Photocatalytic activity of TiO2 films prepared by surfactant-mediated sol–gel methods over commercial polymer substrates

Chemical Engineering Journal 283 (2016) 535–543

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Photocatalytic activity of TiO2 films prepared by surfactant-mediated sol–gel methods over commercial polymer substrates R.M. Cámara a,b, R. Portela a,c,⇑, F. Gutiérrez-Martín d, B. Sánchez a a

Photocatalytic Treatment of Pollutants in Air Group, CIEMAT, Ciudad Universitaria, Madrid, Spain Dept. Forestry Engineering, ETSI Montes, Universidad Politécnica de Madrid, Spain c Spectroscopy and Industial Catalysis (SpeICat). ICP-CSIC, Cantoblanco, Madrid, Spain d Dept. Mechanical, Chemical and Industrial Design Engineering, Universidad Politécnica de Madrid, Spain b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Commercial polymeric supports

coated with TiO2 exhibit photocatalytic activity.  Sol–gel synthesis method is valid for anatase-TiO2 coating at low temperature.  SiO2–TiO2 combination with the help of surfactants improves TiO2 adhesion on PMMA.  PMMA is postulated the best polymer as transparent support in the UV–Vis region.

a r t i c l e

i n f o

Article history: Received 12 May 2015 Received in revised form 20 July 2015 Accepted 21 July 2015 Available online 31 July 2015 Keywords: Photocatalysis TiO2–SiO2 Polymeric substrates Surfactants Trichloroethylene

a b s t r a c t Commercially available transparent polymers are evaluated as support for photoactive TiO2 and tested for their application in gas phase photocatalytic oxidation processes. The polymers were selected on the basis of their optical properties and the TiO2 film adhesion. Two low-temperature surfactant-mediated sol–gel synthesis methods of TiO2–SiO2 nanocomposites are investigated to provide acid TiO2–basic SiO2 interactions for the formation of photocatalytic films of higher quality. These procedures are expected to improve photocatalytic properties compared to those obtained by direct coating of TiO2 on the polymers. One method consists in modifying the polymer surface with a layer of poly(diallyldimethylammonium) chloride, which provides a positively charged surface for the fixation of the alkaline SiO2 sol. The other method is based on reducing the surface tension of the SiO2 sol using perfluorobutane sulfonates. Both methods are compared to direct deposition in terms of homogeneity of the layers and photocatalytic activity for trichloroethylene oxidation. Among the commercial polymers employed poly(methyl methacrylate) processed by sheet moulding showed the best optical properties, TiO2 adhesion, and photocatalytic activity, which was promoted by the deposition of the silica interlayer by both preparation methods. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Polymers are reliable low cost materials with high flexibility in terms of composition and geometry, which makes them an optimal

solution for many different uses [1]. The choice of polymer is usually guided by the chemical, mechanical and photo thermal behavior [2,3], and its capacities may be tuned and broadened through the synthesis of new formulations and the use of additives [4–6].

⇑ Corresponding author at: Spectroscopy and Industial Catalysis (SpeICat). ICP-CSIC, Cantoblanco, Madrid, Spain. Tel.: +34915854873. E-mail address: [email protected] (R. Portela). http://dx.doi.org/10.1016/j.cej.2015.07.080 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

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For instance, specific additives such as UV-stabilizers or surfactants in polymeric matrixes help to prevent photo-oxidation, modify the wettability, and increase interfacial bond strength, but they also alter UV–Vis absorption and matrix degradation properties [7]. Organic polymers are interesting substrates for TiO2 thin films and may have an extensive application in photocatalytic processes [8], especially those transparent to radiation in the semiconductor activation range [9–12]. However, few works deal with their use as it is necessary to optimize low-temperature TiO2 synthesis procedures, especially for thermoplastic polymers. In order to overcome this difficulty, several sol–gel low-temperature routes to nanocrystalline TiO2 particles or thin films have been reported [13–15]. The film adhesion is one of the greatest challenges in polymer -immobilized photocatalysts [16], as most polymeric resins and composites have low surface free energies and lack polar functional groups, which results in poor adhesion properties, particularly with aqueous TiO2 sols [17]. To achieve greater and better fixation of the TiO2 coatings on the polymer, several surface pre-treatments have been reported, like functionalization by plasma [18–21], chemical abrasion [22,23], or deposition of SiO2 [24,25]. The use of surfactants to improve the wettability of polymers has also been investigated [26,27]. The combination of light weight, high strength, chemical inertness, good insulation, and easy processing at low cost, make thermoplastics well suited to many applications where photocatalytic properties, such as pollution control, disinfection and self-cleaning, may add value, e.g. food packaging, agriculture or a variety of enclosures. Industrially produced polymers are rarely present in bibliography as support for TiO2, in spite of the huge veriety of matrix compositions with different additives already in the market than can be valorized. In this work, a wide range of thermoplastic commodities, such as polyolefins, acrylics, polycarbonates, polyesters and polyvinyls, was screened to select the best supporting materials, according to the transmittance in the TiO2 activation range, and immobilize the semiconductor by sol–gel synthesis followed by dip-coating to add value to these commodities. Some of the commercial supporting polymers were specifically chosen because they contain UV stabilizers or hydrophilic surfactants that may improve essential properties for photocatalytic applications. Besides the direct impregnation of titania prepared by sol–gel, two strategies are here investigated to deposit an intermediate silica layer [9,28]. Coating the polymeric surface with SiO2 before TiO2 immobilization may provide several benefits: SiO2 thin-films may contribute to increase the surface area and the affinity for TiO2, and additionally stabilize the photogenerated charges, thus protecting the polymer from photocatalytic degradation, while transmitting UV radiation [29–32]. On the one hand, coating the polymer with poly(diallyldimethylammonium) chloride (PDDA) provides a positively charged surface for the fixation of an alkaline SiO2 sol through CH3-N+(R)4 groups [33]. On the other, perfluorobutane sulfonates (FC), water-soluble surfactants used as wetting and levelling agents, are added to the SiO2 sol to increase the surface tension (c), and thus promote adhesion [34]. Finally, the activity of the photocatalytic films was evaluated in gas phase oxidation tests, with borosilicate glass as reference supporting material for comparison.

2. Materials and methods 2.1. Supports Borosilicate glass was used as reference UV-transparent hydrophilic substrate. The commercial homopolymer and copolymer blends investigated are listed in Table 1. They include polyolefin films used in agriculture such as polyethylene (PE), polypropylene

(PP) and ethylene–vinyl acetate copolymer 4% (EVA); some of these polymers contain additives, like hindered amine light stabilizers (HALS) to prevent photo-oxidation, or hydrophilic surfactants (HS) such as fatty acids of esters and glycerine to increase the polymer surface tension. Other materials studied are poly(methyl-methacrylate) (PMMA), manufactured either by sheet moulding or by extrusion; two polycarbonates (PC) with different structure (compact and cellular); two polyesters, poly(ethylene terephthalate) (PET) and its glycol-modified amorphous copolymer, where 1,4 cyclohexane dimethanol replaces ethylene glycol (PETG) [35]; and three types of vinyl resins: rigid (R) and flexible (F) polyvinyl chloride (PVC), with different plasticizer content, as well as polystyrene (PS). Some additives like antioxidants, necessary to process the materials, are present in all the commercial products. The samples were supplied by Repsol (polyolefins), Wacotech (PET), Nivic (PC, PETG, PS), Ketersa (acrylic, PVC-R) and Manuplast (PVC-F). 2.2. Photocatalytic coatings 2.2.1. TiO2 sol–gel process An acidic TiO2 sol was prepared by adding titanium isopropoxide Ti(i-OPr)4 (Aldrich) to an aqueous solution of nitric acid in proportion 900:6.5:74 (H2O:HNO3:Ti(i-OPr)4) [36]. The sol–gel process is represented in the following scheme 1, where R = CH(CH3)2: Nitric acid has been previously employed for aqueous peptization to obtain stable TiO2 sols containing anatase crystalline phase at near room temperature without need of further calcination at high temperatures [19], and thus was selected for this study. The solution was stirred during 3 days, until a stable and translucent sol was obtained, and then dialyzed to a final pH of 3.5 using cellulosic membranes (3500 MWCO). 2.2.2. SiO2 sol–gel process The basic SiO2 sol was prepared by adding Si(OEt)4 (98%, Aldrich) to an aqueous solution of ammonium hydroxide (340:11.2:50 H2O:NH3:TEOS), then stirred during 2 days until total peptization and dialyzed to a final pH 8.0 using the same membranes. 2.2.3. TiO2 coating For multiple-layer impregnation of the polymers, a dip-coating method using an immersion rate of 1.5 mm s1 was employed. Each single layer was dried for 15 min at 100 °C, except for the most thermal sensitive substrates, which were treated for 1 h at 50 °C (see the glass transition temperature, Tg, in Table 1). The same procedure was employed for the reference borosilicate glass, which was finally calcined at 350 °C. Three series of photocatalysts were prepared, one by direct coating of the polymeric support with three layers of the TiO2 sol (method A), and two placing an intermediate SiO2 layer with the help of two different surfactants: PDDA (method B) and FC (method C). In method B, the samples were submerged for 1 min in 1% v/v aqueous PDDA solution prepared from a 20% water solution of low molecular weight PDDA (100,000–200,000 Da, supplied by Aldrich), then rinsed with deionized water and dried at room temperature (ca. 23 °C, 30% RH), and finally coated with the SiO2 sol and dried at 100 °C during 15 min. Thereafter, the samples were coated with three layers of TiO2 in the same conditions as in method A. In method C, the samples were prepared using the SiO2 sol modified with a surfactant based on perfluorobutane sulfonate (FC-4430, provided by 3 M Corp). FC-4430 was dissolved in 2-propanol (0.1 mg/l) and added to the SiO2 sol in the amount required to obtain a 25 mg/l solution. After coating the polymers

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R.M. Cámara et al. / Chemical Engineering Journal 283 (2016) 535–543 Table 1 Main properties of the synthetic polymers available in the market included in this work. Polymer structure

a b

Polymer name

Sample name

Low density polyethylene

PE

Ethylene–vinyl acetate (4%)

EVA EVA-HS EVA-H

Process/ additivesa

Hydrophilic surf HALSb

Tg (°C)

Density (g cm3)

t (mm)

OD360 (mm1)

c (mN m1)

110

0.92

0.2

0.51

<28

110 110 110

0.92 0.94 n.a.

0.2 0.2 0.2

0.62 0.25 2.54

<28 28 28

150

0.90

0.05

0.82

<28

Polypropylene

PP

Poly(methyl methacrylate)

PMMA-SM PMMA-EP

Sheet moulding Extrusion

120 120

1.18 1.18

4 3

0.02 1

28 28

Polycarbonate

PC-Ce PC-C

Cellular Compact

135 135

n.a. 1.2

0.2 2

8.49 1

28 <28

Poly(ethylene terephthalate)

PET

79

0.05

0.2

0.91

28

Poly(ethylene terephthalate glycol)

PETG

70

1.27

1

0.13

28

Polyvinyl chloride

PVC-R PVC-F

Rigid Flexible, UV stabiliz.

60 60

n.a. 1.4

0.2 3

0.48 0.67

28 28

Polystyrene

PS

Plasticizers

95

1.05

2

0.03

28

All samples contain processing additives. Hindered Amine Light Stabilizers.

Scheme. 1.

with one SiO2-FC layer and drying them at moderate temperatures three layers of TiO2 were deposited, as in method A. 2.3. Characterization Spectroscopic analyses were performed by using a Perkin Elmer Lambda 650S UV–Vis spectrophotometer (200–800 nm) and a FTIR Thermo Nicolet 6700 spectrometer with an attenuated total reflection (ATR) accessory that uses a ZnSe crystal (4000–650 cm1); the spectra were obtained from 64 scans at 4 cm1 resolution and 45° incidence angle. The XRD pattern of TiO2 xerogel dried at 50 °C was recorded on a D5000 diffractometer to analyze the crystallization of TiO2; anatase crystalline mean size was calculated with the Scherrer equation. The effective coating adhesion was studied using a stereoscopic light microscope (Nikon Eclipse 80 i) and a Hitachi scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDX) at accelerating voltage 20 kV; the

samples were initially coated with a conductive layer of graphite for the analysis. Surface tension, c was measured before and after the coatings with ethanol-based test inks (Plasmatreat, Series C, DIN 53364) within the range of 28–92 mN/m. 2.4. Photocatalytic activity The photocatalytic oxidation of trichloroethylene (TCE) in gas-phase was investigated in a continuous flow experimental set-up shown in Fig. 1. The flat photoreactor (L  W  H 120  50  10 mm) was made of stainless steel except for one face, where a window (30 cm2) of borosilicate glass with low iron content is placed for irradiation of the photocatalyst by two UVA fluorescent lamps with maximum emission at 365 nm (Philips Actinic BL/8W). The UVA lamps were supported on the metallic border of the flat photoreactor, at a distance of 1 mm to the surface of the window (3 mm thick) and of 6 mm to the sample surface (20 cm2).

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Fig. 1. Experimental set-up.

0.40

24 20

0.35

16 12 8

0.30

4 0

0.25

3850

3050

2250

1450

650

0.20 0.15 0.10 0.05 0.00 3850

3450

3050

2650

2250

1850

1450

1050

650

Fig. 2. ATR-FTIR spectra of (a) PP and (b) PP with 3 layers of TiO2 (method A). In the inset, FTIR spectra of P25 TiO2 (Degussa).

The photocatalysts were pretreated under irradiation in a continuous flow of dry air during 24 h. Then, a stream from a cylinder containing 250 ppm of TCE in N2 (Air Liquide) was mixed with the adequate flow of dry air to obtain a final mixture with 30 or 60 ppm of pollutant (18.5% and 16% O2, respectively) and flow rates of 100, 200 and 300 ml/min. Flow rate was controlled and pressure, temperature, and UV irradiance were measured by automated equipment; UV irradiance was 10 mW/cm2 and the temperature was 45 °C. The gas composition was continuously monitored in a FTIR spectrometer equipped with a

multiple-reflection gas cell (optical path 2 m) maintained at 110 °C (Thermo Nicolet 6700). The evolution of representative vibrational bands of TCE (965–903 cm1), CO2 (2435–2233 cm1), COCl2 (1873–1780 cm1), CO (2231–2027 cm1), HCl (2885– 2662 cm1) and dichloroacetylchloride (DCAC) (1114–1037 cm1) was analyzed in spectra obtained by accumulation of 64 scans with a resolution of 2 cm1. The actual concentration of TCE, CO2 and COCl2 was evaluated against calibration mixtures provided by Air Liquide. Selectivity to CO2 was calculated as follows:

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and the stretching vibration of O–C–C bond at 1128 and 1099 cm1. The absorption peak at 723 cm1 is assigned to out of plane bending of aromatic C–H bonds. Both PET and PETG spectra also show the typical C–H vibration of the hydrocarbon chain. The spectra of the commercial PMMA-SM, PP, PS and PVC-R were analyzed in a previous work [38]. 3.2. Characterization of TiO2 coatings

Fig. 3. UV–Vis spectra obtained with different number of TiO2 layers deposited by method A onto the commercial poly(propylene) support.

SCO2 ¼

CO2  100 2  ðTCEinlet  TCEoutlet Þ

ð1Þ

3. Results and discussion 3.1. Selection of polymers The UV–Vis absorbance of the raw polymers was measured with the objective of selecting the materials with better transmittance in the appropriate spectral range. For comparative purposes, 360 nm was selected as reference wavelength because it corresponds to the radiance maximum of common lamps used in photocatalysis, and to a significant radiation absorbance by crystalline TiO2 (Eg anatase = 3.2 eV) [37]. In order to compare commercial materials with different thickness (t, mm), the optical density (DO, mm1) at 360 nm was calculated from the corresponding absorbance (A) by Eq. (2), and included in Table 1.

OD360 ¼ A360 =t

ð2Þ

UV–Vis measurements indicate that PVC-F, PMMA-EP and PC sheets transmit less than 6% of the incident radiation at 360 nm. This high absorbance of UV radiation may foster the polymer photo-degradation. Additionally the radiation harvesting by the back of the polymeric sheet, or by successively stacked photocatalytic units of polymer-supported TiO2, is hindered, and thus the photocatalytic activity, because the radiation is absorbed by the support. Among the polymers considered, only nine samples showed both OD360 < 1 and transmittance >70%, and therefore they were selected to be coated with TiO2: PP (91%), PS (86%), PMMA-SM (89%), PVC-R (80%), PET (73%), PETG (74%), PE (79%), EVA (75%) and EVA-HS (89%). It is worth remarking that PMMA-SM and PP have similar good transmittance despite the higher thickness of the former. PMMA-SM also exhibits good mechanical and chemical resistance to aging processes, as demonstrated in a previous work [38]. The polymers selected were characterized by ATR-FTIR (Fig. S1 in supplementary information) to confirm the composition of the commercial samples. The presence of additives has not been detected by this technique. The spectra of polyolefins reveal the typical C–H vibrations of the hydrocarbon chains at 2850– 2950 cm1 (stretch), 1458 cm1 (bend) and 719 cm1 (rock) [39]. Additionally, EVA copolymers show a typical carbonyl group stretch at 1750 cm1 and bands at 1250 and 1050 cm1 corresponding to the O–C(O)–C bond in acetate groups and the C–O bond close to C–C bonds. PET and PETG spectra are similar: they show the characteristic vibration of carbonyl bond in ester group at 1717 cm1, the stretch vibration of O–C(O)–C at 1261 cm1,

The low TiO2 concentration of the sol, along with the low treatment temperature, lead to only a small fraction of crystalline titania on the surface not detectable by XRD of the thin films. Thicker films and/or treated at higher temperatures are needed to use XRD technique [40]. However, XRD pattern of TiO2 xerogel obtained after drying the sol at 50 °C confirms the formation of anatase phase (with minor contribution of brookite) at the temperature employed for drying the coated polymers, with a crystalline domain size of 3.9 nm, in line with previous studies [9]. Therefore, the acidic peptization allows obtaining nanocrystalline anatase at low temperatures, compatible with thermal sensitive substrates. FTIR-ATR spectroscopic characterization of the polymers coated by method A indicates that TiO2 is present on their surface. The modification of propylene IR spectrum after the dip-coating in the sol is observed in Fig. 2: the wide IR absorption band at 650–950 cm1 corresponds to the vibration of Ti–O–Ti bonds [41], and the bands at 2650–3700 cm1 and 1620– 1635 cm1 are characteristic of hydroxyl groups. According to SEM and EDX mapping analysis (not shown), the titania films deposited without pretreatment on PE, EVA and EVA-HS were very irregular. PMMA-SM, PVC-R, PS, PP, PET and PETG, however, revealed a relatively homogeneous coating by direct impregnation of TiO2, so these materials are susceptible of optimization and thus were further characterized. The determination of the weight or thickness of the titania coating on the polymers was complicated because the low affinity of the substrate for the aqueous sol resulted in extremely fine films, and the surface irregularities of the commercial polymers (superficial defects and orientation) made difficult the characterization by techniques such as AFM. The UV–Vis absorption spectra of TiO2–coated substrates were therefore used to roughly estimate the efficacy of the film deposition by dip-coating. The polymers were characterized after each of six consecutive impregnations. The significant decrease of transmittance observed with the deposition of the first three TiO2 layers is not appreciated with subsequent immersions into the sol, as shown for PP in Fig. 3. Hence, three to four layers are enough to achieve the maximum amount of TiO2 that can be directly fixed to the materials, and thus the samples coated with three layers of TiO2 were used to complete the characterization and compare the photocatalytic activity. All the coated samples showed similar transmittance values to the coated reference substrate, borosilicate glass (71%) [19]. 3.3. Characterization of TiO2–SiO2 coatings The surface tension of the polymeric supports is very low, in all cases ca. 28 mN/m or lower, according to ethanol inks tests. This value results from the low polarity of the polymers and is far from the surface tension of water (cwater = 42 mN/m), which would ensure the wettability required to uniformly impregnate the polymer with the TiO2 sol. The surface tension of the EVA copolymer containing HALS or the hydrophilic surfactant is only slightly higher than that of the sample without additives. Thus, even the modification of the polymer matrix by these additives in the manufacture of the commercial material is not enough and the interaction polymer-TiO2 sol must be modified to obtain high quality

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Fig. 4. UV–Vis spectra of the polymers as received (

) and with three TiO2 layers deposited by methods A (

photocatalytic coatings. With this aim, surfactant-mediated TiO2– SiO2 sol–gel methods B and C were developed. The deposition of SiO2 between the polymer and TiO2 by means of surfactant-mediated multi-layer procedures improved TiO2 adhesion. The surface tension increased after SiO2 deposition. EDX mapping of Si and Ti elements indicates that the films obtained with methods B and C were homogeneous on most polymers selected, except for PVC-R, where SiO2 coating by B-method did not homogeneously cover the surface (Si was not detected by EDX in wide areas) and hindered an effective deposition of titania, as the UV–Vis spectrum in Fig. 4 reflects. In addition, EDX results indicate that none of the surfactant-mediated approaches were efficient to coat PE and EVA with SiO2, and thus TiO2 films obtained by methods B and C were as bad as those obtained by method A. TiO2 adhesion was improved in PMMA-SM and PET by both methods. In the case of PMMA-SM the films prepared with methods B and C caused a red-shift of the UV–Vis absorption edge (Fig. 4), which suggests an increase of TiO2 adhesion. 3.4. Photocatalytic oxidation of trichloroethylene All the TiO2-coated polymeric samples present photocatalytic activity for degradation of trichloroethylene due to semiconducting properties of the oxide. In presence of water vapor, direct mineralization of TCE may be accomplished according to Eq. (3) [42]:

C2 HCl3 þ 3=2O2 þ H2 O ! 2CO2 þ 3HCl

ð3Þ

In the operating conditions of the present study, without humidity in the gas feed, the TCE removed cannot be completely mineralized, and the oxidation of TCE proceeds with formation of phosgene via dichloroacetylchloride (Eqs. (4) and (5)) [43]:

C2 HCl3 þ 1=2O2 ! C2 HCl3 O

ð4Þ

C2 HCl3 O þ 1=2O2 ! COCl2 þ HCl þ CO

ð5Þ

), B (

), and C (

).

Accordingly, FTIR spectra of the effluent gas show the formation of phosgene and small amounts of dichloroacetyl chloride (DCAC), CO and HCl, in agreement with the literature [43–46]. Therefore both TCE conversion and selectivity to the mineralization product (CO2) provide information to evaluate the photocatalytic activity of the samples. TCE conversion increases with the number of TiO2 layers deposited on the surface, as shown for PMMA-SM in Fig. 5, though the conversion shift is not significant between 5 and 7 layers. This result is in agreement with the characterization results, which show no significant improvement in UV–Vis radiation absorption at this number of coating layers. A similar maximum number of active layers has been previously reported with an alcoholic sol–gel method [47]. A slight increase in selectivity to CO2 with the number of coating layers is also obtained. In Fig. 6 the performance of the photocatalytic polymers coated with 3 TiO2 layers as a function of the residence time and concentration is compared, in order to determine which polymer is the best support for the semiconductor. As expected, the efficiency of TCE oxidation increases with lower flow rate and pollutant inlet concentration due to the reaction and mass transfer effects, assuming a first-order kinetics as found in other studies for TCE decomposition rates in a fixed bed plug flow reactor [28]. It is observed, especially in PMMA-SM, PS and PET, that both conversion and mineralization yield are favored by higher residence times in the photoreactor, as a result of the higher rate of TCE degradation and CO2 formation. PMMA-SM showed the best conversion results at all pollutant inlet concentrations, but a higher amount of side compounds is formed in the reaction; the drop of selectivity to complete oxidation products (i.e. CO2) as the conversion increases is typical of many chemical processes depending of the reaction mechanism and the operating conditions. PMMA-SM was also the only support that reached better conversion results than borosilicate glass, used as reference material. We note that the reactivity tests were performed as characterization method and operating conditions were not optimized. Further studies are thus

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photocatalysts prepared with method C reach higher values of conversion and mineralization than those prepared with method B at the same residence time. This result reveals a better efficiency in photocatalytic reaction with method C. It is worth noting that PMMA-SM demonstrates excellent properties as TiO2 support for photocatalytic applications, especially with the deposition of a silica interlayer which significantly enhances the quality and performance of the photocatalytic coating. The good transmittance in the UVA obtained with this sample despite of its high thickness, which endows this support with good mechanical resistance, facilitates radiation harvesting in both sides of the sample and in several samples irradiated in the same direction. PMMA-SM also exhibits good photochemical resistance to aging processes, as demonstrated in our previous work [38]. Fig. 5. Effect of the number of TiO2 layers on the photocatalytic activity for TCE oxidation of PMMA-SM coated by method A. [TCE] = 30 ppm.

4. Conclusions

required to likely establish the optimal experimental conditions for TCE degradation, for instance oxygen concentration [42] and RH [43]. The deposition of SiO2 between polymer and TiO2 by means of surfactant-mediated multi-layer procedures improved TiO2 adhesion on all polymers, and consequently their photocatalytic activity, as observed for PMMA in Fig. 6. As the characterization studies predicted, and in agreement with previous studies with other polymers [28], the more efficient TiO2 adhesion and distribution achieved by methods B and C promoted the conversion and mineralization of TCE, to values higher than those obtained with more TiO2 layers. When TCE concentration increases,

Non porous and UV-transparent commercial polymeric substrates can be effectively utilized as supports for TiO2. The acidic sol–gel synthesis method employed in this work is valid for anatase-TiO2 deposition by dip-coating at low temperature onto thermally sensitive substrates. The polymeric supports coated with TiO2 showed similar transmittance values to that of the coated reference substrate, borosilicate glass, and exhibit good photocatalytic activity, as demonstrated in TCE oxidation experiments. The photocatalytic activity can be improved by multilayer deposition up to a limited number of TiO2 layers, and by SiO2–TiO2 combination in mixed films prepared with the help of surfactants (PDDA and FC). The deposition of a silica intermediate layer improves TiO2 adhesion, especially for PMMA-SM, which results

Fig. 6. Photocatalytic activity for TCE oxidation: (a) conversion, and (b) selectivity to CO2, of the selected polymers coated with 3 layers of TiO2 by method A (glass calcined at 350 °C is included as reference support); (c) conversion obtained with PMMA-SM coated by methods A ( ), B ( ) and C ( ); (d) TCE conversion at initial concentration of 60 ppm.

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in higher radiation absorption and photocatalytic performance of the samples. PMMA-SM demonstrates high photocatalytic reactivity for degradation of TCE due to the good interaction with TiO2 and is postulated to be the best commercial polymer to be used as transparent support in the UV–Vis region. The improvement in the photocatalytic activity as a result of the better interaction TiO2-polymer obtained with the deposition of the SiO2 interlayer demonstrated in this study, is similar to the activity enhancement obtained by pretreatment of the polymeric surface by plasma previously reported [19]; thus both approaches facilitate the coating of the polymeric surfaces, resulting in better photocatalytic performance. The thermal sensitivity of organic polymers makes them unsuitable for the use of temporal additives, like surfactants or pore generating agents, which could lead to a further improvement of the photocatalyst performance, as it has been previously observed using glass substrates, but require high-temperature treatments [43]. However, several options can be explored in the future to deposit a higher amount of titania, such as increasing the sol concentration or immersion speed. Additionally, similar sol– gel preparations may be employed to incorporate different metal oxides or their mixtures, such as ZrO2 [48] or visible light-active semiconductors. As a final remark, this research points out that commodity polymers can be valorized by deposition of photocatalytic TiO2. The semiconductor provides them with novel properties and opens up new fields of application for these polymeric hybrids, not only in the area of environmental and chemical engineering, for elimination of airborne chemical pollutants by advanced oxidation processes using solar energy, but also in different areas, e.g. Modified Atmosphere Packaging (MAP), agriculture, construction, automotive industry, etc. that can benefit from this and other photocatalytic properties such as disinfection and self-cleaning. Acknowledgements The authors are gratefully acknowledged to the Spanish Government for the financial support (CMT2011-25093 and Juan de la Cierva postdoctoral contract of R.P.), and to E. Espí (Repsol) for polyolefin samples. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.07.080. References [1] G.M. Wallner, R.W. Lang, Aging of polymeric films for transparent insulation wall applications, Sol. Energy 79 (2005) 603–611. [2] L. Delbreilh, A. Bernès, C. Lacabanne, J. Grenet, J.-M. Saiter, Fragility of a thermoplastic polymer. Influence of main chain rigidity in polycarbonate, Mater. Lett. 59 (2005) 2881–2885. [3] E. Espí, A. Salmerón, A. Fontecha, Y. García, A.I. Real, The effect of different variables on the accelerated and natural weathering of agricultural films, Polym. Degrad. Stab. 92 (2007) 2150–2154. [4] A. Boersma, Predicting the efficiency of antioxidants in polymers, Polym. Degrad. Stab. 91 (2006) 472–478. [5] L. Irusta, A. González, M.J. Fernández-Berridi, J.J. Iruin, J.M. Asúa, I. Albizu, A. Ibarzabal, A. Salmerón, E. Espi, A. Fontecha, Y. García, A.I. Real, Migration of antifog additives in agricultural films of low-density polyethylene and ethylene-vinyl acetate copolymers, J. Appl. Polym. Sci. 111 (2009) 2299–2307. [6] L. López-Vilanova, E. Espí, I. Martinez, J.L.G. Fierro, T. Corrales, F. Catalina, Photostabilization study of ethylene-butyl acrylate copolymers functionalized in the molten state with hindered amine light stabilizers (HALS), Polym. Degrad. Stab. 98 (2013) 2146–2152. [7] R. Yang, P.A. Christensen, T.A. Egerton, J.R. White, A. Maltby, Spectroscopic studies of photodegradation of polyethylene films containing TiO2 nanoparticles, J. Appl. Polym. Sci. 119 (2011) 1330–1338. [8] S. Singh, H. Mahalingam, P.K. Singh, Polymer-supported titanium dioxide photocatalysts for environmental remediation: a review, Appl. Catal. A 462– 463 (2013) 178–195.

[9] R. Portela, B. Sánchez, J.M. Coronado, R. Candal, S. Suárez, Selection of TiO2support: UV-transparent alternatives and long-term use limitations for H2S removal, Catal. Today 129 (2007) 223–230. [10] S.M. Miranda, F.V. Lopes, C. Rodrigues-Silva, S.D. Martins, A.M. Silva, J.L. Faria, R.A. Boaventura, V.J. Vilar, Solar photocatalytic gas-phase degradation of ndecane–a comparative study using cellulose acetate monoliths coated with P25 or sol-gel TiO2 films, Environ. Sci. Pollut. Res. Int. 22 (2015) 820–832. [11] L.X. Pinho, J. Azevedo, S.M. Miranda, J. Ângelo, A. Mendes, V.J.P. Vilar, V. Vasconcelos, R.A.R. Boaventura, Oxidation of microcystin-LR and cylindrospermopsin by heterogeneous photocatalysis using a tubular photoreactor packed with different TiO2 coated supports, Chem. Eng. J. 266 (2015) 100–111. [12] M.S. Curcio, M.P. Oliveira, W.R. Waldman, B. Sánchez, M. Canela, TiO2 sol–gel for formaldehyde photodegradation using polymeric support: photocatalysis efficiency versus material stability, Environ. Sci. Pollut. Res. (2014) 1–10. [13] M. Langlet, A. Kim, M. Audier, C. Guillard, J.M. Herrmann, Liquid phase processing and thin film deposition of titania nanocrystallites for photocatalytic applications on thermally sensitive substrates, J. Mater. Sci. 38 (2003) 3945–3953. [14] F. Shiraishi, N. Kamikariya, Y. Shibata, Photocatalytic decomposition of gaseous HCHO over a titanium dioxide film formed on a hydrophobic PET sheet, J. Chem. Technol. Biotechnol. 86 (2011) 852–857. [15] J. Kasanen, J. Salstela, M. Suvanto, T.T. Pakkanen, Photocatalytic degradation of methylene blue in water solution by multilayer TiO2 coating on HDPE, Appl. Surf. Sci. 258 (2011) 1738–1743. [16] M. Langlet, A. Kim, M. Audier, J.M. Herrmann, Sol–gel preparation of photocatalytic TiO2 films on polymer substrates, J. Sol-Gel. Sci. Technol. 25 (2002) 223–234. [17] F. Awaja, M. Gilbert, G. Kelly, B. Fox, P.J. Pigram, Adhesion of polymers, Prog. Polym. Sci. 34 (2009) 948–968. [18] M. Noeske, J. Degenhardt, S. Strudthoff, U. Lommatzsch, Plasma jet treatment of five polymers at atmospheric pressure: surface modifications and the relevance for adhesion, Int. J. Adhes. Adhes. 24 (2004) 171–177. [19] R.M. Cámara, E. Crespo, R. Portela, S. Suárez, L. Bautista, F. Gutiérrez-Martín, B. Sánchez, Enhanced photocatalytic activity of TiO2 thin films on plasmapretreated organic polymers, Catal. Today 230 (2014) 145–151. [20] W. Su, S. Wang, X. Wang, X. Fu, J. Weng, Plasma pre-treatment and TiO2 coating of PMMA for the improvement of antibacterial properties, Surf. Coat. Technol. 205 (2010) 465–469. [21] E. Bormashenko, G. Whyman, V. Multanen, E. Shulzinger, G. Chaniel, Physical mechanisms of interaction of cold plasma with polymer surfaces, J. Colloid Interface Sci. 448 (2015) 175–179. [22] C. Fonseca, J. Pereña, J. Fatou, A. Bello, Sulphuric acid etching of polyethylene surfaces, J. Mater. Sci. 20 (1985) 3283–3288. [23] W. Liu, L. Sun, Y. Luo, R. Wu, Facile transition from hydrophilicity to superhydrophilicity and superhydrophobicity on aluminum alloy surface by simple acid etching and polymer coating, Appl. Surf. Sci. 280 (2013) 193–200. [24] A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago, T. Minami, Transparent anatase nanocomposite films by the sol–gel process at low temperatures, J. Am. Ceram. Soc. 83 (2000) 229–231. [25] S. Matsuzawa, C. Maneerat, Y. Hayata, T. Hirakawa, N. Negishi, T. Sano, Immobilization of TiO2 nanoparticles on polymeric substrates by using electrostatic interaction in the aqueous phase, Appl. Catal. B 83 (2008) 39–45. [26] U. Cˇernigoj, U.L. Štangar, P. Trebše, U.O. Krašovec, S. Gross, Photocatalytically active TiO2 thin films produced by surfactant-assisted sol–gel processing, Thin Solid Films 495 (2006) 327–332. [27] J. Ruso, E. Blanco, P. Messina, Tuning morphology of mesoporous titanium oxides through fluorinated surfactants-based systems, J. Porous Mater. 20 (2013) 95–105. [28] B. Sánchez, J.M. Coronado, R. Candal, R. Portela, I. Tejedor, M.A. Anderson, D. Tompkins, T. Lee, Preparation of TiO2 coatings on PET monoliths for the photocatalytic elimination of trichloroethylene in the gas phase, Appl. Catal. B 66 (2006) 295–301. [29] Z. Liu, X. Quan, H. Fu, X. Li, K. Yang, Effect of embedded-silica on microstructure and photocatalytic activity of titania prepared by ultrasoundassisted hydrolysis, Appl. Catal. B 52 (2004) 33–40. [30] D. Rats, V. Hajek, L. Martinu, Micro-scratch analysis and mechanical properties of plasma-deposited silicon-based coatings on polymer substrates, Thin Solid Films 340 (1999) 33–39. [31] E. Pabón, J. Retuert, R. Quijada, A. Zarate, TiO2–SiO2 mixed oxides prepared by a combined sol–gel and polymer inclusion method, Microporous Mesoporous Mater. 67 (2004) 195–203. [32] L. Zhou, S. Yan, B. Tian, J. Zhang, M. Anpo, Preparation of TiO2–SiO2 film with high photocatalytic activity on PET substrate, Mater. Lett. 60 (2006) 396–399. [33] X. Zhang, A. Fujishima, M. Jin, A.V. Emeline, T. Murakami, Double-layered TiO2–SiO2 nanostructured films with self-cleaning and antireflective properties, J. Phys. Chem. B 110 (2006) 25142–25148. [34] J. Medina-Valtierra, C. Frausto-Reyes, S. Calixto, P. Bosch, V.H. Lara, The influence of surfactants on the roughness of titania sol–gel films, Mater. Charact. 58 (2007) 233–242. [35] R.B. Dupaix, M.C. Boyce, Finite strain behavior of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG), Polymer 46 (2005) 4827–4838. [36] S. Yamazaki-Nishida, J.K. Nagano, L.A. Phillips, S. Cervera-March, M.A. Anderson, Photocatalytic degradation of trichloroethylene in the gas phase

R.M. Cámara et al. / Chemical Engineering Journal 283 (2016) 535–543

[37]

[38]

[39] [40]

[41]

[42]

[43]

using titanium dioxide pellets, J. Photochem. Photobiol. A: Chem. 70 (1993) 95–99. N. Serpone, Is the band gap of pristine TiO2 narrowed by anion- and cationdoping of titanium dioxide in second-generation photocatalysts?, J Phys. Chem. B 110 (2006) 24287–24293. R.M. Cámara, R. Portela, F. Gutiérrez-Martín, B. Sánchez, Evaluation of several commercial polymers as support for TiO2 in photo-catalytic applications, Global NEST J 3 (2014) 525–535. D. Feldman, Polymer weathering: photo-oxidation, J. Polym. Environ. 10 (2002) 163–173. Y. Chen, D.D. Dionysiou, Correlation of structural properties and film thickness to photocatalytic activity of thick TiO2 films coated on stainless steel, Appl. Catal. B 69 (2006) 24–33. M. Hamadanian, A. Reisi-Vanani, A. Majedi, Sol–gel preparation and characterization of Co/TiO2 nanoparticles: application to the degradation of methyl orange, JICS 7 (2010) S52–S58. H.-H. Ou, S.-L. Lo, Photocatalysis of gaseous trichloroethylene (TCE) over TiO2: The effect of oxygen and relative humidity on the generation of dichloroacetyl chloride (DCAC) and phosgene, J. Hazard. Mater. 146 (2007) 302–308. S. Suárez, N. Arconada, Y. Castro, J.M. Coronado, R. Portela, A. Durán, B. Sánchez, Photocatalytic degradation of TCE in dry and wet air conditions with TiO2 porous thin films, Appl. Catal. B 108–109 (2011) 14–21.

543

[44] W.A. Jacoby, M.R. Nimlos, D.M. Blake, R.D. Noble, C.A. Koval, Products, intermediates, mass balances, and reaction pathways for the oxidation of trichloroethylene in air via heterogeneous photocatalysis, Environ. Sci. Technol. 28 (1994) 1661–1668. [45] S. Suárez, J.M. Coronado, R. Portela, J.C. Martín, M. Yates, P. Ávila, B. Sánchez, On the preparation of TiO2-sepiolite hybrid materials for the photocatalytic degradation of TCE: influence of TiO2 distribution in the mineralization, Environ. Sci. Technol. 42 (2008) 5892–5896. [46] M.D. Hernández-Alonso, S. García-Rodríguez, S. Suárez, R. Portela, B. Sánchez, J.M. Coronado, Operando DRIFTS study of the role of hydroxyls groups in trichloroethylene photo-oxidation over titanate and TiO2 nanostructures, Catal. Today 206 (2013) 32–39. [47] G. Balasubramanian, D.D. Dionysiou, M.T. Suidan, I. Baudin, J.-M. Laine, Evaluating the activities of immobilized TiO2 powder films for the photocatalytic degradation of organic contaminants in water, Appl. Catal. B 47 (2004) 73–84. [48] R. Portela, B. Sánchez, J.M. Coronado, Photocatalytic oxidation of H2S on TiO2 and TiO2–ZrO2 thin films, J. Adv. Oxid. Technol. 10 (2007) 375–380.