Turning things downside up: adsorbate induced water flipping on Pt(111)

Area of Science:

• Surface science
• Physical chemistry
• Materials science

Background:

• Water monolayers on metal surfaces exhibit unique structures.
• The orientation of hydroxyl (OH) groups in water films influences surface properties.

Purpose of the Study:

• To investigate the adsorption behavior of small molecules on a specific water monolayer structure.
• To understand the structural and vibrational changes induced by adsorbates.

Main Methods:

• Molecular beam dosing for controlled gas exposure.
• Infrared reflection absorption spectroscopy (IRAS) for vibrational analysis.
• Temperature programmed desorption (TPD) for studying desorption kinetics.

Main Results:

• The water monolayer on Pt(111) adopts a "down" configuration (H-down) lacking dangling OH bonds.
• Krypton (Kr) and oxygen (O2) adsorption minimally perturb the water structure.
• Nitrogen (N2) and carbon monoxide (CO) induce a reversible "flip" to an "up" configuration (H-up) at low temperatures.
• Small energy barriers exist between H-down and H-up configurations.

Conclusions:

• The H-down configuration of the water monolayer is stabilized by substrate interactions.
• Electrostatic interactions between water and adsorbates mediate the H-down to H-up transition.
• The observed "flipping" is a reversible process sensitive to adsorbate type and temperature.