The surface structure of Fe₃O₄(111) films as studied by CO adsorption

We have studied adsorption of CO on Fe3O4(1 1 1) films grown on a Pt(1 1 1) substrate by temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and high resolution electron energy loss spectroscopy (HREELS). Three adsorption states are observed, from which CO desorbs at ~110, 180, and 230 K. CO adsorbed in these states exhibits stretching frequencies at ~2115–2140, 2080 and 2207 cm_1, respectively. The adsorption results are discussed in terms of different structural models previously reported. We suggest that the Fe3O4(1 1 1) surface is terminated by 1/2 ML of iron, with an outermost 1/4 ML consisting of octahedral Fe2+ cations situated above an 1/4 ML of tetrahedral Fe3+ ions, in agreement with previous theoretical calculations. The most strongly bound CO is assigned to adsorption to Fe3+ cations present on the step edges