Large-scale synthesis and self-organization of silver nanoparticles with Tween 80 as a reductant and stabilizer
© Li et al.; licensee Springer. 2012
Received: 3 September 2012
Accepted: 25 October 2012
Published: 6 November 2012
Tween 80 (polysorbate 80) has been used as a reducing agent and protecting agent to prepare stable water-soluble silver nanoparticles on a large scale through a one-pot process, which is simple and environmentally friendly. Silver ions can accelerate the oxidation of Tween 80 and then get reduced in the reaction process. The well-ordered arrays such as ribbon-like silver nanostructures could be obtained by adjusting the reaction conditions. High-resolution transmission electron microscopy confirms that ribbon-like silver nanostructures (approximately 50 nm in length and approximately 2 μm in width) are composed of a large number of silver nanocrystals with a size range of 2 to 3 nm. In addition, negative absorbance around 320 nm in the UV-visible spectra of silver nanoparticles has been observed, probably owing to the instability of nanosized silver colloids.
KeywordsSilver Nanoparticles Tween 80 Discrete dipole approximation (DDA)
In recent years, nanoparticles (NPs) of noble metals have attracted considerable particular attention due to their unique physical and chemical properties different from those of bulk substances[1–5]. Among those noble metal nanoparticles, silver (Ag) NPs have been mostly studied owing to their high electrical and thermal conductivity as well as their extraordinary optical properties, which lead to extensive application in the fields of catalysis, electronic and optical materials[6–10]. However, the applications mentioned above depend on to a great extent the properties of Ag NPs especially on the particle shape, size, and size distribution[11–13].
There have been some shape-controlled synthetic methods of silver NPs including template-assisted methods, seed-mediated methods, polyol routes, and so on[14–20]. Different silver nanostructures such as spheres, hollow spheres, cubes, wires, rods, tubes, and plates would display different optical phenomena[21, 22]. During the preparation process, silver nanoparticles can be modified and stabilized by surfactants or polymers[23–25]. Surfactants are a kind of organic compounds possessing both hydrophilic polar groups and hydrophobic nonpolar groups. Under certain conditions, the surfactant molecules can form into various structures of ordered agglomerations such as micelles, reversed micelles, microemulsions, vesicles or liquid crystals[26, 27] whose microenvironment can serve as microreactors and templates for the controllable formation of nanomaterials[28–30]. On the other hand, the surface of nanocrystals is composed of different lattices, and different lattices may have different growing rates on its vertical direction. Some kinds of surfactants can stabilize certain lattice faces by hindering their growth, and the proportion of the faster ones will gradually decrease until all will disappear, making most of the surface of the nanocrystals be composed of the more slowly growing ones[31, 32]. Surfactant molecules are also capable of improving the stability of system through static electricity repulsive forces, steric hindrance, and Van der Waals force by absorbing onto the surface of nanomaterials[33, 34].
In most of the previous works, surfactants only acted as stabilizers or protecting agents. Actually, many nonionic surfactants, such as poly-(10)-oxyethylene oleyl ether (Brij 97, Sigma-Aldrich Co., MO, USA), polyoxyethylene-(20)-sorbitan monooleate (Tween 80, Sigma-Aldrich Co.), polyoxyethylene tert-octylphenyl ether (Triton X-100, Sigma-Aldrich Co.), can function as reductants to synthesize noble metal nanoparticles. Premkumar et al. had used Tween 80 as a reductant to react with a gold salt at room temperature and synthesized well-dispersed gold nanoparticles. Luo et al. reported that silver nanoparticles were prepared under mild conditions by exploiting poly(ethylene glycol) as a reducing agent at temperature (>17°C). Debnath et al. showed a solid-state high-speed vibration milling method for the synthesis of Ag NPs, in which poly(vinylpyrrolidone) (PVP) functioned as a reductant. Therefore, using those surfactants or polymers with reductive properties for the synthesis of metal nanoparticles is not only theoretically possible but also practical.
In this study, we have synthesized well-dispersed water-soluble silver nanoparticles using polysorbate 80 (Tween 80) as both the reducing agent and the protecting agent, and performed a systematic study on the formation process of silver nanoparticles, thus, produced. A trace of water in the system is very important to the homogeneity and dispersity of obtained Ag NPs. By changing the temperature, new arrays such as network- and ribbon-shaped self-organization have been observed. In addition, we consider that the formation mechanism of Tween 80-stabilized Ag NPs be similar to the oxidation of nonionic surfactants reported by Currie et al.. The initial step in the oxidation of Tween 80 is the formation of a free radical in α-position to the ether oxygen. Silver ions can accelerate the oxidation process and silver nanoparticles slowly form afterwards.
The one-pot synthesis is clean and green, and the preparation is simple with an easy operation. Tween 80 can act as both a reducing agent and a stabilizer without adding any additional reducing agents into the reaction. Moreover, the method can provide a high concentration of nanosized silver colloids; thus, it may be developed into industrial mass production of noble metal nanoparticles.
Polysorbate 80 (Tween 80) and AgNO3 were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used without further purification. Deionized water was used throughout the work.
Synthesis of silver nanoparticles
In a typical experiment, silver particles were prepared by simply mixing AgNO3 in a state of solid or an aqueous solution with Tween 80. In all systems, the fixed amount of Tween 80 was 2 mL. In the aqueous solution, the silver salt solutions were prepared by dissolving an appropriate amount of AgNO3 in 0.2 mL of water before mixing. The solid ones are designated as the dry systems, and the aqueous solution ones the wet systems. No stirring but shaking was necessary to homogenize the solution.
The mixture was then kept at different temperatures with different reaction times. When the reaction started, the color of the solution turned from yellow to orange, and finally to dark red-brown, which indicated the formation of silver nanoparticles.
After the reaction mixture has cooled to room temperature, a large amount of absolute ethanol was poured into the mixture, which was centrifuged at 10,000 rpm for 20 min. The resulting pellet was dispersed in ethanol by gentle bath sonication, and the suspension was centrifuged again. The dispersion-centrifugation process was repeated three times to wash off the surfactants and the remaining residues. In the final step of centrifugation, the final pellet was dried and then dissolved in deionized water.
The optical properties of silver particles were monitored by UV-vis spectroscopy. The UV-vis absorption spectra were taken at room temperature on a UV-3150 spectrophotometer (Shimadzu, Kyoto, Japan) with a variable wavelength between 200 and 800 nm using a glass cuvette with a 1-cm optical path. A UV/visible spectrum diode-array spectrophotometer (Model HP 8453; Agilent Technologies, Palo Alto, CA) was also employed for some specific samples. All of the UV-vis absorption spectra in this paper, except those mentioned otherwise, were recorded on a UV-3150 spectrophotometer.
The selected area electron diffraction (SAED) pattern, elemental analysis, and transmission electron microscope (TEM) images were acquired on a JEOL 2100F microscope (JEOL Co., Ltd., Tokyo, Japan) operating at an accelerating voltage of 200 kV. All TEM samples were made using aqueous colloids of the metal nanoparticles directly without size selection, which were deposited onto a 230-mesh copper grid covered with Formvar.
The Fourier transform infrared (FT-IR) spectra were collected on an IRPRESTIGE-21 FT-IR spectrophotometer in the wavenumber range of 400 to 4,000 cm−1 at a resolution of 4 cm−1. The samples were prepared in the form of pellets together with KBr. The X-ray powder diffraction (XRD) pattern was recorded using a Rigaku D/max γB-ray diffractometer (Rigaku Corporation, Tokyo, Japan) in transition mode and Cu K radiation (γ = 1.54056 Å). Samples for measurement were prepared by dropping silver colloids on quartz plates and allowing them to dry at 40°C. The X-Ray photoelectron spectroscopy (XPS) was performed on a ESCALAB MKII X-ray photoelectron spectrometer (VG Instruments, CA, USA) using non-monochromatized Mg-Kα X-rays as the excitation source. The binding energies for the samples were calibrated by setting the measured binding energy of C 1s to 284.60 eV. Particle size distribution (PSD) analysis was carried out by manually digitizing the TEM image with Image Tool from which the average size and standard deviation of metal nanoparticles were generated.
Results and discussions
Characterization and study of Tween 80-stabilized silver nanoparticles
Higher resolution spectra of the Ag 3d region in Figure2b showed that the binding energies of Ag 3d5/2 and Ag 3d3/2 of the silver nanoparticles are 367.8 and 373.8 eV, respectively, with a spin-orbit separation of 6.0 eV. These results are essentially the same as those reported for the PVP-stabilized Ag nanoparticles, which indicate a strong interaction between the carboxyl oxygen atoms in the PVP chain and silver nanoparticles[41–43].
In the present study, the binding energies of Ag 3d for the Tween 80-stabilized Ag nanoparticles are between those for metal silver (368.2 eV for Ag 3d5/2 and 374.2 eV for Ag 3d3/2) and for silver (I) oxide (367.5 eV for Ag 3d5/2, 373.5 eV for Ag 3d3/2). In our opinion, this implies the strong coordination of Ag atoms with the oxygen atoms of the carbonyl (C=O) groups in the Tween 80 or oxidized Tween 80 chains. Actually Tween 80 molecules cannot provide enough carbonyl groups to interact with Ag nanoparticles, while the oxidized Tween 80 should contain a sufficient number of carbonyl groups, which further confirms the silver ions-mediated oxidative degradation of Tween 80.
The peak in the C 1s spectrum (shown in Figure2c) can be fitted to several symmetrical peaks with binding energies of 284.6, 286.1, and 288.7 eV, which are assigned to the C 1s of the adventitious reference hydrocarbon, that of carbon –C–O–, and that of carboxyl carbon–O–C=O, respectively[45, 46]. The peak of O 1s (Figure2d) has also been deconvoluted into two bands with binding energies in agreement with those reported in the literature. One of the O 1s curves located at 532.7 eV should possibly be attributed to a combination of the backbone –CH2O–CH2–, plus the oxygen in the carboxyl group (−C=O–) which interacted with surface of the silver nanoparticles.
In agreement with the XPS observation, the interaction of C=O group with the surface of Ag nanoparticles can be further proved by the FT-IR analysis. The relative intensity of the peaks (after background correction and normalization relative to the lowest value around 1,105 cm−1) assigned to C=O in the sample decreases significantly, possibly implying that the oxygen in the carbonyl group along the long Tween 80 chains can provide coordinative saturation of dangling bonds on the surface of the silver nanoparticles, favoring the stability of the Ag nanoparticles in water. Similar result has already been reported for Tween 80 interacting with other kinds of nanoparticles in which the decrease of the relative band intensity and even the disappearance of the C=O absorption peak around 1,735 cm−1 have been observed[49, 50].
Concentration effects on the formation of Tween 80-stabilized silver nanoparticles
In the reaction system, there are only three kinds of reactants, AgNO3, Tween 80, and traces of water without additional reductants. Thus, it is convenient to evaluate the effect of the concentrations of the reactants on the products.
It can also be found that the increase of the amount of AgNO3 in the dry systems could induce a slight widening and a red shift of the characteristic peaks of silver (as shown in Figure5a and the inset), indicating the development of larger crystalline silver nanoparticles, and that the size of particles was not uniform. This can be explained by the poor dispersing of silver nitrate in Tween 80. In the dry system, the solid silver salt could not be dispersed homogeneously in Tween 80. The unreacted salt might be gathered around the new generated silver particles and reduced on their surface, which resulted in a larger size distribution of the as-formed silver nanoparticles.
The insets are the high-resolution transmission electron microscopy image (HRTEM, shown in Figure6b) of a nanoparticle and SAED patterns (shown in Figure6c) taken from silver nanoparticles. The HRTEM gives a well-resolved fringe spacing (0.23 nm), which agrees well with the spacing of the (111) lattice places of silver nanoparticles, and indicates that the nanoparticles are single nanocrystals. From the SAED image, we can see some diffraction rings respectively corresponding to the different lattice of a centered cubic silver nanoparticle that is (111), (200), (220), (311), and (222).
Self-organization of Tween 80-stabilized silver nanoparticles
A correlation between the dissolution rate of the surfactant and the temperature has already been proved to exist. In order to study the effects of temperature on the system, we have carried out the reactions at 90°C and 110°C without changing any other conditions.
The Ag nanoparticles, produced in the wet system with 50 and 100 mg of AgNO3 at 90°C, show two absorption peaks around 410 and 560 nm, respectively, with a long tail at the long-wavelength side of the band (shown in Figure8a,c). The absorption features are in good agreement with the results for the aggregation of polymer-coated silver nanoparticles or the salt-induced silver aggregates[57, 58]. In addition, the red shift of the band in the long-wavelength side indicates that the size of the silver aggregates increase with the concentration of AgNO3[57, 58].
On the other hand, the absorption spectra of the Ag nanoparticles, obtained with 50 and 100 mg of AgNO3 at 110°C in the wet system, exhibit a single sharp, symmetrical peak around 420 nm. While the Ag nanoparticles produced with 20 mg of AgNO3 show an absorption profile with very low absorbance and similar to those observed for silver nanoparticles produced at 90°C, presumably owing to the aggregation of Ag nanoparticles. The silver nanoparticles generated with 20 mg of AgNO3 were more instable than those obtained with high concentration of silver salt, which has also been observed in the silver nanoparticle production at relative lower temperature. The observed behavior can be explained by the slow initial nucleation rate. In detailed speaking, the fewer silver salts existed in the starting stage, the slower the nucleation process occurred, and the fewer nuclei were generated in the nucleation stage. The slow nucleation rate and the fewer nuclei result in the larger-size distribution of Ag nanoparticles, followed by the instability of silver colloids.
Then, in Figure9d, a mixture of different shapes of particles is observed, making sense the particular profile of its absorption peak. Moreover, when the silver salts with high concentration were used in the system at 110°C, the obtained silver nanoparticles have a tendency to self-organize into 2D well-ordered arrays. The formation of these nanoparticle arrays may be attributed to the synergetic effects of interparticle, particle-substrate, and solvent-substrate (wetting) interactions. When the solvent began to evaporate during the sample preparation for TEM characterization, the solvent film forming on the surface of the copper grid became unstable, and small droplets generated. The particles contained in the droplets would be left behind fast at the margin of the merge point after ethanol and would be entirely evaporated, which finally caused the formation of nanonetworks. On the other hand, strong adsorption of dendritic-structured Tween 80 on silver nanoparticles facilitates the cohesive interaction between the particles such as interdigitation and interpenetration, favoring the self-assembly of Ag nanoparticles. Similar arrays, including some hexagonal close-packed arrays, of noble metal nanoparticles have been reported before[64, 65].
A plausible formation process for the silver ribbons is briefly presented as following: It is more likely that the concentrated Tween 80 molecules aggregated into one-dimensional micelles in the presence of silver ions. When the temperature reached about 90°C, a large number of tiny silver nanoparticles generated quickly around the micelles of Tween 80. Subsequently, the tiny silver nanocrystals started to form aggregates, most of them ordering in ribbon shape. The silver nanocrystals could also act as the cross-linker of Tween 80 molecules, which kept the ribbon-shaped aggregates in water after purification. As the reaction proceeded, some tiny particles grew larger because of coalescing or Ostwald ripening. More and more discrete silver nanoparticles with the size of 20 nm or so appeared in the products obtained in the three- or four-day reaction. Only a small fraction of nanoparticles, which had not grown into larger ones, tended to form into quite rigid spindles with bits of stabilizers around them (see Additional file2: Figure S2).
Theoretical studies of the dependence of the UV-vis spectra upon aggregate shape and size of Tween 80-stabilized silver nanoparticles
The simulation methods of calculating the absorption and scattering efficiency usually belong to two categories: the exact and approximated solutions. In this paper, we use DDSCAT7.2, an open-source Fortran 90 software applying the discrete dipole approximation (DDA), to calculate scattering and absorption of electromagnetic waves by targets with arbitrary geometries and complex refractive index.
In our experiment, we have synthesized Ag nanoparticles coated with Tween 80. According to the TEM images of the products, some silver nanoparticles aggregated to form one-dimensional spindle-like structures. The structure was a mixture of large amount of silver nanoparticles and organic coat, so that we may consider it as silver-Tween 80 alloy. Besides, the solution we obtained also contained large amount of dispersive Ag nanoparticles and aggregated molecules of Tween 80. Thus, we established a model this way: calculate the extinction efficiency of a Ag-Tween 80 alloy ellipsoid (Ag/Tween 80 = 7:3), whose ratio of the major axis to minor axis equals to 2:1. Then, mix it with Ag-Tween 80 alloy sphere particles (Ag/Tween 80 = 8:2). Initial parameters are set as effective radius of ellipsoid = 25 nm, effective radius of sphere = 10 nm, and refractive index of ambient medium = 1.33 (H2O).
where xAg means the Ag fraction of Ag-Tween 80 alloy; ω means the frequency of incident light.
where n means complex refractive index.
Figure11 also shows the simulation results of extinction efficiency of the mixed system when the fraction of ellipsoid was decreased while the fraction of sphere was increased. By increasing the fraction of the sphere particles, the extinction intensity of the peak at ca. 410 nm grows a lot. It illustrates the shape of the UV-vis spectra when the amount of AgNO3 equals to 100 mg.
The extinction spectra in Figure11 show more distinct profiles, compared with the real absorption spectra of the silver nanoparticles (shown in Figure8a,c). We found in the simulation that if more Ag-Tween 80 alloy sphere particles in the simulation model, the extinction peaks tend to be less distinct. That is because alloy particles have a larger complex refractive index than the ambient environment, water, but smaller than the complex refractive index of pure silver; thus, the extinction peak appears at the middle area. In addition, we use an ideal model of nanoparticles with uniform size; while the sizes of real silver nanoparticles vary at some extent, this also leads to gradually expanded peaks. Moreover, in the real mixture system, large amounts of Ag nanoparticles, together with Tween 80, are dispersed randomly, making it more difficult to separate these extinction peaks. Similar profile differences between the absorption spectra of the real nanoparticles and the simulated data have also been reported[72, 73].
In summary, we have prepared well-dispersed stabilized silver nanoparticles in which Tween 80 acts as both stabilizer and reductant. Special arrays and aggregates of nanoparticles were found and thoroughly studied, and a possible mechanism for the formation of silver nanoparticles has been proposed. The optical absorption obtained by DDA simulation for the aggregated silver nanoparticles was in good agreement with those from experiments. The one-pot synthesis is clean and easily operated which may be applied to some industrial demands.
Financial supports from the National Science Foundation of China (grant nos.: 20871031, 51073039, 11179015, 51173108, and 91027042) and from the Innovation Program of Shanghai Municipal Education Commission (grant no.: 12ZZ143) are gratefully acknowledged.
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