In this article, polyaniline (PANI) nanoparticles in the uniform size (3.2-10 
nm in diameter) were successfully prepared by ultrasonic irradiation method 
at different times in the presence of hydrochloric acid as a dopant and 
ammonium peroxydisulfate (APS) as an oxidant. The dependence of particle 
size, morphologies, and crystallinity on the time of ultrasonic irradiation 
reaction was studied using transmission electron microscope (TEM) images, 
and X-ray diffraction (XRD). The data indicated that, the particle size was 
increased with increasing the time of reaction. The vibrational structure and 
electronic transition were studied using FTIR and UV-VIS spectroscopic 
techniques. All data confirmed that, the particle size of polyaniline 
nanoparticle was increased with increasing the time of ultrasonic irradiation 
reaction.