J. Phys. Chem. 1995,99, 9869-9875
9869
Gold Particulate Film Formation under Monolayers Kyunghee C. Yi,? Victor Sanchez Mendieta,” Rafael Lopez Castaiiares,$Fiona C. Meldrum,? Changjun WU: and Janos H. Fendler*J Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100, and Departamento de Quimica, Instituto Nacional de Investigaciones Nucleares, Apartado Postal 18-1027, I I801 Mexico, D.F., Mexico Received: July 21, 1994; In Final Form: November 14, 1994@
Gold nanoparticulate films were generated under monolayers, prepared from octadecyl mercaptan (l),N,Ndioctadecyl-N,”-dithioethylammonium bromide (2), and N,N-dioctadecyl-N,N-dimethylammonium bromide (3), by the exposure of aqueous HAuC14 to carbon monoxide and to steady-state irradiation by a 150-W xenon lamp. Absorption spectrophotometric and transmission electron microscopic investigations of the gold particulate films, transferred to solid substrates, indicated the marked influence of the surfactants used to form the monolayer. Those prepared under monolayer 1 had a broad absorption maximum at 580 nm and diameters between 3 and 50 nm. Those formed under monolayer 2 were highly dense and had an absorption maximum at 564 nm. Gold particles formed under monolayer 3 had a very narrow size distribution with a mean diameter of 10 nm and had a broad absorption maximum at 574 nm. Annealing the gold particulate films at high temperature decreased their absorption bandwidth and shifted their maximum to higher energy.
Introduction The unique properties of nanoparticles and nanoparticulate films are being increasingly exploited in a variety of optical, electrical, electrooptical, and catalytic application^.'-^ As part of our overall interest in developing a “wet” colloid-chemical approach to advanced materials synthe~is,~ we have employed monolayers, floating on aqueous solutions, as templates for the in situ generation of semicond~cting~-~ and metallics-I0 nanoparticulate films. There are manifold advantages to using monolayers as templates for nanoparticulate films.3 First, the formation of monolayers from well-characterizedsurfactants (or surfactant mixtures) is well understood. Second, monolayer surface areas and charges are two-dimensionally controllable, and the composition of the aqueous phase is readily varied. This permits the preparation of a large variety of simple and composite nanostructured films by judicious chemical manipulations. Third, monolayers, along with the particulate films grown under them, can be readily transferred onto solid substrates; the 20-25-A-thick surfactant monolayer coating provides a protective barrier against impurities. Judicious selection of the types of precursors and their concentrations and encounter rates, as well as of the surfactants used in forming monolayers, has led to particulate films with desired thicknesses, porosities, and morphologies. Epitaxial growth of lead sulfide” and lead selenide’* nanocrystals has been demonstrated, for example, under monolayers prepared from arachidic acid. The possibility of transferring the nanoparticles and nanoparticulate films to solid substrates, at any stage of their growth under the monolayer, is a particularly attractive feature. It permits characterization and utilization in the solid state. Preparationand morphology-dependent spectroelectrochemical properties have been observed for junctions which were prepared from lead sulfide grown under different monolayers and experimental conditions. Surfactant monolayers have previously been used as matrices under which silver particles were grown by using both elec~
* To whom correspondence should
be addressed.
Syracuse University.
* Instituto Nacional de Investigaciones Nucleares. +
@
Abstract published in Advance ACS Abstracts, March 15, 1995.
trochemical and chemical means8-10 In each of these methods, silver ion reduction occurred at the negatively charged monolayer-aqueous subphase interface. The chemical method involved the exposure of a monolayer-covered, aqueous silver nitrate solution to formaldehyde. The appearance and structure of the chemically generated silver particulate film were found to depend upon the surfactant used in forming the monolayer, the concentration of the silver ions in the subphase, the pH of the subphase, and the exposure time to formaldehyde. In general, silver particulate films with large surface-to-volume ratios could only be formed under monolayers which strongly complexed the silver ion.l0 Generation of gold particulate films under monolayers, prepared from a thiol, a dithiol, and a quaternary ammonium functionalized surfactant by chemical and photochemical reduction, is the subject of the present report. Exposure of a monolayer-covered aqueous solution of chloroauric acid to carbon monoxide or to light (provided by a steady-state xenon lamp or by repetitive laser pulses when the subphase contained 2-propanol and acetone) resulted in gold particulate film formation. Subsequent to their transfer to appropriate substrates, the gold particulate films were characterized by absorption spectra and transmission electron microscopy. Annealing at high temperatures shifted the maxima to higher energies and sharpened the absorption bands of the gold particulate films.
Experimental Section Preparation, purification, and characterization of octadecyl mercaptan [ C H ~ ( C H ~ ) I ~ S11, H ;N,N‘-dioctadecyl-N,”-dithioethylammonium bromide [(CIEH~~)~N+(CH~CH*SH)*B~-; 21, and N,N‘-dioctadecyl-N,N‘-dimethylammonium bromide, [ ( C I ~ H ~ ~ ) * N + ( C H 31 ~ )have ~ B ~been - ; described.I0 Chloroauric acid (HAuCl4-3H20, Sigma), acetone (Fisher), 2-propanol (Fisher), HPLC grade chloroform (Aldrich), sodium polyphosphate (Aldrich), carbon monoxide (Matheson), and argon (Union Carbide) were used as received. Water was purified by using a Millipore Milli-Q filter system provided with a 0.22-pm filter at the outlet. Either a commercial Lauda film balance (surface pressure controlled by a Teflon barrier) or a circular trough (a known
0022-3654/95/2099-9869$O9.00/0 0 1995 American Chemical Society
9870 J. Phys. Chem., Vol. 99, No. 24, I995
amount of the surfactant, dissolved in the spreading solvent, was dispersed on the water surface to give the desired equilibrium surface pressure), enclosed in a Plexiglas hood, was used for monolayer formation. Spreading solutions were prepared by dissolving 1 or 3 in HPLC grade chloroform and 2 in mixtures of ch1oroform:ethanol (9:1 v/v) at concentrations of 1-8 mg/mL. These spreading solutions were established to be the most appropriate by extensive trials of a variety of different solvents and surfactant concentrations. Inert atmosphere was maintained over the trough during the spreading of the surfactant solution, the evaporation of the solvent, and the formation and compression of the monolayer by continuous purging of argon through the Plexiglas hood. The surface of the aqueous solution (the subphase) was cleaned several times prior to monolayer formation by sweeping it with a Teflon barrier. The subphase was deemed clean when the surface pressure increase was less than 0.2 dydcm upon compression to 1/20th of the original area and when the surface pressure remained the same subsequent to aging for several hours. An appropriate amount of the spreading solution, (4-8) x 10'' molecules/mL, was carefully injected onto the cleaned aqueous subphase. The surfactant was compressed at a rate of (2-5) x A2/molecule. Surface pressure (ll)vs surface area (A) measurements commenced 5 -30 min subsequent to monolayer formation. Gold particulate films were prepared under monolayers, present in their solid state, by chemical and photochemical methods. The chemical method involved the exposure of monolayer-covered, aqueous 5.0 x M HAuQ solutions to carbon monoxide under an airtight glass cover! The photochemical method involved the exposure of monolayercovered, aqueous 5.0 x M solutions to a 150-W xenon lamp. At different stages of their growth, the gold particulate films were transferred to well-cleaned (No-chromix, copious amounts of dust-free water) substrates (quartz slides for optical observation and carbon-coated, formvar-covered, 400-mesh copper grids for electron microscopy). No disruption or change in the appearance of the gold particulate films was observed upon transferring them to substrates (by the naked eye and optical microscopy). Gold .particles in solution (in the absence of monolayer coverage) were prepared by the exposure of aqueous, degassed (by argon bubbling and, in some cases, by repetitive freezepump-thaw in a high-vacuum system) 5.0 x M HAuC4 solutions (containing 0.1 M 2-propanol, M acetone, and 5 x M sodium polyphosphate) to repetitive (10 Hz) 308nm, -25-mJ laser pulses. Heating of the gold particulate films was carried out in air in a furnace. Absorption spectra were taken by using either a HewlettPackard 8450A or a Hewlett-Packard 8452A diode array spectrometer. In all experiments, the absorption of the clean quartz substrate was subtracted from the recorded spectra. Dynamic light scattering measurements were performed by using a Model PR 102 spectrometer system (Malvern Instruments). The signal of the PM tube was analyzed by a Model BI-2030AT digital correlator with 72 data channels and an attached IBM-AT computer (Brookhaven Co). A Spectra Physics 2020 argon ion laser (514.4 nm) was used as the light source. The apparent diffusion coefficients were determined by the cumulant technique at a 90" scattering angle. The corresponding apparent hydrodynamic diameters were calculated by the Stokes-Einstein equation. Samples for transmission electron microscopy were prepared by passing 400-mesh, carbon-coated, formvar-covered copper TEM grids vertically through the monolayer and then lifting
Yi et al.
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Wavelength, nm Figure 1. (Top) Hydrodynamic diameters of gold particles generated M HAuC4 solution by the irradiation of an aqueous 5.0 x M sodium (containing 0.1 M 2-propan01, IO-*M acetone, and 5 x polyphosphate)by repetitive (10 Hz)308-nm, -25-mJ laser pulses for 100 min. The solution was degassed on a high-vacuum line by repeated freeze-pump-thaw cycles. (bottom) Absorption spectra of gold particles generated, as described above, by exposure to repetitive laser pulses. The insert shows a plot of absorbancesat 560 nm as a function of irradiation time.
the grids horizontally up from the solution. The grids were examined using a JEOL 2000FX transmission electron microscope operating at 200 kV.
Results and Discussion Gold Particles in Aqueous Solution. The optimal conditions for the photochemical generation of gold particles under monolayers were established by investigating gold particle formation in aqueous solutions. In a typical experiment, an aqueous solution of 5.0 x M HAuC14, containing 0.1 M 2-propanol, 1.0 x M acetone, and 5.0 x M sodium polyphosphate, was degassed on a high-vacuum line by repeated freeze-pump-thaw cycles in a vessel which had a 1.0-cm rectangular spectrophotometric cell attached to it. Exposure of this degassed solution to repetitive (10 Hz) 308-nm, -25-mJ laser pulses resulted in the appearance of an absorption band whose intensity grew and shifted to higher energies with increasing irradiation time (Figure 1). Eighty minutes of irradiation, for example, produced gold particles with an absorption maximum of 547 nm and a bandwidth of 80 nm at half-maximum height (Table 1). This result indicated the photoreduction of gold ions to zerovalent gold nanoparticles with a pronounced surface plasmon absorption band.I4.I5 Acetone served as the sensitizer to absorb the ultraviolet light and to initiate radical formation, via the acetone triplet and subsequent hydrogen atom abstraction from 2-propanol molecules. l 6 The 2-hydroxypropan-2-yl radicals formed and, in turn, transferred electrons to the incipient gold clusters, thereby
Gold Particulate Film Formation under Monolayers
TABLE 1: Absorption Spectra of Gold in Different Systems A,,,, bandwidth at conditions nm half-height, nm aqueous 5 x M HAuC14 containing 0.1 M 2-propanol, 0.01 M acetone, and 5 x M sodium polyphosphate (Ar bubbled) irradiated by 308-nm, -25-mJ, 10-Hz laser pulses for 10 min 556 110 20 min 551 90 40 min 550 82 80 min 547 80 aqueous 5 x M HAuCL containing 0.1 M 2-propanol, 0.01 M acetone, and 5 x M sodium polyphosphate (degassed by repeated freeze-pump-thaw cycles) irradiated by 308-nm, -25-mJ, 10-Hz laser pulses for 3 min 547 88 5 min 547 83 10 min 549 83 15 min 550 83 aqueous 5 x M HAuC14 containing 0.1 M 2-propanol and 0.01 M acetone, coated by a monolayer prepared from 3, and irradiated by a 150-W xenon lamp for lh 590 151 159 2h 58 1 545 125 2 h + annealing at 500 "C for 5 min aqueous-monolayer-coated 5 x M HAuC14 exposed to CO for 1 h monolayers prepared from 1 580 160 5 64 157 monolayers prepared from 2 monolayers prepared from 3 574 180 aqueous 5 x M HAuC14, coated by a monolayer prepared from 1, exposed to CO for 1 h, and annealed at 500 "C for 0 min 580 160 1 min 547 103 3 min 537 86 5 min 525 73 15 min 524 71 aqueous 5 x M HAuC14, coated by a monolayer prepared from 2, exposed to CO for 1 h, and annealed at 140 "C for 0 min 564 157 10 min 563 112 aqueous 5 x M HAuC14, coated by a monolayer prepared from 2, exposed to CO for 1 h, and annealed at 500 "C for 0 min 564 157 5 min 547 72 10 min 539 62 50 min 536 61 aqueous 5 x M HAuC14, coated by a monolayer prepared from 3, exposed to CO for 1 h, and annealed at 500 "C for 0 min 574 180 1 min 540 69 5 min 540 65 10 min 536 61 50 min 538 62 contributing to their growth. Sodium polyphosphate stabilized the gold nanoparticles against aggregation. The mean hydrodynamic diameters of photochemically generated gold particles were determined to be 80 nm (with a polydispersity index of 0.1) by dynamic light scattering (Figure 1). The size distribution of the aqueous gold sols remained unaltered for at least 6 months. Deoxygenating the aqueous 5.0 x M HAuC14 solution, containing 0.1 M 2-propanol, 1.0 x M acetone, and 5.0 x M sodium polyphosphate, by argon bubbling for 60 min led to gold particles whose sizes and absorption spectra were quite similar to those which were prepared on the high vacuum line by repeated freezepump-thaw cycles. These results lent credence to our method of deoxygenation of the monolayer-covered HAC14 solutions by argon purging. Irradiation of an aqueous 5.0 x M HAuC14 solution, in the absence of sodium polyphosphate (but in the presence of 0.1 M 2-propanol and 1.O x M acetone),
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A, A2/molecule Figure 2. Surface pressure (ll)vs surface area (A) isotherms of monolayers, prepared from 1 (c), 2 (b), and 3 (a), on aqueous 5.0 x M HAuC14 solutions.
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Wavelength, nm Figure 3. Absorption spectra of gold particulate films generated by M HAuC14 solution, covered by a exposing an aqueous 5.0 x monolayer prepared from 1 (c), 2 (a), and 3 (b), to CO for an hour. Absorption spectra were taken subsequent to transferring the gold particulate films to quartz slides. led to gold particles whose initial absorption spectra were very broad and which subsequently precipitated. Monolayers on Aqueous HAuC14 Solution. Monolayers formed from a thiol (l), a dithiol (2), and a quaternary ammonium bromide (3) surfactant, over an aqueous 5.0 x M HAuC14 solution, were characterized by surface pressure (n) vs surface area (A) measurements. The isotherms showed the characteristic gaseous, liquid, and solid phases (Figure 2) and yielded headgroup areas of 15, 40, and 64 A2/molecule for 1, 2, and 3, respectively. These values agree well with those determined previously for the corresponding monolayers floating on aqueous electrolyte solutions. Chemical Generation of Gold Particulate Films under Monolayers. Exposing aqueous 5.0 x M HAuC14 solutions, covered by monolayers, to carbon monoxide for 30 min or longer resulted in the development of visible coloration. Transferring the particulate films to quartz slides permitted absorption spectrophotometric measurements. Gold particulate films generated under monolayers prepared from 1, 2, and 3 had broad absorbances with maxima at 580, 564, and 574 nm, respectively (Figure 3), indicating that the reduction of HAuC14 to metallic gold had occurred. The absorption maxima and bandwidths corresponded to those anticipated for colloidal gold
9872 J. Phys. Chem., Vol. 99, No. 24, 1995
particle^.'^.'^ A control experiment was performed, for comparative purposes, in which identical conditions were utilized, but no surfactant monolayer was spread. Transmission electron micrography provided more structural information on the gold formed (Figure 4) and demonstrated that the surfactant in the monolayer was important in influencing the size and morphology of the growing crystals. The gold particles developed under a monolayer of 3 are shown at low and high magnifications in Figure 4, a and b, respectively. They possess a narrow size distribution and diameters of approximately 20 nm. A range of particle morphologies are apparent, including triangular single crystals (labeled 1)" and multiply-twinned particles (MTPs) of icosahedral (labeled 2) and decahedral (labeled 3) form (A-D). MTPs have been generated using a range of protocols, including vacuum dep~sition"-'~and chemical reduction of HAuCL by citrate in aqueous solution.20 The gold nanocrystallites, grown under monolayers of surfactant 2, are shown in Figure 4c,d and are markedly in contrast to those formed under monolayers prepared from 3. The particle size range is again narrow, but the nucleation density is higher and the crystal size is smaller, with diameters approximating to 10 nm. The particles are also of constant morphology, being circular in cross section. The influence of monolayers of surfactant 1 on the development of the gold nanocrystallites is shown in the micrographs of Figure 4e,f. There is a wide range in crystal diameters from 3 to 50 nm, and the larger crystals have irregular morphologies. These particle shapes have been described for gold crystals produced by carbon monoxide reduction of HAuC14.*I In the case of monolayers prepared from 3, uniform particles are produced of morphologies not typically generated by carbon monoxide reduction of H A u C ~ ~ Monolayers .~' of surfactant 2 appear to form a very effective nucleation site for the gold crystallites, as is demonstrated by the high density of crystals present. That these monolayers strongly affect crystal growth is not surprising, considering that the surfactants are positively charged, which will result in a concentration of AuC14- ions at the solution/ monolayer interface. Nucleation and growth will also be further defined by other interfacial features, such as the geometric and stereochemical properties of the substrate and nascent crystals.22 Indeed, it has been demonstrated that both cationic (hexadecylpyridinium chloride) and nonionic (polyoxyethylene nonylphenyl ethers) surfactants in solutionz3 and phosphatidylcholine-based vesicles24can be utilized as substrates with which to control the growth of Au crystals. Although thiol groups are known to bind tenaciously to gold,25this property alone does not guarantee closely controlled crystal growth, as is demonstrated by the large range in the sizes of crystals produced under monolayers of surfactant 1. An electron micrograph of gold crystals produced under the control experiment conditions is shown in Figure 5 . It can be seen that there is a large distribution in particle sizes and that many crystals have coalesced during growth to form complex networks of particles. The influence of the monolayers in mediating the growth of uniform Au crystals and preventing interparticle coalescence is, thus, clearly demonstrated. Photochemical Generation of Gold Particulate Films under Monolayers. Photolysis of a monolayer-coated aqueous solution of HAuC14 also produced colloidal gold particles. M HAuC14 Steady-state irradiation of an aqueous 5 x solution (containing 0.01 M 2-propanol and 0.01 M acetone) under a monolayer prepared from 3 by a 150-W xenon lamp for 1 h led to the development of a broad absorption band with a maximum at 590 nm (Figure 6, Table 1). Longer irradiation
Yi et al. further broadened the absorption spectra, indicating the photoinduced coalescence of the particles f ~ r m e d . ~ ~ . ~ ' Simulation of the Optical Spectra. Optical spectra of gold particles (obtained in the photolysis of an aqueous 5.0 x M HAuC14 solution, containing 0.1 M 2-propanol, 1.0 x lo-* M acetone, and 5.0 x M sodium polyphosphate; Figure 1) and gold particulate films (generated by exposing an aqueous 5.0 x M HAuC14 solution, covered by a monolayer prepared from 1, 2, or 3, to CO for an hour; Figure 3) were calculated by the Mie theory, taking into consideration the sizedependent dielectric functions, E ( w , R ) : ~ * 2
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+
A reasonable agreement was obtained between the experimental and the calculated absorption spectra of gold nanoparticles, photochemically generated in aqueous solutions (Figure 7). No satisfactory fitting could be obtained, however, between the experimental spectra of gold particulate films generated under monolayers and those calculated by the Mie theory, employing eq 2 for the dielectric function and assuming simple spherical gold clusters. A somewhat better fit was obtained by assuming an unrealistically thick (200 A) surfactant coating on the gold particulate films (Figure 8). The presence of a range of particle morphologies in nonuniform roughness (Figure 4) and a complex microenvironment are likely to be responsible for the difficulty in fitting the absorption to relatively simple models. Effects of Annealing. Heating the gold particulate films on quartz substrates dramatically affected their absorption spectra. For example, heating a gold particulate film (obtained upon the exposure of an aqueous 5 x M HAuC14 solution, covered by a monolayer prepared from 1, to CO for 1 h) at 500 "C for 1 min decreased the absorption bandwidth from 150 nm to 103 nm and shifted its maximum from 580 nm to 547 nm (Figure 9). Annealing for longer periods of time further sharpened the gold plasmon band and blue-shifted the spectrum, finally yielding a time constant spectrum. The effects of annealing on the absorption spectra of the differently prepared gold nanocrystallites are summarized in Table 1. Annealing at lower temperatures resulted in much smaller spectral changes. For example, heating the gold nanoparticles (formed under monolayers prepared from 2) at 140 "C for 10 min reduced the bandwidth from 157 nm to 112 nm and shifted its maximum from 564 nm to 563 nm, whereas heating the same sample at 500 "C for 10 min resulted in a bandwidth of 62 nm and maximum of 539 nm (Table 1). A similar, albeit less pronounced, annealing effect was observed for the photochemically prepared gold particles (Table 1). Interpretation of the spectral changes which accompany the annealing of gold nanocrystallites is less than straightforward. Heating metallic particles on substrates is known to increase the grain sizes and the surface roughness in many instances.
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J. Phys. Chem., Vol. 99, No. 24, 1995 9873
Gold Particulate Film Formation under Monolayers I_
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Figure 4. Transmission electron micrographs showing gold nanocrystallites grown under surfactant monolayers prepared from 3 (a and b), 2 (c M HAuCl4. CO was infusdthrough and d), and 1(e and f) at low and high magnification. In all cases, the aqueous subphase contained 5.0 x the compressed monolayer for 1 h, as described in the Experimental Section. The scale bars correspond to 100 nm (a, c, e) and 50 nm (b, d, f). In (b) the arrows indicate single crystals of triangular (1). isohedral (2), and decahedral (3) morphology.
9874 J. Phys. Chem., Vol. 99, No. 24, 1995 , *rs
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WAVELENGTH, nm Figure 7. Experimentally determined (dark points) and calculated (thin lines) absorption spectra of gold particles, generated by the irradiation M HAuC14 solution (containing 0.1 M of aqueous 5.0 x 2-propanol, M acetone, and 5 x M sodium polyphosphate) by repetitive (10 Hz) 308-nm, -25-mJ laser pulses for 5 (l), 10 (2), 15 (3), 20 (4), 25 (5), 30 (6), and 40 min (7). The spectra were calculated by the Mie theory, taking into consideration the sizedependent dielectric functions (eq 2) and using 248 8, for the diameters of the Au particles, 1.67- 1.78 for the refractive indices of the solutions, and A = 5.979.22.
Figure 5. Transmission electron micrograph showing gold particles produced in the control experiment. The experimental conditions are identical to those described in Figure 4, with the exception that no surfactant monolayer was present. Scale bar = 100 nm. 0.05
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Wavelength, nm Figure 6. Absorption spectra of gold particulate films generated by exposing an aqueous 5.0 x low4M HAuCL solution (containing 0.01 M 2-propanol and 0.01 M acetone) to irradiation by a 150-W xenon lamp for 1 h (a), 2 h (b), and 2 h plus a subsequent 5 min of annealing at 500 "C (c). Indeed, annealing of silver particulate films was shown to result in the appearance of interband transitions.IO Changes in interparticle distances and interactions, as well as alterations of the refractive index of the environment, could also manifest themselves in shifts of the position and changes of the intensity and breadth of the metallic surface plasmon bands.3o Transfer-
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WAVELENGTH, nm Figure 8. Experimentally determined (dark points) and calculated (thin lines) absorption spectra of gold particulate films generated by exposing M HAuC4 solution, covered by monolayers an aqueous 5.0 x prepared from 1 (c), 2 (a), and 3 (b), to CO for 1 h. The spectra were calculated by the Mie theory, taking into consideration the sizedependent dielectric functions (eq 2) and using 100-194 8, for the diameters of the Au particles, 200 8, for the thickness of the .surfactant coating, 3.0 + i0.2 for the refractive index of the surfactant coating, 1.67- 1.78 for the refractive indices of the solutions, and A = 1.876.15. ring aqueous 16.0 f 1.2 nm gold sols to organic solvents has recently been shown to result in a dramatic shift of the surface plasmon band ab~orption.~~ Furthermore, the optical properties of the gold nanoparticles correlated well with the refractive index of the solvent and could be modeled with calculations based on Mie's theory.31
Conclusion The formation of gold nanoparticulate films under monolayers upon the chemical or photochemical reduction of aqueous
J. Phys. Chem., Vol. 99, No. 24, 1995 9875
Gold Particulate Film Formation under Monolayers
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Wavelength, nm Figure 9. Absorption spectra of gold particulate films generated by M HAuC14 solution, covered by a exposing an aqueous 5.0 x monolayer prepared from 1, to C O for 1 h. Absorption spectra were taken subsequent to transfemng the gold particulate films to quartz slides and annealing at 500 "C for 0 (a), 1 (b), 5 (c), and 15 min (d).
HAuC14 solutions has been demonstrated by absorption spectrophotometry and electron microscopy. The nature of the surfactant in the monolayer influenced the size, monodispersity, and morphology of the gold particles formed. In general, the nonuniform roughness and the complex microenvironment of the gold particles precluded good fittings of the absorption spectra, calculated by the Mie theory to those determined experimentally.
Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged. References and Notes (1) Ozin, G. A. Adv. Muter. 1992, 4 , 612. (2) Hodes, G. Isr. J. Chem. 1993, 33, 95.
(3) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Advances in Polymer Science Series Vol. 113; Springer-Verlag: Berlin, 1994. (4) Zhao, X. K.; Xu, S.; Fendler, J. H. Langmuir 1991, 7, 520. (5) Zhao, X. K.; Fendler, J. H. Chem. Mater. 1991, 3, 168. (6) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1991, 95, 3716. (7) Fendler, J. H. In Organic Thin Films and Surjiuces; Ulman, A,, Ed.; Academic Press: Boston, in press. (8) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1990, 94, 3384. (9) Kotov, N. A,; Zaniquelli, M. E. D.; Meldrum, F. C.; Fendler, J. H. Langmuir 1993, 9, 3710. (10) Yi, K. C.; H6rvolgyi, Z.; Fendler, J. H. J. Phys. Chem. 1994, 98, 3872. (11) Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (12) Yang, J.; Fendler, J. H.; Jao, T.-C.; Laurion, T. Microsc. Res. Technol. 1994, 27, 402. (13) Tian, Y.; Wu, C.; Kotov, N.; Fendler, J. H. Adv. Muter. 1994, 6 , 959. (14) Optical Properties of Metals; Weaver, J. H., Krafka, C., Lynch, D. W., Koch, E. E., Eds.; Physics Data Series No. 18-2, Vol. 2; Fachinformationszentrum: Karlsruhe, 1981. (15) Johnston, P. B.; Christy, R. W. Phys. Rev. E 1972, 8, 4370. (16) Henglein, A.; Mulvaney, P.; Linnert, T. Furaday Discuss. 1991, 92, 31. (17) Smith, D.; Marks, L. D. J. Cryst. Growth 1981, 54, 433. (18) Marks, L. D.; Smith, D. J. Cryst. Growth 1981, 54, 425. (19) Buffat, P. A,; Flueli, M.; Spycher, R.; Stadelmann, A,; Boret, J. P. Faraday Discuss. Chem. SOC. 1991, 92, 173. (20) Duff, D. G.; Curtis, A. C.; Edwards, P. P.; Jefferson, D. A,; Johnson, B. F. G.; Kirkland, A. I.; Logan, D. E. Angew. Chem. Int. Ed. Engl. 1987, 26, 676. (21) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Chem. SOC.1951, 11, 55. (22) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6 , 9. (23) Esumi, K.; Sato, N.; Torigoe, K.; Meguro, K. J. Colloid Interface Sci. 1992, 149, 295. (24) Meldrum, F. C.; Heywood, B. R.; Mann, S. J. Coll. Int. Sci. 1993, 161, 66. (25) Strong, L.; Whitesides, G. M. Langmuir 1988, 4 , 546. (26) Eckstein, H.; Kreibig, U. Z. Phys. D 1993, 26, 239. (27) Satoh, N.; Hasegawa, H.; Tsujii, K.; Kimura, K. J. Phys. Chem. 1994, 98, 2143. (28) Hovel, H.; Fritz, S.; Hilger, A,; Kreibig, U. Phys. Rev. E 1993,48, 18178. (29) Doremus, R. H. J. Appl. Phys. 1965, 36, 2853. (30) Dusemund, B.; Hoffmann, A,; Salzmann T., Kreibig, U.; Schmid, G. Z. Phys. D 1991, 20, 305. (3 1) Underwood, S . ; Mulvaney, P., Private communication, 1994.
JP94 1867K
