TiO2 functionalization for efficient NOx removal in photoactive cement

Applied Surface Science 319 (2014) 29–36

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TiO2 functionalization for efficient NOx removal in photoactive cement S. Karapati a , T. Giannakopoulou a , N. Todorova a , N. Boukos a , S. Antiohos b , D. Papageorgiou b , E. Chaniotakis b , D. Dimotikali c , C. Trapalis a,∗ a b c

Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Ag. Paraskevi, 15310 Athens, Greece Titan Cement Industry S.A., Elefsina, Greece National Technical University of Athens, Department of Chemical Engineering, 15780 Athens, Greece

a r t i c l e

i n f o

Article history: Received 20 June 2014 Received in revised form 26 July 2014 Accepted 26 July 2014 Available online 1 August 2014 Keywords: Photocatalysis Modified TiO2 Cement Visible light NOx

a b s t r a c t Commercial titania nanoparticles (P25 Evonic-Degussa) were modified with organic compounds oleic acid (OA), oleylamine (OM) and equimolar concentrations of both modifiers (OAOM) through a biphase toluene/water emulsion processing in order to be endowed with hydrophobic properties. Specific molar ratio 3 between modifier and titania powder was used. The modified and the initial P25 photocatalysts were embedded in cement matrix in low percentage loading (2.5, 1, 0.5%). The grafting of the modifier to the titania nanoparticles was verified by thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and Fourier transform infrared spectroscopy (FT-IR). Light absorption measurements revealed that the energy band gap of the photocatalysts was lowered after the modification. The average size (nm) and polydisperse index of the initial and modified P25 were determined via dynamic light scattering (DLS). The photocatalytic activity of the photocatalysts alone and the composite cement specimens was evaluated via ISO standard NOx oxidation procedure. The photocatalytic cements containing modified P25 exhibited 2–5 times better NOx removal than those with non-modified P25 even for the lowest photocatalyst loading. The results were attributed to the hydrophobic properties of the modified titania and its behavior during the incorporation in the cement matrix. This conclusion was confirmed by the SEM/EDX analysis which demonstrated a gradual increase of hydrophobic photocatalyst from the bottom to the surface of the cement specimens. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Nanostructured TiO2 depicts remarkable photocatalytic properties associated with aesthetic durability [1–5], antimicrobial [6–8], air depolluting effects [9–11], water splitting and purification, oil spill remediation, photovoltaics, and photoinduced superhydrophilicity [11–16]. The photocatalyst TiO2 is an inert oxide, highly stable, non toxic, low costing and easily activated in ambient conditions [17,18]. TiO2 possesses high efficiency in degradation of organic pollutants such as VOCs (benzene, toluene, organic chlorides, aldehydes) and inorganic pollutants in liquid as well as in gas phase such as NOx , SOx , NH3 , and CO [10,11,19–21]. Combining TiO2 nanoparticles with cementitious binders is undoubtedly one of the most promising topics in the field of building materials pursuing composite environmentally friendly

∗ Corresponding author. Tel.: +30 210 650 3343; fax: +30 210 651 9430. E-mail addresses: [email protected] (S. Karapati), [email protected] (C. Trapalis). URL: http://www.ims.demokritos.gr (C. Trapalis). http://dx.doi.org/10.1016/j.apsusc.2014.07.162 0169-4332/© 2014 Elsevier B.V. All rights reserved.

materials with multifunctional attributes. TiO2 photoactivity was first noticed when it was embedded in building materials as white pigment in 1929 [22,23]. Currently, the applications of nano-scale photocatalytic TiO2 in the form of films and powders include coatings upon or component into supporting materials (substrates) such as ceramics, glass, paint, cement, and mortar thus improving their resistance in mechanical abrasion, environmental aging and granting photocatalytic performance as well [10]. Moreover, the synergism between the substrate and the TiO2 may even enhance the photocatalytic effect of the TiO2 itself, e.g. the porous structure of the hardened cement binders helps the TiO2 nanoparticles to easily come into contact with the target pollutants, thus facilitating the photocatalytic process. The greatest advantage featuring those photacatalytic products is that the only energy demands in the TiO2 -containing construction material are sunlight, oxygen and water [5,24]. This paper is focused on the contribution of cementitious surfaces loaded with modified TiO2 to NOx depolluting effect, i.e. De-NOx process. The NOx gaseous pollutants accumulated in urban cities mainly from anthropogenic sources such as mobile and stationary combustion sources are in the category of the very toxic air

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S. Karapati et al. / Applied Surface Science 319 (2014) 29–36

Table 1 Chemical composition of the white cement, %. SiO2

Al2 O3

Fe2 O3

CaO

MgO

K2 O

Na2 O

SO3

TiO2

P2 O5

Cl

21.19

3.06

0.26

67.29

1.44

0.36

0.00

0.97

0.12

0.10

0.03

pollutants causing damaging effects to: (a) human health including respiratory infections, health disease, lung cancer and (b) ecosystem through the creation of photochemical smog and acid rain [25,26]. When TiO2 surface is UV-illuminated, the photogenerated electrons and holes react with oxygen molecules and hydroxyl anions from the environment. As a result, highly reactive species are produced according to the following reactions: UV

TiO2 −→h+ + e− ,

(1)

−

−

O2 + e → O2 , −

+

OH + h → −

(2)

OH• ,

(3)

+

O2 + H → HO2 • .

(4)

The produced OH• and HO2 • oxidize the NO gas on the TiO2 surface forming also an intermediate NO2 air phase as follows [26]: NO + OH• → HNO2 ,

(5)

HNO2 + OH• → NO2 + H2 O,

(6)

•

NO + HO2 → NO2

+ OH• ,

NO2 + OH• → HNO3 .

(7) (8)

The final nitrate species are water soluble and can be flushed from the active concrete surface by rain [27]. Despite the above mentioned advantages of the photoactive cements and mortars, the cost of the already available products is still high because of the large outdoor surface areas that need to be covered (buildings, roads). In the case of photocatalytic cements, the hydrophilic nature of TiO2 leads to sedimentation of the photocatalyst in cement matrix reducing thus the exposed surface able to adsorb and oxidize air pollutants. The novelty of this study is the use of hydrophobic titania for incorporation in cement matrix in order to overcome the above referred limitation. The modified photocatalyst due to repulsive hydrophobic forces will be boosted to the cement surface and as a result greater amount of TiO2 will be “available” for NO degradation. Until now, hydrophobic surfactant or other adsorbent (e.g. zeolite) have been used to support TiO2 for efficient degradation of organic pollutants such as decabromodiphenyl ether (BDE 209) and 2-propanol in water [28,29]. As far as we know, this is the first report on incorporation of hydrophobically functionalized TiO2 in cement matrix for enhanced photocatalytic activity in oxidation of NOx (NO and NO2 ) gaseous pollutants. 2. Materials and methods 2.1. Materials and preparation of the samples White cement was cordially provided by Titan Cement Company S.A. The chemical composition of the cement is given in Table 1. Commercial TiO2 powder P25 (Evonic-Degussa) with mixed phase crystal composition of approximately 75% anatase and 25% rutile was used. The organic solvent toluene (purity 99.5%) was purchased from Merck, Germany. The oleic acid (C18 H34 O2 ) and oleylamine (C18 H37 N) were obtained from PRS Panreac and Sigma–Aldrich, respectively. Biphase toluene/water solvent was used for the functionalization procedure.

2.1.1. Preparation of modified P25 nanoparticles 1 g of P25 nanoparticles were dispersed in 100 ml of distilled H2 O. To achieve better homogeneity the suspension was sonicated in tip sonicator (Hielseher, UIP 100hd) functioning at 50% amplitude for 3 min. Separately, specific amount of oleic acid, oleylamine and equimolar concentrations of both modifiers were diluted in toluene and added to the titania suspension (Table 2). The emulsions obtained were kept under vigorous stirring at 60 ◦ C for 18 h. After that, the biphase mixtures were cooled at room temperature until the two phases of water/toluene were clearly separated. Depending on the modifier used different behavior of modified nanoparticles was observed. In case of oleic acid the modified nanoparticles were dispersed in both phases, while when oleylamine was used the particles tend to gather in the organic solvent. In case of equimolar mixtures of oleic acid and oleylamine the modified nanoparticles were sedimented in the aqueous phase. The modified titania nanoparticles were collected by centrifugation (8000 rpm, 3 min), washed twice with acetone and dried at room temperature for 24 h. 2.1.2. Preparation of titania loaded cement Pure P25 or modified nanoparticles and white cement were initially dry mixed. The mixtures contained 2.5, 1 and 0.5% titania. Then, water was added (wt% water/dry mixture = 50%) and the mixtures were stirred at 500 rpm for 3 min. The stirring was stopped for 2 min and repeated at 2000 rpm for 2 min. The final pastes were transferred to moulds (50 mm × 100 mm × 5 mm), covered and let harden for 3 days at room temperature. Finally, cement blocks with smooth surfaces were obtained. The nomination of the bare titania powders and the titania loaded cements investigated in the present work is presented in Table 2. 2.2. Methods of characterization Fourier transform infrared spectroscopy (FT-IR) was used to investigate the presence of bonds between modifiers and titania. Instrument Equinox 55/S, Bruker operating in diffuse reflectance mode was employed. The samples consisted of 95% KBr and 5% photocatalyst were placed in a sampling cup and were scanned in wavenumber range 400–4000 cm−1 . The thermal behavior of modified nanoparticles was determined by thermal gravimetric analysis using Perkin-Elmer Pyris TGA/DTA instrument. Dynamic light scattering technique was employed to determine the particle hydrodynamic diameter. The measurements were carried out in chloroform Table 2 Modified TiO2 nanoparticles and composite cement samples nomination. Titania nanoparticles

Titania loaded cements sample

Modifier MR 3

Sample name

2.5% TiO2 97.5% cement

1% TiO2 99% cement

0.5% TiO2 99.5% cement

– OA OM OAOM

P25 OA-3 OM-3 OAOM-3

WC-P251 WC-OA31 WC-OM31 WCOAOM31

WC-P252 WC-OA32 WC-OM32 WCOAOM32

WC-P253 WC-OA33 WC-OM33 WCOAOM33

MR: molar ratio modifier:P25. OA: oleic acid; OM: oleylamine; OAOM: mixture oleic acid and oleylamine.

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Fig. 1. Experimental setup for evaluation of the photocatalytic activity in NOx removal from air: (1) NO cylinder, (2) compressor, (3–5) gas flow controllers, (6) humidifier, (7) air mixer, (8) light irradiation source, (9) photocatalytic reactor, (10) analyzer, and (11) PC.

media on Zeta-sizer Nanoseries, Malvern instrument. Light absorption measurements in wavelength range 350–800 nm were carried out using the UV UV2100, Shimadzu spectrophotometer so as to estimate the energy gap between valence and conductance band. Morphological characterization of hydrophobic materials was conducted through transmission electron microscopy (TEM, Philips CM20) operated at 200 kV and equipped with the image filter Gatan GIF 200. SEM/EDX micrographs and composition analyses for the cement samples were carried out using FEI Inspect microscope with tungsten filament operating at 25 kV. Finally, the photocatalytic activity of the prepared titania and titania loaded cement samples was examined using NO (initial concentration 1 ppm) oxidation based on ISO DIS 22197/1:2007 standard method. The experimental setup used is schematically presented in Fig. 1. The oxidation process was monitored through the concentrations of the NO and NO2 gases in the gas flow above the photocatalysts. The De-NOx ability was evaluated by the decrease of the NO and NO2 concentrations in the gas media. The photocatalytic activity was determined under UV-A (main peak at 350 nm) and visible light irradiation with intensity 10 W/m2 . The photonic efficiency  of the photocatalytic materials under UV irradiation was evaluated through the ratio of the number of oxidized gas molecules to the number of incident photons for the entire illumination period according to the equation:

 t1 =

t0

AX(ppm)dt qn,p T

(mol/Einstein)

where A is the constant related to flow gas (A = 2.08 × 10−9 mol/s), X is the concentration of NO, NO2 and NOx (sum of NO and NO2 ), qn,p is the photons flux on the sample’s surface, and T (s) is the photocatalytic cycle period. The effectiveness of the photocatalysts under visible light irradiation was evaluated by the percentage difference  (%) in the concentration of the monitored gases, namely decrease of NO, increase of NO2 and decrease of the total NOx concentrations [30]. 3. Results and discussion 3.1. Fourier transform infrared spectroscopy (FT-IR) The FT-IR spectra of the pure, modified photocatalysts and of the surfactants as well are depicted in Fig. 2. The broad intense band in the range 800–400 cm−1 is assigned to the Ti O and Ti O Ti groups [31]. As it can be expected, the characteristic titania peaks are preserved in all modified samples.

Fig. 2. FT-IR spectrum of neat P25 nanoparticles, OA, OM, OAOM surfactants and OA-3, OM-3, OAOM-3 modified TiO2 nanoparticles.

The spectrum of neat oleic acid modifier includes peaks near 2970 cm−1 , 2930 cm−1 and 2850 cm−1 that correspond to stretching vibrations of CH3 and CH2 groups of long alkyl chains of the modifier. Also, characteristic peak at 1714 cm−1 is ascribed to stretching vibration of carbonyl group C O. As for TiO2 modified with oleic acid, the low absorption at 1556 cm−1 and 1400 cm−1 can be attributed to the asymmetric and symmetric stretching bands of COO− group, correspondingly. This result indicates that oleic acid is chemisorbed on TiO2 surface as a carboxylate. Because the wavenumber separation between asymmetric and symmetric stretches is equal to 156 cm−1 , the interaction between carboxylate head and Ti atom corresponds to bridging bidentate [32–36]. The presence of the absorption at 1714 cm−1 in the spectrum of modified TiO2 reflects the fact that some of the oleic acid molecules are also present on TiO2 surface as dimers which are kept together via hydrogen bonding of COOH groups [37]. This can be attributed to incomplete removal of the excessive oleic acid modifier after washing of the modified nanoparticles. The spectrum of neat oleylamine modifier contains the major bands at 3380 and 3305 cm−1 of N H stretching mode, at 3100–2850 cm−1 of C H stretching mode, at 1614 cm−1 N H bending mode, at 1500–1300 cm−1 of C H bending mode, and at 1076 cm−1 of C N stretching mode. As for TiO2 modified with oleylamine, the N H mode at 1614 cm−1 disappears and the two new peaks corresponding to vibrations of NH3 + groups due to NH2 interaction with TiO2 surface appear at 1570 and 1524 cm−1 . More specifically, these peaks correspond to amide bonds, an oxidized form of amine which probably form a hydrogen bonding network around TiO2 surface [38–40]. The main spectral features of the mixture of two modifiers and of the TiO2 modified with the mixture are absence of amide bonds and the appearance of two peaks at 1550 cm−1 and 1407 cm−1 that are indicative of asymmetric and symmetric stretching bands of carboxylate group COO− presented in oleate species. The difference of 139 cm−1 reveals chemisorption of carboxylates to TiO2 surface in form of bridging bidentates similarly to the case of OA modifier. As it was already suggested, the use of both modifiers promotes deprotonation of oleic acid by oleylamine which resulted in an intense grafting of oleic acid molecules to TiO2 surface [37,41,42]. 3.2. Dynamic light scattering (DLS) measurements Fig. 3 depicts the intensity-weighted size distribution of unmodified and hydrophobically modified titania nanoparticles

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Fig. 3. Size distribution of pure P25 photocatalyst in water and OA-3, OM-3, OAOM-3 modified nanoparticles in chloroform solvent.

in different solvents received via DLS technique. The unmodified nanoparticles were dispersed and tested in water. Since hydrophobically modified nanoparticles cannot be dispersed in polar solvents, they were dispersed and tested in non-polar solvent namely chloroform [43]. The concentration of the unmodified and modified photocatalysts in different solvents was chosen to be of 5 mg/L that is justified by long-term stability of the prepared dispersions. The results for mean hydrodynamic diameter and polydisperse index are presented in Table 3. It can be seen that the modification of TiO2 led to the decrease of the mean size with the lowest mean size to appear for the P25 modified with mixture of surfactants (sample OAOM-3). Concerning the particle’s size nonuniformity, the polydisperse index illustrated that modified particles preserved their polydisperse nature.

3.3. Light absorption measurements Fig. 4 shows the diffuse reflection (DR) spectra of the initial P25 and the modified photocatalysts. The standard (STD) BaSO4 reflection curve is also presented. The surface modification of titania nanoparticles exhibited decrease in diffuse reflectance in the whole measured wavelength range between 350 and 800 nm. After application of the Kubelka–Munk conversion, the width of the effective energy band gap was found to be slightly reduced for the modified titania in comparison to the initial P25 as given in the inclusion to Fig. 4. This finding reflects the fact that surface functionalization is in some extend equivalent to surface doping due to alteration of the semiconductor electronic structure through formation of surface states near the bottom/top of its conduction/valence band [44]. The ability of the materials to be activated with lower energy irradiation indicates the possibility for effective NOx oxidation under visible light as well.

Fig. 4. Diffuse reflectance spectra of pure P25, OA-3, OM-3 and OAOM-3 modified nanoparticles.

3.4. Thermal analysis In order to estimate the thermal stability of the pure P25 photocatalyst and the modified nanoparticles, thermal gravimetric (TG) and differential thermal analysis (DTA) were used. The samples were heated in air from 100 to 800 ◦ C with a heating rate of 10 ◦ C/min. The TG and DTA curves of the initial and modified P25 powders are presented in Fig. 5a and b. For the non modified P25 a total weight decrease of ∼3.5% (Table 3) was recorded revealing a good thermal stability of the photocatalysts. As for OA-modified nanoparticles, the total mass reduction was larger, i.e. 6.7% which can be related to the removal of the organic modifier. In the DTA curve, a weak exothermic peak at 350 ◦ C can be observed. Taking into account the fact that the boiling point of oleic acid is 242 ◦ C, it can be suggested that the surfactant’s molecules are not free but attached to the TiO2 nanoparticles by chemical adsorption as discussed in Refs. [45–47]. Concerning the OM-modified nanoparticles there is a well defined mass reduction region (weight loss ∼5%) between 200 and 470 ◦ C and a clear exothermic peak at 470 ◦ C. According to Ref. [40] the weight loss region between 100 and 300 ◦ C can be attributed to decomposition of alkyl chains organic coating. The mass reduction recorded for the OM-3 sample at higher temperatures indicates a possible chemisorption of OM to TiO2 nanoparticles. The next mass reduction region (weight loss ∼1.23%) between 450 and 800 ◦ C is attributed to the total removal of nitrogen head groups as discussed in Refs. [38,40]. As for the OAOM-functionalized nanoparticles, the TG curve is similar to this of the OM with a larger mass reduction of 8.92% for the same temperature region. The DTA curve shows a well defined exothermic peak at 370 ◦ C due to combustion of the chemisorbed OA component [45–47]. There is no combustion of amine groups at temperatures higher than 400 ◦ C which suggests that the OM component has not been attached to the particles that is in agreement with FT-IR results. 3.5. Transmission electron microscopy (TEM)

Table 3 Mean hydrodynamic diameter, polydisperse index values and weight loss of pure and hydrophobically modified P25. Samples nomination

Mean particle size (nm)

Polydisperse index

Weight loss (%)

P25 OA-3 OM-3 OAOM-3

294.8 199.5 170.8 150.7

0.200 0.159 0.236 0.181

3.45 6.70 7.23 8.92

TEM high resolution micrographs of the modified nanoparticles are presented in Fig. 6. For the OA-modified P25 organic coating is not evident. This result correlates with FT-IR and thermal analysis results, showing a rather weak chemical adsorption of surfactant to P25 surface. For the OM-modified TiO2 nanoparticles a thin layer seemed to cover the majority of the particles. Concerning the OAOM-modified P25, some of the particles appeared coated ˚ monolayer. This coating can be attributed to OA with an A-scaled

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Fig. 5. Thermogravimetric (a) and differential thermal (b) analysis curve of P25, OA-3, OM-3 and OAOM-3 powders.

component of the mixed modifier, which remained alone chemisorbed to the TiO2 as revealed by the FT-IR. 3.6. Photocatalytic De-NOx ability

Fig. 6. High resolution TEM micrographs of OA-3, OM-3 and OAOM-3 titania nanoparticles.

The photocatalytic activity of the photocatalysts in oxidation of air pollutants NO and NO2 (NOx ) was evaluated by monitoring the changes in their concentrations under irradiation. The process involves adsorption of NO on TiO2 surface and its successive oxidation to gaseous NO2 and solid nitrates NO3 − resulting in NOx removal, i.e. purification. The results obtained for the modified TiO2 nanoparticles and cement samples are presented in Table 4 and Fig. 7a. For the powders (Table 4), it can be perceived that the modification of P25 with organic compounds OA, OM and OAOM increased significantly its activity in the oxidation of the initial pollutant NO. More importantly, higher values for efficiency under UV () and visible light () were obtained for NOx decrease in comparison to the initial P25 powder. This tendency was preserved for the titania loaded cement samples which is evident from the corresponding histograms. Specifically, for the cement samples containing 2.5% OAfunctionalized TiO2 an increase in NOx removal can be observed in comparison with the cement containing the same quantity bare P25. The increase was more prominent for the cement samples loaded with OM- and OAOM-modified TiO2 . Under UV irradiation it advanced the NO decrease almost 2 times and 2–3 times the NOx removal (Fig. 7a). These remarkable results were connected with the hydrophobic properties of the modified titania nanoparticles. It is our suggestion that when the hydrophobic titania powders are placed in hydrophilic cement environment it would tend to avoid the media and reach the surface. Thus, greater amount of the photocatalytic component would be concentrated on the cement surface and available for NOx oxidation. The better performance of the OM- and OAOM-modified titania can be related to their more prominent lipophilic behavior in comparison to the OA-modified sample revealed by the DLS measurements. Under visible light irradiation the cement samples containing the same (2.5%) quantity OM- and OAOM-modified titania (WC-OM31 and WC-OAOM31) exhibited 4–5 and 5–7 times improvement in NO decrease and NOx removal in comparison to WC-P251, respectively (Fig. 7b). This outcome is related to the lower band gap of the modified nanopartictes revealed by the UV–vis spectroscopy. It should be noted that the efficiency of secondary toxic pollutant NO2 remained decreased for all cement samples used. This is ascribed to a different photocatalytic mechanism featuring the

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Table 4 Efficiency of the pure and modified P25 powders under UV () and visible light () illumination for decrease of NO, increase of NO2 and NOx removal. Gas

Powder P25

NO NO2 NOx

OA-3

OM-3

OAOM-3

UV  (×10−2 )

Vis  (%)

UV  (×10−2 )

Vis  (%)

UV  (×10−2 )

Vis  (%)

UV  (×10−2 )

Vis  (%)

1.07 1.51 0.33

15.86 10.77 5.09

2.1 0.37 1.74

15.12 5.8 9.2

1.67 0.24 1.43

1.53 1.1 3.21

1.79 0.37 1.42

14.94 3.21 10.11

photoactive cement samples. According to the Frost diagram and Ref. [5], the NO2 oxidation state is unstable in a very high pH environment that is characteristic for cementitious materials. At the same time, the oxidation states NO2 − and NO3 − appeared to be thermodynamically stable in alkaline conditions, implying that there is a possible oxidation of NO to NO3 − without passing through the intermediate NO2 state given by Eqs. (5)–(7):

comparison to the cements with 2.5% titania loading. Nevertheless, the activity of the composite cements containing functionalized titania remained higher in regard to the corresponding cements containing non modified P25 (samples WC-P252 and WC-P253). Our hypothesis on the behavior of hydrophobic TiO2 in hydrophilic cement slurry was verified in Section 3.7.

NO + OH• → HNO2 ,

3.7. SEM/EDX results

(9)

HNO2 + OH− → NO2 − + H2 O, −

−

NO2 + 2OH• → NO3 + H2 O.

(10) (11)

If even the unstable NO2 state is formed, it is oxidized to NO3 − as follows: NO2 + OH• → HNO3 ,

(12)

HNO3 + OH− → NO3 − + H2 O.

(13)

The excellent photocatalytic properties of composite nanoparticles lead us to the usage of lower photocatalyst concentration in the cement matrix namely 1% and 0.5% (Fig. 7c and d). It is obvious that the photocatalytic efficiency of all the samples decreased in

Fig. 8 shows SEM/EDX images of white cement samples with modified titania nanoparticles (samples WC-OA31, WC-OM13, WC-OAOM31). From the mapping of the Ti element a gradual distribution of the photocatalyst from the bottom to the upper surface of the cement can be observed. The titania particles are mainly concentrated in large aggregates close to the edges and within or around the pores of the cement. Their presence in the volume of the cement is lower which is attributed to the functionalization of the nanoparticles by modification and their driving to locations where the particles encounter lower surface tension (air-cement boundaries). In the case of the OA modified sample though (Fig. 8a and b), the photocatalysts is enclosed within the cement matrix to a larger extent having as a result the lower photocatalytic activity in

Fig. 7. Photonic efficiency of cement samples including 2.5% (w/w) pure or modified P25 under UV irradiation (a), effectiveness of cement samples with the same TiO2 concentration under visible light irradiation (b), photonic efficiency of cement samples with 1% (w/w) (c) and 0.5% (w/w) (d) pure and modified P25.

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Fig. 8. SEM/EDX images of upper and bottom cement surfaces of WC-OA31 (a), WC-OM31 (b), WC-OAOM31 (c) where the Ti, Ca and Si elements are depicted in yellow, green, and red color, respectively. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

comparison to the OM (Fig. 8c and d) and OAOM modified samples (Fig. 8e and f). 4. Conclusions Hydrophobic titania nanoparticles were prepared by modification of P25 powder with surfactants of OA, OM and mixed modifier OAOM. According to the FT-IR and the thermal analysis the OA molecules were poorly chemisorbed to the TiO2 nanoparticles. The OM was attached to the TiO2 through hydrogen bonding. The use of mixed OAOM modifier resulted in stronger sorption of the OA component to the titania particles.

DLS analysis revealed that while the initial P25 was well dispersible in water, the modified titania particles’ demonstrated lipophilic behavior. The best dispersion in terms of particles diameter and polydisperse index was achieved in the case of OAOM-modified particles. The modified titania powders and titania-loaded cements exhibited remarkable photocatalytic activity in NO oxidation and especially in NOx removal under both UV and visible light irradiation. The results were attributed to the appropriate hydrophobic modification and endowed mobility of the photocatalysts. Cements with high De-NOx ability and low photocatalyst loading were prepared.

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Acknowledgement The financial support from GSRT Projects 09SYN42-925, 10 TUR/1-18-2 and 12CHN-205 is highly appreciated. References [1] C. Kapridaki, P. Maravelaki-Kalaitzaki, TiO2 –SiO2 –PDMS nano-composite hydrophobic coating with self-cleaning properties for marble protection, Prog. Org. Coat. 76 (2013) 400–410. [2] E. Quagliarini, F. Bondioli, G. Battista Goffredo, C. Cordoni, Self-cleaning and depolluting stone surfaces: TiO2 nanoparticles for limestone, Constr. Build. Mater. 37 (2012) 51–57. [3] Y. Wang, L. Lu, H. Yang, Q. Che, Development of high dispersed TiO2 paste for transparent screen-printable self cleaning coating on glass, J. Nanopart. Res. 15 (15) (2013) 1384. [4] J. Kumar, A. Srivastava, A. Bansal, Production of self-cleaning cement using modified titanium dioxide, Int. J. Innov. Res. Sci. Eng. Technol. 2 (2013) 2688–2693. [5] A. Folli, C. Pade, T. Hansen, T. Marco, TiO2 photocatalysis in cementitious system: insights into self-cleaning and depollution chemistry, Cement Concrete Res. 42 (2012) 544–547. [6] J.Y. Kim, C. Park, J. Yoon, Developing a testing method for antimicrobial efficacy on TiO2 photocatalytic products, Environ. Eng. Res. 13 (2008) 136–140. [7] N. Saraschandra, M. Pavithra, A. Sivakumar, Antimicrobial applications of TiO2 coated modified polyethylene (HDPE) films, Arch. Appl. Sci. Res. 5 (2013) 189–194. [8] H. Li, Q. Cui, B. Feng, J. Wang, X. Lu, J. Weng, Antibacterial activity of TiO2 nanotubes: influence of crystal phase, morphology and Ag deposition, Appl. Surf. Sci. 284 (2013) 179–183. [9] M.Z. Guo, C.S. Poon, Photocatalytic NO removal of concrete surface layers intermixed with TiO2 , Build. Environ. 70 (2013) 102–109. [10] J. Chen, S. Kou, C. Poon, Photocatalytic cement-based materials: comparison of nitrogen oxides and toluene removal potentials and evaluation of self cleaning performance, Build. Environ. 46 (2011) 1827–1833. [11] M. Hassan, L. Mohammad, S. Asadi, H. Dyla, S. Cooper III, Sustainable photocatalytic asphalt pavements for mitigation of nitrogen oxide and sulfur dioxide vehicle emissions, J. Mater. Civil Eng. 25 (2013) 365–371. [12] A.R. Khataee, A.R. Amani-Ghadim, M. Rastegar Farajzade, O. Valinazhad Ourang, Photocatalytic activity of nanostructured TiO2 -modified white cement, J. Exp. Nanosci. 6 (2011) 138–148. [13] O.C. Compton, C.H. Mullet, S. Chiang, F.E. Osterloh, A building block approach to photochemical water-splitting catalysts based on layered niobate nanosheets, J. Phys. Chem. 112 (2008) 6202–6208. [14] A. Fujishima, K. Honda, Photolysis-decomposition of water at the surface of an irradiated semiconductor, Nature 238 (1972) 37. [15] F.E. Osterloh, Inorganic materials as catalysts for photochemical splitting of water, Chem. Mater. 20 (2008) 35–54. [16] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State 32 (2004) 33–177. [17] B.X. Wei, L. Zhao, T.J. Wang, H. GaO., H.X. Wu, Y. Jin, Photo-stability of TiO2 particles coated with several transition metal oxides and its measurement by rhodamine-B degradation, Adv. Powder Technol. 24 (2013) 708–713. [18] A.O. Ibhadon, P. Fitzpatrick, Heterogeneous photocatalysis: recent advances and applications, Catalysts 3 (2013) 189–218. [19] M.M. Hassan, H. Dylla, L.N. Mohhamad, T. Rupnow, Methods for the application of titanium dioxide coatings to concrete pavement, Int. J. Pavement Res. Technol. 5 (1) (2014) 12–20. [20] J. Chen, C.S. Poon, Photocatalytic cementitious materials: influence of the microstructure of cement paste on photocatalytic pollution degradation, Environ. Sci. Technol. 43 (2009) 8948–8952. [21] L. Cassar, Photocatalysis of cementitious materials: clean buildings and clean air, MRS Bull. 29 (2004) 328–331. [22] Y.C. Lan, Y.L. Lu, Z.F. Ren, Mini review on photocatalysis of titanium dioxide nanoparticles and their solar applications, Nano Energy 2 (2013) 1031–1045. [23] E. Keidel, Influence of titanium white on the fastness to light of coal-tar days, Farben-Zeitung 34 (1929) 1242–1243. [24] M.Z. Guo, T.C. Ling, C.S. Poon, Nano-TiO2 -based architectural mortar for NO removal and bacterial inactivation: influence of coating and weathering conditions, Cem. Concr. Compos. 36 (2013) 101–108.

[25] M. Kampa, E. Castanas, Human health effects of air pollution, Environ. Pollut. 151 (2008) 362–367. [26] M.V. Sofianou, C. Trapalis, V. Psycharis, N. Boukos, T. Vaimakis, J.G. Yu, W.G. Wang, Study of TiO2 anatase nano and microstructures with dominant {0 0 1} facets for NO oxidation, Environ. Sci. Pollut. Res. 19 (2012) 3719–3726. [27] G. Husken, M. Hunger, H.J.H. Brouwers, Experimental study of photocatalytic concrete products for air purification, Build. Environ. 44 (2009) 2463–2474. [28] T.C. An, J.X. Chen, G.Y. Li, X.J. Ding, G.Y. Sheng, J.M. Fu, B.X. Mai, K.E. O’Shea, Characterization and the photocatalytic activity of TiO2 immobilized hydrophobic montmorillonite photocatalysts: degradation of decabromodiphenyl ether (BDE 209), Catal. Today 139 (2008) 69–76. [29] H. Yamashita, S. Kawasaki, S. Yuan, K. Maekawa, M. Anpo, M. Matsumura, Efficient adsorption and photocatalytic degradation of organic pollutants diluted in water using the fluoride-modified hydrophobic titanium oxide photocatalysts: Ti-containing beta zeolite and TiO2 loaded on HMS mesoporous silica, Catal. Today 126 (2007) 375–381. [30] N. Todorova, T. Vaimakis, D. Petrakis, S. Hishita, N. Boukos, T. Giannakopoulou, M. Giannouri, S. Antiohos, D. Papageorgiou, E. Chaniotakis, C. Trapalis, N and N,S-doped TiO2 photocatalysts and their activity in NOx oxidation, Catal. Today 209 (2013) 41–46. [31] Y. Ohko, Y. Nakamura, N. Negishi, S. Matsuzawa, K. Takeuchi, Photocatalytic oxidation of nitrogen monoxide using TiO2 thin films under continuous UV light illumination, J. Photochem. Photobiol. A: Chem. 205 (2009) 28–33. [32] L. Zhang, R. He, H.C. Gu, Oleic acid coating on the monodisperse magnetite nanoparticles, Appl. Surf. Sci. 253 (2006) 2611–2617. [33] N. Shukla, C. Liu, P.M. Jones, D. Weller, FT-IR study of surfactant bonding to FePt nanoparticles, J. Magn. Magn. Mater. 266 (2003) 178–184. [34] N. Wu, L. Fu, M. Su, M. Aslam, K.C. Wong, V.P. Dravid, Interaction of fatty acid monolayers with cobalt nanoparticles, Nano letters 4 (2004) 383–386. [35] Z. Li, Y. Zhu, Surface modification of SiO2 nanoparticles with oleic acid, Appl. Surf. Sci. 211 (2003) 315–320. [36] Y. Gao, G. Chen, Y. Oli, Z. Zhang, Q. Hue, Study on tribological properties of oleic acid-modified TiO2 nanoparticle in water, Wear 252 (2002) 454–458. [37] M. Klokkenburg, J. Hilhorst, B.H. Erne, Surface analysis of magnetite nanoparticles in cyclohexane solutions of oleic acid and oleylamine, Vib. Spectrosc. 43 (2007) 243–248. [38] X. Liu, M. Atwater, J. Wang, Q. Dai, J. Zou, J.P. Brenan, Q. Huo, A study on gold nanoparticle synthesis using oleylamine as both reducing agent and protecting ligand, J. Nanosci. Nanotechnol. 7 (2007) 3126–3133. [39] M. Andiappan, S. Sundaramoorthy, N. Panda, G. Meiyazhaban, S.B. Winfred, G. Venkataraman, P. Krishna, Electrospun eri silk fibroin scaffold coated with hydroxyapatite for bone tissue engineering applications, Prog. Biomater. 2 (2013) 1–11. [40] M. Menelaou, K. Georgoula, K. Simeonidis, C. Dendrinou-Samara, Evaluation of nickel ferrite nanoparticles coated with oleylamine by NMR relaxation measurements and magnetic hyperthermia, Dalton Trans. 43 (2014) 3626–3636. [41] W. Bu, Z. Chen, F. Chen, J. Shi, Oleic acid/oleylamine cooperativecontrolled crystallization mechanism for monodisperse tetragonal bipyramid NaLa(MoO4 )2 nanocrystals, J. Phys. Chem. C 113 (2009) 12176–12185. [42] S. Verma, D. Pravarthana, One-pot synthesis of highly monodispersed ferrite nanocrystals: surface characterization and magnetic properties, Langmuir 27 (2011) 13189–13197. [43] M. Ijima, M. Kobayakawa, H. Kamiya, Tuning the stability of TiO2 nanoparticles in various solvents by mixed silane alkoxides, J. Colloid Interface Sci. 337 (2009) 61–65. [44] D.Q. Fang, A.L. Rosa, Th. Frauenheim, R.Q. Zhang, Band gap engineering of GaN nanowires by surface functionalization, Appl. Phys. Lett. 94 (2009) 073116. [45] S.Y. Zhao, D.K. Lee, C.W. Kim, H.G. Cha, Y.H. Kim, Y.S. Kang, Synthesis of magnetic nanoparticles of Fe3 O4 and CoFe2 O4 and their surface modification by surfactant adsorption, Bull. Korean Chem. Soc. 27 (2006) 237–242. [46] M. Mahdavi, M. Bin Ahmad, M.J. Haron, F. Namvar, B. Nadi, M. Zaki Ab Rahman, J. Amin, Synthesis, surface modification and characterisation of biocompatible magnetic iron oxide nanoparticles for biomedical applications, Molecules 18 (2013) 7533–7548. [47] R. De Palma, S. Peeters, M.J. Van Bael, H. Van den Rul, K. Bonroy, W. Laureyn, J. Mullens, G. Borghs, G. Maes, Silane ligand exchange to make hydrophobic superparamagnetic nanoparticles water-dispersible, Chem. Mater. 19 (2007) 1821–1831.