Abstract：SO42- 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(SO42-)/n(OH-). To discuss the effect of SO42- 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 SO42- 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 SO42- 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 SO42- ion has a higher affinity than other ions such as Cl- and SO42-, the SO42- ion is able to enter its interlayers. The structure is stabilized due to the strong coordination bonds between SO42- and Ni2＋ in Ni(OH)2-x layers. Also, free Ni2＋ ions in solution would be linked to intercalated SO42- 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  direction. However, this process is not likely to continue when there is high amount of OH-, which may cause high Coulomb repulsion along the  direction.
卢亚骏, 王浩然, 顾煜, 徐岚, 孙晓骏, 邓意达. 水热合成α-Ni(OH)2纳米线的形成机理研究[J]. 化学学报, 2012, 70(16): 1731-1736.
Lu Yajun, Wang Haoran, Gu Yu, Xu Lan, Sun Xiaojun, Deng Yida. Studies on the Growth Mechanism of Hydrothermal Synthesis of α-Ni(OH)2 Nanowires. Acta Chimica Sinica, 2012, 70(16): 1731-1736.