Summary of the antibacterial testing results on sprayed and digital printed tiles.
TiO2 powders can be employed as both photocatalytic and structural materials, leading to applications in external coatings or in interior furnishing devices, including cement mortar, tiles, floorings, and glass supports. The technology of photocatalytic building materials is connected with the widespread production of photocatalytic active tiles. All the techniques proposed in the study involve the employment of nanosized TiO2: this represents a new problem to be dealt with, as inhaling nanoparticles exposes workers during industrial production and people in everyday locations to their dangerousness. Only very recently the employment of microsized TiO2 has been proposed, and the authors in this manuscript report the use of micrometric titania materials, but employing a new deposition technique, which is digital inkjet printing. It represents an improvement of the classical spray coating methods, as it requires piezoelectric heads to precisely direct the deposition of the suspension with an electrostatic field. The mixture contains aqueous/organic components containing micrometric TiO2: to form a suspension, which is printed onto the surface of porcelain grès, large slabs using a digital printer. Many advantages are immediately evident, namely rapid and precise deposition, (almost) no waste of raw materials, thereby highlighting the economy, environmental friendliness, and sustainability of the process. All the materials we obtained have been thoroughly characterized by means of several experimental physico-chemical techniques, such as Raman microspectroscopy and scanning electron microscopy coupled with elemental analysis. Two different model VOCs, ethanol and toluene, and NOx have been selected to test the photocatalytic performances of the abovementioned tiles. Moreover, the antibacterial properties of the tiles have been determined, using Escherichia coli as example. Life cycle assessments (LCAs) for the two processes were modeled for 1 m2 of tiles produced in Modena, Italy. The impact assessments revealed that jet spraying exhibited uniformly greater impacts than digital inkjet printing and that the principal impacts were in human toxicity, cancer effects, freshwater ecotoxicity, and climate change. Most of the impacts were associated with the energy required for the production processes. Further considerations revealed that jet spraying is projected to generate twice as much CO2 and 30% more NOx than digital inkjet printing.
- digital inkjet printing
- jet spraying
- E. coli
- life cycle assessment
- impact assessments
The WHO (World Health Organization) estimates that 12.6 million deaths each year are attributable to unhealthy environments , confirming the urgent need for investment in strategies to reduce environmental risks in cities, homes, and workplaces. Of these deaths, 4.3 million (34%) and 3.7 million (29%) have been linked to exposure to indoor and outdoor pollution, respectively . More broadly, it is considered that 92% of the population lives in areas that are exposed to respirable particulate matter of <2.5 μm in diameter (PM2.5) at an annual mean concentration of >10 μg/m3 (or a 24 h mean of >25 μg/m3), which exceeds the WHO Air Quality Guidelines (AQG). The worst affected areas are China, the Indian subcontinent, the Middle East, and northern Africa.
Particulate and gaseous air pollution results largely from industrial and motor vehicular sources, although these emissions are exacerbated by those generated by heating . The combustion of solid biomass and impure liquid fuels is the main source of pollution leading to health problems, as evidenced by numerous recent studies. For example, in Ethiopia, the average concentration of respirable particulate matter of diameter of <PM2.5 reaches as high as a 24-h mean concentration of 280 μg/m3, indoor CO levels are greater than the regulatory limits for the US, and the average NO2 level is 97 ppb . In Southeast Asia, the large-scale burning of biomass causes heavy emissions of combustion by-products, which photochemically transform to other more dangerous molecules , such as polycyclic aromatic hydrocarbons (PAHs) . Europe is affected mainly by emissions of nitrogen oxides (NOx) . Although outdoor pollution tends to be a focal point, indoor air in the US can be two to five times more harmful than outdoor air . Almost 96% of houses in North America are reported to have at least one indoor air quality problem . Cigarette smoke contains more than 7000 chemicals, of which 69 are known to be carcinogenic and many more to be poisonous . Household items, such as cleaning products, air fresheners, and sanitation products, also contribute to indoor pollution by releasing chemicals. Consequently, there are environmental health imperatives to develop technologies that address these while satisfying other demands in the areas of energy efficiency, economic efficiency, utility, and esthetics.
1.2. Photocatalytic TiO2 and its applications
Semiconductor photocatalysis involves the activation of a photocatalytic material, such as titania (TiO2), by the action of light to produce charge carriers that undergo redox reactions with water and oxygen molecules . This property allows these materials to decompose organic contaminants ideally into CO2 and water, thus allowing applications in self-cleaning, anti-fogging, corrosion prevention, and air and water purification . Although there are many scientific and technical requirements for effective photocatalytic performance , there are also numerous practical challenges that must be met. One of these is catalyst immobilization  because it generally must balance the requirements of achievement of a robust chemical bond to a substrate, the retention of sufficient exposed surface area, and the minimization of contamination from the substrate.
There has been an increase in the sales of TiO2-based products for self-cleaning and air and water purification owing to growing environmental concerns . One of the key product developments was photocatalytic building materials, which appeared on the market at the beginning of the 1990s . TiO2 can be applied either in exterior construction or in interior furnishing materials, including concrete, mortar, tiles, flooring, and glass. In the early 2000s, the sales of photocatalytic building materials accounted for ~60% of the total market for photocatalytic materials . Recent market projections to 2020 foreshadow increasing demand in the construction sector for self-cleaning coatings and other technologies for improving air quality, leading to a global market for photocatalyst-based products of ~$2.9 billion . At present, TiO2-based photocatalyst products constitute 97.6% of the market, and this dominance is a result of TiO2’s low cost, ready availability, and well-documented performance data.
Many studies have demonstrated the effectiveness of photocatalytic materials for pollution abatement of the volatile organic compounds (VOCs) and nitrogen oxides (NOx) that represent widespread risks to health. For example, concrete paving blocks with TiO2 added directly to the mix were able to remove 4.01 mg/h/m2 of NO . However, similar work demonstrated that the efficiency of NOx photodegradation varied significantly according to preparation method and resultant microstructure, with some products achieving 40% photodegradation while others showed almost no effect . A comparison between pure TiO2 and TiO2 embedded in neat hydrated cement revealed that the photocatalytic activity was three to 10 times lower for the latter, which was attributed to the agglomeration of the TiO2 powder . TiO2 also has been applied as surface coatings on roofing tiles and corrugated sheet and found to achieve gaseous toluene removal efficiencies as high as 78 ± 2%, although this was limited by gas concentration and residence time .
It is clear that the permanence and performance of TiO2-containing construction products depend significantly on the method used to immobilize the photocatalyst, whether in the bulk or on the surface. Other factors of importance include the type of substrate and the method of TiO2 application . Hence, there has been a commensurate focus on the design of such building products and their manufacture in order to obtain maximal lifetimes and efficiencies. An additional consideration is the potential alteration of the photocatalyst by fouling, which can block the active sites and reduce or prevent photocatalysis . Finally, there are also potential health issues associated with the formation of chemical intermediates  and the respirability of nanoparticulates .
1.3. TiO2 coatings
One of the most promising areas in photocatalytic building materials is photocatalytically active tiles, which are self-cleaning but also can decompose air pollutants. The standard approach has been to apply TiO2-based materials on ceramic surfaces and to attempt to immobilize the TiO2 at as low a temperature as possible. In the case of spray coating of aqueous dispersions of TiO2 on clay roofing tiles, the heating temperature was 290°C . Chemical vapor deposition (CVD) using gaseous precursors, following by heating at 550°C, also has been investigated  but these films suffered from poor adhesion. By contrast, plasma-enhanced chemical vapor deposition (PECVD), involving reheating at 400°C, yielded high-quality materials . Aerosol flame synthesis (temperature probably <700°C) has been used to produce ultra-fine TiO2 nanoparticles that could be deposited directly on supports . Sol-gel TiO2 precursors have been coated on cement-based materials by dipping and heating at 500°C . TiO2 suspensions in water or alcohol also have been sprayed on limestone with no subsequent heating since the porous microstructure immobilized the powder . All of these approaches used nanosized TiO2.
Although spray coating remains the simplest and least expensive coating technique, recent work  reports the functionalization of commercial glazed ceramic tiles with pigment-grade, microsized, anatase (Kronos 1001) using an industrial-type process. This method offers a reduction in the risk of inhalation as it does not involve the use of nanosized TiO2 powders. The TiO2 powder was deposited by airless jet spraying aqueous suspensions containing a silica-based commercial product as binder. The tiles were dried and then fired at 680°C in an industrial kiln for 80 min (cold-to-cold cycle).
This work reports a new deposition technique, which is digital inkjet printing. Despite the advantages of conventional spray coating methods, digital inkjet printing offers an advantageous, precise, coating technique that utilizes piezoelectric heads to effect directional deposition of an aqueous/organic ink suspension within an electrostatic field . The suspension contained pigment grade, microsized anatase (Kronos 1077), and this was printed on the surface of porcelain grès
2. Materials and methods
2.1. Microsize TiO2 and porcelain grès tiles
The TiO2 powder was Kronos 1077, which is ≥98.5% anatase and has a particle size range of 110–130 nm, a BET surface area of 12 m2/g, a true density of 3800 kg/m3, and an optical indirect band gap of 3.15 eV [35, 36].
The TiO2 was deposited on porcelain grès tiles that were manufactured by GranitiFiandre S.p.A. These are dry-pressed tiles fired at 1200–1300°C. The phase assemblage as such consists of glass, mullite, and quartz.
2.2. Digital inkjet printing and airless jet spraying procedures
Conventional, industrial-scale, airless, jet spray deposition was used for the deposition of coatings, as described in brief previously by the authors . However, with the new, innovative, automated decoration of ceramic surfaces by digital instruments, it has become possible to manufacture products of considerably superior quality. Consequently, this work also reports an investigation of the deposition of coatings of photocatalytic TiO2 by digital inkjet printing using a proprietary ink containing suspended microsized TiO2 particles.
A prototype digital printer manufactured by Projecta S.p.A. was used to apply the suspensions. Following deposition, the tiles were calcined at 680°C for 80 min using an industrial-scale kiln designed to avoid thermal shock. The cooled tiles then were washed with water, brushed vigorously to remove loose particles, and air jet dried. The process is summarized in Figure 1. The water used for degreasing was recovered and purified prior to recycling; any damaged tiles were recycled.
Mineralogical analyses were done by laser Raman microspectroscopy (Raman; inVia Raman Microscope, green laser, 514 nm, 20×, 1.5-μm beam diameter, Renishaw, Wotton-under-Edge, Gloucestershire, UK). Microstructural analyses were done by field emission gun scanning electron microscopy (Field emission gun scanning electron microscopy (FEG-SEM); LEO 1525, 15, or 20 kV accelerating voltage, LEO Electron Microscopy Inc., Thornwood, NY, USA). Elemental analyses were done by paired energy dispersive spectroscopy (EDS; Bruker Quantax, Bruker Italia S.r.l., Milan, Italy).
2.4. Photocatalysis testing
2.4.1. Volatile organic compounds
VOC photodegradation reactions were conducted in a Pyrex glass cylindrical reactor with a diameter of 200 mm and an effective volume of 5 L . A 100-cm2 digital inkjet-printed sample or jet-sprayed sample was placed at the bottom of the reactor and exposed to a 500-W iron halogen lamp (HG 500, 315–400 nm, UV-A, Jelosil, Milan, Italy). The reactor had two entrances, one for VOC loading and the other for sampling by a gas chromatograph (490 Micro GC System, Agilent Technologies Italia S.p.A, Milan, Italy).
Ethanol and toluene were selected as model pollutants, and the initial concentration for both was 400 ppm for each test. The photodegradation tests were conducted for 6 h.
2.4.2. Nitrogen oxides
Two different setups were used—one under static conditions  and the other under flowing conditions . A chemiluminescence NOx monitor (Teledyne API, Model 200E, San Diego, CA, USA) provided online detection of NO and NO2 concentrations. The batch reactor consisted of a Pyrex glass cylinder of volume of 25 L; this is described elsewhere . The tile (2 × 20 cm) was located at the bottom of the cylinder and an overhead 500-W iron halogenide lamp (HG 500, 315–400 nm, UV-A, Jelosil, Milan, Italy) provided irradiation of 20 W/m2. The inlet gas consisted of NO2 (0.6% in N2) mixed with air of relative humidity (RH) 40%, which allowed equilibration between NO and NO2 to be reached quickly. The NO2 concentration was set at 1000 ppb (static) or 250 ppb (flowing) and tests lasted 6 h each. The continuous flow reactor allowed testing of larger tiles (60 × 60 cm) . A Thermo-Hygro Meter (HT-3006A, Metravi, Calcutta, India) measured both temperature and relative humidity, which ranged between 40 and 50%. Two iron halogen lamps (Jelosil, model HG 500), at 770-mm center-to-center distance, which was designed to irradiate the sample surface with the same 20 W/m2 light intensity, were used. The total NOx gas flow was in the range of 140–180 NL/h, which is in accordance with the standard concentration based on the limiting values specified in
2.5. Antibacterial effects
The photocatalytic tiles were tested using Escherichia coli (E. coli, ATCC 8739) according to
A fluorescent UV lamp (18 W, Royal Philips, Amsterdam, Netherlands) was used for the testing at an intensity of 0.25 mW/cm2 for 8 h. A viability count was performed by dilution and plating on nutrient agar incubated at 37°C for 48 h. More details are reported elsewhere .
3. Results and discussion
3.1.1. Field emission gun scanning electron microscopy
The FEG-SEM images shown in Figure 2 reveal a relatively widespread, homogeneous, and thick deposition of TiO2 on the digital inkjet-printed tile surface in comparison to the jet-sprayed tile surface, which shows that voids are present. The EDS data confirmed a greater areal extent of coverage by TiO2 for digital inkjet printing. However, it is significant that the digital inkjet-printed coating exhibits a considerably lower degree of agglomeration than does the jet-sprayed coating. The former can be expected to provide a greater surface area and associated density of photocatalytically active sites. The reason for this difference is the superior dispersion of the TiO2 powder by the inclusion of a dispersant in the ink, which reduced the formation of soft agglomerates. Further, it is likely that the included organic phases played a key role in the development of this microstructure through separation of the particles and consequent surface exposure during pyrolysis. Consequently, digital inkjet printing can be expected to exhibit a superior photocatalytic performance owing to the greater volume of deposited TiO2, the extent of areal coverage of the tile, and exposed surface area.
3.1.2. Laser Raman microspectroscopy
In the optical micrographs taken by Raman, shown in Figure 3, the lighter rougher regions consist of TiO2 and the darker regions are the exposed porcelain grès substrates. These images confirm that the TiO2 deposited by digital inkjet printing is more widespread than that by jet spraying. Figure 4 shows that the Raman spectra of the digital inkjet-printed and the jet-sprayed coatings are significantly different. The intensities of the peaks and the slopes of the baselines indicate the TiO2 content of the coating and the glass content, respectively, for each sample. That is, lower peak intensities indicate a low amount of TiO2 and/or a low degree of crystallinity and the sloped baseline indicates a high glass content in the vitreous tile. The greater peak intensities and flat baseline for the digital inkjet-printed coating confirm the greater extent of coverage of the tile by TiO2, whereas the low peak intensities and sloped baseline for the jet-sprayed coating confirm the low areal distribution of TiO2. Since laser Raman microspectroscopy does not analyze amorphous materials accurately, the peak at 510 cm−1 is attributed to albite feldspar and the peak at 610 cm−1 to α-quartz. The two other small but relatively sharp peaks at 439 and 356 cm−1 could not be identified.
3.2. Photocatalytic testing
3.2.1. VOC photodegradation
Figure 5 shows the UV photodegradation of ethanol and toluene. Both sets of data demonstrate the superior photocatalytic performance of the digital inkjet-printed coating. While similar testing of Kronos 1077 bulk powder has been shown to decompose ethanol completely in 1 h , this work for the deposition of this TiO2 powder as a digital inkjet-printed coating shows that nearly complete decomposition (~97%) requires 6 h. By contrast, the jet-sprayed coating decomposes only ~47% of the ethanol after 6 h. Interestingly, both coatings outperform the powder when decomposing the more stable aromatic toluene ring. While Kronos 1077 bulk powder decomposed only ~43% of toluene after 6 h, the digital inkjet-printed coating decomposes ~84% and the jet-sprayed coating decomposes ~75%. It is probable that the difference in results for ethanol and toluene is that while both involved decomposition by photocatalysis, the latter also included a contribution from direct photolysis, which was enhanced by the greater surface area of the coating exposed to UV in comparison to that of a powder bed. This component may be significant because the difference between the two coating techniques is less than that for ethanol photodegradation.
These data confirm that the photocatalytic performance of the digital inkjet-printed coating is superior to that of the jet-sprayed coating for VOC decomposition. This is a result of the greater extent of coverage, more even areal distribution, a reduced agglomeration, a greater extent of exposed particle surface area, and a greater crystallinity of the TiO2 in the digital inkjet-printed coating.
3.2.2. NOx photodegradation
Figure 5 also shows the UV photodegradation of NOx in both batch and continuous-flow reactors. Again, the digital inkjet-printed coating outperforms the jet-sprayed coating. After 6 h under static conditions, the digital inkjet-printed coating decomposes ~90% of the NOx while the jet-sprayed coating decomposes only ~56%. After 6 h under flowing conditions, the performances were closer, with the digital inkjet-printed and jet-sprayed coatings decomposing ~80 and ~74% of the NOx, respectively. It is clear that the maximal levels of decomposition were attained relatively quickly and they were maintained such that no deactivation was observed.
The larger dimensions (60 × 60 cm) of the tiles and the lower NOx concentration used during the testing under flowing conditions provide a better simulation of real environmental conditions . It may be noted that this digital inkjet-printing technology is capable of coating dimensions as large as 150 by 300 cm.
3.3. Antibacterial testing
Table 1 summarizes the results of the antibacterial testing, which reveals that both types of coatings were highly effective in destroying
|Antibacterial activity of photocatalytic coatings using E. coli ATCC 8739|
|Symbol||Parameter||Units||Uncoated tile 1||Uncoated tile 2|
|N||Number of live bacteria||Cells||1,900,000||1,100,000|
|A||Number of live bacteria after inoculation||Cells/mL||280,000||160,000|
|BD||Number of live bacteria before UV irradiation||Cells/mL||220,000||600,000|
|BL||Number of live bacteria after UV irradiation||Cells/mL||120,000||170,000|
|—||Reduction in bacterial count after UV irradiation||%||45.45||71.67|
|CD||Number of live bacteria before photocatalysis||Cells/mL||180,000||130,000|
|CL||Number of live bacteria after photocatalysis||Cells/mL||10||10|
|—||Reduction in bacterial count after photocatalysis||%||99.99||99.99|
|RL||Antibacterial activity after photocatalysis||—||4.1||4.2|
|ΔR||Antibacterial activity with photocatalysis||—||4.1||3.6|
3.4. Life cycle assessment
In 1993, the Society of Environmental Toxicology and Chemistry (SETAC) standardized the process for life cycle assessment (LCA) to include four components: (1) goal and scope, (2) inventory, (3) impact assessment, and (4) data interpretation . LCA normally is applied to early-stage plants in order to investigate environmental hot spots arising from new technologies  or to establish industrial-scale processes for the comparison of divergent designs and optimization of environmental profiles . In this work, the LCA has been modeled for the production of photocatalytic ceramic tiles by digital inkjet printing and jet spraying.
The approach applies
In order to highlight the differences between the two production processes, LCA comparison calculations were performed using the
|Impact category||Jet sprayed||Digital inkjet printed||Difference (%)|
|Human toxicity, cancer effects||83.83||42.49||49.31|
|Human toxicity, non-cancer effects||19.35||10.62||45.10|
|Mineral, fossil and resource depletion||14.44||3.69||74.41|
|Photochemical ozone formation||9.17||5.04||45.03|
|Ionizing radiation, human health||7.44||5.85||21.42|
|Water resource depletion||4.38||2.76||36.90|
|Ionizing radiation, ecosystems (interim)||0.00||0.00||0.00|
These data show that jet spraying makes consistently greater impacts than digital inkjet printing does, with the most significant impacts being on human toxicity, cancer effects, freshwater ecotoxicity, and climate change. The first two of these categories have been examined in more detail using
Further, the calculation of the CO2 equivalents, carried out by the
The preceding LCA data suggest that the traditional method of jet spraying is considerably inferior to that of digital inkjet printing and that further detailed analysis of each step of the process is likely to improve the process and its outcomes commercially, environmentally, and performatively.
This work reports design, synthesis, and characterization of photocatalytic TiO2-coated tiles fabricated by the conventional jet spraying technique and the new technique of digital inkjet printing. The latter represents a significant improvement over the former, in that (1) it provides a more precise deposition as it utilizes piezoelectric heads to direct the suspension within an electrostatic field, (2) the microsized TiO2 powder strikes a medium that balances the small size advantageous for photocatalytic performance and the ability to resist solubility in the glass bonding to the tile against the large size that inhibits respirability, and (3) the topography of the digital inkjet-printed coating is less amenable to cell attachment and proliferation than those of the jet-sprayed coating.
The performance data show that the digital inkjet-printed coating was significantly more effective than the jet-sprayed coating in ethanol photodegradation. While the same trend was observed for toluene photodegradation, the difference was not as significant owing to direct photolysis of this aromatic compound. Both coatings were highly effective in destroying E. coli but the antibacterial activity with photocatalysis (ΔR) of the digital inkjet-printed coating was significantly superior to that of the jet-sprayed coating. Similarly, the LCA analysis of the impact assessments reveals the multifarious advantages of the digital inkjet-printed tile.
In memory of Benedetta Sacchi, a worthy researcher and a collaborator for years of the UNIMI research group. The authors would like to thank Dr. A. Carletti (Artest, Italy) for the antibacterial testing and Mr. R. Pellini (GranitiFiandre, Italy) for the preparation of the photocatalytic tiles. This research was supported financially by the