The molecular mechanisms by which mechanical energy accelerates a chemical reaction at sliding solid−solid interfaces are not well understood because of the experimental difficulties in monitoring chemical processes and their rates, and in controlling parameters such as interfacial temperature. These issues are addressed by measuring the shear-induced rate of methane formation from the decomposition of adsorbed methyl thiolate species on copper in ultrahigh vacuum (UHV), where the frictional heating is negligible. The effect of a constant force F on the energy profile for thiolate decomposition from density functional theory calculations is modeled by superimposing a linear potential, V(x) = −Fx. This enables the change in activation barrier to be calculated as a function of force. The mechanically induced reaction rate is measured by sliding a ball over a methyl thiolate-covered copper surface from the methane yield measured by a mass spectrometer placed in the UHV chamber. Molecular dynamics simulations reveal that a wide distribution of forces are exerted on the thiolates and comparing the measured methyl thiolate decomposition rate with the rate calculated by assuming a wide force distribution reproduces the experimental data. This reveals that only a small proportion of the adsorbed thiolates experience sufficiently high forces to reduce the activation barrier to reproduce the experimentally measured rate constant.

Quasi-static quantum calculations of the mechanochemical decomposition rate of methyl thiolate species on Cu(100) accurately reproduce the experimental kinetics measured in ultrahigh vacuum by an atomic force microscopy tip.

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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.

It has been shown that the rate of decomposition of methyl thiolate species on copper is accelerated by sliding on a methyl thiolate covered surface in ultrahigh vacuum at room temperature. The reaction produces small gas-phase hydrocarbons and deposits sulfur on the surface. Here, a new ReaxFF potential was developed to enable investigation of the molecular processes that induce this mechanochemical reaction by using density functional theory calculations to tune force field parameters for the model system. Various processes, including volumetric expansion/compression of CuS, CuS, and CuS unit cells; bond dissociation of Cu-S and valence angle bending of Cu-S-C; the binding energies of SCH, CH, and S atoms on a Cu surface; and energy for the decomposition of methyl thiolate molecular species on copper, were used to identify the new ReaxFF parameters. Molecular dynamics simulations of the reactions of adsorbed methyl thiolate species at various temperatures were performed to demonstrate the validity of the new potential and to study the thermal reaction pathways. It was found that reaction is initiated by C-S bond scission, consistent with experiments, and that the resulting methyl species diffuse on the surface and combine to desorb ethane, also as found experimentally.