Indoor air quality has become a critical concern due to the increasing presence of volatile organic compounds (VOCs) emitted from building materials, furniture, cleaning products, and other household sources. These pollutants contribute to health issues such as respiratory irritation, headaches, and long-term chronic conditions. Photocatalytic nanomaterials, particularly titanium dioxide (TiO2), have emerged as a promising solution for degrading VOCs in indoor environments through advanced oxidation processes. This technology leverages light-activated reactions to break down harmful pollutants into harmless byproducts, offering a sustainable and efficient method for air purification.

Photocatalytic oxidation relies on semiconductor nanomaterials that generate electron-hole pairs when exposed to light, typically ultraviolet (UV) or visible wavelengths. TiO2 is the most widely studied photocatalyst due to its chemical stability, non-toxicity, and high reactivity. When UV light with energy equal to or greater than the bandgap of TiO2 (approximately 3.2 eV for anatase phase) strikes the surface, electrons are excited from the valence band to the conduction band, leaving positively charged holes. These charge carriers react with adsorbed oxygen and water molecules to produce reactive oxygen species (ROS) such as hydroxyl radicals (•OH) and superoxide anions (•O2−). These ROS are highly oxidative and capable of breaking down VOCs like formaldehyde, benzene, and toluene into carbon dioxide and water.

The efficiency of photocatalytic VOC degradation depends on several factors, including the crystal phase of TiO2, surface area, light intensity, and humidity. Anatase-phase TiO2 generally exhibits higher photocatalytic activity than rutile due to its electronic structure. Nanostructuring TiO2 into nanoparticles or thin films increases the available surface area for pollutant adsorption and light absorption. Studies have demonstrated that under optimal conditions, TiO2-coated surfaces can achieve VOC degradation efficiencies exceeding 80% for formaldehyde and 70% for benzene within a few hours of exposure to UV light. Humidity plays a dual role; while water molecules are necessary for hydroxyl radical formation, excessive humidity can compete with VOC molecules for adsorption sites, reducing degradation rates.

Reactor design is crucial for implementing photocatalytic air purification in practical settings. Two primary configurations are commonly used: fixed-bed reactors and flow-through reactors. Fixed-bed reactors incorporate TiO2-coated substrates such as glass plates, ceramic tiles, or fibrous mats installed on walls, ceilings, or within HVAC systems. These systems are passive, relying on natural air circulation to bring VOCs into contact with the photocatalytic surface. Flow-through reactors, on the other hand, actively force air through a photocatalytic filter using fans, enhancing mass transfer and degradation rates. These systems are often integrated into standalone air purifiers or building ventilation systems.

Recent advancements have focused on improving visible-light activation of TiO2 to overcome reliance on UV light, which constitutes only a small fraction of indoor lighting. Doping TiO2 with nitrogen, carbon, or transition metals reduces its bandgap, enabling photocatalytic activity under visible light. For instance, nitrogen-doped TiO2 nanoparticles have shown significant formaldehyde degradation under fluorescent lighting, making them suitable for residential and office environments. Another approach involves coupling TiO2 with plasmonic nanoparticles like gold or silver, which enhance light absorption through localized surface plasmon resonance.

Residential applications of photocatalytic nanomaterials include self-cleaning and air-purifying coatings for walls, windows, and furniture. Commercially available products incorporate TiO2 into paints, films, and textiles, providing continuous VOC degradation without requiring additional energy input beyond ambient light. In commercial settings such as hospitals, schools, and offices, photocatalytic air purifiers with integrated UV LEDs or visible-light-activated filters are deployed to maintain high air quality. These systems often combine photocatalytic oxidation with mechanical filtration to capture particulate matter alongside gaseous pollutants.

Long-term performance and maintenance considerations are essential for real-world implementation. Photocatalytic surfaces may experience deactivation due to the accumulation of reaction intermediates or dust deposition, necessitating periodic cleaning or reactivation under UV light. Some studies report a gradual decline in efficiency over several months of continuous use, highlighting the need for durable nanostructured coatings. Additionally, the potential formation of harmful byproducts, such as formaldehyde from incomplete oxidation, must be carefully monitored through proper reactor design and material optimization.

The integration of photocatalytic nanomaterials into building materials represents a forward-looking approach to sustainable indoor air quality management. By embedding TiO2 or modified photocatalysts into construction elements, buildings can passively reduce VOC levels while minimizing energy consumption. Future research directions include the development of hybrid systems combining photocatalysis with adsorption materials like activated carbon or zeolites to enhance pollutant capture and degradation. Advances in nanostructuring and surface engineering will further improve photocatalytic efficiency, enabling broader adoption in both residential and commercial environments.

Photocatalytic air purification using nanomaterials offers a viable and eco-friendly solution to indoor VOC pollution. Through continued innovation in material science and reactor design, this technology holds significant potential for creating healthier indoor environments without relying on energy-intensive filtration methods. The successful deployment of these systems will depend on interdisciplinary collaboration to address challenges related to efficiency, durability, and scalability.