Photocatalytic degradation of azo dyes by ultra-small green synthesized silver nanoparticles

Background: The presence of toxic dyes in wastewater is a serious problem for human health. In this work, the degradation of commercial azo dyes sold over-the counter in Mexico was studied, using as a photocatalyst ultrasmall silver nanoparticles (AgNPs) green synthesized.Methods: For the first time, the successful green synthesis of ultra-small AgNPs using, as both reducing-stabilizing agents (RSa), the aqueous extract of Pelargonium hortorum (Phf) flower or Allium fistulosum (Afs) stem, is reported. AgNPs were characterized by ultraviolet-visible (UV-Vis) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopies, field emission scanning electron microscopy-energy dispersive spectroscopy (FE-SEM-EDS), and field emission transmission electron microscopy (FE-TEM). Significant findings: The morphology of the AgNPs, in both cases, are spherical. AgNPs synthesized with Phf (AgNPs/Phf), which have a mode size of 2 nm (41.9%) and a bandgap of 2.38 eV, resulted in a photocatalytic degradation efficiency of 95%. On the other hand, AgNPs synthesized with Afs (AgNPs/Afs), which have a mode size of 0.6 nm (37.8%) and a bandgap of 2.43 eV, presented a photocatalytic efficiency of 100%. The photocatalytic efficiency and bandgap of the synthesized AgNPs were found to be inversely proportional to the AgNPs size; i.e. the smaller the AgNPs, the higher photocatalytic degradation, and bandgap exhibited.