SO₄^2- ions and OH^- ions play important roles in hydrothermal synthesis of Ni(OH)₂ nanostructures with only inorganic nickel salt and NaOH. The whole process could be divided into 2 parts, the preparation of Ni(OH)₂ gel and its hydrothermal reaction. According to our analysis, the former one is an ionic-reaction which produces amorphous Ni(OH)₂. It is the later one when Ni(OH)₂ gel crystallizes that dominates the polymorph and morphology of final products. The influence of OH^- on morphologies of Ni(OH)₂ has been studied by changing the ratio of n(SO₄^2-)/n(OH^-). To discuss the effect of SO₄^2- ions in hydrothermal system on crystal growth, we wash the gel which is prepared by NiCl₂ and NaOH with the mixture solution of Na₂SO₄ and NiSO₄ for different times, so that Cl^- ions in the gel solution could be replaced by SO₄^2- ions in varying degrees. Then the nanosized Ni(OH)₂ products is prepared by hydrothermal treatment. Also, the samples were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). TEM results show that there will be 2 main nanostructures in different hydrothermal conditions: nanowires and nanosheets. XRD and FTIR results reveals that the nanowires is Ni(SO₄)_0.3(OH)_1.4, one kind of α-Ni(OH)₂, and the nanosheet is β-Ni(OH)₂. We finally got a conclusion that low amount of OH^- and pure SO₄^2- environment is advantageous to synthesis of α-Ni(OH)₂ nanowires. Since α-Ni(OH)₂ is hydroxyl-deficient and consists of stacked Ni(OH)_2-x layers toward which the SO₄^2- ion has a higher affinity than other ions such as Cl^- and SO₄^2-, the SO₄^2- ion is able to enter its interlayers. The structure is stabilized due to the strong coordination bonds between SO₄^2- and Ni^2＋ in Ni(OH)_2-x layers. Also, free Ni^2＋ ions in solution would be linked to intercalated SO₄^2- through coordination bonds, so that new Ni(OH)_2-x layers could form constantly and stack along the c-axis. As a result, there will be a preferential growth in the [001] direction. However, this process is not likely to continue when there is high amount of OH^-, which may cause high Coulomb repulsion along the [001] direction.

Key words: hydrothermal synthesis, α-Ni(OH)₂ nanowires, growth mechanism