There are several substances with photocatalytic properties suitable for obtaining self-cleaning surfaces. In the ceramic sector, the most commonly used is TiO2 (Anatase), but there are many other compounds with similar properties, such as ZnO, CeO2, ZrO2, SnO2, CdS, ZnS, etc.
In simple terms, when the photoactive compound is struck by a photon of sufficient energy, a positive charge is generated on the surface of the crystal. This positive charge reacts with water, leading to the formation of highly reactive radicals that destroy organic pollutants. This action extends to the molecules present in viruses and bacteria, explaining the antibacterial properties associated with these compounds.
Apart from self-cleaning tiles, numerous other products exhibit similar characteristics. For instance, the plastic fabric used in beach chairs contains ZnO to prevent the growth of fungi and bacteria between users. Many glasses used in urban building structures have a self-cleaning external surface, and the same applies to concrete. The addition of Anatase to concrete ensures that building surfaces do not become dirty due to smog or the growth of moss and lichens.
The application of Anatase in ceramics must occur after the tiles have been fired. If applied to raw glaze, it reacts during firing, resulting in the loss of its properties. Typically, Anatase is applied using spray methods, such as airless spraying, and then re-fired in a third firing. Annealing is essential for the crystals to adhere to the enamel surface. Often, the product is applied to the tiles in a specific cooling zone of the kiln. In this zone, a gap between oven modules allows the product to be sprayed onto the very hot tile surfaces.
One drawback of using Anatase is its requirement for substantial light intensity to exert its effect. Additionally, the treated surface loses its effectiveness over time due to the removal of the active layer caused by wear.
Alternatively, surfaces of tiles can acquire virucidal and antibacterial properties through the action of metal ions. Research has shown that certain ions exhibit antibacterial/virucidal actions with the following strength scale: Ag+ > Co2+ ≥ Ni2+ ≥ Al3+ ≥ Zn2+ ≥ Cu3+ = Fe3+ > Mn2+ ≥ Sn2+ ≥ Ba2+ ≥ Mg2+ ≥ Ca2+.
Many everyday surfaces possess antibacterial properties, ranging from stainless steel surfaces in professional kitchens and brass handles to silver cutlery and sanitary masks soaked in silver or copper salts. These examples illustrate how sanitizing properties have been intentionally or unintentionally utilized in various surfaces.
In the ceramic tile sector, Silver ions have often been employed as sanitizing agents. They are frequently applied in the form of granules containing silver or as surface applications on raw enamel using solutions of silver salts. The action of these metal ions is attributed to their ability to bind to proteins in bacterial and viral membranes, causing denaturation and inactivation.
The advantage of Silver antibacterial surfaces over TiO2 surfaces lies in their mechanism of action, which does not require light, and the practically unlimited duration of the sanitizing effect.