Effect of caffeic acid adsorption in controlling the morphology of gold nanoparticles: role of surface coverage and functional groups.

Caffeic acid (CA) is well known for its strong adsorption on metal or metal oxide surfaces mostly due to the catecholic functional group. On the other hand, the detailed adsorption configurations and the effects of functional groups on molecular adsorption have not been clarified yet. In this study, first-principles calculations were implemented to elucidate the adsorption phenomena of CA and its deprotonated forms on Au(100), (110) and (111), and then predict the morphology of Au nanoparticles (AuNPs). The adsorption energetics and configurations were carefully examined by employing van der Waals interactions to take dispersion forces into consideration. It was found that the adsorption strengths and geometries of the adsorbates are significantly changed by the surface coverages, deprotonated forms, and bound functional groups. These changes in adsorption features induce changes in surface energies, thereby resulting in different morphologies of AuNPs. To accelerate the morphology prediction of AuNPs, we demonstrated that the adsorption energy of CA can be rapidly estimated by the sum of the adsorption energies of the effective functional groups. Our results provide not only fundamental information about the adsorption behaviors of organic molecules on metal surfaces, but also insights for application in the customized synthesis of nanoparticles.