Emerging and Promising Multifunctional Nanomaterial for Textile Application Based on Graphitic Carbon Nitride Heterostructure Nanocomposites

Nanocomposites constructed with heterostructures of graphitic carbon nitride (g-C3N4), silver (Ag), and titanium dioxide (TiO2) have emerged as promising nanomaterials for various environmental, energy, and clinical applications. In the field of textiles, Ag and TiO2 are already recognized as essential nanomaterials for the chemical surface and bulk modification of various textile materials, but the application of composites with g-C3N4 as a green and visible-light-active photocatalyst has not yet been fully established. This review provides an overview of the construction of Ag/g-C3N4, TiO2/g-C3N4, and Ag/TiO2/g-C3N4 heterostructures; the mechanisms of their photocatalytic activity; and the application of photocatalytic textile platforms in the photochemical activation of organic synthesis, energy generation, and the removal of various organic pollutants from water. Future prospects for the functionalization of textiles using g-C3N4-containing heterostructures with Ag and TiO2 are highlighted.

The attractiveness of g-C 3 N 4 is directly related to its properties, as it is distinguished as a sustainable, organic, and metal-free two-dimensional conjugated polymeric n-type semiconductor with unique optical and electronic properties, high physicochemical and thermal stability, and high corrosion resistance, in addition to having a high earth abundance and an easy and inexpensive means of fabrication [21]. Due to its mild band gap of about 2.7 eV, g-C 3 N 4 responds to visible light with an optical absorption edge of about 460 nm and, therefore, enables visible-light-driven photocatalytic reactions [22,23]. Pure g-C 3 N 4 consists of carbon and nitrogen elements and is usually prepared through thermal polycondensation from nitrogen-rich precursors, such as melamine, urea, thiourea, Figure 1. Schematic presentation of g-C3N4: photo of g-C3N4 powder (a), SEM image of g-C3N4 nanosheets (b), s-triazine (c), and tri-s-triazine (d) structure of g-C3N4. Reprinted with permission from [19]. Copyright 2020, Elsevier.
Another important strategy to improve the photocatalytic efficiency of g-C 3 N 4 is the formation of g-C 3 N 4 -based binary and ternary heterostructure composites, including doping/loading with noble metals and creating heterojunctions with other organic and inorganic semiconductors [15,28]. Recently, for example, various heterostructure composites, such as Ag/g-C 3 N 4 [29][30][31], Au/g-C 3 N 4 [31][32][33], graphene oxide/g-C 3 N 4 [34][35][36][37], TiO 2 /g-C 3 N 4 [38][39][40][41][42], Ag/TiO 2 /g-C 3 N 4 [43][44][45], TiO 2 /Cu/g-C 3 N 4 [46], TiO 2 /ZrO 2 /g-C 3 N 4 [47], and Bi 2 WO 6 /g-C 3 N 4 /TiO 2 [48] have been successfully prepared, to optimize the optical properties of g-C 3 N 4 and significantly improve its overall photocatalytic activity. g-C 3 N 4 and g-C 3 N 4 -containing heterostructure composites have also become important materials in the field of textiles and can be beneficially used for the degradation of Nanomaterials 2023, 13, 408 3 of 26 pollutants in textile wastewater or for the chemical modification of textile fibers, to create different functionalities [11]. Natural and synthetic textile fibers are an ideal material for the fabrication of textile-based photocatalytic platforms, because they have advantages over other solid substrates, such as flexibility, lightness, porosity, absorptivity, and wearability. However, while the photocatalytic degradation of different dyes through the presence of g-C 3 N 4 alone or its heterostructure composites has become a widely used sustainable strategy for the purification of dye wastewater [11], the functionalization of textile fibers with g-C 3 N 4 -containing materials remains challenging and, therefore, a topic of research [27]. There are 65 publications dealing with the use of g-C 3 N 4 and g-C 3 N 4 heterostructures for textile applications (Source: Web of Science, advanced search query preview: "graphitic carbon nitride or g-C 3 N 4 " and "textile or fabric" in abstracts, assessed on 28 December 2022), but most of these studies deal with the removal of textile dyes from wastewater. Therefore, it is of great importance to investigate the advantages of g-C 3 N 4 and g-C 3 N 4 -containing heterostructure composites as promising "green" materials for textile functionalization.
In our previous review paper [27], we presented g-C 3 N 4 as a new sustainable photocatalyst for textile functionalization, focusing on the textile substrates used, the application methods, and the developed functionalities, such as photocatalytic self-cleaning, antibacterial, and flame-retardant properties, as well as the creation of a textile catalytic platform for water disinfection, removal of various organic pollutants from water, and selective organic matter transformations. To provide additional valuable information on the recent advances in surface and bulk modification of textile fibers by g-C 3 N 4 -containing nanomaterials, this review article focuses on the application of heterostructure nanocomposites of g-C 3 N 4 with Ag and TiO 2 nanoparticles (NPs), as the most popular and widely used nanomaterials for surface and bulk chemical modification of textiles [49]. In the literature, both binary and ternary heterostructure composites, including Ag/g-C 3 N 4 , TiO 2 /g-C 3 N 4 , and Ag/TiO 2 /g-C 3 N 4 have been considered promising functional nanomaterials, because the synergistic effect of the components in the heterostructures leads to the enhanced photocatalytic performance of the composites compared with the single-component materials. In this review, the processes for the synthesis of Ag/g-C 3 N 4 , TiO 2 /g-C 3 N 4 , and Ag/TiO 2 /g-C 3 N 4 nanocomposites are explained, while their potential photocatalytic mechanisms of action and the developed functionalities on textile fibers for photochemical activation of organic synthesis, energy generation, and the removal of various organic pollutants, as well as future prospects, are highlighted.
Two different approaches have been used to prepare Ag/g-C 3 N 4 nanocomposites, namely one-step and two-step processes, the latter of which is more commonly used. In the two-step process [30,[53][54][55][56][57]59,61,63,65,67,[69][70][71][72][73], g-C 3 N 4 is first synthesized from a suitable precursor in the form of bulk material, g-C 3 N 4 nanosheets, or g-C 3 N 4 quantum dots. Then, g-C 3 N 4 is dispersed in the water medium and mixed with AgNO 3 , which serves as a precursor for Ag NPs. Subsequently, Ag NPs are synthesized in the pres-ence of g-C 3 N 4 using various reducing agents, such as NaBH 4 [30,59,61,65], hydrazine hydrate [70], sodium citrate [72], plant extracts [54], and UV light [53,55,63,67,73]. In this case, g-C 3 N 4 serves as a platform for the synthesis of Ag NPs, and its surface is decorated with Ag 0 (Figure 2a). On the other hand, in the one-step process [56,57,62,74], urea or a mixture of melamine and cyanuric acid are used as g-C 3 N 4 precursors and mixed with AgNO 3 in a suitable medium, and the simultaneous synthesis of Ag NPs/g-C 3 N 4 is carried out under appropriate conditions. This synthesis procedure enables the preparation of Ag-doped g-C 3 N 4 .
serves as a precursor for Ag NPs. Subsequently, Ag NPs are synthesized in the presence of g-C3N4 using various reducing agents, such as NaBH4 [30,59,61,65], hydrazine hydrate [70], sodium citrate [72], plant extracts [54], and UV light [53,55,63,67,73]. In this case, g-C3N4 serves as a platform for the synthesis of Ag NPs, and its surface is decorated with Ag 0 (Figure 2a). On the other hand, in the one-step process [56,57,62,74], urea or a mixture of melamine and cyanuric acid are used as g-C3N4 precursors and mixed with AgNO3 in a suitable medium, and the simultaneous synthesis of Ag NPs/g-C3N4 is carried out under appropriate conditions. This synthesis procedure enables the preparation of Ag-doped g-C3N4.
The proposed mechanism behind the photocatalytic activity of the Ag/g-C3N4 nanocomposite is shown in Figure 2b [53,54,56,57,59,63,75,76]. It is believed that the photocatalytic efficiency of the noble metal/semiconductor heterostructure composite is significantly enhanced by the presence of Ag 0 , which acts as a current collector and plasmonic absorber [77]. Ag/g-C3N4 nanocomposite is a visible-light photocatalyst. According to the literature [23], the band gap energy of g-C3N4 is 2.7 eV, with the potentials of the valence band (VB) and conduction band (CB) being 1.4 eV and −1.3 eV, respectively. Irradiation with energy higher than the band gap energy of g-C3N4 results in the excitation of electrons (e -) from VB to CB, leaving holes (h + ) in VB. The photogenerated ein CB of g-C3N4 can be easily transferred to the Ag NPs because the Fermi level of Ag is less negative compared with the CB of g-C3N4. This creates a Schottky barrier that maximizes photoinduced charge carrier separation and prevents recombination of the e --h + pair. The injected eaccumulates on Ag and can easily enter the reduction reactions on the surface, such as the reduction of O2 to superoxide radicals (•O 2 − ) [63] or the reduction of H + to H2 in hydrogen production by water splitting [76]. At the same time, visible light leads to the excitation of e-in the Ag surface layer, resulting in surface plasmon resonance (SPR). The generation of SPR can greatly enhance the photoactivity of the composite through the mechanism of plasmon resonance energy transfer [77], because the intense near-electric field induced by SPR improves the efficiency of charge carrier separation [33] and increases the rate of charge carrier formation in g-C3N4 [44]. On the other hand, h + in VB can directly oxidize different The proposed mechanism behind the photocatalytic activity of the Ag/g-C 3 N 4 nanocomposite is shown in Figure 2b [53,54,56,57,59,63,75,76]. It is believed that the photocatalytic efficiency of the noble metal/semiconductor heterostructure composite is significantly enhanced by the presence of Ag 0 , which acts as a current collector and plasmonic absorber [77].
Ag/g-C 3 N 4 nanocomposite is a visible-light photocatalyst. According to the literature [23], the band gap energy of g-C 3 N 4 is 2.7 eV, with the potentials of the valence band (VB) and conduction band (CB) being 1.4 eV and −1.3 eV, respectively. Irradiation with energy higher than the band gap energy of g-C 3 N 4 results in the excitation of electrons (e − ) from VB to CB, leaving holes (h + ) in VB. The photogenerated e − in CB of g-C 3 N 4 can be easily transferred to the Ag NPs because the Fermi level of Ag is less negative compared with the CB of g-C 3 N 4 . This creates a Schottky barrier that maximizes photoinduced charge carrier separation and prevents recombination of the e − -h + pair. The injected e − accumulates on Ag and can easily enter the reduction reactions on the surface, such as the reduction of O 2 to superoxide radicals (•O − 2 ) [63] or the reduction of H + to H 2 in hydrogen production by water splitting [76]. At the same time, visible light leads to the excitation of e − in the Ag surface layer, resulting in surface plasmon resonance (SPR). The generation of SPR can greatly enhance the photoactivity of the composite through the mechanism of plasmon resonance energy transfer [77], because the intense near-electric field induced by SPR improves the efficiency of charge carrier separation [33] and increases the rate of charge carrier formation in g-C 3 N 4 [44]. On the other hand, h + in VB can directly oxidize different pollutants [57,63], but h + cannot oxidize − OH to give •OH radicals, since the VB edge potential of g-C 3 N 4 is less positive than the standard redox potential of − OH/•OH Nanomaterials 2023, 13, 408 5 of 26 (+1.99 eV) [53,57,63,75]. This means that •OH radicals cannot be directly generated in the photochemical process of g-C 3 N 4 .

Ag/g-C 3 N 4 Nanocomposites for Textile Application
Chemically modified textile substrates with Ag/g-C 3 N 4 nanocomposites have been advantageously used in the photocatalysis of various organic reactions [78,79] and textilebased triboelectric nanogenerators [80]. While the first application involves photochemical activation of organic synthesis without additional reagents, thus providing a more environmentally friendly route for organic chemical conversion, the second application represents an emerging textile-based energy-harvesting device, as a potential power source for wearable electronics.
New functionality was imparted to a polyester (PES) fabric by dip-coating with g-C 3 N 4 nanosheets under ultrasonic treatment, followed by in situ synthesis of Ag NPs in an aqueous solution of AgNO 3 of different concentrations (3-10 wt%), using NaBH 4 as a reducing agent [79]. From the SEM images, it is evident that the micro smooth morphology of the PES fibers ( Figure 3a It is also evident that the application of g-C 3 N 4 nanosheets changed the white color of the PES fabric to brown, but the in situ synthesis of Ag NPs to grey (Figure 3e).
PES coated with g-C 3 N 4 and Ag/g-C 3 N 4 nanocomposites, containing different amounts of Ag 0 from 3 to 10 wt%, was used as a sustainable chemical catalyst for the hydrogenation of 4-nitrophenol, one of the most toxic organic pollutants in industrial wastewater, into the valuable compound 4-aminophenol, using NaBH 4 as the hydride source [79]. Since a one-component g-C 3 N 4 coating on PES fabric does not act as a catalyst for the conversion of 4-nitrophenol into 4-aminophenol, the presence of 3 wt% Ag 0 in the Ag/g-C 3 N 4 nanocomposite resulted in a 30% conversion of 4-nitrophenol into 4-aminophenol after a reaction time of 5 min, and this increased to 90% conversion when the Ag loading was increased to 10 wt%, with an apparent rate constant of 0.462 min −1 , which is more than six-times higher than that of 3 wt% Ag 0 . (Figure 3f). This indicates that Ag NPs facilitated the electron transfer from BH − 4 to 4-nitrophenolate, thus lowering the barrier of activation energy for the reduction of 4-nitrophenol to 4-aminophenol ( Figure 3g). The high recyclability and stability of the catalyst was evidenced by the fact that the catalytic performance of the catalyst was still nearly 90% after 10 cycles. A comparison with some other Ag-based catalysts from the literature clearly showed that the conversion of 4-nitrophenol over Ag/g-C 3 N 4 coated PES exhibited enhanced the catalytic activity and recyclability [79].
Ag NP-decorated g-C 3 N 4 was also used in the bulk modification of polyacrylonitrile nanofibers (PAN NFs) for selective oxidation of styrene, benzylic methylene groups, and benzene into the desired products under visible light irradiation and milder reaction conditions [78]. For this purpose, Ag NPs/g-C 3 N 4 composite and 10 wt% PAN were dispersed in an organic solvent under sonification, to produce a homogeneous polymer solution, which was then electrospun to produce PAN NFs with the embedded Ag NPs/g-C 3 N 4 (Figure 4a,b). TEM micrography showed small dark particles and bulges on the PAN NFs, indicating that the Ag NPs/g-C 3 N 4 was well dispersed on the PAN surface or embedded in the PAN matrix, without agglomerating ( Figure 4b). The as-prepared PAN/Ag NPs/g-C 3 N 4 NFs exhibited a highly porous nature with excellent absorption performance. To optimize the photooxidation reactions, the influence of different parameters, such as the amount of PAN/Ag NPs/g-C 3 N 4 NFs, organic solvents, reaction time, the presence or absence of visible light (domestic bulb (40 w)), and H 2 O 2 as oxidant, on the photocatalytic Nanomaterials 2023, 13, 408 6 of 26 performance was investigated. It was found that the oxidation selectivity and conversion increased with the increase in catalyst concentration and reaction time in the organic solvent acetonitrile, as well as in the presence of light and H 2 O 2 . In this case, a 98% conversion of styrene and 60% selectivity toward styrene oxide (Figure 4c), 98% conversion of ethylbenzene and 99% selectivity toward acetophenone (Figure 4d), as well as an excellent 99% conversion of benzene into the desired phenol, was obtained using H 2 O 2 as oxidant ( Figure 4e) under optimized reaction conditions (Figure 4f). A recyclability study of the PAN/Ag NPs/g-C 3 N 4 NFs for styrene oxidation showed that the catalyst exhibited high reusability activity with a product yield of more than 85% in a repeat test of five runs.  PES coated with g-C3N4 and Ag/g-C3N4 nanocomposites, containing different amounts of Ag 0 from 3 to 10 wt%, was used as a sustainable chemical catalyst for the hydrogenation of 4-nitrophenol, one of the most toxic organic pollutants in industrial wastewater, into the valuable compound 4-aminophenol, using NaBH4 as the hydride source [79]. Since a one-component g-C3N4 coating on PES fabric does not act as a catalyst for the conversion of 4-nitrophenol into 4-aminophenol, the presence of 3 wt% Ag 0 in the  Recently, a textile-based triboelectric nanogenerator (T-TENG) for mechanical energy harvesting was fabricated by depositing an active layer of g-C3N4 nanosheets decorated with Ag NPs on a nylon-coated conductive carbon fabric as a textile backbone (Figure 5ac), using Teflon or polypropylene as a counter triboelectric material [80]. To establish electrical contact, aluminum adhesive tapes were attached on one of the surfaces of these layers, with an extension for electrical contacts. To produce a voltage, the fabric based samples and Teflon were put in repetitive contact-separation mode through mechanical agitation. When the two materials are in contact, charge transfer occurs between the two surfaces, and when the two materials are separated, a current flows through the external cir- Recently, a textile-based triboelectric nanogenerator (T-TENG) for mechanical energy harvesting was fabricated by depositing an active layer of g-C 3 N 4 nanosheets decorated with Ag NPs on a nylon-coated conductive carbon fabric as a textile backbone (Figure 5a-c), using Teflon or polypropylene as a counter triboelectric material [80]. To establish electrical contact, aluminum adhesive tapes were attached on one of the surfaces of these layers, with an extension for electrical contacts. To produce a voltage, the fabric based samples and Teflon were put in repetitive contact-separation mode through mechanical agitation. When the two materials are in contact, charge transfer occurs between the two surfaces, and when the two materials are separated, a current flows through the external circuit, to balance the potential on the two surfaces. Such a configuration of an Ag/g-C 3 N 4 /nylon bi-layer T-TENG generates an open circuit voltage of~200 V, which is more than 10 times higher compared with a bare g-C 3 N 4 nanosheet layer (19 V) and almost four-times higher compared to a g-C 3 N 4 /nylon bi-layer (52 V) (Figure 5d). The thermal stability, as well as the conversion efficiency, at an elevated temperature of up to~65 • C make it a potential candidate for integration into textile-based wearable electronic devices (Figure 5e). The synergistic effect of interfacial charge trapping, the increased surface area and the increased surface charge density in the Ag/g-C 3 N 4 /nylon bi-layer system result in the development of a maximum short circuit current of~1.1 µA, which is about three-times higher than that of g-C 3 N 4 /nylon ( Figure 5f) and delivery of a maximum output power of 3.1 µW/cm 2 , which is higher than that obtained from g-C 3 N 4 /nylon (Figure 5g). When a Ag/g-C 3 N 4 /nylon bi-layer T-TENG was examined upon charging a commercial capacitor (0.26 µF) using a bridge rectifier, the Ag/g-C 3 N 4 /nylon device was able to charge the capacitor to~85 V within 30 s, which is more than 2.7 times higher than the result of a g-C 3 N 4 /nylon device (Figure 5h). In addition, several commercially available LEDs could be driven by the capacitors, which were charged by the impact of the textile-based nanogenerator (insert in Figure 5h). The excellent power generation capability of this fabric coated with an Ag/g-C 3 N 4 /nylon bi-layer indicates its potential applicability for wearable and flexible nanogenerators.
Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of cuit, to balance the potential on the two surfaces. Such a configuration of an Ag/g-C3N4/n lon bi-layer T-TENG generates an open circuit voltage of ~200 V, which is more than times higher compared with a bare g-C3N4 nanosheet layer (19 V) and almost four-tim higher compared to a g-C3N4/nylon bi-layer (52 V) (Figure 5d). The thermal stability, well as the conversion efficiency, at an elevated temperature of up to ~65 °C make it potential candidate for integration into textile-based wearable electronic devices (Figu 5e). The synergistic effect of interfacial charge trapping, the increased surface area and t increased surface charge density in the Ag/g-C3N4/nylon bi-layer system result in the d velopment of a maximum short circuit current of ~1.1μA, which is about three-tim higher than that of g-C3N4/nylon ( Figure 5f) and delivery of a maximum output power ~3.1 μW/cm 2 , which is higher than that obtained from g-C3N4/nylon (Figure 5g). When Ag/g-C3N4/nylon bi-layer T-TENG was examined upon charging a commercial capacit (0.26 μF) using a bridge rectifier, the Ag/g-C3N4/nylon device was able to charge the c pacitor to ~85 V within 30 s, which is more than 2.7 times higher than the result of a C3N4/nylon device (Figure 5h). In addition, several commercially available LEDs could driven by the capacitors, which were charged by the impact of the textile-based nanoge erator (insert in Figure 5h). The excellent power generation capability of this fabric coat with an Ag/g-C3N4/nylon bi-layer indicates its potential applicability for wearable an flexible nanogenerators. voltage generation from TENG operation for different layers with Teflon as a counter triboelectric material under a mechanical impact (d); attachment of TENG to different parts of clothes (e); short circuit current (f), output power density (g), and capacitor charging voltage (h) for g-C 3 N 4 /nylon and Ag/g-C 3 N 4 /nylon based T-TENGs (insets show a closer view of voltage profile and the glowing of LEDs using the charged capacitor). Reprinted with permission from [80]. Copyright 2022, Elsevier.
Two mechanisms have been proposed for the photocatalytic activity of TiO 2 /g-C 3 N 4 nanocomposites, including the Type-II heterojunction (Figure 6a) [82,87,[92][93][94][96][97][98]102] and the direct Z-scheme (Figure 6b) [90,91,95]. In both mechanisms, it is assumed that when g-C 3 N 4 and TiO 2 are excited in the heterojunction by incident UV/visible light of sufficient energy, the photoinduced electrons are transferred from VB to CB, leaving holes in VB. According to the Type-II heterojunction mechanism, the photogenerated electrons can be easily transferred from CB of g-C 3 N 4 to CB of TiO 2 because the CB edge potential of g-C 3 N 4 (−1.3 eV) is more negative than that of TiO 2 (−0.29 eV). At the same time, the photogenerated holes can be transferred from VB of TiO 2 to VB of g-C 3 N 4 because the VB edge potential of TiO 2 (2.91 eV) is more positive than that of g-C 3 N 4 (1.4 eV). In this case, photoinduced electrons accumulate in the CB of TiO 2 for the reduction reaction and photoinduced holes accumulate in the VB of g-C 3 N 4 for the oxidation reaction, which efficiently separates the photogenerated electron-hole pairs and suppresses their recombination [102][103][104]. However, despite the enhanced photocatalytic efficiency of the as-constructed TiO 2 /g-C 3 N 4 nanocomposite, a drawback of the Type-II heterojunction mechanism is attributed to the impairment of redox capability, since the reduction reaction proceeds on TiO 2 with a lower reduction potential compared with g-C 3 N 4 , and the oxidation reaction proceeds on g-C 3 N 4 with a lower oxidation potential compared with TiO 2 [102,103]. Since the holes in the VB of g-C 3 N 4 cannot directly generate OH radicals in the oxidation reaction, this significantly reduces the photocatalytic efficiency of the nanocomposite.

TiO2/g-C3N4 Nanocomposites for Textile Application
TiO2/g-C3N4 nanocomposites were applied to cotton and polyester substrates to develop textile-based photocatalysts for effective purification of emerging liquid, gaseous pollutants, and bacteria [39,[105][106][107]. To this end, TiO2/g-C3N4-cotton and TiO2/g-C3N4polyester composites were constructed and used as solar-driven photocatalysts for the degradation of the antibiotic sulphaquinoxaline (SQX) and the pesticide thiamethoxam ( Figure 7) [106,107]. The preparation of a TiO2@g-C3N4-cotton photocatalyst included binding of the coupling agent (3-Aminopropyl)triethoxysilane (APTES) to the carboxylmodified g-C3N4, to create reactive silanol groups on the g-C3N4 surface. Afterwards, a cotton fabric sample was immersed in the g-C3N4-APTES hydrolysate, followed by squeezing on a two-roll padder and drying at 130 °C, to chemically bind g-C3N4-APTES to the cotton surface. To produce the TiO2@g-C3N4-cotton, a g-C3N4-APTES-cotton sample was immersed in the TiO2 dispersion and maintained at 120 °C for 2 h for hydrothermal reaction, to achieve the deposition of TiO2 on the g-C3N4-APTES-cotton surface ( Figure  7a). In the preparation of the g-C3N4-TiO2@LMPET photocatalyst, low-melting non-woven polyester (LMPET) was immersed in the g-C3N4 dispersion, followed by squeezing and drying at 80 °C, and heat treated at 135 °C, to melt the LMPET sheath to strongly stick g-C3N4. The as-prepared g-C3N4@LMPET was immersed in the TiO2 dispersion and maintained at 120 °C for 2 h for hydrothermal reaction, to achieve the deposition of TiO2 on the g-C3N4@LMPET surface (Figure 7b). In contrast to the Type-II heterojunction mechanism, the direct Z-scheme photocatalysis system assumes a significantly different charge carrier transfer pathway in the TiO 2 /g-C 3 N 4 nanocomposite, although it has the same band structure configuration (Figure 6b) [102]. Indeed, the direct Z-scheme dictates that the existence of an internal electric field, the extra potential barrier, and the Coulomb repulsion hinder the transfer of the photogenerated electrons from the CB of g-C 3 N 4 to the CB of TiO 2 and the photogenerated holes from the VB of TiO 2 to the VB of g-C 3 N 4 , as well as promoting the recombination between the photogenerated electrons in the CB of TiO 2 and the photogenerated holes in the VB of g-C 3 N 4 with a lower redox ability [102,103]. In this case, the electrons and holes are spatially separated on g-C 3 N 4 with the higher reduction potential and TiO 2 with a higher oxidation potential, respectively. The electrons in the CB of g-C 3 N 4 are trapped by O 2 on the nanocomposite surface to form reactive •O − 2 , since the CB edge potential of g-C 3 N 4 (−1.3 eV) is more negative than the redox potential of O 2 /•O − 2 (−0.33 eV and −0.046 eV), while the holes in VB of TiO 2 react with absorbed water to generate reactive •OH radicals [91] because the VB edge potential of TiO 2 (2.91 eV) is more positive than the redox potential of − OH/•OH (+1.99 eV). Compared with the Type-II heterojunction, the direct Z-scheme has a much stronger redox capability to drive photocatalytic reactions [102], which could explain the superior photocatalytic efficiency of TiO 2 /g-C 3 N 4 nanocomposite in the various reduction and oxidation reactions. The presence of the direct Z-scheme instead of the Type-II heterojunction was also confirmed by experiments involving the trapping of •OH and •O − 2 radicals [103]. On the other hand, when irradiated with visible light, when only g-C 3 N 4 can be excited because the energy of the incident light is too low to excite TiO 2 , the reduction reaction on the TiO 2 surface can be indirectly induced via electron transfer from the CB of g-C 3 N 4 to the CB of TiO 2 , leading to effective separation of photogenerated electron-hole pairs and an enhanced photocatalytic activity (Figure 6c) [81,84,96].

TiO 2 /g-C 3 N 4 Nanocomposites for Textile Application
TiO 2 /g-C 3 N 4 nanocomposites were applied to cotton and polyester substrates to develop textile-based photocatalysts for effective purification of emerging liquid, gaseous pollutants, and bacteria [39,[105][106][107]. To this end, TiO 2 /g-C 3 N 4 -cotton and TiO 2 /g-C 3 N 4polyester composites were constructed and used as solar-driven photocatalysts for the degradation of the antibiotic sulphaquinoxaline (SQX) and the pesticide thiamethoxam ( Figure 7) [106,107]. The preparation of a TiO 2 @g-C 3 N 4 -cotton photocatalyst included binding of the coupling agent (3-Aminopropyl)triethoxysilane (APTES) to the carboxylmodified g-C 3 N 4 , to create reactive silanol groups on the g-C 3 N 4 surface. Afterwards, a cotton fabric sample was immersed in the g-C 3 N 4 -APTES hydrolysate, followed by squeezing on a two-roll padder and drying at 130 • C, to chemically bind g-C 3 N 4 -APTES to the cotton surface. To produce the TiO 2 @g-C 3 N 4 -cotton, a g-C 3 N 4 -APTES-cotton sample was immersed in the TiO 2 dispersion and maintained at 120 • C for 2 h for hydrothermal reaction, to achieve the deposition of TiO 2 on the g-C 3 N 4 -APTES-cotton surface (Figure 7a). In the preparation of the g-C 3 N 4 -TiO 2 @LMPET photocatalyst, low-melting non-woven polyester (LMPET) was immersed in the g-C 3 N 4 dispersion, followed by squeezing and drying at 80 • C, and heat treated at 135 • C, to melt the LMPET sheath to strongly stick g-C 3 N 4 . The as-prepared g-C 3 N 4 @LMPET was immersed in the TiO 2 dispersion and maintained at 120 • C for 2 h for hydrothermal reaction, to achieve the deposition of TiO 2 on the g-C 3 N 4 @LMPET surface (Figure 7b).
The incorporation of the TiO 2 /g-C 3 N 4 nanocomposite into the cotton and LMPET significantly changed the morphology of the fibers, resulting in an increased surface roughness of the cotton (Figure 8a-d) [107] as well as LMPET [106]. Photocatalytic activity was studied in the degradation of the antibiotic sulphaquinoxaline (SQX) (Figure 8e,f) and the pesticide thiamethoxam (Figure 8g,h) under sunlight irradiation. It was found that both the TiO 2 @g-C 3 N 4 -cotton and g-C 3 N 4 -TiO 2 @LMPET samples showed excellent photocatalytic performance, resulting in an almost 100% degradation of SQX after 60 and 90 min by the TiO 2 @g-C 3 N 4 -cotton and g-C 3 N 4 -TiO 2 @LMPET samples, respectively, and of thiamethoxam after 150 and 180 min by TiO 2 @g-C 3 N 4 -cotton and g-C 3 N 4 -TiO 2 @LMPET samples, respectively, at pH 7. The photocatalytic performance was sufficiently higher than that of the TiO 2 -cotton and TiO 2 @LMPET samples and that of the g-C 3 N 4 -cotton and g-C 3 N 4 @LMPET samples. The rate of SQX removal by TiO 2 /g-C 3 N 4 was higher than that of thiamethoxam for both textile-based photocatalysts, because thiamethoxazine is a more difficult pollutant to degrade than SQX. The results also showed that the TiO 2 /g-C 3 N 4 composite exhibited better photocatalytic activity in the decomposition of SQX under acidic and neutral conditions and gradually weakened under alkaline conditions (Figure 8i). Both TiO 2 @g-C 3 N 4 -cotton and g-C 3 N 4 -TiO 2 @LMPET photocatalysts maintained excellent catalyst recyclability and stability and could remove 97% SQX after 10 cycles. The incorporation of the TiO2/g-C3N4 nanocomposite into the cotton and LMPET sig nificantly changed the morphology of the fibers, resulting in an increased surface rough ness of the cotton (Figure 8a,b) (Figure 8c,d) [107] as well as LMPET [106]. Photocatalyti activity was studied in the degradation of the antibiotic sulphaquinoxaline (SQX) (Figur 8e,f) and the pesticide thiamethoxam (Figure 8g,h) under sunlight irradiation. It wa found that both the TiO2@g-C3N4-cotton and g-C3N4-TiO2@LMPET samples showed exce lent photocatalytic performance, resulting in an almost 100% degradation of SQX after 6 and 90 min by the TiO2@g-C3N4-cotton and g-C3N4-TiO2@LMPET samples, respectively and of thiamethoxam after 150 and 180 min by TiO2@g-C3N4-cotton and g-C3N The mechanism of the photocatalytic activity of the TiO2/g-C3N4 heterojunction and the behavior of the charge transfer at the interface were discussed based on the photoluminescence spectra (Figure 8j) [106]. The results showed that g-C3N4 exhibited a strong emission peak at about 450 nm, which decreased drastically in the case of the TiO2/g-C3N4 heterojunction. This phenomenon can be explained by the electron transfer from the CB The mechanism of the photocatalytic activity of the TiO 2 /g-C 3 N 4 heterojunction and the behavior of the charge transfer at the interface were discussed based on the photoluminescence spectra (Figure 8j) [106]. The results showed that g-C 3 N 4 exhibited a strong emission peak at about 450 nm, which decreased drastically in the case of the TiO 2 /g-C 3 N 4 heterojunction. This phenomenon can be explained by the electron transfer from the CB of g-C 3 N 4 to the CB of TiO 2 , which efficiently suppresses the recombination of photogenerated electron-hole pairs and improves the photocatalytic performance compared with the single-components TiO 2 and g-C 3 N 4 , indicating a synergistic effect between TiO 2 and g-C 3 N 4 in the heterojunction.
A textile-based photocatalyst was also prepared by constructing a TiO 2 /g-C 3 N 4 coating on cotton fabric using a simple layer-by-layer (LBL) self-assembly strategy, in which the cotton fabric was alternately immersed in the cationic TiO 2 solution and the anionic g-C 3 N 4 solution to obtain two, five, and seven bilayers (BL) [105]. After each immersion, the sample was rinsed and dried. A TiO 2 /g-C 3 N 4 powder composite was also prepared for comparison. The SEM analysis revealed that the self-assembly coating mainly covered the surface of the cotton fibers and significantly increased their roughness (Figure 9a). The mass of the coating increased with the number of BL. The photocatalytic performance of the TiO 2 /g-C 3 N 4 coated cotton fabric was investigated through the degradation rate of RhB dye (Figure 9b) and toluene (Figure 9c) under visible-light irradiation. The higher the degradation rate constant, the higher the photocatalytic performance. From the results, it can be seen that the reaction rate constant, κ, for the degradation of Rhodamine B (RhB) dye gradually increased with the increasing number of BL, and the highest value was reached for the coating with 7 BL, which was much higher than that of TiO 2 /g-C 3 N 4 and TiO 2 powders (Figure 9b). This confirmed that the coupling of TiO 2 with g-C 3 N 4 is an efficient strategy to improve photocatalytic performance and highlights the importance of the cotton fabric as a support for the photodegradation reaction, since the fabric acts as an absorbent for pollution and drives the active species to rapidly absorb and degrade the pollutants. Similar results were obtained for the photodegradation of toluene, with coatings of 2 BL, 5 BL, and 7 BL showing significantly higher degradation compared with the TiO 2 /g-C 3 N 4 powder (Figure 9c). The excellent performance of the 7 BL coated cotton fabric was also demonstrated by the degradation of RhB solution under sunlight, where not only was the RhB solution completely discolored within 4 h but also the coated fabric, indicating RhB degradation in the solution and on the fabric surface (Figure 9d). To investigate the photocatalytic mechanism, a TiO2/g-C3N4 nanocomposite was chemically grafted onto cotton fibers in a hydrothermal process (Figure 10) [39]. In the preparation of cotton fabric loaded with TiO2/g-C3N4 in different mass ratios (C-g-C3N4-TiO2 samples), the in situ synthesis of TiO2 was performed in a solution of presynthesized g-C3N4 nanosheets in the presence of the swollen cotton fibers at 120 °C for 4 h. The results showed that the as-prepared C-g-C3N4-TiO2 samples exhibited fish-like lobes, with densely aggregated nanosized particles of irregular shape (Figure 10a). Chemical grafting of the g-C3N4-TiO2 heterojunction composite with cotton fibers resulted in an Eg of 3.31 To investigate the photocatalytic mechanism, a TiO 2 /g-C 3 N 4 nanocomposite was chemically grafted onto cotton fibers in a hydrothermal process ( Figure 10) [39]. In the preparation of cotton fabric loaded with TiO 2 /g-C 3 N 4 in different mass ratios (C-g-C 3 N 4 -TiO 2 samples), the in situ synthesis of TiO 2 was performed in a solution of presynthesized g-C 3 N 4 nanosheets in the presence of the swollen cotton fibers at 120 • C for 4 h. The results showed that the as-prepared C-g-C 3 N 4 -TiO 2 samples exhibited fish-like lobes, with densely aggregated nanosized particles of irregular shape (Figure 10a). Chemical grafting of the g-C 3 N 4 -TiO 2 heterojunction composite with cotton fibers resulted in an Eg of 3.31 eV, which was red-shifted compared with the Eg of C-TiO 2 fibers of 3.45 eV (Figure 10b), indicating a higher light absorption efficiency. Both calculated Eg values were higher than those of TiO 2 NPs (Eg = 3.2 eV) and g-C 3 N 4 nanosheets (Eg = 2.82 eV), suggesting that cotton affected the energy band structure of the generated composite photocatalysts, which is difficult to explain. Nanomaterials 2023, 13, x FOR PEER REVIEW 17 of 28 that the mass ratio between g-C3N4 and TiO2 in the heterojunction should be carefully selected. Figure 10. FESEM images of C-g-C3N4-TiO2 cotton fibers (a); schematic diagrams of energy band structure for C-TiO2 and C-g-C3N4-TiO2 cotton fibers (b); photodegradation of MO dye solution by the C-g-C3N4-TiO2 cotton fibers (c); trapping experiments for the photodegradation of MO dye solution by C-g-C3N4-TiO2 cotton fibers (d). Reprinted with permission from [39]. Copyright 2021, Springer Link.
The photocatalytic mechanism of action of the C-g-C3N4-TiO2 cotton fibers was determined through trapping experiments (Figure 10d) [39], in which photodegradation of the MO dye solution was performed under irradiation with visible light in the presence of four radical scavengers, i.e., 1,4-benzoquinone (BQ), furfuryl alcohol (FA), tertbutyl alcohol (TBA), and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) to scavenge •O 2 − , singlet oxygen ( 1 O 2 ), •OH and h + , respectively. It was found that the photocatalytic activity of the C-g-C3N4-TiO2 cotton fibers decreased only slightly in the presence of EDTA-2Na, but a significant decrease in photocatalytic capacity was observed after the addition of BQ and FA. This indicates that •O 2 − , generated by the reduction reaction at the surface of the TiO2/g-C3N4 heterojunction, is the most important type of radical for the photocatalytic degradation of MO, followed by 1 O 2 , •OH, and h + . It was concluded that cotton fabric modified with g-C3N4-TiO2 can be repeatedly used to remove organic contaminants. Figure 10. FESEM images of C-g-C 3 N 4 -TiO 2 cotton fibers (a); schematic diagrams of energy band structure for C-TiO 2 and C-g-C 3 N 4 -TiO 2 cotton fibers (b); photodegradation of MO dye solution by the C-g-C 3 N 4 -TiO 2 cotton fibers (c); trapping experiments for the photodegradation of MO dye solution by C-g-C 3 N 4 -TiO 2 cotton fibers (d). Reprinted with permission from [39]. Copyright 2021, Springer Link.
The photocatalytic performance results of the C-g-C 3 N 4 -TiO 2 and C-TiO 2 samples showed that the C-TiO 2 cotton fibers exhibited a poor photodegradation performance for methyl orange (MO) dye solution when irradiated with visible light, with only 10% MO decolorization after 150 min, while the photocatalytic performance of the C-g-C 3 N 4 -TiO 2 cotton fibers was much higher (Figure 10c) [39]. The latter increased with the increase in the mass of the g-C 3 N 4 nanosheets in the TiO 2 /g-C 3 N 4 nanocomposite, from 0.025 to 0.05 g, and reached its maximum with a photocatalytic efficiency about four-times higher than that of the C-TiO 2 cotton fibers. Further increasing the mass of the g-C 3 N 4 nanosheets led to a decrease in the photocatalytic activity of the C-g-C 3 N 4 -TiO 2 cotton fibers, indicating that the mass ratio between g-C 3 N 4 and TiO 2 in the heterojunction should be carefully selected.
The photocatalytic mechanism of action of the C-g-C 3 N 4 -TiO 2 cotton fibers was determined through trapping experiments (Figure 10d) [39], in which photodegradation of the MO dye solution was performed under irradiation with visible light in the presence of four radical scavengers, i.e., 1,4-benzoquinone (BQ), furfuryl alcohol (FA), tertbutyl alcohol (TBA), and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) to scavenge •O − 2 , singlet oxygen ( 1 O 2 ), •OH and h + , respectively. It was found that the photocatalytic activity of the C-g-C 3 N 4 -TiO 2 cotton fibers decreased only slightly in the presence of EDTA-2Na, but a significant decrease in photocatalytic capacity was observed after the addition of BQ and FA. This indicates that •O − 2 , generated by the reduction reaction at the surface of the TiO 2 /g-C 3 N 4 heterojunction, is the most important type of radical for the photocatalytic degradation of MO, followed by 1 O 2 , •OH, and h + . It was concluded that cotton fabric modified with g-C 3 N 4 -TiO 2 can be repeatedly used to remove organic contaminants.

Ag/TiO 2 /g-C 3 N 4 Heterostructure
Preparation and Photocatalytic Mechanism of Ag/TiO 2 /g-C 3 N 4 Nanocomposites The construction of ternary Ag/TiO 2 /g-C 3 N 4 heterostructures represents a very promising strategy for achieving the enhanced photocatalytic performance of semiconductor nanocomposites under visible light irradiation [43,45,[108][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123][124][125][126]. Various approaches for the synthesis of Ag/TiO 2 /g-C 3 N 4 nanocomposites have been reported in the literature. One of these proposes the preparation of a mixture of Ag, TiO 2 and g-C 3 N 4 precursors and the synthesis of an Ag/TiO 2 /melamine nanocomposite at 70 • C, followed by calcination of the nanocomposite at 550 • C, to produce g-C 3 N 4 from melamine (Figure 11a) [126]. Another strategy is to mix previously synthesized TiO 2 and g-C 3 N 4 , followed by the addition of an AgNO 3 precursor and synthesis of Ag NPs in the presence of a g-C 3 N 4 and TiO 2 mixture or previously prepared TiO 2 /g-C 3 N 4 nanocomposite (Figure 11b1,b2) [45,115,123,124]. A TiO 2 /g-C 3 N 4 composite was also prepared by the synthesis of g-C 3 N 4 from a suitable precursor in the presence of TiO 2 or through the synthesis of TiO 2 from a suitable precursor in the presence of g-C 3 N 4 , followed by the synthesis of Ag NPs from an AgNO 3 precursor in a reduction reaction in the presence of TiO 2 /g-C 3 N 4 nanocomposite (Figure 11c1,c2) [108,116,117]. It has also been reported that the Ag/TiO 2 /g-C 3 N 4 nanocomposite was prepared by synthesis of Ag NPs from an AgNO 3 precursor in the presence of TiO 2 and subsequent mixing of the Ag/TiO 2 nanocomposite with previously synthesized g-C 3 N 4 ( Figure 11d) [43,110,113,118,119,121,122].
It should be emphasized that the photocatalytic mechanism of the ternary Ag/TiO 2 /g-C 3 N 4 nanocomposite is very complex and not yet fully understood. Since it is influenced by nanocomposite construction, which is directly related to the synthesis route and the formation of tight interfacial connections between the components in the heterojunction, there are various schematic representations of the photocatalytic mechanisms of Ag/TiO 2 /g-C 3 N 4 in the literature, as well as explanations of the charge carrier transfer. The most commonly proposed mechanisms are shown in Figure 12 [45,116,120,122].
It is proposed that the enhanced photocatalytic performance of Ag/TiO 2 /g-C 3 N 4 is due to the effective Z-scheme mechanism established in the TiO 2 and g-C 3 N 4 heterojunction under UV-and visible-light irradiation, which is supported by the SPR of Ag facilitating charge transfer (Figure 12a) [45,108]. It has been suggested that Ag, as a conductive material, can directly act as a center to combine electrons on the surface of TiO 2 with the holes on g-C 3 N 4 and maintain this remarkable Z-scheme photocatalytic system [45].
in a reduction reaction in the presence of TiO2/g-C3N4 nanocomposite (Figure 11c) [108,116,117]. It has also been reported that the Ag/TiO2/g-C3N4 nanocomposite was prepared by synthesis of Ag NPs from an AgNO3 precursor in the presence of TiO2 and subsequent mixing of the Ag/TiO2 nanocomposite with previously synthesized g-C3N4 (Figure 11d) [43,110,113,118,119,121,122]. Figure 11. Schematic presentation of strategies for the synthesis of the Ag/TiO2/g-C3N4 nanocomposites: synthesis of Ag/TiO2/melamine from a mixture of Ag, TiO2 and g-C3N4 precursors, followed by calcination to produce Ag/TiO2/g-C3N4 (a); preparation of a mixture of TiO2 and g-C3N4, followed by synthesis of Ag NPs (b1); synthesis of TiO2/g-C3N4, followed by synthesis of Ag NPs (b2); synthesis of g-C3N4 in the presence of TiO2, followed by the synthesis of Ag NPs (c1); synthesis of TiO2 in the presence of g-C3N4, followed by the synthesis of Ag NPs (c2); synthesis of Ag NPs in the presence of TiO2, followed by a mixture with g-C3N4 (d). P = precursor.
It should be emphasized that the photocatalytic mechanism of the ternary Ag/TiO2/g-C3N4 nanocomposite is very complex and not yet fully understood. Since it is influenced Figure 11. Schematic presentation of strategies for the synthesis of the Ag/TiO 2 /g-C 3 N 4 nanocomposites: synthesis of Ag/TiO 2 /melamine from a mixture of Ag, TiO 2 and g-C 3 N 4 precursors, followed by calcination to produce Ag/TiO 2 /g-C 3 N 4 (a); preparation of a mixture of TiO 2 and g-C 3 N 4 , followed by synthesis of Ag NPs (b1); synthesis of TiO 2 /g-C 3 N 4 , followed by synthesis of Ag NPs (b2); synthesis of g-C 3 N 4 in the presence of TiO 2 , followed by the synthesis of Ag NPs (c1); synthesis of TiO 2 in the presence of g-C 3 N 4 , followed by the synthesis of Ag NPs (c2); synthesis of Ag NPs in the presence of TiO 2 , followed by a mixture with g-C 3 N 4 (d). P = precursor. anomaterials 2023, 13, x FOR PEER REVIEW 20 of 28 Figure 12. Direct Z-scheme of the photocatalytic mechanism of Ag/TiO2/g-C3N4 heterostructure (a (Reprinted with permission from [45]; Copyright 2022, IWA Publishing); Type II heterojunction o Ag/TiO2/g-C3N4 accompanied by the Schottky barrier (b) (Reprinted with permission from [122] Copyright 2017, Elsevier) and (c) (Reprinted with permission from [116], Copyright 2020, Elsevier) photocatalytic mechanism of Ag/TiO2/g-C3N4 under visible light (d) (Reprinted with permission from [120], Copyright 2015, Elsevier).
In some of the reported studies, only visible light was used as the excitation source and it was suggested that only g-C3N4 absorbs the visible light photons and is excited [118,120,128]. Subsequently, the photogenerated ein the CB of g-C3N4 can be transferred to the CB of TiO2. It has been suggested that the Ag deposited on the surface of TiO2 plays a key role as an electron conduction bridge, to transfer the electron from the CB of g-C3N to the CB of TiO2. The formation of a Schottky barrier at the interface between Ag and TiO2 efficiently enhances the electron transfer to TiO2 and the separation of electron−holes in g-C3N4 [118,120]. At the same time, the SPR effect of Ag contributes significantly to the absorption of visible light in the Ag/TiO2/g-C3N4 nanocomposite. However, it is also believed that the strong electron oscillation in the SPR in Ag under visible light triggers the transfer of energetic electrons from Ag into the TiO2 conduction band, thus shattering the Schottky barrier (Figure 12d) [120]. It is hypothesized that loading of TiO2 with Ag can cause the shift of the Fermi level of Ag to a more negative level and of TiO2 to a more positive level to achieve a new Fermi level equilibrium that allows the transfer of the energetic plasmon electrons of Ag NPs across the energy barrier into the conduction band of TiO2 [115]. In this case, the oxidation reaction occurs at the surface of TiO2, while the holes of g-C3N4 are directly involved in the oxidation reaction. Figure 12. Direct Z-scheme of the photocatalytic mechanism of Ag/TiO 2 /g-C 3 N 4 heterostructure (a) (Reprinted with permission from [45]; Copyright 2022, IWA Publishing); Type II heterojunction of Ag/TiO 2 /g-C 3 N 4 accompanied by the Schottky barrier (b) (Reprinted with permission from [122]; Copyright 2017, Elsevier) and (c) (Reprinted with permission from [116], Copyright 2020, Elsevier); photocatalytic mechanism of Ag/TiO 2 /g-C 3 N 4 under visible light (d) (Reprinted with permission from [120], Copyright 2015, Elsevier).
Another possible photocatalytic mechanism of Ag/TiO 2 /g-C 3 N 4 involves the Type-II mechanism of the TiO 2 /g-C 3 N 4 heterojunction and Schottky barrier formed at the interfaces of Ag/TiO 2 , Ag/gC 3 N 4 or TiO 2 /Ag/g-C 3 N 4 (Figure 12b,c) [116,117,122,125,126]. According to this mechanism, both TiO 2 and g-C 3 N 4 are excited under spectrum solar irradiation, but g-C 3 N 4 mainly absorbs visible light and TiO 2 absorbs UV light. After excitation, photogenerated ecan easily be transferred from the more negative CB of g-C 3 N 4 to the less negative CB of TiO 2 and, at the same time, h + can be easily transferred from the more positive VB of TiO 2 to the less positive VB of g-C 3 N 4 (Type-II mechanism). When Ag is deposited on TiO 2 in TiO 2 /g-C 3 N 4 , ecan be transferred from the CB of TiO 2 and trapped by Ag due to the Schottky barrier formed at the interface of the Ag and TiO 2 (Figure 12b) [117,121,122,124,126]. This promotes the separation of charge carriers and significantly enhances the photocatalytic activity. On the other hand, when Ag is deposited on TiO 2 and g-C 3 N 4 in the ternary heterostructure, Ag captures the electrons from both g-C 3 N 4 and TiO 2 (Figure 12c) [116,117,125]. The effect of the position of the noble metal in TiO 2 /g-C 3 N 4 on the photocatalytic activity was systematically investigated for ternary Pt/TiO 2 /g-C 3 N 4 nanocomposites, and the results showed that the efficiency increased as follows: Pt deposited only on g-C 3 N 4 < Pt deposited on both TiO 2 and g-C 3 N 4 < Pt deposited only on TiO 2 [127].
In some of the reported studies, only visible light was used as the excitation source, and it was suggested that only g-C 3 N 4 absorbs the visible light photons and is excited [118,120,128]. Subsequently, the photogenerated e − in the CB of g-C 3 N 4 can be transferred to the CB of TiO 2 . It has been suggested that the Ag deposited on the surface of TiO 2 plays a key role as an electron conduction bridge, to transfer the electron from the CB of g-C 3 N 4 to the CB of TiO 2 . The formation of a Schottky barrier at the interface between Ag and TiO 2 efficiently enhances the electron transfer to TiO 2 and the separation of electron−holes in g-C 3 N 4 [118,120]. At the same time, the SPR effect of Ag contributes significantly to the absorption of visible light in the Ag/TiO 2 /g-C 3 N 4 nanocomposite. However, it is also believed that the strong electron oscillation in the SPR in Ag under visible light triggers the transfer of energetic electrons from Ag into the TiO 2 conduction band, thus shattering the Schottky barrier (Figure 12d) [120]. It is hypothesized that loading of TiO 2 with Ag can cause the shift of the Fermi level of Ag to a more negative level and of TiO 2 to a more positive level, to achieve a new Fermi level equilibrium that allows the transfer of the energetic plasmon electrons of Ag NPs across the energy barrier into the conduction band of TiO 2 [115]. In this case, the oxidation reaction occurs at the surface of TiO 2 , while the holes of g-C 3 N 4 are directly involved in the oxidation reaction.
In the literature, an Ag/TiO 2 /g-C 3 N 4 ternary nanocomposite has not yet been applied to textile fibers, although it has been established as a powerful nanomaterial for the photocatalytic degradation of various dyes [43,108,113,116,118,121,123,124], phenol [118], acetaldehyde [119], formaldehyde [117], ammonia [109], and carbon dioxide [122]; hard metals, such as hexavalent chromium [108,109]; and uranium from uranium-containing wastewater [45]. Ag/TiO 2 /g-C 3 N 4 nanocomposite has already been used for photocatalytic hydrogen evolution [115,126], solar water oxidation [112], electron transport in organic solar cells [114], vitamin B3 production [110], and as an antibacterial agent [108,111]. Due to its excellent multifunctional properties, the use of Ag/TiO 2 /g-C 3 N 4 nanocomposite for the chemical modification of textile fibers is still a promising research challenge.

Conclusions and Future Perspectives
In this review, binary Ag/g-C 3 N 4 and TiO 2 /g-C 3 N 4 nanostructures and ternary Ag/TiO 2 /g-C 3 N 4 nanostructure were presented as very promising and effective nanocomposites with Schottky, Type II, and Z-scheme mechanisms of photocatalysts. All the above nanocomposites have attracted much attention, due to their ability to initiate and carry out various reduction and oxidation reactions under visible light, and can thus be advantageous when used in environmental remediation, energy storage and conversion, sustainable catalysis, biosensing, and antimicrobial disinfection.
For textile applications, binary Ag/g-C 3 N 4 and TiO 2 /g-C 3 N 4 composites have emerged as promising functional nanomaterials, because the synergistic effect of the components in the heterostructures leads to improved photocatalytic performance of the composites compared with the single-component material itself. The Ag/g-C 3 N 4 nanocomposites not only exhibit enhanced visible-light photocatalytic performance but also an improved antimicrobial performance, due to the excellent antimicrobial activity of Ag. In the preparation of Ag/g-C 3 N 4 nanocomposites, g-C 3 N 4 is surface-decorated with Ag during the in situ synthesis of Ag 0 from AgNO 3 precursor in the suspension of presynthesized g-C 3 N 4 . Another approach is to mix urea, melamine, and/or cyanuric acid with AgNO 3 precursors and then synthesize Ag-doped g-C 3 N 4 under suitable conditions. Compared to bare g-C 3 N 4 , it is believed that the efficiency of the Ag/g-C 3 N 4 nanocomposite is significantly enhanced by the presence of Ag 0 , which acts as a current collector and plasmonic absorber. The Schottky barrier formed at the interface between Ag and g-C 3 N 4 maximizes photoinduced charge carrier separation and prevents electron-hole pair recombination. The nanocomposite photoactivity is further enhanced by plasmon resonance energy transfer, as the intense electric near field induced by SPR improves the efficiency of charge carrier separation. The exceptional photocatalytic performance of Ag/g-C 3 N 4 nanocomposites on textile substrates has already been advantageously used for the photochemical activation of organic syntheses and as a textile-based source for wearable electronics.
The two-semiconductor heterojunction of TiO 2 and g-C 3 N 4 has been recognized as an effective material for various environmental and energy applications. TiO 2 /g-C 3 N 4 was prepared by various synthetic routes, including the facile mixing of prepared TiO 2 and g-C 3 N 4 under suitable conditions, the in situ synthesis of TiO 2 from its precursor in the presence of presynthesized g-C 3 N 4 , or conversely, the in situ synthesis of g-C 3 N 4 from its precursor in the presence of presynthesized TiO 2 . Moreover, the precursors of TiO 2 and g-C 3 N 4 were simultaneously mixed in sol to synthesize the TiO 2 /g-C 3 N 4 heterojunction. An important step in the synthesis process is the calcination of the nanocomposites under suitable conditions, to obtain the desired morphology of the nanocomposite. The unique properties of the TiO 2 /g-C 3 N 4 heterojunction are related to the simultaneous utilization of UV and visible light, resulting in excellent photocatalytic performance under UV-and visible-light irradiation. When TiO 2 /g-C 3 N 4 is excited by incident UV/visible light of sufficient energy, the Type-II heterojunction and the direct Z-scheme charge carrier transfer pathway are adopted in the photocatalytic mechanism of the TiO 2 /g-C 3 N 4 nanocomposite. According to the band edge potentials, the Type-II heterojunction allows the transfer of the photogenerated electrons from the CB of g-C 3 N 4 to the CB of TiO 2 and the photogenerated holes from the VB of TiO 2 to the VB of g-C 3 N 4 . This causes the reduction reaction on TiO 2 to proceed with a lower reduction potential compared with g-C 3 N 4 , and the oxidation reaction on g-C 3 N 4 to proceed with a lower oxidation potential compared to TiO 2 , which is a disadvantage of the Type-II heterojunction mechanism. On the other hand, despite having the same band structure configuration, the direct Z-scheme assumes a much stronger redox capability for the TiO 2 /g-C 3 N 4 heterojunction because it promotes the spatial separation of electrons and holes on g-C 3 N 4 with the higher reduction potential and on TiO 2 with the higher oxidation potential, respectively, and promotes the recombination between the photogenerated electrons in the CB of TiO 2 and the photogenerated holes in the VB of g-C 3 N 4 with the lower redox capability. The superior photocatalytic efficiency of TiO 2 /g-C 3 N 4 nanocomposite has already been beneficially utilized in the development of textile-based photocatalysts for the effective purification of liquid and gaseous pollutants and bacteria.
There is no evidence in the literature that an Ag/TiO 2 /g-C 3 N 4 ternary nanocomposite has been used for textile applications, although it is a promising strategy for the surfaceand bulk-modification of textiles. There are many strategies to prepare Ag/TiO 2 /g-C 3 N 4 heterostructures, including simultaneous synthesis of the nanocomposite from suitable precursors, in situ synthesis of Ag in the presence of a previously synthesized TiO 2 /g-C 3 N 4 composite, and surface decoration of TiO 2 by Ag and subsequent mixing with g-C 3 N 4 . The synthesis pathway directly affects the photocatalytic mechanism of the Ag/TiO 2 /g-C 3 N 4 nanocomposite, which can be explained by the direct Z-scheme or the Type-II mechanisms established in the TiO 2 and g-C 3 N 4 heterojunction, supported by the Schottky barrier and SPR of Ag. The great potential of the Ag/TiO 2 /g-C 3 N 4 ternary nanocomposite lies in its ability to provide multifunctional textile properties, such as photocatalytic self-cleaning, antimicrobial activity, UV protection, conductivity, and thermal stability. Therefore, the development of textile platforms with integrated Ag/TiO 2 /g-C 3 N 4 heterostructures is a major challenge, where the in situ synthesis of Ag/TiO 2 /g-C 3 N 4 in the presence of textile fibers as an stabilizing agent is a priority. Due to the additional requirements imposed on textile materials, the preparation of simultaneously effective, multifunctional, noncytotoxic, and durable chemical modification of textile substrates will certainly be a hot research topic and will open new application opportunities for textile-based Ag/TiO 2 /g-C 3 N 4 nanocomposites.
Funding: This research was funded by the Slovenian Research Agency, Slovenia (Programme P2-0213 Textiles and Ecology, Programme P1-0143 Circulation of substances in the environment, material balance and modeling of environmental processes and risk assessment, Infrastructural Centre RIC UL-NTF, BI-US/22-24-162, and a Grant for the doctoral student D.G.).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.