Effect of pH on the Optical, Structural, Morphological, and Photocatalytic Properties of CdS Thin Films Prepared by Chemical Bath Deposition.
Haji Sozan A SA, Mohammed Raghad Y RY
Cadmium sulfide (CdS) thin films were deposited by chemical bath deposition (CBD) using cadmium acetate as the cadmium source, thiourea as the sulfur source, ammonium acetate as a buffering agent, and ammonium hydroxide as a complexing agent and pH controller. The bath pH was adjusted to 7, 8, 9, and 10 using NH₄OH solution at 80°C for 30 min; no film formation occurred at pH 6, establishing pH 7-10 as the viable deposition window for this system. X-ray diffraction (XRD) data revealed that the films at pH 7 and 10 are amorphous, whereas films at pH 8 and 9 exhibit mixed cubic zinc blende and hexagonal wurtzite crystal structures. The morphological analysis showed that the average particle sizes increased from 22.35 to 38.8 nm with increasing bath pH from 7 to 9, and decreased at pH 10 to 25.69. The optical bandgap decreased from 2.39 to 2.26 then to 2.18 eV for pH 7, 8, and 9, respectively, consistent with the increase in crystallite size and reduced quantum confinement, before reverting to 2.32 eV at pH 10, alongside a corresponding decrease in film thickness. Photocatalytic degradation of methyl orange (MO) dye under UV irradiation (254 nm, 25 W) demonstrated the highest mean efficiency of 25.31% ± 2.35% (k = 0.0048 ± 0.0005 min-1) at pH 8, based on triplicate testing, attributed to its optimal crystallinity, surface morphology, and favorable bandgap of 2.26 eV. The pH 8 film also retained approximately 90.5% of its initial photocatalytic activity after three reuse cycles, confirming good stability. Unlike previous studies that examine these properties in isolation, this work provides an integrated structural, optical, morphological, compositional, and photocatalytic investigation of pH-dependent CBD-grown CdS thin films, further supported by reusability testing and triplicate reproducibility validation across four pH conditions.