Colloidal semiconductor nanoparticles exhibit unique size-dependent properties compared to their bulk counterparts, which can be particularly beneficial for catalytic applications. To develop highly efficient, environmentally-friendly photocatalytic devices it is essential to understand the surface chemical reactivity of nanoscale semiconductor materials and the microscopic structure of the nanoparticle-liquid interface.
Here we use polarimetric angle-resolved second harmonic scattering 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 (photo)catalytic applications.