The semiconducting properties of CdS nanocrystalline thin films prepared by chemical bath deposition. Application to the eosin photodegradation

Abstract

CdS nanocrystalline thin films were synthesized by chemical bath deposition for different deposition times (10–75 min) and the semiconducting and photo-electrochemical properties were investigated. The results indicate that the deposition rate decreases with the deposition time, due to the depletion of Cd2+ and S2− ions near the glass subtract. The structure is characterized by X-ray diffraction; the films crystallize in a cubic blende structure with (111) as a preferential orientation and the crystallite size varies between 13 and 35 nm. The surface morphology, studied by atomic force microscopy, indicates that the growth process occurs through ion by ion mechanism because of the stability of the complex Cd(NH3)4 (~7), the small deposition rate corroborates this result. The UV visible spectra of the films indicate a high transparency (80%). The transmittance data analysis give a variable forbidden band (2.2–2.4 eV), closely related to the deposition time and agree with the disorder range (230–500 meV). The capacitance–voltage measurement exhibits a linear behavior, characteristic of n type conduction, from which a flat band potential of −0.92 VSCE is determined. The Nyquist plot shows a semicircle assigned to the bulk contribution (=2.5 Ω cm2); the center is located below the real axis (−10°), ascribed to a constant phase element (CPE). A fill factor (FF) of 0.37 and energy conversion of 0.38% are obtained under visible illumination. As application, eosin solution (15 ppm) i oxidized under visible light; the best activity is obtained at pH ~6.3. 80% of eosin disappears after 1 h of exposition and the kinetic obeys to a first order model with a rate constant of 4.5 × 10−2 min−1.