There is no doubt as to the fact that photocatalysis is taking an ever more important role in biological processes and in environmental control activities. Besides, ever since the beginning of the 20th century, energy and production of materials were directly and indirectly obtained from sunlight. Moreover, during the course of the century, population increase, the creation of new materials and the increase in oil and nuclear power exploitation for energetic purposes has led to an ever-increasing discontinuity between nature and society requirements. The need for a cleaner environment and for an improved quality of life urge us to conceive an eco-friendly use of light and sun and, within this framework, photochemistry applied to building materials may turn out to represent a very interesting solution, so much so that it may become an integrant part of the strategy aimed at reducing environmental pollution through the use of building materials containing photocatalysts.

For this very purpose, studies have been carried out to identify a system that chiefly includes titanium dioxide in the form of anatase and cement. The results of the tests conducted to date have led to the conclusion that cement materials containing titanium dioxide (TiO2), when irradiated with an adequate luminous intensity, show greater efficiency in oxidizing the organic substances with which they come in contact. Heterogeneous Photocatalysis The technology of heterogeneous photocatalysis is based on the use of a photocatalytic semiconductor. Semiconductors are characterised by a restricted jump between the conduction valence bands (VB and CB respectively).

The absorption of an amount of luminous energy that is greater or equal to that corresponding to the band jump of the above-mentioned semiconductor (Ebg) results in a sudden transfer of electrons from the valence band. This charge transfer introduces conditions of non-balance, which lead to the reduction or to the oxidation of the species absorbed on the surface of the semiconductor. The crystallographic forms of TiO2 are anatase and rutile. These two forms crystallize according to a tetragonal structure that differs in the number of terminal octahedrons TiO6 (4 for anatase and 2 for rutile). This explains the different photocatalytic activity of the two crystallographic forms and the greater reactivity of anatase. Therefore, over the last ten years, scientific and engineering interest for the application of photocatalysis to the study of semiconductor materials has displayed an exponential growth, when one considers that over 200 papers have been published on the subject of water and air treatment only. Indeed, we ought to take into account that we are rapidly switching from a world in which Nature reproduces through sunlight to a world in which materials need to be made inert towards light itself to guarantee their duration; a practical example may be found in the building industry.

Products in concrete, for instance, need to preserve their aesthetic features in time (colour in particular), also in aggressive urban environments, especially in the case of white cement-based cement materials. Indeed, the main cause for deterioration, in these cases, is the collection of coloured organic compounds onto their surface. Inorganic powders adhere to the surface of materials when an organic interface exists or in the event of macroporosity.

Brilliancy tests carried out so far show the aesthetic treatability of concrete elements made in white cement, containing 2% of TiO2 and exposed within an urban environment for five years. The tests carried out in Japan on the activity of TiO2 for ecological purposes has lead to the production of: Self-cleaning glass containing TiO2, developed by the Kanagawa Academy of Science and Technology and by Nipon Soda. A deodorant paper filter containing TiO2, which is four times more active than traditional filters containing active carbon, developed by K.G. Pack. Ceramic materials containing TiO2, developed by Toto, which has registered in a few years over thirty patents related to this class of products (such as self-cleaning crystal glasses and antibacterial ceramics for hospitals). Furthermore, studies are being conducted on TiO2-based catalysts to reduce Oxides of Nitrogen to N2 and O2.

The analyses, studies and tests carried out have therefore demonstrated the photocatalytic activity of cement materials containing TiO2 in the decomposition of organic and inorganic substances; furthermore, the data made available by the literature on the subject confirm the possibility of destroying microorganisms, such as bacteria and viruses, through the photocatalytic action of TiO2. Therefore, the possibility of transferring the photocatalytic activity of TiO2 to cement materials, although still in the studying phase, may lead to extraordinary advantages in the construction of new hospital facilities, by contributing to the solution of the problem represented by the presence of microorganisms.

(Trad. Interpres sas Giussano)