Sophisticated IrO2(110)-based model electrodes are prepared by deposition of a 10 nm thick single-crystalline IrO2(110) layer supported on a structure directing RuO2(110)/Ru(0001) template, exposing a regular array of mesoscopic roof-like structures. With this model electrode together with the dedicated in-situ synchrotron based techniques (SXRD, XRR) and ex-situ characterization techniques (SEM, ToF-SIMS, XPS) the corrosion process of IrO2(110) in acidic environment is studied on different length scales. Potential-induced pitting corrosion starts at 1.48 V vs. SHE and is initiated at so-called surface grain boundaries, where three rotational domains of IrO2(110) meet. The most surprising results is, however, that even when increasing the electrode potential to 1.94 V vs. SHE still 60-70 % of the IrO2 film stays intact down to the mesoscale and atomic scale and no uniform thinning of the IrO2(110) layer is encountered. Neither flat IrO2(110) terraces nor single steps or grain boundaries, where only two rotational domains meet, are attacked. Ultrathin single-crystalline IrO2(110) layers seem to be much more stable in the anodic corrosion than hitherto expected.

The interaction of ultrathin single-crystalline IrO 2 (110) films with the gas phase proceeds via the coordinatively unsaturated sites (cus), in particular Ir cus , the undercoordinated oxygen species on-top O (O ot ) that are coordinated to Ir cus , and bridging O (O br ). With the combination of different experimental techniques, such as thermal desorption spectroscopy, scanning tunneling microscopy (STM), high-resolution core-level spectroscopy (HRCLS), infrared spectroscopy, and first-principles studies employing density functional theory calculations, we are able to elucidate surface properties of single-crystalline IrO 2 (110). We provide spectroscopic fingerprints of the active surface sites of IrO 2 (110). The freshly prepared IrO 2 (110) surface is virtually inactive toward gas-phase molecules. The IrO 2 (110) surface needs to be activated by annealing to 500−600 K under ultrahigh vacuum (UHV) conditions. In the activation step, Ir cus sites are liberated from on-top oxygen (O ot ) and monoatomic Ir metal islands are formed on the surface, leading to the formation of a bifunctional model catalyst. Vacant Ir cus sites of IrO 2 (110) allow for strong interaction and accommodation of molecules from the gas phase. For instance, CO can adsorb atop on Ir cus and water forms a strongly bound water layer on the activated IrO 2 (110) surface. Single-crystalline IrO 2 (110) is thermally not very stable although chemically stable. Chemical reduction of IrO 2 (110) by extensive CO exposure at 473 K is not observed, which is in contrast to the prototypical RuO 2 (110) system.

The growth of a flat, covering, and single-crystalline IrO 2 (110) film with controlled film thickness on a single-crystalline TiO 2 (110) substrate is reported. The preparation starts with a deposition of metallic Ir at room temperature followed by a post-oxidation step performed in an oxygen atmosphere of 10 −4 mbar at 700 K. On this surface, additional Ir can be deposited at 700 K in an oxygen atmosphere of 10 −6 mbar to produce a IrO 2 (110) layer with variable thicknesses. To improve the crystallinity of the resulting IrO 2 (110) layer, the final film was post-oxidized in 10 −4 mbar of O 2 at 700 K for 5 min. The surface-sensitive techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO 2 (110) films and how these films decompose upon annealing under ultrahigh vacuum (UHV) conditions. STM provides evidence that the IrO 2 (110) films start already to reduce at 465 K under UHV conditions. Upon annealing to 605 K under UHV the reduction of IrO 2 intensifies (XPS), but the oxide film can readily be restored by re-oxidation in 10 −4 mbar of O 2 at 700 K. Thermal decomposition at 725 K leads, however, to severe reduction of the IrO 2 (110) layer (XPS, STM) that cannot be restored by a subsequent re-oxidation step. The utility of the IrO 2 (110)−TiO 2 (110) system as model electrodes is exemplified with the electrochemical oxygen evolution reaction in an acidic environment.

A template-assisted growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness is reported that is suitable for use in model catalysis and as model electrodes in electrocatalysis. The template consists of a single-crystalline covering RuO2(110) layer grown on Ru(0001). In the first step, we formed IrO2 seeds on the RuO2(110) layer, which then continue to grow by deposition of Ir in an oxygen atmosphere of 3 × 10–7 mbar at a sample temperature of 700 K. The IrO2 seeds are prepared by depositing nanometer size metallic Ir particles on RuO2(110) (in total 0.3–0.5 monolayer (ML) of Ir) at room temperature. Subsequently, the Ir particles are oxidized in 10–5 mbar of O2 at a sample temperature of 700 K. The techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO2(110) films. Thermal desorption spectroscopy, LEED, and STM provide conclusive evidence that the thermal decomposition of IrO2(110) films already started at 500 K under ultrahigh vacuum conditions.

Down to a cathodic potentials of −1.20 V versus the reversible hydrogen electrode, the structure of IrO2(110) electrodes supported by TiO2(110) is found to be stable by in situ synchrotron-based X-ray diffraction. Such high cathodic potentials should lead to reduction to metallic Ir (Pourbaix diagram). From the IrO2 lattice parameters, determined during cathodic polarization in a H2SO4 electrolyte solution (pH 0.4), it is estimated that the unit cell volume increases by 1% due likely to proton incorporation, which is supported by the lack of significant swelling of the IrO2(110) film derived from X-ray reflectivity experiments. Ex situ X-ray photoelectron spectroscopy suggests that protons are incorporated into the IrO2(110) lattice below −1.0 V, although Ir remains exclusively in the IV+ oxidation state down to −1.20 V. Obviously, further hydrogenation of the lattice oxygen of IrO2(110) toward water is suppressed for kinetic reasons and hints at a rate-determining chemical step that cannot be controlled by the electrode potential.