Atom-by-Atom Growth of CdS Thin Films by an Electrochemical Co-deposition Method: Effects of pH on the Growth Mechanism and Structure

摘要

Thin films of cadmium sulfide (CdS) were prepared on Au(111) by a new electrochemical co-deposition method. The appropriate electrodeposition potentials based on the underpotential deposition potentials (upd) of Cd and S have been determined by the cyclic voltammetric measurements. The films were grown at room temperature from an aqueous solution of CdSO_4, ethylenediaminetetraacetic acid (EDTA), and Na_2S at various pHs. The thin films were characterized by atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray diffraction (XRD), and UV-visible spectroscopy techniques. The Cd/S ratio of electrodeposited thin films was found to be 1:1 determined by electron dispersive spectroscopy (EDS). XRD, STM, and AFM results revealed that the electrosynthesized thin films deposited at -0.520 V (vs Ag/AgCl) have a highly preferential orientation along (110) directions for hexagonal crystal and (111) directions for cubic crystal in the solutions with pH = 4 and pH = 5, respectively. The phase transition from cubic to hexagonal structure was observed with decreasing pH below 5. UV-visible absorption measurements as a function of deposition time indicated that the band gap of the CdS film increases as the deposition time decreases.