Proton-Conducting Films of Nanoscale Ribbons Formed by Exfoliation of the Layer Perovskite H₂SrTa₂O₇

Thin films of nanoscale ribbons derived from the layer perovskite H2SrTa2O7 were grown and characterized as solid-state proton conductors. The ribbons, made by exfoliation of the parent solid with tetra(n-butyl)ammonium hydroxide, were typically 500 nm long, 40 nm wide, and 3 nm thick, with the short axis corresponding to the layer axis of H2SrTa2O7. Powder X-ray diffraction and high-resolution transmission electron microscopy confirmed that the ribbons had similar a-axis unit cell parameters (3.98 ± 0.05 Å) to H2SrTa2O7 (3.87 ± 0.02 Å) and the defect perovskite SrTa2O6 (3.92 ± 0.01 Å) which is the dehydration product of H2SrTa2O7. By using inductively coupled plasma atomic emission spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy, the ribbons were found to have a lower Sr/Ta ratio than H2SrTa2O7. Thin films of these ribbons were grown by layer-by-layer assembly and by spin-coating, and in both cases the long axis had a preferred orientation parallel to the substrate. Electrochemical impedance measurements were done on pellets of H2SrTa2O7 and SrTa2O6 and on films of oriented ribbons measured perpendicular and parallel to the plane of their long axis. At 25 °C, the proton conductivities in humidified air were 1.5 × 10−4 and 1.7 × 10−5 S/cm for pellets containing randomly oriented grains of H2SrTa2O7 and SrTa2O6. The measured conductivities of oriented films of the ribbons were 1.3 × 10−9 and 2.4 × 10−2 S/cm in the perpendicular and parallel directions, respectively, indicating a high proton conductivity along the ribbon axis and very high conductivity anisotropy.