Natzeck, Carsten

Thissen, Peter

Weidler, Peter

Woell, Christof

English

KITopen

KIT – University of the State of Baden-Wuerttemberg andNational Research Center of the Helmholtz AssociationReferencesInfrared Reflection Absorption Spectroscopy (IRRAS)On dielectric surfaces, the surface selectionrule does not apply!Consequences: both s- and p-polarized light cancouple to adsorbate vibrations:C. Natzeck, P. Weidler, C. Wöll, P. Thissen* Using the standard optical path within theIR-spectrometer without any additional optical element Attach spectrometer directly to Ultra High Vacuum (UHV) chamberExperimental Challenge: verylow reflectivity of dielectrics!Investigations of interaction of water and methanol on Wollastonite surface have been done by IRRAS allowingfor consideration of all components of the incident polarized light separately. With the help of first-principles calculations,a model of the Wollastonite as model surface for concrete satisfying most of the discussed points of the IRRAS could be developed.Surprisingly, methanol did not dissociate during adsorption on the Wollastonite at room temperature. This chemical behavior is in sharpcontrast to water, quickly dissociating in contact with CS surfaces as first step of the Metal Proton Exchange Reaction.Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Karlsruhe, GermanyMotivation The surface chemistry of the mineral Wollastonite as a real model sample for Calcium-Silicate (CS) phase has been studied using infraredreflection-absorption spectroscopy under ultrahigh-vacuum conditions (UHV-IRRAS) using water and methanol as probe molecules. This is the first application of IRRAS to a crystalline, yet multi-domain mineral surface of natural origin - The assignment of the vibrationalbands is supported by a first-principles theoretical study.Wollastonite as real model sample  Sample surface breaks into lath-or needle shaped (acicular)particles with preferred orientationalong the b-axisFirst-principles theoretical study [3] C-O-H deformation mode appears in non- und p-polarized mode, but not in s-polarization: gives evidence for a strong orientation of methanol along the b-axis. Molecular methanol is bond on top of the Ca (Ca-O) and gaining additionalorientation along the b-axis by hydrogen-bonding (Si-O-H). The Ca-O-H angle is 84.4°, and the Ca-O-C is 136.6°, explaining best theappearance of the C-O-H deformation mode in s-polarization but not in p-polarization. CH3 umbrella can be associated with the intact OH group of the methanol:the CH3 umbrella mode is split.b-axisCleaning with XPS-monitoring in situ under UHV condition moderate Heating (400 K, 1 h), gentle sputtering (Ar, 15 min) Embedded into resin Grinding / polish steps Removal resinPart of natural stone – mechanical preparation [1]Characterization with XRD specimen is pure Wollastonite andno other mineral phase is presentStructure unit Distance / Å Structure unit Angle / °Ca-OCH3OH 2.32 C-O-H 113.5Si-Otop 1.57 Ca-O-H 84.8Otop-HCH3OH 1.61 Ca-O-C 136.5OCH3OH-HCH3OH 1.01 H-C-O-H 169.8Si-Otretrahedron 1.67Conclusion Calculations are performed with Vienna Atomic Simulation Package (VASP) Basis Set: Plane Waves, Supercell Approximation: Periodic Boundaries Exchange-Correlation Functional: Generalized Gradient Approximation(Perdew and Wang ‘91) Ultrasoft Pseudo Potentials Kinetic Energy Cut-off: 360 eVFig. 8: Side- and top-view of energetically favorable structures modeling the adsorption of CH3OH on Wollastonite(001)Fig. 5: Top view on the clean wollastonite(001) surface. The surface unit cell is highlighted. The arrows indicate the chains formed by Si-O tetrahedra along the crystal b-axis. Grey spheres represent Ca, red spheres represent O and green spheres represent Si[1] R. C. Longo, K. Cho, P. Brüner, A. Welle, A. Gerdes, P. Thissen, ACS  Appl.  Mater.  Interfaces 2015, 7, 4706−4712 [2] Buerger, M. J. Proc. Natl. Acad. Sci. U.S.A. 1956, 42, 113−116[3] N. Giraudo, P. Krolla-Sidenstein, S. Bergdolt, M. Heinle, H. Gliemann, F. Messerschmidt, P. Brüner, P. Thissen, J. Phys. Chem. C, 2015Fig. 2: UHV-IR/XPS-chamber at the IFG: “THEO“* Karlsruhe Institute of Technology, Institute of Functional Interfaces, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, GermanyE-Mail: peter.thissen@kit.eduCalculation ExperimentalVibration mode DFT p-polarization s-polarizationC-O-H-deformation 1380 1360CH3 umbrella 1167 1177CH3 umbrella 1146 1157C-O stretching 1040 1044 1046Water on Wollastonite Surface Water dissociate in interaction with Wollastonite surface – early stage hydrationor Metal Proton Exchange Reaction (MPER): Measurements at 10-10 mbar pressure and RT or 100 K sample temperature beam path: grazing incidence (80°) in direction of preferred orientation (b-axis) Exposures of water or methanol in situ in Langmuir units (L) from 1 mL to 1000 mL Methanol on Wollastonite Surface Methanol does not dissociate in interaction with Wollastonite surface – no protontransfer take place and weakly chemisorbed molecular Methanol is present:1600 1400 1200 1000 800(Si-O) s-polarization p-polarizationAbsorbanceWavenumber / cm-15x10-4(Si-OD) Si-OD stretching vibration is the strongest mode and is formed by transfer of aproton from water no vibrational modes from molecular D2O are observed (D2O at 1250 cm-1) a further OD-binding on Ca species is with 350 cm-1 (Ca-OD) outer limit in comparison to the sensitivity of the Si-OD vibration, the intensity of surfaceO-D (2690 cm-1) is very lowFig. 3: Differential infrared absorption spectra of D2O adsorbed on Wollastonitereferenced to the clean surface before exposure in s- and p-polarization modes1600 1400 1200 1000 800(SiO)(CD3)(CD3)p-polarization1x10-4AbsorbanceWavenumber / cm-1(CO)1600 1400 1200 1000 800(CD3)(SiO)(CD3)s-polarizationWavenumber / cm-11x10-4(CO)1600 1400 1200 1000 800 1 mL 10 mL(SiO)p-polarizationAbsorbanceWavelength / cm-13x10-4(CO)1600 1400 1200 1000 800 1 mL 10 mL(SiO)(CO)umbrellas polarizationWavelength / cm-11x10-4(COH)Fig. 1: Wollastonite crystal representation [3]Fig. 7: Differential infrared absorption spectra of CD3OD (left side) and CH3OH (right side) adsorbed on Wollastonite referenced to the clean surface before exposure in s- and p-polarization modesFig. 6 (C) and (D) present the molecular adsorption of methanol on the clean Wollastonite(001) surface  lead to no Metal Proton Exchange Reaction:no formation of Si-OH or Ca-OH species weakly deformation of Si-O-tetrahedraC) D)A) B) lead to partial leaching of Ca from thenear surface reaction [1] and depend onthe pH value of aqueous environment [2] first step is the proton exchangebetween adsorbed molecular speciesand the mineral surface – leading to theformation of surface OH species lead to strong deformation of Si-O-tetrahedra keeping the same volumeFig. 4: (A) and (B) present the proton exchange during the water adsorption on the clean Wollastonite(001) surface. Gray spheres represent Ca, red spheres represent O, and blue spheres represent Si

Investigation of Calcium-Silicate surfaces with UHV-IRRAS to understanding the first step in the corrosion of concretes

https://publikationen.bibliothek.kit.edu/1000054897/3847952

Investigation of Calcium-Silicate surfaces with UHV-IRRAS to understanding the first step in the corrosion of concretes

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