Semiconductor nano-particles of CdS (about 2 nm) with a hexagonal phase have been prepared at a relatively low temperature (60 C) and short time in micro-emulsion (O/W) under ultrasound. This study presents the effects of ultrasonic irradiation on the formation of CdS nano-particles in micro-emulsion and compares the results with samples prepared without sonication. The nano-particles have been characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energy dispersed analysis of X-ray (EDAX), UV–visible spectroscopy, and surface area measurements (BET). The effect of some important factors such as sonication time, intensity of ultrasound, temperature, and oil fraction was studied on the prepared nano-particles. The particle size can be controlled by applying the ultrasonic waves on the micro-emulsion under proper conditions. It has been postulated that bubble collapse generates high temperature and many nucleation sites which lead to the uniform spherical particles with small size and fast transition phase. Further evidence was obtained by UV–visible absorption and a photoluminescence spectrum. The study of optical properties reveals that the band-gap of the CdS nano-particle decreases with increase of sonication time and reach an approximate constant value. It was also observed that nano-particles grew faster at the beginning of sonication and then reached an almost constant value. The growth of nano-particles at different intervals during sonication was followed by UV– visible spectroscopy.

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