic gold disk.18−22 Following their production, AuNPs are routinely investigated using various characterization techniques, including UV−vis−NIR spectroscopy,5,6 transmission electron micros© 2013 American Chemical Society

Received: June 17, 2013 Revised: August 22, 2013 Published: September 10, 2013 12452

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nm irradiation, samples were prepared to be examined using TEM with a Philips CM 100 electron microscope. A drop of AuNP solution was deposited on a carbon-coated thin bar copper grid and dried at room temperature. The TEM images were captured at an emission energy of 80 kV and magnification of 130 000. All recorded images were analyzed using the Olympus iTEM analysis program to determine particle size distributions. 2.2. X-ray Photoelectron Spectroscopy. Samples synthesized following 300 min of 1064 nm irradiation were also prepared for X-ray photoelectron spectroscopic (XPS) analysis. One drop of the mixture solution was deposited on a clean 6 × 6 mm silicon (Si) wafer. The silicon wafer was dried under vacuum and subsequently fixed to the sample holder using double-sided copper tape to ensure the electrical conductivity of the sample. Photoelectron spectra were recorded at the Soft X-ray Beamline at the Australian Synchrotron (AS) using a SPECS Phoibos 150 hemispherical electron analyzer. Since it is well-known that photochemical reduction can occur in gold samples undergoing XPS,10,32 a series of spectra were recorded over time. Samples were continuously irradiated for a period of 900 s. During this irradiation, 15 sequential XPS spectra were recorded by scanning the electron analyzer from 95 to 70 eV in 0.05 eV increments. Each scan each took 50 s to complete followed by ∼16 s delay before the next scan started. The synchrotron radiation was set to a photon energy of 650 eV (see ref 35) and adjusted to yield an irradiation spot size of ∼600 μm. The electron analyzer pass energy was set to 10 eV, yielding an instrumental resolution of 0.295 eV in terms of resultant electron kinetic energy.35 The calculated photon resolution broadening at this photon energy is 0.08 eV. The total instrumental broadening due to the photon source and the hemispherical analyzer is 0.31 eV. XPS spectra in the region of the Au 4f peaks were calibrated using the Au 4f7/2 peak (84.0 eV) of gold foil.36 We employed an X-ray photon flux at the AS of approximately 1012 photons mm−2 s−1, conditions similar to those recently discussed by Hu et al.37 As we have previously reported,32 under these conditions, it is highly unlikely that we are inducing localized heating of the AuNPs or the substrate. For example, at a photon energy of 650 eV, the total irradiation power is approximately 100 μW over an area of ∼600 μm2. This is insufficient monochromatic X-ray power to result in appreciable thermal damage.32 For each recorded XPS spectrum, the data were deconvoluted to obtain the relative population of each gold oxidation state. This was achieved by fitting the spectra to a sum of peaks using nonlinear leastsquares minimization. A Shirley background was applied to remove the electron scattering background and maintain the intrinsic line shape from the raw data.38,39 A pseudo-Voigt function comprised of the sum of Gaussian (30%) and Lorentzian (70%) functions40 was used, with a full width at half-maximum of all peaks fixed at 1.28 eV. This functional form for the peaks gave the best fit when comparing all scans. The Au 4f spin−orbit (4f7/2 and 4f5/2) energy splitting was fixed at 3.67 eV, but the absolute position was allowed to vary. The ratio of peak areas between the 4f7/2 and 4f 5/2 peaks are fixed to appear as 4:3 due to the (2j + 1) multiplicity of each spin−orbit state.41,42

Laser ablation synthesis in solution (LASiS) is an alternative technique for forming metal nanoparticles in solution.18,19,21,22 The technique involves focused laser irradiation of a bulk metal target in a liquid. Importantly, unlike chemical synthesis methods, nanoparticles can be produced in solution with or without the presence of stabilizers. LASiS is quick and simple and can be a chemically clean, or “green”, nanoparticle production method.28,29 Considerable effort has been directed toward understanding the mechanism of aggregation and size reduction of AuNPs produced via LASiS in various solvents, with a variety of ligands and surfactants present in solution.5,11,30,31 We have previously used the LASiS method to generate AuNPs in aqueous solution, with the anionic surfactant sodium dodecyl sulfate (SDS) present in various concentrations.18,19 In these studies we presented a comprehensive analysis of the kinetics of AuNP production and stability and reported that the AuNPs produced at each concentration of SDS can be modeled by assuming three AuNP size regimes: small (17 nm). Sylvestre and co-workers9 have reported that femtosecondlaser-generated AuNPs in aqueous solution (average diameter ∼40 nm) have a surface comprising Au3+ and Au+ species. Using time-of-flight secondary ion mass spectrometry, these workers determined that surface oxidation results from reactions with water molecules to form Au−O− and Au2O3− structures on the AuNP surface. Motivated by these studies, in this contribution we extend our previous work to investigate the nanosecond LASiS-based production of AuNPs in the presence of the cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) in aqueous solution. We report the size distribution and formation kinetics of the production process, as well as determine the oxidation states of the Au surface atoms, via XPS, of AuNPs that were produced with and without surfactant in solution. Importantly, this study builds upon earlier studies from our laboratory32 and allows us to explore AuNP X-ray photoreduction chemistry under synchrotron soft X-ray exposure as a function of surfactant type and concentration.

2. EXPERIMENTAL METHODS 2.1. Gold Nanoparticle Formation. AuNPs were produced by the LASiS method in a manner similar to our previous work.18,19 A thin gold disk (>99.99%) within a Teflon holder was placed at the bottom of a glass vessel containing 10 mL of either 10−5 or 10−3 M aqueous solutions of CTAB or CTAC. The aqueous critical micelle concentrations (CMC) of CTAB and CTAC are 0.94 and 1.1 mM, respectively. Therefore, the two surfactant concentrations used in these studies correspond to being below and approximately equal to the aqueous CMC.33 On the basis of previous studies, we do not expect a dramatic change in nanoparticle size at the CMC.18,34 All water samples were purified with a Waters Milli-Q system. The immersed gold plate was irradiated with the fundamental output (1064 nm) of a Nd:YAG laser (Quantel YG-981C) which was focused by a plano-convex lens with a focal length of 250 mm. The laser operated at a repetition rate of 10 Hz, with a pulse width of 7 ns (fwhm) and a beam diameter (before focusing) of 8 mm. To prevent the laser ablation process burning a hole through the thin gold disk, a rotation stage rotating at 1 rev min−1 was employed. The laser was operated at a power of 7.5 mJ pulse−1, focused to a spot size of 66 μm, with an ablation fluence of 2.2 × 105 mJ cm−2 pulse−1. Samples were irradiated for up to 300 min. During the experimental procedures above, UV−vis spectra of samples undergoing irradiation were recorded at 10 min intervals with a UV−vis spectrophotometer (Cary, Bio 300). After 300 min of 1064

3. RESULTS AND DISCUSSION 3.1. Production of AuNPs. Absorption spectra of AuNPs produced by 1064 nm irradiation of a gold disk and taken at 20 min intervals (up to 220 min) for each of the 10−5 and 10−3 M CTAB and CTAC solutions are presented in Figure 1. In Figure 2, we present absorbance-normalized variants of the same spectra thus allowing changes in band shape to be discerned.43 Each spectrum shows very similar behavior, with a steady increase in intensity of the distinctive surface plasmon band (SPB) of AuNPs at approximately 525 nm. The position of the wavelength maximum of the SPB (λmax) is a property of average particle size.44 Figure 1 shows that λmax does not appreciably change over the 220 min experimental time scale. It is apparent from Figure 2b−d that at the CTAB concentration 12453

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Figure 1. UV−vis absorption spectra of AuNP samples produced via 1064 nm LASiS in aqueous solution with (a) 10−5 M CTAB, (b) 10−3 M CTAB, (c) 10−5 M CTAC, and (d) 10−3 M CTAC present in solution. For each surfactant solution concentration, spectra are displayed at 20 min intervals up to a maximum irradiation time of 220 min.

Figure 2. Normalized UV−vis absorption spectra of AuNP samples produced via 1064 nm LASiS in aqueous solution with (a) 10−5 M CTAB, (b) 10−3 M CTAB, (c) 10−5 M CTAC, and (d) 10−3 M CTAC present in solution. For each surfactant solution concentration, spectra are displayed at 2 min intervals for the first 10 min and then 20 min intervals up to a maximum irradiation time of 220 min.

of 10−3 M and CTAC concentrations of 10−5 and 10−3 M λmax shifts to the blue by no more than 3 nm over the first 100 min of the laser irradiation, after which time it remains steady. This indicates that these solutions contain particles that are large enough to support the SPB, and furthermore, the average particle size does not change significantly over the duration of the experiment. By contrast, the 10−5 M CTAB solution (Figure 1a) is observed to have a larger blue shift of approximately 12 nm over the first 20 min. Figure 2a highlights a broad, weak absorption feature at longer wavelengths (∼700 nm) at this surfactant concentration in the first 20 min, which we attribute to larger particle sizes being formed. This is based upon previous studies where the wavelength maximum of the SPB shifts to longer wavelengths for larger particle sizes.18,19,44−46 After 20 min, the ∼700 nm feature reduces in intensity relative to the SPB peak. This longer-wavelength feature is less apparent under 10−5 M CTAC conditions (Figure

2c) and not apparent at surfactant concentrations of 10−3 M. The larger particle sizes at surfactant concentrations of 10−5 M are apparent in the data yet to be presented in Figure 4, but the absorption features highlighted in Figure 2 are not apparent on the scale of the spectra presented in Figure 1. The observed λmax values are slightly dependent upon which surfactants are present. After 100 min, both CTAC concentrations show λmax = 523 nm, while the CTAB solutions exhibit a λmax = 526 nm. These are very similar values to those previously reported for pure water (λmax =524 nm) and using SDS as surfactant (λmax = 524 nm for 10−4 M; λmax = 521 nm for 10−2 M).19 The results for differing surfactants and concentrations cannot be used to provide information on the particle sizes since λmax varies with changes in dielectric strength that accompany changes in the surfactant’s identity and concentration.5 All the spectra clearly show the width of the 12454

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surface plasmon band reduces gradually over time, consistent with a reduction in the particle size.47 An understanding on the AuNP formation chemistry is achieved by fitting the time-dependent absorbance data to an appropriate kinetic model using the intensity of the SPB. These data are presented in Figure 3 for each surfactant as well as for

Scheme 1. Model Reaction Scheme Highlighting the Reversible Formation of AuNPs Too Small To Support a Surface Plasmon Band (SPB), AuNP(small); Those Just Large Enough To Support a SPB, AuNP(med); and Those Large Enough To More Easily Support a SPB, AuNP(large), from Bulk Gold as Well as the Interconversion between the Three Forms of Nanoparticle

between each type of AuNP is allowed, in principle. The rationale for employing a three-size AuNP kinetic model is provided in our previous reports18,19 and is based on the wellestablished notion, both experimentally44,49,50 and theoretically,47,48,50 that the AuNP absorption coefficient increases with particle size. The kinetic model presented in Scheme 1 contains up to 12 unique AuNP transformation channels. Numerical integration of the resultant rate expressions18,19 combined with nonlinear least-squares fits to the absorbance data yields the lines of best fit presented in Figure 3. The most important aspect to arise from the data fitting process is that, as with previously reported experiments involving the anionic surfactant SDS,18,19 only four AuNP transformation channels (of the 12 possible) are required to fit the data, namely kl, km, ks, and k−m. Extensive exploration of the kinetic model has determined that these transformation channels are the minimum number required to account for the observed behavior and at each surfactant concentration represents a unique solution with this minimum set of parameters. The rate constants determined in this study are presented in Table 1. It is clear from these data, together with those previously reported by us for anionic surfactants,18,19 that the kinetic model describing AuNP LASiS formation under our experimental conditions is independent of the type of surfactant present in aqueous solution. Representative TEM images together with histograms indicating their associated particle size distributions of AuNP samples prepared in 10−5 and 10−3 M CTAB and in 10−5 and 10−3 M CTAC after 300 min of 1064 nm laser irradiation are presented in Figure 4. To improve the statistics of the particlesize distribution determinations, each histogram was generated by analyzing multiple images, rather than just the image presented above it. Each TEM image shows that the AuNPs have a roughly spherical geometry with a range of particle sizes. In the case of 10−5 M CTAB (Figure 4a), the particle size distribution peaks at 13 nm with a broad distribution of sizes up to ∼45 nm in diameter. The average particle size is 15.5 nm with a standard deviation (1σ) of 10 nm. In the case of 10−3 M CTAB (Figure 4b), the particle size distribution peaks at approximately 10 nm with a relatively broad distribution of sizes out to a maximum of approximately 30 nm. Statistical analysis of the particle size distribution results in an average particle size of 11 nm with a standard deviation (1σ) of 5.1 nm.

Figure 3. Variation in absorbance of the AuNPs in aqueous solution (a) with 10−5 and 10−3 M CTAB and (b) with 10−5 and 10−3 M CTAC present in solution as a function of 10 Hz 1064 nm laser irradiation at the SPB λmax. In both panels data are presented in the case where no surfactant is present. See text for discussion of the phenomena involved.

LASiS performed in pure water as the solvent. (The results of LASiS experiments in pure water have been previously reported18,19 and are included here for comparative purposes.) Immediate inspection of the data in each of panel of Figure 3 indicates that production of AuNPs large enough to contribute to the SPB absorbance is enhanced at the higher CTA+ surfactant concentrations. SPB-supporting AuNP yields are diminished, relative to using pure water as the solvent, at the lower CTA+ concentrations. We have previously reported a model that explains the production of AuNPs via the LASiS approach.18,19 This model is used to fit the experimental data presented in Figure 3, and the quality of the fits is evident by inspection. The basis for the kinetic model is presented in Scheme 1. In brief, the model considers (i) AuNPs of a size too small to support a surface plasmon resonance transition (denoted AuNP(small))it is well established that the minimum size to support a SPB transition in AuNPs is approximately 4 nm,48 (ii) “medium-sized” AuNPs having a weak, but measurable, SPB molar absorbances (denoted AuNP(med)), and (iii) “large” AuNPs with greater SPB molar absorbances (denoted AuNP(large)). Interconversion 12455

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Table 1. Resultant 1064 nm Kinetic Parameters Determined Following a Nonlinear Least-Squares Fit of the Absorbance Data to the Kinetic Model Given in Scheme 1a 10−5 M CTAB −1

kl (pulse ) km (pulse−1) ks (pulse−1) k−m (pulse−1)

(3.05 (1.61 (3.62 (2.21

± ± ± ±

0.07) 0.42) 1.00) 0.70)

× × × ×

10−3 M CTAB −6

10 10−5 10−9 10−4

(7.84 (1.45 (4.74 (1.57

± ± ± ±

0.09) 0.29) 0.25) 0.37)

× × × ×

10−5 M CTAC −6

10 10−5 10−8 10−4

(4.53 (1.19 (1.06 (7.46

± ± ± ±

0.09) 0.18) 0.16) 1.47)

× × × ×

10−3 M CTAC −6

10 10−5 10−8 10−5

(8.44 (1.16 (5.34 (7.78

± ± ± ±

0.12) 0.18) 0.27) 1.60)

× × × ×

10−6 10−5 10−8 10−5

a

All reported uncertainties represent three standard deviations error (3σ). CTAB: cetyltrimethylammonium bromide; CTAC: cetyltrimethylammonium chloride; kl, km, ks, k−m: rate constants for interconversion between the different gold nanoparticle species as defined in Scheme 1. See text for details.

Figure 4. Representative TEM images, together with histograms indicating their associated particle size distributions, of AuNP samples prepared at each surfactant concentration after 300 min of 1064 nm laser irradiation. The horizontal bar in the lower right-hand corner of each image represents 200 nm.

Table 2. Determination of the Concentration of AuNP in Solution and the Number of CTA+ Molecules Available per AuNP following 300 min of Laser Irradiation (See Text for Details) surfactant CTAB CTAC a

CTA+ concn (M) 1 1 1 1

× × × ×

10−5 10−3 10−5 10−3

av AuNP diam (nm) 15.5 11.0 14.0 12.9

AuNP extinction coeff (M−1 cm−1) 1.42 4.42 2.40 3.15

× × × ×

108 108 108 108

AuNP concn (M) 7.09 5.67 6.52 8.76

× × × ×

surface area of av AuNPa (m2)

10−9 10−9 10−9 10−9

7.55 3.80 6.16 5.23

× × × ×

10−16 10−16 10−16 10−16

no. of CTA+ required to cover the AuNP surface

no. of CTA+ molecules available per AuNP

1680 845 1370 1160

1410 176000 1530 114000

These data refer to the surface area of average-sized nanoparticles, as reported in Figure 4.

The sample containing 10−5 M CTAC (Figure 4c) shows a similar maximum in particle size distribution to that of 10−5 M CTAB, with an average particle size of 14 nm and a standard deviation (1σ) of 8.5 nm. Finally, the sample containing 10−3 M CTAC (Figure 4d) shows a similar peak in particle size distribution at 10−5 M CTAC. Here, the average particle size is 12.9 nm with a standard deviation (1σ) of 7.2 nm. The TEM data analysis indicates that there exist only minor differences between the particle size distributions of AuNPs produced in CTAB and CTAC surfactants. In the case of CTAB, where the 10−3 M concentration is just above the aqueous CMC, there is a significant narrowing of the AuNP particle size distribution compared to that obtained under 10−5 M conditions. For CTAC, where 10−3 M is slightly below the aqueous CMC, the particle size distribution is very similar to that obtained at 10−5 M. Overall, the size distributions are very similar to that obtained using pure water (see Figure 2 in ref 19). This is in contrast to AuNPs produced in aqueous SDS solutions, where we have previously reported that AuNPs

produced in solutions containing high SDS concentrations show a marked reduction in particle diameter compared to that obtained in pure water.18,19 We do not, however, attribute the variations in particle size distributions at different surfactant concentrations to the presence of micelles in solution. We have previously reported that there is no apparent influence of the SDS concentration around the CMC on particle size during LASiS.18 The particle size distributions in the presence of CTAB presented in Figure 4a,b are sufficiently similar in appearance at all but the largest particle diameters (greater than ∼25 μm) to suggest that a CMC effect is also not operative with the CTA+ surfactant. Similarly, consistent with our previous work,18 we do not attribute the differences in particle size distributions to partial surfactant coverage of the nanoparticles (see below). Nonetheless, we are unable to comment on the possible role of surfactant bilayers or other structural motifs adsorbed onto the AuNPs contributing to the observed particle size distributions. 12456

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Figure 5. Fitted XPS spectra for the initial scan of gold nanoparticles generated in aqueous solution containing (a) pure water, (b) 10−2 M SDS, (c) 10−5 M CTAB, (d) 10−3 M CTAB, (e) 10−5 M CTAC, and (f) 10−3 M CTAC present in solution. See Experimental Methods section for a discussion of the fitting procedure.

that the same three-size reaction scheme accounts for the observed kinetic behavior exhibited using the cationic surfactants CTAB and CTAC, we now look to undertake a similar analysis to quantify the AuNP size boundaries. We refer the reader to our previous reports to obtain details of the sizedetermination method.18,19 It is important to recognize what the terms “small”, “medium”, and “large” refer to in this model. “Small” nanoparticles are those unable to support the surface plasmon resonance transition, “medium” nanoparticles represent weak surface plasmon absorbers, and “large” nanoparticles represent strong surface plasmon absorbers. Consistent with the smoothly varying particle size distributions presented in Figure 4, the model does not infer that abrupt transitions in surface plasmon absorbances occur at the size boundaries. The first challenge in the size-boundary-determination analysis is that, unlike the 10−2 M SDS situation where a large fraction of the observed AuNPs are smaller than 5 nm, the TEM histograms presented in Figure 4 show that under all concentrations of CTA+ employed relatively few particles are produced in this small size regime. The low number of “small” nanoparticles makes a determination of the small-medium size boundary prone to unacceptable uncertainty. Moreover, as discussed in our previous determination of the boundary ranges, the presence of even a small number of “large” AuNPs in the TEM particle size histograms can yield distorted results,19 and so we therefore fix the small−medium size boundary to 5.5 nm as determined (with high confidence) in our previous report. We now consider the size of the medium−large AuNP boundary. Inspection of the data presented in Figure 1 indicates that the AuNP SPB appears on the “tail” of a broader intervalence band (IVB) transition. As previously reported,19 we account for the absorbance at the SPB λmax in terms of the AuNP IVB absorption coefficient, ε450 nm, as

One possibility to account for the near invariance in AuNP particle size distributions at each CTA+ concentration is that there are insufficient CTA+ molecules to fully coat the nanoparticles with a monolayer of surfactant and thus allow for enhanced particle aggregation. On the basis of the work of Liu and co-workers,51 we are able to determine the sizedependent optical extinction coefficient of AuNPs at each CTA+ concentration and thus the concentration of AuNP in solution from the Beer−Lambert law. Assuming spherical AuNP morphology, we can further estimate the “average” surface area of AuNPs at each CTA+ concentration from the average diameters reported in Figure 4. Lang et al.52 report that the effective size of each CTA+ headgroup in a micelle is approximately 0.45 nm2. From this body of information, we are able to estimate the number of CTA+ molecules required to coat the AuNP with a monolayer of surfactant together with the number of available CTA+ molecules per AuNP in solution after 300 min of irradiation. The results of this analysis are presented in Table 2. It is clear from the data presented in Table 2 that at CTA+ concentrations of 10−5 M there are approximately the required number of surfactant molecules to completely encapsulate the AuNPs, while at 10−3 M concentrations there is a clear excess of CTA+ relative to the number of AuNPs in solution. Thus, particle aggregation due to incomplete surfactant coverage is unlikely to occur. 3.2. Characterizing AuNP Size Regimes. In a previous report from our laboratory, where the LASiS reaction scheme involving “small”, “medium”, and “large” AuNPs was developed (using pure water and SDS solutions to produce the nanoparticles), we also reported on an analysis involving both absorption spectroscopy and TEM microscopy to characterize the size boundaries of each type of AuNP in the reaction scheme.19 This study determined that, in the presence of SDS (an anionic surfactant), the small−medium AuNP size boundary is 5.5 ± 0.5 nm, while the medium−large size boundary is 17 ± 3 nm. Having shown in this current report 12457

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Figure 6. Fitted XPS spectra for the 15th scan of gold nanoparticles generated in aqueous solution containing (a) pure water, (b) 10−2 M SDS, (c) 10−5 M CTAB, (d) 10−3 M CTAB, (e) 10−5 M CTAC, and (f) 10−3 M CTAC present in solution. See Experimental Methods section for a discussion of the fitting procedure.

peaks at 84.0 and 87.7 eV. These are assigned to the 4f7/2 peak and 4f5/2 peaks of Au0, respectively. The ratio of the peak areas is the expected 4:3 ratio based on the spin multiplicity of each spin−orbit state.41,42 AuNPs produced in 10−5 M CTAB (Figure 5c) show a similar spectrum to that of pure water and SDS, whereas 10−3 M CTAB (Figure 5d) displays an extra set of peaks at 86.6 and 90.3 eV. These are assigned to the AuIII 4f7/2 and 4f5/2 chemical species, respectively. When CTAC is used as the surfactant during AuNP formation, the XPS spectrum of 10−5 M CTAC (Figure 5e) shows predominately peaks for Au0 species with a minor contribution of AuIII species. The 10−3 M CTAC sample (Figure 5f) displays both Au0 and AuIII species; however, there is now a larger fraction of the AuIII species present. All XPS spectra were collected from AuNP samples dried on a clean silicon wafer. It is reasonable to question whether the observation of AuIII is as a result of the drying process and therefore not truly indicative of the AuNP surface oxidation states in solution. However, the absence of AuIII in XPS spectra arising from AuNP samples (prepared in an identical manner) consisting of pure water or the anionic surfactant SDS, or aqueous solutions of NaCl or NaBr (these data are presented in Figure 7 and will be discussed later), leads us to conclude that we cannot attribute the observation of AuIII in the XPS spectra presented in Figure 5d−f to an artifact of the drying process. We have previously shown that exposure of a sample of sodium tetrachloroaurate to synchrotron soft X-rays results in AuIII species undergoing reduction to first form the AuI species, followed by further reduction to the Au0 species.32 Figure 6 shows the XPS spectra of the same AuNP samples but after exposure of synchrotron X-rays for 16.5 min (15th scan of each sample). There are minimal differences between the first scan (Figure 5) and 15th scan (Figure 6) for all concentrations except for the 10−3 M CTAC and 10−3 M CTAB samples. The 10−3 M CTAC 15th-scan spectrum (Figure 6f) shows that, over time, there is a production of the AuI oxidation state and a

A λmax = ε450nm S{0.73([AuNP(small)] + [AuNP(medium)] + [AuNP(large)]) + X[AuNP(medium)] + Y [AuNP(large)]}

where the values of X and Y, representing the extinction coefficient ratios of the SPB component of the SPB-supporting nanoparticles (“medium” and “large”, respectively), are determined by a simultaneous nonlinear least-squares fit of all species concentrations and absorbances at both 450 nm and the SPB λmax. Using the three-size LASiS reaction model with the small−medium size boundary fixed at 5.5 nm, the nonlinear least-squares fitting algorithm that determines the LASiS formation rate constants yields an SPB component of the extinction coefficient for “medium” AuNPs of X = 0.56 (compared to 0.61 using SDS)19 and Y = 0.97 for “large” AuNPs (compared to 0.86 using SDS).19 Using these extinction coefficients, the boundary between medium and large AuNPs in the presence of CTAB and CTAC is 13 ± 4 nm (compared to 17 ± 3 nm for SDS). Within experimental uncertainty, this size boundary between weak (“medium” AuNP) and strong (“large” AuNP) surface plasmon resonance absorbers is the same for both anionic and cationic surfactants. 3.3. X-ray Photoelectron Spectroscopy. Additional experiments were undertaken to explore the surface oxidation of AuNPs formed following 1064 nm laser irradiation at each CTAB and CTAC concentration by using synchrotron X-ray photoelectron spectroscopy (XPS). Recall that in order to monitor possible photoreduction of surface Au atoms, for each sample 15 XPS spectra were collected in rapid succession. The initial XPS spectrum recorded for each of the samples are presented in Figure 5. This figure also displays the XPS spectra recorded for AuNPs produced in pure water (acting as a control; Figure 5a), and in 10−2 M SDS solution (allowing comparison with an anionic headgroup surfactant; Figure 5b). Each spectrum shows the photoelectron lines of Au 4f region. XPS spectra for the AuNPs produced in pure water and 10−2 M SDS samples (Figure 5a,b) consist of a single pair of 12458

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Figure 7. Fitted XPS spectra for the initial and 15th scans of gold nanoparticles generated in aqueous solution containing 10−3 M NaBr (a, b) and 10−3 M NaCl (c, d).

diminution in the AuIII species yield (relative to the Au0 intensity). Comparing the XPS spectra in Figures 5f and 6f, approximately 20% of AuIII is photoreduced to AuI after 16.5 min of irradiation. The sample produced in 10−3 M CTAB solution shows a reduction of the amount of AuIII during X-ray exposure (Figures 5d and 6d), although it is interesting that there is no detectable amount of AuI formed in the sample. The lack of observation for the AuI species in this sample may be due to either mechanistic or kinetic factors. For example, the photoreduction process may not proceed through an AuI intermediate state (i.e., the AuIII species reduces directly to Au0); we note that recent reports indicate that direct reduction of AuIII to Au0 is possible.53,54 Alternatively, the rate constant for the AuI → Au0 reduction process may be much faster for nanoparticles produced in the presence of CTAB when compared to those prepared in CTAC solutions. An intriguing possibility to account for the reduction in the AuIII XPS signal intensity as a function of X-ray exposure is the photoinduced aggregation of AuNPs. Andersson and coworkers have undertaken synchrotron XPS studies to investigate a series of chemically synthesized, atomically precise gold clusters immobilized on titania nanoparticles.2 These workers report no evidence of aggregation occurring as a function of X-ray exposure. The small Au cluster sizes, involving 8, 9, 11, and 101 Au atom stoichiometries, would make any Au aggregation channels readily apparent. As such, notwithstanding the differences in sample supports between our study and that of Andersson and co-workers (silicon versus TiO2), we discount AuNP aggregation accounting for the decrease in AuIII signal intensity as a function of X-ray exposure. The appearance of AuIII in the initial XPS scans is observed for gold nanoparticles produced in 10−3 M CTAB, and both 10−5 and 10−3 M CTAC solutions, but not for those produced in pure water or SDS solution. Interestingly, Sylvestre and coworkers have reported a similar experiment on producing AuNPs in pure water using 800 nm LASiS with Ti/sapphire

laser (1 mJ, 1 kHz, 120 fs pulse width) and showed the presence of AuI and AuIII oxidation states of AuNPs which they attributed to Au−O− and Au-CO3 species from water and dissolved atmospheric CO2, respectively.9 The observation of oxidized gold by Sylvester et al. and the absence of these species in our spectra of samples prepared in pure water and aqueous solutions of SDS is possibly due to the different temporal pulse widths of the ablation lasers employed. It is known that the ablation laser pulse width, fluence, and wavelength all affect the mechanism of nanoparticle production.55−57 Specifically, the dynamics of AuNP production are known to be different between nanosecond and femtosecond laser pulses; when using the former, laser energy can be absorbed by the plasma, thereby increasing its internal energy via inverse Bremsstrahlung processes.56,57 Alternatively, we cannot eliminate the possibility that AuIII is formed under all ablation conditions but is only stabilized by the presence of the CTA+ surfactant. The observation of AuIII in the cationic surfactant samples indicates that the laser ablation process produces oxidized gold when performed in an environment containing CTA+ and an anion. To determine if CTA+ is responsible for the oxidation/ stabilization of the gold atoms, or whether the oxidation arises from the anionic species in solution, additional experiments were undertaken where AuNPs were produced via LASiS in 10−3 M NaCl and 10−3 M NaBr solutions (i.e., in the absence of surfactants). The 10−3 M concentration was chosen to correspond with the same concentration of anions present in the highest concentration surfactant solutions. The Au 4f region XPS spectra for the AuNPs produced from these samples are presented in Figure 7. These spectra only exhibit peaks from Au0, clearly showing that Cl− and Br− anions are not responsible for the production of the oxidized gold observed in the XPS spectra of samples containing the cationic surfactants, indicating a role involving the presence of CTA+ in solution during the ablation process. The spectra in Figure 7 also show that X-ray photoreduction is not occurring; only Au0 is present in both the initial and 15th XPS scans. 12459

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It is possible that AuNPs are produced with a partially oxidized surface or that a gold salt (i.e., AuCl4−or AuBr4−) is formed in solution during laser ablation and is subsequently deposited on the silicon substrate. The presence of AuCl4− anions in solution can be readily determined via UV−vis spectroscopy since AuCl4− solutions show absorption maxima at 230 nm58 and 350 nm.27 We observe no UV−vis spectroscopic evidence for any appreciable absorption at 350 nm (the absorbances observed in this region are attributed to broad Au IVB transitions19), suggesting that gold salts are not produced during the LASiS process. Under the XPS experimental conditions employed in this study the photoelectron escape depth is no more than approximately 1 nm.59,60 Therefore, one possibility to account for the observation of oxidized gold is to consider that the surface atoms on the AuNPs are partially oxidized (by the water solvent) during the LASiS process. This has been observed previously by Sylvestre and co-workers9 where time-of-flight static secondary ion mass spectrometry was used to show that during femtosecond LASiS the AuNP surface includes AuO− and CO3Au− moieties. These species were attributed to reactions involving Au atoms with oxygen-containing species present in the laser-generated plasma. Under higher laser fluence nanosecond-LASiS conditions compared to those employed in this study, Mafuné and co-workers61 have reported similar results whereby they attribute AuNP surface oxidation in aqueous solution to the formation of Au−O− sites involving 3.3−6.6% of the surface atoms. In contrast, under the lower laser fluence conditions employed here, we do not observe oxidized gold in nanoparticles produced in pure water, SDS, or sodium salt solutions. Rather, we readily observe oxidized gold in 10−3 M CTA+ solutions, with a minor presence with 10−5 M CTAC. This investigation indicates that higher CTA + concentrations approximating the aqueous critical micelle concentration are required for the low-powered nanosecond LASiS process to generate and stabilize the oxidized surface atoms.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of the Australian Research Council for the laser used in this study. We thank Professor Bill Skinner (University of South Australia) for helpful discussions on XPS data analysis. The XPS experiments were undertaken on the soft X-ray spectroscopy beamline at the Australian Synchrotron, Victoria, Australia.

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