Octahedral SnO2/Graphene Composites with Enhanced Gas-Sensing Performance at Room Temperature

High-Performance Ppb Level NO2 Gas Sensor Based on Colloidal SnO2 Quantum Wires/Ti3C2Tx MXene Composite

Nitrogen dioxide is one origin of air pollution from fossil fuels with the potential to cause great harm to human health in low concentrations. Therefore, low-cost, low-power-consumption sensors for low-concentration NO₂ detection are essential. Herein, heterojunction by SnO₂ quantum wires, a traditional metal oxide NO₂ sensing material, and Ti₃C₂T_x MXene, a novel type of 2D layered material, was synthesized using a simple solvothermal method for enhancing gas-sensing performance and reducing operating temperature. The operating temperature was reduced to 80 °C, with a best performance of 27.8 and a fast response and recovery time (11 s and 23 s, respectively). The SnO₂ and Ti₃C₂T_x MXene composite exhibits high speed and low detection limit due to the construction of the heterojunction with high conductive Ti₃C₂T_x MXene. The selectivity and stability of gas sensors are carried out. This could enable the realization of fast response, high-sensitivity, and selective NO₂ sensing under low operating temperatures.

Light-assisted room temperature gas sensing performance and mechanism of direct Z-scheme MoS2/SnO2 crystal faceted heterojunctions

Light assistance and construction of heterojunctions are both promising means to improve the room temperature gas sensing performance of MoS₂ recently. However, enhancing the separation efficiency of photo-generated carriers at interface and adsorption ability of surface have become the bottleneck problem to further improve the room temperature gas sensing performance of MoS₂-based heterojunctions under light assistance. In the present study, a novel direct Z-scheme MoS_2/SnO₂ heterojunction was designed through crystal facets engineering and its room temperature gas sensing properties under light assistance was studied. It was found that the heterojunction showed outstanding room temperature NO₂ sensing performance with a high response of 208.66 toward 10 ppm NO_2, together with excellent recovery characteristics and selectivity. The gas sensing mechanism study suggested that high-energy {221} crystal facets of SnO₂ and MoS₂ directly formed Z-scheme heterojunction, which could greatly improve the separation efficiency of photo-generated carriers with high redox capacity. Moreover, {221} facets greatly enhanced adsorption ability towards NO₂. This work not only opens up the application of Z-scheme heterojunctions in gas sensing, which will greatly promotes the development of room temperature light-assisted gas sensors, but also provides a new idea for the construction of direct Z-scheme heterojunctions through crystal facets engineering.

MoO3 Nanorods Decorated by PbMoO4 Nanoparticles for Enhanced Trimethylamine Sensing Performances at Low Working Temperature

The gas sensing performance of metal oxides is limited by the lack of conductivity and sensing activity. Inducing the release of more electrons and activating more chemisorbed oxygen ions to participate in the gas sensing reaction can effectively overcome this limitation. The development of a PbMoO₄/MoO₃ heterostructure prepared by the addition of Pb²⁺ ions with MoO₃ nanorods is reported for highly sensitive and selective trimethylamine (TMA) detection. The response of the PbMoO₄/MoO₃ sensor (33.2) to 10 ppm TMA is improved 3-fold compared to the MoO₃ sensor (10.7), and the working temperature is reduced from 170 to 133 °C. The enhanced gas sensing performance and mechanism of PbMoO₄/MoO₃ were demonstrated using the energy band diagram and X-ray photoelectron spectroscopy (XPS) analysis. It is mainly attributed to the following promotion: (1) the induction of Pb²⁺ ions increases the electron density around the Mo element, enabling the decorated MoO₃ to release electrons easily; (2) the formed PbMoO₄/MoO₃ heterojunction endows a high degree of electron transfer at the interface; (3) the formation of the potential barrier causes the device resistance to decrease significantly upon TMA exposure. Finally, the practicability of the sensor was verified by detecting TMA released from Carassius auratus and shrimp to reflect their freshness.

Boosting room-temperature ppb-level NO2 sensing over reduced graphene oxide by co-decoration of α-Fe2O3 and SnO2 nanocrystals

As promising sensing materials, reduced graphene oxide (RGO)-based nanomaterials have drawn considerable attention in the fields of gas monitoring owing to their low operating temperature. However, constructing RGO-based room-temperature gas sensors possessing ppb-level limit of detection with high sensitivity remains challenging. In this work, a series of highly sensitive NO₂ sensors were fabricated using α-Fe₂O₃ and SnO₂ co-decorated RGO hybrids (designated as α-Fe₂O₃/SnO₂-RGO) as sensing materials. They were rationally synthesized by a one-pot hydrothermal method. Compared to SnO₂ modified RGO hybrids (SnO₂-RGO with bandgap of 3.88 eV), the bandgap energy of α-Fe₂O₃/SnO₂-RGO hybrids (3.53 eV) was reduced by adding α-Fe₂O_3; the narrower bandgap facilitated the sensing materials to release more electrons and form more oxygen ions at room temperature. Besides, the high carrier migration of RGO, which served as continuous phase, identical structure with ultrasmall particle size of α-Fe₂O₃ and SnO₂ (about 3-6 nm), and abundant chemisorbed oxygen species on the surface (20.8%) of the sensing materials, as well as their suitable bandgap (3.53 eV) in the sensing materials, significantly improved NO₂ response at room temperature. Among the sensors fabricated, α-Fe₂O₃/SnO₂-RGO-15-based NO₂ sensor had the highest response of 7.4 with a short response time of 59 s towards 1 ppm NO₂; it could even reach a response of 2.6 towards 100 ppb NO₂. Notably, α-Fe₂O₃/SnO₂-RGO-15 sample has excellent capability to recognize NO₂, where the response value (7.4) towards 1 ppm NO₂ is about 7 times higher than that of 100 ppm ammonia and common volatile organic compounds (formaldehyde, toluene, ethanol and acetone). Such NO₂ sensor has superior repeatability with negligible response deviation towards 1 ppm NO₂ for four reversible cycles. This makes it to have a great potential application in the field of NO₂ detection.

Crystal facets effect of tin dioxide nanocrystals on photocatalytic degradation and photo-assisted gas sensing properties

Crystal facets effects have been widely concerned in the field of photocatalysis and gas sensing in recent years. However, little attention have been paid to the crystal facets effect of...

Chemical Surface Adsorption and Trace Detection of Alcohol Gas in Graphene Oxide-Based Acid-Etched SnO2 Aerogels

An acidified SnO₂/rGO aerogel (ASGA) is an attractive contributor in ethanol gas sensing under ultralow concentration because of the sufficient active sites and adsorption pores in SnO₂ and the rGA, respectively. Furthermore, a p-n heterojunction is successfully constructed by the high electron mobility between ASP and rGA to establish a brand-new bandgap of 2.72 eV, where more electrons are released and the surface energy is decreased, to improve the gas sensitivity. The ASGA owns a specific surface area of 256.1 m²/g, far higher than SnO₂ powder (68.7 m²/g), indicating an excellent adsorption performance, so it can acquire more ethanol gas for a redox reaction. For gas-sensing ability, the ASGA exhibits an excellent response of R_a/R_g = 137.4 to 20 ppm of ethanol at the optimum temperature of 210 °C and can reach a response of 1.2 even at the limit detection concentration of 0.25 ppm. After the concentration gradient change test, a nonlinear increase between concentration and sensitivity (S-C curve) is observed, and it indirectly proves the chemical adsorption between ethanol and ASGA, which exhibits charge transfer and improves electron mobility. In addition, a detailed energy band diagram and sensor response diagram jointly depict the gas-sensitive mechanism. Finally, a conversed calculation explains the feasibility of the nonlinear S-C curve from the atomic level, which further verifies the chemical adsorption during the sensing process.

Recent Progress in Graphene Derivatives/Metal Oxides Binary Nanocomposites Based Chemi-resistive Sensors for Disease Diagnosis by Breath Analysis

Background::

The scientific and clinical interest of breath analysis for non-invasive disease diagnosis has been focused by the scientific community over the past decade. This was due to the exhalation of prominent volatile organic compounds (VOCs) corresponding to the metabolic activities in the body and their concentration variation. To identify these biomarkers, various analytical techniques have been used in the past and the threshold concentration was established between a healthy and diseased state. Subsequently, various nanomaterials-based gas sensors were explored for their demand in quantifying these biomarkers for real-time, low cost and portable breathalyzers along with the essential sensor performances.

Methods::

We focus on the classification of graphene derivatives and their composites’ gas sensing efficiency for the application in the development of breathalyzers. The review begins with the feasibility of the application of nanomaterial gas sensors for healthcare applications. Then, we systematically report the gas sensing performance of various graphene derivatives/semiconductor metal oxides (SMO) binary nanocomposites and their optimizing strategies in selective detection of biomarkers specific to diseases. Finally, we provide insights on the challenges, opportunity and future research directions for the development of breathalyzers using other graphene derivatives/SMO binary nanocomposites.

Results::

On the basis of these analyses, graphene and its derivatives/metal oxides based binary nanocomposites have been a choice for gas sensing material owing to their high electrical conductivity and extraordinary thickness-dependent physicochemical properties. Moreover, the presence of oxygen vacancies in SMO does not only alter the conductivity but also accelerates the carrier transport rate and influence the adsorption behavior of target analyte on the sensing materials. Hence researchers are exploring the search of ultrathin graphene and metal oxide counterpart for high sensing performances.

Conclusion::

Their impressive properties compared to their bulk counterpart have been uncovered towards sensitive and selective detection of biomarkers for its use in portable breathalyzers.