Formation mechanisms for hierarchical nickel hydroxide microstructures hydrothermally prepared with different nickel salt precursors

Hierarchical nickel hydroxide nanostructures have been hydrothermally synthesized with only hexamethylenetetramine (HMT) as alkaline source and NiCl2, Ni(NO3)2 and NiSO4 as nickel source, respectively. All samples were verified to be of α-Ni(OH)2 phase. The samples produced with NiCl2 or Ni(NO3)2 resembled each other in structure and morphology: the flower-like porous microspheres with an interesting cavity-in-cavity structural characteristic. The product prepared with NiSO4, however, showed two kinds of structures with distinctly different characteristics. One was the flower-like porous microsphere; the other was like a microsphere of knitted brambles, which was probably first reported. The formation mechanisms were explored and attributed to the multiple roles of HMT and the effects of anions. Different from chloride and nitrate ions, sulfate ions could be adsorbed strongly on the surface of nickel hydroxide, which resulted in the formation of bramble-like structure units. This work suggested that combining the selections of alkaline source and anion could be helpful for controlling the structures and morphologies of transition metal hydroxides.