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Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO2), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced super hydrophilic phenomenon involving TiO2 and its applications are presented.
Keywords: TiO2, photocatalysis, superhydrophylic properties, applications
In the past decades, the use of TiO2 as a photocatalyst has received much attention in the treatment of wastewater and exhaust gases. For water detoxification, mostly particles in the nanometer range are used, because of their large specific surface area. In slurry reactors, such materials show excellent performance, but it is difficult and expensive, to separate the catalyst from the treated water. To overcome this drawback, titania powder has been immobilized or directly synthesized on supports [1,2]. Together with large two-dimensional surfaces, both compact and highly porous particles, such as silica gel or alumina [3-6], have been used as carriers.
Starting in the late 1980s, Fujishima  have been involved in an unfolding story whose main character is the fascinating material titanium dioxide (TiO2). The story began with photo electrochemical solar energy conversion and then shifted into the area of environmental photocatalysis, including self-cleaning surfaces, and most recently into the area of photoinduced hydrophilicity, which involves not only self-cleaning surfaces, but also antifogging ones. One of the most interesting aspects of TiO2 is that the types of photochemistry responsible for photocatalysis and hydrophilicity are completely different, even though both can occur simultaneously on the same surface. In this review, we will briefly trace the development of these two areas.
2. TiO2 Photocatalytically properties
Plants capture the energy from sunlight and thus grow. During this process, they produce oxygen by oxidizing water and reducing carbon dioxide. In other words, the oxidation of water and the reduction of CO2 are achieved with solar energy. By analogy with natural photosynthesis, Fujishima  investigate the photo electrolysis of water using light energy . This approach involves essentially a photochemical battery making use of a photo excited semiconductor (Fig. 1). The basic process of photocatalysis consists of ejecting an electron from the valence band (VB ) to the conduction band (CB )of the TiO semiconductor creating a hole in the valence band. This is due to UV irradiation of TiO with an energy equal or superior to the band gap 3.2 eV. The mechanism is summed up in Fig.1
Figure General mechanism of photocatalysis
The possibility of solar photo electrolysis was demonstrated for the first time with a system in which an n-type TiO2 semiconductor electrode, which was connected though an electrical load to a platinum black counter electrode, was exposed to near-UV light (Fig. 2) . When the surface of the TiO2 electrode was irradiated with light consisting of wavelengths shorter than 415 nm, photocurrent flowed from the platinum counter electrode to the TiO2 electrode through the external circuit. The direction of the current reveals that the oxidation reaction (oxygen evolution) occurs at the TiO2 electrode and the reduction reaction (hydrogen evolution) at the Pt electrode. This fact shows that water can be decomposed, using UV-VIS light, into oxygen and hydrogen, without the application of an external voltage.
Figure 2 Schematic diagram of an electrochemical photocell . (1) n-type TiO2 electrode; (2) platinum black counter electrode; (3) ionically con-ducting separator; (4) gas burette; (5) load resistance and (6) voltmeter.
When a semiconductor electrode is in contact with an electrolyte solution, thermodynamic equilibration takes place at the interface. This may result in the formation of a space-charge layer within a thin surface region of the semiconductor, in which the electronic energy bands are generally bent upwards or downwards, respectively, in the cases of n- and p-type semiconductors. The thickness of the space-charge layer is usually of the order of 1-103 nm, depending on the carrier density and dielectric constant
of the semiconductor. If this electrode receives photons with energies greater than that of the material's band gap, EG, electron-hole pairs are generated and separated in the space charge layer. In the case of an n-type semiconductor, the electric field existing across the space charge layer drives photogenerated holes toward the interfacial region (i.e. solid-liquid) and electrons toward the interior of the electrode and from there to the electrical connection to the external circuit. The reverse process takes place at a p-type semiconductor electrode. If the conduction band energy ECB is higher (i.e. closer to the vacuum level, or more negative on the electrochemical scale) than the hydrogen evolution potential, photogenerated electrons can flow to the counter electrode and reduce protons, resulting in hydrogen gas evolution without an applied potential, although, as shown in Fig. 2, ECB should be at least as negative as - 0.4 V (SHE) in acid solution or -1.2 V (SHE) in alkaline solution. Among the oxide semiconductors, TiO2 (acid), SrTiO3, CaTiO3,KTaO3,Ta2O5 and ZrO2 satisfy this requirement. On the other hand, the employment of an external bias or of a difference in pH between the anolyte and catholyte is required in the case of the other materials in order to achieve hydrogen evolution.
3. TiO2 Super hydrophilic properties
It is one of the unique aspects of TiO2 that there are actually two distinct photo-induced phenomena: the first is the well-known original photocatalytic phenomenon, which leads to the breakdown of organics, and the second, more recently discovered one involves high wet ability. This latter phenomenon it have termed 'superhydrophilicity'. Even though they are intrinsically different processes, they can, and in fact must, take place simultaneously on the same TiO2 surface. Depending upon the composition and the processing, the surface can have more photocatalytic character and less superhydrophilic character, or vice versa.
The superhydrophilicity, has only recently been studied. This effect was actually discovered by
accident in work that was being carried out at the laboratories of TOTO Inc., in 1995. It was found that, if a TiO2 film is prepared with a certain percentage of SiO2, it acquires superhydrophilic properties after UV illumination. In this case, electrons and holes are still produced, but they react in a different way. The electrons tend to reduce the Ti(IV) cations to the Ti(III) state, and the holes oxidize the O2 . anions. In the process, oxygen atoms are ejected, creating oxygen vacancies. Water molecules can then occupy these oxygen vacancies, producing adsorbed OH groups, which tend to make the surface hydrophilic. The longer the surface is illuminated with UV light, the smaller the contact angle for water becomes; after about 30 min or so under a moderate intensity UV light source, the contact angle approaches zero, meaning that water has a tendency to spread perfectly across the surface. In contrast, TiO2 coatings can maintain their hydrophilic properties indefinitely, as long as they are illuminated. Making use of the idea of cleaning by a stream of water, coated windows can be cleaned by rainfall. Other related applications for hydrophilic glass include windows that are easily cleaned by water alone, and anti-fogging or anti-beading windows and mirrors. Beading of rainwater on automobile side-view mirrors can be a serious safety problem, and now this problem has been virtually solved.
4. Applications of TiO2 photocatalytic properties
TiO2 is excellent for photocatalytically breaking down organic compounds. For example, if one puts catalytically active TiO2 powder into a shallow pool of polluted water and allows it to be illuminated with sunlight, the water will gradually become purified.
One of the most important aspects of environmental photocatalysis is the availability of a material such as titanium dioxide, which is close to being an ideal photocatalyst in several respects. For example, it is relatively inexpensive, highly stable chemically, and the photogenerated holes are highly oxidizing. In addition, photogenerated electrons are reducing enough to produce super oxide from dioxide. In order to avoid the use of TiO2 powder, which entails later separation from the water, various researchers began to work on ways of immobilizing TiO2 particles, for example in thin film form.
One of the most important aspects of TiO2 photocatalysis is that, like the photoelectric effect, it depends upon the energy of the incident photons, but, to a first approximation, not on their intensity. Thus, even if these are just a few photons of the required energy, they can induce photocatalysis. This means that even ordinary room light may be sufficient to help to purify the air or to keep the walls clean in the indoor environment, because the amounts of pollutants are typically small. Thus, in a reasonably well lit room, with a total light intensity of 10mWcm-2 , the intensity of UV light with energy exceeding that of the TiO2 band gap would be approximately 1 mWcm-2 . As shown later, assuming a quantum efficiency of 25%, this would be sufficient intensity to decompose a hydrocarbon layer of approximately 1 mm thickness every hour. Most soilage to the interior of buildings comes from organic substances. Photocatalyst are not especially useful for breaking down large volumes of soilage, but they are capable of destroying it as it accumulates. For example, ordinary room light should be sufficient to prevent cigarette smoke residue stains if the catalyst-coated surface in question is clean to begin with. TiO2 photocatalyst thus hold great potential as quiet, unobtrusive self-cleaning materials. In addition, odors that are objectionable to humans are due to compounds which are present only on the order of 10 parts per million by volume (ppmv), and, at these concentrations, the UV light available from ordinary fluorescent lighting should be sufficient to decompose such compounds when TiO2 photocatalyst are present. Naturally, if higher light intensities are available, larger quantities of material can be decomposed, e.g. using the bright lamps that are used in highway tunnels .
It is possible to cite a number of examples of applications of environmental photocatalysis that are already at or near the stage of implementation or commercialization, presented in table 1.
These include the following: 1. photocatalytic indoor air cleaners, 2. photocatalytic automobile air cleaners, 3. photocatalytically self-cleaning ceramic tiles for bathrooms and kitchens, 4. photocatalytically self-cleaning lamp-shades for in-door lighting, 5. photocatalytically self-cleaning Venetian window blinds, 6. photocatalytically self-cleaning glass covers for highway tunnel lamps.
For the moment, it would like to briefly touch on the last topic, that of highly tunnel lamps. Workers at the Toshiba Lighting and Technology Corp. in Japan have been working to develop the technology for self-cleaning cover glasses for such lamps . These lamps are continually exposed to vehicle exhaust fumes and become progressively less transparent over a period of weeks or months, depending upon the traffic. If a TiO2 coating is present on the glass, it stays transparent much longer and does not require such frequent cleaning . The cleaning process is time consuming, expensive and even dangerous, because the tunnel traffic must be diverted so that the cleaning crews can work. One of the problems in the implementation of this type of TiO2 coating on glass is that, unless special measures are taken, the coating as deposited on ordinary soda-lime glass is inactive. This is because the Na cations from the glass diffuse into the TiO2 film when it is fired, with an adverse effect on the subsequent photocatalytic activity. When TiO2 films are prepared on high-purity silica glass, this problem does not arise, so an obvious solution was to deposit a thin intermediate layer of silica prior to depositing the titania film. The silica layer effectively blocks the diffusion of Na cations from the glass into the titania film. Heller and co-workers have solved this problem by acid-treating the glass to remove the Na cations prior to the deposition of the titania film . The tunnel lamp application is a good example of the appropriate use of photocatalytic cleaning technology. The flux of organic contaminants to the surface is more or less balanced with the rate at which the photocatalyst can break them down. In addition, the light source is already built into the system, which is a fortunate situation. This is of course also true of indoor room lighting fixtures, which are also beginning to be marketed in Japan.
The ability of TiO2 films to kill bacteria and other microorganisms is also a very active area for research and development. Such organisms are naturally made up of organic compounds and thus are subject to the same types of decomposition reactions that have already been discussed. One of the specific areas that has been researched is that of the bactericidal effect on Escherichia coli and the detoxification of the E. coli end toxin . TiO2 -coated glass plates have been found to exhibit significant bactericidal and detoxification activity. One of the most important applications of photocatalytic technology now is that of self-sterilizing ceramic tiles for hospitals, particularly operating rooms.. A critical problem in operating rooms is the presence of methi-cillin-resistant Staphylococcus aureus (MRSA). These are bacteria that have built up a resistance to methicillin and related antibiotics. Large quantities of disinfectants must be used to kill these bacteria, and the bactericidal effect is not continuous. In contrast, TiO2 -coated tiles are active continuously, as long as there is illumination. Newer coatings have also been developed that are even active in the dark very small particles of silver are photocatalytically deposited on the surface.
Cancer treatment is one of the most important topics that is associated with photocatalysis. As far back as the mid-1980s, there were interested researcher in using the strong oxidizing power of illuminated TiO2 to kill tumor cells . In the first experiments a polarized, illuminated TiO2 film electrode, and an illuminated TiO2 colloidal suspension was also found to be effective in killing HeLe cells. It was found possible to selectively kill a single cancer cell using polarized, illuminated TiO2 microelectrode . Fujishima and co. were able to conduct animal experiments. They implanted cancer cells under the skin of mice to cause tumors to form. When the size of the tumors grew to about 0.5 cm, they injected a solution containing fine particles of titanium dioxide. After 2 or 3 days, they cut open the skin to expose the tumor and irradiated it. This treatment clearly inhibited the tumor growth . After 13 more days, they repeated the treatment with the titanium dioxide photocatalyst and observed a further marked effect. However, this technique was not effective in stopping a cancer that had grown beyond a certain size. Fujishima developed a device to allow the cancer to be exposed to light while titanium dioxide powder was being added to the tumor. However, because the photocatalytic reactions only occur under illumination, it is possible to selectively destroy cancer cells, if there is a technique available for illumination of the tumor.
5. Applications of TiO2 superhydrophilic properties
There is an extremely wide range of applications for superhydrophilic technology; selected ones are listed in Table 2; and new ones are being contemplated all the time. Here, we will briefly discuss a two typical examples, antifogging surfaces, and self-cleaning building materials. Fogging of the surface of mirrors and glass occurs when humid air condenses, with the formation of many small water droplets, which scatter light. On a superhydrophilic surface, no water droplets are formed. Instead, a uniform film of water can form on the surface, and this film does not scatter light. It is also possible, depending on the humidity, for the water film to be sufficiently thin that it evaporates quickly. The opposite approach, i.e. making water droplets easier to remove by imparting water repellency to the surface of glass, has also been the subject of intense research efforts. This approach also has merit; with it, the water droplets merely roll off the surface, as shown in a recent paper by Nakajima et al.  in which a super hydrophobic surface was reported.
Fujishima have compared the behavior of a superhydrophilic mirror and a normal mirror when exposed to steam. The normal mirror quickly fogs, but the superhydrophilic one retains its clarity. Mirrors with superhydrophilic coatings retain their capacity for photoinduced wetting semi permanently, at least for several years. Various glass products, i.e. mirrors and eyeglasses, can now be imparted with antifogging functions using this new technology, with simple processing and at low cost. In fact, many models of cars are being equipped with antifogging, antibeading superhydrophilic side-view mirrors. Stain-proofing, self-cleaning properties can now be bestowed on many different types of surface by means of the superhydrophilic effect. To take an example, a plastic surface smeared with oil can not normally be cleaned unless one uses detergent. A superhydrophilic surface, even though it is amphiphilic, however, has a higher affinity for water than for oil, that is, when water is in excess, the opposite being true when the oil phase is in excess. Thus, an oil smear on a plastic utensil can be spontaneously released from the surface when it is simply soaked in water. Based on this characteristic, a kitchen exhaust fan which is likely to be covered with oil, could be easily cleaned by water if the fan blades were coated with a superhydrophilic film.
Outdoor applications of this technique are also possible. Next to highways, most of the exterior walls of buildings become soiled from automotive exhaust fumes, which contain oily components. If the building materials are coated with a superhydrophilic photocatalyst, the dirt on the walls can be washed away with rainfall. Alternatively, the walls can be sprayed with water. The susceptibility of an exterior building material to soiling is closely related to its contact angle with water. A material used on the outside walls of a building is actually more likely to be soiled if it is more water-repellent. Thus, plastic is more likely to be soiled than sheet glass or tiles. A water-repellent material like a fluorocarbon plastic is the most likely to be soiled. A superhydrophilic material that shows a water contact angle of zero degrees is far less likely to be smeared than any other material.
A lot of applications become available for TiO2 thin films. The interest in the studying of TiO2 properties is also present at Bacau University. Here we manage to set up an real laboratory in order to prepare and study the most important parameters of TiO2 thin films. Among the other results we have found an important behavior of TiO2 surface and we found an important relation between the surface state density and the adsorption molecules at the surface Fig 3.
Figure 3 The variation of TiO2 thin films electrical conductivity with the gessoes pressure
We found that the electrical conductivity is dependent by the gessoes pressure from the vacuum chamber. We found the empirical relation. The conductivity dependence by the external pressure it is a feature of the unusual properties of TiO2 surface properties. It depends upon the molecular adsorption and is a measurement of the surface electrical properties of TiO2. This behavior was observed only when exposed to air.