[Paper Review] Tribopolymer Formation Mechanism on the RuO$_2$(110) Surface

Jing Yang, Yubo Qi+2 more|arXiv (Cornell University)|Jun 13, 2017

Fuel Cells and Related Materials26 references3 citations

TL;DR

This study reveals that mechanical stress above 15 GPa on RuO₂(110)-Oᶜᵛᵈ surfaces transforms weakly physisorbed benzene into chemisorbed, reactive intermediates that form an oxabicyclic tribopolymer rudiment via H-migration. Using DFT-based compression simulations and transition-state theory, it predicts that normal stress dominates tribopolymer formation rates over gas pressure, with half-lives decreasing exponentially with stress and saturating at high benzene partial pressures.

ABSTRACT

Tribopolymer formed on the contacts of microelectromechanical and nanoelectromechanical system (MEMS/NEMS) devices is a major concern hampering their practical use in information technology. Conductive metal oxides, such as RuO$_2$ and ReO$_3$, have been regarded as promising candidate materials for MEMS/NEMS contacts due to their conductivity, hardness, and relatively chemically inert surfaces. However, recent experimental works demonstrate that trace amounts of polymer could still form on RuO$_2$ surfaces. In this study, we demonstrate the mechanism of this class of unexpected tribopolymer formation by conducting density functional theory based computational compression experiments with benzene as the contamination gas. First, mechanical force during compression changes the benzene molecules from slightly physisorbed to strongly chemisorbed. Further compression causes deformation and chemical linkage of the benzene molecules. Finally, the two contacts detach, with one having a complex organic molecule attached and the other with a more reactive surface. The complex organic molecule, which has an oxabicyclic segment, can be viewed as the rudiment of tribopolymer, and the more reactive surface can trigger the next adsorption--reaction--tribopolymer formation cycle. Based on these results, we also predict tribopolymer formation rates by using transition--state theory and the second--order rate law. This study deepens our understanding of tribopolymer formation (especially on metal oxides) and provides strategies for suppressing tribopolymerization.

Motivation & Objective

• To uncover the mechanistic pathway of unexpected tribopolymer formation on conductive RuO₂(110) surfaces, despite their low reactivity.
• To investigate how mechanical compression alters benzene adsorption from physisorption to chemisorption and induces chemical bonding.
• To model the full mechanical switching cycle (compression and detachment) to identify reactive intermediates and surface changes.
• To quantify tribopolymer formation rates using transition-state theory and second-order rate laws under varying stress and gas pressure.
• To provide design principles for suppressing tribopolymerization in MEMS/NEMS by targeting mechanical load and surface reactivity.

Proposed method

• Density functional theory (DFT) with the Quantum ESPRESSO code and GGA-PBE functional, using norm-conserving pseudopotentials and a 680 eV kinetic energy cutoff.
• Modeling of symmetric RuO₂(110)-Oᶜᵛᵈ slabs with 20 Å vacuum spacing and an 8×8×1 k-point grid, with mechanical stress applied via controlled reduction of supercell height.
• Use of the nudged elastic band (NEB) method to calculate activation energies along the reaction pathway under varying normal stress.
• Application of transition-state theory to compute rate constants: $ k = \frac{k_B T}{h} \exp\left(-\frac{\Delta E(\sigma)}{k_B T}\right) $, where $ \Delta E(\sigma) $ is stress-dependent activation energy.
• Second-order rate law: $ \text{rate} = -\frac{d\theta(t)}{dt} = 2k\theta(t)^2 $, with half-life $ t_{1/2} = \frac{1}{2k\theta_0} $, where $ \theta_0 $ is initial surface coverage.
• Calculation of initial surface coverage $ \theta_0 $ using Langmuir isotherm: $ \theta_0 = \frac{p_{\text{C}_6\text{H}_6}}{p_{\text{C}_6\text{H}_6} + p_0} $, with $ p_0 $ derived from adsorption energy and gas-phase partitioning.

Experimental results

Research questions

• RQ1 How does mechanical compression transform weakly physisorbed benzene into chemically bonded species on RuO₂(110)?
• RQ2 What is the critical normal stress threshold that enables spontaneous tribopolymerization on RuO₂(110) surfaces?
• RQ3 What are the key reaction intermediates formed during the compression-induced polymerization of benzene on RuO₂(110)?
• RQ4 How do benzene partial pressure and applied normal stress influence the rate of tribopolymer formation?
• RQ5 Can the reaction half-life and formation rates be quantitatively predicted using transition-state theory and second-order kinetics?

Key findings

• A critical normal stress threshold of 15.0 GPa is identified, above which the activation barrier for benzene linkage drops to zero, enabling spontaneous polymerization.
• At stresses above 15 GPa, benzene molecules form a reactive biphenyl-like intermediate with an oxabicyclic segment, representing the rudiment of a tribopolymer.
• The reaction half-life decreases exponentially with increasing benzene partial pressure, saturating at pressures ≥1×10⁻⁴ atm due to full surface coverage.
• Normal stress has a stronger influence on reaction rate than gas-phase partial pressure, with rate changes spanning several orders of magnitude with stress variation.
• The activation energy decreases linearly with decreasing supercell length, indicating that higher local stress concentrations reduce the energy barrier for reaction.
• Detachment after compression leaves one contact with a complex organic molecule (oxabicyclic segment) and the other with a more reactive, defect-like surface, enabling a self-sustaining cycle of adsorption–reaction–polymerization.