Effects of Additives on Kinetics, Morphologies and Lead-Sensing Property of Electrodeposited Bismuth Films

This study presents a systematic examination of the effects of bath additives and deposition conditions on the rates of electrodeposition of bismuth, obtained morphologies, and the ability of the bismuth films to detect trace concentrations of lead. Novel morphologies of bismuth are reported for the first time. The bath comprises bismuth nitrate, nitric acid, and a set of additives, viz., citric acid (complexant), poly(vinyl alcohol) (surface inhibitor), and betaine (grain refiner). Rotating disk electrode voltammetry and cyclic voltammetry have been used to determine the mechanism and rates of bismuth electrodeposition. Scanning electron microscopy is used to study deposit morphologies, while X-ray diffraction and X-ray photoelectron spectroscopy have been used to examine crystallinity and composition of the deposited thin films. Even in the presence of additives, it is seen that bismuth deposition is diffusion-controlled process with progressive nucleation growth of crystallites, and the reduction is a single-step, three-electron-transfer, quasi-reversible reaction. The films deposited from the bath without additives comprise micrometer-sized, hexagonal rods with controlled aspect ratios (1.83-2.05). Baths containing citric acid produce films with flower-like structures and cracked grains, but with poor adhesion to copper substrate. Introducing poly(vinyl alcohol) significantly slows down bismuth deposition, increases the number of nuclei, produces cauliflower-like crystallites, and promotes adhesion to copper. Betaine smoothens these crystallites while retaining good adhesion. Pulsing the deposition current promotes growth of existing nuclei. In the absence of additives, fused flat disk-type spindles are seen. In the presence of additives, pulsed deposition results in sea-urchin-like morphologies. Adhesion of bismuth onto copper impacts the ability of the film to detect trace concentration of Pb2+ ions in aqueous solutions using anodic stripping voltammetry. The films obtained from baths with additives through direct current plating show the best sensor response for 50 ppb Pb2+.