Formation and Stabilization of Silver Nanoparticles through Reduction by N,N-Dimethylformamide

Abstract

The reduction of Ag + ions in N,N-dimethylformamide (DMF) is shown to take place spontaneously at room temperature. When no other additives are present in the system, the slow reduction leads to silver deposition on the glass walls of the container, while in the presence of 3-aminopropyltrimethoxysilane (APS) stable dispersions of silver nanoparticles are obtained. Even though the reduction can be performed at room temperature, higher temperatures markedly increase the reaction rate and improve the monodispersity of the colloid. The reduction rate and morphology of the colloidal particles also depend on the ratio [Ag]/[APS]. The reduction takes place both with nitrate and perchlorate as counterions, which confirms that DMF plays the role of a reducing agent. Additionally, when excess APS is used, a thin, homogeneous silica shell is formed on the silver particle surface.One of the main applications of metal nanoparticles is found in catalysis. 1,2 Since most of the reactions to be catalyzed take place in organic solvents, it is desirable to design synthetic methods which lead to the stabilization of metal nanoparticles in such solvents. This can be accomplished through the reduction of the metallic salts or complexes "in situ", or by transfer of aqueous colloids into the organic phase, 3,4 normally after surface modification. 5-7 In both cases, the reduction can be driven by a chemical reducing agent, 8-11 an electron scavenger for the formation of radicals promoted by γ, 12 UV, 13 or ultrasound 14 irradiation, or a thermal method such as prolonged reflux. 15 N,N-Dimethylformamide (DMF) is one of the standard organic compounds used as a solvent for various processes, including the preparation of colloids 16-18 which usually contain metals in their composition. The oxidation of DMF has been studied with respect to the production of hydrogen from water-DMF mixtures 19 and even for the reduction of Ni(IV) to Ni(II) in alkaline medium. 20 These studies show that DMF can be an active reducing agent under suitable conditions. We show here the ability of DMF to reduce Ag + ions to the zerovalent metal, even at room temperature and in the absence of any external agent. Such a redox process should be taken into account when handling solutions of metal ions in DMF, because it can also be of importance for metals other than silver. Furthermore, by means of a silane coupling agent, 21 dispersions of silver nanoparticles can be prepared which remain stable for months. We have also observed that the relative amount of this coupling agent, as well as the temperature, influences noticeably both the reaction rate and the morphology of the colloids obtained. Experimental SectionMaterials. AgNO3, AgClO4, and 3-(aminopropyl)trimethoxysilane (APS) were purchased from Aldrich. N,N-Dimethylformamide (DMF) was Fluka (Pure grade). Ethyl alcohol was Scharlau (Pure Grade). All chemicals were used as received. For the preparation of aqueous solutions, Milli-Q deionized water (18 MΩ) was used.Particle Preparation. Simple addition of aqueous...

A facile one-pot synthesis of blue and red luminescent thiol stabilized gold nanoclusters: a thorough optical and microscopy study

“…N,N 0 -Dimethylformamide (DMF) was used as a solvent as well as a weak reducing agent. It is already known from other reports 24,25 that DMF is able to act as a mild reducing agent at high temperature. In particular, the addition of a gold salt solution into a pre-heated DMF was observed here to perform as a redox reaction that predominantly yielded AuNC and a little or no formation of larger gold nanoparticles and bulk metals as by-products is observed.…”

“…In many previous reports, 24,25 it was believed that, during the process, dimethylformamide (DMF) oxidizes to carbamic acid which in turn at high temperature can easily decompose to dimethyl amine with the evolution of carbon dioxide gas. …”

“…11 Following this seminal work, various methods have been developed so far to synthesize monodisperse gold nanocluster of definite particle sizes by controlling the initial reaction conditions such as the thiol-to-gold ion ratio, reducing agents, temperature and also by modifying the post-synthesis purification processes. Several well defined nanoclusters have been synthesized so far such as Au 20 (SR) 16 , Au 25 (SR) 18 , Au 38 (SR) 24 , Au 102 (SR) 44 , and Au 144 (SR) 60 . [12][13][14][15][16][17] Among all these thiolate protected gold nanocluster, Au 25 (SR) 18 and Au 144 (SR) 60 are most widely studied due to their higher stability in comparison to other thiolated gold nanocluster.…”

“…The synthesis of noble metal nanocluster without the use of external reducing agents was demonstrated in the recent past by Liz-Marzán and co-workers using N,N 0 -dimethylformamide (DMF) as both a mild reducing and stabilizing agent. 24 However, they only succeeded in producing non-luminescent nanoparticles with sizes larger than 5 nm. These noble metal nanoparticles exhibit intense surface plasmon bands in the UV-Vis absorption spectrum.…”

Synthesis of Silver Nanodisks Using Polystyrene Mesospheres as Templates

“…Colloidal Ag particles prepared using DMF are usually spherical with a plasmon absorption band centered at 415 nm. 17,18 When colloidal polystyrene is included in the preparation, a set of color changes is observed. The final UV-vis spectrum (Figure 1) shows bands at λ max ) 340 (weak) and 574 (strong), with other peaks between 430 and 470 nm.…”