Surface Proton Conduction of Sm-Doped CeO_2-δ Thin Film Preferentially Grown on Al₂O₃ (0001)

Sm-doped CeO_2-δ (Ce_0.9Sm_0.1O_2-δ; SDC) thin films were prepared on Al₂O₃ (0001) substrates by radio frequency magnetron sputtering. The prepared thin films were preferentially grown along the [111] direction, with the spacing of the (111) plane (d₁₁₁) expanded by 2.6% to compensate for a lattice mismatch against the substrate. The wet-annealed SDC thin film, with the reduced d₁₁₁ value, exhibited surface protonic conduction in the low-temperature region below 100 °C. The O1s photoemission spectrum exhibits H₂O and OH^- peaks on the SDC surface. These results indicate the presence of physisorbed water layers and the generation of protons on the SDC (111) surface with oxygen vacancies. The protons generated on the SDC surface were conducted through a physisorbed water layer by the Grotthuss mechanism.