Journal of Cultural Heritage 13 (2012) 204–209 Available online at www.sciencedirect.com Original article Smart surfaces for architectural heritage: Preliminary results about the application of TiO2 -based coatings on travertine Enrico Quagliarini a,∗ , Federica Bondioli b , Giovanni B. Goffredo a , Antonio Licciulli c , Placido Munafò a a b c Department of Architecture, Constructions and Structures, Polytechnic University of Marche, via Brecce Bianche, 60131, Ancona, Italy Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, via Vignolese 905, 41100, Modena, Italy Salentec srl, via dell’Esercito 8, 73020, Cavallino (LE), Italy a r t i c l e i n f o Article history: Received 9 May 2011 Accepted 11 October 2011 Available online 15 November 2011 Keywords: Architectural heritage Titanium dioxide Self-cleaning surfaces Photocatalysis Stone surface conservation Color and gloss appearance a b s t r a c t The development and application of self-cleaning treatments on historical and architectural stone surfaces could be a significant improvement in conservation, protection and maintenance of Cultural Heritage. In this paper, a TiO2 -based coating has been investigated in order to evaluate its possible use as a self-cleaning treatment. This coating was obtained by a sol-gel and a hydrothermal (134 ◦ C) processes and then it was applied on travertine (a limestone often used in historical and monumental buildings) in two ways, obtaining a single-layer and a three-layer treatment, respectively. In order to verify its potential use in the field of Cultural Heritage, the maintenance of appearance properties of the treated travertine surfaces was monitored by colour and gloss analyses. Besides, de-pollution and soiling removal tests were carried out under ultraviolet-light exposure to evaluate photo-induced effects and self-cleaning efficiency. Results seem to allow the use of TiO2 -based treatments on historical and architectural surfaces made up by travertine, where de-pollution and self-cleaning photo-induced effects are well evident, maintaining their original visual appearance. Anyway, before applying TiO2 -based coatings as conservative treatments, further tests are needed especially on their durability, that is mandatory for Cultural Heritage applications. On-site test in an urban environment and accelerated test by weatherometer are currently under way. © 2011 Elsevier Masson SAS. All rights reserved. 1. Introduction During the last decades, there has been a strong impulse in developing innovative building materials that could offer extra value in addition to outstanding mechanical properties and workability. In this way, building industry, inspired by nature, has recently shown a great interest in developing easy-to-clean and de-pollution surfaces which can be cleaned by a simple rainfall by the use of nanotechnology. Some models of self-cleaning surfaces are in fact available in nature, such as lotus plant leaves [1] and the exoskeleton and wings of some species of insects [2,3]. In particular, the self-cleaning property of lotus leaves has been researched thoroughly and it has been ascribed to the interdependence between surface roughness, reduced particle adhesion and water repellence of the leaves themselves: the so-called “Lotus Effect”. Transferring this self-cleaning and de-pollution effect to man-made surfaces seems to be possible by the use of photoinduced catalysis and super-hydrophilicity as well. For example, semiconductor photocatalytic materials are able to catalyse the ∗ Corresponding author. Tel.: +39 0712204248; fax: +39 0712204378. E-mail address: e.quagliarini@univpm.it (E. Quagliarini). 1296-2074/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2011.10.002 mineralization of polluting agents, either organic and inorganic, through photochemical phenomena that occur on the surface of the photocatalysts under ultraviolet (UV) irradiation [4–7]. The synergy of photo-induced redox reaction of adsorbed substances and photo-induced super-hydrophilicity is the foundation of self-cleaning application in building and construction materials [8]. Titanium dioxide (TiO2 ) is one of the most used and efficient photocatalytic material and, especially in the field of construction and building materials, it is the most widely used photocatalyst. TiO2 is a common semiconductor material that has three crystal structures: anatase, rutile and brookite, with anatase showing a greater photocatalytic activity than the other types of TiO2 polymorphs. Since the discovery of its photocatalytic efficacy, TiO2 has been used in many researches and practical applications, including water [9] and air purifications [10], self-cleaning and antibacterial effect [11], as cement mortar, in exterior tiles, paving blocks, glass, PVC fabric and titanium itself [12–16]. The extensive use of TiO2 is due to its characteristics: relatively inexpensive, safe, chemically stable, high photocatalytic activity compared with other metal oxide photocatalysts, compatible with traditional construction materials, such as cement, without making any original performance got worse, effective under weak solar irradiation in outdoor environment. Besides, TiO2 can withstand the rigors of E. Quagliarini et al. / Journal of Cultural Heritage 13 (2012) 204–209 harsh environments and its durability is proven by exposure to any environmental stress screening. There are several examples of the use of the TiO2 coatings for surface protection: nano-TiO2 coatings exhibit excellent anticorrosion properties in sterile seawater at the room temperature [17], and TiO2 pigments are often added to improve the light fastness properties of coated paper [18]. Despite its wide use in building industry, the number of papers concerning the application of titanium dioxide in the field of Cultural Heritage is still rather limited. As an example, titanium dioxide can be used as an additive in lime binder to improve the durability of lime-based mortars [19]. In fact, the self-cleaning property and the transparency of nano-TiO2 based materials could play a very important role for monuments, historical buildings and any other architectural surfaces exposed to urban pollution. The TiO2 based coatings could potentially allow an easier maintenance of the original colour and aspect of the historical surfaces thanks to the superhydrophilic and photocatalytical properties of this material, while the abatement of NOX and organic pollutants [20,21] could reduce soiling phenomena and black crusts formation on the surfaces [22]. Recent researches, in fact, report that the use of TiO2 nanoparticles seems to allow to realize transparent coatings [22,23] that could improve the clean ability of historical surfaces without changing their appearance properties, acting in a preventive and less invasive way to preserve their original aspect. Luvidi et al. [22] applied commercially available and experimental TiO2 -based coatings on marble, red limestone and black limestone obtaining a treatment able to photo-degrade soiling without evident changes in appearance aspect of the treated surfaces. In this way, the aim of this paper is to provide some preliminary results about the applicability of TiO2 -based transparent coatings on historical, artistic and architectural stone surfaces, in order to preserve their original appearance properties and to decrease the deposition of pollutants on them, so as to obtain a self-cleaning treatment and better conserve them. 205 The selected material was a travertine stone, a limestone often used as a facing in many monuments, historical building and architectural and artistic surfaces. An aqueous sol of titania, prepared by sol-gel process and successive hydrothermal crystallization, was applied by spray coating to the travertine samples. The sol-gel method is widely used for the titania sol production [24]. At room temperature, it generally leads to the formation of amorphous TiO2 that has to be thermally treated to obtain crystallized powders and coatings. In this work, a hydrothermal process, that directly allows particle crystallization in the liquid phase, was used. The so obtained titania suspension was deposited on the substrate without any additional thermal process [25,26]. This approach was chosen in order to apply a titania coating with high photocatalytic activity on the travertine substrate, with no further crystallization step after the coating deposition [27]. In order to evaluate the potential use of this TiO2 -based coating in the field of Cultural Heritage, different tests were carried out. In particular, colour and gloss variations were investigated to define the aesthetical changes induced by the coating (one of the main paradigm in architectural conservation). Then, in order to assess the de-pollution and self-cleaning properties induced by the coating, the photocatalytic efficiency of the treated surfaces was determined in accordance of Italian UNI rules for cementitious materials (nitrogen oxides (NO) degradation and rhodamine B test method). 2. Phases and methods 2.1. Sample and coating preparation According to Italian UNI standards, prismatic (80 × 80 × 15 mm3 ) test specimens were shaped from the selected stone (travertine). Aqueous TiO2 sols were prepared through sol-gel techniques starting from TPOT (tetrapropyl orthotitanate) as ceramic precursor that was added drop wise to a bihydrate oxalic acid water solution. After 4 hours of vigorous stirring, the obtained titania sol was hydrothermally crystallized at 134 ◦ C, 2 bar, for 30 minutes. The XRD pattern of the titania particles, obtained after a drying step at 60 ◦ C, is reported in Fig. 1a, where the presence of very fine anatase crystals with an estimated (Sherrer’s formula [28]) average diameter of 4 nm is underlined by the XRD peak broadening. The average crystal size, as determined by Dynamic Light Scattering analysis (DLS, Malvern Zetasizer Nano), is around 40 to 50 nm with a narrow grain size distribution (Fig. 1b) indicating a partial aggregation of the primary crystallites. The hydrothermal sol was deposited on the travertine samples through a spray gun with 0.8 mm diameter nozzle. Two different amounts of titania were deposited on the samples: 0.12 g/m2 for one deposition layer (T1) and 0.40 g/m2 for three deposition layers (T2). After the deposition, the samples were dried at about 60 ◦ C for 1 hour. This drying phase of the process is not strictly necessary and it can be avoided in real outdoor use on stone surfaces, since it simply accelerates the normal process of drying. 2.2. Travertine characterisation 2.2.1. Microstructure analysis The surface morphology of the treated stone was observed by SEM analyses, using FEI Quanta-200 instruments, over gold-coated EDS samples. Fig. 1. XRD pattern (a) and Dynamic Light Scattering (DLS) analysis (b) of as synthesized TiO2 particles. 2.2.2. Aesthetical properties The effect of the titania coatings on the visual appearance of stone specimens was monitored by using the CIELab method [29] by a Konica Minolta CM 2600 d spectrophotometer. The method 206 E. Quagliarini et al. / Journal of Cultural Heritage 13 (2012) 204–209 defines a colour through three different parameters, L*, a*, and b*, measuring brightness, red/green and yellow/blue colour intensities, respectively [30]. This method also allows a colour difference (E*) to be defined, based on the following relationship:  2 2 E ∗ = (L∗ ) + (a∗ ) + (b∗ ) 1 2 2 where L*, a* and b* measure the differences in luminosity and in chromaticity between two colours. In this way, the hue variations due to the treatments were determined. For each treatment (T1 and T2), four samples were analysed and the measurements were repeated at least seven times as recommended in UNI EN 15866:2010 [31]. The measurement points were localized by a reference spatial grid to ensure precise repeated measurements in the same points. The gloss of each samples was also measured by a Novo Gloss Trio apparatus (Rophoint Instruments) using 60◦ as standard geometry. Measurements were repeated at least four times for each specimen. All of the previous colorimetric measurements were also repeated after NO degradation test (as described in Photocatalicity section) to evaluate the appearance variation due to UV and NO exposure. 2.2.3. Photocatalicity A NO degradation test was carried out to evaluate the photocatalytic activity of the coatings, following the UNI EN standard for cementitious materials [32]. A flow type photoreactor was used to examine the NO degradation capability of the treated limestone. The samples were placed in a 3-L borosilicate reactor and dry air containing 0.6 ppm of NO was passed through at a rate of 1.5 L/min. After a short period, the samples were irradiated with an UVA lamp (20 W/m2 ) for at least 45 minutes. Untreated specimens were used as reference. The photocatalytic decomposition was monitored, every minute for at least 125 minutes, by a Nitrogen Oxides Analyzer model 8841 (Rancon Instruments). A rhodamine B solution photo-degradation test [22,33,34] was used to assess the photocatalytic activity by the colorimetric method, following the UNI standard for hydraulic binders [35]. A rhodamine B (0.05 ± 0.005 g/L) water solution was put on the samples (0.5 mL per specimen) and then, after complete drying, colorimetric measurements were carried out using an UV spectrophotometer (Color Quality, COROB, I). The samples were subjected to UV radiation using an UV light with wavelength range 325 to 390 nm and light intensity 3.75 ± 0.25 W/m2 . The colour measurements were carried out after 4 and 26 hours of irradiation in order to monitor photocatalytic decomposition over time. Two specimens were analysed for each type of treatment (uncoated, T1 and T2). Due to the red color of rhodamine, only chromatic coordinate a* was used to define the photocatalytic efficiency, which was expressed as: R(t) = a∗ (0) − a∗ (t) × 100 a∗ (0) where a*(0) and a*(t) are the measured values of a* before UV irradiation, and after t hours of radiation exposure, respectively. Since the used test method is specific for hydraulic binders, the results related to porous or rough surfaces, such as that of travertine specimens, should be carefully considered, because absorption and distribution of the applied dye could be not so regular. 3. Results and discussions The sample surfaces obtained after TiO2 deposition were observed by SEM. The images (Fig. 2) show that the coating is formed by irregular aggregation of particles and there is no formation of an uniformly spread film on the limestone surface. The Fig. 2. SEM images (2000×) of limestone surfaces: untreated (a), T1 (b) and T2 (c). 207 E. Quagliarini et al. / Journal of Cultural Heritage 13 (2012) 204–209 Fig. 3. Spectra of limestone surfaces: untreated (a), T1 (b) and T2 (c). microstructure of the travertine surface is substantially unmodified by the titania coating. In Fig. 3, several EDS spectra are reported in order to show the presence of titania locally spread on the surface of the treated limestone in comparison with the no treated one. Table 1 reports the L*, a* and b* values and the average values of E* between the treated and the untreated surfaces. The gloss measures (GU) are also reported in Table 1. From Table 1, it is evident that before UV-light and NO exposure the E* values of the treated samples are greater than 1, thus there is a slightly visible hue variation due to the applied coatings (colour change with E* less than 1 is conventionally not visible to the naked eye). However, this variation is satisfactory in the field of Cultural Heritage [36]. Besides, these hue variations seem to slightly increase as the titania content increases, while there is no effect on gloss value as a result of TiO2 -based coating application. These colour variations measured are anyway lower than those reported about other TiO2 -based treatments on limestone [22]. Colour and gloss measurements were carried out again after UVA and NO exposure. Untreated samples did not show a visible colour variation in reference to the initial colorimetric values (with no exposure), as shown in Table 1. T1-samples instead showed a little bit higher hue variation after UV irradiation and NO exposure (E* from 1.4 to 2.4), while colour change of T2-samples was practically unchanged (E* from 2.0 to 1.9). GU measurements were substantially unaffected by UVA and NO exposure. Taking everything into account, the changes of the aesthetic properties (colour and gloss) of the analysed samples seem to be quite moderate, thus the use of the analysed TiO2 -coatings seems to be aesthetically compatible with historical and architectural travertine surfaces. In order to evaluate the photocatalytic property of the TiO2 based coatings, NO degradation and clean ability test were performed. The results of NO degradation test are reported in Fig. 4, showing NO degradation as a function of time. As regards titania content, untreated samples show no NO degradation (their results are not reported in Fig. 4), while T1- and T2-coatings degrade about 30% and about 50% of NO concentration, respectively. This means that, as expected, the photocatalityc activity is higher where the TiO2 content is greater. Anyway, in this case, it is clear that a higher number of TiO2 layers (and so a higher TiO2 content) does not mean a proportional higher degradation value. This could be explained taking into account that only the surface layer of the coating is fully in touch with NO and it only degrades the polluting substances. The NO-abatement phase starts as the UV lamp is turned on, it achieves its mean value very quickly and it ends as soon as the lamp is turned off: after this period the NO concentration returns to the initial value, so photo-induced de-pollution effect of photocatalicity is well evident. The efficiency and the behaviour of the analysed coatings are similar to what reported from other studies, even though different methods were used [37–39]. In all of these tests, NO degradation stops as the UV exposure ends and NO decomposition values are comparable being their efficiency range from 20% to 45%. Photo-degradation of rhodamine B was monitored to assess selfcleaning efficiency of both the treated and untreated specimens. In Fig. 5, photocalalytic activity R values, as defined before, are reported as a function of UV irradiation time. It is clear that even the uncoated samples show a decrease in their own R values during UV irradiation. This is probably due to the degradation effect of UV-light on rhodamine B. Anyway, treated specimens show higher Table 1 Colorimetric properties (± standard deviation) of the specimens before and after ultraviolet-light irradiation and nitrogen oxide (NO) exposure. Colour changes after ultraviolet-light and nitrogen oxide exposure refer to the values of untreated specimens before ultraviolet-light and nitrogen oxide exposure. Before ultraviolet-light and nitrogen oxide exposure L* a* b* E* GU After ultraviolet-light and nitrogen oxide exposure Untreated T1 T2 Untreated T1 T2 78.5 ± 1.5 3.5 ± 0.3 9.7 ± 0.9 / 2.5 ± 0.2 79.3 ± 1.8 3.3 ± 0.4 8.7 ± 1.0 1.4 2.5 ± 0.2 79.3 ± 1.2 3.2 ± 0.3 8.0 ± 1.2 2.0 2.5 ± 0.1 77.7 ± 1.1 3.5 ± 0.1 9.7 ± 0.5 0.7 2.4 ± 0.2 80.0 ± 1.0 3.1 ± 0.3 8.0 ± 0.9 2.4 2.6 ± 0.3 79.3 ± 1.1 3.2 ± 0.3 8.1 ± 1.7 1.9 2.9 ± 0.2 208 E. Quagliarini et al. / Journal of Cultural Heritage 13 (2012) 204–209 Fig. 4. Nitrogen oxide (NO) degradation as a function of ultraviolet (UV) irradiation time: T1 (a) and T2 (b). Fig. 5. Average photocatalytic activity (R) as a function of ultraviolet (UV) irradiation time (h), measured by colorimetric test method. R values, especially after only 4 hours of exposure. In fact they have a greater slope at a first time then the degradation kinetic becomes slower, while degradation on untreated surface is more uniform. In this way, TiO2 -coatings greatly accelerate the rhodammine B degradation process during the first hours of exposure: after 4 hours, the T1- and T2-R values are, respectively, about 2.5 and 3 times higher than those of the uncoated samples, while after 26 hours of UV-light exposure the degradation value of the untreated samples and that one of the T1-samples are very similar, while the degradation value of the T2-samples is still 17% higher than that of the untreated case. As seen in NO decomposition test, a higher TiO2 content does not imply a proportional higher rhodamine degradation. Even in this case, this could be explained taking into account that just the outer parts of the coating become in touch with dye. The results and the kinetic of photo-degradation of the dye are partly in agreement with those of other rhodammine degradation tests on limestone [22], where after 20 hours of UV irradiation, there was an approximately 30% of rhodammine B degradation on uncoated surfaces, while treated surfaces degraded from 40% to 60% of dye. It is well evident that, especially during the first part of UVlight exposure, treated surfaces greatly accelerate the rhodammine B degradation process, degrading a larger amount of dye than uncoated ones. This behaviour is very important in possible outdoor use of TiO2 coatings, since the self-cleaning effect can be quickly activated by solar-light exposure. appearance. An aqueous sol of titania obtained by sol-gel and successive hydrothermal process was deposited by spray coating on travertine, a limestone often used in historical buildings and architectural surfaces, obtaining two treatments: a single-layer coating and a three-layers one. Results show that the TiO2 -based treatments modify the aesthetic aspect (colour, gloss) of the treated stones in a negligible way and thus having a very limited visual impact. Photocatalytic and self-cleaning activities were also evaluated. From results, it is well evident that the TiO2 coatings can effectively photo-degrade NO during UV-light irradiation, decreasing pollution effect on treated surfaces and that they are able to accelerate the degradation process of the dye under UV exposure. This behaviour can be very important in outdoor use, as long-term solar light exposure is not always available. The deposition of successive TiO2 based layers does not seem to increase photo-induced characteristics (NO and soiling degradation) in proportion to higher titania content, since just the outer layer seems to determine these properties coming in contact with NO flux and dye, respectively. Applying multiple layers of titania coating could lead to no evident benefits at least for short-medium periods, but they surely are more time and money-spending. In conclusion, the transparency of the analysed coatings seems to allow the use of TiO2 -based treatments on historical and architectural surfaces made up by travertine, improving de-pollution and self-cleaning photoinduced effects. Anyway, before applying TiO2 -based coatings as conservative treatments, further tests are needed especially on their durability, that is mandatory for Cultural Heritage applications. On site test in an urban environment and accelerated test by weatherometer are currently under way. Acknowledgement The authors wish to gratefully acknowledge the valuable assistance given by Professor Gabriele Fava (Department of Physics and Engineering of Materials and Environment, 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 to this paper. The authors would also like to thank Salentec srl for its cooperation and the supply and application of titanium dioxide sol. References 4. Conclusions The aim of this paper was to examine the feasibility of using nano-TiO2 -based coating on historic architectural stone surfaces, in order to obtain a self-cleaning treatment able to reduce pollution and deterioration effects preserving their original visual [1] W. Barthlott, C. Neinhuis, Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202 (1997) 1–8. [2] A.R Parker, C.R. 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