B.A. Legg, M. Zhang, J. Heo, E. Nakouzi, J.J. De Yoreo
Pacific Northwest National Laboratory,
Keywords: in situ AFM, high resolution AFM, electrical double layer
Summary:Surface charging and the associated formation of an electrical double layer (EDL) is an important phenomenon at solid-water interfaces, but the EDL response above surfaces with heterogeneous charge distributions remains poorly understood. Understanding EDL structure in these systems is important, since EDL-mediated electrostatic forces can drive complex modes of nanoscale-assembly, such as the oriented-aggregation of nanoparticles or the spontaneous formation of laterally structured surface precipitates. New experimental methods that allow us to map EDL structure around nanoscale charge heterogeneities can thus provide important new insights into mechanisms of nanoscale assembly. Modern in-situ atomic force microscopy (AFM) provides an unpreceded ability to understand EDL behavior around nanoscale charge heterogeneities by combining high-resolution imaging to map-out atomic-scale to nano-scale surface structures with new three-dimensional fast force mapping (FFM) to map the EDL structure. With 3D-FFM, an oscillating probe is rastered both laterally and vertically above the surface. Tracking changes in both phase and amplitude allows one to produce a 3D map of the tip-sample interaction force, which provides insights into EDL structure through DLVO-models of the tip-sample interaction. In this presentation, I will first share results from a study on the EDL structure over boehmite (γ-AlOOH) nanoparticles that display complex microstructural features such as terraces, step-edges, and pits. Tip-sample interactions show pH and ionic-strength dependent interactions that can be linked to local defect-driven surface charging behavior. This work is providing new insights into the face-specific attachment probabilities that drive the oriented aggregation of boehmite nanoparticles. Next, I will share results from a study on the adsorption of multivalent cations (e.g. Mg2+, Al3+ and La3+) on muscovite mica across a range of pH values. These cations are found to display complex adsorption behavior. At intermediate pH values, individual hydrolyzed ions adsorb strongly and increase the surface-potential, which can be detected in 3D-FFM as pH-dependent EDL forces. In certain cases we are able to image individual 3+ cations ions with atomic-resolution AFM, and they show evidence for lateral ordering that is likely driven by local electrostatic repulsion between ions. At higher pH values, the ions form two-dimensional hydroxide films. The properties of the films vary greatly with ion type. While the Mg-hydroxide films form large continuous sheets, the trivalent hydroxides form nanostructured films. FFM provides an estimate of surface charging, and 3D-FFM allows us to characterize how the EDL structure changes between the grown film and the adjacent mica-surface. The results indicate that the Al-hydroxide films are more strongly charged than the Mg-hydroxide films, supporting the hypothesis that the nanostructure is stabilized by electrostatic interactions between the negatively charged substrate and the positively charged film.