High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

It is demonstrated that the sensing characteristics of a semiconductor gas sensor using SnO2 can be improved by controlling fundamental factors which affect its receptor and transducer functions. The transducer function is deeply related with the microstructure of the elements, i.e., the grain size of SnO2 (D) and the depth of the surface space-charge layer (L). The sensitivity is drastically promoted when D is made comparable to or less than 2L, either by control of D for pure SnO2 elements or by control of the Debye length for impurity-doped elements. On the other hand, the receptor function is drastically modified by the introduction of foreign receptors on the surface of SnO2. In the particular cases of Pd and Ag promoters, the oxides (PdO and Ag2O) formed in air interact with the SnO2 surface to produce an electron-deficient space-charge layer, and this contributes much to promoting the gas sensitivity. For a test gas having a specific reactivity, such specificity can be utilized for exploiting gas-selective receptors, as exemplified by CuOSnO2 and La2O3SnO2 elements, which detect H2S and ethanol gas respectively very sensitively.

New approaches for improving semiconductor gas sensors

Many scientific papers have been written concerning gas sensors for different sensor applications using several sensing principles. This review focuses on sensors and sensor systems for gaseous ammonia. Apart from its natural origin, there are many sources of ammonia, like the chemical industry or intensive life-stock. The survey that we present here treats different application areas for ammonia sensors or measurement systems and different techniques available for making selective ammonia sensing devices. When very low concentrations are to be measured, e.g. less than 2 ppb for environmental monitoring and 50 ppb for diagnostic breath analysis, solid-state ammonia sensors are not sensitive enough. In addition, they lack the required selectivity to other gasses that are often available in much higher concentrations. Optical methods that make use of lasers are often expensive and large. Indirect measurement principles have been described in literature that seems very suited as ammonia sensing devices. Such systems are suited for miniaturization and integration to make them suitable for measuring in the small gas volumes that are normally available in medical applications like diagnostic breath analysis equipment.

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Ammonia sensors and their applications - a review

The gas sensors fabricated by using conducting polymers such as polyaniline (PAni), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) as the active layers have been reviewed. This review discusses the sensing mechanism and configurations of the sensors. The factors that affect the performances of the gas sensors are also addressed. The disadvantages of the sensors and a brief prospect in this research field are discussed at the end of the review.

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Gas Sensors Based on Conducting Polymers

Polymers in sensor applications

Due to the dramatic growth in industrial development and population, the natural atmospheric environment has become polluted and is rapidly deteriorating. Thus, the monitoring and control of such pollutants is imperative to prevent environmental disasters. Conventional analytic instruments for this purpose are time consuming, expensive, and seldom used in real-time in the field. As such, a solid-state gas sensor that is compact, robust, with versatile applications and a low cost, could be an equally effective alternative. Accordingly, this paper presents a brief overview of solid-state gas sensors, which can be classified into semiconductor, capacitor, and solid-electrolyte type sensors, based on their sensing mechanisms and a simple NDIR instrument. Furthermore, the sensing properties of solid-state gas sensors to environmental gases, such as NO X , SO X , CO 2 , volatile organic compounds (VOCs), plus certain other gases, are also classified and summarized.

Environmental gas sensors

A sensor array with 10 sensors integrated on a substrate was developed to recognize various kinds and quantities of volatile organic compounds (VOCs), such as benzene, toluene, ethyl alcohol, methyl alcohol, and acetone. The sensor array consists of gas-sensing materials using SnO 2 as the base material, plus a heating element based on a meandered platinum layer, all deposited on the substrate. The sensors on the sensor array are designed to produce a uniform thermal distribution and show a high and broad sensitivity and reproductivity to low concentrations through the usage of nano-sized sensing materials with high surface areas and different additives. By utilizing the sensing signals of the array with an artificial neural network, a recognition system can then be implemented for the classification and quantification of VOCs. The characteristics of the multi-dimensional sensor signals obtained from 10 sensors are analyzed using the principal component analysis (PCA) technique, and a gas pattern recognizer is implemented using a multi-layer neural network with an error-back-propagation learning algorithm. Simulation and experimental results demonstrated that the proposed gas recognition system is effective in identifying VOCs. For real-time processing, a DSP board can be used to implement the proposed VOC recognition system in conjunction with a neural network.

Recognition of volatile organic compounds using SnO2 sensor array and pattern recognition analysis

The influence of temperature (0, 25, and 40 °C) and relative humidity (5, 10, and 20%) of polypyrrole (PPy) sensor were studied. The PPy sensors were prepared by chemical oxidative method. Initial resistance of the PPy sensor was increased with increase of humidity and decrease of temperature, respectively. Each sensitivity to methanol, water vapor, and mixture gas (methanol and water vapor) was measured, and then compared. Methanol and water vapor were recognized to different type gases in PPy sensors, and adsorption of methanol and water vapor was competed on the surface of sensors. The sensitivity was increased under low humidity and low temperature. The fastest response and recovery time was shown at 25 °C. The reproducibility of PPy sensor was excellent at various humidity conditions. The best operating temperature was investigated at 25 °C.

Sensing behaviors of polypyrrole sensor under humidity condition

An array with five SAW sensors using different kinds of polymers was fabricated to detect chemical agents and their gas response characteristics were extensively investigated. The SAW devices with different interdigital transducer (IDT) electrode line widths of 3, 4, 6, 8 and 20 μm, which corresponded to the central frequencies of 264, 198, 132, 99 and 39.6 MHz, respectively, were designed. The IDT electrodes consisted of 100 finger pairs of 200 nm thick aluminum films. The polymers used as the sensing materials were polyisobutylene (PIB), polyepichlorohydrin (PECH), polydimethylsiloxane (PDMS), polybutadiene (PBD) and polyisoprene (PIP). The thin films were coated on a quartz substrate by spin coating technique. Four simulant gases of chemical warfare agents of dimethylmethylphosphonate (DMMP), acetonitrile (CH 3 CN), dichloromethane (CH 2 Cl 2 ) and dichloropentane (DCP) were used as target gases, instead of the real nerve, blood, choking and vesicant agents. After spin coating of PIB and PECH, the substrate was heated at 65 °C in N 2 ambient for 1 h to remove the cyclohexane and ethylacetate, which were used as solvents. PDMS films were heated at 75 °C in a N 2 flow for 2 h to remove the ethylacetate used as a solvent. And PBD and PIP were heated at 60 °C in a N 2 flow for 1 h to remove the benzene used as a solvent. The sensing characteristics of the SAW sensors were measured by using E-5061A network analyzer. The polymer SAW sensor array showed good selectivity to simulant gases. Principal component analysis (PCA) was adapted to classify the chemical warfare agents.

Duk-Dong Lee

Papers

Environmental gas sensors

Recognition of volatile organic compounds using SnO2 sensor array and pattern recognition analysis

Sensing behaviors of polypyrrole sensor under humidity condition

Fabrication of polymer SAW sensor array to classify chemical warfare agents

Effect of substrate on NO2-sensing properties of WO3 thin film gas sensors