Biosynthesis, characterization and photocatalytic activity of ZnO nanoparticles using extracts of Justicia spicigera for the degradation of methylene blue

•ZnO nanoparticles were biosyntehsized with Justicia spicigera extract.•The crystallite size and homogeneity was influenced by the amount of extract used.•The values of the band gap increased as the concentration of extract increased.•Particle size depends on Justicia spicigera extract amount.•The photocatalytic activity presented 90% of MB degradation within 120 min. This paper presents an easy and simple green route method for the biosynthesis of ZnO nanoparticles, which were synthesized using Justicia spicigera extract, in different concentrations (1, 2, 4 and 8% in weight-volume). The samples were characterized by Fourier Transform Infrared (FT-IR) and Visible Ultraviolet Spectrometry (UV-vis) spectrophotometric techniques, furthermore complemented with the use of X-ray Diffraction (XRD), Scanning (SEM) and Transmission Electron Microscopy (TEM). Finally, the different catalysts were tested for the degradation of methylene blue under ultraviolet light. The samples presented the typical band of the Zn-O bond at 618 cm−1. UV-vis studies revealed a characteristic signal at 373-376 nm, confirming the formation of ZnO nanoparticles. The different samples manifest the same hexagonal crystal structure, Wurtzite. These materials have a semi-spherical morphology and sizes ranging from 10 to 100 nm. Regarding photocatalytic activity, the catalysts degraded close to 90% of the dye in 120 min. [Display omitted]