Synthesis of copper nanoparticles catalyzed by pre-formed silver nanoparticles
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- Grouchko, M., Kamyshny, A., Ben-Ami, K. et al. J Nanopart Res (2009) 11: 713. doi:10.1007/s11051-007-9324-5
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Synthesis of well dispersed copper nanoparticles was achieved by reduction of copper nitrate in aqueous solution using hydrazine monohydrate as a reducer in the presence of preformed silver nanoparticles as catalysts. It has been demonstrated that addition of silver nanoparticles to the reaction mixture leads to formation of aqueous dispersion of copper nanoparticles and also results in a drastic reduction in reaction time compared to procedures reported in the literature. The absorption spectrum of the dispersions, HR-TEM and STEM images and XRD pattern indicate the formation of copper nanoparticles with particle size in the range of 5–50 nm.
Copper nanoparticles attracted considerable attention because of their catalytic (Dhas et al. 1998; Vitulli et al. 2002), optical (Huang et al. 1997; Liz-Marzan 2004), and conducting properties (Liu and Bando 2003) originating from an extremely large ratio of surface area to volume and quantum confinement effect. Their synthesis has been achieved via various reduction routes, including radiation methods (Joshi et al. 1998), microemulsion techniques (Lisiecki and Pileni 1993; Pileni et al. 1999) , sonochemical reduction (Kumar et al. 2001), laser ablation (Yeh et al.1999), vacuum vapor deposition (Liz-Marzan 2004), chemical reduction (Wu and Chen 2004; Wei et al. 2005; Wang et al. 2006) etc.
Chemical reduction is one of the most convenient methods for synthesis of metallic nanoparticles, since it yields a large variety of dispersions in terms of their particle characteristics (size, morphology, stability) by varying the different experimental parameters (concentrations, redox potentials (Kamyshny and Magdassi 2006; Goia 2004), temperature, pH (Sergeev 2001) etc).
However, synthesis of stable copper nanoparticles brings difficulties caused by the relatively low Cuo/Cu2+ redox potential (+0.34 V) and by spontaneous oxidation of the nanoparticles at ambient conditions (Xia et al. 2006). For the reduction to take place, the use of strong reducing agents such as hydrazine, sodium borohydride, and hydrogen, in very large excess is required, but the kinetics of nanoparticle formation is still very slow, the exhaustive reduction of precursor ions requires one to several hours (Wu and Chen 2004; Wei et al. 2005). For example, Wu and Chen (Wu and Chen 2004) reported that the reduction of cupric chloride by hydrazine in the presence of ammonia as a complexing agent takes two hours, in spite of the large excess of hydrazine.
To protect copper nanoparticles against oxidation during preparation and storage, in many cases the reaction is performed in nonaqueous media, at low precursor concentration, and under inert atmosphere (Christopher et al. 2003; Hirai et al. 1986).
The reduction of copper ions in solution for coating of a solid substrate by the electroless process is well known. The reduction proceeds while the reaction is catalyzed by a metal, mainly palladium, which is present on the substrate surface, and does not proceed in solution (Shukla and Seal 2003).
Although the catalytic activity of metal nanoparticles in homogeneous and heterogeneous reactions is now well known (Kamyshny and Magdassi 2006), little has been reported on nanoparticle-catalyzed formation of metallic nanoparticles.
Recently, Zhong et al. studied the formation of gold nanoparticles by the reduction of AuCl4− in aqueous solution in the presence of pre-synthesized Pt nanoparticles (Njoki et al. 2006). The measurement of changes in the surface plasmon peak of gold nanoparticles allowed them to assess the reaction kinetics. After comparing the formation rates of gold nanoparticles catalyzed by Pt nanoparticles in the presence and in the absence of hydrogen (H2), it was concluded that the reaction involves the mediation of H species pre-adsorbed on a Pt nanocrystal surface (Pt-H).
Here we report the preparation of copper nanoparticles dispersed in aqueous solution, by reduction of cupric nitrate with hydrazine in presence of dispersed silver nanoparticles as catalysts. We found that the rate of copper nanoparticle formation drastically increases in the presence of silver nanoparticles, while without the silver nanoparticles only a coating of the reaction vessel surface is observed.
Dispersions of silver nanoparticles to be used as catalyst (1 mM) with a particle size in the range of 4–15 nm (according to dynamic light scattering) were synthesized by the reduction of silver nitrate with tri-sodium citrate as described previously (Magdassi et al. 2003).
Copper nanoparticles were obtained by reduction of cupric nitrate (Aldrich) by hydrazine monohydrate (Sigma-Aldrich) in aqueous solution in the presence of a polymeric stabilizer, polyacrylic acid sodium salt with MW of 15,000 (Sokalan PA 40, BASF) while stirring at room temperature. Silver nanoparticles were added to the copper nitrate solution before the addition of hydrazine (a typical procedure was performed by the addition of 0.04 mL 19.6 M N2H4·H2O to an aqueous solution composed of 3.63 mL 43 mM Cu(NO3)2·2.5H2O, 0.3 mL 28 wt% Sokalan PA 40 and 6 mL of 1 mM silver nanoparticles).
The concentrations of silver nanoparticles in the reaction mixture were in the range of 0.0–0.8 mM Ag, while copper nitrate concentration remained constant. Since the reaction was performed in a closed vessel, the excess of hydrazine protects the copper nanoparticles against oxidation (N2H4 + O2 --> 2H2O + N2).
It was found that in the absence of silver nanoparticles, copper nanoparticles were not formed and only a copper deposition on the walls of the reaction vessel was observed (after 2–4 h). However, addition of silver nanoparticles at a concentration as low as 0.18 mM (Ag/Cu atomic ratio = 0.011) yielded a dispersion of copper nanoparticles, while copper deposition on the walls of a reaction vessel was not observed. It was also found that by using this procedure, a very significant decrease in the reaction duration can be achieved compared to procedures reported in the literature.
The obtained dispersions of copper nanoparticles are characterized by a red wine color and have a characteristic absorption peak at ∼560 nm due to the plasmon effect. The evolution of this peak with the reaction duration as presented for a typical reaction in Fig. 1b may be used to follow the reaction kinetics. The appearance of plasmon band 10 min after hydrazine addition indicates the rapid formation of the copper nanoparticles. It was found that the peak reaches its maximal value about 30 min after the reaction was initiated. Despite the absence of the silver plasmon peak around 400 nm due to the very low concentration of silver nanoparticles (the silver plasmon band intensity at the highest concentration of silver nanoparticles is lower than 0.05), STEM analyses reveal the presence of individual silver nanoparticles with an average diameter of 10 nm; according to quantitative STEM analyses performed on typical field view, the average Ag/Cu atomic ratio is 0.05, which is close to the expected atomic ratio (Ag/Cu = 0.028 atomic ratio) according to the initial concentrations of silver and copper.
The effect of catalyst addition was also evaluated at reaction condition similar to that described previously by Wu and Chen (Wu and Chen 2004) in the presence of ammonia. We found that in this case, the addition of silver nanoparticles to the reaction mixture also resulted in rapid formation of copper nanoparticles. For example, the reaction duration decreased from 2 h in the absence of silver nanoparticles to 10 min in the presence of silver nanoparticles at atomic ratio Ag/Cu = 0.1.
In conclusion, we have shown that silver nanoparticles noticeably accelerate the formation of copper nanoparticles by reduction of cupric nitrate in aqueous solution. We assume that the silver nanoparticles act as catalysts in this reaction. The mechanism of metal catalyzed formation of copper nanoparticles is now under investigation.
This project was supported by the European Community Sixth Framework Program through a STREP grant to the SELECTNANO Consortium, Contract No.516922.03/25/2005. We would like to express thanks to our partner, the BASF Polymer Research Laboratories for supplying Sokalan PA 40.