Enhanced Gas-sensing Performance Research Articles

Enhanced H2S gas-sensing performance of α-Fe2O3 nanofibers by optimizing process conditions and loading with reduced graphene oxide

We systematically investigated the effects of process conditions and reduced graphene oxide (RGO) loading on the H2S gas-sensing performance of the α-Fe2O3 nanofibers (NFs) fabricated via on-chip electrospinning. The annealing temperature and precursor solution contents strongly influenced on the morphology and structure of the α-Fe2O3 NFs that accordingly affected on the gas-sensing performance. The optimum process conditions with the annealing temperature of 600 °C and the precursor solution contents of 11 wt% PVA and 4.0 wt% Fe(NO3)3.9H2O led to the α-Fe2O3 NF sensors having a high response of ∼6.1 at 1 ppm H2S gas. The RGO loading further improved the gas response, increasing the response to 1 ppm H2S gas up to ∼9.2. Also, the RGO-loaded α-Fe2O3 NF sensors enhanced their selectivity and detection limit as compared with pure α-Fe2O3 NF sensors. The enhanced gas-sensing performance was attributed to the presence of nanograins, the increase of surface-to-volume ratio and the formation of potential barriers at nanograin homojunctions and RGO/α-Fe2O3 heterojunctions.

Gas Sensors: Enhanced Gas Sensing Performance of Organic Field‐Effect Transistors by Modulating the Dimensions of Triethylsilylethynyl‐Anthradithiophene Microcrystal Arrays (Adv. Mater. Interfaces 4/2020)

In article number 1901696, Wi Hyoung Lee and co-workers report a method to increase sensitivity of the 5,11-bis(triethylsilylethynyl)-anthradithiophene (TES-ADT) gas sensors by adopting TES-ADT crystal arrays of various shapes and dimensions. The patterning and crystallization of spin-cast TES-ADT films are simultaneously achieved by the use of a solvent-containing engraved polydimethylsiloxane mold. The increased sensing performance in patterned TES-ADT film is mainly due to the increased area of crystal edges, which provides a diffusion pathway for gas molecules to arrive at the semiconductor-dielectric interface.

Enhanced Gas Sensing Performance of Organic Field‐Effect Transistors by Modulating the Dimensions of Triethylsilylethynyl‐Anthradithiophene Microcrystal Arrays

AbstractThis paper systematically compares the gas sensing properties of organic field‐effect transistors (OFETs) based on patterned 5,11‐bis(triethylsilylethynyl)anthradithiophene (TES‐ADT) films, by adopting TES‐ADT crystal arrays of various shapes and dimensions. The patterning and crystallization of spin‐cast TES‐ADT layers are achieved by the use of a solvent‐containing engraved polydimethylsiloxane (PDMS) mold. Decreasing width of the TES‐ADT pattern enhances gas sensing performance, as well as field‐effect mobility of OFETs. The decreased grain boundary density at narrower line width contributes to the increase of field‐effect mobility. On the other hand, the increased sensing performance is mainly due to the increased area of crystal edges, which provides a diffusion pathway for gas molecules to arrive at the semiconductor‐dielectric interface. This study provides new perspectives on the diffusion pathway of gas molecules in OFET‐based gas sensor, and will be useful for the design of active channel to boost the gas sensing properties of OFETs.

High-Performance Gas Sensor of Polyaniline/Carbon Nanotube Composites Promoted by Interface Engineering.

Inspired by the enhanced gas-sensing performance by the one-dimensional hierarchical structure, one-dimensional hierarchical polyaniline/multi-walled carbon nanotubes (PANI/CNT) fibers were prepared. Interestingly, the simple heating changed the sensing characteristics of PANI from p-type to n-type and n-type PANI and p-type CNTs form p–n hetero junctions at the core–shell interface of hierarchical PANI/CNT composites. The p-type PANI/CNT (p-PANI/CNT) and n-type PANI/CNT (n-PANI/CNT) performed the higher sensitivity to NO2 and NH3, respectively. The response times of p-PANI/CNT and n-PANI/CNT to 50 ppm of NO2 and NH3 are only 5.2 and 1.8 s, respectively, showing the real-time response. The estimated limit of detection for NO2 and NH3 is as low as to 16.7 and 6.4 ppb, respectively. After three months, the responses of p-PANI/CNT and n-PANI/CNT decreased by 19.1% and 11.3%, respectively. It was found that one-dimensional hierarchical structures and the deeper charge depletion layer enhanced by structural changes of PANI contributed to the sensitive and fast responses to NH3 and NO2. The formation process of the hierarchical PANI/CNT fibers, p–n transition, and the enhanced gas-sensing performance were systematically analyzed. This work also predicts the development prospects of cost-effective, high-performance PANI/CNT-based sensors.

Preparation of gas sensor based on Fe-doped reduced graphene/ZnSnO<sub>3</sub> nanocomposite and optimization mechanism of enhanced gas-sensing performance for formaldehyde gas

Formaldehyde is the main indoor air pollution gas, which seriously affects people’s health. ZnSnO3 is a ternary metal oxide material with excellent gas-sensing performance. In this work, the rGO and Fe-doing are used to optimize ZnSnO3 gas-sensing properties further, and the Fe-doped reduced graphene/ZnSnO3 nanocomposite is prepared by the hydrothermal method. The technologies of X-ray diffraction, Roman, Infrared spectrum, X-ray photoelectron spectrum, scanning electron microscope, transmission electron microscope, UV-visible diffuse-reflectance spectrum, and electron spin-resonance spectroscopy are used to characterize the morphology, structure, chemical composition, defects, and energy level of the products, while the corresponding gas-sensing properties of sensitivity, response-recovery time, optimum working temperature, selectivity, stability, and humidity effects are determined using formaldehyde as a target gas. The results revealed that using the rGO improved the specific surface area and electron mobility, formed p-n heterojunction, caused a drastic change in resistance, reduced the optimum working temperature, and enhanced the gas-sensing properties of ZnSnO3. Meanwhile, the Fe-doping increased the oxygen vacancies defects and promoted the Debye electron depletion layer on the surface of ZnSnO3, which furtherly enhanced the gas-sensing performance of ZnSnO3. This work provides the technical support and theoretical basis for the optimization of ZnSnO3 gas-sensing properties.

A General and Straightforward Route to Noble Metal-Decorated Mesoporous Transition-Metal Oxides with Enhanced Gas Sensing Performance.

Controllable and efficient synthesis of noble metal/transition-metal oxide (TMO) composites with tailored nanostructures and precise components is essential for their application. Herein, a general mercaptosilane-assisted one-pot coassembly approach is developed to synthesize ordered mesoporous TMOs with agglomerated-free noble metal nanoparticles, including Au/WO3 , Au/TiO2 , Au/NbOx , and Pt/WO3 . 3-mercaptopropyl trimethoxysilane is applied as a bridge agent to cohydrolyze with metal oxide precursors by alkoxysilane moieties and interact with the noble metal source (e.g., HAuCl4 and H2 PtCl4 ) by mercapto (SH) groups, resulting in coassembly with poly(ethylene oxide)-b-polystyrene. The noble metal decorated TMO materials exhibit highly ordered mesoporous structure, large pore size (≈14-20 nm), high specific surface area (61-138 m2 g-1 ), and highly dispersed noble metal (e.g., Au and Pt) nanoparticles. In the system of Au/WO3 , in situ generated SiO2 incorporation not only enhances their thermal stability but also induces the formation of ε-phase WO3 promoting gas sensing performance. Owning to its specific compositions and structure, the gas sensor based on Au/WO3 materials possess enhanced ethanol sensing performance with a good response (Rair /Rgas = 36-50 ppm of ethanol), high selectivity, and excellent low-concentration detection capability (down to 50 ppb) at low working temperature (200 °C).

Surface Functionalization of Layered Molybdenum Disulfide for the Selective Detection of Volatile Organic Compounds at Room Temperature.

Semiconducting two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are considered promising sensing materials due to the high surface-to-volume ratio and active sensing sites. However, the reported strategies for 2D TMDCs toward sensing of volatile organic compounds (VOCs) present with some drawbacks. These include high operation temperatures, low gas response, and complex fabrication, limiting the development of room-temperature gas sensors. In this study, 2D MoS2 nanoflakes were prepared by liquid-phase exfoliation, and their surface was functionalized with Au nanoparticles (NPs) through a facile solution mixing method. MoS2 decorated with Au NPs with an average size of 10 nm was used as a material platform for an electrochemical sensor to detect a wide variety of VOCs at room temperature. Through dynamic sensing tests, the enhancement of gas-sensing performance in terms of response and selectivity, especially in detecting oxygen-based VOCs (acetone, ethanol, and 2-propanol), was demonstrated. After Au functionalization, the response of the gas sensor to acetone improved by 131% (changing from 13.7% for pristine MoS2 to 31.6% for MoS2-Au(0.5)). Sensing tests under various relative humidity values (10-80%), bending or long-term conditions, indicated the sound robustness and flexibility of the sensor. Density functional theory simulations suggested that the adsorption energy of VOC molecules on MoS2-Au is significantly higher than that on pristine MoS2, contributing to the gas-sensing enhancement; a VOC-sensing mechanism for Au-decorated MoS2 nanoflakes was proposed for the first time for the highly sensitive and selective detection of oxygen-based VOCs.

Fe-Doped ZnO/Reduced Graphene Oxide Nanocomposite with Synergic Enhanced Gas Sensing Performance for the Effective Detection of Formaldehyde.

Here, we report the synthesis of Fe-doped ZnO/reduced graphene oxide (rGO) nanocomposites for gas sensing applications via a one-pot hydrothermal process. A wide range of characterization techniques were used to confirm the successful fabrication of the nanocomposite material and to determine the surface area, the structural and morphological properties, the chemical composition, and the purity of the samples, such as Brunauer–Emmett–Teller, X-ray diffraction, Fourier transform infrared, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, UV–vis spectroscopy, and X-ray photoelectron spectroscopy techniques. The gas sensing performance to formaldehyde was studied thoroughly in a temperature-controlled test chamber. Compared to that of the bare ZnO and ZnO/rGO nanocomposites, the as-prepared 5 atom % Fe-doped ZnO/rGO nanocomposites presented significantly enhanced gas sensing performance to formaldehyde at relatively low temperatures. Whereas most formaldehyde sensors operate at 150 °C and can detect as low as 100 ppm concentrations, the presented sensor can detect 5 ppm formaldehyde at 120 °C. Its fast response–recovery time, high stability, and high selectivity make it an ideal sensor; however, it can exhibit degenerative gas sensing performance at elevated relative humidity. The enhanced gas sensing mechanism was explained as the synergic effect of rGO and Fe doping. The results demonstrate that Fe doping and decorating the nanocomposite with rGO are promising approaches for achieving a superior gas sensing performance for the development of ZnO gas sensors for the detection of formaldehyde.

Enhanced Gas Sensing Performance of Hydrothermal MoS2 Nanosheets by Post-Annealing in Hydrogen Ambient

Abstract A two-dimensional (2D) MoS2 is considered as a next-generation gas sensor owing to its high surface-to-mass ratio and excellent selectivity. However, promoting the gas-sensing capability requires increasing the presence of active sites such as edges and S vacancies for gas adsorption. Herein, we report a process of post-annealing in a hydrogen atmosphere to improve NO2 gas-sensing performance of hydrothermally grown MoS2 nanosheets. The effects of the post-treatment on the morphology, chemical state, and resultant gas-sensing performance were investigated by varying the annealing temperature (100–800 °C). The annealing treatment eliminated residual organics formed during the hydrothermal process and increased both the S vacancies and crystallinity, without significant oxidation. A gas-sensing test using a resistive-type sensor indicated that the sensitivity for NO2 gas of 500 ppm at room temperature was enhanced from 258% to 451% (p-type response) as the annealing temperature increased to 700 °C, followed by a rapid decrease at 800 °C, owing to the n-type response. These findings demonstrate that post-annealing in a hydrogen ambient is effective at increasing the defective sites of 2D MoS2, and hence, the gas-sensing performance.

Enhanced Gas Sensing Performance of Surface‐Activated MoS2 Nanosheets Made by Hydrothermal Method with Excess Sulfur Precursor

Defect engineering of two‐dimensional (2D) nanostructures is of significant importance for achieving enhanced surface properties in optoelectronic, sensors, and catalytic applications. In this work, the authors study the gas‐sensing properties of 2D molybdenum sulfide (MoS2) nanosheets fabricated using a hydrothermal synthetic route with various nominal S/Mo precursor ratios, in order to generate nano‐domains on the surface. The effect of the excess S precursor on the morphology and the resulting gas sensing performance are investigated by varying the S/Mo precursor ratio in the range of 2–12. Nano‐domains are generated on the MoS2 basal plane for all samples, and their evolution became significant as the S/Mo precursor ratio increases. The presence of the nano‐domains provides additional active edge sites, enhancing the surface‐to‐mass ratio and the resulting gas‐sensing properties. Resistive type gas sensing tests using such 2D MoS2 show that, when the S/Mo precursor ratio is increased from 2 to 12, the sensitivity to 500 ppm NO2 gas at room temperature increased from 27 to 213%.

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

Although high-energy facets on metal oxides are usually active and preferred for gas sensing, it is difficult to expose them according to thermodynamics. In this work, nanocomposites of SnO2 and graphene are prepared by a hydrothermal method. The SnO2 nanoparticles change from a lance shape to an octahedral shape as the concentration of HCl in the solution is increased gradually from 6.5 to 10 vol %. However, the SnO2 nanoparticles have an elongated octahedral shape if the concentration of HCl is increased further. The octahedral SnO2 nanoparticles are mainly surrounded by high-surface-energy {221} facets, thus facilitating gas sensing. First-principles calculation shows that the surface energy and adsorption energy of the {221} facets are larger than those of the stable {110} facets, and so, the gas adsorption capacity on the {221} facets is better. Furthermore, because the Fermi level of the SnO2{221} facet is higher than that of graphene, the electrons are transferred from SnO2 nanoparticles to graphene sheets, enabling effective electron exchange between the composite and external NO2 gas. The excellent gas-sensing properties of the octahedral SnO2/graphene composites are ascribed to the high-surface-energy {221} facets exposed.

Development of ethanol and acetone gas sensing performance of MgCo2O4 nanosensors by clad modified fiber optical method

We have successfully synthesized large scale magnesium cobalt oxide (MgCo2O4) nanosheets (NSs) was synthesized by a facile hydrothermal route. These MgCo2O4 NSs were characterized by X-ray diffractometry, N2 adsorption Brunauer-Emmett-Teller method, scanning electron microscopy and transmission electron microscopy analysis. XRD and TEM results suggest that MgCo2O4 was cubic structure with nanosheets and sizes in the range of 200–250 nm diameter and 10–15 nm thickness. The N2 adsorption–desorption analysis indicates that the BET surface area of MgCo2O4 nanoparticles is calculated to be 98.5 m2/g and the pore size distribution is mostly centered at 30 nm. The MgCo2O4 sensor was exposed two type of reducing gases like ethanol and acetone and the results demonstrates that the sensor showed superior gas sensing performance such as high sensitivity (81 × 10−3 k/Pa), rapid response (15 s) and recovery time (19 s) towards ethanol gas. This could be due to large specific surface area with sufficient sensing active sites, more gas adsorption, and achieving the enhanced gas-sensing performance. The improved gas sensing mechanism of the proposed sensor is also discussed in detail.

Microwave-assisted preparation of hierarchical CuO@rGO nanostructures and their enhanced low-temperature H2S-sensing performance

Hierarchical CuO@rGO nanostructures, consisting of CuO nanoparticles (NPs) and reduced graphene oxide (rGO) nanosheets, were prepared with a normal-pressure microwave-assisted in-situ growth process. In the CuO@rGO composites, CuO NPs with a size range of 4–11 nm are uniformly anchored on rGO surfaces. In contrast, CuO particles of large and non-uniform distribution in the CuO-doped rGO nanosheets (CuO/rGO) were prepared by a conventional hydrothermal method. The CuO@rGO sensors show high response and selectivity to H2S (1–10 ppm) at low operating temperatures (50–150 °C). The mass ratio of copper acetate to graphene oxide during preparation (6–10) is found to highly influence the gas-sensing properties, and the 8-CuO@rGO sample shows the highest H2S-sensing performance at 100 °C. Compared with pristine CuO nanocrystals, the 8-CuO@rGO composite exhibits considerably enhanced gas-sensing performance, such as good response, high selectivity and low optimal temperature. The improved H2S-sensing performance of the CuO@rGO composites can be attributed to the synergistic effect in components and their hierarchical nanostructure.

Enhanced gas-sensing performance of metal@ZnO core–shell nanoparticles towards ppb–ppm level benzene: the role of metal–ZnO hetero-interfaces

The enhanced gas-sensing performances of metal@ZnO are correlated to the work function differences between the contacted metal and ZnO.

Selective Functionalization of High-Resolution Cu₂O Nanopatterns via Galvanic Replacement for Highly Enhanced Gas Sensing Performance.

Recently, high-resolution patterned metal oxide semiconductors (MOS) have gained considerable attention for enhanced gas sensing performance due to their polycrystalline nature, ultrasmall grain size (~5 nm), patternable properties, and high surface-to-volume ratio. Herein, we significantly enhanced the sensing performance of that patterned MOS by galvanic replacement, which allows for selective functionalization on ultrathin Cu2O nanopatterns. Based on the reduction potential energy difference between the base channel material (Cu2O) and the decorated metal ion (Pt2+), Pt could be selectively and precisely decorated onto the desired area of the Cu2O nanochannel array. Overall, the Pt-decorated Cu2O exhibited 11-fold higher NO2 (100 ppm) sensing sensitivity as compared to the non-decorated sensing channel, the while the channel device with excessive Pt doping showed complete loss of sensing properties.

A Simple Absorbent Cotton Biotemplate to Fabricate SnO2 Porous Microtubules and Their Gas-Sensing Properties for Chlorine

SnO2 porous microtubules (PMs) were synthesized by a two-step immersion–calcination method, and absorbent cotton is used as the biotemplate. This method is simple, eco-friendly, and cost-efficient. The absorbent cotton as a biotemplate not only supported the formation of microtubule-like structures but also provided a hypoxia atmosphere to introduce oxygen vacancies in the calcination process. The as-synthesized SnO2 PMs maintain the morphology of absorbent cotton—long, curly, and twisted. The gas-sensing property of the SnO2 PMs sensor was systematically investigated for detection of chlorine (Cl2). On the basis of such porous microtubules, the sensor exhibited excellent sensitivity and selectivity to Cl2 at 200 °C. Compared with SnO2 particles, the gas response (Rgas/Rair) of the SnO2 PMs sensor to 10 ppm of Cl2 at 200 °C was increased about 100 times. Finally, the enhanced gas-sensing performance was associated with the hollow morphology of SnO2 PMs and the formation of abundant oxygen vacancies becaus...

High response and selectivity of single crystalline ZnO nanorods modified by In2O3 nanoparticles for n-butanol gas sensing

In2O3-nanoparticle-modified single crystalline ZnO nanorods with enhanced gas-sensing performance were prepared using a facile sol-gel and chemical precipitation process. In2O3 nanoparticles obtained good dispersibility and were modified on the surface ZnO nanorods. One-dimensional ZnO nanorods served as a matrix for In2O3 nanoparticles, which effectively restrained the agglomeration of the In2O3 nanoparticles and increased the active sites for the sensing reaction. The sensing performance of as-prepared In2O3/ZnO and ZnO sensors was systematically tested using n-butanol as the probe analyte. Test results revealed the In2O3/ZnO sensor response was 104.3 at a concentration of 100 ppm, 3.5 and 5.3 times higher than that of the pure ZnO and In2O3 sensor, respectively. The influence of added In2O3 on sensing performance of the In2O3/ZnO nanocomposites was discussed. Test results showed that the optimal atomic ratio of indium was 3.3%. The response of the In2O3/ZnO sensor reached 6.1 at a concentration of 1 ppm, suggesting good detection of n-butanol. The In2O3/ZnO sensor displayed rapid response-recovery speed along with good selectivity and stability. A possible sensing mechanism for In2O3/ZnO was thus proposed based on experimental data and band structure analysis.

Enhanced gas-sensing performance of graphene by doping transition metal atoms: A first-principles study

By performing the first-principles calculations, we investigated the sensitivity and selectivity of transitional metal (TM, TMSc, Ti, V, Cr and Mn) atoms doped graphene toward NO molecule. We firstly calculated the atomic structures, electronic structures and magnetic properties of TM-doped graphene, then studied the adsorptions of NO, N2 and O2 molecules on the TM-doped graphene. By comparing the change of electrical conductivity and magnetic moments after the adsorption of these molecules, we found that the Sc-, Ti- and Mn-doped graphene are the potential candidates in the applications of gas sensor for detection NO molecule.

Nanoporous and crystal evolution in nickel oxide nanosheets for enhanced gas-sensing performance

Understanding the formation mechanism and control of crystal evolution of nanoporous materials is important in the synthesis of nanoporous materials for enhanced gas-sensing performance. Herein, single crystal nickel hydroxide nanosheets were prepared by hydrothermal reaction between nickel chlorine and ammonium hydroxide. Then, the products were subsequently calcined at 400, 500, 600, and 700 °C in air to form nanoporous NiO nanosheets. Evolution of the nanopores and crystal size of the nanoporous NiO nanosheets were investigated X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption/desorption isotherm and photoluminescence excitation spectroscopy, towards gas-sensing applications. Results showed that the calcination temperature altered the crystallinity, morphology, specific surface area, and the porous structure of the NiO nanosheets. Gas-sensing properties of the synthesized NiO nanosheets towards H2 and H2S were investigated to clarify the effect of material characteristics on gas-sensing performance. The NiO nanosheets calcined at 700 °C for 2 h exhibited the highest response to H2 despite having the largest crystal size and the lowest specific surface area. The response of the porous NiO nanosheet device to H2S gas in air and in nitrogen as carrier was also studied to determine the sensing mechanism.

Actinomorphic flower-like n-ZnO/p-ZnFe2O4 composite and its improved NO2 gas-sensing property

Actinomorphic flower-like ZnO/ZnFe2O4 composite has been successfully synthesized by a facile two-step synthesis method. The as-prepared samples have been characterized by XRD, SEM and TEM. Compared with pure-ZnO sensor, the ZnO/ZnFe2O4-based sensor exhibited remarkably higher response, shorter response-recovery time and more superior selective to low-concentration NO2 at low operating temperature of 200 °C. The mechanism of enhanced gas-sensing performance of the ZnO/ZnFe2O4 composite with p-n heterostructure has been investigated. It indicates that the as-prepared ZnO/ZnFe2O4 composite is promising for NO2 gas sensor.