Bull Eng Geol Environ DOI 10.1007/s10064-015-0820-z

ORIGINAL PAPER

Nanostructured TiO2 for stone coating: assessing compatibility with basic stone’s properties and photocatalytic effectiveness Mariateresa Lettieri1 • Angela Calia1 • Antonio Licciulli2 • Amy E. Marquardt3 Raymond J. Phaneuf3

•

Received: 17 March 2015 / Accepted: 11 November 2015  Springer-Verlag Berlin Heidelberg 2015

Abstract Many building materials have been functionalized to achieve photocatalytic properties, namely selfcleaning and depolluting abilities, through the application of photocatalytic TiO2 nanoparticles to those materials. These nanoparticles are able to preserve building fac¸ades by blocking the deposition of airborne particulates in polluted urban environments, and they are able to purify the air, thus benefiting the environment. In this study, the application of nanostructured TiO2 as a photoactive coating on two types of natural stone was investigated. A TiO2 sol obtained by sol–gel synthesis followed by hydrothermal processing was applied via spray deposition onto a compact limestone and a highly porous calcarenite. The effects of this coating on some basic properties of the stone, such as its color and water absorption, and the photocatalytic effectiveness of the coated surface were then studied. Scanning electron microscopy and energy-dispersive X-ray spectroscopy showed that the coating presented a uniform morphology on both types of stone, with the TiO2 nanoparticles penetrating \1 lm into the stone. The coating was found to be compatible with the properties of the investigated types of stone. Colorimetry indicated that the change in the color of the stone due to the coating was negligible. Measurements of the static contact angle and the results of the capillary water absorption test showed & Angela Calia [email protected] 1

CNR-IBAM (Institute for the Archaeological Heritage), Provinciale Lecce Monteroni, 73100 Lecce, Italy

2

Department of Engineering for Innovation, University of Salento, Via Monteroni, 73100 Lecce, Italy

3

Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA

that photoinduced superhydrophilicity did not increase the amount of the water absorbed by the coated stone. A photodegradation test of rhodamine B demonstrated the self-cleaning ability of the coating on both types of stone. Conversely, the photocatalytic effectiveness of the coating—as measured by a nitrogen oxide abatement test—was found to be higher for the porous calcarenite than for the compact limestone, and to depend on the porosity and roughness of the substrate. Keywords Photocatalytic TiO2  Stone coating  Compact limestone  Highly porous limestone  Physical compatibility  Photocatalytic efficacy

Introduction Atmospheric pollutants have undesirable effects on stone building materials (Fassina 1978; Amoroso and Fassina 1983). Surface accumulation of both inorganic particulates and organic compounds leads to unsightly darkening of stone surfaces, thus altering the appearance of building fac¸ades (Brimblecombe and Grossi 2005; Grossi and Brimblecombe 2007; Grossi et al. 2007). Surface deposits can also act as a repository of dangerous compounds that compromise the durability of the stone. These issues can be particularly important in the conservation of artefacts of historical and architectural value (Young et al. 2003; To¨ro¨k and Prˇikryl 2010; Price and Doehne 2011), as they increase the cost of maintaining buildings that form part of our cultural heritage. Recently, photocatalysis promoted by nanostructured titanium dioxide has been used in the field of building construction to promote the photodecomposition of polluting substances that are absorbed by or deposited on the

123

M. Lettieri et al.

surface; this occurs through a series of oxidative reactions induced by ultraviolet radiation in sunlight (Fujishima et al. 2008; Chen and Poon 2009). Among the allotropes of TiO2, anatase is the most photocatalytically active, although mixed anatase–rutile phases exhibit higher photocatalytic efficiencies than pure anatase (Ohno et al. 2003; Su et al. 2011). The most appealing properties of photocatalytic TiO2 are its abilities to decompose pollutants and to selfclean. Photocatalysis is activated when TiO2, which is a semiconductor, is irradiated with electromagnetic radiation (Mills and Le Hunte 1997). In particular, when a photon of energy (hm) exceeding the bandgap of the photocatalyst excites an electron ðe CB Þ from the valence band (VB) to the conduction band (CB), an electron vacancy ðhþ VB Þ in the VB (i.e., a ‘‘hole’’) is generated: þ TiO2 þ hm ! e CB þ hVB

ð1Þ

These photoexcited electron–hole pairs can migrate to the surface of the catalyst, where they can react with adsorbed water and atmospheric oxygen molecules (Cozzoli et al. 2003), producing hydroxyl radicals ðOHÞ and superoxide radical anions ðO 2 Þ: These highly reactive species are able to accelerate the degradation of a wide range of organic and inorganic molecules (Toma et al. 2004; Agrios and Pichat 2005; Demeestere et al. 2007) in solid, liquid, and gaseous phases, as well as in microorganisms (Kikuchi et al. 1997; Sunada et al. 2003). The decomposition of pollutants through photocatalysis is not the only photochemical effect of activated TiO2. In fact, under UV exposure, TiO2 coatings become superhydrophilic, which decreases their contact angle with water and leads to the creation of a uniform film of water on the treated surface, promoting the removal of agents of degradation. The synergistic effect of these two photoinduced properties (i.e., photocatalysis and superhydrophilicity) is the excellent self-cleaning ability of TiO2 coatings (Guan 2005). During the last few years, many studies have focused on the photocatalytic activity of TiO2 applied onto different building materials, including cement mortar, paving blocks, exterior tiles, glass, and PVC fabric (Chen and Poon 2009). The efficiency of TiO2 coatings at inducing self-cleaning of natural stone has also been investigated recently (Potenza et al. 2007; Luvidi et al. 2010; Licciulli et al. 2011; Quagliarini et al. 2012; Bergamonti et al. 2013, 2014). TiO2-based coatings with the ability to self-clean could make it easier to preserve stone building surfaces, as they could prevent the accumulation of airborne pollutant materials, thus reducing the need for cleaning and maintenance. In addition, TiO2-based coatings covering large areas of stone surfaces on heritage buildings could provide

123

a significant contribution to air purification through the abatement of gas pollutants such as NOx, SOx, and VOCs. From these perspectives, the development of photocatalytic treatments and their application to stone surfaces are timely and appealing challenges. In the building industry, catalytic TiO2 powders are added to the cement or lime of mortars and concretes (Ruot et al. 2009; Ballari et al. 2011; de Melo and Tricheˆs 2012; Folli et al. 2012). Alternatively, photocatalytic coatings are applied to material surfaces (Paz et al. 1995; Diamanti et al. 2008; Bondioli et al. 2009; Ramirez et al. 2010). Photocatalytic TiO2 coatings can be prepared by performing a sol–gel process that produces amorphous titania and then by thermal treatment at high temperature ([450 C) of the coated surface in order to crystallize the anatase and obtain coatings with significant photocatalytic properties (Vicente et al. 2003; Puzenat and Pichat 2003). A heat treatment can be applied to certain substrate materials, such as glass, metals, and ceramic elements, as well as in the manufacture of TiO2 powders for incorporation into mortar, concrete, or ceramic pastes. Photocatalytic TiO2 must be applied as a coating to natural stone; the standard thermal procedure cannot be employed to treat stone as it will induce thermal stress. Nor can it be employed to apply titania onto existing building surfaces since the process must be carried out in situ under outdoor conditions. Instead, the crystallization of anatase by a sol–gel process followed by hydrothermal treatment has made it possible to synthesize TiO2 photocatalyst particles at low temperatures directly in the liquid phase (Wang and Ying 1999; Yin et al. 2001; Licciulli et al. 2011). The resulting sols can be applied onto the surface of a material without the need for any further treatment after the coating has been introduced. In this way, the potential uses of photocatalytic TiO2 can be extended to include in situ applications and treatments of materials such as natural stone. Recently, photoactive anatase was obtained at ambient temperature through the synthesis of a SiO2–crystalline TiO2 nanocomposite in the presence of oxalic acid (Kapridaki et al. 2014). Methods of making a stone surface photocatalytic are of growing interest and novel strategies for obtaining photocatalytic coatings that can be applied to thermally sensitive stone substrates have been devised. Incorporating titania nanoparticles into a silica matrix has been shown to yield photoactive TiO2–SiO2 nanocomposites that are easy to apply using a simple spray process in outdoor conditions; these nanocomposites simultaneously provide additional properties of interest in stone conservation, such as surface strengthening (Pinho and Mosquera 2011; Pinho et al. 2013) and hydrophobicity (Kapridaki and MaravelakiKalaitzaki 2013). They have been obtained by adding a colloidal dispersion of pre-formed titania nanoparticles to a

Nanostructured TiO2 for coating stone: assessing its compatibility with basic stone properties and…

starting sol of silica oligomers in the presence of n-octylamine surfactant (Pinho and Mosquera 2013; Pinho et al. 2015) or by mixing Ti and Si alkoxide precursors in the presence of oxalic acid and organic silica oligomers (Kapridaki et al. 2014). Alternatively, nano-Ti has been dispersed in commercial acrylic (La Russa et al. 2012) or fluorinated polymers (Colangiuli et al. 2015). TiO2 suspensions combined with hydrophobic agents (Luvidi et al. 2010) have also been developed and tested. The immobilization of Ti nanoparticles in these products also helps to suppress the release of the nanoparticles into the environment. In the research documented in the present paper, we investigated the effects of spray coating two types of natural stone with a water-based TiO2 sol prepared by sol–gel and hydrothermal processes. The study aimed to assess the performance of the coating on the two stone substrates. The compatibility of the coating with basic properties of a compact limestone and a highly porous calcarenite was therefore assessed, and the photocatalytic activity of the coating on each type of stone was evaluated. The morphology of the film and the distribution of titania nanoparticles on each stone substrate were investigated. Changes in the color of the stone due to coating deposition and the effects of the photoinduced superhydrophilicity on the wettability and water absorption (i.e., by capillarity) of the stone were measured. Interaction with water is one of the most important factors in stone decay, so an increase in water absorption could have an adverse effect on the stone, limiting the potential applicability of TiO2 photocatalytic coatings to stone. Finally, the self-cleaning abilities of the coated stone surfaces and their effectiveness at abating NOx were measured to assess the photocatalytic activity.

Materials and methods Materials and coating application A compact limestone (CL) and a soft and highly porous calcarenite (PC) were selected as substrates for the TiO2 coating. The former (Trani stone) is widely used in the heritage buildings of the Puglia region (Southern Italy), including minor buildings in many historic towns and significant monuments. These include the precious Romanesque cathedrals, considered to be of high artistic and architectural value, the UNESCO site of Castel del Monte, and the numerous Norman–Swabian castles in the region. This stone is currently used locally but it is also exported to many countries. The soft PC (Lecce stone) is typically used for ornamental purposes in the Baroque architecture of Salento (southern part of the Puglia region), but it is also the traditional building material most commonly used in the area (Calia et al. 2013).

Similar stones can be found around the Mediterranean basin, e.g., in Sicily (Anania et al. 2012) and in the Maltese islands (Cassar 2010). Indeed, compact and highly porous and soft limestones of many varieties are very commonly employed as building and ornamental materials around the world (Smith et al. 2010) due to their high availability, workability, and attractiveness. Samples of each stone measuring 5 9 5 9 1 cm and 5 9 5 9 2 cm were used as substrates for the coating studies. Following the UNI 10921 protocol (UNI Ente Nazionale Italiano di Unificazione 2001), after cutting the samples they were polished with sandpaper, cleaned with a soft brush, and washed with deionized water. Finally, the samples were dried in an oven at 60 ± 5 C for 7 days and stored in a desiccator with silica gel (15 ± 5 % RH). A water-based sol containing crystalline TiO2 nanoparticles (1 wt%) was used. This product was synthesized using a sol–gel and hydrothermal process, starting from titanium tetraisopropoxide (the TiO2 precursor). The required amount of tetrapropyl orthotitanate (TPOT, Sigma–Aldrich, St. Louis, MO, USA; 97 %) was dissolved in a water–oxalic acid (Sigma–Aldrich) mixture by adding TPOT in acidified water dropwise. Heating at 80 C for 2 h was then performed. The sol obtained after the hydrolysis and condensation reactions was kept inside an autoclave at 125 C for 30 min while a pressure of 2 bar was maintained. After the hydrothermal treatment, a white suspension of TiO2 nanocrystals was obtained. Further details about the sol–gel and hydrothermal process employed were provided in a previous paper (Pal et al. 2014). Crystalline phases of the TiO2 powder samples were investigated by X-ray diffraction (XRD) on a Rigaku (Tokyo, Japan) Ultima X-ray diffractometer using CuKa ˚ ) and operating at 40 kV/30 mA radiation (k = 1.5406 A with a step size of 0.02. To perform the measurements, all of the suspensions were dried at 60 C in an oven with air circulation and crushed to powder. A spray-coating procedure was used to apply the TiO2 sol to the stone. A high volume–low pressure (HVLP, model H2000A, AKOKA) spray gun with a 1.00-mm-diameter nozzle, operating at a pressure of 2 bar, was employed. The spray was directed perpendicular to the stone specimens (placed on a surface tilted at 45) from a distance of about 10 cm. During the treatments, the temperature was 23 ± 2 C and the relative humidity (RH) was 50 ± 5 %. For the CL samples, 3.5 mg/cm2 of TiO2 sol were applied; twice this quantity was adopted for the PC specimens. The sol was only applied onto one of the largest faces of each sample (i.e., that measuring 5 9 5 cm). After the application of the sol, the samples were dried in a controlled laboratory environment (23 ± 2 C and 50 ± 5 % RH) and then stored under dark conditions.

123

M. Lettieri et al.

Microscopic analyses Mineralogical and petrographical characteristics of the stones were observed by optical microscopy with transmitted light using a polarizing optical microscope (Eclipse LV 100 PL, Nikon, Tokyo, Japan). Roughness characteristics of the stone surfaces were observed under reflected light. Morphological observations of the coated samples were made via an environmental scanning electron microscope (ESEM); X-ray microanalyses of coated samples were also carried out using this instrument. These investigations were performed on samples without metallization, in low vacuum mode (ambient pressure of 0.6 torr, beam accelerating voltage of 25 kV). An ESEM-XL30 (FEI Company, Hillsboro, OR, USA) instrument equipped with an energydispersive X-ray (EDX) microanalyzer and a gaseous secondary electron (GSE) detector was used for the morphological observations and elemental microanalysis. EDS spectra were acquired in at least three areas (acquisition time 170 s) and the results were averaged. Ti distribution maps were collected via EDS (sampling matrix: 400 9 280 pixels, dwell time: 64 ls, incident energy: 15 kV) from the coated stone surfaces; cross-sections of the CL and PC stones embedded in resin were also used. The EDS Ti distribution maps were analyzed using the National Institutes of Health ImageJ analysis software. Pixel intensity line scans were obtained along a direction perpendicular to the stone surface to obtain a Ti spatial distribution profile. For improved statistics, we averaged 15 adjacent line profiles that were each perpendicular to the original deposition surface and ran along the length of the image. Before averaging, each line profile was fitted to a Gaussian lineshape to locate the stone surface and offset to align the surface peaks. Each of the adjusted profiles across the length of the image was averaged and fitted with a Gaussian with different widths on each side of the peak. The finite line width on the resin side was attributed to statistical variations resulting from the roughness of the surface in the plane perpendicular to the images and the finite penetration of the incident electrons in the EDS measurement (r1), and the larger width on the stone side was due to the convolution of the roughness-associated width with permeation broadening (total width r3). Deconvolution yields the width due to permeation broadening (r2), which is a measure of the penetration of TiO2 into the stone. We assume that both roughness and permeation produce Gaussian broadening; the deconvolution is simple to obtain in this case, and the permeation depth can be calculated by assuming that the two contributions to the lineshape width are independent and add in quadrature, as expressed in Eq. 2.

123

r2 ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r23  r21 :

ð2Þ

Porosimetric analyses The porosity and porosimetric features of the stone substrates in the pore radius range 0.001–100 lm were analyzed by mercury-intrusion porosimetry (MIP). This was done using both Pascal 140 series and Pascal 240 series mercury-intrusion porosimeters (Thermo Finnigan, San Jose, CA, USA) for macropore and micropore measurements, respectively. Determination of the physical properties of the stone before and after coating Colorimetry, measurements of the contact angle of water, and capillary water absorption tests were performed on the samples before the coatings were applied and repeated just after coating to assess the effect of the deposition of the TiO2 film on the stone surface. Measurements of the color of the stone surface (EN 15886; CEN 2010) were made using a CR 300 Chroma Meter colorimeter (Minolta, Tokyo, Japan); ten measurements were carried out on each sample. Color characteristics were referred to the CIELab space, which expresses colors through the parameters L*, a*, and b*, which represent brightness and the red/green and yellow/ blue chromatic intensities, respectively. The color differences (DE*) were calculated using the following equation: h i1=2 DE ¼ ðDL Þ2 þðDa Þ2 þðDb Þ2 : ð3Þ Static contact angle measurements (EN 15802; CEN 2010) were performed on the 5 9 5 9 1 cm samples using a Costech (Pioltello, Italy) apparatus. The results of thirty measurements at different points were averaged to determine both the mean and the standard deviation. The capillarity water absorption test (EN 15801; CEN 2009) was performed on the samples measuring 5 9 5 9 2 cm. Weight measurements during the absorption were taken at 10, 20, 30 min, 1, 2, 4, 6, 8, and 24 h. The amount of absorbed water (Q) was calculated as follows: Qi ¼ ðwi  w0 Þ=S;

ð4Þ

where wi and w0 are the weight of the sample at times ti and t0, respectively; S is the area of the sample exposed to water. The absorption coefficient (AC) represents the slope of the linear part of the absorption curve.

Nanostructured TiO2 for coating stone: assessing its compatibility with basic stone properties and…

Effects of the photoinduced superhydrophilicity of the coating The effects of the photoinduced superhydrophilicity of the coating on the stone surface were investigated by performing water contact-angle measurements and by testing the water absorption of the stone via capillarity after irradiating the samples with light. The light was supplied by a solar simulator (Solarbox 1500e RH, Erichsen Instruments, Hemer, Germany). The radiation source was a xenon lamp (1500 W), which was located in the upper part of the exposure chamber, on the axis of a parabolic mirror that guaranteed uniform irradiation. A glass filter limited the radiation emitted to the wavelength range 275–825 nm, simulating outdoor exposure. Light irradiance and temperature were set at 450 W/m2 and 30 C, respectively. The coated samples were kept in a dark environment for 48 h before exposure in order to avoid photoactivity caused by ambient light. Both static water–stone contact angle measurements and capillary water absorption tests were carried out after 1 h of exposure of the samples to light from the Solarbox source. The measurements began at least 1 h after the end of the period of irradiation, thus allowing sufficient time for the surfaces to cool. Photocatalytic activity The photocatalytic activity of the coatings in terms of selfcleaning and depollution was assessed by performing a rhodamine B (RhB) photodegradation test and by testing for NOx abatement, respectively. The 5 9 5 9 1 cm samples were used for these tests. The degradation of RhB applied to the stone surfaces under irradiation with light was evaluated via color measurements. An aqueous solution of RhB (dye concentration = 0.05 g/l) was spread onto both uncoated and TiO2coated stone; 63 mg/cm2 of dye solution were applied onto the porous stone (PC) while 24 mg/cm2 were applied onto the compact stone (CL). After a drying step (24 h in laboratory), color measurements were carried out. Next, the samples were exposed to light using the solar simulator for a range of irradiation periods (up to 450 min) in order to monitor the photocatalytic decomposition over time. Finally, the color measurements were repeated. Due to the red color of RhB, the evolution of the parameter a* can be used to evaluate the photocatalytic activity in term of self-cleaning (PA) (Quagliarini et al. 2012). This was calculated as     PAt ¼ a0  at = a  a0  100 ð5Þ where a0 and at are the values measured at the beginning of the test and at time t during the irradiation period, respectively; a* is the value obtained by color measurements before the application of the RhB solution.

Next, the efficiency of the TiO2 coatings at degrading nitrogen oxides was evaluated. The specimens were placed in a 3-l Pyrex reactor and dry air containing 0.6 ppm of NOx (45 % NO2 and 55 % NO) was passed through at a rate of 5 l/min. The NOx concentration was monitored with a chemiluminescent NOx analyzer (model 8440, Monitor Labs, Englewood, CO, USA). Measurements were carried out after 30 and 60 min of exposure at different experimental conditions: in the dark (to evaluate the gas adsorption on the coating) and under irradiation with a UVA lamp (Osram Vitalux) at a light intensity of 30 W/m2 (to evaluate the photooxidation). The NOx removal efficiency (NRE) was calculated as the following ratio: NRE ð%Þ ¼ ½ðCin  Cout Þ=Cin   100;

ð6Þ

where Cin is the NOx concentration before entering the reactor and Cout is the final NOx concentration.

Results and discussion Characteristics of the stone supports, TiO2 nanoparticles, and coatings The CL stone studied here is a packstone (Dunham 1962) with grain sizes ranging from fine to medium calcarenite. The texture is grain supported, with grains consisting of micritic intraclasts, peloids, and sporadic fossil remains of bivalves (Fig. 1a). They are very well cemented by sparry calcite and neomorphic spar calcite that fill the intergranular spaces; cementation often occurs as a thick, unselective mosaic in large intergranular cavities. The PC stone studied here is also a packstone, with a grain size of fine calcarenite. The grains are composed of calcareous microfossil debris of mainly planktonic and benthonic foraminifera, with minor fragments of calcareous algae, echinoids, and mollusc shells (Fig. 1b). The groundmass consists of fine calcareous detritus mixed with micrite. Widespread grains of glauconite occur within the groundmass or within the cavities of the microfossils; clastic grains of quartz are also present. The PC stone is densely packed but poorly cemented by small amounts of microcrystalline calcite; it has widespread intergranular and intragranular porosity, with vug-type pores. The stones exhibit different surface morphologies. A smooth profile of the cut surface was observed for CL stone on thin cross-sections, while irregular profiles were exhibited by cut PC stone (Fig. 1a, b). CL stone shows a fine and homogeneous structure; noticeably more of its surface is regular than seen for the PC stone (Fig. 1c, d). The latter shows protrusion of the grains in the detritic structure and cavities between, together with the presence

123

M. Lettieri et al. Fig. 1 a CL stone (packstone) showing a well-cemented grainsupported structure; the smooth profile of the cut surface of the stone sample is shown. b PC stone (packstone), showing a structure mainly consisting of microfossils and fossil remains within a groundmass with a poor microcrystalline cement; the rough profile of the cut surface of the stone is evident (polarizing optical microscope images of thin sections, transmitted light, crossed nicols). c Regular surface morphology of CL stone. d Rough surface of PC stone (optical microscopy images in reflected light)

of pores, resulting in high surface roughness of the PC stone. Our porosimetric analyses showed very different porosity characteristics for the two types of stones analyzed. The porosity measured for CL stone was 2 %. The pore network consists of pores \0.1 lm in radius, with the largest fraction (90 %) of the pore radii lying between 0.007 and 0.05 lm (Fig. 2). PC stone has a very high porosity, 42 %, with larger pore sizes than CL stone [lying mostly (77 %) between radii of 0.3 and 10 lm] and the pore-size distribution tailing off between 0.3 and 0.005 lm. Figure 3 shows the XRD spectrum of the powder sample obtained by drying the TiO2 sol. Nanocrystalline TiO2

Fig. 2 Pore-size distribution curves of CL and PC stone

123

consists of anatase (42 %), rutile (33 %), and brookite (25 %). The weight percentages of these three crystalline phases were calculated according to the following equations (Zhang and Banfield 2000): WA ¼ 0:886IA =ð0:886IA þ IR þ 2:721IB Þ

ð7Þ

WR ¼ IR =ð0:886IA þ IR þ 2:721IB Þ

ð8Þ

WB ¼ 2:721IB =ð0:886IA þ IR þ 2:721IB Þ;

ð9Þ

Fig. 3 XRD spectrum of the powder sample obtained by drying the sol. The hkl planes of anatase (A), rutile (R), and brookite (B) are indicated

Nanostructured TiO2 for coating stone: assessing its compatibility with basic stone properties and…

where WA, WR, and WB are the weight fractions of anatase, rutile, and brookite phases, respectively. IA, IR, and IB are the integrated intensities of the anatase (101), rutile (110), and brookite (121) diffraction peaks. The crystal sizes, calculated using Scherrer’s formula, were 4.03, 10.74, and 5.55 nm for anatase, rutile, and brookite, respectively. Application of the TiO2 coating left the morphological features of the surfaces of both types of stone unchanged, implying high film conformability. The deposited TiO2 was not evident in the ESEM observations (Fig. 4), but Ti was detected by EDS analyses (Fig. 5). A homogeneous distribution was seen on the surface of the CL stone, as supported by distribution maps of this element (Fig. 6a, c). Only sporadic accumulations of Ti were recorded. In the case of the PC stone, the Ti distribution on the surface followed the microstructure of the stone, indicating a decreased presence at pores. Ti was also detected under the surface (Fig. 6b, d); the penetration depth appeared to be affected by the porosity characteristics of the stone substrate. The apparent permeation depth of TiO2 was analyzed as described above in the ‘‘Microscopic analyses’’ section. EDS analysis of a cross-section of the sample implied that TiO2 was present at depths of up to 0.37 lm in the CL. The layer of Ti on the PL was less concentrated: it was detected up to a depth of approximately 0.64 lm.

Compatibility of the coatings with the physical properties of the stones The presence of the TiO2 coating produced negligible changes in the surface colors of the examined stones. The differences in the parameters L*, a*, and b* as well as the variations in color (DE*) between the untreated and coated surfaces (Table 1) were small, and no alteration of the surface appearance upon coating was visible to the naked eye. All of the measured chromatic changes in the coated surfaces were limited, so the studied TiO2 sol appears to be a good candidate for treating stone. Based on our results, preservation of the aesthetic properties of building fac¸ades can be expected following the application of a titania-based coating, meaning that the coating is also compatible with the surfaces of historic artefacts and monuments. Measurements of the static contact angle to assess changes in the wettability of the stone surfaces relating to the superhydrophilic effect of the coatings failed for the PC stone. The water absorption of the uncoated stone was very high and rapid and no drops formed on its surface. For this reason, the static contact angle could not be measured. It remained unmeasurable after treatment with TiO2, as well as after exposure to the solar simulator.

Fig. 4 ESEM images of the stone surfaces: a uncoated CL stone, b coated CL stone, c uncoated PC stone, d coated PC stone

123

M. Lettieri et al.

Fig. 5 EDS spectra acquired on the coated stone surfaces: a CL stone, b PC stone

Fig. 6 EDS distribution maps of Ti: a CL stone surface, b CL cross-section, c PC stone surface, d PC cross-section

Table 1 Mean values of the changes in the color parameters (DL*, Da*, Db*) and the global color changes (DE) upon coating the samples Stone substrate

DL*

CL

-0.14

0.06

1.07

1.08

PC

1.67

-0.34

-1.22

2.10

123

Da*

Db*

DE

In the case of the CL, the contact angles measured for the uncoated and coated stone for samples stored in the dark were comparable (Table 2; Fig. 7a, b). However, 1 h of exposure to the solar simulator was able to impart superhydrophilicity to the treated surfaces: the contact angle values decreased dramatically from 53 to 9, and the deposited drop of water covered the stone surface as a thin film (Fig. 7c).

Nanostructured TiO2 for coating stone: assessing its compatibility with basic stone properties and… Table 2 Contact angle values for uncoated, coated, and irradiated samples of compact limestone

Treatment

Contact angle ()

Before coating

–

54 ± 8

After coating

48 h dc

53 ± 9

1 h solar simulator exposure ? 1 h dc

9±4

1 h solar simulator exposure ? 48 h dc

11 ± 5

dc dark conditions

Fig. 7 Water drops over the CL sample. a Uncoated stone surface, b stone surface with TiO2 coating before exposure to the solar simulator, c stone surface with TiO2 coating 1 h after exposure to the solar simulator, d stone surface with TiO2 coating 48 h after exposure to the solar simulator

Once produced, the superhydrophilicity persisted for a long time, even without any further exposure to light. The same behavior was also observed in a previous study of a TiO2 nanoparticles/fluoropolymer mixture, where the hydrophilic properties of the photocatalytic TiO2 predominated over the hydrophobic properties of the organic polymer (Colangiuli et al. 2015). The contact angle measurement was repeated after 48 h for the samples left in the dark to avoid photoactivity due to ambient light. Again, the measured values were extremely low, and a film of water appeared on the stone (Fig. 7d). Not surprisingly given their dissimilar porosities and porosimetric features, the two types of stone differed greatly in water uptake (i.e., total amount of water absorbed and the absorption rate). Curves for the water absorption of the two stone substrates as a function of the square root of the time are reported in Fig. 8; the mean values of the maximum amount of water absorbed (Q) and of the absorption coefficient are listed in Table 3. The high

porosity of PC stone accounts for the very large amount of water it absorbed, and its ability to rapidly take up water can be linked to the high capillarity of its porous network. Conversely, the CL presented low water absorption and a low absorption rate due to its low porosity. The coated PC stone behaved similarly to the uncoated PC stone. The highest water uptake occurred during the first hour for all samples. The overall amount of water absorbed remained unchanged after the coating was applied. However, during the first stages of the absorption process, a very small decrease in the amount of water absorbed (corresponding to a slightly lower AC value) was measured. No such variation was detected after exposure to light. The results obtained indicate that water absorption was mainly influenced by the porosity characteristics of the substrate, rather than by the presence of the coating. The water absorbed by the compact stone decreased after the application of the TiO2 coating for the

123

M. Lettieri et al. Fig. 8 Curves of water absorption versus square root of the time for CL and PC stone before coating (untreated samples), after coating (coated samples), and after exposure to the light (exposed samples)

Table 3 Mean values of the maximum amount of water absorbed (Qmax) and of the absorption coefficient (AC, as measured by the capillary rise), along with the related standard deviations

CL stone

PC stone 2

Qmax (mg/cm )

2

1/2

AC (mg/cm s )

Qmax (mg/cm2)

AC (mg/cm2 s1/2) 10.06 ± 1.46

Untreated

9.18 ± 1.91

0.038 ± 0.004

506.89 ± 2.64

Coated

6.58 ± 3.80

0.017 ± 0.006

502.79 ± 13.74

9.10 ± 2.58

Exposed

5.69 ± 4.52

0.027 ± 0.014

492 ± 12

9.07 ± 2.67

nonirradiated samples; after 24 h it had dropped by 28 %. The initial rate of water absorption was found to decrease. In contrast to what was seen for the porous stone, the water absorption of the compact stone initially increased after irradiation. The amounts of water absorbed were comparable to those absorbed by the uncoated samples, but after 1 h they matched the data obtained for the nonirradiated samples. This decrease in water absorption following the application of the coating could be due to a reduction in porosity, which would have a greater impact on the porous structure of CL than on that of the PC stone due to the smaller pores present in the CL stone. Fig. 9 Photocatalytic activity (PA) versus irradiation time

Photocatalytic effectiveness of the coated stone surface The TiO2 coating exhibited high PA on both the compact and the porous stone, as indicated by the results of RhB photodegradation tests. This demonstrates that both coated surfaces possess good self-cleaning ability. RhB degradation has been extensively used in other studies (Li et al. 2008; Ruot et al. 2009; Folli et al. 2012; Krishnan et al. 2013) to evaluate the self-cleaning efficiencies of TiO2 coatings, since the structure of RhB resembles those of some airborne particulate pollutants such as polycyclic aromatic hydrocarbons (typical particulate pollutants resulting from combustion processes) (Krishnan et al. 2013). In Fig. 9, the PA is reported as a function of irradiation time, and a comparison of the RhB degradation on the surfaces treated with the TiO2 sol with the corresponding degradation on the untreated stone substrates is shown.

123

Brief exposure to simulated sunlight was sufficient to initiate dye degradation. The highest rate of degradation was measured at the beginning of irradiation; the rate of degradation then gradually decreased. The first 15 min of irradiation caused significant discoloration of the surface, while 2 h of exposure were enough to obtain 70 % degradation. The efficiency of the photocatalytic action at the end of the test was similar for both stones (over 90 %); the discoloration was visible to the naked eye (Fig. 10). RhB degrades to some extent when exposed to UV light, even in the absence of a self-cleaning coating (Wilhelm and Stephan 2007). Consistent with this, we found that the uncoated specimens also showed discoloration of RhB under irradiation, with an almost linear trend observed. However, the presence of a TiO2 coating markedly accelerated the degradation of RhB; indeed, the decomposition process was especially fast at initial exposure when a coating was present.

Nanostructured TiO2 for coating stone: assessing its compatibility with basic stone properties and… Fig. 10 Rhodamine B photodegradation test: coated CL sample (a); coated CL sample stained with RhB before (b) and after (c) 450 min of exposure to the solar simulator; coated PC sample (d); coated PC sample stained with RhB before (e) and after (f) 450 min of exposure to the solar simulator

more porous materials should yield a more extensive layer of TiO2 particles, leading to greater contact with the NOx, which in turn will result in greater degradation of these polluting agents. In our study, the higher degradation efficiency of PC is accounted for, at least qualitatively, by the higher porosity and surface roughness observed for PC stone, as this leads to a larger interaction surface between the stone and the gaseous molecules. Indeed, the adsorption of NOx under dark conditions was greater for the PC than the CL samples.

Fig. 11 NOx removal versus irradiation time, as measured in dark conditions and under UV exposure

Figure 11 shows the NRE measured in the dark and under UV exposure for both the CL and PC stone materials. NOx abatement was clearly induced by exposure to light, since the NOx concentration was only reduced very slightly when the experiment was conducted in dark conditions. A high level of nitrogen oxide photodegradation was measured for PC stone. An abatement of [60 % was achieved after 30 min of exposure. Beyond this, the process slowed, reaching an abatement of 80 % after 1 h. The degradation of NOx was lower for the CL stone, with rates of 38 and 55 % after 30 min and 1 h, respectively. Again, our results in this case showed that it was initially (i.e., in the first 30 min) more efficient. The higher rate of NOx removal measured for the PC stone is in agreement with results reported for concrete substrates (Allen et al. 2008; Poon and Cheung 2007). This is also consistent with the idea that coating rougher and

Conclusions In this study, a TiO2 colloidal sol prepared using a sol– gel technique and a hydrothermal process was sprayed onto two calcareous stone substrates with very different porosities, porosimetric features, and surface roughnesses. Application of the TiO2 coating did not alter the surface morphology of either of the types of stone studied here. A conformable coating was always obtained, and there was no detectable morphological difference between the bare and coated stone in ESEM observations. The coating was found to have negligible impact on the color characteristics of each stone. Elemental EDS microanalysis revealed that a Ti-containing species was present in a spatially homogeneous manner on the surface of each coated stone, and that this species had penetrated into the substrate. The porosity characteristics of the stone affected the distribution of TiO2 within the stone: the penetration depth was larger for the more highly porous stone.

123

M. Lettieri et al.

The photoinduced superhydrophilicity induced by the presence of the TiO2 nanoparticles increased the wettability of the surface of the compact stone. This photoinduced superhydrophilicity, once induced, remained active for a long time, even when there was no further exposure to light. In spite of this effect, the amount of water absorbed through capillarity decreased, probably because the surface porosity of the stone was modified by the deposition of TiO2 onto the stone. Conversely, the application of the TiO2 coating onto the highly porous stone did not significantly change the measured wettability of the surface. It was already high in the absence of the coating and was not measurable because drops did not form on the stone surface. Nor was water absorption through capillarity affected by the presence of the coating. Each of these phenomena were mainly influenced by the characteristics of the substrate rather than by the presence of the TiO2. The application of the coating caused both of the stone substrates to present very high photocatalytic activity in terms of their ability to self-clean. The capacities of the coated stone substrates to degrade RhB (used here to mimic airborne particulate pollutants) under irradiation with light were also very high. However, the coated stone substrates were found to differ significantly in their ability to decompose inorganic pollutants, as measured using an NOx abatement test. The highly porous stone showed greater pollutant decomposition, probably due to its greater surface roughness and porosity, which led to a more extensive coating of TiO2 nanoparticles and thus greater NOx degradation. In conclusion, the TiO2 coating analyzed here was found to be compatible with the features of the stone materials considered in this research. The coating functionalized the stone surfaces with photocatalytic properties. The observed capacity of the coated surfaces to selfclean and depollute upon exposure to light undoubtedly results in enhanced stone performance, since it reduces the need for cleaning and maintenance of building stone surfaces in polluted environments and contributes to air purification. However, these promising initial results need to be supported by further investigations, especially concerning coating durability, which can influence the longterm performance of the coating under real conditions. This issue is the focus of our ongoing research in this field. Acknowledgments This research was supported by the Puglia Funds (FSE-POR Puglia 2006–2013). Amy E. Marquardt and Raymond J. Phaneuf acknowledge support from the National Science Foundation (SCIART #DMR1041809). The authors would like to thank Dr. Maurizio Masieri (CNR-IBAM) for the ESEM morphological analyses and Dr. Piero Negro (Italcementi Laboratory, Brindisi) for performing NOx photodegradation tests.

123

References Agrios AG, Pichat P (2005) State of the art and perspectives on materials and applications of photocatalysis over TiO2. Rev App Electrochem 58:655–663. doi:10.1007/s10800-005-1627-6 Allen NS, Edge M, Verran J, Stratton J, Maltby J, Bygott C (2008) Photocatalytic titania based surfaces: environmental benefits. Polym Degrad Stabil 93:1632–1646. doi:10.1016/j.polymdegrad stab.2008.04.015 Amoroso G, Fassina V (1983) Stone decay and conservation: atmospheric pollution, cleaning, consolidation and protection. Elsevier, Amsterdam Anania L, Badala` A, Barone G, Belfiore CM, Calabro` C, La Russa MF, Mazzoleni P, Pezzino A (2012) The stones in monumental masonry buildings of the ‘‘Val di Noto’’ area: new data on the relationships between petrographic characters and physical– mechanical properties. Constr Build Mater 33:122–132. doi:10. 1016/j.conbuildmat.2011.12.076 Ballari MM, Yu QL, Brouwers HJH (2011) Experimental study of the NO and NO2 degradation by photocatalytically active concrete. Catal Today 161:175–180. doi:10.1016/j.cattod.2010.09.028 Bergamonti L, Alfieri I, Lorenzi A, Montenero A, Predieri G, Barone G, Mazzoleni P, Pasquale S, Lottici PP (2013) Nanocrystalline TiO2 by sol–gel: characterisation and photocatalytic activity on Modica and Comiso stones. Appl Surf Sci 282:165–173. doi:10. 1016/j.apsusc.2013.05.095 Bergamonti L, Alfieri I, Franzo` M, Lorenzi A, Montenero A, Predieri G, Calia A, Lazzarini L, Bersani D, Lottici PP (2014) Synthesis and characterization of nanocrystalline TiO2 with application as photoactive coating on stones. Environ Sci Pollut Res 21:13264–13277. doi:10.1007/s11356-013-2136-5 Bondioli F, Taurino R, Ferrari AM (2009) Functionalization of ceramic tile surface by sol-gel technique. J Colloid Interface Sci 334:195–201. doi:10.1016/j.jcis.2009.02.054 Brimblecombe P, Grossi CM (2005) Aesthetic thresholds and blackening of stone buildings. Sci Total Environ 349:175–189. doi:10.1016/j.scitotenv.2005.01.009 Calia A, Laurenzi Tabasso M, Mecchi AM, Quarta G (2013) The study of stone for conservation purposes: Lecce stone (southern Italy). In: Cassar J, Winter MG, Marker BR, Walton NRG, Entwisle DC, Bromhead EN, Smith JWN (eds) Stone in historic buildings: characterization and performance. Special publication no. 391. Geological Society, London, pp 139–156. doi:10.1144/ SP391.8 Cassar J (2010) The use of limestone in a historic context—the experience of Malta. In: Smith BJ, Gomez-Heras M, Viles HA, Cassar J (eds) Limestone in the built environment, present-day challenges for the preservation of the past. Special publication no. 331. Geological Society, London, pp 13–25 CEN (2009) EN 15801: Conservation of cultural property—test methods—determination of water absorption by capillarity. CEN (European Committee for Standardization), Brussels CEN (2010) EN 15802: Conservation of cultural property—test methods—determination of static contact angle. CEN (European Committee for Standardization), Brussels CEN (2010) EN 15886: Conservation of cultural property—test methods—colour measurement of surfaces. CEN (European Committee for Standardization), Brussels Chen J, Poon CS (2009) Photocatalytic construction and building materials: from fundamentals to applications. Build Environ 44:1899–1906. doi:10.1016/j.buildenv.2009.01.002 Colangiuli D, Calia A, Bianco N (2015) Novel multifunctional coatings with photocatalytic and hydrophobic properties for the preservation of the stone building heritage. Constr Build Mater 93:189–196. doi:10.1016/j.conbuildmat.2015.05.100

Nanostructured TiO2 for coating stone: assessing its compatibility with basic stone properties and… Cozzoli P, Comparelli R, Fanizza E, Curri ML, Agostiano A (2003) Photocatalytic activity of organic-capped anatase TiO2 nanocrystals in homogeneous organic solutions. Mater Sci Eng C 23:707–713. doi:10.1016/j.msec.2003.09.101 de Melo JVS, Tricheˆs G (2012) Evaluation of the influence of environmental conditions on the efficiency of photocatalytic coatings in the degradation of nitrogen oxides (NOx). Build Environ 49:117–123. doi:10.1016/j.buildenv.2011.09.016 Demeestere K, Dewulf J, Van Langenhove H (2007) Heterogeneous photocatalysis as an advanced oxidation process for the abatement of chlorinated, monocyclic aromatic and sulfurous volatile organic compounds in air: state of the art. Crit Rev Environ Sci Technol 37:489–538. doi:10.1080/10643380600966467 Diamanti MV, Ormellese M, Pedeferri MP (2008) Characterization of photocatalytic and superhydrophilic properties of mortars containing titanium dioxide. Cem Concr Res 38:1349–1353. doi:10. 1016/j.cemconres.2008.07.003 Dunham RJ (1962) Classification of carbonate rocks according to depositional textures. In: Ham WE (ed) Classifications of carbonate rocks. AAPG memoirs no. 1. AAPG, Tulsa, pp 108–121 Fassina V (1978) A survey on air pollution and deterioration of stonework in Venice. Atmos Environ 12:2205–2211. doi:10. 1016/0004-6981(78)90176-2 Folli A, Pade D, Bæk Hansen T, Tiziana De Marco T, Macphee DE (2012) TiO2 photocatalysis in cementitious systems: insights into self-cleaning and depollution chemistry. Cem Concr Res 42:539–548. doi:10.1016/j.cemconres.2011.12.001 Fujishima A, Zhangb X, Tryk DA (2008) TiO2 photocatalysis and related surface phenomena. Surf Sci Rep 63:515–582. doi:10. 1016/j.surfrep.2008.10.001 Grossi CM, Brimblecombe P (2007) Effect of long-term changes in air pollution and climate on the decay and blackening of European stone buildings. In: Prˇikryl R, Smith BJ (eds) Building stone decay: from diagnosis to conservation. Special publication no. 271. Geological Society, London, pp 117–130. doi:10.1144/ GSL.SP.2007.271.01.13 Grossi CM, Brimblecombe P, Esbert RM, Alonso FJ (2007) Color changes in architectural limestones from pollution and cleaning. Color Res Appl 32:320–331. doi:10.1002/col.20322 Guan K (2005) Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO2/SiO2 films. Surf Coat Technol 191:155–160. doi:10.1016/j.surfcoat.2004.02.022 Kapridaki C, Maravelaki-Kalaitzaki P (2013) TiO2–SiO2–PDMS nano-composite hydrophobic coating with self-cleaning properties for marble protection. Prog Org Coat 76:400–410. doi:10. 1016/j.porgcoat.2012.10.006 Kapridaki C, Pinho L, Mosquera MJ, Maravelaki-Kalaitzaki P (2014) Producing photoactive, transparent and hydrophobic SiO2– crystalline TiO2 nanocomposites at ambient conditions with application as self-cleaning coatings. Appl Catal B Environ 156–157:416–427. doi:10.1016/j.apcatb.2014.03.042 Kikuchi Y, Sunada K, Iyoda T, Hashimoto K, Fujishima A (1997) Photocatalytic bactericidal effect of TiO2 thin films: dynamic view of the active oxygen species responsible for the effect. J Photochem Photobiol A 106:51–56. doi:10.1016/S10106030(97)00038-5 Krishnan P, Zhang MH, Yu L, Feng H (2013) Photocatalytic degradation of particulate pollutants and self-cleaning performance of TiO2-containing silicate coating and mortar. Constr Build Mater 44:309–316. doi:10.1016/j.conbuildmat.2013.03. 009 La Russa MF, Ruffolo SA, Rovella N, Belfiore CM, Palermo AM, Guzzi MT, Crisci GM (2012) Multifunctional TiO2 coatings for cultural heritage. Prog Org Coat 74:186–191. doi:10.1016/j. porgcoat.2011.12.008

Li Y, Sun S, Ma M, Ouyang Y, Yan W (2008) Kinetic study and model of the photocatalytic degradation of rhodamine B (RhB) by a TiO2-coated activated carbon catalyst: effects of initial RhB content, light intensity and TiO2 content in the catalyst. Chem Eng J 142:147–155. doi:10.1016/j.cej.2008.01.009 Licciulli A, Calia A, Lettieri M, Diso D, Masieri M, Franza S, Amadelli R, Casarano G (2011) Photocatalytic TiO2 coatings on limestone. J Sol Gel Sci Technol 60:437–444. doi:10.1007/ s10971-011-2574-9 Luvidi L, Laguzzi G, Gallese F, Mecchi AM, Sidoti G (2010) Application of TiO2 based coatings on stone surfaces of interest in the field of cultural heritage. In: Ferrari A (ed) Proceedings of 4th International Congress on Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin, vol II. Grafica Elettronica, Napoli, pp 495–500 Mills A, Le Hunte S (1997) An overview of semiconductor photocatalysis. J Photochem Photobiol A 108:1–35. doi:10. 1016/S1010-6030(97)00118-4 Ohno T, Tokieda K, Higashida S, Matsumura M (2003) Synergism between rutile and anatase TiO2 particles in photocatalytic oxidation of naphthalene. Appl Catal A Gen 244:383–391. doi:10.1016/S0926-860X(02)00610-5 Pal S, Laera AM, Licciulli A, Catalano M, Taurino A (2014) Biphase TiO2 microspheres with enhanced photocatalytic activity. Ind Eng Chem Res 53:7931–7938. doi:10.1021/ie404123f Paz Y, Luo Z, Rabenberg L, Haller A (1995) Photooxidative selfcleaning transparent titanium dioxide films on glass. J Mater Res 10:2842–2848. doi:10.1557/JMR.1995.2842 Pinho L, Mosquera MJ (2011) Titania–silica nanocomposite photocatalyst with application in stone self-cleaning. J Phys Chem C 115:22851–22862. doi:10.1021/jp2074623 Pinho L, Mosquera MJ (2013) Photocatalytic activity of TiO2–SiO2 nanocomposites applied to buildings: influence of particle size and loading. Appl Catal B Environ 134–135:205–221. doi:10. 1016/j.apcatb.2013.01.021 Pinho L, Elhaddad F, Facio DS, Mosquera MJ (2013) A novel TiO2– SiO2 nanocomposite converts a very friable stone into a selfcleaning building material. Appl Surf Sci 275:389–396. doi:10. 1016/j.apsusc.2012.10.142 Pinho L, Rojas M, Mosquera MJ (2015) Ag–SiO2–TiO2 nanocomposite coatings with enhanced photoactivity for self-cleaning application on building materials. Appl Catal B Environ 178:44–154. doi:10.1016/j.apcatb.2014.10.002 Poon CS, Cheung E (2007) NO removal efficiency of photocatalytic paving blocks prepared with recycled materials. Constr Build Mater 21:1746–1753. doi:10.1016/j.conbuildmat.2006.05.018 Potenza G, Licciulli A, Diso D, Franza S, Calia A, Lettieri M (2007) Ciccarella G (2007) Surface engineering on natural stone through TiO2 photocatalytic coatings. In: Baglioni P, Cassar L (eds) Proceedings of International RILEM Symposium on Photocatalysis, Environment and Construction Materials—TDP 2007. Rilem, Bagneux, pp 315–322 Price CA, Doehne E (2011) Stone conservation: an overview of current research. The Getty Conservation Institute, Los Angeles Puzenat E, Pichat P (2003) Studying TiO2 coatings on silica-covered glass by O2 photosorption measurements and FTIR–ATR spectrometry: correlation with the self-cleaning efficacy. J Photochem Photobiol A 160:127–133. doi:10.1016/S10106030(03)00231-4 Quagliarini E, Bondioli F, Goffredo GB, Cordoni C, Munafo` P (2012) Self-cleaning and de-polluting stone surfaces: TiO2 nanoparticles for limestone. Constr Build Mater 37:51–57. doi:10.1016/j. conbuildmat.2012.07.006 Ramirez AM, Demeestere K, De Belie N, Mantyla T, Levanen E (2010) Titanium dioxide coated cementitious materials for air purifying purposes: preparation, characterization and toluene

123

M. Lettieri et al. removal potential. Build Environ 45:832–838. doi:10.1016/j. buildenv.2009.09.003 Ruot B, Plassais A, Olive F, Guillot L, Bonafous L (2009) TiO2containing cement pastes and mortars: measurements of the photocatalytic efficiency using a rhodamine B-based colourimetric test. Sol Energy 83:1794–1801. doi:10.1016/j.solener. 2009.05.017 Smith BJ, Gomez-Heras M, Viles HA, Cassar J (2010) Limestone in the built environment: present-day challenges for the preservation of the past. Special publication no. 331. Geological Society, London Su R, Bechstein R, Sø L, Vang RT, Sillassen M, Esbjo¨rnsson B, Palmqvist A, Besenbacher F (2011) How the anatase-to-rutile ratio influences the photoreactivity of TiO2. J Phys Chem C 115:24287–24292. doi:10.1021/jp2086768 Sunada K, Watanabe T, Hashimoto K (2003) Studies on photokilling of bacteria on TiO2 thin film. J Photochem Photobiol A 156:227–233. doi:10.1016/S1010-6030(02)00434-3 Toma FL, Bertrand G, Klein D, Coddet C (2004) Photocatalytic removal of nitrogen oxides via titanium dioxide. Environ Chem Lett 2:117–121. doi:10.1007/s10311-004-0087-2 ´ , Prˇikryl R (2010) Current methods and future trends in To¨ro¨k A testing, durability analyses and provenance studies of natural stones used in historical monuments. Eng Geol 115:139–142. doi:10.1016/j.enggeo.2010.07.003 UNI Ente Nazionale Italiano di Unificazione (2001) UNI 10921: Cultural heritage—natural and artificial stones—water

123

repellents—application on samples and determination of their properties in laboratory. UNI Ente Nazionale Italiano di Unificazione, Milan Vicente JP, Gacoin T, Barboux P, Boilot JP, Rondet M, Gueneau L (2003) Photocatalytic decomposition of fatty stains by TiO2 thin films. Int J Photoenergy 5:95–98. doi:10.1155/ S1110662X03000199 Wang CC, Ying JY (1999) Sol–gel synthesis and hydrothermal processing of anatase and rutile titania nanocrystals. Chem Mater 11:3113–3120. doi:10.1021/cm990180f Wilhelm P, Stephan D (2007) Photodegradation of rhodamine B in aqueous solution via SiO2@TiO2 nano-spheres. J Photochem Photobiol A 185:19–25. doi:10.1016/j.jphotochem.2006.05.003 Yin H, Wada Y, Kitamura T, Kambe S, Murasawa S, Mori H, Sakata T, Yanagida S (2001) Hydrothermal synthesis of nanosized anatase and rutile TiO2 using amorphous phase TiO2. J Mater Chem 11:1694–1703. doi:10.1039/B008974P Young ME, Urquhart DCM, Laing RA (2003) Maintenance and repair issues for stone cleaned sandstone and granite building fac¸ades. Build Environ 38:1125–1131. doi:10.1016/S03601323(03)00084-2 Zhang H, Banfield JF (2000) Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO2. J Phys Chem B 104:3481–3487. doi:10.1021/jp000499j