The structure of Co nanowires deposited at the same potential depends on Co2+ concentration in solution. When depositing at -1.6 V, the formed Co nanowire are hcp phase in 0.356 M solution, a mixture of hcp and fcc phases in 0.53 M solution, almost fcc phase in 0.71 M solution and pure fcc phase in 1.06 M solution. The transient curves show two interesting observations. First, the i(max) increases with increasing concentration of Co2+ ions while the t(m) decreases with increasing concentration. Second, the i(max) and t(m) observed in depositing Co nanowires at -1.6 V in the 0.71 M solution are close to those in depositing Co nanowires at -3.0 V in the 0.356 M solution. A higher i(max) and shorter t(m) can represent a larger N-s (saturation nucleus density). Therefore we believe that the deposition at - 1.6 V in higher concentrations such as 0.71 and 1.067 M can lead to a larger N-s, indicating the formation of smaller critical nuclei. The structure of Co can be determined by the critical nucleus size and smaller critical nuclei favor the formation of fcc Co. Therefore the fcc Co nanowires were observed when depositing in the high concentration solution such as 0.71 and 1.067 M. (C) 2016 Elsevier B.V. All rights reserved.