nanotechnology innovations for the construction industry

Progress in Materials Science 58 (2013) 1056–1102

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Progress in Materials Science

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Nanotechnology innovations for the constructionindustry

0079-6425/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.pmatsci.2013.04.001

⇑ Corresponding author at: Engineering Excellence Group, Laing O’Rourke, Level 2, 97 Rose Street, Chippendale, NSAustralia.

E-mail addresses: [email protected], [email protected] (M.J. Hanus).

Monica J. Hanus ⇑, Andrew T. HarrisEngineering Excellence Group, Laing O’Rourke, Level 2, 97 Rose Street, Chippendale, NSW 2008, AustraliaSchool of Chemical and Biomolecular Engineering, Chemical Engineering Building (J01), University of Sydney, NSW 2006, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 November 2012Received in revised form 14 March 2013Accepted 20 March 2013Available online 6 April 2013

A broad range of challenges faced by the construction industry,ranging from the performance of the materials to environmentaland safety issues, relate to materials and their properties. Recentdevelopments in various areas of nanotechnology show significantpromise in addressing many of these challenges. Research anddevelopments have demonstrated that the application of nanotech-nology can improve the performance of traditional constructionmaterials, such as concrete and steel. Noteworthy improvementsin concrete strength, durability and sustainability are beingachieved with considered use of metal/metal oxide nanoparticlesand engineered nanoparticles (carbon nanotubes and carbon nano-fibres), and environment-responsive anticorrosion coatings formedusing nanoencapsulation techniques are showing promise in labo-ratory settings. Developments in nanotechnology are also improv-ing the accuracy and commercial viability of sensor-basedstructural health monitoring; a task rapidly gaining importance asthe structures that comprise many countries’ most expensiveinvestments near the end of their design life. As energy usage world-wide continues to grow, a focus on the potential for nanotechnologydevelopments to reduce energy consumption has become evident.Research demonstrates that nanotechnology can contribute tonovel cooling systems, and improve the functionality of solar cellsand insulation. A range of nanomaterials are also being used toadd new functionalities, such as self-cleaning properties, to tradi-tional construction industry products, for example paint andcement. First generation products are available on the market andfurther advances are evident in the academic literature.

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Nomenclature

AM1.5G air mass 1.5 global;BOS balance-of-systemsCNF carbon nanofibreCNT carbon nanotubeCH calcium hydroxideC-S-H calcium silicate hydrateC60 buckminsterfullereneDNA deoxyribonucleic acide_ electron E. coli, Escherichia coliEU European Unionh+ ‘‘hole’’ or electron vacancy IR, infraredLEED Leadership in Energy and Environmental DesignMEMS microelectromechanical systemsMRSA methicillin-resistant Staphylococcus aureusMWCNT multi-walled carbon nanotubeOPC ordinary Portland cementPDMS polydimethylsiloxane; PICADA, Photocatalytic Innovative Coverings Application for

Depollution Assessmentppb parts per billion ppm, parts per millionPTFE polytetrafluoroethylenePVC polyvinyl chloride S. aureus, Staphylococcus aureusSEM scanning electron microscope/microscopySWCNT single-walled carbon nanotubeTEM transmission electron microscope/microscopyUV ultraviolet; VOC, volatile organic compound(s)

M.J. Hanus, A.T. Harris / Progress in Materials Science 58 (2013) 1056–1102 1057

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10582. Concrete improvements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1058

2.1. Metal oxide nanoparticle additions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10582.2. Nanocarbon additions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10602.3. Utilisation of industrial waste products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1061

3. Structural health monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1062
3.1. Sensing devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10633.2. Self-sensing concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1064
4. Antimicrobial surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1064
4.1. Antimicrobial metal and metal oxide nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1066
4.1.1. Photocatalytic antimicrobial nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10664.1.2. The effect of metal/metal oxide physicochemical properties on antimicrobial activity 10674.1.3. Antimicrobial coatings from antimicrobial nanoparticles . . . . . . . . . . . . . . . . . . . . . . 1068

4.2. Antimicrobial engineered nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1070
5. Self-cleaning surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1071
5.1. Superhydrophobic surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10725.2. Superhydrophilic surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1075

5.2.1. Applications of superhydrophilic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10776. Air purifying surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10807. Evaporative cooling for building surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10858. Silica aerogel insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10869. Solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109010. Anticorrosion coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093

1058 M.J. Hanus, A.T. Harris / Progress in Materials Science 58 (2013) 1056–1102

11. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095

1. Introduction

Nanotechnology refers to the understanding and manipulation of materials on the nanoscale(<100 nm). At the nanoscale, substance properties are dictated by quantum mechanics. Surface effects,rather than bulk properties, dominate. This can lead to marked changes in material properties and canresult in improved performance and new functionality [1].

Nanotechnology has broad-reaching applications in the construction industry. Nanotechnologymay improve the primary properties of traditional construction materials (e.g. concrete), add func-tionalities to existing materials (e.g. paints/coatings or glass may gain self-cleaning, antimicrobialand pollution reducing properties) and introduce ‘‘new’’ materials to fill existing needs (e.g. silicaaerogels for thin and effective or transparent insulation, or nanoencapsulated corrosion inhibitorsfor steel corrosion protection). Nanotechnology has the potential to reduce the environmental impactand energy intensity of structures, as well as improve safety and decrease costs associated with civilintrastructure. Many nanotechnology-based innovations have far-reaching and significant impactsand are highly regarded during Leadership in Energy and Environmental Design (LEED), Green Star(Australia) or similar accreditations [2].

Nanotechnology and nanomaterials with applications in the construction industry are at variousstages of development ranging from conceptual ideas to commercially available products. Awarenessof nanotechnology applications in construction among industrial personnel is, however, remarkablylow [3–5]. Failure to distinguish between what is available now and what is theoretically possiblein the future has been the cause of many misconceptions about nanotechnology [1,6]. A report by RI-LEM suggested, based on responses by construction industry personnel to their surveys, that nano-technology was perceived by the construction industry as expensive and too complex to explain toclients who want a structure built as soon and as cheaply as possible [3]. This contrasts with the veryrapid growth in awareness and interest in nanotechnology utilisation in many other industrial sectors,such as chemicals, automobiles and energy [3]. The negative perception within the constructionindustry results in both current and future developments with potentially significant impact on boththe success of construction companies and the betterment of society being overlooked.

2. Concrete improvements

Concrete (alongside steel) is the cornerstone of the construction industry. It is estimated that3.3 billion tonnes of cement, the binder in concrete, were produced worldwide in 2010. This is an al-most 8% increase from 2009, and worldwide production of cement continues to increase, spurred onby the growing demand in emerging economies such as China and India [7]. Cement manufacture is,however, an energy intensive process and represents 7% of worldwide energy consumption and 4% ofworldwide industrial CO2 emissions [8]. The incorporation of nanomaterials can improve structuralefficiency, durability and strength of cementitious materials and can thereby assist in improvingthe quality and longevity of structures. The use of nanoscale industrial waste-based cement replace-ments can reduce carbon dioxide emissions associated with concrete production [4,5,8].

2.1. Metal oxide nanoparticle additions

The steel in reinforced concrete structures is susceptible to corrosion and degradation by ions inthe environment (e.g. chloride and sulfate ions), which may permeate through the concrete matrixat rates dependent on the concrete structure. The addition of some metal oxide nanoparticles to


concretes can both reduce the permeability of concrete to ions and increase the strength of concrete,thereby improving durability. The addition of TiO2 nanoparticles [9–11], Al2O3 nanoparticles [12–14],ZrO2 nanoparticles [14], Fe2O3 nanoparticles [12,14–16], SiO2 nanoparticles [11,12,15–20] and metaloxide containing nanoclays [21–24] have all been shown to improve concrete and/or cement mortarproperties. Metal oxide nanoparticle addition accelerates chemical reactions during initial hydrationthus strengthening cement composites. The metal oxide nanoparticles react with CaOH increasingthe amount of calcium silicate hydrate (C-S-H) produced, leading to a more compact microstructure,thereby not only decreasing permeability but also improving mechanical properties [17] such as com-pressive strength, flexural strength and abrasion resistance [18]. For example, Zhang and Li [11] foundthat the addition of 1% (by weight of binder) of 15 nm diameter TiO2 to concrete refined the porestructure and increased the resistance to chloride penetration by 31%. Shekari and Razzaghi [14] foundthat the addition of 1.5% (by weight of cement-based material) of 10–25 nm ZrO2, TiO2, Al2O3 or Fe3O4

increased the compressive strength and reduced chloride penetration of the concrete by 20–80%. It isimportant to examine the impact of these additions on the propensity of the concrete to undergoearly-age cracking. Early-age cracking has been shown to increase with the addition of fine pozzolanssuch as silica fume: SiO2 particles with a mean particle size >100 nm [25,26]. Through-depth cracks, ofcourse, severely compromise improvements in impermeability.

Fig. 1. Strength assessment of self-compacting concrete containing no SiO2 nanoparticles or 4 wt.% SiO2 nanoparticles after 2, 7and 28 days of curing, from [20].


The addition of SiO2 nanoparticles is widely reported to be an effective for strengthening concrete;both normally vibrated concrete [19] and self-compacting concrete [20]. For example, Nazari and Ria-hi [20] report that the compressive, split tensile and flexural strength of the 4 wt.% SiO2 nanoparticleconcrete is, respectively, 1.7, 2.2 and 1.6 times greater than that of the equivalent SiO2 nanoparticle-free concrete after 28 days of curing (Fig. 1). Generally, cement-based materials containing SiO2 nano-particles are stronger than those containing SiO2 fume [19]. This is attributed to the acceleratedcement hydration, increased pozzolanic activity, reduced pore size and improved interfacial bondingbetween the hardened cement paste and aggregate that is associated with the decreased average par-ticle size of the SiO2 [27]. Several cement-based products that contain SiO2 nanoparticles have beencommercially available in recent years, for example Gaia, which was developed by Cognoscible Tech-nologies and Ulmen (Chile) to substitute for SiO2 fume at ready-mixed concrete facilities [28], andNanodur� by Dykerhoff (Germany) [6,29].

2.2. Nanocarbon additions

Nanocarbons [30] (particularly carbon nanotubes (CNTs) and carbon nanofibres (CNFs)) have at-tracted significant attention for the improvement of the strength of cement-based composites. Nano-carbons possess remarkable properties. CNTs have a high tensile strength and Young’s modulus,display elastic behaviour [31] and excellent thermal properties [32]. Indeed, the tensile strength andYoung’s modulus of CNTs are hundreds and tens of times that of steel, respectively, although theirweight per volume of CNTs is a mere fraction that of steel. CNTs reinforce concrete at the nanoscalerather than the macro scale in the manner of traditional steel reinforcement bars. In the mesoporousenvironment of concrete, nanoscale reinforcements can act as fillers to produce a denser, less porousmaterial. Nanoscale reinforcements can also inhibit crack growth in the initial stages (and thus pre-vent crack propagation), enhance the quality of the paste-aggregate interface and increase the amountof high stiffness C-S-H [30,33]. A scanning electron microscope image of CNTs bridging cracks in a hy-drated OPC/CNT composite is shown in Fig. 2.

The predicted reinforcing potential of CNTs/CNFs in a concrete matrix (and indeed in CNT compos-ites in general [34]) is proving difficult to achieve consistently in practice. There is no evidence of CNTsin commercially available construction materials [5]. The results of mechanical testing of CNT/OPC andCNT/concrete composites have been highly variable [30]. Some trials have shown significant improve-ments in compressive strength, Young’s modulus and hardness, while others have shown insignificantchanges in compressive strength and decreases in Young’s modulus. Good concrete improvementsattributed to CNT addition include a 50% increase in compressive strength [35], >600% improvementin Vickers’s hardness at early ages of hydration [36], and >200% increase in Young’s modulus [37]. Re-sults to date have not convincingly shown improved flexural strength. The large variability in the

Fig. 2. CNT bundles crack bridging in a hydrated OPC/CNT composite, from [30].


performance of CNT/concrete and CNT/cement composites is considered a function of the difficultiesin achieving good CNT dispersion [38]. As with all composites, good dispersion of the nanoparticles inthe concrete or cement mortar matrix is vital for improvements in mechanical properties of the com-posite; however, good dispersion is particularly difficult to achieve with nanomaterials with a largeaspect ratio such as CNTs due to their high surface energy and strong interparticle forces (e.g. Vander Waals interactions) [20,39]. If not properly dispersed, the addition nanomaterials to cementitiousmaterials results in a decline in mechanical properties. The presence of aggregated nanoparticles canresult in voids or unreacted ‘‘pockets’’ (since the nanomaterials behave as an activator to promote poz-zolanic reactions), and thus poor dispersion of nanoparticles can result in weak zones or potentialareas for concentrated stresses [20,39,40]. The intrinsically hydrophobic nature of CNTs results in poorinteraction of CNTs with the cement or concrete matrix and this impacts on the cement hydration pro-cess [37]. This effect is exacerbated when the CNTs are bundled due to poorly dispersed.

Compounds that have been reported as suitable for the dispersion of CNTs in cement-based com-posites include polycarboxylate, gum Arabic, sodium linear alkylbenzene sulfonate and polyacrylicacid [35,37,41–44]. Polyacrylic acid is reported to act both as a plasticising agent for the cement pasteand a dispersing agent for the CNTs [44]. The use of nitrogen doped CNTs or prior functionalisation ofCNTs with carboxylic acid groups and sonication was also found to assist with dispersion when used inconjunction with a suitable solvent [34,35,38,41]. Sonication of CNTs and OPC in isopropanol followedby isopropanol removal by drying was found to be effective for dispersion; however, damage to ce-ment grain surfaces and slowing of the initial hydration process was reported [36]. Variations inCNT properties between synthesis batches, and even variations of CNT properties within synthesisbatches, are typical. As such, it is expected that the dispersion that can be achieved is affected bythe source of the CNT sample and the purification method used by the supplier [30,36]. This makescomparisons and generalisations about the effectiveness of dispersion techniques difficult. It must alsobe noted that techniques and methods that result in good dispersion of nanomaterials in cementpastes may not necessarily achieve good dispersions if attempted with a concrete.

The cost of CNTs is currently prohibitively high to allow for the use of CNT/cement composites inlarge concrete structures [30].

Note, however, that although technology used for large-scale CNT manufacture (such as fluidisedbed chemical vapour deposition) typically produces low quality CNTs, these are sufficient for achiev-ing an improvement in concrete properties. Furthermore, the price of CNTs can be expected to con-tinue to fall over the coming years as large-scale CNT manufacturing technology improves andmatures, and production of CNTs increases [32].

2.3. Utilisation of industrial waste products

The use of industrial waste product-based cement replacements, such as fly ash and sewage sludge,have gained attention due to their potential to reduce the cost and environmental impact of cement-based materials [45,46]. The effect of these replacements can also have either a positive or negativeeffect on various concrete properties. For example, although the addition of fly ash to concrete im-proves durability, reduces water requirements, improves finishability characteristics, etc. [47], theearly strength of high fly ash content concrete is low because the pozzolanic reaction of fly ash inthe mortar is slow [45]. In contrast, the addition of SiO2 nanoparticles can activate fly ash, increasingpozzolanic activity significantly, which leads to an increase to both short-term and long-termstrength [45,48] such that the early age strength of the SiO2 nanoparticle/fly ash concrete matchesor exceeds that of normal concrete mixtures [6,49]. The SiO2 nanoparticle additive itself can also besourced from waste materials such as rice husk [50].

The addition of CNTs to fly ash-containing cement mortars has also been shown to compensate forthe loss of strength associated with fly ash addition. The addition of 1 wt.% CNTs to a 20 wt.% fly ash/80 wt.% Ordinary Portland cement (OPC) mortar resulted in the compressive strength after 28 days ofcuring to be 98% of the compressive strength of the OPC, whereas the compressive strength of the20 wt.% fly ash cement mortar without CNTs was 90% of the compressive strength of the OPC [51]. Fur-thermore, CNTs and CNFs have been grown directly on cement components, such as cement particles,silica fume or fly-ash particles, whereby the cement or fly-ash was used as a catalyst for CNT and/or


CNF synthesis either directly or after impregnation of the cement component with a catalyst prior tosynthesis [52–54]. Synthesising CNTs/CNFs directly on cement components can improve the CNT/CNFdispersion in a cement matrix. It can also reduce the number of steps required for CNT/CNF addition tocement because, not only may the need for a CNT/CNF dispersion step be avoided, but the synthesisedCNTs/CNFs do not need to be removed from the substrate on which they form (as is otherwise typi-cally required) [52]. A study by Nasibulin et al. [54] found that although the compressive strengthof a CNT/CNF and cement hybrid material (in which CNTs/CNFs were synthesised on the cement par-ticles) was lower after 7 days of curing, the compression strength after 28 days of curing was doublethat of the CNT/CNF-free cement.

3. Structural health monitoring

Civil infrastructures are generally a country’s most expensive investment, with concrete the mostwidely used material. During the service life of a structure, concrete ages and deteriorates, leading tosubstantial loss of structural integrity and potentially resulting in disasters such as highway bridgecollapses if deterioration is not accurately monitored and rectified. Over 50% of all bridges in theUSA were built before 1940 [55]. At this time, structural health monitoring – the process of imple-menting damage detection and characterisation strategies for engineering structures – is, by and large,done manually by visual inspection. Federal requirements in the USA necessitate local transportauthorities to visually inspect the entire inventory of well over 500,000 highway bridges biennially,and other structures, such as buildings, as needed [56]. Examination of structures by visual inspectionis expensive, time-consuming and labour-intensive. It is also highly subjective and can only considerdamage that is visible on the surface of the structure [57]. Federal spending for the replacement ofstructurally obsolete bridges in the USA is approximately $10 billion per year. The poor accuracy ofassessments by current methods results in the retrofitting or replacement of many bridges that, insome cases, need not be retrofitted or replaced, and the possibility that some bridges needing engi-neering renewal or replacement are not identified [58].

More advanced and accurate methods for structural health monitoring are available and continueto be developed. More advanced structural health monitoring typically involves the analysis of a rangeof measurements from sensors such as displacement transducers, accelerometers, strain gauges andtemperature sensors, which provide an array of real-time information that may be continuouslymonitored from a central location to provide detailed insight into the state of a structure. Structuralhealth monitoring can be performed by: (i) attaching sensors to the surface of a concrete structure,(ii) embedding sensors into concrete structures during construction or, (iii) by constructing structuresor parts of structures using an intrinsically ‘‘smart concrete’’ – a concrete containing a material thatallows self-monitoring of strain and/or other properties. Significant investments have been made intothe development of more effective structural health monitoring systems in recent years [42,59] andstructural health monitoring systems have been installed in some locations e.g. at the Ting Kau, TsingMa and Kap Shui Mun bridges in Hong Kong [60], the Rion-Antirion bridge in Greece [61], the Huey P.Long in the USA [62]. Nanotechnology plays an important role in progressing structural health mon-itoring systems. It is expected that the implementation of advanced structural health monitoring sys-tems will increase as the technology matures and associated costs decrease.

In the context of structural health monitoring, sensing typically refers to the ability to provide anelectrical or optical response to a stress or strain [42] and self-monitoring of structures is performedby measuring the variation of an electrical property (usually electrical resistance) that is correlated tothe increase of stress or strain. Chen and Chung [63] give the following requirements for a structuralsensor:

(a) Wide stress/strain range of detection (from small strains up to failure).(b) Reversible response upon stimulus removal (necessary for repeated use of the sensor).(c) Ease of response measurement (without the need for expensive peripheral equipment).(d) Presence of the sensor having no negative effect on the structural properties of the structure.(e) Chemical stability and durability.(f) Low cost.


3.1. Sensing devices

Commonly used sensors (e.g. optical fibres, piezoelectric sensors, strain gauges) all suffer from highcost, poor durability and the need for expensive peripheral equipment, including electronics and la-sers, and so the use of such sensors in civil structures is uncommon [16,42,63]. Nanotechnology-basedmicroelectromechanical systems (MEMS) sensors have shown some improvements on traditional sen-sors. In general, MEMS technology has led to the development of sensors that are lower in cost, useless power, are compact and easy to install [57]. Emerging nanotechnology-based MEMS sensors havefurther improved capabilities and sensitivities. MEMS sensors can either be surface mounted [64] orembedded in concrete structure itself [65] and the sensors can be used for continuous measurementand monitoring of the strain in structures. Additionally, when embedded in concrete, the sensors canprovide accurate in situ measurements; however, the sensors must be able to withstand harsh condi-tions, including an alkaline environment and internal and external stress present in the concrete.Embedded MEMS sensors can be used for monitoring a range of concrete properties including temper-ature, hydration and moisture, chloride ions, water-related degradation, carbon dioxide andpH [8,65,66]. Ideally, hundreds to millions of these sensors would be embedded in concrete structures;however, both the current state of technology of nanotechnology-based MEMS and cost are major lim-iting factors. In 2008, Norris et al. [66] reported device cost to be �$25/unit with the expectation thatthis cost would decrease to �$1 within a few years.

Nanotechnology used in MEMS sensors for structural health monitoring includes the use of nano-polymer films and CNTs. Norris et al. [66] used a water vapour-sensitive nanopolymer film in a MEMSsensor for the detection of temperature and moisture. CNTs have attracted attention for structuralhealth monitoring applications in surface mounted sensors, embedded sensors and smart concretes.Lebental et al. [8] used highly aligned single-walled CNTs to produce a dense, ultra-thin membranefor a capacitive micromachined ultrasonic transducer that could potentially allow in situ monitoringof hydration and water-related degradation at the microscale (Fig. 3a).

CNT composite (e.g. CNT/cement or CNT/polymer) sensors display piezoelectric behav-iour [43,65,67]. High sensitivity strain sensors (with a sensitivity of 0.004%) that could potentiallybe mass produced have been reported [68]. When these sensors are subjected to mechanical strainor temperature changes [69], the sensor deforms and its effective resistance changes, mainly due to

Fig. 3. Two types of CNT-based sensors for structural health monitoring. (a) A microtransducer containing a vibratingmembrane composed of aligned CNTs, from [8], and, (b) a sensor composed of cement containing CNTs for microcrack detection,from [67].


changes in contact resistance at CNT junctions, where the thickness of the insulating films betweenadjacent CNTs is altered due to the applied load. When microcracks develop in the cement matrix,some percolation branches will be cut off, causing a sudden and sharp change in the electrical resis-tance of the sensor [67]. Sensors containing randomly orientated CNTs, such as those typically pro-duced by chemical vapour deposition (a common large-scale CNT synthesis method), allow for non-directional specific measurements [70]. Saafi et al. [65,67] embedded CNT/cement sensors in concrete.These wireless sensors allowed changes in the electrical resistance of the CNT network (which indi-cated the progress of damage) to be detected and monitored (Fig. 3b). CNT sensors in concrete struc-tures have, however, suffered some drawbacks such as susceptibility to damage by the alkalineenvironment of concrete and high cost [67]. Norris et al. [66] discuss that nanotechnology and MEMScould allow wireless, inexpensive, durable, compact and high-density information collection, process-ing and storage devices for structural health monitoring; however, significant further investigationand development of various aspects including peripheral equipment is required for this to be possible.For example, improvements in the wireless interrogation system (e.g. signal processing, powering,communication, location, orientation, data storage, computation capabilities) are required, as wellas studies of the long-term behaviour and repeatability of such sensors.

3.2. Self-sensing concrete

Additives proven to promote self-sensing in concretes (that is, additives in ‘‘smart concretes’’) aretypically nanoparticles such as Fe2O3 nanoparticles [16] and nanocarbons (e.g. CNFs [42]). When acompressive force is applied to a mortar containing Fe2O3 nanoparticles, the band gap of Fe2O3 nano-particles – a semiconducting material – is narrowed thereby improving conductivity. With the appli-cation of compressive force, the nanoparticles are also compelled to approach each other, whichmakes the tunneling current flows become more intense, and therefore the electrical conductivityof the cement mortar increases gradually [16,71]. CNFs are more conductive than the concrete matrixin with they are embedded, which provides potential for self-sensing. Howser et al. [42] found thatpeaks and troughs in electrical resistance readings matched the peaks and troughs of the applied forceand concrete strain in reinforced, self-consolidating concrete containing well dispersed CNFs. This al-lowed the level of damage to the reinforced concrete columns to be readily detected and assessed, andprovided real-time monitoring of the overall structural integrity. Orientations of the CNFs were ran-dom, and it is reported that the CNFs do not have to touch one another for self-sensing to be possible.The incorporation of carbon black nanoparticle-glass fibre reinforced polymer into concrete has alsobeen shown to produce a concrete that has self-monitoring characteristics, even during concrete cur-ing [72]. Coppola et al. [43] produced a self-sensing mortar by adding CNTs to the mortar; however,sensitivity was low because the CNT concentration used was insufficient for CNT networks to form.

Generally, self-sensing concretes produced by the addition of nanoparticles such as Fe2O3 nanopar-ticles and CNFs address the requirements of structural sensors (as stipulated by Chen and Chung [63])well. They allow a wide range of stresses/strains to be detected and the response is reversible. Mea-surements can be performed easily without expensive peripheral equipment. Not only does the pres-ence of the Fe2O3 nanoparticles and CNFs have no negative effect in structural properties, propertiessuch as compressive strength, flexural strength, and ductility can be enhanced (as was discussed inSection 2).

4. Antimicrobial surfaces

Antimicrobial surfaces have applications in a range of infrastructure including in hospitals, child-care centres, nursing homes, etc., to assist in minimising the spread of disease and reducing the prev-alence of illness and discomfort. Nosocomial infections (hospital-acquired infection) cases have beenincreasing rapidly in recent decades, and the prevalence of antibiotic-resistant pathogens is of partic-ular concern due to difficulties in treating these patients, and their high mortality rates. Two billionpeople worldwide carry Staphylococcus aureus (S. aureus) and up to 53 million of these carry the resis-tant form, methicillin-resistant S. aureus (MRSA) [73]. In 2005, in the USA alone, S. aureus-related


diseases were responsible for nearly 500,000 hospitalisations and 11,000 deaths (with approximately58% of the hospitalisations and 5000 of the deaths attributed to a resistant strain) [74]. In the Euro-pean Union (EU), the European Centre for Disease Prevention and Control reports that about 4 millionpeople acquire nosocomial infections annually (of which 5% are MRSA infections), and about 37,000deaths occur annually as a result of nosocomial infections [75]. Nosocomial infections are estimatedto cost at approximately €5.5 billion per year in the EU [76]. The long periods of hospitalisation re-quired by S. aureus-infected patients, in particular, represents a high cost to public and private healthinsurance systems [77]. Walls, floors and other surfaces provide an easily accessible platform for path-ogenic bacterial to settle and proliferate. Any surface, once touched by an infected person, serves as abreeding ground for such pathogens [78]. It is estimated that 15% of nosocomial infections in the EUoccur due to transmission through inanimate objects [79].

Other often less severe but also prevalent illness and discomfort has also been linked to the growthof microbes in buildings [80]. For example, ‘‘sick building syndrome’’, whereby building occupantsexperience acute health and comfort effects that appear linked to time spent in a building, and ‘‘build-ing related illness’’, whereby illnesses are attributed directly to airborne building contaminants [81].

There are many types of antimicrobial nanoparticles that can be used as the active agent in anti-microbial coatings. Antimicrobial nanoparticles fall broadly into three categories [82]: naturallyoccurring antimicrobial substances (e.g. chitin, chitosan and some peptides), metals/metal oxides(e.g. Ag, TiO2, ZnO, Cu, CuO, MgO) and engineered nanomaterials (e.g. CNTs, fullerenes). Naturallyoccurring antimicrobial substances are, however, generally reported in water treatment and foodpreservation applications rather than in antimicrobial surfaces for infrastructure, and so are not ad-dressed in this review. The antimicrobial activity of the nanoparticles can be achieved through a rangeof different mechanisms. The nanoparticles can interact with microbial cells directly (e.g. by interrupt-ing transmembrane electron transfer, disrupting/penetrating the cell envelope, or oxidising cell com-ponents) and/or by producing secondary products that cause damage (e.g. reactive oxygen species ordissolved heavy metal ions) [82]. The main antimicrobial mechanisms of antimicrobial nanoparticlesare shown in Fig. 4.

Nanoparticle-based antimicrobials can offer significant benefits over conventional chemical, pho-tochemical or physical disinfection methods. Unlike some conventional antimicrobial agents, manyantimicrobial nanomaterials (e.g. Ag nanoparticles and Cu nanoparticles) exhibit strong toxicity to-wards a broad range of microorganisms found in industrial processes [83] and the human body, buta remarkably low toxicity to humans [84,85]. In contrast, many conventional disinfectants are toxicto humans; for example, most antifungal chemicals are non-specific to the organism affected andcan be detrimental to the environment (toxic to plants and animals) [86]. It has also been suggestedthat disinfection with some nanomaterial compounds (e.g. TiO2) can be more effective than conven-tional disinfection (e.g. with chlorine) [87] and, particularly when supported in a substrate, somenanoparticles have demonstrated good stability and long-term activity [84,88]. Opinions are dividedregarding the development of resistance by microbes to nanoparticle-based antimicrobial agents.

Fig. 4. The main antimicrobial mechanisms of nanomaterials, adapted from Li et al. [82].


Some studies report that there has been no evidence of microorganisms developing resistance tonanoparticles and that the development of resistance is unlikely due to the different antimicrobialmechanisms of nanoparticles when compared to that of many conventional antimicrobialagents [84,88]. Others have suggested that silver resistance already occurs extensively, but it is notrecognised due to a lack of testing for silver resistance [89].

Many effective nanoparticle-based antimicrobial agents are confirmed, but few are used in the con-struction industry for cost reasons. At present, Ag and TiO2 are the only commercially used antimicro-bial nanomaterials [90] and Ag nanoparticles are the most widely used [82]. There are many Agnanoparticle-containing fabrics [91] and antimicrobial Ag nanoparticle-containing interior paints [92]are available on the market. Many other commercial silver-based antimicrobial products on the mar-ket currently utilise silver ions or other forms of silver [93]; however, the favourable properties of Agnanoparticles over other forms of Ag (e.g. enduring antimicrobial properties) may see the use of otherforms of Ag phased out in favour of Ag nanoparticles in some applications.

4.1. Antimicrobial metal and metal oxide nanoparticles

The antimicrobial properties of some metals and metal compounds have been known since ancienttimes when, for example, Ag-based compounds were used for water treatment. In recent times, manymetal and metal oxide nanoparticles have been found to be particularly effective antimicrobials, moreso than the corresponding bulk materials due to their greater active surface area. Metal/metal oxidenanoparticles reported to have antimicrobial properties include Ag [84], Cu [84], CuO [94],ZnO [95,96], MgO [97], TiO2 [86]. The relative effectiveness of antimicrobial agents varies, but manyhave proven to be effective against a broad range of bacteria (even antibiotic resistant strains), viruses,fungi [86,89] and algae [83]. For example, the sensitivity of microbes to functional destruction by pho-tocatalytic nanoparticles such as TiO2 is reported to follow the order: viruses > gram-negative bacte-ria > gram-positive bacteria > endospores > yeasts > filamentous fungi [86]. The promising propertieshave prompted scientists and engineers to find cost effective and efficient methods for synthesisingthese antimicrobial nanoparticles and coatings from them.

4.1.1. Photocatalytic antimicrobial nanoparticlesThe antimicrobial properties of photocatalytic nanoparticles such as ZnO and TiO2 nanopartices are

related, at least in part, to their phototcatalytic activity. When irradiated with light of energy greater

Fig. 5. Schematic diagram of the photocatalytic formation of e� and h+ in TiO2 and the possible reaction pathways thereof, from[98].


than or equal to the band gap of the photocatalytic material, an electron (e�) is promoted from thevalence band to the conduction band resulting in the generation of a theoretical positively charged‘‘hole’’ (h+; electron vacancy) in the conduction band. The equation for this process is shown in Eq. (1).

photocatalystþ hv ! photocatalystþ e� þ hþ ð1Þ

where electromagnetic radiation is expressed as the product of Planck’s constant, h, and frequency, m.The h+ and e� are powerful oxidising and reducing agents, respectively. The strong oxidising behav-

iour of h+ enables it to react with OH� dissociated from water (Eq. (2)) to generate the highly oxidisinghydroxyl radical (OH�) (Eq. (3)), while the liberated electrons can diffuse to the surface of the photo-catalyst and reduce oxygen molecules adsorbed from the atmosphere to form reactive oxygen species.The e� reacts with molecular oxygen to form the superoxide anion, O�2 (Eq. (4)). The superoxide anionfurther reacts with H+ dissociated from water to produce HO�2 radicals (Eq. (5)). The reactive oxygenspecies are capable of decomposing microbes into CO2 and H2O [96–98]. The generation of h+ ande� and their possible reaction pathways is demonstrated with TiO2 in Fig. 5.

H2O$ Hþ þ OH� ð2Þhþ þ OH� ! OH� ð3Þe� þ O2 ! O�2 ð4ÞHþ þ O�2 ! HO�2 ð5Þ

Effective use of photocatalytic antimicrobial nanoparticles is limited to environments where thereis sufficient irradiation with light of the required wavelength (388 nm UVA light for TiO2, the cheapestand most abundant photocatalyst). It is, however, noted that incidental irradiation from natural andsome artificial light sources can be sufficient for TiO2 activation [99]. Escherichia coli (E. coli, a commontype of bacteria) cells are deactivated after about 1 h under outdoor UV light intensity and the cells cancompletely disappear after about 1 week of 1 mW/cm2 UV irradiation [100]. Typical indoor UV lightintensity is about several hundred nW/cm2; about three orders of magnitude less than outdoor UVlight intensity, and therefore photocatalytic degradation of microorganisms takes much longer in-doors. Despite this, it has also been shown that a reduction in bacteria numbers can continue in thedark following irradiation [101] and an anti-algal effect, although less than when irradiation was em-ployed, was still observed even without irradiation [83]. This suggests that other antimicrobial mech-anisms unrelated to photocatalysis also contribute to the antimicrobial activity of photocatalyticnanoparticles. Interestingly, a greater inactivation of bacteria and virus has been observed when usingAg+ irradiated with UV-C, UV-A or visible light than when using Ag+, UV-C, UV-A or visible lightalone [102]. This is suggestive of a synergistic effect between some antimicrobial agents andirradiation.

Doping of TiO2 to decrease the band gap, thereby allowing visible light activated photocatalysis, isan active area of research due to the greater indoor photocatalytic activity this promotes. Doping ofTiO2 with noble metals (Ag, Ni, Pt, Au, Ag, Cu, Rh, Pd) and oxides (ZnO, WO3, SiO2, CrO3) or non-metals(C, N, S, P) have been found to be effective [86]. This doping does increase the cost of nanomaterials,but visible light activated TiO2 is commercially available e.g. Kronos vlp 7000 [103] and new visiblelight photocatalysts continue to be developed. A new EU research project, SELFCLEAN (Novel self-cleaning, anti-bacterial coatings, preventing disease transmission on everyday touched surfaces) hasrecently been established, with the aim of developing Sn–Ni matrix-based coatings that incorporatedoped-TiO2 nanoparticles to form visible light-activated antimicrobial surfaces suitable for use in pub-lic places such as hospitals, schools, hotels and public transport [79].

Based on analysis by Future Markets Inc., the use of TiO2 nanoparticles in coatings and paints forsanitisation and elimination of MRSA is considered to be at mature market stage of commercialisation,while other TiO2 photocatalytic antibacterial nanocoatings are at the market entry stage [104].

4.1.2. The effect of metal/metal oxide physicochemical properties on antimicrobial activityCommon metal/metal oxide synthesis methods include chemical reduction (the most common

method for producing nano-Ag particles [89]), mechanochemical processing and physical vapour


synthesis (both used for commercial production of ZnO [96]). Other techniques, for example precipi-tation, thermal decomposition, hydrothermal synthesis [96], may also be used. Conventional nanoma-terial synthesis typically involves using hazardous chemicals, has low material conversion rates, highenergy requirements, and requires purification that results in the wasting of product [93]. Most novelproduction methods are focused on ‘‘green synthesis’’, which aims to replace hazardous chemicalswith non-toxic chemicals, use environmentally benign solvents, and renewable materials [93,105].The synthesis method and conditions have a major influence on nanoparticle size, shape, andmorphology [93].

Size and other physicochemical properties such as shape and crystallinity, have a significant effecton the antimicrobial properties of metal/metal oxide nanoparticles [82,88,96,106]. The manufacture ofnanoparticles with a narrow size distribution, specific shape or crystalinity is therefore of great inter-est. The antimicrobial activity of nanoparticles is inversely dependent on size which is representativeof reactive surface area [88,97]. For example, Ag nanoparticles of <10 nm are more toxic to bacteriasuch as E. coli than larger particles, and Ag nanoparticles 1–10 nm in diameter have been shown toinhibit certain viruses from binding to host cells [82]. Furthermore, triangular silver nanoplates werefound to be more toxic than Ag nanoparticle rods, spheres and Ag+ ions [82,107] because they containmore h111i facets, which are highly reactive [89,107,108]. The crystal structure and phase of thenanoparticles can also be of importance. For example, since the antimicrobial effect of TiO2 is due,at least in part, to its photoactivity, the crystal phase used will affect its antimicrobial properties be-cause the different crystal phases differ in their photocatalytic ability. It is worthwhile noting thatwhich TiO2 phase, or indeed which mixture of phases and in what ratio, is most photocatalytically ac-tive is widely debated [86].

Nanoparticles must be stable in order to remain effective as antimicrobial agents. Long-term sta-bility is of significant importance for antimicrobial nanoparticles used in applications such as paintswhere enduring, long-term antimicrobial activity is highly desirable. As nanoparticles aggregate, theireffectiveness as an antimicrobial diminishes due to the decreased density of atoms available on theputative surface [84,85,89,97]. Aggregation can be avoided by supporting nanoparticles on or in thematrix of other materials e.g. zeolite [109], activated carbon fibres [110–112], CNTs [113–115], fibreglass [116], Ag vanadate nanowires [77], SiO2 [84,117–120] or TiO2 [121,122]. (Supporting non-TiO2

antimicrobial nanoparticles on TiO2 is reported to enhance antimicrobial activity [123].) Many of thesesupport materials can be synthesised with a variety of morphologies – e.g. SiO2 can be made as anamorphous xerogel, aerogel, fibres, monodisperse nanospheres, microparticles, etc. [84]; TiO2 canbe made as nanoparticles or nanotubes [121], etc. – and thus a morphology with properties suitedto a specific application may be selected.

4.1.3. Antimicrobial coatings from antimicrobial nanoparticlesThe addition of Ag nanoparticles to commercial paints imparts antimicrobial properties on the

paint, reportedly even when present in concentrations as low as parts-per-million, and without dete-rioration of the colour of the final products [106,124]. It is reported that the addition of antimicrobialnanoparticles to paints can actually protect the aesthetic appearance of painted surfaces by preventingmicrobial growth that can stain paints and deteriorate paint properties [95]. The addition of 5 vol.%30–40 nm ZnO [95] or 5 wt.% MgO nanoparticles [97] to an interior paint (aqueous acrylic dispersionand modified phenylpropyl type paint, respectively) resulted in potent antibacterial and antifungalproperties. The actual antimicrobial effectiveness is influenced by paint properties including type ofacrylic dispersion, total pigment volume concentration, as well as the morphology of the surface [95].Focusing on Ag and Cu nanoparticles, Zeilecka et al. [84] found that the addition of SiO2 particle-sup-ported Ag or Cu nanoparticles to silicon acrylic emulsion paints improved the antifungal activity of thepaints (Fig. 6-I) and had a negligible effect on the wetability of a painted surface. Kumar et al. [125]developed a potent antimicrobial vegetable oil-based paint with embedded Ag nanoparticles (Fig. 6-II) that was produced in a single step at ambient conditions without using external reagents or largeamounts of energy. Holtz et al. [77] demonstrated that Ag nanoparticles supported on Ag vanadatenanowires dispersed in a commercial water-based paint was effective against MRSA. This water-basedpaint impregnated with 1% (m/v) nano-Ag particles supported on Ag vanadate nanowires showed a4 mm exclusion zone of inhibition against the bacterial growth of MRSA stains (Fig. 6-III).

Fig. 6. I. (a) An SEM image of Cu nanoparticles immobilised on SiO2 particles (Cu/SiO2), (b) a photo showing that algae did notgrow on a surface coated with paint containing Cu/SiO2 (0.5 ppm Cu nanoparticles), and, (c) a photo showing that algae grew ona surface coated with Cu/SiO2-free paint. Adapted from [84]. II. (a) A TEM image of Ag nanoparticles synthesised in drying oils,(b–d) photographs of glass slides onto which aqueous suspensions of S. aureus were sprayed and each black dot corresponds to abacteria colony; (b) is an uncoated slide, (c) is a slide coated in only drying oil paint without nanoparticles, (d) is a slide coatedwith drying oil paint Ag nanoparticles. Adapted from [125]. III. (a) A TEM image of Ag nanoparticles supported on Ag vanadatenanowires, (b and (c) results of inhibition zone tests against MRSA for (b) a glass substrate painted with commercial water-based paint, and, (c) a glass substrate painted with the same paint but with 1% (m/v) Ag vanadate nanowires added. Adaptedfrom [77].


Synthesis process scalability, cost, final product quality and workability are significant consider-ations for the use of antimicrobial nanomaterials in civil structures. Cost effective, scalable synthesismethods are required for the use of these materials to be commercially viable. Synthesis of SiO2–Al2O3

supported Ag nanoparticles in a fluidised-bed reactor – a scalable reactor style – has been reported;


however, the scalability and cost-effectiveness of this process is not explicitly discussed [126]. Thelower cost (and comparable antimicrobial activity) of non-Ag antimicrobial metal nanoparticles suchas Cu, Zn and Mg could attract commercial interest in these materials [90,97]. Effective antimicrobialshave also been formed from metal nanoparticles supported on common fillers such as vermiculites ormontmorillonites [90,127]. For example, Drelich et al. [90] found that the addition of vermiculite sup-ported Cu nanoparticles improved the antibacterial properties of the paint. Vermiculite is an inexpen-sive mineral extensively used as a filler in paints, plastics, fireproof material, and concrete, and thus itis envisaged that the addition of vermiculite supported Cu nanoparticles would have a minimal det-rimental impact on the properties of the paint (other than antimicrobial properties).

4.2. Antimicrobial engineered nanomaterials

Engineered nanocarbons such as buckminsterfullerenes (C60) and CNTs, are proven antibacterialagents, and quantum dots (nanoparticles of a semiconductor material) [128] are also reported to dis-play antimicrobial activity. There is debate regarding the antimicrobial mechanism of engineerednanomaterials; however, particle size is reported to influence the effectiveness of engineered nanocar-bons as antimicrobial agents.

Smaller aggregates of C60 exhibit relatively stronger antibacterial activity and Lyon et al. [129]reported that the increase in toxicity was disproportionately higher than the associated increase inputative surface area. Regarding the antibacterial mechanism of C60, photo-induced [130] and non-photo-induced [131,132] generation of reactive oxygen species by C60 and its derivatives causingdamage such as DNA cleavage [133] and lipid peroxidation [134] have been suggested. Some, how-ever, argue that photo-induced reactive oxygen species generation does not occur, but rather C60

exerts an oxidative effect (independent of reactive oxygen species) at the membrane interface whichhinders respiration [135,136] and antibacterial activity can occur in the absence of light and/oroxygen [137].

The exact mechanism for bacterial inactivation in the presence of CNTs is also poorly understoodand subject to debate. Disruption of intracellular metabolic pathways, oxidative stress [138,139]and physical membrane damage [140] have been discussed as possible mechanisms for CNT toxic-ity [141]. CNT diameter is a key factor governing the antibacterial effects and single-walled carbonnanotubes (SWCNTs) are much more toxic to bacterial cells than multi-walled carbon nanotubes(MWCNTs) (Fig. 7) [141]. Highly purified SWCNTs exhibit strong antimicrobial activity. The likelymechanism is cell membrane damage resulting from direct contact with SWCNT aggregates [140].Kang et al. [141] state that the enhanced bacterial toxicity of SWCNTs may be attributed to: (i) a smal-ler nanotube diameter that facilitates the partitioning and partial penetration of nanotubes into thecell wall; (ii) a larger surface area for contact and interaction with the cell surface; and/or (iii) uniquechemical and electronic properties conveying greater chemical reactivity. Bactericidal cytotoxicity in-creases with improvements in physicochemical properties that enhance CNT-cell contact opportuni-ties [142,143]. For example, CNTs that were uncapped, unbundled, short and well-disperseddisplayed greater toxicity than capped, bundled, longer or less dispersed CNTs [143]. In contrast toconventional antimicrobials, it is expected that microbes are unlikely to develop resistance againstantimicrobial nanomaterials that physically tear microbe membranes such as CNTs. Microbes developresistance against traditional antimicrobial agents by modifying the biochemical pathways targetedby the drugs; however, it would be difficult for microbes to change the composition of their mem-branes so that they could prevent physical damage by sharp nanomaterials [144].

Engineered nanocarbons such as CNTs are also being used as supports for the immobilisation of en-zymes with antimicrobial properties, such as lysostaphin [78], laccase, and chloroperoxidase [145].Lysostaphin enzymes bind to the cell wall of target bacteria and damage the cells [78]. Laccase andchloroperoxidase generate, in a controlled manner, I2 and chlorine-containing compounds, respec-tively. As opposed to the addition of halogens (a class of conventional, broad spectrum antimicrobialagent which generally requires high concentrations of I2 or Cl2 and poses health and environmentalburdens), controlled enzymatic generation of I2 or chlorine-containing compounds offers a potentiallynon-toxic, environmentally friendly alternative to chemical disinfection [145]. Immobilisation of theenzymes on nanocarbons serves to stabilise the enzymes and prevent their release into the

Fig. 7. SEM images of E. coli cells exposed to CNTs; (a) cells incubated with MWCNTs for 60 min, and, (b) cells incubated withSWCNTs for 60 min. From [141].


environment [78]. With respect to CNTs, the molecular curvature stabilises a wide range of en-zymes [146], the high surface area-to-volume ratio enables high loadings without diffusional limita-tions [147] and the large aspect ratio leads to efficient entanglement within the solid matrix,thereby allowing enzymes to be retained [78,148].

Paints containing nanocarbon-supported enzymes can be applied to a broad range of surfaces andmake them sterile over extended operational times [145]. Pangule et al. [78] designed a lysostaphin-MWCNT conjugate that, when incorporated into a paint, was stable during storage for up to 1 monthin dry conditions and was highly efficient at killing MRSA. Grover et al. [145] demonstrated that theincorporation of laccase-discrete polyethylene glycol-MWCNT or chloroperoxidase-MWCNT conju-gates into paint resulted in substantial bactericidal and fungicidal activity and the paints were stablewhen stored for 1 week in dry form. Using a laccase-discrete polyethylene glycol-MWCNT paint com-posite, >99.99% of S. aureus and E. coli were killed within 30 and 60 min, respectively, and >90% and>99% of Bacillus cereus and Bacillus anthracis-DSterne spores were killed within 1 and 2 h, respectively.No bactericidal activity occurred when using a laccase-free, paint formulation. The chloroperoxidase-MWCNT paint composite killed >99.99% of bacteria (S. aureus and E. coli) within 30 min compared to<30% in the additive-free control. The chloroperoxidase-MWCNT paint composite was ineffective inkilling fungal spores.

5. Self-cleaning surfaces

Self-cleaning surfaces have the potential to reduce costs associated with maintaining the cleanappearance of a range of surfaces found in civil infrastructure, including glass surfaces (e.g. glazing,mirrors, lights), concrete surfaces (e.g. pavements and façades) and tiled surfaces (e.g. façades and


roofs). Self-cleaning surfaces can be classified into two broad categories: superhydrophobic surfacesand superhydrophilic surfaces.

5.1. Superhydrophobic surfaces

Hydrophobic surfaces are defined as surfaces upon which a water droplet forms a contact angle of>90�. Superhydrophobic surfaces are a subclass of hydrophobic surfaces upon which water dropletsform contact angles of >150�. Contact angle is defined as the angle between the solid surface andthe tangent line of the liquid phase at the interface of the solid, liquid and gas phases, as illustratedin Fig. 8. In addition to being superhydrophobic, contact angle hysteresis must also be low(<5–10� [149]) if the surface is to exhibit self-cleaning properties [150]. Contact angle hysteresis –the difference between the advancing contact angle and the receding contact angle – representshow easily a droplet will roll off a film. Superhydrophobicity coupled with low contact hysteresis en-able self-cleaning because water beads and rolls off, removing dust and dirt as it does so.

The hydrophobicity of a surface is determined by its surface structure and the surface chemistry orsurface energy. There are many hydrophobic surfaces found in nature: some leaves [151–153] (such as

Fig. 8. The contact angle (h) of a hydrophobic and superhydrophobic surface, adapted from [151].

Fig. 9. SEM micrographs of superhydrophobic surfaces found in nature. The hierarchical structure of the surface of a lotus leaf at(a) low, and, (b) high magnification with (inset) a photograph of the contact angle of water on the lotus leaf surface. The unitarystructure of the underside surface of a ramee leaf at (c) low, and, (d) high magnification with (inset) a photograph of the contactangle of water on the surface. Adapted from [152].


the leaves of lotus, cabbage, rice, taro, ramee, silver ragwort, and Chinese watermelon plants); thewings of butterflies [152]; and the legs of water striders [154]. Their surface structure is generallyeither ‘‘hierarchical’’, with nanometer-scale features upon micron-scale features, or ‘‘unitary’’, where-by the surface consists of a mat of micro- or nano-fibres [152] (Fig. 9). Man-made attempts to producehydrophobic surfaces are often strongly rooted in biomimicry [155] – the mimicking of hydrophobicsurfaces found in nature – and both hierarchical and unitary style surfaces have been produced. Syn-thesis of hydrophobic surfaces typically entails either: (i) controlled surface roughening of a low sur-face energy material to achieve a specified micro and nanostructure; or, (ii) producing surfaces with aspecified microstructure and nanostructure and then modifying the surface chemistry (e.g. with amaterial possessing low surface energy) [152]. In both scenarios the superhydrophobicity is ultimatelyachieved using a combination of high surface roughness and low surface energy.

As discussed in the review of self-cleaning coatings by Ganesh et al. [152], the surface chemistry ofmaterials with desirable micro/nanostructures can be chemically modified using a variety of tech-niques including wet chemical reactions, hydrothermal reactions, electrochemical deposition, lithog-raphy, self-assembly, layer-by-layer assembly, etching, chemical vapour deposition, sol–gel methods,and polymerisation reactions. Common low surface energy materials used to produce hydrophobicsurfaces include silicones such as polydimethylsiloxane (PDMS) and fluorocarbons such as polytetra-fluoroethylene (PTFE or Teflon), although low-density polyethylene, polystyrene, alkylketene, polycar-bonate, polyamide and others have also been used.

Surface roughness increase surface area, which geometrically enhances hydrophobicity accordingto the Wenzel model [156]. Surface roughness also facilitates the entrapment of air at the surface,which allows water droplets to partially sit on air (air is considered to have a water contact angleof 180� [157]) thus enhancing hydrophobicity in accordance with the Cassie-Baxter model [158]. Suit-able surface micro/nanostructures for superhydrophobicity can be created using templates (biologi-cal [153] or otherwise [159]), surface patterning techniques such as etching [160,161] and casting,or by depositing nanoparticles on a surface.

Because particle size renders nanoparticles transparent in the visible spectrum, functionalisednanoparticles have attracted significant attention in the development of transparent superhydropho-bic coatings for self-cleaning glass [162]. Nanomaterials investigated include inorganic nanoparticles(and/or nanotubes) such as SiO2 [163–165] and SiC [166], and engineered nanomaterials such asCNTs [150,157]. The micrometer-scale roughness of superhydrophobic films typically leads to stronglight scattering [164,167]; however, careful placement of nanoparticles simultaneously achieve thesurface roughness required for superhydrophobicity and low contact angle hysteresis, while minimis-ing light scattering [164]. Dip coating [162], layer-by-layer assembly [164,167] and spray coat-ing [168] have all been demonstrated as simple and inexpensive techniques for the formation ofSiO2 nanoparticle-based superhydrophobic coatings over large areas.

The low refractive index of SiO2 nanoparticle-based coatings has also lead to anti-reflective prop-erties when used on glass [164,167]. Bravo et al. [164] performed layer-by-layer deposition of SiO2

nanoparticles on a glass substrate to form superhydrophobic coatings with contact angle hysteresisas low as <5� and optical transmission levels above 90% throughout most of visible region of the spec-trum. Also using a layer-by layer technique on a glass substrate, Li et al. [167] formed silica nanopar-ticle-based films that were initially superhydrophilic, but became superhydrophobic followingtreatment using perfluorooctyltriethoxysilane. Transmittance of the perfluorooctyltriethoxysilane-modified SiO2-coated glass was generally >90%, contact angle hysteresis was <1� and both the trans-mittance and superhydrophobicity were reported to be stable under ambient conditions for over2 months.

Dip coating produces a sparser nanoparticle coating than other nanoparticle coating preparationtechniques such as convective assembly [162]. This can result in a greater surface area, as shown inFig. 10, and superior hydrophobicity. Glass that was dip coated with unfunctionalised SiO2 nanopar-ticles, and subsequently fluorinated and sintered, demonstrated superhydrophobicity, almost 100%transparency in the visible light region and the ability to withstand wear [162].

CNTs are excellent for creating superhydrophobic surfaces. Controlling the density of the CNTsallows control of surface roughness and the CNTs can be functionalised post-synthesis to obtain thesurface chemistry required [169]. Aligned CNTs can be grown directly on a glass substrate. Bu

Fig. 10. I. A schematic illustration of the larger surface area of the sparser SiO2 nanoparticle layer produced by (a) dip coatingthan by (b) convective assembly. II. SEM images of the surfaces of functionalised SiO2 nanoparticle films produced by (a) dipcoating and (b) convective assembly. Higher resolution SEM images and photographs of a water droplet on the respectivesurfaces (inset). III. (a) A photograph of the functionalised, SiO2 dip coated glass that demonstrated good transparency andsuperhydrophobicity. Adapted from [162].


et al. [169] demonstrated that optical transmittance though a superhydrophobic surface consisting ofpost-synthesis functionalised, aligned CNTs grown on a glass substrate can be increased by growingshorter CNTs. Optical transmittance in the visible region when using the shorter (5 lm) CNTs was,however, still low (<20%). Tang et al. produced a translucent superhydrophobic surface using a micro-patterning technique on glass whereby a grid of CNTs were synthesised in �17 � 17 lm cells with�13 lm free space between cells containing CNTs, thereby forming a grid [170] (Fig. 11-I.a). Whilelight mainly passes through the free-space, the surface appears uniform with a transparency of�63% due to the �100 lm resolution of the human eye. Images viewed though the surface do, how-ever, appear blurred at long distances because the micro-pattern array diffracts light (Fig. 11-I.b). Bet-ter visible light transparency has been observed when using randomly orientated CNTs.

A superhydrophobic coating with a transparency of �79% in the visible light region was formedwith a low cost, one step spray coating method using polymer-functionalised randomly orientatedCNTs (Fig. 11-II) [168]. Han et al. [150] produced a transparent and conductive, hydrophobic film froma mat of fluorinated silane solution/randomly orientated CNT hybrid film using a spray coating tech-nique suitable for the treatment of large surface areas. Superior hydrophobicity and transmittance(>150� and >90%, respectively) were observed with the addition of SiO2 nanoparticles (Fig. 11-III).Meng et al. [157] used dip coating to produce a transparent, conductive superhydrophobic coatingfrom a fluorinated, randomly orientated CNT mat on glass. About seven coats proved optimal, achiev-ing water contact angles of 160� and transmittance of �80% in the visible region.

Fig. 11. I. Micro-patterned CNTs on glass: (a) An SEM image of the surface and, (b) an optical microscope image of thecharacters ‘En’ viewed through the micro-patterned CNT/glass surface, adapted from [170]. II. Polymer-functionalised,randomly orientated CNTs film on glass: (a) an SEM image and, (b) a photograph of water droplets on the transparent,superhydrophobic film, adapted from [168]. III. CNT/silane hybrid and SiO2 nanoparticle film spray coated glass: (a) an SEMimage and, (b) a photograph of water droplets on the transparent, superhydrophobic film, adapted from [150].


Self-cleaning hydrophobic paints are potentially valuable in the construction industry for thereduction of costs associated with maintaining building walls and facades. A number of self-cleaningpaints are commercially available e.g. Lotusan� [171] and Deletum 5000 [172]. Some self-cleaningpaints may also be employed as anti-graffiti paints. Hydrophobic surfaces can also be used to reducethe accumulation of snow or eliminate the formation of ice on solid surfaces [152,173], which mayfind applications on roads and windows in regions with sub-0 �C weather conditions; the anti-icingeffect allows the transparency of glass surfaces to be maintained.

5.2. Superhydrophilic surfaces

Hydrophilic surfaces have a water contact angle of <90� and superhydrophilic surfaces are thosewith a contact angle that is close to 0� (Fig. 12). Materials used in hydrophilic surfaces are typically

Fig. 12. The contact angle (h) of a hydrophilic and superhydrophilic surface, adapted from [151].

Fig. 13. The mechanism for photoinduced hydrophilicity of TiO2 by the formation of surface hydroxide groups, from [175].


photocatalytic (e.g. TiO2). Photocatalytically generated h+ oxidise the lattice oxygen at the surface ofthe material resulting in oxygen vacancies that can be filled by adsorbed water. This causes the forma-tion of surface hydroxide groups, which increase the hydrophilicity of the surface, as shown for TiO2 inFig. 13. The static contact angle of a TiO2 surface can drop to almost 0� after UV irradiation [174].

Hydrophillic photocatalytic surfaces have a further benefit since they also facilitate the decompo-sition a broad range of organic pollutants (aromatics, polymers, dyes, surfactants [155]) and manycompounds found in stains on outdoor surfaces when in the presence of oxygen. These stains are typ-ically composite materials originating from atmospheric pollutants that adhere to surfaces by organicbinders such as hydrocarbons and fatty acids (for example, –COOH carboxylic groups bind with cal-cium ions present in concrete), but can also trap atmospheric particles and dusts [175]. Self-cleaningphotocatalytic superhydrophilic coatings can also catalyse the degradation of oily substances to assistremoval [176,177]. This is a key benefit over many self-cleaning surfaces based on a hydrophobic ef-fect since oily contaminants can lead to the loss of hydrophobicity in these [173,177]. The self-clean-ing mechanism of photocatalytic surfaces are, therefore, twofold: many organic molecules aredecomposed to CO2 and H2O when irradiated with UV light; and, when water contacts the surfaceit forms ‘sheets’ and carries away dirt and other contaminants. There is a dual effect whereby the hy-droxyl groups that form on the surfaces not only contribute to superhydrophilicity, but also to the for-mation of hydroxyl radicals that play an important role in the decomposition of organiccompounds [175]. It is noteworthy that the adsorption of organic contaminants can cause a photocat-alytic superhydrophilic surface to become hydrophobic, and therefore sufficient photocatalytic degra-dation of these contaminants may be a requirement for superhydrophilic properties to bemaintained [175]. By producing a photocatalytic surface with a roughness whereby air can intrude be-tween the water droplet and the surface, the wettability of the surface (from hydrophobic to superhy-drophilic) can be controlled by controlling irradiation [100] (Fig. 14). When sufficiently irradiated, thesurface is superhydrophilic due surface hydroxide groups formed as a result of the photocatalytic ef-fect. When in the dark for a sufficient duration, the surface is hydrophobic (e.g. �120�) in accordancewith the Cassie-Baxter model, since the photocatalytic effect is not present.

TiO2 is well-suited to use in self-cleaning coatings. It is cheap and readily available, non-toxic (as abulk material – the toxicity of TiO2 nanoparticles is contended [138]), can be produced in a variety ofmorphologies [176,177,179,180], easy to deposit as a thin film, chemically inert in the absence of UVlight and can easily reach the state of superhydrophobicity upon weak solar irradiation in an ambientatmospheric environment [152,175]. Furthermore, although TiO2 has a high refractive index, evenantireflective self-cleaning coatings – a requirement for applications such as self-cleaning coatings

Fig. 14. A schematic diagram of the effect of UV irradiation on the contact angle of water on photocatalytic surfaces, from [178].


for solar panels – can be produced by supporting the TiO2 nanoparticles on SiO2 particles [181]. Super-hydrophilicity also imparts anti-fogging characteristics which is useful in many applications whereglass is used. Since a thin film forms rather than droplets, the surfaces naturally dry out quicklyand prevent fogging up when in contact with steam or condensation [151,182].

Due to optical property requirements, the glazing industry typically use deposits of smooth, robust,nanocrystalline TiO2 films. Nanoparticles also provide a relatively large surface area per unit volume,which enhances light harvesting, facilitates the diffusion of charge carriers [176] and can be producedusing scalable techniques such as dip coating [183,184]. Transparent and photocatalytic, superhydro-phobic films have been formed from TiO2 with nanoparticle or nanotube morphologies. Kimet al. [176] produced a superhydrophilic nanoparticulate TiO2 layer with 60–90% transmittance inthe visible range. Mor et al. [185] and Tan et al. [179] produced aligned TiO2 nanotube based filmswith �80–95% transmittance of visible light.

Nanoparticle size and film thickness are of considerable importance not only for transparency butalso for photocatalytic activity. A balance between these must be achieved to maximise superhydro-philicity based on photocatalytic activity [152,155,186]: the small particles used to achieve largeeffective surface areas and optical transparency will also have higher rates of electron–hole recombi-nation. Thicker films (up to a few microns in thickness) have shown improved photocatalytic activity,but optical clarity and durability are poor [152,155]. A nanotube TiO2 morphology can offer the ben-efits of high surfaces area (since there are internal and external surfaces available), and the wall thick-ness can be tailored to achieve optical transparency while minimising charge carrier recombination[179]. As such, TiO2 nanotube arrays and even TiO2 nanoparticles demonstrate superior photocatalyticperformance to flat TiO2 surfaces [176,179].

Solution processed nanoparticulate TiO2 coatings are well suited for application to large areas, butthey are generally soft and have low abrasion resistance, which can make them unsuitable for outdooruse [176]. A number of strategies to improve the abrasion resistance/surface hardness of TiO2 nano-particle based coatings on glass have been examined. Kim et al.[176] used laser-induced local meltingof TiO2 nanoparticles onto the glass, which improved surface hardness. There was some reduction inphotocatalytic activity stemming from a reduction in particle surface area, but the films were super-hydrophilic with 60–90% transmission in the visible light region. Yaghoubi et al.[183] achieved max-imum film hardness using TiO2 nanoparticle on a soda glass surface by annealing at 300 �C. Annealingcauses sintering and chemical attachment of the nanoparticles, improves TiO2 crystallinity and facil-itates the removal of volatile compounds that can impede photocatalytic activity for the desired pur-pose. TiO2 films on glass are frequently annealed to a maximum of 500 �C due to the anatase to rutilephase transition and substrate softening effects at higher temperatures; however, Yaghoubiet al. [183] found that mechanical deterioration occurred when films were annealed at >300 �C, pos-sibly due to Na inclusions into the film from the soda glass resulting in low strength Ti–O–Na–O–Tibonds at the neck between the grains. Notably, 300 �C is achievable using heat guns or other low costtechniques and could therefore potentially be done on site, if necessary.

Since the superhydrophilicity of TiO2-based films is photoinduced, the superhydrophilicity of suchfilms can be unstable under some conditions. For example, the utility of superhydrophilic films is lim-ited because their performance is gradually lost in the absence of UV light [187]. Superhydrophilicityindependent of UV irradiation can be achieved by tailoring TiO2 morphology and film structure suchthat superhydrophilicity is no longer a function of photoactivity but structure. Films that maintainedsuperhydrophility even after 240 days in the absence of UV light were produced by Pan et al. [187]from randomly orientated TiO2 nanofibre mats of various morphologies (belts, bead-pierced fibres, fi-bres with pores) such that liquid not only spreads over the surface but penetrates the mat and diffusesinside the material (Fig. 15-I). Chen et al. [188] showed that although a flat TiO2 film required UV irra-diation to induce superhydrophilicity, a film consisting of nanotube arrays of anodic titanium oxide ona the surface of thin films of anatase TiO2 had a capillary effect and were superhydrophilic even in theabsence of UV light (Fig. 15-II).

5.2.1. Applications of superhydrophilic surfacesMany self-cleaning products that work on the principle of hydrophilicity have been commercia-

lised for use in construction, with superhydrophillic self-cleaning surfaces available as tiles, glass,

Fig. 15. I. (a) A schematic diagram of the TiO2 nanofibre film wetting process, (b) a TiO2 nanofibre film with a beaded-piercedfibre morphology, and (c) the �0� contact angle of water and oily substances on the TiO2 nanofibre films regardless of UVirradiation, adapted from [187]. II. (a) An SEM image of the cross-section of nanotube arrays of anodic titanium oxide on thesurface of a thin film of anatase TiO2, (b) an SEM image of the top view of nanotube arrays of anodic titanium oxide on thesurface of a thin film of anatase TiO2 and, (c) the �0� contact angle of water on film in the absence of UV irradiation, adaptedfrom [188].


aluminium siding, plastic films, tent materials, cement, etc. [175,182]. Japan, in particular, has adoptedTiO2-based self-cleaning surfaces, with several thousand tall buildings covered in self-cleaning tiles,widespread use of self-cleaning glass (e.g. 20,000 m2 of self-cleaning glass was used in the Central Ja-pan International Airport), and self-cleaning tent materials used in bus and train stations, sport cen-tres, sunshades in parks, etc. [182]. It is estimated that there are over 2000 companies makingphotocatalytic products, mainly exterior construction materials, with the internal Japanese marketvalues at ¥30 billion in 2003 [189]. It is estimated that, in Europe, 1,000,000 m2 of photocatalytic ce-ment based surface was produced in 2007 [2], and from December 2006 to March 2008 the number ofTiO2-based photocatalytic products (concrete tiles, cement plasters and paints, concrete slabs, etc.) onthe market in Italy increased from 44 to 69 [190].

Self-cleaning glass became available on the consumer market when Pilkington launched Activ™ in2001 [174]. Many other glass manufacturers (e.g. Cardinal Glass Industries, Saint-Gobain, PPG Indus-tries) have introduced other self-cleaning glass products since this time [152]. Pilkington Activ™ com-prises of a hard, thin, transparent, mechanically robust photoactive film of �15 nm thicknanocrystalline TiO2 that is applied to clear float glass using an online chemical vapour deposition

Fig. 16. (a) The Dives in Misericordia, a church constructed of TiO2-containing self-cleaning cement, and (b) the effect of TiO2 andits particle size on the photobleaching of mortar surfaces using rhodamine dye as a staining agent from [191].


process [174]. At the time of its introduction, the use of such films was preferable to TiO2 particle-based films which were not mechanically stable, nor was synthesis highly reproducible [174]. As seenfrom the discussed studies, there have continued to be significant improvements in the properties andpreparation methods of films containing TiO2 nanoparticles, nanofibres and nanotubes since this time,and such films may demonstrate greater activity.

There are limitations to using photocatalytic coatings on transparent organic materials such aspolyvinylchloride (PVC) because, unlike tiles and glass, it cannot withstand the high sintering temper-atures required to anchor the photocatalytic layer. Additionally, an intermediate layer is required toseparate the photocatalyst from the PVC itself since photogenerated electrons and ‘‘holes’’ may de-grade the PVC itself [175].

The addition of nanosized TiO2 to mortars and concretes can create self-cleaning mortars/concretesthat maintain their aesthetic characteristics, such as colour, even in harsh urban environ-ments [175,191]. Structures exploiting these products on exterior surfaces include a school in Mortara,Italy (completed in 1999), the music and art city hall Cité de la Musique in Chambéry, France (com-pleted in 2000) and the Dives in Misericordia church in Rome, Italy (completed in 2003;Fig. 16a) [191]. Long term monitoring shows the photoactive products are performing well. The facadecolour of the Cité de la Musique hall remained almost constant from 2000 to 2005. Only a slight changein colour was observed during 6 years of monitoring the of the Dives in Misericordia church, and colourvariations from inorganic substances could be completely eliminated by washing with water [175].The use of TiO2 nanoparticles (rather than larger TiO2 particles) substantially increases the photoac-tivity of TiO2-containing cements (Fig. 16b) [191].


The market for TiO2 nanoparticle-based self-cleaning products, such as photocatalytic cement addi-tives, and transparent self-cleaning coatings, are currently considered to be at the market entry stageof commercialisation. This is expected to develop into a mature market within 3–5 years [104].

6. Air purifying surfaces

A number of directives have been established in recent decades to impose limit values on pollu-tants such as sulfur oxides (SOx), nitrogen oxides (NOx), particulate matter and lead in ambient air.Some pollution emission targets are being achieved, while other pollutants, such as NOx, which is gen-erated during combustion, remain problematic [192–194]. In large cities, pollutant concentrations atstreet level are often high because the dispersion of automobile exhaust is hindered by surroundingtall buildings – the so called street canyon effect [195]. Indoor air pollution from substances includingNOx, carbon oxides (CO and CO2) and volatile organic compounds (VOCs) from combustion, construc-tion materials and consumer products, is also a health concern. Indoor air pollution levels can exceedthose found outdoors. This is concerning because many modern societies spend long periods of timeindoors, where prolonged exposure to these pollutants have been linked to sick building syn-drome [196]. Air purifying surfaces in the urban environment have consequently attracted attention.The EU currently funds the LIGHT2CAT project [197], which aims to develop highly efficient visiblelight-activated TiO2 for inclusion in concretes for use across Europe (even in areas with low naturallight intensities) for the improvement of ambient air quality.

Photocatalysts can decompose a broad range of oxides and organic compounds found in indoor andoutdoor environments that can cause or contribute to range of health and environmental issues [192].The basic decomposition mechanism involves photocatalyst irradiation to generate radicals that sub-sequently convert pollutants into innocuous compounds [192]. For example, the widely accepted reac-tion pathway for the photocatalytic conversion of NOx compounds nitric oxide (NO) and nitrogendioxide (NO2), is as follows [175,198]. NO diffuses from the environment onto the photocatalyst sur-face where it is oxidised by photocatalytically generated HO�2 or OH� (as described in Eqs. (1)–(5)) toform NO2 (Eqs. (6) and (7)). NO2 (both photocatalytically generated and directly from the atmosphere)also reacts with photocatalytically generated OH� to form nitrate ions (NO�3 ) (Eq. (8)). These are harm-less in the small quantities that form.

NOþHO�2 ! NO2 þ OH� ð6ÞNOþ OH� ! NO2 þHþ ð7ÞNO2 þ OH� ! NO�3 þHþ ð8Þ

The above described reaction pathway and the final, stable products of NOx and those of otherphotocatalytically degradable pollutants has been contended by some researches. Langridgeet al. [199] found that photoenhanced uptake of NO2 onto a TiO2-based self-cleaning window wasaccompanied by the formation of gaseous nitrous acid (HONO). This is a harmful respiratory irritantthat is readily photolysed by solar irradiation to form OH� together with the re-release of NOx asNO. In contrast, Laufs et al. [200] observed that HONO formed on all paints (not just photocatalyticones) in the dark but the photocatalytic paints decomposed the HONO to HNO3=NO�3 upon irradiationwhereas non-photocatalytic paints did not. Auvinen et al. [201], focusing on photocatalytic degrada-tion of VOCs, suggested that although the end products of the chain reactions caused by photocatalyt-ically generated radicals are carbon dioxide and water if complete mineralisation occurs, if completemineralisation does not occur, a great number of stable, harmful side products may form. These con-tradictions demonstrate that further fundamental research on photocatalytic reaction pathways andtheir products may be required. As part of EU project CLEAR-UP, Geiss et al. [202] found that photo-catalyst-containing paints produce particular carbonyls through the photocatalytic breakdown ofpaint constituents. These carbonyls may have adverse effects on health and comfort. This must be ta-ken into consideration in indoor applications in particular, where the replacement rate of air is low.The development of photocatalytic paints optimised to minimise the emission of harmful substanceshas been suggested, particularly where photocatalytic paints are to be used indoors.

Fig. 17. An illustration of the removal of NOx from air by concrete containing photocatalytic particles.


Photocatalysts such as TiO2 can be incorporated into products, for example paints and cements.These can easily be applied on internal walls and ceilings, external surfaces of buildings (walls androofs), roads, public outdoor furniture, footpaths, etc. thereby creating innovative air-purifying sur-faces in our built environment [192,195]. In photocatalytic cements, the NO�3 formed reacts with cal-cium in the cement to form calcium nitrate, a water soluble salt that can be removed by rainwater(Fig. 17). Air pollutants at concentrations of 0.01 ppm to 10 ppm, which correspond to the range fromthose in the ordinary environmental atmosphere to those in highway tunnels, can be efficiently re-moved by such photocatalytic cementitious materials [182].

There are three main application techniques that have been suggested for producing photocatalyticconcrete pavements [203]: (i) application of a thin, photocatalyst-containing cementitious layerwhere the photocatalyst is added as a powder or a suspension during cement making; (ii) applicationof a photocatalyst-containing solution onto the concrete; or (iii) application by sprinkling the photo-catalyst onto the fresh concrete. Each application method has advantages and disadvantages. The useof a coating results in larger proportions of the TiO2 present being irradiated; however, the coated sur-faces are susceptible to loss of TiO2 from wear [204,205]. Thin cementitious layers can be more dura-ble, retaining photocatalytic activity after abrasion and wear [206]; however, the photocatalyticactivity of the embedded nano-sized TiO2 is significantly lower than if the TiO2 was applied as afilm [207]. The photocatalytic activity of TiO2 nanoparticles coated on cementitous materials is alsoreported to be lower than the same TiO2 nanoparticles in suspension. This is possibly due to (i) thereduced effective TiO2 surface area, since part of the nanoparticle surface is in contact with cementi-tious material and hence not available for contact with pollutants; and (ii) the presence of ionic spe-cies that contribute to charge recombination [208].

P25 by Degussa (Germany), a common TiO2 nanopowder that is used in commercially availablephotocatalytic cementitious products [209]. Its excellent photocatalytically activity is attributed toboth the 70:30 mixture of anatase:rutile phase composition and small, �21 nm primary crystal size[190,210]. As with all catalytic processes, TiO2 particles that are finely divided and well dispersed offerthe highest exposure to the surrounding environment, thereby improving photocatalytic effi-ciency [211]. In fact, it has been demonstrated that TiO2 particle size can have a greater effect onNOx degradation rate than the quantity of TiO2 in a cemetitious mortar [198]. However, photocatalystparticle size cannot be optimised in isolation since the particle size also influences the pore structureof cementitious materials.

A study of hardened TiO2-containing cement showed that, although both micrometer and nanome-ter scale TiO2 particles tend to agglomerate, micrometer scale TiO2 particle agglomerates are smaller,have bigger pores and are better dispersed than TiO2 nanoparticles (Fig. 18) [212]. As a result, activephotocatalytic surface area was dependent also on the size of the molecule to be degraded. For exam-ple, NOx (which have relatively small dimensions of 100–200 pm) can easily penetrate both micronand nano-sized TiO2 particle clusters and could therefore utilise the higher specific surface area of

Fig. 18. The structure of hardened cement containing (a) micrometer size TiO2 particles, and, (b) TiO2 nanoparticles, from [212].


nanoparticles to achieve higher rates of decomposition. In contrast, the much larger Rhodamine B(�1.6 nm molecular diameter) cannot utilise the high surface area of nano-sized TiO2 particle clustersbecause it is unable to efficiently penetrate into the 8 nm pores. The larger micro TiO2 structurestherefore actually provided a larger effective surface area for the degradation of this particularmolecule.

Pore shrinkage during curing and the accumulation of contaminants, such as oil, dirt and deicingsalt, can see the performance of photocatalytic concrete decrease with time [213]. Relative humidityalso affects the performance of photocatalytic cementitous materials [193,194,198,203]. At very lowrelative humidity, the absence of water molecules required for the photocatalytic generation of hydro-xyl radicals hinders photocatalytic oxidation. Note, however, since pollution concentrations are typi-cally only in the ppb range, only a very small concentration of hydroxyl radicals (and therefore verylow relative humidity) is required for degradation to proceed [214]. In contrast, at high relativehumidity, water molecules physically block contact of the pollutant with the TiO2 and therefore pol-lutant degradation does not occur. As a general trend, NOx concentration reduction efficiency reducesas relative humidity increases in the 10–80% relative humidity range [193,198,203], although Dyllaet al. [194] report that optimum relative humidity is near 25%. The effect of relative humidity on pho-tocatalytic activity may restrict the geographical locations in which photocatalytic cementitious mate-rials can be effectively utilised.

Reports regarding the effect of pigments on NOx degradation by photocatalytic cementitous mate-rials vary. Some report that red pigments can be added to photocatalytic cementitous materials withnegligible influence on NOx degradation capabilities [198]; others report that red, yellow and blue pig-ments cause NOx degradation to decrease [215]. There are also reports that the addition of metal oxi-des (Fe2O3, Co3O4 and NiO) to TiO2 can, in fact, improve the adsorption of NOx molecules and theircatalytic degradation [190]. These conflicting data make it impossible to draw conclusions on the ef-fect of pigment additives on the efficacy of cementitious photocatalytic materials.

There have been several pilot scale trials of photocatalytic cementitous materials on roads and inmodel street canyons outdoors. From 2002 to 2005, a number of photocatalytic coatings (both mortarsand paints) were developed and tested in collaborations between research centres and industrial part-ners as part of the €3.4 million, EU Photocatalytic Innovative Coverings Application for DepollutionAssessment (PICADA) project [216]. A mortar containing 3% TiO2 was spread on panels that wereplaced on the walls of a model street canyon (2 m wide, 18.2 m long, 5.2 m high) in France and sub-jected to a pollution source containing NOx, SO2, CO, CO2, O2, C6H6 and other VOCs [217] (Fig. 19). NOx

Fig. 19. (a) A schematic diagram of the set up for measuring the influence of the TiO2 content of mortar on NOx concentrationsin a model street canyon. Configuration 1 was used during monitoring the effect of TiO2 mortar on NOx concentrations.Configuration 2 was used as a control, for monitoring the effect of TiO2-free mortar on NOx concentrations. Adapted from [217].(b) A photograph of the model street canyon, from [216].


levels in the TiO2 mortar-treated model street canyon were 36.7–82% lower than in the equivalentmodel street canyon treated with a TiO2-free mortar. In Japan, TiO2 cement coatings have been testedon roads in more than 14 locations (Fig. 20). A �300 m2 test area on the 7th belt highway in Tokyoremoved 50–60 mg of NOx per day, estimated to correspond to NOx emissions from 1000 automobilespassing by [182]. In China, a section of concrete road on federal highway G11 at Zhonghe Toll stationwas coated with a solution containing TiO2 nanoparticles and activated carbon, which penetrated theconcrete [218]. Sampling was conducted under natural conditions over 3 months, and NO and NO2

degradation rates of >37% and >25.84%, respectively, were reported after artificial wearing of the roadsurface.

Photocatalytic pavers have also been tested on roads in Italy, Belgium, England and the Nether-lands. In Bermago, Italy, �12,000 m2 of treated road showed an average day time NOx reduction of45% [175]. NOx abatement was demonstrated over a 10,000 m2 parking lane paved with photocatalyticpavers in Antwerp, Belgium, and after washing with distilled water, the pavers showed no reduction inphotocatalytic efficiency after 2 years [175]. Photocatalytic paving trials in Saville Row, London sawNOx concentrations drop from 110 ppb to 8 ppb [219]. In the 2009 trial on Castorweg Road (Hengelo,

Fig. 20. A photograph of a roadway with an area (the lighter coloured section of road) coated in a photocatalytic TiO2 material,from [182].


Netherlands), NOx concentrations 0.5–1.5 m above the road were 25–50% lower over the �1000 m2 ofphotocatalytic pavers than over normal pavers [220].

Photocatalytic nanoparticles may also been incorporated into indoor paints [214,221,222], com-posites [223], wallpapers [224], or suspension sprays [225] for indoor air purification. The widely usedTiO2 nanoparticles, such as P25, are common in these applications [214,221,222]. Because ambient in-door light may not be sufficient for photoactivity, indoor installations are commonly enhanced withUV lamps [214,226]. There are also indoor paints containing doped TiO2 nanoparticles that are acti-vated by the visible spectrum available [227]. Paint substrates face an additional complication overcementitious materials because the photocatalysts may simultaneously degrade organic and poly-meric paint constituents in addition to pollutants [201,222,228]. The selection of an appropriate paintformulation is therefore very important in order to achieve both photocatalytic activity and paintdurability. For example, in a study of the photocatalytic activity of paint containing TiO2 nanoparticlesover the course of 1 year, Pirola et al. [222] found that a siloxane-based paint lost NOx conversionactivity in a regular way down to a fixed value (from >95% to �55%); however, a silicate-based paintshowed a critical loss in photoactivity after about 150 days, losing photocatalytic activity almost en-tirely. Photocatalytic paint degradation is avoided in regular paints, which contain TiO2 for opacity andwhiteness, by using larger and thus less photoactive particles (see Fig. 21), and using alumina or silicasurface treatments.

Fig. 21. TEM images of, (a) pigmentary TiO2, and, (b) TiO2 nanoparticles from [219].


There are few published studies of photocatalyst-based indoor air purification under unsupple-mented indoor light levels; however, there are studies of trials in real enclosed environments supple-mented with UV lamps. Maggos et al. [226] report on the performance of a white acrylicTiO2-containing paint, developed within the framework of the PICADA project, that was painted onthe 321.9 m2 of ceiling in an artificially closed car park. The closed area was fed with car exhaust gasesand illuminated with UV lamps for 5 h after a steady state was reached. The photocatalytic oxidationof NOx was approximately 20%. This was less than the �90% reduction achieved with the same paintunder laboratory conditions where temperature, and relative humidity were controlled and other or-ganic pollutants present in car exhaust (which can have an inhibitory effect) were absent. Neverthe-less, �20% NOx abatement in a realistic environment is of practical significance. Guerrini fromphotocatalytic product provider Italcement Group [229], report significant NOx concentration reduc-tion after painting a 9000 m2 internal surface of the ‘‘Umberto I’’ tunnel in Rome, Italy with cement-itous photocatalytic paint as part of tunnel refurbishment. The refurbishment included replacingexisting lighting with UV-A emitting lamps to maximise photocatalyst activation. These two changesresulted in absolute NOx concentrations in the tunnel falling by >20%. However, a correction for higheroutdoor NOX concentrations during measurements following the refurbishment showed that NOx deg-radation by the photocatalytic system was 50%. Similar trials have been performed in outdoor envi-ronments where 300 m2 of wall at a school in London was painted with a silicate-based paintcontaining 7.5 wt.% photocatalytic TiO2, which removed �4.5 g of NOx daily [221].

Some TiO2 nanoparticle-based products for air purification are commercially available. The marketis currently considered to be in its infancy, but a mature market is expected to emerge within the nextfew years [104].

7. Evaporative cooling for building surfaces

An evaporative cooling system for buildings has been investigated by Irie et al. [230]. A TiO2 filmapplied to the external surface of the building cause the building surface to become superhydrophilicwhen irradiated by sunlight. Stored rainwater is sprayed onto the surface, where it form a very thin,continuous film that evaporates to cool the building and local surroundings (Fig. 22). Note that the

Fig. 22. An evaporative cooling system: stored rainwater sprinkled over TiO2 coated building surfaces with evaporation drivenby solar energy, from [100].


cooling effect is not derived from the water itself, but by latent heat flux as water evaporates, so thesurface temperature achieved may be lower than that of the water itself.

The superhydrophilicity of the surfaces means that only a small amount of water is needed to pro-duce a very thin, continuous film. This high surface area to volume ratio of the water provides thegreatest contact area for evaporative cooling with minimal water use. In fact, on a vertical buildingsurface with a UV irradiated TiO2 coating, a continuous water layer can form when water film thick-ness is only 0.1 mm, and a water flow of 200 ml/min is sufficient to cover a 1 � 5 m wall.

The temperature drops observed depends on the surface material and its colour. Temperaturedrops of 15 �C and 40–50 �C on window glass and black roof tile surfaces, respectively, have been re-ported. Additionally, mold growth, common on damp surfaces, is avoided on such surfaces due to theantimicrobial effect of the photocatalyst. The use of such surfaces can reduce air conditioner use andthe associated energy consumption, and also reduce heat island phenomena typical in cities and urbanareas.

8. Silica aerogel insulation

In 2005, buildings emitted 8.3 Gt of CO2/yr, accounting for approximately 18% of global greenhousegas emissions and 30% of greenhouse gas emissions in many developed countries [231]. Residentialand commercial retrofit insulation has been found to be one of the most cost effective actions forthe abatement of building greenhouse gas emissions [232,233] since up to 75% of residential dwellingsin 2050 have already been built [234]. However, simply increasing the thickness of traditional insula-tion is often impractical due to increased complexity in building details, decreased net-to-gross floorarea and increased weight of load bearing constructions [235]. A new generation of versatile, efficient,thin, and lightweight insulation is therefore desirable. The development of optically transparent insu-lation is a particular focus. Up to 60% of the total energy loss from a building is from its windows [236]and allowable thermal losses specified in building codes continue to be lowered as governments seekto save energy and reduce emissions [236]. High performance, translucent insulation is also requiredin solar walls and solar collectors; the adoption of which may prove important for countries to reachtheir CO2 emission reduction targets [237]. Nanostructured silica aerogels show significant promise asa suitable material for high performance insulation, even where optical transparency is required.

Silica aerogel consists of a cross-linked internal structure of SiO2 particle chains with a large num-ber of air-filled pores, typically <100 nm in size depending on the purity and fabrication method(Fig. 23) [235]. Derived from a gel in which the liquid component of the gel has been replaced witha gas, the liquid component of the gel was traditionally extracted by supercritical drying. This allowsthe liquid to be slowly drawn off without the solid matrix collapsing due to capillary action, as wouldoccur during conventional evaporation. However, the supercritical drying method does have cost andsafety limitations since high temperatures and pressures are needed to reach the critical point. Safer,ambient pressure drying methods that utilise low-cost precursors for the production of silica aerogelhave been developed in consideration of commercial requirements [238–240].

Fig. 23. (a) A schematic drawing of the structure of silica aerogel, adapted from [241]. (b and c) SEM images of the structure ofsilica aerogels, adapted from [242].

Fig. 24. A comparison of the thermal insulation properties of some commercially available insulation materials (PUR:polyurethane foam; CFC: chloroflurohydrocarbons; EPS: expanded polystyrene; XPS: extruded polystyrene), from [248] citedin [240].

Fig. 25. (a) Granular silica aerogel, from [233], and, (b) monolithic silica aerogel, from [250].


Silica aerogels have high compression strength (1.5–3 bar depending on the bulk of silica is�2200 kg/m3, but the porous structure of silica aerogel results in the density of as low as 3 kg/m3 (just 2.5 times thebulk density of air at ambient conditions). The density of commercially available aerogels for buildingapplications is, however, significantly higher, with overall density typically 70–150 kg/m3 [235].

Aerogels are also excellent insulators because they are composed primarily of gas, which is a poorconductor. Silica aerogels are particularly insulative since the silica matrix itself is also a poor conduc-tor. Convective heat transfer is also minimal because gas cannot easily circulate through the porestructure of the aerogel lattice [235]. The thermal conductivity of aerogels can be further reducedby (i) filling the aerogel with low conductivity gases (generally noble gases); (ii) decreasing the poresize; or (iii) applying a vacuum to the aerogel [235]. The thermal conductivity of a silica aerogel can beas low as �0.02 W/(m K) at ambient conditions when the pores contain air and <0.01 W/(m K) underevacuated conditions (which is lower than a modern wall) [243–246]. Non-translucent silica aerogelswith �0.013 W/(m K) thermal conductivity are commercially available [235,247], and the best com-mercially available translucent silica aerogel glazing products have 0.3 W/(m K) thermal conductivity[236]. Fig. 24 shows a comparison of the thermal conductivity of silica aerogel with other commer-cially available insulation material.

Two different groups of aerogel materials are noted for building applications [235]: (i) granularaerogel-based translucent insulation materials and (ii) transparent monolithic aerogels (Fig. 25).

Fig. 26. (a) The transmittance of a silica aerogel in the ultraviolet, visible and near infrared spectrum, from [252] cited in [235]],(b) a diagram of the cross section of translucent granular aerogel glazing, from [253], (c) translucent aerogel insulation as a highperformance thermal insulation solution for daylighting, from [254], and, (d) translucent insulation used as a curtain wall at theSculpture Building and Gallery, Yale University, from [255].


Monolithic aerogels generally posses superior thermal and optical properties, but are costly to pro-duce, require long processing times and must be protected from tension and moisture [237,249].Granular aerogels consequently dominate the commercial market, despite their inferior performancebecause they are more robust and cheaper and easier to produce on a commercial scale [237,241].

Fig. 27. Examples of applications of silica aerogels in the building industry. (a) A passive solar collector concept, from [256].Thermal insulation is provided by an aerogel layer sandwiched between two panes of glass. The wall of the house is coveredwith a black absorptive material that converts the sun’s rays into heat. Because aerogel is a much better insulator than the wall,most of the heat generated penetrates the wall and enters the house instead of escaping to the outside. In summer a shade couldbe drawn over the absorber, (b) a house retrofitted with an aerogel solar collector, from [237], (c) 1 m2 of aerogel-basedrendering applied with 6 cm thickness in a single application, from [233], (d) commercial fibre-reinforced silica aerogel thermalinsulation for building applications, from [257].


Silica aerogels find a variety of applications in the field of civil infrastructure, including heat insu-lation of buildings – particularly in areas short of space and in areas where optical transparency (e.g.windows) or translucency (e.g. skylights) is required (Fig. 26)) – translucent heat insulators in solarcollectors, and high performance, insulative building render (Fig. 27). Granular aerogels have also beennoted as exceptional reflectors of sound and form excellent sound barrier materials [235,251]. Forexample, Buratti et al. [251] reported that an aluminium framed, double glazed window with granularaerogel in the 15 mm interspace reduced sound transmittance by 3 dB over a comparable windowwith air in the interspace.

Silica aerogels have indeed shown superior performance to conventional glazing in window andsolar collector applications. Burrati et al. [251] reported that, compared to a conventional doubleglazed window with a low-emittance layer, a 55% reduction in heat loss (and 25% reduction in lighttransmittance) occurred when monolithic aerogel was placed in the interspace. Granular aerogel re-sulted in a 25% reduction in heat loss and 66% reduction in light transmission, relative to the sameconventional window. Retrofitting of a 10 mm thick prototype panel, consisting of clear twin–wallpolycarbonate sheet filled with granular aerogel, over a single glazed window was found to reduce


heat loss by 80% (the equivalent of a triple glazed window) without detrimental reductions in lighttransmission [250].

Dowson et al. [237] simulated and tested the in situ performance of a 5.4 m2 solar air collector con-taining granular aerogel, which was incorporated into the external insulation of a mechanicallyventilated end terrace house in London, United Kingdom over a 7 day test period (Fig. 27b). Initial testresults for a 10 mm granular aerogel collector showed significant promise; their model predictedoperational efficiencies of up to 60% representing an annual energy output of 355 kW h/m2/yr and apayback period a low as 4.5 years. Long-term evaluation (2 years) of a 40 mm thick granular aerogelcollector is now being monitored as part of the Retrofit for the Future project in the UnitedKingdom [258].

A render containing granular silica aerogel (Fig. 27c) has been shown to have a thermal conductiv-ity of �0.025 W/(m K) and a water vapour transmission resistance of 4 [233]. These are characteristicsthat are unachievable using existing insulative renders.

Aerogel glazing entered the contemporary glazing market in 2006 as a granular aerogel encapsu-lated between polycarbonate construction panels and weighed less than 20% of the equivalent glassunit and had 200 times more impact strength [259]. Airglass AB (Sweden) produce transparent mono-lithic aerogel in a pilot scale production plant [245,260]. Currently, the maximum size of a crack-freemonolithic aerogel tile is 0.58 m � 0.58 m, limited by the size of the autoclave [245]. Jelle et al. [236]present current summary of commercially available optically transparent and translucent silica aero-gel products in their review of fenestration state-of-the-art. Despite the obvious potential for aerogelproducts in glazing applications, commercially available products are limited.

In terms of the aerogel market in general (not just glazing products), the market is quickly devel-oping. In the 5 years to 2008, the global market for aerogels tripled to $83 million, and is expected toreach $646 million by 2012[261] cited in [235]], with major producers Aspen Aerogels (USA) [262],Cabot Corporation (USA) [237,263] and others [236,237] targeting the civil infrastructure industry.

The price sensitivity of the building industry is a major factor affecting the adoption of aerogelmaterials. Outside of applications that specifically require the outstanding thermal or acoustic insula-tion along with the low weight and/or optical transmittance properties that silica aerogel can provide,the prices of silica aerogel products are currently too high for widespread general adoption to occur.Research must focus on producing silica aerogels with comparable or superior life-cycle costs to gainmarket traction, and even then high capital cost can still be a negating factor where construct onlycontracts are concerned. Research must also address the performance gap between laboratory-pro-duced and commercially available aerogels since at present the thermal conductivity of commercialaerogel products is about ten times higher than pristine, monolithic aerogels that are made inlaboratories.

9. Solar cells

Photovoltaic devices, such as solar cells, present a comparatively clean and renewable method ofsupplying energy. Photovoltaics devices are a multi billion dollar industry that continues to grow,spurred on by the concern of governments and individuals over fuel supplies and carbon emis-sions [264]; however, significant advancements continue to be needed to make photovoltaics costcompetitive with established technologies for energy production [265]. Solar energy is only likely tobecome truly mainstream when the associated costs are comparable to that of other energy sources.Nanotechnology is envisaged to play a vital role in progressing towards this. It is estimated that in2009, approximately 7 GW of photovoltaic solar capacity was installed at a cost of �$39 billion, risingto �$82 billion in 2010 [266]. Conservative projections put worldwide annual photovoltaic solarcapacity at 200 GW in 2020 (others estimate as much as 300 GW), and the related estimated potentialmarket size for nanomaterials is �$1.8 billion (in 2020) [266].

First generation solar cells made of bulk (single crystal or polycrystalline) silicon are costly buthave relatively high efficiencies (Fig. 28). First generation solar cells currently make up over 85% ofthe global photovoltaic device market [267]. Second generation devices attempt to reduce cost byusing less material (e.g. utilising thin film absorbers or solution-phase deposition processes) without

Fig. 28. The cost of electrical power from photovoltaic systems as a function of the total upfront cost and the module powerconversion efficiency. MC, SP, SIII, SI are the manufacturing cost, average selling price, installed cost for a utility-scale system,and installed cost for a residential system, respectively. Installed cost for a utility-scale system (SIII) includes ‘‘balance-of-systems’’ (BOS) cost (which accounts for frames, inverters, battery storage, etc.) for an on-grid system. Installed cost for aresidential system (SI) includes BOS cost for on- and off-grid operation with battery storage. The $/Wp were converted to ¢/kW h assuming a module lifetime of 20 years, a 5% cost of borrowing, a 1% yearly operating (maintenance) cost, and an averagesolar insolation of 200 W/m2 (which is about 5 h of full intensity sunlight/day). Costs are based on 2009 data. The designation of1st, 2nd, or 3rd generation is based on the manufacturing cost and potential module efficiency. From [267].


compromising efficiency. In theory, the efficiency of second generation devices may match that of firstgeneration devices (Fig. 28), but in practice the efficiency of second generation devices is often muchlower [267]. Nanotechnology already plays a key role in second generation devices [267], and thin filmtechnologies account for �6% of the photovoltaics market [264]. Theoretically, third generation solarcells are efficient beyond that of first and second generation devices and are low cost (Fig. 28) and assuch, greater flexibility and applicability is foreseen for third generation devices. Third generation de-vices are under development and not yet commercially available. The efficiency and cost of third gen-eration devices that have been developed are not competitive with that of commercially availablesolar cells at present; however, significant research and advancements continue in this area. Nanom-aterials, such as quantum dots, carbon nanotubes, fullerenes and metal oxides, are likely to play animportant role in third generation devices.

In silicon solar cells, no more than one electron–hole pair is generated per photon absorbed, whichmeans quantum efficiency is <100%. Combined with other losses, the upper theoretical power conver-sion efficiency for a non-concentrated cell limit is 33.7% (referred to as the Shockley–Queisser lim-it) [267]. In practice, the maximum efficiency recorded for the most efficient single junction cellsunder AM1.5G1 light is 25% [267]. One strategy being pursued to increase efficiency in third generationsolar devices is the exploitation of nanomaterials that can generate multiple excitons from the absorp-tion of a single photon. This behaviour has been demonstrated in SWCNTs [269,270] and some quantum-confined nanocrystals, such as inorganic semiconductor nanocrystals known as quantum dots [267].These could theoretically achieve a power conversion efficiency conversion efficiency of 44% in a singlejunction absorbe under unconcentrated AM1.5G. However the current experimental power conversionefficiencies of colloidal quantum dot solar cells are about 5% [265,268], significantly lower than the effi-ciency of commercially available crystalline silicon modules that typically have efficiencies of 12–19% [267].

1 AM1.5G refers to air mass 1.5 global – a standard spectrum for testing which includes both direct and diffusive radiation andaims to account for black body radiation from the sun at the earth’s surface to accounting for attenuation by ozone, moisture, dustand other components in the atmosphere [268].

Fig. 29. A ‘‘solar paint’’ on an optically transparent electrode, from [272].


Despite the continued focus on improving efficiency, it is actually cost that is the major factor insolar power adoption. As noted by Hillhouse et al. [267], on the free market, it is not necessarily effi-ciency but rather the cost of the electricity produced that is important. The cost of first generation so-lar cells continues to decrease, despite the rising cost of polysilicon, and market analysts haveforecasted that first generation solar cell manufacturers may soon achieve manufacturing costs of$2/Wp.2 Second generation solar cells are now being manufactured at a cost of $0.98/Wp [267]. An in-stalled cost of $1.6/Wp is required to beat grid-parity prices for large fractions of continental USA [267].

In the long term, as the cost of nanomaterials themselves decrease, the use of nanomaterials inthird generation devices are expected to lower costs relative to silicon-based systems because thematerials can be cheaply processed and applied using established techniques. For example, a solutionof quantum dots can be coated onto a wide variety of conductive substrates using drop casting, spincoating and ink-jet printing [268]. The size, shape and spatial distribution of nanostructures, as well astheir surface termination and functionalisation with organic ligands can also often be controlled to ahigh degree of accuracy and reproducibility [271]. There are also potentially additional technical andefficiency benefits in using nanomaterials over bulk silicon, such as the ability to tune bandgap and theabsorption spectrum by changing the nanomaterial size, shape and surface termination; excition dis-sociation without application of an external field; increased device efficiency due to efficient multipleexciton generation upon absorption of a single high energy photon; a phonon bottleneck that hinderscarrier relaxation thereby potentially increasing open-circuit voltage, and the possibility of sub-band-gap photon absorption [271]. It is the combination of these control, efficiency, and processing factorsthat theoretically enable third generation solar panels to provide energy at high efficiencies and lowcost.

So-called, ‘‘solar paints’’ – pastes that can easily be applied to a conductive surface and, when ex-posed to light, generate electricity – have also been formed as an alternative to a discrete solar panel(Fig. 29). A solar paint containing TiO2 nanoparticles coated with CdS and CdSe quantum dots was re-cently produced and tested by Genovese et al. [272]. Following application of the paste and annealingat 200 �C, efficiencies of �1% were achieved. This is significantly lower than the highest efficienciesachieved with other quantum dot solar cells (�5%). However, preparation using a paint system is sig-nificantly simpler, requiring hours rather than the 1–2 days for other quantum dot solar cells, and per-formance is expected to improve as active particle structural parameters are optimised. Of course,even with the advent of a solar paint that is efficient, cheap and easy to apply, the need for suitabledevice architecture will limit where such paints may be applied. For instance, to function as a device,an economically viable, suitable conductive surface, methods of attaching electrodes and a suitablecovering will be required.

Nanotechnology is also utilised in solar cell glazing. This is glazing that combines the transparent/translucent properties of glass and the solar energy harvesting properties of solar cells. This is dis-cussed in more detail by Jelle et al. [236], but, in brief, the glazing is sprayed with a coating of siliconnanoparticles that act as solar cells. These products are commercially available from Abakus Solar AG(Germany), PV Glaze (UK), Sapa Building System (Belgium) and others. It is important to note thatthese products have a trade off between visible light transmittance and energy production becausethe latter uses the incoming photons. Electrochromic windows actually benefit from this occurrencebecause they can maximise energy production when in an opaque state and visible light transmission

2 Dollars per watt peak – a measure that accounts for total upfront cost and module efficiency.


is not required. More advanced nanoparticle-based solar glazing products selectively redirect only UVand IR light to traditional solar cells located in the window frames are also under development[273,274].

10. Anticorrosion coatings

The corrosion of materials is reported to cost Western countries approximately 4% of their grossdomestic product, and the corrosion of steel used in construction forms a significant proportion of thiscost [275]. Corrosion prevention strategies often utilise coatings; however, even the best commer-cially available coatings allow oxygen to slowly diffuse to the metal beneath, causing corrosion andultimately, delamination of the coating. Microscopic nicks or pits in the surface also accelerate corro-sion and delamination due to the formation of redox circuits [276].

Anticorrosion coatings traditionally contained hexavalent chromium compounds. Although veryeffective corrosion inhibitors, hexavalent chromium compounds are also toxic carcinogens that canleach from the coating into the environment. Chromate use has subsequently been banned in manycountries – the EU has banned its use in electrical, electronic and automotive industries in the EU since2006/2007 [277] – prompting research into other effective anticorrosion coatings. A range of nanopar-ticles is being explored for use in both passive and active anti-corrosion coatings.

Improvements in corrosion inhibition have been seen when SiO2, Zn, Fe2O3, and halloysite nanoclaynanoparticles have been added to inorganic (e.g. Ni–W coatings [278]), organic coatings (e.g.epoxy [279,280]), and intrinsically conductive polymer coatings [281]. Epoxy coatings in particularare widely used to protect steel reinforcement in concrete structures from corrosion. Significantimprovements in corrosion inhibition have been observed when small quantities of nanoparticlesare well-dispersed in epoxy coatings. The homogeneity of the coatings improve and the porosity ofthe coating decreases, as does diffusion (e.g. of O2 and H2O) through the coating [282]. The improvedbarrier performance is caused by increased cross-linking, decreased total free volume and the morecompact, hydrophobic polymer network formed [279,280]. The integrity of the coating is enhancedas the nanoparticles fill cavities, assist in crack bridging, crack deflection and crack bowing [279].The adherence of the coating to the underlying substrate also improves [279].

Li et al. [280] reported an improvement in the anticorrosion performance of an epoxy coating onmild steel when 2 wt.% SiO2 nanoparticles were added to the coating. When >2 wt.% SiO2 nanoparticleswas added, the number of pores in the coatings was reported to increase, which had a negative impacton corrosion inhibition. Shi et al. [279] reported that in 0.3 wt.% and 3 wt.% NaCl solution, corrosioninhibition with an epoxy coating containing 1 wt.% SiO2, Zn, Fe2O3, or halloysite clay nanoparticlesis, respectively, 638–2365 times and 11–910 times that of epoxy alone. The addition of SiO2 and Znnanoparticles also improved the Young’s modulus of the coating by �1000% and �30%, respectively,although Fe2O3 and halloysite addition resulted in a decrease of �25% and �30%, relative to pureepoxy.

Active self-healing coatings, produced by adding a corrosion-inhibiting compound that is encapsu-lated in a ‘nanocontainer’ into a passive protective film, are also in development. In addition to pro-viding passive corrosion protection, these coatings can rapidly respond to various internal and/orexternal corrosion triggers such as mechanical damage and pH changes. These triggers cause the‘nanocontainers’ in the immediate vicinity to rupture and release corrosion inhibiting agents directlyinto the damaged area to nullify the corrosive attack (Fig. 30a). The ‘nanocontainers’ must be less than300–400 nm in size so that the passive protective properties of the coating are maintained even whensome ‘nanocontainers’ rupture [283]. Because the active materials are only released when triggered,consumption is minimised and leakage into the environment may be prevented. This system has beenexperimentally shown to be more effective than coatings doped directly with corrosion inhibitorwhere consumption of the inhibitor occurs continuously (Fig. 30b).

The ‘nanocontainers’ can be formed by a variety of techniques [283], including: (i) the use of self-organising block copolymers and lipids that can entrap hydrophobic active materials within theircore; (ii) layer-by-layer assembly with polycations, polyanions and nanoparticles as constituents ofthe ‘nanocontainer’; and (iii) the use of ultrasonic waves whereby bubble cavitation interfaces are em-

Fig. 30. (a) A schematic diagram of the self-healing effects of a coating containing nanoencapsulated corrosion inhibitor,adapted from [283]. (b) Photographs of an aluminium alloy coated with; (top) a ZrOx/SiOx sol–gel film directly doped withbenzotriazole after 14 days in 0.005 M NaCl, and, (bottom) a ZrOx/SiOx sol–gel film impregnated with benzotriazole-loaded SiO2

nanocontainers after 14 days in 0.5 M NaCl, from [284].


ployed as reaction zones for the formation of the sensitive shells from organic precursors or preformednanoparticles via polycondensation, polymerisation and particle melting. A range of materials havebeen explored as prospective nanocontainers for corrosion inhibitors, including mixed-oxide nanopar-ticles (e.g. ZrO2/CeO2), b-cyclodextrin-inhibitor complexes, hollow polypropylene fibres, conductingpolyaniline, polyelectrolyte layer-by layer coated SiO2 nanoparticles and halloysite claynanotubes [283,285].

Information on the required active ingredient loading suggests the concentration of corrosioninhibitor should lie in the 20 and 97 wt.% range (Patent 6383271 [286]). At <20 wt.%, a sufficient anti-corrosion effects is not achieved, while at >97 wt.% the polymer material that forms the passive com-ponent of the coating will not be able to form a continuous matrix [287].

11. Conclusion

The construction industry is generally very conservative, favouring traditional materials and tech-nologies. However, traditional materials and technologies cannot satisfy the continued drive by gov-ernments and stakeholders towards the improved safety, sustainability, and performance of buildingsand infrastructure.

Nanotechnology can play an important role in meeting requirements by improving the primaryproperties of materials, such as improving the strength of concrete and improving the thermal insu-lation properties of insulation materials. Nanotechnology can also add new functionalities to existingmaterials and products, as illustrated by antimicrobial, self-cleaning and air-purifying paints and opti-cally transparent insulation.

Nanotechnology can also play a significant role in improving the safety of structures and in reduc-ing the environmental impact and energy intensity associated with buildings and infrastructure. Forexample, metal oxide nanoparticles or CNTs additives can mitigate the adverse effects that ‘environ-mentally friendly’ industrial waste-based cement replacements can have on the physical properties ofconcrete. This increases the range of applications in which these more environmentally friendlyconcretes may be used. Nanotechnology-based sensors may supersede traditional visual inspectionmethods to improve the accuracy of structural health monitoring and reduce labour costs. Nanotech-nology based solar cells, paints, and glazing may broaden the range of areas where solar energyharvesting technology may be applied, and may reduce the cost and improve the efficiency of more


traditional solar cell products. Nanotechnology may also play a role in improving the physical prop-erties and effectiveness of anticorrosion coatings, and reducing the levels of toxic compounds leachingfrom coatings into the surrounding environment.

Nanotechnology-based products and solutions for the construction industry are in various stages ofdevelopment, ranging from conceptual ideas to commercially available products. The leap from labo-ratory-scale successes to a real-world suitability is often a difficult and expensive engineering chal-lenge. Of those that do reach the commercial market, adoption is often hampered by limitedproduct awareness within the industry, the conservatism of the construction industry, and the com-monplace focus on up-front build costs over long-term cost, performance, sustainability and safety.Nevertheless, it is clear that the construction industry will be compelled to embrace new technologiesin order to deliver the safer, more sustainable, and better performing buildings and infrastructure de-manded by stakeholders and this will drive continued research into the development of suitable nano-technologies for the construction industry.

Acknowledgement

The authors wish to acknowledge and thank Dr. Kieran Mackenzie (Engineering Excellence Group,Laing O’Rourke and School of Chemical and Biomolecular Engineering, University of Sydney) for proofreading this manuscript.

References

[1] Holister P. Nanotech: the tiny revolution. CMP Cientifica; July 2002. http://www.nanotech-now.com/CMP-reports/NOR_White_Paper-July2002.pdf.

[2] Guerrini GL, Corazza F. White cement and photocatalysis. Part 2: applications. In: First Arab international conference andexhibition on the uses of white cement, Cairo, Egypt; 28–30 April, 2008.

[3] Zhu W, Bartos PJM, Porro A. Application of nanotechnology in construction. Summary of a state-of-the-art report. MaterStruct 2004;37:649–58.

[4] van Broekhuizen F, van Broekhuizen P. Nano-products in the European construction industry; 2009. http://hesa.etui-rehs.org/uk/newsevents/files/Nano_Executive_%20summary.pdf.

[5] van Broekhuizen P, van Broekhuizen F, Cornlissen R, Reijnders L. Use of nanomaterials in the European constructionindustry and some occupational health aspects thereof. J Nanopart Res 2011;13:447–62.

[6] Forster SP, Olveira S, Seeger S. Nanotechnology in the market: promises and realities. Int J Nanotechnol 2011;8:592–613.[7] van Oss HG. U.S. Geological Survey, Mineral Commodity Summaries; 2011. http://minerals.usgs.gov/minerals/pubs/

commodity/cement/mcs-2011-cemen.pdf.[8] Lebental B, Chainais P, Chenevier P, Chevalier N, Delevoye E, Fabbri J-M, et al. Aligned carbon nanotube based ultrasonic

microtransducers for durability monitoring in civil engineering. Nanotechnology 2011;22:395501.[9] Nazari A, Riahi S. The effects of TiO2 nanoparticles in flexural damage of self-compacting concrete. Int J Damage Mech

2011;20:2049–72.[10] Nazari A, Riahi S. TiO2 nanoparticle effects on physical, thermal and mechanical properties of self compacting concrete

with ground granulated blast furnace slag as binder. Energy Build 2011;43:995–1002.[11] Zhang M-H, Li H. Pore structure and chloride permeability of concrete containing nano-particles for pavement. Constr

Build Mater 2011;25:608–16.[12] Oltulu M, Sahun R. Single and combined effects of nano-SiO2, nano Al2O3 and nano-Fe2O3 powders on compressive

strength and capillary permeability of cement mortar containing silica fume. Mater Sci Eng A 2011;528:7012–9.[13] Nazari A, Riahi S. Improvement compressive strength of concrete in different curing media by Al2O3 nanoparticles. Mater

Sci Eng A 2011;528:1183–91.[14] Shekari AH, Razzaghi MS. Influence of nano particles on durability and mechanical properties of high performance

concrete. Proc Eng 2011;14:3036–41.[15] Li H, Xiao H-G, Yuan J, Ou J. Microstructure of cement mortar with nano-particles. Compos Part B – Eng 2004;35:185–9.[16] Li H, Xiao H-G, Ou J. A study on mechanical and pressure-sensitive properties of cement mortar with nanophase

materials. Cem Concr Res 2004;34:435–8.[17] Senff L, Hotza D, Lucas S, Ferreira VM, Labrincha JA. Effect of nano-SiO2 and nano-TiO2 addition on the rheological

behavior and the hardened properties of cement mortars. Mater Sci Eng 2012;532:354–61.[18] Chen L, Lin DF. Applications of sewage sludge ash and nano-SiO2 to manufacture tile and construction material. Constr

Build Mater 2009;23:3312–20.[19] Jo B-W, Kim C-H, Lim J-H. Characteristics of cement mortar with nano-SiO2 particles. ACI Mater J 2007;104:404–7.[20] Nazari A, Riahi S. The effects of SiO2 nanoparticles on physical and mechanical properties of high strength compacting

concrete. Compos Part B – Eng 2011;42:570–8.[21] Chang T-P, Shih J-Y, Yang K-M, Hsiao T-C. Material properties of Portland cement paste with nano-montmorillonite. J

Mater Sci 2007;42:7478–87.[22] Kuo W-Y, Huang J-S, Lin C-H. Effects of organo-modified montmorillonite on strengths and permeability of cement

mortars. Cem Concr Res 2006;36:886–95.

http://refhub.elsevier.com/S0079-6425(13)00035-2/h0005
















































[23] Kuo W-Y, Huang J-S, Yu B-Y. Evaluation of strengthening through stress relaxation testing of organo-modifiedmontmorillonite reinforced cement mortars. Constr Build Mater 2011;25:2771–6.

[24] He X, Shi X. Chloride permeability and microstructure of portland cement mortars incorporating nanomateials. TransportRes Rec 2008;2070:13–21.

[25] Bentz DP. A review of early-age properties of cement-based materials. Cem Concr Res 2008;38:196–204.[26] Burrows RW, Kepler WF, Hurcomb D, Schaffer J, Sellers JG. Three simple tests for selecting low-crack cement. Cem Concr

Compos 2004;26:509–19.[27] Zhang M-H, Islam J. Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes

of fly ash or slag. Constr Build Mater 2012;29:573–80.[28] Sobolev K, Gutierrez MF. How nanotechnology can change the concrete world. Am Ceram Soc Bull 2005;84:16–9.[29] Dyckerhoff Nanodur

�premium cement for ultra high performance concrete (UHPC). www.dyckerhoff.de.

[30] Raki L, Beaudoin J, Alizadeh R, Makar J, Sato T. Cement and concrete nanoscience and nanotechnology. Materials2010;3:918–42.

[31] Walters DA, Ericson LM, Casavant MJ, Liu J, Colbert DT, Smith KA, et al. Elastic strain of freely suspended single-wallcarbon nanotube ropes. Appl Phys Lett 1999;74:3803–5.

[32] See CH, Harris AT. A review of carbon nanotube synthesis via fluidized-bed chemical vapor deposition. Ind Eng Chem Res2007;46:997–1012.

[33] Konsta-Gdoutos MS, Metaxa ZS, Shah SP. Highly dispersed carbon nanotube reinforced cement based materials. CemConcr Res 2010;40:1052–9.

[34] Calvert P. Nanotube composites: a recipe for strength. Nature 1999;399:210–1.[35] Cwirzen A, Habermehl-Cwirzen K, Penttala V. Surface decoration of carbon nanotubes and mechanical properties of

cement/carbon nanotube composites. Adv Cem Res 2008;20:65–73.[36] Makar J, Chan GW. Growth of cement hydration products on single-walled carbon nanotubes. J Am Ceram Soc

2009;92:1303–10.[37] de Ibarra YS, Gaitero JJ, Erkizia E, Campillo I. Atomic force microscopy and nanoindentation of cement pastes with

nanotube dispersions. Phys Status Solidi A 2006;203:1076–81.[38] Sobolkina A, Mechtcherine V, Khavrus V, Maier D, Mende M, Ritschel M, et al. Dispersion of carbon nanotubes and its

influence on the mechanical properties of the cement matrix. Cem Concr Compos 2012;34:1104–13.[39] Guskos N, Zolnierkiewicz G, Typek J, Blyszko J, Kiernozycki W, Narkiewicz U. Ferromagnetic resonance and compressive

strength study of cement mortars containing carbon encapsulated nickel and iron nanoparticles. Rev Adv Mater Sci2010;23:113–7.

[40] Givi AN, Rashid SA, Aziz FNA, Salleh MAM. Investigations on the development of the permeability properties of binaryblended concrete with nano-SiO2 particles. J Compos Mater 2011;49:1931–8.

[41] Li GY, Wang PM, Zhao X. Mechanical behavior and microstructure of cement composites incorporating surface-treatedmulti-walled carbon nanotubes. Carbon 2005;43:1239–45.

[42] Howser RN, Dhonde HB, Mo YL. Self-sensing of carbon nanofiber concrete columns subjected to reversed cyclic loading.Smart Mater Struct 2011;20:085031.

[43] Coppola L, Buoso A, Corazza F. Electrical properties of carbon nanotubes cement composites for monitoring stressconditions in concrete structures. Appl Mech Mater 2011;82:118–23.

[44] Cwirzen A, Habermehl-Cwirzen K, Nasibulin AG, Kaupinen EI, Mudimela PR, Penttala V. SEM/AFM studies of cementitiousbinder modified by MWCNT and nano-sized Fe needles. Mater Charact 2009;60:735–40.

[45] Li G. Properties of high-volume fly ash concrete incorporating nano-SiO2. Cem Concr Res 2004;34:1043–9.[46] Barrera-Diaz C, Martinez-Barrera G, Gencel O, Bernal-Martinez LA, Brostow W. Processed wastewater sludge for

improvement of mechanical properties of concretes. J Hazard Mater 2011;192:108–15.[47] Swamy RN. Fly ash concrete – potential without misuse. Mater Struct 1990;23:397–411.[48] Lin DF, Lin KL, Chang WC, Luo HL, Cai MQ. Improvements of nano-SiO2 on sludge/fly ash mortar. Waste Manage

2008;28:1081–7.[49] Said AM, Zeidan MS. Enhancing the reactivity of normal fly ash concrete using colloidal nano-silica. Am Concr Inst

2009;267SP:75–86.[50] Rubio E, Rodriguez V, Alcocer SM, Castano VM. Rice husks as source for pozzolan nanomaterials. Mater Res Innov

2011;15:268–70.[51] Chaipanich A, Nochaiya T, Wongkeo W, Torkittikul P. Compressive strength and microstructure of carbon nanotubes-fly

ash cement composite. Mater Sci Eng A 2010;527:1063–7.[52] Munimela PR, Nasibulina LI, Nasibulin AG, Cwirzen A, Valkeapaa M, Habermehl-Cwirzen K, et al. Synthesis of carbon

nanotubes and nanofibers on silica and cement matrix materials. J Nanomater 2009:526128.[53] Dunens OM, MacKenzie KJ, Harris AT. Synthesis of multiwalled carbon nanotubes on fly ash derived catalysts. Environ Sci

Technol 2009;43:7889–94.[54] Nasibulin AG, Shandakov SD, Nasibulina LI, Cwirzen A, Mudimela PR, Habermehl-Cwirzen K, et al. A novel cement-based

hybrid material. New J Phys 2009;11:023013.[55] Stallings JM, Tedesco JW, El-Mihilmy M, McCauley M. Field performance of FRP bridge repairs. J Bridge Eng 2000;5:107–13.[56] Bridge inspection. Nondestructive Testing Resource Centre. http://www.ndt-ed.org/AboutNDT/SelectedApplications/

Bridge_Inspection/Bridge_Inspection.htm.[57] Wang Y, Lynch JP, Law KH. A wireless structural health monitoring system with multithreaded sensing devices: design

and validation. Struct Infrastruct Eng 2007;3:103–20.[58] Chang PC, Flatau A, Liu SC. Review paper: health monitoring of civil infrastructure. Struct Health Monit 2003;2:257–67.[59] Casal Moore N. Smart bridges under development with new federal grant. Michigan Today; 2009. http://

michigantoday.umich.edu/2009/02/story.php?id=7351.[60] Wong K-Y. Current and future bridge health monitoring systems in Hong Kong. TMCA Division, Highways Department,

Government of the Hong Kong Special Administrative Region. www.samco.org/download/ws5/wash.pdf.



































































[61] Stathopoulos-Vlamis A. Structural health monitoring of Rion-Antirion bridge and management in case of special events.Gefyra SA; 2008. www.asecap.com/english/documents/Gefyra_001.pdf.

[62] Bassam A, Schulze B. A case study: structural health monitoring of Huey P. Long bridge. CTL Group; 2011. http://www.ctlgroup.com/insights/detail/89.

[63] Chen P-W, Chung DDL. Concrete as a new stress/strain sensor. Compos Part B – Eng 1996;27B:11–23.[64] Kang I, Schulz MJ, Kim JH, Shanov V, Shi D. A carbon nanotube strain sensor for structural health monitoring. Smart Mater

Struct 2006;15:737–48.[65] Saafi M, Kaabi L, McCoy M, Romine P. Wireless and embedded nanotechnology-based systems for structural integrity

monitoring of civil structures: a feasibility study. Int J Mater Struct Integrity 2010;4:1–24.[66] Norris A, Saafi M, Romine P. Temperature and moisture monitoring in concrete structures using embedded

nanotechnology/microelectromechanical systems (MEMS) sensors. Constr Build Mater 2008;22:111–20.[67] Saafi M. Wireless and embedded carbon nanotube networks for damage detection in concrete structures. Nanotechnology

2009;20:395502.[68] Chang N-K, Su C-C, Chang S-H. Fabrication of single-walled carbon nanotube flexible strain sensors with high sensitivity.

Appl Phys Lett 2008;92:063501.[69] Zuo J, Yao W, Liu X, Qin J. Sensing properties of carbon nanotube–carbon fiber/cement nanocomposites. J Test Eval 2012;40.[70] Dharap P, Li Z, Nagarajaiah S, Barrera EV. Flexural strain sensing using carbon nanotube film. Sensor Rev 2004;24:271–3.[71] Hussain M, Choa Y-H, Niihara K. Fabrication process and electrical behavior of novel pressure-sensitive composites.

Composites Part A 2001;32:1689–96.[72] Nanni F, Ruscito G, Nad L, Gusmano G. Self-sensing nanocomposite CnP-GFRP rods as reinforcement and sensors of

concrete beams. J Intel Mater Syst Struct 2009;20:1615–23.[73] Grundmann H, Aires-de-Sousa M, Boyce J, Tiemersma E. Emergence and resurgence of meticillin-resistant Staphylococcus

aureus as a public-health threat. Lancet 2006;368:874–85.[74] Klein E, Smith DL, Laxminarayan R. Hospitalizations and deaths caused by methicillin-resistant Staphylococcus aureus,

United States, 1999–2005. Emerg Infect Dis 2007;13:1840–6.[75] European Centre for Disease Prevention and Control. Healthcare-associated infections. http://www.ecdc.europa.eu/en/

healthtopics/Healthcare-associated_infections/Pages/index.aspx.[76] Europa. Questions and answers on patient safety, including the prevention and control of healthcare associated

infections; 2008. http://europa.eu/rapid/press-release_MEMO-08-788_en.htm.[77] Holtz RD, Lima BA, Souza Filho AG, Brocchi M, Alves OL. Nanostructured silver vanadate as a promising antibacterial

additive to water-based paints. Nanomed Nanotechnol 2012;8:935–40.[78] Pangule RC, Brooks SJ, Dinu CZ, Bale SS, Salmon SL, Zhu G, et al. Antistaphylococcal nanocomposite films based on

enzyme–nanotube conjugates. ACS Nano 2010;4:3993–4000.[79] European Commission. Novel self-cleaning, anti-bacterial coatings, preventing disease transmission on everyday touched

surfaces. Community Research and Development Information Service. http://cordis.europa.eu/projects/index.cfm?fuseaction=app.details&TXT=Novel+self-cleaning%2C+antibacterial+coatings%2C+preventing+disease+transmission+on+everyday+touched+ surfaces.&FRM=1&STP=10&SIC=&PGA=&CCY=&PCY=&SRC=&LNG=en&REF=106822.

[80] Kuhn DM, Ghannoum MA. Indoor mold, toxigenic fungi, and Stachybotrys chartarum: infectious disease perspective. ClinMicrobiol Rev 2003;16:144–72.

[81] Indoor air facts no 4 (revised) – sick building syndrome. United States Environmental Protection Agency; 1991. http://www.epa.gov/iaq/pdfs/sick_building_factsheet.pdf.

[82] Li Q, Mahendra S, Lyon DY, Brunet L, Liga MV, Li D, et al. Antimicrobial nanomaterials for water disinfection and microbialcontrol: potential applications and implications. Water Res 2008;42:4591–602.

[83] Gladis F, Eggert A, Karsten U, Schumann R. Prevention of biofilm growth on man-made surfaces: evaluation of antialgalactivity of two biocides and photocatalytic nanoparticles. Biofouling 2010;26:89–101.

[84] Zielecka M, Bujnowska W, Kepska B, Wenda M, Piotrowska M. Antimicrobial additives for architectural paints andimpregnates. Prog Org Coat 2011;72:193–201.

[85] Dallas P, Sharma VK, Zboril R. Silver polymeric nanocomposites as advanced antimicrobial agents: classification, syntheticpaths, applications, and perspectives. Adv Colloid Interface 2011;166:119–35.

[86] Markowska-Szczupak A, Ulfig K, Morawski AW. The application of titanium dioxide for deactivation of bioparticulates: anoverview. Catal Today 2011;169:249–57.

[87] Carp O, Huisman CL, Reller A. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem 2004;32:33–177.[88] Huh AJ, Kwon YJ. ‘‘Nanoantibiotics’’: a new paradigm for treating infectious diseases using nanomaterials in the

antibiotics resistant era. J Control Release 2011;156:128–45.[89] Marambio-Jones C, Hoek EMV. A review of the antibacterial effects of silver nanomaterials and potential implications for

human health and the environment. J Nanopart Res 2010;12:1531–51.[90] Drelich J, Li B, Bowen P, Hwang J-Y, Mills O, Hoffman D. Vermiculite decorated with copper nanoparticles: novel

antibacterial hybrid material. Appl Surf Sci 2011;257:9435–43.[91] Benn TM, Westerhoff P. Nanoparticle silver released into water from commercially available sock fabrics. Environ Sci

Technol 2008;42:4133–9.[92] BioniCS. Bioni Hygienic

�. http://bioni.de/index.php?page=produktprogramm_bioni_hygienic&lang=en.

[93] Simoncic B, Tomsic B. Structures of novel antimicrobial agents for textiles – a review. Text Res J 2010;80:1721–37.[94] Pandey P, Merwyn S, Agarwal GS, Tripathi BK, Pant SC. Electrochemical synthesis of multi-armed CuO nanoparticles and

their remarkable bactericidal potential against waterborne bacteria. J Nanopart Res 2012;14:709.[95] Hochmannova L, Vytrasova J. Photocatalytic and antimicrobial effect of interior paints. Prog Org Coat 2010;67:1–5.[96] Espitia PJP. Soares NdFF, Coimbra JSdR, de Andrade NJ, Cruz RS, Medeiros EAA. Zinc oxide nanoparticles: synthesis,

antimicrobial activity and food packaging applications. Food Bioprocess Technol 2012;5:1447–64.[97] Huang L, Li D-Q, Lin Y-J, Wei M, Evans DG, Duan X. Controllable preparation of nano-MgO and investigation of its

bactericidal properties. J Inorg Biochem 2005;99:986–93.[98] Tung WS, Daoud WA. Self-cleaning fibers via nanotechnology: a virtual reality. J Mater Chem 2011;21:7858–69.















































http://bioni.de/index.php?page=produktprogramm_bioni_hygienic&lang=en











[99] Markowska-Szczupak A, Ulfig K, Grzmil B, Morawski AW. A preliminary study on antifungal effect of TiO2-based paints innatural indoor light. Pol J Chem Technol 2010;12:53–7.

[100] Hashimoto K, Irie H, Fujishima A. TiO2 photocatalysis: a historical overview and future prospects. Jpn J Appl Phys2005;44:8269–85.

[101] Rincón A-G, Pulgarin C. Bactericidal action of illuminated TiO2 on pure Escherichia coli and natural bacterial consortia:post irradiation events in the dark and assessment of the effective disinfection time. Appl Catal B 2004;49:99–112.

[102] Kim JY, Lee C, Cho M, Yoon J. Enhanced inactivation of E. coli and MS-2 phage by silver ions combined with UV-A andvisible light radiation. Water Res 2008;42:356–62.

[103] Cleaning with light. The innovation in catalytic pollutant degradation: KRONOS vlp 7000. Kronos. http://www.nl-ind.com/khome.nsf/KRONOS%20vlp%20-%20Cleaning%20with%20light.pdf.

[104] Nanotechnology and nanomaterials commercialisation chart: nanoparticle titanium dioxide (TiO2). Future Markets Inc.;2011. http://www.futuremarketsinc.com/images/Charts/tio2charts.pdf.

[105] Sharma VK, Yngard RA, Lin Y. Silver nanoparticles: green synthesis and their antimicrobial activities. Adv Colloid Interface2009;145:83–96.

[106] Le A-T, Huy PT, Tam LT, Tam PD. Novel silver nanoparticles: synthesis properties and applications. Int J Nanotechnol2011;8:278–90.

[107] Pal S, Tak YK, Song JM. Does the antibacterial activity of silver nanoparticles depend on the shape of the nanoparticle? Astudy of the gram-negative bacterium Escherichia coli. Appl Environ Microbiol 2007;73:1712–20.

[108] Wiley B, Sun Y, Mayers B, Xia Y. Shape-controlled synthesis of metal nanostructures: the case of silver. Chem – Eur J2005;11:454–63.

[109] Shameli K, Ahmad MB, Zargar M, Yunus WM, Ibrahim NA. Fabrication of silver nanoparticles doped in the zeoliteframework and antibacterial activity. Int J Nanomed 2011;6:1833–52.

[110] Oya A, Yoshida S, Abe Y, Iizuka T, Makiyama N. Antibacterial activated carbon fiber derived from phenolic resin containingsilver nitrate. Carbon 1993;31:71–3.

[111] Chen S, Liu J, Zeng H. Structure and antibacterial activity of silver-supporting activated carbon fibres. J Mater Sci2005;40:6223–31.

[112] Kim B-J, Park S-J. Antibacterial behavior of transition-metals-decorated activated carbon fibers. J Colloid Interf Sci2008;325:297–9.

[113] Rangari VK, Mohammad GM, Jeelani S, Hundley A, Vig K, Singh SR, et al. Synthesis of Ag/CNT hybrid nanoparticles andfabrication of their nylon-6 polymer nanocomposite fibers for antimicrobial applications. Nanotechnol 2010;21:095102.

[114] Subbiah RP, Lee H, Veerapandian M, Sadhasivam S, Seo S-W, Yun K. Structural and biological evaluation ofmultifunctional SWCNT-AgNPs-DNA/PVA bio-nanofilm. Anal Bioanal Chem 2011;400:547–60.

[115] Jung JH, Hwang GB, Lee JE, Bae GN. Preparation of airborne Ag/CNT hybrid nanoparticles using an aerosol process andtheir application to antimicrobial air filtration. Langmuir 2011;27:10256–64.

[116] Nangmenyi G, Xao W, Mehrabi S, Mintz E, Economy J. Bactericidal activity of Ag nanoparticle-impregnated fibreglass forwater disinfection. J Water Health 2009;2009:4.

[117] Jeon H-J, Yi S-C, Oh S-G. Preparation and antibacterial effects of Ag–SiO2 thin films by sol–gel method. Biomaterials2003;24:4921–8.

[118] Kim YH, Lee DK, Cha HG, Kim CW, Kang YS. Synthesis and characterization of antibacterial Ag–SiO2 nanocomposite. J PhysChem C 2007;111:3635.

[119] Mahltig B, Gutmann E, Meyer DC, Reibold M, Bund A, Böttcher H. Thermal preparation and stabilization of crystallinesliver particles in SiO2-based coating solutions. J Sol–Gel Sci Technol 2009;49:202–8.

[120] Wang J-X, Wen L-X, Wang Z-H, Chen J-F. Immobilization of silver on hollow silica nanospheres and nanotubes and theirantibacterial effects. Mater Chem Phys 2006;96:90–7.

[121] Guin D, Manorama SV, Latha JNL, Singh S. Photoreduction of silver on bare and colloidal TiO2 nanoparticles/nanotubes:synthesis, characterization, and tested for antibacterial outcome. J Phys Chem C 2007;111:13393–7.

[122] Li H, Duan X, Liu G, Liu X. Photochemical synthesis and characterization of Ag/TiO2 nanotube composites. J Mater Sci2008;43:1669–76.

[123] Niño-Martíez N, Martínez-Castañón GA, Aragón-Piña A, Martínez-Gutierrez F, Martínez-Mendoza JR, Ruiz F.Characterization of silver nanoparticles synthesised on titanium dioxide fine particles. Nanotechnology 2008;19 [artno 065711].

[124] Estrin Y, Khaydarov R, Khaydarov RA, Endres C, Cho SY. Antimicrobial and antibacterial effects of silver nanoparticlessynthesized by novel electrochemical method. In: Proceedings of the 2008 international conference on nanoscience andnanotechnology, ICONN 2008; 2008 [art no 4639241:44-7].

[125] Kumar A, Vemula PK, Ajayan PM, John G. Silver–nanoparticle-embedded antimicrobial paints based on vegetable oil. NatMater 2008;7:236–41.

[126] Soto G, Tiznado H, Cpntreras O, Pérez-Tijerina E, Cruz-Reyes J, Del Valle M, et al. Preparation of Ag/SiO2 nanocompositesusing fluidized bed microwave plasma reactor, and its hydrodesulphurization and Escherichia coli bactericidal activities.Powder Technol 2011;213:55–62.

[127] Valášková M, Hunáková M, Mamulová Kutláková K, Seidlerová J, Capková P, Pazdziora E, et al. Preparation andcharacterization of antibacterial silver/vermiculites and silver/montmorillonites. Geochim Cosmochim Acta2010;74:6287–300.

[128] Lu Z, Li CM, Bao H, Qiao Y, Toh Y, Yang X. Mechanism of antimicrobial activity of CdTe quantum dots. Langmuir2008;24:5445–52.

[129] Lyon DY, Adams LK, Falkner JC, Alvarez PJJ. Antibacterial activity of fullerene water suspensions: effects of preparationmethod and particle size. Environ Sci Technol 2006;40:4360–6.

[130] Kai Y, Komazawa Y, Miyajima A, Miyata N, Yamakoshi Y. [60]Fullerene as a novel photoinduced antibiotic. FullerenesNanotubes Carbon Nanostruct 2003;11:79–87.

[131] Isakovic A, Markovic Z, Todorovic-Markovic B, Nikolic N, Vranjes-Djuric S, Mirkovic M, et al. Distinct cytotoxicmechanisms of pristine versus hydroxylated fullerene. Toxicol Sci 2006;91:173–83.










































































[132] Sayes CM, Fortner JD, Guo W, Lyon DY, Boyd AM, Ausman KD, et al. The differential cytotoxicity of water-solublefullerenes. Nano Lett 2004;4:1881–7.

[133] Takenaka S, Yamashita K, Takagi M, Hatta T, Tsuge O. Photo-induced DNA cleavage by water-soluble cationic fullerenederivatives. Chem Lett 1999:321–2.

[134] Kamat JP, Devasagayam TPA, Priyadarsini KI, Mohan H. Reactive oxygen species mediated membrane damage induced byfullerene derivatives and its possible biological implications. Toxicology 2000;155:55–61.

[135] Lyon DY, Brunet L, Hinkal GW, Wiesner MR, Alvarez PJJ. Antibacterial activity of fullerene water suspensions (nC60) is notdue to ROS-mediated damage. Nano Lett 2008;8:1539–43.

[136] Lyon DY, Alvarez PJJ. Fullerene water suspension (nC60) exerts antibacterial effects via ROS-independent proteinoxidation. Environ Sci Technol 2008;42:8127–32.

[137] Lyon DY, Brown DA, Alvarez PJJ. Implications and potential applications of bactericidal fullerene water suspensions: effectof nC60 concentration, exposure conditions and shelf life. Water Sci Technol 2008;57:1533–8.

[138] Nel A, Xia T, Mädler L, Li N. Toxic potential materials at the nanolevel. Science 2006;311:622–7.[139] Manna SK, Sarkar S, Barr J, Wise K, Barrera EV, Jejelowo O, et al. Single-walled carbon nanotube induces oxidative stress

and activates nuclear transcription factor-jB in human keratinocytes. Nano Lett 2005;5:1676–84.[140] Kang S, Pinault M, Pfefferle LD, Elimelech M. Single-walled carbon nanotubes exhibit strong antimicrobial activity.

Langmuir 2007;23:8670–3.[141] Kang S, Herzberg M, Rodrigues DF, Elimelech M. Antibacterial effects of carbon nanotubes: size does matter! Langmuir

2008;24:6409–13.[142] Liu S, Wei L, Hao L, Fang N, Chang MW, Xu R, et al. Sharper and faster ‘‘nano darts’’ kill more bacteria: a study of

antibacterial activity of individually dispersed pristine single-walled carbon nanotube. ACS Nano 2009;3:3891–902.[143] Kang S, Mauter MS, Elimelech M. Physicochemical determinants of multiwalled carbon nanotube bactericidal

cytotoxicity. Environ Sci Technol 2008;42:7528–34.[144] Coatings xperience: more coatings join the war on germs. JCT CoatingsTech 2008;5:26.[145] Grover N, Borkar IV, Dinu CZ, Kane RS, Dordick JS. Laccase- and chloroperoxidase-nanotube paint composites with

bactericidal and sporicidal activity. Enzyme Microb Technol 2012;50:271–9.[146] Asuri P, Karajanagi SS, Yang H, YIm T-J, Kane RS, Dordick JS. Increasing protein stability through control of the nanoscale

environment. Langmuir 2006;22:5833–6.[147] Asuri P, Bale SS, Pangule RC, Shah DA, Kane RS, Dordick JS. Structure, function, and stability of enzymes covalently

attached to single-walled carbon nanotubes. Langmuir 2007;23:12318–21.[148] Asuri P, Karajanagi SS, Kane RS, Dordick JS. Polymer–nanotube–enzyme composites as active antifouling films. Small

2007;3:50–3.[149] Ma M, Hill RM, Rutledge GC. A review of recent results on superhydrophobic materials based on micro- and nanofibers. J

Adhes Sci Technol 2008;22:1799–817.[150] Han JT, Kim SY, Woo JS, Lee G-W. Transparent, conductive, and superhydrophobic films from stabilized carbon nanotube/

silane sol mixture solution. Adv Mater 2008;20:3724–7.[151] Koch K, Barthlott W. Superhydrophobic and suprhydrophillic plant surfaces: an inspiration for biomimetic materials. Phil

Trans Roy Soc A 2009;367:1487–509.[152] Ganesh VA, Raut HK, Nair AS, Ramakrishna S. A review on self-cleaning coatings. J Mater Chem 2011;21:16304–22.[153] Ogawa T, Ding B, Sone Y, Shiratori S. Super-hydrophobic surfaces of layer-by-layer structured film-coated electrospun

nanofibrous membranes. Nanotechnology 2007;18:165607.[154] Gao X, Jiang L. Water-repellent legs of water striders. Nature 2004;432:36.[155] Parkin IP, Palgrave RG. Self-cleaning coatings. J Mater Chem 2005;15:1689–95.[156] Wenzel RN. Resistance of solid surfaces to wetting by water. Ind Eng Chem 1936;28:988–94.[157] Meng L-Y, Park S-J. Effect of fluorination of carbon nanotubes on superhydrophobic properties of fluoro-based films. J

Colloid Interf Sci 2010;342:559–63.[158] Cassie ABD, Baxter S. Wettability of porous surfaces. Trans Faraday Soc 1944;40:546–51.[159] Cho WK, Choi IS. Fabrication of hairy polymeric films inspired by geckos: wetting and high adhesion properties. Adv

Funct Mater 2008;18:1089–96.[160] Kustandi TS, Samper VD, Yi DK, Ng WS, Neuzil P, Sun W. Self-assembled nanoparticles based fabrication of gecko foot-hair

polymer nanofibres. Adv Funct Mater 2007;17:2211–8.[161] Verma LK, Sakhuja M, Son J, Danner AJ, Yang H, Zeng HC, et al. Self-cleaning and antireflective packaging for solar

modules. Renew Energy 2011;36:2489–93.[162] Ling XY, Phang IY, Vancso GJ, Huskens J, Reinhoudt DN. Stable and transparent superhydrophobic nanoparticle films.

Langmuir 2009;25:3260–3.[163] Guo Y, Wang Q. Facile approach in fabricating superhydrophobic coatings from silica-based nanocomposite. Appl Surf Sci

2010;257:33–6.[164] Bravo J, Zhai L, Wu Z, Cohen RE, Rubner MF. Transparent syperhydrophobic films based on silica nanoparticles. Langmuir

2007;23:7293–8.[165] Soeno T, Inokuchi K, Shiratori S. Ultra-water-repellent surface: fabrication of complicated structure of SiO2 nanoparticles

by electrostatic self-assembled films. Appl Surf Sci 2004;237:543–7.[166] Niu JJ, Wang JN. A novel self-cleaning coating with silicon carbide nanowires. J Phys Chem B 2009;113:2909.[167] Li X, Du X, He J. Self-cleaning antireflective coatings assembled from peculiar mesoporous silica nanoparticles. Langmuir

2010;26:13528–34.[168] Yang J, Zhang Z, Men X, Xu X. Fabrication of stable, transparent and superhydrophobic nanocomposite films with

polystyrene functionalized carbon nanotubes. Appl Surf Sci 2009;255:9244–7.[169] Bu IYY, Oei SP. Hydrophobic vertically aligned carbon nanotubes on Corning glass for self cleaning applications. Appl Surf

Sci 2010;256:6699–704.[170] Tang M, Hong MH, Choo YS, Tang Z, Chua DHC. Super-hydrophobic transparent surface by femtosecond laser micro-

patterned catalyst thin film for carbon nanotube cluster growth. Appl Phys A 2010;101:503–8.











































































[171] Lotusan. http://www.lotusan.de/ (in German).[172] Deleting graffiti. The writing is off the wall. Researchers in Mexico have invented a new type of anti-graffiti paint. The

Economist; 2003. http://www.economist.com/node/2173232.[173] Ding X, Zhou S, Gu G, Wu L. A facile and large-area fabrication method of superhydrophobic self-cleaning fluorinated

polysiloxane/TiO2 nanocomposite coatings with long-term durability. J Mater Chem 2011;21:6161–4.[174] Mills A, Lepre A, Elliott N, Bhopal S, Parkin IP, O’Neill SA. Characterisation of the photocatalst Pilkington Activ™: a

reference film photocatalyst? J Photochem Photobiol A 2003;160:213–24.[175] Chen J, Poon C-S. Photocatalytic construction and building materials: from fundamentals to applications. Build Environ

2009;44:1899–906.[176] Kim J, Kim J, Lee M. Laser-induced enhancement of the surface hardness of nanoparticulate TiO2 self-cleaning layer. Surf

Coat Technol 2010;205:372–6.[177] Mu Q, Li Y, Wang H, Zhang Q. Self-organized TiO2 nanorod arrays on glass substrate for self-cleaning antireflection

coatings. J Colloid Interf Sci 2012;365:308–13.[178] Takata Y, Hidaka S, Cao JM, Nakamura T, Yamamoto H, Masuda M, et al. Effect of surface wettability on boiling and

evaporation. Energy 2005;30:209–20.[179] Tan LK, Kumar MK, An WW, Gao H. Transparent, well-aligned TiO2 nanotube arrays with controllable dimensions on glass

substrates for photocatalytic applications. ACS Appl Mater Interf 2010;2:498–503.[180] Zhang X, Pan JH, Du AJ, Ng J, Sun DD, Leckie JO. Fabrication and photocatalytic activity of porous TiO2 nanowire

microspheres by surfactant-mediated spray drying process. Mater Res Bull 2009;44:1070–6.[181] Zhang X-T, Sato O, Taguchi M, Einaga Y, Murakami T, Fujishima A. Self-cleaning particle coating with antireflection

properties. Chem Mater 2005;17:696–700.[182] Fujishima A, Zhang X, Tyrk DA. TiO2 photocatalysis and related surface phenomena. Surf Sci Rep 2008;63:515–82.[183] Yaghoubi H, Taghavinia N, Alamdari EK, Volinsky AA. Nanomechanical properties of TiO2 granular thin films. ACS Appl

Mater Interf 2010;2:2629–36.[184] Roméas V, Pichat P, Guillard C, Chopin T, Lehaut C. Testing the efficacy and the potential effect on indoor air quality of a

transparent self-cleaning TiO2-coated glass through degradation of a fluoranthene layer. Ind Eng Chem Res1999;38:3878–85.

[185] Mor GK, Varghese OK, Paulose M, Grimes CA. Transparent highly ordered TiO2 nanotube arrays via anodization oftitanium thin films. Adv Funct Mater 2005;15:1291–6.

[186] Kazemi M, Mohammadizadeh MR. Simultaneous improvement of photocatalytic and superhydrophilicity properties ofnano TiO2 thin films. Chem Eng Res Des 2012;90:1473–9.

[187] Pan C, Sun C, Wei H-M, Han G-Z, Zhang J-Z, Fujushima A, et al. Bio-inspired titanium dioxide film with extremely stablesuper-amphilicity. Mater Res Bull 2007;42:1395–401.

[188] Chen C-C, Lin J-S, Diau EW-G, Liu T-F. Self-cleaning characteristics on a thin-film surface with nanotube arrays of anodictitanium oxide. Appl Phys A 2008;92:615–20.

[189] Fujishima A, Zhang X. Titanium dioxide photocatalysis: present situation and future approaches. CR Chim2006;9:750–60.

[190] Roberto M, Bertani R, Cona A, Fiorentin P, Garau G, Maragno C, et al. Photocatalytic building products: innovation andexperimentations. In: SASBE09 – 3rd international conference on smart and sustainable built environments, Delft,Netherlands; 15–19 June, 2009.

[191] Cassar L. Photocatalysis of cementitous materials: clean buildings and clean air. MRS Bull 2004;29:328–31.[192] Ballari MM, Hunger M, Hüsken G, Brouwers HJH. Modelling and experimental study of the NOx photocatalytic

degradation employing concrete pavement with tutanium dioxide. Catal Today 2010;151:71–6.[193] Ballari MM, Yu QL, Brouwers HJH. Experimental study of the NO and NO2 degradation by photocatalytically active

concrete. Catal Today 2011;161:175–80.[194] Dylla H, Hassan MM, Schmitt M, Rupnow T, Mohammad LN. Laboratory Investigation of the effect of mixed nitrogen

dioxide and nitrogen oxide gases on titanium dioxide photocatalytic efficiency in concrete pavements. J Mater Civil Eng2011;23:1087–93.

[195] Chen J, Poon C-S. Photocatalytic activity of titanium dioxide modified concrete materials – influence of utilizing recycledglass cullets as aggregates. J Environ Manage 2009;90:3436–42.

[196] Wang S, Ang HM, Tade MO. Volatile organic compounds in indoor environment and photocatalytic oxidation: state of theart. Environ Int 2007;33:694–705.

[197] Light2Cat. http://www.light2cat.eu/index.php/about-light2cat-2.[198] Hüsken G, Hunger M, Brouwers HJH. Experimental study of photocatalytic concrete products for air purification. Build

Environ 2009;44:2463–74.[199] Langridge JM, Gustafsson RJ, Griffiths PT, Cox RA, Lambert RM, Jones RL. Solar driven nitrous acid formation on building

material surfaces containing titanium dioxide: a concern for air quality in urban areas? Atmos Environ 2009;43:5128–31.[200] Laufs S, Bugeth G, Duttlinger W, Kurtenbach R, Maban M, Thomas C, et al. Conversion of nitrogen oxides on commercial

photocatalytic dispersion paints. Atmos Environ 2010;44:2341–9.[201] Auvinen J, Wirtanen L. The influence of photocatalytic interior paints on indoor air quality. Atmos Environ 2008;42:4101–12.[202] Geiss O, Cacho C, Barrero-Moreno J, Kotzias D. Photocatalytic degradation of organic paint constituents-formation of

carbonyls. Build Environ 2012;48:107–12.[203] Dylla H, Hassan MM, Schmitt M, Rupnow T, Mohammad LN, Wright E. Effects of roadway contaminants on titanium

dioxide photodegradation of nitrogen oxides. Transport Res Rec 2011;3340:22–9.[204] Ramirez AM, Demeestere K, De Belie N, Mäntylä T, Levänen E. Titanium dioxide coated cementitious materials for air

purifying purposes: preparation, characterisation and toluene removal potential. Build Environ 2010;45:832–8.[205] Diamanti MV, Ormellese M, Pedeferri MP. Characterization of photocatalytic and superhydrophilic properties of mortars

containing titanium dioxide. Cem Concr Res 2008;38:1349–53.[206] Hassan MM, Dylla H, Mohammad LN, Rupnow T. Evaluation of the durability of titanium dioxide photocatalyst for

concrete pavement. Constr Build Mater 2010;24:1456–61.














































































[207] Strini A, Cassese S, Schiavi L. Measurement of benzene, toluene, ethylbenzene and o-xylene gas phase photodegradationby titanium dioxide dispersed in cementitious materials using a mixed flow reactor. Appl Catal B 2005;61:90–7.

[208] Rachel A, Subrahmanyam M, Boule P. Comparison of photocatalytic efficiencies of TiO2 in suspended and immobilisedform for the photocatalytic degradation of nitrodenzenesulfonic acids. Appl Catal B 2002;37:301–8.

[209] Cassar L, Pepe C. Paving tiles comprising an hydraulic binder and photocatalyst particles, EP 1600430.[210] Lackhoff M, Prieto X, Nestle N, Dehn F, Niessner R. Photocatalytic activity of semiconductor-modified cement – infuence

of semiconductor type and cement aging. Appl Catal B 2003;43:205–16.[211] Ruot B, Plassais A, Olive F, Guillot L, Bonafous L. TiO2-containing cement pastes and mortars: measurements of the

photocatalytic efficiency using a rhodamine B-based colourimetric test. Sol Energy 2009;83:1794–801.[212] Folli A, Pade C, Hansen TB, De Marco T, Macphee DA. TiO2 photocatalysis in cementatios systems: insights into self-

cleaning and depollution chemistry. Cem Concr Res 2012;42:539–48.[213] Yu JC-M. Deactivation and regeneration of environmentally exposed titanium dioxide (TiO2) based products.

Environmental Protection Department, HKSAR; 2003. http://www.epd.gov.hk/epd/english/environmentinhk/air/studyrpts/files/report-updatedcb_paving_blocks_2003.pdf.

[214] Maggos T, Bartzis JG, Leva P, Kotzias D. Application of photocatalytic technology for NOx removal. Appl Phys A2007;89:81–4.

[215] Enea D, Guerrini GL. Photocatalytic properties of cement-based plasters and paints containing mineral pigments.Transport Res Rec 2010;2141:52–60.

[216] PICADA – Photocatalytic innovative coverings applications for depollution assessment. http://www.picada-project.com/domino/SitePicada/Picada.nsf?OpenDataBase.

[217] Maggos T, Plassais A, Bartzis JG, Vasilakos C, Moussiopoulos N, Bonafous L. Photocatalytic degradation of NOx in a pilotstreet canyon configuration using TiO2-mortar panels. Environ Monit Assess 2008;136:35–44.

[218] Chen M, Chu J-W. NOx photocatalytic degradation on active concrete road surface – from experiment to real-scaleapplication. J Clean Prod 2011;19:1266–72.

[219] Allen NS, Edge M, Verran J, Stratton J, Maltby J, Bygott C. Photocatalytic titania based surfaces: environmental benefits.Polym Degrad Stabil 2008;93:1632–46.

[220] Quick D. On the road to cleaner air with air-purifying, concrete; 2010. http://www.gizmag.com/air-purifying-roads/15638/.

[221] Águia C, Ângelo J, Madeira LM, Mendes A. Influence of photocatalytic paint component on the photoactivity of P25towards NO abatement. Catal Today 2010;151:77–83.

[222] Pirola C, Boffito DC, Vitali S, Bianchi CL. Photocatalytic coatings for building industry: study of 1 year of activity in the NOx

degradation. J Coat Technol Res 2011;9:453–8.[223] Ichiura H, Kitaoka T, Tanaka H. Removal of indoor pollutants under UV irradiation by a composite TiO2-zeolite sheet

prepared using a papermaking technique. Chemosphere 2003;50:79–83.[224] Taoda H, Fukaya M, Watanabe E, Tanaka K. VOC decomposition by photocatalytic wall paper. Mater Sci Forum 2006;510–

512:22–5.[225] Costa A, Chiarello GL, Selli E, Guarino M. Effects of TiO2 based photocatalytic paint on concentrations and emissions of

pollutants and on animal performance in swine weaning unit. J Environ Manage 2012;96:86–90.[226] Maggos T, Bartzis JG, Liakou M, Gobin C. Photocatalytic degradation of NOx gases using TiO2-containing paint: a real scale

study. J Hazard Mater 2007;146:668–73.[227] Active interior paint StoClimasan Color. Sto AG. http://www.climasan.com/33671_EN-Pictures-Brochure_6_Pages.pdf.[228] Marolt T, Škapin AS, Bernard J, Zivec P, Gaberšcek M. Photocatalytic activity of anatase-containing facade coatings. Surf

Coat Technol 2011;206:1355–61.[229] Guerrini GL. Photocatalytic performances in a city tunnel in Rome: NOx monitoring results. Constr Build Mater

2012;27:165–75.[230] Irie H, Sunada K, Hashimoto K. Recent developments in TiO2 photocatalysis: novel applications to interior ecology

materials and energy saving systems. Electrochemistry 2004;72:807.[231] Pathways to a low-carbon economy. Version 2 of the global greenhouse gas abatement cost curve. McKinsey & Company;

2009. https://solutions.mckinsey.com/climatedesk/default.aspx.[232] Power A. Does demolition or refurbishment of old and inefficient homes help to increase our environmental, social and

economic viability? Energy Policy 2008;36:4487–501.[233] Stahl T, Brunner S, Zimmermann M, Ghazi Wakili K. Thermo-hygric properties of a newly developed aerogel based

insulation rendering for both exterior and interior applications. Energy Build 2012;44:114–7.[234] Revertz J. State of the stock – what do we know about existing buildings and their future prospects? Energy Policy

2008;36:4462–70.[235] BaetensR, JelleBP,GustavsenA.Aerogel insulationforbuildingapplications:astate-of-the-artreview.EnergyBuild2011;43:761–9.[236] Jelle BP, Hynd A, Gustavsen A, Arasteh D, Goudey H, Hart R. Fenestration of today and tomorrow: a state-of-the-art review

and future research opportunities. Sol Energy Mater Sol C 2012;96:1–28.[237] Dowson M, Pegg I, Harrison D, Dehouche Z. Predicted and in situ performance of a solar air collector incorporating a

translucent granular aerogel cover. Energy Build 2012;49:173–87.[238] Gao G-M, Miao L-N, Ji G-J, Zou H-F, Gan S-C. Preparation and characterization of silica aerogels from oil shale ash. Mater

Lett 2009;63:2721–4.[239] Gurav JL, Jung I-K, Park H-H, Kang ES, Nadargi DY. Silica aerogel: synthesis and applications. J Nanomater 2010;2010 [art

no. 409310].[240] Hüsing N, Schubert U. Aerogels-airy materials: chemistry, structure, and properties. Angew Chem – Int Ed

1998;37:22–45.[241] Akimov YK. Fields of application of aerogels (review). Instrum Exp Tech+ 2003;46:287–99.[242] Soleimani Dorcheh A, Abbasi MH. Silica aerogels; synthesis properties and characterization. J Mater Process Technol

2008;199:10–26.









































































[243] Scheuerpflug P, Caps R, Büttner D, Fricke J. Apparent thermal conductivity of evacuated SiO2-aerogel tiles under variationof radiative boundary conditions. Int J Heat Mass Transfer 1985;28:2299–306.

[244] Yuan B, Ding S, Wang D, Wang G, Li H. Heat insulation properties of silica aerogel/glass fiber composites fabricated bypress forming. Mater Lett 2012;75:204–6.

[245] Schultz JM, Jensen KI. Evacuated aerogel glazings. Vacuum 2008;82:723–9.[246] Bahaj AS, James PAB, Jentsch MF. Potential of emerging glazing technologies for highly glazed buildings in hot arid

climates. Energy Build 2008;40:720–31.[247] Aerogel provides thermal insulation and fire protection for airplane firewall. Aspen Aerogels. http://www.aerogel.com/

markets/Case_Study_Plane_web.pdf.[248] Schwertfeger FHA. Future special science – the Hoechst magazine, vol. 2; 1997. p. l28.[249] Rubin M, Lampert CM. Transparent silica aerogels for window insulation. Sol Energy Mater 1983;7:393–400.[250] Dowson M, Grogan M, Birk T, Harrision D, Craig S. Streamlined life cycle assessment of transparent silica aerogel made by

supercritical drying. Appl Energy 2011;97:396–404.[251] Buratti C, Moretti E. Experimental performance evaluation of aerogel glazing systems. Appl Energy 2012;97:430–7.[252] Ramakrishnan K, Krishnan A, Shankar V, Srivastava I, Singh A, Radha R. Modern aerogels. http://www.dstuns.iitm.ac.in/.[253] Reim M, Beck A, Körner W, Petricevic R, Gloria M, Weth M, et al. Highly insulating aerogel glazing for solar energy usage.

Sol Energy 2002;72:21–9.[254] Translucent daylighting systems – custom skyroofs™. Kalwall. http://www.kalwall.com/custom.htm.[255] Sculpture building and gallery Yale University NewHaven, CT. Kieran Timberlake. http://kierantimberlake.com/

featured_projects/sculpture_building_1.html.[256] Fricke J. Aerogel. Sci Am 1988;258:92–7.[257] Data sheet, Spaceloft�. High performance insulation for building envelopes. Aspen Aerogels. http://www.aerogel.com/

products/pdf/Spaceloft_DS.pdf.[258] Retrofit for the future. Low energy building database. http://www.retrofitforthefuture.org/.[259] Sadineni SB, Madala S, Boehm RF. Passive building energy savings: a review of building envelope components. Renew

Sust Energy Rev 2011;15:3617–31.[260] Airglass. http://www.airglass.se/.[261] Cagliardi M. Aerogels, report AVM052B. BBC Research; 2009.[262] Aspen Aerogels. http://www.aerogel.com/.[263] Aerogel. Cabot Corporation. http://www.cabot-corp.com/aerogel.[264] Bagnall DM, Boreland M. Photovoltaic technology. Energy Policy 2008;36:4390.[265] Emin S, Singh SP, Han L, Satoh N, Islam A. Colloidal quantum dot solar cells. Sol Energy 2011;85:1264–82.[266] Garland A. Nanosolar. Nanotech Magazine. Future Markets, Inc.; February 2012. p. 4–5.[267] Hillhouse HW, Beard MC. Solar cells from colloidal nanocrystals: fundamentals, materials, devices, and economics. Curr

Opin Colloid Interface 2009;14:245–59.[268] Tang J, Sargent EH. Infrared colloidal quantum dots for photovoltaics: fundamentals and recent progress. Adv Mater

2011;23:12–29.[269] Gabor NM, Zhong Z, Bosnick K, Park J, McEuen PL. Extremely efficient multiple electron–hole pair generation in carbon

nanotube photodiodes. Science 2009;325:1367–71.[270] Wang S, Khafizov M, Tu X, Zheng M, Krauss TD. Multiple excition generation in single-walled carbon nanotubes. Nano

Lett 2010;10:2381–6.[271] Franceschetti A. Nanostructured materials for improved photoconversion. MRS Bull 2011;36:192–7.[272] Genovese MP, Lightcap IV, Kamat PV. Sun-Believable solar paint. A transformative one-step approach for designing

nanocrystalline solar cells. ACS Nano 2012;6:865–72.[273] Tropiglass Technologies Ltd. http://www.tropiglas.com/about/.[274] Chen R-T, Chau JLH, Hwang G-L. Design and fabrication of diffusive solar cell window. Renew Energy 2012;40:24–8.[275] Bohannon J. ‘Smart coatings’ research shows the virtues of superficiality. Science 2005;309:376–7.[276] Paliwoda-Porebska G, Stratmann M, Rohwerder M, Potje-Kamloth K, Lu Y, Pich AZ, et al. On the development of

polypyrrole coatings with self-healing properties for iron corrosion protection. Corros Sci 2005;47:3216–33.[277] Kozhukharov S, Kozhukharov V, Wittmar M, Schem M, Aslan M, Caparrotti H, et al. Protective abilities of nanocomposite

coatings containing Al2O3 nano-particles loaded by CeCl3. Prog Org Coat 2011;71:198–205.[278] Han B, Lu X. Effect of nano-sized CeF3 on microstructure, mechanical, high temperature friction and corrosion behavior of

Ni–W composite coatings. Surf Coat Technol 2009;203:3656–60.[279] Shi X, Nguyen TA, Suo Z, Liu Y, Avci R. Effect of nanoparticles on the anticorrosion and mechanical properties of epoxy

coating. Surf Coat Technol 2009;204:237–45.[280] Li W, Tian H, Hou B. Corrosion performance of epoxy coatings modified by nanoparticulate SiO2. Mater Corros

2012;63:44–53.[281] Olad A, Barati M, Shirmohammadi H. Conductivity and anticorrosion performance of polyaniline/zinc composites:

investigation of zinc particle size and distribution effect. Prog Org Coat 2011;72:599–604.[282] Huang T-C, Su Y-A, Yeh T-C, Huang H-Y, Wu C-P, Huang K-Y, et al. Advanced anticorrosive coatings prepared from

electroactive epoxy-SiO2 hybrid nanocomposite materials. Electrochim Acta 2011;56:6142–9.[283] Shchukin DG, Möhwald H. Self-repairing coatings containing active nanoreserviors. Small 2007;3:926–43.[284] Zheludkevich M, Shchukin DG, Yasakau KA, Möhwald H, Ferreira MGS. Anticorrosion coatings with self-healing effect

based on nanocontainers impregnated with corrosion inhibitor. Chem Mater 2007;19:402–11.[285] Shchukin DG, Zheludkevich M, Yasakau K, Lamaka S, Ferreira VM, Möhwald H. Layer-by-layer assembled nanocontainers

for self-healing corrosion protection. Adv Mater 2006;18:1672–8.[286] Schmidt C. Anti-corrosive coating including a filler with a hollow cellular structure. US patent 6383271; 2002.[287] Lvov YM, Shchukin DG, Möhwald H, Price RR. Halloysite clay nanotubes for controlled release of protective agents. ACS

Nano 2008;2:814–20.





























































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