Self-limited growth of a thin oxide layer on Rh(111): A DFT study

Oxidation is often associated with corrosion, but under the right conditions it can lead to oxide layers which can be applied e.g. as protective layers against corrosion, as insulating layers in microelectronic devices and as catalytic devices[1, 2]. For late transition metals and noble metals (e.g. Pd and Ag), it is now understood that the oxidation proceeds through ultra-thin oxide layers, which are thermodynamically stable at intermediate oxygen potentials and can exhibit astonishing complexity[3–6]. It is still unclear whether the same holds for transition metals and to this end the Rh (111) surface was studied extensively experimentally [7]. To supplement these experimental studies, we performed density functional calculations with the Vienna Ab Initio Simulation Package (VASP) [8] using plane waves and the PAW method [9] as well as generalized gradient approximations [10]. The experimental studies show a Moire like pattern in STM, indicative of a (8×8) oxide layer on a (9×9) supercell of the Rh(111) substrate. This phase is formed at intermediate oxygen pressures, whereas thick corundum-like Rh2O3 is only formed at significantly higher pressures and temperatures. The theoretical calculations indicate that the Moire phase can be rationalised by an ultrathin O-Rh-O trilayer surface oxide. Contrary to the experimental observation, the bulk Rh2O3 oxide is however thermodynamically more stable than ultra-thin layers. This discrepency can be understood by the surface phase diagram shown in Fig. 1(a). It indicates that a single oxygen trilayer on Rh(111) is in fact only metastable, as the trilayer forms only under conditions, where bulk Rh2O3 is already stable (to the right of the thick grey line in Fig. 1(a)). Hence the trilayer is only kinetically stabilised, contrary to the situation on Pd(111) where a Pd5O4 ad-layer is thermodynamically stable for intermediate oxygen potentials [4]. An important hint to the reason of the kinetic stability of the O-Rh-O trilayer is given by the results for the three and four layer thick oxides. Four (three) layer oxides have a lower stability than the single trilayer for oxygen potentials μO <−0.99 eV (μO <−0.78 eV) corresponding to 10 mbar (1 bar) at 800 K. The formation of the bulk oxide must however proceed through thicker oxide layers that present a kinetic barrier for the formation of the bulk oxide at too low chemical potentials. The most favorable structures for 2, 3 and 4 oxygen layers are depicted in Fig. 1(b). In contrast to oxygen atoms on the clean metal, the oxygen atoms at the oxide/metal interface are located preferentially on top of the surface Rh atoms (shifted slightly towards the bridge site). Remarkably, the trilayer termination remains favorable even for thicker oxides. For three oxygen layers (3L) the topmost surface layer contains three Rh atoms, and a fourth single Rh atom is located in the second oxide layer. For four oxygen layers (4L), the trilayer is found at both sides of the oxide, and a single Rh atom interlinks the two O-Rh-O layers. Poster