Environmental and Medical Applications of Tio2 Photocatalysts and Boron-Doped Diamond Electrodes

Received: May 15, 2014 Electrochemistry Accepted: July 8, 2014 Published: September 5, 2014 The Electrochemical Society of Japan http://dx.doi.org/10.5796/electrochemistry.82.720 JOI:DN/JST.JSTAGE/electrochemistry/82.720 Comprehensive Paper (Invited Paper) Electrochemistry, 82(9), 720–725 (2014) Young Researcher Award of The Electrochemical Society of Japan (Sano Award)

Environmental and Medical Applications of TiO2 Photocatalysts and Boron-doped Diamond Electrodes Tsuyoshi OCHIAIa,b,* a Kanagawa Academy of Science and Technology, KSP East 407, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan b Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan * Corresponding author: [email protected]

ABSTRACT This paper summarized the author’s recent studies about effective design of the systems for environmental and dental application of TiO2 photocatalysts and boron-doped diamond (BDD) electrodes. The titanium mesh impregnated photocatalyst, TMiP™ has been fabricated as a novel photocatalytic filter. The high mechanical flexibility of TMiP allows us to design any geometry of modules by combination of UV-sources and the other technologies. In this paper, a practical air-cleaner using the TMiP-plasma synergistic reactor was fabricated and installed in a real-scale smoking room in the office. The amounts of the compounds in tobacco smoke were almost totally removed by the air-cleaner. On the other hands, the author also reported the possibilities of wastewater treatments by electrolysis with BDD electrodes. Based on these studies, a novel pinpoint ozone-water production unit for dental treatment using of BDD microelectrodes was developed. The unit showed almost the same sterilization ability as conventional 20 ppm of aqueous sodium hypochlorite treatment in in vitro assessment in the root-canal of bovine teeth. These results present several key solutions for practical applications of TiO2 photocatalysis and BDD electrolysis. © The Electrochemical Society of Japan, All rights reserved.

Keywords : TiO2 Photocatalysts, Boron-doped Diamond Electrodes, Environmental Puriﬁcation, Dental Treatment

1. Introduction

TiO2 photocatalysts can decompose organics, contaminants, and bacteria into the less harmful CO2 and H2O with its strong oxidation ability under UV light irradiation.1–4 Recently, application of photocatalysts for environmental puriﬁcation has received growing attention. This trend can be observed in the market growth of industries related to photocatalysis (Fig. 1). Under these circum- stance, and by comprehensive reviewing of the research, the author summarized several key requirements for an effective photocatalytic environmental puriﬁcation as follows:5

i. Catalyst immobilization strategy for a cost-effective solid- liquid separation ii. Integrated or coupling system for enhanced photo-mineraliza- Figure 1. (Color online) Market growth of industries related to tion or photo-disinfection kinetics photocatalysis (Source: Photocatalysis Industry Association of Japan). iii. Effective design of photocatalytic reactor system

Especially effective design of photocatalytic reactor system is contaminants and waterborne pathogens efficiently.6,7 Therefore, the most important. Although the operation conditions (pH, TiO2 photocatalysts and BDD electrodes have a great advantage for temperature, and gas or water quality) also affect the efficiency, environmental applications and disinfections. reactive surface area and mass transfer in the system are more Based on these backgrounds, this paper summarizes recent important in many cases. Therefore, the reactor design for effective studies for effective design of the systems for environmental and photocatalytic environmental purification can be classified into two dental application of TiO2 photocatalysts and BDD electrodes. main strategies: (1) enlargement of reactive surface area and (2) improvement of mass transfer. 2. Fabrication and Application of TiO2-coated Ti-mesh as an On the other hands, electrolysis with boron-doped diamond Effective Photocatalytic Filter (BDD) electrodes can also decompose organics. The wide potential window of BDD electrodes makes it possible to generate various In order to meet the above-mentioned requirements for an highly active oxidants such as ozone which can oxidize aqueous effective photocatalytic environmental purification, the titanium

720 Electrochemistry, 82(9), 720–725 (2014)

Figure 2. (Color online) The method for fabrication of titanium-mesh sheet with highly ordered three-dimensional structure modiﬁed with 8 TiO2 nanoparticles, TMiP·.

Figure 3. (Color online) Applications of TMiP for environmental purification.8,12,13,15 Reprinted from Chemical Engineering Journal, 209, T. Ochiai, Y. Hayashi, M. Ito, K. Nakata, T. Murakami, Y. Morito and A. Fujishima, An effective method for a separation of smoking area by using novel photocatalysis-plasma synergistic air-cleaner, 313–317, Copyright (2012), and Chemical Engineering Journal, 218, T. Ochiai, K. Masuko, S. Tago, R. Nakano, Y. Niitsu, G. Kobayashi, K. Horio, K. Nakata, T. Murakami, M. Hara, Y. Nojima, M. Kurano, I. Serizawa, T. Suzuki, M. Ikekita, Y. Morito and A. Fujishima, Development of a hybrid environmental purification unit by using of excimer VUV lamps 16 with TiO2 coated titanium mesh filter, 327–332, Copyright (2013), with permission from Elsevier. mesh impregnated photocatalyst, TMiP· has been fabricated Now various environmental purification units by using TMiP (Fig. 2).8 We have found many advantages of TMiP and its have been fabricated for application to various fields (Fig. 3).8,10–16 usefulness for air or water purification. Because of the presence of Among them, the author proposes a practical air-cleaner using a specific thickness of the TiO2 layer (about several hundred nm) photocatalysis-plasma synergistic reactor in the next section. fabricated by anodizing, the TiO2 nanoparticles were successfully sintered onto its surface without any binder.9 This advantage is 3. The Air-purification Ability of the Photocatalysis-plasma similar to the usual photocatalytic filter by using of ceramic foams, Synergistic Air-cleaner in the Smoking Room17 but TMiP does not break as easily as ceramic foams. Moreover, the high mechanical flexibility of Ti-mesh structure allows us to Figure 4a shows a schematic illustration of a PACT-TMiP manipulate the TMiP. Therefore, any geometry of modules for synergistic reactor. The reactor consists of two essential techno- environmental purification could be designed by combination of logies: plasma assisted catalytic technology (PACT) reactor and the UV-sources and the other technologies such as plasma treatment. TMiP. Basic design and fabrication method of PACT reactor were

721 Electrochemistry, 82(9), 720–725 (2014) described previously.18 A piece of corrugated TMiP was introduced when AC voltages applied (10 kV, peak-to-peak, 25 kHz). Air was in the gap of PACT reactor. The atmospheric dielectric barrier blown through the gap while maintaining a high level of surface discharge between the electrodes of the reactor produces air plasma contact with TMiP and plasma. Figure 4b shows a schematic illustration of a practical air-cleaner which consists of the PACT- fi fi (a) (b) TMiP reactor, a high ef ciency particulate air (HEPA) lter, an airr ozone-cut filter, an activated carbon filter, and a fan. When the fan is inlet turned on, air currents are generated inside the casing from the air electrode HEPA fi dielectric filter air inlet toward the air outlet passed through the lters and the PACT- outlet 3 air plasma PACT- TMiP reactor. Air flow rate was 1200 m /h. AC power TMiP TMiP A schematic illustration of the proposed test method for the supply ozone-e- 10 kV (p-p), air flow cut filterer air current practical air-cleaner is shown in Fig. 4c. The real-scale smoking 25 kHz activatedtedd room with the practical air-cleaner was fabricated in the non- carbonn filtefilterl smoking office room. Air currents are generated inside the office fan room from the air inlets toward the air outlets passed through the smoking room. In this condition, cigarettes were simultaneously (c) burned in sets of eight. Each set of cigarettes were sequentially burned in the smoking room. After 30 minutes from the burning of a first set of eight cigarettes, the amounts of gaseous compounds, total volatile organic compounds (TVOC), and total suspended partic- tobacco ulates (TSP) were measured at near the air inlet (point 1), center of smokek air out the smoking room (point 2), air outlet of the air-cleaner (point 3), (1200 m3/h) and center of the non-smoking room (point 4). An important point is that contaminated air would be treated by the air-cleaner just once, eight air-cleaner cigarettes i.e. this is a “one-pass system”. 0.55 m air in smoking room Figure 5 shows the amounts of the compounds in tobacco smoke non-smoking room 3.6 m (0.2 m/s) at near the air inlet (point 1 in Fig. 4c), center of the smoking room 12 m (point 2 in Fig. 4c), air outlet of the air-cleaner (point 3 in Fig. 4c), Figure 4. (Color online) Schematic illustrations of a plasma- and center of the non-smoking room (point 4 in Fig. 4c). It seems TMiP synergistic reactor (a), a practical air-purifier (b), and the that the amounts of all compounds increased by simultaneously test method in the real-scale smoking room for evaluation of air- burning of the cigarettes in the smoking room (point 2 in Fig. 4c). purification ability of the air-purifier (c). Reprinted from Chemical Especially the amount of nicotine dramatically increased. Interest- Engineering Journal, 209, T. Ochiai, Y. Hayashi, M. Ito, K. Nakata, ingly, the amounts of the compounds were significantly decreased at T. Murakami, Y. Morito and A. Fujishima, An effective method for a air outlet of the air-cleaner (point 3 in Fig. 4c). Finally, the amounts separation of smoking area by using novel photocatalysis-plasma of these compounds were kept at low-level in non-smoking room synergistic air-cleaner, 313–317, Copyright (2012), with permission (point 4 in Fig. 4c). This result indicates that the air-cleaner could from Elsevier.17 decompose and/or remove the compounds efficiently.

Figure 5. (Color online) The amounts of the compounds in tobacco smoke at each measuring points shown in Fig. 4c. Black: near the air inlet (Fig. 4c, point 1); blue: center of the smoking room (Fig. 4c, point 2); red: air outlet of the air cleaner (Fig. 4c, point 3); green: center of the non-smoking room (Fig. 4c, point 4). Reprinted from Chemical Engineering Journal, 209, T. Ochiai, Y. Hayashi, M. Ito, K. Nakata, T. Murakami, Y. Morito and A. Fujishima, An effective method for a separation of smoking area by using novel photocatalysis-plasma synergistic air-cleaner, 313–317, Copyright (2012), with permission from Elsevier.17

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purified water F F F F F F F R

F F F F F F F F Mn2+,Cl-, PFOA etc., H2O - - CO2, F MnO4 , HClO, etc. products Diamond and Related Materials 2011, 20, 64,

decomposition Chemistry Letters 2011, 40, 682 OH, H2O2, O3, etc. BDD (anode) Pt (cathode) H2O organics 100

electrolysis: photocatalysis: 10 -3 -4 k1 = 2.6 10 k1 = 1.0 10 [Lcm-2h-1] [Lcm-2h-1] COD (mg/L)

contaminated water 1 01234 time (h) Electrochemical flow cell for water purification Water Research 2010, 44, 904, (anode: BDD, cathode: Pt, gap: 1 cm, area: 77.4 cm2) Journal of Water and Health 2011, 9, 534.

Figure 6. (Color online) Schematic image of the flow cell reactor using BDD electrode and its applications for wastewater treatments. Reprinted from Water Research, 44, T. Ochiai, K. Nakata, T. Murakami, A. Fujishima, Y. Y. Yao, D. A. Tryk, Y. Kubota, Development of solar-driven electrochemical and photocatalytic water treatment system using a boron-doped diamond electrode and TiO2 photocatalyst, 904–910, Copyright (2010), with permission from Elsevier.24 Figure 7. (Color online) (a) A photograph of solar-driven electrochemical and photocatalytic water treatment system. (b) Schematic Besov et al. reported a similar effective acetone destruction illustration of the system. Reprinted from Water Research, 44, T. by the barrier discharge reactor in combination with photocatalyst Ochiai, K. Nakata, T. Murakami, A. Fujishima, Y. Y. Yao, D. A. 19 carrying support. They concluded that the synergistic effect could Tryk, Y. Kubota, Development of solar-driven electrochemical and be attributed to the following effects: (1) Oxidation or photo- photocatalytic water treatment system using a boron-doped diamond oxidation of the intermediate products over photocatalyst with ozone electrode and TiO2 photocatalyst, 904–910, Copyright (2010), with participation; (2) Increasing of adsorption of intermediate products permission from Elsevier.24 in barrier discharge over the photocatalyst surface. Taken together, present system which realize a better contact among the TMiP surface, the gas phase, and reactive species generated by air-plasma through a pre-filter and stored in a plastic tank. Then, the high-level is effective to have the benefit of plasma enhanced photocatalysis. wastewater was circulated through the electrolysis unit and At the same time, HEPA and activated carbon filters remove the converted to low-level wastewater. The low-level wastewater was compounds. In addition, ozone-cut filter using a MnO2–based circulated through the photocatalysis unit. Finally, the treated water catalyst could not only decompose excess ozone generated by air- was passed through the final-filter and the ion-exchange-filter units. plasma but also produce additional reactive species without injecting Three commercial photovoltaic cells were used to provide electric more energy. Jarrige and Vervisch tested a pulsed corona discharges power for the mechanical systems and the electrolysis. Energy can reactor combined with fixed-bed catalytic post-treatment using charge the batteries to provide electrical power during cloudy 20 a MnO2–based catalyst. They concluded that ozone-promoted periods or at night. A chemical oxygen demand value of cleaning of the catalyst seems to be a promising for air-purification. 106.1 mg dm¹3 for the river water sample decreased to less than Therefore, the arrangement of the filters and the reactor in the 1.0 mg dm¹3 by this system. Moreover, total coliform and standard present air-cleaner is one of the most effective designs for the “one- plate count values estimated by bactericidal tests were not detected pass” system for the separation of smoking area. in the treated water. These results indicate that the treated water is suitable for drinking. 4. Application of BDD Electrode for Wastewater Treatment 5. Pinpoint Ozone-water Production Unit by Using of BDD The author also reported the possibilities of wastewater treat- Microelectrodes for Dental Treatment25 ments by electrolysis with designed flow cell reactor using a BDD electrode (Fig. 6).21–24 A BDD-deposited Nb plate was used as the As mentioned above, the application of BDD electrodes is anode, and a Pt-deposited Ti plate was used as the cathode in the promising for electrolyzing water to produce ozone because of their reactor. Both electrodes had a geometric area of 77.4 cm2, and the superior chemical and dimensional stability, as well as their large electrode gap was 10 mm. The wastewater samples were circulated overpotential for the oxygen evolution reaction.26,27 Thus, electrol- through the reactor by a pump and electrolyzed under galvanostatic ysis with BDD electrodes can disinfect waterborne pathogens condition. Here the author proposes an electrolysis–photocatalysis efficiently.21 On the other hand, ozone has received growing sequential wastewater treatment system using the reactors and the attention as a useful tool for dental treatment because of its 24 TiO2 photocatalyst. Figure 7a shows an overview of the system, disruptive effect on cariogenic bacteria, resulting in elimination of which consists of a pre-filter, an electrolysis unit with the reactors, acidogenic bacteria.28–31 However, while laboratory studies suggest a photocatalysis unit with TiO2 photocatalyst-coated quartz wool, a promising potential of ozone in dentistry, this has not been fully a final-filter and an ion-exchange-filter, and photovoltaic cells. realised in clinical studies to date. Based on these backgrounds, we Figure 7b shows a schematic illustration of the water treatment developed novel pinpoint ozone-water production unit for dental process carried out by the system. High-level wastewater was passed treatment by using of BDD microelectrodes.25 Figure 8 shows a

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Figure 8. (Color online) SEM and schematic image of the pinpoint ozone-water production unit by using of the BDD microelectrode. Figure 9. (Color online) Survival rates of P. gingivalis at a function of treatment time.

SEM image of the unit which has been prepared with reference to teeth. In conclusion, the superiority of BDD microelectrodes over the electrolytic water ejecting apparatus reported by Kitaori et al.32 simple pinpoint ozone-water production unit, in terms of the larger The BDD microelectrodes were prepared by previously reported activities for ozone production and disinfection, was demonstrated. method which has already been established for applications in This research would be attractive to develop a practical unit for in vivo detection.33 The BDD thin film was deposited on the dental treatment. However, further investigations (cultivation, prepared tungsten wire (500 µm) using a microwave plasma assisted improvement of fabrication method, clinical study) are required to chemical vapor deposition system with the polycrystalline diamond bring it to practical stage. grain size being approximately 2 µm. According to the Raman spectrum (data not shown), boron-to-carbon ratio in the BDD 6. Conclusions microelectrode was estimated as almost 1% by comparison of previously reported data.34 A strip of an aluminum foil with an ion- This paper surveyed here hopefully underlines the fact that the μ exchange membrane (Nafion ) was spirally wound around the BDD closely-related TiO2 photocatalysis and BDD electrolysis are microelectrode which was used as an anode. When DC voltage was extremely versatile and can be used in numerous ways to both applied between the anode and the cathode in the water, electrolysis environmental purification functions and disinfection for dental proceeds as follows: treatment. Although numerous research and development have þ proposed, further technological breakthroughs are required for anode ðBDDÞ:3H2O ! O3 þ 6H þ 6e practical applications of TiO photocatalysis and BDD electrolysis. cathode ðAlÞ:6Hþ þ 6e ! 3H 2 2 This paper presented several key solutions to meet these require- The ozone concentration produced by the unit reached 6.4 mg/L ments. For examples, a novel photocatalytic filter based on titanium after 5 min of electrolysis in 0.3 mL of distilled water under mesh sheet and its environmental applications, a practical air-purifier potentiostatic conditions with 7.5 V of the terminal voltage of the by using of photocatalysis-plasma synergistic reactor, and a pinpoint unit. On the other hand, disinfection test was performed at the same ozone-water production unit by using of BDD microelectrodes for condition as above-mentioned electrolysis in 0.3 mL of bacterial dental treatment. In summary, we can expect that the continuous suspensions. Escherichia coli (E. coli) and Enterococcus faecalis improvements of the material property and the reactor design would (E. faecalis) were used as the test bacteria to assess the disinfection create a large number of effective systems. Finally, we feel that, efficiency of the unit. Bacterial densities of E. coli and E. faecalis at TiO2 photocatalysis and BDD electrolysis can realize a healthy and a function of electrolysis time were fitted with a pseudo-first-order comfortable living environment. kinetics given by the following equation: C = Co exp(¹k1t). Where Co is the initial bacterial density and k1 is the observed rate constant. Acknowledgments The values of the k1 were calculated by exponential fitting of the plot to 1.4 and 5.0 min¹1 for E. coli and E. faecalis, respectively. The work was supported in part by the Programs of Special These results suggest that the unit is useful for disinfection in Coordination Funds for Promoting Science and Technology of practical uses. MEXT (Ministry of Education, Culture, Sports, Science and In order to assess the usage of the unit for dental treatment in Technology), JSPS KAKENHI Grant Number 25871218, the biological circumstances, in vitro disinfection test was carried out as Nippon Sheet Glass Foundation for Materials Science and follows. Biofilms of Porphyromonas gingivalis (P. gingivalis) were Engineering, the Tokyo Ohka Foundation for the promotion of formed in root canal of bovine teeth. 0.05–0.1 mL of phosphate Science and Technology, and the Kurata Memorial Hitachi Science buffer saline (PBS) was poured into the root-canal. The unit was and Technology Foundation. inserted into the root-canal. After pre-determined time of electrolysis I’m grateful to Prof. Akira Fujishima (Tokyo University of under potentiostatic conditions with 7.5 V of the terminal voltage Science), Dr. Kazuo Hirota (GC Corporation), Dr. Yuji Hayashi of the unit, bacterial cell viability was checked by conventional (I’m PACT World Co. Ltd.), Dr. Yuko Morito (U-VIX Corporation) alamarBlue· assay. For comparison, usual 20 ppm of NaClO and many collaborators for the experiments, discussions, super- treatment was also performed. Survival rates of P. gingivalis at a visions, and the fruitful joint research. Lastly, I appreciate Ms. Junko function of treatment time were shown in Fig. 9. The unit showed Ochiai who gave me the fundamental lecture of dentistry, almost the same disinfection ability as 20 ppm of aqueous NaOCl suggestions for developing research ideas, and many helpful advices treatment in this in vitro assessment in the root-canal of bovine as a dental hygienist.

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