Enhanced nucleation of Ni nanoparticles on TiN through H3BO3-mediated growth inhibition

In this paper, we have studied the effect of boric acid (H3BO3) on the nucleation of nickel (Ni) nanoparticles on titanium nitride (TiN). We linked the potential transients during galvanostatic deposition to the different regimes of Ni deposition. The two most important regimes, i.e. nucleation and growth regime, were found to depend on the H3BO3 concentration. Indeed, by lowering the amount of H3BO3 the particle density increased (nucleation), while it also resulted in an earlier growth termination. Based on these findings, we propose a growth-inhibited nucleation model to explain the impact of H3BO3 on the formation of Ni nanoparticles. During Ni nucleation the surface pH increases due to local H+ reduction, leading to the formation and adsorption of Ni(OH)2 on the Ni particles. As a result, the growth of the particles is inhibited and new nucleation is promoted. The formation of this inhibiting hydroxide is counteracted by the presence of H3BO3 as it buffers the surface pH. Therefore, a higher particle density is obtained at low H3BO3 concentrations. However, when the concentration of H3BO3 is too low, Ni(OH)2 is already formed in the diffusion layer during the nucleation regime as the onset of H2O reduction occurs sooner. This results in the termination of Ni deposition before the nucleation of Ni is complete yielding almost no deposited Ni at the surface at low H3BO3 concentrations. Therefore, an optimum H3BO3 concentration is needed where the degree of growth-inhibition is high while Ni is still deposited for a sufficient amount of time.