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

We have studied adsorption of CO on Fe3O4(111) films grown on a Pt(111) 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 similar to110, 180, and 230 K. CO adsorbed in these states exhibits stretching frequencies at similar to2115-2140, 2080 and 2207cm(-1), respectively. The adsorption results are discussed in terms of different structural models previously reported. We suggest that the Fe3O4(111) 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. (C) 2004 Elsevier B.V. All rights reserved.