In the last century, the textile and clothing industries have gained revolutionary developments in many aspects. Advanced scientific research has improved the processing methodology of natural fibers as well as the finishing technology for textile products. Tremendous improvements in the physical and/or chemical treatment of textile surfaces and the development of new functional materials for the finishing of textile products have led to the new textile materials. These multifunctional products have a wide variety of applications in many fields, including health, safety and protection, medical and hygiene, clothing, construction, agriculture, transport, electronics, geo-textiles and packaging [1
]. Cotton and some of the synthetic polymers are mostly used as raw materials for the textiles. However, cotton has many advantages over synthetic polymers due to its softness, breathability, biodegradability, high performance and environmentally-friendly nature [2
]. Raw cotton is converted into fibers which are further processed into cotton fabrics either by a weaving or knitting process. The resulting cotton fabrics are subjected to a variety of finishing processes according to the end-product requirements. Finishing processes may be divided into two major steps i.e., (1) physical and/or chemical pretreatment of the surface of the cotton fabrics; (2) application of dyes or other functional materials to the cotton fabrics to get the final products. In the last few decades, surface pretreatment and nano-functionalization of cotton fabrics for multiple applications has been a major focus for researchers. The development of self-cleaning textile fabrics using finishing processes is one of the promising research areas in textile technology.
Self-cleaning is one of the most fascinating natural phenomena, which was observed in lotus plant leaves for the first time, and now many superhydrophobic and self-cleaning surfaces have been developed using this phenomenon [2
]. In general, a self-cleaning surface has the ability to maintain a clean and contamination-free surface either by avoiding the deposition of dust and other pollutants or by decomposing the adsorbed stains and contaminants on the surface. In the former process, the surface is physically and/or chemically treated to develop superomniphobicity, the ability to resist the adsorption of every kind of liquid or other pollutant, to make it dust and pollutant repellent, while in latter case, some photo-active compounds are adsorbed on the surface which decompose the stains and contaminants coming into contact with the surface when exposed to sunlight.
Self-cleaning textile fabrics have been developed by adsorbing photo-active materials on the textile surface. These photo-active materials include BiVO4
], ZnO [6
] and TiO2
]. Among these diverse photo-catalysts, nano-crystalline TiO2
has been preferentially applied because of its photo-stability, high oxidative power, low cost and non-toxicity [9
]. Moreover, a transparent, crystalline and stable coating of TiO2
nano-particles on the textile fabrics has been easily formed by the sol-gel method [8
]. The anatase (TiO2
)-coated cotton fabric shows remarkable photo-catalytic effects for contaminants, microorganisms and dirt, when exposed to ultraviolet light. However, photo-catalytic applications of anatase have been restricted in the ultraviolet range (only 3–5% of the solar spectrum) due to its wide band gap of 3.2 eV and rapid electron-hole recombination. In addition, weak interaction of the TiO2
with textile materials also decreases its practical applications. To increase the attachment of TiO2
to textile materials, pretreatment (physical and/or chemical) of the fabrics has been reported [12
]. Due to the increasing demand for self-cleaning textile products in daily life, visible-light-active TiO2
coatings are prerequisites for scalable applications of self-cleaning textiles. Visible-light-driven, self-cleaning textiles have been produced by doping TiO2
with metals, non-metals or by mixing with other semiconductor metal oxides. Nano-coatings of N–TiO2
] have been applied to the cotton and wool fabrics to get visible-light-driven photo-activity for dirt/stain degradation. Although the electron-hole recombination process has been retarded by the substitution of metal ions in TiO2
nano-crystals, their optical absorption and photo-catalytic activity in the range of visible light are not satisfactory. Dye photo-sensitization of TiO2
nano-particles is another approach to enhance its visible-light-harvesting power. Porphyrin is a synthetic dye, a structural analogue of chlorophyll, with spectral absorption in the near-visible region. Porphyrin and metal-porphyrin derivatives have been applied to TiO2
-coated cotton fabric by post-treatment [20
-coated cotton fabrics exhibit self-cleaning properties under a narrow range of visible light by degrading the dirt/stains; however, its light absorption is only in a narrow range of the ultraviolet or near-visible light spectrum which reduces its photo-activity. Furthermore, the synthesis process of porphyrin is very complicated and costly, which reduces its practical applications on a large scale. Therefore, there is a need for research to develop durable visible-light-driven, self-cleaning textile fabrics to make fruitful use of sunlight for energy harvesting and for practical applications of the textile cotton fabrics.
Phthalocyanine (PC) is a structural analogue of porphyrin, however, its synthesis process is easy and cost effective. The basic PC structure is given in Figure 1
. PC-reactive blue dyes have already been used in the dyeing of textile products. Monomeric metallic PC has characteristic absorption spectra with a Soret band at approximately 350 nm, a small band at 600 nm and a strong absorption peak (Q-band) around 670 nm, with a molar extinction co-efficient of 105
]. This spectral absorption varies by substituting the PC with a variety of substituents at peripheral and non-peripheral positions [25
]. Due to its high thermal and chemical stability, relatively stable triplet excited state, high quantum yield of singlet oxygen, good optical properties and low toxicity, PC compounds have been used as photo-sensitizers as well as photo-catalysts for the degradation of environmentally hazardous compounds such as chlorophenols [26
]. In this study, a (PC) reactive dye (RB-25) was used for the photo-sensitization of TiO2
for the development of self-cleaning cotton fabrics.