Publication date: 21st July 2025
A detailed understanding of the microscopic structure at the semiconductor nanoparticle–liquid interface is essential for optimizing surface-mediated chemical reactions. In particular, both the electrical double layer structure and the surface protonation state can significantly influence photocatalytic reactions by modulating reactant adsorption and electron transfer rates. However, investigating the electrical double layer (EDL) structure and surface acid sites under conditions that closely mimic actual catalytic operation for semiconductor nanoparticles is a technical challenge that requires to probe selectively the few molecular layers of the solid/water interface of a colloidal suspension.
Here, we show that polarimetric angular-resolved second harmonic scattering (AR-SHS) offers a fully optical, non-invasive method to determine surface potential values as well as interfacial water orientation of nanosized metal-oxide particles dispersed in aqueous solutions. By comparison of the surface potential to the zeta potential at different salt concentrations, we are able to establish a microscopic picture of the electrical double layer structure, and follow its evolution with increasing salt concentration. Then, AR-SHS measurements as a function of pH on TiO2 nanoparticles indicate a reversal in water orientation for specific pH values. We demonstrate that the reversal in water orientation indicate transitions between predominantly protonated and deprotonated surface states, providing a direct, optical means to determine surface pKa values from surface susceptibility versus pH data.
Our findings enable a new in situ approach to investigate the structure of the electrical double layer and surface acidity of colloidal nanoparticles, offering deeper insights into fundamental mechanisms affecting photo(electro)chemical processes.
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