Titanium oxide (TiO2) is a semiconductor. In such compounds, the electrons can be in two states: free and bound. The normality of the electron is bound, i.e. it is connected to the ion of the crystal lattice substance, which forms a strong chemical bond. It is necessary to expend more than 3.2 electron volts (eV) of energy to “knock” the electron out of the lattice (comparison: The kinetic energy of a flying mosquito is about one trillion eV).
This is the amount of energy that a quantum of light with a wavelength of less than than 390 nm have. The light quantum “pushes” an electron out of the lattice and forms an electronic gap or simply “the hole”. The electron and the hole move actively within the TiO2 particles. As a result of the movement, they either recombine (meet, “marry” and return in a bound state) or break free at the top surface and are immediately captured by it.
Both the hole and the electron are incredibly reactive.The all-over upper-shell catalyst is a strong oxidation field. The oxygen that comes into contact with the catalyst surface and acquires a free electron, leads to an oxidative radical O-, which can destroy (oxidise) any organic compound.
The hole in turn reacts with the first organic compound encountered on the top of the surface. The hole pulls an electron out of the compound structure and thus its electron is missing, causing the compound to dissolve into water and carbon dioxide.