Materials synthesis
All the reagents used in the experiment were of analytical
grade and were used without further purification. According
to the previous report [31], in a typical procedure, 0.01 mol of
urea was dissolved in 20 mL deionized water under constant
stirring, followed by the addition of 1 mmol Ni(NO3)2$6H2O
and 2 mmol Co(NO3)2$6H2O. The mixture was stirred for
30 min, the transparent solution was transferred to a Teflonlined
autoclave (25 mL capacity). The hydrothermal process
was conducted at 180 C for 6 h. The products were collected
by filtration, washed with deionized water and ethanol, and
finally dried at 60 C for 6 h. Finally, the dried precipitates were
annealed at 400 C under air for 3 h to obtain the hierarchical
porous NiCo2O4 nanosheets.