Molecular insights into the electrical double layer of colloidal oxide particles obtained through second harmonic scattering

Photocatalytic applications play an essential role in the search for alternative energy sources and environmental decontamination techniques. It is of fundamental interest to understand on a molecular level the aqueous solid/liquid interface, where photocatalytic reactions occur, to improve the reaction efficiency.Surface charge, ion diffusion and adsorption are all elements that can have a significant impact on reaction rates and, in the case of nanoparticles, colloidal stability. However, not many experimental methods are able to probe the electrical double layer (EDL) and its peculiar properties, particularly when it comes to colloidal nanoparticles directly dispersed in solution. In this thesis we apply polarimetric angle-resolved second harmonic scattering (AR-SHS) to study the EDL of colloidal oxide particles in aqueous environments. The insulating oxide silica (SiO2) and the semiconducting metal oxide titanium dioxide (TiO2) are investigated and further compared. We show that from AR-SHS measurements, we can extract the surface potential of the particles with respect to bulk liquid and the surface susceptibility, as a measure of interfacial molecular orientation. This can be achieved without assuming a specific charge distribution in the EDL, and hence without employing a model such as the Gouy-Chapman (GC) or the Gouy-Chapman-Stern (GCS). Through electrophoretic mobility measurements, we additionally obtain the zeta potential. Knowing the surface potential, the surface susceptibility and the zeta potential enables us to establish a molecular-level picture of the oxide particle/liquid interface as a function of ionic strength and pH. First, we provide evidence that a compact layer of hydrated ions is formed at the 300 nm amorphous SiO2/aqueous interface at high pH and high NaCl concentrations (> 1mM). Computation of surface charge density values based on our measured surface potential values and using the GC and GCS models are in good agreement with literature values.Next, we study the EDL of 100 nm amorphous TiO2 particles as a function of NaCl concentration and of basic pH. Three regions, reflecting three different phenomena occurring as a function of ionic strength, can be identified. First, inner-sphere adsorption at the lowest concentrations, then formation of a diffuse layer of counterions and finally, accumulation of hydrated counterions near the interface. Similar regions are observed for 100 nm SiO2 particles as a function of NaCl concentration. The TiO2 surface is found to have a stronger affinity for Na+ ions than SiO2.We further evaluate ion-specific effects at the SiO2 and amorphous TiO2 colloidal aqueous interface. The addition of NaCl, RbCl or CaCl2 to the solution leads to relativedifferences in surface potential and in the evolution of the interfacial H-bonding network as a function of ionic strength, which reveal surface and cation- specific preferences for inner- and outer-sphere adsorption.Finally, we demonstrate that AR-SHS measurements as a function of pH can be used to determine the pKas of 100 nm anatase TiO2 particles. Our data reveals a correlation between the change in orientation of interfacial water molecules and the pKas of surface TiO2 groups. This work shows that AR-SHS is a powerful tool to uncover the EDL structure of colloidal oxide particles, thus contributing to a better understanding of interfacial processes important to a variety of (photo-)catalytic applications.