Surface Potential and Interfacial Water Order at the Amorphous TiO₂ Nanoparticle/Aqueous Interface

Colloidal nanoparticles exhibit unique size-dependent properties differing from their bulk counterpart, which can be particularly relevant for catalytic applications. To optimize surface-mediated chemical reactions, the understanding of the microscopic structure of the nanoparticle-liquid interface is of paramount importance. Here we use polarimetric angle-resolved second harmonic scattering (AR-SHS) to determine surface potential values as well as interfacial water orientation of ∼100 nm diameter amorphous TiO₂ nanoparticles dispersed in aqueous solutions, without any initial assumption on the distribution of interfacial charges. We find three regions of different behavior with increasing NaCl concentration. At very low ionic strengths (0-10 μM), the Na⁺ ions are preferentially adsorbed at the TiO₂ surface as inner-sphere complexes. At low ionic strengths (10-100 μM), a distribution of counterions equivalent to a diffuse layer is observed, while at higher ionic strengths (>100 μM), an additional layer of hydrated condensed ions is formed. We find a similar behavior for TiO₂ nanoparticles in solutions of different basic pH. Compared to identically sized SiO₂ nanoparticles, the TiO₂ interface has a higher affinity for Na⁺ ions, which we further confirm with molecular dynamics simulations. With its ability to monitor ion adsorption at the surface with micromolar sensitivity and changes in the surface potential, AR-SHS is a powerful tool to investigate interfacial properties in a variety of catalytic and photocatalytic applications.