Expanding Industrialization of Photocatalysts
Photocatalysts, typified by titanium oxide, promote reactions by absorbing light, and they are effective at deodorization and preventing contamination, in addition to being antibacterial and sterilizing. Industrialization is moving forward rapidly in a variety of fields.
The twenty-first century is being called the environment century. There has been an intensification of global warming as well as air and water pollution associated with industrialization; this is due in a large part to the rapid economic growth of developing countries such as China, first and foremost, and then India and Brazil. In addition, a new strain of influenza has spread throughout the world, becoming a major social issue. Photocatalyst technology is said to be an environmental cleanup technology. Using the natural energy of sunlight, it contributes to the cleaning of air and water, and it is attracting attention as an approach to environmental cleanup by fighting bacteria and providing sterilization to combat influenza and other outbreaks of illness.
Here we will describe the principles of photocatalyst technology, its effects and the progress that has been made in industrializing photocatalyst products. We will also comment on the direction in which things are likely to move in the future.
Photocatalysts are defined as substances that promote reactions by absorbing light without being altered themselves, before or after the reaction. We will take a look at the photocatalytic functions of titanium oxide, which is typical of the mainstream photocatalysts currently in use. When titanium oxide is exposed to a light source such as sunlight or fluorescent light, it exhibits both oxidative decomposition and superhydrophilic properties, which can be effectively used for deodorization, fighting bacteria, preventing contamination, antifogging, and other applications.
Oxidative decomposition means that if titanium oxide is exposed to light, organic substances on its surface will be broken down by oxidation, eventually forming carbon dioxide and water. This has applications in fields such as deodorization, elimination of volatile organic compounds (VOCs), elimination of soiling, antibacterial action and sterilization.
Titanium oxide is said to be an optical semiconductor, and it changes from an insulator to a conductor by absorbing the energy of light. This occurs because electrons are excited from the valence band to the conduction band, causing the formation of holes in the valence band. The excited electrons react with oxygen in the atmosphere forming superoxide anions (•O2-), and the holes react with moisture in the atmosphere producing hydroxyl radicals (•OH). These active oxygen species are extremely reactive, and oxidize and decompose organic substances (Figure 1).
When titanium oxide is exposed to light, its surface exhibits superhydrophilicity, with water contact angles of 5° or less. Although the underlying mechanism is again the formation of electron-hole pairs under light irradiation, the superhydrophilicity is directly the result of the formation of oxygen vacancies by the interaction of holes with oxygen atoms in the crystalline lattice. Such defects form a hydrophilic domain, and since water is easily adsorbed there, general superhydrophilicity results.
These superhydrophilic properties give rise to an antifogging effect. In addition, when surfaces coated with a titanium oxide photocatalyst are exposed to contaminants such as oil, the combination of superhydrophilicity and oxidative decomposition leads to the contamination being washed away by rain water, as shown in Figure 2. Thus, the original clean state of the surface can be maintained, and this is referred to as a "self-cleaning effect".
●Historical background of photocatalysts
The discovery of these photocatalytic properties of titanium oxide began in 1967 when one of the authors, Akira Fujishima, then a graduate student at Tokyo University, noticed the decomposition of water into hydrogen and oxygen in the presence of light irradiation. Taking the discoverers names, this effect was called the Honda-Fujishima effect, and it received worldwide attention with an announcement in Nature in 1972. This discovery led to active research and development related to photocatalysts. This was around the time of the international oil crisis, and there was a great deal of interest in hydrogen-based solar energy production using photocatalysts as an alternative to petroleum. However, this was found to be far from practical because the efficiency was so low. Therefore, Fujishima and others changed directions and focused on the oxidative decomposition capabilities of titanium oxide photocatalysts, concentrating their main research efforts into the development of applications based on these capabilities. On example was deodorization and antibacterial products for bathrooms, and in 1990 joint research was initiated with TOTO Ltd. regarding titanium oxide coatings. It was found that tiles coated with the photocatalyst did not soil easily, and the antibacterial and deodorizing effects were confirmed. In 1994, a photocatalyst antibacterial, stain resistant tile was developed and made practical. In addition, during the joint development period, the photo-excited superhydrophilicity of photocatalysts was discovered. Several companies tried to develop products that capitalized on this, and a rapid expansion took place in the photocatalyst market.
We can see this as one example of the successful cooperation of industry and academia leading to new products and markets.
Based on their oxidative decomposition, superhydrophilic and self-cleaning properties, photocatalysts have found a wide range of applications as shown in Table 1.
Development of products based on photocatalysts has been ongoing in a wide variety of fields. Starting around 1997, photocatalyst products rapidly formed a market focused on Japan, and in recent years the market has also expanded overseas.
In 2006, the Photocatalysis Industry Association of Japan was formed for the purpose of product standardization and approval, and technical training sessions and trade shows have taken place. Figure 3 shows the trend in the photocatalyst market based on a survey by the Photocatalysis Industry Association of Japan. This data represents the sales volume for companies that are members of the Photocatalysis Industry Association of Japan. In FY 2009, the total sales volume was approximately ¥100 billion for the entire global market.
In addition, Figure 4 shows a breakdown of the FY 2008 market by application. As can be seen, exterior materials make up more than half of the photocatalyst market, and they include self-cleaning products such as tiles, window glass, tent films, paint, building materials and billboards.
In addition, patent applications have been made one after another during the formative period of this photocatalyst market. Photocatalyst technology was first discovered in Japan, and this was also where most product development took place. Thus, Japan has both the largest share of the photocatalyst market and the largest number of patent applications, such that it has been called a Japan-originated technology. Figure 5 shows the change in the number of photocatalyst-related patent applications during the period 1984-2007, based on data from the Japan Patent Office. It can be seen that there was a sudden increase in the number of applications starting in 1994 when the superhydrophilicity effect was discovered and that in 1999, there were over 1000 applications in Japan. Following a slight dip, a new peak was reached in 2004, and the number of patent applications has since been dropping. In addition, until 2001, 90% or greater of the applications were made in Japan, but from 2002 on, the trend was toward an increase in the number of applications overseas.
We can see which companies are most actively engaged in photocatalyst development from the number of patent applications by applicant (Table 2) issued by the Japan Patent Office and the home page of the Photocatalysis Industry Association of Japan. Since it is comparatively easy to work in the photocatalyst industry, there is a great deal of participation by small to medium sized businesses, even though such firms do not appear in the table.
Because of their oxidative decomposition capabilities and superhydrophilic properties, photocatalysts have a wide range of applications in air cleaning, water cleaning and antibacterial and sterilization applications, and a large number of products have entered the market. However, some effects of photocatalysts, such as deodorization, VOC elimination, and sterilization, are rather obscure in terms of actual sensation. Taking advantage of this, products with insufficient photocatalytic activity, i.e., phony products, have entered the market, causing a loss of credibility and becoming a barrier to the formation of a healthy photocatalyst market. To combat this, JIS standards began to be established in 2002 for the standardization of performance evaluation methods for products based on titanium oxide photocatalysts, with the Ministry of Economy, Trade and Industry taking the lead, and this standardization was completed by the end of 2008. At the same time, ISO standardization is also being introduced.
Furthermore, the New Energy and Industrial Technology Development Organization (NEDO) has taken the lead in promoting standardization of performance evaluation methods for visible-light-responsive photocatalysts since FY 2007. In addition, the Photocatalysis Industry Association of Japan is formulating standards for photocatalyst products tested by the JIS standardized methods.
Consequently, we can have high expectations that phony products will be eliminated and that a healthy photocatalyst market will develop.
The Project to Create Photocatalyst Industry for Recycling-oriented Society (NEDO project) was started in FY 2007. This has given particular impetus to the research and development of highly sensitive visible light responsive photocatalysts in addition to the development of products. If successful, this project is expected to lead to the development of photocatalyst effects of a magnitude sufficient for practical application to deodorization, VOC elimination and other air cleaning, sterilization and antibacterial scenarios, and development of interior applications in particular.
In addition, there are promising applications to indoor air cleaning, sterilization, antibacterial action, agricultural, water cleaning, and soil cleaning using the visible light responsive photocatalysts described above.
We can also expect a large number of developments in the photocatalyst industry because of the expansion to new fields of business and globalization.
In particular, we feel that, as seen in the developing countries, achieving a comfortable living environment is becoming an important issue, and photocatalyst technology can fulfill an important environment-cleaning role.