Photocatalytic efficiency and self-cleaning properties under visible light of cotton fabrics coated with sensitized TiO2

Applied Catalysis B: Environmental 104 (2011) 361–372

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Photocatalytic efficiency and self-cleaning properties under visible light of cotton fabrics coated with sensitized TiO2 R. Rahal, T. Pigot, D. Foix, S. Lacombe ∗ UMR CNRS 5254, IPREM, Université de Pau et des Pays de l’Adour, Hélioparc, 2 Avenue Président Angot 64053, Pau cedex 09, France

a r t i c l e

i n f o

Article history: Received 11 January 2011 Received in revised form 1 March 2011 Accepted 3 March 2011 Available online 10 March 2011 Keywords: Sensitized TiO2 Photocatalysis Coated cotton textiles Singlet oxygen Self-cleaning

a b s t r a c t A simple and reproducible one-pot process for the elaboration of cotton fabrics coated with sensitized TiO2 was developed. A molecular precursor [Ti(OR)3 (O2 C-AQ)] was prepared starting with anthraquinone-2-carboxylic acid (AQ-COOH) and characterized by FTIR, CPMAS NMR and XPS. Hydrolysis of mixtures of [Ti(OR)3 (O2 C-AQ)] and Ti(OR)4 at low temperature in an aqueous medium leads to pale yellow cotton fabrics together with the corresponding free P-TiO2 /AQ powders. The diffuse reflectance UV spectra confirmed the shift of absorption towards the visible range. From FTIR, CPMAS NMR and XPS analysis of the samples (cotton pieces and powders), it was shown that AQ-COOH was not only adsorbed on Titania but tightly bond through a carboxylate complex as in the molecular precursor. Anatase polymorph was always characterized by XRD even in the absence of a calcination step. Examination by SEM of treated cotton tissues before and after washing showed stable and homogeneous coating of TiO2 particles on the cotton fibers. The photocatalytic properties of the samples were investigated, with special care to visible light activation. Under UV light, acetone mineralization was observed, while under filtered visible light, no acetone mineralization occurred. However efficient singlet oxygen addition to di-n-butyl sulfide was evidenced under visible light. Sulfoxide and sulfone were obtained in better yields using sensitized TiO2 than using un-modified TiO2 or Anthraquinone alone treated fabrics. Optimum results were obtained with low level of sensitizing AQ-COOH relative to TiO2 (8%) and no reactivity improvement was noticed with higher AQ-COOH levels. The cotton pieces coated with sensitized TiO2 also displayed self-cleaning properties towards wine stain, either under solar illumination or even in indoor light. The better efficiency of sensitized TiO2 -coated cotton is accounted for by a synergy effect between TiO2 and AQ-COOH, enhancing the formation of Reactive Oxygen Species (singlet oxygen and/or superoxide radical-anion). However, under these conditions, the production of hydroxyl radical seems to be ruled out. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The development of (multi)-functionalized textiles with specific properties is a field of growing interest. Several textile modifications are possible, such as incorporation of functional additives, chemical grafting of functional additives, or post-equipping textiles with functional coatings. The latter method often relies on sol–gel chemistry, starting with inorganic sols based on nanoparticular modified silica and other metal oxides [1]. Recently, some papers were devoted to TiO2 -functionalized textiles which can be photoactivated, inducing self-cleaning [2–14], UV-blocking [8,15], photo-oxidative [16–19] and/or bactericidal properties [3,9,20]. Besides the most frequently studied photocatalytic textiles treated with a TiO2 -based coating, textiles producing singlet

∗ Corresponding author. Tel.: +33 559 407 579. E-mail address: [email protected] (S. Lacombe). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.03.005

oxygen under irradiation due to incorporation of a photosensitizer (PS) also display interesting bactericidal [21–29] or detoxifying properties [30]. For the first series of textiles treated with a TiO2 coating, one of the main drawbacks is related to the poor overlapping of the TiO2 absorption spectrum with the solar emission spectrum, leading to a maximum activation of the modified textiles in the UV-A range. On the contrary, PS-containing textiles are most often activated in the visible range. It should be emphasized that in the case of TiO2 treated textiles, the oxidative and bactericidal properties are often ascribed to the formation of hydroxyl HO• radicals [19–20], while in the case of PS-containing textiles, singlet oxygen is mainly involved [21–23,28,29]. In both cases membrane damages are expected. In order to enhance the reactivity of TiO2 in the visible range, several chemical modifications are possible: for example coupling with a photosensitizer or with another narrow band-gap semi-conductor absorbing in the visible range, doping with metal impurities, preparing oxygen deficient TiO2 and doping TiO2 with

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non-metal atoms (anion doping). All these methods were recently reviewed by Fujishima et al. [31]. In this paper, we report the elaboration of cotton samples containing either TiO2 or sensitized TiO2 in order to increase its visible range efficiency. Sensitization involved the direct coupling of a sensitizer (anthraquinone 2-carboxylic acid or AQ-COOH) to a TiO2 precursor, Ti(OiPr)4 , by an aqueous one-step process already described by Rahal et al. [32] for the synthesis of para-amino benzoic acid-TiO2 hybrid nanostructures. The choice of this sensitizer (AQ-COOH) was based upon its common use in textile dying industry as a fixing tank agent for various usual dies. Under industrial dying conditions, anthraquinone (AQ) is usually grafted to the fabrics by a reductive/oxidative process [33]. Moreover AQ is a well known PS, capable of producing singlet oxygen by energy transfer under irradiation [34]. Anthraquinone 2-sulfonate was also shown to produce superoxide radical anion O2 •− by electron transfer [34,35] in addition with singlet oxygen [36]. The question of superoxide radical anion formation sensitized by AQ was raised [37–39] but should be possible considering the reduction potential 0 of AQ and O2 (E 0 •− = −0.62 V and EO /O • − = −0.33 V vs SHE in AQ/AQ

2

2

CH3 CN, respectively [37,40]). It should be noted that the value of 0 EO differed significantly in previous papers (between −0.69 /O • − 2

2.2. Study of the photocatalytic activity by photooxydation of di-n-butylsulfide (DBS) This experiment was carried out by adding a tissue (15 cm2 ) in a modified Petri dish (with a septum at the top for injection of DBS, tightly sealed in order to prevent any evaporation of reactants and products. 10 ␮l of DBS solution was injected on the tissue through the septum. Half of the Petri dishes were kept in the dark at 20 ◦ C and the other half were irradiated for 24 h with a device made of four 420 nm lamps (Philips TLD-K 30 W/03) and a 420 nm cut off filter made of a polyester/acrylic sheet (Film anti-UV Sanergies Neutral 240 C, thickness 60 ␮m) placed in an incubator regulated at 20 ◦ C. The distance between the samples and the lamps was 10 cm. The irradiance was measured with a spectroradiometer ILT900-R (total irradiance 5.1 mW/cm2 ). The irradiance spectra of the lamps with and without the anti-UV filter are given in Figure S3, SI. After 24 h, without opening the Petri dish, 5 ml of acetonitile containing cyclododecane (5 × 10−3 M) as internal standard were added through the septum and the washing solutions were analysed by GC (Varian 3900) equipped with Chrompack column CPSil-5CB (30 m, 0.25 mm, 1 ␮m). The same experiment was also performed under indoor light of the laboratory.

2

[41] and −0.53 V [42], with values between −0.47 and −0.60 V depending on pH [43], vs SHE in CH3 CN (taking ESHE = ESCE + 0.25 V). Recently, we showed that anthraquinone, easily incorporated on cotton fabrics by a reductive–oxidative process, could efficiently oxidize di-n-butylsulfide and that the treated cotton pieces had a bactericidal effect when irradiated at 420 nm [44]. The characterization of these modified cotton samples was achieved by various methods in order to demonstrate both TiO2 deposition and the change induced by the photosensitizer. They were also tested under different irradiation conditions in the visible range. They were first evaluated under solvent-free conditions against gaseous acetone mineralization to investigate TiO2 efficiency and second, against alkylsulfides oxidation to check the possible formation of singlet oxygen. Their self-cleaning properties were also investigated. The results obtained are discussed according to the assumed reaction mechanisms.

2. Experimental All the synthetic procedures for preparing the fabrics and powders are detailed in Supplementary Information (SI).

2.1. Study of the photocatalytic activity by mineralization of acetone This experiment was carried out by injecting a defined quantity of acetone (3 ␮l) through a septum into a sealed reactor (350 ml) (corresponding to 2800 ppmv as the initial concentration of acetone) containing the tissue (15 cm2 ). This cylindrical reactor had a circular pyrex window on the top through which it was irradiated. It was linked to a TCD (thermal conductivity detector) gas chromatograph with micro-catharometers (GC-4900 VARIAN), so as to observe simultaneously the increase of CO2 concentration induced by acetone mineralization and the drop of acetone concentration during irradiation. The irradiations were carried out with a device containing four 420 nm lamps (Rayonet RPR 4190-A) with a 420 nm cut off filter (Schott GG420). A picture of the whole device is given in Figure S1, SI. The distance between the samples and the lamps was 10 cm. The irradiance was measured with a spectroradiometer ILT900-R from International Light Technology (total irradiance 4.3 mW/cm2 ). The irradiance spectra of the lamps with and without filter are given in Figures S2, SI.

2.3. Discoloration of red wine stain 500 ␮l of red wine was pipetted on the tissues (treated or non treated). These tissues were irradiated for 24 h with a solar simulator (Oriel Instruments with a 150 W Xe arc lamp) with a 420 nm cut off filter (Schott GG 420). The same experiment was carried out under indoor light without any other irradiation. Half of these samples were covered with black paper for comparison. 2.4. Materials and equipments The diffuse reflectance spectra in the UV–visible range (DRUV) were measured at room temperature with a double beam Cary 5000 spectrophotometer equipped with an 11 cm diameter integrating sphere and a home made powder holder. The diffuse reflectance spectra were corrected vs a white standard (Teflon 55 microns, Aldrich). The Kubelka–Munk model describes the light penetration in porous media with only two parameters: an absorption coefficient, k, and an isotropic scattering coefficient, s (which both have units of cm−1 ) [45]. The Fourier transform infrared (FTIR) spectra of the samples in Nujol for (1) or in KBr pellets for the modified TiO2 powders were recorded using a MAGNA-560 spectrometer at a resolution of 4 cm−1 in an absorption mode using 400 scans. All 1 H HRMAS NMR spectra were recorded on a Bruker Avance instrument operating at 400.13 MHz using a 4 mm HRMAS 1 H/13 C probe head. Powdered samples were packed in a 4-mm zirconia rotors, sealed with Kel-FTM caps and spun at 7 kHz and at a contact time of 500 ms. Chemical shifts were determined relative to tetramethyl silane (TMS) used as control. XPS measurements were carried out with a Kratos Axis Ultra spectrometer, using a focused monochromatized Al K␣ radiation (h = 1486.6 eV). The XPS spectrometer was directly connected through a transfer chamber to an argon dry box, in order to avoid moisture/air exposure of the samples. For the Ag3d5/2 line, the full width at half maximum (FWHM) was 0.58 eV under the recording conditions. The analysed area of the samples was 300 ␮m × 700 ␮m. Peaks were recorded with constant pass energy of 20 eV. The pressure in the analysis chamber was around 5.10−9 mbar. The binding energy scale was calibrated from the hydrocarbon contamination using the C1s peak at 285.0 eV. Core peaks were analysed using a nonlinear Shirley-type background

R. Rahal et al. / Applied Catalysis B: Environmental 104 (2011) 361–372

Ti(OiPr)4

363

AQ-CO 2H

18 hours

RT

[Ti(OiPr) 3 (O 2C-AQ)] + Ti (OiPr) 4 Cotton pieces (a)

(b)

Cotton pieces

Modified cotton pieces + TiO 2 powders

Hydrolysis in 100 ml H2O for 3 hours Fig. 2. Infrared spectra (1800–650 cm−1 ) of AQ-CO2 H, [Ti(OiPr)3 (O2 CAQ)] (1) and Nujol.

Washing Path (a) TiO2/AQ(+)@T & P-TiO2/AQ(+)

Thermal treatment at 80° C 1 hour for the tissues 18 hours for the powders

Path (b) TiO2/AQ(-)@T & P-TiO2/AQ(-)

Fig. 1. Scheme of the synthesis of: path (a) TiO2 /AQ(+)@T and P-TiO2 /AQ(+); path (b) TiO2 /AQ(−)@T and P-TiO2 /AQ(−).

[46]. The peak positions and areas were optimized by a weighted least-squared fitting method using 70% Gaussian, 30% Lorentzian lineshapes. Quantification was performed on the basis of Scofield’s relative sensitivity factors [47]. Thermogravimetric analysis (TGA) was performed using a TA Instruments (Guyancourt, France) apparatus 2950 TGA. The purge rate was 100 ml min−1 , with a flow distribution of 20 ml min−1 to the balance chamber (nitrogen) and 80 ml min−1 to the furnace (oxygen). The heating rate was 5 ◦ C min−1 and all samples were analysed in the powder form. Powder XRD patterns were recorded on an INEL XRG 3000 diffractometer using a curved position-sensitive detector (CPS 120) calibrated with Na2Ca3Al2F14 as standard. The monochromatic ˚ from a long fine focus Cu radiation applied was CuK␣ (1.5406 A) tube operating at 40 kV and 35 mA. Scans were preformed over the 2 range from 5◦ to 115◦ . Accurate unit cell parameters were determined by a least squares refinement from data collected by the diffractometer. Basal spacing distances for the different samples analysed were determined from the position of the d (0 0 3) reflection. Observation of the surface morphology was performed using an environmental scanning electronic microscopy (ESEM Electroscan E3), and the samples were analysed without metallisation, with a voltage acceleration of 25 kV at room temperature.

3. Results The methods of preparation of the various tissues are summarized in Fig. 1 and the names and characteristics of the prepared powders (P) and tissues (T) are summarized in Table 1. In the following samples names, AQ(+) stands for a high AQ-COOH/Ti ratio (the only source of Ti is the precursor [Ti(OiPr)3 (O2 C-AQ)], Path a), while AQ(−) stands for a low AQ-COOH/Ti ratio (the source of Ti is a mixture of Ti(OiPr)4 and of the precursor [Ti(OiPr)3 (O2 C-AQ)], Path b).

3.1. Synthesis and characterization of modified cotton fabrics The synthetic method for the precursor [Ti(OiPr)3 (O2 C-AQ)] (1) preparation was previously reported starting with another aromatic molecule (p-aminobenzoic acid) to synthesize hybrid TiO2 nanoparticles with controlled amounts of organics for cosmetics and ionic separation [32,48]. The same method was used here, using an anthraquinone derivative, AQ-COOH, as AQ is well-known for its photosensitizing properties [34–37,49] and as an industrial dying compound for fixation of dyes on cotton and polyester. The key-step is the preparation of the precursor (1) as a yelloworange powder, obtained from the reaction between equimolar amounts of Ti(OiPr)4 and anthraquinone-2-carboxylic acid (AQCO2 H) in isopropanol at room temperature. The comparison of FT-IR spectra of AQ-CO2 H and (1) (Fig. 2) showed in both cases the presence of the C O stretching band of the quinone moiety at 1677 cm−1 . On going from AQ-CO2 H to (1), the drop of the C O stretching band at 1699 cm−1 indicates the disappearance of the free carboxylic moiety. New sharp bands appears, corresponding to asymmetric stretching as (CO2 − ) at 1608, 1579 cm−1 and symmetric stretching s (CO2 − ) at 1556 and 1479 cm−1 . The value  = [as (CO2 ) − s (CO2 )] smaller than 100 cm−1 suggests a bidentate chelating/bridging coordination [50]. This precursor (1) was used for the synthesis of our modified tissues. Hydrolysis of (1) was performed without any organic solvent: the neat powder (1) for P-TiO2 /AQ(+) and TiO2 /AQ(+)@T or the solution ([Ti(OiPr)3 (O2 C-AQ)] + Ti(OiPr)4 ) for P-TiO2 /AQ(−) and TiO2 /AQ(−)@T, were added to boiling aqueous solution containing 0.01 equivalent of NBu4 Br per atom of titanium and 15 pieces of cotton (15 cm2 each) under vigorous magnetic stirring. An immediate pale yellow precipitate appeared. After heating under reflux, the yellow tissues were removed and the remaining suspension was centrifuged. For UV–visible diffuse reflection spectroscopy, the Kubelka–Munk relation measuring K/S (K and S are respectively the absorption and scattering coefficients of TiO2 ) for thick samples with low optical transmittance allowed the conversion of the reflectance (R) into the equivalent of absorption spectra via F(R) remission function: F(R) =

(1 − R)2 K = S 2R

As expected, the DRUV spectrum of crude cotton presented no absorption band in the UV–visible range (Fig. 3). The TiO2 @T tissue displayed the characteristic cut-off around 380 nm, usually observed with TiO2 . No significant difference between this spec-

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Table 1 Name and characteristics of the prepared samples.

Pure anthraquinone-2-carboxylic acid Prepared from commercial TiO2 Degussa P25 Prepared from a mixture of TiO2 P25 and AQ-COOH. Molar ratio Ti/AQ-COOH = 60 TiO2 prepared from Ti(OiPr)4 Prepared from a mixture of the home-made precursor [Ti(OR)3 (O2 C-AQ)]a with Ti(OiPr)4 . Molar ratio Ti/AQ-COOH = 60 Prepared from the home-made precursor [Ti(OR)3 (O2 C-AQ)]a . Molar ratio Ti/AQ-COOH = 1 Prepared by reductive/oxidative coupling of anthraquinone (AQ) on cotton tissues

Cotton fabrics (T)

AQ-COOH P-TiO2 P25

– TiO2 P25@T

P-TiO2 P25/AQ(−)

TiO2 P25/AQ(−)@T

P-TiO2

TiO2 @T

P-TiO2 /AQ(−)

TiO2 /AQ(−)@T

P-TiO2 /AQ(+)

TiO2 /AQ(+)@T

–

AQ@T

R = iPr.

Fig. 3. DRUV spectra of TiO2 P25/AQ(−)@T, AQ@T.

crude

cotton

samples,

TiO2 @T,

TiO2 AQ(−)@T,

trum and those of TiO2 P25@T (not shown) and TiO2 P25/AQ(−)@T was noticed. A shift towards the visible range up to 500 nm, consistent with the spectrum of AQ@T prepared by conventional reductive/oxidative process, was only observed for the sample prepared with the home made precursor (1), TiO2 /AQ(−)@T. When comparing the tissue TiO2 /AQ(−)@T and the corresponding powder P-TiO2 /AQ(−) (Figure S4, SI), the spectrum of the powder appeared to be more intense than that of the tissue. Both these spectra are strikingly different from the spectrum of AQ-CO2 H: neither the weak n–␲* band at 425 nm, nor the more intense ␲–␲* band at 350 nm of AQ is observed, due to a high amount of TiO2 in these samples (molar ratio Ti/AQ = 60). For all these samples, the spectra were dominated by the characteristic features of TiO2 , more or less shifted towards the visible range. On the other hand, it can be noticed that the spectrum of P-TiO2 /AQ(+) (not shown) was mainly dominated par AQ-COOH feature (Fig.S4, SI). The bandgap energies Eg of the different materials may be estimated using the equation ˛ = A(h − Eg )n /Eg , where ˛ is absorption coefficient, A is a constant, h is the energy of light and n is a constant on the nature of the electron transition [51]. In the case of TiO2 , n = 2 assuming an indirect bandgap [52]. A is proportional to Kubelka–Munk function (F(R)) and the bandgap energy can be obtained from the plots of (F(R)h)1/2 vs h as the intercept at (F(R)h)1/2 = 0 of the extrapolated linear part of the plot (Fig. 4) [53]. The determined bandgaps are summarized in Table 2.

The value obtained for TiO2 @T was consistent with literature data (3.3 eV, [53]), with no significant change for TiO2 P25/AQ(−)@T (3.25 eV). A slight shift towards lower energy (3.14 eV) was only noticed for the sample TiO2 /AQ(−)@T, as already observed with Nor C-doped TiO2 [54], and is mainly due to AQ-COOH absorption (Figure S4-SI). The crystallinity of TiO2 on the modified textile surface was investigated by powder X-ray diffraction (PXRD) with the diffraction angle (2) in the range 20–60 ◦ C. Fig. 5(left) shows the diffractograms of two samples TiO2 @T and TiO2 /AQ(−)@T. Broad and small peaks at  = 25◦ , 38◦ , 48◦ and 55◦ were observed, assigned to the anatase phase of TiO2 (anatase, JCPDS card 21-1272). Note that these TiO2 coated textiles were made at 100 ◦ C in an aqueous medium without any chemical treatment (neither calcination nor acid addition). The intense peak at 23◦ and the broad one at 34◦ constitute the typical XRD pattern of cellulose fibers [10]. XRD diffractograms of P-TiO2 /AQ(−) and P-TiO2 P25/AQ(−) are similar (Fig. 5(right)) and, as previously, the presence of AQ-CO2 H does not affect the crystalline phase of TiO2 , even if the broad peaks of P-TiO2 /AQ(−) compared to those of P-TiO2 P25/AQ(−) indicate that the nanoparticles obtained with our method are smaller (7 nm) [32] than those of TiO2 P25 (25 nm). The IRTF spectra of the powders in KBr pellets are shown in Fig. 6. As for the precursor (1), the drop in intensity of the 1695 cm−1 band, assigned to the C O free carboxylic group, is noticed for P-TiO2 /AQ(+) and P-TiO2 P25/AQ(−). This band even completely disappears for P-TiO2 /AQ(−). On the contrary the quinone band at 1672 cm−1 is observed in all the samples. Broad new bands at 1639 and 1548 cm−1 observed respectively for P-TiO2 /AQ(−) and PTiO2 /AQ(+) are not observed for P-TiO2 P25/AQ(−) and are assigned to as (CO2 − ) and s (CO2 − ) vibrations. For P-TiO2 /AQ(−) we suggest only one structural form of the titanium–carboxylate complex without free carboxylic acid. For the two other samples, mixtures

6 5

SQRT [(K/S)*(Energy)]

a

Powders (P)

TiO2@T 4 TiO2P25/AQ(-)@T 3

TiO2/AQ(-)@T

2 1

Table 2 Determined bandgaps for TiO2 and various AQ-sensitized TiO2 . Sample TiO2 /AQ(−)@T TiO2 P25/AQ(−)@T TiO2 @T

Eg (eV) 3.14 3.25 3.3

0 3,0

3,1

3,2

3,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

E(ev) Fig. 4. Bangap determination using TiO2 P25/AQ(−)@T, TiO2 /AQ(−)@T.

(F(R)*E)1/2

vs

E

plots

for

TiO2 @T,

R. Rahal et al. / Applied Catalysis B: Environmental 104 (2011) 361–372

365

TiO2@T

Anatase Cotton

Counts (a.u.)

Counts (a.u)

P-TiO2/AQ(-) P-TiO2P25/AQ(-)

Anatase Rutile

TiO2/AQ(-)@T

20

25

30

35

40

45

50

55

2θ

60

20

25

30

35

40

45

50

55

60

2θ

Fig. 5. PXRD diffractograms of (left): the cotton tissues TiO2 @T and TiO2 /AQ(−)@T; (right): the powders P-TiO2 /AQ(−) and P-TiO2 P25/AQ(−)

of the adsorbed acid and of the titanium–carboxylate complex are probably obtained. CPMAS RMN 13 C spectra of the two solid samples P-TiO2 /AQ(−) and P-TiO2 /AQ(+) were similar and in good agreement with an anthraquinonic structure. They showed two quinone carbons (C O) at 180 ppm and 179 ppm, close to the corresponding peaks in DMSO-d6 solution of AQ-COOH (182 ppm). However, the peak at 171 ppm attributed to the carbonyl group was significantly shifted relative to the free acid in DMSO-d6 solution (165 ppm). This result is consistent with the involvement of the carboxylate moiety in the bonding with titanium atoms. The thermogravimetric analysis and differential thermal analysis (DTA) curves recorded in air up to 600 ◦ C exhibited weight loss within the temperature range 20–600 ◦ C (Fig. 7). The two materials P-TiO2 /AQ(−) and P-TiO2 /AQ(+) showed one main weight loss (4.9–48.8%, Table 3) above 400 ◦ C corresponding to a chemically bound complex with TiO2 , as it occurs at much higher temperature than for free anthraquinone-2-carboxylic acid (340 ◦ C). Accordingly, the first but weaker weight loss (3.1–11.2%, Table 3) in the range 200–390 ◦ C may be assigned to minor amounts of adsorbed anthraquinone-2-carboxylic acid. The curve obtained with P-TiO2 P25/AQ(−) displays more obviously two ranges of weight loss, indicating the presence of both free and complexed AQ-COOH, consistent with the previous IR data. All these measurements are in agreement with the weight of introduced AQ during the synthesis (Table 3), although some loss of introduced AQ was observed in a more significant extent with P-TiO2 P25/AQ(−) relative to P-TiO2 /AQ(−). This result demonstrates a more efficient incorporation of AQ-COOH on TiO2 with our synthesis method. An accurate examination of the DTA curves (inset of Fig. 8(right)) indicates that the temperature loss occurs for P-TiO2 /AQ(−) at an intermediate temperature (420 ◦ C) between that of AQ-COOH (350 ◦ C) and that of P-TiO2 P25/AQ(−) (460 ◦ C). This result is con-

Fig. 6. Infrared spectra (2000–700 cm−1 ) of AQ-CO2 H; P-TiO2 P25/AQ(−);. PTiO2 /AQ(−) and P-TiO2 /AQ(+) in KBr pellets.

sistent with a stronger bonding of AQ-COOH with TiO2 P25 than with home-made TiO2 . The morphology of the obtained samples is shown in the SEM pictures of the treated cotton pieces in comparison with untreated samples in Fig. 8. The presence of TiO2 is obviously observed around the cotton fibers for both TiO2 P25/AQ(−)@T and TiO2 /AQ(−)@T. Uncontinuous TiO2 nanocristallites appear to be homogeneously deposited on the fibers and not between them. The picture of the fabric before and after washing (Figure S5, SI) in a machine with a detergent containing no bluing dye indicates that the TiO2 coating is resistant to mechanical washing. For the XPS analysis, we first describe the spectra of reference products (Fig. 9), then the spectra of the modified powders (Fig. 10) and finally those of the modified tissues (Figure S6, SI). Quantitative results are summarized in Tables S1–S3, SI. First, the four reference compounds P-TiO2 P25, P-TiO2 , AQCOOH and the raw tissue were studied by XPS (Fig. 9). For both P-TiO2 P25 and P-TiO2 , the Ti2p3/2 peak appears at 458.8 eV, energetic position characteristic of the Ti4+ form. The O1s peak consists of a maximum at around 530.0 eV assigned to the O of the lattice of TiO2 , and of a shoulder at 531.8 eV assigned to the hydroxyl (–OH) of the surface. For AQ-COOH, the C1s peak is made of a maximum at 284.6 eV corresponding to the carbon atoms of the aromatic rings and of smaller peaks at 287.2 eV and 289.1 eV assigned to the carbon atoms of the C O and COOH groups, respectively. The satellite at 290.7 eV is due to the aromatic rings. The O1s highest peak at 531.3 eV is attributed to the oxygen atom of the carbonyl bonds ( O), and the second peak at 533.0 eV is attributed to the acid (–OH) oxygen atom. For the tissue, 3 peaks can be distinguished in the C1s signal. The one at 285.0 eV corresponds to hydrocarbon contamination. The two other are characteristic of the tissue: the highest at 286.8 eV is attributed to the C–O–H and C–C–O bonds, while the smallest at 288.3 eV is correlated to the O–C–O environments. For the O1s peak, all the oxygen atoms are equivalent (C–OH or C–O–C) and appear at 533.1 eV. Four powders were then analysed (Fig. 10): the precursor [Ti(OR)3 (O2 C-AQ)], P-TiO2 P25/AQ(−), P-TiO2 /AQ(−), P-TiO2 /AQ(+). For the precursor, the involvement of the COOH group in the bonding to TiO2 (forming a COO–Ti complex) may be deduced from the following observations: • besides the O1s peak of TiO2 at 529.8 eV and the peak at 531.5 eV corresponding to the C O of AQ-COOH and to the oxygen atoms of OiPr groups, the peak at 533.0 eV (–OH) in AQ-COOH almost disappeared. If the O-H group is now a O–Ti group, as oxygen is the first neighbour of Ti, the influence of its electropositivity is strong. It may thus be assumed that the peak at 533.0 eV of AQ-COOH is strongly shifted and included in the peak at 531.5 eV. • for C1s, besides the peak of the carbon atoms of the aromatic ring at 284.6 eV, and the peak at 287.3 eV assigned to the C O

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120

Derivative weight (% min-1)

0,5

Weight (%)

100 80 60 40

AQ-CO2H P-TiO2P25/AQ(-)

20

P-TiO2/AQ(-) P-TiO2/AQ(+)

0 50

150

250

350

450

550

0 -0,5 0,5

-1

0 -0,5

-1,5

-1 -1,5

-2

-2 350

400

450

500

0,02 0 -0,02 -0,04 -0,06 -0,08 -0,1 -0,12 -0,14 -0,16 -0,18 -0,2 550

-2,5 50

Temperature (°C)

150

250

350

450

550

Temperature (°C)

Fig. 7. TGA (left) and DTA curves (right) of AQ-COOH, P-TiO2 /AQ(+), P-TiO2 P25/AQ(−), P-TiO2 /AQ(−). Table 3 Determination of the percentage weight of introduced AQ-COOH in the modified TiO2 powders deduced from the TGA analysis and comparison with the introduced amount during the synthesis.

Introduced AQ (% weight) Determined AQ (% weight) Temperature range % Weight

P-TiO2 P25/AQ(−)

P-TiO2 /AQ(−)

P-TiO2 /AQ(+)

20 8.9 190–290 3.5

10 8.3 200–340 3.1

76 63.3 280–390 11.2

390–510 5.4

carbon, the last peak present for AQ-COOH at 289.1 eV (attributed to COOH) is shifted to 288.4 eV in the precursor. The influence of Ti electropositivity on the C atom of COOTi is thus slighter than for oxygen atoms. • the atomic ratio deduced from the spectra (O: 27, C:70, Ti:3; Table S2, SI) is consistent with the molecular formula [Ti(OR)3 (O2 C-AQ)].

340–500 5.2

390–540 52.1

For P-TiO2 P25/AQ(−), the signal is dominated by the TiO2 features and AQ-COOH is hardly detectable, in agreement with the large Ti amount (17%, Table S2, SI): • the O1s spectrum presents the same peak positions as P-TiO2 P25 (529.8 eV and 531.5 eV). • For the C1s peak, the peak at 285.0 eV cannot be assigned to AQCOOH (284.6 eV), but instead to hydrocarbon contamination. The

Fig. 8. SEM of TiO2 -coated cotton fabrics at various enlargement.

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Fig. 9. C1s and O1s XPS spectra of the reference compounds P-TiO2 P25, P-TiO2 , AQ-COOH and raw cotton tissue.

two other peaks at 287.7 eV and 290.5 eV are tentatively assigned to –COO and CO3 contamination. To summarize, for P-TiO2 P25/AQ(−), the O1s signal is dominated by the feature of TiO2 , whereas the C1s signal is not characteristic of AQ, but of hydrocarbon contamination. For P-TiO2 /AQ(−), the O1s signal is very similar of the one of TiO2 . This is consistent with the large amount of TiO2 (atomic ratio for Ti atom 20%, Table S2, SI).

For P-TiO2 /AQ(+), the signals of O1s and C1s peaks are dominated by AQ-COOH features. The O1s peak is made of signals at 529.8 eV (TiO2 lattice), 531.3 eV (C O of AQ-COOH) and 532.8 eV. As this latter peak is broader and at lower energy than the one of the OH of AQ-COOH at 533.1 eV, it probably involves a double environment for this carboxylate oxygen (adsorbed AQ-COOH not reacted with TiO2 , and complexed COOTi group appearing at lower binding energy than 533.1 eV of COOH). Besides the C1s peak at 284.6 eV (carbon of the aromatic rings) and at 287.2 eV (carbonyl C O bond),

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Fig. 10. C1s and O1s XPS spectra of the precursor [Ti(OR)3 (O2 C-AQ)], P-TiO2 P25/AQ(−), P-TiO2 /AQ(−) and P-TiO2 /AQ(+).

the peak at 288.8 eV is larger and at lower energy than in AQ-COOH (289.1 eV). This could be due to a complex environment of the C atoms of the COOH and COOTi groups as assumed previously. The XPS spectra (O1s and C1s) of the tissues (Figure S6, SI) show that Ti is present in the proportions given in Table S3, SI with energetic position and width identical to the Ti2p peak of TiO2 ,

characteristic of Ti4+ , as confirmed by the O1s peaks. The O1s and C1s signals of AQ-COOH are hidden by the characteristic features of the tissues, and can only be deduced from the larger peak width and from their relative intensity. It can be noticed that the atomic ratio of Ti grafted on the surface of the tissues is higher for TiO2 /AQ(−)@T (17%) than for TiO2 P25/AQ(−)@T (11%).

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Table 4 Yield of solvent-free oxidation products upon irradiation at 420 nm at 20 ◦ C of modified cotton fabrics in sealed Petri dishes containing DBS (DBSO: di-n-butyl sulfoxide, DBSO2 : di-n-butyl sulfone). Crude tissue DBS DBSO DBSO2

TiO2 @T

100

35 53 12

72 23 5

Table 5 Yield of oxidation products upon irradiation under indoor light at room temperature of modified cotton fabrics in sealed Petri dishes containing DBS (DBSO: di-n-butyl sulfoxide, DBSO2 : di-n-butyl sulfone). TiO2 @T DBS DBSO DBSO2

100 – –

AQ@T 100 – –

TiO2 P25/AQ(−)@T 100 – –

AQ@T

TiO2 /AQ(−)@T 3 80 17

It should be mentioned that control XPS analysis after irradiation did not show any significant modification of the signals, evidencing the stability of the coating.

TiO2 P25/AQ(−)@T 69 29 2

TiO2 /AQ(−)@T 2 34 64

produced (Figure S7, SI), indicating that carbon dioxide originated from acetone, and not from the tissue or from decomposition of AQ on the tissue. It may be noted that the relative abundance of m/z 45/44 peaks decreased from 17% after 18 h under UVA irradiation (fluorescent lamps with maximum emission at 350 nm) to 13% after 48 h under mainly visible irradiation (unfiltered 420 nm lamps) and to less than 1% under neat visible irradiation (420 nm lamps filtered with the Schott GG 420 cut-off filter). These results confirmed that 13 CO2 production was almost inefficient under visible light. From these experiments and from a “blank” experiment without acetone where no carbon dioxide evolution was detected, it may also be concluded that no degradation of the tissues or of anthraquinone was observed with visible light irradiation.

3.2. Photocatalytic tests As the photocatalytic test carried out with TiO2 /AQ(+)@T showed that these fabrics were not active, we focus in the following on TiO2 /AQ(−)@T and P-TiO2 /AQ(−). 3.2.1. Photocatalytic degradation of acetone under visible light irradiation Acetone mineralization photocatalysed by TiO2 is a well known reaction, ultimately giving rise to CO2 [55]. Fig. 11 shows the evolution of the CO2 concentration (relative to the initial ambient CO2 concentration) upon irradiation with the RPR 4190A lamps of the flat reactor containing 3 ␮l of acetone (2500 ppmv) where a piece of cotton tissue (3 cm × 5 cm) was placed. The upper pyrex window was first covered by a Schott GG420 filter, which was removed after 25 h irradiation.

3.2.2. Photocatalytic oxidation of di-n-butylsulfide (DBS) under visible light irradiation (420 nm maximum emission with 420 nm PMMA cut-off filter) Further experiments were carried out in order to assess the photo-activity of the coated cotton fabrics against reactants usually sensitive to singlet oxygen, such as DBS. For these experiments the tissues were placed in modified sealed Petri dishes where the reactants were introduced without any solvent through a septum. After 24 h irradiation with Philips TLD-K 30 W/03 lamps filtered with a sheet of polyester/acrylic sheet in an incubator at 20 ◦ C, 5 ml of acetonitrile were introduced into the Petri dishes and the extraction solutions were analysed by GC and GC/MS. The oxidation reaction gave rise to several products (di-n-butyl sulfoxide DBSO, di-n-butyl sulfone DBSO2 , di-n-butyl disulfide DBDS, n-butyl n-butane thiosulfonate DBSSO2 ), according to Eq. (1). The results are summarized in Table 4.

(1) With the 420 nm cut-off filter (<0.01 mW cm−2 UVA) and whatever the studied sample, CO2 evolution was very weak after 24 h irradiation. The evolution of C (CO2 concentration) with time relative to C0 (initial CO2 concentration i.e. atmospheric CO2 ∼ 400 ppmv) was not more than 1.4 with the most efficient TiO2 /AQ(−)@T. Relative to the initial acetone concentration (2800 ppmv), this means that less than 3% of acetone are mineralized. With the sample TiO2 /AQ(+)@T, no mineralization was observed and the fabrics was strongly colored in orange at the end of irradiation, probably due to AQ oxidation. When the filter was removed (0.13 mW cm−2 UVA), mineralization to CO2 was more efficient with all the samples: it reached 13% with TiO2 P25/@T and TiO2 P25/AQ(−)@T, 8% or less with TiO2 /AQ(−)@T and TiO2 @T. These results show that TiO2 P25, sensitized or not with anthraquinone, is the most efficient under UV light, whereas none of the TiO2 -coated cotton fabrics prepared with laboratorymade precursor mineralize acetone in significant amounts. Control GC-MS experiments under the same conditions with labeled acetone H3 C13 C(O)CH3 showed that 13 CO2 (m/z 45) was

Under these conditions with the untreated tissue or with the same samples kept in the dark, no sulfide oxidation occurred. All the treated tissues gave rise to oxidation, as di-n-butylsulfoxide (DBSO) and di-n-butylsulfone (DBSO2 ) were detected. The efficiency of TiO2 @T can be attributed to unfiltered UV wavelengths with this filter (0.4 mW cm−2 UVA). With AQ@T (6% AQ) and TiO2 P25/AQ(−)@T, the same results were obtained: only 30% of sulfide oxidation was observed in both cases, with major formation of sulfoxide. The best efficiency was obtained with TiO2 /AQ(−)@T as most of the sulfide was mainly converted to sulfone (64%) and sulfoxide (32%) within 24 h. Neither disulfide nor product arising from carbon–sulfur bond cleavage was detected in any of these experiments. The sample TiO2 /AQ(+)@T was not efficient and no oxidation product was detected. These results clearly indicated that singlet oxygen is more efficiently produced from TiO2 /AQ(−)@T than from the other tissues. They also evidenced synergetic effects between AQ and TiO2 when the tissue is prepared with the laboratory made sensitized TiO2 and not with the modified commercial P25 TiO2 . They also show that small amounts (8.3% in weight relative to TiO2 ) of AQCOOH are sufficient to promote this synergetic effect.

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of the wine stain was achieved within 24 h with the sensitized TiO2 -coated cotton fabrics TiO2 /AQ(−)@T. An interesting result was also obtained under indoor light illumination (without any UVA, Figure S8, SI), as the most significant discoloration of the wine stain was achieved within three weeks with TiO2 /AQ(−)@T. The discoloration was less obvious with TiO2 @T and TiO2 P25/AQ(−)@T. 4. Discussion

Fig. 11. Curve of CO2 evolution upon irradiation at 420 nm of gaseous acetone in a sealed reactor with (left) and without (right) the Schott GG420 filter.

These results were confirmed when letting the Petri dishes under indoor light for one week (Table 5): the TiO2 /AQ(−)@T sample was the only one able to achieve complete sulfide conversion to sulfoxide/sulfone. Several successive irradiations under visible light did not significantly modify the photo-oxidation efficiency of the fabrics, although we did not perform long-term stability experiments. This was not the case under UV irradiation, where AQ bleaching was observed. 3.3. Discoloration of wine stain under a solar simulator with a Schott 420 nm cut-off filter The self-cleaning properties of the prepared tissues were investigated by monitoring wine stains discoloration under solar simulator or in indoor light. Six drops of red wine were pipetted on the crude or coated cotton fabrics, irradiated either 24 h under a solar simulator or by natural indoor light of the laboratory for three weeks. Under solar illumination (Fig. 12), total discoloration

The goal of this study was to prepare cotton fabrics coated with sensitized TiO2 . Enhanced properties relative to the photocatalytic TiO2 and to sensitizing anthraquinone were expected [34–38], as anthraquinone-treated cotton fabrics previously showed a small but noticeable oxidation efficiency under irradiation at 420 nm [44]. Even if AQ does not present the most suitable properties as a sensitizer, it is commonly used in textile industry as a tank dying agent [33]. Starting with the published method for the synthesis of hybrid TiO2 nanostructured materials [32], we prepared TiO2 sensitized with anthraquinone-2-carboxylic acid (AQ-COOH) in the form of powders (P-TiO2 /AQ(−) and P-TiO2 /AQ(+)) and tissues (TiO2 /AQ(−)@T and TiO2 /AQ(+)@T) with different AQ/TiO2 ratio, and compared their properties with those of materials obtained by adsorbing AQ-COOH on commercial TiO2 P25 (P-TiO2 P25/AQ(−) and TiO2 P25/AQ(−)@T). It is worth noting that this one-pot synthesis of sensitized TiO2 is carried out in water as the sole solvent and does not need any extensive synthetic steps. The preparation of these samples relies on the synthesis of the laboratory made precursor [Ti(OR)3 (O2 C-AQ)] containing AQ-COOH. This precursor was characterized by IRTF, CPMAS NMR and XPS as a complex between the carboxylic moiety of AQ-COOH and Ti(OR)4 used as titanium source. All the coated tissues contained TiO2 well dispersed around the fibers, and these coatings appeared to be resistant to washing in water (no modification of the DRUV spectrum and of the photocatalytic properties of the fabrics after one run of machine washing at 45 ◦ C with a washing powder containing no fluorescent whiting dye). The introduced [Ti(OR)3 (O2 C-AQ)] modifies the

Fig. 12. Wine stain discoloration on the cotton fabrics under a solar simulator.

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electron donor DBS should be easily oxidized to DBS•+ (Eq. (5)). Oxidation of water to hydroxyl HO• radicals appears unfavourable under these conditions, as no evidence for significant mineraliza• tion was obtained. The reaction between DBS/1 O2 or DBS + /O2 •− would give rise to high yield of sulfoxide and sulfone (Eq. (6)). TiO2 /AQ−→TiO2 /AQ∗ [h+ , e− ]

(2)

hv

TiO2 /AQ∗ [e− ] + 3 O2 → TiO2 /AQ + O2 •− ∗

TiO2 /AQ + O2 ∗

•−

→ TiO2

/AQ• −

+

TiO2 /AQ [h ] + DBS → TiO2

+ O2

(4)

/AQ + DBS• +

(5)

DBS+1 O2 → DBS(O) + DBS(O)2 DBS• + + O2 •− Fig. 13. Proposed mechanical scheme for the synergy effect observed in singlet production by sensitized TiO2 /AQ(−)@T (Visible light is absorbed by the AQ@TiO2 complex).

DRUV, FTIR, and XPS characteristics of the tissues TiO2 /AQ(−)@T relative to TiO2 P25/AQ(−)@T, prepared by simply adding AQ-COOH to TiO2 P25, while keeping the crystallographic anatase phase of TiO2 . The results of FTIR and TGA demonstrate the formation of a tightly bound complex between TiO2 and the carboxylate moiety of AQ. This bond induces a shift of the electronic absorption spectra of the samples (powders P-TiO2 /AQ(−) and tissues TiO2 /AQ(−)@T) towards the visible range (3.14 eV), less significant for the samples P-TiO2 P25/AQ(−) and TiO2 P25/AQ(−)@T (3.25 eV). When irradiated under filtered visible light (totally free of UVA), neither of these samples exhibited a significant mineralization efficiency against acetone (less than 3%). This efficiency slightly increased with unfiltered visible light containing a weak part of UVA (0.13 mW cm−2 ), but remained under 13% after 24 h irradiation. It may be noticed that when using fluorescent lamps with an emission maximum at 350 nm, total acetone mineralization was achieved within 1 h with TiO2 /AQ(−)@T. GC–MS experiments with labeled acetone demonstrated that the evolved carbon dioxide originated from acetone and not from the tissue or from the decomposition of anthraquinone. These results indicate that UVA are necessary to activate the TiO2 part of the complex, leading to acetone mineralization. This reaction, most often proposed as resulting from hydroxyl radicals (HO• ) formation [55], is almost inefficient under visible light with AQ sensitized TiO2 , whatever the AQ/TiO2 ratio and the mode of incorporation of AQ. On the other hand, the TiO2 /AQ(−)@T tissues were very efficient for di-n-butyl sulfide photooxidation under visible light, mainly leading to sulfoxide and sulfone mixtures, whereas all the other samples were much less efficient under similar conditions. The same results were obtained under indoor light. These results demonstrate efficient singlet oxygen production by TiO2 /AQ(−)@T prepared from the laboratory-made precursor [Ti(OR)3 (O2 C-AQ)]. Moreover a synergy effect is noticed with sensitized TiO2 relative to AQ alone. This result is to be compared with recent papers on singlet oxygen formation from doped TiO2 [56–60]. The scheme of the possible reaction path for explaining these results is given in Fig. 13. Tentatively, absorption of a photon in the visible range by the TiO2 /AQ complex should lead to an (e− , h+ ) pair located on the HOMO and LUMO of the AQ moiety (Eq. (2)). The energetic level of the promoted electron being close to the conduction band of TiO2 , charge transfer to TiO2 should be possible. Either the TiO2 conduction band electron or the electron promoted in AQ LUMO is able to reduce ground state 3 O2 to the superoxide radical anion O2 •− (Eq. (3)). This superoxide radical-anion could in turn be oxidized to singlet oxygen by the strongly oxidizing AQ* (Eq. (4)), while strongly

(3)

1

(6)

The synergy effect observed for TiO2 /AQ(−)@T could result from the enhancement of production of superoxide radical-anion, and to its possible easy oxidation to singlet oxygen. It may be recalled that direct singlet oxygen production by Type II mechanism is also known with AQ [34,38]. As it is also well known that superoxide radical can undergoes disproportion reactions to produce small quantities of H2 O2 and HO• radicals (Eqs. (7)–(9)), the effect of this two ROS can nevertheless not be neglected to account for our results. 2O2 •− + H+ → H2 O2 + O2

(7)

H2 O2 + e− → HO• + HO− H2 O2 + O2

•−

→

HO•

(8) −

+ HO + O2

(9)

Bleaching of red wine stains could also be due by the same photosensitized mechanism and/or wine may behave as a photosensitizer injecting electrons to TiO2 conduction band and producing ROS able to destroy the wine components. It is not possible to affirm that 100% of wine stain destruction is only produced by a photocatalytic mechanism, as some moderate bleaching is also observed with un-sensitized TiO2 coated fabrics. 5. Conclusion This study describes a simple and reproducible one-pot process for the elaboration of cotton fabrics coated with TiO2 . The modification of titanium isopropoxide with an anthraquinone derivative as a sensitizer leads to a precursor [Ti(OR)3 (O2 C-AQ)], characterized by FTIR, CPMAS and XPS. Bonding between the organic molecule and titanium atom occurs via a carboxylate complex. Hydrolysis of mixtures of [Ti(OR)3 (O2 C-AQ)] and Ti(OR)4 at low temperature in an aqueous medium leads to pale yellow cotton fabrics together with the corresponding free TiO2 powders. The diffuse reflectance UV spectra confirmed the shift of absorption towards the visible range. From FTIR, CPMAS NMR and TGA analysis of the samples (cotton pieces and powders), it was shown that AQ-COOH was not only adsorbed on Titania but tightly bond through a carboxylate complex as in the molecular precursor. XPS data indicated that TiO2 was grafted on the surface, but AQ-COH was hardly detectable except for the photo-chemically inactive P-TiO2 /AQ(+) with large amount of AQ-COOH (63%). Anatase polymorph of TiO2 was always characterized by XRD even in the absence of a calcination step. Examination by SEM of cotton tissues before and after washing showed stable and homogeneous coating of TiO2 particles on the cotton fibers. Under UV light, acetone mineralization was observed, while under carefully filtered visible light, no acetone mineralization occurred. However efficient singlet oxygen addition to di-nbutyl sulfide was evidenced under visible light. Sulfoxide and sulfone were obtained in better yields using sensitized TiO2 (TiO2 /AQ(−)@T) than using un-modified TiO2 @T or AQ@T alone.

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Optimum results were obtained with low level of sensitizing AQCOOH relative to TiO2 (8%) and no reactivity improvement was noticed with higher AQ-COOH levels. The cotton pieces coated with sensitized TiO2 also displayed self-cleaning properties towards wine stain, either under solar illumination or even in indoor light. The better efficiency of sensitized TiO2 -coated cotton is accounted for by a synergy effect between TiO2 and AQ-COOH, enhancing the formation of Reactive Oxygen Species or ROS (singlet oxygen production and/or superoxide radical-anion). However, under these conditions, the production of hydroxyl radical seems to be ruled out. Further studies devoted to the bactericidal effects of this coated cotton pieces are currently being undertaken, together with their long-term stability under irradiation. The direct characterization of ROS at the gas-solid interface would also help understanding these complex oxidation mechanisms and will be developed in a near future. Acknowledgments The authors acknowledge FEDER and the Conseil Général des Pyrénées Atlantiques for funding this work (research grant), Virginie Pellerin for her kind recording of the SEM pictures and Abdel Khoukh for the CPMAS NMR spectra. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcatb.2011.03.005. References [1] B. Mahltig, H. Haufe, H. Bottcher, J. Mater. Chem. 15 (2005) 4385–4398. [2] W.A. Daoud, S.K. Leung, W.S. Tung, J.H. Xin, K. Cheuk, K. Qi, Chem. Mater. 20 (2008) 1242–1244. [3] K. Qi, W.A. Daoud, J.H. Xin, C.L. Mak, W. Tang, W.P. Cheung, J. Mater. Chem. 16 (2006) 4567–4574. [4] K. Qi, X. Chen, Y. Liu, J.H. Xin, C.L. Mak, W.A. Daoud, J. Mater. Chem. 17 (2007) 3504–3508. [5] W.S. Tung, W.A. Daoud, S.K. Leung, J. Colloid Interface Sci. 339 (2009) 424–433. [6] W.S. Tung, W.A. Daoud, J. Colloid Interface Sci. 326 (2008) 283–288. [7] M.J. Uddin, F. Cesano, D. Scarano, F. Bonino, G. Agostini, G. Spoto, S. Bordiga, A. Zecchina, J. Photochem. Photobiol. A: Chem. 199 (2008) 64–72. [8] M.J. Uddin, F. Cesano, F. Bonino, S. Bordiga, G. Spoto, D. Scarano, A. Zecchina, J. Photochem. Photobiol. A: Chem. 189 (2007) 286–294. [9] D. Wu, M. Long, J. Zhou, W. Cai, X. Zhu, C. Chen, Y. Wu, Surf. Coat. Technol. 203 (2009) 3728–3733. [10] M.I. Mejia, J.M. Marin, G. Restrepo, C. Pulgarin, E. Mielczarski, J. Mielczarski, Y. Arroyo, J.-. Lavanchy, J. Kiwi, Appl. Catal. B: Environ. 91 (2009) 481–488. [11] T. Yuranova, R. Mosteo, J. Bandara, D. Laub, J. Kiwi, J. Mol. Catal. A: Chem. 244 (2006) 160–167. [12] A. Bozzi, T. Yuranova, I. Guasaquillo, D. Laub, J. Kiwi, J. Photochem. Photobiol. A: Chem. 174 (2005) 156–164. [13] A. Bozzi, T. Yuranova, J. Kiwi, J. Photochem. Photobiol. A: Chem. 172 (2005) 27–34. [14] K.T. Meilert, D. Laub, J. Kiwi, J. Mol. Catal. A: Chem. 237 (2005) 101–108. [15] N. Onar, M.F. Ebeoglugil, I. Kayatekin, E. Celik, J. Appl. Polym. Sci. 106 (2007) 514–525. [16] J. Grzechulska-Damszel, A.W. Morawski, Catal. Lett. 127 (2009) 222–225.

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