Mesostructured HfO₂/Al₂O₃ Composite Thin Films with Reduced Leakage Current for Ion-Conducting Devices

Mesoporous hafnium dioxide (HfO₂) thin films (around 20 nm thick) were fabricated by a sol-gel-based spin-coating process, followed by an annealing process at 600 °C to realize the ion-conducting media for the ionics (e.g., Na⁺ and K⁺ for rechargeable ion batteries). Another film of aluminum metal (10 nm thick) was deposited by direct current sputtering to soak into the mesopores. A monitored thermal treatment process at 500 °C in the air yields mesostructured HfO₂/Al₂O₃ composite thin films. However, aluminum dioxide (Al₂O₃) is formed during annealing as an insulating film to reduce the leakage current while retaining the ionic conductivity. The obtained mesostructured HfO₂/Al₂O₃ films show a leakage current at 3.2 × 10^-9 A cm^-2, which is significantly smaller than that of the mesoporous HfO₂ film (1.37 × 10^-5 A cm^-2) or HfO₂/Al film (0.037 A cm^-2) at a bias voltage of 1.0 V, which is enough for ion conduction. In the meantime, among all the thin films, the mesostructured HfO₂/Al₂O₃ composite thin films display the smallest Nyquist arc diameter in 1.0 M KOH electrolyte, implying a lower impedance at the electrode/electrolyte interface and reflecting a better ion diffusion and movement.