Nickel oxide nanoparticles obtained using artitile extracts are black or gray-brown in color (see attached work).
Kumar, M. S., Soundarya, T. L., Nagaraju, G., Raghu, G. K., Rekha, N. D., Alharthi, F. A., & Nirmala, B. (2021). Multifunctional applications of Nickel oxide (NiO) nanoparticles synthesized by facile green combustion method using Limonia acidissima natural fruit juice. Inorganica Chimica Acta, 515, 120059.
The color of nickel oxide (NiO) nanoparticles prepared by green synthesis can vary depending on the specific method and conditions used for synthesis. Generally, NiO nanoparticles appear as green to black in color. However, the exact color can be influenced by factors such as the size, shape, and surface properties of the nanoparticles, as well as the stabilizing agents or capping agents used in the synthesis process.