Experimental and theoretical investigation of the formation of two-dimensional Fe silicate on Pd(111)

A single layer of Fe silicate was grown on Pd(111) and analyzed experimentally and theoretically. Following sequential deposition of SiO and Fe and annealing above 900 K in O2, an incommensurate but well-ordered, low-defect density layer was observed with low-energy electron diffraction and scanning tunneling microscopy (STM). The STM images revealed a moiré pattern due to the lattice mismatch between the relaxed oxide layer and the substrate, while high-resolution images showed a honeycomb structure consistent with a silicate layer with six-membered rings of corner-sharing SiO4 tetrahedra at its surface. Reflection-absorption infrared spectroscopy revealed a single peak at 1050 cm-1 due to Si-O-Fe linkages, while x-ray photoelectron spectroscopy data indicated a Si/Fe ratio of one, that the Fe were all 3+, and that the Si atoms were closest to the surface. Consistent with these experimental observations, first principles theory identified a layer with an overall stoichiometry of Fe2Si2O9 with the six-membered rings of SiO4 tetrahedra at the surface. One of the oxygen atoms appears as a chemisorbed atom on the Pd surface, and, thus, the layer is better described as Fe2Si2O8 atop an oxygen-covered Pd surface. The Fe2Si2O8 is chemically bound to the Pd surface through its oxygen atoms; and the passivation of these bonds by hydrogen was investigated theoretically. Upon hydrogenation, the adsorbed O atom joins the Fe silicate layer and thermodynamic analysis indicates that, at room temperature and H2 pressures below 1 atm, Fe2Si2O9H4 becomes favored. The hydrogenation is accompanied by a substantial increase in the equilibrium distance between the oxide layer and the Pd surface and a drop in the adhesion energy to the surface. Together the results indicate that a highly ordered 2D Fe silicate can be grown on Pd(111) and that subsequent hydrogenation of this layer offers potential to release the 2D material from the growth substrate.