Top Ten Books on Research Methodology

 *¹Rajendrakumar B. Ahirrao,²Sudam D. Chavhan,³Pankaj M. Nahide

*¹Dept.of Physics,Uttamrao Patil Arts & Science College,Dahiwel,Dist.-Dhule,¹Department of Physics, S.G.Patil,Arts,Commerce and Science College,Sakri, ³Department of Physics,Government Polytechnic College, Nandurbar.

Abstract: Efficient air quality monitoring necessitates sensors capable of detecting gaseous pollutants at extremely low concentrations, reaching parts per billion (ppb) and parts per trillion (ppt) levels. These sensors must demonstrate high sensitivity, selectivity, and rapid response/recovery times. Metal oxide semiconductor (MOS) sensors, particularly those employing Zinc Oxide (ZnO), hold significant promise due to their cost-effective manufacturing processes and versatility in detecting various target gases. MOS sensors function by adsorbing target gas molecules onto their surface, leading to electron capture from the conduction band and consequent changes in conductivity. This review synthesizes the latest experimental insights into Metal Oxide gas sensors, focusing prominently on Zinc Oxide (ZnO) thin films. Sensitivity enhancement strategies involve nanostructuring MOS materials to augment porosity and surface-to-volume ratio, while selectivity can be fine-tuned through chemical composition modifications. The paper critically assesses diverse techniques for nanostructuring MOS materials using chemiresistive elements and elucidates how these techniques contribute to tailoring the desired characteristics of gas sensors.

Keywords: Gas sensors, Metal oxide semiconductors, Zinc Oxide (ZnO) thin films, Sensitivity enhancement, Selectivity tuning, Nanostructuring techniques, Chemiresistive materials.

1.Introduction: Poor air quality has become a global concern. No matter who we are, where we live or the state of our health, the quality of the air we breathe each day affects us. Even when we can't see it or smell it, air pollution can still be a threat.  There are many different types of air pollutants – particulate (PM2.5, PM10), greenhouse gases (CO₂, CH₄) and toxic gases (Volatile Organic Compounds (VOCs), CO, NOx, SOx, H₂S).  Although industrial modernization facilitates social, cultural, and economic development globally, it results in the consumption of enormous amounts of energy every day. Consequently, the toxic gas and vapor emissions released into the atmosphere cause air pollution, which has been identified as a major concern in most countries.According to a WHO report, 92% of the global population lives in cities with air pollution exceeding the WHO limits, indicating the need for further monitoring and control of air pollutant gases and vapors [1].

The gas sensors play great importance in determining various gases as they can convert a certain gas volume fraction into electrical signals. The gas sensors expected to be low cost, fast response, high sensitivity, and high selectivity. The gas sensors have been widely used in environmental monitoring, air quality monitoring, vehicle exhaust monitoring, medical diagnosis, food/cosmetics monitoring, and many other fields [2–9].

Conventional gas analyzers, such as mass spectrometry, energy spectrometry and chromatography, are limited by the high cost of the devices [10]. Compared with the analyzers, gas sensors based on metal oxide semiconductors are widely used due to their small size, easy operation, low cost and other benefits. Among the main characteristics of gas sensors, sensitivity and selectivity are the main objects of research. However, as one of the indicators which the costumers are concerned about, stability is not often discussed.

Semiconductor Metal Oxides are widely explored chemiresistive gas sensors [11]. Mostly gas sensors can be of electrochemical or metal-oxide semiconductors in thick-film or in thin film [12]. Metal oxides are favorable for usage in different aspects in domestic, industrial and commercial applications [13]. Applications of gas sensors are seen in wide aspect like monitoring the air quality, detection of toxic and harmful gases, medical field etc. [14].

MOS (Metal Oxide Sensors) sensors detect concentration of various types of gases by measuring the resistance change of the metal oxide due to adsorption of gases. Atmospheric oxygen residing on the MOS surface is reduced by the target gases, allowing more electrons in the conduction band of the metal oxide material. This resistance drop is reversible and varies depending on the reactivity of sensing materials, presence of catalyst materials and working temperature of the sensor.

Many metal oxides are suitable for detecting combustible, reducing, or oxidizing gases by conductive measurements. The following oxides show a gas response in their conductivity: Cr₂O₃,Mn₂O₃, Co₃O₄, NiO, CuO, SrO, In₂O₃, WO₃, TiO₂, V₂O₃, Fe₂O₃, GeO₂, Nb₂O₅, MoO₃, Ta₂O₅, ZnO,La₂O₃, CeO₂, Nd₂O₃ [15]. Metal oxides selected for gas sensors can be determined from their electronic structure. As the electronic structure is the key concern to select metal oxide for sensor [16] the variety of metal oxides are divided into following categories[17].

(1) Transition-metal oxides (Fe₂O₃, NiO, Cr₂O₃, etc.)

(2) Non-transition-metal oxides, which include (a) per-transition-metal oxides (Al₂O₃, etc.) and (b) post-transition-metal oxides (ZnO, SnO₂, etc.).

 

Fig.1. Shows Classification of MOS

But metal oxides having d⁰ and d¹⁰electronic structure is potential enough in the field of gas sensing application. Binary transition metal oxides, TiO₂, WO₃ are having d⁰ and post transition ZnO, SnO₂ are d¹⁰ configured. Some important definitions must have to know as shown in Table.1.

Table:1. The main parameters of a gas sensor.

Parameters	Description
Response	It is defined as a change in some physical properties when the device is exposed to target species.
Selectivity	It is the ability of a gas sensor to detect high sensitivity to a specific gas among various types of gases at the same concentration level.
Sensitivity	It is referred in the graph where slope represents the correlationbetween gas response and the partial pressure of target gas.
Limit of detection	It is the lowest and highest concentration of the target gas that thesensor can detect.
Limit of detection	It is the highest gas concentration that the sensor can detect.
Operating temperature	It refers to the maximum temperature at which the device exhibits its maximum sensitivity in the presence of a target gas.
Repeatability	It is the response cycles of a sensor to be exposed to an analyte gas flow for a long time.
Response time	It is usually defined as the time it takes for gas sensor to respondto a concentration change.
Stability	It is the ability of gas sensors to conserve the output response measurement by a period, the level concentration of gas (ppm) unchanged.
Recovery time	Time measured when the gas sensor response changes in the interval of 90% to 10% when the sensor is exposed to a full-scale concentration of the gas, implying that the sensor exhibits 90% of the saturation value of resistance in seconds.

 

 

2. Historical Background of ZnO:

Impurity in semiconductor alters the various properties of pristine material like resistance ,conductivity etc. In 1953, this effect was demonstrated for Ge [18].Later, it was shown that the conductivity of ZnO thin films heated to ~300°C was sensitive to the presence of traces of reactive gases in the air [04]. Similar properties were reported for SnO₂, with higher stability [19]. These results initiated further development of commercial gas sensors. The early metal oxide-based sensor materials possessed a number of unpleasant characteristics, such as high cross-sensitivity, sensitivity to humidity, long-term signal drift and, slow sensor response. In order to improve sensor performance, a series of various metal-oxide semiconductors have been tested [20] .After testing various materials the most successful investigations were connected with SnO₂, ZnO, and TiO₂. Parallel to this approach, the basic research of metal-oxide materials was carried out in scientific laboratories.

ZnO is widely used as a gas sensing material for detecting the toxic and harmful gases because of its superior physicochemical properties. Firstly, ZnO is an n-type II-VI semiconductor with wide band gap (3.37 eV), large excitation binding energy (60 meV).

The ZnO-based gas sensors have been widely used due to fast response, low detection limit, high selectivity, reliable performance and low manufacturing cost. ZnO belongs to surface sensitive material. When a sensing element is prepared by using ZnO, the gas sensing mechanism can be summarized as adsorption-oxidation-desorption [ 21]

The most extensively explored gas sensors are metal oxide semiconductor (MOS) materials owing to their high selectivity, stability, cost-effectiveness, and simple synthesis techniques[22].

 

Fig.2. Adsorption-oxidation-desorptionmechanism in ZnO.

3.     Properties of ZnO

3.1. Physical Properties of Zinc Oxide

Zinc white crystallises mainly in two forms viz cubic zinc blende and hexagonal wurtzite. The most common and stable structure under ambient conditions is wurtzite. Zincblende can be stabilised by growing zinc oxide on substrates which have a cubic lattice structure. The oxide and zinc centres are tetrahedral.
3.2. Chemical properties of Zinc Oxide

Calamine is a white solid which is insoluble in water and has no smell. Crude zinc oxide appears yellow-grey in colour and exists in a granular solid form with no odour. In its natural form, it is obtained as a mineral zincite which consists of manganese (Mn) and some other impurities which make it appears yellow-red colour. In its crystalline form, it is thermochromic which on heating in the presence of air changes its colour from white to yellow and on cooling it turns white in colour. It is an amphoteric oxide which is insoluble in water. It dissolves in acids and alkalis.

Zinc sulfied, Zinc selenide, Zinc telluride are the other related compounds. The future for high quality Zinc Oxide is certainly going to be fascinating. The potential advances for non-medicinal applications even surpass that of the current medicinal uses. Zinc Oxide Nanorod Sensors, Spintronics, and Piezoelectricity are all very promising fields and ones to keep an eye on in the not too distant future.

4.     Synthesis of ZnO thin films

 

Fig.3. The different synthesis methods of ZnO nanostructures.

There are different methods have been established to synthesize ZnO nanostructures.we proposed the spray pyrolysis [23,24] method for the synthesis of thin films.

 

 

 

 

 

 

 

Fig.4.Schematic sketch of spray pyrolysis system set up.

Numerous materials have been prepared in the form of thin film because of their potential technical value and scientific curiosity in their properties. A number of techniques have been examined in the search for the most reliable and cheapest method of producing thin films.

Fig.5. Schematic sketch of spray pyrolysis system set up.

Spray pyrolysis (SP) technique was initially suggested by Chamberlin and Skarman [25] in 1966 to prepare CdS thin films on glass substrates. Spray pyrolysis involves spraying of an aqueous solution containing soluble salts of the constituent atoms of the desired compounds to the heated substrates. The liquid droplets vaporize before reaching the substrate or react on it after splashing. Doped and mixed films can be prepared very easily, simply by adding to the spray solution a soluble salt of the desired dopants or impurity.

4. Experimental Study of ZnO based gas sensor.

ZnO is an MOS that has been extensively studied as a gas sensor. However, several research groups are investigating the potential of adding elements in the crystal lattice of ZnO as doping materials. The research outcomes revealed that the doping elements change the ZnO structure. For example, doping elements decrease the crystallite size, increase the crystallinity, and modify the ZnO morphology [26-30]. The structural and morphological changes increase the surface-to-volume ratio, creating a more active center at the grain boundaries. Consequently, the sensitivity [31], response/recovery time, selectivity, and working temperature [32] are improved, which, in terms of practical applications, are desirable. Therefore, doping is a viable way of improving the ZnO sensing properties [33]. Given this, various types of doped ZnO gas sensors have been developed [34].

 

 

4.MOS thin film gas sensor:

Grain size is one of the component in nanostructured MOS thin films which has important role in thin film sensors.The films without and without dopant reported small grain size [35].Thin films with smaller grain size are favorable as the increased surface to volume ratio, carrier concentration and enhanced catalytic activity, facilitate its interaction with larger number of gas molecules.

    The thin films synthesized from the different techniques like chemical and physical vapour deposition techniques, etc. [36-39] which includes the electrical, optical and magnetic properties in this context the growth mechanism of thin film is leading to variance in nucleation, growth direction, crystallographic orientation , packing density, interfacial energy and diameter of the crystallite.

The several growth parameters has been reported [40-41] but it was taken long years to understand the thin film growth. Besides the nature and properties of the thin film, the environmental conditions such as operating temperature and humidity determine the sensing performance and characteristics of the thin film gas sensors.The operating temperature at interface determines the sensitivity, stability, response and recovery times [42].Also humidity play important role in sensors response, as the water vapour absorbed on the metal oxide surface affects the electronic and ionic conducting properties of the semiconducting metal oxides.The several terms like gas sensing principles, response and recovery time (defined above),structural parameters,nanostructures, environmental gases also the theory behind the gas sensing principle number of references are available for these terms [43-61].Plenty of literature is published on these topics ,but now a days greater attention is required on the sensing behaviour MOS materials.

Fig.6.Schematic diagram showing a particulate-based thin film deposited on an interdigitated substrate, and the interaction of the grain boundaries after the gas species adsorb on the metal oxide surface.

Left:Illustration of a chemiresistive gas sensor consisting of the metal oxide semiconducting nanomaterials (orange film) deposited on an interdigitated substrate with two finger electrodes providing connections to a circuit for resistance measurement;

Middle: Microstructural characteristics of the MOS film showing the grain boundaries (orange layers),

Right: Ambient oxygen species adsorb on the metal oxide surface, depleting the electrons from the conduction band throughout the material.

5.Parameters of MOS gas sensors

5.1.Stability

The stability is one of the aspects of the sensor.Heating mode affects the stability of sensor. Humidity, operating temperature, target gas, etc.are the factors affecting the stability of the sensor [62]. Continuous high-temperature heating may cause an unexpected crystal growth of sensing materials.

Fig.7. Influencing factors of stability.

The life of sensor depends on several factors the material stability,environmental conditions (humidity, temperature, etc.)and target gas (reducing or oxidizing gas, gas concentration, etc.) are major influencing factors.[63].

However to assess sensor stability, the following two indexes must be tested Stability of the conductivity, which is also known as the sensor’s baseline and Stability of the response. Therefore, from metal oxide sensing materials to advanced devices, the operating stability of gas sensors should be a necessary evaluation index.In abstract five factors determine the stability of semiconductor metal oxide sensors sensitive materials, element doping, ambient humidity, poisoning and component composition of gas sensors.The figure shows key factors impacting stability of sensor.For metal oxide semiconductor gas sensors, the exchange of charge carriers in the gas adsorption and desorption is the cause of the electrical signal change. When the gas-sensitive layer reacts with gas, the conductivity of the film varies. The rate of change in conductivity is related to the concentration of the test gas. Subsequently, the electrical signal is transmitted. The material microstructure needs to be stabilized to maintain a stable sensing performance. Sensitive materials, element doping, ambient humidity and component composition of gas sensors.

5.2.Sensitivity:

Sensitivity can be defined as the change in resistance of the material or the current when the chemical or gas is exposed. It can be mathematically defined as:

The sensor response for n-type semiconductors is defined by

and    R_gas> R_air  (oxidizing gases)

        R_gas< R_air  (reducing gases)     

for oxidizing and reducing gases, respectively.

The effective response is determined by measuring response and recovery time. Response time is defined as the time required to reach the 90% of the maximum value of current during the gas/chemical sensing state. Recovery time is defined as time taken for the the current to reach 10% of its maximum value when the environment switches to ambient air conditions. This definition of sensitivity always leads to positive and comparable values with S = 0 occurring in the case of reference air or in the presence of an undetectable gas species

5.3.Selectivity

Selectivity is the ability of gas sensors to identify a target gas among other different gases. For air quality sensors, selectivity is an important parameter because myriad of gaseous species that are present at the ppb or ppt levels in the ambient air could induce changes in the resistance of the MOS in a similar manner with the target gas. Evidently, for species that are typically in high concentrations in the atmospheric environment (i.e., higher than a few tens/hundreds ppm; e.g., CO₂ and H₂O) it is much easier to develop selective gas sensors as not many other atmospheric compounds are so abundant [64]. For target species that are in the ppb range, selectivity can be improved by modulating the physical (e.g., metal oxide grain size, operating temperature), or the chemical (e.g., doping, surface functionalization) properties of the MOS, using similar strategies to those for enhancing sensor sensitivity. Adding metallic nanoparticles on the surface of the MOSs can provide another means of improving selectivity, among other properties of the sensors. For example ion reduction can be used to form functionalizing nanoparticles on the surface of metal oxides, similarly to the method reported by Wali et al. [65-66].

5.4.Operating Temperature:

As the temperature increases, enhanced thermal motion of the target gas molecules leads to an increase of their diffusion to the bulk MOS. In addition, chemisorption is preferred over physisorption at lower temperatures, forming strong target gas-MOS surface chemical bonds that promote adsorption. This enhanced interaction increases the resistance changes of the MOS and thus the sensitivity of the sensor (cf. Fig. 2). As temperature is further increased, thermal motion of the adsorbed species increases desorption rate, which in turn decreases the sensitivity [67]. Given that these two competing processes have opposite temperature dependences, their dominance determines the optimum operating temperature of the sensors [68].

Fig. 8. Dependence of the sensitivity of MOS gas sensors on the sensor operating temperature.

An increase in temperature initially increases the adsorption (chemisorption and physisorption) of gas species. However, after a specifc threshold (which varies depending on the MOS and the target gas molecule) the high thermal motion of the adsorbed species promotes desorption, which in turn decreases sensor sensitivity.

6.Advanced static Gas sensing system:

 

 

 

 

 

 

 

Fig.9: (a) Circuit used for the gas sensing mechanism (b) Photograph of Advanced gas sensing system which we going to use.

In a static gas-sensing measurement system , the sensor is placed in a gas chamber with an adjustable temperature and humidity. During the gas-sensing test, a predetermined amount of the target gas is injected into the test chamber through the gas inlet by a gas-injection unit. When the sensor resistance becomes stable, the gas chamber is opened for the sensors to recover in air. This procedure is repeated for different gas concentrations and at different temperatures, and a curve indicating the variation of the sensor resistance vs time in different atmospheres is obtained.

6.1.Sensing Mechanism in MOS:

The gas sensing mechanism of MOS is based on the oxygen adsorption model, which assumes that the change in resistance is related to chemisorbed oxygen. In air, oxygen molecules adsorb on the MOS surface and form negatively charged chemisorbed oxygen (, O⁻, O²⁻) by trapping conduction band electrons.The type of chemisorbed oxygen is related to the operating temperature and species of MOS material, which signifcantly determines the sensing performance of the sensing mate‑ rial . However, the temperature interval corresponding to the presence of chemisorbed oxygen ions on metal oxides is not well known and varies from different metal oxides.

Fig.10.Gas sensing function and conduction mechanism for MOS. where the oxygen and carbon monoxide gas can penetrate to interact with each grain.

In general, is usually chemisorbed when the temperature is below 100 °C. Once the temperature is between 100 and 300°C,O⁻ is generally chemisorbed and disappears rapidly. And when the temperature exceeds 300°C, the chemisorbed oxygen is mainly in the form of O²⁻.

In air ∶ O₂(gas) → O₂(ads)

T < 100 ^◦C ∶ O₂(ads) + e− → (ads)

100 ^◦C < T < 300 ^◦C ∶ (ads) + e− → 2O⁻(ads)

T > 300 ^◦C ∶ O⁻(ads) + e− → O²⁻(ads)

As a result of oxygen adsorption, an electron depletion layer with a low electron concentration is formed on the surface of the n-type MOS, which has a higher resistance than the core region due to the reduced number of electrons. While on the surface of the p-type MOS, a hole accumulation layer is formed, which has a lower resistance than the core region of the MOS due to the increased number of holes. As a result of these processes, the resistance of the sensing layer will change signifcantly, which will result in the response of the gas sensor.

The oxygen molecules interact with the metal oxide surface as oxygen anions from the depletion layer around the grains and thereby increases potential barrier. When target gas molecules interact with the metal oxide grains, oxygen anions is desorbed, which makes the majority carrier concentration change inside the oxide layer, which thereby changes conductivity [69].

Fig.11.The general gas sensing mechanism of (a,b) n-type and (c,d) p-type MOS gas sensors in the presence of air and an oxidizing gas.

It is well accepted that the variation of the resistance in the presence of the target gas is the basic mechanism of gas detection by resistive gas sensors.A schematic of the sensing mechanism for n- and p-type MOS sensors in the presence of an oxidizing gas is shown in figure 11. Because of the high electronegativity of oxygen and the extraction of electrons by oxygen species on the surface of the gas sensor, an electron depletion layer is formed on the surface of n-type gas sensors, and a hole accumulation layer is formed on the surface of p-type gas sensors, as shown in figure 11 (a),(c) respectively. When an n-type sensor is exposed to an oxidizing gas, electrons are further extracted, leading to an increase in the width of the electron depletion layer and an increase in the sensor resistance (Fig.11 (b)). In a reducing gas atmosphere, the width of the electron depletion layer decreases, resulting in a decrease in the sensor resistance. Upon exposing p-type MOS gas sensors to an oxidizing gas, more electrons are extracted, resulting in the expansion of the hole accumulation layer and a decrease in the sensor resistance (Fig.11 (d)). For reducing gases, the inverse is true.

7.Factors Affecting the Sensitivity of MOS Gas Sensors

7.1.Effect of Substrate:

The silicon, quartz, glass, and aluminum are the substrate which are used for deposition of the film,which directly affects sensitivity of sensor.Thus, the use of silicon substrate to precipitate films on it gives special different properties to oxides of semi-conductivity materials due to its influences on surface electric charges. Then, its effects on gas interaction with the sensor makes it required in the industry of gas sensors. The silicon etching process is one of the used techniques in the improvement of sensor properties due to the increase of surface area where it determines the regulation of pore size. The glass substrate is known for its optical properties as it has an absorption approach to zero and high transmittance within the visible area so it is used as a substrate to study the optical properties, constructive, and the sensitivity to the prepared film. Also, the proposed silicon acts to improve the sensor. The researcher Rahman et al. studied the sensitivity of the engrafted nano silver oxide prepared by the solution method. The substance was deposited on glass carbon electrodes to give sensitivity with a quick response to methanol in the liquid phase and it gave good and stable allergic results.

7.2.Morphology

The surface morphological studies of the films were analyzed by AFM and SEM techniques.The fig.4dipicts the SEM and AFM of Indium doped ZnO thin film prepared by spray pyrolysis technique. A top view of the images clearly shows that the results are in good agreement with each other for undoped and Indium doped films.

 

 

 

 

 

 

 

Fig. 12. SEM and AFM micrographs of (a) undoped, (b) 5 wt%, (c) 10 wt%, and (d) 15 wt% IZO thin films

The Root mean square values (RMS) of the surface roughness of the films were computed by Nanoscope Analysis using AFM micrographs. Actually Root Mean Square of a surfaces measured microscopic peaks and valleys. The values are in the range 8 to 28 nm indicating the smooth nature of the films. The RMS roughness values are 28.4 nm for undoped, 10.6 nm for 5 wt%, and 10 wt% and 8.18 nm for 15 wt% IZO fifilms. The grains in undoped film exhibit pea-shaped structure and upon increasing indium concentration, the films are taken hexagonal shape as evident from SEM analysis. The 15 wt% IZO film has a large surface area and many grains with well-defifined grain boundaries compared to other films. Consequently it provide more active for gas adsorption which in turn boosts the response toward the target gas [70]. The impact of nanograins on the gas-sensing properties of ZnOnanorods is reported by Kim et al. [71]. The grain boundaries seeks the obstacle in flowing the electron since they act as potential barriers. In the presence of gas, the grain boundaries disappear and the resistance decreases which results in the high resistance modulation along the grain boundaries so geometric consideration involves in sensitivity.Grain size reduction leads to the enhancement of surface area thereby enhancing the performance of the sensing mechanism.In smaller grain size particle, nanoparticles are mostly within the depletion area which increases the sensitivity. And active area of reaction becomes more when the surface area is large. It is also significantly enhances the sensor response. For an example SnO₂ based gas sensor response drastically changes when the particle size becomes smaller than 10 nm [72].

The Control Synthesis of Nano or microsized Particle matters the sensitivity.Depending on the metal oxide semiconductor specific crystallographic orientation gives better result in sensitivity.In an example ZnO, (002) crystallographic orientation shows better sensitivity [73]. Surface to volume ratio is also very important to achieve better sensitivity it depends on grain size and porosity of structure.The ZnO is synthesized and samples are characterized by scanning electron microscope (SEM) which is shown in figure 13(a) and (b) Modification of surface morphology is done by chemical treatment and varying annealing temperature. Figure 4 confirms the formation of hexagonal shape ZnO.Hexagonal structure with high surface to volume ratio results good sensitivity.

 

 

 

 

 

 

 

Fig.13.(a) SEM image of ZnO before chemical treatment (b) SEM image of Hexagonal ZnO nano

The ZnO is an MOS that has been extensively studied as a gas sensor. However, several research groups are investigating the potential of adding elements in the crystal lattice of ZnO as doping materials. The research outcomes revealed that the doping elements change the ZnO structure. For example, doping elements decrease the crystallite size, increase the crystallinity, and modify the ZnO morphology.

7.3.Grain Size and Porosity

In gas sensors, semiconductor nanoparticles are connected to adjacent particles through grain boundaries to form larger aggregates [74].Since the transport of electrons between grains needs to pass through the electron depletion layer, the grain size has a great influence on the conductivity, which in turn affects the gas sensing performance of the material-based gas sensor.

Figure 14. Schematic model of the effect of the crystallite size on the sensitivity of metal-oxide gas sensors: (a) D >> 2L, (b) D≥ 2L, and (c) D < 2L. Reprinted with permission from Ref. [Z]. Copyright 1991 Elsevier.

Let D = Particle size

L= Thickness of the electron depletion layer

(a) If D >> 2L

There is a wide electron channel between the grains, and the gas sensitivity of the material is mainly controlled by the surface of the nanoparticles (boundary control).The change in electrical conductivity depends not only on the particle boundary barrier but also on the cross-sectional area of the channel, and the gas sensitivity of the material is mainly controlled by the contact neck between nanoparticles (neck control)

(b).If D < 2L

The electron depletion layer dominates, the entire nanoparticle is contained in the electron depletion layer (grain control), and the sensitivity of the material is very high. The energy bands are nearly flat throughout the interconnected grain structure, and    there is no significant impediment to the inter-grain charge transport.The small amount of charge gained from the surface reaction results in a large change in the conductivity of the entire structure. Therefore, the smaller grain size is beneficial to improving the sensitivity of the gas sensor. When the grain size is small enough, the crystal becomes very sensitive to the surrounding gas molecules.Smaller is grain size and larger porosity of structure quicker is gas response of the sensor [75].

7.4. Chemical Composition

Sensors based on the two components mixed together are more sensitive than the individual components alone suggesting a synergistic effect between the two components. The choice of the synthesis method and the composition of mixture of precursors used in the synthesis strongly affect the surface of metal oxide and hence its sensing characteristics.

7.5.Temperature and Humidity

Temperature is also a major factor for the metal oxide gas sensors. The gas sensor responses increase and reach their maximums at a certain temperature, and then decreased rapidly with increasing the temperature. This is a common tendency seen in many reports [76-79].In ZnO based sensor the maximum response is found at 200-300⁰C. Sensitivity decreases below or above the specific temperature.

Humidity is an important issue in gas sensor performance. Humidity sometimes lowers the sensitivity. Baseline resistance of the gas sensor decreases when the reaction between the surface oxygen and the water molecules occurs, [80]. So, the sensitivity is decreased. Adsorption of water molecules causes less chemisorption of oxygen species on the ZnO surface as surface area gets decreased. And it effects on sensitivity. Water molecules are basically the barriers against target gas adsorption. So response recovery times increases and sensitivity decreases.

7.8. Doping of Metals

Conductivity is measured by the efficiency of catalytic reactions with the target gas on surface of the sensing material. Doping is an important method to improve gas-sensing performance.Different dopant species may lead to different types of crystallite, defects and electronic properties. Conductivity is measured by the efficiency of catalytic reactions with the target gas on surface of the sensing material. So, the catalytic activity is also a major issue to increase the sensor performance. Doping is the process of addition of different metals in to pure materials.Doping or surface modification by adding metal elements (such as Ag,Au, Pt, Pd, etc.) on the surface of the gas-sensing materials can increase the number of active sites, promote the adsorption/desorption reaction on the surface of the gas-sensing materials, and reduce the reaction activation energy, and reduce the operating temperature, thereby improving the gas-sensing performance [81]. Nobel metal nanoparticle behaves like a catalyst and reduces the activation energy resulting the improved molecular dissociation and reaction [82].

The addition of suitable dopants with appropriate host materials [83]; grain size in the metal oxide thin films can be controlled.Reducing crystallite size might have influenced on the width of space charge region which, aids the chemisorption of gas molecules and sensing process [84]. The mechanism of sensitization or sensitivity (response) enhancement by additives are found to be different in different metal oxide materials.

According to Yamazoe there are to two types of sensitization mechanisms viz chemical and electronic.If we consider the example of Pt doped SnO₂ the chemical sensitization; the pt residing on the metal oxide surface facilitates the chemical reactions between target gas and metal oxide surface through the spill- over mechanism as shown in figure 15 .On the other hand in an example of Ag doped SnO₂  the electrical property of the metal oxide is changed through the change in redox state of the promoter, by acceptor or donor charges from gas molecules in electronic sensitization.

TYPE	Chemical	Electronic
Model
Role of noble Metal	Activation and spill-over of sample gas	Electron donor or acceptor
Origin of gas sensitive property	Change of absorbed oxygen concentration	Change of oxidation state of noble metal
Example	Pt-SnO2	Ag2-O-SnO2,PdO-SnO2

Fig.15. spill over mechanism

Dopants in the metal oxides lowers the energy which is required for chemisorption of gas molecules in the semiconductor surface which in tuners enhances the gas sensitivity therefore adding a suitable dopant can stabilize a particular valence state and increase the electron exchange rate or stabilize the metal oxide against reduction” [85].The choice of the synthesis method and the composition of mixture of precursors used in the synthesis strongly affect the surface of metal oxide and hence its sensing characteristics [86].

 

 

8.Effect on ZnO Sensitivity on doping :

The Ga is another promising metallic element that can be used as a dopant in ZnO-based sensors. Hjiri et al. reported that, incorporation of Ga into the crystal lattice of ZnO notably improves the sensitivity, because it decreases the crystallite size.The increase in sensitivity is because of the decrease in the average crystallite size (approximately 49 nm) of the Ga-doped ZnO nanoparticles compared with that of the undoped ZnO nanoparticles.The advancement compared with the undoped ZnO nanoparticles, because the reduction in crystallite size helps increase the surface area[87].

 In another work, Hjiri et al. analyzed a sensor with similar characteristics. They doped ZnO with different concentrations of Ga (1, 3, and 5 at. %). Figure 5 shows the sensitive values of the Ga-doped ZnO nanoparticles. The maximum sensitivity of the Ga-doped ZnO sensor can be observed at 250^◦C (1 at. %). Thereafter, the sensitivity decreased when the operating temperature increased. A change in the morphology of the particles with increased Ga concentrations was observed.The study also concluded that the solubility limit was 3 at. % Ga in ZnO. When this doping limit is exceeded, the Ga atoms distort the lattice until they cause phase segregation.

Fig.16. Response to 50 ppm CO of the Ga-doped ZnO nanoparticles as a function of the temperature.

TheGa concentration of samples correspond at 0, 1, 3 and 5 at. %. Obtained from reference[88]. Dhahri et al [89] reported that In was used as dopant, In-doped ZnO sensor under a CO gas environment in comparison with the sensitivity of the undoped ZnO sensor. This behavior has been attributed to the most active adsorption sites, assuming the substitution of the Zn²⁺ cation by In³⁺.The authors concluded that the incorporation of In in the crystal lattice changes the oxygen stoichiometric of ZnO and influences the sensing behavior of doped ZnO.

9. Gas-Sensing Devices:

According to different sensing materials and different methods gas sensors can be classified as electrochemical, catalytic combustion, infrared absorption, thermal conductive, solid electrolyte, paramagnetic, and metal oxide semiconductor sensors  [90], depending on the change in electrical resistance with different materials and their compositions. Semiconducting metal oxide, polymers, carbon nanotubes (CNTs), graphene, etc., are mostly used (Figure 17).

 

Fig.17.Semiconductor gas-sensing materials, “Reprinted/adapted Ref.[35] Copyright 2020, Nikolic, M.V. et al.

Gas sensors can also be classified based on optic, acoustic, calorimetric, and chromatography methods.According to Comini [91], a gas sensor can be classified based on the measurement methods, such as (1) DC conductometric gas sensors, (2)  photoluminescence (PL)-based gas sensors, and (3) field effect transistor (FET)-based gas sensors. Table 1 illustrates the comparison of various gas sensors studied by Korotcenkov [92]. Target gas detection by a metal oxide semiconductor has gained wide attention because of its various advantages over other types of sensors. Electrochemical gas sensors have become unpopular due to their short lifespan, limiting their use to only a few applications. Although an optical gas sensor has good sensitivity, an adequate lifespan, good selectivity, and fast response, the cost is high and the size is large. Although metal oxide semiconductor gas sensors are less selective, the low cost and simplicity in fabricating them are the main factors that contribute to their wide usage.

Table 2.Comparison of various types of gas sensors. Reprinted from Ref. [37]. Copyright 2007, Korotcenkov, G.

Type of Gas Sensors
Parameter	SMO Gas Sensors	Catalytic Combustion Gas Sensors	Electrochemical Gas Sensors	Thermal Conductivity Gas Sensors	Infrared Absorption Gas Sensors
Sensitivity	E	G	G	P	E
Accuracy	G	G	G	G	E
Selectivity	F	P	G	P	E
Response Time	E	G	F	G	F
Stability	G	G	P	G	G
Durability	G	G	F	G	E
Maintenance	E	E	G	G	F
Cost	E	E	G	G	F
Suitability to portable Instruments	E	G	F	G	P

E:Exultant,G:Good,P:Poor.

Conclusion:

The review is attempt to study the chosen metal oxide gas sensor(MOS) such as ZnOfor the detection of various gases. The paper survey revealed the different factors for efficiency enhancement of the gas sensor. Subsequently, the sensing mechanism of metal oxide gas sensor has been described to understand the requirement of gas sensor to detect toxic gases. Some other factors which affect the sensitivity of metal oxide gas sensor have also been described. These are grain size, doping with a suitable dopant, effect of surface defects, gas diffusion, effects of heterojunction and thin film-based gas sensor. For materials, the main factors that affect the stability are the uncontrolled grain growth during the service period and the irreversible reactions between the surface and some molecules in the environment or target gases.In brief, sensing properties mainly rely on the surface reaction.It is investigated that hexagonal structural morphology can be obtained by doing chemical treatment and varying annealing temperature. Doping of nobel metal additives with high catalytic activity may enhance the sensitivity because of “spillover effect”.Crystal growth at specific orientation has major effect on sensitivity. On other hand temperature and humidity are also playing an important role in sensitivity. Humidity drastically lowers the sensitivity.

The brief introduction is suitable for the topic chosen.The  review provided an outline of MO ZnO gas sensors. The important parameters of the Metal Oxide which make it ideally suitable for application for gas sensors.Spray pyrolysis technique  for deposition of ZnO thin films described in detail. In this review, the main characteristics of gas sensor are firstly introduced, followed by the preparation methods and properties of ZnO. In addition, the development process and the state of graphene gas sensors are introduced emphatically in terms of structure and performance of the sensor. The doping of material enhances the performance of gas detection significantly. Finally, the clear direction of MO gas sensors for the future is provided according to the latest research results and trends. A brief study on the factors that affect the sensitivity, selectivity and stability of the MO gas sensor was carried out. The doping were more commonly employed ways to improve the sensor performances mentioned significantly.From the study, it is evident that a fair amount of the work was done on the Metal oxide thin film gas sensors in particularly ZnO gas sensor for application. On the whole all sensor types and sensing materials have its limitations such as the sensitivity to all types of target gases and longer response and recovery time. Hence optimizations in terms of fabrication process, sensing material, size, and shape, performance parameters like sensitivity, selectivity, and stability etc. is the key to design a better sensor that will fulfill all our requirements. This review aims to provide recent progress and technology trend on the MO gas sensors for current and future researchers. There is a good scope for the future researcher to perform further work in MO to produce a novel sensor that meets the demands of current and emerging technical applications.It provides direction and ideas for future research.

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