TiO2 Nanosols Applied Directly on Textiles Using Different ... - MDPI

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TiO2 Nanosols Applied Directly on Textiles Using Different Purification Treatments Simona Ortelli, Anna Luisa Costa * and Michele Dondi Received: 16 September 2015; Accepted: 16 November 2015; Published: 24 November 2015 Academic Editor: Fernão Magalhães Institute of Science and Technology for Ceramics—Italian National Research Council, Via Granarolo 64, Faenza (RA) I-48018, Italy; [email protected] (S.O.); [email protected] (M.D.) * Correspondence: [email protected]; Tel.: +39-546-699718; Fax: +39-546-699719

Abstract: Self-cleaning applications using TiO2 coatings on various supporting media have been attracting increasing interest in recent years. This work discusses the issue of self-cleaning textile production on an industrial scale. A method for producing self-cleaning textiles starting from a commercial colloidal nanosuspension (nanosol) of TiO2 is described. Three different treatments were developed for purifying and neutralizing the commercial TiO2 nanosol: washing by ultrafiltration; purifying with an anion exchange resin; and neutralizing in an aqueous solution of ammonium bicarbonate. The different purified TiO2 nanosols were characterized in terms of particle size distribution (using dynamic light scattering), electrical conductivity, and ζ potential (using electrophoretic light scattering). The TiO2 -coated textiles’ functional properties were judged on their photodegradation of rhodamine B (RhB), used as a stain model. The photocatalytic performance of the differently treated TiO2 -coated textiles was compared, revealing the advantages of purification with an anion exchange resin. The study demonstrated the feasibility of applying commercial TiO2 nanosol directly on textile surfaces, overcoming problems of existing methods that limit the industrial scalability of the process. Keywords: nano-TiO2 ; purification process; anion exchange resin; photocatalytic performance

1. Introduction Textile finishing involving the application of inorganic colloidal nanosuspensions (nanosols) can give rise to new fabrics in which the properties of inorganic nanoparticles are transferred to the textiles’ surface [1]. Inorganic nanostructured coatings have a good affinity for fabrics and extend the durability of their function by comparison with conventional methods used to impart various properties to textiles. Their tiny particle size (nanoparticles) also makes them transparent to visible light so their presence does not alter the fabric’s color, “hand” and “breathability”. Textile finishes obtained by applying nanosols can be used to transfer several functional properties to a fabric. For example, Shi, et al. [2] prepared a water-repellent (hydrophobic) coating for cotton textiles from fluorinated diblock copolymers. Xu, et al. [3] used reactive magnetron sputtering to deposit nanoscale TiO2 films on the surface of polyester (PET) nonwovens to obtain antistatic materials. Chattopadhyay and Patel [4] reported manufacturing cotton textiles coated with nano-zinc for its antimicrobial properties. Bozzi, et al. [5] demonstrated that red wine and coffee stains were photo-discolored and mineralized on textiles coated with nano-TiO2 for its self-cleaning properties. In fact, nanostructured TiO2 anatase-based coatings have excellent photocatalytic properties, and are widely used in textile stain removal processes under UV irradiation [6,7]. A straightforward and economical way to make textiles with photocatalytic properties deriving from the application of nanosols (a process called “ceramization”, involving a ceramic nanosol such as TiO2 ) is the dip-pad-dry-cure method [8–11]. In view of a technological transfer to an industrial-scale production of self-cleaning textiles, the Materials 2015, 8, 7988–7996; doi:10.3390/ma8115437

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Materials 2015, 8, 7988–7996

method used to apply a commercial TiO2 nanosol to textiles needs to be simple. Such a method is described in the present work: it focuses on purifying and neutralizing the commercial TiO2 nanosol in order to expand its applicability and improve the photocatalytic performance of the end product. Three different treatments were developed: washing by ultrafiltration, purifying with an anion exchange resin; and neutralizing in an aqueous solution of ammonium bicarbonate. A fundamental aspect to consider is the possibility of changes to physicochemical properties such as pH, surface charge and conductivity as a result of these treatments. This could lead to agglomeration, aggregation or coagulation problems in nanosuspensions, so it is essential to avoid any colloidal destabilization [12,13]. The traditional ultrafiltration method [14,15], already used in our previous works [16,17], was compared here with more innovative approaches involving purification with an anion exchange resin and neutralization after depositing the nano-TiO2 coating. Purified and neutralized samples of TiO2 nanosol were applied directly on the textile using the dip-pad-dry-cure method. The photo-discoloration of rhodamine B (RhB), used as a stain model, was assessed on untreated and treated textiles and the photocatalytic performance of the differently-treated TiO2 coatings on the textile were compared. 2. Experimental 2.1. Materials TiO2 nanosol (NAMA41, 6 wt %), called TAC, was purchased from Colorobbia (Sovigliana, Vinci (FI), Italy). The commercial nanosol was diluted with deionized water to 3 wt %. A soft furnishing fabric was used in this study with a specific weight of 360 g/m2 and a composition of 62% cotton and 38% polyester. The ammonium bicarbonate (purity ě99.0%), rhodamine B (dye content „95%) target dye, and Dowexr 66 anion exchange resin were purchased from Sigma Aldrich (Milano, Italy). 2.2. Methods The commercial TiO2 nanosol (TAC) could not be used as purchased because of its very low pH and very high conductivity (Table 1). The purification treatments were absolutely necessary for two main reasons: (1) the textile substrate is damaged if the acidity falls below pH 3.5 due to acid-catalyzed oxidation phenomena occurring at high curing temperatures; and (2) any residual byproducts of synthesis in the commercial TiO2 nanosol could significantly reduce its photocatalytic activity. The three different treatments applied to the TAC nanosol were: 1. washing by ultrafiltration (TACF); 2. purification with an anion exchange resin (TACR); 3. neutralization of the TAC-coated textile (TACBIC). They are described in detail below. Table 1. Physicochemical characteristics of TiO2 nanosol samples. Sample

Nominal pH

pH *

D50DLS (nm)

Electrical Conductivity (mS/cm)

pHi.e.p.

TAC TACF TACR TACBIC

1.5 4.0 4.5 –

2.9 3.3 4.2 5.0 **

36 42 94 –

1.18 0.25 0.05 –

7.09 6.92 6.91 –

* pH measurement of nanosol (0.1 wt % TiO2 concentration); ** pH measurement onto textile surface.

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2.2.1. Washing by Ultrafiltration (TACF) Ultrafiltration was carried out using a solvent-resistant stirred cell (Merck Millipore, Vimodrone (MI), Italy) and a polymer membrane with a pore size of 100 kDalton that enabled the TiO2 nanoparticles to be retained, thereby increasing the pH while the byproducts of synthesis were removed. Materials The vessel was refilled with water several times until the pH was 4.0. The ultrafiltered 2015, 8, page–page sample (TACF) was so obtained. Ultrafiltration was carried out using a solvent-resistant stirred cell (Merck Millipore, Vimodrone (MI),an Italy) and aExchange polymer membrane with a pore size of 100 kDalton that enabled the 2.2.2. Purification with Anion Resin (TACR) TiO2 nanoparticles to be retained, thereby increasing the pH while the byproducts of synthesis were

This process a weak anion exchange resinthetopH the TiO The resin was removed. involved The vessel adding was refilled with water several times until was 4.0. The ultrafiltered 2 nanosol. sample (TACF) was and so obtained. able to sequester Cl´ ions release OH´ ions with a consequent increase in pH. Once the required pH value had been reached, the resin was removed by simple separation. The pH obtained in the 2.2.2. Purification with an Anion Exchange Resin (TACR) purified sample (TACR) was 4.5. This process involved adding a weak anion exchange resin to the TiO2 nanosol. The resin was able to sequester Cl− ions and release OH− ions with a consequent increase in pH. Once the required 2.2.3. Neutralization of the TAC-Coated Textile (TACBIC) pH value had been reached, the resin was removed by simple separation. The pH obtained in the sample (TACR) was 4.5. directly on the TAC-coated textile before curing (TACBIC). This purified treatment was performed

The process in of applying the commercial TiO2 nanosol (TAC) on the textile using the 2.2.3.consisted Neutralization the TAC-Coated Textile (TACBIC) dip-pad-dry-cure method. Then, an aqueous solution (0.5 M) of ammonium bicarbonate (NH4 HCO3 ) This treatment was performed directly on the TAC-coated textile before curing (TACBIC). The was deposited the TAC-coated using a manual spray-coating technique to neutralize the processonconsisted in applyingtextile the commercial TiO2 nanosol (TAC) on the textile using the acidity of the commercial method. TiO2 nanosol. dip-pad-dry-cure Then, an aqueous solution (0.5 M) of ammonium bicarbonate (NH4HCO3) was deposited on the TAC-coated textile using a manual spray-coating technique to neutralize the acidity of the commercial TiO2 nanosol. 2.3. Dip-Pad-Dry-Cure Method Dip-Pad-Dry-Cure Method in an ultrasound bath for 30 min (15 min with soap and water, Fabric2.3.samples were washed and 15 min with water alone). The fabric thus dipped in and the water, titania nanosol Fabric samples were washed in ansamples ultrasound bathprepared for 30 min were (15 min with soap and left 15 min with water The fabric samples thus prepared were dipped in the titania nanosol (3 wt %) and to soak for 3alone). min, then passed through a two-roller laboratory padder, oven dried at (3 wt for %) and left toat soak 3 min, passed through in a two-roller padder, oven 100 ˝ C, cured 10 min 130for˝ C, andthen finally washed water inlaboratory an ultrasound bathdried for 15 min to at 100 °C, cured for 10 min at 130 °C, and finally washed in water in an ultrasound bath for 15 min remove any nanoparticles not physicochemically adsorbed onto the surface. This dip-pad-dry-cure to remove any nanoparticles not physicochemically adsorbed onto the surface. This dip-pad-dry-cure method is method illustrated in Figure 1. 1. is illustrated in Figure

1. Schematic representation of method. FigureFigure 1. Schematic representation ofthe thedip-pad-dry-cure dip-pad-dry-cure method.

2.4. Characterization of TiO2 Nanosols

2.4. Characterization of TiO2 Nanosols

The phase composition of the commercial TiO2 was ascertained by X-ray diffraction (XRD).

The diffraction patterns were obtained directly on the2TiO 2-based nanosols (TAC, TACF and TACR) The phase composition of the commercial TiO was ascertained by X-ray diffraction (XRD). using a Bragg-Brentano diffractometer (Bruker D8 Advance, Karlsruhe, Germany) operating in a The diffraction patterns were obtained directly on the TiO2 -based nanosols (TAC, TACF and TACR) θ/2θ configuration, with an X-Celeretor detector LynkEye (20°–70°, 2θ range, 0.02 step size, 0.5 s using a Bragg-Brentano diffractometer (Bruker D8 Advance, Germany) in a per step). The particle size distribution of the nanosols (0.01 wt %)Karlsruhe, was measured by dynamicoperating light ˝ –70˝ , 2θ range, 0.02 step size, 0.5 s θ/2θ configuration, with an X-Celeretor detector LynkEye (20 scattering (DLS) through a Zetasizer Nanoseries (Malvern Instruments, Malvern, UK). This provides hydrodynamic of suspended expressed as D50, by which is per step). technique The particle sizethedistribution ofdiameter the nanosols (0.01particles, wt %) was measured dynamic light the value of the particle diameter at 50% of the cumulative distribution. The electrical conductivity scattering (DLS) through a Zetasizer Nanoseries (Malvern Instruments, Malvern, UK). This technique of the three nanosols (0.1 wt %) was measured with a conductometer (AMEL 134, AMEL, Milano, provides the hydrodynamic diameter of suspended particles, expressed as D50, which is the value of Italy). Their ζ potential was examined using electrophoretic light scattering (ELS) (Zetasizer the particle diameter at 50%Instruments, of the cumulative distribution. Theused electrical conductivity of the three Nanoseries—Malvern Malvern, UK). The instrument has an automatic titrating nanosols (0.1 wt %) was measured with a conductometer (AMEL 134, AMEL, Milano, Italy). Their ζ 3 potential was examined using electrophoretic light scattering (ELS) (Zetasizer Nanoseries—Malvern

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Instruments, Malvern, UK). The instrument used has an automatic titrating system for measuring the ζ potential of nanosols as a function of their pH (experimental uncertainty: 1 mV for ζ potential and 0.2 for pH). The measurements were performed on sols at low concentrations (0.1 wt %) to prevent precipitation due to pH changes. The titration was done by adding 0.01 M KOH solution. Three Materials 2015, 8, page–page measurements were obtained for each sample and the average ζ potential values were considered. system for measuring the ζ potential of nanosols as a function of their pH (experimental

2.5. Textile Characterization uncertainty: 1 mV for ζ potential and 0.2 for pH). The measurements were performed on sols at low concentrations (0.1 wt %) to prevent precipitation due to pH changes. The titration was done by

Theadding presence thesolution. coatingThree andmeasurements the amountwere of obtained nano-titania absorbed bythe the fabrics was 0.01 Mof KOH for each sample and average established from values the weight difference using a burn-out technique: 0.5 g of sample was burnt at ζ potential were considered. 800 ˝ C and the residual titania was expressed as a w/w percentage of the TiO2 -coated fabric. 2.5. Textile Characterization

2.6. Photocatalytic Measurements The presence of the coating and the amount of nano-titania absorbed by the fabrics was from the weight difference using a burn-out technique: 0.5 g of sample was burnt at Theestablished pristine fabric sample and titania-coated samples were stained with 0.2 mL droplets of an 800 °C and the residual titania was expressed as a w/w percentage of the TiO2-coated fabric. aqueous solution of rhodamine B (0.07 g/L), chosen as a stain model. After staining, the samples were irradiated with UV at an intensity of 9 W/cm2 (Osram 2.6. Photocatalytic Measurements ULTRA-Vitalux lamp, Munich, Germany). The lamp was switched on 30 min before starting the The pristine fabric sample and titania-coated samples were stained with 0.2 mL droplets of an photocatalytic test to stabilize the power of itschosen emission spectrum. aqueous solution of rhodamine B (0.07 g/L), as a stain model. The distance between the sample and the lamp was kept constant at 25 cm. To assess the of discoloration, the samples After staining, the samples were irradiated with UV atdegree an intensity of 9 W/cm2 (Osram ULTRA-Vitalux lamp, Munich, Germany). The lamp was switched on 30 min before starting the underwent colorimetric measurements before and after UV exposure. All colorimetric measurements photocatalytic test to stabilize the power of its emission spectrum. The distance between the sample were performed with a DRS (Miniscan MSXP4000, Hunter Lab, Reston, VA, USA) in the 400–700 nm and the lamp was kept constant at 25 cm. To assess the degree of discoloration, the samples range (illuminant D65, observer 10˝ ). Color was expressed using CIELab parameters: brightness underwent colorimetric measurements before and after UV exposure. All colorimetric (L*: 100measurements = white 0 =were black) and chroma (a*: (Miniscan red +, green ´; b*:Hunter yellow blue ´). Photocatalytic performed with a DRS MSXP4000, Lab,+,Reston, VA, USA) in performance was judged by assessing the amount of discoloration, expressed in terms of efficiency the 400–700 nm range (illuminant D65, observer 10°). Color was expressed using CIELab (L*:difference 100 = white (∆E*) 0 = black) and chroma red +, green −; b*: yellow blue −). (%). Inparameters: particular,brightness the color between the (a*: samples before and after+, exposure was Photocatalytic calculated as follows:performance was judged by assessing the amount of discoloration, expressed in terms of efficiency (%). In particular, the color between ˚ 2 1/2the samples before and after ˚ 2 difference ∆E˚ “ rp∆L q ` p∆a˚ q2 (∆E*) ` p∆b q s (1) exposure was calculated as follows:

and referred to the pristine sample by the background color of the fabric. (1) Our color 2 + (Δb*) 2]1/2 ΔE* subtracting = [(ΔL*)2 + (Δa*) difference (∆E*%) assessment method is explained in detail in our previous studies [17,18], and and referred to the pristine sample by subtracting the background color of the fabric. Our color correlated with photodegradation efficiency. difference (ΔE*%) assessment method is explained in detail in our previous studies [17,18], and correlated with photodegradation efficiency.

3. Results and Discussion

3. Results and Discussion

3.1. Characterization of TiO2 Nanosols 3.1. Characterization of TiO2 Nanosols

Figure 2 shows the XRD diffractograms of the different samples. The results confirmed that 2 shows the XRD diffractograms of the different confirmed that pH pH changesFigure induced by the purification treatments did notsamples. affect The the results crystalline phases. The main changes induced by the purification treatments did not affect the crystalline phases. The main phase detected was anatase (JCPDS card n. 21-1272) with a small amount of brookite (JCPDS card phase detected was anatase (JCPDS card n. 21-1272) with a small amount of brookite (JCPDS card n. 29-1360). The broad peaks typical ofofnano-sized crystallites were detected all samples. n. 29-1360). The broad peaks typical nano-sized crystallites were detected for allfor samples.

Figure 2. XRD diffractograms of TAC (light gray), TACF (medium gray) and TACR (black);

Figure 2. XRD diffractograms of TAC (light gray), TACF (medium gray) and TACR (A = anatase; B = brookite). (black); (A = anatase; B = brookite). 4

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The three different treatments on the commercial TiO2 nanosol (TAC) enabled an increase in Theparticle three different treatments commercial TiO2 1). nanosol (TAC) enabled an increase in pH pH and and a decreaseoninthe conductivity (Table Materials 2015, 8,size, page–page and particle size, and a decrease in conductivity (Table 1). The DLS data showed an increase in particle diameter in the TACF and TACR samples, caused three treatments on TiO 2 in nanosol (TAC)and enabled ansamples, increase in The The DLS datadifferent showed an increase inthe particle diameter theinTACF TACR caused by a greater degree of agglomeration, and ancommercial associated increase pH. and 3 particle and a decreasevs. in conductivity (Table 1). different by a pH greater degree ofthe agglomeration, and associated increase in pH.purified TiO2 nanosols, from Figure showssize, ζ potential theanpH for the three datathe showed an increase in diameter in the TACF to and TACR TiO samples, caused Figure 3DLS shows ζ pH potential vs. crossover theparticle pH for the three different purified nanosols, from which weThe can identify the i.e.p. at the point from positive negative ζ 2potential values. by a greater degree of agglomeration, and an associated increase in pH. which we can identify pHsimilar, pointinfrom positive to negative ζ potential values. The three curves were the quite butcrossover a slight shift pHi.e.p. towards an acid pH from the TAC i.e.p. at the Figure 3 shows the ζ potential vs. the pH for the three different purified TiO2 nanosols, from The three curves were quite similar, but a slight shift in pH towards an acid pH from the TAC sample to the TACR sample was apparent, i.e., the baseline in i.e.p. pH increased. These data are which we can identify the pHi.e.p. at the crossover point from positive to negative ζ potential values. sample to the TACR sample was apparent, i.e., the baseline pHsurface increased. These data are in agreement agreement with previous reports [16] and indicate a higher acidity due to a higher pH and The three curves were quite similar, but a slight shift in pHi.e.p. towards an acid pH from the TAC with sample previous [16]sample a higher surface aciditypH due to a higher pHdata andare increased increased agglomeration, asand we indicate can from the diameters given in Table 1. in On the to reports the TACR wassee apparent, i.e.,hydrodynamic the baseline increased. These agglomeration, as we can see from the hydrodynamic diameters given in Table 1. On the grounds of grounds of ourwith previous works [16,18], an increase surface acidity wasdue expected to coincide agreement previous reports [16] and indicate ainhigher surface acidity to a higher pH and with ourincrease previous [16,18], an infrom surface acidity was expected to with coincide with anOn increase in increasedinworks agglomeration, as increase we can see hydrodynamic diameters giventhe in Table 1. the data an surface hydrophilicity. The pHthevalues were consistent conductivity grounds of our [16,18], an in surface acidity was expected to(Table coincide with surface hydrophilicity. The works pH values were consistent with the conductivity data The high (Table 1). The highprevious conductivity value forincrease the TAC sample was associated with the 1). presence of an increase in surface hydrophilicity. The values were with of theresidual conductivity data(TAC) conductivity value for TAC sample waspH associated withconsistent the presence byproducts of residual byproducts of the synthesis. The successful purification of the commercial TiO2 nanosol (Table 1). The high conductivity value for the TAC sample was associated with the presence of synthesis. The successful of the commercial TiO2 nanosol (TAC)and wasTACR demonstrated the was demonstrated by thepurification higher pH and lower conductivity of the TACF samples. by There residual byproducts of synthesis.ofThe successful purification of the commercial TiO 2 nanosol (TAC) higher pH and lower conductivity the TACF and TACR samples. There was a good correspondence was a good correspondence between the shift in pHi.e.p. towards a more acid pH and the drop in was demonstrated by the higher pH and lower conductivity of the TACF and TACR samples. There between the shift pHi.e.p. of towards more acid pH and4.the in conductivity a function of pH, conductivity as a in function pH, asa shown in Figure In drop particular, the TACRassample showed a was a good correspondence between the shift in pHi.e.p. towards a more acid pH and the drop in as shown in Figure 4. In particular, the TACR sample showed a very low conductivity, demonstrating very low conductivity, demonstrating that purification with an anion exchange resin was more conductivity as a function of pH, as shown in Figure 4. In particular, the TACR sample showed a that very purification withbyproducts. an anion exchange resin more inexchange removing byproducts. The efficient inlow removing The increase inwas pH achieved the ultrafiltration process conductivity, demonstrating that purification withefficient anwith anion resin was morenever increase in4.0, pH achieved with the ultrafiltration process never exceeded 4.0, whereaslimit. purification exceeded purification with an anion resin has maximum Atnever pH >with 4.5, efficient inwhereas removing byproducts. The increase inexchange pH achieved with thenoultrafiltration process an anion exchange resin has no maximum At pH > resin 4.5, however, the TiOlimit. nanosol underwent exceeded purification withcolloidal an limit. aniondestabilization. exchange has no maximum At pH > 4.5, however, the 4.0, TiOwhereas 2 nanosol underwent 2 however, the TiO2 nanosol underwent colloidal destabilization. colloidal destabilization.

Figure 3. ζ potential vs. pH of TAC, TACF and TACR samples.

Figure 3. ζ potential vs. pH of TAC, TACF and TACR samples. Figure 3. ζ potential vs. pH of TAC, TACF and TACR samples.

Figure 4. Influence of pH on pHi.e.p. and electrical conductivity.

Figure 4. 4. Influence Influence of of pH pH on on pH pHi.e.p. and electrical conductivity. Figure i.e.p. and electrical conductivity. 5

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The anion exchange Materials 2015, 8,resin page–page

efficiently removed byproducts, and Cl´ anions in particular. The

exchange reaction is as follows: The anion resin exchange efficiently removed byproducts, and Cl− anions in particular. The R ´ NH3 OH ` Cl´ Ñ R ´ NH3 Cl ` OH´ (2) exchange reaction is as follows: R-NH3OH + Cl− → R-NH3Cl + OH− (2) It is easy to regenerate the anion exchange resin by washing it with a highly-concentrated NaOH It is easy to regenerate the anion exchange resin by washing it with a highly-concentrated solution [19,20], so purification processes based on ion resin exchange are economical and easily NaOH solution [19,20], so purification processes based on ion resin exchange are economical and scalable, as many papers focusing on large-scale ion exchange processes have demonstrated [21–25]. easily scalable, as many papers focusing on large-scale ion exchange processes have demonstrated [21–25].

3.2. Characterization of Textiles 3.2. Characterization of Textiles A homogeneous coating of nano-TiO2 was obtained, as confirmed by the burn-out test. A A homogeneous coating of nano-TiO2 was obtained, as confirmed by the burn-out test. A white white TiO2 powder agglomerate with an appearance perfectly mimicking the texture of the fabric’s TiO2 powder agglomerate with an appearance perfectly mimicking the texture of the fabric’s fibers fibers was achieved. The amount of TiO2 (about 3 wt %) is consistent with the tested capacity of was achieved. The amount of TiO2 (about 3 wt %) is consistent with the tested capacity of cotton-polyester fabrics toto adsorb ananamount In particular, particular,the theresidual residual cotton-polyester fabrics adsorb amountofofsol solequating equatingto totheir theirweight. weight. In TiOTiO burn-out was 3.0, 3.4, 3.5 and 3.0 wt % in the TAC, TACF, TACR and TACBIC, respectively. 2 after 2 after burn-out was 3.0, 3.4, 3.5 and 3.0 wt % in the TAC, TACF, TACR and TACBIC, respectively. SEM analysis (Figure 5)5)showed in surface surfacemorphology morphology induced presence SEM analysis (Figure showedthe the changes changes in induced by by thethe presence of of TiO nanoparticles confirming the formation of a homogeneous nano-TiO coating on the fabric’s TiO2 2 nanoparticles confirming the formation of a homogeneous nano-TiO2 2coating on the fabric’s surface. Unlike thethe smooth texture the fibers fibers in inthe theTACF-coated TACF-coated surface. Unlike smooth textureofofthe theuncoated uncoatedfiber fiber (Figure (Figure 4a), the fabric (Figure 4b)4b) showed a certain surface TiO22 adhering adheringtotothe the fabric (Figure showed a certain surfaceroughness roughnessdue dueto tothe the thin thin layer layer of TiO textile substrate. textile substrate.

Figure 5. SEM micrographs of: (a) an uncoated fabric fiber and (b) a fabric fiber coated with the Figure 5. SEM micrographs of: (a) an uncoated fabric fiber and (b) a fabric fiber coated with the TACF nanosol. TACF nanosol.

3.3. Photocatalytic Measurements 3.3. Photocatalytic Measurements Photocatalytic activity was assessed in terms of discoloration of a stain caused by an aqueous Photocatalytic activityBwas assessed in terms discoloration a stainwith caused by an aqueous solution of rhodamine on the pristine fabric of and the samplesofcoated differently-treated solution of rhodamine B on the results pristine and the samples coated with differently-treated nano-TiO 2. The photocatalytic arefabric expressed in terms of photochemical efficiency values, nano-TiO . The photocatalytic results are expressed in terms of photochemical efficiency values, calculated taking the uncoated fabric’s photocatalytic efficiency for reference (Figure 6). The various 2 calculated taking the uncoated fabric’s photocatalytic efficiency for activity reference (Figure 6). The various treatments induced an improvement in the fabric’s photocatalytic with the following trend: treatments induced an improvement in the fabric’s photocatalytic with theare following trend: TACR >TACF >TACBIC > TAC. The results of the photocatalyticactivity measurements summarized in Table 2. >TACBIC > TAC. The results of the photocatalytic measurements are summarized TACR >TACF in Table 2. Table 2. Photocatalytic results obtained on TiO2-coated fabric samples. Table 2. Photocatalytic results obtained on TiO2 -coated fabric samples.

Sample TAC Sample TACF TAC TACR TACF TACBIC TACR

Photocatalytic Efficiency (%) Increase in Photocatalytic Efficiency * 68.5 1.54 Photocatalytic Efficiency (%) Increase in Photocatalytic Efficiency * 84.3 1.90 68.5 1.54 92.5 2.08 84.3 1.90 73.2 92.5 2.08 1.65

TACBIC * vis-à-vis the uncoated 73.2 fabric sample (photocatalytic efficiency: 1.65 44.4%). vis-à-vis the uncoated fabric sample (photocatalytic efficiency:in44.4%). As expected, *the presence of residual byproducts of synthesis commercial TiO2 nanosol (TAC) gave rise to a low photocatalytic efficiency. Even after post-neutralization (which increased the pH on the surface of the TiO2-coated fabric), the photocatalytic performance of the TACBIC

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As expected, the presence of residual byproducts of synthesis in commercial TiO2 nanosol (TAC) gave rise to a low photocatalytic efficiency. Even after post-neutralization (which increased the pH Materials 2015, 8, page–page on the surface of the TiO2 -coated fabric), the photocatalytic performance of the TACBIC sample was only slightly than that of thethan TACthat sample. It was probably of residual byproducts sample was better only slightly better of the TAC sample.the It presence was probably the presence of synthesis in the sample that impaired its photocatalytic activity. Better results were obtained with residual byproducts synthesis in the sample that impaired its photocatalytic activity. Better results treatments applied to theapplied nanosol directly (the TACF samples). Despite strong samples). degree of were obtained withdirectly treatments to and the TACR nanosol (the TACF and aTACR agglomeration in the TACR sample, the purification treatment with an anion exchange resin produced Despite a strong degree of agglomeration in the TACR sample, the purification treatment with an the best performance. The main to the improvement in photocatalytic performance anion exchange resin produced thecontributor best performance. The main contributor to the improvement in was therefore the removal of residual byproducts of synthesis, as demonstrated the treated photocatalytic performance was therefore the removal of residual byproducts ofbysynthesis, as sample’s low conductivity andsample’s high pH,low indicative of a high acidity and a consequently high demonstrated by the treated conductivity andsurface high pH, indicative of a high surface hydrophilicity. The photocatalytic results revealed importanceresults of using a purified nanosol in acidity and a consequently high hydrophilicity. The the photocatalytic revealed the importance order to obtain a good end product performance. The neutralization treatment proved less effective of using a purified nanosol in order to obtain a good end product performance. The neutralization in improving photocatalytic performance than the purification treatment (to remove byproducts), as treatment proved less effective in improving photocatalytic performance than the purification treatment theremove weak photoreactivity TACBIC-coated (to byproducts), as of thethe weak photoreactivityfabrics of the demonstrates. TACBIC-coated fabrics demonstrates.

Figure 6. Increase in photocatalytic efficiency as a function of pH (▪) and electrical conductivity (●). Figure 6. Increase in photocatalytic efficiency as a function of pH (‚) and electrical conductivity ( ).

4. Conclusions 4. Conclusions The present work describes a method for applying commercial TiO2 nanosol directly on textiles The present work describes a method for applying commercial TiO2 nanosol directly on textiles with stabilized characteristics. Three different nanosol purification/neutralization treatments were with stabilized characteristics. Three different nanosol purification/neutralization treatments were tested and found fundamental to the success of the self-cleaning textile application. tested and found fundamental to the success of the self-cleaning textile application. The physicochemical properties of the differently-treated TiO2 nanosols showed that a low The physicochemical properties of the differently-treated TiO2 nanosols showed that a low conductivity and a high pH (corresponding to a high surface acidity) enabled fabrics with high conductivity and a high pH (corresponding to a high surface acidity) enabled fabrics with high hydrophilic properties and a good photocatalytic performance to be obtained. hydrophilic properties and a good photocatalytic performance to be obtained. Based on the evidence emerging from our photocatalytic experiments, purification to remove Based on the evidence emerging from our photocatalytic experiments, purification to remove byproducts was more effective than a neutralization treatment. In fact, a purification process byproducts was more effective than a neutralization treatment. In fact, a purification process involving the use of an anion exchange resin proved the most effective treatment. The easy involving the use of an anion exchange resin proved the most effective treatment. The easy scalability scalability of this process, and the opportunity to control the TiO2 nanosols’ physicochemical of this process, and the opportunity to control the TiO nanosols’ physicochemical properties properties (pH and conductivity) make this method very2 promising for the industrialization of (pH and conductivity) make this method very promising for the industrialization of self-cleaning self-cleaning textile applications. textile applications. Acknowledgments: for financial financial support. support. Acknowledgments: The The authors authors are are grateful grateful to to Novaresin Novaresin S.p.A S.p.A for Author Contributions: Dondi andand Simona Ortelli conceived and and designed the Contributions: Anna AnnaLuisa LuisaCosta, Costa,Michele Michele Dondi Simona Ortelli conceived designed the experiments; Simona Ortelli performed the experiments and analyzed the data; Simona Ortelli, experiments; Simona Ortelli performed the experiments and analyzed the data; Simona Ortelli, Anna Luisa Costa Anna Luisa Costa and Michele Dondi wrote the paper. and Michele Dondi wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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