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 liquidnanoparticle 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, Al x O y @SiO 2 , 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-OH 2 ? , 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 Al x O y 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.Keywords X-ray photoelectron spectroscopy Á Near ambient pressure photoemission (NAPP) Á Surface structure Á Solid-liquid interface Á Acid-base equilibrium

Ambient Pressure Photoelectron Spectroscopy: Opportunities in Catalysis from Solids to Liquids and Introducing Time Resolution

“…[159] Agood strategy to enhancethe surface concentration of charged nanoparticles is to add ah ydrophobic ligand,w hich acts as as urfactant allowing the detection of small concentrations. [160] From 2011o nwards, by using liquid microjets of the suspensions, the space distribution of tin oxide (SnO 2 )n anoparticles was characterized, [162] surfacec harging of titania nanoparticles was detected, [163] the surface potentiala tt he solid/liquid interface was measured, [164] Al x O y @SiO 2 core-shell nanoparticles were successfully characterized( surface charging at acidic pH and qualitative depth profile) [165] and an evaluation of the thickness of the electrical double layer as af unction of electrolyte concentration was provided. [166] These examples demonstrate the versatility and strong potentiality of at echnique that gives important insights into the currently poorly understood processes that occur during wet-chemistry synthesis.…”

X-ray Photoelectron Spectroscopy Studies of Nanoparticles Dispersed in Static Liquid

“…Two approaches have been reported in the literature for studying the solid/liquid interface of nanoparticles using XPS. The first utilizes a microjet setup. − In this method, the X-ray beam irradiates a microjet flow (∼50 μm in diameter) containing the suspended nanoparticles. The photoelectrons generated from the nanoparticles travel through the liquid layers and are collected by an energy analyzer.…”

“…The photoelectrons generated from the nanoparticles travel through the liquid layers and are collected by an energy analyzer. Valuable insights about the interface between nanoparticles and liquid have been obtained using this technique by a few groups. − However, the pressure around the microjet in the analyzer chamber during the experiment is roughly 10 –4 to 10 –2 Torr, requiring the use of a differentially pumped energy analyzer instead of a common HV energy analyzer. Second, to prevent clogging of the jet nozzle (<50 μm), a relatively low concentration and small size of nanoparticles are suggested.…”

Detection of suspended nanoparticles with near-ambient pressure x-ray photoelectron spectroscopy

“…To the best of our knowledge, characterisation of suspended nanoparticles with NAP-XPS has so far only been performed with liquid microjet, which was first done on SiO 2based nanoparticles in aqueous solution [12,13]. Core-shell nanoparticles have also been characterised and qualitatively depth profiled by using a liquid microjet [14]. The authors emphasise that a quantitative depth profiling is dependent on better understanding of the inelastic mean free path in water and the nanoparticle distribution at the air-water interface.…”