Molecular dynamics simulations of the formation of Ag nanoparticles assisted by PVP

Understanding the formation mechanisms of nanoparticles is essential for the synthesis of nanomaterials with controlled properties. In solution synthesis, capping agents are used to mediate this process and control the final size and shape of the particles. In this work, the synthesis of silver nanoparticles, with polyvinylpyrrolidone (PVP) as the capping agent, is studied through molecular dynamics simulations. Nucleation of clusters of atoms and subsequent growth to form nanoparticles are analyzed, with focus on the role of PVP. No finite critical nucleus is detected, and amorphous particles seem to form by spinodal growth. In this timescale, PVP seems to have no effect on particle growth, which is ascribed to the competition between the protective effect and "bridging" (where a molecule of PVP is adsorbed to two different clusters, bringing them together). As the process evolves, a sequence of ordered structures appears within the particles: icosahedral, BCC, and FCC, the last one being the equilibrium configuration of bulk silver. In addition, for a low PVP content an apparent acceleration is observed in particle growth after these ordered phases appear, indicating that the growth of ordered particles from the solution is faster than the growth of amorphous particles. For a high PVP content, this acceleration is not observed, indicating that the protective effect prevails on particle growth in this regime. In addition, due to the bridging effect, the final overall configuration is strongly dependent on the PVP content. In the absence of PVP, large but dispersed particles are observed. When the PVP content is low, due to strong bridging, particles form agglomerates (with no strong coalescence in the timescale of simulations). When the PVP content is large enough, particles are smaller in size and do not show a strong tendency to agglomerate.