Influence of the Counterions of Cetyltrimetylammonium Salts on the

Force Curves. The force curves between the gold tip and gold substrate in the aqueous solutions of C16TABr with different concentrations were also acq...
0 downloads 0 Views 546KB Size
Download PDF
J. Phys. Chem. C 2007, 111, 2683-2690

2683

Influence of the Counterions of Cetyltrimetylammonium Salts on the Surfactant Adsorption onto Gold Surfaces and the Formation of Gold Nanoparticles Hideya Kawasaki,* Kouhei Nishimura, and Ryuichi Arakawa Department of Applied Chemistry, Faculty of Engineering, Kansai UniVersity, 3-3-35 Yamate-cho, Suita, Osaka, Japan ReceiVed: October 24, 2006; In Final Form: December 9, 2006

We report the effects of counterion species on the adsorption of cetyltrimetylammonium salts (C16TAX) onto gold surfaces and the subsequent formation of gold nanoparticles in the C16TAX surfactant solution. The surfactant adsorption onto gold surfaces was examined by using quartz crystal microbalance (QCM) and atomic force microscopy (AFM). The counterion species (X) examined in this study were as follows: X ) Br-, Cl-, NO3-, F-, OH-, and SO42-. The adsorption onto the gold surface depended on the counterion species of C16TAXsthe frequency shift of QCM decreased in the following order: X ) Br- > NO3- ≈ SO42- > Cl- > F- > OH-. The chemical reduction of gold(III) in the surfactant solutions of C16TAX with use of 2,2′-iminodiethanol as a mild reducing agent produced anisotropic gold nanoparticles, depending on the surfactant counterion. The high-affinity adsorption of Br-, NO3-, and Cl- on gold surfaces produced anisotropic gold nanoparticles, while only spherical gold nanoparticles were obtained for weakly bound counterions such as SO42-, F-, and OH-. In contrast, the chemical reduction with hydrazine proceeded rapidly such that the shape of the gold nanoparticles was uncontrollable; this resulted in the formation of spherical gold nanoparticles, irrespective of the surfactant counterion.

Introduction Recently, significant efforts have been made to synthesize anisotropic nanostructured metals such as nanorods,1 nanowires,2 nanotubes,3 and nanosheets4 because of their potential applications in catalysis,5 photochemistry,6 sensors,7 tagging,8 and the nanofabrication of optical,9 electronic,10 and magnetic devices.10 It is now widely believed that the preferential adsorption of amphiphilic molecules from solutions onto different crystal faces influences the growth of nanometals into various shapes by controlling the growth rates along different crystal axes.11-13 The adsorption of amphiphilic molecules affects the surface free energies and thereby alters the relative stabilities of the different planes.14,15 To understand the formation mechanism of nanostructured metals with various shapes, it is important to examine the relationship between the adsorption of amphiphilic molecules onto nanometals and the resultant metal nanoparticles. It has been proposed that the growth of gold nanorods is governed by the preferential adsorption of cetyltrimethylammonium bromide (C16TABr) on different crystal faces during the growth.14,16-19 Based on Fourier transform infrared spectrometer (FT-IR) and thermogravimetric analysis, it was reported that the hydrophobic tails of C16TABr interdigitate to form bilayer surface aggregates on gold nanorods.14 Benton et al. studied the adsorption isotherms of a series of alkyltrimetylammonium bromides (CnTABr) in a gold suspension.20,21 The shapes of the isotherms were of the double-plateau type, and the electrophoretic mobility data revealed a surface charge reversal from the negative to positive. Manne et al. reported the direct images of the cationic surfactant aggregates of tetradecyltrimetylammonium bromide (C14TABr) at gold/aqueous solution interfaces by atomic force microscopy (AFM).22 * Address correspondence ipcku.kansai-u.ac.jp.

to

this

author.

E-mail:

hkawa@

These AFM images revealed the ordered domains of fullcylindrical surface aggregates at the interfaces. A model was proposed in which Br- counterions are preferentially adsorbed onto the gold surfaces thereby creating a negatively charged layer. The surfactant adsorption is driven by the electrostatic forces between the cationic headgroups and the negatively charged surface. The resultant surfactant monolayer further induces the formation of full-cylindrical surface aggregates. On the other hand, in the case of cetyltrimethylammonium hydroxide (C16TAOH), the alkyl chains are preferentially adsorbed onto the gold surface resulting in the formation of halfcylindrical surface aggregates.22 Recently, based on liquid chromatography and flow adsorption microcalorimetry, it has also been reported that the adsorption of dodecyltrimetylammonium bromide (C12TABr) from aqueous solutions onto macroporous gold particles involves the formation of an AuBr-/ H+ electrostatic double layer at the gold/solution interface during the initial adsorption, and this is accompanied by a dramatic increase in the solution pH due to the production of C12TAOH in the aqueous phase.23 In the second adsorption, the C12TABr molecules are reversibly adsorbed on the gold surfaces with a head-to-surface orientation; this eventually results in the formation of full-cylindrical surface aggregates. The above-mentioned results indicate that the formation of bilayer-type aggregates (i.e., full-cylindrical surface aggregates or complete bilayer surface aggregates) on gold can be ascribed to the high-affinity adsorption of Br- ions on the gold surface. In this study, we focused on the relationship between the adsorption of cetyltrimetylammonium salts (C16TAX) onto gold surfaces and the resultant synthesis of gold nanoparticles in C16TAX aqueous solutions. First, the adsorption of C16TAX onto gold surfaces was examined by using a quartz crystal microbalance (QCM) and AFM. It has been reported that the adsorption of inorganic salts on gold surfaces increases in the

10.1021/jp066963n CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007

2684 J. Phys. Chem. C, Vol. 111, No. 6, 2007 following order: ClO4- < SO42- < Cl- < Br-.24 Therefore, the counterion species (X) of C16TAX were changed as follows: X ) Br-, Cl-, NO3-, F-, OH-, and SO42-. Second, we synthesized gold nanoparticles in the surfactant solutions of C16TAX with different counterions to study the relationship between the adsorption affinity of C16TAX onto gold surfaces and the size (or shape) of the resultant gold nanoparticles. Experimental Section Chemicals. Tetrachloroauric acid (HAuCl4), cetyltrimethylammonium hydroxide (C16TAOH), cetyltrimethylammonium bromide (C16TABr), cetyltrimethylammonium chloride (C16TACl), hydrazine, and 2,2′-iminodiethanol were obtained from Wako Chemical Co. All other chemicals were obtained from Nacalai Tesuque Co. All the chemicals were analytical grade and used without further purification. Cetyltrimethylammonium nitrate (C16TANO3), cetyltrimethylammonium sulfate [C16TA(SO4)2], and cetyltrimethylammonium fluorine (C16TAF) were obtained from the equimolar mixtures of C16TAOH and corresponding acids (HNO3, H2SO4, and HF). The purity of surfactants used in this work was confirmed by means of matrixassisted laser desorption/ionization mass spectrometry (MALDIMS) (see the Supporting Information). Disk-shaped crystals of Au (111) were purchased from MaTecK Co., Germany. The disc thickness was 1 mm. Gold(III) Reduction. Method A (Hydrazine Reduction). The chemical reduction of gold(III) in the surfactant solutions of C16TAX (X ) Br-, Cl-, NO3-, F-, OH-, and SO42-) was performed in air with use of a hydrazine. Ten milliliters of 10 mM surfactant solutions were placed in small glass vials with wide openings at 25 ( 1 °C. Then, 100 µL of the 0.1 M HAuCl4 aqueous solution was introduced dropwise into the surfactant solution; this was accompanied by vigorous stirring for about 10 min. Following this, 500 µL of the 0.1 M hydrazine aqueous solution was introduced dropwise into the surfactant solution in steps of 20 µL, and thereafter the solution was maintained at 25 ( 1 °C accompanied by vigorous stirring for 10 min. Prior to further analysis, each vial was removed and stored in a dark place. Method B (2,2′-Iminodiethanol Reduction). The chemical reduction of gold(III) in the surfactant solutions of C16TAX (X ) Br-, Cl-, NO3-, F-, OH-, and SO42-) was performed in air with use of 2,2′-iminodiethanol as a mild reducing agent. Two milliliters of 120 mM surfactant solutions were placed in small glass vials with wide openings at 25 ( 1 °C. Then, 100 µL of the 0.125 M HAuCl4 aqueous solution was introduced dropwise to the surfactant solution; this was accompanied by vigorous stirring for 10 min. Following this, 10 µL of 2,2′iminodiethanol was introduced dropwise into the surfactant solution and the solution was thereafter maintained at 25 ( 1 °C accompanied by weak stirring for 1 day. In the case of C16TABr only, 50 µL of 2,2′-iminodiethanol was introduced dropwise to the surfactant solution and the solution was kept for 3 days to reduce the gold ions. Post reduction, prior to further analysis, each vial was removed and stored in a dark place. In the synthesis of gold nanoparticles in the C16TABr solution, C16TABr surfactants did not precipitate upon mixing of the Au(III) salt and C16TABr under the present conditions (the [AuCl4]/surfactant ratio (R) is 0.1 in method A and 0.05 in method B), although the solution changed color from light yellow to darker yellow, which can be taken as an indication of the complex formation.25 The complex ions of C16TA[AuCl4] or C16TA[AuBr4] will be solubilized in the surfactant micelles.26,27 Further increase in the gold concentration induced

Kawasaki et al. the precipitation of the complex. On the other hand, solutions of other C16TAX surfactants became turbid upon mixing of the Au(III) salt and the C16TAX, and precipitation occurred in several hours. The chemical reduction of gold(III) in these C16TAX solutions was performed before the precipitation occurred. Instruments. Spectroscopic Measurements. The UV-vis absorption spectra of gold-containing surfactant solutions were obtained at 25 °C with use of a Jasco Ubest-670 UV/vis spectrometer with a temperature controller. Electron Microscopy (TEM). The TEM was performed on a JEOL-1210 instrument with an acceleration voltage of 110 kV. Drops of the solutions were placed on Formvar-coated grids and were allowed to remain on the grid for a few minutes to facilitate water evaporation. The solutions were diluted with sufficient amounts of water before being examined by TEM. Force CurVes. The force curves between the gold tip and gold substrate in the aqueous solutions of C16TABr with different concentrations were also acquired at 25 ( 2 °C by using the SPA 400 AFM instrument (Seiko Instruments Co, Japan) with an Si3N4 tip coated with Au. Its nominal spring constant was 0.09 Nm-1. Single disk-shaped crystals of Au were used as the gold substrate. The surfaces of the Au disks were flame-annealed before being examined by AFM. Quartz Crystal Microbalance (QCM). The instrument used was a KSV dissipative QCM-Z500 from KSV (Gothenburg). Standard AT-cut quartz crystals had approximately 100 nm thick gold electrodes. The diameter of the crystal was 14 mm. The active area, which is the area that the electrodes on each side of the crystal have in common, was 0.2 cm2. The instrument recorded the fundamental resonance frequency at 5 MHz and the overtones at 15, 25, and 35 MHz. The Peltier heat element in the instrument resulted in a temperature variation of less than (0.1 °C. Adsorption Measured by QCM. Stock solutions were prepared with use of 50 mM C16TAX. By dilution, 14 samples below the critical micelle concentration (cmc) and 3 samples above cmc were obtained for C16TABr. The measurements were initiated with pure water in a measurement chamber. The solution in the chamber was replaced by the solution of 2 mL until no further adsorption could be observed; usually two step changes were sufficient. After 5-10 min, the temperature in the system stabilized. For the next 10 min, a stable frequency was usually observed; this stable frequency was used for calculating the adsorbed mass. As demonstrated by Sauerbrey,28 the change in the mass of the crystal ∆m is linearly dependent on the change in the resonance frequency ∆f:

∆m ) -

Fqυq∆f Fqtq∆f C∆f ))2 f0 n n 2f n

(1)

0

where Fq and νq are the specific density and shear wave velocity in the quartz, respectively; tq is the thickness of the quartz crystal; and f0 is the fundamental resonance frequency. For the crystals used in these experiments, C ) 17.7 ng cm-2 Hz-1. This relationship is valid when the following conditions are fulfilled: the adsorbed mass is evenly distributed over the crystal and ∆m is considerably smaller than the mass of the crystal itself. Further, the adsorbed material must be rigidly attached. Results and Discussion Adsorption of C16TABr on the Gold Surface. The adsorption of C16TABr from the surfactant solution onto gold surfaces is shown in Figure 1a. We examined the adsorption in terms of

Adsorption of Cetyltrimetylammonium Salts onto Au

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2685

Figure 1. (a) Adsorption of C16TABr from the surfactant solution onto gold surfaces in terms of the frequency shift in QCM for the third overtone (15 MHz). (b) The adsorption isotherm of C16TABr from the surfactant solution onto gold surfaces at 25 °C.

Figure 2. Force curves between the gold tip and gold substrate in the aqueous solutions with different concentrations of C16TABr at 25 °C.

the frequency shift of QCM. The measurements were initiated with pure water in the measurement chamber, and then the solution in the chamber was replaced by surfactant solutions with different concentrations. When the water was replaced by 0.003 mM surfactant solution, a sudden change in frequency was observed. This indicates that the C16TABr molecules are adsorbed from the aqueous solution onto gold surfaces at low surfactant concentrations. The frequency shift increases with the surfactant concentrations up to the cmc. The adsorbed amount per area was calculated with eq 1 and the measured frequency shifts for the third overtone. The adsorption isotherm is shown in Figure 1b. The adsorption isotherm of C16TABr is of the double-plateau type, suggesting the progressive buildup of the gradual formation of bilayer-type aggregates. Three distinguishable regions are observed in the adsorption isotherm. In region I, for low surfactant concentrations of less than about 0.03 mM, the adsorption is considered to occur due to the

headgroup-surface electrostatic forces thereby resulting in monolayer adsorption. On the basis of the amount of C16TABr molecules adsorbed at 0.03 mM (70 ng cm-2), the molecular area of C16TABr on gold in the monolayer region is estimated to be about 85 Å2 per molecule. This value is larger than that of C16TABr for the Gibbs monolayer at the air/water interface that is derived from the surface tension (60 Å2 per molecule).29 In region II, the progressive buildup of bilayers occurs with the increase in the surfactant concentration. Finally, bilayertype aggregates are formed in region III near the cmc. Force Curves. To examine the formation process of the bilayer-type aggregates of C16TABr on gold surfaces, the force curves between the gold tip and gold substrate in the aqueous solutions were acquired at different concentrations of C16TABr, as shown in Figure 2. In water, the force curve is purely attractive below a separation of about 5 nm. At a low surfactant concentration of 0.01 mM (region I), the attractive interaction

2686 J. Phys. Chem. C, Vol. 111, No. 6, 2007

Kawasaki et al.

Figure 3. The adsorption of C16TAX onto gold surfaces in terms of the frequency shift of QCM at 25 °C: X ) Br-, Cl-, NO3-, F-, OH-, and SO42-.

force increases and becomes more long range ( NO3≈ SO42- > Cl- > F- > OH-. The adsorbed amounts per area (Γ) calculated with eq 1 are shown in Table 1. It seems that the ordering is partly related to so-called Hofmeister series: SO42-, OH-, F- < Cl- < Br- < NO3- , ClO4- < I-, SCN-. The frequency shifts for C16TANO3 and C16TA2(SO4) are almost the same as that for C16TABr, which forms bilayertype aggregates on gold. This implies that bilayer-type aggregates are also formed for C16TANO3 and C16TA2(SO4).

TABLE 1: Adsorbed Amounts per Area (Γ) of C16TAX Onto the Gold Surfaces at 10 mM Calculated by Using the Frequency Shift of QCM. C16TAX, X ) Br

Cl

F

OH

NO3

SO4

Γa 203 ( 20 150 ( 20 124 (15 97 ( 15 180 (15 177 ( 15 a Units: ng/cm2.

However, the QCM technique cannot differentiate between the adsorbed surfactant films and the solvent bound in the adsorbate. Therefore, we cannot conclude the formation of the bilayertype aggregates for C16TANO3 and C16TA2(SO4) using only the QCM. The frequency shift of C16TAOH is half of that of C16TABr, which forms bilayer-type aggregates on gold surfaces; this implies that C16TAOH forms a monolayer on gold surfaces. This is consistent with the formation of half-cylindrical aggregates of C16TAOH on gold in the AFM study.22 It has been suggested that the presence of hydroxide induces the formation of a neutral AuOH surface layer and not a charged adsorbate layer comprising OH- ions.22,24 Therefore, it can be inferred that there is little interaction between the OH- ions and the gold surface resulting in the formation of the monolayertype aggregate. We also found that that the adsorption of C16TAX proceeded rapidly covering a span of a few seconds (not shown). These results are consistent with those of the previous reports in which, using the QCM technique, it is postulated that the adsorption of C16TABr onto gold proceeds rapidly.30 Effects of Counterion Species of Cetyltrimetylammonium Salts on the Formation of Gold Nanoparticles. In this study, it is found that the adsorption of C16TAX surfactants from a solution onto gold surfaces depends on its counterion species. Therefore, we examined the effects of the counterion species of C16TAX on the formation of gold nanoparticles. Gold nanoparticles can be synthesized by two methods: rapid reduction with a hydrazine (method A) and slow reduction with 2,2′-iminodiethanol (method B).

Adsorption of Cetyltrimetylammonium Salts onto Au

Figure 4. The adsorption spectra of the gold nanoparticles synthesized by the reduction of aqueous AuCl4- in the C16TAX solutions (method A): X ) Br-, Cl-, NO3-, F-, OH-, and SO42-.

Method A. The adsorption spectra of gold nanoparticles exhibit a broad band from the visible to the ultraviolet region at around 530 nm, which is a characteristic of the surface plasmon resonance (SPR) band of spherical gold nanoparticles (Figure 4). These peak maxima weakly depend on the counterion species, except for C16TAOH. For OH-, the plasmon band undergoes a red shift at around 540 nm indicating the presence of particles with the large sizes. The transmission electron microscopy (TEM) image of gold nanoparticles supports the presence of a large number of spherical gold nanoperticles (Figure 5). From histograms in Figure 6, it is seen that the size of the spherical gold particles lies between 10 and 30 nm in

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2687 the case of X ) Br-, NO3-, and SO42 -. On the other hand, for X ) OH-, the size distribution of gold nanoparticles broadens further and their sizes range between 10 and 60 nm. C16TACl surfactants produce gold nanoparticles with smaller sizes (1040 nm), compared to that prepared from C16TAOH. This may be attributed to the fact that the interaction between OH- and gold is weaker than that between Cl- and gold. It appears that the large size of the gold nanoparticles and their broad size distribution in the presence of C16TAOH are due to the weakly bound OH- counterions. However, this explanation is unacceptable for the case of C16TAF, since C16TAF surfactants produce gold nanoparticles of smaller sizes (10-20 nm), irrespective of weaker interactions between F- and gold. This suggests that an additional factor contributes to the formation of gold nanoparticles in the presence of C16TAF. It was found that C16TAF surfactants reduced Au(III) ions without reduction agents from the observation of color changes of solutions from yellow to colorless upon mixing of the Au(III) salt and C16TAF, suggesting reduction of Au(III) to the Au(I) species. This effect of C16TAF promotes a fast reaction rate of the reduction, which might favor the formation of smaller nanoparticles in the C16TAF solution.31 On the basis of the adsorption spectra and TEM observations, we can hypothesize that the reduction of gold(III) in the surfactant solution of C16TAX by method A (reduction with hydrazine) primarily produces spherical gold nanoparticles, irrespective of the counterion species. The effects of the counterion species of C16TAX on the shape of the gold nanoparticles are weak in method A. The chemical reduction with hydrazine proceeds rapidly (within 5 min), uncontrollable

Figure 5. TEM micrographs of the gold nanoparticles synthesized by the reduction of aqueous AuCl4- in the C16TAX solutions (method A): X ) Br-, Cl-, NO3-, F-, OH-, and SO42-.

2688 J. Phys. Chem. C, Vol. 111, No. 6, 2007

Kawasaki et al.

Figure 6. TEM size histograms of the gold nanoparticles synthesized by the reduction of aqueous AuCl4- in the C16TAX solutions (method A): X ) Br-, Cl-, NO3-, F-, OH-, and SO42-.

by the shape of gold nanoparticles, and hence produces spherical nanoparticles. On the other hand, the size and the size distribution of gold nanoparticles are influenced by the counterion species of C16TAX. Method B. In contrast to method A, the shape of the gold nanoparticles synthesized by method B has been found to change drastically with the different counterion species of C16TAX. The general trend derived from the TEM observations is as follows (Figure 7). In the case of X ) Br-, in addition to spheres, gold short rods, triangles, pentagons, and hexagons are dominantly observed. On replacing Br- by Cl-, branched gold nanoparticles (V-shaped bipods and Y-shaped tripods)32 are formed (indicated by arrows). Various sizes and irregular shapes are obtained with NO3-, while spherical gold nanoparticles are mainly produced with OH-, F-, and SO42-. The absorption spectra were acquired for these gold nanoparticles synthesized by method B (Figure 8). For spherical gold nanoparticles, as in the case of OH- and F-, the plasmon band appears at around 520 nm, as expected. For SO42-, the plasmon band of spherical gold nanoparticles undergoes a red shift at around 535 nm due to the large size of the particles. On the other hand, the branched gold nanoparticles formed in the case of Cl- show a spectrum with two resonance peaks: two main peaks at 528 and 830 nm. Similar absorption spectra have been reported in the case of the branched gold nanoparticles.32,33 In the case of anisotropic gold nanoparticles from Br-, three peaks are observed: main peaks at 546 nm, a secondary peak at 607 nm, and a weak peak at around 740 nm as a shoulder. Gold

nanoparticles obtained from NO3- with irregular anisotropic shapes exhibit two peaks: a main peak at 546 nm and a weak peak at around 680 nm as a shoulder. These absorption spectra are consistent with the corresponding TEM images. The shape dependence of gold nanoparticles on the counterion species of C16TAX in method B may be correlated to the affinity of the counterions on the gold surface during the slow crystal growth, which depends on the counterion species. By using QCM, it is observed that the adsorption of C16TAX onto the gold decreases in the following order: Br- > NO3- ≈ SO42> Cl- > F- > OH-. It appears that the high-affinity adsorption of Br-, NO3-, and Cl- onto gold surfaces produces anisotropic gold nanoparticles, while only spherical gold nanoparticles are obtained for weakly bound counterions such as F- and OH-. Although the weak affinity of SO42- on gold surfaces has been reported,24 the solvent bound in the adsorbate of C16TA2(SO4) may contribute to the high-frequency shift of the surfactant on the gold surface. Thus, it is believed that the formation of spherical gold nanoparticles with SO42- is due to the weak affinity of SO42- on the gold surface. The formation mechanism of branched gold nanoparticles (Vshaped bipods and Y-shaped tripods) has been reported, and it is suggested that the final shape of the nanoplates is dependent on the competition between the growth rates along two planes: the {111} and {100} planes.32 If the {111} growth is preferentially inhibited, triangular plates are formed; a similar condition is satisfied when the {100} growth is preferentially inhibited. When the growths of the {111} and {100} planes

Adsorption of Cetyltrimetylammonium Salts onto Au

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2689

Figure 7. TEM micrographs of the gold nanoparticles synthesized by the reduction of aqueous AuCl4- in the C16TAX solutions (method B): X ) Br-, Cl-, NO3-, F-, OH-, and SO42-.

Figure 8. The adsorption spectra of the gold nanoparticles synthesized by the reduction of aqueous AuCl4- in the C16TAX solutions (method B): (a) X ) Br-, Cl-, and NO3-. (b) X ) F-, OH-, and SO42-.

are inhibited simultaneously, there is a high probability of the formation of branched gold nanoparticles. According to this mechanism, in the case of C16TACl, the growth of the {111} and {100} planes may be simultaneously inhibited by the surfactant adsorption of C16TACl, resulting in the formation of branched gold nanoparticles. In the case of C16TABr, the growth rate of one of the two planes may be inhibited due to the preferential adsorption of C16TABr, resulting in the formation of triangular plates. Thus, as the possible mechanism for the shape control of gold nanoparticles in method B, the specific adsorption of surfactants on gold has been proposed. High concentration of HAuCl4 (i.e., the [AuCl4]/surfactant ratio R is large at the constant surfactant concentration of 120 mM) favored the formation of anisotropic gold nanoparticles in method B. For example, branched gold nanoparticles were obtained at the AuCl4/surfactant ratio R ) 0.05 and spherical gold nanoparticles were obtained at R ) 0.01. Although the different R values were applied between method A and method B, this difference does not cause the formation of spherical gold nanoparticles in method A, because spherical gold nanoparticles were also obtained in method A at R ) 0.05 in the surfactant concentration of 120 mM.

In addition to the effect of surfactant counterion species, it is also important to consider the reaction rate of reduction in the formation of gold nanoparticles. The different sizes of gold nanoparticles between hydrazine and 2,2′-iminodiethanol reduction may be explained by the difference in the reaction rate: a strong reduction reaction such as hydrazine promotes a fast reaction rate and favors the formation of smaller nanoparticles, while a weak reduction agent such as 2,2′-iminodiethanol induces a slow reaction rate and favors relatively larger particles.31 It should be noted that there remains as a matter to be discussed further: the application of the adsorptive behaviors of surfactants on different gold materials (gold nanoparticles and gold plates), since it is quite possible that there is a difference between the surface properties of two gold materials, such as the charge state, the size, and the subsequent adsorption capacity. However, we presume that the adsorptive behaviors of surfactants on the gold plate have a deep connection with that of gold nanoparticle from the present results. To further elucidate the mechanism of the shape (or size) dependence of gold nanoparticles on the C16TAX solutions, the affinity of C16TAX surfactants (or inorganic salts containing the corre-

2690 J. Phys. Chem. C, Vol. 111, No. 6, 2007 sponding counterions) on a given basal plane of gold such as the {111} and {100} will need to be examined. Conclusion In this study, we examined the effects of counterion species on the adsorption of cetyltrimetylammonium salts (C16TAX) onto gold surfaces and the formation of gold nanoparticles by using QCM and AFM. The counterion species (X) examined in this study are as follows: X ) Br-, Cl-, NO3-, F-, OH-, and SO42-. The adsorption isotherm by QCM and force curves by AFM reveal that the progressive buildup of the bilayer-type aggregates of C16TABr occurs on the gold surface. The adsorption onto the gold surface above the cmc depends on the counterion species of C16TAXsthe frequency shift of QCM decreases in the following order: X ) Br- ≈ NO3- ≈ SO42- > Cl- > F- > OH-. Therefore, it is considered that the adsorption affinity of the surfactant counterions on gold surfaces determines the degree of adsorption of C16TAX. The chemical reduction of gold(III) in the surfactant solutions of C16TAX with use of 2,2′-iminodiethanol as a mild reducing agent produced anisotropic gold nanoparticles, depending on the surfactant counterion. The high-affinity adsorption of Br-, NO3-, and Cl- on gold surfaces produced anisotropic gold nanoparticles, while only spherical gold nanoparticles were obtained for weakly bound counterions such as SO42-, F-, and OH-. In contrast, the chemical reduction with hydrazine proceeded rapidly, uncontrollable by the shape of gold nanoparticles, and hence produced spherical gold nanoparticles, irrespective of the surfactant counterion species. Acknowledgment. We are grateful to Prof. S. Yamada, Dr. Y. Niidome, and Mr. K. Nishioka (Kyushu University, Japan) for their helpful discussion and certain TEM measurements. This work was partially supported by the CREST of Japan Science and Technology Corporation (JST) for Scientific Research, and in part by the Grant-in-Aid for Scientific Research (Nos. 18710102 and 18310069) from the Monbukagaku-shou, Japan, and by the Research and Development Organization of IndustryUniversity Cooperation from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: The MALDI-MS spectra of C16TAX surfactants with use of the matrix of gold nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (b) Niidome, Y.; Nishioka, K.; Kawasaki, H.; Yamada, S. Chem. Commun. 2003, 2376.

Kawasaki et al. (2) (a) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (b) Han, Y. J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068. (c) Melosh, N. A.; Doukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Science 2003, 300, 112. (d) Kawasaki, H.; Uota, M.; Yoshimura, T.; Fujikawa, D.; Sakai, G.; Kijima, T. J. Colloid Interface Sci. 2006, 300, 149. (3) (a) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater.. 2003, 15, 641. (b) Kijima, T.; Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.; Mouri, S.; Uoyama, S. Angew. Chem., Int. Ed. 2004, 43, 228. (4) (a) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M.; Lyon, A.; Natan, M. J.; Mallouk, T. E. AdV. Mater. 1999, 11,1021. (b) Shirai, M.; Igeta, K.; Arai, M. Chem. Commun. 2003, 623. (c) Song, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, J.; Swol, F. V.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (d) Kawasaki, H.; Uota, M.; Yoshimura, T.; Fujikawa, D.; Sakai, G.; Annaka, M.; Kijima, M. Langmuir 2005, 21, 11468. (5) Hirai, H.; Wakabayashi, H.; Komiyama, M. Chem. Commun. 1983, 1047. (6) Brugger, P. A.; Cuendet, P.; Gratzel, M. J. Am. Chem. Soc. 1981, 103, 2923. (7) Thomas, J. M. Pure Appl. Chem. 1988, 60, 1517. (8) Mulvaney, S. P.; Musick, M. D.; Keating, C. D.; Natan, M. J. Langmuir 2003, 19, 4784. (9) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202. (10) Skomski, R. J. Phys.: Condens. Matter 2003, 15, R841. (11) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (12) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865. (13) Murphy, C. J. Science 2002, 298, 2139. (14) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (15) Wang, Z. L.; Gao, R. P.; Nikoobarkeht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 5417. (16) Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065. (17) Sau, T. K.; Murphy, C. J. Langmuir 2005, 21, 2923. (18) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao. J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (19) Jorge, P.-J.; Isabel, P.-S.; Luis, M. L.; Paul, M. Coord. Chem. ReV. 2005, 249, 1870. (20) Benton, D. P.; Sparks, B. D. Trans. Faraday Soc. 1966, 62, 3244. (21) Benton, D. P.; Sparks, B. D. Trans. Faraday Soc. 1967, 63, 2270. (22) Jaschke, M.; Butt, H.-J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381. (23) Kira´ly, Z.; Findenegg, G. H.; Mastalir, A. Langmuir 2006, 22, 3207. (24) Paik, W.-k.; Genshaw, M. A.; Bockris, O’M. J. Phys. Chem. 1970, 74, 4266. (25) Kameo, A.; Suzuki, A.; Torigoe, K.; Esumi, K. J. Colloid Interface Sci. 2001, 241, 289. (26) Veisz, B.; Kira´ly, Z. Langmuir 2003, 19, 4817. (27) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (28) (a) Sauerbrey, G. Z. Phys. 1959, 155, 206. (b) Knag, M. K.; Sjo¨blom, J.; Øye, G.; Gulbrandsen, E. Colloids Surf. A 2004, 250, 269. (29) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774. (30) Knag, M.; Sjoblm, J.; Gulbrandsen, E. J. Dispersion Sci. Technol. 2005, 26, 207. (31) Reetz, M. T.; Maase, M. AdV. Mater, 1999, 11, 773. (32) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (33) Wu, H.-Y.; Liu, M.; Huang, M. H. J. Phys. Chem. B 2006, 110, 19291.