Langmuir 1997, 13, 3059-3062
Preparation of Hydrophobic Nanometer Gold Particles and Their Optical Absorption in Chloroform Changyu Fan* and Long Jiang Institute of Photographic Chemistry, Academia Sinica, Beijing 100101, China Received February 20, 1996. In Final Form: March 13, 1997
Introduction The preparation of colloidal gold dispersed in an aqueous solution has been extensively studied.1-5 However there have been only a few reports on the preparation of colloidal gold in organic solvent. The preparation of colloidal gold in organic solvent is quite different from that in aqueous solution. Since electrostatic stabilization is generally not as effective in the relative low polarity of the organic medium and the viscosity of the organic solvent is often lower than that of water, there are more frequent encounters between the colloid particles in the organic phase, leading to a more rapid aggregation than in water. In addition, the low solubility of the metal salts in organic solvent makes reduction of the metal ion to free metal unfeasible. Several methods have been reported for the production of metal organosols, such as the solvent extraction/ reduction technique,6,7 the gas evaporation technique,8 inorganic particle preparation in methanol or ethanol with the presence of polymeric stabilizer,9,10 reduction of an organometallic precursor dissolved in ethylene glycol,11 and phase transfer methods.12,13 Some of the methods mentioned above, such as the “ phase transfer method” and “ reduction in methanol and ethanol”, have been used to produce hydrophobic gold particles in our laboratory. However, particles made by these two methods were unsuitable for Langmuir-Blodgett film formation, since they have hydrophilic surfaces. In this report, a new method is presented to produce stable hydrophobic nanometer gold particles in chloroform. With a high concentration of the surfactant CTAB, the gold particles in chloroform were made uniform in size and remain colloidally stable for long periods of time. Their optical absorption peak shifted to blue with a decrease of particle size. Silver nanometer particles in chloroform have also been made with a similar method. The work will be reported elsewhere. Experiment Spectroscopically pure HAuCl4‚4H2O was directly dispersed in analytical grade chloroform to form a 1.75 × 10-4 M chloroauric acid solution. Different amounts of analytical reagent CTAB * Author to whom correspondence should be sent. (1) Turkevich, J.; Stevenson, P. C.; Hiller, J. Discuss. Faraday Soc. 1951, 11, 55. (2) Wilenzik, R. M.; Russell, D. C.; Morriss, R. H.; Marshall, S. W. J. Chem. Phys. 1967, 47, 533. (3) Frens, G. Nature (London), Phys. Sci. 1973, 241, 20. (4) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1986, 110, 82. (5) Baigent, C. L.; Muller, G. Experientia 1980, 36, 472. (6) Meguro, K.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 341. (7) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tana, T.; Torigue, K.; Meguro, K. Langmuir 1991, 7, 457. (8) Kimura, K.; Bandow, S. Bull. Chem. Soc. 1983, 56, 33. (9) Hirai, H. J. Macromol. Sci. Chem. 1979, A13, 633. (10) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci. Chem. 1979, A13, 727. (11) Torigoe, K.; Nakajima,Y.; Esumi, K. J. Phys. Chem. 1993, 97, 8304. (12) Hirai, H.; Aizawa, H.; Shiozaki, H. Chem. Lett. 1992, 1527. (13) Hirai, H.; Aizawa, H. J. Colloid Interface Sci. 1993, 161, 471.
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were added to the chloroauric acid solution. The concentration of CTAB was varied from 2 × 10-1 M, 2 × 10-2 M, and 2 × 10-3 M to 2 × 10-4 M to control the size of gold particles. A 5 × 10-2 M NaBH4 ethanol solution was made without stabilizer. Four 0.4 mL volumes of the NaBH4 in ethanol solution, recently made, were added separately to the four different 10 mL chloroauric acid chloroform solutions rapidly with vigorous stirring. From these four mixtures, a series of different size gold particles with different colors was obtained. As borohydride cannot be dissolved in chloroform, the ethanol solution of borohydride was chosen in this experiment. Superfluous salt and CTAB in the solution were washed out as clean as possible by ultrasonic washing with water several times. The samples for transmission electron microscopy (TEM) were made by dropping the samples onto copper grids and then drying them. The transmission electron microscopic observations were taken with an EM-400 (Philips) instrument. Absorption spectra of the samples were measured by using an HP 8451A spectrometer with the path length of the quartz cell at 10 mm in our experiment.
Result and Discussion Since chloroform has a much lower polarity than water, chloroauric acid does not dissociate in chloroform as it does in water. The tiny amount of crystalline water in HAuCl4‚4H2O changes the polarity of the microenvironment of chloroauric acid, allowing it to be dissolved in chloroform. The high negative potential of borohydride (NaBH4) makes the reducing process of chloroauric acid fast. The fast reducing process is a benefit to formation of small gold particles. Because the surface of gold particles is hydrophilic, the particle surface must be covered by surfactant to make its surface hydrophobic. Also to obtain a colloidally stable particulate system in organic solvent, CTAB was added to the chloroauric acid solution as a stabilizer for the gold particles. By controlling the concentration of CTAB, a series of different size gold particles with different colors was made. The electron micrographs and the particle size distributions corresponding to the four mixtures are shown in Figure 1. As the CTAB concentration decreases from 0.2 M to 0.0002 M, the average diameter of the gold particles increases from 2.6 nm to 13.6 nm. At high concentration of CTAB [i.e., Figure 1a (0.2 M) and b (0.02 M)], the shape of particles is round and the particle size is small and uniform. At low concentration of CTAB [i.e., Figure 1c (0.002 M) and d (0.0002 M)], the particle size is larger and the distribution of particle size is broader comparatively. The plots in Figure 2 show the relationship between the concentration of CTAB, particle size, and the specific surface area of gold particles. The specific surface area Ssp of gold particles was calculated by the following equation:
Ssp ) Sparticles/Nmol ) 4πR2/{[(4πR3/3)/(4πr3/3)]NA} ) 4πr3NA/R (1) where Sparticles is the average surface area of a particle, R is the average radius of gold particles, Nmol is the average mole number of gold atoms in a particle, r is the radius of a gold atom, and NA is Avogadro’s number. Two facts can be observed from the plots in Figure 2: (1) The particle size becomes smaller when the concentration of CTAB was increased. (2) The specific surface area of gold particles is in proportion to the logarithm of CTAB concentration. This means that much more CTAB is needed as the particle size decreases. This phenomenon might be attributed not only to the increase of the particles’ total surface but also to the rise of the surface energy of © 1997 American Chemical Society
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Figure 1. TEM photographs (left) and particle size distribution (right) of four different gold particles.
Notes
Notes
Figure 2. Relationships among concentration of CTAB, particle size, and specific surface area of gold particles.
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Figure 4. Relationship between absorption peak and gold particle size.
phenomenon is due to the medium (i.e., chloroform) effect, which causes the red shift. According to Mie’s theory for small metal particles,14 an absorption peak occurs at a wavelength such that
1(λ) + 2(χ) ) 0
Figure 3. UV absorption spectra of the four kinds of gold particles. Average diameter of particles: (a) 2.6 nm; (b) 3.1 nm; (c) 5.1 nm; (d) 13.6 nm.
the gold particles as the particle size decreases (which made the small particle need much more surfactant to be stabilized). During the formation process of nanometer gold particles, the competition between the particle growth and nucleation was controlled by the diffusion process. When the concentration of CTAB was low, the speed of diffusion was fast and the reduced gold atoms could migrate to the nucleus surface before they formed a new nucleus, giving fewer but larger particles. The time of nucleation and length of the growth period of different nuclei were not the same, giving particles with different sizes at the end of the particle formation process. When the concentration of CTAB was high, the viscosity of the solution was increased, the speed of diffusion was slow, and the reduced gold atoms could not migrate to a nucleus before being nucleated themselves. Nucleation dominated over particle growth, giving small particles. Since nucleation was intense, it was also brief, giving uniform particle size. The UV absorption spectra of these four kinds of gold particles are shown in Figure 3. With the decrease of particle size, the wavelength of the four absorption peaks decreases from 535 nm to 519 nm. The absorption peaks of smaller particles are sharp and with a small blue shift for the smallest particles. The molar absorption coefficient increases when the particle size decreases. These phenomena can be attributed to both quantum size effects and medium effects. Figure 4 shows the relationship between absorption peak wavelength and particle size. A peak of 536 nm for a particle size of 14 nm is much longer than the plasma resonance of gold bulk (520nm). This
(2)
where 1(λ) is the frequency-dependent dielectric function of the metal and 2(χ) is the dielectric constant of the surrounding medium. The increased absorption may be regard as a plasma effect. The incident light polarizes the conduction electron gas of the metal particles, producing an electric moment which oscillates with the wavelength of the light. When the frequency of the light approaches the natural frequency of the electron gas in the particle, a resonant absorption occurs. However, on the basis of the above idea without considering the quantum size effect, the calculated position of the peak was almost constant for particle sizes below 10 nm. We note that Kawabata and Kub (KK)15 have used a quantum mechanical model to calculate the complex dielectric constant 1(λ) as a function of particle size. They regarded the surface not as a source of scattering but rather as a condition that causes a breakup of the conduction band into discrete levels with decreasing sphere size. Ganiere et al.16 have numerically evaluated 1(λ) using the formula given by KK and then substituting it into eq 2; the results obtained were that the position of the absorption peak wavelength increased as the particle diameter increased below 10 nm, referring to Figure 2 in ref 16, the tendencies of the peak varying with the particle size in agreement with those shown in Figure 4. Recently, similar results were obtained by Huang et al.17 after considering the quantum size effects. It may be seen that the absorption peak with a blue shift for small gold particles (d < 6 nm) is mainly due to the quantum size effect. Ruppin18 has pointed out that the interaction between the small particles and the medium should yield a red shift. This is observed in our experiments. To further supplement the above argument, using the parameters for the electron in gold metal given by ref 19, we estimated the mean free path l ) 12.9 nm and the de Broglie wavelength λ ) h/p ) 0.52 nm. Then the diameters of our four sizes of gold (14) Mie. G. Ann. Phys. 1908, 25, 337. (15) Kawabata, A.; Kubo, R. J. Phys. Soc. Jpn. 1966, 21, 1765. (16) Ganiere, J. D.; Rechsteiner, R.; Smithard, M. A. Solid State Commun. 1975, 16, 113. (17) Huang, W. C.; Lue, J. T. Phys. Rev. 1994, B49, 17279. (18) Ruppin, R. Surf. Sci. 1983, 127, 108. (19) Johnson, P. B.; Christy, R. W. Phys. Rev. 1972, B6, 4370.
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particles in units of the de Broglie wavelength are 5.0, 6.0, 9.8, and 26.2, respectively. As an electron is confined to a “ box” with dimensions that approach the de Broglie wavelength, the electron behaves in a wavelike rather than a particlelike way and its energy becomes discrete. This is the reason why the blue shift for a small gold particle is mainly attributed to the quantum size effect. Conclusion We have presented a new method of preparing gold particles in chloroform by which uniform particles with diameters less than 10 nm were obtained and their colloidal stability established. The UV absorption spectra
Notes
showed that as the particle diameter decreases from 14 nm to 3 nm, the wavelength of the resonant peak becomes shorter, and at particle diameters close to 14 nm, it approaches a constant. The blue shift of the absorption peak of small particles is attributed to the quantum size effect, and the red shift for one size particle is attributed to the interaction between the particle and the medium. Acknowledgment. The financial support of Academia Sinica and the National Science Foundation of China is greatly acknowledged. LA960150G
