Silver nanoparticles as pigments for water-based ink-jet inks

Stabilized concentrated citrate-reduced silver nanocolloids for use as pigments in ink-jet inks were prepared. Carboxymethyl cellulose was used as a polymeric stabilizer providing both electrostatic and steric stabilization. X-ray diffraction pattern, optical properties (UV-visible spectroscopy), size (TEM and dynamic light scattering), and zeta potentials of the nanoparticles were studied. It was shown that the product is silver with cubic symmetry. Absorption spectra are characterized, as a rule, by asymmetric absorption bands with maxima at 417-440 nm and shoulders at 350-352 and 380-382 nm. TEM images of unstabilized and stabilized colloids indicate the formation of nanoparticles of different shapes (spheres, hexagons, cubes, and rods) with rather wide size distribution in the range from several nanometers (spheres) up to several hundreds of nanometers (rods). CMC was found to be an effective stabilizer of silver nanoparticles, and the average particle size at CMC concentrations from 0.025 to 0.2 wt % does not exceed 50 nm. Both unstabilized and stabilized silver nanoparticles display negative zeta potentials in the pH range from 2 to 9; the maximal negative values are observed at pH 6-8 (-27 +/- 5 mV for unstabilized and -33 +/- 5 mV for CMC-stabilized colloids, respectively). Concentrated dispersions of silver nanoparticles (1.1 wt % of silver), which were stable for at least 7 months, were prepared by exhausted lyophilization of the freshly prepared colloids followed by redispersion. These nanoparticles in the presence of proper wetting agent, such as Disperbyk, can be used as pigments in ink-jet ink formulations for printing on various substrates (paper, glass, and transparencies). The stabilizing agent, carboxymethyl cellulose, also acts as a binder, providing good adhesion of ink to the substrates.