Proton Conduction Mechanism on Hydrated SnO₂ Nanoparticles: Impedance Spectroscopy and Nuclear Magnetic Resonance Analysis

Cubic SnO2 nanoparticles were prepared via the hydrothermal method, exhibiting preferential orientation on the (110) plane. They absorbed varying degrees of water under different relative humidity conditions. Characterizations using thermogravimetric analysis, X-ray diffraction, electron microscopy, electron paramagnetic resonance, and nuclear magnetic resonance confirmed the presence of a hydration layer (h-layer) on the nanoparticles' surface composed of bridge (b) hydroxyls, terminal (t) hydroxyls, and physically adsorbed water. Complex dielectric spectroscopy identified distinct dielectric losses attributed to the rotation of physically adsorbed water molecules, hydroxyl motion of b-hydroxyls, and proton migration at the interface. The alternating current (AC) conductivity increased from ∼10–7 to 10–4 S m–1 with increasing water content. Double quantum filter spectra and 1H–1H two-dimensional exchange spectra confirmed protons as the primary charge carriers, exchanging between b- and t-hydroxyl protons as well as between b-hydroxyls and physically adsorbed water molecules, facilitating proton transfer. Protons were proposed to migrate via the Grotthuss mechanism on a quasi-two-dimensional system within the hydration layer, with a lower activation energy in the outer water layer, promoting easier migration.