Adsorption of Carboxylic Acids on Magnetite Single Crystal Surfaces

Magnetite is a versatile transition metal oxide with applications in catalysis, biomedical imaging and spintronic devices. Moreover, magnetite nanoparticles covered with oleic acid are formed into novel nanocomposite, hierarchical materials with outstanding mechanical properties in terms of strength and hardness. Despite their strong influence on the stability and properties of the nanocomposites, little is known about interactions that take place at the oxide/organic interface. In order to further tailor and improve these materials, it is necessary to study the controlled adsorption of carboxylic acids on well-defined flat magnetite single crystal surfaces.Following the surface science approach the clean magnetite (111) surface was investigated in this thesis using low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and surface X-ray diffraction (SXRD). The surface was found to be Fe-tet1 terminated after several cycles of argon ion sputtering and annealing in an oxygen atmosphere followed by annealing in ultra-high vacuum (UHV). The surface was homogeneously terminated by a Fe-tet1 layer however, the surface defect concentration, i.e. missing tet1 ironions, was around 35%.The clean magnetite (111) surface was subsequently exposed to formic acid. As the simplest carboxylic acid, formic acid has been used as a probe molecule for longer carbonchain carboxylic acids in the past. The molecule was found to dissociate upon adsorption at room temperature. Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS)revealed two adsorption sites for the formate molecule. For low coverages, the formate molecule was adsorbed at the surface in a chelating bidentate geometry with both formate oxygen atoms bound to a single substrate tetrahedral iron ion and the remaining atomic hydrogen was bound to the oxygen terminated iron defect sites. With increasing coverage, formic acid was adsorbed in a quasi-bidentate configuration with one formate oxygen bound to a tetrahedral iron ion and the other bound to an OH group on the surface. The surface was covered by a (sqrt(3) x sqrt(3)) R30° superstructure at saturation coverage, which was visible with LEED and STM. Both adsorption geometries contributed to the formation of the superstructure.The adsorption of oleic acid on the magnetite (001) surface at room temperature was observed to lift 90% of the (sqrt(2) x sqrt(2)) R45° reconstruction of the clean surface as proven by LEED and SXRD. The lifting mechanism is expected to follow the same path as for the adsorption of atomic hydrogen, water vapor and formic acid on magnetite (001). The adsorption of hydrogen destabilizes the tetrahedral interstitial iron, which subsequently diffuses into one of the two octahedral vacancies. The second vacancy is filled by iron diffusion from deeper layers. FT-IRRAS results revealed that oleic acid molecules adsorbed in a bidentate bridging geometry and the long carbon chain was oriented perpendicular to the surface. With increasing oleic acid coverage, non-dissociated molecules started to adsorb parallel to the surface.The adsorption of oleic acid on the magnetite (111) surface was different from the (001) surface. Both dissociated and non-dissociated molecules were adsorbed in an upright geometryresulting in a higher layer thickness and higher electron density than on the (001) surface as determined by X-ray reflectivity (XRR). When annealing the oleic acid layer at 350 °C, the thickness and electron density of the layer decrease on both magnetite (111) and on (001) as a result of molecule desorption and possible tilt. In addition an atomic roughening of the magnetite substrate occurs.The different adsorption behavior of formic and oleic acid on the (001) and (111) magnetite single crystal surfaces and the subsequent structural changes in the near-surface region of magnetite are important aspects for oleic acid stabilized magnetite nanoparticles. The structural properties of such nanocomposite materials can be tailored by changing the ratio of (111)-type and (001)-type facets hence changing the oleic acid molecule density