**2. Inorganic metal oxide**

**1. Introduction**

174 Descriptive Inorganic Chemistry Researches of Metal Compounds

ide (H<sup>2</sup>

O2

into consideration.

), hydroxyl radical (HO•), superoxide radical (O<sup>2</sup>

nonbiodegradable and persistent pollutants into less toxic and biodegradable ones [10]. Recently, photocatalysis has been recognized as one of the most effective approaches for water treatment using sunlight and other light sources as a driving force [11]. More significantly, this technique can degrade several organic pollutants into less toxic molecules and easily biodegradable compounds without forming secondary pollutants. The use of polymer/metal oxide–based photocatalyst materials to decompose pollutants has been recognised as one of the most promising materials [12], owing to their high quantum dimension effect, low cost, photostability, and low toxicity, small-dimension and surface effect [13]. The chapter starts with a brief introduction on a metal oxide, the synthesis approach, fundamentals, and the characterization techniques, followed by a discussion of the current successes made in the development, modification, and applications of the different nanohybrid polymer/metal oxide–based nanocomposites toward the degradation of pollutants in water resources. Finally, the future perspectives and outlooks are also taken

−•), and ozone (O<sup>3</sup>

) to degrade

Water pollution caused by toxic and hardly-degradable organic and inorganic pollutants posed a severe danger to human well-being and growth. Recently, water pollution has been a significant concern, mostly in areas where people depend on groundwater and surface water for drinking and other domestic purposes [1]. For instance, almost 40% of the world's population has limited access to potable water [2]. The concerns of water shortage have been envisaged to be continued worsen due to the rise in population growth, swiftly improving life standards, rapid modernization, and industrialisation [3]. Shortage of potable water supply is due to the misuse of water resources for irrigation, industry, and domestic purposes in several parts of the world [4]. Industrial wastewaters comprise of several complex organic pollutants such as dyes, oils, detergents, and others, which are known to be carcinogenic to human health and aquatic life [5]. Domestic wastewater containing trace levels of pharmaceuticals, personal care products and others can also induce toxic effects [6]. As a result of the health implication induced by these contaminants, scientists and government authorities are making continuous efforts to address this problem. In the past, the traditional water treatment techniques such as coagulation and adsorption only remove pollutants from water but do not entirely biodegrade them into less toxic compounds [7]. Moreover, water treatment approaches, such as membrane technologies and chemical treatment usually have high operating costs and occasionally produce other poisonous secondary contaminants [8]. Since the water industry is required to produce portable drinking water, there is a need for the development of low-cost and stable approaches to address the day-to-day deterioration of water quality. Among the several approaches used in water treatment, the advanced oxidation processes (AOPs), such as the sonolysis, Fenton reaction, ozonation, and photocatalysis have gained a considerable attention in the removal of pollutants, owing to their simplicity, low cost, high efficiency, easy handling, and good reproducibility [9]. The AOP consist of *in situ* production of nonselective and highly reactive chemical oxidants, such as hydrogen peroxAmong the several groups of nanoparticles, inorganic metal oxide has been of considerable interest from both technological and scientific perspective. When metal oxides are transformed into nanometre scale, they show enhanced hybrid properties compared to the pure materials. Metal oxide adopts several structural geometries with an electronic structure, which can be either semiconductor, metallic, or insulator depending on the nature of the structure. The unique features of metal oxides make them the most miscellaneous class of materials, with optical, electronic, electrical, magnetic, catalytic, and photoelectronic properties covering virtually all aspects of solid state physics and materials science, and can find application such as electroceramics, gas sensing, catalysis, superconductors, and energy conversions [14]. However, there are few limitations in using inorganic metal oxides as an absorbent in water treatment. Reducing inorganic metal oxide to nanoscale size may increase the surface area; however, this increment can make the metal oxide unstable and subsequently turn out to be more susceptible to agglomeration owing to the existence of van der Waals interactions [15]. Due to this interaction, the metal oxide may lose their selectivity, mechanical strength, and high capacity. In addition, low quantum yield owing to the rapid recombination of photogenerated charge carriers and the wide energy gap of some metal oxide limits their application in water treatment [16]. To overcome these limitations, metal oxides are immobilized into other supports, such as polymeric materials [17]. Currently, polymeric nanoparticles are used in the elimination of contaminants from water due to large surface area, tunable surface chemistry, perfect mechanical rigidity, and pore size distribution [15].

#### **2.1. Synthesis of inorganic nanomaterials**

Currently, the synthesis, characterization, and application of inorganic nanomaterials represent a highly active area of scientific research. Nanofabrication is the design and production of materials with chemical and structural restrictions on the nanometer scale [18]. The design of systematic approaches for the synthesis of inorganic nanomaterials has been a major challenge from both industrial and fundamental viewpoints as the first requirement in any study associated with inorganic nanomaterials involves the synthesis and characterization. An in-depth knowledge and understanding of the synthetic method are crucial in order to design hybrid inorganic nanomaterials with unique properties. The general strategy for preparing inorganic nanomaterials in solution is to separate the nucleation and growth of nanocrystals [19]. The synthesis approaches may be categorized into bottom-up and top-down. The top-down approach uses large homogeneous objects and shrinks them down to the nanoscale, while the bottom-up uses the interactions between the small components, such as colloidal particles or molecules to assemble themselves into more discrete and complex nanoscale structures. The bottom-up approach has been accepted as the most promising technique to address several problems related to the top-down approaches [20]. Both techniques have been used to synthesise nanomaterials. Chemical processes, such as chemical vapor deposition, sol-gel, spray pyrolysis, and template synthesis are employed as bottom-up approaches. The properties and structure of the synthesized nanomaterials can be regulated based on the experimental conditions employed. The nature of the method used in the synthesizing the nanomaterials allows control over the doping ratio by different elements, particle size, the degree of particle agglomeration, and particle geometry. For example, the liquid-phase approaches, such as sol-gel, coprecipitation, solvothermal/hydrothermal processing, template syntheses and microemulsion have been very resourceful in synthesizing inorganic nanomaterials owing to their capacity to synthesise several ranges of nanomaterials with control morphology and particle size [21]. Those particle parameters give the synthesized inorganic nanomaterials new chemical and physical properties for different applications.

#### **2.2. Inorganic metal oxide polymer nanocomposites**

A composite involves an immobilization of two or more materials with distinct chemical and physical properties. Composite materials have a magnificent and several practical applications compared to the individual material due to their extraordinary explicit strength and stiffness, corrosion resistance, low density, high thermal insulation, and toughness [22]. The search for improving the properties of composite materials, which are of lower filler size, led to the design of nanocomposites. Nanocomposites are composite materials with nanoscale morphology such as nanotubes, lamellar nanostructure, or nanoparticles as one of the phases [23]. The properties of nanocomposites are influenced by the individual components, the morphology of the system, volume, and shape fraction of the filler as well as the nature of the interphase between the interface of the components [24]. The enhancement of these properties can be accomplished when there are suitable interaction and good dispersion between the matrix and the nanoparticle. Based on the nature of the matrices, nanocomposites have been classified as a metal, carbon, ceramic, and polymer [25]. Among these nanocomposites, the polymerbased has been recognized as the most attractive in several research areas, such as medicine, optoelectronics, engineering, and water remediation due to their distinctive properties emerging from the individual components [26]. The mixing of polymers and inorganic metal oxide has been an active field of research; in particular, the engineering of flexible nanocomposites has received much attention owing to the significant electrical, thermal, mechanical, and magnetic properties compared to the bulk polymers and the inorganic metal oxide [27]. In this nanocomposites, the polymer material provides convenient processing, structural flexibility, photoconductivity, metallic behavior, tunable electronic properties, and efficient luminescence [28], while the inorganic metal oxide offers high carrier mobility's, band gap tunability, thermal, and mechanical stability as well as dielectric and magnetic properties [29]. In addition to the distinctive features, new or improved phenomena can also occur due to the interface between the polymer and inorganic metal oxide [30]. Due to the large surface area, nanocomposites display many variations in their properties compared to the individual component of the metal oxide. The properties and microstructure of the nanocomposites are influenced by the interfacial interaction between the polymer and the inorganic metal oxide, where a wide range of covalent and hydrogen bonds may prevent phase separation [31].

#### **2.3. Characterization techniques of metal oxide polymer nanocomposites**

One of the key features in the design and fabrication of metal oxide/polymer nanocomposites is the in-depth characterization. Characterization of nanocomposites has been mostly centered on the surface analysis methods and conventional characterization techniques designed to determine the topography of surfaces, composition, morphology, crystallinity, shape, and size. Some of the techniques that have been used in the design and fabrication of nanocomposites have been illustrated [32].

conditions employed. The nature of the method used in the synthesizing the nanomaterials allows control over the doping ratio by different elements, particle size, the degree of particle agglomeration, and particle geometry. For example, the liquid-phase approaches, such as sol-gel, coprecipitation, solvothermal/hydrothermal processing, template syntheses and microemulsion have been very resourceful in synthesizing inorganic nanomaterials owing to their capacity to synthesise several ranges of nanomaterials with control morphology and particle size [21]. Those particle parameters give the synthesized inorganic nanomaterials new

A composite involves an immobilization of two or more materials with distinct chemical and physical properties. Composite materials have a magnificent and several practical applications compared to the individual material due to their extraordinary explicit strength and stiffness, corrosion resistance, low density, high thermal insulation, and toughness [22]. The search for improving the properties of composite materials, which are of lower filler size, led to the design of nanocomposites. Nanocomposites are composite materials with nanoscale morphology such as nanotubes, lamellar nanostructure, or nanoparticles as one of the phases [23]. The properties of nanocomposites are influenced by the individual components, the morphology of the system, volume, and shape fraction of the filler as well as the nature of the interphase between the interface of the components [24]. The enhancement of these properties can be accomplished when there are suitable interaction and good dispersion between the matrix and the nanoparticle. Based on the nature of the matrices, nanocomposites have been classified as a metal, carbon, ceramic, and polymer [25]. Among these nanocomposites, the polymerbased has been recognized as the most attractive in several research areas, such as medicine, optoelectronics, engineering, and water remediation due to their distinctive properties emerging from the individual components [26]. The mixing of polymers and inorganic metal oxide has been an active field of research; in particular, the engineering of flexible nanocomposites has received much attention owing to the significant electrical, thermal, mechanical, and magnetic properties compared to the bulk polymers and the inorganic metal oxide [27]. In this nanocomposites, the polymer material provides convenient processing, structural flexibility, photoconductivity, metallic behavior, tunable electronic properties, and efficient luminescence [28], while the inorganic metal oxide offers high carrier mobility's, band gap tunability, thermal, and mechanical stability as well as dielectric and magnetic properties [29]. In addition to the distinctive features, new or improved phenomena can also occur due to the interface between the polymer and inorganic metal oxide [30]. Due to the large surface area, nanocomposites display many variations in their properties compared to the individual component of the metal oxide. The properties and microstructure of the nanocomposites are influenced by the interfacial interaction between the polymer and the inorganic metal oxide, where a wide range of

chemical and physical properties for different applications.

covalent and hydrogen bonds may prevent phase separation [31].

**2.3. Characterization techniques of metal oxide polymer nanocomposites**

One of the key features in the design and fabrication of metal oxide/polymer nanocomposites is the in-depth characterization. Characterization of nanocomposites has been mostly centered

**2.2. Inorganic metal oxide polymer nanocomposites**

176 Descriptive Inorganic Chemistry Researches of Metal Compounds

Raman spectroscopy is an analytical technique which depends on inelastic scattering of monochromatic light by molecules due to the molecular excitation, such as rotation, vibration, and translation. Raman spectroscopy determines the vibrational frequencies of molecules that are Raman active and these frequencies rely on the mass and bond strength of atoms.

Fourier transform infra-red (FT-IR) spectroscopy is a significant analytical method which offers appropriate information on the functional groups and structure of a compound. Since FT-IR determines the stretching vibrations of molecules, it can be utilised in the identification of functional groups present in an unknown inorganic and organic compounds.

X-ray diffraction (XRD) is a nondestructive and versatile technique that gives information on the crystal structure, microstructure, chemical composition, lattice constants, and particle size of a material. XRD technique depends on a constructive interference of a beam of X-ray produced in a certain space of direction.

Scanning electron microscopy (SEM) is an electron microscope that scans materials surface with high-energy rays of an electron. SEM makes use of electrons rather than light to scan the surfaces. SEM determined the shape and morphology of material. In addition, morphological properties such as shape, size, and surface features as well as topological data of materials can be obtained.

Transmission electron microscopy (TEM) depends on the beam of electrons generated from an electron gun to interact with the material. TEM determined agglomeration, observation of defects, the effect of annealing, and dispersion in the matrix.

Ultraviolet (UV)-visible spectroscopy is a spectrophotometric technique, which comprises of the measurement of light photons in the UV-visible region. UV-visible spectroscopy measures the intensity of light before and after it has been passed through the material.

Nuclear magnetic resonance (NMR) spectroscopy relied on the population of magnetic nuclei in an external magnetic field to align the nuclei in a finite and predictable number of orientations. NMR gives information on the environment in which the nuclei of atoms are found.

Atomic force microscopy (AFM) is a scanning probe microscopy which uses a small probe to scan across the material to acquire information on the surface of the material. AFM measured the surface of thin films as well as high loading nanoparticles can also be detected.

Photoluminescence (PL) technique is the instantaneous emission of light from the analyzed materials following optical excitation. The effectiveness of PL signals is determined by the properties of the material and the nature of optical excitation. PL is used to obtain information on the compositional analysis of material, band gap, evaluation of several diode materials as well as defect evaluation of light-emitting materials.

Thermogravimetric analysis (TGA) technique is used to measure the weight of a material as a function of material time or temperature at a constant heat rate. TGA is based on heating a mixture of materials at a high temperature to decompose them in the gas phase. The TGA results are generally obtained as a curve with a percent weight against temperature under controlled atmosphere.
