Surface Chemisorbed Oxygen
- Chemisorbed oxygen concentration is defined as the density of oxygen atoms chemically bonded to a surface, influencing reactivity and electronic structure.
- It is quantified as a monolayer fraction or atomic percentage using techniques like XPS, STM, and LEED, which offer precise insights for catalytic and sensor applications.
- Strong oxygen chemisorption alters charge transfer and work function, affecting defect passivation and phase formation in oxides, semiconductors, and metal surfaces.
Chemisorbed oxygen concentration refers to the density or fraction of oxygen atoms or molecules firmly bound to a solid surface via chemical interactions, in contrast to weaker physisorbed states. The chemisorption process involves the formation of strong bonds—covalent, ionic, or mixed—between oxygen and surface atoms, frequently leading to charge transfer, substantial modifications of electronic states, and activation of new surface reactivity. Chemisorbed oxygen concentration is a key parameter in catalysis, surface sensing, thin film growth, and device interfaces, governing phenomena such as catalytic activity, sensor response, phase transitions, defect passivation, surface oxide formation, and interfacial transport.
1. Adsorption Sites and Energetics
Chemisorption is highly site-specific and energetically favorable only under certain conditions. On ideal crystalline surfaces, the most energetically favorable adsorption sites are typically ‘bridge’ positions—between two cations, adjacent to lattice oxygen ions, or coordinated sites specific to the substrate symmetry. For SrTiO₃(001), atomic oxygen chemisorbs at the bridge position between a surface oxygen and an adjacent metal ion, resulting in substantial binding energies: 2.96 eV (PW-PBE) and 2.03 eV (LCAO) for TiO₂ termination, 3.06 eV (PW-PBE) and 2.43 eV (LCAO) for SrO termination (Alexandrov et al., 2010).
The underlying adsorption energy is defined as: where the factor 1/2 accounts for symmetric surfaces in the simulation cell.
On transition metal oxides such as Sr₃Ru₂O₇(001), O₂ molecules chemisorb as activated superoxo species (O₂⁻), with adsorption energies at low coverage of –0.99 eV per molecule (RPA-corrected) and –1.49 eV near Ca dopants (Mayr-Schmölzer et al., 2018). The energetic preference for chemisorbed oxygen at grain boundaries or on defects is similarly observed for Sb₂Te₃ and carbon-based surfaces: oxygen at Sb–Te boundaries forms Schottky double barriers that filter charge transport (Ghosh et al., 2023); chemisorbed oxygen on O-functionalized HOPG nucleates water/ice clusters with favorable hydrogen bond energetics (Doktor et al., 25 Feb 2025).
2. Charge Transfer and Electronic Structure
Chemisorption is accompanied by pronounced charge redistribution between the surface and oxygen species, commonly generating quasi-molecular entities such as peroxides, superoxides, or bridging dimers. For atomic oxygen adsorbed on SrTiO₃(001), a surface peroxide-like (O₂²⁻) configuration forms via electron density accumulation and orbital overlap between the adatom and a surface oxygen, as demonstrated in Bader/Mulliken analyses and differential charge maps (Alexandrov et al., 2010).
On TiO₂ anatase (101), molecular O₂ is transformed sequentially: O₂ weakly adsorbs, accepts electrons from subsurface vacancies/dopants to become superoxo (O₂⁻), is further reduced to peroxo (O₂²⁻), and ultimately incorporates as a bridging dimer (O₂)ₒ at anionic lattice sites (Setvin et al., 2018).
In 2D semiconductors (WS₂), O₂ chemisorbed at S-vacancies passivates defect-related gap states, suppressing the free electron density and stabilizing excitonic behavior. h-BN encapsulation is critical for “fixating” these chemisorbed molecules, preventing desorption and enabling robust valley polarization (Jung et al., 2022).
3. Coverage Dependence and Measurement
Chemisorbed oxygen concentration is typically reported as a fraction of a monolayer (ML) or atomic percentage (at.%). Quantification is achieved via surface-sensitive spectroscopies (XPS, EELS), electron microscopy, or in some cases, STM counting of individual adatoms. On Cu(100), the saturation of chemisorbed oxygen after O₂ dosing (1×10⁻⁶ Torr, 5 min at 300 °C) is 0.5 ML, confirmed by (√2×2√2) R45° LEED reconstruction (Robinson et al., 2014).
For SnO₂ thin films, in situ XPS reveals a chemisorbed oxygen fraction of 13.5 at.% at 25 °C and 22.5 at.% at 200 °C under 1 mbar O₂. CO exposure reduces this to 8.4 at.% as the chemisorbed oxygen is consumed in the reaction CO + O(chemisorbed) → CO₂ + e⁻ (Ciftyurek et al., 18 Oct 2025).
Molecular dynamics provides additional quantitative descriptors via the mass accommodation coefficient (MAC), representing the fraction of oxygen molecules that become chemisorbed upon impact. For iron oxidation, MAC drops from unity for clean iron to ~0.03 near FeO stoichiometry (Zₒ = 0.5), indicating a self-limiting coverage with high initial reactivity that rapidly declines as oxidation progresses (Thijs et al., 2022).
4. Migration Barriers and Kinetics
The mobility of chemisorbed oxygen is determined by the migration activation energy, which sets the kinetics of surface oxygen transport and bulk incorporation. For SrTiO₃(001), oxygen atom migration along the (001) direction exhibits barriers of ~0.8–1.3 eV, whereas oxygen vacancies migrate with a much lower barrier (~0.14 eV). Hence, the coupling of relatively immobile chemisorbed oxygen and highly mobile vacancies dictates net oxygen flux and penetration into the bulk (Alexandrov et al., 2010). In many metal oxides, healing of vacancies and surface reaction kinetics are controlled more by vacancy dynamics than by individual oxygen atom mobility, with transformation/barrier energies verified by DFT and first-principles dynamics (Setvin et al., 2018).
The growth and diffusion of water clusters on O-HOPG surfaces are similarly governed by the energetic landscape of oxygen–water hydrogen bonds (0.10–0.17 eV per bond); clusters are immobilized when spanning multiple oxygen defects, preventing mobility and favoring nucleation (Doktor et al., 25 Feb 2025).
5. Functional Impact in Catalytic and Electronic Systems
Chemisorbed oxygen concentration is fundamental in controlling surface reactivity for catalysis, gas sensing, thin film growth, and electronic device functionality:
- Catalysis & Sensors: On metal oxides (SnO₂, Co₃O₄, TiO₂), chemisorbed oxygen species are the direct participants in low-temperature oxidation reactions, notably for CO and formaldehyde sensing. On SnO₂ films, high chemisorbed oxygen coverage at ~200°C enables efficient CO oxidation with a rapid sensor response, and the cyclic consumption-regeneration of chemisorbed species underpins reversibility (Ciftyurek et al., 18 Oct 2025). For single-atom Cu sites on Co₃O₄ (Cu₁–Co₃O₄), interfacial Cu–O–Co linkages enhance lattice oxygen activation, accelerating oxidation reactions and dramatically increasing sensor selectivity and response at low temperature (Shin et al., 11 Jun 2025).
- Thin Film Growth: Oxygen pre-dosing of Cu(100) surfaces at sub-monolayer coverages (0.5 ML) dramatically reduces graphene nucleation density (~0.1 islands/μm² cf. ~3 islands/μm²), increases island size, and induces rotational disorder, highlighting the necessity of rigorous oxygen management in CVD processes (Robinson et al., 2014).
- Passivation and Defects: In monolayer WS₂, chemisorbed oxygen at S vacancies passivates gap states, reduces electron density, suppresses exciton annihilation by two orders of magnitude, and stabilizes valley polarization, especially under h-BN encapsulation (Jung et al., 2022). Sb₂Te₃ thin films with oxygen-passivated grain boundaries display improved thermoelectric performance via enhanced Seebeck coefficient and electrical mobility, attributed to energy filtering and band structure modulation at oxygen-enriched interfaces (Ghosh et al., 2023).
- Work Function Modification: On gold surfaces (Au(100), Au(101), Au(111)), oxygen chemisorption induces an increase in work function (Δφ), with a unique behavior for (111) surfaces where subsurface chemisorbed oxygen matches experimental Δφ values. The accompanying reduction in metallicity (LDOS near E_F) further implicates chemisorbed oxygen in tuning electronic structure and catalytic selectivity (Watanabe et al., 25 Nov 2024).
6. Dynamics, Regeneration, and Limiting Factors
Chemisorbed oxygen concentration is inherently dynamic—varying with environmental parameters (temperature, partial pressure), surface structure, defect density, and chemical exposure. For gas sensors, real-time XPS and mass spectrometry track the alternation in chemisorbed oxygen coverage during redox cycles, with maximal sensitivity and reversible response achieved at optimal temperature and oxygen partial pressure (Ciftyurek et al., 18 Oct 2025).
Coverage is self-limiting on many surfaces. For iron combustion, as the oxidation stage Zₒ increases, MAC falls precipitously, curbing further oxygen uptake and moderating thermal evolution, clearly departing from continuum models that assume instantaneous reaction rates (Thijs et al., 2022). On perovskite surfaces, coverage beyond specific threshold (e.g., 1/2 ML for Sr₃Ru₂O₇) leads to significant repulsive interactions and a flat potential energy surface dictating pattern formation and maximal density (Mayr-Schmölzer et al., 2018).
7. Theoretical and Experimental Tools
Measurement and theoretical assessment of chemisorbed oxygen concentration utilize a range of approaches:
| Method | Target Quantity | Example Systems |
|---|---|---|
| In situ XPS | Atomic fraction, bonding state | SnO₂, WS₂, TiO₂ |
| EELS | Defect passivation, hybridization | WS₂, nanocrystals |
| STM | Adatom counting, manipulation | Graphene, HOPG |
| LEED | Surface reconstruction, coverage | Cu(100) |
| Molecular Dynamics | Accommodation coefficients (TAC/MAC) | Iron, combustion |
| DFT/RPA | Binding energies, charge transfer | SrTiO₃, Sr₃Ru₂O₇, Au |
The diverse arsenal of techniques allows high-resolution quantification, mechanistic insight, and validation of theoretical models, delivering a comprehensive picture of chemisorbed oxygen concentration and dynamics across materials applications.
Chemisorbed oxygen concentration is a multifaceted parameter determined by adsorption energetics, charge transfer, kinetic barriers, and surface morphology. It directly affects catalytic turnover, sensor response characteristics, thin film growth, and material passivation in both oxide and non-oxide systems, with its magnitude and behavior precisely elucidated by advanced spectroscopic, computational, and microscopic tools as demonstrated in the arXiv corpus.