3.1. Characterization

. The

#### 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 jetsprayed 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.

Figure 2. FEG-SEM micrographs digital inkjet-printed coating at (A) low magnification and (B) high magnification and jet-sprayed coating at (C) low magnification and (D) high magnification.

#### 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 jetsprayed 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<sup>1</sup> is attributed to albite feldspar and the peak at 610 cm<sup>1</sup> to α-quartz. The two other small but relatively sharp peaks at 439 and 356 cm<sup>1</sup> 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 [37], 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

Figure 4. Laser Raman microspectra of (A) digital inkjet-printed coating and (B) jet-sprayed coating.

Figure 3. Optical micrographs of (A) digital inkjet-printed coating and (B) jet-sprayed coating.

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

http://dx.doi.org/10.5772/intechopen.72790

269

Figure 3. Optical micrographs of (A) digital inkjet-printed coating and (B) jet-sprayed coating.

Figure 4. Laser Raman microspectra of (A) digital inkjet-printed coating and (B) jet-sprayed coating.

#### 3.2. Photocatalytic testing

3.1.2. Laser Raman microspectroscopy

268 Titanium Dioxide - Material for a Sustainable Environment

jet-sprayed coating at (C) low magnification and (D) high magnification.

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 jetsprayed 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<sup>1</sup> is attributed to albite feldspar and the peak at 610 cm<sup>1</sup> to α-quartz. The two other

Figure 2. FEG-SEM micrographs digital inkjet-printed coating at (A) low magnification and (B) high magnification and

small but relatively sharp peaks at 439 and 356 cm<sup>1</sup> could not be identified.

#### 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 [37], 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

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.

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

http://dx.doi.org/10.5772/intechopen.72790

271

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 [40]. It may be noted that this digital inkjet-printing technology is capable of coating

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 (RL), 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

Symbol Parameter Units Uncoated tile 1 Uncoated tile 2

Symbol Parameter Units Digital inkjet-printed tile Jet-sprayed tile

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

Table 1. Summary of the antibacterial testing results on sprayed and digital printed tiles.

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

dimensions as large as 150 by 300 cm.

Antibacterial activity of photocatalytic coatings using E. coli ATCC 8739

3.3. Antibacterial testing

Uncoated porcelain grès tile controls

TiO2-coated porcelain grès tiles

Figure 5. Photodegradation over time by digital inkjet-printed and jet-sprayed coatings of (A) ethanol, (B) toluene, (C) static NOx, and (D) flowing NOx.

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 [40]. It may be noted that this digital inkjet-printing technology is capable of coating dimensions as large as 150 by 300 cm.
