A key property of the molecule TiO2 (titanium dioxide) is found in its interaction with water.
In fact, those little di-hydrogen monoxide (water) molecules love their titanium dioxide cousins so much that when they come into contact they bind so tightly that droplets cannot form.
The term for this is ‘hydrophilia’ (oils and fats are at the other end of the scale – they’re ‘hydrophobic’.) When water and titanium dioxide bond, sparks fly. Metaphorically. We will explain.
Water, or moisture, as we know can harbor a variety of nasties.
We might call them contaminants, pollutants, bacteria, viruses, mould, mildew or cancer-causing Volatile Organic Compounds. These love water too, whether moisture in the air, damp on a surface, puddles or vapour breathed out by passers-by. H2O helps the nasties just as much as it helps us.
Picture this. Water molecules bind with a Titanium dioxide-treated surface. The nasties hitching a ride on the water molecules cannot escape.
They are bathed in light, and TiO2 reacts by generating chemical scavengers known to the chemically-inclined as hydroxyl radicals. These are the sparks we mentioned, the heroes of our story.
These ‘radical’ molecules don’t last long but while in existence they break down organic molecules and microbes that are adsorbed on the titanium-dioxide treated surface. As a result, those airborne nasties get the chop. They are oxidised.
Harmful molecules change. They disappear. They decompose. They disintegrate. They drop dead. They drift away as harmless shadows of what they used to be. The surface self-cleans, and the surrounding air is purified. This is the process of photocatalysis.