Fabrication of a Porous TiO 2-Coated Silica Glass Tube and Its Application for a Handy Water Purification Unit

1 Kanagawa Academy of Science and Technology, KSP building East 407, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan 2 Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 3Optical Communications R&D Laboratories, Sumitomo Electric Industries, Ltd., 1 Taya-cho, Sakae-ku, Yokohama 244-8588, Japan 4R&D General Planning Division, Sumitomo Electric Industries, Ltd., 1-1-3 Shimaya, Konohana-ku, Osaka 554-0024, Japan 5 Kanagawa Academy of Science and Technology, LiSE Lab., 3-25-13 Tonomachi, Kawasaki-ku, Kawasaki, Kanagawa 210-0821, Japan 6Department of Urology, Yokohama City University Graduate School of Medicine, 3-9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan


Introduction
In recent decades, the global human population growth has needed more water.However, there are plenty areas which still need more water purification technologies (especially a water disinfection unit) to use for area residents or industries [1].Various water treatment systems such as solar disinfection, chlorination, and filtration to reduce illness have been studied and realized [2][3][4][5][6][7].Among these technologies, TiO 2 photocatalysis has received growing attention [8,9].However, there is no report about a simple, handy, reusable, and inexpensive photocatalytic water purification unit yet.At the same time, there is also no report about handy photocatalytic unit for the removal of not only bacteria (several micrometers) but also viruses (several ten nanometers) because of the viruses' extremely small size.We have reported that TiO 2 photocatalysts can decompose refractory chemicals [10], gaseous contaminants [11], and waterborne pathogens [12] with their strong oxidation ability [13].Moreover, we also have reported various methods for the design and applications of TiO 2 photocatalyst to maximize its photocatalytic abilities [14][15][16][17].On the other hand, we have succeeded the simple fabrication of novel one-end sealed porous amorphous-silica (a-silica) tubes with large porosity by the outside vapor deposition (OVD) method [18,19].The porous tube is believed to be a good supporting material for gas and/or liquid separation.Based on these backgrounds, now we report a porous TiO 2 -coated a-silica glass tube and its application for a handy water purification unit.The units consist of the porous TiO 2 -coated a-silica glass tubes and small UV lamps were fabricated and evaluated for their biological purification activity by using both E. coli (typical bacteria size species) and Q phage (typical virus size species).

Fabrication of the Porous TiO 2 -Coated a-Silica Glass Tube.
Figure 1 shows the fabrication method of the porous TiO 2coated a-silica glass tube by the OVD method.Fine a-silica particles synthesized by hydrolysis of SiCl 4 in an oxygenhydrogen flame burner were deposited on a rotating Si 3 N 4 rod target with a diameter of 6 mm (Figure 1(a)).After the deposition of a-silica, TiO 2 particles synthesized by hydrolysis of TiCl 4 in the flame burner were deposited onto the porous a-silica glass layer (Figure 1(b)).After the deposition, a one-end sealed porous tube was obtained by pulling out the rod target from the soot body (Figure 1(c)).The external diameter and the length of the obtained porous tube were 8.5 mm and 300 mm, respectively.The morphology of the porous structure was observed with an FE-SEM (S-4800, Hitachi, Tokyo).Samples for cross-section observation were prepared by embedding in resin and then polishing with a cross-section polisher (SM-09010, JEOL, Tokyo).Pore size distribution was measured using a mercury porosimeter (AutoPore III 9420, Micromeritics Instrument, CA).For the structural characterization of the films, Raman spectroscopy excited by 532 nm Nd:YAG laser (LabRAM HR-800, HORIBA JOVIN YVON, Longjumeau, France) was used.For comparison, the porous a-silica glass tube without TiO 2 layer was also fabricated.Japan).E. coli and Q were propagated and assayed by previously described methods [12,20].The aqueous suspensions of E. coli or Q were used as the biologically contaminated water models.In this study, the numbers of E. coli and Q in the suspension were approximately 10 6 colony-forming units per mL (CFU/mL) and 10 9 plaque-forming units per mL (PFU/mL), respectively.Figure 2 shows a handy water purification unit consisting of the porous tube and a pair of super-small-sized cold cathode UV-C lamps (2.5 mW/cm 2 @ 254 nm,  6 mm × 30 mm, Sankyo Denki Co., Ltd., Kanagawa, Japan).The UV intensity at 254 nm at the surface of the porous tube was measured by a UV-radiometer UVR-300 with a sensor head UD-250 (Topcon Corporation, Japan).In a typical run (TiO 2 (+), UV(+)), 4 mL of the Q suspension was poured into the porous TiO 2 -coated a-silica glass tube and was filtered by applying pressure at the filtration rate of 0.4 mL/min for 1 min under UV-C irradiation.Filtered suspension was collected to test tube and assayed by previously described methods [12,20] to analyze the viability of Q.The effective filtration area of the porous tube was approximately 27 cm 2 (the effective filtration length of the porous tube

Results and Discussion
3.1.Characterization.SEM images of the surface and the cross-section of the porous TiO 2 -coated a-silica glass tube are shown in Figures 3(a) and 3(b).The open pore structure was found to be constructed by the sintering process.Figure 3(c) shows a high-magnification secondary electron image (SEI) of cross-section of stacked TiO 2 layers over a-silica layers.White, gray, and black areas in Figure 3(c) represent TiO 2 particles, a-silica particles, and the resin intruded into the pore, respectively.Both TiO 2 and a-silica layers were porous and fit each other on the border.TiO 2 layers' average of the grains cross section area seemed to be smaller than that of a-silica.Stacked TiO 2 layers thickness on a-silica layers was approximately 2 m.This thickness is enough to impart photocatalytic property onto the surface.An average porosity and an average bulk density of the porous tubes were 0.62 and 0.84 g/cm 3 , respectively.We have found that porous tubes with different apparent porosities can be prepared by changing deposition temperature and the average pore diameter slightly and gradually decreased from 0.40 to 0.35 m with decreasing the porosity from 0.64 to 0.39 [19].Based on this insight, the pore diameter of the porous tubes in this research can be estimated to be 0.40 m.
The Raman spectrum of the porous TiO 2 -coated a-silica glass tube is shown in Figure 4.The Raman bands at 138, 235, 446, and 607 cm −1 almost agree with the spectrum of the rutile phase [21].By contrast, anatase phase shows 147, 198, 398, 515, and 640 cm −1 [21].Oh and Ishigaki synthesized TiO 2 nanopowders with various anatase/rutile ratio using in-flight oxidation of TiN powder in a radio frequency thermal plasma reactor and characterized its microstructure by X-ray diffraction and Raman spectroscopy [22].They concluded that O 2 rapidly diffused from the oxidized shell into the TiN core; simultaneously, the evaporation of the particles was accelerated.The vaporized species rapidly solidified into anatase or rutile depending on the ambient O 2 concentration.In this research, Raman spectrum of the porous TiO 2coated a-silica glass tube is similar to the spectrum of the TiO 2 nanopowders with 60 wt% of anatase content prepared by in-flight oxidation of TiN powder under relatively low O 2 concentration (3-4 vol%).Therefore, the Raman spectroscopy indicates that TiO 2 layers in the porous TiO 2 -coated a-silica glass tube are consisted of both rutile and anatase crystals.Repeating heat process with a burner in OVD method seemed to lead some amount of rutile crystals.as 6.6 × 10 6 CFU/mL.There was no E. coli colony on agar plate which incubated filtered E. coli solution drops with a porous a-silica tube or a porous TiO 2 covered a-silica tube without UV-C lamps.Controlling the pressure with a pump makes filtering rate faster without E. coli leakage from the porous tubes.Then, it can be said that the range of pore size 0.40 m of the porous tubes in this research is big enough to let water pass through it and small enough to remove bacteria.However, this pore size of the porous tubes is larger than the viruses' size (viruses are 100 times smaller than bacteria).Thus, in contrast to the physical method of using the porous tubes to retain bacteria, removal of viruses would require a more chemical approach such as electrostatic charge [23].In order to satisfy this requirement, photocatalytic Q removal test was carried out.

Result of Waterborne
Figure 5 shows the result of Q removal test.The Q concentration in the prepared Q solution was determined as 1.6 × 10 9 PFU/mL.Filtering Q solutions by the porous asilica tube (Ti(−), UV(−)) and TiO 2 covered a-silica tube (Ti(+), UV(−)) reduced Q by 97.9% and 97.3%, respectively.The result indicates that filtering Q solutions reduces Q concentration; however, there are still plenty amounts of Q (3.3 × 10 7 and 4.4 × 10 7 PFU/mL, resp.).Nevertheless, there was no much difference between the two filtering features against the Q solution without UV-C lamps.On the other hand, with UV-C lamps turned on, filtering Q solution by the porous a-silica tube (Ti(−), UV(+)) and TiO 2 covered a-silica tube (Ti(+), UV(+)) significantly reduced Q by 99.99973 (5.6-log reduction) and 99.99994% (6.2-log reduction).The result showed that UV-C lamps removed Q effectively while filtering and dropping the Q solution between the lamps.The U.S. Environmental Protection Agency's microbiological reduction requirements for bacteria and viruses are 6-log and 4-log reduction, respectively.Therefore, it is found that UV-C lamps greatly improve the device ability to remove/inactivate Q by inducing of the photocatalysis.
It is well known that anatase TiO 2 exerts higher photocatalytic activity than the rutile one in many reactions [24][25][26].However, there have been a few reports which deal with biocidal activities of TiO 2 with different crystalline structures.Sato and Taya reported that the biocidal activity of TiO 2 particles against bacteriophage MS2 phage was maximized at 70 wt% of anatase ratio in mixture of TiO 2 particles as compared with the activity at 0 and 100 wt% [27].They suggested that the contact between both types of TiO 2 in aggregations caused the enhancement of the quantum yield of TiO 2 suspension and thereby the reactive oxygen species generation, which leads to the encouragement of biocidal activity of the TiO 2 particles.Therefore, optimization of anatase ratio from 60 to 70 wt% in the TiO 2 layer of the tube by controlling the OVD condition is effective for the increased photocatalytic biocidal activity.

Conclusions
A handy water purification unit including a porous TiO 2 -coated a-silica glass tube prepared by the OVD method was investigated.The porous TiO 2 layers were successfully deposited onto porous a-silica glass tube surface with 2 m of thickness.An average porosity and an average bulk density of the porous tubes were 0.62 and 0.84 g/cm 3 , respectively.The pore diameter of the porous tubes was estimated to be 0.40 m.This size was big enough to let water pass through the tubes and small enough to retain E. coli.Raman spectrum of the porous TiO 2 -coated a-silica glass tube indicated that the anatase content of the TiO 2 layers of the tube was estimated to be approximately 60 wt%.The photocatalytic activity of the porous TiO 2 -coated a-silica glass tube with UV-C lamps showed the highest Q reduction efficiency (6.2-log reduction) compared with the filtration by using the porous a-silica glass tube alone (1.7-log reduction), the porous TiO 2coated a-silica glass tube alone (1.7-log reduction), and the porous a-silica glass tube without TiO 2 layer with UV-C lamps (5.6-log reduction).Therefore, a porous TiO 2 -coated a-silica glass tube has great potential as a handy water purification unit.

2 International 4 +Figure 1 :
Figure 1: Schematic diagram of the fabrication method for porous TiO 2 -coated a-silica glass tube by the OVD method.

Figure 2 :
Figure 2: Schematic illustration of a handy water purification unit consisted of the porous tube and a pair of UV-C lamps.

Figure 3 :
Figure 3: SEM images of the surface (a) and the cross-section (b) of the porous TiO 2 -coated a-silica glass tube and a high-magnification SEI of cross-section of stacked TiO 2 layers over a-silica layers (c).

Figure 4 :
Figure 4: Raman spectrum of the surface of the porous TiO 2 -coated a-silica glass tube.