A Review on Synthesis, Characterization and Applications of Cadmium Ferrite and its Doped Variants

Ferrites have gained a lot of attention because of their diverse uses in domains including photocatalytic degradations, gas sensors, electronic devices, organic transformation catalysts, adsorption, and so on. This review focuses on cadmium ferrites and their numerous doped versions' production methodologies, characterization, and applications. The structural, electric, magnetic, and dielectric properties of cadmium ferrites are primarily influenced by the synthesis procedures and circumstances used during preparation. As a result, the main goal of this study was to provide the most often used synthesis processes, such as hydrothermal, co-precipitation, solvothermal, microwave-assisted, micro-emulsion, and solid state, as well as their benefits and drawbacks. Furthermore, the review focuses on the numerous characterization approaches used to investigate features such as optical, structural, magnetic, electric, and dielectric properties of cadmium ferrites. This analysis was further expanded to include applications in some of the most well-studied domains, such as photocatalysis and gas sensing.


INTRODUCTION
Alloy chemistry has gotten a lot of attention because they were first used in 2500 BC, much before the Bronze Age 1 . Two or more elements, at least one of which is a metal, combine in varying amounts to generate a binary or ternary single intermetallic phase that is difficult to differentiate from the base metal used. Because of the lower cost and improved structural, electrical, and magnetic qualities, as produced composites outperform their parent. The term ferrite refers to a category of iron oxide compounds that are ferromagnetic in nature and have high permeability and resistance 2 . The oldest ferrite substance known to prehistoric humans was magnetite (Fe 3 O 4 ). Because of their wide range of applications in numerous fields, ferrites are thoroughly explored. Drug administration, magnetic resonance imaging, gas sensing, magnetic fluids, catalysis, magnetic recorders, transducers, and electromagnetic wave absorbers are just a few of the promising uses 3 [4][5] .
Ferrites are chemical compounds that contain at least one iron(III) ion in their chemical formula. The chemical formula for spinel ferrites is M II Fe III 2 O 4 , where M and Fe represent divalent and trivalent metal ions, respectively. The oxygen ions (blue spheres) in the spinel structure are tightly packed in a face-centered cubic lattice, resulting in two types of interstitial voids for the metal ions, namely tetrahedral (A) (green spheres) and octahedral (B) (red spheres) 6 , as illustrated in reproduced Figure 1.
strategies. Finally, the current research looks at the wide range of uses for cadmium ferrites.

Synthesis
Due to the wide range of uses, there is a lot of research going on in the subject of cadmium ferrite synthesis. Despite this, additional study is needed to synthesize cadmium ferrites that are high in purity, have a regulated size and morphology. There is no complete method for ferrite synthesis published, and most commonly used synthesis procedures have their own strengths and drawbacks. "Bottom-up" and "top-down" synthesis strategies are the two types of synthesis strategies. The atoms/ ions/molecules are put together to manufacture nanoparticles in the "bottom-up" technique, whereas bulk solids are broken down to nano-sizes in the "top-down" approach, as shown in reproduced Fig. 2 10,51 . Fig. 3 depicts a summary of the cadmium ferrites production methods used.
The M II Fe III 2 O 4 spinel ferrite includes 8 tetrahedral and 16 octahedral sites for the residence of divalent and trivalent metal ions 7 . According to the cation distribution (divalent and trivalent) across tetrahedral and octahedral sites, ferrites are classed as Normal, Inverse, or Mixed kinds. Consider the usual formula of ferrite as (M 1-x 4 to shed light on this classification, where x denotes the degree of inversion and ions outside and within the square bracket occupy A and B sites, respectively. The ferrite is known as normal spinel when x = 0 in the preceding formula. The ferrite is known as inverse spinel when x = 1 in the preceding formula. Finally, when 0 < x < 1 is present, the ferrite is known as mixed spinel 8 9 . The current study examined the benefits and drawbacks of the most regularly used cadmium ferrites production techniques. This review also discusses numerous characterization

Co-precipitation
To synthesize uniform size ferrites, it is the most widely used convenient, inexpensive, efficient, and environmentally friendly approach 11 . The aqueous solutions of divalent and trivalent metal ions are combined in a mole ratio of 1:2 in the co-precipitation process. Metal ions in the form of nitrates, sulphates, chlorides, and tartrates, carbonates, oxalates, citrates, or hydroxides are co-precipitated from an aqueous medium as tartrates, carbonates, oxalates, citrates, or hydroxides using the appropriate precipitants. Using NaOH or ammonia solution, this procedure includes a precise and controlled pH change. Following that, the solution is vigorously agitated under inert conditions in the absence or presence of heat. The breakdown temperatures of the precipitate as obtained are lower than those used in solid-state processes. After drying, the precipitate is calcined to the required temperature to generate cadmium ferrite nanoparticles. Despite its advantages, co-precipitation has significant drawbacks, such as difficulty controlling pH. Fig. 4 shows a general schematic diagram for the manufacture of Cd ferrites and their doped variants.
The selected examples of co-precipitations where CdFe 2 O 4 and its doped variation are synthesized are shown in Table 1.

Sol-gel
During the silica synthesis in the midnineteenth century, chemist J. J. Ebelman first published the sol-gel method. It's a well-known bottom-up wet chemical approach 18 . This approach entails two main reactions: (1) hydrolysis of the precursor in an acidic or alkaline medium, and (2) subsequent polycondensation of the hydrolyzed product 19 . To make reactive monomers, partial hydrolysis of metal alkoxides or metal chlorides is used. Following that, through condensation, these monomers form colloid-like oligomers (sol formation). In addition, hydrolysis promotes polymerization, which leads to the production of a three-dimensional matrix (gel formation) 20 . Fig. 5 depicts the primary reactions in the sol-gel technique: metal alkoxide hydrolysis and condensation.
are influenced by morphological and chemical features of the reactants, such as free energy change, surface area, and reactivity 29 . These reactions aren't just for forming complex oxides; they're also used to create sophisticated materials like piezoelectrics. Physical mixing of simple oxides, hydroxide, nitrates, carbonate, oxalates, alkoxides, sulphate, or other metal salts is followed by high temperature treatment, usually between 1000 and 1500°C, to generate a new solid composition with gas evolution 30 . Solid-state processes have several advantages, including the ability to create ferrite materials with minimum knowledge of material science, high yield, low pollution, and large-scale production 31 . However, as compared to the above-mentioned coprecipitation approach, the resulting powder has coarse grain size, strong agglomeration, and hence structures with comparatively large particle size, very low surface area, and low homogeneity, all of which are significant negatives. Because of the high temperatures required for solid state reactions, undesired phases develop from time to time, which can be difficult to regulate. Grinding, on the other hand, can introduce contaminants into the powder, causing crystal structural stresses and so affecting magnetic characteristics 32 . The sol-gel process is widely utilized because it is cost-effective, does not require specific equipment, and operates at a temperature range of 25-200°C, which is significantly lower than that of traditional solid-state reactions. The sol-gel process can be used to make ferrites with precise shapes such as microspheres, fibers, and flower-like structures with a limited size distribution. Aside from these appealing properties, key downsides of this technique include lower purity ferrites due to by-product degradation and the usage of organic solutions, which can be hazardous 21 . Fig. 6 depicts the flow chart for ferrite production using the sol-gel method.
The selected instances of the sol-gel approach where CdFe 2 O 4 and its doped variation are made are included in Table 2.

Solid State Reactions
Solid-state reactions are commonly used to make polycrystalline solids from a mix of solid reactants. Because a mixture of solids does not react at room temperature for a long period of time, a high temperature is used to initiate the reaction (chemical breakdown of reactants). These reactions The effect of ceria addition on the structural characteristics of Cd ferrite was investigated by Abbas K. Saadon 33 . The Cd ferrites were made using the traditional ceramic process, which involved combining Fe 2 O 3 and CdO in a 1:1 ratio with acetone. The blended powder was dried in a 500 0 C oven for 2 hours. The dried and cooled powder was then combined in a ball mill for 1 h before being sintered at 1000 0 C for 2 hours. By ball milling for 1 h with acetone, CeO 2 (0.5, 0.75, 1.0, 1.5, 2.0, and 2.5 wt percent) was blended with synthetic Cd ferrite. Finally, the powder is pressed with 5% PVA to produce a 20mm diameter pellet that is sintered for 3 h at 1200 0 C. XRD and SEM methods were used to characterize the produced composites. was impregnated, dried, and then thermally treated at 500-1200 0 C for 3 hours. Cd nitrate, α-Fe 2 O 3 , and a calculated amount of aluminum and magnesium nitrate were also dissolved in minimal water, mixed, dried at 100 0 C, and then calcined for 3 h at 500-1200 0 C. XRD, DSC, FTIR, SEM, and VSM were used to characterize the pure Cd ferrite and composites formed. P. B. Belavi et al., 36 used the double sintering ceramic process to make Cd 1-x Ni x Fe 2 O 4 (x=0.1, 0.2, 0.3). As a starting material, CdO, NiO, and Fe 2 O 3 were used. In an agate mortar, these precursors were thoroughly combined in the appropriate stoichiometry for several hours. The samples were then pre-sintered at 800°C for 8 h before being cooled to room temperature in the air. After cooling, the samples were milled for one hour. Using a hydraulic press, these homogenized samples were combined with 2-3 drops of PVA as a binder and pressed into a 13mm pellet. Finally, these pellets were sintered for 12 h at 1150°C and cooled to room temperature in the air. XRD, IR, SEM, EDX, and VSM were used to characterize the produced Cd 1-x Ni x Fe 2 O 4 particles. The electrical and dielectric characteristics of the material were also assessed. Figure 7 shows a general schematic diagram for the production of Cd ferrites and their doped variants using a conventional solid-state process.

Microemulsion
Schulman et al., were the first to coin the term "microemulsion" in 1959. A colloidal suspension that is macroscopically homogeneous, isotropic, and thermodynamically stable is referred to as a microemulsion. With the help of a surfactant, two immiscible liquids exist in a single phase in this approach 37   Zaheer Gilani et al., 39 used this approach to make Co 0.5 Cd 0.5 Bi x Fe 2-x O 4 using (x=0.0, 0.05, 0.1, 0.15, 0.2, and 0.25). The microemulsion process is straightforward and can create cadmium ferrite with a homogeneous size distribution and regulated characteristics. Apart from these benefits, this approach has drawbacks such as high cost, limited yield, impurity in adsorbing surfactant on nanoparticle surfaces, and the need for a considerable amount of solvent 40 .

Hydrothermal and Solvothermal method
Roderick Murchison, a British geologist, was the first to create the word "hydrothermal." Inorganic synthesis is dominated by the hydrothermal and solvothermal methods. Chemical reactions in hydrothermal synthesis take place in aqueous solutions above the boiling point of water, but in solvothermal synthesis, non-aqueous solvents are utilized and reactions take place at quite high temperatures. That is to say, hydrothermal and solvothermal procedures are named for the solvent used in the synthesis reaction 41 .
The solutions are subjected to high temperatures (almost 200 0 C) and pressures (usually greater than 125 atm) in high pressure reactors (autoclaves) in this approach 42 . The majority of trivalent transition metal and divalent metal salts are dissolved separately in solvent and then combined in a 2:1 ratio. To make a homogenous solution, solvents such as ethylene glycol or ethanol are added. In a high-pressure reactor, this reaction mixture is subjected to high pressure. The type, particle size, and morphology of ferrite particles are determined by the heating temperature, duration, and pressure 43 . Low cost, low reaction temperature, ecologically friendly, exclusion from further calcination, narrow particle size distribution, product with excellent magnetic behavior, and practicality for large-scale manufacturing are all advantages of this approach 44 . The slowness of the reaction is a major drawback of the hydrothermal approach. Fig. 9 50 and many more.

Spray pyrolysis
Spray pyrolysis is a method of forming a thin coating of cadmium ferrite and its doped variations by spraying a precursor solution over a heated surface. At the deposition temperature, the reactants are chosen in such a way that by-products are volatile 51 . Spray pyrolysis method 52 has three basic steps: precursor solution preparation, aerosol formation and deposition, and synthesis process. This process has several advantages, including cost effectiveness, equipment simplicity, efficiency, and the ability to generate thin films with a wide substrate surface area. Spray pyrolysis has a number of disadvantages, including vapour convection and poor thin film quality 53 .

Fig. 9. General schematic diagram for Cd ferrites and its doped variant synthesis by hydrothermal method
The CdFe 2 O 4 thin film was made by Veerappan Nagarajan and Arunachalam Thayumanavan and used for electro-resistive detection of formaldehyde and ethanol vapours 54 and benzene vapours 55 . K. M. Jadhav et al., 56 synthesized Ni 1-x Cd x Fe 2 O 4 thin films (where x = 0.0-1.0 in steps of 0.2) by using the following experimental parameters: solution concentration 0.1 M, volumetric ratio 1:2, deposition temperature 360 0 C, annealing temperature 500 0 C for 4 hours, spray rate 2 mL/min, distance between substrate and nozzle 25 cm, and air pressure 0.30 MPa.

Characterization Techniques
To investigate the various physicochemical features of cadmium ferrite, various methodologies for characterization of nano ferrite are required. Thermogravimetry and Differential Thermal Analysis

X-ray Diffractometry
Max von Laue was the first to propose the XRD technique in 1912. The XRD technique can be used to analyse attributes such as crystalline grain size, crystalline phase presence, phase composition, XRD density, flaws and stresses present in the crystal, lattice parameter, and unit cell characterisation. This method uses constructive X-ray interference with a crystalline material. X-rays are generated using a cathode ray tube and then filtered to produce monochromatic X-rays in this process. The monochromatic X-ray beam produced is focused on the Cadmium ferrite sample. After that, the diffracted rays' scattering angles and intensities are recognised, processed, and shown as peaks in an XRD graph 57 .
A well-known and simple expression called Debye-Scherrer equation is used to obtain the crystallite size as follows 58 : Where, D=crystallite size (in nm), λ=Xray wavelength (λ=1.5406 Å), θ=Bragg's angle (in radians), β=full width at half maximum of the peak (in radians).
The lattice parameter (a) is calculated by applying following formula 59 : Where, d=interplanar distance, while, h, k and l are the miller indices.
By using lattice parameter value, true density (X-ray density) is calculated by following relation 59 : Where, M=Molecular weight of the sample, N=Avagadro number, a=lattice parameter. Furthermore, the porosity (p) of the ferrite sample is can be calculated as follows 59 : Where, D=apparent density of the sample which is calculated by Archimedes principle i.e. by weighing the ferrite sample and dividing by its apparent volume.
T h e r e p r o d u c e d F i g . 1 0 s h o w s representative XRD of Ni-Zn ferrite synthesized by the ceramic method by P. S. Patil et al., 60 .

Fourier Transform Infrared Spectroscopy
It is the most widely used method for determining the functional groups of produced Cd ferrites. To do this, the sample is bombarded with IR between 400 and 4000 cm -1 , and the absorbance of these radiations by ferrite is measured to determine the molecular structure. It's worth noting that IR radiation rarely generates electrical excitation; instead, it induces vibration excitation, which means that the bond joining atoms or groups of atoms vibrates faster. The FT-IR spectrometer produces an IR spectrum by plotting the substance's absorbance of infrared light against its wavelength 61 . The reproduced Fig. 11 shows a sample FT-IR spectrum of yttrium substituted Cd ferrite prepared by Muhammad Imran Arshad et al., 62 using the co-precipitation method.

UV-Vis Diffuse Reflectance Spectroscopy
The optical band gap is determined using this method. Bonding or non-bonding electrons absorb energy from UV radiation and are stimulated to the anti-bonding orbital in UV spectroscopy. The band gap energy (direct or indirect bandgap, Eg) between the valence and conduction bands of cadmium ferrite is calculated using UV-Vis DRS. Tauc plot is the most commonly used approach for this purpose. The Tauc plot calculates the Eg using the following equation 64 : Where, α=Absorption coefficient, hν=Energy of incident photon, n=Constant which depends upon nature of transition i.e., 2 for direct allowed transition and ½ for indirect allowed transition, K=Energy independent constant, Eg=Band gap energy.
The absorption coefficient (α) can be calculated by following formula: Lastly, by plotting a graph of (αhν) 2 verses hν and extrapolating the linear part of the curve to (αhν) 2 = 0 the direct bandgap (Eg) is determined.

Scanning Electron Microscopy
The morphological characteristics of the resulting nano-ferrite are examined using SEM. It creates images by scanning a ferrite surface with electrons of high energy 65 . When these focused electrons collide with the ferrite surface, they interact with the various atoms in M. Saravanan and T.C. Sabari Girisun 67 synthesized two CdFe 2 O 4 samples using a simple combustion process and annealing at 500 0 C (sample a) and 800 0 C (sample b). With permission, SEM Fig. 13 demonstrates that sample (a) has a porous, spongy, and network-like structure, whereas sample (b) has a spherically homogenized structure. Similarly, A. K. Nandanwar et al., 68 used the Sol-gel micro-oven approach to manufacture Cd 1-x Ni x Fe 2 O 4 samples (x=0.4 and 0.6) whose SEM image is reprinted with permission as Figure 14.

Transmission Electron Microscopy
Another essential approach for studying the morphology and structure of cadmium ferrite is transmission electron microscopy. A high-voltage electron beam is sent through a ferrite sample to create a picture in this approach. Cadmium ferrite samples are typically analyzed as an ultrathin film (100nm) or as a suspension on a carbon-coated copper grid. In a TEM, electrons are created from a tungsten filament cathode and accelerated in the vacuum tube of the microscope. Later, an electromagnetic lens is used to focus electrons into a narrow beam that is conducted through ultrathin ferrite samples and then scatters or strikes the electron beam sensitive fluorescent screen at the microscope's bottom to form a picture 69 . temperature and time in a controlled environment furnace 70 . The TG indicates the temperatures at which reduction, decomposition, and oxidation take place, whereas the DTA determines whether decomposition is endothermic or exothermic. This technique offers several advantages, including the need for a small sample size, minimal cost, and the ability to perform qualitative or quantitative analysis. TGA is a destructive technique, which is one of its key drawbacks 71 . To synthesize pure Cd ferrite samples A, B, and C, an equimolar mixture of Cd nitrate and α-Fe 2 O 3 was impregnated, dried, and then thermally treated at 900, 1100, and 1200 0 C for 3 hours. Agglomeration with particle sizes higher than 100nm can be seen in Cd ferrite calcined at 900 0 C (Fig. 15A). The authors also noticed variations in the size and form of ferrite particles as the calcination temperature increased. The quasi-spherical morphology of Cd ferrite calcined at 1100 and 1200 0 C ( Fig. 15B and C) revealed reduced particle sizes. Fig. 15 is a representative Transmission Electron Microscopy image of Cd ferrite that has been reproduced with permission.

Thermogravimetry and Differential Thermal Analysis
Thermogravimetry is one of the most precise and quick methods for determining the thermal events that occur when cadmium ferrite is heated. Simultaneous thermal analysis (TG-DTA) is a technique for tracking sample mass versus  Authors showed from Fig. 16 total weight loss observed about 28.62% and there is negligible weight loss after 790 0 C which ascribed to the stable phase formation of Cd 0.5 Co 0.5 Fe 2 O 4 nanoparticles. In the DTA curve, the first exothermic peak is observed below 100 0 C which is attributed to evaporation of trapped solvent and absorbed water. Next major exothermic peak is observed at 335 0 C which is ascribed to decomposition of inorganic salts. The prominent weight loss is observed at 628 0 C.

Applications
Cadmium ferrites and their doped versions have piqued researchers' interest due to their unrivalled physicochemical features, including as electrical, magnetic, dielectric, and optical capabilities. Cadmium ferrites and their doped versions are used in a variety of fields.

Sensors
Cadmium ferrites, as well as their doped versions, are widely used in gas and electrochemical sensing. Surface character, particle size, lattice defects, and adsorbed oxygen all influence the gas response of a gas sensing material (Cd ferrite) 73 . Table 3 summarizes some of the selected examples.

Photocatalysis
Because photocatalysts may generate electron-holes, they are frequently used for water pollution treatment, bacterial control, water splitting, and other applications [82][83] . A photocatalyst with a narrow band gap allows solar energy to be converted and used for reduction and oxidation processes 84 . Cadmium ferrites and their doped versions have proven to be outstanding photo-degradation candidates. Table 4 summarizes some of the selected examples.

Other
Cadmium ferrites and their doped variations are used in a variety of fields in addition to the ones described above.
N. Rezlescu et al., 91 used a self-combustion process to make CdFe 2 O 4 . XRD, SEM-EDAX, and BET are used to characterize the generated ferrite material. The Cd ferrite powder was used to investigate the combustion reactions of diluted gasses such as methanol/air, acetone/air, and ethanol/air at temperatures ranging from 20 to 500 degrees Celsius. Acetone, methanol, and ethanol have combustion temperatures of about 425, 375, and 350 degrees Celsius, respectively. S. R. Bhongale et al., 92 used the oxalate co-precipitation method to synthesise Mg x Cd 1-x Fe 2 O 4 (x=0, 0.2, 0.4, 0.6, 0.8, and 1.0) and analysed it using XRD, SEM, VSM, and FT-IR techniques. Synthesized ferrites have grain sizes ranging from 2 to 6.5 m. The authors found that as Mg 2+ content increases, the real component of permittivity (ἐ) drops and the dielectric loss tangent increases up to x=0.6, then decreases. Mg 0.4 Cd 0.6 Fe 2 O 4 sample has highest-10 dB bandwidth.

CONCLUSION
S y n t h e s i s , c h a r a c t e r i z a t i o n , a n d performance evaluation of ferrite materials for various applications have been a growing study area for more than 50 years. This is due to exceptional physicochemical features such as electrical, magnetic, dielectric, and optical properties, among others. Co-precipitation, Sol-gel technique, Solid-state processes, Microemulsion, Hydrothermal and Solvothermal, Spray pyrolysis, and many more methods can be used to make CdFe 2 O 4 and its doped versions; each method has its own set of advantages and disadvantages. The physical properties of synthesized ferrites are largely determined by the synthesis procedures and processing conditions used. As a result, new synthesis procedures are required at this time, with benefits such as efficiency, cost effectiveness, environmental friendliness, uniform and narrow size distribution with best magnetic behavior, large-scale production feasibility, and high yield, among others. XRD, FT-IR, UV-DRS, SEM, TEM, TG-DTA, VSM, and other techniques are used Up-to 100 [90] to characterize the produced CdFe 2 O 4 and its doped variations. Finally, the most well-studied application sectors, such as gas sensing and photocatalysis, are described.