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Building and Environment 71 (2014) 193e203

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Building and Environment

journal homepage: www.elsevier .com/locate/bui ldenv

Durability of self-cleaning TiO2 coatings on fired clay brick façades:Effects of UV exposure and wet & dry cycles

Lorenzo Graziani a, Enrico Quagliarini a, Federica Bondioli b, Marco D’Orazio a,*

aDepartment of Construction, Civil Engineering and Architecture (DICEA), Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, ItalybDipartimento di Ingegneria “Enzo Ferrari”, Università di Modena e Reggio Emilia, via Vignolese 905, 41125 Modena, Italy

a r t i c l e i n f o

Article history:Received 6 June 2013Received in revised form7 October 2013Accepted 9 October 2013

Keywords:TiO2

Self-cleaning façadesContact angleWettabilityFired clay brick

* Corresponding author. Tel.: þ39 (0) 712204587.E-mail addresses: l.graziani@univpm.it (L. Grazi

(E. Quagliarini), federica.bondioli@unimore.it (F. Bon(M. D’Orazio).

0360-1323/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.buildenv.2013.10.005

a b s t r a c t

Environmental pollution is constantly increasing and it causes aesthetical concerns to urban buildingsexposed to the atmosphere. Nanometric titanium dioxide (TiO2) has become a promising photocatalyticmaterial owing to its ability to accelerate degradation of many organic contaminants. Application of TiO2

is rising and it found application on building industry. However, photocatalytic properties of thisnanotechnology strongly depend on substrate morphology and on its nature. Thus, it is not correct toextrapolate photocatalytic activity on different types of substrate. Moreover, very few information isavailable about effectiveness of TiO2 coatings after aging phenomena when applied on different sub-strate. This paper aims to investigate photocatalytic properties of TiO2 applied on clay brick surfaces bothafter deposition and after aging process. TiO2 characterization was carried out by assessing nano-filmmorphology, wettability and self-cleaning efficiency before durability test. Self-cleaning ability wasalso evaluated during aging test in order to evaluate its variation in long term applications. Results showthat photocatalytic efficiency of TiO2 remain stable after aging, thus TiO2 shows a good photocatalyticefficiency when it is applied to clay brick substrate. In the long run, photocatalytic efficiency of clay brickspecimens treated with TiO2 is seven times higher than untreated specimens.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Visible stains occur very frequently on clay building façades.This phenomenon affects the aesthetic properties of the entirebuilding and it increases the maintenance costs. A recent studyconducted in Portugal, aimed to improve the diagnosis and main-tenance technique of building façades, reveals an occurrence ofstains in 36% of buildings inspected [1]. The major cause that pro-duces stains is the surface accumulation of foreign materials ofdiverse nature (organic or inorganic) mainly originated from theatmospheric aerosol pollutants. Due to the presence of water (e.g.rain, condensation), foreign materials can dissolve inside waterand/or penetrate inside the pores of clay façade materials (e.g.bricks, claddings) because of the water absorption processes. In aninitial stage, these anomalies affect only the aesthetics of thefaçade, but, in conjunction with water, chemical componentscontribute to the physical degradation of external surfaces [1].

ani), e.quagliarini@univpm.itdioli), m.dorazio@univpm.it

All rights reserved.

Although ultraviolet radiation (UV) is able to decompose organicstains, this process is extremely slow. Recently, the use of nano-technology is rising and it found applications in the field of buildingconstruction. The use of titanium dioxide (TiO2) as self-cleaningcoating is widely spreading because of its non-toxicity, photo-chemical stability and low cost [2e4]. This nanotechnology is ableto accelerate the cleaning process due to its photocatalytic ability inconjunction with its super-hydrophilicity. Photocatalysis of TiO2nano-coating prevents the building up of the organic/inorganicsubstances that come into contact with the surface, while its super-hydrophilicity helps the cleaning process because it makes thesurface dry faster, and moreover, prevent the undesirable waterstreaking or spotting on the surface [5e7]. TiO2 photo-catalysis isalso a promising technique for air purification [8e14] and thisability is strictly related to the health of inhabitants.

During last years, many studies have investigated the photo-catalytic activity levels of TiO2 applied on different materials, in thefield of new building construction and for cultural heritage pres-ervation [2,4,15e21]. These studies aimed to investigate the pho-tocatalytic properties of TiO2 in short run applications, but very fewstudies investigated on the photocatalytic durability of TiO2 ondifferent substrate. Durability of TiO2 plays an important role inlarge scale application of TiO2 because it can extend in time the

Table 1Specimens identification with the amount of TiO2 applied on the surface.

Samplename

Type oftreatment

TiO2 amount(g/m2)

Average TiO2

amount (g/m2)Standarddeviation

UT/1 Untreated 0.00 e e

UT/2 0.00UT/3 0.00UT/4 0.00S0.5/1 One layer S0.5 0.16 0.17 0.02S0.5/2 0.14S0.5/3 0.19S0.5/4 0.19S1/1 One layer S1 0.25 0.23 0.02S1/2 0.23S1/3 0.22S1/4 0.20

L. Graziani et al. / Building and Environment 71 (2014) 193e203194

beneficial effect of TiO2 toward both internal and external envi-ronment of buildings.

The literature shows that the photocatalytic power of TiO2 de-pends on aging of the substrate (when TiO2 is applied as coating) ormatrix (when TiO2 is intermixed) and seems to decrease as theaging time is increased. Past studies show that, after aging test, theTiO2 ability to remove NOx from the air [22,23] and its self-cleaningability [24e26], reached very low value in relation to the initialvalues. The loss of TiO2 efficiency can be associated to natural agingcoming from the outdoor exposure where nano-coating was sub-jected to the variation of climatic conditions [22,23] and this causesthe particles release and the thickness reduction of the nano-coating [24e26]. Also the effect of UV illumination remains onthe surface of TiO2 nano-coating and this phenomenon cause adeterioration of the TiO2 layer [24,27]. The nature of the substratehas been shown to play an important role in the properties of TiO2nanofilm [28e32]. For this reason, the results of the current studyaimed to photocatalytic activity of TiO2 nano-films cannot neces-sarily be extrapolated to understand photocatalytic activity ondifferent types of substrate. Although clay brick material showed astrong diffusion, to the best of knowledge, poor interest has beenpaid to the self-cleaning ability of TiO2 on clay brick façades and nostudy investigates on its durability on this substrate. The objectiveof this paper is twofold: firstly, TiO2 characterization on clay bricksubstrate was carried out and its compatibility with the materialwas evaluated; secondly, the self-cleaning durability of TiO2 wasassessed by two types of aging tests. In the first aging test speci-mens were subjected to UV radiation, while in the second test,specimens were aged by UV radiation and wet & dry (W&D) cyclesjointly.

2. Materials and methods

2.1. Phases

This paper is divided in two main phases: characterization ofclay brick façades functionalized with TiO2 nanoparticles both attime zero and after aging test. In the first phase, nano-coatingmorphology, wettability and self-cleaning efficiency wereassessed. In the second phase, aging tests were carried out in orderto assess the self-cleaning efficiency after long time UV exposureand wet & dry cycles (W&D).

2.2. Samples and TiO2 application

In this study, TiO2 was applied on clay brick specimens(80� 80� 30 mm3) having an average density equal to 1798 kg/m3

and an average total porosity equal to 21.05% (measured byMicromeritics Autopore III). Surface roughness was also measuredwith a perthometer (Mahr model M4P) with a stylus tip radius of2.0 mm. Ten measurements were made with a cut-off length of0.8 mm following the method described in UNI EN 623-4:2005[33]. The average roughness coefficient Ra was 7.22 mm.

Before TiO2 application, specimens were dried at 80 �C, until thedifference between two subsequent weighing was less than 0.1%(wg/wg). A commercial TiO2 water solution (furnished by Salentec,in the form of nanocystalline anatase suspension) was preparedand deposited on clay brick specimens as proposed in the literature[17e19]. As reported in [18] the suspensionwas composed by a veryfine anatase crystals with an average diameter of 4 nm as estimatedby XRD patterns elaboration. The average crystal size, as deter-mined by dynamic light scattering, is around 40e50 nm with anarrow grain size distribution indicating a partial aggregation ofthe primary crystallites. Two different concentrations were used inthis study. The first concentration (S0.5) had a TiO2 amount equal to

0.5% (wg/vol), while the second concentration (S1) had a concen-tration of 1% (wg/vol). Solutions S0.5 and S1 were applied on fourspecimens each, whereas other four specimens remained untreated(UT) and hence were used as a control. An air spray gun with anozzle of 0.8 mm diameter was used to apply TiO2. Specimens weremanually sprayed from a distance of approximately 250 mm, tobetter simulate a real application on building facades, and subse-quently weighed with an electronic balance (Gibertini modelEU4000AR) to determine the amount of solution applied on thespecimens surface. Results were expressed as g/m2. After TiO2application, specimens were dried at 60 �C for 1 h to accelerate thedrying process, as reported in the literature [17e19]. List of speci-mens and corresponding treatments are shown in Table 1. A coef-ficient of variation of 8.69% and 11.8% were found for S1 and S0.5,respectively, thus indicating a homogeneous application of TiO2 forboth the two treatments.

2.3. Aesthetic properties

A portable colorimeter (Konica Minolta CM 2600d) with a 3 mmaperture was used in order to measure the color of clay brickspecimens before and after the two treatments. Nine measure-ments were repeated for each specimen, as recommended in UNIEN 15886:2010 [34]. Measurements were carried out using adaylight illuminant (D65) and 10� observer angle. Results wereexpressed in CIELAB color space. An 8�8 cm reference spatial gridwas used to ensure precise repeated measurements on the samepoints in subsequent tests.

In order to evaluate the effect of the titania coatings on the vi-sual appearance of clay brick specimens, colorimetric analysis wererepeated on treated specimens. Color difference between originalclay brick specimens and treated specimens was assessed asfollows:

DE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�L*0 � L*

�2 þ �a*0 � a*

�2 þ �b*0 � b*

�2r(1)

where L*0; a*0 and b*0 are the CIELAB coordinates of untreated sam-

ples and L*, a* and b* are the CIELAB coordinates of treated samples.The gloss of each samplewas alsomeasured by a Novo Gloss Trio

apparatus (Rophoint Instruments) using 85� as standard geometry[17]. Measurements were repeated five times for each specimenand then results were averaged. Results are expressed in gloss units(GU).

2.4. Microstructure analysis

In order to study the surface’s morphology and to determine theconcentration and distribution of TiO2 nano-particles on the

L. Graziani et al. / Building and Environment 71 (2014) 193e203 195

surface of both untreated and treated specimens, microscopicanalysis was conducted. In order to obtain a clear signal, a thin goldlayer was deposited on the surface of dried samples by a goldsplutter coater. Scanning electron microscopy (SEM) FEI Quanta-200 instrument equipped with an energy-dispersive X-ray spec-troscopy (EDS) analysis system (Oxford INCA 350) was used toobserve the specimens. SEM observations were carried out on asample for each type of treatment and on an untreated sample. Foreach sample, we increased magnifications from 254� to 16200�.

SEM was also used at the end of accelerated test in order toevaluate the effect of aging on the TiO2 nanocoating. Digital ImageAnalysis (DIA) was used to calculate area occupied by crack on thesurface in order to compare the effect of only UV radiation to theeffect caused by UV þ W&D cycles. Before analysis, a high-passfilter was applied to find edges of the cracks.

2.5. Hydrophilicity

The hydrophilicity of the two TiO2 nano-coatings was evaluatedconsidering both static contact angle and surface water absorption.These analyses were made both with and without UV radiation inorder to study the photo-induced hydrophilicity of the coated bricksurfaces.

Static contact angle analysis were carried out according to UNIEN 15802 [35] by using an OCA-20 apparatus (Data Physics In-strument GmbH, Filderstadt, Germany). This test was performed onthree specimens for each treatment and on three control speci-mens. Fifteen contact angle measurements were performed on thesurface of each specimen (80� 80mm2). A 5 mL dropwas depositedon the surface for each measurement. UV radiation, in the case ofphoto-induced hydrophilicity, was equal to 20 W/m2 for a durationof 50 min [19] and wave length peak of UV lamps is equal to365 nm. Contact angle values were registered at time zero, after5 min from test start and each 10 min until the end of test.

Surface water absorption was evaluated by using the samemethod proposed in the literature [19]. Prismatic specimens(40 � 40 � 30 mm3) were placed on a rack inclined of 10�. Beforetest, specimens were dried in a ventilated oven until reachingconstant mass (difference between two subsequentweighing � 0.1%). Three specimens for each type of treatment andthree untreated ones were used. Distilled water was sprayed everytwo minutes on the surface by using a manual nebulizer from adistance of 30 cm. The total amount of water sprayed during thetest was equal to 50 mL. Water absorption was obtained byweighting the specimens every two minutes after water spry.Water absorption was calculated as follows:

Qi ¼mi �m0

A(2)

where m0 is the weight (in Kg) of the specimens before test, mi isthe weight (in Kg) of the specimen after water spray and A is thesurface of the specimens (in m2).

2.6. Time-zero self-cleaning ability

Self-cleaning ability of the two TiO2 nano-coatings was evalu-ated by considering the degradation of organic dye (Methylene BlueeMB)measured by using a test adapted from the literature [36,37].Although European standards specify the use of Rhodamine B [37],in this paperMBwas chosen because preliminary tests showed thatRhodamine B produces no contrast on the red surface of clay brickspecimens.

Clay brick specimens (80 � 80 � 30 mm3) were stained by0.5 mL MB aqueous solution (MB content: 100 mmol/L) using a

syringe on an area of 2200 mm2. This concentration produces theneeded color variation on red clay brick specimens, and was notaffected by natural discoloration of MB under UV light. Thus, colordiscoloration due to TiO2 can be observed from time zero to the endof durability test. Then, specimens were dried in the dark for 24 h.Next, samples were located on a rack and irradiated by UV light(Philips, model TL Mini Blacklight Blue 8W) working at 365 nm. UVirradiance value was equal to 10 W/m2. Four specimens wereanalyzed for each type of treatment and four untreated specimenswere considered as control ones.

Colorimetric measurements were carried out on the samepoints used for original color detection. The color values weredetected before UV irradiation and after 1, 2, 4 and 24 h of exposure.Although the ISO rule with Rhodamine B specify an UV expositiontime of 26 h [37], in this work self-cleaning test was stopped after24 h because this time allows the right timing of aging cycles.

The chromatic coordinates of CIELAB system L*, a* and b* wereconsidered to assess the discoloration efficiency (R*) during time.

R*Lð%Þ ¼ L*t � L*MB

L*MB

� 100 (3)

R*að%Þ ¼ a*t � a*MB

a*MB

� 100 (4)

R*bð%Þ ¼ b*t � b*MB

b*MB

� 100 (5)

where L*t ; a*t and b*t are the average value of CIELAB coordinates at a

given time and L*MB; a*MB and b*MB are the average value of CIELAB

coordinates after MB deposition.In addition, we considered the contribute of each coordinate by

calculating degradation efficiency as follows [22]:

REð%Þ ¼

���DE*t � DE*0

���DE*0

� 100 (6)

where DE*t is the average color variation between the measuredcolor after 1, 2, 4 and 24 h and the original surface color before MBapplication and DE*0 is the average color variation between themeasured color after MB application and the original surface color.In order to study the significance of results referring to self-cleaning ability, a one-way analysis of variance (ANOVA) was car-ried out, along with the TukeyeKramer honestly significant dif-ference (HSD) using the SYSTAT software [38]. Significance ofdifferences was defined at P-value p < 0.05.

2.7. Aging tests

Accelerated tests were achieved in a laboratory-made system asproposed in literature [7]. The system consisted of a100� 40� 53 cm3 glass chamber containing two aluminiumeglasscomposed racks inclined at 45�. The rackswere positioned front-to-front along the long dimension of the glass chamber to maximizethe number of specimens that could be processed simultaneously(twelve). A scheme of the test set-up used in this study is depictedin Fig. 1.

Two types of aging test were carried out: UV radiation on dryspecimens and UV radiation and W&D cycles according to thescheme in Fig. 2.

Fig. 1. Schematic view of test apparatus a) side view, b) front view.

L. Graziani et al. / Building and Environment 71 (2014) 193e203196

2.7.1. UV radiation aging testIn order to verify the degradation of the coating due to the UV

exposure, UV radiation was provided for 1800 h by two UV lamps(Philips, model TL-D Blacklight Blue 18W, wavelength of 365 nm)which were placed on the lid of the chamber at the same distancefrom the two racks. The samples were irradiated with an averageUVA intensity of 26.7 W/m2 as resulted from measurements madeby a digital light meter (General Tools UV513AB), for 24 h/day. Theglass chamber was obscured with an aluminium foil to avoid theentrance of natural light. By considering UV intensity and aging testduration (1800 h), specimens were irradiated with a total UV in-tensity equal to 173 MJ/m2 corresponding to about 34 months ofnorthern exposed façades and 15 months of southern exposed fa-çades in Mediterranean area [39,40]. Self-cleaning test (as in Sect.2.6) was repeated after 300, 600, 1350 and 1800 h on six specimens(two for each treatment and two as a control). The degradationefficiency RE was then evaluated by equation (6).

2.7.2. UV and wet & dry aging testIn the second type of aging test, W&D cycles were added to the

UV radiation described in Sect. 2.7.1. In order to simulate a possibleaction of the rain, the system was equipped with two sprinklingrails made of PVC tubes with three 2 mm-holes drilled every20 mm, in correspondence of each specimen subjected to W&Dcycles. The rails were mounted approximately 20 mm over thespecimens and connected to a 500 L/h water pump (Blupower)submerged in distilled water with plastic hoses. The water wassprinkled for 15min/cycle, 48 h before the end of each cycle (Fig. 2);considering that flowwas 4 L/h, each samplewas sprinkledwith 1 Lof distilled water every UV cycle. Thus, at the end of test, specimenswere wet with 5 L, corresponding to about 800 mm of rainfall(mean value of one-year rainfall in Italy [41]).

Temperature and relative humidity (RH) inside the glasschamber were recorded every 5 min over the period of the testusing a remote data logger (Lascar Electronics model EL-USB-2).Average temperature in the chamber was about 32 �C, whilemean RH was about 80%. In order to prevent the influence of wateron the color measurement of specimens, 48 h of stabilization timewerewaited before each self-cleaning test. RE were then assessed inthe same way as in Sect. 2.7.1.

3. Results and discussion

3.1. Aesthetical changes due to TiO2 coatings

Averaged results of color measurements with standard de-viations are reported in Table 2. After TiO2 applications, clay bricksbecome a little more white (higher L*) and less red and blue (lowera* and b*). Similar trend can be found in the literature in which a

TiO2eSiO2 nanocomposite solution was applied on a limestone andit produced DE ¼ 4.51 � 1.89 [42].

This phenomenon can be associated to the roughness of thesurface of clay bricks because it causes water detection and de-creases the water absorption speed. In this way, it forms a non-homogeneous thick coating (in relation to dimension of TiO2 par-ticles) and a consequent light whitening [28].

From Table 2, one can see that color change is slightly dependenton the titania content, since color variation DE increases as TiO2content increased, but not in a proportional way.

Table 2 shows that gloss values of S0.5 solutions show no sig-nificant variations after deposition of titania suspension (p-valuebetween UT and S0.5 equal to 0.5969). Thus, specular reflection oftreated surfaces with S0.5 was not altered by the two nano-coating.Contrarily, S1 treatment produce significant variation in the GU,indeed p-value between S1 and UT is equal to 0.0006.

3.2. Nanocoating characterization

Fig. 3 shows microstructure of coating applied on clay bricksubstrate observed by SEM as described in Sect. 2.4. The formationof titania films in both S0.5 and S1 samples were well evident bycomparing images of treated specimens with image of untreatedones. Titanium presence on treated specimens was confirmed byEDS analysis (Fig. 4) where titanium peaks are visible as relatedwith UT specimens. Fig. 4 shows a higher peak in S1 specimen thanin S0.5, confirming that S1 specimens have a major amount of TiO2on the surface.

Moreover, from Fig. 3 it is possible to observe that S1 treatmentproduces a cracked film on the surface, while S0.5 treatment ismore uniform. Micro-crack formation can be associated to theroughness of the surface and to the non-homogeneous thickness ofnano-film that causes the formation of tension during the dryingstep [28]. In fact, substrate roughness promotes retention of TiO2sol on the surface and limits its absorption, thus a thick film wasformed on the surface. The quantity of TiO2 retained on the sub-strate is a function of its surface morphology, where a rough surfaceallows a greater retention of TiO2, resulting in greater deposition ofanatase crystallites and greater thickness [28]. The higher is theTiO2 amount, the higher is the number of crack visible on thesurface.

3.3. Photo-induced hydrophilicity at time zero

Hydrophilicity of TiO2 was assessed by considering both contactangle variation and surface water absorption as described in Sect.2.5. Fig. 5 shows that on average both TiO2 coatings produced aslight reduction of the contact angle before UV exposure (timezero). At the same time UV light was not able to produce a further

Fig. 2. Scheme of the aging tests. UV irradiation time (hours) is on the horizontal axis. The wet cycles are only for specimens subjected to UV þ W&D. Self-cleaning test (as in Sect.2.6) was repeated after each cycle (dark gray rectangles).

Table 2Aesthetical properties of surfaces: chromatic coordinates (CIELAB), color variations(DE) and gloss values (GU).

UT S0.5 S1

L* 50.66 � 0.68 53.97 � 0.90 55.17 � 1.10a* 22.42 � 0.40 20.09 � 0.48 19.32 � 0.78b* 25.67 � 0.25 21.17 � 0.65 20.41 � 0.85DE e 5.57 � 0.80 7.15 � 1.34GU 0.30 � 0.07 0.38 � 0.08 0.72 � 0.19

L. Graziani et al. / Building and Environment 71 (2014) 193e203 197

significant contact angle reduction, indeed p-value between treat-ments is major than 0.05 in each step. This indicates that thebehavior of the untreated specimens is almost the same of thetreated ones. This phenomenon can be associated to themorphology of the specimens’ surface, in particular to the surfaceroughness that limits the super-hydrophilicity of TiO2 because adrop on the surface is not free to distend itself.

Fig. 6 shows the effect of TiO2 on water absorption. It is possibleto see that treated specimens (both S0.5 and S1) have a notablereduction of water absorption when exposed to UV light, while UTspecimens show a small reduction. From Fig. 6 one can see that,after 60 min of UV exposure, water absorption was reduced from0.06 to 0.02 kg/m2 in the case of S1 treatment, while surface waterabsorption of S0.5 was reduced from 0.05 to 0.035 kg/m2. Thereduction of water absorption can be associated to the formation ofa water film over the solid surface that can be easily removed bygravity and the amount of water absorbed by coated surfacesdecreased consequently. After 60 min of UV exposure, the waterabsorption reduction of S1 specimens was twice than S0.5 speci-mens. Thus, a relationship between TiO2 content and surface waterabsorption can be evidenced. The effect of UV on TiO2 is also welldetectable after about 20 min when trends of specimens under UVlight begin to deviate from trends of specimens without UV.

3.4. Self-cleaning efficiency

3.4.1. Time-zeroDegradation of MB stains due to TiO2 is visible by naked eye in

Fig. 7. A light discoloration is also detectable for untreated specimenbecause MB is sensible to UV radiation and it tends to degrade byitself. Anyhow, discoloration on treated specimens (both S0.5 andS1) is higher than on UT specimens.

In order to evaluate the efficiency of TiO2 in a quantitativemanner, the self-cleaning ability of analyzed treatments was

Fig. 3. SEM images (2025�

evaluated as described in Sect. 2.6. Results of this test are shown inFigs. 8 and 9where one can see that the trend of S1 specimens is thesame of S0.5 specimens. Treated surfaces with TiO2 are able todecompose MB, indeed, after 24 h of UV radiation the efficiency isequal to about 25% for both a* and b* coordinates. Results of L*efficiency was not reported because, during self-cleaning process,TiO2 has shown to produce no variation of this value. Statistical test(Table 3) showed that, after 24 h of UV exposure, S0.5 efficiency wasthe same of S1 efficiency (p-value> 0.05). This phenomenon can beassociated to the micro-cracks on the surface of S1 specimens(Fig. 3) that affects photo-induced discoloration and reduce its ef-ficiency. In general, it is possible to say that discoloration of MBoccurs in the first four hours of UV exposure. In the first four hours,efficiency reaches 16% and from 4th hour until 24th hour, it reachesabout 25%. Thus, about 60% of total efficiency occurs in the first fourhours of test. This demonstrates that degradation of MB by TiO2nano-coating is fast and it is able to reach its maximum value after4 h of UV exposure. Moreover, difference between treated speci-mens and UT was significant (p-value < 0.05) and it was about tentimes bigger than UT specimens.

The contribution of every single coordinate can be evaluatedjointly by considering RE as described in Sect. 2.6 (Fig. 10). Results ofthis analysis confirm the self-cleaning ability of TiO2 by consideringeach coordinate separately. Fig. 10 shows that the trend of UTspecimens is almost constant, thus, it is possible to confirm that UVlight was able to produce a negligible discoloration of MB itself. Atthe end of the test, discoloration of dye produced by the photo-catalytic action of nano-coating was eight times higher thanwithout TiO2. Results about self-cleaning characterization showthat TiO2 is able to accelerate the discoloration process when it isapplied on clay brick surfaces. This phenomenon is well visible bynaked eye and it is confirmed by quantitative analysis. According tostandard and literature [37,43], where Rhodamine B is used ascontaminant, a material can be considered photocatalytic if its ef-ficiency reach 20% after 4 h and 50% after 26 h of UV irradiation.From plot in Fig. 10 it can be see that the first condition is checked,while the second condition cannot be verified because the test wasstopped at 24 h. At this time, TiO2 efficiency is about 40% and it isrising. Thus it is possible to conclude that the applied TiO2 coatingallows to obtain photocatalytic clay brick.

3.4.2. Durability of TiO2

In order to study the self-cleaning efficiency of TiO2 during theaging tests, the degradation efficiency RE was assessed after 300,600, 1350 and 1800 h; its values are reported in Fig. 11. In order to

) of clay brick surfaces.

Fig. 4. Spectra of clay brick surfaces.

L. Graziani et al. / Building and Environment 71 (2014) 193e203198

take into account the effect of W&D cycles, the results of the agingtest with only UV exposure were overlapped to the results of theaging test in which W&D cycles were added to UV exposure. Thetrend of RE in the case of UT specimens (both with and withoutW&D cycles) was very lowand about constant during timemeaningthat the aging tests had no effect on the degradation ability of theoriginal substrate. This trend was confirmed by statistical analysis,indeed p-value was always higher than 0.05 at each step.

Considering the efficiency of treated specimens (both S0.5 andS1), it is possible to observe a slight decrease from time zero to300 h and then it remains practically constant until the end of thetest. However, statistical analysis indicated no significant difference(p-value>0.05) between values at time zero and values in otherintervals. Thus, TiO2 maintained its efficiency during time and itsaverage photocatalytic efficiency was about 7e8 times higher thanUT specimens.

A significant observation can be done if we consider thedegradation efficiency REC of each only-UV-cycle, evaluated asfollows:

Fig. 5. Average contact angle values of treated surfaces as a function of ultravioletirradiation time.

RECð%Þ ¼

���DE*i � DE*MB

���*

� 100 (7)

DEMB

where:

- DE*i was the difference between the color of the specimen after300, 600, 1350 and 1800 h of exposure and its starting color(after the coating application for the treated specimens);

- DE*MB was the color variation between the color of the specimenafter eachMB application and its starting color (after the coatingapplication for the treated specimens).

In this way, it is possible to catch the TiO2 photocatalytic effecton MB dye even after a long UV exposure (that is, the time of eachcycle).

The UT specimens, in fact, only reached low REC values, whichwere substantially constant (about 11% on average) after each cycle(as confirmed by statistical analysis), whereas, the treated

Fig. 6. Surface water absorption before UV radiation (crosses) and during UV radiation(circles) for UT specimens (light grey), S0.5 (medium grey) and S1 (black).

Fig. 7. Degradation of MB: sample surfaces for each type of treatment at the beginning of test (0) and after 24 h of UV illumination (24). Samples in the figure are representative offour samples for each type.

L. Graziani et al. / Building and Environment 71 (2014) 193e203 199

specimens reached very high REC values (about 73% on average)which were even in this case practically constant after each cycle(as confirmed by statistical analysis). Fig. 12 clearly states what isreported above: after about 300 h of UV exposure the MB dye isalmost all discolored in the treated specimens, while is substan-tially unchanged in UT ones.

Besides, statistical analysis confirmed that there was no signif-icant difference between S0.5 and S1 specimens. This means thatthe efficiency of the treated specimens keeps on increasing duringlong UV exposure, as expected, being 7e8 times higher than that of

the UT ones, thus confirming the real efficacy of the coating duringtime.

Finally, this test underlines that this TiO2 coatings were notdegraded by the UV long exposure as, instead, founded in otherresearch work [27].

The same comparison cannot be sustained for specimens underW&D cycles because of the water solubility of MB.

The self-cleaning performance of TiO2 was also assessed in termof dye accumulation that produces color changes from the originalcolor of clay brick material (DE). Fig. 13 shows that UT specimens

Fig. 8. Photocatalytic efficiency (Ra*) as a function of ultraviolet (UV) irradiation time(h).

Table 3ANOVA test of results about photocatalytic efficiency (UV time ¼ 24 h).

Efficiency ANOVA between P-value

Ra* S0.5 e S1 0.7953S0.5 e UT 0.0005S1 e UT 0.0010

Rb* S0.5 e S1 0.2212S0.5 e UT 0.0000S1 e UT 0.0001

RE* S0.5 e S1 0.9984S0.5 e UT 0.0004S1 e UT 0.0004

Fig. 9. Photocatalytic efficiency (Rb*) as a function of ultraviolet (UV) irradiation time(h).

Fig. 10. Degradation efficiency of TiO2 (RE) assessed by color variation between testedspecimens and original color of clay brick surface.

Fig. 11. Variation of degradation efficiency of TiO2 (RE) during aging test. Wet speci-mens (circles) are considered separately from specimens with only UV exposure(squares).

Fig. 12. Variation of REC during aging test. Wet specimens (circles) are consideredseparately from specimens with only UV exposure (squares).

L. Graziani et al. / Building and Environment 71 (2014) 193e203200

were not able to prevent dye accumulation, indeed, after theapplication of MB at time zero, DE strongly increased and thenremained about constant during aging test. DE was constant in thecase of UT þ W&D specimens. This means that the action of thewater was able to removeMB dye from the surface of UT specimens.Although water prevent dye accumulation, statistical analysis

Fig. 13. Color variation (DE) due to dye accumulation during aging test. Solid line is thelimit for naked eye perception, dashed line is the limit for cultural heritageapplications.

Table 4Crack extension on the TiO2 nanocoating. Results are expressed in pixel.

Treatment UV radiation UV þ W&D Increase

S0.5 6866 9658 41%S1 11,564 16,321 41%

L. Graziani et al. / Building and Environment 71 (2014) 193e203 201

confirms a significant difference between treated specimens andUT þ W&D specimens, indeed p-value between treated specimensand UTþW&D is minor than 0.05 in all the steps apart at time zero.During durability test, no significant difference (p > 0.05) wasdetected between treated specimens subjected to W&D cycles andtreated specimens without wetting. This phenomenon is incontrast with results from the literature in which the action of the

Fig. 14. Morphology of the coatings after aging. The red ellipses evidence TiO2 agglomeratesreferences to color in this figure legend, the reader is referred to the web version of this ar

water shows to be able to regenerate the self-cleaning properties ofTiO2 [26,44]. Moreover, no significant difference was detected be-tween S0.5 and S1.

This phenomenon can be associated to micro-cracks on thesurface of S1 treatment (Fig. 3) that limits the photocatalytic effi-ciency of nano-coating even if S1 produced a higher anatasedeposition than S0.5 and an higher thickness that is partiallyreached by UV radiation [45]. Graph on Fig. 13 shows that TiO2 wasable to prevent color change due to dye accumulation, indeed DEwas constant during the aging test.

DE between original color of clay brick (DE¼ 0 in Fig. 13) and DEof treated specimens was minor than acceptable DE for culturalheritage application fixed to DE ¼ 5 and it was close to the justnoticeable difference (JND) by naked eye fixed to DE ¼ 2.3 [46].

Results from this paper are comparable with results in theliterature where durability of TiO2 was studied in term of discol-oration of Rhodamine B. In detail was shown that TiO2 maintains itsefficiency after 2500 h of UV exposure when it is applied on silicatespecimens [27]. Instead, when TiO2 was included in cementitiousmatrix, it was affected by carbonation process of cement paste andthis process reduce its efficiency [31,32]. In this study, the substrateis stable and results obtained can be associated to only TiO2

swept by the water flow and stripped off of its initial location. (For interpretation of theticle.)

L. Graziani et al. / Building and Environment 71 (2014) 193e203202

nanocoating in good agreement with that obtained on silicatesubstrate.

By relating time of aging test to real time scale, it is possible tosustain that TiO2 maintains its self-cleaning ability until one year ofexternal exposure at least. By making this consideration, it isnecessary to take into account that environmental parameters inaccelerated test are more demanding than natural environmentactions, thus it seems reasonable to affirm that these TiO2-treat-ments will surely have a greater durability in a real outdoor expo-sure. Although the photocatalytic removal of dyes is a very goodindicator for the preliminary evaluation of self-cleaning potential,further research is necessary before the in-situ performance in or-der to take into account other fouling substances (e.g. microor-ganisms, soot,.) that are commonly found in natural environment.

3.5. Effect of aging on TiO2 nanocoating

The effects of aging cycles on TiO2 nanocoating are visible inFig. 14. Some differences are visible among specimens subjected toonly UV radiation and specimens tested with UV þ W&D cycles(both S0.5 and S1). In the first case, the only UV radiation was notable to cause the physical degradation of nanocoating. A homoge-nous TiO2 nanofilm was visible and thus no significant microstruc-tural variationwas observable between nanocoatingmorphology attime zero (Fig. 3) and after 1800 h of UV exposure (Fig. 14).

Specimens subjected to W&D cycles showed a certain degra-dation of the nanocoating instead. Fig. 14 clearly shows that someagglomerates were swept by thewater flow during accelerated test.Some defects are now present in the coating, leaving now visiblethe substrate that was fully covered before aging (Fig. 3). Thisbehavior is in agreement with the release of TiO2 nanoparticles inthe environment, from the original surface, already observed in theliterature [24,25]. Moreover, the presence of cracks on the film isincreased by theW&D cycles with respect to the dry aging test. Areaoccupied by cracks, calculated as described in Sect. 2.4, was raisedof 41% in both S0.5 and S1 (Table 4).

4. Conclusion

This paper investigated the hydrophilicity and self-cleaningability of TiO2 nano-coatings applied on fired clay brick substrate.In particular, their durability toward organic dye (Methylene Blue)was studied by UV and UV plus wet & dry cycles.

Two different TiO2 concentrations in aqueous solutions appliedby spray coating were tested. A light whitening of the substrate wasrevealed for each treatment.

Wettability of the two TiO2 nano-coatings was assessed. Staticcontact angle and specific surface water absorption analyses werecarried out. From the results, static contact angle was substantiallynot influenced by the presence of titania coatings, but it remainsconstant when specimens were exposed to UV radiance because ofsurface roughness. On the contrary, under UV irradiation, surfacewater absorption reduction is well evident on treated specimenswhile it is negligible on untreated ones. In this way, both treat-ments do not seem to bring to greater water absorption, a potentialsource of damage for brick surfaces.

The self-cleaning ability of the two TiO2 nano-coatings was thenassessed. Both of them greatly accelerate discoloration of organicstain producing an average discoloration of dye 8 times higher thanthe UT specimens. Although certain degradation was occurred, ourfindings indicate that the remaining TiO2 nanoparticles result in apreserved photocatalytic activity at the studied conditions ofcontaminant concentration and UV intensity. Moreover, after theaging tests, each treatment practically maintained the same effi-ciency as at time zero. Besides, under a long UV exposure, the

treated specimens were able to discolor almost all of the dye, whilethe untreated ones practically maintained it. No significant differ-ences can be observed between the two coatings.

In conclusion, TiO2 nano-coatings seem to be able to prevent dyeaccumulation on fired clay brick surfaces during time maintainingtheir photocatalytic properties. This could permit to limit cleaningand maintenance actions during time and their relative high costs.It is important to underline that applying higher titania contentcoatings could lead to no evident benefits since the amount oftitania content deposited through spray coating does not seem toproportionally increase photo-induced properties, since just theouter layer seems to determine these properties, being reallyexposed to ultraviolet light. A limitation of this study is in thesimulation of the real outdoor environment conditions. Otherstudies are in course to overcome this limitation.

Acknowledgments

The authors would like to thank Salentec srl for the supply oftitanium dioxide sol.

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