Article Cite This: Langmuir 2018, 34, 2432−2440
pubs.acs.org/Langmuir
Surface Chemistry Dependence of Mechanochemical Reaction of Adsorbed MoleculesAn Experimental Study on Tribopolymerization of α‑Pinene on Metal, Metal Oxide, and Carbon Surfaces Xin He and Seong H. Kim* Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Mechanochemical reactions between adsorbate molecules sheared at tribological interfaces can induce association of adsorbed molecules, forming oligomeric and polymeric products often called tribopolymers). This study revealed the role or effect of surface chemistry of the solid substrate in mechanochemical polymerization reactions. As a model reactant, α-pinene was chosen because it was known to readily form tribopolymers at the sliding interface of stainless steel under vapor-phase lubrication conditions. Eight different substrate materials were testedpalladium, nickel, copper, stainless steel, gold, silicon oxide, aluminum oxide, and diamond-like carbon (DLC). All metal substrates and DLC were initially covered with surface oxide species formed naturally in air or during the oxidative sample cleaning. It was found that the tribopolymerization yield of α-pinene is much higher on the substrates that can chemisorb α-pinene, compared to the ones on which only physisorption occurs. From the load dependence of the tribopolymerization yield, it was found that the surfaces capable of chemisorption give a smaller critical activation volume for the mechanochemical reaction, compared to the ones capable of physisorption only. On the basis of these observations and infrared spectroscopy analyses of the adsorbed molecules and the produced polymers, it was concluded that the mechanochemical reaction mechanisms might be different between chemically reactive and inert surfaces and that the chemical reactivity of the substrate surface greatly influences the tribochemical polymerization reactions of adsorbed molecules.
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INTRODUCTION
In the case where frictional heating and triboemissions are negligible, one can hypothesize that perturbation of the potential energy of a molecular system under the action of an external mechanical force could substantially lower the activation energy of a chemical reaction, allowing reactions to take place even at low temperature where typical thermal reactions do not occur.11−13,15 In this scheme, the decrease in the activation barrier by a mechanical action can be modeled with a modified Arrhenius-type equation11,13−18
Mechanochemistry refers to reactions activated by mechanical forces.1−3 Compared to conventional thermochemistry, electrochemistry, and photochemistry where reactions are induced by heat, potential bias, or light irradiation, the molecular mechanisms or key parameters governing yield or selectivity of mechanochemical reactions are much less studied and understood. When a substantial amount of frictional heat is generated at high sliding speeds and not dissipated fast enough through conduction to the solid substrate, the thermal activations of interfacial molecules can be considered to play important roles.4,5 When severe wear of solid materials occurs at high applied loads or under poorly lubricated conditions, then reactive surface sites such as dangling bonds or emission of charged particles (electrons or ions) or photons could be involved in the initiation of chemical reactions of molecules present on the wearing surface.6−8 At a relatively low load and/ or under slow sliding speed conditions, frictional heating and triboemissions are insignificant, but mechanochemical reactions can still take place.9−14 © 2018 American Chemical Society
Reaction rate or yield = A ·exp
−(Ea − Em) k bT
(1)
where A is the pre-exponential factor which will vary depending on the unit of the left-hand side of the equation, Ea is the activation barrier of thermal reaction without any external mechanical shear, Em is the amount of decrease in the activation barrier due to mechanical shear, kb is the Boltzmann constant Received: November 1, 2017 Revised: December 25, 2017 Published: January 27, 2018 2432
DOI: 10.1021/acs.langmuir.7b03763 Langmuir 2018, 34, 2432−2440
Article
Langmuir (1.38 × 10−23 J/K), and T is the (average) temperature of the sliding interface. When the mechanical force is applied directly to the molecule along the reaction coordinate, then the mechanical term can be expressed as Em = F·Δx*, where F is the force applied to the specific bond and Δx* is the difference in the bond length at the equilibrium and transition states.17−20 However, it would be impossible to experimentally measure how much force is applied directly along the specific bond of a molecule. Practically, it is more feasible to control the applied pressure or estimate the shear stress in experiments. Alternatively, the mechanical term can be expressed as Em = σ·ΔV*, where σ is the applied stress, and the proportionality constant (ΔV*) has the dimension of volume; therefore, it is often called the critical activation volume.11,13−15,21−25 Recently, we have studied tribopolymerization reactions of three hydrocarbonsα-pinene, pinane, and n-decane.12 These three molecules consist of 10 carbon atoms with different molecular structures. α-Pinene is a bicyclic terpene with a fourmembered ring and one CC double-bond in the sixmembered ring; thus, it has a very high internal strain.26,27 Pinane is the hydrogenated form of pinene; without the CC bond in the six-membered ring, its internal strain is reduced compared to that of α-pinene. n-Decane is a linear alkane without any internal strain in its equilibrium state. When these molecules are adsorbed on the native oxide layer (mostly consisting of Cr2O3) of AISI 440C stainless steel (SS) and sheared at an applied load of 0.4−1.5 GPa at room temperature, all three molecules (even, n-decane) produced polymeric products.12,13 From the applied pressure dependence and the measured friction coefficient, the shear stress was estimated. By analyzing the shear stress dependence of the tribopolymerization yield, it was found that the internal strain of the molecule has a great impact on the critical activation volumeα-pinene has the smallest ΔV* (∼3% of its molar volume, Vmol), pinane has an intermediate value (∼8% of Vmol), and n-decane has the largest value (∼10% of Vmol). In the equilibrium state, molecules are not compressible; then, what is the physical meaning of the magnitude of ΔV*? To study this question, a computational approach was necessary. In another mechanochemical system studied using molecular dynamics (MD) simulations with a reactive force field called ReaxFF, it was found that tribopolymerization reactions of allyl alcohol at a sliding interface of silicon oxide appear to be associated with the deformation of adsorbed molecules.15 This result implies that the physical process governing ΔV* must be a shear-induced deformation of the adsorbed molecules.15 It was also found that such deformations are facilitated when molecules are chemisorbed on the solid surface. This finding suggests that the substrate and the counter-surface are not just an inert boundary or wall that confines the molecules and delivers the mechanical energy from the external mechanical actuator to the molecule being sheared at the interface; they are involved in mechanochemical reactions as an active participant. The involvement of the solid substrate is expected when mechanochemical reactions result in the removal of substrate atoms such as wear of silicon, silicon oxide, silicate glass, and gallium arsenide in humid environments or wear of carbon materials upon rubbing against silicon in vacuum.28−33 However, it is not straightforward to see the involvement of the solid substrate in association reactions among adsorbed molecules by mechanical shear.9,11−13,24 Therefore, this study designed and performed an experimental investigation to test if
the solid surface chemistry plays a critical role in mechanochemical polymerization of adsorbate molecules. αPinene was chosen as a model adsorbate system because it was found to have a good mechanochemical reactivity in previous studies.12,13 Eight different substrates were chosen based on their activity or inertness in various catalytic reactionsPd, Ni, Co, SS, Au, SiO2, Al2O3, and diamond-like carbon (DLC). All experimental works were carried out under ambient pressure conditions; thus, Ni, Co, and SS are covered with native oxide layers (NiO, CuO, and Cr2O3, respectively).34−36 Pd is also covered with a monolayer of oxide.37,38 Even the Au surface could contain oxide species when cleaned with UV/O3.39 The DLC surface is also partially oxidized in ambient air.40 The comparison of tribopolymer yield of α-pinene on these surfaces under vapor-phase lubrication (VPL) conditions clearly revealed the critical role of the surface chemistry in the activation of adsorbed molecules in mechanochemical reactions.
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EXPERIMENTAL METHOD
A custom-built reciprocating ball-on-flat tribometer with an environment control capability was employed to conduct all tribotests. Details of the system were described previously elsewhere.41 Eight substrates were tested in this study. A silicon (100) wafer was purchased from Wafer World, Inc. (West Palm Beach, FL, USA). The silicon wafer surface was covered with 1−2 nm-thick native oxide.42,43 A hydrogenated DLC film (approximately 1 μm thick) was deposited on a silicon wafer at Argonne National Laboratory.44 Details of the DLC deposition were described in a previous paper.45 An aluminum oxide wafer (with the (0001) c-plane) was purchased from University Wafer (Boston, MA, USA). Polycrystalline gold and palladium foils (∼200 μm thick) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Thick copper, nickel, and AISI 440C SS substrates were purchased from McMaster-Carr (Elmhurst, IL, USA). The Cu, Ni, and SS substrates were polished with fine grit sandpapers and then polished with slurry containing 1 μm colloidal alumina. Using optical profilometry (Zygo NewView 7300), the polished surfaces were found to have a root-mean-square (rms) roughness less than 30 nm. All the substrates were cleaned with ethanol followed by a UV/O3 treatment to make sure there is no organic residue before tribotests.46 The counter ball was made of silicon nitride (diameter = 3/32 in.; purchased from McMaster-Carr). Optical profilometry analysis showed that the rms roughness of the ball surface was around 10 nm after the removal of the sphere curvature. Silicon nitride was chosen because its surface is relatively inert in the absence of water.47 It also has high elastic modulus so that its deformation would be small during tribotests. In friction tests, the normal load was adjusted to keep the Hertzian contact pressure at around 0.5 GPa for all materials. The Hertzian deformation depth varied in the range of 20−140 nm, depending on the substrate. For the load dependence test, the normal load was varied from ∼0.4 up to ∼0.9 GPa. The Hertzian deformation depth of each sample was larger than the surface roughness of the sample; thus, two solid surfaces within the nominal contact area were indeed in intimate and conformal contact.48 The sliding speed was 0.4 cm/s, and the sliding span was 2.5 mm. At this low speed, the flash temperature rise was estimated to be less than 5 °C.12 The sliding contact region during the tribotest was continuously purged with a mixture of dry nitrogen stream (with a dew point of −80 °C and
