TiO2 Photocatalysis — Mechanism and Implications
TiO2 absorbs UV light and generates reactive radicals — a problem for outdoor coatings (chalking), but useful for self-cleaning surfaces.
Titanium dioxide is a semiconductor with a band gap of approximately 3.0 eV (rutile) or 3.2 eV (anatase). UV light with wavelength shorter than ~410 nm (rutile) or ~388 nm (anatase) has enough energy to excite electrons from the valence band to the conduction band, creating electron-hole pairs.
These charge carriers are highly reactive. At the TiO2 surface, they can react with adsorbed water and oxygen to produce reactive oxygen species — primarily hydroxyl radicals (•OH) and superoxide anions (O2•−). These radicals are extremely oxidizing — they can break C-C and C-H bonds in organic molecules.
Why this matters for paint: In an exterior coating, the TiO2 pigment is surrounded by polymer binder (typically acrylic, alkyd, polyester, or polyurethane). Under sunlight, the TiO2 generates radicals that attack the binder at the pigment-binder interface. Over time, this degrades the binder, releasing pigment particles to the coating surface. The visible result is chalking — a white, dusty appearance, particularly on horizontal weathered surfaces.
Anatase vs rutile photocatalysis: - Anatase is significantly more photocatalytic than rutile (despite the larger band gap — counterintuitively, anatase has more reactive surface states) - Anatase photocatalysis is roughly 2–3x stronger than uncoated rutile - This is why anatase chalks rapidly in outdoor coatings and is restricted to interior applications
Surface treatment as photocatalysis suppression: Modern TiO2 surface treatment serves explicitly to suppress photocatalysis at the pigment-binder interface: - Alumina coating: modest reduction in photocatalysis - Silica coating: significant reduction - Zirconia coating: dramatic reduction — the gold standard for outdoor durability - Combined Al2O3 + ZrO2 + organic: premium triple-layer architecture (e.g., SEMITI 706, 826D)
A well-surface-treated rutile in a properly-stabilized binder (with UV absorber + HALS) can deliver 15–20 year outdoor service in coil coating applications.
Useful photocatalysis — self-cleaning and air purification: The same photocatalysis that causes chalking can be harnessed deliberately. Surface coatings with anatase TiO2 (typically nano-anatase, no surface treatment) can: - Decompose organic dirt on glass and concrete (self-cleaning facades) - Oxidize NOx and VOCs in air (air-purifying coatings) - Provide antimicrobial activity in tile glazes and ceramics
These applications use uncoated anatase or specifically engineered photocatalytic-active nano TiO2 — opposite design philosophy from coating pigments where photocatalysis is suppressed.
Implications for grade selection: - Outdoor coatings → heavily surface-treated rutile (SEMITI 706, 826D, 880, 996) - Interior coatings → rutile (any surface treatment is enough) - Plastic outdoor service → durable rutile + HALS package - Plastic indoor → any rutile or anatase - Self-cleaning surfaces → uncoated anatase (specialty grade, not standard SEMITI) - Cosmetic sunscreen → nano-rutile with heavy silica coating (deactivated photocatalysis)