Characterization and dye removal capacity of green hydrothermal synthesized ZnO nanoparticles

In the present study, self-assembled ZnO nanoparticles were synthesized via a hydrothermal method utilizing an aqueous extract of mentha pepperita L (peppermint) leaves. The effect of parameters, including the amount of extract, reaction time, and pH was investigated on the structure and morphology of synthesized nanoparticles. X-ray diffraction (XRD) patterns confirmed the formation of pure well-crystallized ZnO nanoparticles using a zinc nitrate solution (50 mM) and peppermint extract in a volume ratio of 6:1 at pH of 8 and 12. However, the morphology and crystallite size of the particles were different in pH of 8 and 12. Fourier transform infrared (FTIR) spectra confirmed the modification of the ZnO surface with peppermint extract components. Analysis of N2 adsorption and desorption cycles showed the increment of surface area by about 5 times using peppermint extract. Scanning electron microscope (SEM) images revealed the formation of a self-assembled sphere structure using peppermint extract. Mechanism of dissolution-precipitation was proposed for green hydrothermal synthesis of ZnO nanoparticles, based on XRD and SEM results at various reaction times. The synthesized ZnO nanoparticles (reaction time of 24 h and pH of 8) were used to remove methylene blue (MB) dye from an aqueous solution, and adsorption and photocatalytic process were studied. The mass of the dye adsorbed per mass of the adsorbent, qt, grew up by increasing the initial dye concentration. Conversely, the photocatalytic degradation percent decreased. In addition, the final adsorption capacity in 50 mL of 30 ppm dye solution was almost the same in 0.01 and 0.05 g of ZnO, although it was smaller in 0.1 g of ZnO. The maximum degradation efficiency was derived by using 0.05 g ZnO. Self-assembled structure, high capacity of adsorption, and excellent photocatalytic properties were unique characteristics of the synthesized ZnO nanoparticles.