Since the first suggestion, during the 1950s, that high-surface-area metal oxides could be used as conductometric gas sensors enormous efforts have been made to enhance both the selectivity and the sensitivity of such devices, and to reduce their operational power requirements. This development has involved the exploration of response mechanisms, the selection of the most appropriate oxide compositions, the fabrication of two-phase ‘hetero-structures’, the addition of metallic catalyst particles and the optimisation of the manner in which the materials are presented to the gas—the structure and the nanostructure of the sensing elements. Far more of the scientific literature has been devoted to seeking such improvements in metal oxide gas sensors than has been directed at all other solid-state gas sensors together. Recent progress in the research and development of metal oxide gas sensor technology is surveyed in this invited review. The advances that have been made are quite spectacular and the results of individual pieces of work are drawn together here so that trends can be seen. Emerging features include: the significance of n-type/p-type switching, the enhancement of sensing performance of materials through the incorporation of secondary components and the advantages of interrogating sensors with alternating current rather than direct current.

https://iopscience.iop.org/article/10.1088/1361-6501/aa7443/pdf

Progress in the development of semiconducting metal oxide gas sensors: a review

Undoped and 0.2-1.0 mol% Rh-doped SnO2 nanofibers were fabricated using electrospinning combined with calcination treatment. The fibrous morphology of these nanofibers were maintained and the grain size of the SnO2 nanocrystals were greatly decreased after Rh doping. Sensors based on these nanofibers were fabricated through a hot pressing mathod. The gas sensing properties of these nanofibers were investigated systematically. The results indicated that the response to 50 ppm acetone of 0.5 mol% Rh-doped SnO2 nanofibers was 60.6, which was 9.6 times higher than that of undoped SnO2 nanofibers. In addition, the Rh-doped SnO2 nanofibers showed a decreased cross-response to ethanol, whereas pure SnO2 naofibers did not show selective detection of ethanol and acetone gases. The doping of Rh ions into SnO2 nanocrystals modulates the electron concentration, and induces the changes of the oxygen vacancies and chemisorbed oxygen of SnO2 naofibers. Thus, the doping of Rh3+ into SnO2 nanofibers should be a promising method for designing and fabricating acetone gas sensor with high gas sensing performance.

Superior acetone gas sensor based on electrospun SnO2 nanofibers by Rh doping

Toxic gases, such as NOx, SOx, H2S and other S-containing gases, cause numerous harmful effects on human health even at very low gas concentrations. Reliable detection of various gases in low concentration is mandatory in the fields such as industrial plants, environmental monitoring, air quality assurance, automotive technologies and so on. In this paper, the recent advances in electrochemical sensors for toxic gas detections were reviewed and summarized with a focus on NO2, SO2 and H2S gas sensors. The recent progress of the detection of each of these toxic gases was categorized by the highly explored sensing materials over the past few decades. The important sensing performance parameters like sensitivity/response, response and recovery times at certain gas concentration and operating temperature for different sensor materials and structures have been summarized and tabulated to provide a thorough performance comparison. A novel metric, sensitivity per ppm/response time ratio has been calculated for each sensor in order to compare the overall sensing performance on the same reference. It is found that hybrid materials-based sensors exhibit the highest average ratio for NO2 gas sensing, whereas GaN and metal-oxide based sensors possess the highest ratio for SO2 and H2S gas sensing, respectively. Recently, significant research efforts have been made exploring new sensor materials, such as graphene and its derivatives, transition metal dichalcogenides (TMDs), GaN, metal-metal oxide nanostructures, solid electrolytes and organic materials to detect the above-mentioned toxic gases. In addition, the contemporary progress in SO2 gas sensors based on zeolite and paper and H2S gas sensors based on colorimetric and metal-organic framework (MOF) structures have also been reviewed. Finally, this work reviewed the recent first principle studies on the interaction between gas molecules and novel promising materials like arsenene, borophene, blue phosphorene, GeSe monolayer and germanene. The goal is to understand the surface interaction mechanism.

Recent Advances in Electrochemical Sensors for Detecting Toxic Gases: NO₂, SO₂ and H₂S.

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Rare-Earth Doping in Nanostructured Inorganic Materials.

Electronic Noses: From Advanced Materials to Sensors Aided with Data Processing

In this paper, the pure and NiO-decorated ZnO flower-like structures with uniform sizes were synthesized by a simple two-step process As the gas sensing materials of oxide semiconductors gas sensors, their sensing properties were investigated systematically The results manifested that the sensor based on 80 at% NiO-decorated ZnO microflowers showed improved gas sensing properties compared with those of pure ZnO microflowers The introduction of NiO is believed to not only change the quantity of chemisorbed oxygen, but also form the p - n junction with ZnO Thus, the functionalization of ZnO with NiO may be a promising method for designing and fabricating the high performance gas sensor

Facile synthesis and gas sensing properties of the flower-like NiO-decorated ZnO microstructures

A NASICON-based mixed-potential gas sensor utilizing the perovskite-type orthoferrite LaxSm1-xFeO3 (x = 0.2, 0.4, 0.5, 0.6, 0.8) sensing electrode (SE) was designed for SO2 detection. The research mainly put emphasis on the effect of different La3+ doping ratios in LaxSm1-xFeO3 on the sensing characteristics, and the highest response value (-86.5 mV) to 1 ppm SO2 was observed for the sensor using La0.5Sm0.5FeO3-SE. The response value of the fabricated sensor displayed segmentally linear relationship with the logarithm of SO2 concentration in the ranges of 0.005-0.2 ppm and 0.2–5 ppm, and the slopes were −8 and −86 mV/decade, respectively. Otherwise, the low detection limit of the present sensor even could reach to 5 ppb with an accurate response value (-8.4 mV). Meanwhile, the sensor also showed good repeatability, long-term stability and selectivity toward other interfering gases. Furthermore, the electrochemical impendence curves indicated the reason that the sensor attached with La0.5Sm0.5FeO3-SE displayed the higher response values than the other sensors. The result indicated that differences of electrocatalytic activity was caused by diverse La3+ doping content in sensing electrode. The sensing mechanism related to the mixed potential was demonstrated by the polarization curve.

Sub-ppb SO2 gas sensor based on NASICON and LaxSm1-xFeO3 sensing electrode

A series of spinel oxides Co 1-x Zn x Fe 2 O 4 (x = 0, 0.3, 0.5, 0.7 and 1) prepared by a facile sol-gel method were used as sensing electrodes of mixed potential type yttria stabilized-zirconia (YSZ)-based gas sensor for detection of acetone. The present study mainly focused on the influence of different proportions of cobalt and zinc in Co 1-x Zn x Fe 2 O 4 −sensing electrode (SE) on the acetone sensing properties for the sensor, and the results showed that when x = 0.5, the fabricated sensor exhibited a largest sensitivity of −63 mV/decade to 5–100 ppm acetone at 650 °C. The response signal of the present sensor to 50 ppm acetone was as large as −112 mV, and even could achieve low detection limit of 300 ppb at 650 °C, which had a faithful response at this concentration. In addition, the present device also displayed good reliability, good cross sensitivities in the presence of various interfering gases, slight sensitive effect to humidity and good stability over 40 days at 650 °C. Moreover, the polarization curve was measured to further demonstrate the mixed potential mechanism.

Mixed potential type sensor based on stabilized zirconia and Co1-xZnx Fe2O4 sensing electrode for detection of acetone

Highly sensitive detection of Pb2+ and Cu2+ based on ZIF-67/MWCNT/Nafion-modified glassy carbon electrode.

In this article, CeO2-based mixed potential type gas sensor attached with sensing electrodes (SEs) consisting of perovskite-type La1-xSrxCoO3 (x = 0.1, 0.2, 0.3 and 0.5) composite oxides has been developed for acetone detection. The effect of different Sr2+ doping content in La1-xSrxCoO3 oxide material and calcination temperature on acetone sensing characteristics was investigated. It was seen that the sensor using La0.8Sr0.2CoO3 calcined at 1000 °C as SE gave the largest sensing signal to acetone in the concentration range of 1–50 ppm at 600 °C among these sensors. The response value (ΔV) of the sensor was sectionally linearly changed with the logarithm of acetone concentration within the ranges of 1–5 ppm and 5–50 ppm, and the sensitivities were −26 and −49 mV/decade, respectively. In addition, it is noteworthy that the response of the sensor to 1 ppm acetone was approximately −8.7 mV. The sensor also exhibited consistent response and recovery characteristics in five cycles, indicating the sensing device had good reproducibility. Besides, the sensor exhibited the highest response to 10 and 50 ppm acetone and less effective response to other tested gases, it indicates that the sensor has good selectivity. Furthermore, the long-term stability for the sensor to 20 ppm acetone was good because that only a slight attenuation in the sensing response was observed during 30 days. The sensing characteristics of the sensor attached with La0.8Sr0.2CoO3-SE sintered at 1000 °C to acetone were explained in terms of electrocatalytic activity changes induced by effects of doped Sr divalent cation.

Tong Liu

Papers

Facile synthesis and gas sensing properties of the flower-like NiO-decorated ZnO microstructures

Sub-ppb SO2 gas sensor based on NASICON and LaxSm1-xFeO3 sensing electrode

Mixed potential type sensor based on stabilized zirconia and Co1-xZnx Fe2O4 sensing electrode for detection of acetone

Highly sensitive detection of Pb2+ and Cu2+ based on ZIF-67/MWCNT/Nafion-modified glassy carbon electrode.

CeO2-based mixed potential type acetone sensor using La1-xSrxCoO3 sensing electrode