Molecular overlayers on ordered substrates have a broad field of applications in catalysis, sensors, molecular electronics, light-to-energy conversion, etc. In particular, bio-inspired oxygen-binding metalated macrocycles, such as iron-phtalocyanines (FePc), are being investigated as viable substitutes for precious metals in catalysis of the Oxygen Reduction Reaction in low-temperature fuel cells. Recent studies of FePc on Ag(110) have shown that sub-monolayer (sub-ML) phases are catalytically active. Remarkably, in oxygen-dosed phases O2 intercalates between the molecules and the surface, thereby substantially changing the Fe magnetic moment. Reversible switching of the Fe magnetic moment in some low-density FePc phases upon an oxygenation - annealing cycle has been demonstrated. In this contribution we report on the structural and magnetic changes along a catalytic cycle of a new dense, “quasi-squared” FePc sub-ML phase (R3) evaporated on Ag(110). Structural changes were monitored by Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED). X-ray absorption spectroscopy (XAS), x-ray lineal polarized absorption (XLPA) and x-ray magnetic circular dichroism (XMCD) experiments at the Fe L2,3 edge were performed on four samples: the as-evaporated phase (R3), two differently oxygenated samples (OX1 and OX2) and the annealed phase (R3ANN). XLPA and XMCD analysis evidence the new R3 phase is catalytically active, however, oxygenation/reduction is less effective than in the previously characterized low-density phase. Sum rules were used to determine the effective spin (mseff/nh) and orbital (mL/nh) magnetic moments per hole as a function of the incidence angle. It is concluded that all characterized phases display planar anisotropy, and the values of mseff/nh are one order of magnitude larger than mL/nh. By oxidation, the isotropic moment undergoes an increase from 7.2x10-2 μB/hole to 1.8x10-1 μB/hole, which is about a factor of 2 smaller than the increase achieved for low-density phase. Our spectroscopic results, in agreement with a combined study of STM and non-contact tuning fork AFM microscopies, show that oxidation of FePc on Ag(110) requires a rotation and displacement of the FePc molecule, which is more difficult to fulfill in denser phases.

Resumen del póster presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016.-- et al.

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