Facet-dependent surface charge and hydration of semiconducting nanoparticles

Anisotropy of shape and surface properties determine the functionality of faceted nanoparticles (NPs) in various contexts including crystal growth, biosensing, facet selective colloidal self-assembly of complex materials, enhanced selective cellular and ions uptake, water splitting for storage of renewable electricity and improved photo/electrocatalytic activity. The characteristic anisotropy, surface proprieties and function of solid-liquid interfaces of crystalline faceted nanoparticles are believed to be essential for their performance but remains poorly understood and difficult to characterize. We use dual scale Atomic Force Microscopy to measure electrostatic and hydration forces of faceted semiconducting SrTiO3 nanoparticles in aqueous electrolyte at variable pH. We demonstrate (i) the ability to quantify strongly facet-dependent surface charges yielding isoelectric points of the dominant {100} and {110} facets that differ by as much as 2 pH units, ii) facet-dependent accumulation of oppositely charged (SiO2) particles, and iii) that atomic scale defects can be resolved but are in fact rare for the samples investigated. Atomically resolved images and facet-dependent hydration structure suggest a microscopic hydration and charge generation mechanism.