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13 pages, 44622 KiB

Open AccessArticle

Cu(OH)₂ and CuO Nanorod Synthesis on Piezoresistive Cantilevers for the Selective Detection of Nitrogen Dioxide

by Laurent Schlur, Manuel Hofer, Ahmad Ahmad, Karine Bonnot, Mathias Holz and Denis Spitzer

Sensors 2018, 18(4), 1108; https://doi.org/10.3390/s18041108 - 5 Apr 2018

Cited by 19 | Viewed by 7789

Abstract

Self-controlled active oscillating microcantilevers with a piezoresistive readout are very promising sensitive sensors, despite their small surface. In order to increase this surface and consequently their sensitivity, we nanostructured them with copper hydroxide (Cu(OH)₂) or with copper oxide (CuO) nanorods. The [...] Read more.

Self-controlled active oscillating microcantilevers with a piezoresistive readout are very promising sensitive sensors, despite their small surface. In order to increase this surface and consequently their sensitivity, we nanostructured them with copper hydroxide (Cu(OH)₂) or with copper oxide (CuO) nanorods. The Cu(OH)₂ rods were grown, on a homogeneous copper layer previously evaporated on the top of the cantilever. The CuO nanorods were further obtained by the annealing of the copper hydroxide nanostructures. Then, these copper based nanorods were used to detect several molecules vapors. The results showed no chemical affinity (no formation of a chemical bond) between the CuO cantilevers and the tested molecules. The cantilever with Cu(OH)₂ nanorods is selective to nitrogen dioxide (NO₂) in presence of humidity. Indeed, among all the tested analytes, copper hydroxide has only an affinity with NO₂. Despite the absence of affinity, the cantilevers could even so condensate explosives (1,3,5-trinitro-1,3,5-triazinane (RDX) and pentaerythritol tetranitrate (PETN) on their surface when the cantilever temperature was lower than the explosives source, allowing their detection. We proved that in condensation conditions, the cantilever surface material has no importance and that the nanostructuration is useless because a raw silicon cantilever detects as well as the nanostructured ones. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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12 pages, 23631 KiB

Open AccessArticle

New Digital Metal-Oxide (MOx) Sensor Platform

by Daniel Rüffer, Felix Hoehne and Johannes Bühler

Sensors 2018, 18(4), 1052; https://doi.org/10.3390/s18041052 - 31 Mar 2018

Cited by 82 | Viewed by 16991

Abstract

The application of metal oxide gas sensors in Internet of Things (IoT) devices and mobile platforms like wearables and mobile phones offers new opportunities for sensing applications. Metal-oxide (MOx) sensors are promising candidates for such applications, thanks to the scientific progresses achieved in [...] Read more.

The application of metal oxide gas sensors in Internet of Things (IoT) devices and mobile platforms like wearables and mobile phones offers new opportunities for sensing applications. Metal-oxide (MOx) sensors are promising candidates for such applications, thanks to the scientific progresses achieved in recent years. For the widespread application of MOx sensors, viable commercial offerings are required. In this publication, the authors show that with the new Sensirion Gas Platform (SGP) a milestone in the commercial application of MOx technology has been reached. The architecture of the new platform and its performance in selected applications are presented. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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12 pages, 21182 KiB

Open AccessArticle

Flexible Gas Sensors Employing Octahedral Indium Oxide Films

by Miriam Alvarado, Èric Navarrete, Alfonso Romero, José Luis Ramírez and Eduard Llobet

Sensors 2018, 18(4), 999; https://doi.org/10.3390/s18040999 - 28 Mar 2018

Cited by 21 | Viewed by 5637

Abstract

Indium oxide octahedral nanopowders were obtained from an ionic precursor compound after an oxidation process conducted under a low-oxygen atmosphere. This method was found to produce contamination-free indium oxide nanomaterial with very similar morphological and crystalline properties to the one produced by vapor-phase [...] Read more.

Indium oxide octahedral nanopowders were obtained from an ionic precursor compound after an oxidation process conducted under a low-oxygen atmosphere. This method was found to produce contamination-free indium oxide nanomaterial with very similar morphological and crystalline properties to the one produced by vapor-phase transport, but at significantly lower temperatures and higher yield. The as-synthesized indium oxide was mixed to an organic vehicle and microdrop deposited to form a film bridging the interdigitated silver electrodes patterned on top of a flexible, polyimide (Kapton^®), substrate. The gas sensing properties of the flexible chemoresistors towards ammonia vapors, hydrogen, and nitrogen dioxide were investigated. It was found that these sensors were remarkably sensitive to nitrogen dioxide at a low operating temperature of 150 °C. These results are consistent with the performance of vapor-phase transport synthesized indium oxide octahedra sensors on rigid, ceramic substrates. Therefore, the results presented here pave the way for the mass production of inexpensive gas sensors onto flexible substrates via additive manufacturing. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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13 pages, 8279 KiB

Open AccessArticle

The Effect of Film Thickness on the Gas Sensing Properties of Ultra-Thin TiO₂ Films Deposited by Atomic Layer Deposition

by Rachel L. Wilson, Cristian Eugen Simion, Christopher S. Blackman, Claire J. Carmalt, Adelina Stanoiu, Francesco Di Maggio and James A. Covington

Sensors 2018, 18(3), 735; https://doi.org/10.3390/s18030735 - 1 Mar 2018

Cited by 59 | Viewed by 11419

Abstract

Analyte sensitivity for gas sensors based on semiconducting metal oxides should be highly dependent on the film thickness, particularly when that thickness is on the order of the Debye length. This thickness dependence has previously been demonstrated for SnO₂ and inferred for [...] Read more.

Analyte sensitivity for gas sensors based on semiconducting metal oxides should be highly dependent on the film thickness, particularly when that thickness is on the order of the Debye length. This thickness dependence has previously been demonstrated for SnO₂ and inferred for TiO₂. In this paper, TiO₂ thin films have been prepared by Atomic Layer Deposition (ALD) using titanium isopropoxide and water as precursors. The deposition process was performed on standard alumina gas sensor platforms and microscope slides (for analysis purposes), at a temperature of 200 °C. The TiO₂ films were exposed to different concentrations of CO, CH₄, NO₂, NH₃ and SO₂ to evaluate their gas sensitivities. These experiments showed that the TiO₂ film thickness played a dominant role within the conduction mechanism and the pattern of response for the electrical resistance towards CH₄ and NH₃ exposure indicated typical n-type semiconducting behavior. The effect of relative humidity on the gas sensitivity has also been demonstrated. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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13 pages, 7282 KiB

Open AccessArticle

Synthesis of ZnMn₂O₄ Nanoparticles by a Microwave-Assisted Colloidal Method and their Evaluation as a Gas Sensor of Propane and Carbon Monoxide

by Juan Pablo Morán-Lázaro, Erwin Said Guillen-López, Florentino López-Urias, Emilio Muñoz-Sandoval, Oscar Blanco-Alonso, Héctor Guillén-Bonilla, Alex Guillén-Bonilla, Verónica María Rodríguez-Betancourtt, Marciano Sanchez-Tizapa and María De la Luz Olvera-Amador

Sensors 2018, 18(3), 701; https://doi.org/10.3390/s18030701 - 27 Feb 2018

Cited by 50 | Viewed by 7791

Abstract

Spinel-type ZnMn₂O₄ nanoparticles were synthesized via a simple and inexpensive microwave-assisted colloidal route. Structural studies by X-ray diffraction showed that a spinel crystal phase of ZnMn₂O₄ was obtained at a calcination temperature of 500 °C, which was [...] Read more.

Spinel-type ZnMn₂O₄ nanoparticles were synthesized via a simple and inexpensive microwave-assisted colloidal route. Structural studies by X-ray diffraction showed that a spinel crystal phase of ZnMn₂O₄ was obtained at a calcination temperature of 500 °C, which was confirmed by Raman and UV-vis characterizations. Spinel-type ZnMn₂O₄ nanoparticles with a size of 41 nm were identified by transmission electron microscopy. Pellet-type sensors were fabricated using ZnMn₂O₄ nanoparticles as sensing material. Sensing measurements were performed by exposing the sensor to different concentrations of propane or carbon monoxide at temperatures in the range from 100 to 300 °C. Measurements performed at an operating temperature of 300 °C revealed a good response to 500 ppm of propane and 300 ppm of carbon monoxide. Hence, ZnMn₂O₄ nanoparticles possess a promising potential in the gas sensors field. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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13 pages, 2670 KiB

Open AccessArticle

Using a Second Order Sigma-Delta Control to Improve the Performance of Metal-Oxide Gas Sensors

by Lukasz Kowalski, Joan Pons-Nin, Eric Navarrete, Eduard Llobet and Manuel Domínguez-Pumar

Sensors 2018, 18(2), 654; https://doi.org/10.3390/s18020654 - 23 Feb 2018

Cited by 7 | Viewed by 5799

Abstract

Controls of surface potential have been proposed to accelerate the time response of MOX gas sensors. These controls use temperature modulations and a feedback loop based on first-order sigma-delta modulators to keep constant the surface potential. Changes in the surrounding gases, therefore, must [...] Read more.

Controls of surface potential have been proposed to accelerate the time response of MOX gas sensors. These controls use temperature modulations and a feedback loop based on first-order sigma-delta modulators to keep constant the surface potential. Changes in the surrounding gases, therefore, must be compensated by average temperature produced by the control loop, which is the new output signal. The purpose of this paper is to present a second order sigma-delta control of the surface potential for gas sensors. With this new control strategy, it is possible to obtain a second order zero of the quantization noise in the output signal. This provides a less noisy control of the surface potential, while at the same time some undesired effects of first order modulators, such as the presence of plateaus, are avoided. Experiments proving these performance improvements are presented using a gas sensor made of tungsten oxide nanowires. Plateau avoidance and second order noise shaping is shown with ethanol measurements. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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16 pages, 4922 KiB

Open AccessArticle

Self-Test Procedures for Gas Sensors Embedded in Microreactor Systems

by Andreas Helwig, Angelika Hackner, Gerhard Müller, Dario Zappa and Giorgio Sberveglieri

Sensors 2018, 18(2), 453; https://doi.org/10.3390/s18020453 - 3 Feb 2018

Cited by 5 | Viewed by 7671

Abstract

Metal oxide (MOX) gas sensors sensitively respond to a wide variety of combustible, explosive and poisonous gases. However, due to the lack of a built-in self-test capability, MOX gas sensors have not yet been able to penetrate safety-critical applications. In the present work [...] Read more.

Metal oxide (MOX) gas sensors sensitively respond to a wide variety of combustible, explosive and poisonous gases. However, due to the lack of a built-in self-test capability, MOX gas sensors have not yet been able to penetrate safety-critical applications. In the present work we report on gas sensing experiments performed on MOX gas sensors embedded in ceramic micro-reaction chambers. With the help of an external micro-pump, such systems can be operated in a periodic manner alternating between flow and no-flow conditions, thus allowing repetitive measurements of the sensor resistances under clean air,

R_{0}

, and under gas exposure,

R_{g a s}

, to be obtained, even under field conditions. With these pairs of resistance values, eventual drifts in the sensor baseline resistance can be detected and drift-corrected values of the relative resistance response

R e s p = (R_{0} - R_{g a s}) / R_{0}

can be determined. Residual poisoning-induced changes in the relative resistance response can be detected by reference to humidity measurements taken with room-temperature-operated capacitive humidity sensors which are insensitive to the poisoning processes operative on heated MOX gas sensors. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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11 pages, 1703 KiB

Open AccessArticle

Effect of Humid Aging on the Oxygen Adsorption in SnO₂ Gas Sensors

by Koichi Suematsu, Nan Ma, Ken Watanabe, Masayoshi Yuasa, Tetsuya Kida and Kengo Shimanoe

Sensors 2018, 18(1), 254; https://doi.org/10.3390/s18010254 - 16 Jan 2018

Cited by 49 | Viewed by 6887

Abstract

To investigate the effect of aging at 580 °C in wet air (humid aging) on the oxygen adsorption on the surface of SnO₂ particles, the electric properties and the sensor response to hydrogen in dry and humid atmospheres for SnO₂ resistive-type [...] Read more.

To investigate the effect of aging at 580 °C in wet air (humid aging) on the oxygen adsorption on the surface of SnO₂ particles, the electric properties and the sensor response to hydrogen in dry and humid atmospheres for SnO₂ resistive-type gas sensors were evaluated. The electric resistance in dry and wet atmospheres at 350 °C was strongly increased by humid aging. From the results of oxygen partial pressure dependence of the electric resistance, the oxygen adsorption equilibrium constants (K₁; for O⁻ adsorption, K₂; for O²⁻ adsorption) were estimated on the basis of the theoretical model of oxygen adsorption. The K₁ and K₂ in dry and wet atmospheres at 350 °C were increased by humid aging at 580 °C, indicating an increase in the adsorption amount of both O⁻ and O²⁻. These results suggest that hydroxyl poisoning on the oxygen adsorption is suppressed by humid aging. The sensor response to hydrogen in dry and wet atmosphere at 350 °C was clearly improved by humid aging. Such an improvement of the sensor response seems to be caused by increasing the oxygen adsorption amount. Thus, the humid aging offers an effective way to improve the sensor response of SnO₂ resistive-type gas sensors in dry and wet atmospheres. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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9 pages, 1882 KiB

Open AccessArticle

Analysis of the Sensing Properties of a Highly Stable and Reproducible Ozone Gas Sensor Based on Amorphous In-Ga-Zn-O Thin Film

by Chiu-Hsien Wu, Guo-Jhen Jiang, Kai-Wei Chang, Zu-Yin Deng, Yu-Ning Li, Kuen-Lin Chen and Chien-Chung Jeng

Sensors 2018, 18(1), 163; https://doi.org/10.3390/s18010163 - 9 Jan 2018

Cited by 22 | Viewed by 5529

Abstract

In this study, the sensing properties of an amorphous indium gallium zinc oxide (a-IGZO) thin film at ozone concentrations from 500 to 5 ppm were investigated. The a-IGZO thin film showed very good reproducibility and stability over three test cycles. The ozone concentration [...] Read more.

In this study, the sensing properties of an amorphous indium gallium zinc oxide (a-IGZO) thin film at ozone concentrations from 500 to 5 ppm were investigated. The a-IGZO thin film showed very good reproducibility and stability over three test cycles. The ozone concentration of 60–70 ppb also showed a good response. The resistance change (ΔR) and sensitivity (S) were linearly dependent on the ozone concentration. The response time (T_90-res), recovery time (T_90-rec), and time constant (τ) showed first-order exponential decay with increasing ozone concentration. The resistance–time curve shows that the maximum resistance change rate (dRg/dt) is proportional to the ozone concentration during the adsorption. The results also show that it is better to sense rapidly and stably at a low ozone concentration using a high light intensity. The ozone concentration can be derived from the resistance change, sensitivity, response time, time constant (τ), and first derivative function of resistance. However, the time of the first derivative function of resistance is shorter than other parameters. The results show that a-IGZO thin films and the first-order differentiation method are promising candidates for use as ozone sensors for practical applications. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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3335 KiB

Open AccessArticle

UV-Enhanced Ethanol Sensing Properties of RF Magnetron-Sputtered ZnO Film

by Jinyu Huang, Yu Du, Quan Wang, Hao Zhang, Youfu Geng, Xuejin Li and Xiaoqing Tian

Sensors 2018, 18(1), 50; https://doi.org/10.3390/s18010050 - 26 Dec 2017

Cited by 13 | Viewed by 4976

Abstract

ZnO film was deposited by the magnetron sputtering method. The thickness of ZnO film is approximately 2 μm. The influence of UV light illumination on C₂H₅OH sensing properties of ZnO film was investigated. Gas sensing results revealed that the [...] Read more.

ZnO film was deposited by the magnetron sputtering method. The thickness of ZnO film is approximately 2 μm. The influence of UV light illumination on C₂H₅OH sensing properties of ZnO film was investigated. Gas sensing results revealed that the UV-illuminated ZnO film displays excellent C₂H₅OH characteristics in terms of high sensitivity, excellent selectivity, rapid response/recovery, and low detection limit down to 0.1 ppm. The excellent sensing performance of the sensor with UV activation could be attributed to the photocatalytic oxidation of ethanol on the surface of the ZnO film, the planar film structure with high utilizing efficiency of UV light, high electron mobility, and a good surface/volume ratio of of ZnO film with a relatively rough and porous surface. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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4097 KiB

Open AccessArticle

New ZnO-Based Glass Ceramic Sensor for H₂ and NO₂ Detection

by Mohamed Hassan, Ahmed S. Afify, Mohamed Ataalla, Daniel Milanese and Jean-Marc Tulliani

Sensors 2017, 17(11), 2538; https://doi.org/10.3390/s17112538 - 3 Nov 2017

Cited by 8 | Viewed by 5439

Abstract

In this study, a glass ceramic with a nominal composition 58ZnO:4Bi₂O₃:4WO₃:33.3B₂O₃ was synthesized by melt quenching technique. A gas sensor was then manufactured using a ZnO sol-gel phase as a permanent binder of the [...] Read more.

In this study, a glass ceramic with a nominal composition 58ZnO:4Bi₂O₃:4WO₃:33.3B₂O₃ was synthesized by melt quenching technique. A gas sensor was then manufactured using a ZnO sol-gel phase as a permanent binder of the glass–ceramic to an alumina substrate having interdigitated electrodes. The film sensitivity towards humidity, NH₃, H₂ and NO₂ was studied at different temperatures. X-ray diffraction technique (XRD), field emission- scanning electron microscopy (FE-SEM) and differential thermal analysis (DTA) were used to characterize the prepared material. Though the response in the sub-ppm NO₂ concentration range was not explored, the observed results are comparable with the latest found in the literature. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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4879 KiB

Open AccessArticle

Stable and Fast-Response Capacitive Humidity Sensors Based on a ZnO Nanopowder/PVP-RGO Multilayer

by Hui Yang, Qiangqiang Ye, Ruixue Zeng, Junkai Zhang, Lei Yue, Ming Xu, Zhi-Jun Qiu and Dongping Wu

Sensors 2017, 17(10), 2415; https://doi.org/10.3390/s17102415 - 23 Oct 2017

Cited by 58 | Viewed by 7295

Abstract

In this paper, capacitive-type humidity sensors were prepared by sequentially drop-coating the aqueous suspensions of zinc oxide (ZnO) nanopowders and polyvinyl pyrrolidone–reduced graphene oxide (PVP-RGO) nanocomposites onto interdigitated electrodes. Significant improvements in both sensitivity and linearity were achieved for the ZnO/PVP-RGO sensors compared [...] Read more.

In this paper, capacitive-type humidity sensors were prepared by sequentially drop-coating the aqueous suspensions of zinc oxide (ZnO) nanopowders and polyvinyl pyrrolidone–reduced graphene oxide (PVP-RGO) nanocomposites onto interdigitated electrodes. Significant improvements in both sensitivity and linearity were achieved for the ZnO/PVP-RGO sensors compared with the PVP-RGO/ZnO, PVP-RGO, and ZnO counterparts. Moreover, the produced ZnO/PVP-RGO sensors exhibited rather small hysteresis, fast response-recovery time, and long-term stability. Based on morphological and structural analyses, it can be inferred that the excellent humidity sensing properties of the ZnO/PVP-RGO sensors may be attributed to the high surface-to-volume ratio of the multilayer structure and the supporting roles of the PVP-RGO nanocomposites. The results in this work hence provide adequate guidelines for designing high-performance humidity sensors that make use of the multilayer structure of semiconductor oxide materials and PVP-RGO nanocomposites. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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7331 KiB

Open AccessArticle

Highly Sensitive and Selective Hydrogen Gas Sensor Using the Mesoporous SnO₂ Modified Layers

by Niuzi Xue, Qinyi Zhang, Shunping Zhang, Pan Zong and Feng Yang

Sensors 2017, 17(10), 2351; https://doi.org/10.3390/s17102351 - 14 Oct 2017

Cited by 44 | Viewed by 7470

Abstract

It is important to improve the sensitivities and selectivities of metal oxide semiconductor (MOS) gas sensors when they are used to monitor the state of hydrogen in aerospace industry and electronic field. In this paper, the ordered mesoporous SnO₂ (m-SnO₂) [...] Read more.

It is important to improve the sensitivities and selectivities of metal oxide semiconductor (MOS) gas sensors when they are used to monitor the state of hydrogen in aerospace industry and electronic field. In this paper, the ordered mesoporous SnO₂ (m-SnO₂) powders were prepared by sol-gel method, and the morphology and structure were characterized by X-ray diffraction analysis (XRD), transmission electron microscope (TEM) and Brunauer–Emmett–Teller (BET). The gas sensors were fabricated using m-SnO₂ as the modified layers on the surface of commercial SnO₂ (c-SnO₂) by screen printing technology, and tested for gas sensing towards ethanol, benzene and hydrogen with operating temperatures ranging from 200 °C to 400 °C. Higher sensitivity was achieved by using the modified m-SnO₂ layers on the c-SnO₂ gas sensor, and it was found that the S(c/m2) sensor exhibited the highest response (Ra/Rg = 22.2) to 1000 ppm hydrogen at 400 °C. In this paper, the mechanism of the sensitivity and selectivity improvement of the gas sensors is also discussed. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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4158 KiB

Open AccessArticle

Ag-Modified In₂O₃ Nanoparticles for Highly Sensitive and Selective Ethanol Alarming

by Jinxiao Wang, Zheng Xie, Yuan Si, Xinyi Liu, Xinyuan Zhou, Jianfeng Yang, Peng Hu, Ning Han, Jun Yang and Yunfa Chen

Sensors 2017, 17(10), 2220; https://doi.org/10.3390/s17102220 - 27 Sep 2017

Cited by 20 | Viewed by 5266

Abstract

Pure In₂O₃ nanoparticles are prepared by a facile precipitation method and are further modified by Ag. The synthesized samples are characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Raman and UV-Vis spectra. The results [...] Read more.

Pure In₂O₃ nanoparticles are prepared by a facile precipitation method and are further modified by Ag. The synthesized samples are characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Raman and UV-Vis spectra. The results show the successful heterojunction formation between Ag and In₂O₃. Gas sensing property measurements show that the 5 mol % Ag-modified In₂O₃ sensor has the response of 67 to 50 ppm ethanol, and fast response and recovery time of 22.3 and 11.7 s. The response is over one magnitude higher than that of pure In₂O₃, which can be attributed to the enhanced catalytic activity of Ag-modified In₂O₃ as compared with the pure one. The mechanism of the gas sensor can be explained by the spillover effect of Ag, which enhances the oxygen adsorption onto the surface of In₂O₃ and thus give rise to the higher activity and larger surface barrier height. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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10861 KiB

Open AccessFeature PaperArticle

Co₃O₄ as p-Type Material for CO Sensing in Humid Air

by Svetlana Vladimirova, Valeriy Krivetskiy, Marina Rumyantseva, Alexander Gaskov, Natalia Mordvinova, Oleg Lebedev, Mikhail Martyshov and Pavel Forsh

Sensors 2017, 17(10), 2216; https://doi.org/10.3390/s17102216 - 27 Sep 2017

Cited by 56 | Viewed by 7662

Abstract

Nanocrystalline cobalt oxide Co₃O₄ has been prepared by precipitation and subsequent thermal decomposition of a carbonate precursor, and has been characterized in detail using XRD, transmission electron microscopy, and FTIR spectroscopy. The sensory characteristics of the material towards carbon monoxide [...] Read more.

Nanocrystalline cobalt oxide Co₃O₄ has been prepared by precipitation and subsequent thermal decomposition of a carbonate precursor, and has been characterized in detail using XRD, transmission electron microscopy, and FTIR spectroscopy. The sensory characteristics of the material towards carbon monoxide in the concentration range 6.7–20 ppm have been examined in both dry and humid air. A sensor signal is achieved in dry air at sufficiently low temperatures T = 80–120 °C, but the increase in relative humidity results in the disappearance of sensor signal in this temperature range. At temperatures above 200 °C the inversion of the sensor signal in dry air was observed. In the temperature interval 180–200 °C the sensor signal toward CO is nearly the same at 0, 20 and 60% r.h. The obtained results are discussed in relation with the specific features of the adsorption of CO, oxygen, and water molecules on the surface of Co₃O₄. The independence of the sensor signal from the air humidity combined with a sufficiently short response time at a moderate operating temperature makes Co₃O₄ a very promising material for CO detection in conditions of variable humidity. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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1384 KiB

Open AccessArticle

CO₂ Sensing Characteristics of a La₂O₃/SnO₂ Stacked Structure with Micromachined Hotplates

by Tatsuya Iwata, Kyosuke Matsuda, Kazuhiro Takahashi and Kazuaki Sawada

Sensors 2017, 17(9), 2156; https://doi.org/10.3390/s17092156 - 20 Sep 2017

Cited by 27 | Viewed by 7076

Abstract

Demand for the detection of carbon dioxide (CO

_{2}

) is increasing in various fields, including air-quality monitoring, healthcare, and agriculture. On the other hand, smart gas sensors, in which micromachined gas sensors are integrated with driving circuits, are desirable toward the development [...] Read more.

Demand for the detection of carbon dioxide (CO

_{2}

) is increasing in various fields, including air-quality monitoring, healthcare, and agriculture. On the other hand, smart gas sensors, in which micromachined gas sensors are integrated with driving circuits, are desirable toward the development of the society of the internet of things. In this study, micromachined hotplate-based CO

_{2}

sensors were fabricated and their characteristics were investigated. The sensors have La

_{2}

O

_{3}

/SnO

_{2}

stacked layers as a sensing material and Pt interdigitated electrodes. A CO

_{2}

response of 2.9 for a CO

_{2}

concentration of 1000 ppm was obtained at 350 °C with low power consumption (approximately 17 mW). A relatively large response was obtained compared with previous studies even though a compact sputtered-SnO

_{2}

film was used. This high response was speculated to be due to a significant contribution of the resistance component near the electrode. Furthermore, CO

_{2}

sensing was successfully performed in the CO

_{2}

range of 200–4000 ppm with at least 200-ppm resolution. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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2512 KiB

Open AccessArticle

The Potentiodynamic Bottom-up Growth of the Tin Oxide Nanostructured Layer for Gas-Analytical Multisensor Array Chips

by Fedor S. Fedorov, Dmitry Podgainov, Alexey Varezhnikov, Andrey Lashkov, Michail Gorshenkov, Igor Burmistrov, Martin Sommer and Victor Sysoev

Sensors 2017, 17(8), 1908; https://doi.org/10.3390/s17081908 - 18 Aug 2017

Cited by 12 | Viewed by 6729

Abstract

We report a deposition of the tin oxide/hydroxide nanostructured layer by the potentiodynamic method from acidic nitrate solutions directly over the substrate, equipped with multiple strip electrodes which is employed as a gas-analytical multisensor array chip. The electrochemical synthesis is set to favor [...] Read more.

We report a deposition of the tin oxide/hydroxide nanostructured layer by the potentiodynamic method from acidic nitrate solutions directly over the substrate, equipped with multiple strip electrodes which is employed as a gas-analytical multisensor array chip. The electrochemical synthesis is set to favor the growth of the tin oxide/hydroxide phase, while the appearance of metallic Sn is suppressed by cycling. The as-synthesized tin oxide/hydroxide layer is characterized by mesoporous morphology with grains, 250–300 nm diameter, which are further crystallized into fine SnO₂ poly-nanocrystals following heating to 300 °C for 24 h just on the chip. The fabricated layer exhibits chemiresistive properties under exposure to organic vapors, which allows the generation of a multisensor vector signal capable of selectively distinguishing various vapors. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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4541 KiB

Open AccessArticle

Insight into the Mechanism of CO Oxidation on WO₃(001) Surfaces for Gas Sensing: A DFT Study

by Hua Jin, Hegen Zhou and Yongfan Zhang

Sensors 2017, 17(8), 1898; https://doi.org/10.3390/s17081898 - 17 Aug 2017

Cited by 20 | Viewed by 5872

Abstract

The mechanism of CO oxidation on the WO₃(001) surface for gas sensing performance has been systematically investigated by means of first principles density functional theory (DFT) calculations. Our results show that the oxidation of CO molecule on the perfect WO₃ [...] Read more.

The mechanism of CO oxidation on the WO₃(001) surface for gas sensing performance has been systematically investigated by means of first principles density functional theory (DFT) calculations. Our results show that the oxidation of CO molecule on the perfect WO₃(001) surface induces the formation of surface oxygen vacancies, which results in an increase of the surface conductance. This defective WO₃(001) surface can be re-oxidized by the O₂ molecules in the atmosphere. During this step, the active O₂⁻ species is generated, accompanied with the obvious charge transfer from the surface to O₂ molecule, and correspondingly, the surface conductivity is reduced. The O₂⁻ species tends to take part in the subsequent reaction with the CO molecule, and after releasing CO₂ molecule, the perfect WO₃(001) surface is finally reproduced. The activation energy barriers and the reaction energies associated with above surface reactions are determined, and from the kinetics viewpoint, the oxidation of CO molecule on the perfect WO₃(001) surface is the rate-limiting step with an activation barrier of about 0.91 eV. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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4582 KiB

Open AccessArticle

Electrospinning Hetero-Nanofibers In₂O₃/SnO₂ of Homotype Heterojunction with High Gas Sensing Activity

by Haiying Du, PengJun Yao, Yanhui Sun, Jing Wang, Huisheng Wang and Naisen Yu

Sensors 2017, 17(8), 1822; https://doi.org/10.3390/s17081822 - 9 Aug 2017

Cited by 40 | Viewed by 7637

Abstract

In₂O₃/SnO₂ composite hetero-nanofibers were synthesized by an electrospinning technique for detecting indoor volatile organic gases. The physical and chemical properties of In₂O₃/SnO₂ hetero-nanofibers were characterized and analyzed by X-ray diffraction (XRD), field emission [...] Read more.

In₂O₃/SnO₂ composite hetero-nanofibers were synthesized by an electrospinning technique for detecting indoor volatile organic gases. The physical and chemical properties of In₂O₃/SnO₂ hetero-nanofibers were characterized and analyzed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Energy Dispersive X-Ray Spectroscopy (EDX), specific surface Brunauer–Emmett–Teller (BET) and X-ray photoelectron spectroscopy (XPS). Gas sensing properties of In₂O₃/SnO₂ composite hetero-nanofibers were measured with six kinds of indoor volatile organic gases in concentration range of 0.5~50 ppm at the operating temperature of 275 °C. The In₂O₃/SnO₂ composite hetero-nanofibers sensor exhibited good formaldehyde sensing properties, which would be attributed to the formation of n-n homotype heterojunction in the In₂O₃/SnO₂ composite hetero-nanofibers. Finally, the sensing mechanism of the In₂O₃/SnO₂ composite hetero-nanofibers was analyzed based on the energy-band principle. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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The Morphologies of the Semiconductor Oxides and Their Gas-Sensing Properties

by Tingting Lin, Xin Lv, Shuang Li and Qingji Wang

Sensors 2017, 17(12), 2779; https://doi.org/10.3390/s17122779 - 30 Nov 2017

Cited by 76 | Viewed by 9416

Abstract

Semiconductor oxide chemoresistive gas sensors are widely used for detecting deleterious gases due to low cost, simple preparation, rapid response and high sensitivity. The performance of gas sensor is greatly affected by the morphology of the semiconductor oxide. There are many semiconductor oxide [...] Read more.

Semiconductor oxide chemoresistive gas sensors are widely used for detecting deleterious gases due to low cost, simple preparation, rapid response and high sensitivity. The performance of gas sensor is greatly affected by the morphology of the semiconductor oxide. There are many semiconductor oxide morphologies, including zero-dimensional, one-dimensional, two-dimensional and three-dimensional ones. The semiconductor oxides with different morphologies significantly enhance the gas-sensing performance. Among the various morphologies, hollow nanostructures and core-shell nanostructures are always the focus of research in the field of gas sensors due to their distinctive structural characteristics and superior performance. Herein the morphologies of semiconductor oxides and their gas-sensing properties are reviewed. This review also proposes a potential strategy for the enhancement of gas-sensing performance in the future. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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Inhomogeneous Oxygen Vacancy Distribution in Semiconductor Gas Sensors: Formation, Migration and Determination on Gas Sensing Characteristics

by Jianqiao Liu, Yinglin Gao, Xu Wu, Guohua Jin, Zhaoxia Zhai and Huan Liu

Sensors 2017, 17(8), 1852; https://doi.org/10.3390/s17081852 - 10 Aug 2017

Cited by 34 | Viewed by 6843

Abstract

The density of oxygen vacancies in semiconductor gas sensors was often assumed to be identical throughout the grain in the numerical discussion of the gas-sensing mechanism of the devices. In contrast, the actual devices had grains with inhomogeneous distribution of oxygen vacancy under [...] Read more.

The density of oxygen vacancies in semiconductor gas sensors was often assumed to be identical throughout the grain in the numerical discussion of the gas-sensing mechanism of the devices. In contrast, the actual devices had grains with inhomogeneous distribution of oxygen vacancy under non-ideal conditions. This conflict between reality and discussion drove us to study the formation and migration of the oxygen defects in semiconductor grains. A model of the gradient-distributed oxygen vacancy was proposed based on the effects of cooling rate and re-annealing on semiconductive thin films. The model established the diffusion equations of oxygen vacancy according to the defect kinetics of diffusion and exclusion. We described that the steady-state and transient-state oxygen vacancy distributions, which were used to calculate the gas-sensing characteristics of the sensor resistance and response to reducing gases under two different conditions. The gradient-distributed oxygen vacancy model had the applications in simulating the sensor performances, such as the power law, the grain size effect and the effect of depletion layer width. Full article

(This article belongs to the Special Issue Gas Sensors based on Semiconducting Metal Oxides)

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