Self-Cleaning and Antibacteric Ceramic Tile Surface

Int. J. Appl. Ceram. Technol., 10 [6] 949–956 (2013) DOI:10.1111/j.1744-7402.2012.02801.x Self-Cleaning and Antibacteric Ceramic Tile Surface Simona de Niederhãusern and Moreno Bondi Dipartimento di Scienze Biomediche, Università di Modena e Reggio Emilia, Via Campi 287, Modena 41125, Italy Federica Bondioli* Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Università di Modena e Reggio Emilia, Via Vignolese 905, Modena 41125, Italy The aim of this investigation was the surface functionalization of industrial ceramic tiles by sol-gel technique to improve at the same time the cleanability and the antibacterial activity of surfaces. This objective was pursued through the design and preparation of nanostructured titania-silver coating that was deposited on glazed, unglazed, and polished tiles by air-brushing. The obtained results showed that the applied coatings are transparent, show a good adhesion, and a remarkable antibacterial activity under the tested conditions. The surface photocatalicity was optimized with the higher thermal treatments (200°C) even if photodegradation process is clearly affected by the sample surface roughness. Introduction The development of new easy-to-clean or even selfcleaning surfaces has recently been under the focus of nanotechnology, that is, by investigating different surface structures or nanocoatings.1,2 However, among the different and interesting properties of nanoparticles, they are characterized by a mean diameter below the *federica.bondioli@unimore.it © 2012 The American Ceramic Society light wavelength and thus they are transparent if applied on a substrate or dispersed in a matrix. This aspect is particularly important for materials, such as ceramic tiles, for which the esthetic aspect is often the parameter that determines the choice or the impression one has of them, which are rarely determined by the particular functional properties. Taking into account the idea to exploit the transparency of nanoparticles, the authors have evaluated different approach to obtain a multifunctional surface for ceramic tiles using both soluble salts solutions3,4 or sol-gel technology.5 In 950 International Journal of Applied Ceramic Technology—de Niederhãusern, Bondi, and Bondioli particular, to improve surface cleanability properties6,7 the photocatalicity of Titanium dioxide (TiO2) nanoparticles has been used. TiO2 is one of the most used and efficient photocatalytic material and, especially in the field of construction and building materials, it is the most widely used. TiO2 is a common semiconductor material that has three crystal structures: anatase, rutile, and brookite, with atanase showing a greater photocatalytic activity than the other types of TiO2 polymorphs. Since the discovery of its photocatalytic efficacy, TiO2 has been used in many researches and practical applications, including water8 and air purifications,9 self-cleaning and anti-bacterial effect, as cement mortar, in exterior tiles, paving blocks, glass, PVC fabric, and to protect the Cultural Heritage surfaces.10–12 The extensive use of TiO2 is due to its characteristics: relatively inexpensive, safe, chemically stable, high photocatalytic activity compared with other metal oxide photocatalysts, effective under weak solar irradiation in outdoor environment. For exemplum, the production of photocatalytically active building materials allows to obtain self-cleaning and self-sterilizing surface that, moreover, might degrade several organic contaminants in the surrounding environment by UV radiation activation.13 In a recent work, the authors reported the possibility of tile surface functionalization using a solgel technique to improve both wear resistance and cleanability of unglazed surfaces.5 A TiO2–SiO2 binary film was deposited on fired tiles by air-brushing to obtain a self-cleaning and self-sterilizing surface. However, the disadvantage of these coatings is that the band gap energy of TiO2 is about 3.2 eV and, therefore, UV illumination is necessary to photoactivate this semiconductor. Silver is so far one of the best known antimicrobial/antifungal agent due to a strong cytotoxic effect toward a broad range of microorganisms and due to its remarkably low human toxicity compared with other heavy metal ions.14 According to various studies, it is believed that the antibacterial activity of silver is due to Ag+ ions that react with proteins by combining the – SH groups of enzymes, which leads to the inactivation of the proteins present in the bacterial/microbial cell membrane and thus destroys the cell by rupturing the wall. In particular this excellent antibacterial activity is not restricted by UV illumination. To take advantage of this property, the aim of this study was oriented in design and synthesis of titania (TiO2) and silver (Ag) multifunctional inorganic Vol. 10, No. 6, 2013 coating, to apply on fine porcelainized stoneware tile, also called porcelainized stoneware or grés, a product used both for internal and external applications in building field. The goal was the obtainment of a transparent physical barrier to enhance the cleanability of tile due to the titania and to assure the antibacterial activity of the coating in any illumination conditions. A few earlier reports on Ag-titania codoped films, mainly on glassy substrate, may be highlighted in this context. Page et al.15 reported sol-gel synthesis of Ag2O –TiO2 films (10 mol% Ag) on microscopic slides (soda-lime glass) and studied their antimicrobial properties after irradiation by UV light (365 nm). They found that Ag-doped titania coatings were significantly more photocatalytically and antimicrobial active than a titania coating. Mukhopadhyay et al.16 developed Ag– TiO2 nanoparticles codoped SiO2 films on crystalline ZrO2 barrier-coated soda-lime glass substrates. The presence of presynthesized anatase nanoparticles helped the silver stabilization and improved the overall antibacterial activity. Regarding ceramic tile, Sun et al.17 deposited Ag–TiO2 thin films on glazed surface by liquid phase deposition method (LPD). The films, prepared starting from ammonium hexauorotitanate (IV) and silver nitrate and annealed at 600°C, showed an excellent antibacterial activity versus Escherichia coli and Staphylococcus aureus. The aim of the present work, therefore, was to analyze the real possibility of creating codoped silver and titania coatings on ceramic tiles taking into account the specific variable of the ceramic substrate (i.e., roughness and finishing) and the industrial ceramic process. In fact, starting from a commercial titania sol-gel suspension in which silver were added as silver nitrate, Ag–TiO2 coatings were deposited on glazed, unglazed, and polished tiles by air-brushing. This deposition technique was chosen, on a laboratory scale, taking into account the industrial applicability and the possible technological solutions necessary to implement these surface treatments in the industrial traditional process. The obtained films were fully characterized to mainly evaluate the effect of silver content and thermal treatment (temperature range 80°C and 200°C) on scratch resistance, cleanability, and antibacterial activity of the coatings. Particular attention was paid to preserve the aesthetical aspect of the final product and the obtained hue variation was evaluated by means of UV–Visible spectroscopy and colorimetric analysis. Self-Cleaning and Antibacterial Tile www.ceramics.org/ACT Experimental Section Coating Preparation A commercial titania sol-gel suspension with a nominal TiO2 content of 1.5 wt% (kindly furnished by NanoSurfaces, Milan, Italy) was mixed with different amount of silver nitrate (AgNO3; Sigma Aldrich, St. Louis, MO). The ratio Ag:TiO2 was fixed to 1:10, 1:20, and 1:30 respectively. For comparison, samples without silver addition were also prepared using only the titania sol-gel suspension. The composition of the studied samples is summarized in Table I. The application of sols was performed by air-brushing on glazed (G), unglazed (UG), and polished (P) gres tiles (kindly provided by Marazzi Group, I). During the airbrushing application, the substrates were kept at a distance of about 14 cm from the air-brush applicator. Each sample was coated with 0.1 ml/cm2 of solution. After coating, the surfaces were dried at room temperature for 24 h and finally heat-treated at 80°C and 200°C for 30 min. Coating Characterizations The effect of the coating on the tile color was determined by performing color measurements on both uncoated and coated tiles by UV–Vis spectroscopy (model Lambda 19, Perkin Elmer, Waltham, MA) using the CIElab method to obtain L*, a*, and b* values.18 The method defines a color through three parameters, L*, a*, and b*, measuring brightness, red/green and yellow/blue color intensities, respectively. The method allows, moreover, to define a color difference as DE*, based on the relationship:  1 DE  ¼ ðDLÞ2 þ ðDaÞ2 þ ðDbÞ2 2 ð1Þ Table I. Composition of the Prepared TitaniaSilver Coatings, Including Heat Temperature Sample ID Ag/TiO2 ratio (wt/wt) Heat temperature (°C) 80_Ti 200_Ti 80_10 80_20 80_30 200_30 No silver No silver 1/10 1/20 1/30 1/30 80 200 80 80 80 200 951 where DL*, Da*, and Db* measure the differences in luminosity and in chromaticity between two color. In this way the hue variation due to the coating step were determined. To evaluate the effect of the coating on the tile appearance, the samples gloss was measured by a Novo Gloss Trio apparatus (Testing Machines, New Castle, DE); measurements were performed by using 60° as standard geometry. The microstructure of the samples was investigated by scanning electron microscopy (SEM) using a XL 30 instrument (Philips, Eindhoven, The Netherlands) coupled with an energy dispersion spectroscopy (EDS, INCA, Oxford Instruments, Oxfordshire, U.K.) equipment over gold-coated samples. The surface morphology of silica-silver films was investigated by atomic force microscopy (AFM) using a Autoprobe CP instrument (Veeco, Plainview, NY). The sample was imaged at many different locations on the surface, to obtain an average value for the roughness. To verify the adhesion of the coatings to the polished tiles, scratch tests (Micro-Combi tester, CSM Instruments, Peseux, Switzerland) with linearly increasing load (0.1N to 30N, scratch speed of 1 mm/min) were performed on the samples using a Rockwell indenter with spherical tip, 100 lm radius. At least, three scratches were performed on each coating, with the minimum distance between two scratches set at 4 mm to achieve results representative of the average response over greater surfaces. The critical loads Lc1 (first crack) and Lc2 (edge spallation) were determined by optical microscopy. To evaluate the photocatalytic activity of the obtained coating, contact angle measurements and cleanability test were performed. Static water contact angles (CA) were measured by the sessile drop method19 using a conventional drop shape technique OCA 20 apparatus (DataPhysics Instrument, Filderstadt, Germany). To avoid any surface contamination, all specimens were rinsed in tetrahydrofuran, THF, and accurately air-dried just before measurement. Static CAs were determined on the basis of at least 10 measurements and a drop volume of 15 lL. All CA measurements were carried out at ambient conditions, under UV irradiation. UV light with wavelength range 325–390 nm and light intensity 5.5 mW/cm2 was used as light source.20 Determination of CA was based on the Young-Laplace equation and performed every 30 s for 10 min. The result was the mean of the drop on five replicate samples. Photo-degradation of methylene blue solution (500 ppm) was used to assess the photocatalytic activity 952 International Journal of Applied Ceramic Technology—de Niederhãusern, Bondi, and Bondioli of the coating.21 The coated tiles were treated with the methylene blue solution, briefly pressed with paper and then irradiated with the UV lamp described above. The photocatalytic decomposition of blue was monitored, every 30 min for 240 min, by color measurement using a portable UV spectrophotometer (Color Quality, Corob Service, Finale Emilia [MO], Italy) equipped with Standard Illuminant D65 and expressed as color variation (DE*) with respect to the initial color of the tile. For the quantitative antibacterial activity evaluation, Japanese Industrial Standard (JIS) Z2801:200022 method was used with slight modification. Briefly 0,4 mL of an appropriate dilution (about 105 colony forming units, CFU) of an overnight cultures (37°C) of Staphylococcus aureus ATCC 6538 and Escherichia coli ATCC 25922 (indicator strains) in Tryptic Soy broth (TSB, Oxoid, Milan, Italy) were spread on the surface of the test pieces (30 mm 9 30 mm) and covered with a polyethylene film. Each test piece was placed in a sterilized petri dish and incubated for 24 h at 35°C temperature and at a relative humidity higher than 90% to prevent drying. The untreated pieces (control) were subjected to the same procedure. To recover the viable cells, the covering film and the test pieces were placed in a sterilized nylon bag with 10 mL of saline solution. The microbial suspensions, opportunely diluted, were sown on Tryptic Soy agar (TSA; Oxoid, Milan, Italy) plates. After 24 h of incubation at 37°C, the colonies were visually counted, and the numbers of colony forming units (CFU) of the survival indicators were recorded and compared with those of untreated samples (control). The value of antibacterial activity was calculated in according to the following formula: R¼ ðA BÞ A 100 where R is the value of antibacterial activity in rate of decrease, A the average number of viable bacteria cells on the untreated test piece after 24 h and B the average number of viable bacteria cells on the antibacterial test piece after 24 h. Results and Discussion In Tables II–IV, DE* between the coated tile surfaces and the untreated tile are reported for the different surfaces finishing. The Tables clearly show that in Vol. 10, No. 6, 2013 Table II. Color Variation (DE*), Gloss Values (GU), First Critical Load (Lc1), and Second Critical Load (Lc2) of the Prepared Coatings on Glazed (G) Tiles Sample ID Gloss DE* (GU) Untreated G tile / G80_Ti 1.1 G200_Ti 1.2 G80_10 1.2 G80_20 1.2 G80_30 1.3 G200_30 1.6 138.6 136.2 135.4 136.6 136.3 136.6 134.8 Lc1 (N) 4.2 5.9 7.3 6.7 5.3 5.8 7.7 ± ± ± ± ± ± ± 0.8 0.3 0.6 1.7 1.6 1.4 2.0 Lc2 (N) 7.5 9.4 11.6 12.6 11.5 12.1 14.1 ± ± ± ± ± ± ± 0.5 0.6 0.8 1.5 0.9 0.1 0.6 Table III. Color Variation (DE*), Gloss Values (GU), First Critical Load (Lc1), and Second Critical Load (Lc2) of the Prepared Coatings on Unglazed (UG) Tiles Sample ID DE* Gloss (GU) Lc1 (N) Untreated UG tile UG80_Ti UG200_Ti UG80_10 UG80_20 UG80_30 UG200_30 / 0.7 0.6 0.6 0.6 0.6 0.8 12.9 12.0 10.1 11.9 11.4 11.6 9.8 n.d. n.d. n.d. n.d. n.d. n.d. n.d. Lc2 (N) 11.7 12.2 13.6 12.5 14.6 13.2 17.6 ± ± ± ± ± ± ± 2.0 1.8 0.8 1.8 0.5 1.5 1.1 n.d., non detectable. Table IV. Color Variation (DE*), Gloss Values (GU), First Critical Load (Lc1), and Second Critical Load (Lc2) of the Prepared Coatings on Polished (P) Tiles Sample ID Gloss DE* (GU) Untreated P tile / P80_Ti 0.8 P200_Ti 0.8 P80_10 0.9 P80_20 0.9 P80_30 0.8 P200_30 1.5 64.6 62.2 61.4 62.7 62.6 62.7 61.3 Lc1 (N) 5.7 6.8 8.1 7.0 6.9 6.9 8.7 ± ± ± ± ± ± ± 0.2 1.8 0.6 2.0 1.7 1.6 1.6 Lc2 (N) 13.4 12.4 13.6 13.6 12.1 13.9 14.9 ± ± ± ± ± ± ± 1.0 0.6 0.3 1.2 0.9 0.3 0.2 www.ceramics.org/ACT Self-Cleaning and Antibacterial Tile all the studied cases the values are in general <1, indicating that there is not an important hue variation due to the applied coatings. This result underlines that the titania and silver nanoparticles are well dispersed in the coatings and thus are effectively transparent. In Tables II–IV, the gloss measured on the prepared samples is also reported for the different surfaces finishing. This parameter is correlated with the surface roughness of the samples. In fact it is higher for the glazed samples (Table II) with respect to the unglazed (Table III) or polished samples (Table IV). Regarding the effect of the coating, the gloss changes are directly correlated with the temperature of the thermal treatment and the gloss value decreases as the temperature is increased. The parameter is not related to the silver content being almost constant as the silver content is decreased. However, it is important to notice that the obtained values are not very different from that of untreated sample indicating that the appearance of the samples is not modified by the deposited coatings. To verify the coating roughness, in Fig. 1 the AFM images obtained on the glazed tiles (G80_30 and 953 G200_30 chosen as representative) are reported. The images clearly evidenced the effect of temperature on the surface roughness that in general increases as the temperature is increased. For a quantitative evaluation, sample roughness was calculated on the basis of 10 profiles 25 lm long for each sample; the obtained results showed an average roughness from 10 to 30 nm as the temperature of the thermal treatment is increased. This behavior allows justifying the change of the sample gloss as the temperature is increased because the surface roughness directly influences the specular light reflected. In Fig. 2, the SEM images of the surface of G200_30 sample, chosen as representative, are reported. The images show a very planar and homogeneous coating on the whole surface. By producing a fracture in the coating (Fig. 2b) it is possible to measure the film thickness that is about 200 nm. To verify the adhesion of the coatings to the tiles, scratch tests with linearly increasing load were performed on the samples. In Tables II–IV the first (coating failure) (a) (a) (b) Fig. 1. Atomic force microscopy images of samples obtained with a Ag:TiO2 ratio of 1:30 heated respectively at 80 (sample 80_30; a) and 200°C (sample 200_30; b). (b) Fig. 2. Scanning electron microscopy images of sample obtained with a Ag:TiO2 ratio of 1:30 heated at 200°C (sample 200_30). 954 International Journal of Applied Ceramic Technology—de Niederhãusern, Bondi, and Bondioli and second (coating detachment) critical load values are reported for the different surfaces finishing. In general, independently on the characteristic of the tile surface (glazed, unglazed, or polished), the obtained results, in comparison with that on untreated tiles, show that the deposition of the nanostructured coatings allows to an increase of the scratch resistance as underlined by the increase of the critical loads. In particular, the silver content does not affect the normal load needed both to scratch and remove the coating from the substrate depending only on the temperature of the thermal treatment. In fact, as the temperature is increased an increase of Lc1 and Lc2 values is observed than can be attributed to a high structural integrity of the coatings related to the temperature of the thermal treatment.23 In agreement with this behavior, a consistent improvement of penetration resistance by increasing temperature can be noted in terms of penetration depth (Pd). In Fig. 3 the behavior of the glazed samples cho- Fig. 3. Penetration depth (Pd) in a progressive load scratch test on the glazed samples. (a) (b) Vol. 10, No. 6, 2013 sen as representative are reported: the slope of the penetration curves decreases progressively with the increase in temperature both in the sample with and without silver. The effect of silver content is not predominant and, in general, the penetration depth decreases as the silver content is increased. Finally, to evaluate the photocatalytic activity of the obtained coating, contact angle measurements and cleanability test were performed. The contact angles of water for the titania-silver films treated at different temperatures as a function of UV-light irradiation time are shown in Fig. 4. All the coatings have a lower CAs with respect to the untreated tile and the value decreases as the irradiation time is increased that is the tile surface becomes more hydrophilic by UV-light irradiation. It is noticed that the CA of 0°, called super hydrophilicity, is observed in general in the samples irradiated for more than 7 min independently on the characteristic of the tile surface. The obtained data clearly show that the silver content does not affect the photo induced hydrophilicity of the coatings due to presence of titania nanoparticles. Regarding the effect of the thermal treatment, the CAs decrease as the annealing temperature is increased suggesting that also the surface roughness plays an important role for surface hydrophilicity of titania-silver mixed films. In particular, taking into account the AFM results, it seems that the photoinduced hydrophilicity is faster as the nanoscale roughness of the surface is increased. Photodegradation of methylene blue solution (500 ppm) was used to assess the photocatalytic activity of the samples. In Fig. 5 the color variations (DE*) as a function of silver content, annealing temperature, and UV-light irradiation time are reported for all the studied samples. It is important to notice that also on the (c) Fig. 4. Contact angle of glazed (G, a), unglazed (UG, b), and polished (P, c) tiles as a function of silver content, annealing temperature and UV irradiation time. Self-Cleaning and Antibacterial Tile www.ceramics.org/ACT (a) 955 (c) (b) Fig. 5. Color variation (DE*) of glazed (G, a), unglazed (UG, b), and polished (P, c) tiles as a function of silver content, annealing temperature, and UV irradiation time. untreated sample there is a slightly color variation probably due to the degradation effect of the UV-light on the methylene blue. All the coatings have, however, a higher DE* with respect to the untreated tile and the value increases as the irradiation time is increased. The behavior is not linear and after approximately 60 min the degradation kinetic changes becoming slower. The obtained data clearly show that, also in this case, the silver content does not affect the photocatalytic activity of the coatings due to presence of titania nanoparticles. Regarding the effect of the thermal treatment, the DE* decrease as the annealing temperature is increased. This behavior is in agreement with the gloss data that decrease as the annealing temperature is increased suggesting the important role of surface roughness in the photodegradation process.24 Relatively to the antimicrobial activity evaluation, the samples obtained with a different Ag:TiO2 ratio (1:10, 1:20 and 1:30) showed, compared with the untreated controls, a considerable efficacy without UV irradiation resulting in all cases in a total killing (reduction of 100%) of the two indicators S. aureus ATCC 6538 and E. coli ATCC 25922. Conclusion The sol-gel process was used in this work for the preparation of coatings for ceramic tiles. This study investigated the effect of annealing temperature and silver content on the final properties of the obtained titania-based films. The coatings are transparent, do not modify the gloss of the uncoated tiles, and show a good adhesion and a remarkable antibacterial activity under the tested conditions. 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