**3.5 Recent novel adsorbents for dye uptake**

The surface functions of electrospun composite nanofibers are crucial for dye removing applications, which depend partly on the chemical groups of the used polymers and can be modified by chemical grafting or loaded absorbents. Novel p(NIPAM-co-MAA)/β-CD nanofibers were fabricated by electrospinning and thermal crosslinking for the application of crystal violet (CV) removal. The porous structure obtained from high-temperature treatment caused a hydrophobic surface, which facilitated the dye removal. The high adsorption capacity was attributed to electrostatic attraction, host-guest interaction of β-cyclodextrin, and hydrophobic forces [40]. Zhang and coauthors synthesized acid-activated sepiolite fibers grafted with amino groups for the adsorption of Congo red (CR) [41]. The Weber and Morris model fitting suggested that the adsorption happened through two stages, which included the initial period involving the external mass transfer and the final stage governed by intra-particle diffusion.

Recently, clay minerals have been intensively studied for the fabrication of clay-polymer composite nanofibers owing to the benefits of low cost, nontoxicity, and good adsorption [42]. Montmorillonite/chitosan/PVA nanofibers were utilized for Basic Blue (BB41) separation. The complex formation between amine groups and cationic dyes governed the adsorption and gave an explanation to the maximum adsorption capacity of the composite material at a pH of 7. At acidic pH, the active sites were occupied by hydrogen ions. Natural calcium alginate with biocompatibility and nontoxicity shows promises in colored water treatment due to possessing carboxyl groups, which can attract cationic dye molecules. Gelatin with amino groups also presents high adsorption performance against dyestuffs. The combination of two materials in the form of composite nanofibers showed good adsorption capacity with improved reusability and regeneration compared to using only calcium alginate nanofibers [43].

#### *Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*


#### **Table 1.**

*Comparison of different composite nanofibers for the adsorptive removal of dyes.*

Owing to the mesoporous structure and the possibility of functionalization, meso-silica has drawn significant interest in the field of dye adsorption. The surface of meso-silica modified with carboxylic acid groups showed affinity toward cationic dyes but presented almost no adsorption for anionic and neutral dyes. The inorganic modification of meso-silica with CuO enhanced the adsorption effects on the cationic dye, which was related to electrostatic forces between CuO and dye molecules [44]. Adsorption capacities of different composite nanofibers for various dyes are listed in **Table 1**.

### **4. Photocatalytic degradation of dyes using composite nanofibers**

#### **4.1 ZnO-loaded nanofibers**

The photodegradation is a light-induced process following the contact of contaminants to the photocatalysts, and its efficiency is substantially governed by the adsorption capacity of photocatalysts. Therefore, the adsorption of pollutants into metal oxides is the prerequisite for efficient photodecomposition, which hints that it is necessary to increase the surface area of adsorbents to give more binding sites and restrict the aggregation. Reducing the sizes of metal oxide to nanoscale and loading them onto the surfaces of nanofibers is a well-studied route to improve

**127**

*Composite Nanofibers: Recent Progress in Adsorptive Removal and Photocatalytic Degradation…*

the photocatalysis. Among different metal oxide semiconductors, ZnO, an n-type semiconductor in the undoped form, has proven to be an immense potential as a photocatalyst owing to its low cost, environmentally benign character, and high quantum efficiency. ZnO structures with the merit of controllable growth into nanoparticles, spindles, nanorods, and flower-like structures, show promises in photocatalytic dye decomposition. However, the nature of the powder form of ZnO makes the recycling and recovery process an arduous task; the issue can be addressed by immobilizing ZnO to nanofibrous membranes. The processes involving electrospinning and heat treatment were straightforward and delivered an outstanding performance [25, 27]. Besides, due to the wide bandgap of 3.37 eV, the photocatalytic activity of ZnO can only be triggered by UV light. Doping with metals, nonmetals, or other semiconductors can affect the ZnO bandgap, resulting in altered photocatalytic performance. Carbon-doped ZnO nanofibers lowered the bandgap energy of ZnO, which enabled the generation of oxygen and hydroxyl radicals to decompose MB under solar light excitation [46]. The stability of ZnO in mediums with different pH is also a hindrance to commercial purposes. Coating with inert oxides, such as TiO2 and SiO2, could show higher photostability and better photolysis due to the passivation of lattice oxygen [47]. In this case, the coating demonstrated remarkably enhanced stability in alkaline and acidic environments

TiO2 is one of the most studied semiconductor materials due to many advantages, including the cost-effectiveness, photocatalytic activity, biocompatibility, nontoxicity, and high stability. It has different forms, such as rutile, brookite, and anatase. The bandgaps of TiO2 are 3.03 and 3.2 eV for rutile and anatase, respectively, and they can be activated by photons in the near UV range (λ < 387 nm). The technique of decorating TiO2 onto nanofibers was a well-applied one to deliver the photocatalytic degradation of organic pollutants and mitigate its drawbacks as spontaneous aggregation and the problem of recovery and recycling. TiO2 embedded CNFs have gained lots of attention in the application of dye elimination by photocatalysis. Liang et al. demonstrated that the CNFs semi-wrapped with TiO2 could maintain consistently high photocatalytic activities against RB after five times [48]. Besides, significant efforts have been made to dope and functionalize TiO2 to trigger the bandgap under the visible light. Qiu et al. presented a novel method of immobilizing Mo/N-codoped TiO2 nanorods onto carbon nanofibers via two facile steps. The composite nanofibers demonstrated superb photocatalytic activity against MB, which suggested that the doping elements exhibited posi-

the photodecomposition confirmed by trapping active species experiments [49]. The doping with other semiconductors has also demonstrated the enhancement in photocatalytic efficiency. Magnetic ZnFe2O4 with a small bandgap of 1.9 eV was successfully integrated into TiO2 nanofibers by hydrothermal technique; the composite nanofibers promote the photoresponse under a broader region of solar

Ion-based materials with the unique characteristic of strong magnetic response,

leading to unprecedented sorption capacity and photocatalytic activities, have shown great promises in water treatment. The sizes and shapes present significant

was believed to be the main active species in

*DOI: http://dx.doi.org/10.5772/intechopen.91201*

as a protective layer.

**4.2 TiO2 composite nanofibers**

tive effects on dye degradation. H<sup>+</sup>

**4.3 Iron-based nanofibrous photocatalysts**

light than TiO2 [50].

*Composite Nanofibers: Recent Progress in Adsorptive Removal and Photocatalytic Degradation… DOI: http://dx.doi.org/10.5772/intechopen.91201*

the photocatalysis. Among different metal oxide semiconductors, ZnO, an n-type semiconductor in the undoped form, has proven to be an immense potential as a photocatalyst owing to its low cost, environmentally benign character, and high quantum efficiency. ZnO structures with the merit of controllable growth into nanoparticles, spindles, nanorods, and flower-like structures, show promises in photocatalytic dye decomposition. However, the nature of the powder form of ZnO makes the recycling and recovery process an arduous task; the issue can be addressed by immobilizing ZnO to nanofibrous membranes. The processes involving electrospinning and heat treatment were straightforward and delivered an outstanding performance [25, 27]. Besides, due to the wide bandgap of 3.37 eV, the photocatalytic activity of ZnO can only be triggered by UV light. Doping with metals, nonmetals, or other semiconductors can affect the ZnO bandgap, resulting in altered photocatalytic performance. Carbon-doped ZnO nanofibers lowered the bandgap energy of ZnO, which enabled the generation of oxygen and hydroxyl radicals to decompose MB under solar light excitation [46]. The stability of ZnO in mediums with different pH is also a hindrance to commercial purposes. Coating with inert oxides, such as TiO2 and SiO2, could show higher photostability and better photolysis due to the passivation of lattice oxygen [47]. In this case, the coating demonstrated remarkably enhanced stability in alkaline and acidic environments as a protective layer.

### **4.2 TiO2 composite nanofibers**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

**No. Adsorbent Dye Adsorption** 

5. Zeolitic imidazolate framework-8

9. NH2 grafted acid-activated sepiolite

10. Meldrum's acid cellulose nanofibers-

11. β-Cyclodextrin modified p(NIPAM-co-

12. Chitosan/polyvinyl alcohol/zeolite

functional polyacrylonitrile nanofibers

7. MOFs grew on silk nanofibers RB

fibers

based PVDF nanofibers

MAA) nanofibers

electrospun nanofibers

14. Chitosan/polyamide nanofibers RB5

15. APAN/Fe3O4–MPA composites nanofiber Indigo

*Comparison of different composite nanofibers for the adsorptive removal of dyes.*

1. PVDF/GO nanofibers MB 621.1 [37] 2. MOF/PAN nanofibers MB 20.68 [8] 3. PVA/PAA/GO-COOH@PDA MB 26.45 [17] 4. Gelatin/alginate composite nanofibers MB 1937 [43]

6. ZIF-8@CS/PVA-ENF MG 1000 [39]

8. CuO-ZnO composite nanofibers CR 126.4 [27]

13. PMMA/zeolite nanofibers MO 95.33 [34]

MB MG

MG

P4R

carmine

**capacity, mg g<sup>−</sup><sup>1</sup>**

36.92 1531.94

> 19 840.2

CR 539.71 [41]

CV 3.984 [21]

CV 1253.78 [40]

MO 153 [33]

456.9 502.4

154.5 [45]

**Reference**

[35]

[23]

[22]

Owing to the mesoporous structure and the possibility of functionalization, meso-silica has drawn significant interest in the field of dye adsorption. The

surface of meso-silica modified with carboxylic acid groups showed affinity toward cationic dyes but presented almost no adsorption for anionic and neutral dyes. The inorganic modification of meso-silica with CuO enhanced the adsorption effects on the cationic dye, which was related to electrostatic forces between CuO and dye molecules [44]. Adsorption capacities of different composite nanofibers for various

**4. Photocatalytic degradation of dyes using composite nanofibers**

The photodegradation is a light-induced process following the contact of contaminants to the photocatalysts, and its efficiency is substantially governed by the adsorption capacity of photocatalysts. Therefore, the adsorption of pollutants into metal oxides is the prerequisite for efficient photodecomposition, which hints that it is necessary to increase the surface area of adsorbents to give more binding sites and restrict the aggregation. Reducing the sizes of metal oxide to nanoscale and loading them onto the surfaces of nanofibers is a well-studied route to improve

**126**

dyes are listed in **Table 1**.

**Table 1.**

**4.1 ZnO-loaded nanofibers**

TiO2 is one of the most studied semiconductor materials due to many advantages, including the cost-effectiveness, photocatalytic activity, biocompatibility, nontoxicity, and high stability. It has different forms, such as rutile, brookite, and anatase. The bandgaps of TiO2 are 3.03 and 3.2 eV for rutile and anatase, respectively, and they can be activated by photons in the near UV range (λ < 387 nm). The technique of decorating TiO2 onto nanofibers was a well-applied one to deliver the photocatalytic degradation of organic pollutants and mitigate its drawbacks as spontaneous aggregation and the problem of recovery and recycling. TiO2 embedded CNFs have gained lots of attention in the application of dye elimination by photocatalysis. Liang et al. demonstrated that the CNFs semi-wrapped with TiO2 could maintain consistently high photocatalytic activities against RB after five times [48]. Besides, significant efforts have been made to dope and functionalize TiO2 to trigger the bandgap under the visible light. Qiu et al. presented a novel method of immobilizing Mo/N-codoped TiO2 nanorods onto carbon nanofibers via two facile steps. The composite nanofibers demonstrated superb photocatalytic activity against MB, which suggested that the doping elements exhibited positive effects on dye degradation. H<sup>+</sup> was believed to be the main active species in the photodecomposition confirmed by trapping active species experiments [49]. The doping with other semiconductors has also demonstrated the enhancement in photocatalytic efficiency. Magnetic ZnFe2O4 with a small bandgap of 1.9 eV was successfully integrated into TiO2 nanofibers by hydrothermal technique; the composite nanofibers promote the photoresponse under a broader region of solar light than TiO2 [50].

#### **4.3 Iron-based nanofibrous photocatalysts**

Ion-based materials with the unique characteristic of strong magnetic response, leading to unprecedented sorption capacity and photocatalytic activities, have shown great promises in water treatment. The sizes and shapes present significant

influences over the magnetic properties of iron oxide nanoparticles due to the changes in magnetic anisotropy. Among magnetic materials, FeO (wustite), Fe3O4 (magnetite), α-Fe2O3 (hematite), β-Fe2O3 (beta phase), γ-Fe2O3 (magnetite), and spinel ferrites (MFe2O4) have been focused on for the multiple applications including catalysis, sensors, and magnetic data storage. α-Fe2O3 presents weak ferromagnetism (saturation magnetization is less than 1 emu g<sup>−</sup><sup>1</sup> ) at room temperature in contrast to γ-Fe2O3 and Fe3O4 (up to 92 emu g<sup>−</sup><sup>1</sup> ). Thus, Fe3O4 and γ-Fe2O3 have been employed extensively to regenerate photocatalysts owing to good magnetic separation [51]. The convenience of separation by using an external magnetic field helps replace the tedious task of filtration and centrifugation for photocatalyst recovery. One prominent advantage of iron oxides is the relative narrow bandgap for the use of visible light activity, which is between 1.9 and 2.5 eV. In comparison to anatase TiO2 (3.03–3.2 eV), which can only harvest light at a wavelength of 387 nm or below in the UV region, iron oxide-based photocatalysts prove to be superior in visible light range. The use of heterogeneous photocatalysts can accelerate the photocatalytic performance of iron oxides as a result of the enhanced visible light activation, better separation of electron-hole pair, and interfacial charge transfer. Bi2MoO6, which possesses a small bandgap (2.5–2.8 eV), was prepared by electrospinning; then the solvothermal method was followed to prepare 1D α-Fe2O3/Bi2MoO6 composite nanofibers [52]. The composite was demonstrated to exhibit enhanced photocatalysis in MB and RB degradation under sunlight irradiation because of the charge separation character of heterogeneous α-Fe2O3 and Bi2MoO6 composite nanomaterials.
