Review—Metal Oxide Chemoresistive Gas Sensing Mechanism, Parameters, and Applications

The economic growth of any country depends upon the MSMEs as it plays a vital role in GDP and employment. The transportation is considered as the lifeline of the country. Hence due to developing countries, the industries and vehicles are continuously increasing to fulfil industrial or domestic requirements. But unfortunately, industries and vehicles emit harmful gases as exhaust to the environment. Which directly or indirectly impact the human health. Fresh and clean air is the prime need of the society. Hence the monitoring of different gas concentrations in the environment is very essential to take preventive steps to control air pollution. The traditional method of monitoring the air quality is very expensive, hence most of the countries have limited air monitoring stations. In the field of nanotechnology, scientists have developed different types of soft metal oxide materials that are capable of sensing different gases at low concentrations and can work in different environmental conditions. For the last 10 years, ferrite-based sensors have the primarily used to detect harmful gases, and pollutants from vehicle exhaust, and environmental pollution monitoring. These soft ferrites have excellent electrical and magnetic properties that can also be tuned according to the requirement of the sensor to increase sensitivity and selectivity. The tuning of ferrite sensors depends upon synthesis technique, optimizing preparation conditions, sintering temperatures, operating temperatures, dopant concentration, etc This paper is based on a deep study of the synthesis techniques of nano-ferrites, different types of gas sensors, gas sensing mechanisms, parameters, and application of chemo-resistive metal oxide gas sensors. The key parameters for the ferrite gas sensors are phase formation, crystallite size, grain size, surface area, selectivity, dopants, sensitivity, gas concentration, operating temperature, and response/recovery time. This review paper also includes the study of different researchers to find the impact of high concentrations of gases like hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), ethylene glycol (CH2OH)2, methane (CH4), ammonia (NH3) liquid petroleum gas (LPG), acetylene (C2H2), and nitrogen oxides (NOx) in the environment and the metal oxide materials selected for the sensor application.

Clean and fresh air is the prime need of society to live healthily.The burning of fossil fuels, industrialization, vehicular emissions etc are the key factors that affect air quality.These all increase the level of hazardous compounds like Nitrogen oxides (NO x ), Methane (CH 4 ), Ammonia (NH 3 ), Acetylene (C 2 H 2 ), Carbon monoxide (CO) and Carbon dioxide (CO 2 ) in the environment. 1The concentration of hazardous gases also increases due to volcanic eruptions, forest fires, microbial decaying processes, combustion engines, industrial processes, vehicular emissions, agricultural activities, and farming.These contaminants are not good for the ecosystem as they cause acid rain and accelerate climate change due to the greenhouse effect.The contaminants are also responsible for various types of diseases like cough, asthma, breathing difficulties, irritation of the respiratory tract, skin and eyes, or an increased risk of a heart attack. 2About 7 million deaths due to air pollution have been reported by WHO (The World Health Organization) in 2016. 3ifferent agencies working globally to develop various techniques to monitor air quality to take preventive action to control air pollution.Most countries are looking for alternate renewable sources of energy like solar energy, wind energy, hydro-energy etc, to fulfil their energy requirements without any harm to the environment.The government of India is also promoting, solar energy, hydro-energy, electrical vehicles etc to reduce vehicular emissions. 4Even with all these developments, still the smoke from industries as well as vehicular emissions needs to be monitored continuously to take preventive measurements. 5Oxygen sensor is used in vehicles and industries like cement industry, food processing plants, etc. 6 Gases like CO 2 , NO x , CO, NH 3 , Cl 2 , LPG, O 2 , CH 4 etc, commonly used in the industries for different applications.Most of the gases are toxic and highly flammable.The leakage of such gases can cause a serious accident.Hence, the sensors are required to monitor the gas concentration in the environment where these gases are stored. 7ue to the high cost, traditional methods cannot be used to monitor the air in different locations.Nanotechnology is an emerging field to fabricate various type of smart materials.The properties of the materials can be tuned by controlling the physical parameter, sintering temperature and the addition of dopants.The  + These ferrites have wide range of applications due to their electrocatalytic properties, magnetic properties, good mechanical hardness, high surface area, ease of processing, and tunable structure. 8anotechnology is applicable in different fields as shown in Fig. 1.The common applications of these ferrites are multidimensional like ferrofluids, high frequency, antimicrobial, water treatment, radar absorption, data storage, humidity sensor, biosensors, catalysis, etc. [9][10][11][12][13][14][15][16] Classification of Sensors Based on Principle of Operation Different types of gas sensors are available in the market now a days.Figure 2 shows the classification of sensors based on principle of operation.
Electrochemical sensors.-Anelectrical signal proportional to the gas concentration is produced by electrochemical sensors in response to the measured gas.The majority of electrochemical gas sensors measure current and generate a current that is directly proportional to the concentration of the gas.The mechanism of operation for electrochemical gas sensors is as follows: the gas flows via a diaphragm that both inhibits condensation and acts as a dustproof barrier.After diffusing through the filter, the gas molecules arrive at the active electrode.Here, the gas is reduced or oxidized, and the current passing through the external circuit changes because of this electrochemical process.The most basic type of sensor based on electrochemical principles is a two-electrode setup.Its working electrode and opposite electrode are coupled to the external circuit with a little resistance and are kept apart by electrolyte.The gas reacts to produce an electric current when it diffuses into the sensor.The current increases with the amount of gas. 17tical sensor.-Theoptical absorption technique is based on the Beer-Lambert law of absorption, which states that.
Where I denote the light intensity entering the sensor or cell, I0 denotes the light intensity entering the system, is the gas absorption coefficient, and L is the optical path-length, which is dependent on the sensor's sensitivity.When gas molecules interact with infrared radiation, they absorb a certain wavelength of infrared light, which causes the gas molecules to vibrate.Every gas molecule has a distinct infrared absorption fingerprint due to its capacity to absorb IR light at different wavelengths.To identify the target gas, the infrared light that is absorbed by the gas passes through the active filters within a specific bandwidth.Thus, the gas concentration of the target gas molecule influences the ratio of transmitted radiation energy to incident energy. 18oustic gas sensor.-Thefirst acoustic gas sensor was developed in 1964.It operates through the detection of bulk acoustic waves (BAWs) in a mass-sensitive piezoelectric quartz crystal resonator.SAW sensors depend on transduction, which is the process of converting input signals into mechanical waves and then back again.The IDTs, or interdigitated transducers that utilize the piezoelectric effect, are what enable this.A transducer and an analyte-sensitive receptor make up an acoustic wave sensor.It is the part in charge of turning the electrical signal from the feedback.Different acoustic wave sensors work with different wave frequencies. 19talytic gas sensor.-Gassensing systems require a suitable catalyst for surface deposition.The coated catalyst quickens the chemical process and raises the sensitivity.A catalyst is an external substance that, without taking part in the reaction, alters the rate of a chemical reaction.As such, it would not alter the response energy; but it has the ability to decrease the activation energy, that's why it is selected to influence the device's selectivity.The ideal function of a catalyst is to enhance the oxidation of one gas while having no influence on another.On the other hand, heat can cause many harmful gasses to oxidize.Catalytic sensors may therefore monitor these gasses precisely.Heat release has the ability to oxidize the majority of hazardous gasses.Catalytic sensors can therefore measure these gasses with accuracy.Based on the heat produced as the gas molecules oxidize over the catalyst surface, the sensor identifies the gas molecules.The detector element (D) and the compensator element (C) comprise the catalytic-type gas sensor.While C is inert, D has catalytic material and is susceptible to flammable gasses.Since catalytic type sensors are specifically made for burning gas, they are widely employed.In commercially available sensors for the catalytic oxidation of hydrocarbons, H2 and CO, noble metals function as active catalysts. 20gnetic gas sensor.-Themagnetic characteristics of the magnetic gas sensor will alter in response to changes in temperature, light, radiation, and pressure within the surrounding environment.This characteristic can be used to develop a wide variety of a reliable and sensitive sensors.Most magnetic probes with reasonably powerful measuring capabilities are used as magnetic gas detectors.The high magnetic characteristics of oxygen are frequently used to measure the concentration of oxygen while testing gases.since high magnetic fields have the ability to pull oxygen in the air.Both magneto-mechanical and thermal magnetic convection oxygen analysis sensors are frequently utilized. 21otoionization (PID) gas sensors.-Gasdetection using photoionization is accomplished via photoionized gas sensors, or PIDs.Elucidatedly, the gas is exposed to UV light produced by an ion lamp, and upon absorbing sufficient ultraviolet light energy, the gas will become ionized.Through the detection of the minuscule current produced by the gas ionization, the measured gas level is determined.With a detection range of 10 ppb to 10,000 ppm, it can identify harmful gasses and volatile organic chemicals.Since PID is extremely sensitive to volatile organic chemicals, many harmful substances contain them. 22miconductor gas sensors.-Adevice that utilizes a semiconductor element as the measuring gas concentration is called a semiconductor gas sensor.The resistance value changes as a result of a redox reaction that the gas has with the semiconductor.The basis of operation involves a redox reaction between the gas and the semiconductor, resulting in a change in the resistance value.The gas adsorbs and reacts on the measuring cell's surface as it flows past it.
A change in conductivity or potential, which is defined by carrier motion, is used to assess the concentrations of gas. 23frared gas sensors.-Theworking principle of infrared gas sensors is that different gases absorb different wavelengths of infrared light; the more gas present in the measurement area, the less light that can pass through.When infrared rays pass through the gas, the rays resonate with the gas molecules and are absorbed by the molecules when the molecules vibrate, which weakens the  transmitted infrared rays.Different gases vibrate at different frequencies; some atoms are very small and light, so they vibrate at high frequencies. 24s chromatography (GC) Sensor.-Aftersampling, a sample is separated into its constituents by the GC component, which is then split into two paths (often by vapour pressure).According to Brattoli  et al. (2011), the chemical detector (CD) path offers chemical analysis of the sample contents, typically using mass spectrometry (MS) and molecular weight.According to Wang et al. (2010), the olfactory channel leads to a human assessor, sometimes known as a "panelist," who offers details about the constituent's olfactory attributes.An extensive characterization of the odors sample is produced by merging the two data streams at the same time.A human assessor can identify compounds for each separated constituent that comes from the GC, as well as gauge the length and strength of the odorous signal. 25,26Advantages and disadvantages of different types of sensors has been tabulated in Table I.
The metal oxide-based sensors provide the best alternate to replace traditional sensors due to their advantages like high sensitivity, small size, durability, and low cost. 34,35inel Ferrite Structure Scientists are becoming more interested in Spinel ferrites among the ferrites due to their applications in a variety of fields.Metal oxide with spinel structure having formula MFe 2 O 4 , where M represent tetrahedral, and Fe represent octahedral site.Figure 3 shows oxygen atoms form 64 tetrahedral and 32 octahedral interstitial sites in spinel ferrites.Each unit contains eight molecules, containing 16 Fe 3+ and 8 M 2+ ions.Because these ions only occupy one-eighth of interstitial sites, there are many vacancies and oxygen defects.36 There are three possible spinel ferrite structures that might arise from the occupancy of M 2+ and Fe 3+ cations between the tetrahedral and octahedral sites.
i. Normal spinel structure ii.Mixed spinel structure iii.Inverse spinel structure In normal spinel structure M 2+ and Fe 3+ occupied in tetrahedral sites and octahedral sites respectively.In the mixed spinel structure Me 2+ and Fe 3+ randomly occupied in both tetrahedral and octahedral sites.In the inverse spinel structure, Fe 3+  Preference for site occupancy depends on the electrostatic contribution to the lattice energy, cation charge, cation radii, and crystal-field effects. 38The properties of spinel ferrites based upon the nature of the divalent cation, the size range of cation radii (40-90 nm), and the variation in ion bonding energy and geometry between the cations and the surrounding oxygen ions in both octahedral and tetrahedral sites. 39Due to tunable properties of spinel ferrites, there is a huge range of applications in different fields.
Metal oxide based sensors.-Metaloxide ferrites are mostly used in the electronic industry due to their electrical and magnetic properties.Scientists have selected different compositions of ferrite materials for sensing different gases. 402][43][44][45] Till now more than 150 types of gases sensed by metal oxide sensors has been reported. 46The resistance of chemoresistive metal oxide sensors varies due to the transfer of electrons and holes in spinel ferrites located in octahedral sites. 47lectrons are the predominant carriers in n-type semiconductors because the Fermi level is closer to the energy level of the conduction band than the valence band.The Fermi level for p-type semiconductors is closer to the valence band's energy level than the conduction band, making holes the dominant carriers.Both oxidizing and reducing type of gases used for industrial, agricultural, medicinal, etc applications and hence the leakage monitoring of these gases is the prime requirement to prevent accidents. 48,49The ntype metal oxides like SnO 2 , 50 ZnO, 51 In 2 O 3 , 52 TiO 2 53 WO 3 54 and ptype metal oxides such as Co 3 O 4 , 55 NiO, 56 CuO, 57 ZrO 2 58 has been reported for sensing of different gases.The number of searched  4.
Synthesis techniques to prepare nano ferrites.-Thereare two methods to synthesize nanoparticles: top to bottom and bottom to top as shown in Fig. 5. Small scale atomic elements combine to form nanoparticles in the bottom to top (bottom-up) method.This is most effective way to produce pure, homogenous, narrowsized nanoparticles.In a top-down method, material in bulk form breaks down to the nanoscale to produce nanoparticles.This method was not favored because the products are not uniform and metaloxide reactions typically need a high temperature.In addition, there may be surface, size distribution, impurities and crystal defects present.The physical and chemical properties of soft ferrites can be controlled by adjusting the amount, position of the metal cations in the crystallographic structure and by using suitable synthesis technique.Commonly used synthesis techniques are co-precipitation method, 59 green synthesis method, 60 solid state reaction, 61 sol-gel, 62,63 hydrothermal, 64 citrate, 65,66 etc. Figure 6 shows different synthesis techniques for some of above-mentioned methods.
Hydrothermal synthesis.-Hydrothermalsynthesis is the grouping of methods used to crystallize substances from hightemperature aqueous solutions at high vapor pressures.With a solvent above room temperature and pressure greater than one atmosphere, the composite materials are created through a heterogeneous chemical reaction in a closed system, producing a highcrystalline and pure material.The autoclave apparatus, as shown in Fig. 7, consists of a stainless-steel chamber lined with Teflon material.Numerous benefits come with this process, including the production of extremely pure homogenous material with symmetrical crystal structures that have unique properties.It also enables low-temperature synthesis, better size control, and greater control over the final product's composition, all of which can be produced in comparatively short amounts of time. 67-precipitation technique.-Co-precipitationtechnique is one of the simple methods for creating metal oxides nanoparticles.This method involves passing a base through a solvent to obtain the metal precipitate in hydroxide from a salt precursor.By adding sodium carbonate to raise the pH level, it facilitates the process of nucleation -the release of cations and anions-and allows for the growth of monodispersed nanoparticles.This self-controlled process aids in the synthesis of these particles.With this method, a flow reactor is used to facilitate rapid and consistent mixing, which can result in the  precursor and base solution's actual temperature.Above-average pore diameter, high specific surface area, miniature particle size, and superior crystallinity can all be obtained using the co-precipitation method.68 Sol gel technique.-Solgel technique is a wet chemical process consists of several steps, including hydrolysis, condensation, metal precursor polymerization reactions, and gel formation that is made up of three-dimensional metal oxide networks.The salient characteristics are enhanced homogeneity, elevated purity, reduced processing temperature, replicability, simplicity, and affordability.After stirring the solution at 80 °C and keeping its pH at 7, the mixture transformed into xerogel and then into loose powder, which was then annealed for 5−6 h at ∼1023 K.The produced nanoferrites can be used for various applications depending on the properties of the nano ferrites.69 Combustion/citrate precursor method.-In the precursor method, metals are mixed at the atomic level in stoichiometric ratios to form a solid compound known as a precursor, which is then heated to a relatively low temperature to break down into the desired product.The technique has the advantage of producing homogenous products with large surface areas and smaller particles, which is desirable for a variety of applications.70 Solid state reaction method.-Thesolid-state reaction technique, also known as the conventional ceramic method, is the most widely used method for producing ferrite materials.This method involves the grinding of oxides, carbonates, oxalates, and other metal compounds, the mixture is heated to a high temperature, the materials are pelletized again, and the process is repeated until the materials reach the desired phase.Despite being a very old and widely used technique, the conventional ceramic method has some disadvantages, such as non-uniform crystallite size and mixed phases because of sintering at a higher temperature and for a longer period.Due to the industrial process's maturity and the readily available and affordable starting materials, ferrites are typically created using the traditional ceramic method.However, it's obvious that the milling process takes a very long time and could leave the material with some impurities.71

Gas Sensing Mechanism
The chemoresistive metal oxide gas sensor and the semiconductor gas sensor both have same mechanism. 72In the presence of gas, metal oxides change their resistance.The sensor's surfaces adsorb oxygen molecules at a temperature ∼100 400 Gas sensing reaction and adsorption of gas is different for different gases.Hence the absorbed oxygen spices are quite important to carry out the reaction in presence of gas.Electrons are captured when oxygen species chemisorb onto metal oxide particles.This results in the formation of a conducting layer from accumulating holes on the surface of a p-type particle or a resistance layer from an electron-depleted space charge on the surface of an n-type particle.For CO 2 gas, the following reaction occurs when gas particles react with chemisorbed oxygen: CO + O ads − → CO 2 + e .− Electrons from oxygen are liberated back to oxide.The space charge layer's conductivity is altered by the release of electrons.The conductivity of n-type oxides rises due to a higher concentration of electrons, but the conductivity of p-type oxides decreases because electrons recombine with holes.The overall electrical resistance of the oxide is altered by the change in conductivity in the space charge layer.The sensitivity rises with increase in the space charge layer thickness.As a result, smaller particle size is beneficial for a higher gas response because the volume of the space charge layer depends on the volume of the gas sensing oxide particle.The charge carrier concentration significantly affects layer thickness.High charge carriers' concentration reduces the thickness of space charge layer. 74According to the Verwey mechanism, charge conduction occurs between transition metal cations from the same element with different oxidation states that are in octahedral sites.The electric conductivity of metal oxides changes when divalent transition metal cations (M 2+ ) in octahedral site get oxidized and form trivalent cations (M 3+ ).After reacting with test gas, trivalent cations are converted back to divalent ones, increasing or decreasing conductivity in n-type or p-type spinel ferrites, respectively.Figure 8 shows the oxygen species chemisorb onto metal oxide surface and electron tapping with gas molecule.
N-type spinel ferrite gas sensors.-N-typespinel ferrite gas sensors before oxygen chemisorption, contain pairs of Fe 2+ and Fe 3+ ions at octahedral site, which are slightly reduced from their stoichiometric composition.Magnetite (Fe 3 O 4 ) is an exception, as it shows n-type conductivity and contains equal levels of Fe 2+ and Fe 3+ in the octahedral site in its stoichiometric form.Nevertheless, due to a lack of thermal stability, it cannot be utilized as a chemical gas sensor.At 200 °C, magnetite nanoparticles permanently change into maghemite -Fe 2 O 3 and subsequently further into the hightemperature polymorph hematite. 75The hopping type of conductivity of e − between iron cations occupying octahedral sites provides charge carrier transport in n-type spinel ferrites.e − conductivity rises with increasing Fe 2+ (Fe 3+ + e − ↔ Fe 2+ ) concentration.Fe 2+ is converted to Fe 3+ after chemisorption of oxygen on the n-type spinel ferrite.An increase in the number of Fe 3+ and Fe 2+ pairs as well as total electrical conductivity results from the reduction of Fe 3+ to Fe 2+ when oxygen reacts with gas.The Fe 2+ content in ZnFe 2 O 4 can be controlled either during synthesis or during the annealing process.In iron-excess zinc ferrites, Fe 3+ is shifted to tetrahedral sites and replaces zinc cations, Fe 2+ incorporates octahedral site leading to the following formula: The amount of Fe 2+ increases as the iron content increases. 76The capacity for oxygen chemisorption increases as Fe 2+ concentrations  increase; however, if Fe 2+ concentrations are too high, the Debye width narrows and the sensor's response to gas is reduced.
Annealing in a reducing atmosphere also used to reduce Fe 3+ to Fe 2+ in accordance with equation: Where V O 2+ is oxygen vacancy and Fe 2 Fe ¢ ¢ is Fe 3+ substitution with Fe 2+ .
Zn 2+ or Cd 2+ preferentially evaporate when annealed at around 1000 °C, producing an excess iron spinel ferrite.The stoichiometry (Zn 1−x Fe 2+x O 4 ) is changed by zinc evaporation, allowing the newly produced Fe 2+ to chemisorb oxygen.However, it is doubtful that heating zinc or cadmium ferrites to high temperatures will result in non-stoichiometry since heat exposure reduces the specific surface area that reduces the reaction with test gas.P-type spinel ferrite gas sensors.-P-typespinel ferrites is having inverted spinel structure and transition M 2+ cations arranged in an octahedral site alongside Fe 3+ .NiFe 2 O 4 is a common example of p-type spinel ferrite.The conductivity in p-type spinel ferrites is due to the hole (h + ) hopping between Ni 2+ and Ni 3+ in octahedral site (Ni 2+ + h + ↔ Ni 3+ ).Nickel has tendency to attract oxygen during synthesis hence form cation vacancies.Ni 2+ oxidizes to Ni 3+ to maintain electrical neutrality in the lattice. 77In Kröger-Vink notation, the lattice oxidation of the divalent nickel cation to Ni 3+ is represented as follows: Where x, * and ″ represents zero, positive charge (holes) and negative charge (electrons) respectively.The nickel ferrite bulk lattice with higher Ni 3+ concentration having lower sensitivity.Because the Ni 3+ does not occupy a higher oxidation state in the spinel structure, a small amount of additional Ni 3+ can be formed by chemisorption of oxygen adsorbates for compositions with high intrinsic Ni 3+ concentrations.In contrast, the stronger oxygen chemisorption occurs in nickel ferrite with a larger Ni 2+ concentration.Oxygen that has been chemisorbed reacts with the test gas, changing the resistance.As a result, higher chemisorbed oxygen concentrations will result in a better gas reaction.Figure 9 depicts that greater Ni 2+ surface concentration in NiFe 2 O 4 is required to enhance the gas reaction.When there is more Fe 2+ than Ni 3+ , electron transport happens because of electron hopping between Fe 3+ + e − ↔ Fe 2+ .
Mixed and substituted spinel ferrites.-Spinelferrites with substitutions include more than two distinct cations.Ni-Zn are a common example of mixed spinel ferrite.The electrical properties of NiFe 2 O 4 are significantly changed by the replacement of Ni 2+ with Zn 2+ .A mixed spinel with uniformly distributed divalent and trivalent ions in tetrahedral and octahedral sites is produced when zinc ions preferentially occupy tetrahedral sites and shift Fe 3+ ions to octahedral sites. 78The Ni-Zn ferrite nanoparticles with additional Zn 2+ become more resistive, and the type of conductivity switches from p-type to n-type.Electrons are the dominant charge carriers in Ni-Zn ferrites.The gas response always changes with the substitution of spinel ferrites.

Parameters of Chemoresistive Gas Sensor
Structural property.-Thegas sensing properties depend upon the morphology of the metal oxide ferrites.The morphology of the ferrites can be controlled by altering the concentration, synthesis techniques, sintering temperature etc. 79 Porous metal oxides materials are preferred for the gas sensor application. 80The pores may be, i. Open to end of the surface on both sides.ii.One end open while other is closed, iii.Pore without opening on the surface.
The pores may be cylindrical, rectangular, split, square, etc, in shape, therefore, pores can be classified in three categories as per the diameter of the pores, i. Microspores (about 2 nm), ii.Mesoporous (between 2-200 nm), and iii.Macrospores (above 200 nm).
Porosity (P) of the material can be calculated as Eq. 12 where V P is the total pore volume, and V is the body volume.In the porous material the active surface is much higher because of the volume of layer available to the gases.Hence for gas sensors, porous materials always preferred for high sensitivity. 81Specific surface is also an important parameter for gas sensor.Specific surface can be calculated from XRD as Eq. 13 S 6 10 13 where S is specific surface area, τ is crystallite size and ρ is density of the sample. 82The materials with smaller grain size and larger surface area are preferred for the high sensitivity.Nano-ferrites with larger surface to volume ratio show higher sensitivity because of better interface between the surface and gas. 83talysts.-Catalystsare preferred in sensor application to increase the reaction rate.These catalysts on the sensor surface easily break the gas molecule bond and increase the interaction between the gas molecule and adsorbed oxygen.The increased oxygen adsorption rate improves the sensitivity of the sensing material.5][86][87] These catalysts reduce energy for gas adsorption, improve selectivity, and reduce response/recovery time.
Dopants.-Dopants in the sensing materials improve the sensing properties like sensitivity, operating temperature, selectivity, reduce response/recovery time.The required morphology like pores, crystallite and particle size can be controlled by using suitable dopant in the sensing material.[90] Sensitivity.-Sensitivity is the variation in the resistance in the presence of gas.Spillover effect and electronic mechanism are the two mechanisms used to increase the sensitivity.Sensitivity increases with increase in surface area.Larger surface area provides the large number of molecules to interact on the surface. 91Sensitivity depends on the nature of the materials as well as the gas concentration.Sensitivity of a sensor can be calculated as Eq.14. 92 S R R % 100 14 where R andR a g are the resistance in air and gas respectively.The resistance of sensor increases in presence of oxidizing gas and reduces in presence of reducing gas.S 1 g > for reducing gas whereas for oxidizing gas S 1. g < Sensitivity of the sensor depends upon the porosity, pore size and specific area.These are the key parameters and scientists are continuously working to control these parameters to make a highly sensitive sensor.Suitable additives and sintering temperature are responsible to control porosity required for the sensor.
Selectivity.-Thesensor response to a specific gas in the presence of other gases is known as selectivity. 93Selectivity depends upon the temperature, dopants, and additive coating (electrodes) to the grain surface.High sensitivity is the results of large amount of test gas absorbed and reaction between gas and oxygen species.The position and gap between the electrodes on the sensor surface affect the sensitivity and selectivity.
Influence of gas concentration on sensor resistance.-Theresponse of the sensor increases with increase in concentration up to certain level.The conductivity (G S ) of the sensor at low concentration C g ( ) of the gas in the atmosphere at constant temperature can be calculated as Eq. 15.
G KC 15 where K and α is constant. 94sponse characteristics.-Responsecharacteristics of a sensor is the relationship between resistance and time in the presence of the gas concentration.The resistance of every sensor changes with gas concentration (ppm) at a specific temperature.The change in resistance in presence of gas concentration is known as rise time whereas the fall time is the time to reach the initial resistant after removal of gas.Thus, rise time and fall time are used to draw response characteristics.The response and recovery time is defined as the time required to change the resistance up to 90% in presence of air and target gas respectively.95 Temperature.-Temperature is an important parameter to increase the sensitivity of the sensor.It increases the speed of chemical reaction by the diffusion of the gas molecules on the surface of the sensor.The activation energy of the chemical reaction depends upon the temperature.At low temperature the response is restricted due to slow rate of diffusion of gas molecule on the surface.Even at high temperature the response is low due to high speed of gas molecules on the surface.Hence the intermediate temperature is preferred to get the maximum sensitivity.96 Adsorption/desorption model.-Theadsorption/desorption model is based on the bulk conductance effect, which is a variation in the work function of a material and the conductivity with the change in the surrounding atmosphere.In the presence of target gas, the adsorption/desorption of gas molecules occur between the gas molecules on the surface of a material.It induces a reaction to ionize gas molecule to transfer electrons or holes between the target gas and the sensor surface.This transfer of holes or electrons can change the density of charge carriers in the sensor that vary the conductivity of sensors.The electric conductivity of the sensor is based on the gas sensing parameters as per Arrhenius Eq. 16.

A E kT P 16
A gas where A is constant determined by the basic conductivity of materials, E the activation energy for conduction characterizing the ionic conduction process, k the Boltzmann constant, T the temperature, P gas is the partial pressure of target gas, and N the empirical constant. 97The sensitivity and operating temperature of the gas sensors are strongly influenced by the activation energy.The sensor sensitivity is inversion of activation energy, as the sensitivity increased the activation energy decreases. 98

Fabrication of Gas Sensors
Resistive metal oxide sensors can be classified in to three categories.Bulk-Pellets, Thin Film, Thick Film as per their applications.
Bulk-pellet.-Suchsensors are produced by synthesis and sintering of suitable oxides.These pellets may be rectangular or cylindrical in shape.Pellet sensors are used for sensing applications at high temperature about 100 °C to 500 °C.Mesoporous materials are preferred for the pellet sensors.The pellet sensors have long stability and are preferred for toxic and flammable gas detectors. 99ick film.-Thesesensors are made by screen printing with a size about mm mm 1 1 .

×
The chemical sensitive powder mixed with organic binder (Poly-Viny Alcohol) is coated on substrates (alumina and ceramic tubes) and consumes less power.The thick film sensors have benefits like low cost, flexible deposition and multiple layers and reheating.Lower degradation temperature of the binder releases fewer toxic gases.The preferred thickness for the thick film sensor is 50-150 nm. 100 Thin film.-Thesensitive powder is deposited on the conventional silicon wafer alumina substrate by techniques like evaporation, sputtering, radio frequency (RF) sputter, pulsed laser deposition (PLD), direct current (DC) sputtering, and thermal evaporation, etc This type of sensor shows high integration performance and consumes a low power.The thickness of the film is between 10-15 nm 101 The sensitivity of the thin film increases with an increase in electrode spacing underneath the film but decreases with an increased spacing of electrodes on the top.The electrode spacing must be larger than the film thickness for good sensitivity. 102

Influence Factors
Temperature.-Theresistance of metal oxide sensors is inversely proportional to temperature and as the temperature increases, resistance decreases.The sensitivity of sensor depends upon the operating temperature of the sensor as per the equitation (16).Pellet sensor has a fixed operating temperature where the response and recovery time both are less. 103midity.-Inthe presence of humidity, the water molecules occupy adsorption site of the sensor surface.The sensor does not absorb gas in this case because of the reduction of the surface area.As a result, the performance of sensor also reduces with reduction in surface area. 104

Testing of Sensor Materials
Various types of gas sensing systems have been reported for metal oxide sensors.The gas sensing system measurement must have the following components: i.A valve and a controller to allow target gas exposed to sensor.ii.A thermostat to control the temperature of the gas sensor.iii.A gas chamber with defined volume.iv.An electronic device with computer to measure and record the change in the resistance in presence and absence of the target gas.v.A vacuum pump to remove air/gas from the chamber.
Figure 10 shows the schematic setup for metal oxide gas sensor.The metal oxide sensor is attached with the transducer and modulator unit with internal connections.The sensing materials sense the presence of gas, and physical reaction on the sensor surface is converted into electrical energy by the transduction material.The strength of observed electrical signal is increased by the modulator.All these units are enclosed in the closed miniaturized package. 105efore testing the pellet sensor, the sensor is heated at a temperature of 80 °C for 30 min to remove the moisture.The prepared sensor is placed on the surface attached with the heaters in a transparent gas chamber with limited volume.The pellet sensor is connected with the circuit.When the resistance of the Sensor in the presence of the air becomes stable then the pre-determined target gas has been introduced through a syringe into the gas chamber.The concentration of the gas is determined by the volume ratio of the injected gas to the gas chamber.The resistance of the sensor changed and becomes stable.
The air is introduced again in the chamber and the resistance changes again.The working temperature of the sensor was adjusted by varying the voltage of the heater. 106,107

Study of Gases
In this article the comparative study of the parameters like composition, synthesis techniques, Morphology, Gas concentration, Temperature, Sensitivity, Response/Recovery time has been done for different gases like, Nitrogen dioxide (NO 2 ), Methane (CH 4 ), Ammonia (NH 3 ), Acetylene (C 2 H 2 ), Carbon monoxide (CO), Liquid Petrolium Gas (LPG), Ethylene glycol, Hydrogen (H 2 ), and Carbon dioxide (CO 2 ).These gases are commonly used for domestic as well as industrial applications.
Carbon dioxide (CO 2 ) gas.-In the atmosphere, the required level of CO 2 is 0.04% or 300-400 ppm.This gas is also known as greenhouse gas as it absorbs the short-wavelength light and helps the plants to grow.The level of CO 2 rises in the atmosphere because of huge consumption of fossil fuels in the vehicles and industries.3][114] Hence, greenhouse gas detection and control, atmospheric environment monitoring, industrial and agricultural carbon emission indicators all together requires a sensor to detect the level of CO 2 in the atmosphere.The reaction of the CO 2 with oxygen is as follows. 115 gas e CO ads 17 In this review paper, eleven CO 2 gas sensor research papers of different compositions has been taken for the study as listed in Table II.Most of the sensors are working at higher temperature and have a higher response/recovery time.Hence, it is required to develop new materials to detect CO 2 gas at low temperature with less response/recovery time.
Liquefied petroleum gas (LPG).-LPG is a fuel gas commonly used in domestic and industrial applications.It is a highly flammable gas that causes serious risk to environment and humans.The composition of LPG is propane (5%-10%), butane (70%-80%), and propylene, butylene, ethylene, and methane (1%-5%) 116,117 Lower explosive limit of LPG is 1.8 vol %. 118 The demand of LPG as well as accident are also increasing day by day.The reaction of LPG with oxygen ions is as follows 119

C H O H O C H O e
The LPG sensor is therefore highly required in homes and industries to prevent accidents due to leakage.The research paper on LPG sensor has been taken for the study as listed in Table II.The papers are showing higher response/recovery time even at high concentration of LPG.To detect LPG gas, the sensors have properties like work at room temperature, sense LPG at low concentration and have low response/recovery time.Hence the researchers have to develop metal oxides with above said properties.
Nitrogen dioxide (NO 2 ).-Nitrogen dioxide (NO 2 ) is a toxic gas produced by refining of petrol, making nitric acid, using explosives, the combustion of coal, gas, oil or wood for industrial and domestic use.It is also produced from welding and vehicular exhaust.1][122] The reaction of NO 2 with oxygen ions is as follows. 123 gas e NO ads 21 It causes acid rain and fog.Therefore, it is essential to develop NO 2 monitor sensors.The research articles have been taken to compare the parameters for the NO 2 gas sensor as listed in Table II.These sensors are working at room temperature but having longer response/recovery time.Hence it is required to develop to develop/ tune the material to reduce the response/recovery time.
Hydrogen gas (H 2 ).-The demand of energy is increasing day by day in all over the world.Hence the consumption of fossil fuel increased as compared to its production.Fossil fuels are nonrenewable sources of energy that increase the level of greenhouse gasses and responsible for climate change.Hydrogen gas is renewable and clean fuel for future generation. 124This gas is having significant heat of combustion without any emission. 125,126H 2 gas is also used for petrol extraction, chemical industries, metal reduction due to its reducing capabilities. 127The storage of hydrogen gas is quite difficult due to its ultra-small molecular size. 128,1291][132] This gas is undetectable by human because it is colorless, odorless, and tasteless in nature. 133,134The reaction between hydrogen and oxygen ionsis as follows 135 Hence, highly efficient H 2 gas sensors are required to detect its leakage during transportation and storage.Research papers based on resistive hydrogen gas sensor has been selected to study the sensors parameter as shown in Table II.It was observed that these sensors are working at high temperature, required more hydrogen concentration to sense, also having long response/ recovery time.For hydrogen gas sensor, these parameters need to reduce to prevent accidents.
Ammonia gas (NH 3 ).-Ammonia(NH 3 ) is commonly used in industries, commerce, production of fertilizers, dyes, pharmaceutical and chemistry industries.NH 3 gas reduces the air quality and biodiversity due to acidification and eutrophication. 138NH 3 is also used as refrigerant in cold storage plants.NH 3 is a colourless toxic gas having pungent smell and is harmful to human health. 1391][142] NH 3 is acts as a precursor to produce foods and fertilizers for the nutritional needs of planetary organisms.NH 3 also used for the synthesis of many pharmaceuticals. 143,144The reaction of NH 3 with oxygen ions is as follows 145,146

(
).-Ethylene glycol is a colorless, odorless and sweetness nature volatile organic compounds (VOCs).0][151] The exposure limit of EG vapour in air should not exceed to 100 ppm. 152The reaction between Ethylene glycol and oxygen ions is as follows 153  Hence to monitor the concentration of Ethylene glycol, highly sensitive gas sensors are required.Research articles on ethylene glycol sensor have been taken to study the parameters as shown in Table II.The material Sn-doped In 2 O 3 228 showing good sensing parameters at low temperature 125 °C as compared to other sensor materials taken for study.

CH
Methane (CH 4 ) gas.-CH 4 is colorless, non-toxic and odorless under normal circumstances.Anaerobic bacterial decomposition activities under water (wetland) are the main source of production of this gas.Anthropogenic sources of methane are the production & combustion of natural gas and coal for energy.CH 4 is the main constituent of natural gas.CH 4 gas is a member of greenhouse gases and is 25 times more powerful than CO 2 gas. 154High concentration of CH 4 gas in atmosphere absorb the infrared radiations and reradiate it back to the Earth surface.Hence, it is responsible for global warming.It is a flammable gas and is explosive at a concentration from 5 to 15% in air.The reaction between methane and oxygen ions is as follows. 155he research articles related to CH 4 gas sensors are taken to study the parameters as tabulated in Table II.These sensors are showing their sensitivity either at higher temperature or at higher concentration.High concentration of CH 4 gas leads to an accident, hence the sensors are required to monitor the concentration of the CH 4 gas in the atmosphere.

CH
Carbon monoxide (CO) gas.-Therequired concentration of CO in the atmosphere is about 80 ppb (part per billion).The main source of CO is the incomplete combustion of carbon containing compounds.Combustion of hydrocarbons commonly take place in Internal Combustion Engines, [156][157][158] thermal power plants 159,160 etc CO is a colorless, tasteless, odorless, poisonous, flammable gas.It mixes with hemoglobin and produce carboxyhemoglobin.This carboxyhemoglobin causes seizure, coma, and fatality in human body. 161,162The reaction between CO and oxygen ions is as follows:  Hence the sensor to monitor the CO gas is highly required.The research papers has been studied to compare parameters tabulated in Table II.
Oxygen (O 2 ) gas.-The third most abundant element on the Earth is oxygen after hydrogen and helium.It is a colorless and odorless gas essential for every living organism.O 2 constitutes 20.95% in the atmosphere.It is continuously replenished by the plants in Earth's atmosphere by photosynthesis. 164Oxygen is the oxidant, high concentration of O 2 accelerate the combustion process.Annually about one hundred million tons of O 2 is extracted from air for industrial uses.Oxygen sensor has wide range of applications to monitor pollution through optimizing industrial boilers, automobile exhaust emission control system, steel, food processing plants, cement industries, biological, and control of chemical processes etc .165 The research papers related to oxygen gas sensors has been selected to study the parameters gas sensors as listed in Table II Where K Eth is the reaction rate constant.The gas has low flash point (−18.15°C) with flamablility limit between 2.5 to 82%. 170his gas is used in metal welding, manufacturing of lithium-ion batteries and conductive plastic. 171C 2 H 2 gas also produced in the faulty tansformer due to heating of insulating oil to a temperature above 1000 °C. 172The transformer faults such as spark discharge, arc discharge, and partial discharge, can be judged by detection of C 2 H 2 gas. 173,174To avoid such kind of accedents the eary detection of the C 2 H 2 gas is highly required.The research papers related to acetelene gas sensers has been taken for the study of sensing perameters as listed in Table II.Acetelene gas is a flamable gas hence it should be sense by the sensor at low concentration.
There are so many gasses in the atmosphere.But the abovementioned gases are closely related to human beings.The high sensitivity sensors are required to monitor these gases.The metal oxide sensors are the low cost, and their properties can be tuned easily by controlling different parameters.Researchers are working worldwide to get the best metal oxide composition for the sensors for different gases.Some metal oxide sensors are listed in the Table II.

Current Challenges and Future Scope
Fresh air is the prime requirement of everyone to live healthy.Worldwide agencies are working to monitor the different gas concentrations in the environment.This article elaborates the metal oxide chemoresistive gas sensors to monitor the gas concentration for industrial and domestic applications.Importance of metal oxides, Gas sensing mechanism and parameters of metal oxide sensors have been studied for the gases like Nitrogen dioxide (NO 2 ), Methane (CH 4 ), Ammonia (NH 3 ), Acetylene (C 2 H 2 ), Carbon monoxide (CO), Acetylene (C 2 H 2 ), Liquid Petrolium Gas (LPG), Ethylene glycol (C 2 H 6 O 2 ), Hydrogen (H 2 ), and Carbon dioxide (CO 2 ).Continuous development of any nation increases the demand of sensors day by day.The level of pollutants is going to increase due to industrialization, vehicular emissions any many other factors.The most promising materials for sensing gases, such as CO, Cl 2 , H 2 , O 2 , CH 4 , H 2 S, NH 3 , gasoline, petrol, LPG, etc, are ferrite gas sensors.A ECS Sensors Plus, 2024 3 013401 multidisciplinary collaborative approach is being used in the search for low-cost sensors with simple processing steps in ferrite gas sensor development.Many factors must be taken into consideration when developing the materials and fabricating the sensors that will be used in gas sensing applications.Some of these are listed as follows.
i. Choosing an appropriate technique to prepare materials with larger surface area and small grain size.ii.An investigation of the potential of additives in prepared ferrites to enhance recovery time, response, selectivity, and sensitivity.iii.Different approaches to material characterization for prepared materials.iv.Examination of how ferrites' structural and magnetic characteristics affect their transport properties.v.A thorough examination of the surface reaction and sensing mechanisms for various ferrous sensors.vi.The role of operating temperature affects surface phenomena enhancement.vii.The sensor's dimensions and form for a given application.viii.The examination of the prepared ferrite sensors' suitability and signal conditioning for industrial, laboratory, and pollution control applications.
These sensors are not only used to monitor the gas concentration in environment but also used in different CNC machines as well as vehicles also.e.g., oxygen sensor is used in the exhaust pipe of vehicle to provide burnet gases information to ECU (electronic control module) which control the ratio of air and fuel in cylinder for better combustion.This oxygen sensor makes a close loop circuit for better fuel efficiency in the automobiles.Metal oxide sensors are having properties like low cost, fast response time, selectivity, sensitivity and can be designed to work at any conditions (toxic or high temperature environment).Metal oxide materials show the poor response to the gases hence it required a lot of research to tune such materials which are highly sensitive as well as selective for a particular gas.Researchers have to search for different materials for a different gas sensing at low temperature as well as with high sensitivity and quick response.Industrial, health sector as well as domestic level demand of such sensors provide a space for researchers to develop extremely sensitive, selective, low powerconsumption metal oxide materials.

Figure 1 .
Figure 1.Application of nanotechnology in different fields.

Figure 2 .
Figure 2. Classification of sensors based on principle of operation.

Figure 4 .
Figure 4.The n-type and p-type metal oxide materials used for gas sensors till December 2023.

2 −− 2 −−
− °C and ionize them to O O , 2 − − and O 2− by taking electrons near the surface of sensor.The ionosorbed species of O O , and O 2− become dominant at temperature 150 like Cl 2 , CH 4 , CO, ethanol, etc, adsorb oxygen from the sensor's surface and extract the electron from the bulk to ionize into O and O which increases the resistance.The reducing gas reacts with the adsorbed O − and releases the electron to the conduction band decreasing the resistance.The change in resistance is proportional to the gas concentration.The adsorption of test gas in the sensor surface and ionization of oxygen from air can be expressed as

Figure 5 .
Figure 5. Top to bottom and bottom to top nanoparticles synthesis techniques.

Figure 9 .
Figure 9. Schematic representation of gas sensing mechanism for nickel ferrite. 1 having some Ni 3+ and 2 don't have Ni 3+ .Hence response of 2 is higher than 1.

Figure 10 .
Figure 10.The schematic setup for metal oxide gas Sensing.
cations equally occupied in both tetrahedral and octahedral sites, whereas Me 2+ cations occupied in octahedral sites.ZnFe 2 O 4 , NiFe 2 O 4 , and Ni 1−x Zn x Fe 2 O 4 are the common examples of normal, inverse and mixed spinel structures respectively.

Table I .
Advantages and disadvantages of different types of sensors.

Table II .
Metal oxide gas sensors with materials and other parameters.Table II.(Continued).Table II.(Continued).
Keeping all in view, the sensors to monitoring of NH 3 gas concentration in the environment is necessary to avoid accidents.Research papers based on NH 3 gas sensor have been taken for the study of the parameters of sensors as listed in TableII.These sensors are showing small response/recovery time for low concentration of NH 3 gas. 163 1687The formation of oxygen ions with temperature is given bellow:168 169n the atmosphere, oxygen present is 400 ppm.Hence all the sensors sensing the O 2 gas above 400 ppm.Optimum temperature and response/ recovery time needs to tune such kind of sensors.Acetylene (C 2 H 2 ) gas.-Acetylene (C 2 H 2 ) Gas is toxic as well as highly flamable.The chemical reaction betweenacetylene gas molecules and oxygen ions can be representedas:169