Phyto-mediated synthesis of nickel oxide (NiO) nanoparticles using leaves’ extract of Syzygium cumini for antioxidant and dyes removal studies from wastewater

•The NiO-NPs were synthesized by using Syzygium cumini extract, through a low-cost, easy and reproducible approach.•Phytochemicals present in the extract are performed as the reducing or stabilizing agent for NiO nanoparticles synthesis.•The % scavenging of DPPH radical was found as 74.83% at 1000 µg concentration of nanoparticles which was found as moderate as compared to BHT.•The synthesized NiO nanoparticles also showed excellent removal efficiency for methylene blue and Congo red dyes.