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

Thin films of nanoscale ribbons derived from the layer perovskite H 2 SrTa 2 O 7 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 H 2 SrTa 2 O 7 . 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 H 2 SrTa 2 O 7 (3.87 ± 0.02 Å) and the defect perovskite SrTa 2 O 6 (3.92 ± 0.01 Å) which is the dehydration product of H 2 SrTa 2 O 7 . 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 H 2 SrTa 2 O 7 . 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 H 2 SrTa 2 O 7 and SrTa 2 O 6 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 H 2 SrTa 2 O 7 and SrTa 2 O 6 . 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.