Special Issue Article TiO2 nanocoatings for architectural heritage: Self-cleaning treatments on historical stone surfaces Proc IMechE Part N: J Nanoengineering and Nanosystems 2014, Vol. 228(1) 2–10 Ó IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1740349913506421 pin.sagepub.com Giovanni B Goffredo1, Enrico Quagliarini1, Federica Bondioli2 and Placido Munafò1 Abstract The development and application of nano-engineered surface treatments on stones could become a useful tool for the realization of smart systems to better preserve and maintain architectural surfaces. Titanium dioxide nanoparticles can be used to realize transparent self-cleaning coatings applicable directly on preexisting surfaces, limiting cleaning actions and conservation processes, thus reducing their costs. The aim of this investigation is to evaluate the potential use of TiO2 on stone surfaces, especially in the field of architectural heritage. An aqueous colloidal dispersion based on titanium dioxide, obtained by sol–gel and hydrothermal processes, was applied by spray coating on travertine, a limestone largely used in buildings, both historical and modern. The maintenance in the original appearance of treated substrates was evaluated monitoring both colour and gloss changes produced by the treatments. Physical changes induced to stone by titanium dioxide were studied by wettability analyses. The efficiency of TiO2 photocatalysis was assessed by depolluting and soiling removal tests under ultraviolet light. The effects of deposited amount of titania on treated surfaces were also evaluated. Obtained results seem to allow the use of selected TiO2 treatments on the selected substrate, travertine, without altering in an evident and harmful way the original properties of limestone. Photoinduced effects (hydrophilicity, degradation of pollutants and decolourization of soiling) are very evident, and the combination of these properties may lead to an actual self-cleaning effect. Keywords Titanium dioxide, self-cleaning, smart surfaces, limestone, nanotechnology Date received: 4 April 2013; accepted: 5 August 2013 Introduction Surface nanoengineering is a promising tool to create new products or to improve several properties of existing materials. New technologies can be used to integrate new features even in natural or traditional elements and applied sciences. The application of nano-treatments on architectural surfaces, especially in the field of cultural heritage, can lead to evident benefits to their conservation since many degrading agents cause their effects starting from the outer layers of architectural materials. Self-cleaning treatments may preserve the original appearance and characteristics of treated surfaces, leading to evident and important benefits in their maintenance, especially in aggressive urban atmosphere. It is possible to develop and realize self-cleaning nanocoatings as a preventive protection system, decreasing the deposition of pollutants and soiling on building stone surfaces and reducing the onset of degradation processes. Titanium dioxide (TiO2) can be used to obtain self-cleaning treatments. This effect is due to its photogenerated characteristics induced by ultraviolet (UV) radiation (including UV rays of natural sunlight): photocatalysis and superhydrophilicity.1–4 Superhydrophilicity decreases water contact angle (CA) till the creation of a uniform, thin water film on treated surfaces and thus preventing the direct contact between external dirt and solid surfaces, allowing an easier removal of stain. Photocatalysis is the acceleration of 1 Department of Civil and Building Engineering and Architecture DICEA, Polytechnic University of Marche, Ancona, Italy 2 Department of Materials and Environmental Engineering DIMA, University of Modena and Reggio Emilia, Modena, Italy Corresponding author: Giovanni B Goffredo, Department of Civil and Building Engineering and Architecture DICEA, Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy. Email: g.b.goffredo@univpm.it 3 Goffredo et al. chemical reactions in the presence of a catalyst activated by light of an appropriate wavelength, and it can be a useful way to photodegrade both organic and inorganic polluting substances, so as to prevent the deposition of soiling on the substrate, moreover reducing the concentration of pollutants in the proximity of the photocatalytic material.5,6 Titanium dioxide is widely spread in different technological fields: food industry, paintings, paper production, cosmetics, water and air purification, antibacterial and self-sterilizing surfaces and building materials.1–4,7–9 In its nanometric forms (especially the most photoactive crystal, anatase), titania is one of the best photocatalysts because of its outstanding properties: high photoactivity, chemical stability, inexpensiveness, non-toxicity and compatibility with traditional construction materials.3,4,7,8 It has been exploited in several self-cleaning products of building industry, such as cement mortars, exterior tiles, paving blocks, glasses, paints, finishing coatings, road-blocks and concrete pavements.4,10–15 Titanium dioxide can be used not only during industrial processes but even on preexisting surfaces (both historical and modern). Transparent self-cleaning coatings16,17 are obtained using TiO2 nanoparticles, thus decreasing the effects of external pollutants without modifying the original aspect of treated substrates, allowing their application for better preservation of architectural surfaces.18–30 The aim of the present work is to evaluate the potential use of TiO2 nano-engineered treatments over building stones, in particular with travertine, a natural limestone. The travertine is a carbonatic stone distinguished by its rough surface and high porosity, widely used in historical buildings and monuments, particularly in the Mediterranean Basin. It is still one of the most frequently used stones in the building industry, especially for fac xades, flooring and external cladding. Materials and methods Coating application and microstructure analysis A previously analysed aqueous colloidal suspension (TiO2 content: 1 wt%), realized by means of sol–gel method,21 was used in this study. The product was deposited directly on travertine surfaces (sample dimensions: 8.0 3 8.0 3 1.5 cm3) by spray coating, thus to obtain two different concentrations: a single-layer (SL) and a three-layer (ML) treatments, having an average deposited titania amount of 0.20 and 0.60 g/m2, respectively. After deposition, specimens were dried in a ventilated oven at about 60 °C for 1 h. This drying phase is not strictly necessary and can be avoided in real outdoor use on stone surfaces since it simply accelerates the drying process. Any other additional or subsequent thermal treatment was avoided since it is not always possible to use thermal processes on outdoor building stones, both in the field of cultural heritage18,20–22,31 and modern architectural surfaces. The coatings applied on stone were observed by using a scanning electron microscope (SEM; FEI Quanta-200 instrument) equipped with an energy dispersive X-ray spectroscopy (EDS; Oxford INCA 350) detector. Maintenance of original aesthetical properties To assess eventual changes in treated stone aspect, both colour and gloss analyses were performed, following European test rules (UNI EN 15866:2010,32 UNI EN ISO 2813:200133). Chromatic values were measured by a Konica Minolta CM-2600d spectrophotometer and defined according to CieL*a*b* colour space, using nine samples for each studied case (untreated (UT), SL and ML). CieL*a*b* model defines all the colours perceivable by naked eye through three parameters, L* (brightness), a* (chromatic values between red and green) and b* (colour intensity in the yellow–blue axis). The colour change between two different surfaces are defined as the distance between two points defined by the three coordinates in the L*a*b* space. Total colour change (DE*) measurements were carried out both before and after deposition of titania suspension on stone surfaces. The specular reflection (gloss) analysis was carried out by the use of a Novo Gloss Trio apparatus (Rophoint Instruments). Photoinduced hydrophilicity and water absorption Static water CA was measured both before and after UV-A light exposure using UNI EN 15802:201034 standard procedure and a OCA 20 apparatus (DataPhysics Instrument GmbH). Static CAs were determined using a drop volume of 5 mL, performing 15 measurements for each test surface, both treated and untreated.34 Water absorption was evaluated by a specific surface water absorption test.22 Test specimens (treated and untreated, 2.5 3 8.0 3 1.5 cm3 obtained from original samples by cut) were dried in a ventilated oven until reaching a constant mass m0 (difference between two different weighings at an interval of 24 h is not greater than 0.1% of the mass of the specimen). Stone samples were placed on a support inclined of 10° from the vertical plane in groups of three, then nebulized water was sprayed on specimens every 2 min and the absorbed amount of water was estimated by weighing (total water amount sprayed: about 45 mL for each group, duration of test: 60 min). The lateral surfaces of each sample were sealed to avoid water absorption from the sides of the stone. To evaluate possible differences arising from TiO2 superhydrophilicity, surface water absorption was monitored both with and without UV illumination (irradiance value: 20 W/m2). The amount of absorbed water was related to the surface area of specimens through Qi = mi  m0 A ð1Þ 4 Proc IMechE Part N: J Nanoengineering and Nanosystems 228(1) in which mi is the weight (kg) of the specimen after i minutes of water spraying, m0 is the original weight of dry specimen (constant mass, kg) and A (m2) is the area of the stone sample exposed to water spraying. Photoactivity evaluation on stone Self-cleaning effect due to photodegradation of soiling was evaluated by the use of rhodamine B35–38 colourimetric test UNI 11259:2008.39 The decolouration of artificial stain (rhodamine B water solution, rhodamine B/water ratio: 0.05 6 0.005 g/L) applied on stone samples through a syringe (0.5 mL of solution per specimen deposited on about 20 cm2, sample dimensions: 8.0 3 8.0 3 1.5 cm3) was monitored by colourimetry (six control points for each specimen) after a 24-h-long drying phase in a dark ambient and after 1, 4 and 24 h of UV-A light exposure (irradiance value: 4.00 W/m2), referring obtained data to original pre-stained state. According to the UNI standard rule, because of the red colour of the dye only chromatic coordinate a* was used to assess the photocatalytic decolouration D* of rhodamine B during the process, defined as follows Dt = at  arB 3100 arB  a0 ð2Þ in which a0 defines the original aspect of travertine samples obtained by previous colour analysis before the deposition of the dye, arB is the mean value of red colour of surfaces after the application and drying of rhodamine B and at is the average value of chromatic coordinate a* after t hours of UV-A light exposure. The depolluting efficiency through photocatalysis was evaluated by degradation of nitrogen oxide (NO).40 The photoactivity was monitored by continuous flow test method under UV exposure, following Italian standard rule for nitrogen oxides (NOX) abatement by photocatalytic cementitious materials (UNI 11247:2010).41 The samples (dimensions: 8.0 3 8.0 3 1.5 cm3) were placed in a 3-L borosilicate reactor, in which dry air containing 0.6 parts per million (ppm) of NO was passed through at a rate of 1.5 L/min. After a short period, the surfaces were exposed to UV irradiation (20 W/m2) for at least 45 min.41 Photocatalytic decomposition was monitored every minute for at least 160 min by a Nitrogen Oxides Analyzer 8841 (Rancon Instruments). The photodegradation of pollutants was measured both before and after the rhodamine B test (without removing the residual stains) in order to better evaluate the efficiency of TiO2 nanocoatings in the presence of by-products difficult to decompose, as expected in real outdoor applications. After the end of rhodamine B test, it was possible to use only one stained specimen for each treatment case. Results and discussion Coating application and microstructure analysis SEM images show that there is no clear formation of a regular film (Figure 1). The presence of titanium dioxide is confirmed by EDS spectra, especially in the case study of ML. Two main differences between the treatments can be noticed. The aggregates of titania nanoparticles are more evident in the case of ML, and the morphology of the coating is more uniform compared to SL, therefore ML treatment covers the travertine substrate and its irregularities in a more homogeneous way. The different amounts of TiO2 deposited on stone surfaces and the diverse nature of the two so-obtained coatings could influence both behaviours and photoactivity of the two treatments. Maintenance of original aesthetical properties Transparency of the coating was evaluated by colour and gloss analyses. In Table 1, the L*, a*, b* and DE* values before (pre) and after (post) treatment are reported. Colour changes depend slightly on titania content (not in a directly proportional way): ML treatment shows an higher colour variation in comparison with SL case, but the difference is negligible. Colour variations in reference to untreated condition are just above the conventional possibility of detection by human naked eye (DE* . 1), and these results are fully satisfactory in the field of architectural heritage since colour changes up to 5 units are considered acceptable.30 As for non-metallic paints and materials, measurement scale of gloss is between 0 (perfectly matt surface) and 100 gloss units (GUs). According to test method, if the specular reflection measured at standard geometry (60°) is below 10 GUs, it is necessary to change the measurement angle to 85°.30,33 Nine samples were used for each treatment condition, performing six measurements for each specimen. The variations were obtained by comparing the different gloss values separately for each case. Gloss (measurement angle: 85°) was almost unchanged by the presence of TiO2 on stone surfaces (Table 1), and the variations are fully satisfactory for stone surfaces.30 Aesthetical changes induced by the application of titania are negligible, and analysed nanocoatings are transparent. Photoinduced hydrophilicity and water absorption Static CA analysis shows that under UV illumination hydrophilic effect of TiO2 is very evident, even if it is not still superhydrophilicity. The determination of static CAs for porous and rough surfaces, such as that of travertine, is not simple, and it is deeply influenced by surface irregularity, so the results should be carefully considered. CA values decrease from about 70° (mostly due to morphology, roughness and physical–chemical Goffredo et al. Figure 1. SEM images (secondary electrons, 10003) and EDS spectra obtained on the travertine: (a) untreated surface, (b) SL surface and (c) ML surface. 5 6 Proc IMechE Part N: J Nanoengineering and Nanosystems 228(1) Table 1. Total colour (n = 9, the results are an average of nine measurements for each sample) and gloss (n = 3, the results are an average of six measurements for each sample) variations obtained for the single-layer and multilayer TiO2 coatings applied on travertine. UT L* a* b* DE* Gloss (GUs) 81.69 6 1.23 2.07 6 0.24 6.84 6 0.70 – 0.98 6 0.41 SL ML Pre Post Pre Post 81.70 6 1.18 2.20 6 0.27 7.84 6 0.94 – – 83.63 6 1.06 2.32 6 0.25 8.26 6 0.78 2.15 1.11 6 0.71 82.28 6 1.03 2.10 6 0.20 7.46 6 0.64 – – 84.54 6 0.80 2.07 6 0.18 6.82 6 0.53 2.50 1.08 6 0.76 UT: untreated; SL: single layer; ML: multilayer. Figure 2. Average static contact angle values under UV light. UT: untreated; SL: single layer; ML: multilayer. properties of travertine substrates) to 21.8° (SL) and 20.7° (ML) after 50 min of UV-A exposure. The behaviour is not monotonic since the decrement rate of CAs is higher during the first minutes of UV light exposure (Figure 2): after 30 min of test procedure, the values are very similar for both treated cases and close to final results. Water drops on untreated surfaces under UV illumination are almost unchanged, and the differences are moderate and mostly due to chemical properties, surface roughness and porosity of the stone substrates. With regard to the surface water absorption, in the absence of UV illumination, there are no clear differences related to the presence of TiO2-based nanocoatings between treated and untreated surfaces (Table 2), but (as already seen for CAs) they seem to be mostly dependent on the physical structure of the stones themselves. As a result of the physical differences between the various samples, standard deviation values are quite high for each kind of treatment. The presence of UV light does not greatly alter the behaviour of untreated surfaces since the average water absorption under UV exposure increases by only 16%: considering the high standard deviation value, the results do not seem to be related to the presence of UV illumination (Table 2). Treated stones absorb a lower amount of water under UV exposure, and the trend of absorption is slower in comparison with their unexposed counterparts. After 1 h of UV light exposure, the average water absorption by SL surfaces is equivalent to 65% of the absorption in the absence of UV irradiation by the same surfaces, while ML water absorption decreases to 57% of the respective unexposed case (Table 2). Moreover, in the presence of UV light, at the end of the test treated, surfaces absorb about 50% of the water absorbed by untreated samples. Water absorption by treated surfaces is more uniform, as a consequence, standard deviation values are much lower in comparison with unexposed analysis or with untreated surfaces under UV illumination, and they are more dependent on the presence of TiO2 than on the physical characteristics (porosity and roughness) of treated stone surfaces. Despite the decrease in static contact 7 Goffredo et al. Table 2. Surface water absorption (Qi) under UV light (n = 3). UT Q2 (kg/m2) Q10 (kg/m2) Q30 (kg/m2) Q60 (kg/m2) SL ML Visible light UV light Visible light UV light Visible light UV light 0.016 6 0.013 0.071 6 0.022 0.167 6 0.046 0.269 6 0.132 0.014 6 0.008 0.093 6 0.043 0.225 6 0.095 0.311 6 0.153 0.021 6 0.008 0.059 6 0.027 0.130 6 0.064 0.220 6 0.108 0.019 6 0.007 0.055 6 0.021 0.129 6 0.015 0.144 6 0.027 0.046 6 0.026 0.114 6 0.043 0.200 6 0.085 0.304 6 0.133 0.013 6 0.005 0.034 60.004 0.080 60.021 0.172 6 0.016 UT: untreated; SL: single layer; ML: multilayer; UV: ultraviolet. Results obtained under mere visible light in laboratory conditions are included for comparison purposes (average values and standard deviations). Figure 3. Decolourization of artificial stain (rhodamine B) under UV illumination (n = 6). UT: untreated; SL: single layer; ML: multilayer. water angles due to hydrophilicity of titania nanoparticles, treated surfaces show no higher absorption at all, since hydrophilicity creates a water film over titania coating that slides away by gravity without being absorbed. As in static CA analysis, the water absorption is not strictly related to the amount of TiO2 in the coating, so the application of multiple layers of titania (needing longer times and higher costs) could not lead to considerable advantages in the short and medium terms. Photoactivity evaluation on stone Decolouration of rhodamine B due to TiO2 nanocoatings exposed to UV-A radiation is very evident, while there is no significant degradation of stain on untreated surfaces under UV illumination. The photocatalytic degradation of stain is more rapid during the first part of UV light exposure (Figure 3): after 4 h, the rhodamine B loses about 50% of its original red colour on both treated surfaces. At the end of the test TiO2-based coatings clearly decolourize most of the stain (up to 75% for ML case). Both treatments show very similar results (considering both mean values and kinetics), especially considering long-term behaviour at the end of the test, and there are not meaningful differences related to the applied amount of TiO2. Anyway, ML treatment shows a more efficient behaviour. The obtained results are very promising, and it is well shown that most part of stain is degraded by treatment and that an important function of titania coatings is to greatly accelerate the decolouration process. The concentration of nitrogen oxide clearly decreases in the presence of titania under UV light. The degradation of original NO concentration (about 0.6 ppm) by SL treatment is over 40%, while ML treatment degrades approximately 50% (Table 3). Untreated case does not show any depolluting effect, and its results are not reported. The differences between SL and ML treatments are evident, and they depend on the amount of deposited titania: ML treatment clearly shows better results. Moreover, the behaviour of ML coatings is very homogeneous, as shown by low standard deviation values (Table 3). The decomposition by partially stained surfaces, both SL and ML, is inferior (Table 3). The presence of rhodamine B residues over travertine surfaces after the self-cleaning test clearly decreases the depolluting ability over the TiO2 coatings. The results 8 Proc IMechE Part N: J Nanoengineering and Nanosystems 228(1) Table 3. Degradation of nitrogen dioxide (NO) under UV light exposure by treated surfaces, both before (n = 2) and after (n =1) decolourization test (average values 6 standard deviations). SL Initial NO (ppm) NO under UV light (ppm) Degradation (%) ML Before rhodamine B test After rhodamine B test Before rhodamine B test After rhodamine B test 0.5975 6 0.0233 0.3528 6 0.0614 41.12 6 7.99 0.5948 0.4035 32.16 0.6099 6 0.0006 0.3006 6 0.0006 50.71 6 0.06 0.6350 0.3514 44.67 UT: untreated; SL: single layer; ML: multilayer; UV: ultraviolet. are mainly due to the traces of rhodamine B which still cover part of the analysed surfaces preventing the direct contact between the air flux containing NO and the coatings. In any case, treated surfaces still show a good depolluting ability, as the dye previously applied was not simply discoloured but degraded. Final results are still related to the analysed treatment: ML coating shows better photodegradation of NO in comparison with SL case. The decrease in photoactivity after the rhodamine B test compared to the results before the application of the dye is lower in the case of ML surfaces (approximately 12%) than for SL case (about 22%). In all analysed cases, NO degradation takes place as soon as the UV lamp is on, and it reaches its maximum value in a few minutes. The photodegradation of NO in a continuous flow finishes at the end of the UV exposure, and the amount of nitrogen oxide rapidly returns to its original values. In regard to photoactivity, as for previous analyses, the results of treated surfaces are partly related to the deposited TiO2 content without showing a behaviour directly proportional to the amount of TiO2 deposited on the surface since just outer layer of TiO2 film gets in contact with external agents (pollutants or soil) thus degrading them.5,6 Conclusion The study verified the compatibility of nano-TiO2 treatments with travertine, a limestone widely used in both historical and modern buildings. The fulfilment of three main requirements was analysed: maintenance of the original aspect of substrate, the absence of adverse effect due to photoinduced wettability and the selfcleaning efficiency. A TiO2-based product was applied through spray coating directly on stone surfaces in two different amounts, obtaining a SL and a ML treatments. The aggregation of TiO2 nanoparticles formed a thin coating without great alteration of the substrate morphology, as shown by microstructure analysis. The coatings are transparent, so they can be applied directly on historical stone surfaces without altering their original aspect. The increment in wettability of treated surfaces under UV light does not generate an increase in water absorption, so the application of titania seems to be compatible with limestone. The effects due to photocatalysis (depolluting and self-cleaning abilities) were well evident, thus making the use of titania nanocoatings promising in the field of preservation of stone surfaces. The deposited amount of TiO2 through spray coating does not seem to increase photoinduced properties in a directly proportional way since only the outer layer is in contact with UV light and external materials to be degraded, such as polluting substances and deposited soiling. The application of multiple layers of titania could not lead to evident benefits at least for short-medium periods. The results are promising, and they seem to allow the possible use of titania coatings for architectural heritage preservation, thus creating new fields of application for this nanotechnology. Acknowledgements The authors wish to gratefully acknowledge the valuable assistance given by Professor Gabriele Fava (Department of Material, Environmental Sciences and Urban Planning SIMAU, Polytechnic University of Marche), for the experimental NO degradation test. The authors would like to thank Dr Daniela Diso and Dr Sergio Franza (Salentec srl) for their cooperation. The authors would also like to thank Salentec srl for its cooperation and the supply of titanium dioxide sol. Declaration of conflicting interests The authors declare that there is no conflict of interest. Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. References 1. Fujishima A and Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972; 238: 37–38. 2. Wang R, Hashimoto K, Fujishima A, et al. Lightinduced amphiphilic surfaces. Nature 1997; 388: 431–432. Goffredo et al. 3. Fujishima A, Rao TN and Tryk DA. TiO2 photocatalysts and diamond electrodes. Electrochim Acta 2000; 45: 4683–4690. 4. Chen J and Poon C. Photocatalytic construction and building materials: from fundamentals to applications. Build Environ 2009; 44: 1899–1906. 5. Ollis D. Kinetics of photocatalyzed film removal on selfcleaning surfaces: simple configurations and useful models. Appl Catal B: Environ 2010; 99: 478–484. 6. Julson AJ and Ollis DF. Kinetics of dye decolorization in an air-solid system. Appl Catal B: Environ 2006; 65: 315–325. 7. Fujishima A, Rao TN and Tryk DA. Titanium dioxide photocatalysis. J Photoch Photobio C 2000; 1: 1–21. 8. Fujishima A and Zhang X. Titanium dioxide photocatalysis: present situation and future approaches. CR Chim 2006; 9: 750–760. 9. Fujishima A, Zhang X and Tryk DA. TiO2 photocatalysis and related surface phenomena. Surf Sci Rep 2008; 63: 515–582. 10. Zhao J and Yang X. Photocatalytic oxidation for indoor air purification: a literature review. Build Environ 2003; 38: 645–654. 11. Diamanti MV, Ormellese M and Pedeferri MP. Characterization of photocatalytic and superhydrophilic properties of mortars containing titanium dioxide. Cement Concrete Res 2008; 38: 1349–1353. 12. Brunella MF, Diamanti MV, Pedeferri MP, et al. Photocatalytic behaviour of different titanium dioxide layers. Thin Solid Films 2007; 515: 6309–6313. 13. Zhao X, Zhao Q, Yu J, et al. Development of multifunctional photoactive self-cleaning glasses. J Non-Cryst Solids 2008; 354: 1424–1430. 14. Kawakami M, Furumura T and Tokushige H. NOx removal effects and physical properties of cement mortar incorporating titanium dioxide powder. In: Proceedings of international RILEM symposium on photocatalysis, environment and construction materials (eds P Baglioni and L Cassar), Florence, 8–9 October 2007, pp.163–170. Bagneux: RILEM. 15. Bondioli F, Taurino R and Ferrari AM. Functionalization of ceramic tile surface by sol–gel technique. J Colloid Interf Sci 2009; 334: 195–201. 16. Nakata K, Sakai M, Ochiai T, et al. Antireflection and self-cleaning properties of a moth-eye-like surface coated with TiO2 particles. Langmuir 2011; 27: 3275–3278. 17. Zhang X, Sato O, Taguchi M, et al. Self-cleaning particle coating with antireflection properties. Chem Mater 2005; 17: 696–700. 18. Potenza G, Licciulli A, Diso D, et al. Surface engineering on natural stone through TiO2 photocatalytic coating. In: Proceedings of international RILEM symposium on photocatalysis, environment and construction materials (eds P Baglioni and L Cassar), Florence, 8–9 October 2007, pp.315–322. Bagneux: RILEM. 19. Luvidi L, Laguzzi G, Gallese F, et al. Application of TiO2 based coatings on stone surfaces of interest in the field of Cultural Heritage. In: Proceedings of 4th international congress on science and technology for the safeguard of Cultural Heritage in the Mediterranean Basin (ed. A Ferrari), vol. 2, Cairo, Egypt, 6–8 December 2010, pp.495–500. Napoli: Grafica Elettronica srl. 20. Licciulli A, Calia A, Lettieri M, et al. Photocatalytic TiO2 coatings on limestone. J Sol-Gel Sci Techn 2011; 60: 437–444. 9 21. Quagliarini E, Bondioli F, Goffredo GB, et al. Smart surfaces for Architectural Heritage: preliminary results about the application of TiO2-based coatings on travertine. J Cult Herit 2012; 13: 204–209. 22. Quagliarini E, Bondioli F, Goffredo GB, et al. Self-cleaning materials on Architectural Heritage: compatibility of photo-induced hydrophilicity of TiO2 coatings on stone surfaces. J Cult Herit 2013; 14: 1–7. 23. Quagliarini E, Bondioli F, Goffredo GB, et al. Selfcleaning and de-polluting stone surfaces: TiO2 nanoparticles for limestone. Constr Build Mater 2012; 37: 51–57. 24. Graziani L, Quagliarini E, Osimani A, et al. Evaluation of inhibitory effect of TiO2 nanocoatings against microalgal growth on clay brick facxades under weak UV exposure conditions. Build Environ 2013; 64: 38–45. 25. Pinho L and Mosquera MJ. Titania-silica nanocomposite photocatalysts with application in stone self-cleaning. J Phys Chem C 2011; 115: 22851–22862. 26. La Russa M, Ruffolo SA, Rovella N, et al. Multifunctional TiO2 coatings for Cultural Heritage. Prog Org Coat 2012; 74: 186–191. 27. Kapridaki C and Maravelaki-Kalaitzaki P. TiO2-SiO2PDMS nano-composite hydrophobic coating with selfcleaning properties for marble protection. Prog Org Coat 2013; 76: 400–410. 28. Pinho L, Elhaddad F, Facio DS, et al. A novel TiO2SiO2 nanocomposite converts a very friable stone into a self-cleaning building material. Appl Surf Sci 2013; 275: 389–396. 29. Pinho L and Mosquera MJ. Photocatalytic activity of TiO2-SiO2 nanocomposites applied to buildings: influence of particle size and loading. Appl Catal B: Environ 2013; 134–135: 205–221. 30. Garcı́a O and Malaga K. Definition of the procedure to determine the suitability and durability of an anti-graffiti product for application on cultural heritage porous materials. J Cult Herit 2012; 13: 77–82. 31. Winkler EM. Stone in architecture: properties, durability. 3rd ed. Berlin: Springer-Verlag, 1994. 32. UNI EN 15866:2010. Conservation of cultural property – test methods – colour measurement of surfaces. 33. UNI EN ISO 2813:2001. Paints and varnishes – determination of specular gloss of non-metallic paint films at 20° 60° and 85°. 34. UNI EN 15802:2010. Conservation of cultural property – test methods – determination of static contact angle. 35. Chen J, Kou S and Poon C. Photocatalytic cement-based materials: comparison of nitrogen oxides and toluene removal potentials and evaluation of self-cleaning performance. Build Environ 2011; 46: 1827–1833. 36. Cassar L, Beeldens A, Pimpinelli N, et al. Photocatalysis of cementitious materials. In: Proceedings of international RILEM symposium on photocatalysis, environment and construction materials (eds P Baglioni and L Cassar), Florence, 8–9 October 2007, pp.131–145. Bagneux: RILEM. 37. Ruot B, Plassais A, Olive F, et al. TiO2-containing cement pastes and mortars: measurements of the photocatalytic efficiency using a rhodamine B-based colourimetric test. Sol Energy 2009; 83: 1794–1801. 38. Stephan D, Wilhelm P and Schmidt M. Photocatalytic degradation of rhodamine B on building materials – influence of substrate and environment. In: Proceedings of 10 international RILEM symposium on photocatalysis, environment and construction materials (eds P Baglioni and L Cassar), Florence, 8–9 October 2007, pp.299–306. Bagneux: RILEM. 39. UNI 11259:2008. Determination of the photocatalytic activity of hydraulic binders – rodammina test method. 40. Hüsken G, Hunger M and Brouwers HJH. Comparative study of cementitious products containing titanium Proc IMechE Part N: J Nanoengineering and Nanosystems 228(1) dioxide as photo-catalyst. In: Proceedings of international RILEM symposium on photocatalysis, environment and construction materials (eds P Baglioni and L Cassar), Florence, 8–9 October 2007, pp.147–154. Bagneux: RILEM. 41. UNI 11247:2010. Determination of the degradation of nitrogen oxides in the air by inorganic photocatalytic materials: continuous flow test method.