OSHA permissible limit for various air pollutants .
Selective detection of gases such as nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), and various volatile organic components (VOCs) is necessary for air quality monitoring. Detection of hydrogen (H2) is equally important as it is a flammable gas and poses serious threat of explosion when exposed to oxygen gas. We have studied the sensing characteristics of these gases using thin film deposited by chemical solution deposition as well as relatively thicker films deposited by atmospheric plasma spray (APS) process. The chapter starts with the sensing mechanism of chemiresistive sensors followed by the definition of gas sensing parameters. Subsequently, we have demonstrated selective NO2 sensing characteristics of zinc oxide-graphene (ZnO-G) multilayered thin film followed by CO and H2 sensing characteristics of ZnO thin film and SnO2 thick film. Cross-sensitivity among CO and H2 gases has been addressed through the analysis of conductance transients with the determination of activation energy, Ea, and heat of adsorption, Q. The concepts of reversible and irreversible sensing have also been discussed in relation to CO and H2 gases. CO2 sensing characteristics of LaFe0.8Co0.2O3 (LFCO)-ZnO thin film have been elucidated. Interference from CO has been addressed with principal component analyses and the ascertaining of Ea and Q values. Additionally, the variation of response with temperature for each gas was simulated to determine distinct parameters for the individual gases. Further, VOC sensing characteristics of copper oxide (CuO) thin film and WO3-SnO2 thick film were investigated. Principal component analysis was performed to discriminate the gases in CuO thin film. The interaction of WO3-SnO2 thick film with various VOCs was found to obey the Freundlich adsorption isotherm based on which Ea and Q values were determined.
- air quality monitoring
- gas sensing
- thin film sensor
- thick film sensor
- Langmuir adsorption isotherm
- Freundlich adsorption isotherm
- reversible sensing
- irreversible sensing
A recent surge in diseases related to poor air quality has made it mandatory to effectively monitor the level of harmful pollutants in the atmosphere. Poor ventilation in indoor spaces as well as vehicular exhausts and emissions from industries poses serious threats to the quality of breathable air. Indoor air quality monitoring has become essential in order to counter the rising incidence of diseases such as sick building syndrome, which occurs in residential and office spaces due to poor ventilation. Indoor air pollutants are mostly volatile organic compounds (VOCs) which include alcohols such as ethanol and methanol and adverse pollutants such as benzene and formaldehyde. Other than this, carbon dioxide (CO2) and nitrogen dioxide (NO2) also pollute indoor living space .
Outdoor air quality is contaminated mainly due to the presence of carbon monoxide (CO), carbon dioxide, nitrogen oxides (NOX), sulfur dioxide (SO2), volatile organic compounds, and particulate matter. Urban sources of pollutants include vehicles and industries. In rural India, many people still use solid fuel such as wood, crop wastes, charcoal, and cow dung in open fires popularly known as “chulhas.” Such cooking practices are highly damaging to human health as they emit huge quantities of CO and NO2 and cause an array of diseases including but not limited to pneumonia, stroke, heart disease, and lung cancer .
Air quality monitoring is thus mandatory to check the levels of various pollutants in the atmosphere. Agencies like Occupational Safety and Health Administration (OSHA) define the permissible exposure limits (PELs) of air pollutants to protect the health of workers. Table 1 shows the permissible limit of various air contaminants.
|Pollutant||Averaging time (h)||Level (ppm)|
Apart from these pollutants, detection of hydrogen (H2) gas is also important. It is used as fuel in space craft and rockets. It is reported that almost all CO sensing materials are cross-sensitive toward H2 as well. It remains a major challenge to address the cross-sensitivity of CO sensor.
To detect these pollutants, one can use chemiresistive type thin or thick film oxide sensors, made from economic precursor materials. The use of oxide sensing materials in thin/thick film form has advantage over commonly used bulk Taguchi-type sensors. Thin/thick film sensors consume less power and also allow miniaturization of gas sensing systems.
The surface of oxide sensors acts as a catalyst to oxidize reducing pollutants (CO, H2, VOCs, etc.) to relatively benign gases (viz., CO forms CO2). Limited attempt has so far been made to study the gas sensing performance of oxide ceramics together with catalytic activity to oxidize the gas sensed . For gas sensing performance, thin film sensor is preferred; however, for catalytic conversion, relatively thicker coating of oxide materials over a large area surface (often curved) is required.
Understanding of gas–solid interaction is of utmost importance to optimize the sensing performance of chemiresistive oxide sensors. Typically, Langmuir adsorption isotherm has been utilized to understand the resistance transients recorded upon exposing the oxide sensor to test gas environment. Through these kinetic analyses, interrelation among sensing layer thickness, its morphology, response %, sensitivity, and response/recovery times can be addressed adequately [5, 6, 7]. One can also estimate the adsorption and desorption energies of the test gas and reaction products, respectively. The estimated energies could be an effective tool to differentiate various types of test gases . Base line stability of the sensing materials is also important to fabricate commercial sensing elements. Interrelation among the surface morphology, response time, and test gas concentration needs to be understood to fine-tune the base line stability [6, 8].
In view to the above, the primary goal of the present chapter is to describe a comprehensive approach to model the resistance/conductance transients based on Langmuir and Freundlich adsorption isotherms to address cross-sensitivity. Modeling the resistance/conductance transients allows us to estimate sensing parameters (such as response %, response/recovery time, etc.) and adsorption/desorption energies of test gas and reaction products. The estimated parameters help us to better understand the effect of the receptor (adsorption of gas molecule on sensing surface), transduction (electronic conduction pathway), and utility functions (effect of thickness and morphology of sensing materials) controlling the sensitivity, stability, and selectivity criteria of thin film type chemiresistive sensors.
The chapter is organized as follows: In the following sections, we have elucidated the mechanism of gas sensing followed by defining various sensing parameters. Subsequently we have briefed the dominant material characteristics influencing the sensing performance of thin film chemiresistive type gas sensors. Next, we have outlined the synthesis procedure of thin as well as thick film sensing elements. The synthesized materials were characterized in terms of their phase purity and microstructure evaluation. Finally, we have outlined the details of static as well as dynamic flow gas sensing measurements to investigate the sensing performance of the synthesized materials.
We have thus prepared thin and thick film sensors to detect NO2, CO, H2, CO2, and VOCs (ethanol, acetone, and isopropanol). Gas sensing mechanism has been described for each gas. Cross-sensitivity has been addressed through conductance transient analysis. The gas–solid interaction has been observed to follow either Langmuir or Freundlich adsorption characteristics. Based on this approach, the activation energy, Ea, and heat of adsorption, Q, were calculated to discriminate the gases. Stability of the gas sensors was also studied with respect to thin and thick films.
2. Salient features of semiconducting oxide gas sensor
Semiconducting metal oxide (SMO) sensors are attractive for lower cost, smaller size, simpler operation principle, durability, and ease of fabrication together with their low concentration of gas detection limit . These sensors change their conductivity when exposed to test gases of different concentrations . However, the sensor operating temperature (∼300°C) needs to be lowered, and cross-sensitivity toward multiple gases needs to be minimized for its wide commercial adaptation. The operating principle and salient features of SMO-based chemiresistive sensors are briefly described as follows:
2.1 Principal of operation
For reducing CO gas sensing, Figure 1 schematically illustrates the operative sensing mechanism. At elevated temperature, when the sensing film is exposed to air, the sensor surface provides active sites for the chemiadsorption of oxygen ions. Thus, oxygen adsorbs on to the sensor surface by accepting electrons from the conduction band of the sensing material. In the operating temperature ranging 100–500°C, are chemiadsorbed  forming an electron depletion layer (EDL). The EDL impedes grain to grain electron migration increasing the surface resistance for an “n”-type sensing material. When the sensor is exposed to a reducing gas such as CO or H2, the adsorbed oxygen ions react with the test gas to release the electrons back to the conduction band of the sensing material. As a result the EDL width is reduced to decrease the surface resistance of the sensor.
In the case of a p-type sensor, since the majority carriers are holes, the oxidation reaction of the reducing test gas and the oxygen ions on the sensor surface would decrease the concentration of holes, increasing the sensor resistance. Assuming n-type and p-type sensors have similar morphologies, the response of p-type and n-type sensor is related by the following relation .
where and is the gas response of n − type sensor.
2.2 Characteristic sensor parameters
Figure 2 illustrates the characteristic sensor parameters one can estimate from the recorded resistance transient of an n-type sensor used for reducing gas sensing.
Response (S) (%): Response is defined by the expression or where and are the measured resistances of the sensor exposed to air and test gas, respectively.
Response time () (s): Time required for a sensor to reach 90% of the maximum response on exposure to test gas .
Recovery time () (s): Time required for a sensor to reach 90% of the original baseline resistance upon removal of the test gas.
Optimum temperature () (°C): The temperature at which the sensor exhibits maximum response.
Selectivity: The ability of a sensor to sense a selective gas preferentially among other test gases.
Stability: Minimum variation of sensor base resistance over a period of time.
Sensitivity: Change of measured signal (resistance, voltage or current) per analyte concentration unit .
2.3 Factors affecting gas sensing performance of thin film sensor
The sensing performance of SMO-based sensor is primarily influenced by the receptor, transducer, and utility functions of thin film sensing elements. The receptor function is influenced by the crystallite size of the sensing material, as at elevated temperature oxygen ions are chemiadsorbed on these crystallites forming a charge-depleted layer. The receptor function controls the interaction of test gases with the sensing elements. The transducer function controls the flow of charges between the electrodes. Finally the utility function controls the diffusion of gases. The parameters that regulate the receptor function of the sensor include composition of the material, surface to volume ratio, and size of the pores and grains. The transducer function is influenced by the grain to grain contact and the phase purity of the material. The composition of the material influences the defect chemistry which in turn changes the nature and concentration of the defects in the sensing material. The surface to volume ratio, pore size and grain size, and grain to grain contact of the sensing material are dependent on the growth condition.
Usually, the sintered block as well as thick film sensors consists of loosely sintered secondary particles made of innumerous tiny primary particles. Generally, the dimension of pores between the primary particles is in microns (termed as macropores), whereas those in between the primary particles are in the order of nanometers (<50 nm) (termed as mesopores). When the sensor is aged at elevated temperature, oxygen gas is diffused through both the macro- (corresponds to molecular diffusion) and mesoporous (corresponds to Knudsen diffusion) regions. Sensor particles are charge depleted due to chemiadsorption of oxygen. During gas sensing, the test gas diffuses through macro- and mesoporous regions and reacts with chemiadsorbed oxygen. The diffusion of gases through the mesoporous regions is known to play an important role during reducing gas sensing using semiconducting oxide-based sensing elements. For compact thin films, the gas-solid interaction mostly takes place at the geometric surface and to a limited extent along grain boundaries. In contrast, the active surface of the porous thin film is much larger than the compact thin films. In both these thin film structures, the key parameters that control the receptor function of the sensing element include its composition, nature of metal-oxygen bonding, specific surface area, porosity, and crystallite size. The transducer function is influenced by grain to grain contact and phase purity of the sensing element, and the utility function will be controlled by the gas diffusion either through grain boundaries or pores. For thin film sensing elements with pore size distribution in the range of 2–50 nm, Knudsen-type gas diffusion seems to be operative. Since the Knudsen diffusion coefficient is related to operating temperature, pore radius, molecular weight of the diffusing gas, and diffusion distance are related to film thickness; it is quite obvious that the sensing performance of a thin film sensor is related to the operating temperature, film thickness, and molecular weight of the test gas. Very limited attempt has so far been made to theoretically investigate the gas transport phenomena within the sensing film and correlate it with its sensing performance.
In the subsequent sections, we have outlined the sensing performance of ZnO-graphene multilayered thin films for NOx sensing. Subsequently, CO and H2 sensing performance of ZnO thin film and SnO2 thick film were investigated. This was followed by CO2 sensing performance of 0.5% LFCO-ZnO thin film and VOC sensing characteristics of copper oxide (CuO) thin film and WO3-SnO2 thick film.
At constant sensor operating temperature, we assumed monolayer coverage of the reducing gas. Assuming Langmuir adsorption kinetics, we have theoretically predicted the response and recovery transients during gas sensing using thin and thick film sensing elements. From an application point of view, marginal base resistance drift of these thin film sensing elements is desirable during repeated response and recovery cycles. Often a significant drift in base line resistance is observed due to the partial recovery of thin and thick film sensing elements. For such partial recovery, the sensing is termed irreversible, whereas for reversible sensing, the base resistance is fully recovered. For a wide range of test gas concentrations and operating temperatures, the response transients of thin film sensing elements are modeled using Langmuir-Hinshelwood reaction mechanism. It is predicted that for irreversible-type sensing, the response time is reduced with the increase in test gas concentration, whereas for reversible sensing, response time is independent of the test gas concentration . The WO3-SnO2 thick film sensor was modeled with Freundlich adsorption isotherm in which the concentration-dependent conductance was ascertained to be nonlinear.
An economic wet chemical synthesis route was adopted to synthesize thin film gas sensing elements. Wet chemical synthesis is also energy efficient, and relatively cheaper precursor materials are used in wet chemical synthesis. We have synthesized the ZnO-graphene (G) multilayer to study its NO2 sensing characteristics. ZnO thin film sol was prepared by dissolving stoichiometric amount of zinc acetate dihydrate (Zn(CH3COO)2.2H2O) in 2-methoxyethanol (2-MOE) solvent with monoethanolamine (MEA) as stabilizer. The final concentration of the precursor solution was ∼0.4 M/L. The precursor solution was heated at ∼80°C for complete dissolution and stabilization. Graphene sheet from crystalline graphite was exfoliated by micromechanical cleavage using facile ultrasonication synthesis . For this purpose, 5 mg of graphite powder was mixed in 5 ml of 2-MOE solution. N-methyl-2-pyrrolidine (NMP) solution was added for stabilization of the graphite sol. This suspension was then ultrasonicated for 3 h at room temperature and aged for 48 h. Multilayered ZnO-G thin films were fabricated by alternate deposition of ZnO and exfoliated graphite sol. Prior to sol deposition, the glass substrates were cleaned in trichloroethylene for 5 min followed by ultrasonic cleaning in the presence of acetone and deionized water. This was followed by drying of the substrates in flowing air. Details of deposition process have been reported elsewhere . The multilayered films were annealed at 500°C for 1 h in air. The sensing performance of ZnO-G multilayered films were characterized using NO2 (0.5–5.0 ppm), CO (50 and 500 ppm), H2 (50 and 500 ppm), and i-C4H10 (50 and 500 ppm) as test gases. The operating temperature was maintained in the range 100–200°C during gas sensing measurements. Additionally, mixed gas sensing behavior for the ZnO-G multilayered films has also been investigated using NO2/CO (5 ppm/50 ppm), NO2/H2 (5 ppm/50 ppm), and NO2/i-C4H10 (5 ppm/50 ppm) gas mixtures.
We have also studied the CO and H2 sensing behavior of ZnO thin film and SnO2 thick film. ZnO thin film was synthesized following the method described above. SnO2 thick film was synthesized using a plasma spray deposition process. Commercial SnO2 powder was used as feedstock material for the plasma spray process. Nitrogen (sans hydrogen) was used as the plasma forming gas and compressed air was used for cooling. Details of the process and equipment can be found elsewhere . Alumina substrates (Ants Ceramic, India) were used for deposition as these are unaffected by the high temperatures generated during the atmospheric plasma spray process.
We have also studied the CO2 sensing behavior of 0.5% LFCO (LaFe0.8Co0.2O3)-ZnO thin film composite. Lanthanum nitrate hexahydrate (La(NO)3.6H2O), iron (III) nitrate nonahydrate (Fe(NO)3.9H2O), and cobalt (II) nitrate hexahydrate (Co(NO)2.6H2O) were dissolved in 2-MOE to prepare LFCO precursor sol with a final solution of 0.2 M via continuous stirring at room temperature. LFCO and ZnO sol were mixed in appropriate proportions to make composites of the composition xLFCO-(1−x) ZnO. The resulting sol was then spin coated on quartz substrate. Each layer was baked at 300°C with an intermittent heating interval of 10 min between layers to burn out the organic moieties. The coating and baking processes were repeated to yield films about 300nm thick. The resultant films were then annealed at 600°C for 2 h.
VOC sensing characteristics of CuO thin film and WO3-SnO2 thick film were measured in a static gas sensing system. CuO sol (0.25 M) was synthesized using copper (II) acetate monohydrate (Cu(COOCH3).H2O) and 2-MOE as precursor materials. Glass substrates used for deposition were cleaned using trichloroethylene, acetone, and deionized water as described earlier. CuO sol was spin coated on glass substrates. Each layer was baked at 300°C for 5 min. The resultant film which was about 200 nm thick was annealed at 600°C for 1 hour to achieve complete crystallization. WO3-SnO2 thick film composite was deposited on alumina substrates using a plasma spray deposition process. The feedstock material consists of WO3 and SnO2 powders in the weight ratio ∼25:75. This ratio was found to be optimum in a series of experiments with bulk pellets with various weight ratios (∼25:75, 50:50, and 75:25) of WO3 and SnO2 .
The resistance change of the thin film sensing elements was measured using gold interdigitated electrode which was sputter coated onto the thin films using a dc sputter coater (Cressington 108, Ted Pella, Inc., USA). High-temperature silver paste was used as electrodes for the thick coatings. An indigenously developed dynamic gas sensing setup was used to measure the gas sensing characteristics of the thin films and thick plasma sprayed coatings. The gas sensing setup consists of a gas chamber connected to mass flow controllers (PR 4000, MKS Instruments, Germany) which control the flow rate of gases. The flow rate is calculated by the relation [12, 16].
where , is the flow rate of test gas, and .
The temperature of the sensor inside the dynamic chamber is controlled with a temperature controller. An electrometer (6517 A, Keithley, USA) is connected to the sensor via gold-coated silver probes. Necessary voltage for electron migration between the grains of the sensor is controlled through the electrometer. The entire gas sensing setup is interfaced with a PC equipped with an RS-232 interface, GPIB card (National Instruments, USA), and LabVIEW 8.5 (National Instruments, USA) software . A schematic of the mentioned gas sensing setup may be found in .
A static gas sensing set was employed to study the VOC (ethanol, acetone, and isopropanol) sensing characteristics of CuO thin film and WO3-SnO2 thick film. Such a setup (also indigenous) consists of a rectangular chamber housing the sensor and the heater which is connected to the temperature controller . The sensor is connected to a microcontroller which in turn is connected to the PC. Details of the system schematic may be found elsewhere . Data (in this case, resistance) is collected at the rate of 1 bit/sec with the help of terminal emulation software. The sensor is heated to the temperature of operation and aged at that temperature to obtain a stable resistance () which is followed by injection of gas. The resistance at which the sensor stabilizes in the presence of gas is termed. The response of the sensor to the test gas in measured in terms of response percent which is defined as [19, 20].
4. Investigations on the gas sensing performance for air quality monitoring
Air quality monitoring involves the monitoring and quantification of various gases present in the atmosphere. Such gases include NOX , CO , CO2 , and VOCs . In addition, flammable gases such as H2 also need detection in order to prevent major disasters . Apart from the detection of low concentration test gases, addressing the cross-sensitivity of the sensors also remains a major research issue. In addition to this, cost of air quality monitoring system is high, and efforts are needed to make it economic for wide uses. Keeping these considerations in mind, we have developed sensors to monitor these gases selectively. For cross-sensitive sensing elements, necessary protocols are developed to address their selectivity. The sensing performances of synthesized sensing elements to detect test gases pertinent to air quality monitoring are described as follows.
4.1 NO2 sensing
The main sources of NO2 emissions are the incomplete burning of fuel and vehicular exhausts. According to a report in The Hindu , one of India’s premier newspapers, winter smog is caused in many parts of northern India due to burning of crop residue. An estimated 240 million tons of NOX (NO and NO2) is generated because of this practice. Figure 3 shows percentage of NOX emissions in India from various sources. Other than this, vehicular exhausts also contribute to NOX formation. NOX, along with other pollutants such as SO2, VOCs, and particulate matter, forms smog as well as acid rain. These are markers of extremely polluted air and can have adverse health effect on humans.
Traditionally, semiconducting metal oxides such as SnO2 , WO3 , ZnO , etc. have been used for NO2 sensing. Recently, composites such as Fe2O3-ZnO  has also gained prominence for selective NO2 sensing. In addition, metal oxide (MO)-graphene (G) composites such as 0.5 wt%G-WO3  and SnO2-RGO  have also been reported as these sensors show high response at low operating temperature. These sensors are also highly selective.
Based on literature study (Table 2), we realized that MO-graphene (G) sensor will be an effective candidate for NO2 sensing study. We have chosen the MO to be ZnO as it is an extremely stable sensor and can be easily modulated with graphene to operate at a low temperature. We have thus prepared a ZnO-G multilayer system for NO2 detection. We have measured NO2 sensing with ZnO which is an n-type semiconductor. It shows optimum response of 26% to 5 ppm NO2 at 150°C when the test temperature range is 100–200°C. Below 150°C, the response to NO2 is almost negligible. However, with the addition of graphene to ZnO layers, the sensing response significantly increases to 894% . These results are presented in Figure 4.
|Sensor material||Form||Gas||Topt (°C)||S%||Selectivity||τres, τrec (s)||Conc. (ppm)||Ref.|
|SnO2-RGO||Nano powder||NO2||30||4.63 (Rg/Ra)||Yes||177, 510||5|||
|SnO2||Hollow sphere||NO2||160||—||Yes||90, 25||5|||
|0.5 wt%G-WO3||Nanopowder||NO2||RT, 250||1.96, 130 (Rg/Ra)||No||25–200||1, 5|||
|ZnO||Nano powder||NO2||350||1.8 (Rg/Ra)||Yes||180, —||1|||
|WO3||Thin film||NO2||300||87||Yes||300, 360||2|||
|ZnO-G||Thin film||NO2||150||894||Yes||150, 315||5|||
The response of ZnO-G multilayer thin film was also measured for 500 ppm H2, CO, and i-butane gases in the said temperature range. It was found that the response to 5 ppm NO2 was far superior to any of the other gases. We have defined the selectivity (κ) as
where is the response % for NO2 and is the response % for other gas (H2, CO, and i-butane).
The selectivity plot for NO2 with respect to H2, CO, and i-butane has been shown in Figure 5. For all the gases, the selectivity factor κ has been found to be greater than 1. Thus we could detect NO2 selectively in the presence of other gases.
4.1.1 NO2 sensing mechanism
The significant enhancement in NO2 sensing may be attributed to the formation of p-n junction between ZnO and graphene. In ZnO-G composite, graphene adheres to ZnO surface to increase the interaction with NO2. Due to the formation of p-n junction, electron flows from graphene to ZnO resulting in an electron-depleted surface. On exposure to air, majority of (dominant species at 150°C) species adsorb on to the ZnO-G surface, leading to the formation of electron-depleted layers. NO2 reacts with this surface to form NO which is explained via the following equations:
The interaction of reducing gases (H2, CO and i-butane) with is poor at 150°C, thus leading to selective detection of NO2. A detailed figure of the sensing mechanism may be found in .
4.1.2 Mixed gas sensing
We felt it important to sense NO2 selectively in a mixed gas environment. Thus, the sensing performance of NO2 was investigated in the presence of 5 ppm NO2 and 50 ppm CO, 5 ppm NO2 and 50 ppm i-butane, and 5 ppm NO2 and 50 ppm H2. The response % histograms are presented in Figure 6. Principal component analyses (PCA) were used to address the cross-sensitivity. PCA involves the fast Fourier transforms (FFT) of the resistance transients of each gas (accomplished using MATLAB®, Mathworks, USA) which are fed into this unsupervised pattern recognition algorithm using STATISTICA-9, StatSoft, USA.
The plots between PC #1 and PC#2 are plotted separately in Figure 7. As shown in the inset of Figure 7a, distinct cluster forms among individual test gases (H2, CO, and iC4H10). Also for NO2-CO, NO2-i-butane, and NO2-H2 mixtures, distinct clustering is readily identified in Figure 7a. PCA is also performed in mixed gases varying NO2 contents in the range of 0.5–5 ppm (Figure 7b). Note that distinct clustering is also achieved in mixed gases with variable NO2 contents, which eventually leads to successful differentiation between them.
4.2 CO and H2 sensing characteristics of thin and thick films
Carbon monoxide is a major air pollutant. Open burning of wastes as well as fuel and biomass burning mainly in rural India is the main source of carbon monoxide. Additionally, fuel adulteration, especially in auto-rickshaws, also adds to the CO in air. On breathing CO-rich polluted air, one might end up inhaling CO as opposed to oxygen. This CO then easily combines with the hemoglobin in blood forming carboxyhemoglobin, which reduces the oxygen-carrying capacity of blood. Additionally, tobacco smoke and heating appliances add to CO pollution of indoor air.
Hydrogen (H2) is a colorless, odorless, and tasteless gas. H2 is also highly flammable with a low ignition energy (0.017 mJ) and low ignition temperature (∼560°C). Hydrogen detection is critical to maintaining the safety of nuclear reactors where the reaction of water with high temperature core or cladding gives rise to evolution of hydrogen in radioactive waste tanks . A small amount of H2 is also produced as a result of methane explosions in coal mines . H2 is used as a liquid propellant for rockets as it is a zero emission fuel. It is also used in the reduction of metallic ores and for methanol production. H2 has a lower explosive limit (LEL) of 4 vol % which makes its detection fairly crucial.
Semiconducting metal oxides such as SnO2, ZnO, WO3, etc. have demonstrated huge research potential when it comes to sensing of reducing gases such as hydrogen, carbon monoxide, hydrogen sulfide, ammonia, and VOCs. However, such materials exhibit poor selectivity, and efforts are on to achieve selectivity either by doping metal oxides or synthesis of composites. A brief review of literature (Table 3) suggests that selectivity has not been addressed for CO and H2 sensing of metal oxides. We have synthesized ZnO thin film via spin coating process and SnO2 thick film via plasma spray deposition process and studied their gas sensing characteristics with respect to H2 and CO gases. We have analyzed the issue of selectivity through conductance transient analyses. The analyses have been performed for both thin and thick films for comparison. Additionally, we have discussed how stability varies in thin and thick film gas sensors when the test gases under consideration are the same.
|Sensor material||Geometry||Gas||Topt (°C)||S%||Selectivity|
|τres, τrec (s)||Conc. (ppm)||Ref.|
|Sm2O3||Thick film||CO||250||1.4 (Rg/Ra)||No||35, 110||5|||
|CuO||Nano-tube||CO||175||3.27 (Rg/Ra)||No||29, 37||1000|||
|In2O3||Thin film||CO||350||45, 85 (Rg/Ra)||No||10– 50||1000|||
|Pd-ZnO||Thin film||CO||150||—||No||17, 23||500|||
|ZnO||Thin film||H2||300||21||No||58, 70||200|||
|In-ZnO||Thin film||H2||300||65||No||35, 64||500|||
|SnO2||Thin film||H2||350||83||No||23, 139||265|||
|ZnO||Thin film||H2||300||68||Yes||53, 67||500|||
|SnO2||Thick film||H2||250||80||Yes||30, 118||500|||
ZnO thin film and SnO2 thick film were investigated for H2 and CO sensing characteristics in the temperature range 350–200°C. The concentration of each gas was fixed at 500 ppm. A maximum response of 68% for 500 ppm H2 and 49% for 500 ppm CO was recorded at 300°C by ZnO thin film. SnO2 thick film recorded maximum response to 500 ppm H2 (81%) and 500 ppm CO (73%) at 250°C. Careful inspection of Figure 8 circumstantiates cross-sensitivity of H2 and CO gases at all temperatures for both ZnO thin film and SnO2 thick film sensors. This necessitates further analysis of the sensing data obtained. Analyses of conductance transients are a well-proven coherent technique to address the problem of cross-sensitivity. Although cross-sensitivity is shown by the sensors at all temperatures, we have performed mathematical analyses only for the optimum temperature of each kind of film. For H2 and CO sensing, the recorded conductance transients of both ZnO (thin film) and SnO2 (thick film) sensors were modeled using Langmuir adsorption kinetics .
4.2.1 Analyses of conductance transients
In a preceding section, we have already outlined the sensing mechanism of n-type semiconducting sensor toward reducing gas sensing. Thus, ambient oxygen physiadsorbs on the sensor surface at ambient temperature (Eq. (7)). At elevated temperatures, the physiadsorbed oxygens are chemiadsorbed (Eq. (8)) by extracting electrons from semiconductor oxide grains. This creates an electron depletion layer (EDL) at the sensor surface [5, 10, 44, 45].
On exposing the sensor to a reducing gas, the chemiadsorbed oxygen reacts with the test gas (R) to yield oxidized reaction products. The reaction schemes and desorption of reaction product (ROgas) is described in Eqs. (9)–(11).
As discussed earlier, when the sensor is exposed to air, oxygen chemisorbs onto the sensor surface which results in the formation of an electron depletion layer (EDL). The EDL formed impedes the transfer of charge carriers from one grain to another. The relation between the height of this potential barrier and the conductance G of the sensor is given by the following Eq. (5):
where eVs is the Schottky barrier, k is the Boltzmann constant, and T is the absolute temperature of operation of the sensor.
As the sensor is exposed to a reducing gas like CO or H2, the chemisorbed oxygen ions react with the gas to reduce the potential barrier between the grains, thus allowing the flow of electrons . This process increases the conductance of the sensor.
The adsorption process of gas molecules on a solid surface was studied by Langmuir who developed an empirical model based on the surface coverage θ. The Langmuir model assumes that (i) the surface on which the gas adsorbs is homogeneous, (ii) a specific number of sites are available for gas adsorption, and once all these sites are occupied, further adsorption is not possible. This is called monolayer adsorption, and (iii) all sites on which gas is adsorbed are equivalent in terms of heat of adsorption. Assuming the above, Langmuir formulated the equation below [5, 15].
where N(t) is the number of gas molecules adsorbed at time t and N* is the total adsorption sites available, and
where P is the gas pressure and b is related to heat of adsorption Q and is defined by the following relation:
where b0 is a constant, Q is the heat of adsorption (Q = Ed − Ea > 0), Ea and Ed are the adsorption and desorption activation energies of gas molecules, R is the gas constant, and T is the operating temperature, . The pre-exponential constant bo depends on υ (frequency of oscillation of adsorbed molecules), Ko (fraction of those molecules with energy > EA, adsorbed on surface), Sarea (the surface area of a single adsorbed molecule), NA (Avogadro number), M (molar mass of the gas), and R (the gas constant).
The amount of gas adsorption is dependent on the nature of the gas sensing surface, the operating temperature, and pressure of the gas. An adsorption isotherm is defined as the study of the amount of gas adsorption as a function of gas pressure. The simplest adsorption isotherm was given by Langmuir in which he assumes no interaction between adsorbed atoms or molecules. At equilibrium, the rate of adsorption is equal to the rate of desorption. Thus, when t → ∞,
Assuming a linear functional dependence between θ(t) and conductance transient (G(t)), the response behavior of the sensor that follows Langmuir model can be written as 
where is the base conductance of the sensor at time t = 0, is the fitting parameter, and is the characteristic response time. This equation will only be valid if all three of Langmuir’s criteria are satisfied.
If Langmuir adsorption isotherm is followed, then variation of /(1-) vs. C should be a linear fit and pass through the origin. The parameter b can be estimated from the slope of the plot.
Using Eq. (18), conductance transients during response at various concentrations for SnO2 sensing element for H2 and CO sensing were fitted to estimate values of , and . Figure 9a shows the fitting of conductance transient of SnO2 thick film at 250°C in response to 500 ppm H2. Figure 9b shows the variation of /(1-) with test gas concentration (C) at 250°C. Since the linear fits for H2 and CO pass through the origin, Langmuir adsorption isotherm is satisfied [5, 47, 48]. The parameter b can be obtained from the inverse of the slope of the plot. Accordingly, the estimated ‘b’ values for H2 (1.5349 × 1010 Pa) and CO (1.7834 × 1011 Pa) have been determined from these plots . From Eq. (15), expression for heat of adsorption (Q) can be written as
The molecular area of nitrogen is approximately 16.2 Å2 as given in . We have considered this as the area per site occupied by the test gas molecule (Sarea). Nitrogen has a molecular weight of 28 g mole−1 which gives Sarea ∼ 16.2 Å2. A similar calculation for carbon monoxide with molecular weight 28.01 g mol−1 yields Sarea to be ∼16.2 Å2. Assuming condensation coefficient Ko = 1 , υ = 1014 Hz , and R = 8.314 JK−1; bo (at T ∼ 523 K) can be estimated for H2 and CO gases using (Eq. (16)).
The ln versus ln C can be fitted linearly to estimate the rate constant and β for H2 and CO sensing where is the fitting constant.
The temperature variation of can be expressed by the following relation (47):
The variation of Ea with concentration for H2 and CO gases is shown in Figure 9d . The activation energy decreases from 134.3 kJ/mol to 131.8 kJ/mol for 500 ppm H2 to 50 ppm H2, whereas the activation energy decreases from 151.05 kJ/mol to 150.07 kJ/mol and from 500 ppm CO to 50 ppm CO for SnO2 thick film sensor.
Similar analyses for the ZnO thin film sensor yield activation energy values of 138.62 and 151.87 kJ/mol for 500 ppm H2 and CO, respectively. Activation energy increases with decreasing concentration for both the gases. The Q values for H2 and CO are 31.48 and 18.48 kJ/mol, respectively. Thus, the activation energy, Ea, and heat of adsorption, Q, may be used to differentiate H2 and CO gases for both thin and thick films.
4.2.2 Stability of thin and thick film gas sensors
Figure 10 shows the schematic of Langmuir-Hinshelwood mechanism  in which the oxygen as well as reducing test gas is initially adsorbed on the sensor surface shown by “A.” An oxidized reaction product is produced as a result of collision among adsorbed oxygen and reducing test species. For efficient formation of an oxidized reaction product “ROgas,” reducing test gas “Rgas” (having concentration “Cg”) and oxygen “O” should cross the energy barrier height shown by “TS” (transition state).
During this investigation, the sensing element was exposed to the test gas (during response) and air (during recovery), respectively. After removal of the test gas, it is expected that the sensor should switch back to the initial base resistance in air. Considering this fact, gas sensing can be classified into reversible and irreversible type. In reversible sensing, the base resistance is fully recovered, whereas for irreversible sensing, the base resistance recovers partially. The mechanism of reversible and irreversible sensing was reported elsewhere . The irreversible sensing can be better understood from the gas–solid interaction. During gas sensor response process, CO and H2 gases are oxidized to CO2 and H2O. These oxidized products are entrapped in the small pores of the sensing element. If the desorption of these products does not occur fully, the base resistance of the sensing element recovers partially. Therefore, a marginal drift in the baseline resistance is observed. The mathematical formulations of irreversible and reversible sensing are described in the subsequent sections.
126.96.36.199 Irreversible sensing model
It is assumed that the site fraction θ on the sensor surface is occupied by adsorbed oxygen ionsand test gas (Rgas) adsorption takes place over remaining available sites. For an “n”-type metal oxide semiconductor gas sensor, reducing gas reacts with chemiadsorbed oxygen to form an oxidized product as represented by Eq. (26).
At constant operating temperature, it was assumed that the total number of active sites (Fθ) can be expressed by the following relation:
It has been assumed that adsorption of the reaction product takes place on the same sites over which reducing test gas (Rgas) adsorbs initially. Since the formation of is the rate-determining step for response process, sensor response is directly proportional to the formation of an oxidized reaction product. Therefore, the sensor response during irreversible sensing can be expressed mathematically by the following relation:
Integrating Eq. (29), we obtain
Simplifying Eq. (31), we can write
where C is the constant of integration.
Therefore Eq. (32) can be written as
Simplifying Eq. (33), we can write
Concentration of at time t can be expressed as
When all active sites (Fθ) are adsorbed by the oxidized reaction product (), maximum sensor response is obtained. Therefore, the sensor response at a time t can be expressed by the following relation:
is termed as the characteristic response time (in seconds) during irreversible sensing.
188.8.131.52 Reversible sensing model
In this case, the equilibrium constant for the sensing reaction (Eq. (38)) is termed as K, and the forward and reverse rate constants are k and k/K. The rate of change of at time t can be written as
Rearranging Eq. (38),
Simplifying Eq. (39), we can write
Integrating Eq. (40), one can write
Simplifying Eq. (41), one can write
Therefore, Eq. (43) can be simplified as
Eq. (44) can be simplified to
Eq. (45) can be further simplified to
The rate of change of at time t can be written as
Sensor response (S(t)) can be written as
where τrev is the characteristic response time during reversible sensing.
Figure 11a,b shows the concentration-dependent resistance transients for H2 and CO sensing for ZnO thin film. These measurements were taken at 300°C by varying the test gas concentration. During these measurements, drift in baseline resistance was observed as shown by the dashed line in Figure 11a,b. Therefore, time-dependent response % for H2 and CO sensing was fitted using irreversible sensing model as given in Eq. (37). Figure 11c shows the typical response % vs. time fitting for 500 ppm H2 and CO sensing. Inset of Figure 11c represents variation of (in seconds) with concentration (in ppm) in which decreases systematically with increase in test gas concentration which is a typical behavior in case of irreversible gas sensing as reported by .
We have also investigated the reversible sensing behavior of plasma sprayed SnO2 thick film sensor . The SnO2 thick film sensor shows marginal drift in baseline resistance when exposed to 20–500 ppm CO and H2. This is shown in Figure 12.
Inset of Figure 12c shows the variation of characteristic response time τrev (s) with concentration (ppm). τrev was almost invariant of the test gas concentration in the range 500–200 ppm. Interestingly, τrev was also invariant of test gas concentration in the range 100–20 ppm. However, the value of τrev was found to be higher in the lower concentration range (100–20 ppm) than that of the higher concentration range (500–200 ppm).
Figure 13 shows the surface micrograph for (a) ZnO thin film and (b) SnO2 thick film sensing elements. ZnO thin film exhibits denser surface morphology than SnO2 thick film as revealed from Figure 13a. In line with this discussion, irreversible sensing behavior for ZnO thin film can be attributed to denser surface morphology. It may be noted here that our results for ZnO thin film are in accordance with . On the other hand, reversible sensing behavior in case of SnO2 thick film can be attributed to its porous surface morphology as revealed from Figure 13b.
4.3 CO2 sensing characteristics
Carbon dioxide concentrations have increased enormously in the atmosphere in the past hundred years or so. In the preindustrial era, CO2 concentration was ∼280 ppm. However, current global standards put CO2 concentrations at an alarming level of 400 ppm mainly arising from deforestation and the burning of fossil fuels. India alone emitted 2299 million tons of CO2 in 2018 according to an International Energy Agency (IEA) report . Most of India’s emissions come from the burning of coal. Increased CO2 emissions have led to global warming—a climate phenomenon that is responsible for the increasing number of storms as well as droughts . In addition, increased levels of CO2 in the atmosphere affect human health severely. Table 4 summarizes the effects on human health with increasing CO2 concentrations. It is thus incumbent that we take necessary steps to detect CO2 concentrations in the atmosphere.
|Concentration of CO2 (ppm)||Effect on human health|
|250–300||Normal outdoor air|
|100–2000||Drowsiness (poor air)|
|2000–5000||Increased heart rate, nausea, headaches|
|>5000||Oxygen deprivation begins|
|>40,000||Serious oxygen deprivation leading to brain damage, coma, and death|
Research on CO2 sensing gained prominence in the late 1980s when researchers such as Toshio Maruyama  and Yasuhiro Shimizu  reported a NASICON (Na3Zr2Si2PO12)-based solid electrolyte gas sensor and an electrochemical K2CO3-polyethylene glycol CO2 sensor, respectively. However, it was soon realized that handling these materials was cumbersome and incorporating these into actual devices posed a serious challenge. Research was thus shifted to resistive-type sensors [54, 55] which were far better in terms of stability and could be easily miniaturized to accommodate in sensing devices. However, such sensors were plagued with poor selectivity and low response to CO2. This prompted the use of mixed oxide or composite sensors [56, 57, 58], the research potential of which is still being investigated [59, 60].
A comprehensive study of literature (elaborated in Table 5) related to CO2 sensing leads us to the fact that formation of p-n junctions is an important criterion for CO2 detection. We have reported CO2 sensing of ∼36% achieved using a LFCO-ZnO composite thin film.
|Sensor material||Form||Gas||Topt (°C)||S%||Selectivity||τres, τrec (s)||Conc. (ppm)||Ref.|
|ZHS microcubes on ZnO nanorods||Thin film||CO2||150||354||No||10, 24||2500|||
|LaBaCo2O5 + δ||Bulk||CO2||300||1.04 (Rg/Ra)||No||—||400|||
|Graphene sheet||Thin film||CO2||60||26||No||8, 10||100|||
|Ag/BaTiO3-CuO||Thin film||CO2||250||28||No||900, 600||5000|||
|LFCO-ZnO||Thin film||CO2||300||36||Yes||300, 630||2500||This work|
CO2 sensing characteristics of LFCO-ZnO composite thin film was measured in the temperature range 350–150°C. The highest response of ∼36% was obtained at 300°C to 2500 ppm CO2, and the response decreases to 12% at 150°C. The response and recovery times recorded at 300°C are 300 and 630 s, respectively. The composite shows n-type sensing behavior which confirms the dominance of ZnO crystallites . The LFCO-ZnO thin film composite is cross-sensitive to CO2. A response of 65% was recorded for 500 ppm CO at 300°C. The composite shows higher response to both CO and CO2 than pure LFCO and ZnO thin films. To address the cross-sensitivity of the composite thin film, principal component analysis of the resistance transients was performed. The relevant features of the resistance transients of the test gases are extracted using fast Fourier transform technique. These features are then used as input parameters for PCA. Figure 14d shows low dispersion of CO and CO2 for the LFCO-ZnO composite thin film after performing PCA. Thus, FFT-PCA is an efficient tool for the differentiation of CO and CO2 gases for LFCO-ZnO thin film.
The LFCO-ZnO composite follows the gas sensing behavior of an n-type semiconductor due to the presence of a large number of ZnO crystallites. Therefore, analyses of the conductance transients of CO and CO2 gases follow Langmuir adsorption mechanism similar to ZnO thin film sensor. Activation energy, Ea, and heat of adsorption, Q, have been calculated in accordance with the theory described in the previous section. Figure 15d represents the variation of Ea with concentration for CO and CO2 gases. It should be noted that each gas possesses distinct activation energy values. The activation energy decreases from 153.83 kJ/mol to 149.67 kJ/mol for 500 ppm CO to 50 ppm CO, whereas the activation energy decreases from 155.93 kJ/mol to 155.28 kJ/mol for 2500 ppm CO2 to 1000 ppm CO2. The variation in Ea is much less in CO2 than in CO which could pertain to the efficiency of CO2 sensing of 0.5% LFCO-ZnO thin film.
As discussed earlier, when the sensor surface is exposed to air, oxygen ions adsorb on the sensor surface which leads to the formation of an electron-depleted layer and hence a potential barrier between two grains. The conductance G at temperature T is given by Eq. (12).
Now, assuming Langmuir adsorption (ideal gas), when sensor surface is exposed to gas, it is covered with a monolayer of gas molecules/atoms. The conductance transients are fitted according to Eq. (18) for the response transient. The recovery transient, however, represents decay of conductance, and it is fitted by the following equation:
Figure 16 shows the conductance transients of CO and CO2 fitted with Eqs. (18) and (51). Table 6 summarizes the fitted parameters. Each parameter is unique to both the gases. It may be noted that the response times to both CO and CO2 are almost similar (∼60 and 68 s, respectively) although the response % is markedly different (64 and 36%, respectively).
|Fitted response parameters||Topt||G0 (Ω−1)||G (Ω−1)||τres (s)|
|Fitted recovery parameters||Topt||G0 (Ω−1)||G (Ω−1)||τrec (s)|
Assuming nonlinear variation of sheet conductance with test gas concentration, the gas sensing response varies with absolute temperature T according to the following relation:
where is the resistance in air, is the resistance in gas, pre-exponential constant, is the sensitivity of the gas, is the Hatta number, is the activation energy of the first order reaction, and is the test gas concentration at the film surface .
The response (S) versus absolute temperature (T) follows a “bell-shaped” dynamic and can be fitted in accordance with Eq. (52) to yield parameters specific to CO and CO2. Figure 17 shows the S versus T fitting for CO and CO2. Sensitivity of the thin film, “n,” is higher for CO2 which shows that the sensor surface is more active toward CO2 adsorption. Addition of 0.5% LFCO to ZnO structure enhances oxygen adsorption on the ZnO surface due to the formation of additional defects. Also, the formation of multiple junctions in the composite leads to the phenomenon of “band bending” which enhances the interaction of CO2 with oxygen ions. It may be noted that CO interferes with CO2 sensing in the composite due to the formation of multiple junctions. This problem is eliminated in the ZnO/LFCO bilayer (not shown) which consists of only a single p-n junction and, thus, is selective to CO2.
Simulated parameters from Figure 17 are summarized in Table 7 where Ea and Ek have units of J mol−1. Significant differences may be noted for the simulated parameters of each gas. Table 7 and Figure 15d corroborate the fact that the activation energy is lower for CO which explains the higher response to CO. It may also be emphasized that the parameters are valid for the entire range of operations of the sensor. The simulation may provide useful insights regarding the maximum and minimum temperature at which the sensor is operative. Further, a logarithmic plot of sensor response S and concentration C yields a value of ∼0.67 for CO and ∼1.56 for CO2 (Figure 18) which is well in agreement with the simulated values. Additionally, it is shown that sensor response decreases sequentially with decreasing concentration as can be seen from Figure 18. The limiting concentration for CO is 50 ppm and that for CO2 is 500 ppm. The detection limit for CO2 is well above the permissible exposure limit (PEL) which is >5000 ppm. Thus, we may conclude that 0.5% LFCO-ZnO composite thin film is a fair CO2 sensor and is well suited for practical applications.
|Fitting parameters||Carbon monoxide||Carbon dioxide|
|m0 = L(√(k/Dk)||1240.78||2.9675E6|
4.3.1 CO2 sensing mechanism of LFCO-ZnO thin film gas sensor
The electrical resistance change in ZnO, which is an n-type conducting material, is attributed to adsorption and desorption kinetics of oxygen on ZnO surface. Chemisorption of molecular oxygen of the type occurs above room temperature on ZnO surface, leading to the formation of electron depletion layer (EDL). As the temperature increases up to ∼300°C, the molecular oxygen dissociates into atomic form and forms a thicker EDL which pushes inside the ZnO core. Homogeneous particles observed in ZnO thin film microstructure (Figure 13a) contribute to the increased rate of oxygen adsorption on ZnO surface. This increases the sensitivity toward CO2 gas as can be observed from Figure 14b. Reaction of carbon dioxide molecules with adsorbed oxygen is given below :
These equations are consistent with the reduction in resistance of ZnO sensor on exposure to CO2. In principle, ZnO sensor should register an increase in resistance on exposure to CO2 as it is an oxidizing gas. However, it was found that ZnO demonstrates typical n-type behavior when brought in contact with CO2. This anomaly can be explained from the defect chemistry and catalytic properties of ZnO. This demands a more detailed study and is currently beyond the scope of this book. Thus, we have confined ourselves to the current discussion of LFCO-ZnO composite thin film sensor.
When n-type ZnO is combined with p-type LFCO (LaFe0.8Co0.2O3) sensor, a p-n junction is created at the interface between the two sensors. On exposure to CO2, the junction pushes further in to the ZnO core. This leads to an enhanced CO2 sensing response as compared to pristine LFCO and ZnO sensors.
The general reaction mechanism of LaFeO3 (LFO) was reported by  in which LFO reacts with CO2 in the presence of moisture to form carbonates. The reaction is given below.
The carbonates thus formed aid in CO2 sensing by reducing the activation energy required for the reaction to occur. Further, equalization of Fermi levels takes place via migration of carriers leading to the formation of p-n junction. On exposure to CO2, hydroxycarbonates form at the junction interface leading to the increase of potential barrier. This increases the concentration of holes at LFCO interface. However, as the concentration of LFCO is only 0.5%, hole accumulation at LFCO surface is impeded, and fall in resistance occurs, and the composite as a whole shows n-type sensing behavior .
SnO2 thick film gas sensor did not show any appreciable CO2 sensing. This is due to the absence of a Schottky contact in between SnO2 thick film and silver electrodes. Investigations on CO2 sensing were not performed for any composite thick film sensor.
4.4 VOC sensing characteristics of thin and thick film
Volatile organic compounds such as ethanol, acetone, and isopropanol pollute auto cabins and indoors of buildings. Long time spent indoors in such an environment ultimately leads to sick building syndrome and also has been found to be linked to leukemia and lymphoma. VOCs are also biomarkers of lung cancer and type 2 diabetes. Ethanol, acetone, and isopropanol have a permissible exposure limit of 1000 ppm over an 8 h work shift as documented by OSHA.
Figure 19 demonstrates the necessity of VOC detection. Traditionally, n-type semiconducting metal oxides such as SnO2, ZnO, WO3, TiO2, In2O3, Fe2O3, etc. have been used for VOC sensing. However, recent research activities show that p-type semiconducting metal oxides such as copper oxide have been gaining popularity in gas sensing mainly due to its ease of fabrication, low cost, high stability, and high response to volatile organic compounds. CuO architectures such as nanorods , nanosheets , nanoparticles , and thin film  play an important role in gas sensing behavior. Thin films, in particular, are preferred over powdered CuO variants due to high stability and ease of device fabrication (Table 8). Additionally, composites have also gained prominence in selective VOC sensing. We have synthesized CuO thin films using solgel technique. Quartz substrate was used to deposit the thin films using spin coating. After heat treatment, gold electrodes were deposited on these films using sputter coating following which the gas sensing measurements were performed in a static gas sensing setup. The VOCs were found to be cross-sensitive. Cross-sensitivity was addressed with principal component analysis. Additionally, we have also investigated the VOC sensing characteristics of WO3-SnO2 thick film. Investigations into various thick films for VOC sensing has been tabulated in Table 9. The gas–solid interaction in this case was modeled with the Freundlich adsorption isotherm which is nonlinear in nature.
|Sensor material||Geometry||Test gas||Topt (°C)||S%||τres, τrec (s)||Mixed gas sensing||Conc. (ppm)||Ref.|
|CuO||Thin film||2-Propanol||225||282||12, 279||No||300|||
|CuO||Powder||Ethanol||230||2.37 (Rg/Ra)||—||No||210||[75, 76]|
|CuO||Nanoparticles||Ethanol||220||2.7 (Rg/Ra)||11, 7||No||0.1|||
The CuO sensors, thus synthesized, were used for detection of ethanol and acetone by varying the operating temperature (200–300°C) and gas concentration (25–300 ppm). It was found that CuO shows high response and excellent stability with respect to both these gases. The response to 300 ppm acetone and ethanol was found to be 168 and 144%, respectively (Figure 12). The response and recovery times were found to be 7 and 248 s for acetone and 3 and 216 s for ethanol, respectively. The static gas sensing setup is similar to a real environment response which explains the delayed recovery times. The response times mentioned are not exact as in a static gas sensing setup, where response to the test gas does not saturate before recovery begins . Mixed gas sensing was also performed with ethanol and acetone (300 ppm/300 ppm) at 300°C. It was found that the response was not affected by the order in which the gases were injected into the static chamber. Principal component analysis was used as a discriminatory method to quantify the gases for mixed gas sensing data (Table 9).
|Sensor material||Geometry||Gas||Topt (°C)||S%||τres, τrec (s)||Selectivity||Conc. (ppm)||Ref.|
|CeOx - SnO2||Thick film||Ethanol||350||95||1.1||Yes||200|||
|Sr-SnO2||Thick film||Ethanol||300||65||2, 7||100|||
|SnO2||Thick film||Ethanol||300||84||8, 340||Yes||300|||
|WO3-SnO2||Thick film||Ethanol||250||73||10, 312||Yes||300|||
4.4.1 Ethanol and acetone sensing mechanism in CuO sensor
CuO is a p-type sensor, and as such the sensing mechanism of a p-type sensor differs slightly to that of an n-type sensor. The major carriers in this case are holes. The concentration of holes changes at the sensor surface due to reaction with the target gas. When the CuO sensor is exposed to air at the operating temperature range of 200–300°C, oxygen takes up electrons from the sensor surface, and the oxygen ions formed chemisorb onto the surface. This increases the density of holes resulting in the decrease of the surface Fermi level. A hole accumulation layer (HAL) at the surface increases the conduction of holes which reduces the baseline resistance of CuO (Ra = resistance in air ambient). The equations involved are listed below .
On exposure to acetone and ethanol, the oxygen ions react with the test gases to produce carbon dioxide and water with the release of electrons. This process reduces the thickness of the hole accumulation layer which increases the sensor resistance (Rg = resistance in gas ambient) .
The probable reaction mechanisms for acetone and ethanol are
In the end, electron hole recombination takes place and the HAL diminishes.
4.4.2 Mixed gas sensing in CuO thin film sensor and principal component analyses
We have also investigated mixed gas sensing for 300 ppm acetone and 300 ppm ethanol. We found that the mixed gas sensing response (205%) is much higher than individual sensing response of acetone (168%) and ethanol (144%), but it was not an addition of the individual responses (Figures 20 and 21). Further, it was found that the order in which the gases were injected into the static chamber had no bearing on the mixed gas sensing response.
This can be attributed to the fact that the gas with the higher reactivity will first interact with the adsorbed O− ions, leaving the next gas to react with less number of the same ions leading to reduced sensing .
The gases can be discriminated by principal component analyses, which quantify the gases in a multi coordinate system. Fast Fourier transform was utilized to extract the important features of the resistance transients of both acetone and ethanol measured at 300°C. These parameters were then fed into a linear unsupervised principal component analyses pattern recognition technique. Figure 22a shows the clustering of ethanol and acetone when injected individually. The clustering does not change when all the resistance transients of 300 ppm each of acetone and ethanol are analyzed (Figure 22b). Thus, PCA can be successfully used to identify the gases in a mixture of acetone and ethanol.
Additionally, we have explored the VOC sensing characteristics of WO3-SnO2 composite thick film sensor . Sensing measurements were made in the range 300–150°C for 300 ppm ethanol, acetone, and isopropanol. The composite thick film was found to be cross-sensitive to each of the gases. To address the cross-sensitivity, the conductance transients of the gases were further analyzed. These conductance transients were obtained at 250°C, and VOC concentration was kept fixed at 300 ppm. Conductance transients for the three VOCs at respective concentration level were fitted using Eq. (18) to estimate the values of parameters such as G0 (mho), G1 (mho), and τres (s). Figure 23b envisages the nonlinear variation of G1/(1-G1) with VOC concentration (C) (symbols). In this case, Langmuir adsorption isotherm is not validated since the variation of G1/(1-G1) with concentration is nonlinear for all the gases and does not pass through the origin. In fact, the results are much closer to the Freundlich adsorption isotherm. The relation between G1 and C in this case obeys the following relation:
From the nonlinear fit, we estimated the values of α and δ for all VOCs under consideration. These results indicate that the sensor surface is not homogeneous and heat of adsorption (Q) is not uniform unlike the Langmuir model.
In case of Langmuir adsorption isotherm, assuming Z = G1/(1-G1), differentiating Eq. (19) we can write
Differentiating Eq. (66), we can write
Assuming Z vs. C slope at a particular point ∼1/b, we can write
Simplification of Eq. (69) yields
As described earlier, considering the surface area, Sarea ∼ 16.2 Å2 for nitrogen (molecular weight = 28 g mol−1), we can easily calculate the surface area for ethanol which is 26.61 Å2 (molecular weight = 46.0 g mol−1). Assuming condensation coefficient Ko = 1 , for all test gases, bo (at T ∼ 523 K) can be estimated using Eq. (71). From Eq. (70) we have estimated Q as a function of VOC concentration at 250°C.
The ln (τres) vs. ln (C) plot was fitted linearly in order to estimate value of β for three VOCs under consideration.
In case of Freundlich adsorption isotherm, temperature-dependent variation of τres can be written as per the following relation:
From Eqs. (70) and (74), we can determine the linearity of Q versus ln C plot as well as the versus ln C plot. Also, we can see that Q decreases with gas concentration, whereas shows the exact opposite behavior. Figure 24b shows the variation of Ea and Q for each VOC concentration (T = 250°C) . From our analysis, we have estimated that for ethanol sensing is lower than that of acetone and isopropanol which justify the larger yield of the composite sensor in terms of ethanol sensing ().
4.4.3 Reproducibility of CuO thin film and WO3-SnO2 thick film
The stability and reproducibility of CuO thin film and WO3-SnO2 thick film composite sensors were studied with respect to acetone and ethanol gases (Figure 25). Excellent baseline recovery was achieved for both the sensors for an extended period of time with marginal change of initial response %. The response % for CuO sensor was recorded for a period of ∼150 days, whereas that for WO3-SnO2 sensor was recorded for ∼500 days. Stable baseline was recorded for CuO thin film sensor, whereas WO3-SnO2 thick film sensor was prone to drift in baseline resistance.
This chapter throws light on detection of common air pollutants including carbon monoxide, carbon dioxide, nitrogen dioxide, volatile organic compounds, and also hydrogen. The permissible exposure limits for these gases act as benchmark for minimum concentration of gas detection. Gas sensing terminologies such as response %, response time, recovery time, selectivity, and stability have been defined. The interaction of semiconducting metal oxide sensor with reducing gas has been illustrated. Factors that affect the performance of a gas sensor, namely, receptor, transducer, and utility functions, have been explained. It was reported that the receptor function is influenced by the grain size of the sensor. The transducer function controls the flow of charge carriers between the electrodes. It is highly dependent on purity of the sensor material. The utility function controls the diffusion processes of gas during response and recovery processes.
A modified wet chemical route was chosen as the method to synthesize sols required for thin film deposition process. The sols were deposited on quartz and glass substrates and annealed at a high temperature. The materials (ZnO-G, ZnO, SnO2, LFCO-ZnO, CuO, and WO3-SnO2) for gas sensors were chosen after extensive literature review and earlier experiments conducted by us in our laboratory.
High response (894%) to 5 ppm NO2 sensing was reported for ZnO-G multilayer as compared to pure ZnO (26%) at 150°C. Also, NO2 detection is selective at 150°C with respect to 500 ppm each of CO, H2, and i-C4H10 gases. The high response and selectivity have been attributed to the presence of p-n junction between ZnO and G grains. The formation of a p-n junction increases the amount of available electrons increasing the gas response. In addition, mixed gas sensing for NO2/CO, NO2/H2, and NO2/i-C4H10 has also been reported. Principal component analyses have been demonstrated as an effective tool for the quantification of these mixed gases. ZnO thin film and SnO2 thick film were investigated for CO and H2 detection. Both materials demonstrated cross-sensitivity to the gases. Cross-sensitivity was addressed through modeling based on Langmuir adsorption mechanism. Activation energy, Ea, and heat of adsorption, Q, were calculated based on the model. Ea and Q values were found to be distinct for each gas and varied as a function of gas concentration. Further, the irreversible nature of ZnO thin film and reversible nature of SnO2 thick film were demonstrated when exposed to a fixed concentration of CO and H2 gases. CO2 sensing characteristics of LFCO-ZnO thin film sensor were investigated. The thin film sensor yields 36% response to 2500 ppm CO2 at 300°C. LFCO-ZnO thin film is cross-sensitive to CO at this temperature. Principal component analyses have been performed to eliminate cross-sensitivity. Further, Langmuir adsorption mechanism is validated in LFCO-ZnO thin film sensor as the variation of conductance and concentration is a straight line passing through origin (not shown). A model has also been developed for the thin film gas sensor relating response with temperature. Modeling of the response with temperature for CO and CO2 sensing yielded parameters unique to both gases. Volatile organic compounds have been detected with a CuO thin film sensor. Both acetone and ethanol show high response with a response time of only a few seconds. The gases have been quantified using principal component analyses. Mixed gas (ethanol/acetone) sensing has also been reported for CuO thin film. The mixed gases have been identified on the basis of the PCA cluster for individual gases. WO3-SnO2 thick film sensor was investigated for ethanol, acetone, and isopropanol sensing characteristics. The thick film sensor yielded highest response to 300 ppm ethanol. Freundlich adsorption mechanism was dominant as the variation of conductance and concentration is nonlinear. Based on this adsorption mechanism, Ea and Q values were estimated for each gas. Both Ea and Q values were found to vary linearly with gas concentration.
The research work was partially supported by research grant obtained from CSIR, Government of India, vide sanction letter no. 03/(1371)/16/EMR-II, dated 10 May 2016 and NNetRa grant vide sanction 5 (1)/2017-NANO dated 28 March 2018, DST/NM/NNetRa/2018 (G)-IIT-KGP dated 21 March 2018.
Conflict of interest
The authors declare no conflict of interest.
|C g||reducing gas concentration|
|EDL||electron depletion layer|
|eV s||Schottky barrier|
|E a||activation energy for gas adsorption during response process|
|E d||activation energy for gas desorption during response process|
|FFT||fast Fourier transform|
|Fθ||total number of active sites|
|G t||conductance transient of the sensor|
|G t response||conductance transient of the sensor during response process|
|G 0||conductance of the sensor in air|
|G 1||conductance of the sensor at site 1|
|HAL||hole accumulation layer|
|K||equilibrium constant for response process|
|K 0||fraction of molecules with energy > E a|
|LFCO||cobalt doped lanthanum ferrite|
|M||molar mass of gas|
|N A||Avogadro number|
|N t||number of gas molecules adsorbed at time t|
|N ∗||total number of available adsorption sites|
|OSHA||Occupational Safety and Health Administration|
|O ads −||chemiadsorbed oxygen|
|O ad − surface||physiadsorbed oxygen|
|PCA||principal component analysis|
|PEL||permissible exposure limit|
|Q||heat of adsorption|
|R||universal gas constant|
|R a||sensor resistance in air|
|R ad||physiadsorbed reducing gas|
|R g||sensor resistance in reducing gas|
|RO ad||adsorbed oxidizing product of reducing gas|
|RO gas||desorbed oxidizing product of reducing gas|
|S||sensor response (%)|
|S area||surface area of a single adsorbed molecule|
|S p||sensor response (%) of p-type semiconductor|
|S n||sensor response (%) of n-type semiconductor|
|SMO||semiconducting metal oxide|
|T opt||operating temperature of sensor|
|VOC||volatile inorganic compound|
|ν||frequency of oscillation of adsorbed gas|
|τ irrev||characteristic response time in irreversible response process|
|τ rev||characteristic response time in reversible response process|
|τ res||response time of sensor|
|τ rec||recovery time of sensor|