Building and Environment 44 (2009) 1899–1906 Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv Photocatalytic construction and building materials: From fundamentals to applications Jun Chen, Chi-sun Poon* Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hum Hom, Hong Kong a r t i c l e i n f o a b s t r a c t Article history: Received 3 November 2008 Received in revised form 15 January 2009 Accepted 15 January 2009 Heterogeneous photocatalysis has been intensively studied in recent decades because it only requires photonic energy to activate the chemical conversion contrasting with conventional catalysis which needs heat for thermo-activation. Over the years, the theories for photochemical activity of photocatalyst including photo-induced redox reaction and super-hydrophilic conversion of TiO2 itself have been established. The progress in academic research significantly promotes its practical applications, including the field of photocatalytic construction and building materials. TiO2 modified building materials are most popular because TiO2 has been traditionally used as a white pigment. The major applications of TiO2 based photocatalytic building materials include environmental pollution remediation, self-cleaning and self-disinfecting. The advantage of using solar light and rainwater as driving force has opened a new domain for environmentally friendly building materials. In this paper, the basic reaction mechanisms on photocatalyst surface under the irradiation of ultraviolet and their corresponding applications in building and construction materials are reviewed. The problems faced in practical applications and the trends for future development are also discussed. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Photocatalysis Titanium dioxide Photocatalytic building materials Air pollution control Self-cleaning Self-disinfecting 1. Introduction Since the discovery of water photo-splitting in a TiO2 anode photochemical cell [1], the fundamentals and applications of photocatalytic oxidation (PCO) reaction have received significant attention. In the past decade, many research studies were devoted to the investigation of the properties of photocatalyst including photocatalytic water and air purifications, self-cleaning and photocatalytic anti-bacterial effect. All these properties can be attributed to two fundamental photochemical phenomena that occur on the surface of photocatalysts under ultraviolet (UV) irradiation. One is the photo-induced redox reaction of adsorbed substances, and the other is the photo-induced super-hydrophilicity. The synergy of these two properties is also the foundation of its application in building and construction materials. In the field of photocatalytic construction and building materials, titanium dioxide (TiO2) is the most widely used photocatalyst. TiO2 is a common semiconductor material which has been used as a white pigment in paints, cosmetics and foodstuff since ancient time [2]. It has three crystal structures: anatase, rutile and brookite. The anatase type is more widely used because it has a higher photoactivity than the other types of TiO2. The extensive use of TiO2 * Corresponding author. Tel.: þ852 2766 6024; fax: þ852 2334 6389. E-mail address: cecspoon@polyu.edu.hk (C.-sun Poon). 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.01.002 in photocatalytic building materials is attributed to the following characteristics: (a) relatively inexpensive, safe, chemically stable; (b) high photocatalytic activity compared with other metal oxide photocatalysts; (c) compatible with traditional construction materials, such as cement, without changing any original performance; (d) effective under weak solar irradiation in ambient atmospheric environment. The use of photocatalysts together with building materials started from the early 1990s. The versatile function of TiO2, which can both serve as photocatalytic materials and structural materials, has facilitated its application in exterior construction materials and interior furnishing materials, such as cement mortar, exterior tiles, paving blocks, glass and PVC fabric. The classification of major TiO2 based construction and building materials is shown in Table 1. The advantages of affixing TiO2 into building materials have attracted many industrial interests. In 2003, the sales of photocatalytic building materials accounted for 60% of the whole photocatalytic market share in Japan [3]. In this paper, the development of fundamental research and applications of TiO2-based photocatalysis in the field of construction and building materials will be introduced. Three major research areas including air purification, self-cleaning and selfdisinfecting are reviewed in details. The problems restricting a larger scale application of the technology and some possible future development are also discussed. 1900 J. Chen, C.-sun Poon / Building and Environment 44 (2009) 1899–1906 Table 1 Classification of TiO2-based photocatalytic construction and building materials Categories Products Function Exterior construction materials Interior furnishing materials Road construction materials Tiles, glass, tents, plastic films, panels Self-cleaning Tiles, wall paper, window blinds, paints, finishing coatings Soundproof walls, tunnel walls, roadblocks, concrete pavements Self-cleaning, anti-bacterial Air-cleaning, selfcleaning, 5. NO diffuses to the surface of TiO2 and is oxidized to NO2 by HO2 radicals. NO þ HO2 /NO2 þ OH 6. Finally NO2 reacts with hydroxyl radicals to form nitric acid. NO2 þ OH /HNO3 2.2. Research-based development and practical applications 2. Air depollution 2.1. Reaction mechanisms It has been demonstrated that both organic pollutants and oxides such as NO, NO2 and SO2 at low concentration levels can be treated by TiO2 under UV irradiation [4]. The mechanism of pollution decomposition is illustrated by Fig. 1. The reaction begins with the irradiation of light over TiO2. When TiO2 absorbs a photon containing the energy equal to or larger than the band gap, an electron will be promoted from the valence band to the conduction band. The activation of the electrons results in the generation of ‘‘holes’’ (electron vacancy) in the valence band. In this reaction, hþ and e are powerful oxidizing and reducing agents respectively. The electron–hole pairs may recombine in a short time or take part in chemical reactions depending on reaction conditions and molecular structures of the semiconductors. The strong oxidation power of hþ enables it to react with water to generate the highly active hydroxyl radical (OH) which is also a powerful oxidant. Most organic air pollutants can be degraded completely by either the hydroxyl radicals or the holes themselves to innocuous final products (e.g. CO2 and H2O). In addition, the reducing power of the electrons can induce the reduction of molecular oxygen (O2) to superoxide (O 2 ). It has been confirmed that the superoxide is almost as effective as the holes and hydroxyl radicals in the chain reactions for the breaking down of organic compounds. An example of photocatalytic conversion of nitric oxide (NO) to nitric acid is illustrated as follows [5,6]: 1. After irradiation of UV light ranging from 300 nm to 400 nm, the photocatalytic reaction begins with the generation of electron–hole pairs. hn TiO2 / hþ þ e The first use of TiO2 to decompose pollution was reported in 1977. It was found that cyanide in wastewater could be degraded by the PCO process to harmless substrates [7]. However, the early work mainly focused on the treatment of wastewater. Only in recent years the removal of trace levels of organic and inorganic contaminants in air using PCO has received much attention as this technology is potentially suitable for air purification in office buildings, homes, cars and aircrafts [8]. 2.2.1. Outdoor air In big cities with dense population the pollutant concentration at street level is quite high because the dispersion of the exhaust generated by a large number of vehicles is hindered by the surrounding tall buildings. For these cities applying TiO2 modified cementitious materials onto the external covering of buildings or roads may be a good supplement to conventional technologies such as catalytic converters fitted on the vehicles for reducing gaseous exhaust emission. Concrete pavement surfaces and external building surfaces are optimal media for applying the photocatalytic materials because the relatively flat configuration of the building materials can facilitate the exposure of the photocatalyst to sunlight. In addition, the nature of cement matrix is particularly suitable for incorporating TiO2 particles and other photo-oxidation products. Under irradiation of solar light, gaseous pollutants can be degraded on the surface of construction materials which can be eventually washed away by rain (Fig. 2). The whole removal process of pollutants is driven by natural energy alone. The depollution effect of photocatalytic cementitious materials has been demonstrated by many laboratory studies [9–15]. Nitrogen oxides (NOx) and volatile organic compounds (VOCs) have been chosen by most studies as representative airborne pollutants due to their potential health risks and ability to generate photochemical smog. The NO removal paving blocks made by waste materials and TiO2 were evaluated by Poon and Cheung [9]. They 2. The hþ reacts with OH dissociated from water to form the hydroxyl radical. UV irradiation hþ þ OH /OH Electron-hole pairs h+ eTiO2 Surface 3. The e reacts with molecular oxygen to form the superoxide anion. Oxidation Reduction e þ O2 /O 2 H2O .OH .O 2 O2 þ 4. The superoxide anion further reacts with H dissociated from water to produce HO2 radicals. Oxidation of pollutant NO VOC  H þ þ O 2 /HO2 NO2 HNO3 CO2+H2O Fig. 1. Pollution removal mechanism of TiO2 photocatalysis. 1901 found that an optimum mix design which incorporated recycled glass, sand, cement and 10% TiO2 achieved 4.01 mg h1 m2 NO removal. A typical performance of NO removal by the photocatalytic paving blocks obtained in the laboratory is shown in Fig. 3. Beeldens [11] also investigated the feasibility of using TiO2 on the surface of pavement blocks. It was reported that the air-cleaning capacity can be enhanced by increasing the surface area, reducing the air flow rate and increasing the turbulence of the pollutant in the test chamber. Hüsken et al. [12] carried out a comparative analysis of different photocatalytic cementitious products in an optimum laboratory conditions. They pointed out that the efficiency with respect to NOx degradation varied significantly, with some products achieving 40% degradation whereas others showing almost no effect. Regarding the degradation of VOCs, Strini et al. [14] used a stirred flow reactor to measure the photodegradation of organic compounds (at ppb level) at the surface of photocatalytic materials. They observed that the photocatalytic activity of pure TiO2 sample was three to ten times greater than the cementitious sample that was prepared with the incorporation of 3% catalyst. The decomposition rate of BTEX was linearly dependent on the concentration of the reactant and the intensity of the irradiation. However, the catalytic activity was not linearly dependent on the TiO2 content in the samples probably because the formation of catalyst clusters in the cementitious paste was influenced by the different viscosities of the paste. Demeestere et al. [15] studied the potential of using TiO2 as a photocatalyst in building materials, i.e. roofing tiles and corrugated sheets, for the removal of toluene from air. It was reported that a toluene removal efficiency of 78  2% and an elimination rate of higher than 100 mg h1 m2 were obtained under an optimal condition. Their results showed that low toluene removal performance occurred at high relative humidity and high inlet concentration, whereas better performance was observed with increased residence time. They also found a decrease of photocatalytic activity by a factor of 2 when the photocatalytic building materials were operated at high pollutant concentration levels ([TOL]in > 76 ppmv). Also, washing the building materials with deionized water could partially regenerate the catalyst activity. Motivated by the promising results of the laboratory scale investigation, several pilot projects have been carried out to verify the effectiveness of the photocatalytic cementitious materials in ambient environment. In Bergamo (Italy), a street in the city centre was re-paved by the photocatalytic concrete paving blocks (total area of about 12,000 m2). Environmental monitoring was conducted on two locations: one was at the area where photocatalytic blocks were laid, and the other was at the extension of the road paved by normal bituminous concrete which was used as a reference. NOx NO NO2 SO2 NO3- SO4- NO3- SO4- Drainage Fig. 2. Illustration of pollutant removal by photocatalytic building materials in natural environment. Concentration / ppb J. Chen, C.-sun Poon / Building and Environment 44 (2009) 1899–1906 1100 1000 900 800 700 600 500 400 300 200 100 0 -100 UV turn on UV turn off NO NO2 NOx 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 Time / min Fig. 3. Photocatalytic NOx removal results of photocatalytic paving block obtained in laboratory. concentration was measured by chemiluminescence analyzers simultaneously on the two sites. A successive air monitoring campaign, lasting two weeks, showed an average NOx abatement of 45% in day time (from 9 am to 5 pm) [16]. A similar project was carried out in Antwerp (Belgium), in which 10,000 m2 photocatalytic pavement blocks was laid on a parking lane. Measurements on the site indicated an evident decrease of the NOx peak concentration due to the presence of the photocatalytic materials. The photocatalytic activity of these blocks was retested in the laboratory after they were in service for 2 years. The results showed that there was no reduction in NOx removal efficiency after washing the paving blocks with distilled water [11]. In Guerville (France), three artificial street canyons were built to evaluate the depollution performance of walls covered with a photocatalytic mortar. Continuous NOx and meteorological measurements were taken. NOx concentrations recorded in the TiO2-treated canyon were 36.7–82.0% lower than the ones observed in the reference canyons [17]. 2.2.2. Indoor air Indoor air pollutants mainly include nitrogen oxides, VOCs and particulates. These pollutants are emitted from different sources such as combustion, construction materials and consumer products. Many VOCs are known to be toxic and carcinogenic. Photocatalytic oxidation (PCO) is one of the most feasible options to improve indoor air quality because of two reasons: (i) PCO can mineralize many organic pollutants to harmless substrates, such as CO2 and H2O, and (ii) the concentration levels of pollutants in indoor environments are usually quite low (at ppb level), so the depollution function of the photocatalyst can be maintained for a long time [18]. The most frequently used building material aimed at removing indoor airborne impurities are photocatalytic paints incorporated in different binders, such as lime, polyorganic siloxane, silica sol– gel and organic binders. Maggos et al. [19] tested the depolluting efficiency of a TiO2-containing paint in an indoor car park under real scale configuration. The ceiling of the car park was painted with a white acrylic TiO2-containing paint. The artificially closed area of the car park was polluted by car exhaust during the testing period. As soon as the system reached a steady state, the UV lamps were turned on for 5 h. Results showed that photocatalytic oxidation of NOx gases was significant. The photocatalytic removal of NO and NO2 was 19% and 20%, respectively. Guarino et al. [20] chose two identical mechanical ventilated farrowing rooms in a swine farm, where NH3 was the main pollutant, to study the pollution removal performance of a TiO2 catalytic paint. Environmental parameters, ventilation rate and gas concentrations were continuously monitored in the catalytic painted room and a reference room. NH3 average concentrations of 5.41 mg m3 (in the reference 1902 J. Chen, C.-sun Poon / Building and Environment 44 (2009) 1899–1906 room without treatment) and 3.76 mg m3 (in the experimental room) have been reported during a full farrowing cycle. Other building materials including composite sheets and wall papers are also modified with photocatalyst to reduce indoor pollutants. Ichiura et al. [21] fabricated a composite TiO2–zeolite sheet using a papermaking technique. The pollutant removal efficiency was tested using toluene and formaldehyde as target indoor pollutants under UV irradiation. The composite sheet with a Ti/ zeolite ratio of 1:4 seemed to be the most effective for the removal of toluene, while no optimum composition was found for formaldehyde due to the high formaldehyde adsorptivity of Y type zeolite. The study showed this kind of sheets had potential to be placed on walls and ceilings for the removal of various indoor pollutants. Taoda et al. [22] developed a photocatalytic wall paper by coating a visible light type photocatalyst on a wall paper. Their experimental results indicated that toluene and acetaldehyde could be decomposed efficiently even under irradiation of a fluorescent lamp, although using UV irradiation had better performance. Regarding indoor environment where additional UV source is not applicable, TiO2 containing building materials can also remove odours. This is because the odour which is sensitive to human noses is caused by chemical substances in the order of 10 parts per million by volume. Weak UV intensity of 1 mW cm2 was found to be sufficient to decompose these substances in the presence of photocatalysts [23]. 2.3. Problems and limitations Although the depollution effect of photocatalytic building and construction materials is evident, it is noticed that there are still unresolved problems when these materials are used in real-life applications. Immobilization of TiO2 by the construction materials can lead to significant loss of the photocatalytic activity. Rachel et al. [24] pointed out that TiO2-cement mixtures and red bricks containing TiO2 were significantly less efficient than TiO2 slurries in decomposing 3-nitrobenzenesulfonic. It is thought that the reduction of active surface and the presence of ionic species, which contributed to the charge recombination, are the reasons for the catalytic activity loss. Lackhoff et al. [25] stated that the carbonation of the TiO2 modified cements led to a noticeable loss in catalytic efficiency over several months because of the changes in cement surface structure. The report published by the Hong Kong Environmental Protection Department claimed that the photocatalytic activity of TiO2 coated paving blocks decreased significantly after 4 month exposure in a downtown area due to the accumulation of contaminants on the block surface [26]. This means periodic servicing (washing or replacement) of the TiO2 materials may be necessary to maintain the pollution reduction effect. It also should be noticed that the photocatalytic air purification function is usually restricted to pollutants which are absorbed on the surface of the construction materials. In widely open spaces, the pollutant removal efficiency may be low as only a small fraction of the pollutants can be trapped. It is believed that the pollution elimination effect is more easily quantified using continuous monitoring data for confined spaces such as canyon streets where dispersion and ventilation are poor [27]. As far as indoor application is concerned, controversy surrounds the question of whether it is safe to apply the photocatalytic materials, especially on the possible health effects of byproducts formed in incomplete photo-oxidation [28]. In addition, the potential impacts of nanomaterials on human health should also be assessed. The particle size of nanoscopic photocatalysts is so small that it is possible it could enter into the human body triggering adverse health effects during the production, transportation, storage, and use [29]. 3. Self-cleaning building materials 3.1. Reaction mechanisms 3.1.1. Decomposition of adsorbed organic substrates Visible stains on building surface are constituted by composite materials mainly originated from the atmospheric aerosol pollutants. Small particles and greasy deposits are adhered to building surface by organic binders such as hydrocarbons and fatty acids [30]. Taking fatty acid molecules for example, their carboxylic groups (–COOH) enable them to stick on building surface via chemical binding with calcium ions present in concrete; on the other hand, their long chains link with other hydrophobic molecules perpendicularly to the surface, resulting in fatty stains which trap many atmospheric particles and dusts. The great redox power of the UV induced electron–hole pair of photocatalyst can decompose the organic binders. An example of degradation of palmitic acid is shown below [30]: hn TiO2 / hþ þ e n-C15 H31  COO Hþ þ hþ /n-C15 H31 —COO þ Hþ /n—C15 H31 þ CO2 þ Hþ After the release of the first carbon atom, the n-C15 H31 radicals are oxidized by OH radicals to an alcohol: n-C15 H31 þ OH /n-C15 H31 OH The produced n-C15 H31 OH undergoes oxidation into aldehyde, and then further to acid n-C14 H29  COOH. Subsequently, a second photoKolbe reaction takes place to release the second CO2. The chain reactions continue until the palmitic acid is completely mineralized to CO2 and H2O. Some side reactions may also be induced to generate volatile compounds, accelerating the oxidation process. 3.1.2. Super-hydrophilicity Super-hydrophilicity is a phenomenon that occurs when a TiO2 film is subjected to UV irradiation a very small water contact angle appears. On this surface, water tends to spread out flat instead of beading up. It has been shown that the reciprocal of the contact angle corresponds to the density of the surface hydroxyl groups reconstructed by UV irradiation [31]. The binding energy between Ti atom and the lattice oxygen atom is weakened by the hole generated after UV irradiation. Therefore, the adsorbed water molecules can break a Ti–O–Ti bond to form two new Ti–OH bonds resulting in super-hydrophilicity (Fig. 4). In fact, TiO2 film is not only hydrophilic but also amphiphilic after UV irradiation. The surface may adsorb both polar and nonpolar liquids. When water is rinsed over the surface, contaminations like oil can be washed away [32]. The macro effect of self-cleaning is, in fact, a combined effect of super-hydrophilicity and degradation of organic deposits. Although the photo-induced super-hydrophilicity and degradation of organic contaminants are different processes, they may take effect simultaneously. It is difficult to distinguish which mechanism is more O Ti Ti O H2O H2O O h+ O h+ O Ti O In the dark Ti Ti O OH OH Ti O UV Irradiation OH OH Ti Ti O Ti O Water absorption Fig. 4. Photo-induced hydrophilic TiO2 surface. J. Chen, C.-sun Poon / Building and Environment 44 (2009) 1899–1906 important for self-cleaning. Also worth noting is the interesting fact that, to some extent, there might be synergetic effect of photocatalysis and super-hydrophilicity promoting self-cleaning. Hydroxyl radical plays an important role in the decomposition of organic compounds. If more hydroxyl groups can occur on the surface of TiO2 due to super-hydrophilicity, the efficiency of degradation of organics may also be improved [33]. On the other hand, the adsorption of organic compounds on the film surface may lead to a conversion of hydrophilic surface to hydrophobic surface. The photocatalytic decomposition of these organic contaminants can restore the super-hydrophilic property [34]. Thus the synergetic effect of photocatalysis and super-hydrophilicity ensure the self-cleaning character of TiO2 film can be maintained continuously. 3.2. Research development and practical applications It is a common phenomenon that the aesthetic and luster of the surface of ordinary buildings are gradually lost with time. The building surface could be soiled by greasy and sticky deposits, which results in a strong adherence of ambient dusts. As a result, dirt built up on the surface reduces the visual appearance. Without constant and proper maintenance, it is difficult to restore the buildings’ aesthetic properties. The applications of self-cleaning building materials provide an excellent solution to this problem. The adsorbed organic soilage can be decomposed to water and CO2, while other residues and dust can be washed away by rainwater. This application can save much time and money spent on cleaning maintenance, particularly for tall buildings where such maintenance may be very difficult and costly. Several research studies have confirmed the effectiveness of this method. Cassar [35] mixed a suitable amount of TiO2 into white cement pastes to endow the structure with photocatalytic function. In order to verify the self-cleaning property, white cement disks were impregnated with a phenanthroquinone solution (0.1 mg/ cm3) showing homogeneous yellow surfaces. After UV irradiation, a rapid restoration of the clean surface was possible for the treated specimens. Similar experiments were carried out under the framework of a European Project PICADA. Two cementitious products, a 10 mm thick rendering (mixture of cement, lime and sand) and a 1 mm thick mineral paint (mixture of cement and fillers), both containing nano-sized TiO2, were developed. The selfcleaning effect of these samples was determined by monitoring the rate of photocatalytic decomposition of an organic dye rhodamine B by colorimetric measurements. The experimental results showed that the samples recovered about 65% of their initial coloration in less than a day of exposure to artificial sunlight [36]. In addition, the validity of self-cleaning cementitious products was demonstrated by colorimetric monitoring of buildings constructed using this kind of materials. A church ‘‘Dives in Misericordia’’ in Rome and a music and art city hall in Chambery (France) were both built with photocatalytic concrete and had been monitored since the beginning of their service life. For the church in Rome, after 6 years monitoring, only a slight different between the external and internal values of the lightness was observed. It was also found that the color variations of the panels caused by inorganic substance could be completely eliminated by washing with water. Regarding the city hall in Chambery, the primary color almost remained constant for approximately 5 years in different positions of the facade (West/ North/East/South) [37]. Besides self-cleaning cementitious materials, TiO2-based selfcleaning exterior building products including tiles and glass have been widely commercialized and applied. About 270 patents have been registered in the photocatalytic technology domain by TOTO Ltd. [38]. Their representative products are white ceramic tiles for exterior walls and home environments. They are fabricated by 1903 spraying a liquid suspension containing TiO2 powder or gel on the surface and then heated to 600–800  C. Through the heat treatment, the TiO2 is sintered and strongly attached to the tile surface forming a micrometer thick layer [39]. The self-cleaning and stain free performance are confirmed by hung up samples outdoor for six months [40]. For interior tiles used in washrooms or bathrooms, soilage and dirt are always a problem. The fatty acids from soap can form chemical bonds with calcium and magnesium in hard water and adhere to the tile surface, which are difficult to clean after the accumulation of dirt. The tiles with TiO2 film surface can break the binding between the organic compounds and the ceramic tiles, which make the washing process easier. Another important commercial product among the photocatalytic building materials is TiO2 based self-cleaning glass. Its successful application is not only due to the self-cleaning function but also strengthened by the light-induced anti-fogging property. Fogging of the surfaces of mirrors or glass happens when steam is cooled down on the surface to form fine water droplets. As droplets fall or form on a hydrophilic surface, they rapidly coalesce to form a water sheet. The visible view behind the glass can still be observed without blockage or distortion. Moreover, the superhydrophilic layer makes the glass dry without leaving the traditional droplet marks [41]. The challenge faced, when coating TiO2 thin film on glass, is that the film should be highly active and stable, while at the same time the optical clarity and appearance cannot be deteriorated [40]. The physical properties and photocatalytic activity of the TiO2 film significantly depend on factors such as calcination temperature, flow rate of the carrier gas and partial pressure of starting materials during the fabrication process [42]. 3.3. Problems and limitations Durability of the performance is one of the most important factors for photocatalytic self-cleaning building materials. For tiles and glass, because of the high temperature treatment, the photocatalyst layer is usually stable and permanent. However, organic building materials, such as PVC (polyvinylchloride), cannot tolerate the high sintering temperature to anchor the photocatalyst layer. Another problem is that organic building materials themselves tend be decomposed by photocatalytic reactions, resulting in not only reduced photocatalytic activity but also structure and strength destructions. Therefore, an intermediate layer must be placed between organic materials and photocatalyst. This dramatically increases the difficulty and manufacturing cost. In current market, it is claimed that various commercial TiO2 containing paints can be coated on the building surface directly to impart self-cleaning capability. These products may demonstrate the self-cleaning effects in a short period of time, but the durability is quite poor. A 5.5 years outdoor exposure test of photocatalytic coating materials showed that most paints made of organics lost photocatalytic capability. Significant degree of chalking has been observed probably due to the decomposition of organic binders [43]. In the case of concrete surfaces, the use of organic admixture for concrete and other cementitious materials must be minimized to avoid possible reduction of the photocatalytic activity. The selfcleaning effect may also be limited due to the physical anchoring of the dirt in large pores. 4. Self-disinfecting building materials 4.1. Reaction mechanisms A number of researches have shown that typical bacteria such as Escherichia coli can be effectively killed by TiO2 under UV irradiation [44,45]. However, the biological inactivation mechanism of 1904 J. Chen, C.-sun Poon / Building and Environment 44 (2009) 1899–1906 illuminated TiO2 is still a debatable subject. Two major explanations have been proposed so far. One explanation attributes the death of microorganism to the attack of chemical species. Hydroxyl radical is assumed to be the lethal chemical species because it is a key factor in the decomposition of pollutants. In the early studies, the decrease of intracellular coenzyme A (CoA) in TiO2-treated cells was detected for various microorganisms. The direct oxidation of CoA that inhibited cell respiration and subsequently caused cell death was proposed as the killing mechanisms [46]. A 14C radioisotope labeling experiment has demonstrated that the carbon content of E. coli could be oxidized to form CO2 with substantial closure of the mass balance, which proved that the organic matter in the whole cells can be completely oxidized [47]. In support of this explanation, it has been reported that the reaction of Fe2þ with H2O2 photo-generated by the TiO2/UV system constituted a supplementary source of OH radicals resulting in the increase of bacterial inactivation rate [48]. However investigations conducted by other researchers concluded that biological structure destruction account for the inactivation of microorganisms. Saito et al. [49] found that TiO2 photocatalytic reaction could cause a significant disorder in cell permeability inducing a fast leakage of potassium ions and a slow leakage of RNA and proteins. Sunada et al. [50] investigated the bactericidal activity of copperdeposited titanium dioxide thin film (Cu/TiO2) under very weak ultraviolet light illumination. The effective bactericidal activity was explained by a two-step process. The first step is the partial decomposition of the outer membrane in the cell envelope by a photocatalytic reaction, followed by the permeation of copper ions into the cytoplasmic membrane. The second step is a disorder of the cytoplasmic membrane caused by the copper ions, which results in a loss of the cell’s integrity. A second killing mode was therefore proposed by Huang et al. [51] who suggested that when microorganisms undergo TiO2 photocatalytic attack, the cell wall will be damaged followed by cytoplasmic membrane damage, leading to a direct intracellular attack. Subsequently, essential functions that rely on intact cell membrane architecture, such as respiratory activity, are lost, and cell death is inevitable. 4.2. Research development and practical application With the increasing concern for human health and quality of life, the use of TiO2 for disinfection becomes more and more important. In the ceramic and building industries, there is a special interest for the photo-induced bactericidal effect of TiO2. This is particularly true when the ceramic is going to be placed in microbiologically sensitive environments, such as medical facilities and food industries where biological contamination must be prevented [52]. Applying antibacterial TiO2 building materials to indoor furnishing have been proved to be an effective way to decrease bacteria counts to negligible levels. It was reported that in an operating room in a hospital the number of bacteria on the wall surface was reduced to zero and the bacteria in the air was decreased significantly as well after installing photocatalytic tiles. The longer term effect was much better than the spraying of disinfectants [2]. Several companies, such as TOTO, Karpery and Biocera, have commercialized the concept of a deposited thin film semiconductor photocatalyst on ceramics as an antimicrobial agent. Their semiconductor photocatalyst thin film ceramic products exhibit both UV light-induced antimicrobial agent and deodorizing properties [53]. The light-induced bactericidal activity of TiO2 can also be used to control biological growth on concrete surfaces. Unsightly stains due to the growth of biofilm may cause the loss of aesthetic beauty particularly at places where design features or maintenance faults result in frequent wetting of the building surface [54]. This also could trigger chemical changes of concrete surfaces and decrease the durability [55]. The photosynthetic algae can only grow where sunlight is available, so that photocatalytic technology is an ideal control method. Linkous et al. [56] employed TiO2 and WO3 as surfacing agents to inhibit the attachment and growth of Oedogonium, a filamentous algae. It was demonstrated that coating cement substrates with a dispersion of 10 wt% TiO2 powder could achieve a 66% reduction in the growth of algae in comparison to the unprotected cement surface. Adding a 1.0 wt% loading of a noble metal such as Pt or Ir to the photocatalyst enabled an 87% reduction. 4.3. Problems and limitations Compared with the other two major applications, less research work has been conducted in the antimicrobial building material area. So far, standardized protocols for evaluating the light-induced anti-bacterial activity have not been established. The stated efficiency of different self-disinfecting products cannot be verified and compared. Moreover, effective and reliable coating techniques are needed to anchor the nano-photocatalysts to interior building surface in the event that the dispersion of fallen nano-particles causes potential health threats. A cost benefits analysis is also needed to further evaluate the applicability of self-disinfecting building materials. 5. New applications Recently a new method of cooling buildings by using the TiO2 photocatalyst was proposed. Using this technology, water is continuously sprinkled onto to the surfaces of buildings which have been coated with TiO2. With solar irradiation, the surface becomes highly hydrophilic due to the coated TiO2, which minimizes the amount of water consumption to form a water film. A very thin water layer of approximately 0.1 mm thickness can cover the whole building with small quantities of water supply (Fig. 5). The building is not cooled by water itself but by the latent heat flux when water evaporates. It has been confirmed the temperature drop was 15  C on window glass and 40–50  C on black roof-tile surfaces on a clear day in the middle of summer. This new application of photocatalytic building materials can result in significant reduction of electricity consumed for air conditioning [57]. The potential of using TiO2 based materials for energy-saving technology is huge. 6. Conclusions This paper gives an overview of the development of photocatalytic construction and building materials from both scientific Latent heat flux by water evaporation Continuous sprinkling water onto TiO2-coated surfaces Fig. 5. Energy-saving system using solar light and stored rainwater [56]. J. Chen, C.-sun Poon / Building and Environment 44 (2009) 1899–1906 and application standpoints. The fundamental research in materials science has provided the basis for the extension of the use of photocatalyst in construction and building materials. The capability of using photocatalytic cementitious materials to reduce urban and indoor pollution level has been confirmed by both laboratory research and field work. Successful commercialization of selfcleaning surfaces including concrete, glass and ceramic products enables buildings to maintain their aesthetic appearance over time. The self-disinfecting function provides a convenient way to achieve a microorganism free environment, which satisfies the high standard hygiene demand in facilities where sterile conditions are crucial. Although the durability and efficiency of the photocatalytic construction and building materials still need improvement, the potential for a wider use of photocatalytic construction and building materials is huge and promising. 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