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

Area of Science:

• Colloid and Surface Science
• Nanomaterials Chemistry
• Physical Chemistry

Background:

• Colloidal nanoparticles possess unique size-dependent properties critical for catalysis.
• Understanding the nanoparticle-liquid interface is essential for optimizing surface-mediated reactions.
• Amorphous titanium dioxide (TiO2) nanoparticles are relevant in various catalytic applications.

Purpose of the Study:

• To investigate the interfacial structure and surface potential of TiO2 nanoparticles in aqueous solutions.
• To determine the orientation of interfacial water molecules without prior assumptions on charge distribution.
• To elucidate the behavior of ions at the TiO2-liquid interface across varying ionic strengths and pH.

Main Methods:

• Polarimetric angle-resolved second harmonic scattering (AR-SHS) was employed to probe the nanoparticle interface.
• AR-SHS was used to determine surface potential and interfacial water orientation.
• Molecular dynamics simulations were utilized for comparative analysis with SiO2 nanoparticles.

Main Results:

• Three distinct regions of ion adsorption were identified with increasing NaCl concentration.
• At low ionic strengths, Na+ ions formed inner-sphere complexes and a diffuse layer.
• At higher ionic strengths, a layer of hydrated condensed ions formed, indicating TiO2's higher affinity for Na+ compared to SiO2.

Conclusions:

• AR-SHS is a sensitive tool for investigating interfacial properties of nanoparticles.
• The study provides a detailed understanding of ion adsorption mechanisms at the TiO2-liquid interface.
• Findings are crucial for advancing catalytic and photocatalytic applications utilizing TiO2 nanoparticles.