A facile direct surfactant-free template-less hydrothermal method is employed for the growth of high surface-area NiO nanoflowers made up of complex and assembled nanosheets network. Field emission scanning electron microscopy revealed that each nanosheet is about 50-60 nm thick. Detailed structural analysis reveals single-crystalline nature of NiO nanoflowers with cubic crystal structure. The optical absorption bands in the wavelength range of 350-800 nm illustrated in terms of ligand field theory. The photoelectrochemical (PEC), water splitting performance on the NiO nanoflowers were also investigated. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.