Molecule–Oxide Bond Formation

Suche

right funCOS 3 will target at the elucidation of reaction mechanisms, kinetics, and energetics when functional molecules are (reactively) linked to oxide surfaces. The chemistry of molecule–surface interactions will be studied by (time-resolved) IRAS and XPS in UHV. Information on molecular orientation, conformation, reaction mechanisms, and chemical transformations of molecules and linker groups will be made available. Ideal oxides, nanostructured model surfaces (oxide/oxide and metal/oxide), and oxide nanostructures will be investigated to address selectivity issues and bridge the gap between simple and complex oxide interfaces elaborated in funCOS 5. Temperature-dependent XPS and IRAS, MB methods and TPD will provide information on the energetics and microkinetics. These methods will also be used in explorative reactivity studies of oxide surfaces with organic layers. Using PM-IRAS, DRIFTS, and high-pressure XPS, the pressure range from UHV to ambient conditions will be covered on both single crystals and nanostructures.

Objectives

In funCOS 3, vibrational spectroscopy and photoelectron spectroscopy will be used to elucidate the elementary mechanisms, energetics, and kinetics of bond formation between functionalized organic molecules and oxide surfaces under UHV conditions. Transferability to ambient conditions and to engineered nanomaterials will be simultaneously explored. We will address the following issues:

  • Resolve the reaction mechanism upon transformation of selected functional groups attached to funCOS Test Molecules during bond formation at oxide surfaces.
  • Quantify the energy of formation and the thermal stability of the surface-bound species.
  • Study the usability of these anchors to link large functional molecules to oxide surfaces and control their molecular orientation and arrangement.
  • Investigate the concept of multiple coordinations through chelating and bifunctional linkers and investigate the influence on molecular orientation, formation barriers, and thermal stability of the surface-bound species.
  • Control the site selectivity of surface-linking reactions, i.e. the potential to address specific surface sites by specific anchor groups on nanostructured oxide surfaces exposing different sites.
  • Find the role of reactive environments by studying the influence of reactants on the linking reactions and, vice versa, the influence of the organic layer on the surface reactivity.

Systems and strategy

We will address the above questions in UHV, primarily combining vibrational spectroscopy (IRAS, including TR IRAS) and XPS (including angle-resolved XPS and high-resolution XPS using synchrotron radiation). Recently, the applicants have demonstrated the ideal complementarity of these techniques in relation to previous cooperation projects [1]. In brief, IRAS provides unprecedented “chemical resolution” to differentiate between surface species, bonding geometries, and sites. XPS yields detailed information on the oxidation state and the chemical environment of the atoms of adsorbed species, also in cases where no information can be obtained from IRAS. In addition, we will apply TPD and MB methods, to address temperature-dependent reactivity. To link the UHV model studies to “real world” applications, we will explore elevated and ambient pressures by PM-IRAS and, if required, HP-XPS as an additional high-pressure method. Also, we will perform studies on engineered nanomaterials using in-situ DRIFTS.

By following a closely synchronized work program, synergies between the applied methods will be maximized. We will start by studying ordered MgO(100) films on Ag(100) in the UHV part, and in parallel compare to MgO nanocube powders provided by funCOS 5. In the second project phase we will proceed to other oxide materials, i.e. Co3O4, and finally to TiO2 surfaces. In parallel, we will start to explore nanostructured oxide surfaces, e.g. defect structures, oxide nanoparticles, and metal nanoparticles supported on these oxides.

As representative selection of anchor groups (R) we will first explore -CN (soft Lewis base, polar), -OH (weak Brønsted acid), -COOH (strong Brønsted acid), and -NH2 (Brønsted/Lewis base), and later proceed to other linkers depending on the obtained results. As test molecules we will start using benzene derivates, e.g. C6H5R, where isotopically labeled C6D5R will be chosen to optimize detection in IR when required. These test molecules will easily allow us to proceed to bifunctional and chelating linkers by investigation of doubly-substituted benzene derivates (C6H4R2, C6H4RR’). From the results obtained, the most promising linkers will be selected and introduced as linking elements into functional units. To this end, we will start by exploring 5,10,15,20-tetraphenylporphyrin (2HTPP) and its linker-substituted derivates. The influence of molecule–oxide bond formation on the molecular orientation, film growth, film stability, and surface reactivity will be explored.

[1]Steinrück, H.-P.; Libuda, J.; Wasserscheid, P.; Cremer, T.; Kolbeck, C.; Laurin, M.; Maier, F.; Sobota, M.; Schulz, P.S.; Stark, M. Adv. Mater. 2011, 23, 2571.