Tin Dioxide-based Gas Sensors Research Articles

(Invited) Role of Surface p-n Heterojunctions in the Gas Sensing with Smox Based Devices - Operando Studies Insights

In applications as well as in research, the majority of chemoresistive gas sensors developed for the detection of reducing gases is based on semiconducting metal oxides (SMOX), which are loaded or doped with an additional compound - typically noble or other transition metals. The introduction of these additives has proven to be the most suitable approach to overcome significant disadvantages of pristine SMOX gas sensing materials, namely the poor long-term stability, lack of selectivity, high operation temperature, i.e. energy consumption, and strong interference of humidity [1-4]. Because of their practical importance the understanding of the chemical and electrical role of additives is an essential topic of gas sensor research. The initial models, established in the 1980s [5-7], suffered because of the inability of the characterization methods to operate in conditions closed to the actual applications ones. Nevertheless, the proposed mechanisms based on solely chemical or electronic sensitization of SMOX by noble metal additives, namely the spillover or Fermi-level control sensitization mechanism, respectively, played an important role in the general advancement of the field. However, these models do not describe the complexity of the sensitization by metal or metal oxide additives, which arises from interplay of the materials reactivity and gas sensing performance and the mutual influence of surface chemistry and the electronic properties of the solid. Thus, in many cases one has to consider both, a chemical and an electronic contribution to the sensitization effect [8-10]. Besides the presence of a separate additive phase, doping, i.e. the presence of impurity atoms in the SMOX’s lattice, additionally influences the chemical and electrical properties of SMOX.Today, the development of in-situ and operando methods provides a large tool-box to probe structural, chemical and electrical properties and processes during gas sensor operation. These developments play a key role in improving the mechanistic understanding of gas sensing with loaded or doped SMOX and this contribution aims to present revised models based on reviewing and discussing current works on sensitization mechanisms in SMOX materials and to describe the complexity of sensitization mechanisms with regard to the chemical and electronic contributions [11]. A summary of findings shows that the sensitization mechanism of an additive will depend on the nature of the additive itself and the properties of the SMOX, i.e. the selection of the materials, as well as the structure of the additive, which can be controlled material synthesis or introduction procedure. In some cases, sensitization mechanisms will be additive, e.g. the introduction of additional electronic states and reactive surface sites by doping; but in other cases, one effect will dominate, e.g. electronically coupled additives, which deactivate the SMOX surface and control its space charge layer.In order to enable a knowledge-based design of SMOX gas sensing materials, it is important to obtain detailed synthesis-structure-function-relationships and concepts of selecting a combination of SMOX and additive, which allow tuning the gas sensing properties. Modern synthesis methods allow an unprecedented control of structural properties and state-of-the-art characterization techniques allow monitoring gas sensing materials under realistic operation conditions. Thus, it is possible to systematically extent synthesis-structure-function-relationships to a large set of gas sensing materials operation conditions and analytes. This will provide a profound basis to rationally design and optimize gas sensing materials based on the understanding of the fundamental mechanisms and how they are linked to the selected materials and structure. Moseley, P. T. Progress in the Development of Semiconducting Metal Oxide Gas Sensors: A Review. Meas. Sci. Technol. 2017, 28 (8), 082001.Yamazoe, N.; Sakai, G.; Shimanoe, K. Oxide Semiconductor Gas Sensors. Catal. Surv. from Asia 2003, 7 (1), 63–75. https://doi.org/10.1023/A:1023436725457.Jaaniso, R.; Tan, O. K. Semiconductor Gas Sensors; Woodhead Publishing Limited, 2013.Ihokura, K.; Watson, J. The Stannic Oxide Gas Sensor: Principles and Applications, first.; CRS Press, Inc., 1994.Yamazoe, N. New Approaches for Improving Semiconductor Gas Sensors. Sensors Actuators B Chem. 1991, 5 (1–4), 7–19. https://doi.org/10.1016/0925-4005(91)80213-4.Matsushima, S.; Teraoka, Y.; Miura, N.; Yamazoe, N. Electronic Interaction between Metal Additives and Tin Dioxide in Tin Dioxide-Based Gas Sensors. Jpn. J. Appl. Phys. 1988, 27 (10), 1798–1802. https://doi.org/10.1143/JJAP.27.1798.Morrison, S. R. Selectivity in Semiconductor Gas Sensors. Sensors and Actuators 1987, 12 (4), 425–440.Yamazoe, N.; Kurokawa, Y.; Seiyama, T. Effects of Additives on Semiconductor Gas Sensors. Sensors Actuators B Chem. 1983, 4, 283–289. https://doi.org/10.1016/0250-6874(83)85034-3.Degler, D.; Müller, S. A.; Doronkin, D. E.; Wang, D.; Grunwaldt, J.-D.; Weimar, U.; Barsan, N. Platinum Loaded Tin Dioxide: A Model System for Unravelling the Interplay between Heterogeneous Catalysis and Gas Sensing. J. Mater. Chem. A 2018, 6 (5), 2034–2046. https://doi.org/10.1039/C7TA08781K.Morrison, S. R. The Chemical Physics of Surfaces; Plenum Press, New York, 1977.D. Degler, U. Weimar, N. Barsan, Current Understanding of the Fundamental Mechanisms of Doped and Loaded Semiconducting Metal-Oxide-Based Gas Sensing Materials, ACS Sensors 4, 9 (2019) 2228-2249 DOI: 10.1021/acssensors.9b00975

Role of vanadium oxide and palladium multiple loading on the sensitivity and recovery kinetics of tin dioxide based gas sensors

Improving the gas sensing properties by Pd and V2O5 co-loading on the SnO2 attributed to the role of each additive.

A novel way for the synthesis of tin dioxide sol–gel derived thin films: Application to O3 detection at ambient temperature

Abstract This paper deals with the development of SnO 2 -based thin sensitive layers dedicated to ozone detection at ambient temperature. Working at ambient temperature drastically differs from the results reported in literature concerning the need of working at high temperature (about 350 °C) for achieving the detection of ozone using tin-dioxide-based gas sensors. The sensitive layers developed in the framework of this study were elaborated using the sol–gel process following aqueous or organic way. Compared to those obtained from the aqueous way, the layers synthesized following the alcoholic way showed high response to O 3 (58 ppb) when the detection was performed at ambient temperature. The results of the physico-chemical characterization and ozone detection using both layers led to the proposition of an ozone detection mechanism using tin dioxide-based gas sensors.

A New Portable Instrument for In Situ Measurement of Atmospheric Methane Mole Fraction by Applying an Improved Tin Dioxide–Based Gas Sensor

Abstract A new portable instrument based on a tin dioxide natural gas leak detector was developed to monitor the atmospheric methane mixing ratio in areas lacking sufficient infrastructure to sustain a conventional measurement system, such as a large power source, carrier gas supply, and temperature-stabilized laboratory. The effect of water vapor, the dominant factor influencing the stable measurement of methane, was examined. Reducing water vapor to less than 10 ppm in the sample air enabled stable and sensitive detection of atmospheric methane. The selectivity of the sensor to other flammable gases was also examined and improved, without additional power consumption, by applying an additional catalytic layer made of platinum-black powder suspended in quartz wool. The developed system is small (W × D × H = 300 mm × 250 mm × 200 mm) and lightweight (4 kg). The normal power consumption of the detector unit, consisting of a detector, a heater for controlling temperature, and an electronic circuit for each device, is less than 10 W. The precision of the tin dioxide–based sensor system for methane measurement in ambient air was compared with that of a conventional system, a gas chromatograph equipped with a flame ionization detector. Diurnal variations were clearly detected, and an excellent linear relationship was found between the two systems, with a standard deviation of 4 ppb (1σ).

Operando DRIFT Measurements on a tin dioxide based gas sensor

Tin dioxide based gas sensors with 1% Pd doping were investigated by using DRIFT spectroscopy and DC resistance measurements in Operando conditions. The surface was characterized at 250∘C in synthetic air and the change of the surface species was monitored during the exposure to hydrogen and deuterium test gases. In this manner it was possible to follow which kind of hydrogen associated compounds are produced/consumed during the reaction with the test gas.

Detection of hydrogen fluoride using SnO 2-based gas sensors: Understanding of the reactional mechanism

Tin dioxide-based gas sensors are very efficient devices for the detection of hydrogen fluoride in trace levels since amounts lower than 50 ppb can be detected. Considering the working temperatures of tin dioxide-based gas sensors which are included between 25 and 500 °C, the best sensitivity was obtained when the sensor's temperature was maintained at 380 °C. In order to explain why high sensitivity is obtained for this temperature and then to understand the reactional mechanism between HF molecules and tin dioxide surfaces, X-ray photoelectron spectroscopy investigations were performed on tin dioxide samples treated with HF vapors at temperatures ranging from 200 to 500 °C. For this temperature range, the comparison between the electrical response curves and the XPS characterization, led to the consideration of two separate temperature ranges, where the interaction mechanism between HF and SnO 2 can be explained. For temperature lower than 380 °C, the adsorption of HF induces the formation of surface hydroxyl groups and SnF 4 species. In that case, the electrical conductance of the sensitive material gradually increases. Beyond this temperature, water vapors desorb from tin dioxide surfaces and then the electrical conductance is lowered. Finally, for both ranges of temperatures, the interaction mechanism occurring at the gas/detector interface is proposed.

Towards a hybrid micro-device allowing the selective detection of hydrogen fluoride vapours in a complex mixture

In this work, design and fabrication of a hybrid micro-device dedicated to the selective detection of hydrogen fluoride vapours is reported. This micro-device is composed by a gas chromatographic micro-column positioned in front of a tin dioxide-based gas sensor. The efficiency of this hybrid micro-system regarding to reproducibility and reversibility for various HF concentrations was investigated. The influence of the HF concentration on the electrical responses of the sensor is also studied. The experimental results show that this hybrid micro-system is efficient since it allows the elution, separation and detection of hydrogen fluoride at a low temperature close to ambient.

Detection of dimethyl methylphosphonate (DMMP) by tin dioxide-based gas sensor: Response curve and understanding of the reactional mechanism

This paper deals with the detection of dimethyl methylphosphonate (DMMP), using tin dioxide-based gas sensors. One of the characteristics of this device is the functioning temperature, which ranges between 25 and 500 °C. The results presented in the first part of this paper concern the study of the thermal degradation of DMMP vapours, in particular the identification and quantification of light and heavy degraded compounds by several analysis techniques (IR, NMR, GC). The second part of this paper is dedicated to the interpretation of the gas sensor's responses during the detection of DMMP and the determination of the origin of the electrical response of the sensor. Finally in the third part of this paper, we propose a reactional mechanism which describes the interactions between DMMP vapours and SnO 2 surface, that occurs during the detection of this phosporous compound.

A selective gas detection micro-device for monitoring the volatile organic compounds pollution

The discrimination of various chemical compounds is generally impossible by using a single tin dioxide-based gas sensor. This research is focused on the development of a hybrid micro-system to get a sensitive and selective portable device for the detection of volatile organic compounds (VOC) in the air. Thus, micro-fabrication technology has been applied to the development of a gas chromatographic micro-column. This system of separation will be positioned upstream of the gas sensor. In this paper, a complete description of the GC micro-column and of the prototype's architecture is given. The gas sample was formed by a gas mixture of benzene, toluene and two isomers of xylene eluted in synthetic air. This prototype, beyond its small size and its great chemical discrimination, provides fast responses and offers best results with very low temperatures of the GC micro-column.

Development of a gas detection micro-device for hydrogen fluoride vapours

This research concentrates on the sensitivity of a tin dioxide-based gas sensor semiconductor with respect to hydrogen fluoride in the atmosphere. This paper is focused on the development of a separation micro-system, which will be positioned in front of the gas sensor, for the potential use in a complex atmosphere. This study deals with the use of traditional techniques of micro-machining on silicon substrates to manufacture chromatographic micro-columns. The manufacture of the micro-channels and formulation of the stationary phase are discussed. Infrared spectroscopy allows following the evolution of the stationary phase with respect to time. We evaluate the thickness of the stationary phase deposited on the GC micro-channel's internal walls close to the micrometer. A solvent extraction test was performed in order to illustrate the use of the device considered.

Use of a chromatographic column to improve the selectivity of the SnO 2 gas sensors: first approach towards a miniaturised device and selective with hydrogen fluoride vapours

Tin dioxide-based gas sensors were laid out at the exit of a chromatographic column for the selective detection of hydrogen fluoride in the atmosphere. After having selected a stationary phase adapted to this particular molecule, the sensor's signals were studied using various experimental parameters. In this paper a complete description of the experimental device and of the electrical responses is given. For the experiment, the gas sample was formed by a gas mixture of HF, O 2 and N 2. It was found that the optimal mode of detection corresponds to the sensitive element (SnO 2) having a temperature near to 500 °C with a carrier gas of oxidising nature (synthetic air). Moreover, it was found that with this experimental configuration, a minimum concentration of about 0.8 ppm was detectable.

Tin dioxide-based gas sensors for detection of hydrogen fluoride in air

This research concentrates on the sensitivity of semiconductor tin dioxide-based gas sensors to hydrogen fluoride in air. After evaluating the characteristic detection temperature, the sensor's signals were studied for different HF concentrations. Despite the corrosive effects of hydrogen fluoride, a reproducibility of the signal was found. Likewise, we did not observe any long-term degradation to the sensor. For the experiment, the sensor was exposed to a gas mixture formed by HF, O 2, N 2 with a constant flow rate of 150 ml min −1. The semiconductor gas sensor reached maximum sensitivity near 380 °C, and a minimum concentration was detected approximately 50 ppb. Moreover, the detection phenomenon appears to be reversible when considering the electrical response under a constant air flow.

Detection of DEMP vapors using SnO 2-based gas sensors: understanding of the chemical reactional mechanism

Tin dioxide-based gas sensors make it possible to detect diethyl methyl phosphonate vapors (DEMP). However, the responses present drifts that can be observed vs. time. Such a problem raises the question of the reliability of these devices. The results presented in this article are two-fold: first, they are concerned with the study of the thermal degradation of DEMP by thermal degradation in an oxidizing atmosphere, and second, they are involved in the characterization of the chemical degradation on the tin dioxide-based gas sensors. Gas chromatography and infrared spectroscopy shows that, from 300 °C on, the thermal degradation of DEMP leads to the formation of ethylene (gas phase) and ethyl methyl phosphonic acid (condensed phase). Given that thermal degradation may occur at the sensor's surface, which can reach temperatures as high as 500 °C, these results showed that responses obtained using the DEMP vapors are principally due to ethylene and the drift in the responses vs. time is probably due to the adsorption of phosphorous compounds to the sensor's surface. The aim of this article is to describe the reactional mechanism of the DEMP interaction at the surfaces of tin dioxide-based sensors for temperatures ranging from 200 to 500 °C.

Verification of a logarithmic model for estimation of gas concentrations in a mixture for a tin oxide gas sensor response

Changes in electrical conductivity due to a reaction occurring between the original adsorbents and the gases present in the surrounding atmosphere were measured for a tin dioxide-based gas sensor. In this study, a model of the gas sensor response using a relationship between the gas concentration in a mixture and the sensor resistance is proposed. The values of coefficients are determined by a least-squares fit of measured data. Using two gas sensors having different characteristics, the concentrations of gases in a mixture can be evaluated. The proposed method has been applied to the evaluation of the gas concentrations in ammonia–ethanol and CO–ethanol mixtures. Furthermore, we verified our model for a gas mixture of carbon monoxide, propane and methane gases, namely for a three-gas system. The results of the present study indicated that the evaluation of gas concentrations in a mixture is feasible using the proposed model of gas sensor response, which can be used as an inexpensive monitor for air pollution.

Investigation of stability and reliability of tin oxide thin-film for integrated micro-machined gas sensor devices

The stability and reliability of SnO2/Pt, SnO2–Cu/Pt and SnO2/Pt/SnO2/Pt sensitive thin-films used for CO gas sensor device are discussed in this paper. The stability and reliability are very important for the tin dioxide-based gas sensor devices when these devices are to be integrated with standard CMOS circuitry. The drift in output voltage of tin dioxide sensing thin-film device must be minimized. The stability in the output of the tin dioxide thin-film sensing resistor is very essential to implement reliable integrated sensor device, because a small drift in baseline led to large change in the biasing current in the subsequent signal processing circuit. Here, we define the baseline as the output voltage across the two-terminal sensor in the absence of the signal. Another important parameter is the sensitivity of the sensor. The sensitivity of the device should be repeatable over large number of operation cycles. The drift in baseline and sensitivity of devices which are fabricated using SnO2/Pt, SnO2–Cu/Pt and SnO2/Pt/SnO2/Pt sensitive thin-films as gas sensitive material were studied. The sensor device based on SnO2–Cu/Pt thin-film shows good sensing characteristics. It was observed that the tin dioxide thin-film cracks during large number of operation cycles. The annealing of these thin-film at higher temperature or longer time induced the cracks in the sensing thin-film. These cracks are responsible for the drift in baseline and sensitivity of the thin-film. The cracks change the resistivity of the sensitive thin-film, which directly impact the device performance and reliability of the device. Our result shows, that SnO2–Cu/Pt thin-film is more stable as compared to SnO2/Pt and SnO2/Pt/SnO2/Pt thin-film. The change in surface structure during the operation of device was investigated using SEM technique. The accelerated thermal tests on the devices were performed to estimate the lifetime and evaluate the stability of the sensor device.

Detection of thiodiglycol vapours using tin dioxide-based gas sensors: study of the interaction mechanism

This paper deals with the detection of an organosulphur compound, the thiodiglycol (TDG), using tin dioxide-based gas sensors. This compound is thermally unstable. The fact that tin dioxide-based gas sensors have working temperatures that can reach 600°C could be the cause of misinterpretations concerning the response obtained during the detection of TDG vapours. The results presented in the first part of this paper concern the study of the thermal degradation of TDG vapours with, in particular, the precise identification and quantification of light degraded compounds by gas chromatography. Next, the results are linked to the interpretation of the gas sensor’s electrical response during the detection of TDG. In order to understand this response, each degraded compound must be analysed in order to obtain its contribution to the overall electrical response.

The response of tin acetylacetonate and tin dioxide-based gas sensors to hydrogen and alcohol vapours

The aim of this work is to investigate the properties of gas sensors with active layers prepared by pulsed laser deposition (PLD) technology. The active layers were deposited on planar sensor chips with interdigital platinum electrodes. The deposition was carried out from tin dioxide and tin acetylacetonate (SnAcAc) targets by KrF excimer laser. In some cases Pd catalyst was sputtered on the surface of the active layer. The ‘as-deposited’ sensors were submitted to heat treatment. The chemical composition of heat treated active layers was studied by XPS method. This method revealed the oxidation state of palladium and the distribution between organic and inorganic phase during the deposition of SnAcAc. The DC responses of the sensors to a reducing atmosphere containing 1000 ppm of hydrogen, methanol, ethanol, n-propanol and n-butanol were also measured. The maximum sensitivity S (ratio of sensor resistances S= R air/ R gas) achieved 67 for hydrogen, 21 for methanol, 50 for ethanol, 71 for n-propanol and 44 for n-butanol. The temperature of maximum sensitivity ( T max) to distinct gases, the influence of molecular weight of detected gas on T max and the influence of molecular weight on the sensor response speed are also discussed.

Selectivity enhancement of SnO 2 gas sensors: simultaneous monitoring of resistances and temperatures

We present results obtained with microstructured tin dioxide-based gas sensors. Thick films with controlled nanocrystallite sizes were deposited on micromachined substrates. Discrimination between CO, CH 4, and C 2H 5OH, and between CO, CH 4 and H 2 in changing humidity conditions was achieved by simultaneously measuring temperature and resistance changes upon gas exposure. These results make it possible to fulfil the American and European safety standards for CO and CH 4 detection with tin dioxide based gas sensors. The physico-chemical origin of temperature and resistance changes are discussed.

Tin dioxide-based gas sensors for SO2 detection: a chemical interpretation of the increase in sensitivity obtained after a primary detection

This paper deals with the study of the adsorption of sulfur dioxide on tin dioxide surfaces. First, using the method proposed by Walling, we studied surface acidity in order to show the effects of sulfur dioxide treatment. The results obtained were compared with those of a previous infrared analyses on sulfate formed during SO2 treatment. At the same time, we also studied the effects of tin dioxide hydration. We demonstrated that sulfur dioxide is adsorbed to form sulfate groups on basic OH groups. This was performed by blocking the acidic and basic sites of the tin dioxide surface via the use of ammonia and boron trifluoride acid, respectively. The analysis of the results obtained have enabled us to propose a reactional mechanism for the formation of sulfate groups and the increase in surface acidity depending on experimental conditions.

Modelling of SO 2 detection by tin dioxide gas sensors

In studying tin dioxide-based gas sensors under sulphur dioxide, we have confirmed phenomena at the gas/semi-conductor interface during the first and subsequent detections. According to Lalauze et al. [1], the tin dioxide gas sensor is modified by the first SO 2 exposure. It appears that modification is due to the adsorption of sulphite and sulphate on the semi-conductor surface. After the first detection, the gas sensor response peaks at 200°C. We have shown that this peak is connected with quantity of SO 2 adsorbed during detection at high temperatures (>200°C), and not low ones. Finally, we propose two mechanisms to explain these results.