Defect-Selective Energy Barrier Crossing during Adsorption of Colloidal Gold Nanoparticles on Zinc Sulfide Crystals under Overall Electrostatic Repulsion

Nanofunctionalized particles are widely used in heterogeneous catalysis, additive manufacturing, and environmental technologies. The electrostatically driven adsorption is an established method for the deposition of colloidal nanoparticles. It is described by the DLVO theory. Here, we investigated the adsorption efficiency and distribution of laser-generated colloidal Au nanoparticles (Au-NPs) on phase-pure and mixed-phase zinc sulfide (ZnS) under overall (apparent) electrostatic attraction and repulsion. For overall electrostatic attraction, a quantitative adsorption and uniform decoration of ZnS with Au-NPs was observed, consistent with the DLVO theory. However, under apparent electrostatic repulsion, 100 and 50% adsorption efficiency of Au-NPs on mixed-phase ZnS was observed despite an overall electrostatic energy barrier (assuming a uniform charge distribution) two to four times higher than the thermal diffusion energy of the Au-NPs, respectively. In contrast, phase-pure ZnS showed a significantly lower adsorption efficiency under similar conditions. Transmission electron microscopy (TEM) showed facet-selective adsorption in the case of apparent electrostatic repulsion with the Au-NPs preferentially binding to the edges of the wurtzite ZnS or the extended surfaces of mixed-phase ZnS. X-ray photoelectron spectroscopy (XPS) suggested that defects with different local surface charges may have served as attractive adsorption sites and need to be considered instead of only assuming overall repulsion as inferred when only comparing the respective ζ-potentials. Our results do not contradict the DLVO theory but infer the importance of considering local deviations of the ζ-potential, e.g., at different facets, extended or point defect clusters, especially when trying to predict the adsorption of colloidal, surfactant-free nanoparticles (e.g., from laser ablation in liquids) to mixed-phase systems under electrostatic repulsion. It also adds a new degree of freedom to potentially tailor nanoparticle-functionalized materials, where only specific local surface sites are being decorated with nanoparticles.