Electrochemical method for the synthesis of silver nanoparticles
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- Khaydarov, R.A., Khaydarov, R.R., Gapurova, O. et al. J Nanopart Res (2009) 11: 1193. doi:10.1007/s11051-008-9513-x
The article deals with a novel electrochemical method of preparing long-lived silver nanoparticles suspended in aqueous solution as well as silver powders. The method does not involve the use of any chemical stabilising agents. The morphology of the silver nanoparticles obtained was studied using transmission electron microscopy, scanning electron microscopy, atomic force microscopy and dynamic light scattering measurements. Silver nanoparticles suspended in water solution that were produced by the present technique are nearly spherical and their size distribution lies in the range of 2 to 20 nm, the average size being about 7 nm. Silver nanoparticles synthesised by the proposed method were sufficiently stable for more than 7 years even under ambient conditions. Silver crystal growth on the surface of the cathode in the electrochemical process used was shown to result in micron-sized structures consisting of agglomerated silver nanoparticles with the sizes below 40 nm.
The last decade has seen the development of a hoist of different methods for the synthesis and characterization of metal nanoparticles and powders thereof (Feldheim and Foss 2002). Owing to their properties being distinctly different from those of the bulk metal, nanoparticles are finding their way into various areas of science and technology (Mazzola 2003; Anselmann 2001; Bönnemann and Richards 2001; Biswas and Wu 2005). In particular, nanoparticles of silver have attracted much attention because of a broad range of their possible applications including medicine (Salata 2004), catalysis (Lewis 1993), textile engineering (Lee and Jeong 2005), biotechnology and bioengineering (Niemeyer 2001), water treatment (Solov’ev et al. 2007), electronics (Li et al. 2005) and optics (Murphy et al. 2005). Many approaches were developed to obtain silver nanoparticles of various shapes and sizes, including laser ablation (Lee et al. 2001), gamma irradiation (Long et al. 2007), electron irradiation (Bogle et al. 2006), chemical reduction by inorganic and organic reducing agents (Bönnemann and Richards 2001), photochemical method (Mallick et al. 2004), microwave processing (Yin et al. 2004), and thermal decomposition of silver oxalate in water and in ethylene glycol (Navaladian et al. 2007). Reetz and Helbig (1994) were the first to describe in detail an electrochemical technique for the synthesis of nanoparticles, in which a metal sheet was anodically dissolved and the intermediate metal salt formed was reduced at the cathode, giving rise to metallic particles stabilised by tetraalkylammonium salts. This work was successfully adopted for the electrochemical synthesis of the silver nanoparticles in acetonitrile containing tetrabutylammonium salts by Rodríguez-Sánchez et al. (2000). Using a similar approach, silver nanoparticles were obtained by potentiostatic or galvanostatic polarisation of silver in ethanol solution by Starowicz et al. 2006). In the work of Yin et al. (2003), Poly(N-vinyl-2-pyrrolidone) (PVP) was essential in the process of electrochemical synthesis of silver particles. The main advantages of the electrochemical methods lie in the high purity of particles and the possibility of the nanoparticle size control by adjusting the current density without a need for expensive equipment or vacuum. The key to the success of electrochemical methods is the right choice of the chemical agents and the process conditions. However, as pointed out earlier (Rodríguez-Sánchez et al. 2000), the method has its limitations, as the deposition of silver on the cathode during the electrochemical process diminishes the effective surface available for particle production. As the entire cathode surface gets covered with the silver electrodeposits, the particle production comes to a halt altogether.
An important issue in the current search for simpler and cost-effective methods for the synthesis of nanoparticles is finding ways to avoid the use of stabilising agents in the electrochemical processes involved. In this article, we present a novel electrochemical method for preparing long-lived silver nanoparticles suspended in aqueous solutions as well as silver powders deposited on the electrodes, which does not involve any chemical stabilising agents.
Materials and methods
The proposed process for obtaining silver nanoparticles is based on the use of an inexpensive two-electrode setup in which the anode and the cathode are made from the bulk Ag metal to be transformed into Ag colloidal particles. We employed two polished silver plates (85 mm × 20 mm × 4 mm) as the anode and the cathode, being vertically placed face-to-face 10 mm apart. The electrodes were immersed in an electrochemical cell filled with 500 mL of distilled water obtained from an ordinary commercially available water distiller (DE-25, Russia). Electrolysis was performed in the temperature range 20–95 °C at a constant voltage of 20 V. Additional technological keys to the electrochemical synthesis of silver nanoparticles lie in changing the polarity of the direct current between the electrodes every 30–300 s, and intensive stirring during the process of electrolysis to inhibit the formation of precipitates. The silver nanoparticle solutions produced in this way were stored under ambient conditions in glass containers.
The concentration of the silver nanoparticles in solutions was determined by neutron activation analysis (Soete et al. 1972). The samples were irradiated in the nuclear reactor of the Institute of Nuclear Physics (Tashkent, Uzbekistan). The product of the nuclear reaction 109Ag(n,γ)110mAg has the half-life T1/2 = 253 days. The silver concentration was determined from the measurements of the intensity of gamma radiation with the energy of 0.657 MeV and 0.884 MeV emitted by 110mAg. A Ge(Li) detector with a resolution of about 1.9 keV at 1.33 MeV and a 6144-channel analyser were used for recording gamma-ray quanta.
Dynamic light scattering (DLS) measurements were carried out at 25 °C using a Zetasizer (ZEN3600, Malvern Instruments, UK) to estimate the size distribution of silver nanoparticles in solutions. The morphology of silver powder deposited on the surface of the cathode during the electrochemical process was observed by field emission scanning electron microscopy (FE-SEM; JSM-6700F, JEOL, Japan). The size and shape of the nanoparticles in solution were determined by transmission electron microscopy (TEM) (LEO-912-OMEGA, Carl Zeiss, Germany). Energy dispersive X-ray spectrometer (EDS) attached to the TEM was used to determine the chemical composition of the samples. The size values were averaged over more than 150 nanoparticles from different TEM micrographs of the same sample.
The silver nanoparticles suspended in aqueous solutions were also imaged by atomic force microscopy (AFM) (Solver P47Bio, NT-MDT Co., Russia) in contact mode using freshly cleaved mica surfaces as substrates.
Results and discussion
As distinct from most non-electrochemical methods (Lee et al. 2001; Long et al. 2007; Bogle et al. 2006; Mallick et al. 2004; Yin et al. 2004; Navaladian et al. 2007), the proposed approach of electrochemical synthesis does not require the use of vacuum chambers, sophisticated preparation or expensive equipment. The process of synthesis proposed here involves three major stages, which are described below.
Stage I: formation of a colloidal solution of silver nanoparticles
Oxidative dissolution of the sacrificial Ag anode:
Release of the oxygen gas:
Ag+ ions migration to the cathode;
Reductive formation of zero-valent Ag atoms on the cathode:
Formation of silver nanoparticles via nucleation and growth due to attractive van der Waals forces between Ag atoms;
Separation of synthesised silver nanoparticles from the cathode and their further migration caused by vigorous stirring of the solution.
The solutions were tested for sizes of suspended silver nanoparticles as soon as current saturation was reached. TEM and DLS studies demonstrated that the average size of silver nanoparticles in the colloidal solution decreases with increase in the current density, as was shown in (Reetz and Helbig 1994) theoretically. The rate of the reaction was shown to increase with decrease in the distance between the electrodes and increase in the voltage. A longer reaction time for each individual trial resulted in a larger average size and a higher concentration of silver nanoparticles. In order to obtain stable silver nanoparticles with a concentration in the range of 20 to 40 mg/L, it was necessary to ensure a reaction time of 50–70 min in the temperature range of 50 to 80 °C.
Stage II: filtration
Stage III: additional treatment
Mean size of silver nanoparticles from different colloidal solution samples
Mean diameter (nm)
Sample A + PVP
Sample B + PVP
24 ± 12
24 ± 12
7 ± 3
7 ± 3
26 ± 12
24 ± 12
9 ± 4
8 ± 4
31 ± 15
26 ± 12
10 ± 4
9 ± 4
40 ± 17
28 ± 13
13 ± 5
11 ± 4
47 ± 20
31 ± 14
19 ± 7
13 ± 5
DLS and TEM measurements showed that an increase in the PVP concentration enhances the stability of colloids. On the other hand, it leads to more viscous colloidal solution potentially complicating the process of impregnating different materials with silver nanoparticles. This effect may be relevant for paints modified with silver nanoparticles, textile fabrics (Jeong et al. 2005; Lee and Jeong 2004) etc., and it needs to be considered in the light of potential industrial applications of the synthesised silver nanoparticles.
Silver nanoparticles obtained in a three-stage process based on the electroreduction of anodically solved silver ions in water have been studied. Various technological keys to improve the output by varying the process conditions have been considered. The morphology of the colloidal silver nanoparticles and the powders deposited on the electrodes was studied using DLS, TEM and SEM measurements. These studies have shown that silver nanoparticles suspended in water solution, which were produced by the present three-stage technique, were nearly spherical, their average size being 7.3 ± 3.1 nm. It was demonstrated by AFM measurements that silver nanoparticles synthesised by the proposed method were sufficiently stable for at least 7 years even under ambient conditions. Furthermore, it was shown that small additions of PVP to silver nanoparticle solutions act to enhance their stability. However, the effect is not very pronounced, and this step does not seem to be critical. The investigation of silver particles grown on the surface of the cathode during the electrochemical process showed that silver powders obtained were ‘bi-modal’: large micron-sized polyhedron-shaped particles and micron-sized silver plates consisting of agglomerated silver nanoparticles with the sizes up to 40 nm. This ‘by-product’ of electrolysis may also be of some use in applications requiring silver powders with a large surface area.
The simplicity of this synthesis route allows low-cost fabrication of large amounts of long-lived silver nanoparticles. No chemical stabilising agents are generally required, which extends the technological viability of the process and industrial applications of the method. We believe that the electrochemical method for the synthesis of silver nanoparticles presented provides an efficient processing route for the fabrication of colloidal solutions of silver nanoparticles in the concentration range of 20 to 40 mg/L or silver nanoparticulates. Depending on whether the process is terminated after Stage II or is continued to include Stage III, the average nanoparticle size of about 20 nm or 7 nm, respectively, can be achieved.
RR Khaydarov acknowledges partial support of this work through the INTAS Fellowship Grant No. 5973 for Young Scientists under the ‘Uzbekistan—INTAS 2006’ program.