One-Step Synthesis of Monodisperse Silver Nanoparticles beneath

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One-Step Synthesis of Monodisperse Silver Nanoparticles beneath Vitamin E Langmuir Monolayers Li Zhang,†,‡ Yuhua Shen,*,†,§ Anjian Xie,†,§ Shikuo Li,† Baokang Jin,† and Qingfeng Zhang† School of Chemistry and Chemical Engineering, Anhui UniVersity, Hefei 230039, P. R. China, Department of Chemistry-Biology, Suzhou College, Suzhou 234000, P. R. China, and State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed: December 2, 2005; In Final Form: February 15, 2006

The monodisperse silver nanoparticles were synthesized by one-step reduction of silver ions in the alkaline subphase beneath vitamin E (VE) Langmuir monolayers. The monolayers and silver nanocomposite LB films were characterized by surface pressure-area (π-A) isotherms, transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), selected area electron diffraction (SAED), Fourier transform infrared transmission spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), respectively. The results showed that the limiting area/VE molecule on different subphases varied. The phenolic groups in the VE molecules were converted to a quinone structure, and the silver ions were mainly reduced to ellipsoidal and spherical nanoparticles. The arrangement of the nanoparticles changed from sparseness to compactness with reaction time. The electron diffraction pattern indicated that the silver nanoparticles were face-centered cubic (fcc) polycrystalline. Silver nanocomposite LB films with excellent quality could be formed on different substrates, indicating that the transfer ratio of monolayer containing silver nanoparticles is close to unity. The dynamic process of reduction of silver ions by VE LB films was also studied through monitoring the conductivity of an Ag2SO4 alkaline solution.

Introduction Recently, the investigation and preparation of noble metal nanoparticles and nanocrystals have drawn considerable attention because of their wide applications.1-3 The size, shape, structure, and morphology of nanoparticles play important roles in modulating their electronic and optical properties. Because silver nanoparticles possess excellent biocompatibility and low toxicity, they have important applications in catalysis,4 in spectroscopic analysis as excellent substrates for surface enhanced Raman scattering (SERS),5,6 and in the field of biology such as antibacterials,7,8 DNA sequencing,9 and so on. Many techniques of synthesizing silver nanoparticles have been reported, such as electrochemical methods,10,11 the chemical reduction of silver ions in reverse micelles,12,13 photochemical methods,14 and the Langmuir-Blodgett technique.15,16 Among these methods, monolayer and Langmuir-Blodgett techniques are the most effective ways in controlling the molecular orientation and packing at a molecular level.17 Not only can Langmuir-Blodgett films fabricate the packing and the thickness of the molecular films in a controlled manner, but also they provide possible templates to control the nucleation and growth of organized inorganic nanoparticles under mild conditions.18 Nanoparticles have been generated in the organized LB films in several ways. One of the approaches is that the metallic ion in the subphase is attracted by oppositely charged Langmuir monolayers through electrostatic interactions and then chemically reacts with external compounds to synthesize * To whom correspondence should be addressed. E-mail: s_yuhua@ 163.com. Tel.: +86-551-5108090. Fax: +86-551-5107342. † Anhui University. ‡ Suzhou College. § Nanjing University.

CdS,19,20 CdSe,21,22 PbS,23,24 and PtS25 nanoparticle films at the air-water interface. Another approach is that nanoparticles are generated photochemically by the ultraviolet decomposition of a precursor compound in a Langmuir monolayer.26 Besides the above two methods, in a slightly different approach, surfactant stabilized colloidal nanocrystals of silver27,28 and magnetite29,30 or capped reverse-micelle-entrapped Ag nanoparticles31 could also form compact, stable monolayer films at the air-water interface. Although these methods do yield fairly monodisperse nanoparticles, the 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.32,33 The way that Langmuir monolayers are used as reducing reagent to prepare metal nanoparticles in one step at the air-water interface has seldom been reported;34 furthermore, the amphiphilic molecules acting as Langmuir monolayers have some toxicity.34 It is generally believed that vitamin E is the most important natural antioxidant present in biological membranes and can protect the membranes from damages induced by lipid peroxidation through their chemical antioxidation.35 Metallic ions being reduced to metal nanoparticles beneath vitamin E (R-tocopherol) Langmuir monolayers has not been reported so far. In this paper, we investigated the reduction of silver ions in the alkaline subphase by VE monolayers, which induced the spontaneous formation of monodisperse silver nanoparticles at the air-water interface in one step. The ordering of Langmuir monolayers rendered the silver nanoparticles organically dispersible. The nanocomposite monolayers could be transferred onto solid substrates, and their structures were characterized by TEM, UV-vis spectra, FTIR transmission spectra, and XPS. It is significant to study the nucleation and growth of metal nanoparticles induced by VE monolayers for understanding the

10.1021/jp0570216 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/11/2006

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Zhang et al. SCHEME 1: Reduction Mechanism of Silver Ions by VE

Figure 1. Molecular structure of vitamin E.

biomineralization mechanism and exploiting new preparations of nanomaterials. Experimental Section Chemicals. Silver sulfate (Ag2SO4), trichloromethane (chloroform, CHCl3), and sodium hydroxide (NaOH) were all AR grade and obtained from Shanghai Reagent Co.; pure R-tocopherol (vitamin E, C29H50O2; structure shown in Figure 1) was purchased from Sigma Chemical Co. and used without further purification. A Milli-Q water purification system was used to produce water with an average resistivity of 18 MΩ cm for all experiments, and its pH was 6.5. Reduction of Silver Ions at the Air-Water Interface by VE Langmuir Monolayers. In a typical experiment, 100 µL of chloroform solution of VE (1.02 mmol/L) was spread slowly onto the surfaces of pure water, 0.1 mmol/L Ag2SO4 aqueous solution (pH 6.5), and 0.1 mmol/L Ag2SO4 alkaline solution (pH was adjusted to 10 using a 1 mol/L NaOH aqueous solution), respectively. Evaporation of the spreading solvent was permitted for 20 min, and then the monolayer was compressed. π-A isotherms were recorded at different times after spreading of the VE monolayer at a compression rate of 20 mm/min in a LB trough of KSV 5000 system (KSV Instrument Ltd.). A standard Wilhelmy plate was used for surface pressure sensing. The accuracy of the surface pressure measurement is 0.1 mN/ m. Reduction of the silver ions at the air-water interface was performed at constant pressure (20 mN/m). After completion of reduction of the silver ions by the VE monolayer, the silver nanocomposite monolayers with different thickness were deposited on different substrates. The quartz, CaF2, and Si(111) substrates were made hydrophobic by depositing three monolayers of arachidic acid prior to transfer of the silver nanocomposite monolayers for UV-vis, FTIR, and XPS measurements, as previously done by Fendler36 and Sastry.37 The transfer ratios were close to unity (0.95-1.01) during both the upward and downward strokes of the substrates. TEM Measurements. At different aging times (2, 5, 8, and 11 h), the silver nanocomposite monolayers were transferred to Formvar-coated 400 mesh copper grids by positioning substrates at an angle of approximately 15 °C with respect to the air-water interface. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on JEM model 100SX and 2010 electron microscope instruments (Japan Electron Co.) operated at accelerating voltages of 80 and 200 kV, respectively. Selected area electron diffraction on the Ag nanoparticles was also carried out. UV-Vis Spectroscopy Studies. UV-vis spectroscopic measurements of silver nanocomposite monolayers deposited on a quartz substrate for 10, 20, 30, 40, and 50 ML LB films (ML ) monolayer) were determined by a TU-1901 model UVvis double beam spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., Beijing, China), and the average transfer ratios were 0.98, 0.98, 0.97, 0.96, and 0.97, respectively. FTIR Measurements. Measurements of 50 ML VE and 50 ML silver nanocomposite LB film were carried out on a Nexus

870 FTIR spectrophotometer with a resolution of 4 cm-1 (American Nicolet Co.). XPS Measurements. XPS measurements of a silver nanocomposite LB film (30 ML) were carried out on a VG ESCALAB MKII instrument at a pressure greater than 10-6 Pa. The general scan, C1s and Ag3d core level spectra were recorded with un-monochromatized Mg KR radiation (photon energy ) 1253.6 eV). Dynamic Measurements. The dynamic process of the silver ions reduced by VE LB films was investigated by monitoring the conductivity of Ag2SO4 alkaline solution. The pure VE LB film (40 ML) was immersed in 125 mL of Ag2SO4 (0.1 mmol/ L) alkaline solution, and the conductivity of this solution as a function of reaction time was recorded by using a DDSJ-308A model conductivity meter (Shanghai Precision & Scientific Instrument Co., Ltd.). All works were carried out in a dust-free box at a temperature of 22 ( 1 °C. Results and Discussion The phenolic groups of VE were ionized into phenolate anions in the alkaline condition, which transferred electrons from themselves to silver ions for the formation of nanoparticles. The oxidation product of VE was identified as R-tocopherolquinone (symbolized VEq).35 According to the classical Gibbs free energy formula, the driving force for the formation of silver nanoparticles (∆Gv) is given by the equation as follows:36

∆Gv ) -RTg/nS

(1)

Here R, Tg, and S are the gas constant, absolute temperature, and supersaturation, respectively. From this equation, it can be concluded that higher supersaturation is helpful for the formation of silver nanoparticles. The negatively charged phenolic group of VE could bind silver ions strongly and thus form a large scale local supersaturation microenvironment, i.e., the strong electric field resulted in low ∆Gv, for the formation of silver nanoparticles. The reduction mechanism of silver ions is shown in Scheme 1. Figure 2 displays the π-A isotherms of VE on different subphases. Figure 2a shows the type of isotherm recorded from the monolayer on the subphase of pure water, which is in agreement with similar reported surface-area pressure isotherms of “nonconventional” materials.37 The inflection of the isotherm at 27 mN/m reflects the collapse of the monolayer. The collapsing point here is recognized in the isotherm where the ratio surface pressure/(area/molecule) (∂π/∂A) starts decreasing due to a phase transition.37 The surface pressure increased slowly along with the monolayer being compressed after the phase transition. This means that collapsed VE molecules did not

Monodisperse Silver Nanoparticles

Figure 2. π-A isotherms of the VE Langmuir monolayer on different subphases: (a) H2O; (b, c) 0.1 mmol/L Ag2SO4 aqueous solutions with pH values 6.5 and 10, respectively.

dissolve in the subphase but stacked as multiplayers on the subphase. This might be owing to the weak polarity of hydroxyl of VE and the hydrophobic function of the close methyl, which led to the weak interaction between the VE and water molecule; as a result, the VE molecules were piled gradually on the monolayer under certain pressures. Figure 2b,c shows the isotherms recorded on the subphase of as-prepared 0.1 mmol/L Ag2SO4 solutions at pH 6.5 and 10, respectively. The limiting area/VE molecule on pure water (Figure 2a) is observed to be ca. 102 Å2, and it decreased to ca. 88 and 100 Å2, respectively, on the surface of solutions on weak acidic solution and alkaline solution. Under alkaline subphase conditions, silver ions in the subphase were attracted by the phenolate anion of VE and then absorbed beneath the monolayer. The static interaction between them shortened the distance of neighboring VE molecules and reduced the limiting area/VE molecule. The ionize degree of the phenolic group of VE on the weak acidic subphase conditions is smaller than that on alkaline subphase conditions, which implied that there were weak interactions between phenolate anion and silver ions on the weak acidic subphase. So the limiting area/VE molecule is larger than that on the alkaline subphase. Figure 2 shows VE molecules could form stable Langmuir monolayers on pure water and silver sulfate solution subphases. π-A isotherms of VE Langmuir monolayers on different suphases as a function of the monolayer spreading time are shown in Figure 3. Figure 3a-c,e,f shows π-A isotherms recorded at time intervals of 20 min and 1, 3, 6, and 10 h after the VE monolayer spreading on the alkaline subphase, respectively. Figure 3d presents the π-A isotherm of VE on the pure water subphase. The limiting areas/VE molecule on the alkaline subphase at the spreading times of 20 min, 1 h, and 3 h were smaller than that of VE on the pure water subphase. At the beginning of the reaction, large amounts of free silver ions existed beneath the VE monolayer with few silver nanoparticles formed. The interaction of silver ions and phenolate anion could weaken the electrostatic repulsion of the phenolate anion, so the VE molecules arrayed more closely on the subphase. After 6 h of reaction, the limiting area/VE molecule on the alkaline subphase increased gradually with increasing reaction time, which was more than that on pure water, and it even reached ca. 111.7 Å2 at a time of 10 h. The reason may be that the more silver nanoparticles could insert into the monolayer, which led to the area/molecule increasing. Under alkaline subphase conditions, a metallic cluster could be seen easily at the interface after about 3-4 h of spreading the monolayer, which then

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Figure 3. π-A isotherms of VE Langmuir monolayers on different subphases as a function of the monolayer spreading time. Subphase silver sulfate alkaline solution: (a) 20 min; (b) 1 h; (c) 3 h; (e) 6 h; (f) 10 h. Subphase pure water: (d) 20 min.

Figure 4. TEM images of the silver nanoparticles formed at the airwater interface as a function of the reaction time (a, 2 h; b, 5 h; c, 8 h; d, 11 h) and a high-resolution TEM image (f, 11 h). (e) Selected area electron diffraction pattern obtained from the nanoparticles shown in image d.

developed with time eventually forming a compact light yellow film of silver nanoparticles floating at the interface after 10 h of spreading the monolayer. Figure 4a-d shows typical micrographs of the silver nanoparticles obtained at the air-water interface after spreading the VE monolayer for 2, 5, 8, and 11 h. There is no detectable change in the average silver nanoparticle size as a function of reaction time. However, the distribution of the nanoparticles changed from sparseness (Figure 4a,b) to compactness (Figure 4c,d). Figure 4d displays many particles that are ellipsoidal and some particles that are spherical. The particles have a size range of 3-14 nm. The small size silver nanoparticles formed at the

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Figure 6. (a) RTIR transmission spectra of pure VE and (b) VEq/ silver nanocomposite LB films. Figure 5. UV-vis absorption spectra of VEq/silver nanocomposite LB films of different thicknesses deposited on quartz substrates (number of monolayers indicated next to the respective curve). The inset of Figure 5 shows a plot of the absorption intensity at 420 nm against the number of monolayers in the VEq/silver nanocomposite LB films on the quartz substrate. The solid line is the nonlinear least-squares fit to the data.

air-water interface show excellent monodispersity. We considered that the ordered headgroups of VE molecules at the airwater interface provided the nucleation sites for Ag nanoparticles as well as prevented nanoparticles aggregation, which made the growth of silver nanoparticles limited and highly localized. The crystalline nature of the silver nanoparticles is revealed by the selected area electron diffraction pattern shown in Figure 4e and the HRTEM image of the silver nanoparticles obtained after the reaction of 11 h shown in Figure 4f. The SAED pattern suggests that the observed four fringe patterns with spacings of 2.360, 2.021, 1.408, and 1.230 Å are consistent with the fcc polycrystalline silver (111), (200), (220), and (311) spacings of 2.359, 2.044, 1.445, and 1.231 Å. The individual sharp spots distributed in the diffraction rings are due to a few larger silver nanoparticles. A high-resolution TEM image (Figure 4f) reveals spherelike particles with the characteristic crystalline order of silver. The fringe spacing measured from the particle indicated by the arrow is around 0.237 nm, which is consistent with the spacing of the (111) lattice planes. These results confirm that the particles formed at the air-water interface are silver nanocrystals. Figure 5 shows the UV-vis spectra recorded from VEq/silver nanocomposite LB films of different thickness. There is a broad absorption band in the range 380-470 nm. According to Mie theory,38 the absorption band is aroused by excitation of surface plasmon vibrations of silver nanoparticles in the LB films. It is related to the concentration, size, shape of particles, and dielectric environment.38,39 Well-dispersed, spherical silver nanoparticles with 10 nm of diameter in solution in general show a sharp surface plasmon resonance at about 400 nm.40,41 The surface plasmon band of the silver nanoparticles in the LB films was red-shifted and broadened. A similar observation was made by Efrima’s42 and Sastry’s group.34,43 These spectral changes could be due to the interactions of the close-packed silver nanoparticles and the change in refractive index of the film relative to the water solution (refractive index of water ) 1.33).34 It is further observed that the absorption peak was red-shifted with increasing film thickness. The maximum absorption peak occurred at 420 nm for 10 ML LB film, and the amount of shift between the 10 ML film and the 50 ML film was more than 25 nm. The above phenomena can be also attributed to interactions between silver nanoparticles and the change of the dielectric environment. The thicker the films were, the more silver nanoparticles could be deposited on the quartz substrates,

which enhanced the interactions between silver nanoparticles. At the same time, silver nanoparticles were closely packed on the substrate to form a two-dimensional superlattice and the dielectric environment changed. Thick VEq/silver nanocomposite LB films would increase the medium dielectric constant of the nanoparticle surroundings39,44 Therefore, a larger red shift appeared as the film thickness increased. Additionally, the plasmon absorption intensity rose with the increase of the film thickness; this indicates a buildup in nanoparticles on the quartz substrate surface. The inset of Figure 5 shows a plot of the absorption intensity at 420 nm against the number of monolayers. This displays that the intensity of the absorption peak increases linearly with the number of VEq/silver nanocomposite monolayers in the LB films. This demonstrates that the density of silver nanoparticles in each layer of the LB film is almost the same and the silver nanocomposite monolayers could be transferred to the substrate successfully. Figure 6 shows the FTIR transmission spectra for pure VE (a) and VEq/silver nanocomposite LB films (b). The peaks centered at 2925 and 2855 cm-1 are attributed to the methylene antisymmetric and symmetric vibrations from the hydrocarbon chains of VE and VEq. The positions of the methylene stretching vibrations are uniform between the two films, but their intensities have little difference. The appearance of a new peak at 1709 cm-1 in the case of the VEq/silver nanocomposite LB monolayers is due to the carbonyl stretching vibrations coming from the quinone after the phenolic group is oxidized. The above results clearly reveal that the phenolic groups in the VE molecules are responsible for reduction of silver ions on the alkaline subphase. The LB film of a 30 ML VEq/Ag nanocomposite was analyzed by XPS. The general scan spectrum of the film showed the presence of strong C1s, O1s, Si2p, and Ag3d core levels with no evidence of impurities (shown in Figure 7a). The nanocomposite LB film was sufficiently thin, and therefore, the Si2p core level spectrum was measured from the substrate. The C1s core level spectrum recorded from the film is shown in Figure 7b, and it could be stripped into four main chemically distinct components at 284.8, 286.2, 287.6, and 288.7 eV. The high BE component observed at 284.7 eV can be assigned to the alkyl carbon of VEq. The 286.2 and 287.6 eV BE peaks are attributed to the carbon bound to the hydroxyl group and carbonyl carbon, respectively. The 288.7 eV BE peak can be assigned to the carboxylate carbon from the three monolayers of arachidic acid deposited on the Si substrate. The core levels were aligned with respect to the C1s BE of 285 eV. The Ag3d3/2 core level spectrum (Figure 7c) recorded from the nanoparticles LB film could be stripped into two main components at 368.6 and 370 eV. Generally speaking, the low BE component is attributed to electron emission from the metallic silver while the high BE component arises from silver ions, indicating that a small

Monodisperse Silver Nanoparticles

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Figure 7. (a) XPS survey, (b) C1s, (c) Ag3d, and (d) AES core level spectra recorded from a 30 ML thick VEq/silver nanocomposite LB film.

tween aging times of 250 and 1500 min. This means that the nucleation point of silver decreased and the silver particles grew gradually along with the reaction. After ca. 1500 min of reaction, the conductivity was constant. This implies that VE was exhausted and silver ions were reduced partially, and once the other pure VE films were immersed in the residual solution again, the conductivity resumed decreasing. This phenomenon farther verified the result that the termination of reaction is due to the exhaustion of the reducer (VE). In addition, the pH value of the solution lowered when the reaction terminated. At the same time, the color of the substrate changed from transparent to gray, which farther testified that the silver ions were reduced to silver nanoparticles. Figure 8. Dynamic curve of the formation process of Ag nanoparticles.

Ag+

fraction of unreduced ions remain bound to the surface of the nanoparticles. There was little difference between the binding energies of Ag and Ag+. The formation of Ag or Ag+ cannot been judged only through the binding energy of Ag3d3/2. The Ag3d auger core level spectrum is observed at 358.1 eV (shown in Figure 7d). The summation of binding energy and auger energy is 726.7 eV, which indicates the formation of Ag.45 The dynamic process of forming silver nanoparticles was analyzed by monitoring the conductivity of 0.1 mmol/L Ag2SO4 alkaline solution (pH 10) after immersing a pure VE LB film of 40 ML. There were several ions including H+, Ag+, Na+, OH-, and SO42- in this solution. According to Scheme 1, the reaction stole the same amount of Ag+ and OH- from solution. Figure 8 shows that the conductivity decreases as a function of reaction time. At the onset of the reaction, it was seen that the conductivity decreased rapidly, silver ions were attracted and reduced by the ionized phenolic group of VE in the LB films, and the nucleation of silver nanoparticles occurred. The decrease of Ag+ and OH- in solution resulted in the decline of conductivity together. The conductivity decreased slowly be-

Conclusion A simple one-step process to synthesize monodisperse silver nanoparticles beneath vitamine E monolayer is reported. The characteristics of the VE monolayer at the air-water interface on different subphases have been investigated. The dynamic process that the silver ions were reduced by VE LB films directly reflected the information about chemical reaction. Excellent quality VEq/silver nanocomposite LB films may be formed on different substrates without significant variation in the nanoparticle density. Acknowledgment. This work is supported by the National Science Foundation of China (Grants 20471001, 20371001) and the Specific Project for Talents of Science and Technology of Universities of Anhui Province (Grant 2005hbz03). References and Notes (1) Yoo, J. W.; Hathcock, D.; El-Sayed, M. A. J. Phys. Chem. A 2002, 106, 2049. (2) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. J.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323.

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