IrO2 Surface Orientation Dictates Oxygen Evolution Pathway

Rutile iridium dioxide (IrO2) is widely recognized as one of the most effective catalysts for the oxygen evolution reaction (OER), a critical process for sustainable hydrogen production through water electrolysis. Despite its superior stability and catalytic activity under acidic conditions, the fundamental origin of IrO2’s performance remains a subject of active scientific debate. Recent investigations have suggested that the OER activity of rutile IrO2 is strongly dependent on its crystallographic surface orientation, driven by the intrinsic structural anisotropy of the rutile lattice. Different crystal faces expose distinct atomic configurations and coordination environments, which can significantly influence adsorption energetics and the specific reaction pathways involved in oxygen generation. A new study published in Advanced Science provides direct experimental evidence that the surface coordination environment governs the dominant OER mechanism, moving beyond idealized computational models to identify the true catalytic pathways operating on IrO2 surfaces.

The research team investigated the surface coordination-governed OER activity of epitaxial IrO2 thin films, establishing a direct correlation between surface atomic structure, electronic structure, and catalytic behavior. To achieve this, they synthesized high-quality IrO2 films with (110), (101), and (001) orientations on rutile TiO2 substrates using radio-frequency magnetron sputtering. X-ray diffraction confirmed the epitaxial growth and out-of-plane orientation of the films, while atomic force microscopy verified their atomic-level flatness. The root-mean-square roughness values for all three films were below 0.5 nm, a level of smoothness significantly lower than that reported for previously studied IrO2 thin films. This high degree of surface quality ensured that the intended orientation-controlled growth was successfully achieved, providing a reliable platform for comparing intrinsic orientation-dependent OER activity.

Understanding the reaction mechanism requires identifying the specific surface terminations, as OER intermediates interact first with exposed surface sites. In IrO2, the connectivity of IrO6 octahedra gives rise to various surface atomic configurations with different Ir-O coordination environments, including 4-, 5-, and 6-fold coordinated surface Ir atoms. The conventional adsorbate evolution mechanism (AEM) assumes that Ir atoms act as the primary adsorption sites for oxygen intermediates. In contrast, alternative pathways such as the quadruple oxygen coupling mechanism (QOCM) involve the direct participation of surface oxygen atoms in O2 formation. The QOCM may provide a more stable oxygen-coupling pathway by promoting O-O bond formation without substantial disruption of the Ir-O framework, which is often a concern with other lattice oxygen mechanisms that can lead to structural degradation.

To identify the actual surface terminations, the researchers employed atomic-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and enhanced annular bright-field (eABF) STEM analysis. HAADF-STEM provides strong Z-contrast, enabling clear visualization of heavy atoms like iridium, while eABF-STEM complements this by enhancing oxygen contrast near the surface. The imaging revealed that the (110) and (101) surfaces are dominated by fully coordinated IrO6 motifs, a finding that deviates from prior reports which described these surfaces as having Ir atoms in IrO5 coordination. In contrast, the (001) surface exhibited a less uniform coordination with slight distortions in the Ir-O octahedra, characterized by a predominance of IrO4 coordination. This higher density of coordinatively unsaturated Ir sites on the (001) surface suggests a different catalytic behavior compared to the (110) and (101) faces.

Electronic structure analysis further clarified the mechanistic implications of these structural differences. Projected density of states calculations showed that the (110) and (101) surfaces possess enhanced O 2p states near the Fermi level, indicating favorable oxygen-involved charge transfer. The (110) surface exhibited the O 2p band center closest to the Fermi level, suggesting that oxygen-involving electron transfer is most favorable on this orientation. Conversely, the (001) surface showed an Ir 5d band center located closer to the Fermi level than the O 2p band center, reflecting a stronger Ir-centered electronic character. These electronic descriptors suggest a qualitative activity trend of (110) > (101) > (001), as enhanced oxygen participation in catalysis may enable oxygen-involved charge transfer beyond the conventional Ir-centered AEM.

To determine whether these electronic tendencies translate into distinct OER pathways, the team compared the reaction energetics of the Ir-centered AEM and the oxygen-centered QOCM. For the (110) and (101) surfaces, the QOCM pathway exhibited substantially lower activation barriers than the AEM. Specifically, the O-O bond formation barrier was calculated to be 0.52 eV for the (110) surface and 0.75 eV for the (101) surface. In contrast, the (001) surface presented a substantially higher barrier of 1.62 eV for O-O bond formation, indicating that this step is kinetically less accessible. The Ir-OOOO-Ir configuration, which is central to the QOCM, was found to be unstable on the (001) surface, leading to O2 evolution proceeding through the AEM pathway with a barrier of 1.52 eV.

Electrochemical measurements were conducted to experimentally test these mechanistic predictions. The OER current densities of the IrO2 films with three distinct crystal orientations were measured in a 0.1 M HClO4 aqueous solution. A clear dependence of OER activity on crystal orientation was observed, with the current density following the order (110) > (101) > (001). This experimental trend was consistent with the calculated activation-barrier sequence, providing strong support for the proposed mechanistic distinction. The current densities were normalized by the AFM-derived 3D surface area to account for differences in surface roughness, ensuring that the comparison reflected intrinsic catalytic activity rather than geometric factors.

Further insight into the OER characteristics was gained by examining the Tafel slopes for each surface orientation. The similar Tafel slopes of the (110) and (101) surfaces are consistent with similar apparent OER kinetics, whereas the distinct Tafel slope of the (001) surface suggests different kinetic behavior. These electrochemical trends align with the computationally predicted distinction, where the (110) and (101) surfaces favor the oxygen-centered QOCM, and the (001) surface is more consistent with an Ir-centered AEM. The study highlights that the surface coordination environment plays a decisive role in determining the reaction mechanism, as the precise surface termination and local coordination environment influence the adsorption sites of OER intermediates.

The findings establish a fundamental framework for understanding orientation-dependent catalytic mechanisms in rutile oxide electrocatalysts. By combining atomic-resolution STEM imaging with electronic-structure and reaction-barrier calculations, the researchers directly identified the surface terminations of IrO2 and established a correlation between surface coordination, electronic structure, and catalytic mechanism. The (110) and (101) surfaces, dominated by fully coordinated IrO6 motifs, exhibit enhanced oxygen-related electronic activity and preferentially favor the oxygen-centered QOCM pathway with low activation barriers. In contrast, the (001) surface, characterized by undercoordinated IrO4 motifs, favors an Ir-centered AEM pathway with a substantially higher activation barrier.

These results provide direct experimental insight into the atomic-scale origin of orientation-dependent OER activity and establish a framework for designing high-performance oxide catalysts through surface-coordination and electronic-structure engineering. While the study provides a robust correlation between surface structure and mechanism, it notes that direct experimental discrimination between AEM and QOCM remains an important challenge. The work underscores the importance of moving beyond idealized assumptions in catalytic studies and emphasizes the need for precise identification of actual surface terminations to clarify the true catalytic pathways operating on complex oxide surfaces. This approach offers a rational strategy for optimizing electrocatalysts by tailoring the surface structure to promote specific, efficient reaction mechanisms.

The experimental design relied on precise material synthesis to isolate the effects of crystal orientation. The researchers grew the iridium dioxide films on titanium dioxide substrates using radio-frequency magnetron sputtering at a temperature of 400°C and a pressure of 30 mTorr. The growth process utilized a target power of 20 W and an oxygen partial pressure of 10%. After the deposition was complete, the samples were cooled in an oxygen atmosphere to maintain the desired oxidation state. This controlled fabrication process was essential for producing atomically flat surfaces that allowed for a direct comparison of the intrinsic catalytic properties of the different crystal faces without interference from uncontrolled surface defects or roughness variations.

Electrochemical testing was performed in a 0.1 M perchloric acid aqueous solution with a pH of 1. The measurement setup included a platinum counter electrode and a saturated silver-silver chloride reference electrode. To ensure accurate potential readings, the measured values were converted to the reversible hydrogen electrode scale. The current density was normalized by the exposed geometric area and further adjusted by the atomic force microscopy-derived roughness factor. This normalization accounted for the actual three-dimensional surface area, ensuring that the reported activity differences reflected true catalytic efficiency rather than variations in the physical surface area of the films.

Theoretical calculations were conducted using density functional theory with the generalized gradient approximation in the Perdew-Burke-Ernzerhof scheme. The computational models included five layers of iridium dioxide for the (110) and (001) orientations, and six layers for the (101) orientation, separated by a 20 Å vacuum layer. Convergence criteria were strictly applied to both energy and force parameters to ensure the reliability of the calculated electronic structures and reaction barriers. The climbing-image nudged elastic band method was employed to determine the activation energy barriers for oxygen-oxygen bond formation, providing a detailed kinetic profile for the proposed reaction pathways.

Scanning transmission electron microscopy was performed at an acceleration voltage of 200 kV using a spherical aberration corrector. The electron probe size was optimized to approximately 0.7 Å to achieve atomic resolution. The high-angle annular dark-field detector collected signals from a semi-angle range of 45 to 180 mrad to visualize the iridium framework. Enhanced annular bright-field imaging was constructed by taking the difference between bright-field signals collected from different regions of the disk. This technique improved the signal-to-noise ratio for light elements, allowing the researchers to distinguish oxygen columns near the surface and confirm the specific coordination environments of the iridium atoms in the outermost layers.

Subject of Research: Surface coordination-dependent oxygen evolution reaction mechanisms in epitaxial rutile IrO2 thin films

Article Title: Surface Coordination‐Dependent Oxygen Evolution Pathways in Rutile IrO2

Article References: Noh, G., Kim, S.-S., Lee, H., Hwang, J., Patton, M., Ericksen, O., Yun, T. G., Heo, S.-H., Lee, S., Hong, Y., Lee, I. S., Lee, D., Eom, C.-B., Lee, J., Choi, M., Chung, S.-Y., & Choi, S.-Y. (2026). Surface Coordination‐Dependent Oxygen Evolution Pathways in Rutile IrO 2. Advanced Science, Article e78589. https://doi.org/10.1002/advs.78589

Image Credits: AI Generated

DOI: 10.1002/advs.78589

Keywords: IrO2, Oxygen Evolution Reaction, Surface Coordination, STEM Imaging, Electrocatalysis, Rutile Oxides, DFT Calculations, Water Electrolysis, Surface, Coordination, Dependent, Oxygen