Self-ordering water molecules at TiO₂ interfaces: Advances in structural classification

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Physics

Background:

• Efficient photocatalysts are crucial for solar hydrogen production via photoelectrochemical (PEC) water splitting.
• Understanding water structuring at metal-oxide surfaces is key to optimizing PEC processes but remains poorly understood.
• Previous interpretations suggested interfacial water layers might be 'ice-like'.

Purpose of the Study:

• To investigate and classify the structure and ordering of water layers at anatase ⟨101⟩ and rutile ⟨110⟩ TiO2 surfaces.
• To distinguish layered water superstructures from bulk-like water configurations.
• To assess whether ordered interfacial water is 'ice-like' or exhibits other structural characteristics.

Main Methods:

• Classical molecular-dynamics simulations were employed to model water-TiO2 interfaces.
• Local order parameters were used to quantitatively analyze and classify water layer structures.
• Comparisons were made to bulk liquid water and ice polymorphs (ice I).

Main Results:

• Distinct layered water structures were identified at both anatase and rutile TiO2 surfaces.
• These interfacial water structures exhibit reduced molecular mobility but do not strictly conform to 'ice-like' models.
• A general framework for classifying condensed-state water interface structures was proposed.

Conclusions:

• The study provides a quantitative classification of water structuring at TiO2 surfaces.
• Interfacial water exhibits unique ordering distinct from bulk ice or liquid water.
• The proposed framework aids in understanding and describing molecular behavior at condensed-phase interfaces.