Optical studies of polyvinylpyrrolidone reduction effect on free and complex metal ions
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Polyvinylpyrrolidone (PVP) reduction effect on free and complex Ag+ and Au3+ ions was studied from optical measurements by adding a metal precursor (K-30), commonly used as a stabilizer, to PVP. It was found that PVP has a strong reduction effect on free ionic metal, such as Ag+ ion in AgNO3, but much weaker on complex ionic metals, AuCl4− in HAuCl4 and Ag(NH3)2+ in Ag(NH3)2OH. This is explained based on the coordinative field of polar group in PVP molecules.
I. INTRODUCTION
Since the time when Faraday presented the first preparation method for metal nanoparticles in an aqueous medium,1 a large number of methods have been developed for the synthesis of metal nanoparticles, involving use of different protecting agents, such as polymers and alkanethiol with different alkyl chain lengths.2–6 Among the polymers available, the synthetic polymer polyvinylpyrrolidone (PVP) is widely used because of its chemical stability, avirulence, and dissolubility in many solvents. Due to its affinity toward metals, PVP polymer was often used to obtain stable dispersions of metal (especially Au, Ag and platinum group) nanoparticles synthesized by many methods, such as irradiation, reflux, and addition of chemical reductants.7–14 Generally, the preparation of PVP polymer-stabilized nanoparticles (through chemical methods) involves two processes: reduction of metal ions into neutral atoms, which form clusters, and coordination of the polymer to the metal clusters. In experiments, the focus is generally fastened on the protective effect of PVP by varying its amount (the molar ratios of metal to PVP usually varies from 0.1 to 4). Even if PVP had a reduction effect on metal ions, such an effect was usually ignored (or could not be detected) because in previous works PVP was added to solution as a stabilizer usually together with reducing agents. Recently, we studied the reduction effect of PVP on metal ions (such as Au3+ and Ag+) simply by mixing PVP with the metal ion solutions from optical measurements and found that PVP has a strong reduction effect on the free metal ions but weak on the complex metal ions. The formation amount of Ag or
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0039 320
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 13 Mar 2015
Au in PVP assumes a roughly linear increase with time, and the formation rate of Ag is three orders of magnitude higher than that of Au in PVP. The details are reported in the following.
II. EXPERIMENTAL
PVP-K30 (molecular weight 40,000), AgNO3 and HCl • AuCl3 • H2O were purchased from the Chemical Reagent Company of Shanghai, China, and used as received. Distilled water was used as the solvent. Ag(NH3)2OH was obtained at our lab through the following reactions 2AgNO3 + 2NH4OH → Ag2O + 2NH4NO3 + H2O , Ag2O + 4NH4OH → 2关Ag(NH3兲2兴OH + 3H2O . AgNO3 (0.2 ml) and Ag(NH3)2(OH) aqueous solutions (both [Ag+] and [Ag(NH3)2]+ are 0.1 M/L) were mixed with 400 mg PVP (molar ratio of Ag/PVP is 2). The pH values were
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