TiO₂ Surface Modification with SACs

• Surface modification of TiO₂ with single-atom catalysts is a strategy that exploits defect sites and metal–support interactions to stabilize isolated metal atoms for enhanced catalytic performance.
• It employs atomic-scale control over metal dispersion, charge transfer, and electronic structure, as evidenced by advanced techniques such as STM, XAS, and XPS.
• The approach optimizes reaction energetics in processes like hydrogen evolution and CO oxidation by manipulating adsorption free energies and defect site physics through controlled ligand and water effects.

Surface modification of TiO₂ with single-atom catalysts (SACs) is a targeted strategy to maximize photocatalytic, electrocatalytic, and heterogeneous catalysis efficiency by exploiting uniquely isolated metal sites, strong metal-support interactions, and local defect engineering. This approach leverages atomic-scale control over metal dispersion, electronic structure, charge transfer, and site stability. The fundamental principles and practical implementations are best understood across several axes: defect site physics, characterization, electronic tailoring, water/ligand effects, and design guidelines for high-performance SAC systems on TiO₂.

1. Atomic-Scale Anchoring and Dispersion Mechanisms

SACs on TiO₂ are stabilized predominantly at intrinsic or extrinsic surface defects. On rutile TiO₂(110), bridging oxygen vacancies (V_O) and subsurface Ti interstitials are prime anchoring sites, while on anatase TiO₂(101), stable V_O sites do not persist at room temperature and instead, subsurface dopants (e.g., Nb, Ti interstitials, termed dark defects) immobilize adatoms (Kraushofer et al., 2022, Puntscher et al., 2023).

The following table gives typical adsorption energies for various metals at defect and stoichiometric sites on rutile TiO₂(110):

Metal Site	E₍ads₎ (eV)	Notes
Au @ V_O	–3.6	Cationic, stable, STM
Pt @ V_O	–4.3	Most tightly bound
Ag @ O_ad	–1.35	Less stable, diffusive
Cu @ O²ᵇʳ	–2.5	Weak at V_O

Adsorption energies (E₍ads₎) are calculated as E_tot(SAC/TiO₂) – E_tot(TiO₂) – E_tot(M), with stronger binding at defect sites. Diffusion barriers for single Pt adatoms range from ΔE‡ = 0.33 eV to 0.86 eV along [001] depending on facet and defect configuration (Kraushofer et al., 2022, Puntscher et al., 2023). Adatom stabilization correlates directly with oxygen affinity: Ir > Ni ≈ Pt > Rh on (101), explaining why Ir₁ remains stable under UHV, while Rh₁ rapidly sinters (Puntscher et al., 2023).

2. Defects, Polarons, and Charge Transfer

Defect engineering is essential for sac stabilization. Reduced TiO₂ surfaces form polarons: localized Ti³⁺ states with excess electrons that migrate to defect sites (Sombut et al., 2022). The charge transfer between polarons and adatoms determines both the oxidation state and binding strength. Quantitatively,

• E₍ads₎ (adsorption energy)
• E₍pol₎ (polaron localization energy)
• Δq (metal–polaron charge transfer, ~+1e to +2e)

Pt and Au strongly couple to polaronic charges at V_O, leading to reduced adatoms (Pt⁻, Au⁻) and deeply negative adsorption energies (Pt₁@V_O: E₍ads₎ = –3.22 eV, Δq = +2e; Au₁@V_O: E₍ads₎ = –2.06 eV, Δq = +1e) (Sombut et al., 2022). Rh shows weak coupling, with polarons remaining localized on Ti sites and Rh favoring hollow site adsorption with less stabilization.

This polaron engineering extends to Janus monolayer geometries of TiO₂, in which noble metals substituted for bridging O alter the electronic structure, induce localized midgap states, and promote charge separation (Asikainen et al., 10 Oct 2024).

3. Advanced Characterization Techniques (XAS, STM, XPS)

Atomic-scale characterization is central to elucidating SAC structure and functionality on TiO₂. Scanning tunneling microscopy (STM) directly images adatom morphology and site occupancy, while X-ray photoelectron spectroscopy (XPS) quantifies oxidation states through binding energy shifts (e.g., Pt 4f₇/₂ at ~71.0 eV, Ni 2p₃/₂ at ~852.7 eV in adatom states) (Puntscher et al., 2023).

X-ray absorption spectroscopy (XAS), encompassing XANES and EXAFS, details:

• Oxidation states (via white-line intensity and edge shifts, ΔE₀)
• Coordination numbers Nᵢ and bond lengths Rᵢ (from EXAFS k-space oscillations)
• Dynamic evolution under in-situ catalytic conditions (e.g., Pt²⁺ ↔ Pt⁰ cycling during CO oxidation at 160°C) (Li et al., 7 Nov 2025)

For bimetallic systems, element-resolved XAS at respective absorption edges reveals interatomic distances and redox interplay, as demonstrated for Cu₁Au₁/TiO₂ and Ru₁Mo₁/TiO₂ (Li et al., 7 Nov 2025).

4. Modifying Electronic Structure and Catalytic Properties

Single-atom doping of TiO₂, including Janus monolayer engineering, allows direct tuning of band structure, charge separation, and adsorption energetics pertinent to solar-driven catalysis and hydrogen evolution (Asikainen et al., 10 Oct 2024). Key findings:

• Doping at the bridging O (Ob) with Ag, Au, Pd, Pt achieves dynamic stability.
• Resulting TiO₂–M structures display reduced band gaps: Pt–TiO₂ (Eg = 0.25 eV), Pd/Ag/Au–TiO₂ exhibit metallic or semimetallic character.
• Bader charge analysis indicates electron transfer from lattice Ti to the noble-metal dopant, with localized magnetic moments induced except for non-magnetic (Pt–TiO₂).
• Adsorption free energy for H (ΔG_H) is near zero for Ag and Pt (ΔG_H = +0.09 eV for Ag–TiO₂, –0.04 eV for Pt–TiO₂), enabling nearly optimal hydrogen evolution overpotentials.
• Internal dipoles up to ΔΦ ≈ 2.0 eV promote electron–hole separation, suppressing recombination.

5. Environmental and Ligand Effects

Water and other co-adsorbates crucially impact adatom stability, dispersion, and catalytic activity:

• Pt: Dispersion unaffected by water vapor (0.025 ML, 2×10⁻⁸ mbar); both clusters and adatoms persist (Puntscher et al., 2023).
• Ni: Presence of water doubles the fraction of Ni₁ single atoms, attributed to OH/H₂O coordination increasing E₍ads₎.
• Ir: Water vapor promotes sintering, converting Ir₁ to large clusters migrating to step edges.
• Rh: Forms clusters regardless of environment; no single-atom stabilization observed.

This shows water can be either beneficial or detrimental, depending on both metal choice and support facet (Puntscher et al., 2023). On rutile, surface hydroxyls and peroxo groups can stabilize normally mobile adatoms by providing additional coordination (Kraushofer et al., 2022).

6. Structure–Function Relationships and Design Guidelines

Optimization of TiO₂–SACs is guided by:

• Maximizing single-atom dispersion and coverage at defect sites.
• Selecting metal(s) and support treatments to match desired redox and electronic structures; Pt, Au for H₂ evolution; Cu, Fe, Co for oxidation or CO₂ reduction (Li et al., 7 Nov 2025).
• Maintaining metal loading below ~1–2 wt% to prevent aggregation while ensuring XAS detectability.
• Tailoring TiO₂ defect density by controlled reduction, doping (Ti interstitials, aliovalent cations), and surface chemistry (OH, O_ad).
• Utilizing advanced XAS cell design for in-situ operando spectroscopy; HERFD-XAS for systems with closely-spaced edges.
• Employing GGA+U DFT, band-structure analysis, and Bader charge calculations for rational design (optimal U_Ti = 4.5 eV, U_M = 2–3 eV, 520 eV cutoff, 13×10×2 k-mesh) (Asikainen et al., 10 Oct 2024).

Quantum efficiency (η), a central performance metric, reflects the ratio of reacted electrons to incident photons: $$\eta = \frac{\text{number of reacted electrons}}{\text{number of incident photons}}$$ (Li et al., 7 Nov 2025). Empirical and computational studies demonstrate that defect-rich, dipole-optimized, and electronically engineered SAC–TiO₂ hybrids yield enhanced η by promoting charge separation, raising visible-light absorption, and offering nearly ideal energetics for target reactions (e.g., HER, CO oxidation).

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Surface modification of TiO₂ with single-atom catalysts is thus governed by interdependent control of defect structure, metal-support charge transfer, environmental interactions, and atomic-scale characterization. Future research will benefit from combined experimental-theoretical approaches targeting dynamic defect engineering and in situ operando analysis of SAC functionality under realistic catalytic conditions (Li et al., 7 Nov 2025, Puntscher et al., 2023, Asikainen et al., 10 Oct 2024, Kraushofer et al., 2022, Sombut et al., 2022).