**Abstract**

Photocatalytic dye degradation has received more attention as an affordable and effective way to treat the dye polluted water. In the present chapter, we are going to discuss; (i) the preparation and photophysical characterization of g-C3N4 intercalated ZnO\Mg-Al LDH, a novel ternary nanocomposite, and (ii) its visible light photocatalytic degradation activity against the methylene blue dye. LDHs are 2D materials composed of "brucite-like" cationic layers where an inclusion of trivalent cations presents an overall positive charge to the nanosheets. g-C3N4 is one of the organic semiconductor photocatalyst which active for several types of reactions such as CO2 reduction, water splitting, and degradation because of its stable, nontoxic, and earth-abundant nature. Mainly, the development of numerous 2D g-C3N4 nanosheets has been extensively used in the field of photocatalyst. By the combination heterojunction with 2D/2D interface can effectively improve the photocatalytic activity. The nitrogen-rich g-C3N4 intercalated ZnO\Mg-Al LDH ternary nanocomposite formation could follow the direct dye degradation process and results enhance the visible light absorption. The enhanced photocatalytic activity is mainly due to the improved charge separation rate and high number of photogenerated electrons. The large number of photogenerated electrons and high charge separation efficiency are effectively influence the dye degradation efficiency.

**Keywords:** layered double hydroxides, graphitic carbon nitride, photocatalytic activity, methylene-blue, ternary nanocomposites, visible light, dye degradation

#### **1. Introduction**

#### **1.1 Water pollution**

The industrial revolution could not avoid its effects on increasing environmental pollution, which pose a life threat to living beings. On the other hand, the increase of population rises the corresponding needs, which in turn result in the increased release of pollutants. The toxic substances from farmhouses, municipalities,

pesticides, and factories are the major sources of water pollution. Organic dyes are one of the major groups of pollutants which are released from textile industrial wastewater. The dye effluent contaminates the surface and groundwater, thereby, making it unfit for drinking and other daily usages. Polluted drinking water can cause serious cariogenic effects on human and other living beings.

contribute in catalytic reactions by forming superoxide and hydroxide radicals

each report put forward some scientific development to its ancestors. The

*LDH Ternary Nanocomposites: g-C3N4 Intercalated ZnO\Mg-Al for Superior…*

heterostructures and organic/inorganic composites [3–7].

The detailed photocatalytic mechanism was shown in **Figure 1**. Several efforts have been conveyed through a variety of materials and methods and it is true, that

photodegradation is one of the cost-effective and easy-to-implement methods, and the materials studied include TiO2, SnO2, ZnO, CuO, and WO3 along with their

The photocatalytic decomposition of pollutants in the real-time application for

responding range, which can only absorb UV light (λ <380 nm), seriously confines the photocatalytic efficiencies. Therefore, it has become a significant problem to develop the photocatalytic SCs with a visible light response for practical applications. Besides, another major task in photocatalysis is the increase in the charge separation efficiency of the photocatalyst and the corresponding photocatalytic efficiency. The separation of the electron–hole pairs can increase the efficiency of photocatalysts. Transition metal oxides (TiO2, ZnO2, SnO2, etc.) have lower photocatalytic efficiency since its wide bandgap and high recombination rate of photogenerated electron-hole pairs. To overcome this difficulty, the development of hetero-nanostructures could offer an enhancement in the photocatalytic efficiency and can act as a better photocatalyst which can degrade various kinds of

Among the numerous photocatalytic materials, ZnO occupied the reasonable research area owing to its whole beneficial characteristics over other materials [8, 9]. Even though, when it comes to commercial developments, the robustness of ZnO needs further developments [10]. Such as, the trapping state (including interstitial and missing atoms/vacancy defect) bolstered loss of excitons, which is basic in oxide-based semiconductors, should be tended to appropriately [11]. One plausibility of accomplishing this is, utilize a better surfactant/capping molecule to passivate the surface traps, which overwhelmingly trigger the charge carrier recombination. On the other hand, such passivation has the opportunity to acting as a barrier for hinders the association between the dye pollutant and active material, which will likewise bring low efficiencies. The development of a ZnO based hybrid photocatalyst comprising of a composite material with suitable band structure would be a better choice towards the concealment of charge carrier recombination and consequent improvement in the

water sanitization requires the use of non-toxic, cheap as well as reproducible resources. The conventionally used wide bandgap (SCs) with limited light

which react with dye molecules [2].

*DOI: http://dx.doi.org/10.5772/intechopen.89325*

persistent organic pollutants.

**Figure 1.**

**105**

photocatalytic dye degradation process [12, 13].

*Schematic illustration of photocatalytic dye degradation.*

The effective handling of increasing environmental pollution is a major challenge for the sustainable progress of modern civilization. With a lack of waste management measures, there is an urgent need in finding efficient ways to treat and decompose the pollutants. Water is a "universal solvent," it can dissolve more substances than any other liquid on earth. It is because of this substantial property, water dissolves most of the pollutants and thus be polluted easily. Quality drinking water is a fundamental right to every human being and most of the countries do not provide drinking water in the WHO standards. Water pollution not only affects the human being, but also every living organism, as there is nothing without water.

Organic dyes used in many industries such as textiles, furniture, chemical, paint, food, and cosmetic industries are the major water pollutants. The organic dyes possess color owing to the following reasons; (i) the dye molecules absorb light in the visible region of the electromagnetic spectrum (400–700 nm), (ii) they have a conjugated structure, i.e. a structure with alternating single and double bonds, (iii) the molecule dye have at least one color bearing chromophore group, and (iv) exhibit resonance of electrons, which is a stabilizing force in organic compounds [1]. The removal of dye molecules is a challenging process because of the enormous variety of functional groups in dissimilar dyes and their different properties. Many techniques like electrochemical coagulation, reverse osmosis, nano-filtration, photocatalytic degradation, adsorption using activated materials etc., are used for the removal of dye from wastewater. Among the various types of approaches adsorption and photocatalytic degradation of chemically stable organic pollutants occupy a prominent place, due to some of the obvious advantages such as costeffectiveness, simplicity of operation besides great efficiency.

#### **1.2 Photocatalysis**

Photocatalysis is a process, which accelerates a photoreaction in the presence of a photocatalyst. Photocatalysis, as a fresh, cheap, environmentally friendly "green" process, offers great potential for environmental protection and energy exchange. The organic pollutants can be effectively decomposed by the semiconductor-based photocatalysts under light irradiation with the photon energies equal or higher to the bandgaps of the photocatalysts. In recent years, the photocatalytic reaction has received increasing attention for environmental applications such as air purification, hazardous material remediation, water disinfection, and water purification. The versatility of the photocatalytic process, for example, photocatalytic degradation of dyes and photoelectrocatalytic reduction of CO2 into hydrocarbon compounds in aqueous semiconductor suspensions, greatly attracted the scientists to work in the field of photocatalysis. The pioneering work of photo-electrochemical water splitting on TiO2 electrode reported by Fujishima and Honda in 1972, has been the initiative in the field of photocatalysis. In this way, the semiconductor based photocatalysis has grown as an ideal green chemistry tool in dealing with the globally concerned energy shortage and environmental pollution issues. In general, a photocatalytic reaction consists of three simple steps; (i) The semiconductor photocatalysts absorb incident photons whose energy (hν) is equal to or more than its bandgap (Eg), resulting in the generation of electron-hole pairs, (ii) The photogenerated charge electrons and holes are separated and transferred to the surface of photocatalysts, and (iii) The photogenerated electrons and holes

#### *LDH Ternary Nanocomposites: g-C3N4 Intercalated ZnO\Mg-Al for Superior… DOI: http://dx.doi.org/10.5772/intechopen.89325*

contribute in catalytic reactions by forming superoxide and hydroxide radicals which react with dye molecules [2].

The detailed photocatalytic mechanism was shown in **Figure 1**. Several efforts have been conveyed through a variety of materials and methods and it is true, that each report put forward some scientific development to its ancestors. The photodegradation is one of the cost-effective and easy-to-implement methods, and the materials studied include TiO2, SnO2, ZnO, CuO, and WO3 along with their heterostructures and organic/inorganic composites [3–7].

The photocatalytic decomposition of pollutants in the real-time application for water sanitization requires the use of non-toxic, cheap as well as reproducible resources. The conventionally used wide bandgap (SCs) with limited light responding range, which can only absorb UV light (λ <380 nm), seriously confines the photocatalytic efficiencies. Therefore, it has become a significant problem to develop the photocatalytic SCs with a visible light response for practical applications. Besides, another major task in photocatalysis is the increase in the charge separation efficiency of the photocatalyst and the corresponding photocatalytic efficiency. The separation of the electron–hole pairs can increase the efficiency of photocatalysts. Transition metal oxides (TiO2, ZnO2, SnO2, etc.) have lower photocatalytic efficiency since its wide bandgap and high recombination rate of photogenerated electron-hole pairs. To overcome this difficulty, the development of hetero-nanostructures could offer an enhancement in the photocatalytic efficiency and can act as a better photocatalyst which can degrade various kinds of persistent organic pollutants.

Among the numerous photocatalytic materials, ZnO occupied the reasonable research area owing to its whole beneficial characteristics over other materials [8, 9]. Even though, when it comes to commercial developments, the robustness of ZnO needs further developments [10]. Such as, the trapping state (including interstitial and missing atoms/vacancy defect) bolstered loss of excitons, which is basic in oxide-based semiconductors, should be tended to appropriately [11]. One plausibility of accomplishing this is, utilize a better surfactant/capping molecule to passivate the surface traps, which overwhelmingly trigger the charge carrier recombination. On the other hand, such passivation has the opportunity to acting as a barrier for hinders the association between the dye pollutant and active material, which will likewise bring low efficiencies. The development of a ZnO based hybrid photocatalyst comprising of a composite material with suitable band structure would be a better choice towards the concealment of charge carrier recombination and consequent improvement in the photocatalytic dye degradation process [12, 13].

**Figure 1.** *Schematic illustration of photocatalytic dye degradation.*

pesticides, and factories are the major sources of water pollution. Organic dyes are one of the major groups of pollutants which are released from textile industrial wastewater. The dye effluent contaminates the surface and groundwater, thereby, making it unfit for drinking and other daily usages. Polluted drinking water can

The effective handling of increasing environmental pollution is a major challenge for the sustainable progress of modern civilization. With a lack of waste management measures, there is an urgent need in finding efficient ways to treat and decompose the pollutants. Water is a "universal solvent," it can dissolve more substances than any other liquid on earth. It is because of this substantial property, water dissolves most of the pollutants and thus be polluted easily. Quality drinking water is a fundamental right to every human being and most of the countries do not provide drinking water in the WHO standards. Water pollution not only affects the human being, but also every living organism, as there is nothing without water. Organic dyes used in many industries such as textiles, furniture, chemical, paint,

food, and cosmetic industries are the major water pollutants. The organic dyes possess color owing to the following reasons; (i) the dye molecules absorb light in the visible region of the electromagnetic spectrum (400–700 nm), (ii) they have a conjugated structure, i.e. a structure with alternating single and double bonds, (iii) the molecule dye have at least one color bearing chromophore group, and (iv) exhibit resonance of electrons, which is a stabilizing force in organic compounds [1]. The removal of dye molecules is a challenging process because of the enormous variety of functional groups in dissimilar dyes and their different properties. Many techniques like electrochemical coagulation, reverse osmosis, nano-filtration, photocatalytic degradation, adsorption using activated materials etc., are used for the removal of dye from wastewater. Among the various types of approaches adsorption and photocatalytic degradation of chemically stable organic pollutants occupy a prominent place, due to some of the obvious advantages such as cost-

Photocatalysis is a process, which accelerates a photoreaction in the presence of a photocatalyst. Photocatalysis, as a fresh, cheap, environmentally friendly "green" process, offers great potential for environmental protection and energy exchange. The organic pollutants can be effectively decomposed by the semiconductor-based photocatalysts under light irradiation with the photon energies equal or higher to the bandgaps of the photocatalysts. In recent years, the photocatalytic reaction has received increasing attention for environmental applications such as air purification, hazardous material remediation, water disinfection, and water purification. The versatility of the photocatalytic process, for example, photocatalytic degradation of dyes and photoelectrocatalytic reduction of CO2 into hydrocarbon compounds in aqueous semiconductor suspensions, greatly attracted the scientists to work in the field of photocatalysis. The pioneering work of photo-electrochemical water splitting on TiO2 electrode reported by Fujishima and Honda in 1972, has been the initiative in the field of photocatalysis. In this way, the semiconductor based photocatalysis has grown as an ideal green chemistry tool in dealing with the globally concerned energy shortage and environmental pollution issues. In general, a photocatalytic reaction consists of three simple steps; (i) The semiconductor photocatalysts absorb incident photons whose energy (hν) is equal to or more than

its bandgap (Eg), resulting in the generation of electron-hole pairs, (ii) The photogenerated charge electrons and holes are separated and transferred to the surface of photocatalysts, and (iii) The photogenerated electrons and holes

cause serious cariogenic effects on human and other living beings.

*Assorted Dimensional Reconfigurable Materials*

effectiveness, simplicity of operation besides great efficiency.

**1.2 Photocatalysis**

**104**

## **1.3 Graphitic carbon nitride (g-C3N4)**

Graphitic carbon nitride (g-C3N4), is a two-dimensional metal-free conjugated crystalline sheet material with a bandgap energy of 2.7 eV, which has concerned exceptional research enthusiasm because of its environmental friendly nature, attractive electronic structure, low-cost excellent thermal and chemical stabilities [14–18]. The conduction and valence band boundaries of g-C3N4, exist at 1.12 and + 1.6 eV, making it active under visible light as an efficient photocatalyst [19–22]. Even though, its implication has drawbacks such as faster recombination of the electron-hole pairs, and agglomeration in most solvents caused by the strong van der Waals attractions between sp2 carbon atoms [23]. 2D g-C3N4, nanosheets have much attention because of their enlarged specific surface area, improved electron– phonon interaction, and enhanced electron mobility along the in-plane direction [24]. Although some developments have been attained, the light-harvesting ability and quantum efficiency of these modified g-C3N4 systems are still poor.

resulted in the enhancement of visible light absorption and improved charge separation to result in the enhanced photocatalytic degradation activity towards the

*LDH Ternary Nanocomposites: g-C3N4 Intercalated ZnO\Mg-Al for Superior…*

The g-C3N4 was prepared by a thermal condensation method using melamine as a precursor. 5 g of melamine was kept in an alumina crucible and thermally treated at 550°C for 3 h in a furnace. The obtained agglomerate residues are ground into fine powder and subjected to hydrochloric acid treatment for 12 h to obtain the g-C3N4 nanosheets. The suspension was centrifuged to separate the residual of g-C3N4 nanosheets. The obtained precipitate product was heated at 60°C for overnight to

In a typical synthesis procedure, Mg-Al LDH was prepared by a facile hydrothermal method. Firstly, 0.05 M of aluminum nitrate and 0.03 M of magnesium chloride were dissolved into 20 ml DDW separately under vigorous magnetic stirring for 10 min. Subsequently, 0.04 M of urea were dissolved into the 10 ml DDW and stirred for 30 min. After that, the precursor and urea solutions were mixed and

suspension was reached 12. The entire solution was transferred into a 100 ml Teflon lined stainless-steel autoclave, followed by heating in an oven under 180°C for 24 h. After the reaction was complete, the autoclave was cooled to room temperature. Finally, the sample was centrifuged and washed with DDW water and dried at 80°C

ZnO nanoparticles were prepared by the hydrothermal method. In this process,

*Pictorial representation for the formation of g-C3N4\ZnO\Mg-Al LDH 2D/2D LDH tertiary nano-composite [34].*

0.2 M of ZnCl2 were dissolved in 100 ml of DDW, and 0.2 M of NaOH were dissolved in 20 ml of DDW separately under constant stirring. After 10 min stirring the above-mentioned solutions were mixed together, and transferred into a 100 ml Teflon liner stainless-steel autoclave, followed by heating in an oven under 180°C

0.2 M of NaOH was added to the above solution mixture until the pH of the

MB dye.

**2. Experimental section**

**2.2 Preparation of Mg-Al LDH**

for overnight to obtain the final product.

**2.3 Preparation of ZnO nanoparticles**

**Figure 2.**

**107**

**2.1 Preparation of g-C3N4 nanosheets**

*DOI: http://dx.doi.org/10.5772/intechopen.89325*

attain the light yellow colored powder of g-C3N4 nanosheets.

For these reasons, various protocols such as surface modification, doping with metal or nonmetal elements and co-polymerization have been actively employed to enhance the photocatalytic performance of g-C3N4. It has high nitrogen content compared to other N-carbon materials, which is capable of creating more active reaction sites that would increase the electron donor/acceptor characteristics. Even after several decades and extensive investigations on several materials, a robust combination of materials and method is still required to vanish away the environment threatening organic pollutants.

#### **1.4 Layered double hydroxides**

Layered double hydroxides (LDH), a new class of lamellar metal hydroxide materials, consist of positively-charged hydrotalcite-like layers with carbonate ions and water molecules in the interlayer galleries [25–27]. Due to the two dimensional (2D) layered structure, LDH has a high explicit surface area, which can help quick ion transfer [26, 28–30]. Dvininov et al. prepared the SnO2/Mg-Al LDH coupling through the thermal treatment, which demonstrated good photocatalytic activity for methylene blue degradation [31]. It was made conceivable by the oxygen reduction and progressive creation of hydroxyl radicals, which are accountable for the degradation. Seftel et al. synthesized Ti incorporated Mg-Al LDH solid which shows better photocatalytic activity due to the isolation of small TiO2 nanoparticles on the LDH surface [32]. Kingshuk Dutta et al. prepared ZnO\Zn-Al LDH nanostructure by hydrothermal method using Al substrate as a template for developing different compositions and morphologies and the author demonstrated the degradation of Congo red dye [33]. Therefore, the LDH is a better candidate to be hybridized with ZnO which will enhance the catalytic activity of photocatalysts.

In any case, to build up a superior photocatalyst, hybridizing the LDH with a material having high conductivity and surface area is one of the hopeful approaches, which can further improve the charge transport proficiency of LDHcomposite. Xiaoya Yuan et al. prepared the g-C3N4\Zn-Al LDH composites through a simple in situ crystallization technique and the as-prepared composite exhibited improved photodecolorization of MB.

In the present work, we have prepared a ternary nanocomposite of g-C3N4 intercalated ZnO\Mg-Al LDH through a hydrothermal technique and studied its photocatalytic activity against the MB dye degradation. The ZnO is attached on the surface also interlayers of the LDHs, and ZnO\Mg-Al LDH are distributed over the surface of g-C3N4 nanosheets. The nitrogen-rich ternary composite formation

resulted in the enhancement of visible light absorption and improved charge separation to result in the enhanced photocatalytic degradation activity towards the MB dye.
