Franziska Traeger

Probing the interaction of the amino acid alanine with the surface of ZnO(1010).

The adsorption modes and stability of the amino acid alanine (NH(2)-CH(CH(3))-COOH) have been studied on the nonpolar single crystal surface of zinc oxide, ZnO(1010), experimentally by X-ray photoelectron spectroscopy (XPS) and computationally using density functional theory (DFT). Deposition at 200 K was found to lead to the formation of multilayers identified by an XPS N1s peak at 401.7 eV assigned to the NH(3)(+) group, a fingerprint of the zwitterionic structure of alanine in the solid state. Heating to 300 K resulted in the removal of most of the multilayers with the remaining surface coverage estimated to 0.4 with respect to Zn cations. At this temperature most of the alanine molecules are found to be deprotonated (dissociated), yielding a carboxylate species (NH(2)-CH(CH(3))-COO(-) (a) + OH (s); where O is surface oxygen, (a) for adsorbed and (s) for surface species). Further heating of the surface resulted in a gradual decrease of the surface coverage and by 500 K a large fraction of adsorbed alanine molecules have desorbed from the surface. Total energy DFT computations of different adsorbate species identified two stable dissociative adsorption modes: bidentate and monodentate. The bidentate species with adsorption energy of 1.75 eV was found to be more stable than the monodentate species by about 0.7 eV.

Formation of weakly bound, ordered adlayers of CO on rutile TiO2(110): A combined experimental and theoretical study

The adsorption of CO on the rutile TiO(2)(110) surface was investigated using He atom scattering (HAS), high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and different types of ab initio electronic structure calculations. The experimental and theoretical results allow to put forward a consistent picture for this rather complicated adsorbate system. At 70 K a (2x1) adlayer with a glide symmetry plane is formed, containing two molecules per unit cell which are tilted in alternate directions by about 20 degrees relative to the surface normal. For this high density phase, the theoretical calculations reveal a substantial repulsion between CO molecules on neighboring lattice sites, in accord with the results of a detailed analysis of the experimental TDS data. The CO binding energy depends strongly on coverage and varies between 0.20 eV for the saturated monolayer and 0.36 eV for isolated molecules. The CO-CO repulsion leads to the desorption of about half of the CO molecules above 70 K and the formation of low density phases. HAS gave no indication of ordered adlayers at these lower coverages. For the internal stretching vibration of the CO molecules a value of 273 meV was determined by HREELS, in very good agreement with the theoretical calculations.

CO2 Adlayers on the Mixed Terminated ZnO(10-10) Surface Studied by He Atom Scattering, Photoelectron Spectroscopy and Ab Initio Electronic Structure Calculations

He-atom scattering (HAS), X-ray photoelectron spectroscopy (XPS and NEXAFS), periodic density functional theory and quantum chemical ab initio cluster calculations have been used for characterizing the adsorption of CO2 on the mixed terminated ZnO(10-10) surface. The HAS data allow to identify three different structural phases of CO2 in the monolayer regime: A high-density (1×1) phase at low temperature, a lower density (2×1) phase at higher temperatures, and in addition an incommensurate phase (which has not been reported before) with a slightly lower coverage and a 2.3-fold periodicity at surface temperatures above 270 K. XPS and NEXAFS measurements performed for the two low-coverage phases show that CO2 is adsorbed in form of a carbonate species. This is supported by the ab initio calculations for CO2 adsorbed on ZnO(10-10). The calculated XP and NEXAFS spectra agree with the experimental data and are only compatible with an adsorbed carbonate species, the presence of physisorbed CO2 can be excluded. In addition, the calculations demonstrate that X-ray emission spectroscopy (XES) is a sensitive method to distinguish between linear CO2 and carbonate.

Adsorbate-induced lifting of substrate relaxation is a general mechanism governing titania surface chemistry

Under ambient conditions, almost all metals are coated by an oxide. These coatings, the result of a chemical reaction, are not passive. Many of them bind, activate and modify adsorbed molecules, processes that are exploited, for example, in heterogeneous catalysis and photochemistry. Here we report an effect of general importance that governs the bonding, structure formation and dissociation of molecules on oxidic substrates. For a specific example, methanol adsorbed on the rutile TiO2(110) single crystal surface, we demonstrate by using a combination of experimental and theoretical techniques that strongly bonding adsorbates can lift surface relaxations beyond their adsorption site, which leads to a significant substrate-mediated interaction between adsorbates. The result is a complex superstructure consisting of pairs of methanol molecules and unoccupied adsorption sites. Infrared spectroscopy reveals that the paired methanol molecules remain intact and do not deprotonate on the defect-free terraces of the rutile TiO2(110) surface.

Diffraction patterns of He atoms from the MgO(100) surface calculated by the close-coupling method

An analysis of He diffraction data for the MgO(001) surface which goes beyond hard-wall eikonal approximations is presented. In a first step, a model potential, for which the form of a corrugated Morse potential is chosen, is set up using the eikonal approximation in connection with an effective corrugation function. The obtained corrugation amplitude is compared to results from density-functional theory calculations of the He–MgO interaction. In a second step, this model potential is used for close-coupling (CC) calculations of He diffraction intensities. A kinematical analysis of the system He/MgO is given. The results on the He diffraction intensities are in good agreement with the experiment.

A theoretical study of the XP and NEXAFS spectra of alanine: gas phase molecule, crystal, and adsorbate at the ZnO(100) surface

The adsorption of alanine on the mixed-terminated ZnO(10 ̅10) surface is studied by means of quantum-chemical ab initio calculations. Using a finite cluster model and the adsorption geometry as obtained both by periodic CPMD and embedded cluster calculations, the C1s, N1s and O1s X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra are calculated for single alanine molecules on ZnO(10 ̅10). These spectra are compared with the spectra calculated for alanine in the gas phase and in its crystalline form and with experimental XPS and NEXAFS data for the isolated alanine molecule and for alanine adsorbed on ZnO(10 ̅10) at multilayer and monolayer coverage. The excellent agreement between the experimental and calculated XP and NEXAFS spectra confirms the calculated adsorption geometry: A single alanine molecule is bound to ZnO(10 ̅10) in a dissociated bidentate form with the two O atoms of the acid group bound to two Zn atoms of the surface and the proton transferred to one O atom of the surface. Other possible structures, such as adsorption of alanine in one of its neutral or zwitterionic forms in which the proton of the -COOH group remains at this group or is transferred to the amino group, can be excluded since they would give rise to quite different XP spectra. In the multilayer coverage regime, on the other hand, alanine is in its crystalline form as is also shown by the analysis of the XP spectra.