Phase Transformation and Water Adsorption Behavior of As-Deposited and Annealed Ru Metal Thin Films Prepared by Atomic Layer Deposition

Surfaces play a central role in catalytic processes, and understanding the transformation of ruthenium metal into ruthenium oxide during annealing is essential for tailoring functional catalytic interfaces. In this study, we systematically investigate ≈22 nm thick Ru metal films deposited by atomic layer deposition (ALD) at 300°C, focusing on their chemical composition, structural evolution, and surface hydration behavior following post-deposition annealing in air from 400 to 600°C. Lab-based and synchrotron X-ray photoelectron spectroscopy (XPS) reveal a gradual conversion from metallic Ru to fully oxidized Ru4+ with increasing annealing temperature, accompanied by a corresponding increase in lattice oxygen. X-ray diffraction (XRD) shows amorphous Ru oxide phases at 400°C and 500°C that evolve into crystalline RuO2 at 600°C, while atomic force microscopy (AFM) indicates enhanced grain growth and surface roughening upon annealing. Ambient-pressure XPS (AP-XPS) under controlled H2O vapor environments (1–17 Torr) demonstrates that samples annealed at 400°C and 500°C exhibit initially high hydroxyl coverage that decreases with increasing water vapor pressure, concurrent with a rise in molecular H2O adsorption. In contrast, the crystalline RuO2 surface formed at 600°C maintains stable hydroxylation and supports increased water uptake. Overall, this work provides fundamental insight into Ru oxide–H2O interactions and establishes design principles for engineering oxide surfaces optimized for electrocatalytic applications.