Estimation of Surface Oxide on Surfactant-Free Gold Nanoparticles

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J. Phys. Chem. C 2007, 111, 17221-17226

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Estimation of Surface Oxide on Surfactant-Free Gold Nanoparticles Laser-Ablated in Water Hitomi Muto, Kunihiro Yamada, Ken Miyajima, and Fumitaka Mafune´ * Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed: July 17, 2007; In Final Form: September 12, 2007

Surfactant-free gold nanoparticles were prepared by laser ablation of a gold metal plate in water. The nanoparticles were characterized by absorption spectroscopy, ζ-potential measurements, and XPS spectroscopy. The nanoparticles are negatively charged because the surface atoms are partially oxidized to Au-O-, according to the literature by Sacher and co-workers (J. Phys. Chem. B, 2004, 108, 16864). We further examined electrostatic interactions between nanoparticle and cationic surfactants. It was found that the surfactant cations attach to the particle surface, neutralizing the particle charge. Taking advantage of the electrostatic interactions, we estimated that 3.3-6.6% of the surface gold atoms was oxidized in water.

Introduction

Experimental Procedures

Gold nanoparticles have attracted considerable attention because they show specific physical and chemical properties that the bulk material do not possess.1-4 Considerable effort has been directed at exploring a variety of methods for gold nanoparticle preparation. Brust and co-workers developed a onepot method to synthesize gold nanoparticles stabilized by dodecanethiol molecules: the sulfur atom in the thiol group binds strongly to the gold atom on the particle surface.5 As a result, the long carbon chains radiate from the surface, which prevent the particles from aggregating in solution. By contrast, citric acid is commonly used to reduce gold ions in water to synthesize gold nanoparticles.6-8 Here, the citrate anions form a charged layer around the particle surface, which provide the particles with electric repulsive forces between them. Hence, in the preparation of nanoparticles, it is critically important to know how the nanoparticles are stabilized in the solvent. Recently, there has been growing interest in laser-based methods to prepare nanoparticles in water.9-18 In a previous report from our laboratory, we demonstrated the formation of surfactant-free noble metal nanoparticles by laser ablation of the bulk materials in water.19 The surfactant-free nanoparticles were found to be very stable in water against aggregation, although they were not stabilized by the surfactants. The ζ-potential measurements on platinum nanoparticles showed that the particles are negatively charged.20 More recently, Sacher and co-workers investigated the surface of gold nanoparticles produced by laser ablation in water by XPS and IR spectroscopy, mass spectrometry, and the ζ-potential measurements.18 They elucidated that the nanoparticles are negatively charged because the particle surface is partially oxidized to Au-O-. In the present study, we investigated the surface of gold nanoparticles by examining the chemical interactions between nanoparticle and cationic or anionic surfactants. We estimated how many anionic sites exist on the particle surface by investigating the stabilities of the nanoparticles in aqueous solutions of the cationic surfactant.

Gold nanoparticles were prepared by laser ablation of a gold metal plate in pure water.12 Here, the metal plate was placed on the bottom of a glass vessel filled with 10 mL of distilled and deionized water. The fundamental of a nanosecond Nd:YAG pulsed laser (1064 nm) operating at 10 Hz with a pulse energy of 80 mJ was focused by a lens having a focal length of 250 mm onto the metal plate. The concentration of the colloidal dispersion was typically 0.12 mM in terms of gold atoms after 36 000 laser shots. Hereafter, when the concentration of nanoparticles is given, it refers to the concentration of atoms dispersed as particles. The colloidal dispersion of surfactant-free gold nanoparticles in water was mixed with a cetyltrimethyl ammonium bromide (CTAB: C16H33N(CH3)3+ Br-) aqueous solution or a sodium dodecyl surfate (SDS: C12H25OSO3- Na+) aqueous solution at the prescribed concentration, and then the mixed solution was vigorously stirred. The ζ-potentials of the particles were measured right after mixing using a Malvern Zetasizer Nano ZS: the mobility of the colloidal particles was measured by electrophoresis. The ζ-potentials were calculated from the electrophoretic mobilities by Henry’s equation. Optical absorption spectra in the UV-vis region ware measured using a Shimadzu UV-1200 spectrometer 1 day after mixing. We recorded the spectra of the supernatant, after the nanoparticles were precipitated. The solution of gold nanoparticles was dried on a copper grid and copper foil for characterization by transmission electron microscopy (TEM) and X-ray electron spectroscopy (XPS), respectively. The size and shape of the nanoparticles were observed by TEM. The TEM images were observed using a JEOL JEM-2010F instrument at a magnification of 500 000150 000. The size distribution was obtained by measuring diameters of more than 3000 particles in sight. The electronic structure was analyzed by XPS, where an Al X-ray at 1253.6 eV was used as the light source.

* Corresponding author. E-mail: [email protected].

Results Figure 1 a shows a TEM image of surfactant-free gold nanoparticles produced by the laser ablation of a metal plate in

10.1021/jp075582m CCC: $37.00 © 2007 American Chemical Society Published on Web 10/26/2007

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Figure 3. ζ-potentials of gold nanoparticles in the SDS (open circle) and CTAB (solid circle) aqueous solutions with different concentrations of SDS and CTAB.

Figure 1. (a) TEM image of surfactant-free gold nanoparticles produced by laser ablation of a gold metal plate in water. (b) Optical absorption spectrum of the colloidal dispersion.

Figure 2. XPS spectrum of the gold nanoparticles, which exhibits two sharp peaks at 84.3 and 88.0 eV assignable to Au 4f7/2 and 4f5/2, respectively.

water. The gold nanoparticles are almost spherical with an average diameter of 11.0 ( 4.0 nm. The particles were stably dispersed in water without a significant precipitation for several months. Figure 1b shows an optical absorption spectrum of the colloidal dispersion. The spectrum exhibits a sharp peak at 520 nm assignable to a surface plasmon band and a broad band in the UV region assignable to an interband transition of the gold nanoparticles. The absorbance in the near-IR region could be enhanced, if the particles aggregate in the colloidal dispersion.21 However, the absorption spectrum does not show any band in the region, which is essentially the same as the band of spherical nanoparticles prepared by chemical reduction of gold ions.6-8 These findings indicate that the surfactant-free gold nanoparticles remain unaggregated in water. Figure 2 shows an XPS spectrum of the gold nanoparticles, which exhibits two sharp peaks at 84.3 and 88.0 eV assignable to Au 4f7/2 and 4f5/2, respectively. In addition, there exist shoulders on the higher energy side of each peak, although their intensities are much lower than the two main peaks. Each shoulder can be deconvoluted into two peaks assignable to Au+

Figure 4. (a) Absorption spectra of gold nanoparticles in aqueous CTAB solution with different concentrations of CTAB. (b) Absorbance at 380 nm of gold nanoparticles in the aqueous CTAB solution plotted as a function of the CTAB concentration. Here, the absorbance of the particles in the CTAB solution, A, is normalized by the absorbance of the original colloidal dispersion, A0. The concentration of the nanoparticles is 0.4 mM in terms of gold atoms.

and Au3+, respectively. The intensity of Au3+ is much lower than that of Au+, and hence, Au3+ contributes only marginally to surface oxidation. The XPS spectrum suggests that the gold

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Figure 5. TEM images of gold nanoparticles in (a) 4 × 10-6 M and (c) 1 × 10-4 M CTAB aqueous solutions and absorption spectra of the particles in (b) 4 × 10-6 M and (d) 1 × 10-4 M CTAB aqueous solutions. The samples were prepared 15 min after the mixing of the colloidal dispersion of surfactant-free nanoparticles with the CTAB aqueous solution.

nanoparticles consist mainly of neutral gold atoms but contain trace levels of gold oxides.18 The surfactant-free gold nanoparticles are very stable in water because they are negatively charged. The ζ-potential of the particles is -30 mV in water (see Figure 3). We added an aqueous solution of an anionic (SDS) or cationic (CTAB) surfactant to the colloidal dispersion of surfactant-free nanoparticles prepared in advance. Figure 3 shows the ζ-potentials of the particles in the aqueous solutions of SDS or CTAB. The ζ-potential remains almost unchanged in dilute SDS solutions (10-8 to 10-3 M). On the other hand, the ζ-potential increases in the CTAB solution with an increase in the CTAB concentration, reaching almost zero at 10-6 to 10-5 M. The ζ-potential then becomes positive when the CTAB concentration exceeds 10-5 M. It should be noted that the ζ-potentials in the 10-6 to 10-5 M CTAB solution are surely close to zero, but our measurments exhibit large error bars due to experimental difficulties: in these CTAB solutions, the gold nanoparticles aggregate gradually. This is the reason why we measure the ζ-potential of the nanoparticles right after mixing. Figure 4a shows the absorption spectra of the gold nanoparticles in the CTAB solutions with different CTAB concentrations 1 day after mixing. We recorded the spectra of the supernatant after the particles precipitated from solution. The spectral intensity decreases with an increase in the CTAB concentration (10-8 to 10-6 M) and then increases with the CTAB concentration (10-5 to 10-3 M). The concentration dependence is more clearly illustrated in Figure 4b, where the absorbance at 380 nm is plotted as a function of the CTAB concentration. As the absorbance at 380 nm is predominantly contributed by the interband transition of gold nanoparticles, it can be used as the measure of the concentration of the nanoparticles dispersed in the solution. The absorbance sharply decreases at a threshold CTAB concentration, levels off at zero above it, and then begins to increase with an increase in the concentration. The changes

of the absorbance with the CTAB concentration resemble the changes of the absolute value of the ζ-potential exhibited in Figure 3. Figure 5a,c shows TEM images of the gold nanoparticles in 4 × 10-6 and 1 × 10-4 M CTAB aqueous solutions, respectively. The TEM samples were prepared 15 min after the mixing of the colloidal dispersion of the surfactant-free nanoparticles with the CTAB aqueous solution because the nanoparticles were precipitated totally at 4 × 10-6 M 1 day after mixing. It is obvious that several nanoparticles have aggregated in the 4 × 10-6 M CTAB solution. The absorption spectrum of the particles in the solution exhibits a characteristic peak at 650 nm, indicating that nanoparticles have aggregated in solution. In the 1 × 10-4 M CTAB solution, there are nanoparticles that remain unaggregated, which is clearly shown in the TEM image and absorption spectrum. These findings indicate that the nanoparticles aggregate with each other exclusively in the 10-6 to 10-5 M CTAB solution. Following aggregation, the nanoparticles precipitate from solution. Discussion Oxidation of Gold Nanoparticles. Gold nanoparticles were produced by the laser ablation of a metal plate in water. Here, we used only distilled and deionized water and a gold metal plate (>99.99%): no stabilizing reagents were added intentionally in the solution. Nevertheless, the produced surfactant-free nanoparticles are very stable against aggregation. Sacher and co-workers prepared gold nanoparticles by laser ablation in water using a femtosecond pulsed laser.18 They showed by both FTIR and SIMS that the gold atoms on the particle surface are partially oxidized to Au-OH or Au-O-. In addition, they elucidated by ζ-potential measurements that the oxygen atoms are negatively charged as Au-O- at pH > 5.8, while they become neutral as Au-OH at pH < 5.8. In the present study, we also measured the ζ-potential of gold nanoparticles produced by using a nanosecond pulsed laser. The

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ζ-potential of the surfactant-free particles in pure water is -30 mV at pH 7. This value indicates that the gold nanoparticle including the core particle and ions adsorbed on the particle surface is negatively charged. The XPS analysis shows that the core particle consists of neutral gold atoms and a small amount of oxidized gold atoms (see Figure 2). These results are in good agreement with those of Sacher and co-workers. They concluded that the gold atoms on the surface of the surfactant-free gold nanoparticle are partially oxidized to Au-OH or Au-O-. There is equilibrium between Au-OH and Au-O-

Au-OH T Au-O- + H+

(1)

which shifts depending on the pH of the solution.18 Stability of Gold Nanoparticles in the CTAB Solution. When CTAB (which dissociates into C16H33N(CH3)3+ (CTA+) and Br- in water) was mixed with the colloidal dispersion, the ζ-potential was found to increase as shown in Figure 3. The increase of the ζ-potential suggests that CTA+ attaches to the particle, and hence, it neutralizes the particle charge. The equilibrium (eq 1) is expected to shift to the right, as CTA+ attaches to the Au-O- site.22 The gold nanoparticle has been completely neutralized when the number of CTA+ ions equals the number of Au-O- sites of each particle. As a result, the particles begin to aggregate in water, and then the aggregates precipitate from solution (see Figure 5). Once the aggregates were formed in the CTAB solution, we were not able to redisperse them. Upon the addition of an excess of CTAB, more CTA+ ions were able to attach to the nanoparticle surface immediately, causing the ζ-potential of the nanoparticles to rise to a positive value. It follows that the gold nanoparticles are dispersed stably in the aqueous solution because the positive charge on the particle surface gives the particles repulsive forces between them. The repulsive forces prevent the particles from aggregating with each other (see Figure 5c). In reality, the slightly charged nanoparticles can also be precipitated, if the Coulomb repulsive forces between the particles are not large enough: the particles that are not totally neutralized by CTA+ in the CTAB solution can aggregate. Similarly, the positively charged particles, which have a little more CTA+ on the particle surface than the anionic sites, can also aggregate. Hence, there is a certain range of the CTAB concentration as shown in Figures 4 and 6, where the nanoparticles are precipitated. Interaction of Nanoparticles with Surfactant. Surface active CTA+ has both a positively charged trimethyl ammonium group (-N(CH3)3+) and a long carbon chain (C16H33-). When CTA+ is added into the colloidal dispersion of the gold nanoparticles, the trimethyl ammonium group attaches to the particle surface with the trimethyl ammonium group pointing inward. In fact, Kawasaki and co-workers investigated the orientation of CTA+ ions on a gold nanoparticle surface by atomic force microscopy, finding that attractive forces operate between the gold tip and the hydrophobic carbon chain sticking out from the particle into the solution phase.25 In the concentrated CTAB aqueous solution, on the other hand, CTA+ attaches to the particle surface with its trimethyl ammonium group pointing outward, forming the bilayers on the particle surface. As a result, the nanoparticle becomes stable in water (see Scheme 1). In contrast, SDS dissociating into Na+ and C12H25OSO4(DS-) in water stabilizes gold nanoparticles in a different manner. As shown in Figure 3, the ζ-potential of the gold nanoparticles in the 10-8 to 10-3 M SDS solution stays almost

Figure 6. (a) Absorbance at 380 nm of gold nanoparticles in aqueous CTAB solution plotted as a function of CTAB concentration. The absorbance of the particles in the CTAB solution, A, is normalized by the absorbance of the colloidal dispersion, A0. The concentrations of the nanoparticles are 0.1 mM (solid circles, red) and 0.4 mM (solid squares, black) in terms of gold atoms. (b) Absorbance at 380 nm of the solution is represented by a color code as functions of both the CTAB and the nanoparticle concentrations.

unchanged, whereas it drops above the critical micelle concentration (cmc). The SDS concentration dependence suggests that a gold nanoparticle can be stabilized by a number of micelles of the negatively charged DS- ions that are formed above the cmc.22 Then, the question to be answered is “Why can the negatively charged gold nanoparticle be stabililzed by the negatively charged micelles?” Ahmadi and co-workers studied the dissociation of methyl red

(CH3)2N-C6H4NNC6H4COOH T (CH3)2N-C6H4NNC6H4COO- + H+ (2) in SDS aqueous solution at different SDS concentrations.23 They elucidated that negative DS- reduces the dissociation of methyl red: the equilibrium in eq 2 shifts to the left especially above the cmc of SDS. Although the equilibrium given by eq 1 does not necessarily shift with the SDS concentration similarly to the equilibrium of methyl red, we can consider that SDS has the nature to reduce the dissociation when micelles are formed. Hence, it is highly likely that the DS- ions are able to cover the gold nanoparticle after the surface Au-O-- has been neutralized to Au-OH in the SDS solution.

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SCHEME 1

Electrostatic Titration. When CTAB is added gradually to the colloidal dispersion, the gold nanoparticles aggregated because the negative charge of the particles was neutralized by the CTA+ ions. Let us estimate the negative charge of a gold nanoparticle by taking advantage of this electrostatic interaction (electrostatic titration):24 the endpoint of the titration was indicated by the precipitation of the gold nanoparticles. In general, there is an equilibrium between ions adsorbed on the particle surface (M+(ad)) and ions dissolved in the solution (M+(s))

M+(s) + NP T M+(ad)-NP

(3)

where NP stands for the adsorption sites of the particles. When the number of the adsorption sites is the same as, or greater than, the number of the CTA+ ions in solution, most of the ions in solution are expected to adsorb onto the particle surface. In contrast, when the number of adsorption sites is less than the number of the ions, some ions remain in the solution, consistent with the equilibrium given by eq 2. To examine this equilibrium, we reduced the concentration of the gold nanoparticles from 0.4 to 0.02 mM. Figure 6 shows the absorbance of the solution at 380 nm as a color code at different CTAB and nanoparticle concentrations. The map shows that gold nanoparticles precipitate from solution in specific concentration ranges, which shift to higher concentrations as the concentration of the particles increases above 0.1 mM. On the other hand, the concentration range stays almost unchanged below the gold concentration of 0.1 mM, indicating that the excess amount of M+(s) stays in the solution below 0.1 mM. The adsorption isotherm of CTAB from solution onto the chemically prepared gold surface was measured by Kawasaki et al.25 They showed that the adsorbed amount per area was 50 ng/cm2 at a CTAB concentration of 7 × 10-6 M.24 The adsorbed amount decreased almost linearly with a decrease in the logarithm of the CTAB concentration. Hence, we estimate ≈25 ng/cm2 at a CTAB concentration of 5 × 10-7 M by the extrapolation, although no data were shown at 5 × 10-7 M in the paper. As the gold nanoparticles were precipitated in the CTAB concentration range of 5-70 × 10-7 M, the gold nanoparticles were totally neutralized when 25-50 ng/cm2 CTAB adsorbed on the surface. The average size of the gold nanoparticles prepared in this study was 11.0 nm, indicating that each particle had a surface area of 4 × 10-12 cm2. Hence,

it follows that 170-330 CTA+ ions are needed to neutralize each particle. Assuming that the gold nanoparticles are icosahedral, there is an average of 5000 gold atoms on the particle surface. These calculations lead to the conclusion that 3.3-6.6% of the surface atoms provides the anionic sites. The CTAB concentration dependences above a gold concentration of 0.1 mM can also be explained by this adsorption (see Figure 6). As each particle (11 nm in diameter) contains an average of 35 000 gold atoms in an assumed icosahedral shape, there are 1016 particles in the 0.4 mM gold nanoparticle solution. Therefore, at least 1.7-3.3 × 1018 CTA+ ions (or 2.8-5.5 × 10-6 M CTA+) are needed to neutralize the particles in solution. This estimation is consistent with the experimental results, indicating that the nanoparticles are totally precipitated in the 3-8 × 10-6 M CTAB aqueous solution. Haruta and co-workers discovered the catalytic reactivity of gold nanoparticles on the metal oxide surface.3,4 They suggested that the close interaction of gold nanoparticles with the oxygen atom of the metal oxide surface is critically important to induce chemical reactivity. In this relation, the gold nanoparticles prepared by laser ablation in water are expected to possess a chemical reactivity, as they have anionic sites such as Au-Oon the surface. This will be the focus of future investigation in our laboratory. Other Surfactants. When CTAB adsorbs on a neutral gold surface, Br- is known to adsorb on the gold surface prior to CTA+. The Br- draws CTA+ to the surface by electrostatic forces.25 This is because Br- has a pronounced affinity for the gold atoms.26 Although cetyltrimethyl ammonium chloride (CTAC: C16H33N(CH3)3+ Cl-) is able to adsorb on the gold surface, the adsorbed amount per area is a half as much as CTAB.25 To explore this relationship, we investigated if a similar mechanism operates on the surface of the negatively charged gold nanoparticles. We added CTAC instead of CTAB to the colloidal dispersion and measured the absorbance of the solution as a function of the concentration. Figure 7 shows the absorbance of the solution at 380 nm after the surfactant-free gold nanoparticles are mixed with CTAC at the prescribed concentration. It was found that the gold nanoparticles were totally precipitated in the CTAC solution at the same concentration as CTAB: there was no counteranion dependence. This fact clearly indicates that the adsorption of CTA+ is not led by the counteranions in this concentration range but more likely proceeded at the Au-O- sites. On the other hand, as shown in

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Figure 7. Absorbance at 380 nm of gold nanoparticles in aqueous CTAC solution plotted as a function of CTAC concentration (solid circles). The CTAB concentration dependence is also shown as open circles for comparison. The absorbance of the particles in the CTAC solution, A, is normalized by the absorbance of the colloidal dispersion, A0.

Conclusion Gold nanoparticles were prepared by laser ablation of a gold metal plate in water. The surfactant-free nanoparticles are very stable in water, although they are not stabilized by a surfactant. The ζ-potential measurement and the XPS analysis indicate that a part of the surface gold atoms of the particle is oxidized, and the particle with the outer Stern layer is negatively charged. It is highly likely that the surface gold atoms are oxidized to AuOH and Au-O-, according to the work of Sacher and co-workers. A cationic surfactant, CTAB, was found to attach to the negatively charged sites of the particle surface, which destabilizes the nanoparticles in water in the dilute CTAB solution. By taking advantage of this electrostatic interaction, we estimated the number of the anionic sites on the particle surface, concluding that 3.3-6.6% of the surface atoms is negatively charged. In the concentrated CTAB solution, on the other hand, the gold nanoparticles are stabilized by the excess amount of CTAB. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology and by collaborative work with the Genesis Research Institute. References and Notes

Figure 8. Absorbance at 380 nm of gold nanoparticles in aqueous (CH3)4N+Br- (C1TAB, blue), C8H17N(CH3)3+Br- (C8TAB, green), C12H25N(CH3)3+Br- (C12TAB, red), or C16H33N(CH3)3+Br- (C16TAB, black) solutions plotted as a function of the surfactant concentrations. The absorbance of the particles in the surfactant solution, A, is normalized by the absorbance of the colloidal dispersion, A0.

Figure 7, the absorbance at 380 nm of the CTAC solution began to rise at 2 × 10-5 M, which is higher than CTAB (6 × 10-6 M). In this higher concentration range, after the gold nanoparticles are neutralized, there is a counteranion dependence in the stability of the gold nanoparticles. Hence, the leading counterions may play important roles in the adsorption of CTA+, once the gold nanoparticles are neutralized. In addition, we used our approach to study several related surfactants (viz., tetramethyl ammonium bromide ((CH3)4N+ Br-), octyltrimethyl ammonium bromide (C8H17N(CH3)3+ Br-), or dodecyltrimethyl ammonium bromide (C12H25N(CH3)3+ Br-)), which each possess a different carbon chain length. It was found that the nanoparticles were also precipitated by the addition of the surfactants as shown in Figure 8: the cationic surfactants are able to neutralize the negatively charged gold nanoparticles. The concentration ranges where the nanoparticles are totally precipitated shift to a lower concentration with an increase in the carbon chain length. This finding indicates that the equilibrium in eq 3 tends to shift to the right as the carbon

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