Effect of pH on the size of CdS nanoparticles synthesized by chemical diffusion across a biological membrane

摘要

Results of the studies on the effect of pH on the size of the CdS nanoparticles synthesized in a chicken egg membrane by the diffusion of precursors are presented in this article. The pH of cadmium acetate in aqueous medium was varied by the addition of ammonium hydroxide. The synthesized CdS nanoparticles were characterized by the optical absorption studies. The particle sizes were estimated from the band gap values using Effective Mass Approximation (EMA). CdS formation is not detected in the acidic medium. In the alkaline medium, the sizes as well as the yield of the CdS formed are strongly dependent on the pH. Smallest particles are formed at weak alkaline medium of pH 8 with lowest Yield under any given concentration and reaction time, due to the lowest concentration of S2- ions. At strong alkaline medium of pH 12, again particle sizes and yield are lower than those at pH 10. This is attributed to the reduction of Cd2+ ion concentration due to the increased formation of Cd(OH)(2) at higher pH values. pH 10 is found to be more favorable for the formation of CdS. The excitonic peaks are observed at the conditions leading to the smallest particles. Thus, a narrow size distribution with very small particles is favored either by decreasing the concentration considerably or by changing the pH at suitable intermediate concentrations. Analysis of some of the samples by Transmission Electron Microscopy (TEM) indicates that the CdS particles have sizes in the nanometer range and the particles are formed in the voids of the membrane.