One-Step Synthesis of Ordered Two-Dimensional Assemblies of Silver

Nov 19, 2004 - The single-step synthesis of silver nanoparticles by the reduction of silver ions present in the subphase under alkaline conditions by ...
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J. Phys. Chem. B 2004, 108, 19269-19275

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One-Step Synthesis of Ordered Two-Dimensional Assemblies of Silver Nanoparticles by the Spontaneous Reduction of Silver Ions by Pentadecylphenol Langmuir Monolayers Anita Swami, PR. Selvakannan, Renu Pasricha, and Murali Sastry* Nanoscience Group, Materials Chemistry DiVision, National Chemical Laboratory, Pune 411 008, India ReceiVed: August 2, 2004; In Final Form: September 10, 2004

The single-step synthesis of silver nanoparticles by the reduction of silver ions present in the subphase under alkaline conditions by 3-pentadecylphenol (3-PDP) Langmuir monolayer and their assembly into ordered two-dimensional structures is described. The reduction of the silver ions occurs by electron transfer from ionized phenol groups of 3-PDP, which then stabilize the particles against aggregation. Similar reduction of aqueous silver ions was carried out at the interface with toluene bearing 3-PDP and yielded monodisperse silver nanoparticles that may be separated as a dry powder and redispersed in a range of organic solvents. The mechanism of reduction of silver ions by 3-PDP under alkaline conditions was studied by UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, transmission electron microscopy (TEM), and chemical analysis by X-ray photoelectron spectroscopy (XPS), and is discussed in detail.

Introduction The exciting electronic and optical properties displayed by metal nanoparticles1 has resulted in their exploitation in a number of applications that include catalysis,2 single electron tunneling devices,3 nonlinear optical devices,4 electron microscopy markers,5 DNA sequencing,6 and the emerging area of plasmonics.7 Silver nanoparticles are being studied in detail and are known to be excellent substrates for surface enhanced Raman scattering (SERS) to probe single molecules,8 and as catalysts.9 A number of reports are available in the literature for the synthesis of silver nanoparticles by the chemical reduction of silver ions by sodium citrate or sodium borohydride,10 reduction in reverse micelles,11 biological methods,12 electrochemical methods,13 photochemical methods,14 radiation methods,15 laser ablation,16 solvent reduction in the presence of surfactants,17 sonochemical methods,18 and reduction at two-phase liquidliquid interfaces.19 It is now well understood that the shape of the nanoparticles plays an important role in modulating their electronic and optical properties and, consequently, synthesis procedures that yield different shapes such as nanodisks,20 nanorods,21 nanowires,22 cubes,23 triangles,24 frames,25 and shells26 are receiving considerable attention. The size, shape, and dielectric constant of the environment surrounding the nanoparticles are the parameters that decide the plasmon resonance frequency of the nanoparticles.27 When nanoparticles are assembled or packed closely in one, two, and three dimensions, there is a coupling between the surface plasmons of the neighboring nanoparticles and this coupling interaction extends up to the length the nanoparticle assembly. In addition to the interaction between a single particle plasmon with light, it is also recognized that the collective or cooperative interaction of all the particles’ plasmons with light results in new optical properties. For example, it was observed that 2D arrays of silver nanoparticles interact coherently and this interaction is sensitive to polarization of light.28 Recently it has * To whom correspondence [email protected].

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been shown that a compact 2D array of dodecanethiol-capped silver nanoparticles made by first drop-coating on carbon substrates can be ordered further in the presence of high magnetic fields. Such arrays of nanoparticles acquire different electronic charges under different illumination and can be used in electron-transfer devices.29 However, assembling 2D arrays of nanoparticles by the drop-coating method suffers from a number of drawbacks such as formation of multilayers or gaps in the monolayers due to uncontrolled solvent evaporation. Another popular approach that often leads to much more compact mono-/multilayers of nanoparticles is based on the Langmuir-Blodgett (LB) technique wherein hydrophobic, organically dispersible nanoparticles are first synthesized,30 purified, rendered more monodisperse by size-selective separation, and then finally organized at the air-water interface.31 In this laboratory, we have begun investigations into the use of Langmuir monolayers of suitable surfactants that would not only assist in assembling nanoparticles at the air-water interface, but also participate in metal ion reduction in a highly localized manner. We believe the full potential of the air-water interface can be achieved in this fashion and have shown that highly anisotropic gold nanoparticles may be obtained by the spontaneous reduction of chloroaurate ions by 4-hexadecylaniline32 and alkylated tyrosine Langmuir monolayers.33 In a slightly different approach, we have shown that hydrophobized gold ions immobilized at the air-water interface may be reduced by subphase reductants to yield anisotropic nanogold.34 Recently we have shown that aqueous silver nanoparticles can be synthesized using the amino acid tyrosine at alkaline pH. Under these conditions, the phenolic groups in tyrosine form phenolate anions that are capable of reducing silver ions.35 Surface capping of the silver nanoparticles by the oxidized tyrosine molecules renders them highly stable.35 Taking a clue from this study, we investigate in this paper the possibility of using Langmuir monolayers of 3-pentadecylphenol (PDP) as two-dimensional reducing agents in the formation of silver nanoparticles. We observe that reduction of silver ions present in the subphase under alkaline conditions by 3-PDP Langmuir monolayers leads

10.1021/jp0465581 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2004

19270 J. Phys. Chem. B, Vol. 108, No. 50, 2004 to the spontaneous formation of highly ordered, large domains of silver nanoparticles at the air-water interface in one step. We have also carried out the reduction of aqueous silver ions by 3-PDP at a liquid-liquid interface and observe the formation of reasonably monodisperse silver nanoparticles in the organic phase (chloroform). Synthesis of silver nanoparticles in the organic phase has been achieved previously either by phase transfer of the nanoparticles first synthesized in the aqueous phase into an organic phase containing surfactants possessing thiol19 or amine30 functional groups or by phase transfer of metal ions into the organic phase and reduction of the ions in the presence of an additional surfactant.36 Though these methods do yield fairly monodisperse organically dispersible nanoparticles,19,30,36 a major drawback is that they are multistep processes and involve the use of a phase transfer agent, an external reducing agent, and an additional capping agent. Furthermore, the surface of the nanoparticles is contaminated by these reagents and often requires additional purification. In the present work, the synthesis of organically dispersible silver nanoparticles of uniform size is achieved in a single step with a water insoluble surfactant containing a phenol group that acts as a reducing (in the presence of KOH) and capping agent. The long hydrocarbon chain attached to the phenol group in 3-PDP renders the nanoparticles’ surface hydrophobic and makes the nanoparticles organically dispersible. Presented below are details of the study. Experimental Details Chemicals. Silver sulfate (Ag2SO4), 3-pentadecylphenol (C21H36O, 3-PDP), and potassium hydroxide (KOH) were obtained from Aldrich Chemicals and used as-received. (A) Reduction of Silver Ions at the Liquid-Liquid Interface. In a typical experiment, 100 mL of 10-4 M aqueous silver sulfate solution was stirred along with 100 mL of 10-2 M chloroform solution of 3-PDP, and 1 mL of 10-1 M solution of KOH was added to the biphasic mixture while stirring. After 5 h stirring, the color of the organic phase turned deep yellow indicating the formation of silver nanoparticles in the organic phase. Faint yellow color was observed in the aqueous phase after reaction indicating formation of a small amount of silver nanoparticles in the aqueous phase as well. When the same reaction was carried out with 5 mL of 10-1 M KOH (overall KOH concentration is 5 × 10-3 M in solution) instead of 1 mL of 10-1 M KOH (overall KOH concentration is 10-3 M in solution), the amount of silver nanoparticles formed in the aqueous phase is higher. Thus it is observed that the amount of nanoparticles formed in the aqueous and organic phases is a function of the concentration of KOH. Reduction of silver ions in both the aqueous and organic phases was followed by the UV-vis spectroscopic measurements as a function of reaction time. After completion of the reaction, the chloroform phase was separated from the aqueous phase and then rotavapped to give hydrophobized silver nanoparticles in the form of powder that is readily redispersible in various organic solvents such as toluene, hexane, etc. (B) Reduction of Silver Ions at the Air-Water Interface by PDP Langmuir Monolayers. In a typical experiment, 100 µL of chloroform solution of 3-PDP (1 mg/mL) was spread on the surface of 10-4 M aqueous Ag2SO4 solution (pH adjusted to 10 using 1 M aqueous KOH solution) as the subphase in a Nima model 611 LB trough. A standard Wilhelmy plate was used for surface pressure sensing. Pressure-area (π-A) isotherms were recorded at room temperature at compression and expansion rates of 50 cm2/minute at different times after

Swami, A. spreading the 3-PDP monolayer. Reduction of silver ions at the air-water interface was monitored by in situ UV-vis spectroscopy measurements of the compressed monolayer (compressed to a surface pressure of 15 mN/m) at different time intervals. After complete reduction of the silver ions by the PDP Langmuir monolayer, multilayer films of the 3-PDP-reduced silver nanoparticles of different thickness were formed by the LB technique at a surface pressure of 15 mN/m and a deposition rate of 20 mm/min with a waiting time of 5 min between dips on carbon-coated transmission electron microscopy (TEM) grids, quartz substrates, and Si (111) substrate for TEM, UV-vis spectroscopy, X-ray diffraction (XRD), and X-ray photoemission spectroscopy (XPS) measurements, respectively. The quartz and Si(111) substrates were hydrophobized by depositing 3 monolayers of lead arachidate prior to transfer of the 3-PDP-reduced silver nanoparticle monolayers. The hydrophobization of the support resulted in better transfer ratios of the nanoparticle monolayers. For the LB films grown on different substrates, monolayer transfer was observed during both the upward and downward strokes of the substrate at close to unity transfer ratio. UV-vis Spectroscopy Studies. In situ UV-vis spectroscopy measurements of the compressed 3-PDP monolayer on the surface of aqueous Ag2SO4 solution were done on an Ocean Optics S2000 spectrometer in the reflectance mode. UV-vis spectroscopic measurements of the silver nanoparticles synthesized in the organic and aqueous phases and LB films of 3-PDP reduced silver nanoparticles deposited on quartz substrate were carried out on a JASCO model V-570 dual-beam spectrophotometer operated at a resolution of 1 nm. TEM Measurements. TEM samples of the silver nanoparticles synthesized in the organic and aqueous solutions were prepared by placing drops over carbon-coated copper grids and allowing the solvent to evaporate. The PDP-reduced silver nanoparticle monolayer was deposited on a carbon-coated copper grid from the LB trough as described above. TEM measurements were performed on a JEOL model 1200EX instrument operated at an accelerating voltage at 120 kV. Fourier Transform Infrared Spectroscopy (FTIR) Measurements. FTIR measurements of 3-PDP and 3-PDP reduced Ag nanoparticles deposited in the form of films on Si(111) substrates were carried out on a Perkin-Elmer FTIR Spectrum One spectrophotometer in the diffuse reflectance mode operating at a resolution of 4 cm-1. Nuclear Magnetic Resonance Spectroscopy Measurements. Purified powder of 3-PDP-reduced silver nanoparticles was dissolved in CDCl3, and the proton (1H) NMR spectra were recorded on a Bruker AC 200 MHz instrument and scanned in the range 0-15 ppm. For comparison, the proton NMR spectrum was also recorded from a solution of pure 3-PDP in CDCl3. XPS Measurements. XPS measurements of a 3-PDP reduced silver nanoparticle film deposited onto a Si(111) substrate by the LB technique (30 ML thickness) were carried out on a VG MicroTech ESCA 3000 instrument at a pressure greater than 10-9 Torr. The general scan and C 1s and Ag 3d core level spectra were recorded with un-monochromatized Mg KR radiation (photon energy ) 1253.6 eV) at a pass energy of 50 eV and electron takeoff angle (angle between electron emission direction and surface plane) of 60°. The overall resolution of measurement is thus 1 eV for the XPS measurements. The core level spectra were background-corrected using the Shirley algorithm,37 and the chemically distinct species were resolved using a nonlinear least squares procedure. The core level binding energies (BEs) were aligned with respect to the C1s binding energy (BE) of 285 eV.

One-Step Synthesis of Silver Nanoparticles

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Figure 1. (A) UV-vis absorption spectra of the silver nanoparticles synthesized in the organic phase at two different KOH concentrations (curve 1, 10-3 M; and curve 2, 5 × 10-3 M) and the silver nanoparticles synthesized in the aqueous phase (KOH concentration 5 × 10-3 M, curve 3). (B) and (C) UV-vis absorption kinetics recorded during the formation of silver nanoparticles in the organic and aqueous phases, respectively (KOH concentration 10-3 M, see text for details).

Results and Discussion Reduction of silver ions by 3-PDP molecules is possible only in the presence of KOH. The function of KOH is to ionize the phenolic group of 3-PDP into phenolate anion, and the resulting electron transfer from these phenolate ions to silver ions leads to the formation of silver nanoparticles capped by the quinone group of the oxidized 3-PDP molecule. When aqueous alkaline silver sulfate solution is mixed with 3-PDP chloroform solution, the 3-PDP molecules are transferred to the aqueous phase under stirring conditions, interact with silver ions in the aqueous phase and reduce them to form silver nanoparticles. Here the concentration of KOH decides the amount of PDP molecules transferred into aqueous phase, which in turn controls the distribution of nanoparticles between the aqueous and organic phases. Therefore, the increase in concentration of KOH allows the reduction of more amounts of silver ions in the aqueous phase and is clearly reflected in the UV-visible spectra of the silver nanoparticles synthesized in organic phase at two different KOH concentrations (Figure 1A). When the overall concentration of KOH in the solution is 10-3 M during reduction, the intensity of the silver surface plasmon resonance in the chloroform phase (curve 1) is much higher and sharper than in the case where the overall KOH concentration is 5 × 10-3 M (curve 2). However in this latter case, there is appreciable formation of silver nanoparticles in the aqueous phase as well (curve 3). The surface plasmon resonance absorption of silver nanoparticles is observed at 415 and 418 nm in the chloroform (curve 1) and aqueous phases (curve 3), respectively. To understand the reduction mechanism of silver ions by the 3-PDP molecules, the kinetics of formation of silver nanoparticles in both aqueous and organic phases was monitored by UV-visible spectroscopy and is shown in Figure 1B and C, respectively (KOH concentration: 10-3 M). In Figure 1B, curve 1 corresponds to the UV-vis spectrum of the as-prepared chloroform solution of 3-PDP while curves 2-6 are the UV-vis spectra recorded from the chloroform solution during stirring with alkaline silver sulfate solution in steps of 1 h of reaction. It is observed that as the time of reaction increases, the surface plasmon resonance intensity increases steeply and saturates after ca. 5 h of reaction. In Figure 1C, curve 1 corresponds to the UV-vis spectrum of the as-prepared silver sulfate solution; curve 2 is the spectrum recorded after addition of KOH to silver sulfate solution and curves 3-6 are the UV-visible spectra recorded at intervals of 1 h during the reaction. The absorption

Figure 2. (A) FTIR spectra recorded from pure 3-PDP (curve 1) and 3-PDP-reduced silver nanoparticles from the organic phase in the biphasic reaction experiment (KOH concentration 10-3 M, curve 2). (B) Proton NMR spectra of pure 3-PDP (curve 1) and 3-PDP-reduced silver nanoparticles formed in the organic phase in the biphasic reaction experiment (KOH concentration 10-3 M; curve 2) after dispersion in CDCl3. The inset shows the structure of 3-PDP before (1) and after oxidation (2) consequent to reduction of the silver ions and formation of silver nanoparticles.

seen at 550 nm immediately after the addition of KOH to the silver sulfate solution (curve 2) is due to the formation of a brown-colored silver hydroxide. During later stages of reaction, the 550 nm absorption band disappears and a new peak appears at 415 nm whose intensity increases as the reaction progresses indicating the formation of silver nanoparticles in the aqueous phase (curves 3-6). The intensity of the surface plasmon vibration band in the silver nanoparticles in all experiments was considerably smaller in the aqueous phase than in the chloroform phase. From the above results, we infer that the aqueous silver ions first electrostatically complex with the ionized 3-PDP molecules at the liquid-liquid interface following which they are reduced to silver nanoparticles and capped by the oxidized 3-PDP molecules. While a large percentage of the silver nanoparticles are sufficiently capped by the 3-PDP molecules to render them hydrophobic and organically dispersible, it is clear that some silver nanoparticles are not fully capped by the 3-PDP molecules and therefore, they are dispersed in the aqueous phase. Figure 2A shows the FTIR spectra recorded from pure 3-PDP (curve 1) and 3-PDP-reduced silver nanoparticles in chloroform

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Figure 3. (A) and (B) Representative TEM images of silver nanoparticles synthesized in the organic phase in the biphasic reaction experiment (KOH concentration 10-3 M) at different magnifications. The selected area electron diffraction pattern recorded from these nanoparticles is shown in the inset of (A). (C) The particle size distribution measured from 100 particles in images (A) and (B) and other similar images; the solid line is a Gaussian fit to the data.

(KOH concentration: 10-3 M, curve 2). The broad peak centered at 3360 cm-1 is attributed to the OH stretching frequency of the phenolic group in the case of 3-PDP (curve 1; scheme 1 in the inset of Figure 2B) and has disappeared in the case of 3-PDP reduced silver nanoparticles (curve 2, scheme 2 in the inset of Figure 2B). The appearance of a peak at 1675 cm-1 (shown by an arrow in curve 2) in the case of the 3-PDPreduced silver nanoparticles is attributed to the carbonyl stretching frequency coming from the semiquinone moiety after oxidation. The above results reveal clearly that the phenolic groups in the 3-PDP molecules are responsible for reduction of silver ions at alkaline pH. The mechanism for the reduction of silver ions by 3-PDP that has been discussed earlier was further confirmed by proton NMR analysis of pure 3-PDP (curve 1) and 3-PDP-reduced silver nanoparticles (curve 2) dissolved in CDCl3 as shown in Figure 2B. Due to oxidation of 3-PDP molecules, the phenyl group is converted into a semi-quinone group; the chemical shift of the quinone protons is usually measured to be in the region of 5-6 ppm (curve 2). In the case of 3-PDP-reduced silver nanoparticles, the appearance of a peak at 5.5 ppm (shown by an arrow in curve 2) indicates that semi-quinone was the oxidized product during the reduction of silver ions; this peak is clearly missing in the proton NMR spectrum of pure 3-PDP (curve 1). The chemical structure of 3-PDP before (1) and after oxidation (2) based on the above observations is shown in the inset of Figure 2B. Representative TEM images of the silver nanoparticles present in chloroform synthesized in the biphasic reaction experiment (KOH concentration: 10-3 M) are shown in Figure 3A and B. It is observed that the silver nanoparticles are present in high concentration (Figure 3A), of almost spherical morphology, and rather uniform in size (Figure 3B). The inset of Figure 3A shows the selected area diffraction pattern recorded from the silver nanoparticles. The ring-like diffraction pattern indicates that the particles are crystalline; the diffraction rings could be indexed based on the fcc structure of silver. The particle size distribution histogram of the silver nanoparticles shown in Figure 3A and B and other micrographs is shown in Figure 3C. From a Gaussian fit to the histogram (solid line), the size of the nanoparticles was determined to be 11.8 ( 1.6 nm. The small standard deviation relative to the average nanoparticle size indicates clearly that the particles are uniform in size. The

Swami, A.

Figure 4. (A) and (B) Representative TEM images of the silver nanoparticles synthesized in the aqueous phase in the biphasic reaction experiment (KOH concentration 10-3 M) at different magnifications. (C) The particle size distribution measured from 100 particles in images (A) and (B) and other similar images; the solid line is a Gaussian fit to the data.

Figure 5. (A) π-A isotherms recorded after the stabilization of the 3-PDP Langmuir monolayer on the surface of: pure water (curve 1); aqueous 1 × 10-4 M Ag2SO4 solution (curve 2); and alkaline 1 × 10-4 M Ag2SO4 (pH adjusted to 10 using 1 M KOH) solution (curve 3). (B) UV-vis spectra of the 3-PDP monolayer on the surface of alkaline 1 × 10-4 M Ag2SO4 (pH 10) solution measured in situ at different time intervals after spreading the monolayer (time indicated next to the respective curve). The curves have been shifted vertically for clarity.

corresponding TEM images recorded from the silver nanoparticles synthesized in the aqueous phase are shown in Figure 4A and B. Unlike their counterparts in the chloroform phase, the silver nanoparticles in the aqueous phase are highly polydisperse in nature and not uniform in shape. The particle size histogram of the aqueous silver nanoparticles is shown in Figure 4C. An analysis of this distribution revealed an average nanoparticle size of 12.3 ( 2.9 nm. It is clear from the above results that 3-PDP can reduce the silver ions under alkaline conditions to form nanoparticles in both aqueous and organic phases. Since 3-PDP is an amphiphilic molecule and can form stable Langmuir monolayers on the surface of water, we were motivated to study the reduction of silver ions at the air-water interface by 3-PDP spread on the surface of aqueous Ag2SO4 solution under alkaline conditions. Figure 5A shows the π-A isotherms recorded after stabilization of the 3-PDP Langmuir monolayer spread on different subphases; curve 1 is the isotherm recorded from the monolayer on the surface of pure water, whereas curves 2 and 3 are the

One-Step Synthesis of Silver Nanoparticles isotherms recorded on the surface of as-prepared aqueous 10-4 M Ag2SO4 solution (pH 6.5) and aqueous 10-4 M Ag2SO4 solution at pH 10, respectively. The pH of the subphase in the latter case was adjusted to 10 using 1 M KOH solution. The liftoff molecular area of 3-PDP on pure water (curve 1) is observed to be ca. 50 Å2/molecule. The liftoff area in the π-A isotherms recorded on the 10-4 M Ag2SO4 solution (pH 6.5, curve 2) and aqueous 10-4 M Ag2SO4 solution at pH 10 (curve 3) increases to ca. 60 Å2/molecule indicating some interaction of the aqueous Ag+ ions with the phenolic headgroup of 3-PDP. The most significant changes in the π-A isotherms are observed in the collapse pressures and hysteresis of the isotherms (compare curves 1-3). The collapse pressure is the least and the hysteresis is the maximum in the case of 3-PDP on 10-4 M Ag2SO4 solution at pH 10, indicating strongest interaction of the monolayer with subphase silver ions under these conditions. Under alkaline subphase conditions, metallic luster could be seen readily at the interface 1-2 h after spreading the monolayer, which then develops as a function of time eventually forming a compact yellow film of silver nanoparticles floating at the interface after about 12 h of spreading the monolayer. In this experiment (10-4 M Ag2SO4 solution at pH 10), we have monitored the reduction of the silver ions at the air-water interface by in situ UV-vis spectroscopic measurements of the monolayer in reflection mode. Figure 5B shows the UV-vis absorption curves obtained from the monolayer of silver nanoparticles recorded at different time intervals after spreading the 3-PDP monolayer (the time at which spectra are recorded is indicated next to the respective curves). There is no absorption observed during the initial stages as shown by the curve recorded 30 min after spreading the monolayer. However, as the reduction proceeds, an absorption band centered at ca. 430 nm is observed (3 h), which arises due to excitation of surface plasmon vibrations in the silver nanoparticles. This absorption band broadens and eventually shifts to ca. 475 nm as seen in the spectrum recorded 12 h after spreading the 3-PDP monolayer. The broadening and red shift in the plasmon absorption band can be attributed to interactions between silver nanoparticles that are assembled on the surface of water as the concentration of the nanoparticles builds up in time.38 The growth and assembly of the spontaneously formed silver nanoparticles at the air-water interface was followed by TEM measurements by depositing a monolayer of the 3-PDP-reduced silver nanoparticles on carbon-coated TEM grids at different times of reaction. Figure 6A, C, E, and G show representative TEM images recorded at low magnification of the silver nanoparticles formed at the air-water interface after 30 min, 3 h, 9 h, and 12 h of spreading the 3-PDP monolayer, while Figure 6B, D, F, and H are the corresponding TEM images recorded at higher magnification. The lower magnification images (A, C, E, and G) reveal that the nanoparticles assemble to form domains whose coverage and size increases from about 200 nm (at time t ) 30 min) to about 10 µm (at time t ) 12 h). The individual nanoparticles and their 2-dimensional assembly in the domains is clearly seen in the higher magnification images (B, D, F, and H). A comparison of the high magnification images shows that there is no detectable change in the average silver nanoparticle size as a function of time of reaction of the 3-PDP Langmuir monolayer with the aqueous silver ions; however, the morphology of the domains does change with time of reaction from one of open, string-like structures (Figure 6B and D) to compact, circular, and hexagonally packed assemblies of nanoparticles (Figure 6F and H). Thus, it is clear from TEM results that the red shift in the surface plasmon resonance

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Figure 6. Representative TEM images of the 3-PDP-reduced silver nanoparticles formed at the air-water interface as a function of time of reaction: t ) 30 min (A) and (B); t ) 3 h (C) and (D); t ) 9 h (E) and (F); and t ) 12 h (G) and (H). The inset of (E) shows the electron diffraction pattern obtained from the nanoparticles shown in image (H). The inset of (G) corresponds to the particle size distribution histogram determined from 100 particles shown in image (H) and other similar images. The solid line is a Gaussian fit to the data.

observed from the 3-PDP-Ag nanoparticle monolayers (Figure 5B) is due to the spontaneous formation and 2D organization of nanoparticles.38 The inset of Figure 6E shows the selected area electron diffraction pattern recorded from the nanoparticles shown in Figure 6H. The characteristic rings in the diffraction pattern reveal the crystalline nature of the nanoparticles and could be indexed as the (111), (200), (220), and (311) allowed Bragg reflections from fcc silver. The inset of Figure 6G shows the particle size distribution histogram measured from 100 particles from Figure 6H and other similar images. The solid line is a Gaussian fit to the particle size distribution histogram and yielded an average particle size of 9.0 ( 1.2 nm. The particle size in the case of the nanoparticles formed at the airwater interface is marginally less than those that are formed in the biphasic reaction experiment (11.8 ( 1.6 nm) and may be due to constrained growth at the air-water interface. The highmagnification TEM images also suggest that the interparticle separation is very uniform; an analysis of the interparticle separations yielded an average value of 1.5 ( 0.2 nm. This value is less than twice the length of 3-PDP molecules calculated empirically, indicating that the silver nanoparticles in the 2-D assemblies are packed with interdigitation of the hydrocarbon

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Figure 7. (A) UV-vis absorption spectra of LB films of 3-PDPreduced silver nanoparticles of different thickness deposited on quartz substrates (film thickness is indicated next to the respective curve). (B) Plot of the intensity of the surface plasmon resonance at 500 nm plotted against the number of monolayers in the silver nanoparticle LB films on quartz substrate. The solid line is the nonlinear least-squares fit to the data.

chains between neighboring particles. Along with the yellow monolayer of silver nanoparticles at the interface, a very faint yellow color was also observed in the subphase indicating that slight reduction of the silver occurs also in the subphase. TEM analysis of this solution shows that the nanoparticles in the subphase are highly polydisperse in size and of very irregular shape (Supporting Information, S1). Hence, the 2D reduction of silver ions at the air-water interface using 3-PDP Langmuir monolayer results in the one-step spontaneous formation of monodisperse silver nanoparticles assembled into close-packed domains that may then be transferred in the form of superlattices onto solid supports by the LB technique. We speculate that the excellent monodispersity of the silver nanoparticles synthesized both at the liquid-liquid interface and at the air-water interface arises due to rapid capping of the silver nanoparticles by oxidized 3-PDP molecules that limits their growth to the initial nucleation stage. This suggests strong interaction of the oxidized 3-PDP molecules with the silver surface. It is well-known that the polydispersity of nanoparticles increases during the growth phase and consequently, limiting nanoparticle growth by surface capping could explain the above results. The fact that the monodispersity of the silver nanoparticles in the aqueous phase was worse than that for the particles in chloroform in the liquidliquid interface experiment provides further support for this hypothesis: insufficient capping of the silver nanoparticles by the oxidized 3-PDP molecules renders them water dispersible and also enables them to grow more readily. Figure 7A shows the UV-vis spectra recorded from silver nanoparticle films of different thickness (number of monolayers is indicated next to the respective curve) deposited on quartz substrates by the LB technique. The surface plasmon band increases monotonically with increasing film thickness (Figure 7A) indicating a build-up in nanoparticles on the quartz substrate surface. Figure 7B shows a plot of the intensity of absorption at λmax (500 nm in this case) against the number of monolayers in the LB film. It is observed that the plasmon absorption intensity increases linearly with the number of silver nanoparticles monolayers in the LB film. This indicates that the density of silver nanoparticles in each of the layers in the LB film is (within detection limits) almost the same. The large red shift in the plasmon resonance band compared to that observed for the silver nanoparticle monolayer (475 nm) is most probably due to a change in the refractive index of the film relative to the air-water interface. A chemical analysis of the 3-PDP-reduced silver nanoparticles was done by XPS measurements of a 30-ML LB film of the

Swami, A.

Figure 8. (A) C 1s and (B) Ag 3d XPS core level spectra recorded from a 30-ML thick PDP-reduced silver nanoparticle LB film deposited on a Si (111) substrate. The solid lines are nonlinear least-squares fits to the data.

nanoparticles deposited on Si(111) substrate. Figure 8A and B show the C1s and Ag 3d core level spectra recorded from this LB film, respectively. The C1s signal could be stripped into three chemically distinct components. The major component at 285 eV binding energy (BE) arises from carbons in the hydrocarbon chains of 3-PDP molecules capping the silver nanoparticles while the other two components at 281.3 and 287 eV can be assigned to the carbons in the aromatic ring of the 3-PDP molecules and carbonyl carbon respectively (scheme 2 in inset of Figure 2B). The Ag 3d core level could be decomposed into two chemically distinct species with Ag 3d5/2 BEs of 368 and 370.2 eV that are assigned to metallic silver (Ag0) and unreduced silver ions (Ag+), respectively, indicating that a small percentage of unreduced silver ions are present in the LB films. Conclusions A simple one-step process for the synthesis of highly ordered, large domains of monodisperse silver nanoparticles by the spontaneous reduction of silver ions under alkaline conditions present in the subphase by 3-pentadecylphenol (PDP) Langmuir monolayers has been illustrated. The reduction of silver ions occurs due to ionization of the phenolic groups in 3-PDP and subsequent electron transfer to silver ions. The oxidized 3-PDP molecules complex with the silver nanoparticles thus formed leading to their assembly at the air-water interface and stabilization in chloroform in the biphasic reaction experiment; the silver nanoparticles formed in chloroform could be separated in the form of a dry powder and readily dispersed in a range of organic solvents. Excellent quality superlattice films of LB of the silver nanoparticles could be formed easily on suitable solid substrates. Acknowledgment. A.S. and PR.S. thank the Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR), Government of India, for financial assistance. This work was partially funded by a grant to M.S. from the DST and is gratefully acknowledged. We thank Dr. A. B. Mandale, Center for Materials Characterization, for assistance with the XPS measurements. Supporting Information Available: TEM images of silver nanoparticles formed in the subphase by the reduction of silver ions by 3-PDP Langmuir monolayers (tiff). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) El-sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Victor, F. P.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (c) Sun, Y.; Xia, Y. Analyst 2003, 128, 686. (d) El-sayed, M. A. Acc. Chem. Res. 2004, 37, 326.

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