Recent Advances in the Synthesis of Metal Oxide Nanofibers and Their Environmental Remediation Applications

Abstract

Recently, wastewater treatment by photocatalytic oxidation processes with metal oxide nanomaterials and nanocomposites such as zinc oxide, titanium dioxide, zirconium dioxide, etc. using ultraviolet (UV) and visible light or even solar energy has added massive research importance. This waste removal technique using nanostructured photocatalysts is well known because of its effectiveness in disintegrating and mineralizing the unsafe organic pollutants such as organic pesticides, organohalogens, PAHs (Polycyclic Aromatic Hydrocarbons), surfactants, microorganisms, and other coloring agents in addition to the prospect of utilizing the solar and UV spectrum. The photocatalysts degrade the pollutants using light energy, which creates energetic electron in the metal oxide and thus generates hydroxyl radical, an oxidative mediator that can oxidize completely the organic pollutant in the wastewater. Altering the morphologies of metal oxide photocatalysts in nanoscale can further improve their photodegradation efficiency. Nanoscale features of the photocatalysts promote enhance light absorption and improved photon harvest property by refining the process of charge carrier generation and recombination at the semiconductor surfaces and in that way boost hydroxyl radicals. The literature covering semiconductor nanomaterials and nanocomposite-assisted photocatalysis—and, among those, metal oxide nanofibers—suggest that this is an attractive route for environmental remediation due to their capability of reaching complete mineralization of organic contaminants under mild reaction conditions such as room temperature and ambient atmospheric pressure with greater degradation performance. The main aim of this review is to highlight the most recent published work in the field of metal oxide nanofibrous photocatalyst-mediated degradation of organic pollutants and unsafe microorganisms present in wastewater. Finally, the recycling and reuse of photocatalysts for viable wastewater purification has also been conferred here and the latest examples given.

1. Introduction

For the treatment of agricultural and industrial wastewater that contains traces of refractory organic compounds such as organic pesticides, surfactants, organohalogens, and coloring agents, wastewater purification technologies are now essential to build economical and more advanced treatment methods. In general, a combination of several approaches gives high treatment efficiency compared with existing treatments. For example, a certain pollutant can hardly be degraded by photolysis with conventional photocatalysts in the ambient environment and treated wastewater may need more steps for total mineralization [1,2,3,4]. However, a combination of several methods, such as synthesizing suitable nanostructured catalysts and optimizing the wavelength of light exposure depending on the selection semiconductor material with favorable bandgap energy, improves the degradation of pollutants from the wastewater [5,6,7].

On the other hand, photocatalysis has an enormous potential for the elimination of organic pollutants from wastewater [6,8,9,10]. However, it is still not in practical use since it has a low oxidation rate. Therefore, a combination of selection of photocatalysts together with light source, which can boost oxidation, is reasonable for the management of hard-to-decompose organic pollutants. The challenging organic pollutants are then expected to decompose more rapidly and thoroughly in the presence of a suitable photocatalyst along with an appropriate light source.

In a broad sense, environmental remediation includes the elimination of agricultural, industrial pollutants, hard-to-decompose organic compounds, bacteria, germs, and fungus present in wastewater, along with cleansing of air pollutants such as volatile organic compounds (VOCs) and nitrogen di-oxide (NOx). Figure 1 illustrates the idea of environmental remediation using photocatalysis.

Last year, Anjum et al. [11] published a comprehensive review article on environmental remediation using different nanomaterials. It is clear from their perspective that electrospun nanofibers and other hybrid nano-membranes are very effective for the efficient removal of organic dyes, heavy metals, and foulants. In a recent study, radially oriented ZnO nanowires were decorated on flexible poly-l-lactide nanofibers for continuous-flow photocatalytic water purification [12]. The photocatalytic decomposition was monitored on various organic pollutant dyes, such as methylene blue, monocrotophos, and diphenylamine under illumination with UV light using this highly flexible hierarchical nanostructure. The same electrospun ZnO- poly-L-lactide nanofibers photocatalysts have been used for the adsorption of Cr(VI) as a crucial step for water purification by Burks et al. [13].

The main objective of this paper is to review the most recently published work in the field of ultraviolet- and visible-light-driven photocatalysis on wastewater treatment. The summary of photocatalytic degradation pollutant dyes, bacteria containing wastewater, and volatile organic compounds (VOC) in air degraded by various researchers using metal oxide nanofibers, including information about the synthesis routes of different sizes of fibers, is presented in

Table 1. Table of metal oxide nanofiber photocatalysts and their environmental remediation applications.

Nanofibers	Fiber Diameter (nm)	Fabrication Method	Light Irradiated	Application	Literature
ZnO	50–150	Electrospinning	UV	Photocatalysis of PAH dyes	Singh et al. [14]
Carbon doped TiO2	25–75	Electrospinning	UV	Photocatalysis of PAH dyes	Mondal et al. [15]
Al2O3–ZrO2/TiO2	150–200	Sol–gel synthesis	UV	Photocatalysis of methyl orange and methylene blue	Hong et al. [16]
C/TiO2	30–50	Electrospinning	UV	Photocatalysis of methyl orange	Reddy et al. [17]
CdS/TiO2	100–140	Electrospinning	UV and visible	Photocatalysis of para-nitrophenol dye	Singh et al. [18]
ZnO/Zn(OH)F	~100	Microfluidic chemical method	UV	Photocatalysis of methylene blue and histidine-rich protein separation	Zhao et al. [19]
Ce1−xZrxO2/SiO2	50–80	Carbon nanofiber (CNF) template-assisted alcohol-thermal procedure	UV	Photocatalysis of methylene blue	Zhang et al. [20]
Al2O3-Mn3O4	~200	Low-temperature stirring	Visible	Photocatalysis of brilliant cresyl blue	Asif et al. [21]
carbon/MWCNT/Fe3O4	100–150	Electrospinning	UV	Simultaneous photocatalysis of of phenol and paracetamol	Akhi et al. [22]
TiO2	50–200	Electrospinning-alkali-acid” combined method.	UV	Photocatalysis of rhodamine B and improved supercapacitance	Wang et al. [23]
graphitic carbon nitride (g-C3N4)	~200	Electrospinning and subsequent hydrothermal treatment	Visible solar	Photocatalysis of antibiotics	Qin et al. [24]
CNT/TiO2	266–292	Electrospinning	UV and visible	Photocatalysis of benzene (gas phase) , methylene blue	Wongaree et al. [25]
SiO2–Bi2WO6	430	Electrospinning Soaking and calcination	UV and visible	Photocatalysis of rhodamine B	Ma et al. [26]
SiO2/CuO	300	Electrospinning Soaking and calcination	UV and visible	Photocatalysis of rhodamine B degradation	Hu et al. [27]
RGO/InVO4	250–400	Electrospinning	Visible	Photocatalysis of rhodamine B	Ma et al. [28]
Ag3PO4/TiO2	100–200	Electrospinning and solution processes	Visible	Photocatalysis of rhodamine B	Xie et al. [29]
WO3	80–100	Electrospinning	Visible	Photocatalysis of methylene blue	Ofori et al. [30]
Fe3O4/TiO2/Ag	10	Sol–gel, hydrothermal method, photoreduction	UV and visible	Photocatalysis of Ampicillin	Zhao et al. [31]
TiO2/ZnS–In2S3	130	Electrospinning, hydrothermal method	Visible	Photocatalysis of rhodamine B	Liu et al. [32]
PAN-ZnO/Ag	702–998	Single-capillary electrospinning, hydrothermal, and reduction	UV	Photocatalysis of methylene blue	Chen et al. [33]
TiO2/ ZnFe2O4	200–300	Hydrothermal	UV and visible	Photo-electrochemical activity	Liang et al. [34]
BiOCl/Bi4Ti3O12	80	Electrospinning technique and solvothermal method	Visible	Photocatalysis of methyl orange and para-nitrophenol	Zhang et al. [35]
ZnO/nickel phthalocyanine	610	Two-step hydrothermal approach	Visible	Photocatalysis of rhodamine B assisted by H2O2	Wang et al. [36]
Polyaniline/CaCu3Ti4O12	30–50	In-situ polymerization	Visible	Photocatalysis of methyl orange, congo red dyes	Kushwaha et al. [37]
Cellulose/TiO2/tetracycline (TC) and phosphomycin	3.5	Green chemistry approach	UV	Antibacterial and photochemical application towards pathogen microorganisms: Staphylococcus aureus and Escherichia coli	Galkina et al. [38]
ZnO	678	Electrospinning, hydrolysis	UV	Photocatalysis of methyl orange	Liu et al. [39]
carbon nanotube/TiO2	~300	Combined sol–gel and electrospinning technique	UV and visible	Indoor benzene, toluene, ethyl benzene and o-xylene (BTEX) purification	Kang et al. [40]
CNT-TiO2	~300	Electrospinning method coupled to hydrothermal treatment	UV and visible	Oxidation of toluene and isopropyl alcohol	Kang et al. [41]
polyamide 6, polystyrene, polyurethane/TiO2	107–350	Electrospinning	Visible	Nitrogen oxide (NOx) removal in air purification	Szatmáry et al. [42]
S-doped TiO2	2000–4000	Electrospinning	Visible	Rhodamine B degradation	Ma et al. [43]
polyaniline-coated TiO2/SiO2	~500	Electrospinning, calcination and in situ polymerization	Visible	Photocatalysis of methyl orange degradation	Liu et al. [44]
p-MoO3 Nanostructures/n-TiO2	~300	Electrospinning method coupled to hydrothermal treatment	UV	Photocatalysis of rhodamine B	Lu et al. [45]
TiO2/carbon/ Ag	250–350	Electrospinning technique and hydrothermal	Visible	Photocatalysis of rhodamine B and methyl orange	Zhang et al. [46]
ZnO−Carbon	400–500	Electrospinning technique and hydrothermal	UV	Photocatalysis of rhodamine B	Mu et al. [47]

. All results using different organic effluents showed that the application of photocatalysis led to efficient and complete degradation and produced simple products that are environmentally safe; however, incomplete mineralization can result in the formation of highly toxic byproducts.

2. Nanofibers

There is a serious disadvantage to nanoparticles: very often they become agglomerated while dispersed in an aqueous medium and the macroscopic properties are compromised [48,49]. However, nanofibers overcome this problem owing to their fibrous form [50]. In addition, the nanoscopic dimension naturally provides a high surface area to volume ratio. This characteristic makes them very attractive in applications where large surface area is necessary [51]. Nanofibers are different to nanoparticles in terms of aspect ratios and not the same as nanotubes because of crystallographic properties. A fibrous morphology is beneficial over a particle morphology in several respects, e.g., the mechanism of photocatalysis, the function of noble metal, the heterojunctions, and the plasmonic properties.

Solid nanofibers with diameters in the submicron to nanometer range can be synthesized from various polymers and consequently have different physicochemical properties and application possibilities. Polymer chains in a nanofiber are associated via a covalent interaction [52]. Nanofibers are distinctive owing to their flexibility in surface functionalization compared to their microfibers, good mechanical strength, high porosity, and enormous surface area-to-volume ratio, etc. [53]. There exist many natural polymeric nanofibers including, gelatin, cellulose, silk fibroin, collagen, keratin, alginate, chitosan, etc., and synthetic polymers such as polycaprolactone (PCL), poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), poly(ethylene-co-vinylacetate) (PEVA), polyurethane (PU), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), etc. [54].

The diameters of nanofibers are mainly determined by the nature of the precursor polymer used, the parameters involved, and the method of fabrication [55]. There are various methods used to synthesize nanofibers, including electrospinning, dry spinning, thermal-induced phase separation, self-assembly, drawing, extrusion, template synthesis, etc. [56]. Among these, electrospinning is the most frequently used technique to produce nanofibers. It has many advantages such as simple and straightforward setup, easy mass production of continuous nanofibers from numerous polymers, and, most interestingly, the ability to produce ultrathin fibers with tunable diameters, compositions, and alignments [57]. This flexibility permits the shape controllability and organization of the fibers on any collector or even in the form of free-standing fibers so that many different structures such as hollow, core-shell, flat ribbon, and even spherical particles shaped by electrospraying can be fabricated on demand to meet the desired application requirements [58,59,60]. Those special nanofiber structures and textures can be exploited in various advanced applications such as energy storage, gas sensors and biosensors, nanofluidics, catalysis and photocatalysis, drug delivery and release, catalytic nano supports, etc. [61,62,63,64,65,66].

3. Electrospinning

The electrospinning method offers a straightforward electrohydrodynamical mechanism [67,68,69] to yield fibers with diameters measuring less than 100 nm [70], even up to 10 nm [71]. Widespread exploration has been done on the electrospinning technique to fabricate nanofibers [72]. Based on the reported literature, the basic electrospinning setup (shown in Figure 2) essentially involved three key parts: a syringe with a metallic needle having a suitable polymeric melt, a high-voltage power supply, and a conducting collector with an adjustable configuration. The process of fabrication of the nanofiber starts when electrically charged particles travel into the polymeric melt through the metallic needle. This originates instability within the polymeric melt because of the induction of electrically charged particles on the polymer drop. Simultaneously, the mutual repulsive force of charged particles originates an electrical force that competes with the surface tension of the polymer, and finally the polymer melt proceeds in the direction of the electric force. An additional escalation in the electric field causes the spherical shaped droplet that emerges from the needle to distort and take on a conical shape. A static or rotating collector assembly, electrically grounded and kept at an optimal distance, collects the ultrafine nanofibers from the conical shaped droplet of the polymeric melt (Taylor cone) because of the high electrical field applied. A steady charged polymer jet from the needle to the collector can be formed only when the polymeric melt has enough cohesive force. Throughout the process, the internal (on the fiber) and external (applied electric field) forces on the charged particles cause the whipping motion of the liquid jet towards the collector. Because of this whipping motion, the polymeric chains within the melt stretch and slide past each other, which ultimately results in the form of fibers with diameters measured in the submicron to nanoscale [73,74].

4. Core-Shell Nanofibers

The spinnablity of a polymeric melt during electrospinning is governed by the viscosity and ionic conductivity solution, and the nature of the solvents, along with the molecular weight and the conformation of the polymer. It is important to note that several polymers are not electrospinnable because of their inadequate solubility in an electrospinning suitable solvent; however, an unspinnable polymer can form nanofibers by co-electrospinning with the help of a spinnable polymer in the same solution [75]. Many dissimilar morphologies of nanofibers, for instance core-shell, hollow, and porous, bi-component structures, could be created with different designs of electrospinning spinnerets and nozzles.

In typical core-shell electrospinning, a coaxial jet is formed by a coaxial nozzle when two dissimilar liquids flow simultaneously through inner and outer capillaries [76]. The free end of the nozzle is connected to a high-voltage electric power supply. The spun nanofibers are consolidated in the course of solvent evaporation and mechanical stretching. The ratio of the solution feeding rate of the two liquid components controls the uniformity and stability of the core and shell flow [77]. Additionally, a few other parameters, such as the applied electric field strength, the shape and size of core-shell capillaries, the volume feed rate of each component, the immiscible nature of the core-shell materials, and their ionic conductivity and viscosity also play an essential role in shaping the uniform formation of core-shell jets and the morphology of the spun nanofibers [78].

5. Hollow and Porous Nanofibers

Hollow nanofibers are targeted for advanced and very specific applications, such as nanofluidics, photocatalysis, solar energy harvest, and hydrogen storage [79]. Typically, two types of techniques, including the direct co-axial spinning method [80] and chemical vapor deposition (CVD) methods, are used for the production of hollow nanofibers [81,82].

To produce hollow nanofibers by the electrospinning method, the co-axial spinning is considered, as shown in Figure 3. In co-axial electrospinning, nanofibers are prepared by following the same process as fabricating core-shell nanofibers; however, the core is removed with a selective solvent etching process at the end of the method [83,84,85]. For example, poly(vinylpyrrolidone) (PVP) and tetra butyltitanate (Ti (OC₄H₉)₄) were electrospun as shell solutions and paraffin oil or mineral oil was poured in as the core material. TiO₂ fibers with a hollow core structure were achieved when the polymer and core materials were removed by selective etching [18]. Figure 4 shows the morphologies of hollow titanium dioxide nanofibers fabricated using the core-shell electrospinning technique, as reported by Sing et al. [18].

In the second CVD method, the first precursor polymer is converted to a nanofiber or a “template” using electrospinning. Then, the spun fibers are coated with either an appropriate polymer or a metal. Lastly, hollow fibers are obtained by removing the template material by selecting solvent etching or by calcining at high temperature in a furnace [83,86,87,88,89]. The diameter and morphology of such hollow nanofibers can also be easily tuned by controlling the electrospinning process parameters [83].

The applicability of porous nanofibers is more comprehensive and broad in contrast with core-shell and hallow nanofibers. Owing to their ultra-high BET surface areas, porous fibers find applications in membranes [90], filtration [91], fuel cells [92], catalysis [93], tissue engineering [94,95], and drug delivery and release [96]. Porous nanofibers can be created with a distinctive topology by choosing precise solvents or a proper mixture of solvents, or polymer mixtures under an optimized environment. One approach could be based on phase separation of the polymeric components in a mixture caused by different evaporation rates. In this method, immiscible polymers are dissolved in a common solvent and electrospun. One of the polymers, either from the core or the shell of the spun fibers, is dissolved to achieve porous nanofibers. There are a few other methods for fabricating porous nanofibers, such as phase separation by vapor, non-solvent and thermally induced phase separation [97,98,99], rapid phase separation [100,101], selective dissolution [102], and selective calcination [102,103].

6. Metal Oxide Nanofibers

Metal oxide nanofibers have huge research importance for both 1D and 2D nanoscopic morphology due to their exclusive electrical and physicochemical properties. A range of uses has been proven in photovoltaic cells, light-emitting diodes, liquid crystal displays, lithium ion batteries, biosensors, and gas sensors.

There are many methods such as melt, colloid, solution, dry and co-axial electrospinning, CVD, PECVD, and electrochemical methods that are used to yield metal oxide and nanofibers [14,15,104,105,106,107,108]. In recent times, electrospun metal oxides’ precursors have garnered much interest for the production of multifunctional nanofibers [109,110]. Figure 5 defines the approach for the synthesis of an electrospun metal oxide nanofiber. There are two ways: either a metal oxide precursor is added to a polymer in the course of electrospinning solution preparation; also, it could be incorporated by dipping spun fibers into a precursor solution afterward.

Generally, calcination of spun metal oxide nanofibers is a common method for large-scale production. Calcination is a two-step process whereby the polymer is eliminated at high temperature in an oxygen atmosphere and an oxidative conversion of the precursor constituent yields a metal oxide by nucleation and growth at high temperature [111,112].

The metal oxide centers can be incorporated into the nanofiber matrix either in situ or ex situ. A suitable organometallic or sol–gel metal oxide precursor has been introduced with electrospinning polymer melt in situ, whereas soaking nanofibers in a solvent comprising the desired metal oxide precursor produces a surface coating in the latter case. There are many works reported for the electrospinning of metal oxide nanofibers either by the sol−gel route [113,114,115,116], or by formation of a polymeric metal oxide colloidal dispersion ex situ [117,118,119]. In the case of electrospinning from an inorganic precursor, the high-temperature calcination process causes a reduction in the diameter of the fibers as the sacrificial polymer template is selectively removed. Consequently, by reason of calcination, the resultant nanofibers become fragile owing to their thinner cross sections and the thermomechanical stress generated from the reduction in size [105,120,121,122]. There have been many efforts made in recent times towards the production of metal oxide nanofibers by the electrospinning route. For example, alumina, zinc oxide, silica, and TiO₂ nanoparticles were mixed with the electrospinning polymer solution [118,123,124,125]. Recently, Sharma’s group used a sol-gel precursor of titanium dioxide, titanium isopropoxide, along with a polyvinylpyrrolidone polymer, for electrospinning and fabricated carbon-doped titania nanofibers, as shown in Figure 6 [15].

In another work, ZnO nanofibers were produced using zinc acetate as an inorganic metal oxide precursor along with sacrificial carrier polymer polyacrylonitrile, which was removed during calcination [14]. Figure 7 shows the morphology of the electrospun ZnO nanofibers.

7. Photocatalysis

Photocatalysis is a potential way to decontaminate amenities, houses, living environments, industrial waste, and numerous organic compounds as well as refractory chlorinated aromatics and more than 27,935 (from Web of Science Core Collection until March 2017) references have been collected on this discipline [126,127,128,129]. As compared to other existing chemical oxidation techniques, photocatalysis may be more efficient since semiconductors photocatalysts are economical and can accomplish the complete mineralizing of various refractory compounds [130]. By further adjusting and developing this technology, the pollution in our air and water can be decreased. One can even decrease the spread of pollutions, infections, and disease such as severe acute respiratory syndrome (SARS) in clinics. This purification technology would be an advantage to the whole world.

Serpone and Emiline have argued that photocatalysis is the speeding up of a photoreaction by the action of a catalyst [131]. An earlier IUPAC article defined photocatalysis as a catalytic reaction containing light absorption on a catalyst or a substrate.

Owing to its capability of generating energetic powerful oxidant species, i.e., OH free radicals, photocatalysis can be treated as an advanced oxidation process (AOP). The photochemical AOPs are light-induced reactions, mainly oxidations that depend on the creation of OH^• by the grouping with supplied oxidants or semiconductor photocatalysts to the system [132]. The photodegradation efficiency of AOPs is significantly improved either by homogeneous or heterogeneous photocatalysis [133]. Heterogeneous photocatalysis hires photocatalyst slurries (for example: ZnO/UV, TiO₂/UV, WO₃, and BiVO₄) for catalysis, while homogeneous photocatalysis (for example: H₂O₂/UV, Fe³⁺/UV) is employed in a single-phase system.

7.1. Mechanism and Kinetics of Photocatalysis by Pure Metal Oxide Nanofibers

The energy difference between the valence band (VB) and the conduction band (CB) in a semiconductor is known as the “Band Gap”. When a metal oxide semiconductor such as TiO₂, ZnO, ZrO₂, SnO₂, CeO₂, etc., absorbs ultraviolet light, it produces an electron–hole pair when the energy is greater than its bandgap energy. The photo-generated electron and hole are negatively and positively charged, respectively. The electron in the conduction band of the photocatalyst gets excited after absorbing photons from light. The electron reacts with an O₂ molecule and yields a superoxide anion. The holes are oxidants, meaning they can oxidize water, which results in the generation of oxygen. The electrons can reduce protons and generate hydrogen. This cycle persists as long as the light energy is accessible.

The chemical reactions in photocatalysis are illuminated as follows with the help of Equations (1)–(9). As ultraviolet light falls on the metal oxide semiconductor photocatalyst, one electron jumps from the valence band to the conduction band, leaving behind a hole. The semiconductors become more efficient if those photo-generated electrons and holes wait before recombining. The electrons in the conduction band react with the O₂ molecule and H⁺ in the surrounding aqueous environment and harvest hydrogen peroxide, which then supplies the OH⁻ ion and highly energized OH^• free radical. Similarly, holes in the valance band react with water to produce an OH^• free radical. Formation of those highly active species helps to decrease the recombination time of the electron-hole pair and thereby offers extra time for the reaction with the pollutants present in the wastewater. Figure 8 shows the idea of photocatalysis by metal oxide nanofibers thorough a pictorial illustration.

The photocatalytic oxidation of pollutants present in wastewater under ultraviolet light can be described as follows [14]:

Metal oxide nanofibers + hυ → electron (e⁻)+ hole (h⁺)

(1)

h⁺ + H₂O → H⁺ + OH^•

(2)

h⁺ + OH⁻ → OH^•

(3)

e⁻ + O₂ → O₂⁻

(4)

2e⁻ + O₂ + 2H⁺ → H₂O₂

(5)

e⁻ + H₂O₂→ OH^• + OH⁻

(6)

Pollutant in wastewater + OH^• → clean water and degradation products

(7)

Pollutant in wastewater + metal oxide nanofibers (h⁺) → oxidation products

(8)

Pollutant in wastewater + metal oxide nanofibers (e⁻) → reduction products

(9)

The recombination process of the electron–hole pair involves additional energy created from the excited electron, which arises as heat. Since the recombination is undesirable and leads to ineffectual photocatalysis, the key aim of the metal oxide photocatalysts is to produce a catalytic reaction among the conduction band excited electrons and oxidant to yield a reduced product. A reaction among the positively charged holes with a reductant is essential to producing an oxidized product. It is worth mentioning that the reduction and oxidation reactions happen at the nanofiber surface from the time when this reaction generates positive holes and negative electrons. In the oxidative reaction, the hole stands with the moisture content on the fiber surface and returns a hydroxyl free radical. Moreover, if more oxygen is provided, this will act as an electron acceptor and delay the recombination rate, which further enhances the degradation of pollutants [15]. Furthermore, the total time period for pollutant degradation rises with a decrease in nanofiber loading since that will not provide enough reaction sites. Figure 9 shows the typical photocatalysis of polyromantic hydrocarbon (PAH) dye anthracene, which is a pollutant present in wastewater [14]. The fluorescence spectra refer to the gradual degradation of the pollutant by assessing its absorption over time.

7.2. Mechanisms of Photocatalysis by Metal/Metal Oxide Composite Nanofibers

Metal oxide nanofibers have spurred enormous research interest in photocatalysis as they have the ability to produce charge carriers when exposed to ultraviolet light. The interesting electron bandgap configurations, exceptional light harvest properties, and efficient charge transport characteristics of the metal oxide nanofibers have made them efficient photocatalysts. However, there are some weaknesses in metal oxide photocatalysts, for instance, quick recombination of photogenerated electron–hole pairs and a wide bandgap, which creates the opportunity to fabricate new composite photocatalysts.

The main objective of engineering composite nanofiber photocatalysts is to adjust the photoelectrochemical properties of the metal oxide counterpart and thus a series of metal–metal oxide and metal oxide–metal oxide composite nanofibers have been prepared to simplify charge recombination in the semiconducting nanofibrous nanostructures [134,135,136,137]. In this context in a noble metal/metal oxide nanofiber, the metal can act as a sink for photoproduced charge carriers and hence helps with interfacial charge transfer processes in composite photocatalysts during the course of photocatalysis [138]. Figure 10 illustrates the electron transfer process in a metal–metal oxide nanofiber photocatalytic system.

These metal–metal oxide hybrid nanofibers can show characteristics of their singular counterparts or produce completely new properties when these two phases are combined together in composite nanofibers. In an ideal situation, photocreated charge carriers could be transferred very quickly from one constituent of the composite to the other due to modified electronic band arrangements, thus decreasing the possibility of radiative electron–hole recombination and allowing their use in chemical reactions for longer [139,140]. Additionally, these metal–metal oxide nanofibers, because they can be designed to have several different facets on their surface, create a higher possibility for the absorption of targeted pollutant molecules during photocatalysis [139,141]. For example, recently research has been done to incorporate noble metals into metal oxides to increase the photocatalytic efficiency of titania [142].

Plasmonic properties for the degradation of organic compounds under visible light were first proved by Ohtani’s group in 2009 [143] (and even earlier by Tatsuma [144], but for other applications—photocurrent). TiO₂ nanofibers codecorated with Au and Pt nanoparticles for plasmon-enhanced photocatalytic activities were reported by Zhang et al. [145]. These attractive features of metal–metal oxide photocatalytic nanofibers motivated many researchers to investigate the impact of metal nanoparticles on semiconductors during photocatalysis.

7.3. Mechanisms of Photocatalysis by Metal Oxide/Metal Oxide Composite Nanofibers

Coupling metal oxides with other metal oxide semiconductors is another widely held method to tune the optical energy band configuration of photocatalysts and increase photon quantum efficiency, since coupled semiconductors can increase the separation rate of photoinduced electron-hole pairs in photocatalysts. Different from usual semiconductors, metal oxide nanofibers have no forbidden energy gap separating occupied and unoccupied levels, but they can excite electrons between bands to generate a high carrier mobility to simplify kinetic charge separation process. As illustrated in Figure 11, well contacted metal oxide–metal oxide nanofibers are advantageous for forming phase junctions. Generally, in metal/metal oxide semiconductor nanofibers, the metal component can trap the photogenerated charge carriers and promotes the interfacial charge-transfer processes, causing quick recombination and therefore hindering photocatalytic efficiency. In recent years, substantial effort has been employed toward combining metal oxide semiconductor nanostructures with suitable metallic oxide materials to synergize their properties [146,147,148,149]. For instance, a metallic metal oxide (Ti₅O₉)–metal oxide (TiO₂) nanocomposite has been developed as the heterojunction to enhance visible-light photocatalytic activity [150]. Compared to the nanoparticles’ nanostructures, the TiO₂/ZnO hybrid nanofiber composites are confirmed as a highly efficient photocatalyst for encouraging applications in the wastewater treatment of organic-polluted water [151].

8. Environmental Remediation Application of Metal Oxide Nanofibers

8.1. Wastewater Treatment

The AOP is a well-known photocatalytic water treatment technique to treat pollutant-containing wastewater, air and various industry effluents and is regarded as one of the most promising methods [152,153]. Figure 12 illustrates how photocatalysis helps degrade organic pollutants in wastewater with the help of metal oxide nanofibers. Common solid metal oxide nanofiber photocatalysts such as TiO₂, ZnO, WO₃, and BiVO₄ are known to have outstanding photocatalytic properties. Among them, TiO₂ and ZnO are the most extensively used due to their superior chemical stability, low toxicity, low cost, and extraordinary photocatalytic activity [154,155].

Recently, ultrafine porous TiO₂ nanofiber diameters in the range of 10 nm were produced from electrospun rice-shaped TiO₂ nanostructures [156]. The rice-shaped TiO₂ achieved by heating the as-spun TiO₂–polyvinyl acetate composite nanofibers in air followed hydrothermal treatment, acid treatment, and low-temperature sintering. The synthesized TiO₂ nanofibers exhibited better photocatalytic activity than the commercial P-25 TiO₂ towards the degradation of methyl orange dye. ZnO nanofibers are also gaining huge attention in wastewater treatment owing to their outstanding photocatalytic properties; for example, Gupta et al. [157] fabricated photocatalytic ZnO nanofibers by the electrospinning method to degrade acid fuchsin dye in natural solar radiations. A review article on electrospun ZnO nanofibers for the photocatalysis of organic-dye-containing wastewater appeared two years ago from Panthi et al. [158]. Due to their great photocatalytic degradation ability, stable physical properties, low cost, reusability, and recyclability, WO₃ nanofibers have also been studied widely in wastewater treatment [159,160]. Early this year, Ma and coworkers constructed novel WO₃/Fe(III) photocatalytic nanofibers that show enhanced photocatalytic activity by means of interfacial charge transfer effect under visible light irradiation [161]. Furthermore, Bi and its oxides are well-known, cost-effective and multifunctional nanostructured materials, and have attracted numerous interest because of their excellent photocatalytic activity [162,163]. In light to this, bismuth vanadate and cerium dioxide (BiVO₄/CeO₂) composite nanofibers were produced by electrospinning and a homogeneous precipitation method with hydrothermal techniques [164]. Their visible-light-driven photocatalysis was evaluated via the degradation of methylene blue, methyl orange, and a mixture of those dyes. Moreover, a series of environmental remediation applications using metal–metal oxide core-shell nanofibrous nanostructures exploited the special advantages of morphology and highly accessible surface areas [165,166]. One-dimensional magnetite/manganese Fe₂O₃ modified by carbon coating and with TiO₂ nanoparticles into porous core–shell composite nanofibers were fabricated via organometallic compounds as templates [167]. These recyclable metal oxide photocatalytic nanofibers showed superior adsorption and photocatalytic properties for the efficient treatment of aqueous dye-containing waste.

8.2. Water Disinfection and Air Cleansing

The photocatalytic cleaning of wastewater using metal oxide nanocatalysts has attracted substantial research interest over the past few decades. Several types of photocatalysts have been tested for a variety of cleaning applications—for instance, bacterial pollutant and air contaminant removal with wastewater treatment and metal oxide nanofibers [168]. The decontamination of microorganisms from wastewater is an essential application of photocatalysis. The photocatalysts for environmental cleansing applications mostly involve either titanium dioxide or zinc oxide; however, they displayed different activities when they reacted with different bacterial systems. Figure 13 explains the mechanisms for microorganism cleansing using photocatalysis. Photocatalysis is often achieved by damaging the membrane of the bacterial cell and inducing oxidative stress by a highly energetic OH^• radical, which is also known as a reactive oxygen species (ROS). This photocatalyst can also attack the bacterial cell wall and release metal ions when needed. For example, a zinc oxide nanofiber can produce zinc ions and a titanium dioxide nanofiber can release titanium. These mechanisms could be more effective when exposed to light illumination. The formation of reactive oxygen species would be significantly greater upon light exposure with photon energy greater than the bandgap energy of the metal oxide nanofiber. However, few metal oxide nanofibers can yield ROS in the absence of photon energy, even upon sub-bandgap radiation.

The interaction between the nanofibers and bacterial cells could be significantly affected due to the entrapment of photo-generated charge carriers on the surface of the photocatalyst if there is any photocorrosion present. This could change the electrostatic attraction to negatively-charged bacterial cell walls since metal ion release from the metal oxide is heavily affected by photocorrosion.

It is desirable to achieve the bacterial disinfection of wastewater under solar or visible light by nanofiber photocatalysts. While ZnO shows strong antibacterial properties under ambient radiation, TiO₂ generally responds under UV radiation [169,170]. Methods for completing the photocatalysis of waterborne microorganisms into visible and solar light energy have been obtained by making composites, design of core-shell type nanostructures, nanoparticle doping, surface wettability patterning, offset printing, or increase of crystal defect [170].

Photocatalytic disinfection of wastewater using a TiO₂–Pt nanocatalyst was first reported by Matsunaga et al. [171]; they showed the degradation of many microorganisms such as Lactobacillus acidophilus (bacteria), Escherichia coli (bacteria), and Saccharomyces cerevisiae (yeast). It was also confirmed that the catalysts have no toxicity towards human cervical cancer cells (HeLa cells) and photocatalysts could be easily recycled and reused for their antibacterial activities. Last year La₂O₃ nanoparticle-doped PAN nanofiber mats were prepared by an electrospinning process [172]. The metal oxide nanofibers were applied for bacterial inactivation based on a feasible antibacterial strategy through phosphate starvation. In addition to this, the polymer/metal oxide composite nanofiber mats were reusable and easy to recycle.

Above and beyond pollutant and microorganism removal from wastewater, metal oxide nanofibers are also effective in the degradation of volatile organic compounds (VOC), nitrogen oxides, ammonia, sulfur oxides, and carbon monoxide, which are also treated as environmental pollutants [168,173,174,175].

8.3. Other Applications

Nanofiber nanostructures are also widely involved in several other applications such as textiles, wound dressings, prosthetic kits, dental care, bioimplants, microsurgical and external corporeal devices, drug delivery, tissue engineering, etc. [67,176,177,178]. In addition, metal oxide nanofibers have revealed a wide spectrum of new applications in energy storage, stem cell research, biomaterial sciences, and biochemistry owing to their favorable morphology and ideal physicochemical properties [104,179,180,181]. It is worth mentioning that Sabba et al. [182] demonstrated a maskless synthesis of TiO₂–nanofiber-based hierarchical structures and used them in solid-state dye-sensitized solar cells. Volatile organic compound-based sensors have been constructed on electrospun crystalline ZnO and CeO₂/ZnO nanofibrous mats and their sensing characteristics have been demonstrated for benzene, propanol, ethanol, and dichloromethane [183]. Metal oxide nanofibers have found application in tissue engineering, where they could be used as a scaffold material that drives cell differentiation and can create an osteogenic environment without the use of exogenous factors [184]. Carbon–cobalt [185] and highly porous ZnCo₂O₄ composite [186] nanofibers were synthesized by the electrospinning technique and exhibit potential application as an anode material for lithium ion batteries. Recently, Ali et al. [187] fabricated a microfluidic immuno-biochip to detect breast cancer biomarkers that is based on a hierarchical composite of porous graphene and electrospun TiO₂ nanofibers. Direct electrospinning of titania nanofibers on a conducting electrode was used to construct a cholesterol biosensor (Figure 14 describes the synthesis and application of nanofibers as an enzymatic biosensor) that can detect esterified cholesterol from human blood with excellent sensitivity (181.6 μA/mg·dL⁻¹/cm²) and rapid detection (20 s) [105]. Additionally, metal oxide nanofibers have found huge importance in immunolabeling, cell and molecular bioimaging, biosensing, treatment of tumor cells, soil nutrient sensing, and numerous photosensitive applications [188,189,190,191,192].

9. Recycling of Nanofiber Photocatalysts

While designing a photocatalytic reactor for wastewater treatment, the combination of a high surface area porous catalytic supporter with suitably planned nanostructured photocatalysts together with their recycling and reuse is a must. There are several types of photoreactors that have been used for wastewater treatment such as cascade photoreactor, downflow contactor reactor, and annular slurry reactor. Photocatalytic reactors for wastewater treatment are of two types, depending on the installed state of the photocatalysts, namely slurry reactors with suspended photocatalysts (Figure 15a) and immobilized reactors with photocatalysts immobilized onto a static inert support (Figure 15b). The difference between these two main candidates is that the former needs an extra downstream separation unit for the recycling and recovery catalysts, whereas the latter permits continuous operation. There is another notable reactor named a tubular continuous flow reactor (shown in Figure 15c). A mechanical peristaltic pump is located between the reactor feed line and the reservoir tank and pumps the polluted water from the main wastewater reservoir to the tubular reaction zone. The reactor tube is of quartz glass coated with nanofiber photocatalysts and designed in such a way that it could harvest a sufficient amount of illuminated light energy for photocatalysis.

The reuse and recycling of photocatalysts is essential since this can reduce the cost of the overall waste management process. Thus, effective engineering is important for creating photocatalysts that could be easily recycled and reused several times. In this context, membrane filtration is a clever exercise for recycling soluble photocatalysts as well. Ultrafiltration and nanofiltration are two familiar membrane filtration techniques for recycling photocatalysts owing to their potential use in enzymatic, organic, and homogeneous catalysis-mediated process intensification at the laboratory level as well as on a larger industrial scale [193,194]. Nanofiltration in continuous flow mode is one more excellent way of recycling nanofiber photocatalysts; however, combining continuous flow with a membrane filtration reactor can be more convenient for efficient wastewater treatment [195]. In this context of recycling, magnetic nanofiber photocatalysts are very useful since they can be easily recycled with an external magnetic field. For example, Zhang et al. [196] synthesized magnetic BiFeO₃ nanofibers with wave node-like morphology by electrospinning. Those nanofibers exhibit highly enhanced photocatalytic properties under visible light. In a recent report, the doping of Fe into a TiO₂/SnO₂ hybrid nanofiber has also been proven to be an exceptional visible-light-driven photocatalyst with much higher photocatalytic activity than pristine TiO₂/SnO₂ hybrid nanofibers [197]. The magnetic property against Fe-loading was characterized, and the corresponding photocatalytic activity under visible light was assessed using Rhodamine B degradation as a model pollutant.

Moreover, these nanostructured photocatalytic nanofibers have strong and active magnetic behavior, which makes their recycling and reuse easy. Santala’s report suggested a direction for the recycling and recovery of electrospun composite magnetic nanofiber photocatalysts using an external magnetic field [198]. A recycling arrangement based on exchangeable water would be very efficient. Photocatalysis in organic, aqueous, and organic/aqueous media permits easy recovery of homogeneous catalyst nanofibers from the product phase.

10. Conclusions

After reviewing hundreds of the latest representative studies on the role of various semiconductor photocatalysts and their synthesis and applications for environmental remediation, we observed that there are many types of metal oxide photocatalysts nanostructures available depending on the choice of fabrication route and the nature of the application. Among them, metal oxide nanofiber morphologies are reasonably stable and could preserve their structural robustness when reused in an aqueous medium towards environmental remediation, bacterial decontamination, and air purification via ultraviolet, visible, or solar light-driven photocatalysis. Electrospinning has been recognized as an efficient technique for the fabrication of metal oxide nanofibers. Various polymers along with their compatible metal oxide precursors have been successfully electrospun into ultrafine fibers in recent times.

It was shown that photocatalytic oxidation using metal oxide semiconductor photocatalysts has significantly improved the ability to degrade organic pollutants under ultraviolet light. The main problem with photocatalytic AOPs lies in the high cost of reagents such as hydrogen peroxide or energy sources such as ultraviolet light and the higher bandgap energy of several semiconductors. For the photocatalytic oxidation process, the energy demand and cost of the metal oxide/UV could be considerably reduced by the use of solar irradiation and superior nanomorphology to access a higher active surface area. Adjusting their electronic, optical, and physicochemical properties can ensure that the metal oxides efficiently degrade pollutant dyes in visible solar radiation.

The synergistic effect, as depicted in Table 1, indicated the advantages of the application of the photocatalytic oxidation process. Photocatalytic oxidation could be proven as an efficient alternative method for the oxidation and elimination of recalcitrant organic species under specific conditions for the management of agricultural and industrial pollutants in wastewater.