Temperature-Regulated Dual-mode Oxidation

• Temperature-regulated dual-mode oxidation mechanisms are processes exhibiting two distinct regimes—typically diffusion-limited and reaction-limited—activated at specific temperature thresholds.
• They occur in diverse systems such as MoS₂ layers, metal alloys, and oxide-coated nanoparticles, where temperature shifts trigger morphological transitions and changes in mass transport.
• Understanding these mechanisms enables precise thermal control in applications ranging from microelectronics and thermal protection to catalysis and combustion in nanomaterials.

Temperature-regulated dual-mode oxidation mechanisms describe systems in which oxidation proceeds via two distinct mechanistic pathways or regimes, with the dominant pathway sharply governed by temperature, often with associated transitions in kinetic behavior, phase, morphology, or atomic transport species. Such mechanisms are prevalent in solid-state, interfacial, and nanoparticulate environments, where both chemical and mass-transport phenomena couple strongly to external thermal driving. The general schema appears throughout metallic alloy oxidation, two-dimensional materials chemistry, oxide-coated nanoparticles, semiconductor processing, and radical–radical gas-phase reactions. Multiple archetypes are evident in the literature, including prototypical studies of MoS₂ heterointerfaces, Ti-based composites, ZrC at ultra-high temperatures, CO-oxidation at electrodes, and combustion-related gas-phase reactions, each illustrating distinctly temperature-steered duality in oxidation pathways.

1. General Principles and Archetypal Dual-mode Behavior

Temperature-regulated dual-mode oxidation arises when multiple oxidation mechanisms are thermally accessible, with at least one regime favored at lower temperatures and another at elevated temperatures. This typically manifests as:

• Distinct rate laws: e.g., parabolic (diffusion-limited) versus linear (interface- or reaction-limited) kinetics, or more complex nucleation-and-growth versus first-order laws.
• Morphological transitions: e.g., formation of discrete etch pits versus oxide islands in 2D materials, or growth of protective versus spalling oxide scales in alloys.
• Dominant species or processes: e.g., switching from molecular to atomic oxidants, or from cation outward to anion inward diffusion.

A unified kinetic expression is often of the form

$\textrm{rate}(T) = \sum_i k_i(T) f_i(T)$

with the $k_i(T)$ and $f_i(T)$ (rate constants and activation factors for each mode) having distinct Arrhenius or non-Arrhenius temperature dependences, such that dominance crosses over as $T$ increases.

2. Interfacial and Layered Systems: MoS₂ Dual-mode Oxidation

In van der Waals layered crystals such as MoS₂, temperature modulates oxidation between two crystallographically controlled pathways (Ryu et al., 2020):

• In-plane, top-surface oxidation ("TE mode"): At $T_{\textrm{ox}} \simeq 320^\circ$C, under moderate oxygen partial pressure, oxidation on the topmost MoS₂ layer yields triangular etch pits (TEs) terminated by zigzag edges. These result from the reaction $$2\ \textrm{MoS}_2 + 7\ \textrm{O}_2 \rightarrow 2\ \textrm{MoO}_3$$ (g) $+ 4\ \textrm{SO}_2$ (g), with volatile products desorbing.
• Out-of-plane, interface-confined oxidation ("TO mode"): Simultaneously, in multilayers directly contacting a SiO₂ substrate, suprathermal diffusion of O₂ through the vdW gap at elevated $T$ produces large triangular oxide islands at the MoS₂:SiO₂ interface (TOs), several times larger than TEs and distinctly elevated (0.3–0.6 nm), with same edge termination but different stacking orientation.
• Critical thermal control: Below $\approx 250^\circ$C, neither mode is accessible; O₂ diffusion in the van der Waals gap is strongly thermally activated and dominates mode accessibility. Both modes exhibit Arrhenius-type kinetics, with interface oxidation sharply accelerating above $T_{\textrm{ox}}\approx 300^\circ$C.
• Substrate effects: SiO₂ provides catalytic or doping enhancement, dramatically accelerating interface-confined oxidation (TO area $\approx 5\times$ TE area), with pronounced suppression on hBN, graphene, or less intimately bound substrates.

This system exemplifies dual-mode oxidation, as tuning $T$ selectively opens (or closes) surface and interface channels, each governed by edge-crystallographic anisotropy and mass-transport barriers unique to its mode (Ryu et al., 2020).

3. Bulk and Composite Alloys: Ti–6Al–4V/(TiC+TiB) and ZrC

In high-temperature ceramics and metallic composites, the interplay between protective oxide integrity and transition to non-protective, rapid oxidation is governed by temperature-activated mechanical and chemical processes.

For Ti–6Al–4V/(TiC+TiB) (Wei et al., 2017):

• Parabolic regime (873 K): Growth of a continuous, adherent Al₂O₃ film is diffusion-limited (Δm² ∝ t), with negligible spallation.
• Intermediate regime (973 K): Partial scale breakdown; kinetics become "quasilinear" (Δmⁿ ∝ t, $1 < n < 2$), with multilayered mixed Al₂O₃/TiO₂ scales and local spallation.
• Linear regime (1073 K): Cyclical, complete spallation of oxide multilayers; kinetics become linear (Δm ∝ t). Mechanical stress at the scale-substrate interface now dominates.

For substoichiometric ZrC (Konnik et al., 2024):

• Diffusion-limited regime ($T \lesssim 1200^\circ$C, $n \approx 0.5$–0.6): Porous, weakly adherent oxide scales; oxidation proceeds via diffusion of O through oxycarbide.
• Mixed/transition regime ($1200^\circ$C $< T < 1400^\circ$C, $n$ increases to $\sim 1$): Increasing contribution of linear kinetics as scales densify and nucleate nitride via carbothermal reduction and concurrent nitridation.
• Linear/nitride-dominated regime ($T > 1400^\circ$C, $n \rightarrow 1$): Nitridation and carbothermal reduction densify the oxide, suppress net mass gain, and phase-stabilize protective t-ZrO₂ (Konnik et al., 2024).

Both cases demonstrate kinetic and morphological transitions as temperature regulates the crossover between slow, protective (diffusion-limited) and rapid, mechanically unstable (interface-limited or reactive) oxidation.

4. Nanoparticles and Membrane-driven Oxidation: Aluminum Nano-energetics

Oxide-coated aluminum nanoparticles exhibit a temperature-regulated dual-mode oxidation with two distinct regimes separated by a sharp critical temperature (Lu et al., 27 Dec 2025):

• "Breathing mode" ($T\lesssim1700$ K): The oxide shell remains intact but undergoes dynamic fluctuations; transient nanoscale channels (1–3 nm wide) periodically open, allowing Al$^{3+}$ outward and O$^{2-}$ inward diffusion. The shell acts as a "gatekeeper," and oxidation is limited by cation diffusion.
• "Rupture mode" ($T\gtrsim2000$ K): Once the shell experiences excessive thermal and mechanical stress, brittle rupture exposes molten Al directly to O₂, producing explosive, transport-unlimited oxidation. The transition threshold is $T_c\approx 1900$–2000 K for $\sim$3 nm shells.

Quantitative analysis demonstrates Al$^{3+}$ diffusion coefficients exceed O$^{2-}$ by 2–4 orders of magnitude across 1000–2000 K, universally establishing Al cation diffusion as the rate-limiting process in both modes (Lu et al., 27 Dec 2025). The coarse-grained dual-mode dynamics provide critical handles on ignition sensitivity and combustion control in energetic nanomaterials.

5. Gas-phase and Surface Reactions: Electronic-State and Site-specific Duality

Temperature-regulated dual-mode oxidation is pronounced in radical gas-phase and surface reactions, often connected to population of distinct adsorbate states, binding sites, or electronic pathways.

a. Multi-state Radical Reactions

For the OH + HO₂ $\rightarrow$ O₂ + H₂O reaction (Zhang et al., 8 Jul 2025):

• Mode 1: Ground-state, low-T regime ($T \lesssim 500$ K): Only HO₂(X $^2$A'') population; three exothermic, barrierless channels dominate, yielding a T-independent or slightly negative rate constant.
• Mode 2: Excited-state, high-T regime ($T \gtrsim 900$ K): Blackbody radiation populates HO₂(A $^2$A'), opening new, weakly endothermic channels. Between 900–1242 K, the rate constant develops a sharp "well" as endothermic branch thresholds are crossed; above ∼1242 K, channels with $E_{a}=0.107$ eV turn on, restoring strong temperature-dependence (Zhang et al., 8 Jul 2025).

b. Electrocatalysis: Site-Dependent Duality

Pt/C-catalyzed CO oxidation (Balasubramanian et al., 2013) demonstrates:

• Weakly-bound (atop) COad: Exponential decay (first-order kinetics), $k = A_w \exp(-E_{a,w}/RT)$, with $E_{a,w}\sim57$ kJ/mol.
• Strongly-bound (multi-coordinated) COad: Nucleation-and-growth control at low $T/E$ ($n\approx2$–3), transitioning to exponential decay as $T$ or $E$ rises and site-heterogeneity collapses ($n\to1$).

Thus, at low temperatures/potentials, oxidation rates are set by the nucleation of OHad on defect-rich sites, whereas at high $T$, oxidation becomes uniformly first-order across all sites.

6. Atomic-scale and Mechanistic Basis: Competing Transport and Reaction Channels

First-principles analyses reveal that dual-mode behavior often originates from temperature-controlled competition between molecular and atomic oxidation, or shift in the dominant mass-transport species. For SiO₂ and SiC (Shen et al., 2012):

Face	Low-T Dominant Oxidant	High-T Dominant Oxidant	Crossover Regime
Si(100)/SiO₂	O₂ (molecular)	O₂ (molecular)	Linear-parabolic law, always O₂
Si-face SiC	Oᵢ (atomic)	Oᵢ (atomic)	O₂ barrier too high, always Oᵢ
C-face SiC	O₂ (defect-assisted)	Oᵢ (atomic)	Crossover at T ≈ 1350°C

Here, the transition in SiC at $T\approx1350^\circ$C from O₂-cracking to Oᵢ-diffusion is dictated by interface bond density and defect concentration, with Arrhenius-type kinetic signatures for both linear and parabolic regimes (Shen et al., 2012). Diffusion coefficients, reaction barriers, and interface permeability determine the temperature threshold for each pathway.

7. Implications, Applications, and Generalizations

Temperature-regulated dual-mode oxidation is a general phenomenon, critical for:

• Device fabrication/control: Enabling patterning strategies and phase-selective oxidation in 2D electronics (MoS₂, WS₂ families) and microelectronic oxides (Ryu et al., 2020, Reidy et al., 2022).
• Thermal protection systems: Establishing usability intervals and failure thresholds of ceramics and alloys in extreme environments (ultra-high-temperature ceramics, energetic alloys) (Wei et al., 2017, Konnik et al., 2024).
• Energetic and catalytic performance: Governing burn rates, ignition control, and catalytic turnover by selecting for diffusion-limited or reaction-limited regimes (Lu et al., 27 Dec 2025, Balasubramanian et al., 2013).
• Fundamental mechanism elucidation: Disentangling site- or state-specific oxidation in heterogeneous catalysis or radical chain reactions, via mechanistic kinetic modeling (Zhang et al., 8 Jul 2025).

Such mechanisms highlight the central role of thermal control in chemically and morphologically patterning interfacial, nanostructured, and bulk materials, and provide quantitative, predictive frameworks adaptable for kinetics engineering across diverse chemical systems.