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In this study, highly hierarchically porous NiO nanosheet arrays were synthesized on glass by a hydrothermal growth method. The effects of temperature and the reactant concentrations on the morphology of the nanosheets were examined. SEM revealed uniform and hierarchical porous NiO nanosheets at 50 mM of a nickel nitrate hexahydrate solution at 90 °C in 2 h. After heat treatment in oxygen at 450 °C for 1 h, the resistivity of the annealed NiO nanosheet samples was five orders of magnitude lower than the as-prepared samples. In addition, the resistivity of the nanosheet arrays decreased significantly with increasing reactant concentration. The hierarchically porous nanosheets synthesized under optimal conditions were 200–400 nm in thickness and 6–8 μm in height. These nanosheets might have potential electrochromic applications. © 2011 Elsevier B.V. All rights reserved. 1