Assimilating the photo-induced functions of TiO 2 -based compounds in textiles: emphasis on the sol-gel process

Abstract

Numerous investigations have recently been conducted to enhance the intrinsic properties of textiles and add new functionalities to textile products. The photocatalytic features of nanoparticles, notably titanium dioxide (TiO₂), have played a pivotal role in this pursuit. This review article presents an overview of the use of pure as well as modified TiO₂ in textiles. In addition, some of TiO₂ modification methods, pertinent photocatalytic mechanisms and potential applications of metallic and non-metallic nanocomposites of TiO₂ in functionalizing textiles, are highlighted. Furthermore, current research accomplishments and future prospects in this field are discussed.

Keywords

titanium dioxide photocatalytic properties textile UV visible light activation

Textile products have been modified with nanomaterials to achieve tailor-made characteristics, particularly in specialized products for sports, healthcare, and defense-related applications.^1–4 TiO₂, Al₂O₃, ZnO, and MgO are among the most commonly used semiconductors in functionalizing textiles.^2,3,5 Other types of nanoparticles, such as nano-clays,^6–11 carbon nanotubes (CNTs),^9,12–16 metal nanoparticles,^9,17,18 ZrO₂,^19,20 and SiO₂,^21–25 have also drawn attention in diverse fields. Among the semiconductors, nano-sized TiO₂ has shown promise due to its UV-induced photocatalytic features. Since the discovery of the photocatalytic properties of TiO₂ and other semiconductors, numerous studies have been conducted to exploit these properties in many industries.^26,27

Titanium dioxide has three distinct crystalline structures, namely anatase, rutile and brookite. The favorable features of TiO₂ such as chemical stability, low cost, accessibility, and non-toxicity have given rise to its wide applications. When TiO₂ is irradiated with light, whose energy is equal to or greater than its band gap (388 nm for anatase), the negative electrons of the valence band [e⁻(cb)] are excited and consequently promoted to the conduction band, leaving positive holes [h⁺(vb)].^28–30 The resultant photo-induced negative electrons and positive holes can trigger a series of reduction and oxidation reactions, respectively.^29,30 The negative electrons react with oxygen molecules, producing superoxide anions (O₂^.–) while the positive holes react with water molecules producing hydroxyl radicals (^.OH). These active species cause oxidation of unwanted adsorbed organic compounds ultimately to water and carbon dioxide. The outstanding features of TiO₂ arise from its high surface area, which facilitates the diffusion of generated electrons/holes before recombination.³¹ In most cases, anatase shows more efficient photocatalytic activities under UV radiation in comparison with those of the rutile and brookite phases. The photocatalytic mechanism of TiO₂ is shown in Figure 1.^27,31–33 Numerous investigations regarding the synthesis and application of TiO₂ nanoparticles in textiles using various methods have been carried out. Of these, the sol-gel process is regarded as one of the most favorable approaches.

Figure 1.

Photocatalytic mechanism of TiO₂.

The sol-gel process in textiles

There are some approaches through which titanium dioxide nanoparticles can be produced and applied to various surfaces. The sol-gel method,³⁴ flame spray pyrolysis,³⁵ chemical vapor deposition,^36,37 pulsed laser deposition,³⁸ ion-assisted electron beam evaporation,³⁹ plasma enhanced metal organic chemical vapor deposition (PE-MOCVD)⁴⁰ and atomic layer deposition^41,42 are noteworthy. Given the importance of the sol-gel process, there have been several studies utilizing this approach. In general, the textile finishing process through the sol-gel method is comprised of several steps, including the preparation of nanosols, coating, drying and then stabilizing the applied nanoparticles on the textile substrate via curing,^43–45 as shown in Figure 2. A nanosol can be defined as a metastable liquid dispersion of nanoparticles that has relatively low viscosity.⁴⁵ The deposition of nanoparticles (e.g. titanium dioxide) on discrete surfaces, such as glass or textiles, has also been investigated.^45–47

Figure 2.

Transformation of precursor to coating layer on textiles based on the sol-gel method.

The use of organic or inorganic precursors to prepare a sol has been reported.⁴⁴ Of common precursors, metal alkoxides have mostly been employed as starting compounds in sol preparation, due to their instant reaction with water.⁴⁴

With respect to textiles, researchers have been meticulous in optimizing the parameters of the finishing process to obtain the highest photocatalytic efficiency and minimize the possible damages of the finishing process to the intrinsic properties of fibers. The rate of hydrolysis and condensation reactions of precursor, processing, and curing conditions are among the most important parameters of textile finishing via the sol-gel method. After curing, a three-dimensional network of nanoparticles is formed on the surface of a textile. Through the sol-gel process, a thin transparent film of nanoparticles with desired particle size can be obtained at low temperatures. As a result, intensive research has been conducted to explore the use of this process to produce crystalline films on various textile substrates.⁴⁸ Although the sol-gel process is one of the most appropriate methods for textile applications, some disadvantages have restricted its use, such as the destructive impact of acids used in sol preparation on the inherent properties of fibers.⁴⁹ Equally important is the thermal resistance of fibers and the temperature required for the formation of photocatalytically active titania films.^49,50 Given that textile fibers have varied thermal stabilities, the temperature and duration of the curing process should be optimized based on the type of fiber.⁴⁵ In the sol-gel method the processing temperature, pH value, solvent nature, type and concentration of the precursor play significant roles in controlling the size, shape, and phase of the crystallites and consequently the photocatalytic properties.^51,52 For instance, it has been reported that low-temperature synthesized titania is more suitable for photocatalytic applications.⁵³ It is noteworthy that based on the pH of the sol, the resultant film has different properties.^44,45 For instance, in acidic conditions, sols with compact configurations are produced, albeit with reduced strength for the resultant networks;⁴⁴ while basic conditions lead to aggregated particles with high porosity.^44,54 The presence of alcohol also has a bearing on the stability of nanosols and it could effectively extend the storage period of sols.⁴⁵ During hydrolysis of precursors, alcohol is produced as a by-product, increasing the miscibility of sol components and consequently a better adhesion on the fiber surface.^44,54 Using the sol-gel process as a simple and cost-effective method, fabrics can be effectively coated with metal oxides, such as TiO₂ nanoparticles. The application process involves dipping the textile in a sol, padding, and thermal treatments for drying and fixation.⁴⁵ Padding the samples serves dual purposes of removing excess uptake of sol and creating an even coating layer on a textile surface. In the sol-gel preparation of TiO₂, the use of precursors, such as titanium alkoxide, titanium tetrachloride, and titanium halogenide, has been reported.^44,55

Photocatalytic applications of TiO₂ in textiles

Functionalizing textiles with TiO₂ colloids

The application of photocatalytic nanoparticles, notably TiO₂, in textiles has been pursued in several studies. Some properties such as self-cleaning, UV protection, hydrophilicity, hydrophobicity, antimicrobial activity and wrinkle resistance, are among the most important functionalities of textiles using nano materials.^56,57 Daoud et al. devised a low-temperature sol-gel process for functionalizing cotton with TiO₂ nanoparticles.⁵⁸ In their study, titanium tetraisopropoxide (TTIP) precursor was added to an aqueous solution containing ethanol to form anatase TiO₂ colloid that was used to produce self-cleaning and UV protective cotton fabrics.^58,59 The prepared sol was applied to the surface of cotton via a dip–pad–dry–cure process.⁵⁸ Furthermore, the ultraviolet protection factor (UPF), a reliable scale to determine the UV protection property of textiles, as well as the UV blocking features of treated samples were evaluated.⁵⁹ A UPF of 50+ is required to achieve an excellent rating of UV protection.^58–60 Figure 3 depicts the UPF range for protective textiles based on the Australia/New Zealand standard.^60,61 Cotton fabrics coated with titanium dioxide nanoparticles showed excellent UPF rating, indicating appropriate stability of coating layer on the surface of fabric.^58,59

Figure 3.

Ultraviolet protection factor scale for textiles.

In order to assess the role of sol preparation temperature, Qi et al.⁵⁸ synthesized TiO₂ sols at three temperatures, 25, 40, and 60℃ and analyzed the resultant photocatalytic activity. The results confirmed that the processing temperature has a direct impact on the properties of the TiO₂ coating on cotton surfaces.⁵⁸ Colloids prepared at higher temperatures showed greater UV protection. This is due to the higher crystallinity of nanoparticles devised at higher temperature.⁵⁸ Conversely, it was observed that low-temperature-prepared TiO₂ showed superior bactericidal properties, which can be attributed to the smaller particle size providing a higher specific surface area.

The self-cleaning property was evaluated by the removal of red wine and concentrated coffee stains on coated cotton fabrics. The removal of stains on coated samples was achieved after 20 h of irradiation using a Suntest solar simulator (Figure 4).⁵⁸ In addition, the durability of the TiO₂ coating was analyzed and the results revealed that the characteristics of coated samples remained intact even after 20 wash cycles.⁵⁸ Hydroxyl groups of cellulose and TiO₂ established covalent bonds during the curing process resulting in a durable coating layer on cotton. ⁵⁸

Figure 4.

Self-cleaning cotton produced based on the sol-gel method:⁵⁸ (a) pristine cotton; (b) TiO₂-coated cotton.

The antimicrobial properties of a uniform layer of TiO₂ deposited on the surface of cellulose fibers were also investigated.⁴⁹ It was reported that TiO₂ particles produced by sol-gel method showed antimicrobial properties against Staphylococcus aureus, a Gram-positive bacteria. Scanning electron microscopy (SEM) images of coated cellulose fibers showed that the size of TiO₂ nanoparticles deposited on the surface of fibers was about 10 nm. Although the mechanical properties of treated samples were similar to untreated ones, some minor drop in inherent properties was inevitable due to the presence of acid in the sol. To preserve the intrinsic mechanical properties of fabrics, the titania coating underwent a drying process followed by a neutralization treatment with sodium carbonate prior to the curing step.⁴⁹ Overall, TiO₂ nanoparticles formed by the sol-gel process show good stability on the surface of cellulose fibers or cotton fabrics.⁴⁹

In addition to cotton, self-cleaning wool fabric has also been achieved.^62,63 Despite their abundance, luxury image and long use in textiles, the full potentials of protein fibers, such as wool and silk, have not yet been realized. This has resulted in numerous investigations not only to modify the properties of protein fibers, but also to overcome their undesirable properties, such as low thermal and chemical stability. In a study by Daoud et al., a TiO₂ nanocoating was conferred on the surface of wool fabrics to impart self-cleaning property.⁶² The photocatalytic activity was assessed based on the red-wine stain degradation on fabrics exposed to simulated sunlight.⁶²

Figure 5.

Color removal of coffee stain on wool fabric coated with TiO₂ sol:⁶³ (a) pristine wool; (b) TiO₂-coated wool.

Similarly, other studies encompassed the diverse aspects and prominent parameters of the sol-gel process.^64–67 Tung and Daoud in a series of studies^63,65–67 optimized and evaluated the parameters pertinent to the sol gel method to produce self-cleaning wool. The effect of acids utilized in the TiO₂ sol preparation on features, such as UV absorption, self-cleaning, and mechanical properties of wool fabrics was investigated.⁶³ Two strong acids, namely hydrochloric acid (HCl) and nitric acid (HNO₃), were used to prepare the sols. In addition, acetic acid was employed to alleviate the reactivity of the TiO₂ precursor and to stabilize the nanoparticles in the sol.⁶⁸ They showed that the UV absorption property of coated fabrics with nitric-acid-based sol is inferior to that of hydrochloric-acid-based sol.⁶³ Both sols showed a strong self-cleaning activity on wool fabrics, degrading red-wine and coffee stains within 20 h of solar-simulated light illumination. However, photoyellowing was observed in nitric-acid-based sol coated wool samples after UV illumination.^63,67 HCl improved the crystallinity of nanoparticles, suggesting that the nature of the acid plays a role in the growth of crystallites rather than the crystalline phase.⁶³ Moreover, the particle size obtained was affected by the type of acid in a way that the hydrochloric-acid-based sol contained relatively smaller particles with an average particle size around 5.3 nm. Figure 5 shows the self-cleaning property of wool fabric coated with hydrochloric-acid-based sol.⁶³

The impact of coating process with acidic colloids on the mechanical properties of fabrics is one of the important parameters that should be considered.⁶³ Coating the wool fabrics with HCl- and HNO₃-based sols reduced the tear strength of wool fabrics by 29% and 37%, respectively. However, the presence of nanoparticles can provide a protective shield for fabric against the destructive impact of UV. A greater reduction of tensile strength was observed for untreated wool fabric after 40 h of exposure to simulated sunlight illumination compared with the samples coated with TiO₂.⁶³

In order to assess the impact of post-coating thermal treatment, wool fabrics have been coated with nanoparticles and cured at 120 and 150℃. For coatings from nitric-acid-based sol, no obvious difference in photocatalytic activity or UPF was observed. Conversely, for coatings from hydrochloric-acid-based-sol, increasing the curing temperature led to a slight reduction of UPF. Coatings from nitric-acid-based-sol blocked both UV-A and UV-B regions while coatings from hydrochloric-acid-based sol showed an efficient UV protection mostly in UV-A region.⁶⁷ Moreover, the color degradation test of methylene blue solution in the presence of treated samples demonstrated that higher temperature could bolster the photocatalytic efficiency of wool fabrics coated with nitric-acid-based sol. Conversely, a reduced discoloring ability was observed for samples coated with hydrochloric-acid-based sol. Furthermore, the curing process did not have a direct effect on fiber’s tensile strength.⁶⁷

The TiO₂ sol preparation time is one of the important factors that should be considered in synthesizing sols.⁶⁷ Tung and Daoud studied two different preparation times, namely 2 and 16 hours.⁶⁷ The longer preparation time, the higher probability of aggregation among nanoparticles, thus reducing the transparency of the sol. The crystallite size of anatase TiO₂ of the 2 and 16 hours prepared sols was 5 and 5.3 nm, respectively.⁶⁵ The negligible difference between the UPF of coated fabrics showed a successful nucleation of TiO₂ nanocrystals within 2 hours. The discoloration of red wine and concentrated coffee stains was used to measure the self-cleaning property of coated wool. Although the results showed an effective color removal capability for both sols, their performance after washing differed. The tensile strength of wool samples coated with 16 hour prepared sol was not significantly different from that of pristine wool. However, some reduction in tear strength of samples coated with 16 hour prepared sol was observed.⁶⁵

Similarly, the effect of precursor concentration on the obtained photocatalytic features has also been investigated. Through increasing the concentration of TiO₂ precursor, larger particles were obtained due to the greater probability of aggregation in the sol.⁶⁶ Coating wool with sols containing high concentrations of TiO₂ nanoparticles enhanced the wettability as well as the self-cleaning property, due to a greater interaction and more deposition of nanoparticles on wool surface.⁶⁶ In addition, it was observed that by increasing the concentration of TiO₂ in the sol, a higher level of protection against UV and greater tensile strength were observed. This could be due to more deposition of nanoparticles and increased diameter of fibers, respectively.⁶⁶ These results suggest that five prominent factors in the sol-gel process should be considered in textile application (Figure 6).

Figure 6.

Key parameters affecting the efficiency of TiO₂ nanosol in textiles.

In a modified sol-gel method, the damaging effects of acids on fibers can be alleviated by decreasing the amount of acid used in the sol.⁶⁹ Cotton samples were treated in a finishing bath with pH of around 5. According to the X-ray diffraction (XRD) pattern of the prepared sol, anatase and brookite phases were detected.⁶⁹ The treated cotton samples retained their self-cleaning property under UV even after 20 wash cycles, suggesting the existence of strong bonding between TiO₂ and the cotton surface.⁶⁹ Moreover, Qi et al. also analyzed the potential impacts of the photocatalytic activities of TiO₂ nanoparticles on the tensile strength of cotton samples.⁷⁰ After 20 h of light illumination on pristine and coated cotton fabric samples, the fabrics tensile strength was measured. It was observed that the presence of TiO₂ nanoparticles on the surface of cotton not only had no destructive effects, but also protected the samples against photo-decomposition. After UV exposure, pristine cotton experienced greater reduction in tensile strength in comparison with coated fabric.⁷⁰

A UV-protective and self-cleaning cotton fabric was produced using sol-gel method by Abidi et al.⁷¹ They used tetrabutyl orthotitanate [Ti(OC₄H₉)₄], absolute ethanol and HCl in the sols synthesis process. TiO₂ sols with four different pH (1.5, 2, 3.5, 4.5) were applied to cotton fabrics and stabilized through a curing process at 150℃ for 5 min. The coated samples were then boiled in a water bath.⁷¹ Samples treated with TiO₂ sol with pH 1.5 showed the greatest discoloration of Cibacron red dye on the fabrics surface. However, the low pH had a negative effect on the mechanical properties of the fabrics. The presence of TiO₂ nanoparticles on fabrics coated with TiO₂ sol (pH 2) photocatalytically decomposed red-wine and coffee stains after 28 h of exposure to UV irradiation.⁷¹ The self-cleaning results were reproducible, even after 18 cycles of home laundering. The washing fastness of coated samples was tested based on AATCC TM 124 (31) standard test method.

Stabilizing nanoparticles on fabrics and resultant features

One of the most important aspects of surface functionalization of textiles with nanoparticles is stabilizing the coating layers to reproduce the obtained features after the washing processes. Modifying some fabric surfaces such as wool–polyamide, polyester and cotton with plasma pretreatment prior to coating with titanium dioxide has been reported to be effective in enhancing the stability of nanoparticles and photocatalytic efficiency.^72–75 The main purpose of such pretreatments is to enrich the fabric surface with negatively charged groups, in order to increase the bonding of TiO₂ nanoparticles with the substrate. It was found that TiO₂ nanoparticles had a greater durability on the surfaces which underwent a plasma pretreatment process.⁷² The pretreatments resulted in activation of fabrics surface, thus increasing the uptake and stability of deposited TiO₂.^73,76

One of the approaches of stabilizing nanoparticles on the fabrics is by using poly-carboxylic acids such as succinic acid, 1,2,3-propanetricarboxylic acid or 1,2,3,4-butanetetra carboxylic acid as chemical spacers in the presence of sodium hypophosphite.^77–79 Given the existence of numerous hydroxyl groups in the configuration of cotton, stable ester linkages between the carboxylic groups and the surface of cotton are formed. This in turn increases the durability of TiO₂ nanoparticles on cotton fabric.⁷⁸ TiO₂ nanoparticles have also been affixed on cotton fabric through co-graft polymerization of 2-hydroxyethyl acrylate (HEA) under γ-ray irradiation.⁸⁰ The applied nanoparticles showed good durability during simulated washing tests, which was equivalent to 150 home laundering cycles.⁸⁰

Stabilizing the nanoparticles on the surface of wool, in comparison with cotton, is more challenging, due mostly to the presence of fewer functional groups, such as hydroxyls, on the surface of wool. This underlines the necessity of using an efficient pretreatment method to increase the stability of nanoparticles on the fibers. Acylation of wool’s keratin with succinic anhydride is an effective pretreatment in increasing the concentration of the negatively charged carboxyl groups on wool surface and hence increasing the adsorption and stability of nanoparticles.^64,81–84 This pretreatment resulted in even distribution and adsorption of nanoparticles and thus greater photocatalytic activity.⁶² Succinic anhydride is a non-toxic substance, one of its two carboxylic groups can react with the hydroxyl groups on the surface of wool, creating ester bonds with wool fabric.^62,78 The other carboxylic group also plays a role as a potential adsorption site for TiO₂ nanoparticles. The affinity of TiO₂ nanoparticles towards the carboxylic groups stems from a strong electrostatic interaction.^62,85 Moreover, the creation of covalent bonds between the carboxylic groups and –NH₂ and –SH groups of wool has also been demonstrated.^62,86 The mechanism of stabilizing TiO₂ nanoparticles on wool through the succinylation process is shown in Figure 7.

Figure 7.

Stabilizing TiO₂ nanoparticles on wool fabric using succinic anhydride.⁶²

Montazer and Pakdel anchored titanium dioxide nanoparticles (P-25) to the surface of wool fabric using carboxylic acids.⁸⁷ The TiO₂ nanoparticles were dispersed in the impregnating bath by ultrasonic waves and stabilized by cross-linking agents, namely citric acid (CA)⁸⁷ and 1,2,3,4-butane tetra carboxylic acid (BTCA)⁷⁹ in the presence of sodium hypophosphite (SHP) (Figure 8). BTCA may be a more effective cross-linking agent than CA due to its four carboxyl groups.⁸⁸ The presence of TiO₂ nanoparticles on the surface of wool provided greater stability against UV, thus decreasing the photodegradation of wool.^79,87 The authors demonstrated that by increasing the concentration of TiO₂ nanoparticles in the impregnating bath, the amount of surface adsorbed particles increased, which provided greater protection against UV. In this approach, a surface oxidation with potassium permanganate in an acidic solution (pH 3) followed by a treatment with sodium bisulfite was introduced as a pretreatment method to improve the uptake of nanoparticles.^79,87,89 The negatively charged carboxyl groups led to a higher deposition of positively charged TiO₂ nanoparticles on the fabrics.⁸⁷

Figure 8.

Cross-linking mechanism of BTCA on wool.⁷⁹

The self-cleaning property of wool fabrics coated with TiO₂ and stabilized with cross-linking agents was evaluated, based on the removal of drink stains such as concentrated coffee, tea, and fruit juice under UV light.⁸⁹ By treating wool fabric with TiO₂ nanoparticles along with carboxylic acids, the hydrophilicity of wool fabrics increased and water droplets were absorbed more rapidly in comparison with pristine wool.⁸⁶ It was also found that UV irradiation of coated samples reduced the water contact angle.⁸⁶ The antimicrobial activity of coated samples with P-25 against two common pathogenic microorganisms, Escherichia coli and S. aureus, was also evaluated.⁹⁰ The maximum reduction for E. coli was around 70%, whereas there was no specific antimicrobial property for S. aureus.^90,91 By increasing the concentration of cross-linking agents (BTCA or CA) in the coating bath, the photocatalytic activity was improved, due to a higher adsorption of nanoparticles on the surface.⁹⁰ Anti-felting characteristics and higher stability against alkali media of wool fabrics coated with TiO₂ nanoparticles were also reported.⁹⁰ Optimizing parameters such as concentration of cross-linking agents, the content of TiO₂ (P-25) and the duration of finishing process, was studied to obtain the highest photocatalytic efficiency.⁷⁹ To this end, the Response Surface Methodology (RSM) was employed for statistical analysis, where the role of all contributing factors on the conferred properties was determined individually.^79,86

In addition to natural textiles, there are some reports on integrating TiO₂ in synthetic fibers. However, increasing the affinity and stability of TiO₂ nanoparticles on synthetic fibers has been rather challenging. Several pretreatment methods such as radio-frequency (RF) plasma, microwave (MW) plasma, and vacuum-UV light irradiation have been used to introduce additional reactive sites on synthetic fabrics for TiO₂ absorption.^92,93 Using plasma pretreatment, negative groups, such as –COO⁻ and –O–O⁻, can be introduced onto the surface of fabric, hence increasing the bonding strength of nanoparticles.⁹⁴

Similarly, fabric pretreatment with alginate to increase the bonding of TiO₂ nanoparticle has been reported.⁹⁵ Having carboxylic groups in its structure, alginate can be useful in the modification of the surface of polyester fabrics. A hydrothermal method has also been examined to produce a layer of nano titanium dioxide on the surface of polyethylene terphetalate (PET) fabric.⁹⁶ The photocatalytic efficacy of nanoparticles on the fabric was confirmed by the photo-degradation of methyl orange and UV absorption of treated fabrics.⁹⁶

Some mechanisms associated in TiO₂-induced features

TiO₂ nanoparticles can be synthesized and applied to the fabrics surface through different approaches. Integrating TiO₂ nanoparticles in textiles brings about some potential capabilities for garments. Basically, the novel features of functionalized textiles are due to the photocatalytic aspects of TiO₂ nanoparticles. As mentioned already, TiO₂ is excited under UV light producing electrons and holes, which in turn generate superoxide anions and hydroxyl radicals, respectively. Through the reaction of the stain adsorbed on fabric with the aforementioned active species, products such as water and carbon dioxide are produced, resulting in fading of the stain color on fabrics.^76,89

The UV absorption mechanism can be explained by the solid band theory of nanoparticles.²⁶ TiO₂ absorbs the energy of UV and the electrons of TiO₂ are excited from the valence band to the conduction band. The generated species may be involved either in a recombination process or in redox reactions on the surface of TiO₂. Consuming the energy of UV by TiO₂ protects the fabric against harmful UV effects.²⁶ Considering the lower band gap of the rutile phase of TiO₂ (3.0 eV) in comparison with anatase one (3.2.eV), the rate of excitation and recombination of electrons is greater for the former. Therefore, rutile has been suggested as a better candidate for UV-protection applications.²⁶

The photo-induced hydrophilicity on fabrics coated with TiO₂ can be explained by the role of positive holes in increasing the concentration of hydroxyl groups on the surface of a photocatalyst.⁹⁷ Some of the photo-generated positive holes react with TiO_2, separating the titanium lattice and oxygen ions through coordinating water molecules.⁹⁷ The coordinated water molecules release protons to alleviate the charge imbalance, leading to the formation of hydroxyl groups. The new hydroxyl groups can increase the surface energy, resulting in higher surface hydrophilicity as well as two-dimensional capillaries on the surface.⁹⁷ Also, the positive holes can oxidize oxygen anions, causing the removal of oxygen molecules from the system and leaving some oxygen vacancies.³² The water molecules occupy the oxygen vacancies increasing the concentration of hydroxyl groups on the surface of photocatalyst.⁹⁸ This process increases the surface hydrophilicity, resulting in fast water absorption.^32,97

The photo-induced antimicrobial activity of TiO₂ is defined by the photocatalytic mechanism of TiO₂ under UV. The active species generated by TiO₂ can attack the cell membrane of microorganisms.^90,97 Under this condition, destruction of cell structure causes leakage of macromolecular compounds, such as proteins, minerals and genetic materials causing the death of cells.⁹⁹

Photocatalytic behavior of TiO₂ nanoparticles can be employed to trigger cross-linking reactions and producing wrinkle-resistance properties in textiles.¹⁰⁰ Using TiO₂ as a co-catalyst, along with SHP and BTCA, can improve the wrinkle-resistance of cotton fabrics,¹⁰¹ where photo-induced properties of TiO₂ accelerate cross-linking reactions of cotton by BTCA (Figure 9).¹⁰² The positive holes of TiO₂ generated under UV confer a positive charge to the carbonyl group of carboxylic acid, subsequent to which ester linkages with hydroxyl groups of cellulosic surface can be formed more efficiently.^102,103 The generated electrons play a role in intensifying the cross-linking mechanism.¹⁰² Photo-induced electrons can react with hydroxyl groups of cellulose facilitating the reaction between cellulose and carboxylic acid.¹⁰² Another reason reported for this new property is that the TiO₂ nanoparticles can penetrate into the amorphous regions of cotton, which reduces the undesirable movements of the cellulose chains. In this situation, intra-molecular hydrogen bonds are not easily affected, increasing the wrinkle recovery of samples.¹⁰¹

Figure 9.

The role of TiO₂ in producing wrinkle-resistant cotton.¹⁰²

There is a great deal of research in which the synthesis and applications of TiO₂ nanoparticles on textiles have been discussed but it is not possible to cover all of them in this review article. Some fellow researchers have focused on other aspects of TiO₂ on textiles, such as photo-bleaching and photo-scouring of wool and cotton,^104,105 moth-proofing of textiles,¹⁰⁶ reducing the photoyellowing rate of powdered wool,¹⁰⁷ increasing the photostability of polypropylene fibers,^108–110 modifying the dyeability of polyester fabrics,¹¹¹ and in situ synthesis of TiO₂ on textiles.¹¹²

Drawbacks of pristine TiO₂ and suggested remedies

The application of pristine TiO₂ nanoparticles in textiles has been the subject of numerous experiments for practical applications. Although TiO₂ has many advantages, there are some inherent problems with its practical applications.¹¹³ The first is its high band gap; it is only photocatalytically active under UV, which forms a very small portion of the solar spectrum.¹¹⁴ Second, the recombination rate of generated negative electrons and positive holes is very high, reducing the efficiency of TiO₂ photocatalytic activity.³³ In order to alleviate these problems, several solutions have been suggested to shift the photocatalytic activity threshold into the visible region (λ > 400 nm). These include (a) doping with metals,^115–118 (b) using blends of TiO₂ with other metal oxides,^119,120 (c) adding some non-metals into the TiO₂ structure and (d) dye sensitization of TiO₂.¹²¹ The following sections outline the application of modified TiO₂ particles in textiles.

Modification of TiO₂

Metal/TiO₂

Metal/TiO₂ as a photocatalyst

As mentioned above, intensive investigations have been conducted to modify the properties of TiO₂. In order to improve the photocatalytic properties of TiO₂, some metals, such as silver,^122,123 gold,^124,125 platinum,¹²⁶ iron,¹²⁷ nickel,^128,129 chromium,¹²⁹ antimony,¹³⁰ and cobalt ions¹³¹ have been employed. The presence of metals has contributed to the enhancement of photocatalytic properties of TiO₂ and shifting its photocatalytic activity threshold to the visible region.^123,132 There are some mechanisms based on which metal and metal ions are able to enhance the photoefficiency of TiO₂ nanoparticles. Metals play a key role in mitigating the undesirable recombination rate of electrons and positive holes by trapping and accumulating the electrons.^133–135 Due to the formation of Schottky barriers, metals act as the accumulation centers for electrons, increasing the separation of electrons and positive holes.¹³⁶ Reducing the energy needed for exciting the electrons of the valence band through introducing some energy impurity levels in the band gap of TiO₂ is another impact of metal dopants, which increases the sensitivity of photocatalysts to visible light.^134,135 In addition, metals undergo surface plasmonic resonance under visible light, thus injecting electrons into the conduction band of TiO₂ and enhancing the efficacy of the photocatalyst.^133,137,138

Among noble metals, silver (Ag) has frequently been studied due to its high efficiency and relatively low cost.¹³⁹ Because of the different work function and Fermi level of TiO₂ and Ag, electrons have a propensity to move towards Ag deposited on the surface of TiO₂.¹⁴⁰ The radius of an ionic metal is important for understanding the mechanism based on which metal improves the functionality of TiO₂. For instance, Ag metal ions are not able to enter into the crystalline lattice of TiO₂ due to their large ionic radius. Therefore, it is expected that silver would be found in the crystal matrix or on the surface of TiO₂.¹³² The efficiency of the nanocomposite depends on the method based on which TiO₂ nanoparticles are doped.^{139,141–143} In addition, the concentration of metals can affect the photocatalytic efficiency, hence finding the optimum doping concentration is important.^123,139 At high concentrations, not only could metals act as stations for electrons, but they could also act as recombination centers, facilitating electron hole recombination.¹⁴⁴

The degradation of dyes has mostly been employed as one of the indicators for the evaluation of the photocatalytic activities of modified photocatalysts.¹³² Sung-Suh et al.¹²³ examined the photocatalytic properties of Ag-loaded TiO₂ by assessing the discoloration rate of rhodamine B (RB) dye under both UV and visible light. They observed that the presence of Ag enhanced the photodegradation rate of RB under visible light by 30%.¹²³ In their study, the role of Ag in increasing the efficiency of TiO₂ under visible light has been examined. Different mechanisms are responsible for enhancing the photocatalytic efficiency under UV and visible light. Under UV, TiO₂ and Ag play a role as a photocatalyst and an electron trap, respectively, which decreases the recombination rate of negative electrons and positive holes.¹²³ While under visible light, Ag and RB have the main contribution in enhancing the photocatalytic efficiency of TiO₂.¹²³ The excitement of Ag under visible light due to the surface plasmon resonance and the effect of dye in intensifying the photocatalytic efficiency play the main roles. Visible light with a wavelength over 470 nm excites the RB adsorbed on the surface of Ag/TiO₂; subsequently, electrons are injected from the excited RB into the conduction band of TiO₂. Electrons are trapped by the Ag metal deposited on the surface of TiO₂, facilitating the reactions between O₂ and electrons leading to greater amount of super oxide anions (O₂^.–) (Figure 10).¹²³ This research claimed that the adsorption of RB on the surface of Ag/TiO₂ would be higher than that on TiO₂ surface, hence higher photocatalytic efficiency.¹²³ Overall, the dyes alone may be employed to enhance the photocatalysis of TiO₂. The particular application of this mechanism in textiles is discussed in the following section.

Figure 10.

The photocatalytic mechanism of metal-TiO₂ particles:¹²³ (b) under UV; (b) under visible light.

In addition to noble metals, doping with rare-earth metals also has an impact on the photocatalytic properties of TiO₂.¹⁴⁵ Through shortening the band gap of TiO₂ by introducing some energy levels, rare-earth metal doping would be helpful to expand the efficiency of TiO₂ into the visible region. Scandium (Sc), yttrium (Y), lanthanum (La) and all of the lanthanide series are categorized as rare-earth metals. It has been reported that due to the large radii of lanthanide ions, they cannot enter the Ti lattice; instead they are adsorbed onto the surface of TiO₂ in the form of oxides.^145,146 Nevertheless, some counter examples, such as samarium and gadolinium, have been reported.^145,147,148 The formation of lanthanide oxides, or integrating with TiO₂ matrix also depends on the employed approach for doping.¹⁴⁵ In view of the lanthanide oxide formation during the calcination step, Ti can insert and supersede the rare-earth metal ions. Concerning gadolinium-doped TiO₂, creation of charge imbalances due to the formation of tetrahedral site would be expected. In order to remove charge imbalance, higher amount of hydroxyl groups would be adsorbed on the surface of modified photocatalyst. This brings about a higher interaction between the hydroxyl groups and positive holes resulting in active hydroxyl radicals, hence a higher photoefficiency.¹⁴⁵ In cases where metal ion can enter the TiO₂ lattice, band-gap modifications occur, shifting the threshold of excitement towards longer wavelength.¹⁴⁸ In doping processes, optimizing the dopant content, type and doping approach should be taken into account.¹⁴⁵

Iron ions can also be effective in enhancing the efficiency of TiO₂ photocatalyst under both UV (320–400 nm) and visible light sources, although controversy remains.¹⁴⁹ Tung and Daoud demonstrated that introducing iron into the TiO₂ lattice could significantly increase the photocatalytic activity under UV.¹⁴⁹ Based on their results, the activity enhancement was much higher under UV than visible light. Increasing the iron’s concentration in the nanocomposite led to a higher efficiency under UV. This was explained by the role of iron in reducing the recombination rate of electrons and holes.¹⁴⁹

Applications of Metal/TiO₂ in textiles

The aforementioned photocatalytic mechanisms of Metal/TiO₂ have been employed to increase the sensitivity of functionalized textiles to the visible light and to enhance the photoefficiency of TiO₂. The visible-light-induced self-cleaning property of cotton fibers treated with Au/TiO₂ nanocomposite has been studied by Uddin et al.¹⁵⁰ An Au/TiO₂ thin layer was formed on the surface of cotton using a low-temperature sol-gel method coupled with a photodeposition process, in which HAuCl₄ was used as gold precursor.¹⁵⁰ It was demonstrated that this nanocomposite is more effective than pure TiO₂ in removing methylene blue stains from the surface of fabric.¹⁵⁰ Transmission electron microscopy (TEM) analysis of synthesized particles revealed that Au nanoparticles with the average diameter of 10–12 nm were dispersed uniformly into the TiO₂ matrix.¹⁵⁰ However, it should be considered that at high concentrations of Au on cotton, the color of fabrics turned to purple restricting the final applications of coated fabrics.¹⁵⁰

Also, applying Ag/TiO₂ nanocomposite to cellulosic fabrics to obtain self-cleaning and antimicrobial functions has been reported.^151,152 Adding the silver into the synthesis process of TiO₂ sol resulted in a smaller size of crystalline TiO₂ nanoparticles. TiO₂ and Ag/TiO₂ nanoparticles had a diameter around 5 and 3.5 nm, respectively.¹⁵¹ In nanocomposites, noble metals, such as Ag and Au, act in part as a sensitizer, thereby extending the activation range of TiO₂ into the visible region.¹⁵⁰ Yuranova et al. compared the bactericidal properties of cotton fabrics treated with Ag and Ag/TiO₂ in both dark and simulator solar light. Ag was affixed on the surface of cotton in order to prevent the growth of microorganisms on wounds. The existence of silver nanoparticles on the surface of cotton changed the color of cotton to yellowish brown.¹⁵³ The diffusion of silver particles with diameter of ∼2–4 nm into the structure of cotton was reported.¹⁵³ The Ag and Ag/TiO₂ treated samples both inhibited the growth of E.coli in dark condition.

In addition gold and silver, iron ions also have been used to enhance the photocatalytic activity of TiO₂ nanoparticles on fabric. Qi et al improved the visible light activity of TiO₂ applied to cotton textiles by doping with Fe³⁺. A low temperature sol-gel process was used to synthesize the Fe doped anatase TiO₂. TTIP and Iron(III) nitrate nonahydrate were employed as TiO₂ and Fe precursors, respectively.¹⁵⁴ The photocatalytic property of cotton fabric treated with Fe/TiO₂ was evaluated based on the discoloration rate of Neolan Blue 2G, coffee and red wine stains under UV and visible light.¹⁵⁴ The obtained results corroborated that adding metals can enhance the photocatalytic activity of TiO₂ nanoparticles on cotton under UV and also visible light.¹⁵⁴ Moreover, increasing the concentration of iron up to an optimal content resulted in higher efficiency. Above the optimal concentration, iron ions transformed to the centers for recombination of electrons and holes, hence decreasing the photocatalytic efficiency.¹⁵⁴

A novel method was employed to stabilize TiO₂ and Ag/TiO₂ nanoparticles on the surface of silk fibers. The surfaces of TiO_2, as well as Ag/TiO₂ nanoparticles, were modified using the antiviral agent 3-(3,4-dihydroxyphenyl) propionic acid (DHBPA). The dimethyloldihydroxyethyleneurea (DMDHEU) resin was grafted onto silk using BTCA as a cross-linking agent. The modified nanoparticles were bonded to the surface of silk fibers through an esterification reaction of the carboxylic groups of DHBPA and the hydroxyl groups of DMDHEU (Figure 11).¹⁵⁵ The establishment of these covalent bonds was confirmed by Fourier transform infrared (FTIR) spectra.¹⁵⁵ Some functionalities, such as UV protection and antimicrobial activity, were evaluated. In terms of UV protection, Ag/TiO₂ coated on the surface of silk showed more efficiency in certain wavelength regions compared with that of pure TiO₂. The antimicrobial property of Ag/TiO₂ on silk was greater than that of pure TiO₂, which emphasizes the role of Ag.¹⁵⁵

Figure 11.

Stabilizing Ag/TiO₂ nanoparticles on silk.¹⁵⁵

Similarly, the antimicrobial activity of wool fabrics, coated with Ag/TiO₂ and pure TiO₂ (Degussa P-25) nanoparticles, stabilized by BTCA and citric acid (CA) cross-linkers, was evaluated.¹⁵⁶ TiO₂ nanoparticles dispersed by ultrasonic waves were doped with silver nitrate under UV illumination producing Ag/TiO₂ nanocomposite. Wool samples treated with Ag and Ag/TiO₂ nanocomposite showed an efficient antimicrobial property against E. coli and S. aureus bacteria.¹⁵⁶ This shows the superior antimicrobial activity of fabrics coated with Ag/TiO₂ nanocomposites in comparison with wool fabric coated with pure TiO₂ (Degussa P-25).^90,156

In situ synthesis of Ag nanoparticles and Ag/TiO₂ nanocomposites on the surface of polyester fabrics has been reported by Allahyarzadeh et al.¹⁵⁷ In this method, simultaneous synthesis of nanoparticles and surface modification of polyester through surface hydrolysis is carried out. Silver precursor (AgNO₃) and TiO₂ nanoparticles (Degussa P-25) were added to a boiling impregnating bath containing NaOH in the presence of polyester fabric for 1 h.¹⁵⁷ The main purpose of hydrolyzing the fabrics surface with an alkali media was to introduce more functional groups on the fabric’s surface to enhance the adsorption of silver and Ag/TiO₂ nanoparticles.¹⁵⁷ The highest rate of photo-induced degradation of methylene blue under daylight illumination was observed on fabric functionalized with Ag/TiO₂.¹⁵⁷ The antibacterial test results demonstrated that wool samples functionalized through the in situ synthesis method showed an excellent efficiency in eliminating E. coli and S. aurous bacteria.¹⁵⁷ Moreover, they analyzed the impact of cetyl trimethyl ammonium bromide (CTAB) addition to the impregnating bath on the self-cleaning property and antibacterial activity of fabrics. These features were improved after the addition of CTAB. This addition enhanced the interactions between the alkali media and the fabrics surface producing more hydroxyl and carboxyl groups, as potential reaction sites with nanoparticles. However, a higher weight loss for polyester fabrics was observed in the presence of CTAB.¹⁵⁷

The functionality of Ag/TiO₂ nanocomposite in removing the odor from the surface of polyester fabrics was also investigated. Ag/TiO₂ nanocomposite was produced under UV illumination in the presence of waterborne polyurethane.¹⁵⁸ The anti-odor property was evaluated based on the decomposition rate of ammonia gas, acetic acid, and trimethylamine under UV. A considerable enhancement in odor removing ability of TiO₂ nanoparticles was observed after doping with silver.¹⁵⁸

TiO₂/SiO₂

One approach to increase the efficiency of TiO₂ is by forming a binary oxide photocatalyst. This modification of the surface characteristics is an effective way to improve the photoefficiency.^159,160 In particular, the modification of TiO₂ as a photocatalyst with silica can result in higher photocatalytic activity. The high surface area, chemical inertness, and transparency to UV of silica make it highly suitable for this application.^161,162

There has been much research in the area of TiO₂/SiO₂ nanocomposites. Anderson and Bard studied the photocatalytic features of TiO₂/SiO₂ employing rhodamine 6G as a test probe.^161,163 The highest photocatalytic activity was observed when the ratio of TiO₂/SiO₂ was 30/70, and the efficiency was almost three times higher than that of pure TiO₂.^161,163 The photocatalytic property of TiO₂/SiO₂ hydrosol was also investigated by Zhang et al.¹⁶⁴ They demonstrated that TiO₂/SiO₂ composite sols caused a higher photodegradation of methylene blue solution compared with pure TiO₂. Guan et al.¹⁶⁵ suggested a correlation between the enhancement of photocatalytic activity in TiO₂/SiO₂ systems and the surface acidity increment. The formation of Ti–O–Si bonds resulted in a positive charge imbalance which created Lewis acid sites.¹⁶⁴ Consequently, the number of hydroxyl groups on the surface of the photocatalyst increased. Moreover, it was observed that the recombination rate of electrons and holes generated under UV in TiO₂/SiO₂ hydrosol systems was lower. This could be related to the role of hydroxide ions adsorbed on the surface of photocatalyst in reacting with positive holes suppressing the recombination rate.¹⁶⁴

There are some publications regarding the chemistry and mechanisms of silica synthesis by sol-gel process.^166–168 The nature of the solvent, pH, the molar ratio between a water and silica precursor, and the type of precursor are influential factors in preparing silica sol.^166,167 Also, the molar ratio between water and precursor affects the rate of hydrolysis and condensation reactions as well as the final applications of the sol.^166,169 For instance, by having linear polymers, sols with low molar ratio are suitable for fiber spinning.^169,170 The hydrolysis and condensation steps of silica are shown in Figure 12.

Figure 12.

Hydrolysis and condensation reactions of silica precursor.¹⁶⁸

Although there are numerous studies on the preparation methods of TiO₂/SiO₂ as well as the influential parameters, rare cases of scientific studies have focused on functionalizing textiles with TiO₂/SiO₂. Cotton fabrics were treated with TiO₂/SiO₂ in an investigation conducted by Qi et al.¹⁷¹ The TiO₂/SiO₂ nanocomposite with a core-shell structure was obtained through deposition of TiO₂ nanoparticles on the outer surface of SiO₂ powder.¹⁷¹ In this study, TTIP and methyltriethoxysilane (MTMS) were employed as precursors of TiO₂ and SiO₂. The average particle sizes for SiO₂ and TiO₂/SiO₂ core-shell were about 500 and 600 nm, respectively.¹⁷¹ TiO₂/SiO₂ showed a higher efficiency in the discoloration of Neolan Blue 2G in comparison with pure TiO₂.¹⁷¹

In other research, cotton fabric was coated with a TiO₂/SiO₂ nanocomposite, which had been prepared by covering the TiO₂ particles with a transparent layer of silica.¹⁷² Silica was used as a barrier layer to prevent the direct contact of photoactive TiO₂ with cellulosic substrate.¹⁷² The particle size of SiO₂ and TiO₂/SiO₂ was 90 and 80–150 nm, respectively. It was found that through the treatment of cotton fabrics with such TiO₂/SiO₂ nanocomposite, a UPF greater than 50+ was obtained, implying the presence of a UV protective layer on the surface of fabrics.¹⁷² Yuranova et al.¹⁷³ applied TiO₂/SiO₂ nanocomposites to the surface of cotton textiles to evaluate the self-cleaning properties. In their preparation of TiO₂/SiO₂ nanocomposites, the optimized ratios were 5.8% (w/w) TiO₂ and 3.9% (w/w) SiO₂. The emission of CO₂ and removal of red wine stain were used as scales to examine the photocatalytic properties.¹⁷³ They claimed that the particle size of TiO₂ and SiO₂ based on this approach was about 4 and 8 nm, respectively. A transparent layer of nanocomposite, with a thickness of 25 nm was deposited on the surface of cotton fibers. In their research study, a silica colloid Ludox SM 30 (ex-Dupont) with a specific surface area of 360 m²/g was used as SiO₂ source. The coated samples were irradiated with Suntest solar simulator for 24 hours, after which the red wine stain on the surface of coated fabric was removed. Based on XRD patterns, the anatase peaks were observed for samples that underwent at least four cycles of coating and thermal treatment. By increasing the thickness of coatings on the surface of textiles, the stiffness increased. A higher amount of CO₂ was emitted from the samples coated with TiO₂/SiO₂ than pure TiO₂, indicating a higher mineralization capability of the nanocomposite.¹⁷³ Although it is true that the active species generated from TiO₂ play the main role in decomposing the stains, excited chromophores of dye molecules under visible light can also come in handy to hasten the photocatalytic mechanisms.¹⁷³ Tannin, which has been known as the chromophore of red wine, can be excited under visible light, injecting negative electrons to the conduction band of TiO₂, thereby bolstering the efficiency of photocatalyst. Furthermore, SiO₂ layer acted as a protective barrier against active species produced by TiO₂.¹⁷³

Similarly, the characteristics of cellulose fiber, coated with TiO₂ and TiO₂/SiO_2, were investigated by Veronovski et al.¹⁷⁴ To determine the role of SiO₂, the difference between sunlight-induced photocatalytic properties of TiO₂ (Degussa P-25) and TiO₂/SiO₂ coated fibers was evaluated. Although the presence of SiO₂ buffer layer reduced the adverse effects of active species on the mechanical properties of fibers, they found that the photoefficiency of TiO₂/SiO₂ was lower than that of TiO₂.¹⁷⁴ Based on this research, the presence of tetra ethylorthosilicate (TEOS) as the precursor of SiO₂ has two positive effects. First, the surface of cellulose can be covered by TiO₂/SiO₂ coating layer through a sol-gel process. In this case, not only were fibers protected by SiO₂ particles against active species, but also the aggregation of TiO₂ particles decreased.¹⁷⁵ Second, the presence of SiO₂ precursor could increase the stability of TiO₂ nanoparticles on the surface of fibers.¹⁷⁴

Pakdel and Daoud synthesized the TiO₂/SiO₂ nanocomposite through a two-step sol-gel method using TTIP and TEOS as the precursors of TiO₂ and SiO₂, respectively.¹⁷⁶ The prepared TiO₂ and SiO₂ sols were mixed based on three molar ratios of Ti/Si including 70/30, 50/50, and 30/70 and then applied to cotton and wool samples.^176,177 It was observed that after incorporating silica, the self-cleaning function on cotton and wool under UV was significantly improved.^176,177 TiO₂/SiO₂ 30/70 followed by 50/50 showed the highest enhancement in coffee stain removal. Also, the application of TiO₂/SiO₂ 50/50 and 30/70 rendered the fabrics superhydrophilic.¹⁷⁷ It was concluded that the presence of silica accelerated the removal of coffee stain under UV exposure while sustaining the inherent mechanical features of fabrics.^176,177

Although it has been demonstrated that SiO₂ can increase the photocatalytic functions of TiO₂, its role depends largely on the method through which the nanoparticles are synthesized and applied to textiles. Moreover, the overall findings germane to the photocatalytic properties of TiO₂/SiO₂, corroborate the synergistic impact of silica on photocatalytic properties of TiO₂.

Non-metal/TiO₂

Another way to increase the visible light sensitization of TiO₂ is through doping with non-metal elements, such as N,^40,178,179 C,^40,180,181 S,^182,183 B,¹⁸⁴ P,¹⁸⁵ and F.¹⁸⁶ There has been much research into the photocatalytic properties of N-doped TiO₂.^{179,187–189} Asahi et al.¹⁷⁹ found that the photocatalytic properties of N-doped TiO₂ were greater than those of pure TiO₂. In addition, they confirmed the ability of nitrogen to mitigate the energy needed to excite TiO₂ and consequently enhance its photocatalytic activities in the visible region.¹⁷⁹ The presence of some mid-gap levels above the valence band of oxygen has been demonstrated in modified photocatalysts. The 2p orbitals of nitrogen can be seen above the valence band narrowing the band gap of photocatalyst (Figure 13).^190,191 Taga revealed that the threshold wavelength to activate N-doped TiO₂ was around 520 nm indicating significant red shift toward the visible region.¹⁹² The amount of CO₂ emitted from photo-degradation of acetaldehyde and toluene was used to evaluate the efficiency of N-doped TiO₂. The photocatalytic activity of modified TiO₂ has been reported based on the decomposition rate of gaseous substances, such as volatile organic compounds and nitrogen oxides.¹⁹³ The photodecomposition of 2-propanol under both UV and visible light was also investigated to assess the impact of nitrogen dopant.¹⁹³ The photo-induced antimicrobial properties of N-doped TiO₂ against some microorganisms, such as E. coli and S. aureus under fluorescent light, has also been demonstrated, revealing higher photocatalytic activities in comparison with pure TiO₂.¹⁹² Additionally, a higher hydrophilicity was observed on surfaces treated with TiO₂–N.

Figure 13.

Band gap modification of TiO₂ by nitrogen doping.¹⁸⁸

Publications on the functionalization of textiles using nitrogen-doped titania is limited.¹⁹² The necessity of a high temperature, preferably above 300℃, for creating a coating layer of N–TiO₂ has prevented the application of this modified photocatalyst to textiles.¹⁹⁴ In a research study, cotton fabric was functionalized with N-TiO₂ sol prepared through the reflux approach at ambient temperature.¹⁹⁴ The photocatalytic property of cotton samples under visible light was investigated by monitoring the degradation rate of methyl orange (MO). It was observed that N-doped TiO₂ led to at least three times higher photodegradation of MO.¹⁹⁴ This was explained by a lower band gap energy of photocatalyst in the presence of nitrogen, which in turn led to the generation of more active species hence a higher efficiency.¹⁹⁴ Following these results, it was observed that through applying AgI nanoparticles to the surface of N/TiO₂-coated cotton fabric, the photocatalytic properties under visible light was enhanced by 2.02 times.¹⁹⁵

Deodorizing natural and synthetic fabrics has been reported using TiO₂–_xN_x. Modified TiO₂ was employed to produce medical outfits, clothes, coats and shirts, drapes, and carpets.¹⁹² This modification method warrants further investigation to examine the role of components in increasing the photocatalytic properties of TiO₂ in textiles.

Dye/TiO₂

Through a dye-sensitization approach, Afzal et al.¹⁹⁶ have produced visible light active self-cleaning cotton fabric. In their study, meso-tetra (4-carboxyphenyl) porphyrin (TCPP) has been self-assembled on cotton fabrics, which had already been coated with TiO₂ sol (Figure 14).¹⁹⁶ It was observed that cotton samples coated with TCPP/TiO₂ showed absorption peaks in the visible region (400–700 nm) due to the presence of TCPP.¹⁹⁶ The XRD patterns also confirmed the presence of anatase crystallites on the surface of cotton. The visible-light induced degradation of coffee stains on the surface of samples coated with TCPP/TiO₂ was superior compared with pure TiO₂. This corroborates the impact of porphyrin dye on improving the visible-light photocatalytic properties of TiO₂. The high capability of visible light absorption in porphyrins stems from delocalized π electrons in their structure.¹⁹⁷ The enhancement of photo-induced activity of dye-modified TiO₂ is ascribed to the injection of electrons from the excited dye molecules into the conduction band of TiO₂. This in turn increases the potential interactions of TiO₂ and oxygen molecules resulting in generation of a higher concentration of superoxide anions hence a higher photocatalytic activity. Figure 15 shows the dye sensitization mechanism of TiO₂.

Figure 14.

Flowchart showing the process of functionalizing cotton fabric.

Figure 15.

The photocatalytic mechanism of dye-synthesized TiO_2.

However, the porphyrin dye did not show strong photostability. Therefore, the importance of selecting an appropriate dye as a photosensitizer is of paramount importance. Overall, a dye should have some basic features to be used as a sensitizer. These features include a high capacity of light absorption notably in the visible region, near-IR and IR, a long lifetime of excited state, a high quantum yield, and compatibility with the electronic configuration of TiO₂.¹⁹¹ In a related study, the impacts of different metals such as Fe(III), Co(II) and Zn(II) on the photocatalytic functions of TCPP/TiO₂ system have been studied.¹⁹⁸ Fe(III) showed the highest performance in increasing the visible-light-induced photocatalytic activities of TCPP/TiO₂ on cotton followed by Co(II) and Zn(II).¹⁹⁸ To improve the TCPP/TiO₂ photo-stability, Cu(II) porphyrin/TiO₂ was introduced as an alternative dye system. Cu/TCPP was synthesized through the refluxing of TCPP and CuCl₂ and then applied to TiO₂-treated cotton. Although CuTCPP on cotton showed a higher photostability than that of TCPP, the visible-light-induced self-cleaning property declined to some extent.¹⁹⁷ For analyzing the photostability, the coated samples were exposed to visible light for 30 h. Based on the results, CuTCPP applied to cotton experienced a 5% of degradation under visible light while 78% degradation was observed for TCPP.¹⁹⁷

The washing fastness of fabrics against different washing media such as detergent, petroleum ether, and water has also been examined. TCPP applied to the fabrics showed 98.98%, 98.84%, and 85.86% retention in the presence of water, petroleum ether, and detergent, respectively.¹⁹⁶ Washing process with detergent caused a 44% reduction in the photocatalytic activity of fabrics under visible light. The washing fastness results for metallized coating layers showed that the photocatalytic activity of sample coated with Fe/TCPP/TiO₂ did not change after being washed. While modified samples with Co and Zn experienced a 5–10% reduction in the photocatalytic activity under visible light.^197,198

Multi-component TiO₂-based nanocomposites

In this class of catalysts, the TiO₂-based binary composites are modified with metals, leading to enhanced properties even under visible light. To the best of the authors’ knowledge, literature dealing with the application of metal/TiO₂/SiO₂ in textiles is scarce. Application of multi-component photocatalysts to textiles can be an interesting topic for further research. Given the noticeable properties of metal catalysts in the nanoscale, researchers have attempted to use metals to produce multi-component nanocomposites.¹⁹⁹ For instance, in metal/TiO₂/SiO₂ systems, the presence of silica can lead to the formation of smaller nanoparticles, increased surface area as well as surface acidity, which are all effective in enhancing the efficiency of TiO₂. Also, the presence of metal can be helpful to shift the absorption threshold to visible region and increasing the lifespan of generated active species.^200,201

Awazu et al. studied the photocatalytic properties of Ag/SiO₂/TiO₂ nanocomposite and focused on the role of surface plasmonic resonance of silver nanoparticles in visible-light-induced photocatalytic properties.²⁰¹ In their research, Ag nanoparticles were coated with a thin sheath of SiO₂ via a sputtering method in order to prevent direct contact of Ag with TiO₂ nanoparticles. Two important parameters, the thickness of the SiO₂ layer and the particles size, were considered.²⁰¹ The TiO₂ layer was spin-coated on Ag/SiO₂, and then the photocatalytic activity was assessed based on methylene blue degradation. It was observed that TiO₂/Ag/SiO₂ systems showed a better photoefficiency compared with pure TiO₂. An inverse relationship between the thickness of silica and photocatalytic enhancement was established. This shows that optimizing the thickness of silica layer on Ag nanoparticles is integral for improving photocatalytic activity based on the plasmon resonance of silver.²⁰¹ The visible light photocatalytic property of Ag/TiO₂/SiO₂ hollow structure has also been reported.¹³⁶ A SiO₂ layer was coated on polystyrene (PS) core with a diameter of 25 nm, then a TiO₂ layer was deposited on the SiO₂ coating. After calcination, the PS core was removed thereby obtaining a hollow structure of TiO₂/SiO₂. Various amounts of Ag were introduced onto the surface of TiO₂/SiO₂ hollow structure.¹³⁶ The role of silica was defined as increasing the surface area and affinity of TiO₂ to the PS core surface. The photocatalytic efficiency of Ag/TiO₂/SiO₂ hollow nanocomposite was investigated based on the decomposition rate of rhodamine blue (RB). Although increasing the content of metal on the surface of photocatalyst enhanced the photodecomposition of RB, the photocatalytic efficiency dropped after a certain loading level of silver. At higher concentrations, Ag nanoparticles played a role as recombination centers of electrons and holes, decreasing the photocatalytic efficacy.¹³⁶ Moreover, an excessive amount of silver can prevent the absorption of light by TiO₂, adequate adsorption of dye on the photocatalyst and decrease the specific surface area. In some research, the role of metal doping in enhancing the photo-induced activities of TiO₂/SiO₂ systems was evaluated.²⁰² TiO₂/SiO₂ was doped with platinum (Pt),²⁰³ silver (Ag), cobalt (Co), and chromium (Cr)²⁰² through photo-deposition and impregnation.

Harifi and Montazer²⁰⁴ have employed co-doping method with Fe(III) ions and Ag nanoparticles for modification of TiO₂ nanoparticles (Degussa P-25). To this end, Fe(NO₃)₃·9H₂O and silver nitrate (AgNO₃) were utilized as the precursors of Fe³⁺ and Ag, respectively.²⁰⁴ The nanocomposites were synthesized through stirring the components in aqueous media followed by UV irradiation. The nanocomposites of Fe³⁺:Ag/TiO₂, Fe³⁺/TiO₂ and Ag/TiO₂ in order, showed higher photocatalytic activity compared with pure TiO₂ under UV and visible light.²⁰⁴

A visible light self-cleaning cotton using Au/TiO₂/SiO₂ nanocomposite has been reported.²⁰⁵ TTIP and TEOS were used as precursors of TiO₂ and SiO₂, respectively, while AuCl₃·HCl·4H₂O was used as the gold precursor.²⁰⁵ Red wine and concentrated coffee stains were used to evaluate the self-cleaning properties of Au/TiO₂/SiO₂ coated samples.²⁰⁵ It was reported that under visible light, the self-cleaning ability was greater than that of TiO₂-treated cotton. The ternary coating layer on the fabric surface showed a suitable washing-fastness and the obtained properties were reproducible even after 30 wash cycles.²⁰⁵ The study showed that light absorption can be augmented by the presence of Au in the region of UV-A and visible light, enhancing the UV-vis vacancies, quantum efficiencies, and red shifting the absorption edge while suppressing the recombination rate.²⁰⁵ Pakdel et al. have also increased the photocatalytic activity of TiO₂ nanoparticles applied to cotton fabric under visible light through incorporating silica and noble metals (Ag, Au, Pt) [206]. The sol-gel method was employed to functionalize the fabric surface with TiO₂, TiO₂/SiO₂ and TiO₂/Metal/SiO₂ systems. It was revealed that under the same experimental condition, samples coated with tertiary nanocomposites showed accelerated removal of coffee stains under visible light.²⁰⁶ However, the obtained results highlight the necessity of optimizing the concentration of metal dopants and silica in the composite systems.

Future prospects and recommendations

Considering the increasing use of functionalized textiles in our daily life, further investigations are needed to tackle the safety aspects of these products. Improving the wash-fastness by stabilizing the applied nanoparticles is a key milestone that should be addressed. This will reduce the potential risks of nanoparticles on human health and the environment. There have been a number of studies attempting to find an appropriate technique to affix the nanoparticles on the textiles, such as grafting,⁸⁰ cross-linking,²⁰⁷ crystallization,²⁰⁸ plasma surface modification,²⁰⁹ among others.

Some barriers, such as lack of adequate wash-fastness and knowledge about the possible impacts of nanomaterials on human health, have currently hindered the practical applications of nanostructured materials in textiles. The exact repercussions of nanomaterials on human health and the environment are hitherto unknown. In addition, potential contamination of water systems and the soil due to the nanoparticles peeling off from the coated surfaces during laundering is one of the concerns. Therefore, defining relevant standards is crucial to prevent latent ramifications on environment and possibly on posterity resources. No carcinogenic impact, respiratory disease, absorption and storage in human organs was observed for TiO₂ nanoparticles.²⁰⁸ Given that the features of materials in the nanoscale are different from those of the bulk, penetration into the blood circulation system or possible inhalation are also important and should not be discounted.²⁰⁸ Furthermore, the impact of TiO₂ nanoparticles on the functionality of organs of test animals has been examined.^{208,210–212} Consecutive exposure to a low dose of TiO₂ nanoparticles has been shown to cause fertility reduction, ovarian inflammation and follicular atresia in female mice.²¹¹ It is therefore essential to consider the potential impacts of functionalized products on human health prior to their mass production.²¹³

Another key objective is increasing the role of visible light in stimulating the nanoparticles on functionalized textiles. Most current research has relied on employing the high energy of UV to activate the photocatalytic properties on fabrics. It is worth bearing in mind that UV constitutes less than 5% of the solar spectrum and around 0.1% of indoor lighting.^145,210 This highlights the necessity of extending the sensitivity of current photoactive products to visible light. Moreover, using longer-wavelength light sources would reduce the harmful impacts on human health of using UV light on textile substrates. Therefore, researchers have been trying to modify the functionalization process of textiles for indoor application. While improving the photocatalytic activities of applied nanoparticles is one of the main goals, maintaining the inherent properties of fibers should also be taken into account. The findings of numerous research groups in functionalizing textiles were mentioned in this manuscript. Some of these publications along with their significance have been summarized in Table 1.

Table 1.

The main significance of some published research about functionalizing textiles

Reference	Sample	Pretreatment	Photocatalyst	Analyzed features	Washing fastness test
Daoud et al.^{49,59,214,215}	Cotton	—	TiO₂-anatase	UV-blocking, antimicrobial activity, photocatalytic property	55 home launderings
Qi et al.^58,94,171	Cotton, polyester	Plasma pretreatment for polyester	TiO₂-anatase, TiO₂/SiO₂	Self-cleaning, bactericidal activity, UV-protection, tearing strength	5 cycles of home laundering
Daoud et al.⁶²	Wool	Acylation in DMF	TiO₂-anatase	Self-cleaning	N/A
Tung and Daoud^{63,65–67,216,217}	Wool	—	TiO₂-anatase	Self-cleaning, UV-protection, mechanical properties, wettability, cytotoxicity	Washing test in petroleum ether
Bozzi et al.^76,92	Cotton, wool–polyamide, polyester	RF-plasma, MW-plasma, and UV-irradiation	TiO₂-rutile, TiO₂ (Degussa-P25) powder	Monitoring the stain degradation process by analyzing released CO₂ amount	20 household washing cycles, sonication of coated fabrics
Yuranova et al.^{72,153,173,218}	Cotton, wool–polyamide, polyester	RF-plasma, MW-plasma, UV-irradiation, Chemical spacers	TiO₂-rutile, TiO₂ (Degussa P25) powder, SiO₂/TiO₂, Ag/TiO₂ and Ag	Self-cleaning and antimicrobial activity	Sonication of coated fabrics
Montazer et al.^{56,79,86,87,89,90,106,156}	Wool	Surface oxidation, cross-linking with polycarboxylic acids	TiO₂ (Degussa-P25) powder, Ag/TiO₂	Preventing photoyellowing, self-cleaning, antimicrobial activity, shrink resistance of wool, alkali solubility, hydrophilicity, moth proofing	N/A
Montazer et al.^111,112,219	Polyester, wool–polyester, and cotton	Enzymatic pretreatment, cross-linking with carboxylic acids, alkaline surface hydrolysis, In situ synthesis	TiO₂ (Degussa P-25) Powder, TiO₂-anatase, Ag/TiO₂	Self-cleaning, antimicrobial, UV-protection	N/A
Veronovski et al.^174,220	Cellulosic substrate	—	TiO₂ and TiO₂/SiO₂	Self-cleaning, low-stress mechanical properties, fabrics handle	N/A
Pakdel et al.^176,177	Wool and Cotton	—	TiO₂-anatse and TiO₂/SiO₂	Self-cleaning, hydrophilicity, photocatalytic activity	N/A
Afzal et al.^196–198	Cotton	—	TCPP/TiO₂, Fe/TCPP/TiO₂, Zn/TCPP/TiO₂, and Cu/TCPP/TiO₂	Self-cleaning, photocatalytic activity	Washing test in water, detergent, petroleum ether
Wang et al.²⁰⁵	Cotton	—	Au/TiO₂/SiO₂	UV-protection, visible-light induced Self-cleaning	20 cycles of home launderings

Conclusion

This review was an overview of recent research findings in the area of functionalizing fibrous materials with TiO₂-based nanoparticles. Much research has been devoted to incorporating pure TiO₂ nanoparticles as efficient photocatalysts in textiles. Moreover, intensive research has been carried out to find a new generation of photocatalysts with higher photocatalytic efficiency, particularly under visible light. We have reviewed some of the modification methods of TiO_2, such as doping with noble metals, silica, non-metals, dyes and their composites, along with their associated mechanisms. We have also examined the potential applications of pure and modified photocatalysts in functionalizing textiles. These modifications have been shown to enhance the functionality of TiO₂-based photocatalysts under UV and visible light. Certain mechanisms, such as suppression of recombination rate of photo-generated charges, mitigation of band-gap energy, and modification of the surface characteristics, effectively enhanced the visible light sensitivity of photocatalysts.

The main research accomplishments were discussed and this has highlighted the need for further investigations into these functionalized products. The potential impacts of nanoparticles on environment, human health and water resources are suggested as key research areas to be investigated.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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Assimilating the photo-induced functions of TiO 2 -based compounds in textiles: emphasis on the sol-gel process

Abstract

Keywords

The sol-gel process in textiles

Photocatalytic applications of TiO2 in textiles

Functionalizing textiles with TiO2 colloids

Stabilizing nanoparticles on fabrics and resultant features

Some mechanisms associated in TiO2-induced features

Drawbacks of pristine TiO2 and suggested remedies

Modification of TiO2

Metal/TiO2

Metal/TiO2 as a photocatalyst

Applications of Metal/TiO2 in textiles

TiO2/SiO2

Non-metal/TiO2

Dye/TiO2

Multi-component TiO2-based nanocomposites

Future prospects and recommendations

Conclusion

Footnotes

Funding

References

Photocatalytic applications of TiO₂ in textiles

Functionalizing textiles with TiO₂ colloids

Some mechanisms associated in TiO₂-induced features

Drawbacks of pristine TiO₂ and suggested remedies

Modification of TiO₂

Metal/TiO₂

Metal/TiO₂ as a photocatalyst

Applications of Metal/TiO₂ in textiles

TiO₂/SiO₂

Non-metal/TiO₂

Dye/TiO₂

Multi-component TiO₂-based nanocomposites