Nanospheres, nanocubes, and nanorods of nickel oxalate: Control of shape and size by surfactant and solvent

The role of surfactant and solvent in the size and morphology of nickel oxalate particles synthesized from reverse micelles was investigated. Nanorods of nickel oxalate with aspect ratios of 5:1 and 6:1 were formed from n-hexane and cyclohexane, respectively. Our studies show that the bulkiness of the solvent molecules leads to larger dimensions of the nanorods. The surface charge on the nanorods also plays an important role in the anisotropic growth of the nanorods. Negative ζ potential values were observed for the nanorods, which may have a bearing on the growth of the rods along the cross-section, especially with surfactant molecules having positively charged headgroups (CTAB). The rodlike morphology could be modified by changing the surfactant. For example, we obtained nanoparticles ∼5 nm in size when the surfactant was changed from CTAB to TX-100, and nanocubes (∼50 nm in dimension) were formed with Tergitol as the surfactant. Our study shows that a larger headgroup of the surfactant (TX-100) provides a greater barrier to interdroplet exchange, leading to small sized particles. The nickel oxalate particles obtained above were decomposed to yield NiO nanoparticles. The size of the oxide nanoparticles depends on the aspect ratio of the precursor rods, which in turn appears to be dependent on the solvent chosen for synthesis.