J Sol-Gel Sci Technol (2011) 60:437–444 DOI 10.1007/s10971-011-2574-9 ORIGINAL PAPER Photocatalytic TiO2 coatings on limestone A. Licciulli • A. Calia • M. Lettieri • D. Diso • M. Masieri • S. Franza • R. Amadelli G. Casarano • Received: 31 March 2011 / Accepted: 29 August 2011 / Published online: 9 September 2011 Ó Springer Science+Business Media, LLC 2011 Abstract The application of photocatalytic coatings on stone has been investigated for providing surface protection and self-cleaning properties. Sol–Gel and hydrothermal processes were used to synthesise TiO2 colloidal suspensions and coatings with enhanced photocatalytic activity without any thermal curing of the coated stone. The stone was a porous limestone (apulian sedimentary carbonatic, calcite stone). Films and powders prepared from TiO2 sols were studied using X-ray diffraction to evaluate the microstructure and identify rutile and anatase phases. A morphological and physical characterisation was carried out on coated and uncoated stone to establish the changes of appearance, colour, water absorption by capillarity and water vapour permeability. The photocatalytic activity of the coated surface was evaluated under UV irradiation through NOx and organics degradation tests. The performances of the synthesised TiO2 sols were compared with commercial TiO2 suspension. Since the coating doesn’t need temperature treatments for activating the photocatalytic properties, the nano-crystalline hydrothermal TiO2 sols seem good candidate for coating applications on stone that cannot be annealed after the coating application. A. Licciulli (&)  G. Casarano Department of Engineering for Innovation, University of Salento, Prov.le Lecce-Monteroni, 73100 Lecce, Italy e-mail: antonio.licciulli@unisalento.it A. Calia  M. Lettieri  M. Masieri CNR-IBAM, Prov.le Lecce-Monteroni, 73100 Lecce, Italy D. Diso  S. Franza Salentec srl, Via dell’esercito 8, 73020 Cavallino, Italy R. Amadelli ISOF CNR, Via L. Borsari 46, 44121 Ferrara, Italy Keywords Photocatalysis TiO2  Synthesis  Self-cleaning  Microstructure  Limestone  Cultural heritage 1 Introduction Titania is considered the most promising photocatalytic material for the degradation of environmental pollutants: it is nontoxic, highly efficient, and very stable under UV [1]. The TiO2 photoactivity is strongly influenced by the microstructure, the presence and concentration of doping elements, the specific surface area, the particle size [2, 3]. Amorphous titania particles have negligible photocatalytic activity, the particle crystallisation is essential for the photoactivity which is generally higher with increasing anatase phase content [3]. Different methods have been used to synthesise titania sols and nanoparticles. One of the most investigated is sol–gel [4–8] but other methods have been studied as well: homogeneous precipitation [9–12] and chemical vapour deposition [13]. Sol–gel method is used for the production of suspended nanoparticles (sols). At room temperature it generally leads to the formation of amorphous TiO2, so that thermal curing is required to crystallise powders and coatings. Thermal curing requires substrate heating, and leads to problems like particle agglomeration, grain growth, phase transformation from anatase to rutile, that decrease the photocatalytic activity of titania [14]. The hydrothermal process allows particle crystallisation directly in the liquid phase. The combination of sol–gel and hydrothermal process could be therefore interesting for the preparation of sols containing crystallised TiO2 photoactive nanoparticles ready to be deposited on the substrate without any additional thermal process [15, 16]. This approach has been undertaken in the 123 438 present work for the application of photocatalytic active coatings on stone. Thermal curing of stones is not allowed because in most cases it is traumatic for natural stone [17]. Thermal curing is also not compatible with previous organic treatments very frequently applied on stone surface for the conservation purposes (protection, consolidation) [18]. This is especially the case of the artefacts of historical and architectural value. De-soiling and de-polluting properties of photocatalytic TiO2 have been exploited along with the self cleaning power on glass surfaces [19]. The application of photocatalytic TiO2 on building facades was the aim of the Picada Project (Photocatalytic Innovative Coverings Applications for Depollution Assessment), within the Competitive and Sustainable Growth European Programme [20]. The addition of TiO2 to lime allows to obtain enhanced carbonation of lime-TiO2 composites and limebased mortars with photocatalytic properties [21]. Photocatalytic earthenware, to be used for outdoor applications, such as roof tiles, floor tile, has also been investigated [22]. So far the possibility to use Titania on buildings stones or for monuments preservation has been scarcely investigated. Many requirements and concerns are involved for any superficial stone treatment in the field of the preservation of historical-architectural heritage [23, 24]. In particular the assessment of the effects of the TiO2 treatments in terms of harmfulness with respect to some characteristics of the stones, as colour, water absorption by capillarity, permeability to water vapour, water wettability is very important for any further investigation. The stone substrate considered is a porous calcarenite named ‘‘pietra leccese’’. This stone is representative of soft and porous materials used in historical building, widely used in Southern Italy, as well as in many countries of the Mediterranean basin. The stone has a carbonatic composition and is mainly made of calcite mineral, with a negligible insoluble residue. Its structure is characterized by a poor degree of cementation, with low grain cohesion. This stone generally shows very high porosity, ranging from 30 to 40%, while exceeding over 40% in decayed stone [23]. In this work a porosity of 40% was measured on the samples used for the experimental tests. Due to these intrinsic characteristics this stone exhibits low durability, being easily affected by chemical, biological and physical decay. The first phase of the work deals with the morphological characterisation of the Titania coatings deposited on the stone using different products. The harmfulness with respect to colour change, water absorption by capillarity, permeability to water vapour, water wettability has been investigated comparing coated and uncoated stones, as well as the efficacy of the self-cleaning properties of the treated stone. To full-fill specific requirements in the field of the preservation of stone materials of monuments, many tests 123 J Sol-Gel Sci Technol (2011) 60:437–444 were performed following the specific protocols in this field, such as Italian Normal Recommendations and UNINormal Standards. 2 Experimental 2.1 Synthesis of titania sols Aqueous colloidal suspensions, also named sols, were prepared from tetrapropyl orthotitanate (TPOT) from Sigma-Aldrich 97% as TiO2 precursor. For the preparation of 1 kg of sol 5.7 g Hydrate oxalic acid (Carlo Erba 99.8%) are dissolved in 957.6 g of deionised water, 37 g of TPOT are added drop wise. The calculated content of TiO2 from such a preparation is 1 wt%. A white precipitate is formed and readily dissolved by stirring and heating in about 2 h. After this process a TiO2 amorphous sol is obtained. In the Teflon-lined autoclave (Mars 5, CEM Corporation) amorphous sol are processed for different dwells at the temperature of 125 °C and at the pressure of 3.5 bar. The heating rate was 2.5 °C/min. The temperature was maintained with the accuracy of ±2 °C. The maximum process time is fixed at 10 min to prevent the anatase–rutile phase transformation. Three sols are obtained: HT01 (125 °C, 3.5 bar for 2.5 min) and HT02 (125 °C, 3.5 bar for 10 min), HT03 (185 °C; 13.5 bar; 10 min). 2.2 Application of the coatings on the stone surface As prepared sols, have been applied on samples of ‘‘pietra leccese’’. The commercial product (TioxoClean, supplied by Tioxoclean, Inc.) was applied and compared with the 2 sols. Tioxoclean is a water-based sol containing 1% of anatase nanoparticles. This product is applied on a wide range of substrates including glass, metal, wood, plastic, concrete, and stone. It is coupled with the primer TioxoGuard, strongly recommended by the supplier for the pretreatment, with the aim to protect the substrates from being degraded by the photocatalytic coating. TioxoGuard is a water-based solution, containing amorphous titania, that forms an inactive coating, acting as a barrier. In this work the coating obtained by the coupled TioxoClean and TioxoGuard products is synthetically named TX. The stone specimens were cleaned with a soft brush and washed with deionized water in order to remove dust deposits. Then they were completely dried by a cyclical procedure: 22 h in oven at 60 °C, followed by 2 h in a desiccator with silica gel (relative humidity R.H. = 10 ± 5%) at room temperature. It was assumed that the dry weight is reached when the difference between two consecutive weighting measures (gathered at 24 h from each other) is less than 0.1% of the original weight of the J Sol-Gel Sci Technol (2011) 60:437–444 439 sample. Before the application of the treatments, the stones were kept at 23 ± 2 °C, 50 ± 5% R.H. for 24 h. Each product was applied on 5 specimens of 5 9 5 9 2 cm and 5 specimens of 5 9 5 9 1 cm. Only one 5 9 5 cm side of each sample was treated. Spray coating was used to apply the titania sols on the stone surface using a HVLP spray gun with 0.8 mm diameter nozzle. The weight of each specimen was measured before and after the treatment to calculate the amount of solution applied. After the application of the products, the samples were kept in laboratory at 23 ± 2 °C and 50 ± 5% relative humidity (R.H.) for 3 days, then they were dried in the oven at 60 °C until the weight stabilisation was achieved; the stabilisation was controlled by periodical measurements of weight. 2.3 Tests and analyses Several tests were carried out in order to assess the harmfulness and the photocatalytic efficiency of the coatings on the stone. 2.3.1 XRD analysis XRD analysis were performed on TiO2 powders obtained from the sols evaporating the water at 60 °C. XRD spectra were obtained with X-ray Diffractometer (Philips PW1729) using Cu Ka radiation (k = 1.5406 Å). Crystallite size was calculated by Sherrer’s equation [25] from the full width at half maximum (FWHM) of the (101) reflection for anatase and the (110) reflection of rutile: d ¼ kk=ðb cos hÞ ð1Þ where d is the crystallite size, k is a constant (0.9 assuming that the particles are spherical), k is the wavelength of the X-ray radiation, b = FWHM and h is the angle of diffraction. The anatase/rutile proportions were measured by the method of Spurr and Myers [26]: WA ¼ 1=½1 þ 1:26ðIR =IA Þ ð2Þ where WA is the weight share of anatase in the mixture, while IA and IR are the integrated intensities of the (101) reflection of anatase and the (110) reflection of rutile. Photocatalytic activity and self-cleaning test The photocatalytic activity of the coatings was evaluated through methyl red (Sigma-Aldrich) decomposition under UV irradiation. The samples were covered with 470 lL of methyl red hydro-alcoholic solution with 7.5 9 10-4 mol/L concentration. The solution was applied on both treated and untreated portions (Fig. 2); coloured samples undergone drying in the oven and then are irradiated by ultraviolet rays with an intensity of 37 W/m2 up to 6.5 h. The degradation activity of the TiO2 coatings was evaluated by colorimetry. The starting colour parameters were taken on both coated and uncoated samples, and their evolution in time was estimated. Photocatalytic NOx oxidation test A flow type photoreactor was used to investigate the NOx degradation capability of the coatings. The samples (5 9 5 9 1 cm3) were irradiated with a light intensity of 30 W/m2 (Osram Vitalux lamp) at the photocatalyst surface. Dry air containing 0.6 ppm of NOx (45% NO2 e 55% NO) was passed through the 3 litres Pyrex reactor at a rate of 5 L/min. The NOx concentration was monitored with a chemiluminescent NOx analyzer (Monitor Labs, Model 8440). The NOx conversion is given by the following ratio: ð%Þ ¼ ½ðCa  Cb Þ=Ca   100 ð3Þ where Ca is NOx concentration before entering the reactor, and Cb is the final NOx concentration. Measurements on natural stone were carried out at different experimental conditions: (1) in the dark without the sample in the reactor (sealing test), (2) in the dark with the sample in the reactor (for evaluating the gas adsorption on the coating), (3) under irradiation with the sample in the reactor (for evaluating the photooxidation). The following tests for the assessment of the harmfulness and efficacy of the coatings on the stone surfaces were performed using the same sample set in order to reduce the influence of the intrinsic variability of the stone characteristics on the results. Colorimetry [27] This test was performed with a Minolta CR 300 Chroma Meter reflectance colorimeter to evaluate the colour change. The chromatic variations due to the application of Titania sols were measured in the CIELab space [28], expressed as: DE ¼ ½ðDL Þ2 þ ðDa Þ2 þ ðDb Þ2 1=2 ð4Þ where DL represents the change in brightness, Da and Db the changes in hue. Morphological characterisation They have been performed by an ESEM—Mod. XL30 (FEI Company), using a GSE detector, in low-vacuum modality (pressure: 0.6 torr; acceleration voltage: 25 kV). Static contact angle measurement [29] Measurements were taken with a Costech contact angle measuring instrument. Capillarity water absorption [30] This test was performed on 5 samples measuring 5 9 5 9 2 cm. The 123 440 J Sol-Gel Sci Technol (2011) 60:437–444 weight measurements during the absorption were taken at 10, 20, 30 min, 1, 2, 4, 6, 8 h, 1, 2, 3, 4, 5, and 7 days. The amount of absorbed water (Q) was calculated as follows: Qi ¼ ðwi  w0 Þ=S ð5Þ where wi and w0 are the weight of the sample at time ti and t0, respectively; S is the area of the sample exposed to the water. The Capillarity Index (CI) was calculated using the following equation: R tf t ðQi Þdt CI ¼ 0 ð6Þ Qtf tf Rt where t0f ðQi Þdt represents the area underneath the absorption curve, Qtf is the amount of water absorbed per surface unit at the final time (tf) of the test (i.e. 7 days). The Absorption Coefficient (AC) represents the slope of the straight part of the absorption curve and it is calculated as: AC ¼ Q30  Q0 pffiffiffiffiffi t30 ð7Þ where Q30 is the amount of water absorbed per surface unit at 30 min; Q0 is the intercept of the line in the straight part of the curve. Water vapour permeability [31] This test was performed on 5 samples measuring 5 9 5 9 1 cm. Permeability to water vapour was calculated as the mass of the water vapour crossing the stone surface unit in 24 h and it is expressed as g/m2. The variation in percentage (DP) of the values measured before and after the treatments was also evaluated. 3 Results and discussion The diffraction patterns of HT01 and HT02 and HT03 powders are reported in Fig. 1 and compared with the patterns from calcined TiO2 powders cured at 400 °C. In Table 1 the processing conditions, the average crystalline size and the phase composition of the powders prepared by hydrothermal process are summarised and compared with the properties of calcined TiO2 powders. From the XRD patterns the influence of the hydrothermal process on the crystallisation of TiO2 powder can be clearly evaluated. From room temperature up to 185 °C, the anatase and rutile peaks result intensified and sharpened significantly with increasing curing temperature and time. The reaction duration reduces the anatase/rutile weight fraction whereas no significant changes in the average 123 Fig. 1 XRD patterns of low temperature titania powders prepared by hydrothermal process Table 1 Microstructural properties of TiO2 solutions Sample Processing conditions Particles size, nm (±10%) Phase composition of synthesized samples HT01 125 °C; 3.5 bar; 2.5 min 3.2 Anatase 66.5% HT02 125 °C; 3.5 bar; 10 min 3.7 Anatase 53.5% HT03 185 °C; 13.5 bar; 10 min 5.8 Anatase 27% Calcined TiO2 400 °C; 1 bar; 2h 8.3 Anatase-80% Rutile 33.5% Rutile 46.5% Rutile 73% Rutile 20% Table 2 Amounts of products applied on stone surfaces Deposited TiO2 (g/m2) Product Amount of solution applied (g/m2) HT01 144.4 ± 14.4 2.888 ± 0.288 HT02 56.8 ± 10.4 1.136 ± 0.208 TioxoGuard 156.0 ± 29.2 TioxoClean 133.6 ± 16.8 1.336 ± 0.168 crystal sizes have been found. Above 185 °C polycrystalline TiO2 is mainly composed by rutile phase. The titania coating is applied by spraying the sols. The corresponding amount of TiO2 deposited was calculated from the amount of the applied solutions since the weight concentration of TiO2 in the sols is known. In Table 2 the solutions sprayed and the corresponding amount of TiO2 deposited on the stone surface are listed. They are expressed as the mean values calculated on 10 specimens; the standard deviation is also reported. J Sol-Gel Sci Technol (2011) 60:437–444 441 Table 3 Colorimetric data of the stone samples before and after the application of the coatings Uncoated samples Mean value SD L a b L a b 77.82 2.156 16.787 79.202 1.8412 14.593 0.87 0.09 0.42 0.83 Uncoated samples Mean value SD SD 0.10 L a b L 77.116 2.185 16.838 78.078 0.98 0.13 0.68 0.72 DE a b 1.9798 15.281 0.07 2.61 0.71 HT02 coated samples Uncoated samples Mean value DE HT01 coated samples 1.85 0.49 DE TX coated samples L a b L a b 78.09 2.066 16.438 77.726 2.071 17.419 0.76 0.06 0.37 0.77 0.04 0.36 1.06 Fig. 2 Morphology of the coatings observed by ESEM a HT02, b 2 wt% HT01, c 1 wt% HT01, d morphology of the TX coating observed by ESEM The results of colour changes DE on the coated and uncoated stone are showed in Table 3. The values of DE show negligible variations before and after the application of all the three different TiO2 products. Their entity is below the minimum value that human eye could appreciates. The morphology of the coating is greatly influenced by the amount of solution sprayed. The relationship between sol gel derived film cracking and film thickness has been studied experimentally by many authors. A critical film thickness below which films is crack-free is generally observed; it is dependent on material properties and experimental condition. For films thicker than this critical thickness the crack spacing was approximately ten times the film thickness. The phenomena is generally explained by different forms of relaxation of the stress in the vicinity of a crack through the film [32]. With reference to the amounts of solution reported in Table 2, discontinuous coating and microcracks, whose dimensions measure until 1 micron (Fig. 2a, b) are formed when titania concentration in the sol equals or exceeds 2 123 442 Fig. 3 Capillarity water absorption curves of stone samples before and after the application of the TiO2 products. a HT01, b HT02, c TX wt%. Additionally, widespread microblisters, with average dimension of 450 nm in diameter, are evident in the coating obtained by the HT01 product. Homogenous and compact coatings are obtained by applying sols with 1 wt% (Fig. 2c). The coating with the TX treatment show a film with the homogeneous and compact distribution, which is characterised by lamellae texture (Fig. 2d). 123 J Sol-Gel Sci Technol (2011) 60:437–444 Due to its intrinsic characteristics, the water absorption by the uncoated stone is very high and rapid and no drop is formed on its surface. For this reason the static contact angle cannot be measured; it still remained not measurable also on the stone treated with titania sols, as expected for the well-known hydrophilic character of the titania treatments. In Fig. 3a–c the curves of the water absorption are reported as a function of the square root of the time. No significant difference can be observed between the treated and untreated samples. All the samples analysed show the most water uptake during the first hour. In Table 4 the mean values of the maximum absorbed water (Q) with the standard deviation are reported. The values of the capillarity index (CI) and the absorption coefficient (AC) are also summarised. The amount of the water absorbed and the capillary index still remain unchanged after the application of the three coating products. A very small decrease is observed only in the case of the TX with regard to the kinetics of the absorption, which corresponds to a lower value of the absorption coefficient. In Table 5 the water vapour permeability is illustrated. Data are expressed as the mean of the permeability values measured before (Pb) and after the treatment (Pa), as well as percentage variation (DP); standard deviation is also reported. Small decreases in water vapour permeability are observed after the application of the coatings, although their entities do not involve a negative performance of the products. The results of the colour measurements for the evaluation of the photocatalytic activity of the products are illustrated in Table 6, in terms of difference between the chromatic parameters of the samples with methyl red before and after the UV irradiation, with reference to the samples without TiO2 coating. The analysis of the colorimetric data shows that the efficacy of the degradation of the methyl red is related to noticeable variations of the a parameter. HT01 coatings have the best photocatalytic activity and the higher difference between the initial and the final a values. In Fig. 4 the a values are reported as a function of the radiating time. It is evident the fast degradation rate of the methyl red during the first hour. The better self cleaning ability of the stone covered by TiO2 sols is evident by the comparison with the a curve of the methyl red directly applied on the stone, without TiO2 coatings; this curve shows a constant rate of degradation during the whole test and a lower final variation of the a . In Fig. 5a, the NOx removal under UV irradiation is reported as function a of the irradiation time during the NOx degradation tests. The experimental apparatus for the NOx control was the same as used by Takeuchi [33]. The photocatalytic activity has J Sol-Gel Sci Technol (2011) 60:437–444 443 Table 4 Parameters related to the capillarity absorption test Coating product Q (mg/cm2) b.t. AC (mg/(cm2 s1/2)) CI a.t. b.t. a.t. b.t. a.t. HT01 580 ± 36 582 ± 35 0.9 0.9 9.7 9.5 HT02 592 ± 17 593 ± 17 0.9 0.9 10.9 10.9 TX 598 ± 25 598 ± 21 0.9 0.9 10.1 9.4 Key Q maximum absorbed water, CI capillarity index, AC absorption coefficient, b.t. before treatment, a.t. after treatment Table 5 Water vapour permeability Product Pb ((g/m2) 24 h) Pa ((g/m2) 24 h) DP (%) -10 ± 5 HT01 262 ± 13 236 ± 13 HT02 251 ± 14 229 ± 2 -7 ± 3 TX 258 ± 14 233 ± 13 -6 ± 3 Table 6 Chromatic variations of the samples before and after the self-cleaning test HT01 HT02 TX Without TiO2 DE 12.92 10.57 9.97 7.90 Da -10.20 -8.67 -7.99 -6.16 Db 4.69 2.88 2.47 2.21 DL 6.39 5.31 5.43 4.42 Fig. 5 a NOx removal in percent from a starting concentration of 0.6 ppm, b NO abatement in percent NO is converted to HNO3 with photo oxidation by way of NO2. hm NO þ 1=2O2 ! NO2 hm 2NO2 þ 1=2O2 þ H2 O ! 2HNO3 NO is almost completely removed after 60 min by samples HT01 and TX. Both NOx and NO abatement curves confirm the higher photocatalytic activity of the sample containing more anatase whose efficiency is comparable with TX sample. Fig. 4 Evolution of the self-cleaning power by the a change as function of UV exposure 4 Conclusions been found in any of the investigated coating type. After 60 min UV irradiation NOx concentration is reduced of 90% in all the experiments. As shown in Fig. 5, the NOx removal of sample HT01 and TX is faster and similar. HT02 exhibits a slower efficiency. The trends in NOx removal is confirmed with the NO concentration measurements (Fig. 5b). Under proper processing conditions, photocatalytic coatings on limestone with selfcleaning and antipollution properties can be obtained without any thermal annealing of the stone. Crystallised titania nanoparticles suspended in water were preliminary obtained by sol–gel and hydrothermal combined process and applied by spray coating. 123 444 The spray coating process represents a practical, cheap process to apply TiO2 coating without significantly changing the morphology and permeability of the porous limestone. This process successfully meets the specific requirements for the surface engineering of natural stone. Both anatase and rutile phases can be obtained and controlled with hydrothermal process by varying pressure, temperature and time. Selfcleaning tests and NOx removal prove that anatase phase is more active so that short heating at relatively low temperature in autoclave is effective to transform a well dispersed and amorphous titania sol into an active crystalline photocatalytic phase. The coatings do not alter the colour of the stone, its water adsorption and vapour permeability. 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