Titanium dioxide (TiO2) has emerged as a leading photocatalyst for water splitting due to its favorable electronic properties, chemical stability, and cost-effectiveness. The process involves using solar energy to drive the dissociation of water into hydrogen and oxygen, offering a sustainable route for clean energy production. The mechanism relies on photoexcitation of electrons from the valence band to the conduction band, creating electron-hole pairs that facilitate redox reactions at the catalyst surface.
Thermodynamically, water splitting is an uphill reaction requiring a minimum Gibbs free energy change of 237 kJ/mol (1.23 eV per electron transferred). TiO2, with a bandgap of approximately 3.0–3.2 eV for the anatase and rutile phases, absorbs ultraviolet light, which constitutes only about 4–5% of the solar spectrum. Upon irradiation, the photogenerated electrons reduce protons (H+) to hydrogen (H2), while the holes oxidize water (H2O) to oxygen (O2). The half-reactions are as follows:
Reduction (Hydrogen Evolution Reaction, HER):
2H+ + 2e− → H2 (E° = 0 V vs. NHE)
Oxidation (Oxygen Evolution Reaction, OER):
2H2O → O2 + 4H+ + 4e− (E° = +1.23 V vs. NHE)
For efficient photocatalysis, the conduction band of TiO2 must be more negative than the H+/H2 redox potential, while the valence band must be more positive than the O2/H2O potential. TiO2 meets these criteria, but its wide bandgap limits visible light absorption. Additionally, rapid recombination of electron-hole pairs reduces the quantum yield, often below 10% for pure TiO2 under UV light.
To mitigate recombination and enhance charge separation, sacrificial agents are employed. These are electron donors (e.g., methanol, ethanol, or triethanolamine) or acceptors (e.g., silver nitrate or persulfate) that scavenge holes or electrons, respectively, suppressing back reactions. For instance, methanol acts as a hole scavenger, oxidizing to formaldehyde and reducing recombination losses, thereby increasing hydrogen evolution rates. However, sacrificial agents are consumed in the process, making the system non-sustainable for large-scale applications.
Surface co-catalysts play a critical role in improving reaction kinetics. Platinum (Pt) is widely used as a HER co-catalyst due to its low overpotential and high work function, which facilitates electron transfer from TiO2 to protons. Pt nanoparticles deposited on TiO2 enhance hydrogen evolution rates by providing active sites for proton reduction. Similarly, iridium oxide (IrO2) or ruthenium oxide (RuO2) can be used as OER co-catalysts to accelerate oxygen evolution. The presence of these co-catalysts reduces activation barriers and improves turnover frequency (TOF), defined as the number of reactions per active site per unit time.
The efficiency of TiO2 photocatalysts is quantified using metrics such as quantum yield (QY) and solar-to-hydrogen (STH) efficiency. Quantum yield represents the ratio of reacted electrons to absorbed photons, expressed as:
QY (%) = (Number of H2 molecules produced × 2 / Number of incident photons) × 100
For TiO2-based systems, QY rarely exceeds 20% under UV light due to losses from recombination and incomplete light absorption. STH efficiency, which measures the fraction of solar energy converted into chemical energy in hydrogen, is typically below 1% for standalone TiO2 systems.
Another key parameter is the turnover frequency (TOF), which indicates the catalytic activity per active site. For Pt-loaded TiO2, TOF values range from 0.1 to 10 h−1 depending on particle size, light intensity, and reaction conditions. Smaller Pt nanoparticles exhibit higher TOF due to increased surface area and improved charge transfer.
Despite its advantages, TiO2 faces intrinsic limitations. The large bandgap restricts light absorption to UV wavelengths, while the high overpotential for OER slows down oxygen evolution kinetics. Surface defects and trap states further exacerbate recombination losses. Strategies such as doping with nitrogen or sulfur narrow the bandgap slightly, but significant improvements require additional modifications beyond pure TiO2.
In summary, TiO2 serves as a robust photocatalyst for water splitting, with its performance enhanced by sacrificial agents and co-catalysts like Pt. However, low quantum yields, limited visible light absorption, and recombination losses hinder its practical deployment. Advances in surface engineering and mechanistic understanding are essential to overcome these challenges and improve the viability of photocatalytic hydrogen production.