Abstract
A comparative study of amine and silver carboxylate adducts [R1COOAg-2(R2NH2)] (R1 = 1, 7, 11; R2 = 8, 12) as a key intermediate in NPs synthesis is carried out via differential scanning calorimetry, solid-state FT-infrared spectroscopy, 13C CP MAS NMR, powder X-ray diffraction and X-ray photoelectron spectroscopy, and various solution NMR spectroscopies (1H and 13C NMR, pulsed field gradient spin-echo NMR, and ROESY). It is proposed that carboxyl moieties in the presence of amine ligands are bound to silver ions via chelating bidentate type of coordination as opposed to bridging bidentate coordination of pure silver carboxylates resulting from the formation of dimeric units. All complexes are packed as lamellar bilayer structures. Silver carboxylate/amine complexes show one first-order melting transition. The evidence presented in this study shows that phase behavior of monovalent metal carboxylates are controlled, mainly, by head group bonding. In solution, insoluble silver salt is stabilized by amine molecules which exist in dynamic equilibrium. Using (bis)amine-silver carboxylate complex as precursor, silver nanoparticles were fabricated. During high-temperature thermolysis, the (bis)amine-carboxylate adduct decomposes to produce silver nanoparticles of small size. NPs are stabilized by strongly interacting carboxylate and trace amounts of amine derived from the silver precursor interacting with carboxylic acid. A corresponding aliphatic amide obtained from silver precursor at high-temperature reaction conditions is not taking part in the stabilization. Combining NMR techniques with FTIR, it was possible to follow an original stabilization mechanism.Graphical abstractThe synthesis of a series (bis)alkylamine silver(I) carboxylate complexes in nonpolar solvents were carried out and fully characterized both in the solid and solution. Carboxyl moieties in the presence of amine ligands are bound to silver ions via chelating bidentate type of coordination. The complexes form layered structures which thermally decompose forming nanoparticles stabilized only by aliphatic carboxylates.
Highlights
Aliphatic carboxylate salts of silver (CnH2n+1COOAg) are well-known precursors for preparation of silver colloids (Abe et al 1998; Wang et al 1999)
Thermolysis of carboxylate metal complex with no use of solvent, stabilizer, or reducing agent should be conducted at high temperatures (~250 °C) since too low temperature leads to a stabilization shell composed of silver ions rather than the fully reduced silver atoms (Shim et al 2008; Szczęsny and Szłyk 2013)
Postulated intermediate amine-silver carboxylate complex was not observed so far; the analogous one was isolated for primary amines
Summary
Aliphatic carboxylate salts of silver (CnH2n+1COOAg) are well-known precursors for preparation of silver colloids (Abe et al 1998; Wang et al 1999). A unique method of preparing Ag NPs was presented by Nakamoto et al, where homologous silver(I) carboxylates and tertiary aliphatic amines in mild reaction conditions result in a good control of silver nanoparticles over the size and distribution (Yamamoto et al 2006). We have shown recently a direct, high yield method for preparing narrow-sized silver nanoparticles by decomposition of silver carboxylate precursor under H2 pressure (3 bar) in a nonpolar solvent at a temperature ~150 °C (Uznanski and Bryszewska 2010). It corresponds to the thermal decomposition of carboxylic acid silver salts at 250 °C, but is faster, reproducible, versatile, and easy to control. Powder X-ray diffraction patterns were collected using a Panalytical X′PERT MPD diffractometer for a 2θ range of 5° to 120° at an angular resolution of 0.05° using Co-Kα (1.7890 Å) radiation
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