In this work we present an electrochemical method to successfully prepare silver nanoparticles using only polyethylene glycol as stabilizer and without any other reactive. Here we study the use of the polymeric stabilizer to allow the introduction of a potential tool to reinforce the control of the size and shape of the nanoparticles throughout the synthesis process. The evolution of the reactions was followed by UV-Vis spectroscopy. The electrode processes were characterized by cyclic voltammetric measurements and the final product was studied by Atomic Force Microscopy, Transmission Electron Microscopy, and X-Ray Diffraction. The influences of the current density, polymer length, and concentration media were analyzed.
Noble metal nanoparticles have been intensely investigated due to their amazing properties such as optical, catalytic, and electric ones that can be controlled depending on the particles size, the size distribution, and shape [
Several methods have been reported for Ag nanoparticles synthesis, including Ag ions chemical reduction in aqueous solutions with or without stabilizing agents [
Particularly, the electrochemical techniques are quite interesting because they allow obtaining particles with a high purity using fast and simple procedures and controlling the particle size easily by adjusting the current density [
Different stabilizers have been used in electrochemical techniques, which include organic monomers as electrostatic stabilizers [
In this work we present an electrochemical method to prepare silver nanoparticles using polyethylene glycol as stabilizer. We study the effect of the stabilizer concentration, polymer chain length, and current density over the particle size and shape. These are common strategies for the control of the particle morphology, but they have dissimilar behavior at different systems, mainly the stabilizer concentration. Also, the use of a polymeric stabilizer allows the introduction of a potential tool to reinforce the control of the size and shape of the nanoparticles by using different polymer chains lengths and so we study this strategy too.
Silver nanoparticles were prepared by electrochemical reduction within a simple two-electrode type cell. The volume of the electrolysis cell was 50 ml. A platinum sheet (
In a typical synthesis, the reaction medium was prepared to obtain an aqueous solution of silver nitrate 2.5 mM with 0.5 to 2% w/v of PEG. The solution was mixed and purged with N2 during 20 minutes. Then, the electrolysis was carried out under ultrasonication at constant current and N2 atmosphere during 30 minutes. The current chosen (7 mA, 10 mA, or 13 mA) was given by adjusting the applied potential.
In order to study the optical behavior of silver colloid, ultraviolet-visible (UV-Vis) absorption spectra of different samples were recorded by a Jasco V-530 UV-Vis spectrophotometer using as reference a corresponding blank sample.
The shapes and sizes of the nanoparticles were determined by Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). AFM images were taken using NanoTec ELECTRONICA equipment in tapping mode configuration with a Si3N4 tip in air at room temperature. In order to prepare the sample for the AFM study, the particles were redissolved in ethanol and drop-cast onto a freshly peeled HOPG substrate. TEM images were taken with Philips EM 301 equipment. Samples were prepared by placing a drop over a TEM grid and leaving it to evaporate the solvent. To study the crystalline structure of the nanoparticles, X-ray Diffraction patterns were recorded by grazing incidence with a 2° incidence angle with an X’Pert Phillips PW 1700 diffractometer using CuK radiation (1.5405 Å) and a graphite monochromator (the step size was of
The cyclic voltammetric measurements (CV) were performed with an EG&G potentiostat/galvanostat in a conventional three-electrode cell at room temperature. A platinum rod embedded in epoxy resin was used as a working electrode and only its cross section was allowed to contact the electrolytes; a platinum microelectrode and a saturated Ag/AgCl electrode were used as the auxiliary electrode and the reference electrode, respectively.
The reaction mixture was characterized in different stages of the reaction progress by UV-Vis spectroscopy. Figure
Absorption spectra of silver nanoparticles prepared using PEG-2000 at a concentration of 1% w/v with a current equal to 10 mA. Initial spectrum (dotted line) and spectra after 15 (dashed line) and 30 minutes of reaction (solid line).
In the electrochemical synthesis of silver nanoparticles, there is a competition between two different cathode surface processes: the silver particle formation and the silver deposition on cathode [
Cyclic voltammograms of 2.5 mM AgNO3 aqueous solutions with 1% w/v (dashed line), 2% w/v (dotted line), and without PEG-2000 (solid line).
The nanoparticles were characterized by XRD, AFM microscopy, and UV-Vis spectroscopy. In Figure
X-Ray Diffraction pattern of silver nanoparticles. The principal position peaks are shown in the figure.
In order to determine the size and shape of the particles, AFM images were registered. Figure
AFM image of PEG protected silver nanoparticles (a) and the corresponding size distribution analysis (b).
TEM images (Figure
TEM images of PEG protected silver nanoparticles.
Furthermore, the morphological characteristics of the particles during different experiments stages were followed by the study of the UV-Vis spectra taking into account the knowing relations between the number and position of the absorption bands and the size and shape of the particles [
In metallic nanoparticles, the size dependence of the surface plasmon absorption is not easily explained as in the case of semiconductor nanoparticles, where a blue shift or a red shift of the absorption onset undoubtedly results from decreasing size or increasing size, respectively.
The peak position of surface resonance is not well suited for discussion of size effect within the intrinsic size region in metallic nanoparticles [
In order to determine the influence of the current density, PEG concentration and polymer chain length on the size and shape of the Ag nanoparticles several experiments were performed to study one by one each parameter of the experimental conditions. After each experiment, a UV-Vis spectrum was taken to analyze the number and position of the absorption bands and then to infer the nanoparticles shape and relative size.
Reetz and Helbig determined that the particle size of Pd clusters obtained by electrochemical reduction can be controlled by variation of the current density [
UV-Vis spectra of Ag nanoparticles obtained by using different current density during the synthesis process. From the bottom to the top: 7 mA, 10 mA and 13 mA. In the figure, the plasmon position is indicated for each spectrum.
The influence of PEG-2000 concentration on the silver particle size was studied through UV-Vis spectroscopy. The synthesis was repeated using PEG-2000 concentration of 0.5, 1, 1.5, and 2% w/v. Figure
UV-Vis spectra of Ag nanoparticles obtained by using different polymer concentrations of PEG-2000 during the synthesis process. From the bottom to the top: 0.5, 1.0, 1.5, and 2.0% w/v. In the figure, the plasmon position is indicated for each spectrum.
A displacement of the maximum of absorption was observed towards the blue with the increase of the concentration of PEG-2000 in the range between 0.5 and 2% w/v. Then, increasing the PEG concentration in the reaction medium, the particle size obtained decreases. Then, it is concluded that it is possible to reinforce the control over the particle size by adjusting the PEG-2000 concentration. This behavior was similar when the same experiment was repeated using PEG-6000 in the range between 1 and 2% w/v (Figure
UV-Vis spectra of Ag nanoparticles obtained by using different polymer concentration of PEG-6000 during the synthesis process. From the bottom to the top: 0.5, 1.0, 1.5, and 2.0% w/v. In the figure, the plasmon position is indicated for each spectrum.
UV-Vis spectra of Ag nanoparticles obtained by using different polymer concentration of PEG-600 during the synthesis. From the bottom to the top: 0.5, 1.0, 1.5, and 2.0% w/v. In the figure, the plasmon position is indicated for each spectrum.
In some cases, when polymeric stabilizers are used, just by changing the chain length of the stabilizer, variations in the form of particles can be produced [
UV-Vis spectra of Ag nanoparticles obtained by using different polymer length during the synthesis process. From the bottom to the top: PEG-6000, PEG-1450, PEG-2000, and PEG-400. In the figure, the plasmon position is indicated for each spectrum.
With all the studies shown during these investigations we obtained stable nanoparticles using PEG as stabilizer and we found the correct experimental parameters that control the morphology. These nanoparticles have the possibility to be used for several purposes, such as bactericide, because they could be manipulated as a powder.
Spherical silver nanoparticles were synthesized by an electrochemical method with PEG as stabilizer. This method is a rapid and eco-friendly technique to obtain silver nanoparticles. PEG is an efficient stabilizer that favours the particle formation over silver deposition on the cathode. The influence of synthesis conditions was studied. The current density is an adequate parameter to control particle size. The PEG chain length does not show effects on the shape of the nanoparticles obtained, which are spherical for all the cases. Nevertheless, with the longer polymer chain length it is possible to control the particle size through the PEG concentration. Thus, the use of polymeric stabilizer of long chain adds an additional tool to control the particle size with respect to other electrochemistry techniques.
The authors thank UNR and CONICET for their institutional and financial support. This work was performed principally with the aid of Grant PICT 20466 from Agencia Nacional de Promoción Científica y Tecnológica, Argentina. The authors thank Dr. Claudia Palopoli (Ph.D.) for the CV measurements and discussion.