Non-toxic complexing agent Tri-sodium citrate's effect on chemical bath deposited ZnS thin films and its growth mechanism

This study demonstrates the growth and characterizations of chemical bath deposited zinc sulfide (ZnS) thin films prepared at pH 10. Aqueous zinc acetate and thiourea were used as precursors along with the non-toxic complexing agent, Na ₃-citrate. The effects of different concentrations of Na ₃-citrate from 0 to 0.2 M on the structural, morphological, compositional, chemical, and optical properties of ZnS thin films were studied. It was revealed through field emission scanning electron microscopy studies that an increase in the concentration of Na ₃-citrate leads to an improvement of the uniformity of the ZnS thin films and decrease in the grain size. Atomic force microscopy showed that the RMS value decreases with an increase in Na ₃-citrate concentration. X-ray diffraction study revealed that crystallinity of ZnS thin films improves upon increasing concentration of Na ₃-citrate and that the films exhibit a hexagonal polycrystalline ZnS phase while deposited with 0.2 and 0.1 M Na ₃-citrate. X-ray photoelectron spectroscopy revealed that the signal intensity decreases for Zn 2p _3/2 and S 2p _1/2 as the concentration of Na ₃-citrate decreases from 0.2 to 0 M. It was shown by ultraviolet-visible spectroscopy that approximately 80% transmission in the visible region and absorption edge shifts towards blue when the concentration of Na ₃-citrate increases from 0 to 0.2 M. The band gap energy of the ZnS film deposited without Na ₃-citrate was found to be 3.53 eV, while it increases from 3.73 to 3.80 eV with a decrease in Na ₃-citrate concentration from 0.2 to 0.025 M. The growth mechanism of CBD-ZnS thin film was found to be dependent on Na ₃-citrate concentration.