Modulation of Structural, Electronic and Electrocatalytic Properties of Pulsed Laser Ablated Ruthenium Oxide Thin Films for Oxygen and Hydrogen Evolution Reactions

This study presents a detailed optimization study of the structural, electronic, and electrocatalytic properties of ruthenium oxide thin films grown using the pulsed laser ablation (PLA) process. The study begins by growing substrate-film interdiffusion-free sub-stoichiometric (RuO_2-x), stoichiometric (RuO₂), and hyper-stochiometric (RuO_2+x) epitaxial ruthenium oxide films with controlled orientation at relatively low deposition temperatures on single-crystal substrates with different crystal structures and surface orientations, such as (110) and (100) oriented rutile TiO₂, (100) oriented perovskite SrTiO₃ and LaAlO₃, and (0001) oriented hexagonal sapphire. The synthesis of the substrate-film interdiffusion-free RuO₂ films has been possible due to the axial velocity of laser-ablated neutral and ionic species as high as ~10⁵ m/s (kinetic energy ~ 3 eV), which compensates for the high substrate temperatures nearly by 50 °C. The next part of the study is focused on the use of x-ray photoelectron spectroscopy (XPS) and Rutherford Backscattering Spectrometry (RBS) measurements to confirm that the oxygen content of RuO₂ films increases from x = 0.5 to x = 1.0 with an increase in oxygen growth pressure from 5 mTorr to 100 mTorr. Further characterization of the RuO₂ films was carried out using x-ray diffraction, atomic force microscopy, four-probe resistivity, spectroscopic ellipsometry, and soft x-ray absorption spectroscopy (XAS) at the O K-edge analysis before and after electrocatalytic property measurements. The study has provided insights to overcome the stability problem of RuO₂ often encountered during the electrocatalytic water oxidation and reduction process. Using the ability to synthesize RuO₂with the same stoichiometry on substrates with different crystal structures, lattice constants, and orientations, we have developed a better understanding of the effect of surface tension and lattice-matched interfacial structures on electrocatalytic oxidation and reduction of water.<br/><br/>This work was supported by a DOE EFRC on the Center for Electrochemical Dynamics and Reactions on Surfaces (CEDARS) via grant # DE-SC0023415. The authors (IK, VC, and DK) also acknowledge the support of the NSF PREM via grant # DMR-2122067 PREM.