Photocatalytic TiO2: From Airless Jet Spray Technology to Digital Inkjet Printing

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 © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. DOI: 10.5772/intechopen.72790 © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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 m 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.


Pollution
The WHO (World Health Organization) estimates that 12.6 million deaths each year are attributable to unhealthy environments [1], 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 [2]. 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 (PM 2.5 )a ta n annual mean concentration of >10 μg/m 3 (or a 24 h mean of >25 μg/m 3 ), 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 [3]. 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 <PM 2.5 reaches as high as a 24-h mean concentration of 280 μg/m 3 , indoor CO levels are greater than the regulatory limits for the US, and the average NO 2 level is 97 ppb [4]. In Southeast Asia, the large-scale burning of biomass causes heavy emissions of combustion by-products, which photochemically transform to other more dangerous molecules [5], such as polycyclic aromatic hydrocarbons (PAHs) [6]. Europe is affected mainly by emissions of nitrogen oxides (NO x ) [ 7 ] . 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 [8]. Almost 96% of houses in North America are reported to have at least one indoor air quality problem [9]. Cigarette smoke contains more than 7000 chemicals, of which 69 are known to be carcinogenic and many more to be poisonous [10]. 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.

Photocatalytic TiO 2 and its applications
Semiconductor photocatalysis involves the activation of a photocatalytic material, such as titania (TiO 2 ), by the action of light to produce charge carriers that undergo redox reactions with water and oxygen molecules [11]. This property allows these materials to decompose organic contaminants ideally into CO 2 and water, thus allowing applications in self-cleaning, anti-fogging, corrosion prevention, and air and water purification [12]. Although there are many scientific and technical requirements for effective photocatalytic performance [13], there are also numerous practical challenges that must be met. One of these is catalyst immobilization [14] 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 TiO 2 -based products for self-cleaning and air and water purification owing to growing environmental concerns [15]. One of the key product developments was photocatalytic building materials, which appeared on the market at the beginning of the 1990s [16]. TiO 2 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 [17]. 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 [18]. At present, TiO 2 -based photocatalyst products constitute 97.6% of the market, and this dominance is a result of TiO 2 '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 (NO x ) that represent widespread risks to health. For example, concrete paving blocks with TiO 2 added directly to the mix were able to remove 4.01 mg/h/m 2 of NO [19]. However, similar work demonstrated that the efficiency of NO x photodegradation varied significantly according to preparation method and resultant microstructure, with some products achieving 40% photodegradation while others showed almost no effect [20]. A comparison between pure TiO 2 and TiO 2 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 TiO 2 powder [21]. TiO 2 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 AE 2%, although this was limited by gas concentration and residence time [22].
It is clear that the permanence and performance of TiO 2 -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 TiO 2 application [23]. 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 [24]. Finally, there are also potential health issues associated with the formation of chemical intermediates [25] and the respirability of nanoparticulates [26].

TiO 2 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 TiO 2 -based materials on ceramic surfaces and to attempt to immobilize the TiO 2 at as low a temperature as possible. In the case of spray coating of aqueous dispersions of TiO 2 on clay roofing tiles, the heating temperature was 290 C [27]. Chemical vapor deposition (CVD) using gaseous precursors, following by heating at 550 C, also has been investigated [28] but these films suffered from poor adhesion. By contrast, plasma-enhanced chemical vapor deposition (PECVD), involving reheating at 400 C, yielded high-quality materials [29]. Aerosol flame synthesis (temperature probably <700 C) has been used to produce ultra-fine TiO 2 nanoparticles that could be deposited directly on supports [30]. Sol-gel TiO 2 precursors have been coated on cement-based materials by dipping and heating at 500 C [31]. TiO 2 suspensions in water or alcohol also have been sprayed on limestone with no subsequent heating since the porous microstructure immobilized the powder [32]. All of these approaches used nanosized TiO 2 .
Although spray coating remains the simplest and least expensive coating technique, recent work [33] 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 TiO 2 powders. The TiO 2 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 Ci na n 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 [34]. The suspension contained pigment grade, microsized anatase (Kronos 1077), and this was printed on the surface of porcelain grès (viz., stoneware) tiles using a digital printer. This process has several advantages, including rapid and precise and more uniform deposition and essentially no waste of raw materials, which are favorable for both the economics of the process and the environment. Moreover, with this technique, slabs of large size up to 120 Â 360 cm can also be activated. The data for the thin films deposited by digital inkjet printing are contrasted with those for the more conventional method of jet spraying.

Microsize TiO 2 and porcelain grès tiles
The TiO 2 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 m 2 /g, a true density of 3800 kg/m 3 , and an optical indirect band gap of 3.15 eV [35,36].
The TiO 2 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.

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 [37]. 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 TiO 2 by digital inkjet printing using a proprietary ink containing suspended microsized TiO 2 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 industrialscale 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.  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.

Nitrogen oxides
Two different setups were used-one under static conditions [39] and the other under flowing conditions [40]. A chemiluminescence NO x monitor (Teledyne API, Model 200E, San Diego, CA, USA) provided online detection of NO and NO 2 concentrations. The batch reactor consisted of a Pyrex glass cylinder of volume of 25 L; this is described elsewhere [39]. The tile (2 Â 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/m 2 . The inlet gas consisted of NO 2 (0.6% in N 2 ) mixed with air of relative humidity (RH) 40%, which allowed equilibration between NO and NO 2 to be reached quickly. The NO 2 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 Â 60 cm) [40]. 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-tocenter distance, which was designed to irradiate the sample surface with the same 20 W/m 2 light intensity, were used.

Results and discussion
3.1. Characterization

Field emission gun scanning electron microscopy
The FEG-SEM images shown in Figure 2 reveal a relatively widespread, homogeneous, and thick deposition of TiO 2 on the digital inkjet-printed tile surface in comparison to the jetsprayed tile surface, which shows that voids are present. The EDS data confirmed a greater areal extent of coverage by TiO 2 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 TiO 2 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 TiO 2 , the extent of areal coverage of the tile, and exposed surface area.

Laser Raman microspectroscopy
In the optical micrographs taken by Raman, shown in Figure 3, the lighter rougher regions consist of TiO 2 and the darker regions are the exposed porcelain grès substrates. These images confirm that the TiO 2 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 jetsprayed coatings are significantly different. The intensities of the peaks and the slopes of the baselines indicate the TiO 2 content of the coating and the glass content, respectively, for each sample. That is, lower peak intensities indicate a low amount of TiO 2 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 TiO 2, whereas the low peak intensities and sloped baseline for the jet-sprayed coating confirm the low areal distribution of TiO 2 . Since laser Raman microspectroscopy does not analyze amorphous materials accurately, the peak at 510 cm À1 is attributed to albite feldspar and the peak at 610 cm À1 to α-quartz. The two other small but relatively sharp peaks at 439 and 356 cm À1 could not be identified. Titanium Dioxide -Material for a Sustainable Environment 268 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 [37], this work for the deposition of this TiO 2 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 decom-poses~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.

VOC 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 TiO 2 in the digital inkjet-printed coating.  Titanium Dioxide -Material for a Sustainable Environment 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 NO x , respectively. It is clear that the maximal levels of decomposition were attained relatively quickly and they were maintained such that no deactivation was observed.

NO x photodegradation
The larger dimensions (60 Â 60 cm) of the tiles and the lower NO x concentration used during the testing under flowing conditions provide a better simulation of real environmental conditions [40]. It may be noted that this digital inkjet-printing technology is capable of coating dimensions as large as 150 by 300 cm. Table 1 summarizes the results of the antibacterial testing, which reveals that both types of coatings were highly effective in destroying E. coli (gram-negative), which is part of the bacterial flora of human and animal intestines and hence a suitable indicator of potential contamination of drinking water and food. These data show that UV irradiation alone is capable of destroying~45-72% of the bacteria while both photocatalytic coatings destroy essentially all of the bacteria. After photocatalysis (R L ), the digital inkjet-printed and jetsprayed coatings are essentially equivalent. However, with photocatalysis (ΔR), the digital inkjet-printed coating clearly outperforms the jet-sprayed coating. Hence, these data are in basic agreement with those for the photodegradation of ethanol and toluene. However, they must be interpreted in light of the different surface topographies, where Figures 2 and 3 show that the jet-sprayed coating provides less homogeneous coverage and hence presents a more uneven surface that is likely to be more amenable to cell attachment and proliferation [41]. A second factor is the sizes of the species that were examined, where Table 2 [42,43] contextualizes the view that the photocatalytic performance is correlated through size similarities between the photocatalyst and the species [42].

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 [44]. LCA normally is applied to early-stage plants in order to investigate environmental hot spots arising from new technologies [45] or to establish industrial-scale processes for the comparison of divergent designs and optimization of environmental profiles [46]. 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 gate-to-gate boundaries using a functional unit of 1 m 2 of tiles produced in Modena, Italy. SimaPro (Version 8.3.0.0) extracted the secondary data from the Ecoinvent 3 database. ILCD 2011 Midpoint + (Version 1.08) and IPCC 2013 GWP 100a methods were used to calculate the impacts, which are given in terms of micro-eco-points (μPt). An eco-point is defined as one thousandth of the total environmental impact caused by a statistical European citizen per year [47]. The LCA was done in accordance with the ISO/TC 207/SC 5 Life Cycle Assessment methodologies for the principles and framework as well as the requirements and guidelines. For the inventory phase, these two processes were divided into substeps.
In order to highlight the differences between the two production processes, LCA comparison calculations were performed using the ILCD 2011 Midpoint + method, weighting the results for the 16 impact categories that were considered. Figure 6 shows the results graphically and Table 3 lists them numerically.

Species Size (nm) Reference
Ethanol 0.437 [43] Toluene 0.564 [43] E. coli 1000 Â 3000 * [42] Viruses 10-300 [42] Bacteria 500-5000 [42] Fungi 5000-15,000 [42] * Rod-like shape.   Ionizing radiation, ecosystems (interim) 0.00 0.00 0.00 Table 3. Numerical data for impact (μPt) assessments of digital inkjet-printed and jet-sprayed coatings. 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 USEtox calculations and the third category also has been examined using IPCC 110a calculations. These analyses reveal that their impacts derive almost entirely from the energy required by the production processes. This explains why jet spraying projects very high impact values since this process also is relatively energy-intensive. It is notable that the impacts of climate change and mineral, fossil and resource depletion reveal differences of >50% between the two production methods. Again, these significant differences are attributed to energy requirements. Three other categories (human toxicity, cancer effects, acidification, and particulate matter) also show nearly the same differential of~50%. The single-score analysis, which is the average of all values in the 16 categories, shows that digital inkjet printing is~46% lower than jet spraying.
Further, the calculation of the CO 2 equivalents, carried out by the IPCC 100a method, demonstrates that jet spraying has a greater impact than digital inkjet printing by a difference of 3.42 kg CO 2 versus 1.63 kg CO 2 . A final point of difference is that the application of the ILCD method reveals that NO x production demonstrates the same trend, with 2.10 g NO x from jet spraying and 1.65 g NO x from digital inkjet printing.
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.

Conclusions
This work reports design, synthesis, and characterization of photocatalytic TiO 2 -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 TiO 2 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.
Titanium Dioxide -Material for a Sustainable Environment 274