Titanium dioxide (TiO2) is a widely studied semiconductor material due to its exceptional photocatalytic properties. Its ability to facilitate redox reactions under ultraviolet (UV) light has made it a cornerstone in applications such as water splitting, air purification, and pollutant degradation. The effectiveness of TiO2 arises from its electronic structure, charge transport characteristics, and surface chemistry. However, its large bandgap limits its absorption to UV light, which constitutes only a small fraction of solar radiation. To overcome this limitation, researchers have explored bandgap engineering, doping strategies, and nanostructuring to enhance its photocatalytic efficiency under visible light.

The photocatalytic activity of TiO2 is governed by its electronic band structure. TiO2 exists in several polymorphs, with anatase, rutile, and brookite being the most common. Anatase is often preferred for photocatalysis due to its higher charge carrier mobility and lower recombination rates compared to rutile. The bandgap of anatase TiO2 is approximately 3.2 eV, while rutile has a slightly narrower bandgap of about 3.0 eV. When TiO2 absorbs photons with energy equal to or greater than its bandgap, electrons are excited from the valence band to the conduction band, leaving behind holes. These photogenerated charge carriers participate in redox reactions at the semiconductor surface. However, rapid electron-hole recombination reduces photocatalytic efficiency. Strategies to mitigate recombination include the use of co-catalysts, heterojunctions, and controlled defect engineering.

Doping TiO2 with foreign elements is a proven method to modify its optical and electronic properties. Nitrogen doping, for instance, introduces mid-gap states above the valence band, effectively narrowing the bandgap and enabling visible light absorption. Nitrogen-doped TiO2 can absorb wavelengths up to 500 nm, significantly extending its photocatalytic activity into the visible spectrum. Similarly, metal ion doping with elements like Fe, Cu, or Ag introduces impurity levels within the bandgap, facilitating electron trapping and reducing recombination rates. Transition metal dopants also enhance surface reactivity by providing additional active sites for adsorption and reaction intermediates. However, excessive doping can introduce recombination centers, degrading performance. Optimal doping concentrations are typically in the range of 0.1 to 2 atomic percent, depending on the dopant and synthesis method.

Surface reactivity plays a critical role in TiO2 photocatalysis. The surface hydroxyl groups on TiO2 act as adsorption sites for reactants and participate in hole scavenging, producing highly reactive hydroxyl radicals. These radicals are potent oxidizers capable of degrading organic pollutants and facilitating water oxidation. The crystallographic orientation of TiO2 surfaces also influences reactivity; anatase (101) facets are more stable but less reactive than (001) facets, which exhibit higher surface energy and catalytic activity. Morphological control through nanostructuring can expose high-energy facets, improving photocatalytic performance. For example, TiO2 nanosheets with dominant (001) facets demonstrate superior activity in dye degradation compared to conventional nanoparticles.

Photocatalytic water splitting is one of the most studied applications of TiO2. Under UV illumination, TiO2 generates electrons and holes that drive the reduction of water to hydrogen and the oxidation of water to oxygen. The overall efficiency of this process is limited by slow kinetics and back-reactions. To enhance hydrogen evolution, platinum nanoparticles are often deposited on TiO2 as co-catalysts, providing efficient proton reduction sites. Oxygen evolution, being a four-electron process, is more challenging and typically requires co-catalysts like iridium oxide or cobalt phosphate. Despite these improvements, the solar-to-hydrogen conversion efficiency of pure TiO2 remains low, prompting research into doped and composite systems.

Pollutant degradation is another major application of TiO2 photocatalysis. Organic contaminants such as dyes, pesticides, and pharmaceuticals can be mineralized into harmless byproducts like CO2 and water through oxidative reactions mediated by photogenerated holes and hydroxyl radicals. The degradation efficiency depends on factors such as pollutant concentration, light intensity, and catalyst loading. For instance, methylene blue, a common model pollutant, can be degraded by over 90% within hours under optimized conditions. Advanced oxidation processes combining TiO2 with H2O2 or ozone further enhance degradation rates by generating additional reactive species.

Recent advances in TiO2 photocatalysis focus on improving charge separation and visible light utilization. Z-scheme heterojunctions, inspired by natural photosynthesis, couple TiO2 with another semiconductor to spatially separate reduction and oxidation sites, minimizing recombination. For example, combining TiO2 with reduced graphene oxide enhances electron extraction and transport. Another approach involves plasmonic enhancement, where noble metal nanoparticles like Au or Ag extend light absorption through localized surface plasmon resonance. These nanoparticles also act as electron sinks, prolonging charge carrier lifetimes.

In summary, TiO2 remains a pivotal material in photocatalysis due to its stability, nontoxicity, and tunable properties. Bandgap engineering through doping and nanostructuring has expanded its functionality into the visible spectrum, while surface modifications and co-catalysts have improved reaction kinetics. Key applications such as water splitting and pollutant degradation benefit from these advancements, though challenges like charge recombination and low solar conversion efficiencies persist. Future research will likely explore novel dopants, defect engineering, and hybrid systems to further optimize TiO2 for sustainable energy and environmental applications.