Efficient Detection of SO2 Gas Using SnO2 Based Sensor Loaded with Metal Oxide Catalysts

Recent Advances on Metal Oxide Based Sensors for Environmental Gas Pollutants Detection

Optimizing materials and associated structures for detecting various environmental gas pollutant concentrations has been a major challenge in environmental sensing technology. Semiconducting metal oxides (SMOs) fabricated at the nanoscale are a class of sensor technology in which metallic species are functionalized with various dopants to modify their chemiresistivity and crystalline scaffolding properties. Studies focused on recent advances of gas sensors utilizing metal oxide nanostructures with a special emphasis on the structure-surface property relationships of some typical n-type and p-type SMOs for efficient gas detection are presented. Strategies to enhance the gas sensor performances are also discussed. These oxide material sensors have several advantages such as ease of handling, portability, and doped-based SMO sensing detection ability of environmental gas pollutants at low temperatures. SMO sensors have displayed excellent sensitivity, selectivity, and robustness. In addition, the hybrid SMO sensors showed exceptional selectivity to some CWAs when irradiated with visible light while also displaying high reversibility and humidity independence. Results showed that TiO2 surfaces can sense 50 ppm SO2 in the presence of UV light and under operating temperatures of 298-473 K. Hybrid SMO displayed excellent gas sensing response. For example, a CuO-ZnO nanoparticle network of a 4:1 vol.% CuO/ZnO ratio exhibited responses three times greater than pure CuO sensors and six times greater than pure ZnO sensors toward H2S. This review provides a critical discussion of modified gas pollutant sensing capabilities of metal oxide nanoparticles under ambient conditions, focusing on reported results during the past two decades on gas pollutants sensing.

Acidic Gas Determination Using Indium Tin Oxide-Based Gas Sensors

This work has presented gas sensors based on indium tin oxide (ITO) for the detection of SO2 and NO2. The ITO gas-sensing material was deposited by radio frequency (RF) magnetron sputtering. The properties of gas sensing could be improved by increasing the ratio of SnO2. The response characteristics of the gas sensor for detecting different concentrations of NO2 and SO2 were investigated. In the detection of NO2, the sensitivity was significantly improved by increasing the SnO2 ratio in ITO by 5%, and the response and recovery time were reduced significantly. However, the sensitivity of the sensor decreased with increasing SO2 concentration. From X-ray photoelectron spectroscopy (XPS) analysis, the gas-sensitive response mechanisms were different in the atmosphere of NO2 and SO2. The NO2 was adsorbed by ITO via physisorption but the SO2 had a chemical reaction with the ITO surface. The gas selectivity, temperature dependence, and environmental humidity of ITO-based gas sensors were systematically analyzed. The high detection sensitivity for acidic gas of the prepared sensor presented great potential for acid rain monitoring.

Metal oxide semiconducting nanomaterials for air quality gas sensors: operating principles, performance, and synthesis techniques

To meet requirements in air quality monitoring, sensors are required that can measure the concentration of gaseous pollutants at concentrations down to the ppb and ppt levels, while at the same time they exhibiting high sensitivity, selectivity, and short response/recovery times. Among the different sensor types, those employing metal oxide semiconductors (MOSs) offer great promises as they can be manufactured in easy/inexpensive ways, and designed to measure the concentration of  a wide range of target gases. MOS sensors rely on the adsorption of target gas molecules on the surface of the sensing material and the consequent capturing of electrons from the conduction band that in turn affects their conductivity. Despite their simplicity and ease of manufacturing, MOS gas sensors are restricted by high limits of detection (LOD; which are typically in the ppm range) as well as poor sensitivity and selectivity. LOD and sensitivity can in principle be addressed by nanostructuring the MOSs, thereby increasing their porosity and surface-to-volume ratio, whereas selectivity can be tailored through their chemical composition. In this paper we provide a critical review of the available techniques for nanostructuring MOSs using chemiresistive materials, and discuss how these can be used to attribute desired properties to the end gas sensors. We start by describing the operating principles of chemiresistive sensors, and key material properties that define their performance. The main part of the paper focuses on the available methods for synthesizing nanostructured MOSs for use in gas sensors. We close by addressing the current needs and provide perspectives for improving sensor performance in ways that can fulfill requirements for air quality monitoring.

Atomic simulation of adsorption of SO2 pollutant by metal (Zn, Be)-oxide and Ni-decorated graphene: a first-principles study

Due to the impact of toxic gases on human health, considerable interest has been shown in detecting noxious air pollutants, particularly sulfur dioxide (SO₂), both experimentally and theoretically. This work provides new insights into the adsorbing (SO₂) molecules on the surface of metal-oxide graphitic structures, i.e., Beryllium-Oxide (BeO), Zinc-Oxide (ZnO), and Ni-decorated graphene applying a first-principles study. Computational analyses suggest that the type of binding of SO₂ molecule on BeO and ZnO sheets is physisorption so that binding energies of -0.405 and -0.154 eV were assigned to ZnO and BeO nanosheets in that order. The adsorption energy of SO₂ on metal oxide sheets was much higher than the pristine graphene. Taking pristine graphene as an adsorbent for SO₂ molecule, it was found that such nanomaterial is not an efficient adsorbent due to the weak interactions (-0.157 eV) and low electron charge transfer (0.042 e) present in SO₂/graphene complex. To overcome this issue, graphene nanosheets decorated with nickel atoms were studied for interaction with SO2 molecules; the results indicate that the SO₂ molecules were chemisorbed on Ni-decorated graphene sheets with an adsorption energy of -2.297 eV. Chemisorption of SO₂ molecules on Ni-decorated graphene sheets was proven by the strong orbital hybridization between Ni 3d and sulfur 3p orbitals in the Projected Density of States (PDOS) plot. This work provides useful information about SO₂ adsorption on Ni-decorated graphene sheets in order to develop a new class of gas sensing devices. Superior chemisorption of SO₂ on Ni-decorated graphene sheets compared to the physical adsorption on BeO and ZnO sheets makes Ni-decorated graphene a potential candidate for detecting SO₂ molecules.

Insight into the effect of the continuous testing and aging on the SO2 sensing characteristics of a YSZ (Yttria-stabilized Zirconia)-based sensor utilizing ZnGa2O4 and Pt electrodes

In this paper, YSZ-based mixed potential SO₂ sensor with ZnGa₂O₄ and Pt electrodes was developed and the effect of the continuous testing and aging process on the sensing characteristics was discussed. The results showed that with this process the response of the sensor to SO₂ performed an opposite direction to that in the sensor's initial state. The reason might be that the PtS produced at the Pt electrode increased the electrochemical catalytic activity of the Pt electrode, leading to the mixed potential of the Pt electrode higher than that of the ZnGa₂O₄ electrode. XPS and EDS results proved that a lot of Pt²⁺ and S^2- were produced at Pt electrode after this process. Moreover, vulcanized sensor also performed similar sensing properties to the above aging sensor, which indicated that the produced PtS should be the reason that the sensor performed reverse deflection on sensing properties. In addition, the sensor after sulfuration can detect 0.05-500 ppm SO₂ with the sensitivity being 5 mV/decade to 0.05-1 ppm and 41 mV/decade to 1-500 ppm. The sensor also had a reliable stability during the continuous measurement.