Charge Carrier Processes and Optical Properties in TiO₂ and TiO₂-Based Heterojunction Photocatalysts: A Review

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H₂) production (e.g., via water splitting or photo-reforming of organic substrates), CO₂ reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO₂) is a "benchmark" photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO₂. We describe the main characteristics and advantages of TiO₂ as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO₂ photoluminescence and on the effect of molecular oxygen (O₂) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO₂ limitations and possible ways to overcome them by discussing some of the "hottest" research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO₂ without any cocatalysts. Discussed topics are highly-reduced "black TiO₂", grey and colored TiO₂, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO₂ is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO₂ composites or heterostructures with metals (e.g., Pt-TiO₂, Au-TiO₂), with other metal oxides (e.g., Cu₂O, NiO, etc.), direct Z-scheme heterojunctions with g-C₃N₄ (graphitic carbon nitride) and dye-sensitized TiO₂.