Influence of the film thickness on the photovoltaic properties of chemically deposited CdS thin films: Application to the photodegradation of orange II

The influence of the thickness on the optical, structural and morphological properties of CdS thin films elaborated by chemical bath deposition is studied. The deposition time varies between 15 and 75 min and the optical measurements show that the films have high optical transmission (85%). The band gap presents a maximum of 2.44 eV for a deposition time of 30 min which corresponds to a lower disorder. The deposition rate (<4.66 nm/min), indicates that ion by ion reaction dominates the CdS growth and as result, smaller crystallite size and thinner films are obtained. The X-ray diffraction reveals a cubic structure and the crystallite size varies from 14 to 21 nm. The scanning electronic microscopy (SEM) and atomic force microscopy (AFM) show homogeneous and continuous thin layers with small rounded cristallite and surface roughness RMS of about 15 nm for the films deposited for 30 min. The Mott–Schottky characteristic indicates n type conductivity with a flat band potential of −0.95 VSCE and an electron density of 1.43×1025 m−3. The Nyquist plot shows the predominant contribution of the grain boundaries; the data are modeled to an equivalent circuit with a constant phase element. As application, orange II is oxidized via radicals and the best photoactivity is obtained at pH~7. 68% of orange II disappears in ~1 h under solar light (AM 1). The oxidation follows a zero order kinetic with a rate constant of 0.11 mol L−1 min−1.