Thermal and Chemical Phase Engineering of Two-Dimensional Ruthenate

Monolayer ruthenate nanosheets obtained by exfoliatinglayeredruthenium oxide exhibit excellent electrical conductivity, redox activity,and catalytic activity, which render them suitable for advanced electronicand energy devices. However, to fully exploit the benefits, we requirefurther structural insights into a complex polymorphic nature anddiversity in relevant electronic states of two-dimensional (2D) ruthenatesystems. In this study, the 2D structures, stability, and electronicstates of 2D ruthenate are investigated on the basis of thermal andchemical phase engineering approaches. We reveal that contrary toa previous report, exfoliation of an oblique 1T phase precursor leadsto nanosheets having an identical phase without exfoliation-inducedphase transition to a 1H phase. The oblique 1T phase in the nanosheetsis found to be metastable and, thus, transforms successively to arectangular 1T phase upon heating. A phase-controllable synthesisvia Co doping affords nanosheets with metastable rectangular and thermallystable hexagonal 1T phases at a Co content of 5-10 and 20 at%,respectively. The 1T phases show metallic electronic states, wherethe d-d optical transitions between the Ru 4d (t(2g)) orbitaldepend on the symmetry of the Ru framework. The Co doping in ruthenatenanosheets unexpectedly suppresses the redox and catalytic activitiesunder acidic conditions. In contrast, the Co2+/3+ redoxpair is activated and produces conductive nanosheets with high electrochemicalcapacitance in an alkaline condition.

Automated summary

The structures, stability, and electronic states of 2D ruthenate were investigated by thermal and chemical methods. Contrary to previous reports, exfoliation of a oblique 1T phase precursor led to nanosheets with the same phase. Co doping resulted in nanosheets with metastable rectangular and thermally stable hexagonal 1T phases. Co doping suppressed the redox and catalytic activities under acidic conditions, but activated the Co2+/3+ redox pair and produced conductive nanosheets with high electrochemical capacitance in alkaline conditions.