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

• Flower-like microspheres with cavity-in-cavity structure and radical openings. • Firstly reported bramble-knitted microspheres prepared with NiSO 4 as nickel source. • Multiple roles of HMT in hydrothermal fabrications of porous Ni(OH) 2 microspheres. • Effects of chloride, nitrate and sulfate ions on the structures and morphologies. Hierarchical nickel hydroxide nanostructures have been hydrothermally synthesized with only hexamethylenetetramine (HMT) as alkaline source and NiCl 2 , Ni(NO 3) 2 and NiSO 4 as nickel source, respectively. All samples were verified to be of α-Ni(OH) 2 phase. The samples produced with NiCl 2 or Ni(NO 3) 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 NiSO 4 , 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. [ABSTRACT FROM AUTHOR]