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Nanofinishes for self-cleaning textiles 9.1 INTRODUCTION: DEFINITION AND HISTORICAL OVERVIEW Self-cleaning clothes with the ability to clean themselves are dreams come true thanks to great efforts made by researchers during the last decades. A path to achieve self-cleaning clothes was actually paved with the aid of living nature. As the first hints to achieve self-cleaning surfaces have been brought by mimicking nature, self-cleaning surfaces are also famous as bioinspired surfaces. In addition to legs of water strider and wings of butterflies, self-cleaning was mainly inspired by leaves of plants, namely, lotus, making it known as lotus effect. Neinhuis and Barthlott were the first who introduced the idea of lotus effect in 1997, arising from the micropapillae structures with the ability to trap a large amount of air, and low surface energy of epicuticular wax crystalloids coating the leaf surface (Sas et al., 2012). Their idea was further developed by Feng et al. (2002) who found out that the surface of leaves has branchlike nanostructures with 124 nm diameter, causing contact angles greater than 160 degrees. Scientific analysis of lotus leaf revealed that a water droplet on the leaf is almost spherical in shape and can roll off easily. This resulted from the combination of a hierarchical surface structure that traps air beneath a water droplet and the hydrophobicity of the surface wax. Moreover, the surface has low adhesion force and low coefficient of friction. Thus, the interfacial area between contamination particles and the surface is very small, resulting in reduced adhesion (Fig. 9.1). On such a surface, water droplets from rainfall can pick up the contamination particles and carry them away when the droplets roll off the surface, leading to self-cleaning properties also known as lotus effect. Both surface morphology and hydrophobic wax material are necessary to create such a superhydrophobic surface on lotus leaves. Moreover, superhydrophobic surfaces with contact angles of more than 150 degrees possess self-cleaning properties only if their contact angle hysteresis and sliding angle are low (Nishimoto and Bhush, 2013). Nanofinishing of Textile Materials https://doi.org/10.1016/B978-0-08-101214-7.00009-1
© 2018 Elsevier Ltd. All rights reserved.
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Water
Water
Leaf
Microstructured surface
Dirt
Fig. 9.1 Lotus effect.
Similar superhydrophobicity and self-cleaning properties have been found in plant leaves such as rice and taro leaves with hierarchical surface structures. Moreover, plant leaves with unitary surface structure such as Ramee leaf and Chinese watermelon indicate superhydrophobicity and self-cleaning properties (Sun et al., 2005). Inspired by lotus effect, researchers have developed self-cleaning textiles by combining low surface energy materials and creating surface roughness. The common low surface energy materials include organic silanes, fluorinated silanes, alkyl amines, and silicates. Popular surface modification methods include wet chemical reactions, self-assembly and sol-gel, layerby-layer (LBL) deposition, polymerization reactions, colloidal template techniques, chemical vapor deposition, plasma treatment, and electrospinning (Gupta and Gulrajani, 2015). Further to lotus effect, self-cleaning has been also achieved by the opposite surface wettability, namely, superhydrophilicity in which water completely covers the surface with a continuous film and washes away the contaminants. This is usually achieved via photocatalysts also benefiting from generating active radicals under light irradiation capable of decomposing organic contaminants (Wang et al., 2015). Semiconductor photocatalysts such as nano TiO2 and ZnO have been mainly used on various textile substrates in the form of nanocoatings or by ex situ or in situ synthetic methods to produce self-cleaning textiles. The treated textiles also benefit from multifunctional activities, including UV protection and antibacterial properties (Montazer and Maali Amiri, 2014). Moreover, there have been numerous efforts to improve the photocatalytic activities of semiconductors using different surface modification methods to produce semiconductors with enhanced activities under solar light irradiation (Zaleska, 2008). After a brief overview of different surface wetting theories, this chapter mainly deals with nanofinishes for producing self-cleaning textiles and we tried to provide the most recent case studies.
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9.2 SURFACE WETTABILITY Depending on surface smoothness or roughness, three theories have been proposed for surface wettability (Callies and Quere, 2005). 1. Young model, which is only valid for smooth surfaces and is based on the correlation between the three interfacial energies per unit area, which are in equilibrium at the droplet (Eq. 9.1) γ sv¼ γ sl + γ lv cos θ (9.1) where θ is contact angle, γ sv and γ sl, γ lv and are the interfacial energies per unit area of the solid-vapor, solid-liquid, and liquid-vapor interfaces, respectively. 2. Wenzel theory, which is based on complete wetting of a rough surface where the liquid penetrates into the grooves of the rough surface (Eq. 9.2). cos θa¼ r cos θs (9.2) where apparent contact angle θa is the true contact angle of the droplet on a plain surface θs multiplied by roughness factor (r), and r is the ratio between the actual rough surface area and the geometric surface area (r¼ Ageometric/ Areal). 3. Cassie and Baxter theory, which describes the nonwetted contact between the liquid and rough surface arising from vapor pockets trapped underneath the liquid in the grooves. Based on this theory when a liquid spreads over a rough porous surface, the solid-vapor interface converted into two new interfaces, solid-liquid and liquid-vapor interfaces (Eq. 9.3). cos θa¼ fs cos θs + fv cos θv
(9.3)
where fs and fv are the fractions of the solid and vapor on the surface. The theories are schematically shown in Fig. 9.2. g1v gsv
(A)
q
gs1
(B)
(C)
Fig. 9.2 (A) Young, (B) Wenzel, and (C) Cassie and Baxter theories for wetting behavior of smooth and roughened surfaces.
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9.3 NANOFINISHES FOR SELF-CLEANING TEXTILES 9.3.1 Self-cleaning textiles based on lotus effect Inspired by lotus effect, self-cleaning textiles could be prepared via two basic approaches (Stegmaier et al., 2008): 1. Introducing nanoscale roughness on textile surface using nanoparticles such as silver, rod arrays such as ZnO nanorods and carbon nanotubes, or by physical surface modification methods such as laser or plasma along with application of low surface energy materials to impart hydrophobicity (Liu et al., 2007; Xu and Cai, 2008; Dastjerdi et al., 2010). 2. Treatment of fabrics having micro- or nanostructure with low surface energy materials. For instance, silicone was coated on microfiber polyester fabric to produce superhydrophobic polyester with self-cleaning properties (Gao and McCarthy, 2006). Some of the recent studies concerning with these two approaches are as follows: Superhydrophobic self-cleaning cotton fabrics were prepared by hydrothermal synthesis of ZnO nanorods followed by coating with dodecyltrimethoxysilane to impart surface roughness and hydrophobicity. Cotton fabric treatment with silver nitrate in presence of potassium hydroxide following by surface hydrophobization with octyltriethoxysilane has been also reported (Xu and Cai, 2008). In situ synthesis of silver nanoparticles on cotton fabric along with treating the fabric with hexadecyltrimethoxysilane was also successful to provide superhydrophobic fabric with self-cleaning properties (Xue et al., 2012). Similar researches have been carried out using carbon nanotubes to create nanoscale roughness on various textile substrates (Liu et al., 2007). Silica nanoparticles have been extensively used to impart superhydrophobic properties into textiles. For instance, polydimethylsiloxane filled with fluorinated alkylsilane functionalized silica nanoparticles and fluorinated alkylsilane were successful to prepare superhydrophobic coating on fabrics (Zhou et al., 2012). Chemical vapor deposition and LBL techniques are among the widely applied methods to produce superhydrophobic textiles. Nanocoating of silicone was deposited on cotton fabric via chemical vapor deposition followed by hydroxylation and polymerization (Li et al., 2007). Polyelectrolyte/silica nanoparticle multilayers were deposited on cotton fabric using LBL method. The process was followed by fluoroalkylsilane treatment to impart superhydrophobicity (Zhao et al., 2010). Modified silica nanoparticles with epoxy functional groups were applied on
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cotton fabric to produce dual-size hierarchical surface structure following by treatment with stearic acid or 1H,1H,2H,2H-perfluorodecyltrichlorosilane or their combination to impart hydrophobicity (Xue et al., 2008). In all cases, durability of the treatment is important and the other involved challenge is the tendency of the treated surfaces to adsorb oily contaminants. These obstacles restricted the feasibility of these methods.
9.3.2 Photoinduced hydrophilicity and self-cleaning: Definition and mechanism As thoroughly discussed in Chapter 2, photocatalysis is based on the irradiation of semiconductors by light with energy equal or greater than the band gap, during which electrons are excited from the valence band to the conduction band and then migrate to the semiconductor surface to produce radicals for contaminants degradation. Accidental finding of a research group in TOTO laboratory revealed that TiO2 films possess superhydrophylic properties under UV light illumination. This phenomenon, which is called photoinduced hydrophilicity, is reversible and the surface becomes hydrophobic in the absence of light. This effect, which was found in 1995, was a pioneering step to achieve self-cleaning surfaces and was first applied to develop selfcleaning glass. Since the introduction of this effect, three different mechanisms have been proposed to support the photoinduced hydrophylicity properties. These include the generation of surface vacancies developed by Wang et al., photoinduced reconstruction of Ti-OH bonds, and photocatalytic decomposition of organic adsorbents (Zhang et al., 2012). However, as none of the proposed mechanisms could completely explain all experimental instances, a combined theory has been developed. Based on the combined theory, which is schematically shown in Fig. 9.3, first step in the photoinduced hydrophilicity is decomposition of organic contaminants by photocatalytic UV-irradiation of semiconductor to obtain a clean surface. This step is followed by electron-hole pair consumption to form oxygen vacancies at which new OH groups are formed causing increased surface energy. In a review written by Zhang et al. (2012) the photoinduced hydrophilicity combined mechanism is shown as Fig. 9.3. 9.3.2.1 Enhanced photocatalytic self-cleaning properties Due to the limitations involved in application of photocatalysts to impart self-cleaning properties, several approaches have been employed to enhance the photocatalytic efficiencies of semiconductors such as TiO2. Nanophotocatalysts with a wide bang gap were only excited via UV light irradiation.
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Fig. 9.3 Mechanism of photoinduced hydrophilicity provided by Zhang et al. (2012). Reproduced with permission from Zhang, L., Dillert, R., Bahnemann, D.W., Vormoor, M., 2012. Photo-induced hydrophilicity and self-cleaning: models and reality. Energy Environ. Sci. 5, 7491–7507. Copyright 2012, The Royal Society of Chemistry.
Moreover, there is a strong tendency for electron-hole pairs to recombine with each other dissipating the energy into heat. Thus, the modification methods have been proposed to develop visible light active semiconductors (Zaleska, 2008). The most versatile procedures are as follows: 1. Doping of photocatalysts such as TiO2 with metal species: Transition metal ions, including Cu, Ni, Mn, Mo, Fe …, and noble metal nanoparticles, such as Ag, Au …, have been successfully applied to enhance the photocatalytic activities of semiconductors such as TiO2 by reducing the recombination rate of charge carriers and producing visible light active photocatalysts. This can be achieved through introducing a new energy level in the band gap of semiconductor or surface Plasmon absorption of electrons in metals such as silver. Moreover, during the visible light irradiation of modified semiconductors, the dye molecules or colored satins such as coffee can be excited by absorbing the light irradiation in the visible region (mostly at maximum absorption wavelength), injecting an electron into the conduction band (and/or surface states) of semiconductor, which was captured by surfaceadsorbed oxygen to generate superoxide radical. Subsequently, stain degradation is facilitated by the generated oxygen radicals. Thus, in visible light photocatalytic reactions, the stain itself helps the degradation process (Montazer et al., 2012; Harifi and Montazer, 2014a). 2. Doping of photocatalysts such as TiO2 with nonmetal species: Doping with nonmetals such as N, C, S enhances the visible light photocatalytic efficiencies of semiconductors due to three different main
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O2
O2·-
e-
e-
CB
hn
Noble metal e.g. Ag VB
h+
(A)
Semiconductor
H2O
OH·
O2·-
CB
O2
Fe3+ + hn → Fe4+ + e-
Fe3+/Fe4+ VB
OH·
hn
H2O
Semiconductor
(B)
Energy level of doped agent such as Fe ions
O2
O2·-
hn (visible) e-
CB
Dye
VB
(C)
Semiconductor
hn
Semiconductor
CB
e-
CB
O2
O2·-
VB
OH·
h+
H2O
VB Semiconductor
(D) hn
CB e- Semiconductor CB VB
OH·
h+
VB
H2O Semiconductor
(E) Fig. 9.4 Enhanced photocatalytic properties by (A) and (B) doping, (C) dye sensitization, (D) coupled dual semiconductors, and (E) capped dual semiconductors.
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reasons, namely, narrowing the band gap, impurity energy levels, and oxygen vacancies (Behzadnia et al., 2015). 3. Sensitization with dyes: Semiconductor sensitization with organic and organometallic dyes is another approach to enhance the visible light photocatalytic activities. For instance, self-assembling of meso-tetra (4-carboxyphenyl) porphyrin (TCPP) monolayers on anatase-coated-cotton fabric with superior visible-light self-cleaning performance in the degradation of Methylene Blue and coffee stains as compared to bare TiO2-coated cotton (Afzal et al., 2012). Based on a recent study carried out by Gaminian and Montazer (2017), polyester fabric treated with sensitized Madder/TiO2 nanoparticles had superior self-cleaning activities under visible light irradiation due to the positive effect of Madder as a natural safe sensitizer for nano TiO2. Through the visible light illumination of dye-sensitized semiconductors, dye molecules are excited generating electron from their ground state (HOMO) to the excited state (LUMO). Subsequently the generated electron was transferred to the conduction band of semiconductor. Formation of superoxide and hydroxyl radicals is the next step to decompose organic contaminants. Therefore, dye molecules on the surface possibly alter the photoresponse from UV to the visible region (Gaminian and Montazer, 2017). 4. Dual semiconductors: Semiconductor combinations could be also beneficial to improve the photocatalytic activities through separation of charge carriers and enhancing the visible light absorption by combining with short band gap semiconductor. These dual systems could be in form of coupled or capped semiconductor combinations, which are different in case of interfacial charge carrier transfer to the surface. SiO2/TiO2, WO3/ TiO2, CdS-TiO2, CdS-ZnO … are some of the combined semiconductors (Pakdel et al., 2013; Harifi and Montazer, 2014b; Gaminian and Montazer, 2015). We tried to schematically summarize the approaches applied for enhancing the photocatalytic activities of semiconductors in Fig. 9.4.
9.3.2.2 TiO2 nanoparticles for self-cleaning textiles Searching through literature, we came up with a vast number of studies dealing with application of TiO2 nanoparticles on various textile substrates mostly cotton, wool, and polyester or cotton/polyester blends to obtain
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self-cleaning activities. These include the nanocoatings and ex situ or in situ synthesis approaches. During the recent years, most studies have been aimed at enhancing the self-cleaning properties of the treated textile substrates by proposing new application methods or synthesis procedures. Moreover, there have been some efforts to increase the durability and attachment of nanoparticles to fibers. This ranges from using spacers, crosslinking agents, physical preactivation methods to in situ sonosynthesis and sonofabrication of nanoparticles. Application of modified TiO2 nanoparticles such as doped photocatalysts and dual semiconductor nanocomposites has been also concerned to intensify the photocatalytic activities. As it was difficult to discuss about all the reported studies, here we tried to provide some examples of case studies. First efforts to prepare self-cleaning cotton fibers with TiO2 nanoparticles dated back to the year 2004 when TiO2 nanocoatings with anatase crystalline structure were prepared on cotton fabrics from tetraisopropoxide using a low-temperature sol-gel process under ambient pressure. Crystallization of titania nanoparticles was obtained during boiling the treated fabric in water for 180 min. Strong acidity of the prepared sols was the main drawback of the applied methods causing tensile strength loss of cotton fibers. To reduce this detrimental effect, Qi et al. (2011) prepared nanocrystalline TiO2 sols with high TiO2 concentration and a very low amount of acid by sol-gel process in an acidic aqueous solution at a low temperature of 60°C under mechanical stirring. TiO2 thin films were produced on cotton fabrics from a colloidal sol by a simple dip-pad-dry-cure process in a short process time. The treated samples had significant self-cleaning performance toward the colorant decomposition and degradation of coffee and curry stains under 4-h UV irradiation. There were many studies reporting the treatment of cotton fabrics in both bleached and mercerized forms with dispersions of commercial TiO2 Degussa P25 or nanocolloidal TiO2 prepared from titanium alkoxides. These methods mainly involved preactivation of fiber surface to provide durable attachment of nanoparticles. For instance, RF-plasma, MW-plasma, and UV-irradiation were applied to introduce negative functional groups to anchor TiO2 on treated cotton fabrics (Bozzi et al., 2005). In other studies, polycarboxylic acids, namely, succinic acid, 1,2,3propanetricarboxylic acid and 1,2,3,4-butanetetracarboxylic acid (BTCA) have been used to form ester bonds acting as spacers to attach TiO2 nanoparticles on cotton fibers. Thus, the carboxylic acids provide the opportunity to form ester bond with hydroxyl groups of cellulose and to anchor TiO2 by
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electrostatic interaction (Harifi and Montazer, 2012). Nazari et al. (2011) treated, bleached, and cationized cotton fabrics with nanotitanium dioxide particles in presence of BTCA. Precationization of fibers with 3-chloro-2-hydroxypropyl was found effective to obtain enhanced selfcleaning properties toward Methylene Blue dye discoloration. Statistical analysis showed that higher amount of TiO2 is not necessarily effective for improved photocatalytic activities, as in higher concentration of nanoparticles the chance for nanoparticles agglomeration is more. Nano TiO2 Degussa P 25 was immobilized on cotton/polyester knitted fabric by citric acid under sonication and discoloration of CI Reactive Black 5 in aqueous solution was successfully achieved by the treated samples. The treated fabrics could be repeatedly used for the dye discoloration due to formation of covalent ester bond in presence of citric acid (Hashemikia and Montazer, 2011). There have been some studies claiming that the oxidative degradation of cellulose and successive cleavage polymer chain occurred under light exposure of cotton fabrics treated with TiO2 due to the generated radical species, causing fabric yellowing. Cotton surface coating with amorphous SiO2 layers prepared from the hydrolysis of tetraethoxysilane (TEOS) following the LBL deposition of previously synthesized TiO2 nanoparticles and use of trialkoxysilanes (OTMS, octyltrimethoxysilane; PTMS, phenyltrimethoxysilane) as interface coupling agents between the cellulose fibers and the deposited TiO2 nanoparticles have been claimed to enhance the chemical resistance of cellulosic chains during the photocatalytic activity of TiO2 nanoparticles (Goncalves et al., 2009). In recent years, sonochemistry has gained wide attraction for ultrasonic polycondensation of Ti–OH or Ti–OR treating textile substrates with TiO2 nanoparticles without requiring subsequent heating of the textile. Perelshtein et al. (2012) were first who used this method for deposition of titanium dioxide nanoparticles with anatase and rutile crystalline structures on cotton fabrics with self-cleaning activities in photodegradation of methylene blue. Their proposed method involved the in situ generation of TiO2 nanoparticles and their simultaneous deposition onto the fabric in a one-step reaction by using ultrasound irradiation promoting the crystallization process of titania due to the high local temperature and pressure generated during the collapse of the acoustic bubble under sonochemical irradiation. Fast migration of the synthesized nanoparticles onto the fabric caused local melting of the fibers at the contact sites and resulted in strong adherence of the nanoparticles to the fabric surface. Akhavan Sadr and Montazer (2014) developed
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the in situ sonosynthesis of nano TiO2 particles on cotton fabric using titanium tetra isopropoxide as a precursor and ultrasonic bath (50 kHz, 50 W). The treated fabric showed excellent self-cleaning properties with no negative effect on the fabric mechanical strength. The best self-cleaning property was obtained by using 9 mL precursor for 4 h sonication at low temperature (75°C). Au/TiO2/SiO2 nanocomposites were used to improve the visible light self-cleaning performance of cotton. Although the prepared nanocomposite was successful, its application was limited due to high cost of gold (Wang et al., 2012). Many research studies have been carried out on the use of Ag/TiO2 nanocomposites on cotton fabrics providing higher stain photodegradation due to the role of silver in trapping the excited electrons reducing charge carrier recombination rate and enhancing the visible light absorption arisen from silver plasmon resonance. Platinum (IV) chloride modified TiO2 (Pt-TiO2) and N-TiO2 (Pt-N-TiO2) nanosols have been synthesized through a low-temperature precipitation-peptization method. The visible light activity was attributed to the surface strongly attached PtCl6 anions, which enable visible light activity of TiO2 through a mechanism of charge transfer from ligand to metal excitation. However, the modification of Pt does not notably improve the performance of N-TiO2 coatings because the surface-adsorbed species on N-TiO2 block the adsorption of PtCl6 anions (Long et al., 2016). Nano TiO2 has been also applied on wool to produce self-cleaning textile, although the application has some difficulties due to low thermal resistance of wool fibers. Montazer and his research group have done many researches to treat wool fabrics with nano TiO2 Degussa. Incorporation of carboxylic acids as crosslinking agents and preactivation of surface with potassium permanganate has been also done to enhance the adsorption of nanoparticles. Self-cleaning efficiencies of the treated samples were assessed toward the discoloration of coffee, fruit juice, concentrated tea, and Methylene Blue dye (Montazer and Pakdel, 2011). Enzymatic pretreatment was also beneficial for enhanced nanoparticles adsorption and subsequent improved self-cleaning properties of wool/polyester fabrics treated with TiO2 Montazer and Seifollahzadeh, 2011). A uniform coating of TiO2/ SiO2 (50:50 and 30:70) nanocomposites was formed on wool to produce self-cleaning and superhydrophilicity. A higher concentration of silica resulted in enhanced self-cleaning (Pakdel et al., 2013). Sonochemistry was also successful to prepare wool fabrics with superior self-cleaning activities. Behzadnia et al. (2014a) presented a novel idea to
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prepare nanocrystalline TiO2 on wool fabric under ambient pressure at 60–65°C using in situ sonosynthesis method. They suggested the higher photocatalytic efficiency of TiO2 nanoparticles prepared by titanium tetra isopoxide in comparison to titanium butoxide as a precursor. The same group also studied the sonosynthesis of N-doped nano TiO2 on wool fabric at low temperature using ammonia (Behzadnia et al., 2014b). Due to lack of functional groups and hydrophobicity of polyester fibers, their successful nanofinishing with TiO2 is dependent on preactivation of surface using chemical or physical surface modification methods. For instance, RF-plasma, MW-plasma, and vacuum-UV light irradiation have been applied as pretreatment allowing the loading of TiO2 by wet chemical techniques in the form of transparent coatings constituted of nanoparticles of diverse sizes. The treated samples possessed significant ability to degrade coffee stains. Recently, simultaneous surface modification of polyester fibers and in situ sonosynthesis of TiO2 nanoparticles have been carried out using ultrasound bath. Hydroxylation of terephthalate occurred by hydroxyl radicals formed during water sonolysis, forming functional groups on polyester surface enhancing nanoparticles adsorption. Self-cleaning activity of sonosynthesized nano TiO2-treated polyester samples toward degradation of Methylene Blue stain was superior to coating of fabric with commercial nano TiO2 (Harifi and Montazer, 2017). 9.3.2.3 ZnO nanoparticles for self-cleaning textiles Up to now, ZnO nanoparticles have been used parallel with TiO2 as ideal photocatalysts. However, incorporation of ZnO nanoparticles into textile materials has been recently gained interest, and the number of studies concerning synthesis and application of nano ZnO particles is not to the level of textiles treated with nano TiO2 particles. Kathirvelu et al. (2010) synthesized nano ZnO by homogeneous phase reaction and then applied on the cotton fabric to obtain the stain-eliminating function. The authors reported that a long time was required to remove stains from the substrate due to large band gap of ZnO making it inefficient to be excited by sunlight. In situ synthesis of ZnO nanoparticles on starch-sized cotton fibers was successful to produce excellent self-cleaning properties against Methylene Blue stain by 2% zinc nitrate and 15% sodium hydroxide using the reducing and stabilizing effect of starch (Khosravian et al., 2015). In situ biosynthesis of zinc oxide nanoparticles on cotton fibers was also reported beneficial to achieve Methylene Blue dye stain self-cleaning properties. In the proposed
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method, Keliab with 15% (v/v) was used as a natural agent to produce ZnO nanoparticles (Aladpoosh and Montazer, 2015). Using triethanol amine as a reducing agent and simultaneous surface modification of polyester fibers resulted in textiles with superior self-cleaning properties due to the photocatalytic activities of ZnO nanoparticles (Poortavasoly et al., 2016). N-doped ZnO/TiO2 nanocomposite was sonosynthesized on wool fabric in ultrasonic bath through hydrolysis of zinc acetate and titanium isopropoxide. Presence of optimal nitrogen and TiO2 on ZnO led to enhanced photocatalytic properties under sunlight light irradiation due to separation of photogenerated electron and holes, reducing the recombination rate. Moreover, sonochemistry created smaller nanoparticles with higher crystallinity resulting in superior self-cleaning properties than conventional sol-gel treatments (Behzadnia et al., 2015). 9.3.2.4 Other nanosemiconductors ZrO2 nanocrystals were successfully synthesized and deposited onto wool fibers using the sol-gel technique at low temperature. Although the treated samples possessed photocatalytic activities toward Methylene Blue and Eosin Yellow dyes, the self-cleaning properties were lower than samples treated with TiO2 nanoparticles in a similar manner. This was attributed to the lower band-gap energy of titania (3.2 eV) comparing with zirconia (4.5 eV) and anatase crystalline structure of TiO2 (Moafi et al., 2010). Nano Cu2O particles were synthesized on cotton fabric using CuSO4 as a precursor and glucose as reducing and capping agent in alkali. The treated fabrics showed photocatalytic activity toward the degradation of Methylene Blue under daylight (Sedighi et al., 2014). Recently, cotton fabric was in situ treated with nanocupric oxide using nanobio method to impart self-cleaning activities (Bashiri Rezaie et al., 2017). 9.3.2.5 Methods for evaluating photocatalytic self-cleaning properties In most studies, self-cleaning properties are assessed by staining the treated samples with organic dirt such as coffee, make up, or dye. Methylene Blue, Methyl Orange, Rhodamine B, and reactive black 5 are the most common model dye compounds. After exposure of samples to light irradiation, the degree of discoloration is evaluated by colorimetric measurements before and after light illumination and is reported as ΔE (Eq. 9.4). 1=2 ΔE ¼ ðΔL ∗ Þ2 + ðΔa∗ Þ2 + ðΔb∗ Þ2 (9.4)
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where ΔL*, Δa*, and Δb* are color coordinates of samples in Lab* color space before and after light exposure. Self-cleaning efficiencies of treated textiles are also evaluated by scanning the samples and measuring the color coordinates in RGB color space using Matlab software (Eq. 9.5). 4 RGB ¼ ðR2 R1 Þ2 + ðB2 B1 Þ2 + ðG2 G1 Þ2 1=2 (9.5) where R1G1B1 and R2G2B2 are color coordinates of samples before and after light illumination, respectively (Harifi and Montazer, 2014a, b). Staining gray scale can be also used to evaluate the self-cleaning effectiveness of the treated samples. In this regard, 5 and 1 refers to maximum and minimum stain degradation, respectively. In some studies, treated textiles are dipped in dye solutions and light irradiated for a definite time duration. Prior to irradiation, the system should be kept in dark for some hours, e.g., 24 h to reach adsorption-desorption equilibrium between treated fabric and dye. Here, photocatalytic efficiencies of the treated samples are evaluated by measuring dye absorbance at maximum wavelength before and after light irradiation (Eq. 9.6). Conversion% ¼ ðA0 At Þ=A0 100 (9.6) where A0 and At are dye absorbance at maximum wavelength before and after t (h) light irradiation, respectively (Harifi and Montazer, 2014a, b).
9.4 CONCLUSION In addition to the published studies concerning with two common approaches for preparing self-cleaning textiles, which have been thoroughly discussed in this chapter, production of surfaces with adjustable hydrophilic and hydrophobic properties for other applications such as separation of oil from water will be a focus of future researches. New photocatalysts such as metal organic frameworks (MOFs) will be also applied on various textile substrates to impart self-cleaning properties. These materials with versatile structures have recently attracted researchers for their photocatalytic efficiencies, although research to enhance their properties together with their stability under photocatalytic reactions needs to be further developed. Graphitic carbon nitride (g-C3N4) is also an attractive low-cost sustainable metal-free photocatalyst with the potential to absorb visible light. Although still not applied on textile substrates, we will see more studies in future using these new photocatalysts. Research will also focus on nanocomposite structures of the new photocatalysts to achieve enhanced properties.
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REFERENCES Afzal, S., Daoud, W.A., Langford, S.J., 2012. Self-cleaning cotton by porphyrin-sensitized visible-light photocatalysis. J. Mater. Chem. 22, 4083–4088. Akhavan Sadr, F., Montazer, M., 2014. In situ sonosynthesis of nano TiO2 on cotton fabric. Ultrason. Sonochem. 21, 681–691. Aladpoosh, R., Montazer, M., 2015. The role of cellulosic chains of cotton in biosynthesis of ZnO nanorods producing multifunctional properties: mechanism, characterizations and features. Carbohydr. Polym. 126, 122–129. Bashiri Rezaie, A., Montazer, M., Mahmoudi Rad, M., 2017. Biosynthesis of nano cupric oxide on cotton using Seidlitzia rosmarinus ashes utilizing bio, photo, acid sensing and leaching properties. Carbohydr. Polym. 177, 1–12. Behzadnia, A., Montazer, M., Rashidi, A., Mahmoudi Rad, M., 2014a. Sonosynthesis of nano TiO2 on wool using titanium isopropoxide or butoxide in acidic media producing multifunctional fabric. Ultrason. Sonochem. 21, 1815–1826. Behzadnia, A., Montazer, M., Rashidi, A., Mahmoudi Rad, M., 2014b. Rapid sonosynthesis of n-doped nano TiO2 on wool fabric at low temperature: introducing self-cleaning, hydrophilicity, antibacterial/antifungal properties with low alkali solubility, yellowness and cytotoxicity. Photochem. Photobiol. 90, 1224–1233. Behzadnia, A., Montazer, M., Mahmoudi Rad, M., 2015. Simultaneous sonosynthesis and sonofabrication of N-doped ZnO/TiO2 core-shell nanocomposite on wool fabric: Introducing various properties specially nano photo bleaching. Ultrason. Sonochem. 27, 10–21. Bozzi, A., Yuranova, T., Guasaquillo, I., Laubb, D., Kiwi, J., 2005. Self-cleaning of modified cotton textiles by TiO2 at low temperatures under daylight irradiation. J. Photochem. Photobiol. A Chem. 174, 156–164. Callies, M., Quere, D., 2005. On water repellency. Soft Mater. 1, 55–61. Dastjerdi, R., Montazer, M., Shahsavan, S., 2010. A novel technique for producing durable multifunctional textiles using nanocomposite coating. Colloids Surf. B: Biointerfaces 81, 32–41. Feng, L., Li, S., Li, Y., Li, H., Zhang, L., Zhai, J., Song, Y., Liu, B., Jiang, L., Zhu, D., 2002. Super-hydrophobic surfaces: From natural to artificial. Adv. Mater. 14, 1857–1860. Gaminian, H., Montazer, M., 2015. Enhanced self-cleaning properties on polyester fabric under visible light through single-step synthesis of cuprous oxide doped nano-TiO2. Photochem. Photobiol. 91, 1078–1087. Gaminian, H., Montazer, M., 2017. Simultaneous nano TiO2 sensitization, application and stabilization on polyester fabric using madder and NaOH producing enhanced selfcleaning with hydrophilic properties under visible light. J. Photochem. Photobiol. A Chem. 332, 158–166. Gao, L., McCarthy, T.J., 2006. Artificial lotus leaf prepared using a 1945 patent and a commercial textile. Langmuir 22, 5969–5973. Goncalves, G., Marques, P.A.A.P., Pinto, R.J.B., Trindade, T., Neto, C.P., 2009. Surface modification of cellulosic fibers for multi-purpose TiO2 based nanocomposites. Compos. Sci. Technol. 69, 1051–1056. Gupta, D., Gulrajani, M.L., 2015. Self-cleaning finishes for textiles. In: Paul, R. (Ed.), Functional Finishes for Textiles: Improving Comfort, Performance and Protection. Woodhead Publishing, Cambridge, UK. Harifi, T., Montazer, M., 2012. Past, present and future prospects of cotton cross-linking: new insight into nano particles. Carbohydr. Polym. 88, 1125–1140. Harifi, T., Montazer, M., 2014a. Fe3+:Ag/TiO2 nanocomposite: synthesis, characterization and photocatalytic activity under UV and visible light irradiation. Appl. Catal. A Gen. 473 (2014), 104–115.
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Harifi, T., Montazer, M., 2014b. A novel magnetic reusable nanocomposite with enhanced photocatalytic activities for dye degradation. Sep. Purif. Technol. 134, 210–219. Harifi, T., Montazer, M., 2017. Application of sonochemical technique for sustainable surface modification of polyester fibers resulting in durable nano-sonofinishing. Ultrason. Sonochem. 37, 158–168. Hashemikia, S., Montazer, M., 2011. Stabilization of nanosized TiO2 particles on knitted cotton/polyester fabric by citric acid for self-cleaning and discoloration of reactive black 5 from waste water. Iran. J. Polym Sci. Technol. 24, 33–42. Kathirvelu, S., D’Souza, L., Dhurai, B., 2010. Study of stain-eliminating textiles using ZnO nanoparticles. J. Text. Inst. 101, 520–526. Khosravian, S., Montazer, M., Malek, R.M.A., Harifi, T., 2015. In situ synthesis of nano ZnO on starch sized cotton introducing nano photo active fabric optimized with response surface methodology. Carbohydr. Polym. 132 (2015), 126–133. Li, S., Xie, H., Zhang, S., Wang, X., 2007. Facile transformation of hydrophilic cellulose into super hydrophobic cellulose. Chem. Commun. (46), 4857–4859. Liu, Y., Tang, J., Wang, R., Lu, H., Li, L., Kong, Y., Qia, K., Xin, J.H., 2007. Artificial lotus leaf structures from assembling carbon nanotubes and their applications in hydrophobic textiles. J. Mater. Chem. 17, 1071–1078. Long, M., Zheng, L., Tana, B., Shu, H., 2016. Photocatalytic self-cleaning cotton fabrics with platinum (IV) chloride modified TiO2 and N-TiO2 coatings. Appl. Surf. Sci. 386, 434–441. Nazari, A., Montazer, M., Moghadam, M.B., Anary-Abbasinejad, M., 2011. Self-cleaning properties of bleached and cationized cotton using nanoTiO2: a statistical approach. Carbohydr. Polym. 83, 1119–1127. Nishimoto, S., Bhush, B., 2013. Bioinspired self-cleaning surfaces with superhydrophobicity, superoleophobicity, and superhydrophilicity. RSC Adv. 3, 671–690. Moafi, H.F., Shojaie, A.F., Zanjanchi, M.A., 2010. The comparison of photocatalytic activity of synthesized TiO2 and ZrO2 nanosize onto wool fibers. Appl. Surf. Sci. 256, 4310–4316. Montazer, M., Behzadnia, A., Bameni Moghadam, M., 2012. Superior self-cleaning features on wool fabric using TiO2/Ag nanocomposite optimized by response surface methodology. J. Appl. Polym. Sci. 125, E356–E363. Montazer, M., Maali Amiri, M., 2014. ZnO nano reactor on textiles and polymers: ex situ and in situ synthesis, application, and characterization. J. Phys. Chem. B 118, 1453–1470. Montazer, M., Pakdel, E., 2011. Functionality of nano titanium dioxide on textiles with future aspects: focus on wool. J. Photochem. Photobiol. C, 293–303. Montazer, M., Seifollahzadeh, S., 2011. Pretreatment of wool/polyester blended fabrics to enhance titanium dioxide nanoparticle adsorption and self-cleaning properties. Color. Technol. 127, 322–327. Pakdel, E., Daoud, W.A., Wang, X., 2013. Self-cleaning and superhydrophilic wool by TiO2/SiO2 nanocomposite. Appl. Surf. Sci. 275, 397–402. Perelshtein, I., Applerot, G., Perkas, N., Grinblat, J., Gedanken, A., 2012. A one-step process for the antimicrobial finishing of textiles with crystalline TiO2 nanoparticles. Chem. Eur. J. 18, 4575–4582. Poortavasoly, H., Montazer, M., Harifi, T., 2016. Aminolysis of polyethylene terephthalate surface along with in situ synthesis and stabilizing ZnO nanoparticles using triethanolamine optimized with response surface methodology. Mater. Sci. Eng. C 58, 495–503. Qi, K., Wang, X., Xin, J.H., 2011. Photocatalytic self-cleaning textiles based on nanocrystalline titanium dioxide. Text. Res. J. 81, 101–110. Sas, I., Gorga, R.E., Joines, J.A., Thone, A.K., 2012. Review on superhydrophobic selfcleaning surfaces produced by electrospinning. J. Polym. Sci. B Polym. Phys. 50, 824–845.
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Sedighi, A., Montazer, M., Samadi, N., 2014. Synthesis of nano Cu2O on cotton: morphological, physical, biological and optical sensing characterizations. Carbohydr. Polym. 110, 489–498. Stegmaier, T., Arnim, V.V., Scherrieble, A., Planc, H., 2008. Self-cleaning textiles using the Lotus effect. In: Abbott, A., Ellison, M. (Eds.), Biologically Inspired Textiles. Woodhead Publishing, Cambridge, UK. Sun, T.L., Feng, L., Gao, X.F., Jiang, L., 2005. Bioinspired surfaces with special wettability. Acc. Chem. Res. 38, 644–652. Wang, R.H., Wang, X.W., Xin, J.H., 2012. Advanced visible-light-driven self-cleaning cotton by Au/TiO2/SiO2 photocatalysts. ACS Appl. Mater. Interfaces 2, 82–85. Wang, J., Zhao, J., Sun, L., Wang, X., 2015. A review on the application of photocatalytic materials on textiles. Text. Res. J. 85, 1104–1118. Xu, B., Cai, Z., 2008. Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic modification. Appl. Surf. Sci. 254, 5899–5904. Xue, C.H., Jia, S.T., Zhang, J., Tian, L., Chen, H.Z., Wang, M., 2008. Preparation of superhydrophobic surfaces on cotton textiles. Sci. Technol. Adv. Mater. 9, 1–7. Xue, C.H., Chen, J., Yin, W., Jia, S.T., Ma, J.Z., 2012. Superhydrophobic conductive textiles with antibacterial property by coating fibers with silver nanoparticles. Appl. Surf. Sci. 258, 2468–2472. Zaleska, A., 2008. Doped-TiO2: a review. Recent Pat. Eng. 2, 157–164. Zhang, L., Dillert, R., Bahnemann, D.W., Vormoor, M., 2012. Photo-induced hydrophilicity and self-cleaning: models and reality. Energy Environ. Sci. 5, 7491–7507. Zhao, Y., Tang, Y., Wang, X., Lin, T., 2010. Superhydrophobic cotton fabric fabricated by electrostatic assembly of silica nanoparticles and its remarkable buoyancy. Appl. Surf. Sci. 256, 6736–6742. Zhou, H., Wang, H., Niu, H., Gestos, A., Wang, X., Lin, T., 2012. Fluoroalkyl silane modified silicone rubber/nanoparticle composite: a super durable, robust superhydrophobic fabric coating. Adv. Mater. 24 (18), 2409–2412.
FURTHER READING Ganesh, V.A., Raut, H.K., Naira, A.S., Ramakrishna, S., 2011. A review on self-cleaning coatings. J. Mater. Chem. A 21, 16304–16322. Ragesh, P., Ganesh, V.A., Naira, S.V., Nai, A.S., 2014. A review on ‘self-cleaning and multifunctional materials’. J. Mater. Chem. A 2, 14773–14797. Schneider, J., Matsuoka, M., Takeuchi, M., Zhang, J., Horiuchi, Y., Anpo, M., Bahnemann, D.W., 2014. Understanding TiO2 photocatalysis: mechanisms and materials. Chem. Rev. 114 (19), 9919–9986.
Nanofinishes for self-cleaning textiles 9.1 INTRODUCTION: DEFINITION AND HISTORICAL OVERVIEW Self-cleaning clothes with the ability to clean themselves are dreams come true thanks to great efforts made by researchers during the last decades. A path to achieve self-cleaning clothes was actually paved with the aid of living nature. As the first hints to achieve self-cleaning surfaces have been brought by mimicking nature, self-cleaning surfaces are also famous as bioinspired surfaces. In addition to legs of water strider and wings of butterflies, self-cleaning was mainly inspired by leaves of plants, namely, lotus, making it known as lotus effect. Neinhuis and Barthlott were the first who introduced the idea of lotus effect in 1997, arising from the micropapillae structures with the ability to trap a large amount of air, and low surface energy of epicuticular wax crystalloids coating the leaf surface (Sas et al., 2012). Their idea was further developed by Feng et al. (2002) who found out that the surface of leaves has branchlike nanostructures with 124 nm diameter, causing contact angles greater than 160 degrees. Scientific analysis of lotus leaf revealed that a water droplet on the leaf is almost spherical in shape and can roll off easily. This resulted from the combination of a hierarchical surface structure that traps air beneath a water droplet and the hydrophobicity of the surface wax. Moreover, the surface has low adhesion force and low coefficient of friction. Thus, the interfacial area between contamination particles and the surface is very small, resulting in reduced adhesion (Fig. 9.1). On such a surface, water droplets from rainfall can pick up the contamination particles and carry them away when the droplets roll off the surface, leading to self-cleaning properties also known as lotus effect. Both surface morphology and hydrophobic wax material are necessary to create such a superhydrophobic surface on lotus leaves. Moreover, superhydrophobic surfaces with contact angles of more than 150 degrees possess self-cleaning properties only if their contact angle hysteresis and sliding angle are low (Nishimoto and Bhush, 2013). Nanofinishing of Textile Materials https://doi.org/10.1016/B978-0-08-101214-7.00009-1
© 2018 Elsevier Ltd. All rights reserved.
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Water
Water
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Fig. 9.1 Lotus effect.
Similar superhydrophobicity and self-cleaning properties have been found in plant leaves such as rice and taro leaves with hierarchical surface structures. Moreover, plant leaves with unitary surface structure such as Ramee leaf and Chinese watermelon indicate superhydrophobicity and self-cleaning properties (Sun et al., 2005). Inspired by lotus effect, researchers have developed self-cleaning textiles by combining low surface energy materials and creating surface roughness. The common low surface energy materials include organic silanes, fluorinated silanes, alkyl amines, and silicates. Popular surface modification methods include wet chemical reactions, self-assembly and sol-gel, layerby-layer (LBL) deposition, polymerization reactions, colloidal template techniques, chemical vapor deposition, plasma treatment, and electrospinning (Gupta and Gulrajani, 2015). Further to lotus effect, self-cleaning has been also achieved by the opposite surface wettability, namely, superhydrophilicity in which water completely covers the surface with a continuous film and washes away the contaminants. This is usually achieved via photocatalysts also benefiting from generating active radicals under light irradiation capable of decomposing organic contaminants (Wang et al., 2015). Semiconductor photocatalysts such as nano TiO2 and ZnO have been mainly used on various textile substrates in the form of nanocoatings or by ex situ or in situ synthetic methods to produce self-cleaning textiles. The treated textiles also benefit from multifunctional activities, including UV protection and antibacterial properties (Montazer and Maali Amiri, 2014). Moreover, there have been numerous efforts to improve the photocatalytic activities of semiconductors using different surface modification methods to produce semiconductors with enhanced activities under solar light irradiation (Zaleska, 2008). After a brief overview of different surface wetting theories, this chapter mainly deals with nanofinishes for producing self-cleaning textiles and we tried to provide the most recent case studies.
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9.2 SURFACE WETTABILITY Depending on surface smoothness or roughness, three theories have been proposed for surface wettability (Callies and Quere, 2005). 1. Young model, which is only valid for smooth surfaces and is based on the correlation between the three interfacial energies per unit area, which are in equilibrium at the droplet (Eq. 9.1) γ sv¼ γ sl + γ lv cos θ (9.1) where θ is contact angle, γ sv and γ sl, γ lv and are the interfacial energies per unit area of the solid-vapor, solid-liquid, and liquid-vapor interfaces, respectively. 2. Wenzel theory, which is based on complete wetting of a rough surface where the liquid penetrates into the grooves of the rough surface (Eq. 9.2). cos θa¼ r cos θs (9.2) where apparent contact angle θa is the true contact angle of the droplet on a plain surface θs multiplied by roughness factor (r), and r is the ratio between the actual rough surface area and the geometric surface area (r¼ Ageometric/ Areal). 3. Cassie and Baxter theory, which describes the nonwetted contact between the liquid and rough surface arising from vapor pockets trapped underneath the liquid in the grooves. Based on this theory when a liquid spreads over a rough porous surface, the solid-vapor interface converted into two new interfaces, solid-liquid and liquid-vapor interfaces (Eq. 9.3). cos θa¼ fs cos θs + fv cos θv
(9.3)
where fs and fv are the fractions of the solid and vapor on the surface. The theories are schematically shown in Fig. 9.2. g1v gsv
(A)
q
gs1
(B)
(C)
Fig. 9.2 (A) Young, (B) Wenzel, and (C) Cassie and Baxter theories for wetting behavior of smooth and roughened surfaces.
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9.3 NANOFINISHES FOR SELF-CLEANING TEXTILES 9.3.1 Self-cleaning textiles based on lotus effect Inspired by lotus effect, self-cleaning textiles could be prepared via two basic approaches (Stegmaier et al., 2008): 1. Introducing nanoscale roughness on textile surface using nanoparticles such as silver, rod arrays such as ZnO nanorods and carbon nanotubes, or by physical surface modification methods such as laser or plasma along with application of low surface energy materials to impart hydrophobicity (Liu et al., 2007; Xu and Cai, 2008; Dastjerdi et al., 2010). 2. Treatment of fabrics having micro- or nanostructure with low surface energy materials. For instance, silicone was coated on microfiber polyester fabric to produce superhydrophobic polyester with self-cleaning properties (Gao and McCarthy, 2006). Some of the recent studies concerning with these two approaches are as follows: Superhydrophobic self-cleaning cotton fabrics were prepared by hydrothermal synthesis of ZnO nanorods followed by coating with dodecyltrimethoxysilane to impart surface roughness and hydrophobicity. Cotton fabric treatment with silver nitrate in presence of potassium hydroxide following by surface hydrophobization with octyltriethoxysilane has been also reported (Xu and Cai, 2008). In situ synthesis of silver nanoparticles on cotton fabric along with treating the fabric with hexadecyltrimethoxysilane was also successful to provide superhydrophobic fabric with self-cleaning properties (Xue et al., 2012). Similar researches have been carried out using carbon nanotubes to create nanoscale roughness on various textile substrates (Liu et al., 2007). Silica nanoparticles have been extensively used to impart superhydrophobic properties into textiles. For instance, polydimethylsiloxane filled with fluorinated alkylsilane functionalized silica nanoparticles and fluorinated alkylsilane were successful to prepare superhydrophobic coating on fabrics (Zhou et al., 2012). Chemical vapor deposition and LBL techniques are among the widely applied methods to produce superhydrophobic textiles. Nanocoating of silicone was deposited on cotton fabric via chemical vapor deposition followed by hydroxylation and polymerization (Li et al., 2007). Polyelectrolyte/silica nanoparticle multilayers were deposited on cotton fabric using LBL method. The process was followed by fluoroalkylsilane treatment to impart superhydrophobicity (Zhao et al., 2010). Modified silica nanoparticles with epoxy functional groups were applied on
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cotton fabric to produce dual-size hierarchical surface structure following by treatment with stearic acid or 1H,1H,2H,2H-perfluorodecyltrichlorosilane or their combination to impart hydrophobicity (Xue et al., 2008). In all cases, durability of the treatment is important and the other involved challenge is the tendency of the treated surfaces to adsorb oily contaminants. These obstacles restricted the feasibility of these methods.
9.3.2 Photoinduced hydrophilicity and self-cleaning: Definition and mechanism As thoroughly discussed in Chapter 2, photocatalysis is based on the irradiation of semiconductors by light with energy equal or greater than the band gap, during which electrons are excited from the valence band to the conduction band and then migrate to the semiconductor surface to produce radicals for contaminants degradation. Accidental finding of a research group in TOTO laboratory revealed that TiO2 films possess superhydrophylic properties under UV light illumination. This phenomenon, which is called photoinduced hydrophilicity, is reversible and the surface becomes hydrophobic in the absence of light. This effect, which was found in 1995, was a pioneering step to achieve self-cleaning surfaces and was first applied to develop selfcleaning glass. Since the introduction of this effect, three different mechanisms have been proposed to support the photoinduced hydrophylicity properties. These include the generation of surface vacancies developed by Wang et al., photoinduced reconstruction of Ti-OH bonds, and photocatalytic decomposition of organic adsorbents (Zhang et al., 2012). However, as none of the proposed mechanisms could completely explain all experimental instances, a combined theory has been developed. Based on the combined theory, which is schematically shown in Fig. 9.3, first step in the photoinduced hydrophilicity is decomposition of organic contaminants by photocatalytic UV-irradiation of semiconductor to obtain a clean surface. This step is followed by electron-hole pair consumption to form oxygen vacancies at which new OH groups are formed causing increased surface energy. In a review written by Zhang et al. (2012) the photoinduced hydrophilicity combined mechanism is shown as Fig. 9.3. 9.3.2.1 Enhanced photocatalytic self-cleaning properties Due to the limitations involved in application of photocatalysts to impart self-cleaning properties, several approaches have been employed to enhance the photocatalytic efficiencies of semiconductors such as TiO2. Nanophotocatalysts with a wide bang gap were only excited via UV light irradiation.
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Fig. 9.3 Mechanism of photoinduced hydrophilicity provided by Zhang et al. (2012). Reproduced with permission from Zhang, L., Dillert, R., Bahnemann, D.W., Vormoor, M., 2012. Photo-induced hydrophilicity and self-cleaning: models and reality. Energy Environ. Sci. 5, 7491–7507. Copyright 2012, The Royal Society of Chemistry.
Moreover, there is a strong tendency for electron-hole pairs to recombine with each other dissipating the energy into heat. Thus, the modification methods have been proposed to develop visible light active semiconductors (Zaleska, 2008). The most versatile procedures are as follows: 1. Doping of photocatalysts such as TiO2 with metal species: Transition metal ions, including Cu, Ni, Mn, Mo, Fe …, and noble metal nanoparticles, such as Ag, Au …, have been successfully applied to enhance the photocatalytic activities of semiconductors such as TiO2 by reducing the recombination rate of charge carriers and producing visible light active photocatalysts. This can be achieved through introducing a new energy level in the band gap of semiconductor or surface Plasmon absorption of electrons in metals such as silver. Moreover, during the visible light irradiation of modified semiconductors, the dye molecules or colored satins such as coffee can be excited by absorbing the light irradiation in the visible region (mostly at maximum absorption wavelength), injecting an electron into the conduction band (and/or surface states) of semiconductor, which was captured by surfaceadsorbed oxygen to generate superoxide radical. Subsequently, stain degradation is facilitated by the generated oxygen radicals. Thus, in visible light photocatalytic reactions, the stain itself helps the degradation process (Montazer et al., 2012; Harifi and Montazer, 2014a). 2. Doping of photocatalysts such as TiO2 with nonmetal species: Doping with nonmetals such as N, C, S enhances the visible light photocatalytic efficiencies of semiconductors due to three different main
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O2
O2·-
e-
e-
CB
hn
Noble metal e.g. Ag VB
h+
(A)
Semiconductor
H2O
OH·
O2·-
CB
O2
Fe3+ + hn → Fe4+ + e-
Fe3+/Fe4+ VB
OH·
hn
H2O
Semiconductor
(B)
Energy level of doped agent such as Fe ions
O2
O2·-
hn (visible) e-
CB
Dye
VB
(C)
Semiconductor
hn
Semiconductor
CB
e-
CB
O2
O2·-
VB
OH·
h+
H2O
VB Semiconductor
(D) hn
CB e- Semiconductor CB VB
OH·
h+
VB
H2O Semiconductor
(E) Fig. 9.4 Enhanced photocatalytic properties by (A) and (B) doping, (C) dye sensitization, (D) coupled dual semiconductors, and (E) capped dual semiconductors.
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reasons, namely, narrowing the band gap, impurity energy levels, and oxygen vacancies (Behzadnia et al., 2015). 3. Sensitization with dyes: Semiconductor sensitization with organic and organometallic dyes is another approach to enhance the visible light photocatalytic activities. For instance, self-assembling of meso-tetra (4-carboxyphenyl) porphyrin (TCPP) monolayers on anatase-coated-cotton fabric with superior visible-light self-cleaning performance in the degradation of Methylene Blue and coffee stains as compared to bare TiO2-coated cotton (Afzal et al., 2012). Based on a recent study carried out by Gaminian and Montazer (2017), polyester fabric treated with sensitized Madder/TiO2 nanoparticles had superior self-cleaning activities under visible light irradiation due to the positive effect of Madder as a natural safe sensitizer for nano TiO2. Through the visible light illumination of dye-sensitized semiconductors, dye molecules are excited generating electron from their ground state (HOMO) to the excited state (LUMO). Subsequently the generated electron was transferred to the conduction band of semiconductor. Formation of superoxide and hydroxyl radicals is the next step to decompose organic contaminants. Therefore, dye molecules on the surface possibly alter the photoresponse from UV to the visible region (Gaminian and Montazer, 2017). 4. Dual semiconductors: Semiconductor combinations could be also beneficial to improve the photocatalytic activities through separation of charge carriers and enhancing the visible light absorption by combining with short band gap semiconductor. These dual systems could be in form of coupled or capped semiconductor combinations, which are different in case of interfacial charge carrier transfer to the surface. SiO2/TiO2, WO3/ TiO2, CdS-TiO2, CdS-ZnO … are some of the combined semiconductors (Pakdel et al., 2013; Harifi and Montazer, 2014b; Gaminian and Montazer, 2015). We tried to schematically summarize the approaches applied for enhancing the photocatalytic activities of semiconductors in Fig. 9.4.
9.3.2.2 TiO2 nanoparticles for self-cleaning textiles Searching through literature, we came up with a vast number of studies dealing with application of TiO2 nanoparticles on various textile substrates mostly cotton, wool, and polyester or cotton/polyester blends to obtain
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self-cleaning activities. These include the nanocoatings and ex situ or in situ synthesis approaches. During the recent years, most studies have been aimed at enhancing the self-cleaning properties of the treated textile substrates by proposing new application methods or synthesis procedures. Moreover, there have been some efforts to increase the durability and attachment of nanoparticles to fibers. This ranges from using spacers, crosslinking agents, physical preactivation methods to in situ sonosynthesis and sonofabrication of nanoparticles. Application of modified TiO2 nanoparticles such as doped photocatalysts and dual semiconductor nanocomposites has been also concerned to intensify the photocatalytic activities. As it was difficult to discuss about all the reported studies, here we tried to provide some examples of case studies. First efforts to prepare self-cleaning cotton fibers with TiO2 nanoparticles dated back to the year 2004 when TiO2 nanocoatings with anatase crystalline structure were prepared on cotton fabrics from tetraisopropoxide using a low-temperature sol-gel process under ambient pressure. Crystallization of titania nanoparticles was obtained during boiling the treated fabric in water for 180 min. Strong acidity of the prepared sols was the main drawback of the applied methods causing tensile strength loss of cotton fibers. To reduce this detrimental effect, Qi et al. (2011) prepared nanocrystalline TiO2 sols with high TiO2 concentration and a very low amount of acid by sol-gel process in an acidic aqueous solution at a low temperature of 60°C under mechanical stirring. TiO2 thin films were produced on cotton fabrics from a colloidal sol by a simple dip-pad-dry-cure process in a short process time. The treated samples had significant self-cleaning performance toward the colorant decomposition and degradation of coffee and curry stains under 4-h UV irradiation. There were many studies reporting the treatment of cotton fabrics in both bleached and mercerized forms with dispersions of commercial TiO2 Degussa P25 or nanocolloidal TiO2 prepared from titanium alkoxides. These methods mainly involved preactivation of fiber surface to provide durable attachment of nanoparticles. For instance, RF-plasma, MW-plasma, and UV-irradiation were applied to introduce negative functional groups to anchor TiO2 on treated cotton fabrics (Bozzi et al., 2005). In other studies, polycarboxylic acids, namely, succinic acid, 1,2,3propanetricarboxylic acid and 1,2,3,4-butanetetracarboxylic acid (BTCA) have been used to form ester bonds acting as spacers to attach TiO2 nanoparticles on cotton fibers. Thus, the carboxylic acids provide the opportunity to form ester bond with hydroxyl groups of cellulose and to anchor TiO2 by
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electrostatic interaction (Harifi and Montazer, 2012). Nazari et al. (2011) treated, bleached, and cationized cotton fabrics with nanotitanium dioxide particles in presence of BTCA. Precationization of fibers with 3-chloro-2-hydroxypropyl was found effective to obtain enhanced selfcleaning properties toward Methylene Blue dye discoloration. Statistical analysis showed that higher amount of TiO2 is not necessarily effective for improved photocatalytic activities, as in higher concentration of nanoparticles the chance for nanoparticles agglomeration is more. Nano TiO2 Degussa P 25 was immobilized on cotton/polyester knitted fabric by citric acid under sonication and discoloration of CI Reactive Black 5 in aqueous solution was successfully achieved by the treated samples. The treated fabrics could be repeatedly used for the dye discoloration due to formation of covalent ester bond in presence of citric acid (Hashemikia and Montazer, 2011). There have been some studies claiming that the oxidative degradation of cellulose and successive cleavage polymer chain occurred under light exposure of cotton fabrics treated with TiO2 due to the generated radical species, causing fabric yellowing. Cotton surface coating with amorphous SiO2 layers prepared from the hydrolysis of tetraethoxysilane (TEOS) following the LBL deposition of previously synthesized TiO2 nanoparticles and use of trialkoxysilanes (OTMS, octyltrimethoxysilane; PTMS, phenyltrimethoxysilane) as interface coupling agents between the cellulose fibers and the deposited TiO2 nanoparticles have been claimed to enhance the chemical resistance of cellulosic chains during the photocatalytic activity of TiO2 nanoparticles (Goncalves et al., 2009). In recent years, sonochemistry has gained wide attraction for ultrasonic polycondensation of Ti–OH or Ti–OR treating textile substrates with TiO2 nanoparticles without requiring subsequent heating of the textile. Perelshtein et al. (2012) were first who used this method for deposition of titanium dioxide nanoparticles with anatase and rutile crystalline structures on cotton fabrics with self-cleaning activities in photodegradation of methylene blue. Their proposed method involved the in situ generation of TiO2 nanoparticles and their simultaneous deposition onto the fabric in a one-step reaction by using ultrasound irradiation promoting the crystallization process of titania due to the high local temperature and pressure generated during the collapse of the acoustic bubble under sonochemical irradiation. Fast migration of the synthesized nanoparticles onto the fabric caused local melting of the fibers at the contact sites and resulted in strong adherence of the nanoparticles to the fabric surface. Akhavan Sadr and Montazer (2014) developed
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the in situ sonosynthesis of nano TiO2 particles on cotton fabric using titanium tetra isopropoxide as a precursor and ultrasonic bath (50 kHz, 50 W). The treated fabric showed excellent self-cleaning properties with no negative effect on the fabric mechanical strength. The best self-cleaning property was obtained by using 9 mL precursor for 4 h sonication at low temperature (75°C). Au/TiO2/SiO2 nanocomposites were used to improve the visible light self-cleaning performance of cotton. Although the prepared nanocomposite was successful, its application was limited due to high cost of gold (Wang et al., 2012). Many research studies have been carried out on the use of Ag/TiO2 nanocomposites on cotton fabrics providing higher stain photodegradation due to the role of silver in trapping the excited electrons reducing charge carrier recombination rate and enhancing the visible light absorption arisen from silver plasmon resonance. Platinum (IV) chloride modified TiO2 (Pt-TiO2) and N-TiO2 (Pt-N-TiO2) nanosols have been synthesized through a low-temperature precipitation-peptization method. The visible light activity was attributed to the surface strongly attached PtCl6 anions, which enable visible light activity of TiO2 through a mechanism of charge transfer from ligand to metal excitation. However, the modification of Pt does not notably improve the performance of N-TiO2 coatings because the surface-adsorbed species on N-TiO2 block the adsorption of PtCl6 anions (Long et al., 2016). Nano TiO2 has been also applied on wool to produce self-cleaning textile, although the application has some difficulties due to low thermal resistance of wool fibers. Montazer and his research group have done many researches to treat wool fabrics with nano TiO2 Degussa. Incorporation of carboxylic acids as crosslinking agents and preactivation of surface with potassium permanganate has been also done to enhance the adsorption of nanoparticles. Self-cleaning efficiencies of the treated samples were assessed toward the discoloration of coffee, fruit juice, concentrated tea, and Methylene Blue dye (Montazer and Pakdel, 2011). Enzymatic pretreatment was also beneficial for enhanced nanoparticles adsorption and subsequent improved self-cleaning properties of wool/polyester fabrics treated with TiO2 Montazer and Seifollahzadeh, 2011). A uniform coating of TiO2/ SiO2 (50:50 and 30:70) nanocomposites was formed on wool to produce self-cleaning and superhydrophilicity. A higher concentration of silica resulted in enhanced self-cleaning (Pakdel et al., 2013). Sonochemistry was also successful to prepare wool fabrics with superior self-cleaning activities. Behzadnia et al. (2014a) presented a novel idea to
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prepare nanocrystalline TiO2 on wool fabric under ambient pressure at 60–65°C using in situ sonosynthesis method. They suggested the higher photocatalytic efficiency of TiO2 nanoparticles prepared by titanium tetra isopoxide in comparison to titanium butoxide as a precursor. The same group also studied the sonosynthesis of N-doped nano TiO2 on wool fabric at low temperature using ammonia (Behzadnia et al., 2014b). Due to lack of functional groups and hydrophobicity of polyester fibers, their successful nanofinishing with TiO2 is dependent on preactivation of surface using chemical or physical surface modification methods. For instance, RF-plasma, MW-plasma, and vacuum-UV light irradiation have been applied as pretreatment allowing the loading of TiO2 by wet chemical techniques in the form of transparent coatings constituted of nanoparticles of diverse sizes. The treated samples possessed significant ability to degrade coffee stains. Recently, simultaneous surface modification of polyester fibers and in situ sonosynthesis of TiO2 nanoparticles have been carried out using ultrasound bath. Hydroxylation of terephthalate occurred by hydroxyl radicals formed during water sonolysis, forming functional groups on polyester surface enhancing nanoparticles adsorption. Self-cleaning activity of sonosynthesized nano TiO2-treated polyester samples toward degradation of Methylene Blue stain was superior to coating of fabric with commercial nano TiO2 (Harifi and Montazer, 2017). 9.3.2.3 ZnO nanoparticles for self-cleaning textiles Up to now, ZnO nanoparticles have been used parallel with TiO2 as ideal photocatalysts. However, incorporation of ZnO nanoparticles into textile materials has been recently gained interest, and the number of studies concerning synthesis and application of nano ZnO particles is not to the level of textiles treated with nano TiO2 particles. Kathirvelu et al. (2010) synthesized nano ZnO by homogeneous phase reaction and then applied on the cotton fabric to obtain the stain-eliminating function. The authors reported that a long time was required to remove stains from the substrate due to large band gap of ZnO making it inefficient to be excited by sunlight. In situ synthesis of ZnO nanoparticles on starch-sized cotton fibers was successful to produce excellent self-cleaning properties against Methylene Blue stain by 2% zinc nitrate and 15% sodium hydroxide using the reducing and stabilizing effect of starch (Khosravian et al., 2015). In situ biosynthesis of zinc oxide nanoparticles on cotton fibers was also reported beneficial to achieve Methylene Blue dye stain self-cleaning properties. In the proposed
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method, Keliab with 15% (v/v) was used as a natural agent to produce ZnO nanoparticles (Aladpoosh and Montazer, 2015). Using triethanol amine as a reducing agent and simultaneous surface modification of polyester fibers resulted in textiles with superior self-cleaning properties due to the photocatalytic activities of ZnO nanoparticles (Poortavasoly et al., 2016). N-doped ZnO/TiO2 nanocomposite was sonosynthesized on wool fabric in ultrasonic bath through hydrolysis of zinc acetate and titanium isopropoxide. Presence of optimal nitrogen and TiO2 on ZnO led to enhanced photocatalytic properties under sunlight light irradiation due to separation of photogenerated electron and holes, reducing the recombination rate. Moreover, sonochemistry created smaller nanoparticles with higher crystallinity resulting in superior self-cleaning properties than conventional sol-gel treatments (Behzadnia et al., 2015). 9.3.2.4 Other nanosemiconductors ZrO2 nanocrystals were successfully synthesized and deposited onto wool fibers using the sol-gel technique at low temperature. Although the treated samples possessed photocatalytic activities toward Methylene Blue and Eosin Yellow dyes, the self-cleaning properties were lower than samples treated with TiO2 nanoparticles in a similar manner. This was attributed to the lower band-gap energy of titania (3.2 eV) comparing with zirconia (4.5 eV) and anatase crystalline structure of TiO2 (Moafi et al., 2010). Nano Cu2O particles were synthesized on cotton fabric using CuSO4 as a precursor and glucose as reducing and capping agent in alkali. The treated fabrics showed photocatalytic activity toward the degradation of Methylene Blue under daylight (Sedighi et al., 2014). Recently, cotton fabric was in situ treated with nanocupric oxide using nanobio method to impart self-cleaning activities (Bashiri Rezaie et al., 2017). 9.3.2.5 Methods for evaluating photocatalytic self-cleaning properties In most studies, self-cleaning properties are assessed by staining the treated samples with organic dirt such as coffee, make up, or dye. Methylene Blue, Methyl Orange, Rhodamine B, and reactive black 5 are the most common model dye compounds. After exposure of samples to light irradiation, the degree of discoloration is evaluated by colorimetric measurements before and after light illumination and is reported as ΔE (Eq. 9.4). 1=2 ΔE ¼ ðΔL ∗ Þ2 + ðΔa∗ Þ2 + ðΔb∗ Þ2 (9.4)
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where ΔL*, Δa*, and Δb* are color coordinates of samples in Lab* color space before and after light exposure. Self-cleaning efficiencies of treated textiles are also evaluated by scanning the samples and measuring the color coordinates in RGB color space using Matlab software (Eq. 9.5). 4 RGB ¼ ðR2 R1 Þ2 + ðB2 B1 Þ2 + ðG2 G1 Þ2 1=2 (9.5) where R1G1B1 and R2G2B2 are color coordinates of samples before and after light illumination, respectively (Harifi and Montazer, 2014a, b). Staining gray scale can be also used to evaluate the self-cleaning effectiveness of the treated samples. In this regard, 5 and 1 refers to maximum and minimum stain degradation, respectively. In some studies, treated textiles are dipped in dye solutions and light irradiated for a definite time duration. Prior to irradiation, the system should be kept in dark for some hours, e.g., 24 h to reach adsorption-desorption equilibrium between treated fabric and dye. Here, photocatalytic efficiencies of the treated samples are evaluated by measuring dye absorbance at maximum wavelength before and after light irradiation (Eq. 9.6). Conversion% ¼ ðA0 At Þ=A0 100 (9.6) where A0 and At are dye absorbance at maximum wavelength before and after t (h) light irradiation, respectively (Harifi and Montazer, 2014a, b).
9.4 CONCLUSION In addition to the published studies concerning with two common approaches for preparing self-cleaning textiles, which have been thoroughly discussed in this chapter, production of surfaces with adjustable hydrophilic and hydrophobic properties for other applications such as separation of oil from water will be a focus of future researches. New photocatalysts such as metal organic frameworks (MOFs) will be also applied on various textile substrates to impart self-cleaning properties. These materials with versatile structures have recently attracted researchers for their photocatalytic efficiencies, although research to enhance their properties together with their stability under photocatalytic reactions needs to be further developed. Graphitic carbon nitride (g-C3N4) is also an attractive low-cost sustainable metal-free photocatalyst with the potential to absorb visible light. Although still not applied on textile substrates, we will see more studies in future using these new photocatalysts. Research will also focus on nanocomposite structures of the new photocatalysts to achieve enhanced properties.
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FURTHER READING Ganesh, V.A., Raut, H.K., Naira, A.S., Ramakrishna, S., 2011. A review on self-cleaning coatings. J. Mater. Chem. A 21, 16304–16322. Ragesh, P., Ganesh, V.A., Naira, S.V., Nai, A.S., 2014. A review on ‘self-cleaning and multifunctional materials’. J. Mater. Chem. A 2, 14773–14797. Schneider, J., Matsuoka, M., Takeuchi, M., Zhang, J., Horiuchi, Y., Anpo, M., Bahnemann, D.W., 2014. Understanding TiO2 photocatalysis: mechanisms and materials. Chem. Rev. 114 (19), 9919–9986.