Zinc and cobalt nano ferrites and their potential applications in environmental sustainability

: Fundamental research and industrially applied research, currently depend on magnetic oxide nanoparticles and ferromagnetic oxides due to their extensive applications for electronic, magnetic, optical sensor and absorptive activities. ferromagnetic substances mainly contain a different type of Ferrite. Ferrite material constitutes, Iron oxide (Fe 2 O 3 ) and divalent metal oxides of Transition metals like cobalt, nickel etc. The key objective of this paper is to study the impending applications of nano - ferrites doped with bivalent transition metals with their properties. Low-Temperature Self-Propagating combustion method was used to synthesize the particles. Synthesized Zinc and Cobalt Nano ferrites from the chemical co-precipitation method were found to be of 20 to 90 nm in average size, and comparative study of their properties using the results of Fourier-transform spectroscopy (FTIR) and X-ray powder diffraction (XRD). The Field Emission Scanning Electron Microscope (FESEM) confirms the conception of ferrite nanoparticles with a structure type cubic spinel. Further comparative Sensing studies showed that the sample displays variation in resistance when gases are passed over the surface, and the change in resistance is observed.


INTRODUCTION
In an era where compactness of various devices is of prime importance, nanomaterials can play a central role [1] . Science is built on a large number of very small nanoparticles. Which are acting as powerhouses of materials [2] . Materials gain a verity of Chemical & physical properties depending on the size and shapes of these nanoparticles [1,3] . Properties, such as electrical and optical, display unique significant differences from the bulk properties [1] .
The small material properties change considerably when they are brought to nano magnitude, of the order of 100 nm or less [4] . Being a particular type of such nanoparticles, the ceramic nanomaterials, had a wide variety of applications [4,5] . They are known for their stability, availability of information on relatively cheaper synthesis techniques [6,7] . Typically, MeFe2O4 is common formula for spinel ferrite (SF). Where Me indicates one or different types of a bivalent transition metal like Mn, Zn, Co, Ni, Cu, etc. [8] [9] ferrite fabrication procedures, Traditionally involve the grinding and calcination of binary metal oxide mixtures repeatedly to obtain pure ferrite phase [10] . These methods often follow the solid-state transformation, which involves high-temperature annealing, ball milling, and other complex sintering protocols leading to the anticipated microstructure and engineered shapes [5] . This also improves homogeneity and particle size distribution, other significant properties of the final product [11] .
Many works are going on metal ions substitution in nano-ferrites. Researchers, getting Interested nowadays in ferrites Substituted with rare-earth ions [12][13][14][15][16] , which have changed the physical as well as chemical (magnetic and electrical) properties of nanoparticles [17] and that intern depends upon nature and ionic radii of element substituted and its particle size [4] . Further applications in magnetism is tailored by decreasing particle size, altering the cation distributions with different ions at the lattice (tetrahedral and octahedral) sites [18] . Specific applications of nano ferrites require prominent properties in nanomaterials [10] . In the case of Ceramics, parameters of Nanostructured ferrites initially decrease to some extent with grinding, and with further milling, it increases [19] . A decrease in the grain size increases magnetization and also found that cation inversion is attributed to change in the lattice and its values [19,20] .
Amongst the category of SF's, CoFe2O4 and ZnFe2O4 have expanded more attention of researchers owing to numerous valuable properties. More chemical stability, extraordinary mechanical hardness, and good coercivity [21] . The properties of CoFe2O4 and ZnFe2O4 can be enhanced by adding metal ions with different valency, coordinating to requirements [22] . The substitution of rare-earth ions in cobalt and Zinc ferrites made the study more interesting [23] .
This is due to their vast applications in many devices such as electromagnetic wave absorbers, inductor cores, converters and many more [24] . By following disciplined fabrication processes, divalent Co 2+ resides at tetrahedral lattice and trivalent Fe 3+ ions at octahedral sites can be tailored. [25] , [26] and we also emphasized from the previous research that, part of Co 2+ ions present at the octahedral sites are responsible for the physical properties of CoFe2O4.That is the preference of accumulation of Co 2+ ions will be influencing the magnetic property, improving the resistive properties and reduces dielectric loss [25,27] .
By combining different types of reactions, extremely controlled shapes (spherical or cubic) can be achieved. Fine quality spinel cobalt ferrite (CoFe2O4) and monodisperse nanocrystals can be synthesized [28] . Owing to controlling nanocrystal, growth shape, can be can also be oscillated between spherical and cubic. Not only this, but also blocking temperature and magnetization of nanoparticles resolute by the size, irrespective of their structure. The nanoparticles structure is a leading factor aimed at the coercivity Factor for nanocrystals due to their superficial anisotropy [29] . And these magnetic nanoparticles due to their high capacity to store information got tremendous technological applications [25,30] The typical spinel structure represented as (M1−xFex) [MxFe2−x]O4 (M is the metal ion) has a 32 O 2− anions, in between these at a particular interstitial sites metal ion are distributed and ideally termed as the face-centered cubic unit cell [31] . In that coordination compounds, round brackets indicate tetrahedral (X site) and square brackets designate the and octahedral (Y site), and x implies the inversion parameter [32] . Based on the composition of divalent metal ions in tetrahedral and octahedral sites, spinel structures are categorized as normal, inverse and partially inverse.
Quantity of these divalent metal ions decided by the synthesis, annealing processes and also on particle size [29] . CoFe2O4 and ZnFe2O4 are spinels, but the arrangement of the ions among the lattice sites I unique.
Another burning issue for social awareness is to control water pollution [33,34] . Our environment is commonly distributed with Heavy metals around [31] . All types of ecological pyramids are affected by these. many industrial effluents and everyday human activities due to the increased technology are the causes [30] . With the knowledge of the literature survey in wastewater treatment attempted to control the toxicity by adsorbing with cobalt and zinc nano ferrites synthesized in our laboratory.
Synthesized Zinc and Cobalt Nano ferrites from the chemical co-precipitation method were confirmed with standard data of literature, found to be of 20 to 90 nm in average size, and comparative study of their properties using the results of FTIR and XRD. The SEM confirms the cubic spinel structure formation of ferrite particles. Absorption [37] and adsorption [38,39] studies are done with laboratory wastewater and conventional laboratory dyes.
Comparative Sensing studies showed that the sample displays variation in resistance when gases are passed over the surface and the change in resistance is observed.

MATERIALS AND METHODS
The present context is to design the synthesis Cobalt and Zinc ferrite and compare its properties by different characterization methods.
Low-Temperature Self-Propagating combustion method was used to synthesize the nanoparticles [40][41][42] . Simple precursors employed were Ammonium Iron (II) Sulfate ((NH4)2Fe(SO4)2·6H2O SDFine-Chem ltd., AR grade), oxalic acid (AR grade)and respective salts of ZnCl2,CoCl2( SDFine-Chem ltd., AR grade), which were weighed for the desired stoichiometric ratio and dissolved in double-distilled water [43] . The mixture, using a magnetic stirrer was stirred for 1 h, a precipitate of constant metal oxalate was obtained at room temperature. The precipitate was heated with the fuel polyvinyl alcohol ([CH2CH(OH)]n) in hot air oven to obtain a colored powdered sample of the nano-ferrites of Zinc and cobalt.

CHARACTERIZATIONS
To analyze the structure of nanoparticles, XRD Diffractometer (Xpert MPD, Make: Philips, Holland) was employed. The XRD patterns were recorded on the X-ray diffractometer at the range of 2θ from 3º to 136º, using Cu target X-Ray tube, Cu Kα (λ = 1.5406 A) radiation with an accelerating voltage of 40 kV. FT-IR study was accomplished in order to confirm the formation of nano particle in the range of 4500-400 cm-1by instrumentation, Using NICOLET 6700, USA instruments. To analyze morphological features of nanoparticles, field emission scanning electron microscope was used at an accelerating voltage With LaB6 filament -2nm 30 KV With W filament 3.5nm at 30 kv by Emission current of 0 to 200 μA and accelerating Voltage: 0.2 to 30 kv.

X-ray diffraction analysis (XRD):
The XRD peaks, structural analysis of CoFe2O4 and ZnFe2O4 nanosamples, were studied with Sherrer's equation, the average particle size obtained varied between 18. 54-18.90 nm based on the 2θ value which was found to be 35.64,40.58.70 and 62.60 respectively. sample possessed a cubic spinel structure [44] . Size and shapes define the XRD peak position and intensity defines the atomic position of the unit cell.
The micro strains by definition, root mean square of the variations in the lattice parameters across the sample is calculated and were found to be 66.14, 71.27, and 81.54, for cobalt ferrites. This implies the decrease of the lattice parameters when the more prominent ion is partially substituted by the smaller one. The XRD patterns are perfectly matched with the standard XRD pattern with reference code ICSD 00-001-1121, indicating purity of the synthesized material.

Figure 1 (a) & 1 (b): XRD peaks of spinel nano ferrites
Similarly, for zinc ferrite, the average particle size obtained varied between 18.63-20.45 nm based on the 2θ value, which was found to be 30. 30, 35.45,43.05, and 56.85, respectively [40] . The micro strains were calculated and were found to be 48. 28

Fourier-Transform -Infrared Spectroscopy (FTIR)
An the tetrahedral and octahedral lattice sites respectively [38] . The vibration observed due to bending was very broad. The reason could be attributed to the distribution of Fe 3+ ions leading to vibrations due to the stretching mode of the (Fe 3+ )-oxygen bond at tetrahedral, and other caused due to (Co 2+ )oxygen vibrations in octahedral sites. shown above, we can infer that the particles are of irregular shape and are agglomerated together to form a bigger particle. There was a considerable trace of carbon seen [45] . The agglomeration of the particles may be due to the hydrophilic nature of the extract added. The traces of carbon observed may be due to the usage of the mica sheet as a substrate for the sample.

Gas sensing mechanism:
Spinal ferrites operate in the same gas sensing mechanism as that of semiconductors.
In spinal ferrites, oxygen chemisorption leading to divalent metal ions also to oxidized in their octahedral sites. Then a change in electrical conductivity occurs due to trapping of electrons.
When gas molecules come in contact with these oxygens, they take back the electrons and shows the sensing activity.

Figure. 4 chemical vapor sensing arrangement
By generating a vacuum in a closed chamber (shown in above figure 4), a setup was made where the probe which was coated with the sample was placed. Gases from various chemicals were made to pass through the chamber with the help of a nozzle from another container. An Arduino UNO was used which was connected to the probe to detect the change in voltages. A program to detect these changes was uploaded to the Arduino UNO. Now, as the probe senses the gas, there is a variation in the voltage, which indicates that gas has been sensed. This variable voltage gives rise to a variable resistance. [19] The sensing was taken at room temperature. The gases that were used are acetone, ethyl alcohol, methanol, and ammonia. Using a Keithley meter, the value of change in resistance (shown in figure 5) was found which can also be calculated using the formula,

= −
Where Rs = required a change in resistance Rr = resistance across reference Vs = supply voltage

Vr = drop in voltage
Thus, for cobalt nano ferrite, resistance values of these gases are in order methanol > acetone > ethanol > ammonia. Then sensitivity of these gases is in reverse order that is methanol < acetone < ethanol < ammonia for cobalt nano ferrite.

Figure 5: (a) & (b) Bar graph of relative sensing properties with different gases
Thus, for zinc nano ferrite, resistance values of these gases are in order, methanol> acetone > ethanol > ammonia. Then sensitivity of these gases is in reverse order that is methanol < acetone < ethanol < ammonia.

Wastewater analysis:
SEM

Hydrogen ion concentration (pH):
As in report pH value decreases from the waste sample, shows acidity of water decreases. The decrease is sharply noted at B4sample. The concentration of hydrogen ions in water determines the pH value. Alkalinity is the capacity of water to neutralize acids. That is it is the measure of the struggle of water to the lowering of pH when acids are added to the water [46] .
Electrical conductivity (EC): EC, of the water, specifies the purity of water. And the number of total dissolved salts (TDS) in water measures the conductivity of water. It is a measure of the capability of water to carriage the electricity. It increases as salinity increases. Fewer ions in the water, less is the EC, purer is the water sample. B4 sample adsorbed the number of ions, appears to be pure.
Electrical conductivity increases with an increase of Chloride, sulphates and Nitrate content.
Chloride, sulphate and nitrate content from wastewater are absorbed by the sample B4, remains with 368mg /l.0.5mg/li and 10mg/l, respectively. It shows that Zn nano ferrite can be used in waste treatment [46] .
Chemical Oxygen Demand the measurement of total oxidizable chemicals in the water. It quantifies the amount of oxidizable pollutants found in wastewater.
The complete discussion reveals that synthesized nano ferrites shows the absorptive capacity.
Which favors that these can be used in wastewater analysis.

Absorption:
The existence of a heavy concentration of constituent at the surface of a solid or liquid a phase is called Adsorption. The reason behind this is the existence of residual forces at the surface of the liquid or solid phase. The constituent on the surface of which adsorption occurs is denoted as Adsorbent. And Adsorbate is which is being adsorbed on substrate [47] .
In this phenomenon, when a gas is adsorbed on a solid surface, ΔS is appears to be negative, as its movement is restricted leading to a decrease in the entropy. Now, this process is natural (spontaneous), that is ΔG is to be negative. But ΔS is becoming negative when adsorption takes place. Thus, according to the thermodynamic relationship of Gibbs free energy, ΔG=ΔH-TΔS adsorption is always exothermic. Van der Waals forces (London Dispersion Forces), a type of weak forces of attraction is responsible for the Adsorption process [48] . The phenomena depend on both the nature of adsorbate and adsorbent along with pH, pressure, and surface area also.
In the case of adsorbents, the size of the pores at the surface and the functional groups are responsible for effective adsorption. Similarly, molecular states, solubility, and functional groups significantly express the nature of adsorbate.
In the current work, we deliberate the adsorption behavior of different concentrations of acetic acid, a weak acid on Synthesized nano ferrites. Different Isotherms are studied for this system and Langmuir model holds good for Zn nano ferrites. Langmuir isotherm explains the adsorption situation where the number of sorption sites available is limited. Because of which adsorption sites become saturated, to maximum capacity.
As acetic acid is a weak acid, it forms OH-groups when it reacts with water and which forms bonds with metal-nanoparticles at the surface. Such reactions act under particular pH of the solutions, further OH group can be acting as a proton donor or acceptor of a proton, and balances the pH of the solution [49] . And according to the references, we found that more alcoholic groups present at adsorption interphase more is the adsorption. FTIR studies have given proof of this phenomenon.
Cobalt and Zinc nano ferrites hold good for the study of adsorption due to porosity.
Absorption of acetic acid at room temperature is studied by absorption isotherm. Acetic acid of different concentrations is prepared by mixing the required amount of distilled water in glass wash bottles. Adsorbents cobalt and zinc nano ferrites are activated by heating them in ovens.
When they become activated with open pores, weighed 1gm, and added to each bottle separately. Now all the four bottles were shaken well in electrical shakers for half an hour [50] and then kept in a water bath for separation for 2hours. After separation, 5ml of each sample is pipetted out carefully and titrated against the standard solution of sodium hydroxide.

Table 3 -Adsorption Details
Langmuir absorption isotherm: The Langmuir isotherm is an actual isotherm for an explanation of monolayer formation on a surface containing a finite number of active adsorptive sites. And further, no more adsorption occurs.
Thus, Langmuir isotherm epitomizes metal ion, distribution between the liquid and solid phases [51] . For n-number of identical sites at the surface, Langmuir isotherm is valid up to monolayer adsorption.
Langmuir model assumes that vacant available sites are of equal size and shape on the surface of the adsorbent and uniform energies of adsorption. Based on assumptions, its equation is given as Ref [52] C0 = initial concentration KL= Langmuir Constant RL = separation factor R 2 = equilibrium sorption Zinc ferrite nanoparticle is showing optimum results for Langmuir isotherm, with equilibrium sorption value very much near to 1.and separation factor which shows the absorptive nature is advantageous that is greater than 0 and less than 1.

Adsorption of Dyes:
The dye industry is one of the most water-consuming industries. Effluents of these industries contain many hazardous chemicals, including dyes. Proper advanced techniques are incorporated to remove these dyes and discharge into water bodies [53,54] . To do so synthesized nano ferrites are examined with some laboratory dyes adsorption study, which may further minimize the risk of industrial effluents.    Table 6. Percentage of adsorption The mechanism of color change by nano ferrites can be predicted. For example, in phenol red, negatively charged ion denoted as HPS−, the proton from the ketone group of nano ferrites is lost, resulting in the yellow. And at the same time towards higher pH (pKa = 7.7), resulting in the red ion denoted as PS2−,as the phenol's hydroxy group loses its proton.

Conclusions:
Single-phase cobalt ferrite nanoparticles and Zinc ferrite nanoparticles are synthesized by the chemical wet method. The observed transmittance bands in the range 400-600 cm -1 in FTIR spectra approve the establishment of spinel ferrites. SEM images reveal the formation of fine particles within the nanometric range. Nano ferrite of cobalt was synthesized with a particle size of around 50 nm. Sensing studies showed that the sample displays variation in resistance when gases are passed over the surface, and the change in resistance is highest for acetone. Nano ferrite of zinc was synthesized with the particle size of around 90 nm. Sensing studies showed that the sample displays variation in resistance when gases are passed over the surface and the gases sensed in decreasing order of the value of change in resistance obtained are methanol, acetone, ethanol, and ammonia, respectively. The motto of this article is to research and development of ferrite structures, cation distributions, and their impact upon the applications of ferrite components in the adsorption also. Sample B4 holds good for wastewater analysis results and follows the Langmuir isotherm forming the single layer adsorption of acetic acid on it. Further dye adsorption studies reveal that zinc nano ferrites hold good results of dye adsorption.