With the improvement of people's living standards and increasing emphasis on environmental protection, higher demands are being placed on the detection of various toxic and harmful gases, the monitoring of air pollution and industrial waste gas, and the detection of food and living environment quality. Gas sensors , as one of the sensory or signal input components, are indispensable. Gas sensors can detect and analyze various gases in real time, possessing advantages such as high sensitivity and short response time. Furthermore, the rapid development of microelectronics, microfabrication technology, and automation and intelligent technologies has made gas sensors smaller, cheaper, and easier to use, leading to their widespread application in military, medical, transportation, environmental protection, quality inspection, anti-counterfeiting, and home applications. However, commercially available gas sensors still have some problems, such as poor selectivity and stability. Further improvements in the performance indicators of gas sensors, the development of new gas-sensitive materials, and the creation of novel gas sensors are receiving increasing attention, with countries worldwide investing heavily in research in this field.
There are many types of gas sensors, and the classification standards vary. Based on the different gas-sensitive materials of the sensors and the different mechanisms and effects of the interaction between the gas-sensitive materials and the gas, they can be mainly divided into semiconductor gas sensors, solid electrolyte gas sensors, contact combustion gas sensors, optical gas sensors, quartz oscillator gas sensors, surface acoustic wave gas sensors, etc.
1 Semiconductor Gas Sensor
Semiconductor gas sensors are classified into metal oxide semiconductor gas sensors and organic semiconductor gas sensors.
1.1 Metal Oxide Semiconductor Gas Sensor
Since the 1960s, metal oxide semiconductor gas sensors have dominated the gas sensor market due to their high sensitivity and rapid response. Early gas sensors primarily used SnO2 and ZnO as gas-sensitive materials. In recent years, several new materials have been researched and developed. Besides a few single metal oxide materials such as WO3, In2O3, TiO2, and Al2O3, the focus of development has been on composite metal oxides and mixed metal oxides, as shown in Table 1. Metal oxide semiconductor sensors can be further divided into resistive and non-resistive types.
1.1.1 Resistive Metal-Oxide-Semiconductor Sensors
SnO2 and ZnO are typical gas-sensitive materials for resistive metal-oxide-semiconductor sensors. They possess both adsorption and catalytic effects and are surface-controlled. However, these semiconductor sensors operate at relatively high temperatures, approximately 200–500°C. To further improve their sensitivity and lower the operating temperature, noble metals (such as Ag, Au, and Pb), activators, and binders such as Al2O3, SiO2, and ZrO2 are typically added to the masterbatch. For example, a ZrO2-SnO2 gas sensor with 1% ZrO2 added exhibits approximately 50 times higher sensitivity for 1×10⁻⁵ H₂S gas compared to a sensor without ZrO2. Adding Pb to SnO2 significantly improves the response time. Surface-doped SnO2/SnO2:Pt bilayer films prepared using powder sputtering technology for CO concentration detection showed a reduction in operating temperature and exhibited high sensitivity within a temperature range from room temperature to 200°C. The selectivity of gas sensors can be improved by adding different additives. Adding Ag to ZnO enhances its sensitivity to flammable gases, adding V₂O₅ increases its sensitivity to Freon, and adding Ga₂O₃ improves its sensitivity to alkanes. Fe₂O₃-based sensors also belong to this category. Nanoscale Fe₂O₃ synthesized using sol-gel and chemical vapor deposition methods exhibits excellent sensitivity to CH₄, H₂, and C₂H₅OH. Adding small amounts of SO₄²⁻ and tetravalent metal ions such as Sn₄⁺ to Fe₂O₃ increases sensitivity by inhibiting grain growth. In recent years, thin-film and integrated circuit technologies have been used to integrate heating elements, temperature sensors, interdigitated electrodes, and gas-sensitive films onto silicon substrates, creating gas-sensitive elements with significantly higher efficiency than conventional polycrystalline films. These sensors are simple in structure, easy to manufacture, and allow for the selection of different sensitive films based on the measured gas, making them a promising new type of semiconductor gas sensor. However, gas-sensitive elements are generally exposed to the atmosphere, and the voltage of the heating element determines the operating temperature of the gas-sensitive element. Therefore, how to eliminate the influence of environmental factors such as humidity and temperature on the measurement has not yet been well resolved.
1.1.2 Non-resistive metal-oxide-semiconductor gas sensor
Non-resistive metal-oxide-semiconductor gas sensors mainly include MOS field-effect transistor gas sensors and diode gas sensors.
The hydrogen-sensitive Pd-gate MOSFET is the earliest developed catalytic metal-gate field-effect gas sensor. When hydrogen reacts with Pd, the threshold voltage of the field-effect transistor changes with the hydrogen concentration, thus detecting hydrogen. This type of gas sensor has a sensitivity to hydrogen at the ppm level and excellent selectivity, but its long-term stability has not yet been well resolved. In addition, Pd-gate MOSFET gas sensors can also detect gases that easily decompose into hydrogen, such as NH3 and H2S. Using YSZ as the gate of a MOSFET and Pt as the metal gate, an oxygen-sensitive MOSFET gas sensor can be fabricated. A. Fuchs et al. used a MOSFET gas sensor with a KI-sensitive film to achieve good O3 detection with excellent resolution in the 20–80 ppb concentration range. By removing the metal gate of the MOSFET and using a La0.7Sr0.3FeO3 nanofilm as the gate, a micron-sized, room-temperature OSFET gas sensor was successfully fabricated for the detection of ethanol.
The principle of transistor-type gas sensors is that the gas adsorbed between the metal and semiconductor interfaces causes changes in the semiconductor's bandgap or the metal's work function. The concentration is determined by observing the changes in the semiconductor's rectification characteristics. A palladium/cadmium sulfide diode sensor can be constructed by evaporating a thin layer of palladium onto indium-doped cadmium sulfide, and can be used to detect hydrogen. In addition, palladium/titanium oxide, palladium/zinc oxide, and platinum/titanium oxide diodes can also be used to fabricate diode sensing elements for hydrogen detection.
1.2 Organic Semiconductor Gas Sensor
Organic semiconductor materials are increasingly valued by researchers at home and abroad due to their ease of handling, simple processing, good room temperature selectivity, low price, ease of integration with microstructure sensors, and ability to be molecularly designed and synthesized according to functional requirements.
Phthalocyanine polymers are representative of organic semiconductor sensitive materials. Their cyclic structure enables electron-transfer relationships between adsorbed gas molecules and organic semiconductors. Different phthalocyanine polymers can be used to fabricate thin-film gas-sensitive elements on detection devices using film-forming techniques such as vacuum sublimation, LB film technology, spin coating, and self-organizing film technology. They can also be used to create sensor arrays for integration with computer pattern recognition technology. Xie Dan et al., based on MOSFETs and the principle of charge-flow capacitors, used a sandwich-type rare-earth metal element protactinium bisphthalocyanine complex Pr[Pc(OC8H17)8]2 as the gas-sensitive material, replacing the gap position in the middle gate. Using LB supramolecular thin film technology, they drew an LB multilayer film, composed of a 1:3 mixture of Pr[Pc(OC8H17)8]2 and octadecyl alcohol (OA), onto a charge-flow field-effect transistor (CFT), forming a novel LB film NO2 gas sensor with a CFT structure. The NO2 detection sensitivity at room temperature can reach 5 ppm. In addition, polypyrrole, anthracene, dinaphthalene, β-carotene, and other organic semiconductor gas-sensitive materials have also attracted attention in recent years.
2 Solid Electrolyte Gas Sensor
Solid electrolytes are a class of solid materials that rely on ions or protons for conduction. The principle of solid electrolyte gas sensors is that the sensitive material generates ions in a specific atmosphere; the migration and conduction of these ions create a potential difference, which is used to determine the gas concentration. Because these sensors exhibit high conductivity, sensitivity, and selectivity at certain temperatures, they are widely used in various fields such as metallurgy, petrochemicals, energy, environmental protection, and aerospace.
ZrO2 oxygen sensors are the most representative solid electrolyte gas sensors. Typically, ZrO2 stabilized by CaO, MgO, or Y2O3 is used as the oxygen ion conductor. They exhibit extremely high sensitivity; the lower limit of measurement for ZrO2 (CaO) sensors at 1000℃ is 10⁻¹³ Pa, with a fast response, enabling continuous tracking and detection. A key characteristic of this type of sensor is that the ions derived from the adsorbed gas in the gas-sensitive material are the same as the mobile ions in the electrolyte, resulting in a simple operating principle.
Currently, research on solid electrolyte gas sensors mainly focuses on two categories: one is sensors where the ions derived from the adsorbed gas in the gas-sensitive material are different from the mobile ions in the electrolyte; the other is sensors where the ions derived from the adsorbed gas in the gas-sensitive material are different from both the mobile ions in the electrolyte and the fixed ions in the material. The principles behind these two categories are relatively complex, and some principles remain unexplained to this day. A small CO2 solid electrolyte gas sensor was fabricated using a composite electrode of NASICON synthesized by the sol-gel method and an auxiliary phase of BaCO3-LiCO3. This device exhibited good linear sensitivity, rapid response recovery, and strong anti-interference capability for CO2. A sensor using NASICON as the solid electrolyte and NaNO2 as the auxiliary electrode showed significantly better sensitivity to NO2 and NO than NaNO2. Various electrolytes, from K2SO4, Na2SO4, Li2SO4, and AgSO4 to NaSiCON, Na-β(β)-Al2O3, and Ag-β-Al2O3, were used as SO2 gas sensors. Solid electrolytes NH-CaCO3 and YST-Au-WO3 were used as NH3 and H2S gas sensors, respectively. Our laboratory fabricated H2O, H2, and SO2 solid electrolyte sensors using single crystals, polycrystalline materials, and LaF3 (CaF2), finding high sensitivity and selectivity. Organic solid electrolytes have also attracted considerable interest from researchers due to their ease of film formation, good elasticity, light weight, ability to form large areas, simple preparation, and readily available raw materials. Common organic solid electrolytes include polyethylene oxide (PEO), uranyl hydrogen phosphate, and Nafion polymers, which are often used as hydrogen ion conductors (proton conductivity) in H2 and water vapor solid electrolyte sensors. Organic gel electrolyte sensors have been used to detect harmful gases such as H2S and PH3 in the air.
3. Contact combustion gas sensor
The principle of contact combustion gas sensors is as follows: when an electric current is applied, the temperature of the gas-sensitive material is approximately 300–600°C. When a combustible gas oxidizes and burns, or oxidizes and burns under the action of a catalyst, the heat of combustion further heats the heating wire, causing a change in its resistance. Measuring this change in resistance allows for the measurement of the gas concentration. The advantages of this type of gas sensor are its good gas selectivity, minimal susceptibility to temperature and humidity, and fast response. It has been widely used in petrochemical plants, mines, bathrooms, and kitchens. Currently, practical contact combustion gas sensors include mass-produced products for detecting H2, LPG, and CH4, followed by products for detecting hydrocarbon and organic solvent vapors. However, they have low sensitivity to low concentrations of combustible gases, and the sensitive element is significantly damaged by catalysts.
4. Optical Gas Sensor
Optical gas sensors are primarily of the spectral absorption type. Their principle is that different gases have different absorption spectra due to differences in their molecular structure, concentration, and energy distribution. This determines the selectivity, discriminative power, and unique determination of gas concentration by spectral absorption gas sensors. If this spectrum can be measured, qualitative and quantitative analysis of the gas can be performed. Currently, various online infrared absorption gas sensors, such as fluid switching type and direct process measurement type, have been developed. In automobile exhaust, the concentrations of CO, CO2, and hydrocarbons, as well as harmful gases SO2 and NO2 in industrial combustion boilers, can all be detected using spectral absorption gas sensors.
Optical gas sensors also include fluorescent and fiber optic chemical material types. Gas molecules, when irradiated with excitation light, enter an excited state and emit fluorescence during their return to the ground state. Since fluorescence intensity is linearly related to the concentration of the analyte gas, fluorescent gas sensors can determine the gas concentration by measuring the fluorescence intensity. Fiber optic chemical material gas sensors involve coating the surface or end face of an optical fiber with a special chemical material. When this material comes into contact with one or more gases, it causes changes in various performance parameters of the optical fiber, such as coupling degree, reflection coefficient, and effective refractive index. These parameters can be detected using methods such as intensity modulation. For example, a palladium film coated on an optical fiber expands upon contact with H2; this gas-induced expansion of the film can be measured by measuring the intensity of the output light from an interferometer.
5 Quartz Resonant Gas Sensor
A quartz resonant gas sensor consists of three parts: a quartz substrate, gold electrodes, and a support. A gas-sensitive film is coated on the electrodes. When molecules of the target gas are adsorbed onto the film, the film's mass increases, thus lowering the resonant frequency of the quartz oscillator. The change in resonant frequency is proportional to the concentration of the target gas. This sensor has a simple structure and high sensitivity, but it can only use gas-sensitive films that operate at room temperature. Using polyethyleneimine (PEI) as the sensitive film, the sensor exhibits excellent gas-sensing characteristics and selectivity for CO2, with a response time of 5 seconds and a recovery time of 2 seconds for a volume of 500 × 10⁻⁶ CO2 gas. Phthalocyanine polymers are also commonly used to fabricate quartz resonant gas sensors. Quartz resonant gas sensors capable of testing NH₃, SO₂, HCl, H₂S, and acetic acid vapor have already been developed.
6 Surface Acoustic Wave Gas Sensor
Surface acoustic wave (SAW) gas sensors have a relatively short history, making them a relative newcomer. Although many challenges remain in practical application, their alignment with the trends of digitalization, integration, and high precision in signal systems has garnered significant attention from many countries worldwide. The propagation speed of SAW is influenced by numerous factors, including ambient temperature, pressure, electromagnetic field, gas properties, the mass of the solid medium, and conductivity. Selecting a suitable sensitive membrane allows for the control of one of these influencing factors. When mass is dominant, the oscillation frequency of the SAW is directly proportional to the density of the gas-sensitive membrane; when conductivity is dominant, the oscillation frequency is inversely proportional to the sheet conductivity of the gas-sensitive membrane. In design, a dual-channel delay line structure is typically employed to compensate for changes in ambient temperature and pressure. Currently, most gas sensors of this type utilize organic membranes as the gas-sensitive material, primarily polyisobutylene and fluorinated polyols, used to detect organic vapors such as styrene and toluene; phthalocyanine polymer films are used to detect gases such as NO₂, NH₃, CO, and SO₂.
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