Xin Zhou,†,‡ Songyi Lee,† Zhaochao Xu,* and Juyoung Yoon*,† †Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 120-750, Republic of Korea ‡Research Center for Chemical Biology, Department of Chemistry, Yanbian University, Yanjii 133002, People’s Republic of China Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Shahekou, Dalian, Liaoning, People’s Republic of China

Recent Progress on the Development of Chemosensors for Gases.

Improvements in the efficiency of combustion within a vehicle can lead to reductions in the emission of harmful pollutants and increased fuel efficiency. Gas sensors have a role to play in this process, since they can provide real time feedback to vehicular fuel and emissions management systems as well as reducing the discrepancy between emissions observed in factory tests and ‘real world’ scenarios. In this review we survey the current state-of-the-art in using porous materials for sensing the gases relevant to automotive emissions. Two broad classes of porous material – zeolites and metal–organic frameworks (MOFs) – are introduced, and their potential for gas sensing is discussed. The adsorptive, spectroscopic and electronic techniques for sensing gases using porous materials are summarised. Examples of the use of zeolites and MOFs in the sensing of water vapour, oxygen, NOx, carbon monoxide and carbon dioxide, hydrocarbons and volatile organic compounds, ammonia, hydrogen sulfide, sulfur dioxide and hydrogen are then detailed. Both types of porous material (zeolites and MOFs) reveal great promise for the fabrication of sensors for exhaust gases and vapours due to high selectivity and sensitivity. The size and shape selectivity of the zeolite and MOF materials are controlled by variation of pore dimensions, chemical composition (hydrophilicity/hydrophobicity), crystal size and orientation, thus enabling detection and differentiation between different gases and vapours.

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Gas sensing using porous materials for automotive applications

Metal-oxide-semiconductor (MOS) based gas sensors have been considered a promising candidate for gas detection over the past few years. However, the sensing properties of MOS-based gas sensors also need to be further enhanced to satisfy the higher requirements for specific applications, such as medical diagnosis based on human breath, gas detection in harsh environments, etc. In these fields, excellent selectivity, low power consumption, a fast response/recovery rate, low humidity dependence and a low limit of detection concentration should be fulfilled simultaneously, which pose great challenges to the MOS-based gas sensors. Recently, in order to improve the sensing performances of MOS-based gas sensors, more and more researchers have carried out extensive research from theory to practice. For a similar purpose, on the basis of the whole fabrication process of gas sensors, this review gives a presentation of the important role of screening and the recent developments in high throughput screening. Subsequently, together with the sensing mechanism, the factors influencing the sensing properties of MOSs involved in material preparation processes were also discussed in detail. It was concluded that the sensing properties of MOSs not only depend on the morphological structure (particle size, morphology, pore size, etc.), but also rely on the defect structure and heterointerface structure (grain boundaries, heterointerfaces, defect concentrations, etc.). Therefore, the material-sensor integration was also introduced to maintain the structural stability in the sensor fabrication process, ensuring the sensing stability of MOS-based gas sensors. Finally, the perspectives of the MOS-based gas sensors in the aspects of fundamental research and the improvements in the sensing properties are pointed out.

Metal-oxide-semiconductor based gas sensors: screening, preparation, and integration

This review highlights recent developments in the synthesis and unconventional applications of nanosized microporous crystals including framework (zeolites) and layered (clays) type materials. Owing to their microporous nature nanosized zeolites and clays exhibit novel properties, different from those of bulk materials. The factors controlling the formation of nanosized microporous crystals are first revised. The most promising approaches from the viewpoint of large-scale production of nanosized zeolites and clays are discussed in depth. The preparation and advanced applications of nanosized zeolites and clays in free (suspension and powder forms) and fixed (films) forms are summarized. Further the review emphasises the non-conventional applications of new porous materials. A comprehensive analysis of the emerging applications of microporous nanosized crystals in the field of semiconductor industry, optical materials, chemical sensors, medicine, cosmetics, and food industry is presented. Finally, the future needs and perspectives of nanosized microporous materials (zeolites and clays) are addressed.

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Nanosized microporous crystals: emerging applications.

Zinc oxide (ZnO) thin films have been widely used as an effective gas sensor element. In the present study, nanostructured thin films of ZnO were prepared by using the simplistic and economical successive ion layer adsorption and reaction (SILAR) technique. The effects of SILAR cycles on the structural, optical, surface morphological and electrical properties of nanostructured ZnO thin films were investigated. Characterization techniques such as XRD, UV-vis, PL, FESEM, and Hall measurement were utilized to study the physical and chemical properties of the synthesized films. XRD confirms the formation of hexagonal phase structural ZnO thin films. FE-SEM analysis reveals the formation of well-dispersed ZnO nanoparticles having sizes of ∼18–40 nm. The SILAR cycles play a key role in the synthesis of nanostructured ZnO thin films and it is found that, with increasing SILAR cycles, the grain size continues increasing. Optical studies confirm the presence of oxygen vacancies in synthesized ZnO thin films. Finally, the ZnO thin films were exposed to NO 2 gas with a concentration of 100 ppb–200 ppm and the resulting resistance transient was recorded. The nanostructured ZnO thin films synthesized at 30 SILAR cycles displays an enhancement of gas sensing performance and exhibit significantly higher responses (∼5% per ppm). Moreover, our ZnO thin-film-based gas sensor is sensitive to very low concentrations of dangerous NO 2 (100 ppb). The sensitive gas sensor used to trace level NO 2 detection, synthesized via simple SILAR route proves the novelty of our work. The present report provides a new direction in fabricating nanostructured ZnO thin films for low-cost and efficient gas sensing applications.

Fabrication of nanostructured ZnO thin films based NO2 gas sensor via SILAR technique

The illicit manufacture of drugs in the 21st century presents a danger to first responders, bystanders and the environment, making its detection important. Electronic noses based on metal oxide semiconducting (MOS) sensors present a potential technology to create devices for such purposes. An array of four thick film MOS gas sensors was fabricated, based on zinc oxide inks. Production took place using a commercially available screen printer, a 3 × 3 mm alumina substrate containing interdigitated electrodes and a platinum heater track. ZnO inks were modified using zeolite β, zeolite Y and mordenite admixtures. The sensors were exposed to four gases commonly found in the clandestine laboratory environment; these were nitrogen dioxide, ethanol, acetone and ammonia. Zeolite modification was found to increase the sensitivity of the sensor, compared to unmodified ZnO sensors, all of which showed strong responses to low ppm concentrations of acetone, ammonia and ethanol and to ppb concentrations of nitrogen dioxide. Machine learning techniques were incorporated to test the selectivity of the sensors. A high level of accuracy was achieved in determining the class of gas observed.

The gas sensing properties of zeolite modified zinc oxide

Port security with a focus on drug trafficking prevention requires inexpensive and portable systems for on-site analysis of containers in order to minimise transit delays. The potential of metal oxide semiconductors for illicit drug detection is explored here. A six-sensor array consisting of WO3 and SnO2 inks was devised. Zeolites H-Y and H-ZSM-5 were incorporated to introduce variations in sensor response. Sensors were tested against acetone, ethanol and toluene as proxies for their use in illicit drug manufacture and against ammonia and nitrogen dioxide as first models of amino- and nitro-containing compounds, given their prevalence in the structural framework of drugs and precursor molecules. Sensor sensitivity and selectivity were greatly enhanced by inclusion of zeolite materials. Admixed sensing materials were found to be particularly sensitive to the gases. Support vector machines were applied to the dataset as classification tools that accurately classified the data according to gas type. The sensing array was successful in targeting and discerning between the tested drug markers. This could be key for illicit drug detection with electronic noses based on MOS technology in the future.

Assessing the potential of metal oxide semiconducting gas sensors for illicit drug detection markers

The gas-sensing properties of orthorhombic and spinel ferrites (LaFeO
 3
, NiFe
 2
O
 4
 and CoFe
 2
O
 4
) prepared by self-propagating high-temperature synthesis (SHS) have been investigated. This is one of the first reports of using an SHS derived powder for gas sensing applications. The gas response of the materials was tested against a wide range of environmentally important gases (ethanol, ethane, ethene, ammonia, propane, and CO) at a range of different operating temperatures. Good gas response behavior was found with excellent selectivity toward ethanol, particularly in the case of the LaFeO
 3
 sensor.

Ferrite Materials Produced From Self-Propagating High-Temperature Synthesis for Gas Sensing Applications

The gas-sensing properties of spinel and orthorhombic ferrites (NiFe2O4, CoFe2O4 and LaFeO3 respectively) prepared by self-propagating high-temperature synthesis (SHS) are reported. This is one of the first reports of using an SHS derived powder for gas sensing applications. The gas response of the materials was investigated against a range of gases (ethanol, ammonia, propane, CO, ethane, ethene) at a variety of operating temperatures. Good gas response behavior was found with excellent selectivity towards ethanol.

Ferrite materials for gas sensing applications

A portable, hand-held instrument for the detection of trace gases uses a battery powered circuit board capable of operating and measuring an array of plural metal oxide semiconductor (MOS) gas sensors of varied sensing material deposition geometry. The instrument includes one unmodified MOS gas sensor (A) and plural MOS sensors (B-E) modified with a crystalline porous solid such as a zeolite or metal oxide material in a varied deposition geometry across the sensor array to provide selectivity to a range of different gases.

Emma Newton

Papers

The gas sensing properties of zeolite modified zinc oxide

Assessing the potential of metal oxide semiconducting gas sensors for illicit drug detection markers

Ferrite Materials Produced From Self-Propagating High-Temperature Synthesis for Gas Sensing Applications

Ferrite materials for gas sensing applications

Hand held device for the detection of trace gases