Structural and Electrocatalytic Studies of Pulsed Laser Deposited Epitaxial RuO₂ Thin Films

Two sets of high-quality epitaxial ruthenium oxide (RuO₂) thin films with different thicknesses were synthesized in situ on cost-competitive sapphire substrates by using a pulsed laser deposition technique. The first set of films, with a thickness of 40 nm and a sheet resistance of 15.7 Ω/□, was prepared using 2100 laser pulses, while the second set of films, with a thickness of 87 nm and a sheet resistance of 6.7 Ω/□, was prepared using 4800 laser pulses. All other deposition parameters were kept the same. The post-deposition structural and morphological measurements showed that both sets of films grew at the same growth rate, had the same crystallinity, similar grain boundary density, and slightly different surface roughness. The thicker RuO₂ films achieved an overpotential of 280 mV for the oxygen evolution reaction at a current density of 100 μA/cm², comparable to or exceeding the performance of films grown on more expensive substrates. Comparatively, the thinner RuO₂ films, which have a significantly higher charge transfer resistance (250 Ω versus 100 Ω for thick films), display a higher overpotential of 320 mV. These results indicate that the lower electrical resistance of thicker films promotes charge transfer through the film body, leading to superior electrocatalytic properties.