We describe a new electrochemical technique - the "H₂ coevolution method" - for electrodepositing nickel particles ranging in diameter from 20 to 500 nm on graphite surfaces. The H₂ coevolution method enabled the size-selective deposition of nickel nanoparticles that are very narrowly dispersed in diameter. Specifically, a relative standard deviation of the diameter, RSD_dia., of 8-15% was routinely obtainable over the entire particle diameter range indicated above. In this particle size regime, this degree of size monodispersity is comparable to the best that has been obtained for the growth of nickel particles in solution.

The importance of H₂ coevolution can be seen in the series of SEM images shown above. Here are shown nickel particles produced at three deposition potentials ranging from -1.2 V vs. MSE (virtually no H₂ evolution occurs at this potential) to -2.0 V (vigorous H₂ evolution).

H₂-coevolution eliminates interparticle coupling in two ways: first, the nucleation, growth, and eventual release of H₂ bubbles on the surface efficiently stirs the plating solution in the immediate vicinity of the growing nickel particles thereby forestalling the onset of interparticle coupling. The convective effect of H₂-coevolution at electrode surfaces has been investigated in some detail by Tobias and coworkers. Secondly, there is indirect evidence that H₂ bubbles "push" nickel nanoparticles on the surface. Specifically, the diameter of individual nickel particles is not correlated with the proximity to neighboring particles on these surfaces prepared using H₂ coevolution. This fact is obvious even in the SEM images shown above.

In addition to size-monodispersity, the nickel particles produced by H₂ coevolution are nanocrystalline: each nanoparticle produced by this method is a nanocrystalline "snowball" of 1-10 nm FCC nickel grains. Evidence of this fact is provided by electron diffraction, high resolution SEM images of these particles, and dark field TEM images in which individual nickel grains may be resolved (See figure below).