Proton Dynamics on Goethite Nanoparticles and Coupling to Electron Transport.

Area of Science:

• Surface chemistry
• Computational materials science
• Geochemistry

Background:

• The surface charge of metal oxide particles dictates their behavior in aqueous solutions.
• This surface charge is critical for mineral transformation, biogeochemical reactions, remediation, and sorption processes.

Purpose of the Study:

• To investigate the spontaneous development of electrostatic potential gradients at the surface of metal oxide nanoparticles.
• To understand how these potential gradients influence charge carrier dynamics within the material.

Main Methods:

• Implementation of replica-exchange constant-pH molecular dynamics simulations.
• Utilized classical molecular dynamics for configurational exploration and Metropolis Monte Carlo for protonation space.
• Employed simulated annealing to escape metastable configurations.

Main Results:

• Electrostatic potential gradients spontaneously arise between intersecting crystal faces and across goethite (α-FeOOH) nanoparticles.
• The magnitude of these potential gradients exceeds Johnson-Nyquist voltage fluctuations.
• Fluctuations in adsorbed proton density continuously repolarize surface potential bias at a rate slower than electron-polaron hopping.

Conclusions:

• Spontaneous surface potential fluctuations on metal oxides are significant.
• These fluctuations are likely to control the net movement of charge carriers in the lattice.
• Findings provide new insights into the surface reactivity and charge transport mechanisms of metal oxides.