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

Abstract

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.

Synthesis and Characterization of Anatase TiO2 Microspheres Self-Assembled by Ultrathin Nanosheets

“…By contrast, in the drop-casting deposition method, the potassium fluorotitanate hydrolyzes first at high temperatures to produce the first-generation anatase nano-TiO 2 and continues to hydrolyze at low temperatures during the drying process to form the second anatase nano-TiO 2 , leading to the formation of many small, xenomorphic, and fragmentary anatase nanocrystals ( Figure 4 ). Although both the methods can synthesize the anatase nanosheet-based microspheres with the high exposed (001) facets, it is evident that the microspheres synthesized by the on-site precipitation hydrothermal method have a more stable structure and thus better physicochemical properties as well as photocatalytic and photoelectric performance [ 1 , 3 ]. In addition, even if the materials for the collection board are different, it seems to have an insignificant influence on the anatase and microsphere’s morphology compared to the temperature, synthesis time, and synthesis method under the same conditions.…”

“…Nanocrystalline titanium dioxide (TiO 2 ) has good chemical and physical stability and excellent photocatalytic performance [ 1 , 2 , 3 ]. Due to its scientific and technological importance, it is widely used in optoelectronic devices, sensors, solar cells, and photonic crystals [ 4 , 5 ].…”

Main Hydrogen Production Processes: An Overview

“…In the last years different configurations have been proposed, including CuS-and NiS-based heterojunctions, titanium dioxide based core-shell structures and periodical structures with excellent adsorption ability, and imogolite hollow cylinders [90], as well as S-scheme heterostructure [91]. Among them, the titanium dioxide based ones are the most interesting, due to the chemical resistance properties, accessibility, and affordability [92]. Moreover, in recent years the scarce activation of titanium dioxide by visible sunlight has been mitigated; novel strategies of doping development of novel composites are allowing to obtain interesting results also in the field of visible light-activated photocatalysis [92].…”

“…Among them, the titanium dioxide based ones are the most interesting, due to the chemical resistance properties, accessibility, and affordability [92]. Moreover, in recent years the scarce activation of titanium dioxide by visible sunlight has been mitigated; novel strategies of doping development of novel composites are allowing to obtain interesting results also in the field of visible light-activated photocatalysis [92]. The addition of different metals to titanium dioxide results in increasing the hydrogen production under visible light.…”

A High-Efficiency TiO2/ZnO Nano-Film with Surface Oxygen Vacancies for Dye Degradation

“…Heterogeneous photocatalysis is a main advanced oxidation process consisting of oxidative reductive reactions induced by the generation of highly reactive free radicals on the surface of photocatalysts in presence of light with energy equal or higher than photocatalytic materials' forbidden bandgap [2]. Among various photocatalytic materials, titanium dioxide (TiO 2 ) is a highly effective photocatalytic semiconductor owing to its prominent photocatalytic performance, superior chemical and thermal stability, low cost, and non-toxicity [3].…”