Recent Aspects of Photocatalytic Technologies for Solar Fuels, Self-Cleaning, and Environmental Cleanup by Akira Fujishima, Kazuya Nakata, Tsuyoshi Ochiai, A. Manivannan, and Donald A. Tryk

ncreasingly severe climatic, energy, and environmental problems warrant the Ineed to continue to develop greenhouse gas-mitigating, energy-producing, energy- saving, environmentally-beneficial technologies. The closely related fields of semiconductor photoelectrochemistry and semiconductor , largely involving , have blossomed during the past forty years since the publication of our initial work on photoelectrochemical water splitting.1 This highly cited paper has provided a foundation for steadily increasing numbers of works on a broad range of topics, including applications such as solar light-induced 2 water splitting (hydrogen production) , CO2 reduction to produce carbonaceous solar fuels,3 water purification, decontamination, and disinfection, as well as new materials and fundamental aspects. Our recent review summarizes a number of photocatalytic applications, including self- cleaning surfaces, anti-fogging surfaces, heat dissipation, corrosion prevention, and visible light-sensitive materials.4 Figure 1 illustrates such a broad range of applications. The topics of “designer” titanium dioxide materials with various levels of Fig. 1. Overview of photocatalytic applications. dimensionality5,6 and photocatalysis for environmental applications7 have also been investigated thoroughly in our laboratory. These processes can be made to occur 3 Dimensionality of Titanium Our early work on photocatalytic and at macroscopic electrodes and thus have a photoelectrochemical8,9 CO reduction is Dioxide Structures 2 strong electrochemical aspect. For example, now continuing at present, in the laboratory 10 in Case 1, the electrons are drawn off to of our colleague, Akihiko Kudo, and in reduce water to hydrogen at a platinum The development of new materials is our own laboratory, both at the electrode, and the holes react with water at strongly desired to obtain higher performance University of Science, as well as by a the titanium dioxide photoanode to generate with respect to photocatalytic properties, number of other groups around the world. 1 molecular oxygen. However, the large band and to find new applications for2 TiO At the outset, we would like to gap (3.0 to 3.2 eV) of unmodified titanium photocatalysis.5 Recently, the preparation emphasize the essential unifying principles dioxide made it too inefficient to produce of TiO2 nanostructures and microstructures of photocatalysis before presenting some hydrogen from sunlight due to the small with interesting morphologies and properties specific examples. After energetic photons number of sufficiently energetic photons. has attracted much attention, including are absorbed in the semiconductor, There have been several efforts to modify spheres, nanorods, fibers, tubes, sheets, and electrons and holes are generated. The TiO2 so that visible light could possibly be interconnected architectures. Many factors, mobile electrons are free to move around, used. In contrast, photocatalytic cleaning including size, specific surface area, pore reaching the surface of the solid, and then can operate with fewer energetic photons, volume, pore structure, crystalline phase, and react with water or oxygen. Similarly, the but it is still desirable to develop visible the exposed surface facets, have important mobile, highly energetic holes reach the light-sensitive materials, which could be effects on the photocatalytic performance; surface and oxidize water and/or organic used indoors. thus the improvement of performance by matter. Thus, there are four simple cases for In this brief overview of some recent adjusting these factors is still a major focus reactions involving the electrons and holes: variations on the theme of photocatalysis, of photocatalysis research. Another factor (Case 1) water-water, (Case 2) oxygen- we describe the design of various that significantly affects the photocatalytic water, (Case 3) water-organic, and (Case forms of titanium dioxide with specific performance of TiO2 materials is the 4) oxygen-organic. Cases 3, 4, and 5 are all dimensionalities, such as zero-dimensional structural dimensionality. Therefore, involved with photocatalytic decomposition (0D), 1D, 2D, and 3D; and second, the materials with appropriate dimensionalities of organics, whereas Case 1 is likely to be development of specific environmentally- enable us to take full advantage of the involved in the photoinduced hydrophilic beneficial applications. unique properties of TiO (Fig. 2). effect (PIHE), as well as photocatalytic 2 water splitting. (continued on next page)

The Electrochemical Society Interface • Summer 2013 51 Fujishima, Nakata, Ochiai, et al. (continued from previous page) process. The crystal morphology depends on very little of this dye remained on the self- conditions such as the amount of PVP used cleaning , indicating high anti-fouling and the hydrothermal method. properties. This is in contrast to the case of

Zero-dimensional nanostructured or Electrospinning can also tailor TiO2 glass coated with TiO2 nanoparticles, which microstructured TiO2 spheres are the most materials with 1D structures, such as fibers retained a significant amount of methylene widely studied and used TiO2-related and tubes, having unique properties and blue. These results are due to the fact that 11,12 materials. Such structures usually advantages for photocatalytic reactions. the self-cleaning glass prepared using TiO2 possess high specific surface area, high pore A co-jetting method has been reported nanosheets has a very smooth surface, volume and pore size, high activity, and low for making hollow TiO2 fibers. By using a which reduces the attachment of methylene density. All of these properties increase the multi-channel nozzle, multi-channel hollow blue molecules, whereas glass prepared accessible surface area and mass transfer TiO2 fibers with zero to three channels can using TiO2 particles has a relatively rough for organic pollutant adsorption, resulting be obtained (Fig. 3b).13 The photocatalytic surface, which promotes the attachment in better photocatalytic performance, performance improves as the number of of methylene blue molecules. Materials since photocatalytic reactions are based on channels increases, which leads to the with this combination of low adhesion chemical reactions on surfaces. In 0D TiO2 specific surface area increase with increasing and photocatalytic properties are good materials, i.e., spheres, the introduction numbers of channels, and therefore multiple candidates for new self-cleaning coatings. of both a hierarchical structure and high- reflections of incident light can be expected. Among the various morphological energy (001) facets offer high photocatalytic The idea of 1D materials, which can structures, 3D interconnected arrangements activity. Recently, we reported mesoporous decrease recombination of electrons and have the potential for producing a new class core–shell spheres composed of small TiO2 holes, has been combined with that of of materials and applications. They are nanocrystals with exposed step-like (001) visible light absorption.14 Nitrogen doping important for practical applications, because and (010) facets, prepared by a method that was found to induce visible-light-responsive 3D hierarchical structures with pores combines electrospraying and hydrothermal photocatalytic activity but lowered the UV- have potentially large surface-to-volume treatments.11 light-responsive photocatalytic activity. ratios, which are an advantage for effective Electrospraying is a technique using Visible-light photocatalytic activity was diffusion pathways for guest species, such a high-voltage electric field to obtain concluded to originate from N 2p levels as organic pollutants, into the framework; microsize or nanosize spheres. In this case, near the valence band. This work on titania this should support efficient purification, 15 core–shell TiO2 spheres are produced by nanobelts has recently been extended. separation, and storage. Furthermore, an electrospraying titanium alkoxide with As a 2D material the nanosheet is a interconnected structure is potentially polyvinylpyrrolidone (PVP)(Fig. 3a). nanosized flake with a flat surface and superior from a practical point of view. For Further, hydrothermal treatment provides high aspect ratio with an extremely small example, at present, almost all photocatalytic a hierarchical structure with tunable pore thickness (1-10 nm). The platelet size could purifiers utilize 2TiO particles coated on size, pore volume, specific surface area, range from submicrometers to several porous structured ceramics; however, TiO2 and percentage of specified crystal facets, tens of micrometers. We reported the particles may be stripped from the ceramic, because PVP forms a network, and the production of a self-cleaning glass using leading to the release of small dust particles 16 amorphous TiO2 particles fill the pores TiO2 nanosheets. The TiO2 nanosheets and degradation of the photocatalytic within the network during electrospraying have low turbidity, strong adhesion to glass, properties. To solve such problems, self- of the core–shell TiO2 spheres. Later, PVP is and high hardness (Fig. 4). Furthermore, supported porous TiO2 frameworks, i.e., removed from the TiO2 spheres (Fig. 3a), and after dip-coating the self-cleaning glass monolithic materials, are suitable candidates TiO2 is crystallized during the hydrothermal in a solution containing methylene blue, for water remediation. Recently, we prepared TiO2 monoliths and evaluated their photocatalytic performance

using methylene blue decolorization. TiO2 monoliths have a porous, interconnected structure, which is advantageous for photocatalytic decolorization. Furthermore,

TiO2 monoliths have a porous structure after calcination at high temperature with sufficiently high hardness. The combination of porous structure and hardness provide Interconnected benefits for water remediation. In actuality, architecture TiO2 monoliths showed good performance Sheet for the photocatalytic decolorization of methylene blue. TiO2 monoliths should Tube realize both hardness and high photocatalytic performance for water remediation.

The dimensionality of the TiO2 Fiber structure affects various properties such as 3D photocatalytic performance, specific surface area, adsorption properties, reflectance, 2D High carrier mobility adhesion, and carrier transportation. TiO2 is Smooth surface the most widely studied photocatalyst, and Sphere High adhesion 1D it is used in numerous applications because Light scattering of its compatibility with modern technology. Nonwoven mat Novel forms of materials can help to develop advanced TiO photocatalysts, which will 0D 2 High specific surface area improve our lives in terms of advanced energy production and environmental protection. Fig. 2. Schematic illustration of structural dimensionality of materials with expected properties. (continued on page 54)

52 The Electrochemical Society Interface • Summer 2013 (a)

(b)

ig F . 3. (a) FE-SEM image of zero-dimensional core-shell TiO2 spheres and SEM image of cut TiO2 spheres; (b) SEM images of multi-channel TiO2 fibers.

The Electrochemical Society Interface • Summer 2013 53 Fujishima, Nakata, Ochiai, et al. (continued from page 52) to fabricate TMiPs is shown in Fig. 5. We The continuous improvement of the have found several advantages of TMiPs material properties and the reactor design and their usefulness for environmental would greatly aid the creation of effective Environmental Purification purification. The high mechanical flexibility environmental purification systems. Thus, of TMiPs allows us to design any geometry it is becoming apparent that photocatalysis Environmental purification, especially of modules for environmental purification has been realized as an important field of indoor air and polluted water, is highly through the combination of UV-sources and of study for a healthy, comfortable living important for human life. The market the other technologies. Our contribution in environment. for photocatalytic technology in photocatalytic environmental purification has experienced encouraging growth, with TMiPs is summarized in a recently Photocatalytic CO Reduction approximately doubling during the last published review article.7 Now, we are 2 ten years, reaching on the order 1 billion focusing on the fabrication of practical Several years after our initial work USD (these data represent the sales environmental purifiers. on solar hydrogen production in the volume for companies that are members One of the recent applications of TMiPs late 1960s and early 1970s, the topic of of the Photocatalysis Industry Association is a photocatalytic-plasma synergistic photocatalytic CO reduction was taken 19 2 of Japan). Recently, the sales volume air-purifier. The purifier consists of two up, an idea that was essentially unknown of “cleanup” applications has increased essential technologies, a plasma-assisted at that time.3 Subsequently, the idea caught greatly. This trend indicates the increasing catalytic technology (PACT) reactor, and on, and several groups became interested numbers of people who are interested the TMiP, to realize the plasma-enhanced (see Ref. 10). We also experimented with in environmental issues. For example, photocatalysis. A practical test for the a photoelectrochemical approach, making the swine influenza outbreak of 2009 deodorization of tobacco smoke was use of high-pressure CO2 at a p-type indium raised serious fears of global pandemic proposed for a life-sized smoking room phosphide photocathode in methanol.8,9 and suddenly increased the sales volume (2.7 × 3.6 × 2.5 m), which included the Recently, the photocatalytic approach has of photocatalytic air-purifiers. The key analysis of selected gaseous pyrolytic and enjoyed a renewed popularity in Japan. One scientific and technical requirements for oxidative decomposition products by using very recent example is the work of Kudo and effective photocatalytic environmental a single-pass system from the smoking room coworkers, who have examined ALa4Ti4O15 purification are (1) catalyst immobilization to a non-smoking room. The amounts of all (A = Ca, Sr, and Ba) photocatalysts strategies, (2) integrated or coupled systems of the contaminants except acetaldehyde with layered perovskite structures.10 The for enhanced photo-oxidation, and (3) were significantly decreased at the air outlet photocatalytic approach has also become effective design of photocatalytic reactor of the air-purifier. Finally, the amounts of popular in other parts of the world, for 17 systems. these compounds were maintained at low example, at the University of Nevada, in To meet these requirements, we levels in the non-smoking room. This result collaborative work with one of the present have fabricated a Ti-mesh impregnated indicates that nearly all of the compounds authors (AM).20 Grimes and coworkers have ® photocatalyst, (TMiP ) by use of anodizing can be decomposed and/or removed by the recently published a review on solar fuel 18 and etching methods. The method used practical single-pass air-purifier. production.21

Fig. 4. Photographs of the samples of glass coated (right) or not coated (left) with niobia nanosheets after a self-cleaning test.

54 The Electrochemical Society Interface • Summer 2013 Fig. 5. Fabrication method of TMiP.18

Concluding Remarks Prize and the Japan Academy Prize and was multidisciplinary areas in materials science named an Honorable Citizen of Kawasaki for more than 28 years in academia. His The work surveyed here hopefully City. He also received the Imperial Invention research interests and expertise include underlines the fact that the closely-related Award and Kanagawa Culture Award in materials for energy conversion and storage photocatalytic and photoelectrochemical 2006. He was recognized as a Cultural involving photoelectrochemical cells, solid approaches are extremely versatile and can Contributor in 2010. He may be reached at oxide fuel cells, electrochemical energy be used in numerous ways to both produce [email protected]. storage devices such as Li, Na, Mg-ion fuels and chemicals from sunlight and to batteries, supercapacitors, thermoelectric carry out various light-induced self-cleaning Kazuya Nakata joined Tohoku University materials, catalysts, etc. Dr. Manivannan and environmental purification functions. as a research fellow of the Japan Society for has been active in performing several the Promotion of Science (JSPS) in 2005, international research collaborations and and then joined the Massachusetts Institute participates as an adviser in the NRC Acknowledgments of Technology in 2006 as a JSPS research Research Associateship Programs. He is fellow. He has been a full-time researcher currently on the editorial board of Materials This work was supported by a Grant- in the photocatalyst group at the Kanagawa Science and Engineering B: Advanced in-Aid for Scientific Research (B) and for Academy of Science and Technology Functional Solid-State Materials. He may Challenging Exploratory Research from (KAST) since December 2007. Also, in be reached at [email protected]. the Ministry of Education, Culture, Sports, September 2010, he joined in the Organic Science and Technology (MEXT), Japan. Solar Cell Assessment Project at KAST. He Donald A. Tryk was a member of the The authors are grateful to Yuko Morito has also been Visiting Associate Professor Ernest B. Yeager Center for Electrochemical (U-VIX Corporation) and Yuji Hayashi at the Tokyo University of Science since Sciences from 1980 to 1995 and then joined (I’m PACT World Co., Ltd.) for support and January 2011. He received the Sano Award Professor Akira Fujishima’s group at the supervision of this work, as well as helpful for Young Scientists of the Electrochemical , staying for six years, advice and useful discussions. Society of Japan in 2012. He may be reached as an Associate Professor for the last three at [email protected]. years. Then, he was a Visiting Professor at About the Authors the University of Puerto Rico and Tokyo Tsuyoshi Ochiai has been a full-time Metropolitan University, before joining Akira Fujishima was appointed Chairman researcher in the Photocatalyst Group at the Fuel Cell Nanomaterials Center of the of the Kanagawa Academy of Science and the Kanagawa Academy of Science and University of Yamanashi in 2008, where Technology (KAST) and Director of the Technology since April 2008. Since April he is a Professor. He may be reached at Functional Materials Research Laboratory 2010, he has also been a part-time lecturer [email protected] of the Central Japan Railway Company in (in chemistry) at the Nippon Institute 2003. He was appointed Professor Emeritus of Technology. Since January 2011, he References of The University of Tokyo and later became has also been a Visiting Researcher of a Special University Professor Emeritus of the Tokyo University of Science. His 1. A. Fujishima and K. Honda, Nature, The University of Tokyo in 2005. He served research interests involve photosynthesis, 238, 37 (1972). as the Chairman for the Chemical Society of photoelectrochemistry, and environmental 2. A. Fujishima, K. Kohayakawa, and Japan from 2006 to 2007 and has been the chemistry. He may be reached at pg-ochiai@ K. Honda, J. Electrochem. Soc., 122, Director of the China Research Center at the newkast.or.jp. 1487 (1975). Japanese Science and Technology Agency 3. T. Inoue, A. Fujishima, S. Konishi, and (JST) since 2008. He has been the President A. Manivannan is a materials scientist at K. Honda, Nature, 277, 637 (1979). of Tokyo University of Science since 2010. the U.S. Department of Energy’s National 4. A. Fujishima, X. Zhang, and D. A. He received the Asahi Prize in 1983, the Energy Technology Laboratory (NETL). He Tryk, Surface Science Reports, 63, Chemical Society of Japan Award in 2000, also currently serves as an Adjunct Professor 515 (2008). the Purple Ribbon Medal (Shijuhosho) in in the Physics Department at West Virginia 2003, and in 2004 he received the Japan University. He has carried out research in (continued on next page)

The Electrochemical Society Interface • Summer 2013 55 Fujishima, Nakata, Ochiai, et al. (continued from previous page) 10. K. Iizuka, T. Wato, Y. Miseki, K. Saito, 16. T. Shichi and K.-i. Katsumata, J. and A. Kudo, J. Am. Chem. Soc., 133, Surface Finishing Soc. Japan, 61, 30 20863 (2011). (2010). 5. K. Nakata and A. Fujishima, J. 11. K. Nakata, M. Sakai, T. Ochiai, 17. M. N. Chong, B. Jin, C. W. K. Chow, Photochemistry and Photobiology T. Murakami, K. Takagi, and A. and C. Saint, Water Research, 44, C: Photochemistry Reviews, 13, 169 Fujishima, Langmuir, 27, 3275 (2011). 2997 (2010). (2012). 12. B. Liu, K. Nakata, M. Sakai, H. Saito, 18. T. Ochiai, T. Hoshi, H. Slimen, K. 6. K. Nakata, T. Ochiai, T. Murakami and T. Ochiai, T. Murakami, K. Takagi, Nakata, T. Murakami, H. Tatejima, Y. A. Fujishima, Electrochim. Acta, 84, and A. Fujishima, Catalysis Science & Koide, A. Houas, T. Horie, Y. Morito, 103 (2012). Technology, 2, 1933 (2012). and A. Fujishima, Catalysis Science & 7. T. Ochiai and A. Fujishima, J. 13. T. Zhao, Z. Liu, K. Nakata, S. Technology, 1, 1324 (2011). Photochemistry and Photobiology Nishimoto, T. Murakami, Y. Zhao, 19. T. Ochiai, Y. Hayashi, M. Ito, K. C: Photochemistry Reviews, 13, 247 L. Jiang, and A. Fujishima, J. Mater. Nakata, T. Murakami, Y. Morito, and (2012). Chem., 20, 5095 (2010). A. Fujishima, Chem. Eng. J., 209, 313 8. K. Hirota, D. A. Tryk, T. Yamamoto, 14. J. Wang, D. N. Tafen, J. P. Lewis, Z. (2012). K. Hashimoto, M. Okawa, and A. Hong, A. Manivannan, M. Zhi, M. Li, 20. K. S. Raja, Y. R. Smith, N. Kondamudi, Fujishima, J. Phys. Chem. B, 102, and N. Wu, J. Am. Chem. Soc., 131, A. Manivannan, M. Misra, and V. 9834 (1998). 12290 (2009). Subramanian, Electrochem. Solid- 9. K. Hirota, D. A. Tryk, K. Hashimoto, 15. N. Wu, J. Wang, D. N. Tafen, H. Wang, State Lett., 14, F5 (2011). M. Okawa, and A. Fujishima, J. J.-G. Zheng, J. P. Lewis, X. Liu, S. S. 21. S. C. Roy, O. K. Varghese, M. Paulose, Electrochem. Soc., 145, L82 (1998). Leonard, and A. Manivannan, J. Amer. and C. A. Grimes, ACS Nano, 4, 1259 Chem. Soc., 132, 6679 (2010). (2010).

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