Self-cleaning and de-polluting stone surfaces: TiO₂ nanoparticles for limestone

Abstract

To evaluate the self-cleaning and de-polluting properties of a transparent titania coating, in this investigation a titanium dioxide based suspension was deposited by spray coating on travertine, a porous limestone largely used as building material. To assess compatibility with stone substrates, microstructure analysis, color and gloss measurements, static contact angle analysis and two different water absorption tests were carried out. Self-cleaning and de-polluting activities were assessed by the use of rhodamine B decoloration and NO_x degradation tests. The results showed good compatibility with stone surfaces and evident efficiency, allowing the use of titania coatings for real outdoor applications.

Introduction

Photocatalysis, that is the acceleration of chemical redox reactions thanks to the synergy of particular materials (usually semiconductor) and solar light, is one of the most promising tool for the handling of different problems in several areas of applied chemistry, including environmental science. In fact photocatalysis allows the photo-decomposition of both organic and inorganic polluting substances absorbed or deposited on photoactive surfaces by redox reactions induced by solar light.

Titanium dioxide (TiO₂) is one of the mostly used photocatalytic material in order to develop innovative materials and solutions in different fields: water and air purification, anti-bacterial and self-sterilizing surfaces, food industry, paintings, paper production, cosmetics and building materials [1], [2], [3], [4], [5], [6], [7]. In construction sector titanium dioxide has been used to realize self-cleaning treatments in a large number of building elements, as cement mortars, exterior tiles, paving blocks, glasses, paints, finishing coatings, road-blocks, concrete pavements [7], [8], [9], [10], [11], [12], [13], [14]. Its widespread use is attributed to its main features: high catalysis efficiency, chemical stability, inexpensiveness, non-toxicity, compatibility with traditional construction materials [3], [4], [5], [7].

Photocatalysis is not the only photochemical effect of TiO₂ activated by ultraviolet irradiation of solar light. Under UV exposure titanium dioxide becomes super-hydrophilic, thus decreasing water contact angle and creating an uniform water film on treated surfaces [1], [2], preventing contact between external dirt and surfaces themselves.

The synergy of these two photo-induced properties (photocatalysis and super-hydrophilicity) is the base of self-cleaning ability of TiO₂ [7], since the formation of a water film over treated surfaces and the photocatalytic degradation of pollutants and external agents could make removal of degradation agents easier bringing to a real self-cleaning effect.

Titanium dioxide shows three different crystal structures: anatase, rutile and brookite. The photocatalytic efficiency of TiO₂ is greatly influenced by its structure: amorphous titania shows very little photoactivity while this property is much more evident in crystal form, especially for anatase nanocrystals. However, among the interesting properties of anatase nanoparticles, they are characterized by a mean diameter below the light wavelength and thus they are transparent if applied on a substrate or dispersed in a matrix. Nanostructured titania films over a solid surfaces can allow to obtain a transparent physical barrier to enhance the cleanibility of treated substrate without altering its aspect.

In this scenario, surface treatments can effectively improve the conservation, protection and maintenance of several types of stone surfaces and elements, since the action of many degradation agents begins from the outer layers of these surfaces. The development and application of transparent self-cleaning coatings could improve the preservation of the original aspect of treated elements, decreasing the deposition of pollutants and soiling on building surfaces and reducing the onset of external degradation processes due to soiling phenomena, thus limiting cleaning and maintenance actions [15], [16], [17], [18], [19], [20], [21].

The aim of this work was to evaluate the possible use of TiO₂-based coatings over architectural stone surfaces, analyzing both their efficiency and the effects induced to treated substrates by their properties. In order to establish efficiency and compatibility of titania coatings to preserve building stones, historical monuments, architectural and artistic stone elements, the travertine, a natural limestone, was selected as the reference substrate. Travertine is a porous carbonatic stone widely used in historical building and monuments in Europe and Mediterranean Basin and it is still one of the most frequently used stone material in modern architecture, especially for façades, flooring and external cladding [18], [19].

Particular attention was paid in the estimation of the consequence of photo-induced hydrophilicity on treated substrate since, especially for treatment on porous materials, higher wettability could bring to higher water absorption, a potential source of damage for porous materials.