Photocatalytic treatment of organic dyes using metal oxides and nanocomposites: A quantitative study

1 Introduction

Water is an essential resource for human development and survival on Earth. It is indeed the basic need of all processes. Whether the users are domestic, industrial, commercial, or agricultural, they all need water for their proper functioning [1]. As a result of the industrial revolution and technological advancement, various new industries are on a global level. These industries also use water as a raw material and waste removal medium. Waste from such units contains toxic substances such as heavy metals, organic toxins, and solid organic wastes [2,3].

Furthermore, due to the population explosion, pure water needs have increased [4]. These factors, i.e., population explosion and industrialization, are significant causes of environmental pollution. Organic substances that are not even toxic are also an environmental hazard. For instance, the discharge of oil in water bodies results in lethal damage to the marine ecosystem. It causes deoxygenation in water and hypoxia in surroundings [5]. Pollutants in water are mainly of anthropogenic origin.

Based on the origin of pollutants, water pollution sources may be direct or indirect sources [6,7]. Fluids of different quality from industries, plants for waste treatment, refineries, etc., are direct sources of water contamination. Usually, the effluents are dumped into freshwater supplies. Water contaminations caused by air, soil, and sewerage systems are the indirect sources of water pollution. Polluted air contains oxides of nitrogen, sulfur, carbon, and greenhouse gases (chlorofluorocarbons), which are emitted from automobiles and industries [8]. These gases cause acid rain and toxify water. Soil pollutants include fertilizers, pesticides, industrial wastes, and domestic wastes. Rainwater seeps through the polluted soil and carries the contaminants to the groundwater. Sewerage systems in urban areas are another indirect source of unclean water. This article mainly focuses on organic pollutant dyes and their photocatalytic degradation through various advanced metal oxide nanoparticles and nanocomposites.

Approximately 300–400 million tons of unprocessed organic pollutants are discharged annually, causing water contamination concerns, particularly near industrial regions [6,9]. Consequently, it is crucial to prioritize the improvement and innovation of environmentally friendly, energy-efficient, and cost-effective technologies for purifying water. To tackle the challenges, developing an innovative and economically viable technology that can effectively remove contaminants from wastewater with minimal energy consumption and chemical usage is essential. To address this need, researchers have directed their efforts toward advanced oxidation processes (AOPs) as reliable alternatives capable of oxidizing and mineralizing a wide range of organic chemicals due to their highly potent and strongly oxidizing radicals, such as hydroxyl radicals (˙OH) and oxide radicals ( O 2 ⋅ – ) [10]. AOPs employ an aqueous phase oxidation technique that harnesses the in situ generation of powerful oxidizing agents to accelerate the oxidation of water and wastewater contaminants [11,12]. AOPs have gained much interest because of their effectiveness in ultimately converting pollutants into less toxic compounds and their capacity to break down toxins quickly and without selectivity [13]. Chemical agents such as metals, O₃, or H₂O₂ are frequently used in these processes, as well as energy sources such as ultraviolet or visible light, ultrasound, electric current, or irradiation. Light-absorbing catalysts (photocatalysts) have demonstrated exciting uses in the degradation of poisonous dyes and the reduction of heavy metal ions to less hazardous forms in recent years. Photocatalysts offer higher catalytic efficiency, shorter reaction times, and the absence of dangerous secondary by-products. Under optimal conditions, solar energy can initiate photocatalysis through photons’ absorption rather than relying on thermal energy. When exposed to UV light, several semiconductors are utilized in photocatalysis, producing highly energetic electron–hole pairs. These electron–hole pairs can generate reactive oxygen species, particularly ˙OH and O 2 ⋅ – radicals, which possess strong oxidizing properties and can migrate to the outer surface, initiating oxidation and reduction reactions [14,15]. This article primarily focuses on degrading organic dyes using metal oxides and nanocomposites through photocatalysis.

2 Organic dyes and their classification

The sense of color comes from the brain’s interpretation of the eye’s stimulation by light. Because of this, color perception may vary based on the viewer. The pigments and dyes are tabulated into variants [16] such as colored, colorless, or fluorescent coloring agents. Pigments are often finely divided solids or powders that are insoluble in a dissolved solution. Dye is a colored soluble substance [17].

In contrast to pigments, where the physical characteristics of the particle (such as particle size and shape) correlate with the color of the pigment, dyes and chemical structures predetermine its color characteristics [18,19]. The primary distinction between dyes and pigments is solubility. Unlike dyes, which become soluble throughout their application procedures, pigments remain essentially insoluble during various application processes [20,21].

Organic dyes are compounds with a chromophore group that absorbs specific wavelengths of light and gives them their characteristic color. It is widely used in various applications, including textiles, food, and cosmetics. Organic dyes can be classified based on their chemical structure, application, and color properties. Here are some of the standard classifications of organic dyes [22]:

1. Azo dyes: Organic dyes with a nitrogen–nitrogen double bond are azo dyes (azo group). They are the most widely used dyes in the textile and food industry.

2. Anthraquinone dyes: These are derived from anthracene, a hydrocarbon containing three fused benzene rings. They are commonly used to produce textiles, paper, and plastic colorants.

3. Phthalocyanine dyes: Phthalocyanine dyes are synthetic organic dyes containing a phthalocyanine ring. They are widely used in the production of inks and pigments.

4. Triarylmethane dyes: They have a central carbon atom bonded to three aryl groups. They are commonly used in the production of colored paper, textiles, and ink.

5. Acid dyes: Acid dyes are water-soluble dyes primarily used for dyeing wool, silk, and nylon. They are typically used in the production of clothing, carpets, and other textiles.

6. Basic dyes: Water-soluble dyes are often used to color acrylic fibers, paper, and leather. They are also used as biological stains in medical and research laboratories.

7. Direct dyes: Direct dyes are water-soluble dyes mostly used to color cotton, silk, and other cellulosic fibers. They are commonly used in the production of clothing and home textiles.

8. Reactive dyes: Cellulosic fibers like cotton and rayon are colored with a type of dye known as a reactive dye, a variety of water-soluble dye. They are utilized frequently in the textile sector and are famous for the brilliant eye-catching colors that they produce.

These are just a few examples of the various types of organic dyes and their classifications. The color and characteristics of dye molecules depend on their chemical structure [23]. As a result, they can be divided into categories based on their chemical composition (functional groups), color, or use characteristics. Acid, basic, direct, azo, naphtha, reactive, mordant, vat, dispersion, and sulfur dyes are among the variants often used in the textile industry; at the moment, azo dyes are the most popular [24,25]. Dyes are often categorized using their molecular charge following dissociation in aqueous-based applications to research their characteristics regarding photo-degradation processes [26]. In Tables 1–3, the chemical structures of several typical dyes widely applied in photo-degradation applications are shown. Based on their chemical structure, they are divided into cationic, neutral, and anionic dyes [26,27].

Table 1

Anionic dyes [28,29,30,31,32,33]

Commercial name	Structure	Mol. wt. (g/mol)	λ max (nm)	Supplier
Acid red 151		454	512	Sigma-Aldrich
Acid orange 7		350	483	Boruta, Poland
Acid yellow 36		375	435	Boruta, Poland
Methyl orange		327	465	Acros Organics
Orange G		452	479	Sigma-Aldrich
Alizarin yellow R		309	385	MP bio-medicals

Table 2

Neutral dyes [30,34,35,36,37,38]

Commercial name	Structure	Mol. wt. (g/mol)	λ max (nm)	Supplier
Neutral red		289	543	Merck
Eosin-MB		661	655	Sigma-Aldrich
Giemsa’s stain		292	650	SRL, India
Sudan III		352	512	Sigma-Aldrich
Oil red O		409	520	Loba Chemie, India
Mordant violet 5		366	590	Merck

Table 3

Cationic dyes [30,31,32,39,40]

Commercial name	Structure	Mol. wt. (g/mol)	λ max (nm)	Supplier
Crystal violet		408	595	Sigma-Aldrich
MB		320	665	Sigma-Aldrich
Rhodamine B		479	554	Sigma-Aldrich
Astrazon orange G		351	492	Santa Cruz, USA
Nile blue A		733	639	Sigma-Aldrich

3 Wastewater treatment

Dyes are organic and hence are rendered as impurities [41]. They produce perilous by-products in wastewater via oxidation, hydrolysis, and other chemical reactions [42]. The question arises: at what concentration do these dyes pose a risk to life and aquatic life? The textile industries significantly contribute to aqua pollution [43,44]. They cause skin sensitization, respiratory and heart problems, and irritation; therefore, their disposed water must be treated per health standards [43,45]. Figure 1 highlights wastewater treatment, sources, and problems caused by polluted water.

Figure 1

Wastewater treatment, sources, and problems caused by polluted water.

Among the treatment methods, the best one is where complete mineralization of the organic components takes place without leaving any harmful by-product at all [46]. These methods include biological, chemical, physical, and AOPs [47,48,49,50]. The type of pollutant and the permitted contamination levels are used to determine which approach is most appropriate. Reducing contamination in industrial discharge and improving the use of treated water in sectors like the petrochemical and building industries are two main drivers for water treatment. Cost-effectiveness is another factor affecting the treatment method choice [51].

4 Photocatalysis

AOPs are promising techniques for wastewater treatment as they convert toxic organic substances into nontoxic components, i.e., water and carbon dioxide [1]. The term photocatalysis is formed by combining two words, photo and catalysis. A catalyst is a reagent that causes the chemical reaction to speed up by usually lowering the activation energy for the substrate. Furthermore, the catalyst has to be recovered at the end of the reaction. The photocatalyst is a substance that does the same but with the help of sunlight.

The best example in this regard is photosynthetic machinery. In the process, chlorophyll acts as the photocatalyst. Both photosynthesis and photocatalysis are analogous in function since both work as light-harvesting systems; chlorophyll absorbs the sunlight and converts water and carbon dioxide to glucose and oxygen. Similarly, photocatalysis yields strong oxidizing agents, transforming the organic matter into water and carbon dioxide in the presence of sunlight, water, and a catalyst, as depicted in Figure 2.

Figure 2

Photocatalyst and chlorophyll.

5 Principle of photocatalysis

Photocatalysis is the amalgam of two words: photochemistry and catalysis. It implies that for the occurrence of a chemical reaction, both light and catalyst are essential [69]. There is a massive difference between a photocatalytic and a mere catalytic reaction. In photocatalytic reactions, only light is the driving factor since it lowers the activation energy of the reaction. In contrast, conventional catalytic reactions use heat or other sources for this purpose. The light source generates electron–hole pairs, which play essential roles in the catalytic degradation of toxic dyes, as shown in Figure 3.

Figure 3

Principle of photocatalysis of organic dyes.

6 Types of photocatalysis

Based on differences in catalyst and substrate phases, photocatalysis is broadly classified into two categories.

6.2 Heterogeneous photocatalysis

In this type of catalysis, both species are in different phases. In the process, electrons and holes are photo-generated on the catalyst’s surface [72]. The reaction proceeds in more than one step. Compared to homogeneous photocatalysis, such reactions are challenging since the catalyst is in a solid phase, while the substrate is in a liquid state. This review aims to look into heterogeneous photocatalysis-based organic pollutant degradation in aqueous media.

For heterogeneous photocatalysis, the redox reaction is facilitated by the photocatalyst. While choosing the semiconductor, specific properties are kept in view, such as the detailed electronic structure having an empty CB and filled VB [73]. With the absorption of a photon possessing band gap energy, the electron from the VB is promoted to CB, leaving a hole behind in the VB [74].

The lifetime of the photo-generated electrons and holes falls in the nanosecond regime [75]. This lifetime is enough for the charge transfer from the semiconductor to the specie adsorbed on the surface of the photocatalytic semiconductor [76]. Some factors help prevent electron–hole recombination. They are the incident light intensity and electronic structure of the semiconductor (Figure 4) [77]. When a photon with an energy equal to or greater than the band gap impacts a semiconductor, it promotes an electron to the CB, leaving a hole in the VB. The generated e- moves to the adsorbed organic species or solvent, due to which holes are generated on the exterior portion, i.e., the surface of the semiconductor. Electron transfer is duly facilitated by the substrate molecules on the catalyst surface [78]. In Figure 4, Path A shows the reduction of the electron-accepting species from the electron donated by the semiconductor surface. As evident from path B, due to the acceptance of the electron, a hole is also transferred to the semiconductor surface, which undergoes recombination with the electron from the donor species. The charge transfer rate via electron and hole depends on the band edge positions of VB and CB. Furthermore, the redox potential level is also crucial in determining the charge transfer rate. Electrons and holes can also undergo recombination in bulk semiconductors (Path D) or on the surface of semiconductors and cause a release of heat (Path C).

Figure 4

Schematic photoexcitation in a solid semiconductor.

Heterogeneous photocatalysis involves the reaction in the adsorbed phase, as well as the entire process, which can be explained in the ensuing steps [72]:

1. The migration of reactants from the bulk of solutions to the photocatalyst’s surface.

2. Incorporation of the reactants from the exterior to the pore structures of the catalysts.

3. At minimal, adsorption of one reactant shall occur.

4. Response in the adsorbed stage.

5. The process of desorption of product.

6. Product movement from the pore structure to the exterior.

7. Movement of the products toward the bulk.

Photoexcitation in semiconductors is described in Figure 4.

7 Photocatalytic activity

The Beer–Lambert law governs photocatalytic activity. This law states “the absorption of a solution is directly proportional to the concentration of the compound” [79]

where A is the absorbance and is a dimensionless term, ε is the molar absorption coefficient (L mol⁻¹ cm⁻¹), c is the concentration, where b is the length of the cuvette in centimeters. The absorbance of the dye was found to be in direct relation to its concentration. The following expression determines photocatalytic degradation efficiency (PDE) [80]:

where A _o denotes the dye’s initial absorbance, and A _t indicates the dye’s absorbance after time “t” is measured by ultraviolet–visible (UV–Vis) spectroscopy [81].

8 Choice of a photocatalyst

Semiconductors that absorb suitable energy light generate electron–hole pairs [82]. Thermodynamically, the VB of the semiconductor must have a higher oxidation potential than the oxidation potential of the dye molecules [77]. Similarly, for photo-reduction, the reduction potential of the CB must be more −ve than the chemical species’ reduction potential. Regarding photocatalysis, the redox potential of the photo-generated VB hole shall be more positive than the water’s oxidation potential to oxidize the water to yield hydroxyl ions [83]. Furthermore, the reduction–oxidation potential of the photo-generated electrons should be more −ve to reduce the “O” to superoxide anion. Semiconductors possessing low band gaps are ideal for photocatalytic degradation.

The following are some pre-requisites for an excellent semiconductor:

• Photoactive,

• Ability to utilize UV–Vis light,

• Photo-stable,

• Chemically and biologically inert, and

• Cost-effective.

Semiconductors, such as ZnS, CdS, CdSe, TiO₂, etc., are thoroughly investigated as suitable catalysts for the resourceful degradation of organic dyes.

9 General mechanism of semiconductor photocatalysis

During photocatalysis, the VB and CB of photocatalysts have active participation. The energy band gap (E _g) is the distance between the VB and CB. The light falling on the photocatalyst should have energy equal to or more than the energy of the band gap. When light falls on the photocatalyst, electrons in the VB become energized and migrate to the CB, leaving behind the positive hole. If recombining electron–hole pairs is avoided, they move toward the catalyst’s surface, combining with adsorbed species [84]. The hydroxyl radical is formed when the hole combines with the water molecule. At the same time, electrons reduce oxygen to superoxide radicals. Hydroxyl radicals attack organic compounds to convert them to degradation products (water and carbon dioxide) [85]. Figure 5 shows the mechanism of photocatalysis.

Figure 5

Mechanism of photocatalysis.

The photocatalysis mechanism involves the following steps:

10 Semiconductor as photocatalysts

Semiconductors are used as heterogeneous photocatalysts; they have filled VBs and vacant CBs [86,87]. The gap between these two bands is called a band gap which is larger in semiconductors than conductors (metals) and smaller in insulators (non-metals). Light energy excites the electron from the VB to the CB, leaving the hole behind. Photo-generated electron–hole pair is known as an exciton. Exciton has less life span, usually in nanoseconds. There are chances of an undesirable process, termed recombination, happening. Recombination is rejoining the photo-generated electron–hole pair and releasing energy as heat. Different factors are being manipulated to avoid recombination and make possible charge transfer to the adsorbed components, e.g., enhancing light intensity and structural changes in semiconductors. Structural features can be manipulated by decreasing the size of the photocatalyst up to the nanoscale. Nanowires, nanotubes, and nanoparticles are currently used for heterogeneous photocatalysis. Similarly, doping and nanocomposites are also promising in this regard. Semiconductors like ZnO [88,89], CdS [90,91], TiO₂ [92], ZnS [93], ZrO₂ [94], and CeO₂ [95,96,97] work as photocatalysts and mineralize organic pollutants into the less harmful state [98]. Every photocatalyst has a different band gap and requires wavelength of light according to the band gap. It could be calculated by using the following formula:

11 Nanotechnology and nanomaterials

Nanotechnology is the science that deals with things at the nanoscale (1–100 nm). Materials having at least one dimension of nanoscale are termed nanomaterials. In 1959, physicist Richard Feynman seeded the concept of nanoscience in his talk, “There is Plenty of Room at the Bottom” by describing a synthesis process via manipulating atoms. This technology gained attention with the discovery of a scanning tunneling microscope in 1981. Later, fullerenes, carbon nanotubes, and metal oxide nanoparticles were discovered [117].

11.2 Fabrication of nanostructures

Nanostructures can be synthesized by using two approaches.

11.2.1 Top-down approach

In this method, bulky materials are reduced in size to the nanometer range [120]. These nanostructures have some defects, like their surfaces are not smooth due to impurities leading to heat and electricity’s low conductivity. Lithography, milling, aerosol spray, gas phase condensation, and atomic force manipulation come in this category. Nanostructures made by this approach find applications in different fields.

11.2.2 Bottom-up approach

This approach is based on fabricating nanostructures from smaller units to combine and make larger units (nanoscale) [120]. It is also called self-assembly. It is inspired by natural biological systems, where essential structures are constructed by chemical forces joining the smaller units. Self-assembled nanostructures have comparatively fewer defects (Figure 6). Different fabrication methods like sol–gel, microwave synthesis, colloidal dispersion methods, vapor phase deposition methods, hydrothermal methods, and combustion processes are included in this approach [121].

Figure 6

Top-down and bottom-up approach to fabrication of nanostructures.

Metal oxide nanoparticles and nanocomposites offer several advantages and disadvantages regarding their physicochemical properties. One benefit is their high surface area-to-volume ratio, which increases reactivity and facilitates efficient catalytic reactions [122]. Furthermore, their compact size enables excellent dispersion and accessibility to target pollutants [123]. Tunable features of metal oxide nanoparticles and nanocomposites, such as band gap and shape, may also be adjusted for specific applications. Furthermore, because of their distinct physicochemical characteristics, they display improved photocatalytic activity, making them excellent candidates for water treatment. However, there are several drawbacks to using these nanomaterials. Agglomeration and aggregation are significant issues that contribute to diminished responsiveness and efficiency.

Additionally, the possibility of metal ion leakage from nanomaterials raises worries about their long-term environmental impact. Furthermore, producing and characterizing metal oxide nanoparticles and nanocomposites can be complicated, necessitating careful control over experimental conditions. While metal oxide nanoparticles and nanocomposites offer numerous advantages regarding their physicochemical properties, carefully considering their limitations and potential risks is necessary for their safe and effective implementation [124].

12 Photocatalysts at nanoscale

Semiconductors at the nanometer scale have a lot of differences from bulk. With the reduction of size, large surface area, electrical and optical properties are being manipulated. The color of nanoparticles and bulk semiconductors is not the same; e.g., gold nanoparticles have a red color [125]. The quantum confinement effect becomes pronounced when the size is reduced to the Bohr radius of the first excitation or becomes equal to the electron’s wavelength [126]. Quantum confinement restricts the random motion of free electrons to quantized energy levels. These quantized energy levels are discrete energy states having fixed energy, resulting in a wide band gap. Semiconductors with particle size reduction lead to blue shift and move toward shorter wavelengths [127]. It happens due to a larger band gap; energy levels of VB slightly move toward low energies while CB vigorously moves to higher ones. At the nanoscale, the surface area is increased, resulting in higher photocatalytic degradation of dyes [71,128,129,130]. A high surface-to-volume ratio enhances the surface phenomena [131]. That is why semiconductor nanoparticles have been extensively used as photocatalyst for dye degradation.

13 Photo-degradation by metal oxide

Photocatalysis is a safe, long-lasting, and ecologically benign method of decomposing or degrading organic contaminants. Since the dangerous molecules are destroyed or changed into nontoxic forms, photocatalytic remediation technology eliminates organic pollutants from contaminated water without leaving toxic leftovers. A complicated oxidative process called photocatalytic remediation occurs when a photocatalyst is present. Energy levels that are equivalent to or higher than the band gap energy between the VB and CB of the photocatalyst are absorbed by photocatalytic materials. Exciting electrons from the VB to the CB with photon absorption results in charge separation, which creates positive holes in the VB [134]. Positive holes either oxidized water to produce hydroxyl radicals (OH^•) or impurities in the CB, while excited electrons reduce the oxygen absorbed by the photocatalyst. The organic pollutants are attacked by the OH^• free radical, which undergoes a series of reactions to completely decompose them into CO₂ and H₂O or transform them into nontoxic and non-hazardous forms.

On the contrary, the photo-generated electron and hole pairs have a strong recombination propensity, reducing photocatalysis’s efficiency. The recombination of photo-generated charge carriers must be minimized or managed to allow photocatalytic reactions. The type and nature of photocatalysts, organic pollutants, the reaction medium’s pH and temperature, the reaction medium’s light source and intensity, the presence of the solvent and sacrificial reagents, and other factors all affect how effectively organic pollutants are degraded by photocatalysis for wastewater remediation. The organic pollutants’ chemical composition and associated functional groups also impacted the photocatalytic reactions.

The photocatalytic degradation of dangerous and toxic organic pollutants into nontoxic and gaseous chemicals has been studied using a wide range of metal oxide/semiconductor-based nanomaterials. As discussed in this section, using metal oxide nanoparticles for photocatalytic degradation of organic contaminants in water remediation applications is advancing.

Hosseini and Saeedi have studied the photo-degradation of Rhodamine B by bismuth oxide (Bi₂O₃) NPs, synthesized by the chemical precipitation method. NPs were well crystallized, and the crystal size was 22.4 nm by characterizing the sample via X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR). Under optimal conditions, i.e., at a catalyst dosage of 12.5 mg/L for 2 h at pH 7, 95% of the dye was degraded [135]. An orthorhombic phase of tin sulfide nanoparticles was synthesized through a sonochemical method at a different frequency and used for photo-degradation of methyl blue dye and concluded that the photocatalytic activity of SnS nanoparticles synthesized at a lower sonication frequency was higher. About 50% of the methyl blue dye was degraded in just 45 min at low frequency, and at high frequency, it took 60 min to degrade 50% of the dye [136]. Chen et al. studied the photo-degradation of MG dye by TiO₂ NPs. The solution containing 0.50 g/L dye and 0.5 g/L TiO₂ was irradiated for 4 h and degraded 99.9%. The results show that the dye’s chromophoric structure cleaves stepwise by forming five different intermediates during the degradation. These intermediates were confirmed by the high-performance liquid chromatography–photodiode array–electrospray ionization mass spectrometry technique [137]. Alinsafi et al. employed titania TiO₂ to degrade industrial dyes like reactive azoic and metal phthalocyanines under solar and UV irradiation. PDE came out to be 74%. They observed that titania was also effective for wastewater with a higher pH value [84]. Qutub et al. synthesized cadmium sulfide nanoparticles (CdS NPs) using different precursors as sulfide sources. The photocatalytic efficiency of the synthesized photocatalysts was studied by degrading acid blue 29 dye under visible light. The enhanced photocatalytic activity was attributed to a reduction in size [138]. Chandran et al. synthesized CdS NPs, and their size was found to be 18 nm, and their shape was spherical. The particles were more stable after 30 days of incubation in a static environment. The degradation of methylene blue (MB) dye under solar irradiation was used to test the photocatalytic activity of CdS NPs. At pH 8, the highest photo-activity was recorded [139]. Jan et al. studied the photocatalytic degradation of acid violet 17 dye by Sr-doped ZnO nanoparticles. They used UV–Vis, XRD, SEM, FT-IR, and energy dispersive X-ray (EDX) to characterize synthesized nanoparticles. The particle size was calculated to be 16 and 21 for ZnO and Sr-ZnO, respectively. The photocatalytic activity is increased with increasing pH of the medium. Almost 88% of the dye was degraded in 140 min of irradiation and using a ZnO photocatalyst [140]. Table 6 displays the photocatalytic degradation of organic dyes by metal oxide.

14 Photo-degradation by nanocomposites

Zeng et al. used nickel/nickel oxide-cellulose filter paper composite (Ni@FP) to degrade methyl orange. The in situ reduction reaction under average temperature and pressure prepared these materials. SEM, X-ray photoelectron spectroscopy (XPS), FT-IR, and XRD characterized the synthesized materials. In this article, they found the PDE of the synthesized Ni@FP material to be 93.40% in just 5 min at pH 8 [141].

Under UV light illumination, Hunge et al. examined the photocatalytic activity of TiO₂@nanodiamond composites. When the 10 ppm solution of bisphenol A was exposed under UV irradiation, the TiO₂@nanodiamond photocatalyst destroyed bisphenol A completely in 100 min [142].

Mohamed et al. investigated malachite green (MG) for photocatalytic degradation by polyacrylonitrile (PAN) nanofiber/biogenic silica composite nanofibers. This photocatalyst was made using PAN nanofibers that were electro-spun and then cross-linked with diatomite and rice husk nano-silica, two types of biogenic silica. SEM, transmission electronic microscopy (TEM), EDX, FT-IR, and XRD techniques were used to characterize the produced materials.

A significant factor that affects photo-degradation effectiveness is pH. The mean pH of the reaction medium significantly influences the surface characteristics of the two biogenic silica membranes. Setting the medium neutral resulted in the most significant photocatalytic degradation, which was reduced by 98% in less than 10 min. Low photo-degradation efficiency is caused by the rivalry between hydrogen ions and MG cations for active sites on the surface of adsorbents in an acidic medium (pH 4). This competition reduced the number of active sites accessible to bind MG cations. The positive charge density on membrane surface sites decreases as the pH rises, which increases the prepared membranes’ capacity to bind MG. The hydroxyl groups of silica remain negatively charged under neutral conditions (pH 7), despite the entire membrane being positively charged. MG was more readily adsorbable due to the electrostatic affinity between the positively charged hydroxyl groups and the negatively charged MG ions [143].

The type and nature of photocatalysts, organic pollutants, the reaction medium’s pH and temperature, the reaction medium’s light source and intensity, the presence of the solvent and sacrificial reagents, and other factors all affect how effectively organic pollutants are degraded by photocatalysis for wastewater remediation. The organic pollutants’ chemical composition and associated functional groups also impacted the photocatalytic reactions. The photocatalytic degradation of dangerous and toxic organic pollutants into nontoxic and gaseous chemicals has been studied using a wide range of metal oxide/semiconductor-based nanomaterials. As discussed in this section, using metal oxide nanoparticles for the photocatalytic degradation of organic contaminants in water remediation applications is advancing. The pH of the dye solution affects how photocatalysis functions. Mainly, pH is essential for the degradability of anionic and cationic dyes. Anionic dye is often degraded by acidic pH, while cationic dye is demonstrated to be degraded by basic pH. In this study, they looked at the relative degradability of cationic and anionic dye while keeping the pH of the solution at 7.2. Compared to pristine TiO₂ at pH 7.2, the graphene–TiO₂ composite showed a 15- and 3.5-fold increase in Congo red and MB dye degradability, respectively. They studied its photocatalytic activity under natural sunlight and UV-filtered sunlight irradiation. They highlight the superior efficacy of the visible portion of the solar spectrum in harvesting composite deterioration and the insignificant impact of the UV band of sunlight [144].

Titanium dioxide nano-particles@nitrogen-doped carbon nanocomposite (TiO₂ NPs@C) was the subject of research by Atchudan et al. The catalysts were made using a low-cost hydrothermal process, and attenuated total reflectance was used to examine their physical and chemical characteristics by field emission SEM, XRD, XPS, ATR-FTIR spectroscopy, EDX, and elemental mapping analysis. We looked into the photocatalytic efficiency of a produced TiO₂ NPs@C nanocomposite in the breakdown of MB when exposed to UV light. The degrading efficiency of the created TiO₂ NPs@C nanocomposite is more significant when compared to bare TiO₂ NPs. The degradation efficiency of the synthesized TiO₂ NPs@C nanocomposite is over 90% when the irradiation period is extended to 40 min, which is much higher than the degradation efficiency of the bare TiO₂ NPs, about 39%. The improved photocatalytic activity of the TiO₂ NPs@C nanocomposite was due to the synergistic effect of TiO₂ NPs and graphene-like carbon. Moreover, carbon obtained from a green, natural source has oxygen- and nitrogen-containing groups on its surface, leading to more hydroxyl and superoxide radicals and effective photocatalytic activity [145].

Using hybrid diatomite/ZnFe layered double hydroxide composites, Zhao et al. examined the degradation of MB and MG under visible light irradiation (DZF). It was simple to create the composites utilizing the co-precipitation approach. In 180 and 150 min, the DZF degraded 94.46% of MB and 97.02% of MG, respectively. Due to the synergistic effects of the quick charge separation rates brought on by the creation of heterostructures between diatomite and pure ZnFe-LDH, strong visible light absorption, wide surface area, and pore volume, the hybrid nanocomposite DZF has a more excellent photo-degradation activity than ZF [146]. Recently, Liu et al. [148], Chen et al. [149], and He et al. [150] examined the treatment of CrVI-containing Mg(OH)₂ nanowaste, photocatalytic performance of S-scheme CdMoO₄/CdO nanosphere, and generation of acyl radicals from form-amides and aldehydes, respectively. Mechanism of electrostatic potential changes regulated via adsorption speciation and combination of catalysis and fluorescence toward the detection of H₂O₂ and 2,4-DNP and fluorophore molecule loaded in Tb-MOF for dual-channel fluorescence chemo-sensor applications are explored by Ma et al. [151] and Li et al. [152,153]. Some other recent developments in chemical neutral organic compounds, metal–organic systems, and photocatalytic degradation of refractory contaminants are reported by Chen et al. [154], Li et al. [155], and Zheng et al. [156]. Wang et al. [157] and Kong et al. [158] highlight the performance of Alcaligenes sp. TB by Pd stimulating to produce membrane adaptation mechanism and enhanced red luminescence in CaAl12O19:Mn4⁺ via doping Ga3⁺ for plant growth lighting, respectively.

A composite catalyst made of ZnFe₂O₄ and ZnO was produced by Zouhier et al. using the solution combustion method. The manufactured catalysts were examined using XRD, XPS, X-ray fluorescence, SEM, TEM, and UV–Vis diffuse spectroscopy. The degradation of two dyes, MB and Remazol Brilliant Blue (RBB), in an aqueous solution under both UV and visible light illumination, was used to test the photocatalytic activities of the catalysts. It was found that the composite showed good photocatalytic activity at basic pH for both MB and RBB using one g/L of the catalyst under UV light. The combination demonstrated exceptional visual efficiency, reaching an 80% conversion of the initial dye concentrations in 2 h, whereas virgin ZnO had no activity under visible illumination. The creation of ZnFe₂O₄ associated with ZnO, which has a narrow band gap and contributes to visible photon absorption with an improved separation path for the photo-generated carriers, is thought to be the cause of the Fe/ZnO sample’s increased visible photocatalytic activity over pure ZnO (Table 7).

This study investigated various parameters for dye photo-degradation, including catalyst dosage, initial dye concentrations, and solution pH. They concluded that increasing the catalyst amount and initial dye concentrations increased the photodegradation efficiency of the catalyst up to a certain point before decreasing. The following is an explanation for these phenomena:

1. When photocatalyst mass increases, more active catalyst surface sites are available. The levels of hydroxyl and superoxide radicals therefore rise. The dye degradation efficiency decreases as the catalyst mass expands because the turbidity of the solution makes it harder for light to penetrate and activate the entire catalyst suspension.

2. The likelihood of contact between the dye molecules and the oxidant species, which would increase the degradation rate, rises as the initial concentrations of the dyes do as well. Unfortunately, when dye concentrations are too high at first, solutions become opaque, which means that photons that must interact with the catalyst surface are caught and absorbed by the dye solution, restricting photon absorption by the catalyst and lowering decolorization efficiency [147].

15 Comparative analysis of the data

Metal oxides and nanocomposites both show photo-degradation efficiency under solar irradiation. But from the comparative analysis in Figure 7, nanocomposites show the best performance compared to metal oxides. Nanocomposites degrade the toxic organic dye in less time, while it takes more time to degrade pollutant organic dye than metal oxide. Besides time duration, the degradation efficiency of the nanocomposites is also high. Nanocomposites degrade almost all toxic dyes, while the percent photo-degradation efficiency of the metal oxide is below 90%.

Figure 7

Comparison of the %PDE by metal oxide and nanocomposite.

In metal oxide, the best performers are the bismuth oxide and titania. Bi₂O₃ degraded 95% of the organic dye in 120 min and functionalized TiO₂ degraded 99.9% toxic dye in 240 min. ZnO and CdS are promising photocatalysts as compared to SnS. Tin sulfide is the worst performer among all these photocatalysts, which degrade only 50% in 60 min. In the case of nanocomposites, the best performer is Ni@FP. Nickel/nickel oxide-cellulose filter paper composite degrades 93% methyl orange in just 5 min of solar irradiation, and it is such quick and immediate photo-degradation of the dye by the nanocomposite. TiO₂@nanodiamond is also a promising photocatalyst, which degrades the organic dye completely in 100 min. Overall, nanocomposites efficiently degrade toxic organic dye.

16 Future perspective of the study

The world is going to industrialize more and more in the future. As a result of its changes, water is getting more and more polluted. So, there is a dire need for wastewater treatment. In this regard, photocatalysis is one of the best nanotechnology wastewater treatments, which treats pollutants and plays a role in wastewater treatment. As mentioned in the above section, nanocomposites are the best photocatalyst compared to metal oxide nanoparticles. In the group of metal oxide, Bi₂O₃ and TiO₂ show more PDE among all. So, in future studies, we can synthesize the nanocomposite of Bi₂O₃@ TiO₂. By synthesizing the composite of Bi₂O₃@ TiO₂, we can predict that it will show the best photocatalytic activity. Besides this, if the synthesized nanocomposite is doped with Nickel atoms, it will show even better results, as the Ni@FP %PDE predicted. So, in future research work, the Ni–Bi₂O₃@ TiO₂ will be synthesized for the photo-degradation of organic dye and wastewater treatment.

17 Conclusion

The comprehensive review of metal oxides and nanocomposites for the photocatalytic degradation of organic dyes highlights their significant role in mitigating water pollution resulting from industrial waste. The comparative analysis of metal oxides and nanocomposites for photocatalytic degradation of organic dyes reveals a significant efficiency advantage in favor of nanocomposites. The data indicate that nanocomposites not only achieve a high %PDE but also require considerably less time to do so. For instance, the Ni@FP stands out with an impressive %PDE of 93 achieved in a mere 5 min of irradiation, showcasing its potential for rapid and effective wastewater treatment TiO₂@nanodiamond composite achieves complete degradation with a %PDE of 100 within 100 min. These findings suggest that incorporating nanocomposites, particularly those like Ni@FP and TiO₂@nanodiamond, could significantly enhance the efficiency and practicality of photocatalytic processes in water treatment applications. The metal oxides, while effective, exhibit lower %PDEs and longer irradiation times, with Bi₂O₃ and two forms of TiO₂ showing varied efficiencies under different conditions. Overall, the trend indicates a promising future for nanocomposite applications in environmental remediation strategies, especially in industrialized settings where rapid and efficient water purification is paramount. Innovations in semiconductor photocatalysis, particularly at the nanoscale, have revealed promising approaches to enhance the degradation rates of toxic dyes, contributing to more effective and sustainable water purification methods. The advancements in the synthesis and characterization of photocatalysts, including the exploration of doping and the creation of hybrid structures, offer a pathway to optimize the photo-degradation process further. The insights provided by this review pave the way for future research focused on developing more efficient and cost-effective photocatalysts. The potential for industrial application of these findings could significantly impact environmental remediation efforts, contributing to the provision of cleaner water resources and the overall improvement of ecosystem health.