Electrodeposition of CoWP film: III. Effect of pH and temperature

Polarisation behaviour and nucleation mechanism of induced co-deposition of cobalt, tungsten and phosphorus from citrate electrolytes containing cobalt sulphate, sodium tungstate and sodium hypophosphite on copper disc electrode at various electrolyte pH values were studied using cyclic voltammetry and chronoamperometry, respectively. It was found that the onset potentials for reduction and oxidation gradually shifted towards more cathodic direction with increase in pH. Co-deposition of cobalt, tungsten and phosphorus occurred under diffusion control and followed instantaneous type of nucleation mechanism. The highest current efficiency (>70%) was found when the CoWP films were deposited from neutral or slightly acidic solution. Larger variation of film composition, cobalt from 48 to 74 at.%, tungsten from 13 to 38 at.% and phosphorus from 8 to 33 at.%, could be made by changing the pH of the electrolyte between 3 and 8. The films deposited from very acidic solutions contained larger amounts of tungsten and phosphorus and were amorphous; while the films deposited from near neutral solutions were polycrystalline and contained hexagonal cobalt. The surfaces of the amorphous films were smoother than those of the polycrystalline films. Needle-like dendritic crystallites were obtained when the films were deposited from basic solution or at elevated temperature.