Cocatalyst-Free Photocatalysis: Intrinsic Charge Transfer

Updated 20 October 2025

Cocatalyst-free photocatalysis is a solar-driven process using semiconductor materials that harness intrinsic defects and band engineering for efficient charge separation without external cocatalysts.
It employs defect engineering, phase interfaces, and compositional entropy in materials such as TiO₂ derivatives and high-entropy oxides to enable targeted water splitting and CO₂ reduction.
Advanced characterization techniques like EPR, TEM, and gas chromatography validate these catalysts’ performance under controlled conditions to achieve nanoscale hydrogen evolution.

Cocatalyst-free photocatalysis is the field of photoinduced chemical reactions—typically solar-driven water splitting or CO₂ reduction—that employ semiconductor or oxide photocatalysts which achieve efficient charge transfer and product evolution without external cocatalysts, such as noble metals (Pt, Au) or engineered electron acceptors. The central goal is to access robust hydrogen evolution, oxidation, or similar transformations by exploiting intrinsic defects, mixed valence states, compositional entropy, phase interfaces, or defect engineering within the photoactive material itself. This approach addresses the cost, scalability, and chemical stability limitations associated with cocatalyst loading, and encompasses a diverse set of materials including TiO₂ derivatives, high-entropy oxides/perovskites, defect-engineered transition metal dichalcogenides, and new oxide semiconductors.

1. Fundamental Principle: Intrinsic Activation and Charge Transfer

At its core, cocatalyst-free photocatalysis seeks to establish efficient interfacial electron transfer from photogenerated excited states to reactant species solely via mechanisms internal to the semiconductor:

Photoexcitation yields electron–hole pairs: $\text{Semiconductor} + h\nu \to e^{-} + h^{+}$ .
The conduction band electron must possess sufficient energy to reduce protons ( $2H^+ + 2e^- \to H_2$ ) or CO₂, while the valence band hole must oxidize water or a sacrificial agent.
Absent cocatalysts, the efficiency hinges on fast charge separation, reduced recombination, and existence of surface or subsurface defect states, oxygen vacancies, mixed valence cations, or favorable band offsets driving directional carrier migration.

Mechanisms for intrinsic activation include:

Surface or subsurface Ti³⁺ centers stabilized by nitrogen or oxygen vacancies in TiO₂ (Zhou et al., 2016, Liu et al., 2016).
Internal metallic phases (e.g., Ti₄O₇ Magnéli) embedded at the nanoscale within anatase providing built-in electron "sinks" and transfer cascades (Domaschke et al., 2020).
Compositional or electronic heterogeneity, as realized in high-entropy oxides and perovskites, giving both electron donor and acceptor sites and tailored band structures (Hidalgo-Jiménez et al., 16 Jan 2025, Hai et al., 17 Oct 2025).
Defect-rich amorphous nanoparticles (e.g., PdSe₂₋ₓ with high selenium vacancy concentration) engineered via laser ablation (Ushkov et al., 29 Jul 2025).

2. Defect Engineering and Stabilization Strategies

Defect engineering underpins most approaches in cocatalyst-free systems. The creation or stabilization of defect centers serves both as charge trapping sites and as catalytic centers:

Material System	Type of Defect or Phase	Stabilization Approach
TiO₂:Ti³⁺:N (oxidized TiN, anatase)	Ti³⁺ centers, N-stabilized	Controlled oxidation of TiN
Hydrogenated anatase	Subsurface/intrinsic Ti(III), voids	High-pressure H₂ treatment
TiO₂ nanotubes (N-implanted)	N-doped Ti³⁺, sub-band-gap states	Low-dose ion implantation
Magnéli-phase (Ti₄O₇/Ti₅O₉ in anatase)	Metallic domains, internal junctions	Aerosol/hydrogen synthesis
PdSe₂₋ₓ nanoparticles	Se vacancies, unsaturated sites	Femtosecond-laser ablation
High-entropy oxides/perovskites	Oxygen vacancies, mixed valence	Chemical and solid-state synthesis

Defect stabilization may involve atomic doping (N, Ga), phase control (anatase vs rutile), or formation of amorphous or hetero-phase shells. For TiO₂ systems, the presence of nitrogen in oxidized TiN directly stabilizes the Ti³⁺ active site via charge transfer ( $\text{N} + \text{Ti}^{3+} \rightleftharpoons \text{N–Ti}^{3+}$ ) (Zhou et al., 2016). In high-entropy oxides, multiple cations with d⁰ and d¹⁰ configurations create a mosaic of charge donor/trap locations that collectively generate and stabilize oxygen vacancies, thus facilitating both UV and visible light absorption and increased catalytic activity (Hidalgo-Jiménez et al., 16 Jan 2025).

3. Junction and Band Engineering: Internal Interfaces and Configurational Entropy

Internal interfaces—either between polymorph domains or via engineered junctions—are essential in promoting directional charge migration and separating electron–hole pairs.

Anatase/Magnéli-phase junctions enable electrons to migrate from the higher conduction band edge of anatase into the metallic band of Ti₄O₇, with $E_{F,\text{Ti}_4\text{O}_7} \approx E_{CB,\text{anatase}} - 0.1–0.2\,eV$ (Domaschke et al., 2020). This internal band offset produces a particle transfer cascade that is analogous to the Schottky junction created by noble metal cocatalysts, but achieved intrinsically.
Homo-junctions between defect-rich and pristine regions of N-implanted TiO₂ nanotubes spatially confine catalytic activity to engineered layers and exploit electric fields to drive efficient charge separation (Zhou et al., 2016).
High configurational entropy in oxide perovskites ((Ba₁/₂Sr₁/₂)(Ti₁/₃Zr₁/₃Hf₁/₃)O₃ etc.) maintains cubic phases and allows fine tuning of tolerance, octahedral factor, and ionic radius deviation, facilitating favorable band alignment for water splitting ( $E_g = E_{CBM} - E_{VBM}$ ), and introducing intrinsic defect states (Hai et al., 17 Oct 2025, Hidalgo-Jiménez et al., 16 Jan 2025).

The combination of compositional disorder and phase boundaries controls the ansatz for carrier transport, recombination rates, and ultimately hydrogen evolution efficiency.

4. Experimental Detection, High-Throughput Screening, and Benchmarking

The detection of cocatalyst-free activity requires highly sensitive quantification techniques due to often ultralow hydrogen evolution rates.

Custom closed-cycle reactors minimize gas dilution and leakage, enabling measurement at nanomoles/hour scale with as little as 0.04 g catalyst (Huaiyu et al., 2022).
Continuous-flow and gas-accumulation detection modes quantify H₂ via gas chromatography, with formulas for calculation:
- Continuous-flow: $V_{H_2} = (V_{H_2-GC} \times R_{flow} \times t) / V_{sampling}$
- Gas-accumulation: $V_{H_2} = [V_{H_2-GC} \times (V_{chamber} + V_{circulation})] / (V_{sampling} \times f_{correction})$
Direct comparison shows that bare TiO₂ (11.4 ± 0.3 nmol/h) can be outperformed by ZnFe₂O₄ and Ca₂PbO₄ (~18–36 nmol/h), even without cocatalysts or bias (Huaiyu et al., 2022), although stability under prolonged illumination depends on material robustness.
Sensitivity in these methods is about three orders of magnitude higher than standard setups.

This approach facilitates rapid experimental validation of computationally predicted photocatalysts and establishes intrinsic benchmarks before further cocatalyst optimization.

5. Material Classes and Representative Systems

Research into cocatalyst-free photocatalysis encompasses several families:

Titanium-Based Oxides

Oxidized TiN nanoparticles (TiO₂:Ti³⁺:N, anatase) with robust defect stabilization (Zhou et al., 2016).
Hydrogenated anatase and anatase/rutile mixtures showing strong synergistic effects due to defect centers and interface junctions (Liu et al., 2016).
Magnéli phases within anatase generating metallic internal junctions (Domaschke et al., 2020).
TiO₂ nanotubes with N-implanted defect layers acting as virtual cocatalysts (Zhou et al., 2016).
Black TiO₂ surfaces require both hydrogenation and crystal damage (facet exposure, ion implantation) to achieve activation; coloration is not a proxy for catalytic properties (Liu et al., 2020).

Perovskites and High-Entropy Oxides

Engineered ABO₃ perovskites with multiple B-site cations (e.g., Ti, Zr, Hf, Sn, Ga, In) for built-in charge carrier mobility and band structure tuning (Hai et al., 17 Oct 2025).
High-entropy oxides (TiZrNbTaGaO₁₀.₅) using mixed d⁰/d¹⁰ cation configurations for donor/trap site integration and oxygen vacancy stabilization (Hidalgo-Jiménez et al., 16 Jan 2025).

Transition Metal Dichalcogenides (TMDCs)

Laser-synthesized amorphous PdSe₂₋ₓ nanoparticles with high selenium vacancy density and surface area, enabling >50-fold activity over crystalline forms (Ushkov et al., 29 Jul 2025).

Emergent Semiconductors

ZnFe₂O₄ and Ca₂PbO₄ showing initial cocatalyst-free H₂ evolution rates exceeding bare TiO₂ (Huaiyu et al., 2022).

6. Mechanistic and Structural Characterization

Characterization of active states and interfaces is achieved via:

Electron paramagnetic resonance (EPR) for Ti³⁺ or vacancy detection (Zhou et al., 2016, Liu et al., 2016, Zhou et al., 2020).
Transmission electron microscopy (TEM/HRTEM) for phase boundary, shell structure, and void detection (Liu et al., 2016, Zhou et al., 2020).
X-ray photoelectron spectroscopy (XPS) to monitor surface hydroxylation and mixed valence (Zhou et al., 2020, Hidalgo-Jiménez et al., 16 Jan 2025).
Raman, X-ray reflectivity (XRR), and photoluminescence (PL) to probe symmetry breaking, layer thickness, and defect-induced transitions (Liu et al., 2020, Liu et al., 2016, Zhou et al., 2020).
Band edge and work function calculations via UPS, aligned to water redox potentials (Hai et al., 17 Oct 2025).

Establishment of defect or junction-linked activity is routinely confirmed via spectroscopic signature, kinetic rate measurement ( $\ln(C_0/C_t) = k t$ ), H₂ quantification, or catalytic selectivity expressions.

7. Implications, Limitations, and Future Prospects

Cocatalyst-free strategies provide avenues to cost reduction, scalable synthesis, durability, and tunable activity via elemental selection, defect engineering, and phase control. Robust activity requires:

Defect states that persist and do not act as charge recombination centers.
Band alignment such that both hydrogen evolution and water oxidation are thermodynamically feasible.
Structural stability under continuous irradiation and aqueous conditions, as corrosion and phase transformation can suppress H₂ evolution.

Open research directions include:

Expansion of high-entropy materials chemistry to design better donor/acceptor state distributions (Hidalgo-Jiménez et al., 16 Jan 2025, Hai et al., 17 Oct 2025).
Mechanistic modeling of charge transfer across mixed-phase and defect-engineered interfaces.
Integration of detection platforms for ultralow activity screening to bridge computation and experiment (Huaiyu et al., 2022).
Applications beyond water splitting, including CO₂ reduction, pollutant degradation, and solar fuels.

A plausible implication is that advances in atomic-scale compositional control and defect stabilization could ultimately provide flexible, cocatalyst-free catalysts for diverse solar-driven chemical conversions, reducing reliance on platinum group metals and improving economic viability.

Cocatalyst-free photocatalysis thus represents a convergence of defect chemistry, band engineering, phase interface control, and materials synthesis, supporting sustainable solar energy conversion via intrinsic charge separation mechanisms.