Charge Carrier Processes and Optical Properties in TiO and TiO-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 (H) production (e.g., via water splitting or photo-reforming of organic substrates), CO reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO) 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 TiO. We describe the main characteristics and advantages of TiO 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 TiO photoluminescence and on the effect of molecular oxygen (O) 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 TiO 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 TiO without any cocatalysts. Discussed topics are highly-reduced "black TiO", grey and colored TiO, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO composites or heterostructures with metals (e.g., Pt-TiO, Au-TiO), with other metal oxides (e.g., CuO, NiO, etc.), direct Z-scheme heterojunctions with g-CN (graphitic carbon nitride) and dye-sensitized TiO.