OTHMAN A.A. Assiut university

Thermal Annealing Induced Phase Transition and UV Irradiation Induced Brightening in CdS Nanoparticles

Co-authors OSMAN M.A., RAHIM Ahmed G. Abdel

CdS nanoparticles were synthesized by co-precipitation method using cadmium acetate Cd (CH3COO)2 and sodium sulfide (Na2S) as ionic precursors. Samples were annealed in air for 3 h in steps of 100 °C in the temperature range, 200 –700 °C in order to study the effect of annealing on the structural and optical properties of CdS nanoparticles and the influence of UV photo-induced effect on the optical properties of the synthesized nanoparticles. CdS nanoparticles are characterized by X-ray diffraction, UV-vis absorption spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The increase in the average crystallite size from 3 to 23 nm as a result of annealing has been estimated from the broadening of x-ray diffraction lines. Annealing induced phase transition from the as-synthesized cubic phase to the hexagonal phase after annealing at Ta = 300 oC, as well as the transition into CdSO3 monoclinic phase after annealing at Ta = 600oC were discussed. Analysis of the UV-Vis optical absorption spectra refers to the validity of direct allowed transition with remarkable decrease in the direct band gap E_g^opt from 3.3 to 2.4 eV at 700 oC with increasing Ta as a result of enhancement of crystallinity and increase in particle size which in turn leads to the reduction of quantum confinement effect. UV irradiation of CdS aqueous solution induces photo-brightening ,i.e increase in both of optical transmission and energy gap .The observed photo-brightening can be interpreted in terms of nanoparticles surface passivation , due to UV photo induced oxidation of CdS nanoparticles.