© 2019 IOP Publishing Ltd. Silver nanoparticles were synthesized by direct exposure of mixtures of silver nitrate and sodium citrate to sunlight. The reactions were performed near noon, at ∼32 °C during winter, varying the precursors ratio. Nanoparticles were characterized by optical absorption spectroscopy, scanning transmission electron microscopy, as well as proton nuclear magnetic resonance spectroscopy. The kinetic of nanoparticles formation was followed by the temporal evolution of absorbance, and analyzed by using Finke-Watzky two-step model, showing good fit to experimental data. A possible reaction mechanism for the formation of silver nanoparticles is proposed based in the experimental results and the kinetic analysis. This mechanism consists of multiple stages, where the formation of complexes between citrate and cationic silver dimers limits the nucleation process and determines the final size and morphology of the nanoparticles. The proposed approach offers a simple, economic and ecological route for the synthesis of silver nanoparticles.