In the ultrasonic field, stable silver colloids were produced by the reduction of AgNO 3 with the protection of PVP using KBH 4 or N 2 H 4 ·H 2 O as reductant. The main factors affecting the morphology of silver nanoparticles, such as distribution of the ultrasonic field, ultrasonic time, ultrasonic power, and the species of reductant, were studied. The silver colloids were identified by TEM and spectrophotometry. The results indicate that the factors such as distribution of the ultrasonic field, ultrasonic time, ultrasonic power, and the species of reductant have a great impact on the morphology of the silver nanoparticles. The size of the silver nanoparticles decreases with the ultrasonic power and ultrasonic time increasing. Ag nanoparticles prepared in standing wave field preferentially grow in a certain direction, which is propitious for forming hexagonal-and spherical-like silver nanoparticles. Monodispersed spherical silver nanoparticles are easily synthesized in the diffusion field. The stability of silver colloid becomes improved by ultrasonic treatment. For example, precipitate is not found after several weeks for the silver colloid prepared with an ultrasonic treatment time of 180 min. The silver nanoparticles prepared without ultrasonic treatment are large spherical-like and hexagonal. Well-dispersed spherical silver particles with a mean size of about 20 nm have been prepared under ultrasonic treatment. Spherical, spherical-like, and hexagonal silver nanoparticles can be obtained by changing the reductants..Keywords ultrasonic, standing wave field, diffusion field, silver colloid Colloidal precious metals refer to the dispersion of precious metal particles with sizes ranging from 1 to 100 nm in fluids. These colloidal particles are small in radius, but big in surface area, so they present many unique properties, like special conductive, antistatic, photographic, antimagnetism, and catalytic behaviors [1]. The colloids have wide applications in a variety of industrial fields, such as, optical material, precious metal slurry, biological engineering, medicine, catalyst and micro-electronics [2]. The preparation and application of the precious metal nanoparticles with macromolecular protection has been an attractive research area. The dispersion of the nanosized particles is a colloidal solution. When utilized for catalytic oxidization of ethylene, dispersion of silver nanoparticles exhibits higher catalytic activity and selectivity than conventional silver powder [3]. The conductivity and uniform dispersity of silver colloids are used in developing antistatic coating, conductive coating, and additives in emerging materials with new functions, showing a broad application prospect in the future.Silver colloid with macromolecule protection can be prepared by chemical reduction [4−7] or light radiation [8−9]. However, no literature is available on its preparation in ultrasonic field. This paper describes the preparation of individually dispersed silver colloids with various morphological features through r...

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