TiO2 nanoparticles have emerged as a leading photocatalyst for the degradation of organic pollutants such as dyes and pesticides due to their chemical stability, non-toxicity, and high photocatalytic activity. The effectiveness of TiO2 in breaking down organic contaminants stems from its ability to generate electron-hole pairs under ultraviolet (UV) light, which subsequently produce reactive oxygen species (ROS) that mineralize pollutants into harmless byproducts. However, several challenges, including rapid charge recombination and limited visible-light absorption, have driven research into bandgap engineering, charge separation strategies, and optimized reactor designs to enhance photocatalytic efficiency.

Bandgap engineering is a critical approach to improving the visible-light activity of TiO2 nanoparticles. The wide bandgap of anatase TiO2 (3.2 eV) restricts its activation to UV light, which constitutes only a small fraction of solar radiation. Doping TiO2 with non-metals such as nitrogen (N) and carbon (C) introduces intermediate energy states within the bandgap, reducing the energy required for electron excitation. Nitrogen doping, for instance, creates localized states above the valence band, enabling visible-light absorption up to 520 nm. Carbon doping, whether substitutional or interstitial, further narrows the bandgap by hybridizing with TiO2’s oxygen 2p orbitals. Studies have shown that C-doped TiO2 exhibits enhanced degradation rates for methylene blue under visible light compared to undoped TiO2. However, excessive doping can introduce recombination centers, diminishing photocatalytic efficiency. Optimal doping concentrations typically range between 0.5% and 2.0% atomic weight, balancing visible-light absorption and charge carrier mobility.

Charge separation strategies are equally vital in mitigating electron-hole recombination, a major limitation in TiO2 photocatalysis. Heterojunction formation with other semiconductors, such as WO3, g-C3N4, or CdS, facilitates the spatial separation of charge carriers by creating built-in electric fields at the interface. Type-II heterojunctions, where the conduction and valence bands of the two materials are staggered, promote electron transfer to one semiconductor and hole migration to the other. For example, TiO2/WO3 heterostructures demonstrate prolonged charge carrier lifetimes due to the migration of electrons from TiO2 to WO3, while holes remain in TiO2. Similarly, Z-scheme systems, which mimic natural photosynthesis, retain high redox potentials by recombining electrons from one semiconductor with holes from another. These configurations have achieved up to a 3-fold increase in degradation rates for organic pollutants like rhodamine B compared to pure TiO2.

Reactor design plays a pivotal role in scaling up TiO2 photocatalysis for practical applications. Slurry reactors, where TiO2 nanoparticles are suspended in the pollutant solution, offer high surface area and uniform light exposure. However, post-treatment separation of nanoparticles poses challenges, often requiring centrifugation or filtration. Immobilized systems address this issue by coating TiO2 onto substrates such as glass beads, quartz fibers, or stainless steel meshes. While these systems simplify catalyst recovery, they suffer from reduced active surface area and potential light penetration limitations. Recent advances include fluidized-bed reactors, which combine the high surface area of slurries with the ease of recovery in immobilized systems. Performance metrics such as quantum yield (the ratio of reacted electrons to absorbed photons) and total organic carbon (TOC) removal are used to evaluate efficiency. Quantum yields for TiO2-based systems typically range from 1% to 10%, depending on the pollutant and light source, while TOC removal rates can exceed 80% for dyes like methyl orange under optimized conditions.

Despite these advancements, several limitations persist. Charge recombination remains a bottleneck, even in doped or heterostructured systems, particularly under high light intensities. Visible-light activation, though improved through doping, often requires sacrificial agents or sensitizers to sustain catalytic activity. Long-term stability is another concern, as photocorrosion or surface fouling can degrade performance over time. Additionally, the formation of intermediate byproducts during degradation may necessitate secondary treatment steps to achieve complete mineralization.

In summary, TiO2 nanoparticles hold significant promise for photocatalytic degradation of organic pollutants, with bandgap engineering and heterojunction strategies addressing key challenges. Reactor designs must balance efficiency and practicality, while performance metrics provide benchmarks for optimization. Overcoming limitations such as recombination and stability will be crucial for the widespread adoption of TiO2 photocatalysis in environmental remediation. Future research may explore advanced doping techniques, novel heterojunction architectures, and hybrid reactor designs to further enhance performance.