![]() ![]() Compared to other heterostructures 20, 21 all calculated interface formation energies are very small (about one order of magnitude smaller than the surface free energies discussed below) and consistently below 30 meV Å −2. This effectively filters out the increasing strain in a coherent MO 2 film of increasing thickness and allows to arrive at a purely interface specific quantity that reflects the intrinsic cost of creating the interface. In the calculation ofĪccording to equation (6) we consistently use the optimized TiO 2 bulk lattice constants in the directions parallel to the interface for all three solid-state terms which fixes the value of In the case of the (111) facet, there are two possible stoichiometric interfaces denoted as t1 and t2, see Figure S1 in the Supporting Information for a description of all interfacial geometries. Results and Discussion Core-shell interfaceĪs starting point of our investigation we report in Table 1 the computed interface formation energies and work of adhesion for epitaxial and stoichiometric IrO 2/TiO 2 and RuO 2/TiO 2 interfaces for all five symmetry inequivalent low-index orientations of rutile, namely (001), (010)/(100), (011)/(101), (110) and (111). At enhanced stability, increased activity and minimized precious metal content, this suggests epitaxial rutile IrO 2/TiO 2 or RuO 2/TiO 2 core-shell nanoparticles as a promising target for future synthesis or advanced deposition endeavors. However, most intriguingly, our ab initio thermodynamics based results additionally indicate an increased stability of such particles under OER operation conditions, as well as an increased activity. Corresponding epitaxial core-shell particles would obviously minimize the precious metal demand. 18, 19 Under more oxidizing synthesis conditions, growth of coherent shell films should instead be feasible. This rationalizes in particular the experimentally observed poor wetting behavior of IrO 2 at the prevalent (110) facet of rutile TiO 2 nanoparticles. Analyzing adhesion, strain and surface energies, we show that prevailing gas-phase synthesis protocols will only be able to stabilize thin films in the few-monolayer regime at some low-index facets of TiO 2 for both IrO 2 and RuO 2. In this study we explore this idea with detailed first-principles calculations. This motivates the idea to instead pursue epitaxial core-shell nanoparticles with thin coherent films of the equally rutile-structured IrO 2 or RuO 2 enclosing the cheap core material. Titanium dioxide exhibits a stable rutile modification. As one example we highlight IrO 2 dispersed on TiO 2, 11- 13 as has also already been commercialized in form of the recent Elyst Ir75 0480 catalyst from Umicore. 5- 10 Generally, though, these have been massively loaded composites with incoherent thick Ir or Ru oxide films or small nanoparticles that use the core material more like a high surface-area support. 3, 4 Within the core-shell concept, large research efforts have therefore been undertaken towards dispersing the precious active oxides on a variety of inexpensive core materials comprising abundant metals, their nitrides, carbides or oxides. 2, 3 Despite the already high efficiency of current generation catalysts, significant further reduction of Ir or Ru mass loading is required when considering that for a prospective hydrogen economy gigantic amounts of electrolysis power will be required. Oxides containing rare Ir and Ru are currently the primary anode electrocatalysts for the oxygen evolution reaction (OER) that exhibit both a reasonably small overpotential and sufficient stability under the harsh acidic PEM operating conditions. Proton-exchange membrane (PEM) water electrolysis 1 is an eminent application area for this concept. Intriguingly, the calculations also predict an enhanced activity and stability of such epitaxial RuO 2/TiO 2 core-shell particles under OER operation.Ĭore-shell nanoparticle morphologies are a powerful and frequently pursued concept in heterogeneous catalysis to reduce the demand of precious active materials. Thermodynamic stability in particular of lattice-matched RuO 2 films is instead indicated for more oxidizing conditions. Resulting from a strong directional dependence of adhesion and strain, a wetting tendency is only obtained for some low-index facets under typical gas-phase synthesis conditions. Here, we employ first-principles density-functional theory (DFT) and ab initio thermodynamics to assess the feasibility of encapsulating a cheap rutile-structured TiO 2 core with coherent, monolayer-thin IrO 2 or RuO 2 films. For a future large-scale application, core-shell nanoparticles are an appealing route to minimize the demand for these precious oxides. Due to their high activity and favorable stability in acidic electrolytes, Ir and Ru oxides are primary catalysts for the oxygen evolution reaction (OER) in proton-exchange membrane (PEM) electrolyzers.
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