High-affinity adsorption leads to molecularly ordered interfaces on TiO₂ in air and solution

Area of Science:

• Materials Science
• Surface Chemistry
• Nanotechnology

Background:

• Titanium dioxide (TiO2) is a widely used photocatalyst.
• Ordered molecular structures observed on TiO2 surfaces lack clear origin.
• Understanding surface interactions is crucial for optimizing photocatalyst performance.

Purpose of the Study:

• To identify the molecular origin of ordered structures on TiO2 surfaces.
• To elucidate the selective adsorption mechanism of atmospheric molecules on TiO2.
• To investigate the impact of these structures on TiO2 photocatalysis.

Main Methods:

• Atomic-scale microscopy (e.g., AFM, STM).
• Surface spectroscopy techniques (e.g., XPS, FTIR).
• Controlled adsorption experiments with varying adsorbates (carboxylic acids, alcohols).

Main Results:

• TiO2 surfaces selectively adsorb atmospheric carboxylic acids, even at low concentrations.
• Carboxylic acids form self-assembled monolayers via bidentate binding.
• Alcohols and other adsorbates are repelled, despite higher concentrations.
• These monolayers exhibit both hydrophobic and water-soluble properties.
• Carboxylate monolayers block undercoordinated cation sites on TiO2.

Conclusions:

• Atmospheric carboxylic acids are responsible for the observed ordered structures on TiO2.
• The selective adsorption is driven by strong bidentate binding of carboxylic acids.
• The self-assembled carboxylate monolayers can modulate TiO2 photocatalytic activity by blocking active sites.