Structure of a Core–Shell Type Colloid Nanoparticle in Aqueous Solution Studied by XPS from a Liquid Microjet

Abstract

With the developments of near ambient pressure photoemission and the liquid microjet, X-ray photoelectron spectroscopy (XPS) measurements at the liquid-nanoparticle interface are now possible. This significant advance allows soft matter physicists working in the field of colloid nanoscience the opportunity to perform surface science experiments long deemed impossible. Here we use XPS in conjunction with a liquid microjet to study the electronic and geometric structures of a core–shell type nanoparticle, AlxOy@SiO2, suspended in aqueous solution. The Al 2p spectrum is consistent with two unique electronic structures that we assign to neutral sites, >Al–OH, at higher kinetic energy (KE) and to protonated species, >Al–OH2+, at lower KE. The presence of excess positive charge on the nanoparticles surface is additionally confirmed by electrophoretic mobility experiments (positive zeta-potentials). Taking advantage of the quantitative nature of XPS we find ~35 % of the AlxOy monolayer is protonated at the pH of the experiment. Finally, we perform additional experiments as a function of photoelectron kinetic energy (depth profiling) and qualitatively determine the Si 2p/Al 2p ratios. We discuss the quantitative limitations to such an experiment in aqueous solution.