Adsorption of H₂O, CO₂ and Xe on Soft Surfaces

Abstract

The interactions of water, carbon dioxide, and Xe with octadecanethiol (C(18)H(37)SH, ODT) self-assembled monolayers (SAMs) were studied under ultrahigh vacuum conditions employing temperature-programmed desorption and optical diffraction measurements. The ODT layer was grown on a 1 nm thick gold film deposited over a Ru(001) single-crystal substrate. The gases used in this report differ in their lateral interactions while adsorbed on ODT-SAM being either repulsive (Xe) or attractive (H(2)O, CO(2)). The activation energies for desorption of the first layer from ODT are E(a) = 3.6 +/- 0.9, 4.1 +/- 0.5, and 8.5 +/- 0.9 kcal/mol for Xe, CO(2), and H(2)O, respectively. Sticking probabilities of the three gases on the soft ODT surface are S(0) = 0.7 +/- 0.1, 0.8 +/- 0.1, and 0.95 +/- 0.05 for xenon, CO(2), and water, respectively, derived from the respective adsorption curves. Optical diffraction studies from multilayer coverage grating of Xe on ODT-SAM have demonstrated that sublimation is a thermodynamically more favorable process over diffusion and wetting. The significantly lower binding energy of the first layers of H(2)O and CO(2) adsorbed on the soft surface of ODT compared to that on clean metals and oxides, reflects generally weak (CO(2)) and hydrophobic (H(2)O) interactions that are important for understanding the behavior of these molecules on interfaces that are found in biological systems.

Selective Ablation of Xe from Silicon Surfaces: Molecular Dynamics Simulations and Experimental Laser Patterning

“…The parameters σ Xe–Xe = 4.05 Å and ε Xe–Xe = 2.1 kJ/mol, suggested in ref , are chosen for Xe–Xe, with the potential cutoff of 2.5σ used in the simulations. The potential predicts the binding energy of 3.3 ± 0.1 kcal/mol for a surface atom on a (001) face of a Xe crystal, which is close to the value of evaporation energy obtained in multilayer Xe desorption experiments, 3.6 kcal/mol. The parameters for Si–Xe interaction, σ Si–Xe = 3.9 Å and ε Si–Xe = 1.6 kJ/mol, were fitted in ref to the sum of the van der Waals radii of Si and Xe and to the experimental adsorption energy of Xe on Si(100)-2 × 1 surface, respectively.…”

“…The experimental setup has been described in detail elsewhere. 9,30 oxide SiO 2 /Si(100) sample is attached via copper rods to a closed cycle helium cryostat (APD) capable of cooling the sample down to 25 K and a heating capability up to 800 K. 30 In order to perform the laser ablation and patterning measurements, a p-polarized Nd:YAG pulsed laser working at the second harmonic wavelength was employed (λ = 532 nm, 5 ns pulse duration). The laser power absorbed by the silicon substrate was kept lower than 18 MW/cm 2 to avoid surface damage.…”

“…The maximum surface temperature is way above the standard desorption temperature (57 K) observed in temperature-programmed desorption (TPD) experiments of Xe from solid surfaces, typically performed at slow heating rates of about 2 K/s and under ultrahigh vacuum conditions. 30,35,48,49 This large difference can be attributed to the difference in the substrate heating rates. Indeed, Xe multilayers ablate from ruthenium surfaces at ∼350 K when the heating is induced by 5 ns pulse irradiation at λ = 532 nm and absorbed laser power of 2.5 MW/cm 2 , while the resistive heating (∼2 K/s) results in multilayer desorption at 57 K. 35 Similarly, the vast difference between the desorption temperature in nanosecond pulsed laser desorption (heating rate of 10 8 K/s) and standard TPD (2 K/s) was observed in MD simulations of water desorption from metallic surfaces.…”

“…The experimental setup has been described in detail elsewhere. , Briefly, a standard ultrahigh vacuum chamber at a base pressure of 5 × 10 –10 mbar, equipped with Ne + sputter gun for sample cleaning and a quadrupole mass spectrometer (QMS, VG SX-200) for exposure and coverage determination and calibration, is used in the experiments. In addition, separate Au, Ag, or Ti deposition sources are attached, with in situ quartz microbalance detector for flux calibration measurements.…”

Microkinetic Modeling and Analysis of CO2 Methanation on Ruthenium

“…The site density is chosen as 1 monolayer (i.e., 1.58 × 10 19 sites/m 2 ) on Ru (0001), which corresponds to 2.623 × 10 –09 mol/cm 2 . The probable values of sticking coefficients for the adsorbing species (CO 2 , CO, H 2 O, CH 4 , H 2 ) are chosen as reported in the literature. − Values of binding energies for most of the species are taken from DFT literature. DFT is one of the most accurate approximations at the microscale that is also practical to use nowadays.…”