Modeling Mechanochemical Reaction Mechanisms.

The mechanochemical reaction between copper and dimethyl disulfide is studied under well-controlled conditions in ultrahigh vacuum (UHV). Reaction is initiated by fast S-S bond scission to form adsorbed methyl thiolate species, and the reaction kinetics are reproduced by two subsequent elementary mechanochemical reaction steps, namely a mechanochemical decomposition of methyl thiolate to deposit sulfur on the surface and evolve small, gas-phase hydrocarbons, and sliding-induced oxidation of the copper by sulfur that regenerates vacant reaction sites. The steady-state reaction kinetics are monitored in situ from the variation in the friction force as the reaction proceeds and modeled using the elementary-step reaction rate constants found for monolayer adsorbates. The analysis yields excellent agreement between the experiment and the kinetic model, as well as correctly predicting the total amount of subsurface sulfur in the film measured using Auger spectroscopy and the sulfur depth distribution measured by angle-resolved X-ray photoelectron spectroscopy.

Adsorption of hydrogen on the surface and sub-surface of Cu(111).

The interaction of atomic hydrogen with the Cu(111) surface was studied by a combined experimental-theoretical approach, using infrared reflection absorption spectroscopy, temperature programmed desorption, and density functional theory (DFT). Adsorption of atomic hydrogen at 160 K is characterized by an anti-absorption mode at 754 cm(-1) and a broadband absorption in the IRRA spectra, related to adsorption of hydrogen on three-fold hollow surface sites and sub-surface sites, and the appearance of a sharp vibrational band at 1151 cm(-1) at high coverage, which is also associated with hydrogen adsorption on the surface. Annealing the hydrogen covered surface up to 200 K results in the disappearance of this vibrational band. Thermal desorption is characterized by a single feature at ∼295 K, with the leading edge at ∼250 K. The disappearance of the sharp Cu-H vibrational band suggests that with increasing temperature the surface hydrogen migrates to sub-surface sites prior to desorption from the surface. The presence of sub-surface hydrogen after annealing to 200 K is further demonstrated by using CO as a surface probe. Changes in the Cu-H vibration intensity are observed when cooling the adsorbed hydrogen at 180 K to 110 K, implying the migration of hydrogen. DFT calculations show that the most stable position for hydrogen adsorption on Cu(111) is on hollow surface sites, but that hydrogen can be trapped in the second sub-surface layer.

Surface chemistry of isopropoxy tetramethyl dioxaborolane on Cu(111).

The surface chemistry of isopropoxy tetramethyl dioxaborolane (ITDB), tetramethyl dioxaborolane (TDB), and 2-propanol is studied on a clean Cu(111) single crystal using temperature-programmed desorption (TPD). 2-Propanol is found to have two competing reactions on the copper surface. Dehydration results in water and propene formation, and dehydrogenation results in the formation of acetone and hydrogen. ITDB directly adsorbed on the surface reacts completely and does not molecularly desorb. TDB and 2-propanol decompose desorbing mainly 2,3-dimethyl 2-butene and acetone, respectively. Both of those products desorb above room temperature and are present in TPDs of ITDB. An additional acetone desorption peak was observed for ITDB at higher temperatures than acetone desorption from 2-propanol. This higher temperature peak at ∼391 K was attributed to two acetone molecules forming from the tetramethyl end group resulting from a stronger bound surface species in ITDB compared to TDB despite their identical end groups. The copper surface seems to be reactive enough toward ITDB at room temperature that a potential boron-containing tribofilm could be produced for copper-copper sliding contacts. Despite their similarities, ITDB and TDB have different surface species present at room temperature, so their tribological properties will be investigated in the future.

Shear properties of potassium chloride films on iron obtained using density functional theory.

Density functional theory is used to calculate the shear strength of a thin KCl film grown epitaxially on an Fe(100) substrate. It is first demonstrated that the calculations accurately reproduce the experimental values of the shear moduli of bulk KCl. The method is then extended to calculating the shear properties of a three-layer slab of KCl on an Fe(100) substrate, where shear is found to take place at the KCl-Fe(100) interface and the KCl essentially moves as a rigid body. The resulting calculated values of the shear strength at zero pressure along the [linear span]10[linear span] (79.8 MPa) and [linear span]11[linear span] (70.3 MPa) directions are in excellent agreement with the experimentally measured value of 65 ± 5 MPa.

Low-temperature, shear-induced tribofilm formation from dimethyl disulfide on copper.

The frictional properties of a sliding copper-copper interface exposed to dimethyl disulfide (DMDS) are measured in UHV under conditions at which the interfacial temperature rise is <1 K. A significant reduction in friction is found from the clean-surface values and sulfur is found on the surface and below the surface in the wear scar region by Auger spectroscopy. Because the interfacial temperature rise under the experimental conditions used to measure friction is very small, tribofilm formation is not thermally induced. The novel, low-temperature tribofilm formation observed here is ascribed to a shear-induced intermixing of the surface layer(s) with the subsurface region as suggested using previous molecular dynamics simulations. Although the tribofilm contains predominantly sulfur, a small amount of carbon is also found in the film.

The surface chemistry of dimethyl disulfide on copper.

The surface chemistry of dimethyl disulfide (DMDS) is studied on a Cu(111) single crystal and a polished copper foil in ultrahigh vacuum as a basis for understanding its tribological chemistry using a combination of temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). Low-energy electron diffraction reveals that the polished foil becomes ordered on heating in vacuo and displays identical surface chemistry to that found on the Cu(111) surface. Dimethyl disulfide reacts with the copper surface at 80 K to form thiolate species. Heating the surface to ∼230 K causes a small portion of the thiolate species to decompose to form methyl groups adsorbed on the surface. Further heating results in methane and C(2) hydrocarbon desorption at ∼426 K, due to a reaction of adsorbed methyl species, to completely remove carbon from the surface and to deposit atomic sulfur.