Preparation and characterization of dc-plated nanocrystalline nickel electrodeposits

The microstructure and electrical transport properties were investigated for d.c.-plated Ni electrodeposits prepared from different types of electrolysing bath with deposition current densities (i(dep)) ranging from 2.5 to 50 A dm(-2) onto either Cu or Ti substrates, which were subsequently removed from the deposits. The contamination of the Ni foils was investigated by several analytical techniques, and the impurity content (mainly Co and Fe) was found to be of the order of a few tenths of an atomic per cent. With decreasing deposition current density, the deposit grain size was found to decrease markedly, e.g. deposits with an average grain size of about 30 nm could be produced with i(dep) = 5 A dm(-2) from a bath containing one nickel salt (NiSO4) only. The surface morphology on the bath side of the electrodeposited Ni foils was characteristic for the type of bath, and its roughness correlated well with the observed grain sizes. The room-temperature resistivity increased and the temperature coefficient of resistivity decreased markedly with decreasing grain size as expected, whereas the shift of the room-temperature thermoelectric power towards less negative values is not yet completely understood. In discussing the factors controlling the deposit grain size, it is concluded that the Ni2+ ion content at the cathode-electrolyte interface and the amount of adsorbed hydrogen at the cathode surface may have a decisive role.