Moving toward Smart Cities: Evaluation of the Self-Cleaning Properties of Si-Based Consolidants Containing Nanocrystalline TiO2 Activated by Either UV-A or UV-B Radiation

polymers Article Moving toward Smart Cities: Evaluation of the Self-Cleaning Properties of Si-Based Consolidants Containing Nanocrystalline TiO2Activated by Either UV-A or UV-B Radiation JoséSantiago Pozo-Antonio 1,2,* , Daniel Noya-Pintos 3and Patricia Sanmartín4 1Dpto. Enxeñaría dos Recursos Naturais e Medio Ambiente, Escola de Enxeñaría de Minas e Enerxía, Campus As Lagoas-Marcosende, University of Vigo, 36310 Vigo, Spain 2CINTECX, University of Vigo, 36310 Vigo, Spain 3Escola Superior de Conservación e Restauración de Bens Culturais de Galicia, 36002 Pontevedra, Spain; [email protected] 4Departamento de Edafoloxía e Química Agrícola, Facultade de Farmacia, Universidade de Santiago de Compostela , 15782 Santiago de Compostela, Spain; [email protected] *Correspondence: [email protected] Received: 12 October 2020; Accepted: 31 October 2020; Published: 2 November 2020   Abstract: This study evaluated the self-cleaning ability and durability of Si-based consolidants (an ethyl silicate consolidant and a consolidant based on nanosized silica) spiked with nanocrystalline TiO 2 activated by either UV-A radiation (spectral region between 340 and 400 nm, and main peak at 365 nm) or UV-B radiation (spectral region between 270 and 420 nm, and main peak at 310 nm). Granite samples were coated with consolidant, to which nanocrystalline TiO 2 was added at different concentrations (0.5, 1, and 3%, by wt.). Diesel soot was then applied to the coated surfaces, and the samples were exposed to UV-A or UV-B radiation for 1650 h. The surface color changes, relative to the color of untreated granite, were determined every 330 h by color spectrophotometry. Slight color changes indicated a recovery of the reference color due to the degradation of the soot. The final surfaces of both the untreated and treated surfaces were compared by stereomicroscopy and scanning electron microscopy. The main findings were that: (1) In general, the consolidant containing nanosized silica induced the most intense photocatalytic activity. In the more compact xerogel coating formed by the nanosized silica, more TiO 2 nanoparticles were available to interact with the radiation. (2) For all consolidant mixtures, soot degradation remained constant or decreased over time, except with ethyl silicate with 0.5 wt % TiO 2 (no self-cleaning capacity). (3) Soot degradation increased with the concentration of TiO 2 . (4) The UV-B radiation was the most effective in terms of soot degradation, except for the surface coated with the ethyl silicate and 3% wt. TiO2. Keywords: self-cleaning; TiO 2 ; consolidant; stone preservation; photocatalysis; smart city; natural and accelerated procedures 1. Introduction Urban designers are currently facing an important challenge involving the application of new technological and management strategies within the smart city model [ 1 ]. This model aspires to use technological solutions to improve the management and efficiency of the urban environment, with the ultimate aim of increasing urban sustainability [ 1 ]. Within this framework, preventing the deposition of fuel-derived soot on stone surfaces in both historic and contemporary buildings (e.g., by decomposition of the organic matter in the soot by applying UV-active nanocrystalline Polymers 2020,12, 2577; doi:10.3390/polym12112577 www.mdpi.com/journal/polymers Polymers 2020,12, 2577 2 of 19 TiO 2 -based coatings) has been a challenge in the last decade [ 2 – 6 ]. The deposition of carbonaceous particles on surfaces in the urban-built fabric contributes to the formation of black crusts, which affect the durability and aesthetic appearance of the buildings [ 7 , 8 ]. On exposure to UV radiation (UV-A preferred to daylight [ 9 ]), TiO 2 nanoparticles in the coatings become photocatalytic, thus enhancing the photo-decomposition of organic matter on the surfaces by redox reactions while also repelling water due to a hydrophobic effect [ 2 , 3 , 6 ]. This approach avoids the drawbacks associated with many cleaning procedures, such as chemical contamination, extraction of grains, and fusion of minerals [ 10 ]. Previous research findings on construction materials (e.g., stone, mortars, and bricks) coated with artificially synthesized substances, such as Printex-U [ 11 ], orange dye [ 12 ], red dyes [ 13 ], blue dye [ 4 , 12 ], and tobacco [ 14 ], rather than with real diesel exhaust particles, were taken into consideration in determining the following key factors regarding the cleaning efficacy of nanocrystalline TiO 2 coatings: (1) The mineralogical composition of the nanoparticle TiO2used. TiO 2 can occur in different crystalline forms: anatase, brookite, and rutile, with anatase exhibiting the highest photocatalytic activity [ 15 , 16 ]. Anatase is an indirect band gap semiconductor, while rutile and brookite are direct band gap semiconductors. Indirect band gap anatase exhibits a longer lifetime of photoexcited electrons and holes than direct band gap rutile and brookite [ 16 ]. Consequently, anatase has a lower average effective mass of photogenerated electrons and holes than rutile and brookite. The low effective mass suggests fast migration of photogenerated electrons and holes from the interior to surface of anatase particle. Therefore, the recombination rate of photogenerated charge carriers is lowest in anatase. In a study with anatase alone and a commercial (78:14:8) anatase:rutile:amorphous-based dispersion, either in water or in ethylene glycol coated on Noto calcarenite and Carrara marble, better results (in terms of the degradation of rhodamine B) were observed on the surfaces coated only with anatase, due to the residual benzyl alcohol molecules anchored on the anatase nanoparticles [4]. (b) The binder and the procedure used to disperse the TiO2. Regarding the matrix in which the TiO 2 nanoparticles are dispersed, coatings with TiO 2 synthesized in water via polyol synthesis were sprayed onto samples of travertine and limestone, yielding satisfactory self-cleaning of rhodamine B, regardless of the number of applications [ 2 ]. In a study with nanocrystalline TiO 2 dispersed in an aqueous suspension of an acrylic polymer on marble and limestone, a consolidant effect was observed in addition to photocatalytic (against methylene blue stains), hydrophobic, and biocidal effects [ 3 ]. In another study, nanocrystalline TiO 2 was mixed with a commercial hydrophobic fluorpolymer on a limestone and exposed for a period of one year in the city of Bari (Italy) [ 5 ]. The components had a synergetic effect on water repellency and self-cleaning properties; however, the latter decreased gradually due to a reduction in photocatalysis from the coating surface, attributed to aging-related modification of the polymers [ 5 ]. Finally, satisfactory results were observed in a study of the degradation of rhodamine B on a travertine with an anatase coating prepared using a sol–gel method [6]. (c) The type of radiation (daylight or UV radiation) applied to the coating in order to activate photocatalysis of the nanocrystalline TiO2 . Regarding UV radiation, as stated above, previous studies have reported the satisfactory response of UV-A radiation (in the spectral region 315–400 nm) in activating photocatalysis [ 2 , 3 , 6 ]. Natural and artificial solar light (including UV-A and UV-B radiation) also displayed efficient photocatalytic activity [ 4 , 5 ]. UV-B (spectral region: 280–315 nm) and UV-C (spectral region: 100–280 nm)-activated TiO 2 have been used to a much lesser extent [ 17 ], and, as far as we are aware, never on coated stone surfaces in the urban fabric. The sun emits all three types of UV radiation, but the UV radiation that reaches the Earth’s surface comprises around 95% UV-A and 5% UV-B. Scientific research on the effect of UV-B radiation on the photocatalysis induced by TiO 2 nanoparticles is justified in the context of global change, in which ozone depletion will lead to an increase in UV-B that reaches the Earth’s surface [18]. The application of nanocrystalline TiO 2 : anatase-based coatings has been proven successful in terms of the degradation of organic dyes. Thus, the addition of TiO 2 nanoparticles to consolidant products can result in multifunctional coatings since, in addition to restoring the cohesion to Polymers 2020,12, 2577 3 of 19 disaggregated stone surfaces, these coatings also show self-cleaning property. As recommended by conservation professionals, a consolidant cannot alter stone properties such as porosity, color, gloss, etc. In conservation of granitic cultural heritage, polymeric consolidants composed by acrylic/methacrylic monomers, unsaturated polyesters, fluorinated polymers, epoxy resins, siloxanes, etc. have been commonly used but unfortunately many of these products modify the original properties of stones. Commonly, alkoxysilanes, such as tetraethoxysilane (TEOS) products are applied due to their ability to form siloxane bonds [ 19 ]. These products polymerize in situ inside the pore through a classic sol–gel process, with a significant increase of the cohesion of the material [ 19 ]. TEOS is composed by monomeric molecules that react with water to form polysiloxane [ 20 ]. It is classified as a silicon-based hybrid polymer since it fits into the definition of polymer by the International Union of Pure and Applied Chemistry (IUPAC) [ 21 ] (a substance composed of macromolecules of high relative molecular mass with a structure based on the repetition of units derived from molecules of low relative molecular mass known as monomers). In order to avoid the cracking registered in the TEOS coatings due to the capillary pressure in the gel inside the pore and the evaporation of the solvent upon the gelation process on the outer surface of the capillary network [ 22 ], recently, the addition of nanoscale SiO 2 particles to polymers has been investigated [ 19 , 23 ]. They have become a suitable alternative because they allow a better safeguard of the stone properties and achieve higher penetration rates [ 19 , 23 ]. Usually, these nanoparticles are embedded in polymers [ 24 – 26 ]. Zendri et al. [ 24 ], applying nanoscale SiO 2 particles with sodium silicate and ethyl silicate on calcium carbonate and quartz, found that this mixture constitutes a gel of amorphous silica after the evaporation of the solvent was transformed into xerogels. Moreover, organic–inorganichybridshave beendeveloped, wherethehydrolysisissufficiently rapid to provide hydrolyzed products that can condense with the polymer component [ 27 – 30 ]. Then, a homogeneous organic–inorganic hybrid gel is obtained and the cracking of the coatings is avoided. In order to create multifunctional products (consolidation and self-cleaning), the addition of nanocrystalline TiO 2 to Si-based consolidants (ethyl silicate or nanosized silica) with particular attention given to the aesthetic effects on granite was investigated [ 31 ]. Tests of three different concentrations of TiO 2 revealed thatthelowest concentration(0.5 wt%) didnotinduce avisible colorchange onthegranite surface, an advantage considering use of the consolidant as an intervention for the preservation of the built cultural heritage. Moreover, it was found that (1) higher penetration rates were identified in the granite coated with nanosized silica colloidal solution, while ethyl silicate was only found in the few first µ m, and (2) the TiO 2 addition seemed to reduce the penetration of the nanosized silica consolidant. However, in this previous study, the self-cleaning property achieved by the TiO 2 -spiked consolidants (with different composition) was not assessed. Therefore, as a follow-up study, the aim of the present research was to investigate the impact of composition of the abovementioned consolidants on the self-cleaning properties of the coatings spiked with different amounts of TiO 2 (0.5, 1, and 3 wt %) applied to a granite commonly used in the architecture in the Northwest Iberian Peninsula. Moreover, untreated samples and samples coated with consolidant without TiO 2 were included for comparative purposes. As previous research on self-cleaning evaluation of TiO 2spiked polymers, regardless of the substrate underneath, used synthesized substances such as Printex-U [ 11 ], orange dye [ 12 ], red dyes [ 13 ], etc. as proxies for real soot, it was necessary from the practical point of view to apply real soot from car engines. Moreover, in order to determine the effectiveness in enhancing the self-cleaning property of the different UV radiations, the traditionally used radiation on these types of studies, UV-A and novelty UV-B radiations, were tested. The self-cleaning efficacy of the treatments was monitored by conducting color measurements every 330 hours during 1650 hours (75 days). At the end of the experiment, the surfaces were inspected by stereomicroscopy and scanning electron microscopy. Polymers 2020,12, 2577 4 of 19 2. Materials and Methods 2.1. Stone The stone used in the study was a commercial granite, Rodas (Figure 1A,B), selected because it is commonly used in the architectonic heritage in the Northwest Iberian Peninsula. It is an alkaline-type granite containing 25% quartz, 31% albite, 26% microcline, 12% muscovite, 6% biotite, and accessory minerals such as apatite, zircon, rutile, sillimanite, chlorite, and opaque [ 32 ]. Its total porosity accessible to water is 6.50 ± 0.2% [ 33 ]. Fifty-five granite slabs of dimensions 4 cm x 4cm x 2cm with dish-cutting finish were used in the tests. Figure 1. ( A ): Micrograph of the reference granite slab, taken with a stereomicroscope. ( B ): Photograph showing how the consolidant was applied. ( C ): Application of soot to a slab coated with the Nano Estel (N) containing 0.5% wt. TiO 2 . ( D – H ): Samples after being covered with the diesel soot. ( I ): Box with the UV-A radiation. See Table 1for explanation of sample labeling. Table 1. Samples used in the experiment. For each condition, n =3. TiO2(wt %) Without Consolidant Exposure to UV-A Exposure to UV-B 0 uA uB TiO2(wt %) With Consolidant Estel1000 Nano Estel UV-A UV-B UV-A UV-B 0 E0%-uA E0%-uB N0%-uA N0%-uB 0.5 E0.5%-uA E0.5%-uB N0.5%-uA N0.5%-uB 1 E1%-uA E1%-uB N1%-uA N1%-uB 3 E3%-uA E3%-uB N3%-uA N3%-uB Polymers 2020,12, 2577 5 of 19 In order to create a physically damaged substrate in need of consolidation, the granite slabs were subjected to heat stress (500 ◦ C) for 12 hours and then cooled with a jet of tap water. The slabs were then left undisturbed under laboratory conditions (15 ± 5 ◦ C and 60 ± 10% Relative Humidity-RH) for two days. The whole process was repeated, thus increasing the total porosity accessible to water to 7.30 ±0.4%, as previously described [33]. 2.2. Consolidant Products Two consolidant products with different compositions were tested following [ 31 ], an ethyl silicate consolidant and a consolidant based on nanosized silica. The layers of the consolidants were found to have a different texture, which may influence the self-cleaning capacity of TiO 2 -spiked consolidants. The consolidant products were provided by C.T.S. ESPAÑA (Getafe, Madrid, Spain) [34]: • Estel 1000 ® (hereinafter E) is composed of tetraethyl orthosilicate diluted in white spirit D40 (70 vol %). This is a ready-to-use colorless liquid, previously used in various scientific studies [ 3 , 35 ]. • Nano Estel ® (hereinafter N) is an aqueous colloidal solution of nanosized silica particles (10–30 nm). This product has been used in previous studies [ 3 , 36 , 37 ]. The nanosized silica particles bind to each other forming a silica xerogel, similar to that formed by ethyl silicate consolidants. The commercial product is concentrated and must be diluted with 1–2 parts of deionized water, as recommended by the supplier. 2.3. Sample Preparation Thetitaniumdioxide (TiO 2 ) usedas photocatalystadditive wascommercialproduct, Aeroxide P-25, from Evonik Resource Efficiency GmbH (Barcelona, Spain) [ 38 ]. The product consists predominantly of nanocrystalline anatase (3/1, anatase/rutile ratio) with specific surface area of 50 m 2 g −1 [ 39 ]. The TiO 2 nanoparticles were rounded, hexagonally shaped, and of diameter of about 30 nm [39]. Table 1showsthe conditionsand thecodesused foreach samplein theexperiment. Thecommercial consolidants were directly mixed with TiO 2 at three different concentrations (0.5, 1, and 3 wt %). Moreover, consolidant without TiO 2 added (0 wt %) was also tested. Therefore, for each consolidant, four formulations were applied (each formulation was applied to six granite slabs). The consolidant products were applied by brush (Figure 1B) until the samples were saturated. Before application of the consolidant, as previously described [ 40 ], the stone surface was pretreated by a single application of ethanol, by brushing, in order to reduce the surface tension and thus enhance penetration of the product. The stone surface was then covered with Japanese craft paper. After four applications (with an interval of 48 hours between each), the slabs were maintained under laboratory conditions (18 ± 5 ◦ C and 50 ± 10% RH), until they reached constant weight (approximately 30 days), in order to ensure polymerization of the products. Seven slabs without consolidant coatings were also included in the study for purposes of comparison; one slab was used as a reference sample and the other six slabs were used in the UV radiation tests. These untreated samples were maintained under laboratory conditions while the coated samples were being prepared. After 30 days, samples were covered by soot collected from the exhaust pipes of diesel cars in the car park of the Mining and Energy Engineering School at the University of Vigo (Vigo, Spain). In order to facilitate adherence of the soot to the surface of the stone, the surfaces were wetted with a cotton swab before the soot was applied with a brush (Figure 1C) until the surfaces were completely black (Figure 1D–H). 2.4. UV Radiation Tests Samples were exposed to one of two different types of UV radiation in closed boxes for 1650 h (75 days, with 2 hours off/day to avoid lamp overheating issues) (Figure 1I): Polymers 2020,12, 2577 6 of 19 UV-A irradiation with three Philips (Koninklijke Philips N.V., Amsterdam, Netherlands) TL-D 18W (Actinic BL 5A Hg) lamps (spectral region 340–400 nm, main peak at 365 nm) UV-B irradiation with three Philips (Koninklijke Philips N.V., Amsterdam, Netherlands) Ultraviolet-B TL-200W/12RS lamps (spectral region 270–420 nm, main peak at 310 nm) The same system of lamps was used in a previous study [ 41 ]. For each irradiation, three lamps were placed 2.5 cm above the samples and separated by 10 cm from each other. 2.5. Analytical Techniques The chemical composition of the diesel soot was characterized by x-ray fluorescence (XRF) using a Siemens SRS 3000 (Siemens AG, Berlin, Germany). This enabled the chemical composition of the major and trace elements to be determined. The soot was then applied to a glass slide for visualization by scanning electron microscopy (SEM) (Philips XL30, Koninklijke Philips N.V., Amsterdam, Netherlands) coupled with an energy dispersive X-ray spectrometry (EDS) (Oxford Inca Energy 300 SEM, Oxfordshire, UK) in backscattered electron (BSE) mode. Carbon-coated samples were visualized at an accelerating potential of 15–20 kV, a working distance of 9–11 mm, and specimen current of 60 mA. The acquisition time for recording EDS spectra, i.e., the dwell time, was 40–60 s. In order to monitor the self-cleaning properties of the coatings, measured as the recovery of the initial color of the granite during the exposure of the samples to the UV radiation, the color of the surfaces was measured with a portable spectrophotometer (Minolta, model CM-700, KONICA MINOLTA, Chiyoda, Tokyo, Japan) equipped with CM-S100w (SpectraMagicTM NX, Chiyoda, Tokyo, Japan) software. Color was expressed in the CIELAB (Commission Internationale de l ´ Eclairage, CIE, Vienna, Austria) color space [ 42 ]. The following parameters were measured: lightness (L*), which varies from 0 (absolute black) to 100 (absolute white); a*, representing the redness–greenness range (+a*: red and − a*: green); b*, associated with yellowness–blueness spectrum (+b*: yellow and − b*: blue). Color measurements were made in specular component excluded (SCE) mode, for a spot diameter of 8 mm with diffuse illumination by means of xenon flash arc lamp and 10-nm diffuse bandwidth, using illuminant D65 at observer angle 10 ◦ . A total of 20 random readings were made on each surface, following [ 43 ]. For each sample, measurements were made on seven different occasions: (1) before being coated with the consolidant (unconsolidated granite slabs) or after being coated with the spiked consolidants (treated granite slabs); (2) three days after soot application; and (3) to (7) at intervals of 330 hours (15 days) during the UV radiation tests. The global color change ( ∆ E* ab ) was then computed as follows [42]: ∆E∗ ab =q(∆L∗)2+ (∆a∗)2+ (∆b∗)2(1) where ∆L* =L*p−L*i, (2) ∆a* =a*p−a*i, (3) ∆b* =b*p−b*i, (4) and where i is the coordinate related to the initial color of the stone (unconsolidated or consolidated with/without nanocrystalline TiO 2 : first measurement) and p is the coordinate at the different measurement times previously indicated. Remains of diesel soot on the sample surface were detected by stereomicroscopy (Nikon SMZ645, Minato, Tokyo, Japan). Micrographs were taken of the uncoated reference slabs, coated slabs prior to soot application, and coated slabs before and after the UV radiation test. The surfaces exposed to UV radiation (UV-A or UV-B) for 1650 h were evaluated using SEM-EDS (FEIThermo Fisher Scientific, Waltham, Massachusetts, United StatesQuanta 200 in BSE mode). Polymers 2020,12, 2577 7 of 19 Carbon-sputtered surfaces were studied using the same optimum observations’ conditions used to characterize the soot. 2.6. Statistical Analysis Color data were subjected to one-way analysis of variance (ANOVA) and Tukey ' s Honestly-significant-difference (HSD) post hoc tests to compare the different treatments. Homogeneity of variance was tested using Levene ' s test, and the normality of residuals was checked using the Shapiro–Wilk test. Statistical significance was established as p <0.05. Statistical tests were conducted using R software (R Development Core Team, Vienna, Austria) for MacOSX [44]. 3. Results The chemical composition of the diesel soot coincided with previously reported characterization data [45]. The major components were C (>94%) and S (4.26%) (Table 2). The other compounds were present at concentrations lower than 0.5%, with those of Si, P, Ca, Fe, Na, and Al being relatively highest. Table 2. Chemical composition of diesel soot used in this study, determined by x-ray fluorescence (XRF) analysis. Compound Content (%) C 94.04 Na 0.17 Mg 0.05 Al 0.15 Si 0.374 P 0.283 S 4.26 Cl 0.026 K 0.0454 Ca 0.205 Ti 0.0394 Cr 0.0678 Mn 0.0015 Fe 0.2 Ni 0.002 Cu 0.0415 Zn 0.0295 Br 0.00056 Sr 0.00069 Ba 0.0035 SEM observation of the diesel soot enabled identification of carbonaceous particles of different sizes mixed with Si-rich particles (Figure 2). Polymers 2020,12, 2577 8 of 19 Figure 2. Micrograph of the diesel soot. The EDS spectra are also shown. * points out the area where the EDS2 was taken. Regarding the color parameters (Figures 3–5, Table S1), the lightness (L*) parameter was the most affected by the application of the soot (Figure 3, Table S1), in all cases with decreases of more than 49 CIELAB units attributed to a notable darkening of the surface (compare Figure 1D–H and Figure 1A). The lowest ∆ L* values (Table S1) were detected for E1%-uB ( ∆ E* ab =49.21 CIELAB units), N3%-uB (∆E*ab =51.98 CIELAB units), and E3%-uA (∆E*ab =51.37 CIELAB units). Figure 3. CIELAB parameter L* (lightness–darkness changes) of the samples during exposure for 1650 hours (75 days) to UV radiation (E-coated samples: ( A ) and N-coated samples: ( B )). Each measurement was made at intervals of 330 hours (15 days), i.e., a total of seven measurements. The first bar of each group represents L* from the samples before being covered with soot, the second bar represents L* from the surface after soot application, and the other bars represent the measurements of the surfaces exposed to UV radiation every 330 hours (15 days). See Table 1for sample labeling. Polymers 2020,12, 2577 9 of 19 Figure 4. CIELAB parameter a* (redness–greenness changes) of the samples during exposure for 1650 hours (75 days) to UV radiation (E-coated samples: ( A ) and N-coated samples: ( B )). Each measurement was made at intervals of 330 hours (15 days), i.e., a total of seven measurements. The first bar of each group represents a* from the samples before being covered with soot, the second bar represents a* from the surface after soot application, and the other bars represent the measurements of the surfaces exposed to UV radiation every 330 hours (15 days). See Table 1for sample labeling. For the unconsolidated surfaces, L* (Figure 3, Table S1), a* (Figure 4, Table S1), and b* (Figure 5, Table S1) did not vary, regardless of the type of UV radiation (uA and uB samples). The same trend was also observed for the surfaces coated with both unmodified consolidants (without TiO 2 ) (E0%-uA, E0%-uB, N0%-uA, and N0%-uB). Regarding the samples with TiO 2 , different responses were identified considering the TiO 2 concentrations and the consolidant used: (1) For the surfaces coated with E spiked with 0.5% wt. TiO 2 (Figure 3A), the slight changes in L* over time were not statistically significant differences for either type of radiation (0.5%-uA and 0.5%-uB). However, for the N-coated surfaces (Figure 3B), an increase in L* was already recorded on the 15th day of radiation exposure (third bar) relative to the measurement after the surface was covered with the soot (second bar of each group). Regarding the standard deviations, for N0.5%-uA, only the final measurement (seventh bar) was statistically significantly different from that obtained before the test (second bar), while for N0.5%-uB, the measurement on the 15th day (third bar) was statistically significantly different from that made on the surface only covered with soot (second bar). (2) Regarding surfaces coated with 1% (wt.) TiO 2 -spiked consolidant, for the N consolidated surfaces, L* increased from the beginning of exposure (Figure 3B), yielding similar values of L* regardless of the radiation at the different times (15, 30, 45, 60, and 75 days). Considering the standard deviations, for the N1%-uA, the measurements were different (with statistically significant differences regarding the measurement of the surface just covered with soot, second bar) only after 75 days (seventh bar) and for the N1%-uB, after 45 days (fifth bar). For the E-coated surfaces (Figure 3A), two different responses were detected: for the surfaces irradiated with UV-A, there were no changes in L* over time, while for the surfaces irradiated with UV-B, the L* increased over time, although not statistically significantly different from the surfaces covered by soot (second bar). Polymers 2020,12, 2577 16 of 19 Moreover, regardless of the consolidant used, soot degradation remained almost constant over time, except for E0.5% (uA and uB), which did not have self-cleaning, and N3%-uB, which showed a soot degradation rate decreased over time. The decrease in photocatalytic activity can be attributed to the aging-related modification of the consolidant, altering its capacity to retain the nanoparticles on the surface and the consequent loss of TiO 2 nanoparticles [ 5 ]. However, as the experiments in the present study were performed in closed boxes, the samples were not subjected to washing by water from precipitation. Thus, the decrease in photocatalytic activity was not as intense as reported in the aforementioned study [ 5 ]. The photocatalytic activity decreases due to the deactivation of photocatalysis when TiO 2 nanoparticles adsorb ions on their surface [ 52 ]. As a consequence, washing or rinsing the surface after a certain time is recommended to recover the photocatalytic activity by reactivating the TiO2sites [5,50]. Regarding the influence of the UV radiation on the self-cleaning achieved by the spiked consolidants, UV-B radiation was the most effective in terms of soot degradation, except on the surface coated with the ethyl silicate and 3 wt % TiO 2 (E3%), with which UV-A achieved better results in terms of soot degradation. Photogeneration of electron-hole pairs occurs when the energy of the incident photon is greater than the band gap of the semiconductor material. In low band gap materials, such as PbS (0.37 eV) and GaAs (1.43 eV), solar energy is sufficient to activate the process. By contrast, in materials with a high band gap, UV radiation is needed for activation. The typical band-gap energy of TiO 2 is between 3–3.2 eV. Thus, at least UV-A radiation (wavelength <400 nm) is needed for its activation [ 53 ], and better results are achieved with shorter wavelength radiation (more energetic according to Planck ' s theory [ 41 ]) such as UV-B (wavelength between 315 nm and 280 nm). Thus, poorer performance is expected when longer wavelength light is applied and vice versa, i.e., the self-cleaning results of TiO 2 are maximized when activated by UV-B radiation. The different performance of E3% may be related to the interaction between the TiO 2 nanoparticles (3 wt %) and the ethyl silicate. One of the limitations of TiO 2 photocatalysis is the agglomeration of TiO2 nanoparticles due to their bonding by the organic ionizable molecules [ 54 , 55 ] from the ethyl silicate. Such agglomeration may prevent the active centers from receiving the light radiation and consequently hamper the photocatalytic activity of TiO2[56]. 5. Conclusions This study evaluated how the self-cleaning properties of different consolidants were affected by composition and by the addition of different concentrations of nanocrystalline TiO 2 . Two consolidants were selected for study: (1) an ethyl silicate and (2) a nanosized silica-based solution, which were combined with three different concentrations (0.5, 1, and 3 wt %) of nanocrystalline TiO 2 . Granite slabs coated with the different formulations were exposed to different types of UV radiation (UV-A and novelty UV-B) for 1650 h in order to evaluate the effect of the TiO 2 concentration on the self-cleaning property and to determine the most effective type of UV radiation. Degradation of real soot deposited on the surfaces was monitored using spectrophotometry and the final surfaces were evaluated by stereomicroscopy and SEM. Although some degree of self-cleaning was detected in all samples, except the sample coated with the ethyl silicate-based consolidant containing 0.5 wt % TiO 2 , the degree of self-cleaning depended on the consolidant composition and the subsequent micro-texture of the coating, the TiO 2 content, and the type of UV radiation applied. Although the soot was not completely degraded and the self-cleaning efficacy increased with the TiO 2 concentration, self-cleaning was more efficient on the slabs coated with the nanosized silica-based consolidant, becausemoreTiO 2 nanoparticleswereavailableto beactivated bythe radiation. The coatings containing this consolidant were more compact with narrower fissures than the ethyl silicate-based coatings. Regardless of the consolidant product, soot degradation rate over time remained constant or decreased over time. UV-B radiation achieved higher soot degradation rates than UV-A radiation. Polymers 2020,12, 2577 17 of 19 The TiO 2 -spiked, nanosized silica-based consolidant mixture with the lowest TiO 2 ( 0.5 wt % ) content could potentially be applied to granite in cultural heritage because (1) the coating did not induce a visible color change, (2) the higher the TiO 2 concentration, the more intense the reduction on the consolidant effect [ 31 ], and (3) the ethyl silicate consolidant spiked with the lowest concentration TiO 2 content did not induce self-cleaning. Further studies should be conducted to evaluate the performance of the selected mixture exposed to outdoor conditions. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/11/2577/s1. Table S1: Color parameters (L*, a*, and b*) and their respective variations during their exposure (15, 30, 45, 60, and 70 days) to the different UV radiations (UV-A: uA and UV-B: uB) considering the uncoated surfaces or those with consolidants (Nano Estel-N and Estel1000-E) and different TiO 2 contents (0, 0.5, 1, and 3%TiO 2 ) as the reference color (i.e., ref in the ID of each sample). Moreover, the color changes of the surfaces after being covered with soot were also determined. Check Table 1for the labeling. Moreover, the global color change ( ∆ E* ab ) was also computed; n=60. Author Contributions: Conceptualization, J.S.P.-A.; methodology, J.S.P.-A.; software, J.S.P.-A., D.N.-P., and P.S.; validation, J.S.P.-A. and P.S. formal analysis, J.S.P.-A. and D.N.-P.; investigation, J.S.P.-A. and D.N.-P.; resources, J.S.P.-A. and P.S.; data curation, J.S.P.-A. and D.N.-P.; writing—original draft preparation, J.S.P.-A.; writing—review and editing, J.S.P.-A. and P.S.; visualization, J.S.P.-A. and P.S.; supervision, J.S.P.-A.; project administration, J.S.P.-A.; funding acquisition, J.S.P.-A. All authors have read and agreed to the published version of the manuscript. Funding: J.S. Pozo-Antonio was supported by the Ministry of Economy and Competitiveness, Government of Spain (grant IJCI-2017-32771). P. Sanmart í n is grateful for the financial support from the Xunta de Galicia (grant ED431C 2018/32). This research was performed within the framework of the teaching innovation group ODS Cities and Citizenship from University of Vigo (Spain). 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