Extraordinary Stability of IrO₂(110) Ultrathin Films Supported on TiO₂(110) under Cathodic Polarization

Abstract

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.

In situ studies of the cathodic stability of single-crystalline IrO2(110) ultrathin films supported on RuO2(110)/Ru(0001) in an acidic environment

“…Table S2). This value for the roughness is in very good agreement with previous studies 31 and shows the excellent structural quality of the electrode preparation to be highly reproducible. Therefore, we infer that the thickness of the IrO2(110) film has not been altered after oxygen evolution for ≈26 h at a current density of 50 mA•cm -2 .…”

“…Since it has been shown previously that the lattice parameters of the (110)-oriented TiO2 unit cell (a = c = 6.496 Å, b = 2.959 Å, and α = β = γ = 90°) are not completely adopted by the IrO2(110) film, i.e. the film exhibits a nonpseudomorphous growth (a = c = 6.44 Å, b = 3.05 Å, and α = β = γ = 90°), 31 the TiO2(110) and IrO2(110) Bragg peaks and CTRs can be discriminated. The lattice parameters of IrO2(110) do not vary with the film thickness.…”

“…Figure S4; OCP ≈ 0.67 V vs. the reversible hydrogen electrode (RHE)), a film thickness of 50 ± 1 Å can be derived which does not vary throughout the galvanostatic hold at 50 mA•cm -2 over ≈26 h since the position of the maxima and minima of the oscillations are preserved. In addition, all XRR scans were fitted employing the software package GenX 32 (v. 2.4.10) within a simple one-layer model 31 and exemplified in Figure 2a for XRR scans at OCP and after ≈26 h of the galvanostatic hold. In Figure 2b the IrO2(110) film thickness is shown as a function of time within the galvanostatic hold that is derived from simulations of the complete set of XRR scans.…”

Mixed RuxIr1−xO2 Supported on Rutile TiO2: Catalytic Methane Combustion, a Model Study

“…Obviously, pure IrO 2 and pure RuO 2 adhere differently to the supporting rutile‐TiO 2 particles. Surface science studies revealed rutile RuO 2 to form a strained pseudomorphic RuO 2 (110) layer adopting the surface lattice constants of the supporting rutile‐TiO 2 (110), [24] while rutile IrO 2 (110) grows with much less strain on rutile‐TiO 2 (110) [25,33] . Since the surface energy of IrO 2 is substantially higher than that of TiO 2 and the interfacial energy IrO 2 /TiO 2 remains high, this may explain why IrO 2 forms particles rather than a wetting film on rutile TiO 2 with a small surface and interface area.…”

“…Surface science studies revealed rutile RuO 2 to form a strained pseudomorphic RuO 2 (110) layer adopting the surface lattice constants of the supporting rutile-TiO 2 (110), [24] while rutile IrO 2 (110) grows with much less strain on rutile-TiO 2 (110). [25,33] Since the surface energy of IrO 2 is substantially higher than that of TiO 2 and the interfacial energy IrO 2 /TiO 2 remains high, this may explain why IrO 2 forms particles rather than a wetting film on rutile TiO 2 with a small surface and interface area. Due to the quasi pseudomorphic growth of RuO 2 on rutile-TiO 2 , the interfacial energy RuO 2 /TiO 2 is low so that RuO 2 is now able to wet partly the TiO 2 particles, despite similar surface energies of RuO 2 and IrO 2 .…”