Pulsed electrodeposition of cobalt nanoparticles on copper: Influence of the operating parameters on size distribution and morphology

Cobalt nanoparticles were synthesized by pulsed electrodeposition on copper substrate. Scanning electron microscopy and image analysis were used to determine morphology and particle size distribution of nanoparticle populations obtained in different operating conditions. After preliminary tests, t_on and t_off were set at 50 and 300 ms respectively to obtain distinct nanoparticles and avoid dendritic structures. Experimental tests were performed according to two partially superimposed factorial designs with two factors at two levels. First factorial design investigated the effect of current density (I =10 and 50 mA/cm²) and discharged cobalt (Q =2.5 x10^-3 and 1.0 x 10^-2C); second factorial design investigated the effect of cobalt concentration (C₀ = 0.01 and 0.1 M) for the same two levels of Q. For optimized value of t_on/t_off, square and hexagonal shaped nanoparticles were obtained. Statistical analysis evidenced that, for C₀ = 0.1 mol/L, current density is the most infl uencing factor on mean size: increasing I from 10 to 50 mA/cm² determined a diminution of mean size of 240 nm. For the same cobalt concentration, increasing the deposition time (Q) determined an increase of mean size of 60 nm. Diminishing the initial cobalt concentration from 0.1 to 0.01 mol/L determined an increase of mean size from 10 nm to 36 nm. For C₀ = 0.01 mol/L nanoparticles grow reaching an optimal size (36 nm) and then, increasing the time of deposition, optimal sized subunits tend to aggregate. As for polydispersity of nanoparticles, statistical tests denoted that increasing I determined significant reduction of variance, while increasing the time of deposition determined a significant increase of variance.